Method, system and device for suppressing higher-order modes

Abstract

Laser diode (3200), comprising: a transverse waveguide (3212) comprising an active layer (3218) between an n-type waveguide layer (3214) and a p-type waveguide layer (3216), the transverse waveguide (3212) being bounded by an n-type cladding layer (3208) on an n-side and by a p-type cladding layer (3210) on a p-side; wherein the transverse waveguide (3212) is orthogonal to a lateral waveguide (3206), wherein the lateral waveguide (3206) is bounded in the lateral direction by a ridge waveguide (3220), wherein the lateral waveguide (3206) further includes an embedded higher-order mode suppression layer (HOMSL, 3204) disposed beneath the p-cladding layer (3210) within the lateral waveguide and within the p-type waveguide layer (3216) or within the n-type waveguide layer (3214) and including a thin layer (3204) of a material having a lower refractive index than the waveguide layer (3214, 3216), wherein the lateral waveguide (3206) is defined in a longitudinal direction at a first end by a highly reflective (HR) coated facet (3224) and is limited at a second end by a partially reflective (PR) coated facet (3226) and wherein the higher-order mode suppression layer (HOMSL, 3204) extends in a longitudinal direction from the HR facet over a length that is smaller than the distance between the HR facet (3224) and the PR facet (3226).

Description

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application claims U.S. Provisional Patent Application No. 62 / 885,946, filed August 13, 2019. TECHNICAL FIELD

[0002] The technology disclosed herein relates to diode lasers, in particular to methods, systems and devices for suppressing higher order modes in diode lasers. BACKGROUND

[0003] Lasers are light-emitting devices. Light emission in a laser is the result of optical amplification by stimulated emission of electromagnetic radiation. Some lasers emit spatially and temporally coherent light, which allows the lasers to emit light in a narrow bandwidth that can be tightly focused over long distances. There is a wide variety of lasers, including gas lasers, chemical lasers, dye lasers, metal vapor lasers, solid-state lasers, and semiconductor lasers. Laser diodes are electrically pumped semiconductor lasers in which an active layer is formed from the pn junction of a semiconductor diode. Laser diodes typically contain an active layer sandwiched between a p-type layer of semiconductor material and an n-type layer of semiconductor material.Many laser diodes are fabricated on a semiconductor substrate, such as gallium arsenide, doped with elements such as aluminum, silicon, zinc, carbon, or selenium to create the n- and p-type semiconductor layers. The active layer is typically undoped gallium indium arsenide and can be only a few nanometers thick.

[0004] Laser diodes are fabricated by growing multiple layers of semiconductor materials on a suitable substrate with a lattice constant that allows the choice of materials to produce the desired emission wavelengths. A typical laser diode contains n-type layers, p-type layers, and an undoped active layer between them, so that when the diode is biased, electrons and holes in the active region layer recombine to produce light. The active layer (quantum wells, quantum wires, or quantum dots, type II quantum wells) is located within a waveguide layer that has a higher refractive index than the surrounding p- and n-doped cladding layers. Light generated by the active layer is confined within the plane of the waveguide.

[0005] A conventional edge-emitting Fabry-Perot broad-area laser diode is designed as a rectangular gain- or index-guided semiconductor structure. Opposing end facets of the waveguide define highly and partially reflective elements to provide feedback for the light oscillation within the resonator. The multilayer laser diode structure extends the length of the laser and has a large width for electrical injection, extending to the opposite side faces, which also extend the length of the laser. The multilayer semiconductor materials are typically designed so that the laser operates in a single mode along the laser's growth direction, and this direction is defined as the fast axis direction.Since a semiconductor laser operates in a single mode along the fast axis, the brightness of a laser diode cannot be further enhanced in this direction; it is, so to speak, diffraction-limited. The distance between the top and bottom surfaces of the multilayer semiconductor laser structure thus forms the minor dimension of the end facets, i.e., the thickness of the stripe, which is typically on the order of micrometers. On the other hand, the width of the multilayer laser structure forms the major dimension of the end facets, i.e., the stripe width, which is typically on the order of many tens of micrometers to hundreds of micrometers. This is referred to as the "slow axis."Since the stripe width is much larger than the light wavelength, the lateral property of an optical field propagating along the optical axis of the waveguide is highly multimodal along the wider stripe dimension, and the corresponding axis is described as the slow axis since the divergence angle is much smaller than the divergence angle related to the fast axis.

[0006] Multimode laser diodes or broad-area lasers (BALs) are used in high-power applications. BALs have multiple slow-axis modes, so their slow-axis beam parameter product (BPP) is higher than that of single-mode laser diodes. Furthermore, as they operate at higher currents, thermal lensing becomes more pronounced, causing a higher index contrast profile in the lateral direction, thus accommodating a larger number of lateral modes. Increasing the lateral divergence angle leads to a deterioration of the lateral BPP and brightness (power: BPP), while simultaneously reducing the slow-axis brightness. This means that the slow-axis brightness decreases, although the power typically increases with higher current.Brightness can be improved in BALs by reducing the emitter width; however, the current at which maximum brightness occurs also decreases at increasingly lower current values. Thus, the maximum output power drops at maximum brightness.

[0007] DE 10 2011 075 502 A1 discloses a laser diode with an anti-waveguide layer. The anti-waveguide layer is arranged laterally of an active, current-carrying region. US Pat. No. 6,118,799 A discloses a semiconductor element having a "dip" in a waveguide layer and current-blocking layers. US 2011 / 0176568 A1 discloses a semiconductor laser diode with a light-absorbing layer for suppressing higher-order modes, which layer is arranged outside a ridge waveguide.

[0008] For power-scaling applications and to reduce the cost per watt in diode laser manufacturing, higher brightness with higher output power per emitter is desirable. SUMMARY

[0009] Disclosed are methods, systems, and devices for reducing the magnitude of the refractive index contrast of a lateral waveguide during operation of a laser diode. This may include a laser diode having a transverse waveguide orthogonal to the lateral waveguide, including an active layer between an n-type waveguide layer and a p-type waveguide layer, the transverse waveguide being defined by an n-type cladding layer on an n-side and a p-type cladding layer on a p-side, and a lateral waveguide being defined in a longitudinal direction at a first end by a highly reflective (HR) coated facet and at a second end by a partially reflective (PR) coated facet, the lateral waveguide further including an embedded higher-order mode suppression layer (HOMSL).which is arranged within the lateral waveguide under the p-cladding or on one or both sides of the lateral waveguide or a combination thereof, wherein the HOMSL extends longitudinally from the HR facet over a length that is less than the distance between the HR facet and the PR facet.

[0010] A refractive index of the HOMSL disposed on one or both sides of the lateral waveguide may, in some examples, be higher than that of the p-type waveguide layer and the p-cladding layer.

[0011] In some examples, a refractive index of the HOMSL disposed within the lateral waveguide may be lower than that of the n-type waveguide layer or the p-type waveguide layer, or combinations thereof.

[0012] A thickness of the HOMSL may be selected based on a magnitude of the refractive index contrast in the lateral waveguide induced by thermal lensing within the lateral waveguide during operation of the laser diode. The magnitude of the refractive index contrast may be in a range of 10 -5 < Δn<10 -3 In some examples, such a lateral waveguide supports fewer than 10 lateral modes or only a single lateral mode.

[0013] In some examples, a thickness of the HOMSL may be selected such that an effective index is reduced on a side of the lateral waveguide extending from the HR facet. The lateral waveguide is bounded in the lateral direction by a ridge waveguide, with the ridge waveguide extending from the HR facet to the PR facet.

[0014] In some examples, the HOMSL overlaps the lateral waveguide by 0-10 µm on each side or by 0-20 µm in total.

[0015] In some examples, the HOMSL located within the lateral waveguide is 0-10 µm laterally narrower than the lateral waveguide on each side or 0-20 µm overall.

[0016] In some examples, the lateral waveguide is bounded in the lateral direction by a ridge waveguide that extends in the longitudinal direction from the PR facet over a length that is less than the distance between the PR facet and the HR facet.

[0017] In some examples, the HOMSL has a reduced transverse waveguide thickness in the active stripe and may be formed by either etching or selectively depositing thicker layers adjacent to the active stripe.

[0018] In some examples, the HOMSL contains gallium arsenide (GaAs), aluminum gallium arsenide (AlGaAs), indium gallium arsenide (InGaAs), indium aluminum gallium arsenide (InAlGaAs), indium gallium phosphide (InGaAsP).

