224 Mc/s optical pulse code modulator

The authors report on an optical modulator for pulse code modulation (PCM) transmission systems operating at a 224 Mc/s bit rate. The modulator uses the electrooptic effect in lithium tantalate (LiTaO2) and requires less than ten milliwatts drive power to a transistor pulse amplifier which supplies the drive voltage for the lithium tantalate. It acts as a gate for the regular train of optical pulses from a laser phase-locked at 224 Mc/s. The modulator is constructed using a single crystal of lithium tantalate in the form of a parallelepiped with dimensions 0.025 x 0.025 x 1.00 cm having an antireflection coating on one end and a dielectric reflecting coating on the other. This modulator exhibits a 15 dB optical power extinction ratio between the pulse and no pulse conditions at the output of the modulator. In the "1" state the modulator exhibits an insertion loss of 3.6 dB - including the contribution of all the associated optical components.