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  • TeO2 Crystals for AO application
  • TeO2 Crystals for AO application

TeO2 Crystals for AO application

  • High figure of merit(M2=793 x 10^-18S^3/g) 
  • Large refractive index (no=2.274, ne=2.430@633nm) 
  • Large Size Capability
  • Broad optical transmission range: 350-5000nm
  • Applications: AOM (Acousto-Optic Modulators), AODF (Acousto-Optic Deflectors), AOTF (acousto-optic tunable filters), FCAOM, etc.
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Modules or Types:

Hangzhou Shalom EO provides TeO2 crystals in the following specifications:

  1. 2 inch, 3 inch, and 4 inch TeO2 crystals boules or ingots with inspection polishing
  2. TeO2 crystals blanks
  3. Laser grade polished and coated TeO2 crystals

 

Basic Properties:

Chemical Formula TeO2 Crystal Structure Tetragonal System, Paratellurite Structure,422 point group
Lattice Parameter a=0.4810nm
c=0.7631nm
Mohs Hardness 4.5
Density 5.99 g/cm3 Melting Point 730°C
Sublimation Point 450°C Thermal Expansion Coefficients α11=21x10^-6/°C
α22=α11
α33=5.5x10^-6/°C
Thermal Conductivity 30 +/- 10% uW/cm/°C
Refractive Index At 632.8nm no=2.26
ne=1.41
At 589nm no=2.274
ne=2.430
Transmission Range 350-5000nm Dielectric Constants (at 20°C) Ε11/Ε0=22.9+/-1.0
Ε11/Ε0=24.7+/-1.5
Elastic Constants (at 20°C) C11 5.57+/-0.13x10^11 dyne/cm^2
C12 5.12+/-0.24x10^11 dyne/cm^2
C13 2.18+/-0.20x10^11 dyne/cm^2
C33 10.58+/-0.24x10^11 dyne/cm^2
C44 2.65+/-0.06x10^11 dyne/cm^2
C66 6.59+/-0.15x10^11 dyne/cm^2
S11 11.5X10^-12 cm^2/dyne
S12 -10.48x10^-12 cm^2/dyne
S13 -0.21x10^-12 cm^2/dyne
S33 1.03x10^-12 cm^2/dyne
S44 3.77x10^-12 cm^2/dyne
S66 1.52x10^-12 cm^2/dyne
Temperature Coefficients of the elastic Constants (/°C) Γ11= -3.0x10^-4
Γ33= -3.0x10^-4
Γ44= -1.3x10^-4
Γ66= --3.3x10^-4
Photo-Elastic Coefficients(at 0.6323μm) P11 ~0.0074
P12 =+0.187
P31 =+0.095
P13 =+0.340
P30 =+0.240
P44 ~-0.17
P66 =-0.0463

TeO2 (Tellurium Dioxide) crystal is one of the most prevalent Acousto-Optic (AO) mediums for manufacturing Acousto-Optic modulation components, which could also be alternatively called Bragg Cells (e.g. Acouto-Optic Modulators, Acousto- Optic Tunable Filters, Acousto-Optic Deflectors).

The working principle of AO crystals is that these optically anisotropic crystals, with substantially large photoelastic coefficients, exhibit alternation of their refractive index whereby the crystals could be regarded as sinusoidal gratings that cause the incident light to diffract on its internal structural planes formed by atoms. In an AO modulation system, the AO crystal is equipped with an RF driver, an acoustic transducer that transfers the input electricity into mechanical oscillations (the transducer could be obtained using piezoelectric crystals such as LiNbO3, which is also available in Shalom EO), and an acoustic wave absorber at the end of the AO crystal media to ensure unhindered light propagation by preventing the formation of stationary waves (or a resonant device for operation mode utilizing stationary waves). 

There are normally two kinds of diffraction regimes, the Raman-Nath-Regime, where one incident light beam is diffracted into several orders, and the Bragg-Regime, where one light beam is refracted into only one order. Bragg regime is much more common than the Raman-Nath regime in applications of AOM since its modulation bandwidth is much wider and its diffraction is much more efficient than the latter. 

The Bragg Regime only takes place when mechanical wavelengths are relatively shorter than in the case of the Raman-Nath-Regime, and when light is incident upon the acoustic wavefronts at a certain angle called the Bragg Angle. In this situation, both the amplitude and phase of the light wave are modulated, and multiple orders of diffracted light interfere with each other, leaving only 0 order diffraction and 1 or -1 order diffraction. Numerically, the Bragg Angle could be formulated as: 


θ=λfa/Va=2θb


Tellurium Dioxide crystal is a colorless, transparent crystal with versatility in the Acousto-Optic and Piezo-Electric realms. The family of TeO2 crystals could be classified into three structure kinds, currently only Gamma TeO2 of the Paratellurite structure could be artificially grown. The grown TeO2 crystal exhibits prevailing optical, acoustic, and piezoelectrical properties. The impressively high figure of merits of TeO2 crystals contributes to wide optical bandwidth from the Visible spectrum to the Near IR spectrum, and optimized diffraction efficiencies to yield the maximum extinction ratio possible. The Acousto-Optic Modulators made of TeO2 crystals could have a resolution that is several times the magnitude of the AOM made from other AO materials given that the apertures are the same. Other advantages of TeO2 include fast rise time, slow shear wave propagation in the <110> orientation and moderate driver power requirements. 


