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Abstract
A high pulse energy Q-switched ytterbium-doped fiber laser (YDFL) based on copper oxide (CuO) nanoparticles as a saturable absorber (SA) is demonstrated. The CuO nanoparticles have been fabricated into a thin film using a liquid-phase exfoliation method then it was integrated into a laser cavity to act as Q-switcher. The proposed Q-switched YDFL operates at a central wavelength of 1035.4 nm with a 3-dB bandwidth of 0.26 nm. The Q-switched laser has a repetition rate range from 57 kHz to 104 kHz by varying the pump power from 179 mW to 226.5 mW, while the pulse width was tuned from 4.5 µs to 2.2 µs. Relatively, high pulse energy around 0.192 µJ was achieved at an average output power of 20 mW. This laser with high pulse energy can be seen as a very promising pulsed laser source in many industrial and optical sensing applications. To the best of our knowledge, this is the first demonstration of a Q-switched fiber laser using CuO-SA in YDF.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Q-switching is a technique that can produce short pulse emission with high pulse energy by modulating the intracavity losses [1]. The Q-switching can be achieved by either active or passive techniques based on the modulator used. The passive Q-switching technique using saturable absorbers (SAs) is much preferred over the active one because it got the advantages of simplicity, more stable, and low cost [1,2]. Widely, saturable absorbers can be classified into artificial SAs (such as Nonlinear Polarization Rotation (NPR) [3,4] and Nonlinear-Optical Loop Mirror (NOLM) [5,6], and real SAs. However, real SAs are largely preferred owing to their excellent merits of more compact and ambient insensitive [7]. The first Q-switched lasers based on real SAs were demonstrated in 1964 [8,9], and since that and on, the development of the passive Q-switched fiber lasers are increasing dramatically through the exploration of new generations of SAs.
During last two decades, many SAs were proposed and intensively investigated for the generation of Q-switched fiber laser such as semiconductor saturable absorber mirror (SESAM) [10], Zero dimensional (0D) nanomaterials [11], one dimensional (1D) (carbon nanotube) [12], and two-dimensional (2D) (graphene [13], transition metal dichalcogenides (TMDs) [14], topological insulators (TIs) [15] and black phosphorous (BP) [16]). Yet, each of those SAs have got some of the limitations that restrict its range of applications [17]. Recently, a new generation of saturable absorbers based on transition metal oxides (TMOs) nanomaterials were exploited owing to their large optical nonlinearity, high optical damage threshold, fast recovery time, suitable modulation depth, and the advantages of good thermal and chemical stability in addition to mechanical strength [17,18]. Those very valuable optical properties of the TMO made them a very promising SAs for pulse generation in fiber laser applications [19,20]. Many TMO nanomaterials such as zinc oxide (ZnO) [17,21], titanium dioxide (TiO2) [22], ferroferric-oxide (Fe3O4) [23], NiO, and Aluminum oxide [24] were successfully utilized as an efficient SAs for the generation of Q-switched and mode-locked pulses in various cavity configurations at different operation wavelengths (1, 1.5, and 2 µm). Moreover, MXene $T{i_3}{C_2}{T_x}$ (T = F, O, or OH) is a new type of 2D material that was used as SA and that got high attention of the researchers because of its high nonlinear optical properties to produce ultrafast laser [25,26]. Chen et al. reported a stable Q-switched ytterbium-doped fiber laser using Fe3O4 SA with repetition rate from 18 kHz to 59 kHz and shortest pulse width of 2.68 µs [27]. Ahmad et al. demonstrated Q-switched based on reduced graphene oxide-sliver SA in 1 and 1.5 µm operation wavelength at 1044.4 nm and 1560 nm respectively [28]. On the other hand, copper oxide (CuO) is a very interesting nanomaterial that also belongs to TMOs family [29]. Due to its very remarkable optical and physical properties, CuO was comprehensively studied and used in various applications including optical switches, field emitters, solar cells, photovoltaic devices, and gas sensors [30–33].
