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At least 1,027 records · Page 57Linked to original sources
PubMed2004

Suppression of dynamic laser speckle signals in multimode fibers of various lengths.

The effects of fiber coupling and fiber length on photocurrent fluctuations are studied when the light of a laser diode transmitted to and from a dynamic turbid medium by a step-index multimode fiber is studied. When the laser light is coupled asymmetrically, filling only the higher-order modes, the photocurrent fluctuations are suppressed significantly when fiber lengths of as much as 16 m are added between the laser and the medium. Addition of as much as 16 m of detection fiber, or any fiber in the case of symmetric light coupling, leads to much less or no suppression of the photocurrent fluctuations.

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PubMed2004

Exact analysis of low-finesse multimode fiber extrinsic Fabry-Perot interferometers.

A straightforward theory is presented to accurately model the light inferences in a low-finesse multimode fiber extrinsic Fabry-Perot (FP) interferometer. The effect on the fringe visibility of the gap length, sensor structure imperfections, and modal power distributions is explored. The analysis is particularly useful in the design and optimization of sensors that use an extrinsic FP cavity as the sensing element.

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PubMed2004

Rigorous comparison of parabolically tapered and conventional multimode-interference-based 3-dB power splitters in InGaAsP/InP waveguides.

Design issues such as optical transmission, interference mechanisms, the splitting ratio, the polarization dependence, and the fabrication tolerances of a compact parabolically tapered multimode-interference (MMI)-based 3-dB power splitter on an InP-based deeply etched ridge waveguide, by use of the finite-element-based beam-propagation method, are presented. The benefits and drawbacks of the use of the tapered structure, in comparison with an untapered MMI-based 3-dB splitter, have also been investigated.

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PubMed2005

Scanning near-field optical microscopy as a tool for the characterization of multimode interference devices.

We report the scanning near-field optical microscopy (SNOM) characterization of a 4 x 4 multimode interference (MMI) device working at a wavelength of 1.55 microm and designed for astronomical signal recombination. A comprehensive analysis of the mapped propagating field is presented. We compare SNOM measurements with beam-propagation-method simulations and thus are able to determine the MMI structure's refractive-index contrast and show that the measured value is higher than the expected value. Further investigation allows us to demonstrate that good care must be taken with the refractive-index profile used in simulation when one deals with low-index contrast structures. We show evidence that a step-index contrast is not suitable for adequate simulation of our structure and present a model that permits good agreement between measured and simulated propagating fields.

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PubMed2005

Compact silicon-on-insulator-based multimode interference coupler with bilevel taper structure.

A novel compact silicon-on-insulator- (SOI-)based multimode interference (MMI) coupler with bilevel taper structures was designed. The MMI section and the S-bend sections of the input-output waveguides are deeply etched. The input-output waveguides connecting to single-mode fibers or other photonic light circuits are etched shallowly to yield single-mode operation. A bilevel taper is introduced in the transition region between the shallowly and deeply etched regions. It is predicted theoretically that this design will not only improve the quality of the self-imaging in the MMI section but will also make the structure compact. Both the excess loss and the nonuniformity of the MMI coupler are reduced. By use of a three-dimensional beam propagation method, the performance of a 1 x 4 MMI coupler based on a SOI is simulated as a numerical example of the novel design. The simulated nonuniformity and the excess loss are approximately 0.0285 and 0.2 dB, respectively.

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PubMed2005

Microbend-induced mode coupling in a graded-index multimode fiber.

We present a theoretical analysis of the mode coupling effects at a microbend along a graded-index (GI) multimode fiber (MMF). By matching the incident and excited mode fields at the microbend, we obtain the coupling coefficients among the guided modes at different microbending conditions. The theoretical results compare well with the experimental results from near-field measurements on a GI MMF subject to microbending. The usefulness of our theory is demonstrated with two applications: (i) an explanation of the operation principle of a wavelength-switchable fiber laser that contains a fiber Bragg grating in a GI MMF and (ii) quantification of the function of a microbend-based mode scrambler.

