1. Introduction

In the process of mining, industrial production and transportation, toxic, explosive and corrosive gases are often produced^[1]. To ensure the quality of products and safety in the production process, real-time monitoring of gas components, concentrations, and other parameters plays an important role^{[2, 3]}. It is worth mentioning that semiconductor lasers emitting at 1.6–1.9 μm with the strong absorption lines of various gases, such as ethylene, methane, hydrogen chloride, and nitric oxide, have gained a lot of interest for working as the light source on gas sensing^[4-6]. Combined with the requirements of gas sensing, several wavelength tunable semiconductor lasers are proposed and fabricated, like distributed feedback (DFB) lasers^{[4, 7, 8]}, distributed Bragg reflector (DBR) lasers^[9-13], and vertical cavity surface emitting lasers (VCSELs)^[14-16]. The mode selection of DFB and DBR lasers is based on Bragg grating, which requires fine etching and regrowth process. The VCSELs are based on the DBR structure composed of multilayer materials. Therefore, these lasers always require either fine fabrication technology or complex epitaxial structure, which caused high fabrication costs. Hybrid cavity lasers^[17-20] can realize wavelength tuning without etching grating, which have attracted people's attention. We noticed that a kind of hybrid cavity semiconductor laser based on a Whispering-Gallery (WG) microcavity and a Fabry–Pérot (FP) cavity, was able to realize a larger wavelength tunable range while with lower cost^[21-23]. Due to short cavity length, it is easy to realize single longitudinal mode operation for microcavity lasers. However, a conventional FP cavity laser is usually multiple longitudinal lasing. The hybrid cavity composed of an FP cavity and a square microcavity has been demonstrated for mode selection by enhancing the mode Q factor for hybrid mode between the WG and FP mode. Stable single-mode operation is realized by injecting currents to the square microcavity and the FP cavity sections at the same time^[24]. However, the existing WG-FP hybrid cavity lasers are basically emitting at the C band, which are used in optical communication but are barely used in gas sensing.

Therefore, in this paper, we report a single-mode WG-FP hybrid cavity laser with a wide wavelength tuning range from 1760.87 to 1773.39 nm. The side-mode suppression ratio (SMSR) for the whole tuning range is over 30 dB. And the lasing output power was over 3.5 mW at 20 °C. Compared with the DFB lasers, DBR lasers, and VCSELs, the hybrid cavity laser does not introduce the grating etching process and complex epitaxial structure, which can reduce the fabrication difficulty and cost. And the over –12.5 nm tuning range incorporates absorption lines of methane and hydrogen chloride^[25], showing a practical perspective for gas sensing with this hybrid-cavity laser.

2. Simulation and fabrication

2.1 Simulation results

Aiming to study the mode characteristics of the WG-FP hybrid cavity laser, a systematic numerical simulation based on the two-dimensional (2D) finite element method (FEM) was adopted^[26]. The setting of simulation parameters is shown in Fig. 1. The refractive indices of the hybrid cavity, the bisbenzocyclobutene (BCB) which surrounds the cavities are 3.2 and 1.54, respectively. In practice, the light emits directly into the air from the end of the FP cavity, therefore, an air layer is added with a refractive index of 1. The WG-FP hybrid cavity laser consists of a square microcavity with the side length a = 20 μm, and an FP cavity with the length L = 360 μm, the width w = 3 μm. The perfectly matched layer (PML) is set as the boundary condition because it can absorb all the scattered light to simulate an infinite space, which is consistent with reality. After building the simulation model, we simulated the mode Q factors, field distributions, and far-field behaviors under different eigenfrequency by calculating the characteristic solution f of Maxwell's characteristic equation.

