Small-feature-size fan-out kinoform etched in GaAs

A binary fan-out kinoform for focusing and splitting an incident beam into a 4 × 4 spot array, with a largest deflection angle of 28°, was designed, fabricated in GaAs, and evaluated. The kinoform was defined in resist with electron-beam lithography and etched into GaAs with chemically assisted ion-beam etching. Light at wavelength 0.98 μm from a single-mode fiber was used to illuminate the kinoform. The efficiency was measured to be 34%, and the uniformity error for the 4×4 spots was 29%. Although the typical feature size of the kinoform is only roughly two wavelengths, we found that the scalar theory of diffraction can be used. A first kinoform was designed with the customary Fresnel-diffraction theory, which was found to be too coarse, resulting in a fan-out exhibiting some distortion. A second kinoform was designed with the more rigorous Fresnel-Kirchhoff expression, and its fan-out shows no distortion.     

Design of fan-out kinoforms in the entire scalar diffraction regime with an optimal-rotation-angle method

An algorithm for the design of diffractive optical phase elements (kinoforms) that give rise to fan-out (i.e., spot) patterns was developed and tested. The algorithm is based on the Helmholtz-Kirchhoff rigorous scalar diffraction integral for the evaluation of the electric field behind the kinoform. The optimization of the kinoform phase modulation is performed with an efficient optimal-rotation-angle method. The algorithm permits any spatial configuration of the locations of the desired spots. For example, the spots (all or some) can be located at large angles to the optical axis (nonparaxial case) or they can be located in the near near field of the kinoform, i.e., where the Fresnel approximation is no longer valid. Two examples of fabricated kinoforms designed with this algorithm are presented.     

Design, fabrication, and evaluation of a multichannel diffractive optic rotary joint

A multichannel diffractive optic rotary joint was designed, fabricated by electron-beam lithography, and evaluated with regard to cross talk and to output signal power variations. High cross-talk margin (>25 dB) and low output signal power variations (<2 dB) were achieved. The sensitivity to input-light-beam wavelength uncertainty was investigated. Two design examples are presented. The first design eliminates cross talk due to unwanted diffraction orders and shows that for a ten-channel joint the wavelength uncertainty of an 850-nm emitting laser must be less than 8 nm. In the second design cross talk due to the second diffraction order is permitted, which results in a tolerance level that is three times better for wavelength uncertainty.     

Laser-machining experiment with an excimer laser and a kinoform

We have used a kinoform to increase the beam-power utilization in an excimer-laser-machining experiment. The kinoform creates the pattern to be machined. Thus less power is wasted on the blocking parts of a shadow mask. To achieve as smooth an intensity profile as possible, the kinoform was also used together with a microlens-array beam homogenizer. We discuss the intensity distributions of the patterns created by the kinoform with and without the beam homogenizer as well as the design of the kinoform and the homogenizer, with emphasis on the relation to the coherence properties of the laser beam.     

Theoretical and experimental properties of a binary linear beam-splitting element with a large fan angle

Abstract

Diffractive beam-splitting elements with a large fan angle of about 45° were realized as binary phase elements for application in a commercial laser device operating at the wavelength of 635 nm. The fan-out elements designated to split a laser beam into a line of 43 equal power spots were fabricated in silica by means of microstructuring techniques and replicated in acrylate by ultraviolet curing. Two different gratings have been designed using scalar unidirectional iterative methods, based on the iterative discrete on-axis and on the direct binary search algorithms. The optical properties of both gratings obtained by these scalar methods were compared with simulations based on rigorous electromagnetic calculations in order to verify and control the application-relevant specifications. The experimentally measured optical performance of the replicated fan-out elements is in good agreement with the theoretical predictions. The complete procedure for realizing the linear beam splitters, that is the design method, the fabrication of the master, and the replication process, will be presented. The optical properties and characteristic data of the replicas will be compared with scalar simulations as well as rigorous calculations; the results will be discussed.     

