For current production lasers, the manufacturers' Web sites often provide basic specifications. For older lasers, it's often difficult to obtain detailed specs so estimates based on physical size, and then testing may be the only option.
All Melles Griot HeNe laser tubes are hard-sealed with essentially unlimited shelf life - 12 years is quoted but for all practical purposes, it is infinite. Most standard tubes have a planar HR mirror with a concave OC mirror with its curvature selected for maximum stability. This long radius hemispherical cavity configuration puts the beam waist at the HR with a slightly diverging beam from the OC. But a compensating curvature on the outer surface of the OC mirror results in a positive lens and the beam that exits the laser is quite well collimated.
The following data came from a variety of sources including an old Melles Griot brochure, the 1999 catalog, and the Melles Griot Web site. Go to "Product Info", "Lasers", "HeNe" or more directly to Melles Griot Lasers, "HeNe". Then click on any location.
Red (632.8 nm):
Minimum e/2 c/2L Supply Nominal Output Beam Diver- Mode Opr/Strt Tube Tube Size Model Power Diam gence Spacing (Rb=75K) Current Diam/Lgth 05-LHR- ----------------------------------------------------------------------------- .4 mW .34 mm 2.40 mR 1360 MHz 1.22/5 kV 3.2 mA 23/118 mm 007 .5 mW .46 mm 1.70 mR 1272 MHz 1.18/5 kV 4.5 mA 25/127 mm 640 .5 mW .47 mm 1.70 mR 1200 MHz 1.29/5 kV 3.3 mA 19/135 mm 002(1) .5 mW .46 mm 1.77 mR 1063 MHz 1.32/5 kV 4.0 mA 25/150 mm 213 .5 mW .49 mm 1.70 mR 1040 MHz 1.25/5 kV 4.5 mA 29/152 mm 700 .8 mW .46 mm 1.77 mR 1063 MHz 1.32/5 kV 4.0 mA 25/150 mm 211 1.0 mW .53 mm 1.50 mR 883 MHz 1.47/8 kV 4.5 mA 29/178 mm 900 1.0 mW .59 mm 1.35 mR 687 MHz 1.79/8 kV 6.5 mA 37/226 mm 111 1.0 mW .66 mm 1.25 mR 683 MHz 1.10/8 kV 3.5 mA 28/227 mm 101 2.0 mW .59 mm 1.35 mR 687 MHz 1.79/10 kV 6.5 mA 37/228 mm 121 2.0 mW .76 mm 1.06 mR 636 MHz 1.71/10 kV 5.0 mA 30/250 mm 073 2.0 mW .72 mm 1.10 mR 612 MHz 1.85/10 kV 6.5 mA 29/255 mm 080 2.5 mW .52 mm 1.53 mR 822 MHz 1.77/10 kV 4.5 mA 25/198 mm 691 4.0 mW .80 mm 1.00 mR 435 MHz 2.35/10 kV 6.5 mA 37/353 mm 140 5.0 mW .80 mm 1.00 mR 438 MHz 2.29/10 kV 6.5 mA 37/353 mm 151 7 mW 1.02 mm .79 mR 373 MHz 2.65/10 kV 6.5 mA 37/410 mm 171 10 mW .65 mm 1.24 mR 341 MHz 2.64/10 kV 6.5 mA 37/440 mm 991 12 mW 1.20 mm 3.40 mR NA-MM 2.09/10 kV 6.5 mA 37/350 mm 185(2) 16 mW 1.47 mm 1.40 mR NA-MM 2.48/10 kV 7.0 mA 37/464 mm 981(2) 17 mW .96 mm .83 mR 267 MHz 3.70/12 kV 7.0 mA 37/600 mm 925 25 mW 1.23 mm .66 mR 165 MHz 5.10/15 kV 8.0 mA 42/930 mm 827 25 mW 1.42 mm 2.40 mR NA-MM 3.20/10 kV 7.0 mA 42/590 mm 831 35 mW 1.23 mm .66 mR 165 MHz 5.10/15 kV 8.0 mA 42/930 mm 927
Notes:
The operating voltage across the tube itself can be found by subtracting the voltage drop across the ballast resistor (I*Rb), from the value listed in the table. Actual starting voltages are typically 3 to 5 times the tube operating voltage (though the specifications may be higher).
Both random and linearly polarized models are available (change the "LHR" to "LHP" for most of those listed above). The only other difference in specifications for red HeNe lasers between these is the price (other-color polarized HeNe lasers tend to have lower output than their similar model randomly polarized counterparts). The price for a complete linearly polarized laser head was 10 to 15 percent higher in a catalog I have so you can imagine how much more the tube itself costs since that price differential is virtually all in the tube (at least in terms of manufacturing cost)!
And speaking of prices, if you have to ask, you can't afford a new HeNe laser! But since you asked, prices (Summer 2002) from Melles Griot vary from around $300 for a 0.5 mW laser head to over $4,000 for one rated at 35 mW (power supply sold separately)! Fortunately, surplus prices tend to be much more reasonable - typically between 5 and 20 percent of these depending on actual age and condition as well as many other factors including your luck in finding a good deal. :)
Green (543.5 nm):
Minimum e/2 c/2L Supply Nominal Output Beam Diver- Mode Opr/Strt Tube Tube Size Model Power Diam gence Spacing (Rb=75K) Current Diam/Lgth 05-LGR- ----------------------------------------------------------------------------- .2 mW .63 mm 1.26 mR 732 MHz 1.56/8 kV 4.5 mA 29/215 mm 025 .2 mW .75 mm .92 mR 373 MHz 2.62/10 kV 6.5 mA 37/410 mm 171 .5 mW .80 mm 1.01 mR 438 MHz 2.39/10 kV 6.5 mA 37/351 mm 141 .5 mW .80 mm 1.01 mR 438 MHz 2.39/10 kV 6.5 mA 37/351 mm 151 .8 mW .89 mm .92 mR 373 MHz 2.62/10 kV 6.5 mA 37/410 mm 173 1.0 mW 1.3 mm 1.00 mR NA-MM 1.87/10 kV 6.5 mA 37/351 mm 161 1.0 mW .80 mm .86 mR 328 MHz 2.75/10 kV 6.5 mA 37/475 mm 293 1.5 mW .86 mm .81 mR 328 MHz 2.75/10 kV 6.5 mA 37/475 mm 193 2.0 mW .86 mm .81 mR 328 MHz 2.75/10 kV 6.5 mA 37/475 mm 393
Yellow (594.1 nm):
Minimum e/2 c/2L Supply Nominal Output Beam Diver- Mode Opr/Strt Tube Tube Size Model Power Diam gence Spacing (Rb=75K) Current Diam/Lgth 05-LYR- ----------------------------------------------------------------------------- .35 mW .63 mm 1.26 mR 732 MHz 1.62/8 kV 4.5 mA 29/215 mm 025 .75 mW .80 mm 1.01 mR 438 MHz 2.43/10 kV 6.5 mA 37/351 mm 151 1.0 mW .75 mm .92 mR 373 MHz 2.59/10 kV 6.5 mA 37/410 mm 171 2.0 mW .75 mm .92 mR 373 MHz 2.59/10 kV 6.5 mA 37/410 mm 173 2.0 mW 1.17 mm 1.00 mR NA-MM 2.09/10 kV 6.5 mA 37/351 mm 161
Orange (611.9 nm):
Minimum e/2 c/2L Supply Nominal Output Beam Diver- Mode Opr/Strt Tube Tube Size Model Power Diam gence Spacing (Rb=75K) Current Diam/Lgth 05-LOR- ----------------------------------------------------------------------------- .5 mW .63 mm 1.26 mR 732 MHz 1.66/8 kV 4.5 mA 29/215 mm 025 2.0 mW .80 mm 1.01 mR 438 MHz 2.49/10 kV 6.5 mA 37/351 mm 151 4.0 mW 1.17 mm 1.00 mR NA-MM 2.07/10 kV 6.5 mA 37/351 mm 161
Infra-Red (1,523 nm):
Minimum e/2 c/2L Supply Nominal Output Beam Diver- Mode Opr/Strt Tube Tube Size Model Power Diam gence Spacing (Rb=75K) Current Diam/Lgth 05-LIR- ----------------------------------------------------------------------------- 0.5 mW 1.26 mm 1.59 mR 438 MHz 2.49/10 kV 6.5 mA 37/351 mm 151 1.0 mW 1.33 mm 1.48 mR 373 MHz 2.97/10 kV 6.0 mA 37/410 mm 171
Infra-Red (3,391 nm):
Minimum e/2 c/2L Supply Nominal Output Beam Diver- Mode Opr/Strt Tube Tube Size Model Power Diam gence Spacing (Rb=75K) Current Diam/Lgth 05-LFR- ----------------------------------------------------------------------------- 1.0 mW .83 mm 1.60 mR 438 MHz 2.50/10 kV 6.0 mA 37/351 mm 151
Note: Some of the listed values for divergence in particular appear to be questionable. For example, for the same beam diameter, diffraction limited divergence should be proportional to wavelength. The discrepency for the 3,391 nm IR tube is particularly striking. Either the divergence or beam diameter are almost certainly incorrect. It probably doesn't matter much though because the 3,391 nm model is no longer manufactured.
Brewster angle window HeNe tubes:
Minimum Supply Supply Nominal Number
Output Voltage Voltage Tube Tube Size Model of
Power Tube Only Rb=68K Current Diam/Lgth 05-LHB- Windows
-------------------------------------------------------------------
1.0 mW 1,430 V 1,870 V 6.5 mA 37/222 mm 270 1
1.0 mW 1,460 V 1,900 V 6.5 mA 37/253 mm 290 2
3.5 mW 1,080 V 1,520 V 6.5 mA 37/265 mm 370 1
4.0 mW 1,030 V 1,470 V 6.5 mA 37/265 mm 570 1
??? mW 1,??? V 1,??? V 6.5 mA 37/265 mm 580 1
6.0 mW 1,430 V 1,870 V 6.5 mA 37/351 mm 670 1
The most common application for one-Brewster HeNe tubes was probably for particle counting since by using an external high quality HR mirror, the intracavity flux can be several watts which makes a speck of anything stand out! (Some large one-Brewster HeNe tubes can do as much as 100 W intracavity, not these!). Passing the air/gas/whatever flow through the cavity of a one-Brewster HeNe laser is similar to passing it through the output beam of a high power laser - at a fraction of the cost (and it's much safer as well since if anything macroscopic in size (like an eyeball or piece of paper) were to block the intracavity beam, lasing simply stops with no damage to vision and no risk of fire!
