Measured and simulated performance of a ceramic micromechanical beam steering device at 94 GHz

We report the first experimental demonstration of a transmission-mode micromechanical beam steering device for use in standoff terahertz imaging and spectroscopy. The device was constructed by laminating laser-cut 96% alumina sheets to form two plates with interlocking rectangular gratings of 762 microm period and was characterized at 94 GHz in a free-space measurement setup with an automated elevation scan. Plate tilts as great as 6 degrees deflected the transmitted beam by 6 degrees for the transverse electric (TE) polarization and by 4 degrees for the transverse magnetic polarization. Finite-difference time-domain simulations of the TE performance were in good agreement with the measurements.     

Reduction of polarization-induced artifacts in grating-based spectrometers

An optical device that converts unpolarized light into a single polarization state is described. The device is based on a polarizing beam splitter that separates the two polarization directions. The beam splitter is combined with two pairs of equilateral prisms that are used to collimate the two beams in terms of both propagation and polarization directions. When it is used in combination with a blazed diffraction grating, this device is shown to effectively remove the polarization dependence of the first-order diffracted power. The device has an insertion loss of approximately 14% for purely s-polarized light. However, for unpolarized light incident upon the two gratings studied here, the increased throughput of the p-polarized component leads to an average relative gain in overall efficiency of 13%-19%, depending on the grating. In collimating the two polarization directions, the device may cause a reduction in spectral resolution for a rectangular entrance slit. As a result, the device is more likely to find use in spectrometers that have a circular aperture, such as that provided by an optical fiber.     

Reconfigurable electro-optical waveguide for optical processing

A highly versatile electro-optically induced waveguide is proposed, and some of its applications are discussed. The In(1-s)Ga(s)As(t)P(1-t)-based device can reconfigure an arbitrary refractive-index profile with high speed by using an array of stripe electrodes deposited along the device. This device can act as a variable fractional Fourier transformer or as a beam shaper. Some nonguiding applications based on a specific refractive-index patterning that is normal to the light-propagation direction, such as phase modulation and beam steering, can also be implemented with this device.     

Laser diagnostics of welding plasma by polarization spectroscopy

The application of polarization spectroscopy (PS) to detect atomic species in an atmospheric pressure welding plasma has been demonstrated. PS spectra of Na atoms, seeded in the shielding gas flow of a gas tungsten arc welding (GTAW) plasma, are presented at different pump beam energies. The nature of the PS technique was found to be very efficient in suppressing the high background emission associated with the welding plasma. The PS spectral profiles appear to be Lorentzian and Lorentzian cubed for high and low pump beam energy, respectively. The effect of beam steering, due to the thermal gradient in the interaction plasma zone, was addressed. It was found that there is 2% unavoidable error in the detectable PS signal.     

Achromatic 90° polarization rotating prisms

Half wave plates are often used to rotate the plane of polarization by 90° at one wavelength. To rotate the plane of polarization over a range of wavelengths, Fresnel rhombs may be used. Fresnel rhombs, however, exhibit significant deviation from precisely 180° retardation over any broad wavelength range. This note describes two prism (or mirror) designs which rotate the incident polarization exactly 90° at all wavelengths. In one case the incident beam direction is deviated by 90°, while in the other case the incident beam is transmitted undeviated.     

Phased-array antenna, maximum-compression, reversible photonic beam former with ternary designs and multiple wavelengths

A novel, switched, photonic delay-line ternary design combined with a wavelength-multiplexed transmit-receive beam-former architecture is proposed. One-dimensional antenna beam steering by use of a single-channel, wavelength-dependent switched photonic delay line in cascade with a multichannel wavelength-independent switched photonic delay line is proposed for hardware-compressed, phased-array antenna control with subarray antenna partitioning. Beam-former architecture extensions to two-dimensional antenna beam steering are described.     

Beam steering experiment with two cascaded ferroelectric liquid-crystal spatial light modulators

The design, construction, and evaluation of a laser beam steerer that uses two binary ferroelectric liquid-crystal (FLC) spatial light modulators (SLMs) operated in conjunction are presented. The system is characterized by having few components and is in principle lossless. Experimentally, a throughput of approximately 20% was achieved. The simple system design was achieved because of the high tilt angle FLC material used in the SLMs, which were specifically designed and manufactured for this study. By coherently imaging the first SLM onto the second SLM, pixel by pixel, we obtained an effective four-level phase structure with a phase step of 90 degrees. An appropriate alignment procedure is presented. The beam steering performance of the system is reported and analyzed.     

