DE102017002874A1 - Fourier spectrometer with a multimode quantum cascade laser, and method for spectroscopic examination of a sample - Google Patents

Abstract

A Fourier spectrometer for spectroscopic examination of a sample comprises a multimode quantum cascade (QCL) laser that configures an active QCL region in a laser cavity configured to generate laser light having emission frequencies according to a plurality of resonator modes of the laser cavity electric stimulation of the active QCL region is configured by means of an electric pumping current, and includes a tuner, with which the resonator modes are adjustable, an interferometer for generating a laser light based interferogram, a detector means for detecting the interferogram after interaction with the sample and for detecting a detector signal containing the detected interferogram within a detector measurement time, and an evaluation device configured to acquire a spectrum of the sample by a Fourier transformation of the detected interferogram wherein the QCL tuning means is configured to periodically spectrally vary the resonator modes with a timing period of less than one minute each in a spectral tuning interval that is at least equal to the spacing of adjacent resonator modes of the laser cavity, the active QCL region for generating the laser light the emission frequencies in the range of 1 THz to 6 THz is configured, wherein the emission frequencies of the laser light cover a spectral emission range of at least 50 GHz, and the detector means is designed for averaging the time of the detector signal over the timing of the tuning of the QCL. A method for the spectroscopic examination of a sample with the spectrometer is also described.

Description

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: FTIR 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 a Interaction with a sample and an evaluation device for Fourier transformation of the measured interferogram.

To achieve the highest possible spectral resolution and sensitivity in a conventional Fourier spectrometer, there is an interest in high-performance, broadband and tunable radiation sources with an emission in the terahertz (THz) spectral range. Although such radiation sources with a broadband emission and a continuous coverage of this emission range are known. They are based, for example, on heated SiC rods or on high-pressure mercury vapor lamps. However, such radiation sources have an insufficient output power for many practical applications. In contrast, due to their high output power, quantum cascade lasers (QCL), in particular THz quantum cascade lasers, are promising radiation sources which, in particular because of their narrowbandness, are suitable for spectroscopic investigations, e.g. for the study of energy transitions in atoms, molecules or solids, are particularly suitable. Single-mode QCL and multi-mode QCL with continuous time emission (CW emission, CW emission) have been proposed.

Single-mode QCLs allow for detuning in a spectral emission range of up to 240 GHz (see Han et al. in "Optics Letters", Vol. 39, pp. 3480-3483, 2014 ). However, the continuous, reproducible adjustment of the single mode QCL resonator mode over the entire emission range while maintaining high optical output power presents a technological challenge. For example, it is known to set a single resonator mode of single mode QCL by adjusting the effective refractive index of the single mode light mode QC 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.

Practical applications of a single mode QCL in continuous operation (cw operation), e.g. B. for measuring a gas absorption line or as a local oscillator in a heterodyne detection of blackbody radiation or radiation from space are limited to narrowband tuning ranges below 6 GHz. Attempts to increase the emission range of a single-mode QCL for practical applications have resulted in limited reproducibility, high complexity of QCL tuning, and low optical output powers.

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. However, 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.

From Wienold et al. is published in "Optics Express" (Vol. 22, pp. 30410-3042424, 2014) describes a broadband THz QCL with a Fabry-Perot resonator, wherein the emission of the QCL is detected with a Fourier spectrometer. The tuning range of the individual laser modes of the QCL covers the mode spacing of the resonator. The continuous coverage is achieved by means of a combined current and temperature setting, ie by means of two control variables. The von Wienold et al. described QCL has drawbacks due to the complexity of the simultaneous current and temperature adjustment and the slow response of the vote.

