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Abstract
We report the theoretical and experimental investigation of a self-starting mode-locked fiber laser with a nanoengineered Tm3+-doped yttrium-alumina-silica (YAS) fiber as the gain medium. The YAS fiber exhibits a higher capability of Tm3+ cluster elimination than commercial silica fibers. The Tm3+ fluorescence properties and YAS dispersion are well characterized. As a result, an efficient picosecond mode-locked fiber laser is demonstrated with a slope efficiency of 14.14% and maximum pulse energy of 1.27 nJ. To the best of our knowledge, this is the first mode-locked fiber laser based on a Tm3+-doped YAS fiber. The experimental observation is also supported by the numerical analysis.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Because of being retina-safe and high degree of water absorption, mode-locked fiber lasers in the 2-µm region have attracted considerable attention in environmental sensing [1], retina-safe laser radar [2], surgery [3], particle acceleration [4], and pump sources for efficient mid-infrared generation [5]. Tm3+ is a suitable ion for producing near 2-µm emission in fibers because of its high quantum efficiency and broad gain bandwidth. Fabricating efficient Tm3+-doped fibers is essential to extend their applications in 2-µm fiber laser sources. Accordingly, germanate, tellurite, and silicate glasses have been actively studied [6–8]. A suitable glass host for efficient Tm3+ emission is nanoengineered Tm3+-doped yttrium–alumina-silica (YAS) glass [1,6,7]. The YAS glass host is mainly formed by A12O3, Y2O3, and SiO2. Al2O3 increases the refractive index of silica by 2.3 × 10−3 per mole and has been used as intermediate core glass network modifier [9]. Al2O3 also reduces the phonon energy of silica glass and thus increases the magnitude of the radiative emission probability of the metastable states of Tm3+. More importantly, the contained five-fold and six-fold coordinated aluminum ion charges compensate the Tm3+ ions and reduce the formation of clusters, resulting in high photodarkening resistance [10]. Y2O3 helps create the nanostructured YAS glass phase within the silicate host and acts as a glass network generator [11–13]. In general, YAS glass is substantially strong and possesses optical transparency from 200 nm to more than 3000 nm [12]. Furthermore, compared with germanate and tellurite glass fibers, SiO2-based YAS glass fiber is more compatible to be spliced with conventional passive silica fibers.
The first nanoengineered Tm3+-doped YAS fiber laser was successfully built by Paul et al. in 2015 [14]. Following this research, dual-wavelength nanoengineered Tm3+-doped YAS fiber lasers based on a single-mode–multi-mode–single-mode fiber structure were also demonstrated [15]. All of these results were achieved in continuous-wave (CW) operation with a maximum efficiency of 26.2%. Additionally, no pulsed Tm3+-doped YAS fiber lasers have been demonstrated. As a step further, we report in this paper the latest progress in efficient CW and mode-locked lasers by using nanoengineered Tm3+-doped YAS fiber lasers.
On the other hand, considerable investigations related to mode-locked Tm3+-doped fiber lasers have been made; however, most research has focused on soliton fiber lasers with a net dispersion from −1 to 1 ps2 [16]. Increased demand for compact nJ-level picosecond 2-µm laser sources has been driven by applications such as polymer processing, gas sensing or monitoring, medical treatment, and biomedical diagnostics [17]. With the advantages of high brightness, compactness, and cost-effectiveness, several narrow-band mode-locked 2-µm fiber laser sources have been developed. Notably, Dai et al. described a mode-locked Tm3+-doped fiber laser with a tunable wavelength and pulse duration [18]. Wang et al. demonstrated a narrow-band picosecond Tm3+-doped fiber laser with a high-average-power all-fiber amplifier [19]. However, most of these studies used commercial Tm3+-doped silica fibers and did not include numerical simulations.
