The loss spectra of slab lightguides with Yb-activated P-doped silica cores are studied in the 320-1050 nm wavelength range. Two or three layer lightguiding structures were synthesized by surface-plasma chemical vapor deposition (SPCVD) of doped SiO2 on the inner surface of a substrate silica tube. The glass in the first and the third layer contained ~3.5 mol. % fluorine and functioned as reflecting cladding. The second (intermediate) layer contained ~0.5 mol. % phosphorus and ytterbium and served as a lightguiding core. Slab lightguides 20 mm in length were cut lengthwise from the wall of the substrate tube with the deposited silica layers before and after their fusing by external heating in the flame of an oxygen-hydrogen burner at a temperature of ~1600°C. The fusing of the synthesized SPCVD amorphous silicon dioxide with phosphorous leads to an increase in the size of clusters constituting YbPO4, which is responsible for the significant increase of the scattering loss of the lightguides.
© 2015 Optical Society of America
The uniformity of the distribution of an activator in a host material is of great importance for the characteristics of solid-state lasers. The problem of doping uniformity becomes especially important in the case of large concentrations of a poorly soluble activator incorporated into the host material due to increases in the cooperative quenching effects of the excited laser level.
The solubility of rare earth oxides in silica is low. However, due to the ability to manufacture silica-based optical fibers and fiber lasers, the application of amorphous silicon dioxide as a host for rare earth oxides is of considerable interest. Adding aluminum and/or phosphorous to silica increases the solubility of rare earth elements . Even in this case, the maximum concentration of rare-earth activators, which would not lead to degradation of the laser parameters, remains significantly small compared to phosphate and tellurite glasses [2–4]. The latter have much lower softening temperatures compared to silica but are less suitable for fiber fabrication. For instance, P2O5 has a melting point 562 °C and TeO2 has a melting point of 733 °C, but the melting point of SiO2 is 1713 °C .
Among the rare earth activators, ytterbium (Yb) has received the most attention. Since the first examples of Yb3+ fiber lasers, their maximum output power demonstrated in experiments has constantly grown and has reached ~10 kW for single mode operation . The possibility of this success is due to the relative simplicity of the energy spectrum of the 4f-electrons in Yb3+ ions, which eliminates many unwanted ways for electron relaxation from a metastable to the ground state  that are intrinsic to other rare earth ions. The absence of alternatives for upconversion transitions in the energy spectrum as well as the small value of the quantum defect in the photoexcitation of the metastable state enable the characteristics of Yb-doped laser materials . At the same time, to create more powerful fiber and waveguide lasers, it is desirable to further increase the concentration of ytterbium in glass, which would reduce the length of the active part of the waveguide and thus reduce the negative influence of the nonlinear effects, which are significant at high densities of optical power.
In thermodynamic equilibrium technologies, the increase of rare earth oxide concentrations in silica is limited by the process of clustering and the increasing probability of cooperative relaxation processes, which in most cases have adverse effects . Therefore, the search for technology to allow a more homogeneous distribution of laser ions in silica at higher concentrations is a topical task.
The plasmachemical technology of silica synthesis by surface-plasma microwave discharge at low pressure, SPCVD , allows transparent, uniformly-doped amorphous silicon dioxide. The glass network formation in the SPCVD technology is the result of chemosorption of diatomic molecules of oxides synthesized in the plasma from halides at substrate temperatures below the temperature of the corresponding glass melt. In SPCVD, a glassy layer is formed, avoiding the stage of melting. It has been shown previously [11, 12] that for erbium ions embedded in silica by SPCVD, there is a subsequent fusing stage, which leads to the formation of the clusters, which was confirmed in experiments on the impact of fusing on the spectra and kinetics of the main (2I13/2 → 2I15/2) and upconversion luminescence of Er3+ ions.
In this paper, we present and discuss experiments on the impact of the fusing stage on the light scattering and absorption in an Yb-doped, silica host with phosphorus, synthesized by SPCVD. A critical point for the type of structure formation of phosphates in general and ytterbium phosphate in particular is the relative concentrations of oxygen and phosphorus . If the oxygen-to-phosphorous ratio is greater than 4, YbPO4 is formed. Previous study of silica fabricated by MCVD showed that the relative content of Yb and P does not affect the structure of clusters formed during the heat treatment of preforms . In all cases, the crystals formed were YbPO4.
The study of nanocrystals (Ln = Y, La-Nd, Sm-Lu) fabricated by the hydrothermal method showed that the increase of the ratio, including the case where Ln = Yb, leads only to acceleration of the process of crystal structure formation .
Thus, based on the results of previous studies, the formation of YbPO4 crystals in our samples is expected. The important point, however, is to estimate the role of the fusing stage in this process.
Section 2 summarizes technological details on sample fabrication and the methods used for their characterization. Rayleigh scattering analysis of the measured loss spectra of slab lightguides before and after fusing is presented in Section 3. Evolution of clusters size and crystallinity caused by material fusing is considered in Section 4.
