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Abstract
Herein, we presented a high energy noise-like (NL) pulse Tm-doped fiber laser (TDFL) system. Relying on the nonlinear amplifying loop mirror (NALM), stable noise-like pulses with coherence spike width of ∼317 fs and envelope width of ∼4.2 ns were obtained from an all polarization-maintaining fiberized oscillator at central wavelength of ∼1946.4 nm with 3 dB bandwidth of ∼24.9 nm. After the amplification in an all-fiberized TDF amplifier system, the maximum average output power of ∼32.8 W and pulse energy of ∼10.1 µJ were obtained, which represents the highest pulse energy of NL pulse at ∼2 µm, to the best of our knowledge. We believe that the high energy NL pulse source has the potential application in mid-infrared supercontinuum generation.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Pulsed laser operated at 2 µm spectral regime has attracted a lot of studies in recent years, due to its exceptional advantages in applications such as surgical procedures, polymer processing, and mid-infrared (mid-IR) supercontinuum generation [1–6]. Noise-like (NL) pulse is a typical pulse type which was always generated in passively mode-locked fiber laser based on “artificial” saturable absorber (SA) (e.g., nonlinear polarization rotation (NPR), nonlinear optical fiber mirror (NOLM), nonlinear amplifying loop mirror (NALM)) [7–9] or “real” SA (e.g., CNTs, Graphene) [10–13]. The NL pulse is actually a pulse bunch, composed by many random ultrashort pulses with different pulse intensities and widths [14,15]. By measuring the autocorrelation trace, a narrow coherence spike (usually femtosecond) is the typical characteristic of such pulse. Meanwhile, the NL pulse usually exhibits a wide emission spectrum (typically several tens of nanometers) [11,16]. Owing to these unique properties, the pulse can enable large pulse energy with moderate peak power, which is suitable for supercontinuum generation and industrial processing [17,18].
Till now, relying on the Tm-doped fiber laser (TDFL), many works about NL pulse generation have been experimentally reported both in normal and anomalous dispersion conditions. By using the NPR technique, G. Sobon et al., X. He et al. and S. Liu et al. have obtained the NL mode-locked pulse with pulse energy of ∼1.3 nJ, ∼17.3 nJ and ∼12.3 nJ, respectively [7,19,20]. As for the nonlinear loop mirror method, J. Li et al. have obtained the NL pulse with pulse energy of ∼249 nJ, mode-locking by NOLM [8]. With the similar cavity, mode-locked NL pulse with pulse energy of ∼252.6 nJ was obtained by H. Ahmad et al. [21]. Relying on the NALM, X. Wang et al. got the NL pulse with pulse energy of ∼97.4 nJ [9]. Besides the “artificial” SA, the “real” SA has also been used for NL pulse generation in TDFL. By utilizing the single-wall carbon nanotube as SA, Q. Q. Wang et al. have gotten the mode-locked NL pulse with pulse energy of ∼1.27 nJ [22]. A graphene-based NL mode-locked TDFL was reported by G. Sobon et al. After the amplification, the maximum pulse energy of ∼51.5 nJ was achieved [11]. However, all the works above were operated in standard single mode fiber, which is sensitive to the fluctuation of external environment. Moreover, the polarization controller is essential to initialize the mode-locking. Thus, the self-starting and repeatability were seriously limited. To solve these issues, all polarization-maintaining (PM) fiber configuration is an available way, which has been verified in previous studies [23–26]. In fact, the mode-locked NL pulse also has been reported with all PM fiber configuration in 2 µm spectral regime. Based on the NALM, M. Michalska and J. Swiderski have realized the NL pulses at central wavelength of ∼1993.6 nm with maximum pulse energy of ∼43.4 nJ [27]. However, two segments of gain fibers and double pump lasers were employed in their oscillator, which increases the complexity and cost. Furthermore, according to the studies above, we noted that the pulse energy of the NL pulse at 2 µm wavelength was mainly stayed at ∼nJ level.
In this manuscript, based on the NALM, a simple all-PM TDF oscillator with one section of gain fiber was presented, which could deliver stable NL pulse with pulse envelope width of ∼4.2 ns and coherence spike width of ∼317 fs at central wavelength of ∼1946.4 nm. Comparing with the “real” SA [28], the NALM-based SA has the advantages of high damage threshold and long-time stability [24]. After the amplification by an all-fiberized TDF amplifier system, the maximum average output power of ∼32.8 W and pulse energy of ∼10 µJ was obtained. To the best of our knowledge, it is the highest pulse energy of NL pulse in 2 µm regime.
