Optical: systems and elements – Optical amplifier – Raman or brillouin process
Reexamination Certificate
2001-08-07
2002-09-24
Moskowitz, Nelson (Department: 3662)
Optical: systems and elements
Optical amplifier
Raman or brillouin process
C359S199200, C359S337000, C359S341310, C359S341330, C372S070000
Reexamination Certificate
active
06456426
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to fiber-optic communications networks, and more particularly, to optical network equipment such as Raman amplifiers in which optical gain is provided using modulated optical pumps.
Fiber-optic networks are used to support voice and data communications. In optical networks that use wavelength division multiplexing, multiple wavelengths of light are used to support multiple communications channels on a single fiber.
Optical amplifiers are used in fiber-optic networks to amplify optical signals. For example, optical amplifiers may be used to amplify optical data signals that have been attenuated by fiber spans and components in fiber-optic links. A typical amplifier may include erbium-doped fiber coils that are pumped with diode lasers. Raman amplifiers may also be used. Discrete Raman amplifiers may use coils of fiber to provide Raman gain. Distributed Raman amplifiers provide gain in the transmission fiber spans that are used to carry optical data signals between network nodes.
The fiber in Raman amplifiers may be pumped by single-wavelength sources such as diode lasers. However, the Raman gain spectrum produced by a single-wavelength source often does not have the spectral shape that is desired.
The gain spectrum of a Raman amplifier may be modified using a spectral filter. For example, a gain equalization filter may be used to produce a relatively flat gain spectrum by introducing optical losses that compensate for the non-flat shape of the Raman gain spectrum. However, the optical losses associated with using the filter consume optical power and tend to increase the noise figure of the Raman amplifier because additional gain is required to offset the loss.
Another approach for pumping Raman amplifiers involves using a Raman pump source based on multiple diode laser pumps, each of which operates at a different pump wavelength. With this type of approach, the diode laser pumps are each driven at an appropriate current to provide a Raman gain contribution. The overall gain of the Raman amplifier is determined by the Raman gain contributions of each of the individual Raman pump lasers.
If a sufficient number of diode laser pumps are used, it may be possible to produce a desired gain shape such as a flat gain shape for the Raman amplifier without using filters. However, the noise figure performance of the shortest wavelength channels may be adversely affected by cross-pump coupling effects.
It is therefore an object of the present invention to provide arrangements for pumping Raman amplifiers that reduce the effects of coupling between pumps.
It is also an object of the present invention to provide Raman amplifiers having modulated laser diode pumps.
SUMMARY OF THE INVENTION
These and other objects of the invention are accomplished in accordance with the present invention by providing optical amplifiers for use in fiber-optic communications links in fiber-optic networks. The fiber-optic links may be used to carry optical data signals in a signal band associated with wavelength-division-multiplexing channels. Multiwavelength Raman pumps may be used to provide Raman gain for the optical data signals in the signal band. The Raman gain may be provided using discrete or distributed Raman amplifiers or hybrid amplifiers that include Raman-pumped fiber and optically-pumped rare-earth-doped fiber such as erbium-doped fiber.
The multiwavelength Raman pumps may include diode lasers for producing Raman pump light. The laser diodes may operate at wavelengths that are about a Stokes shift below the signal band. The laser diodes may be modulated to reduce the effects of energy transfer between Raman pumps. Reducing such pump-to-pump interactions may improve the performance of the amplifier.
Pump power may be conserved by modulating the laser diodes using optical components that are controlled by a control unit. The pump light for the laser diodes may be modulated using an arrangement based on a fiber delay loop and a polarization controller that is modulated at a frequency that matches the delay associated with the fiber loop. With this arrangement, a constant laser diode output power may be transformed into a modulated pump signal with minimal loss of pump power. Optical components based on phase controllers may also be used to modulate the pump light.
In-span isolators with pump bypasses may be used to improve the performance of distributed Raman amplifiers that use modulated pump light. Pump light modulation may help to allow amplifier systems to be upgraded without adversely affecting preinstalled Raman amplifier pumps.
Further features of the invention and its nature and various advantages will be more apparent from the accompanying drawings and the following detailed description of the preferred embodiments.
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Bolshtyansky Maxim A.
Chen Chien-Jen
Wong William S.
Fish & Neave
Moskowitz Nelson
Onetta Inc.
Treyz G. Victor
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