Starting A Kerr-lens Modelocked Laser
Initiation of Kerr-lens modelocking depends on the strength of the nonlinear effect involved. If the laser field builds up in a cavity the laser has to overcome the region of cw operation, which often is favoured by the pumping mechanism. This can be achieved by a very strong Kerr-lensing that is strong enough to modelock due to small changes of the laser field strength (laser field build-up or stochastic fluctuations).
Modelocking can also be started by shifting the optimum focus from the cw-operation to pulsed operation while changing the power density by kicking the end mirror of the resonator cavity (though a piezo mounted, synchronous oscillating end-mirror would be more 'turn key'). Other principles involve different nonlinear effects like saturable absorbers and saturable Bragg reflectors, which induce pulses short enough to initiate the Kerr-lensing process.
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