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Abstract
We report an experimental study of passive harmonic mode-locking in an all-fiber switchable dual-wavelength Er/Yb double-clad laser. The proposed scheme supports single- and dual-wavelength operation of mode-locked pulses with rectangular, h-like and trapezoidal shapes in a noise-like pulse regime. Single-wavelength emissions at λ1 = 1545.1 and λ2 = 1563.6 nm were obtained for pump power values of 9.42 and 6.31 W, achieving pulse durations of up to 18 and 11.8 ns, respectively. At an intermediary pump power of 7.5 W, dual-wavelength emission is obtained and pulses of around 3.59 ns are generated. Additionally, the transition dynamics until 4th-order harmonic mode-locking is also observed. Different laser operation regimes of fundamental and different orders of harmonic mode-locking, with rectangular, h-shaped or trapezoidal shaped pulses are obtained with the same laser configuration with simple and well-defined plates and pump power adjustments.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
The operation characteristics of a passively mode-locked fiber laser (PMLFL) allows the generation of coherent and stable waveforms, including conventional and dispersion-managed solitons [1], and a variety of dissipative solitons [2]. Moreover, quasi-stable patterns of solitons involved in dynamics such as soliton rain [3] and noise-like pulses (NLPs) [4] were also demonstrated, even in symbiotic or multiple-soliton (MS) regimes depending of their packet-forming mechanism or their coherence nature [5]. Due to their stability, compactness and ease of use, nowadays PMLFLs have become fundamental devices in optics and materials laboratories around the world. However, the limited average output power originated from the low gain provided by single-mode active fibers is a challenge to overcome [6]. In this sense, rare-earth-doped double-clad fibers used as gain media has proved to be a reliable option to significantly increase the gain factor and efficiency of single-mode fiber lasers operation by using cladding pumping from high-power multimode sources [7].
In recent years, different laser regimes from harmonic generation [8,9] to the formation of different soliton patterns [10] passing by dissipative soliton resonance (DSR) [11–13], or even NLPs [14–19], have been observed in anomalous-dispersion operation of PMLFLs based on the use of Er/Yb double-clad fiber (EYDCF) as gain medium. Particularly, NLP operation is characterized by the generation of nanosecond (or subnanosecond) packets which encompass thousands of ultrashort inner sub-picosecond pulses whose amplitudes and durations vary randomly. Despite the complexity and variability of their fine structure, generally the NLPs display a very simple and stable overall behavior. The operation characteristics of NLPs lasers include a wide, smooth optical spectrum. In addition, the pulses exhibit a double-scale autocorrelation trace with a wide pedestal and a narrow (sub-picosecond) peak on top of it [20]. NLP laser emission offer the obtaining of high energy pulses in adjustable flat and wide emission spectrum with robustness to dispersion, which makes it attractive for potential applications in different research areas such as supercontinuum generation, medical imaging, and, material processing among others [14,21].
Moreover, stable, compact and flexible optical pulse sources simultaneously operating at multiple wavelengths are promising for potential applications in different areas such as fiber sensing, laser measurement, spectroscopy, and optical communications. However, the reported approaches on mode-locked laser generally focus on single-wavelength emission; scarce research efforts on dual or multi-wavelength PMLFLs have been reported [22]. In this regard and considering the recently growing interest for rectangular pulses generation for pulse energy scaling [23], the generation of dual-wavelength rectangular pulses have been experimentally obtained regardless of the dispersion regime and the gain medium [24–28]. Gou et al. [25] observed rectangular pulses in a dual-wavelength erbium-doped fiber laser by using a topological insulator (TI) as saturable absorber (SA). Shao et al. [26] and Mao et al. [27], reported square pulses from dual-wavelength lasers with ultra-large anomalous net dispersion. Lin et al. [28] demonstrated the generation of dual-wavelength domain-wall rectangular-shaped pulses in a highly nonlinear fiber laser by intracavity birefringence-induced spectral filtering effect. Although in the reported investigations the generation of rectangular pulses is demonstrated, the autocorrelation profile was not analyzed in order to determine the laser regime: DSR or NLP.
