EOI is a well-equipped small consultancy specializing in finding imaginative
solutions to hard measurement problems in very many areas of optics,
electronics, photonics, and mixed-technology systems. As consultants we've
worked with clients of all sizes, from garage startups to universities to the
largest manufacturers to national labs.
We also do extensive expert witness work in optics and electronics cases.
You can see some of the breadth and quality of our research, design, and
engineering work here.
Send me an email and let's discuss your application.
This was a patent and trade secret case concerning lidar (laser radar) technology for self-driving cars and trucks. It was the biggest case I've worked on, with potential damages over $2 billion, and also one of the most fun. I was the defendant's expert on the patent side, and we beat Google--they dropped all their patent assertions. (This was made a lot easier by the fact that Uber wasn't infringing, of course.)
In the spring of 2017, I was approached by lawyers from two technology companies working in civil avionics (instruments for airplanes). I can't say who they were due to NDA restrictions, but the job was an unusual and interesting one. The two companies had been joint development partners, but the relationship had soured and trust had now broken down completely. Both were concerned that the other was misusing intellectual property disclosed during the joint venture, and they asked me to do do an audit to see whether this was in fact true. The situation was made more complicated because one company was several hundred times as large as the other, and of course there was no court-ordered discovery and no one was under oath.
Those are great places to talk about optics and electronics, if
you have a reasonably thick skin.
There are a lot of smart people there who know their stuff and
can help you. The clearer and more concise the question, the more
helpful the answer, in general.
Usenet is generally unmoderated,
which is a good thing in a busy group, because it keeps the
discussion flowing. There are a few flamers, whom it's best to
(I used to hang out in alt.lasers as well, but
the whole crowd moved to photonlexicon, which is
Building Electro-Optical Systems: Making it all Work
This book is an attempt to provide a systematic and accessible presentation of the practical lore of electro-optical instrument design and construction: in other words, it's the book I needed as a graduate student, but couldn't find.
It's intended in the first instance for use by oppressed graduate students in physics and electrical engineering, who have to get
their apparatus working long enough to take some data before they can graduate.
When they do, they’ll find that real-world design work has much the same harassed and overextended flavor, so in the second instance, it’s intended as a self-teaching guide and professional reference for working electro-optical designers.
Now a new laboratory bible for optics researchers has joined the list;
it is Phil Hobbs' Building Electro-Optical Systems: Making It All Work,
aimed at providing "accessible presentation of the practical lore of
electro-optical instrument design and construction."
I predict it will move to the front of the shelf.
This is a wonderfully practical book.... [It] is also a wonderfully entertaining read....
I like this guy's attitude. He points out how most scholars...never write
about the troubles they had getting the apparatus to work right, or the
changes they had to make to get valid data.
Mr. Hobbs talks about exactly that. Good man.
...[I]f you work in this field, you ought to buy this book.
If you don't work in this field, then you should still read it.
I got started in consulting during my 20 years as a Research Staff Member at IBM
T. J. Watson Research Center. I began in the Manufacturing Research
department, building special instruments for unique manufacturing problems for
which commercial solutions did not exist, such as scanned-probe and
solid-immersion microscopy, and in-chamber particle detection. I also did a
fair amount of firefighting, retrofitting s/c equipment for new capabilities and
helping fix problems that were causing immediate revenue loss in manufacturing.
Later I developed new classes of computer input device, advanced scanning
technology, and a new class of photonic detector and switch for optical
interconnection, based on metal-insulator-metal tunnel junctions.
Front of Watson Research
The clear practical emphasis has never left me. One of the reasons that I wrote
Building Electro-Optical Systems was to help people to build
better products and apparatus while staying out of all the potholes
in the road, and that's why I love consulting as well.
I do design consultation, expert witness work
(testifying and consulting in patent and trade secret cases),
contract design engineering, debug, and system bring-up tasks, as
well as training in ultrasensitive detection methods and front end
design. I hold
43 US patents
and several foreign ones, and am thoroughly familiar with the patent
process, both in prosecution (i.e. obtaining a patent) and
litigation. Some of my earlier research and development projects
appear below, and there's also a
list of recent projects for clients.
