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Abstract
We demonstrate, for the first time, use of a stretched single mode-graded-index multimode-single mode fiber (SMF-GIMF-SMF) structure as a saturable absorber (SA) for a passively mode-locked Tm fiber laser. Such an all-fiber SA was based on the nonlinear multimode interference (NL-MMI). Stable fundamentally mode-locking operation was obtained at a pump threshold of 100mW. The output soliton pulses had a center wavelength, spectral width, pulse duration, and repetition rate of 1931 nm, 3.77 nm, 1.2ps, and 19.94 MHz, respectively. Furthermore, the SMF-GIMF-SMF structure can also be used as a filter to tune the laser. Continuously tunable mode-locking was experimentally demonstrated only by varying the stretched length of GIMF. Our results indicate that the stretched SMF-GIMF-SMF structure could serve as a SA together with a bandpass filter, which makes it advantageous for wavelength-tunable mode locking lasers.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Mode-locked fiber lasers in the 2 μm region have attracted considerable attention, owing to their significant potential applications in free space optical communication, long-range light detection and ranging (LIDAR), nonlinear frequency conversion, laser surgery, and material processing [1–5]. Various techniques, such as the material SA based on SESAM, graphene, black phosphorous and transition metal dichalcogenides (such as MoS2 and WS2), nonlinear polarization evolution (NPE) or a nonlinear amplifying loop mirror (NALM), have been developed to achieve mode locking [6–12]. Since mode-locked fiber lasers with different center wavelength have different applications, mode-locked lasers with tunable center wavelength are also in demand to be improved. To date, only a few examples of wavelength tunable mode locked lasers in the 2 μm regime have been reported. The ways to achieve wavelength-tunable operation in passively mode-locked fiber lasers is mainly proposed by incorporating an optical filter into the cavity [13], the NPR technique [14] or some polarization-sensitive saturable absorbers [15,16]. However, to maintain the mode-locking stability in the above-mentioned systems, each wavelength tuning step should be followed by a polarization controller (PC) adjustment which is a rather slow process, and the tuning process is hard to repeat.
In the last few years, there is a strong resurgence of interest in the nonlinear regime of graded-index multimode fibers (GIMFs) due to their unique properties. Various complex nonlinear effects in GIMFs including self-phase modulation, cross phase modulation, four-wave mixing, self-steepening, Raman scattering and co-existed interactively have been studied [17–29], especially the nonlinear multimode interference (NL-MMI) [30,31]. Elham Nazmosadat and Arash Mafi numerically presented comprehensive analysis of the NL-MMI in 2013 for the first time in a short section of GIMF. The proposed SMF-GIMF-SMF structure can be used as a SA to achieve a mode locked fiber laser [31]. Recently, several kinds of SAs based on the NL-MMI have been developed experimentally [34–39]. In our previous work [35], a hybrid structure of step index multimode fiber (SIMF)-GIMF as the SA was used to achieve mode-locked operation in Tm-doped fiber lasers. Wavelength tunable mode-locking was observed by varying the curvature of the SIMF-GIMF structure and the tuning range extended from 1835 nm to 1886 nm. However, the essential requirement on bending operation of the device make it difficult to be applied in practice. Zhaokun Wang et.al. verified an erbium-doped all-fiber soliton oscillator using a stretched GIMF as a SA [39]. The SMF-GIMF-SMF device is induced a tensile length to ensure the fine tuning of GIMF length and achieved SA behavior. A soliton all-fiber oscillator capable of generating wavelength switchable ultrafast pulses with the pulse width of 506 fs at 1572.5 nm and 416 fs at 1591.4 nm was obtained in this letter. On the other hand, the multimode interference effect enables SMF-GIMF-SMF structure as an all-fiber bandpass filter which has been demonstrated both numerically and experimentally [32]. Soon after, Mafi et al. presented a low-loss wavelength-dependent coupler structure by implementing a GIMF segment [33]. Considering the transmission peak is inversely proportional to the length of GIMF, the bandpass wavelength of the filter can be tuned by changing the GIMF length. Thus, the stretched SMF-GIMF-SMF structure could serve as both the bandpass filter and the SA which can achieve wavelength-tunable mode-locking lasers with simple and all-fiber-cavity design. However, a tunable wavelength mode-locking Tm-doped fiber laser employing the stretched GIMF SA has not yet been reported so far, to the best of our knowledge.
