Coherent light generators – Particular active media – Semiconductor
Reexamination Certificate
2000-08-10
2004-10-26
Harvey, Minsun Oh (Department: 2828)
Coherent light generators
Particular active media
Semiconductor
C372S096000
Reexamination Certificate
active
06810053
ABSTRACT:
FIELD OF THE INVENTION
This invention pertains generally to the field of semiconductor diode lasers and particularly to surface-emitting distributed feedback semiconductor lasers.
BACKGROUND OF THE INVENTION
Semiconductor diode lasers are formed of multiple layers of semiconductor materials. The typical semiconductor diode laser includes an n-type layer, a p-type layer and an undoped active layer between them such that when the diode is forward biased electrons and holes recombine in the active region layer with the resulting emission of light. The layers adjacent to the active layer typically have a lower index of refraction than the active layer and form cladding layers that confine the emitted light to the active layer and sometimes to adjacent layers. Semiconductor lasers may be constructed to be either edge emitting or surface emitting. In one type of edge emitting semiconductor laser, crystal facet mirrors are located at opposite edges of the multi-layer structure to provide reflection of the emitted light back and forth in a longitudinal direction, generally in the plane of the layers, to provide lasing action and emission of laser light from one of the facets. Another type of device, which may be designed to be either edge emitting or surface emitting, utilizes distributed feedback structures rather than conventional facets or mirrors, providing feedback for lasing as a result of backward Bragg scattering from periodic variations of the refractive index or the gain or both of the semiconductor laser structure. Such distributed feedback (DFB) lasers are discussed in, e.g., papers by H. Kogelnik, et al., “Coupled-Wave Theory of Distributed Feedback Lasers”, J. Appl. Phys., Vol. 43, No. 5, May 1972, pp. 2327-2335; Charles H. Henry, et al., “Observation of Destructive Interference in the Radiation Loss of Second-Order Distributed Feedback Lasers,” IEEE J. of Quantum Electronics, Vol. QE-21, No. 2, February 1985, pp. 151-153; Roel G. Baets, et al., “On the Distinctive Features of Gain Coupled DFB Lasers and DFB Lasers with Second Order Grating,” IEEE J. of Quantum Electronics, Vol. 29, No. 6, June 1993, pp. 1792-1798; and Klaus David, et al., “Basic Analysis of AR-Coated, Partly Gain-Coupled DFB Lasers: The Standing Wave Effect,” IEEE J. of Quantum Electronics, Vol. QE-28, No. 2, February 1992, pp. 427-433.
Since the early 70's, there has been interest in both theoretical and experimental studies of surface-emitting (SE) grating-coupled distributed-feedback (DFB) lasers. It has been demonstrated that 2
nd
order SE-DFB lasers have attractive features such as dynamic single-mode operation, high output power, integrability with other optical components, and surface emission of light in directions substantially normal to the film waveguide. Second-order gratings provide both reflection of guided waves by means of second-order diffraction as well as radiation of free waves away from the film surfaces as a result of first-order diffraction. However, in surface emitting index coupled distributed feedback (IC-DFB) devices the radiation loss is the mode discriminator, and the mode of least radiation loss is invariably an asymmetric one, so that such devices have operated with a two-peaked anti-symmetric near-field pattern and a corresponding double-lobed far-field beam pattern. The latter feature is obviously not desirable for laser applications, since only half the emission can be used. Furthermore, due to the severe non-uniformity of the guided-field intensity profile, IC-DFB lasers are rather vulnerable to gain spatial hole burning, which in turns causes multimode operation.
Several methods for making the far-field pattern approach that of a single-lobed profile have been proposed. They include the incorporation of a phase-shifting film above the grating structure, chirping the grating structure, S. H. Macomber, “Nonlinear Analysis of Surface-Emitting Distributed Feedback Lasers,” IEEE J. of Quant. Elect., Vol. 26, No. 12, December 1990, pp. 2065-2074, or preferential pumping, Nils W. Carlson, et al., “Mode Discrimination in Distributed Feedback Grating Surface Emitting Lasers Containing a Buried Second-Order Grating,” IEEE J. of Quant. Elect., Vol. 27, No. 6, June 1991, pp. 1746-1752. However, these methods, respectively, result in a non-monolithic structure, off-normal radiation, and reliance on the carrier-induced index depression, a fundamentally unreliable technique. Furthermore, such structures have significantly non-uniform guided-field intensity patterns, which makes them vulnerable to gain spatial hole burning.
Surface emitting, complex coupled distributed feedback (SE-CC-DFB) lasers have been investigated which will fundamentally favor operation in a single-lobed beam that is normal to the surface by utilizing an anti-phase design with excess gain preferentially placed in the low-index regions. See M. Kasraian and D. Botez, Appl. Phys. Lett., Vol. 69, 1996, pp. 2795, et seq. and U.S. Pat. No. 5,727,013. Although ridge-guided devices of this type should be capable of providing 50-100 mW CW power, they are generally unsuitable for high power single-mode applications, since they have non-uniform guided-field intensity profiles, which makes such devices susceptible to multimode operation due to longitudinal gain spatial hole burning at high device levels above threshold. By integrating first-order distributed Bragg reflectors at the ends of the SE-CC-DFB structures, surface emitting devices can be made to lase with both relatively high external differential quantum efficiencies, &eegr;
D
, as well as highly uniform near-field and guided-field intensity profiles. See, J. Lopez, M. Kasraian, D. Botez, “Surface Emitting, Distributed Feedback Diodes Lasers With Uniform Near-Field Intensity Profile,” App. Phys. Lett., Vol. 73, No. 16, 19 Oct. 1998, pp. 2266-2268. The efficiency of such devices is limited, however, due to the lossiness of the grating required to suppress the anti-symmetric mode.
SUMMARY OF THE INVENTION
In accordance with the present invention, a high power surface emitting semiconductor laser operates in a single mode with a single lobe far-field radiation profile. Single lobe surface emission is achieved at high power levels with very high efficiency.
The semiconductor laser of the present invention includes a semiconductor substrate and epitaxial structure on the substrate that includes a layer with an active region at which light emission occurs, and can be constructed of a wide variety of semiconductor laser materials. Upper and lower cladding layers surround the active region layer. The semiconductor structure has upper and lower faces, and edge faces. A distributed feedback grating is incorporated with the epitaxial structure and comprises periodically alternating grating elements to provide optical feedback as a second order grating for a selected wavelength of light generation from the active region. In accordance with the present invention, index coupled distributed feedback grating devices of this type can be formed to operate in a mode in which surface emission of light is obtained in a single lobe far-field pattern, without requiring the utilization of lossy grating elements that have been required in order to suppress the antisymmetric mode. Thus, the semiconductor lasers of the present invention are capable of very high efficiency, permitting devices with high power outputs in the watt range.
In the present invention, the semiconductor laser may be formed to operate in an antisymmetric mode with an off-normal single-lobe beam pattern by forming one of the edge faces to be reflective while the other edge face is formed to be antireflective. Appropriate selection of the value of the reflectivity of the reflective edge face and the phase shift of the grating with respect to the reflective edge face results in single lobe surface light emission.
The invention may also be embodied in a structure in which the distributed feedback grating has a spacing within it defining a phase shift in the grating at a position intermediate the edge faces, prefer
Botez Dan
Lopez James G.
Witjaksono Gunawan
Foley & Lardner
Harvey Minsun Oh
Jackson Cornelius H.
Wisconsin Alumni Research Foundation
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