Tunable diode laser absorption spectroscopy (TDLAS) technology utilizes the narrow linewidth and wavelength tunability of semiconductor lasers to realize gas detection in real time and has the advantages of noncontact, high resolution, high sensitivity, and fast response time^[1–9]. In recent years, many tunable semiconductor lasers suitable as the light source for gas detection systems based on TDLAS technology have been reported^[10–21]. Yu et al. demonstrated widely tunable distributed Bragg reflector (DBR) lasers for multispecies gas sensing. Utilizing these DBR lasers, the water and methane detection experiment has been successfully implemented^[11–13]. Wang et al. designed and implemented a near-infrared methane detection system based on TDLAS technology using a distributed feedback laser at 1654 nm. The system allows real-time display and has the potential of adjustable wavelength scanning for multi-gas detection^[15]. Meng et al. reported a micro-ring resonator laser based on whispering-gallery modes (WGMs) with an annular resonator and an output waveguide at 1746.4 nm. It is a promising light source for methane gas detection^[20].

Methane gas molecules have absorption lines in both near-infrared and mid-infrared bands. The absorption coefficient of the absorption lines of carbon dioxide and water molecules near 1.65 µm is much smaller than that of methane molecules^[22]. Therefore, the 1.65 µm light source is less affected by carbon dioxide and water molecules in the atmosphere, and its transmission loss is small. It is suitable for the long-distance detection of methane gas molecules. Compared with 3.3 µm and 7.66 µm lasers, 1.65 µm lasers have lower temperature requirements, which can reduce the complexity of detection equipment^[23–26]. Close to the optical communication band, the corresponding optical devices and technologies of the 1.65 µm band are more mature.

A laser in the 1.6 µm band is designed as a potential light source for methane gas detection. It is required that the laser operates single mode lasing without mode hopping in the wavelength tuning range covering the methane absorption line at 1653.72 nm. The laser structure of a square microcavity as a WGM microcavity coupled with a Fabry–Pérot (FP) cavity meets the requirements and has the advantages of narrow linewidth, high side mode suppression ratio (SMSR), stable output wavelength, low power consumption, small volume, simple structure, and so on^[27–29]. The coupled cavity laser eliminates the process of the second epitaxial growth method and is fabricated without high-precision lithography technology. It is edge emitting, which is conducive to the integration of light sources and other devices. Therefore, the square-FP coupled cavity laser is suitable as the light source of methane gas detection system based on TDLAS technology.

In this Letter, we report a square-FP coupled cavity semiconductor laser operating at about 1654 nm, corresponding to the methane absorption line. The laser output optical power can reach 7.4 mW with the SMSR about 40 dB, and the linewidth is 5 MHz at 1653.72 nm. The wavelength tunable properties of the coupled cavity laser were analyzed. The lasing wavelength can be tuned by adjusting the FP cavity injection current covering a 2 nm tuning range, and it can also be tuned by adjusting the square microcavity injection current or temperature, respectively. A gas detection system based on TDLAS technology using the coupled cavity laser was demonstrated and used for methane gas detection.

We numerically simulate the field distributions of the magnetic component in hertz (Hz) for transverse electric (TE) polarized modes of a 2D square-FP coupled cavity with an FP cavity length of 300 µm and width of 2.5 µm as well as a square microcavity side length of 15 µm by the finite element method in the commercial software COMSOL Multiphysics 5.0. The coupled cavity with refractive index $n_1=3.2$ is surrounded by benzocyclobutene (BCB) with $n_2=1.54$, as shown in Fig. 1. The square microcavity and the FP cavity operate single mode and multi-mode lasing in the gain bandwidth, respectively, because of different free spectral ranges. When a vertex of the square microcavity and FP cavity are connected, resonance enhancement can be realized at the mode wavelength that meets the resonance conditions of the square microcavity and FP cavity at the same time. Coupled modes with higher $Q$ factor obtained by hybridizing the WGM and FP mode realize high-power directional output via the FP cavity. TE mode distributions of the fundamental, first-order, and second-order modes in the square microcavity coupled with the mode in the FP microcavity are shown in Fig. 2, with mode wavelengths of 1642.8, 1641.47, and 1641.0 nm and $Q$ factors of $1.91\times {10}^4$, $8.59\times {10}^3$, and $6.70\times {10}^3$, respectively. Compared with the first-order and second-order modes, the fundamental mode in the square microcavity coupled with the mode in the FP microcavity can achieve a higher $Q$ factor, which indicates that the coupled cavity laser easily realizes single mode lasing.

