Nanophotonic supercontinuum based mid-infrared dual-comb spectroscopy

TL;DR

This paper introduces a simplified, broadband mid-infrared dual-comb spectroscopy method using nanophotonic supercontinuum generation, enabling high-resolution, rapid molecular detection across a wide spectral range.

Contribution

It presents a novel nanophotonic dispersion engineering technique for flat, ultra-broadband mid-IR comb spectra, advancing dual-comb spectroscopy capabilities.

Findings

Achieved >100,000 comb lines in 2800-3600 1/cm range

Enabled resolution of isotopologues in functional groups

Demonstrated high sensitivity, spectral resolution, and speed

Abstract

High resolution and fast detection of molecular vibrational absorption is important for organic synthesis, pharmaceutical process and environmental monitoring, and is enabled by mid-infrared (mid-IR) laser frequency combs via dual-comb spectroscopy. Here, we demonstrate a novel and highly simplified approach to broadband mid-IR dual-comb spectroscopy via supercontinuum generation, achieved using unprecedented nanophotonic dispersion engineering that allows for flat-envelope, ultra-broadband mid-IR comb spectra. The mid-IR dual-comb has an instantaneous bandwidth covering the functional group region from 2800-3600 1/cm, comprising more than 100,000 comb lines, enabling parallel gas-phase detection with a high sensitivity, spectral resolution, and speed. In addition to the traditional functional groups, their isotopologues are also resolved in the supercontinuum based dual-comb spectroscopy. Our approach combines well established fiber laser combs, digital coherent data averaging, and integrated nonlinear photonics, each in itself a state-of-the-art technology, signalling the emergence of mid-IR dual-comb spectroscopy for use outside of the protected laboratory environment.

Equations

$$\Delta \beta (\omega )$$

$$=\beta (\omega )-(\beta ({\omega }_{\mathrm{s}})+v_{\mathrm{g}}^{-1}(\omega -{\omega }_{\mathrm{s}}))$$

$$\frac{\partial \tilde{E}(\omega ,\mathbf{r})}{\partial z}=-i\beta (\omega )\tilde{E}(\omega ,\mathbf{r})-i\frac{\omega {\chi }^{(3)}}{2cn}\mathcal{F}{\left[{E(t,\mathbf{r})}^3\right]}_{\omega }$$

$$\tilde{E}(\omega ,\mathbf{r})=\tilde{B}(\omega ,x,y)\tilde{A}(\omega ,z)$$

$$\begin{gathered} \frac{\partial \tilde{A}(\omega ,z)}{\partial z}= \\ -i\beta (\omega )\tilde{A}(\omega ,z)-i\frac{\omega }{c}\frac{{\chi }^{(3)}}{2n{A}_{\mathrm{eff}}}\mathcal{F}{\left[{\left|A(t,z)\right|}^{2}A(t,z)\right]}_{\omega } \end{gathered}$$

$$\tilde{A}(\omega >0,z)={\tilde{A}}_{\mathrm{s}}(\Omega ,z)e^{-i{\beta }_{\mathrm{s}}z}+\tilde{\sigma }{e}^{-i\beta ({\omega }_{\mathrm{d}})z}$$

$${\beta }_{\mathrm{s}}(\Omega )=\beta ({\omega }_{\mathrm{s}})+\Omega {\beta }^{(1)}({\omega }_{\mathrm{s}})$$

$$\frac{\partial {\tilde{A}}_{\mathrm{s}}}{\partial z}$$

$$\tilde{\sigma }$$

$$f_{\mathrm{CW}}$$

$$f_{\mathrm{rep}}$$

$$\Delta f_{\mathrm{rep}}=\frac{f_{\mathrm{CW}}}{N(N+1)}$$

$$f_{\mathrm{rep}}\times \frac{f_{\mathrm{rep}}/2}{\Delta f_{\mathrm{rep}}}\approx 100\mathrm{THz}$$

