Multiple mode-locked regimes of an Er/Yb double-clad fiber laser based on NPR

During the last decade, mode-locked (ML) lasers have been attracting attention for their development in fiber-based cavities [1, 2]. Fiber laser technology meets the demands and requirements for a wide range of applications due to the advantages of fiber systems such as compactness, high-quality beam generation, relatively low cost and capacity to produce ultra-short optical pulses with high peak powers and repetition rates. In the framework of photonic applications, pulses of ultra-short duration have become essential in many research areas such as fiber communications, optical instrumentation, and particularly in nonlinear optics for THz wave generation, comb generation, soliton generation [3–6] and many others.

Passively ML (PML) fiber lasers can operate in single-, dual- [7–9] or even multi-wavelength [10] emission. When ML single- or dual-wavelength laser emission is achieved, the laser system can target more complex applications worth of study. In case of PML lasers, the properties of the pulsed emission are determined by the performance characteristics of saturable absorber (SA) materials and devices. In this regard, a wide variety of novel materials have been recently investigated for their use as SA elements in order to obtain ultra-fast and ultra-short pulsed laser emission in fiber lasers, based on the SA nonlinear absorption response [11].

There is a large variety of saturable absorption materials that can be used in PML fiber lasers. In the last decade, SA materials such graphene [12], carbon nanotubes [13], 2D semiconductors materials [14, 15], and topological insulators [16] have been reported. Besides those materials, nonlinear optical phenomena have proved to be useful to emulate the saturable absorbing response in fiber-based systems. Then, artificial SA devices and techniques such as nonlinear optical loop mirrors, nonlinear amplifying loop mirrors (NALM) [17, 18] and nonlinear polarization rotation (NPR) [19] can be implemented in a fiber ring cavity approach along with other in-fiber polarization optical devices for generating of laser pulses with duration from picoseconds to femtoseconds.

In 2000 Okhotnikov and Guina [20] reported single- and dual-wavelength operation in a ML Er-doped fiber laser by using a semiconductor Fabry–Perot saturable absorber. In 2009 Song et al [21] experimentally demonstrated switchable dual-wavelength operation of a PML Er-doped ring fiber laser by NPR effect, achieving ultra-short laser pulses of picoseconds duration. The reported laser operates in dual-wavelength mode with the possibility to switch to each wavelength individually, with wavelength tuning by adjustment of the polarizers. In 2010 Luo et al [22] reported a tunable multi-wavelength ML fiber ring laser based on a birefringence-induced comb filter and NPR technique, achieving from one up to four lasing wavelength lines and pulses of the order of picoseconds. In 2017 Szczepanek et al [23] reported a ML laser using nonlinear polarization evolution technique in polarization maintaining fibers (PMFs), generating femtosecond pulses based on Kerr nonlinearity, where the intensity-dependent angle of rotation and transmission forms an artificial SA.

In the context of PML fiber lasers based on the use of an artificial SA, different laser regimes can be obtained based on the phenomena and effects derived from the SA nonlinear response. One of the nonlinear effects is the optical soliton formation in a dispersive and nonlinear optical fiber. Moreover, the interaction of soliton formations can leads to more complex phenomena in fiber lasers resulting in different regimes such as dissipative solitons, multiple soliton states (crystal, liquid, gas etc) and noise-like pulses (NLPs) [24, 25]. In case of these regimes, temporal and spectral profiles depend on a wide range of intracavity parameters. Hence, a variety of pulse profiles can be achieved by adjusting parameters of the optical cavity such as polarization, dispersion, and gain [26]. Particularly, soliton bound states or soliton molecules are interesting dynamics of solitons [27–31]. Different techniques have been developed to characterize their behavior. This is one of the reasons why studying of ML fiber lasers is still a worthwhile endeavor.

