Showing papers in "Journal of Lightwave Technology in 1990"

Limitations on lightwave communications imposed by optical-fiber nonlinearities

Abstract: Optical nonlinearities in the context of lightwave systems limitations are described. The nature and severity of system degradation due to stimulated Raman scattering, carrier-induced phase noise, stimulated Brillouin scattering, and four-photon mixing are discussed. The system power limitations are plotted as a function of a number of wavelength-multiplexed channels. Methods for scaling these results with changes in system parameters are presented. >

Coherent ultrashort light pulse code-division multiple access communication systems

Abstract: A new technique for encoding and decoding of coherent ultrashort light pulses is analyzed. In particular, the temporal and statistical behavior of pseudonoise bursts generated by spectral phase coding of ultrashort optical pulses is discussed. the analysis is motivated by recent experiments that demonstrate high-resolution spectral phase coding of picosecond and femtosecond pulses and suggest the possibility of ultrahigh speed code-division multiple-access (CDMA) communications using this technique. The evolution of coherent ultrashort pulses into low intensity pseudonoise bursts as a function of the degree of phase coding is traced. The results are utilized to analyze the performance of a proposed CDMA optical communications system based upon encoding and decoding of ultrashort light pulses. The bit error rate (BER) is derived as a function of data rate, number of users, and receiver threshold, and the performance characteristics are discussed for a variety of system parameters. It is found that performance improves greatly with increasing code length. >

Derivation of analytical expressions for the bit-error probability in lightwave systems with optical amplifiers

Abstract: A description is given of a relatively simple derivation of the bit-error probability for a lightwave communications system using an amplitude-shift-keying (ASK) pulse modulation format and employing optical amplifiers such that amplified spontaneous emission noise dominates all other noise sources Mathematically, this noise is represented as a Fourier series expansion with Fourier coefficients that are assumed to be independent Gaussian random variables The bit-error probability is given in a closed analytical form that is derived by the approximate evaluation of several integrals appearing in the analysis The author uses the theory to derive the Gaussian approximation and finds that it overestimates the bit-error rate by one to two orders of magnitude >

Semiconductor lasers for coherent optical fiber communications

Abstract: The current status of semiconductor lasers used in coherent optical fiber communications is reviewed for nonexperts in the field. The issues of spectral purity, tuning, modulation, and advanced fabrication methods for photonic integration are discussed, with examples drawn from current experimental devices. >

Beam expanding fiber using thermal diffusion of the dopant

Abstract: A beam-expanding fiber (BEF) for embedding optical devices has been fabricated by utilizing thermally induced Ge diffusion in silica single-mode fibers (SMFs). The preparation of fiber samples and their heat treatment is described, and the effect of heat on Ge dopant distribution and diffusion is discussed. Modal-intensity distributions were studied and found to confirm the broadening of the modal field distribution after heat treatment of the fiber. Localized heat treatment to obtain BEFs is considered, and device characteristics are discussed. The BEF can arbitrarily change the spot size of a propagating mode without changing the normalized frequency. >

Statistical treatment of the evolution of the principal states of polarization in single-mode fibers

Abstract: A simple relationship is found for the evolution of the principal states of polarization (PSPs) and their differential group delay in fiber links. A simple expression is found, using the relationship, for the probability of the differential group delay (DGD), considering the evolution of the PSPs as a Brownian motion. The theory has been verified experimentally on an optical cable composed of 12 single-mode, shifted-dispersion fibers 2.2-km long. The results show that the DGD grows as the square root of the length when the length of the fiber is far larger than the correlation length of the perturbation. The measured value of DGD can vary substantially in two fibers belonging to the same ensemble, and in the same fiber, considering two frequencies differing by more than 5 nm. >

Performance and protocol improvements for very high speed optical fiber local area networks using a passive star topology

Abstract: Two random-access schemes for a multiuser very-high-speed optical data network based on a star topology are presented and analyzed. The protocols presented do not require any type of carrier sensing, and their performance is independent of a, the normalized propagation time of packets over the optical medium. This property is rather important when dealing with very-high-speed networks which result in high values of a, because the performance of the conventional carrier sensing schemes is poor under such conditions. The protocols outperform the corresponding algorithms of I.M.I. Habbab et al. (1987) for all values of system parameters. >