Most recently, CuO was proposed as a novel SA candidate for an erbium-doped fiber laser (EDFL) demonstration. Many valuable and very interesting advantages such as superconductivity at high temperature, friendly to the environment, high third-order optical nonlinearity, very simple fabrication process, appropriate damage threshold, and ultrafast relaxation and recombination dynamics time in the picosecond range [34–37] have motivated the researchers to utilize CuO as SA for fiber laser application. A passively Q-switched erbium-doped fiber laser (EDFL) was demonstrated by using CuO thin film as SA [38]. The constructed Q-switched laser operates at a wavelength of 1560 nm with a maximum repetition rate and shortest pulse width of 83 kHz and 2.6 µs respectively. The same authors were also successfully used CuO-SA to experimentally demonstrate soliton mode-locked EDFL in C-band, which has a modulation depth of 3.5% and a saturation intensity of 3.3 MW/cm2 [37]. Nonetheless, CuO was not yet reported as a SA to generate optical pulses at 1 µm region. CuO with a smaller bandgap around 1.2 eV [39] when compared to other nanomaterials belong to the same family (for instant ZnO, Al2O3, and TiO2) is more suitable in broadband saturable absorption for near IR pulsed laser system [37,38]. It was also reported that CuO-SA can be an efficient saturable absorber for fiber laser demonstration in 1.5 µm wavelength region [38].
On the other hand, CuO is a p-type semiconducting compound with a monoclinic structure. It has useful properties, such as low cost, relatively stable in terms of both chemical and physical properties, high temperature superconductivity, electron correlation effects, and spin dynamics [40,41]. It has a small indirect bandgap 1-1.5 eV and has been studied for photothermal and photoconductive, gas sensing, and solar conversion applications [42–45]. Besides, CuO is easily blended with polarized liquids (i.e., water) and polymers. Moreover, CuO nanostructures have recently attracted more attention in the research area due to their mechanical, electrical, optical, and magnetic properties [46]. Due to the quantum confinement effect in the nanoscale, optical band gap of CuO nanoparticles has been reported to shift into the blue region which is useful in photovoltaic and photocatalytic applications [47]. CuO particles have shown a higher optical nonlinearity compared to Cu particles [48]. Furthermore, the hybridization of CuO nanoparticles in the presence of polymers as stabilizers is useful to form a thin film of CuO nanoparticles.
In this work, we used a CuO-based saturable absorber in a sample 1-µm ring laser cavity and achieved Q-switched pulsed laser. The Q-switched pulses have high pulse energy and peak power of 0.192 µJ and 82.12 W, respectively. As a result, CuO film could be an excellent candidate for developing ultrafast fiber laser. Moreover, it has a bandgap of 1.78 eV with a simple fabrication process and low-cost. A Q-switched ytterbium-doped fiber laser (YDFL) is demonstrated using CuO as saturable absorber for the first time as far as the authors know.
2. Fabrication process and characterization of CuO thin film
2.1 Fabrication process of CuO thin film
To easily integrate the CuO nanoparticles inside the optical resonator of the laser system, it must be formed into a thin film. The liquid phase exfoliation method was used to prepare the CuO thin film as drawn in Fig. 1. At first, the polymer solution as the host material which is in this case polyethylene alcohol (PVA) was fabricated by solving 1 g of PVA powder in 120 ml of deionized (DI) water. For the mixture to be homogenous, it was stirred for about three hours using a magnetic stirrer. Then, a very small amount of CuO nanoparticles around 5 mg were purchased from Sigma-Aldrich mixed with the host material and again stirred for another three hours. After that, the resultant mixture was poured onto a petri dishes and left for three days at room temperature till it completely dried to form CuO:PVA composite thin film. Finally, the CuO film SA is successfully fabricated. The thickness of the thin film was measured to be around 25 µm using a stylus profiler (KLA Tencor P-6).
It is known that the optical properties of metal nanoparticles can be enhanced by surface plasmon resonance of the nanoparticles which occurs from resonant oscillations of free electrons of the nanoparticle surfaces in the presence of light [49]. On the other hand, the ratio of surface area to volume of the nanoparticles has a significant effect on the optical properties of the CuO due to the high concentrations of the carriers on the surface [48]. The broad surface plasmon resonance peak of embedded CuO nanoparticles in PVA polymer thin film was centered at a wavelength of 1000 nm, as shown in Fig. 4, which means a strong absorption in the infrared region of the spectrum. Moreover, the nonlinear optical response of the thin film can result from electronic and nonelectronic processes [50]. For the electronic process, the small bandgap of CuO:PVA thin film leads to a transition from the ground states to the excitation state by the absorption of low photon energy in the infrared region. Increasing the photon power intensity results in growth in the number of the excitation of CuO molecules until almost all electrons on the outer band transited to higher-level reaching into the saturation process. Furthermore, the nonelectronic responses are related to non-radiative interactions which depend on the temperature mainly in the laser heating for the contribution of the electron transitions [51].
