Optical: systems and elements – Optical amplifier – Optical fiber
Reexamination Certificate
1999-12-01
2001-08-28
Hellner, Mark (Department: 3662)
Optical: systems and elements
Optical amplifier
Optical fiber
C359S341400
Reexamination Certificate
active
06282016
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to optical fiber lasers and amplifiers and, more particularly, to fiber lasers and amplifiers which produce a polarized output without requiring all polarization maintaining components.
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 optical 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. Pump energy that 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. The pump energy is absorbed by the gain medium, and the input signal is amplified by stimulated emission from the gain medium.
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. Thus, a signal propagating on a silica-based optical fiber can be introduced to a silica-based optical fiber amplifier, and amplified by coupling pump energy into the amplifier gain medium.
Fiber amplifiers are generally constructed by adding impurities to (i.e. “doping”) an optical fiber. For a silica-based fiber, such dopants include the elements erbium and ytterbium. For example, one type of fiber amplifier referred to as an erbium (Er) amplifier typically includes a silica fiber having a single-mode core doped with erbium ions (conventionally denoted as Er
3+
). It is well known that an erbium optical fiber amplifier 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 nanometers (nm). Likewise, an amplifier having a silica-based fiber “co-doped” with erbium and ytterbium shows excellent amplification of a 1550 nm optical signal when pumped with a wavelength from about 980 nm to about 1100 nm. A particularly useful pump wavelength is 1060 nm because of the availability of high power solid state laser sources at about 1060 nm. Since 1550 nm is approximately the lowest loss wavelength of conventional single-mode glass fibers, these amplifiers are well-suited for inclusion in fiber systems that propagate optical signals in the wavelength vicinity of 1550 nm.
In certain applications, it is desirable to amplify, or generate, a polarized signal using a fiber amplifier or laser, respectively, while maintaining polarization of the optical signal at the output. While a high-birefringence, polarization maintaining fiber may be used, this fiber is difficult to manufacture and generally unavailable as a rare earth doped fiber. One method of providing polarization-maintaining optical amplification is disclosed in U.S. Pat. No. 5,303,314 to Duling III et al. This patent discloses the use of an amplifier having non-polarization maintaining fiber. A linearly polarized optical signal is directed through the amplifier fiber to a Faraday rotator mirror, where it is reflected, and the orientation of its polarization is shifted by 90°. While passing back through the amplifier fiber, any polarization changes caused by the fiber during the initial pass are undone. The reflected signal is thereafter directed to a polarization beamsplitter, where the polarization shift provided by the Faraday rotator mirror allows the returning signal to be separated from the input signal.
SUMMARY OF THE INVENTION
In accordance with the present invention, an amplified single-polarization optical signal may be provided using a number of different embodiments of the invention. A two-stage embodiment of the invention has a first, low-noise stage and a second, high gain stage. Each stage uses a polarization separator to separate signals having different polarization states. In the preferred version of this embodiment, the polarization separators are polarization beamsplitters. The signal input to each stage has a polarization state that allows it to be transmitted through its respective polarization beamsplitter toward an optical gain medium.
The optical gain medium in the first stage is a low noise amplifier, such as an erbium doped optical fiber amplifier, pumped at a wavelength of 980 nm. The optical gain medium in the second stage is a high gain amplifier, such as an erbium/ytterbium doped optical fiber amplifier, pumped at a wavelength of 1060 nm. In each stage, the optical signal passes through the amplifier, is amplified, and directed toward a polarization shifter and reflector, typically embodied in a Faraday rotator mirror. Each rotator mirror reflects the signal directed to it back toward its respective amplifier, while shifting the polarization state of the signal it reflects. Thus, in each stage the returning signal has a new polarization state, and is amplified again within the amplifier of that stage. Upon returning to its respective beamsplitter, each of the reflected signals, having a shifted polarization state, is directed out of the amplifier stage by its beamsplitter. In the first stage, the output signals from the beamsplitter is input to the second stage. When exiting the second stage. the output signal is directed to a system output port.
One embodiment of the invention has an amplifier using an optical fiber with multiple cladding layers (i.e. a “multi-clad” fiber) for a gain medium. Preferably, this fiber is a double-clad fiber. As with each of the stages of the above embodiment, the double-clad fiber gain medium is used with a polarization beamsplitter and a Faraday rotator mirror. The polarized input signal is directed by the polarization beamsplitter to the core of the double-clad fiber. The double clad fiber is pumped by optical pumping energy directed into the inner cladding of the fiber, and provides amplification of the optical signal that passes through its core. The amplified optical signal is reflected by, and has its polarization state shifted by, the Faraday rotator mirror. The signal is further amplified as it passes again through the double-clad fiber and, having a new polarization state, is directed to an output port by the beamsplitter.
The multi-clad fiber embodiment also applies to a fiber optic laser in which the multi-clad optical fiber is used as a medium for generating optical energy that resonates within a laser cavity. The resonator is formed by two gratings, one highly reflective grating positioned to reflect light returning from the Faraday rotator mirror that is transmitted through the polarization separator, and a second, partially-reflective grating positioned to receive optical energy returning from the Faraday rotator mirror that is reflected by the polarization separator. The partially-reflective grating functions as an output coupler for the laser, and reflects a sufficient amount of the energy that a laser cavity is developed between the two gratings. Light resonating in the cavity oscillates between the two gratings, reflecting off the Faraday rotator mirror, and changing polarization state in each pass through the cavity. Another variation of the polarization-maintaining optical amplifier uses either a single-mode or a double-clad amplifier fiber, but uses a double-clad optical fiber as part of the pumping source. The double-clad fiber is doped and is, itself, pumped by coupling optical energy within the gain spectrum of the doped fiber into the inner cladding of the fiber. The
Freeman Paul
Grubb Stephen G.
MacCormack Stuart
Waarts Robert G.
Hellner Mark
Kudirks & Jobse LLP
SDL Inc.
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