Method and system for computer-based dual-comb broadband spectroscopy

Abstract

The DCS includes a pair of optical frequency combs (FCs) that generate respective outputs at different pulse repetition frequencies (PRFs) in a monitoring regime characterized by free-running FCs. The outputs are combined into a single output, which is split between a sample-interrogation (SI) channel and a reference channel. The reference channel includes a cell with an etalon material having a known etalon spectrum at low pressure. The etalon spectrum includes one or more widely spaced, high-intensity, narrow-width molecular lines. When the cell interacts with one of the beams, it emits a cell signal that is detected by a photodetector. The cell signal is processed in a data processing unit operable to mathematically exclude a single molecular line from the etalon spectrum and correct for the phase change in the excluded line. The corrected phase change is used to recover the desired spectrum of the cell signal and, in turn, recover the desired spectrum of the SI signal.

Description

[Technical Field]

[0001] This disclosure relates to mid-infrared dual-comb spectroscopy (DCS). In particular, this disclosure relates to systems and methods for providing a desired spectrum of a sample interrogation (SI) signal detected in one channel of the DCS by continuously monitoring phase changes in a single molecular beam of etalon material located in the other channel of the DCS. [Background technology]

[0002] Spectroscopy uses light to determine the physical, chemical, or structural properties of materials. Absorption spectroscopy, the subject of this application, is based on identifying which wavelengths of light a material absorbs by measuring the photons it passes through. Mid-infrared spectroscopy relies on light absorption over the spectral region extending from approximately 2 μm to at least 14 μm. The mid-infrared spectral region is important in the identification and analysis of a wide variety of materials. A dominant spectroscopic technique in the mid-infrared is Fourier transform (FIR) IR spectroscopy.

[0003] 1 and 2, one of the techniques used in FR IR spectroscopy is dual-comb spectroscopy (DCS). Generally, DCS uses two frequency combs (FCs), each of which is a coherent broadband light source formed by equally spaced optical frequencies. 1,2 ) The outputs of each comb are combined, and the combined output is passed through the sample to be analyzed, after which the output is detected by a photodetector (PD). Referring to Figure 1, the above results in a repeating series of interference signals in the time domain, with increasingly increasing time differences based on the difference between combs FC1 and FC2. Prominent peaks on the interferogram correspond to the time points at which the two pulses from each comb overlap each other at the PD. Figure 2 shows the frequency domain results of passing the combined pulse train through the sample. Here, mixing the two optical FCs converts them into a single RF comb, so the two combs with slightly different mode spacings enable heterodyne detection.

[0004] While all the known advantages of a MID-IR DCS are realized in accurately and precisely measuring characteristics, a constant PRF for each comb significantly impacts the error and uncertainty of the observed line characteristics, all other things being equal. Therefore, the accuracy of a DCS depends on maintaining the PRF difference between the combs within a desired range, despite the numerous environmental factors that are unavoidable outside of a laboratory, such as vibrations and temperature fluctuations.

[0005] Conventional DCSs use phase-locking of each FC to an etalon laser, such as a single-frequency single-mode (SFSM) CW laser. Conventional DCSs typically require a complex electromechanical setup that includes multiple servo locks and feedback loops combined with the servo locks that are required to achieve phase locking.

[0006] FIG. 3 shows a schematic diagram of one of the known circuit diagrams for a computer-implemented MIR DCS, disclosed in U.S. Patent Application Publication No. 2017 / 0307443 (US'443), which is incorporated herein in its entirety. This MIR DSC does not require the phase-locked electromechanical setup of a conventional DCS. In particular, US'443 teaches a method for correcting envelope frequency offset (CEFO) jitter. This circuit diagram includes at least two FC1 and FC2 combs, each outputting a pulsed beam at a slightly different PRF. The combs are spatially combined in combiners C1 and C2 located in the respective working and reference channels, which each contain a sample S to be analyzed and a reference material R. The reference material R has at least one known spectral line. Specifically, referring to the reference channel, the output from the cell containing the reference material R is detected by a reference photodetector (PD), which outputs a heterodyne-converted signal. The interferogram of the heterodyne converted signal is recorded in the data processing unit DPU, which outputs the signal to sections S1 to S2. nwhere n=2, 3, ..., n-1.

