Construction Guidelines for Scanning Fabry-Perot Interferometer Kit 1

Version 1.60 (28-Jul-2018)

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Sam Goldwasser
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HENESFPI1 contains the key components to construct a Scanning Fabry-Perot Interferometer suitable for looking at the longitudinal modes of a TEM00 red, orange, or yellow laser (633 through 594 nm). 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 kit includes the following:

Required Mechanical Components

A resonant cavity needs to be constructed that can be adjusted precisedly between 40 and 45 mm between the surfaces of the mirrors while maintaining parallelism with their centers on the same axis. One of the mirrors must be mounted on the PZT while the other will be attached to a plate. One or both mirrors must also be adjustable in pan and tilt to align them with respect to each other. With care, a single adjustable mirror is adequate.

The diagram below shows just one example of a suitable design. This is close to the minimal complexity and cost possible using Home-Depot hardware and scrap parts. 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!

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

The confocal cavity SFPI requires that the mirrors be spaced precisely at their RoC, around 43 mm in this case. So, the resonator must have some means of fine adjustment as noted above. Their axes and orientation should be coincident. Slight tilt with respect to each other isn't critical - it just shifts the center point of the spherical cavity. However, an offset may be more detrimental. Once the assembly is complete, it's time to do "first light" with a laser! A single longitudinal mode (single frequency) laser is best for this as it reduces any ambiguity in setting the cavity spacing, but a short normal HeNe (e.g., a JDSU 1508) red alignment laser can be used.

  1. The mirror spacing should be set as close to 43 mm as can be done with physical measurements. (I.e., a machinest's scale and Mark II eyeballs.)

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

  3. Trigger the scope externally using a sync signal from the ramp generator, or the ramp if none is available.

  4. 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.)

  5. Drive the PZT with a 20 to 30 V p-p ramp (or triangle) at 50 to 100 Hz.

  6. 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.

  7. With the scope's vertical sensitivity turned up, watch for any signal from the photodiode that is syncronized to 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. 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. ;-)

SFPI Display of SLM Laser as Cavity Length Approaches Optimum Starting from Too Long

(Cavity length error is approximately: +0.5 mm, +0.25 mm, +0.12 mm, +0.06 mm, +0.03 mm, 0 mm)

The entire sequence would represent a length change of a fraction of 1 mm. The amplitude of the single peak (in this SLM example) would actually increase by a larger amount than shown. Some of these diagrams are 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 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, 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 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).

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