[0019] In examples where the HOMSL is a thinner, low-index layer, it may be formed from AlGaAs along the width of the lateral waveguide in the HOMSL region. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] The accompanying drawings, in which like reference numerals represent like elements, are incorporated in this specification and, together with the description, explain the advantages and principles of the presently disclosed technology. In the drawings, Fig. 1 shows an effective lateral index profile for an exemplary laser diode having a higher-order mode suppression layer disposed adjacent to a lateral waveguide; Fig. Figure 2 shows a lateral index profile and modal modeling for an exemplary laser diode with a higher-order mode suppression layer disposed adjacent to the lateral waveguide; Fig. 3 is a perspective cross-sectional view illustrating a vertical epitaxial layer structure of an exemplary laser diode having a higher order mode suppression layer disposed adjacent to the lateral waveguide; Fig. 4 is a perspective cross-sectional view illustrating a vertical epitaxial layer structure of an exemplary laser diode having a higher order mode suppression layer disposed adjacent to the lateral waveguide; Fig.5 is a perspective cross-sectional view illustrating a vertical epitaxial layer structure of an exemplary laser diode having a higher order mode suppression layer disposed adjacent to the lateral waveguide; Fig. 6 is a perspective cross-sectional view illustrating a vertical epitaxial layer structure of an exemplary laser diode having a higher order mode suppression layer disposed adjacent to the lateral waveguide; Fig. 7 is a plan view illustrating an exemplary lateral waveguide of a broad area laser diode having a higher order mode suppression layer disposed longitudinally adjacent to the lateral waveguide; Fig.8 is a plan view illustrating an exemplary lateral waveguide of a broad area laser diode having a partial length higher order mode suppression layer disposed longitudinally adjacent to the lateral waveguide; Fig. 9 shows a plan view of an exemplary lateral waveguide of a broad area laser diode with a partial length higher order mode suppression layer disposed longitudinally adjacent to the lateral waveguide on only one of its sides; Fig. 10 is a plan view showing an exemplary lateral waveguide of a broad area laser diode having a higher order mode suppression layer disposed longitudinally adjacent to the lateral waveguide; Fig.11 is a plan view showing an exemplary lateral waveguide of a broad area laser diode having a partial length higher order mode suppression layer disposed longitudinally adjacent to the lateral waveguide on only one side; Fig. 12 is a plan view showing an exemplary lateral waveguide of a broad area laser diode having a partial length higher order mode suppression layer disposed longitudinally adjacent to the lateral waveguide; Fig. 13 shows a plan view of an exemplary broad area laser diode lateral waveguide having a partial length higher order mode suppression layer disposed longitudinally to one side adjacent to the expanded laser oscillator waveguide; Fig.14 is a plan view showing an exemplary expanded laser oscillator waveguide (i.e., lateral waveguide) of a laser diode having a higher-order mode suppression layer disposed longitudinally adjacent to the expanded laser oscillator waveguide; Fig. 15 is a plan view showing an exemplary expanded laser oscillator waveguide (i.e., lateral waveguide) of a laser diode having a partial length higher-order mode suppression layer disposed longitudinally adjacent to the expanded laser oscillator waveguide; Fig.16 is a plan view showing an exemplary expanded laser oscillator waveguide (i.e., lateral waveguide) of a laser diode having a partial length higher-order mode suppression layer disposed adjacent to the expanded laser oscillator waveguide in the longitudinal direction on only one side; Fig. 17 is a plan view showing an exemplary expanded laser oscillator waveguide (i.e., lateral waveguide) of a laser diode having a partial length higher-order mode suppression layer disposed longitudinally adjacent to the expanded laser oscillator waveguide; Fig.18 is a plan view showing an exemplary expanded laser oscillator waveguide (i.e., lateral waveguide) of a laser diode having a partial length higher-order mode suppression layer disposed adjacent to the expanded laser oscillator waveguide in the longitudinal direction on only one side; Fig. 19 is a plan view showing an exemplary expanded laser oscillator waveguide (i.e., lateral waveguide) of a laser diode having a partial length higher-order mode suppression layer disposed longitudinally adjacent to the expanded laser oscillator waveguide; Fig.20 is a plan view showing an exemplary expanded laser oscillator waveguide (i.e., lateral waveguide) of a laser diode having a partial length higher-order mode suppression layer disposed adjacent to the expanded laser oscillator waveguide in the longitudinal direction on only one side; Fig. 21A is a perspective cross-sectional view illustrating a vertical epitaxial layer structure of an exemplary laser diode with a customized n-side current injection scheme and a higher-order mode suppression layer disposed longitudinally adjacent to the lateral waveguide; Fig.21B is a perspective cross-sectional view illustrating a vertical epitaxial layer structure of an exemplary laser diode with a customized n-side current injection scheme and a higher-order mode suppression layer disposed adjacent to the lateral waveguide in the longitudinal direction; Fig. 22 is a perspective cross-sectional view illustrating a vertical epitaxial layer structure of an exemplary laser diode with a customized n-side current injection scheme and a higher-order mode suppression layer disposed longitudinally adjacent to the lateral waveguide; Fig.23 is a perspective cross-sectional view illustrating a vertical epitaxial layer structure of an exemplary laser diode with a customized n-side current injection scheme and a higher-order mode suppression layer disposed longitudinally adjacent to the lateral waveguide; Fig. 24 is a perspective cross-sectional view illustrating a vertical epitaxial layer structure of an exemplary laser diode with a customized n-side current injection scheme and a higher-order mode suppression layer disposed longitudinally adjacent to the lateral waveguide; Fig.25 is a perspective cross-sectional view showing a vertical epitaxial layer structure of an exemplary laser diode having a higher order mode suppression layer disposed adjacent to the lateral waveguide in the longitudinal direction; Fig. 26 is a perspective cross-sectional view illustrating a vertical epitaxial layer structure of an exemplary laser diode having a higher order mode suppression layer disposed longitudinally adjacent to the lateral waveguide; Fig. 27 is a perspective cross-sectional view showing a vertical epitaxial layer structure of an exemplary laser diode with an adapted n-side current injection scheme; Fig.28 is a perspective cross-sectional view showing a vertical epitaxial layer structure of an exemplary laser diode with an adapted n-side current injection scheme; Fig. 29 is a perspective cross-sectional view showing a vertical epitaxial layer structure of an exemplary laser diode with an adapted n-side current injection scheme; Fig. Figure 30A is a graph showing a conventional step index profile in the transverse direction of a single-emitter laser diode operating at low power without significant thermal lensing; Fig. 30B is a graph showing a step index profile with an added parabolic profile; Fig. Figure 30C is a graph showing an example “negative” step index profile with an added parabolic profile that models the index profile in the negative compensation region; Fig.31A is a cross-sectional view of a vertical epitaxial layer structure, viewed from the highly reflective (HR) side of an exemplary laser diode, including an embedded higher-order mode suppression layer disposed on adjacent sides of a lateral waveguide and extending longitudinally; Fig. Figure 31B is a cross-sectional view of a vertical epitaxial layer structure, from the partially reflective (PR) side of the Fig. 31A, which includes an embedded layer for suppressing higher-order modes disposed on adjacent sides of a lateral waveguide; Fig.31C is a plan view of an exemplary laser diode having an embedded higher-order mode suppression layer disposed on adjacent sides of a lateral waveguide and extending longitudinally; Fig. 31D is a perspective cross-sectional view of a vertical epitaxial layer structure, as viewed from the highly reflective (HR) side of an exemplary laser diode, including an embedded higher-order mode suppression layer disposed on adjacent sides of a lateral waveguide and extending longitudinally; Fig.32A is a cross-sectional view of a vertical epitaxial layer structure, viewed from the highly reflective (HR) side of an exemplary laser diode, including an embedded higher-order mode suppression layer disposed within a lateral waveguide and extending longitudinally; Fig. 32B is a cross-sectional view of a vertical epitaxial layer structure, from the partially reflective (PR) side of the Fig. 32A, which includes an embedded higher-order mode suppression layer disposed within a lateral waveguide and extending longitudinally; Fig.32C is a plan view of an exemplary laser diode having an embedded higher order mode suppression layer disposed within a lateral waveguide and extending longitudinally; Fig. 32D is a perspective cross-sectional view of a vertical epitaxial layer structure, viewed from the highly reflective (HR) side of an exemplary laser diode, including an embedded higher-order mode suppression layer disposed within and extending longitudinally from a lateral waveguide; Fig.33A is a cross-sectional view of a vertical epitaxial layer structure, viewed from the highly reflective (HR) side of an exemplary laser diode including a higher order mode suppression layer having a reduced thickness waveguide portion disposed within a lateral waveguide and extending longitudinally; Fig. 33B is a cross-sectional view of a vertical epitaxial layer structure, from the partially reflective (PR) side of the Fig. 33A, having a higher-order mode suppression layer having a reduced-thickness waveguide portion disposed within a lateral waveguide and extending longitudinally; Fig.33C is a plan view of an exemplary laser diode with a higher order mode suppression layer having a reduced thickness waveguide portion disposed within a lateral waveguide and extending longitudinally; Fig. 33D is a perspective cross-sectional view of a vertical epitaxial layer structure, viewed from the highly reflective (HR) side of an exemplary laser diode including a higher order mode suppression layer having a reduced thickness waveguide portion disposed within a lateral waveguide and extending longitudinally. Fig.34 is an exemplary diagram 3400 for approximating an optimized thickness of a higher order mode suppression layer to compensate for a specific delta in the index caused by thermal lensing; and Fig. Figure 35 is a graph 3500 illustrating a simulation predicting exemplary far fields of supported modes with and without suppression of higher order modes by compensating for thermal lensing. DETAILED DESCRIPTION

[0021] As used in this application and the claims, the singular forms "a" and "an" include the plural forms unless the context clearly dictates otherwise. Furthermore, the term "includes" has the meaning "comprising." Furthermore, the term "coupled" does not exclude the presence of intermediate elements between the coupled elements.

[0022] The systems, devices, and methods described herein should not be construed as limiting in any way. Rather, the present disclosure is directed to all novel and non-obvious features and aspects of the various disclosed embodiments, alone or in various combinations and subcombinations. The disclosed systems, methods, and devices are not limited to any particular aspect, feature, or combinations thereof, nor do the disclosed systems, methods, and devices require any particular advantage or advantage to be present or problems to be solved. Any principles of operation are intended to facilitate explanation, but the disclosed systems, methods, and devices are not limited to such principles of operation.

[0023] Although the operations of some of the disclosed methods are described in a particular sequential order for convenience of presentation, it should be understood that this type of description involves rearranging them unless the specific language used below requires a particular order. For example, operations described sequentially may, in some cases, be rearranged or performed concurrently. Further, for convenience, the accompanying figures illustrate the various ways in which the disclosed systems, methods, and devices may be used in conjunction with other systems, methods, and devices. In addition, terms such as "generating" and "providing" are occasionally used in the description to describe the disclosed technology. These terms represent high-level abstractions of the actual operations performed.The actual operations corresponding to these terms will vary depending on the particular implementation and will be readily apparent to one of ordinary skill in the art.

[0024] In some examples, values, procedures, or facilities are referred to as "lowest," "best," "minimum," or similar. It is clear that such descriptions are intended to indicate that a selection may be made among many functional alternatives in use, and such selections may not be better, lesser, or otherwise preferable to other selections. Examples are described with reference to directions labeled "up," "down," "upper," "lower," and the like. These terms are used to facilitate description but do not imply any particular spatial orientation.

[0025] As discussed above, it is desirable to efficiently scale power and improve brightness in BALs while minimizing the output power-related penalties. Methods, systems, and devices are described here that aim to achieve higher brightness at higher output power by reducing the slow-axis divergence angle without reducing the emitter width. The goal is to suppress the higher-order slow-axis modes while preserving the lower-order lasing modes.

[0026] The Fig. The laser diodes described in Figures 1 to 29 represent the state of the art. Fig. 31 and Fig. 33 do not fall within the wording of the claims, but are considered helpful for understanding the invention. Higher-order mode suppression layer (HOMSL)

[0027] To overcome the disadvantages of conventional laser diodes discussed above in the "Background" section, a laser diode configured to suppress higher-order modes in the lateral direction (i.e., orthogonal to the propagation direction) while preserving lower-order modes of light is required. This can be achieved by placing higher-order mode suppression structures adjacent to the lateral waveguide of the laser diode. Higher-order mode suppression structures can comprise a variety of materials and can be index-guided, anti-waveguided, and / or high-loss structures.

