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High power top emitting vertical cavity surface emitting laserRelated Patent Categories: Coherent Light Generators, Particular Active Media, Semiconductor, Injection, Monolithic Integrated, Laser Array, With Vertical Output (surface Emission)High power top emitting vertical cavity surface emitting laser description/claimsThe Patent Description & Claims data below is from USPTO Patent Application 20070091960, High power top emitting vertical cavity surface emitting laser. Brief Patent Description - Full Patent Description - Patent Application Claims
[0001] The present invention generally relates to a laser device including a plurality of vertical cavity surface emitting laser (VCSEL) elements, which are provided in a single chip so as to increase the total output power of the laser device compared to a single VCSEL. [0002] VCSEL devices are considered an attractive alternative to conventional edge-emitting laser diodes due to their small size and their potential of being formed in a substantially circular symmetry. Generally, VCSEL devices show a relatively low threshold current, a high modulation efficiency and, if designed so as to emit a substantially circular beam profile, allow to be coupled into optical fibers in a simple fashion. Additionally, the manufacture of VCSEL devices comes along with a parallel and cost-effective production, testing and packaging process, and also offers the possibility of being packed in one and two-dimensional arrays to comply with a plurality of applications such as data communication, sensing applications, and the like. [0003] In other applications, such as laser pumping, free space communication, illumination systems, or other high-power applications, a laser device not only requires a high total output power in the range of several hundred milliwatts, but also necessitates a high output power per chip area to reduce the required chip area and hence the costs per watt output power. In addition, a high wall-plug efficiency is required to keep thermal losses and the requirements on the packaging side low. Consequently, the output power per chip area or the power density and the wall-plug efficiency represent important parameters that may be decisive for the success of VCSELs in such high power applications. Moreover, the wavelength required for specific high power applications may range from visible wavelength for display applications to infrared wavelength for various sensing or pumping applications. For wavelengths that may be transmitted through a corresponding substrate of a VCSEL element, the requirements with respect to high output power have been met by so-called bottom emitting, flipchip-bonded devices having formed therein an oxide aperture. In this respect, it is to be noted that the terms "bottom" and "top" refer to positions or directions with respect to the substrate on which a VCSEL device is formed. Hence, a bottom emitting VCSEL describes a laser device emitting its output power through the substrate. By means of a heat sink, which is closely located to the laser active area, a very efficient heat removal is provided for the bottom emitting configuration so that relatively high output powers may be generated, wherein, however, this technology is limited to the emission of wavelengths for which the substrate is transparent. [0004] In view of this serious drawback it has been proposed to remove the absorbing material in the substrate, wherein issues concerning the reliability and the requirement for additional fabrication steps may render these approaches less than desirable for mass production of VCSEL elements. For this reason, top emitting VCSEL devices represent an attractive possibility for devices emitting at wavelengths corresponding to the absorption range of the substrate. Generally, the output power of a top emitting VCSEL can be increased by enlarging the active area of the VCSEL. This is usually accomplished by correspondingly increasing an aperture that is formed closely to the active area, wherein the aperture typically provides a current confinement and a restriction of the optical field. Frequently, this aperture is formed by an electrically conductive and transparent material layer, the peripheral area of which is selectively oxidized so as to convert the periphery into a non-conductive oxide material. In other approaches, a conductive and transparent material layer may be modified at the periphery by ion implantation so as to reduce the conductivity and the transmittance of the periphery. Presently, top emitting VCSEL devices having an oxide-based aperture seem to be the most promising approach for demanding applications. For example, VCSEL devices having an output wavelength of 980 nm have been fabricated with an oxide aperture size greater than 90 .mu.m, thereby achieving an output power of more than 100 milliwatts. The increase in the output power, however, is accompanied by a significant decrease of the wall-plug efficiency to about half the value of VCSEL devices having a small oxide aperture. Consequently, VCSEL devices having a large active area may not be considered promising for high power applications when a high efficiency is required. [0005] In view of the problems identified above, there exists a need for a VCSEL device that enables high power output with moderately high efficiency without being restricted to transmittance wavelength range of a substrate. [0006] Generally, the present invention addresses the above-specified object by providing a laser device and a method for fabricating the same, wherein