⇦Chinese Physics Letters, 2016, Vol. 33, No. 7, Article code 074207 Stimulated Brillouin Scattering Enhancement Factor Improvement in a 11.6-GHz-Linewidth 1.5-kW Yb-Doped Fiber Amplifier * Guang-Bo Liu(刘广柏)1,2, Yi-Feng Yang(杨依枫)1**, Jian-Hua Wang(王建华)3, Ye Zheng(郑也)1,2, Xiao-Long Chen(陈晓龙)1, Kai Liu(刘恺)1, Chun Zhao(赵纯)1, Yun-Feng Qi(漆云凤)1, Bing He(何兵)1**, Jun Zhou(周军)1** Affiliations 1Shanghai Key Laboratory of All Solid-State Laser and Applied Techniques, Research Center of Space Laser Information Technology, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800 2University of Chinese Academy of Sciences, Beijing 100049 3Department of Space and Command, Academy of Equipment, Beijing 101416 Received 15 April 2016 *Supported by the National Natural Science Foundation of China under Grant Nos U1330134, 61308024 and 11174305, the National High-Technology Research and Development Program of China under Grant No 2014AA041901, and the Shanghai Natural Science Foundation under Grant No 11ZR1441400.
**Corresponding authors. Email: yfyang@siom.ac.cn; bryanho@siom.ac.cn; junzhousd@siom.ac.cn Citation Text: Liu G B, Yang Y F, Wang J H, Zheng Y and Chen X L et al 2016 Chin. Phys. Lett. 33 074207 Abstract The stimulated Brillouin scattering (SBS) threshold enhancement factor in a pure white noise linewidth broadening Yb-doped fiber amplifier (YDFA) with a short large mode area fiber is theoretically and experimentally studied. We demonstrate a 1064.08 nm, 11.6 GHz linewidth, 1.5 kW output power YDFA with an SBS threshold enhancement of $\sim$57 (26 W SBS threshold with single frequency seed). The output beam is near-diffraction limited with a beam quality factor of $M^{2}=1.15$ and a slope efficiency of up to 87%. No SBS or stimulated Raman scattering effects are observed in the whole power range. Further power scaling is limited by the available pump power in our system. DOI:10.1088/0256-307X/33/7/074207 PACS:42.55.Wd, 42.60.Fc, 42.65.Es © 2016 Chinese Physics Society Article Text Yb-doped high power fiber amplifiers with near-diffraction-limited beam quality are well known as a power scalable laser and amplifier architectures,[1-6] which have been found to be widely applicable in laser materials processing, remote sensing, medical imaging, low coherence interferometry, optical coherence tomography, spectral beam combining, and so on.[7] Nevertheless, the fast scaling of average power from single mode narrow linewidth fiber lasers and amplifiers is slowed down due to the fundamental limits arising from nonlinear optical effects of stimulated Brillouin scattering (SBS).[8] Various approaches have been considered in both amplifier parameters and master oscillator configurations, such as acoustically tailored fibers, thermal gradients, strain gradients, advanced large mode area fiber designs, filtered amplified spontaneous emission (ASE) seed sources,[9-12] multi-tone seed sources,[13-16] and linewidth broadening techniques.[17,18] The system structure to vary the fiber parameters is much more complex and is difficult to control, while the laser linewidth broadening technique is easy to accomplish, which is widely used in almost all of the high power monolithic fiber amplifier and fiber amplifier beam combining systems (coherent beam combining and spectral beam combining).[19-24] Behavior of the SBS threshold $P_{\rm th}$ in the case of single frequency linewidths in large mode area (LMA) fiber amplifiers has been well studied,[25,26] theoretical models have shown a relationship of the SBS threshold to the effective area of the LMA fiber ($A_{\rm e}$), the Brillouin gain coefficient of the LMA fiber ($g_{\rm b}$), the effective length of the LMA fiber ($L_{\rm e}$), the spectral width of the seed laser ($\Delta \nu _{\rm p}$), and the Brillouin gain bandwidth ($\Delta \nu_{\rm b})$.