1. Introduction

All-fiber, femtosecond, passively mode-locked lasers have attracted much attention over the last two decades [1–3]. Unfortunately, they tend to have relatively low average output powers and pulse energies, mostly due to limited gain, offered by standard single-mode active fibers [4]. Some researchers carried out experiments with exploiting the advantages of using double clad active fibers in mode-locked lasers. Usefulness of DC active fibers comes with their relatively high gain and efficiency factor, single-mode operation and the possibility to use cheap, high power multimode pumps [5]. Nevertheless, assembly of systems achieving high average output power and pulse energies in in most cases requires complicated resonator configuration and bulk gratings allowing precise chirp control of the generated pulses [6]. These limitations originate from the fact, that in net-anomalous-dispersion, mode-locked soliton lasers a direct relationship between the dispersion and fiber nonlinearity is present. Due to the soliton area theorem, the required balance between these parameters severely limits the pulse energy achieved in standard all-fiber lasers, or extremely complicates the resonator configuration [7]. However, even precise design of DC-active-fiber mode-locked resonators can sometimes lead to unwanted high-harmonic generation due to very high gain and the tendency to high-energy related pulse-braking of such configurations [8,9].

Lasers capable of achieving high energy nanosecond, square-shaped pulses directly from the resonator are also an interesting research field, for example due to applications in laser material processing, medical equipment, laser range finding and high-energy physics experiments [10–15]. Several approaches to the problem of generating intense square-shaped pulses have been proposed, including: cavity-dumping, mode-locking and Q-switching. Convenient way for circumventing the limitations and drawbacks of complicated resonator configurations was predicted by Chang et al. in 2008 [16] and involves designing the passively mode-locked laser resonator with parameters enabling existence of so called Dissipative Soliton Resonances (DSR) effect. This permits the soliton energy to increase virtually indefinitely, while keeping the amplitude at a constant level. Theoretical model describing the formation of square pulses in a Nonlinear Amplifying Loop Mirror (NALM) was published by Zheng et al. in 2008 [17]. After that, a number of publications documenting DSR, nanosecond, square-wave pulse operation of nonlinear polarization rotation (NPR), F8 and even graphene mode-locked lasers has been reported [18–23]. Nevertheless, no reports of utilizing the DSR effects in DC fiber lasers can be found in scientific papers. The vision of combining the advantages of DC active fiber resonators and DSR effects is very promising, and could open new ways of achieving high energy laser pulses directly from a non-complex and cost effective laser resonator configurations.

In this paper we will present first demonstration of a passively mode-locked DC Er:Yb co-doped F8L with resonator parameters permitting DSR pulse evolution.

2. Experimental setup

The laser setup is depicted in Fig. 1. The resonator is built in an ultra-simple all-fiber F8L configuration. The loops are linked with a 3 dB coupler. The right loop is built in a standard NALM configuration. Achieving high gain, and therefore record pulse energies, was possible due to a 5-meter-long DC active fiber co-doped with erbium and ytterbium ions. Pump power delivered from two multimode laser diodes (each with a maximum output power of 9 W) is coupled into the active fiber via a standard pump-beam combiner. Pump light exiting the active fiber is absorbed by a mode stripper (MS). The NALM pulse forming mechanism relies on a disproportion of nonlinearities influencing the pulses propagating in opposite directions [24].

 

Fig. 1 Schematic of the figure-8 laser. CIR – circulator, PC – polarization controller, MS – mode striper, COMB – pump-beam combiner, Er/Yb DC – erbium-ytterbium double clad fiber, SMF28 – single-mode fiber, OSC – oscilloscope, RF – RF spectrum analyzer, OSA – optical spectrum analyzer, P – optical power meter.

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In the presented setup the imbalance in accumulated phase shift for clockwise- and counter-clockwise propagating pulses was achieved by incorporating an additional 200-meter-long piece of SMF28 fiber in the NALM loop (right loop in Fig. 1). The length was experimentally selected to ensure stable mode-locking operation throughout the entire pump power range (above threshold) and minimum nonlinear distortion to the generated high energy pulses. The directional loop (left) consists of only two elements: a polarization controller and a circulator. The circulator plays two key roles. It guarantees unidirectional propagation of pulses, while the 3 port simultaneously serves as an output for the generated laser pulses. In comparison to standard approach (an isolator and a coupler [25,26]) the circulator outputs all optical energy travelling in the prohibited direction (port 2-3 direction). This permitted generating pulses with record energies without exposing the fiber-optic parts to damage. The proposed setup using a fiber circulator exhibited no time-dependent power drifts.

3. Experimental results

The spectrum generated by the laser was monitored simultaneously with an optical power meter, optical spectrum analyzer, oscilloscope and a RF spectrum analyzer. If appropriate position of the polarization controller paddles was ensured, mode-locked pulses were observed at the output of the laser for pump powers above 2 W. After forcing the laser into mode-locked state the position of the polarization controller paddles could be changed in a wide range without perturbation to the pulsed operation (only slight correlation between polarization state and average circulator out-coupled power was recorded). If the paddles were nut-secured in the optimal position, self-starting of mode-locking was achieved during each ON-OFF cycle of the laser, with no alteration to parameters of generated pulses. Figure 2 shows analysis of pulses generated by the laser while being pumped with 5 W of power. The laser emission is centered at 1566 nm with a 6.3 nm 3dB spectral bandwidth.

