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A chirped fiber Bragg grating with triple functions for stable wavelength-tunable Yb-doped fiber laser
Infrared Physics and Technology 102 (2019) 103008
Contents lists available at ScienceDirect
Infrared Physics & Technology journal homepage: www.elsevier.com/locate/infrared
Regular article
A chirped fiber Bragg grating with triple functions for stable wavelengthtunable Yb-doped fiber laser
T
⁎
Huiran Yanga, , Wenlei Lib, Guangwei Chenb a b
School of Science, Xi’an University of Posts and Telecommunications, Xi’an 710121, Shaanxi, People’s Republic of China Chinese Acad Sci, Xi’an Inst Opt & Precis Mech, State Key Lab Transient Opt & Photon, Xi’an 710119, People’s Republic of China
A R T I C LE I N FO
A B S T R A C T
Keywords: Mode-locked laser Chirped fiber Bragg grating Distributed reflection
Chirped fiber Bragg gratings (CFBGs) have been extensively employed as dispersion compensation element in mode-locked fiber lasers. However, multiple functions of CFBG in fiber lasers have not been studied adequately. Here, triple functions of CFBG, dispersion compensation, distributed reflection performance, and stabilizing mode-locked operation, are experimentally investigated. Firstly, the generation of dissipative solitons (DSs) in an all-normal-dispersion Yb-doped fiber laser is reported, while the mode-locked operation is proved to be insufficiently stable. In order to improve performance of the laser, a conventional solitons (CSs) fiber laser is obtained by inserting a CFBG with large anomalous dispersion into the laser cavity. Besides, the distributed reflection performance of CFBG is researched and a wavelength-tunable fiber laser with tuning range of center wavelength from 1033.5 to 1035.5 nm is achieved. It is no doubt that our work opens up venues to create wavelength tunable Yb-doped fiber laser systems with extremely high stability.
1. Introduction Ultrafast pulsed lasers, who play an important role in the fields of materials processing, medicine, microscopy and telecommunication, have attracted extensive attention because of their various advantages such as compactness, low cost, reliability, and high stability [1–9]. Passive mode-locking, regarded as an efficient way to generate ultrafast pulses [10,11], has been achieved by a variety of saturable absorbers (SAs), such as the nonlinear polarization rotation (NPR) technique [12–15], nonlinear optical loop mirrors (NOLMs) [16,17], graphene [18–22], single-walled carbon nanotubes (SWNTs) [6,23–30], transition metal dichalcogenides (TMDs) [31–36], and titanium dioxide [37]. Among them, SWNTs are particularly interesting for pulse generation due to their large modulation depth, broad bandwidth, high optical damage threshold, ultrafast charge carrier relaxation, and easy integration into resonator of fiber laser [38–40]. Intracavity dispersion, besides SA, also affects the operation of mode-locked fiber lasers, to a certain degree. This is a great obstacle to the development of Yb-doped fiber lasers, because, as we all known, standard single-mode fiber only possesses positive group-velocity dispersion at the wavelength of 1.03 μm [41,42]. In the past decade, the performance of Yb-doped fiber lasers has been tremendously improved by managing the intracavity dispersion. The dispersion-management is realized by inserting dispersion compensation elements into the laser ⁎
cavity, such as a grating pair [43,44], hollow-core photonic crystal fiber (PCF) [45], solid-core PCF [46]. Although these elements contribute to achieve stable mode-locked Yb-doped fiber lasers, they have many drawbacks. For example, use of a grating pair in the resonator breaks the all-fiber structure of lasers [43]. It is unavoidable to introduce tremendous loss by inserting a section of PCF into laser cavity, which results from its structure and splicing loss [46]. To overcome defects above, CFBG has been extensively employed as dispersion compensation element in the past decades, due to its simple design and excessive negative dispersion [47–51]. CFBG was first inserted into passively mode-locked fiber laser in 1995 [48]. In previous reports, CFBG is always utilized as a dispersion compensation or wavelength selection element [48–50]. It is worth noting that CFBG also possesses the characteristic of the distributed reflection, and according to this, wavelength-tunable fiber lasers have been reported [3,51]. Fermann et al. have achieved high-energy pulses by employing a CFBG with large amounts of excessive negative dispersion, and realized a wavelength-tunable Er-doped fiber laser with tuning range from 1550 to 1562 nm [51]. A distributed ultrafast Erdoped fiber laser using a CFBG has been proposed and demonstrated experimentally, whose wavelength can be tuned from 1556 to 1564 nm [3]. Zhang et al. have demonstrated an ytterbium-doped mode-locked fiber laser with CFBG as dispersion compensation element, and cavity net dispersion of the laser could be changed from large normal
Corresponding author. E-mail address: [email protected] (H. Yang).
