Patent Application
20220069546
The present invention relates to vertical-cavity semiconductor lasers (VCSELs), and arrays of VCSELs. More particularly, this disclosure relates to VCSELs and VCSEL arrays having controlled distribution of spatial (also referred to as transverse) modes. VCSELs employing structural features configured to control output light distribution can have improved performance in applications including illumination, sensing and communications.
Vertical-cavity surface-emitting lasers (VCSELs) are a class of semiconductor lasers with many applications and offer various advantages when compared to edge-emitting lasers. The planar structure of these devices, configured to provide light emission along an axis that is transverse to the layers of the semiconductor structure, allows on-wafer testing (before dicing and packaging of individual devices or arrays); the ability to form both one-dimensional and two-dimensional arrays; low divergence output beams that facilitate efficient coupling to optical fibers, waveguides and other optical elements; the use of traditional low-cost light emitting diode (LED) packaging technology; as well as integration with electronic, optoelectronic and optical elements, high reliability and high efficiency.
The successful use of VCSELs and VCSEL arrays has been demonstrated in optical-fiber-based data and telecommunication applications (typically over shorter distances of about 1 mile or less, such as in local area networks and data centers, for example), but they are now finding use in a variety of other applications including free-space optical interconnects, sensors, illumination sources for systems such as three-dimensional cameras or gesture recognition systems, dot projectors for structured-light sources, and automotive LIDAR. These devices typically operate at wavelengths of about 850 nm (which light is produced using gallium arsenide (GaAs) quantum-well (QW) active regions), wavelength(s) between about 940 nm and 980 nm (when indium gallium arsenide (InGaAs) QW active regions are employed), and—more recently—wavelength(s) between about 1250 nm and 1600 nm (when devices are structured to utilize dilute nitride QW active regions).
For some applications, such as data communications or sensing, it is generally desirable to provide a VCSEL device characterized by a substantially single-mode operation with an output beam having a substantially circular cross-section. Output power of such single mode operation can be limited, however, and special control is required to enhance the available single-mode-power output. Other applications (such as 3D imaging, illumination, object or gesture recognition, LIDAR, optical coherence tomography, and interference microscopy, for example) can also benefit from an improved mode control, where the output beam has a differently-shaped cross-sectional distribution of irradiance, such as one with a ring shape, or a dumb-bell shape, or where multiple transverse modes exist. Since such applications require higher levels of optical power (ranging from a few tens of milliwatts up to about 10 W), the use of laser arrays instead of single laser device may be preferred. To ensure the devices perform with the desired characteristics, mode-controlling techniques are usually applied to the VCSEL device.
Improvement of single-mode emission in VCSELs was achieved through control of at least one of the spatial distribution of reflectivity and optical loss. (For example, the use of a passive anti-guide region was demonstrated to yield an improved single-mode performance.) Another approach was to spatially modify either the reflective feedback from at least one of the mirrors that are included in the VCSEL structure, or the optical loss for different modes having different spatial distributions. A limitation of both of these techniques, however, is that the maximum achievable single-(spatial)-mode output power is relatively low.
In the case of ring-mode devices, the industrial standards require either the appropriate patterning of the laser reflectors (which can expose aluminum-containing layers, thereby affecting reliability of operation of the laser structure and, quite possibly, current injection) or a complex and involved formation of multiple apertures.
A person of skill readily appreciates that there remains a need to control the spatial mode performance of VCSEL devices, to ensure multimode operation or ring-mode operation. Furthermore, in applications such as imaging or illumination, the spatial output patterns from the devices should overlap in the far-field in order to reduce speckle contrast, which arises from the coherence of laser sources. When coherent light is reflected from a diffused surface, it is as though each point of the surface is emitting a light wave. Generally, all of the reflected light waves have the same frequency, but the phase and amplitude of the light reflected from different parts of the surface will vary. The light will interfere constructively and destructively producing a pattern of light and dark spots that appears random. In an array of VCSELs, while an individual device can be coherent, the individual VCSELs in a laser array are not coherent with each other, and if the emission from the VCSELs overlaps, for example, in the far-field, the speckle contrast for an array decreases as the square root of the number of devices in an array.
Similarly, the speckle contrast is a function of the speckle contrast of an individual laser, and the individual VCSEL speckle contrast can be reduced in devices operating with multiple spatial output spots, filaments and transverse modes. While an individual spatial filament or mode within a device aperture can be fully coherent, the degree of coherence of the superposition of all transverse modes or filaments within a device aperture is reduced, which reduces the speckle contrast produced by a device. The different spatial modes or filaments can also exhibit different wavelengths and thus the linewidth of a multimode VCSEL can be greater than 0.5 nm or 1 nm or 1.5 nm.
Embodiments of the invention provide a vertical cavity surface-emitting laser (VCSEL) structure that includes first and second reflectors; a gain medium between these reflectors; a peripheral material layer having an output aperture in this layer; and at least one confining material layer disposed across the longitudinal axis of the VCSEL structure between the first and second reflectors. Such confining material layer has at least one confining aperture in it. (In a specific case, at least one of the output aperture and the at least one confining aperture may be dimensioned to be between 3 microns and 50 microns.) In addition, the first and second axes (the first being an axis of the output aperture and transverse to a plane of the output aperture, and the second being an axis of the at least one confining aperture and transverse to a plane of at least one confining aperture) do not coincide with one another. In one implementation, the VCSEL structure is configured to satisfy at least one of the following conditions: a) the output aperture is dimensioned to have no more than two axes of symmetry in the plane of the output aperture; a lateral extent (of at least one of the peripheral material layer and at least one confining material layer) considered in a first plane that is transverse to the longitudinal axis is smaller than a lateral extent of the active region in a second plane that is parallel to the first plane; and c) at least one confining layer includes first and second confining layers each of which is disposed between the first and second reflectors, while the first and second confining layers are located on the opposite sides of the gain medium. Alternatively or in addition, the VCSEL structure may have only one axis of symmetry. In substantially any implementation, at least one of the following conditions may be satisfied: i) a value of a lateral offset (measured in a plane parallel to a plane of at least one confining aperture) between the first axis and the second axis is at least 1 micron; and ii) such value of the lateral offset does not exceed 40% of a dimension of the at least one confining aperture at hand. Alternatively or in addition, the peripheral material layer may be configured as a metallic layer providing an electrical contact layer of the VCSEL structure, and/or be dimensioned to include a first peripheral portion and a second portion surrounded by the first peripheral portion. (In such a case, the first peripheral portion may be dimensioned to define a ring-shaped stripe of material while the second portion may be dimensioned to satisfy one of the following conditions: (i) the second portion is configured as radially-extended stripe of material, and (ii) the second portion is configured as a stripe connecting two different sides of a polygonally-shaped first peripheral portion and while the first and second portion is electrically connected at at least one point. Alternatively, in substantially any implementation where the peripheral material layer is dimensioned to include a first peripheral portion and a second portion surrounded by the first peripheral portion, the first peripheral portion may be dimensioned to define a closed upon itself stripe of material having a closed internal perimeter and a closed external perimeter, and the second portion may be dimensioned to cover a geometrical center of the first peripheral portion.)