On the other hand, harmonic mode-locking (HML) is an important technique to increase the pulse repetition rate in pulsed fiber lasers. In reported investigations, passively HML laser operation has been demonstrated by means of a nonlinear amplifying loop mirror [29], nonlinear polarization rotation technique [30], and by using a fast SA such as a semiconductor saturable absorber [31], single-walled carbon nanotubes [32], graphene [33] and TIs [34].
For the first time, to the best of our knowledge, we report rectangular, h-like and trapezoidal noise-like pulse generation from a passively mode-locked Er/Yb-doped double-clad fiber laser based on a nonlinear optical loop mirror (NOLM) as SA, with switchable dual-wavelength fiber laser operation in the anomalous dispersion regime, in which HML appears to be the most usual mode of operation. The proposed figure-eight fiber laser operates in the 1.55-µm wavelength band, based on the use of an EYDCF as gain medium. The wavelength switching or dual-wavelength emission of the generated laser lines is obtained by birefringence adjustments within the laser cavity. Low-order HML and transition dynamics for the bunch of pulses in the 4th-order HML are observed. A wide variety of pulsed regimes and noise-like pulses shapes are obtained with the same laser cavity by well-defined control of the pump power and simple adjustment of quarter-wave retarder (QWR) plates.
2. Experimental setup
The experimental setup for the proposed figure-eight PMLFL is shown in Fig. 1. The right loop of the configuration acts as NOLM whereas the left one is the optical ring oscillator. The gain medium is a 4-m long EYDCF (Nufern, SM-EYDC-6/125-HE) with a first cladding diameter of 125 µm and numerical aperture (NA) of 0.46, and a core diameter of 6 µm with NA of 0.18. The absorption of the first cladding at 915 nm is ~0.75 dB/m and the core absorption near 1535 nm is ~40 dB/m. The EYDCF is pumped by a 25 W multi-mode laser source at 976 nm through a (2 + 1) × 1 pump combiner. The maximum launched pump power was 9.42 W to avoid risk of damage on the fiber components. A 50/50 fiber coupler connects the oscillator with the NOLM which is formed by splicing the ports 3 and 4 through a 120-m spool of SMF-28 fiber twisted at a rate of 7 turns per meter. The NOLM is power-symmetric but polarization imbalance is induced by the quarter-wave retarders (QWR1 and QWR2). Then, QWR1 was properly adjusted in order to obtain stable self-starting mode-locking operation. As a consequence of the absence of an input polarizer in the proposed configuration, the polarization state is not restricted within the cavity [14]. The total cavity length L was estimated as ~151-m, which yields an estimated net cavity dispersion of −2.86 ps2 and a repetition rate of 1.37 MHz. The 15% output port of an 85/15 fiber coupler is used to provide the laser output. A fiber-pigtailed optical isolator (ISO) was inserted within the ring oscillator to ensure unidirectional operation. The laser output power is measured by a thermal optical power meter (Thorlabs PM310D). The output pulses were detected by a high-speed photodiode (12.5 GHz bandwidth and 28 ps rise/fall time) and monitored by a 2.5 GHz bandwidth oscilloscope. The spectrum was measured by an optical spectrum analyzer (OSA, Yokogawa AQ6375) with resolution of 0.05 nm.
