Since the industrial revolution in the 1870s, most of the technological advancements have been electron-driven. In recent years, more and more technologies are switching from electron-driven to photon-driven. For instance, 5G telecommunication technology uses using optical fiber system as a backbone. Aside from telecommunication, photon-driven technologies have been employed in the developments of new energy, super-computer, and other frontier areas. One of the important frontier researches is in ultrafast laser development. These lasers have attracted significant attention for use in material processing, nano-scale imaging, and femtosecond time spectroscopy due to their excellent properties of high pulse energy and low thermal impact. Passive mode-locking based on a saturable absorber (SA) is known as the efficient and economical technology to generate femtosecond pulses. As such, many new SAs have been explored in recent years to develop efficient and low-cost ultrafast lasers for various applications.
Lately, organic materials (OMs) have been utilized in different technology such as bi-stable memory devices, organic thin-film transistors, and solar energy. They exhibit high flexibility in the control process and have significant properties that are favorable in the design of multifunctional molecules. The conducting polymer of poly(3,4-ethylene dioxythiophene) polystyrene sulfonate (PEDOT: PSS) is an important organic material that has been intensively studied in recent years. The PEDOT: PSS has the advantages of flexibility, thermal stability, outstanding film-forming ability, and wet-process preparation. The exploration of OMs in ultrafast fiber laser system is still far behind compared to other low-dimensional materials. In this work, we proposed and demonstrated the employment of OM in mode-locked pulse generation at 1.5 μm region by using spin-coated PEDOT: PSS thin film as SA. To the best of our knowledge, this is the first time that PEDOT: PSS material has been utilized in a fiber laser system to induce soliton pulse generation.
The prepared spin-coated PEDOT: PSS film-based SA was integrated into an Erbium-doped fiber laser (EDFL) cavity to induce mode-locking operation in a 1.5 μm region. A 70 m long standard single-mode fiber (SMF) was added into the cavity to ensure large anomalous dispersion so that the net cavity dispersion in the anomalous regime is about −1.59 ps2. The EDF and SMF, have group velocity dispersion (GVD) of 27.6 ps2/km and – 21.7 ps2/km respectively. The total cavity length is about 83 m, which corresponds to pulse repetition rate of about 2.437 MHz and a round-trip time of 410.4 ns. Polarization controller (PC) and isolator were used to control the nonlinear losses and to force the unidirectional operation of the cavity, respectively. A 90/10 coupler was utilized to extract 10% of the power from the cavity. An optical spectrum analyzer (OSA), oscilloscope, radio frequency (RF) spectrum analyzer, and autocorrelator were used to analyze the characteristic of output pulses.
The output spectrum investigation versus time was analyzed to demonstrate the long-time stability of the mode-locking laser operation. The laser wavelength was taken every 5 min for a total duration of 1 h. All samples were operated at 1567.6 nm with an optical power intensity of −16.8 dBm. All samples were almost identical each time, and this confirms the stability of the pulse laser. Also, this proves that the proposed material can work for a long duration without any pulse disturbance. The radiofrequency (RF) spectrum, which could be utilized to infer the laser stability. The fundamental frequency was obtained at 2.437 MHz with a signal-to-noise ratio (SNR) of 55 dB, indicating relatively low-amplitude fluctuations. The spectrum also shows high cavity harmonics (on a span of 101.3 MHz), implying stable mode-locked operation performance. Both pulse energy and output power are incremented linearly with input LD power. At an input LD power of 295 mW, the average pulse energy and output power were around 2.79 nJ and 6.89 mW, respectively. The maximum peak power was obtained at 1.43 kW. Following the realization of stable mode-locking operation through utilizing the PEDOT: PSS SA device in the EDFL cavity, the same experiment was also conducted using the same setup without the incorporation of SA. No mode-locking operation could be observed at any polarization controller position or pump power level, which confirmed that the saturable absorption behavior is attributed to the PEDOT: PSS material. The result also shows the performance comparison of PEDOT: PSS PVA film SA with other nanomaterial-based SAs. The performance of mode-locking operation using PEDOT: PSS SA is comparable with most of the reported SAs. More importantly, the reported results have proven the potential of organic material for the ultrafast laser application.
In conclusion, the SA film was prepared by spin-coating technology, and it was successfully used to produce soliton mode-locked laser operating at 1567.6 nm with 3-dB bandwidth of 1.55 nm. The PEDOT: PSS PVA SA has a modulation depth of 22% with a saturation intensity of 0.21 . The pulse energy and average output power were realized at 2.79 nJ and 6.89 mW at a pump power of 295 mW. The result indicates that PEDOT: PSS is favorable for potential use in the development of various photonic devices based on nonlinear optics, including ultrafast fiber lasers that operate in a 1.5 μm region.
Author: Prof. Dr. Moh. Yasin, M.Si.
Details of the research can be viewed here:
https://www.sciencedirect.com/search?authors=Moh.%20Yasin&pub=Journal%20of%20Luminescence&cid=271627
