In this manuscript, we demonstrate high power, all-fiberized and polarization-maintained amplifiers with narrow linewidth and near-diffraction-limited beam quality by simultaneously suppressing detrimental stimulated Brillouin scattering (SBS) and mode instability (MI) effects. Compared with strictly single frequency amplification, the SBS threshold is scaled up to 12 dB, 15.4 dB, and higher than 18 dB by subsequently using three-stage cascaded phase modulation systems. Output powers of 477 W, 1040 W, and 1890 W are achieved with full widths at half maximums (FWHMs) of within 6 GHz, ~18.5 GHz, and ~45 GHz, respectively. The MI threshold is increased from ~738 W to 1890 W by coiling the active fiber in the main amplifier. Both the polarization extinction ratio (PER) and beam quality (M2 factor) are maintained well during the power scaling process. To the best of our knowledge, this is the first demonstration of all-fiberized amplifiers with narrow linewidth, near linear polarization, and near-diffraction-limited beam quality at 2 kW power-level.
© 2016 Optical Society of America
High power fiber laser with narrow linewidth and near-diffraction-limited (NDL) beam quality has widely applications in various regimes, such as nonlinear frequency conversion (NFC) , remote communication , beam combination , and gravitational wave detection . As for power scaling of this type of monolithic fiber source, stimulated Brillouin scattering (SBS) effect has been become one of the most primary limitations in previous studies. Over the past decade, several techniques were employed in high power amplifiers for SBS suppression, such as imposing thermal and/or strain distributions [5–8], design special active fiber [9–11], using large core size and/or highly doped active fiber [2, 12], directly using multi-longitudinal-mode oscillator for amplification [13, 14], and employing phase modulation technique [15–19]. Within these SBS suppression techniques, phase modulation technique is a preferable approach to achieve laser sources with narrow linewidth beyond kilowatt-level outputs [16–19]. Notably that 2.3 kW output power with narrow linewidth and NDL beam quality has been presented by using two-stage phase modulation systems to suppress SBS effect quite recently . Despite impressive result demonstrated, it has been shown that the mode instability (MI) effect will become another serious limitation for further high brightness scaling .
A point should be noted is that the polarization states in most of the aforementioned high power demonstrations are stochastic. In fact, except for high brightness operation, linear polarization is also strongly required in many applications of fiber lasers with narrow linewidth. Due to that the effective Brillouin gain in polarization-maintained (PM) fiber is typically higher than that in non-PM fiber, SBS suppression in high power narrow linewidth and PM amplifiers is more challenging [2, 11]. More importantly, previous study shows that the MI threshold in PM amplifier seems to be remarkably lower than that in non-PM amplifier . Consequently, power scaling of fiber lasers with narrow linewidth and near linear polarization by directly using PM amplifiers is more difficult. Nowadays, the record output power of this kind of fiber source has been still remained at 1 kW power-level [1, 20, 21].
In this manuscript, we present narrow linewidth, all-fiberized, and polarization-maintained amplifiers operating at maximum output power of 1.89 kW by simultaneously suppressing the dual impacts of SBS and MI effects. The SBS effect in such high power amplifiers is suppressed by using three-stage phase modulation systems. By subsequently imposing the three-stage phase modulation signals into the seed laser, the SBS threshold is scaled up to 12 dB, 15.4 dB, and higher than 18 dB compared with the single frequency amplification process. Output powers of 477 W, 1040 W, and 1890 W are obtained with full widths at half maximums (FWHMs) of within 6 GHz, ~18.5 GHz, and ~45 GHz, respectively. In the experiment, the MI threshold is increased from ~738 W to 1890 W by simply coiling the active fiber in the main amplifier. The polarization extinction ratio (PER) at maximum output power is measured to be 15.5 dB and low degradation of the far-field intensity distribution is observed in the whole power scaling process. As far as we know, this is the highest demonstration of all-fiberized amplifiers with narrow linewidth, near linear polarization, and NDL beam quality.
