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We experimentally demonstrate the use of a bulk-like, MoSe2-based saturable absorber (SA) as a passive harmonic mode-locker for the production of femtosecond pulses from a fiber laser at a repetition rate of 3.27 GHz. By incorporating a bulk-like, MoSe2/PVA-composite-deposited side-polished fiber as an SA within an erbium-doped-fiber-ring cavity, mode-locked pulses with a temporal width of 737 fs to 798 fs can be readily obtained at various harmonic frequencies. The fundamental resonance frequency and the maximum harmonic-resonance frequency are 15.38 MHz and 3.27 GHz (212th harmonic), respectively. The temporal and spectral characteristics of the output pulses are systematically investigated as a function of the pump power. The output pulses exhibited Gaussian-temporal shapes irrespective of the harmonic order, and even when their spectra possessed hyperbolic-secant shapes. The saturable absorption and harmonic-mode-locking performance of our prepared SA are compared with those of previously demonstrated SAs that are based on other transition metal dichalcogenides (TMDs). To the best of the authors’ knowledge, the repetition rate of 3.27 GHz is the highest frequency that has ever been demonstrated regarding the production of femtosecond pulses from a fiber laser that is based on SA-induced passive harmonic mode-locking.
© 2016 Optical Society of America
1. Introduction
Ultra-fast lasers that operate at GHz-repetition rates are useful in the fields of astronomical frequency-comb generation [1] and optical communication [2]. A commonly used method for the realization of high-speed pulsed lasers that can operate at GHz-repetition rates is active harmonic mode-locking. The incorporation of an external electrical-signal-driven modulator into a cavity using harmonically mode-locked pulses is known to be readily achievable. One concern regarding active harmonic mode-locking, however, is the limited temporal response of the electro-optic modulator that complicates the attainment of femtosecond pulses, even though pulse repetition rates of more than 10 GHz have been readily realized with this technique [3–5]. Another method is passive harmonic mode-locking, for which a passive optical device with a nonlinear optical transmission is employed [6–10]. The passive mode-locking technique has been commonly used to generate femtosecond pulses for a variety of scientific and industrial applications such as optical imaging, optical communication, optical sensing, and laser processing [11–13]. Passive mode-locking is usually implemented with the use of a saturable absorber (SA), which is a passive device with an optical absorption that varies depending on the incident-beam intensity [14, 15]. Even if ultra-short pulses with a sub-picosecond temporal width can readily be realized with this technique, one critical issue has been the difficulty regarding the increase of the pulse repetition rate [16].
Two common approaches have been used for the generation of GHz-level, repetition-rate short pulses, as follows: One is the shortening of the cavity length for a fundamental passive mode-locking [17, 18], and the other is the use of a passive harmonic-mode-locking technique [19–33]. The cavity-shortening-based approach is excellent for the realization of high-speed pulsed lasers; however, the need for a few centimeters of active-medium length with a high optical gain has made this approach more useful for waveguide-type lasers [34–36]. Alternatively, the harmonic-mode-locking approach is advantageous because of the absence of any limitation regarding the active-medium length; in particular, this approach is useful for fiber lasers since most fiber lasers are implemented with a substantial cavity length. To induce passive harmonic mode-locking within a cavity, both saturable absorption and Kerr nonlinearity are commonly required [20, 27]. It has been reported that microfiber-based or side-polished, fiber-based SAs with nonlinear absorption materials deposited can readily produce harmonically mode-locked pulses within a fiberized cavity [20, 25–33].
For decades, compound semiconductors have served as the base materials for the implementation of SAs; however, various limitations such as a narrow operating bandwidth and the requirement of complicated fabrication facilities remain problematic [14]. Replacements for the compound-semiconductor-based SAs have been intensively investigated over the past decade. In 2004, Set et al. reported that carbon nanotubes (CNTs) could be a strong replacement candidate due to a semiconducting-energy-band structure, ultra-fast recovery times, and broadband-operation properties [37]. Broadband saturable-absorption properties were also found in graphene, which is a two-dimensional (2D) hexagonal structure composed of carbon atoms [38, 39]. Recently, topological insulators (TIs) [40–44], gold nanoparticles [45–48], black phosphorous [49–52], and TMDs [53–61] have also been identified as efficient saturable-absorption materials.
