A numerical simulation model for investigating modulation instability in a Raman-assisted fiber-optics system is presented and used. The modulation instability (MI) spectrum and its threshold are numerically analyzed based on this theoretical model. The different MI behaviors between forward and backward Raman pumping are studied in detail, respectively. The MI threshold in the case of forward Raman pumping is lower than that in the case of backward Raman pumping. Experiments are conducted based on the theoretical analysis, and the results agree well with the numerical calculations.
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.
Proc. SPIE. 10323, 25th International Conference on Optical Fiber Sensors
KEYWORDS: Signal to noise ratio, Modulation, Sensors, Single mode fibers, Ocean optics, Sensing systems, Bismuth, Spatial resolution, Acoustics, Temperature metrology
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 preactivated 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.
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.
KEYWORDS: Spatial resolution, Sensors, Signal to noise ratio, Sensing systems, Signal detection, Single mode fibers, Signal processing, Sun, Time metrology, Continuous wave operation
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.
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.
Proc. SPIE. 9679, AOPC 2015: Optical Fiber Sensors and Applications
KEYWORDS: Signal to noise ratio, Continuous wave operation, Modulation, Control systems, Modulators, Signal processing, Sensing systems, Signal detection, Electrooptic modulators
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.
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