Development of a digital astronomical intensity interferometer: laboratory results with thermal light

Abstract

We present measurements of the second-order spatial coherence function of thermal light sources using Hanbury-Brown and Twiss interferometry with a digital correlator. We demonstrate that intensity fluctuations between orthogonal polarizations, or at detector separations greater than the spatial coherence length of the source, are uncorrelated but can be used to reduce systematic noise. The work performed here can readily be applied to existing and future Imaging Air-Cherenkov Telescopes used as star light collectors for stellar intensity interferometry to measure spatial properties of astronomical objects.     

Intensity noise reduction of twin beams by a passive semiconductor medium

Abstract

A passive optical system is proposed to explore the intensity quantum correlation of two twin beams to reduce the photon noise of one of them. It consists of using a semiconductor medium inside an optical cavity, which behaves as a nonlinear medium presenting a crossed Kerr effect. The intensity fluctuations of one beam modify the resonance condition of the cavity for the other beam and therefore its intensity. The medium is described microscopically within the two-level atom model. It is shown that, under typical experimental conditions, this system may produce noise reduction.     

Quantum noise of laser diodes

Abstract

Single-mode laser theory for semiconductor lasers predicts sub-Poissonian light generation for a laser quietly driven far above threshold. Experiments have shown however that only few laser diodes exhibits such reduced intensity noise. We present a review of different mechanisms that have been proposed to explain the excess noise observed in semiconductor lasers, including imperfect anticorrelations in multimode lasers, and Petermann excess noise factor in single-mode lasers.     

Method for Birefringence Measurements with Correction of Errors Due to Multiple Reflection in Anisotropic Plates

Abstract

Birefringence measurements of anisotropic plates using coherent light are affected by errors due to the multiple reflections in the investigated plate. An evaluation method for measuring birefringence is presented, where the intensity distributions of two light beams are used for the correction of these errors. These beams arise from a polarizing beam splitter, which is the analyser of the polarization system. In this paper, a theoretical treatment is given which shows that this method results in more accurate values than the usual methods, especially for highly refractive materials with a small birefringence.     

Broadband Intensity Stabilization of a Diode-pumped Monolithic Miniature Nd: YAG Ring Laser

Abstract

We have built an electronic servo system to reduce the intensity noise in a diode-pumped single-mode monolithic Nd: YAG ring laser with a maximum output power of 350 mW. The servo used two stabilization systems operating simultaneously over two different frequency ranges, each of which detected a fraction of the laser output, amplified and phase shifted the detected signals and used the resultant signals to control the current to the laser diode pump. We achieved a reduction in intensity noise of up to a factor of 30 for frequencies up to about 1 kHz, with some noise reduction observable up to about 30 kHz. At frequencies between around 30 and 200 kHz the intensity noise was increased by a factor of approximately 1·4. In the frequency range 200 to 300 kHz, around the relaxation oscillation frequency of 260 kHz, the intensity noise was suppressed, the reduction factor being approximately 14 at 260 kHz.     

Excess noise of an Er/Yb fiber superfluorescent radiation source

A study is made of the noise in the output intensity of a high-power superfluorescent fiber radiation source generating 20 mW in the 1.55 μm region of the spectrum. The source is constructed in an all-fiber technology from a quartz fiber activated with ytterbium and erbium. It is found that the spectral density of the noise in the frequency range 100–500 kHz is uniform and corresponds to the excess noise of “thermal” sources. It is shown that the noise in the output intensity of the source can be compensated (decreased by an order of magnitude) in a two-channel scheme of photodetection, where it approaches a level close to the photon shot noise. Pis’ma Zh. Tekh. Fiz. 23, 1–6 (December 12, 1997)     

DL-Methionine and Thiourea as Photostabilizers for Saturated Solution of Potassium Iodide

Abstract

Saturated solution of potassium iodide suffers from coloration on exposure to light due to the liberation of iodine. DL-methionine and thiourea in a concentration range from 5 to 25 mg% demonstrated an outstanding photostabilizing action for potassium iodide solution when stored for two hours under high intensity fluorescent light. Decrease of concentration of either DL-methionine or thiourea below 25 mg% appeared to have no effect on their stabilizing action during this short-term storage. Saturated solution of potassium iodide containing 25 mg% of DL-methionine remained colorless after exposure to light for 118 hours, whereas solution containing 25 mg% of thiourea developed only a faint yellow color. The photostability of potassium iodide solution was monitored by measuring the increase in absorbance at 353 nm.     

