Laser cutting machine light from energy optical fibers is a point light source with divergence angle (theta). If we place this point light source in the focus of the optical fiber collimator, we know that the light emitted from the focus of a focusing mirror (which actually uses the focusing mirror in reverse) becomes parallel light after passing through the focusing mirror. The calculation formula is 2F*tag(1/2*theta). If the divergence angle is 10 degrees and F=150 mm, the beam diameter from the collimator is 2*150*tag(5 degrees)=26.2466 mm. After passing through the optical collimator, the laser enters the focusing mirror of the optical fiber cutting machine. Generally speaking, the focal length of the collimator is equal to the energy density ratio after focusing. For example, the focal length of the collimator is 75 mm, the focal length of the focusing mirror is 150 mm, and the focal length of the focusing mirror is 75 150=1/2. This means that the area of the focusing spot after passing through the focusing mirror is compared with the point just coming out of the energy fiber. The area of the light source is doubled, and the energy density is one-second of the original one.

　　一、大家会疑问：能量密度集中不是更好吗？为什么要降低光能量密度呢？主要有几个原因：

　　First, you will ask: is it better to concentrate energy density? Why reduce the energy density of light? There are several main reasons:

　　1、如果聚焦镜的焦距越短，聚焦镜的焦深就越浅，焦深浅就容易导致切不深；

　　1. If the focal length of the focusing lens is shorter, the focal depth of the focusing lens will be shallower, and the shallow focal depth will easily lead to the lack of depth of cut.

　　2、焦距越短，聚焦点越小，切缝就越小，缝小不利于切割出来熔渣的掉落，导致切不透。所以我们一般尽量使用120-150mm之间的焦距去做光纤切割机的聚焦镜。

　　2. The shorter the focal length is, the smaller the focus is, the smaller the slit is. The slit is not conducive to the falling of molten slag, which leads to the impermeability of cutting. So we usually try to use the focal length between 120 and 150 mm to make the focus mirror of the optical fiber cutting machine.

　　二、问题又来了，我们为什么不用长焦距的准直镜？这里涉及到两个方面的原因：

　　Second, the question has come again. Why don't we use a collimator with a long focal length? There are two reasons for this:

　　1、用长焦距的光纤准直镜，需要更大的镜片直径，会导致机械设计比较麻烦；

　　1. The use of long focal length optical fiber collimator requires larger lens diameter, which will lead to more trouble in mechanical design.

　　2、用长焦距的光纤准直镜会导致在聚焦的时候，对光纤切割机的聚焦点很敏感，一旦偏离聚焦镜焦点一点点就会出现切不透的现象出现，这也就是我们一般光纤切割机的聚焦一般在：60-100mm之间的原因。

　　2. The use of long-focal-length collimator will make the focus of the optical fiber cutting machine very sensitive when focusing. Once the focus of the optical fiber cutting machine deviates from the focus of the focus point, there will be a phenomenon of impenetrability, which is the reason why the focus of our general optical fiber cutting machine is generally between: 60-100mm.