This paper reports on the phase-matching properties of LiGaS
2 for upconverting a Nd:YAG laser-pumped KTP and AgGaS
2 optical parametric oscillator (OPO) at mid-IR to the near-IR by mixing with its pump source together with the new Sellmeier equations that provide a good reproduction of the present experimental results as well as the published data points of second-harmonic generation (SHG) and sum-frequency generation (SFG) of a CO
2 laser, a Ti:Al
2O
3 laser-pumped optical parametric amplifier (OPA), and a Nd:YAG laser-pumped OPO in the mid-IR.
This index formula gives the important information that group velocity mismatch (GVM) (Δ
sp = 1/υ
s - 1/υ
p) of LiGaS
2 in the 4 - 11 μm range is 12 ~ 27 fs/mm lower than that of the widely used LiInS
2, which makes it ideal for the upconversion of the mid-IR femtosecond pulses having large spectral bandwidths to the near-IR.