Frequency-Doubled Nd:YAG Lasers - Lee Laser, Inc.

3Mechanisms are devised to raster scan the SHG crystal across the focused laser beam asmore impact locations become unusable. Eventually, the SHG crystal must be replaced. Ifeffect, the SHG crystal becomes a consumable component of the laser.Lee Laser avoids this problem by installing the SHG crystal inside the laser optical resonator.The high level of peak power density that is needed to achieve good wavelength conversion isprovided by the high circulating power of the 1064-nm beam inside the laser optical resonator.There is no need to focus the 1064-nm beam into the SHG crystal, thereby greatly extendingthe lifetime of the SHG crystal.This design provides a variety of useful benefits to the user:1. greater conversion efficiency from 1064 nm to 532 nm, up to 85 %2. superior Q-switched pulse stability; instability at 532 nm about 50 % of instability at 1064nm3. permissible operation at much higher Q-switch frequencies because of better pulsestability4. much longer SHG lifetime; no requirement to raster-scan 1064-nm beam across face ofSHG crystal (LBO) to maintain undamaged beam input site, as do competitorsNew Inline SHG ResonatorRecently Lee Laser developed a new, Inline Resonator design that achieves the same highefficiencyof second harmonic generation, but in a resonator that emits the 532-nmwavelength along the same axis as the 1064-nm emission. The inline SHG resonator alsobenefits from simpler optical alignment.LBO SHG CrystalLee Laser now uses LBO (lithium triborate) as its choice of harmonic generator crystal. LBOis as much as seven (7) times more durable than KTP (potassium titanyl phosphate) againstinternal, bulk damage from laser beam propagation. However, because of LBO's inherentlyless efficient wavelength conversion from 1064 nm to 532 nm (and from 1320 nm to 660 nm),it is necessary to use a much longer crystal. Lee Laser uses LBO crystals that measure 18mm long. The crystal is housed inside a temperature controlled oven that maintains anelevated temperature within 0.1 o C. Precise temperature control of the LBO crystal isvital to maintain a stable level of output beam power. A 2-axis adjustable mount permitsangular tuning of the 532-nm or 660-nm wavelength beam output power.SHG Crystal Protection from Laser DamageIn a frequency-doubled optical resonator, the SHG crystal is the optical element that is mostsensitive to laser damage. This damage can be caused by airborne contamination that hasbeen allowed to settle on the optical surfaces (Lee Laser uses dust hoses to enclose thebeam path of the optical resonator). SHG damage may also be caused from operation of thelaser in a low-pulse-rate mode in which the high pulse energy and short pulse width combineto generate a high peak pulse power condition.