Vcsel with high-contrast grating

Abstract

The invention relates to a VCSEL (10), comprising a substrate (12) on which a semiconductor layer structure (14) is arranged which has a first mirror (16) facing the substrate, a second mirror (20) facing away from the substrate, said second mirror being designed as a distributed Bragg mirror, and an active region (18) between the first mirror (16) and the second mirror (20). The first mirror (16), the active region (18) and the second mirror (20) form an optical resonator in order to generate laser light in the resonator, wherein the laser light is emitted through the first mirror (16) and through the substrate (12) during operation of the VCSEL (10). The optical resonator has a high-contrast grating (28), wherein the high-contrast grating (28) has a higher reflectivity for a first polarisation direction of the laser light than for a second polarisation direction of the laser light perpendicular to the first polarisation direction. The high-contrast grating (28) is arranged on the side of the active region (18) that faces away from the substrate (12). A combined reflectivity of the arrangement consisting of the high-contrast grating (28) and the second mirror (20) is equal to or greater than 99.7% for the first polarisation direction.

Description

VCSEL with high-contrast grid

[0001] The invention relates generally to the field of vertical cavity surface-emitting lasers, abbreviated as VCSELs (vertical cavity surface-emitting lasers). Specifically, the invention relates to a VCSEL configured as a bottom emitter.

[0002] A VCSEL typically features a semiconductor layer structure mounted on a substrate. This semiconductor layer structure comprises multiple semiconductor layers stacked vertically on the substrate. It includes a first mirror, which may be a distributed Bragg mirror (DBR), a second mirror, which may also be a distributed Bragg mirror, and an active region between the first and second mirrors. One of the two mirrors is located on the side of the active region facing the substrate, and the other is located on the side facing away from the substrate.

[0003] The first mirror, the active area, and the second mirror form an optical resonator in which laser light is generated when the active area is pumped, for example, electrically.

[0004] The present invention relates specifically to a VCSEL in which the laser light generated in the optical resonator is emitted through the substrate. Such a VCSEL is also referred to as a bottom emitter. In a bottom emitter, the mirror facing away from the substrate has a higher reflectivity than the mirror facing the substrate. Bottom emitters have the advantage, among others, that the VCSEL can be relatively easily equipped with optics, e.g., a lens array, by integrating the optics into the substrate without having to connect a separate optic to the VCSEL.

[0005] There is currently a need for bottom-emitters whose laser emission has stable polarization to enable the integration of optics into the substrate. Standard optical gratings are typically used for polarization stabilization; however, in a bottom-emitter, these gratings are located on the mirror facing away from the substrate, which has a higher polarization. Reflectivity alone is insufficient. Optical gratings with metallization are also difficult to use, particularly due to potential surface plasmons that cause light absorption.

[0006] US 2015 / 0288146 A1 discloses a vertical cavity surface-emitting laser having a silicon-on-insulator (SOI)-type substrate featuring a high-contrast grating, and designed to connect a VCSEL substructure to the substrate. The VCSEL substructure has an active region and only one mirror, the other mirror being formed by the substrate with the high-contrast grating.

[0007] US 2022 / 0209502 A1 discloses a VCSEL comprising a first mirror, an active region on the first mirror, and a second mirror on the active region, all mounted on a substrate. A high-contrast grating is arranged on the second mirror. The high-contrast grating comprises a first grating element and a second grating element, the first and second grating elements being separated from each other by an air gap.

[0008] The invention is based on the objective of providing a bottom-emitter VCSEL that can emit laser light with stable polarization during operation. In particular, the performance of the VCSEL should remain stable even with respect to manufacturing tolerances.

[0009] According to the invention, this problem is solved by a VCSEL with a substrate on which a semiconductor layer structure is arranged, the structure having a substrate-facing first mirror, a substrate-away second mirror configured as a distributed Bragg mirror, and an active region between the first and second mirrors, wherein the first mirror, the active region, and the second mirror form an optical resonator to generate laser light in the resonator, wherein the laser light is emitted through the first mirror and through the substrate during operation of the VCSEL, and wherein the optical resonator has a high-contrast grating, the high-contrast grating having a higher polarization direction for a first direction of the laser light. has a reflectivity for a second polarization direction of the laser light perpendicular to the first polarization direction, wherein the high-contrast grating is arranged on the side of the active area facing away from the substrate, and wherein a combined reflectivity of the arrangement of the high-contrast grating and the second mirror for the first polarization direction is equal to or greater than 99.7%.

