1. This notice is included in its entirety at the beginning.
   
2. There is no charge except to cover the costs of copying.

Introduction

Wavelengths beyond 633 nm and slightly below 594 nm should work as well, but NOT down to 543 or 532 nm. Performance is best with narrow beam lasers like HeNes but the use of an aperture beam reduction optic should allow the modes of fat beam lasers to be displayed as well. The advantage of a confocal SFPI is that alignment with respect to the laser being tested is much less critical than with a planar-planar SFPI and back-reflections can be off-axis so that the laser is less likely to be destabilized.

The general optical layout is shown below:

For the confocal configuration, L = RoC of the mirrors, which must be exactly equal. In this kit, the PZT ring has been replaced by the "holey" beeper element which is more sensistive and a lot less expensive. There is no need for an output focusing lens because the beam is already very narrow.

The typical parts are shown below:

The kit includes the following:

• Confocal cavity mirrors - Two high quality dielectric mirrors:
  • S1: 99.4-99.8%@633nm with its RoC around 42 mm. These will be from the same production lot so their RoCs should be the identical.
  • S2: AR@633nm.
  • Diameter: 7.75 mm.

  CAUTION: The mirror coating on highly curved surface is extremely delicate. DO NOT TOUCH. These should be clean, requiring at most to have dust blown off with a air bulb. If cleaning is needed, ONLY use laser mirror cleaning techniques! Warranty does not cover damage to mirror surface! I suggest NOT removing the mirrors from the lens tissue until ready to use. They look kind of cool but are just mirrors! :)

  When set up in the confocal configuration where the mirror spacing is equal to the RoC, the Free Spectral Range (FSR) will be approximately 1.75 GHz. The mirror set can be used for other mode degenerate SFPI configurations using different spacings if desired, but the common confocal setup is generally most useful, though 1/2 confocal is more compact and has a larger FSR. This is left as an exercise for the student. More info at Mode Degenerate Fabry-Perot Interferometers.

  The 42 mm RoC mirror substrates are ground with significant "wedge" - the front and back are not parallel. Small wedge is often built into mirrors like this to avoid reflections from the non-mirror surface re-entering the laser, but the amount of wedge here is large enough that the mirror itself would be set at a steep enough angle to matter. So correcting for it is desireable, especially if an adjustable mount is not used for the PZT/rear mirror. Therefore, it should be attached to the PZT at a slight angle to compensate. (This can be avoided by attaching the mirror to the back of the PZT but I prefer the other way since being recessed, it would be almost impossible to clean if that should ever be required.) A piece of thick paper or ACCO™ label ;-) is usually about right if placed under the thinner side.

• PZT disk with center hole - Requires 5-10 V to move through 1 FSR. A standard electronic function generator (e.g., an old Wavetek) can be used as a driver if its p-p voltage is adequate. Or, construct Scanning Fabry-Perot Interferometer Driver 1. Or, add an amplifier to boost its output. (A dual op-amp with a +/-15 V swing will produce up to 60 V p-p with the PZT floating, which is way more than enough. See the schematic for the driver, above.) The PZT is 27 mm in diameter and the standard hole is 4-6 mm in diameter.

  The attachment of the leads to the PZT disk is rather fragile, especially the one soldered to the metallic coated area which is only loosely bonded to the PZT material itself. So it would be a good idea to coat the area with some 5 minute Epoxy to minimize stress, and then to add a strain relief when installed in the SFPI assembly.

  Note that these are what are known as "Drumhead PZTs" which means that rather than expanding and contracting in thickness, they flex like, well, the head of a drum. ;-) Therefore the PZT should be secured ONLY around its perimeter near the edge.

• Silicon photodiode (PD) - This has an active area of around 2x2mm. For mounting, the leads can be put through the holes in a Perf. board and secured with adhesive. Take care soldering as they plastic melts easily.

  The PD can be connected via shielded cable directly to the vertical input of your scope with a 10K ohm load resistor. However, it is better to connect the PD in series with the load resistor and back-bias it with a few volts (e.g., a 9 V battery). With back bias, the response (across the load resistor) will be linear up to several mW. Something along the lines of:

                              Shielded Cable
         R Protect    PD1         Center
     +-----/\/\-------|<|-----------------------+----o Scope input (direct)
     |    1K ohms            +--------------+   |
     |  (Optional)           |    Shield    |   /
     |                       |              |   \ R Load
     |                       |              |   / 10K ohms
     |                       |              |   \
     |     +| | -            |              |   |
     +------||||-------------+              +---+----o Scope Gnd
         B1 | |
  

  Confirm that the PD is responding to light. Room light will suffice, but a better test source is a dimmable LED flashlight. These use Pulse Width Modulation (PWM) to chop the light output and a pulsed waveform should be clearly visible on the scope.

  For initial setup, leave the entire photodiode uncovered. Once there is a signal, installing an aperture to block all but the central 1 mm or so may improve resolution. For low power lasers, a preamp would be beneficial.

Dual polarization option:

For simulataneously monitoring the orthogonally polarized longitudinal modes of a random polarized HeNe or other similar laser, this adds an additional photodiode and a small polarizing beam-splitter cube (PBSC). The PBSC splits the beam to the two photodiodes. The simplest mechanical arrangement is to attach the PDs to the PBSC directly with adhesive such as 5 Minute Epoxy. The PDs should be wired independently and sent to separate channels of your scope. The SFPI "head" will then need to be oriented to align with the polarization axes of your laser.

