In-Cavity Phase Modulator for Integrated DQPSK Transmitters

We present a new concept to implement phase modulation in DQPSK transmitter: moving the physical phase modulator from outside the laser cavity, where it is usually placed, to inside the cavity. On analyzing the new scheme, we find that the in-cavity process of phase-frequency-phase conversion provides a phase gain, which is larger than one up to bit-frequency of about 100 GHz. We supplement the first-hand calculations by numerical simulations based on a travelling-wave model of the field internal to the cavity and find good agreement with theory.