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S band passively Q-switched thulium-fluoride fiber laser based on using gallium selenide saturable absorber
Optics and Laser Technology 107 (2018) 116–121
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S+/S band passively Q-switched thulium-fluoride fiber laser based on using gallium selenide saturable absorber H. Ahmad a,b,⇑, S.A. Reduan a a b
Photonics Research Centre, University of Malaya, 50603 Kuala Lumpur, Malaysia Department of Physics, Faculty of Science and Technology, Airlangga University, Surabaya 60115, Indonesia
a r t i c l e
i n f o
Article history: Received 20 March 2018 Received in revised form 1 May 2018 Accepted 13 May 2018
Keywords: Thulium-fluoride fiber laser Q-switched S band fiber laser Saturable absorber Gallium selenide
a b s t r a c t A Q-switched thulium-fluoride fiber (TFF) laser using gallium selenide (GaSe) as a passive saturable absorber (SA) for operation in the S+/S band is demonstrated. The generated pulses can be tuned from 1454 nm to 1512 nm, giving a tuning range of 58 nm. At 1502 nm, stable Q-switching operation is obtained from a pump power of 76.2 mW and is observed until the maximum pump power of 133.6 mW is reached, with the repetition rate varying from 16.5 kHz to 33.3 kHz and having a narrowest pulse width at 2.7 ms. The highest pulse energy obtained in this work is 51.4 nJ and this is, to the knowledge of the authors, the first time that Q-switching operation in the S+/S band region is obtained from a TFF laser using a GaSe based SA. Ó 2018 Elsevier Ltd. All rights reserved.
1. Introduction Pulsed lasers with high-energy outputs are highly desired for use in a multitude of applications ranging from environmental sensing, scientific research, material processing and medicine [1–4]. Initially, pulsed laser found significant applications in material processing such as in material laser ablation [5] and laser welding [6] but recent advances have seen the development of fiber lasers capable of generating high-energy, long-duration pulses. These lasers are particularly sought after for their capability to generate the desired pulses in a compact, cost-effective and robust platform [7]. Q-switched fiber lasers can generate an output with the afore-mentioned characteristics through active or passive approaches. Actively Q-switched fiber lasers offer a high degree of control over various output pulse parameters, including repetition rates and pulse durations [8,9]. However, the extra components required for active Q-switching are highly sensitive and expensive, thereby limiting the use of actively Q-switched fiber lasers in real-world applications. Fiber lasers that are passively Q-switched on the other hand are significantly less expensive and complex to fabricate and operate, although with less control over the laser’s output parameters [10,11]. These lasers are
⇑ Corresponding author at: Photonics Research Centre, University of Malaya, 50603 Kuala Lumpur, Malaysia. E-mail address: [email protected] (H. Ahmad). https://doi.org/10.1016/j.optlastec.2018.05.028 0030-3992/Ó 2018 Elsevier Ltd. All rights reserved.
realized through the use of saturable absorbers (SA) that are incorporated into the laser system. SAs, including semiconductor saturable absorber mirrors (SESAMs) and more recently 2-dimensional (2D) and 3-dimensional (3D) nanostructures such as carbon nanotubes (CNTs) and graphene [12–14] are typically incorporated into the laser cavity to generate Q-switched outputs, with the latter seeing increasing application due to their low fabrication cost and complexity. In fact, such is the potential for SAs in developing compact, high performance Q-switched fiber lasers that research efforts into developing new SA materials has intensified. This includes the exploration of topological insulators (TIs) including bismuth telluride (Bi2Te3) [15,16] and bismuth selenide (Bi2Se3) [17] as well as transition metal dichalcogenides (TMDs) such as molybdenum sulfide (MoS2) [18], tungsten disulfide (WS2) [19], molybdenum diselenide (MoSe2) [20] and tungsten (WSe2) [21] for the development of a new generation of SAs. Other materials such as black phosphorus [22], zinc oxide [23] and silver nanoparticles [24] are also being explored for their use as SAs, and have been shown to be able to produce a consistent and high performance Q-switched pulse. In this work, gallium selenide (GaSe), a compound formed using a 2D layered metal monochalcogenide, is used as an SA to generate an S band Q-switched output. Typically used for nonlinear optics [25], opto-electronics [26,27] and terahertz systems [28,29], GaSe has a substantial nonlinear optical coefficient, high surface damage threshold, broad transparency range and extremely low optical
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losses [30]. Furthermore, GaSe exhibits interesting optical properties such as up-conversion luminescence [31], making them suitable for optical applications [32]. The GaSe is used as an SA in a thulium-fluoride fiber (TFF) based laser cavity to enable emissions in the S+ and S band in this work. The S band is the telecom wavelength band that covers a region of 1480–1530 nm, while the S+ band region covers a shorter wavelength range of the S band region from 1450 to 1480 nm [33–35]. These new bands are crucial developments towards addressing current demands for bandwidth, which are quickly overwhelming the C and L band bandwidths regions of current telecommunications systems. The development of S+/S band fiber lasers enables the realization of applications such as coarse wavelength-division multiplexing (CWDM) and fiber to the premise (FTTP) systems [33,34]. This would be, to the best of author’s knowledge, the first successful demonstration of Q-switching in a TFF laser cavity using GaSe based SA.
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Fig. 2. Raman spectra of a thin layer of GaSe and bulk GaSe.
The SA is prepared by mechanical exfoliation, a technique that is commonly and widely used by many researchers in this field of study for its simplicity and reliability [22,36]. In this work, a small monochalcogenide (MX) GaSe crystal flake obtained from 2D Semiconductors is carefully placed on one side of scotch tape as shown in Fig. 1. The scotch tape is then folded into half and pressed repeatedly to cause a few layers of the crystal to exfoliate onto the scotch-tape. The portion of the scotch tape with the few GaSe layers is then carefully cut-out from the larger area as illustrated in Fig. 1(b) and (c). The cut out scotch tape section with the exfoliated GaSe thin layer is then carefully attached onto the fiber ferrule. A small amount of index matching gel is used to help stick the GaSe thin layer onto the face of the fiber ferrule. This is given in Fig. 1(d). The fiber ferrule is then connected to another fiber ferrule using an FC/PC adaptor, thus creating the complete SA assembly. The insertion loss of the GaSe thin layer as an SA is around 3.20 dB, whereby the insertion loss of the scotch tape is around 0.02 dB.
