Tunable polarization diversity liquid-crystal wavelength filter

A compact polarization-insensitive liquid crystal filter is demonstrated using the polarization diversity technique. The incoming signal is split into two orthogonally polarized beams with a calcite crystal and the beams are filtered using two regions of the liquid crystal Fabry-Perot filter. The optic axes in these two regions are made orthogonal by controlling the surface structure. It is shown that a wide tuning range of 70 nm is possible with a band-pass of about 0.6 nm. Using arbitrarily polarized input light it is demonstrated that polarization-dependent power fluctuations are less than 1 dB.<>     

Cutoff wavelength determination in single-mode optical fibres by measurement of equalisation wavelength

A simple interferometric method for indirectly determining a value for the cutoff wavelength of single-mode optical fibres is described. The technique is accurate and no stringent requirements are placed on the experimental equipment.     

All-optical wavelength conversion by semiconductor opticalamplifiers

Following a brief introduction to the applications for wavelength conversion and the different available conversion techniques, the paper gives an in depth analysis of cross gain and cross phase wavelength conversion in semiconductor optical amplifiers. The influence of saturation filtering on the bandwidth of the converters is explained and conditions for conversion at 20 Gb/s or more are identified. The cross gain modulation scheme shows extinction ratio degradation for conversion to longer wavelengths. This can be overcome using cross phase modulation in semiconductor optical amplifiers that are integrated into interferometric structures. The first results for monolithic integrated interferometric wavelength converters are reviewed, and the quality of the converted signals is demonstrated by transmission of 10 Gb/s converted signals over 60 km of nondispersion shifted single mode fiber     

Optical wavelength shifting by traveling-wave electroopticmodulation

Optical wavelength shifting of 1.054-nm laser pulses in excess of ± 10 wavenumbers (± 300 GHz) was demonstrated. The wavelength shifter consists of synchronous microwave and optical waveguides fabricated monolithically on LiNbO₃. An optical pulse experiences a constant refractive index gradient that travels with the pulse and causes the wavelength shift     

Underwater image enhancement by wavelength compensation and dehazing

Light scattering and color change are two major sources of distortion for underwater photography. Light scattering is caused by light incident on objects reflected and deflected multiple times by particles present in the water before reaching the camera. This in turn lowers the visibility and contrast of the image captured. Color change corresponds to the varying degrees of attenuation encountered by light traveling in the water with different wavelengths, rendering ambient underwater environments dominated by a bluish tone. No existing underwater processing techniques can handle light scattering and color change distortions suffered by underwater images, and the possible presence of artificial lighting simultaneously. This paper proposes a novel systematic approach to enhance underwater images by a dehazing algorithm, to compensate the attenuation discrepancy along the propagation path, and to take the influence of the possible presence of an artifical light source into consideration. Once the depth map, i.e., distances between the objects and the camera, is estimated, the foreground and background within a scene are segmented. The light intensities of foreground and background are compared to determine whether an artificial light source is employed during the image capturing process. After compensating the effect of artifical light, the haze phenomenon and discrepancy in wavelength attenuation along the underwater propagation path to camera are corrected. Next, the water depth in the image scene is estimated according to the residual energy ratios of different color channels existing in the background light. Based on the amount of attenuation corresponding to each light wavelength, color change compensation is conducted to restore color balance. The performance of the proposed algorithm for wavelength compensation and image dehazing (WCID) is evaluated both objectively and subjectively by utilizing ground-truth color patches and video downloaded from the Youtube website. Both results demonstrate that images with significantly enhanced visibility and superior color fidelity are obtained by the WCID proposed.     

A heuristic wavelength assignment algorithm for multihop WDMnetworks with wavelength routing and wavelength re-use

Presents a heuristic algorithm for effectively assigning a limited number of wavelengths among the access stations of a multihop network wherein the physical medium consists of optical fiber segments which interconnect wavelength-selective optical switches. Such a physical medium permits the limited number of wavelengths to be re-used among the various fiber links, thereby offering very high aggregate capacity. Although the optical connectivity among the access station can be altered by changing the states of the various optical switches, the resulting optical connectivity pattern is constrained by the limitation imposed at the physical level. The authors also study two routing schemes, used to route requests for virtual connections. The heuristic is tested on a realistic traffic model, and the call blocking performance of new requests for virtual connections is studied through extensive simulations and compared against the blocking performance of an ideal infinite capacity centralized switch (lowest possible call blocking caused exclusively by congestion on the finite capacity user input/output links, never by the switch fabric itself). Surprisingly, the authors find that, for a wide range of parameters, the blocking performance of the lightwave network is almost the same as that of the ideal centralized switch. From these results, they conclude that the heuristic algorithm is effective and the routing scheme is efficient     

