Interview

Abstract

Dr Jun-ichi Hashimoto is a researcher at the Transmission Devices Laboratory, Sumitomo Electric Industries Ltd., Japan. Here he tells us about his research and new Electronics Letters paper, ‘Low power-consumption mid-infrared distributed feedback quantum cascade laser for gas-sensing application’ page 549. Dr Jun-ichi Hashimoto Since the late 1980s, our laboratory has been engaged in developing semiconductor devices in the optical communication field of the near infrared region (1.3–1.55 µm). We have manufactured various kinds of commercial products including laser diodes, detectors, and modulators. Recently we started developing optical devices in the mid-infrared (MIR) region. We consider a quantum cascade laser (QCL) to be a future key light source in the MIR region, and we are now focusing our attention on its development. Our developing technologies cover all fields including device design, epitaxial growth, fabrication process, and evaluation system. A QCL is a new semiconductor laser enabling an oscillation in the MIR region. It has a structure similar to a conventional pn-junction laser diode, but the core region (emission region) structure is totally different. The core region of QCL consists of several tens of unit structures stacked together, and each unit structure has an active region serving as a light-emitting region, and an injector region to transport carriers (electrons) into the next active region. Both are made of super-lattice structures. In the core region, the radiative transition occurs between the sub-bands of the conduction band of the active region, which makes it possible to oscillate in the MIR wavelength region. We developed a low power consumption distributed feedback quantum cascade laser (DFB-QCL) applicable to a light source of a portable gas sensor. To reduce its power consumption significantly, we introduced our original high-gain vertical-transition core structure, a buried-hetero waveguiding structure with a high thermal conductivity and a low optical loss, a buried-type grating favourable for a large coupling-coefficient, and a high-reflective Au-coating on the rear facet. As a result, we succeeded in realising a DFB-QCL with a low threshold power-consumption (0.93W) at 20°C. In addition, this QCL demonstrated a sufficient continuous wave (CW) output power (≈20mW), a good singlemode property (single mode suppression ratio>25 dB), and a single-lobed far-field patterns (FFPs) at 20°C. We think this DFB-QCL can be used as a light source of a portable gas sensor. The QCL we developed in this study has low power consumption (∼1W) as mentioned earlier, so it is especially promising as a light source for portable QCL gas sensors. These QCL gas sensors have compact, fast (<1s) and high-sensitive (ppb∼ppt) characteristics which could not be satisfied simultaneously with the conventional MIR gas sensors. We expect these to open up new markets in various fields in the future, such as process and/or emission gas control in factories, environmental gas sensing, breath analysis for medical diagnosis and military applications. Developing our original high-gain vertical-transition core structure proved most challenging. In order to achieve high net optical gain in the core structure design, it is important to increase the carrier transition probability between the sub-bands, as well as reducing the optical loss due to the non-radiative recombination caused predominantly by longitudinal optical phonon (LO phonon) scattering. To address this issue, our structure has a high optical gain due to the vertical-transition of carriers between the sub-bands in the conduction band of the active region. At the same time, the LO-phonon scattering is supressed effectively by optimising the coupled quantum well structure of the active region. We predict that a further power consumption reduction of QCL, such as below 0.5 W, is needed to realise cheaper and smaller mass-marketable gas sensors for consumer use. Therefore, finding a way to reduce the power consumption more drastically seems a first priority. Expanding the wavelength tuning range of QCL is another important issue to improve the detection accuracy of the gas sensor and to increase the kinds of target gas which can be detected simultaneously with one gas sensor. We would like to study the integration of QCL and the related devices striving towards better performance and a novel function such as a wider wavelength tuning range, a higher output power, a narrower beam shape, a light emission in vertical direction, and so on. We are also interested in other wavelength regions having promising markets like MIR region, and Terahertz region is one of the candidates.