定义：利用声光效应控制衍射光的光学调制器。

声光调制器是利用电子驱动信号可以用来控制激光光束的功率，频率或者其空间方向的器件。它利用声光效应，即通过声波机械振荡压力改变折射率。 

 图1：非共振声光调制器的示意图。转换器产生声波，该处光波部分被衍射。这里衍射角被放大
AOM的关键元件是一块透明晶体（或一块玻璃），光在其中传播。与晶体接触的压电转换器用来激发声波，声波的频率在100 MHz量级。光在周期性折射率光栅中传播受到布拉格衍射产生声波，因此AOMs有时也称为布拉格盒。衍射光的频率增加或者减小，增加或减小的值等于声波频率（与声波相对于光束的传播方向有关），并且衍射光的方向稍有变化。（方向的变化很小，如图1，因为声波的波数与光播相比非常小。）衍射光的频率和方向可以通过控制声波的频率进行控制，然而声波功率则受制于光功率。当声波功率足够高时，大于50%的光功率被衍射，极限情况下大于95%的光波被衍射。 

声波可能在晶体另一端被吸收。这种行波结构使其可以达到很宽的调制带宽。其他装置是与声波共振的，利用晶体另一端对声波的强反射。共振效应可以显著提高调制深度（或者降低需要的声波功率），但是会减小调制带宽。 

声光调制器常见的材料为二氧化碲（TeO2），石英晶体和熔融二氧化硅。在材料选择方面有很多标准，包括电光系数，透明范围，光损伤阈值和需要的尺寸。也可以采用不同的声波，最常用的是纵波（压缩）。这样可以得到最高的衍射效率，而衍射效率也与光束的偏振有关。当采用声剪切波（声学振动方向与激光光束相同）时，与偏振方向无关，但是这会降低衍射效率。 

还有在一块芯片上包含多个声光调制器的集成光学器件。可以在铌酸锂（LiNbO3）上集成光学器件，由于它是压电的，因此芯片表面的金属电极可以产生表面声波。这种装置有很多用途，例如，用做可调谐光学滤波器或者光学开关。 

应用 
声光调制器具有很多应用： 

• 它可以用做固态激光器中的Q开关。AOM也称为Q开关，在脉冲产生之前用来关闭激光谐振腔。大多数情况下，只有零阶光束满足激光条件，并且当不需要激光时可以将AOM打开。这时需要衍射损耗（每次往返谐振腔产生的损耗）大于激光器增益。对于高增益激光器（例如，光纤激光器），通常是一阶衍射光束满足激光条件，因此当AOM关闭后，会产生非常大的谐振腔损耗。但是，激光态的损耗也相当高。 
• AOM也可以用做固态激光器的倾斜腔，产生纳秒或超短脉冲。后者情况下，AOM的速度只有当谐振腔比较长时才满足要求，或者需要采用电光调制器。 
• 采用AOM调整谐振腔中往返光的谐振波损耗可以实现主动锁模。 
• AOM可以用做脉冲拾取器来降低脉冲列的脉冲重复速率，为了将脉冲进行后续的放大过程得到很高的脉冲能量。 
• 在激光打印机和其它装置中，AOM可以用来调制激光光束的功率。调制可以为连续的或者数字的（开/关）。 
• AOM会使激光光束频率产生偏移，例如用在各种测量器件中，或者用在利用通过频移光学反馈实现锁模的激光器中。 
• 有些情况下需要利用衍射角与声学频率有关的效应。尤其是，可以扫描出射光束的方向（至少扫描小范围的）来改变调制频率。 

声光调制器的重要性质 
实际应用中挑选声光调制器时可以从以下方面考虑： 

• 材料需要在需要的频率范围内具有很高的透明度，并且需要使寄生反射最小化，例如，采用抗反射涂层。 
• 很多情况下，需要很高的衍射效率。例如，将AOM用做高增益激光器的Q开关时很重要，而在倾斜腔中更加重要。 
• 需要的微波功率同时会影响需要的电功率和冷却装置。 
• 开关时间。 
• 用做移频器时，装置需要工作在很大的微波频率范围内。 
• 很高的光峰值功率情况下需要采用合适的材料和大的开放孔径，可以得到较高的损伤阈值。 

由于需要权衡很多参数，不同应用情况下需要采用不同的材料和工作参数。例如，具有最高的衍射效率的材料并不能得到最高的光损伤阈值。大的模式面积可以提高控制功率能力，但是需要采用更大的晶体或者玻璃片还需要更高的驱动功率，同时还会提高开关时间，开关时间受制于声学跃迁时间。对于快速的声光扫描仪，需要大的模式面积实现高的像素分辨率，但是高的扫描速度又需要较小的模式面积。
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Acronym: AOM

Definition: optical modulators based on the acousto-optic effect

An acousto-optic modulator (AOM) is a device which can be used for controlling the transmitted power of a laser beam with an electrical drive signal. It is based on the acousto-optic effect, i.e. the modification of the refractive index of some crystal or glass material by the oscillating mechanical strain of a sound wave (photoelastic effect).