An AlGaInAs/InP multiple-quantum-well laser wafer is used to fabricate the coupled cavity laser. The laser adopts the structure of numerical simulation above, as shown in Fig. 1. First, a 600 nm ${\mathrm{SiO}}_2$ layer is deposited by plasma-enhanced chemical vapor deposition (PECVD) on the laser wafer. The patterns of the coupled cavity are transferred onto this ${\mathrm{SiO}}_2$ layer using standard photolithography and inductively coupled plasma (ICP) etching techniques, and then the laser wafer is etched at about 4 µm by ICP etching with the patterned ${\mathrm{SiO}}_2$ layer as masks. The residual ${\mathrm{SiO}}_2$ masks on the cavities were removed by using diluted HF solution. Next, a 250 nm ${\mathrm{SiN}}_x$ protecting layer is deposited by PECVD on the wafer, and a divinylsiloxane BCB (DVS-BCB) polymer overcladding layer is subsequently spun onto the ${\mathrm{SiN}}_x$ layer. The hard cured BCB layer is etched by reactive ion etching to expose the top of the microcavity structures. An electrically isolated channel is formed by ICP etching over the ohmic contact layer with the patterned ${\mathrm{SiO}}_2$ mask layer obtained by the same method. A 200 nm ${\mathrm{SiO}}_2$ insulating layer is deposited on the wafer, which can protect the isolated channel. Finally, ${\mathrm{SiO}}_2$ and ${\mathrm{SiN}}_x$ on the top of the microcavity are removed by ICP etching to form an electrical injection window, and the patterned Ti-Pt-Au $\mathrm{p}$-electrode is formed by photolithography and lift-off techniques. An Au-Ge-Ni metallization layer is evaporated on the backside as the $\mathrm{n}$-electrode after the wafer mechanically laps down to a thickness of 120 µm. The laser wafer is cleaved to provide a reflection facet at the end of the FP cavity, with an FP cavity length of about 300 µm. A microscopic image of a laser chip is shown in Fig. 3(a). This laser chip contains two devices. This Letter is completed with only one device—laser A. The laser chip was sintered and wedge bonded on an AlN submount for subsequent testing. Figure 3(b) shows a microscope image of the laser after wedge bonding.

The coupled cavity laser is tested on a semiconductor thermoelectric cooler (TEC), which is fixed at 16°C except for the variable temperature test. A multi-mode fiber is used to collect the output light at the output facet of the FP cavity away from the square microcavity except for the linewidth characterization. The injection currents of the two $\mathrm{p}$-electrodes of the coupled cavity can be adjusted, respectively, when the laser operates. When the square microcavity injection current $I_{\mathrm{SQ}}$ is 20 mA, the output power of the laser at different FP cavity injection currents $I_{\mathrm{FP}}$ is shown in Fig. 4, and the threshold current is 16 mA. The multi-mode fiber coupling output power of the laser almost reaches a maximum of 7.4 mW at $I_{\mathrm{SQ}}=20\mathrm{mA}$ and $I_{\mathrm{FP}}=84\mathrm{mA}$. The output power of the laser with the variation of $I_{\mathrm{SQ}}$ at fixed $I_{\mathrm{FP}}=83\mathrm{mA}$ is shown in the inset of Fig. 4. The output optical power increases suddenly near $I_{\mathrm{SQ}}=0$ because the accumulation of photon generated carrier numbers causes saturation in the square microcavity. Affected by the increase of $I_{\mathrm{SQ}}$, the equivalent reflectance of the square microcavity and the $Q$ factor of the coupled modes increase, which causes the output power to increase until thermal saturation. As $I_{\mathrm{SQ}}$ continues to increase, the output power decreases slightly because of the thermal crosstalk between the square microcavity and FP cavities. The dominant mode wavelength is 1653.72 nm with the SMSR of 36.6 dB, as shown in the optical spectrum in Fig. 5, corresponding to the methane R3 absorption branch in the $2{\mathrm{\nu }}_3$ second harmonic band.

