Instructions for Stabilized Zeeman HeNe Laser Kit 1

Version 1.20 (28-Jul-14)

Sam's Laser FAQ, Copyright © 1994-2018
Samuel M. Goldwasser
--- All Rights Reserved ---

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Introduction

The two-frequency Zeeman-split HeNe laser is the key component in many high precision interferometer-based metrology systems used in applications like stage positioning for semiconductor wafer fabrication. In a nutshell, when an axial magnetic field is applied to a short random polarized HeNe laser tube, a longitudinal mode will be split into right circular and left circular polarized components with a difference in optical frequency of up to a few MHz. When constructing an interferometer, having the difference signal, F2-F1, provides advnatages over a single frequency laser in terms of immunity to noise and elimination of periodic adjustments as the laser ages.

For more information, see:

This document contains notes on the construction of a two-frequency Zeeman HeNe laser using the set of parts provided in ZEEMAN-A1. Note that ZEEMAN-A1 is a superset of HENESTAB1 so a single or dual mode stabilized (non-Zeeman) HeNe laser can also be constructed from these parts if the HeNe laser tube satisfies certain requirements (not all do).


(The style of some parts may vary slightly from what's shown in the photo.)

The following laser components are included (top pane of photo):

The following control components are included (bottom pane of photo):

What you will need to provide:

Organization of two-frequency Zeeman-split HeNe Laser:

The diagram below shows a system similar to what may be constructed using parts in this kit and readily available materials and electronics.

Many variations are possible. The Arduino implements the electronics inside the box. The REF Signal monitor is shown using the waste beam but could also be in the main beam (the polarizer would then need to be at 45 degrees). However, that would require an additional beam sampler plate (not included) which would decrease the output power slightly.

Power and wiring:

The HeNe laser power supply requires a regulated 5-6 VDC power supply at 1.5 A max. A switchmode wall adapter is included in this kit. However, if you'd rather use one of your own, adapters like those for some digital cameras or cell phones should be acceptable, but check to make sure its current rating is adequate and measure its voltage unloaded to confirm it is between 5 and 6 VDC. Jameco, Marlin P. Jones, DigiKey, Mouser, and other electronics distributors will have a suitable unit. Confirm the correct polarity if in doubt. The short red and yellow wires on the HeNe laser power supply brick should be tied together securely (soldered is desireable) and are positive; the short black wire is negative. (The yellow enables the laser when tied to red. For this application, red and yellow should be tied together permanently.)

IMPORTANT: If you ordered the kit with a 24 VDC HeNe laser power supply and adapter, the wiring is slightly different! Using the incorrect wiring may destroy the power supply. The following is for the 5 V version:


HeNe Laser Power Supply Connections

(All black wires are the same - return/Ground. The other unused wires provide approximately +/-9 V at low current intended to power an RS232 transceiver in the original barcode scanner application.)

It is even more essential that the wiring to the HeNe laser tube have the correct polarity and insulated so there can't be shorts or arcs. Reverse polarity may ruin the tube in a few minutes even though it may appear to light up and lase normally. The end of the tube that has the thick glass capillary is the anode, positive, red, and 91K ohm ballast resistor. It has the HIGH VOLTAGE and must be separated from the magnet frame by air or adequate HV insulation. The voltage can be up to 5,000 V when starting. The end of the tube with the aluminum can is the cathode, negative, black (either black wire of the power supply brick. A short or arc can destroy the HeNe laser power supply and perhaps even the wall adapter.

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! After powering down, discharge the power supply and tube capacitance by shorting between the two ends of a tube with a pair of clip leads, attaching one end to the cathode/black wire FIRST! Touching the HV probably won't kill you but throwing the tube across the room as a result of a reflex reaction is bad form and failure of the tube due to being smashed is not included in any warranty.

Make sure all connections are secure before applying power. The tube should light almost instantly. The output power may start relatively low (llke 0.7 mW) and climb to between 0.8 mW and 1.2 mW or more. (This modest increase probably won't even be obvious using only Mark I eyeballs.) It discharge glow and beam should be on continuously with no pulsing, flickering, or sputtering.

