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Abstract
A nanostructured core silica fiber with active and photosensitive areas implemented within the fiber core is demonstrated. The photosensitivity, active and passive properties of the fiber can be independently shaped with this new approach. We show that discrete local doping with active ions in form of nanorods allow to obtain effective laser action as in case of continuous distribution of the ions in the core. Co-existing discrete photosensitive nanostructure of germanium doped silica determine single-mode performance and allow inscription of highly efficient Bragg grating over the entire core area. Each nanostructure do not degrade performance of other one since physical interaction between active and photosensitive areas are removed. As a proof of concept, we have designed and fabricated the nanostructured, ytterbium single-mode silica fiber laser with the Bragg grating inscribed in the entire core area. We demonstrated fiber laser with good quality of generated laser beam (M2=1.1) with lasing efficiency of 44% and inscribed Bragg grating with 98.5% efficiency and -18 dB contrast.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Over the last 30 years, fiber lasers have undergone massive improvement in optical performance, becoming one of the most reliable sources of extremely good quality, high average and peak power beams [1,2]. Owning to the environmental robustness, reliability and alignment-free configuration fiber lasers have been applied in areas ranging from fundamental science, through industrial production to defense applications. The excellent properties of fibers as lasers originate from their waveguide geometry, whereby the fundamental mode is well confined in an active core of the fiber. That defines the beam quality, enables interaction path with the active medium, and allows very large single-pass gains. The capability of high-power operation is also a consequence of the high surface-to-active-volume ratio offered by the fiber geometry, which allows for the dissipation of heat generated during the lasing process.
Simultaneously, the waveguide geometry is also the main source of limitations of modern fiber lasers. Tight confinement of light over long interaction lengths leads to the occurrence of nonlinear effects, which can distort spectral and spatial characteristics of laser emission. Among them, stimulated Brillouin scattering (SBS), stimulated Raman scattering (SRS), self-phase modulation (SPM) and self-focusing (SF) have the most significant impact on laser performance. Since each effect is based on a different physical mechanism, various strategies to mitigate them have been proposed in the literature [3]. The most general approach to simultaneously mitigate the majority of these nonlinear effects is to reduce the density of electric field in the fiber core. This can be achieved by increasing the core diameter, and therefore by extending the fundamental mode (FM) effective area [4] to obtain waveguides known as large-mode-area (LMA) fibers. Although this is a potentially simple solution, it carries a risk of reducing the beam quality due to multimode (MM) propagation and high bending losses. Moreover a main limit of fiber laser is related an transverse mode instability (TMI) effect, which significantly restricts the power performance of the fiber lasers, as identified in 2011 [5]. In recent years, TMI has been perceived as having the strongest negative implications for the development of high-power lasers [6].
To preserve the single-mode (SM) performance and to avoid issues resulting from the multimode propagation, one has to either reduce the index difference between the core and the cladding or to discriminate higher order modes (HOMs). The first approach can be realized, e.g. via the concept of photonic crystal fibers (PCFs), where by adjusting the arrangement of inclusions or holes located in the cladding area one can obtain single-mode operation waveguides with large cores areas and very low numerical apertures (NA). However, such PCFs have to be kept straight in order to prevent high bending losses of the FM [7]. Therefore, a more promising approach is based on the idea of HOM discrimination in multimode LMA fibers. The discrimination can be realized using different mechanisms. One way is to introduce losses to the HOMs selectively by using fiber bending [8].
There are also challenges of technological nature, related to the fiber fabrication processes. The most widely used technique for manufacturing doped fibers is the modified chemical vapor deposition method (MCVD) [9]. The general advantage of the MCVD process is that in fibers fabricated using this method propagation losses are extremely low. The method is not without limitations, though. The main ones include restricted to cylindrical symmetry distributions of dopants, a lack of precise control of the refractive index across the entire profiles caused by the diffusion-driven migrations of impurities, and difficulties with obtaining simultaneously low and homogenous optical contrast between the core and the cladding [10, 11]. To mitigate these factors, several non-CVD techniques were introduced and verified such as, e.g. the modified sol-gel method combined with a high-temperature melting and molding technology [12] or utilizing sintering processes [13].