obtained from the bulk GaSe sample exhibits peaks at 148.6 cm 1, 214.0 cm 1 and 322.6 cm 1 respectively, as would be expected for bulk GaSe and comparable with the previous works [25,38]. It is also important to note that the intensity of the Raman spectrum for the bulk GaSe sample is lower than that of the thin layer GaSe sample, due to the reduction in the GaSe scattering mode that leads to less effective Raman scattering [27]. The thin GaSe film has multiple layers, indicated by the red-shift and Raman peak intensity reduction as well as the peaks between both spectra [25,37]. The nonlinear power-dependent absorbance of the GaSe SA is given in Fig. 3. The twin detector measurement technique is used to characterize the GaSe SA’s nonlinearity [39,40]. A passively mode-locked erbium-doped fiber laser (EDFL) with a repetition rate of 27.9 MHz and 0.70 ps is used as a laser seed. The GaSe based SA’s modulation depth is measured to be about 11.5%, while its saturation intensity is computed at 0.02 MW/cm2. The modulation depth of the GaSe based SA is on par with that of other SAs based on materials such as MoS2 at 9.7% [41] and Bi2Se3 at 11.1% [42]. This shows that the GaSe SA can perform comparably to other works.
3. Characterization of GaSe-SA
4. Q-switched laser design
The Raman spectra of GaSe in both its bulk and thin layer forms is shown in Fig. 2. The spectra were obtained using a Renishaw Raman spectroscope at an excitation wavelength of 532 nm and 1800 l/mm grating. The resulting spectra shows only a single peak the case of the thin layer GaSe as compared to the Raman spectrum of bulk GaSe, in which three distinct peaks can be observed. The single peak of the thin-layer GaSe sample is observed at 319.4 cm 1, and is comparable to the observed by Sidong Lei et al. [25] and Xiang Yuan et al. [37]. On the other hand, the spectrum
The schematic of the proposed Q-switched S+/S band fiber laser with a GaSe based SA is given in Fig. 4. The laser is configured as a ring cavity and uses a 1400 nm FOL1405RTD laser diode (LD) as a pump source. The LD’s output is connected to an optical isolator (ISO) to protect the LD from back reflections, before being connected to the 1400 nm port of a 1400/1500 nm fused wavelength division multiplexer (WDM). The common output of the WDM is now connected to the 14.5 m long Fiberlabs Inc TFF, with an absorption rate of 0.15 dB/m at the wavelength of 1400 nm, as well
Fig. 1. Mechanical exfoliation of GaSe from crystal flake.
Fig. 3. Nonlinear power dependent absorbance of thin layer GaSe SA.
2. Preparation of GaSe-SA
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Fig. 4. Experimental setup of S+/S band fiber ring cavity laser Q-switched using GaSe SA: LD (Laser diode), ISO (Isolator), WDM (Wavelength division multiplexer), TFF (Thulium Fluoride Fiber), TBPF (Tunable bandpass filter) and SA (Saturable absorber).
as a numerical aperture value of 0.26 and a mode-field diameter of 4.5 mm at 1500 nm. The TFF’s output is now channeled through another ISO before reaching a 90:10 optical tap coupler, which captures 10% of the propagating signal for further analysis. The remaining 90% signal continues to propagate within the ring cavity until it reaches the Agiltron Inc FOTF-025121332 tunable bandpass filter (TBPF) which is used to control the lasing wavelength in the cavity. The output of the signal then reaches the SA assembly, where it will become pulsed. Finally, the SA’s output is connected to the WDM’s 1500 nm port, completing the laser cavity. The cavity has a total measured length of 26.5 m. An Anritsu MS9740A Optical Spectrum Analyzer (OSA) is used to examine the spectral characteristics of the generated output, while a Yokogawa DLM2054 Oscilloscope (OSC) and a 1.2 GHz photodetector is used to observe the output pulse train. The signal-tonoise ratio (SNR) of the generated laser is obtained using an Anritsu MS2683 Radio-Frequency Spectrum Analyzer (RFSA) while the output power is monitored using a Thorlabs optical power meter (OPM). 5. S+/S band passively Q-switched characterization Q-switching begins at a pump threshold power of 76.2 mW and is stable until reaching the maximum pump power of 133.6 mW. A pulse train that is typical of Q-switched operation is obtained as the pump power increases. Fig. 5 shows the oscilloscope trace of the Q-switched pulse trains at pump powers of 76 mW, 109 mW and 121 mW. From the figure, the repetition rate increases proportionally with the pump power, from 16.5 kHz at pump power of 76 mW to 33.3 kHz at a pump power of 121 mW. This is a characteristic of Q switching operation [21,23,36], and also validates that mode-locking has not occurred, as the repetition rate of the generated pulses remains fixed against the rising pump power for the case of a mode-locked signal [43]. The Q-switching effect is confirmed to be induced by the GaSe based SA, as no pulsed outputs are generated when the SA is removed. The typical Q-switched output pulse characteristics is given in Fig. 6 taken at an LD pump power of 109 mW. The lasing wavelength of the Q-switching operation is located at 1502 nm which covers a shorter wavelength of the S band region and the measured 3-dB spectral width of the optical spectrum is around 1.0 nm as shown in Fig. 6(a). The corresponding operation has a 3.9 ms fullwidth at half-maximum (FWHM) of a single pulse profile as shown
Fig. 5. Oscilloscope trace of the Q-switching operation at different pump powers Pp of (a) 76 mW, (b) 109 mW, and (c) 121 mW.
in Fig. 6(b). There are also no amplitude modulations seen to occur on the corresponding pulse profile as shown in Fig. 6(b) as well as in the single Q-switched pulse profiles taken at different pump powers. The corresponding RF spectrum is illustrated in Fig. 6(c). with a fundamental frequency and resolution bandwidth of 23.3 kHz and 300 Hz, respectively. The signal-to-noise ratio (SNR) of the generated output is measured to be 52 dB, indicating stable Q-switching operation [21]. The repetition rate and pulse width against a pump power for a signal at 1502 nm is illustrated in Fig. 7(a). From the figure, it can be seen that the repetition rate increases from 16.5 kHz to 33.3 kHz as the pump power rises from 76.2 mW to 133.6 mW. The pulse width decreases almost linearly from 6.9 ms to 3.1 ms as the pump power rises from 76.2 mW to 115.7 mW, after which the pulse width becomes relatively flat and remains unchanged at 2.7 ms from pump power of 118.3 mW until the maximum pump power is reach. It is assumed at this point that the SA has become satu-
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Fig. 7. (a) Repetition rate and pulse width against the pump power and (b) average output power and pulse energy.
