Ultra-Narrow Bandwidth Acousto-Optic Filter

Periodic structures created in crystals by an acoustic wave have a number of advantages compared to conventional devices based on diffraction gratings [1, 2]. They allow changing the period of a volume grating, modulating its... [ view full abstract ]

Periodic structures created in crystals by an acoustic wave have a number of advantages compared to conventional devices based on diffraction gratings [1, 2]. They allow changing the period of a volume grating, modulating its characteristics, and high spectral resolution to be achieved.

In the present paper the resonator formed by two spatially-distributed “mirrors” is studied theoretically. The influence of different apodization functions on diffraction curves of reflection and transmission is studied. The effects of light absorption and moving gratings on the spectral resolution are evaluated. Influence of random changes of the dielectric constant of the medium on the reflection and transmission spectra of AO filter is investigated numerically. Significant increase in spectral resolution of acoustooptic filter due to spatial variation of refractive index of medium is shown.

The line width of the transmittance can reach very small values (Fig. 1). In transparent crystals with low absorption coefficients, the resolution of the filter is Δλ/λ≈10^-11, which is by 5-6 orders of magnitude higher than the resolution of the conventional filters with the same crystal thickness.

Complete elimination of sidelobes and a significant suppression of the “tails” of a diffraction reflection curve are demonstrated. Creation of high contrast optical filters with suppressed spurious sidelobes is important for various applications. This problem is particularly acute in Raman spectroscopy, when there is a need for suppression of laser radiation to measure very weak Raman scattering. 

 Acousto-optic filters with metamaterial inclusions opens up opportunities to create devices of infrared (IR) and terahertz technology, inaccessible to ordinary crystals. Note that a one-dimensional photonic bandgap  structure  with  plasmonic  inclusions  can  be  used  in  a  refractive  index  sensor  in  terahertz domain for optofluidics [3].  

The results are of practical interest and can be used in the development of new diffraction acousto-optic modulators, AO filters and spectrometers, and also in the measurements of gravitational waves.  

References

[1] V. I. Pustovoit, Doklady Physics 44, 132-136 (1999).

[2] N. I. Petrov and V. I. Pustovoit, Laser Physics Letters 14, 115702 (2017).

[3] J. Jose, Optics Letters 42, 470-473 (2017).