Multiwavelength operation with thin diffractive elements

It is generally accepted that diffractive elements designed for multiwavelength operation require deep surface-relief profiles. We show, however, that thin diffractive elements can be designed to operate with more than one wavelength. A novel, to our knowledge, optimization technique is introduced for this purpose. The maximum phase delay is limited to only a few multiples of 2pi, and the element can implement different functions for different wavelengths. Examples with fan-out gratings are discussed.     

Kinoform-only Gaussian-to-rectangle beam shaper for a semiconductor laser

A kinoform that shapes the divergent beam from a semiconductor laser without using any other optical components was designed and fabricated. The kinoform-only concept means that the kinoform must perform both the actual beam shaping as well as focusing the divergent laser beam, correcting for the astigmatism of the laser, and correcting for the spherical aberration of the laser exit window. A rectangular beam of dimensions 1000 μm × 300 μm is formed 42 mm behind the kinoform. Of the total output from the laser, some 50% is incident upon the kinoform, of which ~50% will appear in the rectangular beam. The intensity uniformity error within the rectangle increases from the design value of 8% to 38% because of sensitivity to fabrication errors. The kinoform-only design for beam-shaping applications requires high manufacturing accuracy but is attractive because a system using such a component is easily mounted and aligned and, with the use of kinoform-replication techniques, can be mass produced at low cost.     

Wavelength-multiplexing diffractive phase elements: design, fabrication, and performance evaluation

We report on the wavelength-multiplexing diffractive phase element (WMDPE) capable of generating independent spot patterns for different wavelengths. The iterative method proposed by Bengtsson [Appl. Opt. 37, 1998] for designing a kinoform that produces different patterns for two wavelengths is extended to the WMDPE for multiple wavelengths (more than two wavelengths). Effectiveness of the design algorithm is verified by design and computer simulations on the WMDPE's for four and nine wavelengths. The WMDPE for three wavelengths (441.6, 543.5, and 633 nm) is designed with five phase levels and is fabricated by electron-beam lithography. We observed that the individual spot patterns are reconstructed for the design wavelengths correctly. Performance of the WMDPE is evaluated by computer simulations on the uniformity error, the light efficiency, and the contrast. On the basis of the results, the characteristics of the WMDPE's are discussed in terms of various conditions of fabrication and usage.     

Modified phase function model for kinoform lenses

A more computationally tractable model of the kinoform lenses in hybrid refractive-diffractive systems is proposed by taking into consideration the actual phase function of the kinoform lenses for every wavelength. The principle and outline of this modified model are explained. We compare the results of this approach with the more conventional single order calculation and with the standard diffraction-order expansion by using a practical hybrid optical system example.     

Incoupling waveguide holograms for simultaneous focusing into multiple arbitrary positions

A design method is presented that enables the realization of a novel type of incoupling waveguide hologram (IWGH) that simultaneously focuses the incoupled light to any desired positions in the waveguide. IWGH's, or grating couplers, are gratinglike structures etched into the waveguide surface. They couple the light incident from free space into the waveguide. The grating lines can be dislocated with respect to each other to provide phase modulation of the incoupled light. By use of this phase modulation, novel beam splitting and focusing functions can be built into the IWGH's. The new design algorithm is based on a model that assumes a simple relation between the incident light wave and the locally excited guided wave. This model is used to obtain an efficient formulation of the optimization problem. Four different IWGH's were designed and fabricated in InP for light at 1550-nm wavelength. Experiments confirm that these IWGH's are capable of incoupling the incident wave and simultaneously splitting and focusing the guided wave into multiple positions in the waveguide at different distances from the IWGH.     

Analysis of multimask fabrication errors for diffractive optical elements

As design algorithms for diffractive optical elements improve, the limiting factor becomes the fabrication process. It is hoped a better understanding of fabrication errors will allow elements with greater tolerance to be designed. This is important for high-power laser fiber coupling, where hot spots lead to failure. We model seven different fan-out gratings applying misetch, misalignment, and feature rounding. Our main findings are that misetch can lead to improved results, misalignment is strongly asymmetric, and both the pi and pi/2 masks can dominate misalignment. Rounding has a r(2) dependence and potentially can be incorporated into the design stage. Finally we present some experimental data for misalignment.     

