Abstract
1. Introduction
In the future of ultra-high speed and large-capacity all-optical communication, the laser source used is required to be simple, compact, and have high beam quality, especially with dual functions of mode-locking and multi-wavelength operation. Based on this, multi-wavelength ultrafast photonics has been a very active and important research direction in recent years[
In recent years, three kinds of fiber gratings have been developed: fiber Bragg grating, long-period fiber grating, and ultra-long-period grating (ULPG). Among them, the axial effective index in the fiber is periodically modulated by the pitch of millimeter (mm) order, which is called ULPG. From the point of view of the spectrum, there are a series of discrete attenuation bands and a wide periodic distribution in the transmission spectrum[
The application of long-period gratings in fiber lasers is just beginning[
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Here, we achieved the cascaded multi-wavelength mode-locked EDFL, in which the ULPG can operate as both a mode-locker for pulse shaping and a comb filter for the multi-wavelength lasing simultaneously. These findings indicate that the ULPG could become a good candidate of pulse-shaping devices for ultrafast photonics.
2. Fabrication and Characteristics of the ULPG and Its Saturable Absorption
The as-used ULPG was fabricated by using the fused biconical taper technique, which was similar to our previous report[
Figure 1.Schematic diagram of the experimental setup for fabricating the ULPG sample. The green frame is the diagram of the ULPG sample and a micrograph of a fiber taper.
Next, we provided the transmission spectrum of the as-used ULPG by using the light injection method, as shown in Fig. 2(a). It can be seen that there are two dips centered at 1557.8 nm () and 1593.4 nm () in the transmission spectrum, respectively, which are consistent with the prediction of coupled mode theory. Theoretically, the two adjacent fiber tapers of the ULPG can be regarded as a Mach–Zehnder interferometer, and the core mode and cladding mode are equivalent to its two arms[
Figure 2.Typical optical characteristics and saturable absorption property of the as-used ULPG: (a) transmission spectrum (inset: the photograph when the yellow light is passing out); (b) nonlinear saturable absorption behavior.
In this experiment, the saturable absorption behavior of the ULPG played a key role. To this end, we then investigated it with a balanced twin-detector method. The experiment setup was similar to our previous report[
3. Experimental Setup
To verify the optical property of the prepared ULPG, we transferred it into a ring cavity EDFL. The experimental scheme is illustrated in Fig. 3. The gain medium consisted of a ∼ 4.5 m erbium-doped fiber (Core active L-900, EDF) with a dispersion parameter of ps/(km·nm). The pump source was a laser diode (LD, 980-420-B-FA) centered at 976 nm, and its maximum output power was 420 mW. The unidirectional operation of the laser beam was achieved by utilizing a polarization-independent isolator in the ring cavity. Meanwhile, we could adjust flexibly the polarization states of the laser beam by using a polarization controller (PC). A variable-length of SMF with a dispersion parameter of ∼18 ps/(km·nm) varies between 20 m and 135 m and was used to adjust the cavity dispersion. Moreover, a 980/1550 wavelength-division multiplexer and a 10:90 optical coupler were used to extract the input and output of the laser beam, respectively. The performance parameters of the laser pulse are measured by a power meter, a spectrum analyzer (ANDO AQ-6317B) with a spectral resolution of 0.01 nm, a photodetector (Thorlabs PDA 12.5 GHz) combined with a 1 GHz mixed oscilloscope (Tektronix MDO4054-6, 5 GHz/s), and a commercial autocorrelator (APE, PulseCheck).
Figure 3.Experimental setup. LD, laser diode; WDM, wavelength-division multiplexer; EDF, erbium-doped fiber; ISO, isolator; OC, optical coupler; SMF, single-mode fiber; PC, polarization controller; ULPG, ultra-long-period grating.
4. Results and Discussion
4.1. Multi-wavelength mode-locked pulses generation
Before carrying out the experiment, we tested the operation characteristic of the fiber laser without incorporating the ULPG in the laser cavity. By adjusting the pump strength and the cavity polarization state in a wide range, there is neither mode-locking nor multi-wavelength generation, which excludes the possibility of self-mode-locking and Fabry–Perot cavity effect. Then, the ULPG was inserted into the cavity (Fig. 3); we observed that the CW and soliton state appeared when the output power from the LD was about 30 and 50 mW, respectively. Next, by flexibly rotating the paddles of the PC, we obtained the three-wavelength soliton pulses when the pump power increased to ∼ 60 mW, a length of SMF of 93.5 m, and a corresponding cavity dispersion of . Its optical spectrum and corresponding pulse train are shown in Fig. 4. As seen in Fig. 4(a), there were three wavelengths that appeared in the range of 3 dB on the whole optical spectrum. Clearly, there are two pairs of Kelly sidebands in the spectrum, which are the feature of the soliton pulse emitted from the laser systems. However, these Kelly sidebands are asymmetrical, which may be caused by the wavelength-filtering and dispersion effect in the laser cavity[
Figure 4.Three-wavelength soliton mode-locking operation with a length of SMF of 93.5 m and a corresponding cavity dispersion of
Furthermore, we also obtained the four-wavelength soliton mode-locking operation by flexibly adjusting the polarization states through the PC when the pump power was set to 180 mW. It can be seen from Fig. 5(a) that there were three pairs of Kelly sidebands that appeared on both sides of the optical spectrum, which implied that they were soliton pulses. In experiment, we also provided the autocorrelation trace of the single-soliton pulse using a narrow-band filter. Its pulsewidth is about 7.8 ps. In addition, we provided the pulse train and zoom-in image of a single-pulse profile, as shown in Fig. 5(b). It can be seen that four pulses (marked 1, 2, 3, 4) with a 477 ns period are transmitted in the laser cavity, and the pulse intervals between the two pulses (1 and 2, 2 and 3, 3 and 4, 4 and 1) are about 190 ns, 95 ns, 75 ns, and 117 ns, respectively.
