We report the experimental observations of vector pulse trapping and scalar dissipative soliton in a compact nanotube-mode-locked all-fiber laser for the first time to our best knowledge. The vector pulse exhibits a smooth Gaussian spectral profile without any sidebands. Although two orthogonally polarized components of the vector pulse have different central wavelengths, they copropagate as a unit in the laser cavity with the same speed. The scalar dissipative soliton shows a rectangular spectrum with pulse duration of ~13 ps, and can be compressed to ~320 fs external to the cavity. This flexible laser provides stable, ultrashort vector- and scalar-pulsed sources, which is convenient and attractive for practical applications.
© 2014 Optical Society of America
Temporal soliton in optical fiber is very attractive for fundamental research and practical application in fields of nonlinear optics, ultrafast laser, and optical communications [1–3], as well as material processing. As single-mode fibers (SMFs) have weakly birefringence due to stresses and bends, the vector feature of the solitons should be considered as they propagate along the fiber . It is found that depending on the strength of linear birefringence, different types of vector solitons, such as group velocity-locked vector solitons (GVLVSs) , polarization-rotating vector solitons , and phase-locked vector solitons (PLVSs) , have been discovered in SMFs. The GVLVSs are also known as soliton trapping, which has been theoretically predicted by Menyuk et al . and then experimentally verified by Korolev et al . It is shown that such a vector soliton can be formed because the two orthogonally polarized components shift their central frequencies in opposite directions through the self-phase modulation and cross-phase modulation .
Optical soliton formation and dynamics in mode-locked fiber lasers have been investigated extensively [11–14]. However, different from the soliton formed in fibers, solitons formed in a laser are dissipative nature [15–17]. The formation is mutual interactions among cavity dispersion, fiber nonlinear effect, laser gain, and loss [18–23]. Various techniques, such as nonlinear polarization rotation (NPR) [24–26], nonlinear optical loop mirror (NOLM) , semiconductor saturable absorber mirror (SESAM) [28, 29], single-walled carbon nanotubes (SWNTs) [30–33], graphene , and graphene-nanotube mixtures  have been utilized to realize passive mode locking. Due to the polarization-insensitive nature, SESAMs have the potential to generate vector solitons. Zhao et al. have observed the soliton trapping in an anomalous-dispersion fiber laser, where the optical spectrum exhibits a double set of sidebands corresponding to two orthogonally polarized components . Currently, the pulse trapping in a SESAM mode-locked fiber laser with near-zero dispersion has been in-depth investigated by Mao et al . However, SESAMs require complex and costly clean-based fabrication systems, and suffer from a low optical damage threshold . Novel mode lockers based on SWNTs and graphene have been widely studied and used in fiber lasers due to inherent advantages of ultrafast saturation recovery time, super-broadband saturable absorption, mechanical and environmental robustness, and easy fabrication process over the SESAMs [30–35, 38]. To the best of our knowledge, the study of pulse trapping in SWNTs-based fiber lasers is rare.
In this paper, we propose a compact nanotube-mode-locked laser that delivers trapped vector pulse and scalar dissipative soliton. The vector pulse exhibits a smooth Gaussian spectral profile without any sidebands. The spectral bandwidth and pulse duration are ~8.5 nm and ~500 fs, respectively. Although two polarization components of the vector pulse have different central wavelengths, they copropagate as a unit in the fiber laser. The scalar dissipative soliton exhibits a rectangular spectrum with a bandwidth of ~12 nm. The duration of the dissipative soliton is ~13 ps, and can be further compressed to ~320 fs external to the cavity. We believe the proposed flexible, compact, low-cost fiber laser can find important applications in the future.
