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Optimization investigation for high-power 1034 nm all-fiber narrowband Yb-doped superfluorescent source
Optics Communications 445 (2019) 187–192
Contents lists available at ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
Optimization investigation for high-power 1034 nm all-fiber narrowband Yb-doped superfluorescent source Peng Wu a,b , Baoyin Zhao a ,∗, Wei Zhao a , Zhe Li a , Wei Gao a , Pei Ju a , Gang Li a , Qi Gao a , Yishan Wang a a b
State Key Laboratory of Transient Optics and Photonics, Xi’an Institute of Optics and Precision Mechanics, Chinese Academy of Sciences, Xi’an 710119, China University of Chinese Academy of Sciences, Beijing 100049, China
ARTICLE
INFO
Keywords: Superfluorescent fiber source Amplified spontaneous emission Fiber source
ABSTRACT An optimization for all-fiber narrowband Yb-doped superfluorescent source with a central wavelength shorter than 1040 nm is conducted theoretically and then verified experimentally using steady-state rate equations. Theoretical investigation indicates that with the increase in fiber length, signal power presents one peak value, but the ratio of signal power to output power is decreasing monotonously. Moreover, a filter with high extinction ratio and a gain fiber with high absorption coefficient can have gain suppression on a long wavelength. One all-fiber superfluorescent source is built experimentally on the basis of the theoretically optimized parameters. The source achieves an output power of 214.64 W with a central wavelength of 1034.18 nm and signal–noise ratio of 30 dB. The output power of the source can have further power scaling with considerable available pump power.
1. Introduction Superfluorescent fiber source (SFS) has been used in many application fields, such as spectra, low coherence interferometry, and fiber optic gyroscopes, due to its great advantages of high temporal stability, low coherence, and broad spectrum [1–3]. Specifically, Yb-doped SFS, as an alternative high power source, has been used in spectral beam combining, laser cutting, and pump source [2,4]. In 2015, Liu et al. achieved a 1.5 kW Yb-doped SFS with a central wavelength of 1064.8 nm and full width at half maximum (FWHM) of 0.8 nm [5]. In the same year, Xu et al. used double-cladding fiber and master oscillator power amplifier configuration to scale up the power of the seed source to 1.87 kW with a central wavelength of 1080.7 nm and FWHM of 1.7 nm [2]. In 2016, Xu et al. demonstrated a 2.53 kW broadband Yb-doped SFS with 6.23 nm FWHM linewidth and 1082.08 nm central wavelength [6]. In comparison with the abovementioned Yb-doped SFS with a central wavelength in the 1050–1080 nm region, an SFS with a central wavelength shorter than 1040 nm is required in many application fields and should deserve further attention [4,7–9]. For example, the response coefficient of a laser radar for a 1030 nm source is higher than that for a 1064 nm source [7]; via frequency up-conversion, 1030 nm can be used to achieve a light source with 515 nm wavelength [9]. Moreover, a source with short wavelength has the advantages of small thermal load and high nonlinear effect threshold, which benefit operations under ∗
high output power. In 2011, Schmidt et al. adopted a space structure to amplify the narrowband superfluorescent signal with 1030 nm central wavelength, and the source showed strong suppression on stimulated Brillouin scattering [10]. Generally, an SFS with an all-fiber configuration is preferable due to the requirements for stability and reliability of application fields. However, the low population reversion in doublecladding gain fiber and the red-shift phenomenon of the emission spectra of Yb3+ ions complicate the use of an all-fiber configuration for realizing high-power SFS with a central wavelength shorter than 1040 nm [11]. The difficulty is aggravated by the exact demands on the control over fusion splice joints, feedback from the fiber end facet, and the isolation of backward light. Detailed investigation on how to optimize the parameters of experimental equipment to achieve an allfiber high-power SFS with a wavelength shorter than 1040 nm remains lacking, which can be adverse for further power scaling with an all-fiber configuration. In this study, optimization measures for achieving an all-fiber narrowband high-power SFS with a central wavelength shorter than 1040 nm are evaluated theoretically on the basis of steady-state rate equations. Then, the parameters of gain fiber and filter are optimized in accordance with the amplification efficiency of signal light and the gain suppression on long wavelength. One high-power narrowband SFS with an all-fiber configuration and a central wavelength of 1034.18 nm is achieved.
Corresponding author. E-mail address: [email protected] (B. Zhao).
https://doi.org/10.1016/j.optcom.2019.04.033 Received 25 January 2019; Received in revised form 16 March 2019; Accepted 8 April 2019 Available online 11 April 2019 0030-4018/© 2019 Elsevier B.V. All rights reserved.
P. Wu, B. Zhao, W. Zhao et al.
Optics Communications 445 (2019) 187–192
Fig. 1. Amplification stage.
