An Integrated Offset Sideband Modulation Receiver
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A major challenge in designing an integrated optical transceiver is minimizing the crosstalk between the high power laser driver and the sensitive receiver. Offset sideband modulation (OSBM) is used for the downstream signal to mitigate the crosstalk between the transmitter and the receiver. When detected with a photodiode, the OSBM signal produces a bandpass signal that can be reliably detected in the presence of baseband crosstalk. OSBM also enables the use of more complex modulation formats that include phase modulation such as quadrature phase shift keying (QPSK). An architecture for an integrated offset sideband receiver is developed using a system level analysis. The offset sideband modulated signal is generated using an arbitrary optical waveform generator (AOWG). The signal parameters, including the pulse shaping function and offset frequency have been optimized to ease the requirements of the integrated reciever. The receiver consists of a bandpass transimpedance amplifier (TIA) followed by a four phase Costas loop to provide carrier synchronization and demodulate an OSBM signal with either BPSK or QPSK modulation. A number of components of the integrated OSBM receiver have been designed and fabricated in a 0.13 um CMOS process. A novel bandpass TIA is created using a stagger tuned topology to produce a second order bandpass response. The TIA achieves a transimpedance gain of 56 dBOhms and a bandwidth of 5 GHz centred at 7.5 GHz. The average input referred noise current of the bandpass TIA is 26.5 pA/sqrt(Hz). A four phase Costas loop has been designed to demodulate a 2.5 GSym/s BPSK signal or a 2.5 Gbit/s QPSK signal with a carrier frequency of 7.5 GHz. The Costas loop has a loop bandwidth of approximately 100 MHz. Measured results are presented showing that the Costas loop is capable of demodulating a BPSK signal at 2.5 Gbit/s with a 2^31-1 pseudo random bit sequence (PRBS). Measured results show the Costas loop is also able to demodulate a 2.5 GSym/s QPSK signal with a 2^7-1 PRBS pattern.