December 10, 2019
— updated February 3, 2020
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.
Read more…December 10, 2019
— updated December 9, 2022
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.
Read more…December 10, 2019
— updated December 9, 2022
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.
Read more…July 26, 2018
— updated March 9, 2025
There are widespread shortages of electronics parts at the moment, especially passives. Quoted factory lead times are 40 weeks or thereabouts, and since the industry is capacity-limited, it isn’t clear that the situation is going to get better any time soon, so everybody’s starting to panic. Given all this churn I’ve been spending an unconscionable amount of time lately finding suitable replacements for out-of-stock parts.
High value ceramic caps are the worst–their capacitance drops by at least 60% and at worst 95% at rated voltage, so finding an adequate substitute involves a lot more than the package, value and voltage rating. Most of their data sheets are useless, which is frustrating. However, all is not lost: most makers have websites where you can look at the C(V) curves.
Read more…March 20, 2018
— updated March 9, 2025
From the cutting room floor at Building Electro-Optical Systems, Third Edition:
Photographic Film
Okay, okay. Photographic film isn’t a detector of the sort we’ve been discussing. Film is so out of fashion, so inconvenient. It needs messy chemicals. Getting it to be highly sensitive requires all sorts of 1960s alchemy such as pre-flashing and hypersensitizing in a forming gas or hot hydrogen atmosphere. Why do we care about it at all, in these days of 4k x 4k CMOS imagers?
Read more…