Single Mode Resonators

Abstract

Without additional frequency selecting elements, most lasers utilizing linear resonators emit at several axial modes. A resonator operating in a single transverse mode, exhibits a frequency spectrum which consists of multiple lines. If we neglect the frequency pulling induced by the gain profile, these emission lines are spectrally separated by c ₀/(2L), where L is the optical resonator length and c ₀ is the speed of light in a vacuum. In inhomogeneously broadened lasers, all axial modes are observed that experience a gain high enough to overcome the resonator losses (Fig. 21.1a). Since no interaction between different axial modes occurs, the number of oscillating axial modes increases as the gain is increased. Far above the laser threshold, the number of observed axial modes n can be approximated by:

$$ n = \frac{{\Delta v\,2L}}{{{c_0}}}$$

((21.1))

where Δν is the gain bandwidth of the active medium (see Table 4.2 for bandwidths of common lasers). Inhomogeneous line broadening is found in low pressure gas lasers (HeNe, AR, CO₂) and in semiconductor lasers at high injection currents. In both laser types, typically on the order of 10 axial modes oscillate simultaneously.