https://doi.org/10.1016/j.infrared.2019.103008 Received 23 April 2019; Received in revised form 3 August 2019; Accepted 5 August 2019 Available online 06 August 2019 1350-4495/ © 2019 Elsevier B.V. All rights reserved.
Infrared Physics and Technology 102 (2019) 103008
H. Yang, et al.
Fig. 1. (a) Schematic of mode-locked fiber lasers deliver DSs (without CFBG depicted in the dotted box) and CSs (with CFBG depicted in the dotted box), (b) reflection spectrum of the CFBG. Fig. 2. Mode-locked pulse characteristics of DSs. (a) Optical spectrum (inset: fundamental radio frequency spectrum), (b) spectra recorded for every hour.
dispersion to large anomalous dispersion, depending on the direction of the CFBG in laser cavity [52]. However, the joint effect of the dispersion compensation, distributed reflection and stabilizing mode-locked operation of CFBG in Yb-doped mode-locked fiber lasers is still rarely researched. In this paper, an all-normal-dispersion mode-locked Yb-doped fiber laser based on SWNT SA, emitting dissipative soliton (DS) pulses, is firstly obtained, whose long-term stability is observed by uninterruptedly detecting its operation features. Then, a CFBG, with large anomalous dispersion, is inserted into the laser cavity through a circulator. Conventional solitons (CSs) with standard Kelly sidebands are realized. Based on the distributed reflection performance of the CFBG, a wavelength-tunable fiber laser is achieved, and the tuning range is from 1033.5 to 1035.5 nm.
The SA is fabricated with a mixture of SWNTs solution and polyvinyl alcohol (PVA) solution with approximate volume ratio of 2:1. A freestanding SWNT-PVA composite film is obtained though slow evaporation under ambient temperature, which is subsequently cut into small pieces about 2 mm2. The integrated SWNT-based mode locker is realized by sandwiching one small piece film between two fiber ferrules with a fiber connector. The insert loss of the prepared SMNT-SA is measured as about 2.1 dB.
3. Experimental results and discussions For the all-normal dispersion Yb-doped fiber laser, by increasing the pump power to a suitable value, mode-locking operation can be achieved when the polarization controller is adjusted appropriately. A typical spectrum of DSs displayed in Fig. 2(a), with the characteristic of steep spectral edges, is centered at ~1033 nm with a 3-dB spectral width of about 0.5 nm. The corresponding pulse train depicted in the inset of Fig. 2(a) shows that the separation between adjacent pulses is about 34.6 ns, which coincides with the cavity length of ~7.1 m. With the increase of pump power, pulse splitting is experimentally observed. The long-term stability of single-pulse operation is experimentally investigated as shown in Fig. 2(b), where spectra are recorded for every hour. The mode-locked state maintains for a short time and then gradually disappears. The fiber laser is further investigated by inserting a CFBG with large anomalous dispersion into the laser cavity. By appropriately adjusting the state of the PC, a mode-locking operation is firstly achieved at the pump power of about 175 mW. As shown in Fig. 3(a), the output spectrum with a 3-dB spectral width of about 0.3 nm exhibits symmetrically Kelly sidebands, confirming that the laser operates in the anomalous-dispersion regime and emits CS pulses [53–55]. The autocorrelation trace of the CSs is shown in Fig. 3(b), which gives a pulse
2. Experimental setup Fig. 1(a) shows the setup of Yb-doped fiber laser. The laser, which is pumped by a 980 nm laser diode (LD) with maximum output power of 550 mW, consists of ~0.3 m Yb-doped fiber and ~6.8 m single mode fiber, with the total net cavity dispersion of ~0.129 ps2. SWNT film is used as a SA to generate ultrafast pulses. A multiple-device of polarization-insensitive isolator and wavelength division multiplexer (IWDM) is employed to couple the pumping light into the resonator, and to guarantee the unidirectional propagation of light at the same time. An optical coupler (OC) with 10% output ratio is used to characterize output optical signals. A polarization controller (PC) is employed to adjust the polarization state of light in the cavity. The elements mentioned above make up an all-normal dispersion Yb-doped fiber laser. Then, a CFBG is inserted into the laser cavity through a circulator, which is shown in the dotted box of Fig. 1(a). The dispersion of the CFBG is about 19.6 ps/nm, which adequately guarantees the laser operating in anomalous dispersion regime. Fig. 1(b) displays the reflection spectrum of the CFBG with center wavelength of 1033.5 nm, whose bandwidth is about 7 nm. 2
