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Indium tin oxide coated D-shape fiber as saturable absorber for passively Q-switched erbium-doped fiber laser
Optics and Laser Technology 124 (2020) 105998
Contents lists available at ScienceDirect
Optics and Laser Technology journal homepage: www.elsevier.com/locate/optlastec
Indium tin oxide coated D-shape fiber as saturable absorber for passively Qswitched erbium-doped fiber laser
T
B. Nizamania, A.A.A. Jafryb, M.I.M. Abdul Khudusc, F.A. Memond, A. Shuhaimie, N. Kasimb, ⁎ ⁎ E. Hanafia, M. Yasinf, , S.W. Haruna, a
Photonics Engineering Laboratory, Department of Electrical Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia Department of Physics, Faculty of Science, Universiti Teknologi Malaysia, 81310 Skudai, Johor, Malaysia c Department of Physics, Faculty of Science, University of Malaya, 50603 Kuala Lumpur, Malaysia d Department of Telecommunications Engineering, Mehran University of Engineering & Technology, Jamshoro 76062, Pakistan e Low Dimensional Materials Research Center, Department of Physics, Faculty of Science, University of Malaya, 50603 Kuala Lumpur, Malaysia f Department of Physics, Faculty of Science and Technology, Airlangga University, Surabaya, Indonesia b
H I GH L IG H T S
Erbium-doped fiber laser with indium-tin-oxide (ITO) saturable absorber (SA). • Q-switched was deposited onto D-shaped fiber using electron beam. • ITO Q-switched pulses operates at 1566.4 nm with the highest repetition rate of 52.77 kHz. • The • The pulse energy was 5.68 nJ at 96.4 mW pump power.
A B S T R A C T
We demonstrated a Q-switched Erbium-doped fiber laser (EDFL) using indium-tin-oxide (ITO) deposited onto D-shape fiber as a saturable absorber (SA). The Dshaped fiber was prepared using polishing wheel technique while the ITO was deposited onto the polished surface using electron beam deposition technique to establish excellent evanescent field interaction between the material and light on the surface of the polished region. The SA device was deployed into EDFL ring cavity to generate Q-switched pulses operating at 1566.4 nm. It was able to initiate pulses as short as 2.3 µs with the highest repetition rate of 52.77 kHz. Stability of the SA is proven as it produced stable pulses within the pump power of 55.2–96.4 mW with signal to noise ratio of 58.6 dB. Q-switched EDFL generates pulses with the output power of 300 µW and pulse energy of 5.68 nJ at 96.4 mW pump power. Therefore, ITO deposited onto D-shape fiber can be deployed as SA in EDFL cavity for portable Q-switched laser source.
1. Introduction In recent years, pulsed laser had attracted vast research attention in number of applications such as medical, telecommunication and material processing [1–3]. As compared to continuous-wave, pulsed laser shows few advantages including higher pulse energy and peak power. There are two techniques to initiate pulsed laser which are active and passive means. Active technique requires few additional devices such as mirror, lenses and U-bench units. However, deployment of those devices into laser system introduced extra insertion loss and complexity in the preparation procedure. Therefore, passive technique is frequently used to generate pulsed laser by exploiting saturable absorption mechanism of selected material inside laser cavity. Saturable absorber (SA), material with reduced absorption ability as the intensity of light
⁎
increased are normally used in laser cavity. Carbon-base material (carbon-nanotubes, graphene, graphite, graphene oxide), transition metal-dichalcogenides (MoS2, WS2, MoSe2, WSe2), topological insulators (Bi2Te3, Bi2Se3, Sb2Te3) and 2-dimensional allotrope (black phosphorus) were normally used for realizing Q-switched and modelocked fiber lasers [4–9]. However, most of these materials suffer from some limitations thus the surge for efficient material as SA is still relevant. For starter, carbonnanotubes (CNTs) are used for laser pulsing as it possesses ultrashort recovery time (~500 fs), wide-operating spectrum and high damage threshold [10]. However, chirality and shape dependent bandgap of CNTs provide complexity for SA fabrication. Then, graphene was introduced with unique condensed matter properties and relativistic electron mobility. In fact, ability of graphene to generate pulses in near-
Corresponding author. E-mail addresses: [email protected] (M. Yasin), [email protected] (S.W. Harun).
https://doi.org/10.1016/j.optlastec.2019.105998 Received 7 September 2019; Received in revised form 26 October 2019; Accepted 1 December 2019 0030-3992/ © 2019 Elsevier Ltd. All rights reserved.
Optics and Laser Technology 124 (2020) 105998
B. Nizamani, et al.
Fig. 1. Fabrication of D-shape fiber using polishing wheel technique.
Guo et al. with the highest attainable pump power of 480 mW, which was 187 mW in our case. In fact, the proposed SA owns a modulation depth of 3.5%, which was higher than the previous works, 0.48% [17] and 0.83% [18]. Therefore, D-shape fiber can serve as an optional SA deployment method in generating Q-switched laser source. In authors’ knowledge, this is the first demonstration of Q-switch generation using ITO deposited onto D-shape fiber in all-fiber laser.
