Nonlinear Optics, Quantum Optics, Vol. 51, pp. 265–316 ©2019 Old City Publishing, Inc. Reprints available directly from the publisher Published by license under the OCP Science imprint, Photocopying permitted by license only a member of the Old City Publishing Group. Pulsed Solid State Laser Systems Using ABCD Matrix Method: A Review ,∗ Y. S. NADA1,J.M.EL-AZAB2,S.M.MAIZE1 AND Y. H. E LBASHAR3 1Physics Department, Faculty of Science, Menoufia University, Shebeen-Elkoom, Menoufia, Egypt 2National Institute of laser Enhanced Sciences, Cairo University, Giza, Egypt 3Egypt Nanotechnology Center (EGNC), Cairo University, Giza, Egypt Received: March 12, 2019. Accepted: April 14, 2019. Although the laser diode and fiber laser are better than CW microchip lasers as they have the smallest size, compact design and wide range of emission laser wavelengths, the Q-switched solid state microchip laser is better than laser diode and fiber laser, because the energy storage capacity and laser induced threshold damage of laser diode are less than those of Q-switched microchip laser and long cavity of the fiber laser prevents achieving very short laser pulses. Yb:YAG was chosen as active medium due to its simple energy level scheme, long upper laser level lifetime (951 μs) which is suitable for storing energy, low quantum defects, large emission cross section, and large absorption cross-section for InGaAs laser emission. The combination of Cr4+: YAG and a doped- YAG gain medium, such as Yb:YAG, is particularly attractive from the point of view of an extremely robust device. Since both materials use the same host crystal, YAG, they can be diffusion-bonded to each other in a way that blurs the distinction between a monolithic and a composite- cavity device. Both materials have the same thermal and mechanical properties and the same refractive index, and the bond between them can be sufficiently strong that the composite device acts in all ways as if it were a single crystal. Due to better mechanical and thermal properties compared to the glass and single crystal, transparent ceramics became high power end-pumping lasers candidate in numerous fields. Ceramic active medium can be heavily and homogeneously doped with laser- active ions. Keywords: ABCD Matrix, Pulsed lasers, laser systems, ultra short pulse lasers ∗ Corresponding author: e-mail: [email protected] 265 266 Y. S. NADA et al. 1. INTRODUCTION 1.1. Solid state lasers According to the case of active medium, lasers can be classified into solid, liquid or gas. The first invented laser was solid state laser (Ruby laser) [1]. Laser science has many branches like laser safety, material processing, and measurements, and all these applications depend on the active medium wave- length and switching technique [2–22]. The achieving gain with a given solid state laser system performance, the host material with its unique macroscopic and microscopic properties, the activator ions, and optical pump source should be selected self-consistently [23]. Solid state laser materials must have sharp fluorescent lines, strong absorption band and high quantum efficiency. Solid state laser materials are mainly crystal or glass doped by transitions or rare earth ions, in which optical transitions can occur between states of inner incomplete electron shells, (4f-4f) transition for rare earth and (3d-3d) transi- tion for transition ions doped materials. The laser radiation of solid state laser emits in the range of visible and near –IR spectrum (0.4-3μm) [23, 24]. Solid state laser material can be expressed as a physical system of active ions doped in host materials (crystal or glass). For best understanding of the solid state laser system, it can be approached by interaction among active ions with cer- tain electronic states, that be immersed in the local electrostatic crystal field of the host, a phonon field, and a photon field (see Figure 1) [25]. 1.1.1. Active ions With an efficient absorption of the pump radiation and an efficient emission, the ions will be useful for giving the optical dynamics of laser material at FIGURE 1 Physical system of a solid state laser. PULSED SOLID STATE LASER SYSTEMS 267 FIGURE 2 (a) Weak electron–phonon coupling of RE ions; (b) relatively strong electron–phonon coupling of TM ions described by the configurational coordinate model. certain wavelengths. In the case of ions with excellent absorption and weak emission, or vice versa, the solution is using two ions in the same host mate- rial, the first called sensitizer, which absorbs the pump energy, and the other called activator which emits the laser radiation. This operation takes place by having efficient nonradiative energy transfer from the sensitizer to the activa- tor through the strong overlap of the emission spectrum of the sensitizer and absorption spectrum of the activator [25]. Due to the electronic transitions of the active ions in the local ligand field environment of the host material, the optical spectral properties of a laser material are determined, and the active ions categorized into two main types. The first type is the transition metal ions (TM ions) which have the nonshielded 3d-electrons that can couple eas- ily with the phonons of the surrounding oxygen ligands, resulting in broad (3d-3d) transition bands. The other type are the rare earth ions (RE ions) with 4f-electrons which are shielded by the electrons of 5s and 5p orbitals leading to narrow (4f-4f) transitions, due to the weak interaction with the crystal field produced by the ligands as seen in Figure 2 [25-26]. 