Showing papers in "Journal of Lightwave Technology in 1992"

The shape of fiber tapers

Abstract: A model for the shape of optical fiber tapers, formed by stretching a fiber in a heat source of varying length, is presented. Simple assumptions avoid any need for the techniques of fluid mechanics. It is found that any decreasing shape of taper can be produced. The procedure for calculating the hot-zone length variation required to produce a given shape of taper is described, and is used to indicate how an optical adiabatic taper can be made. A traveling burner tapering system is capable of realizing the model's prediction, and a complete practical procedure for the formation of fiber tapers with any reasonable shape is thus presented. >

Four-wave mixing in an optical fiber in the zero-dispersion wavelength region

Abstract: Fiber four-wave mixing (FWM) in the zero-dispersion wavelength region is described. The phase-matching characteristics are studied in the wavelength region where the first-order chromatic dispersion is zero. The results show that the phase-matching condition is satisfied and FWM light is efficiently generated at particular combinations of input light wavelengths. It is also shown that the deviation of the zero-dispersion wavelength along the fiber length plays an important role in FWM behavior. >

Polarization multiplexing with solitons

Abstract: It is shown both analytically and with numerical simulation, and confirmed experimentally in transmission over distances up to approximately 10000 km, that solitons maintain a high degree of polarization over an ultra-long distance transmission system consisting of birefringent dispersion-shifted fiber segments and erbium amplifiers. Based on that fact, the authors propose a polarization/time division multiplexing technique which should allow the single-wavelength bit-rate capacity of an ultra-long distance soliton transmission system to be doubled with little or no significant increase in bit error rate. >

The finite-difference vector beam propagation method: analysis and assessment

Abstract: The newly developed finite-difference vector beam propagation method (FD-VBPM) is analyzed and assessed for application to two-dimensional waveguide structures. The general formulations for the FD-VBPM are derived from the vector wave equations for the electric fields. The stability criteria, the numerical dissipation, and the dispersion of the finite-difference schemes are analyzed by applying the von Neumann method. Important issues regarding the implementation, such as the choice of reference refractive index, the application of numerical boundary conditions, and the use of numerical solution schemes, are discussed. The FD-VBPM is assessed by calculating the attenuation coefficients and the percentage errors of the propagation constants of the TE and TM modes of a step-index slab waveguide. Several salient features of the FD-VBPM are illustrated. >

Small signal analysis for dispersive optical fiber communication systems

Abstract: A small signal analysis for analyzing the conversion between intensity and phase modulation or noise in a dispersive fiber is given. Using this theory the small signal response of laser diodes with respect to intensity and phase modulation in dispersive optical fibers is derived. Guidelines are also given for considerably reducing the intensity noise if a fiber with a suitable dispersion is used. All analytical calculations are compared with numerical simulations and good agreement is achieved. >

Planar monomode optical couplers based on multimode interference effects

Abstract: The self-imaging property of a homogeneous multimoded planar optical waveguide has been applied in the design of passive planar monomode optical couplers based on multimode interference (MMI). Based on these designs, 3 dB and cross couplers were fabricated in SiO/sub 2//Al/sub 2/O/sub 3//SiO/sub 2/ channel waveguides on Si substrates. Theoretical predictions and experimental results at 1.52- mu m wavelength are presented which demonstrate that MMI couplers offer high performance: on-chip excess loss better than 0.5 dB, high reproducibility, low polarization dependence and small device size. >

Multilayer waveguides: efficient numerical analysis of general structures

Abstract: An efficient numerical method for accurately determining the real and/or complex propagation constants of guided modes and leaky waves in general multilayer waveguides is presented. The method is applicable to any lossless and/or lossy (dielectric, semiconductor, metallic) waveguide structure. The method is based on the argument principle theorem and is capable of extracting all of the zeros of any analytic function in the complex plane. It is applied to solving the multilayer waveguide dispersion equation derived from the well known thin-film transfer matrix theory. Excellent agreement is found with seven previously published results and with results from two limiting cases where the propagating constants can be obtained analytically. >

Bending losses of coated single-mode fibers: a simple approach

Abstract: A new approach for the bending losses of coated optical single-mode fibers is developed based on a modified fiber geometry model, and the result is a simple formula. An improved condition for the peak locations is given, that allows for the wavefront curvature of the leaky mode in the bent fiber, since it is derived by wave optics. Simple explicit relations for both the peak spacings and the peak-to-trough ratio are given. Their asymptotic forms for small bend radii are independent of the fundamental-mode parameters. An interpretation of oscillations in the bend-loss curves in terms of a thin-film filter effect of the triple-layer structure on the lateral leaky-mode radiation fully explains the behavior, especially the loss increase in the peaks compared to the case of an infinite cladding. The agreement with numerical and experimental results of other authors is excellent. >