infrared region (1–2 µm) is proven as it exhibits wide-spectral bandwidth and faster relaxation time (~200 fs) than CNTs [11]. After graphene, few 2-dimensional materials such as transition metal-dichalcogenides (TMDs) are used as a Q-switcher and mode-locker in nearinfrared region due to its high second order susceptibility, strong orbit coupling and high carrier mobility [12,13]. For topological insulators (TIs), they possess excellent electron mobility on the surface instead of insulating behavior inside the lattice. In fact, TIs exhibit high third-order nonlinear coefficients and low saturation threshold which make them compatible for pulse generation in near-infrared region [14]. However, those materials possess wavelength-dependent band gap that provides complexity in material fabrication. Black phosphorus (BPs) is also used as SA in laser cavity due to small band gap and thermo-dynamically stable material [15]. Unfortunately, black phosphorus is a hydrophilic material with high sensitivity to water and air. So, the needs for new material with band gap independent wavelength is crucial to make Q-switched and modelocked laser reasonable and worthy. Theoretical analysis was also reported for a passively Q-switching operation in an erbium-doped fiber laser (EDFL) with a SA in our previous work [16]. Recently, indium tin oxide (ITO) is introduced for the generation of Q-switched and mode-locked [17,18]. ITO which belongs to the group of oxide plasmonic nanocrystals is widely used in solar cell technology, antireflective coatings and heat-reflecting mirror [19]. It possessed excellent electrical conductivity due to low electrical resistivity (2 − 4 × 10−4 Ω cm ). In addition, its optical nonlinearity is dependable on its free charge carrier density [20]. As a result, wavelength operation of ITO can be engineered from visible to infrared. Ability of ITO to operate over broad wavelength region is also attributed by its strong optical susceptibility and ultrafast recovery time (~360 fs) [18]. In addition, variation of wavelength can be achieved by controlling the concentration of tin doping in ITO’s composition. Usually, pulsed laser is generated by sandwiching thin-film of SA in between fiber-ferrule. However, this method introduced parasitic reaction to thin-film and end face of fiber-ferrule [21]. Therefore, in this paper, we proposed Q-switched fiber laser using ITO deposited onto Dshaped fiber as a SA to provide efficient coupling between light and material via evanescent field. The ITO is deposited onto D-shape fiber using electron beam deposition method. It is observed that, ITO-Dshape as SA can generate Q-switched pulses as short as 2.3 µs with highest repetition rate of 52.77 kHz. At a low threshold pump power of 55.2 mW, we obtained a stable Q-switched with comparable laser parameters to [18]. The repetition rate, pulse width, maximum output power, and maximum pulse energy might be enhanced via the availability of laser diode with higher maximum pump power compared to
2. Fabrication of D-shape fiber The fabrication of D-shape fiber was done using noncommercial setup with a dc motor, fixed in a cylindrical PVC shaft, having a rotator which works as a polisher. The shaft was powered with variable output voltage supplier from 0 to 17 V direct current (DC) with maximum power ability to be 12 W and temperature sustainability of up to 130˚C. This had a variable speed to be adjusted manually by using the knob. The sand paper of grit size 1000cw was pasted over the rotator and optimum speed was retained where the fiber started to leave traces over sandpaper. For this purpose, conventional silica fiber SMF-28 having 125 µm cladding diameter was used. A pair of optical fiber holder were already fixed at certain height on both sides to hold the optical fiber before the side polishing process. The optical fiber holder which hold the fiber at both ends were fixed but the rotor over PVC shaft was set to be adjustable manually, according to the requirements before the start of fabrication. The fiber was then connected at one side with Amplified Stimulated Emission (ASE) and the other side connected with Optical Power Meter (OPM) for real time power monitoring during the polishing process, Fig. 1 shows the schematic diagram of the setup. The polishing was done until variation of insertion loss reached from its reference value to 2 dB. This formed a D-shape of fiber which refers as one side of fiber cladding to be chopped of at specific diameter value. After multiple fabrications a good range of D-shape depth to be 92.31 µm was observed at the insertion loss value of 2 dB. D-shape fiber was then picked up over a microscopic glass slide to measure the Dshape diameter using medilux-12 microscope as shown in Fig. 2. The fabrication of D-shape fiber was done in similar manner as of [22,23]. 3. Deposition of ITO over D-shape fiber After fabrication of a D-shape fiber, it was placed above the microscope glass slide facing the D-shape outwards for the precise deposition over D-shape region. Purpose of microscope glass slide was solely to hold the D-shape fiber, to be placed inside chamber for the deposition of ITO. Alignment of the fiber was assured to have the Dshape region to be exposed outwards for deposition. For this purpose, 2
Optics and Laser Technology 124 (2020) 105998
B. Nizamani, et al.
Fig. 2. Microscopic Image of D-shape fiber (side view).
Fig. 3. Microscope Image of ITO coated D-shape fiber (top view).
Fig. 4. Deposition of ITO over D-shape fiber (a) side view (b) axial view after 90˚ cleaving (c) top view.
fiber was taped to fix on glass slide. Then the D-shape fiber along with glass slide was placed into the e-beam deposition chamber for the ITO deposition purpose. The 60 nm ITO deposition was done using e-beam deposition technique at room temperature. Where the e-beam deposition process is evaporation of the material to be deposited over the substrate, the e-beam evaporation process permits current to first pass through a tungsten filament which allows electron emission. High voltage is applied and because of strong magnetic field, electrons are focused into a unified beam and upon arrival this energy of electron beam is transferred to ITO, which cause it to evaporate and deposit over the region of glass slide and D-shape fiber inside the chamber. Thus, the ITO was deposited over D-shape fiber in the electron beam machine at full vacuum condition of 9.0 × 10−6 torr with the rotating speed of metal plate holding D-shape fiber at 5 rpm. Microscopic image of deposited ITO is shown in Fig. 3 where the effect of ITO deposition over the glass slide is meant to be ignored because the light is supposed to pass through the D-shape only and the purpose of glass slide was only to hold the fiber. In further elaboration to Fig. 3, the schematic of ITO over D-shape fiber is illustrated in Fig. 4(a) as the side
view and in Fig. 4(b) as an axial view after cleaving the D-shape region of fiber at 90˚ and Fig. 4(c) as the top view. E-beam deposition technique is used among the other available techniques as the material thickness can be pre-set to a specific value. Fig. 5(a) represents the linear absorption of ITO coated D-shape fiber taken with the optical spectrum analyser (OSA) at the wavelength span of 500 nm from 1150 nm to 1650 nm and the OSA resolution of 1 nm. Nearly a 2 dB spectral absorption was observed at 1.55 µm, validating the ability of ITO coated D-shape fiber to generate Q-switching. Nonlinear absorption of ITO coated D-shape fiber was observed by using balanced twin detector technique and the results are shown in Fig. 5(b). The fiber laser source for the non-linear absorption was a stable mode-locked source with a repetition rate of 1.885 MHz and a pulse width of 3.62 ps at the center wavelength of 1557.7 nm. By fitting data using the formula in Eq. (1) [18], the non-saturable absorption, a modulation depth, and a saturation intensity are 60%, 3.5% and 40.32 MW/cm2 respectively, where T(I) is transmission, ΔT is the modulation depth, Tns is non-saturable absorbance, I and Isat are input intensity and saturation intensity respectively.
