Recently, YCa4O(BO3)3（hereinafter referred to as YCOB）crystal with diameter of 4 inches was successfully grown in Shanghai Institute of Ceramics, Chinese Academy of Sciences (SICCAS). The crystal was in length of 100 mm with the diameter of 4 inches. The boule was a complete single crystal without defect, and could be cut into clear aperture of OPCPA element with size of 90 mm×90 mm which fulfilled the nonlinear optical quality requirements. This is maximum aperture of nonlinear optical elements ever reported among crystals grown by melts growth method. At present, the nonlinear optical crystal which can provide maximum aperture is KH2PO4 (referred to as KDP), grown by aqueous solution method, has been used in laser fusion projects of several countries, the largest device made is in size of 430 mm×430 mm. But its nonlinear optical properties and physico-chemical stability are inferior to YCOB crystal.
YCOB crystal was invented in 1996 by Aka et al of French National Center for Scientific Research. Later, it attracted new attention after researchers found that YCOB had excellent non-linear optical properties, high temperature piezoelectric properties and good chemical stability. Compared with another non-linear optical crystal LiB3O5 (LBO) which is commonly used in high power laser system, YCOB has many advantages as follows: firstly, this crystal is not deliquescence; secondly, its non-linear optical coefficients are 37.5% larger than that of the former; thirdly, its temperature acceptance is 22 times wider; fourthly, although its laser-induced damage threshold is similar to the former, parametric fluorescence is three orders lower; finally, the machinability of YCOB is better with less anisotropy and one order lower of thermal expansion coefficient. YCOB crystal can be grown by Czochralski method, so large aperture YCOB non-linear optical element for high power laser could be fabricated at low cost because of short growth cycle. YCOB non-linear optical elements with large aperture can be used as SHG, THG, OPO, OPA and OPCPA crystals in high energy and high power laser system.
From 2010, researchers in SICCAS made breakthroughs on growth technologies of YCOB crystal with diameter of 1 inch, 2 inches and 3 inches step by step. In 2011, Non-linear optical element with aperture of 60mm×60mm was cut from the YCOB boule, and had been successfully applied to joule-level and broadband non-collinear optical parametric chirped-pulse amplification centered at 800 nm. An amplified signal energy of 3.36 J was generated with compressed pulse duration of 44.3 fs. The results confirm that YCOB crystal is another potential alternative as a final amplifier besides Ti:sapphire in a petawatt laser. In order to generate higher average power of laser, nonlinear optical element with larger aperture is needed, so crystal researchers are requested to grow larger size YCOB both in diameter and in length.
During the study of growth technology of 4 inches YCOB, since both the diameter and length of the crystals are increased, the time needed for growth and the difficulty greatly increased. Through hard work in research, the researchers explored a set of effective comprehensive technology combining various processes, conquer the issues of cleavage, spiral growth, inclusion produce ,etc., made the crystal quality greatly improved, the optical uniformity arrived at 1.48×10-6, the laser induced damage threshold at 10 J/cm2 (532 nm, 3 ns). And finally the 4 inches crystal to cut elements with aperture of 90 mm×90 mm was obtained. Further explore to grow larger size YCOB crystal is possible based on this progress.
Fig.1 The optical homogeneity of YCOB crystal grown in SICCAS
