Laser diodes operating in the 2.3-μm band are attractive for detecting trace carbon oxide (CO) gas because of the strong absorption lines of CO and weak interference from H2O in this band. As a light source for CO-gas detection, we previously developed a 2.3-μm-band distributed feedback (DFB) laser with InAs/InGaAs multiple quantum wells (MQWs) grown on InP. To construct a high-speed with high-accuracy measurement system for CO, the light source must be capable of fast wavelength tuning over a wide range. In this study, we developed InAs/InGaAs distributed Bragg reflector (DBR) laser operating in the 2.3-μm band that uses current injection and has wider and faster wavelength tuning compared with DFB lasers using chip-temperature control. The laser cavity consists of an active region with InAs/InGaAs MQWs, front/rear DBR regions, and phase-control region. The DBR and phase-control regions are monolithically integrated with the active region by using the butt-joint technique. The measured lasing wavelength was 2348.1 nm with an injection current for the active region of 100 mA at room temperature and under continuous wave operation. When the DBR current (front + rear) increased from 0 to 100 mA, the lasing wavelength shifted by -12.0 nm. The estimated refractive index change was 0.51%, which is comparable to that of conventional devices operating in the telecommunication band. Furthermore, by sweeping the currents for the DBRs and phase-control region, the lasing wavelength could be continuously tuned over a range of 4.2 nm, which covers multiple CO absorption lines in the 2.3-μm band.
Electro-absorption modulator (EAM) integrated distributed feedback (EADFB) laser are widely used for 10-, 40- and over 40-km optical communications. In a certain power dissipation condition, there is intrinsic difficulty in increasing the modulated light output power of EADFB lasers, because large insertion loss of the EAM deteriorates the power conversion efficiency. In this study, we investigated an SOA integrated with a EADFB laser to improve the power conversion efficiency of the EADFB laser. The device is called an SOA assisted extended reach EADFB laser, or AXEL for short. For a transmission with a 1.3-μm wavelength, the transmission distance was limited to 40 km because of the large fiber loss of 0.3 dB/km. To overcome this kind of transmission distance limit, we demonstrated a 1.3-μm AXEL with significantly increased power conversion efficiency. In addition, a 25-Gbit/s 80-km transmission by using AXEL and APD-ROSA was firstly achieved beyond the limitation of transmission distance. In contrast, with respect to an L-band wavelength range, a large chromatic dispersion severely limits the transmission distance. Then, we also investigated the AXEL with 1.57 μm wavelength range, and found that the SOA can operate as both optical booster and chirp compensator. Furthermore, the extension of 10-Gbit/s transmission to 80-km and enhancement of modulated light output power to 9.0 dBm were simultaneously achieved by taking advantages of its chirp compensation effect and high power conversion efficiency. The presented results indicate that the AXEL is an attractive candidate for a high-efficiency modulated light source with any wavelength range.
A novel approach is demonstrated for overcoming the trade-off relationship between the power consumption and transmission distance of an electro-absorption modulator integrated with a DFB laser (EADFB laser). We demonstrate that the monolithic integration of a short semiconductor optical amplifier (SOA) with an EADFB laser is effective in overcoming the limitation imposed by the Kramers-Kronig (K-K) relation of the EA modulator, which cannot be overcome with the conventional method of optimizing the MQW structure of the EA modulator. Our approach provides an EADFB laser with two advantages. One is that we can realize a higher optical output power with smaller power consumption than with a conventional EADFB laser by reducing the DFB laser injection current and allocating it to the SOA section. We design the SOA length based on this concept. The other advantage is the chirp compensation of the EA modulator with the SOA. To confirm the validity of this approach, we investigate the SOA length dependence on the basic characteristics. By using an EADFB laser integrated with a 50-μm-long SOA, we achieve a 2 dB increase in the modulated output power compared with a stand-alone EADFB laser with the same power consumption. We realize an extended transmission distance of 5 km at 40 Gbit/s, and a 1.55-μm-wavelength window, which is conventionally achieved for a 2-km SMF transmission with an EADFB laser. These results indicate that this approach is a promising way to realize a high-speed light source with low power consumption for future large capacity optical network systems.
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