1. Introduction

Tm-doped pulse fiber laser has grabbed tremendous attention in recent years due to its prosperous prospect in variety of applications including LIDAR, remote sensing, laser communication, nonlinear frequency conversion, material processing and so on [1–4]. Generally, Q-switching and mode-locking are two main methods to generate pulse laser output. Passively mode-locking is an effective technique to acquire ultrashort pulses [5, 6] and typical techniques for passively mode-locking are semiconductor saturable absorber mirrors (SESAMs) [7], graphene saturable absorbers (GSAs) [8], carbon nanotubes (CNTs) [9], gigahertz acoustic core resonances [10], nonlinear amplifying loop-mirror (NALM) [11, 12] and nonlinear polarization rotation (NPR) [13].

There are two interesting pulse operation states in passively mode-locked fiber lasers: pulse bunching and harmonic mode-locking. Pulse bunching is an important phenomenon in passively mode-locked fiber lasers and has been widely investigated [13–15]. Pulse bunching is mainly attributed to the overdriven nonlinear effects, which induces the energy quantization of the pulses in laser cavity and consequently results in uniform pulse splitting [16, 17] and pulse bundles (PBs) [18–20]. Passively harmonic mode-locked pulse (PHMLP) [21–24] is also attractive due to its special advantages such as capability of generating ultra-high frequency and ultrashort mode-locked pulse train, and convenience to tune the order of harmonic oscillation or the repetition rate of pulses. The repetition rate of the PHMLPs can reach as high as tens of GHz and most of the pulse widths are in the range of picosecond or sub-picosecond level, which makes ultrafast applications such as optical clock synchronization, precision metrology, and pump sources for high-repetition-rate supercontinuum sources feasible employing this PHMLP laser source [10, 23].

However, nanosecond pulse trains in passively mode-locked fiber lasers have attracted relatively much less attention. Actually, nanosecond pulses, especially bursts or bundles with nanosecond pulses and continuous nanosecond pulse train at different repetition rate in Tm-doped fiber lasers (TDFLs), can provide eye-safe laser source and play an indispensable role in variety of applications [25–27] such as micromachining, material processing, medical care, and environment measurements. In passively mode-locked TDFLs, nanosecond-level PBs and PHMLPs have not been investigated yet up to our knowledge.

In this paper, we present the first nanosecond-level PBs and PHMLPs in a passively mode-locked ring TDFL based on NPR technique. Two polarization controllers (PCs) were employed to perform the NPR. Self-starting PBs at fundamental repetition rate of 448.8 kHz with up to 17 subpulses are generated after the PCs are adjusted and fixed. By changing the positions of the PCs and pump power, up to 6th PHMLPs are obtained. The individual pulse width in PBs and PHMLPs are both around 3-5 ns with uniform shape and intensity. Higher order of PHMLPs and wider pulse durations are also generated by adjusting the PCs. The 3dB spectral bandwidth is as wide as 25 nm, which resembles the characters of noise-like pulse.

2. Experimental setup

Figure 1 depicts the sketch of the passively harmonic mode-locked TDFL. The pump power was provided by a home-made 1550 nm fiber laser with maximum output power of 2.4 W. A 1550/2000 nm wavelength division multiplexer (WDM) was employed to launch the pump power into the laser cavity. The gain medium was a length of 4 m Tm-doped single clad fiber with core diameter of 9 μm and clad diameter of 125 μm. The active fiber’s absorption efficiency at 1550 nm was about 20 dB/m. A length of 440 m SMF-28 fiber was included in the cavity to enhance the nonlinear effects. Two polarization controllers (PCs) and a polarization sensitive isolator (ISO) were used to serve as the polarization control components. 10% of the laser power in the cavity was launched out via a 10/90 fiber coupler. The output pulse train was detected by a high speed InGaAs PIN photodetector (PD) with bandwidth of 7 GHz and a digital oscilloscope with bandwidth of 1 GHz. The spectrum was analyzed by an optical spectrum analyzer (OSA) with resolution of 0.05 nm.

 

Fig. 1 Schematic sketch of the passively mode-locked TDFL.

