We report development of ytterbium doped silica fiber with nanostructured core for laser applications. We study influence of non-continuous distributed Yb dopants on gain, beam quality, and fiber laser performance. The fiber core is composed of over 43 thousand nanorods with a central part doped with Yb. The diameter of each nanorod is 72 nm. With this method we obtained a flat refractive index profile with uniformity of 1.3 × 10−4 refractive index unit (RIU) despite the non-uniformity of 1.2 × 10−3 RIU in Yb doped preform rods used for the fiber development. We demonstrate a nanostructured core single-mode fiber laser with 61.8% of slope efficiency, and extremely low numerical aperture 0.027 of generated mode.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
High power fiber lasers have had great impact on industry development over last three decades since double-clad structure was demonstrated . The main advantages of fiber lasers are power scalability, high efficiency, very good beam quality, high reliability and relative simplicity of laser design.
Power scalability and beam quality of fiber lasers are related with the increasing of fundamental mode field area in fiber waveguide, which has significant influence on reducing existing limitations like thresholds for onset of nonlinear effects, material damage or thermal lensing [2,3]. In fibers an increase of the fundamental mode area is directly related to decrease of numerical aperture (NA) of the fiber, which in all-solid fibers is usually the matter of precise controlling of the refractive index [4,5]. Existing technologies of doped glass development have limitations concerning achievable refractive index homogeneity. In widely used Modified Chemical Vapor Deposition (MCVD) method, the highest homogeneity of the doped silica glass is at the level of 2.2×10−4, although to obtain this uniformity, advanced technology for preform development is required and its yield is limited . Standard quality processes offer preforms with uniformity at the level of 10−3 refractive index unit (RIU). Another modified sol-gel synthesis allowed obtaining glass with refractive index fluctuations down to 2×10−4  or even 1×10−4 . Powder sinter technology provides refractive index homogeneity at level of 4×10−4 . Those methods are also limited to symmetrical and step-index like refractive index profiles. Thus the largest mode areas achieved in all-solid fibers, applicable for single mode high power laser performance, did not exceeded 1000 µm2 [4,5,9]. Researchers presented several fiber designs toward all-solid large mode area (LMA) structures, but structures such as Gain Guided Index Antiguide (GG IAG)  or Chirally Coupled Core (CCC)  fibers had too low efficiency for practical use. Another solution was HOM (higher-order mode) fiber. In this kind of setup, a 6000 µm2 effective mode area was achieved for LP0,14 mode in a standard step-index fiber . HOM fiber system required additional elements like long period grating or axicons for mode conversion which have detrimental influence on achievable efficiency in such fiber lasers. Thus HOM fiber was challenging in very high power laser applications. Apart from all-solid design there were structured designs such as LMA photonic crystal fibers (PCF)  or leaky-channel fibers (LCF) [14,15]. They significantly exceeded 100 µm of core size and were used for very high power laser applications. However, industrial use of those air and glass structured fibers is still challenging due to i.e. cutting, splicing or fiber Bragg grating inscription. Remarkable achievements concern all-solid bandgap fibers (AS-PBF) [16,17] and fibers with aperiodic inner-cladding structure . In fact, significant share of high power fiber laser results, reported so far, have been achieved with conventional all-solid index-guiding LMA fibers . All-solid LMA fibers are still favored in the industry due to their compatibility with standard fiber splicing techniques and heat dissipation capability.
Recently we showed an alternative solution for optical fiber design and development with significantly increased mode field area. We proposed the technology of nanostructurization of entire core area, which allows to develop any arbitrary designed continuous refractive index profile distribution with very high precision. This method allows to use moderate quality MCVD preforms with a central dip, noncircular shape and non-uniform distribution of active dopants, as well as to avoid sophisticated low-yield methods of preform development for high quality active fibers. We proved in numerical studies that fiber with nanostructured core with an axicon profile design would bring the fundamental mode area as large as 1530 µm2 , which is the largest mode area achieved in all-solid fiber applicable for very high power fiber laser and amplifiers. According to limits of MCVD and lack of appropriate technology only theoretical studies on refractive index shaping might be found in literature, e.g. Fini et al. proposed the large mode area fiber resistant to bend loss with the core with refractive index hybrid profile that was a compound of parabolic and axicon profiles .