For the experiments, we fabricated samples in the form of channel lightguides. To this end, using the SPCVD method, double- and triple-layer structures were synthesized on the inner surface of a substrate silica tube with an 18 mm in outer diameter and a wall thickness of 1.5 mm. Samples of the channel lightguides were obtained by longitudinal cutting of slabs from the tube wall together with three (for type ‘#1’ samples) or two (for type ‘#2’samples) layers of amorphous silicon dioxide successively deposited on the inner surface. The first and the third layers of 40 μm in thickness contain fluorine to reduce the refractive index of the glass. The second silica layer of ~150 μm in thickness contains ytterbium and phosphorus. Plasmachemical synthesis of the layers was conducted with the help of a plasma column scanning at a frequency of 8 Hz along the section of a substrate tube ~25 cm in length. A vapor-gas mixture of SiCl4 + CF4 + O2 and SiCl4 + POCl3 + YbCl3 + O2 was used in the synthesis of the layers of the reflecting claddings and core, respectively. The temperature of the substrate tube wall during the deposition was approximately 1150 °C.
Two opposite side planes and edges of the slabs cut from the tubes (Fig. 1) were polished and refinished to better than 0.1 μm, and the length and width of the obtained channel lightguides were 2 ± 0.1 cm and 150 ± 15 µm, respectively. The first group of channel lightguides (hereinafter designated “uf” for unfused) were cut from the tube with a non-fused layer structure of glass, which was formed immediately in the course of the plasmachemical deposition. The second group of samples (hereinafter referred to as “f” for fused) was obtained from the same tube, but after outside heat penetration at a surface temperature of 1500-1700°C while spinning in the flame of a longitudinally moving hydrogen-oxygen burner.
The transmission spectra of the channel lightguides were collected using an LS-1 (Ocean Optics) halogen lamp in the visible and near-infrared regions and a DDS-30 (Lomo) deuterium discharge lamp in the UV part of the spectrum. Radiation from the lamps was transferred by lenses in a section of standard optical fiber with a pure silica core 105 μm in diameter and fluorine-doped silica reflection cladding, face-to-face connected to the edge of a lightguide. Radiation from an opposite edge of the lightguide was delivered to the Avantes CCD spectrometer using a piece of a KU-type pure-silica core optical fiber with a core diameter of 20 μm.
X-ray microprobe analysis and SEM images of the samples were obtained with a Quanta-200 electron microscope equipped with an energy dispersion EDAX spectrometer. The X-ray structure analysis and TEM images of the specimens were obtained with a JEOL JEM-2100 electron microscope. The work was performed in the core facility of the Moscow Institute of Physics and Technology.
In Figs. 2(a)-2(c) and 3(a) and 3(b), the SEM images of the side lightguide planes for samples of types #1 and #2, respectively, are presented. Lighter strips in the figures correspond to a higher concentration of Yb with respect to the relatively darker areas. The surfaces of the type #1 samples (particularly the “uf”) have white dots approximately 0.1 μm in size due to surface contamination associated with the process of polishing.
Table 1 summarizes the results of the X-ray microprobe analysis of the samples performed in the areas corresponding to the enlarged images of Figs. 2 and 3. Despite the significant difference in the concentrations of Yb and P in the different samples, the ratio of their molar concentrations in all samples is close to unity. Some of the differences in the chemical composition of samples before and after fusing are caused by variation of the dopant contents along the substrate tube during deposition.
In our experiments, samples of type #1, with a higher concentration of ytterbium, were used for the X-ray analysis, whereas samples of type #2, which have a slightly thicker lightguiding core, were used for measuring the optical characteristics.
Figure 2(d) shows a profile of the relative changes in the concentrations of Yb and P measured along the line marked in Fig. 2(b). This measurement was conducted only for the type #1f sample. Because the diameter of the electron beam probe exceeds the cluster sizes, the X-ray microprobe analysis of the composition on a scale <100 nm does not give accurate values of the elements at each particular point but allows one to observe their signatures along the sample. When building the curves of Fig. 2(d), the intensity of the characteristic lines for Yb and P was normalized to an average corresponding to the tested area, which can be considered to be proportional to the average concentration of the dopants in the sample.
Figure 2(d) shows that the changes in the concentrations of Yb and P are correlated, and their maximum values correspond to the center of the cluster. As mentioned above, the chemical composition (Table 1) indicates that the average molar concentrations of Yb and P in the deposited layer are similar. From the trace of Fig. 2(d), one can conclude that the 1:1 relationship between ytterbium and phosphorus contents remains in the cluster, so it would be logical to assume that the cluster composition is YbPO4.
