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We have demonstrated a new approach for developing very large mode area silica-based microstructured Ytterbium (Yb)-doped fibers. The microstructured region acting as pump cladding around the core is composed by periodically arranged low-index Fluorine-doped silica inclusions with an extremely low filling ratio of 0.088. To the best of our knowledge, we achieved the most accurate controlling on cladding index by 1 × 10−5 via our passively doped cladding (PDC) method. Two fibers with 127μm and 50μm core diameter respectively were fabricated from the same final preform designed by this approach. It is verified that our 50μm core diameter fiber can maintain robust single mode behavior at 1064nm wavelength. The advantage of an all-solid structure along with a much simpler fabrication process makes our approach very suitable for realizing very large mode area fibers for high power fiber laser application.
© 2016 Optical Society of America
1. Introduction
The output power of high power fiber laser systems has been promoted sharply in recent years, which is largely dependent on the advances in specialty fiber design and fabrication allowing impressive mode area scaling [1–4]. However, the further improvement of high power fiber amplifiers is still being limited by two detrimental effects: nonlinearity and mode instabilities (MI) [5,6]. A direct and effective method to suppress the adverse nonlinear processes in active fibers is scaling the mode field area, and shortening fiber length used. Nevertheless, the increase of the core diameter usually comes along with transition from effective single mode to few-mode or multimode guidance. In recent years, effective single mode operation in very large mode area (VLMA) fibers has been demonstrated based on representative fiber structures such as large pitch fibers (LPF) [4,6], chirally coupled cores [7]and distributed filtering fibers [8]. Unfortunately, the high order modes (HOM) discrimination capability of VLMA fibers decreases with larger core sizes, and thermally induced refraction index change produces non-ignorable effect on the actual index profile of fibers [9,10] when they operate at a significant power level, thus allowing the propagation of HOMs, which consequently facilitates MI happen at high power operation.
A few years ago, distributed filtering fibers [8] have been fabricated with symmetric cladding composed of periodic air-holes and germanium (Ge)-doped silica inclusions. It was proved that such fibers can improve mode stability and maintain effective single mode laser emission even at a core diameter of 85μm. More recently, the approach to break the cladding symmetry of LMA fibers has been proposed to enhance the delocalization of HOMs by Ge-doped silica filaments [11] or by arranging background materials with silica or fluorine (F) doped silica inclusions lattice [12], which enable a robust single mode operation of the laser emission. However, the air channels embedded in those fibers increase the complexity on fiber treatment in practical use, and even challenging trouble is to splice these fibers to the other conventional fibers. Especially, the design of all-fiber-schemes for high power fiber lasers or amplifiers will have to be given up when one use an air cladding fiber with glass diameter larger than one millimeter. Moreover, A large number of Ge-doped silica inclusions in pump cladding [8,13] will inevitably capture and guide pump light along fiber length, which wastes pump light energy and increases the difficulty of stripping unabsorbed pump light, and then results in heat dissipation problem, that is another critical problem along with high power fiber amplifiers or lasers.
All solid VLMA leakage channel fibers (LCF) are also investigated in literature [13] with multiple coupled smaller cores in the cladding of all-solid photonic bandgap fibers for further mode area scaling as well as for coiled operations. Unfortunately, a mass of high index lattice in the cladding will also make the fiber meet the issues with pump stripping and heat dissipation when the design is used for active operation in double cladding structure. More recently, research focusing on all-solid active photonic crystal fibers made of silicate glass [14] and phosphate glass [15] has been reported, which extended the materials region for large mode active fibers. Extensive research on silica-based all-glass LCFs were also reported to scale the mode area with different schemes in details [16–19], which features the structure with a very large mode area and a high filling-ratio F-doped silica lattice arranged periodically in pure silica background of fiber claddings. Nevertheless, the high filling-ratio F-doped silica inclusions in cladding of LCFs act as an obviously low index region around active doped core which leads to pump light difficult to pass through the active core. On the contrary, the design with low F-doped silica filling-ratio presented in [20] is more attractive to achieve practically effective single mode large mode area and high pump absorption, as well as easy-to-use for high power fiber amplifier or lasers because of its all-solid structure and similar lattice compared to those rod-type photonic crystal fibers just with air holes replaced by F-doped silica inclusions.
In this paper, we have demonstrated a new approach for developing VLMA silica-based microstructured Yb-doped double cladding fibers. The microstructured region acted as pump cladding around the core is composed by periodically arranged low-index F-doped silica inclusions with an extremely low filling ratio of 0.088. To the best of our knowledge, we achieved the most accurate controlling on cladding index by 1 × 10−5 via our PDC method. Two fibers with 127μm and 50μm core diameter respectively were fabricated from the same final preform designed by this approach. It is verified that our 50μm core diameter fiber can maintain single mode operation at 1064nm wavelength. Finally, it is worth to believe that our approach to make the large mode area fibers is very suitable for high power fiber amplifiers or lasers application.
