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WO2018172391A2 - Spectromètre de fourier comprenant un laser à cascade quantique à plusieurs modes, et procédé servant à examiner par spectroscopie un échantillon - Google Patents

Spectromètre de fourier comprenant un laser à cascade quantique à plusieurs modes, et procédé servant à examiner par spectroscopie un échantillon Download PDF

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Publication number
WO2018172391A2
WO2018172391A2 PCT/EP2018/057105 EP2018057105W WO2018172391A2 WO 2018172391 A2 WO2018172391 A2 WO 2018172391A2 EP 2018057105 W EP2018057105 W EP 2018057105W WO 2018172391 A2 WO2018172391 A2 WO 2018172391A2
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WO
WIPO (PCT)
Prior art keywords
resonator
qcl
detector
laser
interferogram
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PCT/EP2018/057105
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German (de)
English (en)
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WO2018172391A3 (fr
Inventor
Benjamin RÖBEN
Martin Hempel
Xiang LÜ
Klaus Biermann
Lutz SCHROTTKE
Holger T. GRAHN
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Forschungsverbund Berlin E.V.
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Publication of WO2018172391A2 publication Critical patent/WO2018172391A2/fr
Publication of WO2018172391A3 publication Critical patent/WO2018172391A3/fr

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Classifications

    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
    • G01J3/00—Spectrometry; Spectrophotometry; Monochromators; Measuring colours
    • G01J3/28—Investigating the spectrum
    • G01J3/45—Interferometric spectrometry
    • G01J3/453—Interferometric spectrometry by correlation of the amplitudes
    • G01J3/4535—Devices with moving mirror
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
    • G01J3/00—Spectrometry; Spectrophotometry; Monochromators; Measuring colours
    • G01J3/28—Investigating the spectrum
    • G01J3/42—Absorption spectrometry; Double beam spectrometry; Flicker spectrometry; Reflection spectrometry
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S5/00—Semiconductor lasers
    • H01S5/06—Arrangements for controlling the laser output parameters, e.g. by operating on the active medium
    • H01S5/062—Arrangements for controlling the laser output parameters, e.g. by operating on the active medium by varying the potential of the electrodes
    • H01S5/06209—Arrangements for controlling the laser output parameters, e.g. by operating on the active medium by varying the potential of the electrodes in single-section lasers
    • H01S5/06213—Amplitude modulation
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
    • G01J3/00—Spectrometry; Spectrophotometry; Monochromators; Measuring colours
    • G01J3/28—Investigating the spectrum
    • G01J3/42—Absorption spectrometry; Double beam spectrometry; Flicker spectrometry; Reflection spectrometry
    • G01J3/433—Modulation spectrometry; Derivative spectrometry
    • G01J2003/4334—Modulation spectrometry; Derivative spectrometry by modulation of source, e.g. current modulation
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S5/00—Semiconductor lasers
    • H01S5/06—Arrangements for controlling the laser output parameters, e.g. by operating on the active medium
    • H01S5/062—Arrangements for controlling the laser output parameters, e.g. by operating on the active medium by varying the potential of the electrodes
    • H01S5/06209—Arrangements for controlling the laser output parameters, e.g. by operating on the active medium by varying the potential of the electrodes in single-section lasers
    • H01S5/0622—Controlling the frequency of the radiation
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S5/00—Semiconductor lasers
    • H01S5/30—Structure or shape of the active region; Materials used for the active region
    • H01S5/34—Structure or shape of the active region; Materials used for the active region comprising quantum well or superlattice structures, e.g. single quantum well [SQW] lasers, multiple quantum well [MQW] lasers or graded index separate confinement heterostructure [GRINSCH] lasers
    • H01S5/3401—Structure or shape of the active region; Materials used for the active region comprising quantum well or superlattice structures, e.g. single quantum well [SQW] lasers, multiple quantum well [MQW] lasers or graded index separate confinement heterostructure [GRINSCH] lasers having no PN junction, e.g. unipolar lasers, intersubband lasers, quantum cascade lasers

