The fabrication of a Cr-doped fiber using a drawing-tower method with Cr:YAG as the core of the preform is presented. The Cr-doped YAG preform was 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 amplified spontaneous emission spectrum showed a broadband emission of 1.2 to 1.6 μm with the output power density about a few nW/nm. The results indicate that this new Cr-doped fiber may be used as a broadband fiber amplifier to cover the bandwidths in the whole 1.3-1.6 μm range of low-loss and low-dispersion windows of silica fibers.
©2007 Optical Society of America
The breakthrough technology in dry fiber fabrication has opened the possibility for utilizing transmission bandwidths all the way from 1.3 to 1.6 μm in the low-loss region of silica based fibers. The usable spectral band for the number of channels in a wavelength division multiplexing (WDM) system depends on the gain bandwidth of the fiber amplifiers. The well-known erbium (Er)-doped fiber amplifier provides gain in the C band, the L band, and the S band which totaled 140 nm usable spectral bands. The other types of fiber amplifiers, such as thulium (Tm)-doped  and praseodymium (Pr)-doped , operate gain in the S band and in the O band, respectively. However, the gain bandwidths of the Er-doped, Tm-doped, and Prdoped fiber amplifiers cannot fully cover the whole 1.3-1.6 μm range with a single fiber amplifier. It is important to develop a single fiber amplifier which can operate in ultra-broad bandwidth, because such an amplifier can greatly simplify system architecture of optical fiber telecommunications and potentially reduce installation and maintain cost.
Recently, transition-metal-doped materials, such as Cr4+ ions [3, 4] and Ni2+ ions [5, 6], have shown 300-nm broadband emissions. Furthermore, a Cr4+:YAG crystal fiber has been fabricated by the use of a codrawing laser-heated pedestal growth (LHPG)method [7–9] or a drawing-tower technique [10, 11]. The Cr4+-doped fiber amplifier may be employed in the whole 1.3-1.6 μm range. It was achieved up to 10 dB of gross gain at a wavelength of 1.52 μm at a pump power of 0.83 W . The smaller core diameter growth was difficult and the uniformity of the core diameter varied in lengths by the LHPG method. In the drawing-tower technique [10, 11], a better control of the core diameter and the fiber uniformity was fabricated. The Cr-doped fibers had a 9-μm core diameter and a 125-μm cladding diameter. However, the fabricated fiber core was still non-circular and the drawing process was not continuous. The output power density was about a few pW/nm . Due to the fiber uniformity and core circularity issues, the Cr4+-doped fibers fabricated by LHPG or drawing-tower technique are hard to integrate with the standard single-mode fibers (SMFs) for lightwave transmission system applications.
In this paper, we propose to improve the fabrication process of Cr-doped fibers by employing a drawing-tower technique with negative pressure control on the Cr:YAG preform. The negative pressure is varied during the drawing process. This negative pressure control benefits a better core concentricity and uniformity, and good interface between core and clad. In addition, the negative pressure helps collapsing the tube during fabrication and results in continuity of drawing process. The amplified spontaneous emission (ASE) spectrum of Crdoped fibers shows a broadband emission of 1.2 to 1.6 μm with the output power density about a few nW/nm levels. The Cr-doped fibers may have potential for being used as a broadband fiber amplifier to cover the bandwidths in the whole 1.3-1.6 μm range to further increase the transmission capacity of the WDM system for optical communication applications.
2.1 Preform fabrication
The Cr4+-doped YAG/silica preform was fabricated using a rod-in-tube (RIT) method . The inner and outside diameters of silica tube were 12 mm and 32 mm, respectively. The silica tube first tapered in one end to form a cone shape. Then the tube was inserted with a Cr4+-doped YAG crystal rod  to constitute the preform. The silica tube became the cladding when the assembled preform was drawn into fiber by using the drawing-tower method. Figure 1 shows a schematic diagram of a Cr4+-doped YAG preform. The diameter and length of the Cr4+-doped YAG crystal were 3 mm and 5 cm, respectively.
2.2 Fiber drawing
A commercial drawing tower equipped with a graphite-resistant furnace was used to fabricate the Cr-doped fibers, as shown in Fig. 2. The material properties of Cr4+:YAG and SiO2 used in this study were listed in Table I. Due to the different thermal expansion coefficients between the Cr4+-doped YAG crystal and silica, the raise of heating temperature was kept slow and gradation. The preform first was loaded into the center of the furnace, and the temperature was raised from 1100 °C to 1600 °C with a ramp rate of 20 °C/min. Then the temperature was directly raised to 2150 °C from 1600 °C in order to avoid the initial drop of silica being over weight. This was due to the softening point of the silica is around 1667 °C. The initial lump drop of the fiber could be observed from the bottom of furnace. The drawing temperature was set at 2060 °C when the initial lump of preform was dropped. The drawing speed was from 150 to 200 m/min. It was lower than the setting of 1.5 km/min used in drawing of a conventional SMFs.
The silica was softening when the temperature was raised higher than 1667°C. The Cr:YAG crystal, still in solid state, may be gravitated down and resulted in discontinuity of drawing process if the SiO2 tube was too soft and could not bear the weight of Cr:YAG crystal rod. Therefore, it is important to mitigate the detriment effect by Cr:YAG crystal gravity. In order to achieve better interface between core and cladding and keep the shape of the core a better circularity, a negative pressure control on the YAG perform in drawing-tower process  was used, as shown in Fig. 3. The application of negative pressure helps collapsing the tube at proper temperature, carrying the weight of Cr:YAG, maintaining circuitry of core, and keeping continuity of drawing process. The pressure was altered from 100 to 1000 pa depending upon the drawing process and the required core diameter. The gravity of Cr:YAG crystal rod could be counter balanced 10% to 75% with the help of negative pressure control. When the outer diameter of fiber approached to about 180 μm during drawing process, the fiber would pass through a die for coating and curing, as shown in Fig. 2. The Cr-doped fibers final were coated with dual layers of UV curable acrylate in order to maintain pristine surface during take-up and storage.
