Surface-plasma Chemical Vapor Deposition Research Articles

Gd-doped silica fiber fabricated using surface plasmachemical deposition with view to radiation sensor applications

A Gd3+-doped-silica optical fiber free of other codopants in the core was fabricated with the help of surface plasma chemical vapor deposition (SPCVD) for the first time. The SPCVD-process parameters are described as well as the fiber properties in the as-drawn state (absorption and photoluminescence (PL) spectra upon excimer-laser excitation at λ = 276 and 193 nm and PL decay kinetics) and during γ-irradiation at 1–3.5 Gy/s up to 0.5–1.4 kGy (radiation-induced absorption (RIA) and radioluminescence (RL)). The 315-nm RL band is argued to be suitable for real time dose-rate monitoring, although RIA and persistent luminescence can influence the RL intensity. The possibility to suppress RIA on completion of irradiation by thermal annealing at 500 °C during 15 min was demonstrated.

Silicon Oxynitride Thin Film Coating to Lossy Mode Resonance Fiber-Optic Refractometer.

A fiber-optic refractometer for various liquids with refractive indices in the range from 1.33 to 1.43 has been manufactured and tested. The sensor is based on a thin silicon oxynitride (Si3N4-xOx) film coated thinned optic fiber section (taper) obtained in a multimode all-silica optical fiber by chemical etching of the reflective cladding. The film was deposited on the cylindrical surface of the thinned fiber by the surface plasma chemical vapor deposition method (SPCVD). Lossy mode resonance (LMR) was observed in the transmission spectrum of the coated taper at a wavelength dependent on the refractive index of the liquid in which the taper was immersed. We tested the obtained sensors in distilled water, isopropyl alcohol, dimethylformamide, and their aqueous solutions. It was found that with the help of the SPCVD, one can obtain a set of sensors in a single deposition run with the dispersion of sensitivity and spectral position of LMR no more than 5%. Maximum sensitivity of the manufactured sensors to surrounding media refractive index (SMRI) variation exceeds 1090 nm/RIU, which is the highest value recorded to date for a sensor with a non-oxide coating.

Near-Infrared Luminescence of Bismuth in Silica-Based Glasses with Different Additives

The near-infrared (NIR) luminescence spectra and changes caused in them by an exposure to ArF excimer laser radiation were studied in bismuth-doped silica-based glasses containing Al, P or Ge and B additives. Experiments were conducted using specimens in the form of optical waveguides synthesized by surface plasma chemical vapor deposition (SPCVD), using bismuth-doped glass sample as the light-guiding core. Excited by ~808, ~904, and ~970 nm wavelength laser diodes, the NIR luminescence spectra were recorded in the 700–2000 nm wavelength range at temperatures of 105 and 300 K. The UV laser treatment was found to cause changes in both integrated intensity and spectrum shape of NIR luminescence associated with bismuth impurities. The observed changes are discussed with the assumption of photo-induced reconfiguring of different bismuth inclusions, which might present in the glass network in the form of separate ions and atoms, interstitial molecules, and bulk semiconductor nanoclusters.

1030 nm Yb^3+ distributed feedback short cavity silica-based fiber laser

A narrow line distributed feedback (DFB) fiber laser based on a 16 mm long Bragg grating, inscribed in a purpose-made strongly activated and highly photosensitive silica fiber, was fabricated and tested. A fiber preform synthesized by means of surface-plasma chemical vapor deposition (SPCVD), and a standard Bragg grating writing technology using an ArF excimer laser and a π-shifted phase mask, were applied to fiber drawing and grating writing, respectively. A single-mode generation regime of the DFB laser core-pumped at a wavelength of 976 nm was obtained, the measured linewidth being less than 8 kHz.

Luminescence of Yb3+ ions in silica-based glasses synthesized by SPCVD

The spectra and decay kinetics of Yb3+ single-ion and cooperative luminescence in silica-based optical slab waveguides are investigated. The slab waveguides with a high content of Yb and various amounts of P and Al additives to the light-guiding core glass were fabricated on the basis of fused and unfused glassy layers synthesized via surface-plasma chemical vapor deposition (SPCVD). Luminescence was pumped by laser diodes at ∼904 nm and ∼967 nm wavelengths and recorded in the 450–1175 nm spectral band. For the pure silica host doped with Yb, only the influence of cluster sizes on the luminescence decay kinetics is determined. It is found that the profusion of deposited glass with increased Al content favors separation by geometry of the Yb3+ and Tm3+ ions; the latter are present in the glass as an uncontrollable contamination. Evidence was found that at least two different types of Yb clusters were formed in P doped silica as a result of profusion.

