Local Oscillator Beam Research Articles

Measurement of smoothed Wigner phase-space distributions for small-angle scattering in a turbid medium.

We study Wigner phase-space distributions W (x, p) in position (x) and momentum (p) for light undergoing multiple small-angle scattering in a turbid medium. Smoothed Wigner phase-space distributions are measured by using a heterodyne technique that achieves position and momentum resolution determined by the width and the diffraction angle of the local oscillator beam. The sample consists of 5.7-micron-radius polystyrene spheres suspended in a water-glycerol mixture. The momentum distribution of the transmitted light is found to contain a ballistic peak, a narrow diffractive pedestal, and a broad background. The narrow diffractive pedestal is found to decay more slowly than the ballistic peak as the concentration of scatterers is increased. The data are in excellent agreement with a simple theoretical model that explains the behavior of the narrow pedestal by including multiple diffractive scattering and treating large-angle scattering as a loss.

Modeling heterodyne efficiency for coherent laser radar in the presence of aberrations

Heterodyne efficiency of a coherent lidar system reflects the matching of phase and amplitude between a local oscillator (LO) beam and received signal beam and is, therefore, an indicator of system performance. One aspect of a lidar system that affects heterodyne efficiency is aberrations present in optical components. A method for including aberrations in the determination of heterodyne efficiency is presented. The effect of aberrations on heterodyne efficiency is demonstrated by including Seidel aberrations in the mixing of two perfectly matched gaussian beams. Results for this case are presented as animations that illustrate the behavior of the mixing as a function of time. Extension of this method to propagation through lidar optical systems is discussed.

Heterodyne detection of enhanced backscatter

Heterodyne detection has been used to measure the enhanced backscattering from a standard target consisting of a plane mirror positioned behind a moving ground-glass disk (assumed to act as a phase screen). A tilt of the plane mirror in combination with spectral filtering of the detector output allows isolation of the double-scattered component of the light signal. When the illuminating laser and the local oscillator beams are correctly mode matched, the intensity of this signal displays the factor-of-2 increase that denotes full coherent enhancement. This full enhancement is preserved for a wide range of mirror to phase-screen spacing, in contrast with earlier observations in which direct-detection techniques were used.

Optical Monitoring and Diagnotics for Biomedical Applications. Selective Detection Capability of Optical Coherence Imaging through Strongly Scattering Media such as Biological Tissues.

The preservation of polarization and coherence of the signal light transmitted through strongly scattering media such as biological tissues have been examined for the feasibility study of optical coherence imaging in tissues. Our experimental results indicate that the multiply scattered and diffused components of transmitted light through scattering medium become not only randomly depolarized, but spatially incoherent, making them insensitive to optical heterodyne detection that is based on the interference of the signal and local oscillator beams.

Eye-safe large field of view homodyne detection using a photorefractive CdTe:V crystal

We demonstrate a self-adapting homodyne detection system working at a wavelength of 1.55 μm. The system uses a photorefractive CdTe:V crystal as the element that combines local oscillator and signal beams. The device spontaneously adapts to slow phase and direction changes in the incoming light. Using a pump beam illumination of 66 mW cm −2 and a signal power of 25 μW, we detect phase modulations corresponding to an optical path variation of a few tenths of nanometers at modulation frequencies higher than a cut-off frequency of 15 Hz. The performance of the system is not affected when the signal light consists of scattered light with an “étendue” of up to 0.1 mm 2 sr, limited only by the lenses we are currently using. We obtain a detection limit of (6.2 ± 0.4) × 10 −7 nm W Hz . This is only about 20 times above the theoretical limit of an ideal interferometric system. We show how to scale these results to the optimized crystals and higher laser intensities necessary to obtain higher sensitivity (< 10 −7 nm W Hz ) and higher cut-off frequencies (> 1 kHz).

External phase-modulation interferometry

We analyze and test a laboratory benchtop version of a compound interferometric phase sensor, a Michelson interferometer whose output is combined coherently with a phase-modulated local oscillator beam tapped off the Michelson input beam. This configuration models a whole class of external-modulation interferometers designed to shift signals, obscured by low-frequency intensity noise of the light source, into a shot-noise-limited region of the photocurrent spectrum. We find analytically that the shot-noise-limited sensitivity achievable with this system is comparable with that obtained by using internal phase modulation, with both schemes suffering (for different reasons) approximately a 22% sensitivity penalty compared with ideal shot-noise-limited direct detection. Experimentally we achieve true shot-noise-limited sensitivity, and we investigate trade-offs necessitated by commonly encountered nonideal features in any external-modulation system. Our analytic model, which specifically accounts for Michelson fringe contrast, electronic receiver noise, phase-modulation depth, and the local oscillator tap-off fraction, is sufficiently accurate to predict the absolute sensitivity of our benchtop instrument to within 0.5 dB.

