# Research on Narrow Linewidth External Cavity Semiconductor Lasers
## 1 Introduction  
Semiconductor lasers have been applied in many fields, such as high-resolution spectroscopy and broadband communication network systems. Semiconductor lasers need to have the characteristics of a narrow linewidth, high-frequency modulation and wide tunable range at the same time. In other fields, it is required that lasers have the characteristics of narrower output linewidth, larger coherence length, and narrower spatial coherence [1]. By using the external cavity technology, semiconductor lasers can produce stabilized output with a single longitudinal mode and narrow linewidth, and they can also be tuned in the range of tens of nanometers to hundreds of nanometers [2]. At the same time, some other properties of semiconductor lasers are also improved, including a lower threshold, higher output power and larger side mode suppression ratio (SMSR) [3, 4]. These properties meet the requirements of coherent optical communication, coherent detection and other applications of NLECSLs. In 1964, [PERSON] et al. [5] first proposed the external cavity theory of the semiconductor laser. In 1975, [PERSON] et al. [6] reported the compact and relatively inexpensive external cavity structure of the laser with the III-V compound semiconductor.  
An NLECSL includes a semiconductor laser active section and an external cavity. The active section, which typically contains a III-V semiconductor quantum wells structure, is used to provide the optical gain for the whole cavity, and thereby determines the lasingwavelength range. The external cavity is used to select the lasing wavelength, while reducing the linewidth. The natural cleaving surface at both ends of the active section chip is the resonant cavity, which is called the internal cavity or the intrinsic cavity [7]. The cavity composed of the external feedback element and the chip cleaving surface is called the external cavity. Through the external cavity, part of the output light is fed back to the active region for multiple gain, thereby narrowing the linewidth and reducing the phase noise and intensity noise of the lasers [8]. There are many kinds of external feedback components, such as free space bulk diffraction grating, fiber Bragg grating (FBG), waveguide and Fabry-Perot (F-P) cavity and the combination of these components. NLECSLs have many advantages, such as good monochromaticity, high stability, long coherence length, and so on. Therefore, NLECSLs are widely used in the fields of photoelectric detection, coherent communication, precision measurement, optical frequency standards, absorption spectrum measurement and the study of the interaction between lights and matters [9]. In this paper, free space bulk diffraction grating (FSBDG) external cavity structure, waveguide external cavity structure and confocal F-P cavity structure, the three main device structures for achieving NLECELs, are expanded upon. Among them, the confocal F-P cavity can further narrow the linewidth. [PERSON] et al. [10] proposed the mode of optical self-locking with the external single-chip confocal F-P cavity; when the output power exceeds 50 mW, the corresponding Lorentz linewidth is only 15.7 Hz, which is the highest level in the world at present.