Full text

Mid-infrared (mid-IR) is known as one of the most useful wavelength regions for spectroscopy due to the presence of fundamental vibrational transitions in molecules Stuart (2005). Moreover, it has medical potential as human breath contains numerous volatile chemical compounds (VOC), many of which can be associated with diseases Phillips et al. (1999). Presently, mid-IR spectroscopy is primarily based on Fourier transform infrared (FTIR) spectrometers Griffiths and De Haseth (2007) that are bulky and have limited resolution and acquisition time. Over the past decade dual-comb spectroscopy (DCS) has emerged as an approach that can alleviate some of these shortcomings Keilmann et al. (2004); Schliesser et al. (2005). This approach, emerged with the invention of optical frequency combs Udem et al. (2002); Jones et al. (2000), enables fast detection, scanning without moving parts, high resolution spectra, and have no limitation on size as the device length is independent on resolution, contrary to FTIR. DCS has today seen significant advances Coddington et al. (2016), and has also been successfully applied to an increasing portion of the mid-IR spectrum using a number of mid-IR comb sources Schliesser et al. (2012), including quantum cascade lasers (QCLs) Villares et al. (2014), microresonator Kerr frequency combs Yu et al. (2018), difference frequency generation (DFG) Ycas et al. (2018); Chen et al. (2018a), cascaded quadratic nonlinear process Zhou et al. (2017); Timmers et al. (2018), and optical parametric oscillators (OPO) Muraviev et al. (2018). Yet to date, generating phase locked mid-IR frequency combs that exhibit high brightness, broad bandwidth, and fine resolution remains challenging. The most advanced approaches have used DFG that typically requires the synchronization of two laser beams and features limited instantaneous spectral bandwidth. To reach a larger spectral coverage, either mechanically tuning the phase matching of the nonlinear crystal or implementing chirped quasi-phasematching (QPM) is required Ycas et al. (2018). OPO based mid-IR frequency combs have attained some of the broadest spectra to date, allowing for massively parallel sensing of trace molecules Muraviev et al. (2018). Although capable of a miniature size, this approach is mostly based on solid-state laser cavities that contain discrete bulk optics and components.

Here we demonstrate a new approach to mid-IR DCS that offers unprecedented simplicity, based on supercontinuum generation in “coupled” nanophotonic integrated silicon nitride (Si4N4) waveguides, and driven by conventional low-noise fiber-laser-based optical frequency combs in the well developed telecommunication band, as shown schematically in Fig. 1. In particular, we propose a coupled waveguide structure that represents an advanced approach to dispersion engineering, under which the supercontinuum process is along with the mode coupling in the waveguide(Fig. 1(b)), and is tailored to have a large spectral bandwidth and high flatness in the mid-IR (Fig. 1(c)). The dual-comb spectrometer we presented offers state-of-the-art signal to noise, large instantaneous spectral bandwidth in the mid-IR (over 800 ${\rm cm^{-1}}$). It combines both low-noise fiber lasers and photonic integrated waveguides. Each in itself is a well developed technology and is commercially available, and therefore can contribute to high-performance mid-IR DCS.

I Result

Nanophotonic supercontinuum generation with advanced dispersion engineering — Supercontinuum generation is one of the most dramatic nonlinear optical processes Dudley et al. (2006), and has been essential for enabling femtosecond lasers to be self-referenced, making fully stabilized optical frequency combs Cundiff and Ye (2003). Yet it constitutes a way to access ultra-broadband and coherent comb sources potentially imperative to applications. While being extensively studied in the past decade, the supercontinuum process has to date rarely been exploited for mid-IR DCS. Recent advances have been on nanophotonic integrated waveguides, where lithographically tailorable supercontinuum generation is enabled at low pulse energies Xie et al. (2014); Kuyken et al. (2015); Yoon Oh et al. (2017); Guo et al. (2018); Okawachi et al. (2018), and have been applied for laser self-referencing Carlson et al. (2017) and offset frequency detection Mayer et al. (2015), as well as for DFG Mayer et al. (2016). In particular, ${\rm Si_{3}N_{4}}$ waveguides Moss et al. (2013), combining a wide transparency with nanophotonic dispersion engineering, have been demonstrated to support mid-IR frequency combs based on dispersive wave generation from a femtosecond erbium-fiber laser Guo et al. (2018). In this way, they provide access to the high-demand mid-IR range by bridging an efficient and coherent link with well developed fiber laser technology in the near-infrared, amiable for DCS.