2. Design of all-solid VLMA fiber
To ensure the practicability of our fiber in high power fiber amplifiers or lasers, three primary targets should be met by the designed fiber. They are large mode area at the wavelength around 1μm, suitable to employ double cladding pump technique and high pump absorption, and easy to be spliced with conventional fibers. A general view of the refractive index profile of the designed fiber is shown in Fig. 1(a). The fiber concept demonstrated in this work was designed in all-solid microstructure with conventional hexagonal lattice which is arranged by the fundamental units composed with F-doped silica inclusion surrounded by pure silica or passively doped (e.g. Ge-doped) silica as PDC materials, shown in Fig. 1(b). The fundamental units are designed and fabricated in the same one preform. As a contrast, the units used in [12] including high and low index inclusions need to be prepared respectively, which is much discommodious in practical fabrication process. The fiber is supposed to have an unprecedented low filling ratio (e.g. 0.088) of F-doped silica in order to achieve an extremely low index change of the cladding. With the addition of a new freedom degree to adjust the passively doped (such as F, Ge) concentration of the fundamental units, one can accurately control the effective index of PDC obviously beyond the ability of air hole technique to match the refractive index of the actively doped core.
Fig. 1 (a) Schematic refractive index profile of the designed fiber. (b) a general view on the preform cross-section of the proposed all-solid VLMA fiber; the index profile along x direction has been given above. (c) index profile of one of the fabricated F-doped preform used in final fibers in this work, which is corresponding to a part of the fundamental unit. (d) relation of cladding index and filling ratio of F-doped silica: accurate adjusting of the effective index of cladding. Air cladding situation is also calculated for comparison. Insert is partly enlarged.
Figure 1(d) illustrates the theoretical demonstration by finite element method on the ability of our approach associated with PDC method to control the effective index of cladding. The wavelength is set at 1064nm for all the simulations in this work. DeltaF is usually defined as the ratio of index difference between Fluorine doped silica and background materials to that of background materials. DeltaF here stands for the relatively F-doped induced index change compared to pure silica in this simulation.
It can be seen from Fig. 1(d) that PDC method has a further refined ability to adjust the effective index of cladding than that of air hole cladding technique. Air hole cladding technique has only one freedom degree of changing its filling-ratio. However PDC method can further adjust its passively doped concentration to control the index profile of the cladding. For instance, when the filling-ratio of F-doped silica is fixed at 0.088, the effective refractive index of PDC cladding (the refractive index of pure silica is set at 1.45702 used in our simulated model) is changing from 1.457 to 1.4569 when deltaF is set from −0.2% to −1.7% correspondingly (four values of deltaF marked in Fig. 1(d) coming from the four fabricated F-doped preform with index profiles similar to that of Fig. 1(c)). DeltaF of −0.2% can only produce index change by 2 × 10−5 compared to pure silica. Based on these results and our experience in fiber fabrication process, we can ensure that minimum effective index change of 1 × 10−5 can be achieved controllably in practice fiber fabrication process, because the deltaF of −0.05% is easily obtained by our plasma chemical vapor deposition (PCVD) process. As a comparison, our simulation indicates that the index change induced by air hole cladding technique is up to 32 × 10−5 compared to pure silica when air filling ratio is also fixed at 0.088. Thus we achieved the best exactly controlling to cladding index by our PDC method to the best of our knowledge. This is a significant improvement for realizing VLMA single mode fiber that needs a slight and controllable index difference between cladding and its active core.
3. Fiber fabrication and characteristics
The F-doped silica preform was fabricated by PCVD process with a 4.3mm F-doped region diameter (dF = 4.3mm) and deltaF of −1.2%, and a 21mm glass diameter, as refractive index profile shown in Fig. 1(c). Then it was over-cladded with a suitable silica jacket tube to form the preform that was then drawn to the small rods used in stacking process. Meanwhile, the large core Ytterbium doped preform was made by MCVD process with our vapor deposition technique and then a 5.5mm ytterbium-doped core diameter was achieved with 0.03 core numerical aperture (NA) compared to pure silica materials. The core region of this active preform was taken out separately by etching process, which serves as the core rod used in stacking process of the final preform. Much higher NA of active core rod can also be fabricated to be used in this kind of microstructured fibers because the effective refractive index of the cladding designed in this work can be changed controllably to match that of the active core. If necessary, background materials around F-doped silica region can also be passively doped with Ge to adjust the refractive index of cladding. Finally, the final microstructured fiber preform was assembled as the scheme illustrated in Fig. 1(b) using the above mentioned fundamental units and Yb doped core rod by conventional stacking process. Subsequently, such a robust preform could be easily drawn to the final all-solid fibers on a conventional drawing tower just with utilizing negative pressure to collapse the interspaces or gaps between fundamental units and jacket tube.
