Semiconductor laser and lidar system with semiconductor laser and laser system

Abstract

According to embodiments, a semiconductor laser (10) comprises a semiconductor layer arrangement (112) having an active region (115) for generating radiation. The semiconductor laser (10) further has a first resonator mirror (125), a second resonator mirror (130) and an optical resonator (131) arranged between the first resonator mirror (125) and the second resonator mirror (130), which optical resonator extends in a direction parallel to a main surface (111) of the semiconductor layer arrangement (112). The reflectivity R1 of the first resonator mirror is wavelength dependent such that R1 or the product R of R1 and the reflectivity R2 of the second resonator mirror drops from a value R0 in a wavelength range from a target wavelength λ0 of the laser to λ0 + Δλ, wherein Δλ is chosen in accordance with a temperature dependent shift of the emission wavelength.

Description

[0001] This patent application claims priority to German patent application DE 10 2020 205 254.9, the disclosure of which is incorporated herein by reference.

[0002] Laser radar (LIDAR) systems are increasingly used in vehicles, for example for autonomous driving. For example, the laser radar system is used to measure distances or to identify objects. In laser radar systems, a narrow window is often very advantageous for the wavelength used, since in this case solar light as an interfering signal can be effectively suppressed with a narrowband filter. Since in a usual edge-emitting semiconductor laser the emission wavelength varies by more than 0.2 nm / K, a design was investigated with which the wavelength hardly changes even at varying temperatures, i.e. an increase in temperature stability is ensured.

[0003] The present invention is based on the object of providing an improved semiconductor laser and an improved laser radar system.

[0004] According to the embodiments, this object is achieved by the subject matter of the independent claims. Advantageous refinements are specified in the dependent claims.

[0005] According to an embodiment, the semiconductor laser comprises a semiconductor layer arrangement having an active region for generating radiation. The semiconductor laser further has a first resonator mirror, a second resonator mirror and an optical resonator arranged between the first resonator mirror and the second resonator mirror, which extends in a direction parallel to a main surface of the semiconductor layer arrangement. The reflectivity R1 of the first resonator mirror is wavelength-dependent, such that R1 or the product R of R1 and the reflectivity R2 of the second resonator mirror drops from a value R0 in a wavelength range from a target wavelength λ0 of the laser to λ0+Δλ, wherein Δλ is selected in accordance with a temperature-dependent shift of the emission wavelength. For R at least one of the following relationships applies:

[0006] (i) for at least one wavelength λ, R(λ) < 0.3 * R0, with λ0 < λ < λ0+Δλ;

[0007] (ii) for λ = λ0, dR / dλ < k / nm, with k < -0.1%, or for R1 at least one of the following relationships applies:

[0008] (iii) for at least one wavelength λ, R1(λ) < 0.3 * R0, with λ0 < λ < λ0+Δλ;

[0009] (iv) for λ = λ0, dR1 / dλ < k / nm, with k < -0.1%.

[0010] According to further embodiments, k can also be less than or equal to -0.2% or less than or equal to -0.5%.

[0011] For example, Δλ can be in a range of more than 5 nm. For example, Δλ can be more than 10 or more than 30 nm. For example, Δλ can be less than 100 nm, for example less than 80 nm.

[0012] For example, R or R1 can increase to a value R0 for wavelengths smaller than λ0.

[0013] According to other embodiments, the second resonator mirror can also have a wavelength-dependent reflectivity which increases to a local maximum for wavelengths smaller than λ0.

[0014] According to embodiments, the sum of the intrinsic loss of the semiconductor laser and the reflection loss caused by the first and second resonator mirrors can increase for wavelengths λ in a range of λ0< λ < λ0+ Δλ.

[0015] For example, the sum of the intrinsic loss of the semiconductor laser and the reflection loss can be at least half of a curve which corresponds to the product of the confinement factor Γ(λ, T) and the wavelength-dependent gain g(λ, T).

[0016] According to other embodiments, the slope of the sum of the intrinsic loss of the semiconductor laser and the reflection loss can be greater than a curve which corresponds to the product of the confinement factor Γ(λ, T) and the wavelength-dependent gain g(λ, T).

[0017] According to other embodiments, at least one of the following relationships can apply:

[0018] (v) for λ0< λ < λ0+ Δλ, dR / dλ < -g*R(λ) / nm,

[0019] (vi) for λ0< λ < λ0+ Δλ, dR1 / dλ < -g*R1(λ) / nm, where g > 0.03.

[0020] For example, the first resonator mirror can be implemented as a Bragg mirror. For example, the individual layers of the Bragg mirror can comprise AlO, TaO, SiO, TiO, AlN, SiN, Si, NbO, ITO, ZnO, Ag and Al as well as InAlGaAsP, InAlGaN, ZnSe as a semiconductor material. Here, the sequence of the respective layers, the composition and the layer thickness are chosen such that the desired reflectivity is achieved.

[0021] For example, the Bragg mirror can have epitaxially grown semiconductor layers.

[0022] According to embodiments, at least one of the semiconductor layers of the Bragg mirror can be adapted to absorb electromagnetic radiation having a wavelength range smaller than λ0.

[0023] For example, the reflectivity R1 or the product R of the first resonator mirror can be less than 8% in the target wavelength range.

[0024] According to an embodiment, the active region has only a single quantum well structure.

[0025] According to a further embodiment, the active region has a multiple quantum well structure with mutually detuned quantum wells.

[0026] For example, the carrier density in the active region can increase by more than 25% at the laser threshold in a temperature range from 0°C to 60°C.

[0027] According to an embodiment, the emission wavelength of the semiconductor laser decreases with temperature at ambient temperatures greater than 100°C or at temperatures greater than 60°C.

[0028] The semiconductor layer arrangement of the semiconductor laser can have a plurality of laser elements arranged on top of one another and connected to one another via a connecting layer.

[0029] For example, the lateral delimitation of the semiconductor layer arrangement is bevelled such that the generated electromagnetic radiation is emitted via the first main surface of the semiconductor layer arrangement.

[0030] The laser radar system comprises a semiconductor laser as described above.

[0031] A further embodiment relates to a laser system having a semiconductor laser as described above and a further laser, wherein the further laser can be optically pumped by the semiconductor laser.

[0032] The accompanying drawings serve to understand embodiments of the present application. The drawings illustrate embodiments and, together with the description, explain them. Other embodiments and a large number of expected advantages result directly from the following detailed description. The elements and structures shown in the drawings are not necessarily shown to scale relative to one another. Identical reference signs refer to identical or corresponding elements and structures.

[0033] FIG. 1A Elements of an edge-emitting semiconductor laser according to an embodiment are illustrated.

[0034] FIG. 1B Components of an edge-emitting semiconductor laser according to a further embodiment are illustrated.

[0035] FIG. 1C A configuration of a semiconductor laser according to an embodiment is illustrated, wherein emission takes place via a main surface.

[0036] FIG. 1DA configuration of a semiconductor laser according to another embodiment is described, in which emission is made via a main surface.

[0037] FIG. 2A Wavelength characteristics of reflectivity of a resonator according to an embodiment are described.

[0038] FIG. 2B Wavelength characteristics of reflectivity of a resonator according to another embodiment are described.

[0039] FIG. 3A Combined reflection characteristics of a resonator according to another embodiment are described.

[0040] FIG. 3B Reflection characteristics of a resonator according to another embodiment are described.

[0041] FIG. 3C Reflection characteristics of a resonator according to another embodiment are described.

[0042] FIG. 3D Temperature dependence of emission wavelength in a semiconductor laser according to an embodiment is shown.

[0043] FIG. 4A Reflection characteristics of a resonator according to another embodiment are described.

[0044] FIG. 4B A cross-sectional view of a semiconductor laser according to another embodiment is shown.

[0045] FIG. 4C A cross-sectional view of a semiconductor laser according to another embodiment is shown.

[0046] FIG. 5A Reflection characteristics of a resonator according to another embodiment are described.

[0047] FIG. 5B Reflection characteristics of a resonator according to another embodiment are described.

[0048] FIG. 5C Combined reflection characteristics of a resonator according to another embodiment are described.

[0049] FIG. 6A Different characteristics of a first resonator are described.

[0050] FIG. 6B Loss and gain of a laser at different temperatures when using a resonator with approximately constant reflectivity is described.

[0051] FIG. 6C Loss and gain of a laser at different temperatures when using a resonator with a sharp drop in reflectivity is described.

[0052] FIG. 6D Loss and gain of the laser at different temperatures and current intensities are illustrated.

[0053] FIG. 6E Loss and gain of the laser at different temperatures and current intensities are illustrated.

[0054] FIG. 6F Loss and gain of the laser at different temperatures and current intensities are illustrated.

[0055] FIG. 6G Reflection properties of a resonator mirror according to other embodiments are illustrated.

[0056] FIG. 6H Reflection properties of a resonator mirror according to other embodiments are illustrated.

[0057] FIG. 6I Combinations of reflection properties of a resonator mirror according to other embodiments are illustrated.

[0058] FIG. 6K Combinations of reflection properties of a resonator mirror according to other embodiments are illustrated.

[0059] FIG. 7A A cross-sectional view of a semiconductor laser according to an embodiment is shown.

[0060] FIG. 7B Reflectivity of a second resonator mirror according to an embodiment is shown.

[0061] FIG. 7C Reflectivity of a first resonator mirror according to an embodiment is shown.

[0062] FIG. 7D Reflectivity of a first resonator mirror according to an embodiment is shown.

[0063] FIG. 8A Carrier density in the active region at the laser threshold of different semiconductor lasers according to an embodiment is illustrated.

[0064] FIG. 8B Optical gain of different semiconductor lasers according to an embodiment is illustrated.

[0065] FIG. 8C Differential gain of different semiconductor lasers according to an embodiment is illustrated.

[0066] FIG. 9A Emission wavelength as a function of temperature of different semiconductor lasers according to an embodiment is illustrated.

[0067] FIG. 9B Reflectivity as a function of temperature of different semiconductor lasers according to an embodiment is illustrated.

[0068] FIG. 9C The power of different semiconductor lasers as a function of temperature is illustrated according to embodiments.

[0069] FIG. 9D The difference in the temperature dependence of the emission wavelength of different semiconductor lasers is shown.

[0070] FIG. 9E The temperature dependent emission intensity of different semiconductor lasers is shown.

[0071] FIG. 9F Details of the active region are shown.

[0072] FIG. 10A A lidar system according to embodiments is shown.

[0073] FIG. 10B A laser system according to embodiments is illustrated.

[0074] In the following detailed description, reference is made to the accompanying drawings, which form a part hereof, and in which are shown by way of illustration specific embodiments. In this regard, directional terminology, such as "top," "bottom," "front," "back," "leading," "trailing," etc., is used with reference to the orientation of the Figure(s) being described. Because components of embodiments can be positioned in a number of orientations, the directional terminology is used for purposes of illustration only and is in no way limiting.

