Recent Work

Featured Product: LA-22 Low Noise Lab Amplifier

The LA-22 Low-Noise Laboratory Amplifier, 800 Hz–22 MHz, 1.1 nV / √Hz

One problem that comes up again and again in doing measurements is that we need the apparatus to be quieter than the thing we’re measuring, ideally by at least a factor of two.   Besides quiet, it should be wideband, have an accurately known gain that’s flat with frequency, have a clean step response, and generally do its job while keeping itself out of the way.  There’s a wealth of detail in our app note AN-1  on photoreceiver testing.

A 150-Picosecond Time Domain Reflectometer for Under $2

One of the most enjoyable parts of electronics design is getting excellent performance with rock-bottom parts cost.  The right circuit can produce exceptionally good speed, noise, and accuracy specs from very low-cost parts.  A case in point was a project from December 2016: a time-domain reflectometer (TDR) for a liquid level sensing application in industry.

TDRs work by sending short pulses down a transmission line where they bounce off anything that disturbs their propagation.  By measuring the time delay, you can tell how far down the line the disturbance is.  It’s a bit like a one-dimensional radar, except that with TDR you can learn a lot more from the reflection than just its location.  TDRs can find damaged optical fibres, waterlogged sections of coax cable, and many other things of that sort. This application used an air-dielectric coaxial probe built from two metal tubes sticking downwards into a tank, so that the first part of the probe had air as dielectric and the second part had liquid.  The dielectric constant of a liquid is at least 2, whereas air’s is 1.0, so there’s a nice healthy impedance mismatch at the surface to reflect the pulse.  This approach is very rugged and resistant to fouling (you can get all sorts of nameless crud in process water and diesel tanks, for instance).

Silicon Photomultiplier Module Design

Internal Developments

In the last year or two we’ve been doing a lot of work aimed at replacing photomultiplier tubes (PMTs) in instruments, using avalanche photodiodes (APDs) and silicon photomultipliers (SiPMs).  These devices are arrays of single-photon detectors, so they’re also known as multi-pixel photon counters (MPPCs).  Our main application areas include biomedical instruments such as flow cytometers and microplate readers, which have to measure low light levels very precisely but don’t need the ultralow dark current of PMTs. (Follow-on articles will talk about our SiPM work in airborne lidar and SEM cathodoluminescence, as well as on improving the performance of actual PMTs.)

Signal to Noise Ratio and You, Part 2

In Part 1, we discussed ways to get better measurements by improving the signal to noise ratio (SNR), and saw that although it was often a win to measure more slowly and use lowpass filters, going too far actually makes things worse, because of the way noise concentrates at low frequency.  Here we introduce a more sophisticated approach that generally works better: the lock-in amplifier.

We were considering a typical baseband signal, one that goes from near DC to some much higher frequency.  Audio is a typical example, with a bandwidth usually quoted as 20 Hz to 20 kHz.  To escape the low frequency noise, we need to move our signal up in frequency, out of baseband.  In lock-in detection we make the signal periodic in time at some carrier frequency fc chosen to be several times higher than the required bandwidth. This is generally pretty easy to do, as we’ll see, and doing so ensures that none of the signal we care about remains near DC.  Our noise rejection filter now needs to be a narrow bandpass centered at fc,  so as to reject both low-and high-frequency noise.  We’ll also need some means of measuring the amplitude and phase of the AC signal.  That’s more complicated, of course, but with this setup we can narrow the bandwidth as much as we like and still get the full SNR improvement.  A lock-in amplifier is a device for making such narrow-band AC measurements conveniently.  It’s basically a radio that measures the phase and amplitude of its input, so that we recover a lowpass-filtered version of the baseband modulation signal that we care about, with no 1/f noise pollution to worry about.   At this point we need to geek out a little bit and talk about modulation, which is what we mean by moving the signal away from baseband.

Signal to Noise Ratio and You, Part 1

In building an ultrasensitive instrument, we’re always fighting to improve our signal-to-noise ratio (SNR).  The SNR is the ratio of signal power to noise power in the measurement bandwidth, and is limited by noise in the instrument itself and the noise of any background signals, such as the shot noise of the background light or the slight hiss of a microphone.

If the signal is weak, it will have proportionally more noise, so that the apparatus has to be designed to get rid of as much noise as possible.  There are a number of ways to do this.  The best is to get more signal or reduce the noise, for instance by increasing the laser power and using a laser noise canceller, but eventually we hit a practical limit.  At that point, we’re left with several options, all of which boil down to filtering in one form or another.

InGaAs Photodiode Characterization Drift and 1/f Noise at 70-90 °C

Drift and 1/f Noise at 70-90 °C
Sometimes you have to find out things that aren’t in the datasheet, and even the manufacturer may not know.

Thermoacoustic Refrigeration

Thermoacoustic fridges are magic: you heat one end, and the other end gets cold. (Of course you have to sink all that heat from the middle.) They can easily be made long and skinny, and so are a natural for use down drillholes. They’re also made entirely of metal, and have no moving parts, so they will survive bouncing around in the back of a truck.

This was a design study for a general purpose fridge for 2-inch cased holes (38 mm maximum OD) that would solve many of the temperature problems of downhole operation for a wide variety of sensors.

Transdermal glucose detectors based on optical coherence tomography

This was a photon budget for an OCT system—interesting primarily for the effect of path delay in turning FM noise in the superluminescent diode (SLD) into AM noise in the measurement.

Mixed-reality head-mounted projection displays

I chaired a series of formal design reviews for a start-up company making immersive displays with resolution better than the human eye.

All-Optical Downlink for Antisubmarine Warfare (ASW) sonobuoys

Another Della project for the Navy: I did a photon budget that showed that this could be done optically within the power and weight constraints, and would work in bright sun as well as at night. (Optical communications are much harder to intercept or to jam than radio.)

Expert Case: Waymo LLC v. Uber Technologies, Inc., et al.

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.)

Neutral Referee in IP dispute between former joint development partners

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.

Voxpath RS, LLC v. Desay A&V Science and Technology Co. Ltd, et al.

Testifying defense expert representing Samsung in an action for patent infringement concerning optical storage, holographic optical elements, tracking servos, and signal integrity.

Industrial Technology Research Institute v. LG Corporation, et al.

This was another fun one with a lot of reverse engineering.  This time round I was working with the plaintiff, Industrial Technology Research Institute, which is a research lab owned by the Taiwanese government.  It was an action for patent infringement in the focusing and tracking servos of optical disc drives, as well as in the arrangement of the laser sources.  The patent claims at issue concerned the way the magnetic “voice coil” actuators simultaneously adjusted focus, tracking, and tilt, so I needed to take several of the accused products apart and run the head servos by themselves in my lab.  I also had to cut apart some of the coils to show how they were wired, and decap the lasers to show that there were two chips side-by-side in the CD/DVD source and one in the BluRay source.

ThinkOptics, Inc. v. Nintendo of America, et al.

Testifying expert representing ThinkOptics in an action for patent infringement concerning video games, specifically the human interface of the Nintendo Wii.
February 15, 2015: Settled after an inter partes re-examination.