Fig. 6. Typical Q-switching operation characteristics at a pump power of 109 mW. (a) Optical spectrum (k = 1502 nm), (b) single pulse profile of the Q-switching operation, and (c) RF spectrum at the fundamental frequency (f = 23.3 kHz).
rated because the pump power has reached a level in which it allows both the ground state and excited state populations of the SA to become almost equal. This leads to only minimal further absorption, and thus the SA is said at this point to be saturated [44]. The response of the average output power and pulse energy of the output pulses against the pump power is given in Fig. 7(b). From the figure, it can be seen that both the average output power and pulse energy increases almost linearly against the pump power. A maximum average output power of 1.4 mW is obtained at a maximum LD pump power of 133.6 mW with a corresponding pulse energy of 42.8 nJ. It is observed that the Qswitched pulses becomes unstable and disappear as the pump power becomes more than 133.6 mW, although this is typically an indication that the SA has become saturated and auguring well with previous observations [45]. The pulses may also become unstable due to the laser’s longer intracavity length which is sufficient enough to cause the SA to experience thermal instability at higher pump powers [46]. This is confirmed as a pulsed output is
obtained once again as the LD pump power is reduced. Additionally, the pulses’ reappearance indicates that the SA has not yet experienced any thermal damage [45]. Overall, the output of the proposed laser is highly characteristic of a Q-switched output [20,21,24,47]. The output’s lasing wavelength is tuned to determine the ability of the proposed Q-switched to be able to operate in a broadband region, in particular the shorter wavelength zone that is encompassed by the S band. The lasing wavelength is tuned by the TBPF while all other parameters are kept constant, including the pump power which is set at 109 mW. In this regard, tuning the TBPF results in a stable Q-switched pulses obtained from 1454 nm to 1512 nm, covering the S band region, over a wavelength range of 58 nm. This is given in Fig. 8(a). It is interesting to note that no Q-switched pulses can be seen beyond this wavelength range, as the tunable operation range depends greatly on the TBPF’s loss and the gain medium’s bandwidth [48] and also an expected high loss from the TBPF as it is optimized for C band operation. It is expected that an S band optimized TBPF would result in a better tunability range being obtained. The repetition rate and average output power response against the lasing wavelength is shown in Fig. 8(b). while the trend of the pulse energy for the same wavelength range is given in Fig. 8(c). Initially, operation without the TBPF integrated into the cavity results in lasing at 1502 nm, but an immediate shift is observed in the lasing wavelength to 1498 nm with the TBPF integrated into the laser cavity. The shift in the lasing wavelength is believed to be a result of the TBPF’s insertion loss within the laser cavity. From the figure, the lasing wavelength can be shifted to the longer or shorter wavelength regions throughout the tuning process, with the repetition rate, average output power and pulse energy also becoming varied. This is a result of the cavity loss variation as well
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Fig. 9. Q-switching operation stability, with RF spectra measured at 10 min interval for 1 h.
Table 1 S band Q-switched fiber laser by different SAs and gain medium.
Fig. 8. (a) Wavelength tunable Q-switching operation spectra, and (b) trends of repetition rate and average output power and (c) trend of pulse energy against wavelength.
as the insertion loss of the TBPF and also the gain difference of the gain medium at different wavelengths [49]. In this work, maximum single pulse energy at 51.4 nJ during the tunability process is reported. The Q-switching operation stability is investigated by recording the output RF spectra every 10 min for one hour at a pump power of 109 mW and lasing wavelength of 1502 nm. This is shown in Fig. 9. The SNR of the RF spectra is maintained at around 52 dB which shows that the Q switched pulses exhibit stability during the operation, thus making it suitable for photonics applications. Table 1 provides a comparison of the parameters observed in this work against other S band Q switched fiber lasers using by different SAs and different gain mediums. Form Table 1, it can be seen that the system proposed in this work can cover an extended S band region, including shorter S band wavelengths when compared to other works. The tunability range of the proposed system in this work performs significantly better, as opposed to that of other systems that covers only a small
Type of SA
TI: Bi2Te3
MoS2
GaSe
Type of gain medium Operation wavelength (nm) Tuning range (nm) Max pulse energy Repetition rate (kHz) Min pulse width (ms) Ref
EDF 1510.0–1589.1 78.0 1.525 mJ 2.15–12.8 13 [49]
DC-EDF 1484–1492 8 1.2 nJ 27.17–101.17 1.4 [50]
TFF 1454–1512 58 51.4 nJ 16.5–33.3 2.7 This work
region of S band as reported by Chen et al. [49] and Ahmad et al. [50]. Additionally, while the system proposed by Chen et al. [49], successfully generated high energy Q-switched pulses, these pulses are only limited to a small region of S band from 1510 to 1530 nm. Similarly, other systems utilizing the depressed-cladding EDF (DC-EDF) as a gain medium [50] report Q-switching operation that covers only a small region of S band from 1484 nm to 1492 nm with a tuning range 8 nm and low pulse energy of about 1.2 nJ. However, tunability range of Q switching operation on S band fiber laser by Ahmad et al. [51], which ranges from 1454 to 1510 nm is comparable to this works as both systems utilize the TFF as a gain medium but report a different repetition rate range due to different SAs used. Therefore, the generation of Q-switched pulses in S band region, especially shorter wavelength of S band could be achieved with the combination of TFF as gain medium and GaSe as SA. The proposed laser is capable of fulfilling the demand for photonics applications in S band region, especially shorter wavelength of S band.
