Differentiator based frequency detector for 100-Gb/s analog domain DP-QPSK coherent optical receivers

One of the significant issues in coherent optical communication systems is the frequency offset between the carrier laser and the local oscillator (LO) laser. For the synchronization of the phase and frequency between the two lasers a carrier phase recovery and compensation (CPRC) circuit is required. A CPRC circuit that has a phase detector alone may fail to recover the carrier signal if the frequency offset is too large. A frequency detector is needed along with the phase detector in the CPRC circuit for achieving frequency acquisition. We propose a differentiator based FD for correcting the frequency offset. The proposed FD has been designed and simulated using 130 nm BiCMOS technology. A decision-assisted Costas loop based CPRC circuit that has the proposed FD is capable of compensating a frequency offset from -100 MHz to 100 MHz. The circuit is shown to achieve frequency acquisition in a 100-Gb/s dual polarization-quadrature phase shift keying (DP-QPSK) receiver when the laser line width was 200 kHz and the transmission was done over 80 km-long single mode fiber.