We report the generation of tunable single- and dual-wavelength dissipative solitons in an all-normal-dispersion mode-locked Yb-doped fiber laser, to the best of our knowledge, for the first time. Besides single-wavelength mode-locking, dual-wavelength mode-locking was achieved using an in-line birefringence fiber filter with periodic multiple passbands, which not only allows multiple wavelengths to oscillate simultaneously but also performs spectrum modulation on highly chirped dissipative pulse. Furthermore, taking advantage of the tunability of the birefringence fiber filter, wavelength tuning for both single- and dual-wavelength dissipative soliton mode-locking was realized. The dual-wavelength operation is also switchable. The all-fiber dissipative laser with flexible outputs can meet diverse application needs.
©2012 Optical Society of America
Stable and flexible optical pulse sources simultaneously operating at multiple wavelengths have potential applications in optical sensing, optical instrumentation, microwave photonics, optical signal processing, THz generation, and wavelength-division-multiplexed optical transmission systems . Fabry-Perot laser diode (FP-LD) with self-seeding approach or external injection seeding scheme was used to generate the multiwavelength pulses , which required specially designed semiconductor laser cavity and external modulation. Dual-wavelength synchronously mode-locked pulses has been reported in solid lasers such as Ti:sapphire laser  and Nd:CNGG laser . Comparatively, fiber laser is an effective and simple way to generate multiwavelength optical pulses. While actively mode-locked multiwavelength fiber lasers can be achieved by incorporating multiwavelength filters, such as cascaded fiber Bragg gratings and sampled fiber Bragg grating into the cavities [5, 6], additional techniques have to be employed to ensure the pulses pass through the modulator synchronously for stable multiwavelength pulse generation .
Several methods for achieving multiwavelength passive mode locking have been reported. By virtue of the intensity-dependent loss induced by the nonlinear polarization rotation (NPR) technique to suppress the mode competition, stable multiwavelength passively mode-locked operation at room temperature has been obtained with conventional soliton operation under the anomalous-dispersion regime . Recently, a multiwavelength dissipative soliton operation in an erbium-doped fiber laser based on semiconductor saturable absorber mirror (SESAM) was investigated . However, the wavelengths of output pulses were not tunable and switchable because the artificial birefringence filter in the laser cavity was utilized. The generated dual or triple wavelengths were randomly distributed. Using a phase-shifted long-period fiber grating as spectral filter in the laser cavity, a switchable dual-wavelength mode-locked operation was demonstrated by nonlinear polarization evolution . Nevertheless, the wavelength tunability and wavelength-spacing changeability of multiwavelength mode-locking were still defective as before.
In this letter, we demonstrate a simple and all-fiber mode-locked Yb-doped fiber laser with flexible outputs: tunable single-wavelength, tunable and switchable dual-wavelength operation. The characteristic of all-normal-dispersion laser cavity decides the output pulses of laser belong to dissipative solitons. Fiber-based birefringent filter constructed by inserting a section of polarization-maintaining fiber into the cavity is a key part in the fiber laser, which plays roles of multiwavelength filtering and spectrum modulation on highly chirped dissipative pulse. Wavelength tuning for both single- and dual-wavelength dissipative soliton mode-locking was attained, owing to the tunability of the birefringent filter. To the best of our knowledge, no tunable multiwavelength dissipative soliton fiber laser has so far been reported.
2. Experiment and results
The experimental setup of the dual-wavelength dissipative soliton fiber laser is shown in Fig. 1 . It has a ring cavity configuration made of pure normal-dispersion fibers. A piece of 2 m Yb-doped fiber (YDF) which is pumped by a 976 nm laser diode (LD) through a wavelength-division multiplexer (WDM) has absorption of 350 dB/m at 975 nm, group velocity dispersion (GVD) of 35.8 ps2/km at 1060 nm. The pump LD can supply up to 300 mw optical power. The WDM has a specified operating wavelength of 1060 ± 10 nm. A polarization-dependent isolator (PDI) with two polarization controllers (PC) separately located at two sides of the PDI is the mode-locking part, which converts phase modulation to amplitude modulation . An equivalent Lyot birefringence fiber filter is constructed by incorporating a 17.1 cm length of polarization-maintaining fiber (PMF) after the PDI. The corresponding periodic filter bandwidth is 16.4 nm. A 10:90 optical coupler after the YDF is used as the output coupler, which couples 10% of the power out of the ring cavity. The overall fiber length of the laser cavity is 5.5 m, and the estimated total dispersion of the laser cavity is 0.15 ps2 at 1060 nm. We noted that the cavity dispersion has no significant variation in the wavelength range from 1020 nm to 1090 nm.
We firstly used the normalized power transmission function of the birefringent filter to simulate its filtering effect. In the simulation, the length L and birefringence ∆n of the PMF were set as 17.1 cm and 4.0 × 10−4 as the experiment, respectively. The simulated transmission spectra under different polarization states are shown in Fig. 2(a) . The wavelength spacing between two neighboring transmission peaks is 16.4 nm, which is decided by the formula λ2/(∆nL) and has slight wavelength-dependence. And the filtering profile can be tuned within its free spectrum range, due to its intrinsic polarization-dependence .
