DIODE LASER WITH IMPROVED MODE PROFILE

Description

[0001] The present invention relates to a diode laser with an improved mode profile. State of the art

[0002] In general, edge-emitting laser diodes (diode lasers) possess an active layer embedded within semiconductor layers that differ in their band gaps, refractive indices, and doping. The layers above and below the active layer differ primarily in their conduction type (n-type or p-type). Besides ensuring the transport of electrons and holes to the active layer, where they recombine under stimulation to generate laser radiation, these layers guide the laser light vertically. The layers adjacent to the active layer are called waveguide layers, and those adjacent to these waveguide layers are called cladding layers. Typically, the refractive index of the active layer is higher than that of the waveguide layers, and vice versa. However, other configurations are also possible (e.g., Vertical ARROW, Photonic Band Crystal).

[0003] Wide-strip semiconductor lasers, in particular, are highly sought after by industry due to their high-performance capabilities. Stable optical power outputs exceeding 15 W at 15 A diode current can be achieved from a single laser diode across a 100 µm wide strip. A low-divergence diode laser suitable for high-power applications is disclosed, for example, in US 2004 / 0013147 A1.

[0004] Performance data for wide-strip lasers have already been published by numerous authors, with particular attention paid to conventional and extremely double-asymmetric (EDAS) structures. One such EDAS design is known, for example, from US 8,798,109 B2. The main advantages of EDAS-based laser diodes compared to other wide-strip laser concepts are their particularly high efficiency, low optical losses, and low ohmic resistance. However, the behavior of diodes with an EDAS design and a 100 µm wide stripe has so far only been investigated at room temperature. At these temperatures, however, they exhibit conversion efficiencies for powers greater than approximately 10 W that are significantly higher than those achievable with conventional structures, as reported in Hasler et al., Semicond. Sci. Technol. 29, p. 045010 (2014) and Crump et al., IEEE J. Sel. Top. Quant. Electron. See 19(4), 1501211 (2013).It turns out that diodes with EDAS design, due to their narrow p-side waveguide (thinner than 150 nm, n-waveguide thicker than 1 µm), exhibit a significantly lower series resistance (by up to 40%) as well as reduced loss mechanisms (e.g., band edge bending, charge carrier accumulation in the optical waveguide, and voltage- and temperature-driven leakage currents).

[0005] To reduce the significant influence of the p-type waveguide layer on optical loss and ohmic resistance, the EDAS design incorporates a double asymmetry in the waveguide region, shifting the energy of the developing (fundamental) mode towards the n-type side. However, a disadvantage of the EDAS design is that, due to the narrow p-type waveguide, the fundamental mode maximum shifts away from the active region into the n-type region. This reduces the optical mode confinement within the active region, leading to a drastic increase in the threshold current and a corresponding reduction in the maximum conversion efficiency. Furthermore, as explained in T. Kaul et al., Proc. 25th International Semiconductor Laser Conference (ISLC 2016), Kobe, Japan, Sep. 12-15, p. WD4 (2016), high optical mode confinement is also necessary for the realization of temperature-insensitive diode lasers.

[0006] Previous studies emphasize that the mode confinement factor can be increased by shifting the quantum film towards the n-region or by using a multi-quantum-film configuration. In the first case, the aforementioned advantages of an EDAS design are not achieved. In the second case, there is a drastic increase in the transparency current density, resulting in an unavoidable increase in the threshold current and a reduction in conversion efficiency. Therefore, vertical layer structures with only one quantum film are advantageous. Disclosure of the invention

[0007] It is therefore an object of the present invention to provide a diode laser with an improved mode profile which overcomes the described disadvantages of the prior art. In particular, a diode laser according to the invention should be based on an EDAS design with only one quantum film and exhibit a high optical mode confinement, whereby neither the quantum film is shifted into the n-region nor a wider active zone is required.

[0008] These problems are solved according to the invention by the features of claim 1. Advantageous embodiments of the invention are contained in the dependent claims.

[0009] A diode laser according to the invention comprises an n-type first cladding layer, an n-type first waveguide layer arranged on the first cladding layer, an active layer suitable for radiation generation arranged on the first waveguide layer, a p-type second waveguide layer arranged on the active layer, a p-type second cladding layer arranged on the second waveguide layer, wherein an n-type first intermediate layer is formed as a transition region between the first waveguide layer and the active layer, and a p-type second intermediate layer is formed as a transition region between the second waveguide layer and the active layer, wherein the boundaries between the individual layers are determined by the fact that the refractive index profile exhibits a kink at these locations.wherein the first intermediate layer and, optionally, the second intermediate layer are gradient-index layers (GRIN layers) with a continuous refractive index profile, and wherein a maximum refractive index of the first cladding layer is less than a minimum refractive index of the first waveguide layer, a maximum refractive index of the first waveguide layer is less than a minimum refractive index of the first intermediate layer, a maximum refractive index of the first intermediate layer is less than a minimum refractive index of the active layer, a minimum refractive index of the active layer is higher than a maximum refractive index of the second intermediate layer, a minimum refractive index of the second intermediate layer is higher than a maximum refractive index of the second waveguide layer, and a minimum refractive index of the second waveguide layer is higher than a maximum refractive index of the second cladding layer. The diode laser according to the invention is characterized in thatthat an asymmetry ratio of the thickness of the first intermediate layer to the sum of the thickness of the first intermediate layer and the thickness of the second intermediate layer is chosen to be greater than 0.5 such that the maximum mode intensity of a guided fundamental mode is localized within the first intermediate layer. A further variant not covered by the claims is the use of an asymmetry ratio of less than 0.5, which allows a smaller vertical far-field angle without varying the waveguide thickness.