Atomic motion in a standing-wave squeezed light field

Abstract

We investigate atomic motion in the standing-wave field of a cavity driven by coherent light plus broad-band-squeezed vacuum. We assume the bad-cavity limit and adiabatically eliminate the cavity mode, deriving a master equation for the atomic variables alone. From this master equation we study the (one-dimensional) atomic motion both in the semiclassical approximation and using quantum Monte Carlo wavefunction simulations. The light force and momentum diffusion are shown to be strongly dependent on the relative phase between the coherent and squeezed fields and, using a dressedstate analysis, we identify observable effects unique to the reduced quantum noise that characterizes squeezed-vacuum light.     

Experimental Progress in Intracavity Generation of Squeezed Light Using Resonant Atomic Nonlinearities

Abstract

Quantum-noise reduction to levels nearly 1 dB below the standard quantum limit are reported for light frequencies near an atomic-sodium resonance. This squeezed light is generated in an optical confocal cavity using four-wave mixing due to the atomic-sodium resonance nonlinearity. Squeezed light exits the cavity through a partially reflecting mirror and is detected outside the cavity with a balanced homodyne detector. Experimental details are described for minimizing losses in the atomic beam and detection apparatus. Frequency jitter due to the pump laser also plays a key role in the limits for noise reduction. A wideband phenomenological model is used to analyse the results. Good agreement between this model and the experimental results is obtained. A full quantum model also agrees with the results and predicts even larger squeezing at higher pump intensities. Prospects for achieving this larger-noise-reduction regime are good.     

Squeezed Light

Abstract

In this paper we review the current state of progress in research on squeezed light. We discuss the basic theory of squeezing and the nature of quantum noise in optical fields. We examine various atomic sources of squeezed light and discuss phase-sensitive detectors of quantum noise including homodyne and heterodyne detectors. Various successful nonlinear optical methods for generating squeezed light are reviewed. We conclude with a discussion of the possible applications of squeezed light.     

Interference Figures of Biaxial Crystal Plates (Optic Axes Figures)

Abstract

The intensity distribution in the interference field produced by a plane parallel biaxial crystal plate between crossed polarizers and transmitted by convergent light (optic axes figures) is presented as an explicit formula. This has been achieved by a suitable procedure of approximation which consists of treating the crystal as an isotropic medium and using Fresnel's formulae for the transmitted wave amplitudes at both surfaces. The anisotropy of the crystal is taken into account only in the determination of the orthogonal vibration directions and of the refractive indices pertaining to the two waves originating from the double refraction at the first surface of the crystal. The interference figures calculated by this method are in close agreement with the exact results from Rath and Pohl.     

Dichromated Gelatin Reflection Gratings

Abstract

Reflection gratings were recorded in dichromated gelatin by using a non-symmetrical recording geometry. In addition to the primary grating, secondary gratings were also formed, as is generally known. It is found that the secondary gratings assist the structure stability during development in the exposure energy range (50–100) mJ cm^?2. The structure containing these secondary gratings is more rigid than those without and the reflection grating period is more stable during the development process in this exposure energy range, which is most suitable for the transmission grating formation. Secondary gratings also cause unwanted noise, but the noise level is low and the improved stability outweighs the extra noise.     

A Characterization of Light Scattering by Small Dielectric Spheres

Abstract

While using the standard Mie formulae to calculate the intensity distribution of light scattered by a small dielectric sphere, we have noticed some new qualitative results which suggest a boundary between a scattering regime qualitatively similar to the well known Rayleigh scattering distribution and that observed for rather larger particles. This may be of practical use when considering the position of photodetectors in light scattering experiments, for example particle velocimetry. The reason for such a transition has been discovered, using an unusual approximation in which the diameter of a sphere is made very small and its refractive index very large, with their product and the wavelength held constant.     

Image deblurring based on ForIcM: Fourier shrinkage and incomplete measurements

Abstract

In this paper, we propose a new deblurring algorithm, which is based on image reconstruction from incomplete measurements in Fourier domain. Our algorithm has two steps. Firstly, an initial estimator is obtained using Fourier regularised inverse operator. Secondly, parts of the estimator’s Fourier coefficients are saved, and the others are removed to suppress noise energy, and then the remaining coefficients are used to recover image based on the sparse constraints. This image reconstruction problem is an optimisation problem which is solved by a fast algorithm named split Bregman iteration. Our algorithm combines two different regularisation strategies efficiently by applying a selection matrix. The tests using images with different blurs and noise produce good results. The experiment shows that our method gives better performance than many other competitive deblurring methods.     

