Multiple beams can be combined without inducing multifrequency lasing.
An optical beam combiner now under development would make it possible to use the outputs of multiple single-mode laser diodes to pump a neodymium: yttrium aluminum garnet (Nd:YAG) non-planar ring oscillator (NPRO) laser while ensuring that the laser operates at only a single desired frequency. Heretofore, an Nd:YAG NPRO like the present one has been pumped by a single multimode laser-diode beam delivered via an optical fiber. It would be desirable to use multiple pump laser diodes to increase reliability beyond that obtainable from a single pump laser diode. However, as explained below, simplistically coupling multiple multimode laser-diode beams through a fiber-optic combiner would entail a significant reduction in coupling efficiency, and lasing would occur at one or more other frequencies in addition to the single desired frequency.
Figure 1 schematically illustrates the principle of operation of a laser-diode-pumped Nd:YAG NPRO. The laser beam path is confined in a Nd:YAG crystal by means of total internal reflections on the three back facets and a partial-reflection coating on the front facet. The wavelength of the pump beam — 808 nm — is the wavelength most strongly absorbed by the Nd:YAG crystal. The crystal can lase at a wavelength of either 1,064 nm or 1,319 nm — which one depending on the optical coating on the front facet. A thermal lens effect induced by the pump beam enables stable lasing in the lowest-order transverse electromagnetic mode (the TEM00 mode). The frequency of this laser is very stable because of the mechanical stability of the laser crystal and the uni-directional nature of the lasing. The uni-directionality is a result of the combined effects of (1) a Faraday rotation induced by an externally applied magnetic field and (2) polarization associated with non-normal incidence and reflection on the front facet.
In order to restrict lasing to a single frequency, it is necessary to confine the pump beam within the region occupied by the TEM00 mode of the NPRO laser beam near the front facet inside the crystal. In practice, this means that the pump beam must be focused to within a given solid angle (Ω) and area (A). [If a given pump beam has a larger A or larger Ω but its AΩ is equal to or less than the maximum AΩ for single-frequency lasing in the crystal, then an imaging lens can be used to trade A against Ω so that both A and Ω are equal to or smaller than the maximum values for single-frequency lasing. It is possible to do this because it is a basic principle of optics that AΩ is preserved in imaging by a lens.]
The AΩ of a commercial multimode 808-nm laser diode of the type used heretofore is not axisymmetric: instead, it is elliptically distributed about the optical axis and, hence, does not match the circular distribution of a multi-mode fiber of the type used heretofore to deliver a pump beam. As a result of this mismatch, AΩ for the pump beam emerging from the output end of the fiber is increased, typically to near the maximum single-frequency-lasing value in at least one of the planes containing the principal axes of the elliptical distribution. Consequently, it is difficult or impossible to maintain single-frequency lasing when combining the beams from two or more multimode laser diodes.