Ultra-low noise, highly stable single-mode operation, high power, Bragg grating based semiconductor laser
Waveguide Bragg gratings, optical reflectors and lasers including optical reflectors are disclosed. The optical reflectors include a waveguide, perturbations proximate to the waveguide to create a Bragg grating in the waveguide, and a DC index control structure positioned to vary the DC index along at least a portion of the Bragg grating. In laser embodiments, the waveguide may be coupled to the second end of a semiconductor gain element to form an external cavity having an optical length and a cavity phase. The gain element and optical reflector may be monolithically integrated on a substrate or separate structures.
1. A laser comprising:
a semiconductor gain element to generate light in response to a bias current and having
a first end with a high reflectivity forming a first end of a laser cavity and
a second end with a lower reflectivity to allow at least a portion of light to pass through the second end; and
a waveguide coupled to the second end of the semiconductor gain element to form an external cavity having an optical length and a cavity phase,
the waveguide including:
a Bragg grating forming a second end of the laser cavity having an optical length, and
a DC index control structure positioned to vary the DC index along at least a portion of the Bragg grating, wherein a distance between the DC index control structure and the waveguide varies over a length of the Bragg grating.
2. The laser of claim 1 , wherein the DC index control structure provides for a constant DC index along the Bragg grating.
3. The laser of claim 1 , wherein:
the Bragg grating has a reflection spectrum with a long wavelength side,
the optical length of the laser cavity and reflection spectrum of the Bragg grating supporting only a single lasing mode, and
wherein at least one of the bias current and the cavity phase is controlled to position the single lasing mode on the long wavelength side of the reflection spectrum.
4. The laser of claim 1 , wherein the Bragg grating is apodized to control reflection sidemodes.
5. The laser of claim 1 , wherein:
the DC index control structure is positioned to reduce reflections from a Fabry-Perot cavity formed between the Bragg grating and the waveguide.
6. The laser of claim 1 , wherein:
the DC index control structure is comprised of silicon nitride.
7. The laser of claim 1 , wherein the Bragg grating is created by perturbations proximate the waveguide.
8. The laser of claim 7 , wherein the Bragg grating is created by a series of posts proximate the waveguide.
9. The laser of claim 8 , wherein the Bragg grating is created by a series of posts proximate the waveguide positioned to provide a gap between the waveguide and posts chosen to provide a Gaussian amplitude apodization of the Bragg grating.
10. The laser of claim 9 , wherein:
the waveguide and the series of posts are comprised of silicon nitride.
11. The laser of claim 1 , wherein:
the semiconductor gain element and waveguide are monolithically integrated on a single semiconductor substrate comprised of one of silicon, indium phosphide, and gallium arsenide.
12. The laser of claim 1 , where the waveguide is a high refractive index waveguide, where the refractive index ‘n’ is greater than 3.
13. The laser of claim 1 , wherein:
the DC index control structure is apodized proximate to at least one end of the Bragg grating.
14. The laser of claim 1 , wherein:
the waveguide is included in a planar waveguide having a substrate, and wherein
the Bragg grating is formed via perturbation in the substrate proximate the waveguide, and
the DC index control structure is positioned in the substrate.
15. The laser of claim 1 , wherein the Bragg grating has a physical length larger than 10 mm and occupies at least 50% of the optical length of the external cavity.
16. The laser of claim 1 , further comprising:
a first photodetector to monitor light output from the gain element to the external cavity, and
a second photodetector to monitor light reflected by the Bragg grating,
wherein a ratio of the reflected light to the output light provides a feedback signal for locking the lasing mode of the gain element to a specific wavelength reflected by the Bragg grating by varying at least one of a cavity phase control section and the gain element bias current.
17. A waveguide Bragg grating comprising:
a planar substrate including a waveguide, the substrate including perturbations creating a Bragg grating in the waveguide, and
a DC index control structure positioned to vary the DC index along at least a portion of the Bragg grating, wherein a distance between the DC index control structure and the waveguide varies over a length of the Bragg grating.
18. The waveguide Bragg grating of claim 17 , wherein the Bragg grating is created by a series of posts proximate the waveguide positioned to provide a gap between the waveguide and posts chosen to provide a Gaussian amplitude apodization of the Bragg grating.
19. The waveguide Bragg grating of claim 18 , wherein:
the waveguide and the series of posts are comprised of silicon nitride.
20. A laser comprising:
a semiconductor gain element to generate light in response to a bias current and having
a first end with a high reflectivity forming a first end of a laser cavity and
a second end with a lower reflectivity to allow at least a portion of light to pass through the second end; and
an optical reflector including a waveguide in a planar substrate, the substrate including perturbations creating a Bragg grating in the waveguide, and
a DC index control structure positioned to vary the DC index along at least a portion of the Bragg grating, wherein a distance between the DC index control structure and the waveguide varies over a length of the Bragg grating, and
the waveguide coupled to the second end of the semiconductor gain element to form an external cavity having an optical length and a cavity phase.