The LHB models all have HR mirrors that are probably optimal for 632.8 nm (red) though newer versions, at least, may be quite broadband and better than 99.9 percent from 590 to 680 nm so operation at some of the non-632.8 nm wavelengths may be possible. However, older versions may not have such nice HRs.
Other variations on these tubes are also produced (though they may be special order). I was given an 05-LGB-580 which has an HR optimized for 543.5 nm (green). With an external green HR, the behavior is very similar to the red version but with loads of circulating green photons instead of red ones. :) I was told that this tube may have been made for the sole purpose of confirming the quality of the mirrors to be used in normal internal mirror HeNe laser tubes. So, I doubt you could buy 1. Maybe 1,000, but not just 1! Applications for such a tube would be very limited due to the low gain as it stops lasing entirely in a few minutes after cleaning the optics just due to dust settling on the B-window.
The 05-LHB-370, 05-LHB-570, 05-LHB-670, and 05-LHB-580 have wide bores and generally operate with multiple transverse modes to achieve maximum intracavity power in particle counting applications. The 05-LHB-270 and 05-LHB-290 have narrow bores like most conventional HeNe tubes. (The 05-LHB-270 appears physically similar to an 05-LHR-120 except for the Brewster window at one end.) The model 05-LHB-570 is the one-Brewster HeNe tube used in the CLIMET 9048 one-Brewster laser head described in the section: A One-Brewster HeNe Laser Tube. You can't tell from the model numbers but both Melles Griot and Hughes style designs may be used. For example, the 05-LHB-570 looks like a normal Melles Griot tube but with a Brewster angle window frit sealed to the metal end-cap instead of an OC mirror. The 05-LHB-580 looks like a Hughes style tube, but with an optically contacted Brewster window instead of an OC mirror (though some Hughes style polarized HeNe tubes are just one-Brewster tubes with an OC mirror attached to a glass tube that slips over the Brewster stem and is itself glued in place). Thus, the 05-LHB-580 is actually a much higher quality (and more expensive) tube than the 05-LHB-570 but you can't tell this from the catalog listing! Here are diagrams of each type:
One possible explanation of why the Hughes style design is used for the high quality tubes with optically contacted Brewster windows is that since Hughes already produced HeNe tubes with a glass Brewster stem (as noted above), when Melles Griot took over the Hughes HeNe laser product line, making the modifications for the graded seal to accommodate the fused silica Brewster stem (needed to match the expansion coefficient of the fused silica window) was probably easier than starting with a metal end-cap.
Zero degree AR coated window HeNe tubes:
Minimum Supply Supply Nominal Number
Output Voltage Voltage Tube Tube Size Model of
Power Tube Only Rb=68K Current Diam/Lgth 05-WHR- Windows
-------------------------------------------------------------------
4.0 mW 1,030 V 1,470 V 6.5 mA 37/269 mm 570 1
6.0 mW 1,670 V 2,110 V 6.5 mA 37/351 mm 252 2
8.0 mW 1,670 V 2,110 V 6.5 mA 37/351 mm 183 1
Rather than mirrors, one or both ends of these HeNe tubes have optical flats with very high quality AR coatings to permit the use of external mirrors. One advantage of this arrangement is that external optics can be used to control polarization (the output beam of Brewster tubes is always linearly polarized and can't be changed).
The 05-WHR-252 and 05-WHR-183 appear to be identical except for the number of windows - and the loss of 2 mW with the two window version!
Legend for Type: SM=Single mode, TEM00; MM=Multimode, P=Linearly polarized.
Red (632.8 nm):
Model Number
Power Type Head Tube
------------------------------------------------
0.5 mW SM LGR-7656
0.5 mW SM P LGK-7650 LGR-7650
0.5 mW SM LGR-7651
0.5 mW SM LGR-7651A
0.6 mW SM LGK-7655 LGR-7655
0.75 mW SM LGK-7639
0.75-1.0 mW SM LGK-7657
0.8-1.4 mW SM LGR-7655-N
1.0 mW SM LGK-7655-S LGR-7655-S
1.0 mW SM LGK-7641-S
1.2 mW SM LGK-7632 LGR-7632
1.5 mW SM LGR-7649
2.0 mW SM LGR-7621S
2.0 mW SM LGK-7672
2.0 mW SM P LGK-7634 LGR-7634
2.2-3.2 mW SM P LGK-7634
5.0 mW SM LGK-7627 LGR-7627
5.0 mW SM P LGK-7628 LGR-7628
5.0 mW MM LGK-7621-MM LGR-7621-MM
5.2 mW SM P LGK-7628-1 LGR-7628-1
5.5-7.5 mW SM P LGK-7628-L
7.0 mW SM LGK-7627-M
10.0 mW SM LGK-7653-8
10.0 mW SM P LGK-7654-8
10.0 mW MM LGK-7627-MM LGR-7627-MM
12.0 mW? SM LGK-7638
15.0 mW SM LGK-7665
15.0 mW SM P LGK-7665-P
18.0 mW SM LGK-7665-18
18.0 mW SM P LGK-7665-P18
20.0 mW SM LGK-7665-20
20.0 mW MM LGK-7658-7
25.0 mW SM P LGK-7626-L
25.0 mW SM P LGK-7626
25.0 mW SM P LGK-7676-L
28.0 mW SM P LGK-7676
30.0 mW SM P LGK-7626-S
30.0 mW SM P LGK-7676-S
Green (543.5 nm):
Model Number
Power Type Head Tube
---------------------------------------------------
0.5 mW SM LGK-7770 LGR-7770
0.5 mW SM P LGK-7774
0.5 mW SM P LGK-7786-P50
0.75 mW SM P LGK-7786-P75
1.0 mW SM LGK-7785-P100
1.0 mW SM LGK-7770-S
1.05 mW SM P LGK-7786-P
1.2 mW SM LGK-7785-P120
1.5 mW SM LGK-7785-P150
2.0 mW SM LGK-7785-P200
2.5 mW SM LGK-7785-P250
Yellow (594.1 nm):
Power Type Model
-------------------------------------
1.5 mW SM LGK-7511
2.0 mW SM LGK-7512 P
Orange (611.9 nm):
Power Type Model
------------------------------------
2.0 mW SM LGK-7411
More complete specifications are available at the LASOS Web site.
The LGR-7638 laser tube is generally unremarkable except for the reasonably precise three-screw mirror adjuster at the cathode end. There is enough range that as long as you don't lose the beam entirely, it should be low risk to tweak mirror alignment on this laser. In all other respects, the tube looks like a stretch version of shorter Siemens/LASOS bare tubes with two spider bore supports and one square getter.
The following are based on physical measurements of an intact LGK-7638 laser head and my tests of a two samples of somewhat less than pristine samples of the LGR-7638 tube alone. Only one of these came close to new specs with a maximum output power of about 13 mW. Thus the electrical measurements are not likely to be exact, as operating voltage and optimal current may change with use.
The measurements and healthier of the tube samples were provided by Alan Scrimgeour in response to a posting on alt.lasers.
The LGK-7676 resonator consists of 4 full length 5/8 inch diameter rods joined by 10 thick plates. The tube is secured to these plates using sets of 4 screws with padded tips going in from all four sides at most locations. Some sets are adjustable for bore centering and optimizing straightness. The end-plates hold the mirror mounts. Coarse mirror adjustment is via some thinner rods attached to the ends of the main support rods, with pairs of nuts but no springs. This should permit the mirror mounts to be removed and replaced for cleaning of the optics without requiring coarse realignment. Fine mirror alignment uses Allen's head screws to press on rods which slightly warp the aluminum to which the actual mirror cell is attached. It works reasonably well with good sensitivity and repeatability except that the two adjustments at each end aren't quite independent. Note that with this scheme, walking of the mirrors requires turning the screws at the two ends in opposite directions. The fine adjustments are similar to those in the SP-907 but the coarse adjustments for that laser are three spring loaded nuts which means that removing the mirror mounts requires complete realignment (unless you are *really* good at counting turns!).
The entire resonator assembly is mounted on a thick piece of machined L-shaped aluminum fastened with screws at only two locations. However, under about half the thick plates (see above) on both sides of the "L" are adjustment screws to provide some sort of additional support.
The LGK-7676 uses a coaxial tube with about half of its bore exposed (as opposed to the side-arm tube with totally exposed bore used in Spectra-Physics lasers). While this does result in a more compact package (overall dimensions under 3"x3"x39"), there is less space for IR suppression magnets. In fact, the LGK-7676 only has two sets of magnets (in proximity to less than 25 percent of the bore) for this purpose but could definitely use more. Adding moderate strength magnets (greater than refrigerator strength but much less than rare-earth disk drive strength) almost anywhere along the bore - even outside the large gas reservoir - resulted in a noticeable increase in output power - about 1 percent for a single magnet. I would guess that with enough magnets, a 10 to 20 percent boost would be possible.
There are both anode and cathode ballast resistors of 81K and 27K, respectively. The power supply connector has 3 pins - anode, cathode, and Earth ground. But note that this pinout is not the same as on the physically similar connector on Spectra-Physics lasers. Thus, a Spectra-Physics power supply cannot be used on a Siemens/LASOS laser or vice-versa without modification or bad things will happen to the laser head and/or power supply. Check the power supply and laser head wiring to be sure they are compatible if not originally mated!
The sample I tested is an LGK-7676S with a spec'd output power of at least 30 mW. Of course, since I like to spend as little as possible to acquire these things, mine is a high mileage tube which apparently served hard duty in some sort of high speed printer since there was toner all over it. These are turning up on eBay (and possibly from surplus outfits directly), probably being replaced when their output power drops below a certain value by tiny diode lasers. :)
I was able to run it on my SP-255 exciter only by reducing the anode ballast resistor to 60K and removing the cathode ballast resistor entirely. Prior to this surgery, even with the input voltage to the SP-255 at 140 VAC (the upper limit of my Variac), it would only run for a minute or two (and only if it felt like it) and then cut out, not to restart for several minutes. With the modifications, it will now run all day at 120 VAC input, though restarting was sometimes still a bit of a problem until I added circuitry in an external pod to the SP-255 to boost its starting voltage. (Note that this may not be needed for LGK-7676, SP-907/107/127, and similar size lasers with lower mileage tubes.) See the section: Enhancements to SP-255.) The SP-207 should be able to drive this laser without problems but I don't happen to have one of those.