Perturbation effects on mode propagation in highly elliptical core two-mode fibers

Abstract: The propagation of both fundamental and second-order modes in highly elliptical core two-mode fibers under external perturbations is investigated. Theoretical and experimental results are presented for the differential phase shift between two spatial modes in each eigenpolarization and the polarization behavior of each spatial mode. The perturbations include axial strain, radial strain, hydrostatic strain, temperature change, twisting, squeezing, and bending. An application of the modal behavior of these fibers to simultaneous measurement of strain and temperature is described. >

Polarization control for coherent communications

Abstract: Two practical implementations of endless polarization control are described. The first approach uses polarization maintaining fiber transducer, while the second uses an integrated optic lithium niobate device. In order to characterize the lithium niobate device in detail, a technique for analyzing the birefringence as a function of the applied voltages was developed, and the measured performance of the devices is presented. Fractional-wave controllers for both static and endless control applications are considered, and design rules for maximum wavelength-window and minimum loss fiber-loop polarization adjusters are given. Endless control using various configurations of the three types of transducers is discussed, and it is shown how apparently different schemes are closely related. >

Statistical properties of Rayleigh backscattering in single-mode fibers

Abstract: An analytical model that takes into account the influence of the laser linewidth on Rayleigh backscattering is given. For an unmodulated source the power spectral density is found to be identical to the delayed self-homodyne spectrum of the laser. The backscattered signal was measured for different laser linewidths, and the results are compared with the theory. The effects of Rayleigh backscattering noise on fiber gyroscopes and bidirectional optical systems are discussed. >

Numerical simulation of the nonlinear response of a p-i-n photodiode under high illumination

Abstract: A theoretical study and a model for the numerical simulation of the nonlinear electrical response, including the harmonic-generation rate calculation, of a p-i-n InGaAs photodiode under high-illumination conditions are discussed. The device structure is described. An algorithm, which is based on a finite-difference calculation, is used to calculate the temporal electrical response of the device to a microwave optical input signal. The different harmonics in the power spectrum are obtained using the fast Fourier transform (FFT) calculation. This model is a tool for designing the p-i-n photodiode and determining the conditions for its utilization in order to avoid the electrical response nonlinearity. >

Simultaneous measurement of temperature and strain: cross-sensitivity considerations

Abstract: Interferometric sensors using optical fibers as a transduction medium have been shown to be sensitive to a variety of physical measurands. A result of this is that the resolution of a system designed to sense strain, for example, may be compromised by fluctuations in the temperature of the environment. The possibility of simultaneously determining the strain and temperature applied to the same piece of highly birefringent fiber is discussed. Second-order effects are shown to be important for long sensing lengths or in the presence of high strains or temperature changes. The results of experiments carried out to verify the theoretical predictions are also described. >

The effect of four-wave mixing in fibers on optical frequency-division multiplexed systems

Abstract: The limiting effects of four-wave mixing on optical frequency-division multiplexing (OFDM) systems are described. The optical nonlinearity in a single-mode fiber imposes a fundamental limitation on the capacity of optical frequency-division multiplexed systems. In particular, four-wave mixing (FWM) crosstalk may severely degrade the system performance when the fiber input powers are large and/or the channel spacing is too small. Theoretical and experimental results of the effects of FWM in OFDM systems are presented. The theoretical results demonstrate the dependence of FWM on various system parameters. An analysis of FWM in both undirectional and bidirectional transmission systems is included. The receiver sensitivity degradation from FWM crosstalk is measured in a 16-channel coherent system. >

Study of the complex atomic susceptibility of erbium-doped fiber amplifiers

Abstract: A study of the complex atomic susceptibility of erbium-doped fiber amplifiers operating near the 1.53- mu m transition (/sup 4/I/sub 13/2/-/sup 4/I/sub 15/2/) developed from a semiclassical theoretical description is presented. Expressions for the emission cross section sigma /sub e/( lambda ) and absorption cross section sigma /sub a/( lambda ) of erbium:glass as a quasi-three-level laser system with Stark-split sublevel manifolds are derived. The results are used to calculate the cross sections of aluminosilicate erbium-doped fibers at low temperatures (T=4.2, 77 K) and at room temperature. It is demonstrated that an expression for the gain coefficient of the fiber amplifier can be derived from the expression of the susceptibility. Using Kramers-Kronig relations, an expression for the refractive index change in the fiber is derived. The theoretical spectral gain profile of the fiber amplifier and concurrent refractive index changes are analyzed for different pumping regimes. >

