Recent years, dissipative Kerr soliton has become a novel chip-scale coherent light source, which has attracted great attention for the enormous value in soliton physics research and practical applications. Dissipative Kerr soliton has high repetition rate. So it is a challenge to measure the relative parameters, especially the repetition rate fluctuation.
In our work, we figure out a way to get it. Meanwhile, a robust package is necessary for practical applications. Our protocol provides an effective method for on-chip micro-ring resonator packaging, soliton generation, and measurement of repetition rate fluctuation.
To begin, fix the micro-ring resonator with a chip fixture. On a six axis coupling stage, which includes three linear stages with a resolution of 50 nanometers and three angle stages with a resolution of 0.003 degrees, place an eight channel fiber array. Use a 1, 550 nanometer laser as an optical source for real-time monitoring of the coupling efficiency.
Carefully adjust the position of the fiber array. Measure the input power and output power by an optical power meter. Keep the inset loss at the minimum value, typically less than six decibels, corresponding to a coupling loss of less than three decibels per facet.
Use an ultraviolet curved adhesive to glue the micro-ring resonator and fiber array. Place the adhesive on the side edge of the contacting surface to ensure that there is no glue on the optical path. Expose the UV curved adhesive to a UV lamp for 150 seconds and bake in a chamber at 120 degrees Celsius for more than one hour.
Co agglutinate a 10.2 millimeter by 6.05 millimeter thermal electric cooler chip with a maximum power of 3.9 Watts to the base plate of a standard 14 pin butterfly package using silver glue. Sauter the two electrodes of the thermoelectric cooler chip to two pins of the butterfly package. Paste a tungsten plate to the surface of the thermal electric cooler chip using silver glue.
Use the tungsten plate as a heat sink to fill the gap between the thermal electric cooler and the micro-ring resonator. Use an erbium doped fiber amplifier to boost the pump for micro comb generation. Control the polarization state of the pump using a fiber polarization controller.
Connect all the devices using single mode fibers. Fix the wavelength of the pump laser. At 1556.3 nanometers, manually tune the operation temperature through an external commercial thermoelectric cooler controller to above 66 degrees Celsius, which is high enough to move a resonance of the micro-ring resonator to the top of the tungsten plate using silver glue, and fix the pigtail of the fiber array to the output board of the butterfly package to the red side of the pump.
Monitor the output optical spectrum with an optical spectrum analyzer. Detect the output power trace with a three gigahertz photo detector and record with an oscilloscope. Set the output of the erbium doped fiber amplifier to 34 decibel milliwatts, or responding to an on-chip power of 30.5 decibel milliwatts, which ensures that there is enough power coupled into the micro-ring resonator for micro comb generation.
Set the thermistor to two kilohms corresponding to an operating temperature of 66 degrees Celsius, then slowly decrease the operating temperature by increasing the set value of the thermistor. Tune the polarization of the pump by the fiber polarization controller, until a soliton crystal's step is observed at the falling edge of the triangular transmission power trace. When a palm like optical spectrum is observed on the optical spectrum analyzer, stop decreasing the operation temperature.
The value of the thermistor was around at 5.6 kilohms in these experiments. Connect the generated soliton crystals to a tuneable band pass filter to extract an individual comb line. Set the pass band of the tuneable band pass filter to 0.1 nanometers.
Tune its central wavelength over the full C and L band, and set the filter slope to 400 decibels per nanometer. Couple of the selected comb line to an asymmetric mock Zehnder interferometer. Use an acousto-optic modulator to shift the optical frequency in one arm of the asymmetric mock sender interferometer by 200 megahertz.
The optical field in the other arm is delayed by a segment of optical fibers of two kilometers and 25 kilometers. Attach a photodiode to detect the output optical signal and use an electrical spectrum analyzer to analyze the power spectral density spectrum. Tune the central wavelength of the tuneable band pass filter.
Measure the power spectral densities of every comb line. Using same method, measure the power spectral density curves of soliton crystals with a vacancy. Record the three decibel bandwidth of the power spectral density curve, and that'll enter your fit to it piecewise through a Python program.
This figure shows the transmission powertrains while a resonance thermal was tuned across the pump. There was an obvious power step that indicated the generation of soliton crystals. A perfect soliton crystal with 27 solitons is shown here, as well as a soliton crystal with a single vacancy.
The perfect soliton crystals based on a two kilometer and a 25 kilometer delay fiber were observed with power spectral density curves having flat tops, which were caused by the frequency fluctuation within the delay time. The typical optical spectrum of soliton crystals based on a two kilometer and a 25 kilometer delay fiber were fitted piecewise with linear lines plotted in blue. In summary, an on-chip micro-ring resonator is packed in a butterfly cell and a thermal tuning method is proposed generate soliton crystal.
Finally, we use delayed self-heterodyne method to achieve the measurement of the repetition rate fluctuation.