[0010] According to the invention, the first intermediate layer is a GRIN layer. In a first embodiment, the second p-conducting intermediate layer can be a GRIN layer. In a second embodiment, the second p-conducting intermediate layer can be a layer with a constant refractive index or material composition.

[0011] In a preferred embodiment, the thickness of the second waveguide layer is between 3 nm and 350 nm, since at thicknesses greater than 3 nm an electron wave in the active layer can be efficiently kept away from the cladding layer. In a particularly preferred embodiment, the thickness of the second waveguide layer is between 5 nm and 350 nm, more preferably between 5 nm and 150 nm, more preferably between 5 nm and 100 nm, more preferably between 5 nm and 60 nm, and even more preferably between 5 nm and 30 nm.

[0012] A transition region is a region between two layers where the local refractive indices at the facing edges of the two layers are matched (a corresponding intermediate layer can also be called a refractive index matching layer). If the refractive indices at these edges differ, a continuous transition (e.g., a linear spatial refractive index profile) preferably occurs between these indices within the transition region.

[0013] Preferably, a transition zone is a GRIN layer.

[0014] A GRIN layer (GRIN - Gradient Index) is a layer whose refractive index exhibits a gradient along its thickness. Specifically, these are layers where the refractive index increases or decreases from a first value on one side of the layer to a second value not equal to the first value on the opposite side of the layer (perpendicular to the direction of light propagation). This transition is continuous. Preferably, there is a monotonic relationship between the refractive index and the layer thickness, more preferably a polynomial or exponential relationship, and even more preferably a linear relationship.

[0015] Preferably, at least one of the two waveguide layers contains a refractive index gradient. In the case of one or more waveguide layers with a refractive index gradient, the highest value of the refractive index in the respective layer (i.e., the layer with the refractive index gradient) is always used to calculate the difference between the refractive index of a waveguide layer and the refractive index of an associated cladding layer. According to the present invention, a layer (waveguide layer) with a refractive index gradient is understood to be a layer (waveguide layer) whose refractive index varies along the layer thickness axis (perpendicular to the direction of light propagation). Preferably, the refractive index varies from a first value at the first interface (with the cladding layer) to a second value at the second interface (with the intermediate layer).

[0016] All refractive indices refer to the central wavelength of the radiation emitted or amplified by the active layer. The central wavelength of the radiation emitted by the active layer is preferably between 380 nm and 10 µm, more preferably between 380 nm and 1200 nm, more preferably between 700 nm and 1000 nm, and even more preferably between 900 nm and 1000 nm.

[0017] The idea of ​​the present invention is to introduce an additional asymmetric component into the transition region near the active layer, particularly near a single quantum film acting as the active layer. This allows the mode properties, and especially the mode profile or the spatial location of the mode maximum relative to the active layer, to be specifically influenced and controlled. Specifically, a triple asymmetry is implemented in the layer structure for this purpose (asymmetric n- and p-waveguide layers, asymmetric n- and p-cladding layers, and asymmetric n- and p-intermediate layers).

[0018] This mode-influence method is particularly interesting for laser diodes with an EDAS design, since in symmetrical structures, the introduction of such an asymmetric component in the transition regions has a less pronounced influence on the diode behavior due to the centered mode guidance. An EDAS design extended by such an additional asymmetric component is hereinafter also referred to as an Extreme Triple Asymmetric Structure (ETAS). In particular, the concept of the present invention is suitable for leaving the p-waveguide untouched in a laser diode with an EDAS design and modifying the mode confinement in the active layer by means of the additional asymmetric component in the transition regions to the active layer. However, the application of the inventive asymmetric design of the n- and p-side transition regions between the waveguide layers and the active layer is not limited to laser diodes with an EDAS design.

