Accurate measurement of gas components, concentrations, and other parameters plays an important role in many fields. The spectroscopic method, based on the principle of absorption, emission, and scattering of light by gas molecules, with the advantages of non-contact, rapid response, and high selectivity, meeting the requirements of gas measurement under different environmental conditions, is an efficient way for gas sensing^[1].

Nowadays, most semiconductor lasers used for gas sensing are distributed feed-back (DFB) lasers and distributed Bragg reflector (DBR) lasers. However, power consumption and volumes of DFB and DBR lasers are hard to reduce any more, which adversely affect cost reduction and device miniaturization. The microcavity laser with low power consumption and small volume was expected to replace them as a suitable light source. Microcavities can be divided into three types, which are the Fabry–Perot (FP) microcavity, photonic crystal (PC) microcavity, and whispering-gallery (WG) microcavity^[2]. The FP microcavity is formed by reflection between two reflectors, like the vertical-cavity surface-emitting laser (VCSEL). Generally, to achieve an effective reflectance of over 99%, more than 20 pairs of DBRs are required. However, the FP microcavity laser is surface emitting, which is not conducive to the integration of light sources and other devices^[3,4]. The PC microcavity introduces irregular defects in a perfect PC; when the defects are located in the photonic band gap, light waves with wavelengths at this level will be confined to the defect state, so the PC can form a very high-quality (Q) factor local state in a very small area^[5,6]. But, the material growth and device fabrication of these two methods are too complex. Thus, the WG microcavity semiconductor laser with considerably low power consumption, small volume, and simple fabrication process can be considered as the light source of the gas sensing system^[7]. The WG microcavity has a high-Q factor and small cavity size comparable to an optical wavelength. Light reflected from active area on the wall of the microcavity can provide strong optical confinement, which results in very high cavity Q factors and low thresholds. But, most of the existing WG mode (WGM) microcavity lasers are lasing at 1.3 µm and 1.5 µm, as we know, and lasers emitting from 1700 nm to 2000 nm have not been developed yet. However, lasers emitting at the near infrared (NIR) band from 1700 to 2000 nm contain the absorption region of many gas species like HCl, H2O, and HBr, which are of great significance for gas sensing^[8].

In this Letter, a micro-ring resonator laser used for gas sensing is experimentally demonstrated. The laser consists of a resonator with a 15 µm outer radius, 8 µm inner radius, and output waveguide of 3 µm in width. Single-mode lasing was obtained, for the first time, to the best of our knowledge, at about 1746.4 nm at 288 K. The maximum output power is 1.65 mW with a side-mode suppression ratio (SMSR) over 33 dB.

The simulation of the micro-ring resonator laser was based on the finite element method (FEM), which uses a variation of Maxwell equations as the characteristic equations. For example, the characteristic equation of the transverse electric (TE) mode can be described by^[9]∇×(µr−1∇×E)−(εr−iσωε0)k02E=0,(1)where µr is the relative permeability, and εr is the relative dielectric constant. The frequency and Q factor characteristics of the eigen-mode can be obtained by calculating the characteristic solution of the equation f. The Q factor can be defined as Q=Real(f)2|Imag(−f)|.(2)

To forecast the Q factor and mode field distribution of the device, we simulated a micro-ring resonator laser with a 15 µm outer radius, 8 µm inner radius resonator, and a 3 µm wide output waveguide by the two-dimensional (2D) FEM method. The micro-ring resonator and output waveguide with refractive index of 3.2 are confined by air instead of the SiO2 insulating layer and the p-electrode layer, as in real devices, which has a refractive index of 1.54. Simulation result of the Q value varying with different wavelengths is plotted in Fig. 1(a). A single mode emitting at 1746.3 nm is obtained with a higher-Q factor of 1.42×105. Figure 1(b) shows the mode field distribution of 1746.3 nm.

The fabrication process of the micro-ring resonator laser includes materials epitaxy, photolithography, dry etching, wet etching, metal sputtering, etc. All of these processes can be summarized into two steps, which are materials epitaxy and manufacturing process.

Materials of the laser were grown by metal organic chemical vapor deposition (MOCVD) equipment. Table 1 shows the epitaxial structure of the laser. The whole structure was grown on an InP substrate. Referring to the previous research of semiconductor lasers emitting at 1.7–1.8 µm^[10–12], and making full use of the mature manufacturing process of InP/InGaAsP materials, we choose InGaAs/InGaAsP as the active region materials of the laser. The In component of InGaAs quantum wells (QWs) with a center wavelength of 1.75 µm takes about 80%. The lattice mismatch of the quantum well material is 1.9%. The optical gain of the QWs structure was calculated with the variations of wavelength and carrier concentration, as shown in Fig. 2. Growth interruption was an effective way to improve the interface Q of QWs, which are influenced by the large lattice mismatch value. In order to confine the carrier and the optical field in the active layers effectively, the upper and lower separated confinement heterostructure (SCH) layers are formed by 120 nm InGaAsP with a band-gap wavelength of 1.3 µm (1.3 Q), respectively. With lower refractive indices, 1700 nm Zn-doped and 750 nm Si-doped InP layers formed the cladding layer and the buffer layer, which were used for reducing defects and limiting the light field. A 200 nm InGaAs was deposited on the top of the whole structure as a contact layer.

