CN103487392B - Frequency domain cavity ring-down spectroscopy detection apparatus and method - Google Patents

Abstract

The invention provides a frequency domain cavity ring-down spectroscopy detection apparatus and a method. The apparatus comprises a light source, an optical path system and a data process system. The optical path system comprises an introduction fiber collimator, a first reflector, a second reflector and a derivation fiber collimator, which are connected in order. The data process system comprises a derivation fiber, a photoelectric detection circuit and a data processing circuit, which are connected in order. A light source for emitting light wave signals introduces light wave signals into the introduction fiber collimator through the introduction fiber. The frequency domain cavity ring-down spectroscopy detection apparatus and the method provided by the invention utilize low-speed wavelength modulation to realize high-frequency harmonic wave detection, automatic elimination of a plurality of slow-varying interferences, automatic elimination of interferences caused by decrease of resonant cavity fineness.

Description

A kind of frequency domain cavity ring-down spectroscopy detection device and method

Technical field

The invention belongs to spectrographic detection and technical field of optical fiber sensing, it is related to a kind of spectral detection device and method, more particularly, to a kind of frequency domain cavity ring-down spectroscopy detection device and method.

Background technology

In various supersensitive spectrographic detection technology, it is an important class based on the enhanced technology of passive resonant cavity.The common trait of this class method is：Testing sample being placed in resonance intracavity, using resonator cavity, the multiple reflections of light wave being increased with the distance of light and material effect, thus increasing the amplitude of spectral signal.

Following a few class can be divided into based on resonator cavity enhanced spectrographic detection technology：1. traditional cavity ring-down spectroscopy, including light-pulse generator and two kinds of working methods of continuous light source；2. integration chamber output spectrum；3. cavity reinforced absorption spectrum；4. frequency modulation(PFM) light-source chamber ring-down spectroscopy；5. frequency comb cavity ring-down spectroscopy etc..

Formerly one of technology, a light pulse is imported resonator cavity, this pulse meeting multiple reflections between two end faces of resonator cavity, the output of every secondary reflection all can reduce than last.This pulse train exponentially changes in time, and wherein the time constant of decay can reflect the spectral absorption of intracavity testing sample.The shortcoming of this method is, after pulse passes through two high reflectance minute surfaces, only remaining very small part can reach Electro-Optical Sensor Set, increased the difficulty of photodetection.And, in order to detect extremely short pulse and therefrom extract attenuation constant, need the digital circuit of hypervelocity.

Formerly the two of technology, the continuous light wave of narrow linewidth is imported resonator cavity, and after meeting standing-wave condition, rapid cut-out light source, then detects the attenuation curve of Output optical power.This method is actually the intracavity photon lifetime detecting resonator cavity, can calculate the spectral absorption of testing sample by intracavity photon lifetime.This technology is referred to as continuous wave-cavity ring-down spectroscopy, very high measurement sensitivity can be reached, but due to there is the problem of mismatch between light source longitudinal mode resonant cavity longitudinal mode, need the locking injection of light source technology of expensive acousto-optic modulator and high-precision Frequency Stabilization Technique or complexity.

Formerly the three of technology, the light wave that wavelength is modulated injects resonator cavity, uses the chamber length of modulation with PZT resonator cavity it is ensured that resonator cavity longitudinal mode is synchronously changed with light source longitudinal mode simultaneously.This method is a kind of common approach with reference to Wavelength modulation spectroscopy resonant cavity enhanced spectrum.（Wavelength modulation spectroscopy is a kind of wide variety of high sensitivity spectrographic detection technology, very effectively can improve sensitivity further it is often more important that, much gradual interference can be eliminated.）This technology the disadvantage is that, the modulating frequency that can be provided by of piezoelectric ceramics is very low, limit the frequency of wavelength modulation, thus limit the performance of Wavelength modulation spectroscopy performance.In addition, length adjustable resonator cavity in chamber is also expensive.

Formerly the four of technology, the continuous light source of length scanning is injected resonator cavity, by calculating the time integral inverting intracavity spectral absorption of cavity-type BPM process.This method can use the continuous light source of length scanning, be singly can not to combine with Wavelength modulation spectroscopy, and measurement sensitivity is not also high.

Formerly the five of technology, the light wave of length scanning are injected resonator cavity, then the signal of multiple length scanning are done average treatment.This method is referred to as resonator cavity enhanced spectrum although can combine with Wavelength modulation spectroscopy, but this method response speed is slow, and sensitivity is not also high, can only the inexpensive at last, method of low performance.

