Synthesizing gas-filled anti-resonant hollow-core fiber Raman lines enables access to the molecular fingerprint region | Nature Communications

Nature Communications volume 15, Article number: 9427 (2024) Cite this article

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The synthesis of multiple narrow optical spectral lines, precisely and independently tuned across the near- to mid-infrared region, is a pivotal research area that enables selective and real-time detection of trace gas species within complex gas mixtures. However, existing methods for developing such light sources suffer from limited flexibility and very low pulse energy, particularly in the mid-infrared domain. Here, we introduce a concept that is based on the combination of an appropriate design of near-infrared fiber laser pump and cascaded configuration of gas-filled anti-resonant hollow-core fiber technology. This concept enables the synthesis of multiple independently tunable spectral lines, with >1 μJ high pulse energies and a few nanoseconds pulse width in the near- and mid-infrared regions. The number and wavelengths of the generated spectral lines can be dynamically reconfigured. A proof-of-concept laser beam synthesized of two narrow spectral lines at 3.99 µm and 4.25 µm wavelengths is demonstrated and combined with photoacoustic modality for real-time SO2 and CO2 detection. The proposed concept also constitutes a promising way for infrared multispectral microscopic imaging.

The infrared (IR) region, known as the molecular fingerprint region, plays an essential role in a wide arc of applications particularly for trace gas analysis. Diverse light technologies have been used for optical gas detection, and they can be categorized into broadband (e.g., supercontinuum generation) and narrow linewidth (e.g., quantum cascade laser (QCL)) light sources1,2,3,4,5,6. The latter is more suitable for gas detection at trace amounts, because their narrow linewidth can achieve a high sensitivity and accuracy by avoiding cross-interference of different gases with overlapped absorption spectra. Therefore, narrow-linewidth lasers with a broad tunable wavelength range are typically employed for multi-gas detection, and they have been well-developed and commercialized based on different lasing technologies such as external cavity QCL and optical parametric oscillator (OPO)3,7,8,9 (https://www.daylightsolutions.com/products/mircat/)(https://www.toptica.com/products/tunable-diode-lasers/frequency-converted-lasers/topo)(https://aerissensors.com/pico-series/). Nevertheless, all these technologies still suffer from three common limitations: (i) The requirement for high spectral resolution slows down the wavelength tuning speed of the light source, consequently compromising the ability for “real-time” multi-gas monitoring because it can take seconds long time to scan through the necessary wavelength range for detecting different gases; (ii) Further enhancement of the gas sensing sensitivity and the signal-to-noise ratio can be achieved by scaling up the IR laser power/pulse energy10,11,12 using for example the OPO technology, however it is associated large footprint and limited portability8,9; (iii) The broad wavelength tunability relies on the use of volume Bragg grating, which as a free-space optical component compromises the laser robustness against environmental vibrations, humidity, etc. To address these issues, an alternative technology of synthesizing multiple narrow spectral lines has been proposed, where each distinct laser line targets a specific gas absorption line without interference from other gas species. However, developing such a light source remains a major scientific challenge. Conventional laser diodes at different wavelengths can be synthesized into a single-mode silica fiber through wavelength division multiplexer (WDM), but this approach is limited to the near-IR region ( < 2.4 µm)13. To move into the mid-IR region, the only approach is to synthesize multiple QCLs with different emission wavelengths14,15,16 (https://www.cea.fr/cea-tech/leti/Documents/d%C3%A9monstrateurs/Flyer_Photoacoustic%20spectroscopy_num.pdf)17,18. Specifically, the mid-IR photonic integrated circuit (PIC) technology has been recently proposed to synthesize spectral lines from multiple QCL sources16,19. However, QCL technology suffers from low (peak) power of just a few watts20,21,22,23, thus is fundamentally limited towards the generation of high-energy mid-IR pulses, which is important for enhancing the sensitivity and signal-to-noise ratio of gas detection technologies such as cavity ring-down and photoacoustic (PA)12,24. Additionally, this technology cannot dynamically tailor the number and wavelengths of the synthesized spectral lines.

The recent advent of gas-filled anti-resonant hollow-core fiber (ARHCF) Raman laser technology25,26,27,28,29,30,31,32,33,34 is a potential alternative for the synthesis of IR spectral lines with high pulse energy. The long Raman Stokes coefficient of active gases35, combined with the broadband and low-loss light propagation of silica ARHCF36,37, enables efficient Raman Stokes wavelength conversion from the near- to the mid-IR region. As a result, several near- and mid-IR gas-filled fiber Raman lasers have been reported with microjoule-level high energy nanosecond pulses up to 4.4 µm wavelength26,27,38,39,40,41,42. Meanwhile, the Raman gain bandwidth can be as narrow as a few GHz or even hundreds of MHz using specific gases at appropriate pressure43,44,45, thus allowing the generation of narrow Raman spectral line(s)39,46. For example, gas-filled high-energy fiber Raman lasers with few GHz linewidth have already been reported39,46, which are sufficiently narrow to achieve accurate and selective gas sensing because the discrete absorption lines is typically on the order of few GHz at ambient conditions. However, the existing gas-based Raman lasers have only been developed based on single-wavelength pump lasers with pre-defined static Raman line(s) and thus independent wavelength tuning is inherently not possible.

