The measuring method of weak fluorescence spectrum and device
Technical field
The present invention relates to spectrum detection technique, the particularly detection of atom and molecular spectrum and signal processing technology are specially the measuring method and the device of (atom and molecule) weak fluorescence spectrum.
Background technology
People are undertaken by spectral investigation for the understanding overwhelming majority of atom and molecule.From the weak fluorescence spectrum of laser and matter interaction generation, people can obtain the information of atom, molecule and surrounding environment thereof, thereby have obtained using widely in physics, chemistry and biological study.During Laser Measurement excited fluorescent spectrum, keep excitating light strength constant, the tuning continuously light wavelength that excites, and measure sample emitted fluorescence Strength Changes on certain wavelength location.As exciting light wavelength to be chosen in the peak value place of emissive porwer, the then measured intensity of emission spectra that gets is very big.By measure spectrum, obtain not homoatomic and molecule when intensity of emission spectra is maximum correspondence excite light wavelength.This has special function and significance for some industrial application, as measuring the concentration of atom and molecule, can be widely used in practical fields such as gas laser monitoring, aerochemistry constituent analysis.
When fluorescent radiation power is very little, adopt the method for photon counting to measure weak fluorescence spectrum usually.So-called photon counting is to use quantum counter to measure the photon probability distribution of light field.When prior art is measured weak fluorescence spectrum, adopt the external cavity type tunable diode laser to produce laser, on the piezoelectric ceramics of the feedback grating of mirror or exocoel behind this external cavity laser resonator cavity, apply the sweep voltage signal of triangular waveform (serrate), drive its rotation and realize the continuous adjusting of wavelength, thereby make this laser instrument produce the laser of light intensity constant (light intensity is to be determined by the electric current by laser diode), the slow linear change of wavelength, realize complete scan the whole fluorescent line of sample; With this laser radiation sample, use single-photon detector to receive the fluorescent photon that sample sends, single-photon detector is to the incident fluorescence photo response, and the incident photoelectricity that photon arrives is converted into the logic electric pulse output signal of a standard.With counter the number of electric pulse is counted, promptly write down the small time interval in (also claiming the sampling time) τ (because sampling time τ is very short, wavelength at this small time interval τ inner laser can be thought certain value approx) arrive the number of the logic electric pulse of counter, the count results of logic electric pulse is shown (as with oscillograph or directly show on computers), just can obtain the photon counting spectrum (as shown in Figure 3) of sample.-photon number multilist more shows that light intensity is big more.Therefore, the reflection of accompanying drawing 3 essence is the fluorescigenic Strength Changes of sample under different laser frequencies (or wavelength).Because the signal to noise ratio (S/N ratio) of existing measuring method is lower, therefore from accompanying drawing 3, can only find out the fluorescigenic intensity of sample corresponding under a certain frequency roughly, measuring accuracy is lower.
In photon counting spectrum, noise mainly comes from the thermonoise of substrate fluorescence, surround lighting, detector and the quantum shot noise that is risen and fallen and introduced by photon number.Substrate fluorescence and surround lighting can be inhibited by suitable door switch is set, and the thermonoise of detector can be inhibited by the temperature that reduces detector, and the quantum shot noise can't be inhibited in photon counting spectrum.For photon counting spectrum, its photon counting distributes and obeys the Poisson distribution, and photon number rises and falls can be expressed as F=(S τ)
1/2, wherein S is the counting rate of unit interval, τ is the sampling time.When detector works in low temperature environment, and carried out for surround lighting under the situation of light shield, the thermonoise of background stray light and detector can be ignored than the shot noise of signal, and the optimum signal-noise ratio of photon counting spectrum can be expressed as
In order to improve the signal to noise ratio (S/N ratio) of photon counting spectrum, the method that adopts is to increase sampling time τ usually.When sampling time τ was enough big, the mean value S τ that photon counting distributes was very big, the signal to noise ratio (S/N ratio) of photon counting spectrum (S τ)
1/2Also very big.But sample (molecule that for example has bleaching effect) in experiment or measuring system (for example laser instrument of energy and wave length shift) are when stablizing inadequately, and the long sampling time can't realize in actual measurement.Therefore, rely on the method that increases sampling time τ can not really effectively solve the existing low problem of measuring method signal to noise ratio (S/N ratio).
Summary of the invention
The present invention provides a kind of measuring method that can greatly improve the weak fluorescence spectrum of measuring signal to noise ratio (S/N ratio) in order to solve the low problem of measuring method signal to noise ratio (S/N ratio) of existing weak fluorescence spectrum.
