MAX phase based saturable absorber for mode-locked erbium-doped fiber laser

Highlights

• Mode-locked erbium-doped fiber laser with MAX phase Ti₃AlC₂ as saturable absorber.

• The MAX phase embedded in PVA exhibits a saturable absorption of 2%.

• The mode-locked captured owns an ultrashort pulse of 3.68 ps at 1557.77 nm.

• The maximum peak power was 2.2 kW at the pump power of 131 mW.

Abstract

We presented an ultrashort pulse generation in an erbium-doped fiber laser cavity using MAX-PVA as a mode-locker. A thin film of SA was prepared by mixing PVA with Ti₃AlC₂ as a saturable absorber inside the laser cavity. The optical nonlinearity was also demonstrated, revealing a saturable absorption of 2%, a non-saturable absorption of 58.2%, and a saturation intensity of 1.63 MW/cm². The excellencies of MAX-PVA was proven as it initiates ultrafast laser with a duration of 3.68 ps corresponds to a repetition rate of 1.887 MHz. The ease of preparation, together with its superior optical, physical, thermal, and mechanical properties, makes it a favorable SA for pulse generation in an all-fiberized laser cavity.

Introduction

The phenomenon of nonlinear optics (NLO) in a 1-, 2- or 3-dimensional (3D) material is of great interest among physicists and engineers for the past few decades. Thanks to its revelation on the bizarre optical properties of various materials, many devices were well-developed for photonics and optoelectronic applications. Unique NLO mechanisms such as saturable absorption, nonlinear Kerr effects, and electro-optic effects give rise to a powerful ultrafast pulse generation in a laser cavity [1]. The principle of mode-locking in the latter required a gigantic laser configuration with various lenses installed inside the cavity. Parallelly, a Kerr lens mode-locking (KLM) is prone to a misalignment of the mirrors inside the laser oscillator leading to an asymmetrical light ray within the configuration [2]. Those limitations introduced difficulties in laser beam synchronization and frequent components alignment, which makes them a less favorable approach for pulses generation.

Saturable absorber (SA), which acts as a nonlinear optical media in a laser cavity, is frequently utilized as a mode-locker to generate ultrashort pulses. The generation seems elegant as most of the light is confined in an optical fiber core, thus ensure less photon escaped from the laser system. The portability of this device makes it more preferred than two of the earlier laser configurations as it owns a few centimeters sizes. Simple laser set up together with its ability to construct picosecond to femtosecond laser pulses makes it useful for various industrial applications such as biosensing [3], two-photon polymerization [4], micromachining [5], corrective eye surgery [6], and characterization of material [7]. To date, various materials were incorporated as a pulse initiator inside a laser cavity including carbon-based materials [8], transition metal dichalcogenides (TMDCs) [9], topological insulators (TIs) [10], metal nanoparticles (NPs) [11], and 2-dimensional (2D) materials [12].

In 2004, Geim and Novoselov discovered an atomically thin layer of carbon called a few-layer graphene (FLG) [13]. Graphene owns excellent condensed matter properties with ballistic electron mobility, which prevents lattice dislocation and crystal imperfection at high temperatures. The peculiar characteristic of graphene makes them excel in many fields, including photonics. Thus, it is widely used as a SA in the broad near-infrared spectrum due to its overlapped conduction-to-valence band and ultrafast relaxation time (~200 fs) [14], [15], [16]. As promising as it is, graphene owns a few limitations in terms of bandgap alteration ability and low modulation depth (<2.3% per layer) [17]. The latter contributed to a low second-order susceptibility of its structure [18]. In contrast, TMDCs such as molybdenum disulfide (MoS₂) possess a high second-harmonic generation (SHG) and third-harmonic generation (THG) with recovery time nearly as fast as graphene (~30 fs) [19]. Both SHG and THG are strongly related to the thickness of MoS₂, thus, reducing a few layers thickness of such material might enhance the performance of pulses generated [20]. The modification of TMDCs layer can also convert their bandgap from indirect to direct structure, which is suitable for near-infrared laser generation. Conversely, layer-dependent wavelength operation gives rise to a complicated preparation procedure resulting in the development of other 2D based SA. To simplify its synthesizing method, most work using TMDCs exfoliate them mechanically with scotch tape, but non-uniform powder-like TMDCs are difficult to handle [21]. Careful layer control is needed to ensure the same amount of TMDCs used for both experimental and characterization.

