Stable and Reduced-Linewidth Laser Through Active Cancellation of Reflections Without a Magneto-Optic Isolator

Abstract

Integrating photonics with CMOS electronics in silicon is essential to enable chip-scale, electronic-photonic systems that will revolutionize classical and quantum communication and computing systems. However, the lack of an on-silicon isolator, capable of blocking unwanted back reflections and ensuring the stable operation of the laser, precluded many previous demonstrations from providing single-chip solutions. For most optical systems employing a laser, magneto-optic isolators have been indispensable, but such isolators are incompatible with silicon. To stabilize on-chip lasers, reflections-cancellation circuits were proposed as a way to reduce the reflections going back to the laser. Yet, a stable laser against time-varying back reflections was never demonstrated. Here we demonstrate a stable quantum well-distributed feedback (QWDFB) laser against slowly timevarying reflections using a reflections-cancellation circuit (RCC) on a foundry-produced, silicon-photonic (SiP) chip. The optical spectrum and the relative intensity noise (RIN) of the laser when the RCC was running is comparable to when an isolator was used. By accurately locking the laser in a stable optical feedback regime, the RCC further enhances the QWDFB laser performance by reducing its linewidth by a factor of 100, down to 3 kHz. Both results are enabled using novel techniques in the design, calibration, tuning, and control of the proposed SiP RCC. The optical insertion loss of the RCC is less than 1.5 dB for reflections smaller than -20 dB and can yield isolation ranges of up to 64 dB. Our device paves the way towards the mass production of fully integrated, low-cost electronic-photonic silicon chips without attaching magneto-optic isolators between the laser and the SiP chip.

Roadmapping the next generation of silicon photonics

“…On-chip reflection-control approaches (Fig. 2) that can eliminate the need for bulky isolators include carefully designing the photonic components to reduce reflections below the tolerance threshold of the laser, reducing the reflection sensitivity of the laser by using quantum dot gain regions with low linewidth enhancement factor 94 , monolithic integration of Perspective https://doi.org/10.1038/s41467-024-44750-0 magneto-optical materials (e.g., Ce:YIG) 95 , spatiotemporal modulators 96 , or active reflection cancellation circuits 97 . A generalized, low-cost, scalable, on-chip, low-loss, low-power, and compact solution robust to near-(coherent) and far-end (incoherent) modulated multiwavelength reflections remains a research problem.…”

“…For higher power handling, SiN can be used on the PIC. There are means to improve the linewidth of the laser from the typical linewidth of a DFB and also cancel the reflections to improve isolation 97 . 3D techniques with high-Q Si or SiN external cavity support linewidth reduction down to 1 Hz and lower 98,103,104 , more than sufficient for applications such as coherent communication 101 and automotive LIDAR.…”