In this work, we have demonstrated a concept, that first synthesizes multiple spectral lines in the 1 μm gain range of Ytterbium(Yb)-doped fiber, and couples that light into cascaded gas-filled two stage ARHCFs, to convert these lines into IR spectral lines covering a broad wavelength range from near- to mid-IR region. The generated Raman Stokes from the infused gas in the ARHCF plays the role of the “frequency converter”. This design allows the number and wavelengths of Raman lines to be independently and precisely controlled/tuned by controlling the pump spectral lines, thereby accurately targeting the absorption lines of selected gases with high accuracy. Each Raman spectral line has microjoule-level energy at few nanoseconds pulse duration. This unique and promising spectral synthesis technology is fundamentally attributed to the appropriate design of the 1 µm multi-line fiber laser and the combination of the emerging ARHCF technology. The bend loss of the 2nd stage ARHCF is alleviated by using a nested capillary structure design, as compared to a simple tubular ARHCF. This allows the ARHCF to be compactly coiled to a ~ 30 cm total diameter. As a proof-of-concept, this proposed concept is used for the real-time and simultaneous photoacoustic detection of CO2 and SO2 in the mid-IR region.

Figure 1a presents the proposed concept of synthesizing multiple spectral lines. The center part is a cascaded silica ARHCF configuration pumped by a near-IR laser with multiple synthesized reconfigurable spectral lines lying within the gain range of Yb-doped fiber amplifier (typically 1015–1115 nm (https://coherentinc.my.site.com/Coherent/specialty-optical-fibers/PLMA-YDF-25_250-UF?cclcl=en_US)). The wavelengths of these synthesized pump lines are nonlinearly converted based on the rotational/vibrational stimulated Raman scattering (SRS) effect of gases filled in the 1st and 2nd stage ARHCFs. As a result, the synthesized Raman Stokes lines have diverse wavelengths from the near- to mid-IR region. The wavelength of each Raman Stokes line is determined by the wavelength of its corresponding pump line through \(\frac{1}{{\lambda }_{{{{\rm{R}}}}}}=\frac{1}{{\lambda }_{{{{\rm{P}}}}}}-\varOmega\), where \({\lambda }_{{{{\rm{p}}}}}\) and \({\lambda }_{{{{\rm{R}}}}}\) are the optical wavelengths of the pump and Raman Stokes, respectively, \(\varOmega\) is the rotational/vibrational Raman shift coefficient of gas.

a Concept for the synthesis of multiple IR spectral lines and its application on PA detection of multiple gases. λP1…λPn refer to the wavelengths of pump lines, λ1R1…λ1Rn refer to the wavelengths of Raman laser lines output from the 1st gas-filled ARHCF, λ2R1…λ2Rn refer to the wavelengths of Raman laser lines output from the 2nd gas-filled ARHCF. ARHCF: anti-resonant hollow-core fiber. PA: photoacoustic. b Wavelength conversion examples through filling different gas (CH4, CO2 and N2) into the 1st stage ARHCF. The 2nd stage ARHCF is constantly filled with H2 at 30 bar. Left axis shows measured optical spectra including pump lines, and Raman lines output from the H2-filled 2nd stage ARHCF. Right axis shows the absorbance spectra of CO2 and SO2 obtained from the high-resolution transmission molecular absorption database (HITRAN). The absorbance spectra of CO2 and SO2 are used to show that their absorption peaks are aligned with the two mid-IR spectral lines being synthesized with the proposed concept, therefore constituting the foundation of real-time detection of both CO2 and SO2. Details regarding the measured spectra in b are explained in Supplementary Note 1.

The key of this concept is the broad IR wavelength range of the Raman Stokes lines enabling access to the absorption bands of different gases. The wavelength ranges of Raman Stokes and pump laser can be expressed as:

where \({\triangle \lambda }_{{{{\rm{P}}}}}={\lambda }_{{{{\rm{P}}}}\_\max }-{\lambda }_{{{{\rm{P}}}}\_\min }\) is the pump wavelength range, \({\triangle \lambda }_{{{{\rm{R}}}}}={\lambda }_{{{{\rm{R}}}}\_\max }-{\lambda }_{{{{\rm{R}}}}\_\min }\) is the corresponding Raman Stokes wavelength range, \(\varOmega\) is the Raman Stokes shift coefficient. With a large \(\varOmega\) value, the denominator in Eq. (1) is far less than 1, thus making \({\triangle \lambda }_{R}\) much greater than \({\triangle \lambda }_{P}\) (see examples in Supplementary Table 1). In the case of cascading two ARHCFs, \(\varOmega=\,{\varOmega }_{1}+{\varOmega }_{2}\), where \({\varOmega }_{1}\) and \({\varOmega }_{2}\) are the Raman Stokes shift coefficients of gases filled in the 1st and 2nd stage ARHCF, respectively.

