We report the generation of tunable single-, switchable and tunable dual-, and stable triple-wavelength dissipative solitons (DSs) in an all-normal-dispersion mode-locked Yb-doped fiber laser based on a graphene-oxide saturable absorber (GOSA) without additional components (such as optical filter, or fiber grating). The tunable single-wavelength DS have a wide wavelength-tunable range of 16.4 nm. The dual-wavelength DSs not only have a wavelength-tunable range (about 10 nm) but also have variable wavelength spacing (3.8-13.8 nm). The formation dynamics of the triple-wavelength DSs was also investigated experimentally. The different operations of tunable single-, switchable and tunable dual-, and stable triple-wavelength DSs depend on the strength of the cavity birefringence. The simple, compact all-fiber DS laser with lasing wavelength tunability and flexibility can meet great potential for applications.
© 2014 Optical Society of America
Wavelength-tunable and wavelength-switchable passively mode-locked pulsed fiber lasers have attracted much attention due to their potential applications in optical signal processing, biomedical research, fiber sensing, spectroscopy, and wavelength-division-multiplexed (WDM) optical fiber communication systems. Several methods have been used for achieving tunable and switchable multi-wavelength passively mode-locked fiber lasers. Nonlinear polarization rotation (NPR) techniques [1, 2] and nonlinear optical loop mirror (NOLM) [3, 4] have been utilized to achieve wavelength-tunable and switchable operation in fiber ring cavities. It has also been reported that tunable and switchable wavelength passively mode-locked fiber lasers can be obtained based on optical filters [5, 6] and on polarization-maintaining fiber (PMF) [7, 8]. However, the above mentioned tunable and switchable multi-wavelength mode-locking were achieved together with other additional components (such as optical filter, fiber grating, or PMF), which strongly destroy the simple and compact structure of the fiber laser. Recently, the real and novel saturable absorbers (SAs) such as the semiconductor saturable absorber mirrors (SESAMs) [9, 10] and single wall carbon nanotubes (SWNTs) [11, 12] have been utilized to achieve passive mode-locking with multi-wavelength output. Since the first report of graphene-based ultrafast mode-locked fiber laser in 2009 , the research on passively mode-locked fiber laser with graphene as SA is booming [14–18]. Certainly, the tunable single-wavelength and stable triple-wavelength DSs operation based on graphene SA also have been obtained. Zhang et al have reported a tunable single-wavelength erbium-doped DS fiber laser with atomic layer graphene as SA [18, 19]. In addition, the stable triple-wavelength DSs and four-wavelength mode-locking have been generated in fiber lasers based on a graphene-deposited tapered fiber device [19, 20]. Nevertheless, the wavelength tunability and wavelength-spacing changeability of multi-wavelength mode-locking were still need to be improved.
In the paper, we report a simple, compact and all fiber mode-locked Yb-doped fiber laser based on a graphene oxide (GO) SA in all normal dispersion with flexible outputs: tunable single-wavelength, switchable and tunable dual-wavelength, and stable triple-wavelength DSs operations. The induced cavity birefringence through over bending the single mode fibers is responsible for the multi-wavelength filtering. We show that depending on the strength of the cavity birefringence, tunable single-wavelength, switchable and tunable dual-wavelength, and stable triple-wavelength DSs can be formed in the laser. No multi-wavelength DSs mode-locked Yb-doped fiber laser in all normal dispersion based on real SA (like SESAMs, SWNTs, and graphene) has been reported. The simple, compact and all-fiber structure of the DS fiber laser with flexible outputs can meet diverse application needs.
