1. Introduction

Pulsed all-fiber-format laser sources are beneficial for a broad range of applications, mainly including scientific research, industry, and biomedicine, etc. Recently, 2-μm band fiber laser sources have attracted intense research interests due to their distinguished advantages in clear-clear plastic engineering [1], eye-safe surgery [2], 2-5 μm mid-infrared supercontinuum generation [3], >4 μm parametric generation [4], gas sensing [5], and so on. To produce nanosecond-scale pulses, active Q-switching [6] and gain-switching [7] are two typically utilized techniques. Although these techniques have realized high-power and stable operations, they typically require complex electronics controlling system and also lack the ability to achieve various pulse profiles. Alternatively, mode-locked fiber lasers can provide various pulse-shaping regimes based on nonlinear and dispersion mechanisms while requiring the least controlling components. In the nanosecond duration range, dissipative soliton resonance (DSR) has been proved to be an effective way to generate rectangular-shape pulses, although mainly realized in 1- and 1.5-μm bands.

Theoretical prediction and numerical simulation of the DSR applicable to nonlinear dynamical systems were first performed early in 2008 by W. Chang et al [8]. Then they further predicted that the DSR can be formed in both anomalous and normal dispersion regimes [9–11]. Most recently, D. Li also numerically simulated the formation mechanism of DSR in all-normal-dispersion fiber lasers [12]. Considering the high-energy-availability and pulse-duration-scalability, some experimental results on DSR generation have also been demonstrated but mainly in erbium and ytterbium doped fiber lasers. X Li et al first reported the rectangular-shape DSR pulse generation with the anomalous regime by using nonlinear polarization rotation (NPR) technique in an ultra-long cavity erbium-doped single-mode fiber (ESF) laser [13]. They obtained pulse energy high to 715 nJ at 278 kHz pulse repetition rate (PRR). Their pulse duration could also be tuned from 12.8 to 155.4 ns. Similar results were also obtained by other groups, such as what demonstrated in [14–21]. There have also been some results obtained in ESF laser with net normal dispersion regime through dispersion management. In an all-normal-dispersion ESF laser, X. Wu et al realized square-profile DSR pulse generation with up to 281.2 nJ pulse energy [22]. J. Yang et al reported up to 379.2 nJ DSR pulse generation in a net-normal-dispersion ESF fiber laser through dispersion compensation [23]. In the 1-μm band ytterbium-doped fiber lasers with all-normal-dispersion regime, several results on DSR have also been reported. L. Liu et al first demonstrated that DSR phenomenon could exist in Ytterbium-doped single-mode fiber (YSF) lasers, and they obtained 8.8 ps to 22.92 ns tunable output with maximum pulse energy of 3.24 nJ [24]. X. Li investigated the 54 to 91 ns DSR pulses generated in an all-normal-dispersion YSF laser, and achieved an ultra-high optical-to-optical conversion efficiency of ~46% [25]. Similar results can also be seen in [26–32]. Most recently, D. Li et al showed that DSR pulses could be compressed by using a grating pair [33]. In that report, the 63 ps pulse duration generated in DSR regime was compressed down to 760 fs.

For 2-μm band DSR, to the best of our knowledge, only one result has been reported up to now, which was achieved in a thulium-doped single-mode fiber (TSF) laser based on nonlinear amplification loop mirror (NALM) technology and dispersion-management for achieving a net-normal dispersion [34]. Here, to the best of our knowledge, we first report the DSR generation in a Tm-doped double-clad fiber (TDF) laser by using a nonlinear optical loop mirror (NOLM) as the passive mode-locker in all-anomalous-dispersion regime. Furthermore, we first perform high power amplifications of the 2-μm band DSR pulses, and obtain up to ~100.4 W average output power and ~18.1 kW pulse peak power.

