Erbium-Doped Fiber Lasers (EDFLs) are key technologies in telecommunications, medical diagnostics, and material processing due to their efficient operation in the C-band (1530 - 1565 nm) . A central component in these lasers is the saturable absorber (SA), which enables passive Q-switching by adjusting intracavity losses, resulting in high-energy, short-duration pulses . Traditionally, inorganic SAs, such as carbon nanotubes and graphene, have been used in EDFLs because of their excellent nonlinear optical properties [2] . However, there is growing interest in organic materials as sustainable and cost-effective alternatives for SAs. Among these, Polyaniline (PANI), a conductive polymer, has gained attention for its tunable properties, ease of fabrication, and potential environmental benefits [3] .

Fiber lasers operate based on the principle of stimulated emission of radiation. They produce coherent light by amplifying photons in a specific direction, which is essential for precise applications in fields like telecommunications and material processing [4] . The properties of lasers, such as intensity, coherence, and directionality, make them essential for many industries.

A key mechanism in laser operation is stimulated emission, which is influenced by factors like photon number, laser line shape, and interaction between light and matter. These principles are important for understanding advanced laser systems and phenomena such as Purcell enhancement and super radiance in nano-lasers [5] . Additionally, advances in laser machining and femtosecond laser inscription have improved laser precision, impacting efficiency and surface quality [6] .

Q-switching is important for producing short, intense pulses by modulating the laser cavity’s quality factor (Q). Passive Q-switching, which uses saturable absorbers, controls energy storage and releases it as powerful pulses. For example, materials like VS2 nanosheets have shown great potential as SAs in Er-doped fiber lasers, achieving high repetition rates and stability [7] . Ongoing research into materials like TiOxNy thin films is improving the versatility of Q-switching in laser systems [8] .

The C-band (1530 - 1565 nm) is crucial for optical communication because of its low attenuation and high bandwidth, making it ideal for long-distance, high-capacity data transmission. Recent innovations, such as soliton crystal microcombs, show the C-band’s potential to support ultra-high data rates [9] . It also plays a key role in optical modulation techniques, enabling efficient data transmission and supporting secure, high-speed communication technologies [10] .

Saturable absorbers in EDFLs generate short, high-energy pulses by becoming transparent at high intensities, thus controlling intracavity light and enabling pulse output. Studies on 2D materials, such as SnS₂ and TiS₂, show their effective absorption properties and potential to improve the stability and efficiency of Q-switched lasers [11] . Other materials like Mo₂Ti₂C₃Tx MXene are being explored for their high modulation depth and potential for high-performance mid-infrared lasers [12] .

Organic materials like PANI are gaining importance due to their unique properties. PANI, with its high porosity, surface area, and tunable optical properties, has shown promise as an SA in laser systems [13] . Composites of PANI, especially with materials like graphene and carbon nanotubes, offer enhanced conductivity and stability, making them suitable for advanced laser technologies [14] . Organic materials provide several advantages, including low-cost production, eco-friendliness, and tunable optical properties for specific applications, such as organic photodetectors and electrochromic devices [15] .

PANI has proven effective as a saturable absorber in fiber lasers, improving pulse stability and repetition rates. The addition of nanoparticles like nickel has enhanced PANI’s broadband saturable absorption, aiding the development of high-performance pulsed lasers across various wavelengths [16] . These advancements highlight PANI and other organic materials as sustainable and efficient SAs for future laser systems.

A Polyaniline (PANI) thin film was synthesized by combining PVA (Polyvinyl Alcohol) with PANI using the solvent-casting technique. This method involves depositing a solution mixture of PVA and PANI onto a clean petri dish’s surface. PVA was chosen for its excellent film-forming properties, tensile strength, emulsification ease, and high-water solubility, making it an ideal host material. It exhibits minimal absorption of white light and has a high melting temperature of 200˚C, which is suitable for high-intensity laser applications. The fabrication process is cost-effective and hazard-free.

The fabrication process began with preparing a PVA solution: 1 g of PVA powder was dissolved in 120 mL of deionized water at 90˚C for 24 hours with stirring at 300 rpm. PANI powder was then mixed with the PVA solution to create a free-standing thin film. Approximately 30 mg of PANI powder was dissolved in 30 mL of PVA solution and stirred at room temperature for 24 hours, followed by 45 minutes of ultrasonic treatment. This process was repeated twice.

Approximately 5 mL of the PANI-PVA solution was transferred to a petri dish and exposed to ambient air for 48 hours, resulting in a dry PANI-PVA thin film with a diameter of 3 cm and a thickness ranging from 17 - 25 μm. The density variations of the film were controlled by adjusting the volume of the solution used.

Field emission scanning electron microscopy (FESEM) at 1000× magnification was used to analyze the film’s morphology. The SEM images revealed uniform dispersion of PANI particles, which were well-integrated within the polymer matrix. Energy dispersive X-ray (EDX) analysis confirmed the elemental composition, verifying the presence of PANI in the thin film.

The optical characteristics of the synthesized PANI film were examined, showing light absorption characteristics suitable for laser applications. The film exhibited a noticeable increase in absorption around the 1.55 μm wavelength range, indicating its potential for effective interaction in fibre laser systems.

