Application Cases

Experiment Title: Linear Electro-Optic Effect of Orthorhombic Potassium Tantalate Niobate (KTN) Crystals

Testing Equipment: High Voltage Amplifier, He-Ne Laser, Photodetector, Lock-in Amplifier, Computer, etc.

Experiment Process:

Figure 1: Mach-Zehnder Double-Beam Interference Measurement System

The basic principle of the double-beam interference method is shown in Figure 1. A He-Ne laser is used as the light source. The laser beam is split into signal and reference beams by the beam splitter BS1. The signal beam is focused through the sample using a lens. The presence of the sample introduces an additional optical path length to the signal beam, creating a path difference between the two beams, which results in interference at BS2. The interference fringes are magnified by a lens and detected by a photodetector, which converts the optical signal into an electrical signal and transmits it to a lock-in amplifier. The signal is modulated and output by the lock-in amplifier, then amplified by the high voltage amplifier and applied to the crystal. The high voltage amplifier effectively controls the magnitude of the applied electric field. Data acquisition for the entire double-beam interference system is completed by a computer.

To improve the accuracy of the experiment, attenuators are added to the signal and reference beams to equalize their intensities, enhancing the visibility of the interference fringes. Unavoidably, photorefractive effects exist in the material, causing some scattering of the beam as it passes through the crystal, which can broaden the output beam spot. Attenuators are used to weaken the beam before it hits the sample. Additionally, polarizers are introduced into the optical setup to change the polarization state of light in the crystal to measure different tensor elements of the electro-optic coefficient matrix.

In the experiment, a He-Ne laser with an output wavelength of 632.8nm is used. The frequency of the applied sinusoidal electric field is adjusted by the lock-in amplifier, and a high voltage is applied to the KTN single crystal using the high voltage amplifier. The distance between the crystal electrodes is d = 2.17mm, and the length in the light transmission direction is l = 0.79mm. To improve the quality of the light beam, convex lenses with a focal length of 15cm are placed before and after the crystal sample in the signal beam path to reduce the diameter of the light spot on the sample, thereby minimizing the scattering of the transmitted beam.

Figure 2: Schematic Diagram of Experimental Conditions for Orthorhombic Crystals

Experimental Results:

After polarization along the [011] direction, the orthorhombic KTN crystal has its x1 direction along [011]c, x2 direction along [100]c, and x3 direction along [011]c, as shown in Figure 2. During the experiment, the applied electric field is in the [011]c direction, and the light transmission direction is also along [011]c. When the incident light is horizontally polarized, the experimentally measured γ13 = 1.04×10^-11 m/V. When the incident light is vertically polarized, γ33 = 7.80×10^-11 m/V is calculated.

Due to the presence of sub-micron ferroelectric domain structures within the orthorhombic KTN crystal, the small domain size can enhance the electro-polarization, thereby strengthening the electro-optic effect of the orthorhombic KTN ferroelectric single crystal. Therefore, the orthorhombic KTN crystal exhibits excellent electro-optic effects.

High Voltage Amplifier Recommendation: ATA-2161

Figure: Specifications of the ATA-2161 High Voltage Amplifier

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