Another object of the present invention is for providing the device of realizing above-mentioned measuring method.And be purpose only with the hardware configuration that improves this device.
The present invention adopts following technical scheme to realize: the measuring method of weak fluorescence spectrum, adopt the external cavity type tunable diode laser to produce laser, on the piezoelectric ceramics of the feedback grating of mirror or exocoel behind the resonator cavity of external cavity type tunable diode laser, apply the sweep voltage signal of triangular waveform, on the piezoelectric ceramics of the feedback grating of mirror or exocoel behind the external cavity type tunable diode laser resonator cavity, also apply a sine-shaped modulated voltage signal; With the laser radiation sample that laser instrument produces, use single-photon detector to receive the fluorescent photon that sample sends, single-photon detector is to the incident fluorescence photo response, and the incident photoelectricity that photon arrives is converted into the logic electric pulse output signal of a standard; In each sampling time τ, the logic electric pulse is counted (as shown in Figure 5); Count results number-Mo conversion with the logic electric pulse, count results is converted to analog voltage signal (being about to the analog voltage signal of count results linear transformation for being directly proportional with count results), the analog voltage signal of count results is carried out the even frequency multiplication demodulation of sinusoidal waveform modulated voltage signal, obtain tested weak fluorescence spectrum (harmonic signal) (as shown in Figure 6) and fluorescence spectrum is shown.At different fluorescence lifetime, different fluorescence intensities, to preferably analyze, determine the content of used optimal detection harmonic wave by the signal to noise ratio (S/N ratio) of the spectral signal of gained after the different even frequency multiplication demodulation of modulation signal.
External cavity type tunable diode laser general work is in 21 ℃ of constant temperature, and different laser instruments has different airborne centre wavelength, according to the difference of measurand, selects the laser instrument with centre wavelength corresponding with the measurand absorption peak.By superposition modulated sine wave signal on the scanning voltage signal, change the voltage that is added on the laser instrument piezoelectric ceramics, can realize the continuous variation of the several GHz of laser frequency, scan complete absorption line.By the process of modulation and demodulation, only allow that the modulating wave that comprises absorption signal obtains amplifying, and suppressed the background signal of other frequencies, promptly reduce quantum noise, thereby improve the signal to noise ratio (S/N ratio) of spectral signal by the compression noise bandwidth efficient.
In the prior art, irradiating laser can adopt continuous laser or laser pulse sequence.The method that continuous laser is transformed to laser pulse sequence is that continuous laser is passed through acousto-optic modulator, mask the zero level hot spot of acousto-optic modulator, keep the one-level hot spot and enter subsequent optical path, carry out for first-order diffraction light by on the driver module of acousto-optic modulator, applying logic control signal that gating realizes.Under the application scenario of laser pulse sequence irradiation sample, according to the ultimate principle of signals collecting, the repetition period of sampling time τ should be far above the repetition period of laser pulse sequence.And the repetition frequency that preferably makes laser pulse sequence is the integral multiple of the repetition frequency of sampling time τ, to collect count signal more accurately.The same with prior art, the laser among the present invention also can adopt continuous laser or laser pulse sequence.Because the present invention has applied modulated voltage signal, in order fully to collect the variation of modulated voltage signal, same ultimate principle according to signals collecting, under the application scenario of continuous laser irradiation sample, the repetition frequency of sampling time τ should be far above the frequency of modulated voltage signal (cycle of sampling time τ should far below the cycle of modulated voltage signal).Under the application scenario of light pulse sequence irradiation sample, the cycle of sampling time τ should be far below the cycle of modulated voltage signal, and far above repetition period of light pulse sequence.And the repetition frequency that preferably makes light pulse sequence is the integral multiple of the repetition frequency of sampling time τ, and the repetition frequency of sampling time τ is the integral multiple of the repetition frequency of modulated voltage signal.
Among the present invention, the count results of logic electric pulse number-Mo conversion can be adopted multiple known integrated chip.The present invention selects the multifunctional data acquisition card of National Instruments, NI6251 model for use.This multifunctional data acquisition card has the dual-use function of counting and analog-to-digital conversion, and therefore, the counting of logic electric pulse and number-the Mo conversion can be finished on this multifunctional data acquiring capture card, thus simplified structure.The analog voltage signal of count results carries out the even frequency multiplication demodulation of sinusoidal waveform modulated voltage signal and can adopt correlator to finish.
