Passive Coherent Beam Combining Four

Transcript

CHIN. PHYS. LETT. Vol. 31, No. 11 (2014) 114210 Passive Coherent Beam Combining Four Channels of Nanosecond Pulsed Laser Using All-Fiber Feedback Loop * YANG Bao-Lai(杨保来), WANG Xiao-Lin(王小林), MA Peng-Fei(马鹏飞), ZHOU Pu(周朴), SU Rong-Tao(粟荣涛), XU Xiao-Jun(许晓军)** College of Optoelectric Science and Engineering, National University of Defense Technology, Changsha 410073 (Received 30 June 2014) We present an all-fiber configuration of passive beam combining using an all-fiber feedback loop and demonstrate passive coherent combining of a four-channel nanosecond pulsed laser. The output power, pulse characteristics, polarization extinction ratio and far field laser intensities of the combined output laser are investigated. The experimental results validate that passive phasing using an all-fiber feedback loop provides a simple while robust method for coherent beam combining of pulsed lasers. If replacing the fiber coupler used into a high power tapered fused bundle fiber combiner, this scheme is promising to scale the output power up to the kW level. PACS: 42.55.Wd, 42.60.Da, 42.60.Fc DOI: 10.1088/0256-307X/31/11/114210 High power pulsed fiber lasers have many applications in areas such as material processing, nonlinear frequency generation and remote sensing. However, due to the mode instabilities, thermal effect, facet damage and some nonlinear effects, both the peak and average power of a single fiber laser/amplifier are limited currently.[1,2] Nowadays, the coherent combination of pulsed fiber laser is intensively considered to overcome the limitations and further scale the average/peak power.[3−7] Recently, significant progress has been made in the coherent combination of pulsed fiber lasers and several different schemes are employed.[8−16] In 2013, our group reported active phasing coherent beam combination (CBC) of seven all-fiber nanosecond amplifiers in a master oscillator power amplifier (MOPA) configuration by using a single frequency dithering technique.[9] The pulsed laser from seven amplifiers was collimated and tiled side by side in an array with a fill factor of ∼50%. The average and peak power of the combined output laser reached 1.2 kW and 75.1 kW, respectively. However, some energy is detracted to side lobes inevitably due to the dissatisfactory fill factor (∼50%). The filled aperture combining technique avoids the trouble of side lobes and also shows great power scaling potential. In the year of 2013, Arno Klenke et al. realized a femtosecond fiber laser combining four large-pitch fibers by the active phasing coherent polarization beam combination technique; they obtained a pulse energy of 1.3 mJ and a peak power of 1.8 GW at the 400 kHz repetition rate.[2] Even though the active phasing technique has been widely used, it requires complex phase detection and an electronic control device. Relatively, passive phasing CBC with an optical feedback loop has the advan- tages of simplicity, compactness, a high bandwidth and the ability of scaling power up to the kilo-watt level.[5,17] In 2011, Xue et al. demonstrated passive phase locking of four high power Yb-doped fiber amplifiers using an optical feedback loop, achieving 1062 W continuous wave combined laser output. The four channels of the laser beam are also tiled side by side in an array spatially with a fill factor of 66.7%. Side lobes are inevitable as well and the Strehl ratio of the combined output laser is only 0.23 when laser output reaches 1062 W. However, referring to the pulsed laser, passive phasing CBC using an optical feedback loop is still in the low power level, and a coherent combination of more than two channels has not yet been realized. In 2011, Guillot et al. predicted that an optical feedback loop could handle passive phasing CBC with nanosecond pulses.[18] Later in 2012, Liu et al. proved it by realizing passive phasing CBC of two nanosecond fiber amplifiers with hundreds of milliwatt output power.[19] Nevertheless, the two nanosecond laser beams are tiled in line with an 11 mm distance between them while the diameter of the laser beam is only 4 mm, the fill factor is somehow low. As the optical feedback signal is obtained by coupling fraction of output laser beam into a single mode fiber, this may deteriorate the stability and robustness of the system. In this Letter, we present passive phasing CBC of a four-channel nanosecond pulsed laser using an allfiber feedback loop. In our work, the four channels of fiber amplifier are combined by using a fiber coupler, which avoids side lobes. The experimental system realizes the overall all-fiber configuration, not only in the fiber amplifiers, but also in the beam combiner and the feedback loop. The all-fiber configuration strengthens the stability and robustness of the system. * Supported by the Natural Scientific Fund of Hunan Province of China under Grant No 14JJ3004. author. Email: [email protected] © 2014 Chinese Physical Society and IOP Publishing Ltd ** Corresponding 114210-1 CHIN. PHYS. LETT. Vol. 31, No. 11 (2014) 114210 Power meter 80% A1 A2 1T4 Coupler 4T1 Coupler 1T3 Coupler A3 10% PD 10% A4 A0 Power meter AFG EOPM 2T1 50% coupler 50% FA 20% 1T2 coupler 80% Seed laser Fig. 1. Experimental setup. AFG: arbitrary function generator, EOPM: electro-optical phase modulator, PD: photodetector, FA: feedback amplifier. The experimental setup is shown in Fig. 1. A single frequency seed laser with a wavelength of 1064 nm and power of 55 mW is firstly fed into an EOPM via a 2 × 1 coupler (50:50), and is amplified by a preamplifier (A0) to about 180 mW. Then the laser beam is split into four channels by a 1 × 4 splitter. Each channel passes through a Yb-doped polarization maintained fiber amplifier (A1–A4). The length difference between the channels is controlled by fusing passive fiber in shorter channels.