a plurality of VCSEL elements are provided within a specified chip area and are operated in parallel to achieve a high output power. Moreover, the arrangement of the individual VCSEL elements within the specified chip area is based on the inventor's concept that the lateral size of the effective laser active area of each VCSEL element and the pitch of nearest neighbors of each VCSEL element is to be taken into consideration so as to simultaneously obtain a moderate power density, while nevertheless maintaining a high wall-plug efficiency. Since it is known that large active areas in a VCSEL element may drastically compromise the overall efficiency, whereas VCSEL elements having a relatively small aperture exhibit a high wall-plug efficiency, the output power of the laser device may be increased by using plural, smaller-sized, highly efficient VCSEL elements instead of an inefficient single high output power VCSEL element, wherein the effective size of the active area, for instance determined by an aperture, of the individual small-sized VCSEL elements is selected, in combination with the pitch of nearest neighbors, so as to not only maintain a high efficiency, but also provide a high power density. The high power density reduces production cost per Watt output power, whereas the high wall plug efficiency maintains cost for heat management low. [0007] According to one aspect of the present invention, therefore, a laser device comprises a substrate and a plurality of vertical cavity surface emitting laser (VCSEL) elements formed thereon, wherein each of the VCSEL elements has an effective laser active region with a respective defined center point and a defined lateral size. The defined lateral size of each VCSEL element is equal to or less than 30 .mu.m. Furthermore, a pitch between the center points of nearest neighbors of the VCSEL elements is equal to or less than 80 .mu.m. [0008] As will be described later on, the wall-plug efficiency of small sized VCSEL elements is only slightly affected by a variation of a pitch between adjacent VCSEL elements, whereas large sized VCSEL elements exhibit a significant drop in efficiency at a reduced pitch between adjacent elements owing to the mutual thermal heating effect of nearest neighbors. Hence, the lateral size of the effective laser active region in the above-specified range allows to maintain a relatively high efficiency while nevertheless selecting the pitch between nearest neighbors to 80 .mu.m or even less so as to achieve a high power density. [0009] According to a further embodiment, the laser device additionally comprises a first current terminal and a second current terminal, wherein each of the VCSEL elements is connected to the first and the second current terminals. The provision of a commonly used current terminal for the plurality of VCSEL elements allows a parallel operation without unduly occupying precious chip area. [0010] In a further embodiment, each of the VCSEL elements comprises a radiation output window that is disposed opposite to the substrate. Thus, the laser device may advantageously be designed as a top-emitting device, thereby substantially avoiding the drawbacks of currently available high power bottom emitters. Hence, the laser device may be designed for a wide range of output wavelengths without being restricted to the transmittance range of the substrate. [0011] In a further preferred embodiment, each of the VCSEL elements has a configuration that corresponds to the same design. Using the same design for the VCSEL elements facilitates the manufacturing, while still providing a high degree of flexibility in adapting power density and efficiency, since for the same basic laser design the geometry of the VCSEL array may correspondingly be adapted. That is, the same basic VCSEL design may be used and, by varying the number of VCSEL elements, the pitches thereof as well as the effective lateral size, the absolute power, the power density, and the efficiency may all be adjusted within a wide range so as to conform to the requirements of a specified application. [0012] In this respect, it should be noted that the term "design" is to characterize "vertical" properties of the VCSEL element as well as "horizontal" or "lateral" properties of the element. The vertical properties may include the vertical configuration of the VCSEL element, such as the type of laser active region and the semiconductor materials used therein, the type of reflectors formed in the VCSEL element and the materials therefor as well as the manufacturing processes involved, and the like. The vertical properties substantially determine the output wavelength of the device, the output direction, partly the basic efficiency, and the like. On the other hand, the horizontal or lateral properties, such as the lateral dimension, the lateral shape, the effective size of the laser active region, which is typically determined by providing a corresponding aperture close to the active region, and the like, may also significantly influence the efficiency, as previously noted. Thus, considering the above definition, the present invention provides for the potential of adjusting power density and wall-plug efficiency of the laser device by merely modifying horizontal or lateral parameters for any given vertical configuration of the VCSEL elements, independent from any process technology used, which may, of course, be separately optimized to further enhance output power and/or efficiency. [0013] In a further embodiment, the pitch between the nearest neighbors of each VCSEL element is substantially identical for