The success in fabrication of Cr-doped fibers (CDFs) with fluorescence of Cr3+ by powder-in-tube (PIT) method equipped with drawing-tower process is demonstrated. However, the fabrication by using powder-in-tube (PIT) with redrawing technique provides a better solution to improve the concentration of Cr-ion to enhance the fluorescence of CDFs. The Cr-doped powder was composed of CaO-Al2O3-BaCO3-MgO-Cr2O3 as the material of core. The CDFs had a 7 £gm core and a 125 £gm cladding. The transmission loss was 0.27 dB/cm at 1550 nm and a core non-circularity of less than 3% for the CDFs are achieved. The fluorescence intensity of Cr3+ between 800~1200 nm was 6 nW/nm. The optical fiber fabrication processes, whether the preform is made by MCVD(Modified Chemical Vapor Deposition), RIT(Rod-in-Tube) or PIT, the inter-diffusion between core and cladding materials is an inevitable issue at such high fiber drawing temperature. Since SiO2 is amajor component in the cladding, SiO2 will certainly diffuse into the core region and become one of the new constituents in the core. The Cr-doped powder was composed of SiO2- Al2O3-MgO-K2O-TiO2-Cr2O3 as the material of core. The CDFs had a 10 £gm core and a 125 £gm cladding. The fluorescence intensity of Cr4+ between 900~1300 nm was 200 pW/nm. The CDFs were successfully fabricated by using podwer-in-tube with redrawing technique. The demonstration of CDFs makes it possible as a new generation broadband fiber amplifier and a broadband source for high resolution OCT.

The breakthrough technology in dry fiber fabrication has opened the possibility for using fiber bandwidths all the way from 1.3 to 1.6 £gm. However, the fiber amplifier used in commercial product, such as erbium-doped fiber amplifier (EDFA), can not fully cover the whole fiber bandwidths from 1.3 to 1.6 £gm with a single fiber amplifier. Recently, the Cr4+-doped fiber has shown a broadband emission from 1.3 to 1.6 £gm. Therefore, it is interesting to develop a single fiber amplifier which can operate the wide bandwidth of the 1.3 ~ 1.6 £gm emission. In this study, we have successfully fabricated and measured the Cr-doped fibers by using a commercial drawing-tower technique. The Cr-doped YAG preform was firstly fabricated by a rod-in-tube method. By employing a negative pressure control in drawing-tower technique on the YAG preform, the Cr-doped fibers with a better core circularity and uniformity, and good interface between core and cladding were fabricated. The drawing speed was up to 200m/min. The core diameters were 26 and 16 £gm and the non-circularity was smaller than 3%. The spontaneous emission spectrum showed a broadband emission of 1.2 to 1.6 £gm with the output power density about a few nW/nm. The Cr-doped fibers fabricated by drawing tower are beneficial when integrated with the standard single-mode fibers and broadband WDM couplers for lightwave communication systems. Therefore, the Cr-doped fibers may be used as a broadband fiber amplifier to cover the whole 1.3-1.6 £gm range of silica fibers and have a potential for commercial production and application to lightwave communication systems.

