Soliton Pulse Generation with MoS2 Saturable Absorber

We have successfully generated soliton mode-locked pulses in a ring Erbium-doped fiber laser (EDFL) cavity using molybdenum disulfide (MoS2) saturable absorber. The MoS2 thin film was grown onto ITO substrate through electro-deposition process using a mixture of 0.5M H2MoO4 and 0.5M Na2S2O3.5H2O as electrolyte solution. The EDFL produced stable soliton pulses operating at centre wavelength of 1557 nm and repetition rate of 1.88 MHz. The mode-locking was realized by injecting pump power within a range from 77 mW to 124 mW. It generates the maximum output power of 0.7 mW and pulse width of 3.0 ps.


Introduction
Fiber lasers have garnered significant attention due to their inherent advantages in stability, compact design, and reliability, making them particularly promising for diverse applications in the military, nonlinear optics, fiber-optic communications, and biomedical fields [1,2].The quest for ultrashort pulses has led to the widespread adoption of passive mode-locking techniques, primarily relying on saturable absorbers (SAs).Notably, a range of SAs has been investigated, encompassing semiconductor saturable absorber mirrors (SESAMs), carbon nanotubes, and two-dimensional (2D) nanomaterials like graphene [3][4][5].Most of the commercial laser products used SESAM as SA.However, it has some drawbacks such as high cost and narrow working bandwidth.CNT and graphene are considered to provide fast saturable absorption.However, the bandgap of CNTs is dependent on the tube size and graphene suffers from low modulation depth.To overcome the drawbacks of the current SAs, many researches have been carried out to explore new SA materials [6,7].
Recently, many studies have been carried out on transition metal dichalcogenides (TMDs) for use in electronic and photonics applications due to their layer-dependent properties [8,9].TMDs are a kind of novel 2D materials which are commonly deployed in photocatalytic, transistors, photodetector, and gas sensing.Among all TMDs, molybdenum disulfide (MoS 2) demonstrates unique conducting and optical properties, which opened new opportunities to develop new SA based on this material.Furthermore, it has shown a greater nonlinear optical response than graphene.In this paper, we demonstrate a soliton-pulse generation in Erbium-doped fiber laser (EDFL) cavity by using a MoS2 thin film, which was obtained by electrochemical deposition technique.We have successfully grown MoS2 thin film on ITO substrate through electro-deposition process using the precursor solutions of H2MoO4 and Na2S2O3.5H2O.The thin film was integrated into a simple ring EDFL cavity to achieve mode-locking operation.

Experimental arrangement
We have achieved the successful electrodeposition of a MoS2 thin film onto a transparent indium tin oxide (ITO) conductive substrate.The precursors, namely molybdic acid (H2MoO4) and sodium thiosulphate pentahydrate (Na2S2O3.5H2O),both of analytical grade, served as the sources for Mo 4+ and S 2-ions, respectively.The synthesis involved the combination of a solution containing H2MoO4 in ammonia with another solution containing Na2S2O3.5H2O in distilled water, resulting in an electrolyte solution with concentrations of 0.5M H2MoO4 and 0.5M Na2S2O3.5H2O.For the MoS2 synthesis, the electrolyte mixture maintained a ratio of 1:2 between the precursor solutions of H2MoO4 and Na2S2O3.5H2O,aligning with the stoichiometry of the compound.The pH of the reactive mixture was carefully maintained at approximately 9.3 throughout the process.This meticulous control of the synthesis conditions contributes to the reliable fabrication of the MoS2 thin film on the ITO substrate, highlighting the precision and reproducibility of the electrodeposition method employed.
The electrodeposition of the molybdenum disulfide, MoS2 thin film was carried out using an Admiral Squidstat potentiostat.The deposition potential used in this work is set at -1.4 V, which was derived from cyclic voltammetry technique.Fig. 1 (a) shows the image of thin film, which was obtained at deposition time of 5 seconds.The SA was prepared by sandwiching the newly developed MoS2 thin film between two fiber ferrules so that it can be easily integrated into the EDFL cavity to function as a mode-locker.
The MoS2-based Erbium-doped fiber laser (EDFL) configuration, illustrated in Fig. 1(b), featured a 2.4 m long Erbium-doped fiber (EDF) serving as the gain medium and pumped by a 980 nm laser diode.The EDF exhibited an Er 3+ absorption of 90 dB/m at 980 nm.To facilitate optimal performance, a wavelength division multiplexer (WDM) and an isolator were employed for launching the pump light into the EDF and ensuring unidirectional propagation of the oscillating laser, respectively, within the ring configuration.Within the laser cavity, a 100 m long standard single-mode fiber (SMF) with a group velocity dispersion (GVD) of -21.7 ps 2 /km was integrated.This extended fiber length served to enhance cavity nonlinearity and adjust the dispersion to achieve a balance with nonlinearity.The GVD values for both the EDF and WDM were approximately 27.6 ps 2 /km and -48.5 ps 2 /km, respectively.The overall cavity length measured 108 m, resulting in an anomalous net cavity dispersion of -2.2 ps 2 , facilitating the formation of conventional solitons.
A pivotal component in the configuration was the 80:20 coupler, utilized to extract 20% of the output laser while retaining 80% of the light within the cavity to sustain oscillation.The laser's wavelength was evaluated using an optical spectrum analyzer (OSA, Anritsu, MS97010C).To further characterize the laser, measurements in both the time and frequency domains were conducted using an oscilloscope (GWINSTEK, GSP-9300B) and an RF spectrum analyzer (Anritsu, MS2683A), respectively.The average output power of the pulses was determined using an ILX Lightwave OMM-6810B power meter.

