Controlling light with sound
Acousto-optic methods use sound to quickly and precisely control the intensity and deflection angle of laser beams, isolate individual laser pulses, or filter specific wavelengths from broadband light sources. Applications range from microscopy and spectroscopy to micro-material processing in the semiconductor and electronics industries, and even quantum technologies.
At the heart of the system is an acousto-optic crystal. This crystal is excited at high frequencies using a piezoelectric transducer, which converts electrical signals into vibrations. Within the now-vibrating crystal, through which sound waves travel, a diffraction grating forms as a result of the so-called photoelastic effect, diffracting incident light extremely quickly and precisely. The angle of this diffraction can be controlled via the frequency and amplitude of the piezoelectrically induced sound waves. Since no mechanical components or moving masses are involved, the beam deflection takes barely more than a microsecond—with angular accuracies of a few milliradians (mrad). A key advantage of this deflection of light waves using sound waves is also its high dynamics: if the excitation frequency changes, the diffraction grating also changes. Users of acousto-optic deflectors (AODs) can control the direction of the laser beam and the position of the spot electronically with precision and nearly in real time.
The interaction of sound and light waves—and the resulting precision in beam deflection—is in demand wherever laser systems process and inspect tiny structures. In the manufacturing of smartphones, displays, and wafers; in photolithography; in high-resolution microscopy and spectroscopy; and increasingly also to position and control neutral atoms or ions in quantum computers. Here, AODs are used, among other things, to align laser beams split into dozens of beams—which serve as optical tweezers—with micrometer precision and to control them individually. And in the laser drilling of so-called “microvias” in chip and display substrates, AODs align the laser spots with micrometer precision. The LASER exhibitor Gooch & Housego (UK) Ltd. advertises that its AODs typically achieve over 80 percent diffraction efficiency across a scan angle of 30 mrad. Thanks to a high degree of vertical integration and decades of accumulated know-how, the company also supplies AODs tailored to specific customer requirements. Numerous OEM electronics manufacturers use these in their laser micromachining applications.
Acousto-optical modulation, filtering, and switching
In addition to Gooch & Housego, other Laser World of Photonics exhibitors are also supplying these products, including ISOMET UK Ltd., also based in the UK, and its German partner Acal BFi Germany AOD. That said, beam deflection is just one application of acousto-optics. The interaction of sound and light waves also forms the basis for acousto-optic modulators, Q-switches, filters, and frequency shifters. Sometimes they are used to modulate the intensity and frequency of the laser. Other times, they “pick out” only individual pulses from a series of short pulses. For this purpose, acousto-optic switches are used, which, at high frequencies and with the most precise timing, allow only the required pulses to pass through while blocking all others. Acousto-optically tunable filters (AOTFs), on the other hand, filter out individual wavelengths from broadband light sources—here, too, the key lies in the high-frequency excitation of sound waves. In addition to these solutions, Pegasus Optik GmbH also offers fiber-coupled AOMs. These can be used either as frequency shifters or as so-called Q-switches, which allow losses in resonators to be actively controlled. According to the manufacturer, the applied RF signal typically serves to deflect part of the energy flow out of the resonator. In doing so, losses would be significantly increased to prevent the laser from oscillating. As soon as the RF signal is switched off, the losses would drop abruptly, resulting in an intense laser pulse.
AODs where ultra-fast, precise beam steering is required
Unlike passive Bragg gratings, which can be designed for very specific wavelengths, acoustically excited diffraction gratings can be actively controlled. This makes AODs and AOMs particularly interesting for applications where fast, dynamic control of the laser beam is required. Acousto-optic beam steering and shaping is also of interest for material processing with continuous or pulsed lasers, which today often operates at high kilowatt (kW) power levels. This is because it allows laser pulses to be distributed more quickly across the workpiece to prevent local overheating. AODs are also an interesting alternative to galvanometer or piezo scanners for multibeam concepts in which a high-power beam is split into many individual beams for parallel processing. However, this applies more to applications where the size of the scan area is less important than ultra-fast and highly precise beam guidance. When the latter is required, AODs offer a solution with the highest precision and resolution, combined with minimal maintenance requirements and maximum service life.
For a more in-depth exploration of the topic, Wavelength Opto-Electronic in Singapore offers a very clear and accessible overview article online “What are Acousto-Optic Deflectors? All About AODs Explained”.