Transverse mode instability induced by stimulated Brillouin scattering in a pulsed single-frequency large-core fiber amplifier Kyung-Hyun Lee,1,2 Kangin Lee,3 Yonghee Kim,1 Yong-Ho Cha,1,* Gwon Lim,1 Hyunmin Park,1 Hyuck Cho,2 and Do-Young Jeong1 1
Quantum Optics Division, Korea Atomic Energy Research Institute, Daejeon 305-600, South Korea 2
Department of Physics, Chungnam National University, Daejeon 305-764, South Korea 3
Defence R&D Center, Hanwha Corporation, Daejeon 305-156, South Korea *Corresponding author: [email protected]
Received 8 July 2014; revised 28 October 2014; accepted 29 October 2014; posted 30 October 2014 (Doc. ID 214394); published 9 January 2015
We report the observation of transverse mode instability (TMI) in a pulsed single-frequency ytterbiumdoped large-core fiber amplifier in which stimulated Brillouin scattering (SBS) is generated easily owing to the high peak power and narrow linewidth of the laser pulses. It was shown experimentally that the threshold of TMI is almost the same as that of SBS and that the suppression of SBS also increases the threshold of TMI, which indicates that the TMI originates from SBS in the fiber. © 2015 Optical Society of America OCIS codes: (060.2320) Fiber optics amplifiers and oscillators; (060.3510) Lasers, fiber; (060.4370) Nonlinear optics, fibers; (060.5060) Phase modulation; (140.6810) Thermal effects. http://dx.doi.org/10.1364/AO.54.000189
1. Introduction
Fiber lasers have been used in many scientific and industrial applications because they have many attractive characteristics such as a good beam quality, high efficiency, easy maintenance, and wide wavelength selection. Moreover, the power level of fiber lasers has increased very rapidly over the last decade [1], and even multi-kilowatt continuous-wave (CW) fiber lasers have been developed. On the other hand, fiber lasers have been disadvantageous for the generation of high peak power laser pulses because a high laser intensity concentrated in a small area of a fiber core can induce damage to the fibers or cause many undesired nonlinear effects such as self-phase modulation (SPM), stimulated Raman scattering (SRS), stimulated 1559-128X/15/020189-06$15.00/0 © 2015 Optical Society of America
Brillouin scattering (SBS), and four wave mixing (FWM) [2]. The power level of the fiber lasers can be enhanced with a larger core size because fiber damage and nonlinear effects can be avoided with a reduction of the laser intensity in a fiber core. Fibers of a large core, however, cannot guarantee singlemode waveguiding, and thus, the laser beam quality can be degraded. Although fiber bending can be applied for a good beam quality in large-core fiber lasers, it also leads to the reduction of the effective mode-field area in a fiber core [3], which is not suitable for high-power pulsed fiber lasers. The degradation of beam quality in high-power large-core fiber lasers is known to be related with transverse spatial-hole burning (TSHB) [4]. At highpower levels, a strong fundamental-mode (FM) laser beam with a Gaussian spatial distribution in a fiber core depletes the gain only in an inner region of the fiber core, and thus, the outer region of the fiber core that has a higher gain than the inner region can 10 January 2015 / Vol. 54, No. 2 / APPLIED OPTICS
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excite high-order modes (HOMs) preferably because HOMs overlap with the outer region of a fiber core better than a FM. It was recently reported that some high-power large-core fiber lasers show threshold-like transverse mode instability (TMI), which leads to temporal fluctuations of the laser beam shape as well as a degradation of the beam quality [5–7]. The TMI has been explained to be caused by a thermally induced long-period grating (TILPG) created along fibers at high-power levels by the interference of FM with HOMs [8–10]. The TMI has been reported to be typically observed in large-core fiber lasers at a few hundreds of watts of average power or more, and the threshold of the TMI is influenced by many factors such as the wavelengths of the pump and signal, the design of the fiber, and the seed power. In addition, it is noticeable that the TMI for a specific design of