[0007] When each section S is transformed into the frequency domain, the first section S1 is used as a baseline and all other sections are compared to the first section S1. The comparison between the two sections indicates the time delay between these sections and the frequency lag for each section. The calculated time and frequency lag in the reference channel allows the DPU to periodically correct the acquired frequency spectrum caused by CEFO jitter in the working channel.

[0008] In the above-mentioned reference, a seemingly secondary consideration is how the instantaneous PRF difference between combs varies. Based on the teachings of the reference, PRF differences, even when deemed noteworthy, are determined based on comparisons between sections, not within any given section. However, if PRF variations over the duration of a single section are ignored, the level of measured data may be reduced.

[0009] Several correction algorithms have been written and used to control and maintain coherence between combs with varying degrees of success. Therefore, computer-based DCS is a very promising technique, and relatively simple correction algorithms featuring the ability to continuously monitor phase / frequency variations are needed. [Prior art documents] [Patent documents]

[0010] [Patent Document 1] U.S. Patent Application Publication No. 2017 / 0307443 Summary of the Invention [Means for solving the problem]

[0011] The disclosed computational MIR DCS fulfills this requirement by utilizing a molecular reference, i.e., a cell with an etalon material, typically a gas, to calibrate the time-domain data in the signal processing.

[0012] The disclosed DCS operates in two regimes: in one regime, the PRF difference between the two outputs of each FC is periodically controlled, which involves tuning the resonator cavity of at least one of the FCs;

[0013] The other regime continuously monitors the PRF difference while the FCs are free-running. The disclosed configuration associated with the monitoring regime involves combining the outputs from each FC into a single combined DCS output, which is then split into two beams. The beams are further directed along respective sample interrogation (SI) and reference channels. The beam propagating along the reference channel shines on a cell containing an etalon material with a known spectrum containing one or more widely spaced, high-intensity, narrow molecular lines. The linewidth of each molecular line is equal to or smaller than the resolution limit for the spectrometer used in the disclosed MIR DCS. The other beam interacts with the sample being measured, resulting in the emission of a sample interrogation (SI) signal.

[0014] The cell signal emitted from the cell is detected by the PD and has a narrow optical spectrum, which is excluded from the spectrum of the composite DCS output and includes the spectrum of the etalon material. The PD outputs a heterodyne-converted cell signal, and an interferogram of this signal is first recorded and then divided into multiple frames in the time domain. Each frame in the time domain is further mathematically processed to convert it into a corresponding spectrum of the detected cell signal in the RF frequency domain, which is smeared due to the instability of the PRF difference. The ultimate goal of the disclosed system and method is to restore the detected spectrum for each frame of the recorded interferogram in the reference channel to the desired spectrum, and then use the acquired data in the reference channel to restore the desired spectrum of the corresponding frame in the SI channel, which represents the correct measurement.

[0015] The processing of each frame begins with mathematically transforming the interferogram of each frame into the RF frequency domain, thus obtaining the corresponding spectrum. According to the inventive concept, one of the molecular lines in the spectrum of the etalon material is further mathematically excluded from the RF spectrum. For this excluded molecular line, a computer-executable program determines the phase change. Once determined, the phase change is used to correct the interferogram of the frame under investigation, which is then further transformed back into the frequency domain, thereby obtaining the desired spectrum. Finally, the corrected phase change is used to restore the desired spectrum of the corresponding frame in the SI channel.

[0016] According to a feature of the present invention, the phase change within the excluded molecular beam is determined by utilizing one of many standard programs well known to those skilled in the art of computer science. Once the phase change is calculated, the data obtained as a result of this calculation is used to construct an absorption spectrum for the frame under investigation. The constructed absorption spectrum is then matched with a pre-stored absorption spectrum, obtained, for example, during the DCS tuning phase when the FC is phase-locked or mathematically determined. If the comparison results are satisfactory, the calculated phase change is used to reconstruct the desired RF spectrum for the frame under investigation in the reference channel, and for the same reason, the desired RF spectrum for the corresponding frame of the SI signal.

