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Optical Cavities: Free-Space Laser Resonators
Published in Chunlei Guo, Subhash Chandra Singh, Handbook of Laser Technology and Applications, 2021
Mode-locking many cavity modes requires some modulation technique that couples the modes in phase. Acousto-optic modulators are used for active mode-locking of cw laser systems. An acoustic modulation driven at a frequency that matches the resonator mode spacing is established in a transparent material. This modulation produces sidebands on the modes, which couple to the adjacent modes, locking many modes in phase. Saturable absorbers are normally used with higher-peak-power pulsed mode-locked lasers. Saturable absorbers appropriate for mode locking must have short relaxation times much less than the round-trip cavity transit times. As the laser oscillation builds up with random fluctuations, the strongest fluctuation will begin to saturate the absorption. This fluctuation will build up more rapidly and come to dominate the laser oscillation. Gain will be depleted by the strong fluctuation reducing the amplitude of secondary fluctuations. The rapid saturation of the absorption on each cavity transit is a modulation that couples additional modes. The depth of modulation, gain, gain bandwidth and dispersion in the resonator determine the width on the mode-locked pulses. Kerr-lens-mode locking is an additional technique for producing very short mode-locked pulses. This technique uses the optical Kerr effect or change in refraction index that is produced by very high-intensity pulses.
Nonlinear and short pulse effects
Published in John P. Dakin, Robert G. W. Brown, Handbook of Optoelectronics, 2017
As the Kerr lens only focuses the most intense parts of a short pulse, one can use it to build an ultrafast optical switch, which is also illustrated in Figure 9.7. If an aperture is suitably placed in the focus of the Kerr lens, it will introduce losses for cw laser light of low intensity, whereas high intensity light can pass through as it sees the focusing action of the Kerr lens. This kind of self-switching is used in lasers to discriminate short pulses and create losses for any kind of low intensity background. Placed in a cavity, the Kerr lens switch strips off a pulse pedestal, recleaning the pulse on every passage. This is used for generating some of the shortest pulses ever generated from a laser with a method called Kerr-lens mode-locking [59–62].
The Basics of Lasers
Published in Helmut H. Telle, Ángel González Ureña, Laser Spectroscopy and Laser Imaging, 2018
Helmut H. Telle, Ángel González Ureña
Equation 3.26 is derived by calculating the radially dependent (nonlinear) phase changes and then comparing them with those for a lens made from the same optical material. The equation shows that the effect of Kerr lensing increases with laser power. For short, intense laser pulses, the effect gives rise to the so-called Kerr lens mode locking, also known as “self-mode locking.” By careful arrangement of an aperture in the laser cavity (to curtail the weaker intensity parts of the Kerr-focused beam; see Figure 3.23), the Kerr lens plus aperture constitute the equivalent of an ultrafast response-time saturable absorber. Kerr lensing for femtosecond-laser pulse generation was first realized by Spence et al. (1991).
Accurate second Kerr virial coefficient of rare gases from the state-of-the-art ab initio potentials and (hyper)polarizabilities
Published in Molecular Physics, 2020
The Kerr effect, discovered by John Kerr in 1878 [1], describes the refractive-index change of a material when an electric field is applied. The Kerr electro-optic effect has a fast response to the change of an external electric field and is the basis for electronic controlled optical switches. The Kerr optical effect means that the change of refractive index is proportional to the intensity of light. Its most well-known application nowadays is Kerr-lens modelocking. For an ideal gas, Buckingham et al. [2,3] found that the Kerr constant Km is linearly proportional to the gas density ρ: where the coefficient AK (also called the first Kerr virial coefficient) depends on the atomic second hyperpolarizability γ0. With increasing pressures or densities, the deviations from Eq. (1) can be observed and the terms quadratic, cubic and higher in density contribute to Km(ρ): where BK(T) and CK(T) are the second and third Kerr virial coefficients, respectively and T is the temperature.