2. Theoretical model and analysis The typical configuration of one amplification stage is illustrated in Fig. 1, which consists of pump source, gain fiber and combiner. Through the combiner, seed power and pump power is injected into gain fiber and the signal power is boosted up. Cladding pump stripper (CPS) is employed to strip the residual pump power to maintain the safety of system. The propagation and amplification properties of narrowband SFS signal in the gain fiber of amplifier can be simulated by steady-state rate equations, as shown in Eqs. (1)-(3) [12–14]. In Eqs. (1)–(3), 𝑃𝑠 means signal power and 𝑃𝑝 means pump power; 𝑃0 represents the contribution of spontaneous emission; subscript p and s mean the pump light and the signal light; superscript + and − mean the forward and backward propagation direction; 𝜎𝑒 and 𝜎𝑎 are the emission cross section and absorption cross section; 𝛤 is the filling factor; z represents the location in the gain fiber; 𝑁2 and 𝑁1 are the population density in the upper energy level and underground energy level, which meet 𝑁2 (z) + 𝑁1 (z) = N, N is the population density of Yb3+ ions; h, 𝜏, c and A represent Planck constant, spontaneous lifetime, optical speed in vacuum and core area, respectively.
Fig. 2. Intensity distribution of narrowband superfluorescent signal.
𝑑𝑃𝑝 ± (𝑧)
= 𝛤𝑝 (𝜎𝑒𝑝 𝑁2 (z) − 𝜎𝑎𝑝 𝑁1 (z))𝑃𝑝 ± (𝑧) − 𝛼𝑝 𝑃𝑝 ± (𝑧) (1) 𝑑𝑧 ± 𝑑𝑃𝑠 (𝑧, 𝜆) ± = 𝛤𝑠 (𝜆)(𝜎𝑒𝑠 (𝜆)𝑁2 (z) − 𝜎𝑎𝑠 (𝜆)𝑁1 (z))𝑃𝑠± (𝑧) + 𝛤𝑠 (𝜆)𝜎𝑒𝑠 (𝜆)𝑁2 (z)𝑃0 (𝜆) − 𝛼𝑠 𝑃𝑠± (𝑧, 𝜆) 𝑑𝑧
±
(2) 𝑁2 (z) = 𝑁
(P+𝑝 (z)+P−𝑝 (z))𝜎𝑎𝑝 𝛤𝑝 𝜆𝑝 ℎ𝑐𝐴 (P+𝑝 (z)+P−𝑝 (z))(𝜎𝑎𝑝 +𝜎𝑒𝑝 )𝛤𝑝 𝜆𝑝 ℎ𝑐𝐴
+
+
1 ℎ𝑐𝐴
1 ℎ𝑐𝐴
Fig. 3. Simulation results of the first preamplification stage.
∫ 𝜎𝑎 (𝜆)(P+𝑠 (z, 𝜆) + P−𝑠 (z, 𝜆))𝛤𝑠 (𝜆)𝜆𝑑𝜆
∫ (𝜎𝑎𝑠 (𝜆) + 𝜎𝑒𝑠 (𝜆))(P+𝑠 (z, 𝜆) + P−𝑠 (z, 𝜆))𝛤𝑠 (𝜆)𝜆𝑑𝜆 +
1 𝜏
(3)
a slow decrease. When the length continuously increases, the reabsorption of signal light and the gain competition from the long wavelength range become fierce. Thus, the signal power begins to decrease, which leads to the sharp decrease in ratio, as shown in the range of 3.0–5.0 m. As the signal power is nearly equal in the range of 2.0–3.5 m and the length of gain fiber of the amplifier in our lab is 2.3 m, we assume that the length of gain fiber in the first pre-amplification stage is 2.3 m and simulate the second preamplification stage, as shown in Fig. 4. The variations in signal power and ratio with the increase in fiber length show analogous trends as that of the first preamplification stage. The fiber length in the second preamplification stage can be selected in the range of 1.5–2.5 m on the basis of the simulation results shown in Fig. 4.
2.1. Simulation results in preamplification stages The intensity distribution of narrowband superfluorescent signal injected into the preamplification stages is obtained from our lab and shown in Fig. 2. The central wavelength of signal light is around 1034 nm. For maintaining the beam quality, the gain fiber adopted in the preamplification stages is a few-mode fiber. The core/inner cladding diameter of the gain fiber is 10/130 μm. The numerical apertures of the fiber core and inner cladding are 0.075 and 0.46, respectively. The cladding absorption coefficient for the 975 nm pump light is 3.9 dB/m. Fig. 3 depicts the simulation results of the first preamplification stage. The length of the gain fiber in the first preamplification stage is set in the range of 0.5–5.0 m. With the increase in fiber length, output power gradually becomes saturable, and the signal power initially increases and then decreases. The signal power reaches the peak when the fiber length is 2.5 m. On the contrary, the ratio of signal power to output power is monotonously decreasing. This phenomenon can be attributed to the gain competition between the short and long wavelength ranges. During the propagation of signal light, both wavelength ranges can be amplified. In the range of 0.5– 2.5 m, in comparison with the reabsorption, the amplification of signal light is dominant due to the short fiber length and strong pump power distribution. Accordingly, the ratio of signal power to output power has
2.2. Simulation results in the main amplification stage One filter is used to filter the amplified spontaneous emission in the long wavelength range before the output from the second preamplification stage is injected into the main amplifier. 2.2.1. Influence of the extinction ratio of the filter The extinction ratios of the filter are set to 0, 10, 20, and 30 dB. Fig. 5 shows the simulation results. Fig. 5(a) depicts the situation wherein the output from the second preamplification stage is directly injected into the main amplifier. Although the output power of the 188
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Optics Communications 445 (2019) 187–192
2.2.2. Influence of the absorption coefficient of gain fiber Given one filter with an extinction ratio of 30 dB, two common types of gain fibers are considered in the main amplifier. The first type, which is labeled as 20/400 fiber, has core and inner cladding diameters of 20 and 400 μm, respectively. The absorption coefficient for the pump light is approximately 1.2 dB/m. The numerical apertures of the fiber core and inner cladding are approximately 0.06 and 0.46, respectively. Fig. 6 shows the simulation results of the main amplifier with 20/400 fiber. The maximum signal power is approximately 170 W. With the increase in fiber length, the signal power begins to decrease sharply. The second type, which is labeled as 48/400 fiber, has core and inner cladding diameters of 48 and 400 μm, respectively. The absorption coefficient for the pump light is 7 dB/m, which is higher than that of the 20/400 fiber. Fig. 7 depicts the simulation results of the 48/400 fiber. When the length is in the range of 2–5.5 m, the output power is beyond 210 W, and the ratio of signal power to output power is beyond 97%. The maximum signal power is beyond 220 W, and the ratio reaches 99%. On the contrary, the maximum signal power is around 170 W, and the ratio is approximately 94% when the 20/400 fiber is adopted in the main amplifier. As the absorption coefficient of the 20/400 fiber is lower than that of the 48/400 fiber, a sufficient pump power extraction requires a long fiber length, which induces the acute reabsorption of signal light and strengthens the amplification of long wavelength range. Thus, a gain fiber with a high absorption coefficient can benefit in increasing the amplification efficiency of signal light with a short wavelength and the ratio of signal power to output power.
