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Soliton mode-locked thulium-doped fiber laser with cobalt oxide saturable absorber
Optical Fiber Technology 45 (2018) 122–127
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Soliton mode-locked thulium-doped fiber laser with cobalt oxide saturable absorber
T
⁎
H. Ahmada,b, , M.Z. Samiona, N. Yusoffa a b
Photonics Research Centre, University of Malaya, 50603 Kuala Lumpur, Malaysia Visiting Professor at the Department of Physics, Faculty of Science and Technology, Airlangga University, Surabaya 60115, Indonesia
A R T I C LE I N FO
A B S T R A C T
Keywords: Mode-locking Thulium Cobalt oxide Pulse fiber laser
In this work, we propose and demonstrate a mode-locked thulium doped fiber laser (TDFL) with an integrated cobalt oxide (Co3O4) saturable absorber (SA). The SA is fabricated by suspending Co3O4 nanosheets in a polymer film before being secured between two fiber ferules. The laser operates in the anomalous dispersion regime, confirmed by the Kelly sidebands observed in the obtained optical spectrum. Mode-locking operation is obtained at a low threshold pump power of 77.32 mW, with further optimizations made using a polarization controller The laser generates pulses with a repetition rate of 11.36 MHz and a pulse width of 1.39 ps. The generated pulses are highly stable, with a high signal-to-noise ratio of 46.00 dB and minimum power fluctuations indicating a long-term stability. This work demonstrates a simple and a low-cost laser that would have potential applications for operation near the 2.0-µm region, especially for medical applications.
1. Introduction Passively pulsed fiber lasers are highly desired as laser sources due to their potential use in a variety of applications ranging from material processing to remote sensing, microscopy and medicine. The attractiveness of these sources arises from their robust yet compact form factor, which in turn results low fabrication and operating costs [1–5]. To achieve this, fiber lasers are be designed to be either Q-switched or mode-locked, with the latter generating shorter, ultrafast pulses with overall low output energies and the former generating longer and slower pulses but with higher energies [6]. Traditionally, Q-switching and mode-locking in fiber lasers was achieved by actively modulating the losses within the laser cavity active using acousto-optic or electro-optic modulators [7–11]. However, the bulky nature of these modulators, as well as their relatively high cost quickly made this option unsuitable for most real-world applications. As such, research efforts were focused towards the development of passively pulsed devices, which generated a similar output but in a much more compact form factor and at a lower overall cost. This was typically achieved primarily by saturable absorbers (SAs), such as semiconductor saturable absorber mirrors (SESAMs) [12,13] and more recently 2-dimensional (2D) and 3-dimensional (3D) materials such as carbon nanotubes [14–16] and graphene [17–19]. Additionally, recent developments have now seen new 2D and 3D nanomaterials, such as
⁎
metals [20,21], topological insulators (TIs) [22–24], transition metal dichalcogenides (TMDs) [25–27] and even exotic materials such as black phosphorus [28–30] being used to obtain passive mode-locked outputs. The fabrication and the nonlinear optical properties of these low-dimensional materials have been widely investigated and reported [31–33], with particular attention being paid to 2D nanomaterials for advanced opto-electronic applications [34–36] due to their unique properties such as low saturation intensity, broadband absorption properties as well as fast recovery and response times [37–39]. Of late, transition metal oxides (TMOs) have now become the focus of significant research interest for use as SAs due to their unique optical charactersitics, namely their large nonlinear optical response [40]. As a result of their 2D nature, the bandgaps of TMOs can be tuned without altering their physical properties, giving them a wide-operational bandwidth [41]. TMOs such as nickel oxide (NiO), zinc oxide (ZnO), iron (II, III) oxide (Fe3O4), and titanium oxide (TiO2) have all been successfully demonstrated as SAs for the generation of pulses fiber lasers [42–45]. The performance of these lasers is comparable to the ones built using graphene or other 2D material based SAs, thus indicating the potential of TMOs as SAs. In this regard, cobalt oxide (Co3O4) is an attractive material for use as an SA as it has a large nonlinear to linear absorption ratio [40], as well as spectral absorption that stretches to the near-infrared (NIR) region [46]. Recently, the use of Co3O4 as an SA has been reported with the generation of Q-switched pulses at the 1.5-µm
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.yofte.2018.07.012 Received 6 April 2018; Received in revised form 22 May 2018; Accepted 2 July 2018 1068-5200/ © 2018 Elsevier Inc. All rights reserved.
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Fig. 1. (a) XRD pattern of Co3O4 nanosheets , (b) UV–vis spectra of the Co3O4-PEO in solution form with a close-up view of the absorption peaks (inset), and FESEM images of the Co3O4 nanosheets taken at (c) low magnification and (d) at high magnification.
before being centrifuged at 4000 rpm for 20 min to obtain the Co3O4 nanosheets structure, which appear as a black powder precipitate. The Co3O4 nanosheet precipitate is then washed extensively with DI water and absolute ethanol. This is repeated three times before the final precipitate is dried in an oven at 60 °C for 24 h, giving the desired form of a fine black powder. While the nanosheets are the optically active material of the SA, their fine and brittle nature does not allow them to be used in their current form within the fiber cavity. As such, the Co3O4 nanosheets are instead embedded in a Polyethylene-Oxide (PEO) thin film which serves as a host material. The thin film is formed by dissolving 250 mg of PEO powder in 30 mL of DI water, and stirred continuously at 50 °C for 2 h. Approximately 10 mL of the Co3O4 black powder with a density of 5 mg mL−1 is then added into the PEO solution, and the entire mixture stirred for another 2 h. The resulting compound is then transferred onto a petri dish and left to dry in an oven at 60 °C for 24 h, forming a polymer film with Co3O4 nanosheets embedded in the host material. A small portion of the thin film formed is cut out and placed on the face of a fiber ferrule so that it covers the core region. A small amount of indexmatching gel is used to hold the polymer piece in place, and another fiber ferrule is then connected to the first using a standard fiber adaptor. This forms the SA assembly which will be integrated into the cavity of the proposed laser. The synthesized Co3O4 nanosheets are characterized by x-ray diffraction (XRD) using an Empyrean PANalytical x-ray diffractometer with copper Kα radiation at an excitation wavelength of 1.5418 Å. Diffraction peaks at 31.4°, 36.9°, 38.7°, 44.9°, 55.8°, 59.5°, and 65.4° are obtained from the analysis, corresponding to the (2 2 0), (3 3 1), (2 2 2), (4 0 0), (4 2 2), (5 1 1), and (4 4 0) planes of Co3O4 respectively as seen in Fig. 1(a). All peaks are consistent with the JCPDS card 00-042-1467 [53], thereby confirming the successful synthesis of Co3O4. No other significant peaks are detected in the spectrum, implying a high-level purity in the fabricated compound. Furthermore, the narrow and highintensity diffraction peaks observed in the spectrum indicates a high degree of crystallinity in the synthesized Co3O4 compund. In addition to this, ultraviolet–visible (UV–Vis) spectroscopy analysis is also performed on the Co3O4-PEO when still in its solution form. Characterization is carried out using a Varian Cary 50 UV–Vis Spectrophotometer from Agilent Technologies over a wavelength range
wavelength region [47]; however there are no reports yet on Co3O4 being used to generate mode-locked pulses in fiber lasers operating at the longer 2.0-µm wavelength region. In this report, a mode-locked thulium doped fiber laser (TDFL) using a Co3O4 based SA is proposed and demonstrated for operation in the 2.0-µm region. Lasers operating in the 2.0-µm wavelength region are ‘eye-safe’, making them suitable for numerous industrial and medical applications [48,49]. Furthermore, the strong absorption coefficient of this wavelength region in water and gases makes them favorable for various spectroscopy and sensing applications [50–52]. The TDFL is configured with a thulium doped fiber (TDF) in a ring laser cavity as well as an integrated SA. A polarization controller (PC) is also used to optimize the laser's performance. The proposed setup has the advantage of being easy to implement and does not require the use of costly and difficult-to-handle modulators. This, combined with its operation in the eye-safe region, gives the proposed system high potential for real-world applications.
