033109-3 Dooley et al Rev.Sci.Instrum.83,033109(2012) Each element is a common building block of many opti- ferometer.The mode-matching telescope is a set of three sus- cal experiments and not unique to LIGO.However,their pended concave mirrors between the MC and interferometer roles specific to the successful operation of interferometry for that expand the beam from a radius of 1.6 mm at the MC gravitational-wave detection are of interest and demand fur- waist to a radius of 33 mm at the arm cavity waist.The MMT ther attention.Here,we briefly review the purpose of each of should play a passive role by delivering properly shaped light the IO components;further details about the design require- to the interferometer without introducing beam jitter or any ments are in Ref.16 significant aberration that can reduce mode coupling. A.Electro-optic modulator III.THERMAL PROBLEMS IN INITIAL LIGO The Length Sensing and Control (LSC)and Angular The Initial LIGO interferometers were equipped with a Sensing and Control(ASC)subsystems require phase modu- 10 W laser,yet operated with only 7 W input power due lation of the laser light at RF frequencies.This modulation is produced by an EOM,generating sidebands of the laser light to power-related problems with other subsystems.The EOM was located in the 10 W beam and the other components expe- which act as references against which interferometer length and angle changes are measured.17 The sideband light must rienced anywhere up to 7 W power.The 7 W operational limit was not due to the failure of the IO;however,many aspects of be either resonant only in the recycling cavity or not resonant the IO performance did degrade with power. in the interferometer at all.The sidebands must be offset from One of the primary problems of the Initial LIGO IO the carrier by integer multiples of the MC free spectral range (Ref.18)was thermal deflection of the back propagating beam to pass through the MC due to thermally induced refractive index gradients in the FI. A significant beam drift between the interferometer's locked B.Mode cleaner and unlocked states led to clipping of the reflected beam on Stably aligned cavities,limited non-mode-matched the photodiodes used for length and alignment control (see (junk)light,and a frequency and amplitude stabilized laser Fig.3.Our measurements determined a deflection of approx- are key features of any ultra sensitive laser interferometer.The imately 100 urad/W in the FI.This problem was mitigated at MC,at the heart of the IO,plays a major role the time by the design and implementation of an active beam A three-mirror triangular ring cavity,the MC suppresses steering servo on the beam coming from the isolator. laser output not in the fundamental TEMoo mode,serving two There were also known limits to the power the IO could major purposes.It enables the robustness of the ASC because sustain.Thermal lensing in the FI optics began to alter signif- higher order modes would otherwise contaminate the angu- icantly the beam mode at powers greater than 10 W,leading lar sensing signals of the interferometer.Also,all non-TEMoo to a several percent reduction in mode matching to the in- light on the length sensing photodiodes,including those used terferometer.19 Additionally,absorptive FI elements would for the GW readout,contributes shot noise but not signal and create thermal birefringence,degrading the optical efficiency therefore diminishes the signal to noise ratio.The MC is thus and isolation ratio with power.20 The Initial LIGO New Focus largely responsible for achieving an aligned,minimally shot- EOMs had an operational power limit of around 10 W.There noise-limited interferometer. was a high risk of damage to the crystals under the stress of The MC also plays an active role in laser frequency the 0.4 mm radius beam.Also,anisotropic thermal lensing stabilization,17 which is necessary for ensuring that the signal with focal lengths as severe as 3.3 m at 10 W made the EOMs at the anti-symmetric port is due to arm length fluctuations unsuitable for much higher power.Finally,the MC mirrors rather than laser frequency fluctuations.In addition,the MC exhibited high absorption(as much as 24 ppm per mirror) passively suppresses beam jitter at frequencies above 10 Hz. enough that thermal lensing of the MC optics at enhanced LIGO powers would induce higher order modal frequency C.Faraday isolator degeneracy and result in a power-dependent mode mismatch into the interferometer.21.22 In fact,as input power increased Faraday isolators are four-port optical devices which uti- from 1 W to 7 W the mode matching decreased from 90% lize the Faraday effect to allow for non-reciprocal polarization to83%. switching of laser beams.Any backscatter or reflected light In addition to the thermal limitations of the Initial LIGO from the interferometer (due to impedance mismatch,mode IO,optical efficiency in delivering light from the laser into mismatch,non-resonant sidebands,or signal)needs to be di- the interferometer was not optimal.Of the light entering the verted to protect the laser from back propagating light,which IO chain,only 60%remained by the time it reached the power can introduce amplitude and phase noise.This diversion of recycling mirror.Moreover,because at best only 90%of the the reflected light is also necessary for extracting length and light at the recycling mirror was coupled into the arm cavity angular information about the interferometer's cavities.The mode,room was left for improvement in the implementation FI fulfills both needs of the MMT. D.Mode-matching telescope IV.ENHANCED LIGO INPUT OPTICS DESIGN The lowest order MC and arm cavity spatial eigenmodes The Enhanced LIGO IO design addressed the thermal ef- need to be matched for maximal power buildup in the inter- fects that compromised the performance of the Initial LIGO Reuse of AlP Publishing content is subject to the terms at:https://publishing.aip.org/authors/rights-and-per D0wmlo8doP:183.195251.60:Fi.22Apr2016 00:54:10033109-3 Dooley et al. Rev. Sci. Instrum. 