Optical: systems and elements – Optical amplifier – Optical fiber
Reexamination Certificate
1998-11-30
2001-05-22
Hellner, Mark (Department: 3662)
Optical: systems and elements
Optical amplifier
Optical fiber
Reexamination Certificate
active
06236497
ABSTRACT:
TECHNICAL FIELD
This invention is related to an erbium doped fiber amplifier (EDFA). More particularly, this invention relates to an amplifier design which provides for simplified alignment and better matching between components.
BACKGROUND OF THE INVENTION
In various optical communication applications, it is necessary to amplify an optically transmitted data signal. One method of amplifying optical signals is through the use of a doped fiber amplifier, such as an erbium doped fiber amplifier (EDFA). Fiber amplifiers of this type receive pump light of a first wavelength and transfer its energy to an input optical signal of a second wavelength. For example, in an EDFA, erbium doped fibers absorb the energy in applied pump light having a wavelength in the range of 980 nm. The presence of a 1550 nm input signal in the erbium fiber stimulates the fiber to emit the stored energy at the input signal 1550 nm wavelength, thus amplifying the input signal. Although various mechanisms are suitable for coupling the 980 nm pump light into the erbium doped fiber (EDF), the pump light is conventionally supplied by a laser and combined with the 1550 nm signal in the EDFA using a wavelength division multiplexer (WDM). Other dopants, which are typically rare earth elements, may be used to produce fiber amplifiers operating with other pump and signal wavelengths.
In conventional EDFA systems, the 1550 nm input data signal is supplied to the WDM from a 1550 nm single-mode optical fiber. In other words, the fiber has a core region small enough to prevent the data signal from propagating in more than one spatial mode. The pump energy is coupled to the WDM by a second fiber. Conventionally, this fiber is single mode at the shorter pump wavelength (980 nm) and therefore has a narrower core than the first fiber. The 1550 nm and 980 nm light sources are combined by directing them at an appropriate multiplexing device, such as a dichroic mirror or a prism assembly.
The combination of pump and input signal energy is directed into a third (output) optical fiber. The core of the third optical fiber is conventionally chosen to be single mode for both the 980 nm pump energy and the 1550 nm data signal. The output fiber may be erbium doped or may be coupled to a separate erbium doped fiber. The erbium doped fiber absorbs the 980 nm pump light and amplifies the 1550 nm input data signal.
As is apparent, one characteristic of a conventional EDFA is that the optical fibers which carry the pump light are single mode fibers for the shorter pump wavelength. Because of the smaller core size required for single-mode propagation at the shorter wavelength, aligning optical components to receive and/or direct a light signal into the end of the fiber requires a higher degree of precision. Because of this, conventional EDFAs must be manufactured with tighter design tolerances than if a fiber having a wider core were used. In addition, because of the demanding precision required to align the optical components, a relatively large number of devices must be rejected during the manufacturing process. Finally, while a single mode fiber is used to carry the pump light to the WDM, fiber that is single mode at the pump wavelength is not conventionally used on WDM inputs or outputs, which do not carry the pump light. Because different fiber types are used for input to and output from the WDM, a mode mismatch situation occurs, and the accompanying decrease in coupling efficiency results in a noticeable degree of signal loss.
SUMMARY OF THE INVENTION
These and other problems are solved according to the present invention by the use of optical fiber which is single mode for the longer wavelength data signal but multi-mode for the shorter wavelength pump light. Although multi-mode transmission of the pump light results in a loss of information with regard to the position of the pump light within the fiber, in the context of a doped fiber amplifier, such as an EDFA, the pump light is only used to excite the doped fiber. The information loss associated with a multi-mode light transmission is irrelevant so long as the pump light energy is ultimately absorbed.
Advantageously, aligning optical components to transmit and receive light through optical fibers having a core and mode size large enough to permit multi-mode transmission of the pump light is easier than aligning similar components using a conventional optical fiber, which has a core small enough that the pump light propagates in a single mode. Furthermore, using a multi-mode fiber of this type allows the amplifier to utilize the same kind of fiber for input and output signals, thus making it easier to design the intermediate optical lens system and to reduce mode mismatch coupling losses.
According to a preferred embodiment of the invention, the optical coupling between the signal input and output fibers and the WDM are contained in a single housing. Also situated within the housing is a pump laser optical sub-assembly having an output directed into the WDM. The direct coupling of the pump light in this manner eliminates the need to use additional fiber optic cable to couple the pump light to the WDM device, thus reducing the number of components which must be aligned, and therefore fabrication cost.
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Andersen Bo Asp Moller
Bergmann Ernest Eisenhardt
Gaebe Carl Edward
Darby & Darby
Hellner Mark
Lucent Technologies - Inc.
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