Advanced LIGO Thermal Compensation System Preliminary Design

LASER INTERFEROMETER GRAVITATIONAL WAVE OBSERVATORY LIGO Laboratory / LIGO Scientific Collaboration LIGO LIGO-M08xxxx-00-R May 23, 2008 Advanced LIGO Thermal Compensation System Preliminary Design Phil Willems, Aidan Brooks Distribution of this document: LIGO Scientific Collaboration This is an internal working note of the LIGO Project California Institute of Technology LIGO Project – MS 18-34 1200 E California Blvd Pasadena, CA 91125 Phone (626) 395-2129 Fax (626) 304-9834 E-mail: info@ligo.caltech.edu Massachusetts Institute of Technology LIGO Project – NW22-295 185 Albany St Cambridge, MA 02139 Phone (617) 253-4824 Fax (617) 253-7014 E-mail: info@ligo.mit.edu LIGO Hanford Observatory P.O Box 1970 Mail Stop S9-02 Richland WA 99352 Phone 509-372-8106 Fax 509-372-8137 LIGO Livingston Observatory P.O Box 940 Livingston, LA 70754 Phone 225-686-3100 Fax 225-686-7189 http://www.ligo.caltech.edu/ Signature Page LIGO LIGO-M070109-02-R Significant Changes to the Conceptual Design 2.1 Change of Shielded Ring Heater Scope and Design 2.2 Change of Carbon Dioxide Laser Projector Scope 2.3 Choice of the Hartmann Sensor as the ITM/CP Dedicated Sensor 2.4 Choice of Modified Two-Beam Optical Lever as TM HR Surface Dedicated Sensor 2.5 Change in Number and Layout of Dedicated Sensors 2.5.1 No BS Hartmann Sensor 2.5.2 ETM Transmission Hartmann Sensor 2.5.3 Revised ITM/CP Hartmann Probe Beam Injection Paths 5 5 Shielded Ring Heater CO2 Laser Projector 4.1 Overall Design Philosophy 4.2 Dual temperature stabilized shuttered CO2 laser 4.3 Intensity Stabilization System (ISS) 4.3.1 AOM 4.3.2 In-loop and out-of-loop photodiodes 9 4.4 Quad PD and Galvo Mirrors 10 4.5 False Mach-Zehnder Beam Shaping Section 10 4.6 TCS Power Control and Sensing 11 4.7 Beam Shaping Optics 4.7.1 Central Heating Path 4.7.2 Annular Heating Path 11 11 12 4.8 Power Monitor Photodetectors 13 4.9 Annulus Monitor Camera 13 4.10 Anamorphic Prism Pair (folded IFO only) 13 Hartmann Sensor 13 Bull’s-eye Sensor 14 Phase Camera 15 General Operating Procedures 16 Appendix Pink Floyd 16 Signature Page LIGO LIGO-M070109-02-R Cognizant Laser System Engineer _ Caltech Laser Safety Officer Significant Changes to the Conceptual Design There have been several significant changes to TCS since the Conceptual Design We discuss each in turn LIGO LIGO-M070109-02-R 2.1 Change of Shielded Ring Heater Scope and Design The Conceptual Design of Advanced LIGO TCS considered a shielded ring heater similar to that tested by Ryan Lawrence at MIT as the primary means to apply corrective heating to the compensation plate However, this design was incompatible with the quadruple suspension design, for three reasons 1) The quadruple suspension does not provide sufficient optical access for the shielded ring heater to illuminate the CP from all necessary angles 2) The shielded ring heater uses shields to tailor the spatial profile of the heating, which makes it inefficient Assuming nondirectional radiance from the ring heater element, the heater must dissipate ~170W to deliver 6W to the CP Safely dissipating this heat away from the thermally sensitive quad suspension structure in close proximity would be very difficult 3) There is very little space for a shielded ring heater between the CP and the FM in the folded interferometer Since the CO2 laser projector is already being designed to provide sufficient power to the CP, and with a more flexible spatial distribution, the design has been changed to allocate all heating of the CP to the CO2 laser projector The ring heater now serves the role of controlling the radii of curvature of all four TM HR surfaces, by heating a band around the circumference of the mirrors near the AR face This clearly produces a thermal lens in the ITM substrate, and one that provides some compensation of the thermal lens due to self-heating The combined operation of the ring heater and CO laser projector will be discussed in a later section 2.2 Change of Carbon Dioxide Laser Projector Scope As discussed in Section 2.1, the carbon dioxide laser now provides all the compensation heating to the CP 2.3 Choice of the Hartmann Sensor as the ITM/CP Dedicated Sensor The wedge angles and tilts of the ITM and CP not permit any of the AR surfaces of either optic to produce the reference wavefront for the WLISMI sensor, as reflections from these surfaces take an entirely different optical path and are dumped This makes the WLISMI technology impractical for use as the ITM/CP sensor in LIGO For this reason, the Hartmann sensor, which also demonstrates adequate sensitivity, has been chosen 2.4 Choice of Modified Two-Beam Optical Lever as TM HR Surface Dedicated Sensor The change in the TM HR radii of curvature, if not corrected