Source: SMART SCI&TECH

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Acousto-Optic Frequency Shifters: Explained

How Do Acousto-Optic Frequency Shifters Work?

Acousto-optic frequency shifters utilize the phenomenon of light diffraction at a traveling refractive index grating in an acousto-optic modulator. This results in a shift in optical frequency, which can be either plus or minus the acoustic frequency. These devices are commonly used to achieve precise frequency shifts in optical signals.

Operating Principles

Frequency shifters operate by applying drive frequencies typically in the tens to hundreds of megahertz range, sometimes up to 1 GHz. For larger frequency shifts, multiple devices can be cascaded. They can be operated with a fixed or variable drive frequency, with considerations for beam direction changes and methods to minimize such effects.

Optical Input and Device Types

Acousto-optic frequency shifters are often used with laser beams from single-frequency lasers. While most devices are bulk, there are also compact fiber-coupled and all-fiber versions available. Fiber-coupled frequency shifters involve collimating light from an input fiber, passing it through a modulator crystal, and focusing it into an output fiber.

RF Drivers for Frequency Shifters

Frequency shifters are typically operated with constant drive power. Drivers may have fixed or variable frequency options, utilizing voltage-controlled oscillators or direct digital drivers for precision and stability.

Performance Considerations

Key performance factors include the frequency range, diffraction efficiency (typically around 50%), and suppression of the non-shifted beam for applications like interferometry. The range of operation wavelengths may be limited by coatings, and alignment is crucial for optimal performance.

Applications of Frequency Shifters

Frequency shifters find applications in optical frequency metrology, laser Doppler vibrometry, laser spectroscopy, and heterodyne detection systems. They enable precise frequency control and scanning without the need for tunable lasers, offering improved laser noise properties for various applications.

Conclusion

Acousto-optic frequency shifters play a crucial role in manipulating optical frequencies for a range of applications in metrology and spectroscopy. Understanding their operating principles and performance considerations is essential for utilizing them effectively in various optical systems.

Source: MDPI
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