Objective In general, a single-mode polarization-maintaining fiber can serve as the stretcher and the Treacy-type grating pairs can serve as the compressor in the fiber chirped pulse amplification (FCPA) system. Although the positive group delay dispersion (GDD) introduced by the fiber stretcher can be compensated by the negative GDD introduced by the compressor, but the signs of the third-order dispersion (TOD) introduced by the stretcher and the compressor are the same, resulting in the deterioration of pulse quality due to the accumulation of TOD during the pulse amplification process. To solve the above problem, a segment of negative TOD fiber, high-order mode dispersion fiber or chirped fiber Bragg grating (CFBG) is usually introduced into the fiber stretcher to compensate the TOD in the system. However, the residual TOD in the system cannot be accurately measured, and the length of the dispersion fiber introduced in the stretcher can only be estimated. Therefore, several experiments have attempted to obtain the best dispersion compensation effect. However, these dispersion compensation methods are complicated and time-consuming. Here, we report an FCPA system that is composed of a mode-locked fiber oscillator, a pulse stretcher, a pulse picker, a pulse delay unit, an Yb-doped fiber preamplifier, a rod-shaped photonic-crystal fiber main amplifier, and a grating compressor. Among them, the pulse stretcher and compressor are based on block gratings. By finely adjusting the position and incidence angle of the gratings in the stretcher and compressor, a real-time dispersion compensation in the system is realized and a high-quality pulse is obtained. Compared with the other methods, it is a more convenient dispersion compensation method. However, in previous studies, the single-pulse energy of the CPA system with a rod-shaped photonic-crystal fiber served as the gain medium was generally lower than 100 μJ, and the pulse width was limited to 400 fs. To further reduce the pulse width and simultaneously obtain the hundred-microjoule chirped pulse-amplification laser system with high pulse-quality, further research works are done. Subsequently, a nonlinear frequency-doubled experiment is carried out with a BBO crystal.
Methods First, a set of home-made passive mode-locked fiber oscillator based on the nonlinear polarization evolution (NPE) principle was constructed. The output of the oscillator was directed to a Martinez-type pulse stretcher. The stretched pulse duration was measured to be approximately 1 ns. Then, the stretched pulse from the stretcher was amplified step by step through four ytterbium doped fiber preamplifier stages with the forward pumping scheme. Meanwhile, a fiber coupled acoustic optical modulator (AOM) was used to reduce the pulse repetition rate of the system from 62.8 MHz to 1 MHz@500 kHz. The output laser at the fourth fiber preamplifier stage was coupled to that at the main amplifier stage with the backward pumping scheme by means of space coupling. Then, to compress the laser pulse to the femtosecond level, the laser beam was directed to a Treacy-type pulse compressor. High pulse quality and stability femtosecond laser pulses were obtained by controlling the dispersion, which was optimized by finely adjusting the position and incidence angle of the gratings in the stretcher and compressor in the system. At the same time, the second harmonic generation frequency resolution optical gate (SHG-FROG) was used to observe the pulse duration and phase information online. In addition, a nonlinear frequency-doubled experiment was carried out with a BBO crystal. A half wavelength plate (HWP) placed before the BBO crystal was used to adjust the polarization state of the fundamental laser beam. Simultaneously, the phase-matching condition between the fundamental and the frequency-doubled lasers was optimized to obtain a high average power of green laser.
Results and Discussions The mode-locked ytterbium (Yb)-doped fiber oscillator that was designed to have a repetition rate of 62.8 MHz delivered pulses with an average output power of 12 mW, the central wavelength of 1031 nm, and a bandwidth of 15.8 nm (Fig. 2). The output of the oscillator was directed to a Martinez-type pulse stretcher. The stretched pulse duration was measured to be approximately 1 ns (Fig. 4). Owing to the losses from diffraction gratings and multiple reflections on the mirrors, the average power and spectral bandwidth of the stretcher decreased to 2.87 mW and 13.5 nm, respectively (Fig. 2). Then, to ensure the spectral quality, the average power of the stretcher was enhanced step by step through four ytterbium-doped fiber preamplifier stages with the forward pumping scheme and a main amplifier with the backward pumping scheme (Fig. 2). Device parameters for different stages in the system were listed in detail (Table 1). To avoid excessive gain narrowing, the average power of the fourth fiber preamplifier stage was limited to 0.86 W at a pulse repetition rate of 500 kHz and 2.0 W at a pulse repetition rate of 1 MHz, respectively. The optical spectra were recorded after different amplification stages. Only minor gain narrowing effect was visible (Fig. 2). This could be attributed to the distribution of the amplification gain over multiple stages of the amplifiers. With the increase of pumping power, the average powers of the main amplification and compressor were recorded (Fig. 5). The stability of the system was studied to meet the long-term test requirements of some special experiments. During the process of the 2.5 h continuous test, the laser operated normally, the output average power maintained high stability, and the root mean square(RMS) of the power is 0.31%(61.23 W@1 MHz) or 0.21%(50.4 W@500 kHz) (Fig. 6). The beam quality was measured at the average power of 50 W and a pulse repetition rate of 1 MHz in the system, showing a Gaussian distribution of near-field with M2=1.22 (Fig. 6). The pulse duration was characterized by SHG-FROG. At a repetition rate of 1 MHz in the system, a high-quality femtosecond laser pulse with an average power of 61.5 W and pulse duration of 273 fs was obtained (Fig. 7). A nonlinear frequency-doubled experiment was carried out with a BBO crystal, and more than 20 W average green light power at 517 nm was obtained (Fig. 8).
Conclusions The FCPA laser system based on transmission diffraction gratings serving as stretcher and compressor, was experimentally studied. By controlling and optimizing the spectral shape of the seed source, the stretcher and all stages of fiber pre-amplifiers without distortion are used to ensure that the spectral shape of the main amplifier was smooth and the distortion of spectral shape after de-chirped was avoided. To obtain a high-quality pulse, real-time compensation of the dispersion of the system was conducted by finely adjusting the position and incidence angle of the gratings in the stretcher and compressor. Finally, the high pulse quality and stability femtosecond laser with pulse duration of 251.7 fs, average power of 51.5 W, and 103 μJ of energy per pulse at repetition rate of 500 kHz was obtained. At a repetition rate of 1 MHz in the system, a high quality femtosecond laser pulse with average power of 61.5 W and pulse duration of 273 fs was obtained. By carrying out the nonlinear frequency-doubled experiment with a BBO crystal, a green laser with average power of over 20 W at 517 nm was generated. The CPA device will be applied in femtosecond laser processing, medical treatment, national defense, and other fields. The UV light source prepared by this system can be used in photoelectron spectroscopy experiments and has important practical value in the field of material surface science research.