Among the previously mentioned saturable-absorption materials, TMDs are technically interesting 2D materials. Despite a large bandgap, they can provide a wideband saturable absorption [53] due to sub-bandgap absorption, which is caused by defects and edge states [53–61]. TMDs are thin semiconductors that comprise a layer of transition-metal atoms that is sandwiched between two layers of chalcogen atoms. Since the atomic layers are weakly bonded together by van der Waals force, single- or few-layer nanosheets can easily be exfoliated from the layer structure. The typical TMDs are MoS2, WS2, MoSe2, WSe2, and WTe2. The nonlinear saturable-absorption properties of MoS2 and WS2, in particular, have been intensively investigated during recent years [53–61], and so far, the use of MoS2- or WS2-based SAs for the Q-switching or mode-locking of lasers at various operating wavelengths has been extensively demonstrated [57–61]; however, relatively less attention has been paid to the nonlinear saturable-absorption properties of MoSe2, WSe2, and WTe2. Very recently, two investigations were conducted regarding the potential capacity of MoSe2 as a base saturable-absorption material for pulsed lasers; for example, Q-switched lasers that are based on MoSe2-based materials have been demonstrated by Chen et al. and Woodward et al., respectively [62, 63]. Z. Luo et al. reported a mode-locked laser for which a MoSe2-based SA is incorporated [64].
In this work, the use of a bulk-like MoSe2-based SA for the passive harmonic mode-locking of a fiber laser is experimentally demonstrated. Our SA was prepared by depositing a polymer-composite film that comprises a mixture of bulk-like multi-layered MoSe2 and polyvinyl alcohol (PVA) onto the flat side of side-polished fiber. The particles of the MoSe2, which was prepared from bulk MoSe2 crystal using the liquid phase exfoliation method without centrifugation for the ease of the fabrication process, were characterized with atomic force microscopy (AFM), Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS). Unlike the previous works on few-layered MoSe2-based SAs, whereby a fiber, ferrule-based, sandwich-type platform was employed, a side-polished-fiber platform was used together with the bulk-like MoSe2 particles due to its long interaction. Mode-locked pulses with a temporal width of 737 fs to 798 fs were readily obtained from an erbium-doped-fiber-ring cavity at various harmonic frequencies. The fundamental resonance frequency and the maximum harmonic-resonance frequency are 15.38 MHz and 3.27 GHz (212th harmonic), respectively. The characteristics of the output pulses were investigated as a function of the pump power; interestingly, it was found that the output pulses possessed Gaussian-temporal shapes irrespective of the harmonic order, and even if hyperbolic-secant shapes are more compatible with their spectra. The saturable absorption and harmonic mode-locking performance of our prepared SA was compared to those of the SAs that are based on MoS2 and WS2, and our laser operates at the highest harmonic frequency. Notably, and to the best of the authors’ knowledge, there are no reports of a femtosecond fiber laser that is based on SA-induced passive harmonic mode-locking and can operate at repetition rates higher than 3 GHz in the existing literature. Even if it was shown that a CNT-based SA could be used for the 5 GHz passive harmonic mode-locking of a fiber laser, the generation of the femtosecond pulses was still limited to a 2 GHz repetition rate [20]. Furthermore, the recent demonstration of a harmonically mode-locked fiber laser at 5.88 GHz using a graphene-based SA exhibited a pulse width limitation of a few picosecond level [24].
