1. Introduction

Q-switched ytterbium-doped fiber lasers (YDFLs) are of great research interest as they are the most reliable and robust type of fiber lasers [1]. They have an excellent power conversion efficiency and also broad gain bandwidth [2]. Additionally, Q-switched YDFL can be used in various applications such as medicine, manufacturing, sensing and communications to name a few [3,4]. Q-switching can be demonstrated by the insertion of a saturable absorber (SA) inside the cavity to acts as a self-operating Q-switcher [5]. Many types of SAs have been successfully demonstrated so far for various fiber laser applications such as semiconductor saturable absorber mirrors (SESAMs) and carbon nanotubes (CNTs) and graphene. SESAM is considered the most popular SA [6]. However, CNTs suppressed the dominance of SESAMs [7] due to their wider operating wavelength range and easier fabrication process [8]. Graphene, have also become a dominant choice for SAs [9], that is due to its ultrafast recovery time and high damage threshold [10]. However, both CNT and graphene nanomaterials have some disadvantages. CNT suffers from a complex bandgap control which limits its operation in certain applications [11] while graphene has a small optical absorption range and suffer from the difficulty to create an optical bandgap [12].

Transition metal dichalcogenides (TMDs) have attracted much attention as well. They have been used as SAs for fiber laser applications [13,14] , but TMD has a limited performance for a laser operating at the mid-infrared and near-infrared range due to their relatively large bandgap [15]. There has been an increasing concern about the effect of long-term exposure of these nanomaterials as they might be dangerous on human health [16]. On the other hand, organics materials (as a bio-compatible alternative) might have the potential to be used as a high-performance SA [17]. Organics materials have a large and ultrafast nonlinear response and they have broad spectral tunability as well as simple and low-cost fabrication process [18–20]. Furthermore, organic materials are environmentally-friendly and non-hazardous and they could increase the applications in biomedical laser processing and sensing [21]. Moreover, organic materials have the advantages of versatile molecular design and ease of control of physical properties [19]. Due to these attractive features, different areas have utilized organic materials, such as solar cells [22–24], light-emitting diodes [25,26], bistable memory devices [27,28] and organic thin film transistor [29,30].

In present work, we have used an organic material, that is Bis[2-(4,6-difluorophenyl) pyridinato-C2, N] (picolinato) iridium(III) (FIrpic), as SA in YDFL cavity to generate Q-switching pulses in 1 µm region. FIrpic has excellent stability and shows favourable electrochemical properties, additionally, it can be produced by a straightforward fabrication process. FIrpic has a high emission efficiency and suitable energy level and it remains stable during deposition regardless of the surface morphology [31].

2. Experimental setup

FIrpic was embedded in polyvinyl alcohol (PVA) to form a thin film. Pristine PVA and FIrpic-PVA thin films were prepared by solvent-casting technique. A 10 mg of FIrpic was added to 1 ml of deionized (DI) water and stirred for 60 minutes at 50° C. Then two 2 drops of acetone were added in order to distribute the FIrpic inside the solution equally. Next, 5 ml of PVA solution was added to the FIrpic solution. The PVA solution was produced by adding 100 mg of PVA into 10 ml of DI water following by ultra-sonication for 60 minutes at room temperature. The mixture was then stirred for 180 minutes. The mixture was poured into a well-covered borosilicate glass petri dish and left in room temperature for 72 h. The thickness and diameter of the thin film are about 50 µm and 60 mm, respectively. The mass fraction of FIrpic is about 16.7wt%, which was obtained by $\frac{{{w_{FIpric}}}}{{({{w_{FIpric}} + {w_{PVA}}} )}} \times 100$ , where ${w_{FIpric}}$ and ${w_{PVA}}$ are the weights of FIrpic and PVA, respectively [32].

The PVA and FIrpic-PVA thin films were characterized to analyze the chemical groups by using Perkin Elmer Spectrum 400 Fourier Transform Infrared Spectroscopy (FTIR) Spectrometer. The FTIR spectrum recorded within the range of 4000 to 450 cm−1 with a resolution of 2 cm⁻¹. Figure 1 (a) shows the FTIR Spectrum of PVA and FIrpic-PVA thin films. The PVA spectrum peaks at 3298 cm⁻¹ and shows strong and broad absorption at 3000–3600 cm⁻¹, which is attributed to the symmetrical stretching vibration of O–H from the intramolecular and intermolecular hydrogen bonds.

