Showing papers in "Journal of Lightwave Technology in 1991"

Modeling erbium-doped fiber amplifiers

Abstract: Erbium-doped fiber amplifiers are modeled using the propagation and rate equations of a homogeneous two-level laser medium. Numerical methods are used to analyze the effects of optical modes and erbium confinement on amplifier performance, and to calculate both the gain and amplified spontaneous emission (ASE) spectra. Fibers with confined erbium doping are completely characterized from easily measured parameters: the ratio of the linear ion density to fluorescence lifetime, and the absorption of gain spectra. Analytical techniques then allow accurate evaluation of gain, saturation, and noise in low-gain amplifiers (G >

Erbium-doped glasses for fiber amplifiers at 1500 nm

Abstract: Material-dependent properties influencing the performance of fiber amplifiers are reviewed together with the available data for Er/sup 3+/. The major glass types potentially useful in this application are considered and compared to silica. The topics addressed include quenching processes and the solubility of rare-earth ions, transition strengths and bandwidths at the 1500-nm gain transition, and the characteristics at the 800-, 980-, and 1480-nm pump bands. Aluminum is shown to be an extremely useful codopant for silica, improving its ability to dissolve rare-earth ions and providing desirable spectroscopic properties for Er/sup 3+/. For some of the attributes considered, other glasses have advantages over Al silica, but only with respect to gain bandwidth and pumping performance at 800 nm is significantly better than expected from other glass compositions. >

Statistical theory of polarization dispersion in single mode fibers

Abstract: An analytical characterization of polarization dispersion measurements is presented. The authors report the solution of Poole's stochastic dynamical equation for the evolution of the polarization dispersion vector with fiber length. The authors extend this to a more complete description by considering small, second-order dispersion effects through the frequency derivative of the dispersion vector. The complete analytical solution is seen to accord with what were originally empirically derived features of the joint probability distribution of the polarization dispersion vector and its frequency derivatives. Among the analytically determined properties are the Gaussian probability densities of the three components of the dispersion vector, and the hyperbolic secant (soliton shaped) probability densities of the components of the derivative of the dispersion vector. >

On the bit error rate of lightwave systems with optical amplifiers

Abstract: The problem of evaluating the performances of communication systems with optical amplifiers and a wideband optical filter is addressed. Exact probability of error expressions for optical signals in presence of amplifier spontaneous noise and photodetector shot noise are given and compared with those predicted by Gaussian approximations for amplitude shift keying (ASK), frequency shift keying (FSK), or differential phase shift keying (DPSK) modulations, both for ideal photodetectors and for the case where shot noise is significant. >

The first demonstration of an optically steered microwave phased array antenna using true-time-delay

Abstract: The concept of true-time-delay steering for phased array antennas and its advantage over phase shifters in broadband applications are described with particular emphasis on the potential advantages offered by fiber-optic delay lines. The system architecture of the phased array antenna and the design and performance of the fiber-optic time-shifter networks in the antenna are described. The radiation patterns of the phased array antenna at L and X bands are presented. Using semiconductor laser switching to implement the delay times, the absence of beam squint in the antenna pattern and its frequency was switched from L to X band is demonstrated. >

Propagation of signal and noise in concatenated erbium-doped fiber optical amplifiers

Abstract: Signal propagation and noise accumulation in lightwave systems using saturated optical amplifiers as repeaters are analyzed. Numerical simulations of amplified spontaneous emission in concatenated erbium-doped fiber amplifiers indicate that a reach beyond 10000 km is possible with a 1.55- mu m system in the absence of fiber nonlinearities. Distributed optical amplifiers are shown to have low noise, but require higher pump power than lumped amplifiers. Three operating modes of an amplifier lightwave system are identified and their relative signal power efficiency and noise performance are described. >

Wavelength division multiplexing with solitons in ultra-long distance transmission using lumped amplifiers