Numerical aperture of multimode fibers by several methods:resolving differences

An industrywide study among members of the Electronic Industries Association was conducted to document differences among three numerical aperture measurement methods. Results on 12 multimode graded index fibers indicate systematic differences exist among commonly used far-field and index profile techniques. Differences can be explained by a wavelength-dependent factor and choice of definitions. Conversion factors can be used to relate the various methods     

Rapid wavelength stabilising control for fast LD wavelength switching

A novel control scheme for fast laser diode (LD) wavelength switching with rapid stabilisation is proposed, which is the key technology for wavelength-division multiplexing (WDM) data cell transfer networks. The obtained characteristics are suitable for 600 Mbit/s asynchronous transfer mode (ATM) cell transfer networks. The proposed control scheme can be applied to over 10 Gbit/s cell speed networks.<>     

Dynamic routing and wavelength assignment for limited-range wavelength conversion

We propose a dynamic routing and wavelength assignment algorithm in an all-optical network with limited-range wavelength conversions. For each randomly arrived request, the correlation of different wavelengths on different links in all possible routes is described by the wavelength weight in our algorithm. We try to assign wavelengths on each candidate route using a shortest-path algorithm based on the wavelength weight and choose the best as its route and wavelength assignment scheme. Numerical results obtained for NSFNET and a 12-node ring show that our algorithm can greatly reduce the blocking probability and the number of required conversions under uniform traffic or traffic dominated by larger hop routes.     

Flip-flop resolving time test circuit

Integrated circuit flip-flop resolving time parameters, required for calculation of synchronizer performance and reliability, are measured by wafer probing, without the need for dicing or bonding, by incorporation of test structures on an IC along with the flip-flop to be measured. The circuit has five digital inputs, five digital outputs, including one for frequency measurements, and two analog inputs plus power and ground connections. Several delays that are fabricated as part of the test circuit, including a voltage controlled delay with a few picosecond resolution, are calibrated as part of the test procedure by grating them into and out of the delay path of a ring oscillator. Each of the delay values is calculated by subtracting the period of the ring oscillator with the delay omitted, from the period of the ring oscillator with the delay included. A frequency divider is fabricated as part of the test structure to reduce the output of the ring oscillator to less than 200 kHz so no high-frequency inputs of outputs from the IC are required.     

Tuning Bragg wavelength by writing gratings on prestrained fibers

Bragg gratings at a specific wavelength are made using an excimer KrF laser and a phase mask. The wavelength can be varied for fine tuning and multiplexing applications by straining the optical fiber during UV illumination. When the strain is removed, the grating formed is at a smaller wavelength than that dictated by the phase mask for the unstrained fiber. This technique was demonstrated by writing two gratings located at the same point in the optical fiber. The transmission from the first grating was used as a means for in situ absolute wavelength tuning. The second grating made with approximately 0.2% axial strain was at 1534.54 nm. Once the grating was made, the strain was removed from the fiber leaving the second grating tuned to 1532.04 nm, a wavelength shift of 2.5 nm     

All optical wavelength conversion by SOA's in a Mach-Zehnderconfiguration

Penalty free wavelength conversion is demonstrated at 2.5 Gbit/s over a wavelength span of 12 nm by the use of semiconductor optical amplifier (SOA)'s in a Mach-Zehnder configuration. An increase in the extinction ratio is measured for the converted signal compared to the input signal implying signal regeneration as well as wavelength conversion     

用可调谐法布里-珀罗腔测量光纤光栅波长

提出了一种利用压电陶瓷构成的可调谐法布里 -珀罗腔测量光纤光栅反射波长的方案 ,该方案可以直接将波长转换为电信号。阐述了该方案的基本原理 ,并讨论了在设计法布里 -珀罗腔时应考虑的几个问题。     

耦合器衰减实现的两种全光波长变换

在以往对全光波长变换的研究中,都是利用变换光实现波长变换,而利用闲频光的全光波长变换还没有相关报道.文章通过模拟耦合器衰减对全光波长变换的影响,发现在不同的泵浦光与信号光功率条件下,调节耦合器衰减可分别实现利用变换光的全光波长变换和利用闲频光的全光波长变换.     