Formerly the six of technology, with high frequency modulation(PFM) optical source wavelength, make the dominant frequency of light wave and each side frequency just can pass through each longitudinal mode of resonator cavity.This mode has reached very high sensitivity, is highest in all resonator cavity spectrographic techniques.Shortcoming is high frequency modulated inherently costly, but also is there is longitudinal mode mismatch between light source and resonator cavity.

Formerly the seven of technology, with a mode-locked laser as light source, make each frequency content of frequency comb produced by locked mode may be passed through a longitudinal mode of resonator cavity.This solves the too low problem of traditional light-pulse generator cavity-type BPM power, but mode-locked laser itself is not cheap, and there is also the problem of pattern match between light source and resonator cavity.

Content of the invention

In order to solve the technical problem in the presence of background technology, the present invention proposes a kind of frequency domain cavity ring-down spectroscopy detection device and method.Present invention could apply to the fine constant measuring of the spectroscopic assay of laboratory and etalon, and material concentration sensing, strain and stress sensing, temperature sensing etc. that scene is applied.

The technical scheme is that：

1. a kind of frequency domain cavity ring-down spectroscopy detection device, it is characterized in that：Including light source, light path system data processing system；

Importing optical fiber collimator that above-mentioned light path system includes being sequentially connected, the first reflecting mirror, the second reflecting mirror, derive optical fiber collimator；

Derivation optical fiber that above-mentioned data handling system includes being sequentially connected, photodetection circuit, data processing circuit；

The above-mentioned light source for launching lightwave signal passes through importing optical fiber and lightwave signal is imported optical fiber collimator；

Above-mentioned data handling system is passed through to derive the lightwave signal of optical fiber receiving light path system；

Above-mentioned derivation optical fiber is connected with each other with deriving optical fiber collimator.

The lightwave signal of above-mentioned light source transmitting is the lightwave signal through wavelength linear scanning.

2. a kind of frequency domain cavity ring-down spectroscopy detection method of frequency domain cavity ring-down spectroscopy detection device, it is characterized in that：Step includes：

1】Light source sends lightwave signal and imports light path system；

1.1] light source sends lightwave signal through importing optical fiber, reaches and imports optical fiber collimator；

1.2] lightwave signal passes through to import optical fiber collimator entrance light path system；

2】Lightwave signal reflects in light path system；

2.1] lightwave signal passes through to import optical fiber collimator injection, through the first reflecting mirror, is then passed through testing sample and is absorbed by testing sample, reaches the second reflecting mirror；

2.2] the second reflecting mirror exports a part of light wave, simultaneously by most of light wave reflection, is passed through testing sample and is absorbed by the light wave of the second reflecting mirror reflection, reach the first reflecting mirror；First reflecting mirror leaks away a part of light wave, and most of light wave reflection is passed through testing sample, reaches the second reflecting mirror；

2.2] step 2.1 is repeated several times], step 2.2], the output signal eventually passing through the second reflecting mirror is to interfere with each other the lightwave signal of superposition through multiple reflections；

3】Lightwave signal enters data handling system by light path system；

3.1] lightwave signal, again through deriving optical fiber collimator, deriving optical fiber arrival photodetection circuit, is converted into the signal of telecommunication；

3.2] signal of telecommunication does Fourier transformation by data processing circuit and obtains signal of telecommunication frequency spectrum；

4】The top of each harmonic signal in frequency spectrum is coupled together and obtains a curve；By below equation（1）、（2）Curve can be obtained：

$a\left(f\right)\∝\exp [-\frac{f}{{\mathrm{\τ}}_f}]---\left(1\right)$

${\mathrm{\τ}}_f=\frac{L}{C(\mathrm{\αL}-\ln R)}2\stackrel{\·}{v}---\left(2\right)$

Described n is the exponent number of Fourier coefficient；

Described R is the power reflectance of resonator surface reflecting mirror；

Described L is the length of resonator cavity.

Described τ_fIt is the attenuation constant of frequency spectrum；

Described f is the frequency of signal；

5】Obtain the spectral absorbance values of testing sample；

By formula（3）Can be from attenuation constant τ_fIn be finally inversed by the spectral absorption α of test substance；Formula（3）As follows：

$\mathrm{\α}=\frac{1}{L}[\frac{2L\stackrel{\·}{v}}{C{\mathrm{\τ}}_f}+\ln R]---\left(3\right)$

Described C is the light velocity；

Described α is the spectral absorbance values of intracavity testing sample.