I'm expert in the design, debug, and refinement of electrooptical
and mixed-technology systems. I'm also a leading designer of
ultrasensitive optoelectronics and other low noise analog circuitry.
Many of my designs have improved the state of the art by orders of
magnitude in performance, in cost, or both (see below). I've done
groundbreaking work in thermal infrared imaging, in situ
particle detection, computer input devices, simulation software,
spectroscopy, atomic and magnetic force microscopy, solid immersion
microscopy, heterodyne interferometry, trace metal detection,
photolithography, laser scanning, plasmonics, and silicon photonics.
I also have expertise in downhole instruments (especially stabilized
lasers), disk drives, inspection systems, and
semiconductor processing. There's more detail in my resume,
as well as on the patents page, the recent work page and in the
papers linked below.
On the design side, our customers to date are a mix of start-ups and
larger companies, including some of the largest companies in the
electronics, defense, oil-field services, and semiconductor
equipment industries. We've worked as subcontractor or consultant
on four defense contracts, for DARPA, the Office of Naval Research,
and the Army. Our recent work
page has some more details, but of course NDAs and other
contractual obligations limit how much we can say about most of
them. We're also introducing electro-optical instruments in
cooperation with Highland Technology, a cutting-edge instruments
company in California. On the
intellectual property side, I'm a
testifying expert witness in both patent infringement and trade secret
misappropriation cases, with experience doing depositions, declarations, and
expert reports, in both claim construction through validity or invalidity and
infringement or noninfringement.
EOI is well-equipped for
design, prototyping, and testing of optical, electronic, and
mixed-technology systems of many kinds. If you have a tough
technical problem and need the right solution fast, give me a buzz
at the lab, 914-236-3005 (9-6 Eastern time, preferably). As always, the
first hour or two is free, so if you'd like to discuss your application, you're invited to call,
tell a colleague. I'm always interested to hear
what folks are working on.
Antenna-Coupled Tunnel Junctions for Optical Interconnection
My silicon photonics work at IBM centred on the idea of integrating submicron silicon optical waveguides with metal plasmonic antennas and metal-insulator-metal (MIM) tunnel junctions, to build optical detectors and modulators in the 1.55 μm region.
They use a novel plasmonic travelling-wave structure to eliminate the effects of capacitance at optical frequencies—it's the fat metal bit in the middle of the X-shaped antenna arms, shown in the figure at right.
The attraction of this method is that it potentially eliminates the usual drawbacks of silicon photonic switching devices: narrow optical bandwidth, extreme temperature sensitivity, and high drive power. We estimated that it could reach drive power levels of less than 40 microwatts per Gb/s (40 fJ/bit), due to its low voltage swing (100 mV) and sub-femtofarad capacitance.
We built all our own waveguide wafers as well as the ACTJ devices on top.
The details of the junctions and fabrication procedures are
In the second half of 2010, I did a seedling project on these devices with an aerospace
client, for use as infrared pixels, and I expect more uses to develop.
It probably won't be as an imager, though, because we found that the rather low shunt
resistance of these devices makes them quite insensitive at low light levels.
There are a variety of EM simulation schemes in wide use, with different strengths and weaknesses.
For free-space antennas at radio frequency, where dielectrics are simple and metals are
excellent conductors, integral equation schemes such as the method of moments (MoM) win.
At optical frequencies, particularly when metal is involved, partial differential equation
methods are generally better.
The two most common PDE schemes are finite element method (FEM) and finite difference,
time domain (FDTD).
The antenna-coupled tunnel junction work required simulations with very fine resolution
(1 nm) in some places, to represent plasmons and metal surface discontinuities, and a very
large simulation domain, at least 5 μm square by 20 μm long.
This requires multiprocessor capability and subgridding, i.e. different places in the
simulation domain having different cell sizes.
Subgridding is a natural strength of FE, but presents a challenge in FDTD, which naturally
likes uniform cubical grids. On the other hand, mesh generation can be very time consuming,
and FEM doesn't clusterize as well as FDTD and is much harder to get correct.
