Laser Damage Threshold of Ceramic YAG Jean-Franc ¸oisBISSON  ,YangFENG, Akira SHIRAKAWA, Hitoki YONEDA, Jianren LU, Hideki YAGI 1 , Takagimi YANAGITANI 1 and Ken-Ichi UEDA Institute for Laser Science, University of Electro-Communications, 1-5-1 Chofu-Gaoka, Chofu, Tokyo 182-8585, Japan 1 Konoshima Chemical Co., Takuma Works, 80 Koda, Takuma-cho, Mitoyo-gun, Kagawa 769-1103, Japan (Received June 23, 2003; accepted for publication July 5, 2003) Bulk laser damage thresholds of doped and undoped ceramic Y 3 Al 5 O 12 (YAG) materials are reported. These materials were found to resist 100J/cm 2 , 4-ns pulses at 1.064 mm wavelength. Single-crystal YAG materials of similar composition yielded similar damage thresholds. Hence, ceramic microstructure does not contribute to lower damage threshold. Beam-size dependenceofdamagethresholdfluencewasalsostudiedbyrepeatingtheexperimentusingalenswithalongerfocallength. Theevolutionofdamageprobabilitywithlaserfluencewasfoundtostronglydependonthebeamdiameter;however,damage threshold was not found to vary significantly with beam diameter. [DOI: 10.1143/JJAP.42.L1025] KEYWORDS: YAG, ceramics, laser, damage, microstructure, defects Ceramic YAG laser materials have become an attractive alternativetosinglecrystalmaterialsbecauseoftheireaseof manufacture, low cost and scalability. The ceramic YAG fabrication technology 1–3) is now sufficiently mature to produce materials that compare favorably with the best single crystals with respect to key lasing properties. 4–7) Because ceramic materials are easy to fabricate in large sizes, they are particularly well suited for high power applications. Such applications will require the material to withstand high fluences. For instance, design criteria for the laser fusion driver require the laser-induced damage thresh- old (LIDT) to be several times larger than the emission saturation fluence. 8,9) In the nanosecond pulse-width regime, damageisknowntoarisefromdefects,suchasimpurities,or other imperfections that absorb laser radiation or distort the wave front. 10–12) As shown in Fig. 1, ceramic YAG materials, unlike single crystals, are made up of several micron-size grains. Whether grain boundaries contribute to lowertheLIDThasbeensofaranopenquestion.Thiswork intended to provide the first LIDT data for ceramic YAG fromaQ-switchedYAGlaserdeliveringnanosecondpulses. Pulsesof4-nswidthat1.064 mm fromaQ-switchedYAG laser were used for this experiment. Pulse energy was adjusted by using a variable attenuator, consisting of a halfwave plate placed between two polarizers. In order to probe the bulk material and avoid surface damage, tight focusing was required. Lenses of 5-cm or 10-cm focal lengths were used. Beforehand, calibration of the absolute energydensity(J/cm 2 )deliveredtothesamplewasdoneand involved the following steps. First, total pulse energy was monitored by a photodiode receiving a small calibrated fraction of the pulse energy. Then, the beam profile was measured by imaging the beam profile on a CCD camera. The beam profile was found to be nearly Gaussian. Next, a pinhole was adjusted in the middle of the beam waist to measure the average energy density inside an area smaller than the beam diameter. Finally, information on the average energydensityandbeamprofilewerecombinedtoobtainthe maximum on-axis fluence. Beam quality was also estimated beforehand, by measuring the beam divergence: the M 2 parameter 13) value was about two in the both transverse directions. Damage was detected by using a He-Ne laser beam focused on the irradiated site: onset of damage was clearly visible since it caused most of the probe beam to be scattered. Occurrence of damage, as deduced by scattering, was confirmed afterward by observing the irradiated sites under a microscope. The range of fluence levels was chosen as to cover 0% to 100% damage probability. Because of the statistical aspect of damage at the nanosecond scale, at least 30 sites were irradiated at each fluence level, and up to 200 sites were irradiated at fluence levels yielding low damage probabilities. A damage probability at given fluence level wasestimatedfromthefractionofdamagedsitesobtainedat some fluence level. Each site received only one pulse, for each of which the pulse width and pulse energy were monitored. Interval between irradiated sites was 300 mm. Undoped and 0.7%-Nd doped, 1-cm thick polished ceramic samples, were provided by Konoshima Chemical Fig. 1. Pictures of a typical ceramic YAG grain boundaries obtained by scanning electron microscopy (Fig. 1(a)) and Transmission Electron microscopy (Fig. 1(b)).  E-mail address: Bisson@ils.uec.ac.jp Jpn. J. Appl. Phys. Vol. 42 (2003) pp. L 1025–L 1027 Part 2, No. 8B, 15 August 2003 #2003 The Japan Society of Applied Physics L 1025