[27] The SBS threshold is defined as[28] $$\begin{align} P_{\rm th} =\frac{21A_{\rm e}}{g_{\rm b} L_{\rm e}}\Big(1+\frac{\Delta \nu _{\rm p}}{\Delta \nu _{\rm b}}\Big).~~ \tag {1} \end{align} $$ To suppress SBS while keeping narrow linewidth, spectral broadening of the seed laser $\Delta \nu_{\rm p}$ is proved to be an effective method. While a large number of experimental results prove the effectiveness of this approach, the SBS suppression achieved with white noise modulation is smaller than that expected. Recently, a report from Supradeepa[29] demonstrated that the SBS threshold enhancement for short LMA fiber lengths can be significantly smaller than that anticipated from its broadened bandwidth. They reported that this reduction is attributed to contributions from phase mismatched terms in the SBS process when the fiber lengths are short. In the case of a Lorentzian laser lineshape, the ideal enhancement factor $F_{\rm e0}$ is usually defined as $$\begin{align} F_{\rm e0}=1+\frac{\Delta \nu _{\rm p}}{\Delta \nu _{\rm b}}.~~ \tag {2} \end{align} $$ Considering the short fiber lengths, $F_{\rm e0}$ is revised to[29] $$\begin{align} F_{\rm e}=\frac{\Delta \nu _{\rm b}}{\Delta \nu _{\rm b} +\frac{c}{n_{\rm fiber} L_{\rm e}}}\Big(1+\frac{\Delta \nu _{\rm p}}{\Delta \nu _{\rm b}}\Big),~~ \tag {3} \end{align} $$ where $n_{\rm fiber}$ is the effective refractive index of the optical fiber, $c$ is the speed of light in vacuum. From Eqs. (1)-(3), the SBS threshold $P_{\rm th}$ can be given by $$\begin{align} P_{\rm th} =\frac{21A_{\rm e}}{g_{\rm b} L_{\rm e}}\cdot \frac{\Delta \nu _{\rm b}}{\Delta \nu _{\rm b} +\frac{c}{n_{\rm fiber} L_{\rm e}}}\Big(1+\frac{\Delta \nu _{\rm p}}{\Delta \nu _{\rm b}}\Big).~~ \tag {4} \end{align} $$ The effective area of the fiber $A_{\rm e}$ is given by $$\begin{align} A_{\rm e} =\pi w^2,~~ \tag {5} \end{align} $$ where $w$ is Gaussian beam's waist radius, and it is given by[30] $$\begin{align} w=a\cdot \Big({0.65+\frac{1.619}{V^{1.5}}+\frac{2.879}{V^6}}\Big).~~ \tag {6} \end{align} $$ The normalized frequency parameter $V$ is given by $$\begin{align} V=\frac{2\pi a\cdot {\rm NA}}{\lambda _{\rm p}}.~~ \tag {7} \end{align} $$ The Brillouin gain coefficient $g_{\rm b}$ is given by[28] $$\begin{align} g_{\rm b} =\frac{8\pi ^2\gamma _{\rm e}^2}{n_{\rm fiber} \cdot \lambda _{\rm p}^2 \cdot \rho _0 \cdot cv_{_{\rm A}} {\it \Gamma} _{\rm B}}.~~ \tag {8} \end{align} $$ The phonon decay rate ${\it \Gamma} _{\rm B}$ is given by $$\begin{align} {\it \Gamma} _{\rm B} =\frac{2\pi}{\tau _{\rm p}}.~~ \tag {9} \end{align} $$ The parameters used in our simulations are listed in Table 1, which are chosen based on the experimental condition and the typical values of silica fibers. Here $\lambda_{\rm p}$ is the seed laser wavelength, ${\rm NA}$ is the core numerical aperture of the fiber used in our system, $a$ is the radius of the fiber, $\gamma_{\rm e}$ is the electrostrictive constant, $\rho_{0}$ is the fiber density, $v_{_{\rm A}}$ is the acoustic speed, and $\tau_{\rm p}$ is the phonon lifetime. The single frequency seed laser is spectrally broadened to $\Delta \nu_{\rm p}=11.6$ GHz, the Brillouin gain bandwidth $\Delta \nu_{\rm b}$ is 57 MHz, and the SBS threshold $P_{\rm th}$ is calculated to be 2.2 kW, with an SBS threshold enhancement of $\sim$84.6 (26 W SBS threshold with single frequency seed).
Table 1. SBS threshold simulation parameters.