Mode field of the silicon waveguide under the wide ends of the antenna arms in the previous
For E&M fans: Note the bright fringes in the quartz just outside the silicon, where the
Maxwell continuity condition on perpendicular D makes the E field strength jump by a factor
of about 6.
(It only happens on the ends because this mode is TE.)
Lens design programs and circuit simulators have optimization capability—given a decent starting
point (which may be hard to find sometimes), they will automatically adjust the lens
prescriptions or circuit constants to achieve best performance by some criterion set by the user.
POEMS is a very capable FDTD simulator that brings this optimizing capability to the full EM world.
I've mostly been using it to design waveguide-coupled antennas, but it's good for many sorts of
The current version uses either my own clusterized FDTD code, or (for verification) the well-tested
but very much slower Berkeley TEMPEST 6.0 FDTD code for its number crunching engine.
Here's the manual.
The POEMS clusterized FDTD engine is currently working on EOI's SuperMicro server, a 16-core,
64-bit, 150 GFlops AMD Magny-Cours machine with 32 GB of DDR3 ECC 1333 memory and a hardware RAID5
disk array, running CentOS 6.3 Linux.
Currently we're using it to design fluorescence-detection biochips for DNA sequencing, and
recently we used it to perform large scale thin film infrared antenna simulations for a large
Its previous incarnations used a 24-way SMP and a 7-node, 14-processor Opteron cluster.
Scaling performance on these small SMPs and clusters is excellent, with less than 30% deviation
from linear scaling of the single-machine version.
In multicore boxes, this is due to some issues with the Linux thread scheduler, and in clusters,
there is also communications latency over even the fastest Ethernet connections.
The reason my code is so fast is that it precomputes a strategy, which allows a very clean inner
loop iterating over a list of 1-D arrays of identical voxels, whereas, like most FDTD codes,
TEMPEST has a big switch statement inside triply nested loops to decide what to do at each voxel
on each iteration.
That makes optimization and caching much more difficult.
For more details, here's the manual. Send me an e-mail if you have a problem that might benefit from POEMS.
Laser Noise Cancellers
Laser noise is very often the primary limiting factor in making high-accuracy optical intensity
There are ways of making your laser quieter, but they won't get to the shot noise level.
On the other hand, what we actually measure is the photocurrent, not the laser power, and that
we can improve.
Laser Noise Cancellers are extremely
powerful devices that allow us to make shot-noise limited
measurements at baseband, even with very noisy lasers. With zero
adjustments, they will reliably suppress the effects of laser
residual intensity noise (RIN) by 55 or 60 dB from dc to several
megahertz, and with a bit of (optical) tweaking, will do 70 dB or
more at low frequency, which is where it's most needed (see the
picture above, which shows > 70 dB suppression of noise
intermodulation). There's a
New Focus app note
which surveys applications of noise cancellers.
The laser noise canceller has two
operating modes, linear and
The linear mode produces a replica of the photocurrent minus the noise.
The log ratio mode also suppresses the intermodulation of the laser
noise with the signal, allowing (for example) tunable diode laser
spectroscopy to achieve 1-ppm sensitivities even when the laser
power is varying by >30% over a scan line, as shown here.
Laser noise canceller performance, showing the intermodulation
suppression action of the log ratio output.
The desired signal is the 50 kHz central peak, but the laser has
additional unwanted 5 kHz AM, which puts intermodulation
sidebands on the desired signal.
Upper trace: Single-beam TIA mode (comparison beam
replaced with constant current to show actual photocurrent spectrum);
Lower trace: Normal dual-beam ratiometric operation
(comparison beam unblocked), showing spurious intermod suppressed by 60-70 dB.
Note that the sideband energy has been returned to the main peak, as expected.
Elimination of sloping baseline and peak-height variations.
Here's one way the previous figure translates into real measurement benefits.
Top: Raw detected photocurrent, comparison beam blocked.
Spectral features are barely detectable bumps on a huge nonlinear sloping
baseline, even in the detail view.