[0017] According to another feature, the reference value is the phase change of the detected cell signal acquired using the same DCS system but operating in accordance with a mode-locked FC. If the measured value matches the reference value, the deviation of the measured phase change from the reference phase change is determined and corrected. By correcting the phase change, after a sequence of Fourier transform steps, the detected spectrum is corrected and thus restored to the desired spectrum of the frame under investigation in the reference channel. The corrected phase change is then used to obtain the desired spectrum for the corresponding frame in the SI channel, thus ensuring the accuracy of the spectroscopic measurement.

[0018] All of the above and other features of the DCS techniques and configurations of the present invention are structurally and functionally interrelated and are disclosed in greater detail in the following specific description of the present application.

[0019] The following drawings help illustrate the concepts of the present invention. [Brief explanation of the drawings]

[0020] [Figure 1] FIG. 1 illustrates DCS output in the time domain. [Figure 2]FIG. 1 illustrates DCS output in the frequency domain. [Figure 3] FIG. 1 is an exemplary circuit diagram of a known prior art computerized DCS using molecular referencing. [Figure 4] FIG. 1 is a highly schematic diagram of the DCS of the present invention. [Figure 5] 1 is a flowchart of a computer-implemented signal processing algorithm of the present invention. [Figure 6A] FIG. 6 is a computer shot illustrating the steps of the algorithm of FIG. 5. [Figure 6B] FIG. 6 is a computer shot illustrating the steps of the algorithm of FIG. 5. [Figure 6C] FIG. 6 is a computer shot illustrating the steps of the algorithm of FIG. 5. [Figure 6D] FIG. 6 is a computer shot illustrating the steps of the algorithm of FIG. 5. [Figure 6E] FIG. 6 is a computer shot illustrating the steps of the algorithm of FIG. 5. [Figure 6F] FIG. 6 is a computer shot illustrating the steps of the algorithm of FIG. 5. [Figure 6G] FIG. 6 is a computer shot illustrating the steps of the algorithm of FIG. 5. [Figure 6H] FIG. 6 is a computer shot illustrating the steps of the algorithm of FIG. 5. [Figure 7] FIG. 1 is a diagram illustrating the concept of a beat window. DETAILED DESCRIPTION OF THE INVENTION

[0021] A DCS system implementing the method of the present invention operates in a control regime in which the PRF difference between two FCs is periodically adjusted to fall within a desired range, and in a monitoring regime in which the PRF difference is continuously monitored while the FCs are free-running. The system of the present invention is distinguished from known prior art by a combination of structural and signal processing components. The structural component includes a cell having an etalon material that re-emits a cell signal having a known spectrum as a result of interaction with a portion of the DCS output. The signal processing component involves computer-implementable techniques that continuously monitor the phase change in a mathematically excluded single line of the known spectrum to maintain the desired spectrum of the sample interrogation signal emitted by the sample interacting with other portions of the DCS output.

[0022] Figure 4, combined with Figures 5 and 6A-6H, illustrates the method and DCS optical circuit diagram 10 of the present invention, which includes two or more solid-state FCs 12 and 14 at slightly different PRFs 1 and 2, respectively. For example, the PRF 1 of FC 12 can be selected from 80-100 MHz in the radio frequency (RF) domain (12 ns in the time domain), while the PRF difference between the outputs of each FC can be selected, for example, from a 50-100 Hz range. FCs 12 and 14 are similarly configured to output respective femtosecond (fs) pulse trains with substantially uniform pulse widths. However, the concept of the present invention is not compromised even if the amplitudes of the respective pulses in each pair vary by up to 10% relative to each other.

[0023] The spectra of the comb outputs from each of the FCs 12 and 14 are identical to each other, and each spectrum covers the MIR region from 2 μm to at least 14 μm. The MIR combs can be selected from near-IR sources such as fiber lasers, or directly from MIR semiconductor lasers, optical parametric oscillators, and microresonators. Lasers tested in the experimental DCS system include selenium-chromium laser diodes.

