Optical: systems and elements – Optical amplifier – Optical fiber
Reexamination Certificate
2002-08-30
2004-05-18
Hellner, Mark (Department: 3663)
Optical: systems and elements
Optical amplifier
Optical fiber
Reexamination Certificate
active
06738185
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical amplifier capable of amplifying signal light with multiple channels of mutually different wavelengths (WDM signal light), a gain control method for the optical amplifier, and a gain control circuit applicable to the optical amplifier.
2. Description of the Related Art
Utilization of a wavelength division multiplexing (WDM) communication system has been promoted aggressively in recent years in response to demands for increases in communication capacities. Meanwhile, in order to enhance reliability and efficiency of a network, optical add-drop multiplexers (OADM), optical cross-connects (OXC) and the like, which are designed to branch or insert part of the WDM signal light transmitted on the network, are being introduced in such a WDM communication system.
The WDM communication system must cope flexibly with variation in the number of signal channels on the network. Accordingly, flexibility to the variation in the number of signal channels on the network is also required in an optical amplifier such as an erbium-doped fiber amplifier (EDFA), which is widely used as a network constituent in the WDM communication system. Moreover, along with the widespread use of the WDM communication system as described above, it is extremely important to offer optical amplifiers to the market at low prices.
Nevertheless, as a method of controlling amplification gain, conventional optical amplifiers have generally adopted a method of calculating gain by use of a logarithmic amplifier, which is disclosed in Japanese Unexamined Patent Publication No. 2000-40847, for example. To be more concrete, automatic gain control (AGC) for calculating the gain by use of the logarithmic amplifier is conducted based on the following principle.
Specifically, gain of an optical amplifier is represented by the following formula (1):
G
(
dB
)=
P
OUT
(
dBm
)−
P
IN
(
dBm
) (1)
Here, G denotes the gain (unit: dB), P
IN
denotes a logarithmic value (unit: dBm) of inputted optical power, and P
OUT
denotes a logarithmic value (unit: dBm) of outputted optical power.
On an input side of the optical amplifier, an input-side photodetector outputs a voltage proportional to part of the optical power of signal light before amplification, and the voltage outputted from the input-side photodetector is subjected to logarithmic transformation by an input-side logarithmic amplifier (i.e. an output voltage V
1
from the input-side logarithmic amplifier is proportional to the logarithmic value of the optical power detected by the input-side photodetector). On the contrary, an output-side photodetector outputs a voltage proportional to part of the optical power of signal light after amplification, and the voltage outputted from the output-side photodetector is also subjected to logarithmic transformation by an output-side logarithmic amplifier (i.e. an output voltage V
2
from the output-side logarithmic amplifier is proportional to the logarithmic value of the optical power detected by the output-side photodetector). Thereafter, the gain of the optical amplifier is detected by subtraction of the obtained voltages V
1
and V
2
with a difference calculator. The detected gain and target gain are compared by use of a comparator, and gain control of the optical amplifier is conducted by adjustment of pumping light power, for example, such that the detected gain and the target gain coincide approximately with each other.
Meanwhile, in a case of performing automatic level control (ALC) of the optical amplifier, the gain of the optical amplifier is detected as described above, and the outputted optical power is calculated by the following formula (2):
P
OUT
(
dBm
)=
G
(
dB
)+
P
IN
(
dBm
) (2)
Thereafter, the outputted optical power thus obtained and the target outputted optical power are compared, and the pumping light power or the like is controlled such that the above-mentioned power values coincide with each other.
SUMMARY OF THE INVENTION
After the studies of the above-described prior art, the inventor of the present invention has found out the following problems.
In the gain control of the conventional optical amplifier, the voltage outputted from the photodetector is subjected to logarithmic transformation with the logarithmic amplifier, and then the gain detection is performed by use of the difference calculator which is readily feasible. However, the logarithmic amplifier is relatively expensive among electronic components. Moreover, a control circuit including the logarithmic amplifier constitutes a nonlinear control circuit which is difficult to design. For example, when the logarithmic amplifier is used, gain of the control circuit may fluctuate owing to the inputted optical power to be detected (i.e. the gain of the control circuit is increased as the power of the light inputted to the photodetector is reduced). Accordingly, the control circuit may become unstable upon an attempt to speed up in such a case, so that it is difficult to achieve high-speed gain control in a wide dynamic range.
The present invention has been made to solve the foregoing problems. An object of the invention is to provide an optical amplifier capable of achieving high-speed gain control in a wider dynamic range with a simpler constitution (with lower costs), a gain control method for the optical amplifier, and a gain control circuit applicable to the optical amplifier.
An optical amplifier according to the present invention includes an optical amplifier medium, a pumping light source, an input-side coupler (a first coupler) and an output-side coupler (a second coupler) disposed to sandwich the optical amplifier medium, and a gain control circuit. The optical amplifier medium includes an erbium-doped fiber (EDF), for example. The pumping light source supplies pumping light of a predetermined wavelength to the optical amplifier medium. The input-side coupler includes a branch port for separating part of signal light to be inputted to the optical amplifier medium. The output-side coupler includes a branch port for separating part of the signal light amplified in the optical amplifier medium. Moreover, the gain control circuit controls gain of the optical amplifier by use of difference information of power values between the light separated by the input-side coupler and the light separated by the output-side coupler. Here, gain control performed by the gain control circuit includes at least automatic gain control.
The gain control circuit includes an input-side photodetector (a first photodetector), an output-side photodetector (a second photodetector), a comparator (included in a control system), and a drive circuit. The input-side photodetector outputs a voltage having a linear relation with the power of the light separated by the input-side coupler. The output-side photodetector outputs a voltage having a linear relation with the power of the light separated by the output-side coupler. The comparator outputs a difference voltage obtained from the voltages outputted respectively from the input-side photodetector and the output-side photodetector. Moreover, the drive circuit supplies a desired drive current to the pumping light source such as a laser diode in response to the voltage outputted from the comparator.
Particularly, in order to perform the gain control of the optical amplifier, the gain control circuit adjusts an inclination
a
and an intercept
b
in a function (V
o
=a·P
i
+b) for defining the linear relation between inputted optical power P
i
and an outputted voltage V
o
concerning at least one of the input-side photodetector and the output-side photodetector. Here, in order to perform the gain control only of the WDM signal light targeted for amplification, it is preferable that the adjustment of the inclination and the intercept is performed while considering optical power of noise light (mainly ASE light) contained in outputted light of the optical amplifier. Moreover, in order to av
Hellner Mark
McDermott & Will & Emery
Sumitomo Electric Industries Ltd.
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