Bottom: the spectral region of the detail (about 0.5
cm-1) with the log ratio output of the noise canceller. The noise
intermodulation suppression of 70 dB or thereabouts makes the peak heights
independent of the laser power variations, so the lower trace gives sample
absorbance directly, at a sensitivity better than 10-6.
The definitive summary of my early work on noise
cancellers: Six easily-constructed circuits that can improve the SNR of
bright-field laser measurements by as much as 70 dB at low frequency and 40 dB
out to 10 MHz. Bright field measurements at the shot noise limit become much
The circuits are simple to construct (as simple as 1 dual op amp and
3 jellybean transistors), so anyone who can handle a
soldering iron can improve the quality of his laser measurements enormously.
Detailed applications advice is provided.
A word of caution: these circuits are easy to build but
fairly hard to improve. Even if you want some custom tweaks, build
the circuit as described, test it, and
then change it around.
You will have to add the usual 0.1 μF bypass caps on the
supplies, and replace the discontinued Analog Devices MAT04 matched
quad with the MAT14, but no other changes are necessary. Use
ground plane construction, e.g. dead bug on a piece of Cu-clad FR4,
or perf board style on a Vector 8007 protoboard, use nice quiet
power supplies and/or capacitance multipliers, and
don't put the photodiodes on cables. Bring the beams to them, instead.
Other important points: put a polarizer right at the laser to eliminate the
in the orthogonal polarization state, since it doesn't split the same way as
the laser light, causing imbalance in the subtraction; and
whatever you do, don't vignette the beams after splitting them.
(Applied Optics36, 4 pp 903-920 [1 February
In my consulting work, I've extended laser noise canceller technology
out to higher speeds (10-100 MHz) and wider photocurrent ranges.
Use of the log ratio version of the laser
noise canceller to achieve 10-6 absorption sensitivity in
current-tunable diode laser spectroscopy, even when the laser power
changes by 30% over a scan. This is shown by the pictures at right.
Using the well studied rovibrational spectrum of molecular iodine allowed us a simple
demonstration system for a widely
applicable technique, subsequently used in many instruments.
Popular article on the laser noise canceller, from Optics&Photonics
News, April 1991. An updated and somewhat chattier version of the
SPIE paper below, with more discussion of applications.
(Optics & Photonics News 2 4, pp. 17-23
First description of the laser noise canceller, including
comparisons with heterodyne systems and use with helium-neon lasers
showing mode beat suppression by >50 dB.
(Proc SPIE 1376, pp. 216-221 )
Low-Noise Photodiode Front Ends
Photodiodes are essentially perfect transducers—one photon gets you
one electron. So why are photodiode front ends so hard to design
well, and how can we get better results? This paper begins with
a simple set of op-amp transimpedance amplifier (TIA) design rules,
which will guide you in designing your own shot-noise limited front
Op amp TIAs aren't the best design for all purposes, especially in
very low light or with high capacitance photodiodes. For those
somewhat more difficult cases, the paper presents a couple of unusual
cascode transimpedance amplifier and the
bootstrapped cascode transimpedance amplifier.
Sometimes these circuits can get you a factor of 10 in bandwidth
over the best-possible op-amp TIA, while staying at the shot noise
I also have a lot of techniques for very low currents and for higher
speeds at even lower noise that I haven't written about yet. I design a lot of
custom TIAs for customers, so if you have a hard TIA problem, give me a call.
This paper gives a design study of one difficult case (2 μA
of photocurrent, 100 pF photodiode capacitance, and 1 MHz bandwidth
required). It shows how a somewhat unconventional design, the
bootstrapped cascode transimpedance amplifier, allows a 60x
bandwidth improvement over an optimized load resistor, with a SNR
within 1 dB of the shot noise. (Optics & Photonics News12(14), pp 44-47, April 2001)
(given at the University of Colorado, October 2nd, 2006.)
A talk presenting the design lore of noise cancellers and
other low frequency front ends (<100 MHz)
ISICL: In Situ Coherent Lidar for Submicron Particle Detection
Particles in plasma etch chambers are a major source of yield loss
in semiconductor manufacturing. Particles condensing from
the plasma or spalling out of films on the chamber walls are
levitated in the edges of the plasma sheath for long periods, and
then (too often) drop on the wafer when the plasma excitation is
Process control and tool utilization can both be
improved by knowing what's happening inside the chamber while the
process is going on—but how? The plasmas are usually too bright
to look at, and there's only one (poor quality) window in the
typical chamber, so an optical particle detector would have to work
in backscatter, with a huge background.
