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实验首先测试了系统的锁相效果。采用空间延迟线对两路信号光程差进行补偿后达到干涉状态,再基于光纤拉伸器对相位差进行精确补偿。图2(a)是在单路信号都为最高6.1 W时的长时间归一化时域强度起伏。可以看出,锁相前由于光纤放大器相位噪声的影响,以及环境噪声的扰动,系统的输出功率剧烈、随机地抖动。当对两路激光信号相位差进行实时反馈控制后,系统的输出功率趋于稳定,归一化时域强度的平均值为0.9582,标准差为0.0101。图2(b)是锁相前后的功率谱密度,可知系统在低频下的相位噪声得到了有效抑制,相位噪声性能得到了极大改善。系统锁相残差由下列公式[26]计算:
图 2 (a)系统锁相前后的归一化时间强度起伏(开环和闭环);(b)开环和闭环状态下的功率谱密度曲线;(c)~(e)系统开环状态下的合成光斑;(f)系统闭环状态下的合成光斑
Figure 2. (a) Normalized temporal intensity fluctuation before and after phase locking (open loop and closed loop); (b) Power spectral density curves at the open and closed loop; system combined beam profile in (c) -(e) open loop and (f) closed loop
$$ \sigma = 2\sqrt {{V_{RMS}}/{V_{MAX}}} $$ (1) 式中:σ代表系统锁相残差;VRMS为探测器输出电压的均方根误差;VMAX为输出电压最大值。由上式计算的系统在最高合成功率10.9 W下的锁相残差约为λ/31,该光纤拉伸器表现出了较好的稳定性,功率长时间稳定性良好。图2(c)~(e)为系统在开环状态下测得的合成信号强度分布。可以看出,系统没有锁相时,输出光斑同样不稳定、随机变化,但开启相位控制系统后,在闭环状态下输出光斑会趋于稳定,如图2(f)所示。
图3是系统在不同合成功率下的合成效率,合成效率η由下式计算:
$$ \eta = P1/(P1 + 1.01 \times P2) $$ (2) 式中:P1和P2分别为合成信号经PBS3后的透射和反射光功率。在激光放大的过程中,由于单元光束间激光指向、光斑大小及光束发散角的不完美匹配,不同功率下的合成效率在90%左右小幅波动,在最高合成功率10.9 W下,合成效率约为90.1%。
图4(a)是最高功率下单路激光及合成后激光信号的输出光谱。可以看出,两路信号的输出光谱在1035 nm附近存在一定的差异,推测其主要来源于两路放大通道中光学器件损耗的波长相关性,以及相应激光信号所经历的非线性相移差。需要强调的是,尽管非线性相移差会对合成效率造成一定程度的影响,但通过严格控制两路放大器结构的一致性(如光纤长度、输入信号功率以及泵浦功率等),该影响可忽略不计[27]。合成后的光谱中心波长为1036.1 nm,3 dB线宽为6.5 nm。图4(b)是最高功率下单路及合成后脉冲压缩的时域自相关曲线,假定脉冲为高斯型,单路脉冲压缩的脉宽分别为389 fs和414 fs,合成后的信号可压缩至494 fs。压缩效率约为73.3%,考虑到激光重复频率为2 MHz,合成后的压缩信号单脉冲能量为3.99 μJ。
图 4 最高功率下单路及合成后的(a)光谱和(b)压缩脉冲的自相关曲线
Figure 4. (a) Output spectra and (b) autocorrelation trace of the compressed pulse of single channel and combined beam at the highest power
需要指出的是,笔者所在单位在2018年实现了两路超快光纤激光的共孔径相干偏振合成[19],法国巴黎综合理工学院则在2020年实现了61路超快光纤激光的分孔径相干合成[17]。然而,前期工作所使用的光纤拉伸器相位调节范围有限(±18.4λ),因此,主要被利用反馈控制合成系统的相位噪声,而对由光程差漂移引起的较大幅度相位变化则需使用额外的光纤延迟线来补偿,导致系统的控制环路增加、可靠性变差。文中工作所使用的光纤拉伸器具有更大的相位调节范围(±115.5λ),通过结合SPGD算法,基于单个控制环路实现了对光程差与相位噪声的同时有效控制,这使得该系统的紧凑性与可靠性更强。下一步工作将基于该光纤拉伸器实现更高功率、更多合成路数的超快光纤激光相干合成系统。
Coherent polarization beam combination of two ultrafast laser channels based on fiber stretcher phase locking
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摘要: 超快光纤激光相干合成技术是突破飞秒激光单根光纤功率提升受限的有效技术手段。基于光纤拉伸器锁相,并结合随机并行梯度下降(SPGD)算法,成功实现了两路超快激光相干偏振合成。该锁相方案不仅避免了采用常规电光相位调制器对脉冲信号造成的光谱调制,而且可有效降低系统的插入损耗,提高相位调制范围、耐受功率以及前级光源系统的紧凑性与鲁棒性。合成的最高功率为10.9 W,最高功率下合成效率为90.1%,闭环状态下锁相残差为λ/31。实验结果表明,采用光纤拉伸器和SPGD算法组成的相位控制系统,在超快激光相干合成领域具有较强的发展潜力。合成的脉冲在最高功率下可压缩至494 fs,压缩效率为73.3%,对应的单脉冲能量为3.99 μJ。Abstract:
Objective High-power ultrafast fiber lasers have broad applications in the frontier science and industry fields such as high-energy physics, high-order harmonic generation, advanced manufacturing and so on. Currently, the well-known fiber chirped pulse amplification (CPA) scheme has realized kW-level average power and multi-mJ single pulse energy of ultrafast laser, whilst further development is hindered by the influence of nonlinear effects and mode instability. At present, coherent beam combination (CBC) of ultrafast fiber laser is an effective way to break the power limitation of single-channel fiber, and has attracted much research interest. Essentially, the CBC system requires that each channel of amplifiers is phase locked, which is conventionally realized based on the electro-optical effect of lithium niobate, whereas with the compromise of large insertion loss and low damage threshold. In this study, we propose to utilize the fiber stretcher to control the laser phase by stretching the fiber based on piezoelectric ceramics. Compared with the lithium niobate modulator, the fiber stretcher has a larger dynamic adjustment range, lower insertion loss, higher damage threshold, as well as the additional merits of compactness and robustness. Methods The ultrafast fiber laser with a repetition rate of 50 MHz is firstly broadened by a