A systems perspective on digital interconnection technology

Abstract: The work describes, from a switching system designers perspective, present digital interconnection technologies and needs. The discussion begins at the chip-to-chip interconnection level and proceeds to the frame-to-frame level. The authors evaluate the optimal packaging and interconnection technology that should be used in future system designs. The analysis suggests that parallel optical data links based on a laser array technology implemented at the board-to-board level are presently advantageous. This analysis, as well as a detailed description of laser gate arrays, is also included. >

Coherent optical QPSK intradyne system: concept and digital receiver realization

Abstract: A receiver concept based on optical quadrature phase-shift keying (QPSK) and a digital realization of synchronous demodulation including phase synchronization is presented. To keep the signal processing bandwidth low a phase diversity receiver, called an intradyne receiver, with an orthogonal electrical demodulation is proposed. Basic principles of the synchronous orthogonal and digital demodulation are described. After the evaluation of the shot noise limit some aspects of the digital phase-locked loop (PLL) are presented. In a 100-Mb/s transmission system a receiver sensitivity of -51.6 dBm has been measured. The loss in relation to the shot noise limit of -66.3 dBm (18 photons/b) is mostly due to the low local laser power and the influence of the receiver input noise. >

Detailed dynamic model for semiconductor optical amplifiers and their crosstalk and intermodulation distortion

Abstract: An advanced dynamic model for multisection semiconductor optical amplifiers is presented. It accounts for the carrier and field distributions in the longitudinal direction as well as for the facet reflectivities. The model can handle arbitrary time-varying input signals and current modulations. The model is used to assess intermodulation distortion and crosstalk. Cascaded amplifiers are considered, and the crosstalk and intermodulation distortion due to cascaded amplifiers are found to accumulate by adding together in amplitude; this may limit the number or cascaded amplifiers in multichannel systems. Carrier-induced nonlinearities depend strongly on facet reflectivities; for 25 dB of single-pass gain, a reflectivity of 5*10/sup -4/ will result in 3 dB excess distortion. Reduction of intermodulation distortion by use of multisection amplifiers is found to be possible only for small channel separations ( >

Characteristics and reduction of coherent fading noise in Rayleigh backscattering measurement for optical fibers and components

Abstract: The characteristics of fading noise in Rayleigh backscattering measurements made with coherent lightwaves such as in coherent-OTDR (optical time-domain reflectometry) and coherent-OFDR (optical frequency-domain reflectometry) are studied. The effects of frequency shift averaging on fading noise reduction are clarified theoretically, and the relationships between measurement accuracy and other parameters, such as spatial resolution and frequency variation range are derived. The calculated results of loss measurement accuracy are in good agreement with experimental data. The formula can also be applied to low-coherence interferometric OTDR. >

Sideband generation through perturbations to the average soliton model

Abstract: It is shown that the mechanism for sideband formation in erbium mode-locked soliton fiber lasers is a perturbation to the average nonlinear Schrodinger equation (NLSE) allied to a phase matching criterion. Numerical simulations in agreement with the theoretical description are presented. While the problem has been formulated in terms of a linear, or ring cavity, the same underlying process will occur in figure of eight or other configurations. The theory is equally applicable to soliton transmission systems. >

A planar InP/InGaAs avalanche photodiode with floating guard ring and double diffused junction

Abstract: The authors discuss the fabrication, performance, and design of a novel, planar In/sub 0.53/Ga/sub 0.47/ As/InP separate absorption and multiplication region avalanche photodiode (SAM-APD) with floating guard rings and a double Zn diffused junction. The APD, grown by both vapor phase epitaxy and metalorganic vapor phase epitaxy, is observed to have a uniform gain of 85, a minimum primary dark current density of 5*10/sup -6/ A/cm/sup 2/ at 90% of breakdown, and a capacitance of 0.4 pF for a front-side illuminated device. Both experimental and analytical results show that the double-diffused floating guard ring structure prevents edge breakdown, and also greatly reduces the electric field along the semiconductor/insulator surface. The operation mechanisms and the optimum design of the planar APD based on a two-dimensional device model are discussed. >

Narrow-band optical channel-dropping filter

Abstract: Waveguide couplers are combined with lambda /4 shifted distributed feedback (DFB) resonators to produce narrowband channel dropping filters. The bandwidth of the filter can be made much narrower than the stopband of the grating. It is possible to remove the spurious responses of the grating filter by appropriate dispersion characteristics for the coupled waveguides. However, in some practical applications it may not be necessary to do this, if all channels can be accommodated within half the grating bandwidth of the filters. >