ISICL is capable of seeing and mapping individual particles of less than
0.2 μm diameter, as they float around in the plasma, a unique
An interesting combination of homodyne interferometry, laser noise
cancellation, and signal processing allows reliable operation at the
shot noise limit in the face of a coherent background 106
times larger than the signal and an incoherent background
108 times larger.
The techniques I developed for ISICL have proven to be very
widely applicable. My collaborators David Bomse and Daniel Kane at Mesa
Photonics have had a DOE project for a receiver of a similar design in a
spectroscopic sensor for nuclear nonproliferation monitoring, based on
heterodyne detection of sunlight.
Even more recently, with a large industrial
customer, I have been working on an advanced version of ISICL for detecting and
mapping submicron particles moving at extremely high velocity.
Calculations show that with only a watt of laser power, we should be able to see 0.15 μm particles
moving at up to 3 km/s, i.e. Mach 9.
I'll post more as this exciting opportunity develops.
Calculation vs experiment for the photon budget, signal processing, and detection
strategy of the ISICL sensor. Shows that a photon budget is worth
following, even in a complicated instrument used in hostile
(Proc. SPIE2909 1997 P11-20)
Footprints: A $10 Thermal Infrared Imager
A low-resolution thermal camera with competitive sensitivity
(0.13 K NETD) at very low cost. Easily built from scratch—it
requires no special parts, except a screen-printed sheet of
pyroelectric PVDF polymer (as used in automatic porch lights) and a
moulded polyethylene Fresnel lens. This camera achieves a cost
reduction of 2 orders of magnitude ($10 vs $1000) over the
next cheapest, which is a 256-pixel PZT array from Irisys, while
maintaining very good sensitivity. These are from a project called
The design is simple: screen-printed carbon ink on a free-standing
film of PVDF polymer, with a multiplexer made out of ordinary
display LEDs with a few interesting optical and electronic hacks, as
shown in these photos.
Mosaic image from 6 Footprints sensors, showing four people
wandering underneath. Slightly smoothed to reduce the visual noise
from all the little squares.
from the September 2003 issue of
Optics & Photonics News
about the ups and downs of the Footprints project.
(NB the war story link is free; the OPN one requires a log-in.)
We got a bunch of positive press reactions to this, including the NY
Times and the Discovery Channel. We got
a few negative ones too,
but overall the fact that 30 cm pixel size makes everybody anonymous
reassured people. (Nowadays, of course, video surveillance is
ubiquitous, but back in 2000 privacy still existed.)
Movies of the sensor data
(NB: there aren't too many pixels, so you have to resize the video player to see much!)
There are a few dead pixels, and the sensor in the bottom centre has
a bad surface leakage problem, probably due to the very high
humidity—data were taken during a thunderstorm.
Data were taken at 5 frames/s; the length scales are about 10 cm/pixel for
the first, 30 cm/pixel for the second, one and 15 cm for the second.
(AVIs by courtesy of Sharathchandra Pankanti and Robert H. Wolfe.)
Heterodyne Confocal and Solid Immersion Microscopy
Optical phase is a wonderful thing—it can get you good
topographical images of samples with no discernible amplitude
contrast, for example, or allow you to disambiguate phase features
from amplitude ones. My interest in phase-sensitive microscopes
dates back to my graduate work—hence
It gives design
details and the theory of the heterodyne scanning laser microscope,
including the point- and line-spread functions, plus a deconvolution
method that can give resolution equivalent to an ordinary microscope
working at λ0/2—ultraviolet resolution from a
visible-light scope. Operating with a green Ar+2 laser
(514.5 nm) and 0.9 NA, it attained a 10%-90% edge resolution of 90 nm.