fiber amplifier is not very sensitive to the peak power level and shows similar thresholds in terms of the average power regardless of the mode of operation such as CW or pulsed laser. Currently, TSHB and TMI are very severe limiting factors for increasing the average power levels, especially in high-power pulsed fiber lasers in which large fiber cores and a short fiber length are required. In this paper, we report on another kind of TMI occurring in a narrow-linewidth pulsed large-core fiber amplifier at a moderate average power of several watts. The TMI observed in our fiber amplifier shows a threshold-like onset at a certain output power as observed in other TMIs caused by TILPG. However, the TMI threshold in terms of average power is much lower than that caused by TILPG. Moreover, it was found that the TMI in our fiber amplifier depends strongly on the peak power and the linewidth of the laser pulses, which is clearly distinguishable from the TMI caused by TILPG. Because our fiber amplifier has a narrow linewidth of several tens of MHz and a high peak power of several kilowatts, SBS is easily generated, and we suspect that the TMI is caused by SBS in our fiber amplifier. Such TMI caused by SBS can be a very limiting factor for increasing the power level in narrow-linewidth highpower pulsed fiber lasers, which are useful tools for some spectroscopic applications. 2. Experimental Setup
Figure 1 shows the experimental setup of our pulsed single-frequency fiber laser system. A CW distributed feedback (DFB) diode laser with a wavelength of 1070 nm is used as a seed laser source. A seed laser beam of 7 mW is amplified to 70 mW by the use of a semiconductor optical amplifier (SOA). To make a pulse train from the CW laser, we used an acoustic optical modulator (AOM). After passing the AOM, the power of the pulse train is 50 μW with a pulse width of 13 ns and a repetition rate of 150 kHz. The pulse train from the AOM is amplified to 50 mW in an optical fiber amplifier based on a 190
APPLIED OPTICS / Vol. 54, No. 2 / 10 January 2015
Fig. 1. Experimental setup of our pulsed single-frequency fiber laser. DFB, distributed feedback; OI, optical isolator; SOA, semiconductor optical amplifier; AOM, acousto-optic modulator; BPF, bandpass filter; LD, laser diode; PM DC, polarization maintaining double clad.
Yb-doped polarization maintaining (PM) double clad (DC) fiber with a core diameter of 6 μm, a clad diameter of 125 μm, and a length of 6 m. A laser diode of a wavelength of 976 nm is used as a pump laser source and coupled into the clad of the fiber amplifier through a 2 1 × 1 PM multimode combiner. An optical isolator (OI) is installed after the single-mode fiber amplifier for the protection of the front-end part from any backward light generated in the final power amplifier. After the OI, the laser beam passes through one more free-space OI, which is used not only for blocking backward light but also for monitoring the backward SBS signal that originates from a final amplifier. Because a backward SBS signal cannot pass through the free-space OI but is rejected by the input polarizer of the OI, we can characterize the SBS signal by detecting the backward light rejected from the input polarizer. The bandpass filter (BPF) is setup to eliminate an amplified spontaneous emission (ASE). After two OIs and BPF, the laser power decreases to 15 mW, and the laser beam is injected into a final amplifier. The final amplifier is based on a large mode area (LMA) Yb-doped PM DC fiber with a length of 5.5 m, a core diameter of 25 μm, and a clad diameter of 250 μm. The numerical aperture (NA) of the fiber core is 0.07, and the fiber is coiled to a diameter of 20 cm. The fiber end facet is cleaved at an angle of 7°. The pump beam with a wavelength of 976 nm is coupled into the clad of the fiber in the opposite direction to the signal laser beam. The amplified laser beam is reflected on a dichroic mirror, which has a high transmission at 976 nm and a high reflection at 1030–1100 nm. The output beam is enlarged by a lens with a focal length of 30 cm in order to observe the TMI. The temporal pulse shape and pulse width are measured using a photodiode (PD) and an oscilloscope with a high-speed response.