2. Experimental setup
The configuration of the two-color fiber laser is shown in Fig. 1, where the bottom and top parts present the vector pulse laser and scalar soliton laser, respectively. The vector pulse laser has a ring cavity that consists of a piece of 17-m SMF with the group velocity dispersion (GVD) parameter of ~17 ps/km/nm, a polarization-insensitive isolator (PI-ISO) ensuring unidirectional propagation of the signal light, a 10% port of output coupler (OC1) delivering the signal, and a polarization controller (PC1) adjusting the polarization state. The scalar soliton laser is constructed by a piece of 8-m SMF, a polarization-sensitive isolator (PS-ISO), a 10% port of OC2, and a PC2. Both lasers share a segment of 18-m erbium-doped fiber (EDF) with a GVD parameter of ~-16 ps/km/nm, and a packaged SWNT-SA. The fabrication of SWNT-SA film is shown in Ref . The EDF is pumped by a 980-nm laser diode (LD) through a wavelength division multiplexer (WDM). Two optical circulators (CIRs) are used to realize dual ring configuration. An optical spectrum analyzer, a commercial autocorrelator (AC), a digital storage oscilloscope, and a radio-frequency (RF) analyzer are used to monitor the laser outputs simultaneously. The output pulse can be polarization-resolved along two birefringence axes with a polarization beam splitter (PBS) and a PC external to the cavity.
3. Experimental results and discussion
The loss between the vector pulse and scalar soliton fiber laser results from the PCs-induced pressures, bends, and twists in the fiber. When the PC2-induced loss is strong while PC1-induced loss is negligible, light propagates in the cavity from port CIR1→2→3→CIR2→1→2. The length and net dispersion of the cavity are about 35.5 m and −0.001 ps2, respectively, and vector pulse tends to be formed at the bottom ring of the fiber laser. Here, the PI-ISO is polarization-insensitive and ensures free evolution of the pulse polarization. Contrastively, when PC1-induced loss is strong while PC2-induced loss is negligible, light propagates in the cavity from port CIR2→2→3→CIR1→1→2. In this case, the PS-ISO works as a polarizer and defines the soliton polarization at the cavity. The length and net dispersion of the cavity are about 25.6 m and 0.2 ps2, respectively, and scalar dissipative soliton can be easily formed at the top ring of the fiber laser.
Self-started mode locking operation can be established in the laser by adjusting the orientations of the PCs. Figure 2 shows a typical case of the vector pulse measured from OC1 at pump power of 20 mW. The output power of the laser is given as 0.8 mW. The optical spectrum in Fig. 2(a) exhibits a smooth Gaussian profile without any sidebands, which is the typical feature of the stretched-pulse . The central wavelength is ~1560 nm and 3-dB spectral bandwidth is ~8.5 nm. Figure 2(b) shows the corresponding AC trace, which is well fitted by a Gaussian profile, resulting in the pulse duration of ~500 fs. The time-bandwidth product (TBP) of the output pulse is about 0.5, indicating that the pulse is slightly chirped. The minor deviation from the value of 0.45 expected for transform-limited Gaussian pulses may be attributed to uncompensated third-order dispersion . Figure 2(c) shows a typical equally-spaced uniform pulse train, with ~173 ns interval between the two adjacent pulses, thus giving a repetition rate of ~5.78 MHz. It is note that a polarizer is essential for nonlinear polarization evolution (NPE) effect to initiate the passive mode locking [15, 26, 36]. However, no polarization sensitive component is introduced into bottom cavity, which indicates that vector pulse operation is induced by the SWNT-SA rather than the NPE effect.