[0019] In a laser diode with an ETAS design, for an asymmetry where the ratio of the thickness of the first intermediate layer to the sum of the thickness of the first intermediate layer and the thickness of the second intermediate layer is greater than 0.5, the guided fundamental mode shifts towards the active layer or the quantum film. This increases the optical power density of the fundamental mode in the near field as well as the modal confinement in the active layer (mode confinement Γ). Fig. 4 a)For example, it can be seen that the mode confinement Γ can be significantly increased by increasing the thickness of the n-sided interlayer while keeping the thickness of the p-sided interlayer constant. In an exemplary conventional EDAS structure with single quantum films and symmetrical GRIN interlayers, Γ = 0.2% (see M. Winterfeldt et al., Proc. SPIE 9733, 97330O (2016)). In particular, using this ETAS technique, the mode confinement Γ can be almost tripled from 0.2% to 0.6% for this example, without having to modify the p-region of a conventional EDAS laser diode. Thus, all the positive properties of the conventional EDAS design can be retained. Another finding is that with an asymmetrical ETAS setup (see Fig. 4 a) opposite Fig. 4cWith a second GRIN interlayer thickness of 70 nm, an almost identical mode confinement can be achieved as with a more than twice as wide (150 nm) symmetrical EDAS setup. This strategy enables significantly increased conversion efficiencies at high optical powers from semiconductor lasers with an ETAS design and is essential for the market of high-efficiency, high-power laser diodes. The present invention makes it possible to extend the use of laser diodes with an EDAS design to the high-temperature range.

[0020] For laser diodes with an ETAS design not covered by the claims, where the ratio of the thickness of the first interlayer to the sum of the thicknesses of the first and second interlayers is less than 0.5, the additional asymmetric component in the transition regions to the active layer shifts the mode maximum towards the n-region, resulting in a broadening of the emitted near-field profile and a consequent reduction of the vertical far-field profile. With a constant thickness of the p-side waveguide, the lower radiation density at the front facet of the laser diode also allows for increased failure power due to COMD (catastrophic optical mirror damage), as shown in D. Botez, Appl. Phys. Lett. 74(21), 3102 (1999). Furthermore, this allows for a reduction in the far-field angles, as particularly demonstrated in Figure 4 d)It can be seen where a reduction in asymmetry without changing the p-waveguide thickness allows for a reduction of the far field by more than 10°. This does come at the cost of mode confinement, which can be advantageous, for example, in applications operating at low temperatures. Fig. 5 a) It can also be seen that the power curves for the mode inclusions Γ = 0.46% and Γ = 0.54% are very similar, which is why no significant losses in the power characteristics of the laser diodes have to be accepted when improving the far field.

[0021] The ETAS design with triple asymmetry, for example with asymmetric GRIN interlayers, provides new design freedom that can be used either for increased conversion efficiency and reduced temperature sensitivity (due to increased mode confinement) or for greater usable power due to higher COMD performance (corresponding to reduced mode confinement) and lower divergence.

[0022] According to the invention, the sum of the thicknesses of the first waveguide layer and the second waveguide layer is greater than 1 µm, more preferably greater than 1.5 µm, and even more preferably greater than 2 µm. Preferably, the sum of the thicknesses of the first waveguide layer and the second waveguide layer is less than 10 µm, more preferably less than 5 µm, more preferably less than 4 µm, and even more preferably less than 3 µm. This configuration makes it possible to achieve a vertical far field of less than 60° for the fundamental mode, within which 95% of the radiated power is contained or emitted (FF95%). Furthermore, a small proportion of the fundamental mode within the p-type waveguide layer (preferably < 15%, for example, with a 150 nm thick p-type waveguide layer and a total thickness of 1.15 µm) results in the lowest possible optical absorption loss.

[0023] Preferably, the thickness of the second waveguide layer is less than 500 nm, more preferably less than 350 nm, more preferably less than 250 nm, more preferably less than 150 nm, more preferably less than 100 nm and even more preferably less than 50 nm.

[0024] Preferably, the thickness of the second waveguide layer is greater than 5 nm, more preferably greater than 10 nm and even more preferably greater than 25 nm.

[0025] Preferably, the thickness of the first intermediate layer is less than 500 nm, more preferably less than 350 nm, more preferably less than 300 nm, more preferably less than 200 nm, more preferably less than 150 nm, more preferably less than 75 nm, and even more preferably less than 30 nm. Preferably, the thickness of the first intermediate layer is greater than 5 nm, more preferably greater than 10 nm, and even more preferably greater than 25 nm. Preferably, the thickness of the second intermediate layer is less than 500 nm, more preferably less than 350 nm, more preferably less than 300 nm, more preferably less than 150 nm, and even more preferably less than 75 nm. Preferably, the thickness of the second intermediate layer is greater than 5 nm, more preferably greater than 10 nm, and even more preferably greater than 25 nm.