[0028] In one example, the higher-order mode suppression layer (HOMSL) may be located adjacent to the lateral waveguide at or near the back facet and extend less than the full longitudinal length of the waveguide. The HOMSL may include index-guided structures, anti-waveguided structures, and / or high-loss structures. For example, the index-guided, anti-waveguided, and / or high-loss structures may extend up to 20%, or in another example, between 5% and 50%, of the waveguide length measured from the back facet. Extending the HOMSL only a short distance longitudinally minimizes loss for the unsuppressed modes of the laser diode, and the diode can operate more efficiently than if a HOMSL structure extended the entire length.

[0029] In another example, the HOMSL can be disposed adjacent to a flared laser oscillator waveguide and extend either a full longitudinal length of the waveguide or a partial longitudinal length of the waveguide, measured from the back facet. By constructing the diode in this manner, the advantage of reducing higher-order modes by suppressing them with the HOMSL can be combined with the advantages of using a flared laser oscillator waveguide (FLOW), as described in US Patent No. 9,166,369, which is incorporated herein by reference.

[0030] In another example, a laser diode may include an embedded, high-loss, aperiodic high- and low-index structure in the high-index material disposed adjacent to the waveguide along the longitudinal direction. The aperiodic structure may suppress higher-order modes by disproportionately overlapping higher-order modes compared to lower-order modes and / or the fundamental mode. The aperiodic structures are chosen to introduce high losses for higher-order modes but minimize losses for lower-order modes and / or the fundamental mode. The aperiodic structure may be disposed along the entire length of the waveguide along the longitudinal direction or over a shorter length extending from the back facet.The aperiodic structure can also be arranged along the longitudinal direction of a broad area laser (BAL) or a widened laser oscillator waveguide.

[0031] Fig. Figure 1 illustrates the index profile 100, which shows the relative refractive index for an exemplary lateral waveguide with adjacent HOMSLs. A portion 102 of the index profile 100 represents the relative refractive index of the lateral waveguide relative to portions 104, which represent the relative refractive index of the adjacent HOMSLs.

[0032] In one example, the HOMSL is an antiwaveguided structure comprising a variety of materials with a refractive index higher than the refractive index of the waveguide. The HOMSL may comprise a variety of doped and undoped materials. The HOMSL material may be judiciously chosen to optimize efficiency relative to the BPP under the laser operating conditions. Some examples of antiwaveguided HOMSL materials, for example, when the original waveguide contains aluminum gallium arsenide (AlGaAs), include doped gallium arsenide (GaAs), indium gallium arsenide (InGaAs), or the like, or combinations thereof. Other combinations of materials forming the waveguide and the antiwaveguided structures will be contemplated by those skilled in the art, and the claimed subject matter is not limited in this regard.

[0033] Fig.Figure 2 illustrates the index profile 200, which shows the relative refractive index for an exemplary lateral waveguide with a HOSML disposed adjacent to the lateral waveguide and modal modeling for some of the first lateral waveguide modes. A portion 202 of the index profile 200 represents the relative refractive index of the lateral waveguide versus portion 204, which represents the relative refractive indices of the HOMSL. In one example, the HOMSL is an index-guided aperiodic structure containing high- and low-index materials with high loss in the high-index material, which disproportionately overlaps higher-order modes compared to the lower-order / fundamental modes.In one example, the low-index material has a refractive index lower than the effective refractive index of the lateral waveguide, and the high-index material has a refractive index higher than the effective refractive index of the lateral waveguide. Sections 204 show the relative indices of the aperiodic structure with high- and low-index materials. The mean refractive index of the HOMSL may be lower than the modal index, so that the HOMSL is in the index-guided regime rather than the anti-guided regime, but the locally high-index regions pull in the electric field or intensity and interact locally with the material, introducing high loss in the higher-order modes. Modal modeling 206 illustrates the modal behavior of modes 0-4.The higher the modal number, the more overlap the mode has with the high-index region of the HOMSL, so that the loss in the higher-order modes is higher compared to the lower-order modes (210-216) / fundamental mode (208).

[0034] The examples in the Fig. 3-26 show examples of quantum well lasers for simplicity and illustrative purposes. However, various other laser types can be configured to exhibit HOMSL features, such as double heterostructure lasers, interband cascade lasers, Bragg mirror lasers, distributed feedback lasers, quantum cascade lasers, vertical-to-cavity lasers (VCSELs), and / or vertical-to-external-cavity lasers (VECSELs), to name a few. Therefore, the claimed subject matter is not limited to quantum well laser diodes.

[0035] Fig.Figure 3 shows a perspective cross-sectional view illustrating a vertical epitaxial layer structure of an exemplary laser diode with a higher-order mode suppression layer disposed adjacent to the lateral waveguide. In one example, laser diode 300 is a quantum well laser.

[0036] In one example, laser diode 300 is fabricated to include a substrate 304, an n-type semiconductor layer 306, and a p-type semiconductor layer 308. A quantum well 302 is located between the n-type semiconductor layer 306 and the p-type semiconductor layer 308. An n-type cladding layer 310 is disposed outside the n-type semiconductor layer 306. A p-type cladding layer 312 is disposed outside the p-type semiconductor layer 308. An n-type metal contact 314 is disposed on the n-type substrate 304. A p-type metal contact 316 is disposed under the p-type cladding layer 312. The quantum well 302, the n-type semiconductor layer 306, and the p-type semiconductor layer 308 form a transverse waveguide section 318 of the laser diode 300. The boundary of the lateral waveguide 340 is illustrated by dotted lines extending to the n-metal contact 314 along the longitudinal direction.

[0037] The lateral beam size of the diode laser is determined by the width of the active region or the width of the lateral waveguide. Since the width of the waveguide in the lateral direction is significantly larger than the light wavelength, many modes are generated in the lateral direction. The HOMSL 320 is arranged adjacent to the lateral waveguide 340 along the longitudinal direction. Fig. 3, the HOMSL 320 is further disposed between air and the p-type cladding layer 312 outside the transverse waveguide 318. However, the HOMSL 320 is not required to be located there. The HOMSL 320 can be disposed in a variety of positions in the epitaxial structure, and the claimed subject matter is not limited in this regard.

[0038] In one example, the HOMSL 320 includes a high-index material, where the index of the HOMSL 320 is higher than the index of the lateral waveguide 340. The HOMSL 320 is configured to differentially introduce more loss to higher-order modes, thereby suppressing them in the lateral direction (i.e., orthogonal to propagation). The inclusion of this embedded or surface high-index material disproportionately anti-guiding higher-order modes that overlap with the high-index material compared to lower-order / fundamental modes.

[0039] A variety of known materials and methods can be used to fabricate laser diode 300. For example, substrate 304 may contain gallium arsenide (GaAs). N-type semiconductor layer 306, p-type semiconductor layer 308, n-clad 310, and / or p-clad 318 may be grown on GaAs substrate 304 and may contain any of the following materials: indium (In), gallium (Ga), aluminum (Al), arsenic (As), phosphorus (P), gallium arsenide (GaAs), indium phosphide (InP), or the like, or any combination thereof. N-type and p-type layers may be doped with dopants to produce the desired n-type or p-type materials. The claimed subject matter is not limited in this regard.

[0040] The material for the HOMSL 320 can be an absorbing material, so that it absorbs higher-order modes to optimize efficiency relative to the beam parameter product (BPP) under the operating conditions. The material of the HOMSL 320 can be absorbing or non-absorbing.

[0041] Fig. Figure 4 shows a perspective cross-sectional view illustrating a vertical epitaxial layer structure of an exemplary laser diode with a higher-order mode suppression layer disposed adjacent to the lateral waveguide. In one example, the laser diode 400 is a quantum well laser similar to the one shown in Fig.3. However, the laser diode 400 includes a HOMSL 402 comprising an absorbing material. The absorbing material chosen for the HOMSL 402 may include a semiconductor material such as those mentioned above, a metal (e.g., titanium (Ti) or nickel (Ni)), or a semimetal (e.g., tin (Sn) or As). The HOMSL material 402 may be doped or undoped. The HOMSL 402 may be epitaxially grown or deposited on the surface of the substrate 304 rather than embedded. Fig. Figure 5 shows a perspective cross-sectional view illustrating a vertical epitaxial layer structure of an exemplary laser diode with a higher-order mode suppression layer disposed adjacent to the lateral waveguide. In one example, the laser diode 500 is a quantum well laser similar to the one shown in Fig.3. However, the laser diode 500 includes an aperiodic HOMSL structure 502 comprising high- and low-index materials. A low-index material 506 has a lower refractive index than a high-index material 504 and than the effective index of the lateral waveguide 340. The high-index material 504 and the low-index material 506 may include the same materials doped differently to achieve different refractive indices, or they may include completely different materials. For example, the high-index material 504 may include a deposited dielectric or semiconductor, and the low-index material may include air, a dielectric, or semiconductor materials.

[0042] In one example, the low-index and high-index materials extend outward from the sides of the lateral waveguide 314 in an alternating sequence. The low-index material 506 is disposed adjacent to the lateral waveguide 340. The low-index material 506 is closer to the lateral waveguide 340 than the high-index material 340. The high-index material 504 is disposed outside the low-index material 506. The high-index / low-index material pattern may repeat aperiodically several times in the HOMSL structure 502. As mentioned above, the aperiodic structure of the HOMSL 502 may have an average refractive index that is lower than the effective modal index in the lateral waveguide or higher than the modal index of the lateral waveguide.The materials selected for the aperiodic structure of the HOMSL 502 are chosen to introduce high loss for higher order modes, but minimize the loss for lower order / fundamental modes.

[0043] Fig. Figure 6 shows a cross-sectional view illustrating a vertical epitaxial layer structure of an exemplary laser diode with a higher-order mode suppression layer disposed adjacent to the lateral waveguide. In one example, a laser diode 600 is a quantum well laser similar to the one shown in Fig.5. However, the laser diode 600 includes a HOMSL 502 disposed on only one side of the lateral waveguide 340. In one example, the HOMSL 502 is configured to suppress higher-order modes when distributed asymmetrically (as in this configuration) around the waveguide 340 and / or when HOMSL structures are disposed symmetrically around the waveguide 340.

