I needed to measure the height of a free space fiber termination above an optical bench the other day and in the process remembered a question I had been asked in passing. Since I had the pieces of such an experiment in front of me, all I had to do was save a couple of images.
In this case I was looking at the end of a single mode fiber patch cord to set it a particular distance above an optical breadboard.
With the laser turned on at a minimum intensity the light from the fiber looks like this zoomed in image of about 400 x 300 pixels of the whole 1.6 M pixel frame.
The laser spot (white dot) was purposely decentered from the magenta cross that is the origin of the PSM coordinate system so the shape of the spot could be clearly seen to get best focus. The centroid of the spot was measured to 0.2 μm in x and y using a 4x objective on the PSM.
Without touching anything other than turning off the laser source and shining a light past the objective to illuminate the front of the fiber I got this image of the 2.5 mm diameter fiber ferrule with the 125 μm core embedded in its center.
If you squint you can still see the magenta cross and the red scale bar to give a feel for the relative scale of the 2 pictures. The faint darker cross at 45 degrees gives a hint as to how the ceramic ferrule extrusion was made.
GOOD NEWS FOR THE NEW YEAR, THE STANDARD PSM WORKS IN THE NEAR IR
Many times potential customers have asked “How far does the PSM work into the infrared?” I told them the standard PSM works as far out as 1050 nm with the CMOS camera that comes with every new PSM because I have used the PSM there with a fiber coupled external source. But some customers want to go farther into the IR.
PART 1: EXPERIMENTS WITH IR CAMERAS
USING THE PSM TO SEE THROUGH SILICON
Recently we purchased a camera that is sensitive out to 1600 nm and a laser source at 1550 nm to see if we could use the PSM to see through silicon. It works! We did the most simple minded experiment of putting a silicon wafer between the PSM objective and a front surface plane mirror as in the photo.
This shows that with an external laser source and a fairly inexpensive NIR camera the standard PSM is useful for aligning lenses containing silicon optics. It had been my worry that one of the lenses in the PSM, or a beamsplitter coating, would prevent the PSM from working at this wavelength. The standard PSM works just fine with the addition of the NIR C mount camera.
Some people, however, want to look through germanium that becomes transparent at 1900 nm or so. The inexpensive NIR camera does not work for this. But the question for me remained, is there something in the PSM or objective that would block light in the region that germanium transmits. Mark Christenson, a representative of Envisionate offered to get a Xenics Xera 2.35 camera to try out with the PSM. With a 6 mm uncoated germanium window sitting in front of the objective we could see the filament of an incandescent heat lamp, see below.
IMAGE OF A HEAT LAMP FILAMENT VIEWED THROUGH A 6 MM GERMANIUM WINDOW WITH THE PSM
This is hardly a sophisticated experiment but it proves there is nothing in the PSM that prevents use of a camera sensitive out to 2350 nm. This is not an inexpensive camera, but if you already own a PSM, you can simply screw this camera on to the PSM C mount and bring in a fiber coupled source at around 2 um and be ready to align lenses with germanium optics. In this case, Optical Perspectives is not going to purchase the camera for you, but if you have or can borrow a camera, we will provide a PSM so you can demo the idea in your facility.
XENICS XEVA 2.35 CAMERA C MOUNTED TO THE PSM
The picture of the PSM mounted on the camera looks like the tail wagging the dog, but if that is what it takes, this is a solution to looking through germanium optics.
PART 2: PRECISION LOCATION OF CGHS
A METHOD OF PRECISELY LOCATING A COMPUTER GENERATED HOLOGRAM (CGH)
When using computer generated holograms (CGHS) to test aspheres and freeform optics it is essential that the CGH be precisely located relative to the interferometer transmission sphere and the optic under test. This location is often done with 3 balls mounted to the CGH to form a kinematically repeatable method of mounting the CGH. One of the limits to the precision is how well the balls align with the CGH pattern. We have a way of aligning the balls within a micrometer of their optimum location using the PSM provided there is a little planning in the design of the CGH.
In a prior paper* we talked about writing Fresnel zones on CGHs to simulate concave spheres for alignment purposes. If these Fresnel zones are written at the time the CHG null pattern for the asphere is written the zones will be within 10’s of nm of the desired location. We showed in the paper how a PSM is used to position a ball to a μm of the center of the Fresnel zone. As described in the paper the method of attaching the balls is it is awkward to implement.
A much better method is to use readily available Spherically Mounted Retroreflector (SMR) nests (used with laser trackers) to hold the balls. These nests have a magnet, a plane back and a precise cone to hold a precision ½” diameter ball. This makes a stable mount for the ball, it is easy to slide the nest/ball pair into place on the CGH and provides for good bonding to the CGH. The pictures below show the idea.
