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KDP/DKDP Crystals & Applications

Potassium Dihydrogen Phosphate (KDP) is the first commercially available NLO crystal. The discovery of the KDP crystal dates back to 1935 (Busch and Scherrer). However, it was not until the advent of the laser in 1961 that practical nonlinear optical applications became feasible. In 1962, KDP was employed for SHG of ruby laser to produce UV laser. Subsequently, KDP dominated the field of NLO crystals until the emergence of KTP, BBO, and LBO.

Although KDP crystal is a mature material, it is still quite active in the contemporary nonlinear optical crystal market and is irreplaceable in some fields. This longevity can be attributed to several advantages, including a simple growth method, cost-effectiveness, availability of large sizes, high electro-optic coefficients, and UV tranmission. Notably, the oversized KDP/DKDP crystal remains the sole single crystal material appropriate for applications in high power laser engineering, and it is the only single crystal material accessible for inertial confinement fusion (ICF) engineering.

Natalia Zaytseva, a former Russian scientist, became a member of the Lawrence Livermore National Laboratory (LLNL) in 1993. During her tenure, she introduced advanced rapid growth methodologies for KDP and DKDP crystals. As a result of her innovative technology, the production of KDP crystals measuring up to 100x100x100 cm³ has become feasible recently.

The dvantageous features of KDP:

-Good UV transmission

-High damage threshold

-High electro-optic coefficients

-A simple growth method

-Cost-effectiveness

-Availability of oversized crystals

The disadvantages of KDP:

-Low NLO coefficients

-A susceptibility to deliquescence

KDP/DKDP are still among the most commomly used NLO crystal. KDP crystals are typically employed for the processes of doubling, tripling, and quadrupling the output of a Nd:YAG laser at room temperature.

DKDP crystal

Potassium Dideuterium Phosphate (KD*P or DKDP) is isomorph of KDP. The main advantage of DKDP is its higher electro-optic coefficients. It is widely used as electro-optical modulators, Q-switches and Pockels Cells.

KDP and KD*P are hygroscopic, a sealed housing with transmitting windows is recommended for protection of the crystal surfaces.

KDP/KD*P Physical and Optical Properties

Property\Crystal	KDP	KD*P
Chemical Formula	KH2PO4	KD2PO4
Transparency Range	200-1500nm	200-1600nm
Nonlinear-Coefficients	d36=0.44pm/V	d36=0.40pm/V
Refractive-Index@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 λ/2 Voltage	Vp =7.65KV @546nm	Vp =2.98KV @546nm
Absorptance	0.07/cm	0.006/cm
Extinction Ratio	30dB	30dB
Damage Threshold@1064nm 10ns	>5 GW/cm²	>3 GW/cm²

Sellmeier Equations of KDP (λ in μm):

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 KD*P (λ in μm):

no² = 1.9575544 + 0.2901391/( λ² - 0.0281399) - 0.02824391λ² +0.004977826λ⁴
ne² = 1.5005779 + 0.6276034/( λ² - 0.0131558) - 0.01054063λ² +0.002243821λ⁴

Specifications of KDP/KD*P

Dimension Tolerance:

+0/-0.1mm(WxH) and +0.1/-0.2mm thickness

Surface Quality:

10-5 Scratch-dig

Flatness:

<λ/8 @633nm

Chamfer:

0.1mm@45deg.

Clear Aperture:

Central 90% of Diameter

Parallelism:

<10 arc seconds

Perpendicular:

<10 arc minutes

Orientation Angle:

+/-0.25deg.

Damage Threshold:

0.3GW/cm2@1064nm10ns

Coating:

Dual wavelength AR coating: R<0.2%

i. KDP Crystal

ii. DKDP Crystal

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