While any pump wavelength can be selected, we specifically select two pump lines at 1044 and 1060 nm, to demonstrate the concept behind Eq. (1). These two lines are used to pump two-stage cascaded silica ARHCFs (see Supplementary Note 2 for details). The core diameter of the 1st and 2nd stage ARHCFs are 32.8 and 82.0 μm. Since the bend loss of ARHCF is proportional to its core diameter47, the 2nd stage ARHCF is designed and fabricated with a nested anti-resonant hollow-core structure, to alleviate the bend loss31,48,49 (see Supplementary Note 2). With this design, all ARHCFs in this work have been optimized to a ~ 30 cm bend diameter in this experiment (see Supplementary Fig. 8 in Supplementary Note 6). Figure 1b shows the diversity of the generated and synthesized Raman lines over a broad IR range from 1.2 to 4.3 μm, achieved by filling 30 bar H2 into the 2nd stage ARHCF while the 1st stage ARHCF is filled with different gases including CH4, CO2, and N2, respectively. These results show that each pair of Raman Stokes lines has a larger wavelength spacing than the dual pump lines (see details in Supplementary Note 1). For example, synthesized Raman laser lines at 3.99 and 4.25 μm are generated in the case of filling CH4 into the 1st stage ARHCF. The spacing between these two Raman lines is approximately 15 times larger than that of the dual pump lines, therefore Eq. (1) is validated. Furthermore, the dedicated selection of the pump wavelengths makes the synthesized dual Raman lines (at 3.99 and 4.25 μm) exactly overlap with the absorption spectra of SO2 and CO2, respectively, to enable selective and real-time detection of both gases. Another example is to fill CO2 into the 1st stage ARHCF, which, due to the cascaded vibrational SRS effect of CO2, leads to the generation of four mid-IR Raman lines within a broad spectral range at 2478, 2574, 3778, and 4004 nm, respectively, as shown in Supplementary Fig. 1b.

This section consists of (B.1): the synthesis of multiple spectral lines using the proposed concept, and (B.2): the application of the proposed laser source on PA detection of multiple gases. Figure 2a shows the configuration of the entire system.

a Experiment setup consisting of the pump laser, cascaded ARHCFs filled with CH4 and H2, respectively, PA detection, and data acquisition. HWP: half-wave plate. YFA: Yb-doped fiber amplifier. ARHCF: anti-resonant hollow-core fiber. PA: photoacoustic. b Top: pump pulse bursts alternately output between 1044 and 1060 nm; bottom: corresponding Raman pulse bursts alternately output between 3.99 and 4.25 μm from the 2nd stage H2-filled ARHCF. c Independent and precise wavelength tuning of the mid-IR Raman Stokes lines by thermally tuning the wavelengths of their corresponding laser diode seeds at 1 μm region. The right axis shows the absorbance spectra of CO2 and SO2 obtained from HITRAN. Inset of c is the measured beam profile of 4.25 μm Raman laser using a mid-IR camera (S-WinCamD-IR-BB-7.5, DataRay). d Average power monitoring of the Raman laser alternately output between 3.99 and 4.25 μm. e PA pulse excited at 1% CO2 concentration with 4.25 μm Raman laser at 10 Hz repetition rate. f Fourier transformation spectrum of e. (n, m, nz) in f refers to different radial, azimuthal, and longitudinal modes of the resonant acoustic chamber52.

This part presents the results of the pump fiber laser and the cascaded ARHCF Raman laser.

The pump laser adopts an all-fiber polarization-maintained (PM) structure. The seed laser is composed of two linearly polarized distributed feedback narrow-linewidth (few GHz) laser diodes with different wavelengths at 1044 and 1060 nm which are well within the gain range of Yb-doped fiber. These two seeds have a single-mode fiber (PM980) output, and they are synthesized into a single-mode PM fiber through WDM, to seed the subsequent PM Yb-doped fiber amplifiers, as shown in the top of Fig. 2a. The number and wavelength of the laser diode seeds can be dynamically added/removed through fiber splicing. All laser diode seeds are directly modulated, to emit 3.7 ns pulses. Each seed is equipped with a thermoelectric cooler for thermally and independently tuning its wavelength over ~1 nm in the temperature range of 20–35 °C. The amplification module is composed of two pre-amplifiers and a power amplifier using 915 nm laser diodes as pumps (see Methods section). The Yb-doped fiber in the power amplifier has a large core diameter of 25 μm, to suppress the stimulated Brillouin Scattering (SBS) effect. To avoid the gain competition during the amplification, a trigger-delay system is designed to make an alternate output between different spectral lines in the time domain, as shown by an example at the top of Fig. 2b, where each line operates in burst mode. Each burst is composed of several internal pulses, and these internal pulses are equally spaced in the time domain forming a constant internal repetition rate. The software can control the number of pulses within the burst, internal repetition rate, as well as the time interval between adjacent bursts (defined as burst interval). A detailed illustration of the burst operation of the pump laser is provided in Supplementary Note 3. In this work, the burst interval is set to 4 ms, which is sufficiently longer than the upper-level lifetime of the Yb3+ ion (in the order of 1–2 ms) and thus avoids the gain competition of different spectral lines. Such a short time interval guarantees a fast switching of the spectral lines of the pump and thus the subsequent Raman lines (see an example at the bottom of Fig. 2b), thereby constituting the proposed concept of “real-time detection of multiple gases”.

