Holographic Acquisition Research Articles

Snap-shot topography measurement via dual-VCSEL and dual wavelength digital holographic interferometry

In this paper, we propose a dual-wavelength digital holographic interferometry method based on a compact dual vertical-cavity surface-emitting laser (VCSEL) source. The source simultaneously emits light from two highly stabilized coherent light sources with slightly different wavelengths. A highly stabilized and adjustable current source enables the application of digital holographic dual-wavelength techniques to measure the shape of an object with height steps of a few millimeters. The wavelength drift over 12 h over the entire measurement range, which was evaluated using a wavemeter, was smaller than 1 pm. In addition to the low measurement uncertainty at large height jumps, the dual-wavelength digital holographic system distinguishes itself by its robustness to environmental disturbances such as air turbulence, heat load, and/or mechanical vibrations. This is enabled via a fiber-based almost common-path single-shot digital holographic acquisition of the information of the two different wavelengths using angular multiplexing. The experimental setup and data evaluation are discussed, and we present measurements of non-cooperative objects with specular reflective and/or diffuse reflective surfaces having different colors.

Multiwavelength laser designed for single-frame digital holography.

In this paper, we present a tailored multiwavelength Yb-fiber laser source in the 1.03μm spectral region for spatially multiplexed digital holographic acquisitions. The wavelengths with bandwidths below 0.1nm were spectrally separated by approximately 1nm by employing fiber Bragg gratings for spectral control. As a proof of concept, the shape of a cylindrically shaped object with a diameter of 48mm was measured. The holographic acquisition was performed in single-shot dual-wavelength mode with a synthetic wavelength of 1.1mm, and the accuracy was estimated to be 3% of the synthetic wavelength.

Noise and signal scaling factors in digital holography in weak illumination: relationship with shot noise

We have performed off-axis heterodyne holography with very weak illumination by recording holograms of the object with and without object illumination in the same acquisition run. We have experimentally studied how the reconstructed image signal (with illumination) and noise background (without) scale with the holographic acquisition and reconstruction parameters that are the number of frames and the number of pixels of the reconstruction spatial filter. The first parameter is related to the frequency bandwidth of detection in time, the second one to the bandwidth in space. The signal to background ratio varies roughly like the inverse of the bandwidth in time and space. We have also compared the noise background with the theoretical shot-noise background calculated by Monte Carlo simulation. The experimental and Monte Carlo noise background agree very well with each other.

High speed digital holography for density and fluctuation measurements (invited)

The state of the art in electro-optics has advanced to the point where digital holographic acquisition of wavefronts is now possible. Holographic wavefront acquisition provides the phase of the wavefront at every measurement point. This can be done with accuracy on the order of a thousandth of a wavelength, given that there is sufficient care in the design of the system. At wave frequencies which are much greater than the plasma frequency, the plasma index of refraction is linearly proportional to the electron density and wavelength, and the measurement of the phase of a wavefront passing through the plasma gives the chord-integrated density directly for all points measured on the wavefront. High-speed infrared cameras (up to ∼40,000 fps at ∼64×4 pixels) with resolutions up to 640×512 pixels suitable for use with a CO(2) laser are readily available, if expensive.

New developments in ground probing radar: the possibility of reconstructing a holographic image of underground reflectivity

During the last decade GPR has developed rapidly: instruments have become more compact and more digital, the field of application has broadened from non-destructive testing to humanitarian demining. Studies have been carried out to analyse the full information content of the backscattered wavefield trying to go beyond the time and amplitude analysis. In recent years, many researchers have focused on the possibility of applying holographic acquisition and processing to GPR data. This paper proposes a theoretical outline of a holographic acquisition and processing techniques; a block diagram of the proposed holographic radar; the outlines of the design and the realisation of a custom-built full-scale test-site and the results of the first simulations carried out with newly developed software. The basic rules for an optimum choice of the main acquisition parameters are also given together with a discussion of the main advantages and disadvantages of the proposed techniques.

Amplitude point-spread function measurement of high-NA microscope objectives by digital holographic microscopy

We present here a three-dimensional evaluation of the amplitude point-spread function (APSF) of a microscope objective (MO), based on a single holographic acquisition of its pupil wavefront. The aberration function is extracted from this pupil measurements and then inserted in a scalar model of diffraction, allowing one to calculate the distribution of the complex wavefront propagated around the focal point. The accuracy of the results is compared with a direct measurement of the APSF with a second holographic system located in the image plane of the MO. Measurements on a 100 x 1.3 NA MO are presented.