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Abstract

In long distance digital and frequency modulation systems that use optical sources having optical frequency chirp, degradation due to optical dispersion in the transmission fiber is overcome by filtering the laser wavelength representative of one of the binary levels (e.g., binary '0'). An optical source such as a DFB laser is directly frequency modulated by a digital data stream. A first digital level is represented by a first intensity of the modulated carrier corresponding to a carrier wavelength .lambda..sub.1 and a second digital level is represented by a second intensity of the modulated carrier corresponding to a carrier wavelength .lambda..sub.2. A wavelength selective reflector is used to filter the wavelength .lambda..sub.1 from the modulated carrier. The modulated carrier, from which wavelength .lambda..sub.1 has been filtered, is then coupled to an optical communication path for transmission. In one embodiment the wavelength selective reflector is coupled to the laser output using an optical circulator. In another embodiment, a directional coupler is used.

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First Claim