Do You Know The Narrow Line Width Laser Technology?

What do you think of when you think of lasers? A  laser sword wielded by General Grievous in Star Wars? Or one of those deadly laser channels from Resident Evil? Or Scott's laser eyes from X-Men that could burn everything? These "lasers", as they are known, often appear in some science fiction works in the form of high power and high energy, showing great power (see Figure 1).

In real life, lasers have also sneaked into many industries, from medical fields such as laser cosmetology and myopia treatment to industrial manufacturing fields such as laser marking, cutting, and welding, as well as cutting-edge scientific fields such as radar detection, microscopic imaging, and quantum communication that have emerged in recent years. At present, laser technology has made outstanding contributions to promoting the development of national defense security, biomedical, intelligent manufacturing, and information. 

However, do the lasers we need all need to have high power and great "lethality" as shown in the movie?

First of all, we have to look at the basic characteristics of the laser different from the traditional light source, as shown in Figure 2, we mentioned earlier in the film and television works the laser has powerful characteristics, usually proportional to the brightness (power) of the laser, but also reflects the laser has a good directional characteristics.

In addition, monochromaticity and coherence are also the focus of attention. The light emitted by the ordinary light source is usually different in frequency, so it contains a variety of colors, and the various photons emitted by the laser have the same frequency, so it is a very excellent monochromatic light source. Not only that, because the laser's excited radiation photons are consistent in phase, there is a fixed phase relationship between the points on the cross-section of the laser beam under the action of the resonator, so compared to the ordinary light source, the coherence of the laser is also excellent. Combined with the laser's excellent monochromaticity and coherence characteristics, even if there is no high power "halo" of kilowatts and 10,000 watts, lasers can still be widely used in spectral technology, optical measurement, and other fields.

Characteristics of laser different from traditional light sources

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.

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 and 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 the 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. Their structures and output spectra are shown in FIG. 5. Due to the small grating pitch and good wavelength selectivity, it is easy to achieve stable single-frequency narrow line-width 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 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 the 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.

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

3) Solid-state laser

In 1960, the world's first ruby laser was a solid-state laser, characterized by a high output energy and a wider wavelength coverage. The unique spatial structure of solid-state laser makes it more flexible in the design of narrow linewidth output. At present, the main methods implemented include the short cavity method, one-way ring cavity method, intracavity standard method, torsion pendulum mode cavity method, volume Bragg grating method, and seed injection method.

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