3. Loss spectra, Rayleigh scattering
Figure 4 shows the spectral dependence of the attenuation coefficients, expressed over the logarithm of the ratio of the intensities in the output (I) and in the input (I0) of the lightguide:Figs. 2(b) and 3(b). After fusing, the shape of the absorption spectrum of the Yb3+ ions is slightly changed, indicating a change in the nearest neighbor surroundings of the ions.
We now evaluate the change of the cluster size caused by fusing. Assume all clusters to be homogeneous spherical particles of identical composition, and their sizes on average are significantly less than 320 nm: the smallest wavelength of the investigated optical band. Then, to estimate the sizes of the scattering centers, we can use the Rayleigh approximation , in which:
In Fig. 5, the loss spectra of the lightguides and their fitting by Eq. (6) are presented in the logarithm-logarithmic scale in the wavelength band from 320 to 750 nm. The contribution of the absorption by Yb3+ ions to loss is negligible for this wavelength band. According to Eq. (5), the attenuation coefficients for samples with similar concentrations of Yb3+ ions are directly proportional to the volume of the scattering clusters. This means that the growth of clusters leads to an increase of the attenuation coefficient of the Rayleigh scattering, which can be used to estimate the cluster size. If and in the #2f and #2uf samples are similar, i.e., the clusters vary only in volume but do not have significant differences in the chemical composition or the structure, Eq. (7) for fused and unfused can be rewritten as follows:
4. Results and discussion
Figure 6 shows a TEM image of sample #1f. Clusters with a high concentration of ytterbium and phosphorus are clearly seen, with the size of the largest of them exceeding 130 nm. As expected, some clusters are crystallized (Fig. 6(b)). This is evidenced by a set of periodically-placed, straight, dark and light lines separated by a distance of ~5on the TEM image. YbPO4 has a tetragonal crystal system with lattice parameters ‘a’ and ‘b’ equal to 6.816and 5.966 respectively . The second value is close to the period estimated from the image of Fig. 6(b). Even in the “crystallized” clusters, the lattice in the TEM image can be seen only when the crystal planes are “rightly” (in parallel) positioned with respect to the electronic beam. Therefore, we cannot conclude whether all of the clusters or only a part of them are in the crystalline phase. No correlation was found between the size of the observed clusters and the presence of crystalline phases. According to the boundary lines distinguishable in the TEM image of Fig. 6(b), there are at least 8-10 individual crystalline components that participated in the formation of the cluster.
The electron transmission diffraction pattern of a region with clusters is shown in the inset of Fig. 7. The diffraction maxima in the form of bright points specific to a crystalline phase, along with a bright wide halo characteristic of the amorphous phase of silicon dioxide, are clearly seen. Using this electron transmission diffraction pattern, we calculated the angular positions for the major diffraction maxima of the scattered electrons (Fig. 7). Data for the X-ray diffraction for the YbPO4 crystal from  are added to the same plot. The angular position of the main peaks within the margin of error is the same as in the electron transmission diffraction pattern. There are noticeable differences in the relative intensities of the bands. This discrepancy may be due to following:
- 1. Maxima in the directions of 20° and 26° overlap with a broad region associated with amorphous SiO2, which prevents the identification of the diffraction peaks.
- 2. The peak in the direction closest to 35° includes 3 diffraction peaks of YbPO4: 33°, 35° and 37°, which are difficult to resolve because of their small diffraction intensity.
- 3. In the calculations, we took into consideration not the intensity but the total number of diffraction maxima close to a particular angular direction. For simplicity, we considered the intensities of all the diffraction maxima to be equal. The latter, generally speaking, does not correspond to reality and adds uncertainty to the estimation of the diffraction peak intensities.
The resolving power of the apparatus used is insufficient to provide reliable data for unfused sample, so the question remains whether the clusters in unfused material are in a crystalline or an amorphous phase. We now proceed to estimate the cluster size changes caused by fusing.
To do this, we use Eq. (10) and fit the results for the plots in Fig. 5 (values of and ) and the data from the chemical analysis of our samples (values of and form Table 1). We obtain The present evaluation shows that the process of fusing leads to a 12- to 14- fold increase of the average cluster volume. As noted above, according to the TEM image (Fig. 6(b)), crystallized clusters consist of 8-10 parts; this finding is in an agreement with our estimates performed on the basis of the measured Rayleigh scattering spectra before and after fusing.
Using SPCVD technology, channel optical lightguides with a silica core doped with ytterbium and phosphorus were fabricated. The immediate plasma chemical deposition of such glass in the conditions of the SPCVD process yields an amorphous layer with inclusions in the form of clusters containing ytterbium and phosphorus. The fusing of this layer leads to a thirteen fold increase in the average cluster size, causing a sharp rise in the attenuation coefficient in the short wavelength part of the spectrum. At least some of the clusters are not amorphous and have a crystalline structure. Analysis of the electron transmission diffraction pattern and chemical composition suggests that the primary substance in these clusters is crystalline YbPO4.
This work was supported by Russian Foundation for Basic Research, project 14-29-08170.
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