LIGO laser intensity noise suppressionAmaldi 5 July 2003G030320-00-C Flavio NoceraETMXPSLPDMode CleanerPRMBSITMXITMYETMYGravitational wavesignal4 km4 km12 mLIGO interferometerPSLThe Pre-Stabilized Laser (PSL) subsystem is the light source for the LIGO detector as shown in figure 1fig 1. LIGO interferometerThe output of the PSL is modematched into the suspended Mode Cleaner before being injected into the LIGO interferometer.The term pre-stabilized is used because the light receives some frequency and amplitude stabilization prior to injection into the interferometer.1The PSL subsystem includes the LIGO 10 W laser, the Frequency Stabilization Servo (FSS) , the Intensity Stabilization Servo (ISS) and the Pre Mode Cleaner (PMC) servo electronics.LASERThe LIGO laser is a master-oscillator-power-amplifier (MOPA) configuration developed by Lightwave Electronics Corp. under contract with the LIGO Project.The laser output has an with 10-W cw in mode.The control strategy uses the actuators of the master oscillator in order to stabilize the frequency. Power stabilization is achieved by control of the power amplifier output.10 W LaserMaster oscillatorNPROPower amplifierfrequencyactuatorsintensityactuatorsSlow (thermo-electric)fast (PZT)Current shunt(HF)AC current adjust(LF)10 WPockelsCellHigh Frequency(electro-optic)21.12M ≤00TEMIntensity ServoThe Intensity Stabilization performance requirements are summarized belowLight Power Fluctuations in the GW BandPhotons in the laser light induce a source of noise in the interferometer known as radiation pressure noise.This noise arises from the momentum imparted to the mirrors as statistically different numbers of photons reflect off the mirrors in the interferometer.To minimize the movement of the interferometer mirrors due to radiation pressure, the intensity fluctuations of the laser must be stabilized as specified in what follows.The necessary noise (fluctuation) suppression varies with frequency as reported belowThe topology of the intensity stabilization servo designed to meet these goals is shown in figure 33Hz40fforHz1fHz4010PδP(f)8≤<−Hz100fforHz1Hz100f10PδP(f)218≥<−Hz100fHz40forHz110PδP(f)8≤≤<−laser Intensity Servo topologyIntensityPDsIntensityStabilizationServo(ISS)Mode CleanerPre Mode CleanerTo theInterferometerIn-the-loopOut-of-loop(monitor)RIN [dBVrms/rtHz]Freq [Hz]-160-140-12050-50 non-polBS4401001k10kfig 3. Intensity stabilization scheme4A small fraction of the laser light directed towards the interferometer is sampled and split in two equal parts through a 50-50 non-polarized beam splitter. Each part then impinges on a custom designed photodetector.One photodetector is utilized as an independent monitor and measures the actual level of noise suppression, the other provides the sensor input to the intensity servo electronics.SensorThe photodetectors utilized are GAP2000, large area (2 mm active diameter) InGaAs units (Responsivity R ≈ 0.6 at the frequency of interest).The photodiode amplifier is a current-to voltage converter. A 1 kΩ trans-impedance is chosen to be a reasonable compromise between the lower noise limited constraint and the upper power supply limited voltage.The photodetector is reverse biased at 7 V to decrease the value of the parasitic capacitance associated with the photodiode.The photodetector can be either AC or DC coupled to the rest of the servo.We chose DC coupling due to the high electronics gain at DC complicating the long term robustness of the servo.High gain AC coupled servos can have difficulty with the control of DC offsets and drift causing saturation.A block diagram is shown in figure 4.The photodetectors are located on an optical table outside the vacuum environment.5I to V converterLPLPAC to ServoDC couplingAC couplingDC to servoDC offsetlocal monitorPhotodetector BoxPhotodetector HeadI to V converterPhotodetector HeadLPLPAC to ServoDC couplingAC couplingDC to servoDC offsetlocal monitorfig 4. Photodetectors heads and electronics6CRRCThe current shunt is used as the high frequency actuator in the ISS and the AC current adjust actuator is used for low frequency actuation. The AC current adjust actuator has about a factor of 10 more dynamic range than the current shunt and is used for relatively large, slow drifts.ActuatorsThe measured characteristics for both actuators are shown below.The first one shows the transfer function of the ac current adjust. It clearly exhibits multiple poles at frequency below 100 kHz, making the design of a fast, high gain control servo based on this actuator extremely difficult.If this actuator was used alone, it would necessitate direct current modulation of the 20 A diode power supply to achieve intensity stabilization.To address this issue a second actuator, the current shunt,was designed and used.As the name suggests, it shunts a small amount of current aroundthe laser diodes, thus modulating the diode current.It is placed in parallel to the power amplifier pump diodes and is biased at 250 mA quiescent current (see fig 5).Regulation of a smaller current results in a better dynamic responsethan that exhibited by the ac current adjust actuator.Diode bus +Diode bus -2.5 V nom10 Ω sense resistor± 2.5 Vcontrol inputLaser power supplypoweramplifierpumpdiodesfig 5. current shunt actuator 7-125-115-105-95-85-75-65-55-45-35-25102103104105-270-225-180-135-90-4504590135180degPhaseMagnitudedBfrequency [Hz]AC current adjust transfer function8-80-70-60-50-40-30-20-100102103104105-135-90-450degPhaseMagnitudedBfrequency [Hz]current shunt transfer function9Current shunt driving signalLPHPAC current adjust driving signalIntensity Servo BoardThe strategy adopted was to use the ac current adjust at low frequencies due to its higher dynamic range. This has the effect of maintaining the current shunt drive centered within its available dynamic range.While designing the control loop for the ISS, a choice was made to cross over the AC current adjust actuator's gain profile with the current shunt actuator gain profile at about 100 Hz. This choice allows for a gain boost at low frequencies which helps in the critical 40 Hz to 100 Hz region. The overall open loop gain curve for the servo is a simplified locus of points that provides the necessary gain to reduce the free running laser intensity noise to the required level.fig 6. Intensity Servo loop shaping electronicsThere are two significant