Optical amplifier
An optical amplifier of the present invention comprises: first and second optical amplifying sections connected in series to each other between an input port and an output port; a first variable optical attenuator arranged on a former stage of the first optical amplifying section; a second variable optical attenuator arranged between the first and second optical amplifying sections; an optical amplification control section that controls the first and second optical amplifying sections; and an optical attenuation control section that controls the first and second variable optical attenuators. The optical amplification control section controls each of the optical amplifying sections so that a gain of the entirety of the optical amplifier is held constant, and the optical attenuation control section controls attenuation amounts of the variable optical attenuators so that monitor values of the output powers from the variable optical attenuators approximate the same value, provided that a value of the sum of the attenuation amounts of the variable optical attenuators decided according to the signal light input power per one wavelength of a WDM light is held constant. As a result, it becomes possible to provide at a low cost the optical amplifier capable of achieving flat output wavelength characteristics and the favorable NF, irrespective of the signal light input power per one wavelength of the WDM light and the number of wavelengths thereof.
1. An optical amplifier for amplifying a wavelength division multiplexed light containing a plurality of signal lights of different wavelengths, comprising;
an input port and an output port;
first and second optical amplifying sections connected in series to each other between said input port and said output port;
a first variable optical attenuator arranged between said input port and said first optical amplifying section;
a second variable optical attenuator arranged between said first optical amplifying section and said second optical amplifying section;
an optical amplification control section that monitors the power of the wavelength division multiplexed light input to said input port and the power of the wavelength division multiplexed light output from said output port, and controls said first and second optical amplifying sections so that a gain calculated based on monitoring results is held constant; and
an optical attenuation control section that decides a value of the sum of attenuation amounts of said first and second variable optical attenuators according to the signal light input power per one wavelength of the wavelength division multiplexed light input to said input port, and also, monitors the signal light power of the wavelength division multiplexed light output from each of said first and second variable optical attenuators, and controls the attenuation amounts of said first and second variable optical attenuators, so that monitor values of the output powers from said first and second variable optical attenuators approximate the same value, while holding the decided value of the sum of the attenuation amounts constant.
2. An optical amplifier according to claim 1 ,
wherein said first and second optical amplifying sections each comprises: a rare-earth element doped fiber; a pumping light source which generates a pumping light for pumping said rare-earth element doped fiber; and a multiplexer which multiplexes the pumping light output from said pumping light source and the wavelength division multiplexed light, to supply a multiplexed light to said rare-earth element doped fiber.
3. An optical amplifier according to claim 2 ,
wherein said optical amplification control section comprises: an input monitor which monitors the power of the wavelength division multiplexed light input to said input port; an output monitor which monitors the power of the wavelength division multiplexed light output from said output port; an APC circuit which, for said first optical amplifying section, controls said pumping light source so that the power of the pumping light supplied to said rare-earth element doped fiber is held constant; and an AGO circuit which, for said second optical amplifying section, controls said pumping light source so that the gain calculated based on the monitoring results of said input monitor and said output monitor is held constant.
4. An optical amplifier according to claim 2 ,
wherein said optical amplification control section comprises: an input monitor which monitors the power of the wavelength division multiplexed light input to said input port; an output monitor which monitors the power of the wavelength division multiplexed light output from said output port; and an AGO circuit which controls the pumping light power for said first optical amplifying section and the pumping light power for said second optical amplifying section in accordance with a previously set ratio, so that the gain calculated based on the monitoring results of said input monitor and said output monitor is held constant.
5. An optical amplifier according to claim 1 ,
wherein said optical attenuation control section comprises: a variable optical attenuator output power monitor which monitors the signal light power of the wavelength division multiplexed light output from each of said first and second variable optical attenuators; an attenuation amount sum decision circuit which is fed with the signal light input power per one wavelength of the wavelength division multiplexed light input to said input port, and decides a value of the sum of the attenuation amounts of said first and second variable optical attenuators based on said information; a variable optical attenuator output power difference calculating circuit which calculates a difference between the output powers from said first and second variable optical attenuators based on a monitoring result by said variable optical attenuator output power monitor; and an attenuation amount decision circuit which decides the attenuation amounts of said first and second variable optical attenuators, at which an absolute value of the difference calculated by said variable optical attenuator output power difference calculating circuit can mostly approximate zero, while holding the value of the sum of the attenuation amounts decided by said attenuation amount sum decision circuit, and outputs control signals for controlling said first and second variable optical attenuators in accordance with the decided attenuation amounts.
6. An optical amplifier according to claim 1 , further comprising:
a third optical amplifying section connected between said second optical amplifying section and said output port; and
a third variable optical attenuator arranged between said second and third optical amplifying section,
wherein said optical amplification control section controls said first to third optical amplifying section so that a gain calculated based on said monitoring results is held constant, and
said optical attenuation control section decides a value of the sum of attenuation amounts of said first to third variable optical attenuators according to the signal light input power per one wavelength of the wavelength division multiplexed light input to said input port, and also, monitors the signal light power of the wavelength division multiplexed light output from each of said first to third variable optical attenuators, and controls the attenuation amounts of said first to third variable optical attenuators, so that monitor values of the output powers from said first to third variable optical attenuators approximate the same value, while holding the decided value of the sum of the attenuation amounts constant.
7. An optical amplifier according to claim 6 ,
wherein said third optical amplifying section and said third variable optical attenuator are made to be one unit, and a plurality of said units are multistage connected between said second optical amplifying section and said output port.