Hi,
I am not familiar with the laser technology. However, I may use the laser to do my project, so I want to have an overall picture of the typical laser beam diameter to evaluate the viability of my idea.
Let us assume that the separation distance between laser system is at most several millimeters away from my processed material. Under this circumstances, the divergence won't be that significant. What is the smallest spot diameter it can achieve nowadays and what is the typical range?
Using a single mode laser and a microscopic objective, it is possible to realize a beam spot with a minimal diameter of half the wavelength.
However, this is not practical for most applications. The lenses may be damaged by the laser itself or by particles and gasses emitted from the processed material.
The means to prevent the damage depends on your application. Finally, the reachable beam diameter depends on your application.
Dear Chenjin Lu,
For a fiber laser, smallest beam diameter is related with output fiber diameter and optical design such as delivery cable, collimating lens, cutting head..etc It is better to give some information about process ( cutting welding..etc) or laser power. I can tell you the achiaved smallest diameter then.
As was written above by Mr. Yuriy Pilgun, the minimal spot size is dependent on you beam quality. The beam quality (M^2) is a parameter, which shows how far is your beam from ideal Gaussian beam. If M^2 parameter of your beam is 1, you have ideal Gaussian beam, which can be focused to diffraction limit. To estimate the spot size in this case, you can use following formula (Gaussian Beam, Wikipedia) w=λ/πθ, where θ is approximately the beam diameter before lens divided by focal length of the lens, so, θ=D/f. For example, if you beam diameter from the laser is 5 mm, you have green laser (515 nm), and you are focusing with 5 cm focal distance lens, the spot size will be 1.6 um. If the M^2 parameter is not 1, you should multiply the obtained spot size with the value of M^2. For low power fiber laser, the M^2 is almost 1 for most of the cases. For high power laser, it can be much more, so, you should know that value to estimate the plot size. Good luck.
Just as a side comment: apart from standard far field focusing, near field focusing lenses/particles can be used. They can be used to produce features far smaller than the diffraction limit predicted dimension. Use "mono layer microsphere lenses" as key words for a google search, you will find more useful information
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