Coherent light generators – Particular resonant cavity – Distributed feedback
Reexamination Certificate
1998-08-12
2001-12-04
Arroyo, Teresa M. (Department: 2881)
Coherent light generators
Particular resonant cavity
Distributed feedback
C372S070000
Reexamination Certificate
active
06327293
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
The present invention relates in general to optically-pumped, semiconductor (OPS), vertical-cavity, surface-emitting lasers (VCSEL). The invention relates in particular to an OPS VCSEL having an aluminum-free quantum-well (QW) structure, an external cavity-mirror spaced-apart from the quantum-well structure, and an optical pump arrangement which directs pump-radiation into the quantum-well structure through the space between the external cavity mirror and the gain region.
DISCUSSION OF BACKGROUND ART
Compact and efficient lasers with a power output of 0.1 to 1.0 Watts (W) or greater, and having single-transverse-mode output beam have a wide range of applications. Such applications include optical communications, laser printing, and optical storage. These applications typically require beam propagation over a distance which is large compared to the size of the laser, focussing the output of the laser into a small spot, or coupling the output of the laser into a single-mode fiber. By way of example, Erbium-doped fiber amplifiers (EDFAs) used in optical communications systems require between about 0.1 W and 0.5 W of continuous-wave (CW) optical pump power in a single mode fiber.
Vertical-cavity surface-emitting semiconductor lasers inherently provide desired circular cross-section output beams. Small diameter VCSELs, for example, less than about 10 micrometers (&mgr;m) in diameter, operate in a single transverse mode, however, with an output power limited to less than about 10 milliwatts (mW). For larger devices, for example, greater than about 100 &mgr;m diameter, output power can be greater than about 100 mW, however, only in multiple transverse modes. Using an external cavity, i.e., a cavity which is provided by one mirror in contact with a semiconductor gain medium and another mirror spaced-apart from the semiconductor mirror and gain medium, a large diameter (about 120 &mgr;m) VCSEL has been forced to operate in a single transverse mode, however, at an output power of only about 2.4 mW.
The above comments regarding prior-art semiconductor lasers are devices which are electrically pumped, i.e., in which carriers are injected across electrical semiconductor junctions to recombine in active layers and thereby generate laser-radiation. In U.S. Pat. No. 5,461,637 to Mooradian and Kuznetsov, an OPS VCSEL operable in a single transverse mode is described. The laser includes a quantum-well structure which provides a gain region. The Mooradian and Kuznetsov patent teaches that an OPS VCSEL can be made to operate in a single transverse mode by separating cavity mirrors of the laser by a solid body which has a significant thermal coefficient of refractive index. The quantum-well structure also has a thermal coefficient of refractive index. Any absorbed pump-radiation which does not contribute to the gain process heats the quantum-well structure and the solid body adjacent thereto. This heating forms in effect a thermal lens in the body. The thermal lens forces the laser to operate in a single transverse mode. A significant drawback of devices described in the Mooradian and Kuznetsov patent is that pump-radiation must traverse one of the cavity mirrors, and, in one arrangement, the solid body also, in order to reach the gain structure. This, together with the thermal lensing effect, provides for difficulties in providing optics which efficiently match pump-radiation with the laser mode diameter at the quantum-well. Because of this, such lasers can be expected to have low optical efficiency.
Further, the VCSEL of Mooradian and Kusnetsov has a quantum-well region including spacer layers of AlGaAs. Because of this, it can be expected that the VCSEL would be subject to problems of limited lifetime similar to those which have been identified in edge-emitting diode-lasers using AlGaAs layers. There is clearly a need for an OPS VCSEL which can operate efficiently at a high power in a single transverse mode but which also has a long operating lifetime.
SUMMARY OF THE INVENTION
The present invention is directed to providing a vertical-cavity, surface-emitting laser system. In one aspect, a laser system on accordance with the present invention comprises a first mirror, and a semiconductor multilayer epitaxially-grown quantum-well structure on the first mirror. The quantum-well structure has an output-window layer defined as the epitaxially-grown layer furthest from the first mirror.
The quantum-well structure includes a plurality of quantum-well groups each thereof having a compressively stressed quantum-well layer of InGaAs and a spacer layer group and a barrier layer of GaAs between the spacer layer group the quantum-well layer. The spacer layer group includes one of a single layer of In
x
Ga
(1−x)
As
y
P
(1−y)
having a tensile stress, and a layer of GaAs and a tensile stressed layer of GaAs
u
P
(1−u)
.
A second mirror faces the output-window layer and is spaced apart therefrom, thereby defining a gap between the second mirror and the output-window layer. The first and second mirrors define a resonant cavity therebetween of length L.
The laser system includes at least one source of pump-radiation for optically pumping the quantum-well structure. The pump-radiation source is arranged to direct the pump-radiation into the quantum-well structure via the output-window layer thereof.
Preferably, the pump-radiation source is arranged to direct the pump-radiation through the gap between the second mirror and the output-window layer, without passing through said second mirror, then into the quantum-well structure via the output-window layer thereof.
In another aspect of the inventive laser system, the epitaxially-grown quantum-well structure is grown by molecular beam epitaxy. This growth method provides, among other advantages, more flexibility in selecting layer thickness and composition for pump-radiation absorption and stress control.
In yet another aspect of the present invention, layers of the quantum-well structure are aluminum-free. This forgoes the well known advantages of using the lattice matching to GaAs (low stress) properties materials of the AlGaAs system for building the thick structure of a VCSEL, in favor of aluminum-free materials of the InGaAsP system. These materials require significantly more effort in controlling stress for building a thick structure, but offer significant lifetime advantages over the AlGaAs system for lasers constructed therewith.
In one preferred embodiment of a laser in accordance with the present invention, the quantum-well layers are layers of compressively stressed In
x
Ga
(1−x)
As; where 0.0<x<0.3. Each spacer layer group is a single layer of In
x
Ga
(1−x)
As
y
P
(1−y)
having a composition selected to absorb the pump-radiation, and a stress-thickness product selected to balance the stress-thickness product of the quantum-well layer.
In another preferred embodiment of the present invention, the quantum-well layers are layers of compressively stressed In
x
Ga
(1−x)
As, where 0.0<x<0.3. Each spacer layer group includes a layer of GaAs and a layer of GaAs
u
P
(1−u)
. The GaAs layer absorbs the pump-radiation and the layer of GaAs
u
P
(1−u)
has a composition selected such that it has a stress-thickness product selected to balance to balance the stress-thickness product of the quantum-well layer.
The laser, constructed using an uncoated chip and pumped by the output of a 2.0 W, 808 nm diode laser, has been operated at a power of more than 450 mW in the fundamental (TEM
00
) mode. It is believed to be operable in the TEM
00
mode at a power of about 700 mW using an AR coated chip.
REFERENCES:
patent: 3958263 (1976-05-01), Panish
patent: 4630083 (1986-12-01), Yamakoshi
patent: 5131002 (1992-07-01), Mooradian
patent: 5289485 (1994-02-01), Mooradian
patent: 5321253 (1994-06-01), Gorfinkel
patent: 5331654 (1994-07-01), Jewell
patent: 5349596 (1994-09-01), Molva
patent: 5432809 (1995-07-01), Grodzinski et al.
patent: 5446754 (1995-08-01), Jewell
patent: 5461637 (19
Chilla Juan L. A.
Salokatve Arto K.
Arroyo Teresa M.
Coherent Inc.
Inzirillo Gioacchino
Stallman & Pollock LLP
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