Laser applications that demand a high quality with long coherence length are limited by the Gaussian profile of the
fundamental TEM00 mode. Many of these applications require a uniform irradiance profile with a flat phase-front. In
holography, both phase and intensity are critical to the process. Near-field beam shaping optics, also called beam
transformers, re-map an input Gaussian profile to a top-hat profile. The top-hat profile is created at some working
distance away from the shaping element where a corrector element has traditionally been placed in order to flatten the
phase of the top-hat profile and allow it to propagate as a nominally collimated beam. This paper will discuss the theory
to support the use of a diffractive optical element in holography and other applications where the phase is important.
Two different geometric beam shapes will be explored, round and square profiles.
Diffractive beam shaping, using a remapping approach, requires a laser source that is well characterized and stable. Recent advances in the development of fiber lasers have shown stable, high quality, TEM00 single mode performance at powers < 200 Watts for 1090 nm. This paper will give a detailed account of the design and experimental results for a 5.5 mm 1/e2 fiber lasers shaped by an off-axis diffractive beam shaper to produce tophat focused spots < 50 microns. One application of interest is in the area of micro welding of thin stainless steel sheets. Experimental data will be presented for this micro welding application.
High aspect ratio line shapes can be generated using a diffractive remapping beam shaper and a refractive cylinder lens. A typical line shape is 10 mm in the long uniform axis and 0.05 mm or less in the Gaussian axis. The use of a TEM00 allows for a diffraction limited short axis focus while expanding to sizes of greater than 100 mm in the uniform direction. In applications that require scanning, the Gaussian axis is perpendicular to the scan direction and the uniform axis is parallel to the scan direction resulting in a uniform (time averaged) illumination in both axis. Many laser users throw away > 50 % of their laser power to achieve these line shapes, while using this diffractive/refractive hybrid approach > 90 % of the total input energy can be utilized.
We have measured the rc (effective electrooptical coefficient) of pure and doped Ferroelectric Lithium Niobate (LN) using a single beam, null detection polarimeter. The polarimeter is adjustable between two adaptive optics configurations--an iris hard stop beam pattern on the one hand and a diffractive optics generated top-hat beam on the other. We clearly show the need to control thermal heating of LN due to the transmitted laser beam. The required heating control has been implemented using a fabricated metallic heat sink called a "Cold Finger." In addition to its electrooptical properties, LN possesses a combination of unique piezoelectric, pyroelectric, and photorefractive properties. These properties make it suitable for applications in optical devices as frequency doublers, modulators, switches, and filters in communication systems and holographic recording medium. We present the classical microscopic anharmonic oscillator description for generating Pockels coefficients, and briefly describe the polarimetry measurement system. Here, the growth of pure and iron doped lithium Niobate is also described using an Automatic Diameter Control Czochralski Design growth technique. The results of growth, electrooptic measurements, adaptive optics implementation and some physical properties are compared and presented.
Many laser material processing applications depend only on the intensity profile at a given working plane. A single diffractive optic has been shown to produce useful results in the material processing environment where the intensity is the only concern. However, for laser application that depend on the intensity profile and the phase profile together, it seems self-evident that a minimum of two diffractive phase elements is required. Although, this two element approach can be effective, it requires more complex alignment and a higher energy loss for the diffractive system, relative to a single element solution. For demanding laser applications like holography, both phase and intensity are critical to the process. This paper will discuss the theory and experimental data to support the use of a single diffractive optic for use in holography, and other phase critical applications for round uniform shaped beams.
Low power CO2 lasers with average powers of < 300 Watts are being manufactured with near diffraction limited beam quality. These lasers are a good source for diffractive beam shaping systems that can produce uniform round, square and lineshape intensity profiles. Several beam shaping configurations will be presented in this study, along with material processing results using these shaped beam systems. Comparative data will be presented for an on axis vs. off axis design and the merits of each will be considered.
A state-of-the-art laser beam shaper zoom optical system has been developed. This optical device, efficiently converts a 532 nm single-mode Gaussian intensity profile into top hat lines of variable size with peak-to-valley non-uniformities of ≤±8%. This hybrid zoom system is capable of generating line lengths ranging between 0.16 mm to 1 mm, and line widths of up to 15 μm. The lines are of square-like shape in the long axis while they remain Gaussian in the short axis. The system was designed, fabricated, and tested using a laser with a nominal input Gaussian beam waist of 2.25 mm at the 1/e2 intensity points, and a nominal M2 of less than 1.1. Techniques associated with the system alignment during the optical assembly and system integration are discussed. The optical design and test results of the diffractive-optical system are also presented.
A method for converting a gaussian laser beam into a propagating Airy pattern is described using a diffractive optic. This propagating Airy pattern is focused by a lens to obtain a flat top intensity at the focal plane. The technique is based on the idea of Fourier transform pairs and produces a small spot diameter with a useable depth of field. Experimental results will be presented for round and square focused uniform intensity profiles. A focused uniform intensity profile can prove useful for many laser applications.
Diffractive optical elements can be designed to shape and, hence, optimize a laser beam intensity distribution for a specific application. The elimination of spherical and chromatic aberration in CO2 industrial and medical diffractive focusing elements, respectively, is discussed.
Diffractive focusing elements which eliminate spherical aberration in industrial CO2 laser beam delivery systems are being manufactured. The theory of these elements as well as the experimental verification of diffraction limited performance are discussed.
The evolution in design, fabrication, and performance of diffractive optical elements manufactured for use as focusing elements in CO2 industrial lasers is discussed.
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