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High-power tunable sub-nm narrowband near-diffraction-limited superfluorescent fiber source based on a single-lens spectral filter
Optics Communications 463 (2020) 125359
Contents lists available at ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
High-power tunable sub-nm narrowband near-diffraction-limited superfluorescent fiber source based on a single-lens spectral filter Wei Gao a,b,c , Wenhui Fan a,c,d ,∗, Yanpeng Zhang b , Pei Ju a,c , Baoyin Zhao a , Peng Wu a,c , Gang Li a,c , Qi Gao a,c , Zhe Li a,c a
State Key Laboratory of Transient Optics and Photonics, Xi’an Institute of Optics and Precision Mechanics, Chinese Academy of Sciences, Xi’an 710119, China Key Laboratory for Physical Electronics and Devices of the Ministry of Education & Shaanxi Key Laboratory of Information Photonic Technique, Xi’an Jiaotong University, Xi’an 710049, China c University of Chinese Academy of Sciences, Beijing 100049, China d Collaborative Innovation Center of Extreme Optics, Shanxi University, Taiyuan 030006, China b
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Keywords: High-power Tunable Superfluorescent fiber source Single-lens spectral filter
ABSTRACT We propose a method for a high-power tunable sub-nm narrowband near-diffraction-limited superfluorescent fiber source (SFS) based on a single-lens spectral filter (SLSF), which is composed of a grating monochromator and a movable space-fiber coupler. The center wavelength of the SLSF is set to 1063 nm with a full width at half-maximum (FWHM) linewidth of less than 0.08 nm. By utilizing the SLSF and a broadband amplified SFS, a tunable sub-nm narrowband SFS seed source is obtained, and the central wavelength of the sub-nm narrowband SFS seed source can be easily tuned from 1052.4 nm to 1072.8 nm by adjusting the SLSF. After a three-stage amplifier system, the output power of this sub-nm narrowband SFS is boosted to 230W with the FWHM range from 0.10 nm to 0.12 nm. The beam quality factors (M2 ) of the full power sub-nm narrowband SFS at 1060.1 nm is 1.20. The proposed tunable sub-nm SFS has advantages of high spectral resolution, simple configuration, which may have potential applications in the industrial production and scientific research.
1. Introduction High-power and high-brightness superfluorescent fiber sources (SFSs) are highly in great demand in industry and medical treatment for low temporal coherence, high spatial coherence, and high stability. Their extensive applications can be found in optical sensing [1] and fiber gyroscopes [2], ghost imaging [3], medical imaging by optical coherence tomography [4–6], and so on. Furthermore, high-power SFSs have also become an alternative source for materials processing [7] and beam spectral combination [8] due to high beam quality and high nonlinearity threshold [9]. Initially, lots of researchers focused on broadband high-power SFSs. Wang et al. achieved a broadband SFS with the full width at halfmaximum (FWHM) of 40 nm and output power of 110 W based on a novel offset-core-fiber to suppress parasitic lasing [10]. High-power SFS with spectral range from ∼1035 nm to 1085 nm and output power of 122 W was also attained by utilizing two stages amplification [7]. Xu et al. demonstrated an all-fiber master oscillator power amplifier (MOPA) configuration of SFS with the FWHM of 8.1 nm and the output power of 1.01 kW [11]. However, it is noteworthy that narrowband, especially tunable narrowband high-power SFSs would also open up various new applications, such as pumping the optical parametric
oscillator [12], dense beam spectral combination (DBSC) [13,14] and coherent beam combination (CBC) of phase locked emissions [15], and so on. To date, several methods to generate the narrowband or tunable narrowband high-power SFSs have been experimentally demonstrated. Liu et al. reported a narrowband SFS with the FWHM of 1.2 nm and the power of 120 W based on a narrowband fiber Bragg grating [16]. Xu et al. demonstrated a 1.87 kW narrowband all-fiber SFS fiber source with the FWHM of 1.7 nm by utilizing a fiber Bragg grating [17]. Schmidt et al. presented a narrowband space-structured SFS with the FWHM of 12 pm and the output power of 697 W by utilizing two fiber Bragg gratings as spectral filter component [9]. Besides, Wang et al. presented a 250 W tunable narrowband SFS at near 2 μm with the FWHM of ∼1.5–2.0 nm by employing a manual tunable band pass filter [18]. Ye et al. demonstrated a 106 W SFS with both central wavelength and linewidth tunability based on a bandwidthadjustable tunable optical filter and the minimum FWHM of this SFS is ∼0.4 nm [19]. In this paper, we propose and demonstrate a method for a highpower tunable sub-nm narrowband SFS based on a single-lens spectral filter (SLSF). In our experiment, the central wavelength of this SFS can
∗ Corresponding author at: NO.17 Xinxi Road, New Industrial Park, Xi’an Hi-Tech Industrial Development Zone, Xi’an, Shaanxi, P.R.China, 710119 E-mail address: [email protected] (W. Fan).
https://doi.org/10.1016/j.optcom.2020.125359 Received 8 November 2019; Received in revised form 15 January 2020; Accepted 20 January 2020 Available online 22 January 2020 0030-4018/© 2020 Elsevier B.V. All rights reserved.