Fig. 4. Simulation results of the second preamplification stage.
main amplifier can reach approximately 220 W, the maximum power of signal light around 1034 nm is only approximately 100 W, and the ratio of signal power to output power is approximately 0.52. Fig. 5(b), (c) and (d) present the amplification properties of the main amplifier wherein the output from the preamplification stages is filtered using filters with different extinction ratios. When the extinction ratio increases from 10 dB to 30 dB, the maximum power of signal light around 1034 nm increases from 183.4 W to 221.1 W, whereas the maximum output power is above 222.8 W. The corresponding ratio at maximum signal power increases from 0.87 to 0.99. The gain competition from the long wavelength is suppressed greatly in the main amplifier because the filter with high extinction ratio can greatly filter the power distribution in a long wavelength range. Thus, the signal light with a short wavelength can be enhanced with further efficiency, as presented in Fig. 5.
3. Experimental setup Fig. 8 shows the experimental setup, which involves the seed source, preamplification stages, and main amplifier. The seed source provides a narrowband superfluorescent signal with a central wavelength of around 1034 nm, which is generated by one broadband SFS and filter setup. The signal is then injected into one splitter (SPL) with a splitting ratio of 99:1. From this 1% port, the operation state and output
Fig. 5. Amplification properties of the main amplifier (a) without filter; (b) with an extinction ratio of 10 dB; (c) with an extinction ratio of 20 dB; (d) with an extinction ratio of 30 dB. 189
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Optics Communications 445 (2019) 187–192
4. Experimental results and discussions In the seed source, the output power of the narrowband superfluorescent signal is 20.06 mW. Fig. 2 shows the intensity distribution. After the two preamplification stages, the output power is scaled up to 5.09 W. Fig. 9(a) presents the output spectra of the second preamplification stage under different pump power levels. With the increase in pump power, the intensity of amplified spontaneous emission around 1060 nm also increases. This phenomenon is a common difficulty in the amplification process of ∼1030 nm signal and can degrade the amplification properties of the main amplifier. In our experiment, the other filter is used to filter the amplified spontaneous emission at other wavelength regions. Fig. 9(b) displays the spectra before and after the filter, represented by black and red lines, respectively, at the maximum output power of the preamplification stage. After the filter, the output power is 1.623 W, the central wavelength is 1034.18 nm, and the FWHM is 0.71 nm. The signal–noise ratio is approximately 30 dB. Fig. 10 shows a comparison of the spectrum shapes between the simulation and experimental results to verify the rationality of the simulation results. Fig. 10(a) and (b) present the spectra before and after the filter at the maximum output power of the second preamplification stage, respectively. The experimental results show a good agreement with the simulation results. Fig. 11 depicts the evolution of the output spectra of the main amplifier under different pump power levels. Residual pump power exists due to the incomplete absorption of pump light and the defect of stripping cladding pump light. We calculate the signal power and residual pump power on the basis of the spectra and present them in Fig. 12. During amplification, the proportion of residual pump power to output power is below 3.6%, and at the full output power, the proportion decreases to 1.1%. Meanwhile, the slope efficiency gradually increases with the increase in output power, as shown in Fig. 12. This phenomenon is caused by joint action of two factors [2,15]. First, the absorption coefficient of Yb3+ ion for different pump wavelengths varies and peaks at around 975 nm. Second, with the increase in pump power and concomitant temperature, the output spectra of the pump source shift from 968.35 nm to 974.7 nm, as shown in Fig. 11. Then, the absorption coefficient of Yb3+ ion increases, and the pump power is absorbed further efficiently. Therefore, the slope efficiency becomes higher. At the maximum output power, the signal–noise ratio reaches 30 dB, and the signal power is 214.64 W. The optical–optical efficiency is approximately 66.7%. The central wavelength and FWHM are 1034.18 and 0.82 nm, respectively. In comparison with the seed source injected into the main amplifier, the central wavelength remains the same. The FWHM of the output of the main amplifier has a broadening phenomenon due to the SPM effect [5,16]. No saturation phenomenon occurs, as depicted in Fig. 12, and considerable output power can be realized using additional available pump sources. However, the output power cannot be boosted up infinitely with fixed injected power due to the limitation of the amplification factor of the amplifier. Thus, expect for the substantial available pump power, the boost of seed power and the filter that can bear additional power and possess high extinction ratio is the emphasis for achieving additional output power and the research focus of our following work. In addition, the beam quality and beam profile are important for the application of light source with large-core fiber, which will also be further studied in our following work.
