Integrated Low Threshold Mid-infrared Intra-cavity Continuous-Wave Optical Parametric Oscillator

3μm～5μm mid-infrared laser has many important applications in the fields such as atmospheric communication, environmental monitoring, military and medical treatment. However, the conventional laser gain medium is difficult to cover the whole wavelength range from 3μm～5μm . Optical Parametric Oscillator (OPO) as one of the hot topics in this field, has received great attention for potential applications. In this work, we study the intra-cavity optical parametric oscillator (IOPO), which integrates optical parametric process in the laser cavity. As compared with conventional external cavity OPO, the threshold of IOPO is greatly reduced. At the same time, an aperture slot is used to suppress the oscillations of high-order transverse mode. In addition, by placing a KTP crystal in the cavity for the purpose of second harmonic generation (SHG), the relaxation oscillation is suppressed by nonlinear loss. After these improvements, we obtain compact tunable infrared to mid-infrared (2.2μm～4.2μm) continuous source with high-stability. The main results are summarized as following:1. Mid-infrared IOPO research backgrounds are introduced, including several ways to produce mid-infrared laser and basic ideas of building an IOPO.2. The theory of optical parametric oscillator is briefly introduced. And, according to the three-wave coupling equation, the gain coefficient and threshold condition of optical parametric oscillators are discussed. Quasi-phase matching (QPM) based on periodically poled crystals is elaborated. Wavelength tuning is also calculated theoretically.3. The character of Intra-cavity Continuous-Wave Optical Parametric Oscillator(IC-OPO) is also studied. By devising compensative design of thermal lens effect, the pattern matching is improved. Finally, the ICOPO with the threshold of about 1W is gained.4. The problems inherent in IC-OPO such as instability of output power and long-lived bursts of relaxation-oscillations are solved. Firstly, an aperture slot is used to suppress the oscillations of high-order transverse mode. Secondly, by placing a KTP crystal in the cavity for the purpose of SHG, the relaxation oscillation is suppressed by nonlinear loss. After these improvements, an compact tunable infrared to mid-infrared (2.2μm～4.2μm) continuous source with high-stability is obtained. The amplitude stability of output power is 0.52%(Root-Mean-Square). Especially, a series of observations show that the relaxation-oscillation is suppressed effectively.5. Several engineered OPOs have been developed based on our research. With rational design, those engineered OPOs have compact construction and high environmental adaptability, which are of good value in practice.