[0026] A small thickness of the second waveguide layer allows for low optical loss and low ohmic resistance. According to the invention, the difference between the maximum refractive index of the first waveguide layer and the refractive index of the first cladding layer is between 0.04 and 0.01. This advantageously suppresses or completely eliminates overmodes. The difference in refractive indices must be large enough that, during active operation, thermally and electronically induced changes in refractive index, which are typically smaller than 0.01, have no effect on the laser properties. As a result, the fundamental mode is stable with respect to temperature or charge carrier effects (i.e., defined primarily by the waveguide and cladding layers).In a particularly preferred embodiment, the difference between the maximum refractive index of the first waveguide layer and the refractive index of the first cladding layer is between 0.04 and 0.015. It is also preferably that the difference between the maximum refractive index of the first waveguide layer and the refractive index of the first cladding layer is less than or equal to 0.035. Furthermore preferably, the difference between the maximum refractive index of the first waveguide layer and the refractive index of the first cladding layer is greater than or equal to 0.015. Preferably, within the first intermediate layer, the refractive index decreases from the active layer towards the first waveguide layer by at least 0.03 or by 30% of the difference between the maximum refractive index of the first intermediate layer and the minimum refractive index of the first waveguide layer.The n-sided intermediate layer thus bridges at least 0.03 or 30% of the refractive index step between the maximum refractive index of the first intermediate layer and the minimum refractive index of the first waveguide layer. Preferably, the refractive index within the first intermediate layer decreases by at least 0.05 or 50% of said difference, and even more preferably by 0.07 or 70% of said difference. In particular, the decreasing refractive index profile within the first intermediate layer can be a continuously decreasing function of the layer thickness. Preferably, the refractive index within the first intermediate layer decreases monotonically as a function of the layer thickness from the maximum refractive index of the active layer to the minimum refractive index of the first waveguide layer. Preferably, the minimum refractive index of the first waveguide layer is located at the boundary with the first cladding layer.

[0027] Preferably, within the second intermediate layer, the refractive index does not decrease from the active layer towards the second waveguide layer, but rather maintains a constant value throughout the entire second intermediate layer. In an alternative embodiment, the refractive index within the second intermediate layer decreases by at least 9% of the difference between the maximum refractive index of the second intermediate layer and the minimum refractive index of the second waveguide layer. The p-side intermediate layer thus bridges at least 9% of the refractive index step between the maximum refractive index of the second intermediate layer and the minimum refractive index of the second waveguide layer. In particular, the decreasing refractive index profile within the second intermediate layer can be a continuously decreasing function of the layer thickness.Preferably, the refractive index within the second intermediate layer decreases continuously as a function of the layer thickness from the maximum refractive index of the active layer to the minimum refractive index of the second waveguide layer. Preferably, the minimum refractive index of the first waveguide layer is located at the boundary with the second cladding layer.

[0028] Preferably, the waveguide layers and the cladding layers are uniform. Preferably, the uniformity extends over their entire length, which is located between the facets of the laser diode, and even more preferably over their entire extent. Uniformity within the meaning of this application means that the layer parameters (such as chemical composition, doping, layer thickness, refractive index) differ from each other relatively (maximum to minimum) by less than 10%, more preferably less than 5%, and more preferably less than 1% over said extent.

[0029] The preferred material for the first cladding layer is Al x Ga 1-x As; more preferably, the material composition lies between Al 0.28 Ga 0.72 As and Al 0.32 Ga 0.68 As. The first cladding layer preferably has a thickness between 0.5 µm and 4 µm. The preferred material for the first waveguide layer is Al x Ga 1-x As; more preferably, the material composition lies between Al 0.10 Ga 0.90 As and Al 0.32 Ga 0.68 As. The first waveguide layer preferably has a thickness between 0.5 µm and 3 µm. Suitable materials for the first cladding layer and the first waveguide layer include all compound semiconductors consisting of elements from group 3 (Al, Ga, In) and group 5 (N, P, As, and Sb) of the periodic table, which produce the desired difference between the refractive index of the first waveguide layer and the refractive index of the first cladding layer.Preferably, the first waveguide layer and the second waveguide layer are made of the same material. The refractive index of the first cladding layer is preferably lower than the refractive index of the first waveguide layer. The refractive index of the second cladding layer is preferably lower than the refractive index of the second waveguide layer.

[0030] The preferred material for the active layer is an InGaAs MQW (multi-quantum well), more preferably an InGaAs DQW (double-quantum well), and even more preferably an InGaAs SQW (single-quantum well). Possible materials for the active layer include all compound semiconductors consisting of elements from group 3 (Al, Ga, In) and group 5 (N, P, As, and Sb) of the periodic table, e.g., GaAsP, InGaAsP, AllnGaAs, InGaAsNSb.The preferred material of the second waveguide layer is Al x Ga 1-x As, preferably with the smallest possible molar Al content x for a minimum electrical resistance; for a wavelength of 940 nm, at the interface between the second intermediate layer and the second waveguide layer, preferably less than 0.4, more preferably less than 0.3 and even more preferably less than or equal to 0.25, and at the interface between the second waveguide layer and the second cladding layer, preferably less than 0.95, more preferably less than 0.9 and even more preferably less than 0.85.

[0031] Preferably, the active layer has a (total) layer thickness of less than 80 nm, more preferably less than 60 nm, more preferably less than 40 nm and even more preferably less than 20 nm.

[0032] Preferably, the active layer comprises at least one quantum well without barrier layers. In an alternative preferred embodiment, the active layer comprises at least one well layer and at least two barrier layers. Preferably, the thickness of the layer containing the at least one well is between 1 nm and 25 nm, more preferably between 5 and 20 nm. Preferably, the thicknesses of the layer containing the at least one well are uniform and equal. Preferably, the thicknesses of the optionally present barrier layers are uniform and equal. The number of well layers is preferably less than 15, more preferably less than 10, more preferably less than 5, and even more preferably less than 3. Preferably, the total thickness of the active layer (including all well and barrier layers) is less than 80 nm, more preferably less than 60 nm, more preferably less than 40 nm, and even more preferably less than 20 nm.

