2020
OPTICS LETTERS / Vol. 39, No. 7 / April 1, 2014
Fully switchable multiwavelength fiber laser assisted by a random mirror V. DeMiguel-Soto, M. Bravo,* and M. Lopez-Amo Departamento de Ingenieria Electrica y Electronica, Universidad Publica de Navarra, Campus Arrosadia S/N, Pamplona, 31006, Spain *Corresponding author: [email protected] Received January 15, 2014; revised February 27, 2014; accepted March 2, 2014; posted March 3, 2014 (Doc. ID 204871); published March 26, 2014 A real-time switchable and reconfigurable multiwavelength laser has been experimentally carried out. The laser cavity is based on a random distributed mirror and a novel real-time reconfigurable filter mirror structure. The proposed laser has been demonstrated to generate any combination of wavelengths at the 50 and 100 GHz International Telecommunications Union (ITU) grids specifications. By simultaneously using Er-doped fiber and Raman amplification, a 15 nm wide lasing window at the C band can be utilized to create up to 18 different lasing wavelengths into the ITU grid that can be switched automatically and in real time when desired. © 2014 Optical Society of America OCIS codes: (060.0060) Fiber optics and optical communications; (060.3510) Lasers, fiber. http://dx.doi.org/10.1364/OL.39.002020
0146-9592/14/072020-04$15.00/0
spectrum, achieving an emission line as wide as the optical gain spectrum permits. This characteristic allows modulating random lasers without the locking frequency problems of the conventional fiber laser configurations [11]. They also offer high lasing efficiency and are able to reach long distances [12–14]. In this Letter, a fiber laser which utilizes a random mirror has been dynamically filtered by a novel mirrorfiltering structure to achieve a switchable and reconfigurable fiber optic laser. The dynamic filtering is possible by using the Finisar WaveShaper 1000S, which has been used by other authors as well for dynamic equalization of optical pulses for communication purposes [15]. Figure 1 illustrates the schematic setup of the commutable and reconfigurable laser. The laser is based on a linear cavity formed by two mirrors. The first one (right side) is a distributed mirror generated into a ∼2.5 km dispersion compensated fiber reel, and the second one (left side) is a loop mirror created by connecting the input/output ports of a circulator as shown in Fig. 1. This mirror has the function of filtering the cavity to select the spectrum profile of the switchable and reconfigurable multiwavelength random laser. In order to perform this feature, a programmable tunable filter (Finisar WaveShaper 1000S) was used. On the other hand, an erbiumdoped fiber amplifier (EDFA) was also used because the combination of the erbium gain profile and that of the
Raman laser
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Random-distributed feedback (RDFB) based on Rayleigh scattering through Raman amplification has been the subject of intense theoretical and experimental study [1]. Random lasers are characterized by open cavities or mirrorless cavities, which means that, unlike conventional fiber lasers, their principle of operation rely on distributed scattering events along the fiber cavity. Since the demonstration in 2010 of the RDFB based on Rayleigh scattering [2], some research groups are devoted to use its interesting characteristics to propose different lasing structures. Babin and Vatnik have presented a complete overview of the art up to now [3]. This review describes the fundamentals of the RDFB fiber lasers, various lasing schemes, the basic output characteristics, a theoretical model and its experimental demonstration, and potential applications of various RDFB fiber lasers. One of the renowned applications of RDFB is the multiwavelength fiber lasers (MWFLs) which have attracted much interest recently because of their potential in wavelength-division-multiplexing communications, highresolution spectroscopy, fiber optic sensing, and so on [4]. Since 2010, certain researchers has proposed different MWFL solutions based on RDFB and diverse filter structures. RDFB fiber lasers offer great characteristics for MWFL configurations due to their high stability, broadband operation, and low gain competition between emission lines, among others. For example, Pinto and co-workers have a series of works, which solve this problem by using photonic crystal fiber combined with a fiber loop structure as a filter element placed strategically in the RDFB fiber laser cavity [5–8]. Other authors have used fiber Bragg grating (FBG) reflectors in series as wavelength selector [9] or a more complex configuration based on an all-fiber Lyot filter structure placed in the middle of a dual random mirror structure [10]. Finally, RDFB fiber lasers have attracted a lot of attention because of their advantages such as stability and special spectral properties. While conventional lasers have longitudinal modes associated with the cavity length, random lasers, which have infinite distributed cavities along the fiber, excite the whole of the lasing