More importantly, there is also a more general constraint which affects all of the abovementioned fabrication methods. Although various dopants are used for different purposes, not all of their combinations can be used at the same time and in the same glass volume. The first reason is the process of clustering, which can be triggered by the presence of incompatible ions. For instance, too high simultaneous doping of fused silica with Al and Er atoms leads to the formation of aggregates, and thus to the lower laser efficiencies and operation instabilities [14]. Secondly, some of the dopants cancel out each other's effectiveness. For example, phosphorous doping is known to reduce fiber photosensitivity, when some germanium is incorporated [15]. We note that phosphate fibers express photosensitivity after hydrogen loading. In this case inscription of FBG with UV illumination is possible, phosphorous bleaches the absorption band centered at 240 nm and produces a weak band at 210 nm. After hydrogen treatment strong fiber Bragg gratings inscription is possible when ArF (193 nm) rather than KrF (248 nm) pulsed laser is used [16–18].
A direct consequence of dopant mixing is the issue with designing active fibers with arbitrary waveguiding properties since classical dopants cannot be used for refractive index control [14]. Another example is the difficulty with the inscription of fiber Bragg gratings (FBG) into active fibers. Since the standard writing method requires high-level doping of germanium or/and boron atoms, the realizations of fiber lasers constructed using both active and photosensitive medium are scarce. Nevertheless, because the FBGs are the crucial elements for all-fiber cavity lasers, workaround methods are used, with the most common approach of splicing an external FBG to the active fiber [19]. This solution, however, increases the complexity of the system and introduces additional splice losses. Yet another approach is to use the silica fiber with the core consisting of two separate active and photosensitive rings. The central area of the core doped with erbium/ytterbium is surrounded with a germanium doped ring [15] or vice-versa [20]. Then again, as shown in the literature, this type of design either suffers from poor efficiency of the Bragg grating or has a low gain caused by the low overlap between the FM and the active or photosensitive areas. Finally, periodic refractive index changes needed for the FBG can be created by short pulses with high energy radiation. In this approach, there is no need for the photosensitive medium, and studies have demonstrated successful realizations in silica [21], phosphate [22] or fluoride [19, 23] fibers. However, when using this method, the repeatability and control over the grating depth are not as good as those demonstrated with the usage of the photosensitive medium. Overall, it seems that the current development of active fiber lasers is constrained by many different factors and that the methods used to overcome them are not without major limitations.
In this paper, we propose a new strategy to the design and development of active and photosensitive fiber lasers that allows inscription of FBG directly into active core of the fiber. Our approach is based on the concept of nanostructuring, where by the arrangement of subwavelength elements one can obtain arbitrary designed gradient index refractive index profiles [24, 25]. So far, nanostructuring has been successfully used in passive-only fibers, with uniform and parabolic refractive index profile in the core [26–28] and in step index active fibers but without photosensitive dopants [29]. Also more complex symmetrical and non-symmetrical passive refractive index profiles are obtained, however, they are not demonstrated directly in fiber applications [30,31]. Here, we demonstrate that the flexibility of the nanostructuring design allows us to fabricate active fiber lasers and to simultaneously address some of the issues of modern fiber laser optics. Firstly, we show that the fiber waveguiding properties can be engineered independently of the active ones, allowing to fully implement the idea of confined doping. Secondly, we demonstrate that the problem of the dopants incompatibility is mitigated since we separate particular dopants in space. Thirdly, we show that our discretized structure can work similarly well or, under some circumstances, even better than fibers with the continuous distributions of dopants. Finally, we prove that the method permits for a simultaneous obtaining of low and effectively homogenous optical contrast between the core and cladding, with no restrictions regarding the type of symmetry. We also present a detailed modelling studies of independent shaping of active and passive properties of the nanostructured fiber. We also performed numerical analysis of lasing dynamics in fibers with discrete distribution of gain medium. As a proof of concept, we have designed and fabricated an active and photosensitive single-mode silica fiber with incorporated Bragg grating in the entire core area. In the fiber laser setup, for the fiber length of 20 m, we achieved generation with a very good beam quality and the slope efficiency equal 44% in relation to the launched power.
