A proposal for internal electrooptic intensity and frequency modulation of diode lasers

Modulation of a GaAs diode laser by a GaP electro-optic diode modulator internal to the laser cavity is proposed. Application of a reverse voltage to the modulator diode changes the state of polarization of the oscillating mode. One polarization component of the mode is efficiently coupled out of the cavity by reflection off a Brewster angle laser diode face. Solutions of the threshold equations for the mode frequencies, threshold gain, and the amplitude ratio and the relative phase of the polarization components of the oscillating mode exhibit novel behavior. This allows the laser-modulator combination to be used as either an intensity or a frequency modulated source. This frequency modulation regime in a coupling modulation configuration is an unexpected result and can be applied to other types of lasers. Calculations of the modulation characteristics based on measured properties of the GaP modulator diodes are presented. Operated as a Gürs-Müller coupling modulator (intensity modulation) the device could produce a depth of intensity modulation over 50 percent with a bandwidth of 1 to 3.5 GHz while dissipating only 0.1 watt in the modulator. Operated as a pure frequency modulator the device could have a 2.5 GHz bandwidth, again with 0.1 watt modulator dissipation. The modulator diode can also be used to produce pulse-code modulation. Up to one gigabit per second could be produced with the same modulator dissipation.