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We demonstrate a self-starting dual-wavelength mode-locked fiber laser at a 2 μm spectral region by using a fiber taper in a Tm3+-doped ring fiber cavity. The fiber taper fabricated with a flame brushing technique was used as a periodic filter with a modulation depth of ~3.61 dB and a modulation period of ~7.3 nm, respectively. Diverse dual-wavelength regimes including continuous wave (CW)/multi-soliton, soliton/multi-soliton, and soliton/soliton regimes were obtained by adjusting pump power. Wavelength tuning for the dual-wavelength was also precisely controllable through stretching the fiber taper carefully. The tuning range was ~7 nm which was limited by the modulation period of the taper. By inserting a 10.0 m dispersion compensation fiber (DCF) into the fiber cavity, a stable dual-wavelength dissipative-soliton operation was obtained at 2 μm spectral region for the first time.
© 2016 Optical Society of America
1. Introduction
Multi-wavelength passively mode-locked fiber lasers have attracted wide attention in recent years due to their versatile applications in optical fiber sensing, optical signal processing, and wavelength division multiplexing communication [1–3]. Generally, in order to achieve multi-wavelength operation at room temperature, a comb filter such as Loyt filter, Sagnac loop filter, and Mach-Zehnder interferometer should be used as a wavelength selector [1,4–6]. When a comb filter is combined with mode-locking techniques such as nonlinear polarization rotator (NPR), nonlinear optical loop mirror (NOLM) and nonlinear amplified loop mirror (NALM), the multi-wavelength mode-locking becomes possible. A four-wavelength mode-locked fiber laser with the Mach–Zehnder interferometer and the NPR technique has been demonstrated at 1µm wavelength region [7]. To decrease the structural complexity, a simpler birefringence induced comb filter achieved by combing a polarizer and intra-cavity birefringence has been used to realize multi-wavelength mode-locking with the NPR technique [8–11]. In a similar way, by using some polarization-sensitive saturable absorbers as an equivalent polarizer, a birefringence induced comb filter was also achieved and the multi-wavelength mode-locking has been demonstrated [12–15]. Besides, multi-wavelength mode-locking based on the NOLM and NALM techniques have also been reported, in which the comb filter was formed by a birefringence-induced phase difference between two counter-propagating beams in the loop mirror [16,17].
Up to now, most of researches focus on 1 and 1.5 µm wavelength regions, and there are only few reported works at 2 μm wavelength region. Recently, a switchable and tunable dual- and triple- wavelength Tm3+-doped mode-locked fiber laser at 2 μm region has been demonstrated based on the birefringence induced comb filter with the NPR technique [3,18]. A NALM-based mode-locked fiber laser with up to four wavelengths from 1935 to 1965 nm has also been demonstrated with a segment of birefringent fiber [16]. Note that the wavelengths of the pulses using the above methods cannot be controlled precisely since the birefringence induced comb filter will produce a random wavelength-shift with adjustment of polarization controller (PC). To control and tune the center wavelengths of multi-wavelength precisely, a filter insensitive to the light polarization state should be employed. Cascaded fiber Bragg gratings (CFBGs) is a valid way to solve this problem. By incorporating cascaded CFBGs into a Er3+-doped mode-locked fiber laser cavity [19], stable triple-wavelength mode-locking with center wavelengths of 1540, 1550, and 1560 nm has been achieved, which was accurately tunable by stretching the CFBGs. However, fabrication of CFBGs with different center wavelengths requires multiple phase masks which increase the cost.
