VCSEL polarization control by structurally birefringent resonators

Abstract

To provide a method of forming a VCSEL with a structurally birefringent resonator.SOLUTION: A method includes a step of growing a bottom distributed Bragg reflector (DBR) and a first portion of a resonator on a substrate to form a bottom structure including a plurality of layers. One or more anisotropic features are etched on an upper side layer of the bottom structure to create a patterned growth interface. On the patterned growth interface, a remaining portion of the resonator and a top DBR are overgrown to form an epitaxial structure. One or more oxide openings are formed within the epitaxial structure.SELECTED DRAWING: Figure 1

Description

[Background technology]

[0001]

[0001] The limitations and disadvantages of conventional vertical cavity surface emitting lasers will become apparent to those skilled in the art by comparing such conventional approaches with certain aspects of the present method and system discussed in the remainder of this disclosure with reference to the drawings. Summary of the Invention

[0002]

[0002] Systems and methods are provided for producing vertical cavity surface emitting lasers (VCSELs) with birefringent resonators operable to control polarization, substantially as illustrated by and / or described in connection with at least one of the figures, and as more fully discussed in the claims. [Brief explanation of the drawings]

[0003] [Figure 1]

[0003] FIG. 1 illustrates an exemplary VCSEL epitaxial structure with a birefringent cavity, in accordance with various exemplary embodiments of the present disclosure. [Figure 2A]

[0004] FIG. 2A is a diagram illustrating an example of a resonator, according to various exemplary embodiments of the present disclosure. [Figure 2B] FIG. 2B is a diagram illustrating an example of a resonator, according to various exemplary embodiments of the present disclosure. [Figure 3A]

[0005] FIG. 3A illustrates an exemplary variation of effective refractive index with respect to grating fill factor, according to various exemplary embodiments of the present disclosure. [Figure 3B]

[0006] FIG. 3B illustrates an exemplary variation of birefringence intensity with respect to grating fill factor, according to various exemplary embodiments of the present disclosure. [Figure 4A]

[0007] FIG. 4A illustrates example emission wavelengths associated with parallel polarization directions and example emission wavelengths associated with orthogonal polarization directions, according to various exemplary embodiments of the present disclosure. [Figure 4B]

[0008] FIG. 4B is a diagram illustrating example gains associated with the emission wavelengths of FIG. 4A, according to various exemplary embodiments of the present disclosure. [Figure 5A]

[0009] FIG. 5A illustrates an example of an oxide aperture VCSEL with a patterned cavity, where the oxide aperture is above a patterned growth interface, according to various exemplary embodiments of the present disclosure. [Figure 5B]

[0010] FIG. 5B illustrates an example of an oxide aperture VCSEL with a patterned cavity, where the oxide aperture is below the patterned growth interface, according to various exemplary embodiments of the present disclosure. [Figure 6A]

[0011] 1A-1C illustrate examples of combining a patterned resonator with a tunnel junction lithographic aperture by two separate etching and overgrowth steps, according to various exemplary embodiments of the present disclosure. [Figure 6B]

[0012] 10A-10C illustrate an example of combining a patterned resonator with a tunnel junction lithographic aperture by two successive etching steps and one overgrowth step, according to various exemplary embodiments of the present disclosure. [Figure 7A]

[0013] 10A-10C illustrate examples of combining a patterned resonator with a blocking lithographic aperture by two separate etching and overgrowth steps, according to various exemplary embodiments of the present disclosure. [Figure 7B]

[0014] 10A-10C illustrate an example of combining a patterned resonator with a blocking layer lithographic opening by two successive etching steps and one overgrowth step, according to various exemplary embodiments of the present disclosure. [Figure 8]

[0015] 1A-1C illustrate exemplary aperture definition by implantation, according to various exemplary embodiments of the present disclosure. [Figure 9]

[0016] 1A-1C illustrate examples of power versus current and polarization characteristics, according to various exemplary embodiments of the present disclosure. [Figure 10]