Infrared Physics and Technology 102 (2019) 103008
H. Yang, et al.
Intensity (dBm)
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Fig. 5. Spectra of the CSs laser at seven different center wavelengths, achieved by appropriate adjustment of PC.
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operation, the spectra are recorded for every hour under pump power of 175 mW, as shown in Fig. 4(a). The mode-locked spectra with almost the same intensity and 3-dB bandwidth show no obvious change. Furthermore, the fiber laser still operates at the single-pulse state even under the maximal pump power of 550 mW, as nonlinear phase shift accumulated in the laser cavity is insufficient to cause the pulse to break up. And a maximum average output power of about 4.45 mW is achieved. The pulse train with large time scale under pump power of 550 mW is presented in Fig. 4(b). Slightly amplitude fluctuation can be observed, which mainly results from environmental perturbation and back reflection. No obvious amplitude modulation over a wide span of 4 μm indicates no Q-switching instabilities. It is worth noting that the mode-locked state will not disappear, no matter how to adjust the settings of PC. However, the central wavelength of pulses delivered by the proposed fiber laser can be tunes from 1033.5 to 1035.5 nm with the PC states changed, as shown in Fig. 5. The symmetrically Kelly sidebands and the 3-dB spectral bandwidth of these spectra are almost invariable. The tuning range of central wavelength, limited by bandwidth of the CFBG, is quite small in this work, which can be improved by adding a broadband CFBG. Furthermore, no mode-locking pulse can be observed when SMNT-SA is removed from the laser cavity during entire experiment, indicating that SMNT-SA is necessary for the mode-locked operations.
40
Delay time (ps) Fig. 3. Mode-locked pulse characteristics of CSs. (a) Optical spectrum, (b) autocorrelation trace (inset: pulse train).
4. Conclusions In conclusion, the DSs and CSs delivered by Yb-doped fiber laser operating in normal- and anomalous-dispersion regimes are demonstrated, respectively. The stability of the mode-locked fiber laser is significantly improved by adding a CFBG, acting as a dispersion compensation element. In addition, the CFBG can be also regarded as a distributed reflection element. By appropriately adjusting the PC, the center wavelength of the CSs can be tunes from 1033.5 to 1035.5 nm with Kelly sidebands and the 3-dB spectral bandwidth unchanged. We believe that our work provides a promising way for achieving wavelength tunable Yb-doped fiber lasers.
Fig. 4. (a) Spectra recorded for every hour, (b) pulse train with large time scale under pump power of 550 mW.
Declaration of Competing Interest duration of ~6.2 ps by fitting with a sech2 function. The corresponding time bandwidth product is calculated as 0.53, indicating that the pulse is slightly chirped. The pulse train displayed in the inset of Fig. 3(b) shows that the separation between adjacent pulses is 55.8 ns. It is 21.2 ns more than that of DSs, resulting from fiber pigtail of three-port circulator and CFBG. In order to evaluate the long-term stability of mode-locked
The authors declared that there is no conflict of interest. References [1] U. Keller, Recent developments in compact ultrafast lasers, Nature 424 (2003) 831–838. [2] M.E. Fermann, I. Hartl, Ultrafast fibre lasers, Nat. Photonics 7 (2013) 868–874.
