Wilfrid B. Veldkamp

Coherent addition of AlGaAs lasers using microlenses and diffractive coupling

A near single‐lobed far‐field pattern was obtained from coherent operation of a nonevanescently coupled AlGaAs laser diode array. A diffractive microlens array collimated the individual beams to approximate a plane wave, and diffractive coupling from an external cavity mirror provided mutual coherence. A diffraction‐limited far‐field pattern was observed with 82% of the power contained in the central lobe. The method is directly applicable to two‐dimensional laser arrays and can be implemented as a single thin optical element.

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.

Color separation by use of binary optics

We describe the preliminary design, fabrication, and demonstration of an array of micro-optics that is used to separate colors locally on the focal plane. The 64 x 64 array combines 100 microm x 100 microm, F/2 refractive microlenses with a 17-microm period grating. The microlenses concentrate the incoming radiation, while the grating disperses the radiation according to wavelength. The element, with a minimum feature size of 1 microm and total depth of 8 microm, is fabricated on a silicon wafer (for use in the 8-12-microm band) by means of a nonstandard binary-optics process.

Laser beam profile shaping with interlaced binary diffraction gratings

A new CO(2) laser beam profile shaper was designed and tested. With high-power efficiency, it transforms a fundamental mode laser beam profile into a flattop profile at a focal plane. The shaper uses an interlaced binary diffraction grating that modulates the E field both in phase and amplitude and generates an apodized and clipped sinc(x) distribution in the object plane.

Beam profile shaping for laser radars that use detector arrays

The beam shaper we developed shapes the transmit beam of a CO(2) laser radar that uses a linear detector array. It consists of a diffraction grating and an anamorphic prism beam compressor and produces a stretched profile that efficiently and uniformly illuminates the far-field footprint of the detector array. The diffraction grating phase modulates the near field or the laser beam to generate a far-field flattop intensity profile, whereas the compressor produces the necessary profile eccentricity. We have achieved conversion efficiencies in the 70-90% range.

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%.

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.

Simple digital clipped correlator for photon correlation spectroscopy

The detailed design and performance of a digital clipped correlator constructed as an integral part of a photon correlation spectrometer (PCS) is described. The correlator is of a modular design and easily expandable to several hundred channels. Each channel contains a 4‐bit buffer storage, and the overflow bits from the storage are serially transferred into the memory of a multichannel analyzer. The dead time associated with the transfer is negligible. The sampling time of the correlator can be varied from 1 sec to 50 nsec. The principal features of the design are its simplicity, expandability, low cost, and optimal use of commercially available equipment.

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.