1. Field of the Invention
The present invention relates to an optical communication system, and more particularly to a semiconductor laser for use in an optical communication system.
2. Description of the Related Art
Optical communication systems employ wavelength division multiplexing (WDM) mode, which simultaneously transmits to a single optical fiber by dividing a low-loss wavelength band of an optical fiber into a plurality of channels and multiplexing the channels. In high-capacity wavelength division multiplexing (WDM) optical network, the capability to remove and replace selective WDM wavelengths at multiple add/drop nodes across the network is essential for a variety of value-added communication services.
An example of a semiconductor laser with a distributed Bragg reflector (DBR) is disclosed in U.S. Pat. No. 5,748,660 to Franck Delorme et al., entitled xe2x80x9cSAMPLE GRATING DISTRIBUTED BRAGG REFLECTOR LASER, VERY WIDELY MATCHABLE BY PHASE VARIATION AND PROCESS FOR USING THIS LASER,xe2x80x9d which discloses a semiconductor laser comprising an active section and two sample rating sections each positioned on a side of the active section.
An example of a semiconductor laser which is provided with a DBR and is capable of tuning output wavelength in accordance with input current is disclosed in U.S. Pat. No. 5,325,392 to Tohmori Yuichi et al., entitled xe2x80x9cDISTRIBUTED REFLECTOR AND WAVELENGTH-TUNABLE SEMICONDUCTOR LASER,xe2x80x9d which discloses a wavelength-tunable laser provided with a distributed reflector which may be of a distributed reflector type or distributed feedback type and has distributed reflector regions and a phase adjustment region.
The wavelength-tunable lasers are constructed in a mode that uses an active distributed Bragg reflector structure. The Bragg reflector structure is used to change the oscillating wavelength by changing the refractive index of the active distributed reflector as electric current is applied thereto. The wavelength-tunable lasers of this mode are separately provided with a semiconductor optical amplifier (SOA) to perform power boosting. Power boosting is needed because the oscillating wavelength is changed to a shorter wavelength as electric current is applied and the optical output rapidly decreases as the change of wavelength is increased.
Power boosting is possible if a semiconductor optical amplifier is connected to a side of a wavelength-tunable laser. However, a problem may arise, due to the saturation output power level being low and noise being increased due to amplified spontaneous emission caused by internal reflection. There is also a problem in that the length of the device should be increased in order to obtain sufficient gain.
Accordingly, the present invention overcomes or reduces the above-mentioned problems occurring in the prior art glass optical fibers, and one object of the present invention is to provide a wavelength-tunable laser apparatus which is capable of obtaining a high output while suppressing the generation of spontaneous emission, as well as having a broad wavelength-tunable range.
In accordance with principals of the present invention a wavelength-tunable laser apparatus is provided comprising, a distributed reflector section having a first active layer for oscillating light, a first grating for filtering the light generated in the first active layer, and upper and lower clad layers between which the first active layer and the first grating are interposed, a gain section which is formed adjoining to a side of the distributed reflector section and comprises a second active layer disposed between the upper and lower clad layers for adjusting the gain of the light, and a gain-clamped semiconductor optical amplifier section which is formed adjoining to a side of the gain section and comprises a third active layer disposed between the upper and lower clad layers for amplifying a portion of light having a clamped gain, and a second grating for filtering the light passing through the third active layer.