[0044] Fig.7 shows a plan view illustrating an exemplary lateral waveguide of a broad-area laser diode with a higher-order mode suppression layer disposed adjacent to the lateral waveguide along the longitudinal direction. In one example, a broad-area laser diode 700 includes a HOMSL 702 disposed symmetrically around the lateral waveguide. The HOMSL 702 is an aperiodic, higher-order mode suppression layer structure comprising a region of low-index material 706 alternating with a region of high-index material 704. The low-index material 706 has a refractive index lower than the effective refractive index of the materials constituting the lateral waveguide 340. The HOMSL 702 extends the entire length of the lateral waveguide 340 from the back facet 730 to the front facet 732.

[0045] Fig.Figure 8 shows a plan view illustrating an exemplary lateral waveguide of a broad-area laser diode with a higher-order mode suppression layer disposed adjacent to the lateral waveguide along the longitudinal direction. In one example, a broad-area laser 800 includes a truncated HOMSL 802 disposed symmetrically around the lateral waveguide. The HOMSL 802 includes an aperiodic higher-order mode suppression layer comprising a low-index material 806 that interfaces with a high-index material 804 similar to that shown in Fig.5. However, the HOMSL 802 does not extend the full length of the lateral waveguide 340 from the back facet 730 to the front facet 732. Rather, the HOMSL 802 extends only a portion of the length of the lateral waveguide 340, starting from the back facet 730. The back facet 730 is covered with a highly reflective (HR) coating, and the front facet 732 is covered with a partially reflective (PR) coating. The fact that the HOMSL 802 extends only a short length from the rear facet 730 has the advantage that the loss for unsuppressed modes is minimized, so that the laser diode operates efficiently without significantly increasing the loss for lower order / fundamental modes, since the total intensity of the forward and backward propagating fields along the longitudinal direction is smaller towards the rear facet than towards the front facet.Thus, the losses are proportionally smaller.

[0046] Fig. Figure 9 shows a cross-sectional view illustrating an exemplary waveguide of a broad-area laser diode with a higher-order mode suppression layer disposed adjacent to the waveguide. In one example, a broad-area laser 900 includes a truncated HOMSL 902 disposed asymmetrically around the lateral waveguide 340 along the longitudinal direction. The HOMSL 902 includes an aperiodic, higher-order mode suppression layer structure comprising low-index materials 906 that interface with high-index materials 904 similar to Fig.8 alternate. A single HOMSL feature 902 extends only a portion of the length of the lateral waveguide 340 from the rear facet 730. The rear facet 730 is covered with an HR coating, and the front facet 732 is covered with a PR coating. Again, the fact that the HOMSL extends only a short length from the rear facet 730 has the advantage of minimizing the loss for unsuppressed modes, allowing the laser diode to operate efficiently without significantly increasing the loss for lower-order / fundamental modes.

[0047] Fig.10 shows a cross-sectional top view illustrating an exemplary broad-area laser diode waveguide with a higher-order mode suppression layer disposed adjacent to the waveguide. In one example, a broad-area laser 1000 includes a truncated HOMSL 1002 disposed symmetrically about the lateral waveguide 340. The HOMSL 1002 includes a high-index material configured such that its refractive index is greater than the refractive index of the lateral waveguide 340. The high-index material of the HOMSL 1002 is capable of suppressing higher-order modes generated by anti-guiding in the lateral waveguide 340 that overlap with the high-index material of the HOMSL 1000, while having little to no effect on lower-order / fundamental modes generated therein, since there is very little overlap between the lower-order / fundamental modes and the high-index material.

[0048] Fig. Figure 11 shows a cross-sectional plan view illustrating an exemplary broad-area laser diode waveguide with a higher-order mode suppression layer disposed adjacent to the waveguide. In one example, a broad-area laser 1100 includes a truncated HOMSL 1102 disposed asymmetrically around the lateral waveguide. The HOMSL 1102 includes a high-index material similar to that used in Fig.10. The HOMSL 1102 provides antiguiding for the higher-order modes generated in the lateral waveguide 340. The HOMSL 1102 is not required to be symmetrically distributed around the lateral waveguide 340 to provide antiguiding with respect to at least some of the higher-order modes. Furthermore, the single HOMSL feature 1102 extends only a portion of the length of the lateral waveguide 340 from the back facet 730. The back facet 730 is covered with an HR coating, and the front facet 732 is covered with a PR coating.Again, the fact that the HOMSL 1102 extends only a small length from the rear facet 1130 has the advantage of suppressing higher order modes while minimizing the loss for unsuppressed modes, so that the laser diode operates more efficiently without significantly increasing the loss for lower order and / or fundamental modes.

[0049] Fig.5 shows a cross-sectional top view illustrating an exemplary broad-area laser waveguide with a higher-order mode suppression layer disposed adjacent to a waveguide. In one example, a broad-area laser 1200 includes a truncated HOMSL 1202 disposed symmetrically around the lateral waveguide 340. The HOMSL 1202 includes an absorbing material that may be comprised of a semiconductor material in a variety of compositions, dopings, crystallinities, and / or morphologies; semimetals; or metals. The absorbing material functions to suppress higher-order modes in the lateral direction in the lateral waveguide 340 by disproportionately increasing the round-trip loss of the higher-order modes. The HOMSL does not extend the full length of the lateral waveguide 340 starting from the back facet 730.The fact that the HOMSL 1202 extends only a short length from the rear facet 730 has the advantage that the loss for the unsuppressed modes is minimized, so that the laser diode operates more efficiently without significantly increasing the loss for the higher order / fundamental modes.

[0050] Fig. Figure 13 shows a cross-sectional top view illustrating an exemplary expanded laser oscillator waveguide with a higher-order mode suppression layer disposed adjacent to a waveguide. In one example, a broad-area laser 1300 includes a truncated HOMSL 1302 disposed asymmetrically around the lateral waveguide 340. The HOMSL 1302 includes an absorbing material similar to that shown in Fig.12. The HOMSL 1302 disproportionately absorbs higher-order modes in the lateral waveguide 340, suppressing higher-order modes that spatially overlap the HOMSL 1302 to a greater extent. Therefore, the HOMSL 1302 suppresses higher-order modes while minimizing the impact on lower-order modes and the fundamental mode. Furthermore, the single HOMSL feature 1302 extends only a portion of the length of the lateral waveguide 340, starting from the back facet 730. The back facet 1330 is covered with an HR coating, and the front facet 732 is covered with a PR coating.The fact that the HOMSL 1302 extends only a short length from the rear facet 730 has the advantage that higher order modes are suppressed while the loss for the unsuppressed modes is minimized, so that the laser diode operates more efficiently without significantly increasing the loss for lower order / fundamental modes.

[0051] Fig.14 shows a plan view illustrating an exemplary flared laser oscillator waveguide with a higher-order mode suppression layer disposed adjacent to a waveguide. In one example, a laser 1400 includes a flared laser oscillator waveguide (FLOW) 1418, which can be used in place of a rectangular broad-stripe laser waveguide. The FLOW 1440 has a flared current injection region that extends and flares along the longitudinal direction between the back facet 730, which has a highly reflective (HR) coating, and the front facet 732, which has a partially reflective (PR) coating. By reducing the width of the electrically pumped stripe toward the highly reflective facet, the higher-order modes with larger divergence angles are prevented from coupling back into the laser.The slow-axis divergence of the laser is therefore smaller than for a rectangular geometry device with the same width for the partial reflector. Furthermore, light propagating in the expanded current injection region closer to the front PR facet 732 can form a thermal waveguide closer to the width of the narrower rear HR facet 730, resulting in a beam output at the front facet 732 with a beam width significantly narrower than the width of the front facet 732. As a result, the beam parameter product BPP (slow-axis near-field width times slow-axis divergence) is smaller for FLOW devices than for BAL devices. Because the near-field is smaller than the physical width on the front facet 732 side, FLOW devices can be constructed with a larger total area than BAL without compromising the BPP.The increased total pumped area provided by expanding the current injection region serves to reduce the thermal resistance and electrical series resistance in the device, resulting in higher electrical-to-optical power conversion efficiency. Compared to BAL devices, this results in higher output power for a given operating current. Higher power and a smaller BPP lead to increased beam brightness in the slow axis. In addition to broad-area diode lasers, the FLOW concept can also be applied to other types of semiconductor-based Fabry-Perot lasers, such as quantum cascade lasers (QCLs) and interband quantum cascade lasers (IQLs).Broad-area diode lasers with expanded laser oscillator waveguides can find special use, especially in laser diode modules that can be configured for various applications such as fiber coupling and direct pumping.

[0052] In one example, the HOSML 1402 may include an aperiodic structure made of a first layer of low-index material 1406, wherein the index of the low-index material 1406 is less than the effective refractive index of the material forming the FLOW 1440. The HOMSL 1402 further includes a second layer of high-index material 1404, wherein the index of the high-index material 1404 is greater or less than the effective refractive index of the material forming the FLOW 1440. As described above with reference to Fig. As described in section 5, the HOMSL 1402 can be in the index-led or the anti-led regime. Still referring to Fig.14, by coupling the HOMSL 1402 with the FLOW 1440, the higher order mode suppression effects that could be achieved with either the HOMSL 1402 or the FLOW 1440 alone are combined to further reduce the BPP.

[0053] Fig. Figure 15 shows a cross-sectional top view illustrating an exemplary expanded laser oscillator waveguide with a higher-order mode suppression layer disposed adjacent to the waveguide. In one example, a laser diode 1500 includes the FLOW 1440 and a truncated HOMSL 1502 disposed symmetrically around the FLOW 1440. The HOMSL 1502 includes an aperiodic structure comprising a low-index material 1506 and a high-index material 1504 formed as described in Fig.14. In one example, the HOMSL 1502 extends from the back facet 730 only a portion of the length of the FLOW 1440. The HOMSL 1502 is not located near the front facet 730. This architecture combines the suppression of higher-order modes achieved by a HOMSL structure with a FLOW structure, as the HOMSL 1502 extends less than the full length of the FLOW 1440, thereby minimizing the loss for unsuppressed modes, allowing the laser diode to operate efficiently without significantly increasing the loss for the lower-order / fundamental modes, as described above with reference to Fig. 8 is explained.

[0054] Fig.16 shows a cross-sectional top view illustrating an example of a flared laser oscillator waveguide with a higher-order mode suppression layer disposed adjacent to the waveguide. In one example, a laser diode 1600 may include a HOMSL 1602 asymmetrically distributed around a FLOW 1440. The HOMSL 1602 may extend only a partial length of the FLOW 1440 starting from the back facet 730. In one example, a single truncated HOMSL 1602 may operate in combination with the FLOW 1440 to efficiently suppress higher-order modes. Such an architecture is desirable, for example, to save material costs or to accommodate other structures in the epitaxial layer structure of the laser diode 1600.