3. Experimental Results A.
Observations of TMI
When the amplified power in our final amplifier reaches a certain power level, it is observed routinely that the amplified beam profile becomes unstable, as shown in Fig. 2. The beam profile is very stable and close to the fundamental LP01 mode at an output power of less than 3.2 W, as shown in Fig. 2(a). At a slightly increased output power of 3.4 W, however, the beam profile begins to be unstable, as shown in Fig. 2(b). At a higher output power of 3.6 W, the TMI is much stronger and the beam mode appears to oscillate between the LP01 and LP11 modes, as shown in Fig. 2(c). Because our beam profiler is too slow to analyze the fast mode change of the amplified laser beam, we used the technique introduced in Ref. [6] in which a fast PD is used for the detection of the fast TMI. Figure 3(a) shows the experimental setup for the observation of the TMI with a high-speed PD. The amplified laser beam is enlarged by a lens, and a small center portion of the beam passing through a pinhole is detected by a PD. When the beam mode is stable, the center portion signal detected by a PD is also stable. When the beam mode is unstable, however, the center portion signal varies temporally depending on the temporal change in beam shape. In our setup, we use a fast PD with a 1 ns rise/fall time, and the PD signal is measured by a fast oscilloscope with a bandwidth of 1 GHz. Figures 3(b) and 3(c) are the PD signal measured at the same conditions as in Figs. 2(a) and 2(c), respectively. In the absence of TMI, the temporal pulse shape detected by the PD is very stable, as shown in Fig. 3(b). With the strong TMI shown in Fig. 2(c), on the other hand, the PD signal fluctuates strongly in Fig. 3(c).
To quantify the degree of TMI, we use a normalized standard deviation (σ norm ) obtained from the PD signals, which is defined in Ref. [6]; the standard deviation and the average value of the peak of the temporal pulse shape can be measured easily with the PD and the oscilloscope, and we can calculate the normalized standard deviation by dividing the standard deviation by the average value. Figure 4(a) shows σ norm with respect to the amplified average output power at a repetition rate of 150 kHz. One can see that σ norm is nearly zero at a low amplified power of less than 3.2 W because TMI is not observed. At an amplified power above 3.2 W, however, σ norm increases abruptly, which matches well with the results shown in Figs. 2 and 3. We also measured σ norm at various repetition rates, as shown in Fig. 4. As the repetition rate increases, the average output power at which TMI begins to be observed also increases; σ norm measured at repetition rates of 150 kHz, 300 kHz, 500 kHz, and 600 kHz increases abruptly at average amplified output powers of 3.2 W, 5.6 W, 8.8 W, and 10 W, respectively. For a quantitative analysis of the relation between TMI and amplified output power, we defined the TMI threshold as the amplified output power at which the value of the first derivative of the fitted evolution of the normalized standard deviation equals 0.1%/W [6]: dσ norm ≡ 0.1%∕W: (1) dP out Pthreshold Figure 5 shows the TMI threshold in terms of the average and peak output powers with respect to the repetition rate. The TMI threshold in terms of average output power shows an almost linear increase with respect to the repetition rate. On the other hand, the TMI threshold in terms of peak output power is maintained within 1.3–1.8 kW. This indicates strongly that the onset of TMI observed in
Fig. 2. Images of the amplified laser beam from the final amplifier at various amplified powers of (a) 3.2 W, (b) 3.4 W, and (c) 3.6 W. As the amplified power increases, the output beams mode changes from the stable LP01 mode to the unstable mode that oscillates between the LP01 and LP11 modes.
Fig. 3. (a) Experimental setup for the observations of TMI with a fast photodiode. (b) and (c) are temporal pulse shapes measured at the output powers of 3.2 W and 3.6 W, respectively.
Fig. 4. Evolution of the normalized standard deviation, σ norm , with respect to the amplified output power at various repetition rates of (a) 150 kHz, (b) 300 kHz, (c) 500 kHz, and (d) 600 kHz. 10 January 2015 / Vol. 54, No. 2 / APPLIED OPTICS
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B. TMI Induced by SBS
Fig. 5. Thresholds of TMI measured in terms of the average and peak powers at various repetition rates.