In some cases, the implementation of the VCSEL structure of the invention may be configured to generate a light output that provides, in operation, a spatial distribution of irradiance in one of the following forms: a) a ring-shaped distribution of irradiance, and b) a dumb-bell-shaped distribution of irradiance (as defined in a plane transverse to an axis of the light output) and/or to satisfy at least one of the following conditions: i) at least one confining layer present in the structure includes first and second confining layers (the first confining layer having a first confining aperture therein and the second confining layer having a second confining aperture therein); and ii) these first and second confining layers are located on the opposite sides of the gain medium. (Alternatively or in addition, the VCSEL structure may be configured to satisfy at least one of the following conditions: a) a first portion of at least one of these first and second confining layers has a density of oxygen molecules that is lower than that of a second portion of the at least one of the first and second confining layers, and b) a first portion of at least one of the first and second confining layers has electrical resistivity that is lower than that of a second portion of the at least one of the first and second confining layers. Here, the first portion defines a chosen confining aperture, of the first and second confining apertures, and the second portion is located outside of this chosen confining aperture of the first and second confining apertures. Alternatively or in addition, the first and second confining apertures may be formed such that an axis of the first confining aperture and an axis of the second confining aperture may not coincide with one another—in which case there exists a non-zero lateral offset between projections of a center of the first confining aperture and a center of the second confining aperture on a plane that is substantially parallel to a plane of the at least one confining material layer.) In substantially any implementation, at least one of the first and second reflectors of the VCSEL structure may be configured as a distributed Bragg reflector (DBR), and in this case at least one of the first and second confining layers may be disposed within bounds of the DBR.
Embodiments of the invention additionally provide a VCSEL array that includes a plurality of the VCSEL structures each configured according to an implementation described above. In one case, such VCSEL array is structured to satisfy at least one of the following conditions: a) a first VCSEL structure from the plurality is different from a second VCSEL structure from the plurality of the VCSEL structures; b) at least two output apertures (respectively-corresponding to two VCSELs structures of the plurality of the VCSEL structures) have no more than two axes of symmetry each, while an axis of symmetry in this case is defined in a plane of a corresponding aperture; c) each of at least first and second VCSEL structures from the plurality of the VCSEL structures has corresponding output and confining apertures that are not co-axial with one another, and d) a VCSEL structure from the plurality of the VCSEL structures has an output aperture that is rotationally-symmetric, while a confining aperture of such VCSEL structure is not co-axial with the output aperture of this VCSEL structure (from the plurality of the VCSEL structures of the array).
Alternatively or in addition, the implementation of the VCSEL array may include a plurality of lens elements respectively-corresponding to and operably cooperated with the plurality of the constituent VCSEL structures. In this case, i) first and second locations, defined within bounds of the first and second output apertures of respectively-corresponding first and second VCSEL structures, and ii) first and second axes of respectively-corresponding first and second lens elements from the plurality of lens elements may be made shifted with respect to one another in a plane parallel to a layer of a VCSEL structure from the plurality of the VCSEL structures. Alternatively or in addition, at least one of the following conditions may be satisfied: a) longitudinal axes of constituent VCSEL structures of the array form a first spatially-irregular grid of axes, and b) optical axes of lens elements from the plurality of lens elements form a second spatially-irregular grid of axes. Optionally, the plurality of lens elements may be formed on the same substrate and configured as a stand-alone optical component (in which case such plurality of lens elements may be placed, if desired, to be separated from the plurality of the VCSEL structures by this same substrate).
Embodiments of the invention additionally provide a vertical cavity surface-emitting laser (VCSEL) structure that has a longitudinal axis and that includes first and second reflectors; a gain medium between the first and second reflectors; and a peripheral material layer defining an output aperture therein, the output aperture dimensioned to have no more than two axes of symmetry of the output aperture. This structure may additionally contain at least one confining material layer disposed across the longitudinal axis between the first and second reflectors (the confining material layer defining at least one confining aperture in this layer). Here, an axis of the output aperture and an axis of the at least one confining aperture may be made to not coincide with one another.
Substantially in any embodiment, the VCSEL structure of the invention may be configured such that at least one of i) the at least one confining aperture and ii) the output aperture is substantially coaxial with the longitudinal axis of the structure itself; and/or such that a lateral extent of at least one of the peripheral material layer and the at least one confining material layer in a first plane is smaller than a lateral extent of the active regions in a second plane (here, the first plane is defined to be transverse to the longitudinal axis, and the second plane is defined to be parallel to the first plane); and/or such that at least one confining layer present in the structure includes first and second confining layers (each of which is disposed between the first and second reflectors) while the first and second confining layers are located on the opposite sides of the gain medium; and/or such that the VCSEL structure has only one axis of symmetry.
Alternatively or in addition, and in substantially any implementation of the VCSEL structure, the peripheral material layer may be configured as a metallic layer structured as an electrical contact layer of the VCSEL structure, and/or the peripheral material layer may be dimensioned to include a first peripheral portion and a second portion surrounded by the first peripheral portion. (Or, in the alternative, the first peripheral portion may be dimensioned to define a closed upon itself stripe of material having a closed internal perimeter and a closed external perimeter, while the second portion is dimensioned to cover a center of the first peripheral portion. In one specific case, the first peripheral portion may be dimensioned to define a ring-shaped stripe of material and the second portion may be dimensioned to satisfy one of the following conditions: (i) the second portion is configured as radially-extended stripe of material, and (ii) the second portion is configured as a stripe connecting two different sides of a polygonally-shaped first peripheral portion; at the same time, the first and second portions are made electrically connected with one another at at least one point. Furthermore, the first peripheral portion may be dimensioned to define a stripe of material having a closed polygonal perimeter, while the second portion is dimensioned such as to cover a surface area enclosed by the first peripheral portion and to establish electrical contact with at least one side of the polygonal perimeter.)
