## 2 Materials and Methods
### Modulational Instability-Initiated SC Source  
The SCL used in this study was based on modulational instability-initiated SC generation, which uses picosecond or nanosecond pump pulses to generate SC with a high average power and broadband bandwidth [33]. One advantage of the modulational instability initiation is that the SC results in a high degree of spectral flatness as well as implementation using commercially available parts that are much simpler than those used in SC systems relying on mode-locked lasers. A block diagram of our visible and near-infrared SCL is illustrated in Figure 2a, and its elegance lies in the simplicity of the design and construction. The seed laser outputs \(\sim\)0.5-2 ns wide pulses, which are then boosted up in power by passing through a multi-stage fiber amplifier. In this case, the seed laser operates around 1060 nm, and the amplifiers are ytterbium-doped fiber amplifiers (YDFAs). For the two-stage amplifier design, the preamplifier is optimized for noise performance, since any noise introduced in the first stage will be amplified in the second stage. Then, the power amplifier stage is optimized to minimize nonlinearities, since relatively high powers are reached toward the end of the second stage. For example, a cladding-pumped fiber amplifier is used in the second stage so that the larger core size fibers reduce the nonlinear distortion. To reduce the amplified spontaneous emission from the first stage, and to avoid any spurious reflections into the amplifier, a band-pass filter and isolator are used between the two stages. The pumping architecture shown in Figure 2a is called a master oscillator power amplifier (MOPA) configuration. Figure 1: Block diagram of optical set-up for in vitro laboratory measurements. Figure 2: (**a**) Photograph of the 10-W SCL and a block diagram of the SCL used in this study. (**b**) Output spectrum of the super-continuum light source. The output from the MOPA pump is coupled into two relatively short lengths of fiber for the super-continuum generation. As shown in Figure 2a, the first length of fiber is a few meters of standard single-mode fiber (SMF), which is where the modulational instability mechanism is dominant. In the SMF, modulational instability occurs because the peak power may be several kilowatts, and the wavelength of the light falls in the anomalous group velocity dispersion regime, also known as the soliton regime. The high peak powers in the soliton regime are unstable, causing the quasi-CW or nanosecond pulses to break up into a series of shorter pulses. In other words, modulational instability, which is a particular form of parametric amplification, tries to convert the unstable nanosecond pulses into the stable solution for this operating regime, which are optical soliton pulses. Thus, without using mode-locked lasers, the short pulses with high peak power required for super-continuum generation result from using the natural physics in the fiber. The nanosecond pulses from a gain-switched semiconductor seed laser that are then amplified and turned into a series of ~0.5-ps pulses at the output of the SMF using the modulational instability mechanism. The few meters of SMF fiber are much cheaper and simpler than a complicated mode-locked seed laser source, which is used in many commercially available SCLs. The short pulse output from the SMF fiber is then coupled into several meters of nonlinear SC fiber, whose characteristics are selected to generate the SC over the wavelength of interest. The spectral broadening for SC generation results from physical processes within the fiber, including four-wave mixing, self-phase modulation, and the optical Raman effect. For example, self-phase modulation and four-wave mixing leads to broadening of the spectrum more symmetrically around the pump wavelength, while the Raman effect dominates the long wavelength shifting of the spectrum.