Vertical emitting semiconductor laser device for optical data transmission having an outcoupling surface with a mode selective

Abstract

The invention relates to a vertical emitting semiconductor laser device (1), in particular a VCSEL, comprising an upper Bragg mirror (2) and a lower Bragg mirror (3), an active region (4) for generating laser radiation (5) being arranged between the upper Bragg mirror (2) and the lower Bragg mirror (3); wherein the vertical emission semiconductor laser device (1) has an upper outcoupling surface (20) for the laser radiation (5), the upper outcoupling surface (20) having both a mode-selective microstructure (21) and a polarization-selective microstructure (22). The invention also relates to the use of a surface relief on the VCSEL outcoupling surface, to a communication system, and to a method for producing a corresponding vertical emission semiconductor laser device.

Description

Technical Field

[0001] This invention relates to the field of optical communication technology, and in particular to vertical-emitting semiconductor laser devices, especially "vertical cavity surface-emitting lasers" (VCSELs) for optical data transmission. Background Technology

[0002] VCSELs used for optical data transmission are generally known in existing technology. VCSELs used for optical data transmission are also called data communication VCSELs (Datacom VCSELs). One advantage of VCSELs is their ability to provide the highest data transmission performance while consuming low power. In a VCSEL, or VCSEL (Vertical-Cavity Surface-Emitting Laser) laser diode, light is emitted perpendicular to the plane of the semiconductor chip, unlike edge-emitting laser diodes which emit light from one or two sides of the chip.

[0003] US10742000B2 relates to a VCSEL with an elliptical aperture that reduces RIN (random or relative intensity noise). According to the abstract, the VCSEL described therein should include an elliptical oxide aperture located in an oxide region between the active region and the emitter surface, wherein the elliptical aperture has a short radius and a long radius, the radius ratio (short radius) / (long radius) being between 0.6 and 0.8, and wherein the VCSEL has a relative intensity noise (RIN) of less than -140 dB / Hz. The VCSEL may also include an elliptical emitter aperture having the same dimensions as the elliptical oxide aperture.

[0004] Therefore, in order to reduce RIN, the prior art suggests setting an elliptical oxide aperture or oxide septum in the resonant cavity of the laser.

[0005] Against this backdrop, one object of the present invention is to provide a further improved or alternative vertical-emitting semiconductor laser device, particularly a VCSEL, for optical data transmission. The inventors recognize a particular desire to provide a vertical-emitting semiconductor laser device capable of achieving high-speed optical data transmission over a wide range of environmental conditions, especially over a wide temperature range. Furthermore, it is desirable to provide a vertical-emitting semiconductor laser device that can be manufactured at low cost or through simplified process steps and / or high yield. Additionally, it is desirable to improve the lifetime of data communication VCSELs with low RIN. Summary of the Invention

[0006] The subject matter to be protected is defined in the independent claim. Advantageous extensions are described in the dependent claims.

[0007] Therefore, according to a first aspect of the invention, a vertically emitting semiconductor laser device, particularly a VCSEL, is provided, having an upper Bragg mirror and a lower Bragg mirror, wherein an active region for generating laser radiation is arranged between the upper and lower Bragg mirrors; wherein the vertically emitting semiconductor laser device has an upper output coupling surface for laser radiation, wherein the upper output coupling surface has both a mode-selective macrostructure and a polarization-selective microstructure.

[0008] According to another aspect of the invention, it is proposed to use surface relief on the output coupling surface of a VCSEL in a communication system for optical data transmission, wherein the surface relief on the output coupling surface has a mode-selective macrostructure and a polarization-selective microstructure.

[0009] According to another aspect of the invention, a vertically emitting semiconductor laser device, particularly a VCSEL, is proposed, having an upper Bragg mirror and a lower Bragg mirror, wherein an active region for generating laser radiation is arranged between the upper and lower Bragg mirrors; wherein the vertically emitting semiconductor laser device has an upper output coupling surface for laser radiation, wherein the upper output coupling surface has a polarization-selective microstructure, and wherein the semiconductor laser device is configured such that, under high-frequency excitation at a frequency up to 25 GHz, particularly at a frequency of 25 GHz, its light intensity decreases by no more than 3 dB relative to the light intensity under 1 GHz excitation. Optionally, the upper output coupling surface may have a mode-selective macrostructure. It is understood that other features and configurations described in conjunction with the first aspect in this disclosure may also be combined with this aspect.

[0010] According to another aspect of the invention, a communication system is proposed, which has an input terminal for receiving data, a modulator, and a vertically emitting semiconductor laser device as described above and / or below.

[0011] According to another aspect of the invention, a method for manufacturing a vertically emitting semiconductor laser device is further proposed, wherein the method comprises the following steps: providing a semiconductor substrate; depositing a layer sequence for a lower Bragg mirror on the semiconductor substrate; depositing an active layer having an active region for generating laser radiation on the lower Bragg mirror; depositing a layer sequence for an upper Bragg mirror on the active layer; and providing an upper output coupling surface for laser radiation on the upper Bragg mirror, wherein the upper output coupling surface has both a mode-selective macrostructure and a polarization-selective microstructure.

