Large aperture N31 neodymium phosphate laser glass for use in a high power laser facility Download: 1318次
1. Introduction
Large aperture Nd:phosphate laser glass is at the heart of a high power laser system. For high peak power inertial confinement fusion (ICF) facility application, there are many strict technical requirements on laser glass, such as high gain, low nonlinear refractive index, low attenuation at laser wavelength, excellent optical homogeneity, and large laser damage threshold. Large aperture Nd:phosphate laser glasses have been successfully applied in the NIF facility in the United States, and an over 2 MJ ultraviolet laser has already been realized in this largest laser facility[1, 2]. Phosphate glass has good solubility to rare earth ions, medium phonon energy, low nonlinear refractive index, strong ability to resolve platinum particles, and good spectroscopic properties for ions compared with silicate glass. Since the late 1970s, Nd:phosphate laser glass has been developed for high peak power laser facility use. LHG-8 and LG-770 Nd:phosphate laser glasses, which are used in the NIF facility in the United States, were developed and fabricated by Hoya and Schott, with continuous melting technology[3]. KGSS-0180 Nd:phosphate glass was developed in Russia, and it has been used in the four-channel laser system “Luch”[4]. In China, N21 and N31, two types of Nd:phosphate laser glass, have been used in high power laser facilities. N21 Nd:phosphate laser glass was developed in the 1980s in Shanghai Institute of Optics and Fine Mechanics (SIOM), and it was used in Shen Guang I and Shen Guang II high peak power laser facilities[5, 6]. N31 Nd:phosphate laser glass was developed in Shanghai Institute of Optics and Fine Mechanics in the 1990s. It has been applied in the Shen Guang series of high power laser facilities in China for more than ten years[7]. In recent years, different technologies concerning the mass fabrication of large aperture N31 laser glass have been explored[8]. In this paper we present the composition, main properties, and fabrication techniques of large aperture N31 glass.
Table 1. Main parameters of neodymium phosphate laser glasses from Hoya[1], Schott[1], Russia (GOI)[4, 9], and SIOM.
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2. Compositions and properties of N31 phosphate laser glass
Similar to other neodymium phosphate laser glasses used in an ICF facility, N31 glass is a kind of metaphosphate glass composed of . represents an alkali oxide. MO represents an alkaline oxide. represents mixtures of , , and . Up to 5wt% can be easily doped in N31 glass without obvious change of properties besides density and refractive index. Its composition is satisfied for the requirements of mass fabrication and laser facility applications.
A comparison of the main basic properties of N31 with those of LHG-8, LG-770, and
KGSS-0180 glasses is given in table
The stimulated emission cross section of N31 glass is higher than that of LHG-8 glass
and close to that of LG-770 glass. In order to suppress the damage from self focus due
to the optical nonlinear effect at high peak energy fluence, the nonlinear refractive
index n of neodymium phosphate laser glass should be controlled. From
Table
The laser gain curves of N3122 and N3130 glasses, with 2.2wt% and 3.0wt% doping concentration, were detected at various xenon lamp pumping
voltages. The fluorescence lifetime and optical loss at 1053 nm will have an
important influence on the small signal gain. Only samples with an optical loss of 0.1–0.15% and a lifetime of 340 s (for 2.2wt%) or 320 s (for 3.0wt% ) were chosen for measurement. The results, shown in
Figure
Fig. 1. Gain profiles of N3122 and N3130 glass rods of size 8 162 mm at various xenon lamp pumping voltages. N3122 and N3130 correspond to the glasses with concentration of 2.2 wt% and 3.0 wt%.
3. Melting technologies of 400 mm large aperture N31 phosphate laser glass
It is well known that most of the key parameters of laser glass such as fluorescent lifetime, number of platinum inclusions, bubble and optical homogeneity, birefringence, optical attenuation at lasing wavelength, residual , and absorption at 400 nm are determined by the fabrication technology. The fabrication process of a laser glass slab includes melting, forming, rough annealing, fine annealing, and edge cladding. The fabrication technology, especially the melting technology, is very important in ensuring the quality of laser glass. An N31 glass rod with diameter 90 mm and a slab with a clear aperture of 400 mm have been fabricated in SIOM.
The melting technology of N31 glass has been explored since the mid 1990s, and several fabrication technologies concerning pot melting of N31 laser glass were developed in early 2000. A patented pot melting technology has been established instead of traditional two-step melting[10]. The glass melt was directly flowed into a platinum crucible to refine and homogenize after the primary melting and dehydration in a refractory crucible. This patented pot melting method ensures a lower degree of contamination by impurities and high efficiency of dehydration.