[0024] The combiner 16 in Figure 4 optically combines the outputs of the respective FCs 12, 14 into a combined output that is further split into two beams that are guided along the respective sample interrogation and reference channels. The combiner 16 may include a single bulk optical component, such as a prism, that performs the combination and then splits the combined FC output into two time-correlated beams. Alternatively, two different optical components can be used instead of a single prism.

[0025] Referring to FIG. 5 in addition to FIG. 4, the split beams are further guided along their respective channels, with the first beam in the signal interrogation channel shining on the sample 22 being tested in step 32, and the second beam transmitting through a cell containing etalon material in step 34 of FIG. 5. Before entering the cell containing etalon material 24, the second beam is guided through an optical filter F of FIG. 4, such as a diffraction grating 20, which cuts out a narrow spectral region from 1 to 5 (optical) GHz, as indicated by step 34 of FIG. 5. The relative positions of filter F and the cell can be reversed, such that the second beam is first guided through etalon material 24 and then optically filtered out. In either configuration, the optically filtered spectrum of the propagating beam contains the known spectrum of the etalon material.

[0026] Known spectra of low-pressure etalon materials, such as nitric oxide (NO), have etalon spectra with one or a few widely spaced, high-intensity, narrow molecular lines. Each line has a high intensity and a narrow spectral width approaching the resolution limit of the spectrometer used. While a single molecular line could be optically clipped by filter 20, this would entail structural complexity given that the spectral width of this molecular line is preferably approximately 100 MHz. However, this option is not excluded from the disclosed subject matter. The spectral region clipped by optical filter 20 in step 36 of FIG. 5 is substantially bell-shaped (not shown, but known to those skilled in the art), whereby the position of each molecular line relative to the central region of the bell-shaped region is known.

[0027] The DCS strength comes from a massively parallel heterodyne downconversion procedure, which allows information encoded in the optical domain to be directly mapped to the RF domain. In this case, the signal is processed and acquired using a data processing unit (DPU) 30 with an analog-to-digital converter. In this case, the comb outputs from the two matched FCs must be spatially overlapped, spreading the optical beat frequency across the PD bandwidth. To realize this concept, it is essential that the combs 12 and 14 maintain mutual coherence, since any significant drift or variation in PRF, as in this case, would degrade system performance over long timescales. However, as will be further explained, the combs 12 and 14 are free-running in the monitoring regime, causing PRF differences to become unstable, ultimately resulting in significant loss of information. The disclosed DCS system and techniques eliminate this, as will be explained below.

[0028] Referring again to Figures 4 and 5 in light of Figures 6A-6H, a sample interrogation (SI) signal and a cell signal are output by the respective sample 22 and etalon material 24 as a result of the interaction of the respective beams and are detected by PDs 26, 28. Their respective interferograms are time-synchronously recorded in steps 40 and 38, respectively, of Figure 5, one of which is shown in Figure 6A. The recorded interferograms are stored in the memory of the DPU 30 for further signal processing. Figure 6B shows the spectrum of a portion of the interferogram of Figure 6A that has not yet been processed.

[0029] Signal processing, which is particularly important to the monitoring regime of the DCS 10 of the present invention, begins by digitally dividing each interferogram into multiple short, uniform interferograms called frames. Each frame in both channels may be, for example, 10 milliseconds in duration. An interferogram corresponding to a single frame of detected cell signals is shown in FIG. 6C.

[0030] Figure 6D graphically explains the term frame. When the pulses of each FC 12, 14 in Figure 4 are output at different PRFs, two pulses of each comb simultaneously impinge on the input of, for example, PD 28 during a particular period. In the recorded interferogram, this interference is detected as peaks 52, with three peaks 52 shown in succession. Each peak 52 is considered a frame center. The digital division of the interferogram into individual frames in each SI channel and reference channel is time-synchronized; that is, for each frame in the reference channel, there is a time-correlated frame in the SI channel. The number of frames can vary from tens to hundreds, depending on the tradeoff between computer memory and the amount of mathematical transformations. Figure 6A, for example, includes 400 frames.

