Optical: systems and elements – Optical amplifier – Correction of deleterious effects
Reexamination Certificate
2001-03-31
2003-03-18
Hellner, Mark (Department: 3662)
Optical: systems and elements
Optical amplifier
Correction of deleterious effects
C359S337100, C359S337000, C359S341410
Reexamination Certificate
active
06535330
ABSTRACT:
FIELD OF THE INVENTION
This invention generally relates to control systems for optical amplifiers and is specifically concerned with a dynamic gain and tilt controller for an erbium-doped fiber amplifier that monitors amplified spontaneous emission at different points in the amplifier output spectrum to minimize tilt in the output.
BACKGROUND OF THE INVENTION
Erbium-doped fiber amplifiers (EDFAs) are used in optical transmission networks to extend transmission distances and to compensate for losses from various network elements. Such amplifiers typically comprise a pump laser whose output is optically coupled to the input of two or more serially connected coils of erbium-doped optical fiber. In operation, the output of the pump laser excites the atoms of erbium dopant within the serially connected coils of doped fiber. These excited atoms release their excess energy in proportion to the strength of the incoming optical signal, which results in a modulated, amplified output.
While such EDFAs work well for their intended purpose, problems arise when such amplifiers are used to amplify with multi-channel optical signals. Because of mismatches between the differing wavelengths of such multi-channel signals, and the population of dopant atoms at various excitation states, such amplifiers tend to amplify certain channels more than others, resulting in a non-flat gain across the spectrum of optical channels. Such non-uniformities in output amplification across a spectrum of channels is known as tilt. If the tilt in the output is not corrected, the noise, or amplified spontaneous emission (ASE) carried by the least-amplified channels can accumulate over distance to a point where the signal to noise ratio becomes small enough to effectively obliterate the channel.
The prior art has attempted to solve the tilt problem by interposing a gain flattening filter between the two serially connected coils of erbium-doped optical fiber. Such a filter is pre-calibrated at a particular gain level to attenuate the most strongly amplified channels in order to flatten the distribution of amplification across the channel spectrum. Unfortunately, such a technique has proven to be only a partial solution as it does not remove all of the tilt associated with the amplifier output. Moreover, this technique is limited to a single, preselected gain level as the shape of the tilt varies with the gain of the amplifier. To further flatten the amplified output, it has also been proposed to modulate the power input to the pump laser in accordance with the relative amplitudes of ASE power taken from the end points of the output channel spectrum. Since the strength of an amplified channel is proportional to the strength of the ASE in the frequency neighborhood of that channel, the comparison of ASE strength at the end points of the channel spectrum yields a clear indication of the amount of tilt in the amplified output. The combination of a gain flattening filter with such laser power modulation effectively reduces more of the tilt associated with the output signal by changing the population distribution of the various excited states of the dopant in the amplifier coils. Unfortunately, such a technique is likewise effective at only a single gain set point level, which in turn limits its flexibility within an optical transmission system in situations where, for example, a constant output strength is desired while the strength of the signal to be amplified varies.
Accordingly, there is a need for a dynamic controller for a multi-channel optical amplifier that is capable of providing effective tilt control of the amplifier output over any one of a broad range of selectable gain level set points. Ideally, such a controller would be relatively simple in construction, and formed from inexpensive, commercially available optical and electrical components. Finally, such a system should be easily retrofittable onto existing EDFA amplifiers.
SUMMARY OF THE INVENTION
The controller of the invention overcomes the aforementioned shortcomings associated with the prior art by providing a dynamic controller for a multi-channel optical amplifier capable of providing tilt control over a broad gain range of selectable gain levels. To this end, the controller of the invention comprises a variable optical attenuator (VOA) coupled between the input and output of the amplifier, a control circuit connected to the variable optical attenuator for controlling the amount that each optical channel should be attenuated to attain a flat output over the channel spectrum, and a gain controller and a tilt controller connected to an input of the control circuit. The gain controller provides a signal indicative of an average amount of per channel attenuation necessary to achieve a selected gain set point. The tilt controller provides a signal indicative of relative amounts that different channels should be attenuated to reduce tilt.
The gain controller includes a ratio circuit connected to input and output signals of the amplifier for providing a signal indicative of actual amplifier gain, as well as a set point circuit connected to the ratio circuit for providing a signal indicative of any difference between the selected amplifier gain and actual amplifier gain. This difference signal is in turn relayed to the control circuit of the variable optical attenuator.
The tilt controller includes a noise circuit optically connected to the amplifier output for providing a signal indicative of relative strengths of ASE between different channels, as well as a pump laser control circuit for providing a signal indicative of a power level that said laser should be operated at to reduce tilt. The tilt controller further includes a VOA adjustment circuit connected to the output of the pump laser control circuit for providing a signal to the VOA control circuit indicative of relative amounts the different channels should be attenuated to reduce tilt. To further flatten the amplified signal output, the tilt controller may further comprise a dynamic gain flattening filter coupled between the variable optical attenuator and the downstream coil of erbium-doped fiber cable. The dynamic gain flattening filter may have an input connected to an output of the noise monitoring circuit in order to assist the VOA in flattening the amplifier output.
In operation, the system operator selects a set point via the set point circuit of the gain controller. The gain controller in turn instructs the variable optical attenuator controller of the amount each channel should be attenuated in order to achieve the selected gain. Simultaneously, the tilt controller monitors the amount of ASE across two selected points in the channel output spectrum, and proceeds to provide a signal to the control circuit of the variable optical attenuator indicative of relative amounts that different channels should be attenuated to reduce tilt. The control circuit combines the signals received from the gain controller and the tilt controller such that the variable optical attenuator provides a substantially tilt-free output at the selected gain level.
REFERENCES:
patent: 5557442 (1996-09-01), Huber
patent: 6108123 (2000-08-01), Kinoshita
patent: 6111686 (2000-08-01), Toyohara
patent: 6111688 (2000-08-01), Kobayashi et al.
patent: 6115173 (2000-09-01), Tanaka et al.
patent: 6118576 (2000-09-01), Sugiya et al.
patent: 6125583 (2000-10-01), Murray et al.
patent: 6172534 (2001-01-01), Brierley
patent: 6198571 (2001-03-01), Yang
patent: 6201636 (2001-03-01), Noda
patent: 6229643 (2001-05-01), Nakamura
patent: 6233091 (2001-05-01), Kosaka et al.
patent: 6246514 (2001-06-01), Bonnedal et al.
patent: 6256141 (2001-07-01), Kosaka
patent: 6259553 (2001-07-01), Kinoshita
patent: 6266466 (2001-07-01), Nabeyama et al.
patent: 6271962 (2001-08-01), Choi
patent: 6275330 (2001-08-01), Izumi
patent: 6275331 (2001-08-01), Jones et al.
patent: 6282017 (2001-08-01), Kinoshita
patent: 6288836 (2001-09-01), Kawasaki et al.
Yan Sun, et al., “Optical Fiber Amplifiers for WDM Optical,” Bell Labs Technical Journal, pp. 187-206, J
Cowle Gregory J.
Lelic Muhidin
Menon Naresh
Cole Thomas W.
Corning Incorporated
Hellner Mark
Short Svetlana Z.
LandOfFree
Dynamic controller for a multi-channel optical amplifier does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Dynamic controller for a multi-channel optical amplifier, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Dynamic controller for a multi-channel optical amplifier will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3016199