Performance consideration of three-dimensional optoelectronic interconnection for intra-multichip-module clock signal distribution

The structure, fabrication, and theory of a three-dimensional planarized optoelectronic clock signal distribution device, based on a thin light-guiding substrate in conjunction with a two-dimensional polymer holographic grating array, are described. We have demonstrated previously a 25-GHz 1-to-42 (6 x 7) highly parallel fan-out interconnect with a signal-to-noise ratio of 10 dB. We present theoretical research that focuses on generating a globally uniform fan-out distribution. An objective function aimed at equalizing the intensities among the fan-out beams is established. For an arbitrary M x N fan-out distribution, there are M + N + 1 sets of holograms needed to be recorded independently to provide the required equal fan-outs. The efficiency of each hologram is determined precisely. The angular misalignment, wavelength dispersion, and spot-size problems are discussed further, together with their tolerance requirements on the size of the photoreceivers integrated on the multichip modules. Employment of 0.25-pitch gradient index (GRIN) lenses as a collimator and as a focusing element is demonstrated experimentally. Optical beam profile manipulation and packaging tolerance are enhanced greatly with GRIN lenses. Finally, clock skew problems associated with the proposed system are discussed, and schemes to minimize the skew are proposed.     

Imaging spectroscopy for two-dimensional characterization of auroral emissions

A large throughput transmission spectrometer, with a grating on a prism as the diffraction element, has been developed to study altitude distributions of auroral emissions. The imaging spectrometer disperses spectrally in one dimension while spatial information is preserved in the orthogonal direction. The image is projected onto a CCD array detector. Image processing methods have been developed to calibrate for wavelength, uniform field, spectral sensitivity, curvature of field, and spatial mapping. Single images are processed to represent a measured signal brightness in a unit of Rayleighs/pixel, from which area integrations can be made for desired spatial-spectral resolution. System performance is ~1.5-nm resolution over a 450-nm bandwidth (420-870 nm). Two spectrometer systems of this design were operated simultaneously, one with additional optical instruments and an incoherent scatter radar at Sondrestrom, Greenland, and the other at Godhavn, Greenland, which lies 290 km to the northwest and nearly in the magnetic meridian of Sondrestrom. The developed system, calibration method, and examples of performance results are presented.     

Compact bandpass filters in coplanar strip lines

The authors describe a compact bandpass filter design implemented in asymmetric coplanar strip lines. For initial demonstration, it was built on print circuit boards using modified two-pole Butterworth bandpass filter topology. The novel filter design does not use bonding wires, vias or any interconnecting structure. It is not based on quarter wavelength sections either. It uses the creation of an electrode pattern on the coplanar strip electrodes to emulate an equivalent circuit. The pattern is fine-tuned using full-wave simulations. Bandpass filters for 2.4 GHz ISM (Industrial, Scientific and Medical) and 5.15-5.85 GHz UNII (Unlicensed National Information Infrastructure) bands were designed, fabricated and measured. Measurement responses agree well with full-wave simulation results. The electrode pattern can be scaled for different centre frequency and improved with fine-tuning procedure. For chip-level implementation, the filter size can be greatly reduced because of much smaller minimum electrode width     

Narrow-linewidth vertical-cavity surface-emitting lasers for oxygen detection

The use of vertical-cavity surface-emitting lasers (VCSEL's) for optical detection of atmospheric oxygen is described. The VCSEL's were custom designed for single-mode emission in the 763-nm wavelength range, with low noise and narrow optical linewidth. Using standard wavelength modulation spectroscopy and a second-harmonic detection scheme with a 1-m air path, we determined an oxygen concentration resolution of 0.2%. Because of its small size, low power dissipation, and good tunability characteristics, the VCSEL promises to be an attractive light source for use in compact, low-cost optical sensor microsystems for trace gas detection.     

