Optical: systems and elements – Optical amplifier – Optical fiber
Reexamination Certificate
1999-11-16
2001-10-02
Tarcza, Thomas H. (Department: 3662)
Optical: systems and elements
Optical amplifier
Optical fiber
C359S337000
Reexamination Certificate
active
06297903
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to the field of optical signal processing and, more particularly, to the amplification of an optical signal using an optical fiber amplifier.
BACKGROUND OF THE INVENTION
As is known in the art, an optical amplifier is a device that increases the amplitude of an input optical signal fed thereto. If the optical signal at the input to such an amplifier is monochromatic, the output will also be monochromatic, with the same frequency. A conventional fiber amplifier comprises a gain medium, such as a single mode glass fiber having a core doped with a rare earth material, connected to a WDM coupler which provides low insertion loss at both the input signal and pump wavelengths. The input signal is provided, via the coupler, to the medium. Excitation occurs through optical pumping from the pumping source. The pump energy which is within the absorption band of the rare earth dopant is combined with the optical input signal within the coupler and applied to the medium, and an amplified output signal is emitted from the other end of the fiber.
Such amplifiers are typically used in a variety of applications including, but not limited to, amplification of weak optical pulses such as those that have traveled through a long length of optical fiber in communication systems. Optical amplification can take place in a variety of materials including those materials, such as silica, from which optical fibers are formed.
One type of fiber amplifier referred to as an erbium doped fiber amplifier (EDFA) typically includes a silica fiber having a single-mode core doped with erbium (specifically doped with erbium ions conventionally denoted as Er
3+
). It is well known that an EDFA operating in its standard so-called three level mode is capable, when pumped at a wavelength of 980 nanometers (nm), of amplifying optical signals having a wavelength of approximately 1550 nm. Since 1550 nm is the lowest loss wavelength of conventional single-mode glass fibers, erbium amplifiers are well suited for inclusion in fiber systems that propagate optical signals having wavelengths around 1550 nm.
For many communications applications, particularly those involving the use of digital signal transmission, it is necessary to use polarized optical signals. Modulators used for digital signal modulation use changing signal polarization to control the generation of digital pulse, and are therefore polarization dependent. Thus, it is necessary to have polarized optical energy to ensure that the desired signal modulation is accomplished. Using optical signal energy with a known, controlled polarization is therefore highly desirable for optical signal communication applications.
It has been an ongoing pursuit in the field of optical amplifiers to increase the power output of the amplifiers and/or reduce the noise figure (i.e., the additional noise introduced into the input signal by the amplifier itself). It is therefore an object of this invention to provide an optical amplifier having better performance in output power and relative noise than those demonstrated in the prior art.
SUMMARY OF THE INVENTION
In accordance with the present invention, a multiple stage optical amplifier includes a co-propagating fiber amplifier stage (in which the pump energy and signal energy propagate in the same direction), and a counter-propagating amplifier stage (in which the pump energy and signal energy propagate in opposite directions). In a preferred embodiment, the optical signal is coupled into a first amplifier stage along with the pump energy using, for example, a wavelength division multiplexer (WDM). The pump energy may consist of a combination of energy from a plurality of lasers all close in wavelength, the outputs of which are combined using a narrow channel WDM coupler. Typically, the first amplifier stage has a fiber coil doped with rare earth material and is configured to have relatively low noise. To minimize the noise figure, the coil may be relatively short and use a fiber having a relatively small core and a high numerical aperture. The output of this stage is coupled into a second stage which divides the optical signal energy and the remaining pump energy, sending the different wavelengths along a counter-propagating path through a second rare earth doped fiber coil. The second stage is typically a high gain stage and, unlike the first stage, the fiber of the second stage amplifier coil may be relatively long and use a fiber having a relatively large core and a relatively low numerical aperture.
The fiber of each of the co-propagating and counter-propagating fiber amplifiers is preferably doped with erbium (Er
3+
). This provides an amplifier which functions well for boosting the energy of a signal having a wavelength of approximately 1550 nm using pumping energy in the wavelength range of 980 nm. Optical isolators are also used throughout the amplifier, to ensure unidirectional propagation of the optical signal at certain points in the signal path. In particular, isolators may be located between the optical signal input port and the coupler which couples the input signal with the pump energy, in the counter-propagating amplifier path, and in the output path of the amplifier. These isolators are preferably wavelength selective, passing the optical signal (i.e., the wavelength range of 1550 nm), and help prevent unwanted feedback of the optical signal.
In an alternative embodiment of the invention, the multiple stage amplifier provides amplification of a single-wavelength, single-polarization signal. The initial optical signal is developed within a master oscillator having an output coupler formed by a chirped Bragg grating fabricated in a polarization maintaining fiber. The gain fiber of the oscillator is rare earth doped, preferably with erbium/ytterbium (Er/Yb), and it is pumped with optical energy of an appropriate wavelength. For pumping an Er/Yb resonator, a narrow channel WDM coupler is used to combine energy from plurality of pump laser sources all having different wavelengths in the range of 980 nm. A highly reflective grating at the front end of the resonator, along with the aforementioned chirped grating in the polarization maintaining fiber at the output side of the resonator, combine to provide the resonator cavity necessary for producing a single-frequency, single-polarization output.
The output of the master oscillator passes through a co-propagating fiber amplifier and through a counter-propagating fiber amplifier, each of which is preferably erbium doped. Much of the pumping energy which passes through the resonator is used to amplify the optical signal within the two fiber amplifier stages. In order to control the polarization of the optical output in this embodiment, the output of the counter-propagating fiber amplifier stage (typically the output stage) is input to a polarization controller, which passes only a single polarization of the optical signal. This polarization controller is calibrated to account for any polarization shifts which the optical signal undergoes during its propagation through non-polarization maintaining components.
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Fidric Bernard G.
Grubb Stephen G.
Hughes Deandea
Kudirka & Jobse LLP
SDL Inc.
Tarcza Thomas H.
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