W. Gao, W. Fan, Y. Zhang et al.
Optics Communications 463 (2020) 125359
Fig. 1. Schematic of the broadband SFS. YDF: double-cladding Yb-doped fiber; LD: laser diode; ISO: fiber isolator; Collimated ISO: isolator with collimation; CPS: cladding power stripper.
be easily tuned from 1052.4 nm to 1072.8 nm by adjusting the SLSF. A three-stage amplifier system is used to boost the output power to 230 W with the FWHM range from 0.10 nm to 0.12 nm. The beam quality factors (M2 ) of the full power sub-nm narrowband SFS at 1060.1 nm is 1.20. The proposed high-power SFS with high spectral resolution and simple configuration could be of great interest in industrial production and scientific research.
Fig. 2. Schematic of the tunable single-lens spectral filter (SLSF). VBG: volume Bragg grating; 𝛼: emission angle of the beam after VBG; d: spot diameter of the collimated SFS beam; 𝜙: core diameter of the space-fiber coupler; 𝐹 = 𝐹 ′ : the focal length of the single-lens; 𝜃 ≈ 𝑑𝐹 −1 : the divergence angle of the dispersive beam in 𝑦-axis direction after the single-lens; 𝐿 ≈ 𝐹 𝛼: line spot length in the 𝑥-direction after the single-lens.
2. Experimental setup 0.14) is used to make the space-fiber coupler to receive single-mode narrowband SFS. 𝜆 is the central wavelength of the obtained tunable narrowband SFS seed, which can be taken approximately as 𝜆 ≈ 1063 nm. d = 5 mm is spot diameter of the collimated SFS beam produced by the collimated ISO. 𝜃 is the divergence angle of the dispersive beam in 𝑦-axis direction after the single-lens. F is the focal length of the single-lens. The physical meaning of Eq. (1) is that the core diameter of the space-fiber coupler must be larger than spot size on the second focal plane, and the numerical aperture of the space-fiber coupler must also be larger than the divergence angle of the focused beam in the 𝑦axis direction behind the single-lens. According to Eq. (1), F can be calculated as: 17.8 mm ≤ 𝐹 ≤ 85.5 mm. In this experiment, F = 60 mm is taken. As shown in Fig. 3, the narrowband SFS seed from the SLSF is seeded into the power amplifier system, which consists of two preamplifiers (pre-amplifier 2 and pre-amplifier 3) and a main amplifier. In the pre-amplifier 2, a 0.5-m-length single-cladding Yb-doped fiber (SYF, CorActive, YB 401) is used as the gain fiber, which is core-pumped by a single-mode 974 nm LD by a wavelength division multiplexing (WDM). After the pre-amplifier 2, the narrowband SFS is launched into the preamplifier 3. The LD, gain fiber, and combiner in the pre-amplifier 3 are same as that used in the pre-amplifier 1. To further increase the output power of the narrowband SFS, the main amplifier stage is developed by using 24-m-length 25/400 μm (core/cladding) YDF (Nufern, LMA-YDF25/400-VIII). The gain fiber is pumped by 976 nm multimode LDs via a (6+1)×1 combiner. Before the main amplifier, an ISO is also used to prevent backward light from the end facet. The output port of the main amplifier is spliced to quartz end cap to output high power and reduce the end face feedback. Note that the residual pump light is dumped out by cladding power strippers (CPS) in all forward pumped configuration such as pre-amplifier 1, pre-amplifier 3 and the main amplifier.
The experimental configuration for the high-power tunable subnm narrowband SFS contains three components: a broadband SFS, a tunable narrowband SLSF and a power amplifier system. The broadband SFS consists of a backward-pumped broadband SFS seed and pre-amplifier 1 as shown in Fig. 1. A 3.5-m-length double-cladding Yb-doped fiber (YDF, Nufern, LMA-YDF-10/130-VIII) is pumped by a 976 nm laser diode (LD) and a (2+1)×1 combiner to generate a broadband SFS seed. The backward port of broadband SFS seed is sloped 8◦ to prevent feedback from the end facet. A fiber isolator (ISO) is used to protect components by preventing backward light. The broadband SFS seed is launched into a 4.5-m-length double-cladding YDF (Nufern, LMA-YDF-10/130-VIII) by a (2+1)×1 combiner in a forward-pumped pre-amplifier 1. Moreover, a collimated ISO (AFRlaser, HPMFSI-1064-P) is employed to generate a collimated SFS beam and suppress backward light. Then, the collimated SFS beam is launched into the SLSF, which is designed based on a grating monochromator and a movable spacefiber coupler, as shown in Fig. 2(a). A volume Bragg grating (VBG) and a movable space-fiber coupler are placed on the first focal plane and the second focal plane, respectively. The light at any wavelength from the collimated SFS beam in the 𝑦-axis direction is converged into a point on the second focal plane, while the light at different wavelength is converged into different point. By adjusting the beam direction of the collimated SFS and the angle between the VBG and the optical axis of the single-lens, the 1063 nm light output from the collimated SFS beam is converged at the second focal point, indicating the central wavelength of the SLSF set to 1063 nm. Thus, the collimated SFS beam will be converged at a straight line spot on the second focal plane. Furthermore, the diverged beam in the 𝑥-axis direction after VBG dispersion is also collimated because the diverged beam is on the first focal plane. Therefore, the designed SLSF has two properties simultaneously: the broadband beam can be collimated in the 𝑥-axis direction and focused in the 𝑦-axis direction as shown in Fig. 2(b). In addition, the movable space-fiber coupler is mounted on precision multi-axis stages. By moving the space-fiber coupler along the 𝑥-axis direction, we can obtain a tunable narrowband SFS seed with a desired central wavelength. In order to obtain effective coupling for the space-fiber coupler, the following relations must be satisfied [20]:
