Lateral Mode Discrimination Research Articles

Lateral Mode Discrimination in Edge-Emitting Lasers with Spatially Modulated Facet Reflectance

A post-growth technique aimed at spatial modification of facet reflectance of edge-emitting diode lasers has been proposed. It is based on the deposition of an anti-reflection coating and subsequent precise etching with a focused ion beam. The technique allowed suppressing of high-order lateral modes in 10 μm stripe lasers based on ten layers of InAs/InGaAs quantum dots.

Lateral mode discrimination and self-stabilization in ridge waveguide laser diodes

Lateral mode discrimination and output stability is experimentally investigated in ridge waveguide laser diodes, having various residual guide thicknesses outside the ridge region. It is found that a critical residual thickness exists below which the lasers emit in a single mode, with a low threshold current. Above this critical value, the threshold current rises rapidly with the residual guide thickness, and the lasers oscillate simultaneously in the two lowest order lateral modes. Increasing the injected current intensity, in this regime, results in nonlinear light output-current curves, but also improves mode discrimination in favor of the fundamental mode, until single-mode operation is re-established. A simple mechanism is suggested to explain this phenomenon

Enhancement of Lateral Mode Discrimination in Broad-Area VCSELs Using Curved Bragg Mirrors

This paper studies a novel technique for enhancing the single mode properties of large-area oxide-confined vertical cavity surface emitting lasers (VCSELs). It utilizes a nonplanar shadow masked epitaxial growth to spatially detune part of the upper Bragg stack from the lower stack. The resulting mirror reflectivity modulation is used to enhance the discrimination between the lateral modes. Compared to other techniques, this method results in a larger difference in threshold gain between the lateral modes. We have developed a quasi-three-dimensional (3-D) optical model to analyze this laser. Simulation results indicate that even a small curvature can enhance the mode discrimination, and single mode operation may be possible for VCSEL diameters much greater than 2 /spl mu/m.

Tilted mirror for lateral mode discrimination and higher kink-free power in fiber pump lasers

Beam steering arising from interference between the fundamental lateral mode and higher order lateral modes limits the performance of 980-nm power lasers. In this paper, we analyze the feasibility of using a tilted mirror to discriminate against higher order modes. We calculate the reflectivity seen by different modes as a function of the thickness and tilt angle of the coating. Our results suggest that careful engineering of tilted coated mirrors represents a viable technique for improving the performance of power lasers.

Improvement of the lateral-mode discrimination of broad-area diode lasers with a profiled reflectivity output facet

The emission of a broad-area laser always contains several lateral modes (along the junction plane) even at low drive levels. To increase the discrimination against high-order lateral modes, we developed simple techniques for depositing a profiled thin layer of SiO on the output facet of a broad-area laser that has a 75-μm-wide injection current stripe. The profiled coating provided a nearly Gaussian reflectivity in the lateral direction (parallel to the junction plane). The resolved near-field spectra of the uncoated and coated lasers have been compared. The maximum output power in the single-lateral-mode regime was pushed to 25 mW with the profiled coating, compared with a corresponding power of less than 1 mW before deposition. Experimental results have confirmed the behavior predicted by our numerical simulations. This method is scalable to higher-power lasers.

Lateral-mode discrimination in ridge waveguides by misaligned total internal reflection mirrors

Lateral-mode discrimination by misaligned total internal reflection (TIR) mirrors in multimode ridge waveguide structures is proposed. The effects of angular misalignment in TIR mirrors and index steps in ridge waveguides on the modal reflectivity is analyzed by a plane wave expansion method. The modal reflectivity of a high order mode decreases more rapidly than that of a fundamental mode as the angular misalignment of TIR mirrors increases. It is found that a fundamental mode can have high modal reflectivity (/spl sim/0.9) with significant lateral-mode discrimination over high-order modes if the ridge waveguides have large index steps.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Lateral mode discrimination in surface emitting DBR lasers with cylindrical symmetry

The effective reflectivity and the threshold gain of all lateral modes in a surface emitting DBR laser with cylindrical symmetry are analyzed using the coupled-mode approach. Our results indicate that when a cavity is designed for operation in the odd modes, all even modes are effectively eliminated. A small perturbation is introduced into the complex dielectric constant of the active region to suppress the unwanted lateral modes of odd symmetry. The model predicts that lasing will occur in a single mode with a narrow far-field pattern. >

