Optical receiver sensitivity enhancement by single- and dual-band fiber optical parametric amplifier.

A semi-classical model is proposed theoretically and demonstrated experimentally on the optical receiver sensitivity enhancement by single-band (signal or idler) and dual-band (signal and idler) fiber optical parametric amplifier (FOPA). The sensitivity enhancement by single-band is determined by the gain of FOPA and the transmission loss of signal and idler, and it can be further improved by up to 3-dB using amplified signal and phase-conjugated idler together at dual-band configuration. The theoretical results are experimentally verified in both fiber communication and biomedical imaging applications. This detection sensitivity enhancement scheme can be potentially applied in the scenarios where ultrafast broadband signal at low-power level is being handled.

Quadrature and number fluctuations produced by parametric devices driven by pulsed pumps.

Two sets of formulas are derived for the field-quadrature and photon-number fluctuations (variances and correlations) produced by parametric amplifiers and frequency convertors that are driven by pulsed pumps and act on pulsed signals. The first set is based on the Green functions for the underlying parametric processes, whereas the second is based on the associated Schmidt coefficients and modes. These formulas facilitate the modeling and performance optimization of parametric devices used in a wide variety of applications.

Performance investigation of a hybrid fiber optical parametric amplifier.

We demonstrate reduction of four-wave mixing crosstalk using hybrid optical parametric amplifier with erbium-doped fiber amplifier. Crosstalk reduction of more than 13 dB has been achieved while providing 20-dB gain. Noise figures of different amplifier configurations are compared. Bit error rate measurements confirm the hybrid configuration introduces < 0.5 dB and < 1.5 dB power penalties in signal and idler wavelength, respectively, compared with a pure optical parametric amplifier.

Accurate numerical simulation of short fiber optical parametric amplifiers.

We improve the accuracy of numerical simulations for short fiber optical parametric amplifiers (OPAs). Instead of using the usual coarse-step method, we adopt a model for birefringence and dispersion which uses fine-step variations of the parameters. We also improve the split-step Fourier method by exactly treating the nonlinear ellipse rotation terms. We find that results obtained this way for two-pump OPAs can be significantly different from those obtained by using the usual coarse-step fiber model, and/or neglecting ellipse rotation terms.

Storage limit of two-photon-based three-dimensional memories with parallel access.

For systems that use parallel access by simultaneous writing or reading of bits located in an entire common plane, diffraction sets a limit to the storage density that is far smaller than that for sequential operation. Comparable densities can be achieved by using a three-dimensional waveguiding structure.

Mode-coupling analysis of bending losses in IR metallic waveguides.

The bending losses of infrared metallic waveguides are studied by a mode-coupling analysis. Previous results obtained for slab waveguides are confirmed. The high losses observed in practice with circular guides are explained by the near degeneracy between the TE(01) and TM(11) modes. The method is also used to study losses due to nonsteady-state propagation, such as caused by input mismatch or varying bend radii. Circular metallic waveguides with a thin dielectric lining on the inside could perform better than unlined guides, from the points of view of uniform bending losses and single-mode propagation under arbitrary bends.

Discrete Fourier transforms by single-mode star networks.

A single-mode star network, made from polarization-preserving components, can perform the spatial discrete Fourier transform of coherent light patterns presented at the inputs. This can be accomplished with passive components, such as 2 x 2 couplers, and propagation delays. The Hadamard transform can be performed similarly.

Hierarchic and combinatorial star couplers.

An interconnection method for transmissive stars is presented that permits the fabrication of arbitrarily large stars from elementary ones. Being free from multiple-path interference, this synthesis is particularly well suited for single-mode stars. The method can be implemented with any of a number of techniques; in particular, the use of recently developed high-quality 3-dB couplers should result in the synthesis of large single-mode stars with negligible excess loss.

Mirror-folded free-space crossbars with holographic implementation.

Free-space optical crossbars that utilize elements such as beam splitters and holograms can in principle be folded by two parallel plane mirrors and implemented in a single plane, or even in a single line, parallel to the mirrors. Such crossbars, each performing either a processing or an interconnection function, can in turn be cascaded along a common plane or line between a single set of two mirrors to perform a large class of operations. By use of all possible inputs and outputs, novel power-conserving operations can also be performed. We report on the holographic implementation of a reconfigurable 2 × 2 mirror-folded generalized crossbar with a thermoplastic holographic camera; the bypass, the exchange, and the broadcast states are demonstrated.

3 x 3 single-mode star coupler made from 2 x 2 couplers.