[0017] 10A-10C illustrate exemplary variations in degree of linear polarization (DOLP) with respect to grating fill factor and grating refractive index contrast, according to various exemplary embodiments of the present disclosure. [Figure 11]

[0018] 1A-1C illustrate examples of optical power and polarization characteristics versus operating current, according to various exemplary embodiments of the present disclosure. DETAILED DESCRIPTION OF THE INVENTION

[0004]

[0019] A vertical-cavity surface-emitting laser (VCSEL) is a type of semiconductor laser diode with a laser beam emitting perpendicularly from the top surface, as opposed to conventional edge-emitting semiconductor lasers, which emit from a surface formed by cleaving individual chips from a wafer. VCSELs are used in a variety of laser products, including computer mice, fiber optic communications, laser printers, Face ID, and smart glasses.

[0005]

[0020] A polarization direction can be favored in a laser when the associated threshold current is lower than other polarization directions. Once the threshold gain for the desired polarization direction is reached, gain clamping prevents the threshold gain of undesired polarized directions from being reached, fixing the polarization. The threshold gain level is controlled by optical gain coefficients and optical losses. Laser polarization control is achieved by inducing polarization-dependent gain coefficients as the first and second mechanisms, and optical losses as the third mechanism. Such polarization-dependent gain and optical losses do not exist by default in VCSELs due to the surface-emitting and circular geometry of the emitter. Therefore, as described in this disclosure, polarization-dependent gain and optical losses can be induced to control the polarization of a VCSEL.

[0006]

[0021] This disclosure describes polarization control in VCSELs by patterning anisotropic features within the cavity and performing overgrowth. The patterning introduces birefringence into the cavity structure. Birefringence is an optical property of a material whose refractive index depends on the polarization and propagation direction of light. Anisotropic materials are characterized by physical properties that have different values ​​when measured in different directions. Birefringent materials are optically anisotropic.

[0007]

[0022] FIG. 1 illustrates an exemplary VCSEL epitaxial structure with a birefringent cavity, according to various exemplary embodiments of the present disclosure.

[0023] The epitaxy structure depicted in FIG. 1 is formed as follows: In a first growth, a first portion of a cavity 105, including a lower distributed Bragg reflector (DBR) 101 and an active region quantum well (QW) 103, is grown on a substrate. Anisotropic features are then lithographically defined on the upper layer of the substructure and shallowly etched using either dry or wet etching. The interface between the lower and upper layers is located within the cavity 105. The substructure includes the lower DBR 101, the active region QW 103, and the upper layer 105a of the substructure. The upper structure includes the lower layer 105b of the upper structure and the upper DBR 107.

[0008]

[0024] In the top-emitting configuration, the top DBR 107 has a lower reflectivity than the bottom DBR 101. In the bottom-emitting configuration, the bottom DBR 101 has a lower reflectivity compared to the top DBR 107. In the bottom-emitting configuration, the laser beam travels through the substrate.

[0009]

[0025] Examples of anisotropic features include linear gratings. Gratings may also be 2D and contain anisotropic features, e.g., oval / elliptical, rectangular, diamond, or circular / square, with different periodicity in orthogonal directions. Linear gratings may be periodic, quasi-periodic, or can be a random linear lattice.

[0010]

[0026] The remainder of the cavity 105 and the top DBR 107 are overgrown to complete the VCSEL cavity 105 (e.g., a Fabry-Perot cavity). The resulting epitaxial structure can then undergo standard wafer processing to form an oxide-apertured VCSEL emitter. Note that both front-side and back-side emission can be considered with this approach. Different configurations are further described below with respect to Figures 5 and 6.

[0011]

[0027] 2A and 2B illustrate examples of resonators according to various exemplary embodiments of the present disclosure.

[0028] In the first exemplary configuration (FIG. 2A), the overgrown layer has a different material refractive index n1 compared to n0 of the etched layer of the underlying structure. A planar reflector results in a variation in the optical thickness of the cavity that follows the profile of the etched feature.

