Construction Guidelines for Sam's Stabilized HeNe Laser Kits
(HENESTAB1 and HENESTAB2)

Introduction

The following components are included in both HENESTAB1 and HENESTAB2:

• Spectra-Physics 088 or similar HeNe laser tube that has been tested to assure that it is well behaved and not a flipper. Note: These tubes have anode-end output. Do not touch anode end to avoid shocking experience! The guaranteed total output power is at least 1 mW.

• Laser Drive HeNe laser power supply, ballast resistor(s), and tube clips.

• A spool of magnet wire to construct tube heater.

• Polarizing beam splitter cube and two silicon photodiodes to construct the beam sampler assembly.

HENESTAB2 adds the following:

• Blank SGHS2 PCB for controller. A complete set of electronic components to construct SGHS2 may be available in the future.

What you will have to provide:

• A mounting scheme for the tube. The actual suspension should be at least slightly compliant to allow the tube to expand without sticking. A pair of tube mounting brackets are provided but they are too large for a snug fit. Some pieces of foam or bicycle inner tube can be used to fill the gap.

• Adhesive tape to secure the tube heater and its lead wires. I have used clear packaging tape with great success. However, Kapton tape like that used in commercial lasers is readily available inexpensively on eBay and elsewhere.

• Soldering iron, solder, and soldering skills to construct the SGHS2 or other controller and cables.

Laser Head Construction Procedure:

1. Prepare the HeNe laser tube: To assure the best coupling of the heater to the tube walls, remove any aluminum foil covering and/or paper labels that may be present. The aluminum foil can be peeled off after freeing the glued end. Paper labels can be removed with a single edge razor blade or Xacto knife but take care not to hit the glass-to-metal seals at the ends of the tube as they can be damaged. Clean up any adhesive residue with alcohol. Wrap something soft around the mirror mounts and secure with tape to protect the mirrors during the following steps.

2. Wind the heater: This is the trickiest part of the assembly. The winding should be done "bifilar" style - actually a pair of wires wound side-by-side joined at one end with the power applied to the two leads at the other end. This minimizes the magnetic field produced by the coil and also allows both leads to exit at one end of the winding.
   1. The target resistance for the heater is 20 ohms for use with SGHS2 or an SP-117/A/C or MG 05-STP-901 controller. It doesn't have to be precise, but should be within, say, +/-10 percent. For 20 ohms, you'll need a total wire length as follows (total feet/feet in wire pair):
      • #31: 154/77 (130.1 ohms/kft).
      • #32: 122/61 (164.1 ohms/kft).
      • #33: 97/48.5 (206.9 ohms/kft).
      • #34: 77/38.5 (260.1 ohms/kft).
      • #35: 61/30.5 (329.0 ohms/kft).
      • #36: 48/24 (414.8 ohms/kft).

      #34 is the default size included with this kit and there will be some extra wire, so the length will have to be measured.

      If you're using different AWG wire, adjust the length accordingly. Google will easily find an AWG wire data chart. As a practical matter, a wire thicker than #31 may not fit in a single layer and anything above #36 is too thin and fragile. If you want to use a different controller, you're on your own. :)

   2. It's easiest to do the winding in an area where there is an available straight clear path as long as the wire pair. Unwind 50 feet of wire (assuming #36 AWG) and double it so that you have a 25 foot length of wire pair. Temporarily fasten the end with the separate wires to the farthest point of your clear space - to a door knob for example. Carefully extend the wire pair so that it is just taught. Use a piece of tape to attach the loop-end to the HeNe laser tube about 1" from the *anode* (glass) end of the tube. Don't go closer to the anode-end of the tube to avoid the possibility of arcing from the high voltage to the heater.

      For best long term performance, it may be desirable to use high temperature Kapton tape to secure and cover the winding, but good quality clear packing has worked fine for me.

   3. Carefully wind the wire pair around the tube in a single layer by rotating the tube while gently holding the wire. Do this as neatly as possible going toward the cathode-end of the tube. It doesn't have to be perfect - if there is an occasional overlap, that won't hurt anything. But take extreme care not to kink the wire and especially to tug on kinks resulting in breakage. The wire is rather thin and easily snapped. For the 1-1/4" SP-088 tube, there will be about 75 double turns. Try to spread the winding more or less uniformly over the length of the tube with the termination close to the cathode-end of the tube. Secure that end with a piece of tape and then cover the entire winding with a single layer of tape.

