CN105424650A - Quantum cascade laser-based high-speed infrared frequency modulation laser spectrum gas detection system and method - Google Patents
Quantum cascade laser-based high-speed infrared frequency modulation laser spectrum gas detection system and method Download PDFInfo
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Abstract
The invention relates to a quantum cascade laser-based high-speed infrared frequency modulation laser spectrum gas detection system and method. The system comprises a quantum cascade laser spectrum scanning module, a high-speed frequency modulation module, a synchronization module, a gas multi-returning cavity and a frequency modulation spectrum acquisition module. On the basis of a conventional gas detection system, a distributed feedback quantum cascade laser is used, and a mid-infrared laser is utilized to generate intensity-modulated modulated light for irradiating and pulsating an emergent end face of the quantum cascade laser, thus realizing all-optical modulation on the quantum cascade laser; control and quantitative detection on a gas sample are completed by the gas multi-returning cavity; an all-optical modulation gas absorption spectrum is collected by a high-speed mid-infrared detector, a high-speed lock-in amplifier, a high-speed signal collection card and a computer, thus realizing high-speed high-accuracy gas detection. The high-speed infrared frequency modulation laser spectrum gas detection system can be applied to a high-sensitivity infrared laser spectrum technology and trace gas detection.
Description
Technical field
The invention belongs to infrared laser spectroscopy technical field, particularly relate to a kind of high speed infrared laser of frequency modulation spectroscopic gas detection system based on quantum cascade laser and method, this system can realize at a high speed, the sample gas under study of high s/n ratio detects, and can be applicable to the application of high speed, high sensitivity infrared laser spectroscopy technology and Trace gas detection.
Background technology
In existing gas detection technology, laser spectrum gas detect method has highly sensitive feature, quantum cascade laser has that line width, power are high, the advantage such as can work under room temperature environment, especially INFRARED QUANTUM CASCADE LASERS, its output wavelength sweep limit is positioned at the Fingerprint district of gas, especially has huge advantage for Trace gas detection.Direct absorption spectra detection method at present based on quantum cascade laser can realize detecting fast, but its absorption signal is in low-frequency range, and signal to noise ratio (S/N ratio) is low; Also quantum cascade laser frequency modulation spectrum is had to be applied to the report of gas detect at present, although these kind of method improves signal to noise ratio (S/N ratio), improve the detection limit of system, but owing to adopting slope current slow scanning mode, its detection time is relatively long, generally in millisecond magnitude.
Technical background list of references:
[1]G.Hancock,G.Ritchie,J.V.Helden,R.Wallker.“Applicationsofmidinfraredquantumcascadelaserstospectroscopy,”OpticalEngineering,Vol.49,111121(2010).
[2]L.Wang,T.R.Shaples.“Intrapulsequantumcascadelaserspectroscopy:pressureinducedlinebroadeningandshiftinginthev6bandofformaldehyde,”AppliedPhysicsB,Vol.108,pp.427-435(2012).
[3]R.Eichholz,H.Richter,M.Wienold,L.Schrottke.H.T.Grahn,andH.W.Hubers.“FrequencymodulationspectroscopywithaTHzquantumcascadelaser,”OpticalExpress,Vol.21,pp.32199-32206(2013).
[4]TaoYang,ChaoTian,GangChen,R.Martini.“Non-resonantopticalmodulationofquantumcascadelaseranditsapplicationpotentialininfraredspectroscopy,”Proc.ofSPIE,9002(2013).
Summary of the invention
The object of the invention is for the deficiencies in the prior art, and the infrared frequency modulation laser spectrum gas detecting system based on quantum cascade laser of a kind of high speed provided, high sensitivity, low detection limit and method, while raising detection system signal to noise ratio (S/N ratio), realize detecting fast, can 100 nanosecond-several microsecond in the frequency modulation spectrum realized at a high speed obtain, to can be applicable at a high speed, high sensitivity gas detect.
The present invention comes to be realized by the following technical programs:
Based on a high speed infrared laser of frequency modulation spectroscopic gas detection system for quantum cascade laser, it comprises quantum cascade laser spectral scan module, fast frequency modulation module, synchronization module, gas return chamber and frequency modulation spectrum acquisition module more.
