Sound Filters Light

Russian researchers have developed a small, smart and tolerant to vibrations spectrometer, which is equally reliable in the outer space and in oceanic depths. The development was performed with financial support from the Russian Foundation for Basic Research (RFBR) and the Foundation for Assistance to Small Innovative Enterprises (FASIE). The unique device is based on a completely new principle: the light goes through an acoustooptical filter in the device.

Specialists of the Scientific & Technical Center for Unique Instrument-Making produce unique devices with enviable regularity. The new spectrometer developed with support the RFBR and FASIE by a group under the direction of Vladislav Pustovoit, academy member, is also unique. Such spectrometer would not have to spend precious seconds to switch over from, for example, one wave-length to another – this is inevitable evil of similar classical devices for measuring light intensity at different wave-lengths. The device is tolerant to shaking and takes up little space, therefore it can be taken on board the spaceship, let alone a small search aircraft. The device can be even carried in a pocket.

Such a device due to its compactness and high sensitivity can be applied in multiple areas: from industrial processes control through biomedical applications. For example, the device may be useful for ecologists. It allows to determine instantly what unscrupulous enterprises contaminate water and air with. The device can do that in complicated conditions – at a distance, upon quick reciprocal movement of the carrier and the object. Thus, flying over the sea, the pollution source may be quickly found by reflected light. The device would help to find the pipe from which poisonous drainage gets to the sea and to determine what contaminating agents are pored into the sea from that pipe. This can be done without water sampling and other routine analysis.

The device action is based on a new principle – phenomenon of light diffraction on acoustic waves in crystals. It is known that the crystal refraction index is a constant value for each specific transparent medium. However, if a sound-wave acts upon such transparent medium, then the refraction index will slightly change. If a sound-wave diffuses in the crystal, then occurs the space-periodic structure of the medium refraction index alteration, and then the fireballs incident from the outside will diffract on that structure. Diffraction efficiency depends on the sound-wave amplitude. Consequently, arousing a sound-wave in the crystal can manage its optical properties. The spectrometer action is based on this principle.

The critical part of any spectrometer is the optical filter or gitter, which singles out the light of a certain wave-length from the entire radiation spectrum. In the majority of cases, complicated mechanical tunable devices are used for this purpose. Time is always required to switch the device over from one wave-length to another, retuning may take as long as several dozens of seconds.

The device developed by the Scientific & Technical Center for Unique Instrument-Making (Russain Academy of Sciences) does not suffer from sluggishness. Electric signal instantly makes the piezo-crystal plate vibrate at intended frequency, thus arousing a sound-wave in the crystal, and electric signal’s frequency alteration immediately results in alteration of the acoustic wave period. That also results in a quick alteration of the crystal’s optical properties. Thus, a tunable optical filter (the so-called acousto-optical filter) is in place, it is easy to manage and retune.

One more advantage of the optoacoustic spectrometer is high resolution and the ability to register very weak radiation. As such spectrometer has no mechanically moving parts, it is not afraid of vibrations and shaking. Besides, it is much lighter and smaller than ordinary spectrometers. So, it can really be taken on board the spaceship to investigate, for example, the spectrum of sunlight reflected from the ocean surface or the spectrum of any other radiation.