[0033] Preferably, the active layer is uniform. Preferably, the uniformity of the active layer extends over its entire length, which is located between the facets of the laser diode, and even more preferably over its entire extent. Uniformity of the active layer within the meaning of this application means that the layer parameters (such as chemical composition, doping, layer thickness, refractive index) differ relatively (maximum to minimum) from each other by less than 10%, more preferably less than 5%, more preferably less than 1%, and even more preferably not at all over said extent.

[0034] Preferably, the active layer extends over the entire area between a reflection facet and an exit facet. Preferably, an active layer directly contacts both the reflection facet and the exit facet. The active layer can also be spaced from the facets by a distance between 0 µm and 500 µm, for example by implantation, intermixing, or overgrowth, to prevent laser failure due to facet damage.

[0035] Preferably, the surfaces of the reflection facet and the exit facet are planar. Preferably, the surfaces of the reflection facet and the exit facet are arranged parallel to each other. Preferably, the longitudinal axis of the first waveguide layer and the longitudinal axis of the second waveguide layer are perpendicular to the surfaces of the reflection facet and the exit facet. Preferably, a ribbed waveguide is provided for controlling the lateral far field.

[0036] Preferably, the first and second intermediate layers consist of Al x Ga 1-x As. Preferably, the molar Al content x of the first intermediate layer at the interface between the first intermediate layer and the active layer has a value greater than or equal to 0, more preferably greater than 0.05, and even more preferably greater than 0.1. It is also preferred that the molar Al content x of the first intermediate layer at the interface between the first waveguide layer and the first intermediate layer is greater than 0.05, more preferably greater than 0.1, more preferably greater than 0.15, and even more preferably greater than 0.2. For example, the difference between the maximum refractive index of a first waveguide layer (exemplarily Al 0.25 Ga 0.75 As) and the maximum refractive index of a first intermediate layer (exemplary Al 0.15 Ga 0.85 As) at the interface between the first waveguide layer and the active layer is 0.07.This difference value is particularly preferred to be greater than 0.03, more preferred to be greater than 0.05 and even more preferred to be greater than 0.07.

[0037] The formation of a refractive index gradient in at least one of the waveguide layers, preferably the first waveguide layer, is advantageous because this leads to a favorable intensity profile of the fundamental mode and thus to a low lasing threshold. Due to the design according to the invention, it is possible to define the intensity of the fundamental mode within the quantum film without having to adversely adjust the thickness of the p-interlayer or the thickness of the waveguide layers. However, if the fundamental mode lies too far within one of the waveguide layers, the lasing threshold would increase significantly, so the formation of a refractive index gradient can lead to a more favorable positioning of the fundamental mode in the waveguide.

[0038] In a particularly preferred embodiment, the first waveguide layer and the second waveguide layer exhibit a refractive index gradient (e.g., by varying the aluminum content along the layer thickness), wherein, in the case of Al x Ga 1-x As waveguide layers, the molar Al content x of the second waveguide layer can be varied between 10% and 95%, and that of the first waveguide layer between 10% and 32%. Preferably, the refractive index changes linearly from one side to the other in the case of a refractive index gradient.

[0039] Preferably, the refractive index of a waveguide layer is lower at the interface with the cladding layer than at the interface with the intermediate layer. Preferably, the refractive index of a waveguide layer varies continuously from the interface with the cladding layer to the interface with the intermediate layer. Preferably, the refractive index gradient is generated by a material-specific composition profile in the semiconductor. For example, in the material system Al x Ga 1-x As, the refractive index gradient can be generated by a gradient of the molar Al content x. If the second waveguide layer has a composition profile, Al x Ga 1-x As, with a molar Al content between Al 0.15 Ga 0.85 As and Al 0.85 Ga 0.15 As, is preferably used.

[0040] Preferably, the diode laser according to the invention is configured for continuous wave operation. Preferably, the electrical control for the diode laser is configured such that the diode laser operates in continuous wave mode. Preferably, the diode laser is configured as an edge-emitting diode laser. Preferably, the diode laser is configured as an optical amplifier.

[0041] Preferably, a support substrate is provided on which the aforementioned layer structure is built. Preferably, the n-type layers are arranged on the side of the active layer facing the support substrate, whereas the p-type layers are arranged on the side of the active layer facing away from the support substrate. Preferably, the diode laser has contact layers for injecting charge carriers. Brief description of the drawings

