Measuring the coherence function of continuous-wave lasers by a photorefractive grating method

We demonstrate a simple, adjustment-insensitive technique for measuring the temporal coherence function of cw multimode and monomode semiconductor lasers, using two-beam coupling in photorefractive InP and CdTe crystals. The emission spectra of the diodes are measured independently. The coherence functions are also calculated from these spectra and agree with the photorefractive measurements. Coupling of two partially coherent waves in low-speed photorefractive media is described theoretically. The range of the experimental parameters in which the method of coherence measurement is correct is given.     

High-speed 3D imaging using photorefractive holography with novel low-coherence interferometers

The paper reports recent progress in developing high speed 3D imaging systems based on low coherence photorefractive holography with high-speed depth-sectioned imaging at 476 frames per second. It is demonstrated that photorefractive holography can utilize a wide variety of sources of differing spatial and temporal coherence, including a novel all-solid-state broadband laser. Also presented is a novel real-time optical sectioning technique based on structured illumination combined with photorefractive holography that provides real-time optical sectioning when imaging with reflected light or with fluorescence.     

The ABCs of Space-time-coherence Recording in Holography

Abstract

After placing prior work on time-gated holography and coherence-gated holography in a unified perspective, we broaden both with a new coherence theorem which introduces an assymetry between them and allows otherwise-impossible holograms to be formed.     

Fourier-domain holographic optical coherence imaging of tumor spheroids and mouse eye

Fourier-domain holography (FDH) has several advantages over image-domain holography for optical coherence imaging of tissue. Writing the hologram in the Fourier plane significantly reduces background arising from reference light scattered from the photorefractive holographic film. The ability to use FDH is enhanced by the use of a diffuse target, such as scattering tissue, rather than specular targets, because the broader angular distribution from diffuse targets is transformed into a relatively uniform distribution in the Fourier plane. We demonstrate significantly improved performance for Fourier-domain optical coherence imaging on rat osteogenic sarcoma tumor spheroids and mouse eye. The sensitivity is documented at -95 dB.     

Spatial phase information transmission through an optical fiber by coherence function synthesis

Transmission of one-dimensional spatial phase information by low-coherence light through a single-mode optical fiber is experimentally demonstrated by use of space-time conversion at a 4-f Fourier coherence function shaper and time-space conversion with spectral holography. The dispersion during the fiber propagation can be automatically compensated for with spectral holography. However, space-time coupling caused by the transmitter limits the capacity of information transmittable with one coherence function shaping. A significant advantage in the space-time-space conversion with low-coherence light is that an infinite number of signal channels can be multiplexed with a newly invented delay-time division scheme, which can extend this analog transmission to two-dimensional spatial phase patterns.     

Energy transfer induced by femtosecond two-beam coupling in two Kerr liquids

Femtosecond two-beam coupling is demonstrated in N,N′-dimethylformamide and dimethyl sulfoxide by using a time-resolved pump-probe system at a wavelength of 800 nm. As the temporal delay between pulses is varied within the coherence time, transient energy transfer from one pulse to another occurs. Because of the asymmetry of the normalized transmittance, two-photon absorption is considered in the two liquids. A theory based on two-beam coupling in perpendicular linear polarization is used to interpret the experimental results. Good agreement is obtained between the theory and experiment. Moreover, by introducing a valuable variable, the ratio of the relaxation times of the two Kerr liquids is directly characterized.     

Multilocalization and the van Cittert-Zernike theorem. 1. Theory

The complex degree of coherence and the resulting van Cittert-Zernike theorem are employed to analyze the exit of an arbitrary amplitude-division interferometer with two-beam interferences. Considering that the source is a periodic array of spatially incoherent slits and assuming negligible equivalent aberrations and no vignetting, an expression for the complex degree of coherence as a function of the position of an exit point is derived. Formulas for the location, fringe spacing, and fringe localization depth of the multilocalized fringes are given.     