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PubMed2006

Determination of refractive index profiles of Ag+ -Na+ ion-exchange multimode strip waveguides by variable wavefront shear double-refracting interferometry microinterferometry.

The results of experimental determination of multimode strip waveguide refractive index profiles and Ag concentration profiles obtained by using the variable wavefront shear double-refracting interferometry microinterferometer Biolar PI and an electron microprobe are presented. The strip waveguides under investigation are formed in soda lime glass in an external electric-field-assisted Ag+ -Na+ ion-exchange process from the molten AgNO3 salt by use of dielectric masks with channel apertures. A dry electrochemical technique of dielectric mask formation is applied. The influence of waveguide-forming parameters on the shape of Ag concentration profiles and the range of silver diffusion are shown. Changes in the usually assumed boundary conditions of electric-field calculations in ion-exchange numerical modeling are suggested.

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PubMed2006

Implementation of discrete unitary transformations by multimode waveguide holograms.

Integration of holograms into multimode waveguides allows the implementation of arbitrary unitary mode transformations and unitary matrix-vector multiplication. Theoretical analysis is used to justify a design approach to implement specific functions in these devices. Based on this approach, a compact mode-order converter, a Hadamard transformer, and a spatial pattern generator-correlator are proposed and analyzed. Beam propagation simulations are used to verify the theoretical calculations and to address bandwidth, scalability, and fabrication criteria. Optical pattern generators were successfully fabricated using standard photolithographic techniques to demonstrate the feasibility of the devices.

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PubMed2006

Multimode interference devices with input-output ports on the sides.

Multimode interference (MMI) devices are useful for power splitting and for the separation or combination of wavelengths or polarizations, usually in integrated optics. Input-output guides connect to the MMI region by ports. In all previously reported MMI devices, the input and output guides connect only to the ends of the MMI region; i.e., they are end ported. What is believed to be a novel arrangement of the input-output ports on MMI devices is described. By placing input-output ports either partially or entirely on the sides of the MMI region (i.e., side porting), a variety of benefits are achieved and a variety of new devices can be made.

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PubMed2006

Frequency-domain intermodal interferometer for the bandwidth measurement of a multimode fiber.

A new bandwidth measurement technique for a multimode optical fiber (MMF) using a frequency-domain intermodal interferometer is proposed. We have demonstrated that the relative modal delay (RMD) of a MMF can be obtained easily and accurately based on an optical frequency-domain reflectometry (OFDR) technique by using an intermodal interference signal among the excited modes of a MMF. As an example, a photonic crystal fiber with a few modes is prepared and its RMD is measured by using our proposed measurement technique. Measurement results are compared with those from a previously reported frequency-domain method. We have also measured the RMD of a commercial MMF as a practical application and compared our result with the one obtained from a well-known time-domain differential mode delay measurement technique.

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PubMed2004

Modal power decomposition of beam intensity profiles into linearly polarized modes of multimode optical fibers.

We calculate the modal power distribution of a randomly and linearly polarized (LP) multimode beam inside a cylindrical fiber core from knowledge of spatial-intensity profiles of a beam emitted from the fiber. We provide an exact analysis with rigorous proofs that forms the basis for our calculations. The beam from the fiber end is collimated by a spherical lens with a specific focal length. The original LP-mode basis is transformed by the spherical lens and forms another orthogonal basis that describes the free-space beam. By using this basis, we calculate the modal power distribution from the mutual-intensity profile. This is acquired by adopting a well-known mutual-intensity-profile-retrieving technique based on measurements of the intensity patterns several times after two orthogonal cylindrical lenses with varying separation. The feasibility of our decomposition algorithm is demonstrated with simulations.

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PubMed2005

Quantum-statistical analysis of multimode far-infrared and submillimeter-wave astronomical interferometers.