Use of submicron rods to create a fiber core was preliminary considered by Wadsworth et al.  to obtain quasi-uniform doping in large mode area photonic crystal fiber (PCF) and to unify refractive index of doped area with undoped silica. Presented uniformity of ytterbium doped silica core was very limited and relatively low efficiency of 21% was reported. Nanostructured ring was applied in reduced mode overlap (RMO) fiber to study photo darkening effect in single-mode fiber for laser systems [23,24]. RMO fiber had plain germanium doped silica core surrounded with the nanostructured ring, which was created by Yb and Al co-doped silica merged into F co-doped silica material. However nanostructured material was not considered in optical fibers as a medium responsible thoroughly for laser generation and light guiding in a form, which would enable arbitrary shaping of refractive index profile. Velmiskin et al.  attempted to achieve glass chemical homogeneity by multiple stack and draw processing of the active ytterbium-doped phosphosilicate glass rod preforms made by the powder-in-tube method. The authors did not use nanostructurization, since stacked rods were doped within all their volume. Similar to Velmiskin’s approach was presented by Kong et al.  using etched MCVD preforms for repeatable stacking and drawing process. The authors obtained a moderate core uniformity of about 4×10−4 RIU that can be estimated on the provided data.
Worth mentioning is the multifilament approach for LMA fiber lasers, in which the filaments of few micrometers in diameter were applied within the fiber core resulting with the supermode consisting of modes propagated in each filament . In nanostructurization approach we consider subwavelength elements of diameter less than λ/5, in which the propagation conditions are not fulfilled in none of elements. The structure has to be considered as an effective medium, which allows to shape optical properties with high precision.
In this paper, we present the nanostructurization technology applied within whole core of active silica fiber. In laser setup we achieved high quality single mode performance with 61.8% of slope efficiency, which is the best result for active nanostructured core fibers. We used single-mode fiber with the core diameter of 16 µm and fiber length of 4.2 m. We achieved the maximum output power of 3.1 W, which was limited by our currently available pump power. The homogeneity of developed active nanostructured material was 1.3×10−4, hardly achieved in methods suitable for development of rare-earth doped silica. We used IFA100 - the most accurate device in the world - to measure the refractive index profile in the fiber, and we believe that homogeneity of our nanostructure material is much higher, as a result was in the range of device measurement uncertainty.
2. Development of active fiber with nanostructured core
The nanostructured core is composed of two kind of glasses with different refractive indices, arranged with designed pattern, uniform along the fiber. The size of discrete glass areas are of the fraction of the light wavelength in cross-section of the fiber. In practice, refractive index distribution can be treated as continuous and its refractive index profile can be described using Effective Medium Theory (EMT) and Maxwell-Garnett mixing formula . The refractive index of nanostructured material can be treated as a compound of refractive indices of each glass area averaged over a certain neighborhood inside the material, if single glass areas are up to fifth part of radiation wavelength (λ/5) . It results in continuous-like refractive index profile according to the designed distribution of discrete glass areas.
Nanostructurization allows to overcome accuracy limits related to MCVD method, although it can base itself on components developed with standard MCVD technology. In this case the accuracy of determining the refractive index of nanostructured material is higher, than the accuracy of determining the refractive index of each individual glass area due to the averaging over a certain neighborhood. The accuracy increases with increasing number of individual glass elements in the considered nanostructured material .
Manufacturing of nanostructured core fiber is based on the mosaic technique (known as a stack-and-draw), which was originally used for imaging waveguides , or for photonic crystal fibers . We successfully used this technique for development of parabolic nanostructured gradient optical fiber core [33,34], nanostructured GRIN micro-axicon lenses , and micro-vortex  made of soft glasses.
In the present case we manufactured a nanostructured core fiber with an effective step-index profile to study a fundamental interaction of gained beam with nanostructured active medium over long distance, as rare ions doped optical fiber structure. However the main advantage of the proposed nanostructurization technology is a possibility of arbitrary shaping of refractive index profile in the fiber core that allows further dramatic increase of effective mode area and reduction of bend loses in active fibers as we predicted recently for passive fibers with nanostructured core .
