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Saturable absorber incorporating graphene oxide polymer composite through dip coating for mode-locked fiber laser
Optical Materials 100 (2020) 109619
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Saturable absorber incorporating graphene oxide polymer composite through dip coating for mode-locked fiber laser E.K. Ng a, K.Y. Lau b, H.K. Lee c, M.H. Abu Bakar a, Y. Mustapha Kamil c, M.F. Omar d, M. A. Mahdi a, * a
Wireless and Photonics Networks Research Centre, Faculty of Engineering, Universiti Putra Malaysia, 43400, UPM Serdang, Selangor, Malaysia Department of Electronics and Nanoengineering, Aalto University, Espoo, 02150, Finland InLazer Dynamics Sdn Bhd InnoHub Unit, Putra Science Park Universiti Putra Malaysia, 43400, UPM Serdang, Selangor, Malaysia d Physics Department, Faculty of Science, Universiti Teknologi Malaysia, 81310, Skudai, Johor, Malaysia b c
A R T I C L E I N F O
A B S T R A C T
Keywords: fiber lasers Saturable absorber Optical pulses Microfiber
Fabrication of microfiber saturable absorbers (SA) is a tedious process. This work demonstrates a simplified method of microfiber SA fabrication by integrating graphene oxide/polymethylsiloxane (GO/PDMS) using dipcoating technique. The idea is to enhance light-matter interaction between graphene and propagating light modes within the microfiber by enhancing the binding strength and proximity of the graphene-polymer com posite to the microfiber. The implementation of GO/PDMS SA in an erbium-doped fiber laser cavity has suc cessfully generated ultrashort pulses with pulse duration of 629 fs in the C-band emission region. The success of this work contributes to better insight towards simplifying the fabrication technique of microfiber SAs.
1. Introduction Since its discovery, mode-locked fiber lasers have exhibited excep tional potential in facilitating a broad range of scientific applications such as metrology, laser processing, telecommunications and biomed ical diagnostics. Numerous techniques have been proposed for the generation of passive mode-locked fiber laser which includes the deployment of nonlinear optical loop mirror (NOLM), acoustic-optical modulator (AOM), and saturable absorber (SA). The mode-locking scheme generated from NOLM requires a lengthy cavity since strong birefringence is an essential parameter induced by the Kerr nonlinearity of the optical fiber. An AOM provides the advantage of better control over the output pulses. However, this approach has inherent issues which limit the perfor mance, such as modulator bandwidth constraints, bulky size and high manufacturing cost. An alternative to mode-locked fiber laser genera tion is using a SA, an optical component with an optical loss which absorption reduces at high optical intensities. In its early development, SA was demonstrated using semiconductors. Keller et al. [1] proposed the first semiconductor saturable absorber mirror (SESAM) through defect engineering and micro-fabrication growth [2]. However, SESAM has shown many pronounced limitations such as complex fabrication,
cost ineffective, and limited operating bandwidth [3]. This has moti vated studies on a new class of SA using 2D carbon-based materials. Carbon nanotube (CNT), for example, is a cost-effective material which has shown feasible integration into fiber laser systems operating in sub-picosecond regime [4]. Despite, the success, CNT has also shown the requirement of diameter and chirality control for energy bandgap design which can be tedious [5,6]. Graphene, in different forms such as single layer graphene (SLG), multiple layer graphene (MLG), and graphene oxide (GO), have been comprehensively demonstrated as SAs. SLG and MLG are synthesized through chemical vapor deposition [7,8], whereas the GO can be ob tained using the modified Hummers’ method [9]. In contrast to gra phene, GO contains high density oxygen functional groups, which contributes to water and organic solvent solubility [10]. Numerous methods have been employed to deposit the graphene-based materials onto the optical fiber for SA fabrication [11]. For instance, the fiber ferrule structure is the simplest architecture where the graphene is placed in between two fiber ferrules to form the SA. Nevertheless, the low thermal damage threshold of this structure shows the limitation in operating the SA with high pump power. A solution to this issue is by generating an evanescent field on the optical fiber that can interact with graphene-based materials to produce mode-locked laser [12].
* Corresponding author. E-mail address: [email protected] (M.A. Mahdi). https://doi.org/10.1016/j.optmat.2019.109619 Received 7 October 2019; Received in revised form 15 December 2019; Accepted 16 December 2019 Available online 28 December 2019 0925-3467/© 2019 Elsevier B.V. All rights reserved.
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Optical Materials 100 (2020) 109619
In this work, we demonstrate the fabrication and deployment of GO/ PDMS-coated microfiber as an SA. The advantage of this technique lies within the firmly attached GO/PDMS composite around the entire tapered region of the microfiber without the utilization of additional deposition light source. The SA generates the mode-locking operation in the erbium-doped fiber laser (EDFL) cavity with pulse duration of 629 fs. 2. Materials and methods 2.1. Materials
Fig. 2. The measured waist diameter of the fabricated microfiber.
Graphene oxide was obtained from ACS Material LLC, USA while isopropyl alcohol (IPA, 99.5%) from Sigma Aldrich. Poly dimethylsiloxane (PDMS, Sylgard® 184 Silicone Elastomer) was pur chased from Dow Corning. All materials were used as received without further purification.
PDMS composite via dipping method for 20 s. Once coated, the micro fiber was fixed on a grooved sample holder with epoxy glue applied to both ends of the holder as shown in Fig. 3. This platform was placed in a dry cabinet for 24 h to cure at room temperature where the composite will harden and stabilize.
2.2. Preparation of GO/PDMS composite
2.4. Optical characterization of GO/PDMS composite
For the material preparation, 0.5 mg of GO was sonicated (Hielscher UP200s) with 10 ml of IPA at 200 W for 1 h. After sonication, 1.0 g of PDMS was added to the well-dispersed GO solution and the mixture was heated at 80 � C on a hotplate for 16 h with constant stirring. This was to ensure efficient evaporation of IPA to form GO/PDMS composite. Next, 0.1 g of curing agent was added and stirred for 10 min. The as-prepared GO/PDMS composite was degassed for half an hour in a vacuum oven (CONSTANCE VC-6050). Fig. 1 depicts the schematic diagram of the process. For characterization, GO/PDMS was analyzed using Raman spectroscopy and visible near-infrared spectroscopy (SHIMADZU UV3600).
