Optical: systems and elements – Optical frequency converter – Parametric oscillator
Reexamination Certificate
1998-09-01
2001-07-24
Lee, John D. (Department: 2874)
Optical: systems and elements
Optical frequency converter
Parametric oscillator
C359S326000
Reexamination Certificate
active
06266179
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical transmission apparatus, an optical repetition apparatus and an optical transmission method for use in an optical communications system, and in particular to an optical transmission apparatus, an optical repetition apparatus and an optical transmission method in which a signal light is output for which compensation has been made for wavelength dispersion in the transmission path.
2. Description of the Related Art
It is well known that in ultra high speed optical communication systems waveform distortion resulting from the wavelength dispersion characteristics of an optical fiber produces a deterioration in the quality of the transmission. In order to reduce the effect of this wavelength dispersion, optimization of so-called “chirping” is used, where a fluctuation of suitable magnitude and in a suitable direction is generated in the wavelength of the signal light waveform at the rising and falling of the signal. For example, when transmitting a 1.55 &mgr;m light signal through a 1.3 &mgr;m band zero dispersion fiber, by lengthening the wavelength of the signal light at the rising, and shortening the wavelength at the falling, it is possible to create a signal light waveform which retains good form following transmission through the optical fiber.
The aforementioned chirping is conducted by using a device such as a semiconductor light modulator. It is known that with this type of light modulator the phase modulated component which is generated causes the fluctuation in the wavelength of the light (the chirp). The amount of chirping generated can be expressed in terms of an &agr; parameter. The &agr; parameter is a quantity determined by the operating wavelength and the absorption spectra of the light modulator, and the degree of variation in the dispersion of the refractive index. Thus, chirping can be created in the signal light by considering the &agr; parameter and then setting the operating conditions of the light modulator accordingly.
In current optical transmission and optical repetition apparatus, compensation for wavelength dispersion within an optical fiber has been made by using a light modulator as described above to cause a chirp in the signal light which is of an appropriate magnitude for the wavelength dispersion characteristics of the connected optical fiber.
However, the polarity of the appropriate chirp to be created in the signal light will vary depending on the type of optical fiber through which the signal light is to be transmitted. Chirping generated by a semiconductor light modulator however, is generally of a fixed polarity. Consequently, with current optical transmission and optical repetition apparatus, the altering of the polarity of the chirp in the signal light in accordance with the transmission path being used has been problematic.
Techniques for altering the polarity of a chirp include the use of a nonlinear optical phenomenon known as a four wave mixing effect, and this technique has been reported in Japanese Unexamined Patent Publication No. 63-103215 and Japanese Unexamined Patent Publication No. 7-312574.
The technique outlined in the former of the above two publications involves compression of a light pulse by taking a high output light pulse in which a positive chirp has been generated by transmission through an optical fiber, and following conversion via a four wave mixing medium to a light pulse with a negative chirp, re-transmitting the pulse through the optical fiber. The object of this technique then is to achieve compression of the light pulse by utilizing the wavelength dispersion characteristics of the optical fiber, and it is not possible to generate a signal light in which compensation has been made for the wavelength dispersion characteristics of the optical fiber, as is the object of the present invention.
The technique outlined in the latter of the above two publications involves the connection of two optical fibers with equal amounts of wavelength dispersion to either end of a phase conjugated light generating apparatus which utilizes a four wave mixing effect, so that the wavelength dispersion generated in each of the optical fibers is cancelled out. However with this technique, if it becomes necessary to replace one of the optical fibers connected to the phase conjugated light generating apparatus, then in order to maintain equality in the amount of wavelength dispersion before and after the phase conjugated light generating apparatus it is necessary to replace both of the optical fiber. Consequently, dealing with modifications in an optical communication system using this technique is problematic.
SUMMARY OF THE INVENTION
The present invention takes into consideration the above problems with the object of providing an optical transmission apparatus, an optical repetition apparatus and an optical transmission method in which the chirp polarity applied to an output signal light can be modified easily in accordance with the wavelength dispersion characteristics of the connected transmission path.
In order to achieve the above object, an optical transmission apparatus according to the present invention comprises; a signal light generation device for generating a signal light which has been modulated in accordance with a transmission signal, a probe light generation device for generating a probe light of an optical frequency different from the optical frequency of the signal light generated by the signal light generation device, a four wave mixing generation device in which the signal light output from the signal light generation device and the probe light output from the probe light generation device are injected into a medium having a nonlinear optical effect to generate four wave mixing, and a signal light selection device for selecting one of the signal lights output from the four wave mixing generation device in accordance with the wavelength dispersion characteristics of the transmission path and then outputting the selected signal light to the transmission path.
Furthermore, an optical repetition apparatus according to the present invention comprises; a probe light generation device for generating a probe light of a different optical frequency to that of a signal light received from an input transmission path, a four wave mixing generation device in which the signal light received from the input transmission path and the probe light output from the probe light generation device are injected into a medium having a nonlinear optical effect to generate four wave mixing, and a signal light selection device for selecting one of the signal lights output from the four wave mixing generation device in accordance with the wavelength dispersion characteristics of an output transmission path and then outputting the selected signal light to the output transmission path.
With an optical transmission apparatus and an optical repetition apparatus of such a construction, by injecting a signal light and a probe light of different optical frequencies into a medium having a nonlinear optical effect, light resulting from four wave mixing is generated. The light generated by the four wave mixing generation device incorporates signals such as the signal resulting from reversal of the chirp polarity in the signal light, and a signal light which is suitable for compensating for the wavelength dispersion characteristics of the transmission path connected to the apparatus is selected and then output by the signal light selection device.
Consequently, the chirp applied to the signal light and the polarity thereof can be modified easily in accordance with the wavelength dispersion characteristics of the connected transmission path, and so an optical transmission apparatus and an optical repetition apparatus can be provided in which the transmission characteristics can be readily adapted to deal with any modifications to the optical communication system.
Furthermore, with the optical transmission apparatus, preferably an optical frequency control device
Fujitsu Limited
Lee John D.
Staas & Halsey , LLP
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