MoS₂/SiO₂ Composite Saturable Absorber Enables Sub‑Nanosecond Mode‑Locked Er‑Doped Fiber Laser

Abstract

The two‑dimensional semiconductor MoS₂ has emerged as a promising material for ultrafast photonics due to its broadband saturable absorption and high third‑order nonlinearity. In this study, we fabricate a novel MoS₂‑doped sol‑gel glass, forming a MoS₂/SiO₂ composite. The composite exhibits a modulation depth of 3.5 % and a saturable intensity of 20.15 MW cm⁻², while maintaining an optical damage threshold of 3.46 J cm⁻² – markedly higher than conventional semiconductor saturable absorber mirrors. Incorporating the composite into a ring‑cavity Er‑doped fiber (EDF) laser, we achieve two distinct mode‑locking regimes: a conventional soliton state at 90 mW pump power delivering 780 fs pulses, and a stable long‑pulse state across 100–600 mW pump power producing 1.21 ps pulses with a maximum average output of 5.11 mW. These results demonstrate the composite’s viability as a robust, high‑damage‑threshold saturable absorber for fiber‑laser applications.

Introduction

Nonlinear optical materials based on two‑dimensional (2D) structures underpin modern optoelectronic devices. While graphene has proven effective as a broadband modulator, recent work has shown that transition metal dichalcogenides (TMDs) such as MoS₂ can provide superior saturable absorption and higher third‑order nonlinearity. MoS₂ has been integrated into pulsed lasers at wavelengths ranging from 635 nm to 2950 nm, achieving pulse durations from sub‑femtosecond to several picoseconds, and operating at high repetition rates. Traditional fabrication methods—mechanical exfoliation, liquid‑phase exfoliation, CVD, PLD, and magnetron sputtering—each have limitations in scalability, defect density, or thermal robustness. Existing MoS₂ saturable absorbers (SAs) are typically incorporated by sandwiching polymer‑embedded nanomaterials between fiber ferrules or depositing on tapered/d‑shaped fibers, yet these approaches either suffer from low damage thresholds or complex packaging requirements. To address these challenges, we introduce a MoS₂/SiO₂ composite fabricated via the sol‑gel technique, which combines the optical compatibility of silica with the intrinsic nonlinear response of MoS₂, resulting in a durable, high‑damage‑threshold SA.

Methods

MoS₂/SiO₂ Composite Preparation

MoS₂ nanosheets were first dispersed in deionized water by liquid‑phase exfoliation: 1 mg of MoS₂ was sonicated for 6 h at 90 W, followed by centrifugation to obtain a stable solution. Separately, tetraethoxysilane (TEOS), ethanol, and water were mixed to form the sol‑gel precursor. The MoS₂ dispersion was combined with the TEOS mixture and stirred, then acidified with HCl to maintain a low pH. Hydrolysis and polycondensation reactions produced a MoS₂‑doped silica sol, which was stirred at 50 °C for 5 h to prevent cracking and aggregation. The sol was aged at room temperature for 48 h, then dried in a 60 °C chamber for one week, yielding a solid MoS₂/SiO₂ glass.

Fiber Laser Cavity

The ring‑cavity EDF laser (13.3 m total length) incorporated a 1.2‑m Er‑doped fiber as the gain medium, a fiber‑coupled laser diode (650 mW max) as the pump source, and a wavelength‑division multiplexer to combine pump and signal. A polarization‑independent isolator ensured unidirectional operation, and a polarization controller tuned the intracavity state. The MoS₂/SiO₂ composite was sandwiched between two fiber ferrules, and a 10/90 coupler extracted the output. The cavity was optimized to support both conventional soliton and long‑pulse mode‑locking regimes.

Results and Discussion

Composite Characterization

Optical inspection shows a brown composite indicative of MoS₂ integration. SEM images reveal a uniform dispersion of MoS₂ nanosheets within the silica matrix. Energy‑dispersive X‑ray spectroscopy confirms the presence of Mo, S, and Si. Balanced twin‑detector measurements on a 1550 nm, 500 fs, 23 MHz laser yield a modulation depth of 3.5 % and saturable intensity of 20.15 MW cm⁻². A femtosecond Ti:sapphire laser (800 nm, 250 fs) tests the damage threshold, which is measured at 3.46 J cm⁻²—significantly higher than typical SESAMs (≈0.5 mJ cm⁻²).

Mode‑Locked Fiber Laser Performance

At 90 mW pump power, the cavity exhibits conventional soliton mode‑locking with a 6 nm 3‑dB bandwidth centered at 1557 nm. The radio‑frequency spectrum shows a fundamental repetition rate of 15.76 MHz with a 65 dB SNR. Autocorrelation reveals a 1.21 ps FWHM pulse, corresponding to 780 fs sech² pulses. Increasing the pump to 100 mW destabilizes the soliton regime, indicating a narrow operating window.

By adjusting the polarization controller, a second stable mode‑locking state emerges, persisting from 100 to 600 mW. The optical spectrum widens with pump power, maintaining a 4 nm 3‑dB width at 600 mW. The pulse duration extends to 1.97 ps FWHM (1.21 ps sech²). The output power scales linearly with pump, reaching 5.11 mW at 600 mW. This demonstrates the composite’s capability to support both ultrashort and picosecond pulses over a wide power range.

Conclusion

We have successfully fabricated a MoS₂/SiO₂ composite via a low‑temperature sol‑gel process, achieving a modulation depth of 3.5 % and saturable intensity of 20.15 MW cm⁻². When integrated into an EDF laser, the composite delivers 780 fs pulses at 90 mW and 1.21 ps pulses with 5.11 mW average power across 100–600 mW pump. These findings confirm the composite’s high damage threshold, ease of integration, and versatility, positioning it as a promising candidate for next‑generation ultrafast photonic devices.

Abbreviations

2D

Two‑dimensional

CVD

Chemical vapor deposition

EDF

Er‑doped fiber

EDS

Energy dispersive X‑ray spectrometer

FWHM

Full width at half maximum

I_sat

Saturable intensity

LD

Laser diode

LPE

Liquid phase exfoliation

ME

Mechanical exfoliation

MSD

Magnetron sputtering deposition

PC

Polarization controller

PI‑ISO

Polarization independent isolator

PLD

Pulsed laser deposition

SA

Saturable absorber

SESAM

Semiconductor saturable absorber mirror

SNR

Signal‑to‑noise ratio

TEOS

Tetraethoxysilane

TMD

Transition metal dichalcogenide

WDM

Wavelength division multiplexer

ΔT

Modulation depth