Phil Hobbs's Optics Page

Welcome. This page's last significant update was on January 28th, 2008. 
A volume in the John Wiley Series In Pure And Applied Optics, now in its sixth printing:

  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--to be the book I needed as a graduate student."
--From the Preface

Corrections:


Practical lore is useful only when it's correct, so careful attention to detail and prompt reporting of any errata is very important. There aren't too many so far, but nothing is too small to be worth fixing. Errata are listed by the last printing in which they appear

Antenna-Coupled Tunnel Junctions for Optical Interconnection

( 01/28/08)
My colleagues and I are working on a project to combine submicron silicon optical waveguides with metal antennas and metal-insulator-metal (MIM) tunnel junctions, to build optical detectors and modulators in the 1.55 um region. This is the project that POEMS was written for. We've demonstrated the first waveguide-integrated ACTJ detectors, which have achieved a 40-fold increase in both response and sensitivity over any previous ACTJ detector.

The details of the junctions and fabrication procedures are here.

POEMS: Programmable Optimizing Electromagnetic Simulator


    The antenna-coupled tunnel junction work needs 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 microns square by 20 microns long. This requires multiprocessor capability and subgridding, i.e. different places in the simulation domain having different cell sizes. This is a challenge in FDTD, which naturally likes uniform cubical grids.
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. It's currently fully operative, and I've been using it to design waveguide-coupled antennas. The current version uses either the well-tested Berkeley TEMPEST 6.0 FDTD code or my own clusterized FDTD code for its number crunching engine. Conversations about open-sourcing are underway, but for now, all I can give you is the manual.

Current Status (01/28/08):

The FDTD engine is working, and on a 2-processor Xeon machine running a 1.5 GB simulation of a silicon waveguide, 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.
   Extension to medium-sized cluster machines has also been completed: POEMS can now run large simulations on a cluster. My current hardware is a 14-processor Opteron cluster made from IBM eServer 325s and running the .

Scaling performance on this small cluster is excellent, with less than 30% deviation from linear scaling of the single-machine version, due primarily to communications latency over the 1 Gb/s Ethernet connections. The reason for the speed appears to be that my code precomputes a strategy, which allows a very clean inner loop, whereas TEMPEST has a big switch statement inside its inner loop, which makes optimization and caching much more difficult.

Open sourcing is still in the plan but faces some organizational friction: please e-mail me if you'd like to try POEMS.

Laser Noise Cancellers

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. These papers discuss simple circuits (would you believe 1 dual op amp and 3 transistors?) that can improve the quality of laser measurements enormously.
These 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). 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.

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? These papers present garden-variety op-amp transimpedance amplifier (TIA) design rules, so that you can design your own shot-noise limited front end. When that isn't enough, you might want to try an unusual solution, the cascode transimpedance amplifier, or even 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.

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 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 particles 0.2 micron or even smaller, as they float around in the plasma. 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.

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 Footprints (there's also a mirrored version.)

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.

Heterodyne Confocal Microscopy


Optical phase is a wonderful thing--it can get you good topographical images of samples with no discernable 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 half the wavelength--ultraviolet resolution from a visible-light scope. Would you believe 90 nm 10%-90% edge rise intervals with almost no overshoot, at 514.5 nm wavelength? In air?
Read on.
hobbs @ electrooptical.net


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