Integrated CMOS-Compatible Q-Switched Mode-Locked Lasers at 1900Nm with an On-Chip Artificial Saturable Absorber
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Integrated CMOS-compatible Q-switched mode-locked lasers at 1900nm with an on-chip artificial saturable absorber The MIT Faculty has made this article openly available. Please share how this access benefits you. Your story matters. Citation Shtyrkova, Katia, et al. “Integrated CMOS-Compatible Q-Switched Mode-Locked Lasers at 1900nm with an on-Chip Artificial Saturable Absorber.” Optics Express 27, 3 (February 2019): 3542 © 2019 Optical Society of America As Published http://dx.doi.org/10.1364/OE.27.003542 Publisher The Optical Society Version Final published version Citable link https://hdl.handle.net/1721.1/124414 Terms of Use Article is made available in accordance with the publisher's policy and may be subject to US copyright law. Please refer to the publisher's site for terms of use. Vol. 27, No. 3 | 4 Feb 2019 | OPTICS EXPRESS 3542 Integrated CMOS-compatible Q-switched mode-locked lasers at 1900nm with an on-chip artificial saturable absorber 1,2 1 1,3 KATIA SHTYRKOVA, PATRICK T. CALLAHAN, NANXI LI, EMIR SALIH 1,4 1 1 MAGDEN, ALFONSO RUOCCO, DIEDRIK VERMEULEN, FRANZ X. 1,5 1 1 KÄRTNER, MICHAEL R. WATTS, AND ERICH P. IPPEN 1Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139, USA 2Presently with MIT Lincoln Laboratory, 244 Wood Street, Lexington, MA 02421, USA 3John A. Paulson School of Engineering and Applied Sciences, Harvard University, 9 Oxford Street, Cambridge, MA 02138, USA 4Current address: Koç University, College of Engineering, Department of Electrical and Electronics Engineering, Rumelifeneri Yolu Sarıyer 34450 Istanbul, Turkey 5Center for Free-Electron Laser Science at Deutsches Electronen Synchrotron (DESY), Department of Physics and Center for Ultrafast Imaging, University of Hamburg, Hamburg 22761, Germany *[email protected] Abstract: We present a CMOS-compatible, Q-switched mode-locked integrated laser operating at 1.9 µm with a compact footprint of 23.6 × 0.6 × 0.78mm. The Q-switching rate is 720 kHz, the mode-locking rate is 1.2 GHz, and the optical bandwidth is 17nm, which is sufficient to support pulses as short as 215 fs. The laser is fabricated using a silicon nitride on silicon dioxide 300-mm wafer platform, with thulium-doped Al2O3 glass as a gain material deposited over the silicon photonics chip. An integrated Kerr-nonlinearity-based artificial saturable absorber is implemented in silicon nitride. A broadband (over 100 nm) dispersion- compensating grating in silicon nitride provides sufficient anomalous dispersion to compensate for the normal dispersion of the other laser components, enabling femtosecond- level pulses. The laser has no off-chip components with the exception of the optical pump, allowing for easy co-integration of numerous other photonic devices such as supercontinuum generation and frequency doublers which together potentially enable fully on-chip frequency comb generation. © 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement 1. Introduction High repetition-rate ultrafast mode-locked lasers (MLL) have unique advantages for applications such as photonic analog-to-digital converters, comb spectroscopy, optical arbitrary waveform generation and low-noise microwave synthesis. Traditionally, repetition rates beyond 1 GHz were achieved by either active modulation techniques [1,2], which restricted the pulse duration to more than a few picoseconds, with nonlinearity-induced optical bistability where multimode noise suppression was necessary for a stable operation [3,4], or by introducing a semiconductor saturable absorber, in which case the pulse duration also remained more than a few picoseconds [5–7]. Alternatively, passive mode-locking techniques have been shown to generate femtosecond-level pulses at high repetition rates when used with some form of an external repetition-rate multiplier to bring the system into the GHz-level regime [8,9]. Harmonic mode-locking, where several pulses circulate in the cavity at the same time, has also been used to demonstrate high repetition rates [10]. In all of the above cases the architecture of the laser system itself was based on a fiber laser or a diode laser, with the rest of the cavity constructed with free-space optical components. Such systems could significantly benefit from an on-chip implementation which would #353193 https://doi.org/10.1364/OE.27.003542 Journal © 2019 Received 29 Nov 2018; revised 10 Jan 2019; accepted 10 Jan 2019; published 31 Jan 2019 Corrected: 06 February 2019 Vol. 27, No. 3 | 4 Feb 2019 | OPTICS EXPRESS 3543 dramatically reduce the footprint, enable scalable mass-production, eliminate moving elements, and reduce the cost of fabrication. Moreover, the compact size of the gain cavity integrated on chip together with an on-chip passive mode-locking technique could provide GHz-level pulse repetition rates with pulse