Alternatively, a fiber taper designed with a nearly sinusoidal spectral response can be expected to be a comb filter owing to its periodic filter effect caused by multimode interference in the taper’s waist region. Moreover, the stable tunability can also be achieved by carefully stretching the taper [20]. Compared with CFBG, the structure of fiber taper is simpler and easier to be fabricated, especially it is not sensitive the operation wavelength. Unlike the Loyt filter and Sagnac loop filter which are polarization sensitive, the taper’s filter effect is polarization independent which will be experimentally demonstrated later. Fiber taper induced filter effect has been already used to achieve the wavelength-tunable fiber lasers [20,21]. Kieu et al. have reported a wavelength tunable Er3+-doped CW fiber laser from 1546 - 1566.5 nm by stretching a taper [20]. In a similar way, a mode locked Tm3+-doped fiber laser with a tuning range of 50 nm has also been reported [21]. However, multi-wavelength mode-locking based on a fiber taper filter has never been reported to our best knowledge. Although the fiber taper based SAs have been used to achieve multi-wavelength mode-locked operation, the mode competition was still suppressed by the birefringence-induced filtering effect instead of the weak filter effect of the fiber taper itself [12–15]. Therefore, to better suppress the mode competition by a fiber taper itself, the taper should be designed with a proper modulation period and large modulation depth.
In this paper, a fiber taper with a modulation period of 7.3 nm and modulation depth of 3.61 dB was used to achieve passively dual-wavelength mode-locked fiber laser at 2 μm spectral region for the first time. The fiber laser can operate at CW/multi-soliton, soliton/multi-soliton, and soliton/soliton regimes with fixed center wavelengths of 1956.8/1979.2 nm. The dual-wavelength operation was tunable through stretching the fiber taper. Besides, a self-starting dual-wavelength dissipative-soliton operation with same centere wavelengths of 1956.8/1979.2 nm was also obtained by inserting a 10.0 m DCF into the fiber cavity.
2. Experimental setup and results
The fiber taper was fabricated by heating-stretching a piece of SMF-28e fiber from only one side with flame brushing technique. The flame width and maximum pull-speed were ~2 mm and ~10 mm/s, respectively. Figure 1(a)(inset) shows the scanning electron microscope (SEM) image of the taper's waist region with a magnification of 2400. The measured waist diameter is ~7.0 μm. Spectral response of the fiber taper at the wavelength range of 1880 nm to 2020 nm was measured by injecting a home-made Tm3+-doped ASE light, as shown in Fig. 1(a). A polarizer followed by a PC was used to output a polarization-adjustable linearly polarized light. Figure 1(b) shows the measured spectra with/without the fiber taper. The nearly sinusoidal spectral response, which caused by the interference of modes HE11 and HE12 in the waist region [20], indicates its periodical spectral filtering effect. The measured insertion loss, modulation depth and modulation period were ~2.9 dB, ~3.61 dB and ~7.3 nm, respectively. No any wavelength-shift was observed within the resolved limitation of 0.05 nm by adjusting the PC randomly, suggesting that the taper filter is insensitive to the polarization state. As a comparison, the spectral response by removing fiber taper was also measured at the same pump power (see spectrum 2 in Fig. 1(b)). The intensity difference between spectrum 1 and 2 was caused by the fiber taper induced insertion loss. Note that the fiber taper’s waist diameter has a crucial influence on its filter’s performance. To achieve better wavelength selection function, a filter with large modulation depth and low insertion loss isfavored. With the decreasing waist diameter of the taper, the modulation depth will be enhanced while the insertion loss will also be increased. We thus selected the fiber taper with a diameter of 7 μm to make a compromise between the modulation depth and the insertion loss.
Fig. 1 (a) Setup for spectral response measurement of a fiber taper (Inset: SEM image of the taper's waist region), (b) spectrum of the ASE light with/without a fiber taper.
The well-prepared fiber taper was then inserted into a ring mode-locked laser cavity, as shown in Fig. 2. A 2.0 m double-clad Tm3+-doped fiber (Coractive, DCF-TM-10/128) with an octagonal shaped inner cladding with a diameter of 128 µm and a numerical aperture (NA) of 0.45 served as the gain fiber. A 793 nm diode lasers (BWT) with a max output power of 6 W was used to pump the gain fiber through a (2 + 1) × 1 pump combiner (ITF, Canada). 10% port of a 10/90 fiber coupler centered at 2 μm was used to output laser from the cavity. A polarization independent optical isolator with an insertion loss of 0.76 dB at 2 µm (Advanced Photonics, USA) was used to ensure the unidirectional propagation. A PC was used to control the mode-locking performance. A single-wall carbon nanotube (SWCNT) sandwiched between two optical ferrules was used as SA to achieve mode-locked operation due to its broad operation bandwidth for multi-wavelength pulse generation and good stability [22]. The measured modulation depth and nonsaturable loss of the SWCNT SA were 21.2% and 59.6%, respectively.
