Vector soliton fiber laser passively mode locked by few layer black phosphorus-based optical saturable absorber

1. Introduction

The potential photonic applications of two-dimensional (2D) nanomaterials have received rising attentions due to the unique electrical and optical properties of the materials [1–7]. 2D materials have the advantages of wideband absorption, ultrafast carrier dynamics and 2D planar structures. They have found interesting photonic and optoelectronic applications, such as optical modulator [8, 9], photo-detectors [10], polarizer [11], and optical saturable absorbers (SAs) [12–14]. Using 2D materials as a SA to passive mode-lock and/or Q-switch lasers have been frequently reported [15–21]. H. Zhang et al firstly reported using atomic layer graphene as a saturable absorber to mode lock fiber lasers [22], which opened the study of 2D materials as SAs in ultrafast photonics.

Recently, black phosphorus (BP), a novel 2D material consisting of elemental phosphorous atoms, has become a hot research topic [23–28]. BP represents a type of 2D layered crystals with moderate and direct band gap, and the bandgap of BP was found to be thickness dependent [29]. Typical bandgap of BPs varies from 0.3 eV to 2.0 eV, corresponding to that the optical resonance wavelength ranges from ~4 μm to ~600 nm, which is critically important for broadband optoelectronic devices at optical communication band. Lu et al. reported the broadband nonlinear optical response of multilayer black phosphorus [30]. Chen et al. reported the mechanically exfoliated BP for both laser Q-switching and mode locking [31]. Some other groups also verified the saturable absorption feature of BP [32–35]. It is worth mentioning that D. Li et al. have shown that the mechanically exfoliated BP has polarization dependent absorption [33]. J. Sotor et al. demonstrated the polarization sensitive absorption of mechanically exfoliated BPs as well. They also successfully generated ultra-short pulses in a BP mode locked fiber laser [35]. Different from saturable absorbers made of other 2D materials, such as graphene, topological insulator, the BP based saturable absorbers show polarization dependent linear absorption. Polarization sensitive absorption may introduce a polarization selection effect to the laser cavity and prevent the formation of vector pulses. Our previous experimental study showed that under strong pumping, mode locking could self-start in a fiber laser even without placing an intra-cavity SA device if a polarization sensitive component existed instead [36]. Interestingly, such kind of mode-locking operation will only allow for the generation of scalar solitons. These are soliton pulses with fixed polarization. Therefore, it would be interesting to study the soliton polarization dynamics in fiber lasers mode locked by the BP-based SAs, which shows weak linear polarization dependent absorption.

Several drawbacks of the SAs made of the mechanically exfoliated BPs have been discovered recently. For instance, the high power illumination induced thermal effect and optical damage would accelerate the oxidation of the BPs in air. The lack of stability in air and ease of oxidation prevent the BP based SAs from any practical applications [37–40]. Therefore, from the viewpoint of practical applications it becomes necessary to develop a solution to tackle these problems.

In this letter, we demonstrate that BP nanoflakes fabricated by liquid phase exfoliation method [41] can be used as saturable absorbers to generate short pulses in fiber lasers. The BP nanoflakes were simply deposited onto the end facet of a fiber that is connected into the fiber laser cavity. Stable soliton pulses with duration of 670 fs were successfully obtained. The soliton pulses were found to be vector solitons in the cavity. To the best of our knowledge, this is the first demonstration of vector soliton formation in BP mode locked fiber lasers. The experimental results show that BP nanoflakes based SA is an effective polarization independent device for mode-locked fiber lasers.

