1. Field of the Invention
This invention relates to a signal light outputting apparatus suitable for use as a transmission apparatus for transmitting a plurality of signal lights of different wavelengths in an optical transmission system wherein optical communication is performed by wavelength multiplexing transmission, and also to an optical transmission system having a signal light outputting apparatus of the type mentioned.
2. Description of the Related Art
In order to allow communication of signal light between terminal stations spaced from each other, for example, with a sea interposed therebetween, an optical transmission system has been developed wherein an optical fiber cable is laid on the bottom of the sea and signal light is repeated and amplified in a plurality of stages using a plurality of optical amplifiers provided for the optical fiber cable.
An example of such an optical transmission system laid on the bottom of a sea as just described will be described with reference to FIG. 20.
Referring to FIG. 20, the optical transmission system generally denoted at 100 is constructed to effect bidirectional communication between a plurality of terminal stations, and includes, as such plurality of terminal stations, for example, an A station 101, a B station 102, a C station 103 and a D station 104. The A to D stations 101 to 104 are connected to each other by optical transmission lines 107 each formed from an optical fiber via light branching and insertion apparatus 105 and 106.
In particular, as seen from FIG. 20, the A station 101 and the C station 103 are connected to each other with the light branching and insertion apparatus 105 interposed therebetween, and the B station 102 and the D station 104 are connected to each other with the light branching and insertion apparatus 106 interposed therebetween. The A station 101 and the B station 102, the A station 101 and the D station 104, the B station 102 and the C station 103, and the C station 103 and the D station 104, are connected to each other with the light branching and insertion apparatus 105 and 106 interposed therebetween.
Each of the A to D stations 101 to 104 includes a plurality of transmission-reception apparatus each including a transmission apparatus 109 for transmitting signal light of a single wavelength and a reception apparatus 110 for receiving signal light transmitted from the transmission apparatus 109 of a different one of the transmission-reception apparatus.
A plurality of optical amplifiers 108 for amplifying signal light to be transmitted in a plurality of stages are provided for each of the optical transmission lines 107.
In the optical transmission system 100 shown in FIG. 20 and having the construction described above, signal light is communicated among the A to D stations 101 to 104.
On the other hand, in recent years, research and development for an optical transmission line formed from an optical fiber for wavelength multiplexing (WDM) which effects optical wavelength multiplexing transmission and for an optical amplifier for WDM as well as for a light branching and insertion apparatus for WDM have been and are being proceeded.
In an optical transmission system which employs such an optical transmission line or optical amplifier for WDM or light branching and insertion apparatus as just mentioned, a plurality of signal lights of different wavelengths are transmitted by means of a single optical transmission line, and the plurality of signal lights of the different wavelengths transmitted in this manner are amplified by a single optical amplifier and branching (wave separation or demultiplexing) or insertion (wave combination or multiplexing) of the signal light is performed depending upon the wavelength so that the signal lights are transmitted to desired terminal stations.
However, in an optical amplifier for WDM, since the gain of it has a wavelength dependency, when signal lights, for example, of 4 channels are transmitted, even if the ratios of the powers of the signal lights among the channels are equal as seen in FIG. 21(a), the ratios of the powers of the signal light among the channels after the signal lights are amplified in a plurality of stages and transmitted do not exhibit an equal value as seen from FIG. 21(b). It is to be noted that ASE (Amplified Spontaneous Emission) denoted in FIG. 21(b) is noise light produced by the optical amplifiers.
Where the ratios of the powers of the signal light after transmitted among the channels are not equal in this manner, since the S/N ratios of the signal lights are brought out of the mutually equal condition, the transmission characteristic of the signal lights in the optical transmission system is deteriorated.
Therefore, upon transmission of the signal lights, pre-emphasis which varies the ratios of the powers of the signal lights is performed so that the S/N ratios of the signal lights after transmitted may be equal to each other.
For example, if pre-emphasis is performed such that signal light of a wavelength which exhibits a comparatively high gain may have a comparatively low power as seen in FIG. 22(a), then the ratios of the powers of the signal lights after transmitted become equal to each other as seen in FIG. 22(b). Consequently, the S/N ratios of the signal lights can be made equal to each other.
In this instance, if the set condition of the pre-emphasis varies, then the S/N ratios of the signal lights after transmitted vary. Therefore, in order to prevent variation of the set condition of the pre-emphasis, automatic power control (APC) is performed such that back powers of laser diodes (LDs) which output the signal lights are monitored and the output powers of the laser diodes are adjusted in response to the magnitudes of the back powers so that the powers of the signal lights may always have values equal to those set by the pre-emphasis.
However, in a stage following each of such laser diodes, such apparatus as a modulator, a polarization scrambler and so forth are provided, and when signal light from each laser diode is transmitted through those apparatus, the signal light loses its power. The degree of the loss then is different among the different apparatus, and accordingly, the degree of the loss is different among the signal lights from the different laser diodes. Therefore, there is a subject to be solved in that, even if such automatic power control as described above is performed, the powers of the signal lights cannot be controlled so as to be equal to the respective preset values.
An improved apparatus is disclosed in Japanese Patent Laid-Open Application No. H 5-327662 wherein, as forward powers of laser diodes, the powers of signal lights from the laser diodes after the signal lights suffer from loss are monitored and the output powers of the laser diodes are controlled in response to the magnitudes of the powers so that the powers of the signal lights from the laser diodes may be equal to respective preset values.
However, with the automatic power control described above or the apparatus disclosed in Japanese Patent Laid-Open Application No. H 5-327662, control of the powers of signal lights is performed by varying the outputs of laser diodes. Where the outputs of the laser diodes are varied in this manner, since the temperature in the proximity of each laser diode varies, the wavelength of the output signal light from the laser diode varies.
In the ordinary optical transmission system 100 shown in FIG. 20, since the optical transmission lines 107 are provided individually for the signal lights, even if the wavelengths of the output signal lights vary a little, transmission and reception of the signal lights can be performed with certainty. However, in an optical transmission system for WDM wherein optical wavelength multiplexing transmission is performed, if the wavelengths of output signal light vary, then the branching-insertion characteristic of signal light of a light branching and insertion apparatus sometimes varies. Accordingly, the optical transmission system has another subject to be solved in that, in such an instance as just described, transmission and reception of signal light cannot be performed accurately.