# High Optical Feedback Tolerance of a Detuned DBR Laser for 10-Gbps Isolator-Free Operation
## 3 Results
### Static Performance under Feedback
#### 3.1.2 Critical Feedback Value  
To analyze the effect of wavelength detuning on OFT more thoroughly, the critical feedback value was measured, as shown in Figure 4. Figure 4a shows the relationship between the wavelength detuning and critical feedback value. In the low bias current region of (10 mA\(-\)40 mA), the laser was sensitive to optical feedback, showing a low critical feedback value. In this region, the wavelength of the laser should locate within the center of the DBR reflection spectrum, where its tolerance to feedback can be roughly explained using the theory described in [11]. While in the higher current region of (40 mA\(-\)50 mA), where red detuning of the lasing wavelength occurred, the laser was not easily disturbed, and the RIN remained very stable. Next in the current region of (53 mA\(-\)60 mA) following mode hopping, the lasing wavelength was blue-detuned, the laser became sensitive to feedback and a drop in the critical feedback value can be observed. Usually, lasers with high output power and high damping also tend to be more tolerant to optical feedback [11]. To isolate the contribution of the wavelength detuning, we investigated another DBR laser (Laser 2) with a lower mode hopping current in the same batch, so that the influence of high output power and high damping rate can be minimized. The results are shown in Figure 4b. The mode hopping point of this laser was 27 mA, with other characteristics similar to those of the former (Laser 1). For this laser, the red-detuned region at low pump current is located at about several milliamperes below 27 mA. It can be observed that in the red-detuned region before the mode hopping the critical feedback value was significantly enhanced from \(-18\) dB to over \(-9\) dB more dramatically than in Laser 1. In this region, both the output power and the damping rate were not high enough to play a major role in suppressing the feedback noise. Then, after the mode hopping, the critical feedback value suddenly dropped to \(-19\) dB at the beginning of the blue-detuned region, even lower than that of the 10-mA pump. At a higher current, both lasers quickly entered a chaotic state when the feedback was beyond the critical feedback value. The performance of the two lasers showed an identical trend when the lasing wavelength swept across the Bragg peak, which suggests evidently that wavelength detuning has a significant influence on OFT.  
Figure 4: The lasing wavelength and critical feedback value at a different bias current of **(a)** Laser 1 with the mode hoping point at 53 mA, and the inset depicts the detuning relative to the Bragg reflection spectrum. **(b)** Laser 2 with the mode hoping point at 27 mA. Gray shading indicates the critical feedback value exceeded the measurement upper limit of \(-9\) dB.  
The remarkably high feedback tolerance at the red-detuned region of the DBR laser can be a very promising property for isolator-free operation. One can also note that, although the OFT in the blue-detuned region is not as good as that of the red-detuned region, it is still better than that of most DFB lasers [12].