Parameter Value Parameter Value
$\lambda_{\rm p}$ 1064 nm $v_{_{\rm A}}$ 5.96$\times$10$^{3}$ m/s
${\rm NA}$ 0.065 $c$ 3$\times$10$^{8}$ m/s
$a$ 10 μm $\tau_{\rm p}$ 17.5$\times$10$^{-9}$ s
$\gamma_{\rm e}$ 0.902 $\Delta \nu_{\rm p}$ 11.6 GHz
$n_{\rm fiber}$ 1.45 $\Delta \nu_{\rm b}$ 57 MHz
$\rho_{0}$ 2210 kg/m$^{3}$ $L_{\rm e}$ 13 m
In this study, we set up a Yb-doped fiber amplifier (YDFA) consisting of a linewidth broadening master oscillator and a three-stage amplifier chain. The distributed feedback (DFB) seed laser is linearly linewidth broadened to 11.6 GHz level by an electro-optical phase modulator (EOM), and amplified from 5 mW to 1.5 kW by the master oscillator power amplifier (MOPA) configuration. The slope efficiency of the main amplifier is up to 87% and the beam quality factor stays around $M^{2}=1.15$. The FWHM linewidth of the DFB laser is measured by using the delayed self-heterodyne method,[31] as shown in Fig. 1. One arm of the 1:1 fiber coupler is connected to an acousto-optic modulator (AOM) with 200 MHz frequency shifted. The other arm is connected with a 500 m passive fiber (Ge-doped fiber, GDF) for delay. These two beams are combined by another 1:1 fiber coupler. The signal after the collimator (Co) is received by a photoelectric detector (PD) and is measured by an rf spectrum analyzer. The heterodyne spectrum of the DFB laser is shown in Fig. 2, the linewidth is measured to be 0.8 MHz with a Lorentz shape, and the central frequency is 200 MHz corresponding to the frequency shift of AOM.
cpl-33-7-074207-fig1.png
Fig. 1. FWHM linewidth measurement by using the delayed self-heterodyne method.
cpl-33-7-074207-fig2.png
Fig. 2. Heterodyne spectrum of the DFB laser.
Experimental setup of the 1.5 kW narrow-linewidth fiber amplifier based on MOPA configuration is shown in Fig. 3, which consists of a DFB laser, an EOM, a white noise signal (WNS), an rf driver, several isolators and a three-stage YDFA chain. As driving signal for the EOM, we use a GHz-level WNS to generate a uniformly random distribution of voltage fluctuations, which is amplified by the rf driver. The 5 mW spectrally broadened signal laser is amplified to 300 mW by the first stage amplifier. Then it is amplified to 17 W by the second stage amplifier, and 1.5 kW by the main amplifier. Damage of back reflections can be avoided by a double polarization-maintaining isolator (ISO 1) and three polarization-maintaining isolators (ISO 2–4). All the fibers used in the first two-stage amplifiers (PA 1–2) are single-mode polarization-maintaining fibers to accomplish a single-mode output before the main amplifier. The backscattered power and spectrum can be monitored by the power meter (PM) and the optical spectrum analyzer (OSA) with a resolution of 20 pm from port 3 of the circulator (CIRC). The main amplifier is based on an LMA Yb-doped double-cladding fiber (LMA-YDF-20/400) for high-power operation, which is pumped by six 290 W 976 nm laser diodes (LD) through a (6+1)$\times$1 combiner. The active fiber is placed in an aluminum water-cooled heatsink and coiled properly to introduce losses for the high-order modes. The output end of the all-fiber pump stripper (PS) is spliced to a fiber optic cable. The diameter of the laser beam output from the collimator is about 12 mm, corresponding to ${\rm NA}$ = 0.065 of the output fiber and 100 mm focal length of the collimator.
cpl-33-7-074207-fig3.png
Fig. 3. Experimental setup of the 1.5 kW narrow-linewidth fiber amplifier based on the MOPA configuration.
cpl-33-7-074207-fig4.png
Fig. 4. Emission spectrum at output power of 1.5 kW.
cpl-33-7-074207-fig5.png
Fig. 5. Backscattered spectrum and FWHM bandwidth at different output powers.
Emission spectrum at 1.5 kW output power is shown in Fig. 4. The central wavelength is 1064.08 nm with an FWHM bandwidth of 0.0431 nm ($\sim$11.6 GHz). The amplified spontaneous emission (ASE) is suppressed to more than 55 dB. No spectral components at the Raman wavelength ($\sim$1110 nm) are observed, indicating that the output power is still far below the stimulated Raman scattering (SRS) threshold. More than 98% of the 1.5 kW total output power is contained in the spectrum range of 1064.08$\pm$0.3 nm. The backscattered spectrum and FWHM bandwidth at different output powers measured by an OSA are shown in Fig. 5. The spectrum at every power level maintains the same central wavelength of 1064.08 nm, the recorded spectrum does not show any significant contributions for other wavelengths than the seed wavelength. The line shape stays the same at different output powers, and the FWHM bandwidths of the YDFA output spectrum at different powers in our system are around 0.04 nm, indicating that no broadening of the FWHM is observed in the whole power range.
cpl-33-7-074207-fig6.png
Fig. 6. Backscattered power according to the output power.