In substantially any implementation, the VCSEL structure may be configured to generate, in operation, a light output that includes multiple spatial modes; and/or to generate such light output in which a spatial distribution of the light output is not symmetric with respect to the axis of the output aperture; and/or have the output aperture dimensioned to be is between about 3 microns and about 50 microns; and/or to generate light having a spectral bandwidth (or spectral linewidth) of a width that satisfies at least one of the following conditions i) this width is greater than 0.5 nm; b) this width is greater than 1.0 nm; and c) this width greater than 1.5 nm; and/or to generate a light output that has a spatial distribution of irradiance in a form of a ring, as defined in a plane transverse to an axis of the light output (or, alternatively, a spatial distribution of irradiance that has a dumb-bell shape, as defined in the same plane).
In at least one embodiment, a first aperture (chosen from the output aperture and the at least one confining aperture), has a first dimension while a second aperture (from the remaining apertures) has a second dimension, such that a difference between the first and second dimensions satisfies at least one of the following conditions: a) such difference is equal to or smaller than 6 microns; b) such difference is equal to or smaller than 4 microns; and c) such difference is equal to or smaller than 2 microns. In at least one embodiment, the first and second apertures can be formed such that a first axis of the first aperture and a second axis of the second aperture are made substantially parallel to one another and separated by a distance that satisfies at least one of the following conditions: a) such distance is smaller than 40% of a value representing a dimension of the smallest of the first and second apertures; and b) such distance is at least 1 μm. Optionally, the first and second apertures can be formatted to have substantially equal dimensions.
In at least one embodiment—and preferably in any embodiment—the peripheral material layer may be a layer of metal, while the at least one confining material layer is configured to spatially-confine a spatial distribution of current, during the operation of the VCSEL structure, within the at least one confining aperture. In a specific case, there may be first and second confining material layers in the VCSEL structure (the first confining material layer having a first confining aperture and the second confining material layer having a second confining aperture). In this case, the VCSEL structure may be configured to satisfy at least one of the following conditions: a) a first portion of at least one of the first and second confining material layers has a lower density of oxygen molecules than a second portion of the at least one of the first and second confining layers, and b) a first portion of at least one of the first and second confining layers has electrical resistivity that is lower than that of a second portion of the at least one of the first and second confining material layers (while the first portion defines a chosen confining aperture of the first and second confining apertures and the second portion is located outside of such chosen confining aperture).
In substantially any implementation containing first and second confining material layers, at least one of the first and second reflectors may be configured as a distributed Bragg reflector (DBR), and/or at least one of the first and second confining material layers may be disposed within the bounds of the DBR, and/or the first and second confining material layers may be disposed on the opposite sides of the gain medium.
In substantially any implementation, multiple embodiments of the above-described VCSEL structures can be judiciously grouped to form a VCSEL array. Such VCSEL array can be configured to satisfy at least one of the following conditions: a) a first VCSEL structure from the plurality of the constituent VCSEL structures is different from a second VCSEL structure from such plurality; b) at least two output apertures, respectively-corresponding to two VCSELs structures from the plurality, have no more than two axes of symmetry each (here, an axis of symmetry defined in a plane of a corresponding aperture); and c) each of at least first and second VCSEL structures from the plurality has corresponding output and confining apertures that are not co-axial with one another. In at least one embodiment of the array, the array may additionally contain a plurality of lens elements respectively-corresponding to and operably cooperated with the plurality of the constituent VCSEL structures. Here, i) first and second locations, defined within the bounds of the first and second output apertures of respectively-corresponding first and second constituent VCSEL structures of the array, and ii) first and second axes of respectively-corresponding first and second lens elements from the plurality of lens elements can be structured to be spatially shifted—with respect to one another—in a plane parallel to a layer of a VCSEL structure of the array. Alternatively or in addition, in such a VCSEL array at least one of the following conditions may be satisfied: a) longitudinal axes of constituent VCSEL structures form a first spatially-irregular grid of axes, and b) optical axes of lens elements from the plurality of lens elements form a second spatially-irregular grid of axes; and/or the plurality of lens elements is formed on the same substrate to define a stand-alone optical component; and/or such plurality of lens elements is disposed to be separated from the plurality of the VCSEL structures by the very same substrate.
The following Description is made in reference to the Drawings that are used for illustration of but examples of implementations of the idea of the invention, are generally not to scale, and are not intended to limit the scope of the present disclosure. Of the Drawings:
Generally, to facilitate clarity of presentation, not all elements present in one Drawing may necessarily be shown in another.
The following detailed description refers to the accompanying drawings that show, by way of illustration, specific details and embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the present invention. Other embodiments may be utilized, and structural, logical, and electrical changes may be made without departing from the scope of the invention. Various embodiments discussed below are not necessarily mutually exclusive, and sometimes can be appropriately combined. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the embodiments of the present invention is defined only by the appended claims, along with the full scope of equivalents to which such claims are entitled.
Notwithstanding that the numerical ranges and parameters used in the description are approximations, these numerical values in the specific examples are reported as precisely as possible. Any numerical value, however, inherently contains certain errors necessarily resulting from the standard variation found in their respective testing measurements.
In particular, any numerical range recited herein is intended to include all sub-ranges encompassed therein and are inclusive of the range limits. For example, a range of “1 to 10” is intended to include all sub-ranges between (and including) the recited minimum value of about 1 and the recited maximum value of about 10, that is, having a minimum value equal to or greater than about 1 and a maximum value of equal to or less than about 10.
Also, in this application, the use of “or” means “and/or” unless specifically stated otherwise, even though “and/or” may be explicitly used in certain instances.