2. Preparation and characteristic of the MoSe2/PVA SA
MoSe2 particles were prepared by a bath-type sonication of the MoSe2 bulk crystal (MoSe2, HQ graphene) in 20 ml of distilled water. After the sonication for 12 h, the solution was left unperturbed for 24 h to remove any large-sized MoSe2 before the as-sonicated MoSe2 samples were collected from the top of the solution without a centrifugation process. Our group recently reported that the centrifugation process does not affect the performance of a SA [55]. Figures 1(a) and 1(b) show the measured AFM image and line profile of the MoSe2 particle that has been dispersed in the water, respectively. The measured thickness of the layer is ~80 nm, which is relatively thicker than that of the film of the MoSe2 SA in [62–64], implying that the structure of our prepared MoSe2 nano-particles is not mono- or few-layered and can be regarded as bulk-like. To precisely analyze the particle structure, XPS was conducted to obtain the chemical stoichiometry of the MoSe2 layer. The high resolution of the Mo 3d spectrum and the Se 3d spectrum are shown in Figs. 1(c) and 1(d), respectively. The two peaks of Mo 3d5/2 and Mo 3d3/2 are located at ~228.3 eV and ~231.4 eV, respectively, while the adjacent two peaks of Se 3d3/2 and Se 3d5/2 are located at ~53.9 eV and ~54.6 eV, respectively, and these results are consistent with the previously reported measurements [65].
Fig. 1 Measured (a) AFM image and (b) line profile of the MoSe2 particle dispersed in water. Measured XPS (c) Mo 3d spectrum and (d) Se 3d spectrum of the MoSe2 particle dispersed in water.
To form a MoSe2/PVA aqueous solution, a portion of the PVA powder was mixed into the prepared MoSe2 solution; subsequently, the MoSe2/PVA composite was characterized using Raman spectroscopy and an optical linear-absorption measurement after it was placed onto a glass substrate.
The measured Raman spectrum that was excited by a 532 nm laser is shown in Fig. 2(a). The out-of-plane vibrational mode A1g and the tiny in-of-plain vibrational mode E2g peak were found at 241.3 cm−1 and at 284.1 cm−1, respectively [65, 66]. As we expected, the position of the A1g of the MoSe2/PVA is similar to that of the bulk MoSe2 due to a slightly thick layer of the exfoliated MoSe2. Figure 2(b) shows the linear optical absorption of the MoSe2/PVA composite, whereby the two absorption peaks are located at ~710 nm (A) and ~800 nm (B), respectively, which corresponds to the two spin-orbit-split transitions at the K point of the brillouin zone [67, 68]. Further, the broadband absorption can be clearly observed over a wide spectral range from 400 nm to 2000 nm.
For the implementation of an all-fiberized SA for which the prepared bulk-like MoSe2/PVA composite is used, a side-polished-fiber platform for which the evanescent-field interaction between a guided mode in the optical fiber and the deposited saturable-absorption material is used was chosen. The side-polished fiber used in this work was fabricated by our group. The side-polished fiber was prepared by polishing one side of a standard single-mode fiber (SM28) that was affixed onto a V groove of a quartz block. Firstly, the upper cladding of the fiber was roughly grinded by using a polishing machine until the residual cladding thickness of upper side became approximately 30 μm. Then, a fine polishing process was conducted for the fiber until the residual cladding became approximately 11 μm. After polishing, some minor scratches were observed on its flat surface. The distance between the edge of the core and the polished flat side of the fiber was set as ~11 μm. The insertion loss and the polarization-dependent loss (PDL) of the side-polished fiber were measured as ~0.3 dB and ~0.08 dB, respectively. The bulk-like MoSe2/PVA solution was then deposited on the flat side of the side-polished fiber using the solution-drop method, where it dried at room temperature for 24 h. The cross-sectional structure and side-view of the prepared bulk-like, MoSe2-deposited side-polished fiber are shown in Fig. 3(a), while Fig. 3(b) shows a real photo of the prepared side-polished fiber.
