Shot-noise Limited Sensitivity Research Articles

As the benchmark for rigorous ab initio theory

The observed ro-vibrational spectral lines of the simplest polyatomic molecule, H3+, serve as the benchmark to test the most rigorous ab initio theory of intra-molecular dynamics. The ground state equilibrium structure of H3+ is an equilateral triangle, but near the energies of ∼ 9913 cm−1, the so called barrier to linearity, it begins to sample linear configurations for which theoretical calculations are challenging because of the singularity of the Hamiltonian. We present here a continuation of the spectroscopy of H3+ above the barrier to linearity using a Ti:sapphire laser based high sensitivity spectrometer which allowed us to observe the spectrum with near shot-noise limited sensitivity. 121 new lines have been recorded from the near infrared 10 300 cm−1–13 700 cm−1 entering the visible region bringing the total number of transitions above 10 000 cm−1 to 143. The observed spectrum is compared with theoretical predictions. The remaining discrepancies mostly due to vibrational and rotational non-adiabatic effects are discussed.

Experimental optomechanics with silicon micromirrors

We experimentally demonstrate the high-sensitivity optical monitoring of moving micromirrors, made of low-loss dielectric coatings upon silicon resonators of various shapes and sizes. The very high optical finesses obtained () have allowed us to measure the thermal noise of the micromirrors at room temperature with a shot-noise limited sensitivity at the level and to completely characterize their mechanical behavior, in excellent agreement with the results of a finite-element computation. Applications of such optomechanical systems range from quantum optics experiments to the experimental demonstration of the quantum ground state of a macroscopic mechanical resonator.

High-sensitivity monitoring of micromechanical vibration using optical whispering gallery mode resonators

The inherent coupling of optical and mechanical modes in high finesse optical microresonators provides a natural, highly sensitive transduction mechanism for micromechanical vibration. Using homodyne and polarization spectroscopy techniques, we achieve shot-noise limited displacement sensitivities of . In an unprecedented manner, this enables the detection and study of a variety of mechanical modes, which are identified as radial breathing, flexural and torsional modes using three-dimensional finite element modeling. Furthermore, a broadband equivalent displacement noise is measured and found to agree well with models for thermorefractive noise in silica dielectric cavities. Implications for ground-state cooling, displacement sensing and Kerr squeezing are discussed.

Using active resonator impedance matching for shot-noise limited, cavity enhanced amplitude modulated laser absorption spectroscopy

We introduce a closed-loop feedback technique to actively control the coupling condition of an optical cavity, by employing amplitude modulation of the interrogating laser. We show that active impedance matching of the cavity facilitates optimal shot-noise sensing performance in a cavity enhanced system, while its control error signal can be used for intra-cavity absorption or loss signal extraction. We present the first demonstration of this technique with a fiber ring cavity, and achieved shot-noise limited loss sensitivity. We also briefly discuss further use of impedance matching control as a tool for other applications.

Two-Wavelength Grating Interferometry for MEMS Sensors

Diffraction gratings integrated with micro-electro-mechanical-systems (MEMS) offer shot noise limited sub-nanometer displacement detection sensitivities but are limited in detection range for mechanical transducers. A two-wavelength readout method is developed that maintains high sensitivity while increasing the detection range, which is demonstrated using a MEMS spectrometer with integrated diffraction grating. The two-laser illumination extended the detection range from 105 nm to 1.7mum assuming the readout sensitivity is maintained at >50% of the maximum sensitivity.

High-Resolution Magnetometry with a Spinor Bose-Einstein Condensate

We demonstrate a precise magnetic microscope based on direct imaging of the Larmor precession of a 87Rb spinor Bose-Einstein condensate. This magnetometer attains a field sensitivity of 8.3 pT/Hz1/2 over a measurement area of 120 microm2, an improvement over the low-frequency field sensitivity of modern SQUID magnetometers. The achieved phase sensitivity is close to the atom shot-noise limit, estimated as 0.15 pT/Hz1/2 for a unity duty cycle measurement, suggesting the possibilities of spatially resolved spin-squeezed magnetometry. This magnetometer marks a significant application of degenerate atomic gases to metrology.

