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Abstract
We report a large-mode-area neodymium-doped silicate photonic bandgap fiber. The concept of power delocalization rather than confinement loss differentiation was considered to theoretically determine the structure parameters, and the near single mode operation of the fabricated fiber was experimentally demonstrated. An output power of 3.1 W with a slope efficiency of 48% was obtained for a 75-cm-long fiber.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Rare-earth doped fibers have been widely used in fiber laser systems [1], and one of the primary research interests is to enlarge the fiber core diameter and shorten the length. On one hand, by increasing the core diameter, the non-linear effects can be weakened and a higher laser output power can be obtained. For another, fibers with short lengths will decrease the number of stimulated longitudinal modes in single-frequency fiber laser [2,3], and lead to a short pulse duration in Q-switched operation [4], thus increasing the performances of such systems.
Many design strategies have been suggested to scale mode-field area of fibers, such as photonic bandgap fibers (PBGFs) [5–12], photonic crystal fibers (PCFs) [13], large-pitch fibers [14], chirally-coupled-core (CCC) fibers [15], multi-trench fibers [16], distributed modal filtering rod fibers [17], gain-guiding fibers [18], etc. Among them, large-mode-area photonic bandgap fibers (LMA-PBGFs) have attracted much attention in recent years. Compared with other designs, PBGFs can provide unsurpassed higher order modes (HOMs) suppression [7,10,19]. In addition, these fibers also present interesting characteristics such as spectral filtering and chromatic dispersion control [19], which can be useful to mitigate stimulated Raman scattering [20] and undesired ASE emission [21,22]. Up to now, the core diameters of passive and active LMA-PBGFs have extended to ~87 μm [10] and ~60 μm [12], respectively. Besides, a laser output power of 948 W has been obtained with an Yb-doped PBGF [12], which is the maximum single-mode power from micro-structured fibers, demonstrating the significant potential for HOM suppression of these fibers.
Apart from silica glass, soft glasses can be used as the matrix of PCFs because they have higher rare earth solubility and easily adjustable refractive indices. Therefore, the fiber length can be shortened and the nonlinear effects can be weakened. In recent years, the single-mode performances of various designs have been experimentally investigated with silicate and phosphate micro-structured fibers [23–26] by our group. Some attempts have also been tried by using PBGF to realize Nd three-level laser at ~915 nm [27,28], however, no laser output has been obtained because of the low emission cross section as well as the weak pumping light guidance resulting from the single cladding structure of these fibers. This study is our first attempt to obtain single-mode laser output in silicate glass with the PBGF design. To our knowledge, the soft glass rare-earth doped LMA-PBGFs have not been well studied.
The combination of PBGF and soft glass can lead to a robust single-mode fiber with short length. Considering the characteristics of soft glass fibers, the silicate LMA-PBGF in this study shows some differences with the previously published references. First, because of the short length of soft glass fibers in practical use, a straight fiber is considered in simulation. Second, as the core of silicate fiber has a relatively high intrinsic loss of 1–2 dB/m [24,29], the mode differentiation of these fibers is realized by HOM delocalization [17,24–26,30,31] rather than confinement loss (CL) differentiation [5–12]. Additionally, thanks to the widely adjustable refractive indices of soft glass, the outer cladding of this silicate PBGF can be fabricated by low-index glass rather than polymer coating, hence an all-solid structure is realized.
In this study, we demonstrate a near single-mode Nd-doped all-solid double cladding silicate photonic bandgap fiber with 3.1 W output power and 48% slope efficiency. This fiber has a core diameter of 32.2 μm and a length of 75 cm.
2. Experimental
Four kinds of silicate glasses were used to prepare the fiber: the N0312 glass with an Nd doping level of 1.2 wt.%, and the undoped G1, G2 and G3 glasses. The N0312 glass was fabricated by our laboratory, and the G1, G2 and G3 glasses were commercial silicate glass provided by CDGM Glass CO. Ltd. The N0312 glass was prepared by the conventional melt quenching technique, with the composition of SiO2-Na2O-K2O-CaO. The thermal parameters of all the glasses are shown in Table 1, and some parameters of the N0312 glass are presented in Table 2. The refractive indices of G1, G2 and G3 glasses at 1064 nm are 1.5123, 1.5493 and 1.4614 respectively, calculated by the dispersion formulas provided by the glass manufacturer. For each glass, the refractive index values were measured at 14 different wavelengths by V prism method with an accuracy of 5 × 10−5 to obtain its dispersion formula. The refractive index of the N0312 glass at 1060 nm is 1.5122 [32].
Table 1. Thermal Properties of the Glasses
Table 2. Some Parameters of the N0312 Glass
Figure 1 (a) shows the cross-section of the designed fiber. A 7-cell core design was adopted, and several high-index rods were removed from the cladding for HOM delocalization. The fiber core, low-index background cladding and the high-index rods were composed of the N0312, G1 and G2 glasses, respectively. The G3 glass was used as the outer cladding and not shown in this figure.
The fiber was designed by considering two parameters: the diameter of the high-index rod, d; the center-to-center distance between two nearest high-index rods, Λ. The diameter of the high-index rod was calculated with Eq. (1):
where nhigh and nlow are the refractive indices of G2 and G1 glasses at 1064 nm, and V is the normalized frequency [5–7]. The normalized frequency V was set to be 4.65 in this study, which is the center frequency of the third bandgap [6,19,33]. The calculated diameters of the high-index rods were 4.7 μm.