3
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essential elements such as wavelength division multiplexer, isolator, optical coupler and the D-shape fiber coated with 60 nm thick ITO, which were connected using standard single mode fiber with the total cavity length of 13.9 m. The EDF has a numerical aperture of 0.23, absorption coefficient of about 25 dB/m at 980 nm and the core and cladding diameter of 4 µm and 125 µm respectively. An optical isolator was inserted in between EDF and SA device to ensure the unidirectional propagation of light inside the cavity. The ITO coated D-shape fiber connected after the isolator works as a loss modulator. The optical coupler was used to divide the light intensity into the ratio of 90% and 10%. 10% of light is allowed to tap out as the output while the remaining 90% of the intensity is trapped inside the cavity for further oscillation. The output pulses were observed by a fast photodetector, which was then connected to RFSA and oscilloscope for analysis in the time and frequency domain respectively. The output power of the laser was measured by an optical power meter while an OSA with resolution of 0.07 nm was used to measure the output optical spectrum. It is worthy to note that the cavity does not include a polarization controller (PC). It is observed that the polarization has played a negligible role in Q-switching operation of the laser. We observed almost a similar result with the inclusion of the PC inside the cavity. 5. Results and discussion In this work, the EDFL cavity generates continuous-wave at pump power of 30.4 mW while self-started Q-switched appear at pump power of 55.2 mW. The Q-switched laser starts to diminish as the pump power raise above 96.4 mW, only to reappear with the same lasing parameters as the pump power maintained in the range of 55.2–96.4 mW. To verify the capability of ITO-D-shape fiber to initiate pulses in EDFL cavity the ITO-D-shape fiber is replaced with a bare fiber connector. No pulses are generated on the oscilloscope and this indicates that no self-pulsing occur inside EDFL cavity and the Q-switched pulses are generated due to the ITO SA. Fig. 7(a) compares optical spectrum for the continuouswave and Q-switched laser, which was obtained without and with SA, respectively. By implementing ITO-D-shape fiber inside the cavity, center wavelength shifted from 1558.8 nm to 1566.4 nm. This is attributed to the cavity loss which increases with the incorporation of ITO-D-shape fiber. The operating wavelength has shifted to a longer wavelength which has a higher gain to compensate for the loss.
Fig. 5. (a) Linear absorption profile of ITO coated D-shape fiber (b) nonlinear absorption profile of ITO coated D-shape fiber.
T (I ) = 1 − ΔT ∗ exp(−I / Isat ) − Tns
(1)
4. Configuration of EDFL ring cavity Fig. 6 shows the configuration of the proposed Q-switched erbium doped fiber laser (EDFL). The laser employed a ring cavity with 2.4 m long of Erbium-doped fiber (EDF), which was pumped by 980 nm laser diode (LD) as the gain medium. The cavity also consisted of other
Fig. 6. Erbium-doped fiber laser ring cavity with ITO deposited onto D-shaped fiber as SA. 4
Optics and Laser Technology 124 (2020) 105998
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Fig. 8. Q-switched performances: (a) repetition rate and pulse width as a function of pump power. (b) output power and pulse energy as a function of pump power. (c) Stability of the fiber laser.
Fig. 7. Spectral and temporal characteristics: (a) optical spectrum of EDFL with and without ITO-D-shaped fiber, (b) oscilloscope trace, and (c) fundamental frequency of Q-switched EDFL at pump power of 96.4 mW. Inset: enlarged figure of oscilloscope trace with two pulses envelope.
D-shape fiber exhibits output power from 150 µW to the maximum of 300 µW, corresponding to slope efficiency of 0.37% while the pulse energy was calculated to be from 4.14 nJ to the maximum of 5.68 nJ. The stability of Q-switched EDFL using ITO-D-shape was investigated for 200 min, as depicted in Fig. 8(c). Few stable optical spectra were captured at the pump power of 96.4 mW, denotes excellent stability of generated Q-switched. Thus, ITO-D-shape is comparable to ITO deployed on fiber-ferrule surface via drop-casting method. We successfully produced Q-switched with low threshold pump power of 55.2 mW attributed to optimized EDF length. However, there is still a room for repetition rate and pulse width enhancement via the construction of low cavity length and incorporation of highly doped EDF.
Fig. 7(b) shows the typical oscilloscope trace of the Q-switched laser, which was captured within the span of 800 µs at the highest pump power of 96.4 mW. As depicted in the figure, temporal performance of the laser was stable with a pulse width of 2.3 µs, corresponds to repetition rate of 52.77 kHz. The enlarged image of two oscilloscope pulses envelope clearly displayed full-width half maximum of 2.3 µs and peak-to-peak distance between two pulses of 18.95 µs. Fig. 7(c) shows fundamental frequency with signal-to-noise ratio (SNR) of 58.6 dB measured within the span of 500 kHz. The first peak displayed frequency of 52.77 kHz which is in a good agreement with repetition rate captured from the oscilloscope device. The Q-switched performance of EDFL cavity initiated by ITO-Dshape fiber are then evaluated over a variation of pump power. By adjusting the pump power from 55.2 mW to 96.4 mW, pulse width of Qswitched decreases while repetition rate increases as shown in Fig. 8(a). This indicates typical performance of Q-switched in all-fiber laser cavity. Within the same range of pump power, repetition rate of 36.26–52.77 kHz was recorded which is corresponding to pulse width of 3–2.3 µs. As shown in Fig. 8(b), Q-switched laser initiated by the ITO-
6. Conclusion In conclusion, we developed a robust and efficient Q-switched laser source using ITO-D-shape fiber as SA in EDFL cavity. D-shape fiber was fabricated using polishing wheel technique to expose evanescent field on the surface of optical fiber. Then, ITO was successfully deposited onto the D-shape fiber using electron beam deposition to establish efficient interaction between ITO and light via evanescent field. The SA successfully initiated Q-switched laser with the shortest pulse width of 2.3 µs and the highest repetition rate of 52.77 kHz. The deployment of 5
Optics and Laser Technology 124 (2020) 105998
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SA device inside EDFL cavity efficiently generates pulses with SNR of 58.6 dB, output power of 150–300 µW and pulse energy of 4.14–5.68 nJ which is comparable to other SA devices.
[9]
[10]
Declaration of Competing Interest The authors declared that there is no conflict of interest.
[11]
Acknowledgment
[12]
This research is financially supported by the Ministry of Education, Malaysia (Grant No: LR001A-2016A), University of Malaya (Grant No: RP039C-18AFR) and Universiti Teknologi Malaysia under Research University Grant (RUG), Q.J130000.2654.16J01.
[13]
[14]
Appendix A. Supplementary material [15]
Supplementary data to this article can be found online at https:// doi.org/10.1016/j.optlastec.2019.105998.
