Characterization and evasion of backscattered light in the squeezed-light enhanced gravitational wave interferometer GEO 600
F. Bergamin, J. D. Lough, Emil Schreiber, H. Grote, Moritz Mehmet, H. Vahlbruch, C. Affeldt, T. Andrić, Aparna Bisht, Marc Brinkmann, Volker Kringel, Harald Lück, N. Mukund, S. L. Nadji, B. Sorazu, K. A. Strain, Michael Weinert, K. Danzmann
Squeezed light is injected into the dark port of gravitational wave interferometers, in order to reduce the quantum noise. A fraction of the interferometer output light can reach the OPO due to sub-optimal isolation of the squeezing injection path. This backscattered light interacts with squeezed light generation process, introducing additional measurement noise. We present a theoretical description of the noise coupling mechanism and we prove the model with experimental results. We propose a control scheme to achieve a de-amplification of the backscattered light inside the OPO with a consequent reduction of the noise caused by it. The scheme was implemented at the GEO 600 detector and has proven to be crucial in maintaining a good level of quantum noise reduction of the interferometer for high parametric gain of the OPO. In particular, the mitigation of the backscattered light noise helped in reaching 6 dB of quantum noise reduction [Phys. Rev. Lett.126, 041102 (2021)10.1103/PhysRevLett.126.041102]. We show that the impact of backscattered-light-induced noise on the squeezing performance is phenomenologically equivalent to increased phase noise of the squeezing angle control. The results discussed in this paper provide a way for a more accurate estimation of the residual phase noise of the squeezed light field. Finally, the knowledge of the backscattered light noise coupling mechanism is a useful tool to inform the design of the squeezing injection path in terms of path stability and optical isolation.
Squeezed light is injected into the dark port of gravitational wave interferometers, in order to reduce the quantum noise. A fraction of the interferometer output light can reach the OPO due to sub-optimal isolation of the squeezing injection path. This backscattered light interacts with squeezed light generation process, introducing additional measurement noise. We present a theoretical description of the noise coupling mechanism and we prove the model with experimental results. We propose a control scheme to achieve a de-amplification of the backscattered light inside the OPO with a consequent reduction of the noise caused by it. The scheme was implemented at the GEO 600 detector and has proven to be crucial in maintaining a good level of quantum noise reduction of the interferometer for high parametric gain of the OPO. In particular, the mitigation of the backscattered light noise helped in reaching 6 dB of quantum noise reduction [Phys. Rev. Lett.126, 041102 (2021)10.1103/PhysRevLett.126.041102]. We show that the impact of backscattered-light-induced noise on the squeezing performance is phenomenologically equivalent to increased phase noise of the squeezing angle control. The results discussed in this paper provide a way for a more accurate estimation of the residual phase noise of the squeezed light field. Finally, the knowledge of the backscattered light noise coupling mechanism is a useful tool to inform the design of the squeezing injection path in terms of path stability and optical isolation.
Squeezed light is injected into the dark port of gravitational wave interferometers, in order to reduce the quantum noise. A fraction of the interferometer output light can reach the OPO due to sub-optimal isolation of the squeezing injection path. This backscattered light interacts with squeezed light generation process, introducing additional measurement noise. We present a theoretical description of the noise coupling mechanism. We propose a control scheme to achieve a de-amplification of the backscattered light inside the OPO with a consequent reduction of the noise caused by it. The scheme was implemented at the GEO 600 detector and has proven to be crucial in maintaining a good level of quantum noise reduction of the interferometer for high parametric gain of the OPO. In particular, the mitigation of the backscattered light noise helped in reaching 6dB of quantum noise reduction [Phys. Rev. Lett. 126, 041102 (2021)]. The impact of backscattered-light-induced noise on the squeezing performance is phenomenologically equivalent to increased phase noise of the squeezing angle control. The results discussed in this paper provide a way for a more accurate estimation of the residual phase noise of the squeezed light field.
