A switchable and tunable multi-wavelength Tm-doped fiber laser is successfully demonstrated using a filter constructed with two tapered fiber elements in the cavity. The proposed system design uses a low-cost simple filter that allows stable dual, triple, quadruple, and quintuple-wavelength emission operation in the region around 1.9 μm. In the dual wavelength regime, the laser is capable of independently tuning each wavelength. For switching and tuning, a curvature is applied to the tapered fibers.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
The development of switchable and tunable multi-wavelength fiber lasers has grown considerably due to their potential applications in the field of sensing [1,2], wavelength division multiplexing (WDM) for telecommunications systems  and spectroscopy . In particular, for some dual-wavelength fiber laser schemes, each laser line can be separately tuned. This feature makes these lasers attractive for applications like generating frequency-tunable THz radiation [5,6] and coherent anti-Stokes Raman scattering (CARS) imaging .
Recently, thulium doped fiber lasers (TDFL) with emission tuned over a broadband spectrum from 1.8 to 2.1 μm have gained notable interest because the tuning can avoid strong absorption of water, which enables applications in the field of light detection and ranging (LiDAR) [8,9], remote gas sensing [2,10,11], and medical instrumentation [12,13]. To achieve tunable or multi-wavelength operation of fiber lasers, several authors have implemented various techniques such as fiber Bragg gratings [14–17], four-wave mixing [18,19], nonlinear polarization rotation [19–23], and stimulated Brillouin scattering [24,25]. Some of these works have reached wide tuning range and multi-wavelength emission that involve many laser lines. Li et al  reported a broadband, wavelength selectable thulium doped fiber laser operating from 1975 to 2150 nm using fiber Bragg gratings, the tuning wavelength was over intervals around 25 nm in width. Huang et al  demonstrated a multi-wavelength thulium doped fiber laser assisted by four-wave mixing, which achieved up to 36 lasing lines but with notable amplitude variances between the lines. Often these lasers use complex devices in the cavity and require expensive components. An alternative practical and straightforward method to obtain fine tuning and stable multi-wavelength operation at room temperature is by using comb-filters. They are an efficient device that reduces the effect of mode competition. The implementation of these filters allow fine-tuning less than 1 nm and they give uniformly spaced lasing lines. These optical filters generally operate in low pump powers and are based on multimode fibers [26,27], polarization maintaining fibers (PMF) [28,29], photonic crystal fibers (PCF) , and tapered fibers which are probably one of the most attractive filter due to low cost and simple design [31,32]. The main advantage of tapered fibers is the possibility to implement a variety of wavelength selective mechanisms using strain, curvature or pressure. However, it has been demonstrated that also is possible to achieve tuning and switching operation without physical disturbances, only by changing the polarization state into the cavity [32–34]. Additionally, the reduced waist of the tapered fiber allows the propagation of a large evanescent field, which is directly affected by variations of the refractive index of surrounded medium, thus this property can be used to design other tunable schemes. This also can be used for refractive index sensing applications in the 2 μm wavelength region.
In this paper, we experimentally demonstrate the use of two abrupt-tapered fibers implemented as spectral filters for the development of a switchable multi-wavelength and tunable TDFL operating near 2 μm. The fiber tapers were fabricated to take advantage of its spectral intensity, free spectral range (FSR) and transmission spectra. Each taper waist has a 20 μm diameter. The total length of the taper (Lt) was equal to 5.4 mm for the Taper 1 and 6.2 mm for the Taper 2 and were inserted in an arrangement conformed by two 3-dB fiber couplers. To our knowledge, our filter arrangement has not been employed as wavelength-selective mechanism operating near 2 μm. This simple filter allows the generation from one to five laser lines. In the case of the dual-wavelength operation, the filter allows independent tuning of the lines. In contrast with other configurations, our filter is simple and offers the advantage of low-cost fabrication, consistent output and simple implementation that are desirable attributes for practical applications.
Design of the abrupt-tapered fibers
Most of abrupt-tapered fibers used as spectral filters are fabricated using single-mode fibers (SMF) by a standard fusion and pulling technique . This process consists of stretching an uncoated optical fiber while a heat source sweeps a specific region of the fiber. At the end of the fabrication process, a biconical structure composed of a down-taper transition, a uniform tapered waist and an up-taper transition is obtained. The tapers can be fabricated with different profiles depending on the desired application. For the purpose of our experiments, we designed filters with an abrupt tapered profile. In this case, the diameter of the waist region is highly reduced so that the core of the fiber can be neglected. Then, a new air-cladding interface is formed allowing the propagation of the fundamental mode and the cladding modes excited in the down-taper transition, as depicted in Fig. 1. Subsequently, the modes pass the up-taper transition recombining back to the fundamental mode . In this manner, an abrupt-tapered fiber operates as in-fiber interferometer where the interference pattern depends on the phase shift between modes entering to the up-taper transition. Then, the resulting intensity spectrum at the output of the taper can be approximated by the interference of two modes :38–41].
