1. Introduction

Recently, high energy nanosecond pulses, directly generated from a mode-locked fiber laser with giant-chirp, have attracted significant interest. It is easy for giant-chirp lasers to generate nanosecond pulses with wide bandwidth, which is difficult for Q-switching technique. The giant-chirped lasers are suitable for a range of applications, especially for chirped pulse amplification (CPA) systems [1–3]. Such kind of laser may replace the standard oscillator, stretcher, pulse-picker, and one or more preamplifiers from a fiber CPA system [4]. This simplifies the CPA systems to better exploit the benefits of fiber, particularly greater integration and lower lost.

Researches have revealed that all-normal dispersion fiber lasers are capable of generating large pulse energy [5–9]. Pulse energy as high as 31 nJ has been demonstrated [8]. However, all of these ultra short femtosecond and picosecond pulse sources reported above need to be further stretched in CPA systems. Based on that fact, mode-locked fiber lasers with high energy and nanosecond-duration output pulses have been developed. Elongating the cavity with normal dispersion fibers has been proposed to provide such pulses. Pulses with duration of 3 ns and 3.9 μJ per pulse energy at 77 kHz repetition rate have been reported [10]. However, the use of bulk components in the laser cavity sacrifices the advantages of fiber laser, such as compactness, stability and maintenance-free operation. All-fiber lasers with 900 ps, 4.3 nJ and 1.7 ns, 0.18 nJ output pulses have been demonstrated [11, 12]. However, in both fiber lasers described above, the single pulse energy is low and still need further amplification in CPA systems. All-fiber lasers with nanosecond high energy pulses have also been reported. Nanosecond pulses with single pulse energy of 281 nJ as well as 10 ns pulses with single pulse energy up to 4 μJ have been achieved in erbium-doped fiber laser and ytterbium-doped fiber laser, respectively [13, 14]. In all these nanosecond fiber lasers, nanosecond pulses are achieved by employing ultra-long length of fibers with low repetition rate.

In this paper, for the first time to our best knowledge, we report on an all-fiber mode-locked nanosecond laser with chirped fiber gratings (CFGs). A pair of CFGs with large dispersion and negligible nonlinearity is used as replacement of a segment of ultra-long fiber in the cavity. This is experimentally proved a possible way to achieve nanosecond pulses with high energy. The pair of CFGs is carefully designed with small negative net dispersion for intracavity dispersion management. The self-starting, stable mode-locking 1.27 ns pulses with large chirp and pulse energy up to 15 nJ at a repetition rate of 3.48 MHz are directly generated after one CFG with large positive dispersion. The laser is helpful in achieving the compact all-fiber CPA system without the stretcher and one or more preamplifing stages.

2. Experimental setup

The experimental setup is shown in Fig. 1 . Two polarization controllers (PCs) and one polarization-sensitive isolator are responsible for NPE effect. Three parts of 1 m ytterbium-doped fiber (absorption coefficient of 192 dB/m at 978 nm, core diameter of 4.4 μm), which are pumped respectively by three 975nm single mode diodes via wavelength division multiplexers (WDMs), are used to offer enough gain for compensating large loss from one pair of CFGs. Total loss of the pair of CFGs and circulators is estimated to be 14.45 dB. The transmission ports of the pair of CFGs as well as one fiber coupler are used as three output ports. The transmissivity of the output 1, output 2 and output 3 ports is 10%, 25%, and 10% respectively. By optimizing polarization states of the two PCs and changing SMF length, self-starting stable mode-locking operation can be obtained. The total length of the ring cavity is 58.6 m corresponding to 3.48 MHz repetition rate.

 

Fig. 1 Schematic diagram of the experimental setup. PC: Polarization controller; ISO: Isolator; CFG: Chirped fiber Grating; WDM: Wavelength division multiplexer; OC: Optical coupler; SMF: Single mode fiber, YDF: Yb-doped fiber.

