Tunable laser

1. An external laser cavity comprising:

a first ring;

a second ring, wherein the first ring is a waveguide optically coupled to the second ring; and

a Michelson Interferometer filter (MI filter) comprising different path lengths,

wherein the second ring is optically coupled to the Michelson Interferometer filter,

wherein the MI filter is a coarse tuning filter with a free spectrum range (FSR) greater than 6 THz,

wherein the MI filter is used to enhance a side mode suppression of the first ring and the second ring, and

wherein the FSR of the MI filter is greater than twice a compound FSR of the first ring and the second ring.

2. The external laser cavity of claim 1 , wherein a first thermo-optic phase shifter (TOPS) is attached to a first arm of the MI filter.

3. The external laser cavity of claim 2 , wherein the first TOPS is driven together with a second TOPS to change the cavity phase, and in opposition to change the MI wavelength, and

wherein the second TOPS is attached to a second arm of the MI filter.

4. The external laser cavity of claim 1 , wherein the free spectrum range of a reflectivity of the MI filter is between 0.2 and 2.0 times a 3-dB bandwidth of an optical gain in the laser.

5. The external laser cavity of claim 1 , wherein the MI filter has a coupler, a first MI filter arm, a second MI filter arm, and a rejection photodetector,

wherein the first MI filter arm has a first thermo-optic phase shifter (TOPS) heater, and

wherein the second MI filter arm has a second TOPS heater.

6. The external laser cavity of claim 5 , further comprising control logic comprising logic to minimize a photocurrent from the rejection photodetector.

7. The external laser cavity of claim 6 , further comprising a photodetector monitor,

wherein the control logic is configured to minimize the photocurrent from the rejection photodetector by determining a ratio between the photocurrent from the rejection photodetector and a photocurrent from the photodetector monitor.

8. The external laser cavity of claim 7 , wherein the control logic further comprises logic to compare the ratio to a target ratio.

9. The external laser cavity of claim 8 , wherein the control logic further comprises logic to adjust a power level applied to the first TOPS heater of the first MI filter arm and a power level applied to the second TOPS heater of the second MI filter arm.

10. The external laser cavity of claim 1 , wherein the first ring and the second ring are ring resonators.

11. The external laser cavity of claim 1 , wherein the MI filter is further configured to enable a cavity phase tuning.

12. The external laser cavity of claim 1 , wherein the free spectrum range of the MI filter is greater than a free spectrum range of each of the first ring and of the second ring.

13. The external cavity laser of claim 1 , wherein the first ring and the second ring are configured to enable a high-finesse laser filtering control.

14. A method for tuning an external laser cavity, wherein the laser has a Michelson Interferometer filter (MI filter), a first ring, and a second ring, wherein the MI filter has a first thermo-optic phase shifter (TOPS) on a first arm of the MI filter and a rejection photodetector, wherein the MI filter has a free spectral spectrum range (FSR) greater than 6 THz, wherein the FSR of the MI filter is greater than twice a compound FSR of the first ring and the second ring, the method comprising:

minimizing a photocurrent current from the rejection photodetector, wherein the MI filter is coarse tuning filter; and

enhancing a side mode suppression of the first ring and the second ring using the MI filter.

15. The method of claim 14 , wherein minimizing the photocurrent current from the rejection photodetector comprises determining a ratio between the photocurrent current from the rejection photodetector and a photocurrent from a photodetector monitor.

16. The method of claim 15 wherein the method further includes comparing the ratio to a target ratio.

17. The method of claim 14 , the method further comprising:

adjusting a power level of the first TOPS heater of the first MI filter arm and a power level of a second TOPS heater of a second MI filter arm.

18. The method of claim 17 wherein adjusting the power level comprises changing the power level of the first TOPS heater on the first MI filter arm by adding a delta power factor, and changing the power level of the second TOPS heater on the second MI filter arm by subtracting the delta power factor.

19. The method of claim 14 wherein the first ring and the second ring are high-finesse tuning filters.