Basic principle of acoustooptic modulator
Acousto-optic modulator is a device that can control the power, frequency or spatial direction of laser beam by using electronic driving signal. It uses the acoustooptic effect, that is, the refractive index is changed by the mechanical oscillation pressure of the sound wave.
The key component of AOM is a transparent crystal (or glass) in which light travels. The piezoelectric converter in contact with the crystal is used to excite the sound wave, and the frequency of the sound wave is in the order of 100MHz. Light propagates in a periodic index grating and is diffracted by Bragg to produce sound waves, so AOMs are sometimes called Bragg boxes.
The frequency of the scattered light increases or decreases by a value equal to the frequency of the sound wave (related to the propagation direction of the sound wave relative to the beam), and the direction of the scattered light changes slightly. (The change of direction is very small, as shown in the figure, because the wave number of sound waves is very small compared with that of light broadcasting.) The frequency and direction of scattered light can be controlled by controlling the frequency of sound waves, while the power of sound waves is subject to light power. When the sound power is high enough, more than 50% of the light power is diffracted, and more than 95% of the light wave is diffracted in the limit case.
Sound waves may be absorbed at the other end of the crystal. This traveling wave structure enables it to achieve a wide modulation bandwidth. Other devices are resonant with sound waves and use the strong reflection of sound waves at the other end of the crystal. The resonance effect can significantly increase the modulation depth (or reduce the required acoustic power), but will reduce the modulation bandwidth.
The common materials of acoustooptic modulator are tellurium dioxide (TeO2), quartz crystal and fused silica. There are many standards for material selection, including electro-optic coefficient, transparency range, optical damage threshold and required size. Different sound waves can also be used, the most commonly used is longitudinal wave (compression). In this way, the highest diffraction efficiency can be obtained, and the diffraction efficiency is also related to the polarization of the beam. When the acoustic shear wave is used (the acoustic vibration direction is the same as the laser beam), it is independent of the polarization direction, but this will reduce the diffraction efficiency.
There are also integrated optical devices containing multiple acoustooptic modulators on a chip. Optical devices can be integrated on lithium niobate (LiNbO3). Because it is piezoelectric, the metal electrode on the chip surface can generate surface acoustic waves. This device has many uses, for example, as a tunable optical filter or optical switch.
Acousto-optic modulator has many applications:
The acoustooptic modulator (AOM) can be used as an optical shutter (light cycle switch at a set frequency) or a variable attenuator (dynamic control of transmitted light intensity). Under the effect of Bragg diffraction, the product only appears first-order diffraction light. According to the structure type, the acoustooptic modulator of Xinte Optoelectronics can be divided into free-space acoustooptic modulator and fiber-coupled acoustooptic modulator.
AOM can also be used as a tilted cavity of solid-state lasers to generate nanosecond or ultrashort pulses. In the latter case, the speed of AOM can only meet the requirements when the resonant cavity is relatively long, or electro-optic modulator needs to be used. Active mode-locking can be achieved by adjusting the resonant wave loss of the round-trip light in the resonator with AOM. AOM can be used as a pulse pickup to reduce the pulse repetition rate of the pulse train, in order to obtain high pulse energy for the subsequent amplification process of the pulse. In laser printers and other devices, AOM can be used to modulate the power of laser beams. The modulation can be continuous or digital (on/off). AOM can shift the frequency of laser beam, for example, it is used in various measuring devices, or in lasers that realize mode-locking through frequency-shift optical feedback. In some cases, it is necessary to use the effect of diffraction angle related to acoustic frequency. In particular, the modulation frequency can be changed by scanning the direction of the outgoing beam (at least scanning a small range).
An important factor in selecting a modulator is the required speed. This will affect the material selection, modulator design and RF driver to be used. The speed of the modulator is described by the rise time, which determines the speed at which the modulator can respond to the applied RF driver and limits the modulation rate. The rise time is proportional to the time required for the sound wave to pass through the beam, so it is affected by the beam diameter in the modulator.