1. Introduction

Passively mode-locked fiber lasers have advantages of compact configuration, free of alignment, and good stability, which provide an excellent platform for many important applications [1,2]. The passive mode-locking is usually established through the saturable absorber, such as semiconductor saturable absorber mirror (SESAM) [3], nonlinear optical loop mirror [4], nonlinear polarization rotation technique [5,6], carbon nanotube [7], and graphene [8–10]. As a dominant mode-locking technology, SESAM is suffered from a small tuning range and cost-ineffectiveness [8,11]. Recently, single-wall carbon nanotubes (SWNTs) and graphene have been considered as great alternatives of SESAM due to their inherent advantages of the ultrafast recovery time, ease of fabrication, and polarization robustness [7–9]. Set et al. demonstrated that SWNTs had a saturable absorption effect, which can be used to mode-lock lasers [7]. Zhang et al. and Sun et al. reported the mode-locked fiber lasers with graphene-based saturable absorbers, respectively [8,9]. Additionally, particular attention is paid to design carbon-based hybrid films in the fields of supercapacitors and nonlinear optics [12,13]. Mixing two carbon metamaterials gives a benefit that carbon nanotubes can prevent the agglomeration of graphene [12].

Depending on the settings of net cavity dispersion, different types of pulses can be formed in mode-locked fiber lasers [14–23]. In the anomalous dispersion regime, conventional soliton (CS) with spectral sidebands can be formed by balancing the nonlinearity of fiber and the anomalous dispersion [2,15,16]. Currently, dissipative soliton (DS) has been achieved in net-normal and all-normal dispersion fiber lasers [17–19]. In previous works, Liu observed a experimental result that CS and DS were delivered from a unidirectional fiber laser simultaneously [15]. Additionally, Kieu et al. reported a bidirectional passively mode-locked ring laser offering two counter propagating pulses with similar performances [24]. Ouyang et al. presented a bidirectional passively mode-locked fiber laser using a four-port circulator with two different SESAMs [25]. As the cavity lengths in opposite directions were unequal, the characteristics were not identical for lightwaves propagating in opposite directions. However, the aforementioned bidirectional fiber lasers only delivered CSs in both directions.

In this paper, we propose a bidirectional fiber laser mode-locked with a mixture of graphene and SWNTs for the first time. By exploiting two 50/50 couplers, two cavities with different length and dispersion are formed in a fiber laser. By adjusting attenuators and pump power, the fiber laser can deliver two different types of pulses in opposite directions. In the clockwise (CW) direction, the net cavity dispersion is ~0.075 ps² and DS with quasi-rectangular spectrum is achieved. The output DS is highly chirped with the pulse duration of 9.15 ps and the spectral bandwidth of 7.8 nm. In the counter clockwise (CCW) direction, the net cavity dispersion is about −1.02 ps² and CS with spectral sidebands is obtained. The pulse duration and spectral bandwidth of the CS are 1.64 ps and 2.2 nm, respectively. It is demonstrated that the fiber laser can realize switchable DS and CS mode-locking, which is very convenient for practical applications.

2. Experimental setup

A homogeneous graphene oxide (GO) colloid is yielded through dispersing graphite oxide powder in deionized water, which is synthesized through the Hummers method [26]. Adequate ammonia and hydrazine hydrate are used as efficient reductants. Graphene nanosheets are produced by chemically reducing GO [27]. SWNTs with the outside diameter <2 nm are made by the catalytic chemical vapor decomposition method. The mixture is fabricated by the self-assembly of reduced GO nanosheets and SWNTs with a mixing ratio of 1:1 [12]. Polyvinyl alcohol (PVA) is chosen as the host polymer for its mechanical property and solvent compatibility. To obtain the composite films, mixture-PVA dispersion is drop-cast on a Petri disk. Figure 1 shows the absorption spectra of the graphene, SWNTs, and mixture composite films from 900 nm to 2400 nm, which are measured with a spectrophotometer. The absorption spectrum of SWNTs exhibits absorption peaks at ~1 μm and ~2 μm. Graphene composite film has no strong absorption at specific wavelengths. The wavelength-dependent absorption should be attributed to the incomplete reduction of GO [10] [28], which is similar to the results in [8]. The existence of graphene in mixture weakens the absorption selectivity, as demonstrated by the absorption spectrum of the mixture.