[0075] The description of embodiments is not intended to be exhaustive or to be limited to the precise form disclosed. Embodiments can be practiced with modification and alteration, and there are many embodiments within the scope of the claims. Particularly, elements of the embodiments described below can be combined or eliminated, and the

[0076] The lasers described herein are based on semiconductor materials. In general, the expressions "wafer," "semiconductor" or "semiconductor material" used in the following description can include any semiconductor-based structure having a semiconductor surface. Wafer and structure are to be understood as encompassing doped and undoped semiconductors, epitaxial layers of semiconductor on a base semiconductor, as well as other semiconductor structures. For example, layers made of a first semiconductor material can be grown on a growth substrate made of a second semiconductor material, such as a GaAs substrate, a GaN substrate, or a Si substrate, or made of an insulating material, such as a sapphire substrate.

[0077] Depending on the purpose of use, the semiconductor can be based on direct or indirect semiconductor materials. For example, semiconductor materials particularly suitable for generating electromagnetic radiation include, inter alia, nitride semiconductor compounds by means of which, for example, ultraviolet, blue or longer-wavelength light can be generated, such as GaN, InGaN, AIN, AlGaN, AlGaInN, AlGaInBN; phosphide semiconductor compounds by means of which, for example, green or longer-wavelength light can be generated, such as GaAsP, AlGaInP, GaP, AlGaP; and other semiconductor materials, such as GaAs, AlGaAs, InGaAs, AlInGaAs, AlGaAsP, InGaAsP, SiC, ZnSe, ZnO, Ga2O3, diamond, hexagonal BN and combinations of the aforementioned materials. The stoichiometric ratios of the compound semiconductor materials can vary. Other examples of semiconductor materials can include silicon, silicon germanium and germanium. In the context of the present specification, the term "semiconductor" can also encompass organic semiconductor materials.

[0078] The term "substrate" generally includes insulating, conductive or semiconductive substrates.

[0079] The terms "lateral" and "horizontal" as used in the present specification shall describe an orientation or direction which extends substantially parallel to a first surface of a semiconductor body or a substrate. This can be, for example, a surface of a wafer or a die.

[0080] The horizontal direction can be, for example, located in a plane which is perpendicular to the growth direction at the time of layer growth.

[0081] The term "vertical" as used in the present specification shall describe an orientation which extends substantially perpendicular to a first surface of a semiconductor body or a substrate. The vertical direction can be, for example, corresponding to the growth direction at the time of layer growth.

[0082] FIG. 1AComponents of a semiconductor laser 10 according to an embodiment are shown. The semiconductor laser 10 comprises a semiconductor layer arrangement 112. The semiconductor layer arrangement 112 has an active region 115 for generating radiation. The semiconductor laser 10 further has a first resonator mirror 125, a second resonator mirror 130 and a resonator 131 arranged between the first resonator mirror 125 and the second resonator mirror 130. The resonator 131 extends in a direction parallel to a main surface 111 of the semiconductor layer arrangement 112. As will be explained later on, the first resonator mirror 125 has a wavelength dependent reflectivity. The wavelength dependent reflectivity will be explained in more detail later on. For example, the second resonator mirror 130 can have a lower reflectivity than the first resonator mirror 125. The second resonator mirror 130 can likewise have a wavelength dependent reflectivity. The generated laser radiation 135 can be coupled out via the second resonator mirror 130. Alternatively, the first resonator mirror 125 can have a lower reflectivity than the second resonator mirror 130. For example, the generated laser radiation 135 can also be coupled out via the first resonator mirror 125.

[0083] The semiconductor layer arrangement 112 comprises a first semiconductor layer 110 of a first conductivity type, for example p-type, and a second semiconductor layer 120 of a second conductivity type, for example n-type. Further semiconductor layers can be arranged between the substrate 100 and the first semiconductor layer 110. For example, electrical contact elements can additionally be provided in order to apply a voltage to the semiconductor laser. Furthermore, additional semiconductor layers and / or insulating layers can be arranged above the second semiconductor layer 120.

[0084] The substrate 100 can be insulating or can have a semiconductor material. For example, the semiconductor layers can be epitaxially grown above the substrate 100. The active region 115 can be arranged between the first semiconductor layer 110 and the second semiconductor layer 120. The active region 115 can for example have a pn-junction, a double heterostructure, a single quantum well structure (SQW) or a multiple quantum well structure (MQW) to generate radiation. The expression "quantum well structure" does not have any meaning in terms of quantization dimension here. It thus includes quantum wells, quantum wires and quantum dots as well as any combination of these layers.

[0085] The active region 115 is provided by a layer or a sequence of layers, the main surface of which extends perpendicular to the extension direction of the optical resonator 131. Typically, such a semiconductor laser is commonly referred to as edge emitting semiconductor laser. However, as will be explained later on with reference to FIG. 5A to FIG. 7C It is feasible in this configuration to emit electromagnetic radiation via the main surface 111 of the semiconductor layer arrangement 112.

[0086] In FIG. 1BIn the shown arrangement, the three laser elements 1271, 1272, 1273 are arranged on top of each other. Each of the individual laser elements has a first semiconductor layer 110, a second semiconductor layer 120 and an active region 115. Here, the first semiconductor layers can again be of a first conductivity type, while the second semiconductor layers 120 are of a second conductivity type, respectively. The individual laser elements 1271, 1272 and 1273 are connected to each other via connection layers 1281, 1282, respectively. The connection layers 1281, 1282 can be tunnel junctions, respectively, such that an electrical connection is achieved. It goes without saying that more than 3 laser elements 1271, 1272 and 1273 can also be stacked.

[0087] Generally, in the context of the present application, the term "tunnel junction" comprises a sequence of very highly doped semiconductor layers of a first conductivity type and a second conductivity type. For example, a first highly doped layer of the first conductivity type can be arranged in abutment and contact with the first semiconductor layer 110 of the first conductivity type of the laser elements 1271, 1272, 1273. Further, a second highly doped layer of the second conductivity type can be arranged in abutment and contact with the second semiconductor layer 120 of the second conductivity type of the laser elements 1271, 1272, 1273. Optionally, an intermediate layer can be additionally arranged between the highly doped layers. The sequence of very highly doped layers of the first conductivity type and the second conductivity type and optionally the intermediate layer is a tunnel diode. Using the tunnel diode, the individual laser elements 1271, 1272 and 1273 can be connected in series.

[0088] As in FIG. 1A here, a first resonator mirror 125 and a second resonator mirror 130 are also provided. Similar to FIG. 1A the first resonator mirror 125 has a specific wavelength-dependent reflectivity. The generated electromagnetic radiation 135 can be coupled out via the second resonator mirror 130. According to other embodiments, the second resonator mirror 130 can also have a larger reflectivity than the first resonator mirror. For example, the generated electromagnetic radiation 135 can be coupled out via the first resonator mirror 125. According to other embodiments, the second resonator mirror 130 can additionally have a specific wavelength-dependent reflectivity.

[0089] The designs described in the context of the present application can also be applied to the following semiconductor laser, wherein at least a part of the optical resonator 131 extends in a direction parallel to the main surface 111 of the semiconductor layer arrangement 112, but wherein the emission radiation 135 is via the first main surface 111 of the semiconductor layer arrangement 112.

[0090] FIG. 1C A surface emitting semiconductor laser with an optical resonator 131 extending parallel to the main surface 111 is shown according to an embodiment. As FIG. 1CAs shown, the end face of the semiconductor layer arrangement 112 is etched at an angle of 45°. The dielectric layer 138 can be applied directly adjacent to the inclined etched side wall. According to other embodiments, the dielectric carrier substrate 100 can also be directly adjacent to the inclined etched side wall. Due to the difference in refractive index between the semiconductor material of the semiconductor layer arrangement 112 and the dielectric layer 138, a total reflection occurs at the side wall, respectively. As a result, the inclined side wall of the semiconductor layer arrangement 112 acts as a reflective side wall 137.

[0091] The first resonator mirror 125 and the second resonator mirror 130 are arranged above the first main surface 111 of the semiconductor layer arrangement 112, respectively.

[0092] The generated laser light is reflected into the semiconductor layer arrangement having the active region 115 by the first resonator mirror 125 and the second resonator mirror 130, respectively. In FIG. 1C In particular, the plurality of laser elements 1271, 1272 and 1273 are stacked on top of each other and connected to each other via the connection layers 1281, 1282. The connection layers can also be tunnel junctions. The structure of the carrier substrate 100 corresponds to the shape of the semiconductor layer arrangement. For example, the second resonator mirror 130 has a lower reflectivity than the first resonator mirror 125. In a corresponding manner, the generated electromagnetic radiation 135 is coupled out at the position of the second resonator mirror 130 via the first main surface 111.

[0093] For example, the materials of the first semiconductor layer 110 and the second semiconductor layer 120 and the active region can be based on the AlGaAs or GaAs material system and comprise AlGaAs or GaAs semiconductor layers. In this case, the connection layers 1281, 1282 can have a tunnel junction comprising highly doped AlGaAs:Te / C. For example, the tunnel junction can comprise AlGaAs doped with tellurium or carbon, or GaAs doped with tellurium or carbon. The layer thickness of the tunnel junction can be, for example, less than λ / 2, where λ corresponds to the effective wavelength in the respective propagation medium.

[0094] According to FIG. 1D According to the described embodiments, the first resonator mirror 125 can also have a lower reflectivity than the second resonator mirror 130. The generated electromagnetic radiation 135 can be emitted at the position of the first resonator mirror 125 via the first main surface 111. FIG. 1D The other components of the semiconductor laser in FIG. 1C The described components are similar or identical to the components described with reference to

[0095] The semiconductor laser described within the scope of the present disclosure can generally be based on, for example, the AlGaAs, InAlGaAsP or InAlGaN material system.

[0096] Generally, in a semiconductor laser diode carrier pairs are injected into the active region by an electrical current flow. With increasing current the carrier density in the active region increases, which in turn increases the maximum of the spectrum of the optical intensity of the laser light circulating in the resonator of the laser diode. When the modal optical gain in the maximum of the gain spectrum (Γ*g peak ) compensates the internal losses α i and the reflection losses α m , the laser threshold is reached: Γ*g peak,th = α i + α m .

[0097] The associated injection current is the threshold current. At a current above the laser threshold, at a constant temperature of the active region, the carrier density remains approximately constant there due to the strong stimulated recombination. Of course, the temperature of the laser diode increases at an increasing ambient temperature and also the loss power of the laser increases at an increasing current. Generally, the optical gain decreases with temperature at a given carrier density, so that the laser threshold (threshold current, threshold carrier density) increases with increasing temperature.

[0098] The increase in temperature and the increase in carrier density generally reduce the bandgap of the semiconductor, due to the enhanced interaction of the carriers and the change in the lattice constant of the semiconductor crystal. The spectral position of the gain maximum, in turn the emission wavelength, is thereby shifted towards longer wavelengths. The extent to which the emission wavelength shifts with temperature is here related to the emission wavelength or the photon energy. For example, the following relationship applies for the change in bandgap or emission wavelength with temperature: dEg / dT = -0.4 meV / K. Thereby via E = hc / λ one obtains dλ / dT = -λ 2 / hc*dEg / dT.

