BALANCED-DETECTION INTERFEROMETRIC CAVITY-ASSISTED PHOTOTHERMAL SPECTROSCOPY WITHIN A SINGLE CAVITY

Abstract

A method and corresponding apparatus for detecting a molecule, in particular a trace gas species, in a sample using photothermal spectroscopy including the steps of providing a probe laser beam and propagating the probe laser beam to a cavity of a Fabry-Perot interferometer directing the probe laser beam through the sample in the cavity providing an excitation laser beam for heating the sample in the cavity directing the excitation laser beam through the sample in the cavity detecting the transmitted probe laser beam, which was transmitted from the cavity and detecting the reflected probe laser beam, which was reflected from the cavity.

Claims

1. A method for detecting a molecule, in particular a trace gas species, in a sample using photothermal spectroscopy comprising the steps of: providing a probe laser beam and propagating the probe laser beam to a cavity of a Fabry-Perot interferometer; directing the probe laser beam through the sample in the cavity; providing an excitation laser beam for heating the sample in the cavity; directing the excitation laser beam through the sample in the cavity; detecting the transmitted probe laser beam, which was transmitted from the cavity; and detecting the reflected probe laser beam, which was reflected from the cavity.

2. The method according to claim 1, wherein the probe laser beam propagating to the cavity is separated from the reflected probe laser beam by an optical circulator.

3. The method according to claim 1, wherein the probe laser beam is propagated to the cavity at least in a section in an optical fibre.

4. The method according to claim 3, wherein the probe laser beam propagating to the cavity is coupled into the cavity by a fibre-coupled collimator and the reflected probe laser beam is collected by the same fibre-coupled collimator.

5. The method according to claim 1, further comprising tuning the probe laser beam to a frequency, at which the transmitted probe laser beam and the reflected probe laser beam have the same power.

6. The method according to claim 1, further comprising the step of subtracting a transmitted signal corresponding to the transmitted probe laser beam and a reflected signal corresponding to the reflected probe laser beam.

7. The method according to claim 1, further comprising the steps of: adjusting the transmitted probe laser beam by a first attenuator and/or the reflected probe laser beam by a second attenuator such that the transmitted probe laser beam and the reflected probe laser beam have the same power values, prior to detecting the transmitted probe laser beam and the reflected probe laser beam.

8. The method according to claim 1, further comprising the step of: tuning the probe laser beam to a partial-transmission or a partial reflection of one side of a resonance of the cavity.

9. The method according to claim 1, further comprising the steps of: modulating the excitation laser beam wavelength, wherein the modulated excitation laser beam is directed through the sample in the cavity; and detecting a harmonic, in particular a second harmonic, of a modulation of the transmitted probe laser beam and detecting a harmonic, in particular a second harmonic, of a modulation of the reflected probe laser beam.

10. The method according to claim 1, further comprising the steps of: providing a further probe laser beam and propagating the further probe laser beam to a further cavity of the Fabry-Perot interferometer; directing the further probe laser beam through the sample in the further cavity; detecting the transmitted further probe laser beam, which was transmitted from the further cavity; and detecting the reflected further probe laser beam, which was reflected from the further cavity.

11. A photothermal interferometry apparatus for detecting a molecule in a sample, in particular for detecting a trace gas species, comprising: a Fabry-Perot interferometer with a first partially reflective mirror (3), a second partially reflective mirror and a cavity for containing the sample extending between the first mirror and the second mirror; a probe laser (6) for providing a probe laser beam; an excitation laser for passing an excitation laser beam through the cavity such that it intersects with the probe laser beam in the cavity for exciting the molecule in the sample; a first photodetector arranged for detecting a transmitted probe laser beam, which was transmitted from the cavity; and a second photodetector arranged for detecting a reflected transmitted probe laser beam, which was reflected from the cavity.

12. The photothermal interferometry apparatus according to claim 11, comprising an optical circulator arranged for directing the probe laser beam from the probe laser to the cavity and for directing the reflected probe laser beam from the cavity to the second photodetector.

13. The photothermal interferometry apparatus according to claim 11, comprising an optical fibre which is arranged for at least in a section propagating the probe laser beam from the probe laser to the cavity.