Fig. 6. Simulation results of the main amplifier with 20/400 fiber.
Fig. 7. Simulation results of the main amplifier with 48/400 fiber.
spectrum of the narrowband seed source can be monitored during amplification. Preamplification stages consist of two amplification stages, which have the same configuration and components, as shown in Fig. 1. Section 2.1 describes the parameters of gain fiber, and its length is 2.3 m in the preamplification stages. After the second preamplification stage, one isolator (ISO) with an isolation ratio of 30 dB is used to prevent the backward signal. The other filter centered at around 1034 nm is used to filter the amplified spontaneous emission at the other wavelength range, which emerges in the preamplification stages. The main amplifier also has the same configuration, as shown in Fig. 1. To increase the amplification efficiency of signal light and the ratio of signal power to output power, the 48/400 fiber is selected as the gain fiber in the main amplifier and the length is 3 m, with reference to the optimized results in Section 2.2. Furthermore, the diameter of the coiled gain fiber is approximately 20 cm to guarantee the beam quality. One 1.0 m matched Ge-doped fiber is fused with the gain fiber, and the stripping cladding pump light is conducted on that. Then, one homemade quart block head ( QBH) is used to deliver the output from the main amplifier. All systems, except the pump source of the main amplifier, are placed in one water-cooled heat sink with a temperature of 16 ◦ C to prevent possible heat damage. The pump source of the main amplifier is cooled to 21.6 ◦ C to maintain a good operation state.
5. Conclusions In this study, the optimization measures for achieving an all-fiber high-power narrowband SFS with a central wavelength shorter than 1040 nm is investigated through theoretical simulation and experimental verification. Simulation results show that with the increase in fiber length, the signal power initially increases and then decreases, whereas the ratio of signal power to output power is monotonously decreasing. 190
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Optics Communications 445 (2019) 187–192
Fig. 8. Experimental setup.
Fig. 9. (a) Output spectra of the second preamplification stage. (b) Comparison of spectra before and after the filter . (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 10. Comparison between the simulation and experimental results (a) before the filter; (b) after the filter.
Fig. 12. Output power, signal power, and residual pump power of the main amplifier versus pump power.
Fig. 11. Output spectra of the main amplifier under different pump power levels.
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Optics Communications 445 (2019) 187–192
Moreover, a filter with high extinction ratio and a gain fiber with high absorption coefficient can suppress the gain competition from the long wavelength. On the basis of the simulation results, the parameters of gain fiber and filter are optimized and then used in the experiment. One narrowband SFS with an all-fiber configuration and a central wavelength shorter than 1040 nm is achieved. The maximum signal power and central wavelength are 214.64 W and 1034.18 nm, respectively. The optical–optical efficiency, FWHM, and signal–noise ratio are 66.7%, 0.82 nm, and 30 dB, respectively. During amplification, no saturation phenomenon is found, and the output power is limited by the available pump source. These investigation and realization of the high-power 1034 nm narrowband SFS with an all-fiber configuration can broaden the application fields of superfluorescent source.
[4] 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 (2016) 12063–12071. [5] G. Liu, Y. Yang, M. Lei, M. Hu, Y. Zheng, C. Liu, Y. Qi, B. He, J. Zhou, Y. Wei, Q. Lou, 1.5 kW near-diffraction-limited narrowband all-fiber superfluorescent source, Chin. J. Lasers 42 (2015) 1202009-1202001-1202005. [6] J. Xu, H. Xiao, J. Leng, H. Zhang, P. Zhou, J. Chen, 2.53 kW all-fiberized superfluorescent fiber source based on a compact single-stage power-scaling scheme, Laser Phys. Lett 13 (2016) 105101. [7] C. Zhou, Nm high repetition rate nanosecond pulse all fiber amplifier, Chin. J. Lasers 38 (1030) (2011) 0802010. [8] N.A. Naderi, I. Dajani, A. Flores, High-efficiency, kilowatt 1034 nm all-fiber amplifier operating at 11 pm linewidth, Opt. Lett. 41 (2016) 1018–1021. [9] Y. Sun, W. Ke, Y. Feng, Y. Wang, W. Peng, Y. Ma, T. Li, X. Wang, C. Tang, K. Zhang, Nm Kilowatt-level Ytterbium-doped narrow linewidth fiber amplifier, Chin. J. Lasers 43 (1030) (2016) 0601003-0601001. [10] O. Schmidt, M. Rekas, C. Wirth, J. Rothhardt, S. Rhein, A. Kliner, M. Strecker, S. T, J. Limpert, R. Eberhardt, A. Tünnermann, High power narrow-band fiber-based ASE source, Opt. Express 19 (2011) 4421–4427. [11] A.S. Kurkov, V.M. Paramonov, O.I. Medvedkov, Ytterbium fiber laser emitting at 1160 nm, Laser Phys. Lett. 3 (2010) 503–506. [12] I. Kelson, A.A. Hardy, Strongly pumped fiber lasers, IEEE J. Quantum Electron. 34 (1998) 1570–1577. [13] P. Yan, J. Sun, D. Li, M. Gong, Q. Xiao, Studies of central wavelength of highpower all-fiber superfluorescent sources with Yb-doped double-clad fibers, Opt. Commun. 380 (2016) 250–259. [14] P. Wu, Y. Wang, W. Zhao, B. Zhao, X. Miao, Z. Li, W. Gao, J. Zheng, Temperature dependence of Yb-doped superfluorescent fiber source, Infrared Phys. Technol. 90 (2018) 48–52. [15] R. Paschotta, J. Nilsson, A.C. Tropper, D.C. Hanna, Ytterbium-doped fiber amplifiers, IEEE J. Quantum Electron. 33 (1997) 1049–1056. [16] S.I. Kablukov, E.A. Zlobina, E.V. Podivilov, S.A. Babin, Output spectrum of Yb-doped fiber lasers, Opt. Lett. 37 (2012) 2508–2510.