Highly sensitive spectral interferometric four-wave mixing microscopy near the shot noise limit and its combination with two-photon excited fluorescence microscopy

We present spectral interferometric four-wave mixing (FWM) microscopy with a nearly shot-noise limited sensitivity and with the capability of separating FWM signals from fluorescence signals. We analyze the requirements for obtaining the shot-noise limited sensitivity and experimentally achieve the sensitivity that is only 4-dB lower than the shot-noise limit. Moreover, we show that only FWM signals can be extracted through the Fourier filtering even when the FWM spectrum is overlapped and overwhelmed by the fluorescence spectrum. We demonstrate simultaneous acquisition of FWM and two-photon excited fluorescence images of fluorescent monodispersed polystyrene microspheres.

An interferometric sensor for satellite drag-free control

We describe a compact interferometer, under development at the University of Birmingham, that could be employed as a zero-stiffness sensor for drag-free satellites. A prototype bench-top polarization-based homodyne interferometer is discussed that achieves a shot-noise limited displacement sensitivity of 3 × 10−12 m Hz1/2 above 60 Hz. We discuss a wavelength modulation technique that will render the interferometer more robust as it enables the absolute difference in interferometric armlengths to be determined.

A reflection interferometer with transmission-like characteristics as an element of interferometric gravitational wave detectors

It is proposed to replace the Fabry-Perot interferometers incorporated into the combined Fabry-Perot-Michelson interferometer (FPMI) by transmission-like reflection interferometers (TRIs), creating bright (transmission-like) interference fringes in the reflected light. TRIs employ asymmetric metal-dielectric mirrors with a “matching” absorbing layer. The calculations show that the shot noise-limited sensitivity of an FPMI with TRIs is nearly as high as that of a conventional FPMI (for the same quality of dielectric coatings). In the presence of scattered light or when the laser and interferometer modes are not coupled well enough, the new version of the FPMI offers considerable advantages due to the mode-filtering properties of a TRI.

Chip-scale atomic magnetometer

Using the techniques of microelectromechanical systems, we have constructed a small low-power magnetic sensor based on alkali atoms. We use a coherent population trapping resonance to probe the interaction of the atoms’ magnetic moment with a magnetic field, and we detect changes in the magnetic flux density with a sensitivity of 50pTHz−1∕2 at 10Hz. The magnetic sensor has a size of 12mm3 and dissipates 195mW of power. Further improvements in size, power dissipation, and magnetic field sensitivity are immediately foreseeable, and such a device could provide a hand-held battery-operated magnetometer with an atom shot-noise limited sensitivity of 0.05pTHz−1∕2.

Three-dimensional cellular-level imaging using full-field optical coherence tomography

An ultrahigh-resolution full-field optical coherence tomography (OCT) system has been developed for cellular-level imaging of biological media. The system is based on a Linnik interference microscope illuminated with a tungsten halogen lamp, associated with a high-resolution CCD camera. En face tomographic images are produced in real time, with the best spatial resolution ever achieved in OCT (0.7 µm × 0.9 µm, axial × transverse). A shot-noise limited detection sensitivity of 80 dB can be reached with an acquisition time per image of 1 s. Images of animal ophthalmic biopsies and vegetal tissues are shown.

Towards dual recycling with the aid of time and frequency domain simulations

Dual recycling, the combination of the interferometric techniques of power and signal recycling, allows the improvement of the shot noise limited sensitivity of interferometric gravitational wave detectors. GEO 600 is the first km-scale gravitational wave detector using dual recycling. The hardware installation is completed and dual recycling has become a great challenge in terms of commissioning of GEO 600. Simulations show that lock acquisition of the optical system can only be achieved in certain detector states. Thus as we need to start with a locked detector in such a specific state, an appropriate strategy is needed to change the state of detector operation without losing lock. The basic concepts and first results based on time and frequency domain simulations will be presented in this paper.

Method for compensation of thermally induced modal distortions in the input optical components of gravitational wave interferometers

The next generation of interferometric gravitational wave detectors will employ laser powers approaching 200 W to increase shot-noise limited sensitivity. Optical components that transmit the laser light will exhibit increased thermal lensing induced by bulk absorption and concomitant changes in the material refractive index, resulting in significant changes in the modal characteristics of the beam. Key interferometer components such as electro-optic modulators and Faraday isolators are particularly at risk, since they possess relatively large absorption coefficients. We present a method for passive correction of thermally induced optical path length (ΔΛ) changes induced by absorption in transmissive optical components. Our method relies on introducing material in the optical path that possesses a negative index temperature derivative, thereby inducing a compensating opposite ΔΛ. We experimentally demonstrate a factor of 10 reduction in higher order spatial mode generation for terbium gallium garnet, a Faraday isolator material.

