ObjectiveThe 920-nm femtosecond fiber laser is promising in the fields such as biological imaging, nonlinear optics and spectroscopy. With the development of optical fiber technology, the methods for generating 920-nm femtosecond fiber laser can be classified into two types. One is directly lasing at 920 nm using Nd
3+-doped fiber, and the other is nonlinearly shifting. The latter method often faces challenges such as system complexity and low conversion efficiency. To this end, it is of great practical significance to study ultrafast lasers based on Nd
3+-doped fiber. More importantly, increasing average power and reducing noise are highly demanded for frontier applications. Consequently, there is a strong need to develop a high-power, low-noise 920-nm femtosecond fiber laser. To this end, an all-polarization-maintaining (all-PM) figure-9 mode-locked fiber laser based on a nonlinear amplifying loop mirror with subsequent all-PM amplification is proposed.
MethodsA 920-nm all-PM mode-locked fiber laser is constructed. The output pulses from both the reflection and transmission ports of the seed laser are characterized (Fig.2). The output pulse from reflection port is chosen as the low-noise seed for subsequent power amplification, which demonstrates superior performance (Fig.3). Chirped pulse amplification (CPA) technology is employed to increase the average power of output pulse, while effectively suppressing parasitic light at 1.06 µm. This approach delivers a high-power, high signal-to-noise ratio 920-nm laser with stable performance (Fig.4). The beam quality is good, and after de-chirping, the pulse duration is compressed to the femtosecond scale (Fig.5). Finally, the noise performance of each stage of the laser is experimentally characterized, which indicates a low noise performance (Fig.6).
Results and DiscussionsThe mode-locked seed pulses from the reflection and transmission ports of a figure-9 mode-locked laser are characterized. The reflection port exhibits a flatter output spectrum, higher output power, lower timing jitter and integrated relative intensity noise (Fig.2), which is chosen as the low-noise seed for the high-power amplification. After stretching, the seed is amplified in a two-stage fiber amplifier, increasing the average power to 1.455 W, with a signal-to-noise ratio of about 40 dB. The calculated Relative Standard Deviation is 0.21% (Fig.4). After de-chirping, the pulse duration is compressed to 323 fs. The M² at high power is less than 1.2, indicating good beam quality (Fig.5). Measurements show that the timing jitter and integrated relative intensity noise (10 Hz to 1 MHz) of the femtosecond pulses are 745 fs and 0.098%, respectively (Fig.6), demonstrating the excellent noise performance of the high-power femtosecond laser.
ConclusionsA 920-nm all-PM Nd
3+-doped fiber laser based on CPA technology is proposed. The high-power fiber laser uses a compact figure-9 mode-locked laser as the seed laser. The net cavity dispersion is close to 0. The average power, spectrum and noise performance of the output pulse at the reflection port are better than those at the transmission port. Therefore, the pulse at the reflection port is launched for CPA amplification, and the pulse duration is stretched to ~140 ps before power amplification. Finally, the laser delivers an average power of 1.08 W and a pulse duration of 323 fs. This all-PM femtosecond fiber laser has wide application prospects in the fields such as material nonlinear characterization and biological imaging.