FREQUENCY STABILIZATION BY FREQUENCY PULLING FOR SINGLE-MODE OSCILLATION OF HE-NE-LASER AT MAXIMUM INTENSITY

Intense, frequency-stabilized single-mode laser sources are essential in precision distance measurements. We report a new scheme for stabilizing the oscillation of a three-mode laser so that a maximum output intensity can be realized. When one of the modes oscillates at its maximum intensity, the other two oscillate at each side of the central mode in the gain curve. The two intermode beats generated from the adjacent mode outputs are utilized to stabilize the laser frequency. Both the sum and the difference of the two beat frequencies vary because the frequency of each intermode beat varies with respect to cavity expansion via the phenomenon of "frequency pulling." These secondary beat signals are utilized as control signals for stabilizing the laser output frequency. To evaluate the effectiveness of the proposed concept, we compared two possible stabilization techniques involving the addition and subtraction of beat frequencies from the perspective of design simplicity, reliability, and ease of operation. Our investigation has shown that stabilization using the difference of two intermode beat frequencies, is superior. Excellent frequency stability (instability=10-10) of the laser was achieved during experiments. © 1995 American Institute of Physics.