Xiaobo Tu

Vector Brillouin Optical Time-Domain Analysis With Heterodyne Detection and IQ Demodulation Algorithm

This paper reports a vector Brillouin optical time-domain analyzer based on heterodyne detection and IQ demodulation algorithm. This approach employs an acousto-optic frequency shifter to generate a single-frequency reference, which simplifies the probe spectrum and directly features the Brillouin gain and phase shift compared with the phase modulation. In addition, the proposed IQ demodulation algorithm makes signal processing easy to implement and allows fast and real-time demodulation. A proof-of-concept experiment is carried out in an ~1.6-km standard single-mode fiber consisting of two different sections of fibers. Both the Brillouin gain and phase-shift spectrograms are successfully measured with an ~2-m spatial resolution. The temperature dependence values of the Brillouin gain and phase-shift spectra are also measured, and the temperature dependence values of the Brillouin frequency shift show excellent linearity larger than 0.999 and almost identical slopes of 1.166 and 1.159 MHz/°C, respectively.

Low-noise, single-frequency, single-polarization Brillouin/erbium fiber laser

We demonstrate a low-noise Brillouin/erbium fiber laser (BEFL), which uses only 1.5 m polarization-maintaining erbium-doped fiber as both the Brillouin and erbium gain media. This BEFL presents a phase noise of -125 dB/Hz(1/2) at 1 kHz frequency, at 2 mW Brillouin pump (~3 MHz linewidth) power and 200 mW 980 nm pump power. The polarization extinction ratio of the laser output light is 31 dB. Stable (~2 h) single-mode operation is observed. This BEFL presents potential applications in distributed Brillouin fiber sensors, inteferometric fiber sensors, and optical communications.

Long-range BOTDA sensor over 50 km distance employing pre-pumped Simplex coding

We propose and demonstrate a long-range Brillouin optical time-domain analysis (BOTDA) sensor based on the pre-pumped Simplex coding technique. This approach combines Simplex coding with the pulse pre-pump technique, which takes full advantage of the signal-to-noise ratio enhancement provided by optical pulse coding and achieves meter-scale spatial resolution with an unbroadened Brillouin gain spectrum by the use of pre-pumped short pump pulse. Compared to the widely used differential pulse-width pair technique, a comparable performance can be realized while the measurement time is reduced by half. The theoretical analysis of pre-pumped Simplex coding applied to BOTDA systems is presented and a proof-of-concept experiment is carried out along a ~51 km single-mode fiber composed of two fiber segments with slightly different Brillouin frequency shift values. With the proposed technique, Brillouin frequency shifts of the two different fiber sections at the end of the sensing fiber are clearly distinguished, and the obtained spatial resolution and temperature/strain accuracy along the sensing fiber are respectively 1 m and . According to the experimental results, we believe that the BOTDA employing pre-pumped Simplex coding is able to realize extra-long distance sensing without Raman amplification, while keeping a high spatial resolution and measurement accuracy.

Performance analysis of slope-assisted dynamic BOTDA based on Brillouin gain or phase-shift in optical fibers

This paper analyzes the performance of the slope-assisted dynamic BOTDA based on Brillouin gain or phase-shift in an experiment. Dynamic strains with frequency of 60 Hz are successfully measured with an effective sensing rate of 1 kHz over a 46 m sensing fiber in both schemes. The dynamic ranges of these two schemes are measured to be about 47 MHz (940 ), through dynamic strain measurements while linearly sweeping the work point. The optimum work point for Brillouin gain is theoretically and experimentally proved to be , not as commonly known, where corresponds to the Brillouin frequency shift and is the Brillouin linewidth. The distortion factors are also measured to stay in a quite low level in the dynamic range. These results will provide guidelines for practical dynamic strain measurements and to further improve the performance of the slope-assisted BOTDA systems.

Fast-tuning, low-noise, compact Brillouin/erbium fiber laser

We demonstrate a fast-tuning Brillouin/erbium fiber laser (BEFL) using 4 m erbium-doped fiber (EDF) as both the Brillouin and erbium gain media. The BEFL frequency is fast tuned via modulating a piezoelectric transducer on which the EDF is coiled. Over 60 MHz frequency-tuning range is obtained at less than 5 V voltage amplitude and the tuning rates can achieve 48 kHz. This fast-tuning BEFL presents a low phase noise of -124 dB/Hz(1/2) at 1 kHz (normalized to 1 m interferometer optical path difference) with 32 kHz modulating frequency. It presents potential applications in fiber sensors and optical communications.

