AOTF Module
An AOTF module is a packaged Acousto-Optic Tunable Filter system—a solid-state, electronically tunable optical bandpass filter used with broadband sources (especially supercontinuum lasers) or multi-line lasers to select one or more narrow wavelength channels without moving parts.
How it works:
An RF signal drives a piezoelectric transducer bonded to a birefringent crystal (most often TeO₂). The resulting acoustic wave periodically modulates the refractive index, acting as a tunable diffraction grating.
Only optical wavelengths that satisfy the phase-matching (Bragg) condition are efficiently diffracted into a first-order beam; other wavelengths pass through undiffracted.
Changing the RF frequency (typically tens to a few hundred MHz) selects a different optical wavelength. Multiple RF frequencies can be applied at once so several channels can be selected simultaneously. Intensity is controlled by RF power. Switching is limited mainly by acoustic transit time across the beam (often microseconds).
Common geometries are non-collinear (larger acceptance angle, easier beam separation) or quasi-collinear. Output can be free-space collimated or fiber-coupled (often polarization-maintaining). Polarization is usually specified (input linear, output often rotated).
Typical technical characteristics:
Exact numbers depend on crystal cut, aperture, and band, but representative values for VIS–NIR modules used with supercontinuum sources are:
Tuning ranges by crystal: ~400–650 nm (VIS), ~650–1100/1200 nm (NIR1), ~1100–2000+ nm (NIR2); dual-crystal modules cover two bands from one input.
Up to 8 simultaneous independently tunable channels.
Spectral bandwidth (FWHM): typically 1–16 nm (narrower designs exist, e.g. 0.2–0.4 nm in some fiber-coupled or specialized filters).
Diffraction efficiency: often >80–90% in the selected band.
Extinction / contrast: can exceed 50 dB.
Aperture: commonly 2–10 mm (larger for imaging).
RF drive: ~0.1–2 W per channel typical; multi-channel total power limited by heating.
Control: USB + GUI, wavelength scanning, channel stacking, fast switching; some support external TTL modulation (µs–MHz range) for FRAP, ALEX, etc.
High-power variants exist that reduce polarization-related loss when used with unpolarized supercontinuum light.
Applications:
Supercontinuum sources: The most common “AOTF module” use—turn a white-light laser into several independently tunable, intensity-controlled narrowband outputs (free-space or fiber). Used as a flexible multi-wavelength source instead of several discrete lasers.
Confocal / fluorescence microscopy: rapid multi-line selection and intensity control from combined lasers or a supercontinuum.
Hyperspectral and multispectral imaging, spectropolarimetry.
Spectroscopy (fluorescence, Raman, LIBS, process monitoring).
Wavelength tuning of lasers (e.g., Ti:sapphire cavities) and external-cavity designs.
Optical coherence tomography (OCT).
Remote sensing and space instruments (compact, no moving parts, random-access tuning).
Industrial process control and material analysis.
Advantages vs. filter wheels or grating monochromators: microsecond random-access tuning, simultaneous multi-wavelength operation, no vibration or image shift, compact solid-state construction.
Limitations include RF power/heat management for many simultaneous channels, polarization sensitivity, and finite spectral resolution set by crystal length and acoustic parameters.