CRYSMIT OPTICS manufactures China Ultrathin BBO crystals, specifically designed for femtosecond pulse lasers. These crystals are available in thinnest thicknesses of 0.005mm (with 1mm UVFS substrate) and 0.05mm (without substrate). They are ideal for applications such as SHG, SFG, OPA, and DFG with Ultrashort Pulse Laser, effectively avoiding second harmonic pulse broadening. Moreover, we provide free samples and testing reports for BBO nonlinear optical crystals.

                    Fig.1. The Coating curve of ultrathin BBO crystal.                                           Fig.2. The Transmittance curve of ultrathin BBO crystal.

The ultraviolet femtosecond pulse had become a central of focus in many fields, for example ,Laser-spectroscopy, Laser-chemistry, laser nuclear fusion(ICF) and so on. Thiss due to its many advantage such short wavelength, High quantum-efficiency, broad bandwith, high peak-power. The conversion of ultrashort pulses from visible and near-infared to UV is of great interest.

The propagation of a ultrashort optical pulses through the crystal results in a delay of the pulses because of Group Velocities Mismatch (GVM), a duration broadening because of Group Delay Dispersion (GDD) and a frequency chirp. Unfortunately those effects forces to limit nonlinear crystal thickness in frequency generation schemes.For two collinearly propagating pulses with different group velocities their quasistatic interaction length (Lqs) is defined as distance over which they separate by a path equal to the one of the pulses duration (or to the desired pulse duration):  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.