3. Results and discussions
With the proposed configuration, rectangular NLP are generated and then analyzed for single-switchable- and dual-wavelength operations of the passively HML fiber laser. The single- and dual-wavelength operations of the laser are achieved with a proper rotation angle of the QWRs plates. The NOLM provides the SA mechanism to achieve mode-locked laser emission in the proposed configuration. Initially, single-wavelength mode-locked pulse generation was obtained by adjusting the QWR2 at 60 degrees from the point where the maximal low-power transmission of the NOLM was observed. Figure 2 shows the characterization of the laser for this adjustment. As it can be observed, the pump power was varied from 3.1 to 7.75 W, where single-wavelength laser emission at λ1 = 1545.1 nm and stable mode-locked rectangular pulses are observed. Figure 2(a) shows the measured optical spectrum at wavelength λ1 as a function of the pump power variation. The laser emission exhibits an optical spectrum with central wavelength at 1545.1 nm and 3-dB optical bandwidth of ~4 nm. Figure 2(b) shows the generation of stable trains of pulses with uniform time interval of around 730 ns, in agreement with the roundtrip time (T) of ~727 ns calculated from T = nL/c, with c being the speed of light in vacuum, L the cavity length, and n the refractive index of the medium. This operation thus corresponds to fundamental mode-locking. The evolution of the mode-locked pulses is shown in Fig. 2(c). As the pump power is increased in the studied pump power range, the pulse duration continuously increases from 5.8 to 18 ns. An autocorrelator (Femtochrome, FR-103 XL) was used to measure the pulse autocorrelation, as it is shown in Fig. 2(d). The measurement was obtained for the maximum pump power of 7.75 W. From this result, a narrow coherent peak of ~0.6 ps (inset figure) is riding a wide pedestal that extends beyond the 200 ps measurement window of the autocorrelator, which is consistent with typical characteristics of NLPs [9].
Fig. 2 Output pulse characteristics as a function of pump power: (a) Spectrum of the Er/Yb mode-locked fiber laser at 1545.1 nm, (b) stable train of pulses, (c) evolution of rectangular NLP, (d) autocorrelation trace of generated NLPs at maximum pump power. The inset shows a zoom-in of the coherent peak.
After obtaining mode-locking, for low-pump we varied the angle of QWR2 to 27 degrees with respect to the maximal low-power transmission of the NOLM. Then, laser emission switches to the longer wavelength λ2. The pump power range in which stable mode-locked rectangular pulses were observed at λ2 was from 1.1 to 6.31 W. Figure 3(a) shows the measured optical spectrum switched at λ2. The laser output spectrum is centered at 1563.6 nm and has a 3-dB bandwidth of 5 nm. As can be observed in Fig. 3(b), stable trains of pulses were observed in a pump power range from 1.1 to 6.31 W. Again, the pulse trains exhibit a fundamental repetition rate of 1.37 MHz, meaning that a single pulse circulates in the 151-m long cavity. The evolution of the mode-locked pulses is shown in Fig. 3(c), where a rectangular pulse and continuous pulse width variation from 1.2 to 11.88 ns is observed as pump power increases. Figure 3(d) shows the autocorrelation trace measured at 6.16 W pump power. In accordance with the previous results observed in Fig. 2(d), the NLP generation is determined by observing the typical characteristics of the aforementioned regime, such as the smooth spectra and double-scaled autocorrelation trace.
Fig. 3 Output pulse characteristics as a function of pump power: (a) Spectrum of Er/Yb mode-locked fiber laser at 1563.6 nm, (b) stable train of pulses, (c) evolution of rectangular NLPs, (d) autocorrelation trace of generated NLPs at maximum pump power. The inset shows a zoom-in of the coherent peak.
