Gary J. Swanson

Efficient array illuminator using binary-optics phase plates at fractional-Talbot planes

We describe a phase plate based on the fractional-Talbot effect that converts a single expanded laser beam into a regular array of uniformly illuminated apertures with virtually 100% efficiency. The size, spacing, and fill factor of the illuminated aperture grid can be freely chosen to interface with a variety of electro-optic devices. A binaryoptics phase plate is demonstrated that converts uniform illumination into an array of square illumination cells with a fill factor of 1/16.

Coherent laser addition using binary phase gratings

Binary phase diffraction gratings are shown to couple light coherently from a laser array into a single on-axis beam. The diffraction grating, designed to split a single beam into a specific number of equal intensity diffraction orders, is placed inside the cavity formed by the laser array and a common output mirror. The grating superimposes the light beams from the lasers in the array and produces a far-field pattern with the same divergence as that of a single laser. Six GaAlAs lasers from an antireflection-coated linear array were combined with a coupling efficiency of 68.4%. The far field of the combined GaAlAs lasers consisted of a single on-axis Gaussian beam.

Diffractive optical elements for use in infrared systems

Diffractive optical elements have the potential to improve the performance of infrared optical systems. An approach to constructing high quality diffractive elements has been developed using standard integrated circuit techniques. It is possible to implement arbitrary phase profiles since the elements are computer generated.

Coherent summation of laser beams using binary phase gratings

A method for coherent addition of lasers is presented that uses binary phase-only gratings. It is shown that a grating that splits a single beam into N equal orders with high efficiency can be used in reverse to convert N laser beams into a single beam with the same efficiency. Experiments to demonstrate the conversion of seven beams to one beam are performed with a resulting conversion efficiency of 75%. An experimental apparatus is described that adds the power from both two and three He–Ne lasers (wavelength, 3.39 μm) with efficiencies as high as 83%.

Aperture filling of phase-locked laser arrays.

Arrays of lasers that are locked together with the same phase produce far-field patterns with a significant amount of the energy residing in the sidelobes. We describe a technique that channels the majority of the sidelobe energy into the desired central lobe.

Coherent beam addition of GaAlAs lasers by binary phase gratings

A general technique for combining laser beams coherently using binary gratings has been developed. Experiments were performed with lasers from a monolithic linear GaAlAs array. The grating transmittance profile was designed to convert the light beams from the array into a single beam with high efficiency. Optical feedback through the grating locked the lasers together in proper relative phase. The far‐field diffraction pattern of the sum was practically identical in shape to that of a single laser. Coupling efficiencies greater than 80% appear to be feasible with this technique. The method is applicable to a variety of laser systems (gas, solid state, etc.) and is readily extendable to a two‐dimensional array of lasers.

Efficient side-lobe suppression of laser-diode arrays

An optical technique is described which converts multilobed far‐field patterns from in‐phase laser arrays into a single‐lobed pattern. The technique redistributes the output light from the laser array to produce a uniformly illuminated aperture. This filled aperture produces negligible side lobes, and efficiently channels the array power into the main lobe. Experiments performed on a ten‐element Y‐guide laser array show an increase in main lobe power from 51 to 90% of the total array power.

Binary lenses for use at 10.6 micrometers

By combining advances in lithography and electromagnetic grating theory, we recently have demonstrated the ability to produce highly efficient binary gratings and binary lenses for use at 10.6 um. Electromagnetic theory predicts that binary gratings with the proper parameters can achieve a first-order diffraction efficiency of nearly 100%. If the periodicity of the grating is on the order of the radiation wavelength, all of the orders become evanescent except for the zero and positive first orders. By choosing the depth-to-period ratio and duty cycle properly, the zero order can be suppressed, placing virtually all of the incident radiation into the first diffracted order. Theoretical calculations have been done only for constant period gratings. However, assuming a lens pattern to be a minor perturbation of a grating, we succeeded in producing an f/5 binary lens with a diffraction efficiency of 96% at 10.6 um. Furthermore, because of the high efficiency of these elements, it becomes practical to consider using more than a single diffractive element in a system. We have constructed a simple afocal telescope from two binary lenses. The telescope has a 2 in. entrance aperture and a magnification of 5. A final point to be considered is the wavefront quality of these elements. Electron beam machines, which are used to write the lens patterns, are designed to draw the pattern in a raster fashion. This quantization sets a limit on the quality of the lens pattern.

Infrared Applications Of Diffractive Optical Elements

Diffractive optical elements have the potential to improve the performance of infrared optical systems. An approach to constructing high quality diffractive elements has been developed using standard integrated circuit techniques. It is possible to implement arbitrary phase profiles since the elements are computer generated.

Developments In Fabrication Of Binary Optical Elements

We have produced highly efficient binary diffraction gratings -- 94% first order diffraction efficiency measured in the infrared (10.6 μm) on straight binary gratings and on binary holographic off-axis lenses. The observed point-spread functions of these lenses (F/10, f = 25 cm) were limited only by diffraction. To fabricate gratings we adapted IC production techniques. Our work brings together three independent developments: (1) theories on binary diffraction gratings operating in the EM domain, (2) large-scale integration (LSI) advances in pattern generation and substrate depositions, and (3) improvements in reactive ion-beam etching techniques.