Vintage Spectra-Physics HeNe Lasers
Over the years I have collected several lasers and laser related optics and equipment. If am asked what some of the more valued lasers in my collection would be, I would have to say without doubt, the Spectra-Physics Helium-Neon Lasers.
The design of the old plasma tubes, which are termed "Side-Arm" tubes, evoke thoughts of "science-fiction", during the era of the 60s and 70s when science and discovery what at its peak.
Unlike the modern coaxial Helium-Neon Laser tube, the early Spectra-Physics laser tubes featured a separate design where their bores are separated from the glass envelope that (gas reservoir).
In my collection I have a small sample of the SP-060 plasma tubes, The SP-060-4 is rated at 0.5mW and typically used in the early Spectra-Physics 132 and 155 series laser (which as of this writing, replace a tube to obtain a working 155 unit).
The SP-060-5 is a 2.0mW rated tube which typically is used in the Spectra-Physics 133 Laser.
The following image is an extract from the 1974 Spectra-Physics catalogue which details the dimensions of the various SP-060, 063 and 07x series Laser Tubes.
These lasers had "internal" mirrors which were soft sealed onto the plasma tube using epoxy. Unfortunately, the soft sealed tubes were prone to eventual Nitrogen poisoning as the seals would leak and lead to tube failure.
As of this writing, 4 of my SP-060 series tubes are still operational with good discharge colour (if goes pink, indicates Nitrogen poisoning), and still producing laser output.
Spectra-Physics SP-060-5 Laser Tube
Spectra-Physics manufactured a series of high quality scientific variant Helium-Neon lasers which come under the "Stabilite" series, which is a mounting system used in these lasers to provide a stable mounting platform for the lasers tube and minimise external factors to cause fluctuations in laser output. More details on these lasers are available in Sam's Laser FAQs.
The SP-124 was capable of 15mW output, TEMoo mode, beam diameter 1.1 mm, and a divergence of 1.0 mrad. Optionally, the laser was able to be configured for other wavelengths of 611.8nm (orange) and two IR lines of 1150nm & 3391nm. The Resonator length is 701mm, with an overall length of the head at 816mm.
In early 2000, I was asked if I was interested in two old non-functional lasers, for parts. When I received them, they were the open-frame, Spectra-Physics 907.
Spectra-Physics appeared to have released a selection of OEM variants lasers such as the SP-907 and the 107B tube (which is similar to that used in the SP-127 Laser). These are an open-frame laser, however very little is available on the specifics of the SP-907 as such have no details on the designations for the "-1" for example. These tubes are capable of 35mW output using the 082-3x plasma tube, however, may even produce more.
Following some further discussions with Samual Goldwasser (Laser FAQs / aka, "Laser Sam"), he had mentioned that the SP-907 still were a soft-sealed gas tube, contrary to my thinking that they used optically contacted Brewster windows rather than soft-sealed (epoxy), which allows for greater (if not, indefinite) shelf-life. It seems the 907 may have been an older laser before the 107B.
The resonator length of the SP-907 laser is 930mm, with an overall length of 984mm.
Around 2004 or 2005, a fellow laser enthusiast had procured a large frame NEC Helium-Neon laser that he wanted to test and asked if I had anything that may be able to run it up. I embark on a project to research and construct a power supply capable of driving large-frame tubes. I had contacted, Coast Electrical Industries (Illawarra Transformers), a transformer manufacturer on the South Coast to quote and manufacturer two 240v to 1200v step-up transformers with a 100mA rating. These transformers use a "C-Core" form.
The goal was to make two supplies, which comprise of two modules, the first being the AC filter, doubler and multiplier/starter, and the second being the passbank regulator. Only one complete supply was completed, with the passbank sent for testing with the above NEC laser. I never ended up building a second passbank and the supply project put on the shelf.
2007 marked the beginning of major changes in my life, and just before wrapping up the SP-907s, to store them away, I inspected them and attempted to at least fire some high-voltage into them to see if they were gas intact. One of the tubes was defiantly dead with a cracked tube near the main glass cathode envelope, however, the other tube did show some life but no output. Since then, I have not touched the tubes except having to had moved them into long-term storage as life presented some complex challenges.
On 26 September 2021, I decided to revisit SP-907 tubes and see if worth doing anything with them. This time, I used one half of a 15,000V Neon Sign Transformer to fire 7,500v into the tubes.
The dead tube I had marked as dead, however, the "good" tube fired up to my surprise and I was greeted by the beautiful glow of the neon coloured discharge, however, no laser output was observed.
