In the development of narrow linewidth lasers to the present day, the evolution of laser feedback mechanisms has been synonymous with the evolution of laser resonator structures. Below, various configurations of narrow linewidth laser technologies are introduced in the order of the evolution of laser resonators.
Single Main-Cavity Configuration
Single main-cavity lasers can be structurally divided into linear cavities and ring cavities, and by cavity length, into short-cavity and long-cavity structures. Short-cavity lasers feature a large longitudinal mode spacing, which is more advantageous for achieving single longitudinal mode (SLM) operation, but suffer from a broad intrinsic cavity linewidth and difficulty in suppressing noise. Long-cavity structures inherently exhibit narrow linewidth characteristics and allow the integration of diverse optical devices with flexible configurations; however, their technical challenge lies in achieving SLM operation due to the excessively small longitudinal mode spacing.
As a classic configuration of laser main cavities, the linear cavity boasts advantages such as a simple structure, high efficiency, and easy manipulation. Historically, the first true laser beam was generated using an F-P linear cavity structure. With subsequent advancements in science and technology, the F-P structure has been widely adopted in semiconductor lasers, fiber lasers, and solid-state lasers.
The ring cavity is a modification of the classic linear cavity, overcoming the spatial hole-burning drawback of linear cavities by replacing standing-wave fields with traveling waves to achieve cyclic amplification of optical signals. Driven by the development of fiber-optic devices, fiber lasers with flexible all-fiber structures have garnered extensive attention and have become the fastest-growing category of lasers over the past two decades.
Non-planar ring oscillator (NPRO) lasers represent a special traveling-wave laser configuration. Typically, the main cavity of such lasers consists of a monolithic crystal, which regulates the laser polarization state via crystal end-face reflection and an external magnetic field to realize unidirectional laser operation. This design greatly reduces the thermal load of the laser resonator, delivers exceptional stability in wavelength and power, and features narrow linewidth characteristics.
Single External-Cavity Feedback Configuration
Constrained by factors such as excessively short cavity length and high intrinsic loss, F-P linear cavity single-cavity laser configurations based on intra-cavity feedback suffer from limited photon interaction time and difficulty in eliminating spontaneous emission from the gain medium. To address this issue, researchers proposed the single external-cavity feedback configuration. The external cavity functions to prolong photon interaction time and feed filtered photons back into the main cavity, thereby optimizing laser performance and compressing the linewidth. Early simple external-cavity structures based on spatial optics, such as the Littrow and Littman configurations, utilize the spectral dispersion capability of gratings to reinject purified laser signals into the laser main cavity, exerting frequency pulling on the main cavity to achieve linewidth compression. This single external-cavity structure was later extended to fiber lasers and semiconductor lasers.
The technical challenge of single external-cavity feedback laser configurations lies in phase matching between the external cavity and the main cavity. Studies have shown that the spatial phase of the external-cavity feedback signal is critical for determining the laser threshold, frequency, and relative output power, and laser longitudinal modes are highly sensitive to the intensity and phase of the feedback signal.
DBR Laser Configuration
To enhance the stability of laser systems and integrate wavelength-selective devices into the main cavity structure, the DBR configuration was developed. Designed based on the F-P resonator, the DBR resonator replaces the mirrors of the F-P structure with periodic passive Bragg structures to provide optical feedback. Owing to the periodic comb filtering effect of the Bragg structure on laser interference modes, the DBR main cavity inherently possesses filtering characteristics. Combined with the large longitudinal mode spacing afforded by the short-cavity structure, SLM operation is readily achieved. Although the periodic Bragg structure was originally designed solely for wavelength selection, from a cavity-structure perspective, it also represents an evolution of the single-cavity structure with an increased number of feedback surfaces.
Classified by gain medium, DBR lasers include semiconductor lasers and fiber lasers. Semiconductor lasers have a natural advantage in fabrication compatibility with semiconductor materials and micro-nano processing technologies. Many semiconductor manufacturing processes, such as secondary epitaxy, chemical vapor deposition, step photolithography, nanoimprinting, electron beam etching, and ion etching, can be directly applied to the research and fabrication of semiconductor lasers.
DBR fiber lasers emerged later than DBR semiconductor lasers, mainly limited by the development of fiber waveguide processing and high-concentration multi-doping technologies. Currently, common fiber waveguide fabrication techniques include oxygen-defect phase masking and femtosecond laser processing, while high-concentration fiber doping technologies encompass modified chemical vapor deposition (MCVD) and surface plasma chemical vapor deposition (SCVD).