each of the VCSEL elements. In this way, a simple pattern of VCSEL elements may be formed, thereby facilitating the manufacturing process while the total size of the pattern as well as the pitch in combination with the effective lateral size enable the adjustment of the device characteristics, such as total output power, power density, and efficiency. [0014] In a further embodiment, the plurality of VCSEL elements are arranged in a hexagonal pattern, which enables the arrangement of a maximum number of individual VCSEL elements for a given pitch between nearest neighbors and a given available chip area. Hence, the power density may be optimized. [0015] In a further preferred embodiment, each of the VCSEL elements is located on a site of a lattice defined by the VCSEL elements and at least one lattice site is occupied by a bond pad. In this way, a very compact configuration may be achieved that includes a bond pad at the expense of one lattice site, thereby still providing a high power density. [0016] In a further embodiment, the pitch to nearest neighbors of a first VCSEL element differs from the pitch to nearest neighbors of a second VCSEL element. Hence the laser device may comprise an array of VCSEL elements, wherein the pitch may vary at least for two VCSEL elements, thereby providing an increased design flexibility for configuring the laser array. For instance, the laser device may include one or more areas having a reduced heat dissipation capability compared to other device regions. In this case, the power density may be locally adapted to the heat removal capabilities by correspondingly selecting the corresponding pitch of VCSEL elements formed within these areas. Thus, at an area with reduced heat dissipation capabilities, the pitch between nearest neighbors may be increased compared to a region of the laser device that allows improved cooling. [0017] In a further variant, the effective lateral size of at least two VCSEL elements may differ from each other, thereby providing an increased degree of flexibility in adapting the laser device to application requirements. As in the case described above, the effective lateral size may be selected in conformity with the heat removal capabilities of the various device areas so as to avoid an undue heating of specific VCSEL elements. In other embodiments, both the pitch to nearest neighbors and the effective lateral size of at least two VCSEL elements may be varied so as to correspond to specific application requirements. [0018] In a further embodiment, the laser device comprises an aperture layer having formed therein an aperture that substantially determines the defined lateral size of the effective laser active region. The provision of an aperture layer is a well-established technique and therefore allows a high degree of manufacturing compatibility with well-known manufacturing processes. [0019] In a further embodiment, the aperture layer comprises an oxidized periphery to define the aperture within said periphery. As previously noted, the provision of an oxide aperture is presently considered a promising approach for achieving a high efficiency in VCSEL elements. Thus, this technology may advantageously be combined with the lateral configuration of a VCSEL array in accordance with the principles of the present invention so as to accomplish high power density at a high efficiency. [0020] In a further embodiment, a layer stack forming at least a first reflector of each of the VCSEL elements comprises a periphery that is modified by ion implantation so as to define the aperture within the periphery. Thus, the present invention may advantageously be combined with standard fabrication methods for laterally patterning the current distribution and/or the optical confinement of a VCSEL element. [0021] According to a further preferred embodiment, the defined lateral size of the effective laser active region of each VCSEL element is equal to or greater than 10 .mu.m. Based on investigations by the inventors, the above-specified range for the lateral size may provide for high efficiency at a moderate output power, wherein a decrease in efficiency is substantially negligible for the above-specified range of pitches to the nearest neighbors when the VCSEL elements are arranged in an array. [0022] In a further preferred embodiment, the pitch between nearest neighbors of each of the VCSEL elements is equal to or greater than 40 .mu.m. With a minimum pitch of 40 .mu.m, the mutual thermal heating effect of adjacent VCSEL elements may still remain substantially negligible. Moreover, a minimum distance of approximately 40 .mu.m is compatible with presently available process technologies so that the manufacturing of high power density VCSEL devices having a high efficiency is possible by using well-established process techniques. [0023] In one preferred embodiment, the pitch between nearest neighbors of each of the VCSEL elements is in the range of 55 .mu.m-65 .mu.m. A pitch within this range may provide an optimum trade-off between power density and efficiency for a broad class of applications and for a wide variety of VCSEL designs. [0024] In a further embodiment, the effective laser active region has a substantially circular shape. The circular shape of the laser active region, i.e., of the electrically and optically effective laser region, may provide advantages in terms of efficiency and resulting beam profile compared to other geometric configurations. Continue reading about High power top emitting vertical cavity surface emitting laser... 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