Research Projects

Mid-IR of 2–20 µm enables highly sensitive molecular detection due to strong absorption in this range, making them ideal for spectroscopy, medical diagnostics, and environmental monitoring. An efficient pulsed pump source with high peak power and operating wavelength close to the zero-dispersion of the nonlinear medium is the primary requirement for mid-IR generation. A YAG-based laser is in use as a pump for frequency conversion in crystals, glass, ultra-high nonlinear fibers, or photonic crystal fiber to generate broadband in this region. With the advancement of fiber laser technology, mode-locked ytterbium laser at 1 µm or erbium laser at 1.55 µm can be a potential pump for mid-IR generation in all-fiber configurations. The peak power in the range of 10s of kW and pulse width in the range of 30 ps-100s fs maximize the nonlinear interactions for efficient frequency conversion. The pumping wavelength at 2 µm provides 1.5 times higher frequency conversion efficiency compared to the pumping at 1.55 µm due to the higher pump-to-signal conversion efficiency. Hence, the specification of the pump laser is crucial for mid-IR frequency generation. The project aims to design a passive mode-locked thulium fiber laser to generate ultrafast pulses at 2 µm where a physical saturable absorber (SA) is a key component with desirable properties of saturable absorption, modulation depth, non-saturable loss, recovery time, and saturation power density. So far, the widely used SAs are SESAM, carbon nanotubes, and graphene for ultrafast pulse generation. The proposal focuses on the design of a Metallic Plasmonic Saturable Absorber (MPSA) for ultrafast pulses generation at 2 µm through a thulium fiber laser cavity. The MPSA expects a short recovery time in the range of ps and a broad absorption bandwidth of 300-2500 nm as compared to SESAM. A set of gold or silver nanoparticles materials will be fabricated to design the MPSA in order to achieve modulation depth of 4-30%, saturation intensity around 1 MW/cm2, recovery time of 1 ps -10 ns. The flexibility of controlling the size, shape and aspect ratio of the proposed metal nanoparticle results in shifting of the longitudinal surface plasmon resonance peak leading generation of wide wavelength range in the Vis-NIR to mid-IR which is the potential advantage of the proposed MPSA design. The key parameters, such as, the saturable absorption, modulation depth, non-saturable loss, recovery time, and saturation power density will be engineered by controlling the nanoparticles concentration and selecting a suitable film-forming host or dopant medium to extend the saturable absorption in the 2 µm region as such SA-based mode-locked fiber laser is not explored, while only a Q-switched pulse at 1.94 µm has been demonstrated so far. An optimal design of both fiber-coupled MPSA and mode-locked laser cavity can trigger ultrafast pulse generation at 2 µm, suitable for efficient frequency conversion in a nonlinear medium.