The term “lattice-matched”, or similar terms, refer to semiconductor layers for which the in-plane lattice constants of the materials forming the adjoining layers materials (considered in their fully relaxed states) differ by less than 0.6% when the layers are present in thicknesses greater than 100 nm. Further, in devices such as VCSELs with multiple layers forming individual regions (such as mirrors) that are substantially lattice-matched to each other means define the situation when all materials in the junctions, that are present in thicknesses greater than 100 nm and considered in their fully-relaxed stated, have in-plane lattice constants that differ by less than 0.6%. Alternatively, the term substantially lattice-matched or “pseduomorphically strained” may refer to the presence of strain within a layer (which may also be thinner than 100 nm), as would be understood from context of the discussion. As such, base material layers, of a given layered structure, can have strain from 0.1% to 6%, from 0.1% to 5%, from 0.1% to 4%, from 0.1 to 3%, from 0.1% to 2%, or from 0.1% to 1%; or can have strain less than 6%, less than 5%, less than 4%, less than 3%, less than 2%, or less than 1%. Layers made of different materials with a lattice parameter difference, such as a pseudomorphically strained layers, can be grown on top of other lattice matched or strained layers without generating misfit dislocations. The term “strain” generally refers to compressive strain and/or to tensile strain.
In reference to structured mentioned above, schematic cross-sectional views of two typical VCSEL devices are shown in
In
It will be understood that other layers such as current spreading layers and contacting layers can also be included, more than one confinement region at different depths within the device can be used, and that different electrical contacting configurations used, such as intracavity contacts. However, these have not shown for the sake of simplicity in order to clearly to explain key device design elements. Devices designs with concentric apertures and with symmetry are used to provide uniform current injection and optical guiding within the device.
Metal aperture 122′ is typically circular, though can have other shapes, similar to aperture 114′, and has a second dimension that can equal the first dimension of aperture 114′ or be different to the first dimension of aperture 114′. The method used to form the device 100′ and its apertures 114′ and 122′ align the apertures such that they are concentric and have rotational symmetry. Thus for apertures of different size, the overlap region between the different apertures is the same on either side of the aperture. As with device 100 in
As mentioned previously, in certain applications, VCSELs operating with a single spatial mode are desired. However, the aperture sizes for such devices are typically small (around 6 to 8 μm diameter apertures) and have limited single-mode output powers. Larger area devices can offer higher output powers, but these will typically operate with multiple spatial (or transverse) modes. A problem with larger devices is that spatial mode transitions from one pattern of emission to another pattern of emission can occur at different current injections levels and are difficult to be able to control or predict. Larger area devices also operate at higher currents, and resistive heating effects can affect the mode performance of the devices.
Therefore typically, specific structures are required in order to control the device modal performance. Multiple modes exhibiting a ring-shaped pattern can be useful in multimode fiber based optical communications, and controlled spot patterns may also be useful as structured light sources for imaging applications.
Accordingly, a problem of ensuring a multi-mode lasing (that is, producing a light output, containing multiple and/or high-order spatial modes from) of an individual VCSEL device or an array of VCSEL devices, while gaining control of preventing the operation of such device or array of devices in a single-spatial-mode regime is solved by devising a VCSEL structure with at least one of a) a reduced or at least frustrated—as compared with the conventionally-manufactured VCSEL devices—rotational symmetry of the VCSEL structure; and b) a deliberately-introduced transverse or lateral (i.e., along a layer) misalignment between physical apertures defined in different layers of the VCSEL structure and configured to spatially-contain at least one of the spatial distribution of current injection throughout the VCSEL structure and the spatial distribution of the light output generated by the VCSEL device in operation. In one case, the rotational symmetry may be defined with respect to the longitudinal axis of the structure that is transverse to the layers of the structure, while one physical aperture is defined by a spatially-patterned metal layer configured as an electrical contact of the VCSEL structure. Spatial patterning of other VCSEL components (that are not intended to be electrically active, such as for example substrate patterning, or deposition of additional spatially-patterned dielectric layers) may be used in conjunction with the patterned optical output aperture to facilitate the output beam control.
The idea of the invention stems from the realization that fabrication of a practically-functional multimode (or multiple spatial output) VCSEL devices can be achieved through introduction of asymmetry into the structure of the device aperture. In one implementation, for example, the apertures present in the VCSEL layered structure may have no more than two axes of symmetry, and/or the apertures may be non-coaxial with respect to each other or laterally offset with respect to each other.
Implementations or embodiments of the idea of the invention address problems associated with existing multimode VCSEL devices. The judicial configuration of the apertures in VCSEL layers can be achieved through conventional and simple processing steps while avoiding additional complex processing steps that are common in the manufacturing practice and that can affect device reliability.
The substrate 302 is a semiconductor substrate having a corresponding lattice constant. Typically, the substrate 302 can include gallium arsenide (GaAs), or indium phosphide (InP), but other semiconductor substrates such as gallium antimonide (GaSb), germanium (Ge) or an epitaxially grown material (such as a ternary or quaternary semiconductor), or a buffered or composite substrate, such as a rare-earth oxide buffered silicon substrate can also be used. The lattice constant of substrate 302 is judiciously chosen to minimize defects in materials subsequently grown thereon. Substrate 302 may be doped, which allows formation of a contact metal on the lower surface of substrate 302. In some embodiments (not shown), the substrate may be undoped, and a contact layer can be formed on the substrate to facilitate formation of a lower metal contact for the VCSEL.