The spectral extent and efficiency of supercontinuum generation in nanophotonic waveguides critically depends on the dispersion properties. While the geometry (width and height) of the waveguide can be controlled to achieve dispersion engineering, leading to mid-IR dispersive waves Guo et al. (2018), this control poses limitations on achieving a high conversion efficiency and ultra-broadband mid-IR continuum. Here, we employ coupled structures consisting of multiple ${\rm Si_{3}N_{4}}$ waveguide cores, typically dual-core waveguides (cf. Fig. 1(b)). When two waveguide cores are in close proximity, the optical mode propagating in one core is coupled to the other core, which effectively changes its phase, i.e. the propagation constant of the mode (${\beta(\omega)}$, where ${\omega}$ is the angular frequency of the light). In this way, the group velocity dispersion (GVD) is also changed as it corresponds to the frequency-dependent phase change induced by the mode coupling (${{\rm GVD}=\partial^{2}\beta/\partial\omega^{2}}$). Physically, mode coupling leads to the hybridization of mode-field distributions, resulting in a pair of supermodes, namely the symmetric and anti-symmetric superpositions of the original uncoupled waveguide modes Marom et al. (1984). The coupling induced dispersion is then reflected by the phase profile of these supermodes, which is curved to bridge that of the uncoupled modes, and feature an avoided crossing between each other(cf. Fig. 2(a)). Deterministically, anomalous GVD is always produced by the anti-symmetric mode, while normal GVD is by the symmetric mode. In principle, such mode coupling (formally termed the mode hybridization) can be engineered at arbitrary wavelength regions, particularly in the mid-IR where anomalous GVD is essential for tailoring a flattened dispersion landscape, but is hardly accessible in conventional single-core waveguides. The dispersion landscape is defined as the relative propagation constant compared with the pump pulses (assumed as solitons):

[TABLE]

where ${\beta_{\rm s}(\omega)}$ indicates the dispersionless phase profile of the soliton pulse, ${\omega_{\rm s}}$ is the angular frequency of the pump and ${v_{\rm g}}$ is the soliton group velocity.

Figure. 2 illustrates the design of the ${\rm Si_{3}N_{4}}$ dual-core waveguide. The cross-section of the two ${\rm Si_{3}N_{4}}$ cores are separately selected, in which two modes (one from each core) feature the hybridization in the mid-IR region, by matching their propagation constants, (or equivalently by matching the effective refractive index (${n_{\rm eff}}$), since ${\beta=n_{\rm eff}\omega/c}$, where ${c}$ indicates the speed of light in vacuum). For a choice of ${\rm Si_{3}N_{4}}$ core widths of ${w_{1}=1.3~{}{\rm\mu m}}$ and ${w_{2}=3.4~{}{\rm\mu m}}$, respectively, and for an identical core height of ${h=0.85~{}{\rm\mu m}}$, the fundamental ${\rm TE_{00}}$ mode in the narrow core and the ${\rm TE_{10}}$ mode in the wide core have the same ${n_{\rm eff}}$ at the wavelength of ${3200~{}{\rm nm}}$ (Fig. 2(a)). The propagation constant and the mode-field distribution of supermodes are calculated for a coupling gap of ${g=0.8~{}{\rm\mu m}}$, which exhibit a strongly curved phase profile in the mid-IR compared with the original uncoupled mode (the uncoupled ${\rm TE_{00}}$ mode in the narrow core is selected as the reference in this plot). Therefore, strong and dominant anomalous GVD is produced in the mid-IR (at ${\sim 3200~{}{\rm nm}}$, cf. Fig. 2(b)). Counterintuitively, the larger change in dispersion is not achieved by a closer proximity of the two waveguides as this causes the mode hybridization to occur over a larger spectral range such that the frequency-dependency of mode’s phase constant is reduced.

