Today, we will introduce a “monochromatic” laser to the extreme – narrow linewidth laser. Its emergence fills the gaps in many application fields of laser, and in recent years has been widely used in gravitational wave detection, liDAR, distributed sensing, high-speed coherent optical communication and other fields, which is a “mission” that can not be completed only by improving laser power.

What is a narrow linewidth laser?
The term “line width” refers to the spectral line width of the laser in the frequency domain, which is usually quantified in terms of the half-peak full width of the spectrum (FWHM). The linewidth is mainly affected by the spontaneous radiation of excited atoms or ions, phase noise, mechanical vibration of the resonator, temperature jitter and other external factors. The smaller the value of the line width, the higher the purity of the spectrum, that is, the better the monochromaticity of the laser. Lasers with such characteristics usually have very little phase or frequency noise and very little relative intensity noise. At the same time, the smaller the linear width value of the laser, the stronger the corresponding coherence, which is manifested as an extremely long coherence length.

Realization and application of narrow linewidth laser
Limited by the inherent gain linewidth of the working substance of the laser, it is almost impossible to directly realize the output of the narrow linewidth laser by relying on the traditional oscillator itself. In order to realize the operation of narrow linewidth laser, it is usually necessary to use filters, grating and other devices to limit or select the longitudinal modulus in the gain spectrum, increase the net gain difference between the longitudinal modes, so that there are a few or even only one longitudinal mode oscillation in the laser resonator. In this process, it is often necessary to control the influence of noise on the laser output, and minimize the broadening of spectral lines caused by the vibration and temperature changes of the external environment; At the same time, it can also be combined with the analysis of phase or frequency noise spectral density to understand the source of noise and optimize the design of the laser, so as to achieve stable output of the narrow linewidth laser.

Let’s take a look at the realization of narrow linewidth operation of several different categories of lasers.
(1) Semiconductor laser
Semiconductor lasers have the advantages of compact size, high efficiency, long life and economic benefits.
The Fabry-Perot (F-P) optical resonator used in traditional semiconductor lasers generally oscillates in multi-longitudinal mode, and the output line width is relatively wide, so it is necessary to increase the optical feedback to obtain the output of narrow line width.
Distributed feedback (DFB) and Distributed Bragg reflection (DBR) are two typical internal optical feedback semiconductor lasers. Due to the small grating pitch and good wavelength selectivity, it is easy to achieve stable single-frequency narrow linewidth output. The main difference between the two structures is the position of the grating: the DFB structure usually distributes the periodic structure of the Bragg grating throughout the resonator, and the resonator of the DBR is usually composed of the reflection grating structure and the gain region integrated into the end surface. In addition, DFB lasers use embedded gratings with low refractive index contrast and low reflectivity. DBR lasers use surface gratings with high refractive index contrast and high reflectivity. Both structures have a large free spectral range and can perform wavelength tuning without mode jump in the range of a few nanometers, where the DBR laser has a wider tuning range than the DFB laser. In addition, the external cavity optical feedback technology, which uses external optical elements to feedback the outgoing light of the semiconductor laser chip and select the frequency, can also realize the narrow linewidth operation of the semiconductor laser.

(2) Fiber lasers
Fiber lasers have high pump conversion efficiency, good beam quality and high coupling efficiency, which are the hot research topics in the laser field. In the context of the information age, fiber lasers have good compatibility with current optical fiber communication systems in the market. The single-frequency fiber laser with the advantages of narrow line width, low noise and good coherence has become one of the important directions of its development.
Single longitudinal mode operation is the core of fiber laser to achieve narrow line-width output, usually according to the structure of the resonator of single frequency fiber laser can be divided into DFB type, DBR type and ring type. Among them, the working principle of DFB and DBR single-frequency fiber lasers is similar to that of DFB and DBR semiconductor lasers.

As shown in Figure 1, DFB fiber laser is to write distributed Bragg grating into the fiber. Because the working wavelength of the oscillator is affected by the fiber period, the longitudinal mode can be selected through the distributed feedback of the grating. The laser resonator of DBR laser is usually formed by a pair of fiber Bragg gratings, and the single longitudinal mode is mainly selected by narrow band and low reflectivity fiber Bragg gratings. However, because of its long resonator, complex structure and lack of effective frequency discrimination mechanism, ring-shaped cavity is prone to mode hopping, and it is difficult to work stably in constant longitudinal mode for a long time.

Figure 1, Two typical linear structures of single frequency fiber lasers