# High Optical Feedback Tolerance of a Detuned DBR Laser for 10-Gbps Isolator-Free Operation
## 1 Introduction  
In pace with the development of access network and photonic integration circuit (PIC), semiconductor lasers are playing a growing important role for their low-cost and volume-production characteristics. Due to the nonlinear dynamics of semiconductor lasers, approximately \(-40\) dB (\(0.01\%\)) of the reflected light injected back into the cavity will start to destabilize the device [1]. Many phenomena will occur under feedback, including the increase in relative intensity noise (RIN), degradation of side mode suppression ratio, period oscillation, coherent collapse, etc. For cost-sensitive applications such as the access network, an optical isolator has to be co-packaged with the laser chip to prevent impairment from the optical feedback, leading to an increased cost of the transceivers. For PICs, the lack of on-chip isolation solutions will also raise a concern for the stability of the PIC comprised of lasers and other integrated components, where multiple reflections are inevitable. Therefore, the motivation to develop feedback-tolerant lasers is increasingly strong in both practical applications and theoretical research. Many schemes have been proposed to increase the stability of the lasers, which includes partially corrugated waveguide laser [2], quantum dot lasers [3], distributed feedback (DFB)+R lasers [4], two-\(\kappa\) distributed Bragg reflector (DBR) lasers [5], distributed reflector (DR) laser [5; 6], etc. These solutions have shown their effectiveness in resisting optical feedback. Among them, the DBR laser is a very promising candidate for colorless optical network units (ONU) in XG(S)-PON and NG-PON2 [7; 8], due to its wavelength tunability and direct modulation capability [5; 9]. A reflection tolerance of \(-7\) dB has been demonstrated for a DBR laser under static continuouswave (CW) condition [5]. However, reports on its dynamic resistance to optical feedback under direct modulation (DM) condition are still lacking in the literature. In the case of DM, the alternation of laser amplitude and the transient behavior of the laser are new sets of parameters to be considered when external optical feedback is introduced [10]. Therefore, additional research is needed.  
In this paper, we investigated the static CW and dynamic DM performance of a 1.3-\(\upmu\)m DBR laser under optical feedback. The relationship between wavelength detuning and feedback tolerance is examined. The research shows that when the DBR laser is red detuned from the Bragg wavelength, a considerable enhancement in optical feedback tolerance can be obtained. Stable CW operation with an optical feedback level of up to \(-9\) dB has been demonstrated for the red-detuned DBR laser. While for the case of direct modulation at 10 Gbps, a back-to-back (BtB) bit error rate (BER) of \(1\times 10^{-9}\) and \(3\times 10^{-7}\) has been obtained under a feedback level of \(-15\) dB and \(-9\) dB, respectively. After 20 km fiber transmission, the BER still maintains below \(1\times 10^{-7}\) under a feedback level of \(-15\) dB. The power penalty induced by optical feedback is 0.61 dB and 0.75 dB, respectively, for the case of BtB and 20 km fiber transmission under a feedback level of \(-15\) dB.