Improvements in Baseband Fiber Optic Transmission by Superposition of High Frequency Microwave Modulation

It is well known that phase noise fluctuations in the output of a semiconductor laser can produce intensity noise fluctuations upon transmission through a fiber-optic link due to interferometric phase-to-intensity conversion [145–148]. In a single-mode fiber link, the interferometric conversion occurs when multiple reflections occur between a pair of fiber interfaces (Fig. 15.1). Even in the absence of such fiber discontinuities, Rayleigh backscattering in a sufficiently long piece of fiber can cause similar effects [149]. If a laser source is used in a multimode fiber link, the different transverse modes of the fiber interfere with one another and produce the well known “modal noise.” This chapter specifically studies the former case (multiple discrete reflections in a singlemode fiber link) although it is straight forward to extend the formalism to the case of modal noise in multimode fibers. The nature of interferometric noise has been studied in [145–147]. It was shown that this excess noise can cause bit-error-rate floors [69], and the system performance has been evaluated as a function of the number and/or amplitude of the reflections [150].

To the extent that such interferometric noises arise from interference of the laser output with a delayed version of itself, it is obvious that reduction of laser coherence can eliminate these noises. Indeed, there have been proposals and early demonstrations that by applying a high frequency modulation to the laser, the coherence of the laser can be reduced, leading to a reduction of the type of interferometric noise mentioned above. However, except in the extreme case of a very deep modulation (where the laser output is almost pulse-like and each pulse is incoherent with the previous ones), one does not expect the laser output to be rendered totally incoherent by the applied modulation, but instead, the lasing wavelength “chirps” sinusoidally at the modulation frequency. Can one expect interferometric noises to be reduced (or even totally eliminated) under this situation? The following analysis will show that the answer is positive, provided that proper choices of modulation format and parameters are employed.