A Novel Signal Acquisition And Control System Of Continuous Wave Cavity Ring-down Spectroscopy

Cavity ring-down spectroscopy (CRDS) technology as a novel absorptionspectroscopic technique, not only has the capabilities of traditional spectroscopictechniques with real-time, online, in situ analysis, but also has unique advantages:Without the influence of the fluctuations of light source intensity, high sensitivity,self-calibration, the capacity of absolute quantitative measurement. It was studiedand applied extensively to the field of gas concentration detection.In this thesis, a novel control and signal acquisition system of continuous wavecavity ring-down spectroscopy (CW-CRDS) was introduced, expect to get alow-cost, integrated CW-CRDS measurement system. The entire measurementsystem including the control of cavity excitation on and off, and high-speed dataacquisition module and so on, these features are integrated in a circuit board basedon a field programmable gate array (FPGA) chip. For a CRDS system, the cavityexcitation, laser shut off rapidly and the data acquisition are crucial to measurementring-down time. In this system，FPGA and Digital to Analog converter produce atriangular wave to modulate the laser current source, the LDP-3840B, controllingthe laser output frequency, laser frequency changed in a small range, when thefrequency and the cavity mode has resonanted, cavity was excited; Fast triggercircuit was designed to interrupt the laser output to shutdown the cavity rapidly.Each ring-down decay waveform detected by the photoelectric detector（PDA10CS-EC） was delivered to an analog to digital converter (AD9244) whichhas high sampling rate and resolution in order to complete digitization, and the datawill be transformed to the FPGA for buffer processing, and stored in the RAM;FPGA control USB2.0interface chip CY7C68013A to transfer the data to thecomputer, analyzed by software to get the ringdown time.The main content of this thesis include:1Discussing the implementations of CW-CRDS and describing its works basedon the introducing of the principle and knowledge of the CRDS. Describing briefly the characteristics of the devices used in optical system, such as laser diode.2Designed and implemented a fast trigger circuit to shut off the laser output.Designed a high-speed peripheral circuitry of the AD9244and the connection circuitof the FPGA chip.3The various modules on the FPGA are presented in detail, includingmodulation waveform generation module, the gain control of signal amplificationcircuit module, the FIFO data buffer, the RAM and USB interface controller.4The USB communication and computer software have been developed.Finally, these main hardware system are tested.The functions of whole electronic circuit system have been achievedsuccessfully by the hardware testing. The trigger circuit decided successfully thevoltage threshold to output a pulse signal, and the duration of pulse met therequirement; the FPGA system produced successfully a laser electric modulationwaveform; For the data acquisition system(ADC+FPGA+RAM), the speed of signalacquisition can reached65Mbps and the14-bit precision can be sustainedsimultaneously. Besides, during test found that the rate of trigger circuit is not fastenough, the noise of signals has been found in the data acquisition system, theseissues have to be solved by improving and optimizing the circuit design. In addition,because of the limit of the laboratory condition, optical system needs more time tobuild, the electronic system has not been able to joint with the optical system, so theelectric system need a further work.