[16]
References
[17]
[1] S. Shah, T.S. Alster, Laser treatment of dark skin an updated review, Am. J. Clin. Dermatol. 11 (6) (2010) 389–397. [2] P.T. Shih, J. Chen, C.T. Lin, W.J. Jiang, H.S. Huang, P.C. Peng, S. Chi, Optical millimeter-wave signal generation via frequency 12-tupling, J. Lightwave Technol. 28 (1) (2010) 71–78. [3] S. Juodkazis, V. Mizeikis, H. Misawa, Three-dimensional microfabrication of materials by femtosecond lasers for photonics applications, J. Appl. Phys. 106 (5) (2009) 14. [4] J.C. Chiu, C.M. Chang, B.Z. Hsieh, S.C. Lin, C.Y. Yeh, G.R. Lin, C.K. Lee, J.J. Lin, W.H. Cheng, Pulse shortening mode-locked fiber laser by thickness and concentration product of carbon nanotube based saturable absorber, Opt. Express 19 (5) (2011) 4036–4046. [5] R.Z.R.R. Rosdin, A.A.A. Jafry, S.W. Harun, N.F. Zulkipli, Z. Jusoh, M. Yasin, Nanosecond pulsed laser generation with bismuth (III) tellurite saturable absorber, Chalcogenide Lett. 16 (5) (2019) 209–214. [6] A.H.H. Al-Masoodi, M.H.M. Ahmed, A.A. Latiff, H. Arof, S.W. Harun, Q-Switched ytterbium-doped fiber laser using black phosphorus as saturable absorber, Chin. Phys. Lett. 33 (5) (2016) 4. [7] H. Ahmad, S.A. Reduan, S.N. Aidit, Z.C. Tiu, Generation of passively Q-switched fiber laser at 1 mu m by using MoSSe as a saturable absorber, Chin. Opt. Lett. 15 (2) (2017) 5. [8] L.N. Duan, Y.L. Su, Y.G. Wang, L. Li, X. Wang, Y.S. Wang, Passively mode-locked
[18]
[19] [20]
[21]
[22]
[23]
6
erbium-doped fiber laser via a D-shape-fiber-based MoS2 saturable absorber with a very low nonsaturable loss, Chin. Phys. B 25 (2) (2016) 5. R. Rosdin, F. Ahmad, N.M. Ali, S.W. Harun, H. Arof, Q-switched Er-doped fiber laser with low pumping threshold using graphene saturable absorber, Chin. Opt. Lett. 12 (9) (2014) 5. N. Kasim, A.H.H. A-Masoodi, F. Ahmad, Y. Munajat, H. Ahmad, S.W. Harun, Qswitched ytterbium doped fiber laser using multi-walled carbon nanotubes saturable absorber, Chin. Opt. Lett. 12 (3) (2014) 4. Q.L. Bao, H. Zhang, Y. Wang, Z.H. Ni, Y.L. Yan, Z.X. Shen, K.P. Loh, D.Y. Tang, Atomic-layer graphene as a saturable absorber for ultrafast pulsed lasers, Adv. Funct. Mater. 19 (19) (2009) 3077–3083. J. Du, Q.K. Wang, G.B. Jiang, C.W. Xu, C.J. Zhao, Y.J. Xiang, Y. Chen, S.C. Wen, H. Zhang, Ytterbium-doped fiber laser passively mode locked by few-layer Molybdenum Disulfide (MoS2) saturable absorber functioned with evanescent field interaction, Sci. Rep. 4 (2014) 7. H. Ahmad, N.E. Ruslan, M.A. Ismail, S.A. Reduan, C.S.J. Lee, S. Sathiyan, S. Sivabalan, S.W. Harun, Passively Q-switched erbium-doped fiber laser at C-band region based on WS2 saturable absorber, Appl. Optics 55 (5) (2016) 1001–1005. N.N. Xu, H.N. Zhang, W.Q. Yang, X.L. Han, B.Y. Man, High-efficiency passively Qswitched neodymium-doped fiber laser operation at 1360.61 nm with bismuth selenide as saturable absorber, Laser Phys. 28 (12) (2018) 7. M.B. Hisyam, M.F.M. Rusdi, A.A. Latiff, S.W. Harun, Generation of mode-locked ytterbium doped fiber ring laser using few-layer black phosphorus as a saturable absorber, IEEE J. Sel. Top. Quantum Electron. 23 (1) (2017) 5. A. Nady, A.A. Latiff, A. Numan, C.R. Ooi, S.W. Harun, Theoretical and experimental studies on a Q-switching operation in an erbium-doped fiber laser using vanadium oxide as saturable absorber, Laser Phys. 28 (8) (2018) 085106. J. Guo, H.N. Zhang, Z. Li, Y.Q. Sheng, Q.X. Guo, X.L. Han, Y.J. Liu, B.Y. Man, T.Y. Ning, S.Z. Jiang, Dark solitons in erbium-doped fiber lasers based on indium tin oxide as saturable absorbers, Opt. Mater. 78 (2018) 432–437. J. Guo, H.N. Zhang, C. Zhang, Z. Li, Y.Q. Sheng, C.H. Li, X.H. Bao, B.Y. Man, Y. Jiao, S.Z. Jiang, Indium tin oxide nanocrystals as saturable absorbers for passively Qswitched erbium-doped fiber laser, Opt. Mater. Exp. 7 (10) (2017) 3494–3502. J.L. Humphrey, D. Kuciauskas, Optical susceptibilities of supported indium tin oxide thin films, J. Appl. Phys. 100 (11) (2006) 6. H.I. Elim, W. Ji, F. Zhu, Carrier concentration dependence of optical Kerr nonlinearity in indium tin oxide films, Appl. Phys. B – Lasers Opt. 82 (3) (2006) 439–442. D. Steinberg, R.M. Gerosa, F.N. Pellicer, J.D. Zapata, S.H. Domingues, E.A.T. de Souza, L.A.M. Saito, Graphene oxide and reduced graphene oxide as saturable absorbers onto D-shaped fibers for sub 200-fs EDFL mode-locking, Opt. Mater. Exp. 8 (1) (2018) 144–156. M.F.M. Rusdi, M.B.H. Mahyuddin, A.A. Latiff, H. Ahmad, S.W. Harun, Q-switched erbium-doped fiber laser using cadmium selenide coated onto side-polished Dshape fiber as saturable absorber, Chin. Phys. Lett. 35 (10) (2018) 4. A.A.A. Jafry, N. Kasim, Y. Munajat, R.A.M. Yusoff, M.F.M. Rusdi, M.B.H. Mahyuddin, S.W. Harun, H. Arof, R. Apsari, Passively Q-switched erbiumdoped fiber laser utilizing lutetium oxide deposited onto D-shaped fiber as saturable absorber, Optik 193 (2019) 162972.