The saturable absorption of GO/PDMS composite coated microfiber was characterized using the balanced twin detector measurement sys tem as illustrated in Fig. 4. A Menlo Systems M-Fiber pulsed laser source with a central wavelength of 1550 nm, repetition rate of 250 MHz and pulse duration of 117 fs was deployed in this experiment. A variable optical attenuator (VOA) was positioned after the pulsed laser source to tune the input optical intensity. Next, an inline optical isolator was inserted at the output port of the VOA to avoid back reflection into the pulsed laser. A 50:50 optical coupler was employed next to the isolator
2.3. Fabrication of SA with GO/PDMS composite In this work, an adiabatic microfiber was employed in order to utilize the strong evanescent field generated along the entire tapered region (core-cladding interface) as a result of reduced effective refractive index at the core-cladding interface. This allows optimum light-matter inter action with 2D materials coated transversely along the tapered region. In addition, microfiber ensures an all-fiberized laser system that has high thermal damage threshold without sacrificing high optical loss from the tapering process. The fabrication process began by stripping the polymer on the surface of a single-mode fiber followed by the tapering process with a waist diameter of 10.2 μm, as portrayed in Fig. 2. The waist length was 0.5 mm and the total length of the microfiber is 60.5 mm. After fabrication, the microfiber was coated with the prepared GO/
Fig. 3. Deposition of GO/PDMS on a microfiber which is later fixed on a grooved sample holder.
Fig. 1. Liquid phase exfoliation process which begins with (a) sonicating graphene oxide flakes with IPA, (b) mixing GO solution with PDMS, (c) stir and evaporate, (d) adding curing agent into the prepared GO/PDMS composite, (e) degas to obtain (f) final GO/PDMS composite. 2
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Optical Materials 100 (2020) 109619
Fig. 4. Twin detector measurement setup.
to divide the pulsed laser signal into two equal portions, with one arm measuring the power-dependent transmission of GO/PDMS composite coated microfiber (OPM 2) and the other arm for the reference power (OPM 1). Fig. 5 illustrates the schematic diagram of ring-cavity mode-locked EDFL. A section of 5 m long Lucent HP980 EDF with signal absorption coefficient of 3.5 dB/m at 1530 nm was pumped by a 980 nm laser diode (LD) through a 980/1550 nm wavelength division multiplexer (WDM). A polarization independent isolator (ISO) was employed to ensure uni directional operation and to eliminate any back-scattered lights. The GO/PDMS composite coated microfiber-based SA was spliced in be tween ISO and optical coupler (OC). The polarization controller (PC) was used to rotate the polarization state of the laser cavity. Meanwhile, 30% of the laser signal was extracted from the laser cavity for further optical pulse performance analysis through the 70/30 OC whereby the remaining 70% of the laser signal reverted to the laser cavity to com plete the optical loop. The entire laser cavity was composed of 5 m long EDF and 17 m long SMF. The relatively longer SMF than EDF with complementary dispersion coefficient, β2 of 22 ps2/km and 23 ps2/km, respectively was estimated with a net anomalous dispersion of 0.259 ps2.
Fig. 6. Measured Raman spectrum of GO, PDMS, and GO/PDMS composite.
cm 1, 1262.82 cm 1, 1411.28 cm 1, 2154.27 cm 1, 2498.63 cm 1, 2905.32 cm 1, and 2965.13 cm 1. For GO/PDMS composite, both GO and PDMS Raman peaks can be observed which confirm the presence of both compounds. It is important to note that the D-band (1372.10 cm 1) and G-peak (1595.32 cm 1) shift in GO/PDMS composite were caused by strain in PDMS as a result from the curing process [13]. The absorption property of GO/PDMS was analyzed using a UV-VISNIR spectrophotometer. Based on Fig. 7, the GO/PDMS composite pos sesses a very strong broad absorption between 800 nm and 1620 nm. The strong absorption peak at 1550 nm makes this composite an attractive choice for SA to operate as mode-locked lasers in C-band region. The morphology of GO/PDMS composite coated microfiber was characterized using field-emission scanning microscope (FESEM) and energy dispersive X-ray spectroscopy (EDX) using; Zeiss Crossbeam 340 and Oxford Silicon Drift Detector X-Max, respectively. Fig. 8(a) shows the deposition of GO/PDMS composite around the microfiber. The “mountain ridge” structure at the bottom part of the fiber is due to the accumulated GO/PDMS composite during the curing process. Fig. 8(b) illustrates the cross-sectional image of the GO/PDMS composite coated on the microfiber. The thickness of the coated GO/PDMS composite on the microfiber was taken at five different positions which resulted to an average thickness of 737 nm. Fig. 9 depicts the EDX spectrum of the composite-coated microfiber. From the spectrum, the presence of silicon is contributed by the microfiber, whereas the carbon and oxygen ele ments confirm the presence of GO/PDMS composite.
3. Results and discussions 3.1. Characterization of GO/PDMS composite The samples (GO, PDMS, and GO/PDMS composite) were charac terized with Raman spectroscopy (Horiba Scientific - LabRAM HR Evo lution) by using 532 nm laser (Fig. 6). The GO sample (red line) exhibits two peaks at 1367.70 cm 1 and 1587.10 cm 1 which are known to be the D-band and G-band of GO. Meanwhile, PDMS (blue line) produces peaks at 486.69 cm 1, 616.20 cm 1, 708.92 cm 1, 788.22 cm 1, 862.42
Fig. 5. Schematic diagram for mode-locked EDFL setup.
Fig. 7. UV-VIS-NIR absorption spectrum of GO/PDMS composite. 3
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Optical Materials 100 (2020) 109619
Fig. 10. Power-dependent nonlinear saturable absorption curve of GO/PDMS composite coated microfiber.
the generation of shorter pulse duration. Based on published articles, the measured MD is comparable to MD of graphene-polyvinyl alchohol composite SA at 1.3% [14], PDMS/polyethylene terephthalate sandwich-SA between 0.44% and 5.5% [15], and reduced GO-SA at 5.75% [16]. According to Ref. [17], the MD or the absorption change of the SA can be tailored with the number of graphene layers, where thicker graphene layers provide larger MD at the cost of lower damage threshold. The saturation intensity (Isat) is the optical intensity required to reduce the absorption by a factor of 2, which is measured in power per unit area. Lower mode-locking threshold can be achieved with a smaller value of Isat [18]. The Isat of GO/PDMS composite coated microfiber was characterized at 77 MW/cm2. In contrast to previous article with Isat of more than 1000 MW/cm2 [15], the mode-locked pump power threshold is expected to be sufficiently low. 3.3. Laser setup EDFL Fig. 8. FESEM image of (a) top view of coated microfiber, and (b) crosssectional view of coated microfiber.
