Injection seeding. I. Theory

Injection seeding has proven to be a practical method of controlling the spectral output of a laser in an efficient and reliable manner. To achieve satisfactory injection seeding performance, a sufficiently large seed must be employed. To characterize the required seed, an injection seeding theory is developed here which establishes two interrelated concepts, threshold for injection seeding and spectral purity of the laser output. Rather than utilizing numerical techniques to solve the differential equations, closed-form expressions for the threshold are developed for not only common continuous wave injection seeding but pulsed injection seeding as well. In addition, effects of alignment of the seed to the resonator, both in position and angle, and effects of frequency mismatch, or difference between the seed and the resonant frequencies of the resonator, are taken into account. Expressions for the threshold utilize readily measurable experimental parameters. Spectral purity of the laser output and its consequences on the required seed power or energy are also explored