[0012] According to another aspect of the invention, a method for manufacturing a vertically emitting semiconductor laser device is further proposed, wherein the method includes the following steps: providing a semiconductor substrate; depositing a layer sequence for a lower Bragg mirror on the semiconductor substrate; depositing an active layer having an active region for generating laser radiation on the lower Bragg mirror; depositing a layer sequence for an upper Bragg mirror on the active layer; providing an upper output coupling surface for laser radiation on the upper Bragg mirror, wherein the upper output coupling surface has a polarization-selective microstructure, and the semiconductor laser device is configured such that, under high-frequency excitation at a frequency up to 25 GHz, particularly under high-frequency excitation at a frequency of 25 GHz, its light intensity decreases by no more than 3 dB relative to the light intensity under 1 GHz excitation.

[0013] It is understood that additional layers may optionally be provided, and these layers may also be arranged between the layers mentioned above. For example, one or more optional current apertures between the active layer and the upper and / or lower Bragg reflectors.

[0014] The inventors recognized that VCSELs in data communication applications typically operate with relatively high current and high-speed signal modulation. In conventional VCSELs, the required output power is primarily achieved through multimode operation. During this process, competition and degeneracy can occur between multiple modes, which can lead to strong noise (relative intensity noise, RIN). The inventors also recognized that even with a reduced number of modes, similar noise contributions can arise due to unstable polarization operation. In this case, the polarization direction will periodically undergo a so-called "flip". Another issue with conventional solutions is that these solutions sometimes only achieve RIN improvement within a narrow range of operating parameters, rather than over a wide range of operating parameters, especially over a wide temperature range. Furthermore, solutions such as the elliptical current aperture VCSEL arranged at the active layer in a semiconductor layer sequence, as described in US10742000B2 mentioned at the beginning, can be susceptible to process variations during semiconductor manufacturing. Additionally, for the specific applications of data communication VCSELs, the current conduction within the semiconductor may vary over the desired wide current and temperature range.

[0015] According to one aspect of the invention, an optical solution is thus proposed for the upper output coupling surface of a vertically emitting semiconductor laser device with a special configuration, wherein the upper output coupling surface has both a mode-selective macroscopic structure and a polarization-selective microstructure. According to one aspect, the invention aims to reduce the number of competing modes while stabilizing polarization. This is achieved by using surface structures on the output coupling surface of the VCSEL, particularly surface reliefs, which have both spatial macroscopic and polarization-selective structures. In its simplest case, a data communication VCSEL with an elliptical edge relief structure with a polarization grating can be provided.

[0016] The coexistence of multiple modes in a VCSEL is a source of noise. Lateral macrostructures with typical VCSEL mode dimensions can favor or hinder individual modes, thereby reducing mode number and noise. For example, very small macrostructures (a few picometers) can be used to force fundamental mode operation, with a diameter smaller than that of higher-order modes. Similarly, elliptical cross-sections favor modes with stronger extension along their major semi-axis.

[0017] The proposed special configuration of the upper output coupling surface, with its mode-selective macrostructure and polarization-selective microstructure, offers the advantage of improved RIN (Radar Induction) through purely optical means. The inventors recognize that this solution achieves favorable noise behavior, particularly over a wide temperature range. It also prevents the VCSEL from operating in unstable polarization regions under specific operating conditions over a wide current range, while ensuring operation in the desired laser mode, especially at the high modulation frequencies required for data communication VCSELs.

[0018] Unlike the current aperture described in US10742000B2, the structure located above the upper Bragg mirror, or output coupler, has a purely optical function. The upper side of the upper Bragg mirror, or output coupler, can be referred to as the output coupling surface. The output-side DBR of a VCSEL typically terminates in air or a low-refractive-index medium at the antinode of the standing wave field. For example, increasing the layer thickness of the DBR by one-quarter of the wavelength in the medium significantly reduces the reflectivity of the DBR and makes laser action difficult. However, if this additional layer thickness is partially removed, total reflectivity is restored in regions such as etched-back areas. This is advantageous for modes with high intensity in these regions. The upper structure is easily accessible and therefore feasible in practice. It is understood that one or more layers of the upper Bragg mirror can also be considered part of the upper output coupling surface.

[0019] The terms macrostructure and microstructure describe a relative relationship, where a mode-selective macrostructure has a larger structural size than a polarization-selective microstructure. Alternatively, they can be referred to as a first mode-selective structure and a second polarization-selective structure of the output coupling surface. A mode-selective macrostructure of the output coupling surface is a structure configured to favor a finite number of laser modes, particularly where the mode-selective macrostructure is configured to favor a single laser mode. A polarization-selective microstructure is a structure configured to favor the polarization of laser radiation. For example, a polarization-selective microstructure can be constructed as a grating or an optical metastructure.

[0020] According to one aspect of the invention, the upper output coupling surface has a polarization-selective microstructure, and the semiconductor laser device is configured such that its light intensity decreases by no more than 3 dB under high-frequency excitation up to 25 GHz compared to that under 1 GHz excitation. An advantage may be that, due to the smaller decrease in light intensity under 25 GHz excitation compared to 1 GHz excitation, the preamplifier electronics can have a simpler structure, particularly eliminating or at least simplifying modulation frequency-selective gain matching.

