Free-space optical shuffle implementations by use of birefringence-customized modular optics

A stacked integration technique based on polarization optics is studied for implementing shuffle-based interconnection networks with three-dimensional solid-state modules. A basic building block of the proposed scheme consists of a hole-patterned half-wave retarder for birefringence customizations and a calcite slab for subsequent beam deflections. On the basis of a cascade of such building blocks the submodules of various shuffle-family permutations can be implemented. To minimize channel cross talk, we incorporated a collimating-relaying imaging system. To help design birefringence customization, we developed algebraic formulations of folded shuffle operations using separable shuffles. Proof-of-concept experimental results, as well as system design, fabrication, and integration issues, are discussed.     

Free-space wavelength-multiplexed optical scanner

A wavelength-multiplexed optical scanning scheme is proposed for deflecting a free-space optical beam by selection of the wavelength of the light incident on a wavelength-dispersive optical element. With fast tunable lasers or optical filters, this scanner features microsecond domain scan setting speeds and large- diameter apertures of several centimeters or more for subdegree angular scans. Analysis performed indicates an optimum scan range for a given diffraction order and grating period. Limitations include beam-spreading effects based on the varying scanner aperture sizes and the instantaneous information bandwidth of the data-carrying laser beam.     

Free-space wavelength-multiplexed optical scanner demonstration

Experimental demonstration of a no-moving-parts free-space wavelength-multiplexed optical scanner (W-MOS) is presented. With fast tunable lasers or optical filters and planar wavelength dispersive elements such as diffraction gratings, this microsecond-speed scanner enables large several-centimeter apertures for subdegree angular scans. The proposed W-MOS design incorporates a unique optical amplifier and variable optical attenuator combination that enables the calibration and modulation of the scanner response, leading to any desired scanned laser beam power shaping. The experimental setup uses a tunable laser centered at 1560 nm and a 600-grooves/mm blazed reflection grating to accomplish an angular scan of 12.92 degrees as the source is tuned over an 80-nm bandwidth. The values for calculated maximum optical beam divergance, required wavelength resolution, beam-pointing accuracy, and measured scanner insertion loss are 1.076 mrad, 0.172 nm, 0.06 mrad, and 4.88 dB, respectively.     

Pulse-laser electroholography by use of interference fringe patterns captured by a CCD

Digital holography is combined with a pulse-laser electroholographic system for a real-time three-dimensional display. Owing to the one-dimensional characteristics of the Bragg-regime acousto-optic spatial-light modulator, vertical parallax cannot be generated from the acoustic signal propagating along the fan-shaped beam direction of the incident laser. To obtain a proper interference pattern, we attach a horizontal slit to the confocal lens system for recording the fringe data, significantly reducing the bandwidth of the vertical fringe data. When the bandwidth-reduced fringe data are displayed by use of a pulse-laser electroholographic system, the clarity and the quality of the image are found to be appreciably improved.     

Free-space optical communications link budget estimation

This paper describes a new methodology of estimating free-space optical communications link budgets to be expected in conditions of severe turbulence. The approach is derived from observing that the ability of an adaptive optics (AO) system to compensate turbulence along a path is limited by the transmitter and receiver Rayleigh range, proportional to the diameter of the optics squared and inverse of the wavelength of light utilized. The method uses the Fried parameter computed over the range outside of the transmitter and receiver Rayleigh ranges, to calculate the Strehl ratios that yield a reasonable prediction of the light impinging on the receiving telescope aperture and the power coupling into the fiber. Comparisons will be given between theory and field measurements. These comparisons show that AO is most effective within the Rayleigh ranges, or when an atmospheric gradient is present, and lesser so when the total range is much greater than the sum of the Rayleigh ranges.     

Free-space optical communication through a forest canopy

We model the effects of the leaves of mature broadleaf (deciduous) trees on air-to-ground free-space optical communication systems operating through the leaf canopy. The concept of leaf area index (LAI) is reviewed and related to a probabilistic model of foliage consisting of obscuring leaves randomly distributed throughout a treetop layer. Individual leaves are opaque. The expected fractional unobscured area statistic is derived as well as the variance around the expected value. Monte Carlo simulation results confirm the predictions of this probabilistic model. To verify the predictions of the statistical model experimentally, a passive optical technique has been used to make measurements of observed sky illumination in a mature broadleaf environment. The results of the measurements, as a function of zenith angle, provide strong evidence for the applicability of the model, and a single parameter fit to the data reinforces a natural connection to LAI. Specific simulations of signal-to-noise ratio degradation as a function of zenith angle in a specific ground-to-unmanned aerial vehicle communication situation have demonstrated the effect of obscuration on performance.     

