The output optics of an interferometer are designed to direct the signal carrying light exiting the asymmetric port to the photodetector. For a DC readout system, these optics will ideally also reject any light that does not carry signal (‘junk’ light and the RF sidebands). The output optics described here are installed in the 40 m vacuum system to reduce seismic and acoustic noise. This system functioned as a prototype of the DC readout system in Enhanced LIGO. The DC readout sensing chain receives 60% of the light exiting the output port of the interferometer; the remaining
LASER δϕ δν
δϕ
FSS
250 kHz
MC
60kHz
δϕ
Common Mode
20kHz
f2 = 166MHz f=29.5MHz
f=21.5MHz
AO (> 100Hz) MCL
(< 100 Hz) δϕ
13m
40m
20cm
MC2 DSC
< 100Hz
VCO
~ 80MHz
PZT (< 1 kHz)
PC (> 1 kHz)
VCO DC - 100kHz
MCL (< 100 Hz)
Figure 5.4: The laser frequency stabilization system.
40% immediately exits the vacuum and is sent to a traditional RF readout sensing chain to be used for lock acquisition, signal extraction of auxilliary degrees of freedom, and DC/RF comparisons.
5.9.1 Output Mode Cleaner
We use a monolithic, four-mirror bowtie shaped resonant cavity as a mode cleaning cavity for the output port of the interferometer. This output mode cleaner (OMC) serves to reject the RF modulation sidebands and ‘junk light’ exiting the output port of the interferometer, both of which contribute noise but not signal. The key parameters of the OMC are in table5.4. The OMC power transmission (of light not rejected) is 94%; the remainder is lost intracavity.
5.9.1.1 OMC Length Sensing and Control
The output mode cleaner is kept on resonance with the interferometer carrier light via PZT length actuation; one mirror of the cavity is bonded to a 100V/µm PZT stack, which provides the necessary dynamic range (greater than one FSR) to ensure that the cavity can be locked. To sense the OMC length, the PZT is dithered at a frequency outside of the gravitational wave detection band (in this case, 12 kHz) and demodulated coherently. The dither signal and demodulation are done with a ‘digital lock-in’ system in the real-time digital controls system. The servo bandwidth is about 100 Hz.
5.9.2 Output Steering
The angular degrees of freedom of the OMC (beam tilt and displacement relative to the mode of interferometer) are controlled by a pair of PZT actuated steering mirrors situated between the output port of the interferometer and the OMC; sensing of these degrees of freedom is also via dithering.
Limitations on the actuators (i.e., mechanical resonances) require that this dithering remain in the detection band, at 3−4 kHz for the four degrees of freedom. The two loops which stabilize the beam angle (near field) have bandwidth of ∼20 Hz, while the loops which stabilize the position (far field) have bandwidth below 1 Hz.
5.9.3 Output Mode Matching
There is a mode-matching telescope between the last PZT steering mirror and the OMC. This is a 4:1 beam reducing telescope designed to match the beam circulating in the interferometer (with a waist at the ITM) to the cavity mode of the OMC. The curved mirrors have radii ROC1 = 618.4 mm and ROC2 = 150 mm.
The mode matching is at least 95%. This is determined by measuring the OMC visibility in a bright-fringe Michelson with Fabry-P´erot arms, which is 92%. This configuration is a good estimate
Figure 5.5: An MEDM (EPICS) screen for the output mode cleaner control system. Visible are depictions of the feedback filters for OMC LSC and ASC, several monitors, and the ‘digital lock-in’ detectors used to generated and demodulate the dither signals used for sensing and control of the OMC degrees of freedom.
Parameter Value
length 48 cm
mirrors 4
g-factor 0.72
w0 370 µm
FSR 625 MHz
F 210
Tinput 0.014
Toutput 0.014
ROCsmall 1 m
PZT 8.3 nm/V,∼2µm Range
loss 0.1% per round trip
spacer material Cu
fdither 12 kHz
|GL|= 1 ∼100 Hz
Table 5.4: Output mode cleaner parameters.
of the signal carrying mode: since the arm cavities are overcoupled, the reflected beam consists mostly of a leakage field from the arms. The recombination of the two beams in the Michelson then approximates the signal carrying mode. About 3% of the power in this beam is contained in RF sidebands which are promptly reflected by the arms and transmitted by the Michelson, so this establishes the lower bound on the mode matching.
5.9.4 Higher Order Mode Content at Asymmetric Port
Both the RF and DC readout signals can be adversely affected by higher order mode content at the asymmetric port. Figure 5.6 is the result of using the OMC as a mode analyzer, with the interferometer locked in a detuned RSE configuration.
5.9.5 Photodetectors
Light transmitted through the OMC is directed to a pair of photodetectors; the photodiodes are 2 mm InGaAs diodes, with the InGaAs surface exposed (no glass). They are reverse voltage biased and wired in series with 250 Ω wire wound resistors which function as current-to-voltage converters;
this signal is then amplified/whitened by an op-amp filter stage in a non-inverting configuration.
A load resistor is used as the transimpedance stage rather than an op-amp so the op-amp will not have to source the full amount of the photocurrent; while this is not a major concern for the 40 m, it may be for Enhanced and Advanced LIGO. The photodiodes, series resistors, and amplification electronics are all housed in the vacuum, with the electronics housed in a separate vacuum nipple that was filled with Krypton gas for leak detection (it did leak).
The amplified photodetector signal is transmitted differentially through a vacuum feedthrough, further amplified, and acquired digitally for use in the digital control system and measurement of transfer functions.