Fig. 3 (a) Cross-section and side-view of the schematic of the prepared MoSe2/PVA-deposited side-polished fiber and (b) a real photograph.
After deposition, the insertion loss and the PDL of the prepared side-polished fiber were substantially increased up to 1.5 dB and 1.2 dB, respectively. The side-polished fiber used in this work had a larger distance between the core and the flat surface of the polished fiber than the value of the previous reports [69, 70]. During the SA preparation process we found that a side-polished fiber with a distance of ~11 μm between the core and the flat surface gave a reasonable insertion loss when it was deposited with a composite of the MoSe2/PVA composite. As the distance was smaller, the insertion loss became larger for this particular experiment. The insertion loss was found to be more than 20 dB when the distance was ~2 μm. It should be noticed that our saturable absorber was based on MoSe2/PVA composite whose refractive index is different from that of the saturable absorption material of Sb2Te3 used in [69, 70].
To measure the damage threshold of the prepared MoSe2-layer-deposited side-polished fiber, 1 W of a continuous wave (CW) laser beam was used at a wavelength of 1550 nm. The observations indicate that the bulk-like MoSe2/PVA-layer-deposited side-polished fiber was not damaged within the power level of the laser that was used, indicating that the damage threshold must be larger than the optical-power level; however, the exact damage-threshold value could not be measured in our laboratory due to the optical-power limitation of the used laser.
Next, due to a non-negligible PDL, the nonlinear transmission of the proposed SA that is based on the bulk-like MoSe2/PVA-deposited fiber was measured as a function of the incident-peak power for both the transverse electric (TE)-mode and the transverse magnetic (TM)-mode input beam. A ~550 fs mode-locked fiber laser was used at ~1560.2 nm for the nonlinear-transmission measurement. Together with a fixed-pulse repetition rate of 22.26 MHz, the average output power of the laser was adjusted from 0.18 mW to 3.96 mW for the TE-mode input, and from 0.29 mW to 3.23 mW for the TM-mode input. The measured results, which are shown in Fig. 4, were also fitted with the following well-known fitting equation [71]:
where T is transmission, ΔT is modulation depth, I is incident-pulse energy, Isat is saturation energy, and Tns is nonsaturable loss. The estimations regarding the modulation depth and the saturation power for the TE-mode input were ~5.4% and ~26 W, respectively, while they are ~4.2% and ~43 W for the TM-mode input; here, the modulation-depth level is high enough to induce a soliton mode-locking in an optical-fiber-based cavity with a properly managed dispersion profile [72].Fig. 4 Measured nonlinear transmission of the MoSe2/PVA SA for the (a) TE-mode input and (b) TM-mode input.
It is obvious from the Table 1 that the proposed SA exhibited a higher modulation depth than those of the previous works; moreover, the saturation intensity of the proposed SA is low with a low insertion loss that is almost comparable to the micro-fiber-based WS2 SA of [31]. The saturation intensity of 23 MW/cm2 is much larger than that (5.1 MW/cm2) of the CNT-based saturable absorber demonstrated in [73]. The PDL of the proposed SA is 1.2 dB due to the asymmetric structure of the side-polished fiber, and this is lower than that of the WS2-deposited side-polished fiber of [33].
Table 1. Output-performance comparison of our prepared SA and the previously demonstrated TMD-based SAs that are used for harmonic mode-locking.
3. Passive harmonic mode-locking of a fiber laser
The construction of an all-fiberized laser cavity for a harmonically-mode-locked testing of the prepared bulk-like MoSe2/PVA-based SA, as shown in Fig. 5, occurred next. For this experiment, a simple ring-type cavity was adopted together with a 3-m-long EDF (Liekki Er20-4/125, nLight Corporation) gain medium. The EDF, which has a peak absorption of ~20 dB/m at 1530 nm, was pumped using a 980 nm laser diode (LD) via a 980/1550 nm wavelength division multiplexer (WDM), and the maximum pump power is ~400 mW. A polarization controller (PC) was used to optimize the polarization state of the oscillating beam within the cavity. The prepared SA was located between the PC and a 90:10 coupler, the 10% output port of which was used as a laser-output end. All of the components within the cavity were fusion-spliced, the total cavity length is ~13 m, and the cavity dispersion was estimated as −0.15 ps2. An optical spectral analyzer, a 16 GHz real-time oscilloscope combined with an 15 GHz photodetector, a radio frequency (RF) analyzer, and an optical power meter were used to monitor the output characteristics of the laser.