3.3.1. Optical spectra analyzer In the experiment, the pump power of 980 nm LD was increased from 0 to 177.7 mW (maximum value). The output spectrum behavior was observed using an optical spectrum analyzer (Yokogawa AQ6370B) with a resolution bandwidth of 0.02 nm from 1535 nm to 1580 nm. The output spectrum evolution within this pump power range is depicted in Fig. 11(a). The spontaneous emission was generated at a low pumping power of 7.14 mW and exhibited a low peak intensity of maximum 70 dBm across the operating wavelength span. In spontaneous emission, the photons emit stochastically and there is no fixed phase relationship with incoherent signal. As the pump power was increased to 13.53 mW, stimulated emission was generated with the observation of the contin uous wave (CW) laser at the central wavelength of 1557.53 nm. Multiple-mode CW lasers with sharp intensity and narrow width was observed due to the gain hopping of the EDF. By increasing the pumping power to 33.54 mW, the mode-locked laser threshold was achieved with the observation of multiple Kelly’s sidebands. This threshold value is lower than what was reported previously that managed to achieve a threshold of 100 mW with SA saturation intensity of more than 1000 MW/cm2 [15]. The Kelly’s sidebands were imposed due to the phase matching condition between soliton pulse and dispersive waves at the specific frequency of the multiple of 2π to generate the constructive interference between both mechanisms. Therefore, the generation of Kelly’s sidebands validates the soliton operation, which is typically presented in an anomalous dispersion laser cavity. As the pump power increased, the 3-dB bandwidth was broadened, and the strength of Kelly sidebands was also more pronounced. The corresponding optical spec trum at three pump powers as illustrated in inset of Fig. 11(b). Fig. 11(b) shows the enlarged view of the optical spectrum at maximum pump
Fig. 9. EDX spectrum of GO/PDMS composite.
3.2. Non-linear optical properties of GO/PDMS composite The corresponding power-dependent saturable transmission curve for the GO/PDMS composite coated microfiber is portrayed in Fig. 10. The modulation depth (MD) is determined from the difference between the maximum and minimum values of the transmission. The measured MD of this GO/PDMS-coated microfiber is 4.0%. In addition to that, the high MD of 4.0% is due to multi-layered graphene which is required for 4
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Optical Materials 100 (2020) 109619
Fig. 12. Output pulse train of the C-band mode-locked EDFL.
Fig. 11. (a) Mode-locked EDFL spectrum evolution with respect to pump power (b) optical spectra of C-band mode-locked EDFL at different pump powers and enlarged view of the output spectrum at 177.7 mW pump power. Fig. 13. RF spectrum of the C-band mode-locked EDFL.
power of 177.7 mW. The output spectrum indicated no significant peak of CW was observed at the central wavelength. This represents a stable condition of pulse operation assisted by GO/PDMS composite coated microfiber-based SA. For 177.7 mW pump power, the 3-dB bandwidth of the laser was measured at 5.92 nm as portrayed in Fig. 11(b).
The PER of the proposed mode-locked EDFL pulse was measured as 48.71 dB for the first order RF peak as shown in Fig. 13. 3.3.4. Autocorrelator trace Fig. 14 illustrates the autocorrelation trace for pulse duration mea surement of the mode-locked EDFL. The pulse duration of full-width at half maximum (FWHM) point was 970.80 fs. Considering the sech2 profile pulses with decorrelation factor of 0.648, the pulse duration after deconvolution was 629 fs. Previous articles reported the achievement of
3.3.2. Oscilloscope trace For subsequent experiments, the pump power was maintained at maximum value of 177.7 mW. The pulse train of the mode-locked EDFL was taken using a 100 MHz oscilloscope. The spacing time between two pulses was attained at 103 ns, which corresponded to the fundamental pulse repetition rate of 9.65 MHz as shown in Fig. 12. The oscilloscope trace was clean without any noise disturbance which indicated a stable mode-locked operation. Additionally, no pulse breaking of harmonic mode-locking generation was observed until the maximum pump power. The total cavity length (L) of the proposed mode-locked EDFL is 22 m. The relationship between the round-trip time of the laser cavity (t) and the total cavity length is defined by Equation (1): t ¼ Ln/c
(1)
where c is the speed of light in vacuum and n is the silicate fiber core refractive index of 1.4157. 3.3.3. Radio frequency spectrum Fig. 13 shows the radio frequency (RF) spectrum taken with a radio bandwidth resolution of 300 kHz and video bandwidth resolution of 100 kHz. The mode-locked EDFL pulse was measured using an electrical spectrum analyzer (GW Instek GSP-830). Peak-to-pedestal extinction ratio (PER) is defined as the peak to background contrast ratio measured from the peak level of desired signal to the level of background noise.
Fig. 14. Autocorrelation trace of the C-band mode-locked EDFL. 5
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Optical Materials 100 (2020) 109619
shorter pulse duration from 380 to 450 fs [15,19,20] and longer pulse duration from 1.1 to 1.8 ps [16,21,22] with graphene-based microfiber SA. Shorter pulse duration can be achieved by tailoring the net disper sion of the laser cavity [23], suppression of Kelly’s sidebands by rotating PC into different polarization state to disturb the phase-matching con dition between soliton pulse and dispersive medium, and employing two mode-locking mechanisms of SA and nonlinear polarization rotation, simultaneously [24]. The time bandwidth product of the pulse was deduced to 0.46, which showed a slight pulse chirp from the ideal sech2 pulse profile. 3.3.5. Evolution of laser output power and pulse energy Fig. 15 depicts the average output power and pulse energy with 30% signal extracted from the proposed mode-locked EDFL as a function of pump power. The average output power was measured using an optical power meter, whereas the pulse energy was derived by dividing the average output power to the pulse repetition rate. For mode-locked operation, the average output power evolved from 0.12 mW to 2.5 mW, whereas the pulse energy developed from 0.05 nJ to 0.26 nJ. In this work, the pulse energy of 0.26 nJ showed improvement from previously reported methods; fiber ferule GO-SA with 33.7 pJ [25] and graphene-polyvinyl alchohol SA with 44 pJ [26]. Moreover, a research work based on the microfiber SA using graphene reported the pulse energy of 1.26 pJ [22]. It is believed that the developed GO/PDMS microfiber SA can work in higher pulse energy by either shortening the cavity length or integrating square pulse regime generated from dissi pative soliton resonance in a long laser cavity [27].
Fig. 16. Stability evaluation of the mode-locked EDFL; (a) perspective view and (b) top view.