2. Sample preparation and experimental setup
As a SA of graphene derivatives, GO not only has all the characteristics of ultrafast recovery time and broadband saturable absorption, but also much easier and cheaper to obtain. It has been proved that GO is comparable to graphene as an SA . The fabrication method of GO-based SA is called vertical evaporation, and the fabrication process was similar to our previous works [22, 23]. This process can be simply described as follows. Some chemical oxidized graphite was ultrasonically agitated for obtaining GO sheets. Then the prepared GO sheets were poured into ~10 ml 0.1% sodium dodecyl sulfate (SDS) aqueous solution, following by more than ~10 h of ultrasonic agitation and centrifugation. Next, some polyvinylalcohol (PVA) power was poured into the GO solution and ultrasonically agitated for ~3 h at ~90 °C. The final procedure was vertically evaporating the GO/PVA solution lasting for more than ~40 h at ~40 °C. The finally prepared samples are shown in the inset of Fig. 2. It clearly shows that both the concentration and the thickness of the samples decrease from bottom to top, which provides different modulation depths in the laser cavity. The thickness information can be further measured by using a commercial digital micrometer. In detail, the GO exhibits optional thicknesses from 20 to 120 μm. We choose the middle part and cut into a suitable film piece with:1 × 1 mm2 area and:70 μm thickness. The corresponding Raman spectrum and wavelength-dependent transmission spectrum have been measured by the second author Yonggang Wang and can be found in Ref . The wavelength-dependent transmission is a typical feature of GO and was already observed [21, 23]. The phenomenon might be caused by the presence of functional groups containing oxygen, which may absorb shorter wavelengths more possible. The saturable absorption property of the GO was shown in Fig. 1.The laser source used was homebuilt mode-locked oscillator with 5MHz repetition rate and 150 ps pulse duration operating at about 1064 nm. By fitting the curve with the following formula24, 25], for stable tunable mode-locked lasers, the higher modulation depth for SA is required. The results of GOSA give a saturation intensity of ~13.01 MW∕cm2, modulation depth of ~25.31%, and nonsaturable loss of ~43.35%. Both the modulation depth and the nonsaturable loss of the GOSA are lager, which is attributed to the fact that the thickness and the concentration of our GOSA can be chosen flexibly.
The experimental setup of the tunable and switchable multi-wavelength DSs fiber laser is shown in Fig. 2.It has a ring cavity configuration made of pure normal dispersion fibers. A piece of 3.5 m Yb-doped fiber (YDF), which is pumped by a 976 nm laser diode (LD) through a wavelength division multiplexer (WDM) has absorption of 250 dB/m at 975 nm, group velocity dispersion (GVD) of 27.5ps2/km at 1060 nm. The pump LD can supply up to 500 mW optical power. A polarization independent isolator (ISO), placed after the YDF, is used to ensure unidirectional operation and eliminate undesired feedback from the output end facet. A 10/90 fused fiber optical coupler (OC) is used to extract ~10% energy from the cavity for signal detection. A polarization controller (PC) is for matching the polarization states. The prepared GOSA sample is placed between the PC and the WDM coupler. Apart from these components, ~10 m length of SMF-28 is incorporated in the ring cavity just after the 10/90 OC. The GVD parameter of SMF-28 is ~17.7 ps2/km at 1064 nm. The total cavity length is about 14.4 m, and the net dispersion of the laser cavity is estimated about ~0.289 ps2.
In weakly birefringent cavity fiber lasers, where the cavity birefringence induced artificial birefringence filter has a large bandwidth, and the filtering effect of that can normally be ignored. In our experiment, we intentionally introduced strong cavity birefringence into the laser cavity through over bending the 10 m SMF-28 into a loop on a spool that the birefringence induced filtering effect of the cavity becomes ignorable no longer. The diameter of the loop is about 4.5 cm. When rotating the PC in the cavity, the effective gain peak of the laser will be shifted, this eventually leads to the tunable and switchable multi-wavelength DSs operations.
The output laser is monitored by an optical spectrum analyzer (Yokogawa AQ6370B), a radio-frequency (RF) spectrum analyzer (Tektronix RSA3303B), and a 6-GHz digital storage oscilloscope (DSO) together with an 11-GHz photodetector (PD).