We observed the temporal waveform of the output with the oscilloscope setup as we enlarged the pump power. Mode-locked pulses appeared at a fundamental frequency of 15.38 MHz when the pump power reached only ~9.3 mW. Considering our recent experiences with mode-locked erbium-doped-fiber lasers using 2D materials, it was slightly surprising to see the mode-locking phenomenon at such a low pump power. The minimum pump power for the mode-locking in this particular case is two or three times less than that of our previous mode-locking demonstration on an erbium-doped-fiber laser using a WS2 SA [33]. The authors believe that the dominant mechanism for mode-locking in our laser is saturable absorption since our saturable absorber has a low polarization dependent loss (PDL) of 1.2 dB, which is low enough to exclude nonlinear polarization rotation (NPR) effect. Note that Boguslawski et al. mentioned that a 2.7 dB PDL was small enough to exclude the NPR effect [69].
Figure 6(a) shows the measured optical spectrum of the mode-locked pulses at the fundamental resonance frequency with two different fitting curves, wherein the fitting of a hyperbolic-secant curve with the measured spectrum is more compatible than that of a Gaussian curve; here, the center wavelength and 3 dB bandwidth were 1557.3 nm and 5.4 nm, respectively. The oscilloscope trace of the output pulses at the fundamental resonance frequency is shown in Fig. 6(b). The temporal period and pulse repetition rate were 65 ns and ~15.38 MHz, respectively, which is consistent with a round trip of the cavity. The electrical spectra of the output pulses are shown in Figs. 6(c) and 6(d). A clear signal peak was observed at 15.38 MHz and the signal-to-noise ratio (SNR) was measured as ~59.1 dB over a narrow scan of 10 kHz, as shown in Fig. 6(c). The wide-span view over a 200 MHz range that is shown in Fig. 6(d) reconfirms the signal quality. The resolution bandwidth (RB) and video bandwidth (VB) used for the measurements were 30 Hz for the 10 kHz span, whereas they were 3 kHz for the 200 MHz span.
Fig. 6 Measured (a) optical spectrum, (b) oscilloscope trace of the output pulses at the fundamental repetition rate. Measured electrical spectrum of the output pulses at the fundamental repetition rate; (a) a narrow span of 10 kHz (RB: 30 Hz) and (d) a wide span of 200 MHz (RB: 3 kHz).
An autocorrelation measurement was then carried out using a two-photon absorption-based autocorrelator (FR-103PD, Femtochrome) to estimate the pulse width at a fundamental resonance frequency. Figure 7(a) shows the measured autocorrelation traces of the output pulses with a hyperbolic-secant fitting curve, while Fig. 7(b) shows the autocorrelation measurements with a Gaussian fitting curve. It is obvious that, unlike the case of the output optical spectrum where a hyperbolic-secant curve is more compatible, the fit of the Gaussian curve is more compatible than that of the hyperbolic-secant curve with respect to the autocorrelation measurements. The reason for why the fitting-curve discrepancy occurred between the spectral and temporal domains is not clear at the time of the publication of this study; while it is possibly associated with nonlinear chirp that induced by the unbalanced dispersion management between the anomalous and normal dispersions within the fiberized cavity, further investigation needs to be conducted for a better understanding of the underpinning mechanism. Assuming that the shape of the output pulses is Gaussian temporal, the estimated temporal width of the output pulses is ~798 fs. The time-bandwidth product (TBP) of ~0.53 is higher than the value (0.44) of a transform-limited Gausian pulse, indicating that the output pulses are slightly chirped.
