Application of Dual-Frequency Self-Injection Locked DFB Laser for Brillouin Optical Time Domain Analysis

Abstract

Self-injection locking to an external fiber cavity is an efficient technique enabling drastic linewidth narrowing of semiconductor lasers. Recently, we constructed a simple dual-frequency laser source that employs self-injection locking of a DFB laser in the external ring fiber cavity and Brillouin lasing in the same cavity. The laser performance characteristics are on the level of the laser modules commonly used with BOTDA. The use of a laser source operating two frequencies strongly locked through the Brillouin resonance simplifies the BOTDA system, avoiding the use of a broadband electrooptical modulator (EOM) and high-frequency electronics. Here, in a direct comparison with the commercial BOTDA, we explore the capacity of our low-cost solution for BOTDA sensing, demonstrating distributed measurements of the Brillouin frequency shift in a 10 km sensing fiber with a 1.5 m spatial resolution.

Keywords: BOTDA; Brillouin fiber laser; distributed fiber sensing.

Figure A1

Dual-frequency laser sources used as the master-oscillator in BOTDA setup.

Figure A2

Illustration to dual-frequency laser operation. Green background shows the coupled cavity comprising the DFB laser and feedback loop; yellow background marks the ring cavity.

Figure A3

Stokes output power as a function of the pump output power.

Figure A4

Typical oscilloscope traces of the optical error signal (port A), pump, and Stokes output powers and electrical feedback signal, highlighting the response to a pencil knock on the laser box.

Figure A5

The self-heterodyne spectra of the pump (a) and Stokes (b) laser radiation: the measured spectra (black) and their fitting by the Voigt functions (blue). The fitting parameters evaluating the Gaussian and Lorentzian contributions are $$w_G=2.2 \mathrm{k}\mathrm{H}\mathrm{z}$$, $$w_L=540 \mathrm{H}\mathrm{z}$$ and $$w_G=1.7 \mathrm{k}\mathrm{H}\mathrm{z}$$, $$w_L=220 \mathrm{H}\mathrm{z}$$, for the pump and Stokes outputs, respectively.

Figure A6

Noise performance of the laser: (a) phase noise; (b) relative intensity noise (RIN).

Figure 1

The experimental BOTDA setup.

Figure 2

Optical spectra recorded at two laser outputs in (a) linear and (b) decibel scales.

Figure 3

(a) Typical RF beat note spectrum measured with two master-oscillator outputs; (b) drift in the RF beat frequency measured each minute for 60 min.

Figure 4

Optical fiber line for BOTDA testing.

Figure 5

The measured distribution of the Brillouin gain (a) over the whole testing line and (b) over the range of 9.1–9.18 km.

Figure 6

(a) The Brillouin gain spectra (BGS) measured at the fiber points of 9151 and 9157 m exposed to the temperature of ~25 and ~60 °C, respectively; (b) the measured distribution of the Brillouin frequency shift (BFS) over the range of 9.1 to 9.18 km.