Collimating lenses are for point light sources, and the so-called point light sources we see more often in life are: match heads, old-fashioned flashlight bulbs, and lasers coming out of energy optical fibers.
For our industrial laser industry, when we talk about collimating mirrors, we are basically talking about the laser light coming out of the energy transmission fiber. The light coming out of the energy fiber is a point light source with a divergence angle (θ). This parameter can generally be checked.
If we place this point light source at the focus of the optical fiber collimating lens, we know that: the light emitted from the focus of a focusing mirror (the collimating lens actually uses the focusing mirror in reverse), after passing through the focusing lens, becomes It became parallel light.
Many people ask me what is the diameter of the beam that comes out after passing through a certain collimating lens. Today I am here to give you the answer, which is 2F*tag (1/2*θ). If the divergence angle is 10° and F=150mm, then The diameter of the beam coming out of the collimator is =2*150*tag(5°)=26.2466mm.
This formula is of reference significance for selecting galvanometers for welding machines that use optical fiber transmission. Continuing to talk about it is what people in the fiber cutting machine industry want to know.
After passing through the fiber collimating lens, the laser enters the focusing lens of the fiber cutting machine. According to theory, the focal length of the collimating lens ÷ the focal length of the focusing lens = the ratio of the energy density after focusing to the previous density.
For example: the focal length of the collimating lens is 75mm, the focal length of the focusing lens is 150mm, 75÷150=1/2, that is to say, the area of the focused light spot after passing through the focusing lens is twice as large as the area of the point light source that just came out of the energy fiber. , the energy density is 1/2 of the original.
Some people ask, why do we need to reduce the energy density?
Isn't it better to concentrate the energy density? There are several reasons here:
First: If the focal length of the focusing lens is shorter, the focal depth of the focusing lens will be shallower. Shallow focal depth will easily lead to inability to cut deeply.
Second: the shorter the focal length, the smaller the focus point, and the smaller the cutting seam. The small seam is not conducive to the falling of the cut slag, resulting in inability to cut through.
Therefore, we generally try to use a focal length between 120-150mm as the focusing lens of the fiber cutting machine.
In addition, why do we not use long focal length collimating lenses? There are two reasons involved:
First: Using a fiber collimator with a long focal length requires a larger lens diameter, which will make the mechanical design more troublesome;
Second: Using a fiber collimating lens with a long focal length will cause it to be very sensitive to the focus point of the fiber cutting machine when focusing. Once it deviates a little from the focus of the focusing lens, the phenomenon of inability to cut through will occur.
This is why the focus of our general optical fiber cutting machines is generally between 60-100mm. Then let's talk about beam expanders. Beam expanders also have a collimating function, but beam expanders are for light beams (beams with a certain divergence angle).
The light from many lasers on our market is beam, such as: CO2 glass tubes, CO2 radio frequency tubes, lamp-pumped YAG lasers, lasers from fiber lasers with QBH, end-pumped 355nm 532nm 1064nm lasers, etc. ,
The light from these lasers is all beams, and they are not strictly parallel light (when the beam quality M2 of a laser is 1, the light of this laser has no divergence angle, but this can only be an ideal state, in It does not exist in real life. Generally, the M2 coefficient of lasers on the market can reach 1.2, which is already very good).
Next we will talk about why the beam expander can play a collimating role. Everyone knows that the beam expander can expand the beam. In professional terms, it is to expand the beam waist radius, and the beam waist radius and divergence angle of the laser are The product is a fixed value. As the beam waist radius increases (i.e., the beam expands), the divergence angle decreases (to achieve the effect of collimation).
There is a conclusion that after passing through an N-fold beam expander, the divergence angle of the laser beam is reduced to one N-fold of the original. For example, after passing through a 4x beam expander, the divergence angle is reduced to 1/4 of the original. This is why we try to use a beam expander with a larger magnification (provided that the size of the beam after passing through the beam expander does not exceed the spot size of the galvanometer).
The beam expander includes: CO2 beam expander, 532nm beam expander, 355nm beam expander, 1064nm beam expander, 650nm beam expander, the multiples are: 2 2.5 3 4 5 6 8 10 12 16 20 30 50 100 and so on.
The collimating lens includes: collimating lens for fiber welding machine (focal length 100 120 150 180mm); collimating lens for fiber cutting machine: diameter 30f100 collimating lens (two-piece combination), diameter 28f60 collimating lens (two-piece combination), diameter 25.4F75 collimating lens (two-piece combination) and so on.