2. Fabrication and characterization of the SA The Co3O4 nanosheets used in this work are obtained by the reactions of a cobalt (II) acetate tetrahydrate [Co(CH3COO)2·4H2O] precursor, sodium hydroxide (NaOH), and polyethylene oxide (PEO). All reagents are obtained from Sigma-Aldrich while an additional reagent, ammonia (NH3·H2O) with a purity of 25%, is obtained from R&M Chemicals. All chemicals are of analytical grade and used without any further treatment. The Co3O4 nanosheets used for the fabrication of the SA thin-film are synthesized using the facile hydrothermal method. In this method, the cobalt oxide precursor with a molar concentration of 2 mM is first prepared by dissolving a sufficient amount of Co(CH3COO)2·4H2O in deionized (DI) water by ultrasonication. Approximately 13 mL of NaOH with a concentration of 3 mM is then added to the aqueous solution, after which the solution is stirred continuously for another 30 min at room temperature. The obtained solution is then mixed with 2 mL of NH3·H2O using the drop-cast method and stirred at room temperature for another 1 h. The suspended solution is then transferred into a Teflon-lined stainless-steel autoclave and heated in a hot oven at 150 °C for 16 h. After this, the mixture is left to cool at room temperature 123
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of 200 nm to 800 nm. The resulting spectrum is given in Fig. 1(b), with the linear absorption spectrum of the Co3O4-PEO solution exhibiting four absorption peaks at the 233 nm, 286, 469 nm and 748 nm. The peaks centered at 469 nm and 748 nm are attributed to the O11 → Co11 and O11 → Co111 bandgap energy transitions [54], and validate the presence of the Co3O4 nanosheets in the PEO solution. The appearance of another absorption edge at 286 nm is attributed to the semi-crystalline properties of PEO [55], and the combination of the individual Co3O4 and PEO peaks shows the successful formation of the Co3O4-PEO compound. The inset of the figure shows the same plot with a smaller scale, with marker lines indicating the absorption peaks. The morphological structure of the Co3O4 nanosheets are characterized by Field Emission Scanning Electron Microscope (FESEM) analysis. For this analysis, a Hitachi SU8220 FESEM operating at 2.0 kV is used to capture the topographic details on the sample’s surface, with the obtained images shown in Fig. 1(c). At low magnification, the sample is observed to consist of many interconnected nanosheets with irregular sizes, while at higher magnification it can be seen that the sheet-like structures have an average diameter of less than 50 nm as depicted in Fig. 1(d), confirming the nanoscale size of the fabricated particles. Additionally, the image also shows that the fabricated sample consists of thick platelets with a high surface area. For the non-linear absorption characterization of the fabricated SA, the twin detector technique is used to determine the SA’s modulation depth. For this analysis, a homemade mode-locked laser with a repetition rate of 28.169 MHz and a pulse duration of 2.33 ps is used as the seed laser for this measurement. A variable attenuator with a maximum attenuation of 60 dB is used to vary the incident intensity on the SA for the characterization process. The modulation depth of the Co3O4-PEO based SA is calculated to be around 43.7% as depicted in Fig. 2. This is comparable to the performance of other TMOs-based SAs [42,45], and thus shows that the fabricated SA in this work should be able to generate the desired mode-locked output. The saturation intensity and the non-saturable loss of the SA is determined to be 0.003 MW cm−2 and 56.3%, respectively.
Fig. 3. The schematic diagram for the mode-locked laser operation in 2.0-µm.
and 6.2 µm at 2000 nm. Both ends of the TDF are connected to the common ports of the WDMs, with the LD connected to WDM1 configured for forward pumping, while the LD connected to WDM2 operates in the backward pumping configuration. A 2000 nm isolator is connected to the 2000 nm port of WDM2 to ensure unidirectional light propagation in the cavity, and is in turn connected to 90:10 coupler which is used to extract approximately 10% port of the propagating signal for further analysis. A polarization controller (PC), connected to the output of the 90:10 coupler is used to optimize the signal propagating through the cavity. The PC is in turn connected to the SA assembly, which is used to induce mode-locking in the propagating signal and is finally connected to the 2000 nm port of WDM1, thus completing the optical circuit. The laser cavity comprises of only single-mode (SMF-28) fibers and the TDF that is used as the gain medium. Measurement of the fiber lengths gives a total cavity length of 18.3 m, consisting of the 4 m long TDF and approximately 14.4 m of SMF-28 fibers. This consists of the connecting fibers and also the various components used in the cavity. It is important to note that the measured cavity length is only that of the ring cavity of the fiber laser, and does not include any components or the SMF-28 fibers used to inject the pump signal into the ring cavity. The group velocity dispersion (GVD) of the TDF is −22.8 ps2 km−1 at 1958 nm as given by the manufacturer, while the GVD of the SMF-28 fibers are calculated to be −68.4 ps2 km−1 at the same wavelength. Based on the GVD values of the TDF and SMF-28 fibers, the net cavity dispersion, D2 is computed as LTDFGVDTDF + LSMFGVD2 SMF = − 1.0693 ps . As a result of this, the mode-locked laser is expected to operate in the anomalous dispersion regime.