THIS PICTURE SHOWS JUST THE NEST AND A GRADE 5 CHROME STEEL BALL
THIS PICTURE ABOVE SHOWS HOW THE BALL AND NEST ARE POSITIONED OVER THE SMALL FRESNEL ZONE PATTERN THAT ACTS LIKE A CONCAVE MIRROR WITH A RADIUS OF CURVATURE JUST EQUAL TO THE HEIGHT OF THE CENTER OF THE BALL SITTING IN THE NEST
To position the nest/ball pair, the PSM picks up the center of curvature of the Fresnel zone and is adjusted so the reflected spot is well centered on the PSM crosshair. Then the nest/ball pair are slid into place so the ball center is centered on the crosshair. Now the ball center is precisely centered over the Fresnel zone pattern within 1 μm and the nest is cemented in place.
THREE NEST/BALL PAIRS PRECISELY POSITIONED ON A 6” PHOTOMASK SUBSTRATE AND CGH PATTERN
The finished CGH looks like this image with three nest/ball pairs ready to be set in a kinematic mount.
This method has several distinct advantages over some others.
First, the ability to position each of the balls within a μm of the precise location using the CHG pattern and the PSM.
Second, the SMR nest makes a stable platform on which to hold the ball while positioning it prior to cementing as opposed to the method shown in the paper.
Once the nests are bonded the balls are removable so there is a minimum of height above the CGH surface and the nests are held securely in place. An added advantage is that the position of the CGH can be determined either using a CMM and touch probing the balls, or the ball can be replaced with ½” SMRs and located with a laser tracker.
MICROPHOTOGRAPH OF THE CENTER OF THE FRESNEL ZONE PATTERN USED FOR POSITIONING THE NEST/BALL PAIRS
* Parks, R. E., “Optical Alignment using a CGH and an autostigmatic microscope”, Proc SPIE, 10377, 103770B, (2017), available in the Downloads>Bibliography accessible from the button below.
How well can the Point Source Microscope (PSM) find a point in space?
A POTENTIAL CUSTOMER FOR A PSM RECENTLY ASKED “WHAT IS THE ACCURACY/REPEATABILITY OF ALIGNMENT OF OPTICAL AXIS?”
The question is a little ambiguous as to exactly what he was asking for, but my own experience in the lab is that the PSM with a 10x microscope objective can locate a center of curvature or the axis of a Bessel beam to a small fraction of a micrometer.
However, I wanted to give him a more specific answer than my own experience and had to think where the supporting data could be found.
Then I remembered a paper from 3 years ago with data taken by a graduate student at UNC-Charlotte, Jesse Groover. Jesse, whose advisor was John Zeigert, had set up a PSM in a metrology lab looking at an Axicon grating on a Moore Tool Universal Measuring Machine.
THE POINT SOURCE IN THE PSM CREATES A BESSEL BEAM IN REFLECTION FROM THE AXICON GRATING WHEN THE POINT SOURCE IS ON A NORMAL NEAR THE CENTER OF THE GRATING.
If the PSM is centered precisely on the normal to the center of the Axicon grating, the Bessel beam bright core will lie on the origin of the PSM detector. Jesse took repeatability measurements at 3 distances from the grating to see how well the PSM could locate the center of the Bessel beam.
THE DATA HE TOOK IS SHOWN IN THESE TWO CHARTS.
When the PSM is very close to the grating (5 mm) there is almost no deviation from zero. At greater distances there is some deviation presumably due to air turbulence, but in all cases the repeatability is limited to ± 0.3 μm verifying my anecdotal lab experience.
The paper goes on to say “The repeatability of the PSM was evaluated by mounting it on a Moore UMM located in a well-controlled environment. This machine is highly stable with vibrational disturbances and axis positioning repeatability on the nanometer level.
The PSM was focused on the axicon CGH at distances of 5, 70 and 140 mm from the surface, and readings were taken at 10 second intervals over a period of 10 minutes. It can be seen that the PSM measurements are repeatable to within a few tenths of a micron.
Measurements were also taken by jogging the machine away from the initial position and back multiple times. The results are virtually identical, and consistent with the nanometer level positioning repeatability of the Moore UMM.”
THE PAPER CITED IS “COMPUTER GENERATED HOLOGRAMS AS 3-DIMENSIONAL CALIBRATION ARTIFACTS”
A copy of the full paper is available for download here on the Optical Perspectives Group website and can be accessed by registered users with the button immediately below.
If you have not already registered, do so. Then you have access to all the papers on the website about the PSM and optical testing in general under the Bibliography tab. The registration page is here.