After bore straightening and mirror adjustments, I was able to squeeze more than 19 mW out of the laser at a somewhat reduced operating current (10 mA instead of the spec'd 11.5 +/- 0.5 mA). (I'm using the lower current only so I can look forward to increased power by a simple tweak in the future.) It would exceed 20 mW if when fully warmed up, the laser was shut off for 30 seconds and then restarted. But the output power would drop back to its previous level over the course of a minute or so once the "good" gas had time to migrate back out of the bore or something. :)
Here is a chart of some older Spectra-Physics HeNe lasers. Most of these are from a 1988 catalog (along with 1988 prices). Not all information was available, thus the "???" in places. You can go to the Spectra-Physics Web site for current models (which are now quite limited, possibly to the SP-117 frequency/intensity stabilized laser only). But the hobbyist and experimenter is much more likely to acquire the classic ones below (unless very well endowed!). Typical output power when new may have been 50 percent or more greater than the value listed.
Minimum
Laser Wave- Mirrors Output Exciter Original
Model length Int/Ext Power Model Price Description/Comments
------------------------------------------------------------------------------
107 632.8 nm E 30 mW? 207 $ ?,??? Similar to 127 (1)
115 632.8 nm E 5 mW? 200 $ ?,??? RF excited, 24" resntr.
116 632.8 nm E 5 mW? 200? $ ?,??? " " tuning prism
120 632.8 nm E 6 mW 256 $ 1,980 Small lab laser (2)
-01 1,152 nm E 1 mW " $ 2,800 " "
-02 3,391 nm E 1.25 mW " $ 2,800 " "
122 632.8 nm E 5 mW? 253A $ ?,??? Short version of 124 (3)
123 632.8 nm E 10 mW? I $ ?,??? Between 120 and 124
124B 632.8 nm E 15 mW 255 $ 4,900 Popular lab laser (3)
-01 1,152 nm E 2 mW " $ 5,500 " "
-02 3,391 nm E 5 mW " $ 5,500 " "
125A 632.8 nm E 50 mW 261A $ 16,000 Huge-head >125 lbs. (4)
-01 1,152 nm E 10 mW " $ 17,500 more than 6 feet long.
-02 3,391 nm E 10 mW " $ 17,500 " "
127 632.8 nm E 35 mW I $ ??,??? 39 inch resonator (1)
130B 632.8 nm E 1.5 mW I $ ?,??? Self contained (5)
130C 632.8 nm E 1.5 mW I $ ?,??? Self contained
132 632.8 nm I 1.00 mW I $ ??? Self contained (6)
133 632.8 nm I 2.00 mW 233 $ ??? Separate rect. head (5)
134 632.8 nm I 3 mW? I $ ??? Self contained
135 632.8 nm I ??? mW ??? $ ??? Separate rect. head (5)
138 632.8 nm I ??? mW ??? $ ??? Separate cyl. head
142 632.8 nm I 4 mW 248 $ ??? Separate rect. head (5)
143 632.8 nm I 5 mW? ??? $ ???
147 632.8 nm I 8 mW 247? $ ??? Separate cyl. head
155 632.8 nm I 0.5 mW I $ 310 Educational laser (6)
156 632.8 nm I ??? mW I $ ??? " "
157 632.8 nm I 3 mW I $ 525 Self contained
159 632.8 nm I 5 mW I $ 630 " "
102R 632.8 nm I 2 mW 212 $ 610 Cyl. head, rand. pol.
102P 632.8 nm I 1.5 mW " $ ??? Cyl. head, lin. pol.
105R 632.8 nm I 5 mW 215 $ ??? Cyl. head, rand. pol.
105P 632.8 nm I 5 mW " $ ??? Cyl. head, lin. pol.
117A 632.8 nm I 1 mW 217A $ 3,500 Stabilized (7)
118A 632.8 nm I 1 mW 218A $ ?,??? " " "
119A 632.8 nm I 0.2 mW ??? $ ?,??? " "
Notes:
For more details on the popular large-frame Spectra-Physics HeNe lasers, see the next section.
It should be possible to possible to obtain orange (611.9 nm), yellow (593.9 nm), and green (543.5 nm) output with similar modifications (at least for the longer lasers), though the gain of these lines is only a fraction of that for the red or IR lines (1152.3 nm and 3391.3 nm) so output power will be lower.
Some photos of these lasers can be found in the Laser Equipment Gallery under "Spectra-Physics Helium-Neon Lasers".
Note: The specifications for the SP-124A and SP-125, below, were copied from an almost illegible scan of a fax of a copy of the original product brochure. Corrections are welcome! The specifications for the SP-120 were copied from an original user manual which, however, didn't list the tube operating voltage or current, so these values were sort of, well, guessed. :)
Spectra-Physics Laser: SP-120 SP-124B SP-125A
-------------------------------------------------------------------------------
OUTPUT
Wavelength (nm): 632.8 632.8 1152.3 3391.3 632.8 1152.3 3391.3
Minimum Power (mW): 5.0 15 2.5 5.0 50 10 10
BEAM CHARACTERISTICS
Beam Diameter (mm): .65 1.1 1.4 2.5 1.8 2.4 4.1
Beam divergence (mR): 1.7 .75 1.0 1.5 .6 .8 1.4
RESONATOR CHARACTERISTICS
Transverse Mode: TEM00
Degree of Polarization: 1000:1
Angle of Polarization: Vertical (+/-5 Degrees except SP-120, +/-20 Deg.)
Resonator Configuration: Long Radius
Resonator Length (cm): 39 70.1 177.0
Longitudinal
Mode Spacing: 385 MHz 214 MHz 85 MHz
PLASMA TUBE
Plasma Excitation: 3.7 kV, 7 mA 5 kV, 15 mA 6 kV at 25 to 35 mA
(RF Opt: 15 W at 46 MHz)
Starting Method: ~8 kV ~12 kV Trigger pulse on isolated
(Direct from Exciter) bar adjacent to tube.
AMPLITUDE STABILITY
Beam Amplitude Noise: <.3% RMS <.3% RMS <2% RMS (RF: <.5%)
Beam Amplitude Ripple: <.5% RMS <.2% RMS <.5% RMS (RF: <.6%)
Long Term Power Drift: <5% over 8 hours and 10 °C
Warmup Time: 30 Minutes 30 Minutes 1 Hour
ENVIRONMENTAL CAPABILITY
Operating Temperature: 10 to 40 °C
Operating Altitude: Sea Level to 3,000 m (10,000 ft.)
Operating Humidity: Below Dew Point
POWER REQUIREMENTS
Power Supply: 115/230 VAC, 50/60 Hz, +/-10%
Exciter Model (DC): SP-256 (1) SP-255 (2) SP-261A
Input Power: 50 W 125 W 456 W
PHYSICAL CHARACTERISTICS
Laser Head Size: 3.26" (W) x 3.26" (W) x ??? (W) x
3.66" (H) x 3.66" (H) x ??? (H) x
18.48" (L) 32.00" (L) ??? (L)
Laser Head Weight: 7.5 lb 25 lb 100 lb
Power Supply Size: 7.25" (W) x 7.25" (W) x 13" (W) x
3.72" (H) x 3.72" (H) x 6" (H) x
9.88" (D) 9.88" (D) 18" (D)
Power Supply Weight: 7.5 lb 7.5 lb 30 lb
Notes:
Actual power from these lasers may be much more than their ratings would indicate, especially when new: greater than 35 mW for the SP-124B and up to 200 mW (!!) for the SP-125A. (However, I don't know how likely such 'hot' samples, especially of the SP-125A, really were.)
There is also a model 127 (OEM versions: SP-107 and SP-907) with the following partial specifications (632.8 nm). Beam diameter: 1.25 mm, divergence 0.66 mrad, length 38.75", height and width: about 4", power requirements: 5 kV, 11.5 mA, starting voltage: 12 kV + 6 kV pulse. This appears to be the only large-frame Spectra-Physics HeNe laser in current production. See the next section.
Mirror sets for green (543.5 nm), yellow (594.1 nm), and orange (611.9 nm) were available for the longer lasers. (The SP-120 and SP-122 may be too short for the low gain green line.) There were also tunable versions of the SP-125 and possibly others. The SP-116 was a tunable version of the RF excited SP-115. These used a Littrow prism in place of the HR mirror.
See the following sections for more information on these Spectra-Physics lasers.
Even without powering up the laser there are two things that can be inspected to get a rough idea of the tube's health (beyond the overall condition and that it isn't in a million pieces):
The actual plasma tube in these is the SP-082 with various -dash numbers after probably related to the actual output power. I believe higher -dash numbers mean a higher output power tube (at least when new). These laser heads may sometimes be listed based on the tube number but they are the same thing since you really can't buy a tube by itself unless someone was bored and decided to totally disassemble one!
The SP-107/907 resonator is over 38 inches long and of the "Stabilite" design similar to that of the SP-122 and SP-124 but the mirror mounts differ. There is an internal L-shaped structure and outer thinner metal skin. There are two versions, differing the design of their mirror mounts:
In addition to mirror alignment, there are a pair of bore centering brackets about 1/2 and 3/4 of the way relative to the cathode-end of the laser. These have an effect on both output power and beam shape. Carefully tweaking for maximum output power should done in conjunction with mirror alignment.
The bare resonators have no beam centering adjustments and I don't see any on the packaged SP-127.
The tube has a side-mounted cathode chamber like other SP lasers but it is quite oversize - about twice the typical diameter. The ballast resistors (2 at the anode-end, 1 at the cathode-end, all 27K ohms) are mounted externally in glass tubes sealed with rubber and heat-shrink tubing. The power supply connector has 3 pins - cathode, anode, and Earth ground. But note that this pinout is not the same as on the physically similar connector on Siemens/LASOS lasers. Thus, a Spectra-Physics power supply cannot be used on a Siemens/LASOS laser or vice-versa without modification or bad things will happen to the laser head and/or power supply. Check the power supply and laser head wiring to be sure they are compatible if not originally mated!