Clustering of Yb in silica-based glasses synthesized by SPCVD

Light absorption and scattering are studied in silica-based slab lightguides with different contents of aluminum, ytterbium and phosphorous in the silica core. The slab lightguides used in the experiments were fabricated from the structures of doped silica deposited on the inner surface of a substrate silica tube via surface-plasma chemical vapor deposition (SPCVD). The synthesized glasses contained 0.004–0.4 at. % of Yb, as well as various contents of Al and P ranging from 0 to 1 at. %. Loss spectra of the slab lightguides are measured in the 300–1050 nm wavelength band, with the influence of the profusion of deposited glass on the loss spectra investigated by means of substrate tube external processing at a temperature of ∼1600 °C in the flame of a hydrogen-oxygen burner. We found that a combination of the temperature of the inner surface of the substrate tube during the deposition process and the subsequent profusion of the deposited structure essentially influence the distribution uniformity of ytterbium ions in the glass volume. If aluminum and phosphorous are present in the glass, such profusion can influence the formation of ytterbium clusters in opposite ways.

Influence of fusing on the uniformity of the distribution of Yb^3+ ions and the formation of clusters in silica with phosphorus admixture synthesized by SPCVD

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.

Luminescence of unfused 95%SiO2–5%GeO2 amorphous films with fluorine additive: No evidence for presence of GeODC(I) defects found

Photoluminescence (PL) of unfused amorphous germanosilicate films with fluorine additive is studied in 2–8.5eV spectral range. Experiments are based on films deposited on silica substrates by means of the surface-plasma chemical vapor deposition (SPCVD). Films of about 100μm in thickness with “high F” (~4.2wt.%) and “low F” (~0.5wt.%) fluorine content have been fabricated for the experiments. KrF (248nm), ArF (193nm) and F2 (157nm) excimer lasers are used to pump PL. It is found that absorption and luminescence associated with germanium oxygen deficient centers (GeODCs) in “high F” and “low F” films differ. In the “high F” unfused film absorption coefficient of the band at 5eV as well as intensity of the blue PL band at 3.1eV are significantly greater. This film proves features of the so called GeODCs(II), which symbolize twofold coordinated germanium defects in silica network. In the “low F” unfused film absorption band at 5eV is feebly marked. Poorly resolved PL intrinsic to GeODCs(II) can be detected in this film under the KrF laser pump. The most significant PL features are revealed under deeper UV pump by ArF and F2 lasers. Spectral positions of PL bands excited by these lasers correspond to GeODCs(II). However PL decay kinetics dramatically differs from that one intrinsic to GeODCs(II). Noticeable growth of PL intensity caused by permanent (half an hour and more) exposure to ArF and/or F2 lasers takes place, indicating GeODC(II) formation. It is found that considerable body of fluorine additive has the same effect as profusion for “low F” and/or fluorine free germanosilicate amorphous material synthesized by SPCVD. In the “high F” film yield of GeODC(II) PL pumped by the F2 laser remains high. This speaks for the suppression of the competitive 7.6eV absorption band associated with SiODCs(I) by fluorine additive indicating a decrease in the content of this type of defects in the material. High yield of GeODC(II) luminescence pumped by deep UV photons as well as hypothetical similarity of SiODC(I) and GeODC(I) permit one to conclude that GeODCs(II) are the only defects dominating in the materials under study.

Impact of fluorine admixture, hydrogen loading, and exposure to ArF excimer laser on photoluminescence of bismuth defects in amorphous silica

Photoluminescence (PL) excited by ArF (193nm), KrF (248nm) and N2 (337nm) pulsed lasers is studied in bismuth doped unfused silicon dioxide synthesized on silica substrates by surface-plasma chemical vapor deposition (SPCVD). Additive free and fluorinated (F content ~0.4wt.%) amorphous silica are examined as host materials for bismuth. Three typical PL bands peaking at wavelengths of 650nm (orange), 800nm and 1400nm (near infrared, NIR) were observed. It is found that fluorine additive weakly affects PL detail of as deposited samples. However, hydrogen loading completely deactivates NIR PL in the case of fluorine free sample, but only slightly suppresses the NIR band in fluorinated silicon dioxide. In fluorine free sample quenching of orange PL band caused by hydrogen loading is not as great as for the NIR band. By contrast, the NIR band intensity only slightly goes down in fluorinated silica sample, whereas the orange PL band becomes even more intense as a result of hydrogen loading. Exposure of the hydrogen loaded fluorine free sample to photons of ArF excimer laser renews NIR PL. Storage of fluorine free hydrogen loaded sample in oxygen ambience leads to significant increase of the NIR PL band intensity. The effect of hydrogen loading is explained by collisional deactivation of the excited NIR PL centers. Subsequent outcome of H2 molecules accelerated by UV irradiation and/or storing the sample in oxygen ambience leads to PL recovery. Presence of fluorine in silica network somehow prevents NIR PL centers from collisions with interstitial hydrogen molecules. These molecules at the same time considerably neutralize the negative effect of fluorine on the intensity of the orange PL band. No impact of fluorine on either effectiveness of the orange PL band excitation via electron–hole recombination or inactivity of this process for the NIR PL band excitation is observed.

