Mr. Louis C. Christen Profile

Louis C. Christen

at Univ of Southern California

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Proceedings Article | 13 February 2007 Paper

Improving bit error rate through multipath differential demodulation

Yannick Keith Lize, Louis Christen, Scott Nuccio, Alan Willner, Raman Kashyap

Proceedings Volume 6457, 64570F (2007) https://doi.org/10.1117/12.701671

KEYWORDS: Demodulation, Modulation, Receivers, Optical filters, Tolerancing, Logic devices, Free space optics, Error analysis, Phase shift keying, Optical calibration

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Differential phase shift keyed transmission (DPSK) is currently under serious consideration as a deployable datamodulation format for high-capacity optical communication systems due mainly to its 3 dB OSNR advantage over intensity modulation. However DPSK OSNR requirements are still 3 dB higher than its coherent counter part, PSK. Some strategies have been proposed to reduce this penalty through multichip soft detection but the improvement is limited to 0.3dB at BER 10-3. Better performance is expected from other soft-detection schemes using feedback control but the implementation is not straight forward. We present here an optical multipath error correction technique for differentially encoded modulation formats such as differential-phase-shift-keying (DPSK) and differential polarization shift keying (DPolSK) for fiber-based and free-space communication. This multipath error correction method combines optical and electronic logic gates. The scheme can easily be implemented using commercially available interferometers and high speed logic gates and does not require any data overhead therefore does not affect the effective bandwidth of the transmitted data. It is not merely compatible but also complementary to error correction codes commonly used in optical transmission systems such as forward-error-correction (FEC). The technique consists of separating the demodulation at the receiver in multiple paths. Each path consists of a Mach-Zehnder interferometer with an integer bit delay and a different delay is used in each path. Some basic logical operations follow and the three paths are compared using a simple majority vote algorithm. Receiver sensitivity is improved by 0.35 dB in simulations and 1.5 dB experimentally at BER of 10-3.

Proceedings Article | 8 September 2006 Paper

Multi-bit delay differential-phase-shift-keyed optical demodulation

Yannick Keith Lize, Louis Christen, Robert Gomma, Scott Nuccio, Narander Jayachandran, Alan Willner, Raman Kashyap

Proceedings Volume 6343, 63433V (2006) https://doi.org/10.1117/12.708241

KEYWORDS: Interferometers, Demodulation, Receivers, Tolerancing, Dispersion, Polarization, Modulation, Optical filters, Fiber optics, 3D imaging standards

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Differential-phase-shift-keyed optical modulation (DPSK) has generated a lot of attention in fiber optic transmission over the past few years mainly because of its 3dB optical signal-to-noise ratio (OSNR) improvement over standard intensity modulated transmission [1] offering high receiver sensitivity, high tolerance to major nonlinear effects in high-speed transmissions [2], and high tolerance to coherent crosstalk [3]. To demodulate DPSK, a delay-line interferometer is usually employed to provide a one-bit delay such that a bit interferes on the following bit to provide constructive or destructive interference depending on the phase difference [4]. However, with the use of logical pre-coding a multi-bit delay can be used instead of a single-bit delay. It was recently reported that a two- or four-bit delay might be advantageous in allowing polarization interleaving between bits to lessen the detrimental effects of fiber nonlinearities in fiber optic transmission [5-7]. Multi-bit delay was also proposed as a method to correct errors from amplified spontaneous emission (ASE) noise-limited transmission [8-9]. We present here simulation and experimental results on the penalty of using multi-bit delay demodulation for DPSK detection. We present Q-factor degradation as a function of delay and find that the Q penalty scales with 0.5 x delay for integer delays. We also present results of the detrimental effect of spectral filtering from the reduced free-spectral-range (FSR) in a multi-bit delay interferometer. We also find that it exhibits reduced dispersion tolerance. If not taken into account, these important limitations and more stringent tolerance on the frequency offset may reduce the effectiveness of multi-bit delay methods and prohibit practical implementation.

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