[0031] Referring to Figure 6E, which corresponds to step 42 in Figure 5, a single frame of the detected cell signal is Fourier transformed (or any other suitable transform) into the RF frequency domain, i.e., the spectrum of the detected etalon cell signal in Figure 6E. Specifically, the detected spectrum as shown has not yet been mathematically corrected; therefore, the spectrum is blurred by jitter and no molecular lines are clearly identified. The reason for such a spectrum is the unstable PRF difference between the outputs of the respective combs 12, 14 in Figure 4.

[0032] However, since the position of the molecular line relative to the center of the optical filter 20 in Figure 4 is known, it is easy to mathematically truncate any single line in the etalon spectrum. Preferably, but not necessarily, the line L to be excluded is CR is located within the central region of the spectrum in Figure 6E. Transforming the spectrum of the single excluded line into the time domain allows the phase change to be determined based on a reference value. Once the measured phase change is restored to a predetermined or digitally determined reference value, after several transformations as described below, it is first applied to the spectrum of the detected etalon signal in Figure 6E, as shown by step 50 in Figure 5, and then to the spectrum of the SI signal in step 54 in Figure 5. The resulting spectra of the respective cell signal and SI signal for the corresponding frame are shown in Figure 6G and are corrected to the desired spectrum, as described below. The desired spectrum of the SI signal ensures the accuracy of future spectroscopic measurements of the sample.

[0033] Referring to FIG. 6F, the red line represents the already-corrected phase change in the omitted line Lcr of FIG. 6E, corresponding to step 46 of FIG. 5. One possibility for accomplishing this task is to calculate the absorption spectrum of FIG. 6H while determining the phase change using a program known to those skilled in the art. Once the phase change is determined, the computer-executable program uses the determined phase change to generate an absorption spectrum for the entire frame of FIG. 6C. The generated spectrum and a reference spectrum, used as a reference and stored in computer memory, are then mathematically matched to confirm that the determined phase change is acceptable. The reference spectrum can either be mathematically determined or acquired with the FCs 12 and 14 phase-locked, which is typically done during a calibration phase before the device is shipped to a customer.

[0034] Because the excluded molecular beam Lcr in FIG. 6E is very narrow, the reference phase shift and time delay function are similar. The determined phase shift is then used to correct the interferogram in FIG. 6C, as indicated by step 48 in FIG. 5. The interferogram in FIG. 6C is again transformed into the frequency domain and used to reconstruct the desired spectrum for the current frame, as shown in FIG. 6G. The desired spectrum, when compared to the detected spectrum in FIG. 6E, is characterized by well-defined lines, including a center line Lcr and a smooth envelope.

[0035] Alternatively, the phase change of the omitted line Lcr in FIG. 6E can be corrected by comparing the measured phase change with a pre-stored reference phase change, which is considered to be the reference value obtained in a phase-locked DCS. Typically, the device is calibrated before shipping to a customer, and specific characteristics, such as the phase change when the FC is phase-locked, can be stored in computer memory. After restoring the measured phase change to the reference change, which is considered to be the reference value, the desired spectrum in FIG. 6G is obtained in the same manner as the sequence disclosed above.

[0036] Returning to Figure 6F, the blue line represents the frequency change derived from the phase change in the selected excluded line Lcr. The significance of the calculated frequency change should be considered in light of the PRF difference between the outputs of the respective combs 12, 14 in Figure 4. If this determined frequency change exceeds a predefined range within the excluded line, which can be determined empirically or digitally, reliably indicates that the PRF difference between the comb outputs is no longer within the predetermined range associated with the free-running FCs 12, 14, and if the PRF difference between the FCs is not within the predetermined range, the DPU 30 cannot recover the desired spectrum in Figure 6G. The corrupted frame may be ignored and discarded while waiting for the next "good" frame. However, if problems with phase change correction persist, the monitoring regime is interrupted and the system switches to a control regime with a feedback loop 60, as shown in Figure 4, in which a control signal generated by the DPU 30 is coupled to an actuator, such as a piezo motor or stepper motor, in one or both FC combs 12, 14 to adjust the length of the resonator cavity. When the PRF difference returns to within the preset range stored in the computer's memory, DCS 10 returns to the monitoring regime of DCS 10, where FC 12 and FC 14 of FIG. 4 are free-running.