For the development of nanostructured core fiber we used ytterbium doped aluminosilicate rods drawn from MCVD rod preform acquired from Lumentum Inc. No additional content for lowering of photodarkening was applied. The outer diameter of MCVD preform was 15 mm and inner ytterbium doped area diameter was 5 mm with non-uniform refractive index profile and large dip in the center. This is typical preform profile obtained with MCVD method and used in active fibers. The non-uniformity in refractive index distribution in the core are in this case 1.2×10−3 as shown in Fig. 1. The preform was manufactured using F300 Heraeus silica tube.
We etched the preform rod down to 8 mm of outer diameter to achieve the designed average refractive index of the preform of 5.9×10−4 above undoped silica, which would be the effective refractive index of the nanostructured core in the final fiber. For the preferred refractive index value, the single mode regime is expected for fiber with circular shape core of diameter less than 19.5 µm at 1050 nm wavelength. The design of effective step-index profile allows to distribute nanorods in the final structure on regular hexagonal lattice. This approach of nanostructurization differs from those presented by us previously, in which the nanostructure was developed with doped and undoped nanorods, and their target distribution was determined using simulated annealing algorithm, allowing to design any effective refractive index 2D distribution profile from the range between indices of doped and undoped glass [33–36].
MCVD rod preforms used for developing the nanostructured core fiber were doped with ytterbium at level of 8265 ppm by weight, which corresponds to absorption coefficient value of 615 dB/m at 975 nm. Considering the filling factor of the doped and undoped glass areas in MCVD preform used for nanostructured core development, the average concentration of the nanostructured core material was reduced to 3223 ppm by weight, which gave the effective absorption of 240 dB/m at 975 nm.
Selection of a step-index profile of the fiber core allowed us to use repeatable stacking and drawing of the same doped rods. In this case an assembly of intermediate hexagon preform with only 217 rods of Yb3+ doped silica instead of thousands was required to achieve the nanostructured core . In the next step of manufacturing process, the intermediate preform was drawn into 199 hexagonal canes, which were stacked again in hexagonal lattice to form the final core preform. With this method we achieved the core structure consisted of 43183 individual elements, arranged in quasi-circular shape (Fig. 2). This preform can be used to develop fiber with the uniform step index core with the diameter up to 43 µm, since a condition of effective medium for minimum feature size equal to λ/5 is fulfilled for 1 µm wavelength .
Finally, we drew the fiber with a nanostructured core, an octagonal pure silica primary cladding and a polymer secondary cladding (Fig. 3). The nanostructured core had a diameter of 16 µm. The calculated diameter of each single element in nanostructured core was 72 nm, which is small enough to fulfil the λ/5 criterion. Unlike to SEM images of core preform, shown in Fig. 2, the individual ytterbium doped areas in the final fiber core were not observed due to low contrast between doped and undoped silica glasses and diffusion. Temperature of the fiber drawing process was relatively low for silica, i.e. 1910 C degrees, in order to decrease diffusion between doped and undoped glass areas. Similar drawing parameters were set during fabrication of silica fiber made with higher doping level of germanium nanostructured rods, in which the doped and undoped areas were distinguished .
The octagonal cladding has a diameter of 145 µm side-to-side. The fiber was coated with low index polymer to achieve double-clad structure, commonly used in fibers for laser applications . The absolute value of refractive index of polymer at 975 nm was 1.379 to achieve numerical aperture of pump waveguide of NA = 0.45.
Fabrication of the nanostructured core fibers or optical components is technologically feasible as shown in this publication and in several successful developments presented before [33–36]. Drawing procedure based on repeatable stacking of subpreforms reported in this work decreases significantly the manufacturing cost, however it limits a flexibility in development of arbitrary refractive index profiles offered by nanostructurization technology. We assembled the preform manually, but in case of implementation for mass production the use of robotic assembly would be adequate. In summary, the full process of fiber development starting from drawing MCVD preforms, through twice stacking and drawing process, and drawing final active fiber with cleaning treatment took about 40 working hours.