[0021] All features of the subject matter described and claimed herein may be used individually or in combination, are compatible with each other, and are provided for extensions between them, provided that no logical contradiction arises, and are disclosed herein. In the following text, unless otherwise expressly referenced or logically contradictory, any reference to a subject or feature (including the indefinite articles “ein” and “eine” (one) and the definite articles “der”, “die”, “das” (the)), two subjects or two features, or other numbers of subjects or features, should be understood as not excluding the existence of more such subjects and features. Reference marks in the claims should not be construed as restrictive, but are used only to improve the readability of the claims.

[0022] In one configuration, the output coupling surface, possessing both mode-selective macrostructure and polarization-selective microstructure, can be a (purely) optical structure positioned above the upper Bragg mirror. One advantage of this configuration is the ease of access to the upper structure, making it practically feasible. Another advantage is the simple way different mode profiles and / or polarization states can be matched to customer requirements, as only the output surface needs adjustment, not the entire semiconductor layer structure. Existing semiconductor layer structures with an upper Bragg mirror, a lower Bragg mirror, and an active region in between can therefore be reused for different applications. This allows for simple and low-cost fabrication. The upper side of the upper Bragg mirror can be considered as the side positioned in the direction of laser beam radiation, i.e., the direction in which laser radiation is coupled out from the semiconductor laser device.

[0023] The output coupling surface can be constructed as an (optical) relief structure. The mode-selective macrostructure of the output coupling surface can have a first region with a first layer thickness. The polarization-selective microstructure of the output coupling surface can have a second region with a second layer thickness, particularly wherein the second layer thickness is less than the first layer thickness. The difference in layer thickness, or additional layer thickness, and etch-back can be performed in the semiconductor material of the VCSEL, such as in the topmost GaAs layer. Alternatively or additionally, a dielectric layer can be applied followed by structuring. Alternatively or additionally, a dielectric layer stack can also be applied, wherein one layer is selectively etched. Structuring can be performed by photolithography, such as by electron beam lithography, ultraviolet lithography, or nanoimprint lithography.

[0024] This mode-selective macrostructure can be configured to favor a limited number of laser modes. In particular, it can be configured to favor a single laser mode. Furthermore, it can be configured to match the lateral mode structure. One advantage of this configuration is that the emission behavior of the semiconductor laser device can be matched to the desired application. It is understood that the preferred mode does not necessarily have to be the fundamental mode used in "single-mode" VCSELs, but can be any higher-order mode.

[0025] The mode-selective macrostructure can have a first region with a first reflectivity and a second region with a second reflectivity different from the first reflectivity. The output coupling surface can be accordingly configured with a customized reflectivity to facilitate the desired mode.

[0026] The mode-selective macrostructure of the output coupling surface can have a relief with a non-rotationally symmetric profile. Specifically, it can have a relief with an oval profile, particularly an elliptical profile. This corresponding relief structure allows for a reduction in the number of competing modes, especially favoring modes whose geometry matches the non-rotationally symmetric profile, particularly the oval or elliptical relief profile. For example, a relief with an oval profile can have an inner oval region with a smaller layer thickness and an outer region surrounding it with a larger layer thickness. The same applies to other non-rotationally symmetric or elliptical profiles.

[0027] In one extended scheme, the mode-selective macrostructure of the output coupling surface can have a non-rotationally symmetric or oval relief with a contraction. In particular, the mode-selective macrostructure can have a figure-eight profile. This is a particularly advantageous example, where the oval profile with a contraction (especially a figure-eight profile) can also favor other modes besides or alternative to the fundamental mode, but not an uncontrolled large number of modes, but a finite number. Therefore, the mode profile can be advantageously matched to the desired application.

[0028] This mode-selective macrostructure can be constructed as non-rotationally symmetric in a plane parallel to the upper Bragg mirror layer. This mode-selective macrostructure can be configured to break mode degeneracy. One advantage of the configuration with a non-rotationally symmetric macrostructure is that it can counteract mode degeneracy. Another advantage of this configuration may be that it can further stabilize the laser's emission behavior, reducing unwanted transitions between different emission modes and thus improving noise behavior.

[0029] This polarization-selective microstructure can have a grating structure for polarizing the laser. In other words, a surface grating for polarizing the light can be arranged on the output surface. However, it does not exist alone, but in combination with a mode-selective macrostructure. The inventors recognized that the mode-selective macrostructure and the polarization-sensitive microstructure can advantageously work synergistically or produce a synergistic effect. In particular, this combination has proven to be especially advantageous for stability over a wide temperature range.

[0030] This polarization-selective microstructure can have structural dimensions smaller than the laser wavelength. For example, a grating structure with a grating spacing or grating period smaller than the laser wavelength can be configured. However, an optical superstructure with a structure smaller than the laser wavelength can also be configured. One advantage of this configuration may be that, given these structural dimensions of the polarization-selective microstructure, losses can be reduced by avoiding diffraction.

[0031] The mode-selective macrostructure and polarization-selective microstructure can be constructed together as an elliptical surface relief with a grating structure surrounded by the surface relief. For example, as an optical macrostructure, an elliptical recess can be provided on the output coupling surface, and this recessed region has a polarization-selective grating structure. It is understood that instead of a recess, a protrusion or other manner influencing the reflection behavior can also be provided. Generally, the upper output coupling surface can have a macrostructure that contains a fine structure. Here, the macrostructure spatially defines the transverse mode, and the fine structure sets the polarization. An example is an oval or circular structure with a diameter of, for example, 10 picometers, having a grating with a pitch of, for example, 150 nanometers.

