IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 21, NO. 6, NOVEMBER/DECEMBER 2015 1501909 Widely Tunable Narrow-Linewidth Monolithically Integrated External-Cavity Semiconductor Lasers Tin Komljenovic, Sudharsanan Srinivasan, Erik Norberg, Michael Davenport, Gregory Fish, and John E. Bowers, Fellow, IEEE Abstract—We theoretically analyze, design, and measure the In this paper we theoretically analyze, design, and measure performance of a semiconductor laser with a monolithically in- the performance of a semiconductor laser with a monolithically ∼ tegrated external cavity. A 4 cm long on-chip cavity is made integrated external cavity. Controlling the feedback from the possible by a low-loss silicon waveguide platform. We show tuning in excess of 54 nm in the O-band as well as significant reduction in integrated external cavity to the laser is essential to improve laser linewidth due to controlled feedback from the external cavity. the performance, just as with an external cavity laser. The ad- The measured linewidth in full tuning range is below 100 kHz and vantages of monolithic integration are improved stability, no the best results are around 50 kHz. Approaches to further improve anti-reflection coating of the laser facet, no external alignment the performance of such laser architectures are described. of mirrors, and lower losses from mode-mismatch. The length Index Terms—Semiconductor lasers, cavity resonators, laser of the external cavity is around 4 cm, which is an order of mag- tuning, photonic integrated circuits. nitude longer than the laser cavity. By optimizing the feedback from such a cavity, we are able to improve the laser performance and show wide tuning in excess of 54 nm with linewidths below I. INTRODUCTION 100 kHz across the full tuning range, and best linewidths of HOTONIC integration brings a promise of significant cost, 50 kHz. P power and space savings in today’s optical data transmis- The paper is organized as follows. In Section II we briefly sion networks and sensor applications. Monolithic integration outline the theory of laser feedback and link it to specifics of using heterogeneous processes assembles many devices or opti- the monolithically integrated external cavity. In Section III we cal functionalities on a single chip so that all optical connections give a theoretical analysis of the influence of the external cavity are on chip and require no external alignment and has the further and its influence on laser linewidth, modulation response and promise of improved performance, which we demonstrate here. relative intensity noise (RIN). Measured results are presented in Monolithic integration has been demonstrated on both in- Section IV, and finally in Section V, we conclude the paper and dium phosphide (InP) and silicon (Si) substrates. Integration on give several suggestions on further improving the performance Si substrates has been a major area of research in recent years of lasers with a monolithically integrated external cavity. [1] with goals of lower cost and higher volume manufactur- ing. Heterogeneously integrating III-V materials on silicon also improves individual device performance, mainly due to lower II. EXTERNAL CAV I T Y LASERS losses and better lithography. As optical communications shift A. Overview to more complex modulation formats, narrow linewidth lasers become a necessity. For example, a 16-QAM modulation for- Semiconductor laser behavior, as is very well known, can be mat requires a laser linewidth <300 kHz [2]. Furthermore for significantly affected by external optical feedback. In most situ- DWDM based systems, lasers have to be tunable to align to a ations, one wants to suppress the feedback as much as possible, certain grid. Tuning can also be exploited in switching scenarios but controlled feedback can improve certain laser parameters and for improving network resilience to downtime and typically and has therefore been extensively studied. requires tuning across the communication band. Sensors are an- The study of the effects of optical feedback on semiconductor other area that can benefit from tunable, narrow-linewidth lasers. lasers is made more difficult by at least three things [17]: (i) the There have been many results prior to this work addressing this broad gain spectrum which permits higher-order longitudinal need [3]–[16] and we compare to them in the measurement mode interaction with small changes in feedback conditions, section. (ii) strong dependence of crystal refractive index on tempera- ture, and (iii) strong dependence of the gain medium refractive Manuscript received January 27, 2015; revised March 26, 2015; accepted index on the excited carrier density. Furthermore, the effects of April 10, 2015. This work was supported by a DARPA EPHI contract. The work of T. Komljenovic was supported by NEWFELPRO Grant No. 25. optical feedback are expected