[0010] The present invention provides a bottom-emitter VCSEL in which the substrate-facing mirror, preferably of high reflectivity, is combined with a high-contrast grating whose reflectivity for one polarization direction of the laser light generated in the optical resonator is significantly higher than for the polarization direction perpendicular to this polarization direction. A laser mode with a polarization direction or component for which the high-contrast grating provides higher reflectivity is thus amplified in the optical resonator and emitted by the VCSEL, while the laser mode with the polarization direction perpendicular to this polarization direction is suppressed in the optical resonator due to output coupling losses. The VCSEL according to the invention can therefore operate with stabilized polarization.

[0011] The combined reflectivity of the arrangement consisting of the substrate-facing second mirror and the high-contrast grating results from the individual reflectivities of the second mirror and the high-contrast grating, as described below.

[0012] The VCSEL according to the invention has the further advantage that the combined reflectivity of the arrangement consisting of a mirror facing away from the substrate and a high-contrast grating varies only slightly in the case of manufacturing tolerances of the high-contrast grating, which can manifest themselves, for example, in variations in the shape and depth of the struts and grooves of the grating structure. Thus, the desired stable performance of the VCSEL is ensured even with manufacturing inaccuracies.

[0013] Another advantage of using a high-contrast grating on the substrate-facing side of a bottom-emitter VCSEL is that the arrangement of the substrate-facing mirror and the high-contrast grating does not "too The high-contrast grating can be "too open" or "too closed," meaning it may exhibit excessively high reflectivity, for example, due to variations in the shape or depth of the grating structure. However, for a top-emitter VCSEL, where the high-contrast grating is located on the output coupler facing away from the substrate, variations in the grating's reflectivity would pose a significant problem. In a VCSEL, it is crucial that the output coupling operates within a narrow window of, for example, 0.5–1% transmission, while deviations from this result in substantial changes to laser parameters such as efficiency and laser threshold. In other words, in a top-emitter VCSEL, the output coupler, which has a correspondingly lower reflectivity, can easily be "too open" or "too closed" in combination with a high-contrast grating.On the other hand, for a bottom-emitter VCSEL where the high-contrast grating is located on the side of the active area where the substrate-facing mirror is located, the combination of mirror and high-contrast grating can only be "too open," but never "too closed." This allows for a design window in which the combined reflectivity of the substrate-facing mirror and the high-contrast grating is always "closed enough," even with significant variations in the reflectivity of the high-contrast grating due to manufacturing tolerances or inaccuracies of the grating itself.

[0014] As is known to those skilled in the art, a high-contrast grating is an optical structure with raised bars (also called beams) between which grooves are present. The grating material has a pronounced refractive index contrast compared to the material(s) adjacent to the grating material. The grating period of the high-contrast grating is preferably in the range of or below the wavelength of the laser light generated in the optical resonator.

[0015] Preferably, the combined reflectivity of the arrangement consisting of the high-contrast grating and the second mirror for the first polarization direction is equal to or greater than 99.9%.

[0016] This optimally amplifies the preferred polarization direction in the optical resonator and further stabilizes the polarization of the laser emission.

[0017] Preferably, the reflectivity of the second mirror facing away from the substrate is greater than 90%, preferably less than or equal to 98%.

[0018] In this configuration, the mirror facing away from the substrate advantageously exhibits the highest possible reflectivity, which, in combination with the high-contrast grating, ensures the necessary closure of the VCSEL's side facing away from the substrate. The high reflectivity of the mirror facing away from the substrate reduces the manufacturing accuracy requirements for the high-contrast grating, meaning there is a certain manufacturing tolerance window for the grating. In a conventional bottom-emitter VCSEL, the reflectivity of the mirror facing away from the substrate is typically greater than 99.8%. Without the high-contrast grating, a VCSEL with a reflectivity in the range of 90% to 98% at the end opposite the output would not laser or would only do so with very low efficiency.

[0019] Furthermore, preferably the reflectivity of the high-contrast grating for the first polarization direction is equal to or greater than 90%, preferably equal to or greater than 95%.

[0020] On the one hand, it is advantageous that, in conjunction with a substrate-facing mirror with a reflectivity in the aforementioned range of 90% to 98%, a sufficient combined reflectivity of the arrangement of substrate-facing mirror and high-contrast grating results to adequately close the substrate-facing side of the VCSEL, and on the other hand, the high-contrast grating in the aforementioned reflectivity range can be easily manufactured using manageable manufacturing techniques.

