We have experimentally demonstrated square pulses emission from a co-doped Er:Yb double-clad fiber laser operating in anomalous dispersion DSR regime using the nonlinear polarization evolution technique. Stable mode-locked pulses have a repetition rate of 373 kHz with 2.27 µJ energy per pulse under a pumping power of 30 W in cavity. With the increase of pump power, both the duration and the energy of the output square pulses broaden. The experimental results demonstrate that the passively mode-locked fiber laser operating in the anomalous regime can also realize a high-energy pulse, which is different from the conventional low-energy soliton pulse.
© 2016 Optical Society of America
Passive mode locking continues to be one of the most dominant fields of interest in generating ultrashort pulses. From the generation of harmonic pulses to soliton rain, bunches and crystals, different scenarios of soliton interaction and patterns have been studied [1–3]. In net anomalous dispersion regime, the relationship between dispersion and nonlinearity is obvious and the theorem of soliton area plays an important role in the balance of the cavity . It is thus of great importance to find effective ways to maintain a high pulse energy on the output.
On the other hand, optical pulses shaping have attracted considerable research attention for their practical applications in the fields of optical communications, optical sensing systems, ultrafast optics, and material processing. Different kinds of pulse shape, such as Gaussian pulse, sech-like pulse, parabolic pulse, and square pulse have been generated to satisfy different demands. More precisely, the square pulse can be used for potential applications including optical square-wave clocks, optical sensing and laser micromachining [5,6]. Square-wave pulses can be generated thanks to the dissipative soliton resonance (DSR)  and are of particular interest since their energy is not limited by the soliton area theorem.
The first theoretical study of the dissipative soliton resonance phenomenon was developed in 2008 with resolving the complex cubic-quintic Ginzburg-Landau equation . This theoretical model predicts that the soliton energy increases indefinitely with specific parameters while keeping the peak power stable at a certain level. Different operating regimes were explored to increase the pulse energy. The normal dispersion regime favors large pulse energy compared to the anomalous dispersion regime . Another theoretical study shows the existence of the high energy pulses in lasers operating in the anomalous dispersion . Moreover, the energy of the square pulse increases proportionally with the pumping power which allows the pulse duration to widen linearly with pump power. The square-wave pulses were observed in normal and anomalous dispersion . In anomalous dispersion regime, the pulse contradicted the property of the quantization of energy wherein the pulse energy is limited to about 0.1 nJ . The first experimental observation of high energy pulses in ring fiber laser operating in the anomalous dispersion was observed in 2012 . The DSR pulses exhibited an energy of 32 nJ. It has been shown experimentally that the high non-linearity plays an important role in the widening pulse . In fact, the pulse duration changes from 6 ns to 33 ns when the pump power was increased from 70 mW to 365 mW and the square pulses had an energy of 360nJ inside the cavity. The existence of DSR pulses and the transition from sech-like pulse to square pulse by increasing the pump power has been demonstrated in a net anomalous dispersion regime . The pulse duration changes from 0.54 ps to 68.2 ns when the pump power was increased from 31 mW to 350 mW. Similar DSR pulses were demonstrated in Yb-doped fiber lasers but under low pumping power resulting in low energy square pulses . Recently, the existence of high energy dissipative soliton square pulses around 2.13 µJ has been observed in a double clad co-doped Er:Yb mode-locked all fiber figure eight cavity operating in net anomalous dispersion regime using a 3-way circulator to export all the energy from the cavity .
In this paper, we experimentally demonstrate high output energy DSR pulses from a double-clad Er:Yb co-doped fiber laser passively mode locked through nonlinear polarization evolution.