Definitions

  • the invention relates to a Fourier spectrometer equipped with a multimode quantum cascade laser and to a method for spectroscopic examination of a sample with the spectrometer.
  • Applications of the invention are given in spectroscopy.
  • Fourier spectrometers (or: FTI spectrometers, Fourier transform infrared spectrometers) are well-known spectrometers for infrared spectroscopy with a radiation source for generating laser light, an interferometer for generating a laser light-based interferogram, a detector device for detecting the interferogram after one Interaction with a sample and an evaluation device for Fourier transformation of the measured interferogram.
  • Single-mode QCLs allow for tunability in a spectral emission range of up to 240 GHz (see Han et al., Optics Letters, Vol. 39, pp. 3480-3483, 2014).
  • the continuous, reproducible adjustment of the single mode QCL resonator mode over the entire emission range while maintaining a high optical output power represents a technological challenge.
  • To adjust the sonorus mode of a single-mode QCL by the effective refractive index of the light mode of the single-mode QCL is changed by the approach of metallic material by means of piezo-positioner. Not only is this method relatively slow, it also provides low output powers and may also be too complicated for routine applications, such as in spectroscopy.
  • Multi-mode QCLs even have wider spectral coverage than single-mode QCL, e.g. in an emission range from 100 GHz to 300 GHz, and high output power.
  • the emission spectrum of the multimode QCL is composed of a plurality of narrow-band resonator modes between which spectral gaps exist.
  • the width of the spectral gaps is e.g. 20 GHz. Because of these gaps, multimode QCLs have hitherto been of limited use in spectroscopy applications, e.g. in the study of only slightly varying absorption bands. Since the tuning range of each resonator mode is below 6 GHz, conventional multi-mode QCLs for high-resolution spectroscopy have no advantage over single-mode QCLs.
  • a grating spectrometer with a QCL as a radiation source is described by R. Eichholz et al. in Applied Physics Letters (Vol. 99, pp. 141112-1 - 141112-3, 2011).
  • the QCL is operated in modulated continuous-wave mode, ie with a DC current on which a sinusoidal AC current is modulated, whereby a very small modulation of the emission frequency (order of magnitude 0.01 GHz) (modulation spectroscopy). If the emission frequency covers an absorption line of a gas under investigation, the derivative of a maximum is measured at the detector. To record a whole spectrum, the DC current has to be changed step by step.
  • the modulation spectroscopy according to R. Eichholz et al. thus has disadvantages by a relatively long measurement time and suitability primarily for narrow spectral lines.
  • the object of the invention is to provide an improved Fourier spectrometer with a radiation source in the form of a quantum cascade laser (QCL), with which disadvantages of conventional techniques are avoided.
  • the Fourier spectrometer is designed for sensitive absorption spectroscopy (ie high signal-to-noise ratio, SNR, and high dynamic range) including high-resolution transmission and reflection spectroscopy (eg, 0.1 GHz) in the THz range be suitable.
  • the QCL should be characterized in particular by a continuous (ie gapless) spectral coverage of a broad emission range, an increased optical output power, a simple tunability and / or a high reproducibility of the setting of emission frequencies.
  • a Fourier spectrometer for spectroscopic examination of a sample comprising a multimode quantum cascade laser (QCL) having an active QCL region in a laser cavity used to generate laser light with emission frequencies is configured according to a plurality of resonator modes of the laser resonator, an exciter configured to electrically excite the active QCL region by means of an electric pump current, and a tuner with which the resonator modes are adjustable, an interferometer with a beam splitter, a stationary one Interferometer mirror and a movable interferometer mirror, wherein the interferometer is arranged to generate an interferogram based on the laser light, a detector device for detecting the interferogram after an interaction with the sample and for detecting a Detektorigsig nals, which includes the detected interferogram, within a detector measurement time, and an evaluation device for detecting a spectrum of the sample by a Fourier transform of the detected interferogram.
  • QCL quantum cascade laser
  • the laser resonator may comprise a linear resonator, in particular a Fabry-Perot resonator with planar reflectors, or a circular resonator, in particular a ring resonator or a disc resonator.
  • the tuners of the multimode quantum cascade laser for periodic spectral variation of the resonator modes are configured with a timing period less than 1 min each in a spectral tuning interval at least equal to the spacing of adjacent resonator modes of the laser cavity, the active QCL region for generating the The emission frequencies of the laser light cover a spectral emission range of at least 50 GHz, in particular at least 70 GHz, and the detector means for averaging the time of the detector signal over the tuning period of the tuner designed the multi-mode quantum cascade laser.
