Construction Guidelines for Scanning Fabry-Perot Interferometer Kit 2
Version 2.00 (27-Sep-2016)
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HENESFPI2 contains the key components to construct a confocal Scanning
Fabry-Perot Interferometer (SFPI) suitable for displaying 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 benefit of HENESPFI2 is that in addition
to what's in HENESFPI1, it includes parts from Thorlabs for the
major mechanical assembly so that much of the scavanging
for scrap sheet metal and other tid-bits is eliminated. The Thorlabs
"Cage System" provides a rigid frame allowing for initial adjustment
which can then be locked in place. The completed frame and other
parts are shown below.
The standard HENESFPI2-1 kit includes:
- Confocal cavity mirrors - Two high quality mirrors:
- S1: >99.5%@633-594nm, with 43-45mm RoC.
- S2: AR@633nm.
- Diameter: 7.5-8 mm.
CAUTION: The mirror coating on the highly curved surface is extremely delicate.
DO NOT TOUCH with ANYTHING. The mirrors 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! The warranty does not cover damage to mirror
surface! I suggest NOT removing the mirrors from the lens tissue until ready
to install. They are just mirrors! You can purchase similar mirrors
separately for use as decorative aquarium stones, but those tropical fish had
better be really special. :-)
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. This is left as
an exercise for the student. More info at
- PZT disk with center hole - The "drumhead" PZT disk requires 5-10 V
to move through 1 FSR. A standard function generator can be used as a driver
if its output voltage is adequate. A maximum of 20 V p-p is typical. An
amplifier can be added 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.) Or, construct
Sam's Scanning Fabry-Perot Driver 1. See
Schematic of Sam's Scanning Fabry-Perot
Interferometer Driver 1. (Blank PCBs are available.)
- Silicon photodiode - A photodiode with an active
area of around 2x2mm. (Smaller than the one in the photo. A smaller
area is better both for response and resolution. Adding an aperture
could be beneficial as well. This can be connected via shielded cable
directly to the vertical input of your scope with a 10K ohm load resistor.
For low power lasers, a trans-impedance preamp (1 op-amp) may be required.
See the schematic in the link, above.
HENESFPI2 additional parts:
- Mounting plate for PZT/rear mirror and PD: Thorlabs CP02
(SM1-Threaded 30 mm Cage Plate, 0.35", 2 Retaining Rings).
- Mounting plate for front mirror: Thorlabs CP02T (SM1-Threaded
30 mm Cage Plate, 0.50", 2 Retaining Rings).
- Adapter for front mirror: Thorlabs SM1AD8 (Externally SM1-Threaded
Adapter for 8 mm Optic).
- Mounting plate for 1/2" diamaeter focusing lens: Thorlabs CP11
(SM05-Threaded 30 mm Cage Plate, 0.35", Two Retaining Rings).
- Structural rods: Thorlabs ER4-P4 (Cage Assembly Rod, 4" Long,
6 mm, 4 Pack).
- PZT spacer/adapter ring: Attach the PZT to this ring with adhesive.
- Focusing lens: 1-1/2" to 2" focal length positive lens
(installed). The lens may improve the resolution depending on the
input beam diameter and divergence. Adjust the location for optimal
- Piece of Perf. board: Use for mounting photodiode and optinal
connectors. Use as you see fit. (Cut to fit.)
- Additional parts: Depending on availability, there may
be other odds and ends to make your life easier. :)
Note: If you ordered a kit with mirrors other than the default
43 mm/1.7 GHz FSR, the rod length may differ and the focusing lens
will not be included.
What you must provide:
The following are NOT included:
- Three-screw baseplate or adjustable mount for the SFPI head: A
suitable baseplate can be fashioned from a piece of aluminum, plastic,
or even wood scrap and 3 4-40 machine screws with their tips rounded
or sharpened to a point. Or a LARGE kinematic mount.
- Light and dust blocking cover: Once complete, some means must
be provided to both prevent ambient light from affecting the photodiode
as well as to keep dust off the mirrors.
- Miscellaneous hardware: Small screws, washers, etc.
- Adhesive: 5-Minute Epoxy is recommended. DO NOT use
SuperGlue™ (Cyanoacrylate)!!! It can damage optics (and internal
- Wire, solder, etc.: A functioning brain is also recommended. ;-)
Rear Cage Plate Preparation
The CP02 is used to mount the PZT/rear mirror assembly on its front side and
the photodiode and its wires, as well as the PZT wires on its rear side.
The best way to do this is to drill and tap three 2-56 holes on the front
side for screws to secure the PZT/rear mirror assembly and two 2-56 holes
on the back side for the PD/wiring board.
Take care with the photodiode as it's real easy to break off the leads.
I recommend poking it through the Perf. board and immediately securing it
with 5 Minute Epoxy. Then solder fine wires to the leads.
- Use the PZT spacer/adapter ring as a template to locate and drill and tap
three 2-56 holes on the front of the plate approximately equally spaced
around the outside the ring just barely outside the periphery of the ring.
