Jun Ruan

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.

Measurements of aerosol distribution by an elastic-backscatter lidar in summer 2008 in Beijing

Elastic lidar observations of profiles of the aerosol extinction, backscattering coefficients, and the lidar ratio have been performed in Beijing. The elastic lidar transmitts wavelengths of 532 and 355 nm. The measurement altitude can reach up to 6 km. The similarity of the extinction and backscattering profiles suggests a close relation between the mean transmission and reflection properties. The lidar ratio on July 22, 2008 varied from 10 to 30 sr with the mean value of 20 sr. The profiles of the aerosol properties indicate the cirrus at 6-km altitude and a well-mixed boundary layer from July 22 to 24, 2008. The detected boundary layer also agrees well with the high and stable ozone concentration obtained from the differential optical absorption spectroscopy (DOAS) system.

The research of control system for Cesium atomic fountain clock

The control system for Cesium atomic fountain clock is described in detail in this paper. It consists of five parts, automatic digital frequency locking system of extended cavity diode laser (ECDL), the time serial control system, the data acquisition system, the microwave frequency synthesizer control system and the servo control system. With automatic digital frequency locking system, the frequency of ECDL can be locked to any saturated absorption curve automatically and remains locked throughout a series of abrupt disturbance. And moreover, the laser system can relock itself in few seconds when the frequency of the laser is out of range. The time serial control system generate thirteen time serial signals synchronously, including Transistor-Transistor Logic (TTL) signals and arbitrary waveform signals. Moreover, the result shows that these signals are synchronized within 10 μs. The time of flight (TOF) signals are obtained combined with data acquisition system. The resulting Ramsey resonance signal is obtained by sweeping frequency of the synthesizer. The frequency of the 9.2 GHz synthesizer is square-wave modulated by the servo control system so that the frequency of the quartz crystal oscillator is locked to the atomic resonance of the fountain.

Laser Power Stabilization for the Detection of the Populations of the Atomic Double Levels in Cs Fountain Clock

In this paper we present experimental results on the laser power stabilization of the detection system in Cs fountain clock. In order to stabilize the laser power, we have constructed a feedback loop by using an acousto-optic modulator, a photodetector, and a servo controller. The control-loop can make the relative intensity noise of the laser beam reduce from 1.33×10-4 Hz-1/2 to 7.16×10-6 Hz-1/2 at 2 Hz. However, if the beam used in the control-loop is obtained by using PBS, the two detection beams in fountain clock cannot be stabilized at the same time. Therefore, we propose a new scheme by using a piece of uncoated optical glass plate as a splitter to obtain the branch beam used in the control-loop. Experimental results demonstrate that the power stabilities of the two detection beams are both at 10-5 level (based on rms value) for 1.5 hours at the same time.

A New Steering Strategy for UTC(NTSC)

A new steering strategy is used to monitor and control UTC(NTSC) in this paper. A new atomic time scale TA for real-time monitoring UTC(NTSC) is shown firstly, then the New steering strategy and software proceeding are discussed afterwards. Finally, the results of real steering are discussed, and a successful improvement in steering strategy is showed.

Automatic digital frequency locking system of extended cavity diode laser based on Labwindows/CVI

In order to avoiding manual operation during traditional frequency locking. the automatic digital frequency locking method was stated in this paper. Based on the programming language Labwindows/CVI, the data acquisition (DAQ) card was employed to control piezoelectric transducer (PZT) voltage and inject current, the extended cavity diode laser(ECDL) can be locked to any saturated absorption curve automatically. The system also remains locked throughout a series of abrupt disturbance tests. The experiment results show that the high frequency stability of 2.6×10−10 at 1s integration time is achieved in this system, greatly reduces the device cost and increases device flexibility. Meanwhile, it also provides a friendly human-machine interface and a simple operation.

Atomosphere boundary layer height determination and observation from ceilometer measurements over Hefei during the total solar on July 22, 2009 eclipse

Using an improved inexion point method (IIPM), we investigate atmosphere boundary layer (ABL) height evolution over Hefei during the total solar eclipse on July 22, 2009. A lidar ceilometer is used in ground-based observations. Estimations of ABL heights before, during, and after the solar eclipse are analyzed using the IIPM. Results indicate that the IIPM, which is less sensitive to background noise, is more suitable in detecting ABL height and temporal evolution. Data demonstrate that the total solar eclipse resultes in a decrease in ABL height, indicating a suppression of turbulence activity, similar to that observed during the sunset hours. Changes in ABL height are associated with a slow change in temperature, indicating a significant weakening of penetrative convection and a time lag between ABL response and the reduction in solar radiation.

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.

Development of a Dew/Frost Point Temperature Sensor Based on Tunable Diode Laser Absorption Spectroscopy and Its Application in a Cryogenic Wind Tunnel

We have proposed a sensor for real-time and online measurement of dew/frost point temperature using tunable diode laser absorption spectroscopy (TDLAS) technique. Initial experiments have demonstrated its feasibility and technical advantages in comparison to a chilled mirror hygrometer (CMH). The TDLAS sensor we developed has a dew/frost point temperature range from −93 °C to + 14.5 °C, with a measurement uncertainly of less than 2%, and a response time of about 0.8 s, which is much faster than that of the chilled mirror hygrometer (ranging from several minutes to several hours). A TDLAS-based dew/frost point sensor has many advantages, such as rapid and continuous measurements, low frost point temperature sensing, high accuracy, and non-intrusiveness. Such a sensor would be useful for dew/frost point temperature determinations in various applications. In a cryogenic wind tunnel, real-time dew/frost point temperature measurements are helpful in preventing the formation of condensed liquid and ice, which can affect the model geometry and lead to unreliable test data.

Study the phase transient of the RF Mach-Zehnder interferometric switch

The traditional switch exhibits phase discontinuity and transients in few hundreds microradian which will induce the larger microwave leakage shift in the atomic fountain clock. The RF Mach-Zehnder interferometric switch is developed in order to decrease the phase variations without inducing phase discontinuity. To detect the phase variations, the phase transient measurement system is also built and described in detail in this paper. The result shows the peak to peak phase variations of RF Mach-Zehnder interferometric is 10 microradians within the measurement resolution of a few microradians after five days averaging.