3. Experimental setup The schematic of the proposed mode-locked TDFL is given in Fig. 3. The TDFL is configured in a typical ring cavity with two Princeton Lightwave 1560 nm laser diodes (LDs) used as the pump source. Both LDs have a maximum output power of 240 mW and each LD is linked to a 1550 nm optical isolator before being connected to the ring cavity. This is to protect the LDs from any damaging backscattering from the 1550 nm port of the 1550 / 2000 nm wavelength division multiplexers (WDMs). The gain medium used is a 4-meter-long TmDF200 TDF, obtained from OFS, with an absorption of 20 dB m−1 at 1550 nm and a cutoff wavelength of 1350 nm. The TDF has a numerical aperture (NA) of 0.26, while the mode-field diameter of the fiber is 5.0 µm at 1570 nm
4. Results and discussions With fine adjustments made to the PC, mode-locked pulses are obtained at a threshold pump power of 77.32 mW. Fig. 4(a) shows the optical spectrum of the output laser obtained using a Yokogawa AQ6375 optical spectrum analyzer at the threshold pump power. The center wavelength of the laser is 1958.1 nm while its 3-dB bandwidth is measured to be 3.25 nm. The presence of Kelly sidebands in the spectrum indicates that the mode-locked laser operates in the anomalous dispersion regime. Based on the spectrum, the transform-limited width of the generated pulse is calculated to be 1.24 ps using a time-bandwidth product (TBP) value of 0.315 and assuming a hyperbolic-secantsquared (sech2) pulse shape. In order to determine if the generated mode-locked pulses are transform- or non-transform-limited, the position of the Kelly sidebands with respect to the center wavelength (Δλ) is determined both experimentally and theoretically. The value of Δλ could be estimated from the equation [56,57]:
Fig. 2. Curve-fitted nonlinear absorption of the Co3O4-PEO SA. 124
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Fig. 4. (a) Soliton mode-locked fiber laser spectrum, (b) theoretically calculated mth order of Kelly sideband positions in relation to the center wavelength (Δλ) versus the pulse width (τp), (c) radio-frequency spectrum, and (d) pulse train of the mode-locked laser. All output characteristics of the fiber laser are taken at the threshold pump power.
Δλ =
2 ln(1 + 2 ) λ2 2πcτp
τp 4mπ ⎛ |D2 | ⎝ 2 ln(1 + ⎜
2
⎞ −1 2)⎠ ⎟
(1)
where τp, λ, c, and D2 represent the pulse width, center wavelength, speed of light and the net cavity dispersionrespectively. Fig. 4(b) shows the plot of Δλ for the 1st, 2nd, and the 3rd order Kelly sidebands against τp. For the transform-limited pulse width of 1.24 ps, the expected position of the 1st order Kelly’s sideband in relation to the center wavelength is 6.3 nm, while its measured value is 6.1 nm. This slight deviation from the estimated value is attributed to pulse chirping [58], however the magnitude of chirping cannot be determined in this work due to the need to carry out sophisticated measurements which are currently not available [59]. The radio-frequency (RF) spectrum of the mode-locked pulse is obtained using an Anritsu MS2683A radio frequency spectrum analyzer (RFSA) with a 12.5 GHz InGaAs photodetector at the highest resolution bandwidth of 300 Hz. The spectrum obtained is given in Fig. 4(c) and shows a fundamental frequency peak of 11.36 MHz. A high signal-to-noise ratio (SNR) of about 46 dB is measured, indicating the generated output is highly stable [15]. The pulse train generated by the mode-locked laser at a pump power of 77.32 mW is shown in Fig. 4(d), obtained using a Keysight DSOX3102T 1 Ghz oscilloscope. The average period of the pulses in the train is measured to be 88.0 ns, corresponding to a repetition rate of 11.36 MHz. The pulse train has no significant fluctuations in the amplitude, maintaining an almost uniform shape and pulse intensity and shows that the output pulses are highly stable. The repetition rate augurs well with the estimated fundamental frequency for a cavity length of 18.3 nm. Higher repetition rates can be obtained by using shorter SMF-28 lengths to reduce the cavity round trip time. To further determine the stability of the system, the output of the mode-locked TDFL is obtained for a period of 60 min at constant room temperature and without any external perturbations. The output is measured at 5-minute intervals, with the composite spectra shown in Fig. 5. No significant change can be seen in the spectrum obtained, with a consistent spectral curve centered at 1958.1 nm observed over the entire test period. The TDFL’s output shows only minor fluctuations in its peak intensity, maintaining a value of around −25.7 dBm with a fixed pump power of 77.32 mW. This shows that the mode-locked laser
Fig. 5. The stability of soliton spectrum when tested over time.
Fig. 6. Autocorrelation trace generated by the mode-locked fiber laser and its sech2 fitting curve.
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Table 1 Comparison of all-fiber mode-locked thulium doped lasers operating in the 2.0-µm wavelength region. SA
Threshold Pump Power (mW)
Center Wavelength (nm)
Repetition Rate (MHz)
Pulse Width (ps)
Maximum Pulse Energy (pJ)
Reference
Black Phosphorus WSe2 SWCNT Optically-deposited Graphene Graphene Co3O4
300.00 650.00 650.00 130.00 400.00 77.32
1910.00 1863.96 1947.00 1953.30 1900.00 1958.10
36.80 11.36 16.00 16.94 19.70 11.36
0.74 1.16 2.30 2.10 1.90 1.39
40.7 – 18.8 80.0 69.30 22.83
[28] [63] [15] [64] [65] [This work]
comparable to that obtained from the single wall carbon nanotube (SWCNT) based SA, but falls short of others. In this regard, the proposed laser setup can still be used as a seed laser source for optical amplification to achieve the necessary output required for various industrial applications [66].