6. Conclusion A passively Q-switched TFF laser using a GaSe based SA operating in the S+/S-band region is proposed and demonstrated. The laser uses a 14.5-m long TFF to generate a continuous wave lasing output, while a GaSe SA integrated into the fiber laser system is used to generate the Q-switched pulses in S+/S band region. The system is capable of generating Q-switched pulses covering a wavelength region of 1454–1512 nm, with a tuning range of 58 nm when pumped with a pumping power of 109 mw. The tunability is achieved by tuning the TBPF in the laser cavity. Without the TBPF, Q-switching operation is observed to start from a pump power of 76.2 mW and continues to be observed until a pump power of 133.6 mW, with the repetition rate varying between 16.5 kHz and 33.3 kHz and the lasing wavelength at 1502 nm. The maximum pulse energy and the minimum pulse width obtained in this work are 51.4 nJ and 2.7 ms respectively. The highly
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stable Q switched pulses obtained in this work are observed in the RF spectrum over a period of one hour with the SNR value maintained at approximately 52 dB. To the best of author’s knowledge, this work is the first demonstration of Q-switching operation induced in a TFF laser using a GaSe based SA, and has potential for multiple S band photonics applications. Acknowledgement Funding for this research was provided for by the Ministry of Higher Education (MOHE) under the grants LRGS (2015)/NGOD/ UM/KPT as well as the University of Malaya under the grants RU 001 – 20017 and RP 029B – 15AFR. References [1] M.L. Siniaeva, M.N. Siniavsky, V.P. Pashinin, A.A. Mamedov, V.I. Konov, V.V. Kononenko, Laser ablation of dental materials using a microsecond Nd:YAG laser, Laser Phys. 19 (5) (2009) 1056–1060. [2] U. Sharma, C.-S. Kim, J.U. Kang, N.M. Fried, Highly stable tunable dualwavelength Q-switched fiber laser for DIAL applications, Laser Appl. Chem. Environ. Anal. Opt. Soc. Am., 2004, p. MB3. [3] R. Paschotta, R. Haring, E. Gini, H. Melchior, U. Keller, H.L. Offerhaus, D.J. Richardson, Passively Q-switched 0.1-mJ fiber laser system at 1.53 lm, Opt. Lett. 24 (6) (1999) 388–390. [4] M. Xiang, S. Fu, M. Tang, H. Tang, P. Shum, D. Liu, Nyquist WDM superchannel using offset-16QAM and receiver-side digital spectral shaping, Opt. Exp. 22 (14) (2014) 17448–17457. [5] M. Shirk, P. Molian, A review of ultrashort pulsed laser ablation of materials, J. Laser Appl. 10 (1) (1998) 18–28. [6] C. Luo, L. Lin, The application of nanosecond-pulsed laser welding technology in MEMS packaging with a shadow mask, Sens. Actuat. A 97 (2002) 398–404. [7] S. Orazio, Principles of Lasers, Springer, New York, 2010. [8] M. Delgado-Pinar, D. Zalvidea, A. Diez, P. Pérez-Millán, M. Andrés, Q-switching of an all-fiber laser by acousto-optic modulation of a fiber Bragg grating, Opt. Exp. 14 (3) (2006) 1106–1112. [9] D. Zalvidea, N.A. Russo, R. Duchowicz, M. Delgado-Pinar, A. Díez, J.L. Cruz, M.V. Andrés, High-repetition rate acoustic-induced Q-switched all-fiber laser, Opt. Commun. 244 (1–6) (2005) 315–319. [10] H. Ahmad, A.S. Sharbirin, A. Muhamad, M.Z. Samion, S.A. Reduan, A.Z. Zulkifli, M.F. Ismail, Aluminized film as saturable absorber for generating passive Qswitched pulses in the two-micron region, J. Lightwave Technol. 35 (12) (2017) 2470–2475. [11] O. Okhotnikov, A. Grudinin, M. Pessa, Ultra-fast fibre laser systems based on SESAM technology: new horizons and applications, New J. Phys. 6 (1) (2004) 177. [12] A. Martinez, Z. Sun, Nanotube and graphene saturable absorbers for fibre lasers, Nat. Photon 7 (11) (2013) 842–845. [13] D.-P. Zhou, L. Wei, B. Dong, W.-K. Liu, Tunable passively Q-switched erbiumdoped fiber laser with carbon nanotubes as a saturable absorber, IEEE Photon. Technol. Lett. 22 (1) (2010) 9–11. [14] D. Popa, Z. Sun, T. Hasan, F. Torrisi, F. Wang, A. Ferrari, Graphene Q-switched, tunable fiber laser, Appl. Phys. Lett. 98 (7) (2011) 073106. [15] J. Lee, M. Jung, J. Koo, C. Chi, J.H. Lee, Passively Q-switched 1.89-lm fiber laser using a bulk-structured Bi2Te3 topological insulator, IEEE J. Sel. Top. Quantum Electron. 21 (1) (2015) 31–36. [16] J. Lee, J. Koo, C. Chi, J.H. Lee, All-fiberized, passively Q-switched 1.06 lm laser using a bulk-structured Bi2Te3 topological insulator, J. Opt. 16 (8) (2014) 085203. [17] H. Ahmad, M. Salim, M. Soltanian, S.R. Azzuhri, S. Harun, Passively dualwavelength Q-switched ytterbium doped fiber laser using Selenium Bismuth as saturable absorber, J. Mod. Opt. 62 (19) (2015) 1550–1554. [18] S. Wang, H. Yu, H. Zhang, A. Wang, M. Zhao, Y. Chen, L. Mei, J. Wang, Broadband few-layer MoS2 saturable absorbers, Adv. Mater. 26 (21) (2014) 3538–3544. [19] J. Lin, Y. Hu, C. Chen, C. Gu, L. Xu, Wavelength-tunable Yb-doped passively Qswitching fiber laser based on WS 2 saturable absorber, Opt. Exp. 23 (22) (2015) 29059–29064. [20] R. Woodward, R. Howe, T. Runcorn, G. Hu, F. Torrisi, E. Kelleher, T. Hasan, Wideband saturable absorption in few-layer molybdenum diselenide (MoSe 2) for Q-switching Yb- Er-and Tm-doped fiber lasers, Opt. Exp. 23 (15) (2015) 20051–20061. [21] B. Chen, X. Zhang, K. Wu, H. Wang, J. Wang, J. Chen, Q-switched fiber laser based on transition metal dichalcogenides MoS2, MoSe2, WS2, and WSe2, Opt. Exp. 23 (20) (2015) 26723–26737. [22] Y. Chen, G. Jiang, S. Chen, Z. Guo, X. Yu, C. Zhao, H. Zhang, Q. Bao, S. Wen, D. Tang, Mechanically exfoliated black phosphorus as a new saturable absorber for both Q-switching and mode-locking laser operation, Opt. Exp. 23 (10) (2015) 12823–12833. [23] H. Ahmad, C. Lee, M. Ismail, Z. Ali, S. Reduan, N. Ruslan, M. Ismail, S. Harun, Zinc oxide (ZnO) nanoparticles as saturable absorber in passively Q-switched fiber laser, Opt. Commun. 381 (2016) 72–76.
