Photo Tour of EOI's
New Premises:
Photo Tour of EOI's
New Premises:
New Product: PH200 Nanowatt
PhotoreceiverBesides the do-it-yourself photoreceivers below, I am introducing a new line of electro-optical products in cooperation with Highland Technology, a cutting-edge instruments company in San Francisco.
I can often be found on the Usenet groups sci.electronics.design, sci.optics, and alt.lasers.
Those are great places to talk about optics and electronics, if you have a reasonably thick skin. Usenet is generally unmoderated, and some of the folks who post there are entirely immoderate themselves, leading to a signal to noise ratio that's sometimes lower than we'd like. There are also 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.)
You can start on
Google Groups, but for real use you'll need a mailer with decent
filters, such as Thunderbird.
Talk to you there!
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....
-- Prof. A. E. Siegman, Optics & Photonics News
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.A look inside:
-- Bob Pease, Electronic Design
In my years as a physicist and engineer at IBM Research, I have done small amounts of free, nonconfidential consulting for the many people who have asked me. This continues unchanged, but now in addition I am able to take on larger, fee-based engagements as a consultant, including confidential ones. 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 40 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 development projects appear below.
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, quantum computing, inspection systems, and semiconductor processing. There's more detail in my resume, as well as on the patents page and in the papers linked below.
I started EOI in February 2009. On the design side, my customers to date are mainly startup companies: three in biomedical instruments, building spectroscopic and optical coherence tomography (OCT) sensors for glucose, blood alcohol, and heart surgery; a European manufacturer of computer displays; and two others making laser microphones and downhole acoustic, seismic, and gravity sensors; as well as larger firms in the defense, semiconductor equipment, oil services, and consumer products industries. I have been a subcontractor on four DoD contracts, from DARPA and the Office of Naval Research. I am also introducing a line of electro-optical instruments in cooperation with Highland Technology, cutting-edge instruments company in California. On the litigation side, I've done both patent infringement and trade secret misappropriation cases.
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 845-480-2058 (9-6 Eastern time, preferably). As always, the first hour or two is free, so don't hesitate to call, e-mail, or tell a colleague. I'm always interested to hear what folks are working on.
Antenna-Coupled Tunnel Junctions for Optical Interconnection
(Click on image at right for larger version.)
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. This is the project that POEMS was written for. We demonstrated the first waveguide-integrated ACTJ detectors, which have achieved a 40-fold increase in both response and sensitivity over any previous ACTJ detector. We built all our own waveguide wafers as well as the ACTJ devices on top The details of the junctions and fabrication procedures are here. In the second half of 2010, I did some more work on these devices with an aerospace client, for use as infrared pixels, and I expect more uses to develop. |
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| Mode field of the silicon waveguide under the wide ends of the antenna arms in the previous picture. 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.) |
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.
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 EM problems. The current version uses either my own clusterized FDTD code, or (mainly for verification) the well-tested 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 Linux. In the last year, it's been performing large scale thin film infrared antenna simulations for a large aerospace customer, and will be starting on RFID simulations for a medical application. Its previous incarnations used a 24-way SMP and a 7-node, 14-processor Opteron cluster.
On a 2-processor Xeon machine running a 1.5 GB simulation of a silicon waveguide, POEMS runs > 2.5x faster than the single-processor version of TEMPEST 6.0 (Intel C++ 7.1, all optimizations on). It now runs inhomogeneous cubical meshes stably, subgridding by factors of 2, 3, 4, or 5 at any given rectangular box boundary. The subgrids can themselves be subgridded if desired. There remain some residual problems where material boundaries cross mesh-size changes. When this is fixed, POEMS will be the only FDTD code I know of that will run arbitrary cubical subgrids.
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 gigabit Ethernet connections. The reason for the speed appears to be that my code precomputes a strategy, which allows a very clean inner loop iterating over a list of 1-D arrays, whereas TEMPEST has a big switch statement inside its inner loop, making optimization and caching much more difficult.
Please have a look at the manual. and e-mail me if you'd like to try POEMS.
Laser noise is very often the primary limiting factor in making high-accuracy optical intensity measurements. 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 log-ratio. 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.
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.)
(Applied Optics 36, 4 pp 903-920 [1 February 1997]).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 end. 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 solutions, the 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 level. (There are a bunch of other tricks for very low currents and higher speeds 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.)
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
turned off.
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
0.2 micron or even smaller, as they float around in the plasma, a
unique capability.
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
1010 times larger.
The techniques I developed for ISICL have proven to be very
widely applicable; at this writing (late 2011) I'm designing a
heterodyne receiver of a similar design in a spectroscopic sensor
for nuclear nonproliferation monitoring.
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| (Click on image for full resolution version.) 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 micron per cycle, which is λ0/4.4. (Image taken in early 1992 by P. Hobbs.) |
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 this paper. 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.
This work was one of the first applications of modern signal processing to optical microscopy, and led to the invention of solid immersion microscopy after I moved to IBM Yorktown to work in the Manufacturing Research department. (At that time, IBM produced about 25% of the world's semiconductors, and its advanced bipolar logic devices gave rise to lots of fascinating measurement problems.)
Sam Batchelder, Marc Taubenblatt, and I invented the solid immersion 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. In early 1992 I took the first pictures at a numerical aperture (NA) of 2.8, shown at right. These were made by optically contacting a nearly hemispherical silicon lens to the polished back surface of an 8-inch wafer, focusing a 1.319 μm YAG laser beam through it at an NA of 0.8 with a Mitutoyo long working distance IR microscope objective, and mechanically scanning the wafer+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 0.8 to 2.8. Although the Mitutoyo lens was reasonably well corrected at 1.319 μm, its coatings were off by enough that its internal reflections dominated the returned signal. 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.
It turned out that we couldn't image real product wafers easily in the 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, so I used a clean monitor wafer instead. 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; if the pattern repeats, it's probably real. That made the picture rather boring-looking, as you can see, but it's interesting as a technological demonstration. The two frames have the same scan data, of a 15x15 micron area, but the two line scans are taken from positions about a micron apart along the groove. There is repeatable detail with periodicities down to a bit below 0.30 micron (see e.g. the peak detail around pixel 128). That demonstrates a spatial frequency bandwidth of at least 4.4 cycles per wavelength, which may still be a record for a far-field measurement. If it had been a phase sensitive design, I ought to have been able to use the deconvolution algorithm to achieve about 120-nm resolution at 10%-90%—equivalent to an NA of 5.6. (That's asking a lot of the objective, of course, but the heterodyne approach is pretty tolerant of minor imperfections.)
We had a full-scale, 3.2 NA heterodyne confocal instrument design completed, in cooperation with Sira Ltd. of the UK (principal engineers John Gilby and Dan Lobb). This design had a number of interesting features, including what may have been the first technological optical vortex. We planned to make the instrument scan on a 50-nm air bearing. To avoid huge optical losses due to total internal reflection (TIR) at the air gap, the design used tangential polarization, 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.
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 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, which 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 near-death
experience, both our customer and our budget went away later
that year, so the system never got built. (I still have the
drawings.) The Manufacturing Research department was soon disbanded,
and I started working on computer input devices, low cost thermal
imaging, advanced 3-D scanning, and silicon photonics. Fortunately
others have made solid-immersion a useful technique, though as far
as I know, nobody has built one like ours.
