Observations of spontaneous phase locking of TEM00qmodes at 0.063µ

Two types of spontaneous phase locking of the TEM00qmodes at 0.63μ have been observed for a mixed isotope tube (Ne²⁰, Ne²⁰). In both cases the mode locked laser output consisted of pulses which were less than 1 ns in duration. The pulse repetition frequencies were and for the two types. Adjacent mode competition appears to play a role in determining which type occurs. The persistence of the self-locking was adversely affected by simultaneous 3.39μ oscillation. The mode power spectra for the self-locked configuration revealed a dip close to the center of the gain curve. The mode power spectra for the self-locked and the free-running situations were shifted to the high-frequency side of the gain profile, and for a mixed isotope tube this can be accounted for by an asymmetric gain curve. For a pure isotope tube (Ne²⁰) only one type of self-locking was observed, and the power spectra for the self-locked and free-running configurations were shifted to the low-frequency side of the gain curve. On the basis of the experiments performed, it proved possible to calculate the magnitude of the third-order nonlinear susceptibility of the active medium at 0.63μ which, it is believed, was responsible for the spontaneous mode locking. The possibility of employing nonlinear crystals as passive mode locking devices was examined and it was found that the effect required for locking for a piece of deuterated KDP 3 cm in length and placed within the optical cavity was ∼ six orders of magnitude too small.