14. The photothermal interferometry apparatus according to claim 13, comprising a fibre-coupled collimator for coupling the probe laser beam into the cavity and for collecting the reflected probe laser beam.

15. The photothermal interferometry apparatus according to claim 11, wherein the Fabry-Perot interferometer comprises a sample cell for containing the sample, the first mirror and the second mirror being fixed on a first and second side of the sample cell, wherein optionally the sample cell comprises a sample inlet and a sample outlet.

16. The photothermal interferometry apparatus according to claim 11, comprising a subtractor, in particular a differential amplifier, for subtracting a probe laser signal detected by the first photodetector and a reflected probe laser signal detected by the second photodetector.

17. The photothermal interferometer apparatus according to claim 11, comprising a first attenuator arranged in the path of the transmitted probe laser beam between the cavity and the first photodetector and/or a second attenuator arranged in the path of the reflected probe laser beam between the cavity and the second photodetector, in particular arranged in the path of the reflected probe laser beam between the optical circulator and the second photodetector.

18. The photothermal interferometer apparatus according to claim 17, wherein the first attenuator is a variable value attenuator and/or the second attenuator is a variable value attenuator.

19. The photothermal interferometer apparatus according to claim 11, comprising a tuner for tuning the probe laser beam over a given wavelength range.

20. The photothermal interferometer apparatus according to claim 11, comprising: a modulator for modulating the wavelength of the excitation laser beam, the first photodetector being arranged for detecting a modulation of the transmitted probe laser beam, the second photodetector being arranged for detecting a modulation of the reflected probe laser beam; and a control unit arranged for communicating with the first photodetector and the second photodetector and arranged for determining a harmonic, in particular a second harmonic, of the modulation of the transmitted probe laser beam and the reflected probe laser beam, wherein the control unit optionally comprises a lock-in amplifier.

Description

[0072] The invention is further explained with respect to exemplary embodiments thereof.

[0073] FIG. 1 schematically shows a Fabry-Perot interferometer.

[0074] FIG. 2A schematically shows the reflected intensity of the probe laser beam in interferometric cavity-assisted photothermal spectroscopy (ICAPS).

[0075] FIG. 2B schematically shows the transmitted intensity in ICAPS.

[0076] FIG. 3A schematically illustrates excess probe laser noise (frequency fluctuations) of an ICAPS setup.

[0077] FIG. 3B schematically illustrates environmental noise (e.g. sound) of an ICAPS setup.

[0078] FIG. 4 schematically illustrates the principle of balanced-detection ICAPS.

[0079] FIG. 5 schematically illustrates a preferred embodiment of the photothermal interferometer apparatus.

[0080] FIG. 6 schematically illustrates another preferred embodiment of the photothermal interferometer apparatus, which was also used to experimentally verify the functional principle of the present invention.

[0081] FIG. 7 shows the spectra of a sample gas, once acquired according to the present invention and once acquired in a non balanced detection mode.

[0082] FIG. 8 illustrates the improvement in noise achieved by one embodiment the present invention.

[0083] FIG. 9 shows the relationship between the target molecule concentration and the sensor signal.

[0084] FIG. 10 shows the measured signal amplitude as a function of the target molecule concentration.

[0085] FIG. 1 schematically shows a Fabry-Perot interferometer (FPI) 101 with a cavity 102 extending between an input mirror 103 and a second mirror 104, which are both partially transmitting and are spaced at a distance. Monochromatic radiation 106 entering the FPI 101 is partially reflected by the input mirror 103. The transmitted intensity portion is further reflected between the two mirrors 103, 104, forming an infinite series of partial waves in forward and backward direction and thus, a circulating beam 105. With each reflection, intensity is coupled out of the FPI in both directions, i.e. a transmitted beam 107 and a reflected beam 108 leaves the cavity 102.

[0086] The ICAPS operation principle is shown in FIG. 2A for the reflected intensity and in FIG. 2B for the transmitted intensity. In both cases, the frequency of a probe laser (straight line) is tuned near the inflection point on one side of the cavity's resonance, incorporating sample gas at thermal equilibrium (solid trace). Photo-induced heating of the sample by an excitation laser alters the sample's refractive index, which is accompanied by a shift in the transmittance and reflectance with respect to the vacuum wavelength (dotted trace). This shift is monitored by a photodiode via a change in the detected probe laser intensity (I.sub.).