1.1.2. Host material For achieving a highly efficient solid state laser material the active medium with a high optical quality should be fabricated with the absence of absorp- tion and scattering centers. Therefore, choosing the host material depends on its good optical properties, as inhomogeneous propagation of light through the crystal, which leads to poor beam quality, comes from the variation in the refractive index of the host material. Also, choosing the host material 268 Y. S. NADA et al. depends on its good mechanical and thermal properties such as, hardness, thermal conductivity and fracture strength. On the other side, the possibility of scaling the growth of impurity-doped crystal with maintaining high pro- duction should be obtained [23, 25-26]. The active ion properties such as: the size, disparity, valence, and the spectroscopic properties limit the number of useful materials for the solid state lasers. So, the crystal must have lat- tice sites that have local crystal fields of symmetry and strength required for achieving the desired spectroscopic properties and can accept ions with long radiative lifetimes with emission cross sections near 10−20 cm2 [23]. The two main groups of host materials are crystalline solids and glasses. Compared to glasses, the crystalline materials have the advantages of higher thermal con- ductivity, narrower fluorescence linewidths, and larger hardness. However, the crystalline materials show problems of poor optical quality and doping homogeneity as well as narrower absorption lines and limited dimension of laser medium. Rods of glasses up of to 1m in length and over 10 cm in diam- eter and disks of up to 90 cm in diameter and several centimeters thick, which are useful in high energy applications, have been produced with higher opti- cal quality [23]. 1.2. Ceramic laser “Ceramics" is gotten from the Greek keramos, meaning porcelain and pottery. Ceramics are composed of randomly oriented microcrystallites, as shown in Figure 3, so the opaque and translucent cement and clay, used often in tableware, cannot be used as a laser medium. It is due to the existence of many scattering sources, such as grain boundary phases, residual pores and FIGURE 3 Schematic presentation of the microstructure of conventional transparent ceramics, light scat- tering and the attenuation of input power through the ceramic body. Strong scattering owing to (28) a grain boundary, (30) residual pores, (32) secondary phase, (33) double refraction, (5) inclusions and (6) surface roughness in ceramics prohibits applications in optics [6]. PULSED SOLID STATE LASER SYSTEMS 269 FIGURE 4 Brief overview of the fabrication process of highly transparent Nd: YAG ceramics. secondary phases, which give rise to considerable scattering losses that block laser oscillation in the translucent ceramic laser gain medium [27]. In 1995 the first, highly efficient Nd: YAG ceramic laser medium with average grain size of 50 μm, was fabricated by a solid state reaction method using a high purity powder (> 99.99 wt% purity) [28]. Also Nd: YAG mate- rial with average grain size of 10 μm [29] and 3-4 μm [30] was made by using Nanocrystalline Technology and Vacuum Sintering method (NTVS) as illustrated in Figure 4. The first demonstration of polycrystalline Nd-doped YAG ceramics with high conversion efficiency of pore-free polycrystalline Nd:YAG ceramics and high optical quality comparable to the commercial one as well as single crys- tals with new structures were fabricated from ceramic with Solid–State Crys- tal Growth (SSCG) method, as illustrated in Figure 5 [31]; exhibiting a high degree of transparency. Dense Nd:YAG ceramic samples were prepared by conventional sintering and post- Hot Isostatic Pressing (HIP) treatment [32- 33]. As a result of these different techniques, transparent ceramics with very high optical quality were obtained, because. On the macroscopic scale these materials show no refractive index fluctuation or double refraction. On the microscopic one no secondary phases, residual pores or optically inhomoge- neous parts are observed [27]. Comparing ceramic composite and composite crystalline Nd:YAG rod used for high power diode end–pumping laser system 270 Y. S. NADA et al. FIGURE 5 Fabrication process of a polycrystalline and a single crystal of Nd:YAG material. [34-35] a maximum CW laser output power of 113 W, and a maximum opti- cal to optical transformation efficiency of 47% were achieved for ceramic rod.