Here, we demonstrate for the first time an ultrafast, Tm-doped fiber laser mode locked with the use of a stretched SMF-GIMF-SMF structure. The laser was capable of generating 1.2ps pulses centered at a wavelength of 1931.5 nm with a repetition rate of 19.94MHz. In the experiment, the NL-MMI-based SMF-GIMF-SMF SA could provide a modulation depth of 9.5% and a non-saturable loss of 62%. The stretched SMF-GIMF-SMF structure not only acts as an excellent SA for mode-locking, but also as a filter for wavelength selection. By simply tuning the stretched length without changing the polarization states in the laser cavity at a fixed pump power, continuously mode-locking can be obtained. The mode locking at the 2μm waveband contributed to further demonstration of the broadband saturable absorption of the GIMF SA. Meanwhile, the achievement of tunable mode-locking output confirmed that the NL-MMI-based SA could be a perfect combination of filter and mode-locker.
2. Nonlinear optical properties of the stretched GIMF at 2 μm
A tensile stress on the GIMF is applied to increase its length. The GIMF is placed and fixed on the center of two translation stages, and the length of the fiber can be accurately controlled in motion along the axial direction of the fiber to induce a tensile length from 0 to∼1000 μm with a resolution of 5 μm. The length of the GIMF used in the experiment, is ~23.5 cm. In order to characterize the nonlinear optical properties of the stretched GIMF device as a SA for a mode-locked Tm fiber laser, the power-dependent transmission curve was measured according to the method presented in [32]. The optical source used is a home-made SESAM-based mode-locked fiber laser centered at 1941 nm with a repetition rate of 12.59 MHz and 3-dB bandwidth of ∼0.2 nm. It must be pointed out that the nonlinear optical properties of the stretched GIMF device is different at different stretching length [32]. Here, the modulation depth, as well as the nonsaturable absorption loss of our sample was performed at the stretching length of 220 μm. The transmittance increases as the incident power intensity becomes larger due to saturable absorption, as shown in Fig. 1. The measured datas were fitted with a commonly used formula, taken from [39]:
Fig. 1 The measured and fitting nonlinear transmission curve with indicated saturable absorption parameters.
where T is the transmission, α is the modulation depth, I is the input light intensity, Isat is the saturation intensity and αns is the nonsaturable loss. The absorption modulation depth is measured to be 9.5% and the corresponding saturation fluenceis 142.8 μJ/cm2. The nonsaturable losses are at the level of 61.6%. This fitting method agrees well with the experimental data obtained. With such a low saturable intensity, it can be predicted that the mode-locking threshold can be very low. This SA can be used in the fiber laser for producing stable and robust mode-locked pulse trains.
3. Experimental setup
The ring-shaped cavity of the stretched SMF-GIMF-SMF mode-locked fiber laser is schematically illustrated in Fig. 2. A 2 m Tm-doped fiber (Coractive, SCF-TM-8/125) which has a core numerical aperture of 0.15 and absorption coefficient of ∼12dB∕m at 1570 nm is used as the gain medium. The gain fiber is pumped via a 1550/2000 wavelength division multiplexer (WDM) from a 1570nm fiber laser. A 10:90 coupler is used to direct the pulse out from the cavity. The unidirectional oscillation is ensured by the employment of a polarization-independent optical isolator (PI-ISO). A polarization controller (PC) is used to optimize the mode-locking operation as well as the intra-cavity birefringence. The key component, SMF-GIMF-SMF SA, is inserted between the PI-ISO and PC. In total, there is about 6.5m of SMF-28e pigtail fiber in the laser cavity. The whole cavity length is ~10.6m. The laser output is connected to an optical spectrum analyzer (Yokogawa, AQ6375) and a 1GHz (Tektronix, DPO7104C) oscilloscope together with a 12.5GHz photo-detector (EOT, ET-5000F) to allow simultaneous measurement of the spectra and the pulse train by using a 50/50 coupler.
