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Abstract
In this work, the influence of induced losses on the saturable absorption by zinc nanoparticles photodeposited onto the core of an optical fiber end is reported. Samples with different losses were obtained by the photodeposition technique using a continuous wave laser at 1550 nm. The nonlinear absorption of the saturable absorber was characterized by the P-scan technique using a high-gain pulsed erbium-doped fiber amplifier. The results have demonstrated that for optical fibers with variable induced losses by deposited nanoparticles, the modulation depth increases proportionally based on the nonlinear absorption coefficient. With induced losses fixed at 3 dB, it was demonstrated that the modulation depth increased as a function of the optical power used in the photodeposition process. The saturation intensity of the saturable absorber presents small shifts for higher intensities.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Saturable absorption devices have been widely used as photonic devices, such as in pulsed Q-switched laser applications [1–3], mode-locking [4,5], and noise filtering [6]. These devices have been implemented based on dyes, doped crystals with Cr4+ [7], or semiconductor saturable absorber mirror (SESAM) for ultra-short pulse generation [8]. However, the SESAM are more expensive than the saturable absorbers based on nanostructured materials [9].
The development of nanotechnology has enabled the implementation of saturable absorbers (SA) using nanostructured materials in optical fiber cavities for pulsed lasers [10], where graphene and carbon nanotubes have been widely studied. Graphene has been used as an SA device for switching applications and pulse generation, ideal for optical communication applications [11–13]. Both single-wall and multi-wall carbon nanotubes have also been used as devices in SA for short pulse generation and Q-switched lasers for wavelengths from 532 nm to 1550 nm [14–17].
Recently, nanostructured metallic materials have begun drawing attention owing to their nonlinear properties based on the phenomenon of surface plasmon [18, 19]. For instance, the dependence of the extinction coefficient of zinc nanoparticles (ZnNPs) as a function of the wavelength and nanoparticle radius has been reported. The nonlinear behavior of ZnNPs deposited onto the core of an optical fiber with induced losses at 3dB was reported in [19].
In this work, the dependence of the saturable absorption of ZnNPs photodeposited onto the core of an optical fiber end for different induced losses is studied. The dependence of the basic parameters of the saturable absorber regarding the induced losses is characterized. The results obtained in this work may be applicable in optical fiber pulsed lasers, noise filtering, or sensing devices.
2. Experimental description
The ZnNPs photodeposition onto the optical fiber end was carried out by propagating a continuous wave laser radiation at 1550 nm through an optical fiber according to the procedure previously reported in [19]. The experimental setup of the photodeposition technique is shown Fig. 1.
The end face of an optical fiber was introduced into a solution with 10 ml of isopropyl alcohol and 10 mg of zinc powder. The solution was previously mixed by using an ultrasonic bath for 20 min. After that, the solution was left for several hours without movement so that heavier ZnNPs were precipitated. The aim of this process was to obtain a colloidal solution with smaller and uniform nanoparticles.
The study of the nonlinear transmission was performed using a high-gain pulsed Er3+-doped fiber amplifier (HP-EDFA) previously reported in [20]. In this work, the HP-EDFA was set at a frequency of 2 kHz with a temporal pulse duration of 20 ns to provide an output intensity of ∼450 MW/cm2. In this case, the samples prepared by the photodeposition technique were spliced to the HP-EDFA to carry out transmission characterization, as shown in Fig. 2. A power meter was used to measure the output response with the aim to capture the data transmission on a processor.
Fig. 2 Experimental setup to measure the nonlinear transmission of ZnNPs photodeposited onto the optical fiber end.
3. Theory
Optical forces interacting in the photodeposition process of ZnNPs were previously reported in reference [19]. Otherwise, the transmission of the samples was analyzed applying the Beer-Lambert law [21]:
where α0 and β are the linear and nonlinear absorption coefficients, respectively; L is the sample length; and I is the intensity. We considered a saturation model and, therefore, used a hyperbolic approximation [22]. where Isat is the saturation intensity, which is defined as the intensity when the transmission has reached 50% modulation depth.By combining Eqs. (1) and (2), we obtained the transmittance expression:
The material nonlinear susceptibility calculation has the following expression in the international system (SI):
where λ is the wavelength, ε0 is the permittivity in free space, and n0 is the refractive index of the ZnNPs.In this work, it is considered a fast saturable absorber model with response time shorter than the pulse-width of the pulsed source (HP-EDFA).
4. Results
A first set of samples was prepared for a fixed continuous wave laser power of 50 mW at 1550 nm in the photodeposition process. As shown in Table 1, the induced losses by the ZnNPs deposited onto the optical fiber end were 0.5, 1.5, 3 and 5.2 dB, respectively. These losses in transmission were increasing owing to the amount of nanoparticles deposited during the photodeposition process [19].
Table 1. Induced losses in the samples. The input laser power was fixed at 50 mW during the photodeposition process.
The experimental results of the nonlinear characterization corresponding to samples A1–A4 are shown in Fig. 3. In this figure, the dots represent the transmission behavior regarding the incident intensities of the HP-EDFA. The continuous line represents the fitting of the experimental measurement through Eq. (3).
Fig. 3 Nonlinear characterization of ZnNPs photodeposited onto the optical fiber end. Blue solid lines indicate a fit to the data.
In Fig. 3, it is possible to observe that for samples A1, A2, A3, and A4, their initial transmissions were 68%, 50%, 45%, and 25% at low intensities, reaching their saturation levels at a transmission of approximately 90%, 70%, 55%, and 30%, respectively. As a result, the modulation depth for each sample was 22% (sample A1), 18% (sample A2), 10% (sample A3), and 5% (sample A4). In this figure, it also can be seen that the transmission levels were higher for samples with lower induced losses.
