Compact USB SWIR Spectrometers (500–1700 nm) Using InGaAs Sensors

Silicon is the default detector in visible and NIR spectrometers, and it is useful up to about 1100 nm. At that point the band gap runs out: quantum efficiency falls steeply as it approaches 1100 nm and reaches zero beyond it. Everything interesting in the SWIR, the water bands at 1450 nm and the C-H overtones at 1660 to 1750 nm, sits outside its reach.

The standard lattice-matched InGaAs composition covers roughly 900 to 1700 nm with high quantum efficiency and low dark current at room temperature. This is the workhorse for SWIR spectroscopy: it captures the first overtone bands of O-H, C-H and N-H, which is where most quantitative chemistry lives.

Increasing the indium fraction pushes the cut-off out to 2200 nm or 2600 nm, which opens the combination bands. The trade is real: extended compositions have far higher dark current and normally need deep thermoelectric cooling, which adds cost, power and bulk to what was meant to be a USB-powered module.

The d-vision from Sphere Photonics extends InGaAs response down toward 500 nm, so a single 512 pixel array carries visible, NIR and SWIR information at once. In practice that removes the second spectrometer, the second fiber and the job of splicing two calibrations together.

1. Input optics Fiber or slit

A fiber connector or a fixed slit sets what light gets in and at what angles. The slit width is the first term in the resolution budget: narrow it and lines get sharper but photons get scarce, widen it and the reverse. Every downstream component is designed around the image of this slit.

2. Collimating optics Diverging to parallel

A collimating lens turns the diverging cone from the slit into a parallel beam. The grating equation only behaves cleanly for collimated light, so any error here shows up as a broadened, asymmetric line profile across the whole spectrum.

3. Dispersive element Diffraction grating

The grating is the component that decides the instrument’s personality. Groove density sets the angular spread, and therefore the trade between spectral range and resolution. On the d-vision this is the choice that moves resolution between roughly 1.3 nm and 3.1 nm.

4. Focusing optics Spectrum onto pixels

A curved mirror re-images the dispersed slit onto the detector plane, so each wavelength lands on its own pixel. Getting the whole spectrum in focus at once, across a flat sensor, is most of the optical design work in a compact bench.

5. InGaAs linear array 512 pixels

The 512 pixel InGaAs array converts each wavelength band into charge. Because every pixel integrates at the same time, the full spectrum is acquired in one shot: no scanning stage, no moving parts, and no motion artifacts between one end of the spectrum and the other.

6. Readout and USB 16-bit, USB 2.0 / 3.0

A low-noise preamplifier feeds a 16-bit converter, and an FPGA handles timing, sensor control and transfer. USB carries both the data and the power, which is what lets the whole instrument travel in a coat pocket rather than a rack.

Full VIS to SWIR in one acquisition. Visible color, the NIR overtones and the SWIR bands land on the same 512 pixels and share one calibration.

Typical use: Broad survey work, plastics sorting, first-look material screening

Classic SWIR window. Drops the visible to buy back resolution, and still holds every first overtone band worth measuring.

Typical use: Moisture, fat and protein assays, inline process monitoring

Tightened around the O-H and N-H first overtones. Enough resolution to separate bands that overlap on a broadband grating.

Typical use: Water content, amide and protein chemistry, biofluids

The narrowest, sharpest option. Reserve it for a band you already know you care about, because everything outside the window is simply gone.

Typical use: Single-band quantification, laser and fiber characterization

Process and industrial Real-time chemical monitoring

• Reaction kinetics followed in the vessel rather than in the lab queue
• Blend uniformity checked continuously instead of by grab sample
• Inline quality gates that stop a bad batch at the machine

Because the measurement is optical and takes microseconds, the spectrometer becomes part of the control loop instead of part of the paperwork.