[0099] The material system is selected in correspondence with the emission wavelength to be achieved. Correspondingly, the correlation between the shift of the emission wavelength with temperature and the material system used is obtained. For emission wavelengths in the range of 900 to 1000 nm, it is approximately 0.25 to 0.32 nm / K. AlGaInAs material systems are generally used for such emission wavelengths. For wavelengths of around 400 nm, the extent of the shift of the emission wavelength is 0.05 nm / K. InAlGaN material systems are generally used for such emission wavelengths. For wavelengths of around 1250 nm, the extent of the shift of the emission wavelength is 0.5 nm / K. InAlGaAsP material systems are generally used for this wavelength.

[0100] FIG. 2A The wavelength-dependent reflectivity R1 of the first resonator mirror 125 is schematically illustrated. As FIG. 2AAs shown, the reflectivity R1 decreases from a value R0 within a wavelength range from the target wavelength λ0 to λ0 + Δλ. Here, λ0 represents the target wavelength of the semiconductor laser, and Δλ is a measure of the change in wavelength with respect to the observed temperature fluctuations. For example, if the target wavelength of the semiconductor laser is at the emission wavelength at T1 (e.g., 20 °C) and the semiconductor laser emits laser light of a constant wavelength within a temperature range from T1 (20 °C) to T2 (e.g., 60 °C), then λ0 corresponds to the wavelength at T1 (20 °C). Δλ corresponds to the difference between the emission wavelength at T2 (60 °C) and the emission wavelength at T1 (20 °C). Additionally, at least one of the following two relations applies to the reflectivity R1(λ):

[0101] (iii) For λ0 < λ < λ0 + Δλ, R1(λ) < 0.3*R0;

[0102] (iv) For λ = λ0, dRl / dλ < k / nm, where k ≤ -0.1%.

[0103] According to other embodiments, k can be less than or equal to -0.2% or less than or equal to -0.5%.

[0104] For example, in FIG. 2A the relation (iv) is implemented, that is, the reflectivity R1 drops sharply at λ0.

[0105] For example, for wavelengths less than λ0, the reflectivity can be approximately constant within a range near λ0. According to an embodiment, the reflectivity R0 = R1(λ0) can be a local maximum. According to other embodiments, R0 = R1(λ0) can also be a saddle point or can be any point on the trend of the reflectivity R1, for example, located in the downward edge of the reflectivity R1.

[0106] According to an embodiment, the first resonant mirror can also have FIG. 2B the reflectivity as shown. For example, the wavelength-dependent reflectivity can first increase and then decrease. More precisely, the wavelength-dependent reflectivity (different from that shown in FIG. 2A ) is not constant up to the wavelength λ0, but it first increases to a maximum value R0. The wavelength λ at which the reflectivity R1 has a local maximum corresponds to the target wavelength λ0 of the laser. For wavelengths greater than λ0, the reflectivity of the resonant mirror decreases from its local maximum R max down.

[0107] As will be explained in more detail later, it is not necessary for the reflectivity of the resonant mirror to decrease strictly monotonically. For example, the emission wavelength of the laser moves according to the emission wavelength at a rate of 0.3 nm / K with an increase in temperature. This is illustrated in FIG. 2A by an arrow labeled v0. The target wavelength, that is, the wavelength to be achieved by the laser, is in FIG. 2Aand 2B Let λ0 represent this. Ideally, this wavelength corresponds to the wavelength at which the reflectivity R1 has a bend. For example, R0 can be greater than 10%. For example, R0 can be greater than 30% or 40%.

[0108] Through FIG. 2A In cases where reflectivity decreases at longer wavelengths, the corresponding modes are suppressed for electromagnetic radiation with longer wavelengths, resulting in maximum enhancement of the mode at λ0. This ensures the temperature stability of the laser.

[0109] In a similar manner, according to FIG. 2B For wavelengths less than λ0, reflectivity increases with wavelength. In this reflectivity design, the laser mode with the highest reflectivity is preferred, resulting in a stable emission wavelength. Because... FIG. 2B The specific characteristics shown indicate that the emission wavelength can be stabilized despite temperature increases and wavelength drift with temperature.

[0110] like FIG. 2A and FIG. 2B As shown, the reflectivity of the first mirror can decrease linearly from its maximum value R0 with a certain slope. According to other embodiments, the reflectivity can also decrease non-linearly. This is especially true in… FIG. 3C and FIG. 3D This will be discussed in more detail below. In this way, a limit is introduced to reflectivity, which raises the threshold for the formation of laser radiation at that wavelength.

[0111] According to the implementation method, FIG. 2A and FIG. 2B The reflectivity shown can be the reflectivity of the first resonant mirror 125. According to other embodiments, FIG. 2A and FIG. 2B The reflectivity shown can also be a combination of reflectivity and the product of the reflectivity R1 of the first resonant mirror 125 and the reflectivity R2 of the second resonant mirror 130.

[0112] FIG. 3A Other embodiments are described, in which, for example, the first resonant mirror has an increasing reflectivity R1 up to a first local maximum, and the second resonant mirror has an initially constant reflectivity that subsequently decreases. FIG. 3A The left side of the diagram above illustrates the trend of the wavelength-dependent reflectivity R1 of the first resonant mirror 125. The reflectivity initially increases, but does not reach its maximum value at λ3. It reaches its maximum value at λ4. For wavelengths greater than λ4, the reflectivity remains constant.

[0113] FIG. 3AThe lower part shows the trend of the wavelength-dependent reflectivity R2 of the second resonant mirror. Here, the reflectivity is initially constant and decreases at wavelengths between λ3 and λ4. The reflectivity is designed such that for both resonant mirrors, the ranges between λ3 and λ4 overlap, within which the first and second reflectivities each have local maxima.

[0114] As a result, for the entire system FIG. 3A The reflectivity is shown in the right-hand portion. The reflectivity R of the combination of the first and second resonant mirrors is created such that the maximum value of reflectivity R0 exists at the target wavelength λ0. (Refer to...) FIG. 2B A similar method is used to stabilize the wavelength of the semiconductor laser. The reflectivity R of the combination of the first resonant mirror and the second resonant mirror corresponds to the product of the reflectivity R1 of the first resonant mirror 125 and the second reflectivity R2 of the second resonant mirror 130. The first resonant mirror 125 or the second resonant mirror 130 can be a coupling output mirror, respectively.

[0115] FIG. 3B An example of the reflectivity of the first resonant mirror 125 in a semiconductor laser 10 according to another embodiment is shown. It can be seen that the reflectivity drops sharply from a local maximum of 910 to 925 nm to a minimum of 940 to 950 nm. In this way, the emission wavelength of the semiconductor laser is stabilized in the range of 910 to 925 nm. The reflectivity within the considered range is less than 1%. Accordingly, the first resonant mirror 125 here is a coupling output mirror. (The text continues...) FIG. 3B As can be seen, for example, 1000 nm or 850 nm, the reflectivity of the first resonant mirror is greater than that in the wavelength range of 910 nm to 925 nm. Of course, since the maximum gain is within the wavelength range of 900 to 950 nm in the considered temperature range, the larger reflectivity outside this wavelength range does not affect the emission wavelength of the semiconductor laser. The relationship between wavelength-dependent gain and reflectivity will be described in more detail later.

[0116] FIG. 3CThe reflection behavior of the resonator mirrors according to other embodiments is illustrated. Here, the illustrated reflection behavior can be the reflectivity of the first resonator mirror or also the combined reflectivity of the two resonator mirrors within the resonator. For example, the reflectivity can be the product of the reflectivities of the two resonator mirrors, i.e. R = R1 or R = R1 * R2. It applies at the target wavelength that the derivative of the reflectivity R with respect to the wavelength is less than 0, i.e. for wavelengths greater than the target wavelength, the reflectivity at least temporarily decreases. In particular, the reflectivity falls in the range from the target wavelength λ0to the wavelength λ0+ Δλ. For example, it applies that R(λ0) > R(λ0+ Δλ). Here, Δλ is related to the material system used. For example, for a GaAs material system, Δλ can be 20 nm or 30 nm. As discussed previously, it is not necessary for the reflectivity to strictly monotonically decrease.

[0117] For example, the gradient of the slope of the tangent in the target wavelength range, i.e. dR / dλ(λ0)<-0.03 / nm, can be greater than 3%. It can apply according to other embodiments that:

[0118] dR / dλ(λ0)<-0.05 / nm or dR / dλ(λ0)<-0.07 / nm.

[0119] Thus, the gradient can be greater than 5% or even greater than 7%.

[0120] In addition, the course of the reflectivity in the range from λ0to λ0+ Δλ lies below one of the following two courses of R.

[0121] a) for λ0< λ < λ0+ Δλ, R(λ) < 0.3 * R0;

[0122] b) for λ = λ0, dR / dλ < -g * R(λ) / nm

[0123] with g > 0.03, for example g > 0.05 or g > 0.07.

[0124] For example, in FIG. 3C the combined reflectivity R = R1 * R2 or R = R1 can be considered and R0can be greater than 8%, for example greater than or equal to 10%.

[0125] In FIG. 3C these upper limits a), b) for the reflectivity are illustrated for different values of g as an example of the course of the wavelength-dependent reflectivity.

[0126] In general, a very strong gradient at the target wavelength has a very positive effect on the temperature stability. A reflectivity course that approximates a jump or step function as closely as possible at the point λ = λ0leads to a higher temperature stability of the emission wavelength. In this way, the reflection and propagation of light with a wavelength greater than λ0is strongly suppressed. As a result, the wavelength can be stabilized particularly effectively.

[0127] As indicated, the considered wavelength range λ0+ Δλ around the target wavelength is related to the target wavelength. More precisely, for each material system, as a material parameter, the temperature-dependent wavelength shift serves as a basis for the amplification in order to obtain a maximum wavelength shift of the emission wavelength in case of a supposed temperature difference, for example in case of a temperature difference of 80 K. From this, Δλ can be derived.

[0128] For a laser diode emitting, for example, in the red to near-infrared range, a wavelength shift of approximately 0.3 nm / K is taken as a basis in edge-emitting semiconductor lasers. Accordingly, a temperature change of 80 K causes a wavelength shift of 24 nm, so that Δλ lies in the range of 20 to 30 nm. For example, the laser diode can be based on an AlGaInAs material system.

[0129] For a laser diode emitting in the green or blue wavelength range, a wavelength shift of 0.05 nm / K is taken as a basis in edge-emitting semiconductor lasers. Accordingly, a temperature shift of 80 K causes a wavelength shift of 4 nm, where Δλ can lie in the range of 5 to 10 nm. For example, the laser diode can be based on an InGaN material system.

[0130] For a laser diode emitting in the near-infrared to mid-infrared range, for example up to approximately 1550 nm, a wavelength shift of 0.5 nm / K is taken as a basis in edge-emitting semiconductor lasers. Accordingly, a temperature shift of 80 K causes a wavelength shift of 40 nm, where Δλ can lie in the range of 40 to 50 nm. For example, the laser diode can be based on an InP material system.

[0131] FIG. 3D The course of the temperature-dependent emission wavelength is shown. As shown here, an especially large value of g, i.e. an especially strong gradient at the point λ = λ0, leads to a smaller fluctuation of the emission wavelength with temperature.