Moreover, to exploit anomalous GVD in the mid-IR for engineering the supercontinuum, it is imperative to selectively excite the anti-symmetric mode only. This is accomplished by designing the waveguide input section to be a single-core waveguide (with an inverse taper in the beginning Liu et al. (2018a), which excites the ${\rm TE_{00}}$ mode, followed by the dual-core section (cf. Fig. 1(b)). The length of the input section is chosen such that the pulse propagation enters into the dual-core section before signifiant broadening occurs. In this way, the spectral broadening will extend into the mode hybridization region, and excite the anti-symmetric mode in the mid-IR. The designed dispersion landscape is shown in Fig. 2(c), in which the mid-IR portion is particularly tailored and flattened. This is to compare with a conventional single-core waveguide which shows a similar phase matching (i.e. ${\Delta\beta(\omega)=0}$) wavelength for mid-IR dispersive wave Guo et al. (2018); Akhmediev and Karlsson (1995).

We next carried out experiments to investigate the supercontinuum generation in designed ${\rm Si_{3}N_{4}}$ coupled waveguides. The waveguides are fabricated using the photonic Damascene process Pfeiffer et al. (2016), in which thermal annealing steps are critical, in order to reduce the hydrogen content and the related absorption losses Liu et al. (2018b). The waveguide length is ${5~{}{\rm mm}}$ including the inverse taper section at the beginning. In waveguides similar to the design, supercontinuum generation was observed (Fig. 2(d)), which is seeded by the amplified femtosecond fiber laser in the telecom-band (pulse duration ${<70~{}{\rm fs}}$, maximum averaged power ${>350~{}{\rm mW}}$, pulse energy ${>1~{}{\rm nJ}}$, repetition rate ${\sim 250~{}{\rm MHz}}$). Significantly, with respect to the pumped wavelength (i.e. ${1550~{}{\rm nm}}$) the supercontinuum is mostly extended to the long wavelength side, leading to ultra-broadband mid-IR continuum ranging from ${2000-3700~{}{\rm nm}}$, while at the short wavelength side, it features a sharp edge (stopping at ${1000~{}{\rm nm}}$) followed by a dispersive wave in the visible range (at ${600~{}{\rm nm}}$). The spectral envelope exactly reflects the designed dispersion landscape, i.e. the spectrum amplitude is inverse proportional to the relative phase constant. Significantly, filtering out the mid-IR continuum by an edge-filter (cut-on at ${2500~{}{\rm nm}}$), we measured the net power in the mid-IR to be ${1-3~{}{\rm mW}}$, depending on the intensity of the pump. Note that such a power level already comprises the insertion loss of the waveguide and the loss in the light collecting component (e.g. a mid-IR collecting lens that has a transparency of ${\sim 70\%}$). The conversion efficiency is then estimated ${1-5\%}$, producing thereby sufficient power to implement dual-comb spectroscopy. Interestingly, we also observed a narrow-band mid-IR wave generation in the opposite polarization direction to the pump wave, at mode coupling wavelength ${3200~{}{\rm nm}}$ (Fig. 2(d)). This radiation is understood as the result of mode hybridization such that the polarization of the mode in the coupling region is rotated as well.

We also performed numerical simulations of the supercontinuum process on the coupled waveguide and the single-core waveguide, see Fig. 2(e). The simulation is based on the generalized nonlinear Schrödinger equation (cf. SI for equation details), in which the dispersion landscape is of the anti-symmetric mode in the dual-core waveguide. Simulation result confirms that the supercontinuum in the dual-core waveguide does exhibit an enhanced mid-IR generation over a ultra-broadband wavelength span. This spectral structure qualitatively agrees with our experimental results (cf. Fig. 2(d)). It was confirmed that the coupled waveguide, with tailored dispersion landscape, does support broadband mid-IR continuum, while the single-core waveguide supports the narrowband dispersive wave. In addition, the power of the mid-IR wave in the coupled waveguide is also higher than that in the single-core waveguide.

From separate ${\rm Si_{3}N_{4}}$ waveguide chips, similar mid-IR continuum can be generated with similar dual-core waveguides (Fig. 2(f)). A high level of spectral overlap is implemented, which is essential for building up the mid-IR broadband dual-comb spectrometer.