Contents lists available at ScienceDirect
Optics and Laser Technology journal homepage: www.elsevier.com/locate/optlastec
Indium tin oxide coated D-shape fiber as saturable absorber for passively Qswitched erbium-doped fiber laser
T
B. Nizamania, A.A.A. Jafryb, M.I.M. Abdul Khudusc, F.A. Memond, A. Shuhaimie, N. Kasimb, ⁎ ⁎ E. Hanafia, M. Yasinf, , S.W. Haruna, a
Photonics Engineering Laboratory, Department of Electrical Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia Department of Physics, Faculty of Science, Universiti Teknologi Malaysia, 81310 Skudai, Johor, Malaysia c Department of Physics, Faculty of Science, University of Malaya, 50603 Kuala Lumpur, Malaysia d Department of Telecommunications Engineering, Mehran University of Engineering & Technology, Jamshoro 76062, Pakistan e Low Dimensional Materials Research Center, Department of Physics, Faculty of Science, University of Malaya, 50603 Kuala Lumpur, Malaysia f Department of Physics, Faculty of Science and Technology, Airlangga University, Surabaya, Indonesia b
H I GH L IG H T S
Erbium-doped fiber laser with indium-tin-oxide (ITO) saturable absorber (SA). • Q-switched was deposited onto D-shaped fiber using electron beam. • ITO Q-switched pulses operates at 1566.4 nm with the highest repetition rate of 52.77 kHz. • The • The pulse energy was 5.68 nJ at 96.4 mW pump power.
A B S T R A C T
We demonstrated a Q-switched Erbium-doped fiber laser (EDFL) using indium-tin-oxide (ITO) deposited onto D-shape fiber as a saturable absorber (SA). The Dshaped fiber was prepared using polishing wheel technique while the ITO was deposited onto the polished surface using electron beam deposition technique to establish excellent evanescent field interaction between the material and light on the surface of the polished region. The SA device was deployed into EDFL ring cavity to generate Q-switched pulses operating at 1566.4 nm. It was able to initiate pulses as short as 2.3 µs with the highest repetition rate of 52.77 kHz. Stability of the SA is proven as it produced stable pulses within the pump power of 55.2–96.4 mW with signal to noise ratio of 58.6 dB. Q-switched EDFL generates pulses with the output power of 300 µW and pulse energy of 5.68 nJ at 96.4 mW pump power. Therefore, ITO deposited onto D-shape fiber can be deployed as SA in EDFL cavity for portable Q-switched laser source.
1. Introduction In recent years, pulsed laser had attracted vast research attention in number of applications such as medical, telecommunication and material processing [1–3]. As compared to continuous-wave, pulsed laser shows few advantages including higher pulse energy and peak power. There are two techniques to initiate pulsed laser which are active and passive means. Active technique requires few additional devices such as mirror, lenses and U-bench units. However, deployment of those devices into laser system introduced extra insertion loss and complexity in the preparation procedure. Therefore, passive technique is frequently used to generate pulsed laser by exploiting saturable absorption mechanism of selected material inside laser cavity. Saturable absorber (SA), material with reduced absorption ability as the intensity of light
⁎
increased are normally used in laser cavity. Carbon-base material (carbon-nanotubes, graphene, graphite, graphene oxide), transition metal-dichalcogenides (MoS2, WS2, MoSe2, WSe2), topological insulators (Bi2Te3, Bi2Se3, Sb2Te3) and 2-dimensional allotrope (black phosphorus) were normally used for realizing Q-switched and modelocked fiber lasers [4–9]. However, most of these materials suffer from some limitations thus the surge for efficient material as SA is still relevant. For starter, carbonnanotubes (CNTs) are used for laser pulsing as it possesses ultrashort recovery time (~500 fs), wide-operating spectrum and high damage threshold [10]. However, chirality and shape dependent bandgap of CNTs provide complexity for SA fabrication. Then, graphene was introduced with unique condensed matter properties and relativistic electron mobility. In fact, ability of graphene to generate pulses in near-
Corresponding author. E-mail addresses: [email protected] (M. Yasin), [email protected] (S.W. Harun).
https://doi.org/10.1016/j.optlastec.2019.105998 Received 7 September 2019; Received in revised form 26 October 2019; Accepted 1 December 2019 0030-3992/ © 2019 Elsevier Ltd. All rights reserved.
Optics and Laser Technology 124 (2020) 105998
B. Nizamani, et al.
Fig. 1. Fabrication of D-shape fiber using polishing wheel technique.
Guo et al. with the highest attainable pump power of 480 mW, which was 187 mW in our case. In fact, the proposed SA owns a modulation depth of 3.5%, which was higher than the previous works, 0.48% [17] and 0.83% [18]. Therefore, D-shape fiber can serve as an optional SA deployment method in generating Q-switched laser source. In authors’ knowledge, this is the first demonstration of Q-switch generation using ITO deposited onto D-shape fiber in all-fiber laser.