Interferometers with Internal and External Phase Modulation: Experimental and Analytical Comparison

Optical intensity noise in a light source easily degrades the sensitivity of a shot-noise-limited interferometer which is directly detecting low frequency phase or displacement variations. In this paper we describe and compare two experimental methods in which we use high frequency optical phase modulation to shift low frequency phase signals in an interferometer to a shot noise limited region of the photocurrent spectrum. This phase modulation is applied either within the interferometer arms-internal modulation-or in a local oscillator beam tapped off the main interferometer and coherently recombined with the interferometer output-external modulation. he photocurrent is mixed electronically with the high frequency modulating waveform to extract the signal information free from laser intensity noise. In our experiments, we have been able to detect interferometrically low frequency signals with true shot-noise-limited sensitivity. We find, theoretically and experimentally, that the interferometric sensitivity achievable in each scheme depends critically on non-ideal factors, such as imperfect interferometric fringe contrast and electronic noise in the detectors or amplifiers. This paper examines the relative merits and operating requirements of both modulation schemes in practical interferometers.

A simple multiple beam lo coupler for SIS heterodyne arrays

The construction of SIS heterodyne imaging arrays for submillimetre wave-lengths requires multiple coupling of the local oscillator signal. The quasi-optical analogue of a multiple cross-guide coupler, employing 45° beam-splitters successively stacked along the local oscillator beam, allows for individual adjustment of local oscillator power to each channel. We analyse the coupling as a function of the focal ratio of the incident beams and the number of beam widths off-axis through which the coupler is extended, and describe a simple construction method to realise a compact and effective design.

Dynamic photoelastic analysis by optical heterodyne polarimetry

This paper describes the mapping of the spatiotemporal principal stress distribution evolved with time in an epoxy photoelastic sample. In the optical heterodyne polarimeter exploited, the signal beam of light transmitted by the sample under continuously loaded condition is photomixed with the local oscillator beam of light made up of orthogonal linearly polarized two-frequency components. Every pixel of a MOS video camera used generates a beat photocurrent that possesses the two orthogonal field components of the elliptically polarized signal beam. The spatiotemporal principal stress distributions can be uniquely determined simultaneously and independently from these two orthogonal field components, and are successfully mapped in a time-sequential form. The spatial and temporal resolutions in the maps are 0.18 mm and 2.9 ms, respectively.

Optical phase lock loop with a photorefractive optical beam combiner

Two narrow linewidth Nd:YAG unidirectional nonplanar ring oscillator lasers were electronically phase locked using photorefractive two-wave mixing in an iron-doped indium phosphide (InP:Fe) crystal as an optical phase detector. The phase lock loop phase error signal was derived from a low-amplitude sinusoidal phase modulation signal impressed on the strong local oscillator beam at a frequency large compared to the inverse of the photorefractive-grating formation time. A 60-mW pump laser was phase locked to a 10-nW signal laser beam with an rms phase error less than 0.1 rad. No spatial mode matching, phase conjugate mirror configuration or optical coupling between the two laser cavities were required.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Wave-front matching measurement in coherent CO_2 laser-radar

Wave-front matching of the local oscillator beam and the signal beam is of vital importance in optical heterodyne efficiency. A method for aligning the local oscillator beam and the signal beam is described. Mixing efficiency is improved. One can directly observe the matching procedure on an oscilloscope.

A multichannel interferometer for electron density measurements in COMPASS

A compact seven channel interferometer has been designed and built to measure electron density profiles in the COMPASS (compact assembly) tokamak. Two far-infrared (FIR) laser cavities are optically pumped with a single continuous-wave CO2 laser, generating two similar beams at λ=433 μm with a small, tunable difference frequency (0.5–1.0 MHz). The COMPASS facility incorporates a complex set of poloidal field coils close to the vacuum vessel as well as a versatile set of close coupled ‘‘helical’’ resonant magnetic perturbation windings which severely restrict diagnostic access. As a result a novel approach to the optical circuit has been necessary. Wire grid polarizers are used to divide the laser power equally between channels and to overlay probing and local oscillator beams after the probe beams have made a double pass through the plasma. Gaussian beam-mode optics is used to minimize the size of the optical components.

Heterodyne efficiency for a partially coherent optical signal

Heterodyne efficiency is discussed for a partially coherent signal and a coherent local oscillator beam. Both fileds are assumed to have Gaussian amplitude distributions. An input aperture is used to reduce the background noise. As the coherence of the signal decreases, the efficiency also decreases. However, there is a simple relation between the beam parameters and the detector dimensions to maintain optimum efficiency. The effect of the offset of the signal from the detector axis is also discussed, assuming the Gaussian probability of the deviation. In this case, the optimum parameters that give the maximum efficiency change with the average deviation.