Required Mechanical Components

The diagram below shows just one example of a suitable design using Home-Depot hardware and scrap parts. This is actually more complex than needed with everything adjustable. With a short spherical mirror SFPI like this, careful assembly without any user adjustments except to be able to set the cavity spacing precisely will suffice - alignment of the overall SFPI with respect to the laser will be work just fine, thank you. :) The other extreme is to use Newport or Thorlabs optical breadboard components. But if you can afford those, you may not need to be messing around with this kit! But see the Deluxe SFPI kit!

A pair of mounts for 1 inch optics would be best. Use an adapter (available from Thorlabs) to install one of the mirrors in its mount. Attach this to a linear slide on a baseplate with a micrometer adjustment. Glue the other mirror to the PZT center using 3 dabs of 5 Minute Epoxy. DO NOT USE SUPERGLUE!!!!! Once the adhesive has cured, attach the PZT to the other mirror mount so it contacts only around its perimeter (to allow the center to move). Use an insulating pad and fasteners if it is to electrically float. Attach this mount to the baseplate with a spacer (if needed) so the centers of both mounts are precisely in-line.

A focusing lens with a focal point roughly in the center of the cavity (shown in the diagram but not included in the kit) may improve performance under some conditions but is not essential.

Initial Adjustments

1. The mirror spacing should be set as close to 42 mm as can be done with physical measurements. (I.e., a machinist's scale and Mark II eyeballs.)

2. Connect the PZT to your ramp generator. If driven at a kHz or so at 10 or 20 V p-p, a tone should be audible from the PZT confirming that it's working.

3. Connect the photodiode to your scope's vertical input with a resistor of a few kohms across it. (If a proper photodiode preamp is available, that's even better!)

4. Trigger the scope externally using a sync signal from the ramp generator, or the ramp if none is available. Displaying both the photodiode signal and ramp will confirm that the scope is synced properly.

5. Set up the test laser so it is aimed precisely into the center of the input mirror. (The optional lens should probably not be used at this time as it may make things more confusing.)

6. Drive the PZT with a 10 to 20 V p-p ramp (or triangle) at 50 to 100 Hz.

7. Observe where the intra-cavity beam is located on each mirror and adjust alignment so it is more or less centered and tight. Then check the position of the photodiode and adjust it if necessary so the trasmitted beam is centered on it. The room lights should probably be out for all this.

8. With the scope's vertical sensitivity turned up, watch for any signal from the photodiode that is synchronized with the ramp. If the blips go negative, reverse the PD polarity. If your cavity distance and mirror alignment were perfect, the result scanning through two FSRs for a laser with 3 longitudinal modes would look similar to the photo below.

SFPI Display of Melles Griot 05-LHR-151 5 mW HeNe Laser

More likely, the peaks will be smeared out or composed of multiple small blips as in the sequence of graphics below. Or there may be nothing at all. Adjust the spacing of the mirrors in small increments Slowly and then then let it settle down. With any movement, the display will become quite scrambled, so be patient. If going one way makes it worse, go the other way. :) If the initial cavity spacing was within about 1 mm of being optimal, there should be only one place close by where it resolves into a beautiful display like the one above. ;-)

These simulated "screen shots" depict a display spanning 2 FSRs for an SFPI using 42 mm RoC mirrors with 99.5%R as the cavity length approaches optimum. From left-to-right, top-to-bottom, the error is approximately: 0.3 mm, 0.15 mm, 0.07 mm, 0.03 mm, 0.015 mm, 0 mm.

The entire sequence represents a cavity length change of <1 percent but will also depend on the finesse and the mode order (confocal, half confocal, etc.). The higher the finesse, the more critical it will be. In other words you mileage may vary. :) The amplitude of the peaks would actually increase by a much larger amount than shown. Which side the "crud" is on depends on the relationship of the ramp voltage to cavity length, swap if backwards. :) Some of these diagrams are originallly from the Toptica SFPI 100 manual, I hope they won't mind. :)

First time users do not appreciate how precise the spacing needs to be. But it's less than 1/10th the width of a human hair - a few microns. Once close, the only effective way of fine tuning it is with precision screws that change spacing such as would be found in a linear translation stage, a 3-screw pan/tilt mount (which can be easily constructed from scrap parts), or something equivalent.

It's also essential to avoid back-reflections into the laser, which will likely destabilize it and create chaos in the display. The alignment should be adjusted such the the reflections from the SFPI (mostly the front mirror) do NOT enter the laser's aperture. With the confocal cavity SFPI, a slight offset will not significantly affect resolution. Ideally, an optical isolator could be used but they are really pricey.

Using mirrors identical to the ones in the kit, I've seen a finesse at 633 nm of 500 or more, though this depends on all the stars aligning perfectly. :) And I can't guarantee that all samples are that good. But expect a finesse of several hundred with reasonable care. Performance at other wavelengths may not be as good but it should still be usable to below 594 nm (yellow HeNe) and above 650 nm (may actually be better at longer wavelengths).

While the assembly guidelines here assume the standard confocal cavity configuration, there are smaller and larger mirror spacings that are still mode-degenerate making alignment non-critical. Some of these may be useful by trading off size, FSR, and resolution.

For more on SFPIs, see the section: Scanning Fabry-Perot Interferometers of "Sam's Laser FAQ".