our amplifier is determined by the peak power of the amplified laser pulses rather than the average output power. Because the TMI threshold in terms of peak power is almost constant (1.3–1.8 kW) regardless of the repetition rate in the out amplifier, the TMI threshold in terms of average power determined by the product of the amplified pulse energy and the repetition rate increases linearly with respect to the repetition rate, as shown in Fig. 5. Such behavior of the TMI in our amplifier is clearly differentiated from the TMI that originates from TILPG [9,10] in which the onset of TMI is determined mainly by the average output power. We investigated the temporal behavior of TMI by using a slow PD (bandwidth
Quantum Optics Division, Korea Atomic Energy Research Institute, Daejeon 305-600, South Korea 2
Department of Physics, Chungnam National University, Daejeon 305-764, South Korea 3
Defence R&D Center, Hanwha Corporation, Daejeon 305-156, South Korea *Corresponding author: [email protected]
Received 8 July 2014; revised 28 October 2014; accepted 29 October 2014; posted 30 October 2014 (Doc. ID 214394); published 9 January 2015
We report the observation of transverse mode instability (TMI) in a pulsed single-frequency ytterbiumdoped large-core fiber amplifier in which stimulated Brillouin scattering (SBS) is generated easily owing to the high peak power and narrow linewidth of the laser pulses. It was shown experimentally that the threshold of TMI is almost the same as that of SBS and that the suppression of SBS also increases the threshold of TMI, which indicates that the TMI originates from SBS in the fiber. © 2015 Optical Society of America OCIS codes: (060.2320) Fiber optics amplifiers and oscillators; (060.3510) Lasers, fiber; (060.4370) Nonlinear optics, fibers; (060.5060) Phase modulation; (140.6810) Thermal effects. http://dx.doi.org/10.1364/AO.54.000189
1. Introduction
Fiber lasers have been used in many scientific and industrial applications because they have many attractive characteristics such as a good beam quality, high efficiency, easy maintenance, and wide wavelength selection. Moreover, the power level of fiber lasers has increased very rapidly over the last decade [1], and even multi-kilowatt continuous-wave (CW) fiber lasers have been developed. On the other hand, fiber lasers have been disadvantageous for the generation of high peak power laser pulses because a high laser intensity concentrated in a small area of a fiber core can induce damage to the fibers or cause many undesired nonlinear effects such as self-phase modulation (SPM), stimulated Raman scattering (SRS), stimulated 1559-128X/15/020189-06$15.00/0 © 2015 Optical Society of America
Brillouin scattering (SBS), and four wave mixing (FWM) [2]. The power level of the fiber lasers can be enhanced with a larger core size because fiber damage and nonlinear effects can be avoided with a reduction of the laser intensity in a fiber core. Fibers of a large core, however, cannot guarantee singlemode waveguiding, and thus, the laser beam quality can be degraded. Although fiber bending can be applied for a good beam quality in large-core fiber lasers, it also leads to the reduction of the effective mode-field area in a fiber core [3], which is not suitable for high-power pulsed fiber lasers. The degradation of beam quality in high-power large-core fiber lasers is known to be related with transverse spatial-hole burning (TSHB) [4]. At highpower levels, a strong fundamental-mode (FM) laser beam with a Gaussian spatial distribution in a fiber core depletes the gain only in an inner region of the fiber core, and thus, the outer region of the fiber core that has a higher gain than the inner region can 10 January 2015 / Vol. 54, No. 2 / APPLIED OPTICS
189
excite high-order modes (HOMs) preferably because HOMs overlap with the outer region of a fiber core better than a FM. It was recently reported that some high-power large-core fiber lasers show threshold-like transverse mode instability (TMI), which leads to temporal fluctuations of the laser beam shape as well as a degradation of the beam quality [5–7]. The TMI has been explained to be caused by a thermally induced long-period grating (TILPG) created along fibers at high-power levels by the interference of FM with HOMs [8–10]. The TMI has been reported to be typically observed in large-core fiber lasers at a few hundreds of watts of average power or more, and the