1. Field of the Invention
The present invention relates to an optical communication network, and more particularly to an optical fiber amplifier disposed on an optical transmission line.
2. Description of the Related Art
Technical developments in optical fiber amplifiers have accelerated the spread of wavelength division multiplexing transmission systems and networks, because optical fiber amplifiers can increase signal transmitting distance and uniformly compensate for a loss generated in an optical device over a relatively wide range of wavelengths. In wavelength division multiplexing, an optical signal is composed of a plurality of channels having different wavelengths. Conventional wavelength division multiplexing optical fiber amplifiers can be divided into those with automatic gain control (AGC) which maintain in each channel a gain that is held constant against input power variation, and those with automatic power control (APC) which can maintain a constant output power, regardless of the input power or channel. Transmission of signals tends to vary power level by channel. In compensation, gain is varied by channel in a phenomenon known as gain tilt, which remains constant over input power levels for AGC optical fiber amplifiers. A gain tilt-minimized state refers to again flattened state, in which gain is relatively invariant with channel. The higher the gain tilt, the bigger the power difference between channels. Problematically, known APC optical fiber amplifiers cannot maintain a constant gain tilt.
FIG. 1 shows the configuration of an optical fiber amplifier with automatic gain control in the prior art. The optical fiber amplifier comprises first and second tap couplers (TAB) 110, 130, first and second optical detectors 140, 150, an optical amplifying section 120 and a gain control circuit (CTRL) 160.
The first tap coupler 110 has first to third ports. Part of an optical signal inputted to the first port is outputted to the second port. The remaining part of the optical signal is outputted to the third port.
The first optical detector 140 converts an optical signal inputted through the third port of the first tap coupler 110 into an electric signal and outputs the converted signal. Photodiodes (PD) can be used as the first and second optical detectors 140, 150.
The optical amplifying section 120 amplifies and outputs an inputted optical signal. The optical amplifying section 120 may comprise an erbium doped fiber amplifier (EDFA) having an erbium doped fiber (EDF), a pumping light source for pumping the erbium doped optical fiber and a wavelength division multiplexing coupler (WDM coupler) for coupling pumping light outputted from the pumping light source to the erbium doped optical fiber.
The second tap coupler 130 has first to third ports. Part of an optical signal inputted to the first port is outputted to the second port. The remaining part of the optical signal is outputted to the third port.
The second optical detector 150 converts an optical signal inputted through the third port of the second tap coupler into an electric signal and outputs the converted signal.
The gain control circuit 160 compares the powers of electric signals inputted from the first and second optical detectors with each other. Also, the gain control circuit 160 controls the optical amplifying section 120 so that the gains of the channels of the optical signal can be constantly maintained.
FIG. 2 shows power variations in an optical signal on an optical transmission line where a plurality of optical fiber amplifiers are disposed. The optical transmission line includes a plurality of sections which are divided according to the positions (D1, D2, D3, D4) of optical fiber amplifiers. While being sent along the optical transmission line, an optical signal is amplified by each optical fiber amplifier. The power of the optical signal is gradually attenuated until the signal meets a next optical fiber amplifier. In the optical fiber amplifiers, the gains of channels, for example, (P1–P3) or (P2–P4), is constantly maintained. As a result, a transmission loss generated on the optical transmission line is uniformly compensated. An optical signal that has been amplified by an optical fiber amplifier gradually loses power as it propagates along the optical transmission line. The optical signal can be restored to its original power after passing through each optical fiber amplifier having a gain which is set to be identical to the loss of the optical transmission line. However, the capacity of general optical fibers which are used for optical transmission lines gradually degrades over time due to deterioration of the fibers. Capacity can also suffer due to temporary problems on the optical transmission line. Factors such as these can lead to an increase of transmission loss of any of the sections affected. If optical fiber amplifiers having a constant gain are used on an optical transmission line having the properties mentioned above, an optical signal inputted to each optical fiber amplifier will experience gradual power loss and, at the final receiving end, power which has been greatly reduced as compared to the original power. Low power signals are therefore subject to frequent error.
FIG. 3 shows the configuration of an optical fiber amplifier with automatic power control in the prior art. The optical fiber amplifier comprises an optical amplifying section 210, a tap coupler 220, an optical detector 230 and an output power control circuit 240.
In FIG. 3, the optical amplifying section 210 amplifies and outputs an inputted optical signal. The optical amplifying section 210 may comprise an erbium added optical fiber amplifier having an erbium doped optical fiber, a pumping light source for pumping the erbium doped optical fiber and a wavelength division multiplexing coupler for coupling a pumping light outputted from the pumping light source to the erbium doped optical fiber.
The tap coupler 220 has first to third ports. Part of an optical signal inputted to the first port is outputted to the second port. The remaining part of the signal is outputted to the third port.
The optical detector 230 converts an optical signal inputted through the third port of the tap coupler 220 into an electric signal and outputs the converted signal.
The output power control circuit 240 controls the optical amplifying section 210 according to the power of an electric signal inputted from the optical detector 230 so that the output power of the optical amplifying section 210 is urged to a desired value.
FIG. 4 shows power variations in an optical signal on an optical transmission line where a plurality of optical fiber amplifiers are disposed. The optical transmission line includes a plurality of sections which are divided according to the positions (D1, D2, D3, D4) of optical fiber amplifiers. While being sent along the optical transmission line, an optical signal is amplified by each optical fiber amplifier each having a constant output power of P5. The power of the optical signal is gradually attenuated until the signal meets the next optical fiber amplifier. For example, an optical signal has a power of P6 when inputted to an optical fiber amplifier disposed at position D5. When the optical signal is inputted in an optical fiber amplifier at position D8, it has a power of P7 (lower than P6). Since the output power of the optical fiber amplifiers is fixed, an optical signal will have a constant output power of P5 immediately after amplification by each optical fiber amplifier, even if the transmission loss at each section is variable due to deterioration of the optical transmission line. Use of optical fiber amplifiers to produce constant output power, irrespective of the power of an inputted optical signal, avoids deterioration, gradually over time and cumulatively over distance, in the capacity of the optical transmission line. It is also possible to prevent sudden decrease in the capacity of an optical communication network caused by an increase of transmission loss in a certain section. However, the above optical fiber amplifiers are not widely applicable because they cannot maintain a constant gain tilt, which is one of the requirements for a wavelength division multiplexing optical fiber amplifier. Variations in a gain tilt are made when the power of an inputted optical signal is changed during automatic output power control by the optical fiber amplifiers.