durations in the femtosecond regime without any external repetition rate multiplication [11,12]. Recent growth in integrated photonics capabilities enables such an on-chip MLL implementation in principle. Moreover, recently demonstrated wafer-scale electronic-photonic integration enables the possibility of on-chip control loops with nanosecond response time [13,14] which, when designed together with on- chip MLLs, could lead to demonstrations of fully-stabilized and fully-on-chip frequency combs and ultra-low-noise microwave oscillators. Silicon nitride (SiN) on a silicon-on-insulator (SOI) platform is an excellent candidate for on-chip mode-locked lasers due to its low passive waveguide losses, appreciable Kerr nonlinearity, and the lack of two-photon-absorption at near infrared wavelengths [15,16]. Numerous passive photonic components necessary for integrated MLLs have already been demonstrated in this fabrication platform. These devices include wavelength filters/couplers, mode-locking elements, and integrated diffraction gratings [17,18]. The remaining challenge is an integrated electrically pumped light source, either as a continuous wavelength (CW) laser, or as a pump for an on-chip laser. One method used to achieve this involves III-V-based laser bonding to the SOI chip. More commonly an off-chip optical pump source is used for this task. For SiN on SiO2, optically pumped gain may be incorporated on-chip, for example, in a form of a rare-earth-doped film deposited on top of the SiN/SiO2 layers. In fact, such an approach has been successfully developed over the past 10 years [19–21], borrowing from the extensive experienced obtained with rare-earth-based fiber lasers. With a well-developed on- chip gain platform and demonstrated passive components necessary for mode-locking, it is evident that on-chip high repetition rate MLLs will follow a similar development path to that of the conventional lasers, proceeding along the path from a continuous wavelength to Q- switched, then to Q-switch-mode-locked, and ultimately to CW mode-locked devices. The lasers in this work were developed for DARPA’s Direct-On-chip-Digital-Optical Synthesizer (DODOS) program, the goal of which is to create a fully-integrated ultra-stable optical frequency synthesizer in the communications C-band, referenced to an external 10MHz clock source. MIT’s approach utilizes a tunable single frequency laser that is locked to a self-referenced frequency comb, the repetition rate of which is in turn locked to the external RF source. The on-chip octave-spanning continuum generation, second-harmonic generation, and the octave-wide spectral filter required for this approach have previously been reported [22–24]. Here we describe demonstrations of compact optically pumped on-chip Q-switched and mode-locked lasers in a CMOS-compatible Si3N4 platform, with thulium-doped aluminum oxide glass used as a gain material. The lasers operate near 1900nm and have no off-chip components other than the optical pump source. The first demonstration of such a laser was achieved in our group in 2017 [25]. The laser had a footprint of 25 × 12 mm, cavity repetition rate of 680 MHz, a Q-switching rate of about 1 MHz, and pulse duration of 1.2 ps. In this work we demonstrate a significantly improved laser architecture that allows for truly compact devices (23.5 × 0.78 mm) and achieves a cavity repetition rate of 1.2 GHz, with bandwidth that would support 215fs pulses. 2. Laser architecture and fabrication platform The architecture of the MLL is shown in Fig. 1. This laser consists of three gain section segments connected using compact Si3N4 bends. A wide photonic trench is etched in the entire gain region of the laser, shown as the shaded area in Fig. 1. The purpose of the trench is to allow the gain material deposited on top of the chip to come into close contact to Si3N4 layers and form the gain waveguide. This laser is pumped with an external 1614 nm L-band Erbium-doped fiber amplifier (EDFA). The pump light enters the lasing cavity via a Vol. 27, No. 3 | 4 Feb 2019 | OPTICS EXPRESS 3544 pump/signal combiner component (Section 3.1) shown in the upper left in Fig. 1. The gain waveguide consists of three nearly straight, approximately 2cm-long sections where the pump and the signal modes have large overlap with the gain material (Section 3.2). These three gain waveguide sections are connected by compact Si3N4 bends (Section 3.3). Prior to each bend, both the pump and the signal are transferred from a large-mode-area gain waveguide into a silicon nitride layer with a much more confined optical mode. This makes it possible to achieve tight 180-degree bends without significant losses. Following the three gain sections, the 1900nm signal is again transferred to the Si3N4 layer as it enters a Kerr-nonlinearity-based mode-locking element, which we refer to as a nonlinear interferometer (NLI). The NLI acts as a CW reflector for 1900nm at low incident optical power.