Optical: systems and elements – Optical amplifier – Correction of deleterious effects
Reexamination Certificate
2002-03-28
2003-11-18
Hellner, Mark (Department: 3663)
Optical: systems and elements
Optical amplifier
Correction of deleterious effects
C359S341400
Reexamination Certificate
active
06650467
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an erbium-doped fiber amplifier used in a wavelength division multiplexing transmission system, and more particularly to an optical fiber amplifier, which is capable of controlling its gain to maintain the intensity of an output optical signal constant by driving a laser diode with a signal obtained by use of input optical signal filtering.
2. Description of the Prior Art
Wavelength Division Multiplexing (WDM) is a method for transmitting a plurality of optical signals of different wavelengths via a single optical fiber. Since the WDM uses the different wavelengths of optical signals simultaneously when the optical signals are transmitted, a wide bandwidth provided by the optical fiber can be effectively utilized. Therefore, this method is being popularized as next generation optical transmission technology.
An erbium-doped fiber amplifier is a kind of amplifier used in a WDM transmission system. The erbium-doped fiber amplifier is manufactured by doping special material called erbium into the fiber. When erbium is pumped by a laser, a weak optical signal can be amplified by energy released when excited erbium ions return to their original energy level.
Now, a general erbium-doped fiber amplifier will be described with reference to the accompanying drawings.
FIG. 1
is a view showing a configuration of a general erbium-doped fiber amplifier.
Referring to
FIG. 1
, a first amplification stage comprises an erbium-doped fiber
123
, optical couplers
121
and
122
connected to the front and rear of the fiber
123
, and laser diodes
124
and
125
supplying excitation light, i.e., laser light for the optical couplers
121
and
122
. Similarly, a second amplification stage comprises an erbium-doped fiber
153
, optical couplers
151
and
152
connected to the front and rear of the fiber
153
, and laser diodes
154
and
155
supplying excitation light for the optical couplers
151
and
152
. A gain equalization filter
13
and an optical attenuator
14
are sequentially connected between the first and second amplification stages and a laser diode controller
16
produces a driving voltage required for each of laser diodes
124
,
125
,
154
and
155
. The optical couplers
121
and
151
are forward IWDM (Isolation and Wavelength Division Multiplexing) optical couplers and the optical couplers
122
and
152
are backward IWDM optical couplers. Here, the term “forward” means propagated in the same direction as an input optical signal, and the term “backward” means propagated in the opposite direction as an input optical signal.
In an amplification process in the first amplification stage, each of laser diodes
124
and
125
emits predetermined excitation light by the application of driving voltage supplied from the laser diode controller
16
, and the excitation light is inputted to the erbium-doped fiber
123
by each of the optical couplers
121
and
122
. The excitation light excites erbium ions included in the erbium-doped fiber
123
. An optical signal inputted through an input port
11
and the optical coupler
121
can be amplified by energy released when erbium ions excited by the excitation light return to their original energy level. Such amplification is also achieved in the second amplification stage, and a resultant amplified optical signal is provided to the outside via an output port.
The gain equalization filter
13
located between the first and second amplification stages is for maintaining a balance of total gain by extracting a smooth portion of gain of the optical signal amplified in the first and second amplification stages. The optical attenuator
14
optimizes the optical signal by adjusting the intensity of the optical signal inputted into the second amplification stage.
In the above WDM transmission system, the number of channels of the optical signals is varied by capacity variation of the transmission network, errors of transmission channels, and any attachment and detachment of parts due to reconstruction of the transmission network. When the number of channels of the optical signals in use is varied, surviving channels in operation, i.e., remaining optical channels, move to an unwanted state through a transient state according to the characteristics of the erbium-doped fiber used as gain medium in the fiber amplifier, so the instantaneous change of gain and output power occurs, thus causing errors in optical transmission service.
Generally, since the fiber amplifier is constructed by connecting several amplification stages in series, though each of the amplification stages has a small variation of output, significant errors occur when the optical signal passes through the amplification stages in an optical transmission line. Therefore, there is a need to provide a gain control method for controlling the variation of output to be suppressed in a shorter time.
Generally, the erbium-doped fiber has gain inhomogeneity characteristics and cross gain saturation characteristics. The gain inhomogeneity characteristics mean a variation of gain of the surviving channels generated when the wavelength of the surviving channels having constant gain is varied. Gain in the erbium-doped fiber is shared by various optical channels, each having a constant gain value. The cross gain saturation characteristics mean equal distribution between the remaining optical channels generated when some of various optical channels are extinguished.
Therefore, since gain becomes varied according to wavelengths of the surviving channels and their distribution condition due to the gain inhomogeneity characteristics and the cross gain saturation characteristics of the erbium-doped fiber, there is a need to provide a gain control method for compensating for an output imbalance for each channel.
The gain control method of the fiber amplifier includes several methods that are described below.
A first method is to control the gain of the fiber amplifier by detecting input optical signals and adjusting excitation light such that it has a proper level of intensity.
However, although the above method is highest advantageous in terms of costs and operation, it has a problem that a range of control is widened in proportion to the number of channels used in optical transmission and it is required to provide a high speed excitation light control circuit having a higher response speed as the number of fiber amplifiers is increased in a system for remote transmission.
A second method is to control the gain of the fiber amplifier by adjusting the population inversion of the fiber amplifier by operating additional channels having wavelength bands different from those of multi-channel in operation. However, in this case, there is a problem that the additional channels require a high maximum output as the number of channels is increased, and a noise due to a nonlinear phenomenon occurs in the multi-channel optical signal in operation.
A third method is to control the gain of the fiber amplifier optically by inducing a laser emission through an optical feedback of some light outside the wavelength band of the multi-channel optical signals in operation so that the population inversion of the fiber amplifier is maintained. According to this method, the intensity of the fed-back laser-emitted light exhibits a damping oscillation in a transient variation state due to a variation of the intensity of the input optical signals. This damping oscillation is a phenomenon produced by an instantaneous perturbation of the population inversion in equilibrium if the upper-level lifetime of the erbium ions serving as a gain medium for the laser emission is longer than the lifetime of photons in a resonator. If this phenomenon is not removed or controlled to be less than a proper level, there occurs a problem that the surviving channels are badly affected. In addition, since this method requires a complicated circuit design in order to maintain the population inversion, there is a problem that high speed response characteristi
Chu Moo Jung
Chung Hee Sang
Lee Jong Hyun
Lee Jyung Chan
Blakely & Sokoloff, Taylor & Zafman
Electronics and Telecommunications Research Institute
Hellner Mark
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