The cross-sections of the fabricated Yb-doped all-solid double cladding microstructured fibers are depicted in Figs. 2(a)-2(b) with core/glass diameter of 50/400μm and 127/1038μm respectively. They are both drawn from the same preform and coated with low index polymer as the first layer and special high index UV-cured polymer with operation temperature up to 150°C as the second layer coating. The background material of the structure is made of pure silica. The two fibers present average core diameter of about 50μm and 127μm, yielding into a mode field area of 1256μm2 and 6700μm2 respectively. The pitches of two fibers are 16μm and 41μm, as well as F-doped silica inclusion diameters are 1.4μm and 3.6μm respectively. It is noticed that the edge of pump cladding (shown in Figs. 2(a)-2(b), the modified edge of 400μm fiber was eliminated when cutting the fiber) is modified to noncircular in order to disturbe pump light to avoid spiral beam.
Fig. 2 Microscope images of the two fabricated fibers: (a) 127/1038 and (b) 50/400. Part of coating layer is shown in image (b) because of large size of the fiber. Simulated intensity distribution of the fundamental mode (c) and LP11 (d) in the core of the 400μm fiber. (note: seven units forming the fiber core are set just for the convenience of modeling, while the parameters of the fiber core are given by a single active doped rod in our model and fabricated fibers; the squares marked in (c)and (d) are also used for the convenience of calculation)
The mode property of our 400μm fiber (50μm core with 0.03NA, deltaF = −1.2%, filling ratio~0.088) shown in Fig. 2(b) are verified by finite element method, which indicates only four modes inside the fiber core when the fiber is in loose and straight condition. The calculated modal intensity profiles of the first two modes (LP01, LP11) are given in Figs. 2(c)-2(d), and further simulated analysis shows that the overlap of the mode field with the active-doped core area is below 37% for all HOMs and 69% for fundamental mode, which can ensure the preferential amplification and emission of the fundamental mode at 1064nm [6].
The LPF design features a relatively bigger pitch up to several tens of microns, which is beneficial to exacerbate the delocalization of HOMs out of the gain region to the passive region, minimizing their parasitic participation to the amplification process [6]. The design illustrated in Fig. 1 makes it possible to develop new LPFs based on this PDC method. The glass cladding diameter (that is also the pump cladding for our fibers) of this fiber can be easily reduced to a smaller value, such as 200μm or 250μm, just by jacketing the microstructured bundle with a smaller size glass tube during the procedure of stacking the fiber preform. By this approach, the area ratio of the actively doped core to pump cladding can also be increased controllable without introducing any other adverse influence on the fiber fabrication process because of its all-solid structured design. Though probably the fibers cannot be bent when the core diameter is rather larger and must be operate at stress-free condition as a rod-type fiber, it is more important that such fibers are easily spliced to the other conventional fibers using usual fusion splicer because of their small size and flexile property, as well as no air-holes inside the fibers.
It has been shown that even excellent beam quality (M2 < 1.1) in LMA fibers does not guarantee low HOM content, and that the presence of HOMs can lead to significant uncontrollable changes in beam quality and peak intensity [21]. Therefore, we verify mode quality of our 400μm cladding fiber by the method mentioned in [11,21] to prove the single mode behavior under any launching condition. We translated the input beam (100mw CW laser at 1064nm with 6μm core single mode fiber as output) along X direction and Y direction (perpendicular to X-axis) from −30μm to + 30μm attempting to excite any HOM and recorded a series of near field images at 1064nm wavelength. The fiber was bent in 33cm diameter coil with a total length around 4m. As it is clearly shown from Figs. 3(a)-3(b), there is no HOM excited which confirms the robust and effective single mode operation at 1064nm of the fiber. The beam quality was further verified by employing M2 technique for this 400μm cladding fiber (50μm core diameter). The fiber length and coil diameter are the same as the ones in the tests mentioned above. The wavelength of the laser used for the testing is also 1064nm. A CCD camera is deployed to record the output beam profile when the input beams are set at the centre of the fiber. The results are shown in Fig. 3(c). It can be seen that the measured M2 values is around 1.37 and the beam profile captured also indicate a good single-mode operation. When the same experiment of off-set launching testing was done with 1038μm cladding fiber, even the coiled diameter was adjusted to a series of values from 33cm to 100cm, we found that there are always a few HOMs inside the fiber core even when lunching the 1064nm laser at the centre of the fiber core, which indicates multimode feature of the fiber at the cold (not operating as a gain fiber) condition.