The following evening, I decided to bring the tube into the lab and test it out on a Spectra-Physics 255 power supply, I was able to get the tube running off the supply and begin careful tweaking of the adjustments to see if was able to get the tube to lase.
Within a few minutes, I was greeted with flashes of laser light. At this point I stopped and setup the laser power meter. The laser was producing 5mW.
I revisited the laser two days later, and increased output to 9mW. This was becoming a little personal challenge; Next, to crack 10mW output. I continued to work on the laser over the next two days, with some nice results.
Having pushed through the 10mW mark, the laser was edging closer to 18mW, where the next goal was to reach 20mW.
Finally, 20mW was reached and beyond with a somewhat impressive output of 21.50mW. Reaching this, the alignment requires very light touches and a lot of patience, by no means an easy feat when you are dealing literally in wavelength increments of finesse on the alignment.
On 3 October 2021, I continued to work on the laser, this time to characterise the tube current and its behaviour. These old tubes are not in the best of health as soft-sealed tubes do not age well, as mentioned earlier, they become leaky.
With the laser power meter and multi-meter set to go as well as a variac for some additional control, the tube will light with an initial current of 15mW and then climbs as it heats up. The supply can push 17mA through the tube (which is higher than its design spec), however after around 5 min, the tube will flame out (supply finally cannot overcome the voltage / current demand). This occurs due to the tube age and health.
Running off the variac, the tube (from cold), will light and run for a minute or so at around 13mA, before it flames out. At around 250V on the input, I pushed the supply current to 18mA through the tube, however best not run at high current as tube life is reduced (at least for healthy tubes).
Based on specifications for the 107B tube; these run at 5kV, 11.5mA across the tube and a 12kV starting pulse, however, this may or may not differ for the 907 tube.
The laser settled at 21.4mW @ 16.5mA tube current. To try and "improve" the tube, will commence a power on/off cycle which should clean up the gas and stabilise the running of the tube.
On 4th October, with some cycling and adjustments, hit a record output of 22.33mW, 4.95kV @ 17.4mA across the tube.
The model 127 laser is one of Spectra-Physics largest commercial Helium-Neon Lasers with the SP-125 laser being the largest commercial unit made by the company, however also a royal pain to keep properly tuned. These days would be hard pressed to find any 125 lasers still working.
Essentially the 127 is the "scientific version" of the SP-107B with the plasma tube / resonator, power supply and an EMC line filter integrated into a case. At the time the Model 127 Laser was in production, it was Spectra-Physics's flagship in their large-frame Helium-Neon Laser product line. The laser's manufacturing specification is 35mW output, however, do produce significantly more.
The factory specifications are:
35mW Optical Power.
TEM00 output, 500:1 Horizontal polarisation.
1.25mm beam diameter (@1/e2 points) / 0.66mrad beam divergence.
The opportunity to secure such a laser in excellent condition and at a good price is rare, so in late September 2021, I commenced communication with the seller with respects to shipping considerations. This would become the biggest investments made on a laser.
The main drive to procure the laser was based on confirmed output by the seller and the condition of the tube. The Laser had come out of the University of California, Berkeley.
The Laser shipped from Sacramento, California on Friday 15th October 2021 and arrived on Wednesday 20th October 2021 where it was checked, and minor adjustments made, increasing its output to 43.5mW.
At the time of this writing, the laser had been tuned to 43.5mW output, however further work will be conducted to push this to 45mW with further careful adjustments.
An interesting realisation that this Spectra-Physics Model 127 Laser, Serial Number 35062, was manufactured in December 1990, which is in the same year and further, the exact month when I finally purchased and built my first Helium-Neon laser, so it also holds a nostalgic value in quietly marking an achievement.
The 127 features and integrated high-voltage power supply module ("brick") that provides the necessary voltage for starting (12kV) and running (5kV) the tube. The tube current is set to around 11.5mA.
The rear bezel of the unit holds the emission / power indicator lamp, key switch, interlock and input voltage selection switch.
The tube is in excellent condition with no discolouration of the bore, and excellent getters (dark metallic coating inside of the glass cathode envelope).
UPDATE, 19th November 2021: Following additional tweaks and fine adjustments to the resonator, the output of the SP-127 has been increased to 45.9mW.
This output is very impressive considering the technical specifications for the product is 35mW.
A wonderful website that I have come across is the Vintage Laser Archive which features the collection of vintage lasers, perhaps the world's largest by Robert A. Hess in Arizona, USA. Prepare to spend hours reviewing the history of laser development.
Website: Vintage Laser Archive.
- Flavio Spedalieri -
Written: 3rd October 2021
Updated: 22nd Februray 2023
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