Terahertz quantum cascade lasers with sampled lateral gratings for single mode operation
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RESEARCH ARTICLE
Terahertz quantum cascade lasers with sampled lateral gratings for single mode operation Dixiang SHAO, Chen YAO, Zhanglong FU, Wenjian WAN, Ziping LI, Juncheng CAO (✉) Key Laboratory of Terahertz Solid-State Technology, Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences, Shanghai 200050, China
© Higher Education Press 2020
Abstract In this paper, we presented single mode terahertz quantum cascade lasers (THz QCLs) with sampled lateral grating emitting approximately 3.4 THz. Due to strong mode selection, the implementation of sampled lateral grating on THz QCL ridges can result in stable single longitudinal mode emission with a side-mode suppression ratio larger than 20 dB. The measured peak power of the grating laser is improved by about 11.8% compared to the power of devices with uniform distributed feedback gratings. Furthermore, the far-field pattern of the presented device is uninfluenced by grating structures. Keywords terahertz (THz), quantum cascade laser (QCL), sampled lateral grating
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Introduction
Since the first demonstration in 2002, electrically-pumped, compact, semiconductor terahertz quantum cascade lasers (THz QCLs) have experienced rapid development in the last decades [1,2]. For most applications, such as absorption spectroscopy, telecommunication, and heterodyne mixing in astronomy, single longitudinal mode THz QCLs with wide tuning ranges are necessary. First-, second-, and third-order distributed feedback (DFB) gratings are commonly adopted and designed for THz QCLs to achieve single mode operation [3–7]. Among various structures of DFB gratings, lateral DFB gratings can be simultaneously fabricated with ridge waveguides, so that no extra steps are needed for the conventional fabrication process. THz QCLs with first-order lateral DFB gratings have already been demonstrated for semiinsulating surface-plasmon waveguides with single mode Received August 14, 2020; accepted October 1, 2020 E-mail: [email protected]
emission [5]. As an alternative to DFB gratings, the coupled-cavity technique was recently implemented with success in obtaining stable single mode operation for THz QCLs with a side-mode suppression ratio in the 30–40 dB range [8]. On the other hand, THz QCLs with high output power are desirable in most practical applications due to the significant attenuation of THz radiation by water vapor in the atmosphere, which can sometimes decrease the total output power. In this work, sampled lateral grating, which is expansively used in lasers operating in telecom and midinfrared wavelengths [9–12], is used to improve output power and obtain single mode operation for THz QCLs. DFB gratings are periodically blanked in the sampled lateral grating, so the optical loss during light propagation caused by the sudden change of waveguide widths of lateral DFB gratings has been reduced when compared with that of DFB lasers with the same device length. Sampled lateral grating can improve the mode selection of lasers, leading to single longitudin
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