Symmetrically coated pellicle beam splitters for dual quarter-wave retardation in reflection and transmission

A trilayer pellicle that consists of a high-index center layer that is symmetrically coated on both sides by a low-index film can be designed to produce differential reflection and transmission phase shifts of +/- 90 degrees at oblique incidence and equal throughput for the p and the s polarizations. Such a device splits a beam of incident linearly polarized light into two orthogonal circularly polarized components that travel in well-separated angular directions. Examples of infrared dual quarter-wave retarders that use a symmetrically coated Ge pellicle at 77 degrees angle of incidence are presented. A 50-50% splitter requires a symmetric pellicle with at least five layers. Error analysis shows that the thicknesses of the high-index layers must be tightly controlled. These circular polarization beam splitters are intended for operation with a well-collimated light source and can be used as the basis of a novel circular polarization Michelson interferometer.     

Micro-electro-mechanical system-based digitally controlled optical beam profiler

An optical beam profiler is introduced that uses a two-dimensional (2-D) small-tilt micromirror device. Its key features include fast speed, digital control, low polarization sensitivity, and wavelength independence. The use of this 2-D multipixel device opens up the important possibility of realizing several beam profile measurement concepts, such as a moving knife edge, a scanning slit, a moving pinhole, a variable aperture, and a 2-D photodiode array. The experimental proof of the optical beam profiler concept using a 2-D digital micromirror device to simulate the 2-D moving knife edge indicates a small measurement error of 0.19% compared with the expected number based on a Gaussian beam-propagation analysis. Other 2-D pixel arrays such as a liquid-crystal-based 90 degrees polarization rotator sandwiched between crossed polarizers can also be exploited for the optical beam whose polarization direction is known.     

Broadband wide-angle polarization converter for LCD backlight

A novel polarization converter using reflective metallic gratings and a polarization beam splitter is introduced for LCD backlight illumination. These two optical elements form a polarization rotation resonator. Broadband and high optical efficiency of polarization conversion in the visible region is achieved through the resonance of the refracted light and the surface plasmon wave in metallic surface-relief gratings. For wide-angle illumination, the conversion efficiency with arbitrary incident angle is studied. This device can convert unpolarized light to linear polarization with over 85% efficiency.     

Development of high-throughput, polarization-maintaining, near-field probes

A novel, chemical-etching technique produces very high throughput, polarization-maintaining probes for near-field, scanning, optical microscopy (NSOM). The process includes coating the tips with aluminum and forming the apertures with a focused ion beam (FIB). The elliptical core fibers used resulted in elliptical apertures for the probes. The throughput of the probes depends on the incident polarization. For polarization parallel to the minor axis, the tip presents an insertion loss of only 20 dB for aperture widths of 55 nm. Probes have a typical polarization extinction of 100 to 1 in the far field. These tips produced NSOM images of gold dots on a GaAs substrate in reflection mode.     

Ferroelectric liquid-crystal polarization-control devices with a double-layer cell structure

We analyzed the polarization-switching characteristics of a ferroelectric liquid-crystal polarization control device using a double-layer cell structure for free-space optical processing and switching systems. We theoretically derived the optimum arrangement of layered cells for 90° polarization switching. The theory shows that the optimum parameter range for the double-layer cell structure is wider than that for a single-layer cell. We verified our theory by measuring the polarization cross talk of experimental polarization control devices.     

Gas phase sorting of fullerenes, polypeptides and carbon nanotubes

We discuss the Stark deflectometry of micro-modulated molecular beams for the enrichment of biomolecular isomers as well as single-wall carbon nanotubes, and we demonstrate the working principle of this idea with fullerenes. The sorting is based on the species-dependent susceptibility-to-mass ratio χ/m. The device is compatible with a high molecular throughput, and the spatial micro-modulation of the beam permits one to obtain a fine spatial resolution and a high sorting sensitivity.     

Effects of substrate birefringence and tilt on the irradiance and phase patterns of the return beam in magneto-optical disk data storage

Substrate birefringence in a magneto-optical disk system is shown to have a predictable effect on the return beam. The irradiance and phase patterns of the return beam at the exit pupil of the objective lens are calculated and experimentally verified for the cases of no substrate birefringence, birefringence aligned with the incident polarization, and birefringence aligned at 45° to the incident polarization. The irradiance at the exit pupil is also calculated (and experimentally verified) for a grooved substrate for various amounts of substrate tilt.     