is highly stable and is able to sustain long-term mode-locked operation. Pulse width measurement is performed using a pulseCheck 150 USB second harmonic generation (SHG)-based autocorrelator obtained from A.P.E. at a fixed pump power of 77.32 mW. From the sech2 curve profile given in Fig. 6, the pulse width is measured to be 1.39 ps, and when combined with the spectral-bandwidth value of 3.25 nm, a TBP value of 0.353 is obtained. This is slightly higher than the transform limit of 0.315, again indicating that the pulse is chirped and also indicates that the experimentally and theoretically measured Δλ value is not due to negligible error. At the threshold pump power of 77.32 mW, the modelocked laser has an average output power of 0.11 mW, as well as a pulse energy and a peak power of 9.43 pJ and 6.79 W respectively. At the maximum pump power of 89.65 mW, a maximum average output power of 0.26 mW is obtained, with a corresponding maximum pulse energy and a peak power of 22.83 pJ and 16.43 W, respectively. Wang et al. [57] had previously reported the use of a fiber amplifier with an anomalous dispersion to amplify a 2 µm output pulse to induce an SHG signal, but due to the fiber amplifier dispersion the amplified pulse suffers from substantial pulse width broadening. The measurement is conducted using a highly-sensitive detector without the need of an amplifier, allowing the pulse width of the output pulse to be measured without any significant broadening. The authors are confident that this work can be further expanded towards achieving shorter pulse widths using the CO3O4 based SA in a dispersion-managed cavity with a net cavity dispersion close to zero or slightly normal to generate pulses with shorter pulse widths [60]. Since most silica-based fibers have anomalous dispersions at the 2.0-µm region, a normal-dispersion fiber (NDF) could be used in order to compensate the negative dispersions of the other standard fibers, thus improving the generated output. The center wavelength of the mode-locked laser can also be tuned to be closer to the 2.0-µm region by either incorporating a tunable bandpass filter (TBPF), or a tunable Mach-Zehnder filter (TMZF). Between these two, the latter provides an added advantage by maintaining the soliton spectrum of the mode-locked laser in its original form [61]. The use of a TBPF however would suppress the soliton modelocking due the limited bandwidth of the TBPF and hence cause the pulse width to broaden [62]. The performance of the proposed Co3O4-based SA in this work is comparable to that of other SAs used to generate mode-locked pulses in similar fiber laser configurations. The comparison of the performance of these SAs is given in Table 1. It can be seen from Table 1 that the threshold pump power for this work is significantly lower than that of most other works, which typically exceeds 300 mW. This is more than triple the threshold pump power in this work, except for the case of the optically-deposited graphene. This low threshold power is attributed to both the loss insertion loss of the SA as well as the fiber cavity dispersion and the cavity length [64]. The center wavelength of the mode-locked laser in this setup also is the longest among those studied, at 1958.10 nm. The pulse width of 1.39 ps achieved in this work is comparable to those in other works, ranging from 1.16 ps to 2.30 ps except when compared to black phosphorus which has a shorter pulse width of 0.74 ps. This however can be addressed by managing the net cavity dispersion to be slightly normal. The maximum pulse energy generated using the Co3O4 is also
5. Conclusions In this work, a mode-locked TDFL with a Co3O4 based SA is demonstrated for operation in the 2.0-µm region. The Co3O4 based SA is fabricated using Co3O4 nanosheets suspended in a polymer host and sandwiched between two fiber ferrules. The mode-locked laser output has a center wavelength at 1958.1 nm and a 3-dB bandwidth of 3.25 nm, as well as a repetition rate of 11.36 MHz, with a pulse width of 1.39 ps and SNR of 46.0 dB. The laser output is stable when tested over a period of 60 min, with no observable shift in the center wavelength of laser spectrum. The mode-locked laser would have multiple applications for operation at the ‘eye-safe’ region in 2.0-µm. Acknowledgements Funding for this research was provided by the Ministry of Higher Education, Malaysia under the grant LRGS (2015)/NGOD/UM/KPT as well as the University of Malaya under the grant RU 001–20017. References [1] J. Kim, Y. Song, Ultralow-noise mode-locked fiber lasers and frequency combs: principles, status, and applications, Adv. Opt. Photonics 8 (2016) 465–540. [2] S. Nikumb, Q. Chen, C. Li, H. Reshef, H.Y. Zheng, H. Qiu, D. Low, Precision glass machining, drilling and profile cutting by short pulse lasers, Thin Solid Films 477 (2005) 216–221. [3] M. Skorczakowski, J. Swiderski, W. Pichola, P. Nyga, A. Zajac, M. Maciejewska, L. Galecki, J. Kasprzak, S. Gross, A. Heinrichn, T. Bragagna, Mid-infrared Q-switched Er:YAG laser for medical applications, Laser Phys. Lett. 7 (2010) 498–504. [4] J. Kang, S. Tan, A.H. Tang, K.K. Tsia, K.K. Wong, et al., Ultrawide C-and L-band mode-locked erbium-doped fiber ring laser and its application in ultrafast microscopy, CLEO Sci. Innov. (2016) pp. SM4P–2. [5] C.M. Eigenwillig, B.R. Biedermann, G. Palte, R. Huber, K-space linear Fourier domain mode locked laser and applications for optical coherence tomography, Opt. Express 16 (2008) 8916–8937. [6] O. Svelto, Principles of lasers, 2010. [7] P. Hübner, C. Kieleck, S.D. Jackson, M. Eichhorn, High-power actively mode-locked sub-nanosecond Tm3+-doped silica fiber laser, Opt. Lett. 36 (2011) 2483–2485. [8] K. Yin, B. Zhang, W. Yang, H. Chen, S. Chen, J. Hou, Flexible picosecond thuliumdoped fiber laser using the active mode-locking technique, Opt. Lett. 39 (2014) 4259–4262. [9] Y. Zhou, A. Wang, C. Gu, B. Sun, L. Xu, F. Li, D. Chung, Q. Zhan, Actively modelocked all fiber laser with cylindrical vector beam output, Opt. Lett. 41 (2016) 548–550. [10] C. Kneis, B. Donelan, A. Berrou, I. Manek-Hönninger, T. Robin, B. Cadier, M. Eichhorn, C. Kieleck, Actively mode-locked Tm3+-doped silica fiber laser with wavelength-tunable, high average output power, Opt. Lett. 40 (2015) 1464–1467. [11] J. Lee, J.H. Lee, Experimental investigation of the cavity modulation frequency detuning effect in an active harmonically mode-locked fiber laser, JOSA B. 30 (2013) 1479–1485. [12] Z.-C. Luo, A.-P. Luo, W.-C. Xu, Tunable and switchable multiwavelength passively mode-locked fiber laser based on SESAM and inline birefringence comb filter, IEEE Photonics J. 3 (2011) 64–70. [13] R.C. Sharp, D.E. Spock, N. Pan, J. Elliot, 190-fs passively mode-locked thulium fiber laser with a low threshold, Opt. Lett. 21 (1996) 881–883. [14] J. Wang, X. Liang, G. Hu, Z. Zheng, S. Lin, D. Ouyang, X. Wu, P. Yan, S. Ruan,
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Regular Articles
Soliton mode-locked thulium-doped fiber laser with cobalt oxide saturable absorber
T
⁎
H. Ahmada,b, , M.Z. Samiona, N. Yusoffa a b
Photonics Research Centre, University of Malaya, 50603 Kuala Lumpur, Malaysia Visiting Professor at the Department of Physics, Faculty of Science and Technology, Airlangga University, Surabaya 60115, Indonesia
A R T I C LE I N FO
A B S T R A C T
Keywords: Mode-locking Thulium Cobalt oxide Pulse fiber laser
In this work, we propose and demonstrate a mode-locked thulium doped fiber laser (TDFL) with an integrated cobalt oxide (Co3O4) saturable absorber (SA). The SA is fabricated by suspending Co3O4 nanosheets in a polymer film before being secured between two fiber ferules. The laser operates in the anomalous dispersion regime, confirmed by the Kelly sidebands observed in the obtained optical spectrum. Mode-locking operation is obtained at a low threshold pump power of 77.32 mW, with further optimizations made using a polarization controller The laser generates pulses with a repetition rate of 11.36 MHz and a pulse width of 1.39 ps. The generated pulses are highly stable, with a high signal-to-noise ratio of 46.00 dB and minimum power fluctuations indicating a long-term stability. This work demonstrates a simple and a low-cost laser that would have potential applications for operation near the 2.0-µm region, especially for medical applications.
