Quantum-Noise Limited Distance Resolution of Optical Range Imaging Techniques

The most common optical distance imaging methods (triangulation, interferometry and time-of-flight ranging) can all be described in a unified way as linear shift-invariant systems in which the determination of distance corresponds to the measurement of a spatial or temporal phase. Since the ultimate precision of such a phase measurement is limited by quantum noise of the involved photons or photocharges, the eventual distance resolution of the three optical ranging methods depends in the same way on quantum noise. Evidence from literature supports our basic assertion that photon and photocharge numbers have a Poisson distribution under most experimental conditions. This allows us to derive our main result that the quantum-noise limited distance resolution of the three optical ranging methods is proportional to the inverse of the signal’s modulation amplitude times the square root of the background level. The equation for the precision of the three optical distance measurement techniques contains the method’s experimental parameters in a single factor, from which the optimum distance range of the three techniques can easily be deduced: 1 nm–1 m for interferometry, 1 µm–10 m for triangulation, > 0.1 mfor time-of-flight ranging, if visible or near infrared light is used.

Quantum optomechanics at room temperature

Hendrik Schütz

Why are classical theories often sufficient to describe the physics of our world even though everything around us is entirely composed of microscopic quantum systems? The boundary between these two fundamentally dissimilar theories remains an unsolved problem in modern physics. Position measurements of small objects allow us to probe the area where the classical approximation breaks down. In quantum mechanics, Heisenbergâs uncertainty principle dictates that any measurement of the position must be accompanied by measurement induced back-action---in this case manifested as an uncertainty in the momentum. In recent years, cavity optomechanics has become a powerful tool to perform precise position measurements and investigate their fundamental limitations. The utilization of optical micro-cavities greatly enhances the interaction between light and state-of-the-art nanomechanical oscillators. Therefore, quantum mechanical phenomena have been successfully observed in systems far beyond the microscopic world. In such a cavity optomechanical system, the fluctuations in the position of the oscillator are transduced onto the phase of the light, while fluctuations in the amplitude of the light disturb the momentum of the oscillator during the measurement. As a consequence, correlations are established between the amplitude and phase quadrature of the probe light. However, so far, observation of quantum effects has been limited exclusively to cryogenic experiments, and access to the quantum regime at room temperature has remained an elusive goal because the overwhelming amount of thermal motion masks the weak quantum effects. This thesis describes the engineering of a high-performance cavity optomechanical device and presents experimental results showing, for the first time, the broadband effects of quantum back-action at room temperature. The device strongly couples mechanical and optical modes of exceptionally high quality factors to provide a measurement sensitivity

\sim\!10^4

times below the requirement to resolve the zero-point fluctuations of the mechanical oscillator. The quantum back-action is then observed through the correlations created between the probe light and the motion of the nanomechanical oscillator. A so-called âvariational measurementâ, which detects the transmitted light in a homodyne detector tuned close to the amplitude quadrature, resolves the quantum noise due to these correlations at the level of 10% of the thermal noise over more than an octave of Fourier frequencies around mechanical resonance. Moreover, building on this result, an additional experiment demonstrates the ability to achieve quantum enhanced metrology. In this case, the generated quantum correlations are used to cancel quantum noise in the measurement record, which then leads to an improved relative signal-to-noise ratio in measurements of an external force. In conclusion, the successful observation of broadband quantum behavior on a macroscopic object at room temperature is an important milestone in the field of cavity optomechanics. Specifically, this result heralds the rise of optomechanical systems as a platform for quantum physics at room temperature and shows promise for generation of ponderomotive squeezing in room-temperature interferometers.

EPFL2017

Distributed sensing using polarization sensitive optical low-coherence reflectometry and fiber gratings

Dragan Coric

Optical low coherence reflectometry (OLCR) is non-destructive interferometric technique that allows measuring amplitude and phase of the light reflected from the device under test. OLCR is a powerful tool for the characterization of the various optical devices such as fiber gratings and optical waveguides. This thesis work had explored possibilities to combine the OLCR technique with fiber gratings to perform distributed strain and temperature measurements, and to develop novel sensing techniques as an alternative to classical spectral methods. Special attention is devoted to polarization sensitive measurements and techniques, and OLCR techniques that use only amplitude information, as an alternative to more complicated phase sensitive measurements. Using a polarization sensitive system two methods for the measurement of local birefringence of fiber Bragg gratings (FBG) using OLCR were developed. The first technique uses oscillations in the OLCR amplitude signal to directly obtain the beat length and the birefringence. The second method is based on the measurement of the OLCR phase and inverse scattering algorithm. Both methods were compared with birefringence obtained from spectral measurements, and very good agreement was obtained. The indirect method, based on local Bragg grating determination requires two independent measurements and mathematical reconstruction, but provides a very high spatial resolution of ≈ 25 µm. The grating length limits the minimum measurable birefringence in the direct method, but a good sensitivity of 4×10-6 was obtained, which corresponds to a Bragg wavelength shift of 4 pm. The spatial resolution is in the millimeter range in this case. The novel methods were successfully applied for the distributed measurement of birefringence of FBG under diametric load. New types of tunable devices and sensors – fiber Bragg gratings written in high attenuation fibers (HAF) were developed. Active tuning by heating was achieved by optical pumping of a pure single mode fiber without any mechanical or electrical parts, or deposited light absorption coatings. Tuning can be controlled by the applied pump power, the position of the grating, the HAF attenuation level, and the pumping configuration. The proposed design assures a high repeatability and long lifetime. Using OLCR these devices were successfully applied to measure the liquid level with a spatial resolution of 100 micrometers. Liquid level measurements that use both phase and amplitude, or only amplitude were demonstrated. The same principles could be easily applied to design other fiber grating devices (long-period, tilted etc), and sensors based on hot-wire anemometry, like flow or vacuum sensors.

EPFL2007

Single-Photon Synchronous Detection

Edoardo Charbon, Claudio Favi, Ties Jan Henderikus Kluter, Frédéric Monnier, Cristiano Niclass

Phase and intensity of light are detected simultaneously using a fully digital imaging technique: single-photon synchronous detection. This approach has been theoretically and experimentally investigated in this paper. We designed a fully integrated camera implementing the new technique that was fabricated in a 0.35 mu m CMOS technology. The camera demonstrator features a modulated light source, so as to independently capture the time-of-flight of the photons reflected by a target, thereby reconstructing a depth map of the scene. The camera also enables image enhancement of 2D scenes when used in passive mode, where differential maps of the reflection patterns are the basis for advanced image processing algorithms. Extensive testing has shown the suitability of the technique and confirmed phase accuracy predictions. Experimental results showed that the proposed rangefinder method is effective. Distance measurement performance was characterized with a maximum nonlinearity error lower than 12 cm within a range of a few meters. In the same range, the maximum repeatability error was 3.8 cm.

2009