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Optical Coherent Detection and Processing Systems
Published in Le Nguyen Binh, Advanced Digital, 2017
This chapter deals with the analysis and design of coherent receivers with optical phased lock loop and the mixing of optical signals and that of the LO in the optical domain and subsequently by the optoelectronic receivers following this mixing. Thus, both optical mixing and photodetection devices act as the fundamental elements of a coherent optical receiver. Depending on the frequency difference between the lightwave carrier of the optical signals and that of the LO, coherent detection can be termed as heterodyne or homodyne detection. For heterodyne detection, there is a difference in the frequency, and thus the beating signal falls in the passband region in the electronic domain. Thus, all the electronic processing at the front end must be in this passband region. In homodyne detection, on the contrary, there is no frequency difference, and thus the detection is in the baseband of the electronic signal. Both cases would require a locking of the LO and carrier of the signals. An optical phase-locked loop is thus treated in this chapter.
Principles of Optical Time-Domain Reflectometry (OTDR) for Distributed Sensing
Published in Arthur H. Hartog, An Introduction to Distributed Optical Fibre Sensors, 2017
It is also possible to perform a C-OTDR measurement using the same frequency for the probe and the local oscillator, an approach known as homodyne detection [53]. In telecommunications systems, homodyne detection relies on the signal being at a known phase that allows the signal (and associated noise) to be collected only from positive or negative frequencies; in contrast, in heterodyne, noise is collected from upper and lower sidebands. Homodyne detection therefore has a 3 dB sensitivity advantage over heterodyne detection; this does not apply to OTDR because the phase of the signal is fundamentally random. Nonetheless, there are claimed benefits in terms of system simplicity In practice, most of the work on C-OTDR has involved heterodyne, rather than homodyne, detection. This is possibly for practical reasons, such as the fact that the acousto-optical modulators that are commonly used for cutting the probe pulse from the source laser, owing to their excellent extinction ratio, naturally shift the frequency of the modulated pulse and this simplifies the detection process.
H
Published in Philip A. Laplante, Comprehensive Dictionary of Electrical Engineering, 2018
homodyne detection method of measuring the frequency content of an optical signal in which that signal is combined with itself by means of a square-law detector; simpler but sometimes less informative and sensitive representation of the frequency content of a signal than heterodyne detection. homogeneous having all nodes or machines in a multiprocessor or multicomputer be of the same type or class. homogeneous broadening spectral broadening of a transition in a laser medium due to irreversible dephasing processes like spontaneous
An MDPSK homodyne receiver with adaptive phase-diversity
Published in Journal of Modern Optics, 2020
Changqing Cao, Zengyan Wu, Wenrui Zhang, Xiaodong Zeng, Xu Yan, Zhejun Feng, Yutao Liu, Bo Wang
Compared with microwave-band communication systems, homodyne coherent receivers are more appealing owing to their applicability in optical phase-shift keying communication systems, because of the high channel separation, high spectral efficiency, and long transmission distances that they offer. Moreover, an additional advantage offered by optical communication systems is the significant improvement in the security of information transmission (1). As the line width of the optical carrier is very narrow in these systems, the sensitivity to the background light is considerably reduced, making it difficult to intercept. However, the overall performance of this system is seriously impaired by various factors, such as in-plane and quadrature signal (IQ) imbalance and phase differences between the incoming signal and the local oscillator (LO). In fact, the frequency of the signal light in homodyne detection is not necessarily equal to the frequency of the local light, which causes phase noise. Therefore, the random phase difference includes the phase noise caused by the frequency shift and the phase shift between the frequencies. As homodyne coherent receivers are sensitive to phase changes, an optical phase-locked loop (OPLL) (2,3) that includes a feedback system can be used for eliminating these phase differences. Although OPLL allows the phase and frequency of the LO to track the phase and frequency of the incoming signal, the complexity involved with these loops limits their practical application. In order to reduce the number of optical devices, electrical post-processing methods have been proposed for estimating the phase using a digital signal processor (DSP) (4,5). Irshaad Fatadin et al. explored the Gram–Schmidt orthogonalization procedure (GSOP) for the compensation of quadrature imbalance in an optical hybrid system (6). This digital compensation approach has been widely used in coherent optical transmission systems as off-line (7) as well as real-time processing (8). In addition, the differential detection technology can be effectively used to eliminate the phase noise (9). In these methods, the phase and frequency fluctuations of the free-running local oscillator laser in the digital domain are compensated. However, these receivers suffer from imbalance in the phase delay and amplitude between the two branches, which cannot be completely removed in some cases (10–12).