Tunable narrow-linewidth lasers, as one of the core components in modern high?precision optoelectronic systems, have revolutionized many fields including coherent optical communication, LiDAR, microwave photonics and precision metrology. With the trends of high precision and high capacity in modern massive optoelectronic technologies, integrated light sources are required to simultaneously exhibit ultra?narrow linewidth, wide-range tunability, and high-speed operation, which puts forward higher and stricter technical requirements. Traditional chip-scale semiconductor laser technologies have become relatively mature over decades. However, their applicability in high-precision coherent systems is fundamentally constrained by limited photon lifetime and significant carrier-induced nonlinear effects.

With the boom in silicon photonics, silicon-based integrated external?cavity lasers (Si-IECLs) have emerged as promising chip-scale light sources. By combining mature Ⅲ?Ⅴ gain materials with ultra?low?loss external cavities on various integrated photonic platforms, this family of lasers has the potential to surpass traditional semiconductor lasers and even bench-top lasers in terms of coherence, tunability, and agility. Si-IECLs have attracted considerable attention over the past decade, and various architectures and platforms have been introduced and demonstrated in this field to provide remarkable progress in terms of laser performance. We highlight the integration of semiconductor lasers with emerging platforms such as silicon, silicon nitride (Si3N4), and lithium niobate on insulator (LNOI), and the functionality extension in frequency-modulated continuous-wave (FMCW) generation, frequency stabilization, and ultrafast switching capabilities. Therefore, it is important to summarize the existing research to guide the future development of this field.

Progress

Si-IECLs are classified into three structural categories: Vernier ECLs, self-injection locked (SIL) lasers, and extended distributed Bragg reflector (E-DBR) lasers. The development of these lasers has advanced along two main trajectories: the introduction of novel external cavity platform systems for performance enhancement and the adoption of advanced integration processes to improve integration density and large-scale manufacturing capabilities. Vernier ECLs adopt high-Q cascaded microring resonators (MRRs) with the Vernier effect, which are coupled to a Ⅲ?Ⅴ gain chip, providing both wavelength tunability and linewidth reduction. Vernier ECLs stand out because of their ultra-wide tunability, which has achieved a record tuning range of 172 nm. In terms of linewidth performance, Vernier ECLs reach intrinsic linewidths at the sub-10 Hz level. Moreover, benefiting from the fast tuning mechanisms of external cavity, they support rapid wavelength switching and FMCW generation. A SIL laser employs a high-Q MRR to provide feedback for a single-frequency semiconductor laser. Thus, the laser is adaptively locked to the resonance peak of the MRR, achieving a considerable linewidth reduction. So far, studies on SIL lasers have reported a record intrinsic linewidth of 40 mHz that reaches the fiber-laser level. Leveraging the tuning mechanisms and nonlinearity in the external cavity, SIL lasers can generate FMCW light and soliton microcomb, respectively. An E-DBR utilizes a distributed Bragg grating coupled to a Ⅲ?Ⅴ gain chip, featuring structural and operational simplicity. It holds a unique advantage in FMCW generation, with reported tuning speeds up to 1 GHz and chirp rates reaching 20 EHz/s, which are the highest results among FMCW lasers to date to the best of our knowledge . Advances in heterogeneous integration technology have also led to the development of various fully integrated lasers, driving large-scale manufacturing of these devices.

Benefiting from the high coherence, wide tunability, and diverse functionality, Si-IECLs enable numerous applications that require high?accuracy, highly stable optical signals. For coherent optical communications, they support a high modulation rate in numerous wavelength-division multiplexing channels. In addition, these lasers leverage the ultrafast wavelength switching capability for wavelength routing in low-latency optical circuit switching. In LiDAR systems, Si-IECLs are ideally suitable for FMCW LiDAR architectures. Their narrow linewidth and large tuning range significantly enhance the sensitivity and precision of coherent detection, while the synergy between wide tunability and optical phased arrays enables two?dimensional beam scanning. In microwave photonics, the narrow linewidth of Si-IECLs facilitates low-phase?noise microwave generation, and their wide tunability supports broadband signal generation and frequency measurement.

Conclusions and Prospects

Si-IECLs have achieved remarkable breakthroughs, demonstrating comprehensive advantages over traditional semiconductor lasers in both performance metrics and functionality extension. So far, Si-IECLs have demonstrated a record tuning range up to 172 nm, ultra-low linewidth down to fiber-laser level, and ultra-high operation speed up to 1 GHz. Such advances in integrated light sources have dramatically promoted the development of on-chip photonics systems for diverse application scenarios, such as coherent optical communications, LiDAR, and microwave photonics. Looking forward, Si-IECLs still face numerous challenges and development opportunities. Technically, further optimization of heterogeneous integration processes is needed to improve device consistency and yield rates while reducing manufacturing costs, thereby promoting the transition from laboratory demonstrations to industrial production. In terms of functional expansion, lasers will undergo cross-platform large-scale integration with active/passive devices, including modulators and detectors, to form multifunctional photonic chips. Furthermore, next-generation Si-IECLs are expected to play key roles not only in enhancing system performance in high-speed communications, precision measurement, and sensing fields, but also in emerging areas such as quantum information processing, biomedical sensing, and artificial intelligence computing.