Note that there are two beams from the tube. The main beam emerges from the anode-end and may be used as both the main output and for feedback. The waste beam is around 100 times weaker and emerges from the cathode-end. Since the waveplates are required to convert circular to linear polarization, there are two sets of waveplates included in the kit. However, the waste beam can be used for the internal REF detector if desired requiring only a polarizer and eliminating one beam sampler.

Installing the heater:

For the ~1 inch diameter tube, the 2x3" heater should be wrapped around so it covers nearly the entire circumfrance. Thus the 3 inch dimension goes around it. The Kapton heater has an adhesive backing. Peel off the backing paper and stick it on centered between the two end-caps. For convenience (this is sort of arbitrary) orient it with the wire connection point lined up with the tip-off (the small metal tube where the air was removed and the gas was inserted). Press it firmly in place over its entire surface. Route the red wires toward the cathode-end of the tube. Where the wires attach is lumpy. Allow this to poke out a bit rather than squashing it against the tube which may damage the connections. Wrap some high temperature tape around the heater to keep it secure, but only one layer so as to allow heat to pass to the environment.

Determining the polarization axes of the tube (optional):

This step is only really required if it is intended to use the tube in a (non-Zeeman) stabilized laser. When installed in the magnet, the tube orientation is not critical. However, this is a good exercise in becoming aquainted with mode sweep and how the longitudinal modes behave.

NOT ALL TUBES will work for a non-Zeeman one or two mode stabilized laser, at least not trivially. The tube must have well behaved longitudinal modes. Specifically, it should NOT be a "flipper". It's often possible to use flippers for this but adds complication. Since flippers actually seem to work better for Zeeman, so the tube in this kit may be a flipper. But even it if is, experiments and stabilization without the magnets could still be interesting. So, it would be worth determining the polarization axes of the tube and labeling them.

As noted, 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 use in a conventional stabilized laser, the beam sampler can be constructed behind the rear of the tube and use the weak beam. As a side benefit, this is the low voltage end of the tube so shocking experiences will be minimized. For use with the Zeeman laser, a QWP must be inserted between the tube and PBS cube oriented with its optical axis at 45 degrees to X or Y. Alternatively, a beam sampler can be added in the output beam after the polarization has been converted to linear. In principle, using the main beam will have slightly better stability, especially for some tubes but in practice, do whatever is more convenient. I prefer to make use of the waste beam for feedback.

Place the PBS on a support so the main beam passes through its center and a deflected beam shoots off to one side. Eventually the PBS will need to be mounted securely, but for now, a block of wood or stack of CDs and CD boxes will suffice. :)

Use a white card as a screen to observe the beam coming straight through the PBS and the beam being reflected to the side. They will vary in intensity along with the polarized modes coming out the front. Adjust the tube orientation so they alternately go completely dark periodically and label the tube with with that result.

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 the two red 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 007 HeNe Laser Tube During Warmup (Detail)

This shows the mode sweep from a cold start of a tube similar to the type included in the kit.

To provide the feedback signals, one of the photodiodes will be placed behind the PBS cube and the other on the side. Devise mounts for the PDs so each of the beams strikes its respective PD and any reflections do not go back into the tube. They can even be glued to the PBS with the entire assembly mounted at a very slight angle to avoid back-reflections.

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 6 inch tube running at at 3 mA, 900 V, the bore discharge power is just under 3 W. So, 1 W of heater power should be sufficient to allow the laser to stabilize with reasonable immunity to ambient temperature changes. Using a 3 or 4 VDC power supply, it should be possible to simulate the action of an electronic feedback circuit to confirm that stabilization is possible.

A purist might object (due to noise considerations), but this means that a single 5 or 6 VDC power supply could be used for both the HeNe laser power and the stabilizer. However, this kit includes a 12 VDC power supply intended to be used for the heater.

Installing the Magnets:

CAUTION: These are powerful rare earth magnets. Keep them well away from any magnetic media or other magnetic-sensitive objects or devices. Since they are small, flesh squashing isn't a major concern but they still can do damage.

