Lock Acquisition

Lock acquisition scheme has to be planed carefully because it was a big problem in the first generation large-scale interferometers.

Probabilistic lock (a.k.a. lucky lock) won't probably work. Other locking schemes such as the arm pre-locking has to be considered.

Auxiliary Lock System

Also known as green laser lock, this scheme will make use of auxiliary lasers to lock the arm cavities first.

See here.

When you combine two beams with power P, we get the beat note with amplitude of 2 alpha P Sin phi, where alpha is the contrast (0~1) and phi is the relative phase between the fields. Thus, the phase sensitivity is 2 alpha P [W/rad]. e.g. Two 1mW beams geneartes 4mWpp beat if the contrast is 1. The slope is 0.002 alpha W/rad.
For the broadband PD, InGaAs is not relevant because of the high junction capacitance. If we use Si PD (like FFD-100 phi=2.5mm), the responsivity is eta = 0.08 [A/W].
BBPD Transimepedance and the shotnoise intercept current are assumed to be 1e3 [V/A] and 0.3 [mA], from the experience of LIGO BBPD.
Total gain is 1 [V/rad] if we assume alpha and P to be 0.5 and 12.5mW respectively.
Shot noise
- Shot noise of the modulated beam is described by <in^2> = 2 e [I0+1/2 dI Sin(phi)], according to N. Mio and K. Tsubono, Physics LettersA 164 (1992) 255-258.　Does nonstationary part cancel because of the orthogonality of the RF and LO signals???
- Consider only the stationary part, in = sqrt(2 e I0). Thus in/eta/(2 alpha P) = sqrt(e / eta / alpha / P) [rad/rtHz]. This corresponds to 2.5e-8 rad/rtHz.