Food and agriculture O-H, C-H and N-H overtones

• Moisture, fat, protein and carbohydrate estimated without a reagent
• Sorting and grading on a belt at production speed
• Foreign object and contamination screening

Water dominates the 1450 nm region, fats and oils sit in the C-H bands near 1200 nm and 1720 nm, and protein shows through N-H near 1500 nm. One spectrum carries all three.

Materials and recycling Deeper penetration than visible light

• Polymer identification for sorting streams, including black-on-black cases
• Composition and additive checks on incoming feedstock
• Cure state and crosslink monitoring in composites

SWIR light goes past the surface, so pigment and printing matter far less than they do to a color camera. See the polymer demonstration further down this page.

Pharma and biomedical Non-destructive and inline

• Raw material identity checks through the bag or the vial
• Blend endpoint and drying endpoint determination
• Biofluid and tissue phantom work in the near infrared window

Nothing is consumed and nothing is prepared, which means the same unit can be measured again later, and the measurement can sit inside a validated process.

Optics and photonics Direct infrared coverage

• Laser diode and fiber laser spectral characterization
• Fiber and component transmission testing
• Photoluminescence from infrared emitters and quantum dots

Silicon spectrometers simply cannot see telecom wavelengths. An InGaAs array covers 1310 nm and 1550 nm directly, with no upconversion or exotic detector.

Environmental Sensitive to organic bonds

• Hydrocarbon detection on water and on soil
• Water content and purity screening in the field
• Vegetation and moisture indices from a handheld head

Field measurement is where USB power earns its keep: a laptop, a fiber and a lamp is the entire kit.

Overtones are weak echoes of the fundamental vibrations that live in the mid infrared. That weakness is the feature, not the bug: it lets SWIR light pass millimeters into a sample and come back out, so you measure bulk composition rather than a surface film, without the expensive optics and cooled detectors a mid infrared system demands.

O-H bands

Water, alcohols and hydroxyl groups. The 1400 to 1470 nm feature is the single most useful band in the whole window: it is strong, it is broad and it scales with moisture content in almost any matrix.

C-H bands

Every organic material. Aromatic and aliphatic C-H separate cleanly near 1660 nm and 1720 nm, which is exactly what makes polystyrene distinguishable from polypropylene on a sorting line.

N-H bands

Proteins, amides, amines and urea. The 1490 to 1560 nm band is the basis of protein estimation in grain and dairy, and of amide monitoring in polymer and pharmaceutical chemistry.

Three commodity polymers, measured on one d-vision spectrometer through the same optical path. Across most of the window they are nearly indistinguishable, all sitting near 80 percent transmittance. Everything that separates them happens in the last hundred nanometers, which is precisely the region a silicon detector cannot see at all.

PETE, polyethylene terephthalate

The aromatic ring in PET puts a clear absorption near 1660 nm, so transmittance falls from roughly 0.80 to about 0.54 and then climbs back toward 0.74 by 1700 nm. That recovery on the long side is the tell that separates PET from polystyrene, whose absorption is deeper and does not recover inside the window.

PS, polystyrene

Polystyrene carries a pendant benzene ring on every repeat unit, so its aromatic C-H overtone is the strongest of the three: transmittance drops to roughly 0.36 near 1680 nm. There is a second, shallower dip near 1640 nm, and the pair of features together makes polystyrene the easiest of the three to call with confidence.

PP, polypropylene

Polypropylene has no aromatic ring at all, only aliphatic C-H. It stays high and flat where the other two collapse, and its own absorption sits above 1700 nm, past the detector cut-off. Look instead at the shallow C-H second overtone dip near 1195 nm, visible in the full view.

The decision region

Zoomed into 1580 to 1700 nm, the sorting logic becomes a simple depth-and-shape test. Deep and still falling at 1700 nm is polystyrene. Deep with a recovery is PET. Barely moving is polypropylene. At roughly 2.3 nm per pixel across the broadband range, each of those features is still sampled by ten or more pixels, which is why the call survives a noisy conveyor.