Results and discussion
The mode-locked pulse train becomes evident when the pump power is set at or above 96.8mW, and stable operation is sustained up to a pump power of 123.7 mW.In Fig. 2(a), the measured spectrum of the output soliton reveals pairs of Kelly sidebands symmetrically positioned around the central wavelength (1560.4nm).These soliton pulses are a result of the intricate interplay between dispersion and nonlinear effects within the ring laser cavity [10,11].In the absence of the saturable absorber (SA), the laser operates in a continuous wave regime with a wavelength centered at 1561.4 nm.Mode-locking induces spectral broadening due to the self-phase modulation effect.The obtained spectrum suggests that the pulses are Fourier-transform limited and closely approximate a soliton, emerging as solutions to the nonlinear Schrödinger equation (NLSE) through the delicate balance of dispersion and nonlinearity, further enhanced by the presence of a 100 m long single-mode fiber (SMF) within the laser cavity.
Fig. 2(b) displays a measured pulse train characterized by a consistent peak-to-peak period of 536 ns, corresponding to a repetition rate of 1.88 MHz-the inverse of the cavity round time.To assess the operational stability of the soliton pulse, a 100 MHz RF spectrum was obtained (Fig. 2(c)), revealing a fundamental repetition frequency of 1.88 MHz, consistent with the oscilloscope trace.The signal-tonoise ratio (SNR) was determined to be 48.8 dB, affirming the stability of mode-locking.In Fig. 2(d), a pulse trace recorded by the auto-correlator exhibits a full width at half-maximum (FWHM) of approximately 3.0 ps, fitting remarkably well with a sech 2 profile.
The dependence of average output power and pulse energy on pump power variation is depicted in Fig. 3. Clearly, both average output power and pulse energy exhibit nearly linear growth with increasing pump power.The relationship between output power and pump power demonstrates a slope efficiency of 5.5%.At the maximum pump power of 123.7 mW, the mode locked EDFL attains an average output power of 0.79 mW, corresponding to a single pulse energy of 0.42 nJ.To further enhance pulse energy, optimization of the gain medium, extension of the cavity length, and reduction of intracavity loss can be explored.Pulse energy is calculated by dividing the output power by the corresponding repetition rate.
Increasing the optical cavity length reduces the repetition rate of the mode-locked laser, potentially leading to an increase in pulse energy.Additionally, adjusting the output coupler ratio can contribute to higher output power.Soliton mode-locked fiber lasers hold vast potential across material processing, communication, imaging, and scientific research domains.These lasers serve

Conclusion
We have successfully developed MoS2 based SA to function as a mode-locker for generating soliton pulses in EDFL cavity.The electro-deposition method was used to fabricate the MoS2 thin film onto ITO substrate using a mixture of 0.5M H2MoO4 and 0.5M Na2S2O3.5H2O as electrolyte solution.The mode-locked laser can produce stable pulses with 3.0 ps duration at 1.88 MHz repetition rate.This study indicates that the MoS2 based fiber laser can produce soliton pulses, which greatly encourages researcher's enthusiasm in the field of fiber lasers.

Fig. 1 .
Fig. 1.(a) MoS2 thin film coated onto ITO substrate.(b) the schematic diagram of the ring cavity used to produce soliton mode locked EDFL

Fig. 3 .
Fig. 3.The plot of average output power and pulse energy against pump power