3. Results and discussion When the output power of the broadband SFS after pre-amplifier 1 is set to 12 W, the output spectra of that is recorded by using an optical spectrum analyzer (YOKOGAWA-AQ6370D) and shown in Fig. 4(a). The central wavelength and the FWHM are calculated to be 1053.1 nm and 30.9 nm, respectively. Self-pulse or parasitic oscillation is not observed below the output power of 12 W. To obtain a tunable narrowband SFS, a designed tunable narrowband SLSF is used. A series of narrowband SFS with different central wavelengths are obtained by moving the space-fiber coupler along the 𝑥-axis direction as shown in Fig. 2(a). The tunable spectra after the tunable SLSF are shown in
𝜃 𝑑 𝜆𝐹 , 𝑁𝐴 ≥ ≈ (1) 𝜋𝑑 2 2𝐹 where 𝜙 and 𝑁𝐴 are core diameter and numerical aperture of the space-fiber coupler, respectively, as shown in Fig. 2(b). In this experiment, a single-mode fiber (Nufern, 1060-XP, 𝜙 = 5.8 μm, 𝑁𝐴 = 𝜙≥
2
W. Gao, W. Fan, Y. Zhang et al.
Optics Communications 463 (2020) 125359
Fig. 3. Schematic of the power amplifier system. SYF: single-cladding Yb-doped fiber; YDF: double-cladding Yb-doped fiber; LD: laser diode; WDM: wavelength division multiplexing; CPS: cladding power strippers. ISO: fiber isolator.
Fig. 4. (a) Spectra of the broadband SFS after pre-amplifier 1. (b) Spectra of the narrowband SFS after the SLSF.
Fig. 5. (a) Output power of main amplifier with different wavelengths. (b) Output spectra of the main amplifier at the maximum output power.
Fig. 4(b). The wavelengths can be tuned from 1052.4 nm to 1072.8 nm, which is limited by the moving range of the precision multi-axis stages. The output power at different wavelength after the SLSF varies from 0.8 mW to 1.2 mW. All of the FWHMs of tunable spectra are less than 0.08 nm, which firstly decreases and then increases with the increase of the tunable wavelength. This is mainly attributed to the aberration-induced distortion effect in the single-lens system. In fact, according to the monochromator principle, the FWHM of tunable spectra is mainly dominated by the spot size 𝐿 ≈ 𝐹 𝛼 in the 𝑥axis direction as shown in Fig. 2(b). 𝛼 is the emission angle of the beam after VBG, which depends on angular dispersion ability of the VBG. Because the larger L is, the smaller the spectral interval corresponds to the unit length in the 𝑥-axis direction is, thus the smaller the FWHM is. Therefore, the FWHM of the narrowband SFS still can be decreased by increasing the angular dispersion ability 𝛼 of VBG and/or adopting single-lens with longer focal length, indicating this SLSF very flexible and extensible. The narrowband SFS from the SLSF can be amplified to 5 W after the pre-amplifier 2 and the pre-amplifier 3 in turn. The central wavelength and the FWHM of the 5 W narrowband SFS at all wavelengths are maintained. After the main amplifier, the output power at all wavelengths reaches 230 W. Fig. 5(a) shows output power of main amplifier with selected wavelengths. As shown in Fig. 5(a), the amplification is almost
same for different wavelengths and the slope efficiency is ∼73%. It is very important for applications that only required wavelength can be tuned while output power is maintained. The spectra of selected wavelengths at maximum output power are shown in Fig. 5(b). Clearly, the central wavelength is equal to that from the SLSF. The FWHM at different wavelengths varies from 0.10 nm to 0.12 nm. At the maximum output power, the signal-noise ratio can reach over 26 dB and the residual pump power has been almost dumped by CPS. Furthermore, no evidence of the stimulated Raman scattering (SRS) and the stimulated Brillouin scattering (SBS) is observed. Fig. 6(a) shows the output spectra evolutions of the main amplifier at 1060.1 nm with output power increasing. It is found that the central wavelength is almost maintained, whereas the spectral width is gradually symmetrically broadened. The FWHM of the output spectrum at 230 W is 0.11 nm. The spectral broadening process is mainly dominated by self-phase modulation (SPM) at this stage [19,21,22], which causes the growth rate of the spectral peak intensity at the central wavelength gradually decreases with the output power increasing. The output beam quality and pattern is measured with help of the M2 factor measuring instrument (PRIMES–HP-LQM). When the output power is 51 W, the M2 factor is measured to be 1.04 after the main amplifier at operated 1060.1 nm, which is close to the ideal Gauss beam. The M2 factor increases with the increase of output power. The 3
W. Gao, W. Fan, Y. Zhang et al.
Optics Communications 463 (2020) 125359
Fig. 6. (a) Output spectra of main amplifier at 1060.1 nm with different output powers. (b) Output beam quality of main amplifier at 1060.1 nm with the output power of 230 W.
measured beam quality factor of M2 = 1.20 at 230 W is shown in Fig. 6(b) and the beam profile is shown in the right panel. The beam quality degradation can be attributed to generation of few higher-order modes. Anyway, the output beam quality still sufficiently meets the requirements for single mode applications.