[0032] The mode-selective macrostructure and polarization-selective microstructure can be formed by an optical superstructure on the output coupling surface, specifically, where the output coupling surface possesses an optical superstructure, i.e., a spatially varied phase-matched structure. The superstructure on the output coupling surface can substantially correspond to the mode image of the preferred mode. The corresponding optical superstructure can be obtained, for example, through numerical simulation.

[0033] This optical superstructure can be configured to provide a variable effective refractive index. Alternatively or additionally, the optical superstructure can have pillars of variable density, constant height, and elongation less than the wavelength. For example, polarization selection can be achieved through the elongated shape of a single pillar. Providing an optical superstructure with variable density but the same height in the VCSEL laser radiation emission direction may be particularly advantageous, as this can be achieved by structuring layers with a defined height. The layers to be structured can be provided during semiconductor manufacturing, or wafer fabrication. Structuring can be performed using known methods of semiconductor manufacturing. Therefore, efficient, low-cost, and high-quality manufacturing can be achieved. Another advantage of the superstructure over a relief with hard steps or embossed edges may be the avoidance of diffraction at the relief edges, which can lead to losses. Another advantage may be the ability to more precisely match the desired intensity profile of a specific mode, thereby achieving improved selectivity or stability.

[0034] This vertical-emission semiconductor laser device can have (at least) a current aperture (Stromapertur) whose diameter in a plane perpendicular to the laser beam radiation direction is larger than the diameter of the mode-selective macrostructure of the output coupling surface, which has a polarization-selective microstructure. In other words, the diameter measured in a plane parallel to the upper Bragg mirror layer can be perpendicular to the laser beam radiation direction. By making the current stromblende or current device larger than the relief structure or mode-selective macrostructure, the current density within the VCSEL can be reduced. The current can be distributed over a larger area within the semiconductor. In this case, mode selection is therefore not, or not solely, achieved through the current stromblende. More precisely, optical mode selection is achieved through a combination of the macrostructure of the output coupling surface and the polarization-selective microstructure of the output coupling surface. One advantage of this configuration may be that the definition of the current aperture, for example through an oxide stromblende, ion implantation, or a buried tunnel diode, can be independent of the optical structure of the output coupling surface described herein. It is not required that the current aperture precisely correspond to the optical structure or mode. The inventors recognized that current injection contributing to gain can also occur, for example, at offsets of about 1 or a few picometers from the mode, via lateral transport of charge carriers in the quantum well of the active region. This can be advantageously utilized by allowing current injection to be performed comprehensively and over a region slightly larger than the (optical) mode selection range. Thus, despite selecting a spatially narrow mode, the current conducts within a larger diameter. This reduces the current density, thereby significantly improving lifetime. This provides a significant advantage over mode selection using an elliptical current aperture.

[0035] This vertically emitting semiconductor laser device can be a VCSEL for optical data transmission. A VCSEL for optical data transmission can also be called a data communication VCSEL. The inventors recognized that the proposed solution with a specific output coupling surface (which has both a mode-selective macrostructure and a polarization-selective microstructure) is particularly advantageous for VCSELs used for optical data transmission. Data communication VCSELs typically operate over a wide temperature range, thus presenting special requirements that are not present in other applications or in purely research-oriented devices under laboratory conditions.

[0036] In one configuration, the vertically emitting semiconductor laser device has a relative intensity noise (RIN) of less than -80 dB / Hz, particularly less than -100 dB / Hz, particularly less than -120 dB / Hz, and particularly less than -140 dB / Hz. This allows for exceptionally high data transmission rates in optical communication systems. The values ​​shown can be relative intensity noise RINs in a frequency range up to 40 GHz, particularly in the range of 1 GHz to 40 GHz, particularly in the range of 2 GHz to 30 GHz, and particularly in the range of 5 GHz to 25 GHz.

[0037] This vertical-emitting semiconductor laser device can be configured such that, under high-frequency excitation up to 25 GHz, its light intensity decreases by no more than 3 dB compared to that under 1 GHz excitation. Therefore, this vertical-emitting semiconductor laser device can be particularly advantageous for optical data transmission. One advantage may be that, due to the smaller decrease in light intensity under 25 GHz excitation compared to 1 GHz excitation, the preamplifier electronics can have a simpler structure, especially eliminating or at least simplifying modulation frequency-selective gain matching.

[0038] A particularly advantageous application of the proposed vertical-emitting semiconductor laser device is in communication systems for optical data transmission. This communication system, or transmitter for an optical communication system, can have an input terminal for receiving data, a modulator, and a vertical-emitting semiconductor laser device. The input terminal can be understood as an interface for receiving data. The received data can be converted by the modulator and modulated onto the laser emitted by the vertical-emitting semiconductor laser device, for example, in the form of current modulation or voltage modulation. The communication system can also have a receiver with a detector for detecting the modulated laser and a demodulator configured to demodulate the detected laser and convert it into data that can be output through the interface for outputting data.

[0039] The advantages and optional features described in detail above with respect to the first aspect of the invention are correspondingly applicable to other aspects of the invention.