[0132] According to an embodiment, the mirror having a wavelength-dependent reflectivity can be implemented by a Bragg mirror having a plurality of thin dielectric or semiconductor layers.

[0133] Generally, the term "Bragg mirror" comprises any device that reflects incident electromagnetic radiation and has dielectric or semiconductor layers. The Bragg mirror can consist of dielectric or semiconductor layers. According to other embodiments, the Bragg mirror can have additional metal layers. For example, the Bragg mirror can be composed of a sequence of very thin dielectric or semiconductor layers with different refractive indices, respectively. A mixture of dielectric and semiconductor layers is also possible. For example, the layers can alternately have a high refractive index (e.g. n > 1.7) and a low refractive index (e.g. n < 1.7). For example, the layer thickness of a layer pair can be λ / 4, where λ denotes the wavelength of the light to be reflected in the respective medium. The layer that is first seen from the incident light can have a larger layer thickness, e.g. 3λ / 4. Due to the low layer thickness and the difference in the respective refractive indices, the Bragg mirror provides a wavelength-dependent high reflectivity. The Bragg mirror can have, for example, 2 to 50 dielectric or semiconductor layers. The typical layer thickness of the individual layers can be approximately 30 to 90 nm, for example approximately 50 nm. The layer stack can also contain one or two or more layers with a thickness of more than approximately 180 nm, for example more than 200 nm.

[0134] According to other embodiments, the layers of the Bragg mirror can also be realized by epitaxial semiconductor layers and possibly with additional metal layers.

[0135] The materials of the resonant mirror with wavelength-dependent reflectivity can contain, for example, Al2O3, TaO, SiO, TiO, AlN, SiN, Si, NbO, ITO, ZnO, Ag and Al, and as semiconductors InAlGaAsP, InAlGaN, ZnSe materials. Here, the sequence, composition and layer thickness of the respective layers are selected in order to achieve the desired reflectivity.

[0136] According to embodiments, the second resonant mirror 130 can also be realized as a Bragg mirror. The composition, sequence and layer thickness of the individual layers of the second resonant mirror can be selected such that the second resonant mirror also has a wavelength-dependent reflectivity. According to other embodiments, the composition, sequence and layer thickness of the individual layers of the second resonant mirror can also be selected such that the second resonant mirror does not have a wavelength-dependent reflectivity.

[0137] As described above, the improved temperature stability of the emission wavelength can be achieved by a specific design of the reflectivity of the first and possibly second resonant mirror. For example, it can be meaningful for the reflectivity to steeply drop in a specific wavelength range.

[0138] FIG. 4AA schematic diagram of the course of the combined reflectivity R is shown, which can correspond to the product of the reflectivities of the first resonator mirror 125 and the second resonator mirror 130. In the case of an assumed substantially constant reflectivity of the second resonator mirror, the combined reflectivity R can also correspond to the reflectivity of the first resonator mirror. It can be seen that the reflectivity increases sharply at the wavelength λ5 and falls sharply at the wavelength λ6. The strong decrease in the reflectivity at the wavelength λ6 can be caused, for example, by the first resonator mirror being designed as a Bragg mirror accordingly.

[0139] According to an embodiment, the first resonator mirror 125 can be configured as a Bragg mirror and have at least one epitaxially grown semiconductor layer.

[0140] The semiconductor layer can be suitable for absorbing electromagnetic radiation having a wavelength range that is smaller than the target wavelength λ0 of the semiconductor laser. The semiconductor layer can thus be an absorption layer 129. In this way, a steep edge of the reflectivity can be achieved on the side of the smaller wavelengths.

[0141] FIG. 4B A schematic cross-sectional view of a semiconductor laser 10 with horizontal resonators is shown. In the semiconductor laser 10, the generated electromagnetic radiation 135 is emitted via the first main surface 111 of the semiconductor layer arrangement. It can be seen that the first resonator mirror 125 is configured as a Bragg mirror, wherein the Bragg mirror comprises at least one absorption layer 129 having an epitaxially grown semiconductor layer. The first resonator mirror 125 can comprise a plurality of absorption layers 129. The absorption layers can comprise the same or each a different semiconductor material. The second resonator mirror 130 can be configured in the same way or differently. The generated electromagnetic radiation 135 can be coupled out via the first or the second resonator mirror 125, 130.

[0142] FIG. 4C An example of a semiconductor laser 10 is shown, in which the generated electromagnetic radiation 135 is emitted via the side surface 116 of the semiconductor layer arrangement 112. In this case, the first and possibly the second resonator mirror 125, 130 can also be configured as a Bragg mirror. The first and possibly the second resonator mirror 125, 130 can additionally comprise an absorption layer 129 having an epitaxially grown semiconductor layer. The first resonator mirror 125 can comprise a plurality of absorption layers 129. The absorption layers can comprise the same or each a different semiconductor material. The second resonator mirror 130 can be configured in the same way or differently. The generated electromagnetic radiation 135 can be coupled out via the first or the second resonator mirror 125, 130.

[0143] In the described embodiments, the semiconductor material of the absorption layer 129 can comprise, for example, a III-V semiconductor material, for example an AlGalnP or AlInGaAs material system. By adjusting the composition ratio, the band gap can be adjusted to adjust the wavelength range of the absorption. According to other embodiments, the absorption layer 129 can also comprise GaSb. The absorption layer can be configured as a quantum well layer, for example as a single or multiple quantum well layer. According to other embodiments, the absorption layer 129 can also comprise a bulk semiconductor material. For example, every second layer of the Bragg mirror can comprise a semiconductor material and be implemented as an absorption layer 129. For example, the Bragg mirror layer arranged on the semiconductor layer arrangement 112 side can be the absorption layer 129. According to the described embodiments, the semiconductor layer can be doped or undoped. According to embodiments, the Bragg mirror can comprise doped and undoped semiconductor layers. For example, by doping the semiconductor layer, the refractive index of the semiconductor layer can be further changed, so that the refractive index difference of the semiconductor layer can be adjusted according to the characteristics to be implemented of the Bragg mirror. According to embodiments, the layer thickness of the absorption layer can correspond to approximately λ0 / 4, wherein λ0may correspond to the target wavelength of the semiconductor laser. According to other embodiments, the layer thickness of the absorption layer can lie in the range of (λ0- Δλ) / 4 to (λ0+ Δλ) / 4, wherein Δλmay correspond to a change in the emission wavelength in the following temperature range.

[0144] According to other embodiments, the first resonator mirror 125 can be configured as a Bragg mirror with a specific reflection behavior without an absorption layer 129. Furthermore, the second resonator mirror 130 can be configured as a Bragg mirror and comprise an absorption layer 129. In this case, the following effect can be obtained: The second resonator mirror 130 likewise exhibits a wavelength-dependent reflectivity. However, due to the presence of the absorption layer 129, the portion of the radiation that is not reflected is absorbed by the second resonator mirror 130 and, for example, not in the housing of the semiconductor laser.

[0145] FIG. 5A An example of the wavelength-dependent reflectivity of a resonator mirror is shown. As can be seen, there is a wavelength range 161 in which the reflectivity is large, so that a laser operation of the associated semiconductor laser is possible. The resonator mirror is designed as a Bragg mirror with 9 Bragg pairs and a dielectric material with a refractive index of 3.5 and 3.0 for the adjacent layers.

[0146] FIG. 5B An example of the reflectivity of a resonator mirror is shown, which is configured as a Bragg mirror and has an absorption layer 129 as described above. If now the FIG. 5A resonator mirror and FIG. 5B the resonator mirror combination shown in Fig. 6 in a semiconductor laser, the following reflectivity is obtained FIG. 5C is shown.

[0147] As can be seen, electromagnetic radiation with a wavelength smaller than λ3is absorbed. As a result, a reflectivity with a relatively small wavelength range is obtained, in which the reflectivity is sufficient for laser operation. As a result, the emission wavelength of the semiconductor laser can be limited particularly effectively.

[0148] As a result, a narrow and very well-defined wavelength-dependent reflectivity can be achieved. Thereby, laser operation can be achieved in a predetermined wavelength range.

[0149] As described, it is possible to adjust the semiconductor laser stably to a target wavelength by means of the resonator mirror with the wavelength-dependent reflectivity.

[0150] As will be explained below, different semiconductor lasers with the same first resonator reflectivity result in different results, which can be attributed to different interactions between the gain spectrum and the reflectivity.

[0151] Generally, at the laser threshold with an emission wavelength of λ lase it applies that:

[0152] Γ(λ lase ,T)*g(λ lase ,T)(th) = α i (α lase ,T) + α m (λ lase ,T)

[0153] Here, g(th) denotes the material gain at the laser threshold, Γ denotes the confinement factor, α i denotes the intrinsic losses and α m is the reflection loss, wherein

[0154] The loss curve can also be temperature-dependent. However, since this variation is generally relatively low, it is neglected for the explanation below.

[0155] FIG. 6A In curve (1) the reflectivity of a resonator mirror is illustrated, which is only slightly wavelength-dependent. Curve (2) shows the reflectivity of a resonator mirror which decreases strongly with increasing wavelength.

[0156] FIG. 6B The loss of a laser with the reflectivity indicated with (1) in FIG. 6A is illustrated with a solid line. Since this reflectivity only slightly decreases with increasing wavelength, the loss only slightly increases with increasing wavelength.

[0157] The product Γ * g(λ, T) is shown for different temperatures and almost constant current strength with dashed lines. With increasing temperature, the gain curve together with the gain peak moves towards larger wavelengths. At the laser threshold, the product Γ * g(λ, T) takes the value α i + α m .

[0158] FIG. 6B The curves (1) to (3) in Fig. 4 show the product Γ * g(λ, T) for increasing temperatures, respectively. In the shown curves, the laser condition is reached for different wavelengths, respectively. As can be seen, the emission wavelength changes strongly with increasing temperature.

[0159] FIG. 6C The loss of the laser with the reflectivity indicated with (2) in Fig. 2 is illustrated with solid lines. Since this reflectivity drops sharply with increasing wavelength, the loss increases sharply with increasing wavelength. FIG. 6A

[0160] The product Γ * g(λ, T) is shown for different temperatures and current, i.e. carrier density, with dashed lines. With increasing temperature, the gain curve together with the gain peak moves towards larger wavelengths. At the laser threshold, the product Γ * g(λ, T) takes the value α i + α m . When the dashed curves illustrating the product Γ * g(λ, T) touch the loss curve α i + α m , the laser threshold is met.

[0161] FIG. 6C The curves (1) to (3) in Fig. 4 show the product Γ * g(λ, T) for increasing temperatures, respectively. In the shown curves, the laser condition is reached for different wavelengths, respectively. However, since the loss increases sharply with increasing wavelength, the laser threshold has already been reached at a shorter wavelength than the wavelength at which the gain peak exists. Accordingly, the emission wavelength moves less strongly with increasing temperature than shown in Fig. 3. FIG. 6B

[0162] FIG. 6D The loss of the laser is illustrated with solid lines. The loss has a minimum in the wavelength range from λ6to λ7. This loss can be attributed to the corresponding maximum of the reflectivity of the first and second resonator mirrors. The product Γ * g(λ, T) is output for different temperatures and current, i.e. carrier density, with dashed lines. With increasing temperature, the gain curve together with the gain peak moves towards larger wavelengths. At the laser threshold, the product Γ * g(λ, T) takes the value α i + α m .