[3] P. Janassek, S. Blumenstein, W. Elsäßer, Ghost spectroscopy with classical thermal light emitted by a superluminescent diode, Phys. Rev. Appl. 9 (2) (2018) 021001. [4] A.F. Fercher, W. Drexler, C.K. Hitzenberger, T. Lasser, Optical coherence tomography-principles and applications, Rep. Progr. Phys. 66 (2) (2003) 239. [5] B. Redding, P. Ahmadi, V. Mokan, M. Seifert, M.A. Choma, H. Cao, Low-spatialcoherence high-radiance broadband fiber source for speckle free imaging, Opt. Lett. 40 (20) (2015) 4607. [6] I. Trifanov, P. Caldas, L. Neagu, R. Romero, M.O. Berendt, J.A.R. Salcedo, A.G. Podoleanu, A.B.L. Ribeiro, Combined neodymium-ytterbium-doped SFS fiber-optic source for optical coherence tomography applications, IEEE Photon. Technol. Lett. 23 (1) (2011) 21–23. [7] P. Wang, J.K. Sahu, W.A. Clarkson, Power scaling of ytterbium-doped fiber superfluorescent sources, IEEE J. Sel. Top. Quantum Electron. 13 (3) (2007) 580. [8] Y. Zheng, Y. Yang, J. Wang, M. Hu, G. Liu, X. Zhao, X. Chen, K. Liu, C. Zhao, B. He, 10.8 kW spectral beam combination of eight all-fiber superfluorescent sources and their dispersion compensation, Opt. Express 24 (11) (2016) 12063–12071. [9] O. Schmidt, M. Rekas, C. Wirth, J. Rothhardt, S. Rhein, A. Kliner, M. Strecker, T. Schreiber, J. Limpert, R. Eberhardt, High power narrow-band fiber-based ASE source, Opt. Express 19 (5) (2011) 4421-4427. [10] P. Wang, J. Sahu, W. Clarkson, 110 W double-ended ytterbium-doped fiber superfluorescent source with M2 = 1.6, Opt. Lett. 31 (21) (2006) 3116–3118. [11] J. Xu, L. Huang, J. Leng, H. Xiao, S. Guo, P. Zhou, J. Chen, 1.01 kW superfluorescent source in all-fiberized MOPA configuration, Opt. Express 23 (5) (2015) 5485–5490. [12] Y. Shang, J. Xu, P. Wang, X. Li, P. Zhou, X. Xu, Ultra-stable high-power midinfrared optical parametric oscillator pumped by a superfluorescent fiber source, Opt. Express 24 (19) (2016) 21684–21692. [13] E.C. Cheung, J.G. Ho, T.S. McComb, S. Palese, High density spectral beam combination with spatial chirp precompensation, Opt. Express 19 (21) (2011) 20984–20990. [14] D. Drachenberg, I. Divliansky, V. Smirnov, G. Venus, L. Glebov, High-power spectral beam combining of fiber lasers with ultra high-spectral density by thermal tuning of volume Bragg gratings, in: Fiber Lasers VIII: Technology, Systems, and Applications, Vol. 7914, 2011. [15] S.J. McNaught, J.E. Rothenberg, P.A. Thielen, M.G. Wickham, M.E. Weber, G.D. Goodno, Coherent combining of a 1.26-kW fiber amplifier, in: Advanced Solid-State Photonics, Optical Society of America, 2010. [16] J. Liu, K. Liu, F. Tan, P. Wang, High-power thulium-doped all-fiber superfluorescent sources, IEEE J. Sel. Top. Quantum Electron. 20 (5) (2014) 497–502. [17] J. Xu, W. Liu, J. Leng, H. Xiao, S. Guo, P. Zhou, J. Chen, Power scaling of narrowband high-power all-fiber superfluorescent fiber source to 1.87 kW, Opt. Lett. 40 (13) (2015) 2973–2976. [18] X. Wang, X. Jin, P. Zhou, X. Wang, H. Xiao, Z. Liu, High power, widely tunable, narrowband superfluorescent source at 2 μm based on a monolithic Tm-doped fiber amplifier, Opt. Express 23 (3) (2015) 3382–3389. [19] J. Ye, J. Xu, Y. Zhang, J. Song, J. Leng, P. Zhou, Spectrum-manipulable hundredwatt-level high-power superfluorescent fiber source, J. Lightwave Technol. 37 (13) (2019) 3113–3118. [20] K. Kataoka, Estimation of coupling efficiency of optical fiber by far-field method, Opt. Rev. 17 (5) (2010) 476–480. [21] S. Kablukov, E. Zlobina, E. Podivilov, S. Babin, Output spectrum of Yb-doped fiber lasers, Opt. Lett. 37 (13) (2012) 2508–2510. [22] W. Liu, P. Ma, P. Zhou, Z. Jiang, Spectral property optimization for a narrowband-filtered superfluorescent fiber source, Laser Phys. Lett. 15 (2) (2018) 025103.
