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International Conference on Optical Communications and Networks (ICOCN)

Quadrature squeezing of phase modulated signals

Takayuki Kurosu, Mingyi Gao, Karen Solis-Trapala, Shu Namiki

Published Nov 1, 2014• 0 citations
Publisher

Abstract

We present two effective methods to perform quadrature squeezing and their experimental demonstrations performed on phase modulated signals. The first method, which is based on phase sensitive amplification, is used for de-multiplexing the two orthogonal phase components of QPSK signals. The second method, which is a hybrid method combining optical parametric processes and amplitude/phase modulation, is used for simultaneous phase regeneration of two coherent WDM BPSK signals.

DOI
10.1109/icocn.2014.6987115

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We present two effective methods to perform quadrature squeezing and their experimental demonstrations performed on phase modulated signals. The first method, which is based on phase sensitive amplification, is used for de-multiplexing the two orthogonal phase components of QPSK signals. The second method, which is a hybrid method combining optical parametric processes and amplitude/phase modulation, is used for simultaneous phase regeneration of two coherent WDM BPSK signals.
High gain extinction ratio and stable control of the phase in phase sensitive amplification are fundamental to realize either phase regeneration or quadrature squeezing of phase modulated signals in an efficient and robust manner. In this paper, we show that a combination of our previously demonstrated “sideband-assisted” dual-pump phase sensitive amplifier with a gain extinction ratio of more than 25dB, and a phase-locked loop based stabilization technique, enable efficient QPSK quadrature squeezing. Its stable operation is exploited to realize phase de-multiplexing of QPSK signals into BPSK tributaries. The phase de-multiplexed signals are evaluated through measurement of constellation diagrams, eye diagrams and more importantly, BER curves. The de-multiplexed BPSK signals exhibited an OSNR penalty of less than 1dB compared to the back-to-back BPSK signals.
We present a new method to perform phase regeneration of phase encoded signals. In our concept called "hybrid optical phase squeezer (HOPS)," a multilevel phase-quantized signal is synthesized through the coherent addition of a phase-conjugate copy of the signal and a phase harmonic of the signal with a frequency shifter. Unlike the conventional method by phase sensitive amplification, HOPS does not use any optical parametric gain such that only optical elements with low optical nonlinearity are necessary for optical phase quantization. In the proof-of-concept experiment, it is confirmed that a 2-level HOPS can perform quadrature squeezing with an extinction ratio of 40 dB. Simultaneous phase regeneration of two coherent wavelength-division-multiplexed 10.75-Gb/s binary phase-shift keyed signals is successfully demonstrated using a 2-level HOPS based on a semiconductor optical amplifier.
A newly proposed concept, which is called hybrid optical phase squeezer (HOPS), achieves multi-level optical phase quantization through coherent addition of two (dual-wave scheme) or three (triple-wave scheme) optical waves exploiting optical parametric processes and electro-optic modulation. The triple-wave scheme enables signal phase regeneration free from phase-to-amplitude noise transfer, which is inevitable in the dual-wave scheme. By using HOPS in the dual-wave scheme, 3-fold phase-noise reduction was achieved for 24-Gb/s QPSK signals with a slight increase of amplitude noise. On the other hand, HOPS in the triple-wave scheme allowed phase regeneration of 12-Gb/s BPSK signal with a suppression of phase-to-amplitude noise transfer.
We propose and demonstrate a technique to obtain phase preservation in optical processing which is suitable for many four-wave mixing (FWM)-based processing devices, provided they may be modified to incorporate two conjugating FWM stages. It functions by using a first conjugating nonlinear stage to predistort the signal using self-phase modulation (SPM) such that when the signal undergoes further SPM in a second conjugating FWM stage, the two SPM contributions negate each other, resulting in phase preserving output. In this paper, we use the second stage to perform amplitude squeezing through parametric gain saturation and characterize the scheme by regenerating a QPSK signal contaminated with broadband amplitude noise. Experimental analysis of the system with and without predistortion is provided and phase preserving operation using the proposed scheme is confirmed.