[0012]

[0029] In a second exemplary configuration (FIG. 2B), the overgrowth layer has the same refractive index as the etched layer of the underlying structure. If the overgrowth interface is close to one of the reflectors, the etched feature is transferred to the reflector profile. Both configurations result in a variation in the optical thickness of the resonator that follows the etched profile. Due to the variation in the optical thickness of the resonator, the local refractive index can be shifted by Δn. As a result, the refractive index profile follows the etched feature.

[0013]

[0030] The anisotropic profile of refractive index exhibits an equivalent effective refractive index that is anisotropic, i.e., birefringent. The refractive index parallel to the grating, n ∥ (slow axis) is the refractive index n ⊥ (fast axis). The difference between the refractive index of the slow axis and the refractive index of the fast axis (n ∥ -n ⊥ ) is Δn. The ratio (n ∥ -n ⊥ ) / n0 defines the birefringence strength, which depends on the grating geometry and the grating refractive index contrast Δn / n0.

[0014]

[0031] FIG. 3A illustrates an exemplary variation of effective refractive index with respect to grating fill factor, according to various exemplary embodiments of the present disclosure.

[0032] FIG. 3B illustrates an exemplary variation of birefringence intensity with respect to grating fill factor, according to various exemplary embodiments of the present disclosure.

[0015]

[0033] The fill factor (FF) is defined as the ratio of the grating ridge width to the grating period. For a grating refractive index contrast (Δn / n0) of 3%, the birefringence is maximum for a fill factor of about 0.5. The birefringence intensity is estimated to be about 0.02%, which is high enough to take into account the gain clamping mechanism.

[0016]

[0034] In a cavity with a birefringent medium, light polarized parallel and perpendicular to the slow axis propagates due to different refractive indices. Figure 4A shows examples of emission wavelengths associated with parallel polarization directions and perpendicular polarization directions, according to various exemplary embodiments of the present disclosure. Considering that the cavity thickness is the same for both directions, the two directions correspond to two different emission wavelengths λ, as illustrated in Figure 4A. ∥ (slow axis) and λ ⊥ (fast axis). Optical thickness is the product of physical thickness and refractive index. Optical thickness determines the emission wavelength.

[0017]

[0035] FIG. 4B shows an example of gain associated with emission wavelength, according to various exemplary embodiments of the present disclosure. Quantum wells (QWs) typically provide gain that varies with wavelength. Combining the polarization-dependent emission wavelength and wavelength-dependent gain results in a polarization-dependent gain as shown in FIG. 4B. The polarization direction associated with the highest gain coefficient is favored. The favored polarization direction by this first mechanism depends on the gain variation with wavelength. For wavelengths greater than the wavelength at the gain maximum, the gain decreases with increasing wavelength. Thus, polarization along the fast axis (i.e., perpendicular to the grating direction) is favored. Thus, in the first mechanism, the polarization direction is selected by the polarization-dependent gain.

[0018]

[0036] Second, resonators with birefringent properties can exhibit polarization-direction-dependent confinement factors. Light polarized along the fast axis tends to be less confined compared to light polarized along the slow axis. In VCSELs, the transverse gain profile is not constant across the aperture due to current crowding and spatial hole burning. Thus, the slightly less confined light polarized perpendicular to the grating receives more gain and is favored.

[0019]

[0037] In the third mechanism, a linear grating couples light from the vertical direction to the horizontal direction. The coupling efficiency is polarization dependent, and light polarized parallel to the grating ridge direction is more likely to be coupled. This results in additional cavity loss for parallel polarizations. Orthogonal polarizations are thus favored. This mechanism has the advantage of being independent of the gain curve. Thus, this third mechanism uses polarization-dependent optical loss.

[0020]

[0038] Polarization control by anisotropic patterning of the cavity can be combined with different fabrication methods to define the electrical and optical aperture of the VCSEL. The VCSEL aperture can be defined by oxidation, implantation, or lithographically. Lithographic aperture VCSELs can be achieved with subsequent etching and overgrowth of tunnel junctions or blocking layers.