The pulse pump regime50 is adopted for the Yb-doped fiber amplifier module by electrically modulating the 915 nm laser diode pumps, and the intensity and width of the pump pulse can be independently adjusted for different laser lines to be amplified. Each pump pulse corresponds to a signal laser pulse to be amplified. This design brings two key advantages. (i) It suppresses the unwanted amplified spontaneous emission (ASE) without using bandpass filters in the amplification stages, therefore enabling the “reconfigurable property” of synthesizing any number and wavelength of laser diode seeds within the Yb gain range. (ii) Given the varied gain coefficient of the Yb-doped fiber amplifier with respect to wavelength, for different laser lines this design allows for using different pump parameters to maximize their output pulse energies respectively. In this design, the maximum internal repetition rate of the laser burst is limited to ~1 kHz due to the ~2 ms upper-level lifetime of the Yb3+ ion.

After amplification, the maximum pulse energy at 1044 nm and 1060 nm are measured to be ~75 and ~98 μJ, respectively, using a pyroelectric energy meter (PE9-ES-C, Ophir Optronics), while the linewidths are less than 0.2 nm which is important for suppressing the dispersion walk-off effect and improving the efficiency of Raman Stokes generation51. The polarization extinction ratio of the amplified spectral lines is measured to be ~20 dB which slightly varies as a function of the temperature of the corresponding laser diode seed. The detailed characteristics of the pump laser are provided in Supplementary Note 4.

As shown in Fig. 2a, the pump laser is coupled into the 1st stage 7-m long CH4-filled ARHCF through a pair of C-coated plano-convex lenses, to enable the synthesis of a dual Raman lines at 1.50 and 1.53 μm through the 1st order vibrational Raman Stokes generation of CH4. The CH4 pressure is set to 2.5 bar, to achieve a relatively narrow linewidth of ~0.5 nm with a high pulse energy. The average pulse energies of the 1st order vibrational Raman Stokes lines are respectively measured to be 22 μJ at 1.50 μm and 28 μJ at 1.53 μm and their pulse widths are measured to be 3.3 ns due to the short dephasing time of CH4. Details regarding the characterization of the CH4-filled ARHCF Raman laser are included in Supplementary Note 5.

The two Raman Stokes lines output from the 1st stage CH4-filled ARHCF is then coupled into the 2nd stage H2-filled ARHCF with 5 m length. The two Raman lines at 3.99 and 4.25 μm are generated and synthesized through the 1st order vibrational Raman Stokes generation in H2 gas. Due to the pump laser design, the two synchronized Raman laser lines alternately occur in the time domain (see the example at the bottom of Fig. 2b). Figure 2c presents the measured optical spectra. Their linewidths are measured to be ~1.3 nm using an optical spectrometer (Spectro 320, Instrument Systems) with a resolution of 0.3 nm. By thermally tuning their corresponding seed line, the wavelength of each Raman line can be precisely and independently tuned over ~15 nm, to target the absorption lines of SO2 and CO2. Here, the temperatures for the 1044 and 1060 nm seeds are set to 22 °C and 24 °C, respectively, to maximize the absorption coefficient (see Fig. 2c) and thus the excited acoustic signal during the subsequent PA gas detection process. Under this condition, the maximum pulse energies are measured to be 1.7 μJ at 3.99 μm and 2.7 μJ at 4.25 μm, respectively, at 30 bar H2 pressure. They correspond to a quantum efficiency of 8.7% and 11.1% in terms of the pump energies of 75 μJ at 1044 nm and 98 μJ at 1060 nm. Figure 2d presents the power monitoring result using a thermal power meter. In this measurement, the burst of each Raman line in the time domain is composed of 1000 pulses with a 1 kHz internal repetition rate. The periodic switching of the measured power indicates the alternate output between the synthesized mid-IR Raman lines. Note that the result in Fig. 2d shows a 1.5 μJ pulse energy at the 4.25 μm line which is less than the aforementioned 2.7 μJ because of the laser beam attenuation caused by CO2 absorption in the ambient air (see Supplementary Fig. 7c). Details regarding the 2nd stage H2-filled fiber Raman laser are referred to Supplementary Note 6.

This part presents the configuration of the PA detection setup, and the results from the real-time detection of CO2 and SO2.

The Raman laser beam consisting of synthesized 3.99 and 4.25 μm Raman spectral lines is collimated by a CaF2 lens with a 3 cm focal length and then coupled into a resonant acoustic chamber for SO2 and CO2 detection. The acoustic chamber adopts the design in refs. 52,53 (see the longitudinal section of the acoustic chamber at the bottom of Fig. 2a), which has widely used nanosecond pulsed laser as a light source24,52,54. Details regarding the PA detection setup and gas sample preparation are described in Methods. Figure 2e shows a typical acoustic pulse excited by the 4.25 μm gas-filled fiber laser operating at a 10 Hz repetition rate, where 1% CO2 is used by diluting a pure CO2 with pure Ar. It shows a sudden onset of an oscillation followed by a slow decay in amplitude, a typical characteristic of an acoustic wave excited from a nanosecond laser pulse34. The pulse width of the acoustic wave is measured to be ~7 ms. The corresponding Fourier transformation spectrum is shown in Fig. 2f. The sharp peaks are the resonant modes of the acoustic chamber, indicating the broad frequency range of the acoustic pulse excited by the nanosecond Raman laser pulse, which induces the advantage of robust PA performance against the variation of the ambient temperature variation24.