[0087] The different excess noise sources of an ICAPS setup are schematically illustrated in FIG. 3A and 3B: FIG. 3A illustrates excess probe laser noise (frequency fluctuations of the probe laser beam) and FIG. 3B illustrates environmental noise (e.g. sound).

[0088] FIG. 4 schematically illustrates the principle of balanced-detection ICAPS monitoring the reflectance of the interferometer in an all-fiber-coupled probe laser configuration. Solid lines with arrows illustrate the optical signal and its traveling direction; dotted lines the electrical signal. The beam from a probe laser 110 is split by beam splitter 111 into two equal parta sample probe beam 115 and a reference probe beam 116and is coupled by a collimator 113 each into two separate but identical interferometers 114. The sample beam 115, which intersects with an excitation beam 117, probes the photothermal signal, which is superimposed by noise, whereas the reference beam 116 probes only noise. The reflected light is again collected by the collimator 113 and separated from the forward propagating light coming from the probe laser 110 by a circulator 118, routing the beam to a photodiode 119. By subtraction of the two photodiode signals at subtractor 120, the photothermal signal is received along with high rejection of common mode noise.

[0089] FIG. 5 schematically illustrates a preferred embodiment of the photothermal interferometer apparatus 1 for detecting a molecule in a sample, in particular for detecting a trace gas species. Again, optical signals and their traveling directions are indicated by solid lines and arrows, while electrical signals are indicated by dotted lines.

[0090] The apparatus 1 comprises a Fabry-Perot interferometer 2 with a first partially reflective mirror 3, a second partially reflective mirror 4 and a cavity 5 for containing the sample extending between the first mirror 3 and the second mirror 4. The device further comprises a probe laser 6 for providing a probe laser beam 7. Via an optical circulator 8, the probe laser beam is propagated in an optical fibre to a fibre-coupled collimator 9 for coupling the probe laser beam 7 into the cavity 5. Further, the apparatus 1 comprises an excitation laser (not shown) for providing an excitation laser beam 10 such that it passes through the cavity 5 and intersects with the probe laser beam 7 in the cavity 5 for exciting the molecule in the sample.

[0091] The transmitted probe laser beam 11 leaks out of the cavity 5 at the second mirror 4. It is collected by another coupler 12. A first photodetector 13 is arranged for detecting the transmitted probe laser beam 11. Further, the reflected probe laser beam 14 leaks out of the cavity 5 at the first mirror 3. The reflected probe laser beam 14 also comprises the fraction of the probe laser beam 7 that was reflected at the first mirror 3 and not coupled into the cavity 5. The fibre-coupled collimator 9 is also arranged for collecting the reflected probe laser beam 14. The optical circulator 8 is arranged both for directing the probe laser beam 7 from the probe laser 6 to the cavity 5, as mentioned above, as well as for directing the reflected probe laser beam 14 from the cavity 5 to a second photodetector 15, which is arranged for detecting the reflected probe laser beam 14.

[0092] The transmitted signal 16 corresponding to the transmitted probe laser beam 11 detected by the first photodetector 13 over time and the reflected signal 17 corresponding to the reflected probe laser beam 14 detected by the second photodetector 15 over time are illustrated. Both the transmitted signal 16 and the reflected signal 17 carry the photothermal signal, but with opposed signs, while they carry identical probe laser intensity noise.

[0093] The apparatus also comprises a subtractor 18, which in particular is a differential amplifier, for subtracting the transmitted probe laser signal 16 detected by the first photodetector 13 and the reflected probe laser signal 17 detected by the second photodetector 15. The resulting subtracted signal over time is shown in the center right. It carries the photothermal signal without common mode intensity noise. The amplitude of this detected photothermal signal is doubled compared to the probe laser signal 16 or 17. Thus, balanced detection is achieved within one single cavity, reducing the system complexity, influences of cavity drift are eliminated and the detected signal-to-noise ratio is enhanced.