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The dispersion of CFG 1 and CFG 2 is measured to be −40.91 ps/nm and 43.018 ps/nm at 1055 nm respectively by interferometric method based on Michelson interferometer [15]. The total net dispersion of the ring cavity is estimated to be 0.05577 ps² at 1055 nm, which shows the laser operates in the stretched-pulse mode-locking region [16]. As the circulating pulses in the cavity experience large chirp variation due to large positive and negative dispersion from the pair of CFGs, differently chirped pulses can be achieved from these output ports.

3. Experimental results and discussion

Self-starting, stable mode-locked pulses are easily achieved from the ring fiber laser by adjusting the PCs. The stable pulse train and pulse duration, are measured with a high-speed photodetector with 3-dB bandwidth of 45GHz (Model 1014, Newfocus) and a high-speed oscilloscope (Model 8600, LeCroy) with 6-GHz-bandwidth. Figure 2 exhibits a pulse spacing of 287.4 ns, resulting in a pulse repetition rate of 3.48 MHz. The radio-frequency (rf) spectrum of the pulse from output 2 is measured with a frequency spectrum analyzer (Agilent E4447A), as shown in Fig. 3 . It shows that the laser is mode-locked at a fundamental repetition rate of 3.48 MHz and the fundamental rf spectrum has a signal-to-noise ratio of 70 dB. The spectra are measured via an optical spectrum analyzer (AQ-6315A, ANDO).

 

Fig. 2 Pulse train monitored with a high-speed photodetector.

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Fig. 3 (a) Wideband radio-frequency spectrum, 100 MHz span. (b) The fundamental radio-frequency spectrum.

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As shown in Fig. 4 and Fig. 5 , the pulse durations and corresponding spectra from different output ports at a same mode-locking state are illustrated. Figure 4(a) shows the widest pulse of about 1.27 ns generated from the port of output 2. Single pulse energy up to 15 nJ of this nanosecond pulse is measured at three pump powers of 590 mW, 400 mW and 400 mW respectively. 280 ps, and 340 ps pulses with energies of 10.5 nJ and 0.68 nJ are obtained from output 1 and output 3 respectively. Due to a limited scan range (1.6 ns) of the autocorrelator (Pulsecheck SM, APE) in our laboratory, which the maximum measurable pulse duration is 450 ps, we just measure the autocorrelation trace of the picosecond pulse from output 3 to check the pulse quality, as shown in Fig. 4(b). The clean pulse shape of the autocorrelation trace indicates no wave breaking or multi-pulse operation happened. No coherent spikes are observed in the center of the autocorrelation trace. It confirms that the pulses are generated from pure mode-locking operation rather than a noise burst [17–19]. It is obvious that the pulse is strongly broadened by large positive chirp of CFG1 with positive dispersion and thus can contain more energy without wave breaking.

 

Fig. 4 (a) The 1.27 ns pulse from output 2 detected with a 6 GHz sampling oscilloscope. (b) The autocorrelation trace of the 340 ps pulse from output 3.

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Fig. 5 Spectra of the pulses from the three output ports.

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The spectral shape illustrated in Fig. 5 with two peaks at the edges exhibits typical characteristics of all normal-dispersion fiber lasers [5, 20]. Especially, steep sides of spectra from output 2 and output 3 are clear evidence of filtering effect of the two CFGs. The CFGs have a nearly rectangular reflection bandwidth of about 20 nm (from 1046 nm to 1066 nm, centered at 1056 nm). The filtering effect contributes to pulse shaping and stable mode-locking operation. The spectral peak (located around 1053 nm) of the spectrum from output 2 is lower, compared with that from the other output ports. This is due to the fact that the peak at 1053 nm is located in the reflection band of CFG 2, and thus experiences a relatively low transmission (25%). The other peak at about 1065 nm is just at the edge of the reflection band with more energy transmitted.