This works because the interferometer makes it a confocal
microscope, i.e. its amplitude point-spread function is the square
of the illumination PSF. By the convolution theorem of Fourier
transforms, that means that its bandwidth is twice as wide, i.e.
±2NA/λ. A bit of digital filtering turns the resulting
nearly-triangular transfer function into something a bit more
Gaussian-looking, which gives us a factor of 2 resolution
improvement. Unlike the usual image processing ad-hockery, Fourier
filtering makes absolutely no additional assumptions about the
sample; the additional information comes from measuring both phase
and amplitude, which is why you need an interferometer.
Scan of a 15 μm square
region of the lightly sanded surface of a double-side polished monitor wafer, taken from
the back, showing a single groove left by one grit particle and an undamaged area nearby.
The green diagonal line in each frame shows where the corresponding line scan
was taken. Repeatable detail in the two scan lines resolves detail
of no more than 0.3 μm per cycle, which is
λ0/4.4. (Image taken in early 1992 by P. Hobbs.)
Solid Immersion Microscopy
This work was one of
the first applications of modern signal processing to optical
microscopy, and led to what may have been the first invention of
microscopy. After grad school, I moved to IBM Yorktown to work in the
Manufacturing Research department. At that time, IBM produced about
25% of the world's semiconductors. More than that, though, IBM
computers were all emitter-coupled logic (ECL), based on bipolar
transistors, which were the fastest thing on the planet.
The big advantage of ECL was its very high
could drive long wires at speeds double those of contemporary CMOS.
The downsides were power consumption and complexity: a typical ECL
recipe of the day had about 600 process steps, versus about 300 for
CMOS, and the resulting chips consumed more than twice as much
power. Before about 1992, that was a very good trade, and IBM's
advanced bipolar logic devices gave rise to lots of fascinating
Sam Batchelder, Marc Taubenblatt, and I invented a silicon contact microscope in
1989, as a method for inspecting the bottoms of 16-Mb DRAM trench capacitors,
which at the time were very difficult to etch cleanly. (The basic idea for a
silicon contact lens was Sam's—he originally wanted to use an Amici
sphere.) While this was prior to any publication by others on the subject, the
contact or solid immersion microscope was also being developed in the
laboratory of my Stanford Ph.D. advisor, Prof. Gordon Kino, at about the same
time. They took the first solid-immersion pictures, while we were busy trying
to design a manufacturing tool. (See S. M. Mansfield & G. S. Kino, Appl.
Phys. Lett.57, 24, pp 2615 - 2616 (1990).) (The same idea
is called a numerical aperture increasing lens [NAIL] by
other groups. Neither is a very good name, but we're stuck with one or
the other—it's too bad that we can't just call it a contact
lens, because that's what it is.)
In 1989, we had an initial
optical design performed by Prof. Roland Shack of the University of Arizona,
which showed that the hoped-for high resolution could be obtained, at least in
the ray model. Work continued on the real instrument, including studies of how
to make good contact to the back surface of the silicon wafer. (Interestingly a
much earlier, special-purpose contact microscope was developed by C. W.
McCutchen of the University of Cambridge: Appl. Opt.
1, 3, pp 253-259 [May 1962].)
In early 1992 I took the first pictures at a numerical aperture (NA)
of 2.5, shown above. These were made by optically contacting a
steep silicon plano-convex lens to the polished back surface of an
8-inch wafer (so that the centre of the lens was nearly at the front
surface), focusing a 1.319 μm YAG laser beam through it at
an NA of 0.7 with a Mitutoyo long working distance microscope
objective, and mechanically scanning the wafer+contact lens assembly
using a piezo flexure stage. Detection used an unresolved pinhole
in a standard Wilson confocal (Type 2) configuration, so that the
spatial frequency bandwidth is ±2NA/λ (twice as wide as
the illumination spatial frequency bandwidth).
When a spherical beam crosses the concentric lens's surface, its
angular width doesn't change, but the refractive index goes from 1.0
to 3.5, so the NA goes up from 0.7 to 2.5. (There's also a 3.5x
linear magnification, so no free lunch is involved.) Although the
Mitutoyo lens was reasonably well corrected at 1.319 μm,
the coatings on its many elements were horribly mistuned, enough that its
internal reflections dominated the returned signal and caused all sorts of nasty
interference fringes. Fortunately, because of its long working distance, I
could sneak a chopper wheel in between it and the hemisphere; lock-in detection
then recovered the signal from the wafer and rejected the spurious reflections.