Funding This work is supported by the project of National Key Research and Development Program of China (No. 2017YFB1104400). References [1] K. Liu, M. Digonnet, K. Fesler, B.Y. Kim, H.J. Shaw, Superfluorescent single mode Nd:fiber source at 1060 nm, Opt. Fiber Sensors (1988). [2] 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 (2015) 2973–2976. [3] P. Yan, J. Sun, D. Li, X. Wang, Y. Huang, M. Gong, Q. Xiao, 933 W Yb-doped fiber ASE amplifier with 50.4 nm bandwidth, Opt. Express 24 (2016) 19940–19948.
192
Contents lists available at ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
Optimization investigation for high-power 1034 nm all-fiber narrowband Yb-doped superfluorescent source Peng Wu a,b , Baoyin Zhao a ,∗, Wei Zhao a , Zhe Li a , Wei Gao a , Pei Ju a , Gang Li a , Qi Gao a , Yishan Wang a a b
State Key Laboratory of Transient Optics and Photonics, Xi’an Institute of Optics and Precision Mechanics, Chinese Academy of Sciences, Xi’an 710119, China University of Chinese Academy of Sciences, Beijing 100049, China
ARTICLE
INFO
Keywords: Superfluorescent fiber source Amplified spontaneous emission Fiber source
ABSTRACT An optimization for all-fiber narrowband Yb-doped superfluorescent source with a central wavelength shorter than 1040 nm is conducted theoretically and then verified experimentally using steady-state rate equations. Theoretical investigation indicates that with the increase in fiber length, signal power presents one peak value, but the ratio of signal power to output power is decreasing monotonously. Moreover, a filter with high extinction ratio and a gain fiber with high absorption coefficient can have gain suppression on a long wavelength. One all-fiber superfluorescent source is built experimentally on the basis of the theoretically optimized parameters. The source achieves an output power of 214.64 W with a central wavelength of 1034.18 nm and signal–noise ratio of 30 dB. The output power of the source can have further power scaling with considerable available pump power.
1. Introduction Superfluorescent fiber source (SFS) has been used in many application fields, such as spectra, low coherence interferometry, and fiber optic gyroscopes, due to its great advantages of high temporal stability, low coherence, and broad spectrum [1–3]. Specifically, Yb-doped SFS, as an alternative high power source, has been used in spectral beam combining, laser cutting, and pump source [2,4]. In 2015, Liu et al. achieved a 1.5 kW Yb-doped SFS with a central wavelength of 1064.8 nm and full width at half maximum (FWHM) of 0.8 nm [5]. In the same year, Xu et al. used double-cladding fiber and master oscillator power amplifier configuration to scale up the power of the seed source to 1.87 kW with a central wavelength of 1080.7 nm and FWHM of 1.7 nm [2]. In 2016, Xu et al. demonstrated a 2.53 kW broadband Yb-doped SFS with 6.23 nm FWHM linewidth and 1082.08 nm central wavelength [6]. In comparison with the abovementioned Yb-doped SFS with a central wavelength in the 1050–1080 nm region, an SFS with a central wavelength shorter than 1040 nm is required in many application fields and should deserve further attention [4,7–9]. For example, the response coefficient of a laser radar for a 1030 nm source is higher than that for a 1064 nm source [7]; via frequency up-conversion, 1030 nm can be used to achieve a light source with 515 nm wavelength [9]. Moreover, a source with short wavelength has the advantages of small thermal load and high nonlinear effect threshold, which benefit operations under ∗
high output power. In 2011, Schmidt et al. adopted a space structure to amplify the narrowband superfluorescent signal with 1030 nm central wavelength, and the source showed strong suppression on stimulated Brillouin scattering [10]. Generally, an SFS with an all-fiber configuration is preferable due to the requirements for stability and reliability of application fields. However, the low population reversion in doublecladding gain fiber and the red-shift phenomenon of the emission spectra of Yb3+ ions complicate the use of an all-fiber configuration for realizing high-power SFS with a central wavelength shorter than 1040 nm [11]. The difficulty is aggravated by the exact demands on the control over fusion splice joints, feedback from the fiber end facet, and the isolation of backward light. Detailed investigation on how to optimize the parameters of experimental equipment to achieve an allfiber high-power SFS with a wavelength shorter than 1040 nm remains lacking, which can be adverse for further power scaling with an all-fiber configuration. In this study, optimization measures for achieving an all-fiber narrowband high-power SFS with a central wavelength shorter than 1040 nm are evaluated theoretically on the basis of steady-state rate equations. Then, the parameters of gain fiber and filter are optimized in accordance with the amplification efficiency of signal light and the gain suppression on long wavelength. One high-power narrowband SFS with an all-fiber configuration and a central wavelength of 1034.18 nm is achieved.
