**3. Experiment**

The experimental apparatus, shown in **Figure 5**, consists of a pumping source (pulsed, Nd:YAG laser), a seeding source (CW, external cavity diode laser (ECDL)), amplifiers (for both pumping and seeding beams), and the nonlinear crystal (MgO:LiNbO3 ). The pumping source is a diode end-pumped microchip Nd3+:YAG laser passively Q-switched by Cr4+:YAG saturable absorber. This configuration enables the low-order axial and transverse mode laser oscillation, whose linewidth is below 0.009 nm. The laser delivers more than 1.4 MW peak power pulses (energy/pulse, > 0.6 mJ/pulse; duration, ~ 420 ps) at 100 Hz repetition rate with a M2 factor of less than 1.1 [26]. The pumping beam is amplified by two amplifiers in double-pass configurations. Each amplifier has 0.7 at.% doped Nd3+:YAG with a length and diameter of 70 and 3 mm, transversely pumped by 200 W laser diodes (wavelength, 808 nm) in a threefold geometry. Amplified beam is extracted by a polarization beam splitter (PBS). The seeding beam from an ECDL is also amplified by an *Yb*-doped fiber amplifier. Owing to the grating and confocal arrangement, the noncollinear phase-matching condition is satisfied automatically depending on the wavelength of the seeding beam [28]. The diameter of both beams is the same on the High-Brightness and Continuously Tunable Terahertz-Wave Generation http://dx.doi.org/10.5772/intechopen.75038 35

**Figure 5.** Experimental apparatus for an is-TPG.

(SRS or SBS); four-wave mixing; optical rectification (OR); multiphoton absorption; and the Kerr and Pockels effects. Of these, we revealed that the parametric wavelength conversion near the lattice resonance induced by SRS is significantly inhibited by SBS; however, this nonlinear process has long been ignored. In the previous research done by authors, the conversion efficiency in energy from an infrared pumping beam to a terahertz wave was less than 10−7. It has long been thought that this is the limit of the conversion efficiency using paramet-

10–25 ns) [14]. However, when a photon of the pumping beam (1064 nm) creates two photons (idler beam and terahertz wave (100–1000 μm), in principle, the conversion efficiency reaches 10−2–10−3 according to the Manley-Rowe relations because the wavelength of the tera-

pumping beam generates terahertz waves and an idler beams. We calculated both gain coefficient of the SRS and the SBS in the previous condition; the gain coefficient of SBS has 1000 times larger gain than that of the SRS [19–24]. Typically, the SBS gain reaches the steady state within 10 lifetimes of the acoustic phonon of crystal [25], within about 1.5 ns in LiNbO3

For efficient wavelength conversion, the pulse width of the pumping beam should be enough less than this, but the pulse width limits the linewidth of the generated terahertz waves. By applying a single-mode oscillated microchip Nd:YAG laser [26] with a sub-nanosecond (several hundreds of picoseconds) "pulse gap" pulse width [27] as a pumping source, a high-efficiency and narrow-linewidth wavelength conversion can be performed by the SRS without the SBS. Additionally, when the intensity of the pumping beam is too high, secondorder stoke (idler) beams can be generated, which do not contribute to the generation of terahertz waves as they undergo strong absorption. We thus precisely controlled both pumping

The experimental apparatus, shown in **Figure 5**, consists of a pumping source (pulsed, Nd:YAG laser), a seeding source (CW, external cavity diode laser (ECDL)), amplifiers (for both

is a diode end-pumped microchip Nd3+:YAG laser passively Q-switched by Cr4+:YAG saturable absorber. This configuration enables the low-order axial and transverse mode laser oscillation, whose linewidth is below 0.009 nm. The laser delivers more than 1.4 MW peak power pulses

less than 1.1 [26]. The pumping beam is amplified by two amplifiers in double-pass configurations. Each amplifier has 0.7 at.% doped Nd3+:YAG with a length and diameter of 70 and 3 mm, transversely pumped by 200 W laser diodes (wavelength, 808 nm) in a threefold geometry. Amplified beam is extracted by a polarization beam splitter (PBS). The seeding beam from an ECDL is also amplified by an *Yb*-doped fiber amplifier. Owing to the grating and confocal arrangement, the noncollinear phase-matching condition is satisfied automatically depending on the wavelength of the seeding beam [28]. The diameter of both beams is the same on the

(energy/pulse, > 0.6 mJ/pulse; duration, ~ 420 ps) at 100 Hz repetition rate with a M2

experiment, an infrared pumping beam excites acoustic phonons in LiNbO3

and seeding intensity and diameter as well as the nonlinear crystal length.

pumping and seeding beams), and the nonlinear crystal (MgO:LiNbO3

pumped by nanoseconds Nd:YAG lasers (duration,

, and SRS of the

). The pumping source

factor of

[24].

times longer wavelength than that of the pumping beam. In our

ric wavelength conversion using LiNbO3

–103

hertz wave is about 102

34 High Power Laser Systems

**3. Experiment**

crystal input surface. We used a 50-mm-long nonlinear MgO:LiNbO3 crystal with antireflection coating for a pumping beam. A prism made by high-resistivity silicon placed on the output surface of the nonlinear crystal works as an efficient output coupler only for the terahertz waves to avoid the total internal reflection of the terahertz waves on the crystal output side surface. For an optimization of terahertz-wave emission, the pumping region within the nonlinear crystal must be as close as possible to the output surface, because of the large absorption coefficient of the MgO:LiNbO3 crystal in the terahertz range (10–100 cm−1). The distance between the output surface and the beam center was precisely adjusted to obtain a maximum terahertz-wave output, and it was approximately equal to the pumping beam radius. The terahertz-wave output extracted through the Si-prism coupler was collimated, focused, attenuated, modulated, and then measured using a calibrated pyroelectric detector covered by thick black polyethylene sheet. The temporal waveform and linewidth of the terahertz wave were measured by a Schottky barrier diode (SBD) and a pair of scanning metal mesh plates.
