LIGO Document G1400936-v3

Transportation of Ultra-Stable Light via Optical Fiber

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It has been demonstrated that polarization-maintaining single mode optical fiber can be used to transport frequency-stable light. It is desired to transport stable light to other labs in the building to serve as a frequency reference for various experiments investigating different sources of noise in gravitational-wave detec- tors. Stable light has been obtained from ultra-stable Fabry-Perot cavities by use of the Pound-Drever-Hall locking technique. We have mode matched stable light into the fiber and are using a double-pass acousto-optic modulator (AOM) configuration to cancel fiber phase noise. We use a beamsplitter to interfere the stable light and double passed light onto a photodiode, which is connected to a phase-locked loop (PLL) to measure the beat frequency. From there, we analyze the noise in the system by measuring the power spectral density of the PLL control signal with a spectrum analyzer. We have measured the expected dominant sources of noise in the system by using a similar PLL set-up and suppressed them. We cancel the fiber phase noise by locking the optical beat to the signal generator in the PLL.
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