ABSTRACT
The Classical Least Square regression (CLS) is one of the most popular regression methods in FTIR quantitative estimation. However, CLS is the best unbiased estimator only under the assumption that error (noise) in the spectrum has equal variance, which usually is not the case in FTIR. This paper proposed a noise calibration method for FTIR spectrum analysis. Based on measured variance of noise in the FTIR spectrum by computer, the Weighted Least Square regression (WLS) method is used in quantitative estimation. The experiment results showed that the WLS performs much better than CLS in quantitative estimation of VOCs pollution.
ABSTRACT
Femtosecond optical frequency comb, with large spectral range, narrow pulse width, high stability of frequency and many other remarkable properties, has a significant impact on optical frequency metrology, absolute distance measurement and high-precision spectroscopy. The time-domain and frequency-domain properties of femtosecond optical frequency comb is traced back to the microwave frequency standard, making it possible to open up the door to high-precision gas absorption spectrum detection. Femtosecond optical frequency comb spectroscopy has some excellent performances, such as fast measurement, high sensitivity, high resolution, high signal-to-noise ratio and so on. Therefore, more investments on femtosecond optical frequency comb spectroscopy will better make contribution to the environmental protection, industrial production, biological medicine, scientific research and other social fields. High-precision gas absorption spectroscopy with femtosecond optical frequency comb mainly includes frequency comb based cavity ring-down spectroscopy, cavity-enhanced frequency comb spectroscopy and dual-comb multi-heterodyne spectroscopy. Among them, according to the data collection, cavity-enhanced frequency comb spectroscopy can be divided into comb vernier method, virtually imaged phased array method and Fourier transform method. At present, related research has been widely carried out abroad, and domestic research is still in its infancy. This review summarizes main techniques in the high-precision gas absorption spectrum detection based on optical frequency comb, and demonstrates typical experimental schemes for different methods. It also analyzes the advantages and disadvantages of each method, and tracks frontier achievements of main research groups.
ABSTRACT
In view of the complexity of the conventional control of scanning mirror based on a digital-analog hybrid design, this paper focuses on a concise and efficient control method around the completely digital signal. The quadrate-encoder pulses A and B generated by the scanning mirror were sent into CPLD pins where an algorithm on position control and velocity control had been built. As a result, the CPLD will generate two PWM signals. These two signals are regenerated by a digital power chip to drive the voice of motor bundled with the moving mirror. Experiments validated that the stability of velocity of moving mirror is better than 97.2% and the method can improve the SNR of the measured spectrum and guarantee the accuracy of spectral qualitative and quantitative analyses.
ABSTRACT
A method of principal component interactive validation in multi-region is presented. Based on the characteristic peaks of target gas the full spectral region is divided into multiple characteristic regions. In succession the principal component analysis around the pure components spectral matrix in divided characteristic regions is applied. Thus the potential components included in target gas are identified according to the main components number according to 99% larger cumulated contribution and the descending sorting of maximum value of pure component spectrums. After that, the characteristic regions of potential components are compared with the ones of target gas one by one. In this way the real components of target gas are confirmed. Because the principal component analysis was applied in multiple characteristic regions, this method can avoid omitted identification. Owning to the matching validation between the characteristic absorption regions of target gas and the ones of pure component gas this method can avoid additional identification. The simulated and the measured spectra both validate the qualitative ability of this method.
ABSTRACT
Field of view (FOV) is an important contributor for instrumental line shape (ILS) function of high-resolution Fourier transform infrared spectrometer (FTS) only next to maximum optical difference. For the reason of optical design and layout, commonly, the measured FOV located in detector is not regularly rounded as original one. There exits more or less difference between the measured vertical FOV and measured horizontal one. In view of this case, the present paper replaces the generally circular area light source with an elliptical one, which is probably more suitable for actual FTS. At the same time, the factor maximum optical difference was considered. After these, the mathematic and graphic description about ILS function with unequal field of view (UFOV) was given Finally, comparison between measured spectrum with standard monoxide gas and calculated one with equal field of view (EFOV) and UFOV respectively was taken Experimental results show that the spectral residual from UFOV is less fluctuant than those from EFOV and its root mean square value is the smallest. All these indicate that the ILS function from UFOV is more accurate and suited to reflecting the response of high-resolution FTS than the one from EFOV.
