Measurements of noise levels with semiconductor lasers and long lengths of polarization-preserving fiber, as used in typical linear modulator systems, show noise levels that are ~ 10 dB above thermal noise because of laser phase-to-intensity noise conversion. Data are presented for several launching angles into both polarization-preserving and polarizing fiber. There is good agreement with theory for the variation in noise with an output analyzer angle. To reduce the noise either a single frequency source must be used or the polarization-preserving fiber must be replaced by a conventional single-mode fiber.
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