CN103411923A - Normalized noise reduction method adopting two-path tunable laser absorption spectrum technology - Google Patents

Abstract

The invention provides a normalized noise reduction method adopting a two-path tunable laser absorption spectrum technology. The method comprises the following steps: dividing laser into two paths by an optical fiber splitter, wherein one path penetrates through to-be-measured gas and is used for generating measurement signals, the other path penetrates through a route purified by N2 and is used for generating reference signals, and the measurement signals and the reference signals are collected by a multiway synchronous sampling data collection card; performing normalization on the measurement signal and the reference signal and transforming the amplitude values of the measurement signals and the reference signal to be within the range of 0 to 1 through linear function transformation; calculating absorption signals of the gas by utilizing the normalized measurement signals and reference signals, removing ramp signals independent of absorption and leaving the part reflecting absorption. The method has the effects of effectively inhibiting the influence of background noise produced by intensity fluctuation of the laser on the measurement result, effectively improving the measuring accuracy, further reducing the lower limit of measurement and having a broad application prospect.

Description

A kind of normalization noise-reduction method that adopts two-way tunable laser absorption spectroscopy techniques

Technical field

The present invention relates to a kind of noise-reduction method of tunable laser spectroscopy technology, be specifically related to a kind of normalization noise-reduction method that adopts two-way tunable laser absorption spectroscopy techniques.

Background technology

The tunable laser absorption spectroscopy techniques utilizes the absorption line of narrow-linewidth laser scanning gas molecule to be measured, by analyzing concentration and the temperature that is obtained gas to be measured by the laser intensity after gas molecule absorption to be measured.With traditional gas concentration, compare with thermometry, the tunable laser absorption spectroscopy techniques has following advantage: non-cpntact measurement, laser pass gas to be measured and obtain absorption information, measuring process to gas flowfield to be measured without any interference; Measuring speed is fast, and the response time can reach the ms level; Measuring system realizes simple, and optical fiber emission and coupling scheme make it easily adapt to measurement environment and miniaturization; The measurement selectivity is strong, measures the interference that is not subjected to other gas compositions and variation thereof.Therefore, the tunable laser spectroscopy technology is widely used in the fields such as combustion diagnosis and environmental monitoring.For example, patent " in combustion gas turbine, the hot gas temperature of use tunable diode laser is measured " (application number: 201110278846.8) invented a kind of burning gases measurement mechanism in combustion gas turbine that is arranged on based on the tunable laser absorption spectroscopy techniques, for accurately measuring ignition temperature or other hot gas temperature of combustion gas turbine; Patent " tunable diode laser absorption spectroscopy temperature real-time detector " (application number: 201010583531.X) invented a kind of temperature monitor based on the tunable laser absorption spectroscopy techniques, utilize infrared laser beam to pass temperature province to be measured, utilize the characteristic absorpting spectruming line of steam in this zone to obtain in real time the temperature in zone to be measured; Patent " optical texture of laser absorption spectrum atmospheric methane concentration on-line monitoring the instrument " (patent No.: ZL200520071347.1) utilize the tunable laser spectroscopy technology to combine with the long path cell technology and form the on-Line Monitor Device of methane content in atmosphere.

When the concentration that adopts tunable laser absorption spectroscopy techniques measurement gas and temperature, the fluctuating of laser intensity is the one of the main reasons that produces ground unrest.When gas to the absorption of incident light hour, measurement result more easily is subject to the interference of ground unrest, and then causes larger measuring error, has limited detection sensitivity.Paper " research of tuning semiconductor laser absorption spectrum autobalanced detection method " (article numbering: 0253-2239 (2007) 02-0350-4) studied the auto-balanced detection circuit with electron gain compensation mechanism, the output signal of self-equilibrating circuit.Yet the output signal of self-equilibrating circuit has log characteristic, can not directly reflect the concentration of gas to be measured, need to determine fit equation by calibration and retrieving concentration, and calculate the concentration of gas.Because the variation of temperature can change absorption signal, therefore, for the situation of different temperatures, need to re-start demarcation to instrument, this has limited the practical application of the method to a certain extent.

Adopt two-way tunable laser absorption spectroscopy techniques, a road is the measuring beam that passes gas to be measured, and another road is for passing N ₂The path reference beam purified, the ground unrest that fluctuating produces of laser intensity can be reflected in the measuring-signal that the two-way light beam obtains simultaneously, the signal that the two-way light beam is obtained carries out normalized, can by with absorb irrelevant oblique varying signal and deduct, what stay is the part that reflection absorbs, and effectively suppresses the impact on measurement result of ground unrest that the fluctuating of laser intensity produces.In addition, owing in signal processing, measuring-signal not being carried out to nonlinear transformation, therefore, the absorption signal obtained after calculating can be directly used in the calculating of temperature and gas concentration.