threshold of the TMI is influenced by many factors such as the wavelengths of the pump and signal, the design of the fiber, and the seed power. In addition, it is noticeable that the TMI for a specific design of fiber amplifier is not very sensitive to the peak power level and shows similar thresholds in terms of the average power regardless of the mode of operation such as CW or pulsed laser. Currently, TSHB and TMI are very severe limiting factors for increasing the average power levels, especially in high-power pulsed fiber lasers in which large fiber cores and a short fiber length are required. In this paper, we report on another kind of TMI occurring in a narrow-linewidth pulsed large-core fiber amplifier at a moderate average power of several watts. The TMI observed in our fiber amplifier shows a threshold-like onset at a certain output power as observed in other TMIs caused by TILPG. However, the TMI threshold in terms of average power is much lower than that caused by TILPG. Moreover, it was found that the TMI in our fiber amplifier depends strongly on the peak power and the linewidth of the laser pulses, which is clearly distinguishable from the TMI caused by TILPG. Because our fiber amplifier has a narrow linewidth of several tens of MHz and a high peak power of several kilowatts, SBS is easily generated, and we suspect that the TMI is caused by SBS in our fiber amplifier. Such TMI caused by SBS can be a very limiting factor for increasing the power level in narrow-linewidth highpower pulsed fiber lasers, which are useful tools for some spectroscopic applications. 2. Experimental Setup
Figure 1 shows the experimental setup of our pulsed single-frequency fiber laser system. A CW distributed feedback (DFB) diode laser with a wavelength of 1070 nm is used as a seed laser source. A seed laser beam of 7 mW is amplified to 70 mW by the use of a semiconductor optical amplifier (SOA). To make a pulse train from the CW laser, we used an acoustic optical modulator (AOM). After passing the AOM, the power of the pulse train is 50 μW with a pulse width of 13 ns and a repetition rate of 150 kHz. The pulse train from the AOM is amplified to 50 mW in an optical fiber amplifier based on a 190
APPLIED OPTICS / Vol. 54, No. 2 / 10 January 2015
Fig. 1. Experimental setup of our pulsed single-frequency fiber laser. DFB, distributed feedback; OI, optical isolator; SOA, semiconductor optical amplifier; AOM, acousto-optic modulator; BPF, bandpass filter; LD, laser diode; PM DC, polarization maintaining double clad.
Yb-doped polarization maintaining (PM) double clad (DC) fiber with a core diameter of 6 μm, a clad diameter of 125 μm, and a length of 6 m. A laser diode of a wavelength of 976 nm is used as a pump laser source and coupled into the clad of the fiber amplifier through a 2 1 × 1 PM multimode combiner. An optical isolator (OI) is installed after the single-mode fiber amplifier for the protection of the front-end part from any backward light generated in the final power amplifier. After the OI, the laser beam passes through one more free-space OI, which is used not only for blocking backward light but also for monitoring the backward SBS signal that originates from a final amplifier. Because a backward SBS signal cannot pass through the free-space OI but is rejected by the input polarizer of the OI, we can characterize the SBS signal by detecting the backward light rejected from the input polarizer. The bandpass filter (BPF) is setup to eliminate an amplified spontaneous emission (ASE). After two OIs and BPF, the laser power decreases to 15 mW, and the laser beam is injected into a final amplifier. The final amplifier is based on a large mode area (LMA) Yb-doped PM DC fiber with a length of 5.5 m, a core diameter of 25 μm, and a clad diameter of 250 μm. The numerical aperture (NA) of the fiber core is 0.07, and the fiber is coiled to a diameter of 20 cm. The fiber end facet is cleaved at an angle of 7°. The pump beam with a wavelength of 976 nm is coupled into the clad of the fiber in the opposite direction to the signal laser beam. The amplified laser beam is reflected on a dichroic mirror, which has a high transmission at 976 nm and a high reflection at 1030–1100 nm. The output beam is enlarged by a lens with a focal length of 30 cm in order to observe the TMI. The temporal pulse shape and pulse width are measured using a photodiode (PD) and an oscilloscope with a high-speed response.