The fabrication of broadband single-mode Cr-doped silica fibers (SMCDSFs) using the fiber drawing-tower method with the modified rod-in-tube technique is demonstrated for the first time. A preform was assembled by using the grown Cr:YAG rod as core and the silica tube as cladding. The outer and inner diameters of the silica tube are 20 and 7 mm, respectively. The initial dimension of the Cr:YAG crystal rod had a length of 0.03 m and a diameter of 500 £gm. The Cr:YAG crystal was grown into a diameter of a 290 £gm with a length of 0.12 m by the LHPG method. The SMCDSFs had a 6 £gm core and a 125 £gm cladding. The transmission loss was 0.08 dB/cm at 1550 nm. The far-field pattern measurements indicated the single-mode characteristic when the propagation wavelength was longer than 1310 nm. In order to solve the interface of core and cladding, a novel rod-in-tube(RIT) perform was employed by inserting the Cr:YAG crystal rod of 0.03m length and 500 £gm diameter into the silica capillary tube, which had the same diameter with the drilled silica rod. The single-mode Cr-doped fibers had successfully been fabricated and the loss had been reduced to 0.03 dB/cm at 1550 nm with a 5 £gm core and a 125 £gm cladding. Furthermore, the SMCDSFs also had the single-mode characteristic when they operated in the optical communication window. The successful fabrication of SMCDSFs may be one step forward towards the achievement of utilizing the SMCDSFs as ultra-broadband fiber optical amplifiers to cover the bandwidths in the whole 1300 to 1600 nm range of low-loss and low-dispersion windows of silica fibers and a broadband source for enabling high resolution in optical coherence tomography (OCT).

The success in fabrication of Cr-doped fibers (CDFs) with fluorescence of Cr3+ by powder-in-tube (PIT) method equipped with drawing-tower process is demonstrated for the first time. The fluorescence intensity of CDFs by fabricated RIT method is weak because the concentration of Cr-ion in Cr:YAG rod is low. However, the fabrication with powder-in-tube (PIT) provides a better solution to improve the concentration of Cr-ion to enhance the fluorescence of CDFs. The Cr-doped powder was composed of CaO-Al2O3-BaCO3-MgO-Cr2O3 as the material of core and then it was poured into the silica tube with outer diameter of 20 mm and inner diameter of 7 mm (20/7) to yield the perform. The CDFs had a 17.5 £gm core and a 125 £gm cladding. The transmission loss was 0.74 dB/cm at 1550 nm. And the fluorescence intensity of Cr3+ between 800~1200 nm was 50 nW/nm. To reduce transmission loss further, we used multi-tubes to raise the ratio of cladding to core. According to the principle of conservation of mass, the core diameter of CDFs was 5 £gm. The transmission loss was improved more than 50% and it reached to 0.135 dB/cm at 1550 nm. Moreover, a single-mode characteristic of CDF was observed when the propagation wavelengths were longer than 1260 nm. The CDFs were successfully fabricated by using a fiber drawing-tower technique with PIT method. The demonstration of CDFs makes it possible as a new generation broadband fiber amplifier, a tunable NIR fiber laser for sensor applications, and a broadband source for high resolution OCT.

¡@¡@Currently, The Cr-doped fibers are grown by LHPG method or drawing-tower technique. The Cr-doped YAG preform was firstly fabricated by a rod-in-tube method. We have successfully fabricated the Cr-doped fibers by using a commercial drawing-tower technique. By employing a negative pressure control in drawing-tower technique on the YAG preform, the Cr-doped fibers with a better core circularity and uniformity, and good interface between core and cladding were fabricated. The core non-circularity was smaller than 3%, the spontaneous emission spectrum showed the bandwidth that approach to 300 nm, and the output power density level have promoted to a few nW/nm. ¡@In this study, we focused on the analysis of dynamic fluorescent characteristics of Cr-doped fibers in order to improve the quality effectively. The lifetimes of Cr4+ fluorescence and concentration of Cr ions were 1.5 £gs and 510 £gg/g, respectively.The concentration of the Cr ions was less than the Cr-doped fibers grown by LHPG method. The high-resolution micrograph showed that there was nano-crystalline structure in the core surrounded by SiO2 amorphous matrix. These nano-particles gathered at the core and formed micrometer clusters, and therefore resulted in high scattering loss around 1.17dB/cm. ¡@¡@In order to improve the Cr-doped fibers quality, reduce propagation loss, and promote the spontaneous emission power density. We have to decrease the temperature and drawing speed in the drawing process Therefore, the new Cr-doped fibers may have the potential for being used as a new generation broadband fiber amplifier to cover the bandwidth of the entire 1.3-1.6 £gm range which exhibit 300 nm usable spectral bands.