   4. Strip the enamel insulation from about 1/2 inch at the ends of both wires using the heat of a soldering iron and/or an Xacto knife, razor blade, or sand paper. Confirm that the resistance is around 20 ohms.

   5. Solder a length of thin hookup wire to each of the leads and secure these with more tape so that there is no stress on the fine magnet wire.

   6. Cover the tube with 2 or 3 layers of plastic or other thin insulating material. This provides additional thermal isolation and will improve stability and immunity from fluctuations in ambient conditions due to drafts.

3. Prepare the tube mounts (if used): Drill a 1/8" hole in the center of the cathode-end mounting bracket (if used) to enable the waste beam to get to the beam sampler. Note: Depending on your specific requirements, it may be desirable to not use the included brackets and go to a different mounting scheme. For maximum output power, adjust the variable attenuator in the anode-end/output mounting bracket, or remove the attenuator entirely.

4. Prepare the base: There are pegs on the mounting brackets that should fit into corresponding holes in the base. Drill out the keying holes and drill 1/16" pilot holes for the mounting holes. Fasten the mounting brackets to the base. The plastic covers may be unlocked and flipped up so the tube can be installed without removing the brackets.

5. Mount the tube: Install the tube in the mounting brackets making sure that the anode (glass) end is in the bracket with the red wire and the cathode (can) end is in the bracket with the black wire.

   CAUTION: Reverse polarity of the high voltage to the tube may destroy the laser tube after a few minutes of running even though it may appear to be working.

6. Test the laser tube: The power requirements for the included HeNe laser power supply are 20 to 30 VDC (24 VDC nominal) at about 0.5 amp max. Red is plus, black is minus. Yellow is enable and should be tied to black. DC power can be provided by a regulated DC wall adapter, lab power supply, or stack of D-cells. :) Double check wiring before applying power.

   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.

   CAUTION: Incorrect wiring of the DC power supply to the HeNe laser power supply will damage it instantly probably resulting in no regulation and excessive current through the laser tube. Incorrect wiring of the HeNe laser power supply to the tube may damage it after a few minute even though the laser may appear to be working correctly.

   Temporarily place or mount the tube on an insulating surface for initial testing if not in the mounting brackets provided.

   Apply power. The laser should come on almost immediately. The output power will climb slightly (while mode sweeping) as the tube warms up. Confirm that there is a strong beam coming out of the front (anode-end) of the laser tube (bracket with the large hole) and a weak beam coming out the back (cathode-end) of the tube (bracket with the small hole you drilled). If the weak beam is not present, there may be some paint or other covering on the mirror that will need to be carefully removed.

   WARNING: There is over 1,000 volts on the anode-end of the tube while running and up to 10,000 V when starting. And high voltage capacitors in the power supply remain charged for awhile after shutdown!

7. Mount the Polarizing Beam-Splitter (PBS): The PBS needs to be positioned behind the High Reflector (HR) end of the laser tube. If there is a dot on one of the un-polished surfaces of the PBS, the prism on that side should face the tube but it's not critical. One easy way to mount the PBS without using fancy expensive stuff from Newport is to cut a block of wood or plastic to size so that the PBS can sit on top of it with the waste beam passing through its center. The PBS should not be aligned perfectly with the optical axis of the laser - it should be at just enough of an angle so that any reflections do not re-enter the bore of the laser tube. Confirm that the waste beam from the laser tube is split into two parts that exit at right angles to each other. If the PBS cube is polished on only 3 sides, make sure it is oriented so there are two beams exiting the polished sides. As the tube heats up, the relative intensity of these two beams will vary periodically as the longitudinal modes sweep through the neon gain curve.

   The beam passed by the PBS is the P-Mode which is horizontally polarized.
   The beam reflected by the PBS is the S-Mode which is vertically polarized.

   To minimize bore light from hitting the PDs, drill a 0.75 mm to 1 mm hole in an opaque piece of material and glue or tape it to the HR mirror or the mounting bracket before the PBS. Take care not to get any adhesive in the hole.