The total fiber laser cavity was ~8.5 m including 2.0 m Tm3+-doped fiber, 6.5 m SMF-28e pigtail fiber from the pump combiner, isolator, coupler, SWCNT and fiber taper. The anomalous dispersion values of the Tm3+-doped fiber and the SMF-28e fiber at 1.993 µm were about −84 ps2/km and −80 ps2/km respectively [23]. The net dispersion in the cavity was thus estimated to be −0.688 ps2, indicating that the laser was operating at a anomalous dispersion regime. Temporal and spectral profiles of the output pulses were respectively monitored by a InGaAs photodetector (EOT ET-5000F, USA) followed by a 500 MHz digital oscilloscope and an optical spectrum analyzer (Yokogawa AQ6375) with resolution of 0.05 nm. The pulse duration was measured by an interference autocorrelator (APE, Germany).
By adjusting the PC properly, self-starting CW, soliton, and multi-soliton regimes with a fixed center wavelength of 1979.2 nm were respectively obtained at the pump power of 544 mW, 569 mW and 618 mW, of which the spectra are shown in Fig. 3. By further increasing the pump power from 828.7 mW to 992 mW without any adjustment of PC position, a self-starting dual-wavelength operation centered at 1956.8/1979.2 nm with several regimes was obtained. Note that the dual-wavelength separation is nearly three times of the filter modulation period, and no laser was generated between the two wavelengths. It might be caused by uneven distributed gain band of the Tm3+-doped fiber. Figures 4(b) and (c) respectively shows the detailed evolution in spectral and temporal domain as a function of pump power. Absence of the Kelly sidebands in Fig. 4(b) attributes to the bandwidth limitation of fiber taper filter. Initially, the dual-wavelength CW/multi-soliton regime was obtained at the pump power of 828 mW. Spectrum modulation of the multi-soliton is the typical feature of bound-soliton [24], i.e. several solitons were bound together through mutual interaction. Note that the oscilloscope trace only was a large pulse due to the separations of these solitons cannot be resolved by our detection system. Then, the fiber laser switched to the dual-wavelength soliton/multi-soliton regime at the pump power of 830 mW. In this case, the spectral modulation strength of the multi-soliton centered at 1979.2 nm became stronger compared with the previous regimes, and the oscilloscope trace with three pulse trains was observed, suggesting that the bound-soliton breaks into another bound-soliton and a single-soliton [24,25]. By further increasing the pump power, the spectral modulation of the multi-soliton gradually became weak and finally disappeared, and the oscilloscope trace returned to the single pulse profile. Meanwhile, the spectral strength of the single-soliton centered at 1956.8 nm became stronger. We thus deduced that a part of pulse energy was transferred from the multi-soliton at 1979.2 nm to the soliton at 1956.8 nm. Generally, in the single-wavelength regime, decreasing of the pump power for a multi-soliton will reduce the sub-solitons number, and a single-pulse soliton will be formed at a proper pump power [26]. With the same mechanism, the multi-soliton centered at 1979.2 nm inevitably evolved into single-pulse soliton regime because of the reduction of the pulse energy. The fiber laser started to operate at dual-wavelength soliton/soliton regime at the pump power of 879 mW. Details about the properties of the soliton/soliton regime will be discussed later. With further increasing of the pump power, the spectral strength centered at 1956.8/1979.2 nm became stronger/weaker due to more pulse energy was transferred from 1979.2 nm to 1956.8 nm. At the pump power of 905 mW, a multi-soliton/soliton regime instead of soliton/soliton regime appeared because of the pulse-splitting induced by the large pulse energy of the soliton centered at 1956.8 nm. The multi-soliton/soliton regime can be maintained to the pump power of 992 mW, and then the soliton centered at 1979.2 nm evolved to CW regime by slightly increasing the pump power. Additionally, once the mode locking was achieved with the initial adjustment of the PC, the dual-wavelength operation can be achieved by reducing the pump power, and no longer needed to adjust the PC.