The present invention adopts low frequency Sine Modulated voltage as laser modulation signal, with the triangle wave voltage is the laser swept-frequency signal, excite the faint fluorescence of generation by exploring laser light, the photon counting spectrum of the fluorescence that obtains modulated, after converting the photon counting result to corresponding analog voltage signal, obtain the fluorescence spectrum of actual tested faint fluorescence by the harmonic wave demodulation.Suppressed the quantum shot noise in the Photoncounting methods effectively, can effectively suppress the continuous background noise, improve the signal to noise ratio (S/N ratio) of weak fluorescence spectrum, increased the resolution of weak fluorescence spectrum, can be applicable to monatomic, the real-time monitoring of molecule and the measurement of other weak fluorescence spectrums.
Description of drawings
Fig. 1 is for realizing the apparatus structure synoptic diagram of the method for the invention;
The scanning voltage waveform figure that Fig. 2 applies for the laser instrument control end;
Fig. 3 is for only applying the photon counting spectrum of scanning voltage;
Voltage oscillogram after Fig. 4 scans and modulates for the laser instrument control end applies;
Fig. 5 be apply scanning and modulation voltage after photon counting spectrum;
Fig. 6 is the fluorescence spectrum after the harmonic wave demodulation.
Among Fig. 1: 1,2-function signal generator, 3-compositor, 4-external cavity type tunable diode laser, the 5-saturation-absorption spectrum, the 6-acousto-optic modulator, 7-transmissibility of adjustable attenuation piece, 8-sample cell, the 9-single-photon detector, the 10-He-Ne laser instrument, 11-computing machine, 12-correlator, the 13-pulse signal generator, 14-(10
-2) frequency divider, 15-(10
-4) frequency divider, the 16-frequency dividing circuit.
Embodiment
Device to specific implementation measuring method of the present invention is described below, and 1 apparatus structure synoptic diagram is further described in detail measuring method of the present invention in conjunction with the accompanying drawings.This embodiment is used to measure the occasion of caesium steam weak fluorescence spectrum.
The device of realizing measuring method of the present invention comprises external cavity type tunable diode laser 4, sample cell 8, is positioned at the signal generator 2 of the single-photon detector 9 of sample cell side, triangular wave, pulse signal generator 13, the frequency dividing circuit 16 that links to each other with pulse signal generator 13, computing machine 11, the signal output part of single-photon detector 9 links to each other with computing machine, and the output terminal of frequency dividing circuit 16 links to each other with computing machine 11; The function signal generator 1 and the compositor 3 that also comprise sinewave modulation signal, the output of the function signal generator 1 of sinewave modulation signal and the signal generator 2 of triangular wave links to each other with compositor 3 respectively, the voltage controling end of the piezoelectric ceramics of the feedback grating of mirror or exocoel links to each other behind the resonator cavity of the output of compositor and external cavity type tunable diode laser 4, frequency dividing circuit 16 links to each other with the function signal generator 1 of sinewave modulation signal, be provided with multifunctional data acquisition card in the computing machine 11, be connected with correlator 12 with computing machine 11, frequency dividing circuit 16 links to each other with correlator 12.Described frequency dividing circuit 16 is by 10
-2 Frequency divider 14 and 10
-4 Frequency divider 15 constitutes 10
-2 Frequency divider 14 links to each other 10 with computing machine 11
-4 Frequency divider 15 links to each other with correlator 12 with the function signal generator 1 of sinewave modulation signal.Be provided with acousto-optic modulator 6 and transmissibility of adjustable attenuation piece 7 between external cavity type tunable diode laser 4 and sample cell 8, simultaneously, pulse signal generator 13 links to each other with the driver module of acousto-optic modulator 6, with this continuous laser is transformed to laser pulse sequence.
Referring to accompanying drawing 1.Function signal generator 1 (the HP of sine wave output modulation signal, the 33120A type) and output triangular wave signal generator 2 (HP, the 33120A type) output respectively with compositor 3 (MiniCircuit, the ZFRSC-2050 type) links to each other, the output of compositor is applied on the voltage controling end of piezoelectric ceramics of feedback grating of mirror behind external cavity type tunable diode laser 4 (Sacher, the TEC-100 type) resonator cavity or exocoel.Become low light level pulse train after continuous laser process acousto-optic modulator 6 (Crystal Technology, the 3080-122 type) copped wave of laser instrument output and transmissibility of adjustable attenuation piece 7 decay, incident is filled with the sample cell 8 of caesium atom saturated vapour.Single-photon detector 9 (Perkin-Elmer, SPCM-AQR-15 type) is collected fluorescence from the side of sample cell 8, and the photon that detects is converted into the logic electric impulse signal.Multifunctional data acquisition card (the National Instruments that computing machine 11 is built-in, the NI6251 type) the count results linearity that measures is converted into corresponding simulating signal, the output terminal of computing machine 11 simulating signals and correlator 12 (Stanford Research Systems, SR830 type) link to each other.Computing machine 11 handles the harmonic signal of autocorrelator 12, draws testing result.
