Vertical-cavity surface-emitting semiconductor laser arrays

Details Vertical-cavity surface-emitting semiconductor laser arrays Vertical-cavity surface-emitting semiconductor laser arrays

2001-06-13

2003-08-19

Lee, Eddie (Department: 2815)

Coherent light generators

Particular active media

Semiconductor

C372S043010, C372S045013, C372S046012, C372S050121

Reexamination Certificate

active

06608849

ABSTRACT:

FIELD OF THE INVENTION
This invention pertains generally to the field of semiconductor diode lasers and particularly to vertical-cavity surface-emitting laser arrays.
BACKGROUND OF THE INVENTION
Vertical-cavity surface-emitting lasers (VCSELs) have several significant advantages including low-threshold, high-fiber coupling efficiency, and a compact size that is well suited to integration. In addition, VCSELs also have a simplified fabrication process as compared to edge-emitting distributed feedback lasers and allow wafer level characterization, which provides significant advantages in the manufacturing of such devices. A shortcoming of current VCSELs is the relatively low power output for single mode operation. Coupled arrays of VCSELs offer the potential to increase the coherent output power available from single VCSELs. However, stable, high-power diffraction limited beam operation from two-dimensional (2D) VCSEL arrays has not been realized. See, e.g., J. P. Van der Ziel, et al., IEEE J. Quantum Electron., Vol. 26, 1990, pp. 1873, et seq.; M. Orenstein, et al., Appl. Phys. Lett., Vol. 58, 1991, pp. 804, et seq.; R. A. Morgan, et al., Appl. Phys. Lett., Vol. 61, 1992, pp. 1160, et seq. In addition, prior phase-locked 2D VCSEL arrays operate in either an out-of-phase mode or a mixture of various modes, characteristic of weakly index guided arrays, with poor intermodal discrimination. External phase shifters have been used on optically pumped VCSEL arrays to obtain “in-phase mode-like” emission, but with a relatively broad beam. M. E. Warren, et al., Appl. Phys. Lett., Vol. 61, 1992, pp. 1484, et seq.
The limitations encountered in the development of 2D VCSEL arrays parallels that encountered in the development of edge-emitting phase-locked arrays. Weak coupling and poor intermodal discrimination found in evanescently coupled edge-emitting laser arrays has severely limited their single-mode output power. See, K. L. Chen, et al., Appl. Phys. Lett., Vol. 47, 1985, pp. 555, et seq. In contrast, antiguided array structures exhibit strong leaky-wave coupling, leading to high intermodal discrimination. As a result, edge-emitting antiguided arrays have demonstrated coherent output power in the watt range. See, D. Botez, et al., IEEE J. Quantum Electron., Vol. 26, 1990, pp. 482, et seq. The large built-in index step and strong lateral radiation leakage from an antiguide are well suited for array integration. Antiguided VCSELs have been demonstrated by the regrowth of high-index material and a buried heterostructure design (Y. A. Wu, et al., IEEE J. Sel. Top. Quantum Electron., Vol. 1, 1995, pp. 629, et seq.), by selective oxidation (T. H. Oh, et al., IEEE Photonics Technol. Lett., Vol. 10, 1998, pp. 12, et seq.), or by a cavity-induced resonant shifted structure (K. D. Choquette, et al., Electron. Lett., Vol. 34, 1998, pp. 1991, et seq.). Leaky-wave coupling between two antiguided VCSELs (coupled in-phase or out-of-phase) has been demonstrated using structures fabricated with a cavity-induced resonant shift. Serkland, et al., Appl. Phys. Lett., Vol. 75, 1999, pp. 3754, et seq.
SUMMARY OF THE INVENTION
In accordance with the invention, stable, diffraction limited output at high power levels is obtained from an array of antiguided phase-locked vertical cavity surface-emitting laser arrays. The arrays may be fabricated by a selective etching process and regrowth by metalorganic chemical vapor deposition processes. Resonant coupling of the array of elements occurs with interelement spacings corresponding to an odd (or even) integral number of half waves of the antiguided radiation leakage. Interelement loss may be incorporated in the structure to effectively suppress nonresonant modes.
The laser array device in accordance with the invention includes a semiconductor substrate and a multilayer structure on the substrate. The multilayer structure includes a layer with an active region at which light emission occurs, upper and lower layers surrounding the active region layer, upper and lower faces, and electrodes by which voltage can be applied across the multilayer structure and the substrate. At least four core elements are formed in the multilayer structure arranged in a two-dimensional array, the core elements being separated from one another and surrounded by a matrix region formed to have an effective higher index than the core elements for antiguiding of the radiation leakage from the core elements. The matrix region may also include material providing loss for the radiation in the interelement regions. The width of the matrix region separating adjacent core elements is selected to provide resonant coupling between the core elements. The spacing between core elements may be selected to be substantially equal to an odd integral number of half wavelengths of the antiguided radiation leakage for in-phase coupling, or an even integral number of half wavelengths for out-of-phase coupling. The device is preferably close to the resonance condition, but does not need to be exactly resonant. Operation close to resonance provides strong optical coupling throughout the array, resulting in a nearly uniform near-field intensity. The uniform near-field intensity reduces the influence of non-linear effects above laser threshold, helping to suppress the onset of lasing in other modules. Near the resonance condition, a combination of mode dependent edge losses and interelement losses acts to suppress unwanted modes (i.e., nonresonant modes). An upper reflector above the active region layer and a lower reflector below the active region layer provide vertical confinement of the emitted radiation. The matrix region and the upper and lower reflectors are positioned to act upon the light generated in the active region to produce lasing action phase-locked between the core elements to produce emission of light from at least one of the upper and lower faces of the semiconductor structure. Means for confining the current from the electrodes to the array of core elements may be incorporated in the multilayer structure.
The core elements may be formed as a square, and the core elements may be arranged in rows and columns in the array with equal spacing between the core elements in the rows and columns. The core elements may also be arranged in various array geometries in addition to rectangular, e.g., triangular, etc. The laser array may be formed to either be top surface emitting or bottom surface emitting. For top surface emission, an upper electrode formed on the top face of the structure may include a plurality of openings therein each formed above a core element to define the output aperture for the light from each core element. For a bottom emitting laser, one of the electrodes may be formed over the upper face of the semiconductor laser, and another electrode is formed on the lower face over the substrate and has an opening therein under the array for passage of light therethrough to define the output aperture of the laser array. A heat sink may then be mounted in heat conductive contact with the electrode on the upper face of the semiconductor laser array to efficiently remove heat and allow the semiconductor array laser to operate at higher power levels.
The semiconductor laser array may be formed with various semiconductor materials used in the fabrication of semiconductor lasers. Exemplary structures may include substrates of crystalline GaAs with an active region layer formed of a multiple quantum well structure of layers of, e.g., GaInNAs and GaAs or layers of GaAs and InGaAs. The upper and lower reflectors may be distributed Bragg reflectors formed of multiple pairs of layers, e.g., of AlGaAs and AlAs, AlAs and GaAs, etc.
The material of the higher effective index matrix regions may be selected to locally increase the cavity resonance wavelength to provide an effective equivalent increase in the index of the matrix region. For example, the matrix region may include one or more thin spacer layers of material, e.g., GaInP and GaAs, between layers of an upper distributed Bragg reflector to provide t

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