Fig. 2 Schematic of the Tm-doped fiber laser mode-locked by the stretched SMF-GIMF-SMF saturable absorber. TDF: thulium-doped fiber.
4. Results and discussions
4.1 Mode-locking operation
Figure 3 summarizes the mode-locking results obtained from the mode locked fiber laser. Figures 3(a) and 3(b) show the measured output spectrum and the autocorrelation profile of the mode-locked pulse. The measured optical spectrum is centered at 1931.5nm with a 3dB bandwidth of 3.77 nm. The shape with clearly Kelly spectral sidebands certifies good soliton operation in the anomalous dispersion regime. The temporal duration is ∼1.2 ps. The corresponding time–bandwidth product (TBP) of the soliton pulses is calculated to be 0.364 by assuming a hyperbolic secant squared (sech2) pulse profile, which is very close to the transform-limited value of a soliton pulse of 0.315. Figures 3(c) and 3(d) show the pulse train and the radio frequency (RF) spectrum. It could be seen that the pulse train has a 50.2 ns interval between two adjacent pulses, giving an19.94 MHz repetition rate, which verifies that the oscillator is operating at the fundamental mode-locking state. A signal-to-noise (SNR) ratio is at least 62dB above the noise floor, which confirms the high stability of our fiber laser. Once the mode-locking pulse is obtained, the laser could maintain the mode-locking operation until the pump power is decreased to 100mW. At the pump power of 110mW, the maximum output power at the single-pulse operation is measured to be 0.25 mW, corresponding to the pulse energy of 12.5 pJ.
Fig. 3 Mode-locking pulse measurements.(a) Laser spectrum; (b) Autocorrelation race of output pulses. (c) Oscilloscope trace; (d) RF spectrum measure at the fundamental repetition of 19.94MHz.
In order to figure out the pros and cons of our mode-locked Tm fiber laser incorporating a stretched GIMF-based SA relative to the same sort of lasers using SESAM or two-dimensional (2D) materials-based SAs, the output performance of our laser was compared to that of those lasers. Table 1 summarizes the results. The temporal width of the output pulses from our laser is almost comparable to those from the lasers using SESAM or 2D materials-based SAs. The modulation depth of our stretched GIMF-based SA is approximately a half of those of SESAMs [6] and is approximately 2.5 times larger than that of the grapheme and BP-based ones [7,10]. Even if the saturation fluence of our stretched GIMF-based SA is much higher than those of SESAMs and 2D materials-based SAs, its pump threshold is much smaller than that of SESAMs and 2D materials-based SAs which contributes to its all fiber structure. The large nonsaturable losses of our stretched GIMF-based SA may attribute to the large transmission loss of the GIMF fiber at 2μm region. Compared with our previous work [35], the pump threshold of our laser is much lower than that of the laser using the bending GIMF SA. In addition, the stretching on GIMF is done to replace the bending on GIMF to achieve saturable absorption which is easier to be applied in practice.
Table 1. Performance at 2μm wavelength in comparison with others
4.2 Wavelength-tunable Mode-locking operation
It is well known that the SMF-GIMF-SMFstructure can act as a bandpass filter based on the MMI effect. Thus, tunable wavelength mode-locking, resulting from the bandpass filtering of the SMF-GIMF-SMF structure [32], is investigated.