[0021]

[0039] 5A and 5B present two configurations of a patterned resonator with an oxide aperture. For both configurations, fabrication begins with the formation of an epitaxial structure with a patterned resonator, as presented in paragraph 23. The epitaxial structure includes a high aluminum concentration layer. During oxidation of the structure, this layer oxidizes at a faster rate than the rest of the structure. The unoxidized portion of the layer forms an aperture that confines the flow of the operating current and the laser beam.

[0022]

[0040] 5A shows an example of an oxide aperture VCSEL with a patterned cavity, in which the oxide aperture is above the patterned growth interface, according to various exemplary embodiments of the present disclosure. In FIG. 5A, the oxide layer forming the aperture is located above the patterned growth interface.

[0023]

[0041] 5B shows an example of an oxide aperture VCSEL with a patterned cavity, where the oxide aperture is below the patterned growth interface, according to various exemplary embodiments of the present disclosure. In FIG. 5B, the oxide layer is located below the patterned growth interface and is part of the lower epitaxial structure.

[0024]

[0042] 6A and 6B show how to combine a patterned VCSEL resonator with a tunnel junction lithographic aperture. Subsequent etching and overgrowth steps allow for the insertion of buried tunnel junctions on specific areas. The lithographically defined areas with tunnel junctions are significantly more conductive than areas without tunnel junctions, which allows for lateral electrical confinement and defines the VCSEL aperture. Two process flows are possible to introduce anisotropic patterning of the resonator. It is Noh.

[0025]

[0043] FIG. 6A illustrates an example of combining a patterned resonator with a tunnel junction lithographic aperture through two separate etching and overgrowth steps, according to various exemplary embodiments of the present disclosure. In the process flow presented in FIG. 6A, aperture definition and grating patterning are performed in separate etching and growth steps. The process flow begins with the growth of a lower epitaxial structure, including a lower DBR and active region, and terminated by a tunnel junction layer. After the first etching step, the remaining portion of the tunnel junction defines the aperture. A spacer layer is overgrown. A grating for polarization control is patterned in the spacer layer. Finally, an upper epitaxial structure, including an upper DBR, is overgrown.

[0026]

[0044] FIG. 6B illustrates an example of combining a patterned resonator with a tunnel junction lithographic aperture using two sequential etching steps and one overgrowth step, according to various exemplary embodiments of the present disclosure. In the second process flow presented in FIG. 6B, the aperture and grating are defined in two sequential etching steps and one overgrowth. This process flow allows the number of overgrowths to be reduced from two to one. In this case, the bottom epitaxial structure is terminated by the tunnel junction layer and spacer layer. A first etch defined the aperture down to the bottom of the tunnel junction. Then, a shallower etch defines the grating for polarization control. Finally, the top epitaxial structure is overgrown.

[0027]

[0045] 7A and 7B show how to introduce a blocking layer lithographic opening into a patterned resonator. Subsequent etching and overgrowth steps allow for the insertion of a blocking layer over specific areas in the epitaxial structure. The lithographically defined areas with the blocking layer have a significantly higher electrical resistance than areas without the blocking layer, which allows for lateral electrical confinement and defines the VCSEL opening. Two process flows are possible for introducing anisotropic patterning of the resonator.

[0028]

[0046] 7A illustrates an example of combining a patterned cavity with a blocking layer lithographic aperture through two separate etching and overgrowth steps, according to various exemplary embodiments of the present disclosure. In the process flow presented in FIG. 7A, aperture definition and grating patterning are performed in separate etching and growth steps. The process flow begins with the growth of a lower epitaxial structure, including a lower DBR and active region, terminated by a blocking layer. A first etching step of the blocking layer defines the aperture. A spacer layer is overgrown. A grating for polarization control is patterned in the spacer layer. Finally, an upper epitaxial structure, including an upper DBR, is overgrown.