The reflector 304 is shown as a semiconductor DBR formed with a lattice substantially matched to that of the substrate 302. A DBR is a periodic structure formed from alternating layers of materials with different refractive indices that can be used to achieve high reflection within a range of frequencies or wavelengths. The thicknesses of the layers are chosen to be an integer multiple of the quarter wavelength, based on a desired design wavelength Xo. That is, the thickness of a layer is chosen to be an integer multiple of λ0/4n, where n is the refractive index of the material at wavelength λ0. A DBR can include, for example semiconductor materials of Groups III and V of the periodic table such as, for example, AlAs, AlGaAs, GaAs, InAs, GalnAs, AlInAs, InGaP, AlInGaP, InGaP, InGaAsP, GaP, InP, AlP, AlInP, or AlInGaAs. When formed on a GaAs substrate, the DBR is formed using two different compositions for AlGaAs. Reflector 304 can also be doped with an n-type dopant or a p-type dopant to facilitate current conduction through the device structure. A spacer layer 306, such as AlGaAs or AlGaInP may be formed overlying the first mirror 304. Active region 308 is formed overlying spacer layer 306 and includes a material capable of emitting a substantial amount of light at a desired wavelength of operation. It will be understood that active region 308 can include various light emitting structures, such as quantum dots, quantum wells, or the like, which substantially improve a light emitting efficiency of VCSEL 300. For a GaAs substrate, the active region 308 can include a material that can emit light between a wavelength of about 0.62 μm and 1.6 μm. Active region 308 can include more than one material layer, but is illustrated as including a single layer in the preferred embodiment for simplicity and ease of discussion. For example, active region 308 can include GaAs/AlGas or InGaAs/GaAs or AlGaInP/InGaP or GaInNAsSb/GaAsN multiple quantum wells (MQWs). A spacer layer 310, such as AlGaAs or AlGaInP may be formed overlying active region 308. A second reflector 312 may be formed overlying spacer layer 310. Second reflector 312 may also be a DBR and, in this case, may be similar in design to that of the first mirror 304. When formed on a GaAs substrate, the DBR is formed using two different compositions for AlGaAs. Reflector 312 can also be doped with a p-type dopant or an n-type dopant, the doping type being opposite to the doping type of first mirror 304, in order to form a p-n junction and to facilitate current conduction through the device structure. Contact layer 313 is formed on reflector 312 and is a doped semiconductor layer that facilitates electrical connection of the device with a metal contact layer.
In order to ensure the efficient operation of the VCSEL device, the lateral confinement (in a plane that is transverse to the axis z of the local system of the device coordinates) of the current and/or of the optical field (by providing waveguiding, for example) may be required. Accordingly, a confining region or layer 314 within the VCSEL 300 is formed, for example, by structuring the material properties of the embodiment 300 to be different from those of the adjacent regions: to provide optical waveguiding and/or to define a region for current injection such that lasing occurs through an aperture region or opening 316 defined within the confining region 314. Methods of forming the confining region include, but are not limited to, oxidation, ion implantation, semiconductor etching, semiconductor regrowth, deposition of other materials and combinations thereof. In one implementation of the VCSEL 300, the confining region 314 is formed using ion implantation to produce a highly electrically-resistive layer, while at the same time defining the low electrical resistivity aperture area 316 through which current can be directed. It will be understood that other functional layers (such as current spreading layers and contacting layers) can also be appropriately included, that more than one confinement region or layer at different depths within the device 300 can be formed, and that different electrical contacting configurations (such as intracavity electrical contacts) may be used. (These variations from the schematic structure 300 are not shown for simplicity of illustration and in order to clearly to explain key design elements of the device.) The VCSEL structure 300 is shown to contain a first metal contact layer 318 and a second metal contact layer 320. As shown, each of these contact layers is located at a periphery, a boundary of the stack of the layered structure defining the embodiment of VCSEL, and is defined, therefore, as a peripheral layer. Either the first metal contact layer 318 or the second metal contact layer 320 (here, as shown—the layer 320) has an opening or aperture 322 through which light can be emitted by the VCSEL 300 in operation of the device.
Whereas in VCSEL structure 100 of the related art the aperture region 114 and metal aperture are aligned to be coaxial with one another, aperture opening 316 (defined in the confining layer 314) and the aperture 322 in the metal contact layer are offset in at least one lateral direction with respect to one another (that is, the axes of these two apertures are spatially shifted with respect to one another along an axis that is transverse to the direction of the output beam: here—along an x-axis and/or a y-axis). As a result, these two aperture openings are no longer coaxial (with respect to the z-axis). The aperture openings 316 and 322 are typically circular in shape, and can be characterized by corresponding dimensions, such as diameters. In the case when each of the aperture opening 316, 322 is rotationally symmetric, the introduction of the lateral or transverse offset therefore reduces or frustrates the rotational symmetry of the overall device 300. While the diameters of the apertures 316, 322 can be substantially equal, these diameters can also differ from one another. In the example shown in
As a result of the spatially non-uniform current injection profile, the position of the spatial mode is transversely offset from the center of output aperture 322 and/or confining aperture 316. This is due to a carrier-concentration-dependent gain variation across the aperture 316, and guiding effects within the apertures 316 and 322 caused by a combination of a refractive index variation across the active region due to the carrier concentration variation across the aperture 316, (where the refractive index decreases with increasing carrier concentration), and thermal guiding effects associated with resistive losses for the current injection that varies across apertures 316 and 322 (with refractive index increasing with temperature). Under pulsed operating conditions, the carrier-induced anti-guiding effect can cause spatial mode or filament formation in one or more locations within the aperture. With increase of the pulse duration during the operation of the device—and as the device transitions towards the continuous-wave (CW) operation—the thermal effects caused by the resistive heating increase and dominate the guiding mechanisms, thereby allowing the spatial modes or filaments to form in different spatial regions of the aperture 322. Accordingly, by judiciously choosing the current injection conditions, one can gain the control of the (asymmetric) mode or filament formation in different locations within the aperture, thereby making it possible to switch between different spatial output distributions (patterns) of light during the operation of the device. The existence of different spatial patterns (modes or filaments), and the switching between them can be used. In one embodiment, to reduce the spatial coherence of the light-output of the device and to reduce the speckle contrast for a single, individual VCSEL device.
Asymmetry in the structure of a VCSEL device causes a spatially non-uniform current injection that gives rise to a non-uniform carrier concentration. This, in turn, affects the locale or space occupied by lasing mode(s) (or filament(s)). This is shown schematically in
As the injection current provided to the laser device is increased, the carrier concentration becomes substantially pinned at the location of the formation of the lasing mode, while increasing elsewhere. The carrier concentration can even become reduced at the location of mode formation. This is schematically illustrated in
The formation of a higher order transverse mode or spatial filament is depicted schematically in
Under some current injection operating conditions, more than one spatial mode can exist, each of such multiple modes typically occurring at a slightly different wavelength. Consequently, the degree of coherence of the superposition of all transverse modes or filaments within a device aperture (or an array of devices) is reduced, which in turn reduces the speckle contrast produced by a device (or an array of devices). Multi-spatial mode (or multi-filament) operation of the individual VCSEL device (or an array of VCSEL structures) is therefore advantageous for applications including 3D imaging, illumination, object or gesture recognition, LIDAR, optical coherence tomography (OCT) and interference microscopy.