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Full length article
S+/S band passively Q-switched thulium-fluoride fiber laser based on using gallium selenide saturable absorber H. Ahmad a,b,⇑, S.A. Reduan a a b
Photonics Research Centre, University of Malaya, 50603 Kuala Lumpur, Malaysia Department of Physics, Faculty of Science and Technology, Airlangga University, Surabaya 60115, Indonesia
a r t i c l e
i n f o
Article history: Received 20 March 2018 Received in revised form 1 May 2018 Accepted 13 May 2018
Keywords: Thulium-fluoride fiber laser Q-switched S band fiber laser Saturable absorber Gallium selenide
a b s t r a c t A Q-switched thulium-fluoride fiber (TFF) laser using gallium selenide (GaSe) as a passive saturable absorber (SA) for operation in the S+/S band is demonstrated. The generated pulses can be tuned from 1454 nm to 1512 nm, giving a tuning range of 58 nm. At 1502 nm, stable Q-switching operation is obtained from a pump power of 76.2 mW and is observed until the maximum pump power of 133.6 mW is reached, with the repetition rate varying from 16.5 kHz to 33.3 kHz and having a narrowest pulse width at 2.7 ms. The highest pulse energy obtained in this work is 51.4 nJ and this is, to the knowledge of the authors, the first time that Q-switching operation in the S+/S band region is obtained from a TFF laser using a GaSe based SA. Ó 2018 Elsevier Ltd. All rights reserved.
1. Introduction Pulsed lasers with high-energy outputs are highly desired for use in a multitude of applications ranging from environmental sensing, scientific research, material processing and medicine [1–4]. Initially, pulsed laser found significant applications in material processing such as in material laser ablation [5] and laser welding [6] but recent advances have seen the development of fiber lasers capable of generating high-energy, long-duration pulses. These lasers are particularly sought after for their capability to generate the desired pulses in a compact, cost-effective and robust platform [7]. Q-switched fiber lasers can generate an output with the afore-mentioned characteristics through active or passive approaches. Actively Q-switched fiber lasers offer a high degree of control over various output pulse parameters, including repetition rates and pulse durations [8,9]. However, the extra components required for active Q-switching are highly sensitive and expensive, thereby limiting the use of actively Q-switched fiber lasers in real-world applications. Fiber lasers that are passively Q-switched on the other hand are significantly less expensive and complex to fabricate and operate, although with less control over the laser’s output parameters [10,11]. These lasers are
⇑ Corresponding author at: Photonics Research Centre, University of Malaya, 50603 Kuala Lumpur, Malaysia. E-mail address: [email protected] (H. Ahmad). https://doi.org/10.1016/j.optlastec.2018.05.028 0030-3992/Ó 2018 Elsevier Ltd. All rights reserved.
realized through the use of saturable absorbers (SA) that are incorporated into the laser system. SAs, including semiconductor saturable absorber mirrors (SESAMs) and more recently 2-dimensional (2D) and 3-dimensional (3D) nanostructures such as carbon nanotubes (CNTs) and graphene [12–14] are typically incorporated into the laser cavity to generate Q-switched outputs, with the latter seeing increasing application due to their low fabrication cost and complexity. In fact, such is the potential for SAs in developing compact, high performance Q-switched fiber lasers that research efforts into developing new SA materials has intensified. This includes the exploration of topological insulators (TIs) including bismuth telluride (Bi2Te3) [15,16] and bismuth selenide (Bi2Se3) [17] as well as transition metal dichalcogenides (TMDs) such as molybdenum sulfide (MoS2) [18], tungsten disulfide (WS2) [19], molybdenum diselenide (MoSe2) [20] and tungsten (WSe2) [21] for the development of a new generation of SAs. Other materials such as black phosphorus [22], zinc oxide [23] and silver nanoparticles [24] are also being explored for their use as SAs, and have been shown to be able to produce a consistent and high performance Q-switched pulse. In this work, gallium selenide (GaSe), a compound formed using a 2D layered metal monochalcogenide, is used as an SA to generate an S band Q-switched output. Typically used for nonlinear optics [25], opto-electronics [26,27] and terahertz systems [28,29], GaSe has a substantial nonlinear optical coefficient, high surface damage threshold, broad transparency range and extremely low optical
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losses [30]. Furthermore, GaSe exhibits interesting optical properties such as up-conversion luminescence [31], making them suitable for optical applications [32]. The GaSe is used as an SA in a thulium-fluoride fiber (TFF) based laser cavity to enable emissions in the S+ and S band in this work. The S band is the telecom wavelength band that covers a region of 1480–1530 nm, while the S+ band region covers a shorter wavelength range of the S band region from 1450 to 1480 nm [33–35]. These new bands are crucial developments towards addressing current demands for bandwidth, which are quickly overwhelming the C and L band bandwidths regions of current telecommunications systems. The development of S+/S band fiber lasers enables the realization of applications such as coarse wavelength-division multiplexing (CWDM) and fiber to the premise (FTTP) systems [33,34]. This would be, to the best of author’s knowledge, the first successful demonstration of Q-switching in a TFF laser cavity using GaSe based SA.
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Fig. 2. Raman spectra of a thin layer of GaSe and bulk GaSe.
The SA is prepared by mechanical exfoliation, a technique that is commonly and widely used by many researchers in this field of study for its simplicity and reliability [22,36]. In this work, a small monochalcogenide (MX) GaSe crystal flake obtained from 2D Semiconductors is carefully placed on one side of scotch tape as shown in Fig. 1. The scotch tape is then folded into half and pressed repeatedly to cause a few layers of the crystal to exfoliate onto the scotch-tape. The portion of the scotch tape with the few GaSe layers is then carefully cut-out from the larger area as illustrated in Fig. 1(b) and (c). The cut out scotch tape section with the exfoliated GaSe thin layer is then carefully attached onto the fiber ferrule. A small amount of index matching gel is used to help stick the GaSe thin layer onto the face of the fiber ferrule. This is given in Fig. 1(d). The fiber ferrule is then connected to another fiber ferrule using an FC/PC adaptor, thus creating the complete SA assembly. The insertion loss of the GaSe thin layer as an SA is around 3.20 dB, whereby the insertion loss of the scotch tape is around 0.02 dB.
