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3Fiber lasers with self-sweeping frequency effect: physics and applications for sensing tasks
N.R. Poddubrovskii1, A.Yu. Tkachenko1, S.I. Kablukov1, I.A. Lobach1*
1-Institute of Automation andElectrometry SB RAS
Distributed fiber sensors (DFS) are one of the most powerful tools in the field of measurements of physical parameters such as temperature, strain, etc. Among all of the advantages of DFS one can note long sensing range (up to 100 km) and resistance to electromagnetic impact. DFS consist of a fiber-based sensor and an analyzer, with the latter generating probe optical radiation and analyzing the back-scattered signal returning from the sensor. Three main types of light scattering in fibers - Rayleigh, Brillouin, and Raman - are typically used in DFS. The resistance of results to optical losses and as result higher accuracy are achieved in DFS based on spectral analysis of the scattered signal (Rayleigh or Brillouin DFS) in contrast to amplitude one (Raman DFS). A key element of the spectral analyzer in DFS is a tunable laser. To realize such a tunable laser, rather expensive special spectral selectors such as tunable fiber Bragg gratings or diffraction gratings are typically used. An alternative to using such selectors are dynamic gratings (DGs) formed in doped fibers under influence of standing waves formed in a laser cavity [1]. Under certain conditions, a DG formed by the standing wave with one optical frequency can cause lasing at another adjacent optical frequency. When the process repeats multiple times in the same direction, the changes looks like frequency tuning which is known as self-sweeping effect. One of the main features of the self-sweeping lasers is high coherence (linewidth less than 1 MHz) of the laser radiation which is related to high selective property of DG due to its large length. As a result, the self-sweeping lasers are very competitive alternative compared to conventional single-frequency tunable ones in applications demanding tunable radiation, because of their simplicity, high coherence, broad sweeping range (up to several dozens of nm) at various spectral ranges (from 1 to 2.1 ^m). To date, a number of practical applications have already been demonstrated (from sensor interrogation to gas analysis).
The report will review the latest advances in the field of DFS with spectral analysis based on self-sweeping fiber lasers: Rayleigh-based Coherent Optical Frequency Domain Reflectometry (COFDR) and Brillouin Optical Time Domain Analysis (BOTDA). Discreteness of frequency change determined by longitudinal mode structure in the single-frequency self-sweeping lasers is highly attractive for Fourier analysis in COFDR [2]. A series of papers demonstrates that COFDRs based on self-sweeping lasers can be used for various tasks including optical elements characterization, sensing, vibration measurements, and gas analysis at lengths of about ten meters with spatial resolution as high as 200 ^m. At the same time the sensitivity as good as -120dB/mm makes it possible to analyze back-scattered Rayleigh signal. Recently the first demonstration of BOTDA without any microwave devices typically used in similar systems was presented in [3]. Distributed temperature measurements with sensing line length of 25 km, spatial resolution of 10 m, and sensitivity of 2°C was demonstrated in a BOTDA system based on a self-sweeping fiber laser.
The work is supported by Ministry of Science and Higher Education of the Russian Federation (No. 1021062912374-2-1.3.6).
[1] N.R. Poddubrovskii, R.V. Drobyshev, I.A. Lobach, S.I. Kablukov, Fiber Lasers Based on Dynamic Population Gratings in Rare-Earth-Doped Optical Fibers, Photonics, 9, p. 613 (2022).
[2] A.Yu. Tkachenko, I.A. Lobach, S.I. Kablukov, Coherent Optical Frequency Reflectometry Based on a Fiber Self-Scanning Laser: Current Status and Development Prospects (Review), Instrum Exp Tech 66, pp. 730-736, (2023).
[3] N.R. Poddubrovskii, I.A. Lobach, S.I. Kablukov, Microwave-free BOTDA based on a continuous-wave self-sweeping laser, Opt. Lett. 49, pp. 282-285, (2024).