According to MMI theory, the peak wavelength of a MMI device is given as [40,41]
Where nMMF, DMMF, and L correspond to the refractive index, core diameter, and length of the MMF, respectively, and P isthe self-image number. As shown in Eq. (2), the peak wavelength response of the MMI filter can be selected by changing the refractive index, the length, or the diameter of the MMF [40].The combined effect of strain and temperature on the lengthof the MMF is [42,43]
Where β is the temperature coefficient. It can be found that an applied strain ε and a temperature variation ΔT of the MMF can change the length of the MMF [32] based on Eq. (2). The peak wavelength drift coefficient due to thermal effect is about 0.05 nm/°C [44,45], so the effect of temperature on the length of the MMF can be ignored in a short period of time. Given that the effect of temperature on the length of the MMF is almost negligible, the length variation ΔL of GIMF changes almost linearly with the strain ε for a certain MMF. Based on Eqs. (2) and (3), when tensile stress is applied on the MMF, the peak wavelengthwill be tuned by changing the strain on the MMF.
Keeping the PC orientation unchanged, continuously wavelength tunable mode-locking operation was obtained by changing the stretched GIMF length, as shown in Fig. 4. We tested the wavelength tuning range in three samples with different GIMF length (15cm, 23.5cm, 43cm) respectively. The ΔL in Fig. 4 is referred as the extra stretch we imposed to get the tuning range of the wavelengths. The mode-locked wavelength can be tuned by 4.3nm, 5.2nm, 4.9nm respectively. Although the largest ΔL was obtained at the longest GIMF of 43 cm, the largest tuning rang of 5.2 nm was obtained when the GIMF length is ~23.5cm. In order to find out the factors that affect the tuning rang, the central wavelength shift depending on the stretched ratio of GIMF length (the stretched length/the GIMF length) of the three samples mentioned above was illustrated, as shown in Fig. 5. As could be seen from the figure, the tuning rang is proportional to the stretched ratio. Thus, the largest wavelength shift was obtained at 23.5cm GIMF which has the highest stretched ratio.
Fig. 4 Tunable single-wavelength mode locking with the GIMF length of (a)15cm, (b) 23.5cm and (c)43cm.
Because the stretching effect is elastic in nature so the location of the wavelength peaks could be recovered by unstretching the GIMF with the same amount. Thus, we also test the tuning repeatability of such a system. As Fig. 6 shows, when we increase or decrease the stretched GIMF length, the peak wavelength of the mode-locked fiber laser has a good reversible response on the stretched GIMF length in the repeated experiments.The wavelength response was stable, which indicates the stretched GIMF structure can realize bidirectional wavelength tuning. Notably, the wavelength tuning speed is fast because the change on the stretched length of GIMF is instantly effective. In the future, if an electrical controlling system based on a stepper motor is introduced to control the movement of the GIMF, the wavelength tuning precision of such a system can be improved. The repeatable and controllable tuning process makes the stretched GIMF advantageous for tuning the wavelength of the mode-locked lasers.
Fig. 6 Mode locking Peak wavelength with the stretched GIMF length increased and decreased repeatedly.
Finally, the long-term operation stability of the fiber laser was checked by measuring the spectra variation of the output pulses at the pump power of 100 mW and the results are shown in Fig. 7. The measured spectra were recorded every 1 hour for 7 hours under experimental conditions in which the fibers, especially the SA structure, were carefully mounted and environmental perturbations were minimized. Apparently, the spectra are the same, suggesting good long-term stability.We note that the central spectral peak location, spectral bandwidth and spectral strength remained reasonably stable over the time period of recordings. In addition, the fiber laser could achieve self-starting mode-locked operation for several days during our experimental observations.
5. Conclusions
In conclusion, we experimentally demonstrate a wavelength-tunable mode-locking Tm-doped fiber laser with a stretched SMF-GIMF-SMF SA for the first time, to the best of our knowledge. Continuously mode-locking wavelength tuning was achieved by taking advantage of the bandpass filter effect based on the MMI effect of SMF-GIMF-SMF structure. By tuning the stretched GIMF length in the laser cavity, the operating wavelength of mode-locked pulses can be tuned from 1942 nm to1947 nm repetitively and controllably. Such a system provides a compact, user friendly and low cost wavelength tunable ultrashort pulse source in the 2μm region. The easy fabrication and integration of SAs based on the stretched GIMF are extremely valuable for the mode-locking operation at broad wavelength ranges.
Funding
Natural Science Foundation of Zhejiang Province, China (Y18E020026).
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