A grating spectrometer with a QCL as the radiation source is used by R. Eichholz et al. in Applied Physics Letters (Vol. 99, pp. 141112-1 - 141112-3, 2011) described. 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 of 0.01 GHz) is effected (modulation spectroscopy). The emission frequency covers an absorption line of a gas, 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. Specifically, the Fourier spectrometer is expected to be suitable 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 , In particular, QCL should be characterized by continuous (i.e., gapless) spectral coverage of a broad emission range, increased optical output power, ease of tunability, and / or high reproducibility of emission frequency adjustment. Further objects of the invention are to provide an improved method for the spectroscopic examination of a sample with the spectrometer, which avoids the disadvantages and limitations of conventional techniques.

These objects are achieved by a Fourier spectrometer and a method of operating the same having the features of the independent claims. Advantageous embodiments and applications of the invention will become apparent from the dependent claims.

According to a first general aspect of the invention, the above object is achieved by 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 having emission frequencies according to a A plurality of resonator modes of the laser resonator is configured, an exciter, which is configured for electrically exciting the active QCL region by means of an electric pumping current, and a tuner, with which the resonator modes are adjustable, an interferometer with a beam splitter, a stationary interferometer mirror and a movable interferometer mirror, the interferometer being arranged to produce an interferogram based on the laser light, detector means for detecting the interferogram after interacting with the sample and detecting a detector signal, w which includes the detected interferogram within a detector measurement time, and an evaluation device for detecting a spectrum of the sample by a Fourier transformation of the detected interferogram. The laser resonator may comprise a linear resonator, in particular a Fabry-Pérot resonator with planar reflectors, or a circular resonator, in particular a ring resonator or a disc resonator.

According to the invention, 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 according to the invention 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. Advantageously, 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.

Unlike a conventional single-mode QCL, according to the invention, 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. Advantageously, the resonator modes do not have to 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 with conventional single mode QCLs.

In contrast to conventional multimode QCL, the tuning of the resonator modes at least over the mode spacing of adjacent resonator modes of the laser resonator in the timing period is less than 1 min, so that a continuous spectral coverage of the emission range is achieved on average over time than in conventional modulation techniques.

According to a second general aspect of the invention, there is provided a method for spectroscopic examination of a sample with the Fourier spectrometer according to the first general aspect of the invention. 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 temporal average continuously cover the spectral emission range spanned by the resonator modes and outcoupling of the laser light from the laser resonator into the interferometer, generation of the interferogram with the interferometer, interaction of the interferogram with the sample, detection of the interferogram after interaction of the interferogram with the sample and detection of the detector signal containing the detected interferogram within the detector measurement time, and detection of the spectrum of the sample by a Fourier Transformation of the detected interferogram.

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, AlSb / InAs on an InAs substrate, or InGaAs / AllnAsSb, InGaAs / GaAsSb or InGaAs / AllnGaAs 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. 3 to 20 semiconductor layers composed of different band gaps and thicknesses.

The application of multimode QCL for Fourier transform infrared spectroscopy (especially THz spectroscopy) offers advantages in terms of high spectral resolution and tunability in a wide frequency range. Furthermore, the multimode QCL used in the present invention offers advantages over conventional THz sources for Fourier transmission infrared spectroscopy because of the improved signal-to-noise ratio and dynamic range increased by 1 to 2 orders of magnitude.

According to a preferred embodiment of the spectrometer according to the invention, the detector device is designed for the inherent averaging of the detector signal by using a detector with a time constant which is greater than the time-setting period of the tuner. Alternatively or additionally, 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.

Preferably, the timing period of the tuner is less than 1/10, in particular less than 1/1000 of a measurement time of the spectrometer for recording the measurement spectrum (detector measurement time for detection of the detector signal representing the detected interferogram). Advantageously, an influence on the measurement result by the temporal averaging of the detector signal is negligible or completely excluded.

According to a further preferred embodiment of the spectrometer according to the invention, the timing period of the tuner is less than 1 second, in particular less than 0.01 second. Advantageously, 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.

