Investigation of liquid crystal modulators and retroreflectors for laser communication to buoy

This report describes a technique for communication with underwater systems via a buoy on the surface of the sea. The communication takes place on a retrocommunication link, where a free-space laser beam at 1.55 µm is modulated and retro-reflected. The technique is developed by manufacturing and testing of sub-components, by setting up indoor test links, and by field tests with buoy and laser. The modulator is based on glass cells with ferroelectric liquid crystal (FLC). The final cells are manufactured at FOI. The liquid crystal is used to modulate the polarisation state of the laser beam. For the time being, the modulation speed is 20 kHz and the modulation depth is 0.90. Drive cards adapted to the application have been developed. The way the retroreflector moves is affecting the ability to maintain the link. Measurements have therefore been performed indoors on tilted reflectors and field tests have been made with reflectors on a buoy on the surface of the sea. Results show that reflected irradiance from a retroreflector is larger than 50 % within a cone angle of 20 degrees. It was possible to follow a buoy on the sea at a distance of 3.5 km with a laser. Link interruption occurred only in exceptional cases, and it could not be derived to the seaway or atmospheric turbulence. Indoor test links have been used to transfer information up to 180 m. Short text messages were transferred at rates faster than 1 kbit/s.