In the current paper, we describe a passively mode-locked Tm3+-doped YAS fiber laser based on a linear cavity formed by a semiconductor saturable absorber mirror (SESAM) and narrow-band fiber Bragg grating (FBG). Compared with actively mode-locked fiber laser systems, passively mode-locked fiber laser systems are generally considered to produce shorter pulses [20] and are capable of simple, cost-effective, all-fiber formats. Thus far, several materials have been fabricated for use as saturable absorbers (SAs) in Tm3+-doped fiber lasers, such as SESAMs [8], carbon nanotubes [21], graphene [22,23], and topological insulators [24]. Among these SAs, SESAM-based SAs are the most popular because of their flexibility, stability, and self-starting mode-locking features [16,25]. The use of FBG to form the linear cavity enables large laser wavelength tenability while maintaining the stable wavelength tunability, which is required in numerous practical applications. In addition to the demonstration of efficient mode-locked Tm3+-doped YAS fiber lasers, the influence of cavity parameters on mode-locking dynamics is confirmed through numerical simulation. Our results provide a valuable reference for better understanding the mode-locking dynamics of narrow-band mode-locked Tm3+-doped fiber lasers. The mode-locked laser pulses are further amplified to achieve kW-class peak power through a cladding-pumped Tm3+-doped fiber amplifier. Finally, we demonstrate examples of laser marking on fresh fruit and a polymer thin film.
2. Tm3+-doped nanoengineered YAS silica fiber
The nanoengineered Tm3+-doped YAS fiber is manufactured using the modified chemical vapor deposition process in conjunction with the solution doping (SD) technique. Since 1987, the SD technique has been used to incorporate rare earth materials into the silica glass matrix at the core of the optical fibers [26], allowing a spontaneous phase separation of nano-YAS-Tm into the silica glass matrix. The fiber fabrication process has previously been described [27–29]. To determine whether these YAS phases could distribute Tm3+ ions homogeneously with our fabrication process, we performed transmission electron microscope (TEM) analysis, electron diffraction (ED) pattern measurement, and energy dispersive X-ray (EDX) analysis. The TEM analysis confirmed phase separation and nanostructured distribution in the fiber core. As displayed in Fig. 1(a), two separated phases formed within the fiber core. The dark granules were approximately 7–8 nm in diameter and mainly composed of atoms with high atomic mass. The maximum size of the dark granules was limited by the short cooling time (∼1 min) during fiber drawing. The glass outside of the black granules was mainly formed by atoms with low atomic mass. Through the EDX analysis, the Tm, Y, and Al were found to be concentrated mainly within the black granules. The matrix outside the dark granules was constructed mostly of Si oxide and traces of Al, Y, and Tm. The results reveal that nano-YAS phases incorporated most of the Tm ions within themselves, resulting in the creation of several YAS-Tm rich zones in the optical fiber core. The ED pattern presented in the inset of Fig. 1(a) indicates that the black granules are actually amorphous in nature because the sharp ED rings normally observed in crystalline substances did not appear.
Fig. 1. (a) TEM image of the Tm3+-doped fiber. The inset is the ED pattern. (b) Absorption and emission spectra of the Tm3+-doped fiber.
The employed Tm3+-doped YAS glass fiber has a Tm2O3 doping level of 0.48 wt.%, as estimated from the electron probe. In general, the fiber glass host composition not only affects the solubility of the Tm3+ dopant but also slightly affects the absorption and emission spectra. Figure 1(b) shows the measured absorption and emission spectra of the Tm3+-doped YAS glass. An FSL 300 Fiber Spectral Attenuation Spectrometer was used to measure the absorption spectrum. The emission spectra were measured under 1550-nm pumping along the lateral direction using two optical spectrum analyzers with the optical bands of 400- 1650 nm (ANDO 6315A) and 1200-2400 nm (Yokogawa AQ 6370 B). The results indicate that the spectra of Tm3+ ions in YAS glass are fairly similar to those in silica glass [30], except for the relatively short peak absorption wavelength and peak emission wavelength. The feature of stronger absorption near 1600 nm in heavily Tm3+-doped silicate fibers has also been reported [8]. The short peak absorption wavelength enables stronger absorption at the 1.56-µm region, which is beneficial when the Er3+-doped fiber lasers are used as the pump source.
