Coherent light generators – Particular active media – Semiconductor
Reexamination Certificate
1999-03-05
2001-10-02
Scott, Jr., Leon (Department: 2874)
Coherent light generators
Particular active media
Semiconductor
C372S045013, C372S093000, C372S099000, C372S035000, C372S022000
Reexamination Certificate
active
06298076
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
The present invention relates in general to external-cavity optically-pumped semiconductor lasers (hereinafter, OPS-lasers) including a surface-emitting, semiconductor multilayer (periodic) gain-structure. The invention relates in particular to arrangements of such lasers which can provide fundamental laser output-power of about two Watts (2.0 W) or greater, and intracavity frequency-converted arrangements of such lasers which can provide harmonic laser output-power of about one-hundred milliwatts (100 mW) or greater.
DISCUSSION OF BACKGROUND ART
The term OPS-lasers, as used herein, refers to a class of vertical-cavity surface-emitting semiconductor lasers wherein optical gain is provided by recombination of electrical carriers in very thin layers, for example, about 150 Ångstrom units (Å) or less, of a semiconductor material. These layers are generally termed quantum-well (QW) layers or active layers.
In an OPS-laser, several QW layers, for example, about fifteen, are spaced apart by separator layers also of a semiconductor material, but having a higher conduction band energy that the QW layers. This combination of active layers and separator layers may be defined as the gain-structure of the OPS-laser. The layers of the gain-structure are epitaxially grown. On the gain-structure is an epitaxially-grown multilayer mirror-structure, often referred to as a Bragg mirror. The combination of mirror-structure and gain-structure is referred to hereinafter as an OPS-structure.
In an (external cavity) OPS-laser, another (conventional) mirror, serving as an output-coupling mirror is spaced-apart from the OPS-structure, thereby forming a resonant cavity with the mirror-structure of the OPS-structure. The resonant cavity, accordingly, includes the gain-structure of the OPS-structure. The mirror-structure and gain-structure are arranged such that QW layers of the gain-structure are spaced apart by one half-wavelength of the fundamental laser wavelength, and correspond in position with antinodes of a standing-wave of the fundamental laser-radiation in the resonator. The fundamental-wavelength is characteristic of the composition of the QW layers.
Optical pump-radiation (pump-light) is directed into the gain-structure of the OPS-structure and is absorbed by the separator layers of the gain-structure, thereby generating electrical-carriers. The electrical-carriers are trapped in the QW layers of the gain-structure and recombine. Recombination of the electrical-carriers in the QW layers yields electromagnetic radiation of the fundamental-wavelength. This radiation circulates in the resonator and is amplified by the gain-structure thereby generating laser-radiation.
OPS-lasers have often been used in the prior art as a means of conveniently testing QW structures for later use in electrically-pumped semiconductor lasers. More recently, OPS-lasers have been investigated as laser-radiation sources in their own right. The emphasis of such investigation, however, appears to be on providing a compact, even monolithic, device in keeping with the generally compact nature of semiconductor lasers and packaged arrays thereof.
The gain-structure of OPS-structures may be formed from the same wide range of semiconductor-materials/substrate combinations contemplated for diode-lasers. These include, but are not limited to, InGaAsP/InP InGaAs/GaAs, AlGaAs/GaAs, InGaAsP/GaAs and InGaN/Al
2
O
3
, which provide relatively-broad spectra of fundamental-wavelengths in ranges, respectively, of about 960 to 1800 nanometers (nm); 850 to 1100 nm; 700 to 850 nm; 620 to 700 nm; and 425 to 550 nm. There is, of course, some overlap in the ranges. Frequency-multiplication of these fundamental-wavelengths, to the extent that it is practical, could thus provide relatively-broad spectra of radiation ranging from the yellow-green portion of the electromagnetic spectrum well into the ultraviolet portion.
In conventional solid-state lasers, fundamental-wavelengths, and, accordingly, harmonics thereof (produced by frequency-doubling or frequency-mixing) are limited to certain fixed wavelengths characteristic of a particular dopant in a particular crystalline or glassy host, for example, the well-known 1064 nm wavelength of neodymium-doped yttrium aluminum garnet (Nd:YAG). While one of these characteristic wavelengths may be adequate for a particular application, it may not be the optimum wavelength for that application.
OPS-lasers provide a means of generating wavelengths, in a true CW mode of operation, which can closely match the optimum wavelength for many laser applications, in fields such as medicine, optical metrology, optical lithography, and precision laser machining. Prior-art OPS-lasers, however, fall far short of providing adequate power for such applications. It is believed that the highest fundamental output-power that has been reported, to date, for an OPS-laser is 700 mW at a wavelength of about 1000 nm (Kuznetsov, et al., IEEE Photonics Tech. Lett 9, 1063 (1997)). For an intracavity frequency-doubled OPS-laser, it is believed that highest output-power that has been reported is 6 mW at a wavelength of about 488 nm (Alford et al. Technical Digest of the IEEE/OSA Conference on Advanced Solid State Lasers, Boston Mass., Feb. 1-3 1999, pp 182-184). It believed that there has been no report to date of generation of continuous wave (CW) ultraviolet (UV) radiation in an OPS-laser, either directly or by frequency-multiplication.
However flexible an OPS-laser may be in potentially offering a wide selection of wavelengths, in order to be competitive in applications in which solid-state and other lasers are currently used, at least an order-of-magnitude, and preferably two orders-of-magnitude increase in power over that offered by prior-art OPS-lasers is required. This power increase must also be achieved without sacrifice of output-power stability and beam-quality. Further, in order to be applicable in the broadest range of applications the range of OPS-laser wavelengths available at high-power and with high beam-quality must be extended into the UV region of the electromagnetic spectrum, preferably to wavelengths less than 300 nm.
SUMMARY OF THE INVENTION
The present invention is directed to providing high-power OPS-lasers, including high-power OPS-lasers providing ultraviolet radiation, i.e., at wavelengths less than about 425 nm. In one particular aspect, an OPS-laser in accordance with the present invention comprises an OPS-structure having a gain-structure surmounting a mirror-structure. The gain-structure includes a plurality of active layers having pump-light-absorbing layers therebetween. The active layers have a composition selected to provide emission of electromagnetic radiation at a predetermined fundamental-wavelength between about 425 nanometers and 1800 nanometers when optical-pump light is incident on the gain-structure. The mirror-structure includes a plurality of layers of alternating high and low refractive index and having an optical thickness of about one-quarter wavelength of the predetermined wavelength.
A laser-resonator is formed between the mirror-structure of the OPS-structure and a reflector spaced apart therefrom. An optical arrangement is provided for delivering the pump-light to the gain-structure, thereby causing fundamental laser-radiation having the fundamental wavelength to oscillate in the laser-resonator. A heat-sink arrangement is provided for cooling the OPS-structure. An optically-nonlinear crystal is located in the laser-resonator and arranged for frequency-doubling the fundamental laser-radiation, thereby providing frequency-doubled radiation having a wavelength half of the fundamental wavelength.
The laser-resonator, the optically nonlinear-crystal, the OPS-structure, the heat-sink arrangement and the optical pump-light-delivering arrangement are selected and arranged such that the resonator delivers the frequency-doubled radiation as output-radiation having a wavelength between about 212 nanometers and 900 nanometers at an output-power gr
Caprara Andrea
Chilla Juan L.
Spinelli Luis A.
Coherent Inc.
Jr. Leon Scott
Stallman & Pollock LLP
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