Coherent light sources in the mid-infrared (mid-IR) range are eagerly required for gas sensing because a large number of atmospheric gases have intense absorption caused by fundamental vibrational bands in this wavelength region [1]. Various mid-IR lasers have already been reported and employed for spectroscopic investigations. Quantum cascade lasers (QCLs) have been greatly developed [2–4], and a distributed feedback QCL operating at room temperature is commercially available [5]. However, the QCL tuning range is limited. An optical parametric oscillator (OPO) provides a high output power and a wide tunable range [6,7], while the wavelength sweep is slow because of the mechanical tuning of the laser cavity, which includes a nonlinear crystal such as ${\mathrm{LiNbO}}_3$ (LN). Another approach is difference frequency generation (DFG) using periodically poled ${\mathrm{LiNbO}}_3$ (PPLN). It generates a 2–5 μm mid-IR wave where the LN crystal is transparent. We have developed PPLN waveguides by using a direct bonding and cutting technique, and achieved a conversion efficiency of 40%/W [8]. Thanks to a significant development in telecommunication technology we can easily obtain a 1.5-μm wide-range tunable laser and apply it to the DFG for wavelength tuning. A 3-μm DFG wave is often generated by a combination of 1.5- and 1.0-μm lasers. However, the phase matching bandwidth for 1.5-μm wave tuning is narrow, whereas that for 1.0-μm tuning is wide. Using this property, we demonstrated a two discrete wavelength 3-μm DFG laser and measured real-time ${\mathrm{CH}}_4$ isotope ratios where the results were consistent with those determined from mass spectroscopy [9].

Investigations of the transient processes in detonations and explosions require a time scale of ${10}^{-1}$ to ${10}^{-6}\mathrm{s}$. High-speed spectroscopy using rapidly swept laser sources has been developed [10] and it has proven to be a powerful tool for fast real-time spectroscopy, which is one way of evaluating a gas temperature from acquired spectra. A laser that has a narrow linewidth compared with a white source is repeatedly scanned over the spectral range of interest and the time-dependent transmission spectra through the sample gas are recorded with a fast photodetector. When the elapsed time is identified with the source wavelength, the transient signal is converted into an absorption spectrum of the sample. Analyses of the explosion processes typically require a wavelength sweep rate of 4 nm/μs across a 40-nm span at an instantaneous linewidth of $\sim 0.1\mathrm{nm}$ [11]. An optical coherence tomography (OCT) source satisfies these demands. OCT has been extensively developed [12] in the 1.0-, 1.3-, and 1.5-μm regions. These light sources have attained a wide tuning range and a fast swept rate through the integration of micro electro mechanical systems (MEMS) optics. On the other hand, there are no such fast scanning light sources for 3 μm, where the CH stretching vibrations are located, and this would be particularly useful for the diagnosis of gasoline engines. The only fast scanning mid-IR light source ever reported is an OPO, which has attained a tuning range over $900{\mathrm{cm}}^{-1}$ ($\sim 1.67\mathrm{\mu m}$) in 3.36 ms [13].

In this Letter, we report a wide and fast swept mid-IR DFG source that uses an OCT scan laser as the pump source. The sweep span is wider than 100 nm in the 3-μm region, and the sweep rate is 20 kHz (50 μs). We apply the source to sensing of methane (${\mathrm{CH}}_4$) gas and investigate the temperature dependence of the absorption spectra.

Figure 1 shows schematic drawings of the experimental setup for ${\mathrm{CH}}_4$ gas detection. We employ a 1.0-μm-band swept source including a MEMS-based micro-optic swept laser module (ESS320049-00, Exalos) for the pump and a 1.5-μm-band external-cavity tunable laser diode (TSL-210, Santec) for the signal. The signal wavelength is fixed at 1570.5 nm and the signal wave is amplified to 100 mW with an Er-doped fiber amplifier (ErFA11025, Furukawa). The average power of the pump is 15 mW. The pump and signal lights are combined with a wavelength division multiplexing fiber coupler and injected into a PPLN waveguide. The temperature of the PPLN is kept constant with a temperature controller. A 3-μm band output light is collimated with a collimate lens and injected into an absorption cell made of a test tube with a side arm [14]. The average power of the idler wave is about 10 μW. (A peak power is about 70 μW.) This side arm is connected to a vacuum station and enables us to control the pressure of the sample gas inside the absorption cell. The absorption cell has one window at the top and a retroreflector at the bottom. An incident beam is reflected on three facets of the retroreflector and reflected back to the incident window. The beam is transmitted through an optical bandpass filter to detect only mid-IR light and then focused onto a liquid-nitrogen-cooled InSb detector (J10D-M204-R250 U-10, Judson). The detected signal is sent to a preamplifier and an oscilloscope. The total bandwidth of the detector is 100 MHz. The detected signal is synchronized with the trigger signal provided by the OCT scan source.

 

Fig. 1. Experimental setup. ECLD, 1.5-μm-band external-cavity laser diode; SM, single mode fiber; FA, Er-doped fiber amplifier; WDM, wavelength division multiplexing fiber coupler; Col., collimate lens; InSb, liquid nitrogen cooled InSb detector; OBPF, optical bandpass filter; Osc., oscilloscope. Dotted lines indicate electric cables.