[0021] Preferably, the combined reflectivity of the arrangement consisting of the high-contrast grating and the substrate-away second mirror for the second polarization direction is equal to or less than 99.9%, preferably equal to or less than 99.5%, and more preferably equal to or less than 99.0%.

[0022] This measure sufficiently suppresses the "undesired" second polarization direction in the optical resonator due to output coupling losses.

[0023] The reflectivity of the high-contrast grating for the second polarization direction is preferably equal to or less than 90%, preferably equal to or less than 80%, and further preferably equal to or less than 50%.

[0024] The reflectivity of the first mirror facing the substrate is preferably in the range of 98% to 99.6%.

[0025] The phase difference between the reflectivity of the high-contrast grating and the reflectivity of the second mirror facing away from the substrate is less than + / -(2TT / 3) for the first polarization direction, preferably less than + / -(TT / 2), further preferably less than + / -(TT / 4), and further preferably less than + / -(TT / 10).

[0026] It is therefore preferable if the reflectivities of the second mirror facing away from the substrate and the high-contrast grating are in phase with each other as much as possible. In other words, the laser light reflected by the high-contrast grating and the laser light reflected by the second mirror facing away from the substrate should be in phase with each other as much as possible.

[0027] Preferably, the high-contrast grating is arranged on the side of the second mirror facing away from the substrate.

[0028] An advantage here is that the high-contrast grid can be easily manufactured after the basic structure of the semiconductor layer has been produced.

[0029] Preferably, the high-contrast grating is arranged directly on the second mirror.

[0030] The high-contrast grating can be incorporated into a thicker top layer applied to the mirror facing away from the substrate, e.g. by etching the grating structure into the top layer.

[0031] Furthermore preferably a coating is arranged on a substrate-facing surface of the high-contrast grating, in particular wherein the coating has a thickness in the range of 20 to 30 nm, in particular wherein the coating is an ALD layer.

[0032] The coating advantageously protects the semiconductor layer structure of the VCSEL from moisture, thus ensuring long-term reliability of the VCSEL.

[0033] If an ALD layer is applied directly to the substrate-facing surface of the high-contrast grid, and a passivation coating is applied to the ALD layer, the ALD layer can be very thin and, for example, made of aluminum oxide. The thicker passivation coating can consist of one or more layers of SiN. A metal layer can also be placed on the passivation coating or directly on the ALD layer.

[0034] In a practical embodiment, the high-contrast grating has a grating constant of 500 nm or greater, preferably 1000 nm or less.

[0035] The high-contrast grating, with a grating constant in the specified range, can be fabricated not only by nanoprinting or electron beam writing, but also in a particularly cost-effective manner using a UV stepper operating in the deep ultraviolet spectral range. The grating constant of the high-contrast grating is preferably below the wavelength of the laser light in the optical resonator. The emission wavelength of the VCSEL can be in the range of 840 nm to 1500 nm.

[0036] Preferably, the high-contrast grating has a plurality of bars and grooves between adjacent bars, wherein the ratio of bar width to grating constant is in the range of 0.4 to 0.6, preferably in the range of 0.45 to 0.5.

[0037] Such a ratio of ridge width and lattice constant of the high-contrast grating has been observed in connection with an etching depth of the lattice structure of the high-contrast grating of A wavelength of approximately 245 nm has proven suitable for achieving a reflectivity of 95% for the high-contrast grating in the desired first polarization direction. With the same design parameters, a significantly lower reflectivity of less than 50% is advantageously obtained for the "undesired" second polarization direction.

[0038] In a further embodiment, the high-contrast grating can have a smaller extent in a plane parallel to the substrate than the electrically pumped area of ​​the active region of the optical resonator.

[0039] For example, the high-contrast grating can be arranged only in a central region of the electrically pumped area, so that in this configuration the VCSEL only lasers in the fundamental mode, while higher-order lateral laser modes are effectively suppressed due to the absence of the high-contrast grating in the lateral region of the electrically pumped area and the associated output coupling losses.

[0040] The high-contrast grating can be a single high-contrast grating, or the high-contrast grating can comprise two or more separate, spaced-apart high-contrast gratings or segments distributed over the electrically pumped area.

[0041] The latter measure ensures that the VCSEL also operates stably in a single, higher-order laser mode. Specifically, the individual, spaced-apart high-contrast gratings can be positioned in regions of intensity maxima of a desired higher-order laser mode to stabilize the laser emission in that single mode.