Sub-poissonian Light and Photocurrent Shot-noise Suppression in Closed Optoelectronic Loop

Abstract

Photocurrent shot-noise suppression is observed in closed optoelectronic loop below the limit (1 ? η) of standard shot-noise level (SNL) imposed by conventional theory of photodetector with quantum efficiency η. Experimental results on photocurrent noise are presented for different values of feedback strength and quantum efficiency η of in-loop photodiode. It is shown that at fixed feedback strength the suppression factor does not depend on η. Minimum noise spectral density is observed below the SNL by a factor 17·8 (? 12·5 dB). Such large suppression cannot be explained in terms of an ordinary sub-Poissonian light state in the loop. We suggest the concept of an anticorrelation light state, where the light in the loop is described as a non-stationary state with Poissonian quantum fluctuations, and superimposed anticorrelated classical fluctuations. While the degree of noise reduction in the photocurrent can be arbitrary large, the degree of anticorrelation between the classical and quantum fluctuations in the light beam remains limited by the quantum efficiency of the feedback photodiode. Calculations based on this concept are in a good agreement with experimental data on super-shot-noise power, obtained for the beam extracted from the loop by a beam splitter.     

Interplay of CaF2

Abstract

The radiative decay of surface plasmon polaritons has been investigated in an attempt to characterize the surface roughness of Ag films prepared under different conditions. The polaritons were excited by the method of attenuated total reflection of light. The films were deposited on the face of a 60° BK-7 glass prism at a rate that was deliberately fixed in two different ranges (centred on 0·1 and 10 nms^?1) and in some cases a CaF₂ underlayer was used to roughen the film surfaces. The intensity of the scattered light emitted from the opposite face of the films was measured as a function of direction for each using the same sensitivity scale and was correlated with the preparation of the film. It was found that on nominally smooth substrates fast-deposited thinner films give out more light and are deduced to have greater short wavelength (300–600 nm) roughness amplitude. There is also evidence for long wavelenth (7μm) periodic roughness due to the prism substrate itself. On CaF₂ roughened surfaces the light output from the films is further increased and the peak intensity is backward directed with respect to the exciting laser beam direction. Here roughness on a lateral scale of 350 nm is responsible. Also, elastic scattering of surface plasmon polaritons at grain boundaries reduces the light output from fast deposited, small grain, films on CaF₂ roughened surfaces. Overall, a consistent picture of roughness induced radiative polariton decay emerges for all cases studied.     

Interaction of Counter-propagating Elliptically Polarized Light Waves in Isotropic Nonlinear Media

Abstract

The nonlinear interaction of elliptically polarized counter-propagating waves in an isotropic transparent medium is analysed. Self-induced changes in the polarization of the counter-propagating light waves, are described. Using these results, a new type of modulator giving self-induced rotation of the polarization ellipse (SIRPE) is studied. The transmission of counter-propagating waves in the SIRPE modulator is shown to exhibit saturation with increase in light intensity and to be a multiple-valued function of the light intensity.     

Use of AL307 light-emitting diodes as photodetectors for diagnostics of femtosecond light pulses

A nonlinear electrical response has been observed for the first time from AL307 light-emitting diodes exposed to femtosecond light pulses. When used in an unconventional fashion as unbiased photodiodes, these AL307 light-emitting diodes give an electrical response proportional to the square of the recorded radiation intensity of ultrashort light pulses. Autocorrelation functions are given for femtosecond pulses obtained using AL307 light-emitting diodes in the autocorrelator instead of the conventional photodetector and nonlinear crystal system. Pis’ma Zh. Tekh. Fiz. 24, 62–65 (January 12, 1998)     

Dynamics during holographic exposure in photopolymers for single and multiplexed gratings

Abstract

Under the assumption that at low recording intensity the grating formation process is much slower than the free monomer diffusion, we present a model that describes the dynamics of the holographic grating formation in photopolymers. The model indicates how the beam intensity modulation influences the saturation value of the grating modulation, while the average recording intensity is related in a nonlinear fashion to the process time constant. An extension of this model describes and predicts the recording of a single grating in the presence of an additional mutually incoherent light beam, and the simultaneous recording of angularly mutliplexed gratings. The model is validated by comparing with experimental results obtained using DuPont HRF-150-38 photopolymer.     

Imaging of Extended Objects Using an Off-axis Holographic Lens

Abstract

The aim of this paper is to evaluate the performance of an off-axis holographic lens afflicted with third-order spherical aberration and coma using a numerical ray-tracing procedure. The light intensity distribution in the aberration spot observed at the Gaussian image plane has been evaluated for different amounts of coma and for a fixed value of spherical aberration. The images of both an extended coherent and an incoherent edge as well as bar objects have been computed and the results compared with the aberration-free case. All the results are illustrated in graphical form.