The RSE Tabletop Prototype Experiment
4.1 Prototype Design
4.1.3 Electro-Optics
The Laser
The light source is a Lightwave model 126-1064-100, which is a 100 mW NPRO Nd:YAG 1064 nm laser. This model of laser has been measured to have a frequency noise spectrum of roughly 200 Hz/VHZ at 100 Hz.[55] Integrating this down to 1Hz gives an equivalent length noise of roughly w-to fiRMS for cavities on the order of a meter in length, by the relation
of ol
f
= (4.2)It was measured that the displacement noise on the table was ;::: 2 x 10-9 fiRMs, so frequency noise of the laser isn't a concern.
Modulators
The phase modulator is a New Focus 4003, which has an input circuit resonantly tuned to 81 MHz, and roughly 0.2 rad/V modulation depth. The AOM used to generate the SSB is a Brimrose Corporation AMF-27-5-1064 frequency shifter. The AOM requires +19 dBm of input RF power, and advertises 90% efficiency for the first
order diffracted beam. Although this degraded over time to roughly 50%, 80% was achieved initially. It's assumed this was due to drifts in the alignment of the AOM.
Since the amplitude of the single sideband wasn't critical, no attempts were made to re-optimize the alignment.
Faraday Isolators
The isolators are necessary to keep reflected light from one interferometer from cou- pling backwards to a previous interferometer. Specifically, any light going back into the laser resonator tends to cause the laser to either become noisy or go unstable altogether. Also, it was noticed that light reflected from the main interferometer sig- nificantly degraded the control of the Mach-Zender. Isolation is actually accomplished by two components, the Faraday isolator component, which is an Electro-Optics Tech- nology 1845-2 isolator, in conjunction with a CVI QWPM-1064-05-2 half wave plate, to keep the polarization vertical.
Photodiodes
Two different types of photodiodes2 are used to generate the signals used to control the interferometer, which are labeled "RF /DC photodiodes" in Figure 4.1. First, two tuned photodiodes built in-house by the LIGO electronics shop were acquired. The transimpedance electronics of one was tuned to 81 MHz, while the other was tuned to 54 MHz. The RF gains were typically a few hundred V /W. The other two photodiodes used were New Focus model 1811 photodiodes. These have a flat frequency response up to the 125 MHz roll-off frequency. They have very high RF gain, roughly 30000 V /W, as they are designed to operate in very low light experiments. Because of this, neutral density filters usually were needed to keep their RF amplifiers from saturating.
The two New Focus 1811 photodiodes were used as the reflected and pickoff pho- todiodes. The 54 MHz tuned PD is used as the dark port PD, and the 81 MHz tuned PD is used as the Mach-Zender PD. This is somewhat counter to one's notion of how
2In this context, "photodiode" is used to describe both the photodiode element and the associated RF transimpedance electronics.
to do things. The reason is that the gain of the 81 MHz PD was always about an order of magnitude less than the 54 MHz PD, and furthermore it was noticed at some point that the initial tuning of the 81 MHz PD had changed, and whatever advantage the tuning gave to the signal didn't exist anymore. So they were switched.
The measurement of the matrix of discriminants is to be compared to the output of a model, which predicts the watts of signal power at a point in the interferometer, given a normalized displacement of a mirror. Thus, a calibration is needed to char- acterize the gain from the optic which picks the light out of the main beam to the output of the mixer. The matrix will be measured only at the reflected and pickoff 54 and 81 signal ports, so only these calibrations are needed. The calibration was done by sweeping the Michelson through several fringes, and maximizing the measured demodulated signal with the demodulation phase. Eq. (3.9) gives a model of the optical gain, that is the watts of signal expected, based on a measurement of the laser power and modulation depth. The scale factor which converts this to the volts of the measurement gives the calibration which includes all the effects of the pickoff, neutral density filters, the RF gain of the transimpedance electronics, cable and mixer loss, etc. This is shown in Figure 4.2. The uncertainties are primarily due to the noise of
Light input with signal
N.D.
Pickoff:
RF amp PD (\ filter
~- \7 -- ~----
Cable Lens
Voltage output
Figure 4.2: Calibration of photodiode/mixer gains. Modeling the signal input at the pickoff (in watts) and measuring the voltage signal out gives a calibration which takes into account all the objects in the dashed box.
the measurement itself (::; 5%) and the uncertainties in the power contained in the carrier and RF sidebands (::::::: 5%).
I II
Reflected 81I
Reflected 54I
Pickoff 81I
Pickoff 54I I
Readout gain (V /W)II
6.05±0.3I
6.33±0.4I
13.5±0. 7I
26.5± 1.5 \Table 4.1: Calibration of the readout gains of the signal ports.
The "DC photodiodes" in Figure 4.1 are Thorlabs DET410 photodiodes. These only have a biasing battery, and no transimpedance electronics. A 1kD resistor was used as the transimpedance. These photodiodes are used to monitor the light power stored in the arms.
Cameras
Four cameras were used to monitor power build-up, mode shape, and also a little bit as a fiducial for alignment. The cameras are part of the Radio Shack VSS-400 4-channel observation system, and have a decent sensitivity at 1064 nm. The imaging lens on each camera was removed, and the light was incident directly on the CCD element.
Optical Spectrum Analyzer
A Melles-Griot 13 SAE 006 optical spectrum analyzer was used to monitor the power in each of the RF sidebands and the carrier. Monitoring was done in both the power and signal cavities, using the pickoffs in each cavity to sample the light. Since there was only one spectrum analyzer, the light from both pickoffs was directed into the analyzer, while at any one time, one of the paths was blocked to make a measurement.