Absorption and luminescence in amorphous SixGe1-xO2 films fabricated by SPCVD

Optical absorption and photoluminescence of Ge-doped silica films fabricated by the surface-plasma chemical vapor deposition (SPCVD) are studied in the 2–8eV spectral band. The deposited on silica substrate films of about 10μm in thickness are composed as x·GeO2-(1-x)·SiO2 with x ranging from 0.02 to 1. It is found that all as‐deposited films do not luminesce under the excitation by a KrF (5eV) excimer laser, thus indicating lack of oxygen deficient centers (ODCs) in them. After subsequent fusion of silicon containing (x<1) films by a scanning focused CO2 laser beam absorption band centered at 5eV as well as two luminescence bands centered at blue (3.1eV) and UV (4.3eV) wavelengths arise, highlighting the formation of the ODCs. The excitation of unfused SPCVD films by an ArF (6.4eV) excimer laser yields a luminescence spectrum with two bands typical for the ODCs, but with a faster decay kinetics. Intensities of these bands grow up with samples cooling down to a temperature of 80–60K. Unfused films excited by the ArF laser also demonstrate luminescence due to recombination of a trapped charge resulted from the excitation of localized electron states of the glass network. In the unfused GeO2 film luminescence related to a self-trapped exciton (STE) typical for GeO2 crystals with α-quartz structure is observed. The observed STE luminescence can be indicative of the crystalline fraction availability in the film. Whereas GeO2 crystals are known as not containing twofold coordinated germanium, a polycrystalline inclusion in the SPCVD GeO2 film serves as a factor explaining the absence of any spectroscopic manifestation of this type of defects in it even after fusion. On the other hand, lack of STE luminescence in other unfused films with x<1 testifies truly amorphous state of the matter in them.

Formation mechanisms of precursors of radiation-induced color centers during fabrication of silica optical fiber preform

Samples in the form of transverse slices of rods and optical fiber preforms made from the high-hydroxyl KU-1 and low-hydroxyl KS-4V silica by the plasma outside deposition (POD) method are γ-irradiated to a dose of ∼1 MGy (SiO2). Next, the radial dependences of the radiation-induced nonbridging oxygen hole center (NBOHC) and E′-center (three-coordinated silicon) in the samples are constructed by measuring the amplitudes of their 4.8 and 5.8 eV absorption bands, respectively. Based on the analysis of these radial dependences and considering the temperature and duration of the preirradiation heat treatment of the rods and preforms at the POD-installation, we determine the ratio of the oscillator strengths of the above bands and the microscopic thermoinduced processes occurring during preform fabrication and producing precursors of the radiation-induced NBOHC and E′-center. These processes are found to be associated with the escape of either H2 or H2O from neighboring hydroxyl groups, and, therefore, can occur in high-hydroxyl silica only. It is concluded that enhancement of the radiation resistance of high-hydroxyl silica optical fibers requires decreasing the temperature and duration of the preform fabrication process, in particular, changing from the POD-technology to the low-temperature plasmachemical vapor deposition (PCVD) or surface PCVD (SPCVD)-technology.

Peculiarities of Er3+ photoluminescence in halogen-doped amorphous silica

The spectra and photoluminescence kinetics of Er3+ ions embedded in amorphous fluorine- and chlorine-doped silica matrices synthesized by surface-plasma chemical vapor deposition were investigated at 27–300 K. Luminescence was excited with an Ar+ laser at a wavelength of 514.5 nm and with a diode laser at a wavelength of 975 nm. Narrow and well-expressed components of Stark sublevels with a small contribution of inhomogeneous broadening intrinsic to Er3+ ions in crystalline rather than amorphous matrices were revealed and identified in photoluminescence spectra. The structure of Stark sublevels was well-resolved at low temperatures. The presence of the well-resolved Stark structure in spectra was indicative of stable anion complex formation in Er3+ environment presumably associated with halogen incorporation. This environment was formed at a stage of the low-temperature plasma-chemical synthesis and was destroyed at glass fusion.