 

Fig. 1 Absorption spectra for the graphene (red), SWNTs (blue), and mixture of graphene and SWNTs (black) composite films.

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The schematic diagram of the experimental setup is shown in Fig. 2. A section of 18 m erbium-doped fiber (EDF) with the dispersion parameter of −16 ps/nm/km is utilized as the gain medium, and two 980 nm laser diodes (LDs) provide bidirectional pumps via 980/1550 nm wavelength-division-multiplexed (WDM) couplers. Two laser cavities with different length and propagation direction are formed in the fiber laser with two 50/50 optical couplers (OCs), two polarization controllers (PCs), two polarization-independent isolators (PI-ISOs), and two attenuators. The PI-ISOs ensure the unidirectional operation in each branch. PCs are used to tune the linear birefringence of the cavity, and attenuators are employed to control the loss. A 50 m single-mode fiber (SMF) with the dispersion parameter of 17 ps/nm/km is added in the CCW branch. A fused 2 × 2 OC with 10% output ports is used to extract pulses from the cavity. PI-ISOs external to the cavity can prohibit the reflected lightwave from disturbing the signals in fiber laser. The saturable absorber (SA) based on the mixture of graphene and SWNTs is sandwiched by fiber connectors in the ring cavity.

 

Fig. 2 Schematic diagram of the experimental setup. The red arrows show the propagation directions

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3. Experimental results and discussion

The counter propagating lightwaves share the common EDF, pump sources, and SA, while have different pathways between two 50/50 OCs. By adjusting attenuators, the intensity of lightwave in each direction can be precisely controlled. Lightwave in one direction can obtain sufficient gain to form mode-locking operation, but it is suppressed in the other direction. Then the mode-locking operation can become more stable without the disturbance. Adjusting attenuator-2 to increase the loss of CCW branch and relaxing attenuator-1, lightwave tends to propagate from EDF→OC→SA→OC-1→PC-1→attenuator-1→PI-ISO-1→OC-2. In this direction, the net cavity dispersion is ~0.075 ps² and the cavity length is ~29.1 m. Increasing the loss of CW branch and relaxing attenuator-2, lightwave tends to propagate from EDF→OC-2→PC-2→PI-ISO-2→SMF→attenuator-2→OC-1→SA→OC. The net dispersion and length of cavity are about −1.02 ps² and ~79.9 m, respectively. As the cavity dispersion and length are different for two propagation paths, DS and CS can be generated in CW and CCW directions, respectively.

With the total pump power of ~20 mW, bidirectional continuous-wave operations are established. Mode-locking is not observed in both directions, even when the total pump power is increased up to 100 mW. By tuning attenuator-2 to increase the loss, self-started mode-locking in the CW direction can be achieved when the two pump powers are ~19 mW. However, multiple pulses are formed in this case, even including the harmonic multi-pulse operation [29]. By reducing two pump powers to 13 mW, single-pulse mode-locking operation in the CW direction is obtained. As shown in Fig. 3(a), the output spectrum has a quasi-rectangular profile with a bandwidth of 7.8 nm, which is the typical characteristic of DS in the normal dispersion regime. The central wavelength of the spectrum is ~1560 nm. The corresponding autocorrelation trace of the DS is illustrated in Fig. 3(b). The full width at half maximum (FWHM) is 12.9 ps, and the pulse duration is given as 9.15 ps if a Gaussian fit is used. The time-bandwidth product (TBP) is calculated as 8.8, which indicates that the pulse is highly chirped. The pulse duration can be compressed to several hundred fs, similar to the report in [30]. The pulse train depicted in Fig. 3(c) shows that the separation between adjacent pulses is ~142 ns, which is equal to the cavity round-trip time in CW direction. The radio-frequency (RF) spectrum is shown in Fig. 4(d). One can observe that the fundamental repetition rate of DS is ~7.05 MHz. The peak-to-background ratio of the RF spectrum is ~60 dB, implying a good mode-locking stability. Based on the experiment results, we conclude that the fiber laser operates at a stable DS mode-locking state.

 

Fig. 3 Experimental results of DS in CW direction. (a) optical spectrum, (b) autocorrelation trace, (c) oscilloscope trace, and (d) radio-frequency spectrum.