Described quantum cascade laser spectral scan module realizes spectral scan function, is made up of pulse current source and distributed feedback quantum cascade laser.Pulsewidth is provided to be T by pulse current source to distributed feedback quantum cascade laser
p, repetition frequency is ω
rrectangular current pulses sequence, in each current impulse, the sustaining temperature change utilizing electric current to produce in distributed feedback quantum cascade laser cavity, realizes scanning the output infrared wavelength of distributed feedback quantum cascade laser, wavelength scanning range and pulsewidth T
pbe approximated to direct ratio.
Described fast frequency modulation module realizes distributed feedback quantum cascade laser fast frequency modulation function.Be made up of high frequency signal generator, sinusoidal current source, visible ray or near-infrared laser light source, beam collimator and beam-focuser.Described high frequency signal generator, sinusoidal current source and visible ray or near-infrared laser light source are electrically connected successively, described beam collimator and beam-focuser are successively set on the emitting light path of visible ray or near-infrared laser light source, and the convergent point of beam-focuser drops on distributed feedback quantum cascade laser outgoing end face.It is ω that this module produces repetition frequency by high frequency signal generator
m(be greater than ω
r) high frequency trigger signal, controlling sinusoidal current source generation frequency is ω
msinusoidal current, drive visible ray or near-infrared laser light source, producing intensity is ω by frequency
mthe visible ray of varies with sinusoidal function or near infrared modulated beam of light, this light beam is after beam collimator standard, be concentrated in distributed feedback quantum cascade laser outgoing end face by beam-focuser, cause distributed feedback quantum cascade laser to export infrared light wavelength with frequencies omega
msine-wave oscillation, thus the fast frequency modulation realizing distributed feedback quantum cascade laser, the amplitude of output light wavelength vibration is directly proportional to visible ray or near infrared light beam light intensity.
Described synchronization module realizes the rectangular current pulses of pulse current source and the sinusoidal current synchronizing function of sinusoidal current source, and be the output terminal being connected high frequency signal generator by frequency divider, the trigger pip end of frequency divider connects pulse current source.The repetition frequency that high frequency signal generator is exported by frequency divider by this module is ω
mhigh frequency trigger signal, becoming frequency after frequency division is ω
rtrigger pip, using this signal as pulse current source trigger pip, provide pulsewidth to be T to distributed feedback quantum cascade laser
p, repetition frequency is ω
rrectangular current pulses sequence, ensure that the infrared light pulse of distributed feedback quantum cascade laser is synchronous with the Sine Modulated light beam that visible ray or near-infrared laser light source export thus, and then it is synchronous with frequency modulation (PFM) to realize spectral scan.
It is sealing container as detected gas that described gas returns chamber more, by seal chamber, air intake opening, exhausr port, vacuum meter, off-gas pump, incidence window, exit window and catoptron.The described incidence window two ends that are positioned at seal chamber relative to about exit window, also up and down stagger relative with the second catoptron of first catoptron is arranged in seal chamber, and respectively near incidence window and exit window, described air intake opening and exhausr port are arranged on seal chamber, vacuum meter is arranged in seal chamber, and off-gas pump is connected on exhausr port.Gas returns chamber more and returns chamber, the catoptron and catoptron of parallel placement between roundtrip several after, by exit window outgoing by entering gas through warbled distributed feedback quantum cascade laser output infrared beam by incidence window more.Gas is returned chamber more and is bled by off-gas pump, and detected gas is introduced seal chamber by air intake opening, detects gaseous tension by vacuum meter.
Described frequency modulation spectrum acquisition module realizes the collection to the frequency modulation spectrum of gas, is ω by infrared focus device, frequency bandwidth
d(be greater than ω
m) high speed IR detector, frequency bandwidth be ω
l(be greater than ω
m) high speed lock-in amplifier, High-Speed Data Acquisition Board and computing machine composition.Described infrared focus device is just to exit window, and its focus point drops on the test surface of high speed IR detector, and high speed IR detector, high speed lock-in amplifier, High-Speed Data Acquisition Board are connected with computing machine successively electric signal.This module is by the infrared beam of exit window outgoing, and being converged on high speed IR detector by infrared focus device and form output signal, this signal is admitted to high speed lock-in amplifier, and this signal intermediate frequency rate is ω by high speed lock-in amplifier
msignal export and send into High-Speed Data Acquisition Board, data acquisition card output signal is read finally by computing machine, the detected gas infrared frequency modulation spectrum converted in time, the peak-to-peak value of frequency modulation spectrum is directly proportional to the concentration of detected gas, simultaneously, the peak-to-peak value of frequency modulation spectrum is directly proportional to visible ray or near infrared modulated beam of light average power, can be improved detection sensitivity and improved gas concentration detection limit by the average power improving modulated beam of light.