SO2 and CO2 mixture diluted by pure Ar flows through the acoustic chamber for validating the proposed detection approach. Their concentrations are independently controlled with mass flow controllers (MFCs) (see details in Methods). Every pulse burst of the mid-IR Raman laser lines is set to be composed of 1000 periodic Raman pulses, leading to a periodic excitation of the PA signal which is detected in the frequency domain using a digital lock-in amplifier (MFLI 500 kHz, Zurich Instruments). The internal repetition rate of each laser burst is set to 942 Hz, to overlap its 4th-order harmonic frequency component of the acoustic pulse sequence with the first radial mode (100) of the acoustic chamber at 3.77 kHz, as shown in Fig. 2f. In this case, the PA detection combines both the modulation and pulse regimes, leading to an enhanced sensitivity55.

Figure 3a presents the raw acoustic data of the detection of SO2 and CO2 obtained from the lock-in amplifier, where both of their concentrations vary as a function of time. The zoom-in in the inset shows that the signal switches in ~2 s time interval because of the alternative output of Raman laser lines at 3.99 and 4.25 μm. This ~2 s interval is dependent on the response speed of the lock-in amplifier determined by the time constant, which is set to 30 ms. Using a shorter time constant, a quicker response speed can be achieved but at the price of increasing the fluctuations of the measured acoustic signal and thus compromising the detection limit.

a Raw data stream output from the lock-in amplifier, containing the PA signal excited from both CO2 and SO2 with varied concentrations as a function of time. Inset: a zoom-in result of a.b PA signals of CO2 and SO2 extracted in real-time from a.c Real-time PA signal recorded for varied CO2 concentration while constant SO2 concentration at 84 ppm. d,e are linear fits of the PA intensity versus gas concentrations at different repetition rates of 1.00 and 942 kHz, respectively. f The monitoring of PA intensity excited from 1.3 ppm CO2 by increasing the temperature of acoustic chamber from 22 °C to 23.5 °C, using the Raman spectral line at 4.25 μm but with two different repetition rates of 942 Hz and 1 kHz, respectively.

Since the data stream obtained from the lock-in amplifier contains the acoustic information of multiple gases (SO2 and CO2 here), a MATLAB program was developed to separate each other in real-time (see details in Supplementary Note 7). Figure 3b shows the processed result corresponding to Fig. 3a, where the acoustic signals generated by SO2 and CO2 are distinctly separated, allowing for a clear visualization of the evolutionary trend of each gas. A fast-ward video is provided to show this real-time monitoring process (see Supplementary Movie 1). Furthermore, as aforementioned, the narrow linewidth and precise tunability of all Raman laser lines bring the advantage of high selectivity and thus low cross-sensitivity. As an example, we carried out another measurement where the CO2 concentration increased in steps while the SO2 concentration was set to a constant value of 84 ppm. Figure 3c shows the measurement result, where one can see that the acoustic intensity of SO2 is indeed independent of the variation of CO2 concentration.

The sensitivity and detection limit of each gas is evaluated by only operating the corresponding Raman laser line and turning off other spectral lines. In this case, the burst form is replaced by periodic Raman pulses in the continuous form. Figure 3d shows the measured PA intensity at different CO2 and SO2 concentrations, respectively, based on the measured PA data in Supplementary Note 8. The laser’s repetition rate remains at 942 Hz to include the contributions of both modulation and pulse regimes of the PA detection. In Fig. 3d, the linear dependence of the PA intensity on the concentration is in accordance with the results being reported10,24. The fitted slope (i.e., sensitivity) is 103.6 μV/ppm for CO2 while 1.26 μV/ppm for SO2. The different sensitivity is because the absorption coefficient of CO2 at 4.25 μm is around two orders of magnitude higher than SO2 at 3.99 μm (see Fig. 2c). As a comparison, Fig. 3e presents the measured results with the same setting as Fig. 3d but using 1 kHz repetition rate, where its 4th order harmonic is away from any resonant acoustic peaks in Fig. 2f. In this case, the PA detection operates only in the pulse regime, and the sensitivity decreases to 5.7 μV/ppm for CO2 and 0.09 μV/ppm for SO2 (see Fig. 2c). Despite this, the PA detection operating at 1 kHz repetition rate brings the important advantage of high tolerance on the fluctuation of environmental temperature and is thus more robust towards real-world applications, because it excludes the contribution of the modulation regime which is sensitive to temperature24. To validate this statement, we monitored the PA intensity of CO2 at 1.3 ppm concentration over 65 min when the acoustic chamber was heated up from 22 to 23.5 °C (see details in Supplementary Note 9). As shown in Fig. 3f, the PA intensity does maintain a good stability at 1 kHz repetition rate, but shows a significant decrease over time at 942 Hz.