The decay of fluorescence intensity (3F4 to 3H6 of the Tm3+ energy levels) of a 1550-nm core-pumped 5-cm-long fiber sample was carefully measured with an InGaAs photodetector. The pump laser diode was modulated to achieve square-shaped pulses of ms-width, and the fiber sample was angle-cleaved to prevent feedback. As presented in Fig. 2(a), the measured fluorescence relaxation curve showed a single exponential, thus demonstrating negligible concentration quenching. The radiative lifetime of the 3F4 level is 350 µs, which is inside the range of the reported lifetime from 300–650 µs [30,31]. In general, radiative lifetimes of SiO2 glasses tend to be shorter than those of fluoride glasses. The wide range of reported lifetimes for SiO2 glasses could originate from differences in the local environment of the Tm3+ ions such as electron–phonon-coupling strengths or frequencies for the phonons involved in the relaxation process. The relatively short radiative lifetime raises the laser threshold but does not impair the slope efficiency because stimulated emission dominates the nonradiative relaxation once the laser has risen above threshold [31]. The dispersion of the Tm3+-doped YAS fiber was measured with a Mach–Zehnder interferometer. As shown in Fig. 2(b), the zero dispersion is located at 1286 nm.
Fig. 2. (a) Fluorescence decay curve of the 3F4 level of Tm3+ ions in the fiber sample under 1550-nm excitation. (b) Chromatic dispersion curve of the Tm3+-doped YAS fiber.
3. Efficient 1900-nm CW fiber laser
First, we developed a 1567-nm core-pumped 1900-nm fiber laser to evaluate the optical property of the employed Tm3+-doped silicate fiber laser. The experimental setup of the linear laser cavity is presented in Fig. 3. The gain fiber had a core diameter of 13.5 µm and a cladding diameter of 130 µm. The Tm3+ dopants were homogeneously distributed in the core of the optical fiber, which were confirmed using electron probe microanalysis. The measured core numerical aperture was 0.2, which corresponded to the V number of 4.46 at 1.9 µm.
In the current study, the pump source was a lab-made 1567-nm double-clad Er3+-doped fiber laser amplifier with watt-level output power. This pump source was selected for efficient in-band pumping. The corresponding absorption at 1567 nm was 16.7 dB/m. As shown in Fig. 3, the laser cavity was formed at one end by using a 1900-nm HR FBG with a reflectivity greater than 99% and at the other end by using the Fresnel reflection from the other cleaved fiber end, which acted as a 96% output coupler (OC). As listed in the data sheets, the insertion loss of WDM and HR FBG were 0.15 dB and 0.02 dB, respectively. The Tm3+-doped fiber was pigtailed with more than meter-long SMF28 fibers on both side with proper mode matching splices. To minimize the splice loss between two dissimilar fibers, we adopted the thermal expended core method by using a Vytran glass Processor workstation (GPX3400) [32]. With the fusion time of ∼50 seconds, we were able to achieve the lowest splice loss of 0.03 dB between the Tm3+-doped YAS and SMF28 fibers. Therefore, with the proper mode launching and SMF 28 output fiber, the single-mode laser output is expected. Two long-pass filters were used to minimize the noise produced by the 1567-nm residual pump. The 1900-nm laser was targeted not only because of the high emission cross section and high slope efficiency but also because it is a useful laser pumping source for Ho3+-doped fiber lasers [33].
