Laser Heated Pedestal Growth System Research Articles

Few-Mode Cr-Doped Crystalline Core Fibers for Fiber Amplifier

A few-mode Cr-doped crystalline core fiber (FMCDCCF) is demonstrated. The fiber fabrication is a modified laser-heated pedestal growth (LHPG) technique that consists of over cladding a Cr:YAG crystal rod with silica and then fine tuning its core size with multiple core reduction through the LHPG system. The FMCDCCF exhibits a core diameter of 2 μm and a <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">V</i> -value around 3.77 to provide the few-mode characteristics. The optical properties of the FMCDCCF are similar to that of a pristine Cr <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4+</sup> :YAG crystalline. A gross gain of the FMCDCCF is measured around 2.3 dB at a wavelength of 1.4 μm.

Three-dimensional simulation and analysis of heat transfer and flow field in micro-floating zone of LHPG with asymmetrical perturbation

Under symmetrical conditions, the micro-floating zone of a laser-heated pedestal growth (LHPG) system displays a symmetrical double eddy flow field distribution. However, as the perturbation increases, the double eddy flow field distribution changes from symmetry to tilt, resulting in unstable flow field vibration. This study investigated the influence of the molten zone on the shape (at vapor–liquid and solid–liquid interfaces) and flow field distribution resulting from tilting the CO2 laser heating ring and the gravity field. This tilt is caused by spatial perturbations resulting from mounting the source rod and seed on pedestals with an alignment that slightly deviates from the growth axis of the LHPG system. The feasibility of growing crystal fibers using the micro-floating zone of the LHPG in the horizontal plane was further investigated. After selecting YAG as the growth material, we evaluated various reduction ratios, source rod scales, surface tensions, and thermocapillary coefficients for the two types of perturbation. Deviation in the laser heating ring significantly influenced the solid–liquid interface and the symmetry of the flow field in the molten zone. However, as long as the laser heating ring remained close to symmetrical, the influence on the molten zone of the larger deviation in the gravity field was limited. Therefore, growing crystal fibers using the micro-floating zone of the LHPG system in the horizontal plane is possible for materials that possess the correct physical properties at appropriate source rod scales. The three-dimensional simulation of this asymmetry includes the effects of the diameter reduction ratio and laser heating, and also modifies Lan's thermocapillary floating numerical model. The modified model calculates the physical grid through a non-orthogonal body-fitting grid transformation using the control-volume finite-difference method. To enhance the simulation and represent the physical system more accurately, we compared the shape of the molten zone of the simulation and the experiment. In practice, spatial perturbations can be controlled to within a range of approximately 3°. Using a 500-μm-diameter source rod and a 0.5 diameter reduction ratio as the baseline, we compared the flow field distribution in the molten zone with a 300-μm-diameter source rod and a 0.5 diameter reduction ratio as well as with a 500-μm-diameter source rod and a 0.25 diameter reduction ratio. Finally, a LHPG system undergoing horizontal growth was also simulated for a 1000-μm-diameter source rod and a gravity field perpendicular to the direction of crystal growth for further analysis.

Two-Dimensional Simulations on Heat Transfer and Fluid Flow for Yttrium Aluminium Garnet Single-Crystal Fiber in Laser-Heated Pedestal Growth System

Heat transfer and fluid flow in a laser-heated pedestal growth (LHPG) system are analyzed near the deformed interfaces. The global thermal distributions of the crystal fiber, the melt, and the source rod are described by their temperature and axial gradient over a length of ∼10 mm. Compared with the growth of bulk crystal of several centimeters in diameter, natural convection is 6 orders of magnitude smaller owing to the smaller melt volume; therefore, conduction rather than convection determines the temperature distribution in the molten zone. Moreover, thermocapillary convection rather than mass-transfer convection becomes dominant. The symmetry and mass flow rate of the double eddy pattern are significantly influenced by the molten-zone shape owing to the diameter reduction and the surface-tension-temperature coefficient when it is more than 10-4–10-3 dyn cm-1 K-1.

Low-loss propagation in Cr^4+:YAG double-clad crystal fiber fabricated by sapphire tube assisted CDLHPG technique

Cr4+:YAG double-clad crystal fiber with an uniform 10-microm core was fabricated by using a sapphire tube as a heat capacitor to stabilize the power fluctuation of the CO2 laser in the co-drawing laser-heated pedestal growth system. The uniformity of the fiber core showed a factor of 3 improvement compared to that without the use of sapphire tube. The variation of the core diameter is within the +/-1.35-degree adiabatic criterion and has a autocorrelation length of 1.7 mm. The measured propagation loss is only 0.02 dB/cm. The sapphire tube also reduces the vertical temperature gradient during the crystal fiber growth process so the 10-microm crystal core exhibits a smooth perimeter. The sapphire tube assisted system can be applied to the growth of many other optical crystal materials.

Improved laser-heated pedestal growth system for crystal growth in medium and high isostatic pressure environment

A modified apparatus for single-crystal-fiber growth in an isostatic gaseous environment and for a large range of pressure (100 mbar up to 100 bar) using a floating-zone-type method known as laser-heated pedestal growth is described. The difficulties and advantages of its design are discussed through examples of crystal growth of some oxide materials.

Novel implementation of laser heated pedestal growth for the rapid drawing of sapphire fibers

A new version of laser heated pedestal growth system is described which is amenable for the rapid growth of single crystal fibers. The system incorporates a Gaussian reflector to reshape the radial intensity profile of the CO2 laser beam. When He is used as the growth atmosphere, high optical quality sapphire fibers can be produced at rates as high as 2 cm/min with this apparatus.