Fig. 1. (a) Laser frequency stabilization architecture based on a packaged integrated SiN PIC and a negative feedback configuration. The different laser sources (e.g., CH1 at frequency f1) are fed at the input of the PIC and coupled via MMIs to the frequency locker (FL) ring. Two photodiodes, at the output PD1 and PD2, detect the power variations related to each source. A micro-processor uses these variations to stabilize each laser frequency. The coding of intensity optical modulators (M1 to M16) guarantees the multiple detection and stabilization of the input sources; CH1, channel one; MMI, multi-mode interferometer; μP, micro-processor. (b) Laser frequency drifts (Δf) detection based on the generation of an error signal (ΔP) given by the laser emission frequency with respect to the FL resonance. (c) Different laser sources can be locked along different FL resonances whose spacing is defined by the FL free spectral range (FSR). The current implementation allows the connection of 16 input sources.

Fig. 2. (a) Cross section of the Si3N4 waveguide constituting the PIC layout. The waveguide dimensions are set to obtain zero-dispersion operation along the C-band. (b) Main building blocks of the frequency stabilizer PIC. The fabricated chip is composed of 16 inputs, 16 thermo-optic modulators, and a frequency locker (FL) shared among all the input paths and two outputs. The latter are equipped with on-chip photo-detectors. (c) Static characterization of the FL and first modulator drop port transfer function. The frequency is normalized with respect to 193.38 THz and the optical power to 0 dB. The FL FSR is set to be 50 GHz; FSR, free spectral range.

Fig. 3. (a) Thermal tuning of the M1 modulator. The frequency is normalized with respect to 193.38 THz and the optical power to 0 dB. (b) Experimental set-up for the eye diagram analysis. The device under test (M1 modulator) was modulated with a 1 kHz PRBS7 on–off keying (OOK) modulation. The maximum extinction ratio (ER) of 10 dB was read on the oscilloscope for an applied power of ∼30  mW; PRBS7, seven bits pseudo random binary sequence; TLS, tunable laser source; EDFA, erbium-doped fiber amplifier; PC, polarization controller; PBS, polarization beam splitter; DUT, device under test; OSC, oscilloscope.

Fig. 4. Alignment set-up between an external laser and the SiN PIC. (a) A linearly polarized light is emitted by a tunable laser source (TLS). Its light is coupled through a polarization-maintaining (PM) fiber to one input of the PIC (blue square). The circuit was fully packaged providing a thermo-electric cooler (TEC) at the bottom and on-chip photo-detectors at the output. The latter, along with the modulator heaters, were accessed by external pins. (b) Alignment of 16 fibers was guaranteed by butt-coupling the arrayed fibers with the input waveguides of the PIC in order to excite the TE mode.

Fig. 5. (a) A Keysight 8168F commercial laser was fed to one input of the PIC and its frequency was evaluated using a wavelength meter over a 30 min time window. (b) The trend detected at the μP side as the ratio between the outputs of PD2 and PD1 (normalized to the initial value) perfectly matched (c) the shift recorded at the wavelength meter side. TLS, tunable laser source; PIC, photonic integrated circuit; μP, micro-processor; WM, wavelength meter.

Fig. 6. Evaluation of the thermal frequency drift of a Huawei semiconductor laser when the negative feedback is (a) off or (b) on. The related schematics are shown on the left side. On the right side, the respective histograms show blue bars when the frequencies are retained within an absolute value of 50 MHz, while red bars when this limit is crossed. When the algorithm was off (a), the counts of frequency values were evenly distributed over the histogram, showing a continuous drift of the channel (red bar count is often above 10). The histogram resembled a quasi-Gaussian noise when the connection was established [(b), right side] with a few MHz average value (red bar count is below 10). The insets on the right corner depict the related frequency drift over time.

Fig. 7. Two 50 GHz-spaced lasers (CH1 and CH2) were stabilized in frequency through the shown set-up (blue box). The power of the two lasers was coupled via 3 dB couplers, amplified (EDFA), and detected by a fast photodiode (PD) to retrieve the 50 GHz note, generated by the separation of the lasers frequencies. The high-frequency content was then down-converted using an external frequency (fWG) and the base-band signal was read by an RF spectrum analyzer. fWG, waveform generator frequency; RFSA, RF spectrum analyzer; PD, photodiode; EDFA, erbium-doped fiber amplifier.

Fig. 8. Results of the experiments utilized to evaluate the stability of a 50 GHz tone, generated by the separation between the frequencies of the two lasers connected to the PIC, when the algorithm was off (red line) or on (blue line). The following thermal stimulus was applied to one of the lasers: (a) −125  MHz/hour ramp over a period of 2 hours; (b) co-sinusoidal stimulus with 100 s period and 500 MHz peak-to-peak amplitude; (c) 50 s square-like wave with −500 MHz amplitude.

Table 1. Designed and Measured Parameters of the Generic Modulator M_i and of the FL in the PIC^a