by the shielded ring heaters as discussed in Section 2.1, will produce tilts on the mirror surface of order microradian, which is the same level of tilt the optical levers are built to detect The uncertainty in the optical lever is dominated by instability in the optical lever pier Therefore a two-beam optical lever has been designed for the TM HR surface monitor In this design, the pier drift is common to the two beams LIGO LIGO-M070109-02-R and the TM radius of curvature is differential to the two beams This design is presented more fully in Section GOOGOO 2.5 Change in Number and Layout of Dedicated Sensors 2.5.1 No BS Hartmann Sensor The BS in Advanced LIGO will now use very low absorption Suprasil 2001 fused silica as its substrate material Suprasil 2001 has 0.2 ppm/cm absorption, as opposed to the ppm/cm absorption assumed previously This reduces the BS contribution to the thermal lens from ~10% to ~1% of the contribution from the ITM/CP pair, making it negligible enough not to require separate monitoring 2.5.2 ETM Transmission Hartmann Sensor A Hartmann sensor will probe the ETM through its AR surface Its purpose will be to provide a higher resolution monitor of the absorption on the ETM surface for diagnostic purposes 2.5.3 Revised ITM/CP Hartmann Probe Beam Injection Paths Since the Conceptual Design Review of Advanced LIGO, there have been several relevant changes to the Advanced LIGO baseline design First is the adoption of stable recycling cavities, which will be done by incorporating the input and output mode-matching telescopes into the power and signal recycling cavities, respectively Second, and related, are changes in the Core Optics positions and wedge angles Third is the elimination of the large pickoff telescope for IFO sensing signals that the Hartmann sensors would have used These have driven a review of the possibilities for injecting the Hartmann probe beams The new Hartmann probe beam injection/extraction scheme uses the signal cavity telescope mirrors SM2 and SM3 to magnify the beam, and injects/extracts the probe beams through or near SM2 This scheme is described in more detail in Section Shielded Ring Heater A shielded ring heater will surround each ITM and ETM, to provide a band of heating around the circumference of the mirror 40 mm away from its AR surface The primary purpose of the shielded ring heater is to reduce the radius of curvature of the HR surface of its mirror, and thereby maintain the correct arm cavity mode structure Both thermal deformation due to absorbed interferometer beam power and static curvature errors due to incorrect polish can be compensated in this way The thermal flexure of an Advanced LIGO mirror is very closely approximated by a spherical curvature at the HR face if the rear half of the mirror is heated axisymmetrically Precisely how the heat is distributed is not very important, so long as it is axisymmetric Given the close proximity of the compensation plate or reaction mass to the rear face of the test mass, and of the suspension structure elsewhere, the most convenient location for the ring heater is mounted directly to the lower quad suspension structure, on the front face of the octagonal brace surrounding the rear part of the test mass A drawing of the ring heater in this location is given in Figure 1; the schematics for the ring heater and its shield are in DCC WHATEVER and related documents LIGO LIGO-M070109-02-R Figure 1: ring heater shield as installed in quad suspension, prior to installation of test mass The heater itself is a coil of nichrome wire held upon a fused silica former It has a total resistance of 100-1000 ohms, depending upon the wire gauge and number of turns around the former Finiteelement modeling shows that up to 20W of radiant power is needed to provide sufficient correction of the thermal ROC change in a test mass; the resistance range specified allows this power to be supplied to the ring heater by fairly ordinary supplies of 50-140V, 15-.4A The assembly of the quadruple suspension requires that the ring heater be installed into the suspension structure before the test mass as two separate half-circles with a vertical gap between them, which can then be closed into a full circle after the suspension fibers are welded The shielding of the ring heater has a gold inner coating This reflects incandescent heat emitted from the coil outward from the test mass back to the test mass, using the coil power more efficiently This allows the heater to run at lower temperature and protects the suspension structure from direct heating by the coil A design drawing of the ring heater shield is given in Appendix Pink Floyd The mechanical mounting of the ring heater to the shield is via the mounting blocks, which captivate the fused silica former