Coherent light generators – Particular resonant cavity
Reexamination Certificate
2000-02-04
2001-02-20
Scott, Jr., Leon (Department: 2881)
Coherent light generators
Particular resonant cavity
C372S108000, C372S098000, C372S043010, C372S102000
Reexamination Certificate
active
06192062
ABSTRACT:
FIELD OF THE INVENTION
The invention relates generally to the field of external cavity laser sources. In particular, the invention relates to efficient high-power or high-brightness, multi-wavelength external cavity laser sources that use laser arrays and to methods of generating a high-power or high-brightness multi-wavelength overlapping or coaxial optical beam using laser arrays.
BACKGROUND OF THE INVENTION
High-power or high-brightness semiconductor laser sources which have high efficiency are required for a variety of applications including machining, laser pumping and numerous medical procedures. Efficient high brightness semiconductor laser sources are typically achieved by focusing a semiconductor laser beam into an optical fiber having a small etendue (i.e. small product of core diameter and numerical aperture of the fiber).
Prior methods of fiber coupling high-power diode laser arrays, however, require the use of highly multi-mode optical fiber (i.e. large etendue) and, therefore, have relatively low brightness. For example, one commercial product generates 30 Watts of output power from a multi-mode fiber with a core diameter of about 1 mm and a numerical aperture of 0.12.
Numerous other applications require high-power or high-brightness sources. These applications include communications, solid state laser pumping, imaging, printing, and optical switching. Relatively low-power, multi-wavelength integrated and external cavity lasers have been constructed using dispersive elements.
U.S. Pat. No. 5,351,262 to Poguntke et al. describes a multi-wavelength laser having an integrated cavity that is formed on a single substrate. The laser includes a plurality of individually selectable active waveguides, a diffraction grating, and a passive output waveguide. A resonant cavity is formed between the selected active stripe, the diffraction grating, and the passive output waveguide. The geometry of the resonant cavity determines the lasing wavelengths of each of the plurality of active waveguides. The Poguntke laser can only be used to generate relatively low powers because it is integrated on a monolithic substrate and thus has limited heat dissipation.
Farries, et al.,
Tunable Multiwavelength Semiconductor Laser with Single Fibre Output,
Electronic Letters, Vol. 27, No. 17, Aug. 15, 1991, describes a low-power multi-wavelength external cavity laser that uses a diffraction grating. The external cavity comprises a monolithic semiconductor laser array, a diffraction grating, and a single mode fiber loop mirror. The loop mirror includes a 50:50 coupler with two output ports that are fusion spliced to form a Sagnac interferometer.
Because the Farries laser is designed for fiber optic communication systems, it comprises a single mode semiconductor laser array and, therefore, it can only be used to generate relatively low powers. In Farries, the element separation in the semiconductor laser array is only ten microns. The resulting output power into the fiber is only approximately 0.5 mW per element. In addition, because the external cavity of Farries laser is not completely a free space laser cavity, it is relatively inefficient.
U.S. Pat. No. 5,115,444 to Kirkby et al. describes a multi-wavelength external cavity and integrated cavity laser that uses a dispersive element. A set of optical cavities having different frequency bands is formed from a set of individually addressable semiconductor laser amplifiers, each having a single reflecting facet. The cavity includes a common dispersive element and a common semiconductor amplifier having a partially reflecting facet. The Kirkby laser can only be used to generate relatively low powers. The Kirkby integrated cavity laser is formed on a monolithic substrate and thus has limited heat dissipation. The Kirkby external cavity laser uses a common semiconductor amplifier through which all optical beams in the cavity must propagate. Because the common amplifier also has limited heat dissipation, the Kirkby external cavity laser can only generate relatively low power.
U.S. Pat. No. 5,379,310 to Papen et al. describes an external cavity multi-wavelength laser that uses a dispersive element. A cavity is formed from a plurality of semiconductor lasers, a dispersive element and a reflective element. The plurality of semiconductor lasers generates a plurality of optical beams which are deflected by the dispersive element onto the reflective element. The combination of the dispersive element and the curved surface imposes a different resonance condition on each semiconductor laser thereby resulting in each laser lasing at a different wavelength. The Papen laser generates a plurality of parallel output beams; each beam having a different wavelength. The Papen laser is designed for relatively low power applications such as communication systems, data storage, and spectroscopy. Because the Papen laser generates a parallel (not overlapping or coaxial) output beam, it has relatively low brightness.
U.S. Pat. No. 4,923,270 to Carter describes a laser source for optical wavelength multiplexing. The laser source includes an optical assembly and a reflection grating for collimating light emitted by a laser and for refocusing the light onto an output waveguide. The output waveguide is a fiber that incorporates an embedded reflector which is displaced from the end of the fiber. Because the cavity of the laser source is not a completely free space laser cavity, the power, efficiency, and brightness of the laser is limited.
SUMMARY OF THE INVENTION
The present invention relates to high-power or high-brightness, multi-wavelength external cavity lasers that generate an overlapping or coaxial beam. The lasers of the present invention are constructed with a plurality of single-transverse-mode gain elements, a lens, a wavelength dispersive element, and a partially reflecting element. It is a principal object of this invention that if such lasers are constructed with single-traverse-mode gain elements that they generate a nearly ideal spatial mode with minimal cross talk. It is another principal object of this invention that a spatial filter is not necessary in such a laser to eliminate cross-talk if the dispersive element is required to be substantially at a focal length away from the lens.
Accordingly, in one embodiment, the present invention features a high-power, external cavity laser source. At least two optical gain elements are positioned in the cavity. Each gain element generates optical radiation having one of at least a first and a second wavelength and one of at least a first and a second free space optical path, respectively.
An optical element is positioned in the cavity such that its focal plane is approximately located at the at least two optical gain media and such that it intercepts the at least two respective free space optical paths. The optical element in one embodiment, includes a refractive element. In another embodiment, the optical element includes a reflective element. A dispersive element is positioned in the at least two optical paths. In one embodiment, the dispersive element includes a grating. In another embodiment, the optical element and the dispersive element includes a single optical element such as a Rowland-circle grating.
Another aspect of this invention is the use of a free space external cavity with no guided-wave elements. Free space cavities are inherently less lossy than those that include guided wave elements, and therefore are more efficient. In addition, free space external cavities are simpler.
A partially reflecting element is also positioned in the at least two optical paths. In one embodiment, the partially reflecting element is an end face of an optical fiber. The partially reflecting element and the gain elements together form a free space laser cavity that defines the at least first and second wavelength. In operation, the partially reflecting element transmits an overlapping or coaxial beam including radiation having the at least first and second wavelength.
The present invention also features a multi-wavelength,
Fan Tso Yee
Sanchez-Rubio Antonio
Jr. Leon Scott
Massachusetts Institute of Technology
Testa Hurwitz & Thibeault LLP
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