2. Schematically experimental configuration

The whole experimental layout is schematically shown in Fig. 1. The TDF laser oscillator is constructed by a figure-of-eight (F-8) configuration. The gain is provided by ~4-m long TDF with ~3dB/m cladding absorption at 793 nm, which exhibits core and inner-cladding diameter (flat-to-flat) of $d_{co}=10.2$ and $d_{cl}=130$ μm, respectively. A 105/125 fiber-pigtailed 793 nm multi-mode laser diode (LD) with maximum output power of ~12 W is employed as the pump source, of which the emission is coupled into the first clad of the TDF via a (2 + 1) × 1 pump/signal combiner. The unabsorbed pump light is effectively stripped by a self-made cladding-power-stripper (CPS). Then, an isolator with return loss of ~46 dB and insertion loss of ~1.21 dB is used to ensure the unidirectional propagation of the intra-cavity light. The circulator after the isolator plays two roles here: one is to guide counter propagating light out of the cavity, and the other is to add ~23 dB additional return loss on the gain loop. This design ensures that the gain loop is truly unidirectional, and the mode-locking is only attributed to the artificially saturable absorption property of the bidirectional NOLM. To combine together of the gain loop and the NOLM, a four ports fiber coupler with 35/65 splitting ratio is utilized in between. To enhance the nonlinear of the NOLM, ~150-m high nonlinear fiber (HNLF) and ~20-m Nufern SM-1950 fiber are incorporated within. The HNLF exhibits nonlinear coefficient of $\gamma =11.68$ W⁻¹km⁻¹, zero dispersion wavelength (ZDW) of ${\lambda }_0=\text{1}550$ nm, numerical aperture of$NA=0.35$_{, $d_{co}=3.67$} μm, and $d_{cl}=\text{120}\text{.5}$ μm, respectively. The dispersion parameter $D$ of the utilized HNLF at ~1975.56 nm is $D=~9.81$ ps/nm/km, corresponding to group velocity dispersion of ${\beta }_2=-\frac{{\lambda }^2}{2\pi c}D=~-20.31$ ps²/km. Thus, the HNLF is evidently anomalous at the operating wavelength of the TDF oscillator. Considering that all other utilized fibers are also anomalous at 2-μm band, the fiber laser oscillator should operate in all-anomalous regime. The SM-1950 fiber exhibits mode-field diameter (MFD) of ~8 μm at 1950 nm, $d_{co}=7$ μm, $d_{cl}=125$ μm, and core numerical aperture $N{A}_{co}=0.2$. For adjusting the intra-cavity polarization states, two 3-paddle polarization controllers (PC) are incorporated into the two loops, respectively. The light flows propagating in the F-8 cavity are also drawn in Fig. 1, seen from the solid (generated in the gain loop) and dash (returned from the NOLM) arrow lines. It is noted that the clockwise-propagating laser pulses returning from the NOLM can be guided out of the gain loop prior to arriving at the isolator, via a circulator. This design can eliminate the undesired returning light exerted on the isolator, comparing with typical design by using a fiber coupler, such as what used in [15, 23, 34].

 

Fig. 1 Schematic configuration of the high power DSR all-fiber system. TDF: thulium-doped double-clad fiber; CPS: cladding power stripper; LD: laser diode; PC: polarization controller; HNLF: high nonlinear fiber; NOLM: nonlinear optical loop mirror; SM: single mode; MFA: mode-field adapter; LMA: large mode area; DSR: dissipative soliton resonance.

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The first stage amplifier utilizes the same type of fiber components with the oscillator. The gain fiber is also a segment of 10/130 TDF with length of ~4 m. Only one difference here is that we splices two 793-nm LDs with the combiner, for providing enough pump power. To handle hundreds of watts pump and signal power, in the second stage amplifier we utilize ~2.5-m large-mode-area (LMA) TDF as the gain fiber, which exhibits ~9.5-dB/m cladding absorption at 793 nm, $d_{co}=25$ μm, $d_{cl}=250$ μm, $N{A}_{co}=0.09$ and $N{A}_{cl}=0.46$_. Correspondingly, the signal-delivering fiber employed in the combiner and the CPS is a type of matched passive fiber (Nufern FUD-3440). Two high power 793-nm laser modules are utilized as the pump sources. Each can emit laser power up to >100 W via a segment of 200/220 multimode-fiber (MMF). Considering the signal fibers utilized in the first and second amplifiers are much different in sizes, a fiber-tapering-based mode-field-adapter (MFA) with transmission loss of ~0.43 dB is inserted in between to reduce the propagation loss. A 5/95 tap coupler is also inserted to monitor the characteristics of the forward signal light and the possibly backward reflection. The final high-power output from the second amplifier is delivered through an end cap with anti-reflection (AR) coating and angle-cleaving. For reducing the bending loss of the LMA gain fiber, we coiled the fiber with a large bending radius of ~20 cm. To dissipate the heat accumulation during high power operation, we place the coiled LMA-TDF onto an aluminium (Al) plate cooled by circulating chilly water with temperature of ~5°C. The ~5°C operation can result in water condensation on the Al plate due to the surround humidity of the air. But, according to our experiments, the water condensation exerts little influence on the LMA fiber amplifier. This is possibly due to the fact that the 2-μm band signal laser is always confined within the fiber core during propagation, which is far away from the outside polymer coating. The outside coating is only used to form a waveguide to confine the 793 nm pump light within the inner-cladding of the LMA fiber.

