INFLUENCE OF RESONATOR ON SUM-FREQUENCY CW GENERATION POWER IN AN SRS-LASER

Introduction. Laser radiation in the visible range is widely used in scientifi c research, chemistry, biology, medicine, and other areas. Therefore, research and development of all-solid-state lasers that generate effi ciently visible radiation are of great practical interest. (Quasi)-CW solid-state lasers with diode pumping, the cavity of which contains a stimulated-Ramanscattering (SRS) medium that increases the number of frequencies of generated IR radiation and a quadratically nonlinear medium that converts this radiation into the visible range by generating a harmonic and sum-frequency radiation (SFR), occupy a prominent place among such systems [1–7]. This results in a laser system in the cavity of which comparatively powerful beams of radiation exist and interact strongly. The radiation has at least four frequencies, i.e., the pumping, fundamental laser ωL, Stokes ωS, and sum ωG = ωL + ωS or harmonic ωG = 2ωS of the Stokes frequency. Such a system can also generate a harmonic ωG = 2ωL of the fundamental frequency [2, 4] but then there is no need to place an SRS medium in the cavity [8, 9]. Very high cavity gains at the fundamental and Stokes frequencies and a high transparency of the output mirror in the visible region are usually employed in order to obtain high generation effi ciency of visible radiation in the aforementioned systems. It is thought that there is no resonator at the visible radiation frequency, i.e., radiation generated during one pass of the nonlinear medium exits through the output mirror or dual-pass generation occurs if the system has surfaces that are refl ective at this frequency. Relationships describing the dependence of CW generation power of such a system on the pumping, active media, and resonator with no refl ection of the generated radiation at the output mirror have been published [10, 11]. Herein this limitation is lifted, i.e., the relationships defi ne the SFR generation power with a resonator and at the frequency of the generated radiation. Statement of the Problem. Let us examine as before SFR generation of a SRS-laser [10, 11]. Figure 1 shows a conceptual diagram of a laser system consisting of three (two for a self-Raman SRS-laser where media L and S are co-located) active media positioned co-axially in the cavity formed by mirrors M1 and M2. This is the active medium L of the primary laser generating radiation of frequency ωL (fundamental radiation); scattering (SRS) medium S that converts this radiation into Stokes component of frequency ωS; and a quadratically nonlinear medium N in which SFR generation occurs, ωG = ωL + ωS. Use of the third mirror M3 is desirable in order to direct the whole SFR output through a single mirror and avoid its losses in the active and SRS media. This mirror in system a should refl ect well radiation of frequency ωG and pass well radiation of frequencies ωL and ωS. In this instance, mirror M2 is the output mirror. It should pass SFR and be highly refl ective for the fundamental and laser radiation. Mirror M3 is the output mirror in system b and has high transmission for SFR and refl ects radiations ωL and ωS. In this instance, mirror M2 should refl ect well radiation of all three frequencies. The end surfaces of the active media can also fulfi ll to a certain extent the role of mirror M3.