Figure 5.Four-wavelength soliton mode-locking operation with a length of SMF of 93.5 m and a corresponding cavity dispersion of
In the experiment, we also found that, the multi-wavelength mode-locked pulses were polarization-dependent. For example, the four-, five-, and six-wavelength mode-locking operation could be achieved by properly adjusting the polarization states through the PC under the pump power of 210 mW, as shown in Fig. 6. It can be seen from Figs. 6(a)–6(c) that the wavelength intervals between two pulses and the spectral width within 3 dB were about 0.86 nm, 1.15 nm, 0.98 nm, and 0.24 nm, 0.12 nm, 0.12 nm for the four-, five-, and six-wavelength mode-locked pulses, respectively. Moreover, no sidebands appear in the spectrogram, no solitons are formed, and only one pulse is transmitted in the laser cavity. Moreover, Fig. 6(d) shows the corresponding pulse train with a period of about 477 ns.
Figure 6.Multi-wavelength mode-locking operation with a length of SMF of 93.5 m and a corresponding cavity dispersion of
4.2. Effect of cavity dispersion on multi-wavelength mode-locked pulses
Notably, for the triple-wavelength mode-locking operation, as shown in Fig. 4, their Kelly sidebands were different from those of the soliton pulses, implying that their formation could be affected by the dispersion. Thus, we then discussed the effect of cavity dispersion on the multi-wavelength mode-locked pulses. Here, we studied the dynamics of the multi-wavelength mode-locking operation by properly setting the polarization states through the PC when we decreased the length of the SMF () from 93.5 to 20 m. Figure 7 shows the optical spectrum and pulse train of the seven-wavelength mode-locking operation by flexibly adjusting the polarization states through the PC when was about 20 m, and the corresponding cavity dispersion is . It can be seen from Fig. 7(a) that the wavelength interval between two pulses and the bandwidth within 2.98 dB were about 1.05 and 0.24 nm, respectively. As seen in Fig. 7(b), the pulse train exhibited a period of ∼ 119.9 ns, which indicated that the fundamental repetition rate was about 8.34 MHz. In addition, we also provided the evolution of the output spectra of the seven-wavelength mode-locking operation at a 2 h interval over 14 h, as shown in Fig. 7(c), indicating the long-term stability of the laser.
Figure 7.Seven-wavelength mode-locking operation with a length of SMF of 20 m and a corresponding cavity dispersion of
Furthermore, the seven-wavelength mode-locked pulses were also achieved when we increased the length of the SMF () from 20 to 135 m, and the pump strength was about 120 mW, as shown in Fig. 8. As seen from Fig. 8(b), the pulse train exhibited a period of about 680 ns, which implied that the fundamental repetition rate was ∼ 1.47 MHz. Interestingly, we also observed the hybrid soliton pulses with the pump power of 270 mW. Figures 8(c) and 8(d) show the optical spectrum and the corresponding pulse train, respectively. Different from Fig. 6(b), the whole spectrum contained two parts. One part was a typical single-soliton spectrum with three pairs of Kelly sidebands, and its central wavelength and 3 dB spectral width were 1539.6 and ∼1.2 nm, respectively. The other part was a four-wavelength mode-locking state around 1535 nm. Thus, we believe that it may be a five-wavelength mode-locked pulse. Figure 9 illustrates the linear-fitting relation between the average power and the pump power. It can be seen that the maximum average output power and slope efficiency were 7.9 mW, 8.4 mW, 7.5 mW and 2.16%, 2.03%, 2.04% when was 20 m, 93.5 m, and 135 m, respectively. According to the mode-locked theory, the corresponding pulse energies are 0.95 nJ, 4 nJ, and 5.1 nJ, respectively.
Figure 9.Average output power versus the pump power of the laser with different lengths of SMF.
Figure 8.Seven-wavelength and hybrid soliton operation with a length of SMF of 135 m and a corresponding cavity dispersion of
4.3. Discussion
The generation mechanism of multi-wavelength mode-locked pulses in the EDFL based on ULPGs can be explained as follows. As we know, multi-wavelength lasers are mostly based on the spectral-filtering principle, in which a comb filter is often used. In recent years, a large number of studies show that a cascaded long-period fiber grating has the function of comb filtering and has been used to realize multi-wavelength laser operation[
Meanwhile, we find that the ULPG also has the function of mode-locking and can be used as a mode-locker in fiber lasers, similar to long-period fiber gratings. For example, Intrachat and Kutz theoretically discovered passive mode-locking dynamics of a long-period fiber grating based on the mode-coupling theory and predicted that it could be used as a mode-locker[
Figure 10.Mode-locking principle of ULPG. CW, continuous wave.
5. Conclusions
In conclusion, we have demonstrated a cascaded multi-wavelength mode-locked EDFL based on ULPG, in which the ULPG can be used as both a mode-locker and a comb filter to generate multi-wavelength mode-locked pulses. By taking advantage of the dual-function of the ULPG, three-, four-, five-, six-, and seven-wavelength mode-locked pulses are obtained in an all-fiber ring-cavity EDFL under the proper polarization states and pump strength. For the four-wavelength soliton pulse, its pulsewidth is about 7.8 ps. The maximum average output power, pulse energy, and slope efficiency of these pulses are 8.4 mW, 4 nJ, and 2.03%, respectively. Our study suggests that the ULPG could become a good candidate of pulse-shaping devices for a myriad of ultrafast photonic applications.
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