3. DSR pulse generation

The emission characteristics of the TDF oscillator are plotted in Fig. 2. Mode-locked operation can be initiated when the pump power reaches ~2.93 W. Once the laser entering the mode-locked status, the paddles of the two intra-cavity polarization controllers (PCs) can be tuned in relatively wide ranges without breaking the pulsed operation. Figure 2(a) shows that, despite the output spectral peak intensity increasing, the 3dB spectral bandwidth slightly increases from ~16.09 to ~16.78 nm as the enhancement of pump power from ~2.93 W to ~9.58 W. But the central wavelength is always located at ~1975.56 nm. Meanwhile, the corresponding output pulse duration increases from 3.74 ns to 72.19 ns, as seen from Fig. 2(b). The pulse durations were monitored by using a high speed photodiodes-based photodetector with >12.5 GHz bandwidth and 28 ps rise/fall time (ET-5000, 2 μm InGaAs PIN Detector, Electro-Optics Technology, Inc.), combined with a 20 GS/s high-speed oscilloscope with 1-GHz band width (Tektronix, DPO 7104C Digital Phosphor Oscilloscope). Although the output pulse linearly broadens as the pump power increasing due to the accumulated pulse energy, it is noted that the output pulse peak power is always maintained at nearly an equal level of ~0.56 W, caused by an effect typically named as peak power clamping (PPC) [34, 35], which represents a distinguishing feature of DSR operation regime. Here the formation of PPC effect is mainly attributed to the incorporated HNLF in the NOLM, which enhances the average cavity nonlinear parameter ${\gamma }_A$ and limits the available pulse peak power $P_0$ circulating in the fiber laser oscillator due to the relation of $P_0\propto 1/{\gamma }_A$.

 

Fig. 2 Emission characterisctics of the DSR fiber laser oscillator. (a) Output spectra and (b) single pulse envelops with different pump powers; (c) A typical RF spectrum registered with 10 Hz RBW around the pulse repetition rate; (d) A typical recorded RF spectral distribution with 1 kHz RBW and 50 MHz span; (e) RF spectrum evolution as the increasing of pump power; (f) 4 h output power temporal stability of the TDF laser oscillator. RBW: resolution bandwidth; SNR: signal-to-noise ratio; RMS: root mean square.

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We have also measured the radio-frequency (RF) spectrum characteristics of the output pulses by using the high-speed photodetector together with a high resolution spectrum analyzer with resolution bandwidth (RBW) of 1 Hz - 5 MHz and frequency span of 9 kHz - 26.5 GHz (Agilent, E 4407B, ESA-E series spectrum analyzer). Figure 2(c) plots a typical RF spectrum around the fundamental repetition rate of $f_r=1.065$ MHz by using 10-Hz RBW, which gives a signal-to-noise ratio (SNR) exceeding ~55 dB. Figure 2(d) shows the corresponding RF spectral distribution with 50 MHz span by using 1-kHz RBW. Both Figs. 2(c) and 2(d) are recorded with ~9.58 W pump power. It can be observed that the RF is equally spaced by $f_r=1.065$ MHz, determined by the cavity length $L$ via the well-known relation of $f_r=c/\left(nL\right)$, where $c$ is the velocity of light propagating in the vacuum and $n$ is the average refractive index of fiber core utilized in the fiber laser oscillator. Besides that, there is also a much larger modulation-frequency of $f_m=13.85$ MHz exerting on the recorded RF spectral distribution, which originates from the produced native pulse duration of $\tau =72.19$ ns and satisfies a reciprocal relation of $f_m=1/\tau$. Actually, it is found that $f_m$ linearly decreases as $\tau$ increasing when the pump power is enhanced from 2.93 W to 9.58 W, as seen from Fig. 2(e), further reflecting a reciprocal relation between $f_m$ and $\tau$. For a brief summary and clearly to address the modulation pattern, the frequency space $f_r=1.065$ MHz is defined by the overall optical-path length of the fiber cavity, and the modulation frequency $f_m$ exactly equals to the reciprocal of the native pulse duration.

