Laser-heated Pedestal Growth Method Research Articles

Growth of Tm:Lu3Al5O12 single crystal fiber from transparent ceramics by laser-heated pedestal method and its spectral properties

Abstract Tm:Lu3Al5O12 (Tm:LuAG) single crystal fiber (SCF) with a diameter of 0.6 mm and a length of 140 mm was grown from transparent ceramics by laser-heated pedestal growth (LHPG) method. The growth, Tm3+ ion distribution, absorption spectrum, Judd-Ofelt (J-O)parameter and emission spectrum of SCF are discussed in detail. The test results show that the diameter fluctuation of the SCF is less than 2% and the Tm3+ ion distribution is relatively uniform. J-O theory was applied to analyze the absorption spectrum. With the J-O intensity parameters Ωt(t=2,4,6), the emission probabilities, fluorescence branching ratios and radiative lifetimes for the Tm:LuAG SCF and transparent ceramic were all calculated. The stimulated emission cross-section of the3F4→3H6 transition was obtained by using the Fuchtbauer-Ladenburg (F-L) method. After comparison, it is found that the spectral performance of Tm:LuAG SCF has been improved to a certain extent on the basis of transparent ceramics. It has the potential to be applied to fiber lasers.

Cracking mechanism and spectral properties of Er,Yb:CaGdAlO4 crystals grown by the LHPG method

High quality Er,Yb:CaGdAlO4 (Er,Yb:CALGO) single crystals with a diameter around 1 mm were successfully grown and investigated for the first time by the laser heated pedestal growth (LHPG) method.

Wide-range temperature sensor based on enhanced up-conversion luminescence in Er3+/Yb3+ co-doped Y2O3 crystal fiber

Abstract The Y2O3:Er3+/Yb3+ crystal fiber material has been successfully prepared by laser heated pedestal growth method. The X-ray diffraction results indicate that the prepared Y2O3:Er3+/Yb3+ luminescent material, which is optically transparent under the optical microscope, possesses pure Y2O3 crystal phase. The up-conversion luminescence corresponding to the (2H11/2, 4S3/2, 4F9/2) → 4I15/2 transitions of Er3+ ions were recorded under a 976 nm laser diode excitation. The up-conversion emission intensities from the 2H11/2 → 4I15/2 and 4S3/2 → 4I15/2 transitions of Er3+ ions were studied from room temperature to 873 K by fluorescence intensity ratio (FIR) technique. It has been found that the prepared Y2O3:Er3+/Yb3+ crystal fiber has high-efficient up-conversion emissions, and effectively improves the temperature measurement accuracy and range, making Y2O3:Er3+/Yb3+ crystal fiber an interesting candidate for optical temperature sensor.

The characteristics of high-quality Yb:YAG single crystal fibers grown by a LHPG method and the effects of their discoloration.

Single crystal fibers (SCFs), especially ytterbium (Yb) doped crystal fibers, have great potential in the field of high-power lasers. Colorless Yb:YAG single crystal fibers were fabricated using a laser heated pedestal growth (LHPG) method with a diameter fluctuation of less than 2% and a length to diameter ratio greater than 320 : 1. An abnormal color issue exists with respect to Yb:YAG crystals. The origin of coloration was studied via density functional theory, single-crystal X-ray diffraction, XPS and Raman spectroscopy and it was confirmed that the cyan coloration of Yb:YAG crystals is due to oxygen vacancies. Yb:YAG SCFs prepared via the LHPG method could avoid this type of defect due to the large specific surface area and melt convection caused by surface tension. The fundamental properties of the cyan Yb:YAG crystal source rod and colorless Yb:YAG SCFs were systematically investigated. The colorless Yb:YAG SCFs have higher infrared transmittance and thermal conductivity. The distributions of Yb3+ along the radial and axial directions were also measured. Meanwhile we demonstrated the propagation loss and a fiber laser using the colorless Yb:YAG SCFs, obtaining a minimum loss coefficient of 0.008 dB cm−1 and a maximum continuous-wave (CW) output power of 3.62 W. The colorless Yb:YAG SCFs with good thermal conductivity, low propagation loss, wide transparency and uniform ion distribution show promise for acting as the host material in single-mode lasers.

Dynamics of Er3+:YAG thermal radiation spectra near solid-melt interface at single crystal fiber growth process

The evolution of the Er3+:YAG thermal radiation spectra at heating up to the melting point was investigated in the visible range when growing a crystal fiber by the Laser-Heated Pedestal Growth (LHPG) method. The Er3+:YAG melt of about 1 mm thickness radiates like a black body in several spectral bands sometimes different from the crystal absorption lines. The real melt temperature in the middle of the growth zone was estimated as 2620 ± 15 K. The spectral profile of Er3+:YAG emissivity in the visible range for a melt layer of about L ≈ 1 mm at 2620 K is shown in the Figure.

