Quality enhancement of a 355 nm sum-frequency light generated using a quality-deteriorated fundamental beam

This work provides a comprehensive study around the effect of a quality-deteriorated fundamental beam on the quality of a 355 nm wavelength beam experimentally generated using sum-frequency mixing of a second-harmonic generation (SHG) beam and its un-depleted fundamental inside a walk-off (WO) affected BBO nonlinear crystal. A slit-variable aperture is used inside the resonator of a utilized Nd:YAG resonator to generate a wide range of Hermite–Gaussian modes for the fundamental and subsequent SHG and sum-frequency generation (SFG) beams. The M²-factor of interacting beams is simulated based on the reconstruction of the recorded intensities using a recently reported technique known as few-frame decomposition method. It is found that in the worst case, due to the WO occurring in the X-direction, the M²-factor of the SFG beam is converged to 2.5 after 2 mm of crystal length and remained constant in the remaining length when the fundamental M²-factor is set for 4.5. Based on the simulation results, it is shown that the beam quality of SFG beam along the WO direction is independent of the fundamental M²-factor. It is further realized that by changing the focusing factor up to 2.84, again M²-factor of the UV-SFG beam is not exceeded from 2.5. (paper)