DIYing spectrometers is not new. You can make one with your phone camera, and a broken piece from a CD-ROM as a diffraction grating. There are plenty of tutorials for this online. It takes $5 to make one.

However, a silicon based camera (CMOS sensor) only respond up to ~1100 nm in wavelength, and this is a physical limitation —— silicon has a band gap of ~1.1 eV, and you cannot excite electron-hole pairs with wavelength longer than about ~1100 nm. So we need a different semiconductor if we want to measure any light above this wavelength. A popular choice is InGaAs, whose bandgap is tunable down to ~0.4 eV by changing content of indium versus gallium.

But you probably don’t want to know how much a InGaAs camera will cost you… While a silicon-based camera is as cheap as dirt these days, an one-dimensional InGaAs pixel array already costs upper few thousand dollars. Any full-blown IR spectrometer system goes way over $10k, with their fancy thermoelectric cooling and precision gratings (we actually have one in our lab). The reason why they are so expensive is that the target user group are scientific researchers, not consumers.

Since I have been recently interested in laser optics (as a hobby) and wish to DIY some laser systems, which inevitably requires working with near-IR wavelengths (above 1100 nm), I desperately needs a way to analyze what light I’m producing out of my laser crystals. One day I tried to search for InGaAs photodiodes on DigiKey and it turns out that a single InGaAs photodiode sells for ~20 bucks. While its 100x more expensive than a silicon photodiode, I figured that you can make a spectrometer just with this one photodiode, and it’s definitely within reach of DIY. And here it is —— a fiber-coupled IR spectrometer that measures from 800~1600 nm.

Design

A spectrometer mainly consists of four components: slit, diffracting element, detector, and relaying optics between these parts. In our case, the input optical fiber acts as our input slit. I chose to use a 50 um core multi-mode fiber for a compromise between light throughput and spectral resolution. A 2-m long SMA-905 fiber patch cable can be purchased on AliExpress for about $40, or $70 from Thorlabs.

The diffracting element is typically a grating. It is important to choose a grating with the right line density to obtain reasonable dispersion (wavelength per degree of angle) at the designed order of diffraction (1st-order is most commonly used). For this project since the design wavelength is 800 nm ~ 1600nm, a line density of 600 line per mm is about right, and this gives a dispersion of 40° over the designed wavelength range. See below for a plot of angle-of-diffraction for 50° incident angle onto a 600 line/mm grating. The 2nd and 3rd orders are clearly not usable as they overlap with the incident beam.

Next comes the detector. How are we gonna detect light from a range of angles with only a single ‘pixel’ of photodiode? We mount the photodiode on a motor and scan it across! To do this, I purchased a stepper-driven linear stage from Amazon for $50. This little stage is quite well-built and allows a resolution-per-step of 5 um, more than enough for our resolution.

The InGaAs photodiode mentioned above has an active area of ϕ1mm, while the image of the 50 um input slit is gonna be a bit smaller than that. So an output slit also needs to be mounted as close as possible to the photodiode, in order to make sure that the size of the photodiode does not compromise the spectral resolution. This does not need to be of great precision —— I simply used some aluminum masking tape to make a slit of ~0.3 mm wide.

Lastly, we need optics to connect all these components together. It turns out that this part is quite expensive and tricky if you want high spectral resolution as well as high light throughput (which ultimately determines the signal/noise ratio). Basically, starting from the input slit (which can be viewed as a point source), we need to (1) defocus it into a parallel beam, (2) direct it onto the grating, and (3) focus the diffracted beam (which is approximately parallel for each color) onto the sensor. Traditionally, the standard way is an all-mirror configuration known as the Czerny-Turner design. The reason to use mirrors instead of lenses is to minimize chromatic aberrations, so that all wavelengths can be focus onto the same focal plane. However, the alignment of these parabolic mirrors are quite difficult without a real optics bench, so I decided to pursue a different path that is much more friendly for DIYing.

The first step —— defocusing fiber output into parallel beam, can be achieved using a fiber collimator. This is basically a lens pre-aligned with its back focal point right at the interface of the optical fiber core.

One end of this device is already threaded with standard SMA-905 connector, so that the fiber patch cable can be directly connected to it. The other end is free-space output of parallel beam. Very easy to use!

However, the lens used in the collimator does have a chromatic aberration, so that only the bea