Mechanism and characteristics of a fast-tuning Brillouin/erbium fiber laser

A fast-tuning Brillouin/erbium fiber laser (BEFL) is investigated on its mechanism and characteristics in detail, in which a 4 m erbium-doped fiber (EDF) as both the Brillouin gain and linear gain media is coiled on a piezoelectric transducer (PZT) for laser frequency modulations. We demonstrate the fast-tuning mechanism theoretically and experimentally that only the lasing cavity mode is modulated, instead of the previous presumption that the Brillouin frequency shift of the EDF is modulated synchronously with the lasing mode. And the maximum tuning range (~60 MHz) is limited by the bandwidth of the Brillouin gain spectrum. The frequency tuning amplitude is direct proportional to the voltage on the PZT. The tuning rates reach up to 48 kHz. The BEFL keeps high-coherence property under fast frequency modulation. Its phase noise remains about -124 dB/Hz1/2 (normalized to 1 m optical path difference) at 1 kHz under 32 kHz modulations. This fast-tuning BEFL presents a wide range of applications in fiber sensors, optical fiber communications, and so forth.

High-accuracy and long-range Brillouin optical time-domain analysis sensor based on the combination of pulse prepump technique and complementary coding

A Brillouin optical time-domain analysis (BOTDA) sensor that combines the conventional complementary coding with the pulse prepump technique for high-accuracy and long-range distributed sensing is implemented and analyzed. The employment of the complementary coding provides an enhanced signal-to-noise ratio (SNR) of the sensing system and an extended sensing distance, and the measurement time is also reduced compared with a BOTDA sensor using linear coding. The combination of pulse prepump technique enables the establishment of a preactivated acoustic field in each pump pulse of the complementary codeword, which ensures measurements of high spatial resolution and high frequency accuracy. The feasibility of the prepumped complementary coding is analyzed theoretically and experimentally. The experiments are carried out beyond 50-km single-mode fiber, and experimental results show the capabilities of the proposed scheme to achieve 1-m spatial resolution with temperature and strain resolutions equal to ∼1.6°C and ∼32  μϵ, and 2-m spatial resolution with temperature and strain resolutions equal to ∼0.3°C and ∼6  μϵ, respectively. A longer sensing distance with the same spatial resolution and measurement accuracy can be achieved through increasing the code length of the prepumped complementary code.

Impact of dither-based Electro-Optic Modulator bias control on distributed Brillouin sensing system

In most distributed Brillouin sensing systems, it is crucial to keep the long-term stability of the electro-optic modulator (EOM) operating point. The dither-tone based bias control methods are widely adopted in this kind of systems for its robustness and reliability, but the low frequency dither tone (a few kilohertz) added into the dc bias port of the EOM may have a detrimental impact on the sensing performance of the Brillouin sensing system. Experimental results show that the dither frequency should not be set around quarter of the pulse repetition rate or its multiples, and the employed dither amplitude should be in the range of 0.003Vπ to 0.015Vπ (Vπ is the RF half-wave voltage of the EOM), in order to overcome the limitation of dither tone based bias control techniques in BOTDA systems. These results will provide guidelines to improve the performance of the Brillouin sensing systems using dither-based EOM bias control method.

Theoretical and experimental study of suppressing stimulated Brillouin scattering and phase noise in interferometric fiber sensing systems with phase modulation

The effect of phase modulation on the phase noise in remote interferometric fiber sensing systems is analyzed in both theory and experiment. A matching condition between the modulation frequency and optical delay difference is derived to choose the matching modulation frequency. The matching modulation frequency for an interferometer with 5 m optical-path difference is 60 MHz and its integer harmonic frequencies. By matching the condition, the phase modulation method can suppress the stimulated Brillouin scattering in an interferometric fiber sensing system effectively without bringing excess phase noise, which is verified by the experiment. The results indicate that the maximum input power of the system can be increased effectively, which has great potential in the design of remote interferometric fiber-sensing systems.