Summary of the invention

For the impact of Background suppression noise on measurement result effectively, improve and measure sensitivity, and further reduce the measurement lower limit of tunable laser absorption spectroscopy techniques, the invention provides a kind of normalization noise-reduction method that adopts two-way tunable laser absorption spectroscopy techniques.

The technical solution adopted in the present invention is as follows:

Step 1, utilize optical fiber splitter that laser is divided into to two-way, and gas to be measured is passed in a road after the output of collimating mirror collimation, received by a photodetector, produces measuring-signal V _Meas, N is passed in another road after the output of collimating mirror collimation ₂The path purified, received by another photodetector, produces reference signal V _ref, described measuring-signal and reference signal are gathered by Multi-path synchronous sampled data capture card, and are transferred in computing machine;

Step 2, to described measuring-signal V _MeasWith described reference signal V _refCarry out normalized, through the linear function conversion, the amplitude transformation of described measuring-signal and reference signal arrived between [0,1],

$V_{\mathrm{meas}-\mathrm{norm}}=\frac{V_{\mathrm{meas}}-\min \left(V_{\mathrm{meas}}\right)}{\max \left(V_{\mathrm{meas}}\right)-\min \left(V_{\mathrm{meas}}\right)}---\left(1\right)$

$V_{\mathrm{ref}-\mathrm{norm}}=\frac{V_{\mathrm{ref}}-\min \left(V_{\mathrm{ref}}\right)}{\max \left(V_{\mathrm{ref}}\right)-\min \left(V_{\mathrm{ref}}\right)}---\left(2\right)$

V wherein _Meas-normMeasuring-signal after expression normalization, V _Ref-normReference signal after expression normalization, min (V) and max (V) mean respectively minimum value and the maximal value of signal V;

Step 3, the absorption signal α of calculating gas,

$\mathrm{\α}=\frac{V_{\mathrm{ref}-\mathrm{norm}}-V_{\mathrm{meas}-\mathrm{norm}}}{V_{\mathrm{ref}-\mathrm{norm}}}---\left(3\right)$

Effect of the present invention: the normalization noise-reduction method of employing two-way tunable laser absorption spectroscopy techniques of the present invention, by the normalized to measuring-signal and reference signal, oblique varying signal that will be irrelevant with absorption is deducted, what stay is the part that reflection absorbs, and the ground unrest that produces of the fluctuating that effectively suppresses laser intensity is on the impact of measurement result, the sensitivity that has improved measuring system.

The accompanying drawing explanation

Fig. 1 is the measuring system that adopts the normalization noise-reduction method of two-way tunable laser absorption spectroscopy techniques.

Fig. 2 is measuring-signal V _MeasWith reference signal V _ref.

Fig. 3 is the measuring-signal V after normalization _Meas-normWith the reference signal V after normalization _Ref-norm.

Fig. 4 is the absorption signal α that adopts the normalization noise-reduction method acquisition of two-way tunable laser absorption spectroscopy techniques _NormWith the absorption signal α that adopts traditional direct absorption process to obtain _DASContrast.

Fig. 5 is 1000 absorption signal α that adopt traditional direct absorption process to obtain _DASMean value α _AverWith α _Norm, α _DASContrast.

In Fig. 1: the 1-signal generator; The 2-laser controller; The 3-laser instrument; 4-1 * 2 optical fiber splitters; The output channel one of 5-optical fiber splitter; The output channel two of 6-optical fiber splitter; 7-fiber optic collimator mirror; 8-gas to be measured; 9-N ₂The path purified; The 10-photodetector; 11-Multi-path synchronous sampled data capture card; The 12-computing machine.

Embodiment

In the present embodiment, with H ₂The centre frequency of O is 7185.6cm ^-1Spectral line to H in air ₂O is absorbed as example, utilizes the normalization noise-reduction method of described two-way tunable laser absorption spectroscopy techniques to obtain described centre frequency and is 7185.6cm ^-1The absorption signal of spectral line, work as H ₂O to the absorption of incident light hour, with traditional direct absorption process, compare, the normalization noise-reduction method of described two-way tunable laser absorption spectroscopy techniques can suppress the impact on measurement result of ground unrest that fluctuating that laser intensity rises and falls produces effectively, effectively improve measuring accuracy, and further reduced the lower limit of measuring.

The present invention is further illustrated below in conjunction with accompanying drawing:

As shown in Figure 1, system comprises signal generator, laser controller, laser instrument, optical fiber splitter, fiber optic collimator mirror (two), photodetector (two), synchronously sampled data capture card and computing machine to the measuring system of the normalization noise-reduction method of employing two-way tunable laser absorption spectroscopy techniques of the present invention.Wherein, described laser controller is regulated laser frequency and power by changing temperature and electric current.Centered by described laser instrument, frequency is 7185.6cm ^-1Distributed feedback type semiconductor laser, exemplary power 20mW, laser linewidth is less than 10MHz.Described fiber optic collimator mirror output beam diameter is 0.75mm.Effective light-sensitive surface diameter of described photodetector is 1mm, and described photodetector is converted to electric signal by light signal.Described Multi-path synchronous sampled data capture card carries out synchronous acquisition to multiple signals, and is transferred to described Computer Storage.