Abstract: One attractive feature of the all-optical approach to ultra long-distance transmission is that it greatly facilitates wavelength division multiplexing (WDM). It is known that solitons of different velocities are transparent to each other. It is shown, through numerical simulation, that such transparency is also maintained in a system using lumped amplifiers, as long as the length of the collision (the distance the solitons travel down the fiber while passing through each other), is long enough relative to the spacing between amplifiers, or to a possibly longer period of variation in some other parameter, such as the fiber's chromatic dispersion. This result implies the potential for at least several multigigabits-per-second WDM channels spanning just 1 or 2 nm, in a system of transoceanic length (7000-9000 km). >

1/f frequency noise effects on self-heterodyne linewidth measurements

Abstract: The effects of a 1/f frequency noise on self-heterodyne detection are described, and the results are applied to the problem of laser diode linewidth measurement. The self-heterodyne autocorrelation function and power spectrum are evaluated for both the white and the 1/f components of the frequency noise. From numerical analysis, the power spectrum resulting from the 1/f frequency noise is shown to be approximately Gaussian, and an empirical expression is given for its linewidth. These results are applied to the problem of self-heterodyne linewidth measurements for coherent optical communications, and the amount of broadening due to 1/f frequency noise is predicted. >

Long-distance soliton propagation using lumped amplifiers and dispersion shifted fiber

Abstract: It is shown both analytically and by numerical simulation, that solitons can traverse great distances through a chain of lumped amplifiers connecting dispersion shifted fiber spans. The fiber spans can also have large fractional variations in D. The resultant pulse distortions and dispersive wave radiation tend to be negligible, as long as the length scale of the variations in energy and dispersion are short relative to the soliton period. >

Shuffle Net: an application of generalized perfect shuffles to multihop lightwave networks

Abstract: A multihop wavelength-division multiplexing (WDM) approach, referred to as Shuffle Net, for achieving concurrency in distributed lightwave networks is proposed. A Shuffle Net can be configua19 red with each user having as few as one fixed-wavelength transmitter and one fixed-wavelength receiver, avoiding both wavelength-agility and pretransmission-coordination problems. The network can achieve at least 40% of the maximum efficiency possible with wavelength-agile transmitters and receivers. To transmit a packet from one user to another may require routing the packet through intermediate users, each repeating the packet on a new wavelength until the packet is finally transmitted on a wavelength that the destination user receives. For such a multihop lightwave network, the transmit and receive wavelengths must be assigned to users to provide both a path between all users and the efficient utilization of all wavelength channels. >

A wide-FSR waveguide double-ring resonator for optical FDM transmission systems

Abstract: A wide free-spectral-range (FSR) waveguide double-ring resonator (DRR) is investigated for use in optical frequency-division multiplexing (OFDM) transmission systems. It consists of two ring resonators with different radii and three directional couplers. The FSR is determined by the least common multiple of the FSRs of the two ring resonators. In order to determine the parameters of the DRR, transmission equations of the DRR, which have different radii, are derived by using the scattering matrix. The dependence of the transmittance characteristics of the DRR on the amplitude coupling coefficients of directional couplers and on the propagation loss of the ring waveguides is studied. The design guidelines for OFDM transmission systems are described. >

A review of the fabrication and properties of erbium-doped fibers for optical amplifiers

Abstract: Erbium-doped fiber has become the central component of nearly all optical amplifiers. Applications reported include repeaters, power amplifiers, preamplifiers, and distributed amplifiers. To date, nearly all the fiber used in these devices has been silica based and fabricated by variations on the major telecommunications fiber technology. Disadvantages of the silica-based host glass, such as low solubility of the rare-earth ions and narrowband fluorescence, have been carefully addressed and solutions have been found to overcome these potential drawbacks. Details of the current status of fabrication methods, matching particular fibers for specific applications, together with optimizing the fiber for high efficiency, are presented. >

The design of erbium-doped fiber amplifiers

Abstract: An accurate model for the erbium-doped fiber amplifier is presented. The model is used to design the index profile of the doped fiber, optimizing with regard to efficiency for inline- and preamplifiers as well as for power booster amplifiers. The predicted pump efficiencies (maximum gain to pump power ratios) are in agreement with experimental results presented in the literature. The choice of codopant is shown to be very significant for the pump efficiency when pumping in the 0.98 mu m. The pump efficiency in the 0.98- mu m pump band is shown to be twice the pump efficiency in the 1.48- mu m pump band. >

