Low-frequency shift Raman spectroscopy using atomic filters

Brillouin spectroscopy via an atomic line monochromator

Brillouin spectrometers, used for characterizing material mechanical properties, traditionally employ etalons such as Fabry-Pérot interferometers and virtually imaged phased arrays (VIPA) that use spatial dispersion of the spectrum for measurement. Here, we introduce what we believe to be a novel approach to Brillouin spectroscopy using hot atomic vapors. Using laser induced circular dichroism of the rubidium D2 line in a ladder-type configuration, we developed a narrow-band monochromator for Brillouin analysis. Unlike etalon-based spectrometers, atomic line monochromators operate in free-space, facilitating Brillouin spectroscopy integration with microscopy instruments. We report the transmission and spectral resolution performances of the spectrometer and demonstrate Brillouin spectra measurements in liquids.

Band-rejection filter with high extinction ratio using prism-waveguide cascaded coupling system

A band-rejection filter is proposed based on a prism-waveguide cascaded coupling system, which is composed of an equilateral trapezium prism and a deposited multilayer structure. By properly adjusting the thickness of the coupling layer and the light extinction coefficient of the guiding layer, the radiative and intrinsic dampings matching condition could be well satisfied, and then a series of reflectivity dips will appear in the reflectivity wavelength spectrum. Since the module of reflectivity is smaller than one, the extinction ratio of the rejected frequency via the cascaded coupling system is twice as high as that of the single-coupling technology. By narrowing the guiding layer to a micrometer scale, the free spectral range is broad enough to cover the Raman spectrum scattered from the frequently used sample. In addition, the numerically calculated results show that the light in the free spectral range is mostly reflected, with an insertion loss down to 0.45 dB. Compared to previously reported band-rejection filters, it is relatively simple to manufacture our device, which possesses potential applications to help distinguish the Raman signal from the elastic scattering background.

Imaging enhancement based on stimulated Brillouin amplification in optical fiber

The detection of weak optical signals embedded in strong background illumination has broad application prospects. We propose an imaging enhancement method based on stimulated Brillouin scattering (SBS) in a single-mode fiber, which is capable of amplifying the weak optical signal while neglecting the broadband background noise because of its narrow gain bandwidth. In experiment, a high gain of 60 dB was achieved. An imaging enhancement experiment was carried out, where a target which cannot be seen because of transmission loss could be clearly captured with the amplification of SBS in the fiber. Because of the employment of continuous pump rather than a pulsed pump, this system has wide application in the monitoring of non-cooperative targets.

Atomic optical stimulated amplifier with optical filtering of ultra-narrow bandwidth

Taking advantages of ultra-narrow bandwidth and high noise rejection performance of the Faraday anomalous dispersion optical filter (FADOF), simultaneously with the coherent amplification of atomic stimulated emission, we propose a stimulated amplified Faraday anomalous dispersion optical filter (SAFADOF) at cesium 1470 nm. The SAFADOF is able to significantly amplify very weak laser signals and reject noise in order to obtain clean signals in strong background. We show that for a weak signal of 50 pW, the gain factor can be larger than 25000 (44 dB) within a bandwidth as narrow as 13 MHz. Having the ability to amplify weak signals with low background contribution, the SAFADOF finds outstanding potential applications in weak signal detections.