90
EDFA
Fig. 1. Schematic setup for the proposed switchable and reconfigurable multiwavelength random fiber laser. WS: WaveShaper programmable filter. © 2014 Optical Society of America
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The developed program for controlling the programmable filter includes a feature to equalize the power of the emission lines. The equalization procedure of the emission lines is a key to the performance of our system because the equalization defines the whole spectral behavior, enhancing the number of emission lines and, of course, the flatter multiwavelength shape. In this way, the custom program equalizes the lines by comparing the power difference between all the lines and the lowest one and then applying a proportional attenuation to each one until the power difference fulfils a minimum power difference set by the user. The proportional factor is the other value to take into account. It is important that the software does not induce unjustified loss into the cavity, and this factor imposes the equalization accuracy. As is shown in Fig. 2(c), the first line attenuation which corresponds with the lowest line depicted in Fig. 2(a) is almost 0. When the equalization starts, the amplification efficiency improves this line power and fulfills the lasing condition instead of the previous nonequalized result. The equalization process continues until the power difference, as was mentioned, is lower than the threshold set by the user, in this case 0.5 dB. We have to remark that the threshold cannot be lower than the system’s worst stability. Figure 2(c) is the attenuation profile obtained after using the equalization feature of the program with the emission line profile obtained with the 100 GHz grid. The result of employing this profile is presented in Figs. 3(a) and 3(b). Because the 50 GHz grid requires a narrower spectral performance, the equalization process
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Raman allows us to have a broad lasing spectrum (15 nm) and up to 10 dBm output power. The operation mode of the system is as follows. The circulator-based mirror receives the reflected light from the distributed mirror by port 2. Once the light is inside the mirror (between ports 3 and 1), it is filtered by the programmable filter and amplified by the EDFA. Then the light is launched through port 2 toward the distributed mirror. This key part of the structure that depends on this reconfigurable mirror contributes to the stability and enlarges the available bandwidth of the laser. The programmable filter is able to create a custom filter profile up to 40 nm with a minimum width of 0.08 nm. It has a wavelength resolution of 8 pm in the communications C band (1527.4–1567.4 nm), providing an attenuation precision of 0.01 dB in a 35 dB attenuation range. For our purposes, a sequence of comb-shape filters is set for achieving a multiwavelength random mirror-assisted laser. The center of each filter is located at the maximum emission wavelength. The maximum number of emission lines, the lines widths, and the line separation were studied. Although this configuration is able to perform any multiwavelength laser configuration, the International Telecommunications Union (ITU) grid specifications were taken as a reference for this study. 100 and 50 GHz separation distances were tested in the proposed setup. Thus, we have developed a custom program for controlling the programmable filter. This software fully meets the desired requirements. Figure 2 depicts the obtained experimental results from the system. Figures 2(a) and 2(b) are the maximum number of emission lines for wavelength spacings of 100 and 50 GHz, respectively. The achieved width of each emission line was 0.17 nm. This width was obtained by tracking the peak power values while the line width was varied in a controlled way.
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Fig. 2. (a) Maximum number of emission lines for 100 GHz line separation when it is not equalized. (b) Maximum number of emission lines for 50 GHz line separation, either equalized. (c) Attenuation profile to equalize the spectrum depicted in (a) with the result illustrated in Fig. 3(a). (d) High-resolution reading of the lasing wavelengths developed by the BOSA.
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Fig. 3. Equalized results for the 100 GHz ITU grid measured in points “B” (a) and “A” (b) labeled in red in Fig. 1. Different emission lines configurations measured in B: (c) single wavelength; (d) 4 wavelengths; (e) 10 wavelengths; and (f) 11 wavelengths.
OPTICS LETTERS / Vol. 39, No. 7 / April 1, 2014 Output Power (dBm)
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Fig. 5. (a) Multiwavelength random laser stability in comparison with linear cavity laser stability and (b) two emission lines’ power evolution when the filter configuration was changed.
