Adaptive liquid lens actuated by photo-polymer.

An adaptive liquid lens actuated by a photo-polymer is demonstrated. The lens cell consists of a top glass substrate and a bottom plastic slab with two holes: reservoir hole and lens hole, which are sealed with elastic membranes. A photo-sensitive polymer is attached to the membrane of the reservoir hole. Under blue light irradiation, the polymer is bent which exerts a pressure to regulate the curvature of the membrane on the lens hole and then change the focal length of the plano-convex lens. The focal length is tunable from infinity to 21.2 mm in seconds. Non-mechanical driving, easy integration with other optical components and compact system are the key features of this lens.

Broadening the angular tolerance in two-dimensional grating resonance structures at oblique incidence.

Broadening of the angular response of two-dimensional (2D) guided mode resonant spectral filters at oblique incidence is investigated. Coupling into multiple fundamental guided resonant modes having the same propagation constant but propagating in different planes (inherent multiple-plane diffraction by 2D gratings) is shown to significantly broaden the angular tolerance while maintaining narrow linewidth. Resonances have symmetric and broad angular responses when the incident wave is coupled to four resonant modes in a structure with a hexagonal grating pattern. Further broadening is implemented by enhancing the second Bragg diffraction of the 2D grating structure. Resonance with a narrow spectral linewidth (Dlambda(FWHM) approximately 1.6 x 10(-4)lambda(0)) and angularly tolerant to an ~6 mum beam diameter is obtained. A second approach utilizing a dual 2D grating configuration with a second grating on the substrate side is shown to increase the lateral confinement, causing the merging of two successive resonant bands. This results in further improvement of the angular/spectral linewidth ratio by ~80%.

Control of resonance separation over a wide spectral range in multiwavelength resonant grating filters.

The spectral separation of resonances within multiwavelength resonant grating filters is studied through the use of a grating layer with constant structural parameters in combination with single-layer, multimode waveguides and multilayer, multimode waveguides. The use of multilayer, multimode waveguides are shown to provide the ability to control resonance separations for multiwavelength filters from hundreds of nanometers to only a few nanometers.

All-dielectric unidirectional dual-grating output coupler.

A novel concept for an all-dielectric unidirectional output double-grating coupler is proposed and rigorously analyzed. In addition to a superstrate side grating, a second grating is placed on the substrate side. The periodicities of the gratings are chosen such that no propagating diffracted orders are present outside the structure in the superstrate region and only a single order is present outside the structure in the substrate region. The concept provides a robust output coupler requiring neither phase-matching between gratings nor any resonances in the structure, and is very tolerant to potential fabrication errors. Up to 96% coupling efficiency from the substrate-side grating is obtained over a wide range of grating properties.

Multiple wavelength resonant grating filters at oblique incidence with broad angular acceptance.

Multilayer, multimode waveguides are utilized in resonant grating filters having a broadened angular acceptance bandwidth for multiple wavelengths at a single oblique angle of incidence. It is shown that the waveguide grating structure should support a few leaky modes in order to support a multiwavelength resonant filter at oblique incidence with broadened angle acceptance at each wavelength.

Multiline two-dimensional guided-mode resonant filters.

A novel multiline filter using a two-dimensional guided-mode resonant (GMR) filter is proposed. The filter concept utilizes the multiple planes of diffraction produced by the two-dimensional grating. Multiple resonances are obtained by matching the guided modes in the different planes of diffraction to different wavelengths. It is shown that the location and the separation between resonances can be specifically controlled by modifying the periodicity of the grating and the other physical dimensions of the structure. This is in contrast to the one-dimensional GMR filters where the location of the resonances is material dependent. Two-line reflection filter designs with spectral linewidths less than 1 nm and a controllable spectral separation of up to 23% of the short resonance wavelength are presented using rectangular-grid grating GMR structures. Three-line filters are designed in hexagonal-grid grating GMR structures with two independently controllable resonance locations.

Integrated wavelength stabilization of in-plane semiconductor lasers by use of a dual-grating reflector.

We summarize a novel integrated wavelength-stabilization scheme for broad stripe surface-emitting lasers. The method is based on two gratings fabricated on opposite sides of a device in which the first grating disperses light through the substrate to the opposite side, where the second surface has a feedback grating that operates under total internal reflection and in the Littrow condition to provide feedback into the gain medium. Experimental results have been obtained for both high power and a narrow linewidth, showing a CW slope of 0.85 W/A.

All-optical display using photoinduced anisotropy in a bacteriorhodopsin film.

Photoinduced anisotropy in a bacteriorhodopsin film using the pump-probe method was investigated. A diode-pumped second-harmonic YAG laser was used as the pump beam, and three wavelengths, at lambda = 442, 532, 655 nm, from different lasers were used as probe beams. Without the pump beam, the probe light cannot transmit the analyzer to the detector. However, because of photoinduced anisotropy, a portion of the probe light is detected when the pump beam is present. Based on this property, we demonstrate a full-color all-optical display.

Flat-top narrow-band spectral response obtained from cascaded resonant grating reflection filters.