[0055] Fig.17 shows a plan view illustrating an exemplary laser diode waveguide with a higher-order mode suppression layer disposed adjacent to the waveguide. In one example, a laser diode 1700 includes the FLOW 1440 and a HOMSL 1702 disposed symmetrically around the FLOW 1440. The HOMSL 1702 is truncated and extends only a portion of the length of the FLOW 1440, starting from the back facet 730. The HOMSL 1702 comprises a high-index material compared to the material used in the FLOW 1440. By narrowing the width of the electrically pumped stripe of the FLOW 1440 toward the highly reflective facet, the higher-order modes with higher divergence angles are prevented from coupling back into the laser.The shortened HOMSL 1702 provides further suppression of higher-order modes by disproportionately anti-guiding higher-order modes that overlap with the high-index material towards the HR facet.

[0056] Fig. Figure 18 shows a plan view illustrating an exemplary laser waveguide with a higher-order mode suppression layer disposed adjacent to a waveguide. In one example, a laser 1800 includes a truncated HOMSL 1802 distributed asymmetrically around the FLOW 1440. The HOMSL 1802 may comprise a high-index material similar to that shown in Fig.17. The HOMSL 1802 may extend only a partial length from the rear facet 730 of the FLOW 1440. The single, shortened HOMSL 1802 may operate in combination with the FLOW 1440 to efficiently suppress higher-order modes. This architecture is desirable, for example, to save material costs or to accommodate other structures in the epitaxial layer structure of the laser 1800.

[0057] Fig.19 shows a plan view illustrating an exemplary expanded laser oscillator waveguide with a higher-order mode suppression layer disposed adjacent to the waveguide. In one example, a laser diode 1900 includes the FLOW 1440. A HOSML 1902 is symmetrically disposed around the FLOW 1440. The HOMSL 1902 is truncated and extends only a portion of the length of the FLOW 1440 from the back facet 730. The HOMSL 1902 includes absorbing material. As described with reference to Fig. 4, the absorbing material introduces a higher loss preferentially for the higher-order modes and thereby suppresses higher-order modes that spatially overlap with the HOMSL 1902.

[0058] Fig.Figure 20 shows a cross-sectional top view illustrating an exemplary expanded laser oscillator waveguide with a higher-order mode suppression layer disposed adjacent to the waveguide. In one example, a laser 2000 may include a truncated HOMSL 2002 distributed symmetrically around the FLOW 1440. The HOMSL 2002 may comprise an absorbing material similar to that described with reference to Fig. 19. The HOMSL 2002 may extend only a partial length from the rear facet 730 of the FLOW 1440. A single, shortened HOMSL 2002 may operate in combination with the FLOW 1440 to suppress higher-order modes more efficiently than with the HOMSL 2002 or the FLOW 1440 alone. This architecture is desirable, for example, to save material costs or to accommodate other structures in the epitaxial layer structure of the laser 2000. Gain adjustment approach

[0059] In one example, higher-order modes in the lateral waveguide can be reduced by reducing the amount of current reaching the active layer in the lateral waveguide, where higher-order modes are amplified. This can be achieved by gain matching. Traditionally, gain matching involves current injection from the p-side of the heterostructure. However, gain matching from the p-side is subject to inefficiencies. In contrast, gain matching from the n-side creates a diffuse carrier distribution that overlaps more strongly with the fundamental mode and then with the profiles of lower-order modes, thereby providing higher gain to the desired modes and lower gain to the unwanted higher-order modes.

[0060] Fig.Figure 27 shows a perspective cross-sectional view illustrating a vertical epitaxial layer structure of an exemplary laser diode having a customized n-side current injection scheme. In one example, gain adjustment is performed by current injection via an n-type metal contact 2714, which is designed to be narrower than the p-type metal contact 316. The n-type metal contact 2714 is located at the center of the lateral waveguide 2714 in the longitudinal direction, and a cavity 2710 enables a lateral carrier distribution pattern 2702.

[0061] Fig.Figure 28 shows a perspective cross-sectional view illustrating a vertical epitaxial layer structure of an exemplary laser diode having a customized n-side current injection scheme. Laser diode 2800 includes an n-side 2820 and a p-side 2822. The n-side 2820 includes an n-metal contact 2814, the n-cladding layer 310, the n-type semiconductor layer 306, and the n-substrate 304. The p-side includes the p-cladding layer 312, the p-type semiconductor layer 308, and the p-metal contact 316. The transverse waveguide 318 includes the quantum well 302, the n-type semiconductor layer 306, and the p-type semiconductor layer 308. A longitudinal waveguide 2814 may be defined in various ways (e.g., by gain guide, ridge waveguide, or index guide, or the like, or a combination thereof), as illustrated by dotted lines on the top side of the substrate 304 and the n-metal contact 2814.The p-type metal contact 316 is disposed beneath the p-type cladding layer 312. The n-type metal contact 2814 extends along the cavity 2810 of the longitudinal waveguide 2814.

[0062] Gain adjustment from the n-side can be achieved by introducing a lateral carrier distribution pattern 2802 by disposing a narrow strip of n-metal contact 2814 on the n-side 2820 of the laser diode 2800, instead of the conventional metallization of the entire n-side. The thin n-metal contact 2814 can be disposed at a variety of locations relative to the p-metal contact 316. In one example, the n-metal contact 2814 is offset so that its edge is located at the emitter half-plane 2804. Gain adjustment from the n-metal side 2820 is performed, which reduces higher-order modes propagating in the waveguide 2840 by reducing the magnitude of the gain for the higher-order modes.

[0063] In one example, the n-metal contact 2814 may have a variable width along the cavity to modulate the carrier profile in the lateral and longitudinal directions. This is shown in Fig. 28, wherein a width L1 of the n-type metal contact 2814 is smaller than a second width L2. This provides a gain adjustment in the longitudinal direction, reducing the gain for the higher-order modes toward the HR (highly reflective) facet, allowing the laser diode 2800 to operate more efficiently without significantly increasing the loss for lower-order / fundamental modes.

[0064] Fig.Figure 29 shows a perspective cross-sectional view illustrating a vertical epitaxial layer structure of an exemplary laser diode having a customized n-side current injection scheme. In one example, gain adjustment is achieved by current injection via a flared n-metal contact 2914, which is designed to be narrower than p-metal contact 316. The n-metal contact 2914 is narrower on the HR side and wider (but laterally centered) on the PR side. The n-metal contact 2914 is located in the longitudinal center of the lateral waveguide 2914, and a cavity 2910 enables a lateral carrier distribution pattern 2902. Hybrid gain adjustment / HOMSL approach

[0065] Further reduction of higher-order modes in a lateral waveguide can be achieved using a hybrid approach by 1) reducing the propagation of higher-order modes in the waveguide by applying a customized current injection scheme to adjust the gain or to penalize the gain for higher-order modes, thereby depriving them of gain and suppressing the higher-order modes, and 2) inserting a HOMSL adjacent to the lateral waveguide along the longitudinal direction to further suppress higher-order modes generated despite the gain adjustment.

[0066] Fig.Figure 21A shows a perspective cross-sectional view illustrating a vertical epitaxial layer structure of an exemplary laser diode having a customized n-side current injection scheme and a higher-order mode suppression layer disposed adjacent to the lateral waveguide. A laser diode 2100 has an n-side 2120 and a p-side 2122. The n-side 2120 includes an n-metal contact 2114, the n-cladding layer 310, the n-type semiconductor layer 306, and the n-substrate 304. The p-side 2122 includes the p-cladding layer 312, the p-type semiconductor layer 308, the p-metal contact 316, and the HOMSL 320. The waveguide 318 includes the quantum well 302, the n-type semiconductor layer 306, and the p-type semiconductor layer 308. The longitudinal waveguide 2114 can be defined in various ways (e.g., by gain guiding, ridge waveguide, or index guiding, or the like, or a combination thereof).The longitudinal waveguide 2114 includes a cavity 2110, represented by dashed lines on the top side of the substrate 304 and the n-type metal contact 2140. The p-type metal contact 316 is disposed beneath the p-type cladding layer 312. The n-type metal contact 2114 extends along the cavity 2110 of the longitudinal waveguide 2140.

[0067] As discussed above, gain adjustment from the n-side can be achieved by introducing a lateral carrier distribution pattern 2102 by placing the n-metal contact 2114 on the n-side of the laser diode. Fig.21A, the n-type metal contact 2114 is offset so that its edge is located at the emitter half-plane 2104. Carrier injection is performed from the n-type metal side. Furthermore, the HOMSL 320 includes a high-index material symmetrically arranged along the longitudinal direction around the lateral waveguide 2110. This hybrid approach can reduce higher-order modes propagating in the waveguide 2140 by reducing the magnitude of the gain for the higher-order modes and suppressing the higher-order modes across the HOMSL 320.

[0068] In another example, a HOMSL structure arranged symmetrically around the waveguide 2140 may be used instead of a symmetrically arranged HOMSL structure. In one example, an asymmetric HOMSL structure, as shown in the Fig. 9, Fig. 11, Fig. 16, Fig. 18 and Fig.20, in the heterostructure in series with the n-metal contact 2114, as shown in Fig. 21B. This configuration is more efficient than placing the HOMSL structure on the opposite side because it deprives the higher-order modes of gain and is below the threshold carrier density.

[0069] Furthermore, other HOMSL structures can also be used to implement the higher-order mode suppression method described here using the gain-matching / HOMSL hybrid. Instead of using the high-index materials of the HOMSL 320, absorbing and / or aperiodic materials can be used, for example, as described above with respect to the HOMSL 402 of the Fig. 4 and the HOMSL 502 of the Fig.5. Furthermore, the hybrid gain matching / HOMSL approach using p-side gain matching provides higher-order mode suppression that is improved over conventional p-side gain matching or over higher-order mode suppression using a HOMSL structure alone. Fig.Figure 22 shows a perspective cross-sectional view illustrating a vertical epitaxial layer structure of an exemplary laser diode having a customized n-side current injection scheme and a higher-order mode suppression layer disposed adjacent to the waveguide. In one example, an n-type metal contact 2214 is designed to be narrower than the width of the p-type metal contact 316. The n-type metal contact 2214 is centered at the half-plane 2204 of the waveguide 2240. This creates a diffuse carrier distribution along the path labeled 2202 in the lateral direction, creating a carrier density profile that overlaps more strongly primarily with the fundamental mode and then with the profiles of the lower-order modes than the suppressed higher-order modes.This current injection profile is configured to optimize the overlap with the lower-order lateral modes and the fundamental modes by providing higher gain to the desired modes and lower gain to the undesired higher-order modes. Furthermore, the width of the n-metal contact 2214 along the cavity 2210 can be variable to model the amount of carriers injected along the longitudinal direction of the cavity 2210.