[0094] In this embodiment, the Fabry-Perot interferometer 2 comprises a sample cell 19 for containing the sample, the first mirror 3 and the second mirror 4 being fixed on a first and second side of the sample cell 19. The sample cell 19 comprises a sample inlet 20, at which the sample is introduced into the sample cell 19, and a sample outlet 21, at which the sample is drawn out of the sample cell 19.

[0095] FIG. 6 schematically illustrates another preferred embodiment of the photothermal interferometer apparatus 1, which was also used to experimentally verify the functional principle of the present invention. The embodiment shown in FIG. 6 is similar to the one shown in FIG. 5 and essentially comprises all of the elements mentioned in the context of FIG. 5. Therefore, like parts have been given the same reference numerals and only the differences/additions over the embodiment shown in FIG. 5 will be mentioned.

[0096] In FIG. 6, also the excitation laser 22 for providing the excitation laser beam 10 is shown. The apparatus 1 also comprises a first attenuator 23 arranged in the path of the transmitted probe laser beam 11 between the cavity 5 and the first photodetector 13 and/or a second attenuator 24 arranged in the path of the reflected probe laser beam 14 between the cavity 5 and the second photodetector 15, in particular arranged in the path of the reflected probe laser beam 14 between the optical circulator 8 and the second photodetector 15.

[0097] In order to verify the functional principle of the present invention, the metrological figures of merit were investigated using carbon monoxide (CO) as the (target) molecule of the sample. Investigations of the enhancement of the detected photothermal signal, sensitivity, linear response and the noise cancellation performance were performed by recording spectral scans of CO via tuning the QCL frequency across the selected absorption line for balanced and non-balanced detection as well as by recording the noise when the sample cell 19 was flushed with moisturized N.sub.2. Different trace gas concentration levels were obtained by blending a 100 ppmv CO calibration mixture with N.sub.2 via a custom gas mixing system. The N.sub.2 used for dilution was moisturized with water vapor obtaining an absolute humidity of 2.0%. The presence of water vapor influences the response to CO by enhancing the VT energy transfer rate and thus enhances the detected photothermal signal.

[0098] Transient generation of the photothermal signals was performed by applying wavelength modulation (WM) at reduced sample pressure via a powerful continuous wave (CW) distributed feedback (DFB) quantum cascade laser (QCL) as excitation laser 22 emitting at a wavelength around 4.59 m to target strong fundamental absorption features of the sample molecules in the mid-infrared (mid-IR) region. The induced refractive index changes were monitored by the sample probe laser 6 transversely intersecting the excitation beam 22. This layout offers simple beam alignment and avoids any heating of the FPI's first and second mirror 3, 4 by the excitation laser beam 10, thus enabling a simple, robust, and compact gas sensor design. The photo-induced transducer signal was detected within a narrow bandwidth by a lock-in amplifier (LIA) 25 of the control unit 26 at the second harmonic (2f) of the modulation frequency. This 2f-WM scheme is a powerful method for increasing the signal-to-noise ratio as well as the selectivity of a given measurement.

[0099] Refractive index changes were detected via a CW-DFB fiber laser (FL) as probe laser 6 emitting in the vicinity of 1550 nm. This near-infrared region offers mature technology and readily available high-performing optical components. High sensitivity was accomplished by application of interferometers 2 with moderate finesse as well as a small mirror spacing of 1 mm together with strong photo-thermal signal generation by use of high excitation laser intensities. The setup uses an all-fiber-coupled probe laser configuration, probing the reflectance (i.e. reflected probe laser beam 14) and transmittance (i.e. transmitted probe laser beam 11) of the same interferometer 2. The use of optical fibers greatly improves the sensor ruggedness by avoiding free-space probe laser beams and by precluding any possible mismatch in the beam guiding at the interferometer coupling/collecting interface.