[0042] The invention is explained below using exemplary embodiments with reference to the accompanying drawing. The drawing shows: Fig. 1 the refractive index distribution along the layers of a conventional EDAS laser diode according to the prior art and the associated vertical distribution of the mode intensity in the laser diode, Fig. 2A the refractive index distribution along the layers of a first embodiment of an ETAS laser diode according to the invention and the associated vertical distribution of the mode intensity in the laser diode, Fig. 2B the refractive index distribution along the layers of a second embodiment of an ETAS laser diode according to the invention and the associated vertical distribution of the mode intensity in the laser diode, Fig. 3A a laser diode according to the invention in a schematic perspective view, Fig. 3B the laser diode according to the invention made of Fig. 2AIn a schematic cutaway view along an axis transverse to the direction of light propagation, Fig. 3C, the laser diode according to the invention is made of Fig. 2A in a schematic cutaway view along an axis parallel to the direction of light propagation, Fig. 4 the simulated dependencies of the mode confinement (a, c) and the vertical far field (b, d) on the strength of the asymmetry ratio and the layer thickness of the second waveguide layer, and Fig. 5 the power-voltage-current characteristic of three comparable semiconductor laser diodes with different embodiments of an ETAS design according to the invention at 25°C (a) and at 75°C (b). Detailed description of the drawings

[0043] Fig. 1Figure 1 shows the refractive index distribution along the layers of a conventional EDAS laser diode according to the state of the art, as well as the corresponding vertical distribution of the mode intensity in the laser diode. The laser diode has a layered structure with an n-type first cladding layer 14, an n-type first waveguide layer 12 arranged thereon, an n-type first intermediate layer 11 (GRIN layer) arranged thereon, an active layer 10 arranged thereon, a p-type second intermediate layer 15 (GRIN layer) arranged thereon, a p-type second waveguide layer 16 arranged thereon, and a p-type second cladding layer 18 arranged thereon. The aforementioned layers are arranged on a substrate 28. The respective thicknesses of the first intermediate layer 11 and the second intermediate layer 15 are equal, i.e.,The active layer 10 is located centrally between the indicated interfaces of the first intermediate layer 11 with the first waveguide layer 12 and the second intermediate layer 15 with the second waveguide layer 16. In particular, as with all EDAS laser diodes according to the prior art, the asymmetry ratio of the thickness of the first intermediate layer 11 to the sum of the thickness of the first intermediate layer 11 and the thickness of the second intermediate layer 15 is equal to 0.5.

[0044] In the EDAS design shown, the maximum intensity of the fundamental mode is located well outside the active layer (i.e., low mode confinement) and far inside the first waveguide region 12. The mode can only interact with the active layer 10 via its steeply sloping p-side flank at low intensity and small mode overlap.

[0045] Fig. 2AFigure 1 shows the refractive index distribution along the layers of a first embodiment of an ETAS laser diode according to the invention, as well as the corresponding vertical distribution of the mode intensity in the laser diode. The layer structure shown essentially corresponds to that in Figure 2. Fig. 1 The structure shown is as follows. The respective reference symbols apply accordingly. In contrast to the representation in Fig. 1However, in the laser diode shown, an additional asymmetry with respect to the thicknesses of the first intermediate layer 11 (GRIN layer) and the second intermediate layer 15 (GRIN layer) was implemented compared to a conventional EDAS design. While the structure of the second intermediate layer 15 largely corresponds to the conventional EDAS design in the p-side region, the thickness of the first intermediate layer 11 was significantly increased relative to the thickness of the second intermediate layer 15. This means that, in the ETAS design, the active layer 10 is no longer located centrally between the depicted interfaces of the first intermediate layer 11 with the first waveguide layer 12 and the second intermediate layer 15 with the second waveguide layer 16. In particular, the asymmetry ratio of the thickness of the first intermediate layer 11 to the sum of the thicknesses of the first intermediate layer 11 and the second intermediate layer 15 is greater than 0.5 in the illustration.

[0046] In the depicted ETAS design, the maximum intensity of the fundamental mode shifts significantly towards the active layer 10 (i.e., high mode confinement). In particular, the diagram shows that a large portion of the mode intensity is localized within the first intermediate layer 11. This contrasts with the diagram in Fig. 1 The mode profile shown exhibits a smaller mode extent (lower mode volume) and a higher spatial overlap with the active layer 10, along with an increased maximum intensity at the mode's peak. The mode can therefore interact much more strongly with the active layer 10. Mode confinement is significantly increased.

[0047] Fig. 2B Figure 1 shows the refractive index distribution along the layers of a second embodiment of an ETAS laser diode according to the invention, as well as the corresponding vertical distribution of the mode intensity in the laser diode. The representation essentially corresponds to that in Figure 2. Fig. 2B The illustration shown. The respective reference numerals apply accordingly. In contrast to the first embodiment according to Fig. 2A The p-type second intermediate layer 15 is a layer with a constant refractive index. The n-type first intermediate layer 11, on the other hand, is designed as a GRIN layer.

[0048] Fig. 3A-3CFigure 1 shows a laser diode according to the invention in perspective and section views. The laser diode according to the invention has a vertical layer structure with a substrate 28, an n-type first cladding layer 14 arranged thereon, an n-type first waveguide layer 12 arranged thereon, an n-type first intermediate layer 11 arranged thereon, an active layer 10 arranged thereon, a p-type second intermediate layer 15 arranged thereon, a p-type second waveguide layer 16 arranged thereon, and a p-type second cladding layer 18 arranged thereon. A first contact for injecting charge carriers 30 and a second contact for injecting charge carriers 32 are also shown by way of example. The exact position of the first intermediate layer 11, the second intermediate layer 15, and the active layer 10 can be seen in the enlarged view in Figure 2. Fig. 3B can be taken.

