Tunable laser

1. An apparatus for adjusting a phase noise of a laser, the apparatus comprising:

a laser with a waveguide;

a tap optically connected to the waveguide;

a photodetector optically connected to the tap;

a rejection photodetector optically connected to the waveguide;

circuitry to measure a photocurrent of the rejection photodetector;

an intracavity ring resonator; and

a feedback loop that adjusts a refractive index of the intracavity ring resonator such that the photocurrent is held at a non-zero target value.

2. The apparatus of claim 1 , further comprising a second intracavity ring resonator wherein the second intracavity ring resonator has a second refractive index.

3. The apparatus of claim 2 , wherein:

the feedback loop further adjusts the refractive index by:

normalizing a target value;

determining a difference between an output from the first rejection photodetector and the normalized target value;

multiplying the difference by an alpha factor; and

providing the multiplied difference to a thermal-optic phase shifter (TOPS); and

a second feedback loop adjusts the second refractive index by:

normalizing a second target value.

4. The apparatus of claim 3 , further comprising a second rejection photodetector, and wherein the second feedback loop further adjusts the second refractive index by:

determining a second difference between an output from the second rejection photodetector and the normalized second target value;

multiplying the second difference by a second alpha factor; and

providing the multiplied second difference to the TOPS.

5. The apparatus of claim 2 , further comprising:

a second rejection photodetector;

circuitry to measure a photocurrent of the second rejection photodetector; and

a second feedback loop that adjusts the second refractive index of at least a part of the second intracavity ring resonator such that a photocurrent of the second intracavity ring resonator is held at a non-zero value.

6. The apparatus of claim 2 further comprising:

a second waveguide;

a second rejection photodetector optically connected to the second waveguide;

circuitry to measure a photocurrent of the second rejection photodetector; and

a second feedback loop that adjusts the second refractive index to hold a second photocurrent at a second target value.

7. The apparatus of claim 6 , wherein:

the feedback loop adjusts the refractive index by:

normalizing an output from the rejection photodetector;

determining a difference between the normalized output and a target value;

multiplying the difference by an alpha factor; and

providing the multiplied difference to a thermal-optic phase shifter (TOPS); and

the second feedback loop adjusts the second refractive index by:

normalizing the second target value;

determining a second difference between an output from the second rejection photodetector and the normalized second target value;

multiplying the second difference by a second alpha factor; and

providing the multiplied second difference to the TOPS.

8. The apparatus of claim 7 , further comprising:

a third intracavity ring resonator with a third waveguide, wherein:

the third intracavity ring resonator has a third rejection photodetector optically connected to the third waveguide; wherein the third intracavity ring resonator has a third refractive index;

circuitry to measure a photocurrent of the third rejection photodetector; and

a third feedback loop that adjusts the third refractive index to hold a third photocurrent at a third target value.

9. The apparatus of claim 6 , wherein:

the feedback loop adjusts the refractive index by:

normalizing an output from the rejection photodetector;

determining a difference between the normalized output and a target value;

multiplying the difference by an alpha factor; and

providing the multiplied difference to a thermal-optic phase shifter (TOPS); and

the second feedback loop adjusts the second refractive index by:

normalizing an output from the second rejection photodetector;

determining a second difference between the normalized output from the second rejection photodetector and the second target value;

multiplying the second difference by a second alpha factor; and

providing the multiplied second difference to a second thermal-optic phase shifter (TOPS).

10. The apparatus of claim 6 , wherein:

the feedback loop adjusts the refractive index by:

normalizing a target value;

determining a difference between an output from the rejection photodetector and the normalized target value;

multiplying the difference by an alpha factor; and

providing the multiplied difference to a thermal-optic phase shifter (TOPS); and

the second feedback loop adjusts the second refractive index by:

normalizing the second target value;

determining a second difference between an output from the second rejection photodetector and the normalized second target value;

multiplying the second difference by a second alpha factor; and

providing the multiplied second difference to a second thermal-optic phase shifter (TOPS).

11. The apparatus of claim 6 , wherein:

the feedback loop adjusts the refractive index by:

normalizing an output from the rejection photodetector;

determining a difference between the normalized output and a target value;

multiplying the difference by an alpha factor; and

providing the multiplied difference to a thermal-optic phase shifter (TOPS); and

the second feedback loop adjusts the second refractive index by:

normalizing the second target value;

determining a second difference between an output from the second rejection photodetector and the normalized second target value;

multiplying the second difference by a second alpha factor; and

providing the multiplied second difference to a second thermal-optic phase shifter (TOPS).