was longer, showing high output power instabilities along it. Therefore, the next results are in regards only to the 100 GHz grid. The multiwavelength laser was also measured with a high-resolution Brillouin optical spectrum analyzer (BOSA) at point A. This result is depicted in Fig. 2(d). It shows an almost Gaussian distribution in each line shape. With this result the FWHM has been measured with a result of 72.8 pm and a distance between peaks of 0.789 nm, which fits perfectly with the 100 GHz ITU grid specification. Figure 3 shows different configurations of the filter already equalized for a maximum number of emission lines of 18, when the distributed mirror was illuminated with a 1.6 W Raman pump as well as for the results in Fig. 2. They were measured at two different points of the laser structure. The plot in Fig. 3(b) was measured at point A, which extracts part of the laser power output using a 90∶10 coupler. It was captured by using the same filter configuration that we also used in Fig. 3(a), which was measured at point B, as well as the rest of the plots of Fig. 3. The high power difference is visible because the measurements at point B detect all the cavity power; meanwhile, only 10% of the cavity power is detected at point A. The sole advantage of using point A as the output port is that the filter removes all the extra noise and spurious emissions as four-wave mixing contributions, which affects measurements in B. Additionally, Fig. 3 illustrates the reconfigurability of the proposed system, showing different emission line patterns already equalized. Likewise, the output power evolution versus the Raman pump power was measured in order to demonstrate the lasing behavior of the structure [Fig. 4(a)]. Due to the EDFA extra gain, the lasing point starts earlier as expected, around 0.4 W of Raman laser pump power. In order to justify the 1.6 W used for the measurements, in Fig. 4(b) we show the evolution of the stability of the output power against the Raman pump (measured for the higher and lower peaks of the achieved spectrum). The minimum power instability is located around
1.5 W. Although the system efficiency at that point is lower than when the Raman pump power is 0.6 W, the stability is higher, as depicted in Fig. 5(a) in red. This plot shows a comparison between the stability of a proposed distributed mirror-based cavity and an EDFA-based linear cavity. It shows the high stability of the laser when the distributed mirror is used despite of the high instabilities achieved with the linear configuration, due to the gain competition between emission lines shown in this last case. Thus, the use of the Raman-based distributed mirror is crucial, making this structure viable. Figure 5(b) shows the laser stability evolution when an emission line’s configuration was changed. The measurements were obtained by changing from the configuration depicted in Fig. 3(e) to the configuration depicted in Fig. 3(f). The traces correspond to the emission lines, which are present in both configurations. This result is expected to determine the commutation time, which is less than the optical spectrum analyzer sweep time of 3 s. In conclusion, the operation of a new switchable and reconfigurable MWFL has been experimentally demonstrated. The output spectrum’s high stability, broadband response, reconfigurability, versatility, and high power are the main characteristics shown by this laser. A maximum band of 15 nm can be reconfigured with a minimum distance of 50 GHz between lasing channels. Better operation has been demonstrated with an emission line distance of 100 GHz. Thus, this structure can be easily equalized and switched and presents high stability, fulfilling the 100 GHz ITU grid specification for telecommunication purposes. On the other hand, other researchers have previously proposed previously other switchable multiwavelength laser approaches based on erbium-doped fiber ring cavities and fiber optical switching [16]. This work improves some features of these previous works, such as the number of emission lines, flexibility, bandwidth, low gain competition between lines, and high output powers. Finally, the structure could also be used as a versatile
April 1, 2014 / Vol. 39, No. 7 / OPTICS LETTERS
source for interrogation systems in fiber optic sensor applications [17]. This work was supported by the Spanish Government project TEC2010-20224-C02-01. References 1. Y. J. Rao and W. L. Zhang, in Proceedings of Optical Communications and Networks (ICOCN) (IEEE, 2013), paper 6617202. 2. S. K. Turitsyn, S. A. Babin, A. E. El-Taher, P. Harper, D. V. Churkin, S. I. Kablukov, J. D. Ania-Castañón, V. Karalekas, and E. V. Podivilov, Nat. Photonics 4, 231 (2010). 3. S. A. Babin and I. D. Vatnik, Optoelectron. Instrum. Data Process. 49, 323 (2013). 4. R. A. Perez-Herrera and M. Lopez-Amo, in Current Developments in Optical Fiber Technology, S. W. Harun, ed. (Intech Open, 2013), pp. 449–479. 5. A. M. R. Pinto, O. Frazão, J. Santos, and M. Lopez-Amo, Appl. Phys. B 99, 391 (2010). 6. A. M. R. Pinto and M. Lopez-Amo, Appl. Phys. B 103, 771 (2011). 7. A. M. R. Pinto, O. Frazão, J. L. Santos, and M. Lopez-Amo, J. Lightwave Technol. 29, 1482 (2011).
