Commit dc607288 authored by Phil Jones's avatar Phil Jones

Adding function for filter cavity optimisation.

parent f6256af2
import numpy as np
def filter_cavity_parameters(P, lambda0, m, Titm, Tsrm, L, loss=0):
"""
Compute the optimal filter cavity detuning phase and input mirror
transmissivity for frequency-dependent squeezing in a tuned, dual-recycled
interferometer.
P - Arm cavity circulating power [W]
lambda0 - main laser wavelength [m]
m - mass of test masses (mirrors) [kg]
Titm - Power transmissivity of input test mass
Tsrm - Power transmissivity of signal recycling mirror
L - Filter cavity length [m]
loss - Filter cavity round-trip power loss
See Kwee et al. `Decoherence and degradation of squeezed states in
quantum filter cavities` Physical Review D, 2014, 90, 062006
"""
c = 299792458.0
w0 = 2 * np.pi * c / lambda0
FSR = c / (2.0 * L)
# Approximate frequency at which IFO reaches SQL
# (i.e. radiation pressure noise = shot noise)
w_sql_0 = 8 / c * np.sqrt(P * w0 / (m * Titm))
w_sql = np.sqrt(Tsrm) / (1 + np.sqrt(1 - Tsrm)) * w_sql_0
if loss > 0:
epsilon = 4 / (
2 + np.sqrt(2 + 2 * np.sqrt(1 + (2 * w_sql / (FSR * loss)) ** 4))
)
else:
epsilon = 0
# Filter cavity bandwidth
gamma_f = (
np.sqrt(2 / ((2 - epsilon) * np.sqrt(1 - epsilon)))
* w_sql
/ (2 * np.pi * np.sqrt(2))
)
# Detuning
delta = np.sqrt(1 - epsilon) * gamma_f
phi = -delta / FSR * 180.0
T = 2.0 * gamma_f / FSR * 2 * np.pi
return phi, T
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