The detection limit is estimated using the “propagation of errors model” in ref. 56, which incorporates the errors in the analyte measurements including the blank sample. The calculation is based on the measured data points in Fig. 3d, e as well as the standard deviation and mean value of the blank sample provided in Supplementary Note 10. The detection limit is calculated to be 31.2 ppb for CO2 and 2.7 ppm for SO2 at 942 Hz repetition rate, while 329 ppb for CO2 and 19 ppm for SO2 at a 1 kHz repetition rate. Given the measured 3.24 m−1 attenuation coefficient of the 4.25 μm Raman laser in the ambient air with ~400 ppm concentration (see Supplementary Fig. 7c) and 105 mHz noise equivalent bandwidth of the lock-in amplifier, the normalized noise equivalent absorption (NNEA) coefficient is estimated to be 1.1 × 10−4 Wcm−1Hz−1/2 at 942 Hz and 1.2 × 10−3 Wcm−1Hz−1/2 at 1 kHz repetition rate. The NNEA for SO2 detection cannot be estimated because its absorption coefficient is too weak to be measured at ppm concentration in this experiment.

In our investigation, setting a longer time constant for the lock-in amplifier can further mitigate the fluctuation of the measured acoustic data and thus can reach a lower detection limit. This statement is supported by the Allan deviation shown in Fig. 4 which is calculated from the 30 min background acoustic signal shown in Supplementary Note 10. All the Allan deviations keep decreasing toward longer integration time. These results indicate that the synthesized Raman spectral lines have good stability (i.e., no detected drift within the 30 min monitoring of photoacoustic data), therefore a lower detection limit can be achieved by setting a longer time constant for the lock-in amplifier29,57. Note that the Allan deviation with a longer integration time ( >10 min) can be obtained based on a longer duration of photoacoustic data recording ( >30 min). With a sufficiently long integration time, the decreasing trend of all the Allan deviation curves will finally stop at a certain/critical integration time and then change to an increasing trend, because of the practical drifts of laser and photoacoustic data. The critical integration time, which marks the transition between the decrease and increase of the Allan deviation, can be used to estimate the lowest detection limit of the selected gas. In addition, the good long-term stability of the laser-PA system also ensures good repeatability of the gas detection, which has been experimentally validated in Supplementary Note 11.

These Allan deviations are calculated at different repetition rates (942 Hz and 1 kHz) and wavelengths (3.99 and 4.25 μm) of the proposed gas-filled fiber Raman laser.

Neverthless, it should be noted that, using a longer time constant prolongs the response time of detecting multiple gases, thereby compromising the proposed concept of “real-time detection”. An alternative solution is to enhance the sensitivity of the PA detection by further narrowing the Raman laser’s linewidth and boosting its pulse energy because it can further enhance the intensity of the excited acoustic waves by concentrating more laser line energy into one of the absorption lines of the selected gas. Furthermore, a narrow linewidth can suppress the cross-sensitivity and thus achieve high accuracy/selectivity in absorption-based gas spectroscopy because the absorption spectra of target gases typically overlap with each other and with other possible background gases. In this work, the linewidth of the mid-IR Raman laser from the 2nd stage H2-filled ARHCF is measured to be ~1.3 nm. Since CH4 filled in the 1st ARHCF is known with a relatively large vibrational Raman gain width ( ~ 600 pm at 2 bar)45,58, the primary factor for the further compression of the laser’s linewidth is to replace CH4 with another gas with a narrower Raman gain width. An example is to use the rovibrational SRS effect at the Q-branch of the ν1 band of the ν1/2ν2 Fermi dyad of CO2 which has a much narrower Raman gain width (few pm at 2 bar) than CH444,59,60. The feasibility of using CO2 is supported by the measured spectra presented in Supplementary Fig. 1b, but the wavelength selection of Raman lines needs to take the disturbance of IR absorption bands of CO2 into consideration. Reducing the gas pressure can narrow the linewidth of the Raman laser as well. However, gas pressure reduction is associated with a decrease in the Raman gain coefficient, and thus requires further optimization on reducing the optical loss of ARHCFs. Narrowing the pump linewidth is an additional option, but it involves the suppression of the SBS effect in the Yb-doped fiber amplification stages.

The proposed laser is also adaptive to other absorption-based gas detection modalities. For example, it can be combined with multi-pass cell or cavity ring-down technologies for conducting direct absorption gas spectroscopy in the time domain. In this case, the switching time from one to another gas species could be as short as a few milliseconds, owing to the rapid switching between different Raman laser lines, as illustrated in Fig. 2b.