[0042] Further advantages and features will become apparent from the following description and the attached drawings.

[0043] It is understood that the aforementioned and subsequently explained characteristics exist not only in the combinations specified, but also in other combinations. Combinations or standalone applications can be used without leaving the scope of the invention.

[0044] Exemplary embodiments of the invention are shown in the drawing and are described in more detail below with reference to them. The drawing shows: Fig. 1 schematically shows a sketch of an exemplary embodiment of a VCSEL; Fig. 2 schematically shows a sketch of another embodiment of a VCSEL; Fig. 3 schematically shows a sketch of another embodiment of a VCSEL; Figures 4A and 4B each show a diagram of the reflectivities of a high-contrast grating as a function of the grating width and the ratio of grating width to grating constant of the high-contrast grating, where Fig. 4A shows the reflectivity for a first polarization direction and Fig. 4B shows the reflectivity for a second polarization direction perpendicular to the first polarization direction; Fig. 5 is a diagram showing a combined reflectivity of the arrangement. shows the high-contrast grating and substrate-facing mirror of the VCSEL as a function of the phase difference of the reflectivities of the high-contrast grating and the substrate-facing mirror; Fig. 6 shows a diagram illustrating the combined reflectivity for the first polarization direction as a function of the reflectivity of the high-contrast grating when this varies, for example, due to manufacturing tolerances; Fig. 7 schematically shows a sketch of another embodiment of a VCSEL; Fig. 8 schematically shows a sketch of another embodiment of a VCSEL; Fig. 9 schematically shows a sketch of another embodiment of a VCSEL; Fig. 10 shows a top view of a light emission side of a VCSEL with a virtual superposition of the laser emission with individual high-contrast gratings or segments; Fig. 11 schematically shows a representation of a linear active region of a VCSEL and individual high-contrast gratings or segments; Fig. 12 schematically shows a representation of an annular active area of ​​a VCSEL and individual high-contrast gratings or segments in a first embodiment; Fig. 13 schematically shows a representation of an annular active area of ​​a VCSEL and individual high-contrast gratings or segments in a second embodiment; Fig. 14 schematically shows a representation of a rectangular active area of ​​a VCSEL with rounded corners and individual high-contrast gratings or segments in a second embodiment. Fig. 15 schematically shows a representation as in Fig. 15, but with chamfered corners of the active area; and Figs. 16 to 18 schematically show further embodiments of active areas and individual high-contrast grids or segments.

[0045] Fig. 1 schematically shows the structure of a VCSEL 10 according to the invention. The VCSEL 10 has a substrate 12 on which a semiconductor layer structure 14 is arranged. The semiconductor layer structure 14 consists of a plurality of semiconductor layers, not shown individually. The semiconductor layer structure 14 can, for example, be composed of the material system gallium arsenide / aluminum gallium arsenide.

[0046] The semiconductor layer structure 14 has a substrate-facing first mirror 16, which can in particular be configured as a distributed Bragg mirror (DBR). A DBR typically has a plurality of semiconductor layer pairs, each pair comprising a semiconductor layer with a low refractive index and a semiconductor layer with a high refractive index.

[0047] The reflectivity of the substrate-facing first mirror 16 is preferably in the range of 98% to 99.6%.

[0048] The semiconductor layer structure 14 further features an active region 18. The active region 18 can have one or more quantum wells, as is known in the field of VCSELs.

[0049] The semiconductor layer structure 14 further comprises a wide mirror 20 facing away from the substrate, which is configured as a DBR. The active region 18 is arranged between the mirror 16 facing the substrate and the mirror 20 facing away from the substrate. The mirror 16, the active region 18, and the mirror 20 form an optical resonator in which laser light is generated during operation of the VCSEL 10. To excite the VCSEL 10 to laser emission, the active region 18 is pumped, preferably electrically. For this purpose, the VCSEL 10 has an electrical contact arrangement with electrical contacts or electrodes 22 and 24. The electrical contact 24 is shown here as a ring contact.

[0050] The mirror 20 facing away from the substrate can be p-doped, and the mirror 16 facing the substrate can be n-doped, so that a pn junction is formed on both sides of the active region 18. The contact 24 can be a p-contact, and the contact 22 an n-contact. The substrate 12 can be made of an n-doped semiconductor material, but can also be undoped and therefore not or only slightly conductive, in which case the contact 22, e.g. via a contacting layer, directly contacts the substrate-facing mirror 16.

