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ABCD Matrix

Infrared Photonics

Infrared photonics is the generation, guidance, manipulation, and detection of infrared light—wavelengths longer than the visible (~0.78 µm) out to roughly 1 mm. In lasers and photonics it covers sources, waveguides, coatings, nonlinear devices, and detectors designed for those bands, where ordinary silica-glass and silicon-visible technology often fails and molecular “fingerprint” absorption becomes useful.


The infrared is usually split into:


  • Near-IR (NIR / IR-A): ~0.75–1.4 µm


  • Short-wave IR (SWIR / IR-B): ~1.4–3 µm


  • Mid-wave IR (MWIR / mid-IR): ~3–8 µm (sometimes the whole 2–20 µm “molecular fingerprint” region is called mid-IR)


  • Long-wave IR (LWIR): ~8–15 µm


  • Far-IR / THz: beyond ~15–50 µm toward 1 mm


ISO 20473 puts mid-IR at 3–50 µm; communities differ, so papers always state the range they mean.


Why IR is a distinct photonics field:


Photon energy is lower than in the visible, so many semiconductors that work at 1 µm become transparent or noisy at 5–10 µm. Silica fiber loss rises steeply past ~2 µm. Thermal radiation from room-temperature objects peaks in the LWIR (~10 µm), which is both a source of background and a sensing opportunity. 


Water, CO₂, and most organic molecules have strong fundamental vibrational bands in the mid-IR, so spectroscopy there is far more specific than overtone work in the NIR.


Materials and devices therefore change with band: fused silica and InGaAs dominate NIR/SWIR; chalcogenides, fluorides, III–V cascades, and HgCdTe dominate mid- and long-wave IR.


Sources:


  • NIR: diode lasers, Nd/Yb/Er fiber and solid-state lasers (0.8–1.6 µm)—the workhorses of pumping, telecom, and industrial processing.


  • SWIR–mid-IR solid-state / fiber: Tm, Ho, Er, Cr:ZnSe/ZnS, Fe:ZnSe; fluoride and chalcogenide fibers (~2–4 µm).


  • Quantum cascade lasers (QCLs) and interband cascade lasers (ICLs): electrically pumped semiconductor sources covering ~3–12 µm (QCLs farther into the LWIR) with room-temperature CW operation in many bands.


  • CO₂ lasers at 9–11 µm for high-power processing.


  • Nonlinear conversion: OPOs, DFG, and supercontinuum from near-IR pumps into the mid-IR.


  • Thermal / LED emitters for illumination and low-cost sensing.


Detectors and passives:


Photonic detectors include InGaAs (NIR/SWIR), extended InGaAs and InSb (SWIR/MWIR), HgCdTe, type-II superlattices, QCDs, and ICIPs. Thermal detectors (bolometers, pyroelectrics) cover LWIR imaging without cryogenics at the cost of speed. 


Waveguides and fibers use silica (to ~2 µm), fluoride, chalcogenide, or hollow-core designs. Coatings and substrates (ZnSe, Ge, Si, CaF₂, diamond) must be chosen for transparency and LIDT at the operating wavelength.


Applications:


  • Telecom and datacom: 1310 / 1550 nm fiber links; emerging mid-IR research fibers.


  • Industrial lasers: cutting, welding, and marking at 1 µm; plastics and organics at 2 µm and 10.6 µm.


  • Trace-gas and chemical sensing: tunable QCL/ICL spectroscopy of CO₂, CH₄, NOₓ, VOCs, explosives, and breath markers.


  • Thermal imaging and night vision: MWIR/LWIR cameras for defense, process monitoring, and building inspection.


  • Medical: tissue ablation near water peaks (~3 µm, 6 µm), diagnostics, and photothermal therapies.


  • Defense: IR countermeasures, missile seekers, and directed infrared energy.


  • Astronomy, remote sensing, and free-space links in atmospheric windows (e.g. 3–5 µm and 8–12 µm).


Infrared photonics is therefore the set of sources, optics, and detectors that extend laser and photonic techniques from the telecom near-IR through the molecular and thermal infrared, using materials and device physics that visible and NIR platforms do not provide.

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