We demonstrate the compression of noise-like pulses in an Yb-doped fiber master-oscillator power-amplifier (MOPA). The seed source of the MOPA is an NPR mode locked fiber laser delivering 5.94-ps dissipative soliton pulses with a repetition rate of 37.48 MHz. After amplification in the Yb-doped fiber amplifier, stable noise-like pulses with maximum power of 5 W are obtained. Subsequently a grating pair is used to tailor the spectrum and compensate the dispersion of the amplified noise-like pulses. The pedestal of de-convolution autocorrelation trace is compressed from 6.5 ps to 920 fs. To the best of our knowledge, this is the first time that the pedestal of a noise-like pulse is compressed to femtosecond region.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Mode locked ultrashort pulses can be used in various research fields. In the past decades, researchers have investigated different mode-locking regimes extensively, such as solitons, similaritons, and dissipative solitons (DSs). Due to the limitation of the mode locking mechanism, single pulse energy is usually limited, thus many research efforts have been made to obtain high single pulse energy. Then noise-like pulse (NLP) has attracted much attention for its potential to obtain high-energy ultrafast pulse since it was firstly discovered by Horowitz et al. experimentally in 1997 .
Many factors can lead to the formation of NLPs, including fiber birefringence , nonlinearity instability , soliton collapsing , or the peak power clamping effect . NLPs can be considered as a large pulse pedestal composing by a number of sub-pulses with different phases and amplitudes. Fine structures of NLPs randomly vary with power . An NLP typically has a broad spectrum and its autocorrelation trace has a unique shape, which has a thin, high spike on a broad pedestal. This is also a common method to judge the NLP. Recent experimental results have shown that the pedestals and spikes in the autocorrelation correspond to the pulse width of the NLP pedestal and the average pulse width of the pulse within the pedestal, respectively. The ratio of peak to pedestal is related to the density of pulses in fine structures . Furthermore, NLPs have low temporal coherence compared to other mode-locked pulses.
For these characteristics, NLPs have important applications in the fields such as supercontinuum generation [7–10], material micromachining [11,12], and low-intensity coherent interference [13,14]. Typical method to obtain NLP is a mode-locked fiber laser with the nonlinear polarization rotation (NPR) technique. After adjusting the wave plate angle, the transmittance of the NPR will be changed. Once most of the instantaneous power of the pulse enters the reverse saturable absorption region of the NPR, the NLPs can be easily obtained [1,3–5,15–21]. Besides, there are many ways to generate NLPs, such as mode-locked fiber lasers based on a nonlinear optical loop mirror [22–24], nonlinear amplifying loop mirror [25,26], SESAM , and other saturable absorbers [28–30]. One research interest of NLPs is to obtain narrower pulses, thus a narrower pedestal pulse width is desirable.
We have demonstrated that NLPs have compressible characteristics and obtained the narrowest 14.5 fs spike (the pedestal is 3.7 ps) in experiment previously , which is close to the smallest autocorrelation pedestal of 2.1 ps  at 1 μm. In this paper, we improve the performance the system. An NPR mode-locked fiber laser is used to generate dissipative-soliton pulses as seed, and then is injected into an Yb-doped fiber amplifier. As a result, high-power, noise-like pulse is obtained. The NLPs are compressed by a grating pair. By optimizing the gratings’ separation and pump power, we finally generated a shortest width of 920-fs pedestal (62-fs spike). The single pulse energy is approximately 130 nJ at a repetition rate of 37.5 MHz. To the best of our knowledge, this is the first time that the pedestal of a noise-like pulse is compressed to femtosecond region.
2. Experimental setup
The experimental setup is shown in Fig. 1. The laser is composed of three stages: an Yb-doped fiber laser, an Yb-doped fiber amplifier, and a grating pair compressor. Figure 1(a) shows the seed source: an NPR mode-locked, all-normal-dispersion Yb-doped fiber laser, which is pumped by a single-mode laser diode (LD) with maximum output power of 600 mW. In the cavity the length of fiber is about 5 m and the space length is 20 cm. The birefringence filter has a 3-dB spectral width of 12 nm and the output coupler (OC) uses a 30% coupling output. Figure 1(b) shows the schematic of the nonlinear Yb-doped fiber amplifier pumped by an LD with output power up to 9 W. The gain fiber is a piece of 5.3-m double-clad Yb-doped fiber (LMA-YDF-10/130 VIII, Nufern). The pump geometry is forward-pump in order to generate noise-like pulses more easily after the injection of seed pulses. Figure 1(c) demonstrates a grating pair with groove density of 1000 grooves/mm (LSFSG-1000-1010-94, LightSmyth) used to compress the amplified noise-like pulses.