3. Characterization of nanostructured core fiber
We measured the refractive index profile of manufactured fiber using standard telecom optical fiber analyzer (IFA-100, Interfiber Analysis Inc.) at a wavelength of 633 nm. According to theoretical criterion of λ/5 the size of a single element in the nanostructured core for 633nm wavelength operation should be smaller than 126.6 nm, which was successfully fulfilled in the manufactured fiber with single nanorods with 72 nm of diameter. We measured the difference between refractive indices within the nanostructured core of 1.3×10−4, as shown in Fig. 4, which is one of the best results achieved with current technologies [4,6,7,8], although we used a MCVD preform with refractive index variation of 1.2×10−3 in the core area. We suppose that homogeneity of our nanostructure material is much higher than measured. Accuracy of measurement is limited by the sensitivity of the state-of-the-art IFA100 instrument used for this purpose, which is in the range ± 1×10−4, according to producer . The average refractive index difference between the core and cladding is 5.7×10−4 above undoped silica, which corresponds well with designed value of 5.9×10−4, since difference between designed and measured values are within uncertainty range of the performed measurements.
Next, we characterized also stress in the developed fibers. It is commonly known, that stress in optical fiber have an impact on refractive index profile. Axial stress is usually applied in birefringent fibers to control propagation constants of orthogonally polarized modes [39,40]. Distribution of stress in nanostructured core active fibers was not measured previously. For measurements we used IFA-100 Multiwavelength Optical Fiber Analyzer, which is standard tool used to determine the residual axial stress in telecommunication optical fibers . Measurements of stress profile in investigated fiber (Fig. 5) showed local values below 9 MPa, which were at level of standard telecommunication fiber SMF-28. The stress influence on refractive index profile can be neglected.
It should be noted, that the obtained effective properties of the core is not a result of homogenous distribution of Yb3+ ions, but stems from nanostructured construction of the core with inclusions of doped areas of sizes satisfying the λ/5 condition [29,34].
The nanostructured core fiber demonstrated in this work is very sensitive to bending due to the low difference of refractive indices between the core and cladding. High sensitivity to bending of step-index fibers with low index difference was reported in publications indicating critical bending of several tens of centimeters [4,5]. In our case we noticed significant loss of fundamental mode, if fiber was bent less than 100 cm in diameter. Bending losses are related to the low contrast between the core and cladding but not directly to nanostructuring. However nanostructuring may contribute significantly to confinement losses. Due to experienced bending losses, the confinement losses cannot be properly measured in long samples. It is also important to note that nanostructured core fiber was developed in research grade technological conditions, therefore the influence of nanostructurization, material losses and technological conditions on fiber transmission cannot be assessed separately. Influence of nanostructurization on fiber performance requires further studies using high quality glass components and clean –room technology conditions.
4. Verification of laser performance of nanostructured core fiber
The developed nanostructured core fiber was examined in standard laboratory laser system setup (Fig. 6). The laser cavity was formed by Fresnel reflection of 3.4% from the surface of the end faced of the fiber and by a butt coupled mirror, with high reflectivity (HR = 99%) for pump and generation wavelength. For pumping we used a multimode laser diode operating at 973.5 nm (4 nm spectrum) pigtailed with a 100 µm core fiber with numerical aperture of NA = 0.22 and top-hat profile of the output beam. Laser diode was driven using standard Laser Diode Driver with build-in diode temperature controller (Thorlabs ITC4020). We used an optical system formed by pair of identical aspherical lenses (f1 = 20 mm) to couple efficiently the pump beam into the internal cladding of the examined fiber. We used dichroic mirror (98% reflectance for the wavelengths above 1020 nm and 97% transmission for the wavelengths below 985 nm) to separate the laser output from the pump radiation. The dichroic mirror was set at the angle of 15° with respect to the laser output incident radiation. The coupling loss between pump delivery system and the internal cladding of the examined fiber, was only a 3.4% related to Fresnel reflection losses.
Referring to the absorption spectrum in the bulk doped silica, the spectrum of the pump diode radiation and the ratio of the doped area to the internal cladding area in the fiber, the unsaturated pump absorption in the fiber was estimated as 2.49 dB/m according to standard formula concerning double-clad structures .