[0051] The VCSEL 10 is specifically configured as a bottom emitter. This means that the laser light generated in the optical resonator during the operation of the VCSEL 10 is emitted through the substrate 12, as illustrated by arrow 26.

[0052] The optical resonator of the bottom-emitter VCSEL 10 features a high-contrast grating 28 (hereinafter also referred to as HCG), which is described in more detail below. In the preferred embodiment, the HCG 28 is arranged directly on the second mirror 20, specifically on its side facing away from the substrate 12.

[0053] In the fabrication of the VCSEL 10, the semiconductor layer structure 14 can first be epitaxially grown on the substrate 12 by depositing the individual layers of the semiconductor layer structure 14. While in a conventional bottom-emitter VCSEL the substrate-facing mirror 20 is formed with a number of semiconductor layer pairs such that the reflectivity of the mirror is greater than 99.8%, in the VCSEL 10 the substrate-facing mirror 20 is formed with a smaller number of semiconductor layer pairs, so that the reflectivity of the substrate-facing mirror 20 is preferably in the range of 90% to 98%. With a reflectivity in the aforementioned range, the VCSEL 10 would not laser without the HCG 28, or at best with very low efficiency, since too high a proportion of the laser light generated in the optical resonator would be coupled out by the mirror 20.

[0054] The HCG 28 can be manufactured by growing another layer, e.g., a comparatively thick top layer, on the side of the mirror 20 facing away from the substrate 12, into which the high-contrast grating 28 is then incorporated. In this process, particularly by etching the top layer, a The grid structure of the HCG 28 is generated with beams or struts and trenches between immediately adjacent struts. In Fig. 1, the high-contrast grid 28 is indicated by a small number of beams or struts 28a and trenches 28b between immediately adjacent struts 28a. In practice, the number of struts 28a and trenches 28b is many times higher than shown in Fig. 1. The strut width, the depth of the trenches 28b, the height of the struts 28a, the grid period, and the ratio of strut width to grid period are also not shown to scale in the drawing. Design parameters of the HCG 28 are described later.

[0055] Figures 2 and 3 show modified embodiments of a VCSEL 10 compared to Figure 1, of which only the differences from the VCSEL 10 in Figure 1 are described below. In the embodiment shown in Figure 2, an additional layer 30 is applied to the high-contrast grating 28. The layer 30 can be made of a dielectric material, e.g., SiN, and in particular be a thick layer. Alternatively or additionally, a thin layer, e.g., with a layer thickness of 20–30 nm, in particular an ALD layer, can be applied directly to the high-contrast grating 28, and the thicker layer 30 made of dielectric material is then applied to it. Alternatively, according to the embodiment shown in Fig. 3, the high-contrast grid 28 or the entire substrate-facing side of the VCSEL 10 can be coated with a thin layer 32, e.g. in the form of an ALD layer, followed by a metal layer 34. The thin layer 32 can consist of aluminum oxide.

[0056] All the aforementioned coating measures serve to protect the semiconductor layer structure 14 from moisture and contamination, thus ensuring good long-term reliability of the VCSEL 10.

[0057] The following describes embodiments of the high-contrast grating 28 and its interaction with the substrate-facing mirror 20.

[0058] The arrangement consisting of the substrate-facing mirror 20 and the HCG 28 has a combined reflectivity R, which results from the individual reflectivities Rs of the mirror 20 and RH of the HCG 28 according to the following equation:

[0059] R = ((R s 1 / 2 - H 1 / 2 ) 2 +4 R s 1 / 2 H 1 / 2 sin 2 «))) I ((1 - R s 1 / 2 H 1 / 2 ) 2 +4 R s 1 / 2 H 1 / 2 sin n 2 <|>) (1)

[0060] where sin 2<|) is a phase term which is 1 if the reflectivities Rs and RH are in phase, i.e. the light reflected at mirror 20 and the light reflected at HCG 28 are in phase.

[0061] Preferably, the combined reflectivity of the arrangement consisting of the high-contrast grating 28 and the mirror 20 facing away from the substrate is equal to or greater than 99.7% for the first polarization direction to be stabilized. Even more preferably, the combined reflectivity of the arrangement consisting of the high-contrast grating 28 and the mirror 20 facing away from the substrate is equal to or greater than 99.9% for the polarization direction to be stabilized. The reflectivity of the second mirror facing away from the substrate is, as already mentioned above, preferably greater than 90% and less than or equal to 98%.