Furthermore, erbium as rare-earth dopant in fibers has been of great importance since the development of the first Er-doped fiber amplifier in the 90 s [32]. Since then, many efforts have been made in order to enhance the performance of Erbium-doped fiber (EDF) amplifiers as the fiber technology expands. In this regard, the appearance of the double clad fibers [33] alongside with laser diode (LD) technology have enabled the development of novel fiber lasers and fiber amplifiers covering a wide range of applications which require high power handle in multimode fiber systems where beam quality can be compromised. However, a drawback of EDF systems is the high concentration effect which can cause clustering in pairs leading to population inversion reduction [34]. In order to avoid the clustering effect, EDFs are usually co-doped with ytterbium dopant ions. In this case, the energy transfer between erbium and ytterbium along with the LD provides an efficient pumping mechanism which leads to provide higher output powers from fiber lasers. The double-clad fiber design allows cladding pumping techniques in which the LD power is guided through the inner cladding whereas the signal propagates in the core where the active ions are present to produce the energy transfer between Erbium and Ytterbium. Then, the laser performance can be enhanced. Therefore, utilizing an Er/Yb co-doped fiber as the gain medium is also a reliable alternative for designing PML fiber lasers since strong fiber nonlinearities can be exploited with high power in optical systems. In 2016 Semaan et al [35] reported a passive ML figure-of-eight fiber laser using a double-clad Er/Yb fiber (EYDCF) as gain medium, achieving width and amplitude tunable dissipative soliton resonance (DSR) square pulses. In 2016 Krzempek et al [36] reported an EYDCF ML fiber laser by using a NALM to generate DSR pulses with a maximum duration of 455 ns. Du et al [37] reported ML pulses in the DSR regime in a lineal cavity.

In this paper we exploit the use of EYDCFs as gain medium and the NPR technique as adjustable SA mechanism in order to modulate the losses within a fiber ring cavity fiber laser. As a result, generation of ultra-fast pulses organized in soliton molecules is obtained. By carefully adjusting the polarization state of a couple of wave retarders, a wave retarder position can be found in which steady emission of quasi continuous wave (CW) pulses, molecules of solitons, and single-wavelength NLPs ML regimes are observed.

The experimental setup of the proposed ML fiber laser is shown in figure 1. The laser cavity includes a 1.5 m long segment of EYDCF (CorActive, DCF-EY-10/128) used as gain medium. The EYDCF is pumped by a multi-mode laser source at 976 nm through a (2 + 1) × 1 pump combiner. Unidirectional propagation of the laser is achieved by using a polarization-dependent optical isolator (PD-ISO) within the laser cavity. A half-wave retarder (HWR) plate was used to control the angle of the polarization ellipse at the PD-ISO input. A quarter-wave retarder (QWR) plate was used to control the states of the mode-locking performance. A 70/30 coupler provides the laser output port used to characterize the laser operation.

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The total length of the laser cavity of approximately 9.5 m, corresponds to the 1.5 m EYDCF, the 5.5 m of SMF-28 single-mode fiber (SMF) covering the fiber components and the 1 m long pigtails of polarization maintaining fiber (PMF) incorporated on each side of the PD-ISO. The anomalous dispersion values of the EYDCF, PM fiber, and SMF-28 are −19 ps² km⁻¹, −23 ps² km⁻¹ and −21 ps² km⁻¹, respectively. Then, the cavity net dispersion was calculated as −0.199 ps².

An initial set of measurements was performed to test the performance and characteristics of the fiber laser. The experimental results showed that ML operation can be achieved by the proper rotation of the HWR and the QWR, yielding a variety of regimes and pulse characteristics, such as quasi-CW, molecules of solitons, and NLP emissions. Therefore, we have structured our analysis into three sections, one for each observed regime.

Quasi-CW emission is achieved by properly adjusting the rotation angle of the HWR and the QWR. Figure 2(a) shows the measured optical spectrum evolution with a resolution of 50 pm (OSA, Yokogawa AQ6375). For this measurement the pump power was fixed at 3 W and the output spectrum was recorded every 5 min over a total time window of 45 min. The spectrum shows two CW lines centered at 1544.55 nm and 1544.68 nm.

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The width of a single pulse was determined by measuring the autocorrelation train as it is shown in figure 2(b). The inset of figure 2(b) shows a zoomed view of the trace were the pulse is located. As it can be observed the measured width is of 5.69 ps. The pulse train was measured using a 12.5 GHz bandwidth detector with 28 ps rise/fall time (Electro-Optics Technology, Inc. ET-5000) and recorded on a 20 GHz oscilloscope, see figure 2(c). The measured repetition rate is of 21.7 MHz, which allows determining that the optical oscillator operation is of one pulse per round trip. Once the position of the wave retarders is fixed, the laser self-starts when the pump power reaches the level of 3 W. The radio frequency characteristics of the laser pulse is investigated by measuring RF spectrum of the laser emission (Agilent E 4407B ESA-E spectrum analyzer) with resolution of 10 Hz, as it is shown in figure 2(d). The fundamental peak is located at the repetition rate of 21.7 MHz which exhibits signal-to-noise ratio (SNR) of ∼36.6 dB, which indicates a relatively steady ML pulse train.