2. Concept of nanostructured active fiber with Bragg gratings
The nanostructured core in optical fibers is composed of a few thousand of subwavelength rods with similar diameter ordered in the hexagonal lattice. When the rods are made of two or more types of glasses with different refractive indices, their distribution determines the local refractive index experienced by a light beam. Nanostructuring allows to obtain any arbitrary refractive index profile [26, 28, 31]. The effective refractive index values can be calculated locally based on Maxwell-Garnett effective medium theory [25]. This approach works properly if the size of a single nanostructured component is much smaller than the wavelength. Usually, it is accepted that it should be smaller than λ/2π [25]. However, we have verified experimentally that in the case of nanostructured fibers and with diffusion existing, we can treat a discrete refractive index profile as a continuous one if the size of nanostructured rods in the core is comparable to λ/2.5 [26].
We consider a structure of the fiber with a core composed of germanium oxide and ytterbium doped silica nanorods distributed in the core (Fig. 1). Germanium oxide nanorods are photosensitive and they have higher refractive index than silica. Their roles are shaping refractive index distribution and create medium for Bragg grating inscription. Ytterbium doped silica nanorods create an area with gain to enable laser action. Development of core composed of ytterbium doped silica and germanium oxide nanorods allows development of fibers with large gain and simultaneously high photosensitivity in the same area. An inscribed Bragg grating can fully overlap with active area of the core.
Fig. 1. A concept of nanostructured fiber with active and photosensitive core for integrated fiber lasers. The fiber core is composed of interleaved silica, germanium oxide and ytterbium doped silica nanorods. Germanium oxide nanorods are used to shape refractive index distribution and inscribe Bragg gratings. Ytterbium doped silica nanorods create an area with gain to enable laser action
Ytterbium-doped silica is a preferable lasing material for high power lasers and generation/amplification of wavelength-tunable ultra-short optical pulses, since it offers broad gain bandwidth, high optical conversion efficiency and large saturation optical flux [1]. Together with these power-handling capabilities, ytterbium allows for the operation with pump wavelengths very close to the signal wavelength, which results in a reduction of the quantum defect heating [10]. It is worth stressing that also in case of the ytterbium ions the laser transition changes its property over the gain bandwidth [32,33], similarly to glasses doped with other ions such as erbium or neodymium. A spectroscopic study of relaxation oscillation frequency depending on the occupation of the terminal level clearly shows that the laser transition evolves from quasi three-level to four-level at the long-wavelength tail of the gain spectrum of ytterbium fiber (from around 1060 nm [33]).
3. Modelling of independent shaping of active and passive properties of the nanostructured fiber
To demonstrate that the proposed approach allows shaping the active area of the fiber independently of its guiding properties we designed three SM fibers with identical guiding properties and the same mode propagation constant β=n·k, where n is the effective refractive index of the propagated mode and k=2π/λ. In all cases considered, the fiber core has a diameter of 8.6 μm, mode field diameter of 11.43 μm and a numerical aperture of 0.044 at 1060 nm wavelength. These parameters correspond to the typical passive germanium doped fiber with step-index profile and the refractive indices difference of 17.5×10−4 (what corresponds to GeO2 concentration of 1.1 mol.% [34, 35]). Such classical step-index design, with the presence of only one dopant (germanium) in the core, is the first one among the core geometries considered here (Fig. 2(a), fiber no. 1). Since this fiber is passive, it serves us as a reference for comparing the guiding properties of the other two fibers.
Fig. 2. Three types of considered fibers with the same core size and different internal structure: (a) step-index fiber with homogenously Ge doped core, (b) nanostructured fiber with regularly distributed Ge and Yb doped nanorods in the core, (c) nanostructured fiber with active Yb doped nanorods located only in the center of the core and with Ge doped nanorods with two different doping levels distributed across the entire core area. Each dataset has in the upper row map with the distribution of the nanorods (left) and electric field distribution of the fundamental mode; the white ring indicates the physical boundaries of the nanostructured core (right). In the lower row, we present the mode profile (red region) and the profile of the refractive index (black line). All fibers have identical guiding properties, which matched to the step-index fiber of core radius equal 8.6 μm and refractive index contrast between the core and the cladding equal 17.5×10−4.