Fig. 3 Near field mode profile of 400μm cladding fiber at 1064nm under misaligning the launching condition. (a) X direction and (b) Y direction. (c) Beam quality measurement of the 400μm cladding fiber at 1064nm.
The laser behavior has been characterized by testing a 4 m length of our 400μm cladding fiber in a laser cavity with a 1064nm seed laser, with 100ps pulse duration (PD) and 500 kHz repetition frequency (FR), shown in Fig. 4(a). The evolution of the extracted laser power with respect to the absorbed pump power is shown in Fig. 4(b). The slope efficiency has reached to 77%. Meanwhile, a measured pump absorption coefficient of 3dB/m at 976nm was achieved by using the common cut-back method. The 1038μm cladding fiber has also been measured with the same scheme. For this fiber, the laser has a slope efficiency of 75% as shown in Fig. 4(b), and the pump absorption coefficient is measured to be 3.1dB/m at 976nm.
Fig. 4 Experimental setup (a) used for measuring laser efficiency and pump absorption, (b) the measured output laser power as a function of the absorbed pumping power of 400μm cladding fiber.
When mode profiles of 400μm cladding fiber is tested by a CMOS camera behind an f = 8mm aspheric lens that is located at output of the fiber under testing in active configuration as Fig. 4(a). Though the fiber was disturbed by stress (touch and bend the fibers), the mode profiles at near field show stable and robust performance. The core mode distribution out of the active fiber, which was bent in 33cm diameter coil, was measured to be a steady pattern as depicted in Figs. 5(a)-5(b) without striping cladding modes out of the fiber. Then the same measurement is done with the 1038μm cladding fiber in loose condition under active configuration, and the similar performance is observed. The core mode distribution pattern of the 127μm core fiber was also recorded as Figs. 5(c)-5(d), which shows a fundamental-like distribution than the performance obtained in the cold test condition. Possible reason for this distinction is the mode competition effect in an operating fiber amplifier. It must be pointed out that the beam quality of our 127μm core fiber is distinctly influenced by the input beam quality and bending conditions. If the better beam quality is expected, a truly single mode beam should be used to excite only the fundamental mode of this VLMA fiber that is in stress-free condition. We choose a passively single mode fiber with 10μm core diameter and 0.08 NA as the input in our experiment.
Fig. 5 (a) Near field mode profile and (b) its intensity distribution along x direction of 400μm cladding fiber, (c) near field mode profile and (d) its intensity distribution along x direction of 1038μm cladding fiber.
The relatively low cladding pump absorption coefficient is mainly induced by the lower ratio of doped core to cladding (e.g. compared with the fiber in reference [8]), insufficient doping concentration, and non-optimized shape of pump cladding. We have drawn a short piece of the fabricated Yb-doped preform into a single clad fiber (core/clad, or a/b, 40μm /125μm) to check its core absorption before it is used in our final microstructured preform. Core absorption coefficient around 260dB/m at 976nm was measured by the commercial supercontinuum laser from YSL photonics and the conventional OSA based on standard cut-back technique. Then the cladding absorption coefficient at 976nm is estimated to 4dB/m according to the formula below [3,22]:
Where is pump absorption coefficient, is core absorption coefficient. It is assumed that the outer edge of the fiber is sufficiently irregular to frustrate the propagation of skew rays that never intersect active core.
The optimal index-matching between active core and pump cladding is the critical issue on active VLMA fibers and will be relatively easy to come true based on our all-solid microstructured PDC fiber design, because the refractive index profile of cladding can be adjusted accurately to match that of active doped core.
4. Conclusions
We demonstrated a new approach for developing VLMA silica-based double cladding Yb-doped microstructured fibers via PDC method. Although the index-matching and pump absorption are not yet perfect, the most accurate controlling of cladding refractive index up to 1 × 10−5 is achieved in this work. Moreover, an efficient single mode operation was obtained from our 50μm core fiber. However, the 127μm core fiber could not be verified in single mode operation at present structure design.
Besides, compared to air-hole-based photonic crystal fibers, all-solid designed fibers are easy to be cut and spliced with standard optical fibers. Practically, the control of the fiber parameters during the drawing process is much simpler and the development of parameters similar to the designed ones is straightforward, which allows for a very high production yield. Though further improvements are needed for this kind of fibers, it is believed that PDC approach will be an attractive and practical technology to realize fibers suitable for high power fiber amplifiers and lasers. Prospectively, the PDC approach combined with the stack-draw method can be applied to achieve multi-core fibers [23,24] with the all-solid structure aiming to the extremely high-power coherent fiber laser system, which would pave the way of new gain fiber fabrication and drive the research of fiber laser applications.
Acknowledgments
This work was financially supported by the National Natural Science Foundation of China (Grant NO. 61535009).
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