Optical amplifying method

#The present invention relates to an optical amplifier applied to an optical transmission apparatus such as an optical transmitter/receiver apparatus or an optical repeater in an optical communication system.

#In the prior art, a dispersion compensating fiber is used in an optical receiver or an optical transmitter in order to suppress a waveform distortion of an optical signal due to a waveform dispersion of a transmission line fiber, but because of its large loss, it is essential to use it with an optical amplifier for compensating the loss. This technique is disclosed in OSA Optical Fiber Communication Conference, 1992, pp. 367-370.

#FIG. 14 shows a configuration of an optical fiber transmission system which uses a prior art dispersion compensating optical transmitter 100 and a prior art dispersion compensating optical receiver 200. The optical transmitter 100 comprises an erbium doped optical fiber amplifier 101a, a dispersion compensating fiber 103a and an electro-optical converter 104. The optical receiver 200 comprises erbium doped optical fiber amplifiers 201a and 201b, optical band-pass filters 202a and 202b, dispersion compensating fibers 203a and 203b and a photo-electric converter 205. Losses of the dispersion compensating fibers used are 3.1 dB, 10.6 dB and 5.3 dB, respectively for a light signal level. In order to compensate for the losses, a total of three erbium doped optical fibers are used, which amplify the signal lights by using separate pumping light sources. A characteristic of the optical fiber amplifier when the dispersion compensating fiber is added is that a noise figure is increased by a loss when the dispersion compensating fiber is arranged in a preceding stage, and a light output is decreased by the loss when the dispersion compensating fiber is arranged in a succeeding stage.

#FIG. 1 shows a configuration of a first embodiment of an optical amplifier,

#FIG. 2 illustrates an effect of the optical amplifier of the first embodiment,

#FIG. 3 shows a configuration of a second embodiment of the optical amplifier,

#An embodiment of the present invention is now explained in conjunction with the accompanying drawings. FIG. 1 shows a configuration of an optical amplifier of a first embodiment of the present invention. An optical amplifier AO1 comprises rare earth doped fibers 1a and 1b which are optical amplifying media, wavelength multiplexing/demultiplexing unit 2 and 3, optical isolators 4a and 4b, a pumping light source 5 and a light signal characteristic compensation unit 6. The light signal characteristic compensation unit 6 may be a high dispersion fiber having a reverse dispersion property or an optical resonator such as ethron, which compensates for the dispersion of a transmission line optical fiber, and for a wavelength multiplexing transmission system, it may be a wavelength dependent equalizing optical filter having an optical amplifying medium gain. In FIG. 1, a signal light wavelength is λs shown by a solid line arrow and a pumping light wavelength λp is shown by a broken line arrow. The signal light and the pumping light are applied to the rare earth doped optical fiber 1a through the optical isolator 4a and the wavelength multiplexing/demultiplexing unit 2, and through the wavelength multiplexing/demultiplexing unit 2, respectively, so that the signal light is amplified. For a port which is not used by the wavelength multiplexing/demultiplexing unit 2, it is terminated by an oblique end of the optical fiber. The signal light then passes -through the wavelength multiplexing/demultiplexing unit 3, the light signal characteristic compensation unit 6 and the wavelength multiplexing/demultiplexing unit 3 in sequence, and the pumping light is applied to a next stage rare earth doped optical fiber 1b through only the wavelength multiplexing/ demultiplexing unit 3 and the signal light is again amplified. Normally, in the rare earth doped optical fiber 1b, a signal input is large because the signal light has already been amplified by the rare earth doped optical fiber 1a and it is in a gain saturation state and the gain is low. In the configuration of the present embodiment, however, since only the signal light suffers from the loss by the light signal characteristic compensation unit 6, the signal input is lowered and the rare earth doped optical fiber 1b approaches the non-saturation state and the gain rises. As a result, the loss of the light signal characteristics compensation unit 6 is compensated by the rare earth doped optical fiber 1b and the gain as viewed by the overall optical amplifier AO1 is not lowered by the loss compared with non-loss state of the light signal characteristic compensation unit 6.

#Referring to FIG. 2, an effect of the first embodiment is explained. The optical amplifier of the configuration of FIG. 1 is actually constructed and a gain and a noise figure (only a beat noise component between a signal and an amplified spontaneously emitted light is considered) are actually measured with respect to an input signal light power. A signal light wavelength is 1552 nm, and a pumping light wavelength is 980 nm. In order to examine an effect of the loss of the light signal characteristic compensation unit 6, optical attenuators with losses of 5 dB and 10 dB are inserted. A mark □ is for the loss of 5 dB by the optical attenuator, a mark □ is for the loss of 10 dB by the optical attenuator and a mark □ is for non-insertion of the optical attenuator (0 dB of loss). A pumping light power applied to the rare earth doped optical fiber 1a is 50 mW constant irrespective of the presence or absence of the loss. When the input signal light power is smaller than −20 dBm, the reduction of gain for the 5 dB loss is approximately 2 dB, and the reduction of gain for the 10 dB loss is approximately 4 dB, which is less than one half of the loss. On the other hand, the noise figure is substantially constant around 5 dB for the respective losses. The present experiment shows that, in the optical amplifier of the first embodiment of the present invention, the loss of the light signal characteristic compensation unit can be compensated without increasing the pumping light power. It further indicates that no significant change appears in the noise figure.

#FIG. 3 shows a configuration of a second embodiment of the optical amplifier of the present invention. An optical amplifier A02 comprises rare earth doped optical fibers 1a and 1b which are optical amplifying media, wavelength multiplexing/demultiplexing units 2, 3a and 3b, optical isolators 4a and 4b, a pumping light source 5 and a light signal characteristic compensation unit 6. A signal light and a pumping light are applied to the rare earth doped optical fiber 1a through the optical isolator 4a and the wavelength multiplexing/demultiplexing unit 2 and through the wavelength multiplexing/demultiplexing unit 2, respectively, and the signal light is amplified. The signal light then passes through the wavelength multiplexing/demultiplexing unit 3a, the light signal characteristic compensation unit 6 and the wavelength multiplexing/demultiplexing unit 3b in sequence, and the pumping light is applied to the next stage rare earth doped optical fiber 1b through only the wavelength multiplexing/demultiplexing units 3a and 3b so that the signal light is amplified again. In the present embodiment, the same effect as that of the first embodiment is attained.