Threshold fluences for conditioning, fatigue and damage effects of DKDP crystals
Introduction
Potassium dihydrogen phosphate (KH2PO4 or KDP) and its deuterated analog (DKDP) are the only nonlinear materials for frequency conversion in current large-aperture, high-power laser systems. Laser-Induced Damage (LID) in KDP has been a major issue since it sets the upper limit of fluence below which a laser system can be operated reliably [[1], [2], [3]]. Much research has been conducted to investigate LID mechanism and to establish methods for increasing the damage performance of KDP crystals [[4], [5], [6], [7]]. Among the methods for increasing damage resistance of KDP/DKDP, laser conditioning by pre-exposure to sub-threshold of damage fluence is very effective and promising for increasing damage resistance of KDP/DKDP crystals [[8], [9], [10]]. Preliminary explanations have been attempted by Feit et al. [11] who suggested that the increase in LIDT is attributed to decreased size of damage precursors due to sub-threshold laser fluence of irradiation. As for the characteristics of laser conditioning, DeMange et al. [12,13] have performed the experimental investigation in detail and some significant experimental phenomena have been observed. It is shown that laser conditioning efficiency becomes increasingly improved as the conditioning fluence is augmented as long as no damage appears throughout the course of conditioning. Their results also show that for a given set of conditioning pulse parameters, the conditioning efficiency increased as a function of conditioning pulse number. However, LIDT was observed to decrease with increasing pulse number even though sub-threshold fluence was utilized to irradiate the materials, which is often called “fatigue effects” laser-induced damage. The fatigue effect has been observed and studied in some transparent materials such as glasses, LBO and KDP crystals [[14], [15], [16], [17]]. The fatigue effect is a real bottleneck for many high power laser systems since the systems operate in multiple pulse mode and materials will be subjected to a large number of laser pulses. A detailed review of this subject concerning experimental data and proposed multi-pulse mechanisms has been presented by Chmel [18]. Multiple pulse laser damage in transparent materials remains incompletely understood and at present there is not a commonly accepted and demonstrated mechanism for the multi-pulse sub-threshold laser damage.
In both laser conditioning and the fatigue effect process, materials are exposed to sub-threshold fluence laser pulses before laser-induced damage occurs. However, laser conditioning increases the LIDT of materials while the fatigue effect decreases the LIDT. To investigate the similarity and difference between laser conditioning and fatigue effect, experiments were performed on DKDP crystal at 1ω(1064 nm) and 3ω(355 nm). In this work firstly, the 1-on-1 LIDT of DKDP at 1ω and 3ω was measured. Then S-on-1 damage tests were carried out to observe the fatigue effect in DKDP; where S represents the number of laser pulses. Finally, laser conditioning experiments were performed to investigate the influence of conditioning fluence and pulse number on conditioning efficiency. The experimental results show that as long as no damage appears during laser conditioning, conditioning efficiency becomes increasingly improved (can increase up to 1.5 times) as either the conditioning fluence is augmented or the pulse number is increased. As to fatigue effects, the LIDT of DKDP decreases (about 30%) with increasing pulse number when DKDP was exposed to multiple pulses in our experiments.
Section snippets
Experimental setup
The samples used in all the experiments in this work are rapid growth DKDP tripler (uncoated), 50 mm × 50 mm × 10 mm. Both surfaces of each DKDP sample were cut by single point diamond fly-cutting technique. The cutting depth was controlled at ∼1 μm while the feed rate of the tool relative to DKDP sample was 60 μm/s so that the cutting process operates in ductile regime and there were no cracks on the machined surface. The samples were cleaned with compressed air to wipe the possible dust. The
1-On-1 damage test
In this experiment, a 1-on-1 damage test was performed at 1ω and 3ω to measure the 1-on-1 LIDT of the DKDP crystal. The testing laser Laser1 was run at 1ω, with a ∼400 μm spot diameter, 10ns pulse duration, 1 Hz frequency; and at tripled 3ω, with a 375 μm spot diameter, 10ns pulse duration, and 1 Hz. Fig. 3 shows the damage probability as a function of testing fluence (fit equations can be found in Refs. [15,16]). According to the results, the 1-on-1 damage threshold of DKDP is at ∼17 J/cm2 at
Discussion
Merkle et al. [18,23,24] investigated the fatigue effect in fused silica and found that there exists a “safe” (non-damaging) fluence at which large number of pulses could produce no macroscopically damage. According to the results of S-on-1 damage test, we believe that there also exists a “safe” fluence for DKDP crystal. Table 1 shows the S-on-1 damage threshold measured at 1ω (1064 nm) and 3ω (355 nm), S = 10, 100, 1000, 2000 and 3000. It can be seen that S-on-1 LIDT decreases with increasing
Conclusion
In this paper, experiments were carried out to observe the phenomena of laser conditioning and fatigue effect in DKDP crystal at 1ω (1063 nm laser) and 3ω (355 nm laser). The experimental results show that conditioning effects becomes increasingly effective (can increase to 1.5 times) as the conditioning fluence is augmented or the pulse number is increased as long as no damage appears during laser conditioning. When exposed to multiple pulses, the S-on-1 LIDT of DKDP decreases (about 30%) with
Declaration of interests
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
The authors declare the following financial interests/personal relationships which may be considered as potential competing interests.
Author agreements
I, Yaguo Li, on behalf of my co-workers, am submitting our MS to the journal Optical Materials. The content in the work have not been published in part or in whole previously. All the authors agree the order of the authors and agree to submit the work.
Acknowledgements
The authors acknowledge the funding of Science Challenge Project (JCKY2016212A506-0503), Foundation for the Development of Science & Technology of China Academy of Engineering Physics (2015B0203032), National Natural Science Foundation of China (51505444), Outstanding Youth Talents Project (2017-JCJQ-ZQ-024), Foundation for Youth Talents of LFRC, CAEP (LFRC-PD012), Funding for Scientific Research Activities of Chinese Returnees.
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Present address: Sichuan Wisepride Industry, Chengdu 611743, China.