Two-beam interference experiments in the frequencydomain to measure the complex degree of spectral coherence

Abstract

Correlation-induced spectral changes in two-beam interference experiments in the space-frequency domain are used to determine the amplitude and phase of the complex degree of spectral coherence. The spectral modifications are observed either as a shift in the peak wavelength or as sinusoidal modulations within the bandwidth of the white light spectrum due to the complex degree of spectral coherence of the secondary source and the path difference between the interfering beams. These correlation-induced spectral changes were analysed using a theoretical model to establish the behaviour of the real and imaginary parts of the complex degree of spectral coherence over the entire visible region of the spectrum.     

一种具有自取向极化效应的有机光折变体系

在用真空热压-饱和蒸汽扩散法制备的PVK-PBA/DR1/TNF有机聚合物体系中,观察到了无外电场作用下的较强的光折变效应.通过对样品的双波耦合、四波混频实验中,改变电场强度、电场方向或不加外电场而只改变光强,观察到的衍射效率的变化情况以及测量样品在单纯光照诱导下的光学非线性系数等一系列实验,可以认为光照时在该有机聚合物体系中由DR1分子两端的推、吸电子基团与光生分离的正负电荷相互作用而引发了"自极化",从而产生周期性空间电场,最终产生自增强光折变效应.另外进行了图象存储实验,获得了理想的结果.本研究对发展零外场聚合物光折变体系有重要的参考意义.     

Some Practical Uses of Laser Beam Photography in Engineering

Some applications of laser beams are discussed which take advantage of both the coherence properties and the intense collimated beam. The beam forms an excellent pointer and, combining coherence, enables interferometry to be carried out under difficult circumstances. Full advantage of laser coherence is taken in holography enabling rapid visualisation of surface strain and vibration. In these laser applications, the information is recorded photographically. Common emui- sions will record a laser beam track, but holograms are coded recordings on high resolution emulsions, to construct a three dimensional image in space.     

Geometrical Optics

A previous paper [3] demonstrated that, in principle, holography is a promising technique for use in the measurement of coherence. Its usefulness in practice for obtaining reasonable quantities of accurate data had still to be established. In this paper some of the considerations which lead to an effective design for a holographic arrangement for measuring coherence are discussed. Of principal importance among these considerations are the total amount and accuracy of the information that can be obtained from the hologram. Examples are given of suitable holographic arrangements, and procedures are described to enable the accumulation of reasonably accurate coherence data. Graphs which plot the measured distribution of coherence for two pulsed lasers are shown.     

Incoherent Soft X-ray Holography

Abstract

A long wavelength extension of coded aperture imaging is considered in the context of a possible application in soft X-ray microscopy. It is shown that zone plate coded aperture imaging extends naturally to longer wavelength applications and this is demonstrated experimentally. For a given source of incoherent radiation and exposure time, it is shown that incoherent techniques produce images with a substantially greater signal-to-noise ratio than coherent holography. Incoherent holography also has substantially reduced temporal coherence requirements. The practical implementation of the technique is briefly discussed.     

Electronic channel fringe holography for depth and delay measurements

Electronic spectral holography in the form developed by Shih [Ph.D. dissertation, University Microfilms, Ann Arbor, Mich. (1995)] is adapted to various applications, including optical coherence tomography in scattering media, contouring of surfaces, and optical fiber mode examination.     

Single-cavity, multiline laser system with large coherence length

We present a new approach of generating the several wavelengths required for color holography with coherence lengths in the range of several meters. Our proposed laser system consists of an argon-ion laser, which is equipped with broadband optics. Its main lasing lines include 457, 488, and 514 nm. Sufficient coherence length is achieved by means of an intracavity etalon. We report single-frequency operation at several competing wavelengths and the successful recording of multicolor holograms with the described laser system.     