Cascaded identical resonant grating reflection filters are shown to exhibit flattened spectral responses when the individual filter elements are cascaded pi out of phase. Off resonance, the net response of a cascaded arrangement is given approximately by the sum of the individual filter responses. Cascading filters pi out of phase thus result in a reduction in the off-resonance reflection levels and correspondingly an increase in the spectral bandpass ratio. The spectral bandpass ratio is a figure of merit used to gauge the flatness of a response and is defined as the ratio of the linewidth at an efficiency of 90% to the linewidth at an efficiency of 10%. Cascading two and three filters in this manner results in respective increases in the spectral bandpass ratio of three times and more than five times that of a single filter.

Normally incident resonant grating reflection filters for efficient narrow-band spectral filtering of finite beams.

Resonant grating reflection filters operating at normal incidence are investigated. At normal incidence these structures are shown to have characteristics different from those of structures operated at oblique incidence. We show how higher-order diffraction within the structure laterally confines the incident energy about the point of incidence and results in a broadened angular selectivity. Multimode structures are shown to exhibit broader angular selectivities and narrower spectral linewidths than those of single-mode structures, achieving angular selectivities hundreds of times broader than what could be obtained at oblique incidence. When compared with that for oblique incidence, the increase in angular selectivity is shown to greatly improve the performance of these filters for spectrally filtering finite incident beams.

Interference approach applied to dual-grating dielectric resonant grating reflection filters.

The resonant mechanisms associated with dual-grating dielectric resonant grating reflection filters are described by use of an interference approach. These structures consist of two modulated regions of equal period separated by a higher-index film region. We show that the spectral linewidth is dependent on the separation between the modulated regions and can range from theoretically zero to approximately four times what would be obtained by use of a single-grating geometry.

Diffractive optics and micro-optics: introduction to the feature issue.

This issue of Applied Optics features 19 articles related to the fabrication and the design of diffractive and micro-optics as well as their applications. A companion feature in the Journal of the Optical Society of America A includes papers on the modeling of diffractive elements.

Hybrid two-dimensional subwavelength surface-relief grating-mesh structures.

The homogeneous behavior of periodic two-dimensional subwavelength surface-relief structures that contain both gratings and meshes (inverse gratings) are investigated. It is shown that when effective indices are synthesized near the higher index (substrate region), mesh structures yield larger feature sizes compared with their grating counterparts, whereas grating structures yield larger feature sizes when effective indices are synthesized near the lower index (incident region). For each type of structure investigated, a relation between the parameters of the structure and an effective refractive index is determined. It is shown that an equal area occupied by the high- or low-index media within the grating cell does not, in general, result in equal effective indices. The effective index of the grating is shown to be characterized by both the shape (local distribution) and the area of the high- or low-index medium within the unit grating cell. Finally, the advantages of subwavelength gratings and meshes are combined to produce hybrid grating-mesh structures that are less demanding on the fabrication process.

Diffractive optics: an introduction to the feature issue.

This special feature of Applied Optics contains 20 papers on the design, applications, and fabrication of diffractive optics. The companion feature in the Journal of the Optical Society of America A (May 1995) contains papers on diffractive optics modeling. Many of these papers result from presentations at the second OSA topical meeting on diffractive optics, June 6-9, 1994, in Rochester, New York.

Effects of process errors on the diffraction characteristics of binary dielectric gratings.

The effects of fabrication errors on the predicted performance of surface-relief phase gratings are analyzed with a rigorous vector diffraction technique. For binary elements, errors in the dimensions of the profile [depth, linewidth (fill factor), and grating period], as well as errors in the shape of the profile, are investigated. It is shown that the dimension errors do not have a significant effect on grating performance when the grating is designed for either maximum or minimum diffraction efficiency. A trapezoid is used to model the shape error of the profile. For the first time, design rules that significantly reduce the effects of any shape error are presented.

Real-time image transmission and interferometry through a distorting medium using two phase conjugators.

A phase-conjugate geometry with two photorefractive crystals is demonstrated. It can be used for real-time one- or two-way image transmission through a distorting medium and for real-time holographic interferometry.

Zero-reflectivity homogeneous layers and high spatialfrequency surface-reliefgratings on lossy materials.

The required thickness and complex refractive index of single homogeneous layers on lossy substrates to produce zero reflectivity are calculated by a rigorous impedance matching approach. The analysis is applicable to both TE and TM polarization and to any angle of incidence. The filling factor and groove depth of a rectangular-groove grating, equivalent to a single homogeneous lossy layer in the long-wavelength limit, are calculated. The method reduces to that previously found for dielectric surface-relief gratings in the limit of no losses. The antireflection behavior of the gratings is verified using the rigorous (without approximations) coupled-wave analysis of metallic surface-relief grating diffraction. It is shown that multiple zeroreflectivity solutions exist for both TE and TM polarizations and for any angle of incidence for an arbitrary complex-refractive-index substrate. Example zero-reflectivity gold gratings for incident free space wavelengths from 0.44 to 12.0 microm are presented.