To explore the influence of square microcavity on lasing mode selection, mode Q factor that varied with different mode wavelengths was simulated, as shown in Fig. 2, where the Q factor can be defined as ^[27]:

$$Q = \frac{{{\text{Re}}\left( {{f}} \right)}}{{{{2}}\left| {{\text{Im}}\left( {{{ - f}}} \right)} \right|}} .$$

(1)

Referring to Fig. 2(a)，the mode Q factor in the WG-FP hybrid cavity fluctuated periodically, and the Q factors were significant at the mode wavelengths of 1747.8, 1764.8, and 1782.2 nm. Based on the relationship between the gain coefficient g and the imaginary part of the refractive index Im(n) ^[28]:

$$g = - \frac{{2\omega \rm{Im}\left( n \right)}}{c} = - \frac{{4\pi \rm{Im}\left( n \right)}}{\lambda } ,$$

(2)

where ω is the angular frequency of light waves, c is the speed of light, and λ is the wavelength of light waves.

The relationship between Q factor and gain of the mode wavelength at 1764.8 nm was explored. By changing the imaginary part of the refractive index of the square microcavity Im(n_WG), while keeping the imaginary part of the refractive index of the FP cavity Im(n_FP) = 0, the change of the mode Q factor was studied with the imaginary part of the refractive index of the square microcavity changed. As shown in Fig. 2(b), as Im(n_WG) decreased from 3 × 10^-5 to –3 × 10^-5, which means the gain coefficient increased from –2.1 to 2.1 cm^–1, the mode Q factor near the mode wavelength of 1764.8 nm increased with the improvement of the gain coefficient, which showed low sensitivity to the variation of Im(n_WG) at other mode wavelengths. We infer that the mode at 1764.8 nm is the coupling mode of the square microcavity and the FP cavity.

To study the center wavelength tuning ability of the laser, we changed the real part of the refractive index of the square cavity Re(n_WG) to characterize the change of the laser when the working current is applied. As shown in Fig. 3, the mode center wavelengths with the change of the index Δn_WG were simulated. It can be found that as the refractive index increased, the mode center wavelength of the laser gradually redshifted.

2.2 Fabrication process

In this paper, the materials of the devices were grown by metal organic chemical vapor deposition (MOCVD) equipment. Combined with the experiences of material epitaxy in our group^{[5, 7, 10]}, we chose InGaAs/InGaAsP multi-quantum wells (MQWs) as the active region and the whole material structure was epitaxially grown on an n-type InP substrate. The strained MQW consisted of four pairs of 9.5 nm compressively strained InGaAs wells and 12 nm five tensile strained InGaAsP barriers embedded in two undoped InGaAsP (λ_PL = 1.3 μm, where PL stands for photoluminescence) separate confinement layers (SCH). A 1700 nm InP layer with a lower refractive index used for limiting the light field, formed the upper cladding layer. A 200 nm InGaAs was deposited on the top of the whole structure as a contact layer.

Compared with DFB and DBR lasers, the WG-FP hybrid cavity laser does not need to etch grating, which reduces regrowth processes, simplifies the fabrication process, and reduces the cost. The fabrication process of the WG-FP hybrid cavity laser is shown in Fig. 4. Standard contacting photolithography and the inductively coupled plasma (ICP) etching method were adopted to ensure the quality of the cavity. As shown in Fig. 5(a), the sidewall was supposed to be vertical and smooth. To reduce the light leakage to the substrate, the etching depth was greater than 4.5 μm, which meant the quantum well region was exposed. SiO₂ and BCB layers were deposited and coated to protect the active region from oxidation and electric leakage, and a large area of reactive ion etching (RIE) was performed then to expose the top of the contact layer for depositing the electrode metal. To guarantee the two cavities will be electrically isolated from each other, the p-InGaAs contact layer should be removed. Therefore, a shallow isolation trench of about 300 nm in depth and 5 μm in width was formed by wet etching. This depth is far from the active region and the waveguide layer, which will not affect the mode characteristics of the device. At last, Ti/Au and AuGeNi/Au were deposited to form p and n type electrodes respectively. The microscope image of the fabricated WG-FP hybrid cavity laser was shown in Fig. 5(b), two rectangular p-type electrodes are used for current injection into the two cavities separately.