High purity raw materials with Fe, Cu, Cr, Ni, and V trace elements (less than 3 ppm) have been domestically fabricated. Through the controlling of the purity of raw materials and melting processing, the total amount of transition metal oxides is less than 10 ppm in N31 glass. Research has been done on the effect of Fe and Cu impurities on the optical attenuation of N31 glass[11, 12]. It is found that the most harmful trace element is copper ions, which can seriously affect the optical absorption at laser wavelength even at an amount of only several tens of ppb in glass[12]. The high purity raw materials and patented pot melting technology make the attenuation at lasing wavelength lower than 0.15% . Using the reactive atmosphere processing (RAP) dehydration method[6, 13], the absorption at 3000 can be controlled to be less than in N31 glass produced by pot melting. Platinum inclusions were removed by controlling the redox condition of glass melting in order to ionize the metallic platinum[14].
The pot melting efficiency is too low to manufacture thousands of laser glass slabs. Since 2006, research on continuous melting technology of N31 glass has been carried out. The continuous melting technology of large aperture laser glass is more complicated than that of traditional optical glass due to its special technical parameters. Based on the matured laser glass fabrication technology of pot melting[10] as well as theoretical and small scale experimental modeling[15, 16], we successfully developed a continuous melting technology for N31 laser glass[17].
The continuous melting line of N31 laser glass consists of an interconnected melter,
conditioner, refiner, homogenizer, forming, and annealing lehr. Figure
Fig. 2. Large size N31 glass running from the annealing lehr of a continuous melting line.
Table 2. Parameters of mass production N3135 glass.
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Table
In the measurement of the single shot laser induced bulk damage threshold (bulk LIDT), ISO 11254-1 was taken as a standard[18]. Because the polished surface is the weakest part of the glass sample under laser radiation, the laser beam was focused into the sample by a lens with 100 mm focal length to evaluate the bulk LIDT. The spot size was measured with a beam analyzer, and the effective spot area was about . The test error of the spot size is about 25% because of the highly convergent beam. The laser energy on the target was controlled by an attenuator, and the pulse energy was recorded for each shot by an energy meter from a split-off portion of the beam. The fluctuation of output energy was 3%. A He–Ne laser was used to monitor the glass sample, and high sensitivity on-line damage detection was performed by using a microscopy system. Under the condition of 12 ns pulse width, the damage threshold is around 85.2–86.2 . Accordingly, the calculated threshold for N3135 neodymium phosphate laser glass is around 42.6-43.2 under a 3 ns laser radiation.
Fig. 3. Gain coefficient of 400 mm aperture N3135 glass slabs from pot melting and continuous melting.
Figure
A typical 633 nm transmitted wavefront of 400 mm aperture N3135
laser glass, which was measured by Zygo interferometer with a test aperture of
600 mm, is shown in Figure
Fig. 4. 633 nm transmitted wavefront of a N3135 glass slab of size 810 460 . (a) The left part of 400 and (b) the right part of 400 .
4. Edge cladding of N31 phosphate laser glass
Edge cladding is an important technology to suppress the amplified stimulated emission ASE and to ensure the gain properties of large size Nd:phosphate glass. ion doped phosphate glass with a precise refractive index match to N31 laser glass has been designed as a cladding glass. A kind of self-developed epoxy adhesive agent with precise refractive index match to both the laser glass and the cladding glass is used to bond these two glasses. It provides an adhesive strength of 18 MPa. This adhesive agent has been tested to be highly resistant to high intensity pump and laser power as well as humid environments in the polishing process.
The doping level is limited by the temperature rise at the interface between the cladding glass and the laser glass. This temperature rise is due to strong absorption of ASE energy of a laser pulse. The temperature rise after a laser shot can be expressed by
A patented edge cladding technique has been developed for large aperture N31 laser glass[20]. It has been applied in the fabrication of large aperture N31 laser glass slab. More than 10,000 shots of high energy fluence have been applied in our edge cladding N31 glass slab in the Shen Guang facility.
Fig. 5. The stress distribution of 400 mm aperture N31 glass slab before and after cladding.
The residual stress caused by edge cladding is kept small by the proper choice of
adhesive agent and its curing parameters. Figure
Figure
5. Conclusions
The main composition and properties of N31 Nd-doped phosphate laser glass are reported. Three key techniques for laser glass fabrication (pot melting, continuous melting, and edge cladding) have been developed in Shanghai Institute of Optics and Fine Mechanics. The glass parameters and laser gain of N3135 phosphate glass produced by continuous melting are almost the same as those obtained by pot melting. 400 mm clear aperture N31 glass slabs with high quality have been fabricated for building the Shen Guang high peak power laser facilities in China.
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Article Outline
Lili Hu, Shubin Chen, Jingping Tang, Biao Wang, Tao Meng, Wei Chen, Lei Wen, Junjiang Hu, Shunguang Li, Yongchun Xu, Yasi Jiang, Junzhou Zhang, Zhonghong Jiang. Large aperture N31 neodymium phosphate laser glass for use in a high power laser facility[J]. High Power Laser Science and Engineering, 2014, 2(1): 010000e1.