Chapter 4 Characterization of Cr 4+ :YAG crystal fibers
4.2 Composition analysis
4.2.1 Single crystal fiber
Since doping concentration influences the absorption coefficient, an EPMA (JEOL JXA-8900R) was used to measure the doping profile across the fiber cross section.
Table 4.2 uses the different absorption coefficients of various samples obtained from CASIX. After EPMA measurement, the average doping concentrations of Cr4+:YAG raw material corresponded to various source rods were shown in the third and forth columns.
Table 4.2. Average doping concentration of Cr
4+:YAG raw materials.
Raw material dimension of Cr4+:YAG (mm3)
Absorption coefficient, α at 1064 nm (cm-1)
Ave. Cr2O3
concentration (wt.%)
Ave. CaO concentration
(wt.%)
2 × 2 × 30 4.5 0.226 0.018
2 × 2 × 30 2.5 0.172 0.005
2 × 2 × 30 0.5 0.141 0.005
0.5 × 0.5 × 30 > 4.5 0.237 0.038
0.3 × 0.3 × 30 > 4.5 0.269 0.036
Figure 4.9 shows doping profile with multiple regrowth with the same raw source rod. It started from a 2-mm raw source rod (α = 4.5 cm-1) with a 3 mm/min growth speed. For 0.12 mm diameter fiber, multiple regrowth processes were performed. The doping profile of raw source rod is uniform. After the first growth, the doping profile of fiber cross section with 0.92 mm diameter had a gradient distribution. The doping concentration of Cr ions at the fiber center is lower than that at the edge. On the contrary, the doping concentration of calcium becomes higher near the center portion.
This phenomenon is attributed to the signs of segregation coefficients. The average doping concentration of fibers with multiple regrowths were listed in Table 4.3. Only a slight decrease in the average CaO concentration was observed, typically less than 10%, but the average Cr2O3 concentration was decreased quickly when reducing the fiber diameter. In addition, the doping profiles of fiber cross sections with different growth speeds after the first growth were shown in Fig. 4.10. It shows that lower growth speed results in flatter distribution compared to higher growth speed. Here the fiber diameters are all 0.92 mm. From the measurement of about 30 samples with different growth conditions, an empirical formula was derived for the average Cr2O3
concentration of the crystal fiber after growth,
( )
1 2 1368 0
3 0
2
/ /
/
. C v r D
CCrO =
, when v/r <1.7 (4.11)
,where C0 is the average Cr2O3 concentration of the raw material. v and r are the growth speed and pull/push speed ratio, respectively. D is the diameter of source rod.
Figure 4.11 shows the fitting result using Eq. (4.11). It has good match to experiment data. Therefore, crystal fiber with higher growth speed and lower pull/push speed ratio will have higher chromium concentration due to the chromium ions have less diffusion time from the source rod to the fiber. Also, crystal fiber grown with a larger diameter of the source rod will have higher chromium concentration due to the chromium ions have longer diffusion time from the molten zone center to the circumference and then evaporating to the air.
0 20 40 60 80 100
Table 4.3. Average doping concentrations of Cr
4+:YAG crystal fibers.
Cross section of Cr4+:YAG
Fig. 4.10. Doping profiles of fiber cross section with various growth speeds.
1.0 0.8 0.6 0.4 0.2 0.0
Curve of empirical equation
Cr 2O 3 concentration (wt.%)
Fiber diameter (μm)
Fig. 4.11. Average Cr2O3 concentration of fibers with different multiple regrowths and the curve of empirical equation.
4.2.2 Double-clad crystal fiber
The compositions of Cr:YAG-silica single-clad fiber (SCF) and Cr4+:YAG DCF were also measured by the EPMA. Figure 4.12 shows the major composition of the end-face line scan. In Fig. 4.12 (a), the cladding of the fiber is almost SiO2 (> 99.9 wt.%), but the core area is composed of mixtures of SiO2 and YAG. The average SiO2
concentration in the core area is around 64.9 wt.%. By the conservation of mass, the radius of inter-diffusion core rw can be estimated as follow
( )
where rw0 is the core radius of inserted YAG without inter-diffusion, w and (1-w) are the average weight percent of SiO2 and YAG in the core area, respectively. The densities of fused-silica, df, and YAG, dY, are 2.20 and 4.56 g/cm3, respectively. The core diameter of the SCF is 99 μm as shown in the Fig. 4.12 (a), and the estimated core diameter is 103 μm.
(a)
0 20 40 60 80 100
0 20 40 60 80 100
Concentration (wt.%)
Percent diameter (%) SiO2 Y2O3 Al2O3
0 20 40 60 80 100
0 20 40 60 80 100
Concentration (wt.%)
Percent diametr (%) SiO2 Y2O3 Al2O3
(b)
Fig. 4.12. (a) and (b) are the major composition of the Cr4+:YAG-silica SCF and Cr4+:YAG DCF by EPMA measurement. The inset images are the polished end face.
Figure 4.12 (b) shows the distribution of major compositions by line scanning the DCF cross-section. The inset shows the polished end face of the DCF. The core, inner, and outer cladding diameters are 25 μm, 100 μm, and 320 μm, respectively. The measurement result shows that the compositions of core and outer cladding are almost YAG and SiO2, respectively. But the compositions of the inner cladding are mixtures of SiO2 and YAG. Slight increase of the SiO2 concentration near the core is due to the softening point of fused silica is much lower than the melting point of YAG so that the in-diffusion of SiO2 is blocked at the inner cladding-core boundary. The average SiO2
concentration in the inner cladding is 39.8 wt.%, which can be used to estimate the
inner cladding diameter when the core diameter is known. Using the lever rule, the inner cladding radius, ri, can be expressed as follows,