Abstract
This report introduces a novel time resolved Brillouin spectrometer, consisting of an opto-acoustic transducer which resides on the tip of a single-mode optical fiber of arbitrary length with 125 μm outer diameter and 5 μm sensing diameter. Demonstrated here are proof of concept spectroscopic measurements - shifts in Brillouin frequency - with sensitivities of 41±3MHz/%wt and 2.5±0.6 MHz/°C for changes in water-salinity and water-temperature, respectively, and an interpolated frequency resolution of 9±2 MHz. The technique benefits from low-cost raw materials, scalable fabrication, scalable pixel density, easy alignment, and data acquisition speeds down to 0.4 s: traits which make this compatible with in vivo applications.
Highlights
Brillouin spectroscopy is a well-established optical technique for characterizing and imaging mechanical properties of various photoelastic media [1,2,3]
We present in this report a proof of concept for a novel phonon probe, which permits time resolved Brillouin spectroscopic measurements to be made from the tip of a single-mode optical fiber without the need for a lens
The optical design for an all-fiber variant of a pump-probe system was centered around maximizing signal to noise ratio (SNR), given that the laser spot-sizes were to be dictated by the 5 μm diameter core of a single-mode optical fiber (125 μm diameter cladding)
Summary
Brillouin spectroscopy is a well-established optical technique for characterizing and imaging mechanical properties of various photoelastic media [1,2,3]. In the field of photoacoustics, there has been a flurry of development in optical fiber-based ultrasonic transmitters (multi-mode) and receivers (single-mode), which, when combined, create a powerful endoscopic tool for in vivo tissue imaging with ∼ 20 MHz ultrasound [5]. A similar dual-fiber (both single-mode) concept has been implemented for performing spontaneous Brillouin spectroscopy in the GHz-range on liquid samples [6]. This technique is challenging with a single-mode fiber since it relies on an inherently inefficient incoherent scattering process and contends with strong background signals from the glass of the fiber. The role of optical fibers in GHz-frequency ultrasonic applications has mainly been one of collecting scattered light, creating a confocal pinhole, or as a way to simplify alignment [7]
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