Particularly, the spectral scan of described distributed feedback quantum cascade laser, be adopt squared-pulse current, pulse current source provides one higher than the pulsed drive current sequence of threshold value for distributed feedback quantum cascade laser, and drive current is a pulse width T
ptime in laser instrument internal cavity continuous heating, the effective cavity length of laser instrument is caused to increase, by the tired collection of current generates heat in current pulse duration, the cavity temperature of knots modification qc laser, thus increase its equivalent tank cavity length, cause output wavelength to increase in time and become large, realizing the rapid scanning of spectrum, its pulse width usually between 100 nanoseconds to 10 microseconds not etc.
Described visible ray that distributed feedback quantum cascade laser is modulated or near infrared modulated beam of light, the photon energy of its correspondence is more than or equal to the less semiconductor material band-gap energy of band gap in quantum cascade laser, modulated beam of light is in quantum cascade laser resonator cavity, conduction band is transitted to by exciting valence-band electrons, form free electron and hole, knots modification qc laser resonator cavity refractive index, and then knots modification qc laser resonator cavity effective cavity length, thus knots modification qc laser exports infrared wavelength or light frequency, realize the frequency modulation (PFM) to distributed feedback quantum cascade laser.
The process that described synchronization module realizes the sinusoidal current of the rectangular current pulses of pulse current source and sinusoidal current source synchronous is: producing repetition frequency by high frequency signal generator is ω
m(be greater than ω
r) high frequency trigger signal, controlling sinusoidal current source generation frequency is ω
msinusoidal current, drive visible ray or near-infrared laser light source.Repetition frequency high frequency signal generator exported by frequency divider is ω
mhigh frequency trigger signal, becoming frequency is ω
rtrigger pip, using this signal as pulse current source trigger pip, provide pulsewidth to be T to distributed feedback quantum cascade laser
p, repetition frequency is ω
rrectangular current pulses sequence, ensure that the infrared light pulse of distributed feedback quantum cascade laser is synchronous with the Sine Modulated light beam that visible ray or near-infrared laser light source (5) export thus, and then it is synchronous with frequency modulation (PFM) to realize spectral scan.
Adopt above-mentioned quantum cascade laser to carry out high speed infrared laser of frequency modulation spectroscopic gas to detect, can realize that the fast frequency modulation to the rapid scanning of quantum cascade laser spectrum and quantum cascade laser, laser pulse signal are synchronous with modulation signal, the quantitative detection of sample gas under study, gas infrared frequency modulation spectrum detects fast, specific as follows:
(1) rapid scanning of quantum cascade laser of the present invention is realized by Current Heating mode, under distributed feedback quantum cascade laser is operated in this pulse mode, pulse current source provides one higher than the pulsed drive current sequence of threshold value for distributed feedback quantum cascade laser, and drive current is a pulse width T
ptime in laser instrument internal cavity continuous heating, the effective cavity length of laser instrument is caused to increase, thus make laser output wavelength that " red shift " occur, and then realize quanta cascade rapid spectrum scanning, pulse width generally can control within a hundreds of nanosecond, and pulse width can regulate according to real needs.
(2) the fast frequency modulation of quantum cascade laser of the present invention is visible ray or the near infrared light laser beam of high speed sinusoidal variations by intensity, quantum cascade laser outgoing end face is converged in by focus device, it is made in quantum cascade laser resonator cavity, valence-band electrons to be excited to laser instrument conduction band, increase conduction band electron concentration and valence band hole concentration, and then make quantum cascade laser resonator cavity equivalent refractive index produce high speed sine-wave oscillation, thus cause the high speed sine-wave oscillation of output wavelength, realize the fast frequency modulation of quantum cascade laser.
(3) emphasis of the present invention have employed synchronization module to realize the synchronous of quantum cascade laser infrared laser pulses and visible ray or near-infrared laser modulated beam of light, the high frequency sinusoidal signal that high frequency signal generator produces is divided into two-way and exports by synchronization module, one tunnel, for triggering visible ray or near infrared laser power supply, produces visible ray or near-infrared laser output that intensity is same frequency sinusoidal variations; Sinusoidal signal is shaped to the square-wave signal of same frequency by comparer of separately leading up to, again by the square-wave signal frequency division after shaping, and stablize the square-wave signal level after frequency division by mu balanced circuit, adopt this square-wave signal as the trigger pip of pulse current source, for distributed feedback quantum cascade laser provides pulsed drive current; Realize the synchronous of quantum cascade laser infrared laser pulses and visible ray or near-infrared laser modulated beam of light thus.