2. Synthesis and characterization of black phosphorous nanoflakes

The liquid phase exfoliation (LPE) is considered as a simple but effective technique to synthesize 2D nanomaterials from the layered bulk crystals towards the few-layer structures. The bulk black phosphorus is firstly immersed into a liquid, typically N-methyl-2-pyrrolidone (NMP), and then the 2D layer materials could be obtained through ultrasonic exfoliation. To form few-layer BP nanosheets, bulk black phosphorus was exfoliated in NMP (1 mg/mL in NMP) using bath ultrasonication (operating at 40 kHz frequency and 100% power) for four hours to conduct the liquid exfoliation of the bulk BP. The temperature of the bath was maintained below 30°C throughout using water-cooling. After exfoliation, solution was centrifuged at 3000 rpm for 10 minutes in order to remove any non-exfoliated bulk BP.

To better characterize the prepared few-layer BPs, Raman spectrum measurement was carried out by a Horiba Jobin-Yvon Lab Ram HR VIS high-resolution confocal Raman microscope equipped with a 633 nm laser. The measured results are presented in Fig. 1(a). Three Raman peaks corresponding to one out-of-plane vibration mode A¹g and two in-plane vibration modes B²g and A²g are located at 362.3, 439.4 and 467.1 cm⁻¹, respectively. In order to study the morphologies of the as-prepared few-layer BPs, the transmission electron microscope (TEM) image and atomic force microscopy (AFM) image were provided. Transmission electron microscopy (TEM) was used to further probe the few-layer phosphorene produced by liquid exfoliation. As can be seen in Fig. 1(b), the prepared BPs could be clearly identified to be layered structure with a large size of several micrometers. Atomic force microscopy (AFM) of exfoliated phosphorene on Si/SiO₂ substrates revealed the presence of a range of shapes and thickness of phosphorene, the AFM images shown in Fig. 1(c). Height-profiling of the nanosheet be marked shown in Fig. 1(d), revealed a thickness between 2.8~3.8 nm.

 

Fig. 1 Characterizations of BP: (a) Corresponding Raman spectrum of BP; (b) TEM images; (c) AFM images of few-layer BPs; (d) Height profiles of the section marked in (c).

Download Full Size | PPT Slide | PDF

The BP nanoflakes were collected by burette and carefully transferred on to the facet of the fiber pigtail. After the BP nanoflakes are dried, the fiber pigtail with BP was connected into the ring laser cavity. Figure 2 shows the light absorption properties of the BP nanoflakes. Figure 2(a) presents the spectral absorbance of the BP, which was measured by an ultraviolet spectrophotometer. The operation wavelength of the spectrum ranges from 350~1600 nm. There are two peaks observed at ~1200 nm and ~1400 nm. Figure 2(b) shows the transmittance of the BP at different input powers. The incident light is a home-made pulsed fiber laser source operating at 1550 nm with a pulse width of ~800 fs and the pulse energy is ~100 pJ. The non-saturable loss of BP is ~40%, the modulation depth is ~21% and the saturation influence is ~12 MW/cm².

 

Fig. 2 (a) Linear absorbance of the BP; (b) Measured nonlinear absorption of the BP: Transmittance of BP versus power of incident light

Download Full Size | PPT Slide | PDF

3. Experimental setup

The fiber laser setup is schematically sketched in Fig. 3. The laser was pumped by a high power Raman fiber laser (KPS-BT2-RFL-1480-60-FA) of wavelength 1480 nm and its maximum pump power is 5 W. The pumping light was coupled into a fused WDM coupler into the laser cavity. The fiber laser is reversely pumped to isolate the pumping light from circulating in the laser. The laser cavity is a ring that consists of 1.2 m erbium-doped fiber (EDF OFS-80) with group velocity dispersion (GVD) of −48 ps/nm/km. Intra-cavity passive components are all made by single mode fibers (SMF) and connected by SMFs with GVD parameter of 18 ps/nm/km, the total length of SMF is around 22.8 m. The cavity is thus dispersion managed with net anomalous dispersion. A 10% fiber output coupler was used to output the signal, a polarization independent isolator was used to force the unidirectional operation of the ring cavity, and an intra-cavity polarization controller (PC) was used to fine tune the linear cavity birefringence. All these passive components are polarization independent. The BP nanoflakes were deposited on the end facet of the optical fiber that forms the SA. The vector soliton spectra of the laser were measured with an optical spectrum analyser (Yokogawa AQ6375). A 33 GHz real-time high speed oscilloscope (Agilent DSO-X 92804A) together with two 25 GHz photo-detectors was used to monitor the vector solitons with two orthogonal polarization components emitted by the fiber laser.

