Construction Guidelines for the Selectable FSR Scanning Fabry-Perot Interferometer Kit 1

Version 1.10 (25-Jan-2022)

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Sam Goldwasser
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The Selectable FSR Scanning Fabry-Perot Interformeter is for those who would like to explore the finer points of spherical mirror cavities. While the confocal cavity where the mirror spacing is set to be equal to the RoC of both mirrors is discussed in every text on the subject, there is usually no mention of the fact that there are a large number of other spacings that may have advantages depending on the objective. This kit uses the same mirrors as in the Basic SFPI YOR 590-650 nm kit and Deluxe SFPI YOR 590-650 nm kit (and a few others) on eBay, but utilizes a ball bearing rail and micro-positioner along with adjustable mirror mounts to facilitate setting the cavity spacing over the entire range of stable resonances for the 42 cm RoC mirrors.

Several of the resonances are suitable for looking at the longitudinal modes of a TEM00 red, orange, or yellow laser (650 through 594 nm). For this application the SFPI may also be called a "Laser Spectrum Analyzer". The same configuration may also be used as a tunable etalon or optical frequency filter.

Wavelengths beyond 650 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.

Here are the available spherical resonances up to N=10 sorted by increasing mirror spacing. Mirror spacings of 1.0 or less are often more useful being able to either reduce the size of an instrument or provide a larger FSR than the confocal spacing. Mirror spacings of more than 1.0 may be useful to provide smaller FSRs. The Planar cavity 1-0 at the top with the same mirror spacing as the confocal cavity (d=RoC=4.2 cm) would have an FSR of 2.0 x CFSR, but is much more difficult to use. (while this instrument could be loaded with planar mirrors and have a continuous range of FSRs, alignment would be really tricky.)

                                        <------ 4.2 cm RoC ----->
      Num         <---- Relative ---->    d     FSR    FWHM  Fin-  FSR Rel to
  N k Rep  d/r    FSR    FWHM  Finesse  (cm)   (GHz)   (MHz) esse   Confocal
  1 0  1  0.000  1.000   1.000  1.000   4.200  3.569   11.9   300  2.00 x CFSR
 10 1 10  0.049  2.043  20.432  0.100   0.206  7.292  243.1   30
  9 1  9  0.060  1.842  16.582  0.111   0.253  6.576  197.3   33
  8 1  8  0.076  1.642  13.137  0.125   0.320  5.861  156.3   38
  7 1  7  0.099  1.443  10.098  0.143   0.416  5.149  120.1   43
  6 1  6  0.134  1.244   7.464  0.167   0.563  4.441   88.8   50
  5 1  5  0.191  1.047   5.236  0.200   0.802  3.738   62.3   60
  9 2  9  0.234  0.475   4.274  0.111   0.983  1.695   50.9   33
  4 1  4  0.293  0.854   3.414  0.250   1.230  3.046   40.6   75
  7 2  7  0.377  0.379   2.656  0.143   1.581  1.354   31.6   43
 10 3 10  0.412  0.243   2.426  0.100   1.731  0.866   28.9   30
  3 1  3  0.500  0.667   2.000  0.333   2.100  2.379   23.8  100
  8 3  8  0.617  0.202   1.620  0.125   2.593  0.723   19.3   38
  5 2  5  0.691  0.289   1.447  0.200   2.902  1.033   17.2   60
  7 3  7  0.777  0.184   1.286  0.143   3.265  0.656   15.3   43
  9 4  9  0.826  0.134   1.210  0.111   3.471  0.480   14.4   33
  2 1  2  1.000  0.500   1.000  0.500   4.200  1.785   11.9  150  Confocal 2-1
  9 5  9  1.174  0.095   0.852  0.111   4.929  0.338   19.1   33
  7 4  7  1.223  0.117   0.818  0.143   5.135  0.417    9.7   43
  5 3  5  1.309  0.153   0.764  0.200   5.498  0.545    9.1   60
  8 5  8  1.383  0.090   0.723  0.125   5.807  0.315    8.6   38
  3 2  3  1.500  0.222   0.667  0.333   6.300  0.793    7.9  100
 10 7 10  1.588  0.063   0.630  0.100   6.669  0.225    7.5   30
  7 5  7  1.623  0.088   0.616  0.143   6.819  0.314    7.3   43
  4 3  4  1.707  0.146   0.586  0.250   7.170  0.523    7.0   75
  9 7  9  1.766  0.063   0.566  0.111   7.417  0.225    6.7   33
  5 4  5  1.809  0.111   0.553  0.200   7.598  0.395    6.6   60
  6 5  6  1.866  0.089   0.536  0.167   7.837  0.319    6.4   50
  7 6  7  1.901  0.075   0.526  0.143   7.984  0.268    6.3   43
  8 7  8  1.924  0.065   0.520  0.125   8.080  0.232    6.2   38
  9 8  9  1.940  0.057   0.516  0.111   8.147  0.204    6.1   33
  1 1  1  2.000  0.500   0.500  1.000   8.400  1.785    5.9  300  Spherical 1-1

Much more info at Mode Degenerate Fabry-Perot Interferometers.

But back to basics. ;-) The general optical layout of a confocal cavity spherical SFPI (which is a special case of the range of mode degenerate spacings) 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.

Semi-X-ray drawings of the top and side views of the Selectable FSR SFPI with the mirror spacing set at approximately the confocal spacing are shown below:

Selectable FSR SFPI using MGN12 Rail (left) and Thorlabs Rail (right)

And the version with the Thorlabs rail before installing the PZT, mirrors, and detector.


Prototype of Selectable FSR SFPI using Thorlabs Rail

The rails allow one of the mirrors to be positioned from the mirrors nearly touching to over more than 3.5 inches (8.89 cm), which is the spherical spacing and the largest one that is stable. The carrier may be locked in position so that fine adjustments can be performed using the micropositioner. The kinematic mirror mounts enable the alignment to be fine tuned.

The MGN12 ball bearing rail allows for very smooth movement but is more complex to assemble and locking is not quite as rigid as with the Thorlabs RC1 carrier. Everything is secured with screws except for the joint between the linear slide and MGN15 carriage, which may use 5 minute Epoxy. I was not able to drill a mounting hole in the steel carriage block. :(

Note that since the spacing isn't fixed, a focusing lens is NOT part of the kit since it's focal length depends to some extent on the cavity spacing. However, one can be added externally or attached to the laser if desired. In practice, the benefits of a lens may not be that dramatic for narrow beam lasers. (The Micro and Nano SFPIs don't have focusing lenses and work fine.)

The typical parts are shown below:

The kit includes the following:

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. Both photodiodes can be installed on the detector PCB with one to the side for the S-polarized component and the other over the PBSC for the P-polarized component. The PD outputs feed load resistors or a preamp and are sent to separate channels of your scope. The laser will then need to be oriented so its polarization axes align with the SFPI head.

Initial Adjustments

The best FSR to start with is the normal confocal one where the cavity length is equal to the RoC of the mirrors.

The confocal cavity SFPI requires that the mirrors be spaced precisely at their RoC, around 42 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. A DIODE LASER WILL PROBABLY NOT WORK as most are not even close to single mode.

  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".