The essential difference between a single mode laser and a multimode laser is that there is one and only one mode in the output beam of a single-mode laser, while there are multiple modes in the output beam mode of a multimode laser. Thus, we can judge whether the laser is single mode or multimode by M2 (beam quality). According to the difference of M2, laser can be divided into 3 types. Laser with M2＜1.3 is pure single mode laser, M2 between 1.3 and 2.0 is quasi single mode laser, while M2＞2.0 is regarded as multimode laser.
The different modes will directly affect the spatial distribution of energy. From the perspective of energy distribution, the single-mode laser presents a typical Gaussian distribution, while the multimode laser presents a typical flat-topped type distribution which the energy is relatively average, and it is composed of multiple modes superimposed, so the beam quality is not as good as that of single mode. The energy of single-mode laser is relatively concentrated. After the same laser power is focused by the same optical system, the energy of single mode laser at the focal point is much higher than that of multimode laser. By comparing their beam quality, focus (or defocus) diameter of spot, divergence, and focus (or defocus) power density, it can be seen that single mode lasers are better than multimode lasers among these specifications. What’s more, due to the existing of Higher order modes within multimode lasers, compared to single mode lasers, the multimode laser has greater dispersion and more loss of energy. So multimode lasers are not good choice for long-distance transmission.
However, single mode laser is not superior to multimode laser in some aspects. The fiber core of traditional single-mode laser is very thin and its diameter is very small (generally less than 10 μm). The damage threshold of optical fiber is very low, so it is difficult to bear large energy light wave. The fiber laser needs the signal laser and pump light to transmit in the core at the same time, so the output power of the fiber laser is very low, and the end area of the fiber is small, so it is difficult to carry out the pump light coupling, which makes the optical to optical conversion efficiency of the fiber laser very low. Because the fiber core of single mode laser is thin, the nonlinear effect of single-mode laser is large. Multimode laser increases the diameter of fiber core on the basis of single mode laser, so that there are multiple modes in the fiber and support multiple modes transmission at the same time. After the coherent superposition of multiple mode fields in multimode laser, the mode field distribution in the whole fiber core is relatively flat, which can reduce the self focusing effect in the process of optical transmission. The transmission distance of high-power signal light in the fiber can be relatively long and with high stability. Multimode laser not only improves the damage threshold, but also reduces the laser power density by increasing the cross-sectional area of the fiber, so as to effectively suppress the nonlinear effect in the fiber.
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