The first detections of gravitational waves have opened an exciting new field of astronomy. One of the most fundamental limitations for the sensitivity of current and future interferometric gravitational-wave detectors is imposed by the quantum nature of light: Quantum vacuum fluctuations entering the interferometer through the readout port will contribute to the detection noise, at high frequencies in the form of shot noise and at low frequencies by radiation pressure noise. A promising way to reduce this quantum noise is the injection of squeezed states of light that have a lower uncertainty in one quadrature than the vacuum state. The GEO 600 gravitational-wave detector demonstrated the use of squeezed light in 2010 and it is now the first detector to routinely apply squeezing to improve its sensitivity beyond the limits set by classical quantum shot noise. This thesis details the practical aspects of long-term stable and efficient squeezed-light integration in a large-scale gravitational-wave detector. Imperfections that can limit the amount of observable non-classical noise improvement, such as optical losses and phase fluctuations, were studied in detail and methods for their mitigation were developed. Novel control schemes for the active stabilisation of the squeezed light field's phase and alignment were one main focus of the investigations. At the same time, important experience was gathered in the operation of the squeezed light source over long timescales. Over the course of the thesis work, improvements were implemented that significantly increased the performance of the squeezed-light application. Squeezing was injected with an overall duty cycle of 88%, reaching a noise reduction of up to 4.4 dB, corresponding to a 40% lowered shot-noise level. This work has firmly established the practical application of squeezing as a mature technology. The gained knowledge will directly inform the implementation of squeezed light for all future gravitational-wave detectors.
Photon shot noise, arising from the quantum-mechanical nature of the light, currently limits the sensitivity of all the gravitational wave observatories at frequencies above one kilohertz. We report a successful application of squeezed vacuum states of light at the GEO 600 observatory and demonstrate for the first time a reduction of quantum noise up to 6.03±0.02 dB in a kilometer scale interferometer. This is equivalent at high frequencies to increasing the laser power circulating in the interferometer by a factor of 4. Achieving this milestone, a key goal for the upgrades of the advanced detectors required a better understanding of the noise sources and losses and implementation of robust control schemes to mitigate their contributions. In particular, we address the optical losses from beam propagation, phase noise from the squeezing ellipse, and backscattered light from the squeezed light source. The expertise gained from this work carried out at GEO 600 provides insight toward the implementation of 10 dB of squeezing envisioned for third-generation gravitational wave detectors.
The quantum noise of the light field is a fundamental noise source in interferometric gravitational wave detectors. Injected squeezed light is capable of reducing the quantum noise contribution to the detector noise floor to values that surpass the so-called Standard-Quantum-Limit (SQL). In particular, squeezed light is useful for the detection of gravitational waves at high frequencies where interferometers are typically shot-noise limited, although the SQL might not be beaten in this case. We theoretically analyze the quantum noise of the signal-recycled laser interferometric gravitational-wave detector GEO600 with additional input and output optics, namely frequency-dependent squeezing of the vacuum state of light entering the dark port and frequency-dependent homodyne detection. We focus on the frequency range between 1 kHz and 10 kHz, where, although signal recycled, the detector is still shot-noise limited. It is found that the GEO600 detector with present design parameters will benefit from frequency dependent squeezed light. Assuming a squeezing strength of -6 dB in quantum noise variance, the interferometer will become thermal noise limited up to 4 kHz without further reduction of bandwidth. At higher frequencies the linear noise spectral density of GEO600 will still be dominated by shot-noise and improved by a factor of 10^{6dB/20dB}~2 according to the squeezing strength assumed. The interferometer might reach a strain sensitivity of 6x10^{-23} above 1 kHz (tunable) with a bandwidth of around 350 Hz. We propose a scheme to implement the desired frequency dependent squeezing by introducing an additional optical component to GEO600s signal-recycling cavity.