激光倍频三波长减反射膜

提出Nd:YAG激光二倍频,三倍频和主激光三波长同时实现减反射膜系设计方法。该方法成功地设计出不同折射率基片SiO_2,LiNiO_3和K_9玻璃在350nm, 530nm,1060nm三个波长带剩余反射率小于0.02％的减反射膜。文中给出设计理论和实例。并用所设计的膜系进行镀制实验,得到较好的结果。对实测曲线与计算曲线不一致性进行了分析。     

Error reduction of quantized kinoforms by means of increasing the kinoform size

There are two kinds of method that utilize the redundancy in kinoform domains for reducing the reconstruction errors of quantized kinoforms. One is the iterative-dummy area (IDA) method, which increases the kinoform size indirectly by the addition of a dummy area to the desired image. The other is the interlacing technique (IT), which increases the kinoform size directly by the interlacing of a number of subkinoforms whose sizes are the same as the desired image. We compare the error reduction of quantized kinoforms between these two methods. Simulation results show that reconstruction errors from the IT method can be reduced further and faster than those from the IDA method when the kinoform size is increased to larger than 4 x 4 times the size of the desired image.     

Wavelength-dependent diffraction patterns from a liquid crystal display

Liquid crystal displays (LCDs) are invaluable for a variety of optical applications, including the encoding of programmable diffractive optical elements. The pixel structure in these devices produces a set of diffracted orders of which the central order is the strongest. In most devices that we have examined, the intensity distribution of the diffraction pattern is independent of the wavelength of the illuminating light. Recently we have been examining the performance of LCDs having very small pixel sizes. We compare results for two devices from the same manufacturer. One of them exhibits the normal behavior. For the other, we find surprisingly strong wavelength dependence. The diffraction pattern varies from having most of the energy in the zero order for long wavelengths to having the energy distributed among 50-60 orders as the wavelength decreases. We attribute this behavior to a phase structure over each pixel. We analyze this behavior using a simple two-dimensional model that qualitatively explains the phenomenon. These results can be viewed in two ways--on the positive side this behavior might lead to optical logic or fan-out applications. On the negative side, there is less intensity available in the normally used zero order.     

Bulk acousto-optic wavelength agile filter module for a wavelength-multiplexed optical scanner

An acousto-optic tunable filter-based wavelength-selection module with features optimized for a wavelength-multiplexed optical scanner (W-MOS) is proposed and demonstrated. The W-MOS produces high-speed multiple scan beams if it is engaged with an agile tunable source with multiwavelength generation capability. In particular, the proposed fiber-connected module features high-speed, low-loss, narrow-linewidth, and single-multiple wavelength selection by means of radio frequency drive signal control for single- or multiple-beam scan operations. The unique module offers input laser beam power control that in turn delivers the desired scanned laser beam power shaping. Experimental results match module design theory and demonstrate a fast 5.4-micros wavelength selection speed, a low (1.53-dB) fiber-to-fiber optical insertion loss, a 5.55-nm 3-dB spectral width, and a 1500-1600-nm agile wavelength operational band.     

Kinoform design with an optimal-rotation-angle method

Kinoforms (i.e., computer-generated phase holograms) are designed with a new algorithm, the optimalrotation- angle method, in the paraxial domain. This is a direct Fourier method (i.e., no inverse transform is performed) in which the height of the kinoform relief in each discrete point is chosen so that the diffraction efficiency is increased. The optimal-rotation-angle algorithm has a straightforward geometrical interpretation. It yields excellent results close to, or better than, those obtained with other state-of-the-art methods. The optimal-rotation-angle algorithm can easily be modified to take different restraints into account; as an example, phase-swing-restricted kinoforms, which distribute the light into a number of equally bright spots (so called fan-outs), were designed. The phase-swing restriction lowers the efficiency, but the uniformity can still be made almost perfect.