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Fig. 4 Experimental results of CS in CCW direction. (a) optical spectrum, (b) autocorrelation trace, (c) oscilloscope trace, and (d) RF spectrum.

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The propagation of pulses in CCW direction can be achieved by tuning attenuator-1 to increase the loss. Appropriately adjusting PC-2 and pump powers, stable single-pulse mode-locking operation is obtained. Figure 4(a) shows the output spectrum of CS, which has a bandwidth of 2.2 nm. The soliton features of the pulses are reflected by the appearance of spectral sidebands on the spectrum. It stems from the constructive interference between the soliton and dispersive waves [2,15,16]. The corresponding autocorrelation trace is demonstrated in Fig. 4(b). The FWHM is 2.52 ps, and the pulse duration is estimated as 1.64 ps with a sech² fit. Consequently, the TBP of CS is calculated as 0.44, indicating that the output CS is slightly chirped. It is worth noting that the fundamental repetition rates of CS and DS are different for their distinct cavity lengths. As depicted in Fig. 4(d), the fundamental repetition rate of pulses in CCW direction is given as 2.57 MHz. The oscilloscope trace in Fig. 4(c) shows that the separation between adjacent pulses is ~389 ns.

The proposed fiber laser can generate DS and CS in CW and CCW directions by adjusting the intracavity attenuators, respectively. As the net cavity dispersion in both directions differ from each other, the pulses in CW and CCW directions have distinct formation mechanisms. During the amplification in EDF, the pulse duration and spectral bandwidth are broadened due to the effects of normal dispersion, self-phase modulation, and other nonlinearities [18,23]. In the normal dispersion regime, the spectral filtering effect could cut off the temporal and spectral wings of the highly chirped DS, which results in the steep spectral edges [20–22]. However, the spectral filtering effect almost has no contribution to the formation of pulses in the anomalous regime, because the spectral width of CS is much narrower than that of the gain spectrum [2]. The evolution of CS is mainly determined by the interaction between anomalous dispersion and nonlinearity, which has been demonstrated by the nonlinear Schrödinger equation [2,15]. Consequently, the output DS and CS exhibit different characteristics such as spectral profile, pulse duration, and chirp.

The experimental results show that the proposed laser does not deliver the DS and CS if a graphene-based SA is used. The reasons are as the following. Without SWNTs, graphene is prone to agglomerate in the fabrication procedure, which results in high nonsaturable absorption loss and low modulation depth. The absorption loss of graphene is ~65%, which is much larger than that of mixture (~50%), as shown in Fig. 1. After passing through two 50/50 OCs (Fig. 2), >75% of pulse energy is lost. In addition, ~10% of energy is outputted by the 10/90 OC. As a result, the intracavity loss of laser is more than 93% if a grapheme film is used as SA, but it is about 88% if the mixture is used. The total cavity loss of the grapheme-based laser is so large that it is difficult to achieve self-consistent mode-locking operation.

In experiments, simultaneous mode-locking operations in both directions are not realized. This can be partially attributed to the cavity asymmetry in CW and CCW directions. For instance, the cavity length, dispersion, and propagation path are unequal for the CS and DS [26]. In addition, it is difficult to provide simultaneous amplification for two counter propagating pulses due to the gain competition effect. However, the unidirectional mode-locking operation in CW or CCW direction can be easily achieved alternatively. Compared to traditional unidirectional ring fiber lasers, the proposed fiber laser is capable of emitting two distinct kinds of solitons in a fiber laser. And the two counter propagating pulses can be switched by adjusting attenuators. Our scheme has significantly reduced the cost, and it is more convenient than employing two independent fiber lasers. Additionally, by inserting spectral filter in each branch, wavelength-tunable DS and CS can be obtained in the laser.

4. Conclusions

We have proposed a bidirectional passively-mode-locked fiber laser based on the mixture of graphene and SWNTs. Two different types of pulses are generated in opposite directions by adjusting the intracavity loss. In the CW direction, the net cavity dispersion is normal and then DS with the quasi-rectangular spectrum is achieved. In the CCW direction, the net cavity dispersion is anomalous and CS with spectral sidebands is obtained. The DS and CS approximately have the same central wavelength, while they exhibit distinct optical spectra, pulse durations, and repetition rates owing to the distinct formation mechanisms. We believe this all-fiber switchable fiber laser can find more applications in the future.