The slope efficiency of the output power against the pump power is 87%. Figure 6 shows the backward power versus the output power measured through port 3 of the circulator, the line shows a linear growth, and the maximal backward power is 78 mW, which is 0.0052% of the output power 1.5 kW. There have been several definitions for the SBS threshold in fiber amplifiers recently, including the backward power at a time-averaged reflectivity over several transit times in the range of 0.01%–1%[17,23] of the total output power. Considering that the 0.0052% reflectivity of our system is less than 0.01%, and there is no nonlinear growth trend, we can conclude that there is no observation of SBS effect in this system. Recently, Anderson et al.[23] reported that through WNS, the optical signal was modulated at several FHWM bandwidths between 0.225 GHz and 1.47 GHz, and a maximum measured enhancement factor of 18.0 for an optical bandwidth of 1.47 GHz is achieved. Meanwhile in our YDFA system, we obtained 26 W output power with single frequency seed limited by SBS, and 1.5 kW output power with a linewidth broadening master oscillator seed limited by the available pump power. The results demonstrate the SBS threshold enhancement factor of $\sim$57. To the best of our knowledge, this is the highest SBS threshold enhancement factor recorded in the pure white noise linewidth broadening YDFA. Since the SBS threshold $P_{\rm th}$ is calculated to be 2.2 kW, with an SBS threshold enhancement of $\sim$84.6, we may achieve a higher output power with much more available pump power in our system. Measured beam quality factor from the fiber amplifier at 1.5 kW output power is shown in Fig. 7. The beam quality factor $M^{2}$ is measured to be 1.15 (1.122 for the $x$-axis and 1.170 for the $y$-axis), and stays below this value for the whole power range, indicating that no mode instability behavior occurs.
cpl-33-7-074207-fig7.png
Fig. 7. Measured beam quality factor at 1.5 kW output power.
In summary, we have demonstrated a narrow linewidth, all-fiber three-stage YDFA chain seeded by a WNS phase modulated linewidth broadening single frequency laser. The 11.6 GHz linewidth is obtained by the linewidth control of the DFB master oscillation through a combination of a WNS and an EOM. Compared with the 26 W SBS-limited output power of our system when seeded by a single frequency laser, the output power is boosted up to 1.5 kW with a slope efficiency of 87%, and the SBS threshold is increased more than 57 times. To the best of our knowledge, this is the highest SBS threshold enhancement factor recorded in the pure white noise linewidth broadening YDFA. The output beam is near-diffraction limited with an excellent beam quality factor of $M^{2}=1.15$ at the maximum output power. No SBS or SRS is observed in the whole power range. Considering the theoretical and experimental results of our system, further power scaling is limited by the available pump power in our system. References A kW Continuous-Wave Ytterbium-Doped All-Fiber Laser Oscillator with Domestic Fiber Components and Gain Fiber1.5 kW Near-Diffraction-Limited Narrowband All-Fiber Superfluorescent SourceHigh power fiber lasers: current status and future perspectives [Invited]Ytterbium-doped large-core fiber laser with 1.36 kW continuous-wave output powerStudy on Passive Coherent Beam Combination Technology of High Power Fiber Laser ArraysInfluence of Temperature Distribution on Stimulated Brillouin Scattering in High Power Single-Frequency Fiber AmplifiersHundred-Watt-Level All-Fiber Thulium-Doped Fiber Laser and Superfluorescent SourceHigh power narrow-band fiber-based ASE source101 kW superfluorescent source in all-fiberized MOPA configurationInvestigation of Nonlinear Effects in Multitone-Driven Narrow-Linewidth High-Power AmplifiersA 275-W Multitone Driven All-Fiber Amplifier Seeded by a Phase-Modulated Single-Frequency Laser for Coherent Beam CombiningPump-limited, 203 W, single-frequency monolithic fiber amplifier based on laser gain competitionOutput spectrum of Yb-doped fiber lasersA theoretical study of transient stimulated Brillouin scattering in optical fibers seeded with phase-modulated lightActive phase and polarization locking of a 14 kW fiber amplifier560 W all fiber and polarization-maintaining amplifier with narrow linewidth and near-diffraction-limited beam qualitySPIE ProceedingsHigh average power spectral beam combining of four fiber amplifiers to 82 kWComparison of phase modulation schemes for coherently combined fiber amplifiersStimulated Brillouin scattering in optical fibersInput power limits of single-mode optical fibers due to stimulated Brillouin scattering in optical communication systemsStimulated Brillouin scattering thresholds in optical fibers for lasers linewidth broadened with noiseLoss Analysis of Single-Mode Fiber SplicesExperimental study of an ultra narrow linewidth f iber laser by injection locking
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