Electro-optic beam-steering device based on a lanthanum-modified lead zirconate titanate ceramic wafer

We demonstrate a lanthanum-modified lead zirconate titanate ceramic-based electro-optic beam-steering device that has a 3 mm × 3 mm working area. A series of resistors were made by evaporation of chromium onto the substrate to produce and control the required voltage distribution among the electrodes. A steering angle of 0.04° was obtained with an applied voltage of 700 V. Design considerations, computer simulations, and experimental results are presented.     

Optical phased-array beam steering controlled by wavelength

A scheme for optical phased-array beam steering controlled by wavelength is proposed. In this scheme, the optical scanning device consists of arrayed optical waveguides with specific length differences, by which the desired phase slope that results in optical beam steering is formed at the ends of the waveguides and can be changed by varying the optical wavelength. By introducing the concept of irregularly spaced arrays, sidelobes can be dramatically suppressed regardless of large center-to-center interelement spacing. The absolute phase difference between adjacent elements plays a vital role in optical beam steering, and the relation between the nonuniform length difference and the corresponding center-to-center spacing among elements is found.     

High Transmission LiF Prism Monochromator for Separation of VUV Wavelengths in Frequency Mixing Experiments

Abstract

We describe a simple, high throughput, LiF prism monochromator for the vacuum ultraviolet (VUV). The instrument throughput is 30% at 145 nm and the output can be tuned within the range 120–185 nm. The very high dispersion of LiF in the VUV means that neither narrow slits nor high precision are required. The suitability of this instrument for separation of coherent VUV radiation, generated by four-wave mixing, from the intense fundamental laser radiation (at 212 nm and the visible) is demonstrated. General advantages in throughput, polarization and resistance to radiation damage of such a prism instrument compared to a diffraction grating monochromator are discussed.     

Single-mirror beam steering system: analysis and synthesis of high-order conic-section scan patterns

The vector approach introduced in an early paper for modeling mirror-scanning devices [Y. Li, Appl. Opt. 34, 6417 (1995)] provides the basis of a rigorous study of the scan field generated by a single-mirror beam steering system, in which a hinged movable mirror is able to turn about a fixed pivot point to steering a single laser beam. Because of fewer constraints on mirror angular motion, the system may behave like a true point source for both vector and raster scanning applications. After a summary of the expressions for scan fields generated under different conditions, some fundamental and advanced topics of the single-mirror system are addressed: (1) basic parameters of high-order conic-section scan patterns, (2) scanning spot kinematics, (3) effect of input offset and pixels distortions on two-dimensional images displayed on screens of different formats, (4) mapping and its inverse between the mirror vector space and the scan vector space, and (5) single-mirror beam steering system as a one-element reflective and continuous image zooming device.     

High-resolution optical angle sensors: approaching the diffraction limit to the sensitivity

We carry out a detailed analysis of angle-sensitive devices based on the critical-angle effect. We consider their use in measuring small angular deflections of a laser beam. We establish the diffraction limit to the sensitivity for optical-angle sensors based on reflection and transmission of a laser beam. We find that this limit is identical to that of the triangulation scheme when using a position-sensitive detector or the autocollimation scheme. We analyze the main proposals to date of optical-angle sensors based on the critical-angle effect, focusing on their maximum sensitivity and their polarization dependence in practical conditions. We propose and analyze theoretically a novel and simple angle-sensitive device for sensing optical-beam deflections with very low polarization dependence and a maximum sensitivity close to the diffraction limit when used with typical laser beams. We discuss the basic principles for designing this type of device, provide numerical results, and point out a convenient fabrication procedure.     

Spin-polarized light-emitting diodes with Mn-doped InAs quantum dot nanomagnets as a spin aligner

We have fabricated and characterized surface-emitting, spin-polarized light-emitting diodes with a Mn-doped InAs dilute magnetic quantum dot spin-injector and contact region grown by low-temperature molecular beam epitaxy, and an In(0.4)Ga(0.6)As quantum dot active region. Energy-dispersive X-ray and electron energy loss spectroscopies performed on individual dots indicate that the Mn atoms incorporate within the dots themselves. Circularly polarized light is observed up to 160 K with a maximum degree of circular polarization of 5.8% measured at 28 K, indicating high-temperature spin injection and device operation.