  • the periodic variation of the resonator modes in the spectral tuning interval of the multimode QCL in the Fourier spectrometer comprises a continuous spectral shift of the resonator modes through the tuning interval.
  • the tuning interval includes the entire spectral gain profile of QCL or a portion of it.
  • the multimode QCL is tunable so that the spectral gaps between the resonator modes are closed over time.
  • the spectral gaps between the active resonator modes are closed, i. H.
  • the resonator modes which have an overlap with the gain profile of the QCL, therefore oscillate in the course of the laser process and determine the emission frequencies of the QCL.
  • the tuning of the QCL is performed spectrally continuously and periodically with the timing period less than 1 min.
  • a single resonator mode not only a single resonator mode but a plurality of resonator modes are concurrently tuned which cover the spectral emission range of the multimode QCL.
  • the resonator modes need not be shifted through the entire emission range, but only over the distance of adjacent resonator modes of the laser resonator of the multimode QCL, so that lower tuning requirements exist than conventional single mode QCL.
  • the tuning of the resonator modes takes place at least beyond the mode spacing of adjacent resonator modes of the laser resonator in the time tuning period less than 1 min, so that a continuous spectral coverage of the emission range is achieved on average over time, compared to conventional modulation techniques.
  • the inspection method comprises the steps of electrically exciting the active QCL region of the multimode QCL by means of an electric pumping current, wherein the resonator modes having the timing period less than 1 min are periodically varied in the spectral tuning interval at least equal to the spacing of adjacent resonator modes of the laser resonator is such that the emission frequencies of the laser light on a time average continuously cover the spectral emission range spanned by the resonator modes, and coupling the laser light from the laser resonator into the interferometer, generating the interferogram with the interferometer, interaction of the interferogram with the sample , Detecting the interferogram after the interaction of the interferogram with the sample and detection of the detector signal, which includes the detected interferogram, within the detector measurement time, and detection of the spectrum of the sample by a Fouri he transformation of the
  • the semiconductor heterostructure of the QCL active region of the multimode QCL is preferably realized with at least one of the following material systems: GalnAs / AllnAs on an InP substrate, GaAs / AIAs on a GaAs substrate, GaAs / AIGaAs on a GaAs substrate, AISb / InAs on an InAs substrate, or InGaAs / AllnAsSb, InGaAs / GaAsSb or InGaAs / AlInGaAs on InP substrates.
  • the semiconductor heterostructures each include a plurality, e.g. B. 1000 to 2000, semiconductor layers consisting of z. B. 50 to 300 identical repetitions of a sequence of z. B.
  • the detector means is designed for the inherent averaging of the detector signal by using a detector with a time constant which is greater than the time-voting period of the voting means.
  • the detector device for time averaging of the detector signal circuit components for signal averaging, z.
  • a boxcar integrator which may be configured for operation in synchronism with the periodic variation of the resonator modes, or a lock-in amplifier, which may be configured for operation in synchronism with the periodic variation of the resonator modes.
  • the timing period of the tuning device is less than 1/10, in particular less than 1/1000 of a measuring time of the spectrometer for recording the measuring spectrum (detector measuring time for detecting the detector signal, which represents the detected interferogram).
  • a measuring time of the spectrometer for recording the measuring spectrum detector measuring time for detecting the detector signal, which represents the detected interferogram.
  • an influence on the measurement result by the temporal averaging of the detector signal is negligible or completely excluded.
  • the tuning period of the tuner is less than 1 second, in particular less than 0.01 second.
  • the measurement is considerably accelerated with the Fourier spectrometer.
  • the tuning of the resonator modes of the multimode QCL used in accordance with the invention is preferably based on one of the mechanisms mentioned below.
  • the tuner is configured to periodically vary the pumping current of the QCL.
  • the variation of the pumping current is gradual, not erratic, e.g. B. ramped.
  • the tuning of the resonator modes preferably takes place according to a linear time function or a sinusoidal time function.
  • the linear time function is preferably in the form of a triangle function with ascending and descending ramps.
