1. Field of the Invention
This invention relates to a wavelength dispersion compensation apparatus for compensating for wavelength dispersion occurring in light signals during transmission through optical fibers, and particularly relates to a wavelength dispersion compensation apparatus for a wavelength multiplex transmission system.
2. Background Art
An optical fiber for transmitting light signals possesses the property that the transmission times of light signals differ depending upon the wavelength of the light signal. Thus, in the case when a light signal is transmitted through optical fibers, the signal waveform is likely to spread depending upon the transmission distance. This phenomenon is called the xe2x80x9cwavelength dispersionxe2x80x9d.
Optical fibers possess a characteristic wavelength called zero-dispersion wavelength. If the wavelength of a light signal conforms with the zero dispersion wavelength, wavelength dispersion will not occur irrespective of the transmission length. Therefore, when a light signal with a single wavelength is transmitted, it is possible to prevent degradation of the SIN ratio by conforming the wavelength of the light signal with the zero-dispersion wavelength of the optical fiber.
However, if the wavelength of the light signal does not conform with the zero dispersion wavelength of the optical fiber used for transmission of the light signal, measures must be taken to compensate the wavelength dispersion which occurs during the transmission of the light signal. In order to compensate for wavelength dispersion, a measure is generally taken, in which, a particular dispersion compensating fiber is provided which generates a reverse wavelength dispersion of the light signal, and the light signal is first passed through the particular dispersion compensating fiber before the light signal is sent to the optical fiber forming the transmission line.
For wavelength multiplex transmission systems to transmit light signals through an optical fiber line by multiplex light signals having different wavelengths, the problem of wavelength dispersion becomes more complicated. That is, the amount of dispersion of the wavelength dispersion increases with increasing differences between the wavelengths of light signals and the zero dispersion wavelength, and wavelength multiplex signals passing through the optical fiber undergo different wavelength dispersions corresponding to the wavelengths of the light signals.
Conventionally, the wavelength dispersion compensation apparatus shown in FIG. 2 has been used. The wavelength dispersion compensation apparatus shown in FIG. 2 is used in a wavelength multiplex transmission system for transmitting a wavelength multiplex signal which composes of light signals with wavelengths from xcex1 to xcexn. To this wavelength dispersion compensation apparatus, individual light signals before multiplexing with the wavelengths from xcex1 to xcexn are admitted.
In this example, the wavelength xcex1 is closest to the zero dispersion wavelength of the optical fiber used for the wavelength multiplex transmission, and the difference in wavelength from the zero dispersion wavelength increases in the order of xcex2, xcex3, . . . , xcexn. Thus, if a wavelength multiplex signal is transmitted without compensation through the transmitting optical fiber, the amount of wavelength dispersion will be smallest for the light signal with the wavelength of xcex1, and the amount of the wavelength dispersion generated in each light signal increases in the order of xcex2, xcex3, . . . ,xcexn.
In the wavelength dispersion compensation apparatus, the light signal with a wavelength of xcex1 is made to pass a single dispersion compensation fiber DCF1, and the light signal with the wavelength of xcex2 is made to pass two dispersion compensation fibers DCF1, . . . , and the light signal with the wavelength of xcexn is made to pass n pieces of dispersion compensation fibers. By making each light signal having the wavelength from xcex1xcx9cxcexn pass through appropriate number of dispersion compensation fibers, each light signal s subjected to wavelength dispersion which is reverse in the direction to that of the transmission, and the absolute value of which is the same as that undergone during transmission.
These light signals with wavelengths from xcex1 to xcexn, after passing through the dispersion compensation fibers, are amplified by respective light amplifiers AMP1 to AMPn, multiplexed by the arrayed waveguide grating AWG to produce a wavelength multiplex signal, and the multiplex signal is transmitted by the transmission optical fiber.
In the above transmission system, during transmission, the light signals with wavelengths from xcex1 to xcexn will experience light dispersions corresponding to the wavelength differences between the wavelength of the light signals and the zero dispersion wavelength. However, as described above, the wavelength dispersions are compensated for in the course of passing through respective dispersion compensation fibers. Therefore, as a result, it becomes possible to transmit a wavelength multiplex signal without experiencing wavelength dispersion.
Since the hereinabove described device for compensating the wavelength dispersion compensates for wavelength dispersion by the use of a number of dispersion compensating fibers for each light signal before the wavelength multiplexing, a large number and a large quantity of dispersion compensating fibers are required, so that the problem arises that a large space is required for accommodating the dispersion compensation fibers. Furthermore, another problem arises that, since the dispersion compensation fibers are expensive, the wavelength dispersion compensation apparatus becomes also expensive.
It is therefore an object of the present invention to provide a wavelength dispersion compensation apparatus for a multiplex wavelength transmission system, which does not need a large number of dispersion compensation fibers and which can be constructed at low cost.
The wavelength dispersion compensation apparatus of the present invention compensates for wavelength dispersion of a wavelength multiplex signals, when a wavelength multiplex signal is transmitted through an optical fiber of the wavelength multiplex transmission system, wherein said wavelength dispersion compensation apparatus comprises:
a plurality of dispersion compensation fibers for transmitting said wavelength multiple signal or individual light signals which compose said wavelength multiplex signal in sequence and for producing the wavelength dispersion for said individual light signals in order to compensate for the wavelength dispersion which will occur during transmission through the transmission optical fiber;
a plurality of light branching means, disposed at the output ends of each of said plurality of dispersion compensation fibers, for separating and extracting a light signal with a specific wavelength from the light signals forming the wavelength multiplex signal output after passing through said dispersion compensation fibers; and
a wavelength multiplexing means for amplifying light signals separated and extracted by said light branching means to a specific level, for generating a wavelength multiplex signal by multiplexing the light signals after amplification, and the wavelength multiplex signal is supplied to the transmission optical fibers.