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 wavelength locker comprising:
an athermal asymmetric Mach-Zehnder interferometer (AMZI) comprising an input coupler, two waveguide arms coupled to the input coupler at a first end of the waveguide arms, an output coupler coupled to the two waveguide arms at a second end of the waveguide arms and providing at least two output ports, and an active tuning element disposed in one of the waveguide arms and configured to adjust an optical-path-length difference between the two waveguide arms;
at least two photodetectors each placed at a respective one of the at least two output ports, the at least two photodetectors forming, together with the output coupler, a balanced receiver;
memory storing a target setting of the active tuning element, the target setting corresponding to a balanced photocurrent measured with the balanced receiver being substantially zero at a specified locking frequency; and
circuitry configured to set the active tuning element to the target setting, and to tune a frequency of a light source coupling light via the input coupler into both waveguide arms of the AMZI to the specified locking frequency by bringing the balanced photocurrent measured with the balanced receiver to substantially zero.
2. The wavelength locker of claim 1 , wherein the AMZI and the photodetectors are integrated in a photonic integrated circuit.
3. The wavelength locker of claim 1 , wherein the active tuning element comprises a heater.
4. The wavelength locker of claim 1 , further comprising at least one of a temperature sensor or a strain gauge, the memory storing temperature-dependent or strain-dependent target settings for multiple temperatures of the AMZI or multiple levels of strain in the AMZI, or the wavelength locker further comprising circuitry to adjust the target setting based on a measured temperature or strain.
5. The wavelength locker of claim 1 , wherein the AMZI and the balanced receiver form a first filter, the wavelength locker further comprising a second filter including a second AMZI and a second balanced receiver, wherein a filter period of the first filter is greater than a filter period of the second filter and a frequency error of the first filter is smaller than the filter period of the second filter.
6. The wavelength locker of claim 5 , wherein the filter period of the first filter is at least five times greater than the filter period of the second filter.