Fig. 7 Measured autocorrelation traces of the output pulses at the fundamental resonance frequency. (a) Hyperbolic-secant-curve fitting and (b) Gaussian-curve fitting.
Next, the pump power was increased to observe the harmonic mode-locking. It should be noted that the experiment was carried out within a pump-power range of 0 mW to 400 mW due to the limitation of the maximum pump power that was used in this work. The half-wave plate of the PC within the laser cavity was slightly adjusted to generate stable harmonically mode-locked pulses, when the pump power was enlarged. Fig. 8 shows the characteristics of the 212th harmonic pulses, which were obtained at the maximum pump power of ~400 mW. The measured optical spectrum is shown together with two different fitting curves in Fig. 8(a). Further, like the pulses at the fundamental resonance frequency, the fit of a hyperbolic-secant curve is more compatible with the measured spectrum than that of a Gaussian curve. The estimates of the center wavelength and the 3 dB bandwidth were 1557.3 nm and 5.1 nm, respectively. The oscilloscope trace of the harmonically mode-locked 212th pulses is shown in Fig. 8(b). The temporal period and its corresponding pulse repetition rate were 0.3 ns and 3.27 GHz, respectively. Figures 8(c) and 8(d) show the measured electrical spectra of the harmonic pulses over a wide span of 12 GHz and a narrow span of 10 kHz, respectively. A strong electrical-signal peak was clearly observed at 3.27 GHz. The measured SNR and supermode suppression ratio (SMSR) were measured as ~61.9 dB and ~38.2 dB, respectively. The difference of the noise floor level in Figs. 8(c) and 8(d) can be attributed to VB difference. The RB and VB used for the measurements were 30 Hz for the 10 kHz span, whereas they were 30 kHz for the 12 GHz span.
Fig. 8 Measured (a) optical spectrum and (b) oscilloscope traces of the 212th-harmonic-output optical pulses. Measured electrical spectra of the 212th-harmonic-output pulses; (c) narrow span of 10 kHz (RB: 30 Hz) and (d) wide span of 12 GHz (RB: 30 kHz).
We also measured the autocorrelation traces of the output pulses at the maximum harmonic order (212th) that corresponds to a harmonic frequency of 3.27 GHz. Figure 9(a) shows the traces of the output pulses with a hyperbolic-secant curve, while Fig. 9(b) shows the autocorrelation measurements with a Gaussian curve. Like the case of the pulses at the fundamental resonance frequency, and according to our expectation, the measured auctocorrelation trace of the output pulses was more effectively fitted by a Gaussian curve than by a hyperbolic-secant curve. Assuming that the shape of the output pulses is Gaussian temporal, the estimates of the temporal width and the TBP of the output pulses are 751 fs and ~0.473, respectively.
Fig. 9 Measured autocorrelation traces of the output pulses at the maximum harmonic order. (a) Hyperbolic-secant-curve fitting and (b) Gaussian-curve fitting.
The variation of both the pulse-repetition rate and the corresponding harmonic order of the output pulses depending on the pump power were then measured. Figure 10(a) shows the measured pulse repetition rate and the harmonic order (n) of the output pulses as a function of the pump power. The pulse repetition rate increased from 15.38 MHz to 3.27 GHz as the pump power was enlarged from ~9.3 mW to ~400 mW. According to the linear line fitting, the estimate of the pump harmonic efficiency is ~8.64 MHz/mW, as shown in Fig. 10(a). The type of linear relation that is indicated between the pulse repetition rate and the pump power was observed up to a pump power of 300 mW; however, the pump harmonic efficiency became slightly smaller at higher pump powers. The average output power and the pulse energy of the output pulses as a function of the harmonic order were then measured, as shown in Fig. 10(b). The average output power gradually increased from ~0.23 mW to ~22.8 mW with an increase of the pump power, whereas the pulse energy decreased from ~14.6 pJ to ~5.9 pJ. The harmonic pulses at orders (n) higher than 100 maintained their energy almost constantly between ~5.9 pJ and ~6.7 pJ. Although the characteristics of the laser could not be observed due to the limitation of the pump power, we believe that further increases of the pump power beyond 400 mW could result in increases of both the pulse repetition rate and the harmonic order.
