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 of manufacturing an integrated wavelength locker module, the method comprising:
on a semiconductor substrate, creating a PIC comprising a tunable light source and a wavelength locker, the wavelength locker comprising an AMZI with two waveguide arms and at least two photodetectors for measuring at least two respective optical interference signals at an output of the AMZI; and
depositing metals near the AMZI to form a strain gauge and a temperature sensor.
2. The method of claim 1 , wherein, for the strain gauge, two metals having different respective temperature coefficients of resistance are deposited.
3. The method of claim 2 , wherein the two metals for the strain gauge are platinum and nickel-chromium.
4. The method of claim 1 , wherein, for the temperature sensor, a metal having a temperature coefficient of resistance above 1000 ppm/C is deposited.
5. The method of claim 1 , further comprising encapsulating the strain gauge and temperature sensor in a dielectric.
6. The method of claim 1 , further comprising:
creating one or more electronic control chips including memory for storing one or more parameters associated with a locking condition and processing circuitry configured to adjust the one or more parameters based on temperature and strain measurements with the temperature sensor and strain gauge, respectively, and to tune a frequency of light coupled into the AMZI, based on a feedback parameter derived from the measured optical interference signals, to satisfy the locking condition with the adjusted one or more parameters; and
bonding the PIC and the one or more electronic control chips to a common substrate to form the integrated wavelength locker module.
7. The method of claim 6 , wherein the memory stores target parameter values or correction coefficients for a plurality of temperatures and strains, and wherein the processing circuitry is configured to select one of the stored target parameter values or correction coefficients based on the temperature and strain measurements.
8. The method of claim 6 , wherein the one or more parameters comprise a target parameter, and wherein the processing circuitry is configured to computationally adjust a stored value of the target parameter associated with a nominal temperature and a nominal strain based on the temperature and strain measurements using a stored functional dependence of the target parameter on temperature and strain.
9. The method of claim 6 , wherein the feedback parameter is a filter phase of the AMZI and the one or more parameters comprise a target filter phase associated with a specified locking frequency, the processing circuitry being configured to adjust the target filter phase based on the temperature and strain measurements, wherein the locking condition is satisfied if the filter phase matches the target filter phase.
10. The method of claim 6 , wherein the AMZI includes in one of the waveguide arms an active tuning element configured to adjust an optical-path-length difference between the two waveguide arms, wherein the one or more parameters comprise a target setting of the active tuning element that is associated with a specified locking frequency, wherein the processing circuitry is configured to adjust the target setting based on the temperature and strain measurements, wherein the feedback parameter is a balanced photocurrent measured with the at least two photodetectors, and wherein the locking condition is satisfied if the balanced photocurrent is substantially zero when a setting of the active tuning element matches the target setting.
11. The method of claim 6 , further comprising, following the bonding, calibrating the integrated wavelength locker module.
12. The method of claim 11 , wherein calibrating the integrated wavelength locker module comprises:
fitting the common substrate into a mating socket of a fixed-temperature test bed;
coupling light at a wavelength of interest into the wavelength locker; and
determining the one or more parameters associated with the locking condition and storing the determined one or more parameters in the memory.
13. The method of claim 12 , wherein determining the one or more parameters comprises, while light at the wavelength of interest is coupled into the wavelength locker, measuring photocurrents at the two or more photodetectors.
14. The method of claim 13 , wherein the one or more parameters stored in the memory comprise at least one of the measured photocurrents or one or more filter parameters computed from the measured photocurrents.
15. The method of claim 13 , wherein determining the one or more parameters further comprises tuning an active tuning element in one of the waveguide arms until the measured photocurrents have reached desired values, and wherein the one or more parameters stored in the memory comprise a tuning-element setting corresponding to the photocurrents having reached the desired values.
16. The method of claim 11 , further comprising, following the calibrating, assembling the integrated wavelength locker module onto a printed circuit board or high-density interconnection substrate to form an optical assembly, and re-calibrating the integrated wavelength locker module to compensate for any change in strain resulting from the assembling.
17. The method of claim 1 , further comprising:
creating, on the semiconductor substrate, memory storing one or more parameters associated with a locking condition and processing circuitry configured to adjust the one or more parameters based on temperature and strain measurements with the temperature sensor and strain gauge, respectively, and to tune a frequency of light coupled into the AMZI, based on a feedback parameter derived from measured optical interference signals, to satisfy the locking condition with the adjusted one or more parameters.
18. The method of claim 1 , wherein creating the tunable light source and the wavelength locker comprises a series of lithographic patterning, etching, and deposition steps.
19. The method of claim 1 , wherein the AMZI further comprises an output coupler providing at least two output ports at which the at least two optical interference signals exit the AMZI, and wherein the output coupler of the AMZI and the at least two photodetectors are configured as a coherent receiver in which relative phase shifts imparted between two signals being interfered to form the at least two optical interference signals differ between at least two of the output ports by a value that is not a multiple of 180°.
20. The method of claim 19 , wherein the AMZI comprises and output coupler providing four output ports and the wavelength locker comprises four respective photodetectors, and wherein the output coupler and the four photodetectors are configured as a 90-degree hybrid optical receiver measuring balanced in-phase and quadrature signals.