3. Experimental Results A.
Observations of TMI
When the amplified power in our final amplifier reaches a certain power level, it is observed routinely that the amplified beam profile becomes unstable, as shown in Fig. 2. The beam profile is very stable and close to the fundamental LP01 mode at an output power of less than 3.2 W, as shown in Fig. 2(a). At a slightly increased output power of 3.4 W, however, the beam profile begins to be unstable, as shown in Fig. 2(b). At a higher output power of 3.6 W, the TMI is much stronger and the beam mode appears to oscillate between the LP01 and LP11 modes, as shown in Fig. 2(c). Because our beam profiler is too slow to analyze the fast mode change of the amplified laser beam, we used the technique introduced in Ref. [6] in which a fast PD is used for the detection of the fast TMI. Figure 3(a) shows the experimental setup for the observation of the TMI with a high-speed PD. The amplified laser beam is enlarged by a lens, and a small center portion of the beam passing through a pinhole is detected by a PD. When the beam mode is stable, the center portion signal detected by a PD is also stable. When the beam mode is unstable, however, the center portion signal varies temporally depending on the temporal change in beam shape. In our setup, we use a fast PD with a 1 ns rise/fall time, and the PD signal is measured by a fast oscilloscope with a bandwidth of 1 GHz. Figures 3(b) and 3(c) are the PD signal measured at the same conditions as in Figs. 2(a) and 2(c), respectively. In the absence of TMI, the temporal pulse shape detected by the PD is very stable, as shown in Fig. 3(b). With the strong TMI shown in Fig. 2(c), on the other hand, the PD signal fluctuates strongly in Fig. 3(c).
To quantify the degree of TMI, we use a normalized standard deviation (σ norm ) obtained from the PD signals, which is defined in Ref. [6]; the standard deviation and the average value of the peak of the temporal pulse shape can be measured easily with the PD and the oscilloscope, and we can calculate the normalized standard deviation by dividing the standard deviation by the average value. Figure 4(a) shows σ norm with respect to the amplified average output power at a repetition rate of 150 kHz. One can see that σ norm is nearly zero at a low amplified power of less than 3.2 W because TMI is not observed. At an amplified power above 3.2 W, however, σ norm increases abruptly, which matches well with the results shown in Figs. 2 and 3. We also measured σ norm at various repetition rates, as shown in Fig. 4. As the repetition rate increases, the average output power at which TMI begins to be observed also increases; σ norm measured at repetition rates of 150 kHz, 300 kHz, 500 kHz, and 600 kHz increases abruptly at average amplified output powers of 3.2 W, 5.6 W, 8.8 W, and 10 W, respectively. For a quantitative analysis of the relation between TMI and amplified output power, we defined the TMI threshold as the amplified output power at which the value of the first derivative of the fitted evolution of the normalized standard deviation equals 0.1%/W [6]: dσ norm ≡ 0.1%∕W: (1) dP out Pthreshold Figure 5 shows the TMI threshold in terms of the average and peak output powers with respect to the repetition rate. The TMI threshold in terms of average output power shows an almost linear increase with respect to the repetition rate. On the other hand, the TMI threshold in terms of peak output power is maintained within 1.3–1.8 kW. This indicates strongly that the onset of TMI observed in
Fig. 2. Images of the amplified laser beam from the final amplifier at various amplified powers of (a) 3.2 W, (b) 3.4 W, and (c) 3.6 W. As the amplified power increases, the output beams mode changes from the stable LP01 mode to the unstable mode that oscillates between the LP01 and LP11 modes.
Fig. 3. (a) Experimental setup for the observations of TMI with a fast photodiode. (b) and (c) are temporal pulse shapes measured at the output powers of 3.2 W and 3.6 W, respectively.
Fig. 4. Evolution of the normalized standard deviation, σ norm , with respect to the amplified output power at various repetition rates of (a) 150 kHz, (b) 300 kHz, (c) 500 kHz, and (d) 600 kHz. 10 January 2015 / Vol. 54, No. 2 / APPLIED OPTICS
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B. TMI Induced by SBS
Fig. 5. Thresholds of TMI measured in terms of the average and peak powers at various repetition rates.
our amplifier is determined by the peak power of the amplified laser pulses rather than the average output power. Because the TMI threshold in terms of peak power is almost constant (1.3–1.8 kW) regardless of the repetition rate in the out amplifier, the TMI threshold in terms of average power determined by the product of the amplified pulse energy and the repetition rate increases linearly with respect to the repetition rate, as shown in Fig. 5. Such behavior of the TMI in our amplifier is clearly differentiated from the TMI that originates from TILPG [9,10] in which the onset of TMI is determined mainly by the average output power. We investigated the temporal behavior of TMI by using a slow PD (bandwidth