Corresponding author. E-mail address: [email protected] (B. Zhao).
https://doi.org/10.1016/j.optcom.2019.04.033 Received 25 January 2019; Received in revised form 16 March 2019; Accepted 8 April 2019 Available online 11 April 2019 0030-4018/© 2019 Elsevier B.V. All rights reserved.
P. Wu, B. Zhao, W. Zhao et al.
Optics Communications 445 (2019) 187–192
Fig. 1. Amplification stage.
2. Theoretical model and analysis The typical configuration of one amplification stage is illustrated in Fig. 1, which consists of pump source, gain fiber and combiner. Through the combiner, seed power and pump power is injected into gain fiber and the signal power is boosted up. Cladding pump stripper (CPS) is employed to strip the residual pump power to maintain the safety of system. The propagation and amplification properties of narrowband SFS signal in the gain fiber of amplifier can be simulated by steady-state rate equations, as shown in Eqs. (1)-(3) [12–14]. In Eqs. (1)–(3), 𝑃𝑠 means signal power and 𝑃𝑝 means pump power; 𝑃0 represents the contribution of spontaneous emission; subscript p and s mean the pump light and the signal light; superscript + and − mean the forward and backward propagation direction; 𝜎𝑒 and 𝜎𝑎 are the emission cross section and absorption cross section; 𝛤 is the filling factor; z represents the location in the gain fiber; 𝑁2 and 𝑁1 are the population density in the upper energy level and underground energy level, which meet 𝑁2 (z) + 𝑁1 (z) = N, N is the population density of Yb3+ ions; h, 𝜏, c and A represent Planck constant, spontaneous lifetime, optical speed in vacuum and core area, respectively.
Fig. 2. Intensity distribution of narrowband superfluorescent signal.
𝑑𝑃𝑝 ± (𝑧)
= 𝛤𝑝 (𝜎𝑒𝑝 𝑁2 (z) − 𝜎𝑎𝑝 𝑁1 (z))𝑃𝑝 ± (𝑧) − 𝛼𝑝 𝑃𝑝 ± (𝑧) (1) 𝑑𝑧 ± 𝑑𝑃𝑠 (𝑧, 𝜆) ± = 𝛤𝑠 (𝜆)(𝜎𝑒𝑠 (𝜆)𝑁2 (z) − 𝜎𝑎𝑠 (𝜆)𝑁1 (z))𝑃𝑠± (𝑧) + 𝛤𝑠 (𝜆)𝜎𝑒𝑠 (𝜆)𝑁2 (z)𝑃0 (𝜆) − 𝛼𝑠 𝑃𝑠± (𝑧, 𝜆) 𝑑𝑧
±
(2) 𝑁2 (z) = 𝑁
(P+𝑝 (z)+P−𝑝 (z))𝜎𝑎𝑝 𝛤𝑝 𝜆𝑝 ℎ𝑐𝐴 (P+𝑝 (z)+P−𝑝 (z))(𝜎𝑎𝑝 +𝜎𝑒𝑝 )𝛤𝑝 𝜆𝑝 ℎ𝑐𝐴
+
+
1 ℎ𝑐𝐴
1 ℎ𝑐𝐴
Fig. 3. Simulation results of the first preamplification stage.
∫ 𝜎𝑎 (𝜆)(P+𝑠 (z, 𝜆) + P−𝑠 (z, 𝜆))𝛤𝑠 (𝜆)𝜆𝑑𝜆
∫ (𝜎𝑎𝑠 (𝜆) + 𝜎𝑒𝑠 (𝜆))(P+𝑠 (z, 𝜆) + P−𝑠 (z, 𝜆))𝛤𝑠 (𝜆)𝜆𝑑𝜆 +
1 𝜏
(3)
a slow decrease. When the length continuously increases, the reabsorption of signal light and the gain competition from the long wavelength range become fierce. Thus, the signal power begins to decrease, which leads to the sharp decrease in ratio, as shown in the range of 3.0–5.0 m. As the signal power is nearly equal in the range of 2.0–3.5 m and the length of gain fiber of the amplifier in our lab is 2.3 m, we assume that the length of gain fiber in the first pre-amplification stage is 2.3 m and simulate the second preamplification stage, as shown in Fig. 4. The variations in signal power and ratio with the increase in fiber length show analogous trends as that of the first preamplification stage. The fiber length in the second preamplification stage can be selected in the range of 1.5–2.5 m on the basis of the simulation results shown in Fig. 4.