Growth and characterization of Yb:Ho:YAG single crystal fiber

High quality Yb and Ho co-doped Y3Al5O12 single crystal fibers have been successfully grown by the laser heated pedestal growth method of up to 124 mm in length and 450 μm in diameter for the first time. The results of inductively coupled plasma-atomic emission spectrometry analysis, X-ray diffraction and Raman spectroscopy reveal that the lattice structure and doping concentrations of the SCF are the same as that of the bulk. Scanning electron microscopy microphotographs shows that the fibers only have minor diameter fluctuations within 0.5%.

Compact and sensitive Er3+/Yb3+ co-doped YAG single crystal optical fiber thermometry based on up-conversion luminescence

Compact and sensitive Er3+/Yb3+ co-doped YAG single crystal optical fiber thermometry based on up-conversion (UC) luminescence is presented. The thermal probe is a YAG single crystal fiber with end Er3+/Yb3+ co-doped grown by laser heated pedestal growth method. Excited by a 976nm laser diode, the UC fluorescence intensity ratio (FIR) of the Er3+ ions in two emission bands (2H11/2,4S3/2→4I15/2) was investigated in the temperature range from 298K to 723K. The results indicate that the maximum temperature sensitivity is approximately 0.00486K−1 at 577K. Thus, the Er3+/Yb3+ co-doped YAG single crystal fiber has potential application in optical thermometry by the FIR technique. Furthermore, the thermal probe has compact structure and high thermal stability, making it a more convenient and effective optical fiber temperature sensor.

Theoretical study of the temperature distribution in high power gain fiber of gradient doping

Thermal effect in the gain fiber is one of the main factors which restrict the power improvement of high power fiber amplifiers. Previous studies have shown that the temperature distribution is closely related to the doping concentration along the gain fiber. In order to reduce the maximum temperature of the gain fiber, we propose to use doping concentration varying along the gain fiber as a method to disperse the thermal effect of the fiber laser and improve the laser output power. Based on the rate equation model and thermal conduction model, the thermal distributions and output powers of several different gradient doping gain fibers are simulated in the cases where the output powers are approximately the same. Our study shows that compared with the conventional constant doping gain fiber, linear doping of the rare earth ion along the gain fiber can reduce the maximum temperature of the gain fiber as well as the temperature of the fusion point greatly, thus improving the stabilities of the fusion point and the fiber laser amplifier. In the case of cosinoidal doping, the gain fiber can not only reduce the temperature of the fusion point but also make the temperature have a periodic distribution along the gain fiber, which can suppress the stimulated Brillouin scattering effect effectively. The exponential doping of the gain fiber can also reduce the maximum temperature and the temperature of the fusion point, which is beneficial to the further scaling of the fiber laser output power. At the same time, it can make the gain of the signal light have a uniform distribution along the gain fiber, which suppresses the mode instability effect and improves the output beam quality of the fiber laser. These conclusions also hold true when the pump power changes. Therefore, the gradient doping of the gain fiber proposed in this paper can optimize the temperature distribution along the fiber and improve the stability of the fusion point. Besides, it can improve the beam quality of the output laser and suppress the nonlinear effect and mode instability effect. The results indicate that the gradient doping of the gain fiber is an effective and feasible way to improve the output power of fiber amplifier. Last but not the least, it is possible to produce the gradient doping gain fiber by the laser heated pedestal growth method and the direct nanoparticle deposition technique. The investigation can present a reference for designing the gain fiber in high-power fiber laser systems.

The influence of oxygen partial pressure in the growth atmosphere on the coloration of SrTiO3single crystal fibers

A variety of SrTiO3 (STO) fibers have been grown by the laser-heated pedestal growth method to determine the influence of the oxygen partial pressure in the growth atmosphere and the composition of the feed material on the coloration of SrTiO3. Crystals grown in oxygen rich environments are typically brown and show high Sr/Ti-ratios. Decreasing oxygen partial pressure leads to less coloration and decreasing Sr/Ti-ratios. Growth under very low oxygen partial pressure produces crystals with blueish and greyish tints. By using a feed rod with Sr-excess in an atmosphere that contains about one vol% oxygen, colorless STO fibers can be produced. The main reason for the low transmittance in crystals grown in high oxygen partial pressures is probably off-stoichiometry and the formation of scattering centers from clustered vacancies. The formation energy of the metal vacancies is considerably higher in oxygen poor environments which hinders the formation of the scattering centers. This phenomenon is further prevented by Sr-excess in the feed material that compensates SrO(g) evaporating from the melt.

Broadband and high-brightness light source: glass-clad Ti:sapphire crystal fiber.

