Optical communications – Transmitter
Reexamination Certificate
2000-08-14
2004-06-29
Negash, Kinfe-Michael (Department: 2633)
Optical communications
Transmitter
C398S198000, C398S196000, C372S032000, C372S034000
Reexamination Certificate
active
06757499
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical transmitter and more particularly to an optical transmitter a wavelength multiplexing optical communication system and an optical network using a plurality of pieces of signal light.
2. Related Art
Wavelength multiplexing optical communication, which transmits an optical signal with a plurality of wavelengths along the same optical transmission line, is an important technology for achieving higher performance and lower cost of communication technology and optical information processing technology. A first major object from this point of view is to control the wavelength of a light source for a plurality of channels, and a second major object is to provide an optical modulation means for achieving rapid optical modulation.
Controlling the wavelength of a light source for a plurality of channels in a wavelength multiplexing optical communication system is one of the most important techniques. The wavelength c)r frequency of each channel used is now precisely determined by an international standardization (ITU) at an interval of 50 GHz to 100 GHz (about 0.4 nm to 0.8 nm)
Light sources considered for wavelength multiplexing optical communication with wavelengths controlled in the above manner include a wavelength variable type light source wherein a plurality of channels are covered by using a semiconductor laser device with a wavelength-variable function, and a wavelength selection type light source wherein a plurality of semiconductor laser devices with different wavelengths are used. Realization of these techniques requires realization of a wavelength variable type semiconductor laser that makes it possible to sweep wavelengths efficiently, readily, and continuously, as well as a wavelength control technique for controlling a wavelength on the ITU grid with high reproducibility, or a wavelength stabilization technique for fixing a wavelength on the ITU grid over a long period of time.
On the other hand, the typical transmission speed in a long-distance main line system is currently 2.5 Gb/s to 10 Gb/s. Because of this high-speed transmission, the optical modulation system needs to be an external modulation system, in which optical modulation is performed outside of the light-emitting device, rather than a system in which modulation is performed directly by the light-emitting device. Thus the optical modulation means for achieving rapid optical modulation requires the form of a modulator-integrated light source, in which an optical modulator is monolithically integrated with a laser device. A wavelength selection type light source which, from this point of view, monolithically integrates a plurality of semiconductor laser devices having different wavelengths with an optical multiplexer, an electric field absorption type optical modulator, and the like has been reported. In this example, however, there is a problem in that the characteristics of the optical modulator are changed when the wavelength of the laser light led to the optical modulator is varied.
Incidentally, this type of wavelength multiplexing communication light source is reported in the Institute of Electronics, Information and Communication Engineers general convention SC-3-5, 1999.
As for the above-mentioned first object of controlling the wavelength of a light source for a plurality of channels, one of the major problems is how to set the wavelength of a semiconductor laser device to be used as the light source at a standard value. Currently the standard value is practically provided by the ITU grid. In order to solve the problem, the following two measures are provided for a current system. One is to monitor the wavelength of each channel and provide a wavelength stabilizing means for the light source, which feeds back the correction based on the result of monitoring to the drive of the light source. The other is to provide a spare light source device for each channel in case an individual channel device of such a complex light source should fail.
With the background of these techniques, each of a plurality of DFB (Distributed Feedback) laser devices that corresponds to each channel in the light source used for a wavelength multiplexing optical communication system needs to be built so as to fit into a specified narrow wavelength range. Therefore, there is a major problem in the yields in terms of device manufacturing.
This problem of wavelength control is a major obstacle to the achievement of smaller size and lower cost of an optical transmitter, and will also be a major object to further reduction in channel intervals and the increase in the number of channels in the future.
Next the above-mentioned second object of providing a means for rapid modulation will be dealt with. The characteristics of a current optical modulation means change mainly for the following reason. Optical modulators formed by semiconductor materials include electric field absorption type modulators, optical interference type modulators, and phase modulators. The optical modulation characteristics of these optical modulators formed by semiconductor materials, however, depend heavily on the difference between the band gap energy of the semiconductor material and the energy of the signal light (amount of detuning). Therefore, as the wavelength of the signal light is changed, the amount of detuning of the optical modulator is greatly changed.
In addition, chirping in particular becomes a problem in the case of long-distance transmission. It is desirable from this point of view that the chirping parameter of an optical modulator can be varied according to the transmission specifications. In the case of an electric field absorption type optical modulator, the chirping parameter can be changed by offset voltage. In this case, however, the laser light output is reduced especially when the offset voltage is increased. This is caused by optical absorption in the optical wave guide of the optical modulator.
A first object of the present invention is to provide an optical transmitter that will solve problems such as the wavelength control of a light source, the provision of an optical modulation means for achieving rapid optical modulation, and the control of the chirping characteristics of an optical modulator.
A second object of the present invention is to provide an optical transmitter suitable for realizing a light source for wavelength multiplexing communication with high reproducibility and provide a wavelength multiplexing transmission apparatus using the optical transmitter. The second object of the present invention will be achieved by using a simple method. The method will also enable minute tuning in optical communication, and enable especially the tuning of a wavelength standard channel that conforms to the ITU grid.
A third object of the present invention is to provide an optical transmitter suitable for long-distance transmission and a simple means for realizing the transmitter. More specifically, it controls the chirping parameter of a light source including an optical modulator.
The present invention will be useful especially for a complex light source using a semiconductor laser device with an external modulator monolithically integrated.
SUMMARY OF THE INVENTION
First the basic technical concept of the present invention will be described. The basic configuration of the present invention is a combination of a wavelength variable type light source or an integrated type multiple wavelength light source and an optical modulator. The optical combination form of the light source and the optical modulator may be either a form in which they are separated from each other hybrid integration) or a form that monolithically integrates each other (monolithic integration). Incidentally, the configuration itself in which a light source that emits light with a plurality of wavelengths and an optical modulator are integrated on a single substrate is already known. Specifically, in such an apparatus, part of the output light from a wavelength variable
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