[0163] FIG. 6D ​​Curves (1) to (5) in the figure show the product Γ*g(λ,T) for increasing temperature and current, respectively. FIG. 6D In curve (1), the laser threshold has not yet been reached at temperature T1, and in curve (2), the laser threshold has not yet been reached at temperature T2. In curve (3), laser emission occurs at wavelength λ0 = λ7 at temperature T3. If the temperature continues to rise to T4, emission will still occur at wavelength λ0 = λ7, as shown by curve (4), due to the specific shape of the path of Γ*g(λ,T4). In particular, as long as the loss curve (α) remains constant... i +α m The emission wavelength remains stable as the gain curve Γ*g(λ,T) rises more strongly than that of the gain curve. As a result, the threshold condition is not forced to stabilize at the wavelength of the gain peak, but rather at λ0. According to curve (5), the emission wavelength also remains stable at temperature T5.

[0164] A wider gain spectrum means it is further away from the gain branch and reaches the threshold condition earlier, either near or at the target wavelength (regardless of temperature). Conversely, a narrower gain spectrum results in a higher gain peak at lower carrier densities and reaches the threshold at lower current intensities. However, this also results in a narrower amplification spectrum for the grain.

[0165] This is FIG. 6E The following explanation is provided. It is assumed here that the semiconductor laser has a first resonant mirror with the same reflectivity as the resonant mirrors of the semiconductor laser, and the characteristics of this first resonant mirror have been referenced. FIG. 6D Discussions were held. For example, for... FIG. 6D The first resonant mirror and used for FIG. 6E The first resonant mirror is the same. With FIG. 6D Unlike the situation discussed earlier, semiconductor lasers here have a narrower gain spectrum.

[0166] FIG. 6E Curves (1) to (5) in the figure also show the product Γ*g(λ,T) for increasing temperature and current, respectively. FIG. 6E In curve (1), the laser threshold has not yet been reached at temperature T1, and in curve (2), it has not yet been reached at temperature T2. In curve (3), at temperature T3, laser emission occurs at wavelength λ0 = λ7. If the temperature continues to rise to T4, then, as shown by curve (4), the specific shape for the path of Γ*g(λ,T4) now occurs at wavelength λ > λ7. Due to the loss curve (α i +α mThe gain curve Γ*g(λ,T) of )(λ,T) does not rise as strongly, so the emission wavelength shifts towards longer wavelengths. As a result, the threshold condition shifts towards the wavelength of the gain branch, i.e., towards the direction of maximum gain. According to curve (5), at temperature T5, laser emission occurs at significantly longer wavelengths. Due to the narrower gain bandwidth, wavelength stabilization only occurs at wavelengths greater than λ,T. FIG. 6D Interventions can be carried out within a smaller temperature range.

[0167] Therefore, regardless of the loss curve, the emission wavelength remains near the peak gain wavelength. As a result, the emission wavelength varies more strongly with temperature.

[0168] For comparison, FIG. 6F It shows where α i +α m The curves shown by solid lines are less steep at the edges, representing the different reflectivity behavior of the first resonant mirror. The loss has a minimum in the wavelength range of λ6 to λ7. This loss can be attributed to the corresponding maximum reflectivity of the first and second resonant mirrors. Curves (1) to (3) show the product Γ*g(λ,T) with respect to each increase in temperature and current using dashed lines. As temperature increases, Γ*g(λ,T) shifts towards longer wavelengths. FIG. 6F In curve (1), the laser threshold has not yet been reached at temperature T1. In curve (2), the laser threshold is reached at temperature T2, and the product Γ*g(λ,T) takes the value α. i +α m Since Γ*g(λ,T) shifts further into the long-wavelength range with subsequent temperatures and the slope of the gain curve is greater than that of the loss α, i +α m The slope is steeper, so the emission wavelength increases with increasing temperature.

[0169] Therefore, according to the implementation, the slope of the loss curve can be considered in addition to the gain curve in order to stabilize the laser wavelength by utilizing temperature changes.

[0170] According to the implementation, in a semiconductor laser, the sum of the inherent loss of the semiconductor laser and the reflection loss caused by the first and second resonant mirrors can be increased for wavelength λ in the range λ0 < λ < λ0 + Δλ.

[0171] For example, the sum of the inherent loss and reflection loss of a semiconductor laser can rise at least half the slope of the following curve, which corresponds to the product of the limiting factor Γ(λ,T) and the wavelength-dependent gain g(λ,T).

[0172] According to other embodiments, the sum of the intrinsic losses and the reflection losses of the semiconductor laser can rise with a higher slope than, for example, the curve corresponding to the product of the confinement factor Γ(λ, T) and the wavelength-dependent gain g(λ, T).

[0173] As explained above with reference to FIG. 6D , 6E and 6F, in this case, the emission wavelength can be stabilized over a relatively wide temperature range, for example, over a temperature range of -40°C to 140°C.

[0174] For example, the relatively wide gain spectrum and thus also the product of the confinement factor Γ(λ, T) and the wavelength-dependent gain g(λ, T) can also rise less steeply by the active region having a smaller number of quantum well structures. A wider gain spectrum can also be achieved by a shorter resonator length, for example, a resonator length of less than 1.5 mm, low reflectivity of the second and / or first resonator mirror, and by a high carrier density at the laser threshold. As a result, for example, a wide gain spectrum can be achieved at or above the laser threshold.

[0175] Examples for adjusting the reflectivity of the first and possibly second resonator mirror are explained below.

[0176] FIG. 6G The course of the reflectivity R of, for example, the first resonator mirror 125 is shown, or as the product of the reflectivities of the first and second resonator mirrors of the semiconductor laser according to the embodiments. As can be seen, here the reflectivity is also at a relatively high level over a wide wavelength range. For example, R0 can lie in the range of 0.05 to 0.15. The width, i.e., the difference between, for example, λ7 and λ8, for example, can correspond to the width of the target wavelength spectrum.

[0177] FIG. 6H The course of the reflectivity R of, for example, the first resonator mirror 125 is shown, or as the product of the reflectivities of the first and second resonator mirrors of the semiconductor laser according to the other embodiments. As can be recognized, here the reflectivity is also at a relatively high level over a narrow wavelength range. For example, the range of R0 can lie in the range of 0.05 to 0.15. The width, i.e., the difference between, for example, λ7 and λ8, can correspond to the width of the target wavelength spectrum.

[0178] In FIG. 6G and 6H , for wavelengths smaller than λ7 or larger than λ8, the reflectivity is relatively low, for example, smaller than 0.01 or larger than 0.005, in the range in which the laser gain is still greater than 0.

[0179] For example, in FIG. 6GThe shown reflectivity course can be achieved by a suitable combination of the reflectivities of the first and second resonator mirrors.

[0180] This is illustrated in FIG. 6I In FIG. 6I , the reflectivity R1 of the first resonator mirror is shown in the upper part, and the reflectivity R2 of the second resonator mirror is shown in the lower part. R1 and R2 initially rise steeply to a maximum at a wavelength λ7, and fall at λ8. In the wavelength range smaller than λ7 and larger than λ8, local maxima having a non-zero, however low, reflectivity occur. This reflectivity is set at different wavelength ranges for R1 and R2, and the reflectivity falls to 0 in the range between the local maxima. If the combined reflectivity is now formed as the product of R1 and R2, the value of this reflectivity is 0 for wavelengths smaller than λ7 and larger than λ8. At λ7, the combined reflectivity increases to a higher value than at R1 or R2. As FIG. 6G is shown in at λ8, the combined reflectivity falls here to zero.

[0181] FIG. 6H For example, FIG. 6K the shown reflectivity course can be achieved by a suitable combination of the reflectivities of the first and second resonator mirrors, as is illustrated in

[0182] FIG. 6K The left-hand part shows the reflectivity R1 of the first resonator mirror, and the right-hand part shows the reflectivity R2 of the second resonator mirror. R1 initially begins to increase at a wavelength λ7. In this range, R2 is at a maximum constant level. As a result, the combined reflectivity increases in this range. The maximum of the reflectivity R1 and also the maximum of the combined reflectivity are at a wavelength λ9. From the wavelength λ9, the reflectivity R1 stretches at a maximum constant level and falls again at λ8. From the wavelength λ9, the reflectivity R2 of the second resonator mirror falls to the value 0 at λ8. As a result, the combined reflectivity falls from λ9 and reaches the value zero at λ8.

[0183] According to other embodiments, as explained with reference to Figures 4 to FIG. 5C a steep course of the reflectivity can be achieved by incorporating an absorption layer in the Bragg mirror.

[0184] FIG. 7AA schematic cross-sectional view of a semiconductor laser according to an embodiment is shown, wherein the first resonator mirror 125 with wavelength dependent reflectivity has a lower reflectivity than the second resonator mirror 130. The first resonator mirror 125 is thus the outcoupling mirror for coupling out the generated electromagnetic radiation 135. The semiconductor laser 10 has a first semiconductor layer 110 of a first conductivity type, e.g. p-conducting, and a second semiconductor layer 120 of a second conductivity type, e.g. n-conducting. The semiconductor laser further comprises an active region 15 arranged between the first and the second semiconductor layer. The semiconductor laser 10 is based on an AlGaInAs / GaAs material system. For example, the active region can comprise a plurality, e.g. 3, laser elements arranged on top of each other, which have a double quantum well structure through which a wavelength of e.g. 905 nm can be emitted. An optical resonator 131 is arranged between the first resonator mirror 125 and the second resonator mirror 130. The optical resonator 131 extends in a direction parallel to the first main surface 111 of the semiconductor layer arrangement 112. For example, the length of the optical resonator 131 can be less than 1.5 mm, e.g. less than 1200 pm, e.g. about 600 pm.

[0185] The first contact element 117 is arranged in electrical contact with the first semiconductor layer 110. The second contact element 118 is arranged in electrical contact with the second semiconductor layer 120.

[0186] The first resonator mirror 125 has a reflectivity which is explained in more detail below with reference to FIG. 7C the structure and the reflectivity.

[0187] The embodiments described herein are based on the consideration that in case the low reflectivity of the first resonator mirror 125 is less than 10%, e.g. less than 1%, a blue shift of the emission wavelength can occur. This blue shift can be attributed to an increase of the carrier density at the laser threshold with increasing temperature. This can be attributed to a decrease of the gain with increasing temperature. As a result, the gain maximum moves to shorter wavelengths with increasing temperature. According to the embodiments described herein, this effect is combined with a suitably adjusted reflective behavior of the wavelength specific character, such that the emission wavelength of the semiconductor laser is stabilized.

[0188] For example, the second resonator mirror 130 can have a reflectivity of more than 96% in the considered wavelength range. For example, the second resonator mirror 130 can be configured as a Bragg mirror and comprise more than 2 layer pairs, e.g. 3 layer pairs. For example, the layers of the second resonator mirror 130 can comprise a suitable dielectric material and silicon.

