Fiber Optic Tutorials
Impact of Nonlinear Effects on Coherent and Delay Demodulation Optical Receivers
All nonlinear effects discussed in the impact of fiber nonlinearity tutorial in the context of intensity modulation/direct detection (IM/DD) systems have the potential of limiting the performance of coherent or self-coherent lightwave systems, depending on the optical power launched into the fiber. The impact of stimulated Brillouin scattering (SBS) depends on both the modulation format and the bit rate, and its effects on coherent systems have been studied extensively. The impact of stimulated Raman scattering on WDM coherent systems is less severe compared with the IM/DD systems if the information is encoded in the carrier phase because the Raman-induced power transfer depends only on the channel power. On the other hand, self- (SPM) and cross-phase modulations (XPM) play a much more important role because they convert intensity fluctuations into phase fluctuations. Because of the nonlinear origin of such fluctuations, the phase noise induced by them is known as the nonlinear phase noise. This section focuses mainly on this kind of noise.
Sensitivity Degradation Mechanisms in Practical Coherent and Delay Demodulation Optical Receivers
The discussion of shot noise and bit-error rate of coherent and delay demodulation in the previous tutorial assumed ideal operating conditions in which system performance is only limited by shot noise. Several other noise sources degrade the receiver sensitivity in practical coherent systems. In this tutorial, we consider a few important sensitivity degradation mechanisms and also discuss the techniques used to improve the performance with a proper receiver des
Shot Noise and Bit-Error Rate (BER) for Coherent Demodulation and Delay Demodulation
The signal-to-noise ratio (SNR) and the resulting BER for a specific modulation format depend on the demodulation scheme employed. This is so because the noise added to the signal is different for different demodulation schemes. In this tutorial we consider the shot-noise limit and discuss BER for the three demodulation schemes discussed in the demodulation schemes tutorial. In the next tutorial we will focus on a more realistic situation in which system performance is limited by other noise sources introduced by lasers and optical amplifiers employed along the fiber link.
Demodulation Schemes (Coherent Demodulation and Delay Demodulation)
The use of phase encoding requires substantial changes at the receiver end. Conversion of the received optical signal into an electrical form suitable for reconstructing the original bit stream is called demodulation. When information is coded into phase of the optical carrier, direct detection cannot be used for demodulation because all phase information is lost during the detection process. Two techniques known as coherent demodulation and delay demodulation are used to convert phase information into intensity variations. As discussed in the coherent detection tutorial, coherent detection makes use of a local oscillator and can be implemented in two flavors known as homodyne and heterodyne schemes. Although simple in concept, homodyne detection is difficult to implement in practice, as it requires a local oscillator whose frequency matches the carrier frequency exactly and whose phase is locked to the incoming signal using an optical phase-locked loop. Heterodyne detection simplifies the receiver design but the electrical signal oscillates at microwave frequencies and must be demodulated to the baseband using techniques similar to those developed for microwave communication systems. In this section, we discuss three demodulation schemes used in practice.
Advanced Modulation Formats
Lightwave systems discussed so far are based on a simple digital modulation scheme in which an electrical binary bit stream modulates the intensity of an optical carrier inside an optical transmitter (on-off keying or OOK). The resulting optical signal, after its transmission through the fiber link, falls directly on an optical receiver that converts it to the original digital signal in the electrical domain. Such a scheme is referred to as intensity modulation with direct detection (IM/DD). Many alternative schemes, well known in the context of radio and microwave communication systems, transmit information by modulating both the amplitude and the phase of a carrier wave. Although the use of such modulation formats for optical systems was considered in the 1980s, it was only after year 2000 that phase modulation of optical carriers attracted renewed attention, motivated mainly by its potential for improving the spectral efficiency of WDM systems. Depending on the receiver design, such systems can be classified into two categories. In coherent lightwave systems, transmitted signal is detected using homodyne or heterodyne detection requiring a local oscillator. In the so-called self-coherent systems, the received signal is first processed optically to transfer phase information into intensity modulations and then sent to a direct-detection receiver.