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Optical Fiber Communication Conference

Novel optical thresholder based on second harmonic generation in long periodically poled lithium niobate for ultrashort pulse optical code division multiple-access

Zheng Zheng, Andrew M. Weiner

Published Nov 7, 2002• 0 citations
Publisher

Abstract

We report a novel yet practical optical thresholding scheme for ultrashort pulse CDMA using second harmonic generation in a long PPLN crystal, with a demonstrated contrast ratio of /spl sim/30 dB and the potential for low power operation.
DOI
10.1109/ofc.2000.868601

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Summary form only given.There been increasing interest in realizing optical signal processing functions in optical communication systems based on nonlinear optical parametric processes. In the proposed ultrashort pulse optical code-division multiple access (CDMA) systems, in order to realize multiplexing and demultiplexing for multichannel operation, a nonlinear element is needed to distinguish between correctly decoded signals and incorrectly decoded multiaccess interference, which have comparable average optical power but carry different spectral information. It had been proposed to realize all-optical recognition of optical phase-coded ultrashort waveforms by using second-harmonic generation in long nonlinear crystals, which could have advantages in contrast ratio, operating power and detection efficiency over other proposed schemes. While high contrast ratios have been demonstrated based on such a scheme using bulk periodically poled lithium niobate (PPLN) crystals, the high efficiency has not yet been demonstrated. We use a simple combination of PPLN SHG waveguides and conventional photodetectors to realize all-optical recognition of spectrally phase coded optical pulses, which shows the potential of much higher conversion efficiency compared to previous bulk experiments, which is crucial for real-time operation with a high signal to noise ratio.
We demonstrate a novel all-optical scheme for measuring the correlation of spectrally phase-coded ultrashort optical waveforms that uses second-harmonic generation (SHG) in long, periodically poled lithium niobate crystals. The SHG yield can be controlled over a range of ~30 dB, depending on the correlation of the applied phase codes. Such a spectral phase correlator has applications for ultrashort-pulse optical code-division multiple-access networking and could serve as a nonlinear optical but classical analog for certain schemes for coherent quantum control of multiphoton processes.
We determine the theoretical singly resonant second-harmonic generation squeezing performance of several cavity designs and nonlinear materials. We show that, for the doubling of the 1064-nm output of Nd:YAG lasers, monolithic cavities made from periodically poled LiNbO3 (PPLN) will produce the largest amount of squeezing. We also present experimental results for a free-space cavity with a PPLN sample that yielded slightly less than 0.6 dB of measured squeezing. The cavity performance as a function of input power agrees with our theoretical predictions and shows the superior performance of periodically poled materials for the production of bright squeezed light.
This work reports on the generation, modulation, and detection of temporally coherent pulses in periodically-poled lithium niobate (PPLN) configured as an optical parametric amplifier (OPA).The OPA was pumped at 1 µm, by a 10 kHz, 1 ns, Nd:YAG laser.The cw seeding sources for the OPA were two separate 1550 nm lasers.One source had a short coherence length of .3km, the other a long coherence length of 95 km.The generated pulse trains where phase modulated by using a set of mirrors on a single axis motorized stage.A 10 km fiber optic delay line was used to simulate free space propagation.A homodyne, coherent optical detection system, utilizing the in-phase and quadrature components of the circularly polarized local oscillator, was built to detect the amplitude and phase modulated pulse trains.We report the detection of the modulated signal for the long coherence length source and the loss of the modulated signal for the short coherence length source, demonstrating that the OPA process can transfer the temporal coherence (or coherence length) of the input signal to the pulse-amplified signal