1. Introduction

Pulsed laser sources operating at ∼2.1 μm have found use in a broad range of applications including X-ray generation, medical procedures, transparent plastic welding, and Light Detection and Ranging (LIDAR) systems [1]. In recent years mode-locked, all-fiber lasers basing on holmium-doped fibers began to replace Optical Parametric Chirped-Pulse Amplification (OPCPA) systems, Ho:YAG or Ho:YLF solid state lasers previously used to generate ultrashort pulses in this spectral range [2]. Before saturable absorption in nanomaterials started to be extensively researched, passive mode-locking was achieved with the use of SESAMs [3] and artificial saturable absorbers such as nonlinear polarization rotation [4]. Since 2004, ultrashort pulse generation in 1, 1.5 and 2 μm spectral regions was demonstrated in fiber lasers mode-locked with carbon nanotubes (CNT) [5], graphene [6–8], topological insulators (TI) [9–11], transition metal dichalcogenides (TMD) [12–14], and black phosphorus [15–21].

Black phosphorus (BP) is a two-dimensional (2D) material consisting of phosphorene monolayers bounded with van der Waals forces. The most distinctive features of BP are its energy bandgap scalable from 0.3 eV (bulk BP) to 2 eV (one phosphorene layer) and high electron mobility. Its applications include transistors, photodetectors, optical modulators, and photovoltaics [22,23]. Similarly to graphene and other 2D materials, saturable absorption in BP reaches mid-infrared wavelengths. Due to the scalable bandgap, tuning the materials operating wavelength band and modulation depth is possible [24]. Saturable absorption in BP has been documented in 400 – 2100 nm range [25,26], and studies that utilized holmium/praseodymium co-doped fluoride fibers have verified its usability at nearly 3 μm [27]. It has been experimentally shown that BP is capable of serving as a saturable absorber in ytterbium [15,16], erbium [17–20] and thulium [21] doped fiber lasers. Up to date only a few types of saturable absorbers such as graphene [28,29], carbon nanotubes [30], SESAMs [31], and nonlinear polarization rotation [32, 33] have been successfully used in several demonstrations of holmium doped fiber lasers.

In this work we report for the first time that BP can be used as a saturable absorber to mode-lock an all-fiber holmium-doped fiber laser. Ultrashort soliton pulses centered at 2094 nm are demonstrated. We have also investigated harmonic mode-locking regime of the laser achieving up to 290 MHz repetition frequency at 10th harmonic.

2. Black phosphorus preparation and characterization

The black phosphorus saturable absorber (BPSA) was fabricated by depositing solution of exfoliated BP dispersed in N-methyl-2-pyrrolidone (NMP) on the tip of fiber connector, and drying. The obtained connector was coupled with a clean one through a fiber adapter, forming a fiberized BPSA which can be spliced into the laser cavity.

In our experiments, the BP solution was made using commercially available bulk BP (99.998%, Smart-Elements, Austria) and NMP (>99.0%, anhydrous, Aladdin Co., Inc.). All the other reagents were of analytical purity and used as received without further purification. In order to achieve two-dimensional BP, ultrasound probe sonication method was used. Briefly, 10 mg bulk BP were exfoliated by a sonicator in 30 ml NMP for 6 hours. The ultrasound probe worked at the power of 900 W and every period it worked for 2 s with an interval of 4 s. To avoid the oxidation of BP at high temperature, ice bags were used to keep the temperature under 20 C. The turbid liquid was centrifuged at the speed of 3000 rpm for 30 minutes and the two thirds of the top supernatant was collected for follow-up utilization.