8. Determine the polarization axes of the tube: While watching the output of the tube as it warms up from the end or side of the beam-splitter (preferably on a laser power meter), adjust the orientation of the tube so that the variation is maximized. For a 9 to 10 inch tube, the power won't go to zero but should vary by more than 50 percent. One polarization axis is often aligned with the "tip-off" (for unknown reasons) but not always. So this may be a starting point.

   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 roughly 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. The default warmup heater power for SGHS2 is much larger - 3 or 4 W - to provide a margin.

9. Mount the photodiodes: Each photodiode should be installed so that its respective beam hits approximately in the center. As with the PBS, it's better to orient the PDs at a slight angle so that any reflections does not re-enter the tube. The red plastic strip the PDs come on can be cut in half and used to mount them via a screw through a hole drilled in each one. Or, separate the PDs from the red strip with a thin blade and use hot-melt glue! Use a multimeter to determine the anode and cathode of the PDs.

10. Make the cable and attach the connector: The cable can be of a convenient length, but not more than about 3 feet. Solder thin flexible hookup wire to the 2 PDs and to the appropriate pins on a DB9M connector. Then solder the heater wires likewise. (Twisted pairs pairs for each are recommended.) Add a jumper from pins 1 to 6 for interlock if used with the 117/A or 05-STP-901 controller. Use a cable clamp so the cable doesn't stress the heater winding or 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 Preheat 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:

• Sam's Stabilized HeNe Laser 1 (SG-HS1). This can be assembled with about $1 of electronic components.

• Sam's Stabilized HeNe Laser 2 (SG-HS2). Guidelines for assembling the SGHS2 kit can be found at Instructions for Stabilized HeNe Laser Controller Kit 2 (under development). A blank PCB is available.

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

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 Laser FAQ chapter: Commercial HeNe Lasers.

• All the parts for these circuits are commonaly available from an electronics distributor like Jameco, Digikey, or Mouser. Nothing is critical and substituting parts like different dual or quad op-amps from your junk box should be just fine.

• Contruct the circuit on a Perf. board or protyping board. Layout is not critical as the frequencies are low. If you'd like to make a PCB, that's fine also but not necessary.

• It will probably be desirable to make the feedback resistors in the first amplifier stage(s) either adjustable, or selected for the specific laser tube waste beam power to maximize and equalize the amplitude of the mode signals.

• With the circuit set to go, first check the response of the first amplifier stage(s) to the photodiode mode signals. Drive the heater at 1/2 to 2/3 of full power and watch the mode sweep. When a full cycle takes more than 20 or 30 seconds, it is probably time to close the feedback loop. If the stabilization is working, the modes will go through a part of one cycle forward or backward, and then the heater power will adjust so they stop. The desired stable point is where the heater power is at 20-30 percent of max. If it stabilizes too high, the tube is too hot; too low and the tube is too cool.

Enhancements/experiments:

• The laser tube HR mirror may be ground with no wedge so that there will be reflections from its outer surface back into the laser tube. This will both result in etalon efffects which will result in a slight ripple in output power (in addition to mode sweep) as the tube warms up and may reduce the overall stability. To test for wedge, project the beam from the HR-end of the laser onto a white surface. If there is no lower intensity ghost spot, then there is no wedge. A simple fix is to add a glass plate at a small angle (a few degrees) using clear 5 minute Epoxy or UV cure optical adhesive. Index matching adhesive is best but almost any type will be close enough to greatly reduce the retroreflections. DON'T use Crazy glue (cyanacrylic) or hard Epoxy!!! as these may damage the optics and/or are more difficult to remove if desired.

  An alternative that may be good enough is to coat the mirror glass with a thin layer of clear Silicone or 5 minute Epoxy. If done carefully, this will not distort the waste beam very much but will provie a non-uniform reflective surface that will greatly reduce the etalon effects and may help some with back-reflections into the bore.

• Deliberately introduce retroreflections and determine how they effect both the gross locking behavior and short and long term stability.

• Experiment with the amount of thermal insulation over the tube. The lsaer will stabilize with just the heater and nothing else, but by thermal insulation, the effects of the environment can be reduced substantially. (However, too much isolation will force the lock point to be excessively high.) Adding a temperature controlled enclosure could help even more.

• Look into how the performance is affected by the sophistication of the controller, and with respect to 1 mode or 2 mode feedback schemes.

-- end V1.02 --