Introduction

Broadband optical sources find a wide range of inter-disciplinary application areas encircling science, medicine, defence, engineering etc. They have long been used owing to their spectroscopic applications. Recently, these sources have found their place in defence applications also, the most notable application being the directed infrared countermeasure (DIRCM) system and light detection and ranging (LIDAR) for detection of chemical and biological agents (an application that falls in the domain of spectroscopy). Tunability in the wavelength can be achieved by a number of techniques. Common tunable lasers include dye laser, tunable Excimer lasers, tunable solid state lasers, semiconductor lasers, free electron lasers (FEL) etc. However, for wide tunability, researchers have started exploiting the second order optical non-linearity associated with certain materials by means of optical parametric processes. The arrival of new non-linear optical materials as alternatives to the traditional oxides and silver-containing compounds has allowed development of compact and efficient optical parametric oscillators (OPO) with tunability extending from mid-infrared (mid-IR) to far-infrared wavelengths (also known as terahertz waves).

Optical parametric generation (OPG) essentially relies on phase matching. Phase matching can either be achieved by exploiting inherent birefringence of the material, in which case it is termed as birefringent phase matching (BPM), or by quasi phase matching (QPM), which is based on periodically reversing the non-linear coefficient of the material[1]. In QPM, the accumulated phase mismatch is offset by modulating the second order non-linear coefficient (d33) at a period that is twice the coherence length[1]. The distinct advantages of QPM over BPM have encouraged researchers to develop periodically poled materials. One of the most popular materials for accompli-shing QPM is lithium niobate (LN). A most efficient and commonly used technique to realize QPM structure is electric field poling (E-field poling) in which a high electric field greater than the material's coercive field is applied across the thickness of the crystal[1−2]. For example, periodically poled lithium niobate (PPLN) can be fabricated with this technique and it has been widely used for broadband mid-IR generation using OPO[2].

Recently, there has been a lot of interest in THz generation using PPLN. Although conventional PPLN may be used for THz generation using optical rectification of femtosecond pulses[3], the geometries of PPLN grating for THz generation using nanosecond laser is significantly different.