Few years, research on the physical and optical properties of black phosphorus (BPs) has exploded [22], [23]. Thanks to its excellence charge-carrier mobility (10⁵ cm²/Vs) and thermodynamically stable phases, numerous semiconductors application had been developed [24]. Its ability to generate pulses in the near-infrared region is proven, as a naturally occurring BPs covers wide near-infrared region due to a suitable bandgap of such allotrope (0.3 eV). Unfortunately, BPs' compatibility as a SA diminishes over time as it is very sensitive to the environment. Exposure of BPs to air for a few hours might reduce its saturable absorption properties, making it useless for pulse generation. Lately, metal NPs are utilized as a SA inside an all-fiberized laser cavity. They own a large surface plasmon resonance (SPR), high third-order nonlinearity, high-damage threshold, and fast response time, which contributes to a wide operating wavelength in the visible and infrared region. Alike 2Ds, metal NPs require special preparation procedure that alters their shape thus introduced nonlinearity in near-infrared region. Gold (Au) and silver (Ag) in the form of nanorods [25], nanosphere [26], nanocrystal [27], nanoparticles [28], etc are syntheses using seed-mediated growth and electron-beam deposition. Both techniques require complex experimental equipment, thus they are expensive and tricky. Therefore, the surge for a new SA material which is not only easy to prepare, yet preserve the excellent nonlinear optical properties, wide operating bandwidth, and high damage threshold is still essential.

Recently, an extensive study on a newly synthesis 2D material, MXene, has been widespread [29], [30]. The general formula for MXene is M_n+1X_nT_x where M is any material belongs to transition metal group (Sc, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, etc.), X is a carbon or/and nitrogen with n = 1, 2, 3, and T represents a face terminations such as hydroxyl, oxygen, or fluorine (T = -OH, -O, -F) with x refers to the number of terminal groups. Ti₃C₂T_x, one of MXene, exhibits an excellent nonlinear absorption alike graphene with two orders of magnitude larger than MoS₂ and BPs [31], indicates its fast-optical switching capability. Not only that, Ti₃C₂T_x holds a high damage threshold of 70 mJ/cm², comparable to most metal NPs [32]. Therefore, avalanche research interest had revealed the potential of MXene as a Q-switcher and mode-locker in a 1-, 1.3-, 1.55- and 2-µm laser cavity [33], [34], [35], [36]. However, the ability of its parent, MAX phase (layered metal carbides and nitrides) for the generation of pulses in a near-infrared region is not fully explored. To clarify, MAX phase (bulk) is the early version of MXene (2D) before the termination of A-group element (such as Aluminium) from the composition. It is as unique as its precursor with good electrical conductivity, thermodynamically stable nanolaminates, high damage tolerance at room temperature, good mechanical strength, and excellence oxidation-resistance [37], [38], [39]. The latter seems favorable for pulse generation as it did not easily oxidize in the air due to a dense alumina layer within the material. Unlike 2Ds’ includes MXene, MAX phase is synthesis based on a simple preparation method, a mixture of MAX phase with polyvinyl alcohol (PVA) was magnetically stirred to produce a SA device for pulse generation. Lee et al. [40] demonstrated Q-switched using MAX phase, Ti₂AlC-PVA drop-casted onto a fiber-ferrule as a pulse initiator in an erbium-doped fiber laser cavity. Recently, Ahmad et al. demonstrated Ti₃AlC₂ as a passive Q-switcher in EDFL cavity [41]. Here, we presented for the first of our knowledge, mode-locked erbium-doped fiber laser using MAX phase, Ti₃AlC₂-PVA as a saturable absorber with the generation of a stable pulse as short as 3.68 ps corresponds to a repetition rate of 1.887 MHz. We proved MAX-PVAs’ excellence optical properties as we obtained a modulation depth of 2%, slightly higher than MXene [31], with a low saturation intensity of 1.63 MW/cm², denotes its ability to saturate light at a low pump power.