In general, in all normal dispersion fiber lasers, for pulse mode-locking operation in ns-scale, an intracavity spectral filter is needed as a pulse shaper to stabilize mode-locking. With the help of long-fiber nonlinearity and strong chirp induced by the normal dispersion cavity, the semiconductor saturable absorber mirror (SESAM) as well as single wall nanotube (SWNT) saturable absorber can perform equivalently to a spectral filter and stabilize the pulse spectrum in the laser cavity, thus achieving stable mode-locking [11, 12]. In our early experiment, only CFG 1 providing large normal dispersion with negligible nonlinearity was employed in the ring cavity and no mode-locking could be obtained. This confirms that the additional nonlinearity of the long-fiber is required to maintain the giant chirped pulse mode-locking. To achieve stable mode-locking without the use of long-fiber, we introduce CFG 2 which provides large negative dispersion to compress the pulse and enhance the intracavity nonlinearity. The compressed pulse with duration of 340 ps is obtained from output 3.

The mode-locking stability is checked via a spectrum analyzer for more than 5 hours. The spectrum profile stays almost unchanged, if the fibers are kept undisturbed and without acute variation of environmental temperature. We believe that the CFGs filtering effect and stretched-pulse mode-locking regime contribute to the stable mode-locking operation. The environmental stability can be improved via polarization-maintaining fiber based laser cavity and real saturable absorbers such as carbon nanotubes.

In the process of experiment, we turned the cavity length from 28 m to 75 m and the net dispersion changed from −0.62332 ps² to 0.41973 ps² at 1055 nm correspondingly. Mode-locking could still be obtained in such a dispersion region. However, as the absolute value of the net dispersion in the cavity increased over 0.13 ps², mode-locking became unstable in our fiber laser. It is believed that stable mode-locking can only be obtained with the balance of dispersion and nonlinearity. It was also found that once stable mode-locking was obtained, the pump powers could be decreased to a relatively low level while maintaining mode-locking. The width of the spectrum as well as pulse duration stayed almost unchanged with the decrease of pump powers.

For the long cavity nanosecond fiber lasers, the intracavity fiber of ultra-long length induces a high nonlinear phase shift in the optical pulse, and may thus lead to wave-breaking or multiple-pulsing phenomena [21, 22]. To reduce the intracavity nonlinearity and maintain stable mode-locking in such fiber lasers, peak power of the pulse should be limited to a reasonable level, which thereby results in relatively low single pulse energy [11, 12]. In our nanosecond fiber laser, by employing CFG with negligible nonlinearity instead of a segment of ultra-long fiber, the intracavity nonlinearity is effectively confined to a lower level and the pulse energy is possible to be further increased. The single pulse energy of the nanosecond pulse reaches 15 nJ. Peak power of the pulse is below the threshold of stimulated Raman scattering (SRS), as confirmed by the output spectrum. It is believed that the pulse energy in our fiber laser can be further increased by injecting higher pump powers or employing CFGs with lower loss.

Strongly chirped pulses have been suggested as suitable for compression by anomalous dispersion compensation [9], and compression has been demonstrated in a short cavity [4]. The time-bandwidth product of the 1.27 ns pulse is estimated to be 1180, which indicates giant chirp. The nanosecond pulse is dechirped partly to picosecond pulse of 340 ps in the cavity mainly by CFG2 of negative dispersion, indicating partial pulse compressibility. However, dechirping such giant chirped pulse outside the cavity requires careful design of compressor and needs further work.

4. Conclusion

In conclusion, we demonstrated a self-starting all-fiber Yb-doped nanosecond laser. The dispersion management was performed by two carefully designed chirped fiber gratings in the ring oscillator. Self-starting, stable mode-locked, 1.27 ns pulses with single pulse energy up to 15 nJ at a repetition rate of 3.48 MHz were directly generated without external pulse stretching or amplification. This is, to the best of our knowledge, the first result of nanosecond pulse generation directly from an all-fiber mode-locked giant-chirped laser with CFGs. This fiber laser may be beneficial to all-fiber CPA systems. The pulse energy and pulse duration could be further increased via employing CFGs with lower loss and larger dispersion. The strong nonlinear effects that lead to wave-breaking may limit the single pulse energy. Further work will be devoted to the compression of the nanosecond pulse with large positive chirp.