Polarization Sagnac interferometer with a common-path local oscillator for heterodyne detection

We describe a free-space common-path polarization Sagnac interferometer. The polarization Sagnac interferometer is used in a symmetric fashion with detection on the dark fringe of the interference to avoid photodetector saturation when high-powered illumination is used. By modulating the carrier field that is reflected from the interferometer, we generate a local oscillator for heterodyne detection of the signal field. We calculate the effect of optical element misalignments and imperfect polarization on the shot-noise–limited sensitivity of the interferometer. The predictions are experimentally verified with a 2-m arm-length tabletop polarization Sagnac interferometer.

Electrooptical sampling using 1.55-μm self-seeded semiconductor laser with soliton pulse compression

We report on a compact, reliable and easy to use electrooptic sampling system based on a self-seeded semiconductor laser having a temporal resolution of <1 ps and a shot noise limited sensitivity of 1 mV. With an operating wavelength of 1.55 /spl mu/m and 0.5-ps time jitter of the optical source, it is particularly suited for ultrahigh-speed devices and integrated circuits. For demonstration, results for an ultrafast metal-semiconductor-metal photodetector are presented.

The design and sensitivity of microwave frequency optical heterodyne receivers

Recent advances in high speed photodetector and microwave receiver technology make microwave frequency optical heterodyning an attractive approach for the detection of a number of coherent Raman and Brillouin scattering experiments. We have therefore analyzed the sensitivity of microwave frequency optical heterodyne receivers. Experimental tests on a visible wavelength receiver operating at 13.5 GHz confirm the expectation of shot noise limited sensitivity. The relative merits of microwave frequency optical heterodyne detection and the alternative Fabry–Pérot interferometry approach are discussed.

Detectivity of shot‐noise‐limited interferometric systems measuring weak gaseous absorption

Three alternative interferometric-calorimetry systems that can be used to detect extremely small concentrations of trace gases are compared for their sensitivity limits due to both shot noise and vibrational noise. The three systems are (1) a pulsed system based on excitation by a repetitively pulsed heating laser; (2) a continuous-wave (cw) acoustic system, based on excitation by a cw heating laser whose intensity is modulated at an acoustic resonance frequency of the absorption cell; and (3) a cw isobaric (optical homodyne) system, based on a cw heating laser whose intensity is modulated at a frequency that is well below the acoustic cutoff of the absorption cell. It is shown that the shot-noiselimited sensitivities of the three systems are comparable, but that the potential presence of vibrational noise strongly favors the pulsed system.

Electrooptic sampling evaluation of 1.5 μm metal-semiconductor-metal photodiodes by soliton compressed semiconductor laser pulses

An electrooptic sampling system using soliton compressed pulses from a gain switched laser diode was assembled and used to characterize an InGaAs metal-semiconductor-metal photodetector at the important optical fiber wavelength of 1.55 microns. The measured rise time of the photodiode pulse was 6.5 ps. The pump and probe pulses had a FWHM of 0.6 ps as measured by autocorrelation. The sampling system rise time was evaluated to be 2.0 ps, yielding the estimated intrinsic photodiode rise time of 2.3 ps. By introducing a differential detection scheme and optical filtering, a shot noise limited sensitivity of 0.6 mV/Sqrt[Hz] was achieved. These results represent the fastest rise time, and most sensitive semiconductor laser based, electrooptic sampling system reported to date. >

Optical low coherence reflectometer using [3/spl times/3] fiber coupler

This letter reports that a high-sensitivity OLCR incorporating a balanced detection scheme can be constructed using only one [3/spl times/3] fiber coupler, instead of a cascade connection of a few [2/spl times/2] fiber couplers. This is because two ports on the input side of the coupler can be used to perform balanced detection. A shot-noise limited sensitivity of -152 dB could be achieved without paying special attention to reducing reflection at the local oscillator port.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Phase-noise and shot-noise limited operations of low coherence optical time domain reflectometry

This letter shows theoretically and experimentally that Fresnel end reflection of the waveguide under test degrades the sensitivity of low coherence optical time domain reflectometry (OTDR). Optical mixing of end reflection and reference light in the OTDR produces the phase noise in proportion to end reflectivity. With the balanced detection technique, the excess photon noise is subdued and the phase noise becomes the dominant source of sensitivity degradation. At 3.2% end reflection and 300 μA mean photocurrent, the noise floor due to the phase noise is −138 dB/Hz. By reducing the fiber end reflection with matching oil, a shot-noise limited sensitivity of −140 dB at a 3 Hz bandwidth has been demonstrated at submillimeter resolution.