The other two fiber designs have a nanostructured core composed of 3 types of nanorods: pure silica, germanium-doped silica and ytterbium-doped silica. There are 1759 nanorods in the core in total what corresponds to 43 elements on diagonal. All nanorods have a similar diameter of 200 nm, and they are ordered in hexagonal lattice according to the arbitrary pattern. The silica nanorods are uniform over their cross-section. Geometrical and optical parameters of germanium and ytterbium-doped nanorods are determined by parameters of rod preforms developed with standard MCVD technique that we acquired from the vendor, and were used in the experimental part of this for nanostructured core fiber fabrication. Germanium doped silica nanorods are composed of silica cladding and germanium doped inner diameter of 186 nm. Ytterbium-doped silica nanorods are composed of silica cladding and ytterbium-doped inner diameter of 76 nm (refractive index equals to 1.4523 at 1060 nm wavelength).
Both nanostructured fibers (Figs. 2(b) and 2(c)) have passive germanium and active ytterbium-doped nanorods in the core, but they differ by their internal distribution. In the case of fiber no. 2 (Fig. 2(b)), both types of nanorods are spread equally over the core. Since the doped areas do not fill the entire core interior (only the regions proportional to the inner doped areas of nanorods) in order to obtain the same effective refractive index profile and thus identical guiding properties as for the reference fiber no 1. the Ge doping level in nanorods has to be increased till it corresponds to refractive index value 1.4579 (Ge concentration equal 5.5 mol.%) at 1060 nm wavelength. Fiber no. 3 has ytterbium-doped nanorods distributed only in the center of the core (Fig. 2(c)). Thus, the fiber gain is highly confined in the area close to the optical axis. To compensate inequality in the spatial distribution of the ytterbium and resulting change in the effective refractive index profile, the Ge concentration had to be further raised in nanorods located in the outer regions of the core. Numerical calculations indicated that in these nanorods the Ge doping should give the refractive index equal to 1.4593 (Ge concentration equal 6.4 mol.%) at 1060 nm wavelength.
In this way, we have designed two single-mode nanostructured fibers with different effective profiles of active dopants, but exactly the same guiding properties in terms of effective mode area, propagation constant and numerical aperture (NA) as the reference classical fiber. This show high flexibility of the nanostructuring design and indicates that the active and photosensitive profiles of the fiber can be shaped independently.
4. Development of nanostructured active and photosensitive fiber
In the development of the nanostructured core fiber fiber we used a set of germanium-doped silica rods and ytterbium-doped silica rods drawn from rod performs acquired from OptaCore (presently Lumentum Inc.). We used one MCVD-fabricated, 10 cm long ytterbium-doped aluminosilicate rod preform, with outside diameter of 15.1 mm, and with the diameter of the doped core of 5.7 mm with similar properties along the rod preform. The rod preform was fabricated using F300 Heraeus silica. According to the vendor’s information, the rod was doped with ytterbium oxide at 0.265 mol.%, which corresponded to absorption coefficient value of 1149 dB/m at a wavelength of 975 nm.