In order to guarantee the strict synchronism of whole device, (SRS, DG535) Shu Chu frequency is f to pulse signal generator 13
RepThe fundamental frequency signal of=0.98MHz has obtained two class frequencys and has been respectively f through frequency dividing circuit
Rep/ 10
2And f
Rep/ 10
4Clock signal.First class frequency is f
Rep/ 10
2Clock signal be connected on the built-in data collecting card of computing machine 11, be used to be provided with sampling time τ=10 of counter
2/ f
RepSecond class frequency is f
Rep/ 10
4Clock signal be used for the external trigger synchronizing signal of function generator 2.Function generator 2 output frequencies are f
Rep/ 10
4Sinusoidal modulation signal, the triangular signal of the 0.5Hz of and function generator 1 output is superimposed by totalizer, is loaded into the modulation and the frequency sweep that are used for laser wavelength on the piezoelectric ceramics of laser instrument 4.
In this example, the external cavity type diode laser works in 21 ℃ of constant temperature, and airborne centre wavelength is 852.1nm.Change the voltage that is added on the laser instrument piezoelectric ceramics, can realize the continuous variation of the several GHz of laser frequency, scan complete absorption line.In sample cell, be evacuated to 133.3 * 10
-6Fill behind the Pa with the simple metal caesium, with caesium atom saturated vapour as tested gas.The caesium atom vapor is influenced by the temperature broadening mainly under room temperature and ultralow pressure state, and absorption signal presents Gaussian lineshape.Write down the saturated absorption spectrum of Cs atom D2 line in the experiment, be used for frequency reference.
The quantum efficiency of single-photon detector 9 at the 852nm place of using in this example is~48%, and the dead time is 50ns, on average calculates mentally digit rate and is lower than 50cps.The laser that the He-Ne laser instrument sends is used to simulate the ground unrest under the different background light intensity.
The fluorescence that sample cell sends is after the process detector is converted into the logic electric impulse signal, by the counting module collection (shown in Figure 5) of data collecting card.But and these digital signal linearities have been converted the simulating signal of demodulation to, the multiplication factor g=0.3mV/ counting of digital-to-analog conversion.Simulating signal input correlator is used for even-order harmonic nf (n=2,4,6) demodulation.The instrument control and the data acquisition of whole device are finished by the computer under the corresponding software support.
In this example, the harmonic wave of demodulation gained is chosen as even-order harmonic, and this is because the peak of even-order harmonic is consistent with the peak of direct absorption signal (not having modulation), thereby is convenient to analyze.For odd harmonic, directly the peak correspondence of absorption signal is the null position of harmonic wave, does not consider.
Fig. 2 is the used sweep voltage of wavelength of regulating laser instrument continuously, Fig. 3 is for applying the photon counting spectrum that obtains when sweep voltage does not apply modulation voltage on laser instrument, the voltage signal of Fig. 4 when being added to modulation voltage on the sweep voltage, the photon counting spectrum that Fig. 5 is added to and obtains on the laser instrument for modulation voltage.
Among Fig. 6, (a) and (b) and secondary, four times and the sixth harmonic signal (c) representing respectively to detect.Record the signal noise ratio SNR=6.1 (Fig. 3) of the fluorescence spectrum that existing Photoncounting methods obtains, the signal noise ratio that corresponding secondary, four times and fifth overtone are surveyed is respectively SNR=2305, SNR=2267, SNR=1780.The modulation broadening of higher hamonic wave obviously reduces, but signal to noise ratio (S/N ratio) is not further improved.In this example, use second harmonic more to help improving signal to noise ratio (S/N ratio).The photon counting that this example has effectively been eliminated in the photon counting spectrum by the photon counting modulated spectrum rises and falls, and has improved the signal to noise ratio (S/N ratio) of photon counting spectrum, the sensitivity that has improved weak fluorescence spectrum.
Can estimate from this routine measurement result, minimum detectable photon number is 5 * 10
-4Photon/pulse, this standard are far below the 0.1 photon/pulse under the common practical condition, and therefore this method is applicable to nearly all weak fluorescence spectrum measurement occasion.