What are the factors of a telescope that influence the output power (in watts) of a laser beam passed through it, regardless of the laser's wavelength? Is there any relation in between laser power to telescope aperture ?
The Conservation of Energy rule applies to a laser beam passing through a system or subsystem such as a telescope. Most of time, laser beam energy or power passing through a telescope will obey the following version of the conservation of energy rule:
P = T + R + A + S
Here “P” is the power input to the telescope and “T” is the power of the beam that is observed exiting the telescope. The losses are then “R” for reflection losses, “A” for absorption losses, and “S” is for scattering losses.
Reflection losses occur at each lens surface - these are specular reflections.
Scattering losses are just that, scattered light. Scattering losses occur in the bulk substrate and coatings of the lenses and are dependent on the wavelength of light and substrate/coating materials.
Scattering will also occur when a laser beam is too large for the telescope aperture. When this occurs a portion of the beam striking the telescope outside the lens clear aperture will be scattered.
Some of this light hitting the telescope body will also be absorbed in the housing.
Finally, absorption losses occur in even the best optics. Anytime a light or laser beam passes through an optic, a (hopefully 😊) small portion of the laser beam will be absorbed in the coatings and substrate materials of the lenses.
Second question: “Is there any relation in between laser power to telescope aperture?” For gaussian laser beams this is simple to answer using the power through an aperture equation. Wikipedia provides a good explanation of this at:
https://en.wikipedia.org/wiki/Gaussian_beam
See specifically the section titled “Power through an aperture.” Of course, other types of laser beams require optical modeling in most cases.
This is by no means a comprehensive review, but I hope it helps a little!
You will also find some interesting information about lasers and optics at
www.lenskit.app
If your interest is in gaussian beam stuff, click through to:
https://www.lenskit.app/posts/gausspost
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Gary
I would just add to Gary Herrit's answer that damage is the ultimate limiting factor in output power. Each optical element in the telescope has a damage threshold. At some laser power each element will damage, and once damage starts it usually becomes worse and spreads to other optical elements. The element with the lowest damage threshold is the weakest link of the optical train. Damage threshold depends upon a number of factors such as optical material used, laser wavelength, surface quality, coatings, beam quality, etc.
You’re welcome. I would add that these comments only refer to theoretical considerations. If you are measuring a real laser beam with power or energy meters, then these types of measurements might not perfectly follow the theory.
For example, if you are measuring laser power before and after a telescope, you will likely record a different power level. Most laser power meters are sensitive to the beam diameter - even though manufacturers may claim they aren’t.
I can say from experience this is the case for pyroelectric and thermal power meters that I have used for CO2 and 1 micron fiber lasers. Good luck with your work!
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