In our experiment, the tapers were fabricated with a Fujikura FSM-100P + arc fusion splicer. The Fujikura fiber processing software (FPS) sets the waist length, the waist diameter, and the transition length (the angle of taper). The waist length and the diameter for both tapered fibers (the Taper 1 and the Taper 2) were set at 4 mm and 20 μm, respectively, see Fig. 1. The down and up-taper transitions lengths were symmetric and set to 0.7 mm for the Taper 1 and 1.1 mm for the Taper 2. Longer tapered transitions will result in a narrower FSR.
We inserted these tapers in a Mach-Zehnder interferometer (MZI) configuration composed of two 3-dB couplers, as shown in Fig. 2(a). The transmission of the filter arrangement is the sum of the transmissions of the Taper 1 and the Taper 2 because the coherence length of the laser emission is much shorter than the difference between the lengths of MZI arms. The typical dependence of the transmission on the wavelength for an abrupt-tapered fiber is principally a sinusoidal shape. Thus, the transmission of our two-tapered fiber device arrangement is closely represented as the sum of two sinusoidal functions and the resultant transmission depends on the relative phase difference between the sinusoidal functions. Figures 2(b)-2(e) show four examples of the sum of transmissions for different phase shifts where the blue line represents transmission of the Taper 1, the red line represents transmission of the Taper 2, and the black line represents the resulting transmission of the filter. As can be observed we can adjust the filter by phase shifting to have one, two, three and four equal maxima of the filter’s transmission in which simultaneous lasing may occur. Experimentally, we introduce the phase shift by inducing a curvature on the tapered fibers using an xyz translation stage.
The experimental taper transmissions do not have an exact sinusoidal dependence; however, it is close and we may expect similar to theoretical variance of the filter transmission depending on the phase shift.
For the measurement of the transmission, an amplified spontaneous emission (ASE) source from thulium-doped fiber (TDF) was launched into the input end of the taper and the emission of the output end was monitored by an optical spectrum analyzer (OSA, Yokogawa AQ6375). Figure 3(a) shows the transmission for each tapered fiber when they were aligned to straight position by using two xyz translation stages. The transmission was obtained as the ratio between the output spectrum and the spectrum of the ASE of the TDF. In Fig. 3(a) we observed that the transmission exhibits close to sinusoidal wavelength dependence with a relatively large FSR for this kind of filter. The FSR for Taper 1 (blue line) is ~60 nm and for Taper 2 (red line) it is ~45 nm. The insertion losses for both tapers were around 45% at the wavelength region of this experiment. Figure 3(b) shows the sum of the transmissions of the tapers (blue line) and the measured transmission of the total filter arrangement (black line). As can be seen, the experimental transmission of the arrangement coincides well with the sum of the transmissions of the Taper 1 and the Taper 2. The pump power used for the transmission measurements was 129 mW.
Experimental setup of the TDFL
A schematic of the proposed laser is illustrated in Fig. 4. We used a ring cavity configuration. The pump source is an Er/Yb double-clad fiber laser (EYDCFL) with a wavelength centered at 1567 nm and a maximum power of 3.5 W. The output of EYDCFL is connected to a 3.5-m long Tm-doped single clad fiber (TDF Coractive: SCF-TM-8/125) through a 1550/2000 nm wavelength division multiplexer (WDM). The output of the TDF is connected to the Coupler 1, which splits the radiation between the two abrupt-tapered fibers. Each taper is fixed on both ends over the xyz translation stages, so that the tapers were suspended in air to prevent any undesired physical contact. One translation stage remained fixed while the other was moved in horizontal backwards or forwards direction. At the output of each taper was positioned an optical post providing the adjustable curvature of the fiber that was used as variable optical attenuator (VOA). One of the ports of the Coupler 2 is spliced to a 2 μm isolator ensuring unidirectional light propagation in clockwise direction. Other port of the Coupler 2 provides the laser output, which is monitored by an OSA, with a spectral resolution of 0.05 nm. The pump power was 657 mW during all experiment.