[0037] After acquiring and storing multiple frames with desired spectra, it is necessary to reduce noise and thus increase the signal-to-noise ratio (SNR). The basic idea of ​​averaging for spectral noise reduction is the same as arithmetic averaging to obtain an average value. For example, averaging can be performed in the time domain by processing the interferograms of each stored frame. It is clear that noise reduction depends on the total number of frames. The more frames there are, the better the SNR. However, interferograms contain a large amount of data, which can overload computer memory. Meanwhile, averaging the spectra of each frame requires extensive calculations and increases processing time. For example, when the frames constituting the interferogram of FIG. 6A are stored, a compromise can be made by processing, for example, 100 frames based on each interferogram. The remaining frames can be addressed based on their respective spectra.

[0038] After accomplishing the signal processing disclosed above, a reference phase, or rather a time delay change in each frame of the etalon material cell signal, is applied to the corresponding frame of the sample interrogation material. After restoring the entire spectrum of the sample signal to the desired spectrum, the accuracy of the spectroscopic measurements is guaranteed using the techniques disclosed above.

[0039] FIG. 7 illustrates yet another feature of the present invention, where the disclosed computational techniques prevent uncertainties caused by conversion between RF and optical frequencies. When the outputs of the combs 12 and 14 are mathematically transformed, initially, the pairs of solid and dashed lines corresponding to each output are nearly aligned and virtually indistinguishable on a spectrometer. However, as the combs maintain their different PRFs, the dashed line representing the output of, for example, FC14, moves continuously away from its corresponding solid line. As the distance between these two lines increases, the dashed line for comb 14 eventually reaches and overlaps with the next solid line, and the sequence continues, with the distance between the dashed line and the next solid line increasing continuously. The interval in which the dashed lines cover the distance between the first and second solid lines is called the first window of the beat.

[0040] Therefore, the optical PRF of each FC 12, 14 is selected so that the entire optical spectrum of the DCS output falls within a first window of the beat. Operating within this first window, rather than any subsequent window, involves simply and precisely determining the correspondence between RF and optical frequencies. Subsequent windows would otherwise require additional electronics and complex computational techniques to find the correspondence between RF and optical frequencies.

[0041] The features disclosed herein in accordance with the present invention are not limited in their application to the details of construction and arrangements of components set forth in the following description or illustrated in the accompanying drawings. These features may assume other embodiments and may be implemented or carried out in various ways. Examples of specific implementations are provided herein for illustrative purposes only and are not intended to be limiting. In particular, acts, components, elements, and features discussed in connection with optical circuit diagrams and signal processing systems are not intended to be excluded from a similar role in any other embodiment.

[0042] Also, the phrases and terminology used herein are for purposes of description and should not be considered limiting. References in the singular or plural are not intended to limit the presently disclosed systems or methods, their components, acts, or elements. Additionally, in the event of inconsistent terminology usage between this document and a document incorporated herein by reference, the terminology usage in the incorporated reference supplements the terminology usage in this document, and in the event of conflicting terminology usage, the terminology usage in this document takes precedence.

[0043] Thus, while certain features of at least one example have been described, various alternatives, modifications, and improvements will readily occur to those skilled in the art. For example, the examples disclosed herein may be applicable in other contexts. Such alternatives, modifications, and improvements are part of this disclosure. Accordingly, the foregoing description and drawings are by way of example only. [Explanation of symbols]

[0044] 10 DCS optical circuit diagram 12, 14 FC 16 Combiner 20 Diffraction gratings, optical filters 22 Samples 24 Etalon materials 26, 28 PD 30 Data Processing Unit, DPU 52 Peak 60 Feedback Loop

