Abstract
In this work, we present an enhanced design for a Brillouin ring laser (BRL), which employs a double resonant cavity (DRC) with short fiber length, paired with a heterodyne-based wavelength-locking system, to be employed as a pump-probe source for Brillouin sensing. The enhanced source is compared to traditional long-cavity pump-probe source, showing a significantly lower relative intensity noise (~-145 dB/Hz in the whole 0-800 MHz range), a narrow linewidth (10 kHz), and large tunability features, resulting in an effective pump-probe source in BOTDA systems, with an excellent pump-probe frequency stability (~200 Hz), which is uncommon for fiber lasers. The enhanced source showed an improved signal-to-noise ratio (SNR) of about 22 dB with respect to standard BRL schemes, resulting in an improved temperature/strain resolution in BOTDA applications up to 5.5 dB, with respect to previous high-noise BRL designs.
Highlights
Brillouin optical time domain analysis (BOTDA) has acquired a high degree of interest in the last few years, allowing for accurate measurements of strain and temperature along the length of an optical fiber [1]
It is to note that, unlike in the phase-locked loop (PLL) and optical sideband generation (OSB) layouts, Brillouin ring laser (BRL) resonators add a beneficial linewidth narrowing effect on the probe signal extracted from the cavity [13], which can be used to further improve Brillouin frequency shift (BFS) and temperature/strain resolution [14]
We reported on an enhanced performance fiber Brillouin ring laser (BRL) exploiting a doubly resonant cavity (DRC) using a short single-mode fiber length, combined with an optical wavelength locking technique based on heterodyne detection
Summary
Brillouin optical time domain analysis (BOTDA) has acquired a high degree of interest in the last few years, allowing for accurate measurements of strain and temperature along the length of an optical fiber [1]. 2. Double resonant cavity (DRC) In BOTDA systems, in order to accurately reconstruct the Brillouin gain spectrum along the sensing fiber and evaluate the BFS providing temperature/strain information, it is necessary to tune the pump-probe frequency shift with high accuracy and to ensure that such shift will remain stable for the duration of the measurement. In the DRC-BRL shown, the pump is provided by a frequency stabilized DFB laser whose operating wavelength matches the ring resonant frequency and is coupled to an optical circulator (OC) and through an optical coupler (C1), enters the fiber ring cavity, which is given by a single-mode fiber spool (SMF) (< 10 m length). The doubly resonant SC-BRL shows a lasing threshold power of approximately 10 dBm and an extracted power of 1,5 mW (in correspondence to a DFB pump power of about 14 dBm), that is adequate for use as a probe signal in BOTDA applications
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