Abstract
Distortions caused by system components and by fundamental physical
phenomena can limit the performance of photonic time-stretch ADCs. Here we
use a combination of time-stretch linearization & equalization,
DC-offset subtraction, and operation in a linear propagation regime to
improve the signal-to-noise-and-distortion ratio by 17 dB for a 2-channel
time-stretch ADC testbed and therein obtain noise-limited performance of
6–7 ENOB over a 10-GHz RF input bandwidth. Time-stretch
linearization & equalization corrects for dispersion mismatches
among testbed components by applying time-shifts calculated from component
group delays to output ADC samples. DC-offset subtraction removes static
errors due to insertion loss imbalances and Mach–Zehnder modulator
bias offsets. If optical power levels are too high, nonlinear fiber
propagation lowers the frequencies of dispersion-induced nulls in the RF
transfer function and causes higher-order signal distortions. The 2-channel
testbed can be directly scaled to a practical continuous-time system with
the addition of more sub-aperture wavelength channels (total of 13 channels
and 42 nm of optical bandwidth for a 90 MHz laser repetition rate). Adaptive
online and fixed pre-calibrated stitching methods are demonstrated for
joining data from one wavelength channel to the next.
© 2010 IEEE
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