According to a first preferred tuning mechanism, 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. in "Applied Physics Letters", Vol. 90, p. 141114 (2007) , or H. Zhang et al. in "Journal of Applied Physics", Vol. 108, p. 93104 (2010) ). A changed optical gain (or a changed loss) always gives a modification of the refractive index, since both values are coupled via the Kramers-Kronig relationships. In contrast to a temperature-based tuning, this is advantageously a very fast process because it is coupled to the build-up of the gain in multimode QCL. Therefore, this mechanism is compatible with fast tuning at 300 kHz since it can be provided for the multimode QCL used in the present invention. Depending on the material used, the gain profile and its dependence on the pumping current, the achievable tuning range can vary.

The periodic variation of the pumping current (time function of the pumping current) can be characterized by periodic interruptions of the pumping current (pumping current = 0) in each case between two phases of the ramped variation of the pumping current, in particular as is known from the pulsed operation of conventional QCL. The duration of the interruption (shutdown) of the pumping current is chosen as a function of the available cooling capacity of the QCL cooling.

According to a second tuning mechanism, 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.

According to a third variant, 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. Preferably, however, the tuning means of the Fourier spectrometer is configured for periodic spectral variation of the resonator modes with only a single control variable.

• 1(a) bis 1(d): schematische Emissionsspektren eines Mehrmoden-QCL mit variierenden Resonatormoden;
• 1(e): eine schematische Illustration eines Fourier-Spektrometers gemäß einer bevorzugten Ausführungsform der Erfindung;
• 2(a) und 2(b): Kurvendarstellungen zur Illustration experimenteller Ergebnisse mit einem Mehrmoden-QCL gemäß einer bevorzugten Ausführungsform der Erfindung;
• 3(a) bis 3(c): weitere Kurvendarstellungen zur Illustration von Testergebnissen mit dem Mehrmoden-QCL gemäß einer bevorzugten Ausführungsform der Erfindung; und
• 4: ein Emissionsspektrum eines weiteren Beispiels eines erfindungsgemäßen Mehrmoden-QCL.

Further details and advantages of the invention will be described below with reference to the accompanying drawings. Show it:

• 1 (a) to 1 (d) : schematic emission spectra of a multimode QCL with varying resonator modes;
• 1 (e) Fig. 1 is a schematic illustration of a Fourier spectrometer according to a preferred embodiment of the invention;
• 2 (a) and 2 B) : Graphs illustrating experimental results with a multi-mode QCL according to a preferred embodiment of the invention;
• 3 (a) to 3 (c) : further graphs illustrating test results with the multimode QCL according to a preferred embodiment of the invention; and
• 4 : An emission spectrum of another example of a multimode QCL according to the invention.

Details of the invention are described below with particular reference to the tuning of the multimode QCL and a Fourier spectrometer equipped with the multimode QCL according to the invention. It is emphasized that 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, particularly based on irradiation of the active region with electromagnetic radiation having an energy greater than the energy of the band gaps of the semiconductor material used for the active QCL region or the substrate or a length variation of the laser resonator feasible.

Details of the structure of a multi-mode QCL will not be described because it can be realized as in a conventional multi-mode QCL. By way of example, reference is made to a QCL formed by a GaAs / AlAs heterostructure on a GaAs substrate. However, 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, e.g. on a carrier substrate connected to a pumping power source for injecting a pumping current.

wobei L_opt = nL die optische Weglänge im Laserresonator ist. L_opt wird durch die Brechzahl der Resonatormode n im Laserresonator und die geometrische Länge des Laserresonators bestimmt. Die Ordnung der FP-Resonatormode M repräsentiert die Anzahl von Intensitätsmaxima der stehenden Welle im Laserresonator und beträgt mindestens 100 für typische Resonatorlängen. 1 (a) schematically shows a section of a typical FP spectrum comprising a plurality of equally spaced resonator modes. In practice, the number of active resonator modes is, for example, 10. The spectral positions of the FP resonator modes are given by $${\nu }_M=\frac{cM}{2L_{\text{opt}}}$$

where 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.