2. Experimental Setup
The experimental setup of high power all-fiberized and polarization-maintained amplifiers with narrow linewidth is shown in Fig. 1, which is based on conventional master oscillator power amplification (MOPA) structure. The master oscillator (MO) is a linear-polarized, single-frequency (line-width <20 kHz) laser with output power of 40 mW and central wavelength of 1064.4 nm, which is based on an ultra-short-cavity configuration . Output power of the seed laser is firstly amplified to be 150 mW by using a PM pre-amplifier (P-AI). After P-AI, three-stage phase modulation systems (PMSs) are used to broaden the linewidth of the seed for SBS suppression. The PMSs include three cascaded phase modulators and three sine-signal generators (SG1, SG2, and SG3 shown in Fig. 1). The modulation frequencies and depths generated by SG1, SG2, and SG3 are 17 GHz with 8.9 V peak-vale (PV) voltage, 6 GHz with 11.2 V PV voltage, and 100 MHz with 36 V PV voltage, respectively. The typical half-wave voltages of the three cascaded phase modulators are 4 V, 4V, and 2.2 V, respectively. The output power after PMSs is measured to be about 20 mW, and the power loss is mainly attributed to the insertion loss of the three phase modulators. Then, the linewidth-broadened seed laser is subsequently amplified to be 0.5 W and 20 W by using two-stage PM preamplifiers (P-AII and P-AIII), respectively. At the rear end ports of P-AI, P-AII, and P-AIII, three PM isolators (ISO1, ISO2, and ISO3) are incorporated into the MOPA structure to block off the backward powers from the following amplifications. After ISO3, the pre-amplified laser is coupled through a PM fiber coupler (PM-C) to the main amplifier for further power scaling. The coupling ratios of the out port of the PM-C to backward monitor port and injected signal port are 0.1% and 99.9%, respectively. The main function of the PM-C is to split a small portion of the backward power for diagnosing the SBS effect during the power scaling process.
The main amplifier is pumped by using five wavelength-stabilized, 500 W power-level laser diodes (LDs) with 976 nm central wavelength via a (6 + 1) × 1 PM pump combiner. The active fiber in this stage is large mode area (LMA) and double clad PM Yb-doped fiber with a core diameter of 20 μm and an inner cladding diameter of 400 μm. The cladding absorption coefficient is about 1.7 dB/m at 976 nm and 8.5 m long active fiber is employed for high power scaling. About 1 m long PM passive fiber and a high power fiber end-cap with~1.5 m long PM passive fiber are successively fused to the rear end of the active fiber for power delivering. The core and inner cladding diameters of the two pieces of passive fibers are remained the same as the active fiber. In the 1 m long passive fiber, ~40 cm high-index gel section is made for stripping out the residual pump and cladding light. The delivering laser through the fiber end-cap is collimated into free space by using a high power beam collimator.
3. Experimental Results and Discussions
3.1 Power scaling of the strictly single frequency seed
For comparison of the SBS suppression effects with cascaded phase modulation systems, we firstly investigate the power scaling ability of the high power MOPA structure with strictly single frequency seed. In this situation, all the three sine-wave signal generators are turned off. The actual backward power as a function of the output power is shown in Fig. 2. From Fig. 2, it is shown that nonlinear increase of the backward power occurs when the output power is beyond 30 W. This is attributed to the fact that the signal power dramatically transforms into the Stokes light due to the SBS effect. The SBS threshold in this manuscript is defined as the output power of the MOPA architecture before nonlinear increase of the backward power. Thus, the SBS threshold with strictly single frequency amplification is about 30 W in our experiment.
3.2 Power scaling of the seed by just imposing one stage phase modulation signal
In this section, we investigate the SBS suppression effect by just imposing the phase modulation signal of SG3 to broaden the linewidth of the single frequency seed. From our previous study , the broadened linewidth of the seed in this situation is well within 6 GHz. The output power and actual backward power as a function of the absorbed 976 nm pump power are shown in Fig. 3. As shown in Fig. 3, output power of 477 W can be attained with an optical to optical conversion efficiency of ~73% before nonlinear increase of the backward power, which indicates that the SBS threshold is about 477 W in the experiment. Compared with strictly single frequency amplification, the SBS threshold is scaled up to 12 dB by just employing the phase modulation signal of SG3.
