acousto-optic modulator drivers (original) (raw)

Acronym: AOM drivers

Definition: electronic drivers for operating acousto-optic modulators

Category: photonic devices

• electronics for photonics
  • modulator drivers
    * Pockels cell drivers
    * acousto-optic modulator drivers

Related: modulator driversacousto-optic modulatorselectronics for photonics

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DOI: 10.61835/mh0 Cite the article: BibTex BibLaTex plain textHTML Link to this page! LinkedIn

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Contents

What are Acousto-optic Modulator Drivers?

An acousto-optic modulator (AOM) is driven by an intense RF wave applied to a piezoelectric transducer. The RF wave must be generated with a suitable driver, which is typically placed outside the actual modulator.

Functional Requirements on AOM Drivers

The functional requirements depend on the device type and application:

• The RF frequency is typically at frequencies of tens to hundreds of MHz, and must match the used AOM. It is often fixed, but it needs to be variable in a substantial range for devices like acousto-optic deflectors (with variable deflection angles) and some acousto-optic frequency shifters and acousto-optic tunable filters.
• The RF power typically ranges from a few hundred milliwatts to several watts.
  • In some cases — for example, for Q-switching of lasers or for pulse picking –, it is constant and can only be switched on or off.
  • In other cases, the RF power can be continuously adjusted in a substantial range via an analog input voltage, or possibly via a digital input. An external arbitrary waveform generator may be connected to the input of a simple AOM driver, or such a device may be integrated into a more sophisticated driver.

Building Blocks of an AOM Driver

An AOM driver consists of several main blocks:

Oscillator

There is a high-frequency oscillator, generating a stable sinusoidal signal with the required frequency.

For a fixed frequency, a crystal oscillator is a common solution. In some cases, however, the oscillator frequency needs to be tunable; one then needs a voltage-controlled oscillator (VCO) or a Direct Digital Synthesizer (DDS). The latter approach provides substantially better frequency stability. Some devices offer FPGA-based arbitrary waveform generation.

Control Interface

A control interface allows one to turn the oscillator on or off (e.g. with a TTL signal), and possibly to tune its frequency or power with an analog input signal. Some devices have an advanced digital interface, e.g. USB or Ethernet.

For analog control, different responses concerning RF power versus signal input are possible:

• The RF amplitude usually scales linearly with the signal input voltage. This implies that the acoustic wave amplitude will be proportional to the signal input voltage.
• The RF power (proportional to the RF amplitude squared) could scale linearly with the signal input voltage. In that case, the diffraction efficiency is proportional to the signal voltage as long as the applied RF power is small — but there is saturation of the diffraction efficiency at higher drive powers. (It is approximately proportional to ($\sin^2(C \sqrt{P_\textrm{RF}})$).)
• Some drivers are designed to provide an exponential response of RF power to the control voltage. This is useful when a wide dynamic range of control is desired, making it easier to fine-tune low-power (and thus low-intensity) output and span several orders of magnitude in output power.

Note that both AOMs and RF amplifiers have nonlinearities — as mentioned already concerning the finally relevant diffraction efficiency. To achieve high-fidelity analog modulation, some driver designs apply signal predistortion so as to compensate for the anticipated nonlinear response of the power amplifier and the AOM crystal, resulting in a more linear overall system response.

Power Amplifier

A power amplifier raises the RF power to the required level. Although it is usually integrated into the driver, there are also stand-alone broadband RF amplifiers, particularly for applications where one drives multiple modulators and/or requires particularly high powers.

There are different power amplifier architectures. Class A amplifiers provide high linearity and low signal distortion, but are relatively inefficient (typically 20–35%), and thus require more electrical power and generate more heat. Higher classes like B to F are optimized for substantially higher efficiency (sometimes >90%), but can have substantially larger signal distortions. Class AB can provide a good compromise between efficiency and linearity.

Some amplifiers are designed for a specific operation frequency, but there are also broadband power amplifiers, usable across wide frequency ranges (often several MHz to several GHz), with flat gain and consistent performance throughout. These are especially valuable for special applications where frequency agility, multi-frequency operation, or frequency-chirped signals are required.

Impedance Matching and RF Cable Connections

RF drivers require impedance matching to the modulator input (typically 50 Ω), as otherwise a substantial part of the power would be reflected back to the amplifier. Amplifier and modulator must be connected using suitable RF cables.

Usually, there is a single RF output for connecting a single modulator, typically with a SMA or BNC connector.

Cooling

Due to the substantial dissipated heat (possibly several watts), a high-power driver is usually mounted on a metal plate for effective conductive cooling. Fan-assisted or even water cooling is less common, as conductive cooling is usually sufficient even for 10 W RF output or more.

Frequently Asked Questions

This FAQ section was generated with AI based on the article content and has been reviewed by the article’s author (RP).

What is an acousto-optic modulator (AOM) driver?

An AOM driver is an electronic device that generates the intense radio frequency (RF) wave required to operate an acousto-optic modulator. It supplies an RF signal at the correct frequency and power level to the modulator's piezoelectric transducer.

What are the key functional parameters of an AOM driver?

The main parameters are the RF frequency, typically tens to hundreds of MHz, and the RF output power, ranging from milliwatts to several watts. Both frequency and power can be fixed, switchable, or continuously variable, depending on the application.

What are the main building blocks of an AOM driver?

An AOM driver typically consists of an oscillator (e.g., a crystal oscillator or DDS) to generate the RF signal, a control interface (e.g., TTL or analog input) to modulate it, and a power amplifier to boost the RF power to the required level.

How can the RF output of an AOM driver be controlled?

The RF output is controlled via a dedicated interface. A digital signal (like TTL) can switch the output on and off. An analog input voltage can continuously vary the RF power, often with a linear or exponential response, to achieve analog intensity modulation.

What is the role of different amplifier classes (e.g., Class A, AB) in AOM drivers?

Different amplifier classes offer trade-offs between linearity and efficiency. Class A amplifiers provide high linearity with low distortion but are inefficient. Higher classes like AB offer a compromise or, like Class B-F, prioritize much higher efficiency at the cost of linearity.

Why is impedance matching important for AOM drivers?

Proper impedance matching between the driver and the AOM (typically 50 Ω) is crucial for efficient power transfer. A mismatch causes RF power to be reflected, which reduces the power delivered to the AOM and can potentially damage the driver's amplifier.

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