Fig. 10 (a) Harmonic-mode-locking frequency of the output pulses as a function of the pump power. (b) Measured average optical power and pulse energy of the output pulses as a function of the harmonic order (n).
Next, the pulse width was measured at various harmonic orders, and Fig. 11 shows the measured temporal widths of the output pulses at various harmonic orders. Even when the harmonic order was increased, the output pulses maintained their temporal width between 737 fs and 798 fs. The minimum and maximum pulse widths were found at the 1st order and the 212th order, respectively.
Lastly, the output performance of the proposed harmonically mode-locked laser was compared to those of the previously demonstrated harmonically mode-locked fiber lasers for which TMD-based SAs are incorporated, and the results are summarized in Table 2; here, the repetition rate of the proposed laser is the highest. In terms of the pump harmonic efficiency, the fiber laser that is based on MoS2 shows a more favorable performance than the proposed laser; however, the output pulse width of the laser is limited to a three-picosecond level. Our laser exhibited a pump harmonic efficiency better than the WS2-based lasers but was inferior to the MoS2-based laser. The proposed laser shows a minimum pump power value for mode-locking that is almost comparable to that of the laser for which a MoS2-based SA is used. Further, it is obvious from the table that, in terms of the minimum pump power for mode-locking, maximum repetition rate, and pump harmonic efficiency, our laser outperforms the harmonically mode-locked fiber laser for which a WS2-based SA is used.
Table 2. Output performance comparison of our fiber laser with the previous demonstrated harmonically mode-locked fiber lasers incorporating TMD-based saturable absorbers
4. Conclusion
We have experimentally demonstrated a harmonically mode-locked, all-fiberized laser that can produce femtosecond pulses at a repetition rate of 3.27 GHz, whereby a bulk-like, MoSe2/PVA-deposited side-polished fiber was used as a fiberized SA; furthermore, stable harmonically mode-locked pulses with a temporal width of 737 fs to 798 fs could readily be generated from an erbium-doped-fiber cavity at various harmonic orders. The maximum harmonic order was 212th and its corresponding repetition rate was measured as 3.27 GHz; notably, the repetition rate of 3.27 GHz is the highest frequency at which femtosecond pulses can be produced from a fiber laser that is based on passive harmonic mode-locking. An interesting finding of this work is the exhibition of Gaussian-temporal shapes by the output pulses irrespective of the harmonic order, and even when their spectra possessed hyperbolic-secant shapes. A further study including a frequency resolved optical gating (FROG) measurement needs to be conducted to precisely ascertain the underlying reason regarding the discrepancy between the spectral and temporal domains.
We believe that this experimental demonstration indicates the potential of the bulk-like, MoSe2/PVA-composite SA deposited onto a side-polished fiber regarding its capacity as an efficient SA for the harmonic mode-locking of a fiber laser.
Acknowledgments
This work was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF-2015R1A2A2A11000907), Republic of Korea. This work was supported by the Industrial Strategic Technology Development Program (10048690) funded by the Ministry of Trade, Industry & Energy, Republic of Korea. This work was supported by the Korean Ministry of Trade, Industry and Energy within the project, “Development of Process & Equipment Technology to Engrave Roll Molds with 10-micron Scale Line Width using a Pulse-width Tunable Ultrafast Laser (10048726)”. The authors thank Mr. Hanyong Park for his contribution to the fabrication of a side-polished fiber.
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