Results and discussion
Switchable multi-wavelength laser
The characterization of the laser began at 0 μm of horizontal displacement of both translation stages, i. e. the tapers were set in a straight position. In this point, a single wavelength is generated at 1954 nm with the side-mode suppression ratio (SMSR) of 51 dB, as shown in Fig. 5(a). To switch to the dual-wavelength regime, only the Taper 1 was curved with the translation stage displacement of 50 μm. The output spectrum depicted in Fig. 5(b) shows the two laser lines with wavelength of 1866.5 nm with the SMSR of 52 dB and wavelength of 1952.4 nm with the SMSR of 51 dB, the Δλ was 85.9 nm. Three laser lines were generated at the displacement of the Taper 1 by 254 μm. The wavelengths were measured as 1866.2, 1900 and 1953.9 nm with the SMSR around of 52 dB. The maximum Δλ for this case was 87.7 nm as it is observed in Fig. 5(c). Furthermore, multi-wavelength lasing with four and five lines was observed when the Taper 2 was also curved. For four and five line generation was fundamental the adjustment of the VOA to overcome the mode competition. For quadruple-wavelength regime, the Taper 1 was fixed at 254 μm of displacement and the Taper 2 was displaced by 423 μm. In this case, laser lines were observed at 1840.6, 1887.3, 1929.5 and 1976.7 nm, the SMSR was estimated as 48 dB with a minimum peak value of 45 dB for 1929.5 nm; Δλ was around 136.1 nm, see Fig. 5(d). Finally, quintuple-wavelength regime was observed by increasing the displacement of the Taper 2 to 465 μm; the Taper 1 was fixed at 254 μm of displacement. As it is observed in Fig. 5(e), laser lines were located at 1840.6, 1874.8, 1887.3, 1929.7, and 1976.6 nm. The mode competition was stronger for this regime and we cannot get good stability even using the VOA. Others laser lines could be generated with different displacements but showing higher instabilities and remarkable differences in spectral intensities.
To demonstrate the stability of the laser, we measured the spectra over a 50 min time span with measurements at intervals of 5 min. Figure 6(a) shows the stability of dual-wavelength generation. The maximum power fluctuation was around of 2 dBm. For triple and quadruple-wavelength, the maximum variation of the output power was estimated as 3.2 and 3.3 dBm, as is shown in Figs. 6(b), and 6(c), respectively. These power fluctuations could be caused by pump power fluctuations, vibrations of the components, and also by changes of the surrounding temperature that can affect the fiber taper characteristics.
Tunable dual-wavelength laser
An interesting feature of the proposed laser is the possibility of the independent tuning of the laser lines in dual-wavelength regime. Note that this feature was presented only by setting the tapered fibers under a specific curvature condition. In this case, the Taper 1 was set to straight position and the Taper 2 was displaced by 82.5 μm. The laser lines generated at this point, (λ1 and λ2) were located at 1844.7 and 1930.3 nm with a wavelength separation Δλ ~85.6 nm. The estimated SMSR was 54 dB, as shown in Fig. 7.
For tuning λ1 the Taper 2 was held fixed while one end of the Taper 1 was displaced in steps up to 38 μm. This displacement caused the shift of λ1 from 1844.7 to 1835.1 nm, while λ2 remained fixed at 1930.3 nm, see Fig. 8(a). The tuning limit was reached when the laser line switches to the next peak of the periodic dependence of the filter transmission. The dependence of the wavelength shift on the displacement was nearly linear with the tuning range of 9.6 nm, see Fig. 8(b). The SMSR was measured as 54 dB with a maximum output power difference between the tuned lines of ~0.9 dB. The inset in Fig. 8(b) shows the direction of displacement for the Taper 1.
The Taper 1 and the Taper 2 were returned to the initial positions where dual-wavelength is generated. To tune the wavelength λ2, the Taper 1 fiber remains fixed, while the Taper 2 fiber was moved toward the direction of 0 μm displacement by up to 19 μm, which decreases the curvature of the taper. We observed a linear tuning of the λ2 wavelength from 1930.3 to 1934.3 nm, while λ1 remained fixed, see Fig. 9(a). When the tuning limit was reached, the dual-wavelength regime switched to single-wavelength operation. The dependence of the wavelength shift on the displacement was nearly linear with tuning range of 4 nm, see Fig. 9(b). The inset shows the direction of the Taper 2 displacement to obtain tuning of λ2. The estimated SMSR was around 55.2 dB with a maximum output power difference between the tuned wavelengths of ~0.8 dB. Although it is possible to generate dual-wavelength emission over different spectral ranges, we found that only the case discussed above has a linear tuning with high stability.