High-brightness near-infrared broadband amplified spontaneous emission (ASE) was generated by glass-clad Ti:sapphire crystal fibers, which were developed using the co-drawing laser-heated pedestal growth method. As much as 29.2 mW of ASE power was generated using 520 nm laser diodes as the excitation source on an a-cut, 18 μm core-diameter Ti:sapphire crystal fiber (CF). The 3 dB bandwidth was 163.8 nm, and the radiance was 53.94 W·mm(-2) sr(-1). The propagation loss of the glass-clad sapphire CF measured using the cutback method was 0.017 cm(-1) at 780 nm. For single-mode applications, more than 100 μW of power was coupled into a SM600 single-mode fiber.

Optical phonon characteristics of an orthorhombic-transformed polymorph of CaTa2O6 single crystal fibre

Infrared-reflectivity spectroscopy and micro-Raman scattering were used to determine the optical phonon features of orthorhombic calcium tantalite (CaTa2O6) single crystal fibres. The fibres, obtained by the Laser-Heated Pedestal Growth method, grew into an ordered cubic structure . Long-time annealing was used to induce a polymorphic transformation to an aeschynite orthorhombic structure (Pnma space group). The phase transformation led to the appearance of structural domains and micro-cracks, responsible for diffuse scattering and depolarization of the scattered light in the visible range, but not in the infrared region. Thus, polarized infrared spectroscopy could be performed within oriented single domains, with an appropriate microscope, allowing us to determine all relevant polar phonons of the orthorhombic CaTa2O6. The obtained phononic dielectric response, = 22.4 and <Qu × f > = 86 × 103 GHz, shows the appropriateness of the material for microwave applications. Totally symmetric Raman modes could be resolved by polarization, after re-polishing the cracked sample surface.

Polymorphic-Induced Transformations in CaTa2O6 Single-Crystal Fibers Obtained by Laser-Heated Pedestal Growth

Calcium tantalite (CaTa2O6) single crystal fibers were obtained by the laser-heated pedestal growth method (LHPG). At room temperature, this material can present three polymorphic modifications. The rapid crystallization inherent to the LHPG method produced samples within the Pm3̅ space group, with some chemical disorder. In order to check for polymorphic-induced transformations, the CaTa2O6 fibers have been submitted to different thermal treatments and investigated by micro-Raman spectroscopy. For short annealing times (15 min) at 1200 °C, the cubic modification was maintained, though with an improved crystalline quality, as evidenced by the enhanced inelastic scattered intensity (by ca. 250%) and narrowing of Raman bands. The polarized Raman spectra respected very well the predicted symmetries and the selection rules for this cubic modification. On the other hand, long annealing times (24 h) at 1200 °C led to a complete (irreversible) polymorphic transformation. The Raman bands became still more intense (ca. 15 times larger than for the as-grown fibers), narrower, and several new modes appeared. Also, the spectra became unpolarized, demonstrating a polycrystalline nature of the transformed crystals. The observed Raman modes could be fully assigned to an orthorhombic modification of CaTa2O6 belonging to the Pnma space group.

Microstructure of the yttria doped Al2O3–ZrO2 eutectic fibers grown by the laser heated pedestal growth (LHPG) method

In this work we study the influence of the pulling rate, the concentration and the annealing on the microstructure of the yttria doped Al2O3/ZrO2 eutectic fibers grown by the laser heated pedestal growth method. The fibers microstructure has been characterized by scanning electron microscope (SEM). The microstructural pattern was irregular Chinese-script for two concentrations of Y2O3 with a cellular arrangement for the higher concentration. We note formation of the host at the borders of the quasi-hexagonal structure with a cellular diameter depending on the pulling rate. The size of the cells decreased when the pulling rate increased. The annealing improved significantly the arrangements of the cells in a homogeneous manner.

Cladding YAG crystal fibers with high-index glasses for reducing the number of guided modes

Yttrium aluminium garnet (YAG) crystal fibers with a core diameter of 40 μm were cladded by high index glasses using the co-drawing laser-heated pedestal growth method. Due to the extremely large cooling rates in the fabrication processes, unexpected and phenomenally large index drops of 0.018 and at least 0.02 were found from the as-grown capillary and the YAG crystal fiber cladding compared with bulk N-SF57’s, respectively. The high-index glass cladding is effective in reducing the number of guided modes, and the intensity profiles of the crystal fiber show there are only four guided modes at 532 nm.

Concentration dependence of upconversion emission from Er:YAG fibers

Fibers of Er:YAG have been grown using the laser-heated pedestal growth method. Fibers with Er concentrations ranging from 0.5 to 10 mol. % have been produced and excitation at 965 nm has been used to produce intense upconversion emission at green and red wavelengths. The dependence of ground state absorption and upconversion emission on dopant concentration has been studied and shown to exhibit lower levels of self-absorption than previously reported. At Er concentrations of 5 mol. % and above; however, the variation of upconversion emission intensity with pump power deviates from theoretical predictions, exhibiting saturation behavior consistent with concentration quenching.