  • the effect of varying the pumping current is to modify the refractive index of the active region by so-called "frequency pulling" (or: frequency pulling, cavity pulling, see LA Dunbar et al., Applied Physics Letters, Vol. 141114 (2007), or H. Zhang et al., Journal of Applied Physics, Vol. 108, pp.
  • the duration of the interruption (shutdown) of the pumping current is chosen as a function of the available cooling capacity of the QCL cooling.
  • the tuner is for illuminating the multimode quantum cascade laser, in particular the active QCL region or a substrate of the active QCL region, with electro-magnetic radiation having an energy greater than the energy of the band gaps of the semiconductor materials used the active QCL region or substrate is configured with periodically varying amplitude, in particular, electromagnetic radiation in the visible or near infrared spectral region.
  • the tuning means for periodically varying the length of the laser resonator, z. B. using an external cavity with sliding mirror be set up.
  • the mentioned mechanisms for tuning the resonator modes of the multimode QCL used in accordance with the invention can be used in combination.
  • the tuning means of the Fourier spectrometer is configured for periodic spectral variation of the resonator modes with only a single control variable.
  • Fig. 1 (e) is a schematic illustration of a Fourier spectrometer according to a preferred embodiment of the invention
  • Figures 2 (a) and 2 (b) are graphs illustrating experimental results with a
  • Multi-mode QCL according to a preferred embodiment of the invention
  • FIGS. 3 (a) to 3 (c) are further graphs illustrating test results with FIG.
  • FIG. 4 shows an emission spectrum of a further example of a multimode QCL according to the invention.
  • the application of the invention is not limited to the exemplified materials and tuning mechanisms. Rather, the invention is in modified applications with other materials and with other tuning mechanisms, in particular based on irradiation of the active region with electromagnetic radiation having an energy greater than the energy of the band gaps of the used semiconductor materials for the active QCL region or Substrate is or a length variation of the laser resonator feasible.
  • a multi-mode QCL will not be described because it can be implemented as in a conventional multi-mode QCL.
  • a QCL formed by a GaAs / AIAs heterostructure on a GaAs substrate As described, reference is made to a QCL formed by a GaAs / AIAs heterostructure on a GaAs substrate.
  • the invention is not limited to this combination of semiconductors, but can be realized in accordance with other semiconductors. Details of the Fourier spectrometer and its operation are not described as far as they are known per se from conventional Fourier spectrometers
  • the multimode QCL is based, for example, on a Fabry-Perot laser resonator (FP laser resonator). This is formed by the active region of the QCL whose end faces form the reflectors of the FP laser resonator.
  • the active region comprises, in a manner known per se, a heterostructure of semiconductor layers, for example on a carrier substrate, which is connected to a pumping current source for injecting a pumping current.
  • Figure 1 (a) shows schematically a section of a typical FP spectrum comprising a plurality of equally spaced resonator modes. In practice, the number of active resonator modes is eg 10. The spectral positions of the FP resonator modes are given by cM
  • L opt nL is the optical path length in the laser resonator.
  • L opt is determined by the refractive index of the resonator n in the laser resonator and the geometric length of the laser resonator.
  • the order of the FP resonator mode M represents the number of intensity maxima of the standing wave in the laser resonator and is at least 100 for typical resonator lengths.
  • Adjacent FP resonator modes move virtually the same amount because for large mode orders dvM + i is approximately equal to dvM.
  • the entire continuously-covered frequency range is the product of the mode spacing and the number of active cavity modes, so that approximately the entire width of the gain profile of the active region is reached.
  • FIG. 1 (d) shows that a broadband, gapless emission spectrum is achieved over the time average when the optical path length increases continuously in a ramp over a time range ⁇ , thereby continuously shifting the resonator modes, ie tuning them, and the spectral power p is averaged over this time range ,
  • This process is also referred to as gapless time-averaged wideband operation (G-TAWB operation).
  • Figure 1 (e) shows schematically an embodiment of a spectrometer in the form of an FTIR spectrometer equipped with the multimode QCL according to the invention.
  • the tuning of Lop t is realized by ramp-shaped variations of the pump current with tuning periods in the range of eg 0.1 ⁇ 5 to 10 ⁇ 5.
  • the intensity variation detector is sensitive to a time scale of 10 ms, which is the time scale of the alternating design. and destructive interference due to the movement of the interferometer mirror of the FTIR (moveable mirror) spectrometer. Therefore, the detector means can not follow the signal variations contained in the emission spectrum of the QCL due to the fast tuning, so that the time average of a quasi-continuous spectrum is detected.