 

Fig. 1. (a) FTIR spectra (b) FTIR fingerprint region of FIrpic-PVA and PVA thin films

Download Full Size | PDF

The peaks in the PVA spectrum at 2925, 2852, 1729, 1089, 1023 and 839 cm⁻¹ correspond to stretching vibration of CH2, CH, C = O, C-O-C, C-O and C-C, respectively, while the peaks at 1437, 1370, 1242, 946 and 605 cm⁻¹ are attributed to the CH2 bending, (CH + OH) bending, CH wagging and CH2 rocking and (OH) wagging of PVA, respectively [32–35]. FIrpic-PVA thin film spectrum shows two peaks at 1641 and 1602 cm⁻¹ which belong to the stretching vibration of C = C and C = N in the FIpric molecules, respectively [36–38]. The fingerprint region of FTIR spectrum (1500-500 cm⁻¹) is illustrated in Fig. 1(b). The figure shows vibration peaks of small intensities which is due to the small fraction of the organic material. The peaks at the range of 1400−1000 cm⁻¹ correspond to C-F, C-N and C-O stretching vibration, while the peaks at the range between 1000-500 cm⁻¹ are attributed to the bending vibration of C-H and O-H bonds [39–41].

Figure 2(a) shows the optical absorbance spectrum of FIrpic-PVA thin film. The figure shows two asymmetric and broaden peaks centred at 388 and 1047 nm, which could belong to the absorption band of FIrpic and PVA doped with FIrpic molecules, respectively. The absorption band of FIrpic in CH2Cl2 solution was reported by Baranoff and Curchod to be at 256 nm [31]. The redshift of the FIrpic absorption band is attributed to its embedment in PVA polymer and the formation of the microparticles during fabrication. The PVA absorption band comes from the doped material. The optical band gap (${E_g}$) of the thin film can be obtained from ${({\alpha hv} )^n} = B({hv - {E_g}} )$, $\alpha $ is the absorption value, $hv$ is the photon energy, B is a constant, and n = 2 (as for direct transition). We can obtain the optical band gap which is equal to $h\nu $ (through linear extrapolation at the horizontal axis of ${({\alpha h\nu } )^2}$ against $h\nu $). The FIpric:PVA thin film shows two band gaps at about 2.73 and 2.14 eV, as can be seen in Fig. 2(b). These band gaps are attributed to the PVA doped by FIrpic and micro-particles of FIrpic. The bandgap of the PVA can be modified when it is doped with other materials to be 6.28 eV as reported in Ref. [42]. So, 16.7wt% of FIrpic significantly decreases the band gap of the PVA.

 

Fig. 2. (a) Optical absorption spectrum of FIrpic-PVA thin film (b) the calculated optical band gap (c) SEM image of FIrpic-PVA thin film (d) The power-dependent transmission of FIrpic-PVA

Download Full Size | PDF

Figure 2(c) shows SEM image of FIrpic-PVA thin film. The image shows a homogenous surface with some micron-sized particles in the range of 2−12 µm, the homogenous surface indicates a uniform distribution of FIrpic molecules along the polymer, while the particles might be attributed to FIrpic agglomeration powder. The saturable absorption was investigated as well to determine the nonlinear transmission performance of the prepared FIrpic-PVA thin film. The saturation intensity as well as modulation depth of the FIrpic-PVA are 5.5 MW/cm² and 12.8%, respectively as shown in Fig. 2(d).

Figure 3 illustrates the cavity setup. A 1.3 m of ytterbium-doped fiber (YDF) was used as a gain medium. The YDF has doping level and ion absorption of 1500 ppm and 280 dB/m at 920 nm, respectively. The fiber laser was pumped with a 980-nm laser diode (LD) through a 980/1064 wavelength division multiplexer (WDM). A small piece of 1 mm² of the fabricated FIrpic: PVA thin film was sandwiched between two FC/PC connectors in order to function as Q-switcher. An isolator was utilized to maintain unidirectional light propagation in the cavity, while a 50/50 optical coupler (OC) was used to output 50% of the light and keep 50% to oscillate inside the cavity. The rest of the cavity consists of standard SMF fiber making the cavity length about 5.5 m.