Design of a single-mode linear-cavity erbium fiber laser utilizing Bragg reflectors

Abstract: The design constraints required to achieve single-longitudinal-mode operation from a standing-wave, homogeneously broadened, three-level, fiber laser which utilizes intracore Bragg reflectors for cavity feedback are presented. In particular, a closed-form solution to the laser rate equations is found which bounds the region in which only single longitudinal mode operation is supported. The principal resonator geometry to be considered is a two-reflector cavity which utilizes only Bragg reflectors for feedback. Analytical and experimental results are discussed. >

All-optical CDMA using 'quasi-prime' codes

Abstract: The use of fiber-optic lattices for the generation of quasi-prime codes is described, and their application to CDMA communications is discussed. Theoretical and experimental results (including typical autocorrelation and crosscorrelation performance) are presented for the programmable all-optical generation and decoding of such codes using electrooptically switchable lattices. >

Optical loss property of silica-based single-mode fibers

Abstract: The optical loss property of silica fibers has been investigated theoretically and experimentally based on their Rayleigh scattering and absorption losses. The Rayleigh scattering loss for fibers has been estimated using Rayleigh scattering coefficients and power distribution in the fiber. The Rayleigh scattering coefficients are measured for preforms prepared for fiber fabrication and are discussed for GeO/sub 2/-doped and F-doped glasses. The relationship between the optical loss and fiber parameters is clarified. Moreover, the loss increase due to residual stress which occurs during the drawing process is simulated. The optical loss limitations for GeO/sub 2/-doped and pure silica core fibers are shown. >

Wet etching of proton-exchanged lithium niobate-a novel processing technique

Abstract: A novel and simple method to etch lithium niobate is reported. It was found that proton exchanged areas on the c/sup +/-face of the crystal can be etched away using a mixture of HF:HNO/sub 3/ while the unexchanged regions are left untouched. The method can be used as a tool for studies of the proton exchange process, as well as to form three-dimensional structures. >

Frequency domain analysis of an optical FM discriminator

Abstract: Measurement of optical phase and frequency deviations using a frequency-domain network analysis approach is described. The frequency domain transfer functions are given which relate the conversion of optical phase, frequency, and intensity modulation into photodetector current after passing through a quadrature biased Mach-Zehnder interferometer. It is shown that the error term in the phase and frequency transfer functions depends only to second order on any accompanying intensity modulation. Experimental data are given illustrating the analytical results. >

Strained quantum wells for polarization-independent electrooptic waveguide switches

Abstract: A polarization-independent quantum well waveguide switch is demonstrated. By altering the composition and hence the degree of built-in strain, the bandgap of In/sub 1-x/Ga/sub x/As/InP quantum wells is engineered to produce equal field-induced refractive-index change in TE and TM polarizations. At the same time, the enhanced electrooptic effects characteristic of unstrained quantum wells are maintained, such that the voltage-length product for switching is only 3 V-mm. >

On the transfer matrix method for distributed-feedback waveguide devices

Abstract: The local normal mode transfer matrix method (TMM) is introduced and developed for the rigorous analysis of distributed feedback (DFB) waveguide devices. This approach is valid for relatively large grating depth and index difference and applicable to both TE and TM modes. Remarkably simple analytical expressions for the coupling coefficients and the power reflectivity are derived and the correspondence between the transfer matrix method and the coupled-mode theory (CMT) is established. Comparisons with the coupled-mode theory have been carried out and some salient features of the DFB structures are discussed. The natural links among the transfer matrix method, the Bloch wave approach, and the coupled-mode theory are also revealed and discussed. The analysis demonstrates unambiguously the power of the transfer matrix method and its overall advantages in terms of not only accuracy and scope of validity but also simplicity and insightfulness. >

Statistical properties of the impulse response function of double-carrier multiplication avalanche photodiodes including the effect of dead space

Abstract: The statistical properties of the impulse response function of double-carrier multiplication avalanche photodiodes (APDs) are determined, including the effect of dead space, i.e., the minimum distance that a newly generated carrier must travel in order to acquire sufficient energy to become capable of causing an impact ionization. Recurrence equations are derived for the first and second moments and the probability distribution function of a set of random variables that are related, in a deterministic way, to the random impulse response function of the APD. The equations are solved numerically to produce the mean impulse response, the standard deviation, and the signal-to-noise ratio (SNR), all as functions of time. >

