Coherent light generators – Particular active media – Semiconductor
Reexamination Certificate
1999-11-23
2003-11-11
Leung, Quyen (Department: 2828)
Coherent light generators
Particular active media
Semiconductor
C372S098000, C372S099000
Reexamination Certificate
active
06647046
ABSTRACT:
BACKGROUND OF THE INVENTION
Rare-earth-doped fiber amplifiers have been increasingly deployed in fiber optic signal transmission systems, with Raman-based amplification systems being tested for commercial deployment. Previously, over long distance fiber optic transmission links, the optical signal was detected at periodic distances by an opto-electronic detector and converted into an electrical signal, which was then used to drive a laser diode to in effect regenerate the optical signal for retransmission over the next section of the link. The distance between these opto-electronic systems was dictated by the attenuation at the signal frequencies of the fiber, and if any one of these opto-electronic devices failed, the entire optical transmission link failed. In contrast, fiber amplification systems enable optical signals to be re-amplified without conversion to electrical signals. With the advent of fiber amplification systems, the distance between electro-optical devices was no longer attenuation-limited.
Under the model, the fiber amplifiers are distributed along the link to amplify the optical signals or counter pumping into regular fiber is implemented with Raman pump arrays at the link terminus. Opto-electronic devices are only provided along the link at distances beyond which chromatic dispersion and other effects would impair signal demodulation.
Another advantage associated with the use of fiber amplification in optical transmission links is related to their broad gain spectrum. This feature makes dense wavelength division multiplexed systems realistic since multiple channels can be simultaneously amplified using a single fiber amplifier. Currently, WDM systems using fiber amplification have been deployed with 50-100 channels, with even larger channel systems being proposed.
The fiber amplification systems require relatively few components. They comprise a rare-earth-doped fiber or possible standard fiber in the case of Raman systems. Commonly, erbium-doped fiber amplifiers are used since they have a gain spectrum surrounding 1550 nanometers (nm), where there is a transmission window in commonly deployed silica optic fiber. Erbium-doped fiber amplification systems are usually pumped by laser diode pumps, operating at 980 nm or 1480 nm, while Raman systems use a broad range of pump wavelengths, ranging from about 1300 nm to 1600 nm.
Clean signal amplification is achieved by closely regulating the pump light. In broad-bandwidth WDM amplifiers, both power and wavelength stability of pump lasers should satisfy stringent requirements to avoid noise insertion into the signal frequencies. Additionally, shifting of pump wavelength can ruin gain flatness of fiber amplifiers. This robs power away from some channels and degrades their bit error rates. Typically, the wavelength of pump lasers increases slightly with increasing laser drive current as a fundamental result of the increase in junction temperature. The gain peak of semiconductor lasers typically shifts about 0.3 nm/° C. These wavelength changes can induce changes in the gain spectrum of fiber amplifiers. See K. W. Bennet, et al, “980 nm band pump wavelength tuning of the gain spectrum of EDFAs”, OSA Annual Mtg., PD4-1, 1997. If the laser wavelength is controlled mainly by the semiconductor gain spectrum, wavelength control can be inadequate.
In some implementations spectral shifting of pump lasers is controlled by the use of fiber Bragg gratings between the laser pump chip and the fiber amplifier. These have the effect of helping to lock the emission wavelengths of the pump laser system. These fiber gratings, however, are expensive and complicate the deployment of the pump laser modules in amplifier systems.
Other methods have been used in the past to control the wavelength of semiconductor lasers. The earliest etalon stabilization techniques used an angled etalon, which serves as a transmissive filter. A reflection was provided by a mirror (or mirrors) on the opposite side of the etalon. This method, however, is not capable of providing a compact, integrated structure. See Berg, U.S. Pat. No. 4,081,760; Danielmeyer, U.S. Pat. No. 3,628,173.
External reflective mirrors or gratings (with filter and mirror integrated) have been used to feed a portion of the laser light back into the laser. External reflectors or gratings can induce instabilities in the output power due to multi-cavity effects. Abrupt mode hopping and larger power fluctuation often are induced due to cavity mode competition interacting with semiconductor laser nonlinearity. This in addition to the added complexity, positional sensitivity, temperature sensitivity, nonlinear power-current curves, and higher cost make this method generally inadequate for pump lasers.
Another configuration called the cleaved-coupled cavity (C
3
) laser uses a cleave or etched groove to create a second resonant cavity. It is difficult to apply protective coating to the internal facets of the cleaved cavity. As a result, oxidation of the internal cleaved surfaces causes deterioration of the laser. Moreover, the spacing between reflective surfaces is hard to control within a fraction of a percent. The C
3
laser also has the added complication of a third electrode which must be driven to avoid semiconductor loss. See Allen, et al., U.S. Pat. No. 4,284,963; Scifres, et al., U.S. Pat. No. 4,358,851; Craig, et al., U.S. Pat. No. 5,185,754.
Another method uses a grating (or corrugation) within the semiconductor to achieve narrow band selectivity. This has been done with a distributed Bragg reflector (DBR) grating, which reflects a narrow band of light at one end of the cavity, or with a distributed feedback (DFB) grating, which fills the length of the laser. These both add significant complexity to the chip fabrication. Neither has shown adequate reliability and mode control to date at the powers needed for pump lasers.
Still another method uses an etalon that is selected to transmit only light in a desired narrow frequency band. Thin &lgr;/2 layers are used as spacers between multi-layer dielectric or single-layer metallic stacks. This is an example of a resonant etalon. The etalon is designed for maximum transmission at the desired wavelength. This geometry of etalon is also unable to provide subsequent trimming of the wavelength and it presents very low selectivity of the wavelength and has unproven ability to hit the desired design wavelength. See Jansen, et. al., U.S. Pat. No. 5,629,954.
Another structure has used dielectric coatings on an adjacent body attached with a resilient material to achieve single mode operation. See Smith, et al., U.S. Pat. No. 4,805,185. There are additional single mode lasers that use reflective structures that incorporate a spatially periodic structure adjacent to the laser. See Miller, et al, U.S. Pat. No. 4,675,873.
SUMMARY OF THE INVENTION
For pump lasers, single mode operation is not generally required; wavelength stabilization within a narrow band is desired. The previously described techniques do not provide for a wavelength selective element integrated directly on the laser. An integrated wavelength control element is preferable since it does not induce the additional closely spaced cavity modes that an external cavity does. Over the past few years, coating processes have improved significantly, making adequate low-stress, thick-film, reliable deposition possible directly on the semiconductor
Historically, the term etalon has referred to a Fabry-Perot etalon or interferometer. It includes a plane-parallel plate of thickness L
2
and index n
e
which is bounded on each side by a partial reflector. See A. Yariv, Optical Electronics, 3
rd
Edition, Holt, Rinehart, and Winston Inc., chapter 4, 1985 and L. A. Coldren and S. W. Corzine, Diode Lasers and Photonic Circuits, John Wiley & Sons, Inc, pp 73-85, 1995. An etalon is a resonator.
More generally, etalon devices have been commonly used to restrict the longitudinal mode of operation. These intra-cavity structures typically have periodic spectral transmission peaks. These transmission peaks fa
Agon Juliana
Corning Lasertron, Inc.
Hamilton Brook Smith & Reynolds PC
Jackson Cornelius H
Leung Quyen
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