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8. A. M. R. Pinto, M. Bravo, M. Fernandez-Vallejo, M. LopezAmo, J. Kobelke, and K. Schuster, Opt. Express 19, 11906 (2011). 9. A. E. El-Taher, P. Harper, B. Babin, D. Churkin, E. Podivilov, J. D. Ania-Castañón, and S. K. Turitsyn, Opt. Lett. 36, 130 (2011). 10. S. Sugavanam, Z. Yan, V. Kamynin, A. S. Kurkov, L. Zhang, and D. V. Churkin, Opt. Express 22, 2839 (2014). 11. M. Bravo, M. Fernandez-Vallejo, and M. Lopez-Amo, Opt. Lett. 38, 9 (2013). 12. D. Vatnik, D. V. Churkin, and S. A. Babin, Opt. Express 20, 28033 (2012). 13. A. E. El-Taher, M. Alcon-Camas, S. A. Babin, P. Harper, J. D. Ania-Castañón, and S. K. Turitsyn, Opt. Lett. 35, 1100 (2010). 14. H. Martins, M. B. Marques, and O. Frazão, Opt. Express 19, 19 (2012). 15. R. Schmogrow, S. Ben-Ezra, P. C. Schindler, B. Nebendahl, C. Koos, W. Freude, and J. Leuthold, J. Lightwave Technol. 31, 2570 (2013). 16. R. A. Perez-Herrera, M. Fernandez-Vallejo, S. Diaz, M. A. Quintela, M. Lopez-Amo, and J. M. Lopez-Higuera, Opt. Fiber Technol. 16, 205 (2010). 17. R. A. Perez-Herrera, S. Diaz, M. Fernandez-Vallejo, M. Lopez-Amo, M. A. Quintela, and J. M. Lopez-Higuera, Proc. SPIE 7503, 75031Y (2009).
OPTICS LETTERS / Vol. 39, No. 7 / April 1, 2014
Fully switchable multiwavelength fiber laser assisted by a random mirror V. DeMiguel-Soto, M. Bravo,* and M. Lopez-Amo Departamento de Ingenieria Electrica y Electronica, Universidad Publica de Navarra, Campus Arrosadia S/N, Pamplona, 31006, Spain *Corresponding author: [email protected] Received January 15, 2014; revised February 27, 2014; accepted March 2, 2014; posted March 3, 2014 (Doc. ID 204871); published March 26, 2014 A real-time switchable and reconfigurable multiwavelength laser has been experimentally carried out. The laser cavity is based on a random distributed mirror and a novel real-time reconfigurable filter mirror structure. The proposed laser has been demonstrated to generate any combination of wavelengths at the 50 and 100 GHz International Telecommunications Union (ITU) grids specifications. By simultaneously using Er-doped fiber and Raman amplification, a 15 nm wide lasing window at the C band can be utilized to create up to 18 different lasing wavelengths into the ITU grid that can be switched automatically and in real time when desired. © 2014 Optical Society of America OCIS codes: (060.0060) Fiber optics and optical communications; (060.3510) Lasers, fiber. http://dx.doi.org/10.1364/OL.39.002020
0146-9592/14/072020-04$15.00/0
spectrum, achieving an emission line as wide as the optical gain spectrum permits. This characteristic allows modulating random lasers without the locking frequency problems of the conventional fiber laser configurations [11]. They also offer high lasing efficiency and are able to reach long distances [12–14]. In this Letter, a fiber laser which utilizes a random mirror has been dynamically filtered by a novel mirrorfiltering structure to achieve a switchable and reconfigurable fiber optic laser. The dynamic filtering is possible by using the Finisar WaveShaper 1000S, which has been used by other authors as well for dynamic equalization of optical pulses for communication purposes [15]. Figure 1 illustrates the schematic setup of the commutable and reconfigurable laser. The laser is based on a linear cavity formed by two mirrors. The first one (right side) is a distributed mirror generated into a ∼2.5 km dispersion compensated fiber reel, and the second one (left side) is a loop mirror created by connecting the input/output ports of a circulator as shown in Fig. 1. This mirror has the function of filtering the cavity to select the spectrum profile of the switchable and reconfigurable multiwavelength random laser. In order to perform this feature, a programmable tunable filter (Finisar WaveShaper 1000S) was used. On the other hand, an erbiumdoped fiber amplifier (EDFA) was also used because the combination of the erbium gain profile and that of the
Raman laser
WS
IN
A 10
2 3
DCF WDM
B
1
90:10
Random-distributed feedback (RDFB) based on Rayleigh scattering through Raman amplification has been the subject of intense theoretical and experimental study [1]. Random lasers are characterized by open cavities or mirrorless cavities, which means that, unlike conventional fiber lasers, their principle of operation rely on distributed scattering events along the fiber cavity. Since the demonstration in 2010 of the RDFB based on Rayleigh scattering [2], some research groups are devoted to use its interesting characteristics to propose different lasing structures. Babin and Vatnik have presented a complete overview of the art up to now [3]. This review describes the fundamentals of the RDFB fiber lasers, various lasing schemes, the basic output characteristics, a theoretical model and its experimental demonstration, and potential applications of various RDFB fiber lasers. One of the renowned applications of RDFB is the multiwavelength fiber lasers (MWFLs) which have attracted much interest recently because of their potential in