[0040] It is understood that the features described above and below can be used not only in the respective combinations, but also in other combinations or individually, without departing from the scope of the invention. Attached Figure Description

[0041] Embodiments of various aspects of the present invention are shown in the accompanying drawings and are described in detail below. The drawings show: Figure 1 A first schematic top view of an upper output coupling surface having a mode-selective macrostructure and a polarization-selective microstructure for a vertically emitting semiconductor laser device. Figure 2 A second schematic diagram showing a top view of the upper output coupling surface of a vertically emitting semiconductor laser device, having a mode-selective macrostructure and a polarization-selective microstructure. Figure 3 A schematic cross-sectional view of a vertically emitting semiconductor laser device is shown. Figure 4 A schematic diagram of a chip having a vertically emitting semiconductor laser device is shown; Figure 5 A schematic diagram of the communication system is shown; Figure 6 A flowchart illustrating a method for manufacturing a vertically emitting semiconductor laser device is shown. Detailed Implementation

[0042] The same reference numerals used in the accompanying drawings denote the same or at least functionally identical elements. The terms “upper,” “lower,” “left,” and “right,” and the directional indications derived therefrom, such as “top,” refer to the writing / reading direction of the figure number “Figure.” associated with the drawing, which is printed on the drawing plane below the drawing. The horizontal direction is parallel to the writing direction of “Figure.”, and the vertical direction is perpendicular to the writing direction of “Figure.”. The writing direction is based on a horizontal left-to-right writing style, i.e., primarily from left to right, as in Latin, English, and German, for example.

[0043] Figure 1 and Figure 2 Schematic top views of the upper output coupling surface with mode-selective macrostructure and polarization-selective microstructure for a vertically emitting semiconductor laser device are shown respectively. Figure 3 A schematic cross-sectional view of a vertically emitting semiconductor laser device is shown, in which an exemplary layer structure can be seen. Figures 1 to 3 They will be described together below.

[0044] Figure 3The vertically emitting semiconductor laser device 1 shown is a VCSEL. This VCSEL 1 has an upper Bragg reflector 2 and a lower Bragg reflector 3. An active region 4 for generating laser radiation 5 is arranged between the upper Bragg reflector 2 and the lower Bragg reflector 3. The basic structure of a VCSEL is well known to those skilled in the art. The Bragg reflectors may each have a layer sequence with different refractive indices and provide a resonant cavity or laser cavity for the laser radiation generated in the active region 4. The upper Bragg reflector is constructed as a partial reflector, through which the laser radiation 5 can be coupled out in the light emission direction 6. The light emission direction 6 is in... Figures 1 to 3 The numbers are represented by arrow 6.

[0045] In the illustrated embodiment, a vertically emitting semiconductor laser device 1 having a mesa 11 is shown on substrate 12. It is understood that this disclosure is not limited to the mesa structure, and other configurations of the semiconductor laser device 1 are contemplated. For example, a planar configuration having one or more semiconductor laser devices 1 is contemplated, particularly an array of multiple semiconductor laser devices 1. The multiple semiconductor laser devices 1 are preferably spaced apart from each other by trenches, for example, to avoid crosstalk.

[0046] For electrical contact, such as Figure 3 As shown in the cross-sectional view, an upper connecting contact 13 (also called a top contact) and a lower connecting contact 14 (also called a bottom contact or substrate contact) can be provided. The upper connecting contact 13 can be configured, for example, as an electrode that is at least partially annular or C-shaped.

[0047] like Figure 3 As shown, the vertically emitting semiconductor laser device 1 has an upper output coupling surface 20 for laser radiation 5, wherein the upper output coupling surface 20 has both a mode-selective macrostructure 21 and a polarization-selective microstructure 22. An exemplary embodiment of the upper output coupling surface 20 is described in... Figure 1 and Figure 2 The top view shows the output coupling surface 20, which has a mode-selective macrostructure 21 and a polarization-selective microstructure 22. It is an optical structure arranged on the upper side of the upper Bragg mirror 2.

[0048] The mode-selective macroscopic structure 21 is configured to favor a limited number of laser modes, especially a single laser mode. Figure 1 The top view shows an exemplary geometry that can favor a single laser mode. However, geometries that favor a limited number of laser modes can also be chosen. An example is shown in... Figure 2 It is shown in the top view.

[0049] exist Figure 1In the illustrated embodiment, the mode-selective macrostructure 21 of the output coupling surface 20 is constructed as a relief with an oval profile 23, particularly an elliptical profile. This allows for targeted matching of the lateral extension of the mode with the desired optical layer in a simple manner to accommodate the desired mode profile.

[0050] exist Figure 2 In the illustrated embodiment, the mode-selective macrostructure 21 of the output coupling surface 20 has an oval relief 24 with a contraction 25. In particular, the mode-selective macrostructure 21 can have a figure-eight profile, such as... Figure 2 The image shows a horizontal figure-eight shape.

[0051] exist Figure 1 and Figure 2 In the illustrated embodiment, the inner region (i.e., the region located within contour 23 or 24) and the outer region (i.e., the region located outside contour 23 or 24) can have, for example, different layer thicknesses, resulting in different layer thicknesses and / or different reflectivities near the upper Bragg mirror 2, and thus affecting lateral mode propagation. (See cross-sectional view) Figure 3 As shown, for example, the layer thickness within contours 23 and 24 can be reduced. Therefore, a relief with an oval contour can have an inner oval region with a smaller layer thickness and an outer region surrounding it with a larger layer thickness. Regarding the upper output coupling surface, Figure 3 The cross-sectional view can be regarded as along Figure 1 The cross section of line AA in the middle.

