Spectrometers are used for applications such as chemical analysis, remote sensing, environmental measuring, and optical measurements. Most spectrometers utilize prisms and gratings, where the dispersion of light depends upon the angle and rotation of the grating. But gratings and other moving parts are undesirable in spectrometers because they cause unwanted vibrations, they can go out of alignment, and they are difficult to maintain. In addition, the spectral range and resolution of traditional spectrometers are limited.
To combat these disadvantages, electronically tunable optical filters with no moving parts were developed using novel acousto-optic (AO) technology. These acousto-optic tunable filters (AOTF) are being utilized in the design of new spectroscopic instruments as replacements for conventional grating-technology-based optical instruments.
Acousto-optics involves the interaction of sound and light in dielectric material. When sound propagates through a solid or liquid, compressions are created in the material that cause variable refraction of the passing light, pulling color features from it. Acousto-optic tunable filters are solid-state electronically tunable optical filters that select precise wavelengths by applying the appropriate RF frequency. Filters can be used to pass light with either a single wavelength or multiple wavelengths, depending on the RF signal applied. A high-frequency electrical signal is converted into an ultrasonic wave by a piezoelectric oscillator (transducer) bonded to an AO medium. Sound waves travel through the medium and diffract light. Compared to traditional filters, AOTFs offer fast and agile tunability, and narrow spectral bandwidth selection.
Utilizing AOTF technology, scientists at the Army Research Laboratory (ARL) developed acousto-optic-based portable spectrometers for remote sensing of chemical and biological agents. The spectrometers do not have moving parts, are vibration-insensitive with high-frequency selection sensitivity, and will allow the user to select a range of colors in the visible and near-IR spectral regions. The spectrometers have resolution high enough to perform real-time analytical chemical measurements in the field or on a moving platform.
ARL's spectrometers are compact and robust, making them useful for work in the field, as well as in the laboratory. The inventors also developed a spectroscopic accessory that allows for more efficient collection of light, when only a fraction of light can be collected with traditional systems. ARL's spectrometers can be used to carry out both fluorescence and Raman spectrosopic measurements with only minor adjustments to the system, which was previously impossible using only a single system. Whereas highly trained specialists are needed to run experiments with traditional spectrometers, ARL's devices can be operated by nonspecialists.
The AOTF-based spectrometers at ARL can be used for spaceborne environmental monitoring, detection of forest fires, underwater monitoring of gases, water quality monitoring, monitoring of contaminants in the ground, the detection of cancer, drug interdiction, and airport security. They may also be used to perform real-time process control and quality assurance in pharmaceutical manufacturing, paper manufacturing, food processing, and optics and semiconductor manufacturing.