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Tapered geometry semiconductor optical sources yielding high-power and narrow far-field patterns have proven to be extremely successful, not least because of the relatively simple fabrication requirements. Almost all of the publications to date concentrate on tapers where the width increases linearly along the length of the device. A common feature of the output from such devices is a quadratic phase curvature [1]. In order to achieve the desired, narrow far-field profiles this phase curvature is usually removed by using an appropriate lens external to the device. However, such an arrangement is expensive and it is desirable to design device structures which produce high powers and acceptably narrow far-fields without the use of a lens.Two categories of devices have been reported -(i) those without any built-in lateral index step;(ii) those with a built-in lateral index step. This paper will provide persuasive evidence that a parabolic taper, with a built-in lateral index step, is a simple and convenient structure which significantly reduces the quadratic phase curvature so that a nearly diffraction-limited far-field is obtained without the use of a lens. A model has been developed to analyse tapered device structures. The important features of the model will be provided along with computed results for the output characteristics of a travelling-wave amplifier and a single-taper laser. In addition, a comparison of the characteristics of linear and parabolic taper Bow-Tie lasers will be presented to clearly demonstrate the advantages of using the parabolic shape. The model is based on a piece-wise constant (longitudinally segmented, stepped width) and local mode expansion analysis. Since built-in, lateral index-step devices are considered it is justifiably assumed that the refractive index change due to the carrier distribution is small so that the mode shapes remain unchanged. However, of paramount importance are the effects of the carrier distribution on the complex propagation constants of the local modes. In this model the propagation constants are calculated using a perturbation analysis. It is important to note that the total field in the device may still change due to the carriers at any longitudinal position. The model is thus compact and efficient in computation time. Near diffraction-limited lateral far-field profiles, directly from the facet, with FWHM less than 2° are predicted for a 2mm long bow-tie laser 30jnm wide at the facets. An increase in slope efficiency for the parabolic shape compared to the linear shape is predicted for bow-tie lasers due to improved coupling of the field reflected at the facet into the narrow section of the device. These results show that parabolically tapered bow-tie lasers should be useful as high-power semiconductor optical sources producing narrow output fields. A model has been developed to analyse tapered device structures. The important features of the model will be provided along with computed results for the output characteristics of a travelling-wave amplifier and a single-taper laser. In addition, a comparison of the characteristics of linear and parabolic taper Bow-Tie lasers will be presented to clearly demonstrate the advantages of using the parabolic shape. The model is based on a piece-wise constant (longitudinally segmented, stepped width) and local mode expansion analysis. Since built-in, lateral index-step devices are considered it is justifiably assumed that the refractive index change due to the carrier distribution is small so that the mode shapes remain unchanged. However, of paramount importance are the effects of the carrier distribution on the complex propagation constants of the local modes. In this model the propagation constants are calculated using a perturbation analysis. It is important to note that the total field in the device may still change due to the carriers at any longitudinal position. The model is thus compact and efficient in computation time. Near diffraction-limited lateral far-field profiles, directly from the facet, with FWHM less than 2° are predicted for a 2mm long bow-tie laser 30jnm wide at the facets. An increase in slope efficiency for the parabolic shape compared to the linear shape is predicted for bow-tie lasers due to improved coupling of the field reflected at the facet into the narrow section of the device. These results show that parabolically tapered bow-tie lasers should be useful as high-power semiconductor optical sources producing narrow output fields.
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