3.3 Power scaling of the seed by imposing two cascaded phase modulation signals
For further SBS suppression, we simultaneously impose the phase modulation signals of SG2 and SG3 to broaden the linewidth of the single frequency seed. Output power scaling characteristic along with the absorbed pump power in the main amplifier is shown in Fig. 4(a). As shown in Fig. 4(a), the output power is increased near linearly when the pump power is below 960 W while abnormal increase trend is observed when the pump power is higher than 960 W. Specifically, the optical to optical efficiency decreases from 76% to 72.4% when the pump power is increased from 960 W to 997 W. Further investigation of the temporal instability of the output beam by using an InGaAs photo-detector (PD) with 150 MHz electro-optical bandwidth, abrupt temporal instabilities are observed when the pump power is beyond 960 W. The normalized time-serial signals collected by the PD at pump powers of 960 W and 997 W are shown in Fig. 4(b) and their Fourier spectral distributions are shown in Fig. 4(c). From the experimental results shown in Fig. 4(b), the standard deviations of the temporal signals are respectively calculated to be 0.79% and 2.38% at the two specific pump powers, which is increased more than 3 times. As shown in Fig. 4(c), compared with the Fourier spectral distribution at pump power of 960 W, some noise-like protuberances exist within the frequency range of 0-5 kHz at pump power of 997 W. According to the temporal and Fourier spectral characteristics of MI effect , we confirm that the MI effect occurs when the pump power is beyond 960 W. The MI threshold is defined as the output power at pump power of 960 W, which is 738 W in the experiment.
It is to be noted that the active fiber in the main amplifier is loose coiling with radius of about 0.4 m in the above experiments. As shown above, the MI limited output power is just about 738 W with this loose coiling radius. In order to further power scaling, MI suppression technique is strongly required in our experiment. According to the theoretical and experimental studies [24–27], coiling active fiber to increase the relative losses of higher order modes is a simple and effective method to suppress MI effect in practice. More impressively, as for the active fiber used in the present setup, our theoretical analysis shows that the MI threshold can be increased to be three times by coiling the active fiber with the radius of ~5.5cm . Thus, we reconstruct the main amplifier in the PM MOPA structure by coiling the active fiber with radius of ~5.5 cm and re-scale the output power of the narrow linewidth PM amplifiers. The output power and actual backward power as a function of the absorbed 976nm pump power are shown in Fig. 5(a). As shown in Fig. 5(a), the backward power is increased dramatically when the pump power is beyond 1365 W, which indicates that the SBS effect occurs at this pump power-level. At 1365 W pump power, 1040 W output power is achieved with an optical to optical efficiency of 75.6%, which indicates that the SBS threshold is scaled up to 15.4 dB compared with the single frequency amplification process. Figure 5(b) shows the optical spectrum of the PM amplifiers at 1040 W output power, which is measured by using an optical spectrum analyzer with resolution of 0.02 nm. As shown in Fig. 5(b), higher than 38 dB signal-to-noise ratio (SNR) is attained with a resolution-limited full width at half maximum (FWHM) of ~0.07 nm (18.5 GHz). The normalized time-serial signal and the corresponding Fourier spectral distribution at 1040 W output power are shown in Figs. 5(c) and 5(d), respectively. From Figs. 5(c) and 5(d), it is shown that the temporal characteristic is quite stable without any noise-like protuberances in the Fourier spectral regime, which indicates that the MI effect is suppressed effectively at 1040 W output power.