chirped fiber Bragg grating, and then reduced to a repetition rate of 2 MHz by a pulse picker. After a single-mode amplifier, the pulsed laser signal is divided into two channels. Then, the average power is scaled to 6.1 W through two parallel-configured polarization-maintaining fiber amplifiers. For one of the channels, a spatial delay line consisting of a polarizing beam splitter prism, a quarter wave plate and a mirror placed on a high precision displacement platform is inserted in front of the main amplifier to effectively compensate the optical path difference between the two channels. The amplified lasers are collimated and combined through the polarization beam combining mirror. The combined laser is sampled by a photodetector, processed by the phase-locked control system, and converted into a voltage signal, which is fed back to the fiber stretcher to realize effective phase locking (Fig.1). Results and Discussions The effective coherent polarization beam combination of two ultrafast fiber lasers is realized based on the fiber stretcher and the stochastic parallel gradient descent (SPGD) algorithm. The highest combined power is 10.9 W with a combining efficiency of 90.1 % (Fig.3). According to the normalized temporal intensity fluctuation before and after phase locking at the highest power, the phase noise of the system is effectively suppressed in the closed-loop state with a phase residue error of λ/31, and the output power shows good long-term stability. When the system is in the open-loop state, the output beam profile is unstable and changes randomly. However, after the phase control system is turned on, the beam profile tends to be stable (Fig.2). The central wavelength and 3 dB bandwidth of the combined beam at the highest power are 1 036.1 nm and 6.5 nm, respectively. The combined beam can be compressed to 494 fs (assuming the pulse is Gaussian profile) with a compression efficiency of 73.3% (Fig.4). Conclusions In this study, the coherent polarization beam combination of two ultrafast laser channels is successfully realized based on fiber stretcher and SPGD algorithm. Compared with the conventional electro-optical phase modulator, the fiber stretcher not only avoids the spectral modulation of the pulse signal, but also has smaller insertion loss, larger phase adjustment range and higher damage threshold. In the experiment, the highest combined power is 10.9 W with a combining efficiency of 90.1%, and the phase residue error is about λ/31 in the closed-loop state. The combined beam can be compressed to 494 fs with a compression efficiency of 73.3%, and the corresponding single pulse energy is 3.99 μJ. The above experimental results verify the feasibility of the fiber stretcher to control the phase in fiber CBC system. The next step involves expanding the system to more channels and higher combined power. -
图 1 (a)两路超快激光相干偏振合成实验装置图。CFBG:啁啾光纤布拉格光栅;Pre-amp:预放大器;HWP:半波片;PBS:偏振分束棱镜;QWP:1/4波片;L1~L4:透镜;M1~M4:高反镜; PBC:偏振合束镜;PD:光电探测器;PM:功率计;种子激光的(b)输出光谱和(c)脉冲宽度
Figure 1. Experimental setup of two ultrafast laser channels coherent polarization beam combination. CFBG:chirped fiber Bragg grating; Pre-amp: pre-amplifier; HWP: half-wave plate; PBS: polarization beam splitter; QWP: quarter-wave plate; L1-L4: lens; M1-M4, high-reflection mirror; PBC: polarization beam combiner; PD: photodiode; PM: power meter; (b) output spectrum and (c) pulse width of the seed laser
图 2 (a)系统锁相前后的归一化时间强度起伏(开环和闭环);(b)开环和闭环状态下的功率谱密度曲线;(c)~(e)系统开环状态下的合成光斑;(f)系统闭环状态下的合成光斑
Figure 2. (a) Normalized temporal intensity fluctuation before and after phase locking (open loop and closed loop); (b) Power spectral density curves at the open and closed loop; system combined beam profile in (c) -(e) open loop and (f) closed loop
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