CW THz Wave Generation System with Diode Laser Pumping

Abstract

This chapter contains the work of continuous-wave (CW) terahertz (THz) radiation; the THz waveform generation, propagation and detection. THz technology attracts increasing interest due to its versatile application possibilities in medical imaging, spectroscopy, THz communication, nondestructive screening, and identification of many chemical elements. For such applications, it is necessary to realize compact and cost-efficient THz sources, which emit a broadband spectrum on the one hand, and can be tuned in frequency with a narrow linewidth on the other hand. Thus, the realization of a compact, cost-efficient and frequency-tunable CW THz radiation source was presented in this chapter. The principle of THz wave generation is by exciting the phononpolariton in bulk GaP crystal on the basis of nonlinear optical method. The theoretical concept of difference frequency generation (DFG) on the basis of non-linear optics is well known. The generation of widely tunable CW singlefrequency THz waves from GaP based on laser diode (LD) pumping is described in this chapter. DFG method is a parametric process based on second-order nonlinearities has recently been proved to be one of the most promising techniques for the efficient generation of widely tunable, monochromatic, high-power and coherent THz wave with a suitable combination of light sources and nonlinear optical (NLO) crystals. The region of the electromagnetic spectrum from 0.1 THz to 10 THz is very attractive because THz waves have a variety of applications in several fields. In 1963, Nishizawa [1], [2] proposed the generation of THz waves via resonance of phonons and molecular vibrations in compound semiconductors, following the realization of a GaP semiconductor laser [3], [4]. An electromagnetic wave with a frequency of 12.1 THz was generated from GaAs pumped by a GaP Raman laser at a power of 3W [5]. Our group succeeded in generating wide frequencytunable high power THz wave signals from GaP with Q-switched pulse pumping [6]. Also, time domain THz sources have been developed based on femto-second pulsed lasers [7-10]. Compared to pulsed THz sources, continuous-wave (CW) THz sources provide a THz spectrum with a narrower line width and a higher spectral THz power [11, 12]. The potential applications of CW THz waves include high resolution THz spectroscopy, multichannel telecommunications and imaging technologies [13-19]. Several CW THz wave sources such as, Gunn diodes, TUNNETT diodes, backward wave oscillators, CO2-laser pumped gas lasers, sources based on nonlinear optical difference frequency generation(DFG) [20], optical parametric oscillators [21], free-electron lasers, quantum cascade lasers [13,14] and photo mixers [15-16] have been developed.