Photonic Time-Stretched Analog-to-Digital Converter Amenable to Continuous-Time Operation Based on Polarization Modulation With Balanced Detection Scheme

In this paper, we show analytically and experimentally that a polarization modulator which supports TE and TM modes of opposite phase modulation indexes can be utilized to reject dispersion-induced even-order distortions in a photonic Time-Stretched Analog-to-Digital Converter (TS-ADC). The output of the polarization modulator propagates through a single dispersive channel. This makes the present scheme amenable to continuous operation. Based on the virtual time gating principle, the continuous-time RF signal is time-stretched by a factor of 4 and segmented into four channels prior to digitization. For a single channel, differential operation is achieved by using a polarization beam-splitter that generates complementary pulses which are fed to a balanced detector. The differential operation helps to reject dispersion-induced even-order distortions and the balanced detection assists in the suppression of second-order distortion as well as improving the signal-to-noise ratio (SNR) by 6 dB. Using a 10 bit electronic ADC with a sampling rate of 2 GSamples/s, we demonstrate digitization of RF signals up to a frequency of 950 MHz and obtain
${\sim} {\hbox {3.56}}$
effective number of bits (ENOB) with a single channel at
${\sim} {\hbox {31.6}}\hbox{\%}$
of the electronic ADC's peak-to-peak full scale voltage. With adequate backend digitizing hardware, a four-channel continuous-time TS-ADC with a sampling rate of 8 GSamples/s can be realized to handle RF frequencies as high as 4 GHz.