2. All-PM fiberized oscillator and the output properties
The all-PM fiberized oscillator is presented in Fig. 1. Two fiber loops were contained in the “figure-8” resonator, which were connected by a PM optical coupler (OC) with splitting ratio of 40:60. The NALM was located at the left, which was constructed by a section of ∼2.7 m PM Tm-doped fiber (PM-TSF 9/125) and a section of ∼52 m PM single mode fiber (SMF, PM-1550XP). The gain fiber was pumped by a fiber laser central at ∼1550 nm via a PM 1550/1950nm wavelength division multiplexer (WDM). The long passive fiber was used for the accumulation of nonlinear phase difference. In the right fiber loop, a PM isolator (ISO) with fast axis blocking imposed the unidirectional propagation of the laser in the loop, and a 2×1 OC with output ratio of ∼20% utilized for laser outputting. All the PM fibers were panda-type, which were spliced by a PM fiber fusion splicer (FSM-100P+, Fujikura). The total cavity length and dispersion are ∼64 m and ∼-4.3 ps2, respectively.
The mode-locked operation can be self-started, when the incident pump power exceeds the threshold of ∼1.31 W. Figure 2 shows the evolutions of optical spectrum, pulse envelope and radio frequency (RF) spectrum with the pump power. The optical spectrum was measured by an optical spectrum analyzer (AQ6375, Yokogawa), and the pulse envelope and RF spectrum were recorded by a 12.5 GHz photodetector (ET-5000F, EOT) with a 4 GHz bandwidth oscilloscope (WaveRunner 9000, Teledyne Lecroy) and a 26.5 GHz RF spectrum analyzer (N9020B, Keysight), respectively. As the pump power increasing from ∼1.31 W to ∼1.89 W, the 3 dB bandwidth of the spectrum was slightly broadened from ∼21.4 nm to ∼24.9 nm and no significant variation in optical spectrum shape was observed. The broadening of the spectrum was the result of the enhancement of nonlinear effect (e. g. self-phase modulation (SPM)) as the increasing of the pump power. Similar tendencies were also observed in the pulse envelope. The width of the pulse envelope was increased from ∼2.5 ns to ∼4.2 ns. The characteristics of optical spectrum and pulse envelope strongly imply that the laser operate at NL mode-locking [8,21]. By measuring the RF spectrum, the large modulation frequencies of ∼394 MHz, ∼330 MHz ∼270 MHz, and ∼238 MHz were observed, respectively. The modulation frequencies were well corresponding to the envelope widths of ∼2.5 ns, ∼3 ns, ∼3.7 ns and ∼4.2 ns with the relationship of $f = {\raise0.7ex\hbox{$1$} \!\mathord{\left/ {\vphantom {1 \tau }} \right.}\!\lower0.7ex\hbox{$\tau $}}$ [29], which further verifies the width of the pulse envelope. In the spectrum, besides the signal wavelength, another two peaks at wavelengths of ∼2226 nm, ∼2369 nm were also observed. However, the peaks are not coincided with the Raman shift, which may be the gain peaks of the TSF. Anyhow, the power ratios of the two peaks are both less than 0.05%, which can be ignored.
Fig. 2. Output properties of the oscillator at different pump powers, (a)∼1.31; (b) ∼1.51 W; (c) ∼1.7 W; (d) ∼1.89 W
The detailed output characteristics of the mode-locked pulse at pump power of ∼1.89 W were shown in Fig. 3. The laser has a central wavelength of ∼1946.4 nm with 3 dB bandwidth of ∼24.9 nm, as shown in Fig. 3(a). The dips on the spectrum were mainly caused by the strong water absorption peaks around 1920nm. Figure 3(b) shows the pulse envelope and pulse train, which was directly recorded from the oscilloscope. The pulse envelope has a wide width of ∼4.2 ns, which was mainly caused by the long cavity length. The pulse train in a large time span of∼5 ms was given in Fig. 3(b) inset. The low amplitude fluctuation indicates a stably mode-locked operation. By using a commercial autocorrelator (Pulsecheck 600, APE), the autocorrelation trace of the pulse with different scanning ranges was measured, as shown in Fig. 3(c) and Supplement 1, Fig. S1(a). A narrow coherence spike riding on the pedestal further verifies the NL mode-locking. With the Guass fitting, an ultra-short spike has a full width at half maximum (FWHM) of ∼317 fs. In addition, the RF spectrum of the NL pulse was measured, as given in Fig. 3(d). The frequency of ∼3.228 MHz was determined by the cavity length of ∼64 m, which means the fundamental mode-locking of the laser. The signal to noise ratio (SNR) of ∼71 dB is larger than the results in the previous works, which indicated the low amplitude noise of the sub-pulses in the NL pulse envelope [8,9,27]. But the pedestal of the RF spectrum may indicate a slight fluctuation on pulse duration [27]. The evolutions of average output power and pulse energy of the oscillator were plotted in Fig. 4(a). The maximum pulse energy of ∼25.6 nJ was calculated with average output power of ∼82.6 mW. Moreover, the stability of average output power was measured, as shown in Fig. 4(b). During the 10 hours monitoring, the standard deviation of the average output power was ∼0.31 mW, which corresponds to the power fluctuation of ∼0.38%.