[0049] Furthermore, the laser diode according to the invention has at its lateral opposite ends a reflection facet 20 with a high reflectivity for the central wavelength of the radiation emitted by the active layer 10 and an exit facet 22 with a reflectivity that allows the radiation to be coupled out. The reflectivity of the reflection facet 20 is preferably greater than 0.8, more preferably greater than 0.9, and even more preferably greater than 0.99. The reflectivity of the exit facet 22 is preferably less than the reflectivity of the reflection facet 20. The facets 20 and 22 form a cavity, so that laser operation can be achieved.

[0050] The specific structure of the in the Fig. 3A-3CIn the preferred embodiment shown, a diode laser has an active layer 10 with a central emission wavelength of 940 nm, fabricated from an InGaAs quantum film with a thickness of 5.4 nm. The cladding, waveguide, and intermediate layers 11, 12, 14, 15, 16, and 18 are made of Al x Ga 1-x As. For the waveguides 12 and 16, the molar Al content x in Al x Ga 1-x As is preferably 26% and 25%, respectively. For the cladding layers 14 and 18, the molar Al content x in Al x Ga 1-x As is preferably 30% and 70%, respectively. For the intermediate layers 11 and 15 (GRIN layers), the molar Al content x in Al x Ga 1-x As at the respective interface with the active layer 10 is preferably 15%.In the preferred embodiment, the layer thickness of the n-conducting first cladding layer 14 is 1.65 µm, the layer thickness of the n-conducting first waveguide layer 12 is 2.5 µm, the layer thickness of the n-conducting first intermediate layer 11 is 350 nm, the layer thickness of the p-conducting second intermediate layer 15 is 70 nm, the layer thickness of the p-conducting second waveguide layer 16 is 150 nm and the layer thickness of the p-conducting second cladding layer 18 is 800 nm.

[0051] In an alternative preferred embodiment, the first waveguide layer 12 and the second waveguide layer 16 have a refractive index gradient (by varying the aluminium content along the layer thickness), wherein the aluminium content in the second waveguide layer 16 is varied between 25% and 70% and in the first waveguide layer 12 between 26% and 30%.

[0052] Fig. 4Figure 1 shows the simulated dependencies of mode confinement (a, c) and the vertical far field (b, d) on the strength of the asymmetry ratio and the thickness of the second waveguide layer. The cladding, waveguide, and intermediate layers 11, 12, 14, 15, 16, 18 are assumed to consist of Al x Ga 1-x As, with the thickness of the p-type second waveguide layer 16 being 70 nm (a, b) and 150 nm (c, d), respectively. The values ​​for a Fig. 1 The structure shown, in accordance with the state of the art, is in Fig. 4 (a, b) each marked with a ∘ ("circle"), while the values ​​for a structure according to the invention according to the first embodiment according to Fig. 2 are marked with a □ ("square"). All further information on the simulation parameters can be found in the description in the Fig. 3A-3C The laser diode shown according to the invention can be taken from the laser diode shown.

[0053] The simulation was investigated for different thicknesses of the p-type second interlayer 15 (70 nm (a, b), 150 nm (c, d)) while varying the thickness of the n-type first interlayer 11 (10 nm - 350 nm) and with different molar Al fractions x at the interface between the n-type first interlayer 11 and the first waveguide layer 12. The molar Al fraction x at the interface between the p-type second interlayer 15 and the second waveguide layer 16 is consistently 25% (or 0.25). The active layer consists of a quantum film with a thickness of 5.4 nm. The circles in diagrams a) and b) correspond to those in Fig. 1 The vertical structure of a conventional EDAS laser diode is shown. The squares in diagrams a) and b) correspond to the one in Fig. 2shown vertical structure of an EDAS laser diode extended according to the invention with an additional asymmetry component in the area of ​​the active layer 10 (ETAS laser diode).

[0054] Fig. 5 Figure 1 shows the power-voltage-current characteristics of three comparable semiconductor laser diodes with different embodiments of an ETAS design according to the invention at 25°C (a) and at 75°C (b). In the individual ETAS laser diodes, the mode confinement was varied according to the invention via the asymmetry ratio. These are wide-band semiconductor lasers for high-power operation (Pout > 15 W). If such a semiconductor laser is operated with a conversion efficiency of 55% at the operating point of 15 W, using an industrially standard passive cooling system with a coolant temperature of 25°C, the temperature of the active zone can be assumed to be approximately 62°C, assuming an average thermal resistance of 3 K / W.

[0055] Evaluating the characteristic curves of the three laser diodes with ETAS design at 25°C (a) and 75°C (b) reveals significant differences in their respective power characteristics. All three tested vertical structures have a comparable p-layer architecture, as evidenced by the fact that the voltage characteristics of the three diodes do not differ significantly.

