Pendahuluan
Q-switching technique has gained important research interest because it possesses the ability to produce long pulses with unique properties. This type of pulses has potential applications in several fields like medicine, nonlinear study, and material processing. The Q-switched lasers usually show pulse rates in the kilohertz (kHz) regime whereas pulsing durations range from microsecond (μs) to nanosecond (ns). They are normally realized utilizing a passive technique based on a saturable absorber (SA), which possesses many advantages including low cost and high efficiency. Up till now, various types of materials have been proposed as SA, which include semiconductor saturable absorber mirrors (SESAMs), graphene, carbon nanotubes (CNTs), topological insulators (TIs), and transition metal dichalcogenides (TMDs). Among these materials, SESAMs were widely utilized for Qswitching pulses generation since the pulsing performance can be delicately controlled through its absorption parameters. However, they suffer from a narrow operational wavelength range and complicated fabrication processes.
On the other hand, organic materials (OMs) have significant properties including wettability, viscosity, and elasticity. Also, they have units of photosensitive molecular, and their properties can be manipulated through light. Oms have been utilized in many systems and they are already inserted into curved displays such as smartphone screens in the form of organic light-emitting diodes (OLEDs) and memory devices. Also, these materials were used in solar energy systems and act as an emissive electroluminescent layer. The poly(3,4 ethylenedioxythiophene): poly(4-styrenesulfonate) (PEDOT: PSS) is one of the most significant OMs that have been intensively studied. The PEDOT: PSS has superb film-forming capability, wet-process ability, flexibility, thermal stability, and transparency. Besides, PEDOT: PSS has high conductivity which makes it is suitable for various applications like flexible displays, smart sensors, solid electrolytic capacitors, and wearable electronics. In this paper, we proposed and demonstrated a Q-switched erbium-doped fiber laser (EDFL) using a newly developed PEDOT: PSS PVA film, which was fabricated based on a casting technique as SA. The prepared PEDOT: PSS film has optical bandgaps of 4.1 eV and 3.3 eV and a modulation depth of 40%.
Metode dan Hasil
The film SA was fabricated by depositing a layer of the material onto a PVA film. At first, the PVA film was prepared using a casting technique. 1 g of PVA powder was resolved into 100 ml of distilled water. To fully resolve the material, the solution was stirred in an ultrasonic stirrer for about an hour. 5 ml of homogeneous PVA solution was poured into the petri dish and desiccation at room temperature for 72 hours to make the PVA thin film. Then, PEDOT: PSS solution was prepared by dissolving 1 mg of PEDOT: PSS into 10 ml of deionized (DI) water at 60o C temperature for about 60 minutes. By the course of the process, one drop of acetone was supplemented for the solution to assist in dissolving the polymer mixture. After that, the dissolved PEDOT: PSS was poured onto the prepared PVA thin film and desiccated at 45ó C temperature for about 30 minutes to form a thin layer of PEDOT: PSS onto the polymer film.
To test the Q-switching ability in the 1.5 μm region, the fabricated PEDOT: PSS film was integrated into an Erbiumdoped fiber laser (EDFL) cavity. The cavity configuration contains 2 m long of Erbium-doped fiber (EDF) with a maximum erbium ion absorption of 23 dB/m at 980 nm as the active medium. The net cavity dispersion is around -0.197 ps2 . The EDF was characterized to has cladding and core sizes of 125 μm and 4 μm, respectively with a numerical aperture of 0.16. A 980 nm LD was utilized to pump gain medium through 980/1550 nm wavelength division multiplexer (WDM), which was placed among isolator and LD in cavity configuration. The isolator has been used to evade the reflection issue and helps unidirectional propagation. The polarization controller (PC) was also utilized to specify the polarization state into the ring cavity. In this experiment, A 50:50 coupler was incorporated after SA, which lets 50 % of the power in the laser cavity. Then, a small piece of the SA was placed among an optical isolator and 50:50 coupler into cavity configuration to produce a Q-switched pulse with a total cavity length is 8.7 m. The cavity length and its arrangement were chosen to enable nanosecond pulse generation. The optical spectrum analyzer (OSA) (AQ6370C, YOKOGAWA) with a spectral resolution of 0.07 nm was utilized for the wavelength analysis of the Q-switched laser and an oscilloscope (OSC) (INSTEK GDS-3352) utilized to investigate the output pulse train through a photo-detector (PD), power meter (OPM) (Thorlabs: PM100D), and radio frequency spectrum analyzer (RFSA) (Anritsu: MS2683A) were used to measure output power and investigate the laser stability.
The lasing and pulse generation of the EDFL cavity was investigated by changing the 980 nm input LD power. At first, the EDFL produced a continuous wave (CW) laser as the input LD power reached 10 mW. However, when the input LD power was further increased to 107 mW, it started to generate self-starting Q-switched pulses. As the input LD power was increased to 245 mW, the PEDOT: PSS-based Q-switched laser operation was maintained stably without thermal damage. When pump power was incremented to more than 245 mW, the Q-switched pulse train was disappeared. The CW operation was observed again as no pulsing was shown in the OSC trace because of the excessive saturation of the PEDOT: PSS/PVA film and similar cases have been previously reported [58, 59]. Also, the optical damage threshold of SA film was investigated by increasing pump power above 335 mW. After that, the pump power was decremented to 245 mW, the Q-switched pulses were re-established. Therefore, we can confirm that optical damage threshold to SA film is more than 335 mW pump power. The oscilloscope trace of the Q-switching pulse train obtained by PEDOT: PSS SA at input power LD of 245 mW. The pulse period is gauged to be around 10.78 μs, which matches a pulse rate of 92.76 kHz. The pulse train also indicates a uniform peak intensity without any distortion or inconsistency which indicates the high stability of the Q-switching operation. When the SA film was removed from the ring cavity the Q-switched pulse disappeared. There was no pulse observed through OSC trace even though the input power was incremented to the maximum sustainable limit of the laser diode. This confirms the Q-switched pulse was generated by our SA film. The results display the single pulse envelop of the Q-switching laser, which indicates a pulse duration of 912 ns. The results also shows the optical output spectrum of Q-switching operation at input LD power of 245 mW. It is found that the laser worked at a centre wavelength of 1564.89 nm with a 3-dB spectral bandwidth of 1.54 nm. The result shows the long-term evaluation of Q-switched EDFL was measured every 5 min in one hour. As no change or shift in the emission spectra was observed for each of central wavelength and intensity. The results show that the PEDOT: PSS-based SA was stable for generating Q-switched in a 1565 nm region.
In conclusion, The SA film was prepared by casting method and it was successfully used to produce a stable Q-switched EDFL functioned at 1564.89 nm with 3-dB bandwidth of 1.54 nm. The SA film has a modulation depth of 40 %. As the input power adjusted from 107 mW to 245 mW, the pulse rate could be widely tuned from 68.46 kHz to 92.76 kHz, whereas its pulse duration reduced from 3.5 μs to 0.912 μs. The maximum pulse energy and average output power were obtained at 222.83 nJ and 20.67 mW, respectively. The result indicates that PEDOT: PSS has promising potential for utilizing in the development of different nonlinear photonic systems like Q-switched Erbium-doped fiber laser.
Penulis: Prof. Dr. Moh. Yasin, Drs., M.Si.