1. Introduction Passively pulsed fiber lasers are highly desired as laser sources due to their potential use in a variety of applications ranging from material processing to remote sensing, microscopy and medicine. The attractiveness of these sources arises from their robust yet compact form factor, which in turn results low fabrication and operating costs [1–5]. To achieve this, fiber lasers are be designed to be either Q-switched or mode-locked, with the latter generating shorter, ultrafast pulses with overall low output energies and the former generating longer and slower pulses but with higher energies [6]. Traditionally, Q-switching and mode-locking in fiber lasers was achieved by actively modulating the losses within the laser cavity active using acousto-optic or electro-optic modulators [7–11]. However, the bulky nature of these modulators, as well as their relatively high cost quickly made this option unsuitable for most real-world applications. As such, research efforts were focused towards the development of passively pulsed devices, which generated a similar output but in a much more compact form factor and at a lower overall cost. This was typically achieved primarily by saturable absorbers (SAs), such as semiconductor saturable absorber mirrors (SESAMs) [12,13] and more recently 2-dimensional (2D) and 3-dimensional (3D) materials such as carbon nanotubes [14–16] and graphene [17–19]. Additionally, recent developments have now seen new 2D and 3D nanomaterials, such as
⁎
metals [20,21], topological insulators (TIs) [22–24], transition metal dichalcogenides (TMDs) [25–27] and even exotic materials such as black phosphorus [28–30] being used to obtain passive mode-locked outputs. The fabrication and the nonlinear optical properties of these low-dimensional materials have been widely investigated and reported [31–33], with particular attention being paid to 2D nanomaterials for advanced opto-electronic applications [34–36] due to their unique properties such as low saturation intensity, broadband absorption properties as well as fast recovery and response times [37–39]. Of late, transition metal oxides (TMOs) have now become the focus of significant research interest for use as SAs due to their unique optical charactersitics, namely their large nonlinear optical response [40]. As a result of their 2D nature, the bandgaps of TMOs can be tuned without altering their physical properties, giving them a wide-operational bandwidth [41]. TMOs such as nickel oxide (NiO), zinc oxide (ZnO), iron (II, III) oxide (Fe3O4), and titanium oxide (TiO2) have all been successfully demonstrated as SAs for the generation of pulses fiber lasers [42–45]. The performance of these lasers is comparable to the ones built using graphene or other 2D material based SAs, thus indicating the potential of TMOs as SAs. In this regard, cobalt oxide (Co3O4) is an attractive material for use as an SA as it has a large nonlinear to linear absorption ratio [40], as well as spectral absorption that stretches to the near-infrared (NIR) region [46]. Recently, the use of Co3O4 as an SA has been reported with the generation of Q-switched pulses at the 1.5-µm
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.yofte.2018.07.012 Received 6 April 2018; Received in revised form 22 May 2018; Accepted 2 July 2018 1068-5200/ © 2018 Elsevier Inc. All rights reserved.
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Fig. 1. (a) XRD pattern of Co3O4 nanosheets , (b) UV–vis spectra of the Co3O4-PEO in solution form with a close-up view of the absorption peaks (inset), and FESEM images of the Co3O4 nanosheets taken at (c) low magnification and (d) at high magnification.
before being centrifuged at 4000 rpm for 20 min to obtain the Co3O4 nanosheets structure, which appear as a black powder precipitate. The Co3O4 nanosheet precipitate is then washed extensively with DI water and absolute ethanol. This is repeated three times before the final precipitate is dried in an oven at 60 °C for 24 h, giving the desired form of a fine black powder. While the nanosheets are the optically active material of the SA, their fine and brittle nature does not allow them to be used in their current form within the fiber cavity. As such, the Co3O4 nanosheets are instead embedded in a Polyethylene-Oxide (PEO) thin film which serves as a host material. The thin film is formed by dissolving 250 mg of PEO powder in 30 mL of DI water, and stirred continuously at 50 °C for 2 h. Approximately 10 mL of the Co3O4 black powder with a density of 5 mg mL−1 is then added into the PEO solution, and the entire mixture stirred for another 2 h. The resulting compound is then transferred onto a petri dish and left to dry in an oven at 60 °C for 24 h, forming a polymer film with Co3O4 nanosheets embedded in the host material. A small portion of the thin film formed is cut out and placed on the face of a fiber ferrule so that it covers the core region. A small amount of indexmatching gel is used to hold the polymer piece in place, and another fiber ferrule is then connected to the first using a standard fiber adaptor. This forms the SA assembly which will be integrated into the cavity of the proposed laser. The synthesized Co3O4 nanosheets are characterized by x-ray diffraction (XRD) using an Empyrean PANalytical x-ray diffractometer with copper Kα radiation at an excitation wavelength of 1.5418 Å. Diffraction peaks at 31.4°, 36.9°, 38.7°, 44.9°, 55.8°, 59.5°, and 65.4° are obtained from the analysis, corresponding to the (2 2 0), (3 3 1), (2 2 2), (4 0 0), (4 2 2), (5 1 1), and (4 4 0) planes of Co3O4 respectively as seen in Fig. 1(a). All peaks are consistent with the JCPDS card 00-042-1467 [53], thereby confirming the successful synthesis of Co3O4. No other significant peaks are detected in the spectrum, implying a high-level purity in the fabricated compound. Furthermore, the narrow and highintensity diffraction peaks observed in the spectrum indicates a high degree of crystallinity in the synthesized Co3O4 compund. In addition to this, ultraviolet–visible (UV–Vis) spectroscopy analysis is also performed on the Co3O4-PEO when still in its solution form. Characterization is carried out using a Varian Cary 50 UV–Vis Spectrophotometer from Agilent Technologies over a wavelength range
wavelength region [47]; however there are no reports yet on Co3O4 being used to generate mode-locked pulses in fiber lasers operating at the longer 2.0-µm wavelength region. In this report, a mode-locked thulium doped fiber laser (TDFL) using a Co3O4 based SA is proposed and demonstrated for operation in the 2.0-µm region. Lasers operating in the 2.0-µm wavelength region are ‘eye-safe’, making them suitable for numerous industrial and medical applications [48,49]. Furthermore, the strong absorption coefficient of this wavelength region in water and gases makes them favorable for various spectroscopy and sensing applications [50–52]. The TDFL is configured with a thulium doped fiber (TDF) in a ring laser cavity as well as an integrated SA. A polarization controller (PC) is also used to optimize the laser's performance. The proposed setup has the advantage of being easy to implement and does not require the use of costly and difficult-to-handle modulators. This, combined with its operation in the eye-safe region, gives the proposed system high potential for real-world applications.