[0070] Fig.Figure 23 shows a perspective cross-sectional view illustrating a vertical epitaxial layer structure of an exemplary laser diode having a customized n-side current injection scheme and a higher-order mode suppression layer disposed adjacent to the waveguide. In one example, gain adjustment is achieved by current injection via an n-type metal contact 2314 offset from the p-type metal contact 316. The HOMSL 402 (see Figure 23) Fig. 4) is arranged symmetrically around the lateral waveguide 2314 along the longitudinal direction and contains an absorbing material. In one example, the HOMSL may be n- or p-doped GaAs, or ordered or disordered InGaAs with a band gap smaller than the lasing wavelength.

[0071] Fig.Figure 24 shows a perspective cross-sectional view illustrating a vertical epitaxial layer structure of an exemplary laser diode having a customized n-side current injection scheme and a higher-order mode suppression layer adjacent to the waveguide. In one example, gain adjustment is achieved by current injection via an n-type metal contact 2414 configured to be narrower than the p-type metal contact 316. The n-type metal contact 2414 is disposed at the center of the lateral waveguide 2414 and the cavity 2410. The HOMS1 402 (see Figure 24). Fig. 4) is arranged symmetrically around the lateral waveguide 2414 and contains an absorbing material.

[0072] Fig.Figure 25 shows a cross-sectional view illustrating a vertical epitaxial layer structure of an exemplary laser diode having a matched n-side current injection scheme and a higher-order mode suppression layer disposed adjacent to the waveguide. In one example, gain matching is performed via an n-type metal contact 2514 offset from the p-type metal contact 316. The HOMSL 502 (see Figure 25) Fig. 5), which has an aperiodic high-index layer 504 and a low-index layer 506, is arranged symmetrically around the lateral waveguide 2514.

[0073] Fig.Figure 26 shows a cross-sectional view illustrating a vertical epitaxial layer structure of an exemplary laser diode having a customized n-side current injection scheme and a higher-order mode suppression layer disposed adjacent to the waveguide. In one example, gain adjustment is performed by current injection via an n-type metal contact 2614 configured to be narrower than the p-type metal contact 316. The n-type metal contact 2614 is disposed at the center of the waveguide 2640 and a cavity 2610. The HOMSL 502 (see Figure 26) Fig. 5) is arranged symmetrically around the longitudinal waveguide 2640 and contains an absorbing material. Features of the embedded HOMSL to compensate for thermal lensing

[0074] Index guiding and gain guiding are the predominant confinement mechanisms used to confine lateral optical modes (i.e., transverse to the slow axis) in broad-area laser cavities. Confinement of vertical modes (i.e., transverse to the fast axis) is typically achieved with index guiding using n-type and p-type cladding layers of predetermined refractive indices. In general, lateral optical modes supported by a semiconductor laser cavity exhibiting a variation of zero lateral refractive index in a currentless, cold state are gain guided during a powered state because the injected current induces a lateral index variation between electrically pumped and unpumped regions.

[0075] During high-power operation, a lateral thermal gradient causes a lateral index difference between the lateral waveguide and the cladding, which is associated with thermal lensing. The magnitude of a positive lateral index contrast caused by thermal lensing can depend on various cavity properties and parameters, including length, semiconductor layer thicknesses, carrier densities, active layer type and thickness, emitter / reflector widths, gain, operating wavelength, the amount of waste heat generated by the diode laser, and heat transfer between the diode junction and the heat sink, etc., as will be readily appreciated by one skilled in the art.

[0076] The slow-axis divergence angle of an output beam emitted from a semiconductor device can be significantly influenced by the lateral waveguide confinement near the highly reflective facet. Index guiding (induced by thermal lensing) in the lateral waveguide near the highly reflective facet can promote unwanted higher-order lateral modes, resulting in slow-axis divergence and reduced beam quality.

[0077] These examples describe methods, systems, and devices designed to reduce the occurrence of higher-order lateral modes in device operation caused by high-index contrast-induced thermal lensing. The increased waveguide index contrast resulting from thermal lensing is offset or compensated by forming an index compensation region on the HR side of the Fabry-Perot cavity. This reduces the magnitude of the index contrast-induced thermal lensing.

[0078] Fig. Figure 30A is a graph 3002 showing a conventional step index profile in the lateral direction of a single emitter laser diode operating at low power without significant thermal lensing. Fig.Figure 30B is a graph 3004 showing a step index profile in the lateral direction of a single-emitter laser diode with a parabolic profile added. This is an index model for a conventional waveguide profile with thermal lensing during higher-power operation. This shows how the profile evolves under thermal lensing at high operating current. Fig. Figure 30C is a graph 3006 showing a potential index profile in the lateral direction of a single-emitter laser diode that provides compensation for thermal lensing during higher-power operation. The "negative" step index profile, with a parabolic profile added, models the index profile in the negatively compensated region. The thermal lensing-related index profile creates a weakly index-guided region under operating conditions.

[0079] As described in more detail below, the methods for achieving such a profile include introducing higher refractive index lateral features adjacent to and / or overlapping the lateral waveguide, reducing the transverse waveguide thickness in the active stripe, and / or introducing a thin low-index layer within the transverse waveguide.

[0080] The Fig. 31A - 31D show different views of a laser diode 3100.

[0081] Fig.31A is a cross-sectional view of a vertical epitaxial layer structure of an exemplary laser diode 3100 having an embedded higher-order mode suppression layer (HOMSL) 3104 disposed on adjacent sides of a lateral waveguide 3106. The laser diode 3100 may have various geometries and configurations and may include various arrangements of p-type, n-type, active, capped, and dielectric layers. The term "embedded" is intended to refer to a higher-order mode suppression layer and / or a feature disposed between layers within the epitaxial layer structure of the laser diode 3100 (or other laser diodes described herein).However, in some examples, the higher order mode suppression layer or feature may not be embedded and may operate in the same or similar manner as the embedded higher order mode suppression layer or feature.

[0082] In one example, laser diode 3100 is depicted in a simplified epitaxial structure and includes an n-type cladding layer 3108 and a p-type cladding layer 3110, between which a transverse waveguide or cavity 3112 is formed. Transverse waveguide 3112 is orthogonal to lateral waveguide 3106 and includes an n-type waveguide layer 3114, a p-type waveguide layer 3116, or an active layer 3118. Active layer 3118 typically contains one or more quantum wells, but other configurations are possible, including pn junctions, homostructures, heterostructures, double heterostructures, quantum wires, quantum dots, etc.The p-type cladding layer 3110 can be etched into various shapes to form ridge structures 3120 or mesas that either extend the entire longitudinal length of the diode 3100 or extend only a partial length of the diode 3100 starting from the partial reflector (PR) 3122 side. Additionally, one or more dielectric and / or cap layers (not shown) can be formed on the laser diode 3100 to conduct current through the active layer 3118.

[0083] Fig. 31B is one of the partially reflecting (PR) side 3122 of the Fig. 30A is a cross-sectional view of a vertical epitaxial layer structure of the exemplary laser diode 3100 shown.

[0084] The lateral waveguide 3106 is bounded in the longitudinal direction at a first end by the HR-coated facet 3124 and at a second end by a PR-coated facet 3126. In one example, the HOMSL 3104 is formed below the p-type cladding layer 3110 on one or both sides of the lateral waveguide. The HOMSL 3104 extends from the HR-coated facet 3124 in the longitudinal direction by a length less than the distance between the HR facet and the PR facet. The embedded HOMSL 3104 does not extend to the PR side 3122 and is therefore not visible from the PR side 3122.

[0085] Fig. 31C is a top view of an exemplary laser diode 3100 including embedded HOMSL features 3104 disposed on adjacent sides of a lateral waveguide 3106 extending longitudinally from the HR facet 3124.

[0086] During high-power operation of the laser diode 3100, the temperature profile resulting from lateral heat propagation leads to thermal lensing in the lateral waveguide 3106, causing a refractive index contrast in the lateral waveguide 3106. In one example, partial or full compensation for the increased refractive index contrast in the lateral waveguide 3106 can be achieved by strategically locating the HOMSL features 3104 adjacent to the lateral waveguide 3106 on one or both lateral sides. The HOMSL 3104 is formed between the p-type waveguide layer 3116 and the p-type cladding layer 3110. Alternatively, the HOMSL 3104 can also be formed between the n-type waveguide layer 3114 and the n-type cladding layer 3108.The HOMSL 3104 may also be configured to slightly overlap the lateral waveguide 3106 to compensate for a mode size mismatch between the two regions or for other reasons.

[0087] The addition of HOMSL structures 3104 (e.g., with GaAs) to a BAL can be accomplished using a variety of techniques known per se to those skilled in the art, and the claimed subject matter is not limited in this regard. For example, a modified mask and etch can be used to form a HOMSL 3104 with selected offsets from the ridge waveguide 3120. The high-index HOMSL features 3104 located at the edges of the ridge waveguide 3120 can disproportionately overlap higher-order modes, but the mismatch in mode size of identical-order modes between the HR side and the PR side means that the offset between the oxide ridge 3120 and the HOMSL 3104 in the lateral direction is chosen to avoid excessive losses by coupling out the supported lasing modes.

[0088] Various simulations indicate that for different orders of magnitude of index contrast, the trend in mode size is predictable, regardless of the order. The order of magnitude of the index contrasts lies in a range of 10-5<Δn<10-3. The predicted mismatch in the size of the lateral waveguide 3106 (or the overlap in the HOMSL 3104 across the lateral waveguide) can be in a range of 2-6 μm on each side or 4-12 μm overall.

[0089] In another example, the HOMSL 3104 is configured to laterally overlap the lateral waveguide 3106 to compensate for a mismatch in the lateral mode region between a HOMSL region 3104 and an index-guided region by a value between 0-10µm on each side or 0-20µm in total.

[0090] Fig.31D is a perspective cross-sectional view of a vertical epitaxial layer structure, viewed from the HR side 3102 of an exemplary laser diode 3100 having an embedded HOMSL 3104 disposed on adjacent sides of a lateral waveguide 3106 and extending longitudinally from the HR-coated facet 3124.