The injection currents of the two $\mathrm{p}$-electrodes of the coupled cavity, $I_{\mathrm{SQ}}$ and $I_{\mathrm{FP}}$, are adjusted, respectively, to characterize the wavelength tunability of the laser. Optical spectra and SMSR of the laser with the variation of $I_{\mathrm{FP}}$ at fixed $I_{\mathrm{SQ}}=20\mathrm{mA}$ are shown in Fig. 6(a). The wavelength tuning range is 2 nm from 1652.68 to 1654.70 nm by increasing $I_{\mathrm{FP}}$ from 66 to 98 mA with the step of 1 mA, covering the methane absorption line at 1653.72 nm, and the wavelength tuning rate of the current $I_{\mathrm{FP}}$ is 0.06 nm/mA. The laser operates single mode lasing without mode hopping with the SMSR almost above 40 dB. Optical spectra of the laser with the variation of $I_{\mathrm{SQ}}$ at fixed $I_{\mathrm{FP}}=83\mathrm{mA}$ are shown in Fig. 6(b). The lasing wavelength is tuned from 1653.66 to 1653.74 nm by increasing $I_{\mathrm{SQ}}$ from 20 to 22 mA with the SMSR at about 42 dB, and the wavelength tuning rate of the current $I_{\mathrm{SQ}}$ is 0.04 nm/mA.

The lasing wavelength can also be tuned by adjusting the temperature except for adjusting the injection current, and it is redshifted with increasing temperature. This is because the energy band structure and Fermi distribution in semiconductors are temperature dependent, and the active region gain peak and material refractive index change with temperature. For the coupled cavity laser, the lasing wavelength shift with temperature depends on material refractive index variation, rather than the gain spectrum shift. Optical spectra and SMSR of the laser with temperature variation of the TEC at $I_{\mathrm{SQ}}=20\mathrm{mA}$ and $I_{\mathrm{FP}}=84\mathrm{mA}$ are shown in Fig. 7. The wavelength tuning range is 3.8 nm from 1652.43 to 1656.22 nm by increasing temperature of the TEC from 4.3°C to 37.7°C, and the wavelength tuning rate of the temperature is 0.11 nm/K. The laser operates single mode lasing without mode hopping, and the SMSR decreases from 40 dB to 35 dB.

The linewidth of the coupled cavity laser is characterized at $I_{\mathrm{SQ}}=20\mathrm{mA}$ and $I_{\mathrm{FP}}=84\mathrm{mA}$, corresponding to the methane absorption line 1653.72 nm ($\mathrm{R}3\text{-}2{\mathrm{\nu }}_3$) using the delay self-heterodyning method. The linewidth is a half of FWHM of 5 MHz, as shown in Fig. 8, which is much smaller than the FWHM of the methane absorption line.

A gas detection system based on TDLAS technology using the coupled cavity laser proposed in this Letter was built for methane gas detection. A white cell is used as the gas absorption cell. The sample gas is methane with a concentration of 1001 ppm (parts per million), and its equilibrium gas is nitrogen. The gas cell is filled with air before the sample gas is inlet. The lasing wavelength is tuned from 1653.65 to 1653.8 nm by adjusting $I_{\mathrm{FP}}$, $I_{\mathrm{SQ}}$, or temperature of the TEC, respectively. The output light of the coupled cavity laser is inlet from one end of the gas cell through a single mode fiber and collected by a detector at the other end of the gas cell. When the lasing wavelength is the same as the position of the methane absorption line, the gas molecules will absorb photons and move to a higher energy level, and the laser energy will decay at the same time. Methane gas detection can be realized by comparing the output optical power after absorption with the initial output optical power. Figure 9 shows the ratio of the difference of the output optical power received by the detector before and after sample gas inletting to the output optical power before sample gas inletting, that is, $(P_{\mathrm{air}}-P_{\mathrm{CH}4})/P_{\mathrm{air}}$ as the lasing wavelength varies. The methane gas inlet caused a significant decrease in the output optical power at the methane absorption line of 1653.72 nm ($\mathrm{R}3\text{-}2{\mathrm{\nu }}_3$).

In this Letter, methane gas detection is successfully demonstrated utilizing a square-FP coupled cavity laser. The coupled cavity laser operates at about 1654 nm, corresponding to the methane absorption line. The laser can operate single mode lasing with the SMSR about 40 dB and the linewidth of 5 MHz, and optical power can reach 7.4 mW. The lasing wavelength can be tuned by adjusting $I_{\mathrm{FP}}$, $I_{\mathrm{SQ}}$, or temperature of the TEC, respectively, and the laser operates single mode lasing without mode hopping. The fabricated square-FP coupled cavity laser is a high-performance laser source for methane gas detection.