Showing papers in "Journal of Lightwave Technology in 2001"

Modified quadratic congruence codes for fiber Bragg-grating-based spectral-amplitude-coding optical CDMA systems

Abstract: We have constructed a series of new code families for the spectral-amplitude-coding optical code-division multiple-access (CDMA) system, and proposed new transmitter and receiver structures based on tunable chirped fiber Bragg gratings (FBGs). The proposed system has been analyzed by taking into account the effects of phase-induced intensity noise, shot noise, and thermal noise. We have also compared the performance of this system with that of a former system where a Hadamard code is used. It has been shown that the new code families can suppress the intensity noise effectively and improve the system performance significantly. When the effective power is large (i.e., >-10 dBm), the intensity noise is the main factor that limits the system performance. When the effective power is not sufficiently large, thermal and shot noise sources become the main limiting factors and the effect of thermal noise is much larger than that of shot noise.

Pump to signal RIN transfer in Raman fiber amplifiers

Abstract: We present an analytical model and measurements of the pump to signal relative intensity noise (RIN) transfer characteristics of copumped and counterpumped Raman amplifiers. We give a quantitative analysis of noise averaging from chromatic dispersion in copumped systems and from different propagation directions in counterpumped systems. It is shown that nondispersion-shifted fibers and other fibers with high dispersion between the signal and pump wavelengths give a greater potential for use in copumped Raman amplified systems.

Wavelength division multiplexing and demultiplexing with photonic crystal waveguide couplers

Abstract: A multiplexer-demultiplexer (MUX-DEMUX) based on PC waveguide couplers is proposed, and its wavelength demultiplexing properties are theoretically investigated. First, a two-channel MUX-DEMUX is designed and characterized, and then, by cascading, two stages of photonic crystal (PC) waveguide couplers with different coupling coefficients are constructed. The device sizes are expected to be drastically reduced from a scale of a few tens of micrometers to a scale of a few hundreds of micrometers in a MUX-DEMUX.

High-temperature stability of long-period fiber gratings produced using an electric arc

Abstract: We report, for the first time to our knowledge, the excellent optical performance of long-period gratings written in three standard fibers and in two nonstandard fibers, namely, S-doped and N-doped fibers, using the flexible and low-cost electric arc technique The fundamental mechanism for their formation is proposed The grating thermal behavior and the effect of fiber preannealing prior to writing have been investigated We have demonstrated a good stability of these gratings at high temperatures

Corrugated long-period fiber gratings as strain, torsion, and bending sensors

Abstract: We present a novel corrugated long-period fiber grating whose transmission spectra are highly sensitive to the applied tensile strain, torsion, and bending due to the periodical index modulation created and changed by these mechanic forces. The induced index modulation can also be experimentally characterized by using a built-in fiber Bragg grating (FBG). The long period fiber gratings possess the following unique properties when used as sensors. As a tensile strain sensor, its resonance loss varies but resonance wavelength remains stable. As a torsion sensor, the wavelength varies with the applied twist rate. As a bending sensor, the cladding-mode resonance grows with the bending curvature.

Increase of the SBS threshold in a short highly nonlinear fiber by applying a temperature distribution

Abstract: We evaluate numerically and experimentally the stimulated Brillouin scattering (SBS) threshold increase for a short, highly nonlinear GeO/sub 2/-doped fiber by applying different temperature distributions along the fiber. The temperature coefficient for the Brillouin frequency downshift is measured to 1.2 MHz//spl deg/C. A threefold SBS threshold increase is obtained for a 100-m long highly nonlinear fiber with a 140/spl deg/C temperature gradient. The proposed scheme is implemented in a wavelength converter based on fiber optical four-wave mixing (FWM). The SBS suppression scheme shows negligible influence on the FWM efficiency as well as the wavelength conversion bandwidth. The temperature coefficient for the zero dispersion wavelength is measured to 0.062 nm//spl deg/C.

