Yiben Cui

An approach of open-path gas sensor based on tunable diode laser absorption spectroscopy

Tunable diode laser absorption spectroscopy (TDLAS) is a new method to detect trace-gas qualitatively or quantificationally based on the scan characteristic of the diode laser to obtain the absorption spectroscopy in the characteristic absorption region. A time-sharing scanning open-path TDLAS system using two near infrared distributed feedback (DFB) tunable diode lasers is designed to detect CH4 and H2S in leakage of natural gas. A low-cost Fresnel lens is used in this system as receiving optics which receives the laser beam reflected by a solid corner cube reflector with a distance of up to about 60 m. High sensitivity is achieved by means of wavelength-modulation spectroscopy with second-harmonic detection. The minimum detection limits of 1.1 ppm·m for CH4 and 15 ppm·m for H2S are demonstrated with a total optical path of 120 m. The simulation monitoring experiment of nature gas leakage was carried out with this system. According to the receiving light efficiency of optical system and detectable minimum light intensity of detection, the detectable optical path of the system can achieve 1-2 km. The sensor is suitable for natural gas leakage monitoring application.

H2S detection by tunable diode laser absorption spectroscopy

Hydrogen Sulfide is a highly toxic and flammable gas. It is generated as a common by-product of many industrial processes. Early detection of H2S at concentrations of 10 ppm in air is essential to prevent its toxic influence at higher concentrations. Tunable diode Laser based gas detectors are now being used in a wide variety of applications for safety and environmental interest. Their characteristics like non-intrusive, high selectivity, high sensitivity, and fast response time make them more appealing than conventional point sensors for a reliable H2S detection. A long open-path H2S sensor based on tunable diode laser absorption spectroscopy has been developed, using telecommunication near infrared distributed feed-back(DFB) tunable diode laser combining with wavelength modulation spectroscopy technology, a detection limit of 20 ppmldrm has been demonstrated, an on-board reference cell provides on-line sensor calibration. The sensor is very suitable for large area field H2S monitoring application.

Study on the cloud layer height and properties in Hefei observed by lidar

A co-axial transmission elastic-backscattered lidar aiming to detect the optical properties of the clouds is presented in this paper. The modular co-axial design can guarantee the consistency of the transmitting part and the receiving part. In practice a specific diaphragm is used to suppress the stray light of the primary mirror and background light to improve SNR of the backscattered signal in the daytime. So the near ground signal must be corrected with the appropriate overlap factor. A Licel transient recorder is used for data acquisition in analog and photon counting combined in one acquisition system. With the 15 MHz sampling rate, the spatial resolution of 10 m can be attained. The control over the transient recorder and the treatment of the data is performed on a PC. After getting the correctional backscattered signal, retrieving and analyzing the extinction coefficient profile, the cloud base, cloud peak and related optical parameters of the clouds can be confirmed. In order to testify the feasibility of our lidar, it was implemented with a Finland ceilometer Vaisala simultaneously in May in 2008 in Hefei. Results show the lidar system is stable and the data is reliable.

The measurement of the H2S in the pre-desulfurization of natural gas in the Shengli oil field with the TDL

Hydrogen Sulfide, with the character of erosion and strong toxicity, is a kind of associated gas of nature gas. How to measure and monitor the hydrogen sulfide concentration becomes an important issue to be solved in nature gas transfer-process. Online measurement for the hydrogen sulfide concentration before the desulphurization remains very difficult in Bonan gas gathering station of SINOPEC Shengli Oil Field (SOF).TDL(Tunable Diode Laser) can relative easily select the absorption line of the detecting gas without the interference from other gas thus make the rapid and accurate hydrogen sulfide measurement a possible. In this paper, a hydrogen sulfide measurement system is designed and then be carried out in Bonan gas gathering station of (SOF) .The implemented experiments showed the system effectively solved some problems such as overfall, temperature and pressure. After comparing the hydrogen sulfide online detection sensor of the TDLAS industry with the long hydrogen sulfide detection tube, the linear fitted relationship with the correlation coefficient of 99.96% between them was attained. In order to meet the requests of industrial field anti-explosion, the open-path optical coupling technology was performed in china for the first time. All the results demonstrate that the system will be put into use and enjoy a high application in the near future.