Elliptical-core two mode optical-fiber sensor implementation methods

Abstract: Experimental methods for the practical implementation of few-mode elliptical-core sensors are described. Techniques for desensitizing the lead-in and lead-out fibers are discussed, and results of a vibration sensor embedded in a graphite-epoxy composite are presented. A scheme using a single-mode elliptical-core fiber as the lead-in fiber and an offset circular-core single-mode fiber as the lead-out fiber is successfully implemented. Detection techniques for few-mode fiber sensors are reviewed, and a novel fringe-counting method to unambiguously detect the amplitude and direction of dynamic strain is reported. >

Performance of directly modulated DFB lasers in 10-Gb/s ASK, FSK, and DPSK lightwave systems

Abstract: A comparison is presented of the performance of amplitude-shift-keying (ASK), frequency-shift keying (FSK), and differential-phase-shift-keying (DPSK) lightwave systems which operate at 10 Gb/s with directly modulated 1550-nm distributed feedback (DFB) laser transmitters and conventional 1310-nm dispersion-optimized fiber. Computer modeling techniques were used to accurately simulate the amplitude modulation response and the frequency modulation response of DBF lasers. The system performance is evaluated from simulated eye patterns for both direct and heterodyne detection. With the narrow-optical spectral widths of these signal formats, fiber chromatic dispersion limits up to 70 km were obtained for transmission at 1550-nm using conventional 1310-nm optimized fiber. >

A 100-channel optical FDM transmission/distribution at 622 Mb/s over 50 km

Abstract: A 100-channel optical frequency-division multiplexing (FDM) transmission/distribution experiment at 622 Mb/s is demonstrated for a fiber length of 50 km. The feasibility of a polarization-insensitive waveguide frequency selection switch for 10-GHz intervals and a frequency-shift-keying (FSK) direct-detection scheme employing a Mach-Zehnder filter is verified. The demodulation circuit employs a Mach-Zehnder filter and a balanced receiver, which utilizes optical power more efficiently than the Fabry-Perot filter. No receiver sensitivity degradation is observed due to interchannel crosstalk of the 128-channel tunable waveguide frequency selection switch (FS-SW) or fiber four-wave mixing for transmissions over a 50-km-long nondispersion-shifted (NDS) fiber and a 26-km-long dispersion-shifted (DS) fiber. >

Analysis of laser phase noise to intensity noise conversion by chromatic dispersion in intensity modulation and direct detection optical-fiber transmission

Abstract: Laser phase noise conversion to intensity noise due to fiber chromatic dispersion is analyzed by deriving the noise power spectral density. Theory predicts that the phase-modulation-amplitude-modulation conversion noise is a principal limiting factor of the gigabit-per-second nonregenerative transmission using an external modulator when the linewidth of the laser transmitter is above several tens of megahertz and the total chromatic dispersion of fibers exceeds several thousand picoseconds per nanometer. This fact is confirmed by the 2.4-Gb/s transmission experiments using multiple inline Er-doped fiber amplifiers. The system penalty due to this noise in the intensity modulation and direct detection (IM-DD) optical transmission using an external modulator is evaluated. >

Multichannel coherent FSK experiments using subcarrier multiplexing techniques

Abstract: The results for coherent digital subcarrier multiplexed (SCM) lightwave system experiments are described. A total of 20 frequency-shift-keyed (FSK) channels at 100 Mb/s each were transmitted on one optical carrier using microwave subcarriers in a multioctave configuration. A complete description of the system performance, including carrier-to-noise ratio, crosstalk, intermodulation distortion, and receiver sensitivity, is given. With a phase modulation index of beta =0.13, a receiver sensitivity of -27 dBm was obtained, representing a 14-dB improvement over a 20-channel direct detection SCM system. Crosstalk due to adjacent channels is negligible with a channel spacing of twice the data rate (200 MHz). Theoretical and experimental results are compared, and conditions for optimal performance as a function of channel number and phase modulation index are discussed. >

