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11Comb generation in fiber laser with integrated ring microcavity
Yu. Gladush1*, A. Mkrtchyan1, A. Netrusova1, M. Mishevsky1, Z. Ali, N. Dmitriev, S. Minkov, I. Bilenko2, A. Nasibulin1
1- Skolkovo institute of science and technology, Moscow, Russia 2- Russian quantum center, Moscow, Russia
Optical combs generator, a device that provide a wide spectrum consisting of narrow equidistant lines, finds many applications, including optical clocks, coherent communications, optical coherence tomography, dual comb spectroscopy, etc. Traditional source of the comb is a mode-locked laser, for which a distance between comb lines (FSR) is defined by length of the resonator and usually reaches tens of MHz. Another approach for comb generation is based on the integrated microring resonators with high quality factor exceeding 106 [1]. Here the microring is pumped by CW light in one of the resonances which leads to the comb formation due to four wave mixing mechanism. This type of comb usually provides FSR from 50 GHz to 1 THz, but suffers from low CW to comb conversion efficiency, requires expensive sweeping laser and suffers from low stability.
One of the solutions to overcome these limitations suggests integrating microcavity into the fiber laser resonator. By merging the properties of fiber laser and microcavity it was demonstrated 150 nm wide soliton combs with 75% pump to comb efficiency [2]. In these work authors used four-port ring resonator inside fiber laser ring cavity. Emission from the fiber laser propagated through one buss waveguide to the microcavity and returned from another drop-port waveguide.
We suggest significant simplification of the scheme by utilizing the two-port integrated chip with microcavity working as a reflecting mirror in fiber laser resonator (see Fig. 1a). Here laser resonator is formed by the gold mirror on one side and microring resonator on the other. In the microring directly propagating comb give rise to the counter propagating comb due to Rayleigh scattering, which goes back into the laser cavity forming an optical feedback. With this approach we show robust self-starting soliton comb generation with spectral width more than 400 nm, which greatly exceeds erbium amplification window (see Fig. lb).
Gold mirror
PM Er-doped fiber
Laser diode
PM-WDM 980/1550
Integrated microcavity mirror
PM-lensed fiber
Fig. 1. (a) Scheme of the fiber laser with microring cavity (not in scale); (b) Spectrum of the optical comb on the output of laser.
[1] T.J. Kippenberg, R. Holzwarth, S.A. Diddams, Microresonator-Based Optical Frequency Combs, Science 80., 332, 6029, 555-559, (2012).
[2] H. Bao, et al, Laser cavity-soliton microcombs, Nat. Photonics 136, 13, 6, pp. 384-389, (2019).