In order to characterize the operation stability of the TDF oscillator, we have also further measured the output power pumped at ~9.58 W for 4 hours, as shown in Fig. 2(f). Based on the measured power data, the root mean square (RMS) can be calculated to be ~0.294 mW. To divide the RMS value by the mean value of the power of ~43.13 mW, the obtained relative variation is only ~0.68%, which can be an effective parameter to quantify the stability of the TDF laser. The small relative variation indicates that the TDF laser oscillator exhibits high temporal stability.

4. High-power amplifying characteristics of the DSR pulses

To obtain high power output, we perform two stages amplification on the DSR pulses at different durations. For the first stage amplifier with ~16 W pump power, ~2.6 to ~3.68 W average output power can be obtained when the pulse duration increases from ~3.74 to ~72.19 ns, corresponding to pulse peak power ranging from ~47.9 to ~652.8 W. These peak power levels are not high enough to initiate any nonlinear effects during the pulse-propagation in the 10-μm fiber core, and no significant change can be observed on the recorded spectral profiles, which is beneficial for the succeeding stage amplification.

On the second amplifier, we launch ~207-W maximum pump power into the LMA-TDF. The finally achieved spectral and temporal characteristics are shown in Fig. 3. From the spectra plotted in Fig. 3(a), it can be seen that the pulses with duration of ~3.74 ns experience strong spectral broadening from ~1936 to ~2480 nm, indicating that intensive nonlinear effects occur and supercontinuum (SC) is formed with too high a pulse peak power of ~18.1 kW. For characterizing the nonlinear property, nonlinear phase is an important parameter. The well known formula to calculate the nonlinear phase is ${\phi }_{NL}=\gamma P_0{L}_{eff}$, where $\gamma$is the nonlinear coefficient of the utilized fiber, $P_0=~18.1$ kW is the pulse peak power after amplification, and $L_{eff}$ is the effective length for the propagating fiber. The nonlinear coefficient $\gamma$ can be further expressed as $\gamma =\frac{2\pi n_2}{\lambda A_{eff}}$_{, where $n_2$} is the Kerr nonlinear coefficient that is typically around $2.6\times {10}^{-20}$ m_{2/W for silica fiber, and $A_{eff}$} is the effective mode-field area. After the LMA-TDF, the previously mentioned 25/250 passive fiber is used to construct the pigtails of the CPS and the end cap, of which the total length is ~3.2 m. For the passive fiber, $L_{eff}$ approximately equals its actual fiber length $L_{eff}\approx L=~3.2$ m if the fiber transmission loss is neglected. For the LMA-TDF, however, the effective length can be calculated as $L_{eff}=\frac{1-\exp \left(-gL\right)}{g}L\approx \frac{4.343}{G(dB)}L=\frac{4.343}{14.4}\times 2.5\approx 0.754$ m, where $g$ is the gain per unit length and $G(dB)$ is the total gain of the LMA-TDF expressed in decibels. So the total effective length is ~3.954 m that the amplified pulses will propagate through. According to these relations, we can calculate that, at ~18.1 kW peak power, the nonlinear phase exceeds ~12.06 rad, so nonlinearities are to be expected.

 

Fig. 3 Spectral and temporal delivering characteristics from the 2-stage amplified DSR fiber laser system. (a) Spectra and (b) single pulse envelops of different pulse durations; (c) A typical amplified pulse train recorded with 100-μs span with single pulse duration of ~72.19 ns.

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For longer pulse with lower peak power, the corresponding spectrum rapidly narrows down due to the weakening and even vanishing of nonlinear effect, and when the pulse duration increases to ~21.01 ns, only minor spectral broadening can be observed compared with the oscillator spectrum, i.e. from ~16.09 to ~17.75 nm. When the pulse duration further increases, spectral broadening no more occurs. Alternatively, spectral narrowing effect can be observed, which indicates that no nonlinear effect can be initiated due to the low pulse peak power. Such as for the pulse duration of ~37.85 ns, the corresponding spectrum narrows from ~16.11 nm to ~12.92 nm after amplification, corresponding to 3.19 nm spectral narrowing; and for the pulse duration of ~72.19 ns, the spectral narrowing effect becomes even more severe, i.e. from ~16.78 to ~11.19 nm, narrower by 5.59 nm. The spectral narrowing effect probably comes from the fact that the leading edge is amplified stronger compared with other parts of the pulse envelop. Meanwhile, the spectral region corresponding to the pulse leading edge also obtains stronger amplification. Comparatively, much weaker amplification can only be achieved on the left spectral region, which results in that the whole spectral profile of the pulse becomes narrower after amplification. In case the pulse encounters self-phase-modulation (SPM), the SPM effect in combination with anomalous dispersive fiber also leads to a spectral narrowing effect. Similar narrowing effects in 2-μm band fiber amplifiers were also observed by H. Hoogland et al [36].