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3. Experimental results and analysis

The laser can operate in continuous wave regime when the pump power is about 500 mW, which is the threshold of this laser. The relatively high threshold of this TDFL is mainly attributed to the high loss of the SMF-28 fiber at 2 μm. Passively mode-locking can be achieved when the pump power is above the threshold and the positions of two PCs are adjusted. PBs are generated by carefully adjusting the positions of the PCs and increasing the pump power, as shown in Fig. 2. Actually, by finely adjusting the PCs, PBs with 1-17 subpulses can all be achieved. It can be found out that the subpulses’ parameters (such as intensity, width and shape) are nearly uniform, despite the not exactly identical spacings between the adjacent subpulses and weak intensity fluctuation. The weak intensity fluctuation may be caused by the competition between the adjacent pulses and the experimental environment noise such as the vibration of the fiber and the turbulence of the air. The PBs can be attributed to the pulse split in the laser cavity due to overdriven nonlinear effects. Actually, in passively mode-locking fiber lasers employing nonlinear polarization rotation, the PBs and/or the multipulses operation in the laser can be attributed to the nonlinear effects in the cavity, peak power limiting effect of the laser cavity, gain competition between the multiple pulses and weakly birefringent effects of the fiber [19]. Increasing the pump power and carefully adjusting the PCs, the nonlinear effects in the laser cavity was notably enhanced and multipulses were generated [18–20]. Due to the fiber loss and the low output proportion (10%), the output power of this TDFL is only 10 mW. By exchanging the 10% output port of the coupler with the 90% one, the output power can be increased to about 95 mW, but the consequent subpulses’ number of PBs and order of PHMLPs decrease evidently compared with that of the previous configuration. The main reason is that the low output proportion (10%) results in the relatively high laser intensity in the cavity and hence enhances the nonlinear effects, which is fundamental for pulse splitting and generation of the PBs and high order PHMLPs. So the 10% port was chosen as the output port in order to achieve PBs with more subpulses and PHMLPs with higher order. Actually, the relatively low output power could be scaled up by either power amplification or launching more pump power into the cavity. The side band signal to noise ratio in the experiment is rather high since no obvious noise is observed from the results. Furthermore, the pulse operation of the laser is stable and no evident timing jitter occurs.

 

Fig. 2 PBs in the passively mode-locked TDFL.

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Figure 3 shows the digital oscilloscope traces of the PBs with 14 subpulses. It can be concluded that the PBs oscillating regularly at fundamental repetition rate of 448.8 kHz (determined by the cavity length), meanwhile the subpulses have the repetition rate of 28.7 MHz, which is about the 64th harmonic wave of the fundamental frequency. The width of each subpulses are uniform and measured to be about 3.7 ns. The quite broaden pulse width compared with other passively mode-locked pulses’ widths may be attributed to the much long cavity length and the quite anomalous dispersion of the SMF-28 fiber for 2 μm. The PB’s stability is favorable and this nanosecond PBs may find important applications where pulse bursts are required. We believe that PBs with more subpulses can be generated by further increasing the pump power.

 

Fig. 3 Oscilloscope traces of the PBs in the passively mode-locked TDFL. (a): train of PBs; (b): one PB with 14 subpulses.

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Figure 4 shows the oscilloscope traces of single pulse in the TDFL. Actually, most of the pulse widths in this TDFL are around 3.7 ns (3.5-5 ns) as shown in the left part of Fig. 4. By adjusting the positions of the PCs, pulse with width of about 304 ns at fundamental frequency can also be obtained, as shown in the right part of Fig. 4.

 

Fig. 4 Oscilloscope traces of individual pulse in the passively mode-locked TDFL.

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Finely adjusting the positions of the PCs and scaling up the pump power, PHMLPs can be obtained, as shown in Fig. 5. When individual pulse oscillation at fundamental frequency of 448.8 kHz was built up, the positions of PCs were fixed. Thereafter the pump power was increased slowly, and the higher order harmonic mode-locked pulse train could be generated. The order of the harmonic mode-locked pulses increases as the pump power rises, while the pulse shape and intensity remains almost the same. The order of the harmonic mode-locking is limited by the available pump power and relatively long cavity length.

 

Fig. 5 Harmonic mode-locked pulse trains in TDFL. F: fundamental frequency.

