LASER INTERFEROMETER GRAVITATIONAL WAVE OBSERVATORY. LIGO Scientific Collaboration. Enhanced LIGO. R Adhikari. Distribution of this draft:

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1 LASER INTERFEROMETER GRAVITATIONAL WAVE OBSERVATORY LIGO LIGO Scientific Collaboration LIGO T I May 006 Enhanced LIGO R Adhikari Distribution of this draft: LIGO Scientific Collaboration This is an internal working note of the LIGO Project. California Institute of Technology LIGO Project / MS 5 33 Pasadena, CA 95 Pho: (66) Fax: (66) info@ligo.caltech.edu Massachusetts Institute of Technology LIGO Project / NW7 6 Cambridge, MA 039 Pho: (67) Fax: (67) info@ligo.mit.edu LIGO Hanford Observatory P.O. Box 970 Mail Stop S9 0 Richland, WA 9935 Pho: (509) Fax: (509) LIGO Livingston Observatory P.O. Box 940 Livingston, LA Pho: (5) Fax: (5)

2 OVERVIEW This document presents the baseline plan for enhancing aspects of the LIGO detectors in the time period between the end of the fifth Science Run (S5) and the start of Advanced LIGO. All of the main hardware improvements are direct implementations of Advanced LIGO technologies and techniques. This strategy allows us to test full scale prototypes of the Advanced LIGO system in a low noise environment. The principal change is an increase in the laser power aimed at increasing the sensitivity in the bucket by a factor of ~.5. To take advantage of the increased laser power, the dark port sensing system will be moved in vacuum onto a seismically isolated platform and a monolithic filter cavity will be installed on the dark port beam to clean up the light. The plan described here implements these improvements on only the two 4km interferometers. A staged installation schedule will allow the commissioning team at Livingston to discover problems in time to inform the work being done at Hanford. Post S5 tasks for the Hanford km instrument are still being discussed and are outside the scope of this document. Astrophysical motivations for this sensitivity improvement are detailed in T0505 [] and in Appendix A. TIMELINE and STATUS NO 4QW Other interferometers in operation (GEO, Virgo) 05 S5 4Q 4Q 4Q ~ years 4 4Q yrs 09 S6 4Q 0 Adv LIG O

3 . Interferometer Noise in S5 The S5 Science Run started off with the interferometers at the sensitivity goal set for the start of the run. Further characterization of the instruments throughout the run led to further improvement of ~30% in the sensitivity to neutron star inspirals. The following plot shows the differential arm displacement noise of the 4km interferometers (H & L) and also the sensitivity goal from the Science Requirements Document (SRD). Figure : Best sensitivity of the 4km IFOs. NS/NS range is 4+ Mpc. The excess noise below 00 Hz is not well understood. It is further discussed in Section 3.8.

4 3 DETECTOR ENHANCEMENTS In this section, the major detector enhancements are described. 3. Increased Laser Power To increase the laser power a new Master Oscillator / Power Amplifier (MOPA) will be installed. These new units will be provided by Laser Zentrum Hanover (LZH) []. These MOPAs will provide W in the TEM00 mode; around 3x more than our existing MOPA. Figure : (left) Schematic of the LZH laser. Nd:YVO4 rods are pumped via fiber from remote pump diodes. (right) Beam profile of the MOPA output; > 97% of the power is TEM00. Another attractive feature of these lasers is that they are the front end for the 00 W Advanced LIGO laser (also being developed by LZH) []. Besides the improvement in the interferometer sensitivity, we will also gain valuable experience by debugging such a substantial piece of the Advanced LIGO PSL system. The free running noise of this system, in the lab, is somewhat less than the Lightwave MOPAs we currently have. The laser amplitude and frequency actuators are similar (true?) enough to the existing ones that we envisage no substantial changes in the laser stabilization topology, although the circuits we use now may need to be modified.

5 3. Dark Port Sensing System There are three major components to the new dark port sensing scheme: o DC Readout of the gravitational wave signal (as opposed to RF heterodyning) o An Output Mode Cleaner (OMC) cavity to remove junk light before detection o All in vacuum detection hardware (optical table, photodetectors, auto alignment) The DC readout scheme (with OMC) is being prototyped at the Caltech 40m lab in the summer of 006. This includes all of the same hardware which will be needed at the observatories and all of the design work so far has been done keeping in mind the requirements for the post S5 enhancement as well as Advanced LIGO. 3.. DC Readout DC Readout is the baseline scheme for Advanced LIGO [3]. The current interferometers use an RF readout scheme [4]; a local oscillator field, shifted by ~30 MHz from the carrier, is present at the dark port. The resulting beat signal is synchronously demodulated to recover the gravitational wave signal. In the DC scheme, the arm cavities are shifted slightly off resonance, which shifts the signal at the dark port slightly from the dark fringe. The power at the dark port is then a linear readout of the differential arm length. There are a number of technical advantages to using this scheme. The coupling from several technical noise sources is reduced: laser frequency noise, power recycling cavity length noise, RF oscillator noise, etc. 3.. Output Mode Cleaner To take advantage of the DC scheme, we also employ an OMC. This filter cavity strips off all of the RF sidebands as well as the higher order transverse modes which come from a contrast defect. The removal of all of this 'excess' light reduces the shot noise level. The baseline is the same as for Advanced LIGO: a short (~0 cm), monolithic, ring cavity with a Finesse of ~300. There are many design trade offs to explore and more detailed modeling of the various noise mechanisms is needed. This is an ongoing effort.

