Shilin Sun

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.

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.

The effect of modulation instability on the interferometric fiber sensing systems

An experiment is presented to study the effect of modulation instability (MI) on the interferometric fiber sensing system. It is found that MI induces serious phase noise in the system. The MI suppression methods in the interferometric fiber sensing systems are discussed in the paper.

Optimization of output signal power spectrum in Raman-assisted WDM system

We analyze the output signal power spectrum with different initial input signal power spectrum in a long-haul wavelength-division-multiplexed system with backward multiple-pump Raman amplifiers. The genetic algorithm is used to optimize the output signal power. The results using fixed pump wavelengths are compared with the case in which the pump wavelengths are allowed to vary. We experimentally show that the optimal design algorithm is proper.

Long-range distributed temperature sensing with sub-meter scale spatial resolution based on BOTDA employing pre-pumped Golay coding

The BGS broadening caused by short pump pulse would degrade the SNR enhancement provided by the optical pulse coding and strongly reduce the measurement accuracy of coded BOTDA sensors, especially at long sensing ranges with sub-meter spatial resolution. In this work, to overcome this trade-off between spatial resolution and measurement accuracy, we implement a BOTDA sensor employing pre-pumped Golay coding with optimized pre-pumped pulse power. The proposed scheme fully exploits the SNR enhancement provided by the coding and the benefits from the pre-activated acoustic field established by the pre-pumped pulse, achieving distributed temperature measurements with 70 cm spatial resolution and temperature resolution better than 1.5 °C throughout the 50 km sensing fiber.

Optimization of Raman-assisted wavelength-division-multiplexing system with arbitrary input signal power spectrum

Abstract. We analyze the method to uniform the output signal power spectrum for a long-haul wavelength-division-multiplexing (WDM) system using backward multipump Raman amplifiers with arbitrary initial input signal power spectrum. A genetic algorithm is used to optimize the output signal power. The theoretical results show that using variable pump wavelengths is better than using fixed pump wavelengths to decrease the spectral maximum power difference. An experiment is conducted based on the theoretical analysis. The results agree well with the numerical calculations.

High-coherence light extraction through a compact Brillouin/erbium fiber laser

High-coherence light is stringently demanded in high-accuracy interferometric optical fiber sensors, where the phase noise of the light source greatly affects the sensitivity of the whole system. Distributed-feedback laser diodes with a phase noise of -80 ~ -90 dB/Hz1/2 at 1 kHz (with 1 m optical path difference) is now easily obtained, but the interferometric fiber sensors requires the laser source with the phase noise lower than -100 dB/Hz1/2. Lasers with ultra-low-noise usually require complicated and sophisticated techniques. We propose a novel structure to realize high-coherence light extraction through a compact Brillouin/erbium fiber laser (BEFL) which uses a length of 4 m erbium-doped fiber as both the Brillouin and linear gain media. The phase noise of the Brillouin pump light is greatly smoothed and suppressed after being transferred to the Brillouin Stokes light. High-coherence light with the phase noise of about -104 dB/Hz1/2 at 1 kHz is extracted through the compact BEFL from a commercialized laser diode with the phase noise of about -89 dB/Hz1/2. The capability of phase noise suppression in the compact BEFL presents much importance especially in large-array interferometric fiber sensor systems.