[0091] In one example, HR side 3102 extends from HR-coated facet 3124 to approximately a midpoint 3130 (dashed line). PR side 3122 extends from PR-coated facet 3126 to approximately the midpoint 3130.

[0092] Under operating conditions, the HR side 3102 experiences thermal lensing, but when suppressed or compensated by the HOMSL 3104, this area is weakly index-guided; therefore, it has an index profile that approximates that in Fig.30C. Likewise, the PR side 3122 contains an index-guided area that also experiences a lensing effect during operation without suppression or compensation by the HOMSL 3104 and therefore approximates the Fig. 30B shown index profile.

[0093] To reduce the magnitude of thermal lensing induced by the index contrast on the HR side 3102 of the laser diode 3100, the HOMSL 3104 may contain a material with a higher refractive index than the surrounding materials. The relative refractive index (n) for the HOMSL 3104 with respect to the transverse waveguide layers is n HOMSL 3104 > n p-Typ - Mantelschicht-3110 / n n-Typ-Mantelschicht 3108 > n p-Typ-Wellenleiterschicht 3116 / n n-Typ-Wellenleiterschicht .

[0094] In addition, to reduce the magnitude of the index contrast induced by thermal lensing on the HR side 3102, the refractive index and thickness 3128 of the HOMSL 3104 are carefully selected. The choice of thickness 3128 should be based on its effect on the magnitude of the refractive index contrast (i.e., the potential effective index contrast on the HR side 3102 of the lateral waveguide 3106). A specific thickness 3128 of the HOMSL 3104, a threshold thickness, or a thickness range (collectively referred to herein as "thickness") that sufficiently reduces the magnitude of the index contrast-induced thermal lensing on the HR side 3102 can be identified through a variety of methods, including simulation, experiments, reference tables, and predictive analysis, among others.

[0095] The thickness 3128 determines the Δn on the HR side 3102 relative to the thermal lensing and reduces the guiding capability of the lateral waveguide 3106 in the region near the HOMSL 3104. In various examples, the thicknesses can be chosen to largely compensate for the lensing, thus creating a weakly index-guided region. This allows for weak index guiding of a few lateral modes or, in extreme cases, only a single lateral mode at high operating current on the HR side 3102. Such a weakly guided region can support 10 or fewer lateral modes or even just one mode. As a result, the lateral waveguide 3106 supports fewer lateral modes on the HR side, enabling a reduction in slow-axis divergence and higher brightness compared to conventional BALs.

[0096] Since the HOMSL is designed to largely compensate for thermal lensing during high power operation, there may effectively be an anti-guided region on the HR side 3102 for low power operation, which transitions to a weakly index-guided region with the onset of thermal lensing during higher power operation.

[0097] Fig. 32A- Fig. 32D show different views of a laser diode 3200.

[0098] Fig. 32A is a cross-sectional view of a vertical epitaxial layer structure of an exemplary laser diode 3200 including an embedded HOMSL 3204.

[0099] Laser diode 3200 is shown in a simplified epitaxial structure and includes an n-type cladding layer 3208 and a p-type cladding layer 3210, between which a transverse waveguide or cavity 3212 is formed. Transverse waveguide 3212 is orthogonal to lateral waveguide 3206 and includes an n-type waveguide layer 3214, a p-type waveguide layer 3216, and an active layer 3218. Ridge structures 3220 may extend over part or all of the longitudinal length of diode 3200 between HR side 3202 and PR side 3222.

[0100] The HOMSL 3204 is disposed within the transverse waveguide 3212 within the p-type waveguide layer 3216. Alternatively, the HOMSL 3204 may be disposed within the n-type waveguide layer 3214.

[0101] The HOMSL 3204 contains a thin layer of a lower-index material. Although the HOMSL 3204 is disposed within the transverse waveguide 3212, it is also located within the lateral waveguide 3206; therefore, the effective index of the lateral waveguide 3206 is reduced by the presence of the lower-index HOMSL 3204. The index of the lateral cladding 3210 remains unchanged.

[0102] Fig. 32B is a cross-sectional view of a vertical epitaxial layer structure, from the PR side 3222 of the Fig.32A, which includes the HOMSL 3204 disposed within the lateral waveguide 3206. The lateral waveguide 3206 is bounded in the longitudinal direction at a first end by the HR-coated facet 3224 and at a second end by a PR-coated facet 3226. In one example, the HOMSL 3204 is embedded in a p-type waveguide layer 3216 (or n-type waveguide layer 3214). However, because the HOMSL 3204 extends in the longitudinal direction from the HR-coated facet 3224 for a length that is less than the distance between the HR facet and the PR facet, it is not visible in the epitaxial layers as viewed from the PR side 3222.

[0103] Fig.32C is a top view of an exemplary laser diode 3200 including an embedded HOMSL 3204 disposed within a lateral waveguide and extending in the longitudinal direction.

[0104] During high-power operation of the laser diode 3200, the temperature profile resulting from lateral heat propagation leads to thermal lensing in the lateral waveguide 3206, inducing a refractive index contrast in the lateral waveguide 3206. In one example, partial or full compensation for the increased refractive index contrast in the lateral waveguide 3206 can be achieved by strategically placing a HOMSL 3204 within the lateral waveguide 3206. The HOMSL 3204 can be 0-10 μm laterally narrower than the lateral waveguide 3206 by 0-10 μm on each side or 0-20 μm overall.

[0105] The addition of the HOMSL structures 3204 (e.g., with AlGaAs) to a BAL can be accomplished by a variety of methods known to those skilled in the art, and the claimed subject matter is not limited in this respect.

[0106] Fig. 32D is a perspective cross-sectional view of a vertical epitaxial layer structure, viewed from the HR side 3202 of an exemplary laser diode 3200, including an embedded HOMSL 3204 disposed within the lateral waveguide 3206 and extending in the longitudinal direction from the HR-coated facet 3224. In one example, the HR side 3202 extends from the HR-coated facet 3224 to approximately the midpoint 3230 (dashed line). The PR side 3222 extends from the PR-coated facet 3226 to approximately the midpoint 3230.

[0107] Under operating conditions, the HR side 3202 experiences thermal lensing, but with suppression or compensation by the HOMSL 3204, in which the index contrast is reduced by incorporating a lower-index material into the lateral waveguide 3206, the region of the HR side 3202 becomes weakly index-guided. To reduce the magnitude of index contrast-induced thermal lensing on the HR side 3202, the HOMSL 3204 may contain a material with a lower index than the surrounding materials. The relative refractive index (n) for the HOMSL 3204 with respect to the transverse waveguide layers is: nLow-index layer <np−Wellenleiter / nn−Wellenleiter<np−Mantel / nn−Mantel.

[0108] To reduce the magnitude of the thermal lensing effect induced by the index contrast on the HR side 3202, the thickness 3228 and the refractive index of the HOMSL 3204 are carefully selected, similar to the HOMSL 3104. The choice of the thickness 3228 is based on its effect on the magnitude of the refractive index contrast (i.e., the potential effective index contrast on the HR side 3202 of the lateral waveguide 3206). A specific thickness 3228 of the HOMSL 3204 that sufficiently reduces the magnitude of the thermal lensing effect induced by the index contrast on the HR side 3202 can be identified through a variety of methods, including simulation, experiments, reference tables, and predictive analysis, among others.

[0109] The thickness 3228 determines the Δn on the HR side 3202 in relation to the thermal lensing and reduces the guiding capability of the lateral waveguide 3206 in the region near the HOMSL 3204. In various examples, the thicknesses can be chosen to largely compensate for the thermal lensing to create a weakly index-guided region. This allows for weak index guiding of a few lateral modes or, in extreme cases, only a single lateral mode at high operating current on the HR side 3202. Such a weakly guided region can support 10 or fewer lateral modes or even just a single mode. As a result, the lateral waveguide 3206 on the HR side 3202 supports fewer lateral modes, enabling a reduction in slow-axis divergence and increased brightness compared to conventional BALs.

[0110] In various examples, the HOMSL 3104 and the HOMSL 3204 may be formed from a variety of materials known to those skilled in the art that sufficiently satisfy the dimensional and relative refractive indices constraints disclosed herein to reduce the magnitude of index contrast-induced thermal lensing in the lateral waveguide, and the claimed subject matter is not limited in this regard. For example, the HOMSL 3104 and / or the HOMSL 3204 may be formed from a variety of materials, including gallium arsenide (GaAs), aluminum gallium arsenide (AlGaAs), indium gallium arsenide (InGaAs), indium aluminum gallium arsenide (InAlGaAs), and / or indium gallium phosphide (InGaAsP).

[0111] The Fig. 33A- Fig. 33D show different views of a laser diode 3300.

[0112] Fig.33A is a cross-sectional view of a vertical epitaxial layer structure of an exemplary laser diode 3300 including a reduced-thickness transverse wavelength section, referred to herein as HOMSL 3304. The laser diode 3300 is shown in a simplified epitaxial structure and includes an n-type cladding layer 3308 and a p-type cladding layer 3310, between which a transverse waveguide or cavity 3312 is formed. The transverse waveguide 3312 is orthogonal to the lateral waveguide 3306 and includes an n-type waveguide layer 3314, a p-type waveguide layer 3316, and an active layer 3318. Ridge structures 3320 may extend over part or all of the longitudinal length of the diode 3300 between the HR side 3202 and the PR side 3222. The HOMSL 3304 may be formed within the transverse waveguide 3312 in the p-type waveguide layer 16.Alternatively, a reduced-thickness transverse waveguide section (HOMSL) 3304 may be formed in the n-type waveguide layer 3314. The HOMSL feature 3304 may be formed as a groove, notch, channel, or the like, or a combination thereof.

[0113] Although the HOMSL 3304 is disposed in the transverse waveguide 3212 in the p-type waveguide layer 3316, it is also located within the lateral waveguide 3306; the effective index of the lateral waveguide 3306 is therefore reduced by the presence of the HOMSL 3304.

[0114] Fig. 33B is a cross-sectional view of a vertical epitaxial layer structure, from the PR side 3322 of the Fig.33A, which includes the HOMSL 3304 disposed within the lateral waveguide 3306. The lateral waveguide 3306 is bounded in a longitudinal direction at a first end by the HR-coated facet 3324 and at a second end by the PR-coated facet 3326. In one example, the HOMSL 3304 is formed in the p-type waveguide layer 3316 (or the n-type waveguide layer 3314). However, because the HOMSL 3304 extends in the longitudinal direction from the HR-coated facet 3324 for a length that is less than the distance between the HR facet 3324 and the PR facet 3326, it is not visible in the epitaxial layers as viewed from the PR side 3322.

