What is the difference between the collimator and the beam expander of laser cutting machine?
Collimators are aimed at point light sources, and the so-called point light sources we see more in life, such as: match heads lit, old flashlight bulbs, lasers from energy fibers. For our industrial laser industry, the collimator is basically about the laser coming out of the energy transmission fiber.
The light from the energy fiber is a point light source with divergence angle（ θ) Yes, this parameter can generally be found. If we place this point light source on the focal point of the optical fiber collimator, we know that the light emitted from the focal point of a focusing lens (which actually uses the focusing lens in reverse) becomes a parallel light after passing through the focusing lens. Many people ask me what the diameter of the beam is after passing a collimator. Today, Jiatai is here to give you the answer, which is 2f*tag (1/2* θ)， If the divergence angle is 10 °, f=150mm, the beam diameter from the collimator =2*150*tag (5 °) =26.2466mm. This formula is of reference significance to the selection of galvanometer for welding machine using optical fiber transmission. Let's move on to what people in the fiber cutting machine industry want to know. After passing through the optical fiber collimator, the laser enters the focusing lens of the optical fiber cutting machine. Theoretically, the focal length of the collimator ÷ focal length of the focusing lens = the density ratio of the energy density after focusing relative to the previous density. For example, the focal length of the collimator is 75mm, the focal length of the focusing lens is 150mm, 75 ÷ 150=1/2, that is, the area of the focusing spot after passing through the focusing lens is twice that of the point light source just coming out of the energy fiber, and the energy density is 1/2 of the original.
Someone asked, why should we reduce the energy density? Isn't it better to concentrate the energy density? There are several reasons:
First, if the focal length of the focusing lens is shorter, the focal depth of the focusing lens will be shallower, and shallow focal depth will easily lead to poor cutting depth.
Second: the shorter the focal length is, the smaller the focal point is, the smaller the cutting seam is. A small seam is not conducive to the falling of molten slag cut out, resulting in impervious cutting. Therefore, we generally try to use the focal length between 120-150mm to make the focusing lens of the optical fiber cutting machine. In addition, why don't we use a long focal length collimator? There are two reasons: first, using a long focal length optical fiber collimator requires a larger lens diameter, which will lead to more trouble in mechanical design. Second, using a long focal length optical fiber collimator will cause it to be very sensitive to the focus point of the optical fiber cutting machine when focusing, and once it deviates from the focus of the focusing lens a little bit, it will appear impervious. This is why the focus of our general optical fiber cutting machine is generally between 60-100mm.
Then let's talk about the beam expander. The beam expander also has the function of collimation, but the beam expander is for beams (beams with a certain divergence angle). Many lasers in our market produce light beams, such as CO2 glass tubes, CO2 RF tubes, lamp pumped YAG lasers, lasers from fiber lasers with QBH, end pumped 355nm 532nm 1064nm lasers, etc. the light from these lasers is light beams, And it is not strictly parallel light (when the beam quality m2 of a laser is 1, the light of the laser has no divergence angle, but this can only be an ideal state, which does not exist in real life. Generally, it is very good if the M2 coefficient of the laser on the market can reach 1.2). Now let's talk about why the beam expander can play the role of collimation. As we all know, the beam expander can expand the beam. In a professional way, it is to expand the beam waist radius, and the product of the beam waist radius and the divergence angle of the laser is a fixed value. If the beam waist radius increases (i.e. beam expansion), the divergence angle decreases (to achieve the role of collimation). There is a conclusion, after an n-fold beam expander, The divergence angle of the laser beam is reduced to one nth of the original. For example, after a 4x beam expander, the divergence angle will be reduced to 1/4 of the original. This is also the reason why we try to use a larger multiple of the beam expander (provided that the size of the beam after passing through the beam expander does not exceed the size of the galvanometer spot).
The beam expander includes: CO2 beam expander, 532nm beam expander, 355nm beam expander, 1064nm beam expander, 650nm beam expander. The multiples are: 2.5 3 4 5 6 8 10 12 20 30 50 100 and so on.
Collimator includes: collimator of optical fiber welding machine (focal length 100, 120, 150, 180mm); Collimator of optical fiber cutting machine: diameter 30f100 collimator (two-piece combination), diameter 28f60 collimator (two-piece combination), diameter 25.4f75 collimator (two-piece combination), etc.