It is an advantage of the invention that：

1. the present invention achieves the Harmonic Detection of high frequency using the modulation of low speed wavelength.Due to there is much individual resonator cavity longitudinal modes in the range of length scanning, so the frequency of output pulse signal is equivalent to a lot of times of rate of scanning.In general high-frequency harmonic detects the advantage with signal to noise ratio aspect, and the modulation of low speed wavelength is easier to realize；

2. the present invention achieves the perfect adaptation of resonator cavity enhanced spectrum and Wavelength modulation spectroscopy using cheap cost.Formerly it is intended to various shortcomings are all suffered from the effort with reference to both approaches in technology, such as muting sensitivity, high price, be not suitable for live application, complex operation etc.；

3. method proposed by the invention can eliminate multiple gradual interference automatically.The impact of the ambient interferences such as the decline of light path coupling efficiency, the fluctuation of light source mean power, detection circuit decrease in efficiency due to combining Wavelength modulation spectroscopy, can be eliminated.

4. method proposed by the present invention can eliminate the interference that the decline of resonance fineness of cavity causes automatically, and this problem annoyings the spectrographic detection technology based on resonator cavity of routine always.Due to combining Wavelength modulation spectroscopy, it is a relation being multiplied that resonance fineness of cavity declines between the signal that the interference producing is produced with spectral absorption, can be eliminated by taking the logarithm or calculating the ratio between each harmonic.

Brief description

Fig. 1 is the structural representation of the present invention；

Fig. 2 is the output schematic diagram that wavelength of the present invention passes through after resonator cavity according to the light source that sawtooth fashion scans；

Fig. 3 a is the optical power signals schematic diagram of resonator cavity output in the present invention；

Fig. 3 b is the optical power signals spectrogram of resonator cavity output in the present invention；

Fig. 4 is that the embodiment of the present invention 1 realizes gas sensing schematic diagram using optical fibre ring resonator cavity；

Fig. 5 is that the embodiment of the present invention 2 realizes strain sensing schematic diagram using linear type fiber resonance cavity；

Fig. 6 is that the embodiment of the present invention 3 realizes temperature sensing schematic diagram using wave guide ring shaped resonant cavity；

Wherein, 1- light source, 2- import optical fiber, 3- importing optical fiber collimator, 4- first reflecting mirror, 5- testing sample, 6- second reflecting mirror, 7- crust of the device, 8- derivation optical fiber collimator, 9- derivation optical fiber, 10- photodetection circuit, 11- data processing circuit.

Specific embodiment

Referring to Fig. 1-5, spectrographic detection comprises the following steps that：

1】Light source sends lightwave signal and imports light path system；

1.1] light source sends lightwave signal through importing optical fiber, reaches and imports optical fiber collimator；

1.2] lightwave signal passes through to import optical fiber collimator entrance light path system；

2】Lightwave signal reflects in light path system；

2.1] lightwave signal passes through to import optical fiber collimator injection, through the first reflecting mirror, is then passed through testing sample and is absorbed by testing sample, reaches the second reflecting mirror；

2.2] the second reflecting mirror exports a part of light wave, simultaneously by most of light wave reflection, is passed through testing sample and is absorbed by the light wave of the second reflecting mirror reflection, reach the first reflecting mirror；First reflecting mirror leaks away a part of light wave, and most of light wave reflection is passed through testing sample, reaches the second reflecting mirror；

2.2] step 2.1 is repeated several times], step 2.2], the output signal eventually passing through the second reflecting mirror is to interfere with each other the lightwave signal of superposition through multiple reflections；

3】Lightwave signal enters data handling system by light path system；

3.1] lightwave signal, again through deriving optical fiber collimator, deriving optical fiber arrival photodetection circuit, is converted into the signal of telecommunication；

3.2] signal of telecommunication does Fourier transformation by data processing circuit and obtains signal of telecommunication frequency spectrum；

4】The top of each harmonic signal in frequency spectrum is coupled together and obtains a curve；By below equation（1）、（2）Curve can be obtained：

$a\left(f\right)\∝\exp [-\frac{f}{{\mathrm{\τ}}_f}]---\left(1\right)$

${\mathrm{\τ}}_f=\frac{L}{C(\mathrm{\αL}-\ln R)}2\stackrel{\·}{v}---\left(2\right)$

Described n is the exponent number of Fourier coefficient；

Described R is the power reflectance of resonator surface reflecting mirror；

Described L is the length of resonator cavity.