(4) influx and translocation of sample gas under study of the present invention detects is return chamber to realize by airtight gas more.Gas to be measured returns chamber quantum cascade laser output mid-infrared laser through frequency modulation (PFM) after by air pump suction more and enters in seal chamber by incidence window, and after realizing multiple reflections by two catoptrons at two ends in this cavity, carry modulating light spectrum information and penetrate seal chamber by exit window.
(5) the employing high speed IR detector of high speed infrared laser of frequency modulation spectrum of the present invention obtains the quantum cascade laser infrared signal carrying gas modulating light spectrum information, utilizes lock-in amplifier to extract modulating frequency ω
mcorresponding signal also exports, then by High-Speed Data Acquisition Board, the gas infrared frequency modulation spectrum final by computer acquisition, the detected gas infrared frequency modulation spectrum converted in time, the peak-to-peak value of frequency modulation spectrum and the concentration of gas to be measured linear, concentration is directly proportional.Meanwhile, the peak value of frequency modulation spectrum is directly proportional to visible ray or near infrared modulated beam of light average power, can be improved detection sensitivity and improved gas concentration detection limit by the average power improving modulated beam of light.By detecting the gas infrared frequency modulation spectrum to be measured of concentration known, set up the relation curve between gas concentration and modulating light spectrum signal peak-to-peak value.By this relation curve, the quantitative detection of unknown concentration gas can be realized.Because obtained signal is in high frequency, and most noise is in low frequency, can improve the signal to noise ratio (S/N ratio) of system greatly.
In sum, the present invention utilizes the mode of Current Heating quantum cascade laser to realize the rapid scanning of spectrum, and by light modulated that the is visible or modulation of infrared laser output intensity, fast frequency modulation is realized to quantum cascade laser, it is synchronous with the modulated beam of light that visible or infrared laser export that synchronization module realizes quantum cascade laser infrared laser pulses, quantum cascade laser mid-infrared laser through fast frequency modulation passes through to return chamber more, multiple reflections is completed to strengthen gas absorption by two catoptrons of its inside, the quantum cascade laser mid-infrared light bundle carrying gas modulating light spectrum information is received by high speed IR detector, frequency modulation spectrum is wherein extracted by lock-in amplifier, and pass through high-speed collection card, what read in computing machine gas concentration carries out quantitative test.This distributed feedback quantum cascade laser work in the pulsing mode, in a pulse width (between 100 nanoseconds to 10 microseconds not etc.), realize the infrared frequency modulation spectral detection to gas, infrared pulse width is only a hundreds of nanosecond, therefore its detection speed is fast, highly sensitive, adopt frequency modulation spectrum detection sensitivity can be promoted a more than order of magnitude.
Accompanying drawing explanation
Fig. 1 is based on the high speed infrared laser of frequency modulation spectroscopic gas detection system schematic diagram of quantum cascade laser;
Fig. 2 is quantum cascade laser spectrum rapid scanning schematic diagram;
Fig. 3 is the full optical modulation schematic diagram of quantum cascade laser;
Fig. 4 is mid-infrared laser schematic diagram synchronous with near-infrared laser;
Fig. 5 is that gas returns chamber top plan view more;
Fig. 6 is full optical modulation CO gas absorption spectra figure;
Fig. 7 be CO gas concentration respectively with the graph of a relation of direct absorption spectra intensity and high speed infrared laser of frequency modulation spectral intensity;
Fig. 8 is the peak value of frequency modulation spectrum and the graph of a relation of visible ray or near infrared modulated beam of light average power.
Embodiment
Below in conjunction with accompanying drawing, technical scheme of the present invention is further described.
As shown in Figure 1, the high speed infrared laser of frequency modulation spectroscopic gas detection system based on quantum cascade laser that the present invention proposes comprises quantum cascade laser spectral scan module, fast frequency modulation module, synchronization module, gas return chamber and frequency modulation spectrum acquisition module more.Its concrete structure is as follows:
Quantum cascade laser spectral scan module: its function is all realize spectral scan, is made up of pulse current source 1 and distributed feedback quantum cascade laser 2, provides pulsewidth to be T by pulse current source 1 to distributed feedback quantum cascade laser 2
p, repetition frequency is ω
rrectangular current pulses sequence, in each current impulse, the sustaining temperature change utilizing electric current to produce in distributed feedback quantum cascade laser cavity, realizes scanning the output infrared wavelength of distributed feedback quantum cascade laser, wavelength scanning range and pulsewidth T
pbe approximated to direct ratio.