[0056] At a heat sink temperature of 25°C (a), no significant power saturation mechanisms are observed with respect to mode confinement. However, the threshold current is higher for structures with smaller mode confinement, which already results in a considerable efficiency loss at room temperature. Higher threshold currents lead to higher charge carrier densities within the active layer and in the waveguide. This effect results in increased photon absorption by free charge carriers and excessive heat generation, leading to a self-reinforcing effect. The band edges near the quantum film are bent, increasing leakage current rates and further amplifying charge carrier accumulation in the waveguide.

[0057] If we now consider the same diodes at an increased heat sink temperature at the operating point of 75°C (b), it becomes apparent that the aforementioned saturation mechanisms, dependent on mode confinement and current, are activated. This suggests that for high-power operation at high operating temperatures, it is particularly advantageous to aim for a large mode confinement in order to reduce the charge carrier densities, while keeping the p-waveguide as narrow as possible. Reference symbol list

[0058] 10 Active layer 11 First intermediate layer (n-type) 12 First waveguide layer (n-type) 14 First cladding layer (n-type) 15 Second intermediate layer (p-type) 16 Second waveguide layer (p-type) 18 Second cladding layer (p-type) 20 Reflection facet 22 Exit facet 28 Substrate 30 First contact for charge carrier injection 32 Second contact for charge carrier injection xmolar Al content (Al x Ga 1-x As) ΓMode inclusion

Claims

1. A diode laser, comprising: a) an n-type first cladding layer (14), b) an n-type first waveguide layer (12) arranged on the first cladding layer (14), c) an active layer (10) suitable for radiation generation and arranged on the first waveguide layer (12), d) a p-type second waveguide layer (16) arranged on the active layer (10), e) a p-type second cladding layer (18) arranged on the second waveguide layer (16), f) wherein an n-type first intermediate layer (11) is formed as a transition region between the first waveguide layer (12) and the active layer (10), and g) a p-type second intermediate layer (15) is formed as a transition region between the second waveguide layer (16) and the active layer (10), h) wherein boundaries between the individual layers are determined by a fact that a refractive index curve at these locations has a kink, i) wherein the first intermediate layer (11) and, optionally, the second intermediate layer (15) are gradient index layers with a continuous refractive index curve, j) wherein a maximum refractive index of the first cladding layer (14) is less than a minimum refractive index of the first waveguide layer (12), a maximum refractive index of the first waveguide layer (12) is less than a minimum refractive index of the first intermediate layer (11), a maximum refractive index of the first intermediate layer (11) is less than a minimum refractive index of the active layer (10), the minimum refractive index of the active layer (10) is higher than a maximum refractive index of the second intermediate layer (15), a minimum refractive index of the second intermediate layer (15) is higher than a maximum refractive index of the second waveguide layer (16), and a minimum refractive index of the second waveguide layer (16) is higher than a maximum refractive index of the second cladding layer (18), k) wherein a sum of a layer thickness of the first waveguide layer (12) and a layer thickness of the second waveguide layer (16) is greater than 1 µm, wherein the layer thickness of the second waveguide layer (16) is less than 350 nm, and l) wherein a difference between the maximum refractive index of the first waveguide layer (12) and a refractive index of the first cladding layer (14) is between 0.04 and 0.01, characterized in that m) an asymmetry ratio of a layer thickness of the first intermediate layer (11) to a sum of a layer thickness of the first intermediate layer (11) and a layer thickness of the second intermediate layer (15) is selected to be greater than 0.5 such that a maximum mode intensity of a guided fundamental mode is located within the first intermediate layer (11).

2. The diode laser according to any one of the preceding claims, wherein the layer thickness of the second waveguide layer (16) is less than 150 nm.

3. The diode laser according to any one of the preceding claims, wherein the layer thickness of the first intermediate layer (11) and the layer thickness of the second intermediate layer (15) are less than 350 nm.

4. The diode laser according to any one of the preceding claims, wherein within the first intermediate layer (11), a refractive index, starting from the active layer (10), decreases in a direction of the first waveguide layer (12) by at least 0.03 or by 30% of a difference between the maximum refractive index of the first intermediate layer (10) and the minimum refractive index of the first waveguide layer (12).

5. The diode laser according to any one of the preceding claims, wherein the active layer (10) has a layer thickness of less than 80 nm.

6. The diode laser according to any one of the preceding claims, wherein the active layer (10) has at least one quantum well.

7. The diode laser according to any one of the preceding claims, wherein the first intermediate layer (11) and the second intermediate layer (15) consist of AlxGa1-xAs.

8. The diode laser according to claim 7, wherein a molar Al content (x) of the first intermediate layer (11) at an interface between the first intermediate layer (11) and the active layer (10) is greater than 0.

9. The diode laser according to claim 7 or 8, wherein a molar Al content (x) of the first intermediate layer (11) at an interface between the first waveguide layer (12) and the first intermediate layer (11) is greater than 0.05.

10. The diode laser according to any one of the preceding claims, wherein at least one of the waveguide layers (12, 16) has a refractive index gradient.

11. The diode laser according to any one of the preceding claims, wherein the diode laser is configured as an edge-emitting diode laser or as an optical amplifier.