We propose a variant of the heterodyne holography scheme, which combines the properties of off-axis and phase-shifting holography. This scheme makes it possible filter-off numerically the zero order image alias, and the technical noise of the reference. It is then possible to record and reconstruct holographic images at an extremely low signal level. We show experimentally that the sensitivity of the method is limited only by the quantum nature of photons.Digital holography is a fast-growing research field that has drawn increasing attention [1]. The main advantage of digital holography is that, contrarily to holography with photographic plates [2], the holograms are recorded by a CCD and the image is digitally reconstructed by a computer, avoiding photographic processing [3]. Off-axis holography [4] is the oldest and the simplest configuration adapted to digital holography [5,6,7].In off-axis digital holography, as well as in photographic plates holography, the reference or local oscillator (LO) beam is angularly tilted with respect to the object observation axis. It is then possible to record, with a single hologram, the two quadratures of the object complex field. However, the object field of view is reduced, since one must avoid the overlapping of the image with the conjugate image alias [8].Phase shifting digital holography [9, 10] records several images with different phase for the LO beam. It is then possible to obtain the two quadratures of the field in an on axis configuration even though the conjugate image alias and the true image overlap, because aliases can be removed by taking images differences.In this paper, we propose a digital holography technique that combines off-axis geometry (introduced by Schnars et al. [6]), with the use of a sequence of images obtained with different phase shifts of the LO beam to record the hologram in amplitude and phase (as proposed by Yamaguchi et al. [9]). To get precise phase shift, Le Clerc's et al.[10] heterodyne technique is used. Using a spatial filtering method (Cuche's et al. [8]), the zero order image, and the noise attached to it, is filtered-off numerically. As shown experimentally, this combination of techniques makes it possible to record and reconstruct holographic images at a very low level of signal: 1 photo electron of signal per reconstructed image pixel during a whole sequence of 12 images (≃ 1 s). This corresponds to the ultimate quantum limit.The setup is shown on Fig.1. It is similar to the one used in [10,11]. The main laser L is a Sanyo DL-7140-201 diode laser (λ = 785 nm, 50 mW for 95 mA of current). It is split into an illumination beam (frequency ω L , complex field E L ), and in a LO beam (ω LO , E LO ). The object we want to image is an USAF target in transmission, which is back illuminated. The object signal field E is not shifted in frequency (ω L ). A set of optical attenuators A (grey neutral filter) is used to reduce the illumination. The CCD camera (PCO Pixelfly digital camera: 12 bit, frame rate ω CCD = 12.5 Hz, acquisition time T ...
We present a new and simple method to obtain ultrasound modulated optical tomography images in thick biological tissues with the use of a photorefractive crystal. The technique offers the advantage of spatially adapting the output speckle wavefront by analysing the signal diffracted by the interference pattern between this output field and a reference beam, recorded inside the photorefractive crystal. Averaging out due to random phases of the speckle grains vanishes, and we can use a fast single photodetector to measure the ultrasound modulated optical contrast. This technique offers a promising way to make direct measurements within the decorrelation time scale of living tissues.
Holographic rendering of off-axis intensity digital holograms is discussed. A review of some of the main numerical processing methods, based either on the Fourier transform interpretation of the propagation integral or on its linear system counterpart, is reported. Less common methods such as adjustable magnification reconstruction schemes and Fresnelet decomposition are presented and applied to the digital treatment of off-axis holograms. The influence of experimental parameters on the classical hologram reconstruction methods is assessed, offering guidelines for optimal image rendering regarding the hologram recording conditions
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