3. Experimental results and discussion

The hybrid-cavity was mounted on a Cu submount with the p-side upward and was tested under continuous-wave (CW) at 20 °C. To ensure that the two cavities were electrically isolated from each other, we tested the I–V curve. The isolation resistance is about 25 kΩ as calculated, which was enough to achieve the electrical isolation^[21]. For a WG-FP hybrid cavity laser with dimensions of a = 20 μm, L = 360 μm, w = 3 μm, the output power was collected by an integrating sphere. Light power varied with the injection currents of the FP cavity (I_FP) curves were plotted in Fig. 6 (a) under the injection currents of the WG cavity (I_WG) at 0, 2, 5, and 10 mA. Owing to the WG cavity did not work when I_WG = 0 mA, the laser did not emit. The threshold current I_th reduced from 42 to 30 mA and the output power increased from 2.1 to 3.6 mW, as shown in Fig. 6(a), when I_FP increased to 60 mA and I_WG increased from 2 to 10 mA, which showed the positive feedback effect from the square microcavity. This was due to the improvement of the mode Q factor for the hybrid mode, which was in accord with the trend of simulation in Fig. 2. The slope efficiencies were estimated to be about 0.116 W/A when I_WG = 10 mA, I_FP = 60 mA, respectively. The dependence of threshold current on temperature was shown in the inset of Fig. 6(b). Basing on the empirical relation, I_th = I₀exp(T/T₀), a characteristic temperature (T₀) was calculated as 44.3 K at 20 °C after fitting.

The emitting light was partially collected by a tapered fiber and transmitted to an optical spectrum analyzer with a resolution of 0.05 nm through a single-mode fiber. Lasing spectrum measured for the laser under I_FP = 80 mA, I_WG = 70 mA was shown in Fig. 6(b). The laser emitted as a single mode with the side-mode suppression ratio (SMSR) of 33.79 dB. As shown in Figs. 7(a) and 7(c), when I_FP = 80 mA, the center wavelength redshifted from 1761.52 to 1771.63 nm (10.11 nm) with the SMSRs over 28 dB, as the I_WG continuously increased from 5 to 88 mA. As shown in Figs. 7(b) and 7(d), when I_WG = 35 mA, the center wavelength redshifted from 1762.32 to 1763.05 nm (0.73 nm) with SMSRs over 32 dB, as the I_FP continuously increased from 40 to 80 mA. Owing to the small size of the square microcavity, it is more sensitive to the changes of temperature and refractive index. Therefore, the square cavity has a greater influence on the wavelength change which can be used for coarse tuning, while the FP cavity can be used for fine-tuning.

By varying the injected current of the square cavity and FP cavity simultaneously, the wavelength tuning range of 12.52 nm from 1760.87 to 1773.39 nm was realized, and the SMSRs were above 30 dB for the whole course. The superimposed lasing spectrum is shown in Fig. 8. Compared with the previous works^{[5, 10, 25, 27]}, the wavelength tuning range has been improved.

4. Conclusion

In conclusion, a hybrid-cavity laser consisting of a square Whispering-Gallery microcavity and a Fabry–Pérot was demonstrated. The output power was over 3.5 mW and a wavelength tuning range over 12.5 nm were obtained. Besides, the laser performed single-mode emitting with the side-mode suppression ratio over 30 dB. Light-emitting range from 1760.87 to 1773.39 nm corresponds to the absorption lines of methane and hydrogen chloride. Moreover, grating etching process and complex epitaxial structure were not introduced into the fabrication process, which reduced the manufacturing difficulty and cost while achieving a wide wavelength tuning range as well. Thus, the device yield will be improved, and it is more conducive to production. Therefore, the WG-FP hybrid cavity laser illustrated a practical perspective for certain purposes of gas sensing.

Acknowledgements

This work was supported by the National Key Research and Development Program of China (Grant No. 2018YFA0209001), the Key Project of Frontier Science Research Project of CAS (Grant No. QYZDY-SSW-JSC021), and the Strategic Priority Research Program of CAS (Grant No. XDB43020202).