wavelength-division-multiplexing communications, highresolution spectroscopy, fiber optic sensing, and so on [4]. Since 2010, certain researchers has proposed different MWFL solutions based on RDFB and diverse filter structures. RDFB fiber lasers offer great characteristics for MWFL configurations due to their high stability, broadband operation, and low gain competition between emission lines, among others. For example, Pinto and co-workers have a series of works, which solve this problem by using photonic crystal fiber combined with a fiber loop structure as a filter element placed strategically in the RDFB fiber laser cavity [5–8]. Other authors have used fiber Bragg grating (FBG) reflectors in series as wavelength selector [9] or a more complex configuration based on an all-fiber Lyot filter structure placed in the middle of a dual random mirror structure [10]. Finally, RDFB fiber lasers have attracted a lot of attention because of their advantages such as stability and special spectral properties. While conventional lasers have longitudinal modes associated with the cavity length, random lasers, which have infinite distributed cavities along the fiber, excite the whole of the lasing
90
EDFA
Fig. 1. Schematic setup for the proposed switchable and reconfigurable multiwavelength random fiber laser. WS: WaveShaper programmable filter. © 2014 Optical Society of America
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The developed program for controlling the programmable filter includes a feature to equalize the power of the emission lines. The equalization procedure of the emission lines is a key to the performance of our system because the equalization defines the whole spectral behavior, enhancing the number of emission lines and, of course, the flatter multiwavelength shape. In this way, the custom program equalizes the lines by comparing the power difference between all the lines and the lowest one and then applying a proportional attenuation to each one until the power difference fulfils a minimum power difference set by the user. The proportional factor is the other value to take into account. It is important that the software does not induce unjustified loss into the cavity, and this factor imposes the equalization accuracy. As is shown in Fig. 2(c), the first line attenuation which corresponds with the lowest line depicted in Fig. 2(a) is almost 0. When the equalization starts, the amplification efficiency improves this line power and fulfills the lasing condition instead of the previous nonequalized result. The equalization process continues until the power difference, as was mentioned, is lower than the threshold set by the user, in this case 0.5 dB. We have to remark that the threshold cannot be lower than the system’s worst stability. Figure 2(c) is the attenuation profile obtained after using the equalization feature of the program with the emission line profile obtained with the 100 GHz grid. The result of employing this profile is presented in Figs. 3(a) and 3(b). Because the 50 GHz grid requires a narrower spectral performance, the equalization process
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Raman allows us to have a broad lasing spectrum (15 nm) and up to 10 dBm output power. The operation mode of the system is as follows. The circulator-based mirror receives the reflected light from the distributed mirror by port 2. Once the light is inside the mirror (between ports 3 and 1), it is filtered by the programmable filter and amplified by the EDFA. Then the light is launched through port 2 toward the distributed mirror. This key part of the structure that depends on this reconfigurable mirror contributes to the stability and enlarges the available bandwidth of the laser. The programmable filter is able to create a custom filter profile up to 40 nm with a minimum width of 0.08 nm. It has a wavelength resolution of 8 pm in the communications C band (1527.4–1567.4 nm), providing an attenuation precision of 0.01 dB in a 35 dB attenuation range. For our purposes, a sequence of comb-shape filters is set for achieving a multiwavelength random mirror-assisted laser. The center of each filter is located at the maximum emission wavelength. The maximum number of emission lines, the lines widths, and the line separation were studied. Although this configuration is able to perform any multiwavelength laser configuration, the International Telecommunications Union (ITU) grid specifications were taken as a reference for this study. 100 and 50 GHz separation distances were tested in the proposed setup. Thus, we have developed a custom program for controlling the programmable filter. This software fully meets the desired requirements. Figure 2 depicts the obtained experimental results from the system. Figures 2(a) and 2(b) are the maximum number of emission lines for wavelength spacings of 100 and 50 GHz, respectively. The achieved width of each emission line was 0.17 nm. This width was obtained by tracking the peak power values while the line width was varied in a controlled way.
