LIGO Document T1200429-v1

Arm Length Stabilisation - FIBRE Phase-Locked-Loop

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T - Technical notes
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For the ALS we lock the auxiliary laser in the end-stations to the PSL frequency via an optical fibre. The 1064~nm light of the light transmitted through the PSL Reference Cavity is injected into an optical fibre, which is then sent to the ALS Fibre Distribution Chassis. Inside this chassis, the light will be frequency shifted via an AOM. After the AOM the fibre is split into three paths, two to the end-stations and one spare for later use.

On the ALS table in the end-stations the fibre output, approximately 300~\( \mu W \), is combined with the 1046~nm light of the prometheus laser. The resulting beatnote, from an RF photo detector, will be tuned to 40~MHz by adjusting the temperature of the prometheus laser. The beatnote signal will be mixed with a VCO at 39.7~MHz to obtain en error-signal for feedback to the laser PZT and temperature. A 'Common-mode Board' is used to implement the control servo. In this note the configuration of the CMB is described.

For the One Arm Test, a separate reference cavity was set up in the optics lab in the OSB. The transmitted light of the reference cavity was injected into a fibre and sent to the Y-end station. Approximately 300~\( \mu W \) came out of the fibre on the ALS-Y table. The optical power of the local oscillator was adjusted until the input monitor read approx. -25~dB (the monitor reads 20~dB less then the actual RF input).

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