No abstract

Diffraction characteristics of high-spatial-frequency (HSF) gratings are evaluated for application to polarization-selective computer-generated holograms by the use of two different approaches: second-order effective-medium theory (EMT) and rigorous coupled-wave analysis (RCWA). The reflectivities and the phase differences for TE- and TM-polarized waves are investigated in terms of various input parameters, and results obtained with second-order EMT and RCWA are compared. It is shown that although the reflection characteristics can be accurately modeled with the second-order EMT, the phase difference created by form birefringence for TE- and TM-polarized waves requires the use of a more rigorous, RCWA approach. The design of HSF gratings in terms of their form birefringence and reflectivity properties is discussed in conjunction with polarization-selective computer-generated holograms. A specific design optimization example furnishes a grating profile that provides a trade-off between the largest form birefrin gence and the lowest reflectivities.

Polarizing beam splitters that use the anisotropic spectral reflectivity (ASR) characteristic of high-spatialfrequency multilayer binary gratings have been designed, fabricated, and characterized. Using the ASR effect with rigorous coupled-wave analysis, we design an optical element that is transparent for TM polarization and reflective for TE polarization at an arbitrary incidence angle and operational wavelength. The experiments with the fabricated element demonstrate a high efficiency (Ͼ97%), with polarization extinction ratios higher than 220:1 at a wavelength of 1.523 m over a 20°angular bandwidth by means of the ASR characteristics of the device. These ASR devices combine many useful characteristics, such as compactness, low insertion loss, high efficiency, and broad angular and spectral bandwidth operations.

A 490-nm-deep nanostructure with a period of 200 nm was fabricated in a GaAs substrate by use of electron-beam lithography and dry-etching techniques. The form birefringence of this microstructure was studied numerically with rigorous coupled-wave analysis and compared with experimental measurements at a wavelength of 920 nm. The numerically predicted phase retardation of 163.3± was found to be in close agreement with the experimentally measured result of 162.5 ± , thereby verifying the validity of our numerical modeling. The fabricated microstructures show extremely large artif icial anisotropy compared with that available in naturally birefringent materials and are useful for numerous polarization optics applications. © 1995 Optical Society of AmericaThe form-birefringence or artificial-birefringence effect occurs when the period of such microstructures is much less than the wavelength of the incident optical field and the far field of the transmitted radiation will possess only zero-order diffraction. The two prevalent approaches to characterize such artif icial dielectric properties of the microstructured boundary use the effective medium theory 1 and rigorous coupledwave analysis 2,3 (RCWA). In this study we choose to use RCWA because the simpler effective medium theory does not provide accurate results when the microstructured grating period approaches the wavelength of the radiation. 3,4Form-birefringent nanostructures (FBN's) have several unique properties 3 that make them superior to naturally birefringent materials: (i) A high value for the strength of form birefringence, Dn͞n, can be obtained by the selection of substrate dielectric materials with a large refractive-index difference (here Dn and n are the difference and the average effective indices of refraction, respectively, for the two orthogonal polarizations); for example, a high-spatial-frequency surface-relief grating of rectangular profile on a GaAs substrate provides a Dn͞n value of ϳ0.63, which is much larger than those found for naturally birefringent materials (e.g., for calcite the value of Dn͞n is ϳ0.1).(ii) The magnitude of form birefringence, Dn, can be adjusted by variation of the duty ratio as well as of the shape of the microstructures. Such FBN's are useful for constructing polarization-selective beam splitters 6,7 and generalpurpose polarization-selective diffractive optical elements such as birefringent computer-generated holograms 8 (BCGH's). A BCGH is a general-purpose diffractive optical element that has two independent though arbitrary impulse responses for the two orthogonal linear polarizations. BCGH elements are useful in various applications. 8 In its original design 9 a BCGH consists of two surface-relief substrates with at least one of them birefringent. The two independent etch depths of the BCGH element provide the two degrees of freedom necessary to encode the two independent phase functions. However, the BCGH fabrication process can be simplif ied by use of a single FBN made of an isotropic substrate. One can obtai...

We propose a design of an optical switch on a silicon chip comprising a 5 × 5 array of cascaded waveguide-crossing-coupled microring resonator-based switches for photonic networks-on-chip applications. We adopt our recently demonstrated design of multimode-interference (MMI)-based wire waveguide crossings, instead of conventional plain waveguide crossings, for the merits of low loss and low crosstalk. The microring resonator is integrated with a lateral p-i-n diode for carrier-injection-based GHz-speed on-off switching. All 25 microring resonators are assumed to be identical within a relatively wide resonance line width. The optical circuit switch can employ a single wavelength channel or multiple wavelength channels that are spaced by the microring resonator free spectral range. We analyze the potential performance of the proposed photonic network in terms of (i) light path cross-connections loss budget, and (ii) DC on-off power consumption for establishing a light path. As a proof-of-concept, our initial experiments on cascaded passive silicon MMI-crossing-coupled microring resonators demonstrate 3.6-Gbit/s non-return-to-zero data transmissions at on-and off-resonance wavelengths.