3 The main application of lithium tantalate crystal

3.2 Oscillator

An oscillator is an energy conversion device that converts DC power into AC power with a certain frequency. This circuit is called an oscillation circuit. The oscillator achieves free oscillation through the mutual conversion between magnetic field energy and electric field energy.

Oscillators are divided into RC oscillators, LC oscillators and crystal oscillators. The crystal oscillator has a piezoelectric effect, and the crystal will deform when a voltage is applied to the two poles of the wafer. When the chip is deformed by external force, a voltage will be generated on the metal piece. Crystal oscillators are widely used in communication radio stations, GPS, satellite communications, remote control mobile devices, mobile phone transmitters and high-end frequency counters because of their highly stable frequency AC signals. Crystal oscillators usually use crystals that can convert electrical energy and mechanical energy into each other to provide stable and accurate single-frequency oscillation. Most of the wafers currently used are quartz semiconductor materials. Quartz has a small temperature coefficient and good temperature stability. However, quartz has low electromechanical coupling coefficient, so it is difficult for filters made of quartz materials to achieve high frequencies and large bandwidths. In order to improve the chip type, miniaturization and high frequency of oscillators, many experts have used tantalate in recent years. Lithium wafers were used to the research work of oscillators, and the device performance was good.

Wang Yan et al. designed a low phase noise voltage-controlled oscillator based on lithium tantalate crystal. They analyzed and calculated the loaded quality factor (QL) of the oscillator, and conducted experiments on the phase noise. They found that the tuning range of this oscillator is wider than that of the bulk acoustic wave oscillator. The voltage frequency tuning of the this oscillator was realized by adding voltage-controlled components. A lithium tantalate voltage-controlled oscillator was designed and its performance was tested. Its phase noise level was -85 dBc/Hz @10 Hz, -145 dBc/Hz @ 1 kHz, and the added voltage control component can realize voltage frequency tuning. Test results show that this lithium tantalate oscillator can achieve low phase noise characteristics over a wide tuning range. Sun Bo et al. started from the principle of stimulated electromagnetic coupler scattering and studied the operating characteristics of a terahertz wave parametric oscillator composed of lithium tantalate crystal. They found that during the stimulated scattering process, the inherent characteristics of the symmetry lattice vibration mode will limit the operating performance of the terahertz wave parametric oscillator of the lithium tantalate crystal in terms of frequency tuning, gain, etc. However, since the lithium tantalate crystal itself has good nonlinear optical properties, it is still possible to fully realize the high-performance operation of the lithium tantalate crystal terahertz wave parametric oscillator by methods such as wavelength pumping light, increasing pumping energy, and shortening the resonant cavity length of the terahertz wave parametric oscillator. So lithium tantalate crystal is a good working medium of terahertz wave parametric oscillator. Sukeert et al. designed an oscillator based on periodically polarized MgO-doped homogeneous lithium tantalate (MgO:LT, www.wisoptic.com), which provides continuous tuning at room temperature through simple mechanical translation of the crystal. For a 29 mm long crystal, the oscillator provides an average idle power of 131 mW at wavelength 1476.5 nm with input power 1.8 W, repetition frequency 25 kHz, and slope efficiency 11.3%. Bulk damage is observed in the periodically polarized MgO doped lithium tantalate crystal when the power is greater than 1.8 W  at long-term operation.