Effect of fiber nonlinearity on long-distance transmission

Abstract: The effect of the weak refractive-index nonlinearity of optical fibers on pulse shape is investigated using computer simulations of long-distance transmission. Fiber losses are canceled by periodically spaced optical amplifiers whose spontaneous emission noise is, however, not included in the simulations. The analysis is confined to normal pulses and does not consider solitons. Several conclusions are drawn. (1) If wavelength division multiplexing (WDM) of two channels is used in a uniform fiber without dispersion fluctuations, catastrophic buildup of four-wave mixing occurs if one primary channel is located at the zero-dispersion wavelength. (2) If two pulses with different carrier frequencies collide in a uniform fiber with no gain or loss discontinuities, their four-wave mixing products reach a peak during complete pulse overlap, but this spurious power dies away as the pulses separate. (3) Two-channel WDM transmission of light modulated in amplitude-shift keying format appears feasible at 2.5 GB/s over distances of 7500 km. >

Polarization mode dispersion of short and long single-mode fibers

Abstract: Polarization mode dispersion (PMD) in short and long single-mode fibers was measured by a polarization-maintaining Michelson interferometer. A nonnegligible PMD was found in some standard fibers. The sensitivity enables PMD to measure the bend-induced PMD of a fiber rolled on a 28-cm diameter drum. A theoretical model for PMD with random mode coupling is developed, and an explicit equation for the time-of-flight distribution is presented. Comparison between measurements on short and long fibers with residual birefringence leads to an estimation of the coupling length on the order of 20-30 m. >

Single-channel operation in very long nonlinear fibers with optical amplifiers at zero dispersion

Abstract: The author investigates, by means of computer simulations, the performance of a very long, single-channel optical fiber system operating very close to the zero-dispersion wavelength of the fiber. Fiber losses are compensated by optical amplifiers. The optical signal is filtered after each amplifier, is passed through a final optical filter prior to square-law detection, and is finally filtered electrically. It is found that such a system does not work well if the fiber dispersion is strictly constant and if the carrier wavelength of the modulated signal coincides with the zero-dispersion wavelength of the fiber. As a result, the optical signal spectrum spreads to many times its initial width so that power is lost in the optical filters and the signal-to-noise ratio is degraded by the need for admitting a wider band of noise to the receiver. >

A single-mode fiber with chromatic dispersion varying along the length

Abstract: A method for fabrication of a novel type of optical fiber with dispersion varying along the fiber length is described. The method takes into account the calculated dependence of fiber dispersion on fiber core diameter for the measured profile of the preform and the desirable dispersion dependence on the fiber length. The main optical parameters of the drawn fiber are theoretically studied and experimentally measured. The fibers are of great interest for nonlinear fiber optics. Such applications of the fibers, such as high-quality soliton pulse compression, soliton pulsewidth stabilization through compensation of losses, and generation of a high-repetition-rate train of practically uninteracting solitons, are considered. >

Fiber-optic Fabry-Perot temperature sensor using a low-coherence light source

Abstract: An interferometric fiber-optic sensor using a light-emitting diode (LED) as the optical source is analyzed and demonstrated. The sensor arrangement employs two Fabry-Perot interferometers (FPIs) in series, one for sensing and one which serves as a reference. The optical output from the LED is spectrally modulated by reflection from the sensing FPI. Then, reflection or transmission by the reference FPI produces an interferometric beat response similar to that observed when a laser is used with the sensing interferometer alone. Best fringe visibility is obtained when the optical path lengths of the two interferometers are matched, and the fringes disappear when the path length difference becomes substantially greater than the coherence length of the LED. >

Calculation of bit-error probability for a lightwave system with optical amplifiers and post-detection Gaussian noise