The compactness and robustness of the light source are other important factors for practical applications. The proposed concept has the potential to be developed into a compact and robust all-fiber structure through fiber splicing involving ARHCFs61,62,63. On the other hand, since the Yb-doped fiber laser pump can be coiled with a diameter of less than 10 cm to preserve a compact volume (~12 × 20 × 4 cm in this work), the volume of the optical module is mainly determined by the bend diameter of the 2nd stage ARHCF, which was set to ~30 cm to maintain a relatively low bend low at the near-IR region. It is possible to further reduce the bend diameter and the bend loss by adding more than one nested capillary for the 2nd ARHCF, but at the price of increasing the fabrication complexity48. Alternately, because the bend loss of the ARHCF is proportional to the fiber's core diameter and inversely proportional to wavelength (see Supplementary Note 2), it is possible to further reduce the bend diameter of the 2nd stage ARHCF by moving its pump from 1.5 μm to longer wavelength such as 2 μm. In this case, the 1st order vibrational Raman Stokes of H2 can be employed in the 1st stage ARHCF, to efficiently convert the wavelength from the ~1 μm Yb-doped fiber pump region to ~2 μm region64.

Owing to its ability to emit multiple wavelength-selective spectral lines with high pulse energies, the proposed concept is also promising for multi-spectral microscopic imaging, to achieve selective and simultaneous imaging of different biological macromolecules. For instance, photoacoustic imaging widely adopts the conventional wavelength tunable nanosecond laser as a light source (e.g., OPO and external cavity QCL). However, these laser technologies are impractical for simultaneously imaging multiple biological macromolecules due to the limited wavelength scanning speed typically at hundreds of nanometers per second65,66. Spectral synthesis can effectively address this issue, but the PIC-QCL technology mentioned in the introduction part cannot provide sufficient pulse energy at nanojoule level with few nanoseconds pulse widths, which is however preferable for photoacoustic imaging. These requirements can be fulfilled with the proposed concept. In addition, the proposed Yb-doped fiber pump laser design allows for scaling the internal repetition rate of the burst mode to hundreds kHz or MHz level, to further boost the speed of photoacoustic imaging. This can be achieved by amplifying each signal laser burst with a single long pump pulse instead of a pump pulse burst used in this current design. In this case, ASE suppression would not be a significant issue because of the high repetition rate. The intensity of adjacent long pump pulses can be independently controlled, to maximize the pulse energy of signal laser bursts at different wavelengths.

In summary, we have proposed a concept for the synthesis of multiple narrow and intense spectral lines over a broad near- and mid-IR range. The suggested concept provides the possibility of reconfiguring the number and wavelengths of all spectral lines according to the targeting gases. The proposed approach is expected to push the state-of-the-art in multi-gas detection a step forward in terms of selectivity, sensitivity, and response time. A gas-filled fiber Raman laser synthesizing two independently and precisely tunable narrow spectral lines at 3.99 and 4.25 μm is developed and used as a proof-of-concept for the PA detection of SO2 and CO2 in the ppm and ppb concentration range. Moreover, recent research shows that the transmission range of ARHCF based on soft-glass materials can be extended to the far-IR region, indicating that the proposed concept has significant potential to be further exploited67,68,69,70,71,72. Therefore, the proposed concept opens a promising way for a wide range of applications including IR trace gas spectroscopy and multi-spectral microscopy.

The 1044/1060 nm laser diode seeds being combined into a single PM fiber for optical amplification have the same linear polarization direction aligned to one of the birefringence axes of the PM Yb-doped fiber amplifiers. The amplification module adopts two stages pre-amplifiers and a power amplifier. The core diameters of Yb-doped active PM fibers used in the 1st–3rd stage amplifiers are 6 μm (PM-YSF-HI-HP, Coherent), 10 μm (PLMA-YDF-10/125, Coherent), and 25 μm (PLMA-YDF-25/250, Coherent), respectively. Optical isolators are used after each amplification stage, to block backward detrimental light. The output fiber of the power amplifier is angle-cleaved to mitigate back-reflection. 915 nm laser diodes are used as pumps. They are electrically modulated to emit pulses with width > 50 μs, to support the pulse pump regime. All pulsed laser diode pumps are synchronized to provide the required energy for the amplification of the signal laser pulses.

The pulse width of the seed laser is optimized to 3.7 ns. The detrimental SBS effect becomes significant during the amplification when the signal pulse width > ~4 ns, although a long pulse width is preferable in terms of suppressing the transient Raman regime and enhancing the efficiency of the 2nd stage H2-filled ARHCF Raman laser generation. After amplification, the laser’s average power is less than 300 mW, and a heat sink with cooling fans is used for heat dissipation.

The amplification part of the pump laser is integrated into a compact size of ~12 × 20 × 4 cm. Each laser diode seed (including the temperature controller) has a size of ~4 × 4 × 1 cm. All laser seeds are separated from each other and are connected to the amplification module through WDM with PM980 fibers.

The ARHCF is fabricated with the stack-and-draw method. Custom-made high-pressure gas cells were developed to seal the ARHCFs for gas filling and light coupling. In the 1–30 bar pressure range, the test shows that the gas cell has an excellent gas sealing ability (leakage speed is measured to be ~0.01 bar/hour at 30 bar, and ~10−4 bar/hour at 2 bar), which ensures the Raman laser’s long-term stability. Gases that are used for the ARHCF setup for Raman laser generation have a high purity of >99.9999%.

