arXiv subjects

Li Ju

Publications and source records attributed to Li Ju.

45 records · Page 3Linked to original sources

arXiv2015

Parametric Instability in Long Optical Cavities and Suppression by Dynamic Transverse Mode Frequency Modulation

Three mode parametric instability has been predicted in Advanced gravitational wave detectors. Here we present the first observation of this phenomenon in a large scale suspended optical cavity designed to be comparable to those of advanced gravitational wave detectors. Our results show that previous modelling assumptions that transverse optical modes are stable in frequency except for frequency drifts on a thermal deformation time scale is unlikely to be valid for suspended mass optical cavities. We demonstrate that mirror figure errors cause a dependence of transverse mode offset frequency on spot position. Combined with low frequency residual motion of suspended mirrors, this leads to transverse mode frequency modulation which suppresses the effective parametric gain. We show that this gain suppression mechanism can be enhanced by laser spot dithering or fast thermal modulation. Using Advanced LIGO test mass data and thermal modelling we show that gain suppression factors of 10-20 could be achieved for individual modes, sufficient to greatly ameliorate the parametric instability problem.

physics.optics↗

arXiv2014

Three mode interaction noise in laser interferometer gravitational wave detectors

Triply resonant three mode interactions in long optical cavities have been shown to lead to enhanced scattering of carrier light by the ultrasonic acoustic modes of the test mass mirrors. At high optical power, this can lead to parametric instability (parametric gain R > 1) for a few acoustic modes with strong spectral and spatial overlap. Numerous ?~10^3 acoustic modes of the test masses are predicted to have R > 10^2. Experimental studies have shown that such modes also strongly scatter the carrier light, enabling very sensitive readout of the acoustic modes. The 3-mode scattering from the thermal fluctuation of large population of ultrasonic modes would causes random changes in occupation number of the carrier light and cavity transverse optical modes. Because the thermal fluctuation time scale (set by the acoustic mode relaxation times) is typically a few seconds, the noise spectrum from thermally induced photon number fluctuations is strongly peaked at low frequency. The noise level depends on the acoustic mode structure and acoustic losses of the test masses, the transverse optical mode spectrum of the optical cavities and on the test mass temperature. We theoretically investigate the possible effect of this noise and show that in advanced detectors under construction three mode interaction noise is below the standard quantum limit, but could set limits on future low frequency detectors that aim to exceed the free mass standard quantum limit.

physics.optics↗

arXiv2014

Classical demonstration of frequency dependent noise ellipse rotation using Optomechanically Induced Transparency

Cavities with extremely narrow linewidth of 10-100 Hz are required for realizing frequency dependent squeezing to enable gravitational wave detectors to surpass the free mass standard quantum limit over a broad frequency range. Hundred-meter-scale high finesse cavities have been proposed for this purpose. Optomechanically induced transparency (OMIT) enables the creation of optomechanical cavities in which the linewidth limit is set by the extremely narrow linewidth of a high Q factor mechanical resonator. Using an 85mm OMIT cavity with a silicon nitride membrane, we demonstrate a tunable linewidth from 3Hz up to several hundred Hz and frequency dependent noise ellipse rotation using classical light with squeezed added noise to simulate quantum squeezed light. The frequency dependent noise ellipse angle is rotated in close agreement with predictions.

quant-ph↗

arXiv2013

Observation of Three-Mode Parametric Instability

Three-mode parametric interactions can occur in triply-resonant opto-mechanical systems in which two orthogonal optical modes are coupled with an appropriate mechanical mode. Using an optical cavity with a membrane inside, we report the first observation of three-mode parametric instability in a Fabry-Perot cavity, a phenomenon predicted to occur in long baseline advanced gravitational wave detectors. We present a large signal model for the phenomenon, which predicts exponential growth of mechanical oscillation followed by saturation. Our experimental results are consistent with this model. Contrary to expectations, parametric instability does not lead to loss of cavity lock, a fact which may make it easier to implement control techniques in Advanced gravitational wave detectors.

physics.optics↗

arXiv2013

Radiation pressure excitation of test mass ultrasonic modes via three mode opto-acoustic interactions in a suspended Fabry-Perot cavity