 

Fig. 3 A schematic of the experimental setup. PC: Polarization controller; EDF: Erbium doped fiber; ISO: isolator; WDM: Wavelength-division multiplexer; OC: Optical coupler; BP: Black phosphorus; OSA: Optical spectrum analyser; BS: beam splitter.

Download Full Size | PPT Slide | PDF

4. Experimental results

The mode locking self started as long as the pump power was increased to ~200 mW. Figure 4 depicts the mode-locking performance of the fiber laser. As shown in Fig. 4(a), the optical spectrum of the mode-locked pulses is centered at ~1559.5 nm. The 3-dB bandwidth of the spectrum is ~3.8 nm. Appearance of the Kelly sidebands on the spectrum, which is a typical feature of the soliton fiber lasers with net anomalous dispersion, indicates that the mode locked pulses are solitons. To identify the pulse duration of the solitons, autocorrelation trace of the pulses was measured, as shown in Fig. 4(b). The pulse width is ~670 fs if the sech² intensity profile was assumed. Considering that the bandwidth of the spectrum is 3.8 nm, the time-bandwidth product is ~0.32, indicating that the mode-locked pulses are almost transform limited sech²–profiled pulses. Figure 4(c) shows the radio frequency (RF) spectrum of the mode locked pulses. The fundamental repetition rate of the pulse train is ~8.77 MHz. A signal to noise ratio of ~60 dB was found. Broad span frequency spectrum up to 500 MHz is shown in the inset. There is no obvious spectral modulation, confirming that the pulse train is stable. To test the damage threshold of the BP-SA, the pump power versus output of the fiber laser was measured, as shown in Fig. 4(d). When the pump power is larger than 200 mW, the laser operates in the soliton regime. It is to note that in the soliton regime, increasing the pump power will also increase the numbers of the solitons. Once a soliton is formed in the cavity, it is stable. It is shown that the output power stably increases as the pump power increases, indicating that the passive mode locking in the laser is robust. The BP mode locking can be kept until the pump power is as high as 1.5 W.

 

Fig. 4 Mode locking performance of BP as mode locker in the erbium-doped fiber laser. (a) Optical spectrum; (b) Autocorrelation trace; (c) Radio frequency (RF) spectrum at the fundamental repetition rate f₀. The resolution bandwidth is 300 Hz. Inset: spectrum in 500 MHz scale with resolution of 1000 Hz. (d) Pump power versus output power of the mode-locked fiber laser.

Download Full Size | PPT Slide | PDF

A typical oscilloscope trace of the mode-locked pulse-train is shown in Fig. 5. Initially, multiple pulses are always observed as long as the mode locking occurs. Similar to those previous reported multiple solitons in SA mode locked fiber lasers [42, 43], moving multiple solitons tend to attract each other and form a bunch. Finally, a stable state of soliton bunch is observed, as shown in Fig. 5(a). By carefully decreasing the pump power, the number of solitons in the bunch can be reduced one by one, until only one soliton remained in the cavity, as shown in Fig. 5(b). The time between adjacent single pulses is ~115 ns, which coincides with the round trip time of the ring cavity, which confirms the pulses are generated from mode locking mechanism.

 

Fig. 5 Mode locking pulses train of erbium-doped fiber laser. (a) Soliton bunch. Inset: details of the bunch; (b) Single soliton state.

