1. Introduction

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 [1] and praseodymium (Pr)-doped [2], 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 [7]. 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 [11]. 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. Fabrication

2.1 Preform fabrication

The Cr⁴⁺-doped YAG/silica preform was fabricated using a rod-in-tube (RIT) method [12]. 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 [13] 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.

 

Fig. 1. Cr⁴⁺-doped YAG preform.

Download Full Size | PDF

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.

 

Fig. 2. Drawing tower set-up.

Download Full Size | PDF

 

Fig. 3. Negative pressure control.

Download Full Size | PDF

Table I. The material properties of Cr⁴⁺:YAG and SiO2.

View Table

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 [14] 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 [9]. Figure 6 shows the refractive-index profiles of the fiber with and without pressure control. The solid line shows a refractive-index profile with n_core = 1.55 and an index difference of Δ = 6.06% with negative pressure control process. The dash line shows a refractive-index profile with n_core = 1.5 and an index difference of Δ = 2.93% without negative pressure control process [11]. 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.

 

Fig. 4. Photographs of the polished end (a) with, and (b) without negative pressure control.

Download Full Size | PDF

 

Fig. 5. The laser scanning confocal microscope for index and fluorescence mappings.

Download Full Size | PDF

 

Fig. 6. The refractive index profiles of the Cr-doped fibers.

Download Full Size | PDF

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 [7]. 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.

 

Fig. 7. The X-ray diffraction pattern of (a) a Cr-doped fiber, and (b) Cr:YAG crystal.

Download Full Size | PDF

It is well known that Cr ions have several oxidation states [15], 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 [16], 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 [11]. 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.

 

Fig. 8. (a). The reflected fluorescence spectrum, and (b) the ASE spectrum of the Cr-doped fiber.

Download Full Size | PDF

4. Conclusion

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 n_core=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.