An thin LBO crystal is a valuable nonlinear crystal used to convert the frequency of ultrafast lasers. When employing it for this purpose, a critical factor to consider is the impact of group velocity (Vg) mismatch on pulse broadening. This mismatch arises due to the dispersion in the crystal. To prevent pulse broadening, it's crucial to limit the crystal's thickness to stay below the maximum length (Lmax). This length is determined by dividing the pulsewidth by the inverse group velocity mismatch : Lqs = τ/GVM ; where GVM is the group velocity mismatch and τ is the duration of the pulse. GVM calculations are presented for the most popular Type 1 phase matching applications for different crystals in Table 1.

Table 1. Group velocity mismatch between shortest and longest wave pulse for Type 1 phase matching

Optimal BBO, LBO, KDP crystal  thicknesses which are limited by GVM for Type  1 SHG of 800 nm at different fundamental  pulse duration are presented in the Table 2.

Table 2. Quasistatic interaction length for Type 1 SHG of 800 nm

For infrared light (such as 1064nm) and visible light (such as 532nm) in nonlinear crystals, the group velocity mismatch value is usually on the order of 0.1~1ps/mm. This shows that for a 10 mm nonlinear crystal, the group velocity mismatch will have a significant impact on the frequency conversion process for 10 ps pulses, and especially for femtosecond pulses. For this reason, shorter crystals are required for short pulses, and higher light intensities are required to maintain high conversion efficiencies. And the maximum value of light intensity is limited by effects such as optical damage, then the group velocity mismatch will limit the highest efficiency of short-pulse nonlinear frequency conversion.

Group velocity mismatch is also important for Raman amplifiers with short pulses in fibers. For example, when a picosecond optical pump of 1064nm is used to amplify a pulse of 1110nm, since the group velocity mismatch value of a large mode area fiber is similar to that of a Quartz crystal , the group velocity mismatch will reach 1.1ps/m for the above wavelengths. This means that for a pulse of 1 ps, time-domain walk-off will occur after 1 m in the fiber, and therefore, the effective Raman gain will decrease.