obtained from the bulk GaSe sample exhibits peaks at 148.6 cm 1, 214.0 cm 1 and 322.6 cm 1 respectively, as would be expected for bulk GaSe and comparable with the previous works [25,38]. It is also important to note that the intensity of the Raman spectrum for the bulk GaSe sample is lower than that of the thin layer GaSe sample, due to the reduction in the GaSe scattering mode that leads to less effective Raman scattering [27]. The thin GaSe film has multiple layers, indicated by the red-shift and Raman peak intensity reduction as well as the peaks between both spectra [25,37]. The nonlinear power-dependent absorbance of the GaSe SA is given in Fig. 3. The twin detector measurement technique is used to characterize the GaSe SA’s nonlinearity [39,40]. A passively mode-locked erbium-doped fiber laser (EDFL) with a repetition rate of 27.9 MHz and 0.70 ps is used as a laser seed. The GaSe based SA’s modulation depth is measured to be about 11.5%, while its saturation intensity is computed at 0.02 MW/cm2. The modulation depth of the GaSe based SA is on par with that of other SAs based on materials such as MoS2 at 9.7% [41] and Bi2Se3 at 11.1% [42]. This shows that the GaSe SA can perform comparably to other works.
3. Characterization of GaSe-SA
4. Q-switched laser design
The Raman spectra of GaSe in both its bulk and thin layer forms is shown in Fig. 2. The spectra were obtained using a Renishaw Raman spectroscope at an excitation wavelength of 532 nm and 1800 l/mm grating. The resulting spectra shows only a single peak the case of the thin layer GaSe as compared to the Raman spectrum of bulk GaSe, in which three distinct peaks can be observed. The single peak of the thin-layer GaSe sample is observed at 319.4 cm 1, and is comparable to the observed by Sidong Lei et al. [25] and Xiang Yuan et al. [37]. On the other hand, the spectrum
The schematic of the proposed Q-switched S+/S band fiber laser with a GaSe based SA is given in Fig. 4. The laser is configured as a ring cavity and uses a 1400 nm FOL1405RTD laser diode (LD) as a pump source. The LD’s output is connected to an optical isolator (ISO) to protect the LD from back reflections, before being connected to the 1400 nm port of a 1400/1500 nm fused wavelength division multiplexer (WDM). The common output of the WDM is now connected to the 14.5 m long Fiberlabs Inc TFF, with an absorption rate of 0.15 dB/m at the wavelength of 1400 nm, as well
Fig. 1. Mechanical exfoliation of GaSe from crystal flake.
Fig. 3. Nonlinear power dependent absorbance of thin layer GaSe SA.
2. Preparation of GaSe-SA
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Fig. 4. Experimental setup of S+/S band fiber ring cavity laser Q-switched using GaSe SA: LD (Laser diode), ISO (Isolator), WDM (Wavelength division multiplexer), TFF (Thulium Fluoride Fiber), TBPF (Tunable bandpass filter) and SA (Saturable absorber).
as a numerical aperture value of 0.26 and a mode-field diameter of 4.5 mm at 1500 nm. The TFF’s output is now channeled through another ISO before reaching a 90:10 optical tap coupler, which captures 10% of the propagating signal for further analysis. The remaining 90% signal continues to propagate within the ring cavity until it reaches the Agiltron Inc FOTF-025121332 tunable bandpass filter (TBPF) which is used to control the lasing wavelength in the cavity. The output of the signal then reaches the SA assembly, where it will become pulsed. Finally, the SA’s output is connected to the WDM’s 1500 nm port, completing the laser cavity. The cavity has a total measured length of 26.5 m. An Anritsu MS9740A Optical Spectrum Analyzer (OSA) is used to examine the spectral characteristics of the generated output, while a Yokogawa DLM2054 Oscilloscope (OSC) and a 1.2 GHz photodetector is used to observe the output pulse train. The signal-tonoise ratio (SNR) of the generated laser is obtained using an Anritsu MS2683 Radio-Frequency Spectrum Analyzer (RFSA) while the output power is monitored using a Thorlabs optical power meter (OPM). 5. S+/S band passively Q-switched characterization Q-switching begins at a pump threshold power of 76.2 mW and is stable until reaching the maximum pump power of 133.6 mW. A pulse train that is typical of Q-switched operation is obtained as the pump power increases. Fig. 5 shows the oscilloscope trace of the Q-switched pulse trains at pump powers of 76 mW, 109 mW and 121 mW. From the figure, the repetition rate increases proportionally with the pump power, from 16.5 kHz at pump power of 76 mW to 33.3 kHz at a pump power of 121 mW. This is a characteristic of Q switching operation [21,23,36], and also validates that mode-locking has not occurred, as the repetition rate of the generated pulses remains fixed against the rising pump power for the case of a mode-locked signal [43]. The Q-switching effect is confirmed to be induced by the GaSe based SA, as no pulsed outputs are generated when the SA is removed. The typical Q-switched output pulse characteristics is given in Fig. 6 taken at an LD pump power of 109 mW. The lasing wavelength of the Q-switching operation is located at 1502 nm which covers a shorter wavelength of the S band region and the measured 3-dB spectral width of the optical spectrum is around 1.0 nm as shown in Fig. 6(a). The corresponding operation has a 3.9 ms fullwidth at half-maximum (FWHM) of a single pulse profile as shown
Fig. 5. Oscilloscope trace of the Q-switching operation at different pump powers Pp of (a) 76 mW, (b) 109 mW, and (c) 121 mW.
in Fig. 6(b). There are also no amplitude modulations seen to occur on the corresponding pulse profile as shown in Fig. 6(b) as well as in the single Q-switched pulse profiles taken at different pump powers. The corresponding RF spectrum is illustrated in Fig. 6(c). with a fundamental frequency and resolution bandwidth of 23.3 kHz and 300 Hz, respectively. The signal-to-noise ratio (SNR) of the generated output is measured to be 52 dB, indicating stable Q-switching operation [21]. The repetition rate and pulse width against a pump power for a signal at 1502 nm is illustrated in Fig. 7(a). From the figure, it can be seen that the repetition rate increases from 16.5 kHz to 33.3 kHz as the pump power rises from 76.2 mW to 133.6 mW. The pulse width decreases almost linearly from 6.9 ms to 3.1 ms as the pump power rises from 76.2 mW to 115.7 mW, after which the pulse width becomes relatively flat and remains unchanged at 2.7 ms from pump power of 118.3 mW until the maximum pump power is reach. It is assumed at this point that the SA has become satu-
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Fig. 7. (a) Repetition rate and pulse width against the pump power and (b) average output power and pulse energy.