to differ depending on the dis- T. Komljenovic, S. Srinivasan, M. Davenport, and J. E. Bowers are with tance between the laser diode and the external reflector. For the University of California Santa Barbara, Santa Barbara, CA 93106 USA distances smaller than the coherence length, the laser and the (e-mail: [email protected]; [email protected]; davenport000@ gmail.com; [email protected]). external reflector behave as a compound cavity. Depending on E. Norberg and G. Fish are with Aurrion Inc., Goleta, CA 93117 USA specific conditions, optical feedback has been shown to make (e-mail: [email protected]; greg.fi[email protected]). the laser multi-stable, have hysteresis phenomena, enhance, pro- Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. long or suppress the relaxation oscillation in the transient out- Digital Object Identifier 10.1109/JSTQE.2015.2422752 put, improve the laser’s performance through noise suppression, 1077-260X © 2015 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications standards/publications/rights/index.html for more information. 1501909 IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 21, NO. 6, NOVEMBER/DECEMBER 2015 reduced nonlinear distortion and modulation bandwidth en- hancement and, what is most relevant to our study, improve the laser linewidth [18]–[23]. The effects of feedback are typically divided into five dis- tinct regimes with well-defined transitions [24]. The classical paper from 1980 has been revisited recently and expanded [25]. It is shown that the change of amplitude signal (ΔE) always accompanies that of frequency change (Δv) due to feedback. The dependence is sinusoidal for weak feedback (ΔE ≈ E0 · cos(2kL),2kL being the optical phase associated with distance to external reflector) and becomes more com- plicated leading to switching, hysteresis and chaos for higher levels of feedback. The full dynamics are complex and depend Fig. 1. A schematic view of a tunable laser design with integrated external on many laser parameters such as gain, loss, photon and car- cavity. Tuners are yellow (two phase sections and two rings for wide tuning), and SOAs are dark orange (SOA1 is the laser active section, and SOA2 is used as rier lifetime and, above all, the linewidth enhancement factor an ON/OFF switch or to control the level of feedback). The monitor photodiode αH . Nevertheless, we utilize this dependence of laser output (MPD) in blue is used to measure the laser output power for adjustment of laser power (P ≈ E2 ) on the phase of feedback signal as an easier parameters. way of estimating feedback by monitoring the output power via integrated monitor photodiode. The feedback behavior can further be separated in four dis- lasing wavelength is determined in the tuning section compris- tinct regions: short versus long cavities and coherent versus ing two ring resonators and a cavity phase section, all of which incoherent feedback. The latter condition relates the external are controlled by thermal phase tuners. The front loop mirror cavity length L to the coherence length Lcoh of the unperturbed (at the output of the laser) has a 10% power reflection and the source. In our case, the feedback is always coherent in all cases, output of the laser is terminated at the facet at an angle of 7° to as the external cavity length is ∼4 cm in silicon (around 15 cm minimize reflections. The back loop mirror, after the wavelength in air). It is interesting to point out that semiconductor lasers tuning section, has a power reflection of 60%, which couples are also sensitive to incoherent return signal as it can deplete part of the light to the external cavity. In order to allow for a carriers changing both the gain and the phase inside the cavity. long external cavity, a low-loss waveguide platform is needed. The distinction between short and long cavities is made based Here we utilize optimized silicon waveguides with a loss of ∼ on the relation between the relaxation resonance frequency (fR ) 0.67 dB/cm. The external cavity is 4 cm long and has its own and the external cavity frequency [26] (for short cavities the phase adjustment section and gain section (SOA2). As the prop- length L<c/(2fR )), and the oscillation regime is dependent agation loss in Si waveguides is very low, there is quite strong upon the phase of the external path length. For long cavities (L> feedback from the external cavity (∼5 dB loss for round-trip). c/(2fR )), no dependence of the oscillation regime on phase The feedback is present even when the SOA2 is reverse-biased, should be observed. We return to the distinction of short and possibly from reflections at the tapers to the gain region or due long cavities in the results section. to insufficient attenuation of the reverse-biased SOA2. We re- For narrow-linewidth operation one would typically want to turn to this topic in more detail in the measurement section.