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Infrared Physics and Technology 102 (2019) 103008
H. Yang, et al.
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4
Contents lists available at ScienceDirect
Infrared Physics & Technology journal homepage: www.elsevier.com/locate/infrared
Regular article
A chirped fiber Bragg grating with triple functions for stable wavelengthtunable Yb-doped fiber laser
T
⁎
Huiran Yanga, , Wenlei Lib, Guangwei Chenb a b
School of Science, Xi’an University of Posts and Telecommunications, Xi’an 710121, Shaanxi, People’s Republic of China Chinese Acad Sci, Xi’an Inst Opt & Precis Mech, State Key Lab Transient Opt & Photon, Xi’an 710119, People’s Republic of China
A R T I C LE I N FO
A B S T R A C T
Keywords: Mode-locked laser Chirped fiber Bragg grating Distributed reflection
Chirped fiber Bragg gratings (CFBGs) have been extensively employed as dispersion compensation element in mode-locked fiber lasers. However, multiple functions of CFBG in fiber lasers have not been studied adequately. Here, triple functions of CFBG, dispersion compensation, distributed reflection performance, and stabilizing mode-locked operation, are experimentally investigated. Firstly, the generation of dissipative solitons (DSs) in an all-normal-dispersion Yb-doped fiber laser is reported, while the mode-locked operation is proved to be insufficiently stable. In order to improve performance of the laser, a conventional solitons (CSs) fiber laser is obtained by inserting a CFBG with large anomalous dispersion into the laser cavity. Besides, the distributed reflection performance of CFBG is researched and a wavelength-tunable fiber laser with tuning range of center wavelength from 1033.5 to 1035.5 nm is achieved. It is no doubt that our work opens up venues to create wavelength tunable Yb-doped fiber laser systems with extremely high stability.
1. Introduction Ultrafast pulsed lasers, who play an important role in the fields of materials processing, medicine, microscopy and telecommunication, have attracted extensive attention because of their various advantages such as compactness, low cost, reliability, and high stability [1–9]. Passive mode-locking, regarded as an efficient way to generate ultrafast pulses [10,11], has been achieved by a variety of saturable absorbers (SAs), such as the nonlinear polarization rotation (NPR) technique [12–15], nonlinear optical loop mirrors (NOLMs) [16,17], graphene [18–22], single-walled carbon nanotubes (SWNTs) [6,23–30], transition metal dichalcogenides (TMDs) [31–36], and titanium dioxide [37]. Among them, SWNTs are particularly interesting for pulse generation due to their large modulation depth, broad bandwidth, high optical damage threshold, ultrafast charge carrier relaxation, and easy integration into resonator of fiber laser [38–40]. Intracavity dispersion, besides SA, also affects the operation of mode-locked fiber lasers, to a certain degree. This is a great obstacle to the development of Yb-doped fiber lasers, because, as we all known, standard single-mode fiber only possesses positive group-velocity dispersion at the wavelength of 1.03 μm [41,42]. In the past decade, the performance of Yb-doped fiber lasers has been tremendously improved by managing the intracavity dispersion. The dispersion-management is realized by inserting dispersion compensation elements into the laser ⁎
cavity, such as a grating pair [43,44], hollow-core photonic crystal fiber (PCF) [45], solid-core PCF [46]. Although these elements contribute to achieve stable mode-locked Yb-doped fiber lasers, they have many drawbacks. For example, use of a grating pair in the resonator breaks the all-fiber structure of lasers [43]. It is unavoidable to introduce tremendous loss by inserting a section of PCF into laser cavity, which results from its structure and splicing loss [46]. To overcome defects above, CFBG has been extensively employed as dispersion compensation element in the past decades, due to its simple design and excessive negative dispersion [47–51]. CFBG was first inserted into passively mode-locked fiber laser in 1995 [48]. In previous reports, CFBG is always utilized as a dispersion compensation or wavelength selection element [48–50]. It is worth noting that CFBG also possesses the characteristic of the distributed reflection, and according to this, wavelength-tunable fiber lasers have been reported [3,51]. Fermann et al. have achieved high-energy pulses by employing a CFBG with large amounts of excessive negative dispersion, and realized a wavelength-tunable Er-doped fiber laser with tuning range from 1550 to 1562 nm [51]. A distributed ultrafast Erdoped fiber laser using a CFBG has been proposed and demonstrated experimentally, whose wavelength can be tuned from 1556 to 1564 nm [3]. Zhang et al. have demonstrated an ytterbium-doped mode-locked fiber laser with CFBG as dispersion compensation element, and cavity net dispersion of the laser could be changed from large normal
Corresponding author. E-mail address: [email protected] (H. Yang).