[0062] The reflectivity of the high-contrast grating 28 for the first polarization direction to be stabilized is preferably equal to or greater than 90%, more preferably equal to or greater than 95%.

[0063] For the second polarization direction to be suppressed, the combined reflectivity of the arrangement consisting of the high-contrast grating 28 and the second mirror 20 is equal to or less than 99.9%, preferably equal to or less than 99.5%, and more preferably equal to or less than 99.0%. The reflectivity of the high-contrast grating 28 for the second polarization direction is equal to or less than 90%, preferably equal to or less than 80%, and further preferably equal to or less than 50%.

[0064] The reflectivities of the high-contrast grating 28 and the substrate-facing mirror 20 should be in phase with each other as closely as possible, with a phase difference of at least less than + / - (TT / 2), further preferably less than + / -(TT / 4), and further preferably less than + / -(TT / 10).

[0065] The following describes a design example for the high-contrast grating 28.

[0066] As mentioned above, the high-contrast grating 28 has a plurality of bars 28a and trenches 28b between adjacent bars 28a. Fig. 4A shows a diagram that plots, in a dot pattern, the reflectivities of the high-contrast grating 28 for the desired first polarization direction as a function of parameter combinations. The ordinate includes the depth H of the trenches 28b between adjacent bars 28a, and the abscissa represents the ratio DC of the width of the bars 28a to the grating constant of the high-contrast grating 28. The depth H is given in a range of 0.23 pm to 0.27 pm. The ratio DC is given in a range of 0.45 to 0.75. For manufacturing reasons, the grating period of the high-contrast grating 28 is preferably larger than 500 nm. Fig. 4B shows the reflectivity of the high-contrast grating for the second polarization direction, which is perpendicular to the first polarization direction and is to be suppressed.Decreasing point density in both diagrams indicates increasing reflectivity of the high-contrast grating.

[0067] According to Fig. 4A, for the first polarization direction, the maximum reflectivity of the high-contrast grating 28 is approximately 45% to 50% of the ratio DC of the grating width to the grating period, and at a depth H of the trenches 28b of approximately 245 nm, amounting to 95%. This point of the design parameter combination is marked with an arrow 44 in Fig. 4A. According to Fig. 4B, at the same point of the design parameter combination, marked with an arrow 45 in Fig. 4B, the reflectivity of the high-contrast grating 28 for the second polarization direction, perpendicular to the first, is less than 50%.For a reflectivity RH of the high-contrast grating 28 of 95% and a reflectivity Rs of the substrate-away second mirror 20 of 94%, equation (1) yields a combined reflectivity RPI for the first polarization direction of 99.92% when the reflectivities Rs and RH are in phase, while the combined reflectivity RP2 for the second polarization direction perpendicular to the first polarization direction is only 98.94% due to the low reflectivity RH of <50%, even when the reflectivities RH and Rs are in phase.

[0068] Fig. 5 shows the combined reflectivity RPI for the first polarization direction and the combined reflectivity RP2 for the polarization direction perpendicular to it for the point of the design parameter combination previously described with reference to Figs. 4A and 4B, as a function of the phase shift between the reflectivities of the high-contrast Grating 28 and the substrate-facing mirror 20. When the reflectivities RH of the high-contrast grating 28 and the substrate-facing mirror 20 are in phase, the combined reflectivity for the first polarization direction P1 is 99.92% as stated above, while the combined reflectivity RP2 for the second polarization direction is only 98.94% when the phase between the reflectivities of the high-contrast grating 28 and the substrate-facing mirror 20 is zero.

[0069] As shown in Fig. 5, the phase difference between the reflectivities of the high-contrast grating 28 and the substrate-facing mirror 20 should be around 0; however, a certain tolerance window is required due to manufacturing tolerances of the high-contrast grating 28. These manufacturing tolerances can consist of etch depth variations of the high-contrast grating 28 of, for example, + / - 20 nm, which corresponds to approximately 1 / 10 of the wavelength of the laser light in the optical resonator for a VCSEL with an emission wavelength of 980 nm.

[0070] Fig. 6 shows the combined reflectivity RPI for the first polarization direction as a function of a varying reflectivity RH of the high-contrast grating 28 due to possible manufacturing tolerances of the high-contrast grating 28. As can be seen from Fig. 6, even when the reflectivity RH is reduced to 90%, the combined reflectivity RPI is still above 99.83%, and if the reflectivity RH were greater than that corresponding to the target design of the high-contrast grating 28, this would not pose a technical problem, since the high-contrast grating 28 together with the mirror 20 facing away from the substrate forms the closed side of the optical resonator.