  • the QCL can be adjusted to adjust one of the resonator modes at any desired emission frequency within the emission range.
  • QCL can not emit a continuous spectrum at a given time.
  • the QCL is tuned to a tuning period ⁇ while the detector signal from the spectrometer detector is averaged over that tuning period, as shown in Fig. 1 (d). If the tuning period ⁇ is significantly lower, e.g.
  • the FP resonator modes are tuned far enough to close the spectral gaps between them.
  • the mode spacing is e.g. in FP laser resonators with a length of 3 mm around 10 GHz. The tuning interval over the mode spacing is achieved depending on the actual QCL used and the operating conditions (see in particular Fig. 4 and description thereof).
  • the Fourier spectrometer 100 includes a multimode QCL 10, an interferometer 20, a detector device 30, and an evaluation device 40.
  • the components 20, 30, and 40 of the schematically shown device are e.g. B. constructed as a spectrometer Bruker type IFS 120 HR.
  • the multimode QCL 10 includes an active QCL region in a laser resonator 11, an excitation device in the form of a pumping current source 12 and a tuner 13, the part the pumping power source 12 is.
  • the active QCL region is z. B. constructed as described by L. Schrottke et al. 108, pp. 102102 (2016))
  • the active QCL region has, for example, a width of approximately 120 ⁇ m and a length of approximately 3 mm
  • the active QCL region is designed to generate laser light with emission frequencies according to the resonator modes of the laser resonator 11.
  • the tuner 13 is included in the pumping current source 12 and designed to produce a periodically varying pumping current.
  • the variation of the pumping current with the pumping current source 12 follows, for example, a ram-shaped time function (triangular pattern).
  • the pump current source 12 and the tuner 13 comprise, for example, an ILX Lightwave LDP-3840 driver source with a pulse width of 1 ⁇ 5 and a trigger frequency of 300 kHz.
  • the rectangular pulse shape is converted to a triangular pulse shape that gives a ramp time function of the pumping current.
  • the QCL 10 is equipped with a cooler (not shown) for setting an operating temperature of the QCL.
  • the operating temperature is preferably less than 200 K, in particular less than 100 K, particularly preferably in the range of 5 K to 10 K.
  • the operating temperature is set, for example, with a He cryostat or a Stirling cooler.
  • QCL 10 is mounted on the cooled carrier (cooled finger) of a 10 K continuous flow cryostat in front of the external emission port of spectrometer 100.
  • the volume between the output window of the cryostat and the entrance window of the evacuated spectrometer 100 is purged with nitrogen to reduce the absorption by humidity.
  • the interferometer 20 is formed by a beam splitter 21, a stationary interferometer mirror 22 and a movable interferometer mirror 23 which are positioned in Michelson arrangement. By periodically moving the movable interferometer mirror 23, as known from conventional Fourier spectrometers, an interferogram based on the laser light of the QCL 10 is generated.
  • the interferogram is subjected to an interaction with the sample 1 to be examined at the output of the interferometer 20.
  • the sample 1 is z. B. in a gas cell or a liquid cuvette.
  • the detector device 30 is arranged to detect the interferogram after interaction with the sample 1.
  • a detector signal is detected which contains the detected interferogram.
  • the detector measuring time for detecting a complete detector signal is z. B. 400 seconds.
  • the detector signal is time averaged to compensate for the variations in emission frequencies of the QCL 10.
  • the z. B. comprises a computer unit, the detector signal is subjected to a Fourier transform, resulting in the immediate spectrum of the sample 1.
  • the time averaging of the detector signal is realized, for example, by using in the spectrometer 100 a slow pyroelectric detector which can not follow the rapid intensity changes of the ramped time function of the pumping current.
  • the sensor of the detector is made of DTGS (deuterated triglycine sulfate) with the highest sensitivity for intensity variations between 10 Hz and 100 Hz. This corresponds to typical frequencies of the intensity variation due to the continuously periodically moving one
  • FIG. 1 The maximum resolution of the spectrometer 100 used (based, for example, on the Bruker IFS 120 HR device) is 0.105 GHz (0.0035 cm 1 ).
  • Figure 2 (a) shows the emission spectrum of the multimode QCL in G-TAWB operation at the highest resolution of the spectrometer (0.105 GHz) in relative units. Continuous coverage in an emission range of 72 GHz (from 4,686 THz to 4,758 THz) was achieved. To minimize any broadening effect in the spectrum, boxcar apodization, Mertz phase correction, and an aperture of 8 mm are used. The spectrum is not smooth, because not only the position of the resonator modes but also their power and number is a function of the pumping current.