The longitudinal section of the acoustic chamber is shown at the bottom of Fig. 2a. The main body of the acoustic chamber is a hollow-core cylinder with a 5.15 cm internal radius and 10.3 cm length52,53. Acoustic filter elements are mounted at each end of the chamber, to suppress ambient acoustic noise at few resonant acoustic frequencies. All components of the acoustic chamber are made of stainless steel, and their inner surfaces are well-polished to ensure the uniformity of the acoustic reflections. The two ends of the acoustic chamber are equipped with two sapphire optical windows with an anti-reflection coating at the mid-IR region, to provide low-loss access for the developed Raman spectral lines. A 1/2” condenser low-noise microphone (4955, Brüel & Kjær) is placed halfway between the ends of the acoustic cylinder for recording excited acoustic waves. The front surface of the microphone is aligned with the inner surface of the acoustic cylinder. The microphone has an integrated low-noise amplifier, which is followed by a conditioning amplifier with the functions of both supplying power to the microphone and amplifying the acoustic signal being recorded (1708, Brüel & Kjær). The final output acoustic data from the microphone is recorded by either a high-speed digitizer (M4i4421-x8, SPECTRUM) or a lock-in amplifier (MFLI 500 kHz, Zurich Instruments).

The preparation of the gas sample is based on commercial pressurized gas products including 100 ppm CO2, pure CO2, 100 ppm SO2, 5000 ppm SO2, and pure Ar (Air Liquide A/S). Ar is used as the gas diluent. MFCs calibrated with pure Ar are used to regulate the flow rate of these commercial gases. Gases output from all MFCs are combined into a single gas hose, which is connected to a gas chamber to uniformly mix the concentrations. The gas concentration is regulated by controlling the flow rate with MFCs. The gas chamber’s output is directed into a T-shaped hose splitter, to divide the gas flow into two ports, A and B. Port B is linked to the laboratory’s ventilation point through a needle valve, which serves to control the flow rate in port A. The flow rate in port A is monitored by a mass flow meter (MFM), and then the MFM directs the prepared gas sample into the acoustic chamber via a gas inlet port, where the gas sample flows through the main body of the acoustic chamber and exits through the other gas outlet. In this experiment, the gas flow rate is ≤300 sccm, to mitigate the acoustic noise rising from the turbulence of the gas flow.

The SO2 and CO2 concentration values used in this work (e.g., Fig. 3b, c) are calculated using the flow rates of factory-calibrated MFCs. Since MFCs are commercial products, the calculated concentrations are assumed to be accurate.

Further information on research design is available in the Nature Portfolio Reporting Summary linked to this article.

All data generated in this work have been deposited in the Figshare database.

Matlab code is available from the corresponding authors upon reasonable request.

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This work is supported by the Danmarks Frie Forskningsfond Hi-SPEC project (Grant No. 8022-00091B), VILLUM Fonden (Grant No. 36063, Grant No. 40964), LUNDBECK Fonden (Grant No. R346-2020-1924, R276-2018-869), and US ARO (Grant No. W911NF-19-1-0426). We thank Martin Nielsen (affiliated with DTU Space) for fabricating the gas cells and the PA chamber.

Lujun Hong

Present address: Institute of Space Science and Technology, Nanchang University, Nanchang, China

DTU Electro, Department of Electrical and Photonics Engineering, Technical University of Denmark, Kongens Lyngby, Denmark

Yazhou Wang, Lujun Hong, Cuiling Zhang & Christos Markos

CREOL, The College of Optics and Photonics, University of Central Florida, Orlando, USA

Joseph Wahlen, J. E. Antonio-Lopez & Rodrigo Amezcua-Correa

NKT Photonics A/S, Birkerød, Denmark

Manoj K. Dasa

Microsoft Azure, Unit 7, The Quadrangle, Romsey, UK

Abubakar I. Adamu

NORBLIS ApS, Virum, Denmark

Christos Markos

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Y.W. and C.M. conceived the concept. Y.W. designed and developed the whole experiment system under the assistance of C.M.,M.K.D. and A.I. A.J.E.A.,J.W and R.A.C. designed and fabricated ARHCFs. C.M. measured the scanning electron microscope images of ARHCFs. Y.W. and L.H. conducted the gas detection experiments. Y.W. performed the data analysis and conducted the loss simulation of ARHCFs using COMSOL software. Y.W prepared the figures and manuscript under the discussion with C.M.,M.K.D. and A.I.A.C.Z. improved the figure quality and prepared the manuscript format. Further experiments, revisions and response to reviewers’ comments were prepared and addressed by Y.W.,C.M. with suggestions from M.K.D.,A.I.A. and R.A.C.

Correspondence to Yazhou Wang or Christos Markos.

The authors declare no competing interests.

Nature Communications thanks Wei Ding and the other, anonymous, reviewer(s) for their contribution to the peer review of this work. A peer review file is available.

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Wang, Y., Hong, L., Zhang, C. et al. Synthesizing gas-filled anti-resonant hollow-core fiber Raman lines enables access to the molecular fingerprint region. Nat Commun 15, 9427 (2024). https://doi.org/10.1038/s41467-024-52589-8

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DOI: https://doi.org/10.1038/s41467-024-52589-8

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