Benachbarte FP-Resonatormoden bewegen sich praktisch um denselben Betrag, da für große Moden-Ordnungen dv_M+1 annähernd gleich dv_M ist.If L _opt is varied by an amount dL _opt , e.g. B. by a variation of the pump current, the frequency of a FP resonator changes according to $$\text{d}{\nu }_M=-\frac{cM}{2L_{\text{opt}}^2}\text{d}{L}_{\text{opt}}$$

Adjacent FP resonator modes move virtually the same amount because for large mode orders dv _{M + 1 is} approximately equal to dv _M.

When the common displacement of all the resonator modes reaches the mode spacing, as in FIGS 1 (b) and 1 (c) is illustrated, 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 achieved.

1 (d) shows that a broadband, gapless emission spectrum is achieved in the time average, when the optical path length increases continuously in a ramp over a time range .DELTA.t, thereby the resonator modes continuously shifted, so tuned, 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).

1 (e) 1 schematically shows an embodiment of a spectrometer in the form of an FTIR spectrometer equipped with the multimode QCL according to the invention. The tuning of L _opt is realized by ramp-shaped variations of the pump current with tuning periods in the range of, for example, 0.1 μs to 10 μs. In contrast, the intensity variation detector is sensitive to a time scale of 10 ms, which corresponds to the time scale of alternating constructive and destructive interference due to the movement of the FTIR (moveable mirror) interferometer mirror. 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.

By continuously tuning the resonator modes of the multimode QCL which is at least equal to the spacing of the resonator modes, the QCL can be adjusted to adjust one of the resonator modes at any desired emission frequency within the emission range. However, unlike conventional broadband sources for spectrometers, QCL can not emit a continuous spectrum at a given time. To replace a conventional radiation source in a spectrometer, the QCL is tuned to a tuning period At, while the detector signal from the spectrometer detector is averaged over that tuning period, as in FIG 1 (d) is shown.

If the tuning period Δt is significantly lower, e.g. is smaller than 1/100, preferably smaller than 1/1000, as the measuring time of the spectrometer for taking a measuring spectrum, the emission spectrum appears continuous due to the averaging so that the G-TAWB operation is obtained.

In order to continuously cover the emission region spanned by the resonator modes, the FP resonator modes are tuned far enough to close the spectral gaps between them. The mode spacing is approximately 10 GHz, for example in FP laser resonators with a length of 3 mm. The tuning interval over the mode spacing is achieved depending on the particular QCL used and the operating conditions (see in particular 4 and their description).

Concrete embodiments of the multimode QCL according to the invention and the Fourier spectrometer according to the invention 100 , eg according to 1 (e) are realized as follows, for example. The Fourier spectrometer 100 includes a multi-mode QCL 10 , an interferometer 20 , a detector device 30 and an evaluation device 40 , The components 20 . 30 and 40 of the device shown schematically z. As in a Bruker IFS spectrometer 120 HR built.

The multimode QCL 10 contains an active QCL region in a laser cavity 11 , An excitation device in the form of a pumping power source 12 and a tuner 13 that part of Pump current source 12 is. The active QCL region is z. B. constructed as they are from L. Schrottke et al. in "Applied Physics Letters", Vol. 108, p. 102102 (2016) ) is described. The active QCL region has z. B. a width of about 120 microns and a length of about 3 mm. The active QCL region is designed to generate laser light having emission frequencies in accordance with the resonator modes of the laser resonator 11.

For periodic tuning of the resonator modes is the tuner 13 contained in the pumping power source 12 and designed to generate a periodically varying pumping current. The variation of the pumping current with the pumping current source 12 follows, for example, a ramp-shaped time function (triangular pattern). The pumping power source 12 and the tuner 13 include, for example, an ILX Lightwave LDP- type driver source. 3840 with a pulse width of 1 μs and a trigger frequency of 300 kHz. Using a smoothing capacitor, the rectangular pulse shape is converted to a triangular pulse shape which gives a ramp time function of the pumping current.