  • FIG. 2 (b) shows the QCL emission spectrum of G-TAWB operation recorded at a spectral resolution of 0.3 GHz, a scanner speed of 10 kHz, and an aperture of 8 mm as compared to a high-power thermal source based on a heated SiC Rod, which is part of the commercially available spectrometer.
  • the QCL according to the invention exhibits a power output that is 1 to 2 orders of magnitude higher.
  • the spectrum of the thermal source is due to its low output power, which corresponds to the magnitude of the detector noise because of the high resolution, not smooth.
  • the thermal source power is comparable to that of a high pressure mercury lamp.
  • An important advantage of the inventive spectrometer 100 is the improved signal-to-noise ratio (SNR), which is a measure of the ability to detect a weak signal from a noisy detector signal.
  • SNR can be acquired from several consecutively recorded spectra taken under the same conditions, dividing the mean by the standard deviation for each frequency point.
  • Figure 3 (a) shows the SNR of the QCL 10 used in the present invention and the conventional thermal source calculated from ten successive measurements.
  • the QCL 10 Similar to the power of the emission spectra, the QCL 10 exhibits a frequency dependent varying SNR, which results from the varying power. In the case of the thermal source, its low output power, which is comparable to the detector noise, results in a low SNR. Over the entire frequency range, the QCL 10 SNR is significantly higher than the thermal source SNR.
  • the SNR of the QCL 10 according to the invention can be further increased by minimizing fluctuations in the operation of the tuner, in particular fluctuations in the pump flow.
  • the dynamic range In addition to the SNR, there is another important parameter of the spectrometer 100 in the dynamic range.
  • the detector noise is detected while the radiation from the radiation source is blocked.
  • the dynamic range can then be calculated by dividing the emission spectrum by the determined noise level. It is therefore a measure of maximum absorption that can be distinguished from noise.
  • the QCL 10 and the thermal source at the inlet opening of the sample chamber has been blocked whose low thermal emissivity ensures that no further thermal radiation is incident on the detector 30.
  • the dynamic range is between 50 and 750 depending on the frequency considered. This in turn represents a significant advantage over the thermal source, which has a dynamic range of about 3.5.
  • the output power of the multimode QCL 10 used in accordance with the invention may vary depending on the emission frequency. Advantageously, this does not affect the application of the multimode QCL 10 in the spectrometer 100, as will be illustrated below with reference to FIG.
  • Figure 3 (b) shows the transmission spectra taken with the multimode QCL 10 of the present invention and a conventional thermal source, respectively.
  • the spectral resolution is 0.3 GHz (0.01 cm -1 ), the scanning speed is 7.5 kHz and the measuring aperture is 8 mm
  • the transmission spectra were obtained by passing a spectrum recorded with the gas cell in the sample chamber of the spectrometer 100 through a then without the Gas cell recorded spectrum is divided.
  • the measurement using the multimode QCL 10 as a radiation source clearly shows the transmission spectrum of water vapor with only a spectrally slowly changing transmission curve.
  • the output power was too low compared to the noise level of the detector to make a significant measurement.
  • the measurements were repeated with a reduced noise level by changing the spectral resolution from 0.3 to 1.5 GHz and the mirror speed from 7.5 to 2.2 kHz.
  • Figure 3 (c) shows the resulting transmission spectra.
  • two measurements are shown in Figure 3 (c), each showing the QCL and the thermal source (th.s.).
  • the spectra in Figure 3 (c) show evaluable results using the thermal source.
  • the difference between the two nominally identical transmission spectra and the occurrence of negative values between 4.73 and 4.74 THz show that the rapidly varying modulations in the spectra are caused by noise.
  • the transmission spectra are smooth and highly reproducible using the multimode QCL 10 of the present invention as the radiation source, regardless of the frequency-varying output power.
  • Figure 4 shows the emission spectrum of another embodiment of a multimode QCL 10 used in accordance with the invention in G-TAWB operation which is similar in multilevel QCL in terms of wavefunction structure, center frequency, and waveguide / laser cavity dimensions to the embodiment described above.
  • the highest possible resolution (0.105 GHz), boxcar apodization, Mertz phase correction, and 8 mm aperture was used to acquire the spectrum in Figure 4 to allow any broadening effect in the spectrum minimize.
  • Figure 4 shows the emission spectrum of the multimode QCL 10 whose active region has AlAs barriers. The spectrum has a lower bandwidth, but continuously covers a frequency range of 60 GHz with a narrow gap at 4,724 THz. By optimizing the heterostructure, in particular its layer thicknesses, this gap can be closed.