Albert Einstein's General Theory of Relativity predicts the existence of ripples in space- time, the so-called gravitational waves (GWs). In the past two decades, an international array of ground-based, kilometer-scale Michelson interferometers has been set up aiming at a rst observation. These Earth-bound GW detectors target signals at audio frequen- cies in a band of ca. 10 Hz { 10 kHz. At Fourier frequencies above 1 kHz, their sensitivity is limited by quantum noise. This shot-noise arises from the zero-point uctuations of the electromagnetic eld, the so-called vacuum uctuations , which enter the interferometer through its anti-symmetric signal port. Future generations of GW observatories will even be limited by quantum noise almost over their entire detection band [ ET ]. A `classical' solution to further improve the signal-to-shot-noise ratio (SNR) of the observatories consists in an increase of the circulating light power. A higher optical load inside the interferometer will, however, lead to thermally-induced beam distortion as well as to an increased radiation pressure noise level. This will eectively result in an upper limit for the optical light power employable in future GW observatory generations. An alternative approach to improve the SNR is based on the injection of so-called squeezed states of light. Such a light eld has a characteristic non-classical noise distribution in the eld quadratures. Injected from the signal port, the squeezed state replaces the vacuum state, thereby reducing the interferometer's quantum noise. The rst part of this thesis discusses the setup of a squeezed-light laser for the German- British GW observatory GEO 600. During characterization of the device, a noise reduc- tion of up to 9.6 dB below the vacuum noise level was observed in the frequency band of 10 Hz {10 kHz, setting a new benchmark for squeezing observed at audio frequencies. Corrected for detection losses, this value furthermore corresponds to more than 11 dB of squeezing available for injection into the dark port of GEO 600. Situated on a 1 : 15 m 1 : 35 m optical breadboard, the device features transportability and can be operated independently of the GW interferometer. A key feature of the squeezed-light laser is its unprecedented long-term stability. The longest characterization measurements lasted for 20 h with an overall duty cycle of more than 99 %. In the second part of this thesis, the implementation of squeezed light into GEO 600 is presented along with the results achieved. The observatory shot noise was reduced by up to 3.4 dB, corresponding to an improvement in sensitivity by a factor of up to 1.5. For isotropically distributed GW sources, this corresponds to an increase in detection probability by 1.5 3 3.4. This makes GEO 600 the rst large-scale GW interferometer with a non-classically enhanced sensitivity. Based on the performance of the squeezed- light laser, a sensitivity improvement by a factor of two, corresponding to an equivalent optical power increase by a factor of four, seems feasible in the near future.
The ability to directly detect gravitational waves will open a completely new branch of astronomy to view the Universe, one that is inaccessible to electromagnetic-based astronomy. First generation ground-based interferometric gravitational-wave detectors have achieved strain sensitivities of order one part in ten to the twenty-one, at 100 Hz detection frequency. A new generation of detectors are under construction, designed to improve on the sensitivity of the first-generation detectors by a factor of 10. The quantum nature of light will broadly limit the sensitivity of these new instruments. This quantum noise will originate from the quantum vacuum fluctuations that enter the unused port of the interferometer. One of the most promising options for reducing the quantum noise impact and further increasing the sensitivity is applying quantum squeezed vacuum states. These squeezed states have lower noise in one quadrature than the vacuum state. By replacing the quantum vacuum fluctuations entering the interferometer with squeezed vacuum states, the quantum noise impact is reduced. This thesis firstly details the development of a squeezed light source that produces squeezed states applicable for enhancing interferometric gravitational-wave detectors. A doubly-resonant, travelling-wave bow-tie cavity squeezed light source is presented. For the first time, greater than 10 dB of quantum noise suppression across the gravitational-wave detection band is directly observed, and 11.6 dB of quantum noise suppression is observed above 200 Hz. This squeezing cavity design also has benefits with intrinsic isolation to backscattered light. Experiments that quantify this isolation are reported. The properties affecting squeezing magnitude and low-frequency squeezing measurement are discussed. In addition, a modified squeezing-ellipse-phase control technique for squeezed vacuum states is presented. This thesis secondly presents results from the LIGO Squeezed Light Injection Experiment, undertaken to test squeezed light injection into a 4 km interferometric gravitational-wave detector. The results of the experiment show the first measurement of squeezing-enhancement in a 4 km gravitational-wave detector, with 2.15 dB measured above 250 Hz. This represents the best sensitivity to gravitational waves yet achieved at these frequencies by any single gravitational-wave detector to date. An unknown area was whether the addition of a squeezer would introduce noise couplings that degrade the crucial low frequency sensitivity. The results demonstrate that injected squeezed states are compatible with low frequency gravitational-wave measurement. The characterisation of squeezing-injection optical losses and fluctuations of the squeezing angle are also reported. The knowledge and processes gained, from both the squeezed light source development work and the LIGO Squeezed Light Injection Experiment, will inform the design, planning and implementation of squeezed states in future gravitational-wave detectors.