Optical: systems and elements – Optical amplifier – Optical fiber
Reexamination Certificate
2001-06-19
2004-06-15
Black, Thomas G. (Department: 3663)
Optical: systems and elements
Optical amplifier
Optical fiber
C359S341420
Reexamination Certificate
active
06751014
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to optical devices such as lasers, and fiber optic data transmission systems employing the same, and particularly to a novel wavelength-locked loop servo-control circuit for optimizing performance of optical amplifiers.
2. Description of the Prior Art
Wavelength Division Multiplexing (WDM) and Dense Wavelength Division Multiplexing (DWDM) are light-wave application technologies that enable multiple wavelengths (colors of light) to be paralleled into the same optical fiber with each wavelength potentially assigned its own data diagnostics. Currently, WDM and DWDM products combine many different data links over a single pair of optical fibers by re-modulating the data onto a set of lasers, which are tuned to a very specific wavelength (within 0.8 nm tolerance, following industry standards). On current products, up to 32 wavelengths of light can be combined over a single fiber link with more wavelengths contemplated for future applications. The wavelengths are combined by passing light through a series of thin film interference filters, which consist of multi-layer coatings on a glass substrate, pigtailed with optical fibers. The filters combine multiple wavelengths into a single fiber path, and also separate them again at the far end of the multiplexed link. Filters may also be used at intermediate points to add or drop wavelength channels from the optical network.
Optical amplifiers are used to extend the distance of transmitted signals in fiber optic networks. This enables optical signals to be amplified without incurring the additional latency (and computer performance impact) of an optical-to-optical conversion. Optical amplifiers additionally offer lower timing jitter in some cases, and improved performance on long links.
In Wavelength Division Multiplexing (WDM) applications, optical amplifiers must be implemented to amplify many wavelengths spaced closely together at the same time. The basic function of the optical amplifier is to accept an input optical signal and amplify it without converting the signal to electrical form. An Erbium Doped Fiber Amplifier (hereinafter “EDFA”) is one type of optical amplifier that relies on stimulated emission of light at the proper signal wavelength. For example, as shown in
FIG. 1
, the basic EDFA
99
receives an input optical signal
120
, which may consist of multiple wavelengths near the 1550 nm passband. This input signal
120
is passed through a circulator or optical isolator
130
to remove unwanted noise, and then enters via coupler
150
, a section of optical fiber
170
several meters in length that is doped with Erbium ions. The principles of operation for the EDFA itself are well known: the Erbium is excited to an elevated energy state by a laser diode pump (LD pump)
110
, similar to an optically pumped laser system. The LD pump
110
particularly generates a pump laser signal that is coupled to the erbium doped optical fiber
170
through coupler
150
. The 1550 nm signal passing through this fiber produces stimulated emission of light at the same wavelengths as the 1550 nm signals, increasing their amplitude by up to 20 dB or more before the signals
190
exit the optical amplifier. By altering the doping of the erbium fiber
170
, amplification can be obtained for industry standard C-band and L-band wavelengths. The amplifier gain is proportional to the intensity of the pump laser diode.
It is the case however, that EDFAs also produce background noise in the form of light which is not at the desired wavelength; this results from amplification of random photons outside the signal bandwidth, or spontaneous emission of photons within the EDFA which are subsequently amplified. This phenomena is known as Amplified Spontaneous Emission (ASE) noise. Most optical amplifiers have a strong ASE peak around a wavelength of 1533 nm, with weaker effects at other wavelengths. For this reason, commercial EDFAs are designed with a filter to remove ASE at this wavelength. However, as it is not possible to make an ideal bandpass filter at a specific wavelength, this approach does not remove all the ASE from an amplifier. Consequently, ASE builds up in a network with many stages of amplification, and is a limiting factor in the design of long links. As ASE is proportional to the amplifier gain, it would be highly desirable to provide a system and method for controlling and limiting ASE so that the gain of optical amplifiers may be increased resulting in increased link distances in optical networks.
As noted above, an EDFA operates on the same principle as an optically pumped laser; it consists of a relatively short (about 10 meters) section of fiber doped with a controlled amount of erbium ions. When this fiber is pumped at high power (10 to 300 mW) with light at the proper wavelength (e.g., 980 nm or 1480 nm wavelengths) the erbium ions absorb the light and are excited to a higher energy state. Another incident photon around 1550 nm wavelength will cause stimulated emission of light at the same wavelength, phase, and direction of travel as the incident signal. EDFAs are often characterized by their gain coefficient, defined as the small signal gain divided by the pump power. As the input power is increased, the total gain of the EDFA will slowly decrease; at some point, the EDFA enters gain saturation, and further increases to the input power cease to result in any increase in output power. Since the EDFA does not distort the signal, unlike electronic amplifiers, they are often used in gain saturation. The gain curve of a typical EDFA is not uniform over different wavelengths; for example, the gain at 1560 nm is about twice as large as the gain at 1540 nm. This can be a problem when operating wavelength division multiplexing (WDM) systems; some channels will be strongly amplified and dominate over other channels that are lost in the noise. Furthermore, a significant complication with EDFAs is that their gain profile changes with input signal power levels. Thus, for example, in a WDM system the amplifier response may become nonuniform (different channels have different effective gain) when channels are added or dropped from the fiber. As optical amplifiers do not amplify all wavelengths equally, some form of equalization is required in order to achieve a flat gain across all channels.
Current methods for providing gain equalization include adding an extra WDM channel locally at the EDFA to absorb excess power (gain clamping), and manipulating either the fiber doping or core structure. The most commonly used method today in commercial EDFAs is to manually adjust the gain whenever channels are added or dropped from the network, using variable optical attenuators. However, this type of manual intervention is not desirable in large, complicated networks.
It would thus be highly desirable to provide a system and method of automatic gain control (AGC) for optical amplifiers which automatically provides for the adjustment of the optical gain when there is a change in the power of the input signal.
Furthermore, it would be highly desirable to provide a system and method of automatic gain control (AGC) for optical amplifiers that enables dynamic adjustment of the optical output power of the pump laser diode in response to changing conditions on the input fiber link.
Moreover, it would be desirable to provide the capability of manually adjusting EDFA optical amplifier gain from a remote location.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a system and method for automatically varying the pump laser power to the optical amplifier, and perform gain equalization for an optical amplifier using a feedback control loop.
It is another object of the present invention to provide a servo/feedback loop for an EDFA optical amplifier implemented in a WDM system, that enables dynamic adjustment of the pump laser power for performing gain equalization across one or more wavelengths in response to changing conditions on the inp
DeCusatis Casimer M.
Jacobowitz Lawrence
Black Thomas G.
Cunningham Stephen
Townsend, Esq. Tiffany L.
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