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Fig. 3. Equalized results for the 100 GHz ITU grid measured in points “B” (a) and “A” (b) labeled in red in Fig. 1. Different emission lines configurations measured in B: (c) single wavelength; (d) 4 wavelengths; (e) 10 wavelengths; and (f) 11 wavelengths.
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Fig. 5. (a) Multiwavelength random laser stability in comparison with linear cavity laser stability and (b) two emission lines’ power evolution when the filter configuration was changed.
was longer, showing high output power instabilities along it. Therefore, the next results are in regards only to the 100 GHz grid. The multiwavelength laser was also measured with a high-resolution Brillouin optical spectrum analyzer (BOSA) at point A. This result is depicted in Fig. 2(d). It shows an almost Gaussian distribution in each line shape. With this result the FWHM has been measured with a result of 72.8 pm and a distance between peaks of 0.789 nm, which fits perfectly with the 100 GHz ITU grid specification. Figure 3 shows different configurations of the filter already equalized for a maximum number of emission lines of 18, when the distributed mirror was illuminated with a 1.6 W Raman pump as well as for the results in Fig. 2. They were measured at two different points of the laser structure. The plot in Fig. 3(b) was measured at point A, which extracts part of the laser power output using a 90∶10 coupler. It was captured by using the same filter configuration that we also used in Fig. 3(a), which was measured at point B, as well as the rest of the plots of Fig. 3. The high power difference is visible because the measurements at point B detect all the cavity power; meanwhile, only 10% of the cavity power is detected at point A. The sole advantage of using point A as the output port is that the filter removes all the extra noise and spurious emissions as four-wave mixing contributions, which affects measurements in B. Additionally, Fig. 3 illustrates the reconfigurability of the proposed system, showing different emission line patterns already equalized. Likewise, the output power evolution versus the Raman pump power was measured in order to demonstrate the lasing behavior of the structure [Fig. 4(a)]. Due to the EDFA extra gain, the lasing point starts earlier as expected, around 0.4 W of Raman laser pump power. In order to justify the 1.6 W used for the measurements, in Fig. 4(b) we show the evolution of the stability of the output power against the Raman pump (measured for the higher and lower peaks of the achieved spectrum). The minimum power instability is located around
1.5 W. Although the system efficiency at that point is lower than when the Raman pump power is 0.6 W, the stability is higher, as depicted in Fig. 5(a) in red. This plot shows a comparison between the stability of a proposed distributed mirror-based cavity and an EDFA-based linear cavity. It shows the high stability of the laser when the distributed mirror is used despite of the high instabilities achieved with the linear configuration, due to the gain competition between emission lines shown in this last case. Thus, the use of the Raman-based distributed mirror is crucial, making this structure viable. Figure 5(b) shows the laser stability evolution when an emission line’s configuration was changed. The measurements were obtained by changing from the configuration depicted in Fig. 3(e) to the configuration depicted in Fig. 3(f). The traces correspond to the emission lines, which are present in both configurations. This result is expected to determine the commutation time, which is less than the optical spectrum analyzer sweep time of 3 s. In conclusion, the operation of a new switchable and reconfigurable MWFL has been experimentally demonstrated. The output spectrum’s high stability, broadband response, reconfigurability, versatility, and high power are the main characteristics shown by this laser. A maximum band of 15 nm can be reconfigured with a minimum distance of 50 GHz between lasing channels. Better operation has been demonstrated with an emission line distance of 100 GHz. Thus, this structure can be easily equalized and switched and presents high stability, fulfilling the 100 GHz ITU grid specification for telecommunication purposes. On the other hand, other researchers have previously proposed previously other switchable multiwavelength laser approaches based on erbium-doped fiber ring cavities and fiber optical switching [16]. This work improves some features of these previous works, such as the number of emission lines, flexibility, bandwidth, low gain competition between lines, and high output powers. Finally, the structure could also be used as a versatile
April 1, 2014 / Vol. 39, No. 7 / OPTICS LETTERS
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