Optical wavelength converters

Devices and techniques for shifting the frequency/wavelength of an optical carrier wave are reviewed, with particular reference to applications in wavelength/frequency division multiplexed optical communication systems. Considered first are devices which make use of semiconductor laser amplifiers or oscillators. Some of these techniques come close to satisfying the requirements for a suitable wavelength shifter, though none appear to offer the potential for a transparent network independent of data modulation format. A second main category, using nonlinear optical materials, promises to satisfy the requirements for frequency translation in a network which is fully transparent to the modulation format. However, the conversion efficiencies currently available appear too low for cost-effective communication systems engineering so that further advances in nonlinear optical materials/techniques are still required. It seems likely that guided wave techniques will provide the best means of improving the efficiency by increasing the interaction length and the optical intensity for a given power. Schematic examples of such a converter are discussed. All of the existing techniques have some deficiencies compared to an ideal system so that wavelength conversion for optical communication systems is still a problem in search of a good solution     

Logarithmic wavelength demultiplexers

A general approach for a full 1 /spl times/ N demultiplexer using a tree of filter stages is proposed. The device architecture is compact, requiring at least N - 1 filter stages, and flexible, as each filter stage can be arbitrarily designed, with the only constraint of the half-band power property. The filters can be realized using any optical filtering techniques as thin-film interference, Bragg gratings, or planar delay-line circuits. The performances of the proposed architecture are illustrated with respect to different lattice-form finite impulse response (FIR) and infinite impulse response (IIR) filter stages, showing that the demultiplexer inherently presents low crosstalk and flat passband. A design example of a 1 /spl times/ 4 demultiplexer consisting of three all-pass (AP) filters is compared with a generalized Mach-Zehnder interferometer (MZI) with four AP filters in its arms, showing that the two approaches achieve similar results.     

Long wavelength VLBI

The application of very-long baseline interferometry (VLBI) techniques at meter and decameter wavelengths has proven to be a unique tool for studying a number of different phenomena. Because of the high angular resolution which we achieve with VLBI, candidate objects for study include the compact nuclei and "jets" associated with quasars and radio galaxies, pulsars, small knots in galactic supernova remnants, and nonthermal planetary sources. Our investigations have been aimed at determining the emission spectra of these objects, measuring their apparent angular size, and mapping their structure. We expect that these properties will be interrelated, particularly at or below the frequencies at which their spectra peak. As various components of a radio source become self-absorbed the apparent angular structure must change quite radically. Concomitant investigations of transmission properties of the various media which intervene between the source and the observer must be carried out. Often these investigations of the interstellar, interplanetary, and ionospheric media are as interesting as the study of the radio sources themselves.     

Time diversity by pitch-synchronized interpolation

In mobile communications such as automobile telephone systems, an instantaneous interruption is caused by rapid fading, and speech quality is markedly deteriorated. A pitch-synchronized interpolation method is proposed to improve this quality. This method utilizes the fact that most speech signal regions have a pitch period. In this method, when an instantaneous interruption is detected at the receiver, a speech signal, which includes instantaneous interruption noise, is deleted and interpolated by repetition of the speech signal received one pitch interval wave before the instantaneous interruption. This method can be applied to receivers used in analog and digital transmission systems. A receiver using this method has been constructed, and it was shown that a 10-dB carrier-to-noise ratio (CNR) gain can be obtained by this receiver.     

WDM光网络中支持QoS的波长分配算法

基于在波长可变光网络中的波长分层图模型,本文提出了一种在动态业务情况下的支持不同QoS要求的路由波长分配算法,根据客户层业务不同的QoS要求,通过分层图模型中参数的不同取值,对其光路建立请求区别对待,提供不同级别的光路建立.仿真结果表明该算法具有很好的性能,提高了全网的资源利用率,满足了客户层业务在建立光通路时不同的QoS要求对网络阻塞率的要求.