Multiwavelength fiber laser sources with Bragg-grating sensor multiplexing capability

Abstract: Two erbium-doped fiber ring lasers (EDFRLs) wvith simultaneous emission at four different wavelengths are demonstrated. Both systems employ fiber Bragg gratings (FBGs) to select the operation wavelengths within the ring. The sensing capability of the FBGs has been taken advantage of, allowing for the sources to be used as sensor multiplexing schemes. The first system employs four FBGs in a tree filter topology, achieving four output channels with -5 dBm power each. The second system comprises an in-line filtering topology with active fiber segments within the filter. This second source yields 2-dBm output signals and allows for a higher number of lines to be easily added to the system. A comparison between both topologies is carried out, and their capability for sensor multiplexing is demonstrated.

Polarization and dispersive properties of elliptical-hole photonic crystal fibers

Abstract: We survey the properties of photonic crystal fibers with elliptical air holes, examining mode shapes, birefringence, group velocity walkoff and dispersion, and cutoff conditions. We find new types of behavior for each quantity and demonstrate the possibility achieving large birefringence with zero walkoff in the single-mode regime. We show that the dispersive properties of the vector modes are closely tied to those of the so-called fundamental space filling modes, and that at long wavelengths, the fibers exhibit a single-polarization single-mode regime of propagation without the presence of material anisotropy.

A comparative study of the performance of seven- and 63-chip optical code-division multiple-access encoders and decoders based on superstructured fiber Bragg gratings

Abstract: We report a range of elementary optical coding and decoding experiments employing superstructured fiber Bragg grating (SSFBG) components: first, we perform a comparative study of the relative merits of bipolar and unipolar coding: decoding schemes and show that the SSFBG approach allows high-quality unipolar and bipolar coding. A performance close to that-theoretically predicted for seven-chip, 160-Gchip/s M-sequence codes is obtained. Second, we report the fabrication and performance of 63-chip, 160-Gchip/s, bipolar Gold sequence grating pairs. These codes are at least eight times longer than those generated by any other scheme based on fiber grating technology so far reported. Last, we describe a range of transmission system experiments for both the seven- and 63-bit bipolar grating pairs. Error-free performance is obtained over transmission distances of /spl sim/25 km of standard fiber. In addition, we have demonstrated error-free performance under multiuser operation (two simultaneous users). Our results highlight the precision and flexibility of our particular grating writing process and show that SSFBG technology represents a promising technology not just for optical code division multiple access (OCDMA) but also for an extended range of other pulse-shaping optical processing applications.

Self-calibrated interferometric-intensity-based optical fiber sensors

Abstract: This paper presents self-calibrated interferometric-intensity-based optical fiber sensors, which combine for the first time fiber interferometry and intensity-based devices into a single sensor system. The sensor involves an extrinsic Fabry-Perot (FP) interferometric cavity. The broadband light returned from the FP cavity is split into two channels in such a way that one channel has a coherence length much longer than the doubled air-gap separation in the sensor so the FP generates effective interference, while the coherence length in the other channel is so short that no effective interference takes place. As a result, the optical signal in the channel with a long coherence length yields information about the FP cavity length while the signal in the other channel is proportional only to the source power, fiber attenuation, and other optical loss factors in the optical path. To eliminate fringe direction ambiguity and relative measurement limitations associated with interferometric sensors, the sensor is designed such that it is operated over the linear range between a valley and a peak of one interference fringe in the first channel. Moreover, the ratiometric signal-processing method is applied for the signals in the two channels to obtain self-calibrating measurement to compensate for all unwanted factors, including source power variations and fiber bending losses. Various pressure and temperature sensors based on the self-calibrated interferometric/intensity-based scheme are designed, fabricated, and tested. Experimental results show that a resolution as high as 0.02% of full scale can be obtained for both the pressure and temperature measurements.

