Wavelength locker
Conventionally, wavelength locking and monitoring has been achieved used various components, including calibrated etalon filters, gratings, and arrays of color filters, which offer fairly bulky solutions that require complicated controls. An improved on-chip wavelength monitor comprises: a combination comb filter comprising a plurality of comb filters, each for receiving a test beams, and each comb filter including a substantially different FSR, e.g. 10× to 20× the next closest FSR. A controller dithers a phase tuning section of each comb filter to generate a maximum or minimum output in a corresponding photodetector indicative of the wavelength of the test signal.
1. A wavelength monitor comprising:
a laser source for generating a laser signal;
a tap for tapping a portion of the laser signal forming a test signal;
a splitter for splitting the test signal into a plurality of test beams;
a combination comb filter comprising a plurality of comb filters, each for receiving a respective one of the test beams, each comb filter including a different FSR, the plurality of comb filters comprising a first filter comprising a first FSR and a first largest detection wavelength range, a second filter comprising a second FSR and a second intermediate detection wavelength range, and a third filter comprising a third FSR and a third smallest detection wavelength range;
at least one photodetector for measuring output from each comb filter;
a phase tuning section for each comb filter for tuning each of the plurality of comb filters; and
a controller for controlling the phase tuning section of each comb filter to generate a maximum or minimum output in a corresponding one of the least one photodetector indicative of a wavelength of the test signal;
wherein the controller is configured to tune the first filter to determine that the wavelength of the test signal is within the first largest detection wavelength range; tune the second filter to determine that the wavelength of the test signal is within the second intermediate detection wavelength range within the first largest detection wavelength range; and tune the third filter to determine that the wavelength of the test signal is within the third smallest detection wavelength range within the second intermediate detection wavelength range; and
wherein the controller is configured to determine the wavelength of the test signal using values of pre-calibrated electrical signals used by each phase tuning section to tune each of the plurality of comb filters; and
wherein the controller is configured to tune the laser source based on the wavelength of the test signal.
2. The wavelength monitor according to claim 1 , wherein each comb filter comprises a Mach-Zehnder interferometer.
3. The wavelength monitor according to claim 2 , wherein each Mach-Zehnder interferometer comprises first and second arms comprised of different materials, thereby substantially balancing a first change in refractive index with temperature x a length of the first arm with a change in refractive index with temperature x a length of the second arm to mitigate a thermal second imbalance between the first and second arms.
4. The wavelength monitor according to claim 3 , wherein the first arm comprises Silicon and the second arm comprises Silicon Nitride.
5. The wavelength monitor according to claim 2 , wherein each Mach-Zehnder interferometer comprises first and second arms; and wherein the first arm includes a different shape than the second arm, thereby substantially balancing a first change in refractive index with temperature×a length of the first arm with a second change in refractive index with temperature×a length of the second arm to mitigate a thermal imbalance between the first and second arms.
6. The wavelength monitor according to claim 5 , wherein the first arm includes a strip waveguide, and the second arm comprises a rib waveguide.
7. The wavelength monitor according to claim 5 , wherein the first arm includes a width that is wider than a width of the second arm.
8. The wavelength monitor according to claim 2 , wherein each Mach-Zehnder interferometer comprises first and second arms; and wherein the first arm includes a first polarization rotator proximate a beginning thereof for rotating a polarization of light in the first arm to different than light in the second arm, and a second polarization rotator proximate an end thereof for rotating the polarization of the light in the first arm to the same as the light in the second arm, thereby substantially balancing a first change in refractive index with temperature×a length of the first arm with a second change in refractive index with temperature×a length of the second arm to mitigate a thermal imbalance between the first and second arms.
9. The wavelength monitor according to claim 1 , wherein each comb filter comprise a ring resonator.
10. The wavelength monitor according to claim 9 , wherein each ring resonator comprises a drop port and a through port; and wherein one of the at least one photodetector is coupled to each drop port to determine when a wavelength of the test beam corresponds to a resonant wavelength of the ring resonator and thereby substantially passes through to the photodetector coupled to the drop port.
11. The wavelength monitor according to claim 10 , wherein another one of the at least one photodetector is coupled to each through port to determine when a wavelength of the test beam corresponds to the resonant wavelength of the ring resonator and thereby does not substantially pass through to the photodetector coupled to the through port.
12. The wavelength monitor according to claim 9 , wherein each ring resonator comprises waveguides with positive and negative thermal coefficients on each side thereof to mitigate a thermal imbalance between each side.
13. The wavelength monitor according to claim 1 ,
wherein the second FSR of the second filter is at least 10 times larger than the first FSR of the first filter; and
wherein the third FSR of the third filter is at least 10 times larger than the second FSR of the second filter.
14. The wavelength monitor according to claim 13 , wherein the first FSR of the first filter is between 10 GHz to 40 GHz;
wherein the second FSR of the second filter is between 100 GHz to 800 GHz; and
wherein the third FSR of the third filter is between 1000 GHz to 16000 GHz.
15. The wavelength monitor according to claim 13 , wherein the phase tuning section of the first filter provides wavelength accuracy of at least 10 to 30 times finer than the first FSR of the first filter.
16. The wavelength monitor according to claim 1 , further comprising a temperature sensor for compensating for thermal effects on the plurality of comb filters.
17. The wavelength monitor according to claim 16 , wherein the temperature sensor comprises at least three temperature sensors.
18. The wavelength monitor according to claim 17 , wherein each temperature sensor comprises two diodes with different lengths in order to make differential detection and achieve higher reading accuracy.