The nonlinear absorption (β) and third-order nonlinear susceptibility [Im(χ(3))] calculated for samples A1 to A4 are shown in Table 2. Here, the values of the third-order nonlinear susceptibility decreased slightly from −3.789 × 10−7 to −3.044 × 10−7 esu. In our opinion, this is because, despite having different induced power losses in the photodeposition process, the input laser power was fixed at 50 mW. This is indicative that the deposited nanoparticles have approximately similar sizes according to [19], obtaining similar saturation currents and maintaining very similar values of the nonlinear susceptibility. The modulation depth behavior obtained for each sample can be attributed mainly to the induced losses, resulting in a decrease modulation depth by the increase induced losses and vice versa.
Table 2. Values of the nonlinear absorption coefficient (β), third-order nonlinear susceptibility [Im(χ(3))], and modulation depth for samples with induced losses.
Figure 4(a) shows only the fit of each experimental curve obtained in Fig. 3, and Fig. 4(b) represents the modulation depth and the nonlinear susceptibility as a function of induced losses. In this figure, it can be observed that the modulation depth tended to decrease when the values of the induced losses increased; accordingly, the optical nonlinearity increased to the highest third-order values.
Fig. 4 (a) Curves fitting for nonlinear characterization of samples; (b) modulation depth and nonlinear susceptibility regarding induced losses on the samples.
Additionally, four samples with losses fixed at 3 dB (50%) were obtained for different input laser power levels in the photodeposition process of ZnNPs as shown in Table 3.
Table 3. Induced losses of 50% (3 dB) in samples at different Pin in photodeposition.
Figure 5 shows the experimental results of the ZnNPs deposited onto the core of an optical fiber for samples B1–B4. Figure 5(a) shows the response of sample B1, which has a transmission at low power of 46% with a saturation level of approximately 53%, producing a modulation depth of 7%. In Fig. 5(b), the response of sample B2 has a low transmission of 40% and a saturation level of approximately 53%, which results in a modulation depth of 13%. Figure 5(c) shows the results corresponding to sample B3, which has a low transmission of 37%, reaching its saturation level at 52% and obtaining a modulation depth of 15%. Sample B4 has a transmission at low intensities of 31%, approximately 52% saturation, giving a modulation depth of 21% as shown in Fig. 5(d).
Fig. 5 Nonlinear characterization of samples B1 to B4 with losses fixed at 3 dB. Blue solid lines indicate a fit to the data.
The nonlinear parameters (β) and [Im(χ(3))] calculated for samples B1–B4 are shown in Table 4. In Fig. 5, the blue solid lines represent a fit to the experimental curves obtained, using Eq. (3).
Table 4. Values of the nonlinear absorption coefficient, third-order nonlinear susceptibility, and modulation depth for samples B1–B4
The saturation intensity presents a shift to higher saturation intensities. The nonlinear parameter values present an increase inversely proportional to the input power, because the saturation intensity depends on the input power for photodeposition, as shown in Fig. 6(a).
Fig. 6 (a) Curve fitting of nonlinear characterization corresponding to samples B1 to B4. (b) Modulation depth and nonlinear susceptibility as a function of input power. Losses are fixed at 3dB.
In the case of samples with ZnNPs photodeposited with losses fixed at 3 dB, the saturation level is maintained with the transmission approximately 50% and 55% because the resulting losses are equal. Values of the third-order nonlinear susceptibility show an increase from −3.041 × 10−7 to −6.774 × 10−7 esu, a greater shift than samples A1–A4 with different induced losses. This behavior can be attributed to the different input pump power in the process of nanoparticles photodeposition, with the aim to maintain the losses at 3dB in each sample. Therefore, the sizes of the photodeposited ZnNPs were different for each sample, causing a shift in the saturation for each size of the nanoparticles and then changing the values of the nonlinear susceptibility.
The modulation depth and nonlinear susceptibility performance regarding the input power during the photodeposition process are shown in Fig. 6(b). It is observed that with increasing input power, the modulation depth is reduced, accordingly the optical nonlinearity decreases to the lowest third-order values.
According to the results obtained in this work, it can be seen that it is possible to obtain transmission levels and modulation depths adjustable for a ZnNPs-based SA by adjusting the induced losses or the input power applied during the photodeposition process. These results could be of use in filters in optical communication systems, sensing devices, mode-locking fiber laser or Q-switched fiber laser [23]. It is noteworthy that the samples are repeatable without detachment of ZnNPs that may alter the results.
5. Conclusions
The present work reported a study on the saturable absorption of photodeposited zinc nanoparticles onto the core of an optical fiber. The photodeposition technique was used to carry out the implementation of optical fibers with different transmissions due to the induced losses by the nanoparticles onto the core. The results showed that the nonlinear parameters of the nonlinear absorption coefficient and third-order nonlinear susceptibility increased proportionally with the induced losses. However, the modulation depth took higher values when the induced losses were very low; this effect can be used for applications in noise-filtering devices. On the other hand, the results showed that, with constant induced losses in the optical fiber, the properties of absorption coefficient and nonlinear susceptibility increased when the laser power used to implement the sample increased. It should be noted that under these conditions, it is possible to select the saturation intensity, which can be useful for the design of Q-switched optical fiber lasers.
Funding
Benemérita Universidad Autónoma de Puebla VIEP grant (00019); PROMEP Red Temática de Colaboración 2016, “Nanociencia y Nanotecnología”.
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