A PBS and a PC external the cavity are used to resolve the vector pulse. Along the vertical-axis of the PBS, at the appropriate PC orientation, spectrum with maximum intensity and a longer central wavelength can be achieved. Meanwhile, the horizontally polarized pulse is delivered from the other axis of PBS. As shown in Fig. 2(a), the two orthogonal polarization components of vector pulse have almost the same spectral intensity. The slight asymmetry between two spectra may be attributed to unsymmetrical gain spectrum of EDF . We note that the two orthogonally polarized components exhibit distinct central wavelengths. The center of the horizontal polarization component is ~1559 nm, while that of the vertical polarization component is ~1563 nm. Different from the soliton trapping formed in large net negative cavity dispersion regime , the center wavelength separation of the two polarization-resolved spectra is as large as ~4 nm. The corresponding 3-dB bandwidths are slightly different, the horizontal component is ~8 nm and the vertical one is ~7 nm. The insets of Fig. 2(a) show RF spectra of polarization-resolved components. Both fundamental frequency are 5.78436 MHz with signal-to-noise ratios of ~70 dB. The AC traces in Fig. 2(b) show that the two orthogonally polarized pulses exhibit nearly identical intensity. If a Gaussian fit is assumed, the durations of horizontal and vertical pulses are ~600 fs and ~700 fs, respectively. The polarization feature of the vector pulse is further reflected in Figs. 2(d) and 2(e). In this case, the oscilloscope traces of two polarization-resolved pulses are uniform without any modulation, which is the typical characteristics of group-velocity locked vector pulse . These experimental results suggest that the pulse trapping can be attributed to the dynamic balance among fiber birefringence, GVD, and frequency shift. The product of the frequency shift and cavity dispersion must be moderate to compensate the polarization dispersion induced by the fiber birefringence. Therefore, for a certain birefringence, the frequency shift should be much larger than that in the large anomalous regime. However, the frequency shift cannot be increased infinitely due to the limitation of the gain bandwidth. The result is different from dual-wavelength mode-locked pulses that have two independent repetition rates . Carefully adjusting the orientation of the intra-cavity PC, the two orthogonal polarization components with difference of power can also be obtained.
By adjusting the orientation of the intracavity PC2, the scalar dissipative soliton is obtained from OC2 by fixing the pump power, as shown in Fig. 3. The output power of the laser is given as 1 mW. The spectrum of dissipative soliton exhibits characteristic steep spectral edges. The spectrum of the dissipative soliton is centered at ~1563 nm with a bandwidth of ~12 nm. From the measured AC trace, the pulse duration is estimated as ~13 ps by using a Gaussian fitting. Therefore, the TBP is calculated as 19, indicating that the dissipative soliton is strongly chirped. The pulse can be further compressed to ~320 fs by exploiting a 20-m-long SMF. The compression operation is similar to our previous report in detail . The corresponding TBP is given as ~0.47, which is close to the value of the transform limit. The dissipative soliton has identical pulse intensity with ~125 ns interval between the two adjacent pulses on the oscilloscope trace. After resolved by the PBS, the spectra intensity difference between the two orthogonally polarized components is larger than 15 dB, which indicates that the dissipative soliton after the PS-ISO is a scalar pulse, as shown in Fig. 3(a). The intensity difference of RF spectra in Fig. 3(d) agrees well with that of the optical spectra. Although we expect the SWNT to dominate the starting and stabilizing of the pulse, the operation of the top laser is sensitive to the setting of the PC. This suggests that NPE also plays some role in the pulse-shaping.
We have proposed a vector- and scalar-pulse source emitted from a compact SWNT-based mode-locking fiber laser for the first time to our best knowledge. The vector pulse is characterized by the smooth Gaussian spectral profile without any sidebands. The bandwidth and duration of the vector pulse are ~8.5 nm and ~500 fs, respectively. Two orthogonal polarization components of vector pulse have distinct central wavelengths while copropagating as a unit in the fiber laser. Contrastively, the scalar dissipative soliton exhibits a rectangular spectrum with the bandwidth of ~12 nm. The duration of the dissipative soliton is ~13 ps, and can be further compressed to ~320 fs external to the cavity. This flexible all-fiber-based laser can provide stable, ultrafast vector- and scalar-pulses which is useful for various applications.
This work was supported by the National Natural Science Foundation of China under Grants 10874239, 10604066, 61223007, and 11204368.
Corresponding author (X. Liu). Tel.: + 862988881560; fax: + 862988887603; electronic mail: firstname.lastname@example.org and email@example.com.
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