Semiconductor laser signals and noise in fiber grating systems

Citation

McAdams, Matthew S. (1998) Semiconductor laser signals and noise in fiber grating systems. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/9tcw-nq16. https://resolver.caltech.edu/CaltechETD:etd-06092005-145119

Abstract

This thesis describes the effect of transmission through a fiber Bragg grating on the signal and noise properties of semiconductor laser light. We show that fiber gratings can be used to increase the modulation response of a laser, to improve the response of a laser/fiber system, or to decrease the intensity noise of a continuous-wave laser signal. The effects are the result of dispersive propagation and frequency discrimination, and depend on the nature of the grating and the laser dynamic properties. This connection is developed first by deriving the dynamic properties of semiconductor lasers, including the direct current modulation response, frequency chirp, and laser noise. The effect of propagation through an arbitrary medium is derived, with the conclusion that both dispersion and frequency discrimination result in conversion of frequency modulation into amplitude modulation and vice versa. These general results are applied to laser modulation and noise spectra to derive the transfer functions for dispersive optical fiber. Next we detail the experimental characterization of laser dynamics, from which we can determine the important laser parameters. We follow this with a discussion of fiber Bragg gratings and show that the phase of the grating transmittance, which is important in changing the characteristics of the signals being transmitted, can be inferred numerically from a measurement of the intensity transmission. Finally, we unite these topics with the demonstration of a 7 dB increase in the laser response at frequencies up to 25 GHz in transmission through a fiber grating. The result is well predicted by a numerical Fourier domain analysis of the laser signal and fiber grating. In addition, we investigate the effect of a fiber grating on the relative intensity noise (RIN) of a laser, showing that a model of grating as a linear frequency discriminator is sufficient for explaining much of the results. We show there exist conditions under which a grating can reduce the RIN, which depend on the phase relationship between correlated intensity and frequency fluctuations. We demonstrate a 2 dB reduction in RIN at frequencies up to 15 GHz. The combination of these effects is used in calculating signal-to-noise ratios for real systems incorporating gratings, and in showing that gratings can re-narrow pulses broadened by fiber dispersion.

Thesis Files