Ultrastable THz generation based on frequency mixing of output beams from coupled optical parametric oscillators

When adhesive free bonded (AFB) periodically-inverted potassium titanyl phosphate (KTiOPO₄, KTP) plates were used as a nonlinear medium inside a cavity, coupled optical parametric oscillators (OPOs) were implemented [1]. As a result, two signal waves and two idler waves can be simultaneously generated. In singly resonant OPOs, two idler waves were used to efficiently generate THz waves [2]. When a bulk KTP crystal is inserted in such a cavity, a new pair of the signal and idler waves can be generated. When we mix three signal waves or three idler waves in a nonlinear medium, one in principle expects the generation of three THz frequencies. Here, we have investigated the generation of THz waves by mixing three idler frequencies generated by coupled OPOs in a nonlinear medium.