Tunable laser with directional coupler
A tunable laser has a first mirror, a second mirror, a gain medium, and a directional coupler. The first mirror and the second mirror form an optical resonator. The gain medium and the directional coupler are, at least partially, in an optical path of the optical resonator. The first mirror and the second mirror comprise binary super gratings. Both the first mirror and the second mirror have high reflectivity. The directional coupler provides an output coupler for the tunable laser.
1. A tunable laser comprising:
a first wavelength selective element characterized by a first reflectance spectrum;
a second wavelength selective element characterized by a second reflectance spectrum, wherein the first wavelength selective element and the second wavelength selective element form an optical resonator;
a gain medium between the first wavelength selective element and the second wavelength selective element; and
a directional coupler between the first wavelength selective element and the second wavelength selective element, wherein the directional coupler provides an output coupler for the laser.
2. The tunable laser of claim 1 , wherein:
the first wavelength selective element is configured to have a first reflectance value for a first reflectance peak;
the second wavelength selective element is configured to have a second reflectance value for a second reflectance peak; and
there is at least some overlap of between frequencies in the first reflectance peak and the second reflectance peak.
3. The tunable laser of claim 2 , wherein the first reflectance value and the second reflectance value are greater than 80%.
4. The tunable laser of claim 2 , wherein:
the first reflectance value and the second reflectance value are greater than 80%; and
there is not more than 10% difference in reflectance between the first reflectance value and the second reflectance value.
5. The tunable laser of claim 2 , wherein:
the first reflectance value and the second reflectance value are greater than 90%; and
there is not more than 5% reflectance difference between the first reflectance value and the second reflectance value.
6. The tunable laser of claim 2 , wherein the first reflectance value and the second reflectance value are greater than 98%.
7. The tunable laser of claim 6 , wherein the first reflectance value and the second reflectance value are greater than 99%.
8. The tunable laser of claim 1 , wherein the first wavelength selective element is a binary super grating.
9. The tunable laser of claim 1 , further comprising a phase adjuster that is configured to modify an optical path length of the optical resonator by heating a portion of a waveguide of the optical resonator.
10. The tunable laser of claim 1 , further comprising an optical sensor, wherein the optical sensor is optically coupled to the directional coupler.
11. The tunable laser of claim 1 , wherein the first wavelength selective element, the second wavelength selective element, the gain medium, and the directional coupler are disposed on a substrate.
12. The tunable laser of claim 11 , wherein:
the first wavelength selective element, the second wavelength selective element, the directional coupler comprise silicon; and
the gain medium comprises a III-V compound.
13. The tunable laser of claim 1 , further comprising a heating element coupled to the first wavelength selective element, wherein the first reflectance spectrum is adjustable as a function of temperature of the heating element.
14. The tunable laser of claim 1 , wherein the directional coupler has a core that has a thickness between 0.5 and 2.5 μm.
15. A method of operating a tunable laser, the method comprising:
tuning a first wavelength selective element;
tuning a second wavelength selective element, wherein the first wavelength selective element and the second wavelength selective element form an optical resonator;
generating optical emission from a gain medium disposed between the first wavelength selective element and the second wavelength selective element;
guiding the optical emission to a directional coupler;
transmitting a first portion of the optical emission out of the optical resonator using the directional coupler; and
transmitting a second portion of the optical emission to the first wavelength selective element using the directional coupler.
16. The method of operating the tunable laser of claim 15 , wherein:
the first wavelength selective element comprises a binary super grating (BSG) having a first number of super periods;
the second wavelength selective element comprises a BSG having a second number of super periods; and
the first number of super periods equals the second number of super periods.
17. The method of operating the tunable laser of claim 15 , wherein the first wavelength selective element is tuned by changing a temperature of a heating element attached to the first wavelength selective element.
18. The method of operating the tunable laser of claim 15 , wherein:
the first wavelength selective element comprises a binary super grating with a reflectance value greater than 98%; and
the second wavelength selective element comprises a binary super grating with a reflectance value greater than 98%.
19. The method of operating the tunable laser of claim 15 , wherein:
the first wavelength selective element, the second wavelength selective element, the directional coupler comprise silicon; and
the gain medium comprises a III-V compound.
20. The method of operating the tunable laser of claim 15 , further comprising transmitting a third portion of the optical emission to an optical sensor.