2. Fabrication and characterization of the SA The Co3O4 nanosheets used in this work are obtained by the reactions of a cobalt (II) acetate tetrahydrate [Co(CH3COO)2·4H2O] precursor, sodium hydroxide (NaOH), and polyethylene oxide (PEO). All reagents are obtained from Sigma-Aldrich while an additional reagent, ammonia (NH3·H2O) with a purity of 25%, is obtained from R&M Chemicals. All chemicals are of analytical grade and used without any further treatment. The Co3O4 nanosheets used for the fabrication of the SA thin-film are synthesized using the facile hydrothermal method. In this method, the cobalt oxide precursor with a molar concentration of 2 mM is first prepared by dissolving a sufficient amount of Co(CH3COO)2·4H2O in deionized (DI) water by ultrasonication. Approximately 13 mL of NaOH with a concentration of 3 mM is then added to the aqueous solution, after which the solution is stirred continuously for another 30 min at room temperature. The obtained solution is then mixed with 2 mL of NH3·H2O using the drop-cast method and stirred at room temperature for another 1 h. The suspended solution is then transferred into a Teflon-lined stainless-steel autoclave and heated in a hot oven at 150 °C for 16 h. After this, the mixture is left to cool at room temperature 123
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of 200 nm to 800 nm. The resulting spectrum is given in Fig. 1(b), with the linear absorption spectrum of the Co3O4-PEO solution exhibiting four absorption peaks at the 233 nm, 286, 469 nm and 748 nm. The peaks centered at 469 nm and 748 nm are attributed to the O11 → Co11 and O11 → Co111 bandgap energy transitions [54], and validate the presence of the Co3O4 nanosheets in the PEO solution. The appearance of another absorption edge at 286 nm is attributed to the semi-crystalline properties of PEO [55], and the combination of the individual Co3O4 and PEO peaks shows the successful formation of the Co3O4-PEO compound. The inset of the figure shows the same plot with a smaller scale, with marker lines indicating the absorption peaks. The morphological structure of the Co3O4 nanosheets are characterized by Field Emission Scanning Electron Microscope (FESEM) analysis. For this analysis, a Hitachi SU8220 FESEM operating at 2.0 kV is used to capture the topographic details on the sample’s surface, with the obtained images shown in Fig. 1(c). At low magnification, the sample is observed to consist of many interconnected nanosheets with irregular sizes, while at higher magnification it can be seen that the sheet-like structures have an average diameter of less than 50 nm as depicted in Fig. 1(d), confirming the nanoscale size of the fabricated particles. Additionally, the image also shows that the fabricated sample consists of thick platelets with a high surface area. For the non-linear absorption characterization of the fabricated SA, the twin detector technique is used to determine the SA’s modulation depth. For this analysis, a homemade mode-locked laser with a repetition rate of 28.169 MHz and a pulse duration of 2.33 ps is used as the seed laser for this measurement. A variable attenuator with a maximum attenuation of 60 dB is used to vary the incident intensity on the SA for the characterization process. The modulation depth of the Co3O4-PEO based SA is calculated to be around 43.7% as depicted in Fig. 2. This is comparable to the performance of other TMOs-based SAs [42,45], and thus shows that the fabricated SA in this work should be able to generate the desired mode-locked output. The saturation intensity and the non-saturable loss of the SA is determined to be 0.003 MW cm−2 and 56.3%, respectively.
Fig. 3. The schematic diagram for the mode-locked laser operation in 2.0-µm.
and 6.2 µm at 2000 nm. Both ends of the TDF are connected to the common ports of the WDMs, with the LD connected to WDM1 configured for forward pumping, while the LD connected to WDM2 operates in the backward pumping configuration. A 2000 nm isolator is connected to the 2000 nm port of WDM2 to ensure unidirectional light propagation in the cavity, and is in turn connected to 90:10 coupler which is used to extract approximately 10% port of the propagating signal for further analysis. A polarization controller (PC), connected to the output of the 90:10 coupler is used to optimize the signal propagating through the cavity. The PC is in turn connected to the SA assembly, which is used to induce mode-locking in the propagating signal and is finally connected to the 2000 nm port of WDM1, thus completing the optical circuit. The laser cavity comprises of only single-mode (SMF-28) fibers and the TDF that is used as the gain medium. Measurement of the fiber lengths gives a total cavity length of 18.3 m, consisting of the 4 m long TDF and approximately 14.4 m of SMF-28 fibers. This consists of the connecting fibers and also the various components used in the cavity. It is important to note that the measured cavity length is only that of the ring cavity of the fiber laser, and does not include any components or the SMF-28 fibers used to inject the pump signal into the ring cavity. The group velocity dispersion (GVD) of the TDF is −22.8 ps2 km−1 at 1958 nm as given by the manufacturer, while the GVD of the SMF-28 fibers are calculated to be −68.4 ps2 km−1 at the same wavelength. Based on the GVD values of the TDF and SMF-28 fibers, the net cavity dispersion, D2 is computed as LTDFGVDTDF + LSMFGVD2 SMF = − 1.0693 ps . As a result of this, the mode-locked laser is expected to operate in the anomalous dispersion regime.