Optimum filter bandwidths for optically preamplified NRZ receivers

Abstract: We present a comprehensive treatment of optically preamplified direct detection receivers for non-return-to-zero (NRZ) and return-to-zero (RZ) on/off keying modulation, taking into account the influence of different (N)RZ optical pulse shapes, specified at the receiver input, and filter transfer functions; optical Fabry-Perot filters (FPFs) and Bragg gratings as well as electrical fifth-order Bessel and first-order RC low-pass filters are considered. We determine optimum optical and electrical filter bandwidths and analyze the impact of bandwidth deviations on receiver sensitivity. Optimum receiver performance relies on a balance between noise and intersymbol interference (ISI) for NRZ transmission, while for RZ reception detection noise has to be traded against filter-induced signal energy rejection. Both for NRZ and 33% duty cycle RZ, optical filter bandwidths of around twice the data rate are found to be optimum. Receivers using RZ coding are shown to closely approach the quantum limit, and thus to outperform NRZ-based systems by several decibels. We further analyze the impact of important degrading effects on receiver sensitivity and optimum receiver bandwidths, including receiver noise, finite extinction ratio, chirp, and optical carrier frequency (or optical filter center frequency) fluctuations.

Tapered couplers for efficient interfacing between dielectric and photonic crystal waveguides

Abstract: We design tapered waveguide junctions for coupling between photonic crystal and traditional dielectric waveguides and evaluate their transmission efficiency. While the transmission efficiency is less than 60% using no taper, the tapered couplers have over 90% power transmission. We show that different types of couplers are needed for efficient coupling into and out of photonic crystal waveguides.

Fabrication and characterization of narrow-band Bragg-reflection filters in silicon-on-insulator ridge waveguides

Abstract: We describe the design, fabrication and measurement of an integrated-optical Bragg grating filter, operating at a freespace wavelength of 1543 nm, based upon a silicon-on-insulator (SOI) ridge waveguide. The measured spectral response for a 4-mm long grating has a bandwidth of 15 GHz (0.12 nm), and shows good agreement with theoretical predictions.

Rectangular pulse generation based on pulse reshaping using a superstructured fiber Bragg grating

Abstract: We present a technique for the shaping of short pulses based on the use of superstructured fiber Bragg gratings (SSFBGs). We apply this technique to demonstrate the generation of 20-ps rectangular pulses by phase and amplitude profiling of 2.5-ps soliton pulses. Numerical calculations validate our experimental findings.

200-nm-bandwidth fiber optical amplifier combining parametric and Raman gain

Abstract: Theory shows that the gain bandwidth of a one-pump fiber optical parametric amplifier (OPA) using highly nonlinear fiber (HNLF) could be more than 200 nm. Under these circumstances, the OPA gain would overlap the pump-induced Raman gain. We have studied the combined effects of OPA and Raman gain theoretically and experimentally. The experimental results demonstrate a 200-nm bandwidth from a single fiber-optical amplifier and also verify that the influence of the Raman effect is relatively small, as predicted by the theory.

Tensile and compressive strain measurement in the lab and field with the distributed Brillouin scattering sensor

Abstract: The structural strain measurement of tension and compression in the steel beam was demonstrated with a distributed fiber-optic sensor system based on Brillouin scattering. The experiments were conducted both in the laboratory and outdoors. When it is in the outdoor environment, the temperature compensation has been taken into account for the strain measurement due to sunlight radiation. The compressive strain has been measured, without needing pretension on the fiber with a Brillouin scattering-based distributed sensor system, when the fiber is glued to the steel beam at every point. The dynamic range in the strain measurement has been increased, due to the elimination of the pretension requirement. The spatial resolution of the strain measurement is 0.5 m. The strain measurement accuracy is /spl plusmn/10 /spl mu//spl epsi/(/spl mu/m/m) in the laboratory environment with nonuniform-distributed strain. With uniform strain distribution, the strain accuracy for this system can be. /spl sim//spl plusmn/5 /spl mu//spl epsi/. These results were achieved with the introductions of a computer-controlled polarization controller, a fast digitizer-signal averager, a pulse duration control, and the electrical optical modulator bias setting in the software.

