Fiber AOTF with record large FSR and its application as an NIR spectrometer

Abstract

A fiber acousto-optic tunable filter (FAOTF) is developed for near infrared (NIR) spectroscopy applications. The FAOTF is built on a 7-cm long cladding etched high numerical aperture (NA) single mode fiber. By tailoring the dispersion of the fiber, the core mode to higher order cladding mode coupling of the FAOTF is completely suppressed, leaving only LP₀₁ mode (the core mode) to LP₁₁ mode (the first order cladding mode) coupling enabled. This technique enables the FAOTF to operate with a record large free spectral range (FSR) of >700nm, which is required by NIR spectroscopy applications. Other features of the fiber AOTF include a large wavelength tuning range of >500nm (from 1700nm to 2200nm), a narrow bandwidth of <5.5nm, a low insertion loss of <0.2dB, and a small electrical power consumption of <100mW. For test purposes, the fiber AOTF spectrometer is utilized to measure the output spectrum of a super-luminescence diode (SLD) light source emitting at around 2100nm. In comparison with the same spectrum measured with a PbS array spectrometer, the spectrum acquired with the AOTF spectrometer shows much better resolution and reveals fine spectral features of the SLD light source.