3. Measurements and results
Two kilometers of the Cr-doped fibers have been drawn by using a commercial drawing tower with negative pressure control. Figure 4(a) shows a fiber end face with a 125-μm cladding and a 16-μm core. It indicates that the core shape was more circular than the previous work of the Cr-doped fibers fabricated by drawing tower without negative pressure control [10,11], as shown in Fig. 4(b) with a 9-μm core diameter. Smaller core diameter of the Cr-doped fibers may be fabricated by using different diameter of the silica tube and Cr4+-doped YAG crystal rod. The refractive index and Cr4+ fluorescence intensity profiles were measured by using a laser scanning confocal microscope with a 635-nm DFB laser and a 1064-nm Yb fiber laser. A 40x objective lens with a NA of 0.65 was used to focus incoming laser beam to achieve 1-μm lateral spatial resolution. A measurement setup of Cr-doped fiber is shown in Fig. 5 . Figure 6 shows the refractive-index profiles of the fiber with and without pressure control. The solid line shows a refractive-index profile with ncore = 1.55 and an index difference of Δ = 6.06% with negative pressure control process. The dash line shows a refractive-index profile with ncore = 1.5 and an index difference of Δ = 2.93% without negative pressure control process . Due to the core and cladding were formed by inter-diffusion between YAG and SiO2, the refractive index of core was small than that of YAG.
An X-ray diffraction pattern of a Cr-doped fiber is shown in Fig. 7(a). It shows a mixed crystal in the core compared with Cr:YAG crystal, as shown in Fig. 7(b). The core of the Crdoped fiber was not a single crystal and the sapphire was a nano crystals mixed in silica matrix . In this study, the Cr-doped fibers fabricated by the drawing-tower had good core uniformity and circularity which benefits for better splicing with the standards SMFs and broadband WDM couplers for lightwave communication applications.
It is well known that Cr ions have several oxidation states , such as Cr2+, Cr3+, Cr4+, Cr5+, and Cr6+. Cr3+ is the major Cr oxidation state in YAG. The Cr3+:YAG fluorescence ranges from 0.65 to 0.75 μm , whereas the 1.2 to 1.6 μm optical spectra are dominated by Cr4+. Therefore, to measure the fluorescence spectrum of Cr-doped fibers, a 1064-nm Yb fiber laser was used as the light sources. The fluorescence spectrum of the Cr-doped fiber was excited by 1064-nm in wavelength with the initial power of 0.4 W. The fluorescence spectrum of previous Cr-doped fibers fabricated by drawing tower without pressure control was pumped by 800-nm Ti-sapphire laser with the power of 0.5 W. Figure 8(a) shows the reflected fluorescence spectrum of the Cr-doped fiber. The Cr4+ fluorescence spectrum showed a peak emission about 1.15 μm. In Fig. 8(b), the ASE spectrum of the Cr-doped fiber through a 3.0- cm propagation length shows a broadband emission from 1.2 to 1.55 μm with a peak emission about 1.4 μm and the output power density about a few nW/nm levels. This was due to that the re-absorption of Cr3+ fluorescence caused the re-emission of Cr4+ ions to become apparent when the pumping light travels through a longer length of the Cr-doped fiber . The output power density about a few nW/nm levels was higher than the previous work of a few pW/nm levels fabricated by drawing tower without negative pressure control [10, 11]. The absorption coefficient at 1064 nm was about 0.39 cm-1. Due to the quasi-three-level nature of the Cr doped fiber, there was some absorption in the 1300-1600 nm range. The combined loss coefficients from absorption and propagation loss were 0.34, and 0.08 cm-1 at 1300 nm and 1550 nm, respectively. In order to obtain a higher output power density level in fluorescence, an improvement in the fabrication of the Cr-doped fibers is necessary and currently under investigation.
In summary, we have successfully fabricated broadband Cr-doped fibers by using a RIT method and a commercial drawing-tower technique with negative pressure control. The Crdoped fibers had a 16-μm core and a 125-μm cladding. The refractive index was ncore=1.55 with Δ = 6.06%. The ASE spectra of Cr-doped fiber showed a broadband emission from 1.2 to 1.55 μm with the power densities of a few nW/nm levels. The advantages of using unique negative pressure control in drawing-tower technique to fabricate the Cr-doped crystalline fibers were a better core concentricity and uniformity, and good interface between core and cladding. These Cr-doped fibers are beneficial when integrate with the standard SMFs and broadband WDM couplers for lightwave transmission systems. This study makes it one step forwards to fabricate a better Cr-doped fiber which has potential for being used as a new broadband fiber amplifier to cover the bandwidths in the whole low-loss window of silica fibers. More than 10 microwatt/nm emission have been generated from Cr:YAG crystal fibers. The next approach to improve the emission efficiency is to reduce the drawing temperature in order to have more polycrystalline YAG in core. Furthermore, in silica environment, there are many variations of tetrahedral sites, which are responsible for the generation of Cr4+ ions. To identify and generate higher quantum efficiency tetrahedral sites of the Cr-doped fibers, an optimized drawing process is necessary and will be pursued in a separate study.
This work was partially supported by the Department of Industrial Technology of MOEA under the Contract 95-EC-17-A-07-S1-025, the National Science Council under the Contract NSC95-2221-E-110-004, and the MOE Program of the Aim for the Top University Plan.
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