Multiplexed Fibre-optic Sensors Using Ring Interferometers

Abstract

We describe a number of multiplexing techniques which may be used for fibre-optic sensors in which the sensing element is a ring resonator. The basic optical unit comprises a sensing ring resonator connected to a second interrogating ring resonator of nearly equal length, and illuminated by a source of short coherence length. A number of such units may be combined to form an array, using coherence multiplexing techniques. Using such an arrangement, the high finesse of the ring resonator may be exploited to yield a sensor of increased sensitivity over that obtainable with a two-beam interferometer. When a high-coherence source is used, then multiplexing may be accomplished using frequency-division techniques. However, in this case, the resonators must have low finesse, in order to avoid cross-talk. We also show that when a source of moderate coherence length is used, it is possible to use both frequency division and coherence multiplexing simultaneously, with the potential of realizing a large sensor array.     

Holography with Light Beams of Finite Spectral Width

The effects of spectral coherence on holographic images have been analysed in terms of the Fresnel-transform formulation of the process of holography. In this paper the case of very rough object or diffuse illumination has not been considered. The results have been obtained as contrast modulation in the image of a spatial frequency component in the object. The effect of polychromaticity in the recording beam is seen to be more severe than that due to the polychromaticity in the reconstruction beam. Fourier-transform holography seems to yield a better image. With the lensless Fourier-transform hologram recorded on monochromatic light, white light reconstruction seems to be possible if the illuminating beam is suitably dispersed before it falls on the hologram.     

Fourier-domain holography in photorefractive quantum-well films

Fourier-domain holography (FDH) is investigated as a candidate for holographic optical coherence imaging to produce real-time images of structure inside living tissue and turbid media. The effects of spatial filtering, the background intensity distributions, and the role of background noise in determining dynamic range are evaluated for both FDH and image-domain holography (IDH). The grating washout effect in FDH (edge enhancement) is removed by use of a vibrating diffuser that consequently improves the image quality. By comparing holographic images and background images of FDH and IDH we show that FDH provides a higher dynamic range and a higher image quality than IDH for this specific application of imaging diffuse volumetric objects.     

Suppression of phase ambiguity in digital holography by using partial coherence or specimen rotation

In this paper we present two approaches for extracting the surface profile as well as obtaining 3D imaging of near field objects by usage of partial coherence and digital holography. In the first approach a light source with given temporal partial coherence is used to illuminate a near field object. The reflected light is interfered with the reference source. By computing the local contrast of the generated fringes one may estimate the 3D topography and the profile of the object. This approach extracts the 3D information from a single image, and its accuracy does not depend on triangulation angle like in fringe projection methods. The second approach is tomography based. There we illuminate the object from several slightly different angles, and for each we compute the wrapped phase using digital holography techniques. Combining the wrapped phase estimation from several points of projection allows calculating the unwrapped phase and therefore the true profile of even a phase-only object. Increasing the number of points of view decreases the relative error of the estimated profile.     

Experimental geometry for simultaneous beam characterization and sample imaging allowing for pink beam Fourier transform holography or coherent diffractive imaging

One consequence of the self-amplified stimulated emission process used to generate x rays in free electron lasers (FELs) is the intrinsic shot-to-shot variance in the wavelength and temporal coherence. In order to optimize the results from diffractive imaging experiments at FEL sources, it will be advantageous to acquire a means of collecting coherence and spectral information simultaneously with the diffraction pattern from the sample we wish to study. We present a holographic mask geometry, including a grating structure, which can be used to extract both temporal and spatial coherence information alongside the sample scatter from each individual FEL shot and also allows for the real space reconstruction of the sample using either Fourier transform holography or iterative phase retrieval.     

Signal-to-noise ratio study of full-field fourier-domain optical coherence tomography

We report a new approach in optical coherence tomography (OCT) called full-field Fourier-domain OCT (3F-OCT). A three-dimensional image of a sample is obtained by digital reconstruction of a three-dimensional data cube, acquired with a Fourier holography recording system, illuminated with a swept source. We present a theoretical and experimental study of the signal-to-noise ratio of the 3F-OCT approach versus serial image acquisition (flying-spot OCT) approach.