Fig. 4 (a) Spectra and (b) temporal profiles of the dual-wavelength mode-locking pulses as a function of the pump power.
The dual-wavelength mode-locking also shows a good tunability through careful stretching the fiber taper with a pair of fiber clamps. Figure 5(a) shows three output spectra with different stretching lengths at the pump power of 890 mW. The maximum tuning range was ~7 nm, which was nearly equal to the taper’s modulation period. Spectrum 1 corresponds to the dual-wavelength mode-locking without stretching the fiber taper. The output power and slope efficiency were 8.08 mW and ~1.7%, respectively. The center wavelengths and FWHMs were 1956.8/1979.2 nm and 2.32/2.04 nm, respectively. By slowly increasing the stretching length, the spectrum would be blue-shifted to spectra 2 and 3 with center wavelengths of 1953.8/1976.2 nm and 1949.6/1972.0 nm and fixed separation of 22.4 nm between two center wavelengths. The measured FWHM for spectra 2 and 3 were 2.51/2.08 nm and 1.95/2.05 nm, respectively. The small change of the measured FWHMs during stretching the fiber taper might originate from the wavelength-dependent gain. By further stretching the fiber taper slowly, the center wavelengths would be repeatly tuned from ~1953/1976 nm to ~1949/1972 nm due to the periodic spectral response of the taper. Once we relaxed the stretched fiber taper slowly, the spectrum would re-shift to the initial position, suggesting that the taper stretched within the elastic region. The inset of Fig. 5(b) shows the corresponding oscilloscope trace of spectrum 3, indicating that there are two solitons propagating in the cavity with different energy. For better clarity, a snapshot of the dual-wavelength mode-locked oscilloscope trace with scan range of 700 ns is also presented in the Fig. 5(b). It is observed that once one pulse train was triggered, another pulse train would moved randomly on the oscilloscope screen, indicating that the two pulse trains have different propagated velocities [1]. Figure 5(c) shows the corresponding fundamental RF spectrum with a scanning range of 3.3 kHz and a resolution of 10 Hz. The RF signal-to-noise ratio (SNR) of 71.45/71.73 dB shows the good stability of the dual-wavelength mode-locking operation . The measured repetition rates of 23.482562/ 23.485195 MHz match well with the cavity length. The fiber laser can also be switched between dual-wavelength and single-wavelength mode-locking by adjusting the PC, and their center wavelengths were insensitive to the PC state, indicating that the filter effect surely induced by the fiber taper rather than the intra-cavity birefringence. Figure 5(d) shows a measured spectrum of a single-wavelength soliton with a FWHM of 2.65 nm and a center wavelength of 1949.6 nm at the pump power of 890 mW, which switched from the dual-wavelength mode-locking corresponding to the spectrum 3 in Fig. 5(a). Inset of Fig. 5(d) shows the measured interference and intensity autocorrelation trace of the mode-locked pulses at a scanning range of 16 ps. The FWHM is 5.18 ps corresponding to the pulse duration of 3.35 ps when sech2-pulse fit is assumed. The calculated time-bandwidth-product of 0.70 suggesting that the soliton centered at 1943.6 nm was chirped. Another soliton centered at 1972.0 nm was also observed in this case by carefully adjusting the PC.
Fig. 5 Dual-wavelength soliton operation: (a) tuning characteristic by stretching the fiber taper, (b) pulse waveform, inset: snapshot of the oscilloscope trace, (c) RF spectrums with scanning range of 3.3 KHz and 900 MHz (inset), and (d) single-wavelength soliton and corresponding autocorrelation trace by adjusting PC (inset).