Multimode interference couplers with tunable power splitting ratios

Abstract: New, compact multimode interference couplers with tunable power splitting ratios have been realized. Experiments show large tuning ranges. Such couplers are needed to optimize ON-OFF ratios in interferometric devices and may find applications as extremely compact switches.

Improved approaches for cost-effective traffic grooming in WDM ring networks: ILP formulations and single-hop and multihop connections

Abstract: Traffic grooming is the term used to describe how different traffic streams are packed into higher speed streams. In a synchronous optical network-wavelength division multiplexing (SONET-WDM) ring network, each wavelength can carry several lower-rate traffic streams in time division (TDM) fashion. The traffic demand, which is an integer multiple of the timeslot capacity, between any two nodes is established on several TDM virtual connections. A virtual connection needs to be added and dropped only at the two end nodes of the connection; as a result, the electronic add-drop multiplexers (ADMs) at intermediate nodes (if there are any) will electronically bypass this timeslot. Instead of having an ADM on every wavelength at every node, it may be possible to have some nodes on some wavelength where no add-drop is needed on any timeslot; thus, the total number of ADMs in the network (and, hence, the network cost) can be reduced. Under the static traffic pattern, the savings can be maximized by carefully packing the virtual connections into wavelengths. In this work, we allow arbitrary (nonuniform) traffic and we present a formal mathematical definition of the problem, which turns out to be an integer linear program (ILP). Then, we propose a simulated-annealing-based heuristic algorithm for the case where all the traffic is carried on directly connected virtual connections (referred to as the single-hop case). Next, we study the case where a hub node is used to bridge traffic from different wavelengths (referred to as the multihop case). We find the following main results. The simulated-annealing-based approach has been found to achieve the best results, so far, in most cases, relative to other comparable approaches proposed in the literature. In general, a multihop approach can achieve better equipment savings when the traffic-grooming ratio is large, but it consumes more bandwidth.

Full-vectorial finite element beam propagation method with perfectly matched layers for anisotropic optical waveguides

Abstract: Perfectly matched layer (PML) boundary conditions are incorporated into the full-vectorial beam propagation method (BPM) based on a finite element scheme for the three-dimensional (3-D) anisotropic optical waveguide analysis. In the present approach, edge elements based on linear-tangential and quadratic-normal vector basis functions are used for the transverse field components. To show the validity and usefulness of this approach, numerical examples are shown for Gaussian beam propagation in proton-exchanged LiNbO/sub 3/ optical waveguides. Numerical accuracy of the present PML boundary condition is investigated in detail by comparing the results with those of the conventional absorbing boundary condition (ABC).

Packet loss and delay performance of feedback and feed-forward arrayed-waveguide gratings-based optical packet switches with WDM inputs-outputs

Abstract: This paper analyzes the packet loss and delay performance of an arrayed-waveguide-grating-based (AWG) optical packet switch developed within the EPSRC-funded project WASPNET (wavelength switched packet network). Two node designs are proposed based on feedback and feed-forward strategies, using sharing among multiple wavelengths to assist in contention resolution. The feedback configuration allows packet priority routing at the expense of using a larger AWG. An analytical framework has been established to compute the packet loss probability and delay under Bernoulli traffic, justified by simulation. A packet loss probability of less than 10/sup -9/ was obtained with a buffer depth per wavelength of 10 for a switch size of 16 inputs-outputs, four wavelengths per input at a uniform Bernoulli traffic load of 0.8 per wavelength. The mean delay is less than 0.5 timeslots at the same buffer depth per wavelength.

Propagation loss in single-mode GaAs-AlGaAs microring resonators: measurement and model

Abstract: We report propagation loss measurements in single-mode GaAs-AlGaAs racetrack microresonators with bending radii from 2.7 /spl mu/m to 9.7 /spl mu/m. The experimental data were found to be in good agreement with a physical-loss model which accounts for the bending loss, the scattering loss due to surface roughness on the waveguide sidewalls, and the transition loss at the straight-to-bend waveguide junctions. The model also enables us to identify the dominant loss mechanisms in semiconductor microcavities. We found that for racetracks with large bending radii (greater than 4 /spl mu/m, in our case) the loss due to surface-roughness scattering in the curved waveguides dominates, whereas for small-radius rings, the modal mismatch at the straight-to-bend waveguide junctions causes the biggest loss. This result suggests that circular-shaped rings are preferable in the realization of ultrasmall low-loss microcavities. We also show that the round-trip propagation loss in small-radius racetrack microresonators can be minimized by introducing a lateral offset at the straight-to-bend waveguide junctions.