Fast frequency modulation module: its function realizes modulating distributed feedback quantum cascade laser 2 fast frequency.It is made up of high frequency signal generator 3, sinusoidal current source 4, visible ray or near-infrared laser light source 5, beam collimator 6 and beam-focuser 7.Wherein high frequency signal generator 3, sinusoidal current source 4 and visible ray or near-infrared laser light source 5 are electrically connected successively, beam collimator 6 and beam-focuser 7 are successively set on the emitting light path of visible ray or near-infrared laser light source 5, the convergent point of beam-focuser 7 drops on distributed feedback quantum cascade laser 2 outgoing end face, and the frequency of the high frequency trigger signal that high frequency signal generator 3 produces is ω
m.Wherein ω
mbe greater than ω
r.
Synchronization module: its function is realize the rectangular current pulses of pulse current source 1 and the sinusoidal current of sinusoidal current source 4 synchronous.It connects the output terminal of high frequency signal generator 3 by frequency divider 8, and the trigger pip end of frequency divider 8 connects pulse current source 1 and realizes.
Gas returns chamber more: the sealing container being detected gas, comprise seal chamber 9, air intake opening 10, exhausr port 11, vacuum meter 12, off-gas pump 13, incidence window 14, exit window 15, first catoptron 16 and the second catoptron 17, incidence window 14 two ends that are positioned at seal chamber 9 relative to exit window about 15, also up and down stagger relative with both the second catoptrons 17 of first catoptron 16 is arranged in seal chamber 9, and respectively near incidence window 14 and exit window 15, air intake opening 10 and exhausr port 11 are arranged on seal chamber 9, vacuum meter 12 is arranged in seal chamber 9, off-gas pump 13 is connected on exhausr port 11.
Frequency modulation spectrum acquisition module: function realizes the collection to the frequency modulation spectrum of gas.It is ω by infrared focus device 18, frequency bandwidth
d, and ω
dbe greater than ω
mhigh speed IR detector 19, frequency bandwidth be ω
l, and ω
lbe greater than ω
mhigh speed lock-in amplifier 20, High-Speed Data Acquisition Board 21 and computing machine 22 form.Described infrared focus device 18 is just to exit window 15, and its focus point drops on the test surface of high speed IR detector 19, and high speed IR detector 19, high speed lock-in amplifier 20, High-Speed Data Acquisition Board 21 are connected with computing machine 22 successively electric signal.
The gas detect implementation method of native system is as follows:
Based on the signal that the high speed infrared laser of frequency modulation spectroscopic gas detection system of quantum cascade laser utilizes high frequency signal generator 3 to produce, one tunnel, for controlling sinusoidal current source 4 pairs of visible rays or near infrared laser 5 provides driving sinusoidal current, produces visible ray or near infrared modulated beam of light that light intensity is sinusoidal variations; The signal that high frequency signal generator 3 produces, another road is supplied to frequency divider 8, frequency divider 8 is by after signal shaping, frequency division, for triggering quantum cascade laser pulse current source 1, for distributed feedback quantum cascade laser 2 provides pulsed drive current, produce mid-infrared light pulse, for quantum cascade laser, it provides constant temperature working environment to thermostat 23.Visible ray or near infrared modulated beam of light are first collimated by beam collimator 6, then are converged in by focuser 7 on the active area of distributed feedback quantum cascade laser 2 outgoing end face, make distributed feedback quantum cascade laser 2 produce fast frequency modulation.To enter through incidence window 14 through warbled mid-infrared light and return chamber seal chamber 9 more, the vacuum tightness of seal chamber 9 is controlled by exhausr port 11 and off-gas pump 13, sample gas under study returns chamber by air intake opening 10 insufflation gas more, and measures its actual charge by vacuum meter 12.Infrared multiple reflections in realizing by regulating the one the second catoptrons 16,17, increases gas and effectively absorbs light path; Mid-infrared laser is penetrated by exit window 15 and focuses on high speed IR detector 19 by infrared focus device 18, and infrared eye 19 outputs signal sends into high speed lock-in amplifier 20, and is obtained the frequency modulation spectrum of gas to be detected by computing machine 22 by capture card 21.