Claims

1. 1. A computerized mid-infrared dual-comb spectroscopy (DCS) system operating in a control regime and a monitoring regime, the DCS system operating in the monitoring regime comprising: a free-running frequency comb (FC) that generates respective outputs at offset pulse repetition frequencies (PRFs) relative to one another within a preset PRF range, the outputs being optically combined into a system output, the system output being split into first and second beams that propagate along respective sample interrogation (SI) and reference optical channels; a cell having an etalon material positioned in the reference optical channel and interacting with the first beam to output a cell signal comprising an etalon spectrum of the etalon material; a first photodetector (PD) that captures the cell signal and outputs a heterodyne converted cell signal having an interferogram that is recorded; a data processing unit (DPU), slicing the interferogram into a sequence of uniform frames; mathematically transforming each frame in the time domain into a corresponding etalon cell spectrum in the frequency domain; Excluding a single molecular line from each etalon cell spectrum, determining a phase change for each of the rejected molecular beams; comparing the determined phase change for each excluded molecular beam with a reference value, thereby correcting the determined phase change, if necessary; a data processing unit (DPU) for processing the heterodyne transformed cell signals by recovering each etalon cell spectrum of the desired etalon cell spectrum using the corrected determined phase change; A mid-infrared DCS system comprising:

2. 10. The mid-infrared DCS system of claim 1, further comprising: a cell having a sample under test that outputs a sample interrogation (SI) signal when transmitting the second beam in the SI channel; and a second PD that receives the SI signal and outputs another heterodyne converted signal, wherein an interferogram of the another heterodyne converted signal is recorded and sliced ​​into a plurality of frames, and the plurality of frames are time-correlated with each frame of the interferogram of the heterodyne converted cell signal.

3. 3. The mid-infrared DCS system of claim 2, wherein the DPU is operable to form an absorption spectrum based on the determined phase change for each frame of the heterodyne converted signal in the reference optical channel and compare the formed absorption spectrum with a pre-stored reference absorption spectrum, the pre-stored absorption spectrum being the reference value.

4. The mid-infrared DCS system of claim 2 , wherein the reference value is a phase change in the excluded line acquired with the FCs phase-locked to one another.

5. 3. The mid-infrared DCS system of claim 2, wherein for each frame, the DPU is operable to correct the interferogram of the heterodyne converted cell signal using the corrected phase change, and the DPU is further operable to convert the corrected interferogram of each frame into the corresponding desired cell spectrum of the cell signal.

6. 6. The mid-infrared DCS system of claim 5, wherein the corrected phase change in the excluded molecular line in each frame of the individual frames of the interferogram of the cell signal is applied to a corresponding frame of the interferogram of the sample signal.

7. 10. The mid-infrared DCS system of claim 1, further comprising optical filters in the reference optical channel positioned before and after the cell having an etalon material to optically exclude a narrow spectral region of the spectrum of one of the beams, the excluded spectral region including the etalon cell spectrum of the etalon material.

8. The mid-infrared DCS system of claim 2 , wherein the PRF of each FC is selected such that the optical spectrum of the beam falls within a first window of a beam.

9. 8. The mid-infrared DCS system of claim 7, wherein the optically excluded spectral region has a bell shape with a central region occupied by a single selected molecular beam.

10. The mid-infrared DCS system of claim 2 , wherein each of the frames corresponds to the simultaneous incidence of a pair of pulses of the respective outputs of the FC on the first and second PDs.

11. 2. The mid-infrared DCS system of claim 1, wherein the DPU is operable to induce a frequency change from the corrected phase change in the excluded molecular beam to control a preset range of the PRF difference between the outputs of each FC.

12. 12. The mid-infrared DCS system of claim 11, wherein the FCs are solid-state femtosecond lasers that produce respective outputs having uniform spectra in the 2-14 μm spectral range.

