Open Access
Add to BibSonomy
Abstract
This research experimentally demonstrates a switchable, single-wavelength, thulium-doped fiber laser based on the cascading of a multimode–single-mode–multimode (MSM) fiber filter and a two-mode fiber (TMF) filter. When the MSM fiber filter suffers from bending, the blue-shift of the output spectrum can be obtained. A switchable lasing wavelength output is realized by bending the MSM fiber filter to cover different channels of the TMF filter. The output wavelength can be switched from 1982.54 to 1938.81 nm with an optical signal-to-noise ratio of higher than 40 dB. The wavelength interval of the switchable output is an integral multiple of the wavelength interval of the TMF filter. The stability of the output wavelength was tested within 60 min, and the wavelength shift and output power fluctuation were found to be less than 0.01 nm and 0.31 dB, respectively, which demonstrates a stable output performance.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
The interest in thulium-doped fiber lasers (TDFLs) has been greatly sparked in recent years. Compared with erbium- or ytterbium-doped fiber, thulium-doped fiber (TDF) exhibits the advantage of a wide gain spectrum range, which makes it suitable for high-capacity optical communication systems [1]. Due to their favorable absorption in water, TDFLs can also be applied in many medical applications, especially low-invasive surgery [2]. Moreover, the applications of gas-sensing, light detection and ranging (LIDAR), and wavelength-routing networks can be conveniently satisfied by TDFLs [3,4]. Existing research on TDFLs is mainly focused on multi-wavelength, single-longitudinal, high-power, and mode-lock outputs [5–8].
In addition, a switchable single-wavelength output is one of the most important characteristics of TDFLs, and affords them the ability to be implemented in fiber-sensing, optical instrumentation, and free-space optical communication. Multiple techniques have been proposed to achieve a switchable single-wavelength output. For instance, in 2013, an ultra-wideband tunable TDFL was proposed by utilizing a tunable filter, and the tuning range was greater than 200 nm; however, the optical signal-to-noise ratio (OSNR) was ∼40 dB, which can be further improved [9]. A switchable single-wavelength output can be realized by tuning the polarization controller (PC) to change the polarization state of the cavity, and this has been investigated in detail by the present research group and others [10–13]. However, manually controlled PCs demonstrate a random tuning characteristic, and the output channel cannot be precisely controlled. While fiber Bragg gratings and external diffraction gratings could be used to realize a precisely controlled, switchable, single-wavelength output [14–17], the fabrication of these gratings is costly. Based on the liquid level of the index-matching fluid and multimode interference, a tunable TDFL has been demonstrated [18]; however, the implementation method is impractical for real applications. In 2019, an in-fiber, acousto-optic, tunable bandpass filter was used to accomplish a tunable TDFL with a wide tuning range of 211.5 nm [19]; however, it requires the incorporation of two independent signal sources. Thus, a passive, compact, cost-effective, and easily fabricated tunable filter should be investigated for the facilitation of switchable TDFLs.
Different approaches have been adopted to stabilize the output wavelength, such as intensity-dependent loss (IDL) [20], the polarization hole-burning (PHB) effect [21], four-wave mixing (FWM) [22], and balancing the gain and loss in the cavity [23], among others. To realize the IDL, PHB, and FWM approaches, extra optical elements must be incorporated into the cavity. The gain and loss in the cavity can be controlled by a tunable optical fiber filter, which can simplify the configuration of the TDFL. Recently, the in-line compact multimode–single-mode–multimode (MSM) fiber filter has been extensively investigated for use in sensing applications [24–26], and its characteristics of being cost-effective, having a compact structure, and being easy to fabricate may facilitate a switchable TDFL.
In this research, an MSM fiber filter was adopted to act as a tunable filter to adjust the gain and loss of the cavity. Together with a uniform comb-like filter, a switchable single-wavelength output was obtained by bending the MSM filter. First, the transmission characteristics of the MSM filter were experimentally investigated in detail with different fabrication parameters. Second, a cascaded fiber filter was constructed with the selected MSM filter and a two-mode fiber (TMF) filter. The tunable characteristics of the cascaded filter were studied under different bending statuses. Based on the analysis, a switchable single-wavelength TDFL was proposed and realized, and the stability and repeatability of the fiber laser were tested. Finally, the output performance is discussed with respect to the stability, tuning range, and tuning step of the fiber laser.