After obtaining the quasi-CW emission, the rotation of the HWR and the QWR were carefully adjusted to generate stationary soliton molecules. The performance of the generated pulses of soliton molecules is shown in figure 3. As can be observed in figure 3(a), the soliton molecule spectrum is centered at the wavelength of 1544 nm and exhibits typical modulation of soliton molecules with a period of 0.49 nm, which corresponds to the soliton separation of 15.38 ps (see figure 3(b)). From figures 3(a) and (b) it can be observed that soliton molecule state with a pulse pair is obtained [38]. The pulse width of the individual solitons in the pair is ∼5.9 ps. The two side peaks in the autocorrelation trace correspond to the two-soliton molecules [39–41].

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In order to achieve NLP regime of the ML laser emission, the HWR and QWR plates were adjusted, in addition with a required slight mechanical stimulation on the EYDCF. The measurement of a smooth optical spectrum and of a double-scale autocorrelation trace reveals the presence of the NLP regime. Once ML lasing is achieved in NLP regime, the pulse characteristics cannot be varied through wave retarders adjustments. Instead, if the positions of the retarders are varied, NLP regime is lost and is not recovered through mechanical stimulation unless the wave retarders are rotated back into the initial ML NLP regime positions. When the NLP regime is lost, mode-locking is obtained with the characteristics of quasi-CW operation or soliton molecules, as observed in figures 2 and 3, respectively. The ML emission of the laser in NLP regime is shown in figure 4. The NLP regime exhibits pulse trains with constant repetition rate of 46 ns, which matches the round-trip time of the 9.5 m long cavity (see figure 4(a)). The autocorrelation function shown in figure 4(b) presents a double-scaled structure, with a narrow peak of ∼210 fs (see inset of figure 4(b)) riding a wide pedestal with roughly triangular shape extending over the 200 ps measurement window of the autocorrelator. A broadband spectrum extending over 70 nm is obtained, with a central wavelength near 1545 nm and a full width at half maximum (FWHM) bandwidth of ∼20 nm as shown in figure 4(c). The spectrum presents a periodic modulation, which is due to the filtering effect introduced by the PMF and PD-ISO. The measured RF spectrum is shown in figure 4(d), the fundamental peak is located at the cavity repetition rate of 21.7 MHz with SNR of ∼37 dB. Although lateral peaks, corresponding to relaxation oscillations, are observed at 140 kHz on each side, their peak amplitude is 30 dB below the main intensity peak. The obtained results corroborate typical noise-like characteristic of the generated pulses.

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In conclusion, we experimentally investigated the generation of quasi-CW, molecules of solitons, and NLP ML regimes in a ring cavity EYDCF laser by using NPR technique. The different ML regimes are obtained by the proper rotation adjustment of a HWR and a QWR. Quasi-CW ML operation with pulse width of 5.69 ps is achieved. The quasi-CW operation was verified by its frequency spectrum whose fundamental peak exhibited repetition rate of 21.7 MHz and SNR of ∼37 dB. Soliton molecules with typical modulation of soliton molecules with a state of soliton pair formed by two pulses were observed. The wavelength modulation of 0.49 nm of the soliton molecules corresponds to 15.38 ps temporal separation. Finally, ML laser emission in NLP regime was obtained. Pulse trains with period of 46 ns were observed. Double-scale autocorrelation reveals a peak of ∼210 fs riding a wide pedestal with a triangular shape of the order of 200 ps. This work contributes to verify and experimentally understand the broad range of regimes emerging from the PML fiber lasers based on the use of a EYDCF as gain medium.

This work was supported by CONACyT project grant CB-256401. M Durán-Sánchez wants to thanks to Cátedras CONACyT program. R I Alvarez-Tamayo was supported by Fondo de Investigación UPAEP 2020 project.

All data that support the findings of this study are included within the article (and any supplementary files).