rod in the 0.197” through holes when they are bolted to the rest of the shield The smaller 0.079” though holes are pass-throughs for the nichrome wire- these LIGO LIGO-M070109-02-R are insulated with fused silica tubing glued into place with vacuum epoxy The slots in the shield allow the ring heater to be moved up and down during the suspension assembly The cabling for the ring heater attaches to spade lugs crimped onto the nichrome wire ends after they are threaded through the mounting blocks This cabling consists of three bundled 28AWG multistrand wires per heater end (two per half-circle, four in all) from Cooner Wire Company (part no CZ1105) with crimped spade lugs at the ring heater ends and vacuum-compatible D-sub connectors for electrical coupling to outside the vacuum The lengths of these cables will be determined by the distances to the electrical feedthroughs in the BSC chambers, which are not currently known, but should be roughly meters as was used in the LASTI prototype This cabling snakes up the suspension support structure and across the seismic isolation on its way to the feedthough Attachment points for cable anchors are provided on the suspension support structures Any thermal compensator works by altering the phase profile of the optic, and so fluctuations in the compensation power can inject noise into the interferometer The ring heater is no exception, even though it does not act on the test mass HR surface Therefore, the power delivered to the ring heater must be quiet to some level The flexure of the test mass under thermoelastic expansion in a thin surface layer under the ring heater due to 1W fluctuations at 100 Hz were modeled in COMSOL The motion of the center of the HR surface relative to the average motion of the whole mass in the model gave the injected noise motion to be 9.47x10 -16 m From the TCS DRD and Test Mass Suspension DRD, the noise requirement on TCS at 100 Hz must be less than 10 -24 m/Hz Although this requirement is overly strict and should be relaxed, it implies a TCS power noise of 10 -9 W out of 11W delivered power, or a RIN of 10-10/Hz Such a noise level is impossible to measure, but should be simple to achieve, since the ring heater thermally averages fluctuations in the power delivered to it The thermal time constant of the Advanced LIGO ring heater has not been measured, but that used by Ryan Lawrence is similar in construction, and had a measured time constant of 300 s At 100 Hz, the expected 1/f rolloff in output power fluctuations implies that the power supply to the ring heater needs an output power stability of 3x10-6/Hz The commissioning of the ring heater should include a measurement of the transfer function between supplied heater power and mirror surface displacement, and the supply power should be monitored with sufficient sensitivity to detect glitches capable of detection at the output port LIGO LIGO-M070109-02-R CO2 Laser Projector 4.1 Overall Design Philosophy Figure shows the layout of the CO laser projector This design contains elements from the Enhanced LIGO TCS projector and the Virgo TCS projector The Enhanced LIGO TCS projector served not only to compensate Enhanced LIGO but also as a prototype of the Advanced LIGO design Numerous unforeseen difficulties were discovered in commissioning the projector, leading to several changes in its layout, with more to be made for Advanced LIGO The originally proposed layout can be found in LIGO-G080004-00, and the current layout is given in Figure SNEG Slow distortions of the annular heat pattern and slow drift of the central and annular pattern pointing were the primary difficulties encountered in the Enhanced LIGO projectors Beam pointing variations were generally the cause of these difficulties Improvements to individual components in the projector resolved many of these problems, and these are discussed below in the descriptions of those components The optical layout can be designed in such a way as to minimize the sensitivity of the delivered heating pattern to these drifts, by reimaging the aperture plane of any component that can deflect the beam to the aperture plane of the beam shaping optics, and then to the CP face This was not practical within the constraints of the Enhanced LIGO projector, but this has been done in the Advanced LIGO projector as much as possible Thermal sensitivity of the CO2 laser proved to be another significant source of trouble, causing the beam to wander and the output power to fluctuate The Advanced LIGO projector design includes more active thermal management of the laser temperature, provision for stable, uninterrupted laser operation, and isolation of the laser’s thermal environment from the rest of the projector, allowing scientists to work on the projector without