In one implementation the metal aperture 422 is made circular, though it can have other shapes in related embodiments (similarly to the aperture 414), and can be dimensioned such as to equal, in size, to the aperture 414 (or, alternatively, be sized differently from the aperture 414).
The method used to form the device 400 and its apertures 414 and 422 introduces a lateral offset in at least one direction, such that the apertures such are non-concentric and the device does not have rotational symmetry. In the example shown, aperture 422 is larger than aperture 414. Consequently, this leads to a first offset 424 on one side of the aperture, where the metal 420 overlaps with aperture 416, and a second offset 426 on the opposite side of the aperture, where the offset between the metal 420 and the aperture 416 is larger than if the apertures were aligned concentrically (or coaxially with respect to axis z). The effect is to create a non-uniform current injection profile into the active region of VCSEL 400, as described above for VCSEL 300, in order to produce multiple transverse modes or output filaments.
The operational effect, produced by the lack of mutually-coaxial alignment between at least two of the three apertures in the VCSEL device structure that does not have rotational symmetry, is that a spatial profile of the current injection into the active region of the VCSEL 500 is non-uniform, as was already described above in reference to
A person of skill in the art will readily appreciate, therefore, that single, individual VCSEL devices employing aperture offsets, according to the idea of the invention, can generate multiple transverse (or spatial) modes, thereby improving or reducing, as was already discussed, the speckle contrast associated with the light output of a conventionally-structured single VCSEL source. In many imaging applications, a combination of multiple VCSELs in either 1-dimensional or 2-dimensional arrays may be required. In a VCSEL array formed using the “aperture offset” designs, the offset between the aperture(s) in the confinement layer(s) and the metal aperture(s) in the peripheral layer(s) can be changed from one device to another, such that the spatial mode properties of different devices within the same array are dissimilar. In the so-constructed array of VCSELs, some of the constituent VCSEL devices may still have output metal apertures and the apertures in the confining layers that are coaxial with one another at least because the modal behavior of the light output produced by such constituent VCSELs is still different from that of the light output produced by other devices the apertures of which are not coaxial. In an array of VCSELs, the inclusion of constituent devices with at least two different aperture offsets (also referred to as aperture patterns) can be used to vary the spatial mode pattern across an array, and to further improve speckle contrast.
A person of skill in the art will also appreciate, that devices employing aperture offsets, according to the idea of the invention, can also include vertical external-cavity surface-emitting lasers (VECSELs) configured to generate multiple transverse (or spatial) modes, thereby improving or reducing, as was already discussed, the speckle contrast associated with the light output of a conventionally-structured single VECSEL source. In a VECSEL, an aperture may be implemented, for example in an external reflector or mirror that forms the extended cavity.
Embodiments of VCSEL devices of the invention are dimensioned and referred to in relation to the size of the smallest confinement aperture within a given device, which is usually formed by oxide confinement, or ion implantation, as previously described. VCSELs can have apertures between about 3 μm and 50 μm wide, and the apertures are typically circularly shaped, though they may also be formed to be square, rectangular, or elliptical. In some embodiments, the “VCSEL size” (in terms of the confinement aperture width) is between about 6 μm and about 25 μm, or between about 8 μm and about 20 μm. The metal aperture through which light is emitted (the output aperture) is appropriately dimensioned to be between about 3 μm and about 60 μm wide, or between about 8 μm and about 30 μm wide, or between about 10 μm and about 24 μm wide in related embodiment and typically has the same shape as that of the confinement aperture. For a top emitting device (that is, an embodiment in which the output aperture is formed in the metal contact layer at the top of the overall structure—such as the layer 520 of the embodiment 500, for example) , the output aperture typically has the same width as that of the confinement aperture, or is larger than the confinement aperture by, for example, 6 μm or less. For a bottom emitting device (that is, a device in which the output aperture is formed in the metal contact layer such as the layer 518 of the embodiment 500, for example), where current can be injected and spread through the substrate, the output aperture can be 10 μm or even up to 20 μm larger than the confinement aperture.
VCSEL structures have a first aperture (chosen from a combination of the output aperture and the at least one confining aperture) that has a first dimension, and a second aperture (chosen, from the same combination) that has a second dimension. A difference between the first and second dimensions satisfies at least one of the following conditions: a) this difference is equal to or smaller than 6 μm; b) this difference is equal to or smaller than 4 μm; and c) this different is equal to or smaller than 2 μm.
The lateral offset between the center of the confinement aperture and the center of the output aperture of an embodiment of a single, individual VCSEL device—which can be represented as a transverse offset between the respective axes of these two apertures, perpendicular to the layers in which the apertures are formed—can be at least 1 μm and up to 40% of the dimension of the confinement aperture. As a non-limiting example, for a 10 μm diameter device, the lateral offset of the metal output aperture with respect to the confinement aperture can be between about 1 μm and 4 μm. In some embodiments, the offset between the aperture centers can be up to 30% or 20% of the width of the confining aperture.
In some embodiments, devices configured according to an embodiment of the invention are structured to emit light from at least two different spatial locations within the same output aperture.
In some embodiments, devices configured according to an embodiment of the invention are devised to generate light with a spectral width (full-width-half-maximum, FWHM) of greater than 0.5 nm, or greater than 1 nm, or greater than 1.5 nm.
In a specific embodiment of the array 600, the first aperture is the output aperture(s) of the constituent devices can be aperture(s) made in metal contact layer(s) corresponding to the light-emitting surface(s) of the constituent VCSEL devices. The second aperture is the at least one of the confining aperture(s) formed in the internal (to the structure of the device) layer configured to confine the spatial distribution of current within the VCSEL structure during the operation of the array. In another specific embodiment, both the first and second apertures are confining apertures that are internal to the structure of the VCSEL device and that are oxide-confined apertures or ion-implanted apertures.
In one embodiment, the device A has a first aperture 602A that is a metal (output) aperture 602A, and a second aperture 604A that is a confining aperture. The apertures of the constituent devices B through L are defined in a similar fashion. As shown, the sizes (and shapes) of the first apertures of different constituent devices are substantially equal to one another and the sizes (and shapes) of the second apertures of the individual constituent devices are substantially equal to one another, while a given first aperture is larger than the respectively corresponding second aperture. Generally, however, the corresponding aperture sizes for constituent devices in the array do not have to be the same.