The magnets can be arranged in a variety of ways. The one that has been tested and found to be satisfactory is 7 sets of 7 magnets spaced equally surrounding the tube. They must all be facing the same way, N-S field direction-wise. Whether N or S faces the output isn't critical but will flip the error signal so once a direction has been selected, stick with it. This arrangement of magnets should result in a peak split (or beat) frequency of 0.9-1.1 MHz. However, it's possible that an arrangement of, say, 10 sets of 5 magnets might be even better. Or if a higher split frequency is desired, obtain some more magnets. That's for you to determine if interested. :)

In order for the magnets to cooperate and not jump out of position, each "stick" should be wrapped with 1" Kapton (preferably) or packing tape. Then they can be secured surrounding the tube using slices of bicycle inner tube or tape. Provide adequate insulation so there can be no chance of the high voltage at the anode-end of the tube arcing to the magnets even if they shift position. A sheet of the type of thick clear plastic often used to package grocery items works well here as shown (slightly squashed) below.


Seven Sets of Seven Rare Earth Magnets Secured to Plastic Sleave

The tube can be slipped inside and be close to the magnets without fear of short circuits or arcing.

It may be possible to increase the effective magnetic field - and thus the split frequency - by adding pole pieces at both ends of the magnet array - soft iron rings or the like. However, care must be taken not to boost the field too much. There is there a limit to how high the split frequency can go before "rogue" longitudinal modes are produced resulting in the inability to be locked and poor performance in an interferometer. With the six inch tube and 7x7 magnet array there should be none. A Scanning Fabry-Perot Interferometer (SFPI) can be used to confirm rogue mode-free behavior. Much more on rogue modes via the links, above.


Seven Sets of Seven Rare Earth Magnets Secured to Tube

This shows a similar tube inside the magnets but with electrical connections for an HP laser.

Alternatively, the tube can be installed with insulation inside a non-ferrous cylinder as in the complete Zeeman laser shown below.

Rare earth magnets are more sensitive to heat than Alnico magnets, so if installing a heater surrounding the tube, keep the array of magnets slightly away from it with air or plastic in between.

When powered with the magnets in place, the appearance of the glow of the tube and laser beam won't be noticeably different by eye, though power variation during mode sweep and actual output power may change enough to be easily detected. The fun will begin when using simple instruments. Here they are actually secured to an aluminum cylinder inside which the tube and heater (well insulated) is installed.

The Quarter WavePlates (QWPs):

The raw output of the HeNe laser tube in the Zeeman magnet consists of lasing modes that are left and right circularly polarized. For these to be useful for stabilization feedback and in an interferometer, it is necessary to convert them to orthogonal linearly polarized modes. This is normally done with a Quarter WavePlate (QWP) oriented with its optical axis at 45 degrees to the base (X and Y axes).

A pair of optical quality mica QWPs is included in the kit. THESE ARE VERY THIN AND FRAGILE so extreme care must be exercised in handling. In the envelopes in which they are packaged they are virtually invisible. It is recommended that the first thing to be done is to mount them on a supports like metal washers with a few dabs of adhesive around the periphery. The optical axis may not be readily apparent if it is marked at all, so the orientation will generally need to be determined by testing while monitoring the mode behavior of the transmitted beam(s). This will be done using µSLC1 and/or a laser power meter or equivalent.

If using the main beam for both feedback and output as in the diagram, a single QWP will suffice to convert to linearly polarization. But if using the waste beam for feedback as in the photo of the completed laser, QWPs will need to be installed at both ends of the tube.

Constructing the beat detector:

  1. Determine the anode and cathode of one of the photodiodes with a multimeter. (I can never remember which way it goes!)

  2. Wire the PD to a 9 V battery or the 5 VDC power pack and load resistor as shown:

                                      PD
              +5 to 9 VDC o-----------|<|---------+--------o Output
                                     C   A        |
                                                  /
                                               R1 \         Scope or
                                                  /         Counter
                                                  \
                                                  |
                   Return o-----------------------+--------o Ground
    

The suggested value for R1 is 2.7K which is a compromise between largest amplitude and frequency response. Feel free to experiment with higher or lower values.

Use this detector to view the Zeeman beat or measure its frequency from the tube in magnet. A polarizer must be placed in front of the PD but without the waveplates, its orientation doesn't matter.

