Plastic recording media for holography at 10.6 microm

Digital holographic interferometry with CO2 lasers and diffuse illumination applied to large space reflector metrology [Invited]

Digital holographic interferometry in the long-wave infrared domain has been developed by combining a CO(2) 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.

Nondestructive testing by using long-wave infrared interferometric techniques with CO2 lasers and microbolometer arrays

We describe three different interferometric techniques (electronic speckle pattern interferometry, digital holographic interferometry, and digital shearography), using a long-wave infrared radiation produced by a CO(2) laser and recorded on a microbolometer array. Experimental results showing how these methods can be used for nondestructive testing are presented. Advantages and disadvantages of these approaches are discussed.

Electronic speckle pattern interferometry and digital holographic interferometry with microbolometer arrays at 10.6 μm

Electronic speckle pattern interferometry and digital holographic interferometry are investigated at long infrared wavelengths. Using such wavelengths allows one to extend the measurement range and decrease the sensitivity of the techniques to external perturbations. We discuss the behavior of reflection by the object surfaces due to the long wavelength. We have developed different experimental configurations associating a CO(2) laser emitting at 10.6 μm and microbolometer arrays. Phase-shifting in-plane and out-of-plane electronic speckle pattern interferometry and lensless digital holographic interferometry are demonstrated on rotation measurements of a solid object.

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

We demonstrate the optical reconstruction in the visible range (0.532 microm) of digital holograms recorded at long IR wavelengths (10.6 microm) 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.

Digital Holographic Interferometry by Using Long Wave Infrared Radiation (CO2 Laser)

We show how digital holographic interferometry in the Long Wave InfraRed spectral range (LWIR) can be used for the investigation of mechanical structures. The 10.6 μm radiation is produced by a CO2 Laser. Experimental results showing that the method can be used to locate defects in a panel are presented and advantages and disadvantages of this approach are discussed.

Relief gratings and microlenses fabricated with silicone

Divergent microlenses and low-spatial-frequency interference gratings have been fabricated in low-cost silicone layers. The size of the microlenses ranges from approximately 1 mm to 100 microm while spatial frequencies of interference gratings range from 4 to 18 l/mm. The fabrication method involves the recording of spatial distributions of mid-IR light. At recording time silicone layers are in a gel state. Then layers are cured by heat. The final silicone layers are transparent and rigid.

Infrared recording of transient and permanent interference gratings with commercial plastics

Two commercial plastics have been used to record infrared interference patterns when a CO(2) laser is used as a light source. Recording of transient and permanent interference gratings with different spatial frequencies was carried out. The diffraction efficiency for red light as a function of time for different exposure times is shown; maximum diffraction efficiencies attained were of the order of 30%. The possibility of using one of these plastics as an optical switch is shown.

Infrared recording with gelatin films

Gelatin films glued to an O-ring are proposed as a new procedure to record interference patterns when a CO(2) laser is used as an infrared light source. With this method the unwanted effects given by asubstrate, on which the thin recording film is laid, are avoided. To characterize the medium, interferometric studies including the recording of diffraction gratings have been done. Diffraction efficiencies of ~30% have been obtained when coherent red light (0.6328-microm wavelength) was sent normally to the gratings. An example of an infrared recorded hologram is shown.

Infrared laser photography with silver-halide emulsion

Infrared sensitization of photographic emulsion response to visible light is a useful technique for visualizing IR laser beams. In this work, it is demonstrated that this sensitization results from heating by the IR beam. Investigation of the dependence of sensitization by pulsed CO(2) laser (10.6-microm) preexposures on visible background density, IR fluence, visible wavelength, delay between exposures, and spatial frequency are described. Infrared fluences as low as 10 mJ/cm(2), a dynamic range greater than 100, and a resolution better than 50 lines/mm were observed. Suggestions for further improvements of the technique are made.

Holographie en infrarouge sur de minces couches d'huile

Oil films of approximately 1-microm thickness on glass plates are used as a holographic recording medium at a 10.6-microm wavelength. This dynamic recording medium (on which the recording is not permanent) shows diffraction efficiencies up to 20% when reconstructing at 632.8 nm, the spatial frequency at the recording being as high as ~50 cycles/mm. Furthermore, the response time of these oil films when recording ir can be as small as 2 msec so that ir holography of moving objects is possible in real time.