Laser operation with two controlled transverse modes

Abstract

In a laser resonator operating with many transverse modes, the emerging output beam quality is relatively poor. Improvement of the beam quality is typically obtained by inserting an aperture inside the resonator so as to reduce the effective radius of the gain medium, until only the optimal fundamental mode of Gaussian shape exists. Unfortunately, this results in a significant reduction of the output power. It is possible to operate a laser with a single high-order transverse mode by applying intra-cavity elements, such as apertures or phase elements. Such high-order modes exploit a relatively large volume of the gain medium, so the output power is relatively high. However, the intensity distribution of a single high-order mode usually consists of some low-intensity regions, resulting in inefficient utilization of the gain medium, with respect to the multi-mode operation. We demonstrate a novel resonator configuration in which it is possible to select two different transverse modes, each of which has a specific polarization, and can be manipulated separately. The intensity distributions of these modes can be chosen to be complementary, i.e. the peaks of the first mode would fall on the valleys of the second mode. Thus, the gain medium can be exploited more efficiently. This results in an increase in the total output power, yet with better beam quality than with the multi-mode operation.