High-power spatially coherent operation of unstable resonator semiconductor lasers with regrown lens trains

We have obtained high-power spatially coherent operation in wide-stripe InGaAs/GaAs/AlGaAs semiconductor lasers using a monolithic unstable resonator [consisting of diverging lens elements incorporated above an asymmetric graded-index separate confinement heterostructure (AGRIN-SCH)]. The fabrication involves MOCVD regrowth after wet-chemical etching of lens-like patterns in a GaAs layer above the active region. Pulsed output powers of 175 and 490 mW have been obtained in 170- and 100-μm-wide lasers, respectively, with spatial coherence in the near field exceeding 60%. We observe good lateral mode discrimination up to 3.5 times threshold in the 100-μm stripes with a round-trip magnification of 6.4.

Lateral mode discrimination in AlGaInP selectively buried ridge waveguide lasers

The guiding mechanism of (AlxGa1−x)0.5In0.5P visible lasers is analysed using a simple effective index approach and is found to be analogous to that of AlGaAs channelled substrate planar lasers. Outside the ridge, the bound mode is leaky, resulting in a real lateral index guide. Furthermore, the loss outside the ridge provides good discrimination against oscillation of higher order modes. The best ridge widths, based on the tradeoff of low loss in the fundamental mode along with high lateral mode discrimination, are in the range 5–7 μm.

Lateral-mode discrimination in broad-area semiconductor lasers with a smooth spatially filtering output facet

We present numerical simulations of broad-area semiconductor lasers with smooth spatial filtering at the output facet. The beam-propagation and Prony methods are used to calculate the beam characteristics and discrimination against higher-order lateral modes of the laser devices. With an output facet having a Gaussian reflectivity profile, the fundamental lateral mode is favored. Discrimination between the two lowest-order modes increases significantly with narrow reflectivity profiles.

Leaky-guided channeled substrate planar (LCSP) laser with reduced substrate radiation and heating

A semiconductor laser design, the leaky-guided channeled substrate planar (LCSP) structure, is presented. The structure with a buffer layer in the region outside the channel provides a built-in negative effective index step. The transverse mode is well damped in the buffer layer, and the substrate radiation in the region outside the channel is virtually eliminated. As a result, the significant heating and facet degradation in the region outside the channel that is related to the substrate radiation in conventional CSP lasers is minimized. The calculated local temperature rise in the region outside the channel of the CSP laser is about 45 degrees C for an output power level of 100 mW and is in reasonably good agreement with previously experimental results. Analysis using the complex domain effective index method shows that the LCSP structure provides better lateral mode discrimination than either gain-guided or positive index step structures. The LCSP structure with reduced heating and strong lateral coupling effect may be useful for high-power linear array lasers. >

Lateral mode discrimination and control in high-power single-mode diode lasers of the large-optical-cavity (LOC) type

A comprehensive study of lateral mode discrimination and control in weak-laterally-confining large-optical-cavity (LOC)-type structures is presented. The analysis centers on two types of CDH-LOC laser structures: type A, which supports only the fundamental lateral mode in both the passive and active regimes; and type B, which supports several lateral modes in the passive regime and only the fundamental mode in the active regime. The transverse confinement factor <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\Gamma</tex> is peaked in the center of the structure and varies significantly across the lasing region for both device types. In the passive regime it is found that the effective-index (lateral) profile is a W-shaped waveguide for type A devices and a positive-index waveguide for type B devices. A discussion and analysis of losses in W-guides is also presented. Under carrier injection (i.e., active regime) the evolution of W-guides in CDH-LOC structures is presented as a function of increasing current density up to lasing threshold. For both type A and type B devices the effective-index profiles and corresponding lateral far-field patterns are analyzed as a function of threshold mode gain. Carrier-induced bulk-index depressions are found to be in the -0.02 to -0.04 range depending on the value of the threshold mode gain. The corresponding antiguiding parameter, <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">R = k_{o} \deltan/\deltag</tex> , takes values in the -3 to -4 range, which imply values between 6 and 8 for the linewidth enhancement factor α. It is found that by controlling the threshold mode gain (i.e., changing the device length and/or its facet(s) reflectivity) devices of the same cross-sectional geometry can be made to lase either multimode (spatially) or in the fundamental mode.