2. Experimental setup
For years, long fiber cavities interested in experiments to generate high energy pulses in normal dispersion regime. It has also been used to investigate the nonlinear effect in the generation of high energy DSR pulses in anomalous dispersion regime . The laser setup is shown in Fig. 1. It is based on a simple all fiber ring cavity configuration. To achieve high gain and record output power, we use a C-band double clad V-groove Er:Yb 10 W fiber amplifier from Keopsys. The maximum achievable output power in continuous lasing operation is 10 W, ensured by the 40 W pumping power. Several laser diodes operating at 980 nm pump a 5m-long double-clad fiber that has a second order dispersion of −0.021ps2/m. To ensure large anomalous dispersion together with large pulses duration, we add a piece of 505 m single mode fiber (SMF) so that the net cavity dispersion in the anomalous regime is about −11.872ps2. The total cavity length is about 536 m, including 528 m of SMF with a second order dispersion of −0.022ps2/m. The round trip time of the cavity is 2.681 µs corresponding to a free spectral range of 373 kHz. Two polarization controller (PC) were used with a polarization sensitive isolator to control the nonlinear losses. A polarization-insensitive isolator (PI-ISO) was employed to force unidirectional operation of the cavity. A 10/90 output coupler is used to extract 10% of the power from the cavity. Let us note that the output fiber coupler suffers from large insertion losses of 7 dB which greatly lowers the output pulse energy. The 10 W amplifier requires a threshold input signal to start. This is a security requirement for protecting the internal optic and electronic circuitry. The external optical signal is delivered by a homemade continuous wave tunable fiber laser with 800 mW maximum output power , and is directly injected in the cavity through a 50/50 fiber coupler. Let us mention that once the 10 W laser is operating, the external laser is switched off. In addition, the 10 W amplifier has 2 distinct operating regimes according to the datasheet: the low power regime which requires a minimum input of 17 dBm and a high power regime for which the gain of the amplifier must be below 13 dB. The output intensity is measured using a high-speed photodetector (TIA-1200), and visualized with a fast oscilloscope (Tektronix TDS 6124C, 12 GHz, 40 GSa). Also the output power is measured using a high power wattmeter detector (Coherent Molectron PM500AD). The spectral properties are analyzed with an optical spectrum analyzer (Anritsu MS 9710C) and the pulse duration is measured with an optical autocorrelator with a scanning range of ± 100 ps (Femtochrome FR-103 XL). An electronic spectrum analyzer (Rohde & Schwarz FSP Spectrum Analyzer 9 kHz to 13.6 GHz) is used to characterize the radio frequency spectrum of the laser.
3. Experimental results and discussion
While we visualize the optical spectrum, temporal trace and the RF spectrum, we monitor the output power using the wattmeter. The nonlinear polarization evolution technique is used to obtain the passive mode locking. By carefully adjusting the polarization controllers we retrieve mode-locked pulses at a pumping power of Ppump = 4W showing a conventional mode locking with sech pulse profile. After having the laser stabilized in mode locked regime, we then slightly adjust the paddles of the polarization controller and by increasing the pump power to 4.8 W, we obtain DSR regime. If the polarization controllers are altered during the experiment, we observe other regimes such as multiple solitons  or multiple square pulses. Figure 2(a) shows the optical spectrum trace of the generated square pulses under 5.4 W of pumping power. Such two peaks spectrum has been previously observed in DSR regime but no physical explanation was given . The first band is centered around 1566 nm with 8.5 nm of spectral bandwidth at −3 dB while the second band is centered around 1618 nm with 7 nm of spectral bandwidth at −3 dB. The spectral distance between the central wavelengths is 45 nm. The noise appearing in the wings of the spectrum is linked to the analyzer optical sensitivity characteristics. The autocorrelation trace of the square pulse not represented here, points out a constant level because of the low scanning range compared to the pulse duration.
Figure 2(b) depicts the temporal trace of the generated pulses. We notice a peak in the leading edge of the pulse. This behavior has been noticed before but no further investigation has been made in order to understand its nature . As expected the pulse has a square-shape with a duration of 34 ns. In order to confirm the existence of only one square pulse and that we did not record the envelop of a bunch of short pulses, we have verified that there isn’t a coherent peak with a large pedestal and no fine structure in the autocorrelation spectrum. In addition, our detection apparatus allows us to resolve about 75 ps which is sufficient to visualize any bunch of solitons. According to Fig. 2(c), the RF spectrum shows a signal to noise ratio above 50 dB. The inset exhibits a characteristic modulation with a period of 28 MHz corresponding to the duration of the generated pulses for 5.4 W of pumping power.