[0189] FIG. 7B The wavelength dependent reflectivity of the second resonator mirror 130 is shown. As can be seen, the reflectivity of about 96% first increases with increasing wavelength and then remains at a value of about 98.5%. This value is reached at about 905 nm.

[0190] FIG. 7C The wavelength-dependent reflectivity is shown for different mirrors. The reflectivity shown with (1) drops linearly from about 12% at about 880 nm to about 7.3% at 960 nm. The reflectivity shown with (2) remains approximately constant over the considered wavelength range of 880 nm to 960 nm, about 1%. For example, a mirror showing the reflectance behavior shown with (2) can have multiple pairs of AlO / TaO layers.

[0191] The reflectivity shown with (3) drops from a value of about 5% at 880 nm to a value of 1% at 905 nm and remains below 1% over the wavelength range up to 960 nm. A first characteristic value for characterizing the reflectivity is here the so-called base point λ F , at which the reflectivity drops to 1%. In the reflectivity of 1%, the effect of the wavelength blue shift can be identified unambiguously, as this is discussed later. The base point λ F3 of curve (3) is here 905 nm. Another characteristic value is the negative slope, which is reached for example at a reflectivity of 1% to 2%, i.e. Δ3= ΔR% / Δλ3~ 0.11% / nm. The first resonator mirror having the reflectivity shown in curve (3) is configured as a Bragg mirror having a suitable combination of multiple thin layers such that this specific wavelength-dependent reflectivity is obtained.

[0192] The reflectivity shown with (4) drops from a value of about 9.8% at 880 nm to a value of 1% at 920 nm and remains below 1% over the wavelength range up to 960 nm. Thus, the base point λ F4 of curve (4) is here 920 nm. The negative slope of the reflectivity of 1% to 2% is Δ4= ΔR% / Δλ4~ 0.19% / nm. The first resonator mirror having the reflectivity shown in curve (4) is configured as a Bragg mirror having a suitable combination of multiple thin layers such that this specific wavelength-dependent reflectivity is obtained.

[0193] The reflectivity shown with (5) drops from a value of about 11.3% at 890 nm to a value of 1% at 935 nm and remains below 1% over the wavelength range up to 960 nm. The base point λ F5 of curve (5) is here 935 nm. The negative slope of the reflectivity of 1% to 2% is Δ5= ΔR% / Δλ5~ 0.16% / nm. The first resonator mirror having the reflectivity shown in curve (5) is configured as a Bragg mirror having a suitable combination of multiple thin layers such that this specific wavelength-dependent reflectivity is obtained.

[0194] Here, the reflectivity of the first resonator in the target wavelength range λ0applies that: for λ = λ0, dR / dλ < k / nm, with k < -0.1 %. In contrast to e.g. the behavior shown in FIG. 2A or 2B, here the reflectivity at λ = λ0does not drop from a local maximum, but for wavelengths smaller than λ0the reflectivity has already dropped significantly. This distinct different behavior of the reflectivity with wavelength can be attributed to the observation that the reflectivity in a range of less than 12 % is observed. In a semiconductor laser with a first resonator having a reflectivity in this range, the wavelength stabilization is achieved via effects different from the implementation of the behavior shown e.g. in the reflectivity of the first resonator FIG. 2A and 2B Here, the reflectivity of the first resonator in the target wavelength range λ0applies that: for λ = λ0, dR / dλ < k / nm, with k < -0.1 %. In contrast to e.g. the behavior shown in

[0195] Generally, e.g. the reflectivity of the first resonator 125 in the target wavelength range of the semiconductor laser 10 is less than 10 %, according to other embodiments less than 5 %.

[0196] In the following, further characteristics of the semiconductor laser 10 having the structure shown in FIG. 7A are explained, wherein the first resonator 125 has the different structures discussed with respect to FIG. 7C respectively.

[0197] FIG. 7D The reflectivity of the first resonator 125 is illustrated over a larger wavelength range of 850 nm to 1000 nm, the first resonator having the different structures as discussed above, respectively. It can be seen that for larger wavelengths the reflectivity can increase again. Of course, the reflectivity should stretch such that a sufficiently wide "cut-off range" of the first resonator is ensured. For example, the wavelength at which the reflectivity becomes > 1 % can be chosen according to the gain bandwidth of the active region such that e.g. this wavelength lies in a range having a very low gain. Thereby, it can be avoided that lasing is induced in this higher wavelength range. In FIG. 7D the reflectivity is numbered with respect to the resonator whose reflectivity is shown in FIG. 7C .

[0198] FIG. 8A For the semiconductor laser having the structure shown in FIG. 7A the temperature dependent calculated carrier density at the laser threshold Nthis shown, wherein the first resonator has the characteristics and associated structures described with respect to FIG. 7C respectively. As can be seen, the semiconductor laser having the structure as shown in FIG. 7CThe first resonator mirror 125 with the reflectivity explained in the middle curve (1) has the lowest carrier density Nth at the laser threshold and has the lowest correlation of the carrier density Nth with the temperature. This can be attributed to the low reflection losses which occur due to the high and wavelength-independent reflectivity of the first resonator mirror. As a result, a low threshold gain occurs which leads to a relatively low carrier density. In general, the charge carrier density Nth at the laser threshold increases with increasing temperature. This can be attributed to the gain decreasing with increasing temperature.

[0199] In the first resonator mirror with the reflectivity characterized by the curve (2), the carrier number at the laser threshold remains constantly at a relatively low value. Here, in the range from -40°C to +40°C, the carrier number at the laser threshold is greater than in the first resonator mirrors with the reflectivity shown in the curves (3) to (5).

[0200] In the first resonator mirror with the reflectivity characterized by the curve (3), the reflectivity at a wavelength of 905 nm has already reached a relatively low reflectivity of 1%. Here, the carrier density Nth at the laser threshold is up to 50°C identical to in the curve (2), but higher than in the curves (1), (4) and (5). In the temperature range greater than about 50°C, the carrier density at the laser threshold is higher than in the curve (2) and also higher than in the curves (1), (4) and (5).

[0201] As a whole from the comparison of the curves (1) to (5) it follows that the carrier density Nth at the laser threshold correlates more strongly with the temperature according to the curves (2) to (5) than according to the curve (1). Accordingly, for the first resonator mirrors with the structures which produce the reflectivity according to the curves (2) to (5) in the range from -40°C to +40°C, the carrier density Nth at the laser threshold is greater than for the resonator according to the curve (1). FIG. 7C

[0202] In general, in the curves (3) to (5), the carrier density at the laser threshold increases by more than 25% in the temperature range from 0 to 60°C, for example by more than 30% or 35%.

[0203] Due to the increase in the carrier density Nth at the laser threshold with the temperature, a distinct blue shift occurs with the resonators according to the curves (2) to (5) compared to the resonator according to the curve (1).

[0204] It is generally sought that the carrier density Nth at the laser threshold is increased so much that the blue shift resulting therefrom compensates the red shift caused by the bandgap reduction so that the emission wavelength can be stabilized over a wide temperature range.

[0205] ​Here, the lower the differential gain dg / dN, the stronger the increase in the carrier density at the laser threshold, which is caused by the gain reduction due to the temperature increase. The spectrum gain maximum related to the carrier density approximately follows the empirical formula g = g0*ln(N / Ntr), where the gain coefficient is g0and the transparent carrier density is Ntr. The differential gain follows as dg / dN = g0 / N. This differential gain rapidly decreases with increasing carrier density. Thus, the change in the carrier density at the laser threshold with temperature rapidly increases with the initial threshold carrier density. In this context, the initial threshold carrier density denotes the threshold carrier density at a suitable reference temperature, for example room temperature or the lower limit of the temperature range considered, for example -40°C.

[0206] In conventional laser diodes, the gain and the differential gain are usually maximized as much as possible to achieve high efficiency. According to embodiments, the wavelength-stabilized laser diodes described here are designed such that they have an increased carrier density at the laser threshold compared to conventional laser diodes and thus only a small differential gain. The stabilization of the wavelength leads to an efficiency loss. The efficiency loss can be limited by appropriate design and optimization.

[0207] According to embodiments, for example, the active region can only have a single quantum well structure. FIG. 8B A graph showing the optical gain related to the carrier density is shown for semiconductor lasers with a single quantum well structure (curve (1)), a double quantum well structure (curve (2)) and a triple quantum well structure (curve (3)).

[0208] FIG. 8C A graph showing the differential gain related to the carrier density is shown for semiconductor lasers with a single quantum well structure (curve (1)), a double quantum well structure (curve (2)) and a triple quantum well structure (curve (3)).

[0209] It can be seen that the differential gain decreases for a lower number of quantum films, i.e. for a single quantum well structure, compared to a multiple quantum well structure. Accordingly, a low number of quantum films or quantum well structures is advantageous for the wavelength stabilization.

[0210] In general, the optical gain can be increased by a higher number of quantum films, whereby the efficiency can be increased. In contrast, as described here, the temperature stability can be improved by using a single quantum well structure.

[0211] FIG. 9A Experimental data showing the emission wavelength of a pulsed laser related to the heat sink temperature of the pulsed laser in a temperature range from -40°C to 120°C are shown. The layer structure and the active region of the semiconductor laser are configured such that the emission wavelength is 905 nm, for example at 20°C. As FIG. 1BAs shown, the semiconductor laser has a plurality of laser elements stacked on top of each other. The active region of the semiconductor laser has, for example, a multi-quantum well structure. For example, as shown in Fig. 6, three laser elements each having a double-quantum well structure are stacked on top of each other. A current intensity of 40 A is fed into the semiconductor laser, for example, by applying a respective voltage to the first and second contact elements 117, 118. FIG. 1B As shown, three laser elements each having a double-quantum well structure are stacked on top of each other. A current intensity of 40 A is fed into the semiconductor laser, for example, by applying a respective voltage to the first and second contact elements 117, 118. FIG. 9A Five different curves are shown, in which a semiconductor laser having the reference FIG. 7C characteristics and layer structure discussed above is used.

[0212] Curve (1) shows the wavelength of a semiconductor laser having a first resonator mirror 125 whose reflectivity linearly decreases from a reflectivity of 12% at 880 nm to a reflectivity of about 7.2% at 960 nm. The emission wavelength of this semiconductor laser almost linearly increases from a value of about 892 nm at -40°C to a value of about 935 nm at 120°C. In the ranges 1 and 2 from -40°C to 60°C, this corresponds to an expected redshift of about 0.27 nm / K due to the decreasing bandgap with temperature. In the range 3 from 80°C to 120°C, this effect is masked to a small extent by a blueshift. The blueshift results from an increasing carrier density Nth at the laser threshold, as is also shown in Fig. 8. FIG. 7C Curve (2) shows the wavelength of a semiconductor laser having a first resonator mirror 125 whose reflectivity is approximately constant at 1% in the range from 880 to 960 nm. The emission wavelength of this semiconductor laser continuously increases from a value of about 888 nm at -40°C to a value of about 913 nm at 100°C. In the range 1 from -40°C to 0°C, this increase corresponds to a redshift with about 0.27 nm / K. This redshift results from the decreasing bandgap with increasing temperature. In the range 2 from 20 to 60°C, this redshift is compensated to a small extent by a blueshift. The blueshift results from an increasing carrier density Nth at the laser threshold, as is also shown in Fig. 8. In the range 3 from 80°C to 120°C, the wavelength decreases slightly due to a blueshift of the amplification (gain). The blueshift results from an increasing carrier density Nth at the laser threshold.