Usually, an AOM is understood to be an intensity modulator; other acousto-optic devices are suitable for shifting the optical frequency (→ acousto-optic frequency shifter) or the spatial direction (acousto-optic deflectors).

The key element of an AOM is a transparent crystal (or piece of glass) through which the light propagates. A piezoelectric transducer attached to the crystal obtains a strong oscillating electrical signal from an RF driver (often via an impedance matching device). The piezo transducer excites a sound wave with a frequency of the order of 100 MHz and with an acoustic wavelength which is typically between 10 μm and 100 μm and an acoustic power e.g. of the order of 1 W to 10 W. The intense sound wave generates a traveling strain wave in the material. Through the photo-elastic effect, that leads to a traveling refractive index grating, at which light can experience Bragg diffraction; therefore, AOMs are sometimes called Bragg cells.

Figure 1: Schematic setup of a non-resonant acousto-optic modulator. A transducer generates a sound wave, at which a light beam is partially diffracted. The diffraction angle is exaggerated; it is normally only of the order of 1°.

For a very short interaction length in the modulator, one would operate in the Raman–Nath regime, where multiple diffraction orders are obtained. However, most AOMs operate in the Bragg regime, where there is a substantial diffraction efficiency for the first diffraction order and hardly any diffraction into other orders.

The optical frequency of the diffracted beam is increased or decreased by the frequency of the sound wave (depending on the propagation direction of the acoustic wave relative to the beam) and propagates in a slightly different direction. (The change in direction is smaller than shown in Figure 1, because the wavenumber of the sound wave is very small compared with that of the light beam.) The frequency and direction of the diffracted beam depend on the frequency of the sound wave, whereas the acoustic power is the control for the diffracted optical power. For most applications, the slight change of optical frequency is irrelevant.

The diffraction process may or may not be polarization-dependent, depending on the device designs (use of longitudinal or shear waves, isotropic or anisotropic material etc.). Further, the output polarization is the same as the input polarization for devices with an isotropic interaction, while for anisotropic modulators it is different, and these devices then work only for the correct input polarization. For AOMs, the use of longitudinal (compression) waves is most common, where the diffraction efficiency is strongly polarization-dependent. Polarization-independent operation can be obtained when using acoustic shear waves with the acoustic movement in the direction of the laser beam. As shear waves are typically slower, one then also obtains a larger Bragg angle, i.e., a better separation of the diffrated beam, which may be advantageous.

Typically, an AOM is placed in a small box, having two holes or optical windows on opposite sides for the laser beam going through, and a connector for the RF driver. Sometimes that box is placed on a rotating table for precise rotational adjustment.

Diffraction Efficiency and Contrast Ratio

For small acoustic powers, the diffraction efficiency is proportional to the acoustic power; for higher powers, it saturates. For sufficiently high acoustic power, more than 50% of the optical power can be diffracted – in extreme cases, even more than 95% diffraction efficiency is achieved. High diffraction efficiencies are easier to achieve for short optical wavelengths.

The contrast ratio is defined as the ratio of maximum and minimum transmitted power. The latter may be limited by scattering. For the diffracted beam, the contrast ratio can be quite high (order of 1000), but the maximum transmission is then limited by the diffraction efficiency. A high maximum throughput is obtained for the non-diffracted (zero-order) beam, but in that case the contrast ratio is much lower.

Traveling-wave and Resonant Designs

The acoustic wave may be absorbed at the other end of the crystal (which is often cut at some angle to avoid standing-wave effects due to residual reflections). Such a traveling-wave geometry makes it possible to achieve a broad modulation bandwidth of many megahertz; it is ultimately limited by the single-pass propagation time of the acoustic wave through the region of the light beam. Other devices are resonant for the sound wave, exploiting the strong reflection of the acoustic wave at the other end of the crystal. The resonant enhancement can greatly increase the modulation strength (or decrease the required acoustic power), but strongly reduces the modulation bandwidth.

Used Acousto-optic Materials

Common materials for acousto-optic devices are tellurium dioxide (TeO₂), crystalline quartz and fused silica; one also uses chalcogenide glasses (often flint glasses), indium phosphide and germanium – the latter two for infrared applications. For high frequency signal processing devices, materials like lithium niobate and gallium phosphide can be used. There are manifold criteria for the choice of the material, including the elasto-optic coefficients (there are actually different acousto-optic figure-of-merit values), the acoustic attenuation coefficient, the sound velocity, the transparency range, the optical damage threshold, and the required size.