Fig. 3. NL pulse characteristics at pump power of ∼1.89 W, (a) optical spectrum; (b) pulse envelope (inset, pulse train in a large time span); (c) autocorrelation trace; (d) RF spectrum
Fig. 4. (a) The functions of output power and pulse energy with pump power; (b) the monitoring of average output power in 10 hours with time interval of 10s
3. TDF amplifier system and the amplifying characteristics of the NL pulses
The TDF amplifier system is shown in Fig. 5. Three stages amplifiers were contained in the system. The 1st stage amplifier was constructed by a segment of ∼3 m long Tm-doped single-mode fiber (TSF-9/125, Nufern), two WDMs and an isolator. The front WDM is used for coupling the pump laser and the other one is for leaking out the residual pump power. In the 2nd stage amplifier, the gain fiber was a section of double-clad Tm-doped fiber (DC-TDF-10/130, Nufern) with length of ∼3.5 m, which was pumped by a ∼793 nm LD via a combiner. A cladding power stripper was utilized for strapping the unabsorbed pump power, and an isolator was placed behind the stripper for the unidirectional propagation of the pulse laser. After that, the pre-amplified laser was coupled into the 3rd stage amplifier thought a mode field adaptor (MFA). The 3rd stage amplifier has the similar structure with the 2nd stage amplifier, but all the fibers are large mode area (LMA) fiber with diameters of core and inner cladding of ∼25 µm and ∼400 µm, respectively. A piece of LMA Tm-doped fiber (LMA-TDF-25/400-HE, Nufern) with length of ∼3.3 m was used as gain fiber, and a ∼793 nm LD with maximum output power of ∼80 W was employed as pump source. The pump laser was coupled into the gain fiber by a (6 + 1) × 1 combiner, and the residual pump was stripped by a commercial CPS with passive fiber length of ∼1.5 m. The end of the fiber was cleaved with an angle of ∼8° to eliminate the Fresnel reflection and use for laser outputting. The whole fiber amplifier system was fixed on the water-cooled panel with temperature of ∼10°C.
For the amplification, the pump power of the seed was fixed at ∼1.89 W, which corresponds to the output power of ∼82.6 mW. The output properties after the 1st and 2nd stage amplifiers were presented in Figs. 6(a) and 6(b), respectively. The 1st stage amplifier provides the output power of ∼337 mW, corresponding to the pulse energy of ∼104.4 nJ. Comparing with the seed pulse, the optical spectrum, pulse envelope and coherence spike were well maintained, as shown in Fig. 6(a) and Supplement 1, Figs. S1(b). After the amplification by the 2nd stage amplifier, the average output power and pulse energy were reached to ∼2.99 W and ∼0.93 µJ. We noted that the 3 dB bandwidth of the optical spectrum has a slight narrowing, as presented in Fig. 6(b). Such a narrowing was thought as the result of mismatch of gain spectrum between double-clad TDF and TSF, since the gain spectrum of the double-clad TDF has a red shift, as seen in Supplement 1, Fig. S2. Similar narrowing was also observed in Ref. [11] with the same gain fiber. The width of the pulse envelope was still ∼4 ns, but the pedestal has a slight broadening caused by the amplification. Moreover, the spike FWHM was broadened to ∼372 fs, as seen in Fig. 6(b). Owing to the special pulse and spectral structure of the NL pulse, the broadening mechanism of NL pulse was complex, in which the dispersion and spectral were considered as the two key factors [30]. Herein, the broadening of the coherence spike was most likely caused by the dispersion, since the amplifier operated in net abnormal dispersion and no notable spectrum broadening was observed.