2. Operational principle
2.1 MSM fiber filter
The MSM fiber filter is shown in Fig. 1. It was constructed by sandwiching a section of single-mode fiber (SMF) into two sections of multimode fiber (MMF, SI2012-J, YOFC). The core/cladding diameter of the MMF was 105/125 µm, and the numerical aperture was 0.15. The lead-in SMF was used to illuminate MMF1, and the lead-out SMF was used to guide the light beam out. The fabrication of the MSM fiber filter was conducted as follows. First, a section of MMF was fusion-spliced with the lead-in SMF in a core-alignment manner. Second, the redundant section of MMF was cut using a fiber cleaver with a resolution of 1 mm to obtain the desired length of MMF1. Third, via the same method, the desired length of the SMF was fusion-spliced at the right part of MMF1. By repeating the first and second steps, a cascaded structure of MMF2 and the lead-out SMF was obtained. Finally, the two cascaded structures were fusion-spliced to form the MSM fiber filter.
MMF1 and MMF2 served as the mode-splitter and mode-combiner, respectively. The two equivalent arms of a Mach–Zehnder interferometer were constructed by the core and cladding of the SMF between MMF1 and MMF2. When the emitting light from the lead-in SMF is injected into MMF1, due to the core-aligned splicing, only circular symmetric high-order modes (LP0m, m = 2, 3, 4…) can be excited and propagate through the cladding of the SMF. The core mode and cladding modes interfere with each other at the left splicing point of MMF2, and the multi-channel optical spectrum can be observed after the lead-out SMF.
The phase difference between the core mode and the k-th cladding mode can be expressed as
where $\Delta \textrm{n}_{\textrm{eff}}^\textrm{k}$ is the effective refractive index difference between the core and the k-th cladding modes. Ls is the length of the SMF, and λ represent the wavelength. By considering the excitation of only one dominant cladding mode, the transmission equation of the MSM filter is where ${\textrm{I}_{\textrm{co}}}$ and ${\textrm{I}_{\textrm{cl}}}$ are the intensities of the core and dominant cladding modes, respectively, and $\Delta {\textrm{n}_{\textrm{eff}}}$ is the effective refractive index difference of the core and dominant cladding modes. Additionally, the free spectral range (FSR) can be expressed as $\textrm{FSR}\; = {\mathrm{\lambda }^\textrm{2}}/({\Delta {\textrm{n}_{\textrm{eff}}}{\textrm{L}_\textrm{s}}} )$.In the experiment, when the length of the SMF was 137 mm, different lengths of MMF were symmetrically spliced at both sides of the SMF. The obtained relative transmission was calculated by subtracting the amplified spontaneous emission spectrum of the TDF from the transmission spectrum of the MSM fiber filter, and the results are presented in Fig. 2(a). The FSR remained unchanged when different lengths of MMF were adopted. However, the insertion loss (IL) and extinction ratio (ER) varied. The IL and ER with respect to the length of MMF are presented in Fig. 2(c). In general, it is necessary for an optical fiber filter to have a small IL and a high ER; thus, the length of the MMF was fixed at 3 mm in the subsequent experiment.
Fig. 2. Relative transmission of the MSM filter with respect to different lengths of (a) MMF and (b) SMF; ER and IL values of the MSM filter with respect to different lengths of (c) MMF and (d) SMF.
Different lengths of SMF were spliced between MMF1 and MMF2, and the length of the MMF was fixed at 3 mm. The relative transmission is exhibited in Fig. 2(b). When the lengths of the SMF were 10, 30, 50, and 80 mm, a uniform multi-channel filter could be obtained, which demonstrates that there existed two dominant modes that interfered with each other. According to the definition of the FSR, it is inversely proportional to the length of the SMF. Thus, with the increase of the length of the SMF, a narrower channel spacing was obtained. When the length of the SMF was 200 mm, inhomogeneous transmission was observed, which indicates that other transmission modes also contributed to the formation of the comb-like filter. This phenomenon has been observed in previous research [26]. However, the FSR of the filter when the length of the SMF was 200 mm was found to be in accordance with the trend of the calculation formula of the FSR, which demonstrates that the two dominant modes that interfered with each other remained unchanged. The relationship between the FSR and the length of the SMF is exhibited in the bottom-right area of Fig. 2(b). The FSR values obtained in the experiment were found to be in good agreement with the simulation results. Figure 2(d) presents the ER and IL with respect to the length of the SMF when the length of the MMF was fixed at 3 mm. A tunable filter requires a wide FSR, high ER, and low IL; thus, the length of the SMF was fixed at 10 mm in the subsequent experiment, and the total length of the MSM filter was 16 mm.
