1,721,180 research outputs found
Loss calculations in weakly-guiding optical dielectric waveguides
No full textThe application of perturbation theory to a three-layer weakly-guiding slab waveguide composed of lossy dielectric media yields a simple formula for the attenuation coefficient alpha of a guided mode: α = (Σ3i=1 αi Pi) / (Σ3i=1 Pi), where αi, Pi are respectively the loss coefficient and model power in region i (i = 1,2,3). It is shown that this result can also be obtained from arguments based purely on geometric optics. The result is easily extended to apply to circularly-symmetric optical fibres where it yields confirmation of an earlier approximation for the power ratios Pi/Σ2i=1 Pi .</sub
The cladded parabolic-index profile waveguide: analysis and application to stripe-geometry lasers
No full textA two-dimensional real waveguide having a parabolic distribution of dielectric permittivity in the core region, flanked by cladding layers of constant refractive index is proposed as a model for a class of stripe-geometry injection lasers. The model is analysed by a series-solution method and results are given in terms of plots of normalized propagation constant, near-field filling factor, and far-field half-angle, all versus normalized frequency. These results are compared with those for the alternative models of (a) a homogeneous core waveguide, and (b) a medium whose dielectric permittivity varies quadratically with position throughout the whole of space. Apart from the series-solution method which was employed, a survey is included of other techniques available for analysing this waveguide model. These include perturbation theory, variational analysis, an exact solution of the scalar wave equation, and the WKB approximation
Longitudinal field control in stripe-geometry lasers
No full textThe theory of optical confinement by a gain-guiding mechanism in stripe-geometry lasers has been extended to include the effects of longitudinal inhomogeneity of the optical field and the gain coefficient. A proposal is made for utilising these effects to improve field control and efficiency in lasers with a.r.-coated facets
A unified approach to the problems of semiconductor laser theory
No full textThis chapter is concerned with some of the fundamental problems of semi-conductor laser theory; more specifically, let us ask the questions: (i) which problem remain to be solved? (ii) of the remainder, which solutions are the most appropriate for given situations? (iii) how much influence on laser development have the theoretical solutions had?A conventional starting-point would be to discuss the historical progress of the laser from the early days of a low-power, high-threshold device requiring pulsed operation and/or a low-temperature environment, to the current situation of a wide variety of laser structures adapted for CW room-temperature operation, good mode control, ease of modulation, etc. A brief history of theoretical topics should include (chronologically) the reasons for high thresholds, the temperature dependence of threshold current, the role of dielectric waveguiding, the problem of heat dissipation, the gain-current relationship, transient behaviour (noise, resonance frequencies, 'spiking', modulation, time delays, Q-switching), electromagnetic mode control, carrier transport phenomena and many other topics
Transient time-averaged spectra of rapidly-modulated semiconductor lasers
Full text linkUsing the computed solution of multimode rate equations, and including wavelength-chirping effects via the dependence of the refractive index on carrier concentration, it is possible to calculate time-averaged spectra of rapidly-modulated lasers. The averaging time may be used to simulate the effects of measuring equipment with a specific time response. The results show a characteristic line structure within the dominant longitudinal mode spectrum, and are in qualitative agreement with experimental measurements reported by other author
On the accuracy of WKB approximation in optical dielectric waveguides
No full textEigenvalues and field distributions for optical waveguides obtained from the WKB approximation have been compared with those found from geometrical optics and from more rigorous weakly-guiding LP mode theories in the cases of the step-index and parabolic-index profiles. In all cases it is found that the zero-order WKB approximation yields very accurate eigenvalues provided care is taken in the choice of phase factors in the eigenvalue equation. Expressions are deduced for the required phase factors for guides of arbitrary index profile in both two- and three-dimensions, and physical interpretations are given in terms of ray optics. The first-order WKB field distributions are found to give good agreement with the mode fields everywhere except in the vicinity of the caustics
Optimal design of monomode transmitter module
No full textA simple model of an external cavity is used to investigate optimum configurations for achieving a high degree of spectral purity in one mode. A new figure of merit for monomode behaviour is defined
A proposal for a new approach to heterojunction theory
No full textThe Anderson theory of heterojunctions is reviewed with particular reference to the derivation of the affinity rule. We consider in detail the fundamental assumptions on which the Anderson model is based, analyzing arguments which either confirm or dispute these underlying ideas. An alternate model is then proposed. and again discuss the arguments for and against. Our conclusion is that further work must be done both theoretically and experimentally before a definite determination can be made as to the more appropriate of these two theories
Computer-simulated transient evolution of 1.55µm laser spectra
No full textThe transient evolution of 1.55µm-wavelength laser spectra has been calculated from numerical solution of the multimode rate equations including the effects of spontaneous emission into the modes. A simple analytical model for gain spectrum has been used which assumes recombination between parabolic bands with no k-selection. The results show the characteristic relaxation oscillations in each mode together with competition effects between the modes which occur with a rather slower time scale. The principal parameter which determines the power sharing and competition between the modes is the displacement in wavelength of the steady-state peak gain from the nearest Fabry-Perot mode. The relaxation oscillations and modal competition are shown on a 16mm cine film in a form that can be readily assessed
Picosecond spectra of gain-switched quaternary lasers
No full textDetailed analysis of picosecond pulse generation in 1.55µm lasers is reported. Large dynamic line width (~100 GHz) is predicted for lasers driven to 10 times threshold. Spectra emitted by lasers biased below and above threshold are compared, and the spectral purity of the optical pulse is shown to improve considerably when a pre-bias condition above threshold is chosen
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