For Raman scattering measurement, the ethyl alcohol turbid liquid of BP was dropped onto a Si/SiO₂ chip substrate (300 nm SiO₂) and then dried in a vacuum drying oven at 100 °C. Raman scattering was performed on a Horiba Jobin-Yvon LabRam HR-VIS high-resolution confocal Raman microscope equipped with 633 nm laser as the excitation source at room temperature and a XYZ motorized sample stage controlled by LabSpec software. Raman peak positions were calibrated with respect to the standard silicon Raman peak (520.7 cm⁻¹). Figure 1(a) presents Raman spectra of BP. Three peaks are located at 362.3, 439.2 and 466.7 cm⁻¹, which are ascribed to the out-of-plane phonon mode ${\text{A}}_g^1$, and in-plane modes B_2g and ${\text{A}}_g^2$ of BP, respectively [34,35].

 

Fig. 1 (a) Raman spectra of BP. The insets depict atomic displacements of the Raman active modes in BP [36]. (b) Atomic force microscopy image of BP flakes. (c) Cross section profiles of three BP flakes.

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For atomic force microscope (AFM) measurement presented in Fig. 1(b), 0.5 ml liquid supernatant was centrifuged at the speed of 4500 rpm for 30 minutes and then the sediment was dispersed in absolute ethyl alcohol. The measured cross-sections shown in Fig. 1(c) indicate that the flakes are ∼16–25 nm thick, which corresponds to 8–13 phosphorene layers (assuming 2 nm per layer [26]). The morphologies of the sediment were obtained by a field emission scanning electron microscopy (Hitachi SU8010) and an AFM (Bruker, Dimension Icon).

Linear transmission of the BPSA measured with the use of a broadband white light source is presented on inset of Fig. 2(a). It proves that BP can work as a relatively low loss saturable absorber in the near- and mid-infrared spectral range. Nonlinear optical parameters of the BPSA were characterized using a fiberized equivalent of a Z-scan setup described previously in [37]. The measurement of power-dependent transmission shown in Fig. 2(a) was conducted at 1550 nm due to unavailability of a measurement setup operating at 2 μm. The acquired experimental data were fitted using a theoretical model appliable to slow saturable absorbers [3], which can be justified by the short length of generated pulses (1.3 ps) in comparison to multilayer BP’s relaxation time [26,38]. The BPSA parameters obtained from the fit are 0.67% of modulation depth, saturation fluence of 194 μJ/cm² and 40.2% of non-saturable losses. Due to insufficient pumping power, the sample did not saturate in the measured fluence range.

 

Fig. 2 (a) Saturable absorption of a BPSA. Inset: Transmittance of the BPSA. (b) Experimental setup of the mode-locked, holmium-doped fiber laser.

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The measured linear absorption of ∼40% is agreeable with theoretical amount of losses introduced by 14-layer BP flake (39.2% assuming 2.8% of absorption per monolayer [39]). Previous works that investigated absorption properties of BP films of similar thicknesses (25 nm) have reported similar modulation depths [24]. The measured saturation fluence is relatively low in comparison with values measured at 1100nm [24], which is consistent with reports stating that BPs saturation fluence decreases with increase of wavelength [26].

3. Experimental setup and results

Setup of the oscillator and its pumping system is presented in Fig. 2(b). The pump consists of a multi-mode 793 nm semiconductor laser diode, and an in-house made thulium-doped fiber laser (TDFL) capable of generating up to 3 W of 1940 nm light at 11 W of pumping power.

The pumping signal is introduced to the laser cavity through a 1950/2080 nm wavelength division multiplexer (WDM). As a gain medium 1.8 m of double-clad holmium-doped fiber (Nufern SM-HDF-10/130) is used in core-pumping configuration. The isolator ensures unidirectional light propagation in the cavity. Since the cavity is built with standard single-mode fibers and components, a fiberized polarization controller (PC) is added to aid in initiating mode-locking operation. The light was guided from the cavity using 40% output coupler (OC).

Below the threshold of ∼1 W of pumping power, the laser operated in CW mode. After increasing the pumping power and adjusting the polarization controller, mode-locking was achieved. To get rid of parasitic CW components presented on inset of Fig. 3(a), the pumping power had to be lowered to ∼750 mW. The resulting optical spectrum is shown in Fig. 3(a). During our experiments, Q-switching operation of the laser was not observed. The emitted solitons were centered at 2094 nm with full width half maximum (FWHM) of 4.2 nm. Kelly sidebands characteristic to lasers operating in all-anomalous dispersion regime are present. Autocorrelation trace of the laser pulse along with a theoretical sech² fitting is shown in Fig. 3(b).