3. Results and discussion
In the experiment, when the pump power was increased to about 90 mW, the single-, dual-, triple-wavelength CW lasing operation can be easily obtained by changing the PC. Single-wavelength mode-locking output of DS can be initially achieved at an incident pump power of about ~110 mW, the stable state could be maintained to the pump power of ~260 mW. Figure 3 shows the output characteristics of a typical single-wavelength mode-locked DS state obtained under 204 mW pump power. Figure 3(a) is the spectrum of single-wavelength DS. The output spectrum is centered at 1059.7 nm. The spectrum exhibits steep edges, which is the typical feature of DS in all-normal-dispersion fiber lasers . The spectral edge-to-edge bandwidth is 1.95 nm. The corresponding pulse duration is 340 ps, as shown in Fig. 3(b). The time-bandwidth-product is 177.12, indicating that the pulses have large chirps in the cavity. Figure 3(c) shows the oscilloscope trace, and the corresponding RF spectrum is shown in Fig. 3(d) with a span range of 650 MHz. The inset of Fig. 3(d) correspondings to the fundamental frequency with a signal-to-noise ratio (SNR) of high than 65 dB. It is clear that the repetition rate of the pulse train is about ~14.20 MHz, corresponding to the cavity round-trip time of ~70.39 ns. The average output power is about ~2.1 mW, corresponding to the single pulse energy of about ~147.9 pJ. Owing to the tunable characteristic of the cavity birefringence fiber filter, the lasing wavelength of the single-wavelength mode-locked DS can be continuously tuned just through adjusting the PC. The tuning results with tuning wavelength range of 16.4 nm from 1051.8 nm to 1068.2 nm referring to center wavelength of the pulses are shown in Fig. 4.During the whole tuning range, the spectral bandwidth of the pulse duration had not very remarkable variation.
Keeping the pump power fixed, dual-wavelength DSs can also be obtained from our laser by rotating the PC. Figure 5 shows the output characteristics of a typical dual-wavelength mode-locked DSs state. Figure 5(a) is the typical spectrum of dual-wavelength DSs. The center wavelengths of the two separated steep-edge spectra are 1058.5 nm and 1065.7 nm, respectively. Thus, the wavelength separation is about ~7.2 nm. Both of the spectral bandwidth is 1.65 nm. Figure 5(b) shows the synchronously oscilloscope trace of the dual-wavelength DSs. There are two dissipative solitons propagating with different group velocities in the cavity. Therefore, the oscilloscope trace represents a random distribution. The pulse width of every pulse is more than 1 ns. The corresponding RF spectrum of dual-wavelength DSs is demonstrated in Fig. 5(c). The signal-to-noise ratios of two operations are high than 62 dB. Different from the single-wavelength DS mode-locked operation that has only a fundamental repetition rate, the dual-wavelength operation exhibits two fundamental repetition rates corresponding to two mode-locking states. The fundamental repetition rates for 1058.5 nm and 1065.7 nm are 14.2084 MHz and 14.2092 MHz with a resolution bandwidth of ~5 Hz. Therefore, the spacing between the two RF spectra is 800 Hz. The dual-wavelength DSs can be switched by changing the PC. As shown in Fig. 5(d), the spectrum of mode-locked operation at 1060.7 nm is accompanied with a sideband at about ~1066.7 nm (black curve). With the PC changing, the sideband intensity is gradually strengthened and the mode-locked operation finally turns to be at 1066.7 nm with a sideband at about ~1060.7 nm (red curve). The evolution is reversible. Additionally, taking advantage of the tunability of the cavity birefringence fiber filter, dual-wavelength DSs mode-locking is also tunable, as shown in Fig. 6(a).The wavelength spacing almost retained the same, while the two lasing wavelengths simultaneously shifted about ~9 nm wavelength range, sustaining stable mode-locking. In addition, the wavelength spacing of the dual-wavelength DSs also can be varied from about 4 nm to 14 nm, as shown in Fig. 6(b).
