PhD Defense: "Ultra-Compact Integrated Coherent Receiver for High Linearity RF Photonic Links"

Uppiliappan Krishnamachari

June 9th (Thursday), 9:00am
Engineering Science Building (ESB), Room 1001

Radio frequency (RF) photonic links employing phase modulation can exhibit high dynamic range provided the received RF signal can be linearly demodulated.The linearity of conventional interferometer based coherent receivers is limited by their sinusoidal response. A balanced receiver with negative feedback to a reference tracking modulator is developed in this work. In order to achieve high linearity, the tracking modulator is used to reduce the net phase swing across the demodulator such that the interferometer response is restricted to its linear regime without penalty to the signal to noise ratio (SNR). In order to achieve linear phase tracking at high frequencies, high loop gain and loop bandwidth are required. This means the physical delay of the feedback loop must be sufficiently short enough to prevent loop instability at higher frequencies. Monolithic integration of the receiver components is thus used to realize a compact receiver architecture.

In this work, a monolithically integrated coherent receiver with negative feedback to suppress the nonlinearity arising from phase modulation has been successfully demonstrated. The photonic integrated circuit (PIC) consists of an ultra-compact frustrated total internal reflection (FTIR) trench coupler integrated with multiple quantum well (MQW) reference phase modulators and a balanced uni-travelling carrier (UTC) photodetector pair. The trench coupler contributes less than 1 um to the optical path length, and is shown to exhibit 50:50 splitting. The compactness allows a device geometry that enables flip chip hybrid integration of the PIC with an electronic integrated circuit (EIC) that contains the transconductance amplifier and filter for the feedback loop. Use of the ultra-compact trench and the flip chip integration limits the loop delay to ~10 ps. The resultant 3-dB bandwidth of the receiver was measured to be 1.5 GHz. The shot-noise limited SFDR at 300 MHz is 123 dB Hz^2/3.

Hosted by: Prof. Larry Coldren