Construction Guidelines for Scanning Fabry-Perot Interferometer Kit 1
Version 1.50 (21-Dec-2017)
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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:
- Confocal cavity mirrors - Two high quality dielectric mirrors:
- S1: 99.4-99.8%@633nm with 40-45mm RoC (matched RoCs).
- S2: AR@633nm.
- Diameter: 7.5-8mm.
CAUTION: 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 read to
use. They 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. This is left as
an exercise for the student. More info at
- PZT disk with center hole - Requires 15-30 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.)
- 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.
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.
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 centeral 1 mm or so
will improve resolution. For low power lasers, a preamp may be required.
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.
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.
- 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.)
- 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!)
- Trigger the scope externally using a sync signal from the ramp generator,
or the ramp if none is available.
- 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.)
- Drive the PZT with a 20 to 30 V p-p ramp (or triangle) at 50 to 100 Hz.
- 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.
- 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. :)
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".