[9] The output power of each amplifier (A1–A4) is adjusted to about 100 mW. The lasers from four amplifiers are firstly combined by a 4×1 coupler and then spit by a 1×3 splitter. From the 1 × 3 splitter, 80% of the main output power is sent to the power meter, 10% to the photodetector (Thorlabs, PDA10CF), while the other 10% is fed into a feedback amplifier (FA) as passive phasing feedback. The amplified feedback laser is split by another 1 × 2 splitter (80:20), 80% is sent to the EOPM via the 2×1 coupler (50:50), while the other 20% is measured by a power meter. By monitoring the power in the 20% port of 1 × 2 splitter (80:20), we can estimate the power of the amplified feedback laser fed into the 2 × 1 coupler (50:50). All the fiber used in the amplifiers and the couplers is polarization maintained single mode fiber with a core diameter of 6 µm and a cladding diameter of 125 µm. Polarization maintained isolators are used in each amplifier to avoid the return light. Firstly, we investigate the power of the system without a signal driving the EOPM, namely the system is running in the continuous wave state. When only one of the four channels (A1–A4) is turned on, due to the insert loss of the 4 × 1 coupler, the power detected in the 80% port of the 1 × 3 splitter is only about 15 mW. When all four channels of the amplifiers (A1–A4) are turned on and the FA is off, there is no feedback laser injected and the system is in the non-phase locking state. We say that the system is in open loop. In this case, the power of the output laser fluctuates timely, with a mean value of 58.1 mW, as a result of the phase noise in each amplifier. When we turn on FA and the feedback is amplified enough to saturate the pre-amplifier A0, the system establishes an all-fiber feedback loop. Then we extinguish the seed laser and the system realizes self-organized passive phase locking in the all-fiber ring cavity. We say that the system is in closed loop. The output power of the system becomes quite stable and improves a lot. When the power of the feedback laser injected into the 2 × 1 coupler (50:50) is adjusted equal to the power of seed laser, output power of the amplifiers (A0, A1– A4) remains the same as the output power in the open loop. The power detected in the 80% port of the 1 × 3 splitter in the closed loop is about 243 mW, which is 4.2 times of the power in the open loop state. The output powers of the system in the open and closed loops are depicted in Fig. 2, in which the power data are acquired with a time interval of 0.1 s. 250 Output power (mW) Moreover, the number of passively phased nanosecond pulsed lasers using the all-fiber feedback loop is scaled to four channels as well. When the all-fiber feedback loop is established and the electro-optical phase modulator (EOPM) is driven by the finite sine signal, we can obtain a coherently combined pulsed laser with a pulse width of 19.6 ns and a repetition frequency of 4.28 MHz. To the best of our knowledge, this is the first time passive phasing of a coherently combined nanosecond pulsed laser has been achieved with phase modulation by an EOPM using an all-fiber optical feedback loop. Open loop 200 Closed loop 150 100 50 0 0 50 100 150 Time (s) Fig. 2. Power of the combined output laser in the continuous wave state. Power fluctuation in the open loop and power improvement in the closed loop can be explained by using the 4 × 1 coupler as a combiner. If four channels of incoherent laser are injected into the 4 × 1 coupler, the output power should be stable and equals a quarter of the total injected power without considering insert loss. However, in the open loop, the combined four channels of laser are amplified from the same single frequency seed laser and have good coherence with each other. The combined output power fluctuates due to the coherent adding and irregular phase perturbation caused by the environment. Taking the insert loss (74%) into consideration, the average output power should be a quarter of the total transmitted input power. Then the power of the 80% port of the 1 × 3 splitter should be 59.2 mW, which is quite close 114210-2 CHIN. PHYS. LETT. Vol. 31, No. 11 (2014) 114210 Normalized intensity 1 (a) 150 Open loop Closed loop 100 50 0 0 10 20 30 40 50 Extinction ratio (dB) average output power in the closed loop is 2.7 times of the average output power in the open loop. Compared with the output power in the continuous-wave domain, the combined laser in pulsed state endures a slight drop in average power. As the optical path of the four channels of fiber amplifier is not matched very well, the slight drop in average power, we think, may be the result of dissatisfactory pulse synchronization between the four channels. We also measure the polarization extinction ratio of the output laser using an extinction ratio meter (Thorlabs, ERM100). The polarization extinction ratio of the output laser is about 29.6 dB, shown in Fig. 4(b). As all the fiber, isolators, couplers and splitters are polarization maintained, the output laser of each channel of amplifier is linearly polarized. The measured polarization extinction ratio