Instructions for Stabilized HeNe Laser Kit 2

Version 1.00 (14-Aug-09)

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Samuel M. Goldwasser
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This document contains notes on the construction of a frequency or intensity stabilized HeNe laser using the set of parts provided in HENESTAB1 or HENESTAB2. The following components are included in HENESTAB1.5:

What you will have to provide:

Power and wiring:

The HeNe laser power supply requires 21-31 VDC at about 0.5 A. A common wall adapter with these ratings will suffice, though I would recommend one that's regulated, say 24 VDC. Jameco, Marlin P. Jones, DigiKey, Mouser, and other electronics distributors will have a suitable unit. Confirm the correct polarity if in doubt. Red is positive; black is negative. Yellow enables the laser when tied to black. For this application, black and yellow should be tied together permanently.

It is even more essential that the wiring to the HeNe laser tube have the correct polarity. Reverse polarity will ruin the tube in a few minutes even though it may appear to light up and lase normally. With the two clip sizes provided, it's hard to mess this up but I've seen people try! The end of the tube that has the thick glass capillary is the anode, positive, red. The end of the tube with the aluminum can is the cathode, negative, black. :)

Make sure all connections are secure before applying power. The tube should light within a couple seconds. The output power starts at about 1.1 mW and will climb to about 1.3 or 1.4 mW when warmed up.

WARNING: There is over 1,000 V present while running and over 5,000 V at startup on the anode mirror mount of the tube! Stay clear!

Determining the polarization axes of the tube:

Note that the laser tube actually produces 2 beams: A strong one out the anode-end (front) and a much weaker one out the cathode-end (rear). For the stabilized laser, the slip-on beam sampler goes on the rear of the tube and uses the weak beam. If using Brewster plate sampling, they go in the main beam.

With the tube is powered, place a continuous reading laser power meter in the output beam. Use a polarizer to identify the orientation of the polarized modes of the tube. This will be the angle of the polarizer where the variation in power due to mode sweep is maximized, and the axis 90 degrees from that. One of these often more or less lines up with the exhaust tip-off (metal tube with red cover at cathode end of tube).

The power varies because the longitudinal modes of the laser cavity are moving through the neon gain curve as the tube expands due to heating. The rougly bell-shaped gain curve results in gain variation depending on its height. If 5-10 VDC is applied to the heater (between red and black wires), the rate of the mode sweep will greatly increase since the tube is expanding faster.

As the tube/heater combination approaches thermal equilibrium where the power input from the electrical discharge in the bore of the laser tube and heater power are balanced by heat loss to the environment, the mode sweep will slow down and eventually stop. If power is removed from the heater at that time, the discharge heat alone will no longer be able to sustain the same temperature, the tube will start to cool, and the mode sweep will reverse.

Plot of Spectra-Physics 088 HeNe Laser Tube During Warmup (Detail)

For thermal stabilization to be effective, what is desired is where a modest amount of heater power is needed to be at thermal equilibrium. Perhaps 20-30 percent of the power in the bore discharge. For the 088 tube, the bore discharge power is about 4 W. So, 1 W of heater power should be sufficient to allow the laser to stabilize with reasonable immunity to ambient temperature changes.

Checking the beam sampler:

Cut a piece of a sticky black label or other similar opaque material to be about the same size as the mirror glass at the rear of the tube (order of 6-8 mm or 1/4 inch). (Use a Magic Marker to turn a white label black if needed so it's more or less opaque.) Use a tiny drill bit or similar tool to make a clean 0.7 to 1 mm hole in it. With the laser powered, stick this aperture over the rear end mirror so only the actual weak waste beam passes through it. The purpose of the aperture is to block "bore light" from affecting the photodiodes in the beam sampler. (The tube provided will probably already have something like this in place but you might want to improve upon it.)

Attach a microammeter, or multimeter set to measure current, between the common (center) and one of the photodiode signal leads (red or white). Slide the beam sampler assembly onto the cathode-end (rear) mirror mount. As the modes cycle through the neon gain curve, the current produced by the photodiode will vary in the same way as the corresponding polarized modes in the output beam. Record the minimum and maximum current.

Note: It may be necessary to cover the area of the beam sampler with something opaque to prevent room light from interfering with the photodiodes.

Closing the loop:

To stabilize the laser so that the position of the modes is under automatic control requires some electronics to first run the tube in "Preheat Mode" so that the temperature of the tube/heater combination levels off somewhat above ambient, and then to "Lock Mode" to allow the output of one or both photodiodes to take control.

If you're willing to switch from Prehat to Lock mode manually, the required circuit can be as simple as 2 basic electronic components - a resistor and a power MOSFET. This won't have superior performance but is quick and easy to get working and therefore will provide nearly immediate gratification. :)

Much more sophisticated approahes are possible including a fully digital control system with wireless Internet access and data logging. :) But an intermediate level of complexity similar to that used in most commercial stabilized HeNe lasers is certainly well within the reach of someone with a moderate knowledge of electronics. This would use a couple op-amps to to act as a transimpedance amplifier for the photodiodes and implement Proportional Integral (PI) control loop.

Assuming a basic but not totally minimal approach, one of the following should be suitable as a introductory exercise in laser stabilization:

Photo of Sam's SP-117 Compatible Stabilized HeNe Laser Head and Controller SG-HS2

This laser uses a hand-wound heater but is otherwise similar to the kit.

Note that both these circuits have the photodiodes paralleled with opposing polarity with a single op-amp rather than separate preamps and a difference amp. To do it this way will require rewiring the beam sampler.

For more detailed descriptions and other options, see the sections of the Laser FAQ starting with: Inexpensive Home-Built Frequency or Intensity Stabilized HeNe Laser.

Or, use parts of the circuitry of a commercial stabilized HeNe laser like the Spectra-Physics 117 or Coherent 200. There are complete schematics of these in the chapter: Commercial HeNe Lasers.


Sam's Laser FAQ, Copyright © 1994-2021, Samuel M. Goldwasser, All Rights Reserved.
I may be contacted via the
Sci.Electronics.Repair FAQ Email Links Page.