Flat-top response in one-dimensional magnetic photonic bandgap structures with Faraday rotation enhancement

Abstract: The transmission and Faraday rotation characteristics of one-dimensional photonic crystals in cerium-substituted yttrium iron garnet (Ce:YIG) with multiple defects in the optical bandgap are studied theoretically at /spl lambda/ = 1.55 /spl mu/m. It is found that the interdefect spacing can be adjusted to yield a flat top response, with close to 100% transmission and 45/spl deg/ Faraday rotation, for film structures as thin as 30 to 35 /spl mu/m. This is better than a three-fold reduction in thickness compared to the best Ce:YIG films for comparable rotations, and may allow a considerable reduction in size in manufactured optical isolators. Transmission bands as wide as 7 nm are predicted, which constitutes a considerable improvement over previously reported bandwidths for magnetic photonic crystals. Diffraction across the structure corresponds to a longer optical path length than the thickness of the film, calling for the use of guided optics to minimize insertion losses in integrated devices. The basis for the flat-top transmission in ferrite photonic crystals is presented and discussed.

Large-scale multiplexing of interferometric fiber-optic sensors using TDM and DWDM

Abstract: This paper describes multiplexing schemes for interferometric fiber sensors based on time-division multiplexed and dense wavelength-division multiplexing using optical add/drop multiplexers. The results of an experimental arrangement, which is based on one of the architectures, is also presented. Topics include a discussion of the noise sources in the system, dynamic range, and a characterization of the distributed feedback fiber laser source noise. We show the crosstalk levels in the experimental arrangement to be between -47 and -76 dB depending on the mechanism involved. The multiplexing schemes demonstrate the potential to address at least 192 interferometric sensors through two fibers based on a system with six wavelengths with a phase resolution less than 20 /spl mu/rad//spl radic/Hz. For application to sonar arrays, our analysis has shown that hydrophones multiplexed in this type of architecture would achieve ambient acoustic noise-limited pressure resolution with an in-water dynamic range up to 135 dB at frequencies up to 10 kHz. In general, these architectures would find application in systems requiring very large numbers of sensors with a minimum of telemetry cabling required.

Low loss and high extinction ratio strictly nonblocking 16/spl times/16 thermooptic matrix switch on 6-in wafer using silica-based planar lightwave circuit technology

Abstract: We describe a silica-based 16/spl times/16 strictly nonblocking thermooptic matrix switch with a low loss and a high extinction ratio. This matrix switch, which employs a double Mach-Zehnder interferometer (MZI) switching unit and a matrix arrangement to reduce the total waveguide length, is fabricated with 0.75% refractive index difference waveguides on a 6-in silicon wafer using silica-based planar lightwave circuit (PLC) technology. We obtained an average insertion loss of 6.6 dB and an average extinction ratio of 53 dB in the worst polarization case. The operating wavelength bandwidth completely covers the gain band of practical erbium-doped fiber amplifiers (EDFAs). The total power consumption needed for operation is reduced to 17 W by employing a phase-trimming technique which eliminates the phase-error in the interferometer switching unit.

Modeling the fabrication of hollow fibers: capillary drawing

Abstract: A method for modeling the fabrication of small-scale hollow glass capillaries is developed. The model is based on an asymptotic analysis of the Navier-Stokes equations, which yields a simple closed-form solution for this problem. We demonstrate the validity of this approach using experimental data and use it to make predictions for a range of regimes of interest for the development of microstructured optical fiber technology.