A dual-wavelength dissipative-soliton operation was also obtained by inserting a 10.0 m DCF (OFS, LP980) with a dispersion value of ~89 ps2/km at 1.993 µm. The net dispersion value in the cavity was estimated to be ~0.202 ps2, suggesting that the laser operated at a normal dispersion region. Initially, CW, dissipative-soliton, and bound- dissipative-soliton regimes centered at 1979.2nm were respectively obtained at the pump power of 593 mW, 618 mW and 637 mW by properly setting the PC. Note that the bound- dissipative-soliton characterized by the spectrum modulation [27]. Then, the fiber laser switched to several dual-wavelength regimes by increasing the pump power from 668 mW to 717 mW, including CW/multi-dissipative-soliton, double bound-dissipative-soliton, and double dissipative-soliton regimes. Figures 6(a) and (b) shows the evolution of spectrum and corresponding oscilloscope trace with the pump power. The centre wavelengths of the dual-wavelength operation were 1956.8/1979.2 nm, which are same as that at anomalous dispersion regime. The characteristic steep spectral edge of optical spectrum is the typical feature of dissipative-soliton. Figure 6(c) shows the spectrum of the dual-wavelength dissipative-soliton at the pump power of 709 mW. The measured spectral FWHM were 2.21/1.38 nm, respectively. Figure 6(d) shows the corresponding fundamental repetition rates with a scanning range of 500 Hz and a resolution of 1 Hz. The measured RF signal-to-noise ratios (SNR) and repetition rates were 74.1/72.0 dB and 10.769885/10.770201 MHz, respectively. Additionally, the dual-wavelength dissipative-soliton regime could also be tuned by stretching the fiber taper.
Fig. 6 Dual-wavelength dissipative mode-locked operation: evolution at (a) spectral domain and (b) temporal domain; (c) optical spectrum, (d) RF spectrum with scanning range of 500 Hz (Inset: corresponding pulse train) at the pump power of 709 mW.
To investigate the long-term stability of the dual-wavelength mode-locked fiber laser, we monitored for 24 hours’ continuous operation under laboratory condition. The oscillator can maintain the dual-wavelength mode-locked operation and was not sensitive to the environmental fluctuations. During the long-term operation, the fiber taper was sealed in a closed clean plastic box with a size of ~8 cm to avoid the dust-induced degradation. Pigtails of the fiber taper were exported from two slits located at the left and right sides of the box. Additionally, when we removed the fiber taper, the fiber laser only operated at single-wavelength regime no matter how we adjusted the PC and pump power, suggesting that the dual-wavelength operation crucially depends on the fiber taper.
3. Conclusion
In summary, we have experimentally demonstrated a switchable and tunable dual-wavelength mode-locked fiber laser by using a fiber taper in a Tm3+-doped SWCNT-based ring fiber cavity. Advantages of the proposed method include controllable center wavelength, compact structure, low cost, and stability of operation. The achieved dual-wavelength mode-locked fiber laser centered at 1956.8/1979.2 nm has a FWHM bandwidth of 2.32/2.04 nm. The center wavelengths of the dual-wavelength oscillator can be precisely tuned through stretching the fiber taper. By adjusting the PC, a single-wavelength mode-locking can be obtained. Other dual-wavelength regimes including CW/multi-soliton, soliton/multi-soliton was also achieved by changing the pump power. Moreover, a dual-wavelength dissipative-soliton has been obtained for the first time at the 2 μm spectral region by using a 10.0 m DCF.
Acknowledgments
This work was supported by National Nature Science Foundation of China (NSFC) (Grant No. 61377042, 61435003, 61307070 and 61421002); Program for New Century Excellent Talents in University of Ministry of Education of China (Grant No.NCET-13-0094); Open Fund of State Key Laboratory of Advanced Optical Communication Systems and Networks (Grant No.2015GZKF004); Open Found of Key Laboratory of High Energy Laser Science and Technology, CAEP (Grant No.2013005580); Open Found of Key Laboratory of Specialty Fiber Optics and Optical Access Networks (Grant No.2014KLS003).
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