Fig. 6. (a) Output properties at the 1st stage amplifier; (b) Output properties at the 2nd stage amplifier
After that, the NL pulse laser was further boosted in the 3rd stage amplifier. The output characteristics of the average power and pulse energy were plotted in Figs. 7(a) and 7(b), respectively. As the enhancement of the incident pump power, the average output power was increased linearly with slope efficiency of ∼41.4%. At the maximum pump power of ∼79.3 W, the average output power of ∼32.8 W was obtained, which corresponds to the pulse energy of ∼10.2 µJ. Figure 8 exhibits the evolution of optical spectrum with pump power. As can be seen that the optical spectrum has experienced a serious broadening at a high pump power. At the maximum output power of ∼32.8 W, a wide optical spectrum ranges from ∼1920nm to ∼2300 nm was obtained, which indicates the strong nonlinear effects. The detailed nonlinear effects for the spectrum broadening were considered as follows. In our experiment, the whole system was operated in net anomalous dispersion regime. Thus, the modulation instability (MI), Raman induced self-frequency shift (RISFS) and SPM were taken the mainly responsibility for the broadening [31]. The MI provides the symmetric gains and induces the breakup of the pulse, which restrains the spectrum broadening to the short wavelength. As the power increasing, the RISFS was progressively produced and used for the spectrum extension toward the long wavelength [32]. Moreover, the SPM works for the smooth of the spectrum [3]. With the assumption of nanosecond pulse of the NL pulse, the peak power of the pulse was calculated to be ∼2.5 kW. According to the previous work, the peak power was enough to cause the intensive nonlinear effect and seriously spectral broadening [11,33]. The pulse properties were also measured at maximum output power, as given in Fig. 9. No significant changes were observed in pulse envelope, but the spike FWHM was further broadened to ∼597 fs. Moreover, by measuring the autocorrelation trace in 150 ps scanning range (Supplement 1, Fig. S1(d)), an obvious background fluctuation around the spike was observed. Besides the dispersion, the strong nonlinear effects caused sub-pulse splitting in the NL bundles was also considered as the reason for the broadening and deterioration of the coherence spike, since the coherence spike is a coherent artifact that derives from the substructure of NL pulses instead of the duration of the pulse envelope [16]. The RF spectra of the final output was presented in Fig. 9(c). The SNR of the fundamental frequency is ∼64 dBm. Owing to the amplification, the SNR of the pulse has a feeble decreasing, but the pulse also shows a good stability. For further checking the power stability, the average output power at final output was monitored for 4 hours, as shown in Fig. 9(d). The power fluctuation of ∼0.42% indicates an excellent power stability of the whole fiber laser system.
Fig. 7. The evolutions of average output power and pulse energy vs. pump power after the 3rd stage amplifier
Fig. 9. The output characteristics at output power of ∼32.8 W (a) pulse envelope (inset is the corresponding pulse train); (b) autocorrelation trace; (c) the RF spectra; (d) power stability in 4 hours
4. Conclusion
In this paper, we have experimentally demonstrated a high energy NL pulse source at 2 µm spectrum regime. The NL pulse was originally generated from an all-PM fiberized oscillator mode-locked by NALM, which centered at wavelength of ∼1946.4 nm with envelope width of ∼4 ns and coherence spike width of ∼317 fs. Moreover, the seed pulse has exhibited a good stability with SNR of ∼71 dB and power fluctuation less than ∼0.4%. After the amplification in an all-fiberized TDF amplifier system, the maximum average output power of ∼32.8 W and the pulse energy of ∼10.2 µJ were obtained, which represented the highest pulse energy of NL pulse at 2 µm wavelength, to the best of our knowledge. The NL pulse source may be suitable for some applications, such as mid-IR supercontinuum generation and polymer processing.
Funding
National Natural Science Foundation of China (61905146); Shenzhen Technology Development Program (JSGG20190819175801678, JSGG20191129105838333); Guangdong Provincial Department of Education Youth Innovation Talent Project (2018KQNCX400); Free Exploration Project of Shenzhen Basic Research (JCYJ20180301171044707); China Postdoctoral Science Foundation (2020M682864).
Acknowledgments
We thank Dr. X. Luo for fruitful discussions.
Disclosures
The authors declare no conflicts of interest.
Supplemental document
See Supplement 1 for supporting content.
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