Polarization modulated direct detection optical transmission systems

Abstract: A novel polarization modulated direct detection (PM-DD) system suitable both for binary and multilevel transmission is presented. At the transmitter the optical field is polarization modulated by a standard modulator. The receiver is based on the estimation of the Stokes parameters of the received optical field by means of a direct-detection optical front end and baseband electrical processing. The Poincare sphere rotation induced by the fiber is compensated by means of a purely electronic algorithm and the decision is performed in the Stokes space. The system performance is evaluated by an analytical model when the only relevant noise source is the receiver thermal noise and when erbium-doped optical amplifiers introduce amplified spontaneous emission (ASE) noise. The system is completely compatible with a direct-detection-based optical network, and it is possible to implement efficient multilevel modulation formats. >

Hybrid integration of semiconductor lasers with Si-based single-mode ridge waveguides

Abstract: Hybridization is a convenient way for providing silicon-based integrated optical circuits with a laser source. An entirely passive alignment technique exclusively based on the high precision obtainable by lithography and by semiconductor processes was examined. A novel way of cleaving the laser chips by self-aligned cleaving grooves was employed. It resulted in 15.5-dB coupling losses between the laser diode and the single-mode buried ridge waveguide. >

A collisionless multiple access protocol for a wavelength division multiplexed star-coupled configuration: architecture and performance analysis

Abstract: A collisionless wavelength-division multiple-access (WDMA) protocol for a passive star-coupled photonic network is introduced and shown to possess significant performance and flexibility advantages. A performance modeling technique based on a semi-Markov analytic model, which eliminates many of the unrealistic assumptions of past approaches, is introduced. The performance of the protocol is analyzed using this model and discrete-event simulation. Control channel access arbitration is achieved through time-division multiplexing (TDM), enabling all active nodes to transmit once every control cycle. The long synchronization delays typical of TDM systems are significantly reduced, because the control cycle length is proportional to the control packet size rather than the data packet size. The protocol eliminates packet collision and variable-sized data packets are supported without utilization degradation. >

In-line fiber Fabry-Perot interferometer with high-reflectance internal mirrors

Abstract: A finesse of 21 has been measured for a fiber Fabry-Perot interferometer (FFPI) with multilayer TiO/sub 2//SiO/sub 2/ internal mirrors. Experimental reflectance and transmittance data for the thermally tuned interferometer were fit with calculated curves using values of 86% for the reflectance and 7.2% (0.33 dB) for the excess optical loss of each mirror. Applications in sensing and communications are discussed. >

Index-and-dimensional taper and its application to photonic devices

Abstract: A mode-field-converting optical channel waveguide with tapered refractive index and cross-sectional dimensions is proposed. In this waveguide, both the refractive index and cross-sectional dimensions of a core are adiabatically varied along a propagation direction in such a way that the normalized frequency V of the waveguide may be kept constant. Since radiation loss caused by waveguide imperfections is strongly dependent on the V value, the waveguide has good mode-field-converting capability without causing high radiation loss. Layer thickness and index of a core of a silica waveguide were tapered by modulating a raw material gas flow in a flame hydrolysis deposition method, and a channel width taper was formed by a photolithographical technique. In order to confirm the usefulness, taper waveguides have been used (1) to achieve good field matching between a fiber and a guided-wave optical switch, and (2) to fabricate a fiber-compatible monolithic 32*32 star coupler in a small chip. >

InP/GaInAsP guided-wave phase modulators based on carrier-induced effects: theory and experiment

Abstract: An overview of the different contributions due to carrier-induced effects that appear in InP/GaInAsP guided-wave phase modulators is given. The authors review and calculate the band-filling effect in such devices and point out optimized structures. The fabrication of phase modulators and directional coupler switches based on a GaInAsP/InP heterostructure is described. These devices exhibit modulation characteristics in good agreement with the calculations having a phase modulation efficiency as high as 11 degrees /V mm and low optical losses. >

Ultrafast photonic ATM switch with optical output buffers

Abstract: An ultrafast photonic asynchronous transfer mode (ATM) (ULPHA) switch based on a time-division broadcast-and-select network with optical output buffers is presented. The ULPHA switch has an ultra-high throughput and excellent traffic characteristics, since it utilizes ultrashort optical pulses for cell signals and avoids cell contentions by novel optical output buffers. Feasibility studies show that an 80*80 ULPHA switch with 1-Gb/s input/output is possible by applying the present technology, and that more than 1 Tb/s is possible by making a three-stage network using such switches. As an experimental demonstration, 4-bit 40-Gb/s optical cells were generated and certain cells were selected at an output on a self-routing basis. With its high throughput and excellent traffic considerations, the ULPHA switch is a strong candidate for a future large-capacity optical switching node. >