 

Fig. 2 Laser emission registered at 5 W of pump power. (a) optical spectrum in decibel scale; inset shows spectrum in linear scale, (b) RF spectrum registered with 1 Hz BW; inset shows RF spectrum in a 150 MHz span, (c) temporal profile of the generated square pulses, (d) pulse-train recorded during 40 μs.

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The increase in noise registered beyond 1590 nm is connected with the OSA sensitivity characteristic. According to the registered RF spectrum (Fig. 2(b)) the signal-to-noise ratio (SNR) exceeds 59 dB, which is better than results obtained for similar setups generating high-energy pulses directly from the cavity [27]. The inset in Fig. 2(b) shows the RF spectrum in a 150 MHz span. The observed characteristic modulation with period of 17.7 MHz originates directly from the duration of generated pulses, which for 5 W of pump power was ~57 ns. The “period” of the modulation had a direct linear correlation to the pump related increase in pulse duration. The repetition frequency of the constructed laser was ~800 kHz, which corresponds to a ~255 m long cavity. The resonator consists of only single-mode fiber based components, the additional SMF28 fiber spool (with dispersion of −21 ps²/km [2]) and a 5 m piece of DC active fiber (with similar dispersion to the SMF, equal to −19 ps²/km [9]). Total net dispersion of the resonator is calculated to be −5.345 ps². The graphs depicted in Fig. 3 clearly show that the average output power, pulse energy and duration can be linearly tuned with pump power, while maintaining relatively stable peak power. The achieved pulse tuning ranges from 20 ns to 170 ns. At maximum pump power (17 W) the laser generated 170 ns square-shaped pulses with an average power of 1.7 W. Having a repetition frequency of 800 kHz this corresponds to a pulse energy of 2.13 μJ, with peak power of 11 W.

 

Fig. 3 Average output power, pulse energy (a), pulse duration and peak power (b) plotted versus pump power.

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Up to our best knowledge, this is the highest pulse energy recorded directly at the output of an all-fiber figure-8 DSR laser using DC active fiber. Worth noticing is the fact that under high pump powers no signs of pulse braking, noise-like or higher harmonic operation were observed [28]. The linearity of graph Fig. 3(a), and no signs of its kinking, suggests that the laser would be capable of reaching higher average output powers, consequently pulse energies, with additional pump power delivered to the active fiber.

According to Mei [27], the PPC effect can be explained by investigating the NALM transmission function as a function of peak pulse power. This allows to analyze the loop as a nonlinear switching device, which operation depends on the circulating power, due to the SPM effects occurring in the fibers. During the first milliseconds after switching the laser on, the pulse oscillating in the cavity increases in duration until it reaches a poin in which its peak power saturates the gain. At this point the peak power of the pulse is “clamped” in a region of maximum transmission of the loop. Therefore, a change of gain (pump power delivered to the active fiber) in a resonator with a “clamped” pulse will influence the duration of the pulse, keeping the peak power at a relatively stable level (Fig. 3(b)). Stable peak power can be also observed by analyzing the amplitude of detected pulses on an oscilloscope (Fig. 4).

 

Fig. 4 Pulse duration tuning via pump power change. Insets at each graph present corresponding optical spectra.

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Although the average output power increased, the detector signal amplitude remained nearly constant (around 13 mV) over the entire pump tuning range. Varying the pump power had no observable influence on the generated optical spectra or the SNR parameter.

4. Conclusions

The presented work was the first demonstration of an all-fiber DC mode-locked laser exploiting DSR effects to obtain record 2.13 μJ square-shaped pulses directly from the resonator. Experimental verification included constructing a figure-8 laser with an 5 meter long Er:Yb active fiber. At 17 W of pump power the laser generated 2.13 μJ pulses with 1.7 W of average power, 800 kHz repetition rate and peak power of 11 W. The duration of the generated pulses could be tuned within a 150 ns range simply by changing the pump power delivered to the active fiber due to proper cavity design favoring PPC pulse shaping effect. We predict that constructing the laser with multi-port pump-beam combiner, allowing delivering higher pump power, should enable achieving square-wave pulses with energies suitable for supercontinuum generation in nonlinear fibers. Further improvements will involve experimental verification of the possibility to construct a dual-wavelength version of the laser. Continuous Wave (CW) versions of dual-wavelength operation of Er:Yb configurations were presented in our previous work [29,30].Positive results will enable building a compact all-fiber source ideal for Difference Frequency Generation (DFG) of pulses in the Mid-IR wavelength region. Moreover, we plan to investigate the possibility of compressing the pulses using bulk grating compressor or adequate dispersion compensating fibers, as proposed by Chang et al. [31].