Three-mode parametric-instabilities risk stable operation of gravitational-wave detectors. Instabilities occur through time varying radiation-pressure distributions, derived from beating between two optical modes, exciting mirror acoustic modes in Fabry-Perot cavities. Here we report the first demonstration of radiation-pressure driving of ultrasonic-acoustic modes via pairs of optical modes in gravitational-wave type optical cavities. In this experiment ~0.4W of TEM01 mode and ~1kW of TEM00 mode circulated inside the cavity, an ~181.6kHz excitation was observed with amplitude ~5E-13m. The results verify the driving force term in Braginsky's parametric instability feedback model. The interaction parametric gain was (3.8+/-0.5)E-3 and mass-ratio scaled opto-acoustic overlap 2.7+/-0.4.

physics.optics↗

arXiv2013

High quality factor mg-scale silicon mechanical resonators for 3-mode optoacoustic parametric amplifiers

Milligram-scale resonators have been shown to be suitable for the creation of 3-mode optoacoustic parametric amplifiers, based on a phenomena first predicted for advanced gravitational-wave detectors. To achieve practical optoacoustic parametric amplification, high quality factor resonators are required. We present millimetre-scale silicon resonators designed to exhibit a torsional vibration mode with a frequency in the 10^5 - 10^6 Hz range, for observation of 3-mode optoacoustic interactions in a compact table-top system. Our design incorporates an isolation stage and minimizes the acoustic loss from optical coating. We observe a quality factor of 7.5 x 10^5 for a mode frequency of 401.5 kHz, at room temperature and pressure of 10^-3 Pa. We confirmed the mode shape by mapping the amplitude response across the resonator and comparing to finite element modelling. This study contributes towards the development of 3-mode optoacoustic parametric amplifiers for use in novel high-sensitivity signal transducers and quantum measurement experiments.

physics.ins-det↗

arXiv2008

Direct Amplification of Sound By Light

A new device, the opto-acoustic parametric amplifier (OAPA), is introduced, which is analogous to an optical parametric amplifier (OPA). The radiation pressure reaction of light on the reflective surface of an acoustic resonator provides nonlinearity similar to the Kerr effect in the OPA. The OAPA can be tuned to operate in two regimes. In the positive gain regime, an acoustic signal is directly amplified. In the negative gain regime, we show that the acoustic resonator can be cooled to the quantum ground state.

gr-qc↗

arXiv2008

Direct string magnetic gradiometer for space applications

Recently, a novel Direct String Magnetic Gradiometer (DSMG) has been developed, where a vibrating wire, driven by an AC current, is used as a single sensitive element. It is designed to directly measure the local off-diagonal components of the magnetic gradient tensor, Bxz, Byz and Bxy, provided the distance to an object creating magnetic anomalies is much larger than the length of the string. This requirement is well satisfied in space, if the sensor is deployed from a satellite platform orbiting near the planet under surveillance. Current instruments operating at $1{kPa}$ pressure achieve sensitivity of $1.8 \times 10^{-10}{T}/{m}$ in the band $0.0001{Hz}$ to $0.1{Hz}$. In this paper we show that proposed modifications to the current gradiometer design, specifically aimed at the deployment in space, could have a magnetic gradient sensitivity better than $10^{-13}{T}/{m}/\sqrt{Hz}$ in the frequency range of interest for specific missions both for fundamental research and for such applications as geophysical exploration on Mars and other solar system planets. Also, by combining a few single-axis magnetic gradiometer modules, it is possible to deploy a full tensor magnetic gradiometer.

physics.ins-det↗

arXiv2008

Reducing the magnetic susceptibility of parts in a magnetic gradiometer

In this paper we report a detailed investigation of a number of different materials commonly used in precision instrumentation in the view of using them as critical components in the magnetic gradiometer. The materials requirement inside a magnetic gradiometer is stringent because the presence of magnetic susceptible material will introduce intrinsic errors into the device. Many commercial grade non-magnetic materials still have unacceptably high levels of volume magnetic susceptibility between 0.001 and 0.0001. It is shown here that machining with steel tools can further increase the susceptibility by up to an order of magnitude. The ability of an acid wash to remove this contamination is also reported. Washing in acid is shown to reduce the variation of volume susceptibility in several commercial grade plastics which already have low values of susceptibility.

cond-mat.mtrl-sci↗