Retrocommunication with ferroelectric liquid crystal modulators - preliminary results

We have demonstrated a free-space laser communication link based on retromodulation. The communication is difficult to overhear and highly directional. A potential application is in an all-optical link from air to submarines. A text message was transmitted from a retromodulator to a transceiver by one-way communication. The transfer rate was 10 kbit/s and the distance was 8 m. The modulator utilised a ferroelectric liquid crystal (FLC) cell, which was manufactured at FOI. The optical anisotropic property of the smectic C* phase was used for amplitude modulation. The modulator response (optical contrast) was measured with respect to drive amplitude, frequency, temperature, and different FLC materials. The optical contrast was greater than 3 in a frequency range up to 20 kHz. At higher frequencies, the response was distorted due to limited switch time of the FLC. The transceiver was arranged using fibre-optic components, which made a flexible system. It consisted of a 1.55-micro m semiconductor laser, an optical circulator, a fibre coupling port, a beam expander and a detector. A link budget was calculated for the combined transceiver-retromodulator system. The communication performance was limited by 42 dB attenuation (low signal-to-noise ratio) and distortion of modulator response. Important factors affecting the link performance included the FLC material and drive voltage; cell thickness and temperature; overall attenuation; precise alignment of fibre port and beam expander; and laser beam divergence.