Stabilization of an InP Mode-Locked Laser PIC Through Simultaneous Optical Filtering and Self-Injection Locking Using a Fabry-Perot Etalon

We report experimental studies of a self-injection locking technique with optically filtered feedback and its influence on optical linewidth, RF linewidth, carrier side-band suppression and frequency stability of a monolithically integrated InP passive mode-locked laser. We use a Fabry-Perot etalon as the photonic filter placed in an external fiber feedback loop for filtering the mode-locked laser output and subsequently self-injection locking. Using this technique, we demonstrate optical axial mode linewidth narrowing by a factor of 100x from 600 MHz to 6 MHz, while improving the RF linewidth by a factor of 50x from 109 kHz to 2.2 kHz. The external cavity modes arising from the fiber feedback loop were suppressed by 20 dB, while the phase noise at 200 kHz offset from the carrier is suppressed by 35 dB. Finally, we implement a coupled optoelectronic loop by using the photo-detected output from the self-injection locked laser and provide feedback to drive the integrated electro-absorption modulator in the laser cavity. The coupled optoelectronic loop is referenced to an external RF source to transfer the stability and further improve the noise performance of the self-injection locked laser. The Allan deviation at 10 s averaging time is 5 × 10
⁻¹²
, showing an improvement of 5 orders of magnitude over passive mode-locking operation.