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Idler wave power is defined as [15]

 

Fig. 2. Pump and signal wavelength combination curves that satisfy $\mathrm{\Delta }k=0$ (center, red) and [$\sin (\mathrm{\Delta }kL/2)/(\mathrm{\Delta }kL/2){]}^2=0.5$ (upper and lower, blue) for a QPM bulk LN device with QPM periods of 30.3 μm. LN temperature is 19°C and $L=50\mathrm{mm}$. Horizontal straight line is 1570.5 nm and vertical straight line is 1064 nm. $\mathrm{\Delta }{\lambda }_j$ is the bandwidth in wavelength ($j=\mathrm{p},\mathrm{s},\mathrm{i}$). The subscripts indicate pump, signal and idler, respectively. When the margin of the DFG conversion is half of the peak power, the bandwidth is 129 nm in the 3-μm region when the signal at 1570.5 nm is constant, while that for the bandwidth of 1064 nm is only 7 nm. The cross shows the $\mathrm{\Delta }k=0$ position against the signal at 1570.5 nm when we take the waveguide dispersion into account.

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Fig. 3. Measured phase matching spectra. LN temperature is constant at 18.8°C. The thick (red) curve indicates a fixed signal wavelength. Pump wave is repeatedly swept. Swept time is the upper horizontal axis. The gray curve shows the phase matching curve of the fixed pump wavelength. The upper horizontal axis is only for the thick curve.

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Then, we checked the linewidth of the DFG source. We introduced methane gas into the absorption cell and controlled the pressure at 9.7 kPa (73 Torr). The signal wavelength was 1570.504 nm and the LN temperature was 18.8°C. Figure 4 shows the obtained methane spectrum with an absorption length of 40 cm. We successfully observed the fine structure from P(6) to R(11) of the fundamental band ${\nu }_3$ of methane. The number in parentheses is the rotational angular momentum of the vibrational ground state of methane. This spectrum is averaged over 8 times, however, it should be noted that one sweep time for observing the overall ro-vibrational band is less than 10 μs. The time of the corresponding full width at half-maximum (FWHM) of one R-branch absorption line, R(0), is 18 ns (Fig. 4, inset), which corresponds to $0.6{\mathrm{cm}}^{-1}$ (18 GHz) (FWHM) by calibrating time with wavenumber from HITRAN2012 [18]. The main linewidth factors of a sample are the pressure and Doppler widths. The pressure width is 460 MHz [19] and the Doppler width is 280 MHz (FWHM) at room temperature. On the other hand, the rise time corresponding to the signal bandwidth of the detection, 100 MHz, is 3.5 ns, which corresponds to a linewidth of $0.12{\mathrm{cm}}^{-1}$ (3.6 GHz) (FWHM). The coherence length of the source is defined as [20]

 

Fig. 4. Fine structure of the fundamental band ${\nu }_3$ of methane. This spectrum was acquired by averaging 8 times. The inset is the arrowed absorption line [R(0)] expanded.

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Figure 5(a) shows the normalized spectrum of methane at room temperature. This spectrum is obtained by dividing the Fig. 4 spectrum by the spectrum without the sample. Figure 6(a) shows the result when the absorption cell is cooled with liquid nitrogen. The horizontal axis shows the idler wavenumber and is calibrated to the methane lines. We were able to confirm the linearity between the wavenumber and the elapsed time and to assign the entire measured spectrum beyond the $200{\mathrm{cm}}^{-1}$ wavenumber region to that in the HITRAN2012 database [18]. Silva and Lindsay reported a spectrum over $900{\mathrm{cm}}^{-1}$ using an OPO [13]. However, the linewidth of the OPO is $4.5{\mathrm{cm}}^{-1}$, which is 7 times wider than in this work. Figure 5(b) shows the calculated spectra of methane for 296 K. We assumed a Lorentzian lineshape for the spectra and the parameters we used included a linewidth of $0.4{\mathrm{cm}}^{-1}$ (FWHM), a pressure of 9.3 kPa (70 Torr), and an absorption length of 40 cm. The intensities were calculated using the JavaHAWKS program [21]. Figure 6(b) shows a spectrum calculated on the assumption that the light is transmitted in a sequence through a 30 cm cell at 77 K and a 10 cm cell at 296 K, because the absorption cell was cooled over only 3/4 of its length. When we compare the spectra, we found a good agreement between the experimental results and the calculation, especially in the 2970 to $3100{\mathrm{cm}}^{-1}$ wavelength region.

 

Fig. 5. Normalized (a) and calculated (b) spectra for methane ${\nu }_3$ band spectra: (a) is measured at room temperature; (b) is assumed a Lorentzian lineshape with a linewidth $0.2{\mathrm{cm}}^{-1}$, a pressure of 70 Torr, and an absorption length of 40 cm at 296 K.

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Fig. 6. Normalized (a) and calculated (b) spectra for methane ${\nu }_3$ band spectra: (a) is measured by cooling the absorption cell with liquid nitrogen; (b) is assumed a Lorentzian lineshape with a linewidth $0.2{\mathrm{cm}}^{-1}$, a pressure of 70 Torr, and an absorption length of 30 cm at 77 K and 10 cm at 296 K.

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In summary, we have reported a wide and fast swept mid-IR DFG source in the 3-μm region where we used an OCT scan laser as the pump source. We have demonstrated methane (${\mathrm{CH}}_4$) gas detection using the developed spectrometer and revealed broadband characteristics as wide as 123 nm with a scan time of less than 10 μs. We also demonstrated a preliminary gas temperature analysis technique using a liquid nitrogen cooled gas cell. In this work, we demonstrated only the broadband property of the light source, however, this source has a fast sweep ability up to 20 kHz. We conclude that this newly developed light source would be beneficial for real time chemical species detection and the analysis of transient combustion processes such as those of engines and gas turbines.