Absorption and scattering in bismuth-doped optical fibers

The optical-loss spectra of bismuth-doped aluminosilicate glass core optical fibers have been investigated in the spectral range 200–1700 nm. Absorption (active and inactive) and scattering losses have been separated. The optical fibers under study were fabricated by methods of modified chemical vapor deposition and surface-plasma chemical vapor deposition.

Fabrication of x-ray zone plates by surface-plasma chemical vapor deposition

A new cost-efficient sputter-slice technology for hard x-ray (10-30 keV) Fresnel zone plates fabrication, imposing no limitation to aspect ratio, is proposed. By means of a plasma chemical process, SiO(2)/Si(1-x)Ge(x)O(2) glassy film multilayer structures are deposited on a lateral surface of a silica rod, outermost layers being as thin as 100 nm. It has been shown by numerical simulation that for x=0.2 germanium fraction, 100-300 microm zone plate thickness and the number of zones of about 1000, first order diffraction efficiency as high as 20%-30% at the energy of approximately 20 keV can be achieved.

Features of Er3+ luminescence in fluorine-doped amorphous silicon dioxide fabricated by low-temperature plasma CVD

Spectra and photoluminescence kinetics of Er3+ ions incorporated in an amorphous host of fluorine-doped silicon dioxide synthesized by surface plasma chemical vapor deposition (SPCVD) are investigated at temperatures of 27–300K. Luminescence is excited with an Ar+ laser at a wavelength of 514.5nm and with a diode laser at a wavelength of 975nm. Narrow and well-defined components of Stark manifolds with a small contribution from inhomogeneous broadening intrinsic to Er3+ ions in crystalline, but not amorphous, hosts are revealed and identified in photoluminescence spectra. The structure of the Stark manifolds is well-resolved at low temperatures. The presence of the well-resolved Stark structure in the spectra is indicative of stable anion complexes formating the Er3+ local neighborhood presumably associated with fluorine incorporation. This neighborhood is formed during the low temperature plasma chemical synthesis and is destroyed upon glass fusion.

Nano-scale compositional lamination of doped silica glass deposited in surface discharge plasma of SPCVD technology

The kinetics of metal halogenide mixture transformation into oxides was studied with the help of optical emission spectroscopy (OES) in the microwave-induced discharge of surface plasma chemical vapor deposition (SPCVD) process. The structure of germanium-doped silica layer deposited in plasmachemical synthesis of preforms for optical fiber drawing was also investigated. It was shown that deposited glass has a periodical structure of alternate layers with different glass composition. Observation experiments on the oxide formation kinetics in plasma column were performed for the interpretation of the lamination effect. It was demonstrated that the lamination effect is related to a non-uniform distribution of the corresponding monoxides along the plasma column. This lamination results from the competition of different chemical oxidation reactions for oxygen. On the basis of the results obtained, a set of chemical reactions responsible for the formation of multicomponent glasses in SPCVD-type processes is proposed.

Erratum: Separation of germanium and silicon oxides by plasma-chemical deposition of germanosilicate glass in a moving plasma column [Tech. phys. lett. 25, 530–532 (1999)

The optical transmission spectrum of germanosilicate glass deposited by surface plasma chemical vapor deposition on the inner surface of a quartz tube revealed interference resonances typical of multilayer dielectric coationgs with alternating refractive indices. It is shown that this effect can be attributed to the longitudinal inhomogeneity of the plasma composition and specifically to an axial shift of the concentration maxima of germanium and silicon oxide. As a plasma having a nonuniform composition moves along the tube, a layer of glass is formed with a strong transverse germanium concentration gradient. It is established that in surface plasma chemical vapor deposition the axial separation of the regions of deposition of the silicon and germanium oxides increases if the glass synthesized under conditions of oxygen deficiency.

Separation of germanium and silicon oxides by plasma-chemical deposition of germanosilicate glass in a moving plasma column

The optical transmission spectrum of germanosilicate glass deposited by surface plasma chemical vapor deposition on the inner surface of a quartz tube revealed interference resonances typical of multilayer dielectric coatings with alternating refractive indices. It is shown that this effect can be attributed to the longitudinal inhomogeneity of the plasma composition and specifically to an axial shift of the concentration maxima of germanium and silicon oxide. As a plasma having a nonuniform composition moves along the tube, a layer of glass is formed with a strong transverse germanium concentration gradient. It is established that in surface plasma chemical vapor deposition the axial separation of the regions of deposition of the silicon and germanium oxides increases if the glass is synthesized under conditions of oxygen deficiency.