Fiber-optic communication systems form the high-capacity transport infrastructure that enables global broadband data services and advanced Internet applications. The desire for higher per-fiber transport capacities and, at the same time, the drive for lower costs per end-to-end transmitted information bit has led to optically routed networks with high spectral efficiencies. Among other enabling technologies, advanced optical modulation formats have become key to the design of modern wavelength division multiplexed (WDM) fiber systems. In this paper, we review optical modulation formats in the broader context of optically routed WDM networks. We discuss the generation and detection of multigigabit/s intensity- and phase-modulated formats, and highlight their resilience to key impairments found in optical networking, such as optical amplifier noise, multipath interference, chromatic dispersion, polarization-mode dispersion, WDM crosstalk, concatenated optical filtering, and fiber nonlinearity

Advanced Optical Modulation Formats

Publisher Summary
This chapter provides a taxonomy of optical modulation formats along with experimental techniques for realizing them. High-speed optical transmission systems are increasingly building on digital signal processing and digital communication techniques that are well established in radio- frequency (RF) communication systems, including efficient forward error connection (FEC), advanced modulation, line coding, polarization multiplexing, digital signal processing at both transmitter and receiver, and coherent detection. Nevertheless, an optically routed network exhibits important fundamental as well as technological differences from an RF system, and not all techniques that have proven successful in RF communication should be expected to work well for optical networks. The chapter makes clear the substantial distinctions between design conditions for optical and RF applications, namely, the presence of nonlinear distortions in the optical fiber communication channel. As a consequence, and in stark contrast to classical RF systems, the performance of an optical communication link exhibits a maximum at a certain signal power level, which represents the optimum trade-off between optical amplifier noise and fiber Kerr nonlinearity. Demodulation concepts for coherent and delay demodulation are also covered analytically. The conversion process of an (optical) bandpass signal to an (electrical) baseband signal is called demodulation and relies on photodetectors as its key component. Photodetectors used in optical fiber communications are implemented as reverse-biased semiconductor pin diodes. Advanced modulation formats that are robust to transmission impairments or able to improve spectral efficiency are being considered for next-generation lightwave systems.

2 – Advanced optical modulation formats

An original analysis, facilitated by the use of Gaussian approximation (GA) and Chernoff bound (CB) techniques, is presented for an optically preamplified intersatellite pulse position modulation (PPM) receiver employing maximum likelihood detection (MLD). The theoretical results, from calculations performed at a wavelength of 1.54 /spl mu/m and bit rate of 25 Mb/s demonstrate that the receiver is approximately 1.5 dB more sensitive than an earlier optically preamplified PPM receiver and also more sensitive than the fundamental limit of optically preamplified on-off keyed (OOK) NRZ signalling. Thus, it is demonstrated that the proposed receiver configuration is a strong contender for implementation in future laser intersatellite communication systems.

An optically preamplified intersatellite PPM receiver employing maximum likelihood detection

Sensitive multi-rate optical communication is experimentally investigated on static and fading channels at data rates from 52 Mbps to 311 Mbps using 16-ary pulse-position-modulation, channel coding and interleaving, and a near-optimal thresholded preamplified receiver.

Demonstration of multi-rate thresholded preamplified 16-ary pulse-position-modulation

A method and system for generating single and multi-channel optical analog pulse-position modulation waveforms are disclosed. The system comprises a frequency modulator, a continuous wave optical source, and an optical modulator. The frequency modulator receives the analog signal and converts the analog signal into a frequency modulated signal. The optical modulator modulates the continuous wave optical source with the frequency modulated signal, to produce the optical pulse-position modulated signal. The method comprises converting an analog input signal into a frequency modulated signal amplifying the frequency modulated signal and producing a large signal therein, modulating a continuous wave optical source with the large signal, therein producing the optical pulse-position modulation signal, and compressing the optical pulse-position modulation signal, therein increasing a processing gain.

Generating methods for single and multi-channel wideband optical analog pulse positioned waveforms

An original scheme for the utilisation of optically preamplified (EDFA) PPM receivers in intersatellite laser communications is presented. In order to facilitate high bit rate operation an analysis is presented for a PPM system employing threshold detection. When operating at the STM-1 bit rate of 155 Mbit/s and using conservative parameters the authors' model demonstrates a possible receiver sensitivity of 13.5 photons/bit (at a BER of 10/sup -6/) in the presence of background noise, thus indicating the impressive potential of the proposed receiver formation for use in satellite communications. The authors further demonstrate that their threshold detection PPM scheme can obtain receiver sensitivities up to 12.3 dB-better than an equivalent EDFA OOK NRZ satellite receiver, for which new results are also presented. Furthermore the results indicate that the PPM system may perform at sensitivities beyond the fundamental optically preamplified OOK NRZ limit of 27.3 photons/bit. >

J.M. Senior

Papers

Novel laser intersatellite communication system employing optically preamplified PPM receivers