Stepping across a row of array the constituent devices from device A to device D, for example—it can be seen that the lateral offset between the first and second apertures is systematically changed along the row. Stepping down a column of the array (for example, from device A to device I), it can be seen that the lateral offset between the first and second apertures is also systematically changed in the direction of the column. Thus, the constituent devices within a given array can be configured to have non-coaxial apertures that have different offsets with respect to one another, thereby causing different spatial profiles of current injection for the constituent lasers and hence different content of spatial modes in outputs from different constituent lasers. In this example, while the offsets between the apertures of the constituent devices are generally systematically varied in the x-direction and/or the y-direction across the overall array, it is possible that at least one constituent device in the array has coaxially-aligned first and second aligned apertures, while at least one constituent device in the array has non-coaxial first and second apertures. As a result, the different spatial modes at the output from the different lasers of the array form spatially-multimode light emission with differing patterns, which patterns can be judiciously manipulated by changing offsets between the axes of the first and second apertures to achieve differently-shaped output beams from the VCSEL-arrays, as will be described later.
In some examples, the aperture offsets can be designed according to a desired pattern in order to control beam output from various locations of the device array. At least one device in the array (or segment of an array) has non-concentrically aligned apertures. In other examples, the devices and their apertures are irregularly spaced. In other examples, the array of devices can be segmented such that different portions of the array can be used separately from each other or can be used together, according to the illumination requirements (such as required power or patterning or direction of the beam) for a given application.
The asymmetry of the metal contact layer in which such output aperture(s) are formed can, on one hand, affect the current injection profile during the operation of the device but can also be used to occlude parts of the are through which the light output is delivered that would otherwise normally be transparent to allow light emission in a regular non-patterned aperture situation.
For example,
The apertures in
Accordingly, in at least one embodiment of the VCSEL structure, the first peripheral portion (of the material layer that defines an output aperture of the VCSEL structure) may be dimensioned to define a ring-shaped stripe of material and the second portion of such material layer is dimensioned to satisfy one of the following conditions: (i) the second portion is configured as radially-extended stripe of material, and (ii) the second portion is configured as a stripe connecting two different sides of a polygonally-shaped first peripheral portion. At the same time, in at least one embodiment the second portion is configured to be electrically connected with the first peripheral portion at at least one point. Alternatively or in addition, in at least one embodiment of the VCSEL structure, the first peripheral portion (of the material layer that defines an output aperture of the VCSEL structure) may be dimensioned as a stripe of material having a closed polygonal perimeter, while the second portion is dimensioned to cover a surface area and establish electrical contact with at least one side of the polygonal perimeter.
It will be appreciated by a person of skill in the art, that the appropriate positioning and dimensioning of portions 906B, 906C, 906D and 906F is used to generally suppress light emission at those locations of the output surface of the laser structure where the metal contact layer is deposited, and, therefore, enforce the transverse mode formation to occur where the metal contact layer is not present. In specific examples of
The ability to cause multimode spatial emission with suppressed emission at the center of an output aperture structure (effectively, at a longitudinal axis of a given VCSEL) can be useful in a variety of applications, including multimode optical fiber communication based on offset launch schemes, and in reducing speckle formation in operation of imaging systems.
In a related embodiment, an array is formed of individual VCSELs having at least two different output-aperture shapes (for example—the shaped presented in
Referring again to the example of the VCSEL array shown in
It therefore becomes clear to a person of ordinary skill in the art that with the use of embodiments of the invention illustrated in
Individual VCSEL devices and/or VCSEL arrays structured according to the idea of the present invention can further be integrated or operably-cooperated with auxiliary optical elements—such as microlenses or optical filters, for example example. In doing so, in some embodiments the microlenses may be structured to form a separate, stand-alone array of lens elements overlying (and combined with) the VCSEL array on the side producing light output. For example, microlenses may be fabricated on a separate transparent substrate that is attached and aligned to the overall laser array. In other embodiments, the microlenses may be formed on the backside of the surface of the substrate by using a number of different processes known to one of ordinary skill in the art. (One technique for forming such microlenses involves a photolithography process that defines a lens element with photoresist shaped as a cylinder or otherwise, which is then melted onto the substrate before having the lens shape transferred to the substrate through an etching process. The etch may be a Chlorine (Cl) based dry etch adjusted for or approaching an even etch selectivity between the substrate material and the photoresist, so as to etch both materials at close to or at the same rate. The photolithographic step used to create the lens elements is accomplished, for example, using a backside wafer alignment system common in the industry.
Regardless of whether the geometrical arrangement of the constituent VCSEL devices in a given VCSEL array is regular or irregular (and regardless of whether the array of corresponding output apertures is geometrically regular or irregular), the fact that at least some constituent VCSEL devices in the array are intentionally dimensioned to produce laser outputs with transverse distributions of irradiance that are asymmetric with respect to the centers of the corresponding output apertures causes the array of the output beams be, generally, irregularly spaced. As a result, when the array of the output apertures (of the VCSEL array) is aligned with the array of microlenses, the beams of light defined as the throughput of the array of microlenses can also be offset with respect to longitudinal axes of the constituent microlenses. Alternatively, even in the case when the output beams from the lasers of the VCSEL array are substantially-regularly spaced with respect to one another, the corresponding array of microlenses can be chosen to be geometrically irregular. Such variability affords the user to manipulate and modify the spatial distribution of the VCSEL array useful light output, based on a combination of the driving conditions of the chosen lasers and/or the chosen subsets of lasers within the VCSEL array that are driven such that the resulting, overall output beam from the VCSEL array equipped with the microlenses is focused and scanned over different spatial regions of interest.