Figure 7 shows the laser output power versus the launched pump power for 4.2 m long fiber, which was effectively straight according to high sensitivity to bending. The slope efficiency achieved in that fiber was 61.8%, which proves that it is feasible to achieve efficient laser based on nanostructured core fiber. The maximum power of the single-mode laser output was 3.1 W. For pumping we used the setup with maximum output pump power of 5.8 W. Power scaling requires the use of appropriate pump power setup. The laser action threshold was 0.8 W of launched power. The calculated mode field diameter (MFD) was 19.7 µm (Aeff = 306 µm2).
The laser line at threshold was 0.2 nm wide with a central wavelength of 1045.3nm, as shown in Fig. 8. Upon increasing of the pump power to maximum achievable power, we obtained a spectrum composed of several longitudinal modes with different frequencies, but at the fundamental transverse mode. In this case the output spectrum was about 10 nm wide (1041-1051 nm), as shown in Fig. 9. This type of relatively broad spectrum of fiber laser generation is typical when non-selective refractive mirrors are used in the fiber resonator . The use of frequency selective elements in the laser setup, e.g. Bragg grating, would result in narrowing of the laser spectrum.
A Gaussian-like single mode pattern was observed in the far field pattern of the laser beam. Measured M2 for the generated beam was below 1.1, which confirms single mode performance of the fiber laser.
We measured the numerical aperture of the output laser beam using standard method based on measurement of mode field diameter in far field . Very low numerical aperture of NA = 0.027 was measured for the generated beam, which is in exact agreement with calculated numerical aperture of fundamental mode in the far field. In this model we assumed a fiber with standard step-index core and refractive index difference between the core and the cladding equal to 5.9×10−4, which corresponds to the effective parameters of developed nanostructured core fiber.
The nanostructured fiber has significantly more complex internal structure than standard step-index fiber. Despite this, the light scattering for the tested nanostructured fiber and for a step-index fiber developed with standard MCVD technology with a core of the same diameter was at a similar level. Due to diffusion in the nanostructured core, there are no sharp boundaries between doped and undoped areas. Thus the light scattering is limited in nanostructure and it should not be significant source of additional loss, harmful for laser performance.
It is important to note that standard fibers for high power laser applications have large cores as reported in [4,5], and our test fiber does not overcome the performance of those fibers. The presented nanostructured core fiber does not compete with state-of-the-art fibers if MFD or achievable power is considered. Our goal was to obtain a fiber with nanostructured core as a proof-of–concept of fibers with perspectives for refractive index and gain profile precise shaping.
We reported on active optical material, which exhibits effective properties on the example of double clad silica fiber with active Yb doped nanostructured core for laser systems. The fiber core was composed of over 40 thousands Yb doped silica nanorods with a diameter of 72 nm ordered in a hexagonal lattice.
We proved that despite of the number discrete nanorods within the fiber core, we are able to develop very good quality of nanostructured material using standard stack-and-draw technique with moderate clean room facility and moderate quality of MCVD Yb doped preforms. The proposed method allows to reduce non-uniformity of refractive index in the fiber core by one order of magnitude. We showed that due to nanostructurization an uniformity of MCVD preform can be neglected and fiber with uniformity refractive index distribution of 1.3×10−4 can be obtained.
We showed that nanostructurization does not bring any additional stress in the fiber which might influence its refractive index distribution. As a proof-of-concept a fiber laser with 61.8% of gain efficiency was presented. This result showed that nanostructurization of active gain does not bring any negative effects in terms of fiber laser efficiency.
Although in this paper we demonstrated a nanostructured fiber with highly uniform refractive index distribution (1.3×10−4), the main advantage of the proposed nanostructurization technology is a possibility of arbitrary shaping of refractive index profile in the fiber core that allows further dramatic increase of effective mode area and reduction of bend loses in active fibers as we predicted for passive fibers with nanostructured core .
The nanostructurization technology opens new opportunities in development of active fibers with arbitrary refractive index distribution and contribute to development very large mode area active fibers dedicated to high power fiber lasers, where MCVD technology cannot be directly and efficiently applied.
Fundacja na rzecz Nauki Polskiej (POIR.04.04.00-1C74/16).
POIR.04.04.00-1C74/16 operated within the Foundation for Polish Science Team Programme co-financed by the European Regional Development Fund under Smart Growth Operational Programme (SG OP), Priority Axis IV.
M.B., M.D.: Fibrain corporation (E) in the field of IFA-100 measurements.
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