2.2 Characterization of CuO thin film
The surface morphology of the CuO:PVA thin film is presented in Fig. 2(a). The FESEM image is clearly seen a smooth film with good dispersion of CuO nanoparticles along the thin film. Figure 2(b) showed the histogram of CuO nanoparticles dispersion and the average size of these particles is around 38 nm with a standard deviation of 5 nm. The small standard deviation reveals the uniform size of the nanoparticles. Thus, the Liquid-phase exfoliation method with using the host materials is a suitable method to distribute the CuO nanoparticles in the thin film.
Fig. 2. (a) FESEM image of the CuO thin film. (b) Particle size distribution histogram of CuO nanoparticles and black solid line following the Gaussian fit distribution.
The optical properties of CuO:PVA composite thin film were examined by Raman and UV-VIS-NIR spectroscopies. Figure 3 shows the Raman spectrum, which was obtained by launching a 514 nm laser beam into the CuO:PVA thin film. In the experiment, laser power and exposure time were fixed at 1 mW, and 30 s, respectively. Two peaks are clearly observed at 2935 and 1444 cm−1 which belong to PVA material [17]. Enlarging the spectrum inside the black empty box is presented in an inset figure. Three Raman shifts at around 631, 361, and 292 cm−1 are corresponding to the active modes of CuO of two Bg and one Ag modes, respectively [52,53].
Fig. 3. Raman spectrum of PVA:CuO thin film. An inset figure is an enlarging and smoothing spectrum at range of 100-800 cm−1.
Figure 4 shows the UV-VIS-NIR absorption spectrum of CuO:PVA thin film. A broad absorbance band is at about 900-1300 nm. The appearance of the broadband at the near-infrared region is attributed to the size effect [54]. As known, CuO is a p-type semiconductor and the bandgap, ${E_g}$, of the CuO:PVA thin film can be obtained from the direct band gap equation ${({\alpha hv} )^2} = B({hv - {E_g}} )$, which is shown smooth curve for direct transition equation, by extrapolating $hv$ to $\alpha = 0$. Where $hv\; $ is the photon energy, B is a constant relative to the material, and $\alpha $ is the absorption coefficient. The absorption coefficient can be calculated by Beer-Lambert’s law as $(v )= 2.303 \times Abs(\lambda )/d$, where d is the thin film thickness. The calculated band gap was about 1.78 eV which is slightly bigger than the bulk CuO (1.5 eV) due to the quantum confinement effect of the nanoscale and the formation of surface defects by the PVA molecular [42,55,56].
Fig. 4. Optical absorption spectrum of CuO:PVA thin film. An inset figure is the calculated optical band gap obtained by extrapolation to $\alpha $ = 0.
The nonlinear optical absorption property of CuO film was measured using a twin-detector balance technique to investigate its saturable absorption profile as shown in Fig. 5. A mode-locked fiber laser was used as an input pulse source with a repetition rate of 18 MHz and a pulse width of 2.2 ps. It was linked to a low-dispersion optical amplifier, a variable attenuator, and then to a 50:50 dB coupler. The first port of the coupler was utilized for the power-dependent absorption records of CuO film and the second port was used as a reference. The experimental data for nonlinear optical absorption was prepared depending on two-level SA model of α(I)=αs/(1+I/Isat)+αns, where α(I) is the absorption coefficient, αs is the modulation depth, I is the input intensity, Isat is the saturation intensity, and αns is the non-saturation loss, respectively [57]. Thus, the modulation depth is 4.8% with a saturation intensity of 9 MW/cm2.
3. Ring laser configuration
The ring cavity configuration of the Q-switched YDFL based on CuO saturable absorber is drawn in Fig. 6. The YDF was pumped by a 980 nm laser diode (LD) via a 980/1060 wavelength division multiplexer (WDM). The gain medium of the laser was a 2 m length YDF with an absorption of 23 dB/m at 980 nm, whereas the numerical aperture (NA), group velocity dispersion (GVD), and the core and cladding diameters are 0.16, 27.6 ps2 / km, 4 µm, 125 µm, respectively. The unidirectionality of the light inside the optical resonator is ensured by using a polarization-insensitive isolator which is connected after the gain medium directly. Then, the CuO -SA is integrated between the isolator and 3 dB optical coupler. The SA is simply formed by inserting the pre-fabricated CuO:PVA between two fiber ferrules. The SA insertion loss was measured to be approximately 0.8 dB. Around 50% of the light was feedback to the cavity through the 1060 nm port of the WDM, and the other 50% used as an output of the laser. The cavity length of the proposed laser was approximately 5 m. The output laser analyzed using an optical spectrum analyzer (OSA) with a spectral resolution of 0.07 nm, 500-MHz oscilloscope (OSC) via a photodetector, and a 7.8 GHz RF spectrum.