3. Results and discussions
The oscillator can output alternatively dissipative soliton or noise-like pulse by adjusting the rotation angle of the space devices (half-wave plate, quarter-wave plate and birefringent filter). Since it has been confirmed that dissipative soliton pulses can evolve into noise-like pulses in a fiber amplifier , and in our experiment the state of dissipative solitons is more stable than that of noise-like pulses, thus dissipative soliton pulses are selected as the seed source and injected into the all-fiber Yb-doped fiber amplifier. In order to ensure the mode-locking stability and large output power of the seed, the pump power of oscillator is optimized and set as 400 mW for the self-starting of each boot while meeting the requirements. The output power of seed source is 42 mW. The output characteristics are measured and the results are show in Fig. 2. Figure 2(a) shows the spectrum of the seed pulse and the center wavelength is 1029.2 nm, and the 3-dB spectral width is 14.5 nm (YOKOGAWA-AQ6370C). The mode locking is dissipative soliton mode-locking for the spectrum has two typical Kelly sidebands. Figure 2(b) is the RF spectrum of the seed laser (Agilent E4447A), and the repetition rate is deduced to be 37.48 MHz, which agrees well with the cavity length. The signal-to-noise ratio (SNR) is about 85 dB, which shows the mode locking is very stable. Figure 2(c) is the autocorrelation trace of the seed pulse, and the pulse duration is 5.94 ps.
The seed pulse is injected into an all-fiber Yb-doped amplifier, noise-like pulses are generated, and the evolution has been explained theoretically and confirmed in experiment . The output power of amplified pulses was measured (Newport 1916-C) and result is shown in Fig. 3. The maximum output power is 5.06 W at the pump power of 9 W with a slope efficiency of about 58%, which demonstrates a high conversion efficiency of the amplifier system. The corresponding single pulse energy is calculated to be 135 nJ.
Since the output spectrum is related to pump power in a nonlinear amplifier, we measure the spectrum of output pulses at pump power from 1W to 9 W and Fig. 4 shows the results. It is evident that Raman phenomena appear in the spectra, due to the nonlinear effect in the pulse with the pump power increasing. When the power exceeds 4 W, pulse energy is mainly concentrated on the first-order Raman spectrum (near 1072 nm). When the pump power is larger than 6 W, the second-order Raman spectrum (near 1134 nm) appears. When pump power is about 7 W, the first-order Raman spectrum is strongest. When the pump power exceeds 7 W, second-order Raman spectrum is enhanced significantly and still grows with the increase of pump power.
The amplified NLP is then injected in to a grating pair as shown in Fig. 5, where grating2 is placed in a two-axis translation stage so that it can be finely translated in both directions. Since diffractive spectrum is very wide beyond the capacity of grating, thus part of the spectral components has to be abandoned. Thus the grating pair can be used both as a spectral filter and compressor. Since second-order Raman spectrum is more difficult to compress compared to first-order Raman spectrum, we deliberately select the first-order Raman spectrum for compression and try to filter the second-order Raman spectrum out by adjusting the position the grating2.
The dependence of spectrum on pump power as shown in Fig. 4 displays that the intensity of first-order Raman spectrum is largest at the pump power between 7 W-8 W, and the corresponding spectral width of the first-order Raman is widest. Therefore, the pump power of the amplifier is set 7 W-8 W at the interval of 0.2 W respectively, and the amplified NLPs are spectrally filtered and compressed by finely tuning the separation and position of grating2. The corresponding autocorrelation traces are recorded by an autocorrelator (FR 103-XL, FEMTOCHROME). When pump power is fixed in the experiment, the autocorrelation trace changes evidently by adjusting the separation between Grating1 and Grating2 attributed to the change of dispersion introduced by the grating pair. There exists an optimum separation to obtain the shortest pulse width, and the pulse width would increase with the deviation from the optimum separation. The shortest autocorrelation traces with respect to pump powers are shown in Fig. 6, which exhibits typical autocorrelation shape of NLPs, i.e. a pedestal with a sharp spike. When the pump power is 7.4 W, the noise-like pulse has a 920-fs pedestal and 62-fs spike, which is the shortest pedestal to date for noise-like pulses near 1 μm. It is noteworthy that the spike’s pulse width of 62.3 fs is larger than previous report of 14.5 fs . The spike’s pulse width corresponds to the overall spectrum. However, grating2 cuts off partial spectra components and leads to a narrower spectrum. As a result the spike is stretched accordingly. Thus there is a tradeoff in the compression of pedestal. And the experiment confirms that with fine spectral filtering and dispersion compensation, noise-like pulse can be compressed to femtosecond region.