83, 033109 (2012) Each element is a common building block of many opti￾cal experiments and not unique to LIGO. However, their roles specific to the successful operation of interferometry for gravitational-wave detection are of interest and demand fur￾ther attention. Here, we briefly review the purpose of each of the IO components; further details about the design require￾ments are in Ref. 16. A. Electro-optic modulator The Length Sensing and Control (LSC) and Angular Sensing and Control (ASC) subsystems require phase modu￾lation of the laser light at RF frequencies. This modulation is produced by an EOM, generating sidebands of the laser light which act as references against which interferometer length and angle changes are measured. 17 The sideband light must be either resonant only in the recycling cavity or not resonant in the interferometer at all. The sidebands must be offset from the carrier by integer multiples of the MC free spectral range to pass through the MC. B. Mode cleaner Stably aligned cavities, limited non-mode-matched (junk) light, and a frequency and amplitude stabilized laser are key features of any ultra sensitive laser interferometer. The MC, at the heart of the IO, plays a major role. A three-mirror triangular ring cavity, the MC suppresses laser output not in the fundamental TEM00 mode, serving two major purposes. It enables the robustness of the ASC because higher order modes would otherwise contaminate the angu￾lar sensing signals of the interferometer. Also, all non-TEM00 light on the length sensing photodiodes, including those used for the GW readout, contributes shot noise but not signal and therefore diminishes the signal to noise ratio. The MC is thus largely responsible for achieving an aligned, minimally shot￾noise-limited interferometer. The MC also plays an active role in laser frequency stabilization,17 which is necessary for ensuring that the signal at the anti-symmetric port is due to arm length fluctuations rather than laser frequency fluctuations. In addition, the MC passively suppresses beam jitter at frequencies above 10 Hz. C. Faraday isolator Faraday isolators are four-port optical devices which uti￾lize the Faraday effect to allow for non-reciprocal polarization switching of laser beams. Any backscatter or reflected light from the interferometer (due to impedance mismatch, mode mismatch, non-resonant sidebands, or signal) needs to be di￾verted to protect the laser from back propagating light, which can introduce amplitude and phase noise. This diversion of the reflected light is also necessary for extracting length and angular information about the interferometer’s cavities. The FI fulfills both needs. D. Mode-matching telescope The lowest order MC and arm cavity spatial eigenmodes need to be matched for maximal power buildup in the inter￾ferometer. The mode-matching telescope is a set of three sus￾pended concave mirrors between the MC and interferometer that expand the beam from a radius of 1.6 mm at the MC waist to a radius of 33 mm at the arm cavity waist. The MMT should play a passive role by delivering properly shaped light to the interferometer without introducing beam jitter or any significant aberration that can reduce mode coupling. III. THERMAL PROBLEMS IN INITIAL LIGO The Initial LIGO interferometers were equipped with a 10 W laser, yet operated with only 7 W input power due to power-related problems with other subsystems. The EOM was located in the 10 W beam and the other components expe￾rienced anywhere up to 7 W power. The 7 W operational limit was not due to the failure of the IO; however, many aspects of the IO performance did degrade with power. One of the primary problems of the Initial LIGO IO (Ref. 18) was thermal deflection of the back propagating beam due to thermally induced refractive index gradients in the FI. A significant beam drift between the interferometer’s locked and unlocked states led to clipping of the reflected beam on the photodiodes used for length and alignment control (see Fig. 3. Our measurements determined a deflection of approx￾imately 100 μrad/W in the FI. This problem was mitigated at the time by the design and implementation of an active beam steering servo on the beam coming from the isolator. There were also known limits to the power the IO could sustain. Thermal lensing in the FI optics began to alter signif￾icantly the beam mode at powers greater than 10 W, leading to a several percent reduction in mode matching to the in￾terferometer. 19 Additionally, absorptive FI elements would create thermal birefringence, degrading the optical efficiency and isolation ratio with power.20 The Initial LIGO New Focus EOMs had an operational power limit of around 10 W. There was a high risk of damage to the crystals under the stress of the 0.4 mm radius beam. Also, anisotropic thermal lensing with focal lengths as severe as 3.3 m at 10 W made the EOMs unsuitable for much higher power. Finally, the MC mirrors exhibited high absorption (as much as 24 ppm per mirror)— enough that thermal lensing of the MC optics at enhanced LIGO powers would induce higher order modal frequency degeneracy and result in a power-dependent mode mismatch into the interferometer.21, 22 In fact, as input power increased from 1 W to 7 W the mode matching decreased from 90% to 83%. In addition to the thermal limitations of the Initial LIGO IO, optical efficiency in delivering light from the laser into the interferometer was not optimal. Of the light entering the IO chain, only 60% remained by the time it reached the power recycling mirror. Moreover, because at best only 90% of the light at the recycling mirror was coupled into the arm cavity mode, room was left for improvement in the implementation of the MMT. IV. ENHANCED LIGO INPUT OPTICS DESIGN The Enhanced LIGO IO design addressed the thermal ef￾fects that compromised the performance of the Initial LIGO Reuse of AIP Publishing content is subject to the terms at: https://publishing.aip.org/authors/rights-and-permissions. Download to IP: 183.195.251.6 On: Fri, 22 Apr 2016 00:54:10