# Broadband Passively Mode-Locked Fiber Laser with DNA Aqueous Solution as Saturable Absorber
## 3 Results  
The optical spectrum and oscilloscope trace of the output pulses are displayed in Figure 4. The spectrum measured by the OSA has a central wavelength of 1563 nm with a 3 dB bandwidth of 3.9 nm (Figure 4a). The oscilloscope trace (Figure 4b) shows a fundamental repetition rate of 14.11 MHz and a pulse train of 70.87 ns corresponding to a total cavity length of 14.3 m.  
In the frequency domain, the RF spectrum (Figure 4d) indicates the stability of the laser; the signal-to-noise ratio of the mode-locked pulses is approximately 50 dB. The autocorrelation trace (Figure 4c) shows a measured pulse trace that is well fitted with the Seth profile. The pulse width estimated using the fitting curve is approximately 822 fs. These results successfully demonstrate the ultrafast mode-locked laser with the proposed etched fiber-based liquid DNA SA. As mentioned earlier, the average output power of our ultrafast laser is 7.5 mW when the pump 980 nm LD power is 280 mW, which can be improved by optimizing the optical gain (i.e., EDF/YDF configurations) and out-coupling ratio (through optical coupler and/or isolator). The lasing characteristics of the 1 \(\upmu\)m laser with an ytterbium-doped fiber are illustrated in Figure 5. Mode-locking behavior is observed at a pump LD power ranging from 185 to 247 mW. In this regime, with the increase in the pump power, the pulse duration decreases, and a more stable mode-locked laser is obtained. At a pump power of 247 mW, the center wavelength of the laser is 1037 nm and the 3 dB bandwidth is approximately 5 nm. The optical spectrum (Figure 5a) presents a typical dissipative soliton, and the fundamental repetition rate of 20.16 MHz derived from the pulse train in Figure 5b matches well with the entire cavity length of 10 m. The pulse duration estimated using an 80 GHz high-speed oscilloscope and 12.5 GHz high-speed photodetector is 250 ps (Figure 5c). Figure 4: Soliton pulse train of the 1.5 \(\upmu\)m passively mode-locked fiber laser: (**a**) optical spectrum; (**b**) oscilloscope trace; (**c**) autocorrelation trace; (**d**) RF spectrum. This is comparable with the other dissipative solitons in 1 \(\upmu\)m wavelength pulsed lasers demonstrated with various solid-state SAs [27; 28; 29], and 250 ps pulse duration is clearly in a mode-locked laser range rather than Q-switching. The RF spectrum (Figure 5d) shows a signal-to-noise ratio of 55 dB, which directly proves the obtainment of stable mode-locked pulses at a high pump power. The mode-locked laser emission of the 1 \(\upmu\)m laser was measured over time, and the optical spectrum change is presented in Figure 6. In addition, the pulse duration and 3 dB bandwidth are depicted as a function of time. These results establish that our mode-locked laser exhibits virtually identical lasing characteristics over time, even at high pump power operation, due to the thermal stability of the aqueous DNA SA. The laser maintains its characteristics not only at a time scale of a few hundred hours but also over several weeks, as long as the liquid-phase DNA SA is perfectly passivated such that the solution does not suffer from evaporation. Our system exhibits stable mode locking and is free from thermal damage and/or evaporation of the SA component at a relatively high operating power because the DNA SA is an aqueous solution with large specific heat. As a result, both our 1 \(\upmu\)m and 1.5 \(\upmu\)m laser exhibit virtually identical laser characteristics after six weeks under ambient conditions, proving that the liquid DNA SA mode-locked laser has superior long-term stability as long as the liquid DNA SA is perfectly sealed and intact. Figure 5: Characteristics of the 1 \(\upmu\)m mode-locked pulse laser at a pump power of 247 mW: (**a**) optical spectrum; (**b**) oscilloscope trace; (**c**) high-speed oscilloscope trace and estimated pulse duration; (**d**) RF spectrum.