Author: csadmin

In general, computer generated holograms (CGHs) and plane Fresnel mirrors (and lenses), made by the same techniques as CGHs, have optical “datums” or foci that are “rigidly attached” to the CGH or Fresnel plane substrate and move in six degrees of freedom with the substrate.

THIS CONCEPT IS MORE EASILY SEEN BY CONSIDERING A GRATING PATTERN FOR A FRESNEL SPHERICAL MIRROR AS SHOWN IN FIG. 1. 

Fig. 1 Chrome on fused silica Fresnel zone pattern for a spherical mirror. The extraneous artifacts that look like contamination is contamination from incomplete cleaning of the pattern

This two-dimensional Fresnel zone pattern will reimage a point source placed at its center of curvature exactly the same as if it were a three dimensional solid spherical mirror and will re-image the point source in transmission as though it were a solid lens. It is obvious that if this pattern moves in three degrees of translation and two degrees of tilt the center of curvature will move with the substrate. For a Fresnel mirror of an off-axis ellipse or hyperbola with two foci not on a line perpendicular to the substrate, the foci will move in all six degrees of freedom with the substrate.

Thus the center of curvature or foci move with the substrate as though they were rigidly attached even though you cannot see or physically touch the foci. The foci are only visible by putting a point source of light at one focus and viewing the reflected point image at the other. The CGHs behave the same way. The aspheric wavefront they produce moves with the substrate. If you know where the substrate is in space (and the design of the pattern) you know by analysis where the foci are and vice versa.

THE PROBLEM IS HOW DO YOU RELATE THE FOCI TO THE SUBSTRATE SO THE FOCI OR ASPHERIC CGH PATTERN ARE PRECISELY LOCATED WHERE YOUR OPTICAL DESIGN SPECIFIES. 

One solution is to print spherical Fresnel zone patterns at known locations relative to the main pattern during the same process as the main pattern is written. Then you know the centers of curvature of the Fresnel patterns relative to the main pattern with a precision on the order of tens of nanometers. Now the challenge is to turn these virtual centers of curvature into something physical that can be probed with a coordinate measuring machine, a laser tracker or to serve as seats for a kinematic mount.

We follow an idea first described by Laura Coyle¹ where steel balls were centered on the spherical Fresnel mirror patterns but modify the concept to make it what we think is more practical to implement. What we will describe is not the only method. A commercial vendor of CGHs is using another method that is an offshoot of the Coyle method².

Instead of mounting balls directly to the CGH substrate, a method we found awkward and tedious³, we attach spherically mounted retroreflector (SMR) nests to the substrate over the Fresnel patterns to give a more solid mounting method and to reduce the final height of the attachment except for when metrology is needed.

Fig. 2. A ½” Grade 5 steel ball seated in a ½” SMR nest manufactured so the center of the ball is ½” above the bottom of the nest

Because this variety of SMR nest is made so that it holds the center of the ball ½” above the bottom of the nest within 10 μm, we specify that the Fresnel pattern over which the ball/nest pair are mounted have a ½” radius of curvature. We also specify where the centers of the Fresnel patterns are located in the plane of the CGH to the main pattern. This means we know where the Fresnel pattern centers of curvature are relative to the main pattern to tens of nm in the plane of the substrate and ± 5 μm perpendicular to the substrate.

FIG. 3 SHOWS AN EXAMPLE OF THE FRESNEL OFF-AXIS CONIC TO WHICH WE WILL ATTACH THE BALL/NEST PAIRS. 

Fig. 3. Off-axis plane Fresnel conic mirror

The square patterns in the corners of the CGH in Fig. 3 are the patterns the vendor would use to position balls. The circular patterns just inside the square patterns are the spherical mirror patterns we will use. 

In order to attach the nest/ball pairs the CGH is held firmly on a vacuum chuck beneath a PSM focused on the pattern. The CGH is tapped gently to center the Fresnel pattern (Fig. 1) under the PSM. Then the PSM is raised ½” to pick up the center of curvature of the Fresnel pattern. By first centering the PSM on the pattern itself it is easy to pick up the center of curvature because it is guaranteed to be in the PSM field of view.

WITH THE CENTER OF CURVATURE IN THE FIELD OF VIEW, THE SET REF POS BUTTON IN THE PSM SOFTWARE IS CLICKED TO CENTER THE ELECTRONIC CROSSHAIR ON THE RETURN SPOT AS IN FIG. 4. 

Fig. 4 Screenshot of the electronic crosshair centered on the return spot to 0.1 and 0.4 μm in x and y respectively

WITH THE CGH STILL FIRMLY HELD, SEE FIG. 5, A BALL/NEST PAIR ARE SLID ONTO THE CGH AND ROUGHLY CENTERED OVER THE FRESNEL PATTERN, FIG. 6. 