During the tuning process, the VOA was carefully adjusted to balance gain and loss in the laser cavity in order to stabilize the intensity of λ1 and λ2 emission lines.
The proposed configuration of the TDFL based on an arrangement of two abrupt-tapered fiber filters demonstrates stable switchable and tunable laser operation. Due to differences in the FSR´s and the advantage that the tapered fiber ends can be independently moved, it was possible to support multi-wavelength switching. Design and fabrication of tapered fiber filters is feasible to be implemented in laser cavities due to ease of fabrication, high reliability and low cost. Moreover, this scheme can be considered as a robust switching mechanism since the switching process was performed many times and the output remained reproducible. This was demonstrated during several backward and forward displacements from 0 to 465 μm. Additionally, the use of an optical post as VOA proved to be an effective method to balance the gain and loss and reduce the mode competition contributing to making our proposed design simple, and producing operation in a stable and efficient manner. We have experimentally demonstrated switching and tuning features by inducing curvatures in abrupt-tapered fibers; however, other interesting advantage of this kind of structure is the possibility to perform switching or tuning by inducing strain, pressure, temperature and by changing the refractive index of its surrounding medium demonstrating a wide variety of configurations for this type of filters.
A simple switchable and tunable multi-wavelength TDFL based on a filter using two abrupt-tapered fibers is experimentally demonstrated. The switching and tuning is performed through curvatures induced to two abrupt-tapered fibers. Single, dual, triple, quadruple and quintuple-wavelengths were generated near 2 μm. The maximum Δλ achieved in the switching regime was around 136.1 nm. Moreover, under the specific curvature conditions, the dual-wavelength operation with independently tunable lines was demonstrated. The tuning wavelength range of λ1 was from 1844.7 to 1835.1 nm with a maximum output power difference between the tuned lines of ~0.9 dB. The tuning range of λ2, was from 1930.3 to 1934.3 nm with a maximum output power difference between the tuned wavelengths of ~0.8 dB. The proposed TDFL configuration is practical, simple and with the possibility to be enhanced by optimizing the tapered fiber filters.
Consejo Nacional de Ciencia y Tecnología project 237855.
M. V. Hernández-Arriaga and H. Santiago-Hernández thank Consejo Nacional de Ciencia y Tecnología postdoctoral fellows. The authors thank J. W. Haus for fruitful discussions and careful reading of the manuscript.
References and links
1. S. Diaz, “Stable dual-wavelength erbium fiber ring laser with optical feedback for remote sensing,” J. Ligthwave Technol. 34(19), 4591–4595 (2016). [CrossRef]
2. A. Pal, S. Y. Chen, R. Sen, T. Sun, and K. T. V. Grattan, “A high-Q low threshold thulium-doped silica microsphere laser in the 2 μm wavelength region designed for gas sensing applications,” Laser Phys. Lett. 10(8), 085101 (2013). [CrossRef]
3. X. L. Zhang, K. J. Zhou, N. Q. Ngo, T. H. Tan, and W. C. Poon, “Multi-wavelength fiber source with equal frequency spacing,” Laser Phys. 20(7), 1625–1628 (2010). [CrossRef]
4. J. Wang, W. Zhang, L. Li, and Q. Yu, “Breath ammonia detection based on tunable fiber laser photoacoustic spectroscopy,” Appl. Phys. B 103(2), 263–269 (2011). [CrossRef]
5. T. Tiess, M. Becker, M. Rothhardt, H. Bartelt, and M. Jäger, “Independently tunable dual-wavelength fiber oscillator with synchronized pulsed emission based on a theta ring cavity and a fiber Bragg grating array,” Opt. Express 25(22), 26393–26404 (2017). [CrossRef] [PubMed]