4. Conclusions In conclusion, we proposed a method for a high-power tunable sub-nm narrowband near-diffraction-limited SFS based on a singlelens spectral filter. In the experiment, this SLSF consists of a grating monochromator and a movable space-fiber coupler, which has the features of high spectral resolution, simple configuration and high flexibility. Using the SLSF, a tunable narrowband SFS seed from an amplified broadband SFS is easily obtained. The wavelengths are easily tuned from 1052.4 nm to 1072.8 nm by adjusting the spatial position of the space-fiber coupler. With employing a MOPA configuration, the maximum power at different wavelengths is 230 W. The FWHM of the source at full power varies from 0.10 nm to 0.12 nm. No evidence of the SRS and SBS is observed. The beam quality factors (M2 ) of the full power SFS at 1060.1 nm is 1.20. Moreover, the SFS may still have the potential to power scaling by increasing the pump power. The presented high-power tunable sub-nm narrowband neardiffraction-limited SFS may generate great interest in the industrial production and scientific research. CRediT authorship contribution statement Wei Gao: Conceptualization, Methodology, Writing - original draft, Writing - review & editing. Wenhui Fan: Project administration, Writing - review & editing. Yanpeng Zhang: Supervision. Pei Ju: Methodology. Baoyin Zhao: Resources. Peng Wu: Investigation. Gang Li: Formal analysis. Qi Gao: Data curation. Zhe Li: Visualization. Acknowledgments This work is financially supported by Key R & D Program of Shaanxi Province under Grant no. 2018ZDXM-GY-060, National Natural Science Foundation of China under Grant no. 61675230 and Equipment Pre-research Foundation of China under Grant no. 61406190302. References [1] S. Martin-Lopez, M. Gonzalez-Herraez, A. Carrasco-Sanz, F. Vanholsbeeck, S. Coen, H. Fernandez, J. Solis, P. Corredera, M.L. Hernanz, Broadband spectrally flat and high power density light source for fibre sensing purposes, Meas. Sci. Technol. 17 (5) (2006) 1014–1019. [2] B. Lee, Review of the present status of optical fiber sensors, Opt. Fiber Technol. 9 (2) (2003) 57–79. 4
Contents lists available at ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
High-power tunable sub-nm narrowband near-diffraction-limited superfluorescent fiber source based on a single-lens spectral filter Wei Gao a,b,c , Wenhui Fan a,c,d ,∗, Yanpeng Zhang b , Pei Ju a,c , Baoyin Zhao a , Peng Wu a,c , Gang Li a,c , Qi Gao a,c , Zhe Li a,c a
State Key Laboratory of Transient Optics and Photonics, Xi’an Institute of Optics and Precision Mechanics, Chinese Academy of Sciences, Xi’an 710119, China Key Laboratory for Physical Electronics and Devices of the Ministry of Education & Shaanxi Key Laboratory of Information Photonic Technique, Xi’an Jiaotong University, Xi’an 710049, China c University of Chinese Academy of Sciences, Beijing 100049, China d Collaborative Innovation Center of Extreme Optics, Shanxi University, Taiyuan 030006, China b
ARTICLE
INFO
Keywords: High-power Tunable Superfluorescent fiber source Single-lens spectral filter
ABSTRACT We propose a method for a high-power tunable sub-nm narrowband near-diffraction-limited superfluorescent fiber source (SFS) based on a single-lens spectral filter (SLSF), which is composed of a grating monochromator and a movable space-fiber coupler. The center wavelength of the SLSF is set to 1063 nm with a full width at half-maximum (FWHM) linewidth of less than 0.08 nm. By utilizing the SLSF and a broadband amplified SFS, a tunable sub-nm narrowband SFS seed source is obtained, and the central wavelength of the sub-nm narrowband SFS seed source can be easily tuned from 1052.4 nm to 1072.8 nm by adjusting the SLSF. After a three-stage amplifier system, the output power of this sub-nm narrowband SFS is boosted to 230W with the FWHM range from 0.10 nm to 0.12 nm. The beam quality factors (M2 ) of the full power sub-nm narrowband SFS at 1060.1 nm is 1.20. The proposed tunable sub-nm SFS has advantages of high spectral resolution, simple configuration, which may have potential applications in the industrial production and scientific research.
