Fig. 1: Schematic of the hybrid integrated laser system.
From: Low-noise frequency-agile photonic integrated lasers for coherent ranging
a Principle of laser linewidth narrowing via laser self-injection locking. The laser frequency tuning is realised by applying a sweeping electrical signal on the monolithically integrated AlN actuator. b Optical micrograph showing the DFB laser butt-coupled to the Si3N4 photonic chip. c Photo of the experimental setup with DFB laser (left, mounted on piezoelectric stage), Si3N4 chip (middle), output lensed fibre, probes for piezoactuator (top). d False-coloured scanning electron microscope (SEM) image of the sample cross-section, showing the piezoelectric actuator integrated on the Si3N4 photonic circuit. The piezoelectric actuator is composed of Al (yellow), AlN (green) and Mo (red) layers on top of Si3N4 buried in SiO2 cladding. e Spectrogram showing laser frequency change upon the linear tuning of the diode current, measured for 190.7 GHz FSR microresonator, dashed areas correspond to the range where the laser is self-injection locked (featured with minimal lasing frequency fluctuations). f Schematic of the tuning of the laser frequency. Different voltage levels applied to the piezoactuator correspond to different microresonator resonance frequencies, thus leading to the different frequencies of the laser when the laser current is in the range for self-injection locking. g Spectrogram of laser frequency change upon the linear tuning of the cavity resonance by piezoelectric actuator, measured for 190.7 GHz FSR microresonator, dashed areas correspond to the range where the laser is self-injection locked to the shifting resonance. h Schematic of linear laser frequency tuning with integrated piezoactuator. By applying the triangular voltage ramp to the piezoactuator, we transduce the cavity resonance shift induced by the piezoactuator to the triangular laser frequency change while operating inside the locking range.