In order to obtain dual-wavelength mode-locked pulse operation, the rotation position of QWR2 was set to 18 degrees from the position where the maximal transmission for low-power of the NOLM was obtained. Then, simultaneous measurements of the optical spectrum and the temporal profiles for dual-wavelength operation of the laser were obtained. Stable laser pulses in dual-wavelength operation were observed in a range of pump power between 1.66 and 7.5 W. As can be observed in Fig. 4(a), the optical spectrum reveals the generation of two simultaneous laser emissions with peaks centered at 1545.1 and 1563.6 nm, with 3-dB bandwidths of 5 and 8.2 nm, respectively. These laser lines correspond to the laser wavelengths λ1 and λ2 obtained in switchable single-wavelength operation. At low pump power levels the gain peak of λ1 is higher than for λ2, however, with the increase of pump power a preference to generate the longer wavelength λ2 is observed. Next, the characteristics of each laser line in dual-wavelength operation were investigated by using a fiber optical loop mirror (FOLM) with high birefringence (Hi-Bi) fiber loop as spectral filter. The length of the Hi-Bi fiber of 14 cm was chosen to achieve a FOLM free spectral range of 40 nm, then, the spectrum of the FOLM was wavelength tuned to obtain maximal transmission at λ2 and maximal reflection at λ1 [35]. For simultaneous measurement of both laser wavelengths in dual-wavelength laser operation, a fiber optical circulator (OC) was used together with the FOLM. The port 1 of the OC was connected at the laser output. The port 2 of the OC was connected to the input port of the FOLM. Then, the port 3 of the OC was used to measure the reflection of the FOLM (channel 1) whereas the transmission is measured at the output port of the FOLM (channel 2). The same configuration was previously used and described in ref [36]. Figures 4(b) and 4(c) show the measured optical spectra filtered around λ1 and λ2, respectively, as a function of pump power. Then, by maintaining fixed the pump power at 6.3 W, the optical pulses of each filtered laser emission (λ2 in blue and λ1 in red) were measured simultaneously with two photodetectors through the filter, as shown in Fig. 4(d). A time delay of 23.4 ns between both pulses is observed. The evolution of mode-locked pulses filtered at λ1 is shown in Fig. 4(e). With the increase of pump power, the pulse width varies in a range from 0.61 to 3.56 ns. The pulse envelope exhibits an h-like shape, recently observed by Zhao et al. [37]. Figure 4(f) shows the evolution of the pulse envelope filtered around λ2 at different pump powers. As the pump power is incremented, the pulse width varies from 0.59 to 3.19 ns. In this case, there is a noticeable change in the temporal profile of the optical pulse, it changes from its original h-shape to a trapezoidal-shape at the maximum pump power.
Fig. 4 Output laser characteristics for dual-wavelength generation: (a) spectrum of the dual-wavelength emission. Spectrum of the laser output for filtered wavelength (b) λ1 = 1545.1 nm and (c) λ2 = 1563.6 nm. (d) Simultaneous measurement of the filtered pulses profile. Evolution of the rectangular NLPs as a function of the pump power for (e) λ1 = 1545.1 nm and (f) λ2 = 1563.6 nm.
Figure 5 shows the train of pulses and autocorrelation traces for dual-wavelength operation. Stable pulse trains with fundamental repetition rate of 1.375 MHz are depicted in Figs. 5(a) and 5(b) for filtered λ1 and λ2, respectively, measured over a pump power range from 1.66 to 5.82 W. Figures 5(c) and 5(d) show the autocorrelation traces for filtered λ1 and λ2, respectively, corresponding to the generated pulses trains shown in Fig. 5(a) and Fig. 5(b) at a fixed pump power of 5.82 W. From these measurements one can observe a narrow peak riding a wide pedestal, suggesting that both filtered spectral components operate in the NLP regime. For λ1 the width of the autocorrelation peak is measured as ~0.36 ps, whereas for λ2 the peak duration is of ~0.29 ps.
Fig. 5 Pulse trains of filtered (a) λ1 and (b) λ2 at different pump power levels in a dual-wavelength operation. Autocorrelation traces of the NLPs for (c) λ1 and (d) λ2, respectively.
