用于激光频率梳的 GHz 蓝光超连续谱产生

Objective This study addresses the challenge of generating high-quality, visible-to-blue supercontinuum (SC) spectra using GHz-repetition-rate femtosecond laser frequency combs. While GHz combs offer advantages in mode spacing and system integration, especially for astronomical spectrograph calibration and precision metrology, their low single-pulse energy limits efficient spectral broadening. To overcome this, we develop and experimentally demonstrate a pre-chirped, dual-stage nonlinear fiber amplification scheme, achieving SC generation from 440 nm to 1500 nm at nJ-level pulse energies. The results provide a stable, broadband light source for advanced GHz frequency comb applications and pave the way for future extension into violet and ultraviolet (UV) regimes. Methods The experiment employs a 1 GHz mode-locked fiber laser as the seed source, producing 54 fs pulses with a 33 nm bandwidth. A portion of the output is amplified using a two-stage nonlinear fiber amplifier. The first stage, based on a single-mode polarization-maintaining (PM) fiber, boosts the power from 22 mW to 146 mW. The second stage, using a double-cladding PM fiber, further increases the output to 1.93 W. Pre-chirp control and precise dispersion management enable pulse compression to 110 fs and enhance nonlinear interactions (Fig. 4 and Table 1). The compressed pulses are injected into a tapered photonic crystal fiber (PCF), engineered to shift the zero-dispersion wavelength and support efficient supercontinuum (SC) generation extending into the blue. The resulting SC spans 440‒1500 nm. Spectral characteristics are measured using optical spectrum analyzers (Fig. 5), and the SC stability is assessed through long-term spectral recording and power monitoring over 6 h (Fig. 6). Results and Discussions The experiment successfully generates a broadband SC spectrum covering 440 nm to 1500 nm with an average power of 1.5 W and a pulse energy of 1.5 nJ. The compressed pulses exhibit a 3 dB bandwidth of 28.8 nm and a pulse duration of 110 fs (Fig. 4). The SC spectrum shows a uniform power distribution across the visible range, with the blue region (440‒550 nm) achieving over 20 mW power (Fig. 5). Stability measurements over 6 h demonstrate minimal power fluctuation (<1 mW) and consistent spectral profiles (Fig. 6). Notably, increasing the pump power to 11 W extends the spectrum to 420 nm but introduces a dip in the 570‒700 nm range due to soliton self-frequency shift (SSFS) effects. These results highlight the effectiveness of the proposed amplification and spectral broadening strategies in achieving high-quality SC generation under low-energy conditions. SC spanning 440 nm to 1500 nm is achieved using a GHz repetition-rate system at pulse energies of only 1.5 nJ and pulse durations of 110 fs after compression. The visible portion (>50 mW), including 20 mW in the 440 ‒ 550 nm blue region, exhibits spectral flatness with intensity variation within 20 dB [Fig. 5(a)]. The system second-order dispersion (SOD) is compensated to near zero, while the third-order dispersion (TOD) is self-compensated through nonlinear phase accumulation during amplification. As a result, the compressed pulses exhibit high peak powers, minimal sidelobes, and clean temporal profiles [Table 1 and Fig. 4(b)]. As pump power increases, the appearance of spectral dips (e.g., 570‒700 nm) is attributed to SSFS, which red-shifts the pulse center wavelength and disrupts the phase-matching conditions for dispersive wave generation in this band. This mechanism highlights the importance of managing soliton dynamics and dispersion design for SC uniformity [Fig. 5(b)]. The visible (440‒900 nm) SC output power remains stable within <1 mW fluctuation over 6 h continuous operation at 9 W pump power. Spectral shape and peak positions show no significant drift, demonstrating the system thermal and mechanical stability for metrology applications (Fig. 6). Conclusions This work presents an effective strategy for generating broadband supercontinuum light from a GHz-repetition-rate femtosecond fiber laser using pre-chirped, dual-stage nonlinear amplification. Despite the low pulse energy (~1.5 nJ), the system delivers 110 fs pulses and achieves spectral broadening from 440 nm to 1500 nm, with ~20 mW power in the blue region. The resulted spectrum is flat, stable, and well-suited for high-precision applications such as astronomical spectrograph calibration. These results confirm the viability of nonlinear amplification and dispersion control at high repetition rates and establish a scalable approach for extending GHz frequency combs into the violet and ultraviolet. This work provides a solid technical foundation for advancing GHz comb systems in precision metrology and broadband spectroscopy.