3. Experimental setup The schematic of the proposed mode-locked TDFL is given in Fig. 3. The TDFL is configured in a typical ring cavity with two Princeton Lightwave 1560 nm laser diodes (LDs) used as the pump source. Both LDs have a maximum output power of 240 mW and each LD is linked to a 1550 nm optical isolator before being connected to the ring cavity. This is to protect the LDs from any damaging backscattering from the 1550 nm port of the 1550 / 2000 nm wavelength division multiplexers (WDMs). The gain medium used is a 4-meter-long TmDF200 TDF, obtained from OFS, with an absorption of 20 dB m−1 at 1550 nm and a cutoff wavelength of 1350 nm. The TDF has a numerical aperture (NA) of 0.26, while the mode-field diameter of the fiber is 5.0 µm at 1570 nm
4. Results and discussions With fine adjustments made to the PC, mode-locked pulses are obtained at a threshold pump power of 77.32 mW. Fig. 4(a) shows the optical spectrum of the output laser obtained using a Yokogawa AQ6375 optical spectrum analyzer at the threshold pump power. The center wavelength of the laser is 1958.1 nm while its 3-dB bandwidth is measured to be 3.25 nm. The presence of Kelly sidebands in the spectrum indicates that the mode-locked laser operates in the anomalous dispersion regime. Based on the spectrum, the transform-limited width of the generated pulse is calculated to be 1.24 ps using a time-bandwidth product (TBP) value of 0.315 and assuming a hyperbolic-secantsquared (sech2) pulse shape. In order to determine if the generated mode-locked pulses are transform- or non-transform-limited, the position of the Kelly sidebands with respect to the center wavelength (Δλ) is determined both experimentally and theoretically. The value of Δλ could be estimated from the equation [56,57]:
Fig. 2. Curve-fitted nonlinear absorption of the Co3O4-PEO SA. 124
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Fig. 4. (a) Soliton mode-locked fiber laser spectrum, (b) theoretically calculated mth order of Kelly sideband positions in relation to the center wavelength (Δλ) versus the pulse width (τp), (c) radio-frequency spectrum, and (d) pulse train of the mode-locked laser. All output characteristics of the fiber laser are taken at the threshold pump power.
Δλ =
2 ln(1 + 2 ) λ2 2πcτp
τp 4mπ ⎛ |D2 | ⎝ 2 ln(1 + ⎜
2
⎞ −1 2)⎠ ⎟
(1)
where τp, λ, c, and D2 represent the pulse width, center wavelength, speed of light and the net cavity dispersionrespectively. Fig. 4(b) shows the plot of Δλ for the 1st, 2nd, and the 3rd order Kelly sidebands against τp. For the transform-limited pulse width of 1.24 ps, the expected position of the 1st order Kelly’s sideband in relation to the center wavelength is 6.3 nm, while its measured value is 6.1 nm. This slight deviation from the estimated value is attributed to pulse chirping [58], however the magnitude of chirping cannot be determined in this work due to the need to carry out sophisticated measurements which are currently not available [59]. The radio-frequency (RF) spectrum of the mode-locked pulse is obtained using an Anritsu MS2683A radio frequency spectrum analyzer (RFSA) with a 12.5 GHz InGaAs photodetector at the highest resolution bandwidth of 300 Hz. The spectrum obtained is given in Fig. 4(c) and shows a fundamental frequency peak of 11.36 MHz. A high signal-to-noise ratio (SNR) of about 46 dB is measured, indicating the generated output is highly stable [15]. The pulse train generated by the mode-locked laser at a pump power of 77.32 mW is shown in Fig. 4(d), obtained using a Keysight DSOX3102T 1 Ghz oscilloscope. The average period of the pulses in the train is measured to be 88.0 ns, corresponding to a repetition rate of 11.36 MHz. The pulse train has no significant fluctuations in the amplitude, maintaining an almost uniform shape and pulse intensity and shows that the output pulses are highly stable. The repetition rate augurs well with the estimated fundamental frequency for a cavity length of 18.3 nm. Higher repetition rates can be obtained by using shorter SMF-28 lengths to reduce the cavity round trip time. To further determine the stability of the system, the output of the mode-locked TDFL is obtained for a period of 60 min at constant room temperature and without any external perturbations. The output is measured at 5-minute intervals, with the composite spectra shown in Fig. 5. No significant change can be seen in the spectrum obtained, with a consistent spectral curve centered at 1958.1 nm observed over the entire test period. The TDFL’s output shows only minor fluctuations in its peak intensity, maintaining a value of around −25.7 dBm with a fixed pump power of 77.32 mW. This shows that the mode-locked laser
Fig. 5. The stability of soliton spectrum when tested over time.
Fig. 6. Autocorrelation trace generated by the mode-locked fiber laser and its sech2 fitting curve.
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Table 1 Comparison of all-fiber mode-locked thulium doped lasers operating in the 2.0-µm wavelength region. SA
Threshold Pump Power (mW)
Center Wavelength (nm)
Repetition Rate (MHz)
Pulse Width (ps)
Maximum Pulse Energy (pJ)
Reference
Black Phosphorus WSe2 SWCNT Optically-deposited Graphene Graphene Co3O4
300.00 650.00 650.00 130.00 400.00 77.32
1910.00 1863.96 1947.00 1953.30 1900.00 1958.10
36.80 11.36 16.00 16.94 19.70 11.36
0.74 1.16 2.30 2.10 1.90 1.39
40.7 – 18.8 80.0 69.30 22.83
[28] [63] [15] [64] [65] [This work]
comparable to that obtained from the single wall carbon nanotube (SWCNT) based SA, but falls short of others. In this regard, the proposed laser setup can still be used as a seed laser source for optical amplification to achieve the necessary output required for various industrial applications [66].