2.2 Two-mode fiber filter and the characteristics of the cascaded filter
To construct a cascaded filter, another uniform comb-like filter with a smaller FSR should be incorporated. In this study, the TMF filter was used, which has been investigated in detail in the authors’ previous work [27], and the structure is exhibited in Fig. 3. It was manufactured by sandwiching a section of the TMF into two sections of the SMF with offset-splicing at both sides of the TMF.
Similar to the principle of the MSM filter, the principle of the TMF filter is based on mode interference. However, the modes that interfere with each other in the TMF filter are the core modes (LP01 and LP11) in the TMF. The transmission of the TMF filter can be calculated with an equation similar to Eq. (2). In the experiment, the length of the TMF was 4 m, and the relative transmission is presented in Fig. 4(a). The ER is 8.4 dB. The FSR is 1.3 nm, and the IL is ∼7.5 dB.
Fig. 4. (a) Relative transmission of the TMF filter; (b) relative transmission of the cascaded MSM filter and TMF filter.
Figure 4(b) exhibits the relative transmission of the cascaded MSM filter and the TMF filter. The transmission spectrum of the TMF filter is modulated by the transmission spectrum of the MSM filter; thus, a modulated relative transmission can be obtained with a non-uniform transmissivity. When it is applied in the ring cavity, a laser with high gain can be emitted. Due to the compact structure of the MSM filter, it is very convenient to fix the filter on the translation stages using fiber clamps, as shown in the experimental configuration. One of the translation stages is fixed, while the other is movable. When the fiber suffers from bending as the movable translation stage moves to the + z-direction over a distance of δz, a refractive index difference between the interfering modes can be generated, and the blue-shift of the transmission spectrum can be observed, as shown in Fig. 4(b). This characteristic is feasible for the facilitation of a precisely switchable TDFL.
3. Experimental configuration
Based on the analysis of the filters, the experimental setup is displayed in Fig. 5. The pump light from a 793-nm laser diode with a multimode pigtail was injected into the ring cavity via a (2 + 1)x1 fiber combiner (FC). The backward-pumped double-clad TDF (SM-TDF-10P-130M, Nufern) used as the gain medium was 1.85 m in length. An isolator was inserted into the ring cavity to enforce counter-clockwise transmission. The polarization state in the cavity was controlled by a three-loop PC. Two filters were cascaded and fusion-spliced to select the desired lasing channel. The emitted light was monitored using an optical spectrum analyzer (OSA, AQ6375, Yokogawa) via the 10% port of the optical coupler (OC) with a coupling ratio of 90:10, and the 90% port of the OC was connected to the isolator to enclose the ring cavity.
Fig. 5. Experimental configuration of the TDFL. FC: fiber combiner, TDF: thulium-doped fiber, ISO: isolator, OC: optical coupler, TMF: two-mode fiber, PC: polarization controller.
4. Experimental results and discussion
4.1 Experimental results
During the experiment, the pump power was set as 3.58 W. When the MSM filter was kept straight (δz = 0 mm), a single-wavelength output at 1982.54 nm was obtained by properly adjusting the PC, and the result is shown in Fig. 6(a). The OSNR was ∼48.4 dB. The stability was tested within 60 min with time intervals of 5 min. The repeated scans of the output spectra were recorded, and are shown in the inset of Fig. 6(a). Neither an obvious optical power fluctuation nor a wavelength shift was observed. To further quantify the stability, the optical power fluctuation and wavelength shift with respect to time are presented in Fig. 6(b). The maximum wavelength shift was 0.01 nm and the optical power fluctuation was less than 0.251 dB, which demonstrates a stable output performance.
Fig. 6. Single-wavelength output at (a) δz = 0 and (c) δz = 0.21 mm; the wavelength shift and output power fluctuation of (b) 1982.54 nm (d) 1951.31 nm.