[0100] The embodiment of FIG. 6 uses a single air-spaced optical cavity 5, consisting of two fused silica plates (1052 mm) on which dielectric-coated mirrors with a reflectivity of R=0.989 are deposited as first and second mirror 3, 4. The mirrors 3, 4 are separated by spacers of 1 mm thickness. The cavity 5 was simultaneously used as the transducer for monitoring induced changes in the refractive index, as well as the reference to apply balanced detection. Photothermal-induced refractive index changes inside the cavity 5 were monitored via a fiber-coupled, single-mode tunable CW-DFB-FL (probe laser 6). The probe laser 6 emitted a probe laser beam 7 at a wavelength of 1550 nm with a constant optical output power of 40 mW; its wavelength could be thermally tuned within a total range of 1.2 nm by a laser driver 38. The fiber-coupled output beam (probe laser beam 7) of the probe laser 6 was routed through a fiber-coupled optical circulator 8 whose corresponding port was coupled to a pigtailed gradient-index (GRIN) fiber-optic collimator 9 (working distance, WD=15 mm, beam diameter at WD=0.5 mm FWHM). This collimator 9 served to couple the forward traveling light into the cavity 5 and the reflected, backward travelling light (i.e. the reflected probe laser beam 14 again into the fiber.

[0101] The reflected light 14 was separated from the forward traveling light by the circulator 8 and sent to the second photodetector 15. The transmitted probe laser beam 11 was also coupled by a further coupler 12 into an optical fiber and sent to the second photodetector 13. Both the first and the second photodetector 13, 15 comprise a gallium indium arsenide (GaInAs) positive intrinsic negative junction (PIN) photodiode amplifying the signal via a trans-impedance amplifier (TIA, not shown). The intensities of these individually transmitted and reflected probe laser beams 11, 14 were adjusted by fiber-coupled attenuators 23, 24 ahead of the photodetectors 13, 15 to avoid saturation. At the sensor's operation point the intensity of the transmitted and reflected probe laser beam 11, 14 was identical. This yielded the same response of intensity noise in both channels.

[0102] The electronic outputs of the photodiodes 13, 15 were passed to a 4th order Gaussian high-pass filter (which is one element with the subtractor 18) with a 3 dB cut-off frequency of 200 Hz and a low-noise differential amplifier (as subtractor 18) with a gain of 100, whose output was fed into a lock-in amplifier (LIA) 25. The probe laser emission frequency was maintained at the operation point of the cavity's (5) resonance via a slow feedback circuit (mHz), by using the DC component of the first photodetector 13, which monitored the transmitted probe laser beam intensity. By monitoring the DC-component and adjusting the probe laser frequency, any drift of the transducer, e.g., due to temperature or changing sample gas composition, or drift of the emitted laser frequency itself was automatically compensated. The interferometer 2 was fixed into a compact and gas-tight aluminium sample cell 19. Transmission of the probe laser beam 7 was enabled directly by the interferometer substrates and a fused silica window, respectively, transmission of the QCL beam (excitation laser beam 10) through the sample cell 19 was enabled by two CaF2 windows 27. Sample gas exchange was performed via sample gas in-and outlets 20, 21. The outer dimensions of the sample cell 19 were 321830 mm with a total inner sample gas volume of a few cm.sup.3.

[0103] Selective heating of the sample gas inside the interferometer 2 was performed by using a collimated, high heat load (HHL) packaged CW-DFB-QCL excitation laser 22 emitting at a wavelength of 4.59 m, whose frequency could be tuned by varying the QCL temperature via injection current and temperature control by a Peltier element by a laser driver 39. The QCL output beam (excitation laser beam 10) was focused by a plano-convex CaF2 lens 28 (f=50 mm) between the two mirrors 3, 4 forming the cavity 5 to induce strong photothermal excitation via the high laser intensity, intersecting the standing wave of the probe laser beam 7 in the transverse direction.

[0104] The sensor platform was based on photothermal sample excitation via wavelength modulation and detection of the second harmonic (2f) by demodulation of the alternating current (AC) component of the differentially amplified photodetector signals 16, 17, i.e., the balanced signal, using an LIA 25. The digitized electronic signals were transferred to a computer 29 via data acquisition and processing unit 33 for further data processing in a LabVIEW-based program.

[0105] The QCL output beam was split by a beam splitter 30 (97:3), whose low power part was guided through a reference cell 31 filled with CO in N.sub.2 at reduced pressure, and finally onto a pyroelectric photodetector 32. The reference gas cell 31 and the photodetector 32 were used as the reference channel to monitor the emitted excitation laser 22 wavelength feeding the detector 32 signal to another LIA 34. The ICAPS detection was performed in scan mode, where spectra of the sample gas were acquired by slowly tuning (mHz) the excitation laser frequency over the desired spectral range around the target absorption line through a change of the DC injection current component using a sawtooth function. To implement the WM technique, the emission wavelength of the excitation laser 22 was modulated by adding a sinusoidal function to the DC injection current input. The detected probe laser beam intensity was modulated when the temperature of the gas inside the cavity 5 was altered via absorption of the excitation laser radiation by the target molecules.

