# Broadband Passively Mode-Locked Fiber Laser with DNA Aqueous Solution as Saturable Absorber
## 2 Materials and Methods  
The liquid DNA solution used in this study was prepared using commercially purchased crude DNA powder extracted from salmon testes (Sigma-Aldrich, single-stranded, molecular mass = 1.3 \(\times\) 106 Da, approximately 200 bases, %G-C content = 41.2%) mixed in DI water at a concentration of 1.2 wt%. After sonification of the DNA solution at 60 \({}^{\circ}\)C for an hour, the solution exhibited a clear and slightly yellowish color without precipitation. Figure 1 illustrates the manufacturing process and cross-section of the DNA SA fabricated using etched fiber. Initially, the polymer protection of the single-mode optical fiber (SMF-28(r), Corning(r)) was removed from a 1 cm section in the middle of the fiber. The cladding of this section was subsequently wet etched by immersing in 30% hydrofluoric acid solution, which selectively etched the fiber cladding for 2 h until the total diameter of the fiber observed by a microscope was 15 microns. For the 1 um wavelength mode-locked laser with Yb-doped fiber laser gain material, the fiber diameter was etched down to 8.5 microns because evanescent field interaction between the DNA solution and shorter-wavelength light can be obtained with a thinner cladding configuration. The etched part was sealed with a capillary tube containing aqueous DNA solution, where the DNA was used as the SA. Unlike D-shaped fibers (or side-polished fibers), the etched fiber has a relatively low intrinsic optical loss of 12% due to an intact fiber core. The transmittance change in the fabricated DNA SA was measured with an 80 fs pulse laser (Spectra-Physics, repetition rate = 80 MHz, wavelength = 1550 nm) input source directed to the SA through a variable optical attenuator and a polarization controller, and a power meter. The measured nonlinear transmission of the DNA SA is depicted in Figure 2. The nonsaturable transmission is 87.73% when the intensity of light is 1500 MW/cm\({}^{2}\) or less. The low intrinsic loss is not only due to the intact fiber core as mentioned above, but also the DNA aqueous solution. While a light scattering from the dried DNA film on D-shaped fiber is inevitable, our aqueous solution is free from the scattering issue. This is a significant improvement over existing technologies based on D-shaped fibers. The transmission of the DNA SA component increases to 92.73% as the input light intensity increases. Based on this transmission measurement, the modulation depth of the DNA SA is estimated to be approximately 5%, which is higher than that previously reported, and this is mainly due to the lower optical loss and elongated light-SA interaction of the etched fiber configuration. The modulation depth can be somewhat modified, controlling  
Figure 1: Fabrication process and cross-section of the DNA SA. the final diameter of the etched fiber. The larger modulation depth with the thinner etched fiber gives a higher optical loss, resulting in a loss of the mode locked laser. The most reliable mode-locked laser was obtained in the optimal conditions described previously. Even though the modulation depth of 5% is slightly higher than the typical numbers from most mode-locked lasers, it is still far from the Q-switching regime, thus this result derived from trial and error is reasonable. The experimental setup of the fiber laser system, including the liquid DNA SA, is shown in Figure 3. The ring cavity of the 1.5 um laser with a 4.5 m erbium-doped fiber (EDF) as the gain medium is pumped using a 980 nm laser diode (LD) through a 980/1550 nm wavelength division multiplexing (WDM) coupler; replacement of the gain medium with a 5 m yterbium-doped fiber (YDF) and the corresponding components (dimensions of the ring cavity/components were optimized for each of the 1.5 um and 1 um laser schemes) switches the entire system to a 1 um laser. Note that the DNA solutions utilized in both systems are identical, even though the etched fiber dimensions are tailored to optimize light-DNA interaction.