When the MSM filter was bent (δz = 0.21 mm), a shorter wavelength at 1951.31 nm with an OSNR of ∼44.3 dB was obtained, as shown in Fig. 6(c). The degenerate OSNR may have originated from the loss induced by the bending of the MSM filter. The output spectrum was recorded every 5 min for a total of 13 times, and the spectra are shown in the inset of Fig. 6(c). The relationships of the wavelength and output power with time are demonstrated in Fig. 6(d). The wavelength shift and optical power fluctuation were less than 0.01 nm and 0.31 dB, respectively. The optical power fluctuation was slightly larger than that of the output channel at 1982.54 nm, which indicates that the bending of the MSM filter may be more sensitive to the environmental surroundings.
When the movable stage moved toward the + z-direction over a distance of δz, the output wavelength exhibited a blue-shift, as shown in Fig. 7(a). The wavelength interval is an integral multiple of the wavelength interval of the TMF filter. At a specific position of δz, a corresponding lasing channel can be obtained. It should be noted that, during the adjustment of δz, the PC was kept unchanged. When δz was adjusted from 0 to 0.29 mm with a step of 0.01 mm, the lasing channel was switched from 1982.54 to 1938.81 nm. The repeatability was also tested in the experiment, and the results are shown in Fig. 7(b). An approximately linear relationship between the output wavelength and δz was observed. The experimental results exhibited in Fig. 7(b) demonstrate the good repeatability of the TDFL.
In the experiment, the tuning range was ∼43.73 nm, which is far less than the FSR (83 nm) of the MSM filter. This is mainly determined by the bending of the MSM filter. The shortest wavelength at 1938.81 nm was obtained at δz = 0.29 mm. With the further increase of δz, due to the optical loss induced by bending, no lasing channel was obtained. It should be highlighted that, during the experiment, an MSM filter with a narrower FSR was adopted when the length of the SMF was 30 or 137 mm, and a stable single-wavelength output could not be obtained. The possible reasons for this are as follows. First, the envelope enforced on the transmission spectrum of the TMF filter depressed the wavelength competition to some extent. Moreover, although the tunable filter was formed by multimode interference, the two dominant modes caused the transmission spectrum of the MSM filter to be very uniform, and, together with the narrowed FSR, more intense wavelength competition occurred. This led to the unstable single-wavelength output.
4.2 Discussion
When the MSM fiber filter suffers from bending, the effective refractive index difference of the core and dominant cladding modes can be expressed as [28]
where $\Delta {\textrm{n}_{\textrm{eff0}}}$ and k represent the effective refractive index difference when the MSM fiber filter under straight state and the strain refractive index coefficient, respectively, and d is the distance of the core and cladding. When the structure under bending state, the outer/inner layer of the fiber suffers from tension/compression, and the mode field distribution of cross section shifts to the convex direction, which leads to the change of the refractive index difference [29]. The resonant dip wavelength can be expressed asA stable single-wavelength output was obtained via the balance of the gain and loss in the cavity by moving the movable stage. During the experiment, when δz was increased or decreased, the MSM filter was suspended in open air. Compared with the straight state of the filter, the bending of the MSM filter was found to be more vulnerable to the vibrations in the environmental surroundings. Temperature also has effects on the performance of MSM filter and may be a possible laser-tuning mechanism. However, since two sections of common MMF and a section of SMF were adopted in the MSM filter, the wavelength drift coefficient of the filter induced by the thermal effect is only ∼0.05 nm/°C [13,26]. Further considering the additional active temperature control equipment needed, the temperature induced wavelength tuning is more complex and inefficiency. Thus, in this work the tunability was realized by bending of the MSM filter, and to further improve the stability of the proposed fiber laser, the filter should be glued on a cantilever arm or elastic steel, and well-packaged in an environment with a constant temperature. Noted that, by bending other parts of the cavity, the optical loss increases and the blued-shift of the output spectrum can not be obtained
To further characterize the TDFL, a TMF filter with a wavelength interval of ∼4 nm and an IL of ∼1.35 dB was adopted, and the other parts of the laser were unchanged. The laboratory-prepared TMF filter was fabricated by offset-splicing a section of TMF (∼1.41 m) between two sections of SMF. A switchable single-wavelength output was obtained, and the single-wavelength output when δz = 0 mm is shown in Fig. 8(a). The OSNR was ∼55 dB, which is higher than that shown in Fig. 6(a). The high OSNR may have benefitted from the lower IL. By tuning the movable stage toward the + z-direction, a switchable single-wavelength output was obtained, and the results are displayed in Fig. 8(b). A lasing channel at 1946.91 nm was obtained when δz = 0.24 mm. With the further increase of δz, a longer wavelength was obtained, which indicates a new circle of the switchable output. However, the loss introduced by bending the MSM filter reduced the OSNR of the lasing channel. Thus, it is not suggested to continue increasing δz. The repeatability was also tested, and the results are presented in Fig. 8(c). The wavelength difference between the increase and decrease of δz was less than 0.01 nm, which indicates good repeatability. In the experiment, a dual-wavelength output was obtained when δz = 0.3 mm, as shown in Fig. 8(d). The large wavelength interval of the TMF filter may alleviate the wavelength competition to some extent, and the dual-wavelength output can work stably in a short period of time. The dual-wavelength lasing can be suppressed by adjusting the PC to balance the gain and loss in the cavity [11,12], and it is possible to induce dual/multi-wavelength lasing operation in the other filter configuration [23].
