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.
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