Abstract: The author's previously published results (ibid., vol.8, p.1816-23, 1990) which dealt with the bit-error probability of a lightwave communication system whose noise contribution was exclusively attributed to spontaneous emission noise of optical amplifiers are extended. The earlier theory was based on an NRZ (nonreturn-to-zero) modulation format with zero signal power assigned to logical zeros. The extension is based on the same mathematical technique but includes Gaussian noise originating after the signal has been converted to an electrical current. This noise could be thermal noise, shot noise, or a combination of both. It is no longer assumed that the pulse representing a logical one fills its time slot completely. Now, the optical pulses can be much shorter than the bit period (solitons), the power level of logical zeros need not vanish, and the thermal noise of the postdetection electronic circuit is taken into account. >

High-resolution OCDR for testing integrated-optical waveguides: dispersion-corrupted experimental data corrected by a numerical algorithm

Abstract: A numerical algorithm is presented and applied to dispersion-corrupted experimental data taken by coherence domain reflectometric (OCDR) measurements. It results in a retrieval of micrometer resolution and an enhanced dynamic range. High-resolution OCDR measurements may be seriously impaired by dispersion effects in the waveguides under test and rugged source spectra. In the investigation (Ti:LiNbO/sub 3/ single-mode waveguide, 16 mm long; superluminescent diode source at lambda /sub 0/=820 nm) the spatial resolution progressively deteriorates along the waveguide by about one order of magnitude (from 6 to 60 mu m). The algorithm was applied to the experimental data and successfully removed the dispersive broadening of the signatures. >

Polarization-independent interferometric optical-time-domain reflectometer

Abstract: A polarization-independent interferometric optical-time-domain reflectometer is proposed and demonstrated. The experimental setup is composed of a superluminescent diode and a fiber coupler which combines a polarization-maintaining fiber and a conventional single-mode fiber to achieve a polarization-independent spatial profile of the reflectance in a single-mode fiber or waveguide sample. The polarization independence and the reliability of the measured reflectivity, were confirmed, and the measurement of loss and birefringence in an optical waveguide from its reflectance profile is demonstrated. The influence of dispersion in the fibers on the spatial resolution of the reflectometer is also discussed. >

Effects of fiber nonlinearities and amplifier spacing on ultra-long distance transmission

Abstract: It is shown that it should be possible to send error-free signals at a 2.5-Gb rate (or higher) over distances of at least 9000 km using an amplitude shift keying (ASK) soliton modulation system. To accomplish this, the amplifiers must be kept close enough that their power gain is less than 10 dB. (It is noted that timing jitter and other noise effects measured in recent soliton transmission experiments carried out at low D and with amplifier spacing of 25 km are in close accord with predictions of this work). Frequency division multiplexing of several channels over the same fiber should also be possible, as solitons of different frequencies interact very weakly, provided the distance over which they pass through one another is large compared to the amplifier spacing. >

Slotted Aloha and reservation Aloha protocols for very high-speed optical fiber local area networks using passive star topology

Abstract: A number of cases of slotted Aloha protocols for high-speed optical fiber local area networks (LANs) are presented and analyzed. The results are compared to the cases of Slotted Aloha/Aloha schemes presented by I.M.I. Habab et al., (1987) and N. Mehravori (1990). The proposed schemes were found to offer better throughput and delay characteristics. These cases are also compared with Aloha/N-server and CSMA/N-server protocols presented by Habab and Mehravori. Some of the considered protocols offer better throughput when the data packet lengths are small. Two cases of reservation Aloha protocols are presented and analyzed. >

Performance comparison between optoelectronic and VLSI multistage interconnection networks

Abstract: The performance characteristics of optoelectronic and VLSI multistage interconnection networks are compared The bases of the comparison include speed, bandwidth, power consumption, and footprint area The communication network used in the comparison is a synchronous packet-switched multistage interconnection network built from 2*2 bit-serial switching elements CMOS technology was used in the VLSI implementation, and it is assumed that the entire network resides on a single chip Regular free-space optical interconnects are used in the optoelectronic implementation The results show that for large networks optoelectronics offers higher speed and lower area than VLSI Based on the assumed technology parameters, optoelectronics outperforms VLSI in bandwidth for network sizes above 256 >

Annealing of linear birefringence in single-mode fiber coils: application to optical fiber current sensors