- Locate a pair of suitable holes in the Perf. board and use them as
a template to drill and tap 2-56 holes on the rear of the plate to hold it
If you don't have a
drill-press and are not familiar with using small drill bits and taps, it's
probably best to consider some alternative. It's really easy to put holes in
the wrong place at strange angles and to break drill bits and taps, which
ends up making a mess because at the very least, it often impossible to
extract the broken pieces. (Don't ask how I know.)
The PZT/rear mirror assembly and Perf. board can be attached to the cage plate
with 5 minute Epoxy, which is soft enough that it can be removed
and replaced if the need should arise.
PZT and back mirror
Prepare the aluminum spacer ring and attach the PZT disk to it.
(Using the ring isn't essential but will make mounting to the Thorlabs plate
slightly easier and provide a path for the PZT leads inside the plate
to exit out back.)
Using a narrow file, create a notch wide and deep enough on one surface of
the mounting ring for the PZT leads to pass through between the ring and
Use 5 Minute Epoxy to attach the PZT disk to the aluminum spacer ring
around its perimeter. The active (coated) side of the PZT disk should
be facing up.
The back cavity mirror must be glued to the PZT.
(The following applies to the standard 43 mm RoC/1.7 GHz mirrors. Other types
may not have wedge and thus may not require this extra step.)
The 43 mm RoC mirror substrates are ground with significant "wedge" - the
front and back are not parallel. Small wedge is often built in to mirrors
like this but the amount of wedge here is large enough that 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 it to the back of
the PZT but I prefer the other way since being recessed, it's almost
impossible to clean if that should ever be required.) Carefully remove
the mirror from its protective lens tissue. Locate the thinnest part and
put a mark on the side with a pencil. The currect angle can be obtained
by putting a tiny piece of standard Avery sticky label under it on the
thin side. :) (Very scientific, huh?) A 1x5 mm piece of label will
Center the mirror on the PZT disk taking care to orient it so the mark
you added lines up with the bump. While holding it in position with
a clean cotton swab, apply the tiniest amount 5 minute Epoxy in 3 locations
around its perimeter. Just enough to secure it. Once the Epoxy sets up,
additional adhesive can be added to increase the strength, but there really
isn't much stress on this thing and too much adhesive may restrict the
motion of the PZT..
Set this assembly aside covered so the mirror doesn't collect dust or
The front cavity mirror is installed in the SM1AD8 adapter. Remove one of
the retaining rings. Slip one of the cavity mirrors in, coated face first.
Something compliant between the back of the mirror and second ring is
required to (1) prevent crunching of the mirror (which is bad form) and
(2) to assure that the front face seats square. A rubber O-ring is
ideal, but a piece of thin wire insulation (without the wire) can be curled
up to fit. Screw the second retaining ring in so it is just snug. Don't
overtighten. Be particularly careful not to touch the coated surface and
not to ding either surface while tightening the ring. (Tighten from the
AR-coated side - damage to it is less of a disaster!)
Remove one retaining ring (if present) from the CP02T cage plate and adjust
the other one so it is just flush with the front of the CP02T.
Screw the SM1AD8 adapter in until it is 1 or 2 turns away from the retaining
Focusing lens (if used)
A focusing lens with a focal point roughly in the center of the cavity
(included in the standard Deluxe kit) may improve performance under some
conditions but is not essential. If this isn't already installed,
remove one retaining ring from the CP11,
slip in the lens, and install the retaining ring just snug. If you can't
find it in the package, it may already be installed. :)
Completed SFPI on Three-Screw Adjustable Base (not included)
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. With the Thorlabs cage parts,
coarse adjustment is provided by sliding the front mirror along the bars.
Fine adjustment is done by rotating the threaded adapter in which the
front mirror is mounted. The axes and orientation of the mirrors should
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. Thus care in assembly with
respect to where the mirror mounted on the PZT and where the PZT is
mounter are important. 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 synchronized 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
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. :)
If there is no evidence of any response related to the laser, confirm that
the PZT is actually beging driven - set the function generator to a few
kHz and listen to the PZT! There should be a very audible tone. Check
the output of your function generator and connections if there is none.
And check that the photodiode responds to light.
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. :) With no angular adjustment, the centerlines of the two
mirrors should also be as coincident as practical to really peak the finesse.
This requires either super-careful installation
of the glued mirror, or fine adjustment of the position of the PZT
assembly on the cage plate. In addition, down to a point,
narrower beam (e.g, 1 mm) generally results in higher finesse.
A wide beam can be put through an aperture (though the signal will be reduced.)
Expect a finesse of several hundred at 633 nm with reasonable care.
Performance at other wavelengths is not as good but it should still be
usable to below 594 nm (yellow HeNe) and above 650 nm (might even be better
at some longer wavelengths).
Once it's near optimal using the sliding adjustment, tighten the set
screws to secure the cage plate. Then fine tune further by rotating
the SM1AD8 adapter while pressing it *away* from the locking ring.
Keep in mind that since it's likely the mirror isn't perfectly centered,
the alignment will also change slightly as the SM1AD8 adapter is rotated,
so also peak alignment as you are doing this. (May require 3 or 4 hands.)
Then tighten the locking ring to secure it.
For more on SFPIs, see the section:
Scanning Fabry-Perot Interferometers
of "Sam's Lsaer FAQ".