disturbing the laser These aspects of the design are discussed in Section 4.2 Many interferometer operators have requested the capability to remotely align the TCS projector, due the frequent need to realign it during Enhanced LIGO commissioning Remote steering of a 35W laser beam can present a safety hazard if not done correctly, but we think that a safe design is practical The optical layout includes remotely steerable optics in two places: a pair of galvo LIGO LIGO-M070109-02-R mirrors between the beam shaping optics and the laser/AOM, and piezoactuators on the periscope mirror that directs the projector beam to the CP 4.2 Dual temperature stabilized shuttered CO2 laser The drift of the beam pointing has largely been traced to the Synrad 48-2 laser, whose output is very sensitive to its thermal state, likely due to flexure of the laser’s optical resonator It is essential that the laser be left on continuously when possible, and at a stable temperature Preliminary attempts at LHO to stabilize the temperature of the CO lasers by using the chillers to actuate on the temperature of the laser cooling water has proved to be very successful in stabilizing the output power of the CO2 lasers, and drift has been reduced significantly In Advanced LIGO, the laser will be enclosed in a lightproof box, which will be stabilized to within 0.05K of a setpoint slightly above room temperature The beam will exit the box through a 1” diameter AR-coated ZnSe window When beam is not needed, a shutter on the box will divert the beam to a water-cooled beam dump This beam dump and the beam path to it will also be in a lightproof enclosure, but not part of the thermally stabilized laser enclosure The shutter will be latched in the closed position by a hasp This gives the laser Class I status, allowing it to be left on continuously with the beam contained, while the projector table is in a laser safe environment The laser temperature will be controlled by its chiller, which will be configured to stabilize the temperature of a thermistor on the laser head, rather than stabilize the temperature of the coolant reservoir in the chiller itself Additionally, the CO2 laser will have its own Thermoflex 1400 chiller A separate chiller will be used to cool the other components of the CO laser projector table (AOM, AOM driver, and beam dumps) These components collectively dissipate much less heat, are less thermally sensitive, and present a fluctuating heat load that would complicate the laser chiller operation if they added to its line Together they generate ~75W of waste heat which can be removed with a much smaller chiller 4.3 Intensity Stabilization System (ISS) This will be the same as the ISS in eLIGO TCS [T070224-00] Briefly, an AOM diverts a small fraction (~5-10%) of the laser beam to a dump, and a photodetector monitors intensity fluctuations on the through beam through a pickoff Fluctuation error signals are amplified, filtered, and fed back to the AOM drive power to divert more or less of the beam as needed 4.3.1 AOM The baseline AOM is the IntraAction AGD406-B1 used in Initial and Enhanced LIGO There is evidence1 of variation in the beam pointing of the undeflected beam through the eLIGO TCS AOM when the AOM drive voltage is changed Figure shows this effect Therefore, the AOM level should not be used to vary the ISS gain without then confirming the alignment of the optical layout We may want to test a different AOM for Advanced LIGO, such as the Isomet LS600-1011 As noted, the optical layout reimages the AOM to the first axicon, rendering the system insensitive to this pointing shift to first order LHO ilog: Wed Mar 2009 LIGO LIGO-M070109-02-R AOM at 1.1V (at LHO) AOM at 3.0V (at LHO) Figure 2: The annulus distribution at LHO for different AOM voltages 4.3.2 In-loop and out-of-loop photodiodes Advanced LIGO will use the same style of HgCdTe photodetectors used in Enhanced LIGO These are Vigo PVM-10.6 PDs, which have a dark noise level 98.5% Meller Optics catalog ZnSe lens (~6) >98.5% Meller Optics catalog AOM (1) >83% Intraaction spec for insertion loss + 5% diffraction efficiency 14 LIGO LIGO-M070109-02-R Pickoff mirror (2) 95% Directed Light, Inc quote Polarizer beam splitter (2) 98% II-VI catalog Central beam path (1) 1W Needed to provide central heating equal to self-heating of IFO Power control (waveplate + 98% polarizer) (1) (waveplate) Mirror (~6) >98% Total ~55% (polarizer)*98% II-VI catalog Newport catalog Table 1: efficiency of carbon dioxide laser projector elements Note that the overall efficiency of 55% (not including the axicons) is comparable to that of the Enhanced LIGO projectors The amount of TCS power required for optimal compensation of 0.5 ppm absorption at full power is 5W; with the factor of two excess capacity