First Confocal Images at NA=2.5
It turned out that we couldn't image real product wafers
easily in our off-line lab, because there were fairly thick oxide
and nitride films on the back of the wafer that made most of the
light evanescent, as well as a lot of embedded particles which made
the back surfaces too rough for good contact. Instead, I used a
clean monitor wafer. A featureless flat surface isn't the most
informative sample, of course, so I scratched the far side once,
gently, with fine sandpaper. That produced a set of long fine
grooves whose repeatable cross-section allows a good sanity check
for a raster-scanned measurement; if the pattern repeats, it's
probably real. It made the scenery rather boring, as you can see,
but it's interesting as a technological demonstration.
The two frames have the same scan
data, of a 15x15 μm area, but the two line scans are taken from
positions about 5 μm apart along the groove, shown as the green
lines on the images. There is repeatable detail with periodicities
down to a bit below 0.30 μm (see e.g. the peak
detail around pixel 128). That demonstrates a spatial frequency
bandwidth of at least 4.4 cycles per wavelength, clearly more than
the Type 1 limit of 2.5 cycles per wavelength at this NA. If it had
been a phase sensitive design, I ought to have been able to use the
deconvolution algorithm to achieve about 140-nm resolution at
10%-90%—equivalent to an NA of 5.0. (That's asking a lot of
the objective, of course, but the heterodyne approach is pretty
tolerant of minor imperfections.)
A Manufacturing Instrument
In 1Q 1990, we put out a request for bids, and by early 1992 we had a
full-scale, 3.2 NA scanning heterodyne confocal instrument design completed, in
cooperation with some very smart folks from Sira Ltd. of the UK: John Gilby,
Dan Lobb, and Robert Renton. This design had a number of interesting features,
including what may have been the first technological optical vortex. To allow
easy navigation, we planned to make the instrument scan on a 50-nm air bearing.
This was somewhat difficult, because at an NA above 1.0, the light in the air gap
is evanescent. Not just a bit evanescent, either—at NA=3.2 the intensity falls off by
1/e2 in about 40 nm.
Fortunately, we found a
trick for this: it turns out that the falloff is very different in the two
linear polarizations, with s polarization falling off n times more
slowly than p. Thus we could avoid huge optical losses due to total
internal reflection (TIR) at the air gap by using
i.e. the light incident on the air gap was always
s-polarized. This was done by using a segmented half-wave
plate shaped like an 8-petal daisy. Normally, surface reflections
are reduced by using
p-polarized light, as in polarized sunglasses, but with TIR,
s-polarized is much better. We hoped to do measurements at
an equivalent NA of 6.4, which would be pretty slick even today.
Optical Vortex and Daisy Waveplate
Along the way, we found that tangential polarization in the pupil leads to a very ugly
focused spot with an amplitude null in the centre, i.e. an optical
vortex. (We didn't think it was anything special at the time, except that it
was spoiling our measurement.) We designed around this problem by using a
circumferentially graded coating on the daisy wave plate to apply a
one-cycle-per-revolution phase delay around the pupil circumference. That got
rid of the vortex and improved the PSF a great deal: instead of a null, the
field had a circularly polarized peak in the center, about as sharp as a normal
linearly-polarized one, so by early 1992, everything looked good.
Unfortunately, as a result of IBM's
, both our customer and our budget went
away later that year, so the system never got built. (I still have
all the drawings.)
IBM had started making computers out of CMOS like everyone
else, so Manufacturing Research was soon absorbed into another
department with a software and services mission. For the next 15
years, I worked very happily on ultrasensitive instruments
for semiconductor process control, computer input devices, low cost
thermal imaging, advanced 3-D scanning, and especially silicon
photonics, and never came back to contact microscopy.
Fortunately others have made it a useful technique, though as far as
I know, nobody has built one like ours.