In case of the germanium silica rod preform, doped at 5.5 mol.%, the outer dimension and dimension of the doped core were respectively 27.9 mm and 25.9 mm. We used about 6 cm of the rod preform with similar properties along the preform for nanostructured fiber development. The central area of the germanium and ytterbium-doped preforms had the average refractive index values, respectively, of 82 × 10−4 and 26 × 10−4 above the undoped silica (Heraeus F300) in peripheral parts of rod preforms. The average refractive index values for both the ytterbium and germanium doped rod preforms were evaluated based on the refractive index profiles obtained from the vendor. The final nanostructured fiber was fabricated using the stack-and-draw approach [36]. This method was adapted to the assembly of a few to several thousands of submicrometre rods of two or more types of glass, into complex structures (layouts) and it has been successfully used for the development of gradient index micro-optical components as, micro-axicon lenses [31], micro-vortices [30], or GRIN parabolic microlenses [24]. The method of assembly is similar to the one commonly used for photonic crystal fiber development, although it requires additional one step of assembly to obtain the final preform. Assembly of the preform is done manually and takes in total about 3 working days. In future, a robotic assembly can be applied to reduce time for preform development. Three types of glass were used for the fabricated fiber: undoped silica rods and tubes (Heraeus F300), ytterbium-doped rods and germanium doped rods. In particular, we used 1759 rods to develop the core: 1320 silica rods doped with ytterbium (active medium) and 439 rods with germanium oxide (photosensitive medium) with the parameters corresponding to the ones presented in Fig. 3(b). All rods were arranged in a hexagonal lattice according to a design to form the circular core preform. The basic cell of the honeycomb pattern was created by one germanium doped rod surrounded by six ytterbium-doped rods and was repeated throughout the preform core. This resulted in a core with effective parameters corresponding to the step-index profile (Fig. 2), although its actual refractive index profile is not uniform (Fig. 3). In the next step, the preform was drawn into core subpreform. One core subpreform was placed into the octagonal shape silica tube, creating the final preform, which was after then drawn to the final fiber.
Fig. 3. SEM images of fabricated fibers. The cross-sections of manufactured Yb and Ge doped silica fiber (same parameters as fiber no. 2 in numerical investigations) without polymer cladding: (a) the overall view of octagonal shape optical fiber, (b) nanostructured core, (c) nanostructure inside the fiber core with Ge doped silica nanorods, Yb doped areas are not visible due to the low contrast. The SEM of the cross-section of manufactured Yb and Ge doped silica nanostructured core subpreform: d) the overall view of subpreform, e) the part of subpreform with visible germanium doped bright hexagonal shape areas and ytterbium darker doped circular areas between them arranged in honeycomb pattern, the background material is undoped silica.
The Scanning Electron Microscope (SEM) image of fabricated fiber clearly identifies individual areas of germanium oxide. Presence of ytterbium doped silica cannot be verified simultaneously with SEM image since a difference of refractive indices between silica and ytterbium doped silica is too small. In Figs. 3(d) and 3(e), we show the cross-section of the intermediate fabrication stage product, called a subpreform, in which the germanium and ytterbium-doped areas are clearly visible. It is noticeable that the germanium doped areas are hexagonal in shape, although crated with circular shape rods. The change in shape is due to the presence of the interstitial holes between stacked rods in the preform were to be filled due to glass viscosity and forces present during the drawing process, resulting with transforming circular rods into actually hexagonal rods. The same mechanism concerned Yb doped rods, but according to larger amount of undoped silica around doped rod in Yb in comparison with germanium rods, the influence on Yb doped rod cores were much weaker, allowing to behave more circular shape of Yb doped rod cores.
We achieved the fiber core with a designed diameter of 8.6 µm. The maximum diameter of individual Ge doped glass areas in the nanostructured core was 186 nm, which fulfilled the effective medium criterion of λ/5 diameter of elements in nanostructured material designed for 1 μm wavelength. The final fiber had an inner cladding diameter of 112 μm corner-to-corner, as it was prepared in an octagonal shape to increase the pump absorption [37]. The silica fiber was surrounded with a low-index polymer to create the waveguide for pump radiation of NA=0.45 at 975 nm. This led to creating the final standard double-clad laser fiber structure [37]. It is important feature since for end users this nanostructured core fiber can be treated similarly to any active all-solid step index double clad fiber. All complex structures that allow to obtain high gain and photosensitivity is hidden inside the core and it do not require any special treatment during cleaving, splicing or laser construction.
