Ultralow 192 Hz RF linewidth optoelectronic oscillator based on the optical feedback of mode-locked laser diodes Mohsin Haji, Lianping Hou, Anthony E. Kelly, Jehan Akbar, John H. Marsh, John M. Arnold, and Charles N. Ironside School of Engineering, University of Glasgow, Glasgow, G12 8LT, UK m.haji.1@research.gla.ac.uk Abstract: We present an ultralow RF linewidth (192 Hz) and subpicosecond phase noise (260 fs) using a passively mode-locked quantum-well laser with feedback via a dual optical fiber loop. OCIS codes: (060.2340) Fiber optics components; (060.5060) Phase modulation; (140.5960) Semiconductor lasers; (230.5590) Quantum-well devices; (320.7090) Ultrafast lasers. 1. Introduction Optical self seeding feedback techniques can be used to improve the noise characteristics of passively mode- locked laser diodes (MLLD) [1, 2]. External cavities such as fiber optic cables can increase the phase memory of the MLLD and subsequently improve the timing jitter [3]. In this work, an improved optical feedback architecture is proposed using a dual optical fiber loop delay as a cavity extension of the mode-locked laser. A composite cavity is included for suppressing supermode noise artifacts presented due to harmonic mode-locking effects [4]. Using this configuration, we achieve a RF linewidth of as little as 192 Hz, which is the lowest reported linewidth of any high frequency (>1 GHz) passively mode-locked laser to date, making it promising for the development of low cost, high frequency optoelectronic oscillators. 2. Experimental configuration The experimental setup for the dual optical feedback loop is shown in figure 1. The MLLD was a 20 GHz two- section MLLD, fabricated on a three quantum-well AlGaInAs/InP epitaxial structure [5]. The output of the laser was coupled into a lensed fiber which was fed through a circulator to minimize back reflections. A 3 dB fiber coupler was then used to split the output into two paths, one towards an optical delay line, and the other towards a dispersion shifted erbium doped fiber amplifier (DS-EDFA) followed by a length of dispersion shifted fiber (DSF) before being recombined via a second 3 dB coupler. The signal was then fed into an optical attenuator before being coupled back into the SA end of the MLLD cavity with the TE polarization maximized. The additional output port of the 3 dB coupler was used for subsequent spectral analysis. The total length of the composite cavity (inner fiber loop) was ~22 m, and the outer fiber loop was ~66 m, of which ~48 m was dispersion shifted via the EDFA, attenuator, and an additional length of DSF, resulting in a total dispersion of approximately 200 fs/nm. Fig. 1. Experimental configuration of the dual loop optoelectronic oscillator. (SA: saturable absorber, circ.: optical circulator, DSF: dispersion shifted fiber, atten.: optical attenuator). 3. Results The laser was passively mode-locked when the gain section was forward biased with 83 mA and the SA section was reverse biased with 2.9 V. Figure 2(a) shows the RF spectrum of the output of the dual optical feedback loop with an uncorrelated composite cavity length, (i.e. the modes associated with the inner loop were not aligned to the modes of the outer loop). When the composite cavity length was optimized using the variable optical delay line,