We demonstrate the optical reconstruction in the visible range (0.532μm) of digital holograms recorded at long IR wavelengths (10.6μm) by means of a spatial light modulator. By using an integrated recording-reconstruction system, it is, in fact, feasible to achieve direct imaging of holograms acquired outside the visible range, i.e., in the IR spectrum. By choosing a Fourier recording configuration, the reconstructed image, obtained at about a 20 times shorter wavelength than the acquisition image, exhibits minor aberrations, which do not significantly affect the optical reconstruction. The high NA achievable at a long IR wavelength allows us to image large objects at reasonable distances.

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Optical reconstruction of digital holograms recorded at 10.6 μm: route for 3D imaging at long infrared wavelengths

Digital holographic interferometry in the long-wave infrared domain has been developed by combining a CO2 laser and a microbolometer array The long wavelength allows large deformation measurements, which are of interest in the case of large space reflectors undergoing thermal changes when in orbit We review holography at such wavelengths and present some specific aspects related to this spectral range on our measurements For the design of our digital holographic interferometer, we studied the possibility of illuminating specular objects by a reflective diffuser We discuss the development of the interferometer and the results obtained on a representative space reflector, first in the laboratory and then during vacuum cryogenic test

Digital holographic interferometry with CO2 lasers and diffuse illumination applied to large space reflectors metrology

ew Tech n iq u es in D ig ial H o lo g ap h y E d ied b y P acal P iart This book is devoted to special techniques in digital holography. The authors establish a synthetic state of the art of important advances in the field of digital holography. They are interested in detailing advances related to fundamentals of digital holography, in-line holography, digital color holographic metrology, digital holographic microscopy, infrared holography and special techniques in full-field vibrometry.

New Techniques in Digital Holography

A search for recording media suitable for holography at wavelengths comparable to the 10.6 μm of a CO2 laser has led to the successful use of Saran Wrap plastic films and of thick acrylic plates. Holographic phase recording has been made with a 1-W single mode cw CO2 laser yielding incident intensities up to 3.5 W/cm2. Holographic reconstruction at a wavelength of 632.8 nm has given a diffraction efficiency up to 20%. At a wavelength of 10.6 μm, reconstruction by reflection on the silver-coated surface of an acrylic plate had a diffraction efficiency of 0.5%. It was observed that the recording is less linear on acrylic plates than on Saran Wrap films.

Plastic recording media for holography at 10.6 μm

Two-dimensional holographic interferometry has been accomplished at wavelengths near 3 μm The source is a pulsed hydrogen fluoride oscillator-amplifier system Transverse excitation is achieved with a fast bank discharging into a simple electrode configuration The spatial detector is a thin film of bismuth, exposed by local evaporation by a single line of the HF laser emission spectrum Double exposure interferograms covering areas up to ~10 cm2 in the object plane have been obtained with single-line laser energies of less than a joule

Pulsed HF Laser Holographic Interferometry.

Holograms are produced on bismuth and paraffin films with pulsed TEA CO2 laser light. Sensitivity of both films is about 0.3 J/cm2. Local resolution proved to be much better than 70 lines/mm. Capability of this holographic technique for interferometric measurements is demonstrated.

Holography and Holographic Interferometry with Pulsed High-Power Infrared Lasers

Holographic interferograms are produced on gelatine films with pulsed HF laser light (λ ≈ 3 μm). The sensitivity is about 2 J/cm2, and the resolution at least 300 lines/mm. The spectrum of the HF laser is measured by means of holography.

G. Decker

Papers

Holography and Holographic Interferometry with Pulsed High-Power Infrared Lasers

Pulsed High-Power HF Laser Holography with Gelatine Detectors