Fig. 8. (a) Single-wavelength output at 1988.71 nm; (b) switchable output and (c) repeatability of the TDFL; (d) dual-wavelength output of the TDFL.
Comparisons of the proposed fiber laser under different parameters with other works was carried out and the results are shown in Table 1. The proposed TDFL exbibits the minimum wavelength shift and power fluctuation, which demonstrates the superior stability. In addition, the lasing range is larger than that in [11], [13] and [20], comparable with that in [18] and [31]. However, the switchable output in [18] is based on the change of liquid level, which is not applicable from a practical point of view, and the number of translation stages used in [31] doubled compared with this work, which complicates the experimental setup. In all, the proposed TDFL exhibits favorable stability, large lasing range and simple configuration.
Table 1. Comparison of the proposed TDFL under different parameters with other works.
5. Conclusion
In this research, a stable, precisely controlled, and switchable single-wavelength TDFL was proposed and experimentally demonstrated. Two mode interference filters with the advantages of easy fabrication and cost-effectiveness were cascaded to select the desired lasing channel. By bending the cascaded filter, a precisely controlled lasing channel was obtained. The stability of the output wavelength was monitored by an OSA within 60 min without temperature control or vibration isolation, and the wavelength shift and output power fluctuation were less than 0.01 nm and 0.31 dB, respectively. The repeatability of the TDFL was also investigated by adjusting the movable stage both forward and backward. The fiber laser was found to demonstrate good repeatability with a wavelength fluctuation of less than 0.01 nm. In the future, enlarging the lasing range and further improving the OSNR deserve further dedicated investigations. The proposed fiber laser may have applications in optical fiber-sensing or free-space optical communication systems.
Funding
Fundamental Research Funds for the Central Universities (2020YJS009); National Natural Science Foundation of China (61620106014, 61775128, 61827818, 61975009, 61975049).
Disclosures
No conflict of interest exits in the submission of this manuscript.