1. Introduction High-power and high-brightness superfluorescent fiber sources (SFSs) are highly in great demand in industry and medical treatment for low temporal coherence, high spatial coherence, and high stability. Their extensive applications can be found in optical sensing [1] and fiber gyroscopes [2], ghost imaging [3], medical imaging by optical coherence tomography [4–6], and so on. Furthermore, high-power SFSs have also become an alternative source for materials processing [7] and beam spectral combination [8] due to high beam quality and high nonlinearity threshold [9]. Initially, lots of researchers focused on broadband high-power SFSs. Wang et al. achieved a broadband SFS with the full width at halfmaximum (FWHM) of 40 nm and output power of 110 W based on a novel offset-core-fiber to suppress parasitic lasing [10]. High-power SFS with spectral range from ∼1035 nm to 1085 nm and output power of 122 W was also attained by utilizing two stages amplification [7]. Xu et al. demonstrated an all-fiber master oscillator power amplifier (MOPA) configuration of SFS with the FWHM of 8.1 nm and the output power of 1.01 kW [11]. However, it is noteworthy that narrowband, especially tunable narrowband high-power SFSs would also open up various new applications, such as pumping the optical parametric
oscillator [12], dense beam spectral combination (DBSC) [13,14] and coherent beam combination (CBC) of phase locked emissions [15], and so on. To date, several methods to generate the narrowband or tunable narrowband high-power SFSs have been experimentally demonstrated. Liu et al. reported a narrowband SFS with the FWHM of 1.2 nm and the power of 120 W based on a narrowband fiber Bragg grating [16]. Xu et al. demonstrated a 1.87 kW narrowband all-fiber SFS fiber source with the FWHM of 1.7 nm by utilizing a fiber Bragg grating [17]. Schmidt et al. presented a narrowband space-structured SFS with the FWHM of 12 pm and the output power of 697 W by utilizing two fiber Bragg gratings as spectral filter component [9]. Besides, Wang et al. presented a 250 W tunable narrowband SFS at near 2 μm with the FWHM of ∼1.5–2.0 nm by employing a manual tunable band pass filter [18]. Ye et al. demonstrated a 106 W SFS with both central wavelength and linewidth tunability based on a bandwidthadjustable tunable optical filter and the minimum FWHM of this SFS is ∼0.4 nm [19]. In this paper, we propose and demonstrate a method for a highpower tunable sub-nm narrowband SFS based on a single-lens spectral filter (SLSF). In our experiment, the central wavelength of this SFS can
∗ Corresponding author at: NO.17 Xinxi Road, New Industrial Park, Xi’an Hi-Tech Industrial Development Zone, Xi’an, Shaanxi, P.R.China, 710119 E-mail address: [email protected] (W. Fan).
https://doi.org/10.1016/j.optcom.2020.125359 Received 8 November 2019; Received in revised form 15 January 2020; Accepted 20 January 2020 Available online 22 January 2020 0030-4018/© 2020 Elsevier B.V. All rights reserved.
W. Gao, W. Fan, Y. Zhang et al.
Optics Communications 463 (2020) 125359
Fig. 1. Schematic of the broadband SFS. YDF: double-cladding Yb-doped fiber; LD: laser diode; ISO: fiber isolator; Collimated ISO: isolator with collimation; CPS: cladding power stripper.
be easily tuned from 1052.4 nm to 1072.8 nm by adjusting the SLSF. A three-stage amplifier system is used to boost the output power to 230 W with the FWHM range from 0.10 nm to 0.12 nm. The beam quality factors (M2 ) of the full power sub-nm narrowband SFS at 1060.1 nm is 1.20. The proposed high-power SFS with high spectral resolution and simple configuration could be of great interest in industrial production and scientific research.
Fig. 2. Schematic of the tunable single-lens spectral filter (SLSF). VBG: volume Bragg grating; 𝛼: emission angle of the beam after VBG; d: spot diameter of the collimated SFS beam; 𝜙: core diameter of the space-fiber coupler; 𝐹 = 𝐹 ′ : the focal length of the single-lens; 𝜃 ≈ 𝑑𝐹 −1 : the divergence angle of the dispersive beam in 𝑦-axis direction after the single-lens; 𝐿 ≈ 𝐹 𝛼: line spot length in the 𝑥-direction after the single-lens.
2. Experimental setup 0.14) is used to make the space-fiber coupler to receive single-mode narrowband SFS. 𝜆 is the central wavelength of the obtained tunable narrowband SFS seed, which can be taken approximately as 𝜆 ≈ 1063 nm. d = 5 mm is spot diameter of the collimated SFS beam produced by the collimated ISO. 𝜃 is the divergence angle of the dispersive beam in 𝑦-axis direction after the single-lens. F is the focal length of the single-lens. The physical meaning of Eq. (1) is that the core diameter of the space-fiber coupler must be larger than spot size on the second focal plane, and the numerical aperture of the space-fiber coupler must also be larger than the divergence angle of the focused beam in the 𝑦axis direction behind the single-lens. According to Eq. (1), F can be calculated as: 17.8 mm ≤ 𝐹 ≤ 85.5 mm. In this experiment, F = 60 mm is taken. As shown in Fig. 3, the narrowband SFS seed from the SLSF is seeded into the power amplifier system, which consists of two preamplifiers (pre-amplifier 2 and pre-amplifier 3) and a main amplifier. In the pre-amplifier 2, a 0.5-m-length single-cladding Yb-doped fiber (SYF, CorActive, YB 401) is used as the gain fiber, which is core-pumped by a single-mode 974 nm LD by a wavelength division multiplexing (WDM). After the pre-amplifier 2, the narrowband SFS is launched into the preamplifier 3. The LD, gain fiber, and combiner in the pre-amplifier 3 are same as that used in the pre-amplifier 1. To further increase the output power of the narrowband SFS, the main amplifier stage is developed by using 24-m-length 25/400 μm (core/cladding) YDF (Nufern, LMA-YDF25/400-VIII). The gain fiber is pumped by 976 nm multimode LDs via a (6+1)×1 combiner. Before the main amplifier, an ISO is also used to prevent backward light from the end facet. The output port of the main amplifier is spliced to quartz end cap to output high power and reduce the end face feedback. Note that the residual pump light is dumped out by cladding power strippers (CPS) in all forward pumped configuration such as pre-amplifier 1, pre-amplifier 3 and the main amplifier.