High-resolution transmission electron microscopy analysis of the microstructures of Cr4+:Y3Al5O12 double-clad crystal fibers prepared by LHPG method

In order to unravel the relation of the nanostructure of Cr4+:Y3Al5O12 (Cr4+:YAG) crystal fibers controlled by the laser-heated pedestal growth (LHPG) technique with different growth conditions, the nanostructural analysis of Cr:YAG crystal fiber was investigated by high-resolution transmission electron microscopy (HRTEM), which offers nano-scale information in the core region and inner/outer cladding interface and the mechanism of inter-diffusion process during growth. HRTEM analysis indicates that the core has high crystallinity and a sharp core/inner-cladding interface. γ-Al2O3 nanocrystals were formed during YAG/SiO2 inter-diffusion. To further enhance the Cr4+ concentration in YAG crystal fiber, the Cr4+ ions were replenished using an electron gun to deposit Cr2O3 and divalent-ion oxide (CaO or MgO) on the source rod circumference before LHPG. It was observed that Ca2+ diffuses into the source rod more efficiently than Mg2+, while generating fewer defects and stacking faults.

Laser heated pedestal growth of potassium lithium niobate for UV generation

Potassium lithium niobate (KLN) is a nonlinear optical material with a high nonlinearity. It has the potential to improve the performance and reduce the cost of blue and UV lasers. KLN crystals are not commercially viable because growth by traditional techniques is not possible. In an effort to develop commercially viable KLN, single crystals of the material were grown by the laser heated pedestal growth method (LHPG) with compositions of x=0.02, 0.06 and 0.2 following K3Li2−xNb5+xO15+2x. Noncritical phase matching at 20°C for previously unreported compositions of x=0.02 and 0.06 was measured at 795nm and 805nm, respectively. Overall, the results suggest that single crystal KLN can be used for SHG into the UV region of the spectrum and can be developed into a commercially viable nonlinear optical material.

Raman and Infrared Phonon Features in a Designed Cubic Polymorph of CaTa2O6

At room temperature, calcium tantalite (CaTa2O6) is known to exhibit three stable polymorphs, depending on the crystal growth conditions. In this work, the laser-heated pedestal growth method was used to obtain CaTa2O6 single crystal fibers in its quenched (ordered) cubic polymorph, attractive for optical applications. X-ray diffraction, Raman scattering and infrared spectroscopy showed that the fibers grew into the centrosymmetrical Pm3 space group. The growth axis was determined as [001]. The cylindrical fibers (20 mm long and 400 μm wide) showed good optical quality, without cracks or striations, but with small facets, parallel to crystal edges or to in-plane edge bicetrices. A comprehensive set of the optical phonons for this cubic structure was obtained, consisting of 13 (of 16 foreseen) triply degenerate polar phonons, and all the 21 predicted Raman modes. By using special scattering geometries, the Raman bands were attributed to their corresponding irreducible representations. In general, the phono...

Ytterbium-Doped Yettrium Aluminum Garnet Crystal Fiber Multipass Ring Laser

We report the growth of ytterbium-doped yettrium aluminum garnet (Yb:YAG) crystal fibers by the laser-heated pedestal growth (LHPG) method. The characteristics of Yb:YAG crystal fibers were analyzed by electron probe microanalyzer (EPMA) and X-ray diffraction measurements. The small crystal fiber diameter makes it effective for heat removal from a high-Yb3+-concentration and quasi-three-level gain medium. The length of this crystal-fiber-based high absorption can be shorter, which is eminently suitable for a multipass ring cavity to maintain cavity stability and mode symmetry. We successfully demonstrated a two-mirror multipass ring laser with 54.7% slope efficiency, which is higher than the 50.3% for a bulk Yb:YAG laser.

High-Luminance White-Light Point Source Using Ce,Sm : YAG Double-Clad Crystal Fiber

Ce and Sm ions codoped yttrium aluminum garnet (Ce,Sm : YAG) double-clad crystal fibers (DCFs) were successfully fabricated using the codrawing laser-heated pedestal growth method. At a pump wavelength of 446 nm, white-light point source was produced from a 10--core DCF. The amplified spontaneous emissions and the residual pump resulted in a directional high brightness point source. A luminance of 4.5 × 109 cd/m2 was generated. The luminous flux of the point white light was 6.2 lm. When coupled to a 200-core multimode fiber, the output power and coupling efficiency were 4.8 mW and 36.8%, respectively. Point white-light sources are useful for biomedical endoscopic application and long-range directive illumination. The generated broadband yellow emission can also be served as the light source for optical coherence tomography system. Using the 91-nm yellow emission, an axial resolution of 1.5-optical coherence tomography can be achieved for ultrahigh-resolution ocular and industrial applications.