Fig. 6. Typical Q-switching operation characteristics at a pump power of 109 mW. (a) Optical spectrum (k = 1502 nm), (b) single pulse profile of the Q-switching operation, and (c) RF spectrum at the fundamental frequency (f = 23.3 kHz).
rated because the pump power has reached a level in which it allows both the ground state and excited state populations of the SA to become almost equal. This leads to only minimal further absorption, and thus the SA is said at this point to be saturated [44]. The response of the average output power and pulse energy of the output pulses against the pump power is given in Fig. 7(b). From the figure, it can be seen that both the average output power and pulse energy increases almost linearly against the pump power. A maximum average output power of 1.4 mW is obtained at a maximum LD pump power of 133.6 mW with a corresponding pulse energy of 42.8 nJ. It is observed that the Qswitched pulses becomes unstable and disappear as the pump power becomes more than 133.6 mW, although this is typically an indication that the SA has become saturated and auguring well with previous observations [45]. The pulses may also become unstable due to the laser’s longer intracavity length which is sufficient enough to cause the SA to experience thermal instability at higher pump powers [46]. This is confirmed as a pulsed output is
obtained once again as the LD pump power is reduced. Additionally, the pulses’ reappearance indicates that the SA has not yet experienced any thermal damage [45]. Overall, the output of the proposed laser is highly characteristic of a Q-switched output [20,21,24,47]. The output’s lasing wavelength is tuned to determine the ability of the proposed Q-switched to be able to operate in a broadband region, in particular the shorter wavelength zone that is encompassed by the S band. The lasing wavelength is tuned by the TBPF while all other parameters are kept constant, including the pump power which is set at 109 mW. In this regard, tuning the TBPF results in a stable Q-switched pulses obtained from 1454 nm to 1512 nm, covering the S band region, over a wavelength range of 58 nm. This is given in Fig. 8(a). It is interesting to note that no Q-switched pulses can be seen beyond this wavelength range, as the tunable operation range depends greatly on the TBPF’s loss and the gain medium’s bandwidth [48] and also an expected high loss from the TBPF as it is optimized for C band operation. It is expected that an S band optimized TBPF would result in a better tunability range being obtained. The repetition rate and average output power response against the lasing wavelength is shown in Fig. 8(b). while the trend of the pulse energy for the same wavelength range is given in Fig. 8(c). Initially, operation without the TBPF integrated into the cavity results in lasing at 1502 nm, but an immediate shift is observed in the lasing wavelength to 1498 nm with the TBPF integrated into the laser cavity. The shift in the lasing wavelength is believed to be a result of the TBPF’s insertion loss within the laser cavity. From the figure, the lasing wavelength can be shifted to the longer or shorter wavelength regions throughout the tuning process, with the repetition rate, average output power and pulse energy also becoming varied. This is a result of the cavity loss variation as well
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Fig. 9. Q-switching operation stability, with RF spectra measured at 10 min interval for 1 h.
Table 1 S band Q-switched fiber laser by different SAs and gain medium.
Fig. 8. (a) Wavelength tunable Q-switching operation spectra, and (b) trends of repetition rate and average output power and (c) trend of pulse energy against wavelength.
as the insertion loss of the TBPF and also the gain difference of the gain medium at different wavelengths [49]. In this work, maximum single pulse energy at 51.4 nJ during the tunability process is reported. The Q-switching operation stability is investigated by recording the output RF spectra every 10 min for one hour at a pump power of 109 mW and lasing wavelength of 1502 nm. This is shown in Fig. 9. The SNR of the RF spectra is maintained at around 52 dB which shows that the Q switched pulses exhibit stability during the operation, thus making it suitable for photonics applications. Table 1 provides a comparison of the parameters observed in this work against other S band Q switched fiber lasers using by different SAs and different gain mediums. Form Table 1, it can be seen that the system proposed in this work can cover an extended S band region, including shorter S band wavelengths when compared to other works. The tunability range of the proposed system in this work performs significantly better, as opposed to that of other systems that covers only a small
Type of SA
TI: Bi2Te3
MoS2
GaSe
Type of gain medium Operation wavelength (nm) Tuning range (nm) Max pulse energy Repetition rate (kHz) Min pulse width (ms) Ref
EDF 1510.0–1589.1 78.0 1.525 mJ 2.15–12.8 13 [49]
DC-EDF 1484–1492 8 1.2 nJ 27.17–101.17 1.4 [50]
TFF 1454–1512 58 51.4 nJ 16.5–33.3 2.7 This work
region of S band as reported by Chen et al. [49] and Ahmad et al. [50]. Additionally, while the system proposed by Chen et al. [49], successfully generated high energy Q-switched pulses, these pulses are only limited to a small region of S band from 1510 to 1530 nm. Similarly, other systems utilizing the depressed-cladding EDF (DC-EDF) as a gain medium [50] report Q-switching operation that covers only a small region of S band from 1484 nm to 1492 nm with a tuning range 8 nm and low pulse energy of about 1.2 nJ. However, tunability range of Q switching operation on S band fiber laser by Ahmad et al. [51], which ranges from 1454 to 1510 nm is comparable to this works as both systems utilize the TFF as a gain medium but report a different repetition rate range due to different SAs used. Therefore, the generation of Q-switched pulses in S band region, especially shorter wavelength of S band could be achieved with the combination of TFF as gain medium and GaSe as SA. The proposed laser is capable of fulfilling the demand for photonics applications in S band region, especially shorter wavelength of S band.