256-channel bidirectional optical interconnect using VCSELs and photodiodes on CMOS

Abstract: Two-dimensional parallel optical interconnects (2-D-POIs) are capable of providing large connectivity between elements in computing and switching systems Using this technology we have demonstrated a bidirectional optical interconnect between two printed circuit boards containing optoelectronic (OE) very large scale integration (VLSI) circuits The OE-VLSI circuits were constructed using vertical cavity surface emitting lasers (VCSELs) and photodiodes (PDs) flip-chip bump-bonded to a 035-/spl mu/m complementary metal-oxide-semiconductor (CMOS) chip The CMOS was comprised of 256 laser driver circuits, 256 receiver circuits, and the corresponding buffering and control circuits required to operate the large transceiver array This is the first system, to our knowledge, to send bidirectional data optically between OE-VLSI chips that have both VCSELs and photodiodes cointegrated on the same substrate

All-optical analog-to-digital converters, hardlimiters, and logic gates

Abstract: The authors propose and analyze the optical signal processing functionality of periodic structures consisting of alternating layers of materials possessing different Kerr nonlinearities. They explore structure-materials-performance relationships in all-optical analog-to-digital converters, hardlimiters, and AND and OR gates. They show that their proposed analog-to-digital converters can extract a binary word from multilevel optical signals in a single bit interval. They also propose a family of optical limiters whose output signal clamps to a set upper logic level for any input value exceeding a chosen threshold. They explore the performance of an all-optical logic gate whose forward-directed output implements a binary AND and whose backward-directed output implements an OR function.

Analysis of signal degree of polarization degradation used as control signal for optical polarization mode dispersion compensation

Abstract: The basic property of degree of polarization (DOP) degradation of optical signal induced by polarization mode dispersion (PMD) in high-speed optical fiber transmission is investigated in detail. The DOP of the optical signal reflects the degree of waveform degradation caused by PMD, therefore, it is proposed to be used as the control signal judging the best compensation point for the optical adaptive PMD compensation techniques. However, the signal DOP is not only affected by PMD, but also by various factors, such as the modulation format, modulator chirp, fiber nonlinearity, amplified spontaneous emission (ASE), and so on. We use numerical simulations and experiments to explore the basic DOP property to detect PMD with these factors. We also show that using the signal DOP as control signal is especially useful for the optical duo-binary modulation because of its high sensitivity and wide PMD detection range.

A novel structure for the intrinsic Fabry-Perot fiber-optic temperature sensor

Abstract: A novel structure of the intrinsic Fabry-Perot interference (IFPI) fiber temperature sensor is presented. The sensor uses two different core diameter fibers and produces a reflective mirror by fusing uncoated bare fibers. This procedure not only solves the problem of controlling thickness and reflectance of the thin film but also provides easier and cheaper technologies for IFPI fiber sensors. Theoretical and experimental aspects of the intrinsic Fabry-Perot cavity are described. Both theoretical and experimental results from this novel structure show good agreement with those from the traditional Fabry-Perot fiber sensor.

Real-time location of multiple time-varying strain disturbances, acting over a 40-km fiber section, using a novel dual-Sagnac interferometer

Abstract: Updated results using a novel sensing architecture based on a Sagnac interferometer are presented and, for the first time, real-time separation and positioning of multiple disturbances has been realized. A 40-km long dual-Sagnac sensor was formed by spectral slicing of light from a single, broad-band erbium-doped-fiber super-luminescent source and wavelength division multiplexed (WDM) routing around the loop to form an inherently low loss system. Independent active phase biasing of each Sagnac was employed, allowing the use of a single optical detector. The effects of residual optical cross talk between the two Sagnacs has been accurately modeled, allowing resulting errors to be corrected. The new system has capability for narrow-band fast Fourier transform (FFT) analysis of detected disturbance signals, and hence their separation in the frequency domain. For audio-frequency excitation, an average positional resolution of 100 m over a 40-km length was achieved with a postdetection signal processing bandwidth of 8 Hz.