SWIR Grating Spectrometer Proof of Concept

This was a prototype of a transcutaneous (through-the-skin) sensor for blood glucose and blood alcohol.

Optical and optomechanical design, detection and control electronics and software, prototype construction: first spectra were taken 5/16/2013, and technology transfer to a contract engineering firm was essentially completed 6/14/2013.

Most notable was the schedule requirement: from a standing start, in less than 6 weeks' work, I did a complete photon budget, designed and built all of the optics and electronics, wrote all the software, integrated and shipped the system.  It worked great.

Because of the schedule, the prototype was built mostly out of stuff I had in my drawer. That meant that all the coatings were mistuned, which cost a lot of light, but it nevertheless works very well. There are a few amusing features.
For instance, the grating is mounted on a servomotor intended for radio-controlled airplanes. While rather unorthodox, this is actually a pretty sweet solution for a lowish-resolution spectrometer—you get a rare-earth magnet motor, titanium gear train, magnetic position encoder, and servo control electronics in a 2-cubic-inch device that costs $139 in quantity one. It has to be designed out of the actual product, because it isn't quite good enough at making very small moves repeatably, and besides, it wouldn't do to show the Food & Drug Administration a device built with toy parts!

If you want to try this, I suggest putting an optical position sensor on the grating, e.g. by putting a barefoot diode laser next to the slit and detecting the specular reflection with a lateral-effect photodiode or a webcam sensor. That way, minor nonrepeatability of the tuning doesn't turn the slopes into absorption error. Human tissue has a huge absorption slope in the SWIR—about 2 AU in 100 nanometres—so this is a real issue.