An optical network made from two 3-dB and one 1.76-dB 2 x 2 single-mode couplers can perform as a 3 x 3 star coupler. One version is particularly suitable for polarization-preserving components and requires adjustment of the phase in one arm; it can also perform the three-point discrete Fourier transform by suitable adjustment of other phases. Experimental verification of the principle is presented.

Transverse magnetic loss of metallic whispering gallery waveguides at 10.6 microm.

The TM losses of metallic whispering gallery waveguides in the IR have been calculated for a range of angles of incidence. The allowed angles of incidence have been calculated from the caustic of the rays associated with particular modes. In many practical situations the pseudo-Brewster angle is large compared with the allowed ray angles, so that the usual assumptions must be reconsidered. In that case we find that the TM losses are much lower than generally assumed and that the TM and TE loss ratio is independent of the wall material. Experimental verification of these conclusions is presented.

Matched filtering for efficient cleanup of strongly distorted beams.

Efficient one-way cleanup of laser beams can be accomplished by holographic matched filtering. Severe amplitude and phase distortions, such as those caused by passage through multimode fibers or ground-glass diffusers, can be compensated for, with theoretical efficiencies of the order of 50% for light in a single state of polarization. Experiments have confirmed the improvement in beam quality.

Efficient white laser illuminators for plastic optical fibers.

Multiwavelength lasers are used to compensate for color distortion in transmission of light through plastic optical fibers. Efficient methods to eliminate the speckle effect and to generate multicolor laser light are described.

Backscattering patterns of step index optical fibers: angular width.

Angular widths of laser backscattering patterns from optical fibers with thin claddings due to rays reflected by core-cladding interface are investigated. Calculated and measured values are presented and found to be in good agreement. A summary of different backscattering patterns from clad fibers is given.

Simultaneous oscillation of competing laser transitions.

The conditions necessary to obtain simultaneous oscillation of two laser transitions sharing a common energy level are examined. The general form of the equations is the same for all three possible cases, and it is shown that the operating point should lie within the area limited by the axes of coordinates and two straight lines in the transmittance plane. Parameters determining those lines can either be calculated or measured experimentally; their slopes depend only upon the transition parameters and are thus the same for all lasers operating on given transitions.

Misalignment in imaging multifibers.

A statistical approach is used to characterize image transmission through misaligned multifibers, i.e., differently arranged at the input and output faces, leading to a spatially invariant line spread function. The resulting MTF is the product of the MTF for the aligned system and a function characterizing stochastic departures from alignment. The case of circular fibers with Gaussian misalignment is treated theoretically and is found to account satisfactorily for experimentally observed results.

Selective broadcast interconnection: a novel scheme for fiber-optic local-area networks.

We introduce a passive, unswitched scheme for directly interconnecting N stations, each of which has C transmitters and receivers. Implementations using fiber optics with spatial multiplexing and optionally wavelength multiplexing are discussed. This scheme utilizes the same resources as standard topologies with C parallel buses but outperforms them in two respects: (1) the aggregate throughput is proportional to C(2) rather than to C and (2) the power of each transmitter need reach only N/C, instead of N, receivers.

Coupling of rigid to flexible imaging multifibers.

By magnifying the high-quality image produced by a rigid miniature endoscope onto a large flexible multifiber, an instrument has been developed which provides both high resolution and flexibility. It is shown theoretically, and verified experimentally, that the MTF of a system of coupled multifibers is the product of the individual MTFs. Measurements show that the coupled instruments outperform state-of-the-art all-flexible systems. The technique should greatly facilitate observation, display, and photography in percutaneous endoscopy and in a variety of industrial applications.

Continuous-wave fiber optical parametric oscillator.

We report continuous-wave operation of singly resonant fiber optical parametric oscillators. In a cavity formed by 100 m of highly nonlinear fiber and two fiber Bragg gratings, the pump power threshold was 240 mW; the output wavelength could be tuned over 80 nm by tuning of the pump. We also obtained an internal conversion efficiency of 30%, compared with the maximum theoretical value of 50%, by use of a 1-km-long cavity.

92% pump depletion in a continuous-wave one-pump fiber optical parametric amplifier.

Theory shows that near-complete pump depletion can be obtained in uniform fiber-optic parametric amplifiers (OPA's) for a particular phase-matching condition. We have demonstrated 92% pump depletion in a cw fiber OPA, with a 200-mW pump at 1560 nm in an 11-km-long dispersion-shifted fiber.