Heterodyne Holography Research Articles

Digital holography with ultimate sensitivity

We propose a variant of the heterodyne holography scheme that combines the properties of off-axis and phase-shifting holography. This scheme makes it possible to 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.

Playing with speckle in acousto-optic imaging

Using light to image in-depth structures and to quantify them in terms of optical (absorption, scattering) properties is a goal for physicists. We will recall the approaches which overcome the strong loss of the photons directional memory in scattering media and their limitations in terms of depth and resolution. Then we will consider hybrid methods which couple acoustics and optics to reach the optical contrast with the acoustic resolution: opto-acoustics, where a locally absorbed laser pulse generates an acoustic signal whose position and depth can be localized by an array of piezoelectric detectors, and acousto-optics (A.O.), where an ultrasonic beam modulates the speckle. We will focus on the principle of A.O. signal generation for tagging spatially selected photons and describe various approaches based on numerical heterodyne holography or heterodyne holography using a photo-refractive beam splitter. To get enough signals through thick tissues (e.g., breast) in a short time we use these techniques at the shot noise limit. Finally we show images revealing optical contrasts in artificial structures or in tissues.

Synthetic-aperture experiment in the visible with on-axis digital heterodyne holography

We have developed a new on-axis digital holographic technique, heterodyne holography. The resolution of this technique is limited mainly by the amount of data recorded on two-dimensional photodetectors, i.e., the number of pixels and their size. We demonstrate that it is possible to increase the resolution linearly with the amount of recorded data by aperture synthesis as done in the radar technique but with an optical holographic field.

Numerical heterodyne holography with two-dimensional photodetector arrays

We present an original heterodyne holography method for digital holography that relies on two-dimensional heterodyne detection to record the phase and the amplitude of a field. The technique has been tested on objects as much as 13 mm in size. Consistency checks were performed, and high-resolution images were computed. We show the requirement for a spatial filter to select properly sampled near-axis photons. Heterodyne holography is superior to off-axis digital holography for both field of view and resolution.

Imaging properties of scanning holographic microscopy

Scanning heterodyne holography is an alternative way of capturing three-dimensional information on a scattering or fluorescent object. We analyze the properties of the images obtained by this novel imaging process. We describe the possibility of varying the coherence of the system from a process linear in amplitude to a process linear in intensity by changing the detection mode. We illustrate numerically the properties of the three-dimensional point-spread function of the system and compare it with that of a conventional imaging system with equal numerical aperture. We describe how it is possible, by an appropriate choice of the reconstruction algorithm, to obtain an ideal transfer function equal to unity up to the cutoff frequency, even in the presence of aberrations. Some practical implementation issues are also discussed.

Three-dimensional location of fluorescent inhomogeneities in turbid media by scanning heterodyne holography

We describe and illustrate experimentally a method aimed at the three-dimensional (3-D) imaging of fluorescent inhomogeneities embedded in a turbid medium. The method utilizes incoherent scanning holography to capture 3-D information in a single two-dimensional scan and phase-sensitive heterodyne detection to reject multiply scattered light and to produce a single-sideband holographic record. The 3-D imaging capability of the method is illustrated by an example.

Effect of scintillation noise in heterodyne speckle photogrammetry

Heterodyne interferometry provides an accurate method of measuring specklegram halo fringes for strain analysis; however, when fringe contrast is less than unity, the uncorrelated portions of the fields generate a background scintillation that is a random function of specklegram and detector positions. In practice, this scintillation effect establishes the actual limits on the utility of heterodyne phase measurements, regardless of the accuracy with which they can be made. This effect has been analyzed for heterodyne holography, and these results are applied here to the derivation of a fundamental relationship between image resolution, gage length, strain resolution, and parameters related to fringe contrast and the readout system.

Heterodyne interferometry and holography for vibration measurements

Two frequency or heterodyne interferometry transforms the optical interference phase into the phase of an electronically detectable beat‐frequency signal. This allows to use powerful signal processing for increased sensitivity far below the optical wavelength λ and high temporal resolution, especially for vibration analysis. The principle and the properties of this heterodyne technique will be illustrated by presenting laser interferometry for displacement and velocity measurements. With an extended version [R. Dändliker and J.‐F. Willemin, Opt. Lett. 6, 165–167 (1981)] amplitude and phase of in‐plane and out‐of‐plane microvibrations (amplitudes 1 nm, spatial resolution 30 μm) at frequencies from 5 kHz to 5 MHz can be analyzed in real time. Experimental results for piezoelectric ultrasound transducers will be presented. Two‐reference‐beam holographic interferometry allows to use the heterodyne technique for accurate fringe evaluation and interpolation (λ/1000), since the two object fields are stored and accessible independently by the corresponding reference beam [R. Dändliker, Progress in Optics (North‐Holland, Amsterdam, 1980), Vol. 17, pp. 1–84.]. For vibration analysis, however, the object has to be recorded holographically by two or more coherent light pulses, synchronized to the object movement, using pulsed lasers or stroboscopic light modulation. Experimental results of deformation measurements by heterodyne holographic interferometry will be presented. The strength and weakness of this technique applied to vibration analysis will be discussed in particular.