In general, laser efficiency can be influenced by several parameters. To evaluate the efficiency of the 1900-nm Tm3+-doped YAS fiber laser, we used LIEKKI Application Designer v4.0 for numerical modeling. The aforementioned optical parameters of the Tm3+-doped YAS fiber were incorporated into the simulation. Both the simulated and experimentally measured laser output powers are plotted as functions of launched pump power in Fig. 4(a). The experimental data are in good agreement with simulation with the simulation results. The measured slope efficiency of 72.4% verified the suitable optical properties of the gain fiber. The output spectrum of the fiber laser with its peak at 1900nm and 3-dB bandwidth of 0.42 nm is presented in the inset of Fig. 4. The laser output spectrum is mainly controlled by the employed HR FBG. Figure 4(b) illustrates the laser slope efficiency and pump threshold powers as a function of gain fiber length. The results indicate that the optimal fiber length is approximately 3 m and that the laser threshold gradually increases as gain fiber lengths increase because of reabsorption. The numerical simulation confirmed that a slope efficiency of 73.5% can be achieved. The ∼9% mismatch with the quantum defect efficiency (82%) may originate from device loss.
Fig. 4. (a) Laser output power as a function of the launched pump power. The black solid curve and the blue dots represent the theoretical simulation results and the experimental data, respectively. The inset is the measured laser output spectrum. (b) Simulated laser slope efficiency as a function of gain fiber length.
4. Narrow-band mode-locked Tm3+-doped YAS fiber laser
To verify the capability of the fabricated gain fiber for high-peak-power generation, we used the same Tm3+-doped YAS fiber to build a simple linear cavity mode-locked fiber laser. The experimental setup of the all-fiber mode-locked Tm3+-doped YAS laser is presented in Fig. 5. The setup consists of a 4-m section of the Tm3+-doped YAS fiber placed in a linear optical cavity formed at one end by a SESAM and at the other end by a chirped FBG, which acts as 40% OC at 1980nm. The FBG has a 3-dB bandwidth of 0.47 nm and normal dispersion coefficient of −0.83 ps/nm at 1980nm. The Tm3+-doped fiber is core-pumped with the lab-made 1567-nm Er3+-doped fiber laser through a wavelength Division Multiplexer (WDM) placed on the FBG side. The 4-m gain fiber was chosen to provide sufficient pump absorption and favorable stability during the mode-locking operation. The mode-locking mechanism in the Tm3+-doped YAS fiber laser is controlled using the SESAM, which has an unsaturated reflection value of 73% and a modulation depth of 16%. The recovery time, saturation fluence, and group delay dispersion at 1980nm of the SESAM are 10 ps, 70 µJ/cm2, and −1500 fs2, respectively. Because of the use of the chirped grating, the laser is operated in the normal dispersion regime.
The average output power of the mode-locked Tm3+-doped fiber laser is plotted as a function of pump power in Fig. 6(a). First, the fiber laser began with Q-switched mode-locked lasing and was then transferred to CW mode-locked operation under a pump power of 254 mW. The average output power increased linearly to a maximum of 18.35 mW at a pump power of 313 mW. The corresponding slope efficiency of the CW mode-locking operation was 14.14%, which to our knowledge is the highest slope efficiency from a mode-locked Tm3+-doped mode-locked fiber laser. The relatively low efficiency compared with that of the aforementioned CW Tm3+-doped YAS fiber laser could be due to the loss of the SESAM and the higher cavity reflection. When the pump power was further increased, the single-mode-locked pulses split into two pulses and became unstable. The peak laser wavelength and FWHM were mainly determined by the FBG reflection spectrum.
Fig. 6. (a) Laser output power as a function of pump power. (b) Oscilloscope trace under the pump power of 313 mW.
