Deep UV generation in a femtosecond optical parametric oscillator through multistage cascading

We report efficient generation of ultrashort laser pulses in the deep ultraviolet (DUV) from a quasi-phase-matched (QPM) optical parametric oscillator (OPO) based on a periodically poled KTP (PPKTP) crystal. Up-conversion through frequency-doubling of the signal pulses (2ω_S) and sum-frequency generation between the pump and signal pulses (ω_P + ω_S) produces femtosecond pulses in the green and in the blue, respectively. A third-order QPM is responsible for the green-pulse generation and a fifth-order QPM for the blue. The cavity mirrors have two high-reflection bands, which support the oscillation of the OPO signal (ω_S) in the near infrared and the sum-frequency generation of ω_P + ω_S in the blue, respectively. The interaction between the blue and green pulses enables further cascading by a sum-frequency generation process, producing the DUV pulses. Cavity-length detuning facilitates the large tuning range of the DUV generation.