Replication techniques for diffractive optical elements

We present a method for reproducing diffractive optical elements in quantity. The method is compatible with VLSI microfabrication techniques and involves generating a gray-scale mask. The gray-scale mask is employed in an optical aligner to expose an analog photoresist on any environmentally durable substrate, e.g., glass, quartz, semiconductor, or metal, one exposure for each diffractive optical element. After copies of the mask on the photoresist are developed, many substrates can be processed in parallel in a chemically assisted ion-beam etcher to transfer the microstructures on the analog resists simultaneously onto the surfaces of the substrates.

Cost-effective mass fabrication of multilevel diffractive optical elements by use of a single optical exposure with a gray-scale mask on high-energy beam-sensitive glass.

Continuous profile writing by electron and optical lithography

The refractive and the diffractive properties of planar micro-optical elements are investigated. The transition between purely refractive and purely diffractive planar microlenses is numerically simulated for the example of differently designed phase-matched Fresnel elements. Results obtained from numerical simulations and experiments show that the refractive and diffractive types exhibit a distinctly different behavior in the presence of small fabrication errors or wavelength deviations. Based on these results, design rules for various applications, including low- and high-numerical-aperture lenses and hybrid refractive–diffractive elements, are derived. For a high-numerical-aperture (f/# = 1.0) lens the experimental characterization of the irradiance distribution in the image space is presented and shown to agree well with theoretical predictions.

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Refractive and diffractive properties of planar micro-optical elements.

Soldering technology for optoelectronic packaging is reviewed by studying modules in four categories: solder assembly with no precision self-alignments, and self-aligned solder assembly with no, one or two mechanical stops. There have been at least 60 papers and 8 U.S. patents published between 1990 and 1995. In addition to die-attachments, soldering technology has been successfully demonstrated for precision alignments. However, some packaging issues may hamper the progress of its manufacturing insertion for wide applications. Four of the issues to be discussed are solder materials, fluxless reflow, design, and reliability. More studies on these issues are needed to support the advancement of optoelectronic packaging for low-cost, high-performance and high-reliability modules.

Soldering technology for optoelectronic packaging

For optical interconnects to become a mature technology they must be amenable to electronic packaging technology. Two main obstacles to including free-space optical interconnects are alignment and heat-dissipation issues. Here we study the issues of alignment tolerancing that are due to assembly and manufacturing variations (passive-element tolerancing) over long board-level distances (>10 cm) for free-space optical interconnects. We also combine these variations with active optoelectronic device variations (active-element tolerancing). We demonstrate a computer-aided analysis procedure that permits one to determine both active- and passive-element tolerances needed to achieve some system-level specification, such as yield or cost. The procedure that we employ relies on developing a detailed design of the system to be studied in a standard optical design program, such as CODE V. Using information from this model, we can determine the integrated power falling on the detector, which we term optical throughput, by performing Gaussian propagation or general Fresnel propagation (if significant vignetting occurs). This optical throughput can be used to determine system-level performance criteria, such as bit-error rate. With this computer-aided analysis technique, a sensitivity analysis of all the variations under study is made on a system with realistic board-level interconnect distances to find each perturbation's relative effects (with other perturbations set to 0) on the power falling on the detector. This information is used to set initial tolerances for subsequent tolerancing analysis and design runs. A tolerancing analysis by Monte Carlo techniques is applied to determine if the yield or cost (yield is defined as the percentage of systems that have acceptable system performance) is acceptable. With a technique called parametric sampling, a subsequent tolerancing design run can be applied to optimize this yield or cost with little increase in computation. We study a design example and show that most of the tolerances can be achieved with current technology.

Tolerancing of board-level-free-space optical interconnects

One of the general requirements of a computer-aided design system is the existence of efficient (in data size and running time) algorithms that are generally reliable for the broadest range of design instances. The restricted data formats of the electron-beam machines impose difficulties in developing algorithms for the design of diffractive optical elements (DOE's) and computer-generated holograms (CGH's). Issues that are related to the development of CGH algorithms for e-beam fabrication of DOE's and CGH's are discussed. We define the problems the CGH algorithms need to solve, then introduce general curve drawing algorithms for the e-beam data generation of diffractive optical components. An efficient algorithm for general aspherical DOE's is proposed. Actual design and fabrication examples are also presented.

Efficient encoding algorithms for computer-aided design of diffractive optical elements by the use of electron-beam fabrication

An introduction to current research in computer-aided design (CAD) and packaging for optoelectronic systems utilizing free space optical interconnects is presented. New CAD tools are presented that allow layout of multistage networks based on CGH fabrication limitations and the subsequent synthesis of the interconnect holograms. Recent work involving double-sided alignment of a single substrate as well as alignment techniques between two different substrates is discussed. Several packaged systems utilizing photorefractive crystals to minimize alignment problems are also presented.

Computer aided design and packaging optoelectronic systems with free space optical interconnects

Due to the stringent requirements on the lateral alignment and the large number of steps required to fabricate diffractive optics by typical microlithographic techniques, the need for more efficient methods to fabricate these elements has arisen. This paper discusses the use of electron beam lithography in fabricating diffractive optical elements by direct e-beam alignment and direct write into the electron beam resist. Many of the practical advantages and disadvantages of each method will be pointed out. In particular, current research and future research directions into such direct write problems as the proximity effect, hologram ruggedness, and lengthy exposure times will be addressed.

Diffractive optics fabricated by electron-beam direct write methods

Optoelectronic systems based on space-variant optics give great freedom to the system designer in terms of interconnect topologies. One feature of space-variant systems is that they can achieve a high interconnect density. However, this density is achieved by having large arrays of diffractive elements with very small apertures relative to the propagation distances involved. Thus diffraction losses from the finite apertures can significantly affect power throughput for these types of systems, regardless of the diffractive efficiencies of the optical elements involved. Therefore it is desirable that this loss be minimized. We present several space-variant optical interconnect design methods (for both one-to-one and fan-out interconnects) and compare them in terms of power throughput for diffraction-limited interconnect distances. Both numerical simulations and experimental results are presented.

David Zaleta

Papers

Tolerancing of board-level-free-space optical interconnects

Efficient encoding algorithms for computer-aided design of diffractive optical elements by the use of electron-beam fabrication

Computer aided design and packaging optoelectronic systems with free space optical interconnects

Diffractive optics fabricated by electron-beam direct write methods

Design methods for space-variant optical interconnections to achieve optimum power throughput