Graphene-chitin bio-composite polymer based mode locker at 2 micron region

Abstract

In the field of pulsed fiber laser, graphene is a well-known two-dimensional (2D) material for its excellent optical properties. An alternative approach to the existing method, graphene based filament originally intended for 3-dimensional (3D) printing was used as starting material. Coupled with a newly introduced chitin nanofiber as the host polymer, it was demonstrated and reported as passive mode locker at 2-micron region. The conventional soliton operated at operating wavelength of 1982.7 nm with repetition rate of 11.35 MHz. The produced average output power, pulse width, time-bandwidth product (TBP) and signal to noise ratio (SNR) was 76.83 µW, 1.88 ps (HAC200), 0.416 and 43 dB, respectively. When the pulse was amplified with 5.4 dB of Thulium doped fiber amplifier (TDFA), the average output power increased to 3.43 mW and produced a broad operating wavelength around the 2-micron region. At the same repetition rate of 11.35 MHz, the measured pulse width, SNR, pulse energy and peak power of 7.033 ps (pulseCheck 150), 42.0 dB, 0.30 nJ, and 42.98 W, are obtained, respectively. High power laser operation in this region can find applications in medical field and sensors technology.

Introduction

The ever-growing industries of multiple sorts have widely used fiber lasers for their relatively high output power and design compactness. Besides its evident applications in laboratories worldwide, fiber lasers have been used for material processing such as cutting and welding [1], [2], medical diagnostics [3], as well as sensor system technologies [4]. Only made possible by the use of rare-earth ions such as Ytterbium, Thulium, and Erbium as the gain mediums, fiber lasers have seen many achievements since they were first invented in 1961 [5], [6].

Eye-safe region in the range of 1600–2100 nm in particular is highly sought after for its wide range of applications ranging from remote gas sensing to medical surgery [7], [8]. The strong absorption lines of water particles at 2-micron region has made it expedient to applications in medical field such as laser surgery, photo dermatology and tissue ablation [9]. The newest finding of its feasible applications in the telecommunication networks made possible due to its close bandwidth ranges to that of Erbium-doped fiber’s telecommunication wavelength [10], [11] has also contributed to its increasing marketability. Research on pulsed laser generations, in particular, is still lacking in the 2-micron region compared to that of the matured telecommunication wavelength at 1.5-micron region. Laser generation through passive Q-switching technique has been utilized tremendously owing to the simplicity and flexibility in design that it can offer. Passive pulsed fiber laser generation requires a saturable absorber (SA) instead of using a bulky external control element as intra-cavity modulator. On account of this preferable prospect, many materials have been investigated as SA materials, ranging from black phosphorous, topological insulator, to metal nanoparticles [12], [13], [14], [15].

Two-dimensional (2D) materials amongst other lower dimensional materials, despite of reports on their relatively low-damage resistance and poor long-term stability [16] had proven to be excellent materials as SAs in fiber laser generations. Recent years has shown a grown interest in the advantage of optical properties through the structural thinness offerable by 2D materials. 2D perovskite nanosheets, for example, was reported to possess stronger saturable absorption and large modulation depth as compared to its bulk counterpart [17], [18]. Titanium disulphide, on another note, was also reported to possess strong light absorption properties, as well as a remarkable electrical stability making it an excellent SA material to generate pulsed laser in the telecommunication region [13]. Part of a group with broader range of bandgaps, antimonene has also gain attention for its excellent thermal conductivity and high carrier mobility. It has also been reported to be particularly excellent in optical signal processing [14]. However, these materials, while has been proven to generate ultrafast lasers in the telecommunication region, their flexibility in the medical safe wavelength are still lacking in reports. Carbon based SAs, part of the 2D materials family, in particular, have long surpassed its predecessors in regards to having lower saturation intensity, broadband operations, and speedy pulse recovery time [19], [20]. Interestingly, within the carbon family, graphene was found to have a much lower saturation intensity, high carrier mobility, and greater saturable absorption when compared to that of carbon nanotubes [21], as well as higher damage threshold allowing for high power operation. In addition, the gapless linear dispersion of Dirac electrons in graphene enables tunable operation without having to physically change the graphene structure [22], [23], [24].

Graphene has been exploited in many ways in researches especially in the photonic field. Ultrafast fiber laser has seen graphene with excellent SA potentials. In investigating so, graphene has been prepared through various means to ease its integration in laser systems. Conventional method of producing graphene through mechanical exfoliation or atomic force microscopy (AFM) cantilever was reported to successfully produce few-layer graphene. Nonetheless, the graphene was still produced with ~10 nm in thickness, equivalent to 30 layer graphene [25]. Chemical vapour deposition (CVD) of graphene was first reported in an ultrafast fiber laser in 2009 [21] in which Bao et al. reported an atomic layer graphene based mode-locked fiber laser in the C-band region. Even supposing its ability to produce graphene with low defects rate and good uniformity, graphene produced through CVD and solution method was always burdened by the scrupulous yet necessary additional step of transfer process [26]. A simpler and more efficient method introduced through optical deposition of graphene instituted a more efficient and safer way of handling graphene nanostructures [21]. However, deformation- and distortion-prone characteristic of 2D materials associated with optical deposition method of SA integration was alarming. Martinez et al. [27] reported an improved performance of graphene SA through mechanical exfoliation method, a way to physically obtain multi-layered graphene, as compared to using optical deposition method, with the advantage of being simpler and having less time-consuming fabrication procedure. Nevertheless, this method is limited by the intricacies to control the size and number of layers of the produced graphene flakes. Another graphene synthesis method is the graphitization of hexagonal silicon carbide (SiC) crystals which involves high temperature of around 1500 °C as reported by Emtsev et al. which produced irregular graphene layers with wrinkled surface and restricted mobility of graphene carriers [28]. Derivatives of graphene such as graphene oxide (GO) and reduced graphene oxide (rGO) have also exhibited excellent optical properties. Unlike graphene, their hydrophilic property owing to the oxidation process of graphene can serve a wider range of researches and applications. However, the fabrication of GO and rGO involves a more complicated process and the use of hazardous material such as potassium permanganate (KMnO₄) requires the process to be done by an expert. Furthermore, the further reduction process of GO to produce rGO added to the complex process was proven to serve little to no effect to its performance as an SA [29].