Figure 6(b) depicts the oscilloscope trace of the mode-locked laser pulse trains at a pump power of 313 mW. The time interval of the sequential pulse is approximately 69.2 ns, which corresponds to the round-trip time of the 7.2-m cavity. The relatively long cavity is due to the long, pig-tailed fibers of the fiber-based devices. The time trace of the mode-locked pulse train is displayed with a longer time scale in the inset of Fig. 6(b), which depicts relatively stable mode-locking pulses with relatively low amplitude fluctuation. To test the self-starting mode-locking operation, we gradually reverted the pump power to 0 mW and subsequently returned the power to 313 mW. The results indicate that the mode-locked laser exhibits excellent self-start repeatability. In addition, the mode-locking operation was stable for several hours. The calculated pulse energy is illustrated as a function of pump power in Fig. 6(a). At a pump power of 313 mW, the signal reached maximum pulse energy of 1.27 nJ.
Figures 7(a) and (b) show the radio frequency (RF) spectrum of the mode-locked laser under a pump power of 313 mW. A fundamental repetition rate of 14.45 MHz was observed. The 60.2-dB signal-to-noise ratio in Fig. 7(a) confirms the uniform and stable pulse operation. The inset of Fig. 7(b) is the wideband RF spectrum up to 1 GHz. With the absence of spectral modulation in the RF spectrum, the laser operated excellently in the CW mode-locking regime.
Fig. 7. (a) RF spectrum at fundamental repetition rate under a resolution of 100 kHz. (b) RF spectrum of harmonics with a scanning range up to 100 MHz and 1 GHz (inset).
The evolution of the autocorrelation traces of the mode-locked pulse along the pump power increase is illustrated in Fig. 8(a). It has been experimental and numerical investigated by Wang et al., the employ of narrow-bandwidth spectral filter allows for amplifier similariton (AS) operation in normal dispersion fiber lasers. These AS pulses have parabolic temporal profiles [34,35]. The inset of Fig. 8(b) is one example that can successfully be fitted to a parabolic temporal profile. With a deconvolution factor of 1.43 [36], the derived full-width at half maximum (FWHM) pulse widths as a function of pump power are presented in Fig. 8(a), which indicates that the pulse width slightly decreases as the pump power increases from 44.53 to 23.0 ps. The reduction of pulse duration along the increase of pump level was also reported in a narrow bandwidth mode-locked Er3+-doped fiber laser [37]. The corresponding peak powers are also listed in Fig. 8(b). The peak powers increase linearly to a maximum of 55.21 W as the pump powers increase.
Fig. 8. (a) Autocorrelator trace. (b) FWHM pulse widths and peak powers as a function of pump power.
Figure 9(a) displays the optical spectrum of the single-pulse mode-locked laser under different pump powers. The laser output spectrum is mainly controlled through the FBG grating. As presented, the measured central wavelength peak is fixed at 1979.9 nm and the 3-dB bandwidths of 0.42–0.44 nm. The corresponding time bandwidth products (TBPs) under various pump powers are presented in Fig. 9(b). For this laser, the smallest TBP is 0.78, indicating that the laser was chirped.
Based on Dai’s results, the mode-locked laser bandwidth is primarily controlled by the filter bandwidth [18]. To examine the narrow-band picosecond AS pulse generation and further understand the pulse features, we processed numerical simulations by using the commercial simulation tool Fiberdesk. This tool solves the modified nonlinear Schrödinger equation, given as Eq. (1), through the split-step Fourier method to determine the evolution of the fiber laser pulses [38].
In Eq. (1), A is the slowly varying pulse envelope in a retarded time frame, β2 is the group-velocity-dispersion parameter, γ is the nonlinearity parameter, and g is the gain coefficient. In this simulation, higher order dispersion is neglected. By applying the parameters listed in Table 1, we successfully verified the output pulse performance at the time and space domains. Figure 10(a) indicates the simulated 3-dB pulse duration and peak power under different pump powers. Figure 10(b) and (c) indicate the parabolic fitted pulse and laser output spectrum. The numerical simulation results are in good agreement with the experimental data. The decrease of the pulse duration with the increasing pump power may be caused by the change of cavity nonlinearity and dispersion.
Fig. 10. (a) Simulated laser temporal profiles under different pump powers. (b) Parabolic fitted laser temporal profiles and (c) spectrum of laser pulses under a pump power of 313 mW.
