Use Of Diffractive Optical Elements Research Articles

Power amplification for petawatt Ti: Sapphire lasers: New strategies for high fluence pumping

One of the major bottlenecks when we pump large Ti:Sapphire crystals, to reach Petawatt-level laser amplification, is the careful control of the spatial energy distribution of Nd:Glass pump lasers. Commercially available nanosecond Nd:Glass and Nd:YAG lasers exhibit poor spatial profile quality especially in the near and in the intermediate field, which can lead to local hot spots, responsible of damages in crystals, and parasitic transverse lasing enhancement, strongly dependent on the profile of the pump beam. For these reasons, it is mandatory to keep the pump beam intensity profile as flat as possible on the pumped crystal. To guarantee the best pumping conditions we are investigating the combined use of DOE (diffractive optical elements) and optical smoothing techniques. In parallel we are starting a study on laser induced damages mechanisms in crystal. With DOE and microlens arrays we plan to guarantee to the beam a supergaussian shape. Simulation and first experiments with both optical systems show that a flat top spatial profile with less than 10% fluctuations and a 8th order supergaussian is possible with the present technology.Optical smoothing will keep the beam free of hot spots. We especially focused on the smoothing techniques involving optical fibers. This is the first time to our knowledge that this technique is applied to the pumping beams forTi:Sapphire systems. A deep study of laser-crystal interaction will allow us to fully understand the damages created by hot spots. The knowledge of the phenomena involved in laser damages on Ti:Sapphire is mandatory to control the pumping processes and thresholds. In conclusion, mixing the advantages of these different approaches to overcome this bottleneck will allow us to amplify in a safety way femtosecond laser beams to the Petawatt level using Ti:Sapphire crystals.

Implementation of Fresnel full complex-amplitude digital holograms

The use of diffractive optical elements (DOEs) is increasing for several industrial applications, such as beam shaping and optical filtering. Most elements modulate the phase of the incoming light or its amplitude, but not both. To overcome this limitation, we developed a full complex-amplitude modulation DOE. We employed well-established integrated circuit fabrication steps to fabricate the devices at relatively low cost and with high precision. Using this approach, the new element's optical performances are improved even for near-field operations. With this device it is possible to obtain the total control of the zeroth order transmitted light, resulting in low-noise reconstructed images.

Design, fabrication, and characterization of a full complex-amplitude modulation diffractive optical element

The use of diffractive optical elements (DOEs) is increasing for several industrial applications. Most elements modulate the phase of incoming light or its amplitude, but not both. The phase modulation DOE is the most popular because it has a high diffraction efficiency. However, the phase-only limitation may reduce the freedom in the element design, increasing the design complexity for a desired optimal solution. To overcome this limitation, a novel, full complex-amplitude modulation DOE is presented. This element allows full control over both phase and amplitude modulation of any optical wave front. This flexibility introduces more freedom in the element design and improves the element's optical performance, even in a near-field operation regime. The phase grating of the element was fabricated in an amorphous hydrogenated carbon film. The amplitude modulation was obtained by patterning a reflective aluminum thin film, which was deposited on top of the phase grating. The apertures in the metal film determine the quantity of transmitted light. The use of a reflective layer in the fabrication decreases the risk of laser-induced damage since no absorption is involved in the process. With this device it is possible to obtain extremely efficient spatial filtering and reconstruct low noise images.

Diffractive optical elements as raster-image generators

The use of diffractive optical elements (DOEs) to generate complex raster images for a primarily artistic purpose is dealt with. Aspects of human vision that are relevant for the design of such elements are discussed. A design method based on an iterative Fourier transform algorithm and extended with elements from the direct-binary-search and the simulated-annealing algorithms is described. The proposed method provides a large set of parameters that can be adjusted freely to optimize it for any given design task. For demonstration a phase-only DOE was designed that generates an image of a Chinese dragon as a diffraction pattern. It was realized as a surface-relief element on a planar substrate through multilevel binary lithography and reactive-ion etching. Experimental tests confirm the usefulness of the design and the fabrication procedures to achieve excellent image quality.

Testing of rod objects by grazing incidence interferometry: theory

Interferometry at grazing incidence allows one to perform macroscopic shape tests of rough surfaces with submicrometer precision. The highest measurement accuracy is achieved with a null test: the object wave front is adapted to the ideal surface under test through the use of diffractive optical elements. Deviations between the ideal and the real surface shape result in characteristic phase distributions at the interferometer output. Rod objects with rather arbitrary cross-section profiles are especially suitable for this type of measurement. The design of appropriate test wave fronts can be carried out by means of geometrical considerations. Expressions describing aberrations that are due to misalignments of the work piece can easily be derived. Misalignment aberrations and genuine surface deviations are separated by least-squares fitting.

Synthetic infrared spectra

We have designed, microfabricated, and characterized a diffractive optical element that reproduces the infrared spectrum of HF from 3600 to 4300 cm(-1) . The reflection-mode diffractive optic consists of 4096 lines, each 4.5mum wide, at 16 discrete depths relative to the substrate from 0 to 1.2 mum and was fabricated upon a silicon wafer by anisotropic reactive ion-beam etching in a four-mask-level process. We envisage the use of diffractive optical elements of this type as the basis for a new class of miniaturized, remote chemical sensor systems based on correlation spectroscopy.

Infrared zoom lenses in the 1990s

The breadth of activity in the field of infrared zoom lenses is international in scope. Representative infrared zoom lens systems of the 1990s from the continents of North America, Europe, and Asia and the Middle East are presented. Design considerations unique to the infrared spectrum such as athermalization are given special attention. The various approaches to solving these design problems are described along with the operating characteristics of these systems. Applications include forward-looking infrared scanning systems and target simulators. Tolerance considerations during manufacture are presented. Innovative reflective zoom systems are also considered. Development trends for the next several years are discussed, including the use of diffractive optical elements in infrared zoom lens systems.

Diffraction-free beams generated with programmable spatial light modulators

Diffraction-free beams are generated by the use of diffractive optical elements written on a spatial light modulator (SLM). By alteration of the pattern placed on the SLM, the axis of propagation can be both shifted laterally and rotated. We present experimental results obtained by use of a magneto-optic SLM.