2.1. Simulation results in preamplification stages The intensity distribution of narrowband superfluorescent signal injected into the preamplification stages is obtained from our lab and shown in Fig. 2. The central wavelength of signal light is around 1034 nm. For maintaining the beam quality, the gain fiber adopted in the preamplification stages is a few-mode fiber. The core/inner cladding diameter of the gain fiber is 10/130 μm. The numerical apertures of the fiber core and inner cladding are 0.075 and 0.46, respectively. The cladding absorption coefficient for the 975 nm pump light is 3.9 dB/m. Fig. 3 depicts the simulation results of the first preamplification stage. The length of the gain fiber in the first preamplification stage is set in the range of 0.5–5.0 m. With the increase in fiber length, output power gradually becomes saturable, and the signal power initially increases and then decreases. The signal power reaches the peak when the fiber length is 2.5 m. On the contrary, the ratio of signal power to output power is monotonously decreasing. This phenomenon can be attributed to the gain competition between the short and long wavelength ranges. During the propagation of signal light, both wavelength ranges can be amplified. In the range of 0.5– 2.5 m, in comparison with the reabsorption, the amplification of signal light is dominant due to the short fiber length and strong pump power distribution. Accordingly, the ratio of signal power to output power has
2.2. Simulation results in the main amplification stage One filter is used to filter the amplified spontaneous emission in the long wavelength range before the output from the second preamplification stage is injected into the main amplifier. 2.2.1. Influence of the extinction ratio of the filter The extinction ratios of the filter are set to 0, 10, 20, and 30 dB. Fig. 5 shows the simulation results. Fig. 5(a) depicts the situation wherein the output from the second preamplification stage is directly injected into the main amplifier. Although the output power of the 188
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2.2.2. Influence of the absorption coefficient of gain fiber Given one filter with an extinction ratio of 30 dB, two common types of gain fibers are considered in the main amplifier. The first type, which is labeled as 20/400 fiber, has core and inner cladding diameters of 20 and 400 μm, respectively. The absorption coefficient for the pump light is approximately 1.2 dB/m. The numerical apertures of the fiber core and inner cladding are approximately 0.06 and 0.46, respectively. Fig. 6 shows the simulation results of the main amplifier with 20/400 fiber. The maximum signal power is approximately 170 W. With the increase in fiber length, the signal power begins to decrease sharply. The second type, which is labeled as 48/400 fiber, has core and inner cladding diameters of 48 and 400 μm, respectively. The absorption coefficient for the pump light is 7 dB/m, which is higher than that of the 20/400 fiber. Fig. 7 depicts the simulation results of the 48/400 fiber. When the length is in the range of 2–5.5 m, the output power is beyond 210 W, and the ratio of signal power to output power is beyond 97%. The maximum signal power is beyond 220 W, and the ratio reaches 99%. On the contrary, the maximum signal power is around 170 W, and the ratio is approximately 94% when the 20/400 fiber is adopted in the main amplifier. As the absorption coefficient of the 20/400 fiber is lower than that of the 48/400 fiber, a sufficient pump power extraction requires a long fiber length, which induces the acute reabsorption of signal light and strengthens the amplification of long wavelength range. Thus, a gain fiber with a high absorption coefficient can benefit in increasing the amplification efficiency of signal light with a short wavelength and the ratio of signal power to output power.
Fig. 4. Simulation results of the second preamplification stage.
main amplifier can reach approximately 220 W, the maximum power of signal light around 1034 nm is only approximately 100 W, and the ratio of signal power to output power is approximately 0.52. Fig. 5(b), (c) and (d) present the amplification properties of the main amplifier wherein the output from the preamplification stages is filtered using filters with different extinction ratios. When the extinction ratio increases from 10 dB to 30 dB, the maximum power of signal light around 1034 nm increases from 183.4 W to 221.1 W, whereas the maximum output power is above 222.8 W. The corresponding ratio at maximum signal power increases from 0.87 to 0.99. The gain competition from the long wavelength is suppressed greatly in the main amplifier because the filter with high extinction ratio can greatly filter the power distribution in a long wavelength range. Thus, the signal light with a short wavelength can be enhanced with further efficiency, as presented in Fig. 5.