We present a detailed quantum-statistical model of multimode far-infrared and submillimeter-wave astronomical interferometers. The scheme identifies explicitly the optical modes associated with each telescope and uses these to trace the quantum-statistical properties of the field from a source through the telescopes, through the beam combiners, and onto the detectors. The scheme can be used with any optical configuration, and elegant expressions result for the average rate at which photons are detected by the pixels of an imaging array, the mean-square fluctuations in the rates, and the correlations between the fluctuations in the rates of different pixels. Numerous extensions to the basic technique are possible.

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PubMed2003

Direct imaging of multimode interference in a channel waveguide.

By use of a near-field scanning optical microscope in collection mode, multimode interference was directly measured in an annealed proton-exchanged LiNbO3 waveguide. Periodic transitions from a single-peaked Gaussianlike intensity distribution to a double-peaked intensity distribution were observed. The intensity distribution along the waveguide was calculated, and the results agree well with the experimental observation.

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PubMed2003

Near-field scanning optical microscopy imaging of multimode interference.

By use of a near-field scanning optical microscope (NSOM) in collection mode, the intensity distribution along a 2 x 2 multimode interference coupler was directly imaged as a function of wavelength. Although calculations can predict the general trend of wavelength dependence and the approximate positions of multiple images in the coupler, the accuracy is poor because of uncertainties in the waveguide width. We show that direct imaging using a NSOM bypasses calculational uncertainties and proves to be a powerful technique for studying these waveguide devices.

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PubMed2003

Waveguide lenses with multimode interference for low-loss slab propagation.

It is shown that a nearly ideal two-dimensional focusing Gaussian beam can be synthesized by use of a linear combination of the two lowest-order even modes of an optical waveguide. This property can be used to couple laterally guided modes across slab waveguide regions with low loss. The technique is illustrated by use of a conventional multimode interference (MMI) geometry, in which the MMI coupler transforms the fundamental mode of an initial waveguide into a focusing Gaussian beam, which is then fed to a slab region. Two-dimensional beam propagation simulations show that the beam does not initially diverge in the slab region, but rather comes to a focus. A second MMI coupler then transforms the diverging beam back to the initial mode. A structure is designed that can couple the fundamental mode of a 9-microm-wide waveguide across an 88-microm-long slab region with only a 0.036-dB loss. This technique can be applied to improve the performance of small-angle waveguide crossings and integrated turning mirrors.

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PubMed2003

Multimode reverse-symmetry waveguide sensor for broad-range refractometry.

A sensor design that uses a grating-coupled multimode planar optical waveguide for absolute detection of the refractive indices of liquids or gases is presented. The waveguide consists of a simple 50-microm-thick, free-standing glass plate with air beneath it and the sample to be analyzed on top of it. This design offers a simple alternative to the well-known monomode waveguides, and, owing to its so-called reverse-symmetry configuration, a large detection range can be obtained.

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PubMed2004

Wavelength stabilization and spectrum narrowing of high-power multimode laser diodes and arrays by use of volume Bragg gratings.

Spectral line narrowing (by a factor of 8) and stabilization of the emission wavelength (by a factor of 30) of multimode high-power laser diodes and arrays is demonstrated by use of volume Bragg gratings fabricated in high-stability inorganic photorefractive glasses. Applications include stabilization of pump laser diodes and arrays for solid-state lasers and metal-vapor lasers, spin hyperpolarization of noble gases used in medical imaging, and others.

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PubMed2004

Multimode pumped continuous-wave solid-state Raman laser.

We demonstrate the continuous-wave operation of a solid-state Raman laser containing a barium nitrate crystal as the Raman medium. The Raman laser, which has a singly resonant cavity, is pumped by multimode radiation. The Raman oscillation threshold is reached at approximately 2 W of pump power. As much as 500 kW/cm2 of Stokes power density at 60-kW/cm2 pump power density is obtained in the cavity.

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