As shown in Figure 2, in figure, dotted line represents that quantum cascade laser is operated in pulse width is T
plight signal, the spectral scan scope of quantum cascade laser within a burst length is measured by F-P infrared standard tool, sine moving curve shown in figure, without warbled quantum cascade laser pulsed light beam, without gas absorption, directly by F-P infrared standard tool, the signal obtained by high speed IR detector.Wavelength variable quantity in curve between two adjacent crests or trough equals the Free Spectral Range of F-P infrared standard tool, can obtain the relative wavenumbers value that in quantum cascade laser pulse wave number sweep limit and pulse, each moment is corresponding thus by three rank fitting of a polynomials.Finally by the absorption peak position in quantum cascade laser sweep limit of known gas, the absolute wavelength that in paired pulses, each moment is corresponding or wave number are demarcated.
As shown in Figure 3, quantum cascade laser is fixed on metal heat sink.Visible ray or near-infrared laser modulated beam of light are converged on the outgoing end face of quantum cascade laser, semiconductor material valence-band electrons in quantum cascade laser resonator cavity is caused to transit to conduction band, change the carrier concentration of conduction band (electronics) and valence band (hole), realize infrared output wavelength in quantum cascade laser to change with the change of visible ray or near infrared modulated beam of light intensity, thus realize the frequency modulation (PFM) of quantum cascade laser.
As shown in Figure 4, being through infrared output pulse sequence and intensity in the quantum cascade laser is synchronously sinusoidal variations visible ray or near infrared light beam.
Gas as shown in Figure 5 returns chamber more, the mid-infrared laser that quantum cascade laser exports after full optical modulation is incident by the central vertical of incidence window 14, and just in time vertical irradiation at catoptron 17 edge of seal chamber 9 other end, make infrared beam return chamber roundtrip at gas as much as possible by regulating the catoptron 16,17 at seal chamber two ends more, thus reaching the object increasing available gas absorption light path as much as possible, exit window 15 center that the quantum cascade laser mid-infrared light bundle finally carrying modulating light spectrum information returns chamber by gas more is penetrated.
As shown in Figure 6, be infrared frequency modulation laser spectrum by the variable concentrations CO gas obtained based on the high speed infrared laser of frequency modulation spectroscopic gas detection system of quantum cascade laser.
As shown in Figure 7, be CO gas concentration respectively with the relation of direct absorption spectra intensity and high speed infrared laser of frequency modulation spectral intensity, reflect relative to direct absorption spectra thus, detection sensitivity can be promoted a more than order of magnitude by high speed infrared laser of frequency modulation spectrum.
As shown in Figure 8, the peak-to-peak value of frequency modulation spectrum is directly proportional to visible ray or near infrared modulated beam of light average power, can be improved detection sensitivity and improved gas concentration detection limit by the average power improving modulated beam of light.
The full light modulated gas absorption spectrum detection system of distributed feedback quantum cascade laser provided by the invention, can be applied to high speed, high-precision gas detect.
Claims (5)
1., based on a high speed infrared laser of frequency modulation spectroscopic gas detection system for quantum cascade laser, it comprises quantum cascade laser spectral scan module, fast frequency modulation module, synchronization module, gas return chamber and frequency modulation spectrum acquisition module more; It is characterized in that:
Described quantum cascade laser spectral scan module realizes spectral scan function, be made up of pulse current source (1) and distributed feedback quantum cascade laser (2), provide pulsewidth to be T by pulse current source (1) to distributed feedback quantum cascade laser (2)
p, repetition frequency is ω
rrectangular current pulses sequence, in each current impulse, the sustaining temperature change utilizing electric current to produce in distributed feedback quantum cascade laser cavity, realizes scanning the output infrared wavelength of distributed feedback quantum cascade laser, wavelength scanning range and pulsewidth T
pbe approximated to direct ratio;
Described fast frequency modulation module realizes distributed feedback quantum cascade laser (2) fast frequency modulation function; Be made up of high frequency signal generator (3), sinusoidal current source (4), visible ray or near-infrared laser light source (5), beam collimator (6) and beam-focuser (7); Described high frequency signal generator (3), sinusoidal current source (4) and visible ray or near-infrared laser light source (5) are electrically connected successively, described beam collimator (6) and beam-focuser (7) are successively set on the emitting light path of visible ray or near-infrared laser light source (5), and the convergent point of beam-focuser (7) drops on distributed feedback quantum cascade laser (2) outgoing end face; Producing repetition frequency by high frequency signal generator (3) is ω