Described τ_fIt is the attenuation constant of frequency spectrum；

Described f is the frequency of signal；

5】Obtain the spectral absorbance values of testing sample；

By formula（3）Can be from attenuation constant τ_fIn be finally inversed by the spectral absorption α of test substance；Formula（3）As follows：

$\mathrm{\α}=\frac{1}{L}[\frac{2L\stackrel{\·}{v}}{C{\mathrm{\τ}}_f}+\ln R]---\left(3\right)$

Described C is the light velocity；

Described α is the spectral absorbance values of intracavity testing sample.

Referring to Fig. 2, because the wavelength of light source 1 is linear scanning, as shown in the jaggies in λ 0t face.If certain moment, optical source wavelength is just directed at the passband in transmission spectrum, and the luminous power of output is just high；If being not aligned with passband, the luminous power of output will be very low.Therefore, after the light wave of linear scanning passes through resonator cavity, its power also can become pectination over time.

Optical signal from reflecting mirror 6 output is the light wave carrying out algorithm for power modulation according to comb function form, and this optical signal through deriving optical fiber collimator 8, deriving optical fiber 9 arrival photodetection circuit 10, is converted into the signal of telecommunication again；

Because optical signal is that the output of photodetector 10 is also comb function, as shown in Figure 3 a according to comb function amplitude modulation(PAM)；Data processing circuit 11 does Fourier transformation to the signal of this pectination or similar algorithm obtains its frequency spectrum, as shown in Figure 3 b.

The top of each harmonic signal in frequency spectrum is coupled together and obtains a curve, such as Fig. 3（b）Middle dotted line.According to the expression formula that theory analysis understands this curve it is：

$a\left(f\right)\∝\exp [-\frac{f}{{\mathrm{\τ}}_f}]---\left(1\right)$

Wherein, τ_fIt is the attenuation constant of frequency spectrum, f is the frequency of signal.There is relation：

${\mathrm{\τ}}_f=\frac{L}{C(\mathrm{\αL}-\ln R)}2\stackrel{\·}{v}---\left(2\right)$

Wherein C be the light velocity,It is the derivative to the time for the light frequency, that is, length scanning speed, α are the spectral absorption of intracavity testing sample.

Using formula（2）Can be easy to from attenuation constant τ_fIn be finally inversed by the spectral absorption α of testing sample.I.e.

$\mathrm{\α}=\frac{1}{L}[\frac{2L\stackrel{\·}{v}}{C{\mathrm{\τ}}_f}+\ln R]---\left(3\right)$

Embodiment 1 is as shown in figure 4, the light wave that the LASER Light Source 1 of linear length scanning sends enters the one arm of the fiber coupler 3 of high splitting ratio through importing optical fiber 2, subsequently into sensitive optical fibre 4.The effect of sensitive optical fibre 4 is to make light wave contact with the testing sample in environment, thus the light energy of a part can be absorbed by the spectrum of material.Sensitive optical fibre 4 can have a variety of forms, such as D-shaped optical fiber, photonic crystal fiber, LPFG, conical fiber etc., and the cross section that in figure depicts D-shaped optical fiber is used as example.Enter a light arm of fiber coupler 5 through the light wave of sensitive optical fibre, wherein sub-fraction light energy exports derivation optical fiber 7, and most of light energy is assigned to optical fiber 6.Light wave in optical fiber 6 again passes by bonder 3, sensitive optical fibre 4, returns to bonder 5, forms a loop.The output of system stability is exactly the result of the light wave multiple stacking in this loop.Bonder 3, bonder 5, sensitive optical fibre 4, optical fiber 6 together constitute fiber annular resonant cavity.The light wave of resonator cavity output enters photodetection circuit 8, finally processes, by data processing circuit 9, the concentration obtaining under test gas material.

Embodiment 2 is as shown in Figure 5.Each it is inserted with highly reflecting films, as the end mirror of resonator cavity in the joints of optical fibre 3 and the joints of optical fibre 8.Adapter 3,8 and optical fiber 4 together constitute linear type fiber resonance cavity.The light wave that the LASER Light Source 1 of linear length scanning sends is through importing optical fiber 2, then enters resonator cavity by the joints of optical fibre 3, exports from the joints of optical fibre 8 through optical fiber 4.Again through deriving optical fiber 9, photodetection circuit 10, finally reach data processing circuit 11.The cavity 4 of fiber resonance cavity is fixed on workpiece for measurement 6 by fixing device 5 and 7.When workpiece 6 produces strain, optical fiber 4 also can be stretched therewith, and the chamber length of resonator cavity becomes big, leads to attenuation constant τ_fChange.By measuring τ_fChange, the dependent variable of workpiece for measurement can be perceived.