is highly stable and is able to sustain long-term mode-locked operation. Pulse width measurement is performed using a pulseCheck 150 USB second harmonic generation (SHG)-based autocorrelator obtained from A.P.E. at a fixed pump power of 77.32 mW. From the sech2 curve profile given in Fig. 6, the pulse width is measured to be 1.39 ps, and when combined with the spectral-bandwidth value of 3.25 nm, a TBP value of 0.353 is obtained. This is slightly higher than the transform limit of 0.315, again indicating that the pulse is chirped and also indicates that the experimentally and theoretically measured Δλ value is not due to negligible error. At the threshold pump power of 77.32 mW, the modelocked laser has an average output power of 0.11 mW, as well as a pulse energy and a peak power of 9.43 pJ and 6.79 W respectively. At the maximum pump power of 89.65 mW, a maximum average output power of 0.26 mW is obtained, with a corresponding maximum pulse energy and a peak power of 22.83 pJ and 16.43 W, respectively. Wang et al. [57] had previously reported the use of a fiber amplifier with an anomalous dispersion to amplify a 2 µm output pulse to induce an SHG signal, but due to the fiber amplifier dispersion the amplified pulse suffers from substantial pulse width broadening. The measurement is conducted using a highly-sensitive detector without the need of an amplifier, allowing the pulse width of the output pulse to be measured without any significant broadening. The authors are confident that this work can be further expanded towards achieving shorter pulse widths using the CO3O4 based SA in a dispersion-managed cavity with a net cavity dispersion close to zero or slightly normal to generate pulses with shorter pulse widths [60]. Since most silica-based fibers have anomalous dispersions at the 2.0-µm region, a normal-dispersion fiber (NDF) could be used in order to compensate the negative dispersions of the other standard fibers, thus improving the generated output. The center wavelength of the mode-locked laser can also be tuned to be closer to the 2.0-µm region by either incorporating a tunable bandpass filter (TBPF), or a tunable Mach-Zehnder filter (TMZF). Between these two, the latter provides an added advantage by maintaining the soliton spectrum of the mode-locked laser in its original form [61]. The use of a TBPF however would suppress the soliton modelocking due the limited bandwidth of the TBPF and hence cause the pulse width to broaden [62]. The performance of the proposed Co3O4-based SA in this work is comparable to that of other SAs used to generate mode-locked pulses in similar fiber laser configurations. The comparison of the performance of these SAs is given in Table 1. It can be seen from Table 1 that the threshold pump power for this work is significantly lower than that of most other works, which typically exceeds 300 mW. This is more than triple the threshold pump power in this work, except for the case of the optically-deposited graphene. This low threshold power is attributed to both the loss insertion loss of the SA as well as the fiber cavity dispersion and the cavity length [64]. The center wavelength of the mode-locked laser in this setup also is the longest among those studied, at 1958.10 nm. The pulse width of 1.39 ps achieved in this work is comparable to those in other works, ranging from 1.16 ps to 2.30 ps except when compared to black phosphorus which has a shorter pulse width of 0.74 ps. This however can be addressed by managing the net cavity dispersion to be slightly normal. The maximum pulse energy generated using the Co3O4 is also
5. Conclusions In this work, a mode-locked TDFL with a Co3O4 based SA is demonstrated for operation in the 2.0-µm region. The Co3O4 based SA is fabricated using Co3O4 nanosheets suspended in a polymer host and sandwiched between two fiber ferrules. The mode-locked laser output has a center wavelength at 1958.1 nm and a 3-dB bandwidth of 3.25 nm, as well as a repetition rate of 11.36 MHz, with a pulse width of 1.39 ps and SNR of 46.0 dB. The laser output is stable when tested over a period of 60 min, with no observable shift in the center wavelength of laser spectrum. The mode-locked laser would have multiple applications for operation at the ‘eye-safe’ region in 2.0-µm. Acknowledgements Funding for this research was provided by the Ministry of Higher Education, Malaysia under the grant LRGS (2015)/NGOD/UM/KPT as well as the University of Malaya under the grant RU 001–20017. References [1] J. Kim, Y. Song, Ultralow-noise mode-locked fiber lasers and frequency combs: principles, status, and applications, Adv. Opt. Photonics 8 (2016) 465–540. [2] S. Nikumb, Q. Chen, C. Li, H. Reshef, H.Y. Zheng, H. Qiu, D. Low, Precision glass machining, drilling and profile cutting by short pulse lasers, Thin Solid Films 477 (2005) 216–221. [3] M. Skorczakowski, J. Swiderski, W. Pichola, P. Nyga, A. Zajac, M. Maciejewska, L. Galecki, J. Kasprzak, S. Gross, A. Heinrichn, T. Bragagna, Mid-infrared Q-switched Er:YAG laser for medical applications, Laser Phys. Lett. 7 (2010) 498–504. [4] J. Kang, S. Tan, A.H. Tang, K.K. Tsia, K.K. Wong, et al., Ultrawide C-and L-band mode-locked erbium-doped fiber ring laser and its application in ultrafast microscopy, CLEO Sci. Innov. (2016) pp. SM4P–2. [5] C.M. Eigenwillig, B.R. Biedermann, G. Palte, R. Huber, K-space linear Fourier domain mode locked laser and applications for optical coherence tomography, Opt. Express 16 (2008) 8916–8937. [6] O. Svelto, Principles of lasers, 2010. [7] P. Hübner, C. Kieleck, S.D. Jackson, M. Eichhorn, High-power actively mode-locked sub-nanosecond Tm3+-doped silica fiber laser, Opt. Lett. 36 (2011) 2483–2485. [8] K. Yin, B. Zhang, W. Yang, H. Chen, S. Chen, J. Hou, Flexible picosecond thuliumdoped fiber laser using the active mode-locking technique, Opt. Lett. 39 (2014) 4259–4262. [9] Y. Zhou, A. Wang, C. Gu, B. Sun, L. Xu, F. Li, D. Chung, Q. Zhan, Actively modelocked all fiber laser with cylindrical vector beam output, Opt. Lett. 41 (2016) 548–550. [10] C. Kneis, B. Donelan, A. Berrou, I. Manek-Hönninger, T. Robin, B. Cadier, M. Eichhorn, C. Kieleck, Actively mode-locked Tm3+-doped silica fiber laser with wavelength-tunable, high average output power, Opt. Lett. 40 (2015) 1464–1467. [11] J. Lee, J.H. Lee, Experimental investigation of the cavity modulation frequency detuning effect in an active harmonically mode-locked fiber laser, JOSA B. 30 (2013) 1479–1485. [12] Z.-C. Luo, A.-P. Luo, W.-C. Xu, Tunable and switchable multiwavelength passively mode-locked fiber laser based on SESAM and inline birefringence comb filter, IEEE Photonics J. 3 (2011) 64–70. [13] R.C. Sharp, D.E. Spock, N. Pan, J. Elliot, 190-fs passively mode-locked thulium fiber laser with a low threshold, Opt. Lett. 21 (1996) 881–883. [14] J. Wang, X. Liang, G. Hu, Z. Zheng, S. Lin, D. Ouyang, X. Wu, P. Yan, S. Ruan,
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