KDP (KH2PO4), DKDP or D*KDP (KD2PO4) and ADP (NH4H2PO4) are widely used as the second, third and fourth harmonic generators for Nd:YAG and Nd:YLF lasers. The crystals are also widely used for electro-optical applications as Q-switches for Nd:YAG, Nd:YLF, Ti:Sapphire and alexandrite lasers, as well as for pockels cells and as for acousto-optical applications. The most commonly used electro-optical crystal is DKDP with a deuteration more than 98%. These crystals are grown by a water-solution method and can be grown up to very large size. Therefore, they are as low-cost and large-size finished non-linear components available.
For frequency-doubling (SHG) and -tripling (THG) of Nd:YAG laser at 1064 nm, both type I and type II phase-matchings can be employed for KDP and DKDP. For frequency - quadrupling (4HG, output at 266 nm) of Nd:YAG laser, KDP crystal is normally recommended.
| KDP | KD*P | |
| Chemical Formula | KH2PO4 | KD2 PO4 |
| Transparency Range | 200-1500nm | 200-1600nm |
| Nonlinear Coefficients | d36=0.44pm/V | d36=0.40pm/V |
| Refractive Index (at 1064nm) | no=1.4938, ne=1.4599 | no=1.4948, ne=1.4554 |
| Electro-Optical Coefficients |
r41=8.8pm/V r63=10.3pm/V |
r41=8.8pm/V r63=25pm/V |
| Longitudinal Half-Wave Voltage: | V∏=7.65KV(λ=546nm) | V∏=2.98KV(λ=546nm) |
| Absorptance: | 0.07/cm | 0.006/cm |
| Optical Damage Threshold: | >5 GW/cm 2 | >3 GW/cm 2 |
| Extinction Ratio: | 30dB | 30dB |
| Sellmeier Equations of KDP: | ||
|
no2= 2.259276+0.01008956/(λ2-0.012942625)+ 13.005522λ2/(λ2-400) ne2= 2.132668+0.008637494/(λ2-0.012281043)+ 3.2279924λ2/(λ2- 400) | ||
| Sellmeier Equations of DKDP | ||
|
no2=1.9575544+0.2901391λ2/(λ2- 0.0281399)-0.02824391λ2+0.004977826λ4 ne2=1.5005779+0.6276034λ2/(λ2- 0.0131558)-0.01054063λ2+0.002243821λ4 | ||