[0029]

[0047] FIG. 7B illustrates an example of combining a patterned cavity with a blocking lithographic aperture by two successive etching steps and one overgrowth step, according to various exemplary embodiments of the present disclosure. In the second process flow presented in FIG. 7B, the aperture and grating are defined in two successive etching steps and one overgrowth. This process flow allows the number of overgrowths to be reduced from two to one. In this case, the lower epitaxial structure is terminated by a blocking layer and a spacer layer. A first etch defined the aperture down to the bottom of the blocking layer. Then, a shallower etch defines the grating. Finally, the upper epitaxial structure is overgrown.

[0030]

[0048] 8 shows an example of combining a patterned resonator with an aperture defined by ion implantation. Fabrication begins with the formation of an epitaxial structure with a patterned resonator, as presented in paragraph 23 (above). In the following steps, ion implantation electrically isolates regions of the epitaxial structure. The non-implanted regions confine the flow of operating current and define the VCSEL aperture.

[0031]

[0049] 9 shows an example of power versus current and polarization characteristics according to various exemplary embodiments of the present disclosure. The three graphs in FIG. 9 represent optical power, DOLP, and azimuth angle.

[0032]

[0050] Figure 9 shows the impact of introducing a linear grating into the cavity on the optical versus current and polarization characteristics. In the absence of a grating, the degree of linear polarization (DOLP) reaches a high value of 95% (polarization extinction ratio (PER) 13 dB) at threshold, which is attributed to single-mode operation. As the operating current increases and higher-order modes emerge, DOPL decreases significantly, indicating poor polarization control. With the presence of a linear grating in the cavity, DOLP remains at >99% (PER >20 dB) from threshold until the device begins to roll over. Azimuthal angular measurements show that the linear polarization direction remains perpendicular to the grating direction. This demonstrates the effective linear polarization stability resulting from the linear grating in the cavity.

[0033]

[0051] 10 shows an example variation of DOLP with respect to grating fill factor and grating refractive index contrast according to various exemplary embodiments of the present disclosure. The grating refractive index contrast is estimated from the emission wavelength difference from the unetched (FF=1) and maximally etched (FF=0) cavity emitters and using the following relationship:

[0034] Δn / n0=Δλ / λ0

[0052] Thus, the deeper the etch depth, the greater the grating refractive index contrast, resulting in a stronger birefringence. Effective control of polarization is demonstrated for grating fill factors ranging from 0.2 to 0.3 when the grating refractive index contrast is 2%. For grating refractive index contrasts of 3%, the fill factor range for stable polarization is wider, from 0.1 to 0.5. As the grating refractive index contrast becomes larger, the manufacturing tolerances on the grating fill factor are relaxed.

[0035]

[0053] FIG. 11 shows an example of optical power and polarization characteristics versus operating current according to various exemplary embodiments of the present disclosure. The three graphs in FIG. 11 represent optical power, DOLP, and azimuth angle. FIG. 11 shows the change in optical power and polarization characteristics versus operating current for a VCSEL device with a grating under backside temperatures ranging from 25° C. to 75° C. Polarization remains stable in terms of intensity (DOLP>99%, PER>20 dB) and direction (perpendicular to the grating) throughout the entire operating range investigated. This demonstrates the robustness of polarization control by cavity patterning. The exemplary device reference in FIG. 11 is a 6 μm diameter VCSEL with a cavity grating having a 1 μm period and a fill factor of 0.2.

[0036]

[0054] Although the present methods and / or systems have been described with reference to certain embodiments, it will be understood by those skilled in the art that various modifications may be made and equivalents substituted without departing from the scope of the present methods and / or systems. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the present disclosure without departing from the scope of the present disclosure. Therefore, it is not to be understood that the present methods and / or systems are limited to the particular embodiments disclosed, but the present methods and / or systems may be modified in accordance with the accompanying drawings. It is intended to include all embodiments that fall within the scope of the appended claims. [Explanation of symbols]

[0037] 101 Lower Distributed Bragg Reflector (DBR), Lower DBR 103 Active region quantum well (QW), active region QW 105 Resonator 105a Upper layer of substructure 105b Lower layer of the superstructure 107 Upper DBR

Claims

1. A substructure, a lower distributed Bragg reflector (DBR); a first portion of a resonator; a patterned growth interface etched onto the upper layer of the substructure; a substructure including:

1. An epitaxial structure comprising: a second portion of the resonator operatively coupled to the patterned growth interface; and an upper DBR operatively coupled to the patterned growth interface; an epitaxial structure comprising: One or more openings; wherein one or more features of the patterned growth interface are transferred to a profile of the top DBR.

