Читать книгу Processing of Ceramics - Группа авторов - Страница 23

Figure 2.4a is a photograph of the world's first highly efficient laser oscillation using the Nd:YAG ceramic developed by the author in December 1991. On this day, because the CW (continuous oscillation) equipment was malfunctioning and also the sample was not optically polished and was not AR (Anti‐reflection) coated, an 808 nm LD light was input from the end surface of the disk‐shaped ceramic sample, and Ar gas laser excitation was applied from the side. Then, the Quasi‐CW laser was oscillated by using this setup. A pulsed oscillation signal was obtained from the oscillator, and this became the world's first invention of a high‐efficiency ceramic laser. Figure 2.4b shows the ceramic sample tested with a CW laser oscillator at a later date, and the slope efficiency was 28%. The optical loss of this ceramic was about 0.9%/cm @ 1 μm, but at that time this sample was an unprecedented ultra‐high‐quality ceramic. Since the laser gain length was not sufficiently long, the leakage of LD light was also large, and the excitation technique at that time was also insufficient, so the slope efficiency could reach only 28%. However, having the same laser oscillation characteristics comparable to the commercially available Nd:YAG single crystal which was used as a reference means that the material properties of these ceramics are “the highest level” as a laser gain medium at that time. There is no doubt about this. It is not a mistake that confirming the first laser oscillation here is 1991, because the publication in Japan was in 1994, and the publication in the US journal was in 1995. Since the late 1990s, the interest and importance of ceramics lasers have been recognized in the US, and research on ceramics lasers has been conducted worldwide. Representative results presented by the author and others are described in the following.

Figure 2.4 (a) First demonstration of Q‐CW laser oscillator using Nd:YAG ceramics in December 1991 and pulse laser oscillation spectrum from Nd:YAG ceramics. (b) Incident power vs. output of cw‐laser from Nd:YAG ceramics pumped by diode laser.

Figure 2.5a shows the oscillation characteristics of 7%Yb:YAG ceramics. Although the quantum limit efficiency of this material is 91%, the slope efficiency reaches 84% even when excited by a laser diode (LD) at 940 nm [5]. This material has a thickness of about 1 mm, and there is a leakage of pumped LD light at 940 nm; hence, if the horizontal axis is regarded as an absorbed power, the slope efficiency may reach nearly 88%. This means that the optical loss of the material is very small, and high laser conversion efficiency has been realized. Figure 2.5b shows a laser oscillation test using a thin‐disk type 10%Yb:YAG (Ф12 × t0.15 mm). The maximum output achieved is 1.8 kW, and the slope efficiency is 74%, which is very high [6].

Figure 2.5 (a) Laser performance of 7%Yb:YAG ceramics pumped by 940 nm LD.

Source: Pawlowski et al. [5].

(b) High‐power laser generation of 10%Yb:YAG thin‐disk ceramics without sintering additive.

Source: Ikesue and Aung [6].

By the way, a trace amount of SiO₂ sintering aid is added to almost all garnet type laser ceramics developed in the past. The sintering aid is very effective for making the material transparent, lowering the sintering temperature, and shortening the sintering time, but it also has disadvantages. The author pointed out the disadvantages of the sintering aid, especially the problem of heat generation (especially for high‐power laser operation) in the wavelength region of 1 μm or longer. Laser ceramics that do not use sintering aids are ideal, but other than this result [6, 7] has not been reported. Due to the development of high‐purity ceramics, recent research has confirmed that the output exceeds 5.5 kW from a single Ф10 × 0.15 mm thin disk (excitation area is Ф8 × 0.15 mm) although it is a part of an unpublished work. In addition, the power density (output per 1 cm³ of gain medium) reaches about 0.36 MW/cm³ (nearly 1 MW/cm³ in the latest case), it was just the era of super high output power by ceramic laser technology. Although there are reports (literature) that laser oscillation of 10 kW was successful using an Yb:YAG single crystal from a thin‐disk shape [8], it is necessary to investigate with the same resonator when comparing laser characteristics between the laser materials. In our experiments, it was possible to generate 1 kW laser even with a single crystal material, but we confirmed that cracks occasionally occurred during oscillation. Although the grain boundaries of ceramics have been considered disadvantageous for optical properties, this is also simple speculation. On the contrary, we must also consider the development of a laser gain medium with strong laser damage property using the grain boundaries inherent in ceramics.

Figure 2.6a is a photograph of the appearance of 0.6%Nd:YAG (slab) ceramics before polishing. This slab was made by bonding three samples of 55 × 55 × 6 mm size. The slab after polishing is 40 × 155 × t2.5 mm, and both surfaces are polished at the Brewster angle (the angle where there is no surface scattering when the laser passes through the gain medium). Figure 2.6b shows the input/output characteristics of the laser when this material is side pumped by a high‐power LD of 808 nm. The maximum output was 4.3 kW, and the slope efficiency was 37%. Although this slab is a monolithic structure fabricated by diffusion bonding, the bonding interface after this 4.3 kW laser generation test was no damage at all. From this result, it is shown that the bonding cross‐sectional area with respect to the maximum output of the ceramic laser gain medium can sufficiently withstand a laser power density of 4.3 kW/cm².

Figure 2.6 (a) Appearance of 0.6%Nd:YAG slab ceramics (165 × 55 × 6 mm) which bonded three pieces of Nd:YAG ceramics with 55 × 55 × t6 mm. (b) Input of 808 nm diode laser vs. output powder of 1.06 μm laser from ceramics.

In garnet‐based materials, disordered solid solution gain media are interesting materials. Figure 2.7 shows the emission spectrum of a laser medium with Nd:Y₃Sc_xAl_5−xO₁₂ composition in which the x value is serially changed from 0 to 2.0. That is, if Al sites in the YAG material are substituted with Sc, spectral shift, and broadening occur. This spectral shift is very effective for making the laser tunable and short pulse oscillation.

Figure 2.8a shows a setup of the laser oscillator for 15%Yb:Y₃Sc₁Al₄O₁₂ ceramic (Ф10 mm, t = 1 mm) which was used as the laser oscillation medium, and a commercially available Ti:Sapphire laser was used as the excitation source. Figure 2.8b shows the CW laser oscillation characteristics obtained from the laser oscillator. The efficiency depends on the transmittance of the output coupler, and a maximum slope efficiency of 72% was obtained at T = 20%. Figure 2.8c shows an oscillation wave type of a short pulse laser obtained by mode‐locked oscillation. The oscillation wavelength width was 4.1 nm, and the pulse width was 280 fs. The spectral linewidth of Yb:YSAG ceramics is wider than Yb:YAG (similar to Nd:YSAG), making it easier to generate short‐pulse lasers.

Figure 2.7 Emission cross sections of Nd:Y₃Sc_xAl_5−xO₁₂ ceramics (x = 0.3–2.0) and Nd:YAG single crystal (x = 0) at room temperature.

Source: Sato et al. [9], Image courtesy of The Ceramic Society of Japan.