infrared region (1–2 µm) is proven as it exhibits wide-spectral bandwidth and faster relaxation time (~200 fs) than CNTs [11]. After graphene, few 2-dimensional materials such as transition metal-dichalcogenides (TMDs) are used as a Q-switcher and mode-locker in nearinfrared region due to its high second order susceptibility, strong orbit coupling and high carrier mobility [12,13]. For topological insulators (TIs), they possess excellent electron mobility on the surface instead of insulating behavior inside the lattice. In fact, TIs exhibit high third-order nonlinear coefficients and low saturation threshold which make them compatible for pulse generation in near-infrared region [14]. However, those materials possess wavelength-dependent band gap that provides complexity in material fabrication. Black phosphorus (BPs) is also used as SA in laser cavity due to small band gap and thermo-dynamically stable material [15]. Unfortunately, black phosphorus is a hydrophilic material with high sensitivity to water and air. So, the needs for new material with band gap independent wavelength is crucial to make Q-switched and modelocked laser reasonable and worthy. Theoretical analysis was also reported for a passively Q-switching operation in an erbium-doped fiber laser (EDFL) with a SA in our previous work [16]. Recently, indium tin oxide (ITO) is introduced for the generation of Q-switched and mode-locked [17,18]. ITO which belongs to the group of oxide plasmonic nanocrystals is widely used in solar cell technology, antireflective coatings and heat-reflecting mirror [19]. It possessed excellent electrical conductivity due to low electrical resistivity (2 − 4 × 10−4 Ω cm ). In addition, its optical nonlinearity is dependable on its free charge carrier density [20]. As a result, wavelength operation of ITO can be engineered from visible to infrared. Ability of ITO to operate over broad wavelength region is also attributed by its strong optical susceptibility and ultrafast recovery time (~360 fs) [18]. In addition, variation of wavelength can be achieved by controlling the concentration of tin doping in ITO’s composition. Usually, pulsed laser is generated by sandwiching thin-film of SA in between fiber-ferrule. However, this method introduced parasitic reaction to thin-film and end face of fiber-ferrule [21]. Therefore, in this paper, we proposed Q-switched fiber laser using ITO deposited onto Dshaped fiber as a SA to provide efficient coupling between light and material via evanescent field. The ITO is deposited onto D-shape fiber using electron beam deposition method. It is observed that, ITO-Dshape as SA can generate Q-switched pulses as short as 2.3 µs with highest repetition rate of 52.77 kHz. At a low threshold pump power of 55.2 mW, we obtained a stable Q-switched with comparable laser parameters to [18]. The repetition rate, pulse width, maximum output power, and maximum pulse energy might be enhanced via the availability of laser diode with higher maximum pump power compared to
2. Fabrication of D-shape fiber The fabrication of D-shape fiber was done using noncommercial setup with a dc motor, fixed in a cylindrical PVC shaft, having a rotator which works as a polisher. The shaft was powered with variable output voltage supplier from 0 to 17 V direct current (DC) with maximum power ability to be 12 W and temperature sustainability of up to 130˚C. This had a variable speed to be adjusted manually by using the knob. The sand paper of grit size 1000cw was pasted over the rotator and optimum speed was retained where the fiber started to leave traces over sandpaper. For this purpose, conventional silica fiber SMF-28 having 125 µm cladding diameter was used. A pair of optical fiber holder were already fixed at certain height on both sides to hold the optical fiber before the side polishing process. The optical fiber holder which hold the fiber at both ends were fixed but the rotor over PVC shaft was set to be adjustable manually, according to the requirements before the start of fabrication. The fiber was then connected at one side with Amplified Stimulated Emission (ASE) and the other side connected with Optical Power Meter (OPM) for real time power monitoring during the polishing process, Fig. 1 shows the schematic diagram of the setup. The polishing was done until variation of insertion loss reached from its reference value to 2 dB. This formed a D-shape of fiber which refers as one side of fiber cladding to be chopped of at specific diameter value. After multiple fabrications a good range of D-shape depth to be 92.31 µm was observed at the insertion loss value of 2 dB. D-shape fiber was then picked up over a microscopic glass slide to measure the Dshape diameter using medilux-12 microscope as shown in Fig. 2. The fabrication of D-shape fiber was done in similar manner as of [22,23]. 3. Deposition of ITO over D-shape fiber After fabrication of a D-shape fiber, it was placed above the microscope glass slide facing the D-shape outwards for the precise deposition over D-shape region. Purpose of microscope glass slide was solely to hold the D-shape fiber, to be placed inside chamber for the deposition of ITO. Alignment of the fiber was assured to have the Dshape region to be exposed outwards for deposition. For this purpose, 2
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Fig. 2. Microscopic Image of D-shape fiber (side view).
Fig. 3. Microscope Image of ITO coated D-shape fiber (top view).
Fig. 4. Deposition of ITO over D-shape fiber (a) side view (b) axial view after 90˚ cleaving (c) top view.
fiber was taped to fix on glass slide. Then the D-shape fiber along with glass slide was placed into the e-beam deposition chamber for the ITO deposition purpose. The 60 nm ITO deposition was done using e-beam deposition technique at room temperature. Where the e-beam deposition process is evaporation of the material to be deposited over the substrate, the e-beam evaporation process permits current to first pass through a tungsten filament which allows electron emission. High voltage is applied and because of strong magnetic field, electrons are focused into a unified beam and upon arrival this energy of electron beam is transferred to ITO, which cause it to evaporate and deposit over the region of glass slide and D-shape fiber inside the chamber. Thus, the ITO was deposited over D-shape fiber in the electron beam machine at full vacuum condition of 9.0 × 10−6 torr with the rotating speed of metal plate holding D-shape fiber at 5 rpm. Microscopic image of deposited ITO is shown in Fig. 3 where the effect of ITO deposition over the glass slide is meant to be ignored because the light is supposed to pass through the D-shape only and the purpose of glass slide was only to hold the fiber. In further elaboration to Fig. 3, the schematic of ITO over D-shape fiber is illustrated in Fig. 4(a) as the side
view and in Fig. 4(b) as an axial view after cleaving the D-shape region of fiber at 90˚ and Fig. 4(c) as the top view. E-beam deposition technique is used among the other available techniques as the material thickness can be pre-set to a specific value. Fig. 5(a) represents the linear absorption of ITO coated D-shape fiber taken with the optical spectrum analyser (OSA) at the wavelength span of 500 nm from 1150 nm to 1650 nm and the OSA resolution of 1 nm. Nearly a 2 dB spectral absorption was observed at 1.55 µm, validating the ability of ITO coated D-shape fiber to generate Q-switching. Nonlinear absorption of ITO coated D-shape fiber was observed by using balanced twin detector technique and the results are shown in Fig. 5(b). The fiber laser source for the non-linear absorption was a stable mode-locked source with a repetition rate of 1.885 MHz and a pulse width of 3.62 ps at the center wavelength of 1557.7 nm. By fitting data using the formula in Eq. (1) [18], the non-saturable absorption, a modulation depth, and a saturation intensity are 60%, 3.5% and 40.32 MW/cm2 respectively, where T(I) is transmission, ΔT is the modulation depth, Tns is non-saturable absorbance, I and Isat are input intensity and saturation intensity respectively.