Etching high-Q resonators with a vertical and smooth sidewall plays the core role in the whole production process of the device. Since the wet etching method always has directional selectivity and will lead to a lateral etching, the microcavity was etched by the inductively coupled plasma (ICP) etching method. As a hard mask for etching, 500 nm SiO2 was deposited by plasma-enhanced chemical-vapor deposition (PECVD) on the InGaAs contact layer. The micro-ring resonator and a short output waveguide pattern were transferred onto the SiO2 layer using standard photolithography and ICP etching techniques. Under the cover of a SiO2 mask, the underlying material was etched by ICP as well. To reduce optical absorption losses and ensure lasing Q, the etching depth needs to be about 4.5 µm. The cross-sectional view scanning electron microscope (SEM) image of the etching result is shown in Fig. 3(a). After ICP etching, the next significant production process is electrode preparation. To efficiently pump the WGM region, the electrode contact is just over the resonator. A Ti–Au p electrode layer was formed over the wafer by magnetron sputtering, and the electrode pattern was defined by wet etching. The wafer was thinned down to 150 µm, and an Au–Ge–Ni–Au later was formed as an n-type contact on the back of the InP substrate by metal evaporation. After rapid thermal annealing, the device was completed. Figure 3(b) shows the top-view SEM image of the finished device.

The laser was tested on a thermoelectric cooler (TEC), and the temperature was controlled at 288 K. Light power and voltage versus the injection current (PIV) were collected by an integrating sphere, and the PIV curves under 40 mA are plotted in Fig. 4. The light power can come up to about 1.65 mW when the injection current was 40 mA, as shown in Fig. 4. But, the threshold current of the laser cannot be confirmed through these PIV curves. Because of the small laser volume, the spontaneous emission rate is high; almost all of the spontaneous emissions are coupled to the single lasing mode. In addition, due to the high value of the Q factor, most of the spontaneous emission light is confined in the microcavity, resulting in a high intensity. Therefore, the power-current curve does not have an inflection point, so the threshold current cannot be confirmed^[13].

The lasing spectra were measured by an optical spectrum analyzer with a resolution of 0.05 nm. Light lasing from the device was partially coupled into a tapered fiber and transmitted to the optical spectrum analyzer through a multimode optical fiber. The lasing spectrum at an injection current of 40 mA is shown in Fig. 5. As the figure shows, the central wavelength is 1746.4 nm with SMSR of 33.33 dB. Obviously, only fundamental transverse mode lasing is achieved because the high-order transverse modes are completely suppressed by the ring resonator. To characterize the stability of the device, the spectra under different inject currents were tested. When the injection current varies from 20 mA to 50 mA, with an interval of 10 mA, the central wavelengths are 1745.18 nm, 1745.73 nm, 1746.37 nm, and 1747.07 nm, with the SMSR of 19.30 dB, 31.93 dB, 33.33 dB, and 25.09 dB, respectively, as shown in Fig. 5. The change is 0.063 nm per milliampere as calculated.

Compared with DFB and DBR lasers^[14], whose volumes are hundreds of millimeters and threshold currents are at least tens of milliamps, the volume of our WGM micro-ring resonator laser is much smaller, and the threshold current is lower too. Compared with other WGM micro-resonator lasers^[15–19], our device has a higher lasing power. Thus, we think the low-power consumption, small volume laser is competitive and has potential in gas sensing.

In conclusion, a WGM micro-ring resonator laser used for gas sensing is experimentally demonstrated. The laser is based on multi-InGaAs/InGaAsP-QWs and an InP substrate. It consists of a resonator with a 15 µm outer radius and 8 µm inner radius, and a 3 µm wide output waveguide. The maximum output power is 1.65 mW at 288 K, as shown in the PIV curve. The lasing spectrum shows that a single-mode lasing was obtained, for the first time, to the best of our knowledge, at about 1746.4 nm at 288 K, with an SMSR over 33 dB.

Compared with other lasers like DFB and DBR lasers, the WG micro-ring resonator laser has a much smaller volume and much lower threshold current, making it promising in gas sensing.