Yanjie Zhao

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.

The relative intensity noise and relaxation oscillation characteristics of a distributed-feedback fiber laser

The relative intensity noise (RIN) and relaxation oscillation characteristics of distributed-feedback fiber lasers (DFB-FLs) have been investigated theoretically and experimentally. The effects of Er3+ doping concentration and phase-shift grating parameters such as the coupling coefficient on the RIN and relaxation oscillation characteristics of DFB-FLs are presented by an analytical model. Also, four DFB-FLs with dissimilar coupling coefficients are fabricated and tested to confirm the theoretical results. In addition, the influence of the external environment and the contribution of the pump power fluctuations are analyzed in detail by experiments.

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.

High-resolution fiber carbon monoxide sensing system and its data processing

Carbon monoxide is one of the important gases need to be detected in coal mine safety. Detection technology based on signature gas is the primary means of spontaneous combustion forecasting of coal goaf area. Because of the high accuracy requirement of CO concentration in the coal mining applications, we had to introduce more data processig methods to improve the signal-to-noise ratio (SNR), finally to achieve the requirements of coal mining. Therefore, we used three data processing methods to eliminate noises of the CO sensing system which based on the tunable diode laser absorption spectroscopy (TDLAS): Fourier transform, least-squares fitting and Kalman filter. The results show that the combination of three data processing methods had a good inhibitory effect of random noise and interference fringes, etc. and significantly improved the system detection accuracy, the minimum detectable spectral absorption rate could be increased by an order of magnitude. So this high-resolution fiber CO sensing system can better meet the needs of coal mine safety.

All-Optical Remote Sensing Monitoring Technique of Methane Concentration Based on Tunable Diode Laser Absorption Spectroscopy

A novel all fiber optic sensing monitoring technique is researched for remotely detecting the concentration of methane gas. The triangular signal is used to modulate the light wavelength of the DFB laser. The narrow absorption line of the laser eliminates the cross sensitivity to moisture, ethane or other gases. With the use of the optical fiber links between monitor and sensor header, the system can provide remote and online monitor information. Since the sensor head is made without any active electrical components, it is intrinsically safe when used in the hazard environment such as coal mines. The results show that the monitoring technique has a great application in the methane extraction pipe-line, coal mines and the environment detection.

Effect of temperature on methane gas concentration by tunable diode laser absorption spectroscopy

The effect of temperature on gas absorption characteristics is obvious. When the tunable diode laser absorption spectroscopy (TDLAS) technique is applied to monitor the methane gas concentration, the influence of temperature must be correction in order to measure the gas concentration accurately. Based on HITRAN database, the absorption characteristics of methane near infrared spectrum with temperature have been researched. Then a multiplexed fiber optic methane monitoring system is designed and the distributed feedback (DFB) laser is employed as laser source. The effect of the temperature on the system is emphasized. The results show that it obviously improved concentration precision by compensating retrieval concentration. It is valuable references for the gas detection in practical applications.

High-resolution fiber methane sensor based on diode laser and its data processing

Tunable diode laser absorption spectroscopy (TDLAS) based optical fiber methane sensing technology has a number of advantages compared with conventional electronic methane sensor device, such as high Precision, passive and intrinsically safe in explosive and hazardous environment as well as immune to electro-magnetic interference. In order to accurately measure the oxidation rate of coal mine ventilation air methane oxidizer system, and Meet requirements for accurate measurements to the oxidizer exhaust emissions, A Fiber optic methane monitor based on Distributed feedback tunable diode (DFB) laser of 1.65um central wavelength is demonstrated. We use a reflective chamber of only a 10cm effective optical path as sensing gas cell. By the data processing of fitting baseline method, we remove the effect of the baseline tilt of background Spectral. The system achieves 0 to 0.1% measure range and 5.8*10E-6 minimum detection sensitivity, and meets the requirements of high accuracy, real-time measure to the oxidizer exhaust emissions.

Optical fiber oxygen sensor based on DFB laser absorption spectroscopy

This paper describes an optical fiber oxygen sensor based on wavelength scanning and spectrum absorption technique. An open path optical gas chamber is employed to analyze the absorption lines of oxygen in visible region. The oxygen sensor works in the concentration range 0-100% oxygen with good performance in stability and sensitivity. The precious is less 0.5% over 30 hours in atmosphere. The results show that it will have a great potential application in the harsh environments.