Fiber-coupled lasers: single-mode and multi-mode

Fiber laser is a laser with rare-earth-doped glass fiber as a gain medium. Fiber lasers can be developed based on fiber amplifiers. Under the action of pumping light, the high power density is easily formed in the fiber, resulting in the "particle number inversion" of the laser energy level of the laser working substance, and the laser oscillation output can be formed by adding a positive feedback loop (forming a resonant cavity) appropriately.

Fiber laser is composed of pump source, multi-mode coupler (beam combination), fiber grating, active fiber, beam calibration output module, and passive fiber (energy output fiber). Its application scope is very broad, including fiber optic communications, laser space distance communication, industry, shipbuilding, automobile manufacturing, laser engraving, laser marking laser cutting, printing, metal, nonmetal drilling in rollers/cutting/welding, brazing welding, water quenching, cladding, and depth), military defense security, medical instrument equipment, large-scale infrastructure construction, As the pump source of other lasers.  

Fiber coupled lasers are distinguished by single-mode and multi-mode. When there is only one pump module inside the laser, it is called a single-mode laser, and multiple pump modules are combined to let multiple pump beams into the active fiber through the beam splitter, so obtain a higher power beam. This kind of multi-module combination laser is a multi-mode laser. Usually,  in fiber laser products, single-mode lasers are mostly medium and small power, while high-power products are mostly multi-mode lasers.

Differences between single-mode and multi-mode:

(1) Beam quality: the single-mode fiber core is relatively thin, sending out a typical Gaussian beam, energy is very concentrated, similar to a steep mountain, beam quality is also better than multi-mode; Multi-mode is equivalent to the combination of multiple Gaussian beams, so the energy distribution is similar to an inverted cup, relatively average, of course, the beam quality is worse than that of single-mode.

(2) Application performance: according to different characteristics of performance, single-mode，and multi-mode in the specific application direction are also different. For example, in the 1mm and below stainless steel/carbon steel sheet cutting, single-mode processing efficiency is significantly better than multi-mode (single mode 15%-20% faster), and the cutting quality is similar; In thick plate cutting of 2mm and above, high power multimode lasers perform better both in quality and efficiency.

(3) laser welding field: heat conduction welding, the single-mode laser can get more uniform and smooth weld, so some thin material welding using single-mode laser, such as soft package power battery group when the pole ear overlapping welding; In deep fusion welding, the multimode laser can obtain a weld with a better aspect ratio, such as the welding of square power battery bus-Bar.

It is worth noting that single-mode and multi-mode lasers are an important basis for selecting fiber lasers. Fiber laser, with its high electro-optical conversion efficiency, high stability, high beam quality， and low-cost advantages, in the field of laser processing is more and more popular with the market, and as a kind of extremely high-quality light source, costs are declining in recent years, so the traditional solid and gas laser market has been replaced by fiber laser.