The experimental configuration for the high-power tunable subnm narrowband SFS contains three components: a broadband SFS, a tunable narrowband SLSF and a power amplifier system. The broadband SFS consists of a backward-pumped broadband SFS seed and pre-amplifier 1 as shown in Fig. 1. A 3.5-m-length double-cladding Yb-doped fiber (YDF, Nufern, LMA-YDF-10/130-VIII) is pumped by a 976 nm laser diode (LD) and a (2+1)×1 combiner to generate a broadband SFS seed. The backward port of broadband SFS seed is sloped 8◦ to prevent feedback from the end facet. A fiber isolator (ISO) is used to protect components by preventing backward light. The broadband SFS seed is launched into a 4.5-m-length double-cladding YDF (Nufern, LMA-YDF-10/130-VIII) by a (2+1)×1 combiner in a forward-pumped pre-amplifier 1. Moreover, a collimated ISO (AFRlaser, HPMFSI-1064-P) is employed to generate a collimated SFS beam and suppress backward light. Then, the collimated SFS beam is launched into the SLSF, which is designed based on a grating monochromator and a movable spacefiber coupler, as shown in Fig. 2(a). A volume Bragg grating (VBG) and a movable space-fiber coupler are placed on the first focal plane and the second focal plane, respectively. The light at any wavelength from the collimated SFS beam in the 𝑦-axis direction is converged into a point on the second focal plane, while the light at different wavelength is converged into different point. By adjusting the beam direction of the collimated SFS and the angle between the VBG and the optical axis of the single-lens, the 1063 nm light output from the collimated SFS beam is converged at the second focal point, indicating the central wavelength of the SLSF set to 1063 nm. Thus, the collimated SFS beam will be converged at a straight line spot on the second focal plane. Furthermore, the diverged beam in the 𝑥-axis direction after VBG dispersion is also collimated because the diverged beam is on the first focal plane. Therefore, the designed SLSF has two properties simultaneously: the broadband beam can be collimated in the 𝑥-axis direction and focused in the 𝑦-axis direction as shown in Fig. 2(b). In addition, the movable space-fiber coupler is mounted on precision multi-axis stages. By moving the space-fiber coupler along the 𝑥-axis direction, we can obtain a tunable narrowband SFS seed with a desired central wavelength. In order to obtain effective coupling for the space-fiber coupler, the following relations must be satisfied [20]:
3. Results and discussion When the output power of the broadband SFS after pre-amplifier 1 is set to 12 W, the output spectra of that is recorded by using an optical spectrum analyzer (YOKOGAWA-AQ6370D) and shown in Fig. 4(a). The central wavelength and the FWHM are calculated to be 1053.1 nm and 30.9 nm, respectively. Self-pulse or parasitic oscillation is not observed below the output power of 12 W. To obtain a tunable narrowband SFS, a designed tunable narrowband SLSF is used. A series of narrowband SFS with different central wavelengths are obtained by moving the space-fiber coupler along the 𝑥-axis direction as shown in Fig. 2(a). The tunable spectra after the tunable SLSF are shown in
𝜃 𝑑 𝜆𝐹 , 𝑁𝐴 ≥ ≈ (1) 𝜋𝑑 2 2𝐹 where 𝜙 and 𝑁𝐴 are core diameter and numerical aperture of the space-fiber coupler, respectively, as shown in Fig. 2(b). In this experiment, a single-mode fiber (Nufern, 1060-XP, 𝜙 = 5.8 μm, 𝑁𝐴 = 𝜙≥
2
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Fig. 3. Schematic of the power amplifier system. SYF: single-cladding Yb-doped fiber; YDF: double-cladding Yb-doped fiber; LD: laser diode; WDM: wavelength division multiplexing; CPS: cladding power strippers. ISO: fiber isolator.
Fig. 4. (a) Spectra of the broadband SFS after pre-amplifier 1. (b) Spectra of the narrowband SFS after the SLSF.
Fig. 5. (a) Output power of main amplifier with different wavelengths. (b) Output spectra of the main amplifier at the maximum output power.