To confirm that 7.4 W is the most appropriate pump power, we measured the spectra after compression at pump power of 1 W-9 W and the results are shown in Fig. 7, which shows that when pump power is less than 7 W, the compressed spectral width is narrow. When the pump power exceeds 8 W, second-order Raman begins to generate, and the energy starts to concentrate on the second-order Raman spectrum. When the pump power is between 7 W-8 W, the spectrum has a relatively broader FWHM of first-order Raman spectrum and second-order Raman effect is relatively weak, so the pump power between 7 W-8 W is the most suitable for the NLPs’ compression. Since the experiment mainly compresses the first-order Raman spectrum, the unfavorable second-order Raman components can be further filtered by optimizing position of the grating.
The autocorrelation at pump power of 7.4 W before compression is shown in Fig. 8, which shows that the pedestal width of the NLP before compression is 6.5 ps and the spike width is 98 fs. Compared with the pedestal width of 920 fs and spike width of 62 fs after compression, the pulse width is clearly shortened, which proves that with fine tuning of the grating pair, noise-like pulses can be compressed to femtosecond domain. Note that the compression ratio of pedestal and spike is not equal since the spike corresponds to wider spectrum than the pedestal. In experiment, only limited spectral range can be dispersion compensated ideally, thus the pedestal corresponding to narrower spectrum can meet the requirement more easily than the spike. So it is more difficult to compress the spike than the pedestal and their compression ratio is not equal.
The RF spectrum after compression at pump power of 7.4 W is shown in Fig. 9. The repetition rate of 37.48 MHz is the same as the oscillator and SNR is about 78 dB, slightly lower than that of oscillator, which is possibly attributed to the influence of anomalous nonlinear frequency and amplified spontaneous emission in the amplification. However, the long-term radio-frequency spectrum with span of 1 GHz indicates that the compressed pulsed output is very stable.
The output power after compression is also measured as shown in Fig. 10. The maximum output power of femtosecond NLPs is 1.55 W at pump power of 9 W with corresponding pulse energy of 41 nJ. It should be noted that the output power after compression is significantly smaller than before compression attributed to three factors. The first factor is the considerable diffractive loss of the grating. The second factor lies in the fact that a number of nonlinear frequencies are filtered by the grating for favorable compression. The third factor is the reflective loss introduced by the collimating mirrors with limited bandwidth in the compressor. However, the output power of 1.55 W is enough to meet the requirements of supercontinuum generation, material micromachining and other relative applications.
High-power NLPs are generated in an Yb-doped fiber master-oscillator power-amplifier. A fiber laser as seed delivers dissipative soliton pulses with pulse width of 5.94 ps and repetition rate of 37.48 MHz. After amplification in the Yb-doped fiber amplifier, stable NLPs are generated with maximum output power of 5 W at pump power of 9 W. The NLP has a pedestal width of 6.5 ps and spike width of 98 fs, and the single pulse energy is 135 nJ. The amplified NLPs are compressed by a grating pair used both as spectral filter and compressor by adjusted the position of the second grating. The autocorrelation trace of compressed pulse shows a 920-fs pedestal width and 62.3-fs spike, which demonstrates the compressibility of noise-like pulses. To the best of our knowledge, this is the first time that the pedestal of an NLP is compressed to femtosecond region near 1 μm. The output power of compressed NLPs is 1.55 W at pump power of 9 W, corresponding to pulse energy of 41 nJ. We believe this femtosecond noise-like pulsed laser can be applied in supercontinuum generation, material micromachining, low-intensity coherent interference and other relative research fields.
National Natural Science Foundation of China (NSFC) (Grant No. 61575011) and 973 Program (Grant No. 2013CB922404).
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