Objective High-speed modulated semiconductor lasers are important light sources for high -capacity optical communication systems. Compared with externally modulated lasers, such as electro-absorption modulated distributed feedback (DFB) lasers, directly modulated DFB lasers have several advantages, including a simple structure, low cost, and low power consumption. A higher speed of data transmission of an optical communication system can be obtained using DFB lasers with a higher direct modulation bandwidth in the following ways. First, high -gain active materials such as InGaAlAs/InP multi -quantum wells (MQWs) can be used for the fabrication of lasers. Subsequently, a short laser cavity length can be used to realize a short photon lifetime in the cavity. Because of their important applications, high-speed directly -modulated InGaAlAs/InP MQW DFB lasers have been widely studied. However, to obtain a high modulation bandwidth, the length of the active region for most reported lasers must be less than 150 mu m. A small active length leads to a high facet loss, and thus, a low optical power output. In addition, a small length results in high resistance, which leads to a strong self -heating effect. In this paper, we report high-speed directly -modulated DFB lasers integrated with a distributed Bragg reflector (DBR) section working at 1.3-mu m wavelength. For the cavity length of 200 mu m, the 3 -dB small signal direct modulation bandwidth of the laser is larger than 29 GHz.Methods This device is fabricated via two-step lower pressure metal organic chemical vapor deposition (MOCVD) growth. In the first step, the active layer, a multi -quantum well structure comprising nine 1.2% compressively strained InGaAlAs wells and ten 0.2% tensile -strained InGaAlAs barriers, is grown. The thicknesses of each well and barrier are 4 nm and 10 nm, respectively. A 50-nm-thick InGaAlAs graded index layer and a 50-nm-thick InAlAs laser are grown on both sides of the MQW layer. A 60-nm-thick InGaAsP layer is grown on the upper InAlAs layer for grating fabrication. After a uniform grating is fabricated using electron beam lithography and dry etching, an InP cladding layer and an InGaAs contact layer are grown in the second growth step. Figure 2 shows a schematic of the cross-section structure and an optical graph of the fabricated laser. The laser has a 1.7 -p.m -wide ridge waveguide structure and consists of a 200 -p.m -long DFB section and 130 -p.m -long DBR section. The gratings of the two sections have the same period and etching depth. To obtain a high single -mode yield, a lambda /4 phase -shift structure is placed in the middle of the DFB section. The light emitted from the DFB section is reflected back by the DBR section, which helps increase the optical power. Moreover, the feedback from the DBR section can further increase the yield of single -mode emission. As shown in Fig. 2(a), the two sections of the device have the same InGaAlAs MQWs, which greatly simplifies device fabrication.conResults and Discussions The threshold current of the laser at 20 degrees C is 12 mA. The optical power is 18 mW at the injection current of 100 mA [Fig. 3(a)]. The emission wavelength of the laser is approximately 1320 nm. The side -mode suppression ratio of the optical spectra is larger than 50 dB when the DFB current increases from 30 mA to 90 mA [Fig. 3(b)]. At 20 degrees C, the 3 -dB small signal direct modulation bandwidth of the laser is 20 GHz at the injection current of 60 mA and increases to 29 GHz when the current increases to 90 mA [Fig. 4(a)]. By fitting the experimental modulation response curve, the intrinsic modulation bandwidth is found to be 35.4 GHz at the current of 94 mA, indicating that the modulation bandwidth of the laser can be further enhanced by optimizing the laser design and fabrication process. The frequency of the relaxation oscillations, which is also obtained by fitting the experimental modulation response data, increases with the injection current at the rate of 0.25 GHz/mA. 25-Gbit/s nonreturn to zero (NRZ) data transmissions using the laser are conducted. Data patterns having a 215 - 1 length are generated by a commercial pulse pattern generator. Clear open eye views can be obtained after 10 km transmission (Fig. 5). The bit error rate (BER) performance of the transmission is also analyzed. The 10 -km transmission power penalty of obtaining BER of 10-10 is less than 1 dB at both 20 degrees C and 80 degrees C (Fig. 6).Conclusions A high-speed directly -modulated DFB laser integrated with a DBR section working at 1.3- p.m wavelength is fabricated using InGaAlAs/InP multi -quantum wells as the active material. For the cavity length of 200 p.m, the 3 -dB small signal direct modulation bandwidth of the laser is larger than 29 GHz. Under 25-Gbit/s NRZ data direct modulation, the power penalty to obtain the BER of 10-10 after single -mode fiber transmission of 10 km is less than 1 dB at both 20 degrees C and 80 degrees C. A longer active region length of the device is beneficial for improving the output slope efficiency and reducing the adverse effects of current heating. The fabricated device is a promising light source for short -reach, high -capacity optical communication systems.