The deposition methods used in previous studies on graphene-based microfiber SA for C-band mode-locked fiber lasers (as tabulated in Table 1) are either more complex or induce uncontrollable in homogeneity as compared to our work. For instance, sandwiching and covering thin film around the microfiber had shown some restrictions in covering the entire tapered region [19, 21, 28]. Thereafter, a graphene-clad microfiber saturable absorber was pro posed by wrapping the graphene layer around the microfiber, which resulted to loosely attached graphene layers due to the absence of magnesium fluoride (MgF2) substrate [29]. Next, optical deposition technique was demonstrated to fabricate the SA. In this technique, numerous sources were employed such as amplified spontaneous emis sion [30,31] and continuous wave laser [22] to enhance the evanescent field interaction with the graphene materials. Nonetheless, this tech nique used a bulky optical deposition system which imposed cost inef fective drawback in contrast to conventional deposition approach. Up to date, graphene-polymer composites have yet to be thoroughly explored. The advantage of this method addresses the limitations of prior research works including better graphene-polymer coverage around the micro fiber, stronger attachment and simple fabrication technique.
3.3.6. Pulsed laser stability The stability of the proposed SA for mode-locked operation was conducted for 60 min with results recorded at an interval of 2 min. For this experiment, the polarization state was optimized and fixed at a constant orientation within the observation time. The mode-locked fiber laser was pumped at the maximum power of 177.7 mW. The measured spectra for the entire 60 min are depicted in Fig. 16. Based on the measurement, the central wavelength and spectral bandwidth are stable throughout the observation period. There are no significant changes on the optical spectrum of the mode-locked EDFL. However, at 20 min, a slight shift on the lasing wavelength was observed. This could be due to the perturbation induced by the laboratory environment such as tem perature drift and vibration. In order to mitigate this issue, the experi ment must be conducted in a more controlled environment such as a closed chamber to minimize the temperature effect. In addition, the integration of an isolated optical table can avoid any vibration effect. Overall, under controlled conditions, proposed mode-locked fiber laser was able to generate a stable output as shown by Fig. 16(a) and (b).
4. Conclusion In conclusion, this work has successfully demonstrated a GO/PDMS composite coated microfiber-based SA for C-band mode-locked fiber laser. The saturable absorber was fabricated by coating the GO/PDMS composite around the tapered region of the microfiber on a groove. The SA showed obvious D and G characteristic peaks for GO as well as strong absorption peak at 1550 nm. This SA serves as the mode-locker in an EDFL with the central wavelength, spectral bandwidth, repetition rate, pulse duration, PER, and pulse energy of 1557.05 nm, 5.92 nm, 9.65 MHz, 629 fs, 48.71 dB, and 0.26 nJ, respectively at maximum pump power of 177.7 mW. The stability of the SA was evaluated with stable laser output over an observation period of 60 min. The success of this work will contribute to a better method of fabricating SA and a more efficient development of mode-locked laser technology. Fig. 15. Average output power and pulse energy as a function of pump powers. 6
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Optical Materials 100 (2020) 109619
Table 1 Summary of this research work in comparison to previous work. Ref
Incorporation Method
Center Wavelength (nm)
3-dB Bandwidth (nm)
Pulse Width (ps)
[28] [19]
Covering graphene thin film on the top surface of the microfiber on an MgF2 substrate. Covering graphene thin film between PDMS layer and the top surface of the microfiber on an MgF2 substrate. Sandwiching graphene thin film between PDMS layer and the top surface of the microfiber on a PDMS substrate. Wrapping the entire microfiber with the graphene thin film without a substrate. Dripping the graphene/DMF-composite on a glass slide where the microfiber situated with an amplified spontaneous emission source Dripping the graphene/DMF-composite on a glass slide where the microfiber situated with a 17 mW amplified spontaneous emission source Dripping the GO/PNIPAAm-composite on a U-shaped metal mount where the microfiber situated with a 16.5 mW laser source Coating the GO/PDMS composite around the tapered region of the microfiber on a groove.
1563.8 1560
8.3 –
1.94 0.38
1559
2.3
1.10
1531.3 1530.38
2.08 1.55
1.21 350
1559
4.3
0.68
1557.56
0.2
15.70
1557.05
5.92
0.63
[21] [29] [30] [31] [22] This work
Author contributions
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E.K. Ng designed, performed the experiments, analyzed data and drafted the manuscript. K.Y. Lau designed the experiments and co-wrote the paper. H.K. Lee and M.F. Omar performed the material characterization analyses. M.H. Abu Bakar conducted analyses on laser performance. Y. Mustapha Kamil performed material preparation and device fabrication. M.A. Mahdi supervised the research. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgement This work was funded by the Ministry of Education Malaysia under Fundamental Research Grant Scheme, FRGS/1/2017/TK04/UPM/01/2. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.optmat.2019.109619. References [1] M. Haiml, R. Grange, U. Keller, Optical characterization of semiconductor saturable absorbers, Appl. Phys. B 79 (2004) 331–339, https://doi.org/10.1007/ s00340-004-1535-1. [2] D.H. Sutter, L. Gallmann, N. Matuschek, F. Morier-Genoud, V. Scheuer, G. Angelow, T. Tschudi, G. Steinmeyer, U. Keller, Sub-6-fs pulses from a SESAMassisted Kerr-lens modelocked Ti:sapphire laser: at the frontiers of ultrashort pulse generation, Appl. Phys. B 70 (2000) 5–12, https://doi.org/10.1007/ s003400000308. [3] X. He, Z.B. Liu, D.N. Wang, M. Yang, T.Y. Hu, J.G. Tian, Saturable absorber based on graphene-covered-microfiber, IEEE Photonics Technol. Lett. 25 (2013) 1392–1394, https://doi.org/10.1109/LPT.2013.2266114. [4] A. Martinez, Z. Sun, Nanotube and graphene saturable absorbers for fibre lasers, Nat. Photonics 7 (2013) 842–845, https://doi.org/10.1038/nphoton.2013.304. [5] H. Zhang, Q. Bao, D. Tang, L. Zhao, K. Loh, Large energy soliton erbium-doped fiber laser with a graphene-polymer composite mode locker, Appl. Phys. Lett. 95 (2009) 1–4, https://doi.org/10.1063/1.3244206. [6] Y.W. Song, S.Y. Jang, W.S. Han, M.K. Bae, Graphene mode-lockers for fiber lasers functioned with evanescent field interaction, Appl. Phys. Lett. 96 (2010) 1–4, https://doi.org/10.1063/1.3309669. [7] B. Hu, H. Ago, Y. Ito, K. Kawahara, M. Tsuji, E. Magome, K. Sumitani, N. Mizuta, K. I. Ikeda, S. Mizuno, Epitaxial growth of large-area single-layer graphene over Cu (1 1 1)/sapphire by atmospheric pressure CVD, Carbon 50 (2012) 57–65, https://doi. org/10.1016/j.carbon.2011.08.002.
