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Current location: Lenses&Mirrors
| Lithium niobate is a ferroelectric material suitable for a variety of applications. Its versatility is made possible by the excellent electro-optic, nonlinear, and piezoelectric properties of the intrinsic material. It is one of the most thoroughly characterized electro-optic materials, and crystal growing techniques consistently produce large crystals of high perfection.
Applications that utilize the large electro-optic coefficients of lithium niobate are optical modulation and Q-switching of infrared wavelengths. Because the crystal is nonhygroscopic and has a low half-wave voltage, it is often the material of choice for Q-switches in military applications. The crystal can be operated in a Q-switch configuration with zero residual birefringence and with an electric field that is transverse to the direction of light propagation. Because piezoelectric ringing can be severe, piezoelectrically damped designs can be very useful. The damage threshold of the intrinsic material at 1.06 microns with a 10 nsec pulse is approximately 3 J/cm2. With appropriate AR coatings, a surface damage threshold of 300-500 MW/cm2 can be achieved for the same conditions.
Applications that use the large nonlinear d coefficient of LiNbO3 include optical parametric oscillaton, difference frequency mixing to generate tunable infrared wavelengths, and second harmonic generation. With a broad spectral transmission, which ranges from 0.4 µm to 5.0 µm with an OH- absorption at 2.87 µm, a large negative birefringence, and a large nonlinear coefficient, phasematching is an effective way to generate tunable wavelengths over a broad wavelength range.
Lithium niobate is particularly effective for second harmonic generation of low power laser diodes in the 1.3 to 1.55 µm range.
For infrared generation by difference frequency mixing, the peak power limit is considerably lower than for 1.064 µm, being about 40 MW/cm2Efficiencies for difference frequency mixing generally are smaller than shg efficicncies with KDP or BBO, which is due to the lower peak powers that can be tolerated by the crystal and the fact that the longer wavelength photons that are generated in the process are less energetic. Typical powers for 10 nanosecond long pulses with 5 mm diameter beams are 30 mJ/pulse of 0.640 µm minus 40 mJ/pulse of 1.064 µm to produce 2.5 mJ/pulse at 1.54 µm, and 32 mJ/pulse of 0.532 µm minus 32 mJ/pulse of 0.640 µm to produce 0.25 mJ/pulse at 3.42 µm.
Basic Properties of LiNbO3:
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Crystal Structure: |
Trigonal, Space group R3C, Point group 3m |
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Cell Parameters: |
a=5.148Å , c=13.863Å |
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Melting Point: |
1253° C |
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Curie Temperature: |
1140° C |
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Mohs Hardness: |
5 |
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Density: |
4.64 g/cm3 |
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Deliquescence |
None |
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Optical Homogeneity |
~ 5x10-5/cm |
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Transparency Range |
420nm-5200nm |
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Absorption Coefficient: |
~0.1%/cm @ 1064nm |
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Refractive indices at 1064nm: |
ne= 2.146, no = 2.220 @ 1300 nm
ne = 2.156, no = 2.232 @ 1064 nm
ne = 2.203, no = 2.286 @ 632.8 nm |
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Thermal Expansion Coef. (@ 25¡ãC) |
//a, 2.0x10-6/K |
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//c, 16.7x10-6/K |
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Thermal Conductivity Coefficient: |
38 W/m/K at 250C |
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Thermal Optical Coefficient: |
dno/dT=-0.874x10-6/K at 1.4m m
dne/dT=39.073x10-6/K at 1.4m m |
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The Sellmeier equations
( l in m m) |
no2 = 4.9048 + 0.11768/(l 2-0.04750)-0.027169l 2
ne2 = 4.5820 + 0.099169/(l 2-0.04443)-0.021950l 2 |
Nonlinear Optical Properties:
| NLO Coefficients |
d33 = 34.4 pm/V
d31 = d15 = 5.95 pm/V
d22 = 3.07 pm/V |
| Efficiency NLO coefficients |
deff =5.7 pm/V or ~14.6 x d36 (KDP) for frequency doubling 1300 nm;
deff =5.3 pm/V or ~13.6 x d36 (KDP) for OPO pumped at 1064 nm;
deff =17.6 pm/V or ~45 x d36 (KDP) for quasi-phase-matched structure. |
| Electro-Optic Coefficients |
gT33 = 32 pm/V, gS33 = 31 pm/V,
gT31 =10 pm/V, gS31=8.6 pm/V,
gT22 = 6.8 pm/V, gS22= 3.4 pm/V, |
| Half-Wave Voltage, DC |
Electrical field ||z, light ^z: |
Electrical field|| x or y, light ||z: |
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3.03 KV |
4.02 KV |
| Damage Threshold |
100 MW/cm2 (10 ns, 1064nm) |
Fe:LiNbO3 and MgO:LiNbO3 crystals is available too. The MgO: LiNbO3 has similar effective nonlinear coefficients to pure LiNbO3. Its Sellmeier equations (for MgO dopant 7 mol%) are:
no2 = 4.8762+ 0.11554/(l 2-0.04674)-0.033119l 2
ne2 = 4.5469+ 0.094779/(l 2-0.04439)-0.026721l 2
LiNbO3’s Specifications
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Material: |
Laser grade LiNbO3 |
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Orientation: |
±0.5 o |
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Dimensional Tolerance: |
±0.1mm |
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Surface quality: |
20/10 Scratch/Dig per MIL-O-13830B |
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Flatness: |
l/8 at 633 nm |
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Perpendicularity: |
5 arc min |
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Parallelism: |
better than 20 arc sec |
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Clear Aperture: |
> Central 90% |
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AR Coating: |
AR coating with R < 0.2% at center wavelength |
Note: The other specification for LiNbO3 crystals are available. |
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