 

Fig. 3 (a) Optical spectrum. Inset: Spectrum with CW component. (b) Pulse autocorrelation.

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The measured pulse duration of 1.3 ps corresponds to time-bandwidth product of 0.373, indicating slight chirping of the pulse. This is caused by pulse broadening in ∼50 cm of fiber between the output coupler and an autocorrelator. Stable RF spectrum recorded in a 3 GHz span is shown in Fig. 4(a). Figure 4(b) depicts the repetition frequency of 29.1 MHz, which indicates that physical length of the laser cavity is ∼7.2 m. Small peaks around the repetition frequency imply that the pulse train is slightly modulated. The signal-to-noise ratio is better than 55 dB. The average laser output power during fundamental mode-locking operation was of 11 mW at 750 mW of pumping power, which corresponds to pulse energy of 379 pJ.

 

Fig. 4 (a) RF spectrum measured in 3 GHz span. (b) Fundamental frequency of 29.1 MHz.

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After increasing the pumping power beyond 1 W and adjusting the polarization controller, harmonic mode-locking operation of the laser can be achieved. During our experiments we have observed up to 10th harmonic corresponding to 290 MHz, which is shown in Fig. 5(a). Signal-to-noise ratio is greater than 40 dB, which is comparable to previous reports on harmonic mode-locking [40]. Figure 5(b) shows that the optical spectrum at the 10th harmonic exhibits typical soliton-like shape with FWHM of 3.4 nm. Inset of Fig. 5(b) depicts measured autocorrelation time of 2.43 ps, which implies pulse duration of 1.6 ps. The measurements were conducted at pumping power of 1.1 W, and the maximum average output power and pulse energy were 67 mW and 231 pJ, respectively. After further increases of pumping power up to 2 W, higher harmonics were observed. At this point the lasers operation was unstable. In all experiments no signs of damage or degradation of the BPSA were recorded.

 

Fig. 5 Performance of the laser mode-locked at 290 MHz (10th harmonic): (a) RF spectrum. (b) Optical spectrum with FWHM of 3.4 nm. Inset: Autocorrelation of the laser pulse.

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Performance of the laser was characterized using the following measurement devices: optical spectrum analyzer (Yokogawa AQ6375), radio frequency spectrum analyzer with 3.6 GHz band-width (Keysight EXA N9010A) coupled with a 16 GHz photodiode (Discovery Semiconductors DSC2-50S), and an autocorrelator (Femtochrome FR-103XL).

4. Summary and conclusions

Summarizing, we have demonstrated that BP can effectively serve as a saturable absorber in all-fiber lasers operating at ∼2.1 μm. Solvent of bulk BP was fabricated using liquid phase exfoliation, and deposited on a fiber connector forming a fiberized saturable absorber. The BP layer placed between connectors was well isolated from ambient conditions, and no degradation of the absorber and laser performance was observed. The laser was capable of generating optical solitons centered at 2094 nm with pulse energy and duration of 379 pJ and 1.3 ps, respectively. Operation in harmonic mode-locking regime with repetition frequencies reaching 290 MHz (10th harmonic) was observed. To the best of our knowledge, this is the first demonstration of mode-locking a holmium-doped fiber laser with the use of a BP saturable absorber.

This work fits into research on nanomaterial-based SAs, which is now mostly dominated by graphene, CNT and TMD. We believe that BP can join this group. To make it possible, BPs stability and manufacturing technology must improve and gain reproducibility. From our perspective the main factor limiting parameters of BP-based mode-locked fiber lasers is the BPSAs low modulation depth. In order to reach performance of graphene, CNT and TMD-based setups, solutions to reach modulation depths of ∼10% should be developed. This will allow for generation of much shorter and higher energy pulses in balanced and normal dispersion regimes.