[0052] Additional layer thickness and etch-back can be performed on the semiconductor material of the VCSEL, such as in the topmost GaAs layer. Alternatively, a dielectric layer can be applied followed by structuring. Alternatively, a dielectric layer stack can be applied, in which one layer is selectively etched. Structuring is performed by photolithography, such as by electron beam lithography, ultraviolet lithography, or nanoimprint lithography.

[0053] like Figure 1 and 2 As shown in the top view, the mode-selective macrostructure 21 lies in a plane parallel to the upper Bragg mirror layer 2 (therefore in Figure 1 and Figure 2 The component (parallel to the drawing plane) is preferably constructed as non-rotationally symmetric and configured to break the degeneracy of the mode. This can further reduce the RIN of the laser radiation emitted by the semiconductor laser element.

[0054] exist Figures 1 to 3In the illustrated embodiment, the contours 23 and 24 of the mode-selective macrostructure surround the region having the polarization-selective microstructure 22. In the illustrated non-limiting embodiment, the polarization-selective microstructure 22 is configured as a grating structure for polarizing the laser. Here, the polarization-selective microstructure has a structural size smaller than the laser wavelength. For example, the grating structure 26 can be provided by photolithography to provide a plurality of parallel gratings in the upper output coupling surface 20 of the semiconductor laser device 1. The grating period or the distance between adjacent gratings is preferably smaller than the wavelength of the laser radiation. This avoids or reduces scattering losses. The mode-selective macrostructure 21 and the polarization-selective microstructure 22 are together configured as an elliptical surface relief with an enclosed grating structure.

[0055] As shown in the cross-sectional diagram Figure 3 As shown, the vertically emitting semiconductor laser device may optionally have a current aperture 7. In the illustrated embodiment, this current aperture is arranged between the active layer 4 and the upper Bragg mirror 2. However, other arrangements are also conceivable in the layer structure. A key feature is that the current aperture 7 has a diameter in a plane perpendicular to the radiation direction 6 of the laser beam 5 that is larger than the diameter of the mode-selective macrostructure 21 of the output coupling surface 20 having the polarization-selective microstructure 22. The size of the current aperture 7 is... Figure 1 and Figure 2 As shown in the top view, and larger than the mode-selective macrostructure 21, where a rectangular current aperture 7 is exemplarily provided. Therefore, the lateral advantage for the desired mode is more limited by the mode-selective macrostructure 21 of the output coupling surface, i.e., achieved through changes in optical properties rather than through a contraction of the current aperture 7. By making the current separator, or current device, larger than the relief structure, or mode-selective macrostructure, the current density within the VCSEL can be reduced. The current can be electrically distributed over a larger area within the semiconductor. In this case, mode selection is therefore not, or not solely, achieved through the current separator. More precisely, optical mode selection is achieved through a combination of the macrostructure of the output coupling surface and the polarization-selective microstructure of the output coupling surface. One advantage of this configuration may be that the definition of the current aperture, for example through an oxide separator, ion implantation, or a buried tunnel diode, can be independent of the optical structure of the output coupling surface described herein. It is not required that the current aperture precisely correspond to the optical structure or mode. The inventors recognized that current injection contributing to the gain could also occur, for example, at offsets of about 1 or a few picometers from the mode, via lateral transport of charge carriers in the quantum well of the active region. This could be advantageously utilized by injecting current comprehensively over a region slightly larger than the (optical) mode selection range. Thus, despite selecting a spatially narrow mode, the current conducts within a larger diameter. This reduces the current density, thereby significantly improving the lifetime.

[0056] Understandably, the choice of specific structure is relatively free, but should follow typical lateral pattern structures. These structures can be combined with current apertures of (slightly) different shapes. Simple examples include: (a) a circular current barrier with an elliptical or annular output surface; (b) a basically oval output surface, possibly with additional contractions; (c) a rectangular current aperture with an elliptical output surface or an output surface with multiple maximum values, such as... Figure 1 and Figure 2 As shown in the example; (d) a circular or rectangular current aperture with a superstructure on the output surface, which substantially corresponds to the mode image of the preferred mode.

[0057] exist Figure 3 In another variant of the vertically emitting semiconductor laser device 1 shown, a mode-selective macrostructure is an optional feature. The vertically emitting semiconductor laser device 1 can be a VCSEL having an upper Bragg mirror 2 and a lower Bragg mirror 3, wherein an active region 4 for generating laser radiation 5 is arranged between the upper Bragg mirror 2 and the lower Bragg mirror 3. The vertically emitting semiconductor laser device 1 has an upper output coupling surface 20 for the laser radiation 5. This upper output coupling surface 20 has a polarization-selective microstructure 22. The semiconductor laser device 1 is configured such that, under high-frequency excitation up to 25 GHz, particularly at a frequency of 25 GHz, its light intensity decreases by no more than 3 dB relative to the light intensity under 1 GHz excitation.