Fiber-coupled and Integrated-optical AOMs

Although most AOMs are bulk devices, there are also compact fiber-coupled versions (fiber-pigtailed AOMs). Light from the input fiber is first collimated, then sent through the modulator crystal and finally focused into the output fiber. The insertion loss is typically around 3 dB.

There are also integrated-optical devices containing one or more acousto-optic modulators on a chip. This is possible, e.g., with integrated optics on lithium niobate (LiNbO₃), as that material is piezoelectric, so that a surface-acoustic wave can be generated via metallic electrodes on the chip surface. Such devices can be used in many ways, e.g. as tunable optical filters or optical switches.

RF Drivers for AOMs

If an acousto-optic modulator is used as an amplitude modulator or an active Q-switch, the used electronic driver is usually a device operating with a fixed modulation frequency but a variable amplitude. The amplitude is often controlled with a analog input voltage or with a digital input signal (for on/off modulation).

The required RF drive power is substantial (sometimes several watts), and particularly for long optical wavelengths often not high enough to achieve a high diffraction efficiency.

Important Properties of Acousto-optic Modulators

Various aspects can be essential for the selection of an acousto-optic modulator for some application:

• The material should have a high transparency at the relevant wavelengths, and parasitic reflections should be minimized e.g. with anti-reflection coatings.
• In many cases, a high diffraction efficiency is important. For example, this matters when using the AOM as a Q switch in a high-gain laser, and even more so for cavity dumping. The required RF power influences both the electric power demands and cooling issues. It is lower for acousto-optic materials with high elasto-optic coefficients.
• Depending on the device design, the diffraction efficiency can be polarization dependent.
• For intracavity laser applications like Q switching and mode locking, and particularly for high-power applications, AOMs with low parasitic absorption are required, possibly also a high damage threshold for laser pulses. Large aperture are often required for high power levels. For applications concerning ultrashort pulses, chromatic dispersion and optical nonlinearities can be important.
• The input aperture size limited the usable beam radius. AOMs for large beams are more expensive (because more of the expensive crystal material is required), and tentatively they are slower (see below) and need more RF power.
• The switching time is critical for some applications (e.g. Q switching and particularly cavity dumping). It is limited by the finite velocity of sound in the acousto-optic medium. This implies that an AOM switching a laser beam with large diameter is necessarily slow. One may operate such a modulator with a focused laser beam of reduced diameter, but the diffraction efficiency may decrease due to the increased beam divergence.

Note that the diffraction efficiency depends nonlinearly on the acoustic drive power. For an effectively linear response, a suitable pre-distortion of the drive signal is required.

For acousto-optic frequency shifters and acousto-optic deflectors, other aspects can come into play. For example, a low velocity of sound is advantageous for achieving a wide range of beam angles.

Due to various trade-offs, quite different materials and operation parameters are used in different applications. For example, the materials with highest diffraction efficiencies are not those with the highest optical damage threshold. A large mode area can increase the power handling capability, but requires the use of a larger crystal or glass piece and a higher drive power, and also increases the switching time, which is limited by the acoustic transit time. For fast acousto-optic beam scanners, a large mode area is required for achieving a high pixel resolution, whereas a smaller mode area is required for a high scanning speed.

Applications of AOMs

Acousto-optic modulators find many applications:

• They are used for Q switching of solid-state lasers. The AOM, here more specifically called an acousto-optic Q switch, then serves to block the laser resonator before the pulse is generated. In most cases, the zero-order (not diffracted) beam is used under lasing conditions, and the AOM is turned on when lasing should be prohibited. This requires that the caused diffraction losses (possibly for two passes per resonator round trip) are higher than the laser gain. For more details, see the article on acousto-optic Q switches.
• AOMs can also be used for cavity dumping of solid-state lasers, generating either nanosecond or ultrashort pulses. In the latter case, the speed of an AOM is sufficient only in the case of a relatively long laser resonator; an electro-optic modulator may otherwise be required.
• Active mode locking is often performed with an AOM for modulating the resonator losses at the round-trip frequency or a multiple thereof.
• An AOM can be used as a pulse picker for reducing the pulse repetition rate of a pulse train, e.g. in order to allow for subsequent amplification of pulses to high pulse energies.
• In laser printers and other devices, an AOM can be used for modulating the power of a laser beam. The modulation may be continuous or digital (on/off).
• In a noise eater device, the diffraction losses may be controlled with a feedback circuit such that the transmitted power has induced intensity noise.
• AOMs can be used as external modulators in certain laser communications systems.

Other Acousto-optic Devices

Other acousto-optic devices are based on the same kind of setup, but not used for power modulation, and therefore normally not called acousto-optic modulators:

• Acousto-optic frequency shifters can be used for shifting the optical frequency, e.g. in optical frequency metrology.
• Acousto-optic deflectors are used for eletrically controllable deflection of laser beams.
• Acousto-optic tunable filters are tunable bandpass filters, e.g. for wavelength tuning of lasers.

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