By fixing the polarization controllers, the square-wave pulse is stretched with the increase of the pump power while the peak power remains nearly constant. The evolution of the pulse duration and pulse output energy for fixed polarization controllers is presented in Fig. 3. We notice that the pulse duration and the pulse energy can be quasi-linearly increased with the pumping power without significantly affecting the peak power.
In fact, the graphs in Fig. 3(a) and Fig. 3(b) can be divided into 3 regions. For Ppump ≤ 7W the amplifier operates in the low power regime and the evolution of both the pulse duration and energy is nearly linear. For 7 ≤ Ppump ≤ 26W the amplifier operates in high power regime for which the evolution is still linear but with a different slope. Finally, for Ppump ≥ 26W although there is no modification of the operating regime of the amplifier, there is the emergence of a CW component which requires an adjustment of the polarization controllers to remove it. Such readjustment leads to a variation of the nonlinear losses and then to a modification of the pulse characteristics in particular the peak power.
The highest achieved pulse energy is 2.27 µJ and is connected to the fiber coupler used as the output port of the laser. This study is done with a 10/90 output coupler that has a significant 7dB insertion losses coupled to fiber connectors that have around 7.6 dB of insertion losses. Also after this coupler we use a second 30/70 coupler to measure 30% of the output power with a wattmeter. Taking into account all these additional losses occurring after the laser output, our pulse output energy would be as high as 6.5 µJ. So by optimizing the fiber components the output energy will be multiplied by a factor of about 3. The achieved generated square-wave pulses are tuned in a range from 28 ns to 190 ns as shown in Fig. 4. We notice that the peak present in the square-wave pulse as mentioned before under low pumping power, disappears while we increase the latter power.
At a maximum pump power of 30 W, the laser generated a 190 ns square pulse with an output power of 850 mW. At a repetition of 373 kHz, the output pulse energy is 2.27 µJ with 11.5 W of peak power. This is by far the highest output pulse energy ever reported in a double clad Er-doped fiber laser passively mode-locked through nonlinear polarization evolution. It is worth to mention that during our experiment there was no pulse breaking, nor pulse bunching or higher harmonic operation till we passed the 26 W threshold of pump power. In fact, a further increase in the pumping power (above 30 W) destabilizes the system and we observe wave breaking even by carefully readjusting the polarization controllers. This is not in agreement with theoretical predictions .
The evolution of the optical spectrum at various pumping power is shown in Fig. 5. The spectral intensity increases slightly with the pump power while the 3 dB bandwidths of the spectra remain nearly invariable. By increasing the pump power while fixing the polarization controllers under the stable mode-locking operating regime, no significant change in the optical spectrum is observed.
In conclusion, we have experimentally generated a 2.27 µJ DSR pulse with a square wave profile in a fiber laser cavity operating in a net anomalous dispersion regime. The experimental setup is exploiting the nonlinear polarization evolution technique with a 536 meter long and with the use of a co-doped Er:Yb double clad laser. While pumping at a power of 29 W, the laser generated 2.27 µJ pulses that have an average output power of 850 mW, 373 kHz repetition rate and a peak power of 11.5W. The pulse duration can be tuned within a 168 ns range, and with the output energy, they both increase linearly as evidence of the dissipative soliton resonance in the net anomalous dispersion regime. The insertion losses of the 10% output coupler with the 30% coupler have a big impact on the measured output power, and by carefully optimizing the cavity losses we could multiply the energy pulse by a factor of about 3, reaching a very high energy pulse of 6.5µJ.
This work has been partially supported by grant from the National Natural Science Foundation of China (11504081).
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