Design optimization for efficient erbium-doped fiber amplifiers

Abstract: The gain and pumping efficiency of aluminosilicate erbium-doped fiber amplifiers (EDFAs) are analyzed as a function of guiding parameters and Er-doping profile for two pump wavelengths of lambda /sub p/=980 nm and lambda /sub p/=1.47 mu m. Three designs of fiber-amplifier waveguides are considered: one with the same mode size as standard 1.5- mu m communication fibers (type 1); one with the same mode size as standard 1.5- mu m dispersion-shifted fibers (type 2); and one with mode size smaller than those of communication fibers (type 3). For the 1.47- mu m pump, fundamental LP/sub 01/ mode excitation is assumed, while for the lambda /sub p/=980-nm pump, concurrent excitation of LP/sub 11/ modes is considered. It is shown that excitation of higher-order pump modes at 980 nm does not significantly affect the amplifier gain performance. The effect of concentrating the Er/sup 3+/ doping near the center of the fiber core is shown to increase the amplifier gain coefficients by a factor of 1.5 to 2. >

Four-channel wavelength division multiplexers and bandpass filters based on elliptical Bragg reflectors

Abstract: Four-channel multiplexers and four-wavelength bandpass filters based on elliptical Bragg reflectors (EBRs) are discussed. The channel spacing is 50 AA near 1.56 mu m. The EBRs are narrowband elliptical mirrors that can refocus light from an input waveguide to any one of a number of output waveguides. Spectrally, they perform similarly to Bragg reflectors. The devices were fabricated on silicon using silica-based waveguide technology. The elliptical gratings with 0.53- mu m periods were patterned using deep UV spatial frequency doubling photolithography. Multiplexers with single filtering and double filtering were demonstrated. With single filtering, the fiber-to-fiber insertion loss was 3.0+or-0.5 dB, and the crosstalk was -20 dB; with double filtering, the insertion loss was 4.0+or-0.5 dB, and the crosstalk was -30 dB. About 2 dB of this loss was due to coupling between the fibers and the waveguide. The additive loss associated with the EBR, including propagation, Bragg reflection, and coupling between input and output waveguides, was about 1 dB. >

Surface plasmon polaritons in thin metal films and their role in fiber optic polarizing devices

Abstract: The polarization-selective evanescent field coupling of an optical wave in a 1.3- mu m, fiber to surface, plasmon polaritons supported by a thin aluminium film was investigated theoretically and experimentally. Good agreement was observed between the theoretically predicted conditions for efficient coupling and the experimentally determined conditions for high TM/TE extinction ratio of the optical field. The use of silver and chrome in place of aluminum was investigated experimentally at 1.3 mu m, and 0.63- mu m devices using aluminium were also studied. The results led to the realization of readily manufacturable, high-extinction-ratio (>50 dB) low-loss ( >

Fast, sensitive magnetic-field sensors based on the Faraday effect in YIG

Abstract: Magnetic-field sensors based on the Faraday effect in ferrimagnetic iron garnets are investigated in terms of their sensitivity, speed, and directionality. Signal-to-noise measurements at 80 Hz on small (typically 5-mm-diameter*3-mm-long) samples of yttrium iron garnet (YIG) yield noise equivalent magnetic fields of 10 nT/ square root Hz. Frequency-response measurements exhibit virtually flat response to approximately 700 MHz. >

Modulation and demodulation techniques in optical heterodyne PSK transmission systems

