40  GHz pulse source based on cross-phase modulation-induced focusing in normally dispersive optical fibers

Abstract

We theoretically and experimentally investigate the design of a high-repetition rate source delivering well-separated optical pulses due to the nonlinear compression of a dualfrequency beat signal within a cavity-less normally dispersive fiber-based setup. This system is well described by a set of two coupled nonlinear Schrödinger equations for which the traditional normally dispersive defocusing regime is turned in a focusing temporal lens through a degenerated cross-phase modulation process (XPM). More precisely, the temporal compression of the initial beating is performed by the combined effects of normal dispersion and XPM-induced nonlinear phase shift yield by an intense beat-signal on its weak out-of-phase replica co-propagating with orthogonal polarizations. This adiabatic reshaping process allows us to experimentally demonstrate the generation of a 40-GHz well-separated 3.3-ps pulse train at 1550 nm in a 5-km long normally dispersive fiber.The ability to design optical pulse sources at repetition rates of tens of GHz is of a high interest in many fields of photonics including optical communications, sampling, metrology, clocking, sensing, spectral comb or arbitrary waveform generation. In order to develop alternative solutions to traditional mode-locked fiber lasers, numerous cavity-less scenarios have been investigated based on the nonlinear reshaping within optical fibers of an initial beat signal into a train of well-separated pulses [1]. Basically, the nonlinear temporal compression of the initial beating is induced through the focusing regime of the nonlinear Schrödinger equation (NLS), taking advantage of the interplay between the nonlinear Kerr effect and the anomalous dispersive regime [2]. This particular technique has been demonstrated in a wide range of fiber arrangements to produce pedestal-free pulse trains at various repetition rates, ranging from GHz to several THz [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16] Nonetheless, it is noteworthy that even though various techniques of nonlinear compression of a beat signal have been reported in anomalous dispersive optical fibers, only a few exist for normally dispersive fibers. For instance, in two-stage techniques, it is known that an incident optical pulse can be first chirped through self-phase modulation, cross-phase modulation or through an opto-electronic phase modulator and then subsequently compressed by means of a dispersive element inducing an opposite sign of chirp such as a grating or a suitably designed fiber segment [17][18][19]. However, a less exploited process combining both the nonlinear reshaping and chirp compensation stages can be provided in the defocusing regime of the NLS equation by means of a degenerate cross-phase modulation phenomenon [20][21][22][23][24]. Indeed, it was shown that a XPM-induced focusing of a probe beam can occur in the defocusing regime when it co-propagates orthogonally polarized and bounded between two intense pump pulses travelling at the same group velocity [20][21][22]. In this configu...

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