Route to mode locking in a three-mode He-Ne 3.39-μm laser including chaos in the secondary beat frequency

A helium-neon laser at 3.39 μm has been operated under cavity and discharge conditions permitting three modes to simultaneously oscillate for some regions of cavity tuning. Both mode-locked and -unlocked regions have been found. For unlocked three-mode operation, the laser output may include a secondary beat frequency. As the cavity length is tuned to the mode-locked region, this secondary beat frequency approaches zero and displays critical fluctuations at the transition to mode-locked operation. As the discharge current is increased, for fixed cavity length, the peaks in the power spectrum representing the secondary beat and its harmonics undergo a 100-fold broadening enroute to a broad chaotic spectrum before the laser reaches a mode-locked, low-noise configuration. This appears to be the onset of a deterministically chaotic behavior of the coupled modes that is distinct from the critical fluctuations. Details of cavity tuning and current variation on the laser output, including hysterisis effects, are reported here.