Showing papers on "Mode scrambler published in 1995"

Fiber devices and sensors based on multimode fiber Bragg gratings

Abstract: New sensing and diagnostic methods are implemented with multimode fiber Bragg gratings (MMFBG) and methods for sensor addressing and multiplexing are described. Real time structural strain and vibration monitoring using optical time domain reflectometry (OTDR) is discussed. The first all-mode, wave optics calculations of the reflectivity of Bragg gratings in highly multimode optical fibers are presented, including the effects of individual mode power changes due to microbending transducer loss upstream from the grating. Two categories of multimode fiber gratings are possible, independent mode and mode coupling, and the fabrication differences between these types of gratings are described. The reflectivity properties of independent mode multimode fiber gratings are examined and compared to single mode fiber gratings. In particular, multimode fiber gratings offer added flexibility in grating design and performance characteristics compared to single mode fiber gratings, because the reflectivity response may be tuned by the spatial periodicity, length, core size, numerical aperture, and mode coupling characteristics of the grating.

Adiabatic reflection Y-coupler apparatus

Abstract: A wavelength selective structure is coupled to an adiabatic Y-coupler via a multimode section which supports both symmetric and antisymmetric modes. One single mode branch of the coupler converts guided light to a symmetric mode, whereas the other single mode branch converts guided light to an anti-symmetric mode. The structure, which includes a pair of single mode waveguide arms coupled to the common section and a reflection device (such as a grating or ROR) located in each arm, converts reflected light from a symmetric mode to an anti-symmetric mode and conversely. Applications described include a channel dropping fiber and channel balancing apparatus for WDM systems, and a dispersion compensator for fiber optic systems.

Selective excitation of fiber-modes using surface plasmons

Abstract: Experimental and theoretical results are presented on a surface plasmon fiber device that provides selective excitation of individual modes in a dual-mode optical fiber. Using prism coupling to probe this device, good modal discrimination was obtained experimentally for the LP/sub 1/1 fiber mode. The LP/sub 1/1 modal discrimination, however, was found to be inherently limited by the field distribution characteristics of the dual-mode fiber. Applications are to fixed and tunable mode tap or couplers. >

Single mode optical fiber sensors

Abstract: Single mode fibers are used for sensing when extreme sensitivity is required or when a well defined polarization of light is needed at a remote sensing point. Most sensors which use single mode fibers are of the intrinsic type (i.e. the action of the measurand on the light occurs within the fiber itself). The sensitivity advantage of single mode fibers arises because they permit the user to construct guided wave interferometers directly from the fiber itself so as to measure small phase changes in light transmitted through the measuring region. This is achieved by comparing the phase of a light wave which has traversed a sensing path with the phase of another light wave originating from the same source but arriving via a protected, reference path. The phase difference can be measured with a sensitivity of ∼10-6 of a wavelength [1] and the pathlength for the measuring interaction can be millions of wavelengths long. This leads to a possible measurement resolution for the optical path of one in 1012! Simultaneously, the absence of free space optical paths between sources and detectors eliminates slow alignment drifts which could easily occur if bulk-optical interferometers had been used. In practice, single mode fiber sensors tend to need very stable, highly coherent sources with low phase noise in order to gain full advantage of their potential sensitivity. When such sources are used, absolute calibration of phase difference is normally not possible and a range limit arises from the periodic nature of the interferometer output. These points will be explained later in this chapter. Recently, both of these problems have been avoided by using sources emitting in a broad wavelength range, with some compromise regarding the ultimate sensitivity achievable with any particular sensor. The concluding part of the chapter will be devoted to such sensors. Another important point to understand is that this type of sensor ultimately measures optical pathlength. Anything which changes the pathlength will therefore produce a signal. Since there are a multitude of effects which can affect the optical pathlength through a fiber, great care must always be taken to reduce or to compensate for these unwanted changes.

Image pick-up apparatus with expanded dynamic range

Abstract: An image pick-up apparatus includes: an image sensing device for converting an optical image into an electrical signal, the image sensing device including a photoelectric conversion part and a vertical transfer part; a mode switching circuit for switching an operation mode between a frame mode and field mode; and a control circuit for controlling the bias level of a vertical transfer pulse depending on the operation mode selected via the mode switching circuit; thereby expanding the dynamic range of the apparatus.

Double-cylindrical cavity multiple-device structure for stabilized high-output power combiners

Abstract: As a method to overcome the moding problem by injection locking while the number of devices is increased when multiple device power combining is carried out with a cylindrical cavity, a power combiner of a double-cavity multiple-device configuration was proposed in which the oversized cavity with the desired mode of TMOnO (n ⩾ 2) is connected by a coaxial line to a fundamental mode cavity with the same frequency of the TMOnO Mode. First, by means of the modal analysis using the equivalent circuit model of the oscillator, the condition for sustaining the desired mode, the condition for suppression of the undesired mode, and the condition for existence of the double mode are obtained so that the range of stable growth of each mode is found. It was shown that the suppression of the undesired mode can be accomplished and the stable oscillation of the desired mode becomes easier by the present configuration. Further, as the oversized cavity, the TM020-mode octuple-device cavity and the TM030-mode 15-device cavity were used in the power-combining experiments. The power-combining efficiency of 91 and 95.8 percent was accomplished. With respect to the growth range of each mode, the theory and the experiment agreed qualitatively.