Active mode locking in cw neodymium garnet lasers

An experimental investigation was made of mode locking in a cw neodymium garnet laser when the resonator losses were modulated at the intermode frequency using an acoustooptic modulator. The dependence of the parameters of ultrashort pulses on the detuning between the modulation frequency and the intermode frequency of the resonator was determined. It was found that in the case of a laser in which all the components were wedge-shaped (so that there was no frequency selection) an increase in the detuning caused the pulse duration to increase faster and the pulse phase to vary more slowly than predicted theoretically. A special study was made of fluctuations of the output radiation when the detuning was due to mode locking. The dependence of the width of the mode locking zone (i.e., of the range of the detuning in which there were no additional fluctuations of the radiation) on the depth of modulation was determined and the results indicated that intracavity selectors produced a strong increase in the width of the mode locking zone. When the components inside the laser were not wedge-shaped, stable mode locking could be readily achieved by the use of a quartz oscillator supplying the modulator and by making the laser resonator from a material characterized by a small thermal expansion coefficient, whereas in the case of a laser in which all the elements were wedge-shaped it was very difficult to achieve stable mode locking by this method.