Contents lists available at ScienceDirect Optics and Laser Technology journal homepage: www.elsevier.com/locate/optlastec Full length article Recent advances in stabilization of mode-locked quantum dash lasers at 1.55 µm by dual-loop optical feedback Haroon Asghar a,b, ⁎ , Ehsan Sooudi c , Muhammad Aslam Baig b , John Gerard McInerney d,e a Atomic and Molecular Physics Laboratory, Department of Physics, Quaid-i-Azam University, Islamabad 45320, Pakistan b National Center for Physics, Quaid-i-Azam University Campus, Islamabad 45320, Pakistan c Infinera Corporation, 140 Caspian Court, Sunnyvale, CA 94089, USA d Department of Physics and Tyndall National Institute, University College Cork, Western Road, Cork, Ireland e College of Optical Sciences, University of Arizona, Tucson, AZ 85721, USA HIGHLIGHTS • Stabilization of self-mode-locked two-section Quantum-Dash Lasers using dual-loop optical feedback schemes. • Measurement of feedback ratio required to stabilize the QDash mode-locked lasers. • Determination of RF linewidth vs delay and its comparison for various feedback confgurations. • Identifcation of optimal feedback confguration required to stabilize the Quantuum Dash Mode-locked lasers. ARTICLEINFO Keywords: Mode-locked lasers Single-loop feedback Dual-loops optical feedback RF-linewidth Integrated timing jitter ABSTRACT This paper reports the efectiveness of a variety of single- and dual-loop optical feedback arrangements on stability of self-mode-locked two-section quantum dash lasers emitting at ≈1.55 µm and operating at 21 GHz repetition rate. We describe reduction of RF linewidth and timing jitter using fve distinct schemes, including single and dual loops with symmetric and asymmetric lengths, and with balanced and unbalanced feedback ratios. All feedback schemes described are efective in stabilizing the pulse trains from SML QDash lasers, but some require precisely tuned resonance between loop delay and laser cavity. We show balanced asymmetric dual-loop optical feedback is the most robust, cost-efective and low-noise method to stabilize and control pulses from mode-locked lasers and optoelectronic oscillators. 1. Introduction Semiconductor Mode-Locked Lasers (MLLs) with low timing jitter and high repetition rates are very valuable as coherent frequency comb sources for coherent optical fber communications [1], signal proces- sing [2], arbitrary waveform generation [3], all-optical signal proces- sing [4], millimeter wave generation [5], Lidar and other emerging applications. Quantum Dash (QDash) MLLs have attracted recent at- tention due to the many advantages of semiconductor lasers (reliability, efciency, ease of pumping, wide range of wavelengths accessible) plus broad gain spectrum, high thermal stability, low threshold and ultrafast charge carrier dynamics [6]. However, semiconductor lasers are strongly infuenced by external optical feedback and have complex dynamics such as self-pulsation, multistability [7,8], chaos [9,10], and low-frequency fuctuations [11]. In the early 1990’s, Solgaard and Lau [11] demonstrated that weak external optical feedback (−22 dB) with phase matching can be highly benefcial for dynamic characteristics of semiconductor MLLs, reduction in RF linewidth of nearly two orders of magnitude compared to free-running conditions is possible. It was also determined that the efects of optical feedback change periodically as the length of the feedback loop is varied. Picosecond pulse widths from semiconductor MLLs have been demonstrated routinely [12]; although these pulses typically have signifcant chirp and poor timing jitter. The latter parameter is especially important in most applications. To im- prove timing jitter in nanostructured semiconductor MLLs, several ex- perimental methods such as coupled optoelectronic oscillators (OEOs) [13–16], hybrid mode-locking [17], injection-locking [18–21] and ex- ternal optical feedback [22–27] have been proposed and demonstrated. Optoelectronic feedback signifcantly improves the integrated timing jitter by converting optical oscillations into electrical oscillations from a https://doi.org/10.1016/j.optlastec.2019.105884 Received 26 April 2019; Received in revised form 29 July 2019; Accepted 29 September 2019 ⁎ Corresponding author at: Atomic and Molecular Physics Laboratory, Department of Physics, Quaid-i-Azam University, Islamabad 45320, Pakistan. E-mail addresses: haroon.asghar92@gmail.com, haroon.asghar@qau.edu.pk (H. Asghar). Optics and Laser Technology 122 (2020) 105884 Available online 14 October 2019 0030-3992/ © 2019 Elsevier Ltd. All rights reserved. T