[0213] Curve (3) shows the wavelength of a semiconductor laser having a first resonator mirror 125 whose reflectivity is approximately constant at 1% in the range from 880 to 960 nm. The emission wavelength of this semiconductor laser continuously increases from a value of about 888 nm at -40°C to a value of about 913 nm at 100°C. In the range 1 from -40°C to 0°C, this increase corresponds to a redshift with about 0.27 nm / K. This redshift results from the decreasing bandgap with increasing temperature. In the range 2 from 20 to 60°C, this redshift is compensated to a small extent by a blueshift. The blueshift results from an increasing carrier density Nth at the laser threshold, as is also shown in Fig. 8. In the range 3 from 80°C to 120°C, the wavelength decreases slightly due to a blueshift of the amplification (gain). The blueshift results from an increasing carrier density Nth at the laser threshold.

[0214] Curve (3) shows the wavelength of a semiconductor laser having a first resonator mirror 125 whose reflectivity is approximately constant at 1% in the range from 880 to 960 nm. The emission wavelength of this semiconductor laser continuously increases from a value of about 888 nm at -40°C to a value of about 913 nm at 100°C. In the range 1 from -40°C to 0°C, this increase corresponds to a redshift with about 0.27 nm / K. This redshift results from the decreasing bandgap with increasing temperature. In the range 2 from 20 to 60°C, this redshift is compensated to a small extent by a blueshift. The blueshift results from an increasing carrier density Nth at the laser threshold, as is also shown in Fig. 8. In the range 3 from 80°C to 120°C, the wavelength decreases slightly due to a blueshift of the amplification (gain). The blueshift results from an increasing carrier density Nth at the laser threshold. FIG. 7CThe emission wavelength of the semiconductor laser increases only slightly in a range 1 from -40°C to 0°C, wherein the increase approximately corresponds to 0.1 nm / K. The increase results from the steep negative edge of the reflectivity in the low wavelength range below 905 nm. In a range 2 from 20 to 60°C, a redshift reduction results due to the specific reflectivity of the mirror. In range 2, the maximum emission wavelength has been reached. From a temperature of 60°C and in a range 3 from 80 to 120°C, the wavelength decreases significantly due to a blueshift of the gain. The blueshift results in turn from an increase of the carrier density Nth at the laser threshold.

[0215] Curve (4) shows the wavelength of a semiconductor laser with a first resonator mirror 125 whose reflectivity behaves as explained in FIG. 7C The emission wavelength of the semiconductor laser first increases in a range 1 from -40°C to 0°C, wherein the increase corresponds to a redshift of approximately 0.27 nm / K. The redshift results in turn from a decrease of the bandgap with increasing temperature. In a range 2 from 20 to 60°C, a reduced redshift with 0.067 nm / K results due to the specific reflectivity of the first resonator mirror. In a range 3 from 80 to 120°C, the wavelength decreases due to a blueshift of the gain. The blueshift results from an increased carrier density Nth at the laser threshold.

[0216] Curve (5) shows the wavelength of a semiconductor laser with a first resonator mirror 125 whose reflectivity behaves as explained in FIG. 7C The emission wavelength of the semiconductor laser first increases in a range 1 from -40°C to 0°C, wherein the increase corresponds to a redshift of approximately 0.27 nm / K. The redshift results in turn from a decrease of the bandgap with increasing temperature. In a range 2 from 20 to 60°C, a reduced redshift with 0.067 nm / K results due to the specific reflectivity of the first resonator mirror. In a range 3 from 80 to 120°C, the wavelength decreases due to a blueshift of the gain. The blueshift results from an increased carrier density Nth at the laser threshold.

[0217] If it is based on the semiconductor laser that a stable wavelength should be emitted in a temperature range from 0 to 60°C, it can be identified that a semiconductor laser with a first resonator mirror 125 with a reflectivity according to curve (3), (4) or (5) results in good results, as shown in FIG. 9B .

[0218] FIG. 7C shows the reflectivity of the first resonator mirror in relation to the temperature. For a semiconductor laser with a reference FIG. 7CThe first resonator mirror of the discussed properties and layer structure yields the reflectivity. The reflectivity here is the reflectivity at the operating wavelength associated with the respective temperature.

[0219] As can be seen from the curve (1): In the resonator mirror 125, the reflectivity for the amplification redshifts slightly with increasing temperature, the reflectivity of which linearly decreases from 12% at 880 nm to about 7.2% at 960 nm.

[0220] Due to the resonator mirror structure according to curve (2) (reflectivity approximately constant with wavelength), its reflectivity is also approximately independent of temperature.

[0221] For the resonator mirrors according to curves (4) and (5), the reflectivity decreases in range 1 from -40 to 0°C. The wavelength approaches the respective base point of the mirror. In range 2 from 20 to 60°C, the reflectivity is constant. Here, the wavelength changes similarly to a mirror redshift. In range 3 from 80 to 120°C, the reflectivity increases due to a blue shift caused by a high carrier density Nth at the threshold of the laser.

[0222] For the resonator mirror according to curve (3), the reflectivity is approximately constant in ranges 1 and 2. Here, the following negative slope Δ3 discussed below can be identified. FIG. 9C A large negative slope Δ3 is discussed. Only a slight redshift occurs.

[0223] Since the resonator mirrors according to curves (3) to (5) have a reflectivity that increases in range 3 due to a blue shift, these resonator mirrors are suitable for achieving good optical efficiency data.

[0224] If a low sensitivity to optical feedback is desired, the high reflectivity of the resonator mirror according to curve (5) is advantageous.

[0225] FIG. 7C The optical output power of semiconductor lasers with different first resonator mirrors 125 is shown as explained with reference to FIG. 9D The output power is determined at 60°C and a uniform current.

[0226] As follows from the comparison: The optical output peak power is slightly lower in the use of the first resonator mirrors according to curves (3) to (5) relative to the resonator mirror according to curve (1). It also becomes apparent that the resonator mirror according to curve (2) with constant low reflectivity has the lowest efficiency. From this it follows that the resonator mirrors according to curves (3) to (5) can cause a wavelength stability of the semiconductor laser without any significant loss of efficiency.

[0227] FIG. 7C For semiconductor lasers with different first resonator mirrors 125, a graph of the emission wavelength as a function of temperature is shown. Here, the use of the first resonator mirrors according to curves (1) to (5) is explained with reference to FIG. 7CThe first resonator mirror 125 discussed and the layer structure. The change of the emission wavelength with temperature corresponds to the difference of the emission wavelength at 25°C and 80°C divided by the temperature at which the output power is 10 W.

[0228] The resonator mirror according to variant zero is a resonator mirror whose reflectivity is constant at 7%. FIG. 7C The resonator mirror according to variant one is a resonator mirror whose reflectivity is constant at 7%. The resonator mirror according to variant two is a resonator mirror whose reflectivity is constant at 5% and the resonator mirror according to variant three is a resonator mirror whose reflectivity is constant at 3%. The resonator mirror according to variant four is a resonator mirror whose reflectivity is shown in curve (2) in FIG. 7C The resonator mirror according to variant five is a resonator mirror whose reflectivity is shown in curve (5) in FIG. 7C The resonator mirror according to variant six is a resonator mirror whose reflectivity is shown in curve (4) in FIG. 7C The resonator mirror according to variant seven is a resonator mirror whose reflectivity is shown in curve (3) in FIG. 7C The resonator mirror according to variant seven is a resonator mirror whose reflectivity is shown in curve (3) in

[0229] As can be identified: The change of the emission wavelength with temperature according to variant one is highest at about 0.27 nm / K and decreases down to variant seven with a value around 0. The change of the emission wavelength according to variant one corresponds to the change of the bandgap with temperature.

[0230] As described above, an increased temperature stability of the emission wavelength can be achieved by using a first resonator mirror 125 with the shown properties. It is accordingly feasible to realize the first resonator mirror by a specific facet coating which is anyway part of the edge emitting semiconductor laser. As a result, an increased temperature stability can be achieved in a cost- and effort-neutral way.

[0231] Furthermore, when using a first resonator mirror with a steep drop towards longer wavelengths, for example according to as explained by curves (3) to (5) in FIG. 9E For example, the drop-off of the long wavelength thus becomes steeper and thereby the linewidth of the spectrum becomes smaller compared to the emission spectrum of a laser with a first resonator mirror reflectivity according to curve (1). For example, the bandwidth of the emission can be reduced by more than 15%. The effect is more pronounced at higher temperatures.

[0232] FIG. 7C The emission spectrum of a semiconductor laser with a first resonator mirror according to curve (3) in FIG. 7C is shown in the upper region at 20°C (solid line) and 85°C (dashed line). For comparison, the lower region shows the emission spectrum of a semiconductor laser with a first resonator mirror according to curve (1) in FIG. 9Ethe emission spectrum of the semiconductor laser of the first resonator mirror of curve (1) in the upper region at 20°C (solid line) and 85°C (dashed line). As can be seen, a slight wavelength shift towards lower wavelengths of the spectrum (< 5 nm) occurs in the upper region compared to in the lower region FIG. 9E FIG. 7A to FIG. 9E In addition, the half-value width (line width at 0.5 of the intensity) is reduced by more than 15%. For example, the half-value width of the spectrum at 85°C of the spectrum is 7 nm in the upper region and 8.5 nm in the lower region.

[0233] In some applications, the reduced bandwidth of the emission can cause still improved performance. For example, the semiconductor laser 10 can be used for optically writing a pump of a solid state laser. Here, for example, the pump efficiency can be increased when pumping a narrow absorption line of a Nd:YAG or Yb:YAG fiber laser. In a laser radar system with a source, a filter and a detector, a reduced bandwidth of the emission of the source can also be advantageous.

[0234] As described, due to the edge shape of the edge filter, the reflection loss rises rapidly at temperature, in turn wavelength, increase, and the threshold carrier density is strongly increased, which in turn causes a blue shift over the entire temperature range and a certain degree of self-compensation of the wavelength shift. Within this range, the effective reflectivity remains almost constant, and the emission wavelength follows to a certain extent the temperature-dependent spectral position of the reflectivity of the first resonator mirror 125 (about 0.07 nm / K). Thereby, the robustness and the manufacturability of the semiconductor laser are higher compared to solutions which depend on the specific resonator conditions, etc. In the use of laser diodes, it is often required that the emission wavelength does not leave a predefined range over a predefined temperature range (e.g. 0 to 85°C or -40°C to 120°C). With a corresponding design, the red-shift overcompensation effect additionally achieved by the described measures can even produce a blue shift at high temperatures, which makes the usable temperature range for wavelength stabilization particularly large. The wavelength change with temperature is positive at low temperatures, negative at high temperatures, and the emission wavelength has a maximum at medium temperatures. In addition to increasing the temperature range available for wavelength stabilization, the wavelength reduction at high temperatures produces an increase in the reflectivity of the first resonator mirror by the low-pass-like form of the wavelength-dependent reflectivity. Thereby, the efficiency of the laser is increased just at high temperatures compared to lasers with a flat characteristic of the reflectivity of the first resonator mirror. Wavelength stabilization via adjustment of the reflectivity of the first resonator mirror is achieved by modifying the facets with a lower reflectivity. As a result, it also works for lasers with an arbitrary number of laser elements 1271, 1272,... stacked on top of each other.