Fig. 5 CGH held on a vacuum chuck under the PSM centered on the Fresnel pattern.

Fig. 6  Ball/nest pair sitting on the CGH over the Fresnel pattern.

BY GENTLY TAPPING THE BALL/NEST PAIR THE REFLECTED SPOT FROM THE BALL CENTER CAN BE CENTERED ON THE CROSSHAIR TO LESS THAN 1 ΜM. IT TAKES A MINUTE AT MOST TO POSITION THE BALL/NEST PAIR TO THIS PRECISION. THE SCREENSHOT IN FIG. 7 SHOWS THE RESULT OF THIS ALIGNMENT TO 0.4 AND 0.4 ΜM IN X AND Y RESPECTIVELY. 

Fig. 7 Reflected spot from the ball center centered on the electronic hair.

With the ball/nest pair centered on the crosshair now carefully add drops of cement at the nest/CGH interface while checking the PSM software that you have not disturbed the alignment of the pair. Five minute epoxy is a good choice for cement because it gives you a small time window in case the pair moves. Also, the epoxy will not “set” in 5 minutes. More like 10 to 15 minutes before it is safe to move the CGH to the next Fresnel pattern. After an overnight cure you may want to add a little addition epoxy to fully secure the nests. Fig. 8 gives an idea of what the drops of cement might look like.

Also, the cement tends to pull out into a thin hair when you pull your applicator away from the drop of cement. Make sure the hair does not fall on the main CGH pattern. It may be well to protect the pattern before cementing.

Fig. 8 Drops of cement at the base of the nest to secure the nest to the CGH.

In all it will take about an hour to secure all four ball/nest pairs to a CGH. It would be wise to wait a day before removing the balls from the nests as the balls are held in with a magnet and it requires some force to remove the balls. 

When finished you will have four balls attached to the substrate within < 1 μm each of their ideal location in the plane of the CGH and within ± 5 μm perpendicular to the substrate. If more precision is required perpendicular to the substrate the nests can be lapped on fine silicon carbide lapping paper until the required ball/nest height match is met. The PSM can be used to determine this height by looking in at the side of the ball where there is the higher lateral sensitivity.

¹  L. E. Coyle, M. Dubin, and J. H. Burge, “Locating computer generated holograms in 3D using precision aligned SMRs,” in Classical Optics 2014, OSA Technical Digest (online) (Optical Society of America, 2014), paper OTh1B.2.
https://www.osapublishing.org/abstract.cfm?URI=OFT-2014-OTh1B.2

² Arizona Optical Metrology LLC, http://www.cghnulls.com

³ Parks, R. E., “Optical alignment using a CGH and an autostigmatic microscope “, Proc. SPIE, 10377, 103770B (2017)

⁴ Hubbs Machine & Mfg. Inc., https://hubbsmachine.com/

Recently, a client asked how well can you focus if you really had to do better? I did not know but it was easy to do an experiment with our centering station that has a motorized stage and the ability to log data as the stage moves.

It is easily demonstrated that the PSM lateral sensitivity to centroiding on a return reflection from a center of curvature is better than 1 μm with a 10x objective. In the usual case the sensitivity is better than 0.2 μm. However, the sensitivity axially, or in the direction of focus is less sensitive, typically ± 2-3 μm judging by the size and shape of the image on the video screen.

WAS I SURPRISED BY THE RESULTS! 

Using our centering station I got repeatability of better than 0.1 μm using the center of a 1/8” steel ball as my mirror and a 10x objective with a NA of 0.25 on the PSM. Classically, the depth of focus is λ/2*NA or about 0.635/.5 = 1.27 μm in this case.

To do the experiment to find focus you have to adjust the shutter speed on the PSM so there are about 10-15 pixels above threshold at what appears from the video screen to be best focus. The area designation on the PSM control panel is a count of the number of pixels above threshold so the illumination is easy to set to get the right number of pixels. Then you scan through where you expect best focus while logging the number of pixels above threshold as you scan. These data are saved to a file that is then copied to Excel. When the data of number of pixels versus scan position are plotted as in the graph below you can fit a second order polynomial as shown in the equation on the chart.

Remembering that we can find the maximum of the curve by taking the derivative and setting it to zero, we get -2*81691*x -707493 = 0, or x = 4.3303 mm. When this scan was repeated another 4 times I got 4.3303, 4.3302, 4.3303 and 4.3301 mm as the scan position at maximum pixels above threshold. This represents repeatability of less than 0.1 μm even though the data are rather noisy due to the small number of pixels used in the data.

This experiment points out the advantage of using a digital camera on the PSM. Without the ability to digitize the intensity at each pixel it would be impossible for this process to work. 

The exact set of parameters used in this experiment may not be optimized, but this data shows the remarkable sort of focus repeatability that can be achieved with the PSM. It may be possible to do even better.

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.