A comparison on the pulse evolution as a function of pump power is shown in Fig. 6 for single-wavelength operation in switching mode and dual-wavelength operation for λ1 and λ2. As can be observed, in case of single-wavelength laser operation, the pulse duration varies from 5.8 to 18 ns at λ1 over a pump power range from 3.1 to 7.75 W. The pulse duration at λ2 varies from 1.2 to 11.88 ns over a pump power range from 1.1 to 6.31 W. For filtered wavelengths in dual-wavelength laser operation, when the pump power increased from 1.66 to 5.82 W the pulse duration for λ1 and λ2 varies from 0.61 to 3.56 ns and from 0.59 to 3.19 ns, respectively. For single-wavelength operation, the pulse duration increases with a similar linear fitting slope of ~2.65 and ~2.17 ns/W for λ1 and λ2, respectively. However, as the pump power increases in dual-wavelength operation, the rate of pulse duration increase is significantly lower than the one observed in single-wavelength operation, being of ~0.66 and ~0.55 ns/W for filtered λ1 and λ2, respectively. We consider that this behavior of the pulse duration evolution is related with the different pulse shapes obtained in each regime: rectangular pulses for single-wavelength operation in contrast to h-like and trapezium-shaped pulses for filtered λ1 and λ2 in dual-wavelength laser generation.
Fig. 6 Evolution of pulse duration for single- and dual-wavelength operations as a function of the pump power.
Besides fundamental mode-locking, the laser could also be operated in HML regimes. Figure 7 shows the pulse trains at different harmonic orders for single- and dual-wavelength laser operations. When the laser operates at pump power levels over the 6.01 and 8.45W range, the square-wave pulse packet splits leading to harmonic laser operation regime. In case of single-wavelength operation, when the laser emission is switched to λ1, pulses trains corresponding to 2nd, 3rd, and 4th harmonic orders of mode-locked pulses could be achieved with pump power levels of 7.98, 8.24, and 8.45 W, respectively, as it can be observed in Fig. 7(a). Similarly, for single-wavelength operation switched to λ2, pulses of the 2nd, 3rd, and 4th harmonic orders are respectively observed with pump power levels of 6.56, 6.79, and 7 W (Fig. 7(b)). In case of dual-wavelength laser generation, the multiple pulses were observed by separate measurements using the FOLM as spectral filter. Figure 7(c) shows the harmonic pulses obtained for filtered λ1. h-shaped pulses of the 2nd and 3rd harmonic orders were observed with pump power levels of 6.07 W, and 6.31 W, respectively. Similarly, for filtered λ2, trapezium-shaped pulses corresponding to the 2nd and 3rd harmonic mode-locking orders are achieved for pump power levels of 6.07 W, and 6.31 W, respectively (Fig. 7(d)). The inset of each figure is a close-up on the pulse profile evolution over the different mode-locking orders. In all cases, the pulse duration decreases as the harmonic order increases. The autocorrelation traces of the harmonic mode-locked pulses were also measured and indicate that these regimes possess a similar behavior to the noise-like pulse cases reported in single-wavelength operation.
Fig. 7 Harmonic mode-locked pulse of different orders achieved with increasing pump power. Single wavelength (a) λ1, (b) λ2, and dual-wavelength filtering (c) λ1 and (d) λ2.
4. Conclusion
We have experimentally demonstrated the formation of NLPs in a mode-locked EYDCF laser with single- and dual-wavelength laser operation. By incorporating a NOLM as SA, the laser operates in anomalous dispersion regime to achieve mode-locking of the proposed laser setup. By properly adjusting the QWRs, stable single-wavelength or dual-wavelength laser operation was obtained. We experimentally demonstrated the emission of stable rectangular NLPs with fundamental repetition rate of 1.38 MHz in switchable single-wavelength laser operation. Besides h-like and trapezium-shaped NLPs are observed for filtered λ1 and λ2, respectively, in dual-wavelength laser operation. Besides the fundamental mode-locking order, the transition dynamics from a bunched state of pulses to 4th order HML for switchable wavelength were observed. We experimentally demonstrated the obtaining of a wide variety of mode-locked laser operation regimes for different NLP shapes by simple QWR plates and pump power adjustments in the same laser cavity.
Funding
Consejo Nacional de Ciencia y Tecnología (CB-256401).
Acknowledgments
M. Durán-Sánchez and R. I. Álvarez-Tamayo would like to thank the Cátedras CONACyT program.
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