Role of modal distribution in determining power backscattered from fibres with diameter perturbations

Certain results of a theory of the backscattering process in multimode fibres are discussed. Calculated near-field intensities of backscattered waves are presented. The dependence of the backscattered power on diameter perturbations is calculated and shown to agree with the empirically determined relation.     

The MLC assay as a test for HLA-D region compatibility between patients and unrelated donors: results of a national marrow donor program involving multiple centers

Mixed lymphocyte culture (MLC) assays from 763 patients and prospective unrelated marrow transplant donors identified through the National Marrow Donor Program (NMDP) were analyzed for their overall utility in measuring HLA-D region compatibility. The assays were performed at 31 different transplant centers (range, 1 to 197; median, 9 assays per center). A total of 325/763 (42.6%) of the tests were judged to be uninterpretable, either due to lack of sufficient control cells included in the assay (89 tests) or to insufficient reactivity by patient and/or donor cells (236 tests). Among the 438 tests that could be interpreted, HLA-Dw phenotyping with HLA-D homozygous cells was performed for a subset of 190. The relative response (RR) values from these 190 tests, however, were not clearly separable into distinct populations; i.e., RR values corresponding to Dw identity versus nonidentity between patient and donor could not be reliably discriminated. The predictive value of a nonreactive MLC for Dw identity was calculated to be 0.91 for RRs of < or = 20%, while the predictive value of a reactive MLC for Dw nonidentity was 0.35 for RRs of > 20%. These results, based on an analysis of data submitted from multiple transplant centers testing patients who had a variety of hematologic disorders, suggest that the MLC assay is a relatively imprecise method for determining HLA-D region compatibility between patient and prospective unrelated marrow donor.     

Scattering induced crosstalk in active directional couplers

Scattering from defects in an integrated optical device causes phase randomization and depolarization. Scattered light can be recaptured by the waveguides of the device and perturbs modal fields. This leads to crosstalk in directional coupler switches. A defect-scattering-induced crosstalk model is developed and crosstalk in one-, two-, and three-electrode directional couplers is investigated with the model. The number of independent electrode voltages needed to tune out crosstalk is studied. Simulations show that scattering-induced crosstalk can be tuned out completely in active directional couplers with two independent electrode voltages. When modal differential loss and unequal taper coupling are taken into account, two independent electrode voltages are insufficient to tune out the crosstalk, whereas three independent electrode voltages are sufficient. This agrees with the conclusion from previous three-electrode directional coupler experiments     

Eigenmode analysis of symmetric parallel waveguide couplers

An analysis of a laterally symmetric parallel waveguide structure has been carried out by using symmetric and antisymmetric eigenmodes in the coupled mode equations. As opposed to modes of each individual guide, eigenmodes are orthogonal and therefore comply with coupled mode formalism. Of particular interest has been the popular configurationZ-cut LiNbO3with Wave propagation along the crystalY-axis. The theory shows that output versus voltage characteristics are nonperiodic in voltage, and furthermore, they are asymmetric if the eigenmodes are not equally excited. The uniform electrode coupler can easily be explained in terms of eigenmode coupling and interference.     

Semiconductor laser stabilization by external optical feedback

A report is presented on a general theory describing the effect of external optical feedback on the steady-state noise characteristics of single-mode semiconductor lasers. The theory is valid for arbitrarily strong feedback and arbitrary optical feedback configuration and spectrum. A generalized Langevin rate equation is derived. The equation is, in general, infinite order in d/dt constituting an infinite-order correction to the low-frequency weak-feedback analysis. The general formalism includes relaxation oscillations and permits analysis of the effect of feedback on the laser linewidth, frequency noise, relative intensity noise, and the relaxation oscillation sidebands in the field spectrum. The theory is applied to two important feedback configurations: the laser coupled to a single mirror and the laser coupled to a high-Q cavity     

Analysis of an experimental technique for determining Van der Polparameters of a transistor oscillator

The Van der Pol (VDP) model of a transistor oscillator describes the behaviour of the oscillator with three parameters. When operating in steady state, only two parameters can be determined by spectrum analysis, these being the oscillation frequency and amplitude of oscillation. In this paper, a technique for measuring the other VDP parameter is examined. In this approach, a periodically modulated voltage is added to the bias of the oscillator to perturb the operational state. A theoretical derivation shows that the power spectrum of the perturbed oscillator contains additional information for determination of the other VDP parameter. A simple analytical perturbation formula predicts the oscillator's response to the ramped bias. Our experimental results agree with the analytical perturbation solution and therefore, this allows one to read off the other VDP parameter from the experimental data. The VDP model allows one to predict the behaviour of coupled transistor oscillators more accurately and simply than does the traditional large-signal model of the transistor. This VDP model will simplify oscillator array design since the number of parameters needed to describe each oscillator is reduced from that which would be required using a large-signal circuit model     

Remote Laboratories for Optical Circuits

This paper presents a remote laboratory implementation for an optical circuits course. The process from design and implementation towards assessment and continuous improvement phase is outlined. The design of the experiments involved the research on remote access technology and the investigation of best practices for an experimental setup. The first pilot implementations were used to determine the possible shortfalls during an ldquoIntroduction to an Optical Communicationsrdquo course in fall 2004 at the University of Colorado, Boulder, and an ldquoOptical Circuitsrdquo course in fall 2005 at the University of Houston, Houston, TX. After improvements, two experiments were pilot tested during spring 2006 at the University of Houston. Assessment of learning outcomes and teaching methods were performed. The remote access methods, in addition to the delivery of the class and lectures, are presented in the paper. Delivery of a class with remote laboratories and videotaped lectures are also outlined under future directions of this project.     

Experimentally verifiable modeling of coplanar waveguidediscontinuities

A general technique for obtaining the frequency dependent scattering parameters of open waveguiding structures is discussed. The first step of the analysis is an iterative solution for the charge distribution on the electrodes which, in our case, uses the “straight line” solution, the one first derived by Maxwell, as the starting value. A calibrated optical sampling technique allows for direct verification of the validity of the quasi-static charge distribution for structures in which the dielectric layers are also electrooptic. (Common waveguiding dielectrics such as GaAs and InP are sufficiently electrooptic to yield more than sufficient signal-to-noise ratios for accurate verification.) In cases where the quasi-static solution is valid, it is shown that the full dynamics of the propagation problem can be recovered from an equivalent nonuniform transmission line, the parameters of which can be determined from the phase velocity and impedance distribution defined by the static charge distribution. Here, we present analysis of planar discontinuities in coplanar waveguides (CPW), but the method can be modified and applied to include active devices, as well as three-dimensional discontinuities, such as airbridges. The method is based on an iterative solution of the quasi-static charge distribution using successive over relaxation and the dynamics are introduced via the Riccati equation. Additional measurements performed on the “in-house” fabricated passive circuits using an HP8510 network analyzer verify the accuracy of the dynamical part of the method