IR suppression magnets are placed at every available location on two sides of the bore. Thin rubber boots seal the space between the Brewster windows and mirrors but these can be pushed back to permit cleaning of the windows and mirrors in-place (barely and not recommended unless the resonator has been previously disassembled as the optics stay quite clean). Some of these lasers include a metal cover and electrical heaters to decrease the warmup time required to achieve rated power and stability. CAUTION: The part of the rubber boots surrounding the tube are easily torn if the boots are removed since they tend to stick to the tube.
Depending on specific model, the SP-107/127/907 has a minimum output power of 25 or 35 mW but may do much more when new. The following is from a Spectra-Physics datasheet. Only the specs for the red version are shown but any of the other HeNe lasing wavelengths (except possibly 3.391 nm which may require a wider bore tube and removal of the IR suppression magnets) should be possible by substituting appropriate optics. A yellow or green version would be nice. :)
Spectra-Physics Laser: SP-107B
-----------------------------------------------------------------
OUTPUT
Wavelength (nm): 632.8
Minimum Power (mW): 25 or 35
BEAM CHARACTERISTICS
Beam Diameter (mm): 1.25
Beam divergence (mR): 0.66
RESONATOR CHARACTERISTICS
Transverse Mode: TEM00
Degree of Polarization: 500:1
Angle of Polarization: Horizontal (+/-5 Degrees)
Resonator Configuration: Long Radius
Beam Waist Location: Outer surface of output mirror
Resonator Length (cm): 95
Longitudinal Mode Spacing: 161 MHz
PLASMA TUBE
Type: Hard-seal, cathode in side-arm
Operating Voltage: 5 (+/- 0.4) kV, 11.5 (+/- 0.5) mA
Starting Voltage: ~15 kV
Lifetime: Greater than 20,000 hours
AMPLITUDE STABILITY
Beam Amplitude Noise: <1% RMS
Beam Amplitude Ripple: <1% RMS
Warmup Time: 20 Minutes (95% power)
ENVIRONMENTAL CAPABILITY
Operating Temperature: 10 to 50 °C
Operating Humidity: 5-90% non-condensing
POWER REQUIREMENTS
Power Supply: SP-207A (110/220 VAC +/- 10%)
SP-207A-1 (100/200 VAC +/- 10%)
SP-207B (90-130 VAC or 180-260 VAC)
PHYSICAL CHARACTERISTICS
Laser Head Size: 3.7" (W) x 3.7" (H) x 38.75" (L)
Laser Head Weight: 23 lb
Power Supply Size: 2.4" (W) x 1.4" (H) x 10" (L)
Power Supply Weight: 3 lb
It is possible to run these lasers on the smaller linear SP-255 exciter but starting may be erratic or not work at all (at least for non-pristine tubes) unless the AC line voltage is increased to 125 VAC for starting (it can then be backed off somewhat while operating). A bleeder resistor of 200M ohms or so rated for 15 kV can be installed to discharge the power supply capacitors after shutdown as starting of the longer SP-107/127/907 tube apparently requires the voltage to rise from close to 0 V to start reliably on the SP-255's whimpy starter. An alternative and better solution is to add a passive boost circuit to the starting multiplier of the SP-255. This can be in an external pod requiring no modifications to the exciter itself. Note that the added starting voltage may not be needed for LGK-7676, SP-907/107/127, and similar size lasers with lower mileage tubes. If your laser starts reliably, don't worry about it. Otherwise, see the section: Enhancements to SP-255.) Make sure the laser head frame is securely connected to the power supply (and earth) ground. Since the operating voltage and current are well within the capabilities of the SP-255, the laser and power supply should both be happy once started (though the AC line voltage may still need to be slightly above 115 VAC to minimize drop out/restarts if there are line dips, expecially for a high mileage tube which may have increased operating voltage). Changing the jumpers to use one of the lower line voltage taps on the SP-255's power transformer would probably help in a marginal case (low line voltage, or a laser with a higher HeNe tube voltage or higher ballast resistance) where regulation can't be maintained with adequate current without using a Variac to boost line voltage.
The laser tube is about 20 inches long with separate bore and gas chambers side-by-side. The bore uses rather thin glass tubing and is a very large diameter for a HeNe laser - about 3 to 4 mm ID - consistent with early HeNe laser technology. The laser head is nicely mounted with lots of fine machined hardware. It has no IR suppression magnets. There are two RF connectors on the side for the Spectra-Physics model 200 RF-type power supply. One of the connectors is for the actual RF signal; the other is for starting. There is an impedance matching network located under the "tube deck". This consists of a series LC circuit (C is adjustable for peaking the tuning) between the RF input and case with the output taken from the junction of the L and C. The RF drives a dozen or so electrodes with alternating polarities in close proximity to the tube bore. The start connection goes to the input of a potted transformer which produces a several kV pulse when the "Start button" on the exciter is pressed. The starting pulse goes to a separate small electrode clamped near the center of the tube bore.
The laser has external adjustable mirrors mounted on the very solid precision milled black anodized aluminum box support structure. Both mirrors have screw adjustments for coarse alignment not accessible from outside the case without removing the end-plates. The front mirror also has external fine adjustments in X and Y via two precision Lufkin micrometers and the rear mirror is mounted on a precision slide with an external micrometer adjustment for mirror separation (try to find that on any modern laser!). I don't know if the intent of this axial adjustment (over 1/2" of travel) was to fine tune the longitudinal or transverse modes or both. Since the resonator frame would experience little if any heating (and expansion), the micrometer could be used to center a longitudinal mode and maximize output for this low gain laser. In addition, the larger movement could possibly be used to select a particular transverse mode pattern, though actually achieving TEM00 operation in such a wide bore laser might not be possible.
The power supply for the SP-115 is a high quality 15 to 25 watt 40.68 MHz RF source consisting of a crystal controlled oscillator and a power amplifier using a 4x150 tube. All active elements are tubes, of course, but out of character for the era, the oscillator and driver are built on a printed circuit board. Overall, the system looks like something straight out of the ARRL Handbook (which is probably where the design came from!).
Not surprisingly, on the sample I have, the tube has leaked and only produces a weak purple glow when the RF is turned on. The getter has the "white cloud of death" syndrome and without an aluminum can cathode, there is no possibility of getter action anywhere else. (Not that a tube this far gone would have any chance of revival in any case. The tube would make an ideal candidate for refilling since the vacuum could be breeched by cutting the exhaust nipples at either end of the gas ballast without contaminating the Brewster windows.) The SP-200 does do a nice job of lighting 20 W fluorescent lamps and most likely screwing up radio reception in the neighborhood. :)
There was also a Spectra-Physics model 116 laser which appears similar but has a tuning prism to enable wavelength selection. It goes without saying that a working sample of an SP-116 would be a real prize. :)
The cylindrical SP-117A laser head has the normal strange SP high voltage cable/connector. Although I haven't yet seen it, I assume this mates with a conventional HeNe laser power supply inside the controller box. The laser head will run happily on the SP-248 exciter or any other HeNe laser power supply compatible with a 3 to 5 mW HeNe laser tubes. But of course, there will be no mode stabilization. So, it behaves more or less the same as other similar size HeNe lasers.
In addition, there is a DB9 that is used for the control functions. It includes connections for a heater to adjust the cavity length of the HeNe laser tube, a pair of photodiode outputs for feedback mode stabilization, and an interlock.
I have looked at the longitudinal modes of the SP-117A laser using my home-built scanning Fabry-Perot interferometer. The head was powered by an SP-248 exciter so there was no mode stabilization. As the tube heated and expanded, the modes would cycle under the gain curve (normal behavior for any HeNe laser!). It oscillates on at most two modes with a tendency for there to be two modes of reasonably equal amplitude. A single mode appeared for a relatively small portion of the cycle but I assume that is actually the desired result when using the feedback system. Having at most 2 modes is unusual for a HeNe laser of this power as 3 or 4 modes would be expected based on the tube length.
This laser seems to be interesting in another respect: While the typical ordinary HeNe laser, the modes roughly follow the profile of the gain curve as they traverse it, with this tube, the mode on one side will tend to disappear and reappear on the other side of the gain curve relatively abruptly. I don't know whether this behavior is a peculiarity or a feature but it seems like it could be beneficial. ;-)
(There is also an SP-118A but I'm not sure what the difference is.)
Photos of a SP-120 laser head and the SP-120 resonator and tube can be found in the Laser Equipment Gallery (version 1.85 or higher) under "Spectra-Physics Helium-Neon Lasers". The complete user manual for the SP-120 laser with SP-256 exciter can be found at Lasers.757.org, Manuals. On the one sample of the SP-256 exciter that I've seen, the current was set for 7.2 mA. However, I don't know if this is the default optimum setting for the SP-120 laser or whether it had been tweaked. (The specs list 7 mA at 3.7 kV.) There is also an SP-120S. It has been suggested that this simply means "with shutter" but I don't know for sure.
The resonator uses three-screw adjustable mirror mounts for coarse alignment (tweaking these is a true pain!). Fine alignment is done via a pair of hex screw pan/tilt adjustments at each end which actually shifts the tube X-Y position without affecting the mirror. These are accessible via a pair of holes visible once the circular bezel/optics mount is unscrewed. It is possible to replace the tube in about 5 minutes without requiring major mirror re-alignment (no need to touch the coarse adjustments, only the tube centering).
The resonator is constructed from 3 pieces of thick very nicely machined aluminum stock - an L-channel and 2 end-plates bolted together to form a very rigid structure. It is supported at only three points and essentially floats inside the outer case (the "Stabilite" name as discussed for the SP-124 laser, below) which isolates the resonator from external stress (or so it is claimed). So, the clunking you hear when changing the position of the laser head is normal.
CAUTION: Unless the tube has been removed, there should be no need to clean the optics. Since there is no way to clean the Brewster windows with the tube in place and no way to clean the mirrors without removing them, it is a royal pain to be avoided. Remove, clean, install, test and tweak, repeat until output power comes back to what it was before attempting this stunt. :)
The one I obtained also used the strange SP-253A exciter - a switchmode power supply which sends medium voltage AC to a voltage multiplier/boost module in the laser head. See the end of the next section for more on this. There is also an SP-123 which appears similar but with an internal power supply.