13. 1. A method for operating a mid-infrared DCS system functioning in a control regime for periodically controlling a PRF difference between two FCs and a DCS, and in a monitoring regime for continuously monitoring said PRF difference using a free-running FC, said method of operating said DCS in said monitoring regime comprising: combining the outputs of each FC into a composite output and splitting the composite output into two beams having uniform spectra; directing the split beams along respective sample interrogation channels and reference channels through a cell having a sample to be tested and an etalon material, thereby generating respective sample interrogation (SI) signals and cell signals, the cell signals including a spectrum of the etalon material; detecting the SI signals and cell signals by respective PDs outputting respective heterodyne converted SI signals and cell signals from which interferograms are recorded; slicing each of said interferograms of the respective heterodyne SI signals and cell signals into a sequence of uniform frames; mathematically transforming the frame into a corresponding spectrum containing one or more molecular lines of the spectrum of the etalon material; and subtracting a single molecular line from the corresponding spectrum; processing each frame of the heterodyne converted cell signal by determining a phase change in the excluded single molecular line based on a reference value, thereby restoring the spectrum of each frame to a desired spectrum; A method comprising:

14. The method of claim 13 , wherein the frames of the interferogram of the heterodyne transformed cell signal are time-correlated with corresponding frames of the heterodyne transformed SI signal.

15. 15. The method of claim 14, wherein the step of determining the phase change in the excluded single molecular line for each frame comprises calculating an absorption spectrum of the cell signal until the calculated absorption spectrum matches a predetermined absorption spectrum corresponding to the reference value.

16. 15. The method of claim 14, comprising matching the determined phase change with a stored phase change, the stored phase change being the reference value.

17. 15. The method of claim 14, further comprising correcting the interferogram using the corrected phase change of the cell signal in the excluded molecular line for each frame and converting the corrected interferogram into the desired corresponding spectrum of the cell signal.

18. 20. The method of claim 17, further comprising averaging the frame of the cell signal by summing a plurality of the corrected interferograms and a plurality of desired spectra of the cell signal.

19. 15. The method of claim 14, further comprising optically excluding the beam in the reference channel to cull a spectral region corresponding to the etalon material spectrum, wherein the single molecular beam is located within a central region of the spectral region.

20. interrupting the monitoring regime, thereby switching the DCS to the control regime; The method of claim 14 , further comprising adjusting the resonator cavity of at least one of the FCs if the PRF difference between the respective FCs deviates outside a desired range.

21. 1. A computerized mid-infrared dual-comb spectroscopy (DCS) system operating in a control regime and a monitoring regime, the DCS system operating in the monitoring regime comprising: a free-running frequency comb (FC) that generates respective outputs at offset pulse repetition frequencies (PRFs) relative to one another, the outputs being optically combined into a system output, the system output being split into first and second beams that propagate along respective sample interrogation (SI) and reference optical channels; a cell having an etalon material positioned in the reference optical channel and interacting with the first beam to output a cell signal comprising an etalon spectrum of the etalon material; a data processing unit (DPU) for receiving the cell signal, excluding a single molecular line from the etalon cell spectrum; determining a phase change in the excluded cell spectrum; a data processing unit (DPU) running a program to compare the determined phase change with a reference value and, if necessary, correct the determined phase change used to restore a desired etalon spectrum of the cell signal; A mid-infrared DCS system comprising:

22. 22. The mid-infrared DCS system of claim 21, further comprising: a cell having a sample under test that outputs a sample interrogation (SI) signal when transmitting the second beam through the SI channel; and a second PD that receives the SI signal, wherein the corrected phase change in the excluded molecular beam of the cell signal is used to restore the desired spectrum of the SI signal.

23. 22. The mid-infrared DCS system of claim 21 , wherein the reference value comprises a predetermined absorption spectrum, and the DPU is operable to form a measured absorption spectrum based on the predetermined phase change in an excluded molecular line of the etalon spectrum.

24. 22. The mid-infrared DCS system of claim 21, wherein the reference value is a phase change in the excluded line acquired with the FCs phase-locked with respect to each other.

25. 22. The mid-infrared DCS system of claim 21, wherein in the control regime, the DPU executes a program to generate signals coupled to one or both actuators of respective FCs, the FCs operable to modify a length of a respective resonator cavity of the FCs when the PRF difference between the outputs of the respective FCs is detected to be outside a predetermined range.

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