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:
|
Crystal
Structure:
|
Trigonal,
Space group R3C, Point group 3m
|
|
Cell
Parameters:
|
a=5.148Å
, c=13.863Å
|
|
Melting
Point:
|
1253°
C
|
|
Curie
Temperature:
|
1140°
C
|
|
Mohs
Hardness:
|
5
|
|
Density:
|
4.64
g/cm3
|
|
Deliquescence
|
None
|
|
Optical
Homogeneity
|
~ 5x10-5/cm
|
|
Transparency
Range
|
420nm-5200nm
|
|
Absorption
Coefficient:
|
~0.1%/cm
@ 1064nm
|
|
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
|
| Thermal
Expansion Coef. (@ 25¡ãC)
|
//a,
2.0x10-6/K
|
| //c,
16.7x10-6/K
|
|
Thermal
Conductivity Coefficient:
|
38
W/m/K at 250C
|
|
Thermal
Optical Coefficient:
|
dno/dT=-0.874x10-6/K
at 1.4m m
dne/dT=39.073x10-6/K at
1.4m m
|
|
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: |
| 3.03 KV
|
4.02 KV
|
| Damage Threshold |
100 MW/cm2
(10 ns, 1064nm) |
he 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
| Material: |
Laser
grade LiNbO3 |
| Orientation: |
±0.5
o |
| Dimensional
Tolerance:
|
±0.1mm |
| Surface
quality:
|
20/10
Scratch/Dig per MIL-O-13830B |
| Flatness:
|
l/8
at 633 nm
|
| Perpendicularity:
|
5
arc min |
| Parallelism: |
better
than 20 arc sec
|
| Clear
Aperture:
|
>
Central 90%
|
| AR
Coating:
|
AR
coating with R < 0.2% at center wavelength |
Note: The other specification
for LiNbO3 crystals are available from CRYSTECH.