The QCL 10 is equipped with a cooling device (not shown), with which an operating temperature of the QCL is set. 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.

In a preferred embodiment, the QCL 10 on the cooled support (cooled finger) of a 10 K continuous flow cryostat in front of the spectrometer's external emission port 100 assembled. The volume between the output window of the cryostat and the input window of the evacuated spectrometer 100 is purged with nitrogen to reduce absorption by humidity.

The interferometer 20 is through a beam splitter 21 , a stationary interferometer mirror 22 and a movable interferometer mirror 23 formed, which are positioned in Michelson arrangement. By periodic movement of the movable interferometer mirror 23 is, as is known from conventional Fourier spectrometers, on the laser light of the QCL 10 based interferogram generated.

The interferogram is at the output of the interferometer 20 an interaction with the sample to be examined 1 subjected. The sample 1 is z. B. arranged in a gas cell or a liquid cuvette.

The detector device 30 is to detect the interferogram after interaction with the sample 1 arranged. 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 averaged over time to match the variations in the emission frequencies of the QCL 10 compensate. With the evaluation device 40 that z. B. includes a computer unit, the detector signal is subjected to a Fourier transform, which directly the spectrum of the sample 1 results.

The time averaging of the detector signal is realized, for example, by the spectrometer 100 a slow pyroelectric detector is used, which can not follow the rapid intensity changes of the ramped time function of the pumping current. For example, 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 mirror 23 of the interferometer 20 in the spectrometer 100 , The maximum resolution of the spectrometer used 100 (based eg on the device of type Bruker IFS 120 HR) is 0.105 GHz (0.0035 cm ^-1 ).

2 (a) shows the emission spectrum of 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. For this reason, in the illustrated example, only in a part of the extending from 4.6 to 4.8 THz 200 GHz broad emission spectrum, the in the side image of 2 (a) is shown, achieved a complete coverage.

2 B) shows the emission spectrum of QCL recorded at a spectral resolution of 0.3 GHz, a scanner speed of 10 kHz and an aperture of 8 mm in G-TAWB operation as compared to a high-power thermal source based on a heated SiC rod of the commercially available spectrometer. In comparison with the thermal source, the QCL according to the invention exhibits a power output that is 1 to 2 orders of magnitude higher. Furthermore, the spectrum of the thermal source is due to their low output power, which corresponds to the magnitude of the detector noise because of the high resolution, not smooth. At a frequency of about 4.7 THz, the thermal source power is comparable to that of a high pressure mercury lamp.

An important advantage of the spectrometer according to the invention 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. The SNR can be acquired from several consecutively recorded spectra acquired under the same conditions, dividing the mean by the standard deviation for each frequency point. 3 (a) shows the SNR of the QCL used in the invention 10 and the conventional thermal source, each calculated from ten consecutive measurements. Similar to the power of the emission spectra shows in the QCL 10 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. In the entire frequency range, the SNR of the QCL 10 significantly higher than the SNR of the thermal source.

Advantageously, the SNR of the QCL 10 be further increased by fluctuations in the operation of the tuner, in particular fluctuations in the pumping current can be minimized.

In addition to the SNR, there is another important parameter of the spectrometer in the dynamic range 100 , To calculate 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 detected noise level. It is therefore a measure of maximum absorption that can be distinguished from noise.

To the noise level of the detector 30 to capture, became the QCL 10 and the thermal source is blocked at the inlet opening of the sample chamber, eg with an aluminum foil, whose low thermal emissivity ensures that there is no further thermal radiation on the detector 30 falls. With a determined noise level of the recorded spectra in the range of 0.04 relative units and a variation of the QCL output power in the range of 2 to 30 relative units ( 2 B) ), 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 used in the invention 10 may vary depending on the emission frequency. Advantageously, this is the application of the multimode QCL 10 in the spectrometer 100 not impaired, as described below with reference to 3 is pictured.