Abstract: Annealing procedures that greatly reduce linear birefringence in single-mode fiber coils are described. These procedures have been successfully applied to coils ranging from 5 mm to 10 cm in diameter and up to 200 or more turns. They involve temperature cycles that last 3-4 days and reach maximum temperatures of about 850 degrees C. The residual birefringence and induced loss, are minimized by proper selection of fiber. The primary application of these coils is optical fiber current sensors, where they yield small sensors that are more stable than those achieved by other techniques. A current sensor with a temperature stability of +8.4*10/sup -5//K over the range from -75 to +145 degrees C has been demonstrated. This is approximately 20% greater than the temperature dependence of the Verdet constant. Packaging degrades the stability, but a packaged sensor coil with a temperature stability of about +1.6+10/sup -4//K over the range from -20 to +120 degrees C has also been demonstrated. >

Helical-grating two-mode fiber spatial-mode coupler

Abstract: The authors investigate a helical-grating geometry for coupling the spatial modes in a two-mode fiber. The feasibility of making fiber grating filters based on this geometry is investigated. By adding mode strippers at the input and output of a helical-grating spatial-mode coupler, an optical notch filter is demonstrated with 0.3-dB waveguide loss, 24-dB extinction, and 24-nm spectral width. The effect of small ellipticity in the fiber core is also investigated, and it is shown that polarization-insensitive coupling can be achieved in a fiber with a sufficiently circular core. >

Integrated optical receivers using MSM detectors

Abstract: The performance of a metal-semiconductor-metal (MSM) detector when combined with preamplifier electronics is discussed, and it is noted that certain advantages due to its horizontal surface can result in receivers with fundamentally higher performance than that possible with vertical p-i-n detector designs. The sensitivity of optical receivers using a MSM detector is analyzed, and a set of scaling rules is used to compare a MSM receiver's sensitivity with that of a similar receiver using a vertical p-i-n photodiode structure. Assuming identical preamplifier designs it is shown that the MSM-based receiver, except at very high bit rates, can in principle consistently outperform its p-i-n counterpart with as much as a 6-dB improvement in sensitivity. >

High-isolation polarization-independent quasi-optical circulator

Abstract: A high-isolation polarization-independent optical circulator is reported. The circulator structure can sharply reduce isolation dependency on both the rotation error of a rotator and on extinction ratio of a polarizing beam splitter. As a result, the circulator has high isolation over a wide wavelength and temperature range. A 4-port circulator coupled with single-mode fibers was fabricated. Its measured insertion loss and isolation were 31.5 dB, respectively. Consequently, a loss ratio of over 29.7 dB has been realized. The isolation (which is free from reflections) was measured to be >45.1 dB. It is confirmed that the wavelength range for this kind of isolation over 40-dB is very wide. The measured range was wider than the measurement system limit of 90 nm. >

Random access protocols for high-speed interprocessor communication based on an optical passive star topology

Abstract: A multiple-instruction multiple-data (MIMD) distributed memory parallel computer system environment is considered. Media access control protocols that maintain good performance with high capacity optical channels are investigated. Three examples of star-coupled structures are introduced, one of which exhibits optical self-routing. Self-routing single-step optically interconnected communication structures can be designed through the incorporation of agile laser diode sources and wavelength tunable optical filters in a wavelength-division multiple-access environment. Intermediary latencies typical of MIMD distributed memory systems are eliminated. The degree and diameter of the resulting structures are dramatically reduced, and the complexity of the communication subsystem is reduced since intermediate buffering and routing of packets are eliminated. >

A broad-band single polarization optical fiber

Abstract: The authors report on a single-mode optical fiber that transmits one and only one polarization state of the fundamental mode over a broad wavelength band (13%). The good broadband performance is attributed to the fiber's refractive-index profile, which mathematically allows only one polarization state to cutoff. The authors discuss general design principles and show that the mode field diameter is a qualitative indicator of the fiber's performance. The fiber's guided polarization state is similar to the fundamental mode of a step-index fiber in its low attenuation, 5.5 dB/km, and small mode diameter, 5.8 mu m (both measurements at 840 nm). Most important, the fiber shows a broad single polarization band, 13%, a feature well described by the change in mode field diameter with wavelength of an ideal, circularly symmetry, W-profile fiber. >