Mohammad H. Rahnavard

Optimization of tapering profile in IC-DFB and DBR lasers

Performance parameters of tapered distributed feedback (DFB) and distributed Bragg reflector (DBR) lasers are compared with earlier configurations, such as uniform grating DFB, (lambda) /4 phase-shifted DFB and uniform grating DBR lasers. Parameters such as oscillation conditions (above and below threshold), mode pattern along the laser length, output power, effects of nonzero end reflections on oscillation conditions, yield of the gain, linewidth an differential quantum efficiency are obtained for tapered DFB and DBR lasers. To find oscillation conditions a nonlinear model is used, which considers gain saturation due to longitudinal field intensity variation along the laser. Grating shape is of rectangular type. For tapering profile, a class of polynomial curves of hyperbolic form with a degree of freedom is used. For the analysis F-matrix approach is applied. For purpose of comparison, a constant effective coupling coefficient for all structures is defined. Some new important results have been obtained which show that, tapered DFB lasers are superior to phase shifted DFB lasers. Improvement of tapered DBR over uniform DBR depends on the type of performance parameter.

Reflected millimeter power from moving-strip-illuminated semiconductor panel

Moving spot illuminated semiconductor panels are used as millimeter-wave image converters. To determine the performance of this system, it is required to know the response of illuminated semiconductor panels as a function of scanning velocity, width of the strip, time and so forth, is studied.

Improving the performance of the semiconductor panel used in millimeter and IR radar by shaping illuminating spot properties

An optical processing technique is introduced. This technique can improve the resolution of any scanning imaging system, that extract information by using the electronic behavior of materials. In this optical processing technique proper transparency, and adjusting the scanning light intensity are used. This technique can improve the performance of digital image enhancer. We implement our idea on a familiar millimeter image converter.

Optimization of first-order and second-order trapezoidal gratings used in DFB and DBR lasers

Radiation coupled mode theory with first order perturbation is used to investigate the optimum shape and depth in trapezoidal gratings (which includes rectangular and triangular cases) for radiative and nonradiative applications. Optimum trapezoidal grating shape for nonradiative purposes is obtained for both DFB and DBR lasers. For radiative purposes Optimum triangular grating are obtained for DBR lasers.

Laser-scanning semiconductor panels and their use in IR and millimeter wave radar

A moving spot illuminated semiconductor panel is used to convert millimeter wave images to visible displays. The response of semiconductors to moving spot illumination is important in this method. In this paper the response of a semiconductor panel to a moving Gaussian (laser) spot is considered in detail. Initially, the profile of excess carrier in the bulk of the semiconductor panel for Gaussian illumination vs. position, scanning velocity, width of the semiconductor panel, etc., are studied. Using the expression for excess carrier, the single path attenuation of a millimeter wave through moving Gaussian spot illuminated semiconductor panel vs. standard deviation of Gaussian spot and scanning velocity is studied.

Laser scanning semiconductor panels and their use in IR and millimeter-wave radar

A method of conversion of millimeter wave images to visual displays is the use of semiconductor panel under scanning light spot. In this method of conversion, the behavior of semiconductor panel under moving spot of light is important. Performance of this system has been under investigation for last three decades. In this paper excess millimeter wave attenuation through semiconductor panel, due to moving rectangular illuminated spot, with arbitrary intensity profile across the length of the spot is formulated. Numerical calculation is done, for uniform and linearly graded cases. Effects of scanning velocity and spot dimensions on the excess millimeter wave attenuation are considered. It is shown that, with proper choice of parameters, higher system resolution is attainable with linearly graded intensity.

Orthogonal coupled-mode theory for rectangular dielectric waveguides

In this paper two dimensional coupled mode theory is presented. Analyzing the modal behavior, coupling coefficients for various systems could be obtained. It is a natural extension of one dimensional coupled mode theory discussed vastly in the last two decades. A general theory for gain coupling devices, and grating assisted co-directional coupler is presented in this paper. The reason that this theory has not been implemented so far is that, the coupled mode profiles can not be obtained easily. We propose an approximate method based on the Green function for Helmholtz equation for perfect guide. The approximate approach is based on the azimuthal effective index method. This is a good approximation in the weak guidance regime. Coupling coefficients can be obtained from the coupled mode profiles in the grating region in a manner similar to conventional one dimensional theory approach. Numerical results are presented for single dielectric waveguides with periodic corrugations. The theory can also analyze grating-assisted directional couplers, and gain coupled distributed feedback lasers. Furthermore, it can be easily modified to handle the same structures used in other fields of interest.

Effect of index tapering on characteristics of DFB and DBR lasers

ABSTRACT Some critical parameters of uniform and tapered DFB and DBR lasers are calculated and compared to each other. We haveshown that tapering improves some of them including internal quantum efficiency, modal selectivity and yield. Howeverspectral purity of tapered strictures would be worsen. We show that tapering is an ideal choice when the designer desires tomake the device asymmetric.Keywords: Quasi-periodic gratings, DFB and DBR lasers 1. INTRODUCTION Although, DFB laser now is accepted as a standard light source in long haul optical communication networks, there is atendency to improve its characteristic through manipulating its basic structure. In conventional DFB and DBR lasers,usually uniform gratings are used. These periodic structures cause coupling between the incident and reflected wavesprovided that the propagation constant is close to the Braggs conditions. Tapering of periodic structure is a commonpractice in microwave region of spectrum. Matching is the main aim of using tapering there. However in DFB and DBRlasers, there is not an obvious reason for using tapered structures. Index tapering is implemented by reducing the gratingdepth gradually. It causes mainly a variation of coupling coefficient through the device length. Also it causes the variationof local propagation constant. By linear tapering we intend that the depth of grating reduces linearly with distance alongthe device length. Similarly exponential tapering indicates exponential grating depth variation. In this paper we considerlinear tapering exclusively.Tapering is a tailoring between grating depth and length. It is a known fact that coupling will increase if gratingdepth increases. Furthermore, coupling will be more strong for longer gratings. Shallow grating with long length can bereplaced virtually with a deep short grating. In this regard, tapering does nothing; we can replaced it with an equivalentuniform one. Even this equivalence is a drawback of tapering, since the uniform grating with equivalent coupling usuallyis smaller in length. However, as we will show the superiority of tapering for DFB and DBR lasers is on the other aspects.

Solution of Helmholtz's equation in multilayered dielectric waveguide with periodic surface corrugation

Dielectric waveguide with periodic surface corrugation are used in distributed feedback lasers and DBR lasers. In this paper the boundary element method (BEM) has been used to analyze 2D dielectric periodic corrugated waveguides. It is a very efficient method for analysis of this type of structure. The computational method relies on the numerical solution of the integral wave equation inside the grating region. This formalism has distinct advantages over the more traditional ones, especially when the boundary conditions are imposed through a collocation (point-matching) technique. The unknown field quantities together with all the boundary conditions of the problem are explicitly incorporated in the defining equation. For the problem at hand, the boundary conditions on the longitudinal interfaces of the grating layer are functionally known because of the Floquet expansion of the fields in the uniform layers above and below it. On the other hand, the boundary conditions for the interface between the periodic unit cells are naturally provided by Floquets theorem and continuity requirements. Thus the method can be applied in a rather straightforward way towards a rigorous solution of the periodic problem, without any a priori assumptions, within a user specified accuracy. The BEM is a natural choice for this problem because we seek the field solution only on the grating layer interfaces in order to set up a transverse resonant-type characteristic equation for propagating mode. In this paper electromagnetic field and coupling coefficient for multi- layer dielectric waveguide is calculated.