DFB Laser Configuration
Another resonator structure based on Bragg gratings is the DFB configuration. The DFB laser main cavity integrates the Bragg structure with the active region and introduces a phase-shift region at the center of the structure for wavelength selection. As shown in Fig. 3(b), this configuration features a higher degree of integration and structural unity, and mitigates issues such as severe wavelength drift and mode hopping in DBR structures, making it the most stable and practical laser configuration at the present stage.
The technical challenge of DFB lasers lies in the fabrication of grating structures. There are two primary methods for grating fabrication in DBR semiconductor lasers: secondary epitaxy and surface etching. Regrown grating feedback (RGF)-DFB semiconductor lasers employ secondary epitaxy and photolithography to grow a set of low-refractive-index gratings in the active region. This method preserves the active layer structure with low loss, facilitating the fabrication of high-Q resonators. Surface grating (SG)-DFB semiconductor lasers involve directly etching a grating layer on the surface of the active region. This approach is more complex, requiring precise adjustment according to the active region material and doping ions, and exhibits higher loss, yet offers stronger optical confinement and higher mode suppression capability.
Similar to DBR fiber lasers, DFB fiber lasers rely on advances in fiber waveguide processing and high-concentration doped fiber technologies. Compared with DBR fiber lasers, DFB fiber lasers pose greater challenges in grating fabrication due to the wavelength absorption characteristics of rare-earth ions.
Composite Feedback External Cavity
Short-cavity main-cavity lasers such as DFB and DBR have limited intra-cavity photon interaction time, making deep linewidth compression difficult. To further compress the linewidth and suppress noise, such short-cavity main-cavity configurations are often combined with external-cavity structures for performance optimization. Common external-cavity structures include spatial external cavities, fiber external cavities, and waveguide external cavities. Prior to the development of fiber-optic devices and waveguide structures, external cavities were predominantly composed of spatial optics combined with discrete optical components. Among these, grating-based spatial external-cavity feedback structures mainly adopt the Littrow and Littman designs, typically consisting of a laser gain cavity, coupling lenses, and a diffraction grating. The grating, as the feedback element, enables wavelength tuning, mode selection, and linewidth compression.
In addition, spatial external-cavity feedback structures can incorporate a range of optical filtering devices, such as F-P etalons, acousto-optic/electro-optic tunable filters, and interferometers. These filtering devices inherently possess mode-selection capabilities and can replace gratings; certain high-Q F-P etalons even outperform reflective gratings in spectral narrowing and linewidth compression.
With the advancement of fiber-optic device technology, replacing spatial optical structures with highly integrated, robust fiber waveguides or fiber devices represents an effective strategy for improving laser system stability. Fiber external cavities are usually constructed by splicing fiber devices to form an all-fiber structure, offering high integration, ease of maintenance, and strong immunity to interference. Fiber external-cavity feedback structures can be simple fiber loop feedback, or all-fiber resonators, FBGs, fiber F-P cavities, and WGM resonators.
Narrow linewidth lasers with integrated waveguide external-cavity feedback structures have attracted widespread attention due to their smaller package size and more stable performance. Essentially, waveguide external-cavity feedback follows the same technical principles as fiber external-cavity feedback, but the diversity of semiconductor materials and micro-nano processing technologies enable more compact and stable laser systems, enhancing the practicality of waveguide external-cavity feedback narrow linewidth lasers. Commonly used semiconductor laser materials include Si, Si₃N₄, and III-V compounds.
Optoelectronic Oscillation Laser Configuration
The optoelectronic oscillation laser configuration is a special feedback laser architecture, where the feedback signal is typically an electrical signal or simultaneous optoelectronic feedback. The earliest optoelectronic feedback technology applied to lasers was the PDH frequency stabilization technique, which uses electrical negative feedback to adjust the cavity length and lock the laser frequency to reference spectra, such as high-Q resonator modes and cold-atom absorption lines. Through negative feedback tuning, the laser resonator can match the laser operating state in real time, reducing frequency instability to the order of 10⁻¹⁷. However, electrical feedback suffers from significant limitations, including slow response speed and overly complex servo systems involving extensive circuitry. These factors result in high technical difficulty, stringent control precision, and high costs for laser systems. Furthermore, the system’s strong dependence on reference sources strictly confines the laser wavelength to specific frequency points, further restricting its practical applicability.