3. Experimental setup Fig. 8 shows the experimental setup, which involves the seed source, preamplification stages, and main amplifier. The seed source provides a narrowband superfluorescent signal with a central wavelength of around 1034 nm, which is generated by one broadband SFS and filter setup. The signal is then injected into one splitter (SPL) with a splitting ratio of 99:1. From this 1% port, the operation state and output
Fig. 5. Amplification properties of the main amplifier (a) without filter; (b) with an extinction ratio of 10 dB; (c) with an extinction ratio of 20 dB; (d) with an extinction ratio of 30 dB. 189
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4. Experimental results and discussions In the seed source, the output power of the narrowband superfluorescent signal is 20.06 mW. Fig. 2 shows the intensity distribution. After the two preamplification stages, the output power is scaled up to 5.09 W. Fig. 9(a) presents the output spectra of the second preamplification stage under different pump power levels. With the increase in pump power, the intensity of amplified spontaneous emission around 1060 nm also increases. This phenomenon is a common difficulty in the amplification process of ∼1030 nm signal and can degrade the amplification properties of the main amplifier. In our experiment, the other filter is used to filter the amplified spontaneous emission at other wavelength regions. Fig. 9(b) displays the spectra before and after the filter, represented by black and red lines, respectively, at the maximum output power of the preamplification stage. After the filter, the output power is 1.623 W, the central wavelength is 1034.18 nm, and the FWHM is 0.71 nm. The signal–noise ratio is approximately 30 dB. Fig. 10 shows a comparison of the spectrum shapes between the simulation and experimental results to verify the rationality of the simulation results. Fig. 10(a) and (b) present the spectra before and after the filter at the maximum output power of the second preamplification stage, respectively. The experimental results show a good agreement with the simulation results. Fig. 11 depicts the evolution of the output spectra of the main amplifier under different pump power levels. Residual pump power exists due to the incomplete absorption of pump light and the defect of stripping cladding pump light. We calculate the signal power and residual pump power on the basis of the spectra and present them in Fig. 12. During amplification, the proportion of residual pump power to output power is below 3.6%, and at the full output power, the proportion decreases to 1.1%. Meanwhile, the slope efficiency gradually increases with the increase in output power, as shown in Fig. 12. This phenomenon is caused by joint action of two factors [2,15]. First, the absorption coefficient of Yb3+ ion for different pump wavelengths varies and peaks at around 975 nm. Second, with the increase in pump power and concomitant temperature, the output spectra of the pump source shift from 968.35 nm to 974.7 nm, as shown in Fig. 11. Then, the absorption coefficient of Yb3+ ion increases, and the pump power is absorbed further efficiently. Therefore, the slope efficiency becomes higher. At the maximum output power, the signal–noise ratio reaches 30 dB, and the signal power is 214.64 W. The optical–optical efficiency is approximately 66.7%. The central wavelength and FWHM are 1034.18 and 0.82 nm, respectively. In comparison with the seed source injected into the main amplifier, the central wavelength remains the same. The FWHM of the output of the main amplifier has a broadening phenomenon due to the SPM effect [5,16]. No saturation phenomenon occurs, as depicted in Fig. 12, and considerable output power can be realized using additional available pump sources. However, the output power cannot be boosted up infinitely with fixed injected power due to the limitation of the amplification factor of the amplifier. Thus, expect for the substantial available pump power, the boost of seed power and the filter that can bear additional power and possess high extinction ratio is the emphasis for achieving additional output power and the research focus of our following work. In addition, the beam quality and beam profile are important for the application of light source with large-core fiber, which will also be further studied in our following work.
Fig. 6. Simulation results of the main amplifier with 20/400 fiber.
Fig. 7. Simulation results of the main amplifier with 48/400 fiber.
spectrum of the narrowband seed source can be monitored during amplification. Preamplification stages consist of two amplification stages, which have the same configuration and components, as shown in Fig. 1. Section 2.1 describes the parameters of gain fiber, and its length is 2.3 m in the preamplification stages. After the second preamplification stage, one isolator (ISO) with an isolation ratio of 30 dB is used to prevent the backward signal. The other filter centered at around 1034 nm is used to filter the amplified spontaneous emission at the other wavelength range, which emerges in the preamplification stages. The main amplifier also has the same configuration, as shown in Fig. 1. To increase the amplification efficiency of signal light and the ratio of signal power to output power, the 48/400 fiber is selected as the gain fiber in the main amplifier and the length is 3 m, with reference to the optimized results in Section 2.2. Furthermore, the diameter of the coiled gain fiber is approximately 20 cm to guarantee the beam quality. One 1.0 m matched Ge-doped fiber is fused with the gain fiber, and the stripping cladding pump light is conducted on that. Then, one homemade quart block head ( QBH) is used to deliver the output from the main amplifier. All systems, except the pump source of the main amplifier, are placed in one water-cooled heat sink with a temperature of 16 ◦ C to prevent possible heat damage. The pump source of the main amplifier is cooled to 21.6 ◦ C to maintain a good operation state.
5. Conclusions In this study, the optimization measures for achieving an all-fiber high-power narrowband SFS with a central wavelength shorter than 1040 nm is investigated through theoretical simulation and experimental verification. Simulation results show that with the increase in fiber length, the signal power initially increases and then decreases, whereas the ratio of signal power to output power is monotonously decreasing. 190
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Fig. 8. Experimental setup.
Fig. 9. (a) Output spectra of the second preamplification stage. (b) Comparison of spectra before and after the filter . (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 10. Comparison between the simulation and experimental results (a) before the filter; (b) after the filter.
Fig. 12. Output power, signal power, and residual pump power of the main amplifier versus pump power.
Fig. 11. Output spectra of the main amplifier under different pump power levels.
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Moreover, a filter with high extinction ratio and a gain fiber with high absorption coefficient can suppress the gain competition from the long wavelength. On the basis of the simulation results, the parameters of gain fiber and filter are optimized and then used in the experiment. One narrowband SFS with an all-fiber configuration and a central wavelength shorter than 1040 nm is achieved. The maximum signal power and central wavelength are 214.64 W and 1034.18 nm, respectively. The optical–optical efficiency, FWHM, and signal–noise ratio are 66.7%, 0.82 nm, and 30 dB, respectively. During amplification, no saturation phenomenon is found, and the output power is limited by the available pump source. These investigation and realization of the high-power 1034 nm narrowband SFS with an all-fiber configuration can broaden the application fields of superfluorescent source.
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