Polarization heterodyne interferometry using another local oscillator beam

This paper describes a novel polarization heterodyne interferometer with another local oscillator beam for the precision measurement of phase retardation. The excess phase deviation from an ideal linear retardation output always exists in the conventional polarization heterodyne interferometer due to the cross-coupling between the orthogonal two-frequency components propagating in a common path. The precision measurement of retardation is limited to such an excess phase deviation. The polarization heterodyne interferometer developed can eliminate almost completely the excess phase deviation. Elimination of the excess phase deviation is demonstrated experimentally.

Optical Heterodyne Polarimeter for Studying Space-and Time-dependent State of Polarization of Light

Abstract This paper describes an optical heterodyne polarimeter with a photodetector array by which the space- and time-dependent state of polarization (SOP) of light can be determined. Since no optical components for polarization control are used, the time response of the polarimeter is free from such components, but is basically limited to the frequency bandwidth of the photodetector array used. The signal and local oscillator beams are coherently photomixed to generate a beat photocurrent at every pixel of the photodetector array. The orthogonal linearly polarized two-frequency components of the local oscillator beam are superimposed with their respective counterpart orthogonally decomposed components of the elliptically polarized signal beam. The generated beat-photocurrent offers the significant physical parameters required for the determination of the space- and time-dependent SOP. The performance principle of the polarimeter is explained and confirmed experimentally.

Optical heterodyne processing method of mechanically induced phase fluctuations of guided modes in a birefringent single-mode fiber

A method is described for carrying out statistical processing of the individual phases of all the modes guided in a perturbed birefringent single-mode fiber. The method is based on optical heterodyne detection processes using a signal beam with orthogonal linearly polarized two-frequency components and a local oscillator beam. All the time-varying phases of the guided modes are taken out separately and processed to offer their power spectra and correlation functions from which the phase fluctuations can be evaluated statistically.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Correlations and Excess Noise in One-port and Balanced Two-port Homodyne Detection

Abstract The fluctuations in the processes of one-port and balanced two-port homodyning are calculated in the operator picture for a weak signal and a strong local oscillator beam where the last is contaminated with Gaussian excess noise. For such a mixture of coherent and chaotic light, we construct a physical model with the help of a linear laser amplifier. To be able to measure phase-dependent quantum fluctuations like squeezing with one-port homodyning, demands a Poisson-like photon distribution of the local oscillator and stringent requirements on its intensity. The Poisson-like photon distribution can also be achieved by a very low reflectivity which is equivalent to strong attenuation. These conditions are relaxed for balanced two-port homodyning but certain less stringent consequences of the excess noise are derived to occur.

Squeezed-light generation with an incoherent pump.

We report generating squeezed light via parametric down conversion of a Q-switched and frequency-doubled multi-longitudinal-mode Nd-doped yttrium-aluminum-garnet laser. A local oscillator with appropriate temporal incoherence is needed to homodyne detect the generated squeezed light. As a first approximation, we derive the local-oscillator beam from the same multimode laser whose harmonic is used to pump the parametric down-conversion process. 0.08\ifmmode\pm\else\textpm\fi{}0.2 dB of squeezing is routinely observed.

Antenna measurements of open-structure Schottky mixers and determination of optical elements for a heterodyne system at 184, 214 and 287 μm

Antenna patterns of open structure Schottky mixers were measured at 184, 214 and 287 μm. For use in a heterodyne detector system the antenna pattern must be matched to the signal beam, for example the astronomical signal of a telescope, and the optically coupled local oscillator beam. The procedure of determining a lens system is described and the successful application in astronomical observations with the Kuiper Airborne Observatory (KAO) is shown.

Polarization Matching Mixer In Coherent Optical Communications Systems

The polarization matching mixer (U.S. Patent No. 4,723,315) is a device that uses polarization beamsplitters to first combine local oscillator and signal light beams into an orthogonal-polarization mixed beam and then split this beam into two matched-polarization mixed beams ideally suited for differential detection. It has potential advantages for coherent communications systems since it offers improvements in optical efficiency through sensing and controlling the polarization state of the incoming signal beam in the receiver. In multiple-wavelength laser diode systems, it may make it possible to increase the number of channels used within a given wavelength band by allowing fine pointing, polarization, and wavelength sensing through diffraction grating dispersers. This paper discusses the polarization matching mixer, how it works, how it compares to the use of amplitude beamsplitters, and how it can be used to reduce signal loss due to degradation in signal polarization. Its potential application to multiple-wavelength laser diode systems is briefly discussed. The paper closes with a discussion of high efficiency polarization beamsplitters.