[0235] As described with reference to FIG. 9F ​As described, the reflectivity of the first resonator mirror 125 is wavelength dependent and less than 10% or less than 8% in the target wavelength range. For example, the reflectivity of the second resonator mirror 130 is greater than 95%. For example, according to embodiments, the reflectivity of the second resonator mirror 130 is not wavelength dependent or only to a small extent. In this way, the coupling out of the available optical output power at the second resonator mirror 130 is suppressed as much as possible. Furthermore, uncontrolled or harmful absorption of the optical output power in the housing can be avoided.

[0236] According to embodiments, the length of the optical resonator 131 can be less than 1 mm, for example less than 700 pm. In this way, the reflection losses can be increased compared to the length of the resonator 131 in which amplification occurs.

[0237] According to embodiments, further features can be implemented to increase the effect of the wavelength blueshift with increasing temperature. For example, the additional features can cause a differential gain that is small. Furthermore, features can be implemented to broaden the gain and thus reduce the gain maximum.

[0238] For example, the active region can have a multi-quantum well structure with mutually detuned quantum wells.

[0239] FIG. 7A An example of the course of the conduction band and the valence band in the active region of a semiconductor laser according to embodiments is shown, for example in the horizontal direction perpendicular to the extension direction of the optical resonator 131. For example, three quantum wells are contained in the associated active region. The composition and / or layer thickness of the individual layers are selected such that the energy difference between the respective energy levels in the conduction band and the valence band is respectively slightly different. For example, the transition energies E1, E2, E3 can be associated with the quantum wells shown respectively. Here, the transition energies E1, E2, E3 are respectively slightly different. For example, the respectively emitted wavelengths can differ by more than 5 nm, for example by more than 10 nm.

[0240] In this way, the wavelength-dependent gain can be broadened. Furthermore, the gain maximum can be reduced. As a result, the blueshift is increased. Accordingly, the temperature stability of the semiconductor laser can be further improved. Furthermore, the manufacture and yield of the semiconductor laser can be improved.

[0241] According to further embodiments, additional measures can be taken to broaden the optical losses alpha_i. Thereby, the carrier density Nth at the laser threshold can be further increased.

[0242] Even if the semiconductor laser according to FIG. 7B to 9F is described with respect to the features of the semiconductor laser according to FIG. 1B it is clear that the described features and designs can also be used for example in FIG. 10A ,1C and the semiconductor laser shown in 1D. Furthermore, the described design can also be used for semiconductor lasers of other material systems.

[0243] FIG. 10B A schematic setup of a lidar system 150 in which the described semiconductor laser 10 can be used is shown. The laser radiation of a typical pulse emitted by the semiconductor laser 10 is emitted, for example, by means of collimating optics 157 and a deflection / scanning unit 154. The object beam 153 is radiated onto an object 156 and reflected by it. Here, the reflected beam 155 is formed. The reflected beam 155 is conveyed to a detector 160 by means of receiving optics 152. The distance of the object 156 can be determined from the time difference between the sending of the laser pulse and the reception of the laser pulse.

[0244] For the semiconductor laser to be able to work with a stable wavelength even at variable temperatures, it is possible to use narrow-band detectors. For example, the detector can use a narrow wavelength window of less than 10 nm or less than 5 nm or even less than 1 nm. As a result, the influence of solar radiation can be reduced and the signal-to-noise ratio can be increased. As a result, less power out of the laser is required to measure the same distance. As a result, the performance of the system is improved overall. Furthermore, the laser can be operated with less power in order to perform the same measurement with an unchanged signal-to-noise ratio. As a result, the energy consumption is reduced. Furthermore, the described semiconductor laser can be manufactured in a simple manner.

[0245] ​ A laser system 164 with a semiconductor laser 10 as described above and a further laser 165, which can be optically pumped by the semiconductor laser 10, is shown. For example, the further laser 165 can be a solid-state laser or a fiber laser. For example, the laser can have a very narrow absorption spectrum. Since the semiconductor laser 10 is wavelength-temperature over a large temperature range depending on the embodiment as described above, it can be used to optically pump a laser having a very narrow absorption spectrum.

[0246] According to further embodiments, the described semiconductor laser can also be used for laser welding or laser cutting.

[0247] Although specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that any arrangement which is calculated to achieve the same purpose can be substituted for the specific embodiments shown and that it is the claims and their equivalents that are intended to cover all adaptations and modifications. This application is intended to cover any adaptations or variations of the specific embodiments discussed herein. Therefore, it is intended that this application be limited only by the claims and the equivalents thereof.

[0248] List of reference signs

[0249] 10 semiconductor laser

[0250] 100 carrier substrate

[0251] 110 first semiconductor layer

[0252] 111 first main surface

[0253] 112 semiconductor layer arrangement

[0254] 115 active region

[0255] 116 side surface

[0256] 117 first contact element

[0257] 118 second contact element

[0258] 120 second semiconductor layer

[0259] 125 first resonator mirror

[0260] 1271 first laser element

[0261] 1272 second laser element

[0262] 1273 third laser element

[0263] 1281 first connecting layer

[0264] 1282 second connecting layer

[0265] 129 absorbing layer

[0266] 130 second resonator mirror

[0267] 131 resonator

[0268] 135 emitted laser radiation

[0269] 137 reflective side wall

[0270] 138 dielectric layer

[0271] 140 absorbing element

[0272] 141 transparent insulating layer

[0273] 142 transparent dielectric layer

[0274] 143 absorbing layer

[0275] 144 first resonator mirror

[0276] 146 second resonator mirror

[0277] 150 lidar system

[0278] 151 beam splitter

[0279] 152 receiving optical device

[0280] 153 object beam

[0281] 154 deflection / scanning unit

[0282] 155 reflected beam

[0283] 156 object

[0284] 157 collimating optical device

[0285] 160 detector

[0286] 161 wavelength range with sufficient reflectivity

[0287] 164 laser system

[0288] 165 further laser

Claims

1. A semiconductor laser (10) having a semiconductor layer arrangement (112) with an active region (115) for generating radiation, a first resonator mirror (125), a second resonator mirror (130) and an optical resonator (131) arranged between the first resonator mirror (125) and the second resonator mirror (130), the optical resonator extending in a direction parallel to a main surface (111) of the semiconductor layer arrangement (112), wherein the reflectivity R1 of the first resonator mirror (125) is wavelength dependent such that R1 or a product R of R1 and the reflectivity R2 of the second resonator mirror (130) drops from a value R0 in a wavelength range from a target wavelength of the laser λ0 to λ0 + Δλ, wherein Δλ is chosen according to a temperature dependent shift of the emission wavelength and is less than 100 nm, and wherein for R at least one of the following relations applies: (i) for at least one wavelength λ, R(λ) < 0.3 * R0, with λ0 < λ < λ0 + Δλ; (ii) for λ = λ0, dR / dλ < k / nm with k < -0.1%, or for R1 at least one of the following relations applies: (iii) for at least one wavelength λ, R1(λ) < 0.3 * R0, with λ0 < λ < λ0 + Δλ; (iv) for λ = λ0, dR1 / dλ < k / nm, with k < -0.1%, and wherein R or R1 increases with increasing wavelength for wavelengths smaller than λ0 to a value R0.

2. The semiconductor laser (10) according to claim 1, wherein the second resonator mirror (130) has a wavelength dependent reflectivity which increases to a local maximum for wavelengths smaller than λ0.

3. The semiconductor laser (10) according to one of the preceding claims, wherein k < -0.2% applies.

4. The semiconductor laser (10) according to claim 1 or 2, wherein at least one of the following relations applies (v) for λ0 < λ < λ0 + Δλ, dR / dλ < -g * R(λ) / nm; (vi) for λ0 < λ < λ0 + Δλ, dR1 / dλ < -g * R1(λ) / nm, with g > 0.

03.

5. The semiconductor laser (10) according to claim 1, wherein the sum of the intrinsic loss of the semiconductor laser and the reflection loss caused by the first resonator mirror and the second resonator mirror increases for wavelengths λ in the range of λ0 < λ < λ0 + Δλ.

6. The semiconductor laser (10) according to claim 5, wherein the slope of the sum of the intrinsic loss of the semiconductor laser and the reflection loss caused by the first resonator mirror and the second resonator mirror for wavelengths λ in the range of λ0 < λ < λ0 + Δλ is at least half of a curve corresponding to the product of a confinement factor Γ(λ, T) and a wavelength dependent gain g(λ, T).

7. The semiconductor laser (10) according to claim 6, wherein a slope of the sum of the intrinsic loss of the semiconductor laser and the reflection loss caused by the first and second resonator mirrors for wavelengths λ in the range of λ0< λ < λ0+ Δλ is larger than a curve corresponding to the product of the confinement factor Γ(λ, T) and the wavelength dependent gain g(λ, T).

8. The semiconductor laser (10) according to claim 1 or 2, wherein the first resonator mirror (125) is implemented as a Bragg mirror.

9. The semiconductor laser (10) according to claim 8, wherein the Bragg mirror has epitaxially grown semiconductor layers.

10. The semiconductor laser (10) according to claim 9, wherein at least one of the semiconductor layers of the Bragg mirror is adapted to absorb electromagnetic radiation having a wavelength range smaller than λ0.

11. The semiconductor laser (10) according to claim 1 or 2, wherein the reflectivity R1 of the first resonator mirror or the product R is smaller than 8% in the range of the target wavelength.

12. The semiconductor laser (10) according to claim 1 or 2, wherein the active region (115) has only a single quantum well structure.

13. The semiconductor laser (10) according to claim 1 or 2, wherein the active region has a multiple quantum well structure with mutually detuned quantum wells.

14. The semiconductor laser (10) according to claim 1 or 2, wherein the carrier density of the laser threshold increases by more than 25% in a temperature range from 0°C to 60°C.

15. The semiconductor laser (10) according to claim 1 or 2, the emission wavelength of the semiconductor laser decreases with temperature at temperatures larger than 100°C.

16. The semiconductor laser (10) according to claim 15, the emission wavelength of the semiconductor laser decreases with temperature at temperatures larger than 60°C.

17. The semiconductor laser (10) according to claim 1 or 2, wherein the semiconductor layer arrangement (112) has a plurality of laser elements (1271, 1272, 1273) arranged on top of each other and connected to each other via a connection layer (1281, 1282).

18. The semiconductor laser (10) according to claim 1 or 2, wherein a lateral delimitation of the semiconductor layer arrangement is beveled such that the emission of the generated electromagnetic radiation (135) takes place via a first main surface (111) of the semiconductor layer arrangement (112).

19. A lidar system (150) having a semiconductor laser (10) according to claim 1.

20. A laser system (164) having a semiconductor laser (10) according to claim 1 and a further laser (165), wherein the further laser (165) is optically pumpable by the semiconductor laser (10).

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