6. Conclusion A passively Q-switched TFF laser using a GaSe based SA operating in the S+/S-band region is proposed and demonstrated. The laser uses a 14.5-m long TFF to generate a continuous wave lasing output, while a GaSe SA integrated into the fiber laser system is used to generate the Q-switched pulses in S+/S band region. The system is capable of generating Q-switched pulses covering a wavelength region of 1454–1512 nm, with a tuning range of 58 nm when pumped with a pumping power of 109 mw. The tunability is achieved by tuning the TBPF in the laser cavity. Without the TBPF, Q-switching operation is observed to start from a pump power of 76.2 mW and continues to be observed until a pump power of 133.6 mW, with the repetition rate varying between 16.5 kHz and 33.3 kHz and the lasing wavelength at 1502 nm. The maximum pulse energy and the minimum pulse width obtained in this work are 51.4 nJ and 2.7 ms respectively. The highly
H. Ahmad, S.A. Reduan / Optics and Laser Technology 107 (2018) 116–121
stable Q switched pulses obtained in this work are observed in the RF spectrum over a period of one hour with the SNR value maintained at approximately 52 dB. To the best of author’s knowledge, this work is the first demonstration of Q-switching operation induced in a TFF laser using a GaSe based SA, and has potential for multiple S band photonics applications. Acknowledgement Funding for this research was provided for by the Ministry of Higher Education (MOHE) under the grants LRGS (2015)/NGOD/ UM/KPT as well as the University of Malaya under the grants RU 001 – 20017 and RP 029B – 15AFR. References [1] M.L. Siniaeva, M.N. Siniavsky, V.P. Pashinin, A.A. Mamedov, V.I. Konov, V.V. Kononenko, Laser ablation of dental materials using a microsecond Nd:YAG laser, Laser Phys. 19 (5) (2009) 1056–1060. [2] U. Sharma, C.-S. Kim, J.U. Kang, N.M. Fried, Highly stable tunable dualwavelength Q-switched fiber laser for DIAL applications, Laser Appl. Chem. Environ. Anal. Opt. Soc. Am., 2004, p. MB3. [3] R. Paschotta, R. Haring, E. Gini, H. Melchior, U. Keller, H.L. Offerhaus, D.J. Richardson, Passively Q-switched 0.1-mJ fiber laser system at 1.53 lm, Opt. Lett. 24 (6) (1999) 388–390. [4] M. Xiang, S. Fu, M. Tang, H. Tang, P. Shum, D. Liu, Nyquist WDM superchannel using offset-16QAM and receiver-side digital spectral shaping, Opt. Exp. 22 (14) (2014) 17448–17457. [5] M. Shirk, P. Molian, A review of ultrashort pulsed laser ablation of materials, J. Laser Appl. 10 (1) (1998) 18–28. [6] C. Luo, L. Lin, The application of nanosecond-pulsed laser welding technology in MEMS packaging with a shadow mask, Sens. Actuat. A 97 (2002) 398–404. [7] S. Orazio, Principles of Lasers, Springer, New York, 2010. [8] M. Delgado-Pinar, D. Zalvidea, A. Diez, P. Pérez-Millán, M. Andrés, Q-switching of an all-fiber laser by acousto-optic modulation of a fiber Bragg grating, Opt. Exp. 14 (3) (2006) 1106–1112. [9] D. Zalvidea, N.A. Russo, R. Duchowicz, M. Delgado-Pinar, A. Díez, J.L. Cruz, M.V. Andrés, High-repetition rate acoustic-induced Q-switched all-fiber laser, Opt. Commun. 244 (1–6) (2005) 315–319. [10] H. Ahmad, A.S. Sharbirin, A. Muhamad, M.Z. Samion, S.A. Reduan, A.Z. Zulkifli, M.F. Ismail, Aluminized film as saturable absorber for generating passive Qswitched pulses in the two-micron region, J. Lightwave Technol. 35 (12) (2017) 2470–2475. [11] O. Okhotnikov, A. Grudinin, M. Pessa, Ultra-fast fibre laser systems based on SESAM technology: new horizons and applications, New J. Phys. 6 (1) (2004) 177. [12] A. Martinez, Z. Sun, Nanotube and graphene saturable absorbers for fibre lasers, Nat. Photon 7 (11) (2013) 842–845. [13] D.-P. Zhou, L. Wei, B. Dong, W.-K. Liu, Tunable passively Q-switched erbiumdoped fiber laser with carbon nanotubes as a saturable absorber, IEEE Photon. Technol. Lett. 22 (1) (2010) 9–11. [14] D. Popa, Z. Sun, T. Hasan, F. Torrisi, F. Wang, A. Ferrari, Graphene Q-switched, tunable fiber laser, Appl. Phys. Lett. 98 (7) (2011) 073106. [15] J. Lee, M. Jung, J. Koo, C. Chi, J.H. Lee, Passively Q-switched 1.89-lm fiber laser using a bulk-structured Bi2Te3 topological insulator, IEEE J. Sel. Top. Quantum Electron. 21 (1) (2015) 31–36. [16] J. Lee, J. Koo, C. Chi, J.H. Lee, All-fiberized, passively Q-switched 1.06 lm laser using a bulk-structured Bi2Te3 topological insulator, J. Opt. 16 (8) (2014) 085203. [17] H. Ahmad, M. Salim, M. Soltanian, S.R. Azzuhri, S. Harun, Passively dualwavelength Q-switched ytterbium doped fiber laser using Selenium Bismuth as saturable absorber, J. Mod. Opt. 62 (19) (2015) 1550–1554. [18] S. Wang, H. Yu, H. Zhang, A. Wang, M. Zhao, Y. Chen, L. Mei, J. Wang, Broadband few-layer MoS2 saturable absorbers, Adv. Mater. 26 (21) (2014) 3538–3544. [19] J. Lin, Y. Hu, C. Chen, C. Gu, L. Xu, Wavelength-tunable Yb-doped passively Qswitching fiber laser based on WS 2 saturable absorber, Opt. Exp. 23 (22) (2015) 29059–29064. [20] R. Woodward, R. Howe, T. Runcorn, G. Hu, F. Torrisi, E. Kelleher, T. Hasan, Wideband saturable absorption in few-layer molybdenum diselenide (MoSe 2) for Q-switching Yb- Er-and Tm-doped fiber lasers, Opt. Exp. 23 (15) (2015) 20051–20061. [21] B. Chen, X. Zhang, K. Wu, H. Wang, J. Wang, J. Chen, Q-switched fiber laser based on transition metal dichalcogenides MoS2, MoSe2, WS2, and WSe2, Opt. Exp. 23 (20) (2015) 26723–26737. [22] Y. Chen, G. Jiang, S. Chen, Z. Guo, X. Yu, C. Zhao, H. Zhang, Q. Bao, S. Wen, D. Tang, Mechanically exfoliated black phosphorus as a new saturable absorber for both Q-switching and mode-locking laser operation, Opt. Exp. 23 (10) (2015) 12823–12833. [23] H. Ahmad, C. Lee, M. Ismail, Z. Ali, S. Reduan, N. Ruslan, M. Ismail, S. Harun, Zinc oxide (ZnO) nanoparticles as saturable absorber in passively Q-switched fiber laser, Opt. Commun. 381 (2016) 72–76.
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