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Hysteresis phenomenon of the harmonic mode-locked pulse train’s frequency versus pump power is observed when the PCs’ positions are fixed, as shown in Fig. 6. The pulse frequencies in Fig. 6 correspond to the 1-6 order harmonic mode-locked frequencies. It is clear that the pulse frequency jumps to a higher one when the pump power increases to a certain threshold, and the maximum pulse frequency reaches 6th order harmonic mode-locked frequency when the pump power surpasses 2.25 W. When the pump power decreases gradually from the maximum value of 2.4 W, stable 6th order harmonic mode-locked pulse train maintains until the pump power drops below 1.83 W, which is much lower than the previous frequency jumping threshold of 2.25 W. This hysteresis phenomenon continues and fades until the pump power falls below the lasing threshold. In addition, hysteresis on output power and number of subpulses in PBs are also observed in this TDFL as demonstrated in other passively mode-locked fiber lasers [20, 28].

 

Fig. 6 Hysteresis phenomenon of the harmonic mode-locked pulse train’s frequency versus pump power.

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The order of harmonic mode-locking is not limited to 6th, higher order harmonic mode-locked pulses can also be achieved by finely adjusting the PCs. The results are shown in Fig. 7, where the pump power is fixed at 2.1 W. From the traces in Fig. 7 one cannot only see the pulse train at fundamental frequency with 304 ns pulse width aforementioned in Fig. 4, but also see the pulse train at frequency of 6.904 MHz with pulse width of 4.5 ns. Note that the frequency of 6.904 MHz is about 15.4 times of the fundamental frequency of 448.8 kHz. This nanosecond pulse train with fractional time of fundamental repetition rate (15.4 times) may be associated with the dynamic nonlinear saturation effect induced by NPR and the unusual temporal and spectral characters of this passively mode-locked pulse train, since most of the passively mode-locked pulses have rather short pulse width (picosecond, for example) and the spectrum is usually either with side-lobes which indicates the soliton mode operation [29], or with smooth and much wide spectrum which suggests the noise-like mode operation [29, 30].

 

Fig. 7 Oscilloscope traces of pulses with different frequencies and widths in TDFL.

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Figure 8 depicts the spectra of the passively mode-locked TDFL in three cases: continuous wave operation, random pulse operation (the pulse repetition rate is not stable at all and hence the pulse appears like “random” pulse) and regular pulse operation (the pulse repetition rate is very stable). It shows that at continuous wave operation regime, the spectrum’s bandwidth is narrow (~1.3 nm). In addition, the central wavelength of the continuous wave lasing can be altered in a range of about 60 nm (1920-1980 nm) by changing the positions of the PCs. The spectrum broadens to several tens nanometers when the PCs are adjusting to generate pulses with random frequency but identical intensity and shape. Keep on adjusting the PCs properly, regular pulse trains (PBs, PHMLPs) are obtained. The 10dB spectral bandwidth can reach as large as ~40 nm, and the 3dB bandwidth is about 25 nm. The pulse train’s broad spectrum has somewhat the character of noise-like pulse, but it is not very smooth. Furthermore, the pulse width is several nanoseconds, which is different from the ultrashort noise-like pulse as demonstrated [29, 30]. These results may be caused by the interaction and competition between the multiple pulses and the polarization modes in the rather long fiber cavity. This broadband spectral property of the harmonic mode-locked pulses makes it a promising laser source for applications such as supercontinuum generation. Furthermore, the pulse width with several nanoseconds enables power amplification more feasible and convenient to make the practical application of this pulse source available.

 

Fig. 8 Spectra of different cases in TDFL.

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4. Conclusion

In conclusion, we present a passively mode-locked TDFL with nanosecond-level PBs and PHMLPs based on NPR. The PBs at fundamental repetition rate of 448.8 kHz can contain the maximum pulses of 17 with uniform pulse shape and intensity. PHMLPs with 6th order and even higher order are obtained with pulse width around 3-5 ns. The maximum repetition rate of the PHMLPs can reach 6.9 MHz, much higher repetition rate can be achieved by shortening the cavity length and/or increasing the pump power. The 3dB spectral bandwidth is about 25 nm, which resembles that of noise-like pulses. The temporal and spectral characters of this TDFL enable efficient power amplification and variety of practical applications. In further endeavors, the tunability and output power are to be improved to enhance the laser source’s practicability in applications. The low loss SM2000 fiber may be employed to lower the threshold of the fiber laser.