Download Full Size | PPT Slide | PDF

To study the vector nature of the BP mode locked soliton pulses, a polarization beam splitter was connected to the laser output and carefully aligned so that the two orthogonal polarization components of the laser emission could be measured simultaneously. The polarization resolved output of the fiber laser is shown in Fig. 6. Figure 6(a) shows the polarization resolved optical spectra. It is noticed that apart from the Kelly sidebands which are similar in both polarizations, there are extra sidebands exhibited as peak (see vertical axis) and dip (see horizontal axis) on the spectrum, indicating the existence of the energy exchange between the two orthogonal polarizations [44], which could be only observed in the spectra of vector solitons. Under the spectrum of Fig. 6(a), one can also observe multiple vector solitons or soliton bunches in the oscilloscope traces. Figure 6(b) shows a typical oscilloscope trace of multiple vector soliton pulses in 400 ns scale in which three round trip times are captured. The two traces correspond to the output from the two orthogonal polarization components. The soliton pulses of the two orthogonal polarization components are synchronized, indicating that the vector solitons are with the same group velocity. The intensities of the pulses do not change in each round trip time, confirming that the polarization of the vector solitons are polarization locked [45]. We have also measured the autocorrelation traces of the pulses along the two axes. It was found that their autocorrelation traces are similar to that shown in Fig. 4(b).

 

Fig. 6 Polarization locked vector soliton observation of the mode-locked fiber laser. (a) Optical spectrum of vector soliton: (b) Multiple vector soliton pulse train detected by oscilloscope.

Download Full Size | PPT Slide | PDF

Under multiple vector soliton operation, by carefully tuning the paddles of the PC, bound vector solitons, which is a special structure of two solitons binding together with fixed separation and moving in the cavity, can be obtained. The formation of bound states of vector solitons is a result of direct interaction between the vector solitons. Experimental results of bound solitons are shown in Fig. 7. Figure 7(a) shows the optical spectra of the bound states of vector solitons. Periodic spectral modulation, which is a typical feature of the bound solitons [46], is obviously observed on the spectra. The interval between the adjacent spectral modulation peaks is around 1.7 nm, which corresponds to 5.5 ps in the time domain. Note that apart from the Kelly sidebands, extra sidebands caused by the coherent coupling between the two orthogonal polarization components, which are indicated by the arrows in the figure, are also observable. Figure 7(b) shows the measured autocorrelation trace of the bound solitons. The pulse width of the solitons are still ~670 fs. The measured soliton separation in the bound state is ~5.5 ps, which coincides with the modulation period observed on the optical spectra of Fig. 7(a). We have also measured the polarization resolved autocorrelation traces. There is no obvious difference in pulse shape or pulse duration. Figure 7(c) shows the pulse train observed in oscilloscope. As the separation between the pulses are too small to be resolved by the oscilloscope. Only “one pulse” can be observed in each round trip time. Figure 7(d) shows the measured RF spectrum of the bound solitons. A ~55 dB S/N ratio was found, indicating the bound solitons state is stable in the laser cavity.

 

Fig. 7 Bound states of vector soliton in the mode-locked fiber laser. (a) Polarization resolved optical spectrum of bound vector soliton; (b) Measured autocorrelation trace of bound states of solitons; (c) Corresponding pulse train; (d) Corresponding radio frequency spectrum. Resolution: 1000 Hz. Inset resolution: 300 Hz.

Download Full Size | PPT Slide | PDF

5. Summary

In summary, few-layer BP nanoflakes were fabricated using the LPE method and the saturable absorption features of the BP nanoflakes-based SAs were experimentally investigated. Self-started mode locking of an erbium-doped fiber laser operating at ~1550 nm with a BP nanoflakes-based SA was experimentally demonstrated. We show that due to the polarization independent property of the BP nanoflakes-based SAs, vector solitons were formed in the fiber lasers. Moreover, vector soliton bunch and bound states of the vector solitons were also experimentally observed. Our results show that the BP nanoflakes-based SAs are polarization independent and suitable for vector soliton fiber laser research.