Abstract
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 950MHz 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 4GHz.
© 2011 IEEE
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