specified in the Design Requirements, this rises to 10W Assuming a 25W laser injected into the projector, there will be 12.8W of power available, and the annular beam shaping optics must therefore be 78% efficient In practice, the Synrad 48-2 lasers in Enhanced LIGO have been providing more than 30 W of power, reducing the required efficiency to 63% This efficiency may prove difficult to achieve in practice, and suggests that the system be capable of upgrade to a 50W Synrad 48-5 laser Hartmann Sensor Twin-beam Optical Lever Sensor Bull’s-eye Sensor The Hartmann sensor provides a powerful diagnostic of the thermal lenses in individual mirrors of the interferometer However, the fundamental figure of merit for the quality of the thermal compensation is the spatial mode structure of the interferometer beam itself The bull’s-eye sensor provides a simple diagnostic of the overlap of the RF sidebands with the carrier light When correctly installed, bull’s-eye sensor measures the amplitude of the LG 10 mode in the RF sideband fields of the power recycling cavity, in the Laguerre-Gauss basis where the carrier field is in the LG00 mode For small variations of the RF sideband waist size and location relative to the carrier, these are the leading terms in the modal expansion of the RF field Full derivations of the bull’s-eye sensor signal are available elsewhere 23 Originally developed for characterization of mode-matching into optical cavities, Stefan Ballmer adapted the sensor for use in servo control of “Input Optics Final Design,” R Adhikari et al., LIGO document LIGO T980009-01-D, section 9.3 “Determination and optimization of mode matching into optical cavities by heterodyne detection,” G Mueller et al., Optics Letters 25 (2000), pp 266-8 15 LIGO LIGO-M070109-02-R TCS in initial LIGO.4 We not anticipate that the bull’s-eye sensor will be used in a closed loop for TCS control in Advanced LIGO, but only for diagnostic purposes during commissioning In Advanced LIGO, there will be two pairs of RF sidebands, circulating within the coupled power and signal recycling cavities These RF frequencies (9 and 45 MHz) are different from the 24 MHz used in initial LIGO However, the bull’s-eye sensor uses the same electronics as the WFS detectors, which are being adapted to Advanced LIGO radio frequencies, and these frequencies are well within the bandwidth of the photodiode The bull’s-eye sensor will use either the ITMX PO beam or REFL beam to sample the recycling cavity fields In either case, the bull’s-eye sensor will reside on an optical table outside the vacuum with a telescope to produce a beam with the appropriate waist size and Gouy phase DESCRIBE THE TELESCOPE FOR THE BES Contacted Mike Smith TYPICAL WAVEFRONT SIGNALS FOR THE BES REQUIRES DETAILED MODEL Contacted Muzammil Phase Camera The phase camera allows for a fuller resolution will allow for full characterization of the structure of the various optical fields at pickoff points of the interferometer where thermal lensing effects are visible: REFL, ASP, and POX The design for the phase camera will be nearly identical to the design used in initial LIGO The primary difference will be to change the demodulation to the RF sideband frequencies used in Advanced LIGO A reference optical field will be picked off from the pre-stabilized laser just before the diagnostic breadboard, frequency-shifted with an AOM, and coupled into a singlemode optical fiber for delivery to the appropriate ISCT, recoupling into free space, combination with the interferometer pickoff beam, and sensing with the phase camera An advanced phase camera design is currently being studied, using a fast, shuttered CCD camera to collect the demodulated light fields rather than scanning galvos and an RF PD Such a design would readily be accommodated into Advanced LIGO as a future upgrade General Operating Procedures 10 Appendix Pink Floyd “LIGO interferometer operating at design sensitivity with application to gravitational radiometry,” Stefan Ballmer, Ph.D thesis, MIT, 2006, section 2.8.4 K Goda et al., “Frequency Resolving Spatiotemporal Wavefront Sensor,” Optics Letters 29, pp.1452-1454 (2004) 16 LIGO LIGO-M070109-02-R 17 ... TCS since the Conceptual Design We discuss each in turn LIGO LIGO-M070109-02-R 2.1 Change of Shielded Ring Heater Scope and Design The Conceptual Design of Advanced LIGO TCS considered a shielded... Probe Beam Injection Paths Since the Conceptual Design Review of Advanced LIGO, there have been several relevant changes to the Advanced LIGO baseline design First is the adoption of stable recycling... been designed for the TM HR surface monitor In this design, the pier drift is common to the two beams LIGO LIGO-M070109-02-R and the TM radius of curvature is differential to the two beams This design