[0071] With reference to Figures 7 to 9, further possible configurations of the VCSEL 10 are described.

[0072] Fig. 7 shows an embodiment of a VCSEL 10, wherein in Fig. 7 an additional current limiting orifice 52, e.g. in the form of an oxide orifice, is shown, which is omitted in Figures 1 to 3, but may also be present there.

[0073] The current-limiting aperture 52 limits the pumped region of the active region 18. In Fig. 7, the high-contrast grating 28 has a smaller extent in a plane parallel to the substrate 12 than the pumped region within the current-limiting aperture 52. This allows higher-order lateral laser modes to be suppressed due to coupling losses in the regions where the HCG 28 is not present, so that the VCSEL 10, for example, emits only in the fundamental mode.

[0074] Fig. 8 shows an embodiment of a VCSEL 10 in which the high-contrast grating 28 comprises two or more individual, spaced-apart high-contrast gratings or segments 29a, 29b, 29c, distributed over the electrically pumped region located within the current-limiting aperture 52. In such an embodiment, the laser emission of the VCSEL 10 can be stabilized to a single higher-order laser mode.

[0075] Fig. 9 shows another embodiment of a VCSEL 10, in which the high-contrast grating 28 again comprises individual, spaced-apart high-contrast gratings or segments 29a to 29h, which are arranged at positions along the electrical pumping region located within the current-limiting aperture 52, where a desired mode exhibits intensity maxima. The intensity maxima are illustrated in Fig. 9 by circles 56a to 56h. The aspect described above will be further explained below with reference to Figures 10 to 18. Fig. 10 shows a top view of a light emission side of a VCSEL, with individual high-contrast gratings or segments 29a–29g virtually superimposed on the areas of intensity maxima or high intensity of the laser emission. In the areas of the individual high-contrast gratings or segments 29a–29g, the reflectivity of the side of the VCSEL opposite the output coupling side is high. These areas, marked by arrows 60 and 62 as examples for HCGs 29f and 29g, contain the high-intensity regions of the selected desired laser mode. The remaining areas, marked by arrows 64 and 66 as examples, are areas where The intensity of the selected laser mode is low. Therefore, in these mode-suppression regions, where no HCG is present, no amplification occurs due to high coupling losses. This is also shown schematically in Fig. 11, which shows the active area 18 of a VCSEL and individual high-contrast gratings or segments 29a - 29h projected onto it. This aspect can be realized not only in a VCSEL 10 with a linear, strip-shaped geometry of the laser-emitting active area as shown in Fig. 11, but also in VCSELs with other geometries of the active area. For example, Fig. 12 shows, in a further embodiment, an annular active area 18 and individual high-contrast gratings or segments 29i, i = 1, ..., n, where n is an integer greater than 1 and describes the number of HCGs or HCG segments. In the embodiment shown in Fig. 12, the structures of the HCGs or HCG segments, i.e., the webs 28a and grooves 28b, are rotated relative to each other following the course of the active area 18. In an embodiment shown in Fig. 13, where the active area is formed in an annular shape as in Fig. 12, the webs 28a and grooves 28b of the HCGs or the HCG segments 29i are oriented in the same way relative to each other. Fig. 14 shows an embodiment of a rectangular-ring-shaped active area 18 with rounded corners, as well as individual HCGs or HCG segments 29i, whose webs 28a and grooves 28b are oriented the same relative to each other on two parallel sides of the four sides of the active area 18, while they are oriented perpendicular to each other on two perpendicular sides of the four sides. At the rounded corners of the active area 18, the webs 28a and grooves 28b are oriented at an angle of 45° to the webs and grooves 28a, 28b of the adjacent HCGs or HCG segments. Fig. 15 shows a modification of the embodiment in Fig. 14 with corners beveled by 45° and an identical orientation of the webs 28a and grooves 28b of all HCGs or HCG segments 29i relative to each other. Figures 16 to 18 show further embodiments of active regions of VCSELs with HCGs or HCG segments 29i. The embodiments in Figures 16 to 18 correspond to the embodiments in Figures 10 and 11, but instead of a linear strip-shaped geometry, they have a circular geometry in Figure 16, a rectangular or square geometry in Figure 17, and a rectangular or square geometry with rounded corners in Figure 18. The webs 28a and grooves 28b of the HCGs or HCG segments 29i are oriented the same relative to each other in all three embodiments. In the embodiments described above, the HCGs or HCG segments 29i are located at positions of the active pumped area 18 where a selected laser mode has a high intensity.

