5.5.1
MOPA
The main laser is a Lightwave electronics 10 watt CW, 1064 nm Nd:YAG laser in a MOPA (Master Oscillator Power Amplifier) configuration. The master oscillator is a non-planar ring oscillator
DARM SRC Satellite amplifier Length Sensing & Control AA/WHITE I & Q Demodulator Digital OMC Controller ADC AA DAC AI/DEWHITE PZT DRIVER AS OMC ADC Digital Suspension Controller AA/WHITE AI/DEWHITE COIL DRIVER ADC DAC PD Interface OSEM Interface OSEM
Figure 5.3: A simplified real-time digital control network diagram. The solid lines indicate analog signals, while the dashed lines indicate digital communication via the reflective memory network. Digital elements are yellow, analog elements are white, and the interfaces (ADC/DAC) are blended.
(NPRO), pumped by diodes emitting at 808 nm. The NPRO emits a single mode, linearly polarized Gaussian beam at 500 mW, which is double passed through four diode pumped amplifying rods in the power amplifier. Attached to NPRO are a Peltier device which provides a slow, large dynamic range frequency actuation (5 GHz/V,.0.1 Hz) and a PZT which provides a faster actuation (5 MHz/V,
.100 kHz). The MOPA in the 40 m is currently operating at 2.7 W of output power, due to age related deterioration (most likely the PA pump diodes failing).
5.5.2
Power Stabilization
The laser power is stabilized by a system called the Intensity Stabilization Servo (ISS), which senses the power and feeds back via current-shunt that modulates the current supplying power to the pump diodes in the MOPA power amplifier. The bandwidth of this system is∼60 kHz.
5.5.3
Frequency Stabilization
The laser frequency is stabilized by a hierarchical system of servos, including the Frequency Sta- bilization System (FSS), the mode cleaner servo (MC), and the common mode servo (CM). This system is shown schematically in figure5.4. The FSS stabilizes the laser frequency to a frequency reference cavity (RC,F ∼9500) a standard PDH setup, with the primary actuator being a Pockels cell (PC) at the output of the MOPA, up to a bandwidth of 250 kHz (although the system is designed for 500kHz). The PC has a limited range, so the loop also actuates on the PZT attached to the NPRO crystal, which provides the primary actuation below a few kHz. Finally, a perl script adjusts
the NPRO crystal temperature to prevent the PZT drive from saturating.
As shown in figure 5.4, a frequency shifting device (a double passed acousto-optic modulator) shifts the frequency of the beam going toward the reference cavity, with the shift frequency supplied by a voltage-controlled oscillator (VCO). Without any applied voltage the VCO oscillates at 80 MHz, and so the laser light entering the reference cavity is actually shifted in frequency from that of the main laser field by 160 MHz. By applying a voltage to the VCO the laser frequency can be shifted while the RC stays exactly on resonance. The FSS can then be used as a frequency actuator, limited by the FSS bandwidth, and with a range determined by the limits of the VCO (±7 MHz) rather than the linewidth of the RC (∼76 kHz).
5.5.4
Pre-Mode Cleaner
A triangular mode cleaning cavity with a fixed glass spacer serves to reduce the higher-order-mode content of the beam, and also passively filters noise out of the beam above the cavity pole, which is approximately 450 kHz. The main purpose of this cavity is to reduce laser noise at the frequencies where RF sidebands will be applied for interferometer sensing and control (cf. section5.6.1).
5.6
Input Optics
5.6.1
Phase Modulations
A pair of New Focus model 4003 electro-optic modulators (EOMs) are used to apply phase mod- ulation sidebands to the laser light for use in interferometer sensing and control. These phase modulation sidebands are used as local oscillator fields in the variants of PDH sensing used for global interferometer length sensing and control (more in chapter 6). The electrical signals used to drive the Pockels cells come from a set of IFR2023A signal generators which are kept in phase using the 10 MHz output of one as an external clock for the others. Two sets of RF sidebands are applied for the LSC, calledf1(at 33.2 MHz) andf2(at 166 MHz). There are three more IFR2023A
generators, which are used to provide modulations at 29.5 MHz (for the input mode cleaner, more in section 5.6.2) and atf2+f1 (199 MHz) and f2−f1 (133 MHz). The last two signals are used as
electronic local oscillators for a heterodyne scheme; there are no RF sidebands at those frequencies. 5.6.1.1 A Mach-Zehnder interferometer for non-cascaded RF sidebands
The length sensing and control scheme for the 40 m (described in chapter6) depends on sensing the optical beats between different sets of RF sidebands (i.e.,f1andf2); such signals are calleddouble
demodulation signals (DDM). DDM signals can be mixed down to the audio band (cf. section3.5.1)
Applying two phase modulations in series yields sidebands on the sidebands. This second set of sidebands appears, relative to the carrier frequency, at the difference frequency between the two sidebands. These sidebands-on-sidebands corrupt the purity of the DDM signals, causing them to be heavily influenced by carrier light, and thus less useful in isolating the short degrees of freedom from motion in the long degrees of freedom [76]. To avoid this, the phase modulation sidebands are applied in parallel, in the arms of a Mach-Zehnder interferometer. This means the RF sidebands are not true phase modulations (cf. equation (3.26)), as the carrier field is recombined at the output of the Mach-Zehnder but not the phase modulation sidebands. The resulting ‘modulation depth’ is a factor of 2 lower at the output of the MZ than in the arm. The resulting spectrum of light, which is injected into the mode cleaner, (next section) is cartooned in figure6.1(a).
5.6.2
Input Mode Cleaner
The beam exiting the PSL is injected into the vacuum and passed through a suspended, triangular mode cleaning cavity. The mode cleaner cavity parameters are shown in figure5.1.
The input mode cleaner serves two purposes: (1) it passively stabilizes the beam above the cavity pole frequency; (2) it provides further higher-order-mode suppression beyond that of the PMC; (3) it provides a quieter frequency reference than the RC above∼100 Hz.
The mode cleaner servo stabilizes the mode cleaner to match the laser frequency below 100 Hz, by applying a force to the MC2 mirror. Above 100 Hz, the MC length error signal is fed back to the VCO in the PSL, thus stabilizing the laser frequency by matching it to the mode cleaner length (cf. figure5.4). The bandwidth of the mode cleaner servo is∼60 kHz.
The mode cleaner macroscopic length is roughly matched to the RF modulation frequencies, so that the RF sidebands fall on a free-spectral range of the mode cleaner and can thus be transmitted to the interferometer. The exact RF frequencies (down to∼5 Hz) are then set to be exactly resonant in the mode cleaner, the precise length of which is measured using a technique similar to the one described in [77].
5.6.3
Input Isolation, Mode Matching, and Steering
The beam exiting the MC is passed through a Faraday Isolater (FI) and then through a fixed mode matching telescope which expands the beam to match the mode in the arm cavities. The beam is then reflected off two PZT actuated tip-tilt steering mirrors which provide input pointing to the interferometers. These PZT steering mirrors have in principle a useful actuation bandwidth up to ∼800 Hz; a series of mechanical resonances in the 1−10 kHz regime limit their utility above this frequency.