Free-space optical interconnections with liquid-crystal microprism arrays

Liquid-crystal microprism arrays are shown to be useful for providing electrically controlled alignment of optical beams and fixed various free-space optical interconnections. They can deflect closely spaced micro-optical beams individually to any position with high transmittance (95%), high deflection angle (~10°), and low voltage (<2.8 V(rms)). Various fixed optical interconnections can be made simply by changes in the voltages applied to the microprism.     

Free-space optical relay for the interconnection of multimode fibers

We present results from a system that shows that multimode fibers can be used for both the input and the output of a free-space optical system. The system consists of plastic microlenses integrated with plastic optomechanical components that are suitable for low-cost fabrication and assembly. Such a system opens up opportunities to construct large repeaters and switches for multigigabit ethernet applications by integration with two-dimensional arrays of optoelectronic devices. We demonstrate a 2.5-Gbit/s transmission rate by using commercial vertical-cavity surface-emitting lasers coupled to 62.5-mum core fibers. We consider the design constraints and the capabilities of custom optical modules suitable for mass production.     

Free-space optical module configuration using a guide-frame assembly method

A new assembly method is described for easy construction of optical modules consisting of guide frames, spacer frames, and a housing frame. This method is used to assemble a two-dimensional optical-fiber collimator and a digital discrete correlator, which are fundamental parts of free-space optical computing systems. We show that a multistage optical system can be constructed simply by stacking of several optical functional blocks. Moreover, these compact modules do not need a conventional optical bench, they are compact, and assembly time is reduced. We demonstrated by experiment that the accuracy of optical modules assembled with this method is within the specifications of the optical system.     

Generation of continuously wavelength-tunable optical short pulses by use of two self-seeded Fabry-Perot laser diodes and an optical switch

Generation of wavelength-tunable optical short pulses by use of two self-seeded Fabry-Perot laser diodes in a parallel configuration is described. The system supports continuous wavelength tuning in a relatively wide range of 42 nm. The side-mode suppression ratio achieved is 35 dB across the entire wavelength tuning range. The system is convenient for continuous wavelength tuning.     

Compact optical crossbar switch

A novel compact holographic crossbar architecture based on planar optics is presented. It consists of a pair of identical planar holographic elements, a two-dimensional array (N x N) transmission mask, a one-dimensional array (N) of input light sources, and a one-dimensional array (N) of detectors. Each planar element contains two cylindrical holographic lenses, both of which are recorded on a single glass substrate. The design of the overall compact configuration is presented along with experimental results.     

Free-space optical crossbar network integrated in a single block of LiNbO3 crystal

A free-space optical crossbar network integrated in a single block of LiNbO(3) crystal is proposed, which consists of stages of 2×2 switches making use of the electro-optic effect of crystal and in-between routing devices for permutation based on double refraction and internal double reflection on interfaces. Two basic configurations are suggested. A control algorithm for the crossbar network is discussed, which may control a nonblocking interconnection between any input and output. The integrated crossbar network is low energy loss, nonblocking, easy to assemble, and insensitive to environment. A 3×3 crossbar network is designed and the experiment is demonstrated.     

Design of polarisation-independent bidirectional optical switch

Optical switching plays a vital role in all optical communication and network. Various optical switchings have been developed in recent years as a way to improve reliability problems and to build small size, robust integrated devices. A novel low crosstalk, high switching speed, compact in structure, efficient in performance, polarisation independent, and reversible 2 × 2 optical switching is proposed using a phase spatial light modulator (PSLM), a polarising beam-splitter (PBS), a mirror, a half-wave plate (HWP) and a quarter-wave plate (QWP). The optical switching should be helpful in the design of a large-scale switch matrix, and show considerable promise in the future of fibre optic networks.     

Interferometric optical switch enhanced by a multi-resonance ring resonator structure

We propose a novel configuration for optical switches by the use of two coupled ring resonators acting as a phase-shifting element in a Mach–Zehnder interferometer. Because the ring structure is multi-resonant within one period of its phase response, light at any frequency within the period can be addressed by tuning the device across a small frequency interval. This enables the use of low voltages for electro-optic control of the switch, allowing for a tunable photonic switching device that operates at 1?V voltage levels.     

An integrated electrochromic nanoplasmonic optical switch

We demonstrate an electrochemically driven optical switch based on absorption modulation of surface plasmon polaritons (SPPs) propagating in a metallic nanoslit waveguide containing nanocrystals of electrochromic Prussian Blue dye. Optical transmission modulation of ～96% is achieved by electrochemically switching the dye between its oxidized and reduced states using voltages below 1 V. High spatial overlap and long interaction length between the SPP and the active material are achieved by preferential growth of PB nanocrystals on the nanoslit sidewalls. The resulting orthogonalization between the directions of light propagation and that of charge transport from the electrolyte to ultrathin active material inside the nanoslit waveguide offers significant promise for the realization of electrochromic devices with record switching speeds.