Increasing the pump power to 225 mW, the stable triple-wavelength DSs mode-locking can also be obtained experimentally through careful control of the cavity polarizations, as shown in Fig. 7.Figure 7(a) shows the typical spectrum of stable triple-wavelength DSs, the center wavelengths of the three separated steep-edge spectra are 1056.5 nm, 1062.3 nm, and 1069.5 nm, respectively. The edge-to-edge bandwidths of three separated mode-locking pulse spectra are 0.85 nm, 0.91 nm, and 0.80 nm, respectively. Due to the effect of mode competition, the bandwidths of the three separated mode-locked pulse spectra are narrower than that of the single-wavelength and dual-wavelength mode-locked pulse spectra. Figure 7(b) shows the synchronously oscilloscope trace of the triple-wavelength DSs. There are three dissipative solitons coexist in the cavity, and the pulse energies of each soliton are slightly different with different pulse heights. Similar to the oscilloscope trace of the dual-wavelength DSs, the oscilloscope trace of the triple-wavelength DSs also represents a random distribution. The pulse width of every pulse is also more than 1 ns. In addition, the formation dynamics of the triple-wavelength DSs was investigated experimentally. Keeping the pump power at 225 mW unchanged, the triple-wavelength DSs can be developed from single-wavelength DS by adjusting the PC, as shown in Fig. 8.There have been two sidebands at 1063.5 nm and 1069.5 nm when the single-wavelength DS at 1057.3 nm formed from CW lasing, as shown in polarization orientation 1. The phenomenon is induced by the cavity birefringence fiber filter. The two sidebands will evolve into two CW lasing operations on polarization orientation 2. Then, on polarization orientation 3, dual-wavelength DSs and a CW lasing operation have been formed. Eventually, the triple-wavelength DSs state was obtained on polarization orientation 4. Thus, the process shows that the triple-wavelength DSs state is developed from CW lasing operation. Certainly, this evolution is reversible.
In the experiment, to verify whether the multi-wavelength DSs mode-locking is purely contributed by the GOSA, the GOSA was purposely removed out of the laser cavity. In this case, no mode-locked pulses could be observed though the pump power was changed and the PC was rotated. That is to say the GO is the only SA in the laser cavity. The mechanism for tunable single-wavelength, switchable and tunable dual-wavelength, and stable triple-wavelength DSs can be explained as follows: due to adjust the PC can lead to different birefringence of the cavity, so different transmission distribution of spectrum can be obtained. There is a fiber birefringence filter in the cavity. The relationship between the peak wavelength spacing and the birefringence of the cavity is decided by the formula, where is the central wavelength, is the cavity length and is the strength of the birefringence [8, 9, 12]. The birefringence of the SMF-28 is about 10−5, the cavity length is about 14.4 m, so the peak wavelength spacing is calculated about 7.8 nm. Considering that the SMF-28 is bended into a loop and the PC is rotated, the actually birefringence is a little different from the theoretical one. So, the experimental peak wavelength spacing is a little different from the calculated one of 7.8 nm. Additionally, considering that SMF-28 is not single-mode at 1064nm, transverse mode mixing will probably occur between the LP01 and LP11 mode. The single-mode-multimode-single-mode (SMS) interference bandpass filters by bending fiber might also play a small role in the experiment [27, 28].
In summary, we have experimentally obtained the tunable single-, switchable and tunable dual-, and stable triple-wavelength dissipative soliton generation from an all-normal-dispersion Yb-doped fiber laser based on a GOSA, for the first time. The tunable single-wavelength DS have a wide wavelength-tunable range of 16.4 nm. The dual-wavelength DSs not only have a wavelength-tunable range (about 10 nm) but also have variable wavelength spacing (3.8-13.8 nm). The formation dynamics of the triple-wavelength DSs was also investigated experimentally. The induced cavity birefringence through over bending the single mode fibers is responsible for the multi-wavelength filtering. We show that depending on the strength of the cavity birefringence, tunable single-wavelength, switchable and tunable dual-wavelength, and stable triple-wavelength DSs can be formed in the laser. The simple, compact and all fiber multi-wavelength DSs mode-locked fiber laser with flexible outputs can meet many potential applications.
This research was supported by the NSFC (61275144), Natural science fund of Guangdong province (S2013010012235), the foundation for scientific and technical innovation in Higher Education of Guangdong (2013KJCX0161), the Improvement and Development Project of Shenzhen Key Lab (ZDSY20120612094924467), the Science and technology project of Shenzhen City (JCYJ20120613172042264, JCYJ20130329142040731), Natural Science Foundation of SZU (No.201221).
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