The SP-124 laser head is a box about 76 mm (H) x 76 mm (W) x 813 mm (L) (3" x 3" x 32"), nicely massive for its size. There are threaded beam apertures at both ends though the HR is backed by a solid aluminum plate so I don't think much light would ever get through that even if there was leakage through the mirror!
This is one of SP's "Stabilite" series lasers. This approach to frequency stabilization is based on a mounting system that employs optimally located pivots in an attempt to minimize the coupling of gravitational and vibrational torques and other distorting forces to the resonator cavity itself. In the SP-124, most of the mass of the laser head is in such an optimally mounted heavy solid frame with roughly an L cross section that runs nearly the full distance between the mirror mounts and attached to each of them at three points.
Adjustments accessible externally at each end of the laser allow the beam alignment (X and Y) to be tweaked very accurately by moving the entire optics chassis relative to the head mounting studs (which accept 6-32 screws or rubber feet). The adjustment scheme is sort of interesting (to me, at least): A V-shaped block (bolted to the rosonator and case) sits between a pair of wedges (part of the mounting stud assembly) that can be moved up and down via a pair of screws (call them A and B) and retained in position by a stiff spring. Rotating both A and B equally in the same direction moves the beam in Y; rotating A and B equally in opposite directions moves it in X. The setting may then be locked.
The external mirror HeNe tube is clamped in rubber mounts at its ends and also stabilized at the 1/3 and 2/3 (approximately) positions. Metal bellows join the tube mount brackets to the mirror mounts and, in conjunction with the rubber seals, prevent dust and dirt from getting on the inside surfaces of the mirrors and on the Brewster windows. The mirror mounts have hex head bolts for adjustments with set screws to prevent their settings from changing over time. An additional metal bellows joins the OC to the treaded output aperture.
The HeNe tube itself is a bare capillary about 7 mm OD with a 1.1 mm ID (no, I didn't measure it - just trust the specs!). The cathode, getter assembly, and HeNe gas reservoir is in a side-arm at the output-end of the laser bent to run parallel to the bore. It is about 32 mm x 178 mm (1-1/4" x 7") with the 'can' electrode nearly filling the glass envelope. The anode is (naturally) at the other end of the bore along with the three 9.8K ohm (5 W at least) ballast resistors also in a parallel side-arm inside the gas envelope as apparently is the case with other Spectra-Physics lasers of this era. Interesting, they are just ordinary Ohmite power resistors. I guess this approach does reduce problems with high voltage insulation breakdown but it would be a shame if the laser went bad because a $.50 resistor failed and could not be easily replaced! The total value of about 30K ohms would seem to be rather low but might have been selected to match the needs of the SP-253A exciter (see below) or additional external ballast resistors may be required. The SP-124B version of this laser may use a more normal 81K ballast resistance.
A series of relatively weak (e.g., refrigerator note holder strength) ceramic magnets 14 mm (W) x 22 mm (L) x 6 mm (H) (9/16" x 7/8" x 5/16") are mounted in close proximity under (15 magnets) and on one side (24 magnets) all along the length of the bore wherever they fit. (See the section: Magnets in High Power or Precision HeNe Laser Heads for an explanation of their purpose.) The approximate arrangement is shown below. I may have the poles backwards (which is of course irrelevant). A cheap pocket compass came in handy to determine the pole configuration!: The magnets were positioned with their broad faces about 2 mm from the bore.
Magnets N_S_N_S_N_S_N_S_N_S S_N_S_N_S_N_S_N N_S_N_S_N_S_N_S_N
on side |_|_|_|_|_|_|_|_|_| |_|_|_|_|_|_|_| |_|_|_|_|_|_|_|_|
(24)
-------------------------------------------------------------
HR end ============================================================= OC end
of bore ------------------------------------------------------------- of bore
Magnets N_S_N S_N_S N_S N_S S_N S_N S_N S_N S_N S_N_S N_S_N
below |_|_| |_|_| |_| |_| |_| |_| |_| |_| |_| |_|_| |_|_|
(15)
N_S_N +-----+-----+
Where: |_|_| = 2 adjacent ceramic magnets: |N S|S N|
+-----+-----+
I assume that the only reason there aren't 24 magnets below the tube is that
the holes in the Stabilite frame got in the way.
Apparently, there must have been a couple of power supply options for the SP-124. Most of these lasers appear to use the Spectra-Physics Model 255 Exciter (SP-255). This is a traditional HeNe power supply providing operating and start voltage through a high voltage BNC connector. However, the laser I have apparently is supposed to use an SP-253A Exciter, a model for which no one (including Spectra-Physics) seems to have any information or even acknowledge exists though I have since found out that the SP-122 laser, a model slightly shorter than the SP-120 but built more along the lines of an SP-124, may have also used the SP-253A (possibly a slightly different version or at least different jumper options). For more information on what I have found out so far about the exciter, see the section: Spectra-Physics Model 253A Exciter (SP-253A).
Unfortunately, on the system I obtained, the boost/start module (which is what I assume was supposed to be inside the head to attach to the exciter) had been ripped out with the cable just chopped off and thus I can't even determine what was there originally. So, I removed the multiconductor cable and replaced it with a HV coax (terminated with a standard Alden connector) and wired it directly to the tube anode terminal and chassis ground (recall that the ballast resistors are inside the tube. Yes, I know, the 30K ballast resistance may be too low for use with the SP-255!)
Using my SP-255 to power the head, I get a nice pink glow in the bore (more red than orange indicating a rise in pressure from slow leakage over the years) but as expected, no coherent light. The low ballast resistance is fine as far as maintaining a stable discharge (I don't know if this would still be the case if the gas pressure in the tube were correct). Maybe someday in the far distant future after that hot place freezes over AND those pigs start flying, I will get around to regassing the tube! :)
The SP-125A tube has a common cathode in the middle of the tube with two anodes, one at each end. The dual discharges are driven from its SP-261A Exciter which provides 6 kV at up to 35 mA. The SP-250 Exciter is also compatible with this laser.
With a bit of rewiring of the laser head, one could feed the anodes separately reducing the individual current requirements so that a pair of power supplies similar to the SP-255 could be used. With this sort of scheme, it should also be possible to selectively power only one of the discharge paths for reduced beam output if desired. Yes, I know, why would you ever want *less* power? :)
Two sets of ballast resistors in the laser head totaling 87K ohms (75K+12K) provide the operating voltage to each of the anodes of the dual discharge tube. They are located between the anodes and chassis ground (The SP-261A's output is negative with respect to ground. Thus, ground is the positive supply voltage). The HeNe tube's single cathode is attached directly to the negative output of the SP-261A.
The starter operates in a manner similar to that of the method of triggering the xenon flashlamp in a typical electronic flash unit or solid state laser power supply - by pulsing an external electrode in close proximity to the HeNe tube bore. The whole tube is supported by metal rods which are insulated from the cavity structure by nylon disks. One of the rods is the trigger electrode. The starter runs off a voltage from the 75K/12K ohm taps of both ballast resistors ORed together so that it repeatedly generates a trigger pulse until BOTH discharges have been successfully initiated.
The SP-261A also has a low power RF output (this isn't the same as the RF power supply option mentioned below) which drives a pair of plates in proximity to the HeNe tube. The RF is supposed to stabilize the laser power (presumably by some sort of discharge dithering process). However, the RF apparently also results in interference with local radio stations. :(
An RF power supply option is/was also available. (Possibly some version of the SP-200 though the specs don't quite match for the one I have. See the section: Spectra-Physics Model 200 Exciter (SP-200).) This would replace theSP-261A and starter entirely by driving the tube directly with radio frequency energy - 15 W at 46 MHz. Note the greatly reduced power to the tube compared to the 150 to 210 W for the DC discharge! The drive is applied via coax from a BNC connector on the back of the laser to a resonant circuit about midway in the laser head. The two phases of the output of the resonant circuit connect to a pair of 0.1 inch diameter bars running the length of the tube about 0.6 inches from the centerline suspended from insulators.
Unfortunately, many SP-125s that appear as surplus are not good for more than long boat anchors (or as a parts unit for salvage of the optics and frame). Unless the tube has been replaced relatively recently, being soft-seal, it has likely leaked to the point at which the getter can no longer clean up the contamination. Refilling is the only option and that cost would make what you paid for the laser look like pocket change. And, refilling a HeNe tube is generally not a realistic basement activity. So, if you come across an SP-125 at a low price, unless it is guaranteed to lase, buyer beware. An SP-125 sold "as-is" almost certainly means the seller couldn't get it to work (not that everything possible wasn't tried) since they likely know it is worth 10 times as much in operating condition!
Also see the section: Spectra-Physics 120, 124, and 125 HeNe Laser Specifications and Spectra-Physics Model 261A Exciter (SP-261A).
(From: Marco Lauschmann (mla@sbk-ks.de).)
The SP125A is absolutely beautiful with much chrome and a metallic blue cover! It is nearly 2 meters long and looks like an older large-frame argon ion laser. A Spectra-Physica scientist noticed that this device will deliver twice the rated power with no problems. Others have claimed as much as 200 mW for the red (632.8 nm) model!
The tube inside the lasers in the photos is the typical small Spectra-Physics side-arm type (like those in the SP-155 and other similar lasers also shown on the Web page above) but with Brewster windows instead of mirrors. However, earlier versions may look a bit different with a side-arm for the anode as well and really early versions (SP-130, no B) actually used a heated filament for the cathode (though for some reason, the schematic of the SP130 with the heated filament is dated slightly later than the schematic of the SP130B with the cold cathode design).
Based on the length of the tube, I would have expected its output power to be in the 2 to 5 mW range. However, from the specifications in the manual, it turns out to be only 0.75 mW when used with the hemispherical mirror configuration (planar and 30 cm radius of curvature), but capable of a TEM00 beam despite its wide bore (2.5 mm). With a confocal configuration using a pair of 30 cm mirrors, the beam is multimode (non-TEM00) and output power may be as much as 1.5 mW.
When I obtained the first of these lasers (the one in the top two photos), the tube actually still lit up but there was no output beam. At first I thought it might even have a chance of working since the discharge color looked sort of reasonable, though somewhat less intense than I would have expected. Fiddling with the optics didn't yield any positive results. And then, when I wasn't looking, the discharge went out! As best I can tell, a crack must have opened somewhere in the tube and it is now at much higher pressure or up to air - bummer! I can find no visible damage or any evidence of this except that it won't start even on a much larger HeNe power supply and shows no signs of a glow from an RF source. So far, the getter hasn't changed color.