In the experiment, when the pump power was increased to about 40 mw, single-, dual- or triple-wavelength CW lasing operation was easily observed. Figure 2(b) gives an example of triple-wavelength CW laser operation. By further increasing the pump power and properly adjusting the orientations of the two PCs, we could obtain single- and dual-wavelength mode-locking output of dissipative soliton. Figure 3 shows the output characteristics of a typical single-wavelength mode-locked dissipative soliton state obtained under 127 mW pump power. Figure 3(a) is the spectrum on a linear scale, and Fig. 3(b) is the spectrum on a logarithmic scale. The spectra exhibit steep edges, as the typical feature of all-normal-dispersion or net-normal-dispersion laser [13–15]. The spectral edge-to-edge bandwidth is 11.5 nm. It is visible that there is a sideband on each side of the steep-edge pulse spectrum, which is induced by the periodical filtering of the birefringence fiber filter and inversely indicates the role of spectral filtering on the dissipative soliton. Figures 3(c) and 3(d) show the corresponding pulse train and autocorrelation trace. The fundamental repetition rate of the laser is 36.0 MHz. The pulse duration is 5.4 ps (Gaussian pulse profile assumed), which gives a time-bandwidth product of 3.1. Owing to the tunable characteristic of the birefringence fiber filter, the lasing wavelength of the single-wavelength mode-locked dissipative soliton can be tuned through adjusting the PC. The tuning results with tuning wavelength range of 16.7 nm from 1065.6 nm to 1048.9 nm referring to center wavelength of the pulses, which overtakes the free spectrum range 16.4 nm of the filter, are shown in Fig. 4 . For clarity, the spectra are shifted along vertical axis direction with different offset, which is also applied on Figs. 6 and 7. During the whole tuning range, the spectral bandwidth and pulse duration had not remarkable variation.
Dual-wavelength dissipative soliton can also be obtained from our fiber laser. Figure 5 shows the output characteristics of a typical dual-wavelength mode-locked dissipative soliton state obtained under 200 mW pump power. Figure 5(a) is the spectrum on a linear scale, and Fig. 5(b) is the spectrum on a logarithmic scale. The center wavelengths of the two separated steep-edge spectra are 1048.7 nm and 1064.9 nm, respectively. Thus, the wavelength separation is about 16.2 nm, which matches well the filtering period of the intra-cavity birefringence fiber filter [16–18]. Their spectral bandwidths are 10.1 nm and 7.2 nm, respectively. The output spectrum is slightly polarization-dependent. Two pulse trains of different central wavelengths usually propagate with different group velocities in the cavity. Therefore, they will distribute randomly on the oscilloscope. In our experiment, synchronously dual-wavelength mode locking could be obtained through carefully tuning the orientation of the PC so as to control the relative intensity and phase of the dual-wavelength soliton pulses. The synchronously mode-locked dual-wavelength pulse trains are presented in Fig. 5(c). As can be seen in Fig. 5(c), in one cavity period two mode-locked pulses with fixed soliton separation were observed, which indicates they have the same group velocity traveling around the laser cavity. The autocorrelation trace was measured using an autocorrelator with long scanning range, as shown in Fig. 5(d). There is no optical beating between the dual-color mode-locked pulses in the autocorrelation trace, as they were temporally synchronized but separated from each other. Compared with single-wavelength operation, the autocorrelation trace of the dual-color mode-locked pulse became broader, due to the autocorrelation superposition of pulses at two wavelengths. To further confirm that the pulses observed in our experiment are the dual-wavelength pulses, and not multiple pulses induced by energy quantization , one wavelength was filtered out by using a bandpass filter. The measurement in time domain revealed there was only one pulse in the cavity in this case.
Additionally, the formation dynamics of the dual-wavelength dissipative soliton was investigated experimentally. Once the dual-wavelength dissipative was obtained under appropriate pump power (200 mW) and polarization, we decreased the pump power by a step of 25 mW, while kept polarization fixed. It was found that the dual-wavelength dissipative soliton turned to sing-wavelength dissipative soliton with CW at the other wavelength, then sing-wavelength dissipative soliton with narrower spectral bandwidth, finally two CWs, in sequence. The results are shown in Fig. 6(a) . This evolution is reversible. That is to say, the dual-wavelength dissipative soliton is developed from two CWs lasing, in contrast with single-wavelength dissipative soliton evolved from single CW lasing. This further proves the filtering role of the birefringence fiber filter. Note that the CW with sing-wavelength dissipative soliton appearing on the aforementioned process can be suppressed through adjusting the polarization. And the mode-locking wavelength and the CW lasing wavelength can be exchanged, as shown in Fig. 6(b). Thus, the dual-wavelength dissipative soliton has switchability.
Taking advantage of the tunability of the birefringence fiber filter, dual-wavelength dissipative soliton mode-locking is also tunable, as shown in Fig. 7 . The wavelength spacing retained the same, while the two lasing wavelengths simultaneously shifted with 7.7 nm wavelength range, sustaining stable mode locking. Therefore, the currently presented dissipative soliton fiber laser with all-fiber configuration has flexible output characteristics: tunable single- and dual-wavelength, switchable dual-wavelength dissipative soliton. It was experimentally confirmed that the two separate mode-locked wavelengths were initially developed from two CWs, and the tunability were experimentally implemented and theoretically explained. Moreover, the wavelength spacing of the dual-wavelength operation can be chosen by changing the used PMF length.
In conclusion, we have demonstrated the single- and dual-wavelength dissipative soliton generation from an all-normal-dispersion Yb-doped fiber laser with an in-line birefringence fiber filter. Tuning for both single-wavelength and dual-wavelength dissipative soliton operation was achieved, to the best of our knowledge, for the first time. It is attributed to the polarization-tunable characteristic of the birefringence fiber filter, which was verified by numerical simulation. As a result, our experimental results showed 16.7 nm tuning range for single-wavelength dissipative pulse and 7.7 nm for dual-wavelength dissipative pulse with wavelength spacing of 16.2 nm deiced by the used PMF length. The switchable dual-wavelength operation and formation dynamics have also been investigated. Such all-fiber dissipative soliton laser with lasing wavelength tunability and flexibility has great potential for applications.
This work is supported by the National Natural Science Foundation of China (60807014 and 60967002), and the Science Foundation of Jiangxi Provincial Department of Education (GJJ12170).
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