Nanophotonic supercontinuum based mid-IR dual-comb spectrometer — The schematic setup of the nanophotonic supercontinuum based mid-IR dual-comb spectrometer is shown in Fig. 1(a). The pump source consists of two ultra-low noise femtosecond fiber lasers with sub-mHz individual linewidth (FC-1500-ULN from Menlo Systems, wavelength ${\sim 1550~{}{\rm nm}}$, repetition rate ${f_{\rm rep}\sim 250~{}{\rm MHz}}$). Both lasers have the carrier-offset frequency locked via self-referencing Jones et al. (2000); Cundiff and Ye (2003), and one comb mode optically locked to a shared reference laser (at ${\sim 1541~{}{\rm nm}}$, the laser is free-running and a daily shift in frequency is ${{\mathcal{O}}(10~{}{\rm MHz})}$). The locked mode index is different by one, which leads to a small difference in the repetition rate, i.e. ${\Delta f_{\rm rep}\approx 320~{}{\rm Hz}}$, and in principle allows the dual-comb spectrometer to cover a large span in the optical window, i.e. ${\sim 100~{}{\rm THz}}$ (cf. details in the SI). In principle, the mid-IR frequency comb from the supercontinuum process is viewed as the spectral extension of the frequency comb structure of the original pump source, therefore it inherits the full properties of the source comb. Based on such a configuration, a phase-resolved mid-IR dual-comb spectrometer was built up, with one mid-IR comb passing through a gas cell for gas-phase detection, and the other comb serving as the reference. After detection, the two combs are interfered on a mid-IR photodetector (VIGO PV-4TE, mercury cadmium telluride (${\rm HgCdTe}$) detector). In the presence of a difference in the repetition rate, they generate a radio frequency (RF) comb composed of distinguishable heterodyne beats between pairs of optical comb teeth. In the time domain, it corresponds to a periodic interferogram pattern that can be directly recorded by the detector. The data acquisition was implemented by a field programmable gate array (FPGA). Real-time coherent averaging process is also enabled with a computer for multiple sets of signal (cf. SI for coherent averaging details). The normalized signal-to-noise ratio of our spectrometer has a peak value of ${25/\sqrt{\rm s}}$ at the region of ${3400~{}{\rm cm^{-1}}}$ (where the spectral intensity is strongest). The averaged signal-to-noise ratio is estimated as ${10/\sqrt{\rm s}}$. Therefore, we can conclude a figure of merit of ${1.0\times 10^{6}/\sqrt{\rm s}}$ for our ultra-broadband mid-IR dual-comb spectrometer, which is mostly limited by the RIN on the mid-IR frequency combs (cf. details in SI). Although this figure of merit does not reach the shot-noise limit, it is comparable to reported results in other works, which mostly for DFG-based dual-comb spectrometers is ${1-6\times 10^{6}/\sqrt{\rm s}}$.

