Optical: systems and elements – Optical amplifier – Optical fiber
Reexamination Certificate
2002-01-11
2003-12-30
Cushlinski, Jr., William A. (Department: 3663)
Optical: systems and elements
Optical amplifier
Optical fiber
C359S337100, C359S349000, C359S337110, C359S341410, C359S341400
Reexamination Certificate
active
06671085
ABSTRACT:
FIELD OF THE INVENTION
The invention relates generally to optical amplifiers in optical communication systems. More specifically, the invention relates to optical amplifiers for large-capacity dense wavelength-division multiplexing (DWDM).
BACKGROUND OF THE INVENTION
Current and future high-speed, high capacity dense wavelength-division multiplexing (DWDM) communication systems have to handle two particular types of user services: multimedia services to multiple users, and select-cast data transport from user-to-user or from region-to-region. A dynamic reconfigurable multi-wavelength channel add/drop function at the user nodes can efficiently process the information of these two types of services, with minimum electronics at the access node, at lower system cost [see for example A. R. Moral et al. “Optical Data Networking: Protocols, technologies, and architectures for next generation optical transport networks and optical internetworks”, J. LightWave Technol. vol. 18, 2000 pp. 1855-1870]. Fiber optical amplifiers will be used in these WDM networks to compensate for insertion loss of optical switches and transmission loss in optical fibers. When the network is reconfigured and wavelength channels are added or dropped, cross-gain saturation in fiber amplifiers will induce power transients in the surviving channels, which can cause service impairment not known in electronically switched networks. As fiber amplifiers saturate on a total power basis, addition or removal of channels in a multi-wavelength network will tend to perturb other channels that share all or part of the route. The power of the surviving channels should be maintained constant in order to prevent unacceptable error bursts if the surviving channel power becomes too low to preserve adequate eye opening or exceeds thresholds for optical nonlinearities.
For DWDM applications, all channels going through the same amplifier gain medium should have as low a noise figure (NF) and as high a gain as possible. In addition to gain and NF requirements, WDM amplifiers must also conform to tight specifications with respect to multichannel gain flatness, gain-tilt, and channel add/drop response. Gain variation is the main concern in designing amplifiers because the gain profile of an optical amplifier changes with its gain. Existing commercial gain-flattened DWDM amplifiers in the current market typically use passively gain-flattening filters. Passively gain-flattened DWDM amplifiers are usually designed for a specific gain requirement, i.e., a well-defined output level for a well-defined input level. They often operate under automatic gain control in the system, meaning that when the input is changed, the output is also changed proportionally, with the gain remaining fixed. This feature fits well in systems where the power level of all channels is fixed but not in cases where channels are added or dropped from an amplified system.
In many situations, the channel-power that is input into a DWDM amplifier, is not constant. If channels have to be switched, re-routed, or transported from one point to multiple points, then the channel power arriving at the entrance of a DWDM amplifier is not constant. If there is to be no degradation in system performance, then all channels must be at approximately the same power level at the DWDM amplifier output, independent of the input power. Thus, a DWDM amplifier must be able to provide a variable gain, without affecting the amplification uniformity across all channels. Alternatively, there may be situations where the input levels do not change, but instead the channels may be required to be routed along a different path with a larger loss. In such cases, the flexibility of increasing the amplifier gain may be required, again without compromising the gain uniformity. The problem is that for a passively gain-flattened DWDM amplifier, if gain changes over the certain small dynamic range, gain shape will change and the corresponding NF may increase. A passively gain-flattened amplifier is inadequate for the varying and demanding DWDM environment.
To solve the above problems, the DWDM amplifier must be actively gain controlled. Many dynamic gain-flattened DWDM amplifiers have been investigated recently [S. K. Yun, et al., Dynamic erbium-doped fiber amplifier based on active gain flattening with fiber acousto-optic tunable filter, IEEE Photon. Technol. Lett., vol.11, 1999, pp.1229-1231]. [B. J. Offrein, et al., Adaptive gain equalizer in high-index-contrast SiON technology, IEEE Photon. Technol. Lett., Vol.12, 2000, pp. 504-506]. [J. C. Chiao, et al., Liquid-crystal optical harmonic equalizers, The Proceeding of the 27th European Conference on Optical communication, October, 2001]. [K. Wundke, et al., A fiber-based, slope adjustable filter for EDFA gain tilt control, The Proceeding of the 27th European Conference on Optical communication, October, 2001]. [T. Kitabayashi, et al., Novel gain-tilt free L-band EDFA using Thulium-doped fiber, The Proceeding of the 27th European Conference on Optical communication, October, 2001]. All the above methods have limitations both in the dynamic gain range and the response time, which makes them unsuitable the future DWDM networking systems.
SUMMARY OF THE INVENTION
Dynamic gain-flattened fiber amplifiers with ultra-wide dynamic gain range and very fast response time across an operational wavelength range with a very flat wavelength response regardless of channel count or channel power level are provided. The maximum variable range of the gain level is preferably 30 dB or larger with very fast response time across the whole C- or L-band wavelength range.
One broad aspect of the invention provides a switchable dynamic gain-flattened optical amplifier with a wide dynamic gain range. An optical signal is first amplified through common amplification such that the gain is approximately common to all channels of the optical signal. Further amplification is then achieved through distinct amplification wherein the optical signal is routed through one of N parallel amplification paths each having its own fixed gain. Each distinct amplification of N parallel paths has a passive gain flattening filter (GFF) to flatten the output power profile across the whole operational wavelength range within a certain flatness requirement (for example, <±0.5 dB). Then the amplified signals are passed through a common variable optical attenuator (VOA) preferably having an attenuating range from 0 dB to L-dB.
Preferably, the value of the gain of the common amplification plus values of the fixed gain on the paths have been designed to satisfy the following relationship: G
1
=G
2
−L=G
3
−2L= . . . =G
N
−(N−1)L, where G
i
is the common gain plus the fixed gain of the i-th path (i=1, 2, 3, . . . N) and L is the maximum attenuating range of the common VOA. The total adjustable gain range of the dynamic gain-flattened optical amplifier in this case will be NL.
In order to self-adjust quickly and respond to changes in input conditions and/or operating conditions of the optical amplifier and output requirements while maintaining gain flatness and a low noise figure (NF) over a broad optical bandwidth and a wide range of gain levels, the switchable dynamic gain-flattened optical amplifier preferably makes use of two optical switches, one at the input to the N parallel amplification paths and one at the output of the N parallel amplification paths, to allow switching in and out one of the gain-flattening filters and gain mediums in parallel.
Preferably, a control function is provided to control the switchable gain amplifier. This involves controlling which of the paths an input signal should be routed through, and involves controlling the gain of the variable optical attenuator. A required overall gain may be input from a networking management system, and the control function makes adjustments to the switchable gain amplifier to best achieve the required overall gain
Lu Zhenguo
So Vincent
BTI Photonics Inc.
Cunningham Stephen
Cushlinski, Jr. William A.
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