High power integrated laser for microwave photonics Jörn P. Epping1, Ruud M. Oldenbeuving1, Dimitri Geskus1, Ilka Visscher1, Robert Grootjans1, Chris G.H. Roeloffzen1, and René G. Heideman1 1LioniX International BV, P.O. Box 456, 7500 AL, Enschede, The Netherlands. j.p.epping@lionix-int.com Abstract: We present a hybrid integrated laser with two gain sections coupled to one tunable cavity. The resulting laser has a record on-chip power of up to 20.7 dBm and an intrinsic linewidth of 320 Hz. © 2020 The Author(s) 1. Introduction Tunable semiconductor lasers with a narrow linewidth have found a wide range of important applications, reaching from coherent communications, remote optical sensing such as Lidar to arising applications such as microwave photonics (MWP). Especially, integrated microwave photonics [1,2] aims to achieve RF-to-RF signal processing in the optical domain with a small footprint while offering wide bandwidths. Recently, we developed a hybrid integrated photonic platform [3] based on active InP components such as gain sections, modulators, and detectors with passive silicon nitride-based TriPleX enabling low loss optical signal processing due to its low propagation losses (< 0.1 dB/cm). The combination of these two platforms enables fully integrated RF-to-RF analog photonic links (APL). The link gain of an APL quadratically depends on the sensitivity of the modulator, responsivity of the photodetector (PD) as well as the optical power at the receiving PD [2]. Therefore, to achieve an efficient APL with a high link gain an optical input power of typically more than 20 dBm is needed. Furthermore, high optical powers can keep the noise figure at a minimum by making the use of additional amplifiers obsolete. However, so far current hybrid as well as heterogeneously integrated lasers lack sufficient optical power to achieve such efficient APLs. Here, we present, to the best of our knowledge, the first hybrid integrated laser consisting of more than one InP gain sections sharing a common laser mirror. The resulting on-chip power of 20.7 dBm using two gain sections at a pump current of twice 300 mA is the highest optical power of a hybrid integrated laser to date. Furthermore, the laser shows a low intrinsic linewidth of 320 Hz as well as a low relative intensity noise (<-160 dBc/Hz), which should enable future APL with a high link gain and a low noise level. 2. Device design The scheme and a photograph of the dual-gain section hybrid integrated laser is shown in Fig.1 (a) and (b), respectively. The laser comprises of two InP reflective semiconductor optical amplifier (RSOA) chips and a low- loss TriPleX frequency selective mirror chip. The frequency selective mirror is based on a well-known principle of Vernier mirror with two micro-resonators of slightly different length and, hence, free-spectral ranges of 208 GHz and 215 GHz. To achieve a higher optical output power the mirror chip is coupled to two individual InP RSOAs with a length of 700 µm instead of one [4]. The coupling losses from the InP RSOAs to TriPleX laser cavity are estimated to be about 1 dB. Each of the RSOAs has an individual electrical connection to apply a pump current. On the TriPleX chip, thermo-optic tuners are used to set the frequency selective mirror, to control the phases of the RSOAs within the cavity, two tunable Mach-Zehnder couplers are used to combine both output coupling of the cavity and the power in the output waveguide. The output waveguides are coupled to a fiber array with standard polarization maintaining fibers with a coupling loss of 0.5 dB. Figure 1 a) Conceptual design of the dual gain laser cavity consisting of a tunable dual-ring cavity, two gain section with phase tuners, and two tunable Mach-Zehnder couplers (TC) to control the output coupling and output power. b) Photograph of the assembled dual gain laser.