solid‐state lasers, research, and development on various lasers have been carried out with the discovery of ruby laser by Dr. Maiman in 1960 as a trigger. Among them, continuous‐wave laser oscillation at room temperature using Nd:YAG single crystal by Dr. Guseic in 1964 and tunable ultra‐short pulse laser using Ti:Sapphire single crystal by Dr. P. F. Moulton in 1982 are technically excellent, and it is still applied industrially.
Solid‐state laser gain medium is used as a so‐called “coherence converter of light” that converts white light into coherent light; hence, the obtained yield (conversion efficiency) of coherent light is obtained with respect to excitation light energy decreases. However, the important thing is the nature of coherence light. As the energy density is remarkably high when collimated, and even small energy can be applied to high‐precision processing technologies such as laser marking and cutting. Since the laser amplifies the coherent light inside the gain medium while injecting the excitation light, very low optical scattering and very good optical uniformity are required as a laser gain medium. For this reason, glass or single crystals have been used conventionally. Since the former can synthesize a large laser gain medium, it is generally used for drivers in laser fusion, but because of its low thermal conductivity, it is difficult to repeat lasing and continuous oscillation of the laser. Single crystal materials are excellent in thermo‐mechanical properties and can be easily used for industrial laser because they can oscillate in continuous‐wave and pulsed laser mode. However, it is difficult to manufacture gain media on large scale, and it is also difficult to generate large output power.
When a transition metal element such as Cr, Ti, etc. or a lanthanide rare earth element such as Nd, Yb, Er, etc. is added to a laser gain material, these laser‐active elements absorb light energy of a specific wavelength and then emit fluorescence. Strong monochromatic light obtained by stimulated emission of the fluorescent rays inside the optical resonator is laser. Conventionally, white light such as a flash lamp or an arc lamp was used as an excitation source, but the absorption wavelength band of the laser active element in the laser host is very narrow and only a very small part of the white light can be converted into laser light. Therefore, the conversion efficiency was very low.
In recent years, by using a semiconductor laser (for example, a GaAlAs type laser having a wavelength of 808 nm that can excite an Nd type laser) which can excite particularly the absorption wavelength band of the laser‐active element at pinpoint, light (excitation light)‐to‐light (Laser in gain media) conversion efficiency can be significantly improved. For example, the conversion efficiency in the case of Nd:YAG single‐crystal medium by conventional lamp excitation is about 3%, whereas by the semiconductor laser excitation system high efficiency up to 60% level can be achieved. It is a common knowledge to say that the solid laser gain medium is “optical grade single crystal with extremely low scattering.” Therefore, the suggestion of a sintered body (ceramics) to use in high‐tech optical technology was out of the question. Can you imagine how the ceramic including numerous scattering sources such as residual pores, secondary phases, and grain boundaries, etc. was able to use as a laser gain medium? It was a reckless challenge until it can contribute to laser science. Details are described in this chapter.
2.2 Principle of Laser Generation
2.2.1 Spontaneous Emission
First, the fundamental part of the laser such as fluorescence generation and stimulated emission phenomenon will be explained. Transition metals such as Cr, Ti, Co, etc. or lanthanide rare earth elements such as Nd, Yb, etc. which participate in light emission are added to a general solid‐state laser gain medium. The light emission of Nd and Yb is described as a typical example relating to these laser emissions. Yb and Nd are known to be typical three and four‐level lasers, respectively.
Figure 2.1 shows an energy diagram of three and four‐level lasers. In a three‐level laser, when Yb electrons in the ground state are excited with 940 nm LD light, electrons are pumped up to the excitation level in Figure 2.1a, and the excited electrons are electronically transited to 2F5/2 at the lower level. Spontaneous emission (fluorescence emission) occurs from this level, but electrons return to the lower level 2F7/2 (ground state). This electron transition is called three‐level laser because it transits from the ground state ⇒ excitation ⇒ laser transition level as three steps. Due to the progress of LD (laser diode) technology in recent years, the LD of 970 nm has also been developed, and quantum limit efficiency (energy of generated laser/energy of input LD light) becomes 91% when excited with this LD.
Figure 2.1 Schematic energy diagram of (a) three‐level laser (Yb‐doped) and (b) four level laser (Nd‐doped).
Nd system is called a four‐level laser because it has four stages: ground state ⇒ excitation level ⇒ upper level ⇒ lower level ⇒ ground state (see Figure 2.1b). The laser cannot oscillate unless a population inversion (described later) finally occurs. In the case of a three‐level laser, the lower level is equal to the ground state; and in order to trigger population inversion, it is necessary to raise more than half of the doped Yb atoms to the excitation level, so that strong excitation is required. Therefore, LD excitation is an effective means. On the other hand, in the case of the Nd four‐level system, since the lower level is not in the ground state, the population inversion is easily generated, and laser can be easily generated even with a flash lamp. In the case of Nd, when the electrons in the ground state are excited to 4F5/2, the excited electrons are automatically firstly transferred to 4F3/2 to generate fluorescence. Since Nd has three lower levels, the electrons excited at 4F3/2 are transferred into three different lower levels; 4F3/2 ⇒ 4I9/2 (emission at 946 nm), 4F3/2 ⇒ 4I11/2 (emission at 1064 nm), and 4F3/2 ⇒ 4I13/2 (emission of 1.3 μm). The transition probabilities are 0.25, 0.60, and 0.15, respectively. Since the Nd system has four levels, strong excitation is not required, and a laser can be easily generated even with a xenon lamp excitation. However, since it is white light, the energy conversion efficiency is as low as about 3%. As in the Yb system, an 808 nm LD can pump electrons to 4F5/2 energy level in the Nd system; recently, it has become possible to pump directly to 4F3/2 energy level with an 885 nm LD to increase the pumping efficiency. Since the quantum limit efficiency of the Nd system is 74%, although laser generation is easy, there is a drawback that the efficiency is inferior to the Yb system.
2.2.2 Stimulated Emission and Laser Generation
As described above, fluorescence can be generated at four levels or three levels, but this fluorescence is merely a spontaneous emission phenomenon. LASER stands for “Light amplification by stimulated emission of radiation,” so it is necessary to understand how stimulated emission and light amplification are performed. When a laser gain medium is irradiated with a white light (flash lamp), electrons present in the laser element are brought into an excited state to generate fluorescence (spontaneous emission). As shown in Figure 2.2a, if a reflecting mirror (100% reflection) and an output mirror (reflectance 98–90%) are provided on the opposite surfaces of the laser gain medium (both ends are optically polished), spontaneous emission in random directions will occur. A part of the spontaneous emission light has directionality (that is, reciprocal reflection toward mirrors installed at both ends), and when the excitation light is irradiated from the outside, the spontaneous emission
