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Encyclopedia of Laser Physics and Technology

Kerr Lens

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Definition: a lensing effect arising from the Kerr nonlinearity

When a short optical pulse propagates through a nonlinear medium, the Kerr effect leads to a phase delay which is largest on the beam axis (where the optical intensity is highest) and smaller outside the axis. As a consequence, the wavefronts are deformed, so that the pulse is focused (assuming a positive nonlinear index n2). This effect is called self-focusing and has important implications for passive mode locking of lasers (→ Kerr lens mode locking) and for optical damage of media (catastrophic self-focusing). For negative n2, the nonlinearity is self-defocusing.

When a Gaussian beam with optical power P and beam radius w propagates through a thin piece (thickness d) of a nonlinear medium with nonlinear index n2, the dioptric power (inverse focal length) of the Kerr lens is

strength of Kerr lens

when considering only the phase changes near the beam axis in a parabolic approximation.

The equation shows that for a given optical power Kerr lensing becomes more important for stronger beam focusing.

See also: Kerr effect, self-focusing, Kerr lens mode locking, self-phase modulation

Categories: nonlinear optics, physical foundations


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