The development trend of narrow linewidth laser

The development trend of narrow linewidth laser
The evolution of laser feedback mode in narrow linewidth laser is the evolution of laser resonant cavity structure. Below, we will introduce various configurations of narrow linewidth laser technologies in the order of evolution of laser resonators.

1. Single main cavity configuration. This type of laser can be divided into linear cavity (classical configuration, simple and efficient structure) and annular cavity (overcoming spatial hole burning and using traveling wave field). Non planar ring resonator (NPRO) is specifically mentioned in the ring resonator, which is a special and highly stable traveling wave field laser. From the perspective of cavity length, it can be divided into short cavities (easy to implement single longitudinal mode SLM, but with wide intrinsic line width and high noise) and long cavities (inherently narrow line width, but implementing SLM operation is a technical difficulty).

2. Single external cavity feedback configuration. This configuration is proposed to solve the problems of short photon interaction time and difficult elimination of spontaneous emission in a single main cavity, by filtering and feeding back photons through an external cavity to compress the linewidth. Early classic structures included Littrow and Littman Metcalf type external cavities utilizing gratings. The technical difficulty of this configuration lies in the phase matching between the main cavity and the outer cavity.
3. Two integrated main cavity configurations based on Bragg gratings:

DFB laser configuration: Combining Bragg structure with active region and introducing phase shift region, it has higher integration, stability and practicality, and improves the wavelength drift of DBR. The technical difficulty lies in grating processing (such as the secondary epitaxial RGF-DFB and surface etching SG-DFB methods of semiconductor DFB).
DBR laser configuration: replaces traditional mirrors with periodic passive Bragg structures, which have filtering characteristics and are easy to implement SLM with short cavities. According to the gain medium, it can be divided into semiconductor DBR (with good process compatibility) and fiber DBR (relying on fiber processing and doping technology).

In order to further compress the linewidth of the short cavity main cavity (such as DFB/DBR), a composite outer cavity structure will be used. The external cavity form has evolved with the development of technology:
Space external cavity: early main forms, including grating (Littrow/Littman) and various optical filters (such as F-P standard).
Fiber optic external cavity: using all fiber optic devices (such as fiber optic circuits, FBGs, fiber optic F-P cavities, etc.), the integration and anti-interference ability are stronger.
External waveguide cavity: Micro nano processing based on semiconductor materials such as Si and Si3N4, making the system more compact and stable.

Finally, this article introduces the configuration of optoelectronic oscillating lasers, which is a special form of feedback, such as PDH frequency stabilization technology. By using electrical negative feedback to lock the laser frequency to a highly stable reference source, extremely high frequency stability can be achieved. However, the system is complex, costly, and wavelength flexibility is limited.