[0115] Fig.33C is a top view of an exemplary laser diode 3200 including a HOMSL 3304 disposed within a lateral waveguide and extending in the longitudinal direction from the HR-coated facet 3324 for a length less than the distance between the HR facet 3324 and the PR facet 3326.

[0116] During high-power operation of the laser diode 3300, the temperature profile resulting from lateral heat propagation leads to thermal lensing in the lateral waveguide 3306, inducing a refractive index contrast in the lateral waveguide 3306. In one example, partial or full compensation for the increased refractive index contrast in the lateral waveguide 3306 can be achieved by strategically placing the HOMSL 3304 within the lateral waveguide 3306. A groove for the HOMSL 3304 can be formed to be 0-10 µm laterally narrower than the lateral waveguide 3306 by 0-10 µm on each side or 0-20 µm overall.

[0117] Adding a HOMSL 3304-related groove, notch, channel, etc. to a BAL can be accomplished using a variety of methods known to those skilled in the art, and the claimed subject matter is not limited in this regard. For example, the HOMSL 3304 can be formed by etching or selectively depositing thicker layers adjacent to the active stripe.

[0118] Fig.33D is a perspective cross-sectional view of a vertical epitaxial layer structure, viewed from the HR side 3302 of an exemplary laser diode 3300, including an embedded HOMSL 3304 disposed within the lateral waveguide 3306 and extending in the longitudinal direction from the HR-coated facet 3324. In one example, the HR side 3302 extends from the HR-coated facet 3324 to approximately the center 3330 (dashed line). The PR side 3322 extends from the PR-coated facet 3326 to approximately the center 3330.

[0119] Under operating conditions, the HR side 3302 experiences thermal lensing, however, with the suppression or compensation by the HOMSL 3304 by providing the section with thinner transverse wavelength thickness, the region of the HR side 3302 becomes weakly index-guided, thereby reducing the magnitude of any index contrast-induced thermal lensing on the HR side 3302. In one example, the smaller the thickness of the transverse waveguide 3312, the higher the effective refractive index, allowing the lateral waveguide 3306 to have a lower refractive index than the lateral cladding.

[0120] To reduce the magnitude of the thermal lensing effect induced by the index contrast on the HR side 3302, the thickness 3328 is carefully selected, similar to the HOMSL 3104. The choice of the thickness 3328 is based on its effect on the magnitude of the refractive index contrast (i.e., the potential effective index contrast on the HR side 3302 of the lateral waveguide 3306). A specific thickness 3328 of the HOMSL 3304 that sufficiently reduces the magnitude of the thermal lensing effect induced by the index contrast on the HR side 3302 can be identified through a variety of methods, including simulation, experiments, reference tables, and predictive analysis, among others.

[0121] Thickness 3328 determines the Δn on HR side 3302 relative to the thermal lensing and reduces the guiding capability of lateral waveguide 3306 in the region near HOMSL 3304. In various examples, the thicknesses can be chosen to largely compensate for the thermal lensing to implement a weakly index-guided region. This allows weak index guiding of a few lateral modes or, in extreme cases, only a single lateral mode at high operating current on HR side 33002. Such a weakly guided region can support 10 or fewer lateral modes or even just one mode. As a result, lateral waveguide 3306 on HR side 3302 supports fewer lateral modes, enabling a reduction in slow-axis divergence and increased brightness compared to conventional BALs. SIMULATIONS AND EXAMPLES

[0122] The HOMSL 3104 and the HOMSL 3204 can each be manufactured by a variety of methods known to those skilled in the art, and the claimed subject matter is not limited in this respect. For manufacturing, it may be necessary to determine a suitable thickness of the HOMSL 3104 sufficient to reduce the index contrast on the HR side. This relative difference in Δn can be considered for a simulation calculation of the relationship between the thickness of the HOMSL 3104 and Δn.

[0123] Fig.34 is a graph 3400 showing a simulation of the influence of thickness on Δn to approximate an optimized thickness of an embedded HOMSL 3104 to compensate for a specific delta in the index caused by thermal lensing. The specific HOMSL 3104 can be made of GaAs and is formed between the p-type waveguide layer 3116 and the p-type cladding layer 3110 with a refractive index lower than that of GaAs. A negative Δn means a negative lateral index difference between the lateral waveguide and the cladding. Using the graph 3400, the thickness of the HOMSL 3104 can be optimized by approximation to compensate for a specific delta in the index caused by thermal lensing. Although the simulation is specific to the Fig.34, there are other methods to achieve the weakly index-guided HR side 3102 of the emitter. These include reducing the thickness of the transverse waveguide in the active stripes. Both approaches would achieve a reduced index contrast between the lateral waveguide and the cladding.

[0124] The approximate index compensation required for the thermal lensing effect is determined using the simulated beam propagation of a waveguide and the simultaneously simulated far field.

[0125] Fig.Figure 35 is a graph 3500 showing a simulation predicting the far fields of supported modes with or without suppression of higher-order modes by compensating for thermal lensing. The specific example is a BAL with a 200 μm lateral waveguide width and a typical step index and thermal lensing. Reducing the supported modes implies a reduction in the far field of ~10.9 degrees to ~4.3 degrees. The slow-axis far-field divergence of a variety of BALs with HOMSLs under different step indices and thermal lensing is simulated and summarized in the table below: start Waveguide width Δn "Therma" parabola In total HOM Wide field1 / e^2 to index index index M degree Multimode 100 0.0004 0 0.0004 - 6.7 Fashion of the highest order 100 0.0004 0.0004 0.0008 12 8.9 Fashion of the highest order 100 0.0004 0.0004 0.0008 13 9.3 Fashion of the highest order 100 0.0004 0.0008 0.0012 15 11.2 Fashion of the highest order 100 0.0004 0.001 0.0014 16 12.3 Fashion of the highest order 100 -0.001 0.0012 0.0002 1 5.2 Fashion of the highest order 100 -0.0008 0.001 0.0002 1 4.85 Multimode 200 0.0004 0 0.0004 - 6.35 Fashion of the highest order 200 0.0004 0.0004 0.0008 26 9.1 Fashion of the highest order 200 0.0004 0.0008 0.0012 31 10.9 Fashion of the highest order 200 0.0004 0.0012 0.0016 35 12.55 Fashion of the highest order 200 -0.0004 0.0012 0.0008 17 9.1 Fashion of the highest order 200 -0.0008 0.0012 0.0004 7 6.3 Fashion of the highest order 200 -0.0006 0.0008 0.0002 3 4.3 Fashion of the highest order 200 -0.0007 0.0008 0.0001 1 3.3

[0126] As described above, a flat index step is used to determine the index contrast required to match the far-field at low operating power. The HOM column denotes the order of the highest-order predicted mode.

[0127] The above description of laser diodes, epitaxial layer structures, various features / structures within the epitaxial layers, and waveguides are merely examples and are for illustrative purposes; other structures and features or combinations of structures and / or features are conceivable and are within the scope of the disclosed subject matter; the claimed subject matter is not limited in this respect.

[0128] Having described and illustrated the general and specific principles of examples of the presently disclosed technology, it should be apparent that the examples may be modified in arrangement and detail without departing from these principles. We claim all modifications and variations that are within the spirit and scope of the following claims.

Claims

[1] Laser diode (3200), comprising: a transverse waveguide (3212) comprising an active layer (3218) between an n-type waveguide layer (3214) and a p-type waveguide layer (3216), the transverse waveguide (3212) being bounded by an n-type cladding layer (3208) on an n-side and by a p-type cladding layer (3210) on a p-side; wherein the transverse waveguide (3212) is orthogonal to a lateral waveguide (3206), wherein the lateral waveguide (3206) is bounded in the lateral direction by a ridge waveguide (3220), wherein the lateral waveguide (3206) further includes an embedded higher-order mode suppression layer (HOMSL, 3204) disposed beneath the p-cladding layer (3210) within the lateral waveguide and within the p-type waveguide layer (3216) or within the n-type waveguide layer (3214) and including a thin layer (3204) of a material having a lower refractive index than the waveguide layer (3214, 3216), wherein the lateral waveguide (3206) is defined in a longitudinal direction at a first end by a highly reflective (HR) coated facet (3224) and is limited at a second end by a partially reflective (PR) coated facet (3226) and wherein the higher-order mode suppression layer (HOMSL, 3204) extends in a longitudinal direction from the HR facet over a length that is smaller than the distance between the HR facet (3224) and the PR facet (3226). [2] The laser diode (3200) of claim 1, wherein a thickness of the higher order mode suppression layer (HOMSL, 3204) is selected based on a magnitude of the refractive index contrast in the lateral waveguide (3206) induced by thermal lensing within the lateral waveguide (3206) during operation of the laser diode (3200). [3] The laser diode (3200) of claim 2, wherein the thickness of the higher order mode suppression layer (HOMSL, 3204) is further selected to reduce the magnitude of the refractive index contrast of the lateral waveguide (3206) during operation. [4] Laser diode (3200) according to claim 3, wherein the magnitude of the refractive index contrast is in the range of 10 -5 < Δn < 10 -3 is. [5] The laser diode (3200) of claim 3, wherein the lateral waveguide (3206) supports fewer than ten lateral modes. [6] The laser diode (3200) of claim 3, wherein the lateral waveguide (3206) supports a single lateral mode. [7] The laser diode (3200) of claim 1, wherein a thickness of the higher-order mode suppression layer (HOMSL, 3204) is selected to reduce an effective index on a side of the lateral waveguide (3206) extending from the HR facet (3224). [8] The laser diode (3200) of claim 1, wherein the higher order mode suppression layer (HOMSL, 3204) comprises gallium arsenide (GaAs), aluminum gallium arsenide (AlGaAs), indium gallium arsenide (InGaAs), indium aluminum gallium arsenide (InAlGaAs), or indium gallium phosphide (InGaAsP). [9] A laser diode (3200) according to any one of the preceding claims, wherein the higher order mode suppression layer (HOMSL, 3204) laterally overlaps the lateral waveguide (3206) by 0-10 µm on each side or by 0-20 µm in total. [10] The laser diode (3200) of claim 1, wherein the higher order mode suppression layer (HOMSL, 3204) disposed within the lateral waveguide (3206) is laterally narrower than the lateral waveguide (3206) by 0-10 µm on each side or by 0-20 µm overall.

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