Description of a system based on the DFB laser for natural gas leakage in the long optical-path of the tunnel

The leakage of methane and sulfureted hydrogen in the tunnel may threaten the environment, safety and lead to damage and casualties. In this paper, a system based on the distribution feedback laser is introduced to detect the leakage natural gas in the tunnel. The system can be used in the tunnel with the length over 2km. In the system, two approaches, namely the direct absorption and wavelength modulation is both adopted to acquire the concentration of methane and sulfureted hydrogen. The results show that the remote sensing can reach up to two kilometers with the detectability of methane 2ppm and sulfureted hydrogen 20ppm respectively. This system is carried out in the Puguang Gas filed for the first time and satisfactory results are obtained, making up the gap in such field as detecting natural gas in domestic market.

Direct absorption measurement of ambient nitric oxide based on room-temperature pulsed quantum cascade laser

The Mid-infrared lasers are very effective for high-sensitive trace gas detection for the fundamental absorption lines of most gases. The quantum cascade (QC) laser, with the advantages of high power, wide tuning range and room-temperature operating conditions, is one of the ideal mid-IR sources developed in these years. The technology combing the room-temperature pulsed quantum cascade laser and the long-path multipass absorption cell can satisfy the need for the sensitivity, selectivity and rapid response. A set of experimental apparatus for the direct absorption measurement of ambient nitric oxide(NO) with the room-temperature pulsed QC laser was presented. With this system a NO concentration of 188ppb was demonstrated, along with an estimated detection limit of 4ppb. The results show that this experimental apparatus can fulfill the detection of NO concentration in ambient air.

A broadly tunable mid-infrared laser system based on difference frequency generation in periodically poled lithium niobate

A room-temperature broadly tunable mid-infrared laser system based on quasi-phase-matched (QPM) difference frequency generation (DFG) in a periodically poled LiNbO3(PPLN) crystal is developed. The continuous wave mid-infrared coherent radiation from 3.2 to 3.7µm with a maximum output power of about 1µW has been generated. Performance characteristics of the mid-infrared laser system has been carried out.

Exhaust gas monitoring based on absorption spectroscopy in the process industry

This non-invasive gas monitor for exhaust gas monitoring must has high reliability and requires little maintenance. Monitor for in-situ measurements using tunable diode laser absorption spectroscopy (TDLAS) in the near infrared, can meet these requirements. TDLAS has evolved over the past decade from a laboratory especially to an accepted, robust and reliable technology for trace gas sensing. With the features of tunability and narrow linewidth of the distributed feedback (DFB) diode laser and by precisely tuning the laser output wavelength to a single isolated absorption line of the gas, TDLAS technique can be utilized to measure gas concentration with high sensitivity. Typical applications for monitoring of H2S, NH3, HC1 and HF are described here together by wavelength modulation spectroscopy with second-harmonic(WMS-2F) detection. This paper will illustrate the problems related to on-line applications, in particular, the overfall effects, automatic light intensity correction, temperature correction, which impacted on absorption coefficient and give details of how effect of automatic correction is necessary. The system mainly includes optics and electronics, optical system mainly composed of fiber, fiber coupler and beam expander, the electron part has been placed in safe analysis room not together with the optical part. Laser merely passes through one-meter-long pipes by the fiber coupling technology, so the system itself has anti-explosion. The results of the system are also presented in the end, the systems response time is only 0.5s, and can be achieved below 1×10−5 the detection limit at the volume fraction, it can entirely replace the traditional methods of detection exhaust gas in the process industry.

Development of a broadly tunable mid-infrared laser source for highly sensitive multiple trace gas detection

A room-temperature broadly tunable mid-infrared difference frequency laser source for highly sensitive trace gas detection has been developed recently in our laboratory. The mid-infrared laser system is based on quasi-phase-matched (QPM) difference frequency generation (DFG) in a multi-grating, temperature-controlled periodically poled LiNbO3 (PPLN) crystal and employs two near-infrared diode lasers as pump sources. The mid-infrared coherent radiation generated was tunable from 3.2 to 3.7 mum with an output power of about 100 muW. By changing one of the pump laser head with another wavelength range we can readily obtain other needed mid-infrared wavelength range cover. Performance of the mid-infrared laser system and its application to highly sensitive spectroscopic detection of CH4, HCI, CH2O and NO2 has been carried out. A multi-reflection White cell was used in the experiment gaining ppb-level sensitivity. The DFG laser system has the features of compact, room-temperature operation, narrow line-width and broadly continuous tunable range for potential applications in industry and environmental monitoring.