Also, the feasibility of spectroscopic measurements using the multimode QCL used in the invention 10 To show a transmission spectrum of a gas cell was taken, which is filled with water vapor-saturated air at atmospheric pressure (length of the gas cell: 10 cm). In this case, a weakly varying transmittance curve is expected due to the strong pressure broadening of the water absorption lines. The absence of spectrally rapidly varying curve sections in the transmission spectrum then serves as a sign that the structured emission spectrum of the multimode QCL 10 does not affect the accuracy of the measurement.

3 (b) shows the transmission spectra with the multimode QCL according to the invention 10 or a conventional thermal source were recorded. The spectral resolution is 0.3 GHz ( 12:01 cm ^-1 ), the scanner speed is 7.5 kHz and the measuring aperture is 8 mm. The transmission spectra were obtained by placing a gas cell in the sample chamber of the spectrometer 100 Spectrum is divided by a subsequently recorded without the gas cell spectrum.

The measurement in which the multimode QCL 10 is used as a radiation source, clearly shows the transmission spectrum of water vapor with only a spectrally slowly changing transmission curve. Using the thermal source, the output power was too low compared to the noise level of the detector to make a significant measurement. To confirm the accuracy of the result obtained, 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.

3 (c) shows the resulting transmission spectra. To show the reproducibility of the measurement are in 3 (c) two measurements are shown, each showing the QCL and the thermal source (th.s.).

In contrast to the measurement at high spectral resolution, the spectra in FIG 3 (c) evaluable results using the thermal source. However, the difference between the two nominally identical transmission spectra and the occurrence of negative values between 4.73 and 4.74 THz shows that the rapidly varying modulations in the spectra are caused by noise. In contrast to the strong modulations due to noise when using the thermal source, the transmission spectra are when using the multimode QCL according to the invention 10 as a radiation source regardless of the frequency-dependent varying output power smooth and easy to reproduce.

4 shows the emission spectrum of another embodiment of a multi-mode QCL used in the invention 10 in G-TAWB operation which, in terms of its wave function structure, center frequency, and waveguide / laser cavity dimensions, is similar to the multi-mode QCL embodiment described above. As in 2 (a) was for recording the spectrum in 4 the highest possible resolution (0.105 GHz), the boxcar apodization, the Mertz phase correction, and an 8 mm aperture used to minimize any broadening effect in the spectrum. 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.

The features of the invention disclosed in the foregoing description, drawings and claims may be significant to the realization of the invention in its various forms both individually and in combination or sub-combination.

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited non-patent literature

• Han et al. in "Optics Letters", Vol. 39, pp. 3480-3483, 2014 [0004]
• Wienold et al. is published in "Optics Express" (Vol. 22, pp. 30410-30424, 2014). [0007]
• R. Eichholz et al. in Applied Physics Letters (Vol. 99, pp. 141112-1 - 141112-3, 2011) [0008]
• L.A. Dunbar et al. in "Applied Physics Letters", Vol. 90, p. 141114 (2007) [0023]
• H. Zhang et al. in "Journal of Applied Physics", Vol. 108, p. 93104 (2010) [0023]
• L. Schrottke et al. in Applied Physics Letters, Vol. 108, p. 102102 (2016) [0041]

Claims (16)