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essential elements such as wavelength division multiplexer, isolator, optical coupler and the D-shape fiber coated with 60 nm thick ITO, which were connected using standard single mode fiber with the total cavity length of 13.9 m. The EDF has a numerical aperture of 0.23, absorption coefficient of about 25 dB/m at 980 nm and the core and cladding diameter of 4 µm and 125 µm respectively. An optical isolator was inserted in between EDF and SA device to ensure the unidirectional propagation of light inside the cavity. The ITO coated D-shape fiber connected after the isolator works as a loss modulator. The optical coupler was used to divide the light intensity into the ratio of 90% and 10%. 10% of light is allowed to tap out as the output while the remaining 90% of the intensity is trapped inside the cavity for further oscillation. The output pulses were observed by a fast photodetector, which was then connected to RFSA and oscilloscope for analysis in the time and frequency domain respectively. The output power of the laser was measured by an optical power meter while an OSA with resolution of 0.07 nm was used to measure the output optical spectrum. It is worthy to note that the cavity does not include a polarization controller (PC). It is observed that the polarization has played a negligible role in Q-switching operation of the laser. We observed almost a similar result with the inclusion of the PC inside the cavity. 5. Results and discussion In this work, the EDFL cavity generates continuous-wave at pump power of 30.4 mW while self-started Q-switched appear at pump power of 55.2 mW. The Q-switched laser starts to diminish as the pump power raise above 96.4 mW, only to reappear with the same lasing parameters as the pump power maintained in the range of 55.2–96.4 mW. To verify the capability of ITO-D-shape fiber to initiate pulses in EDFL cavity the ITO-D-shape fiber is replaced with a bare fiber connector. No pulses are generated on the oscilloscope and this indicates that no self-pulsing occur inside EDFL cavity and the Q-switched pulses are generated due to the ITO SA. Fig. 7(a) compares optical spectrum for the continuouswave and Q-switched laser, which was obtained without and with SA, respectively. By implementing ITO-D-shape fiber inside the cavity, center wavelength shifted from 1558.8 nm to 1566.4 nm. This is attributed to the cavity loss which increases with the incorporation of ITO-D-shape fiber. The operating wavelength has shifted to a longer wavelength which has a higher gain to compensate for the loss.
Fig. 5. (a) Linear absorption profile of ITO coated D-shape fiber (b) nonlinear absorption profile of ITO coated D-shape fiber.
T (I ) = 1 − ΔT ∗ exp(−I / Isat ) − Tns
(1)
4. Configuration of EDFL ring cavity Fig. 6 shows the configuration of the proposed Q-switched erbium doped fiber laser (EDFL). The laser employed a ring cavity with 2.4 m long of Erbium-doped fiber (EDF), which was pumped by 980 nm laser diode (LD) as the gain medium. The cavity also consisted of other
Fig. 6. Erbium-doped fiber laser ring cavity with ITO deposited onto D-shaped fiber as SA. 4
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Fig. 8. Q-switched performances: (a) repetition rate and pulse width as a function of pump power. (b) output power and pulse energy as a function of pump power. (c) Stability of the fiber laser.
Fig. 7. Spectral and temporal characteristics: (a) optical spectrum of EDFL with and without ITO-D-shaped fiber, (b) oscilloscope trace, and (c) fundamental frequency of Q-switched EDFL at pump power of 96.4 mW. Inset: enlarged figure of oscilloscope trace with two pulses envelope.
D-shape fiber exhibits output power from 150 µW to the maximum of 300 µW, corresponding to slope efficiency of 0.37% while the pulse energy was calculated to be from 4.14 nJ to the maximum of 5.68 nJ. The stability of Q-switched EDFL using ITO-D-shape was investigated for 200 min, as depicted in Fig. 8(c). Few stable optical spectra were captured at the pump power of 96.4 mW, denotes excellent stability of generated Q-switched. Thus, ITO-D-shape is comparable to ITO deployed on fiber-ferrule surface via drop-casting method. We successfully produced Q-switched with low threshold pump power of 55.2 mW attributed to optimized EDF length. However, there is still a room for repetition rate and pulse width enhancement via the construction of low cavity length and incorporation of highly doped EDF.
Fig. 7(b) shows the typical oscilloscope trace of the Q-switched laser, which was captured within the span of 800 µs at the highest pump power of 96.4 mW. As depicted in the figure, temporal performance of the laser was stable with a pulse width of 2.3 µs, corresponds to repetition rate of 52.77 kHz. The enlarged image of two oscilloscope pulses envelope clearly displayed full-width half maximum of 2.3 µs and peak-to-peak distance between two pulses of 18.95 µs. Fig. 7(c) shows fundamental frequency with signal-to-noise ratio (SNR) of 58.6 dB measured within the span of 500 kHz. The first peak displayed frequency of 52.77 kHz which is in a good agreement with repetition rate captured from the oscilloscope device. The Q-switched performance of EDFL cavity initiated by ITO-Dshape fiber are then evaluated over a variation of pump power. By adjusting the pump power from 55.2 mW to 96.4 mW, pulse width of Qswitched decreases while repetition rate increases as shown in Fig. 8(a). This indicates typical performance of Q-switched in all-fiber laser cavity. Within the same range of pump power, repetition rate of 36.26–52.77 kHz was recorded which is corresponding to pulse width of 3–2.3 µs. As shown in Fig. 8(b), Q-switched laser initiated by the ITO-
6. Conclusion In conclusion, we developed a robust and efficient Q-switched laser source using ITO-D-shape fiber as SA in EDFL cavity. D-shape fiber was fabricated using polishing wheel technique to expose evanescent field on the surface of optical fiber. Then, ITO was successfully deposited onto the D-shape fiber using electron beam deposition to establish efficient interaction between ITO and light via evanescent field. The SA successfully initiated Q-switched laser with the shortest pulse width of 2.3 µs and the highest repetition rate of 52.77 kHz. The deployment of 5
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SA device inside EDFL cavity efficiently generates pulses with SNR of 58.6 dB, output power of 150–300 µW and pulse energy of 4.14–5.68 nJ which is comparable to other SA devices.
[9]
[10]
Declaration of Competing Interest The authors declared that there is no conflict of interest.
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Acknowledgment
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This research is financially supported by the Ministry of Education, Malaysia (Grant No: LR001A-2016A), University of Malaya (Grant No: RP039C-18AFR) and Universiti Teknologi Malaysia under Research University Grant (RUG), Q.J130000.2654.16J01.
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Appendix A. Supplementary material [15]
Supplementary data to this article can be found online at https:// doi.org/10.1016/j.optlastec.2019.105998.