To this end,
To illustrate the possibility of achieving different output beam characteristics,
To fabricate embodiments of semiconductor optoelectronic devices structured according to the idea of the invention, a plurality of layers can be deposited on an appropriate substrate in a first-materials-deposition chamber. The plurality of layers may include etch-stop layers; release layers (i.e., layers designed to release the semiconductor layers from the substrate when a specific process sequence, such as chemical etching, is applied); contact layers such as lateral conduction layers; buffer layers; layers forming reflectors or mirror structures, and/or or other semiconductor layers. For example, the sequence of layers deposited can include buffer layer(s), then a lateral conduction or contact layer(s), and then layer(s) forming a reflector of the VCSEL structure. Next, the substrate can be transferred to a second-materials-deposition chamber, where a cavity region and an active region are formed on top of the existing, already-deposited semiconductor layers. The substrate may then be transferred to either the first-materials-deposition chamber or to a third-materials-deposition chamber for deposition of additional mirror layer(s) and contact layers. Tunnel junctions may also be formed in some implementations.
The movement or repositioning/relocation of the substrate and semiconductor layers from one deposition chamber to another chamber is referred to as transfer. The transfer may be carried out in vacuum, at atmospheric pressure in air or another gaseous environment, or in an environment having mixed characteristics. The transfer may further be organized between materials deposition chambers in one location, which may or may not be interconnected in some way, or may involve transporting the substrate and semiconductor layers between different locations, which is known as transport. Transport may be done with the substrate and semiconductor layers sealed under vacuum, surrounded by nitrogen or another gas, or surrounded by air. Additional semiconductor, insulating or other layers may be used as surface protection during transfer or transport, and removed after transfer or transport before further deposition.
For example, a dilute nitride active region and cavity region can be deposited in a first-materials-deposition chamber, while the AlGaAs/GaAs DBRs and other structural layers can be deposited in a second-materials-deposition chamber. To fabricate VCSEL devices discussed in this disclosure, some or all of the layers of a cavity region, including a dilute nitride based active region can be deposited with the use of molecular beam epitaxy (MBE) on one deposition chamber, and the remaining layers of the laser can be deposited with the use of chemical vapor deposition (CVD) in another materials deposition chamber.
In some embodiments, a surfactant, such as Sb or Bi, may be used when depositing any of the layers of the device. A small fraction of the surfactant may also incorporate within a layer.
A semiconductor device comprising a dilute nitride layer can be subjected to one or more thermal annealing treatments after growth. For example, a thermal annealing treatment includes the application of a temperature in a range from about 400° C. to about 1,000° C. for a duration between about 10 microseconds and about 10 hours. Thermal annealing may be performed in an atmosphere that includes air, nitrogen, arsenic, arsine, phosphorus, phosphine, hydrogen, forming gas, oxygen, helium, or any combination of the preceding materials.
The invention as recited in claims appended to this disclosure is intended to be assessed in light of the disclosure as a whole, including features disclosed in related art to which reference is made.
For the purposes of this disclosure and the appended claims, the use of the terms “substantially”, “approximately”, “about” and similar terms in reference to a descriptor of a value, element, property or characteristic at hand is intended to emphasize that the value, element, property, or characteristic referred to, while not necessarily being exactly as stated, would nevertheless be considered, for practical purposes, as stated by a person of skill in the art. These terms, as applied to a specified characteristic or quality descriptor means “mostly”, “mainly”, “considerably”, “by and large”, “essentially”, “to great or significant extent”, “largely but not necessarily wholly the same” such as to reasonably denote language of approximation and describe the specified characteristic or descriptor so that its scope would be understood by a person of ordinary skill in the art. In one specific case, the terms “approximately”, “substantially”, and “about”, when used in reference to a numerical value, represent a range of plus or minus 20% with respect to the specified value, more preferably plus or minus 10%, even more preferably plus or minus 5%, most preferably plus or minus 2% with respect to the specified value. As a non-limiting example, two values being “substantially equal” to one another implies that the difference between the two values may be within the range of +/- 20% of the value itself, preferably within the +/−10% range of the value itself, more preferably within the range of +/−5% of the value itself, and even more preferably within the range of +/−2% or less of the value itself. The term “substantially equivalent” may be used in the same fashion.
The use of these terms in describing a chosen characteristic or concept neither implies nor provides any basis for indefiniteness and for adding a numerical limitation to the specified characteristic or descriptor. As understood by a skilled artisan, the practical deviation of the exact value or characteristic of such value, element, or property from that stated falls and may vary within a numerical range defined by an experimental measurement error that is typical when using a measurement method accepted in the art for such purposes.
Although specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that any arrangement that is devised to achieve the same purpose may be substituted for the specific embodiments shown. In a related embodiment, for example, a VCSEL structure is provided that has a longitudinal axis and that includes first and second reflectors; a gain medium between the first and second reflectors; and a peripheral material layer defining an output aperture therein (the output aperture dimensioned to have no more than two axes of symmetry of the output aperture; here, the peripheral material layer is a metallic layer configured as an electrical contact layer of the VCSEL structure, and the peripheral material layer is dimensioned to include a first peripheral portion and a second portion surrounded by the first peripheral portion). Such VCSEL structure may be configured to produce, in operation, a light output having a spatial distribution of intensity in one of the following forms: a) a ring-shaped distribution of intensity, and b) a dumb-bell-shaped distribution of intensity, as defined in a plane transverse to an axis of the light output, while an axis of the output aperture and an axis of the at least one internal aperture may be configured to not coincide with one another, and/or while a lateral extent of at least one of the peripheral material layer and the at least one confining material layer (in a first plane that is transverse to the longitudinal axis) may be chosen to be smaller than a lateral extent of the active region (in a second plane that is parallel to the first plane).
Overall, this application is intended to cover any adaptations or variations of embodiments of the present invention. It is to be understood that the above description is intended to be illustrative, and not restrictive, and that the phraseology or terminology employed herein is for the purpose of description and not of limitation. Combinations of the above embodiments and other embodiments will be apparent to those of skill in the art upon studying the above description. The scope of the present invention includes any other applications in which embodiment of the above structures and fabrication methods are used. The scope of the embodiments of the present invention should be determined with reference to claims associated with these embodiments, along with the full scope of equivalents to which such claims are entitled.
This international application claims priority from and benefit of the U.S. Provisional Patent Application No. 62/793,557 filed on Jan. 17, 2019, the disclosure of which is incorporated by reference herein in its entirety.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/US2020/012771 | 1/8/2020 | WO | 00 |
| Number | Date | Country | |
|---|---|---|---|
| 62793557 | Jan 2019 | US |