[0058] Figure 4 A schematic diagram of a chip 40 having a vertically emitting semiconductor laser device 1 is shown. The chip 40 has a carrier 41 on which the semiconductor laser device 1 is disposed. Connection contacts 42 and 43 can be provided here, which are connected to respective electrical connection contacts 13 and 14 of the semiconductor laser device 1 via bonding lines 44 and 45. Providing such a chip facilitates operation of the vertically emitting semiconductor laser device 1. Optionally, the chip 40 may have other functional elements, such as amplifiers, modulators, or protection circuitry, such as ESD protection elements.

[0059] Figure 5A schematic diagram of a transmitter 50 for a communication system, or an optical communication system, is shown. This transmitter has an input terminal 51 for receiving data, a modulator 52, and a vertically emitting semiconductor laser device 1 or a chip 40 incorporating such a device. As mentioned earlier, the input terminal 51 can be understood as an interface for receiving data. The received data can be converted by the modulator 52 and modulated onto the laser 5 emitted by the vertically emitting semiconductor laser device 1, for example, in the form of current modulation or voltage modulation. The communication system may also have a receiver (not shown) with a detector for detecting the modulated laser and a demodulator configured to demodulate the detected laser and convert it into data that can be output through the interface for outputting data.

[0060] Figure 6 A flowchart of a method 100 for fabricating a vertically emitting semiconductor laser device is shown. In a first step S101, a semiconductor substrate 12 is provided. In a second step S102, a layer sequence for a lower Bragg mirror is deposited on the semiconductor substrate. In a third step S103, an active layer having an active region for generating laser radiation is deposited on the lower Bragg mirror. In a fourth step S104, a layer sequence for an upper Bragg mirror is deposited on the active layer. In a first variant of the method for fabricating a vertically emitting semiconductor laser device, in a fifth step S105, an upper output coupling surface for laser radiation is provided on the upper Bragg mirror, wherein the upper output coupling surface has both a mode-selective macrostructure and a polarization-selective microstructure. Such an output coupling surface can be provided by applying more layers and structuring them, or by structuring one or more existing layers (e.g., the upper layer of the upper Bragg mirror). In a second variant of the method for manufacturing a vertically emitting semiconductor laser device, in the fifth step S105, an upper output coupling surface for laser radiation is provided on the upper Bragg mirror, wherein the upper output coupling surface has a polarization-selective microstructure, and the semiconductor laser device is configured such that its light intensity decreases by no more than 3 dB under high-frequency excitation at a maximum frequency of 25 GHz relative to the light intensity under 1 GHz excitation.

[0061] According to one configuration, an elliptical surface relief with a grating structure can be applied to reduce the noise (RIN) of VCSELs used in data communication. This reduces the number of modes and avoids unstable polarization regions over a wide temperature and current range. By setting the size of the grating structure to be smaller than the wavelength, scattering losses can be avoided. The grating can also be constructed as an optical metastructure.

[0062] In summary, one or more aspects of the present invention offer particular advantages in providing a further improved or alternative vertical-emitting semiconductor laser device, especially a VCSEL, for optical data transmission. One advantage may be the ability to provide a vertical-emitting semiconductor laser device capable of achieving high-speed optical data transmission over a wide range of environmental conditions, particularly a wide temperature range. Alternatively or additionally, an advantage may be the ability to provide a vertical-emitting semiconductor laser device that can be manufactured at low cost or through simplified process steps and / or with high yield (high output). Alternatively or additionally, an advantage may be the ability to provide a vertical-emitting semiconductor laser device capable of improving the lifetime of data communication VCSELs with low RIN.

Claims

1. A vertically emitting semiconductor laser device (1), particularly a VCSEL, having an upper Bragg mirror (2) and a lower Bragg mirror (3), wherein, An active region (4) for generating laser radiation (5) is arranged between the upper Bragg mirror (2) and the lower Bragg mirror (3); characterized in that the vertically emitting semiconductor laser device (1) has an upper output coupling surface (20) for the laser radiation (5), wherein the upper output coupling surface (20) has both a mode-selective macrostructure (21) and a polarization-selective microstructure (22).

2. The vertically emitting semiconductor laser device (1) according to claim 1, wherein, The output coupling surface (20), having the mode-selective macrostructure (21) and the polarization-selective microstructure (22), is an optical structure arranged on the upper side of the upper Bragg mirror (2).

3. The vertically emitting semiconductor laser device (1) according to any one of the preceding claims, wherein, The output coupling surface (20) is constructed as an embossed structure, wherein the mode-selective macrostructure (21) of the output coupling surface (20) has a first region with a first layer thickness, and wherein the polarization-selective microstructure (22) of the output coupling surface (20) has a second region with a second layer thickness, and in particular, wherein the second layer thickness is less than the first layer thickness.

4. The vertically emitting semiconductor laser device (1) according to any one of the preceding claims, wherein, The mode-selective macrostructure (21) is configured to favor a limited number of laser modes, and in particular, the mode-selective macrostructure (21) is configured to favor a single laser mode.

5. The vertically emitting semiconductor laser device (1) according to any one of the preceding claims, wherein, The mode-selective macrostructure (21) has a first region with a first reflectivity and a second region with a second reflectivity different from the first reflectivity.

6. The vertically emitting semiconductor laser device (1) according to any one of the preceding claims, wherein, The mode-selective macrostructure (21) of the output coupling surface (20) has a relief with a non-rotationally symmetric profile (23), especially an oval profile (23), and particularly an elliptical profile.