https://doi.org/10.1016/j.infrared.2019.103008 Received 23 April 2019; Received in revised form 3 August 2019; Accepted 5 August 2019 Available online 06 August 2019 1350-4495/ © 2019 Elsevier B.V. All rights reserved.
Infrared Physics and Technology 102 (2019) 103008
H. Yang, et al.
Fig. 1. (a) Schematic of mode-locked fiber lasers deliver DSs (without CFBG depicted in the dotted box) and CSs (with CFBG depicted in the dotted box), (b) reflection spectrum of the CFBG. Fig. 2. Mode-locked pulse characteristics of DSs. (a) Optical spectrum (inset: fundamental radio frequency spectrum), (b) spectra recorded for every hour.
dispersion to large anomalous dispersion, depending on the direction of the CFBG in laser cavity [52]. However, the joint effect of the dispersion compensation, distributed reflection and stabilizing mode-locked operation of CFBG in Yb-doped mode-locked fiber lasers is still rarely researched. In this paper, an all-normal-dispersion mode-locked Yb-doped fiber laser based on SWNT SA, emitting dissipative soliton (DS) pulses, is firstly obtained, whose long-term stability is observed by uninterruptedly detecting its operation features. Then, a CFBG, with large anomalous dispersion, is inserted into the laser cavity through a circulator. Conventional solitons (CSs) with standard Kelly sidebands are realized. Based on the distributed reflection performance of the CFBG, a wavelength-tunable fiber laser is achieved, and the tuning range is from 1033.5 to 1035.5 nm.
The SA is fabricated with a mixture of SWNTs solution and polyvinyl alcohol (PVA) solution with approximate volume ratio of 2:1. A freestanding SWNT-PVA composite film is obtained though slow evaporation under ambient temperature, which is subsequently cut into small pieces about 2 mm2. The integrated SWNT-based mode locker is realized by sandwiching one small piece film between two fiber ferrules with a fiber connector. The insert loss of the prepared SMNT-SA is measured as about 2.1 dB.