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Oxide as saturable absorbers for Er-doped passively mode-locked fiber laser, Opt. Express 20 (2012) 19463–19473, https://doi.org/10.1364/OE.20.019463. [26] D. Popa, Z. Sun, F. Torrisi, T. Hasan, F. Wang, A.C. Ferrari, Sub 200 fs pulse generation from a graphene mode-locked fiber laser, Appl. Phys. Lett. 97 (2010) 203106, https://doi.org/10.1063/1.3517251. [27] H. Ahmad, S.N. Aidit, Z.C. Tiu, Dissipative soliton resonance in a passively modelocked praseodymium fiber laser, Opt. Laser. Technol. 112 (2019) 20–25, https:// doi.org/10.1016/j.optlastec.2018.10.056. [28] R. Wang, Y. Liu, M. Jiang, X. Xu, H. Wu, Y. Tian, J. Bai, Z. Ren, Passively Qswitched and mode-locked fiber laser research based on graphene saturable absorbers, Opt. Quant. Electron. 49 (2017) 137, https://doi.org/10.1007/s11082017-0982-y.
[29] X.M. Liu, H.R. Yang, Y.D. Cui, G.W. Chen, Y. Yang, X.Q. Wu, X.K. Yao, D.D. Han, X. X. Han, C. Zeng, J. Guo, Graphene-clad microfibre saturable absorber for ultrafast fibre lasers, Sci. Rep. 6 (2016) 26024, https://doi.org/10.1038/srep26024. [30] A.P. Luo, P.F. Zhu, H. Liu, X.W. Zheng, N. Zhao, M. Liu, H. Cui, Z.C. Luo, W.C. Xu, Microfiber-based, highly nonlinear graphene saturable absorber for formation of versatile structural soliton molecules in a fiber laser, Opt. Express 22 (2014) 27019–27025, https://doi.org/10.1364/OE.22.027019. [31] P.F. Zhu, Z.B. Lin, Q.Y. Ning, Z.R. Cai, X.B. Xing, J. Liu, W.C. Chen, Z.C. Luo, A. P. Luo, W.C. Xu, Passive harmonic mode-locking in a fiber laser by using a microfiber-based graphene saturable absorber, Laser Phys. Lett. 10 (2013) 105107, https://doi.org/10.1088/1612-2011/10/10/105107.
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Saturable absorber incorporating graphene oxide polymer composite through dip coating for mode-locked fiber laser E.K. Ng a, K.Y. Lau b, H.K. Lee c, M.H. Abu Bakar a, Y. Mustapha Kamil c, M.F. Omar d, M. A. Mahdi a, * a
Wireless and Photonics Networks Research Centre, Faculty of Engineering, Universiti Putra Malaysia, 43400, UPM Serdang, Selangor, Malaysia Department of Electronics and Nanoengineering, Aalto University, Espoo, 02150, Finland InLazer Dynamics Sdn Bhd InnoHub Unit, Putra Science Park Universiti Putra Malaysia, 43400, UPM Serdang, Selangor, Malaysia d Physics Department, Faculty of Science, Universiti Teknologi Malaysia, 81310, Skudai, Johor, Malaysia b c
A R T I C L E I N F O
A B S T R A C T
Keywords: fiber lasers Saturable absorber Optical pulses Microfiber
Fabrication of microfiber saturable absorbers (SA) is a tedious process. This work demonstrates a simplified method of microfiber SA fabrication by integrating graphene oxide/polymethylsiloxane (GO/PDMS) using dipcoating technique. The idea is to enhance light-matter interaction between graphene and propagating light modes within the microfiber by enhancing the binding strength and proximity of the graphene-polymer com posite to the microfiber. The implementation of GO/PDMS SA in an erbium-doped fiber laser cavity has suc cessfully generated ultrashort pulses with pulse duration of 629 fs in the C-band emission region. The success of this work contributes to better insight towards simplifying the fabrication technique of microfiber SAs.
1. Introduction Since its discovery, mode-locked fiber lasers have exhibited excep tional potential in facilitating a broad range of scientific applications such as metrology, laser processing, telecommunications and biomed ical diagnostics. Numerous techniques have been proposed for the generation of passive mode-locked fiber laser which includes the deployment of nonlinear optical loop mirror (NOLM), acoustic-optical modulator (AOM), and saturable absorber (SA). The mode-locking scheme generated from NOLM requires a lengthy cavity since strong birefringence is an essential parameter induced by the Kerr nonlinearity of the optical fiber. An AOM provides the advantage of better control over the output pulses. However, this approach has inherent issues which limit the perfor mance, such as modulator bandwidth constraints, bulky size and high manufacturing cost. An alternative to mode-locked fiber laser genera tion is using a SA, an optical component with an optical loss which absorption reduces at high optical intensities. In its early development, SA was demonstrated using semiconductors. Keller et al. [1] proposed the first semiconductor saturable absorber mirror (SESAM) through defect engineering and micro-fabrication growth [2]. However, SESAM has shown many pronounced limitations such as complex fabrication,
cost ineffective, and limited operating bandwidth [3]. This has moti vated studies on a new class of SA using 2D carbon-based materials. Carbon nanotube (CNT), for example, is a cost-effective material which has shown feasible integration into fiber laser systems operating in sub-picosecond regime [4]. Despite, the success, CNT has also shown the requirement of diameter and chirality control for energy bandgap design which can be tedious [5,6]. Graphene, in different forms such as single layer graphene (SLG), multiple layer graphene (MLG), and graphene oxide (GO), have been comprehensively demonstrated as SAs. SLG and MLG are synthesized through chemical vapor deposition [7,8], whereas the GO can be ob tained using the modified Hummers’ method [9]. In contrast to gra phene, GO contains high density oxygen functional groups, which contributes to water and organic solvent solubility [10]. Numerous methods have been employed to deposit the graphene-based materials onto the optical fiber for SA fabrication [11]. For instance, the fiber ferrule structure is the simplest architecture where the graphene is placed in between two fiber ferrules to form the SA. Nevertheless, the low thermal damage threshold of this structure shows the limitation in operating the SA with high pump power. A solution to this issue is by generating an evanescent field on the optical fiber that can interact with graphene-based materials to produce mode-locked laser [12].