mhigh frequency trigger signal, wherein ω
mbe greater than ω
r, controlling sinusoidal current source (4) generation frequency is ω
msinusoidal current, drive visible ray or near-infrared laser light source (5), producing intensity is ω by frequency
mthe visible ray of varies with sinusoidal function or near infrared modulated beam of light, this light beam is after beam collimator (6) standard, be concentrated in distributed feedback quantum cascade laser (2) outgoing end face by beam-focuser (7), cause distributed feedback quantum cascade laser (2) to export infrared light wavelength with frequencies omega
msine-wave oscillation, thus the fast frequency modulation realizing distributed feedback quantum cascade laser (2), the amplitude of output light wavelength vibration is directly proportional to visible ray or near infrared light beam light intensity;
Described synchronization module realizes the rectangular current pulses of pulse current source (1) and the sinusoidal current synchronizing function of sinusoidal current source (4), it is the output terminal being connected high frequency signal generator (3) by frequency divider (8), and the trigger pip end of frequency divider (8) connects pulse current source (1); The repetition frequency that high frequency signal generator (3) produces is ω by synchronization module
mhigh frequency sinusoidal signal is divided into two-way and exports, and a road, for triggering visible ray or near infrared laser power supply, produces visible ray or near-infrared laser output that intensity is same frequency sinusoidal variations; Sinusoidal signal is shaped to the square-wave signal of same frequency by comparer of separately leading up to, then by the square-wave signal frequency division after shaping, becoming frequency is ω
rtrigger pip, and stablize the square-wave signal level after frequency division by mu balanced circuit, adopt this square-wave signal as the trigger pip of pulse current source (1), for distributed feedback quantum cascade laser (2) provides pulsewidth to be T
p, repetition frequency is ω
rrectangular current pulses sequence, i.e. pulsed drive current, ensure that the infrared light pulse of distributed feedback quantum cascade laser (2) is synchronous with the Sine Modulated light beam that visible ray or near-infrared laser light source (5) export thus, and then it is synchronous with frequency modulation (PFM) to realize spectral scan;
Described gas returns the sealing container of chamber as detected gas more, comprise seal chamber (9), air intake opening (10), exhausr port (11), vacuum meter (12), off-gas pump (13), incidence window (14), exit window (15), first catoptron (16) and the second catoptron (17), described incidence window (14) and the relative two ends being positioned at seal chamber (9), exit window (15) left and right, first catoptron (16) and both relative also staggering up and down of the second catoptron (17) are arranged in seal chamber (9), and respectively near incidence window (14) and exit window (15), described air intake opening (10) and exhausr port (11) are arranged on seal chamber (9), vacuum meter (12) is arranged in seal chamber (9), off-gas pump (13) is connected on exhausr port (11), bled by off-gas pump (13), detected gas is introduced seal chamber (9) by air intake opening (10), detect gaseous tension by vacuum meter (12), return chamber by entering gas through warbled distributed feedback quantum cascade laser (2) output infrared beam by incidence window (14) more, between first catoptron (16) and the second catoptron (17) of parallel placement after roundtrip several, by exit window (15) outgoing,
Described frequency modulation spectrum acquisition module realizes the collection to the frequency modulation spectrum of gas, is ω by infrared focus device (18), frequency bandwidth
d, and ω
dbe greater than ω
mhigh speed IR detector (19), frequency bandwidth be ω
l, and ω
lbe greater than ω
mhigh speed lock-in amplifier (20), High-Speed Data Acquisition Board (21) and computing machine (22) form; Described infrared focus device (18) is just to exit window (15), and it is coaxial with exiting infrared light bundle, its focus point drops on the test surface of high speed IR detector (19), and high speed IR detector (19), high speed lock-in amplifier (20), High-Speed Data Acquisition Board (21) are connected with computing machine (22) successively electric signal; By the infrared beam of exit window (15) outgoing, converge to high speed IR detector (19) by infrared focus device (18) go up and form output signal, this signal is admitted to high speed lock-in amplifier (20), and this signal intermediate frequency rate is ω by high speed lock-in amplifier (20)
msignal export and send into High-Speed Data Acquisition Board (21), data acquisition card (21) output signal is read finally by computing machine (22), the detected gas infrared frequency modulation spectrum converted in time, the peak-to-peak value of frequency modulation spectrum is directly proportional to the concentration of detected gas.
