Application Cases

Research Direction:
The material used in the temperature-tunable multi-wavelength filter studied in this paper is periodically poled lithium niobate (PPLN). In recent years, in-depth research on quasi-phase-matching materials with periodic domain inversion structures (also known as optical superlattices), such as PPLN and periodically poled lithium tantalate (PPLT), has led to the development of various techniques for achieving periodic domain structures. These include crystal growth technology, electron beam scanning, impurity ion diffusion, and the room-temperature pulsed external electric field method. Among these, the room-temperature pulsed external electric field method stands out for its simplicity and practicality. Additionally, this method enables the transition from periodic to aperiodic domain structures.

Lithium niobate crystals are ferroelectric crystals, and several techniques have been developed to achieve periodic polarization inversion structures on them. These include Ti diffusion from the positive domain surface of LiNbO₃, Li ion out-diffusion, SiO₂ coating followed by heat treatment, and domain inversion induced by proton exchange on the positive domain surface of LiNbO₃ crystals at 230°C. Another method is ferroelectric domain inversion via external electric field polarization at room temperature [12]. The first few techniques typically result in domain inversion only in shallow triangular regions near the crystal surface, and their processes require specific temperatures. In contrast, the room-temperature electric field polarization method for ferroelectric domain inversion was first proposed in 1991 and successfully implemented in the laboratory in 1993. With the application of integrated optical processes and lithography technology, the preparation and application of electric field-poled PPLN have advanced rapidly over the past decade.

Experimental Objective:
To study the optoelectronic properties of lithium niobate under periodic polarization induced by an external electric field, providing a foundation and preliminary verification for subsequent experiments.

Test Equipment:
ATA-2161 high-voltage amplifier, signal generator, oscilloscope, light source, tunable resonant light source, high-speed response power meter, spectrum analyzer, etc.

Experimental Process:
A pulsed signal with an output voltage of 2 Vpp and a frequency of 40 kHz was generated by the signal generator. The ATA-2161 high-voltage amplifier was used to adjust the amplification factor, producing an output of 800 Vpp, which was applied across the crystal. The polarization direction of the lithium niobate crystal was altered, and its optoelectronic properties were studied. The experimental block diagram is shown in Figure 1.

Figure 1: Block Diagram of the PPLN Experiment Using the Pulsed External Electric Field Method

Experimental Results:
Typically, for periodically electric field-poled lithium niobate crystals, the polarization direction of the domains (i.e., the optical axis direction) is as shown in Figure 2-1. Before polarization, the ferroelectric domains of the crystal are all aligned in the +Z direction. After polarization, in the PPLN crystal, the polarization direction in the positive domains remains along the +Z direction, while in the negative domains, it aligns along the -Z direction, as shown in Figure 2-2. The polarization directions of the two are exactly 180° opposite. However, this structure does not affect the optical axis direction of the crystal. The optical axis directions of both positive and negative domains should remain along the Z direction. Only when an electric field is applied in the Y direction will the optical axes of the positive and negative domains deflect in different directions, thereby producing the Solc filtering effect.

Figure 2-1: Schematic Diagram of Polarization Direction Before Electric Field Poling

Figure 2-2: Schematic Diagram of Polarization Direction After Pulsed External Electric Field Poling

Recommended Voltage Amplifier: ATA-2161

Figure: ATA-2161 High-Voltage Amplifier Specifications

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