External resonator-type semiconductor laser element and optical element
A ring resonator is connected to an optical amplifier. The ring resonator and optical amplifier are contained within the optical path of an optical resonator formed by a first and second reflector. The optical coupler branches part of the light conducting from the optical amplifier to the ring resonator within the optical resonator off to an output optical waveguide.
1. An external resonator-type semiconductor laser element, comprising:
an optical amplifier;
a ring resonator optically connected to the optical amplifier;
a first and second reflector constituting an optical resonator that contains the ring resonator and optical amplifier within its optical path;
an output optical waveguide; and
an optical coupler that branches part of a light conducting from the optical amplifier to the ring resonator within the optical resonator off to the output optical waveguide.
2. An external resonator-type semiconductor laser element as described in claim 1 , wherein the second reflector is placed at a position where the reflection of the light output from the drop port of the ring resonator can occur and comprises a distributed Bragg reflector.
3. An external resonator-type semiconductor laser element as described in claim 2 , wherein:
the optical coupler is a directional coupler whose coupling coefficient has a wavelength dependence;
the optical amplifier has a maximum gain at a longer wavelength than the resonant wavelength determined by the ring resonator and second reflector; and
the optical coupler has a characteristic such that its coupling coefficient increases as the wavelength lengthens past the resonant wavelength.
4. An external resonator-type semiconductor laser element as described in claim 2 , wherein:
the optical coupler is a directional coupler whose coupling coefficient has a wavelength dependence;
the optical amplifier has a maximum gain at a shorter wavelength than the resonant wavelength determined by the ring resonator and second reflector; and
the optical coupler has a characteristic such that its coupling coefficient increases as the wavelength shortens past the resonant wavelength.
5. An external resonator-type semiconductor laser element as described in claim 1 , wherein the ring resonator, optical coupler, second reflector and output optical waveguide are disposed on the same substrate.
6. An external resonator-type semiconductor laser element described in claim 5 , further comprising a ring modulator disposed on the substrate and connected to the output optical waveguide, wherein the operating wavelength of the ring modulator is synchronized with the resonant wavelength of the ring resonator.