Table 1. Fiberdesk Simulation Parameters
5. 793-nm cladding-pumped Tm3+-doped fiber amplifier
To further increase power of the 1980-nm mode-locked fiber laser, we built a 793-nm cladding-pumped Tm3+-doped silicate fiber amplifier. The amplifier configuration is shown in Fig. 11(a). The gain medium employed was a 5-m commercial Tm3+-doped fiber (Nufern SM-TDF-10P/130-HE) with core, pedestal, and cladding diameters of 10, 22, and 125 µm, respectively. An isolator was employed to minimize reflection and maintain mode-locking stability. To avoid surface damage caused by high peak power, we produced an angle cap by splicing a coreless 130-µm fiber to the end of the Tm3+-doped fiber. Figure 11(b) displays the signal output power as a function of 793-nm pump power. A maximum output power of 622 mW was achieved with an input power of 5.08 W. The amplifier efficiency was 25.9% under high-power operation. The relatively low slope efficiency may have occurred due to the high splice loss between the pedestal Tm3+-doped gain fiber and the SMF28 passive fibers. The corresponding pulse energies under different pump powers are presented in Fig. 10(b), in which a maximum pulse energy of 46.0 nJ and a peak power of 1.87 kW were reached.
Fig. 11. (a) Experimental setup of the high-peak-power Tm3+-doped fiber laser. (b) Measured amplifier output power as a function of launched pump power. (c) Laser marks on the PEDOT:PSS thin film and grape.
Because of their strong OH absorption, high-peak-power Tm3+-doped fiber laser sources are suitable for polymer micromachining [39]. Similarly, soft fruit labeling with Tm3+-doped fiber laser sources would be more efficient and compact than that with traditional CO2 laser sources [40]. We used the developed laser source to demonstrate its laser marking capability on a PEDOT:PSS thin film and laser labeling on a fresh grape. The single-mode output laser beam was imaged to obtain a beam diameter of 15 µm on each sample. As displayed in Fig. 11(c), with high intensity pulses, laser marking can be achieved in one second. Additional theoretical analysis will be presented in further research.
6. Conclusions
We successfully developed efficient CW and mode-locked fiber lasers by using a nanoengineered Tm3+-doped YAS fiber. This new type of host fiber exhibits the improved capability of cluster elimination compared with commercial Tm3+-doped silica fibers. We experimentally demonstrated an efficient 1900-nm all-fiber CW laser to confirm its high-quality material and optical characteristics. Crucial material characteristics of the Tm3+-doped YAS fiber were reported, and efficient 1900-nm laser operation was achieved with maximum efficiency of 72.4% and an average power of 160 mW. The CW laser performance was further confirmed by the simulation results. Furthermore, we present the first study on Tm3+-doped YAS fiber for mode-locked laser operation. The 23.0-ps narrow-bandwidth mode-locked pulses were generated with a laser slope efficiency of 14.14%, a repetition rate of 14.45 MHz, and maximum pulse energy of 1.27 nJ. All results were further verified through theoretical simulation based on the modified nonlinear Schrödinger equation. Finally, a 793-nm cladding-pumped Tm3+-doped fiber amplifier was used for power scaling, resulting in maximum pulse energy of 46 nJ. The 1.87-kW peak-power-mode-locked pulses were successfully employed for laser marking on a PEDOT:PSS thin film and fresh grape. The results indicate that Tm3+-doped YAS fiber exhibits optical and mechanical properties suitable for efficient and cost-effective mode-locked fiber lasers.
Funding
Ministry of Science and Technology, Taiwan (MOST-108-2221-E-027 -096 -MY2).
Acknowledgments
The authors thank Professor Zong-Liang Tseng for providing the PEDOT:PSS thin film. The authors would also like to thank the Department of Science and Technology, New Delhi for partial grants supporting this study.
Disclosures
The authors declare no conflicts of interest.
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