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Implementation of adaptive optics into a femtosecond laser chain

Valdma, Sandhra-Mirella LU (2015) FYSM60 20151
Department of Physics
Abstract
Optical aberrations are the main cause for reduced focusability and low peak intensity in laser beams. All the applications that rely on high peak power and/or small focus spot size, loose efficiency when optical aberrations are present. They can be reduced by using specially shaped optical elements, but all aberrations cannot be avoided completely.
In this thesis we used a new approach for reducing aberrations in a femtosecond laser beam. Usually, aberrations have to be corrected by using a deformable mirror with large diameter to avoid damage to the mirror. When the beam is expanded the peak intensity is reduced and the aberrations can be corrected. Deformable optics is normally not used for femtosecond lasers with kHz repetition rate... (More)
Optical aberrations are the main cause for reduced focusability and low peak intensity in laser beams. All the applications that rely on high peak power and/or small focus spot size, loose efficiency when optical aberrations are present. They can be reduced by using specially shaped optical elements, but all aberrations cannot be avoided completely.
In this thesis we used a new approach for reducing aberrations in a femtosecond laser beam. Usually, aberrations have to be corrected by using a deformable mirror with large diameter to avoid damage to the mirror. When the beam is expanded the peak intensity is reduced and the aberrations can be corrected. Deformable optics is normally not used for femtosecond lasers with kHz repetition rate and mJ pulse energy, because the price for implementation is often too high. We used a small deformable mirror, developed for applications in microscopy, with custom made ultrafast coating in order to correct aberrations in the femtosecond laser. Prior embedding the deformable mirror into the laser chain, we investigated its properties with a HeNe laser and an uncompressed femtosecond laser beam. The aberrations and focusability of the laser beam were measured with a Shack-Hartmann wavefront sensor and a CCD camera. Finally, the deformable mirror was integrated into the laser chain to reduce aberrations.
We demonstrated the measurement and correction of optical aberrations in a femtosecond laser chain. The high-order aberrations introduced by the deformable mirror itself currently prevent from increasing the focused intensity. Nevertheless, the method we proposed is promising and should be further investigated. (Less)
Popular Abstract
A Mirror into a Thousand Pieces

Imagine if we could shape the light in any way we want with a mirror consisting of small mirrors. The ability to do so would improve and help develop laser-based applications, thus making the technology more widespread. Today we use lasers everywhere: medicine, industrial manufacturing, micro-drilling and fundamental research etc.
These applications require high intensity that can be produced by femtosecond lasers. Such lasers have very short pulses and high intensity. Femtosecond lasers are complex and consist of many optical elements (lenses, mirrors etc.), which, due to their inherent imperfections, cause distortions in the laser beam. These distortions, also called aberrations, are unwanted because... (More)
A Mirror into a Thousand Pieces

Imagine if we could shape the light in any way we want with a mirror consisting of small mirrors. The ability to do so would improve and help develop laser-based applications, thus making the technology more widespread. Today we use lasers everywhere: medicine, industrial manufacturing, micro-drilling and fundamental research etc.
These applications require high intensity that can be produced by femtosecond lasers. Such lasers have very short pulses and high intensity. Femtosecond lasers are complex and consist of many optical elements (lenses, mirrors etc.), which, due to their inherent imperfections, cause distortions in the laser beam. These distortions, also called aberrations, are unwanted because they reduce the the overall quality of the beam.
With passive optical elements, such as specially shaped lenses etc., we are able to reduce some of the aberrations. However to improve the quality of an unstable and distorted laser beam, we will need something smarter. A solution for that is adaptive optics.
One could think of an adaptive optics system as a human visual system. There is a sensor measuring the light (aberrations) - retina in the eye, a computer calculating the shifts to produce a sharp image - the brain, and a deforming element to change the incoming light accordingly - the eye lens with muscles. When the image is not sharp the shape of the lens will change until optimum image is produced on the retina.
Usually an adaptive optics system consists of a deformable mirror, a wavefront sensor and a computer. Many small segments that can be moved up and down, thus creating a wanted shape for the mirror, make a deformable mirror. Upon reflection from this kind of mirror aberrations can be corrected and the overall beam quality improved.
These adaptive optics systems are usually very expensive for high intensity lasers. Usually the deformable mirror must have a large diameter to avoid damage from a high intensity laser beam. However, in this study we investigated a much cheaper option. We used a deformable mirror with a small diameter and a special coating and placed it in a femtosecond laser to correct aberrations.
We learned, that with this technique, it is possible to reduce aberrations in the femtosecond laser beam and although the resulting output was not ideal, it proved that the concept of using adaptive optics to correct high intensity femtosecond laser is solid. (Less)
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author
Valdma, Sandhra-Mirella LU
supervisor
organization
course
FYSM60 20151
year
type
H2 - Master's Degree (Two Years)
subject
keywords
wavefornt correction, deformable mirror, femtosecond laser, wavefront sensor, shack-hartmann, adaptive optics
language
English
id
7854984
date added to LUP
2015-09-05 17:45:06
date last changed
2015-09-05 17:45:06
@misc{7854984,
  abstract     = {Optical aberrations are the main cause for reduced focusability and low peak intensity in laser beams. All the applications that rely on high peak power and/or small focus spot size, loose efficiency when optical aberrations are present. They can be reduced by using specially shaped optical elements, but all aberrations cannot be avoided completely. 
In this thesis we used a new approach for reducing aberrations in a femtosecond laser beam. Usually, aberrations have to be corrected by using a deformable mirror with large diameter to avoid damage to the mirror. When the beam is expanded the peak intensity is reduced and the aberrations can be corrected. Deformable optics is normally not used for femtosecond lasers with kHz repetition rate and mJ pulse energy, because the price for implementation is often too high. We used a small deformable mirror, developed for applications in microscopy, with custom made ultrafast coating in order to correct aberrations in the femtosecond laser. Prior embedding the deformable mirror into the laser chain, we investigated its properties with a HeNe laser and an uncompressed femtosecond laser beam. The aberrations and focusability of the laser beam were measured with a Shack-Hartmann wavefront sensor and a CCD camera. Finally, the deformable mirror was integrated into the laser chain to reduce aberrations. 
We demonstrated the measurement and correction of optical aberrations in a femtosecond laser chain. The high-order aberrations introduced by the deformable mirror itself currently prevent from increasing the focused intensity. Nevertheless, the method we proposed is promising and should be further investigated.},
  author       = {Valdma, Sandhra-Mirella},
  keyword      = {wavefornt correction,deformable mirror,femtosecond laser,wavefront sensor,shack-hartmann,adaptive optics},
  language     = {eng},
  note         = {Student Paper},
  title        = {Implementation of adaptive optics into a femtosecond laser chain},
  year         = {2015},
}