The distance between the nearest two rods, Λ, was determined by two criteria. First, as the typical intrinsic loss of the silicate core is 1–2 dB/m, a CL of less than 1 dB/m for the fundamental mode (FM) can be acceptable. Second, the overlap factor [17,24–26,30,31] (OF) difference (ΔOF = OFFM - OFHOM) between the FM and all HOMs should be greater than 30% [17,24–26,30], which guarantees single mode transmission of the designed fiber.
The calculation was performed with the commercially available Finite Difference Time Domain software MODE Solutions (v6.6.0, Lumerical Corp.). The anisotropic perfectly matched layer (PML) was used for CL and OF calculation. The CL of each mode was derived from the imaginative part of the complex refractive obtained by the software, and the OF can be calculated with Eq. (2):
where Ac and Ap are the areas of the doped core and pump waveguide respectively, I is the electric field intensity of the calculated mode, and ds is the cross-section of integration. More than the first 200 guided modes were calculated for each structure to find the HOM with the largest OF (HOMmax). The OF evolutions of FM, LP11 mode and HOMmax are shown in Fig. 1 (b). According to the criteria elucidated above, single mode laser transmission can be realized within the d/Λ range of 0.32–0.36. A d/Λ value of 0.36 was adopted in this study.
The fiber preform was prepared by stack-and-draw method. As the designed d/Λ value was 0.36, a G2 rod with diameter of 8.7 mm, and a G1 tube with inner diameter of 8.7 mm and outer diameter of 24 mm, were fabricated, with the fabrication tolerances of −0.1/0 mm, 0/0.1 mm and ± 0.05 mm respectively. After sonically cleaned with ethanol, the G1 tube and G2 rod were drawn into rods with 0.995 mm in diameter by the rod-in-tube method. Besides, a N0312 rod, a G1 rod and a G3 rod with diameters of 24 ± 0.05 mm were also fabricated and drawn into rods with diameters of 2.46 mm, 0.995 mm and 0.995 mm, respectively. The diameter variations of all the thin rods were within ± 5 μm. Then, the thin rods were closely arranged according to the designed fiber structure, and the obtained fiber preform was fed into the fiber fabrication tower and drawn into fibers with the diameter of 226 ± 5 μm.
Figure 2 presents the cross-section of the fabricated fiber. The core of the fiber is composed of N0312 glass. The grey background cladding and the white high-index rods are composed of G1 and G2 glass, respectively. The G3 glass is used as the black outer cladding. In addition, the G1 glass is used as the jacket of the fiber. The diameter of the active core is 32.2 μm and d/Λ value is 0.362. The inner cladding has a numerical aperture (NA) value of 0.39, a flat-to-flat diameter of 125 μm, and a corner-to-corner diameter of 145 μm.
We used a fiber with 75 cm in length and an outer diameter of 226 μm to build the laser. The fiber was cladding pumped with a commercial fiber-coupled diode laser (nLIGHT, United States) operating at 793 nm out of a 400 μm core (NA = 0.22), with the maximum pump power of 35 W. The output pumping beam was aligned by a collimating lens with a NA of 0.25, and then coupled into the fiber cladding through another aspherical lens with a NA of 0.3. The coupling efficiency was 30% for the launched pump power. The cavity was constructed by a butt-coupled dichroic mirror with high reflectivity (99%) at 1064 nm and a perpendicularly cleaved fiber end with 4.2% Fresnel reflectivity. The laser set-up is shown in Fig. 3. The doped part has a loss of ~2 dB/m at 1310 nm [24,29] and a pumping absorption coefficient of 7.7 ± 0.5 dB/m measured by the cut-back method.
Fig. 3 Laser setup used to characterize the laser performance. PM: power meter. OSA: optical spectrum analyzer.
3. Results and discussion
Figure 4 shows the laser performance of the fiber. As Fig. 4(a) presents, a maximum power of 3.1 W is extracted from a 75-cm-long fiber. The slope efficiencies relative to the launched and absorbed pump power are 39% and 48% respectively, and the residual 793-nm laser power is 1.8 W at the highest pump power. No saturation of laser output power is observed in this figure, indicating that the maximum output power of our laser is limited by the maximum pump power. The laser spectrum is displayed in Fig. 4(b). As this figure presents, several longitudinal modes exist ranging from 1060 to 1070 nm, and the center wavelength is at 1064 nm.
The near-field pattern of the fiber end-face is shown in Fig. 5. As Fig. 5 shows, the light in the high-index rods is much brighter than elsewhere, resulting from the residual 793-nm pumping light. After the pumping light is filtered by a 980-nm long-pass filter, a hexagonal near field pattern of the output laser at ~1064 nm can be observed.
Fig. 5 Near-field pattern of fiber end-face obtained by CCD. Inset: near field pattern of output laser beam.
The beam quality (M2) factor was measured by the Thorlabs BP109-IR2 beam profiler and shown in Fig. 6. The measured values are 1.19 and 1.23 at X and Y orientations respectively. The accuracy of the M2 factor is within 0.05. Considering the noncircular pattern of the FM, the M2 factor of 1.21 can demonstrate the near single-mode operation of this fiber.
4. Conclusion
In summary, we theoretically designed and experimentally realized an Nd-doped large- mode-area all-solid silicate photonic bandgap fiber, with an active core diameter of 32.2 μm. The HOM suppression was realized by HOM delocalization rather than confinement loss differentiation, determined by the relatively high intrinsic loss of soft glass. A laser output of 3.1 W with a slope efficiency of 48% was obtained for a 75-cm-long fiber.
Funding
Natural Science Foundation of Shanghai (No. 17ZR1434000); China Postdoctoral Science Foundation (No. 2016M601653); National Natural Science Foundation of China (No. 61775224).
Acknowledgments
This work was supported by the R&D Center for High Power Laser Components, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences.
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