6. M. Y. Jeon, N. Kim, J. Shin, J. S. Jeong, S. P. Han, C. W. Lee, Y. A. Leem, D. S. Yee, H. S. Chun, and K. H. Park, “Widely tunable dual-wavelength Er3+-doped fiber laser for tunable continuous-wave terahertz radiation,” Opt. Express 18(12), 12291–12297 (2010). [CrossRef] [PubMed]
7. T. Gottschall, T. Meyer, M. Baumgartl, C. Jauregui, M. Schmitt, J. Popp, J. Limpert, and A. Tunnermann, “Fiber-based light sources for biomedical applications of coherent anti-Stokes Raman scattering microscopy,” Laser Photonics Rev. 9(5), 435–451 (2015). [CrossRef]
8. K. Yin, R. Zhu, B. Zhang, T. Jiang, S. Chen, and J. Hou, “Ultrahigh-brightness, spectrally-flat, short-wave infrared supercontinuum source for long-range atmospheric applications,” Opt. Express 24(18), 20010–20020 (2016). [CrossRef] [PubMed]
10. A. Ghosh, A. S. Roy, S. D. Chowdhury, R. Sen, and A. Pal, “All-fiber tunable ring laser source near 2 μm designed for CO2 sensing,” Sens. Actuators B Chem. 235, 547–553 (2016). [CrossRef]
11. K. Bremer, A. Pal, S. Yao, E. Lewis, R. Sen, T. Sun, and K. T. V. Grattan, “Sensitive detection of CO2 implementing tunable thulium-doped all-fiber laser,” Appl. Opt. 52(17), 3957–3963 (2013). [CrossRef] [PubMed]
13. N. M. Fried, G. A. Lagoda, N. J. Scott, L. M. Su, and A. L. Burnett, “Noncontact stimulation of the cavernous nerves in the rat prostate using a tunable-wavelength thulium fiber laser,” J. Endourol. 22(3), 409–414 (2008). [CrossRef] [PubMed]
14. S. Liu, F. Yan, W. Peng, T. Feng, Z. Dong, and G. Chang, “Tunable dual-wavelength thulium-doped fiber laser by employing a HB-FBG,” IEEE Photonics Technol. Lett. 26(18), 1809–1812 (2014). [CrossRef]
15. 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 1925 nm and 2200 nm,” Opt. Express 22(5), 5387–5399 (2014). [CrossRef] [PubMed]
16. M. Durán-Sánchez, R. I. Álvarez-Tamayo, B. Posada-Ramírez, B. Ibarra-Escamilla, E. A. Kuzin, J. L. Cruz, and M. V. Andrés, “Tunable dual-wavelength thulium-doped fiber laser based on FBGs and a Hi-Bi FOLM,” IEEE Photonics Technol. Lett. 29(21), 1820–1823 (2017). [CrossRef]
17. S. Liu, F. Yan, T. Feng, B. Wu, Z. Dong, and G. K. Chang, “Switchable and spacing-tunable dual-wavelength thulium-doped silica fiber laser based on a nonlinear amplifier loop mirror,” Appl. Opt. 53(24), 5522–5526 (2014). [CrossRef] [PubMed]
18. Y. Wei, X. Yang, B. Mao, Y. Lu, X. Zhou, M. Bi, and G. Yang, “Channel-spacing tunable multiwavelength thulium-doped fiber laser based on four-wave mixing effect in a high nonlinear fiber,” Microw. Opt. Technol. Lett. 58(2), 337–339 (2016). [CrossRef]
19. 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] [PubMed]
20. Z. Yan, X. Li, Y. Tang, P. P. Shum, X. Yu, Y. Zhang, and Q. J. Wang, “Tunable and switchable dual-wavelength Tm-doped mode-locked fiber laser by nonlinear polarization evolution,” Opt. Express 23(4), 4369–4376 (2015). [CrossRef] [PubMed]
21. H. B. Sun, X. M. Liu, Y. K. Gong, X. H. Li, and L. R. Wang, “Broadly tunable dual-wavelength erbium-doped ring fiber laser based on a high-birefringence fiber loop mirror,” Laser Phys. 20(2), 522–527 (2010). [CrossRef]
22. X. Li, X. Liu, D. Mao, X. Hu, and H. Lu, “Tunable and switchable multiwavelength fiber lasers with broadband range based on nonlinear polarization rotation technique,” Opt. Eng. 49(9), 094303 (2010). [CrossRef]
23. X. H. Li, Y. G. Wang, Y. S. Wang, X. H. Hu, W. Zhao, X. L. Liu, J. Yu, C. X. Gao, W. Zhang, Z. Yang, C. Li, and D. Y. Shen, “Wavelength-switchable and wavelength-tunable all-normal-dispersion mode-locked Yb-doped fiber laser based on single-walled carbon nanotube wall paper absorber,” IEEE Photonics J. 4(1), 234–241 (2012). [CrossRef]