Fourier spectrometer (100) configured to spectroscopically examine a sample (1), comprising - a multimode quantum cascade laser (QCL) (10) having an active QCL region in a laser cavity (11) for generating laser light is configured with emission frequencies according to a plurality of resonator modes of the laser resonator (11), an excitation means (12) configured to electrically excite the active QCL region by means of an electric pumping current, and tuning means (13) for adjusting the resonator modes an interferometer (20) having a beam splitter (21), a stationary interferometer mirror (22) and a movable interferometer mirror (23), the interferometer (20) being arranged to produce an interferogram based on the laser light, - A detector means (30) for detecting the interferogram after an interaction with the sample (1) and for detecting a Detect orsignals containing the detected interferogram is configured within a detector measurement time, and - an evaluation device (40) configured to detect a spectrum of the sample (1) by a Fourier transform of the detected interferogram, characterized in that - the tuner (13) of the multimode quantum cascade laser (10) is configured for periodic spectral variation of the resonator modes 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 (11), - the active QCL Is configured to generate the laser light with the emission frequencies in the range of 1 THz to 6 THz, wherein the emission frequencies of the laser light cover a spectral emission range of at least 50 GHz, and - the detector means (30) for averaging the time of the detector signal over the time Abs Timmperiode of the tuner (13) of the multimode quantum cascade laser (10) is designed. Fourier spectrometer according to Claim 1 in which - the detector means (30) is adapted for inherently averaging the detector signal by use of a detector having a time constant greater than the timing period. Fourier spectrometer according to Claim 1 in which - the detector device (30) for the time averaging of the detector signal circuit components for the signal averaging. Fourier spectrometer according to Claim 3 in which - the circuit components for the signal averaging comprise a boxcar integrator or a lock-in amplifier. Fourier spectrometer according to one of the preceding claims, in which - The timing period of the tuning device (13) is equal to or less than 1/10 of the detector measuring time of the detector device (30). Fourier spectrometer according to Claim 5 in which - the timing period of the tuner (13) is equal to or less than 1/1000 of the detector measurement time of the detector means (30). Fourier spectrometer according to one of the preceding claims, in which - The timing period of the tuner (13) is less than 1 second. Fourier spectrometer according to Claim 7 in which - the timing period of the tuner (13) is less than 0.01 second. Fourier spectrometer according to one of the preceding claims, in which - The tuning device (13) is configured for periodically varying the pumping current. Fourier spectrometer according to one of the preceding claims, in which - The tuning means (13) for illuminating the multimode quantum cascade laser (10), in particular the active QCL region or the 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 for the active QCL region or the substrate, in particular electromagnetic radiation in the visible or near infrared spectral range, is configured with periodically varying amplitude. Fourier spectrometer according to one of the preceding claims, in which - the tuner (13) is configured to periodically vary the length of the laser resonator (11). Fourier spectrometer according to one of the preceding claims, in which - The tuner (13) is configured for periodic spectral variation of the resonator modes with only a single control variable. Fourier spectrometer according to one of the preceding claims, in which the multimode quantum cascade laser (10) has at least one of the following features: - The laser resonator (11) comprises a linear resonator, in particular a Fabry-Pérot resonator, or a circular resonator, in particular a ring resonator, and - The spectral emission range extends over at least 70 GHz. A method of operating a Fourier spectrometer (100) according to any one of the preceding claims, comprising the steps electrical stimulation of the active QCL region by means of an electric pumping current, wherein the resonator modes with the timing period less than 1 min are periodically varied in the spectral tuning interval at least equal to the distance of adjacent resonator modes of the laser resonator (11), so that the Emission frequencies of the laser light in the temporal average continuously cover the spectral emission range, which is spanned by the resonator modes, and - Coupling of the laser light from the laser resonator (11) in the interferometer (20), Generation of the interferogram with the interferometer (20), Interaction of the interferogram with the sample (1), - Detection of the interferogram after the interaction of the interferogram with the sample (1) and detection of the detector signal, which includes the detected interferogram, within the detector measurement time, and - Detecting the spectrum of the sample (1) by a Fourier transform of the detected interferogram. Method according to Claim 14 in which - the timing period is equal to or less than 1/10 of the detector measurement time for detecting the detector signal including the detected interferogram. Method according to Claim 15 in which - the timing period is equal to or less than 1/1000 of the detector measurement time for detecting the detector signal including the detected interferogram.

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