Claims

Patent claims 1. VCSEL, comprising a substrate (12) on which a semiconductor layer structure (14) is arranged, the structure having a substrate-facing first mirror (16), a substrate-away second mirror (20) configured as a distributed Bragg mirror, and an active region (18) between the first mirror (16) and the second mirror (20), wherein the first mirror (16), the active region (18), and the second mirror (20) form an optical resonator to generate laser light in the resonator, wherein the laser light is emitted through the first mirror (16) and through the substrate (12) during operation of the VCSEL (10), and wherein the optical resonator comprises a high-contrast grating (28), the high-contrast grating (28) having a higher reflectivity for a first polarization direction of the laser light than for a second polarization direction of the laser light perpendicular to the first polarization direction.wherein the high-contrast grating (28) is arranged on the side of the active area (18) facing away from the substrate (12), and wherein the combined reflectivity of the arrangement consisting of the high-contrast grating (28) and the second mirror (20) for the first polarization direction is equal to or greater than 99.7%.

2. VCSEL according to claim 1, wherein the combined reflectivity of the arrangement consisting of the high-contrast grating (28) and the second mirror (20) for the first polarization direction is equal to or greater than 99.9%.

3. VCSEL according to claim 1 or 2, wherein the reflectivity of the second mirror (20) is greater than 90%, preferably less than or equal to 98%.

4. VCSEL according to any one of claims 1 to 3, wherein the reflectivity of the high-contrast grating (28) for the first polarization direction is equal to or greater than 90%, preferably equal to or greater than 95%.

5. VCSEL according to any one of claims 1 to 4, wherein the combined reflectivity of the arrangement consisting of the high-contrast grating (28) and the second mirror (20) for the The second polarization direction is equal to or less than 99.9%, preferably equal to or less than 99.5%, and more preferably equal to or less than 99.0%.

6. VCSEL according to any one of claims 1 to 5, wherein the reflectivity of the high-contrast grating (28) for the second polarization direction is equal to or less than 90%, preferably equal to or less than 80%, further preferably equal to or less than 50%.

7. VCSEL according to any one of claims 1 to 6, wherein the phase difference of the reflectivity of the high-contrast grating (28) and the reflectivity of the second mirror (20) for the first polarization direction is less than + / -(2TT / 3), preferably less than + / -(TT / 2), further preferably less than + / -(TT / 4), further preferably less than + / -(TT / 10).

8. VCSEL according to any one of claims 1 to 7, wherein the high-contrast grating (28) is arranged on the substrate-away side of the second mirror (20).

9. VCSEL according to claim 8, wherein the high-contrast grating (28) is arranged directly on the second mirror (20).

10. VCSEL according to any one of claims 1 to 9, wherein a coating is arranged on a substrate-facing surface of the high-contrast grating (28), in particular wherein the coating has a thickness in the range of 20 to 30 nm, in particular wherein the coating is an ALD layer.

11. VCSEL according to claim 10, wherein a passivation coating and / or a metal layer is arranged on the coating.

12. VCSEL according to any one of claims 1 to 11, wherein the high-contrast grating (28) has a grating constant of 500 nm or greater, preferably 1000 nm or less.

13. VCSEL according to claim 12, wherein the high-contrast grating (28) has a plurality of bars (28a) and trenches (28b) between adjacent bars (28a), and wherein the ratio of bar width to grating constant is in the range of 0.4 to 0.6, preferably in the range of 0.45 to 0.

5.

14. VCSEL according to any one of claims 1 to 13, wherein the active region (18) has an electrically pumped region, in particular wherein the high-contrast grating (28) has a smaller extent in a plane parallel to the substrate (12) than the pumped region.

15. VCSEL according to claim 14, wherein the high-contrast grating (28) is a single high-contrast grating.

16. VCSEL according to claim 14, wherein the high-contrast grating (28) comprises two or more separate, spaced-apart high-contrast gratings or segments (29i) distributed over the electrically pumped area.

17. VCSEL according to claim 16, wherein the two or more individual high-contrast gratings or segments (29i) are arranged at positions along the pumped area where a desired mode has intensity maxima.

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