Jiasheng Ni

Fiber methane gas sensor and its application in methane outburst prediction in coal mine

Fiber optic methane gas detecting system based on distributed feedback (DFB) laser wavelength scanning technique is demonstrated. Wavelength scan of methane absorption peak at 1665.9 nm is realized by saw tooth modulation of current which is injected to DFB laser. A reference methane gas cell is used to find the methane absorption peak around 1666 nm, and normalization is used to reduce the outside affection such as power drift, fiber loss. Concentration is got by arithmetic processing absorption coefficient of the methane gas. In-situ test is carried out in coalmine and long time precision of 0.05% is achieved. Some spot data of coal mine is introduced. By the system, methane outburst can be measured.

Novel gas sensor combined active fiber loop ring-down and dual wavelengths differential absorption method

A novel active fiber loop ring-down gas sensor combined with dual wavelengths differential absorption method is proposed. Two Distributed Feedback Laser Diodes (DFB LDs) with different wavelengths are employed. One LD whose wavelength covered with the absorption line of target gas is used for sensing. Another LD whose wavelength is centered outside the absorption line is used for reference. The gas absorption loss can be obtained by differencing the reference signal and sensing signal. Compared with traditional method of one wavelength employed, it can eliminate the influence of the cavity loss variety and photoelectric device drift in the system efficiently. An Erbium Doped Fiber Amplifier (EDFA) with Automatic Gain Control (AGC) is used to compensate the loss of the light in the ring-down cavity, which will increase the cavity round trips and improve the precision of gas detection. And two fiber Bragg gratings (FBGs) are employed to get rid of amplified spontaneous emission (ASE) spectrum noise as filters. The calibrating ethyne samples of different concentrations are measured with a 65 mm long gas cell in order to evaluate the effect of reference. The results show the relative deviation is found to be less than ± 0.4% of 0.1% ethyne when a certain additional loss from 0 to 1.2dB is introduced to the cavity and the relative deviation of measured concentration is less than ± 0.5% over 24 hours.

Research on a novel composite structure Er 3+ -doped DBR fiber laser with a π-phase shifted FBG

A simple composite cavity structure Er³⁺-doped fiber laser was proposed and demonstrated experimentally. The resonant cavity consists of a pair of uniform fiber Bragg gratings (FBGs) and a π-phase shifted FBG. By introducing the π-phase shifted FBG into the cavity as the selective wavelength component, it can increase the effective length of the laser cavity and suppress the multi-longitudinal modes simultaneously. The narrow linewidth of 900 Hz and low RIN of -95 dB/Hz were obtained. And the lasing wavelength was rather stable with the pump power changing. The SMRS was more than 67 dB. The results show that the proposed fiber laser has a good performance and considerable potential application for fiber sensor and optical communication.

Fiber optic vibration sensor based on over-coupled fused coupler

The theory and experiment results of fiber optic vibration sensor based on over-coupled fused coupler (OCFC) is described, 2 novel scheme to form single-end sensor is presented, i.e. OCFC sensor with a reflection mirror and OCFC fiber loop sensor, the transmission spectrum of these OCFC sensor when applied with different strains is investigated and interrogation system is developed, the output of these sensors when applied with vibration and shock is shown. We demonstrate that the OCFC fiber loop sensor is feasible to have practical use to measure vibration signal because the sensor is easy to be fabricated, it has good low and high frequency response and the interrogation system is simple, low cost.

Fiber-optic vibration sensor system

A vibration measuring system based on a matched-fiber Bragg grating (FBG) is demonstrated, and the cross sensitivity of the temperature and strain was reduced by packaging the matched-sensing and interrogation FBG in the same shell, theory, system structure, and experimental results are presented. The experimental results demonstrated that the system has a good response to the 8–80 Hz vibration signal; it responds well to an acceleration of as low as 0.05 m/s2, the system was deployed in a coalmine, and good experimental result were received. Because the system has the advantage of intrinsic safety and an easy multiplex, it has good prospects in the mining and petrochemical industry.

A low cost mutiplexed 1.331 μm spectroscopic remote methane sensor system

A remote multiplexed methane sensor system taking advantage of low cost 1.33 1&mgr;m SLD is demonstrated. it has long time stabilization, immunize from the influence of the temperature changing. Reference light is adopted in the system to reduce the affection of attenuation caused by long distance transmission of light in fiber and the drift of the SLD output power. Resolution of 0.01% (10-s time constant) is obtained by introducing intensity modulation and Lock-In-Amp.

Optic fiber pulse-diagnosis sensor of traditional Chinese medicine

The wrist-pulse is a kind of signals, from which a lot of physiological and pathological status of patients are deduced according to traditional Chinese medicine theories. This paper designs a new optic fiber wrist-pulse sensor that based on a group of FBGs. Sensitivity of the optic fiber wrist-pulse measurement system reaches 0.05% FS and the range reaches 50kPa. Frequency response is from 0 Hz to 5 kHz. A group of typical pulse signal is given out in the paper to compare different status of patient. It will improve quantification of pulse diagnosis greatly.

Linewidth narrowing and polarization control of erbium-doped fiber laser by self-injection locking

A novel and simple self-injection locking (SIL) configuration of distributed feedback (DFB) fiber laser is presented. By injecting the feedback light into the cavity of the fiber laser, it can realize that the line-width can be narrowed by half compared with the same system without SIL. Meanwhile the stable single-polarization lasing is achieved and the degree of polarization is improved from 0.165 to 0.989. Then the self-pulsation behavior of the DFB fiber laser is investigated in the injection-locked and unlocked system respectively and the output power fluctuation is reduced from about 4 to 0.1 dBm by injection locking. And the power output characteristics of the DFB fiber laser are also researched by changing the pump current. With increasing the pump current from 30 to 275 mA, the output power of the DFB fiber laser increases linearly and the injection-locked output increases slightly more quickly than the unlocked output. The results have a great potential application in optical communication and high sensitive fiber sensors.

External optical feedback effects on stability of asymmetric DFB-FL and isolation method

The effects of external optical feedback mainly introduced by Rayleigh backscattering (RB) in long lead fiber, reflection of fiber end facet, and the fusion splices on the stability of asymmetric distributed feedback fiber laser (DFB-FL) have been investigated. Theoretical analysis based on the compound cavity model is presented, and the critical length Lp,c of lead fiber is obtained to evaluate the tolerance of DFB-FL to RB. Analysis and experimental results show that RB is the main factor causing the instability. The critical length for the shorter end of our asymmetric DFB-FL is measured to be about 170 m. To isolate the laser from the effect of RB, we propose a method using isolator and WDMs, which is experimentally proved to be an efficient way.

Fiber laser sensor interrogation system development and test

The development and test of the fiber laser interrogation system is demonstrated in this paper. Unbalanced Michelson interferometer and digital phase-generated carrier (PGC) demodulation are adopted by the interrogation system. Interferometric interrogation principle and overall interrogation system are described. The interrogation system consists of digital PGC demodulation algorithm, data acquisition, photodiode amplifier and low-pass filter, PGC phase carrier generation and unbalanced Michelson interferometer. Realization methods and the performance of the above system components are discussed in detail. In order to simulate the wavelength shift induced by measurand, a standard wavelength shift signal generator is set up and its frequency response is tested. Interrogation experiment of acoustics signal in water is carried out, and the frequency response of the uncoated DFB fiber laser is obtained.