A Hadamard-multiplexed spectrometer implemented using an acousto-optic tunable filter

This paper presents a Hadamard-multiplexed spectrometer based on an acousto-optic tunable filter (AOTF). In a multiplexed instrument multiple signals are measured simultaneously. The signals are combined in such a way that subsequent processing can recover the individual signal values. The multiplexing technique used in this instrument is based on the Hadamard matrix. The Hadamard encoding process can be performed by an AOTF. The AOTF transmits an optical wavelength in response to an applied acoustic signal in the filter. The transmission efficiency is a nonlinear function of power and varies with frequency. Dynamic range and constraints on maximum power place a limit on the number of signals that can be simultaneously applied. Under a maximum-total-power constraint, the power per frequency can be traded off for maximum multiplex advantage, due to the nonlinearity of the power transmission relationship. Determination of the optimal power allocations and the number of frequencies to multiplex are considered. An expandable, PC-based spectrometer has been developed to illustrate this technique. The current implementation provides four-frequency multiplexing and collects the spectra as multiplexed blocks. The AOTF is controlled via VHF synthesizers. The synthesizers are IEEE-485 bussed modules based on a hybrid DDS/PLL technique. The modular design allows convenient system expansion