24. X. Wang, P. Zhou, X. Wang, H. Xiao, and L. Si, “Multiwavelength Brillouin-Thulium fiber laser,” IEEE Photonics J. 6(1), 1500507 (2014). [CrossRef]
25. K. Hu, I. V. Kabakova, S. Lefrancois, D. D. Hudson, S. He, and B. J. Eggleton, “Hybrid Brillouin/thulium multiwavelength fiber laser with switchable single- and double-Brillouin-frequency spacing,” Opt. Express 22(26), 31884–31892 (2014). [CrossRef] [PubMed]
26. H. Ahmad, A. S. Sharbirin, M. Z. Samion, and M. F. Ismail, “All-fiber multimode interferometer for the generation of a switchable multi-wavelength thulium-doped fiber laser,” Appl. Opt. 56(21), 5865–5870 (2017). [CrossRef] [PubMed]
27. 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]
28. W. He, L. Zhu, M. Dong, and F. Luo, “Tunable and switchable thulium-doped fiber laser utilizing Sagnac loops incorporating two-stage polarization maintaining fibers,” Opt. Fiber Technol. 29, 65–69 (2016). [CrossRef]
29. S. Liu, F. Yan, F. Ting, L. Zhang, Z. Bai, W. Han, and H. Zhou, “Multi-wavelength thulium-doped fiber laser using a fiber-based Lyot filter,” IEEE Photonics Technol. Lett. 28(8), 864–867 (2016). [CrossRef]
30. M. R. K. Soltanian, H. Ahmad, A. Khodaie, I. S. Amiri, M. F. Ismail, and S. W. Harun, “A stable dual-wavelength thulium-doped fiber laser at 1.9 μm using photonic crystal fiber,” Sci. Rep. 5(1), 14537 (2015). [CrossRef] [PubMed]
31. M. V. Hernández-Arriaga, M. Durán-Sánchez, B. Ibarra-Escamilla, R. I. Álvarez-Tamayo, H. Santiago-Hernández, M. Bello-Jiménez, and E. A. Kuzin, “Tunable thulium-doped fiber laser based on an abrupt-tapered in-fiber interferometer,” J. Opt. 19(11), 115704 (2017). [CrossRef]
32. M. F. Ismail, M. Dernaika, A. Khodaei, S. W. Harun, and H. Ahmad, “Tunable dual-wavelength thulium-doped fiber laser at 1.8 μm region using spatial-mode beating,” J. Mod. Opt. 62(11), 892–896 (2015). [CrossRef]
33. A. A. Jasim, M. Dernaika, S. W. Harun, and H. Ahmad, “A switchable figure eight erbium-doped fiber laser based on inter-modal beating by means of non-adiabatic microfiber,” J. Ligthwave Technol. 33(2), 528–534 (2015). [CrossRef]
34. H. Ahmad, M. A. M. Salim, S. R. Azzuhri, M. Z. Zulkifli, and S. W. Harun, “Dual wavelength single longitudinal mode Ytterbium-doped fiber laser using a dual-tapered Mach-Zehnder interferometer,” J. Eur. Opt. Soc.- Rapid 10, 15013 (2015). [CrossRef]
35. K. Jedrzejewiski, “Biconical fused taper – a universal fibre devices technology,” Opto-Electron. Rev. 8(2), 153–159 (2000).
36. J. D. Love, W. M. Henry, W. J. Stewart, R. J. Black, S. Lacroix, and F. Gonthier, “Tapered single-mode fibres and devices. Part 1: Adiabaticity criteria,” in IEE Proceedings-J. Optoelectronics 138(5), 343–354 (1991). [CrossRef]
38. W. B. Ji, Y. C. Tan, B. Lin, S. C. Tjin, and K. K. Chow, “Nonadiabatically tapered microfiber sensor with ultrashort waist,” IEEE Photonics Technol. Lett. 26(22), 2303–2306 (2014). [CrossRef]
39. B. Musa, Y. M. Kamil, M. H. Abu Bakar, A. S. M. Noor, A. Ismail, and M. A. Mahdi, “Effect of taper parameters on free spectral range of non-adiabatic tapered optical fibers for sensing applications,” Microw. Opt. Technol. Lett. 58(4), 798–803 (2016). [CrossRef]
40. T. K. Yadav, M. A. Mustapa, M. H. Abu Bakar, and M. A. Mahdi, “Study of single mode tapered fiber-optic interferometer of different waist diameters and its application as a temperature sensor,” J. Europ. Opt. Soc. Rap. Public. 9, 14024 (2014). [CrossRef]
41. A. Stiebeiner, R. Garcia-Fernandez, and A. Rauschenbeutel, “Design and optimization of broadband tapered optical fibers with a nanofiber waist,” Opt. Express 18(22), 22677–22685 (2010). [CrossRef] [PubMed]