# High Optical Feedback Tolerance of a Detuned DBR Laser for 10-Gbps Isolator-Free Operation
## 2 Device Structure and Experimental Setup  
The DBR laser was composed of a 180-\(\upmu\)m-long gain section, a 100-\(\upmu\)m-long phase section, and a 280-\(\upmu\)m-long DBR section, separated by isolation regions of 20 \(\upmu\)m, as shown in Figure 1a. The AlGaInAs/InP material system was chosen for the gain section for its better carrier confinement and the higher characteristic temperature. The InGaAsP bulk structure was chosen for the passive sections (the phase and DBR sections) for its low absorption loss. The integration of the above two was realized by a butt-joint process. The structure was grown by a three-step metalorganic chemical vapor deposition (MOCVD). In the first-step growth, 9 pairs of strained multiple-quantum-wells (MQWs) were sandwiched between 100-nm-thick separate-confinement heterojunction (SCH) on an n-type InP substrate. After that, the region designed as the passive sections was removed by wet etching to prepare for the butt-joint process. In the second-step growth, a 400-nm-thick core layer was grown to fill the passive sections. After that, the 25-nm-deep Bragg grating pitch was defined by e-beam lithography on part of the core layer. In the final third-step growth, a 2.2-\(\upmu\)m-thick InP cladding and InGaAs contact layer were grown on the whole wafer. A 2-\(\upmu\)m-wide double trench was fabricated by wet etching. The p-electrode and n-electrode were both made by magnetron sputtering. The laser chip was cleaved and mounted on a high-frequency heat sink for testing.  
The output power and spectrum of the DBR laser were measured at room temperature (25 \({}^{\circ}\)C), as shown in Figure 1b,c. The DBR laser had a threshold current of about 9 mA and a slope efficiency of 0.37 W/A, with output power over 20 mW at a pump current of 60 mA. The light current curve shows a kink at 53 mA due to mode hopping. The optical spectrum plotted in Figure 1c records the whole mode hopping process in preparation for investigating the OFT of the DBR laser. As shown in Figure 1c, the lasing wavelength redshifted continuously with the increase of gain current until the mode-hopping point at 53 mA, where a sudden blueshift occurred. The state before and after mode-hopping respectively correspond to a lasing wavelength on the red and blue flank of the Bragg reflection spectrum. Data are representative of similar results obtained from independent tests performed on dozens of lasers in the same batch, except that the mode hopping points are randomly distributed in the whole range.  
Figure 2 shows the schematic measurement setup for the OFT evaluation. The optical feedback is achieved by using a circulator and a variable attenuator. The light emitted from the laser is coupled into the circulator through a tapered fiber and then reinjected back into the laser cavity after a variable attenuator, a 3 dB coupler, and a polarization controller. By adjusting the variable attenuator, the feedback level can be controlled. The overall length of the feedback path is about 7 m, and the maximum feedback strength can reach \(-9\) dB with the roundtrip coupling loss of the tapered fiber as low as \(-6\) dB. All the experiment data are recorded under the worst RIN and transmission performance by controlling the feedback polarization. During the static test, the injection current is provided by a direct current (DC) source. During the modulation test, nonreturn-zero (NRZ) signal generated by a pulse pattern generator (PPG) is applied to the laser through a bias tee. The eye diagram is tested by the sampling oscilloscope, and the BER is tested by the BER Tester after the optical signal is received by a commercial SFP+ transceiver.