3. Experimental results and discussions For the all-normal dispersion Yb-doped fiber laser, by increasing the pump power to a suitable value, mode-locking operation can be achieved when the polarization controller is adjusted appropriately. A typical spectrum of DSs displayed in Fig. 2(a), with the characteristic of steep spectral edges, is centered at ~1033 nm with a 3-dB spectral width of about 0.5 nm. The corresponding pulse train depicted in the inset of Fig. 2(a) shows that the separation between adjacent pulses is about 34.6 ns, which coincides with the cavity length of ~7.1 m. With the increase of pump power, pulse splitting is experimentally observed. The long-term stability of single-pulse operation is experimentally investigated as shown in Fig. 2(b), where spectra are recorded for every hour. The mode-locked state maintains for a short time and then gradually disappears. The fiber laser is further investigated by inserting a CFBG with large anomalous dispersion into the laser cavity. By appropriately adjusting the state of the PC, a mode-locking operation is firstly achieved at the pump power of about 175 mW. As shown in Fig. 3(a), the output spectrum with a 3-dB spectral width of about 0.3 nm exhibits symmetrically Kelly sidebands, confirming that the laser operates in the anomalous-dispersion regime and emits CS pulses [53–55]. The autocorrelation trace of the CSs is shown in Fig. 3(b), which gives a pulse
2. Experimental setup Fig. 1(a) shows the setup of Yb-doped fiber laser. The laser, which is pumped by a 980 nm laser diode (LD) with maximum output power of 550 mW, consists of ~0.3 m Yb-doped fiber and ~6.8 m single mode fiber, with the total net cavity dispersion of ~0.129 ps2. SWNT film is used as a SA to generate ultrafast pulses. A multiple-device of polarization-insensitive isolator and wavelength division multiplexer (IWDM) is employed to couple the pumping light into the resonator, and to guarantee the unidirectional propagation of light at the same time. An optical coupler (OC) with 10% output ratio is used to characterize output optical signals. A polarization controller (PC) is employed to adjust the polarization state of light in the cavity. The elements mentioned above make up an all-normal dispersion Yb-doped fiber laser. Then, a CFBG is inserted into the laser cavity through a circulator, which is shown in the dotted box of Fig. 1(a). The dispersion of the CFBG is about 19.6 ps/nm, which adequately guarantees the laser operating in anomalous dispersion regime. Fig. 1(b) displays the reflection spectrum of the CFBG with center wavelength of 1033.5 nm, whose bandwidth is about 7 nm. 2
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operation, the spectra are recorded for every hour under pump power of 175 mW, as shown in Fig. 4(a). The mode-locked spectra with almost the same intensity and 3-dB bandwidth show no obvious change. Furthermore, the fiber laser still operates at the single-pulse state even under the maximal pump power of 550 mW, as nonlinear phase shift accumulated in the laser cavity is insufficient to cause the pulse to break up. And a maximum average output power of about 4.45 mW is achieved. The pulse train with large time scale under pump power of 550 mW is presented in Fig. 4(b). Slightly amplitude fluctuation can be observed, which mainly results from environmental perturbation and back reflection. No obvious amplitude modulation over a wide span of 4 μm indicates no Q-switching instabilities. It is worth noting that the mode-locked state will not disappear, no matter how to adjust the settings of PC. However, the central wavelength of pulses delivered by the proposed fiber laser can be tunes from 1033.5 to 1035.5 nm with the PC states changed, as shown in Fig. 5. The symmetrically Kelly sidebands and the 3-dB spectral bandwidth of these spectra are almost invariable. The tuning range of central wavelength, limited by bandwidth of the CFBG, is quite small in this work, which can be improved by adding a broadband CFBG. Furthermore, no mode-locking pulse can be observed when SMNT-SA is removed from the laser cavity during entire experiment, indicating that SMNT-SA is necessary for the mode-locked operations.
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Delay time (ps) Fig. 3. Mode-locked pulse characteristics of CSs. (a) Optical spectrum, (b) autocorrelation trace (inset: pulse train).
4. Conclusions In conclusion, the DSs and CSs delivered by Yb-doped fiber laser operating in normal- and anomalous-dispersion regimes are demonstrated, respectively. The stability of the mode-locked fiber laser is significantly improved by adding a CFBG, acting as a dispersion compensation element. In addition, the CFBG can be also regarded as a distributed reflection element. By appropriately adjusting the PC, the center wavelength of the CSs can be tunes from 1033.5 to 1035.5 nm with Kelly sidebands and the 3-dB spectral bandwidth unchanged. We believe that our work provides a promising way for achieving wavelength tunable Yb-doped fiber lasers.
Fig. 4. (a) Spectra recorded for every hour, (b) pulse train with large time scale under pump power of 550 mW.
Declaration of Competing Interest duration of ~6.2 ps by fitting with a sech2 function. The corresponding time bandwidth product is calculated as 0.53, indicating that the pulse is slightly chirped. The pulse train displayed in the inset of Fig. 3(b) shows that the separation between adjacent pulses is 55.8 ns. It is 21.2 ns more than that of DSs, resulting from fiber pigtail of three-port circulator and CFBG. In order to evaluate the long-term stability of mode-locked
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