Abstract: Modulation and demodulation techniques are described for an optical PSK heterodyne transmission system operating at 560 Mb/s and 1.2 Gb/s. Performance limitations affecting the receiver sensitivity in a 1.2-Gb/s DPSK system, such as laser phase noise, phase modulation depth, IF center frequency deviation, and local laser power, are studied. High receiver sensitivities for PSK systems were achieved. The applicability of the Mach-Zehnder modulator as a phase modulator for 1.2-Gb/s DPSK is also demonstrated. A 1.2-Gb/s DPSK transmission of over 100 km, using polarization diversity with novel polarization-insensitive automatic frequency control in an attempt to overcome signal fading caused by polarization fluctuation in the transmitting fiber, is also described. A receiver sensitivity of less than -42.8 dBm and varying within 1.4 dB for all states of polarization was achieved. A multichannel high-definition TV (HDTV) transmission experiment using a DPSK polarization-diversity tunable receiver is described. >

Fabrication of high-concentration rare-earth doped optical fibers using chelates

Abstract: A process which uses rare-earth complexes of 2,2,6,6,-tetramethyl-3,5-heptanedione for uniform incorporation of rare earths in the cores of optical fibers is demonstrated. It is a vapor-phase transport process capable of producing fibers having a rare-earth content of at least 11 wt.% and easily lends itself to incorporation of multiple rare-earth dopants. The process gives high rare-earth concentration with low loss and is readily adaptable to multiple rare-earth doping. High-concentration fibers, 1.0 wt.% Nd/sub 2/O/sub 3/, necessary for the double-clad fiber laser configuration with low loss, less than 10 dB/km, have been fabricated and shown to have efficiencies approaching quantum limits when pumping well above threshold. >

Bending effects in biconically tapered single-mode fibers

Abstract: Biconically tapered single-mode fibers were fabricated, and their characteristics were studied experimentally. The optical throughput was measured as the fiber was being pulled to produce the required radial profile. The tapered single-mode fiber was bent in a simple fixture, and the optical throughput was measured as a function of the bending angle. It is seen that the bending resulted in very strong oscillations of the optical power as the bend angle was varied. At the point when the light in the core was minimal, the cladding region was bright, indicating that the light has moved to the cladding. The propagation characteristics of the tapered single-mode fiber subjected to bending are analyzed using a stepwise approximation. A simple sensor based on this principle is discussed. >

A distributed fiber optic sensor based on cladding fluorescence

Abstract: The fiber for the sensor is formed by cladding fused silica during drawing with polydimethyl siloxane into which an organic fluorescent dye, 9, 10-diphenylanthracene, has been dissolved. Upon side illumination at a wavelength within the excitation range of the dye, the cladding fluoresces; some of this fluorescence is coupled into guided modes in the fiber core through the evanescent fields of these modes. In the presence of oxygen, fluorescent emission by the dye is diminished. For the sensor described, the rubbery liquidlike nature of the polydimethyl siloxane cladding allows rapid diffusion of gases, and the intensity of the guided fluorescence is observed to drop by 30% in less than 5 s when the ambient atmosphere changes from pure nitrogen to pure oxygen. The advantages of this sensing technique, and some of the possibilities for new sensors based on this principle, are discussed. >

A new transfer matrix formalism for the analysis of fiber ring resonators: compound coupled structures for FDMA demultiplexing

Abstract: A method of analyzing fiber-ring resonators which is based on the transfer matrix technique is presented. The method is described and used to analyze known structures and compound coupled-ring resonators. Application of these structures as demultiplexers in FDMA (frequency-division multiple-access) systems is discussed. >

A photonic variable RF delay line for phased array antennas

Abstract: It is shown that by applying spatial frequency-dependent phase compensation in an optical heterodyne process a variable RF delay line can be synthesized over a prescribed frequency band. Experimental results which demonstrate the performance of the delay line with regard to both maximum delay and resolution over a broad bandwidth are presented. A spatially integrated optical system is proposed for control of phased array antennas. The integrated system provides mechanical stability essentially eliminates the drift problems associated with freespace optical systems, and can provide high-packing density. The approach uses a class of spatial light modulator known as a deformable mirror device and leads to a steerable arbitrary antenna radiation pattern of the true time-delay type. >

Amplifier induced crosstalk in multichannel optical networks

Abstract: The effect of crosstalk introduced due to gain saturation in an optical amplifier when it is used for amplifying multiple channels in a wavelength division multiplexed (WDM) network employing ON-OFF keying with direct detection is studied. The system power penalty is quantified as a function of the amplifier input power, the number of channels, and the extinction ratio. >