7. The vertically emitting semiconductor laser device (1) according to any one of the preceding claims, wherein, The mode-selective macrostructure (21) of the output coupling surface (20) has an oval relief (24) with a contraction (25), and in particular, the mode-selective macrostructure has a figure-eight profile.

8. The vertically emitting semiconductor laser device (1) according to any one of the preceding claims, wherein, The mode-selective macrostructure (21) is constructed as non-rotationally symmetric in a plane parallel to the upper Bragg mirror layer and is configured to break mode degeneracy.

9. The vertically emitting semiconductor laser device (1) according to any one of the preceding claims, wherein, The polarization-selective microstructure (22) has a grating structure (26) for polarizing the laser (5).

10. The vertically emitting semiconductor laser device (1) according to any one of the preceding claims, wherein, The polarization-selective microstructure (22) has a structural size smaller than the laser wavelength of the laser (5).

11. The vertically emitting semiconductor laser device (1) according to any one of the preceding claims, wherein, The mode-selective macrostructure (21) and polarization-selective microstructure (22) are constructed together as an elliptical surface relief (23) with an enclosed grating structure (26).

12. The vertically emitting semiconductor laser device (1) according to any one of the preceding claims, wherein, The mode-selective macrostructure (21) and polarization-selective microstructure (22) are formed by an optical superstructure of the output coupling surface (20), wherein the output coupling surface has an optical superstructure, i.e., a structure with spatially varying phase matching.

13. The vertically emitting semiconductor laser device (1) according to claim 12, wherein, The optical superstructure is configured to provide a variable effective refractive index, wherein the optical superstructure has pillars of variable density, constant height, and extension less than the wavelength.

14. The vertically emitting semiconductor laser device (1) according to any one of the preceding claims, wherein, The vertically emitting semiconductor laser device (1) has a current aperture (7), wherein the diameter of the current aperture (7) in a plane perpendicular to the radiation direction (6) of the laser beam (5) is greater than the diameter of the mode-selective macrostructure (21, 23) of the output coupling surface (20) having a polarization-selective microstructure (22).

15. The vertically emitting semiconductor laser device (1) according to any one of the preceding claims, wherein, The vertical emission semiconductor laser device (1) is a VCSEL used for optical data transmission.

16. The vertically emitting semiconductor laser device (1) according to any one of the preceding claims, wherein, The vertically emitting semiconductor laser device (1) has a relative intensity noise (RIN) of less than -80 dB / Hz, especially less than -100 dB / Hz, especially less than -120 dB / Hz, especially less than -140 dB / Hz.

17. The vertically emitting semiconductor laser device (1) according to any one of the preceding claims, wherein, The vertical emission semiconductor laser device (1) is configured such that its light intensity decreases by no more than 3dB under high-frequency excitation at a maximum frequency of 25GHz compared to the light intensity under 1GHz excitation.

18. The use of surface reliefs on the output coupling surface (20) of a VCSEL in a communication system (50) for optical data transmission, wherein, The surface relief of the output coupling surface (20) has a mode-selective macrostructure (21) and a polarization-selective microstructure (22).

19. A vertically emitting semiconductor laser device (1), particularly a VCSEL, having an upper Bragg mirror (2) and a lower Bragg mirror (3), wherein, An active region (4) for generating laser radiation (5) is arranged between the upper Bragg reflector (2) and the lower Bragg reflector (3); characterized in that the vertically emitting semiconductor laser device (1) has an upper output coupling surface (20) for laser radiation (5), wherein the upper output coupling surface (20) has a polarization-selective microstructure (22), and the semiconductor laser device is configured such that its light intensity decreases by no more than 3 dB under high-frequency excitation at a maximum frequency of 25 GHz relative to the light intensity under 1 GHz excitation.

20. A communication system (50) having an input terminal (51) for receiving data, a modulator (52) and a vertically emitting semiconductor laser device (1) according to any one of the preceding claims.

21. A method (100) for manufacturing a vertically emitting semiconductor laser device, wherein, The method includes the following steps: Provide (S101) semiconductor substrate; A layer sequence for a lower Bragg mirror is deposited on the semiconductor substrate (S102). An active layer having an active region for generating laser radiation is deposited on the lower Bragg mirror (S103). A layer sequence for the upper Bragg mirror is deposited on the active layer (S104). An upper output coupling surface for laser radiation is provided on the upper Bragg mirror, wherein the upper output coupling surface has both a mode-selective macrostructure and a polarization-selective microstructure (S105).

22. A method (100) for manufacturing a vertically emitting semiconductor laser device, wherein, The method includes the following steps: Provide (S101) semiconductor substrate; A layer sequence for a lower Bragg mirror is deposited on the semiconductor substrate (S102). An active layer having an active region for generating laser radiation is deposited on the lower Bragg mirror (S103). A layer sequence for the upper Bragg mirror is deposited on the active layer (S104). An upper output coupling surface for laser radiation is provided on the upper Bragg reflector, wherein the upper output coupling surface has a polarization-selective microstructure, and the semiconductor laser device is configured such that its light intensity decreases by no more than 3 dB under high-frequency excitation at a maximum frequency of 25 GHz relative to the light intensity under 1 GHz excitation (S105).

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