Step 1, determine that by demarcating described centre frequency is 7185.6cm ^-1The design temperature of described laser controller corresponding to spectral line, by the output current of the described signal generator output frequency described laser controller of triangular modulation that is 500Hz, change the frequency of described laser instrument.By in the laser coupled to 1 of laser instrument output * 2 optical fiber splitters, the outlet of the optical fiber of optical fiber splitter is described fiber optic collimator mirror.Laser in the output channel one of described optical fiber splitter passes air after described fiber optic collimator mirror collimation, received by described photodetector, produces measuring-signal V _MeasLaser in the output channel two of optical fiber splitter passes through N after described fiber optic collimator mirror collimation ₂The path purified, namely do not pass air, received by another described photodetector, produces reference signal V _ref.Described measuring-signal V _MeasWith described reference signal V _refAs shown in Figure 2.

Step 2, to described measuring-signal V _MeasWith described reference signal V _refCarry out normalized, through the linear function conversion, by described measuring-signal V _MeasWith reference signal V _refAmplitude transformation between [0,1],

$V_{\mathrm{meas}-\mathrm{norm}}=\frac{V_{\mathrm{meas}}-\min \left(V_{\mathrm{meas}}\right)}{\max \left(V_{\mathrm{meas}}\right)-\min \left(V_{\mathrm{meas}}\right)}---\left(1\right)$

$V_{\mathrm{ref}-\mathrm{norm}}=\frac{V_{\mathrm{ref}}-\min \left(V_{\mathrm{ref}}\right)}{\max \left(V_{\mathrm{ref}}\right)-\min \left(V_{\mathrm{ref}}\right)}---\left(2\right)$

V wherein _Meas-normMeasuring-signal after expression normalization, V _Ref-normReference signal after expression normalization, min (V) and max (V) mean respectively minimum value and the maximal value of signal V, described measuring-signal V _MeasWith described reference signal V _refThe normalization result as shown in Figure 3;

Step 3, the absorption signal α of calculating gas _Norm, will with absorb irrelevant oblique varying signal deduction, and suppress the impact on measurement result of ground unrest that the fluctuating of laser intensity produces.

${\mathrm{\α}}_{\mathrm{norm}}=\frac{V_{\mathrm{ref}-\mathrm{norm}}-V_{\mathrm{meas}-\mathrm{norm}}}{V_{\mathrm{ref}-\mathrm{norm}}}---\left(3\right)$

The absorption signal α of the normalization noise-reduction method acquisition of two-way tunable laser absorption spectroscopy techniques will be adopted _NormThe absorption signal α obtained with traditional direct absorption process _DASContrast, as shown in Figure 4, described absorption signal α _NormFlatness obviously be better than α _DASFlatness.In order further to verify the correctness of the normalization noise-reduction method that adopts two-way tunable laser absorption spectroscopy techniques, by described α _Norm, α _DASThe absorption signal α obtained with 1000 described traditional direct absorption processes _DASMean value α _AverCompare, as shown in Figure 5, α _AverWith α _NormDegree of agreement be better than α _AverWith α _DASDegree of agreement.In sum, compare with described traditional direct absorption process, adopt the effectively impact of Background suppression noise on measurement result of normalization noise-reduction method of two-way tunable laser absorption spectroscopy techniques, improve the precision of measuring, and further reduce the lower limit of measuring.

Above description to the present invention and embodiment thereof, be not limited to this, shown in accompanying drawing, is only one of embodiments of the present invention.In the situation that do not break away from the invention aim, without designing and the similar structure of this technical scheme or embodiment, all belong to protection domain of the present invention with creating.

Claims (1)

1. normalization noise-reduction method that adopts two-way tunable laser absorption spectroscopy techniques, the method comprises the following steps:

Step 1, utilize optical fiber splitter that laser is divided into to two-way, and gas to be measured is passed in a road after the output of collimating mirror collimation, received by a photodetector, produces measuring-signal V _Meas, N is passed in another road after the output of collimating mirror collimation ₂The path purified, received by another photodetector, produces reference signal V _ref, described measuring-signal and reference signal are gathered by Multi-path synchronous sampled data capture card, and are transferred in computing machine;

Step 2, to described measuring-signal V _MeasWith described reference signal V _refCarry out normalized, through the linear function conversion, the amplitude transformation of described measuring-signal and reference signal arrived between [0,1],

V wherein _Meas-normMeasuring-signal after expression normalization, V _Ref-normReference signal after expression normalization, min (V) and max (V) mean respectively minimum value and the maximal value of signal V;

Step 3, calculate the absorption signal α of gas, will with absorb irrelevant oblique varying signal deduction, what stay is the part that reflection absorbs, and suppresses the impact on measurement result of ground unrest that the fluctuating of laser intensity produces.

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