Fig. 4(b). The wavelengths can be tuned from 1052.4 nm to 1072.8 nm, which is limited by the moving range of the precision multi-axis stages. The output power at different wavelength after the SLSF varies from 0.8 mW to 1.2 mW. All of the FWHMs of tunable spectra are less than 0.08 nm, which firstly decreases and then increases with the increase of the tunable wavelength. This is mainly attributed to the aberration-induced distortion effect in the single-lens system. In fact, according to the monochromator principle, the FWHM of tunable spectra is mainly dominated by the spot size 𝐿 ≈ 𝐹 𝛼 in the 𝑥axis direction as shown in Fig. 2(b). 𝛼 is the emission angle of the beam after VBG, which depends on angular dispersion ability of the VBG. Because the larger L is, the smaller the spectral interval corresponds to the unit length in the 𝑥-axis direction is, thus the smaller the FWHM is. Therefore, the FWHM of the narrowband SFS still can be decreased by increasing the angular dispersion ability 𝛼 of VBG and/or adopting single-lens with longer focal length, indicating this SLSF very flexible and extensible. The narrowband SFS from the SLSF can be amplified to 5 W after the pre-amplifier 2 and the pre-amplifier 3 in turn. The central wavelength and the FWHM of the 5 W narrowband SFS at all wavelengths are maintained. After the main amplifier, the output power at all wavelengths reaches 230 W. Fig. 5(a) shows output power of main amplifier with selected wavelengths. As shown in Fig. 5(a), the amplification is almost
same for different wavelengths and the slope efficiency is ∼73%. It is very important for applications that only required wavelength can be tuned while output power is maintained. The spectra of selected wavelengths at maximum output power are shown in Fig. 5(b). Clearly, the central wavelength is equal to that from the SLSF. The FWHM at different wavelengths varies from 0.10 nm to 0.12 nm. At the maximum output power, the signal-noise ratio can reach over 26 dB and the residual pump power has been almost dumped by CPS. Furthermore, no evidence of the stimulated Raman scattering (SRS) and the stimulated Brillouin scattering (SBS) is observed. Fig. 6(a) shows the output spectra evolutions of the main amplifier at 1060.1 nm with output power increasing. It is found that the central wavelength is almost maintained, whereas the spectral width is gradually symmetrically broadened. The FWHM of the output spectrum at 230 W is 0.11 nm. The spectral broadening process is mainly dominated by self-phase modulation (SPM) at this stage [19,21,22], which causes the growth rate of the spectral peak intensity at the central wavelength gradually decreases with the output power increasing. The output beam quality and pattern is measured with help of the M2 factor measuring instrument (PRIMES–HP-LQM). When the output power is 51 W, the M2 factor is measured to be 1.04 after the main amplifier at operated 1060.1 nm, which is close to the ideal Gauss beam. The M2 factor increases with the increase of output power. The 3
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Fig. 6. (a) Output spectra of main amplifier at 1060.1 nm with different output powers. (b) Output beam quality of main amplifier at 1060.1 nm with the output power of 230 W.
measured beam quality factor of M2 = 1.20 at 230 W is shown in Fig. 6(b) and the beam profile is shown in the right panel. The beam quality degradation can be attributed to generation of few higher-order modes. Anyway, the output beam quality still sufficiently meets the requirements for single mode applications.
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4. Conclusions In conclusion, we proposed a method for a high-power tunable sub-nm narrowband near-diffraction-limited SFS based on a singlelens spectral filter. In the experiment, this SLSF consists of a grating monochromator and a movable space-fiber coupler, which has the features of high spectral resolution, simple configuration and high flexibility. Using the SLSF, a tunable narrowband SFS seed from an amplified broadband SFS is easily obtained. The wavelengths are easily tuned from 1052.4 nm to 1072.8 nm by adjusting the spatial position of the space-fiber coupler. With employing a MOPA configuration, the maximum power at different wavelengths is 230 W. The FWHM of the source at full power varies from 0.10 nm to 0.12 nm. No evidence of the SRS and SBS is observed. The beam quality factors (M2 ) of the full power SFS at 1060.1 nm is 1.20. Moreover, the SFS may still have the potential to power scaling by increasing the pump power. The presented high-power tunable sub-nm narrowband neardiffraction-limited SFS may generate great interest in the industrial production and scientific research. CRediT authorship contribution statement Wei Gao: Conceptualization, Methodology, Writing - original draft, Writing - review & editing. Wenhui Fan: Project administration, Writing - review & editing. Yanpeng Zhang: Supervision. Pei Ju: Methodology. Baoyin Zhao: Resources. Peng Wu: Investigation. Gang Li: Formal analysis. Qi Gao: Data curation. Zhe Li: Visualization. Acknowledgments This work is financially supported by Key R & D Program of Shaanxi Province under Grant no. 2018ZDXM-GY-060, National Natural Science Foundation of China under Grant no. 61675230 and Equipment Pre-research Foundation of China under Grant no. 61406190302. References [1] S. Martin-Lopez, M. Gonzalez-Herraez, A. Carrasco-Sanz, F. Vanholsbeeck, S. Coen, H. Fernandez, J. Solis, P. Corredera, M.L. Hernanz, Broadband spectrally flat and high power density light source for fibre sensing purposes, Meas. Sci. Technol. 17 (5) (2006) 1014–1019. [2] B. Lee, Review of the present status of optical fiber sensors, Opt. Fiber Technol. 9 (2) (2003) 57–79. 4