4. Result and discussion
A small piece of the fabricated CuO:PVA thin film was cut and inserted inside the optical resonator of the produced laser to serve as a passive Q-switcher. After integrating the CuO-SA in the laser system and gradually increasing the pump power, a CW lasing started at a power of 100 mW. Then by further increasing the pump power to 179 mW, self-started Q-switching pulses were clearly seen. The optical spectrum of the demonstrated Q-switched YDFL based CuO-SA is shown in Fig. 7. It can be seen that only a single wavelength was obtained and the laser operates at a central wavelength of 1035.4 nm with a 3-dB spectral bandwidth (Δλ) of 0.26 nm. The oscilloscope trace of the generated Q-switched pulses is shown in Figs. 8(a)–8(c), with very stable pulse trains at three different pump powers of 179, 192.8, and 226.5 mW with repetition rates around 57, 73, and 104 kHz, respectively. It is worthwhile to note that the Q-switching operation remained stable even as the pump power changes, with no damage threshold of CuO-SA was observed. This observation indicates that the damage threshold of the SA was higher than the available maximum pump power that was applied to the laser cavity.
The quality of the released Q-switched pulses depends mainly on the SA and pump power. At the threshold pump power, the Q-switched pulses started with a repetition rate of 57 kHz and it was increasing by increasing the pump power until it reaches 104 kHz at the maximum pump power of 226.5 mW. On the other hand, the pulse width was decreased from 4.5 µs to 2.2 µs. This increment of the repetition rate and decrement of the pulse width of the laser pulses with the pump power is the main property of the Q-switching operation. When the pump power increased, the energy required to saturate the SA will be stored faster in the laser cavity and consequently lead to a rise in the repetition rate and reduce the width of the pulse. This reverse variation of the repetition rate and pulse width with the increment of the pump power is illustrated in Fig. 9. The output power and the pulse energy were also investigated with the variation of the pump power as shown in Fig. 10. The output power was linearly increased from 10.3 mW to a maximum value of 20 mW by increasing the pump power from 179 mW to 226.5 mW. This mainly related to the ytterbium ion in the gain medium, where more electrons can be excited to the higher energy level when increasing the pump power, and that consequently increase the output power of the generated Q-switched pulses [58]. By further increasing the pump power higher than 226.5 mW, the pulse started to disappear due to the saturated state of the SA. While the maximum released pulse energy was about 0.192µJ at the maximum pump power of 226.5 mW.
For the signal to noise ratio (SNR) investigation, a radio frequency (RF) analyzer was then used to investigate the stability of the Q-switched pulse as shown in Fig. 11. Good stability was observed with SNR of 52 dB at the maximum repetition rate of 104 kHz. No presence of any pulse operation on the oscilloscope or RF analyzer before inserting and after removing the CuO-SA in the laser system at any pump power which confirms the only attribution of the SA in generating the Q-switched pulses. Finally, the performance of the CuO-SA in releasing Q-switched pulses from YDFL was compared with the performance of other TMOs materials such as Zinc oxide (ZnO), Iron Oxide (Fe3O4), cobalt oxide (Co3O4), Titanium dioxide (TiO2), Nickel oxide (NiO), and Vanadium oxides (V2O5) as detailed in Table 1. From the comparison, we can see that we have achieved the highest pulse energy, repetition rate, and output power over all other TMOs materials. These results make CuO the best TMOs materials as a SA for releasing Q-switching pulses from YDFL. Such laser with high pulses energy, output power, and repetition rate is very promising for various applications such as fiber sensors, laser cutting, laser welding, laser engraving, and many other industrial applications [59].
Table 1. Performance comparison for various Q-switched YDFLs using TMOs
5. Conclusion
CuO nanomaterial was formed into a thin film using the liquid phase exfoliation method and used as a saturable absorber for generating high energy Q-switched pulses in YDFL. The demonstrated laser operates with a central wavelength of 1035.4 nm. The repetition rate was varying from 57 kHz to 104 kHz and pulse width from 4.5 µs to 2.2 µs by rising the pump power from 179 to 226.5 mW. The maximum output power was measured to be 20 mW, and the maximum calculated pulse energy was 0.192 µJ. We believe that the constructed laser can be a potential light source in industrial and optical sensing applications.
Disclosures
The authors declare no conflicts of interest
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