Optical: systems and elements – Optical amplifier – Free electron
Reexamination Certificate
2002-03-15
2004-07-27
Hellner, Mark (Department: 3663)
Optical: systems and elements
Optical amplifier
Free electron
C359S341330, C359S341310
Reexamination Certificate
active
06768577
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a tunable multimode wavelength division multiplex Raman pump and amplifier, and a system, method, and computer program product for controlling the same.
2. Discussion of the Background
With the explosion of the information age has come a demand for larger data transmission capacity for optical communication systems. Conventionally, optical communication systems transmitted data on a single optical fiber using a single wavelength of light (e.g., 1310 nm or 1550 nm). Signals at these wavelengths were desirable since they have reduced light absorption properties for optical fibers. However, in order to increase the data transmission capacity of these single fiber systems, it was necessary to increase the number of optical fibers laid on a transmission route which greatly increased the cost of optical fiber networks.
To mitigate this problem, wavelength division multiplexing (WDM) optical communications systems such as the dense wavelength division multiplexing (DWDM) system have become desirable. In a WDM system, a plurality of optical signals, each having a different wavelength, can be transmitted simultaneously through a single optical fiber.
Optical fiber communication systems transmit optical signals over considerable distances. However, the signal strength of the optical signals attenuates with distance because of absorption and scattering. Signal strength attenuation ultimately results in signal reception degradation if the signal strength is not kept above background noise (or other sources of noise) by a predetermined amount. Amplifiers are used to keep the signal strength above background noise by a predetermined amount. In general, there are two approaches to amplifying an optical signal: the first, is to use an electronic repeater, which converts the optical signal into an electric signal, amplifies the electrical signal, and then converts the amplified electrical signal back into an optical signal for further transmission along an optical fiber; the second, is to amplify the optical signal itself. Two types of amplifiers that can be used to amplify an optical signal according to the second approach are rare earth doped fiber amplifiers such as erbium doped fiber amplifiers (EDFA), and Raman amplifiers.
EDFAs are currently the most widely used optical amplifiers for WDM systems and are effective and reliable for optically amplifying WDM signals. However, EDFAs have an amplification bandwidth that is limited in range, and produce a wavelength-dependent gain profile. These two characteristics of EDFAs are undesirable for WDM signals, which are spectrally distributed, since a non-uniform amount of gain will be applied to the various WDM channels, depending on the wavelength of the channels. To offset this effect, a gain flattening filter may be used to obtain a uniform or flat gain profile (having a gain deviation of less than 1 dB) across the entire communication band. The gain flattening filter is designed to have a loss profile having a shape that is the inverse of the shape of the gain profile. Gain flattening filters, however, are limited to a particular gain profile, and are not dynamically adjustable to compensate for changes in a magnitude of the gain of the EDFA. Therefore, a flat gain profile cannot be maintained when the gain of the EDFA is changed, or if the attributes of the communications network are changed, such as by adding more WDM signals. In addition, the gain flattening filter decreases the total amount of power launched into an optical fiber.
Raman amplifiers use a phenomenon known as Stimulated Raman Scattering (SRS) of light within an optical fiber to achieve a gain in a particular wavelength band. The inelastic scattering process generates an optical phonon and a co-propagating Stokes wave, light that is downshifted in frequency from the pump light by an amount equal to the phonon frequency (i.e. total energy is conserved). In silica fibers, the peak SRS gain occurs at about 13 THz below the pump light frequency (or conversely, at a wavelength that is longer than a wavelength of the light pumped into the optical fiber by about 100 nm). Since Raman amplification is a scattering process, unassociated with the resonance properties of any particular material, one can generate a Raman gain spectrum for pump light at any wavelength. Therefore, changing a wavelength of the pump light, changes the wavelength at which a peak gain is applied to WDM signals, thereby amplifying some WDM signals more than others. By multiplexing several different pump wavelengths into the same fiber, one can generate a reasonably flat gain spectrum over an arbitrary bandwidth. Because Raman amplifiers require a greater pumping power to obtain the same gain as an EDFA, Raman amplifiers have primarily been used in signal wavelength bands outside of the amplification bandwidth of EDFAs.
Although a Raman amplifier amplifies a signal over a wide wavelength band, the gain of a Raman amplifier is relatively small and, therefore, it is preferable to use a high output laser device as a pumping source. However, increasing the output power of a single mode (or frequency) pumping source beyond a certain threshold leads to undesirable stimulated Brillouin scattering and increased noise at high peak power values. As recognized by the present inventors, to prevent this problem, a multimode laser device is preferably used as a pumping source in a Raman amplifier. A multimode laser has a plurality of oscillating longitudinal modes, each providing output power at less than the threshold at which stimulated Brillouin occurs. A multimode laser can provide a sufficient amount of output power to achieve Raman amplification distributed over the various modes (i.e., wavelengths of output light), as opposed to providing the power all at a single wavelength.
To control the wavelength of the light emitted from the pumping source, and therefore, determine what wavelength of signal will be amplified, it is well known to use fiber gratings. A fiber grating selectively reflects certain wavelengths of light causing a laser beam of a specific wavelength to be output. Fiber gratings are known to be included in the core of an optical fiber, separate from the laser device itself.
Spectroscopy is one application for using a tunable fiber grating is described in U.S. Pat. No. 6,188,705, the entire contents of which being incorporated herein by reference. A tunable fiber grating is described as being coupled to a quasi-monochromatic light source for selecting a single frequency of light output from the light source. Although the light source is capable of producing multiple modes of light, the tunable fiber grating is configured so that only a single frequency output of the light source results. As explained in U.S. Pat. No. 6,188,705, for spectroscopy applications, signal frequency operation is highly desirable, if not required, in order to detect substances with narrow bandwidth absorption lines ('705 patent, col. 1, lines 54-57). A wavelength tuning mechanism may tune the fiber grating by way of temperature change, compression, or through the application of stress of strain ('705 patent, col. 6, lines 14-42).
The present inventors have recognized, however, that the single frequency aspect of the tunable fiber grating described in the '705 patent would render it unsuitable for use as a Raman amplifier pump device because, as discussed above, multimode pump sources are superior to single frequency pump sources when used in Raman applications.
As described in Bruce, E. “Tunable Lasers,” IEEE Spectrum, pages 35-39, February 2002, there are a variety of other types of tunable lasers made for use in WDM systems, although the primary application is for generating a WDM signal at a particular frequency (or wavelength). As recognized by the present inventors, since none of these tunable lasers are made for operation as a Raman pump source that intentionally outputs light at more than one frequency, it is unclear from th
Eggleton Benjamin J.
Emori Yoshihiro
Fujisaki Akira
Kimura Toshio
Namiki Shu
Fitel USA Corp.
Hellner Mark
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