* Corresponding author. E-mail address: [email protected] (M.A. Mahdi). https://doi.org/10.1016/j.optmat.2019.109619 Received 7 October 2019; Received in revised form 15 December 2019; Accepted 16 December 2019 Available online 28 December 2019 0925-3467/© 2019 Elsevier B.V. All rights reserved.
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In this work, we demonstrate the fabrication and deployment of GO/ PDMS-coated microfiber as an SA. The advantage of this technique lies within the firmly attached GO/PDMS composite around the entire tapered region of the microfiber without the utilization of additional deposition light source. The SA generates the mode-locking operation in the erbium-doped fiber laser (EDFL) cavity with pulse duration of 629 fs. 2. Materials and methods 2.1. Materials
Fig. 2. The measured waist diameter of the fabricated microfiber.
Graphene oxide was obtained from ACS Material LLC, USA while isopropyl alcohol (IPA, 99.5%) from Sigma Aldrich. Poly dimethylsiloxane (PDMS, Sylgard® 184 Silicone Elastomer) was pur chased from Dow Corning. All materials were used as received without further purification.
PDMS composite via dipping method for 20 s. Once coated, the micro fiber was fixed on a grooved sample holder with epoxy glue applied to both ends of the holder as shown in Fig. 3. This platform was placed in a dry cabinet for 24 h to cure at room temperature where the composite will harden and stabilize.
2.2. Preparation of GO/PDMS composite
2.4. Optical characterization of GO/PDMS composite
For the material preparation, 0.5 mg of GO was sonicated (Hielscher UP200s) with 10 ml of IPA at 200 W for 1 h. After sonication, 1.0 g of PDMS was added to the well-dispersed GO solution and the mixture was heated at 80 � C on a hotplate for 16 h with constant stirring. This was to ensure efficient evaporation of IPA to form GO/PDMS composite. Next, 0.1 g of curing agent was added and stirred for 10 min. The as-prepared GO/PDMS composite was degassed for half an hour in a vacuum oven (CONSTANCE VC-6050). Fig. 1 depicts the schematic diagram of the process. For characterization, GO/PDMS was analyzed using Raman spectroscopy and visible near-infrared spectroscopy (SHIMADZU UV3600).
The saturable absorption of GO/PDMS composite coated microfiber was characterized using the balanced twin detector measurement sys tem as illustrated in Fig. 4. A Menlo Systems M-Fiber pulsed laser source with a central wavelength of 1550 nm, repetition rate of 250 MHz and pulse duration of 117 fs was deployed in this experiment. A variable optical attenuator (VOA) was positioned after the pulsed laser source to tune the input optical intensity. Next, an inline optical isolator was inserted at the output port of the VOA to avoid back reflection into the pulsed laser. A 50:50 optical coupler was employed next to the isolator
2.3. Fabrication of SA with GO/PDMS composite In this work, an adiabatic microfiber was employed in order to utilize the strong evanescent field generated along the entire tapered region (core-cladding interface) as a result of reduced effective refractive index at the core-cladding interface. This allows optimum light-matter inter action with 2D materials coated transversely along the tapered region. In addition, microfiber ensures an all-fiberized laser system that has high thermal damage threshold without sacrificing high optical loss from the tapering process. The fabrication process began by stripping the polymer on the surface of a single-mode fiber followed by the tapering process with a waist diameter of 10.2 μm, as portrayed in Fig. 2. The waist length was 0.5 mm and the total length of the microfiber is 60.5 mm. After fabrication, the microfiber was coated with the prepared GO/
Fig. 3. Deposition of GO/PDMS on a microfiber which is later fixed on a grooved sample holder.
Fig. 1. Liquid phase exfoliation process which begins with (a) sonicating graphene oxide flakes with IPA, (b) mixing GO solution with PDMS, (c) stir and evaporate, (d) adding curing agent into the prepared GO/PDMS composite, (e) degas to obtain (f) final GO/PDMS composite. 2
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Fig. 4. Twin detector measurement setup.
to divide the pulsed laser signal into two equal portions, with one arm measuring the power-dependent transmission of GO/PDMS composite coated microfiber (OPM 2) and the other arm for the reference power (OPM 1). Fig. 5 illustrates the schematic diagram of ring-cavity mode-locked EDFL. A section of 5 m long Lucent HP980 EDF with signal absorption coefficient of 3.5 dB/m at 1530 nm was pumped by a 980 nm laser diode (LD) through a 980/1550 nm wavelength division multiplexer (WDM). A polarization independent isolator (ISO) was employed to ensure uni directional operation and to eliminate any back-scattered lights. The GO/PDMS composite coated microfiber-based SA was spliced in be tween ISO and optical coupler (OC). The polarization controller (PC) was used to rotate the polarization state of the laser cavity. Meanwhile, 30% of the laser signal was extracted from the laser cavity for further optical pulse performance analysis through the 70/30 OC whereby the remaining 70% of the laser signal reverted to the laser cavity to com plete the optical loop. The entire laser cavity was composed of 5 m long EDF and 17 m long SMF. The relatively longer SMF than EDF with complementary dispersion coefficient, β2 of 22 ps2/km and 23 ps2/km, respectively was estimated with a net anomalous dispersion of 0.259 ps2.
Fig. 6. Measured Raman spectrum of GO, PDMS, and GO/PDMS composite.
cm 1, 1262.82 cm 1, 1411.28 cm 1, 2154.27 cm 1, 2498.63 cm 1, 2905.32 cm 1, and 2965.13 cm 1. For GO/PDMS composite, both GO and PDMS Raman peaks can be observed which confirm the presence of both compounds. It is important to note that the D-band (1372.10 cm 1) and G-peak (1595.32 cm 1) shift in GO/PDMS composite were caused by strain in PDMS as a result from the curing process [13]. The absorption property of GO/PDMS was analyzed using a UV-VISNIR spectrophotometer. Based on Fig. 7, the GO/PDMS composite pos sesses a very strong broad absorption between 800 nm and 1620 nm. The strong absorption peak at 1550 nm makes this composite an attractive choice for SA to operate as mode-locked lasers in C-band region. The morphology of GO/PDMS composite coated microfiber was characterized using field-emission scanning microscope (FESEM) and energy dispersive X-ray spectroscopy (EDX) using; Zeiss Crossbeam 340 and Oxford Silicon Drift Detector X-Max, respectively. Fig. 8(a) shows the deposition of GO/PDMS composite around the microfiber. The “mountain ridge” structure at the bottom part of the fiber is due to the accumulated GO/PDMS composite during the curing process. Fig. 8(b) illustrates the cross-sectional image of the GO/PDMS composite coated on the microfiber. The thickness of the coated GO/PDMS composite on the microfiber was taken at five different positions which resulted to an average thickness of 737 nm. Fig. 9 depicts the EDX spectrum of the composite-coated microfiber. From the spectrum, the presence of silicon is contributed by the microfiber, whereas the carbon and oxygen ele ments confirm the presence of GO/PDMS composite.
