The development of new, compact mid-infrared light sources is critical to enable biomedical sensing applications in resource-limited environments. Here, we review progress in fiber-based mid-IR sources, which are ideally suited for clinical environments due to their compact size and waveguide format. We first discuss recent developments in mid-IR supercontinuum sources, which exploit nonlinear optic phenomena in highly nonlinear materials (pumped by ultrashort pulse lasers) to generate broadband spectra. An emerging alternative approach is then presented, based on broadly tunable mid-IR fiber lasers, using the promising dysprosium ion to achieve orders of magnitude higher spectral power density than typical supercontinua. By employing an acousto-optic tunable filter for wavelength tuning, an electronically controlled swept-wavelength mid-IR fiber laser is developed, which is applied for absorption spectroscopy of ammonia (NH<sub>3</sub>), an important biomarker, with 0.3 nm resolution and 40 ms acquisition time.
We explore the potential of a new mid-infrared laser transition in praseodymium-doped fluoride fiber for emission around 3.4 μm, which can be conveniently pumped by 0.975 μm diodes via ytterbium sensitizer co-doping. Optimal cavity designs are determined through spectroscopic measurements and numerical modeling, suggesting that practical diode-pumped watt-level mid-infrared fiber sources beyond 3 μm could be achieved.
In this work silicon pillar waveguides have been proposed to exploit the entire transparent window of silicon. These geometries posses a broad and at dispersion (from 2 to 6 μm) with four zero dispersion wavelengths. We calculate supercontinuum generation spanning over two octaves (2 to >8 μm) with long wavelengths interacting weakly with the lossy substrate. These structures have higher mode confinement in the silicon - away from the substrate, which makes them substrate independent and are promising for exploring new nonlinear phenomena and highly sensitive molecular sensing over the entire silicon's transparency range
The linear and nonlinear optical response of SiGe waveguides in the mid-infrared are experimentally measured. By cutback measurements we find the linear losses to be less than 1.5dB/cm between 3μm and 5μm, with a record low loss of 0.5dB/cm at a wavelength of 4.75μm. By launching picosecond pulses between 3.25μm and 4.75μm into the waveguides and measuring both their self-phase modulation and nonlinear transmission we find that nonlinear losses can be significant in this wavelength range due to free-carrier absorption induced by multi-photon absorption. This should be considered when engineering SiGe photonic devices for nonlinear applications in the mid-IR.
We present progress on 3 laser systems operating near 3 μm: a continuous-wave (cw) widely tunable narrow linewidth
system, a high peak power Q-switched system, and a passively mode-locked system. The cw system emitted average
powers of < 1 W over a wavelength range of 130 nm, with a spectral width of < 1 nm. The Q-switched system produced
pulses with 33 ns and peak power of 576 W. The mode-locked system produced ~20 ps pulses at a repetition rate of 27