Specialty Optical Fibers for THz Generation: A Review

This chapter describes the state-of-the-art sources of terahertz (THz) radiation, with focus on all-optical fiber-based sources for THz generation. THz technology based on the optical fiber platform is expected to be most attractive for day-to-day applications. Though optical fibers have been considered before for low-loss guidance of THz radiation, their nonlinear effects can also be exploited for THz generation. We discuss how a glass-based legacy step index fiber can be used to make a THz source. However, high absorption losses of silica glass in the THz regime and a small overlap between the modes at the optical and THz frequencies limit the THz generation efficiency to a level below 0.01%. Next, we discuss our design of a THz source based on a plastic fiber. By exploiting the nonlinear parametric process of four-wave mixing (FWM) in an appropriately designed microstructured-core double-clad plastic fiber (MC-DCPF), both the loss and the modal overlap issues can be overcome to a great extent. The microstructure geometry of this fiber allows for fine tuning of the required phase matching condition, group-velocity dispersion, and nonlinear properties at the optical pump wavelength. By using such a MC-DCPF, we show that a THz wave at a frequency near 3 THz can be generated by using two commercially available high-power lasers. The high-power CO2 laser acts as the pump and a CO laser of much lower power acts as a seed for the FWM process. Numerical simulations reveal that more than 30 W of THz power within a bandwidth of 2.13 GHz can be generated at the end of a 65 m long fiber when 1 kW of CO2 laser power is launched together with 20 W of CO laser power. A conversion efficiency of 30% is possible for a loss-less configuration, but efficiency of > 10% is achievable even in the presence of material losses. Recent results show that further optimization of such plastic microstructured fibers can provide conversion efficiencies close to 45%. As an alternative, we have focused on the use of plastic fibers and discussed a design criterion that is promising for realizing large output powers with a relatively high efficiency.