2. A substructure, a lower distributed Bragg reflector (DBR); a first portion of a resonator; a patterned growth interface etched onto the upper layer of the substructure; a substructure including:

1. An epitaxial structure comprising: a second portion of the resonator operatively coupled to the patterned growth interface; and an upper DBR operatively coupled to the patterned growth interface; an epitaxial structure comprising: One or more openings; 1. A laser structure comprising: a first growth interface; a second growth interface; a second growth interface; a third growth interface; a fourth growth interface; a fourth growth interface; a fourth growth interface; a fourth growth interface; a fifth growth interface; a fifth growth interface; a fifth growth interface; a fifth growth interface; a sixth ...

3. A substructure, a lower distributed Bragg reflector (DBR); a first portion of a resonator; a patterned growth interface etched onto the upper layer of the substructure; a substructure including:

1. An epitaxial structure comprising: a second portion of the resonator operatively coupled to the patterned growth interface; and an upper DBR operatively coupled to the patterned growth interface; an epitaxial structure comprising: One or more openings; wherein the patterned growth interface determines birefringence strength according to the ratio of grating ridge width to grating period.

4. A substructure, a lower distributed Bragg reflector (DBR); a first portion of a resonator; a patterned growth interface etched onto the upper layer of the substructure; a substructure including:

1. An epitaxial structure comprising: a second portion of the resonator operatively coupled to the patterned growth interface; and an upper DBR operatively coupled to the patterned growth interface; an epitaxial structure comprising: One or more openings; wherein the one or more apertures are formed by growing an oxide layer below the patterned growth interface as part of the substructure.

5. A substructure, a lower distributed Bragg reflector (DBR); a first portion of a resonator; a patterned growth interface etched onto the upper layer of the substructure; a substructure including:

1. An epitaxial structure comprising: a second portion of the resonator operatively coupled to the patterned growth interface; and an upper DBR operatively coupled to the patterned growth interface; an epitaxial structure comprising: One or more openings; 1. A laser structure comprising: a patterned vertical cavity surface emitting laser (VCSEL) cavity having a tunnel junction lithographic aperture.

6. A substructure, a lower distributed Bragg reflector (DBR); a first portion of a resonator; a patterned growth interface etched onto the upper layer of the substructure; a substructure including:

1. An epitaxial structure comprising: a second portion of the resonator operatively coupled to the patterned growth interface; and an upper DBR operatively coupled to the patterned growth interface; an epitaxial structure comprising: One or more openings; 1. A laser structure comprising: a substructure including a lithographic aperture;

7. 7. A laser structure according to any one of claims 1 to 6, wherein the first portion of the cavity comprises a plurality of active region quantum wells.

8. 7. A laser structure according to any one of claims 1 to 6, wherein the patterned growth interface is wet etched or dry etched.

9. 7. A laser structure according to any one of claims 1 to 6, wherein the patterned growth interface comprises a linear grating.

10. 7. A laser structure according to any one of claims 1 to 6, wherein the substructure has a different material refractive index than the epitaxial structure.

11. 7. The laser structure of claim 1, wherein the patterned growth interface comprises a grating characterized by a grating ridge width and a grating period.

12. 7. A laser structure according to claim 1, wherein the one or more openings are formed by growing an oxide layer above the patterned growth interface as part of the top growth of the epitaxial structure.

13. 7. A laser structure according to any one of claims 1 to 6, wherein the upper layer of the substructure is a spacer layer.

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