3. Results and discussions 3.1. Characterization of GO/PDMS composite The samples (GO, PDMS, and GO/PDMS composite) were charac terized with Raman spectroscopy (Horiba Scientific - LabRAM HR Evo lution) by using 532 nm laser (Fig. 6). The GO sample (red line) exhibits two peaks at 1367.70 cm 1 and 1587.10 cm 1 which are known to be the D-band and G-band of GO. Meanwhile, PDMS (blue line) produces peaks at 486.69 cm 1, 616.20 cm 1, 708.92 cm 1, 788.22 cm 1, 862.42
Fig. 5. Schematic diagram for mode-locked EDFL setup.
Fig. 7. UV-VIS-NIR absorption spectrum of GO/PDMS composite. 3
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Fig. 10. Power-dependent nonlinear saturable absorption curve of GO/PDMS composite coated microfiber.
the generation of shorter pulse duration. Based on published articles, the measured MD is comparable to MD of graphene-polyvinyl alchohol composite SA at 1.3% [14], PDMS/polyethylene terephthalate sandwich-SA between 0.44% and 5.5% [15], and reduced GO-SA at 5.75% [16]. According to Ref. [17], the MD or the absorption change of the SA can be tailored with the number of graphene layers, where thicker graphene layers provide larger MD at the cost of lower damage threshold. The saturation intensity (Isat) is the optical intensity required to reduce the absorption by a factor of 2, which is measured in power per unit area. Lower mode-locking threshold can be achieved with a smaller value of Isat [18]. The Isat of GO/PDMS composite coated microfiber was characterized at 77 MW/cm2. In contrast to previous article with Isat of more than 1000 MW/cm2 [15], the mode-locked pump power threshold is expected to be sufficiently low. 3.3. Laser setup EDFL Fig. 8. FESEM image of (a) top view of coated microfiber, and (b) crosssectional view of coated microfiber.
3.3.1. Optical spectra analyzer In the experiment, the pump power of 980 nm LD was increased from 0 to 177.7 mW (maximum value). The output spectrum behavior was observed using an optical spectrum analyzer (Yokogawa AQ6370B) with a resolution bandwidth of 0.02 nm from 1535 nm to 1580 nm. The output spectrum evolution within this pump power range is depicted in Fig. 11(a). The spontaneous emission was generated at a low pumping power of 7.14 mW and exhibited a low peak intensity of maximum 70 dBm across the operating wavelength span. In spontaneous emission, the photons emit stochastically and there is no fixed phase relationship with incoherent signal. As the pump power was increased to 13.53 mW, stimulated emission was generated with the observation of the contin uous wave (CW) laser at the central wavelength of 1557.53 nm. Multiple-mode CW lasers with sharp intensity and narrow width was observed due to the gain hopping of the EDF. By increasing the pumping power to 33.54 mW, the mode-locked laser threshold was achieved with the observation of multiple Kelly’s sidebands. This threshold value is lower than what was reported previously that managed to achieve a threshold of 100 mW with SA saturation intensity of more than 1000 MW/cm2 [15]. The Kelly’s sidebands were imposed due to the phase matching condition between soliton pulse and dispersive waves at the specific frequency of the multiple of 2π to generate the constructive interference between both mechanisms. Therefore, the generation of Kelly’s sidebands validates the soliton operation, which is typically presented in an anomalous dispersion laser cavity. As the pump power increased, the 3-dB bandwidth was broadened, and the strength of Kelly sidebands was also more pronounced. The corresponding optical spec trum at three pump powers as illustrated in inset of Fig. 11(b). Fig. 11(b) shows the enlarged view of the optical spectrum at maximum pump
Fig. 9. EDX spectrum of GO/PDMS composite.
3.2. Non-linear optical properties of GO/PDMS composite The corresponding power-dependent saturable transmission curve for the GO/PDMS composite coated microfiber is portrayed in Fig. 10. The modulation depth (MD) is determined from the difference between the maximum and minimum values of the transmission. The measured MD of this GO/PDMS-coated microfiber is 4.0%. In addition to that, the high MD of 4.0% is due to multi-layered graphene which is required for 4
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Fig. 12. Output pulse train of the C-band mode-locked EDFL.
Fig. 11. (a) Mode-locked EDFL spectrum evolution with respect to pump power (b) optical spectra of C-band mode-locked EDFL at different pump powers and enlarged view of the output spectrum at 177.7 mW pump power. Fig. 13. RF spectrum of the C-band mode-locked EDFL.
power of 177.7 mW. The output spectrum indicated no significant peak of CW was observed at the central wavelength. This represents a stable condition of pulse operation assisted by GO/PDMS composite coated microfiber-based SA. For 177.7 mW pump power, the 3-dB bandwidth of the laser was measured at 5.92 nm as portrayed in Fig. 11(b).
The PER of the proposed mode-locked EDFL pulse was measured as 48.71 dB for the first order RF peak as shown in Fig. 13. 3.3.4. Autocorrelator trace Fig. 14 illustrates the autocorrelation trace for pulse duration mea surement of the mode-locked EDFL. The pulse duration of full-width at half maximum (FWHM) point was 970.80 fs. Considering the sech2 profile pulses with decorrelation factor of 0.648, the pulse duration after deconvolution was 629 fs. Previous articles reported the achievement of
3.3.2. Oscilloscope trace For subsequent experiments, the pump power was maintained at maximum value of 177.7 mW. The pulse train of the mode-locked EDFL was taken using a 100 MHz oscilloscope. The spacing time between two pulses was attained at 103 ns, which corresponded to the fundamental pulse repetition rate of 9.65 MHz as shown in Fig. 12. The oscilloscope trace was clean without any noise disturbance which indicated a stable mode-locked operation. Additionally, no pulse breaking of harmonic mode-locking generation was observed until the maximum pump power. The total cavity length (L) of the proposed mode-locked EDFL is 22 m. The relationship between the round-trip time of the laser cavity (t) and the total cavity length is defined by Equation (1): t ¼ Ln/c
(1)
where c is the speed of light in vacuum and n is the silicate fiber core refractive index of 1.4157. 3.3.3. Radio frequency spectrum Fig. 13 shows the radio frequency (RF) spectrum taken with a radio bandwidth resolution of 300 kHz and video bandwidth resolution of 100 kHz. The mode-locked EDFL pulse was measured using an electrical spectrum analyzer (GW Instek GSP-830). Peak-to-pedestal extinction ratio (PER) is defined as the peak to background contrast ratio measured from the peak level of desired signal to the level of background noise.