I don't think this laser was ever really alive - the tube was probably gasy or helium deficient or something but I still can't explain what happened. The only place it could have leaked that I can't see is under the anode connection which is kind of potted but there shouldn't have been any heat there to cause such a problem.
And to compound my disappointment, I dinged the OC removing the tube. Enough of it may be left to still work but the optics appear to be soft-coated as the AR coating came off totally by just barely touching it. However, that still hurts. Sometimes, you just have one of those days. :(
The laser in the third photo was DOA with an up-to-air tube, seriously damaged mirrors (coatings mostly gone), and evidence of prior dissection attempts (cut wires, etc.). The tube in that one is probably one of the earliest non-heated filament types with a small cathode and separate side-arm for the anode.
However, I have since obtained a third SP-130B which originally had a red/blue discharge. But while running for a few hours, the color gradually changed to a mostly correct white-ish red-orange. And, with an optics cleaning and alignment, this SP-130B actually lases. The output power is not up to spec - about 0.25 mW at maximum current (it's rated at 0.75 mW) - but that's still a bit amazing considering its age. See the section: Reviving a Spectra-Physics Model 130B Antique Laser for details.
The internal power supply accounts for much of the weight and most of the height of the box and consists of:
There is no actual starter - the open circuit voltage of the power supply is about 5,000 VDC but drops to around 1,500 VDC under load.
For more info and schematics, see the section: Spectra-Physics Model 130 HeNe Laser Power Supply (SP-130).
Now, the question becomes: Do I leave the dead ones intact as examples of antique lasers or replace their tube and optics with modern 3 mW barcode scanner tubes (about the largest that would fit height-wise, a 1 inch diameter tube) to have working lasers? I guess there's nothing special about 3 mW HeNe lasers so leaving them intact would be the best option. And, it would be a shame to only have 3 mW when the power supply is easily capable of driving at least a 5 mW tube. In order to do a test with an SP098-2 barcode scanner tube (actual output: 2.8 mW), I had to add 500K ohms of ballast resistance in addition to what is built into the power supply to get the current low enough so the adjustment would include the optimum current setting. (I can hear the antique connoisseurs breathing a collective sigh of relief!) Who knows, maybe someone will drop replacement tubes and mirrors in my lap someday! Hint, hint. :)
There is also an SP-130C laser which is virtually identical in construction and function, except for the lack of an external current adjust pot.
Photos of a typical SP-155 can be found in the Laser Equipment Gallery under "Spectra-Physics Helium-Neon Lasers".
The HeNe laser tube is the classic Spectra-Physics side-arm design but with the anode electrode mounted about halfway along the length of the bore. The same tube with the anode mounted at the end would produce around 4 to 5 mW. In fact, the Spectra-Physics 157 (3 mW) and 159 (4 mW) lasers are virtually identical except for the tube's anode location and the use of a larger power supply. (The SP-156 may be similar but I haven't seen one to confirm.)
The power supply for the SP-155 is a basic transformer/doubler/multiplier design with a single transistor current regulator. The power supply on later versions of the SP-157 and SP-159 lasers may be a potted brick instead of a discrete PCB but all of the SP-155 lasers appear to retain the older quaint power supply design. :)
Note that other manufacturers sell (or have sold) lasers identical in appearance to the SP-155. For example, there is a Uniphase model 115ASL-1 and a Liconix L-388 (even though it is made by Uniphase). However, these use a hard-seal Uniphase barcode scanner HeNe tube (similar to a model 098 with a tiny collimating lens attached to its OC to reduce divergence) rather than the fancy Spectra-Physics side-arm tube we know and love. But their power supplies are similar or identical to that used in the SP-155. (There is also a Spectra-Physics model 155ASL which is physically identical to the Uniphase and Liconix lasers except for the name on the front. I assume it has the same construction though I haven't seen the insides of one up close and personal.)
Also see the section: Spectra-Physics Model 155 HeNe Laser Power Supply (SP-155).
Here are some specifications for a couple of REO tunable lasers. Two models were listed in their 1992 catalog though only the LSTP-1010 shows up in a recent listing and on the REO Laser Products Page. As expected, both are linearly polarized (500:1) since they use a tuning prism external to the laser tube:
Note the line at 604.6 nm (orange/yellow) which is almost never seen in other other-color HeNe lasers (at least isn't supposed to be there). :)
There was also an LSTP0050 which should have similar wavelength specs but the output powers are unknown.
And for only $4,050.00 plus shipping and handling, you can now buy your very own LSTP-1010 through Edmunds Industrial Optics. :)
(From: Lynn Strickland (stricks760@earthlink.net).)
The five lines are 543.5, 594.1, 604.6, 611.9, and the common (red) 632.8 nm. You might see a flash at 629.4 nm and at 640.1 nm, but nothing to write home about. The 629 and 640 nm lines are so weak, and so close to 633 that they're sometimes hard to distinguish. There should be nothing at the IR lines (1,153, 1,523 or 3,391 nm).
As originally designed, these lasers used a Brewster window tube with a Littrow prism as the wavelength selection mechanism. The tube's internal mirror was a broad band output coupler. Don't know if it's changed, but I doubt it.
The fundamental design issue is that the optimum Bore-to-Mode Ratio (BMR) for green is much higher than for red. (BMR is the ratio of limiting aperture size to mode radius. To get TEM00 operation for green, the optimal number is about 4.2, for red it's about 3.5.) If you know the wavelength, mirror curvature, and spacing, you can calculate the mode radius at any point in the cavity. The capillary bore serves as the limiting aperture, so adjusting bore length and bore diameter sets the BMR, which in turn determines transverse mode purity.
Thus, if you optimize the BMR for green power (which you have to do), the red is under-apertured, and has something like 50% off-axis modes. It's getting close to a doughnut-mode.
REO builds some of the highest 'Q' Brewster tubes in the world (probably THE highest), exclusively for the company, Particle Measuring Systems (PMS). REO and PMS used to be one in the same, but the owner sold off the particle counter biz a few years back, for something like $75 million. They now have some sort of supply agreement. The REO tubes aren't the most robust or mechanically stable, but if you get them packaged right, probably some of the highest power you can get from a given tube length. This is mostly due to coatings (all Ion Beam Sputtered), and a super-polishing process they have for substrates. As they say, it's all done with mirrors. ;)
A green Brewster tube IS a bitch! The original REO (PMS) tube was a 5 mW size - about 15" long. They did a soft-seal on the B-window; because it's fused silica. Don't know if they've gone to optical contacting/graded seal now - I'd hope so.
I think REO added a 7 mW, maybe even a 10 mW size for power. I recall seeing some longer ones at a trade show. As for cavity power, I've seen an REO B-tube with 2 HRs do almost 45 Watts of intra-cavity circulating power. They're probably higher than that now. These puppies are like $1,700 each in volume and only sold to PMS - pretty hard to come by.
(From: Sam.)
There is a weak line at 635.2 nm which could also show up as its gain is higher than that of the 594.1 nm and 604.6 nm lines. 640.1 nm is actually quite strong - next in line after 632.8 nm. See the section: Instant HeNe Laser Theory for a listing.
Here is a photo of the PMS One-Brewster HeNe Laser Tube and a closeup of the Littrow Prism Tuning Assembly from PMS Tunable HeNe Laser showing its proximity to the one-Brewster tube's Brewster window. There are adjustments for wavelength and transverse (alignment). The Littrow prism is the shiny thing at the far left. The Brewster window is next to it. There is normally a tight fitting metal cover to keep out dust which has been removed to take the photo. Except for the high quality internal OC mirror and window, the HeNe tube itself isn't that much different from the common variety, though the metal envelope - typical of PMS/REO tubes - may help stability. It does have a heater coil on the OC mirror mount, presumably to decrease warmup time. (These heaters are on some but not all PMS HeNe laser tubes.) The resistance is around 31 ohms and it runs on 9 VAC from a small transformer. The rest of this laser is unremarkable - a brick power supply and case. :)
One problem that limits power in the REO tunable HeNe laser are losses through the Brewster window of the 1-B tube. The Brewster angle is only correct at a single wavelength so there will still be some Fresnel (reflection) at all the others. And, even super polished fused silica isn't perfect so there will still be some scatter. If these could be eliminated, the available power at all wavelengths would increase but this would be especially dramatic for the very weak 543.5 nm (green) and 594.1 (yellow). So, what I suggest is to place the tuning prism inside the tube envelope mounted on a two-axis bearing. Coupling through the glass can be via a pair of magnets to adjust tuning (pitch) and transverse mirror alignment (yaw). This is quite simple mechanically. Even simpler would be to attach the tuning prism assembly via a flexible metal bellows. In either case, 2 of 3 Brewster surfaces are eliminated from the intracavity beam path. The 3rd one is for the Littrow prism which unfortunately cannot be eliminated unless a high efficiency grating could substitute for the prism. Dust collecting on the optics is also, of course, no longer a problem. :)
Rated CDRH New Melles Griot Coherent Model Wavelength Power Power Power Length Model Number ------------------------------------------------------------------------------ 21-2090-000 632.8 nm (Red) 10 mW 30 mW 17 mW 19.05" 05-LHR-991 31-2330-000 594.1 nm (Yellow) 2 mW 10 mW 5.7 mW 17.95" 05-LYR-173 31-2772-000 543.5 nm (Green) 2 mW 5 mW 2.4 mW 20.09" 05-LGR-393
There is little doubt that these laser heads are actually manufactured by Melles Griot. Everything about them is identical except for the manufacturer name and model number printed on the label. Aside from very minor differences that may be typos (mine or theirs), the specifications listed on the Coherent Web site also match. The "CDRH Power" is what is listed on the safety sticker. The "New Power" was measured on samples of these laser heads that appear to have never been used, or have seen very little use.