Embodiment 3 is as shown in Figure 6.Wave guide ring shaped resonant cavity is to have made input straight waveguide 3, output straight waveguide 5 and disc waveguide 4 on substrate 9.The distance between straight waveguide 3 and disc waveguide 4 very little, can lead to a part of light energy to be transferred to disc waveguide；The part energy of disc waveguide in the same manner 4 also can be transferred to straight waveguide 5；Light wave constantly runs in disc waveguide, and final output is to interfere the result being added.The light wave that the LASER Light Source 1 of linear length scanning sends, through Transmission Fibers 2, enters resonator cavity by straight waveguide 3.The light wave of multiple-beam interference exports from derivation straight line fiber waveguide 5, through Transmission Fibers 6, enters photodetection circuit 7, finally reaches data processing circuit 8.Whole waveguide resonant cavity is placed in environment to be measured as the sensing element of temperature sensing, and the change of ambient temperature can lead to the change of the length of resonator cavity, is finally reflected in attenuation constant τ_fOn, by measuring τ_fAmbient temperature can be calculated.

Claims (1)

1. a kind of frequency domain cavity ring-down spectroscopy detection method it is characterised in that：Step includes：

1】Light source sends lightwave signal and imports light path system；

1.1] light source sends lightwave signal through importing optical fiber, reaches and imports optical fiber collimator；

1.2] lightwave signal passes through to import optical fiber collimator entrance light path system；

2】Lightwave signal reflects in light path system；

2.1] lightwave signal passes through to import optical fiber collimator injection, through the first reflecting mirror, then Absorb through testing sample and by testing sample, reach the second reflecting mirror；

2.2] the second reflecting mirror exports a part of light wave, simultaneously by most of light wave reflection, quilt The light wave of the second reflecting mirror reflection passes through testing sample and is absorbed, and reaches the first reflecting mirror； First reflecting mirror leaks away a part of light wave, and most of light wave reflection is passed through testing sample, Reach the second reflecting mirror；

2.3] step 2.1 is repeated several times], step 2.2], eventually pass through the defeated of the second reflecting mirror Going out signal is the lightwave signal interfering with each other superposition through multiple reflections；

3】Lightwave signal enters data handling system by light path system；

3.1] lightwave signal is again through deriving optical fiber collimator, deriving optical fiber arrival photodetection Circuit, is converted into the signal of telecommunication；

3.2] signal of telecommunication does Fourier transformation by data processing circuit and obtains signal of telecommunication frequency spectrum；

4】The top of each harmonic signal in frequency spectrum is coupled together and obtains a curve；Pass through Below equation (1), (2) can obtain curve：

$a\left(f\right)\∝\exp \[-\frac{f}{{\mathrm{\τ}}_f}\]---\left(1\right)$

Described a (f) is the change with signal frequency for the envelope of signal spectrum；

Described τ_fIt is the attenuation constant of frequency spectrum；

Described f is the frequency of signal；

${\mathrm{\τ}}_f=\frac{L}{C(\mathrm{\α}L-\ln R)}2\stackrel{\·}{v}---\left(2\right)$

Described L is the light path of resonance intracavity；

Described α is the spectral absorbance values of intracavity testing sample；

Described R is the power reflectance of resonator surface reflecting mirror；

DescribedThe frequency scan rate of light wave, i.e. the light frequency of unit interval change；

Described C is the light velocity；

5】Obtain the spectral absorbance values of testing sample；

Can be from attenuation constant τ by formula (3)_fIn be finally inversed by the spectral absorption of test substance α；Formula (3) is as follows：

$\mathrm{\α}=\frac{1}{L}\[\frac{2L\stackrel{\·}{v}}{{\mathrm{C\τ}}_f}+\ln R\]---\left(3\right)$

Described C is the light velocity；

Described L is the light path of resonance intracavity；

Described R is the power reflectance of resonator surface reflecting mirror；

DescribedThe frequency scan rate of light wave, i.e. the light frequency of unit interval change；

Described α is the spectral absorbance values of intracavity testing sample.