2. the high speed infrared laser of frequency modulation spectroscopic gas detection system based on quantum cascade laser according to claim 1, it is characterized in that, the spectral scan of described distributed feedback quantum cascade laser (2), adopt squared-pulse current, pulse current source provides one higher than the pulsed drive current sequence of threshold value for distributed feedback quantum cascade laser, and drive current is a pulse width T
ptime in laser instrument internal cavity continuous heating, the effective cavity length of laser instrument is caused to increase, by the tired collection of current generates heat in current pulse duration, the cavity temperature of knots modification qc laser, thus increase its equivalent tank cavity length, cause output wavelength to increase in time and become large, realizing the rapid scanning of spectrum, its pulse width usually between 100 nanoseconds to 10 microseconds not etc.
3. the high speed infrared laser of frequency modulation spectroscopic gas detection system based on quantum cascade laser according to claim 1, it is characterized in that, described visible ray that distributed feedback quantum cascade laser (2) is modulated or near infrared modulated beam of light, the photon energy of its correspondence is more than or equal to the less semiconductor material band-gap energy of band gap in quantum cascade laser, modulated beam of light is in quantum cascade laser resonator cavity, conduction band is transitted to by exciting valence-band electrons, form free electron and hole, knots modification qc laser resonator cavity refractive index, and then knots modification qc laser resonator cavity effective cavity length, thus knots modification qc laser exports infrared wavelength or light frequency, realize the frequency modulation (PFM) to distributed feedback quantum cascade laser.
4. the high speed infrared laser of frequency modulation spectroscopic gas detection system based on quantum cascade laser according to claim 1, it is characterized in that, the process that described synchronization module realizes the sinusoidal current of the rectangular current pulses of pulse current source (1) and sinusoidal current source (4) synchronous is: producing repetition frequency by high frequency signal generator (3) is ω
mhigh frequency trigger signal, wherein ω
mbe greater than ω
r, controlling sinusoidal current source (4) generation frequency is ω
msinusoidal current, drive visible ray or near-infrared laser light source (5); Be ω by frequency divider (8) by the repetition frequency that high frequency signal generator (3) exports
mhigh frequency trigger signal, becoming frequency is ω
rtrigger pip, using this signal as pulse current source (1) trigger pip, provide pulsewidth to be T to distributed feedback quantum cascade laser (2)
p, repetition frequency is ω
rrectangular current pulses sequence, ensure that the infrared light pulse of distributed feedback quantum cascade laser (2) is synchronous with the Sine Modulated light beam that visible ray or near-infrared laser light source (5) export thus, and then it is synchronous with frequency modulation (PFM) to realize spectral scan.
5. utilize system described in claim 1-4 to realize the method for gas detect, it is characterized in that, described method is the signal utilizing high frequency signal generator (3) to produce, one tunnel provides driving sinusoidal current for controlling sinusoidal current source (4) to visible ray or near infrared laser (5), produces visible ray or near infrared modulated beam of light that light intensity is sinusoidal variations; The signal that high frequency signal generator (3) produces, another road is supplied to frequency divider (8), frequency divider (8) is by after signal shaping, frequency division, for triggering the pulse current source (1) of quantum cascade laser, for distributed feedback quantum cascade laser (2) provides pulsed drive current, produce mid-infrared light pulse, for quantum cascade laser, it provides constant temperature working environment to thermostat (23) simultaneously; The visible ray that visible ray or near infrared laser (5) send or near infrared modulated beam of light are first collimated by beam collimator (6), converged in by focuser (7) on the active area of distributed feedback quantum cascade laser (2) outgoing end face again, make distributed feedback quantum cascade laser (2) produce fast frequency modulation; To enter through incidence window (14) through warbled mid-infrared light and return chamber seal chamber (9) more, the vacuum tightness of seal chamber (9) is controlled by exhausr port (11) and off-gas pump (13), sample gas under study returns chamber by air intake opening (10) insufflation gas more, and measures its actual charge by vacuum meter (12); Infrared multiple reflections in realizing by regulating first, second catoptron (16,17), increases gas and effectively absorbs light path; Mid-infrared laser is penetrated by exit window 15 and focuses on high speed IR detector (19) by infrared focus device (18), infrared eye (19) output signal sends into high speed lock-in amplifier (20), and is obtained the frequency modulation spectrum of gas to be detected by computing machine (22) by capture card (21).
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