References
1. K. Yin, R. Zhu, B. Zhang, G. Liu, P. Zhou, and J. Hou, “300 W-level, wavelength-widely-tunable, all-fiber integrated thulium-doped fiber laser,” Opt. Express 24(10), 11085–11090 (2016). [CrossRef]
2. M. Michalska, W. Brojek, Z. Rybak, P. Sznelewski, M. Mamajek, and J. Swiderski, “Highly stable, efficient Tm-doped fiber laser—a potential scalpel for low invasive surgery,” Laser Phys. Lett. 13(11), 115101 (2016). [CrossRef]
3. F. J. McAleavey, J. O’Gorman, J. F. Donegan, B. D. MacCraith, J. Hegarty, and G. Maźe, “Narrow linewidth, tunable Tm3+-doped fluoride fiber laser for optical-based hydrocarbon gas sensing,” IEEE J. Quantum Electron. 3(4), 1103–1111 (1997). [CrossRef]
4. J. Geng, Q. Wang, Y. Lee, and S. Jiang, “Development of eye-safe fiber lasers near 2 µm,” IEEE J. Sel. Top. Quantum Electron. 20(5), 150–160 (2014). [CrossRef]
5. X. Wang, Y. Zhu, P. Zhou, X. Wand, H. Xiao, and L. Si, “Tunable, multiwavelength Tm-doped fiber laser based on polarization rotation and four-wave-mixing effect,” Opt. Express 21(22), 25977–25984 (2013). [CrossRef]
6. Q. Zhang, Y. Hou, X. Wang, W. Song, X. Chen, W. Bin, J. Li, C. Zhao, and P. Wang, “5 W ultra-low-noise 2 µm single-frequency fiber laser for next-generation gravitational wave detectors,” Opt. Lett. 45(17), 4911–4914 (2020). [CrossRef]
7. Z. Zhang, A. J. Boyland, J. K. Sahu, W. A. Clarkson, and M. Ibsen, “High-power single-frequency thulium-doped fiber DBR laser at 1943nm,” IEEE Photonics Technol. Lett. 23(7), 417–419 (2011). [CrossRef]
8. M. A. Chernysheva, A. A. Krylov, N. R. Arutyunyan, A. S. Pozharov, E. D. Obraztsova, and E. M. Dianov, “SESAM and SWCNT mode-locked all-fiber thulium-doped lasers based on the nonlinear amplifying loop mirror,” IEEE J. Sel. Top. Quantum Electron. 20(5), 448–455 (2014). [CrossRef]
9. Z. Li, S. U. Alam, Y. Jung, A. M. Heidt, and D. J. Richardson, “All-fiber, ultra-wideband tunable laser at 2 µm,” Opt. Lett. 38(22), 4739–4742 (2013). [CrossRef]
10. M. Wang, Y. Huang, L. Yu, Z. Song, D. Liang, and S. Ruan, “Multiwavelength thulium-doped fiber laser using a micro fiber-optic Fabry–Perot interferometer,” IEEE Photonics J. 10(4), 1–8 (2018). [CrossRef]
11. Q. Qin, F. Yan, Y. Liu, L. Zhang, Y. Guo, W. Han, Z. Bai, T. Feng, and H. Zhou, “Isolator-free unidirectional dual-wavelength thulium-doped fiber laser assisted by a two-mode fiber filter,” Opt. Laser Technol. 134, 106638 (2021). [CrossRef]
12. Y. Guo, F. Yan, T. Feng, L. Zhang, Q. Qin, W. Han, Z. Bai, H. Zhou, and Y. Suo, “Switchable multi-wavelength thulium-doped fiber laser using four-mode fiber based Sagnac loop filter,” IEEE Photonics J. 12(2), 1–10 (2020). [CrossRef]
13. P. Zhang, T. Wang, W. Ma, K. Dong, and H. Jiang, “Tunable multiwavelength Tm-doped fiber laser based on the multimode interference effect,” Appl. Opt. 54(15), 4667–4671 (2015). [CrossRef]
14. J. Li, Z. Sun, H. Luo, Z. Yan, K. Zhou, Y. Liu, and L. Zhang, “Wide wavelength selectable all-fiber thulium doped fiber laser between 1925nm and 2200 nm,” Opt. Express 22(5), 5387–5399 (2014). [CrossRef]
15. L. Zhang, F. Yan, T. Feng, W. Han, Y. Bai, Z. Bai, D. Cheng, H. Zhou, and Y. Suo, “Wavelength-tunable thulium-doped fiber laser with sampled fiber Bragg gratings,” Opt. Laser Technol. 120, 105707 (2019). [CrossRef]
16. F. Liu, P. Liu, X. Feng, C. Wang, Z. Yan, and Z. Zhang, “Tandem-pumped, tunable thulium-doped fiber laser in 2.1 µm wavelength region,” Opt. Express 27(6), 8283–8290 (2019). [CrossRef]
17. S. Chen, Y. Jung, S. U. Alam, J. Wang, R. Sidharthan, D. Ho, S. Jain, S. Yoo, and D. J. Richardson, “Ultra-wideband operation of a tunable thulium fibre laser offering tunability from 1679–1992nm,” in Proceedings of IEEE European Conference on Optical Communication (IEEE, 2017), pp. 1–3.
