Optical waveguides – With optical coupler – Particular coupling function
Reexamination Certificate
1999-02-04
2001-12-04
Lee, John D. (Department: 2874)
Optical waveguides
With optical coupler
Particular coupling function
C359S341430, C359S345000, C372S006000, C372S032000, C372S075000
Reexamination Certificate
active
06327402
ABSTRACT:
TECHNICAL FIELD
The present invention relates to lightwave transmission systems, and, more particularly, to wavelength division multiplexed (WDM) lightwave transmission systems employing optical fiber amplifiers.
BACKGROUND OF THE INVENTION
Current wavelength division multiplexed (WDM) lightwave communication systems strive for maximum transmission capacity by spacing optical channels as closely as possible, typically about or less than a nanometer (nm). Additionally, in such lightwave communication systems, Erbium (Er) doped optical fiber amplifiers are typically used to maintain the amplitude and integrity of the optical signals over long distance spans. While transmission capacity is greatly increased, it cannot be overlooked that stable lasing transmission wavelengths are critical to system operation inasmuch as any drift of the laser's wavelength readily causes signals from one optical channel to cross into another. As such, current designs for WDM communication systems specify relatively stable, narrow linewidth transmission lasers, typically employing external fiber gratings to stabilize the lasing wavelengths with variations in temperature.
In such communication systems, a factor affecting the optical gain of the Er-doped fiber amplifier (EDFA) is the wavelength used to optically pump the optical signals. Due to its higher gain efficiency, it is preferable to pump near the 980 nm absorption band of the Erbium. For reliability considerations, however, several low power pump lasers may be employed to generate the required pump power levels.
In current WDM lightwave communication systems, pump lasers are also required to lase precisely at predetermined wavelengths. One drawback to this design approach is the precise wavelength stabilization requirements for the pump lasers.
SUMMARY OF THE INVENTION
The present invention is directed to a novel wavelength division multiplexed (WDM) lightwave communication system having allocated wide bands of wavelengths (about a few nanometers) within which the respective optical pump light employed to amplify the optical signals must stay. As the temperature of the pump laser varies, its lasing wavelength is allowed to wander anywhere within its allocated wavelength band. Preferably, each pump laser is wavelength stabilized within its respective allocated wavelength band by means of a fiber Bragg grating having an appropriately wide spectral width. Extending the lasing wavelength range of the pump laser advantageously, in turn, may be used to extend the temperature range over which the pump laser can be operated or locked.
In one preferred embodiment, a wavelength division multiplexed (WDM) lightwave communication system carries the desired optical signals over eight independent optical channels. Each of the eight optical channels is allocated a unique band of wavelengths to transmit information over an optical transmission fiber. The corresponding optical channels, generated by a plurality of transmission lasers, are multiplexed into a single stream and coupled into the fiber using a coupler.
The lightwave communication system preferably employs an Erbium (Er) doped optical fiber amplifier to maintain the amplitude and integrity of the optical signals over substantially the entire span of the fiber. One or more pump lasers nominally lasing within the 980 nm absorption band of the Er-doped fiber, are optically coupled to the fiber for co-directionally pumping the optical signals. Preferably, WDM couplers are used for coupling the pump beams from each corresponding pump laser to the fiber over which the optical signals are propagating while simultaneously allowing the transmitted optical signals to propagate through the fiber.
Wide bands of wavelengths of about a few nanometers, are allocated within which the respective wavelengths of the pump lasers must stay. For example, three corresponding wide bands of wavelengths of about three nanometers wide, falling within a portion of the 980 nm absorption band of the Erbrium doped fiber, may be allocated to the pump lasers. These bands of wavelengths herein referred to as “pump wavebands” are allocated within preferably a center portion of the 980 nm absorption band, thereby ensuring a substantially uniform optical gain over the allocated pump wavebands.
Preferably, the pump beams wander only within a smaller portion of the wavebands to facilitate combining and filtering the pump beams into and out of the fiber with some high degree of precision. Accordingly, there are guard bands of about 5% or more of the width of the pump wavebands near each edge of the allocated bands where the pump beams should not reside. Also, the allocated pump wavebands are preferably of equal width, and thus their widths depend on the number of pump lasers employed.
Although the lasing wavelengths of the pump lasers can wander anywhere within their allocated pump wavebands, each pump laser is still preferably wavelength stabilized within its respective allocated pump waveband by means of a fiber Bragg grating, but having an appropriately wide spectral width that corresponds to about the width of its respective allocated waveband. These wide pump wavebands, however, can be used advantageously to extend the temperature range over which the external fiber bragg grating can stabilize the pump laser.
At lower temperatures, so long as laser's gain peak wavelength is within the laser's locking range, the pump laser's lasing wavelength locks to the grating's left edge wavelength. As the temperature now varies and the gain peak wavelength shifts right and within the grating's reflection band, the lasing wavelength of the pump laser coincides with its gain peak wavelength, until it similarly locks onto the grating's other edge wavelength. In this manner, the pump laser's locking range is extended the temperature equivalent of about the spectral width of the reflection band, which is a resulting effect of allowing the lasing wavelengths to wander within the allocated pump wavebands.
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De La Rosa J.
Lee John D.
Lucent Technologies - Inc.
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