Operation Modes of TeO2 Crystals:

There are mainly three operation modes of the Acousto-Optic Modulators developed on the basis of TeO2 crystals: The first is the slow shear mode propagating in the [110] direction and making displacements in the [110] direction, the advantage of using this mode is low acoustic attenuation, and tremendous figure of merit relative to this orientation (M2=n^6P^2/ρv^3=793x10^-18 S^3/g). The second, longitudinal wave in the [001] orientation, referring to the modulation bandwidth, the figure of merit is relatively high (M1=n^7P^2/ρv=142x10^-7cm^2·S/g). The third is where the shear wave propagates in the [001] plane, with an angle of 35.9° to the x-axis, and making displacements in the [001 ]direction. The character of this mode is zero temperature sensitivity and a rather high figure of merit (M=200x10^-18 S^3/g). It is also worth noting that acoustic attenuation of sound waves traveling in the [110] direction is comparatively higher than the sound waves traveling in the [001] direction. More detailed data are given in the tables below.

Acoustic Velocities of TeO2 Crystals for Various Operation Modes (Measured at 20℃):

Operation Mode

Direction

Corresponding Elastic Stiffness Constants (Cij)

Acoustic Velocity

(10^5cm/s)

Propagation

Displacement

Longitudinal

[100]

-

C11

3.051+/-0.012

Shear

[100]

[010]

C66

3.317+/-0.009

Longitudinal

[001]

-

C33

4.202+/-0.01

Shear

[001]

Random

C44

2.104+/-0.006

Shear

[110]

[110]

(C11-C12)/2

0.616+/-0.003

Shear

[101]

[101]

C

2.101+/-0.006

Note: C={(C11+C33+C44)-[(C11-C33)^2+4(C13+C44)^2]^1/2}/4


The Relationship between the Propagation of the Acoustic Wave and the Acoustic Attenuation:

Operation Mode

Propagation Direction

Velocity

(10^5cm/s)

Temperature Coefficients

(10^-6/℃)

Acoustic Attenuation

(dB/cm)

S

[110]

0.62

211

6(100MHz)

L

[001]

4.20

-117

2.5(500MHz)

S

35.9° to the x-axis

0.93

0

~5


Applications:

AOM (Acousto-Optic Modulators)

An Acousto-Optic Modulator (AOM) allows laser beam shuttering and various kinds of light modulations including Q-switching of Nd;YAG lasers, cavity dumping of Ar lasers, or Mode lockers, pulse pickers, laser Doppler velocimetry,  LIDAR, etc.

AODF (Acousto-Optic Deflectors)

An Acousto-Optic Deflector is a component for precise spatial control of the output light, or continuous/random light scanning within a certain angle. In fact, an AOM and an AOD are intrinsically the same. If one only alters the power output of the RF driver, while maintaining the wavelength of the mechanical wave, so that the position of the diffracted light beams remain the same but their intensity is modulated, in this situation the component is called an AOM. In contrast, if one only alters the wavelength of the mechanical wave, so that the intensity of the output light is not affected but causes changes to the directions of the diffracted light, in this situation the component is called an AOD. Therefore, an acousto-optic modulator could equally function as an acousto-optic modulator simply by altering the way one manipulates the RF driver.

For the acousto-optic deflectors, the main concerns include resolution(both static and dynamic), access time, etc. Materials with a large refractive index and slow mechanical wave propagation velocity are generally more advantageous. TeO2 crystal is the most ubiquitous material for AODs, featuring a quite decent large size capability and exceptional figure of merit which contribute to a high resolution. 

AOTF (Acousto-Optic Tunable Filters) and PCAOM (Poly-Chromatic Acousto-Optic Modulators) AOTF and PCAOM for light show systems

Because the Bragg Angle varies with the change in wavelength, an AOM could be utilized to select light of certain wavelengths from a light source of a significant spectrum. Such a component is called an AOTF or a PCAOM. TeO2 operating in the shear mode is an excellent crystal material for manufacturing AOTF. The large figure of merit of Tellurium Dioxide reduces the obligatory power supply to the RF driver, with the design of a compensation prism to reduce the separation angle. 

FCAOM (Fiber-Coupled Acousto-Optic Modulator) for telecommunications

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