Integrated wavelength locker
Described are various configurations of integrated wavelength lockers including asymmetric Mach-Zehnder interferometers (AMZIs) and associated detectors. Various embodiments provide improved wavelength-locking accuracy by using an active tuning element in the AMZI to achieve an operational position with high locking sensitivity, a coherent receiver to reduce the frequency-dependence of the locking sensitivity, and/or a temperature sensor and/or strain gauge to computationally correct for the effect of temperature or strain changes.
1. A method for locking a frequency of a light source of a photonic integrated circuit using an integrated wavelength locker comprising an asymmetric Mach-Zehnder interferometer (AMZI) with an active tuning element in one interferometer arm, the method comprising:
coupling light emitted by the light source into the AMZI at an input of the AMZI;
adjusting a setting of the active tuning element to match a target setting stored in memory, the target setting being associated with a specified locking frequency;
measuring a balanced photocurrent at an output of the AMZI; and
tuning a frequency of the light source until the measured balanced photocurrent is substantially zero,
wherein the target setting corresponds to a setting of the active tuning element that, when the frequency of the light source matches the specified locking frequency, results in a substantially zero balanced photocurrent.
2. The method of claim 1 , wherein the active tuning element comprises a heater and the setting being adjusted comprises a heater power.
3. The method of claim 1 , further comprising measuring at least one of a temperature of the AMZI or a strain in the AMZI, and adjusting the setting of the active tuning element, based on the measured temperature or strain, prior to tuning the frequency of the light source to bring the balanced photocurrent to substantially zero.
4. The method of claim 3 , wherein the setting of the active tuning element is adjusted based on the measured temperature or strain by selecting the target setting based on the measured temperature strain from among a plurality of target settings stored for multiple temperatures or levels of strain.
5. The method of claim 3 , wherein the setting of the active tuning element is computationally adjusted based on the measured temperature or strain.
6. The method of claim 1 , further comprising calibrating the integrated wavelength locker prior to locking the frequency of the light source by:
tuning the frequency of the light source, based on an external reference signal having the specified locking frequency, until the frequency of the light source matches the specified locking frequency; and
while the frequency of the light source matches the specified locking frequency, tuning the setting of the active tuning element until a balanced photocurrent measured at the output of the AMZI is substantially zero, and then storing that setting as the target setting in memory.
7. The method of claim 1 , wherein the AMZI forms part of a first filter, the integrated wavelength locker comprising a second filter with a second AMZI, a filter period of the second AMZI being smaller than a filter period of the first AMZI, the frequency of the light source being tuned in the first filter up to a frequency error no greater than the filter period of the second AMZI, the method further comprising, following coarse-tuning the frequency of the light source with the first filter, fine-tuning the frequency of the light source with the second filter by:
coupling light emitted by the light source into the second AMZI at an input of the second AMZI; and
while the setting of the active tuning element match the target setting stored in memory, measuring a second balanced photocurrent at an output of the second AMZI and tuning the frequency of the light source until the measured second balanced photocurrent is substantially zero.
8. The method of claim 1 , further comprising applying a frequency dither to the light source.