In addition, it is found that many of the reported mode-locked lasers of carbon based SAs in the two-micron region required additional component, i.e. polarization controller (PC) to help initiate mode-locking operation. Wang et al. [30] reported a CVD graphene grown on copper foils based SA in a mode-locked laser yielding a shortest pulse width of 3.8 ns with the aid of a narrow-band fiber Bragg grating and a PC. GO based mode-locker in the 2 µm region was also reported oscillating at 1920.8 nm with a pulse duration of 1.14 ps [31]. Ahmad et al. [32] reported an rGO and titanium dioxide (TiO₂) based SA to produce a soliton mode-locking by evanescent field interaction and PC adjustment. A carbon nanotube film based SA with a high pulse energy of 26.8 nJ was reported by Wang et al. [33]. The ring cavity employed two PCs to aid the mode-locking operation of the laser. This work demonstrated a soliton mode-locking based on a graphene-chitin SA with comparable shortest pulse width and high output power as compared to similar works using carbon based materials as SAs. The soliton was also yielded at a significantly lower pump power, with high stability. Sharbirin et al. [34] reported a mode-locked thulium doped fiber laser (TDFL) using Mach-Zehnder interferometric filter at a much higher threshold input pump power of 150.4 mW.

Graphene as SAs has been prepared by mixing it in a host polymer matrix for easier in-cavity integration. Researchers have often opted for polyvinyl alcohol (PVA) in SA fabrication procedures [35], [36], [37], [38]. As compared to other synthetic polymer such as polyethylene oxide (PEO), PVA has a higher melting point of around 180 °C [39] which is vital in pulsed laser generation. Besides PVA and PEO, polymethylmethacrylate (PMMA), polystyrene (PS) and polycarbonate (PC) among many others have been used with the same purpose. CVD graphene on PMMA as SA has been reported to produce mode-locked lasing operation with 1.2 ps pulses [40]. Despite the many choices of polymers on the market, PVA and cellulose derivatives are preferred for their affinity towards concentrated graphene aqueous solution which is better for optical density with lower non-saturable absorption losses [41]. A graphene in PVA host polymer was reported to yield a mode-locked Thulium-Ytterbium do-coped fiber laser (TYDFL) with pulse width and pulse energy of 52.85 ps and 1190.5 pJ, respectively [42]. Another was reported by Zhang et al., [43] producing a repetition rate and output power of 6.46 MHz and 2 mW, respectively.

Chitin nanofiber (ChNF), fabricated from chitin, an alternative bio-host polymer is newly introduced in this work. Chitin (C₈H₁₃O₅N)_n was first discovered in 1811 [44], and has since been used for many applications such as in medicine, manufacturing industries, and wastewater treatment [44], [45]. ChFN was derived from the fungal origin and more hydrophobic due to its acetylated nature. This can help in hydrophobic graphene distribution (minimize agglomeration) and made percolation network already possible in minimal graphene concentration. Other than that, ChFN is resistant and durable towards higher acidity and harmful environment, making it practical for the production of films to use in a higher power and temperature laser operations, a solution of which current SA is facing where the performance of the SA is limited by their host polymers lower heat resistance. With this advantage, chitin is used in the SA fabrication process to produce a graphene-ChNF based SA for pulsed laser generation. In telecommunication applications specifically, a polymer with C-F overtones with low absorption losses at the desired wavelength is better in terms of stability [41]. This biological based material is chosen as an alternative to current conventional synthetic host polymers due to its competitive production cost, non-toxicity, water solubility, compatibility with nanomaterials, biodegradability and bioadsorbability [46]. Produced from natural resources such as plants and shells, large scale production of ChNF at low cost is highly feasible. Although chitin is no stranger in agricultural, material, and biomedical research, optical communication [47], [48], [49] has not dipped its toes into this novel material until recently when we first reported its use in the generation of a Q-switched pulsed fiber laser in the 1.5 µm region [50].

In this work, ChNF has been applied as host polymer to produce graphene based SA to produce a mode-locking operation in the 2.0 µm region. The ChNF was derived from mushrooms and combined with graphene based 3D printer filament. The TDFL yielded a lasing operation with a repetition rate and pulse width of 11.35 MHz and 7.033 ps, respectively. A high peak power was also obtained at 42.95 W. The use of ChNF in this work will contribute to a more sustainable and environmental friendly option for host polymer materials, especially in the optical communication research field.