The HeNe laser tube is powered from a standard Laser Drive 6.5 mA, 2,100 V power supply brick via a HV BNC connector. There is no special control or regulation of this supply - it's turned on by the main power switch. But some thoughtful engineer included a high resistance bleeder to discharge the HV caps in the power supply brick after power is removed. :)
The HeNe laser tube itself looks like a standard Melles Griot (not made by Coherent!) model, probably a 05-LHR-120 specially selected to produce only 2 longitudinal modes. It may also be filled with isotopically pure gases. The tube itself probably puts out more than 2 mW but the polarizing and beam sampling optics sucks up some of it. In addition, depending on the particular version, there is either a dielectric filter or polarizing filter in the end-cap. The dielectric filter cuts the output by about half but the this can be varied by 10 percent or so (though I'm not sure if this is intentional). The polarizing filter allows continuous adjustment of output power. (In both cases, the adjustment is done by loosening a set-screw and rotating the end-cap). According to the CDRH sticker, the output beam is supposed to be less than 1 mW. Given the wide swings in output power during warmup (see below), even with 50 percent attenuation, the peak output power may approach 1 mW.
There is a thin film heater between the tube and laser head cylinder. A pair of photosensors monitor orthogonal polarized outputs from the tube. The controller monitors the lasing modes and maintain cavity length using the heater so that a pair of orthogonally polarized longitudinal modes straddle the gain curve. The beam sensor assembly can be rotated to align the photosensors with the 2 orthogonal lasing modes as this is arbitrary from tube to tube, but probably remains fixed for the life of the tube.
The user controls consist of one (1) power switch. There are indicators for AC power and Status. After a warmup period of 20 minutes or so for the laser head to reach operating temperature, the Status indicator will change from Wait (red) to Ready (green). Doing anything that causes lock to be lost will result in a shorter delay of a couple minutes to re-establish it.
The internal circuitry of the controller box is relatively simple and includes a 741 op-amp and LM311 voltage comparator along with a TO5 power transistor to drive the heater.
Here is the pinout of the circular control connector as determined by my measurements. There may be errors.
Pins Wire Color Function Comments -------------------------------------------------------------------------- 1,2 Blk/Wht Heater Pwr ~22 ohms 3,4 Blk/Red Temp Sense? ~700 ohms at 25 °C; Increases with heat 5,6 Blk/Blu Photodiode 1 Anode is pin 5; Approximately 250 uA max 7,8 Blk/Grn Photodiode 2 Anode is pin 8; Approximately 50 uA max
It would appear that the difference in sensitivities is the way it's supposed to be since this was similar on 3 heads. The controller and laser head are normally a matched pair so I assume there are adjustments inside the controller to equalize the responses.
I picked up a controller and 3 laser heads in two separate eBay auctions for a grand total of $22.50 + shipping. The serial number on one of the heads matched that of the controller and while this head was initially hard to start, after running it for awhile on my HeNe laser test supply, it now starts normally.
The controller originally had a dead HeNe laser power supply brick which is likely the reason it was taken out of service. I replaced that with an Aerotech LSS-5(6.5) which seems to be happy enough. Using a laser power meter, one of the two modes of the laser (the one present in the output beam) could be seen cycling up and down between about 0.60 and 1.40 mW with the orientation of the beam sensor assembly adjusted for maximum peak power. Each cycle took longer and longer as the tube warmed up to operating temperature, helped along by the heater. After about 15 minutes, it would appear to try to "catch" at certain power levels but couldn't quite remain there. (This behavior may have had nothing to do with the feedback control though.) Then suddenly, after about 20 minutes, the Ready light came on and a few seconds later, it locked rock stable at 0.95 mW. :) A second laser head behaved in a similar manner but with a slightly higher final output power of 1.02 mW. No adjustments were needed inside the controller despite the fact that the second head's serial number didn't match the controller's serial number. Possibly, even better stability or slightly higher stabilized output power could be achieved with some fine tuning. (The 1.02 mW head actually had higher peak power than the 0.95 mW head. The difference is probably in part due to the photodiode sensitivities.) With the fixed filter end-caps installed, the output power dropped to around 0.50 mW. I rather suspect that these are normal power levels for this system. A third head (which I haven't tested because its cables were cut) had the adjustable polarizer in its end-cap. With that installed on either working head, the output power could be continuously adjusted from near 0 mW to about 1 mW.
Note that the Ready light comes on and then the laser locks in at the proper phase of the next mode cycle. So, basically the pea brain in the controller (no actual CPU of any kind!) decides that conditions are suitable and enables the feedback loop. Perhaps it's based on a combination of temperature and cycle duration or something. :) I've also seen the ready light come on even if the laser doesn't start and when one of the previously locked heads was plugged back in after a few minutes of cooling. In the latter case, the laser was indeed locked though it might not have been able to maintain it continuously since the tube was probably no longer really warm enough.
Red (632.8 nm):
Output Size
Model Power (LxWxH) Applications Price
--------------------------------------------------------------------------
ML800 0.8 239x72x74 Student use deomonstrations $389.00
ML810 0.8 239x72x74 Student use demonstrations $399.00
ML811 0.5 181x33x47 Pointer, CE approved $399.00
ML855 5.0 540x72x74 Lecture demos, research, holography $899.00
ML868 0.8 328x72x74 Modulated, lecture demos, communication $489.00
ML869 1.5 328x72x74 Modulated, lecture demos, communication $499.00
Green (543.5 nm):
Output Size
Model Power (LxWxH) Applications Price
--------------------------------------------------------------------------
ML815 0.08 181x33x47 CE approved $719.00
The first is the HP-5501B laser head from the HP-5501A Laser Interferometry Measurement System. Position/distance resolution down to better than 10 nm (that's nanometer as in 0.000000001 meter!) were possible with this equipment. Of course, only the laser remains) but the specifications say something about the frequency stability of the laser head. (Note that the HP-5501B laser head appears to use a very different laser tube than the HP-5501A laserhead, described below.)
One other thing that is most interesting is that the original list price from the HP catalog for the laser head alone is about $9,000 (now over $12,000)!
(From: Angel Vilaseca (100604.1242@compuserve.com).)
Here is a quick description of the unit:
I have other HeNe lasers but this one really seems to be a class (or several!) above all others...
The label on the unit says:
HENE GAS LASER, Hewlett-Packard, P.N. 05517-60501 Date of mfg. 4-12-93, Date of instl. 4-19-93, Ser. no. 591-3 Made in USA, Licensed by Patiex Corporation, under patent no. 4,704,583.
(From: Sam.)
The Patent is rather interesting but I'm not sure it relates directly to this laser.
There are photos of some version of the HP-5501A and HP-5501B laser heads in the Laser Equipment Gallery (Version 2.01 or higher) under "Assorted Helium-Neon Lasers". The tube used in that HP-5501A matches my strange tube but not the description above which appears to be more like the tube in the HP-5517A (though not identical). But perhaps there is at least one other type as the HP-5501B photos would seem to imply. I think this likely older 5501 tube looks much cooler than the newer HP-5517 versions. :)
The HP-5501 laser head lases in two modes that are polarized orthogonal to each other. These are split and sent down different paths. The two beams rather than creating an interference pattern are beat together to and sent to a detector that outputs a difference signal. If the difference between the two beam paths changes by one wavelength of the laser (about 632.8 nm but accurate to many significant digits!), the phase of the difference signal will change by 360 degrees. The laser outputs a reference signal from beating the signals together internally. This is compared to the detector signal and an electronics package counts off the phase shifts and uses it to determine the distance traveled. The laser is supposed to have an accuracy on the order of 10 parts per billion over the life of the instrument.
(From: Wong Sy Ming (siming@singnet.com.sg).)
I picked up a HP-5517A laser head for S$50 (that's about US$30) and I have to say it's an extremely fine piece of equipment, about the same as the HP-5501B. The datasheet (which may be found by searching for "5517" on the Agilent Web Site) claims a "vacuum wavelength stability" of 0.002 ppm(!!!) over 1 hour and 0.02 ppm over it's entire 50,000 hour lifetime. Quite incredible, isn't it? It also says it has a wavelength of 632.991372 nm and a wavelength accuracy of 0.1 ppm. (that's for the "consumer grade" model, the "military calibrated" one is 0.02 ppm).
I got a rather more complete version than the one above. It came within its original casing, an inverter and a whole lot of electronics (don't know what they were for so I just took them out).
The tube is really non-standard, it has only one thick white HV wire coming out of the back and two smaller wires (red and purple, just like the HP 5501B) and the tube connects to the HV power supply through only the ONE HV cable (for the anode). I discovered later (by poking around with a separate little inverter power supply) that the not-so-obvious cathode connection is via the red wire.
The two smaller wires are connected to a "connector board" (that's what it says on the PCB) which has a big multiway connector on it, but I just ignored it and connected a 12 VDC power supply to a 470 uF or so capacitor on the board, and the tube lights up! It states a maximum power of 1 mW but the beam looks much brighter than that (probably due to the magnets along the tube which were drawing all my tools to them).
The power supply is a Laser Drive, Inc. model 111-ADJ-1, which appears to be adjustable (due to the model number and the presence of a third wire which goes to a small preset on the PCB) but I didn't fiddle with that. It only takes 0.5 A at 12 VDC which is quite incredible. CAUTION: Do NOT just connect a 12 VDC power supply to the two red and black wires from the power supply or you will get quite a nasty shock. I don't know why.
I wasn't able to trace where the two smaller wires from the tube went. The tube also has additional optics to expand the beam size to 6mm.
(From: Sam.)
The two unmarked wires and that stuff you removed were needed to actually obtain the incredible stability that HP (now Agilent) claims. I think you got a shock playing with the power supply because the HV return is via the black input wire since there is no second HV connection to the supply.
These are called "Continuous Wave Two Frequency Lasers" or more specifically: "Helium-Neon Lasers with Automatically Tuned Zeeman-Split Two-Frequency Output". They have an extremely precise wavelength of: 632.991384 nm and 0.002 ppm short term wavelength stability.
A diagram of the general approach is shown in Interferometer Using Two Frequency HeNe Laesr.