Mid-IR gas-phase spectroscopic results We next applied the DCS for gas phase spectroscopy. Figure 3(a) shows the result of mid-IR dual-comb gas-phase spectroscopy, where the spectral coverage is ${2800-3600~{}{\rm cm^{-1}}}$ (${\sim 25~{}{\rm THz}}$) corresponding to the number of comb teeth of ${100,000}$. The dynamical range is ${>40~{}{\rm dB}}$ supported by sufficient power (${>1~{}{\rm mW}}$) in the mid-IR. This spectrum is retrieved from a coherent averaged temporal interferogram pattern which in the Fourier domain corresponds to the RF comb constructed by the interference of the two optical frequency combs. The gas cell (length ${104~{}{\rm cm}}$) was constructed with wedged sapphire windows to avoid etalon effects, and filled with a low concentration of methane and acetylene as targeted gas species, and nitrogen as the buffer gas. The overall pressure in the cell is ${1~{}{\rm atm}}$. To extract the absorption spectrum, we measure first the spectrum through the sample gas ${T}$, then purge the cell, fill it back to the original pressure with pure nitrogen, and measure the reference spectrum ${T_{0}}$. The spectral absorbance is then determined as ${{\rm-ln}(T/T_{0})}$. The gas concentrations are extracted from the absorption spectrum via a nonlinear least square fitting with the data (line center frequency, line intensity, pressure broadening and shift coefficients) from HITRAN 2016 database. The HITRAN phase spectra are calculated from the Kramers-Kronig transformation of the absorption spectra. The retrieved gas absorbance and phase spectra agree very well with HITRAN database (Fig. 3(b,c)). In addition the water vapor in the circumstance was also detected as ${\sim 1.8~{}\%}$. Moreover, the performance of our supercontinuum-based mid-IR dual-comb spectroscopy was also benchmarked by successful detection of natural isotopologues of methane (Fig. 4), i.e. ${\rm{}^{12}CH_{4}}$, ${\rm{}^{13}CH_{4}}$ and ${\rm{}^{12}CH_{3}D}$. For such a measurement, the gas cell is operated at low pressure such that the collisional broadening of spectral lines is reduced and those corresponding to isotopologues (i.e. ${\rm{}^{13}CH_{4}}$, natural abundance ${1.11\%}$; ${\rm{}^{12}CH_{3}D}$, ${0.06\%}$) can be resolved as separated from traditional elements (${\rm{}^{12}CH_{4}}$). In experiments, we set the pressure of the gas cell to be ${\sim 0.1~{}{\rm atm}}$ and the methane concentration to ${\sim 12.5\%}$. At this pressure, the full-width-at-half-maximum spectral linewidth of methane is reduced to ${\sim 480~{}{\rm MHz}}$, which is both sufficient for separating isotopes, and resolvable by our sub-Doppler resolution (i.e., 250 MHz, determined by the mode spacing) of the mid-IR frequency comb. The capability of identifying natural abundance of isotopes is of high importance as it provides signatures in earth science as well as in cosmology.

II Discussion

We have demonstrated a high-performance mid-IR dual-comb spectrometer based on the supercontinuum process in nanophotonic integrated nonlinear ${\rm Si_{3}N_{4}}$ waveguides. The proposed coupled waveguide structure has revealed unprecedented ways of performing dispersion engineering, which can lead to flat-envelope, ultra-broadband mid-IR frequency combs. Such a supercontinuum based dual-comb spectrometer has not only detected traditional chemical species, but can also trace their isotopologues. Our approach combines fiber-laser combs technology with nanophotonic integrated devices, each in itself a well established technology, yet it exhibits a performance competitive to DFG-based dual-comb spectrometers, which is amiable for applications outside the protected laboratory environment. In addition, this approach can benefit from superior laser stabilization methods, e.g. adaptive laser stabilization Ideguchi et al. (2014), frequency alignment Millot et al. (2016) or feed forward locking Chen et al. (2018b). At the moment, the long wavelength edge of our spectrometer is limited to ${4.0~{}{\rm\mu m}}$, which is mostly as a result of the ${\rm SiO_{2}}$ cladding in the fabrication of ${\rm Si_{3}N_{4}}$ waveguides. Although ${\rm Si_{3}N_{4}}$ shows a much larger transparency window reaching the beginning of the backbone region (${5~{}{\rm\mu m}}$), mode coupling will expose the propagating light mostly to the cladding and therefore feature strong loss in ${\rm SiO_{2}}$. Such a problem can be solved with air-cladding waveguides or substrates that are mid-IR transparent (e.g. sapphire).

Note: During the preparation of this work, DCS in the near IR (${<2500~{}{\rm nm}}$) with two ${\rm Si_{3}N_{4}}$ single-core waveguides was reported Baumann et al. (2018).

Acknowledgements.

Authors acknowledge Miles Anderson for fruitful discussions and suggestions regarding the manuscript. This publication was supported by Contract W31P4Q-16-1-0002 (SCOUT) from the Defense Advanced Research Projects Agency (DARPA), Defense Sciences Office (DSO), and the Swiss National Science Foundation under grant agreement 163864. W.W. acknowledge funding from the European Union’s H2020 research and innovation programme under grant agreement No. 753749 (SOLISYNTH). F.Y. and L.T. acknowledge funding from the Swiss National Science Foundation under grant agreement No.200021_178895. Nanophotonic Si3N4 waveguide chips were fabricated at the Center for MicroNanoTechnology (CMi) at EPFL.