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References
[17]
[1] S. Shah, T.S. Alster, Laser treatment of dark skin an updated review, Am. J. Clin. Dermatol. 11 (6) (2010) 389–397. [2] P.T. Shih, J. Chen, C.T. Lin, W.J. Jiang, H.S. Huang, P.C. Peng, S. Chi, Optical millimeter-wave signal generation via frequency 12-tupling, J. Lightwave Technol. 28 (1) (2010) 71–78. [3] S. Juodkazis, V. Mizeikis, H. Misawa, Three-dimensional microfabrication of materials by femtosecond lasers for photonics applications, J. Appl. Phys. 106 (5) (2009) 14. [4] J.C. Chiu, C.M. Chang, B.Z. Hsieh, S.C. Lin, C.Y. Yeh, G.R. Lin, C.K. Lee, J.J. Lin, W.H. Cheng, Pulse shortening mode-locked fiber laser by thickness and concentration product of carbon nanotube based saturable absorber, Opt. Express 19 (5) (2011) 4036–4046. [5] R.Z.R.R. Rosdin, A.A.A. Jafry, S.W. Harun, N.F. Zulkipli, Z. Jusoh, M. Yasin, Nanosecond pulsed laser generation with bismuth (III) tellurite saturable absorber, Chalcogenide Lett. 16 (5) (2019) 209–214. [6] A.H.H. Al-Masoodi, M.H.M. Ahmed, A.A. Latiff, H. Arof, S.W. Harun, Q-Switched ytterbium-doped fiber laser using black phosphorus as saturable absorber, Chin. Phys. Lett. 33 (5) (2016) 4. [7] H. Ahmad, S.A. Reduan, S.N. Aidit, Z.C. Tiu, Generation of passively Q-switched fiber laser at 1 mu m by using MoSSe as a saturable absorber, Chin. Opt. Lett. 15 (2) (2017) 5. [8] L.N. Duan, Y.L. Su, Y.G. Wang, L. Li, X. Wang, Y.S. Wang, Passively mode-locked
[18]
[19] [20]
[21]
[22]
[23]
6
erbium-doped fiber laser via a D-shape-fiber-based MoS2 saturable absorber with a very low nonsaturable loss, Chin. Phys. B 25 (2) (2016) 5. R. Rosdin, F. Ahmad, N.M. Ali, S.W. Harun, H. Arof, Q-switched Er-doped fiber laser with low pumping threshold using graphene saturable absorber, Chin. Opt. Lett. 12 (9) (2014) 5. N. Kasim, A.H.H. A-Masoodi, F. Ahmad, Y. Munajat, H. Ahmad, S.W. Harun, Qswitched ytterbium doped fiber laser using multi-walled carbon nanotubes saturable absorber, Chin. Opt. Lett. 12 (3) (2014) 4. Q.L. Bao, H. Zhang, Y. Wang, Z.H. Ni, Y.L. Yan, Z.X. Shen, K.P. Loh, D.Y. Tang, Atomic-layer graphene as a saturable absorber for ultrafast pulsed lasers, Adv. Funct. Mater. 19 (19) (2009) 3077–3083. J. Du, Q.K. Wang, G.B. Jiang, C.W. Xu, C.J. Zhao, Y.J. Xiang, Y. Chen, S.C. Wen, H. Zhang, Ytterbium-doped fiber laser passively mode locked by few-layer Molybdenum Disulfide (MoS2) saturable absorber functioned with evanescent field interaction, Sci. Rep. 4 (2014) 7. H. Ahmad, N.E. Ruslan, M.A. Ismail, S.A. Reduan, C.S.J. Lee, S. Sathiyan, S. Sivabalan, S.W. Harun, Passively Q-switched erbium-doped fiber laser at C-band region based on WS2 saturable absorber, Appl. Optics 55 (5) (2016) 1001–1005. N.N. Xu, H.N. Zhang, W.Q. Yang, X.L. Han, B.Y. Man, High-efficiency passively Qswitched neodymium-doped fiber laser operation at 1360.61 nm with bismuth selenide as saturable absorber, Laser Phys. 28 (12) (2018) 7. M.B. Hisyam, M.F.M. Rusdi, A.A. Latiff, S.W. Harun, Generation of mode-locked ytterbium doped fiber ring laser using few-layer black phosphorus as a saturable absorber, IEEE J. Sel. Top. Quantum Electron. 23 (1) (2017) 5. A. Nady, A.A. Latiff, A. Numan, C.R. Ooi, S.W. Harun, Theoretical and experimental studies on a Q-switching operation in an erbium-doped fiber laser using vanadium oxide as saturable absorber, Laser Phys. 28 (8) (2018) 085106. J. Guo, H.N. Zhang, Z. Li, Y.Q. Sheng, Q.X. Guo, X.L. Han, Y.J. Liu, B.Y. Man, T.Y. Ning, S.Z. Jiang, Dark solitons in erbium-doped fiber lasers based on indium tin oxide as saturable absorbers, Opt. Mater. 78 (2018) 432–437. J. Guo, H.N. Zhang, C. Zhang, Z. Li, Y.Q. Sheng, C.H. Li, X.H. Bao, B.Y. Man, Y. Jiao, S.Z. Jiang, Indium tin oxide nanocrystals as saturable absorbers for passively Qswitched erbium-doped fiber laser, Opt. Mater. Exp. 7 (10) (2017) 3494–3502. J.L. Humphrey, D. Kuciauskas, Optical susceptibilities of supported indium tin oxide thin films, J. Appl. Phys. 100 (11) (2006) 6. H.I. Elim, W. Ji, F. Zhu, Carrier concentration dependence of optical Kerr nonlinearity in indium tin oxide films, Appl. Phys. B – Lasers Opt. 82 (3) (2006) 439–442. D. Steinberg, R.M. Gerosa, F.N. Pellicer, J.D. Zapata, S.H. Domingues, E.A.T. de Souza, L.A.M. Saito, Graphene oxide and reduced graphene oxide as saturable absorbers onto D-shaped fibers for sub 200-fs EDFL mode-locking, Opt. Mater. Exp. 8 (1) (2018) 144–156. M.F.M. Rusdi, M.B.H. Mahyuddin, A.A. Latiff, H. Ahmad, S.W. Harun, Q-switched erbium-doped fiber laser using cadmium selenide coated onto side-polished Dshape fiber as saturable absorber, Chin. Phys. Lett. 35 (10) (2018) 4. A.A.A. Jafry, N. Kasim, Y. Munajat, R.A.M. Yusoff, M.F.M. Rusdi, M.B.H. Mahyuddin, S.W. Harun, H. Arof, R. Apsari, Passively Q-switched erbiumdoped fiber laser utilizing lutetium oxide deposited onto D-shaped fiber as saturable absorber, Optik 193 (2019) 162972.