of the combined output laser demonstrates that a combined output laser maintains polarization quite well. Output power (mW) to the average power measured (58.1 mW). When the system is in the closed loop and the combined laser is completely phase locked, the power of the output laser should be four times higher according to the coupling mechanism analyzed in Ref. [20], which is consistent with the measured output power. In the experiment, the 4.2 times power improvement from the open loop to the closed loop is slightly higher than the theoretical four times, which is thought here to be the result of the power of the feedback slightly exceeding the power of the seed laser. The slight power measured error in the 20% port of the 1 × 2 splitter (80:20) will lead to four times the error in the power of the feedback laser injected into the 2 × 1 coupler (50:50). These power measurement errors may cause a slight improvement in the power of the combined output laser. In previous works, an electro-optical amplitude modulator (EOAM) or acousto-optical modulator (AOM) is usually employed to modulate the continuous wave laser into the pulsed laser, which have notable insert loss. In our work, the pulsed laser is realized by introducing phase modulation with an electrooptical phase modulator (EOPM). When the EOPM is driven by a sine signal with a frequency of 4.277 MHz and an amplitude of 7.05 V, the combined output laser is no longer a continuous wave while in the pulse state. The pulse shape is recorded by a photodetector, as shown in Fig. 3. The pulse width is 19.6 ns and the repetition frequency is 4.28 MHz. When the all-fiber feedback loop is established, the configuration can be seen as a complex ring cavity, in which four channels of fiber amplifier are inserted. The appearance of the pulsed laser, we deem, is attributed to active mode locking induced by phase modulation in the ring cavity. When finite phase modulation corresponding to the complex ring cavity is introduced, the active mode locking pulsed laser comes into being. 31 (b) 30 29 28 0 Time (s) 20 40 60 80 Time (s) Fig. 4. (a) Power of the combined output pulsed laser in the open and closed loops, and (b) the polarization extinction ratio of the output laser in the closed loop. (a) (b) (c) (d) (b) (a) 0.8 0.6 19.6 ns 0.4 Fig. 5. Far field laser intensity of combined output laser: (a)–(c) open loops, and (d) the closed loop. 0.2 0 1 2 Time (ms) 3 0 50 100 150 200 Time (ns) Fig. 3. Pulse shape of the output pulsed laser (a) several pulses, and (b) single pulse. Similar to the continuous wave state, when the feedback laser is amplified and the seed laser is extinguished, passive phase locking of the four channels of nanosecond pulsed laser is realized. The output power of the pulsed laser is shown in Fig. 4(a). The average output power in the open loop fluctuates with a mean value of 59 mW, while the output power in the closed loop state becomes stable and reaches 162 mW. The The far field laser intensities of the combined output laser in the open and closed loops are also investigated, as shown in Fig. 5. Because all fibers used in the experiment are single mode fibers, all the far field laser intensities observed are a fundamental-mode beam pattern. When the system is in the open loop, the laser intensity fluctuates timely; three typical measured far field laser intensities are shown in Figs. 5(a)– (c). When the system is in the closed loop, the far field laser intensity becomes quite stable and the intensity improves, as shown in Fig. 5(d). Compared with CBC by using an optical feedback loop employing a spatial 114210-3 CHIN. PHYS. LETT. Vol. 31, No. 11 (2014) 114210 configuration and tiled aperture, the all-fiber configuration employing a coupler as the beam combiner has the advantages of simplicity, stability and no side lobes reducing the energy. In conclusion, we have established an all-fiber configuration of passive phasing CBC using an all-fiber feedback loop and realized passive coherent combining four nanosecond pulsed lasers modulated by EOPM for the first time, to the best of our knowledge. The adoption of the fiber coupler as the combiner eliminates the problem of side lobes, and the all-fiber configuration ensures stability and robustness. With passive phasing, the averaged output power of the combined nanosecond pulsed laser improves to be 2.7 times of the averaged output power without phasing. The measured polarization extinction ratio demonstrates that the combining system maintains polarization well. This scheme provides an easy while robust way to obtain a combined pulsed laser with a nanosecond width. In this experiment, the output power is low due to the power handling ability of the combining output fiber coupler. If we replace the fiber coupler with a high power tapered fused bundle (TFB) fiber combiner, this method can even scale power up to the kW level with a relatively good beam quality. References [1] Eidam T, Wirth C, Jauregui C and Stutzki F 2011 Opt. Express 19 13218 [2] Klenke A, Breitkopf S, Kienel M and Gottschall T 2013 Opt. Lett. 38 2283 [3] Wang X L, Leng J Y, Zhou P, Ma Y X et al 2012 Appl. Phys. B 48 785 [4] Yu C X, Augst S J, Redmond S M and Goldizen K C 2011 Opt. Lett. 36 2686 [5] Xue Y H, He B, Zhou J, Li Z et al 2011 Chin. Phys. Lett. 28 054212 [6] Wang B S, Mies E, Minden M and Sanchez A 2009 Opt. Lett. 34 863 [7] Li J, Duan K, Wang Y, Zhao W et al 2008 IEEE Photon. Technol. 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