Coherent light generators – Particular active media – Semiconductor
Reexamination Certificate
2000-12-13
2003-12-02
Ip, Paul (Department: 2828)
Coherent light generators
Particular active media
Semiconductor
Reexamination Certificate
active
06658034
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a vertical external cavity surface emitting lasers (“VECSELs”) excited by optical or electrical pumping. More particularly, the present invention relates to VECSELs which are tunable, which may be made stable at one of a selectable plurality of modes, and which are more readily fabricated and with less complexity than prior approaches.
BACKGROUND OF THE INVENTION
Practical semiconductors lasers generally follow two basic architectures. The first laser type has an in-plane cavity, and the second laser type has a vertical cavity, a so-called vertical-cavity surface-emitting laser or “VCSEL”. If the optical resonance cavity is formed externally of the semiconductor structure, the laser is known as a vertical external cavity surface-emitting laser or “VESCEL”.
One known drawback of in-plane diode lasers, and most particularly the Fabry-Perot type, is that it has a tendency to mode-hop, i.e. to hop unpredictably to a different mode (wavelength) in spite of a constant pumping current. As the current is increased, there are wavelengths at which the mode hopping becomes uncontrollable. Moreover, diode lasers may show hysteresis, in that mode hopping may occur at different wavelengths during control current increases than at control current decreases. Another issue with in-plane diode lasers is the transverse optical beam profile is typically elliptical rather than circular, and has high divergence, increasing the complexity of coupling the laser energy into an optical fiber. Precision gluing of tiny aspheric lenses at the laser-fiber interface is often required.
Furthermore, such lasers only have about 30 to 35 dB of side mode suppression. If the side modes are not well enough controlled, the laser may excite two or three adjacent communications channels, resulting in unwanted interference.
VCSELs include semiconductor structures which have multiple layers epitaxially grown upon a semiconductor wafer or substrate, typically gallium arsenide. The layers comprise Bragg or dielectric-layer mirrors which sandwich layers comprising quantum well active regions. Within the VCSEL, photons emitted by the quantum wells are reflected between the mirrors and are then emitted vertically from the wafer surface. VCSEL lasers typically have a circular dot geometry with lateral dimensions of a few microns. The emitting aperture of a few microns facilitates direct-coupling to optical fibers or other simple optics, since a narrow aperture typically supports only a single lateral mode (TEM
00
) of the resulting optical waveguide, but is sufficiently wide to provide an emerging optical beam with a relatively small diffraction angle. The typical power does not exceed 3 mW in TEM
00
. Recently, a 1.3 micron VCSEL was said to be developed by Sandia National Laboratories in conjunction with Cielo Communications, Inc. According to a news report, “This new VCSEL is made mostly from stacks of layers of semiconductor materials common in shorter wavelength lasers . . . aluminium gallium arsenide and gallium arsenide. The Sandia team added to this structure a small amount of a new material, indium gallium arsenide nitride (InGaAsN), which was initially developed by Hitachi of Japan in the mid 1990s. The InGaAsN causes the VCSEL's operating wavelength to fall into a range that makes it useable in high-speed Internet connections.” (“First ever 1.3 micron VCSEL on GaAs”, Optics.Org Industry News, posted Jun. 16, 2000.) One of the characteristics of VCSELs is that the laser cavity is formed entirely within the semiconductor structure.
As mentioned above, if a cavity is formed which is external to the semiconductor structure having the quantum well active region, the laser is known as a VECSEL. One example of an optically-pumped VECSEL is described in IEEE Photonics Technology Letters Vol. 9, No. 8 pp 1063-1066 and in WO 00/10234, the disclosures thereof being incorporated herein by reference. The disclosed VECSEL includes an epitaxially-grown semiconductor structure or chip having a multiple-layer mirror structure integrated with a multiple-layer quantum-well structure which provides a gain medium, and an external mirror forming a resonant cavity with the integrated semiconductor multilayer mirror. Optical pumping radiation is directed at the quantum-well and pump-absorbing layers. The quantum-well layers release photons in response to the pumping energy, and the external cavity is dimensioned to result in laser energy output at approximately 976 nm in response to pumping energy at approximately 808 nm. Because this VECSEL operates in the visible light spectrum, the active gain medium is made to be aluminum-free, since aluminum ions tend to diffuse in visible light lasers. Accordingly, the quantum-well and pump-radiation absorbing layers are aluminum-free layers of alloys of gallium arsenide and indium gallium arsenide phosphide (GaAs/InGaAsP).
One approach for tuning a VECSEL is described in a paper by D. Vakhshoori, P. Tayebati, Chih-Cheng Lu, M. Azimi, P. Wang, Jiang-Huai Zhou and E. Canoglu entitled, “2 mW CW single mode operation of a tunable 1550 nm vertical cavity surface emitting laser with 50 nm tuning range”, published in Electronics Letters, Vol. 35, No. 11, May 27, 1999, pp. 1-2, the disclosure thereof being incorporated herein by reference. The VECSEL structure described in this note comprises an indium phosphide substrate carrying an epitaxially-grown 1.55_m multiple quantum well system. A via is formed through the bottom of the substrate and a thermally conductive multilayer mirror is deposited into the via to form the bottom mirror of the cavity. A support post structure and a top membrane having a multilayer top mirror structure is formed on top of the active region. The radius of curvature of approximately 300 &mgr;m of the top mirror results in a stable optical resonator cavity as well as a pumping-exit window. To achieve tuning, a voltage is applied between the top membrane and the bottom mirror. The electrostatic force generated will pull the top mirror toward the bottom mirror, reducing the cavity length and also reducing the laser wavelength. With a 980 nm laser pump at 40 mW, a TEM
00
single mode output at approximately 2 mW is achieved by the VECSEL. A tuning voltage from 0 volts to 40 volts changes the VECSEL's output wavelength from 1564 to 1514 nm. One drawback of the VECSEL described in this note is the fabrication complexity. Another drawback is that the VECSEL must be continuously regulated by a voltage control loop in order to maintain the VECSEL at the desired wavelength.
VECSELs may have as a gain structure a few microns thick multiple quantum well active region sandwiched between a bottom Bragg minor grown on a semiconductor substrate, and an epitaxially grown antireflection coating or dielectric coating. An external high reflectivity dielectric concave minor is then added to form an external optical cavity. Co-inventors Garnache, Kachanov and Stockel of the present invention have previously reported in a paper entitled “High-sensitivity intracavity laser absorption spectroscopy with vertical-external-cavity surface-emitting semiconductor lasers”, Optics Letters, Vol. 24, No. 12, Jun. 15, 1999, pp. 826-828 (the disclosure of which is incorporated herein by reference),that an optically pumped multiple-quantum-well (“MQW”) VECSEL is an excellent candidate for use in high sensitivity intracavity laser absorption spectroscopy (“ICLAS”). In the ICLAS method an absorbent analyte is placed inside an external cavity of a broadband laser with homogeneously broadened gain. In the setup reported in this paper, a VECSEL was grown by molecular beam epitaxy on a 0.5 mm gallium arsenide substrate. The bottom stack was a standard Bragg mirror comprising 30.5 pairs of aluminum arsenide/aluminum gallium arsenide quarter-wave layers having a measured reflectance of 99.96 percent at a design wavelength of 1030 nm. The MQW active region comprised two sets of three (2×3) strained 8 nm indium gallium arsenide quantum wells separated b
Garnache Arnaud
Katchanov Alexandre
Stoeckel Frederick
Ip Paul
Lumen Intellectual Property Services Inc.
Picarro, Inc.
Zahn Jeffrey
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