Coherent light generators – Particular active media – Semiconductor
Reexamination Certificate
1999-01-18
2001-12-11
Davie, James W. (Department: 2881)
Coherent light generators
Particular active media
Semiconductor
C372S006000, C372S029020, C372S031000, C372S075000
Reexamination Certificate
active
06330264
ABSTRACT:
BACKGROUND OF THE INVENTION
Rare-earth-doped fiber amplifiers have been increasingly deployed in fiber optic signal transmission systems. Previously, over long distance fiber optic transmission links, the optical signal was detected at periodic distances by an opto-electronic detector, converted into an electrical signal, which was then used to drive a laser diode to in effect regenerate the optical signal for retransmission over the next section of the link. The distance between these opto-electronic systems was dictated by the attenuation at the signal frequencies of the fiber, and if any one of these opto-electronic devices failed, the entire optical transmission link failed. With the advent of fiber amplifiers, the distance between electro-optical devices changed from attenuation-limited to dispersion limited, however.
Under the model, the fiber amplifiers are distributed along the link to amplify the signal to counter its attenuation. Opto-electronic devices are only provided along the link at distances beyond which problematic chromatic dispersion and other effects would impair signal demodulation.
Another advantage associated with the use of fiber amplifiers in optical transmission links is related to their broad gain spectrum. This feature makes dense wavelength division multiplexed systems realistic since an amplifier can simultaneously amplify multiple channels traveling through the same optical fiber. Currently, WDM systems using fiber amplifier amplification have been deployed with 50-100 channels, with even larger channel systems being proposed.
The fiber amplifier systems require relatively few components. They comprise a rare-earth-doped fiber. Commonly, erbium-doped fiber amplifiers are used since they have a gain spectrum surrounding 1550 nanometers (nm), where there is a transmission window in commonly deployed fiber optic cabling. The fiber amplifier is anti-reflection coated at both its input and output facets to ensure that it behaves as an amplifier and not a laser, and isolators are also used, typically at the fiber output. Such fiber amplifiers are usually pumped by laser diode pumps, operating at 980 nm or 1480 nm have been used, but 980 nm pumping appears to becoming increasingly the standard due to certain stabilities in the fiber at this frequency and the laser diode availability. Clean signal amplification is further achieved by closely regulating the pump light. Typically, the pumps are tuned for temporal and spectral stability. Fiber grating stabilization is also used in some systems.
SUMMARY OF THE INVENTION
To further increase the amount of data carried by a single fiber and distances over which signal can be transmitted, noise inserted into the signal frequencies by the amplifier must be minimized. Even with pump stabilization and efforts to better anti-reflection coat the fiber amplifier's facets, the noise in the fiber amplifier system is still not insubstantial. Research has shown that some of this noise results from spurious reflections at the signal frequencies in the laser pump systems. Light originating in the optical fiber leaks into the fiber pigtails and is apparently reflected by the pump laser's output facets to return back to the fiber amplifier where it is now substantially out-of-phase with the optical signal, thus contributing to the noise figure.
The present invention resolves these spurious reflections by providing a laser pump system that is dissipative at the signal frequencies. Thus, spurious reflections at the signal frequencies from the laser pumps are reduced into the fiber amplifier.
In general, according to one aspect, the invention features a wavelength division multiplexing system. This system features a fiber amplifier, which amplifies at signal frequencies of wavelength-separated channels, and a pump system. The pump system comprises a laser pump in which the output facet is coated to be anti-reflective, or transmissive, at the signal frequencies. The pump system further comprises a fiber pigtail having a micro-lens for collecting light from the laser pump and transmitting the pump light to the fiber amplifier.
Finally, according to still another aspect, the invention features a 980 nm laser diode comprising an output facet, which is transmissive at approximately 1550 nm.
The above and other features of the invention including various novel details of construction and combinations of parts, and other advantages, will now be more particularly de:scribed with reference to the accompanying drawings and pointed out in the claims. It will be understood that the particular method and device embodying the invention are shown by way of illustration and not as a limitation of the invention. The principles and features of this invention may be employed in various and numerous embodiments without departing from the scope of the invention.
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Corning Lasertron, Inc.
Davie James W.
Hamilton Brook Smith & Reynolds P.C.
Inzirillo Gioacchino
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