6 3..3 In-vacuum Hardware The Initial LIGO experience has taught us that placing any of the interferometer's sensors outside of the vacuum introduces a large susceptibility to environmental noise (acoustics, seismic) as well as dust, etc. The chief motivation in moving towards an in vacuum, isolated platform is to reduce the coupling of these noise sources.this requires the development of a few new techniques: in vac low noise, DC photodetection. etc. HAM5 and HAM6 are both empty in the current interferometer layout. After S5, the plan is to insert a vacuum flange with a Brewster angle window between HAM6 and the beamtube connecting HAM5 and HAM6. Figure 3: HAM5 (left) and HAM6 (right) A major part of the in vacuum hardware will be the introduction of an Advanced LIGO HAM isolation table in HAM6. Although this is probably more isolation than is required for the kind of beam jitter we expect from the initial LIGO interferometer, it is another good opportunity to commission, ahead of time, another Advanced LIGO sub system. The HAM isolation mechanics and control systems will be prototyped at LASTI and also at LLO in the Staging Building.

7 3.3 High Power Related Issues A 3x increase in the laser power requires upgrades in a few of the auxillary optics systems. Most notably in the Input Optics (Electro optic modulators and Faraday Isolator) and in the Thermal Compensation System (TCS) for the test masses. The Preliminary Design [5] for the Advanced LIGO Modulators and Isolators describes in detail the proposed upgrades to be made to the initial LIGO hardware Electro-optic Modulators (EOM) The new EOM design uses a crystal of RTP instead of Mg:LiNO3. This has a much lower absorption at 064 nm Faraday Isolator (FI) The initial LIGO Faraday Isolators exhibit some thermal lensing leading to a significant beam drift between the interferometer's locked and unlocked states. This has been mitigated somewhat by with the use of active beam steering on the beam rejected by the isolator. The Advanced LIGO Faraday design solves this problem at the source through the use of negative dn/dt materials, etc Thermal Compensation System (TCS) The TCS currently compensates for excess absorption in the bulk of the ITM and in the HR coatings of the ITMs and ETMs. By projecting an annular pattern of 0 micron light from a CO laser, a compensating thermal lens is induced in the bulk of the ITMs. Extrapolating from the higher power compensation levels employed now, one can roughly predict the amount of CO power necessary to compensate for a 3 4x increase in absorbed power (allows for some margin). [Include a table using the TCS snapshot values] Radiation Pressure Instabilities At high circulating powers, Fabry Perot cavities become unstable to small angular misalignments [6]. Preliminary estimates of the effect on the initial LIGO cavities indicate that a 3x increase in the laser power will move the unstable mode frequency close to the edge of the range of the control systems. An adaptive control system needs to be developed to dynamically compensate for the power dependent stiffness.

8 3.4 Mystery noise / Upconversion The low frequency region of the strain noise spectrum shows a slight excess which seems to come from a variety of nonlinear noise generating mechanisms. Electronics, scattering, charging, etc. 3.5 Misc. Tasks Cleaning the MC New PMC Changing EQ Stops Bias module electronics