[0106] The pressure and flow of the sample gas inside the sample cell 19 were controlled and maintained by using a metering valve, pressure sensor 35, pressure controller 36, and mini diaphragm vacuum pump 37. The metrological figures of merit for the presented apparatus 1 were investigated by employing a modulation frequency of f.sub.mod=297 Hz, a modulation depth of =0.09 cm.sup.1, an LIA time constant set to =1 s, and a sawtooth excitation laser tuning frequency of f=6.67 mHz. The absolute pressure and flow of the sample gas was kept constant at p=850 mbar and u=25 mL min.sup.1.

[0107] To investigate the enhancement of the detected photothermal signal via balanced-detection within a single cavity 5 two spectra of 10 ppmv CO in moisturized N.sub.2 were acquired: once in the balanced-detection mode, i.e. according to the present invention, and once in the non balanced-detection mode (see FIG. 7). (Non balanced-detection refers to the use of only the transmitted or the reflected signal. In this setup, the reflected signal was used.) The results show an improved signal by a factor of approximately 1.9 using balanced-detection. The discrepancy from theoretical enhancement by a factor of 2 occurs due to a slightly reduced total height of the reflected resonance profile.

[0108] In particular, FIG. 7 shows the 2f-WM ICAPS sensor response for non balanced-detection and balanced-detection within a single cavity when the excitation laser 22 was tuned across the targeted absorption band centered at 2179.77 cm.sup.1 at an absolute pressure of 850 mbar.

[0109] To investigate the noise cancellation performance of the present invention (labelled as balanced-detection (within a single cavity)) and thus the improvement in the signal-to-noise ratio of the balanced-detection scheme, the noise floor of the sensor was recorded for a total duration of 30 min when the cell was flushed with moisturized N.sub.2. Comparison of the calculated standard deviation of the measured data show a noise reduction by a factor of approximately 9 for the balanced-detection scheme, see FIG. 8. In particular, FIG. 8 shows the 2f-WM ICAPS sensor response for non balanced-detection and balanced-detection within a single cavity for moisturized N.sub.2 when the excitation laser 22 was kept at 2179.77 cm.sup.1 at an absolute sample pressure of 850mbar.

[0110] Based on the signal amplitudes for 10 ppmv CO and the standard deviations of the noise level for moisturized N.sub.2, a signal-to-noise ratio of 226 and 3816 was calculated for non balanced-detection and balanced-detection, respectively. By applying the present invention an improvement in the signal-to-noise ratio by a factor of 16.9 was achieved, which yielded a lo minimum detection limit (MDL) of 2.6 ppbv for an acquisition time of 1 s. This improvement in the signal-to-noise ratio is composed by the enhancement in the detected signal (1.88) and the improvement in noise (9), when employing balanced-detection ICAPS within a single cavity.

[0111] The selective response and linearity of the sensor response to various concentrations of CO in moisturized N.sub.2 was verified by recording 2f-WM spectra for six different trace gas levels (1,2, 4, 6, 8 and 10 parts per million by volume, ppbm) as well as the noise floor of the sensor for moisturized N.sub.2 (see FIG. 9). The measured data for each concentration level yielded excellent linearity between signal amplitudes and the CO concentrations (see inset of FIG. 10).

[0112] In particular, FIG. 9 shows the 2f-WM single-cavity balanced-detection ICAPS sensor response for six different CO gas concentrations in moisturized N.sub.2 (absolute humidity=2.0% H.sub.2O), as well as the sensor noise floor for moisturized N.sub.2, recorded when the QCL frequency was tuned over the targeted absorption band centered at 2179.77 cm.sup.1 at an absolute pressure of 850 mbar. FIG. 10 shows the measured signal amplitudes as a function of Co concentration, showing linear sensor performance to varying sample gas concentration levels.