A permanent magnet does the Zeeman splitting resulting in a pair of circularly polarized outputs at two very slightly different frequencies, F1 and F2 (difference of between 1.5 and 4 MHz depending on model and specific sample). The distance between the mirrors in the HP-5517 is feedback controlled by a heating coil wrapped around the bore to force the laser tube to maintain the position of the lasing line within the doppler broadened gain curve. I assume that a wave plate somewhere in the optical path converts the circular polarized output to orthogonal polarized components which are used externally. F1 is reflected from the thing being measured or tested (e.g., disk drive servo writer) and F2 is reflected from a fixed reference. The difference frequencies (F1-F2) and (F1-F2)+dF1 are then analyzed to determine precise position, velocity, or whatever. This approach has lower noise, greater stability, and is therefore more accurate than the common single frequency interferometer. By using cavity length control to lock the difference frequency to a known reference, the actual optical wavelength/frequency can be set very accurately. Using the MHz range beat signals makes straightforward signal processing and is more immune to noise than the baseband optical signals.
There are some photos of the HP-5517 laser head as well as two versions of the HP-5501 HeNe laser (HP-5501A and HP-5501B) with descriptions) in the Laser Equipment Gallery (Version 2.01 or higher) under "Assorted Helium-Neon Lasers".
(From: Michael (phlatlyne@attbi.com).)
The HP-5501A and HP-5517A are dual frequency interferometers where the two frequencies come from the Zeeman splitting of the energy levels. The laser produces two frequencies polarized normal to each other which are beat together internally to create a reference signal (which is just the difference between the two laser frequencies and is about 1.5 to 2 Mhz for the HP-5501A and HP-5517A and a bit higher on the newer ones). The beam is then sent to a beam splitter outside the laser which sends the vertical polarization one way and horizontal the other. One of these frequencies will be the reference beam, the other will be reflected from the object whose distance is being measured. The configuration looks a lot like a Michelson interferometer but the beam splitter is different. When the two beams are recombined and beat together, the resulting beat signal can be compared against the reference signal from the rear panel of the laser head. If you crunch through the math you will see that if the object being measured moves through a distance of one wavelength, the phase of the beat signal will move through one complete cycle (2*pi).
Both the HeNe laser power supply and piezo power supply bricks run off the -15 VDC power supply. An interlock switch (easily defeated) prevents operation with the cover removed. In the HP-5500C, the HeNe laser power supply has an additional input that may be a current adjust signal, and the piezo driver power supply provides 0 to 1.5 kVDC when fed a control voltage of 0 to 15 VDC relative to the negative input. However, in the HP-5501A, the potted power supply bricks have no inputs other than power. Rather, current and voltage control are accomplished by externally regulating the input current.
The output of the laser tube is passed through a 1/4 wave waveplate to convert the circular polarization to linear polarization, and then through a 1/2 wave waveplate to rotate the linear polarization by an arbitrary, but fixed angle to line the two linearly polarized components up with subsequent optics. The beam is then expanded to about 7 mm and passed through an angled partially reflecting plate located just beyond the collimating lens on the laser tube assembly. This deflects about 30 percent of the beam to a polarizing beamsplitter which sends each component to its own photosensor to provide the frequency control feedback. A control loop uses these signals to adjust the PZT, and thus resonator length, so that the two signals are of equal amplitude. The difference of the two signals is the frequency/phase reference.
The HP-5501A laser head requires +15 VDC and -15 VDC for power. (There is also a +5 VDC pin but it is an output according to the manual.) The two voltages (and common) are all that is needed to operate the laser but the interlock switch must be closed prior to applying power. It will lock even if not connected to the interferometer. At least I assume it's locked though I haven't yet looked at the output with a photodiode or scanning Fabry-Perot interferometer - after a few seconds, the "Retune" LED goes off, similar to if the "Retune" button is pressed. I have since acquired an operation and service manual for the HP 5501A laser head which confirms the information above.
The HP-5501B uses a very different HeNe laser tube of more conventional contruction but which has an internal heating coil for cavity tuning. The HP-5501A and HP-5501B are in the same size case and look very similar externally. See Interior of the HP-5501B Laser Head - Left Side and Interior of the HP-5501B Laser Head - Right Side.
Like the HP-5501A, the HP-5501B also requires only +/-15 VDC to power up. There is no case interlock on the laser I have, though one is shown in the manual so I assume this is either an addition or deletion depending on version. When power is applied, at first, only the +/-15 VDC power LEDs come on. After 2 or 3 minutes, the "Ready" LED begins to flash at about a 1 second rate. After another minute or so, the "Laser On" LED comes on and the beam appears. Finally, a minute or so after that, the Ready LED comes on solid and remains that way. Specific times for one test beginning from a cold start at an ambient temperature of about 65 °F were: (min:sec) 3:15, 1:35, and 0:48. The first of the times is called "preheat" and is determined by how long it takes for what HP calls the "laser rod" to reach operating temperature. The laser rod is the large glass bore of the laser tube to which the mirrors are clamped at either end. It thus controls cavity length. The temperature is sensed by disabling the heater drive and measuring the resistance of the heater coil every 25.6 seconds. The warmup is much shorter if the laser is restarted after having been running: 1:00, 1:20, and 0:50. Only after the Ready LED is on solid, do the reference signals appear. The HP-5501B adjusts the cavity length so that the two polarized components of the beam (the Zeeman split longitudinal modes) have equal power. Interestingly, there is only one photodiode sensor which is alternatively switched between beams using a liquid crystal polarization rotator. A sample-and-hold then outputs to the error amplifier of the optical mode control feedback loop.
There are two outputs of about 5 to 6 V p-p (centered about 0 V), 180 degrees out of phase. For this laser, the reference frequency is about 1.80 MHz. There is no need for a "Retune" button as with the PZT based system of the HP-5501A. Also unlike the HP-5501A, there are no other signals to or from the HP-5501B (no large connector), only the +5 VDC output on the power connector, and a fused +15 VDC output on the reference connector.
There were two problems with the particular laser I have that I had to deal with. The first is that the tube won't stay on stably at the 3.5 mA setting (fixed) of the power supply but works fine at 4 mA and still produced adequate output power. Such a condition is usually due to the tube having been run for a long time, which wouldn't be surprising with a surplus HP-5501B laser head. Since the existing power supply has no current adjustment, I needed to find a similar size HeNe laser power supply brick (1"x1.5"x4" or smaller) that will run on 15 VDC to replace it that can be set for 4 to 4.5 mA. The tube seemed healthy enough otherwise. I installed one that runs the tube at 4 mA but draws more DC input current than the original, and possibly for that reason, the controller aborts and resets after about 1 second when it turns the laser on. For now, to get around this, I have connected the HeNe laser power supply directly to the raw -15 VDC and added a transistor to drive its enable input when the original laser power turns on. That appeared to work fine. But after replacing the cover (or what of it I have - there is no front plate - which I would like to obtain), the laser tube wouldn't come on. :( I discovered that it needed the room light to start! This is a relatively rare malady for HeNe laser tubes but more common for neon lamps and glow-tube fluorescent lamp starters. So, there is now a decorative red LED shining on the back of the tube which is lit when the laser is powered. An HeNe laser power supply with a higher starting voltage would probably make this kludge, oops, feature, unnecessary. But no one will ever know about it. :)
After thinking about this, I came to the conclusion that the two mica (or whatever) pieces might act together as an adjustable waveplate where their orientations with respect to each other control the relative delay of their e and o axes, and their overall orientation determines the orientation of any linear polarization components in the output beam. (The Zeeman split beam directly out of the HeNe laser tube should be circularly polarized.) If this were set up for a 1/4 wavelength retardation, these could be used convert the circular polarization of the Zeeman split beam back into linear polarizations that could be separated out with a polarizing beamsplitter at the detectors. In fact, having acquired an operation and service manual for the HP 5501A (which functions in a similar manner), this is exactly how it works.
If two modes were oscillating simultaneously as might be the case with the reasonably short HeNe laser tube in the HP-5517, then it should be possible to recover their two polarizations and use this in the temperature control feedback loop to stabilize the difference frequency - which ultimately determines the accuracy of the interferometer measurements.
One interesting characteristic of the HP-5517 HeNe laser tube I have is that the difference frequency only appears for perhaps 10 percent of the time as it heats up - possibly only when the dominant longitudinal mode is near the center of the gain curve. I don't know if this is the normal behavior for these lasers but suspect that is is based on experiments with common barcode scanner HeNe laser tubes inside strong magnetic fields. With those as well, the beat frequency would come and go as the tube heated and expanded with this effect becoming more pronounced when the magnetic field encompassed the entire tube as it does with the HP-5517.
Experimenting with and without the presence of the mystery optics showed some effect. With them removed, there was absolutely no indication of a polarization preference in the output beam at any time. When the optics were installed and aligned to the original blue paint, the symmetry of the beat waveform, if nothing else, was polarization dependent. In addition, just after the output beat appear as well as just before it disappeared, the polarizer would suppress the beat entirely when oriented so that its axis was parallel or at 90 degrees to the axis defined by the blue paint. These perhaps weren't quite as dramatic as the effects I was hoping for but confirmed some of the speculation at least.
Thankfully, regardless of the looks of the power supply box, the actual power supply is a Laser Drive brick, 2900 V at 7.8 to 8.3 mA, adjustable. It was set all the way up on my sample. In fact, I tested the laser head using an SP-255 on a Variac and the laser will output at least 5 percent more power at 9 mA, which is as high as I went. I don't know whether they simply provided a whimpy power supply or if 8.3 mA is spec'd to be the optimum current for the tube.
The RMM355 is multi-transverse mode laser, which in itself is a novelty. This sample has an output power of about 40 mW. (Yes, that's 40 mW.) The beam is generally circular with what is basically a top-hat profile (more or less flat with ripples), has a divergence of about 2.5 mR, and is random polarized. The model number includes "R" and "MM", and possibly the "35" indicates a rated output power of 35 mW. But then what is the extra "5" for?
The laser head is a rectangular aluminum extrusion, capped at both ends with what look like black Plexiglas plates apparently attached with adhesive which has so far resisted my best efforts to remove them. The head is about 2-3/16x2-3/16x34-3/4 inches but the actual tube may be closer to only 27 inches long based on the location of the holes where the rubber potting compound was injected to hold it in place. There are no screws or mounting holes anywhere. I don't know the manufacturer of the actual laser tube but assume it has internal mirrors. There is a simple hole in the output cap for the beam to pass (no shutter) and a hole in the rear plate for the Alden cable. I'm sure I could get more power by aligning the mirrors but it doesn't look like there is any easy non-destructive way of getting inside at either end. They seem to be glued all too well. What's the fun without tweakability! :)