3.4 Power scaling of the seed by imposing three cascaded phase modulation signals
As shown in section 3.3, with effective MI suppression technique, further power scaling of the PM amplifiers is still limited by SBS effect. In this section, we add all the three cascaded phase modulation signals to broaden the linewidth of the seed for further SBS suppression. In this situation, the power scaling process with the injected pump power is shown in Fig. 6(a), and the backward power as a function of the output power is shown in Fig. 6(b). As shown in Fig. 6(a), 1890 W output power is ultimately achieved with a linear-fitting slope efficiency of 74%. Figure 6(b) shows that the backward power is also increased near linearly with a slope efficiency of ~2.4% in the power scaling process, which denotes that SBS effect is suppressed effectively in such high power PM amplifiers. Compared with the single frequency amplification process, the SBS threshold is increased to be more than 18 dB in this experiment. At 1890 W output power, the normalized time-serial signal and the corresponding Fourier spectral distribution are also measured in the experiment, which are shown in Figs. 6(c) and 6(d), respectively. Experimental results in Figs. 6(c) and 6(d) denote that the PM amplifiers are operated at MI-free state. Consequently, we conclude that the MI threshold can be scaled to be more than 2.5 times (from 738 W to more than 1890 W) by tight coiling the active fiber to suppress MI effect, which is compatible with our previous theoretical analysis .
The emission spectra of the PM amplifiers at 102 W and 1890 W are shown in Fig. 7(a). From Fig. 7(a), it is shown that the residual pump power, the amplified spontaneous emission (ASE), and the stimulated Raman scattering (SRS) effect are not observed with SNR of ~44dB at 1890 W. The spectral details at 102 W and 1890 W are shown in the inset of Fig. 7(a). The resolution-limited FWHMs are measured to be 0.19 nm (51 GHz) at 102 W and 0.17 nm (45 GHz) at 1890 W, respectively. Besides, the spectral linewidths within 20 dB are measured to be 0.84 nm at 102 W and 0.7 nm at 1890 W, respectively. This linewidth narrowing effect is attributed to the gain competition effects in the power scaling process. Figure 7(b) shows the captured far field intensity distributions of the PM amplifiers at output powers of 552 W, 1048W, 1520 W and 1890 W, respectively. As shown in Fig. 7(b), low degradation of the far- field intensity distribution is observed during the power scaling process. At 1520 W output power, the beam quality (M2 factor) of the high power PM amplifiers is investigated for long time (15 minutes) operation. Five groups of M2 data (measured by using M2-200) are subsequently obtained during the observation time and the beam quality is maintained well during the whole investigation process. Figure 7(c) gives a typical M2 measurement result, which shows that the beam quality of the output beam is near-diffraction-limited (M2x ~1.19, M2y ~1.27). Besides, the output power of the PM amplifiers is stable during the observation time of 15 minutes. Figure 7(d) shows the polarization extinction ratio (PER) as a function of the output power, which is measured by using assemble components of a half-wavelength plate and a polarization beam combiner operating at central wavelength of 1064 nm. As shown in Fig. 7(d), the measured PER changes between 15.5 dB (97.2%) and 20.8 dB (99.2%), which just fluctuates within 2% during the power scaling process. At maximum output power, the PER of the PM amplifiers is measured to be 15.5 dB (97.2%).
High power, all fiberized and polarization-maintained amplifiers with narrow linewidth and NDL beam quality are presented based on a conventional MOPA configuration. During the power scaling process, the SBS effect is effectively suppressed by subsequently using three-stage cascaded phase modulation systems and the MI effect is managed by simply coiling the active fiber in the main amplifier. With increase of phase modulation signals, the SBS threshold is scaled up to 12 dB, 15.4 dB, and higher than 18 dB compared with the strictly single frequency amplification process. Output powers of 477 W, 1040 W, and 1890 W are achieved with FWHMs of within 6 GHz, 18.5 GHz, and 45 GHz, respectively. The MI threshold is increased from 738 W to 1890 W, which is scaled up to 2.5 times in the experiment. The PER just fluctuates within 2% during the power scaling process and as high as 15.5 dB is obtained at 1890 W output power. Low degradation of the far-field intensity distribution is observed along with increase of output power. At 1520 W, the PM amplifiers are operated stably without degradation of beam quality (M2 factor) for long time observation. The M2 factor is measured to be within 1.3 (M2x ~1.19, M2y ~1.27) at 1520 W output power. To the best of our knowledge, this is the first demonstration of all-fiberized amplifiers at 2 kW power-level with the characteristics of narrow linewidth, near linear polarization, and NDL beam quality.
This research is sponsored by the National Natural Science Foundation of China (NO. 11274386) and the innovation projects of Hunan Province and National University of Defense Technology for graduate students.
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