A Stark-effect modulator for CO? laser free-space communications

by 1979- Holloman, Ryan Lane

A Stark-effect modulator prototype has been designed and constructed to transmit an RF signal upon an infrared carrier wave produced by a CO2 laser. The anticipated future use of this prototype will be to integrate it into an all-weather long-wavelength infrared (LWIR) free-space communication link. This communication system is expected to be capable of operation at 2.5 Gbits/sec over a range of > 6 kilometers. The Stark-effect modulator prototype is ideal to replace existing microwave and radio communication links used currently. Also, the Stark-effect modulator is a better choice than electro-optic crystal modulators and acousto-optic Bragg cell modulators because it operates at higher frequencies and is less costly. The Stark-effect occurs when an electric field is applied to a gas molecule that has a substantial polarizability. The electric field changes the spacing of the gas molecule energy levels. The gas molecule used is deuterated ammonia (NH2D) because it has a molecular absorption resonance near the 10.59 micron wavelength line of the CO2 laser emission. The modulator is a dielectric waveguide that is constructed of borosilicate glass, measuring 36-cm in length and having an inner diameter of 1.8-mm and an outer diameter of 3-mm. The modulator has been characterized successfully, with an experimental result agreeing with a predicted model. For the first time, an RF signal has been successfully applied to the Stark-effect modulator and successfully transmitted and detected by a nearby receiver. iii