GET MORE FUNCTIONALITY FROM YOUR PSM BY SAVING LARGE SAMPLES OF DATA
In the PSM Align software there is a means of saving the locations of up to four x, y spot locations at a time as a part of the Threshold tab.
Once the spots are individually tagged with the cursor, their x, y locations, as seen on the Threshold tab, can be saved to a .csv file.
While this is a handy feature, some users might wish for a method of saving spot locations as an object is scanned, or saved over a time period when doing a drift test to see how alignment changes with temperature changes in the lab.
All you have to do is ask us and we will send a link to upgrade your software, gratis (see below).
LCS-PSM ALIGN SOFTWARE
As you are probably aware, Optical Perspectives sells a centering station with a vertical column that moves the PSM up and down the optical axis of the system being aligned.
As a part of that operation the LCS-PSM Align software saves the spot and vertical column locations to a .csv as the stage moves.
This software is not supplied with the purchase of just a PSM because there is a bit of a learning curve getting familiar with the PSM.
However, once familiar with the PSM and software it is not much of a jump to take advantage of the LCS-PSM software.
While the software is designed to synchronize the spot location data with the movement of the Centering Station stage, the software is written in a way that it can be fooled to take data points at constant time intervals. This means that if you want to monitor the drift in alignment it is possible with the LCS-PSM software.
SYNCHRONIZE YOUR DATA
Further, while the software was meant to be synchronized with a stage motion, since it takes data at a constant time interval, if you have a scanning system that operates at a constant velocity, the software will take data as though it were synchronized.
This means that scan data can be saved without going through all the coding compatibility issues to do true synchronization.
Just start the LSC-PSM scan and then start your independent but constant velocity scan. The software will create a file of x, y spot location versus time.
After transferring the PSM .csv file to Excel or similar software, you can turn the time base into distance scanned and have a graph of spot position versus distance.
THIS GRAPH IS AN EXAMPLE OF DATA TAKEN THIS WAY.
In this chart the LCS-PSM scan was started at 0 degrees while the table rotation was started at 45 degrees and stopped at 315 degrees and then the PSM software scan was stopped at 360 degrees.
It is clear the table was not moving from 0 to 45 degrees and ended it rotation at 315 degrees because the spot motion ceased except for a little noise at the sub-μm level.
While the table was rotating there is a continuous plot of the small following error between the point on the table and the tool motion that was supposed to follow the table rotation.
There is also clear evidence of a shift at start up and shutdown as well as possible backlash as the follower changed directions.
This type of scanning information is easily gathered by the PSM following a high quality steel ball at the focus of the PSM objective.
It does not require compatibility between machine tool and PSM software. The only requirement is that the scan motion run at a constant velocity so the time base is easily converted to a distance, or angle as in this case.
HOW TO OBTAIN THE LCS-PSM SOFTWARE
If you have an application where you would like to use your PSM to follow a constant velocity motion or drift over time, use the form below to ask and we will send you a link to the LCS-PSM software.
NOTE: The PSM part of the software is identical to what you already have, the new part enables the captures of scan data versus a time base.
I am sorry to say I did not get to meet any of you personally at either the SPIE Optics and Photonics show in San Diego or the American Society for Precision Engineering Exhibit scheduled for Minneapolis in October.
THE CENTERING STATION DOES PRECISION CENTERING WITHOUT THE NEED OF A ROTARY TABLE. THIS MAKES CENTERING SIMPLER AND FASTER, INCREASING PRODUCTIVITY.
While I call it a centering station you can also think of it as a vertical optical bench so gravity helps when you insert optical elements. In this way of thinking you can easily obtain first order lens parameters quickly from PSM reading and the motorized vertical stage with 1 micron resolution.
I find the station so useful that I am constantly changing from one set up to another as inquires come in to make a measurement or see if a particular assembly performs as expected. The breadboard work table makes it particularly easy to assemble test fixturing from standard catalog opto-mechanical fixtures usually found in most optics labs.
ACCESSORIES FOR THE PSM
Speaking of common opto-mechanical hardware, we have a variety of accessories for the PSM in the webstore including Ronchi gratings for spatial calibration of the PSM, precision measured wedged windows for angle calibration and Axicon gratings for creating Bessel beams.
We will shortly be adding Bessel beam projectors and long working distance objectives that work with the PSM. The objectives are designed to maintain sensitivity while substantially increasing the working distance.
If you have an idea that would make the PSM work better for you, let us know. We can always add it as a new product and make your life in the lab easier.
Keep safe, Bob
You can see our exhibit of the Axicon Grating Centering Station here.