18. X. Ma, D. Chen, Q. Shi, G. Feng, and J. Yang, “Widely tunable thulium-doped fiber laser based on multimode interference with a large no-core fiber,” J. Lightwave Technol. 32(19), 3234–3238 (2014). [CrossRef]
19. E. Hernández Escobar, M. Bello Jiménez, A. Camarillo Avilés, R. López Estopier, O. Pottiez, M. Durán Sánchez, B. Ibarra Escamilla, and M. V. Andrés, “Experimental study of an in-fiber acousto-optic tunable bandpass filter for single-and dual-wavelength operation in a thulium-doped fiber laser,” Opt. Express 27(26), 38602–38613 (2019). [CrossRef]
20. Q. Zhao, L. Pei, J. Zheng, M. Tang, Y. Xie, J. Li, and T. Ning, “Switchable multi-wavelength erbium-doped fiber laser with adjustable wavelength interval,” J. Lightwave Technol. 37(15), 3784–3790 (2019). [CrossRef]
21. T. Feng, M. Wang, X. Wang, F. Yan, Y. Suo, and X. S. Yao, “Switchable 0.612-nm-spaced dual-wavelength fiber laser with sub-kHz linewidth, ultra-high OSNR, ultra-low RIN, and orthogonal polarization outputs,” J. Lightwave Technol. 37(13), 3173–3182 (2019). [CrossRef]
22. T. Huang, X. Li, P. P. Shum, Q. J. Wang, X. Shao, L. Wang, H. Li, Z. Wu, and X. Dong, “All-fiber multiwavelength thulium-doped laser assisted by four-wave mixing in highly germania-doped fiber,” Opt. Express 23(1), 340–348 (2015). [CrossRef]
23. B. Ibarra-Escamilla, M. V. Hernández-Arriaga, M. Duránsánchez, H. Santiago-Hernández, M. Bello-Jiménez, E. Rivera Pérez, L. A. Rodríguez-Morales, and E. A. Kuzin, “Abrupt-tapered fiber filter arrangement for a switchable multi-wavelength and tunable Tm-doped fiber laser,” Opt. Express 26(12), 14894–14904 (2018). [CrossRef]
24. P. Niu, J. Jiang, S. Wang, K. Liu, Z. Ma, Y. Zhang, W. Chen, and T. Liu, “Optical fiber laser refractometer based on an open microcavity Mach-Zehnder interferometer with an ultra-low detection limit,” Opt. Express 28(21), 30570–30585 (2020). [CrossRef]
25. T. Sun, Z. Liu, Y. Liu, Y. Zhang, Z. Jing, and W. Peng, “All-fiber liquid-level sensor based on in-line MSM Fiber Structure,” Photonic Sens. 4, 1–7 (2020). [CrossRef]
26. B. Yin, Y. Li, Z. Liu, S. Feng, Y. Bai, Y. Xu, and S. Jian, “Investigation on a compact in-line multimode-single-mode-multimode fiber structure,” Opt. Laser Technol. 80, 16–21 (2016). [CrossRef]
27. Q. Qin, F. Yan, Y. Liu, Y. Guo, L. Zhang, T. Li, T. Feng, and H. Zhou, “Investigation of a multiwavelength thulium-doped fiber laser incorporating a two-mode fiber filter,” Infrared Phys. Technol. 108, 103360 (2020). [CrossRef]
28. L. Niu, C. Zhao, H. Gong, Y. Li, and S. Jin, “Curvature sensor based on two cascading abrupt-tapers modal interferometer in single mode fiber,” Opt. Commun. 333, 11–15 (2014). [CrossRef]
29. Y. Wu, L. Pei, W. Jin, Y. Jiang, Y. Yang, Y. Shen, and S. Jian, “Highly sensitive curvature sensor based on asymmetrical twin core fiber and multimode fiber,” Opt. Laser Technol. 92, 74–79 (2017). [CrossRef]
30. B. Yang, Y. Niu, B. Yang, Y. Hu, L. Dai, Y. Yin, and M. Ding, “High sensitivity curvature sensor with intensity demodulation based on singlemode-tapered multimode-singlemode fiber,” IEEE Sens. J. 18(3), 1063–1072 (2018). [CrossRef]
31. E. Chen, S. Liu, P. Lu, J. Zhang, and Z. Lian, “Tunable 2 µm fiber laser utilizing a modified Sagnac filter incorporating cascaded polarization maintaining fibers,” IEEE Photonics J. 12(1), 1–7 (2020). [CrossRef]