Fig. 14. Autocorrelation trace of the C-band mode-locked EDFL. 5
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shorter pulse duration from 380 to 450 fs [15,19,20] and longer pulse duration from 1.1 to 1.8 ps [16,21,22] with graphene-based microfiber SA. Shorter pulse duration can be achieved by tailoring the net disper sion of the laser cavity [23], suppression of Kelly’s sidebands by rotating PC into different polarization state to disturb the phase-matching con dition between soliton pulse and dispersive medium, and employing two mode-locking mechanisms of SA and nonlinear polarization rotation, simultaneously [24]. The time bandwidth product of the pulse was deduced to 0.46, which showed a slight pulse chirp from the ideal sech2 pulse profile. 3.3.5. Evolution of laser output power and pulse energy Fig. 15 depicts the average output power and pulse energy with 30% signal extracted from the proposed mode-locked EDFL as a function of pump power. The average output power was measured using an optical power meter, whereas the pulse energy was derived by dividing the average output power to the pulse repetition rate. For mode-locked operation, the average output power evolved from 0.12 mW to 2.5 mW, whereas the pulse energy developed from 0.05 nJ to 0.26 nJ. In this work, the pulse energy of 0.26 nJ showed improvement from previously reported methods; fiber ferule GO-SA with 33.7 pJ [25] and graphene-polyvinyl alchohol SA with 44 pJ [26]. Moreover, a research work based on the microfiber SA using graphene reported the pulse energy of 1.26 pJ [22]. It is believed that the developed GO/PDMS microfiber SA can work in higher pulse energy by either shortening the cavity length or integrating square pulse regime generated from dissi pative soliton resonance in a long laser cavity [27].
Fig. 16. Stability evaluation of the mode-locked EDFL; (a) perspective view and (b) top view.
The deposition methods used in previous studies on graphene-based microfiber SA for C-band mode-locked fiber lasers (as tabulated in Table 1) are either more complex or induce uncontrollable in homogeneity as compared to our work. For instance, sandwiching and covering thin film around the microfiber had shown some restrictions in covering the entire tapered region [19, 21, 28]. Thereafter, a graphene-clad microfiber saturable absorber was pro posed by wrapping the graphene layer around the microfiber, which resulted to loosely attached graphene layers due to the absence of magnesium fluoride (MgF2) substrate [29]. Next, optical deposition technique was demonstrated to fabricate the SA. In this technique, numerous sources were employed such as amplified spontaneous emis sion [30,31] and continuous wave laser [22] to enhance the evanescent field interaction with the graphene materials. Nonetheless, this tech nique used a bulky optical deposition system which imposed cost inef fective drawback in contrast to conventional deposition approach. Up to date, graphene-polymer composites have yet to be thoroughly explored. The advantage of this method addresses the limitations of prior research works including better graphene-polymer coverage around the micro fiber, stronger attachment and simple fabrication technique.
3.3.6. Pulsed laser stability The stability of the proposed SA for mode-locked operation was conducted for 60 min with results recorded at an interval of 2 min. For this experiment, the polarization state was optimized and fixed at a constant orientation within the observation time. The mode-locked fiber laser was pumped at the maximum power of 177.7 mW. The measured spectra for the entire 60 min are depicted in Fig. 16. Based on the measurement, the central wavelength and spectral bandwidth are stable throughout the observation period. There are no significant changes on the optical spectrum of the mode-locked EDFL. However, at 20 min, a slight shift on the lasing wavelength was observed. This could be due to the perturbation induced by the laboratory environment such as tem perature drift and vibration. In order to mitigate this issue, the experi ment must be conducted in a more controlled environment such as a closed chamber to minimize the temperature effect. In addition, the integration of an isolated optical table can avoid any vibration effect. Overall, under controlled conditions, proposed mode-locked fiber laser was able to generate a stable output as shown by Fig. 16(a) and (b).
4. Conclusion In conclusion, this work has successfully demonstrated a GO/PDMS composite coated microfiber-based SA for C-band mode-locked fiber laser. The saturable absorber was fabricated by coating the GO/PDMS composite around the tapered region of the microfiber on a groove. The SA showed obvious D and G characteristic peaks for GO as well as strong absorption peak at 1550 nm. This SA serves as the mode-locker in an EDFL with the central wavelength, spectral bandwidth, repetition rate, pulse duration, PER, and pulse energy of 1557.05 nm, 5.92 nm, 9.65 MHz, 629 fs, 48.71 dB, and 0.26 nJ, respectively at maximum pump power of 177.7 mW. The stability of the SA was evaluated with stable laser output over an observation period of 60 min. The success of this work will contribute to a better method of fabricating SA and a more efficient development of mode-locked laser technology. Fig. 15. Average output power and pulse energy as a function of pump powers. 6
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Optical Materials 100 (2020) 109619
Table 1 Summary of this research work in comparison to previous work. Ref
Incorporation Method
Center Wavelength (nm)
3-dB Bandwidth (nm)
Pulse Width (ps)
[28] [19]
Covering graphene thin film on the top surface of the microfiber on an MgF2 substrate. Covering graphene thin film between PDMS layer and the top surface of the microfiber on an MgF2 substrate. Sandwiching graphene thin film between PDMS layer and the top surface of the microfiber on a PDMS substrate. Wrapping the entire microfiber with the graphene thin film without a substrate. Dripping the graphene/DMF-composite on a glass slide where the microfiber situated with an amplified spontaneous emission source Dripping the graphene/DMF-composite on a glass slide where the microfiber situated with a 17 mW amplified spontaneous emission source Dripping the GO/PNIPAAm-composite on a U-shaped metal mount where the microfiber situated with a 16.5 mW laser source Coating the GO/PDMS composite around the tapered region of the microfiber on a groove.
1563.8 1560
8.3 –
1.94 0.38
1559
2.3
1.10
1531.3 1530.38
2.08 1.55
1.21 350
1559
4.3
0.68
1557.56
0.2
15.70
1557.05
5.92
0.63
[21] [29] [30] [31] [22] This work
Author contributions
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