Non-linear saturable absorption behaviour of the PANI thin film was investigated using a twin-balanced detector technique. The absorption properties were analyzed with a mode-locked laser operating at a repetition rate of 1.885 MHz and a pulse width of 3.62 ps. A 3 dB coupler was used to direct part of the pulse toward the saturable absorber, while the rest passed through a bare single-mode optical fibre. The data fitting confirmed that the film exhibited significant modulation depth, with a saturation intensity (Isat) of 10 MW/cm². The thin films are typically around 50 μm thick and the recovery time is estimated to be around 1 picosecond [17] .

This study reveals that adjusting the film thickness can further enhance the modulation depth and reduce non-saturable absorption. The PANI film exhibited a high quality of saturable absorption, remaining within the thermal damage threshold, and demonstrated promising performance for use as a saturable absorber in fibre laser systems ( Figure 1 ).

The 1.55 µm Q-Switched Erbium-Doped Fiber Laser (EDFL) system was carefully designed to ensure efficient and stable operation. The setup begins with a 980 nm laser diode, which acts as the primary pump source. The output from the diode is directed into a 980 nm wavelength division multiplexer (WDM) that separates the light into 980 nm and 1550 nm wavelengths. The 980 nm light is then passed into an erbium-doped fiber (EDF), which amplifies the signal. The 1.8-meter-long EDF was chosen for its stability and efficiency.

An isolator is included to ensure unidirectional light flow and prevent back reflections, which could destabilize the system or damage the laser diode. The optical path includes FC/PC fiber ferrules, connected via a clean fiber adapter, with a 2 mm × 2 mm organic material thin-film saturable absorber (SA) placed between the components. This SA modulates the light within the cavity, enabling Q-switching. An index-matching gel is used to reduce unwanted reflections, ensuring optimal coupling.

The system also features a 90:10 optical coupler to control light distribution within the cavity. As shown in Figure 2 , 90% of the light is sent back into the cavity at 1550 nm, while the remaining 10% is used for diagnostic measurements. The cavity length and placement of the components are optimized for stability and efficiency, with the system operating in the anomalous dispersion region to ensure stable pulse generation.

Several diagnostic tools are used to monitor the system’s performance. A 300 MHz digital storage oscilloscope and a spectrum analyzer are used to observe the pulse laser’s time and frequency domain features. A high-resolution optical spectrum analyzer captures continuous-wave and Q-switched spectral data, while an optical power meter measures the laser diode’s output at different pump settings.

The setup underwent iterative tuning to ensure precise alignment and optimal component performance. This careful calibration resulted in a reliable, high-performance Q-switched EDFL system, capable of producing stable laser pulses for advanced applications. The study of pump powers in Erbium-Doped Fiber Amplifiers (EDFAs) is crucial for understanding the effective power utilized for amplification, as it is influenced by factors such as coupling efficiency and fiber transmission losses. The pump powers in the study reflect the optical power launched into the EDF, not the output from the diode, which is a significant distinction. Measurement uncertainty, often ranging from 1% to 5% [19] , is a critical aspect that can affect the reported pump powers, and future studies should include these uncertainties to enhance reproducibility and reliability ( Figure 3 ).

The results presented in this section reveal the effects of varying pump power on the performance metrics of PANI, providing insights into its suitability for different laser applications. Key findings, including optical spectrum characteristics, pulse energy, output power, and SNR, demonstrate how PANI performs as a saturable absorber in Q-switched EDFLs. This discussion highlights the strengths and limitations of PANI, offering valuable perspectives on its potential in fiber laser systems, particularly in applications where pulse stability and moderate power output are crucial. The derivation of pulse energy and peak power from the oscilloscope data and the 90:10 coupler ratio is essential to accurately assess the performance of the Q-switched Erbium-Doped Fiber Laser (EDFL) system. The oscilloscope measures the instantaneous peak voltage of the pulse, which, when combined with the system’s known parameters, allows for the calculation of peak power. The 90:10 coupler ratio plays a crucial role, as it divides the power between the output and the reference. In this case, 90% of the power is directed towards the main output, and 10% is used for diagnostic measurements. Pulse energy can be derived using the relationship between the average power (P_av) and the repetition rate (f) as E = P_avg/f, Where P_avg is the average power, and f is the repetition rate of the laser[citation]. The average power can be derived from the measured voltage and the coupler ratio. In addition, Peak power is calculated using the formula, P_peak = E/τ, Where E is the pulse energy and τ is the pulse duration [20] . The pulse duration can be calculated from the repetition rate.

This study examined the use of Polyaniline (PANI) as a saturable absorber (SA) in Q-switched Erbium-Doped Fiber Lasers (EDFLs) showed that PANI performs well, achieving a maximum pulse energy of 52 nJ and a repetition rate of 78.13 kHz at the highest pump power. The Signal-to-Noise Ratio (SNR) was 60.4 dB, indicating stable performance. PANI proved to be a reliable alternative for certain applications, offering an eco-friendly and cost-effective solution compared to traditional inorganic materials. This research highlights the potential of organic materials, particularly PANI, for use in fiber laser systems, making it a promising choice for telecommunications, biological imaging, and other laser-based technologies. While PANI showed promise, its pulse energy and SNR performance could be further improved. Additionally, its long-term stability in high-power systems requires further exploration. Future research should focus on enhancing PANI’s performance and exploring its use in real-world applications ( Figures 4-9 ) ( Table 1 ).

The authors would like to thank Universiti Teknikal Malaysia Melaka (UTeM) lecturers and all who contributed to the successful completion of this research.