Characteristics of mode-coupled lasers

Experimental studies of lasers with mode coupling will be described. Forced mode coupling was achieved through the use of an ultrasonic diffraction grating placed inside the cavity as described by Hargrove, et al. Experimental data was obtained with He-Ne and argon lasers whose axial mode spacing was equal to 100 Mc. When sufficient mode coupling is introduced into the cavity, the laser output consists of a periodic train of subnanosecond width pulses without an appreciable reduction in average output power. The ratio of peak power with mode coupling to the average power is approximately equal to the number of coupled modes. A typical number of 20 coupled modes results in a peak power of 20 times the average power. The theoretical pulse width was approximately 10 nanoseconds divided by the number of coupled modes. Pulse widths of ∼ 0.5 nanoseconds have been observed with a high-speed photomultiplier consistent with theory. In addition, it was observed that mode coupling can occur naturally in a laser without the Use of the internal modulator. The experimental conditions necessary for obtaining a self-pulsing laser will be discussed. A theoretical explanation, based upon some preliminary experiments, will be described.