Coherent light generators – Particular resonant cavity – Distributed feedback
Reexamination Certificate
2000-09-22
2004-05-25
Davie, James (Department: 2828)
Coherent light generators
Particular resonant cavity
Distributed feedback
C372S020000, C372S070000, C372S099000
Reexamination Certificate
active
06741629
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to an optically-pumped external cavity surface emitting laser emitting single spatial and longitudinal mode radiation at selected wavelengths over a frequency comb. More particularly described are the laser design, manufacturing and assembly processes for optical fiber telecommunications.
BACKGROUND OF THE INVENTION
Practical semiconductor 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 “VECSEL”. Electrically pumped diode lasers are most frequently of the in-plane cavity type. Necessary optical feedback within the in-plane type is most frequently provided by simple cleaved-facet mirrors at each end of the optical cavity. The reflectance of such cleaved mirrors, while sufficient is not very high, and laser energy is emitted through the cleaved mirrors to the external ambient at opposed edges of the structure, giving rise to “edge-emitting” diode lasers. Such relatively simple structures are sometimes referred to as Fabry-Perot diode lasers. Epitaxial patterning of a grating pattern along a top surface of the edge-emitting diode laser is frequently provided to set a design wavelength, resulting in a distributed feedback diode laser or “DFB”.
In-plane electrically pumped (PIN diode) lasers, such as DFB lasers, are typically single mode, and are also typically tunable continuously across some wavelength band from near-infrared and into the visible light spectrum. Rapid tuning may be carried out by controlling the electrical pumping current, while slow tuning may be carried out by controlling the temperature of the laser via a heat sink and a thermal cooler/heater arrangement. In-plane lasers have many known uses including optical wavelength absorption spectroscopy, storage, printing and telecommunications. In-plane lasers are frequently employed within telecommunications systems using optical fiber as the information transfer medium. Conventionally, multiple channels are carried through a single optical fiber, and it is therefore necessary when using a Fabry-Perot diode laser or a DFB laser as the illuminating source to regulate the wavelength of the transmitting laser in order to stay on a selected channel.
In order to keep a diode laser tuned to a desired wavelength, complex current and thermal control loops must be provided to stabilize the laser at the desired wavelength, particularly as the laser ages during usage. Also, there is no absolute wavelength stabilization within these in-plane lasers, and the emission wavelength may drift, without careful feedback control, during usage and over the useful lifetime of the laser device. This tendency to drift or change characteristics with temperature and over time puts extremely stringent conditions on the materials used to make the laser.
One known drawback of in-plane diode lasers, and most particularly the Fabry-Perot type, is that it can manifest a tendency to mode-hop. Mode-hopping basically means that for a given pumping current, unexpectedly the laser can hop to a completely different mode (wavelength). As the current is increased, there are wavelengths at which the mode hopping or wavelength jumping becomes uncontrollable. Moreover, diode lasers may manifest a hysteresis, in that mode hopping may occur at different wavelengths during control current increases than the mod-hopping wavelengths encountered during control current decreases. Another drawback of in-plane diode lasers is that output power is inextricably intertwined with active region temperature and pumping current. Another issue with in-plane diode lasers is that the transverse optical beam profile is typically elliptical rather than circular and has high divergence, increasing the complexity of coupling the laser energy into the optical fiber, such as with precision gluing of tiny aspheric lenses at the laser-fiber interface.
Dense wavelength division multiplexing (DWDM) for optical fiber telecommunications applications require optical transmitters that can be tuned to any frequency in the standard ITU “grid” (wavelength comb) with a relative frequency error not greater than ten percent of the ITU channel spacing. This requirement implies that an optical transmitter laser has extreme frequency stability as well as broad tunability. For a 12.5 GHz channel spacing, the transmitter must have 1.25 GHz of absolute accuracy and frequency (wavelength) stability. Such control of the lasing frequency cannot be achieved with existing DFB lasers without complex electronic control and frequently carried out diagnostics. Furthermore, compensation algorithms must be developed in the laser control to handle the DFB's known aging processes, which is often unpredictable.
Another requirement for an ideal DWDM optical transmitter is that a single laser can cover all of the DWDM channels, and that it can be reliably and reproducibly set to any one of the standard channel frequencies. Practically, a laser source will only have a limited tuning range, which covers only a fraction of the full ITU grid. Existing telecommunications DFB lasers have limited tunability; and, the temperature tuning coefficient of telecom DFB lasers is typically 0.09 nm/° C. For a DFB laser thermal operating range of +20° C., or 40° C. total temperature differential, one DFB laser could only be expected to cover a wavelength range of 3.6 nm (or about 460 GHz, representing only four channel coverage with 100 GHz channel spacing or 36 channel coverage with 25 GHz spacing) provided that necessary accuracy in wavelength could be achieved.
In addition, DFB lasers only have about 30 to 35 dB of side mode suppression. If the side modes are not sufficiently controlled, the laser may excite two or three adjacent communications channels, resulting in unwanted interference. Because of these drawbacks, the telecommunications industry has recently turned to VCSELs.
Micro-cavity VCSELs include semiconductor structures which have multiple layers epitaxially grown upon a semiconductor wafer/substrate, typically Gallium Arsenide or Indium Phosphide. The layers comprise semiconductor or dielectric Bragg mirrors which sandwich layers comprising quantum well active regions. Within the VCSEL photons emitted by the quantum wells bounce between the mirrors and then are emitted vertically from the wafer surface. The VCSEL type laser naturally has 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 the 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. 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 . . . aluminum 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 June 16, 2000). One of the characteristics of micro-cavity VCSELs is that the laser cavity is formed entirely within the semiconductor structure. One drawback of such VCSELs is that they do not generate very much power, on the order of 3 mw for a small aperture of 5 &mgr;, for example. Also, there is transverse s
Garnache Arnaud
Katchanov Alexandre
Paldus Barbara
Romanini Daniele
Stoeckel Frederic
Blueleaf, Inc.
Burkhard Herbert G.
Davie James
Schipper John F.
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