9 4 Schedule Summary: Begin installation at LLO, Fall of '07 with significant support from LHO staff. Stagger the installation on H by several months. Complete major hardware installation in Livingston Vent part 3 weeks ) HAM6 flange / window (Rus W, Harry O, Joe G) ) HAM4 telescope re-alignments (Mike S, Betsy, Dan H) 3) Faraday Isolator (UF + Malik, KenF, RupalA) 4) ITM Re-alignments w/ PAMs (Gary T, Joe H) 5) ITM Arm Cavity Baffles (Gary T, Danny S) 6) Drag Wipe the MC (UF + Betsy) 7) ISS Pickoff move (part of FI install) 8) New Laser electronics installation (Ken W, Peter K, Mike F, Rus W) (laser electronics work continues through pump down) Pump Down 6 weeks ) HAM installation (Harry O, Hugh, Corey, Brian O, Joe G) ) OMC + HAM6 Optics (Valera, Dan H, Cheryl, Keita) 3) New EOM (UF + Ken F, Rupal A) 4) PMC redo (UF + Ken F, Rupal A, Rick S, Justin G) 5) Vent Ends -> (Rus W) ETM Baffles (Gary T, Danny S) ETM Re-align w/ PAMs (Gary T, Danny S) 6) ISCT Floating prep (if necessary) (Robert S, Joe H, Doug L) 7) SUS Bias Modules (Rich A, Mike F, Rai) 8) HAM6 Electronics (Valera, Rich A, Ken W) Commissioning -8 Weeks ) DC Readout debugging (Valera, Keita, Dan H, Ken W) (MICH / starts before arms are open) ) IFO Locking (on RF) 3) Full Noise debugging (re-establish S5 sensitivity) Laser Install -- 4 weeks ) Install (LZH, Rupal A, Rick S, Doug C) ) Servo Tune Up (Rupal A, Valera, Rick S, Ken F) High Power / DC Readout Commissioning 0 Weeks ) Noise Hunting ) Highpower RF (for REFL, PO if necessary) ) New ASC code (optical springs, copied from 40m MC) 3) Lower Noise Coil Drivers (Rai, Rich A, Ken W)

10 4. Hanford - Run w/ Virgo ends or change to run with only Virgo & H Vent part 3 weeks ) HAM6 flange / window (Kyle R, John W) ) HAM4 telescope re-alignments (Mike S, Betsy, Corey) 3) Faraday Isolator (UF + Malik) 4) ITM Re-alignments w/ PAMs (Doug C, Betsy) 5) ITM Arm Cavity Baffles (Doug C, Betsy) 6) Drag Wipe the MC (UF + Corey, Betsy) 7) ISS Pickoff move (part of FI install) 8) New Laser electronics installation (Josh M, Richard M, Rick S, Justin G) (laser electronics work continues through pump down) Pump Down 6 weeks ) HAM installation (Hugh R, Corey G, Brian O) ) OMC + HAM6 Optics (Cheryl, Keita) 3) New EOM (UF + Rick S, Rupal A) 4) PMC redo (UF + Rick S, Rupal A, Justin G) 5) Vent Ends -> (John W) ETM Baffles (Doug C, Betsy) ETM Re-align w/ PAMs (Doug C, Betsy) 6) ISCT Floating prep (if necessary) (Robert S, Ski) 7) SUS Bias Modules (Rich A, Josh M) 8) HAM6 Electronics (Vern S, Rich A, Richard M) Commissioning - Weeks ) DC Readout debugging (Keita + grad students) (MICH/PRC starts before arms are open) ) IFO Locking (on RF) 3) Full Noise debugging (re-establish S5 sensitivity) Laser Install -- 3 weeks ) Install (LZH, Rupal A, Rick S, Doug C) ) Servo Tune Up (Rupal A, Justin G, Rick S) High Power / DC Readout Commissioning 6 Weeks ) Highpower RF (for REFL, PO if necessary) ) New ASC code (optical springs, copied from 40m MC) 3) Lower Noise Coil Drivers (Richard M, Josh M)

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12 5 Budget total for Qty Power Increase EO modulators Faraday isolators PSL/IO optics Laser Infrastructure for laser Laser controls PMC mirrors Subtotal unit cost, $ ifos, $ R&D Vacuum hardware for HAM6 detection Isolation plate Gate valves Ion pump setup Turbo pump setup Windows Subtotal Output mode cleaner OMC cavity bodies OMC mirrors OMC suspension Opto-mechanical HW Electronics Subtotal HAM6 Seismic isolation, single stage ISI ISI instruments ISI mechanics Support structure ISI electronics Subtotal ITM beam tube baffles SUS Bias modules Total covered by AdLIGO R&D Not covered by AdLIGO R&D Table : Budget for post S5 enhancements, IFOs

13 6 References [] "Nd:YV04 Amplifier System", B. Schulz, March '06 LSC Meeting, 00/ [] "AdvLIGO Laser Status", L. Winkleman, March '06 LSC Meeting, 00/ [3] "Frequency and Intensity Noise", K. Somiya, Internal Technical Document [4] "Readout and Control of a Power recycled Interferometric Gravitational wave Antenna", D. Sigg, H. Rong, P. Fritschel, M. Zucker, R. Bork, N. Mavalvala, D. Ouimette, G. Gonzalez, [5] "Upgrading the Input Optics for High Power Operation", UF LIGO Group, E D, 00/ [6] "Optical Torques in Suspended Fabry Perot Interferometers", J. Sidles, D. Sigg, C