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  • Spectrometry And Color Measurement (AREA)

Abstract

L'invention concerne un spectromètre de Fourier servant à examiner par spectroscopie un échantillon. Le spectromètre de Fourier comprend un laser à cascade quantique (QCL) à plusieurs modes qui contient une zone QCL active dans un résonateur laser qui est configurée pour générer une lumière laser présentant des fréquences d'émission conformément à une pluralité de modes du résonateur laser, un système d'excitation qui est configuré pour exciter de manière électrique la zone QCL active au moyen d'un courant de pompage électrique, et un système de syntonisation qui permet de régler les modes de résonateur ; un interféromètre servant à générer un interférogramme à base de lumière laser ; un système détecteur servant à détecter l'interférogramme après une interaction avec l'échantillon et servant à détecter un signal de détecteur, qui renferme l'interférogramme détecté, pendant une durée de mesure de détecteur ; et un système d'analyse qui est configuré pour détecter un spectre de l'échantillon par une transformée de Fourier de l'interférogramme détecté. Le système de syntonisation du laser QCL est configuré pour faire varier périodiquement de manière spectrale les modes de résonateur avec une période temporelle de syntonisation inférieure à 1 min respectivement dans un intervalle de syntonisation spectral qui est au moins égal à l'espacement entre des modes contigus du résonateur laser. La zone QCL active est configurée pour générer la lumière laser présentant des fréquences d'émission situées dans la plage allant de 1 THz à 6 THz. Les fréquences d'émission de la lumière laser couvrent une plage d'émissions spectrale d'au moins 50 GHz. Le système détecteur est mis au point pour pondérer dans le temps le signal de détecteur sur la période temporelle de syntonisation du système de syntonisation du laser QCL. L'invention concerne un procédé servant à examiner par spectroscopie un échantillon à l'aide du spectromètre.
PCT/EP2018/057105 2017-03-22 2018-03-21 Spectromètre de fourier comprenant un laser à cascade quantique à plusieurs modes, et procédé servant à examiner par spectroscopie un échantillon WO2018172391A2 (fr)

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DE102017002874.5A DE102017002874B4 (de) 2017-03-22 2017-03-22 Fourier-Spektrometer mit einem Mehrmoden-Quantenkaskadenlaser, und Verfahren zur spektroskopischen Untersuchung einer Probe
DE102017002874.5 2017-03-22

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WO2018172391A2 true WO2018172391A2 (fr) 2018-09-27
WO2018172391A3 WO2018172391A3 (fr) 2018-11-29

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CN114062311A (zh) * 2020-08-06 2022-02-18 昱辉激光科技(泰州)有限公司 量子级联激光光谱仪

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WO2015163965A2 (fr) * 2014-02-04 2015-10-29 Board Of Regents, The University Of Texas System Source de rayonnement térahertz accordable monolithique utilisant un mélange de fréquence non linéaire dans des lasers à cascade quantique

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* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CN114062311A (zh) * 2020-08-06 2022-02-18 昱辉激光科技(泰州)有限公司 量子级联激光光谱仪

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WO2018172391A3 (fr) 2018-11-29

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