The delivered single-pulse envelopes from the final amplifier are plotted in Fig. 3(b). It can be seen that each pulse maintains the same duration after amplification, but the pulse peak power lowers down as the pulse duration increases. Another phenomenon can be observed is that, for each single-pulse envelope, the intensity rapidly decreases towards the trailing edge viewed on the top profile, indicating that the amplification mainly occurs at the leading edge. Figure 3(c) further gives a captured pulse train in 100-μs span when operating at 100.4 W average power and ~72.19-ns duration.

The average output power delivered from the second amplifier versus incident pump power is plotted in Fig. 4. For the pulses with duration of ~3.74 ns, significant nonlinear effects can be initiated during the amplification and the power-enhancing rate slows down, as the average output power increases to higher than 40 W, i.e. the peak power>10.04 kW. The maximum average output power can only reach ~72.1 W at the highest pump power of ~207 W, while the coating of the output end cap reaches temperature higher than 300 °C, which is caused by the strong absorption of the AR-coating material at long-wavelengths approaching mid-infrared region. So high a temperature would make the end cap be in high risk if for long-term operation. Despite the rapid heating of the end cap, we have also observed the temperature rises to more than 90 °C with the whole fiber. The heating of the whole fiber might come from the fact that there is a lot of Raman shifted light generated for the short pulses. During stimulated Raman scattering the light is frequency-shifted to longer wavelengths. The difference in energy between the Raman shifted light and original light dissipates into heat. When the pulse duration is long enough, taking the range of ~21.01 to ~72.19 ns for instance, the average output power increases almost linearly with the rise of incident pump power. The slope efficiency only experiences a minor change from 47.23% to 47.68% as the pulse duration increasing from ~21.01 to ~72.19 ns; meanwhile, the maximum output average power increases from ~72.1 W to ~100.4 W with the same pump power of ~207 W. However, the obtained pulse peak power decreases from ~18.1 kW to ~0.94 kW when the pulse duration increases from ~3.74 ns to ~72.19 ns. The inset of Fig. 4 also shows the maximum output power recorded by a laser energy/power meter (EPM 2000, Coherent, Inc.).

 

Fig. 4 Average output power of the LMA TDF amplifier with the increase of incident pump power. Insert: the highest average output power recorded by a power meter.

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The output beam characteristics have also been measured by using a laser beam diagnostics (PY-III-C-A, Spiricon, Inc.), as shown in Fig. 5. Figure 5(a) shows the measured beam widths along both x- and y-axis directions versus the z-axis positions around the focusing spot, which can give the laser beam quality factor M² of 1.26 and 1.32 in x and y directions, respectively. Simultaneously, the beam asymmetry is measured to be ~1.01. Figures 5(b) and 5(c) show the captured 2D and 3D beam profiles, indicating that the beam intensity should be confined in the lowest LP₀₁ mode, although the LMA-TDF can also support some power distributing in the LP₁₁ mode considering that the normalized frequency of the LMA-TDF at the operating wavelength $\lambda$ is $V=\pi d_{co}\cdot NA/\lambda =3.578$.

 

Fig. 5 Output beam characteristics measured by a laser beam diagnostics. (a) The measured laser beam quality factor (M²); Beam profile around the focus performed in (b) 2D and (c) 3D, respectively.

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5. Conclusion

In conclusion, we have proposed and demonstrated the DSR operation in a TDF laser based on a HNLF-incorporated NOLM, and have further scaled the average power up to ~100.4 W and the pulse peak power up to ~18.1 kW. To the best of our knowledge, the DSR generation and amplification are first achieved in a double clad fiber laser system operating at 2-μm band and in all-anomalous regime, which also represents the highest power level of 2-μm DSR pulses at the moment. Our result also indicates that the DSR-based TDF MOPA system can provide a high power all-fiber-format laser source at 2-μm band with tunable pulse duration and peak power.