Note that while µSLC1 has a split (or "REF") frequency input which will display in the Main Window of the GUI, it requires a TTL-compatible signal. This can be generated by an HP/Agilent optical receiver or an equivalent home-built circuit. The kit currently doesn't include parts for that, sorry, but may in the future. A simple circuit that should be suitable can be found at Teletrac 150 Reference Receiver Schematic. Either REF or REF- can be sent to the µSLC1 REF input.

Test the tube for Zeeman behavior:

Temporarily install the tube inside the magnet array with adequate insulation to prevent shorts or arcing at the anode end. It should light up within a few seconds, and to the unaided eye, will appear to lase normally. Aim the mian beam through the polarizer to the PD.

     

Tube Installed in Zeeman Magnet for Testing (Left) and Video of Typical Beat Signal (Right)

This shows the bare tube snug with its heater inside a plastic sleeve with 7 sets of 7 rare earth magnets secured to it with slices of bicycle inner tube, along with a polarizer and a built up version of the beat detector in the schematic, above, with a swing arm and 9 V battery inside. The position of the magnets along the tube was selected to maximize their overlap with the active bore discharge, resulting in the highest peak split frequency. As the tube warms up and the cavity length increases, a beat will appear periodically during mode sweep at a frequency peaking around 1 MHz for the 6 inch tube and the set of 49 rare earth magnets. The beat will probably be present between 50 and 75 percent of the cycle, and will change frequency somewhat during this time. With a greater magnetic field, the percentage of the cycle over which there is a beat decreases. For example, it may be less than 10 percent for an HP-5517D laser which locks with a ~3.75 MHz split frequency.

If 5-10 VDC is applied to the heater, the rate of the mode sweep will greatly increase since the tube is expanding faster. The short video on the right shows the output of the back biased photodiode behind a polarizer using the rig in the photo on the left with around 10 V applied to the heater shortly after the tube was powered on.

Mounting the tube in the Zeeman magnet:

Now mount the tube/heater assembly more permanently inside the set of magnets making sure to provide adequate electrical insulation. There should only be minimal thermal insulation - one layer of Kapton tape. The tube needs to be able to dissipate heat to the environment for the stabilization to be effective. In addition, it needs to be able to expand and contract so it should NOT be attached rigidly. But since the total change in length is a fraction of 1 mm, this should not be a problem.

Eventually, the QWPs will need to be added but that can wait until there is a convenient way of monitoring the outputs.

Closing the loop:

To stabilize the laser so that the position of the modes is under automatic control with a Zeeman beat 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. :)

However, this kit includes components to construct a fully automatic controller using an Atmega Nano 3.0 Arduino-compatible microprocessor platform. Assembly of the electronics and operation of the µSLC1 Windows GUI may be found in the Micro Stablized Laser Controller 1 (µSLC1) Installation and Operation Manual.

However, unlike the non-Zeeman case where one can lock on either side of the gain curve depending on the polarity of the error signal, with the Zeeman laser, there is only one place to obtain a beat. In other words, it's possible to lock so that the two components - F1 and F2 - are equal in two locations. One is where the Zeeman-split longitudinal mode is between the split gain curves and there will be a beat. The other is where there are actually two separate (non-Zeeman) longitudinal modes with equal amplitude but far far away at the longitudinal mode spacing of the laser tube. In that case, there will be no beat without a fancy high speed detector since the frequency would be over 1 GHz.

Adjusting the QWPa:

Perpare to install the QWP between the tube and PBS beam sampler. While monitoring the horizontal and vertical mode signals from the PBS, rotate the QWP until the two signals are most different. They should look something like the screen shot below.


µSLC1 Display Showing Zeeman Mode Behavior Near End of Locking

The ragged green and orange plots of of the modes is characteristic of Axial Zeeman behavior. The QWP orientation should be set to maximize their amplitude.

If using both the waste beam for feedback, test and adjust both QWPs and label their orientation. Then transfer one to the main beam using the same orientation.

Enhancements/experiments:


Sam's Laser FAQ, Copyright © 1994-2018, Samuel M. Goldwasser, All Rights Reserved.
I may be contacted via the
Sci.Electronics.Repair FAQ Email Links Page.