Adaptive Optics in Lasers

Typical applications of ultra-high-power femtosecond lasers include precision drilling and surface micro-machining of metals, and micro-structuring of transparent materials [1,2].  Because the absorption in a transparent material is nonlinear, it can be confined to a very small volume by tight focusing, and the absorbing volume can be located in the bulk of the material without any damage to the surface [3].  The extent of the structural change produced by femtosecond laser pulses can even be smaller than the focal volume.  This feature makes possible direct writing of optical waveguides [4], and three-dimensional binary data storage [5].

However, high peak-power pulsed lasers are difficult to focus close to the diffraction limit because of aberrations that induce deviations from a perfect spatial wave-front.  The sources of these aberrations include thermally induced and nonlinear optical distortions, and static distortions such as those introduced by gratings used in chirped-pulse amplification (CPA).

A spatially clean beam is desirable to achieve the highest possible intensity on-target, and to minimize the energy deposited outside the central focus.  One way to achieve this is to correct the wave-front using an adaptive optical element such as a deformable mirror.

We are working in collaboration with the National Centre for Laser Applications (NCLA) to correct the aberrations in a high-power laser system.  The wave-front will be measured with a Hartmann-Shack wave-front sensor, and corrected with a 37-channel deformable membrane mirror [6].  The necessary optics will be mounted on a vertical breadboard which delivers the femtosecond pulses to a precision machining stage as shown the figure.

Figure 1Experimental  setup for deformable-mirror wave-front correction.

The deformable mirror has been tested with a FISBA OPTIK μPhase® HR digital interferometer [7].  The figure below shows typical live-camera images of the interference fringes as the voltage to the various actuators is varied.  These particular images were recorded for about 55° angle of incidence.  This interferometer serves as a very useful tool with which to calibrate the performance of the wave-front sensor.

Figure 2 Fisba testing of Okotech 37-channel deformable mirror

References

[1]        S. Nolte, G. Kamlage, F. Korte, T. Bauer, T. Wagner, A. Ostendorf, C. Fallnich, and H. Welling, Advanced Engineering Materials 2, No. 23 (2000).

[2]        C. B. Schaffer, A. Brodeur, J. F. Garcia, and E. Mazur, Opt. Lett. 26, No. 2, 93-95 (2001).

[3]        E. N. Glezer and E.Mazur, Appl. Phys. Lett. 71, No. 7, 882-884 (1997).

[4]        S. Nolte, M. Will, B. N. Chichkov, and A. Tünnermann, Proc. SPIE

Vol. 4637, 188-196 (2002).

[5]        E. N. Glezer, M. Milosavljevic, L. Huang, R. J. Finlay, T.-H. Her,

J. P.Callan, and E.Mazur, Opt. Lett. 21, No. 24, 2023-2025 (1996).

[6]        http://www.okotech.com/

[7]        http://www.fisba.com/