An acousto-optic deflector is an important optical component that works based on the acousto-optic effect, and plays a great role in laser scanning systems due to its excellent performance. The bandwidth is an important performance indicator for acousto-optic deflectors and determines the operating frequency range of these devices. Conventional acousto-optic deflectors of single transducer are simple and easy to fabricate. However, these devices have a limitation in operating bandwidths. The main acoustic beams in the acousto-optic deflectors of single transducer are fixed under changed driving frequency, resulting in a narrow bandwidth. By increasing the number of transducers, the direction of the main acoustic beam can be changed with the frequency to improve the operating bandwidth of the acousto-optic deflector. Meanwhile, other methods should be investigated to further improve the bandwidth of acousto-optic deflectors and maintain high diffraction efficiency.

We propose a method to both increase the bandwidth of the acousto-optic deflector and maintain high diffraction efficiency. In the aspect of the Bragg bandwidth, the acousto-optic deflector of two transducers with delay lines has a phase difference with frequency, and the main acoustic beam can track the Bragg angle to improve the bandwidth. First, the relationship between the incident angle, center distance and tracking frequency in the acousto-optic deflector of two transducers with delay lines can be obtained based on the principle of acoustic beam steering. Then, the Bragg bandwidth model in the acousto-optic deflector of two transducers with delay lines is built based on the theory of Bragg loss. Finally, the acoustic energy utilization in the Bragg bandwidth model of the acousto-optic deflector of two transducers with delay lines is enhanced by adjusting the tracking frequency position, with a reasonable balance between the diffraction efficiency and the bandwidth realized. In the aspect of the transducer bandwidth, the sandwich matching layer model is proposed to increase the transducer bandwidth. First, the matching layer material is selected according to the theory of acoustic impedance matching and the conditions of actual sputtering. Then, based on the theory of transmission lines and Mason’s equivalent circuits, the matching layer can be considered as the transmission matrix to calculate the transducer bandwidth. Finally, considering the adhesive strength of the bonding layers and the thinning work of the piezoelectric layers, the matching layer thickness is controlled within a certain range in our study, and the effect of the variation in the matching layer thickness on the transducer bandwidth is studied. Meanwhile, the bandwidth of the acousto-optic deflector of the single transducer based on the traditional single matching layer, and the bandwidth of acousto-optic deflectors of the single transducer, two transducers, and two transducers with delay lines based on the sandwich matching layers are experimentally tested in our study to further verify the effect of the bandwidth.

Due to the fixed main acoustic beam, when the frequency changes, the acoustic energy in the acousto-optic deflector of the single transducer decreases rapidly under changed frequency [Fig. 7(b)], leading to a narrower 3 dB bandwidth (Fig. 14). The acousto-optic deflector of two transducers yields two energy-symmetric acoustic beams in the acousto-optic crystal due to a fixed phase difference of π [Fig. 2(b)]. Since Bragg diffraction employs only one of the acoustic beams, the acoustic energy utilization in the acousto-optic deflector of two transducers is lower [Fig. 7(c)]. The acoustic energy in the acousto-optic deflector of two transducers with delay lines is not symmetrically distributed. Most of the acoustic energy is concentrated in the +1 order acoustic beams [Figs. 6(d)?(f)], which improves the acoustic energy utilization over the acousto-optic deflector of two transducers [Fig. 7(c)]. In the aspect of the Bragg bandwidth, the traditional Bragg bandwidth is designed for a large 3 dB Bragg bandwidth, but it causes lower acoustic energy utilization [Fig. 7(a)]. In contrast, we improve the acoustic energy utilization in the Bragg bandwidth model of the acousto-optic deflector of two transducers with delay lines [Fig. 7(a)], which allows the acousto-optic deflector of two transducers with delay lines to increase the 3 dB Bragg bandwidth and ensure the acoustic energy utilization [Fig. 7(b)]. In the aspect of the transducer bandwidth, the sandwich matching layers provide a larger 3 dB transducer bandwidth, and it is less affected by the variation in the matching layer thickness [Fig. 9(b)]. Experiments on acousto-optic deflectors have shown that the sandwich matching layers improve the 3 dB bandwidth by 14% over the traditional single matching layers (Fig. 14). While keeping the peak diffraction efficiency, the acousto-optic deflector of two transducers with delay lines improves the 3 dB bandwidth by 25% over the acousto-optic deflectors of single transducer and by 7% over the acousto-optic deflector of two transducers (Fig. 14). As a result, with the peak diffraction efficiency maintained, the acousto-optic deflector of two transducers with delay lines based on the sandwich matching layers improves the 3 dB bandwidth by 43% over that of the single transducer based on the traditional single matching layer (Fig. 14). To increase the bandwidth of the acousto-optic deflector even further, we should design a larger transducer bandwidth to provide a wider frequency range of electro-acoustic energy conversion, with the specific device and the actual process conditions taken into account. Additionally, the effect on the bandwidth of the acousto-optic deflectors should be verified via experiments.

The 3 dB bandwidth of the acousto-optic deflector is improved in the aspects of the Bragg bandwidth and transducer bandwidth. In the aspect of the Bragg bandwidth, we analyze the acoustic beam steering in the acousto-optic deflector of two transducers with delay lines, propose a method that takes the diffraction efficiency and the bandwidth into account, and build a Bragg bandwidth model in the acousto-optic deflector of two transducers with delay lines. In the aspect of the transducer bandwidth, our study designs the sandwich matching layers according to the acoustic impedance matching theory and actual sputtering conditions to improve the transducer bandwidth. Simulations and experiments show that the developed acousto-optic deflector of two transducers with delay lines broadens the operating bandwidth while ensuring peak diffraction efficiency. Compared with the traditional single matching layers, the proposed sandwich matching layers improve the operating bandwidth of the acousto-optic deflector. Finally, the acousto-optic deflector of two transducers with delay lines based on the sandwich matching layers helps yield a significant improvement in the operating bandwidth.