 

Fig. 3. Cavity Setup for YDFL laser

Download Full Size | PDF

3. Results and discussion

For the initial phases of the experiment, the laser operated in continuous wave (CW) regime at an incident pump power of 60 mW. Stable Q-switched operation was then observed when the LD power was raised to a threshold value of 86 mW and maintained its operation as the LD power was further increased up 147 mW. The spectrum of Q-switched fibre laser was measured by an optical spectrum analyzer. The central wavelength and 3-dB bandwidth were 1067 nm and 0.14 nm, respectively, as shown in Fig. 4(a).

 

Fig. 4. characteristics of Q-switched YDFL. (a) Output optical spectrum, (b) pulse train at 86 mW and shows and repetition rate of 46.2 kHz, inset figure shows a single pulse profile with a pulse width of 4.9 µs, (c) pulse train at 117 mW and shows and repetition rate of 54.6 kHz, inset figure shows a single pulse profile with a pulse width of 3.2 µs, (d) pulse train at147 mW and shows and repetition rate of 59.1 kHz, inset figure shows a pulse profile with a pulse width of 2.4 µs, (e) pulse width duration and repetition rate against input incident power, (f) pulse energy and output power versus input incident power, (g) peak power versus input power and (h) RF spectrum at maximum input incident power.

Download Full Size | PDF

The pulse trains of the Q-switching operation were recorded through 350-MHz oscilloscope together with a 2-GHz photodetector. Figure 4(b), (c) and (d) shows the output pulse trains of the Q-switching operation at different pump power inputs of 86, 117 and 147 mW, the figure show pulse widths of 4.9 µs, 3.2 µs and 2.4 µs, respectively. The decreasing pulse duration is due to gain compression in the cavity [43]. The pulse train shows uniform pulse peak intensity with no fluctuation nor distortion during this evolution process, which proves the high stability of the Q-switching operation. The inset in each figure shows a single-pulse profile and pulse width. The fiber laser showed a decrement in the pulse width (from 4.9 µs to 2.4 µs) and an increment in the repetition rate (from 46.2 kHz to 59.1 kHz) as the LD power was raised from threshold value to 147 mW, see Fig. 4(e). The pulse width can be reduced further through shortening the overall cavity length and enhance the cavity birefringence [44].

The pulse energy and average output power were relatively high, as illustrated in Fig. 4(f). That is due to the low inter-cavity loss and the low insertion loss and high performance of the SA [43], additionally, the 50:50 OC extract a large portion of signal power from the cavity. The pulse energy and the output power were increased from 195 to 465 nJ and 9 to 27.5 mW respectively, as the incident input power was increased from 86 mW to 147 mW. Figure 4(g) Illustrates the peak power performance of the fiber laser versus incident pump power, the peak power was relatively high and showed a linear increment from 39.8 to 193.9 mW.

The radio frequency (RF) spectrum was investigated (at 147 mW LD power) through 7.8 GHz RF spectrum analyzer, see Fig. 4(h). The inset figure shows the signal to noise ratio (SNR) of 66.4 dB, which indicates the stability of the fibre laser. It is also worth to notice that the proposed Q-switched laser operated stably in the laboratory condition for at least 48 hours without any noticeable degradation of performances such as the SNR. Based on the absorption spectrum of Fig. 2(a), it is expected that the Q-switching operation could also be achieved at longer wavelength region. It is worthy to mention that we have also repeated the experiment by incorporating the FIrpic SA in an erbium-doped fiber laser (EDFL) cavity. It is found that the Q-switching operation was also achieved in the EDFL cavity to operate at 1.5 µm wavelength region.

4. Conclusion

We demonstrated passive Q-switch fiber laser by using FIrpic as SA in 1 µm region. The SA thin film was prepared through the solvent-casting technique and was inserted between two FC/PC to act as Q-switcher in YDFL cavity. A stable passively Q-switched operation was achieved at a central wavelength of 1067 nm. The pulse width duration reduced from 4.9 to 2.4 and the repetition rate increased from 46.2 to 59.1 kHz as the incident power was varied between 86 to 147 mW. The peak power and pulse energy were relatively high and increased linearly to maximum values of 193.9 mW and 465 nJ, respectively. The results suggest that FIrpic, as an organic, bio-compatible and non- hazardous martial, can be used as SA in a YDFL cavity.