Considering the optical properties of Ge and Yb doped rods used for manufacturing we estimated that the effective refractive index difference between nanostructured core and cladding was 17.5×10−4, as used for numerical calculations presented in Section 3. The refractive index profile of developed fiber was measured using standard telecom optical fiber analyzer (IFA-100, Interfiber Analysis Inc.) at 633 nm wavelength (Fig. 4). The measured difference of effective refractive indices of core and cladding was 18.2×10−4, which is in good agreement with the designed value with consideration of the accuracy of IFA-100 instrument of ±1×10−4. We note that lateral resolution of the fiber analyzer is too low to show internal nanostructure of the fiber. According to the achieved core size of 8.6 µm, the cut-off wavelength of fundamental mode performance in investigated fiber was indicated at 803 nm. We experimentally analyzed the modal properties of the fabricated fiber. For characterization we prepared a similar fiber as described above but with a high index coating that could effectively suppress cladding modes. Light from a supercontinuum source filtered with several bandpass filters with the central wavelengths from 450 to 1100 nm was coupled to the nanostructured fiber section of about 1.5 m, and the mode field distribution was registered at the fiber output with the use of CMOS camera. Next, we changed the coupling conditions to excite the higher-order modes in the fiber. We experimentally established that the cut-off of the fundamental mode was around the wavelength of 750 nm, which is in good agreement with the theoretical value. The difference can be due to the determination of the core size, and quasi-circular shape of the core, which was not considered theoretically. Based on the obtained experimental results confirmed by numerical calculations, we can conclude that for the wavelengths of 973.5 nm (pump) and 1064 nm (laser) the structure operates as a single-mode fiber, as it was designed.
Fig. 4. Relative averaged refractive index profile of the developed fiber with Yb and Ge doped silica nanostructured core.
Considering the concentration of ytterbium ions in MCVD preform used for nanostructured core development (data in the beginning of Section 4), and the fill factor of the ytterbium doped rods within nanostructure, the average concentration of the ytterbium oxide in nanostructured core was reduced to 0.028 mol.%, which corresponded to absorption coefficient value of 123 dB/m at 975 nm. Based on the absorption spectrum of the ytterbium doped silica rod preform, the ratio of the doped area to the internal cladding area of the final fiber, and the spectrum of the pump diode radiation used in the experiment discussed in Section 6, the unsaturated pump absorption in the final fiber was calculated at 0.66 dB/m. The pump absorption coefficient (αCP) was obtained with the following equation αCP = αco(Aco/Acl), where αco is the average absorption coefficient of the core, Aco and Acl are respectively core area and cladding area [37]. The average absorption coefficient of the core takes into consideration the fill factor of ytterbium doped areas in the nanostructured core, and spectrum of used pump diode at 973.5 nm with spectral width of 4 nm.
5. Photosensitive properties of the fiber and Bragg grating inscription
The presence of GeO2 dopants in the fiber core is needed for the efficient FBG inscription with UV irradiation since fiber UV photosensitivity and permanent refractive index changes are required [38]. Higher dopant levels lead to enhancement of photosensitivity, but at the same time, they cause the increase in contrast between the core and the cladding, which limits a single mode performance in LMA fibers. During UV-irradiation, several physical processes happen simultaneously, and the relative contribution of the mechanisms can vary depending on the Ge dopant level, hydrogen/deuterium concentration in the fiber and irradiation conditions [39]. The enhancement of photosensitivity due to hydrogenation is minimal in fibers with low Ge concentrations, but the improvement can reach orders of magnitude in highly doped fibers [40]. A concept of nanorods allows to obtain a high local concentration of GeO2 dopants in subwavelength region and reduced effective refractive index for entire core area [28].
To analyze and compare the process of grating inscription in active nanostructured fibers with writing in passive classical step-index equivalent, we performed a series of numerical simulations. The schemes of considered structures were presented earlier in Figs. 3(a) and 3(b). The simulations were carried out for Bragg gratings consisting of 27500 periods, which corresponds to the ∼1 cm long FBG with Bragg wavelength of 1060 nm. The refractive index photosensitivity for hydrogen-loaded glass as a nonlinear function of doping level is presented with the red line in Fig. 5. It was calculated based on the model given by Konstantaki et al. [41]. In the case of the step-index fiber where the concentration of germanium in the core is 1.1 mol.%, the illumination with light of energy density of 600 J/cm2 results in the change of the refractive index in the core at the level of Δn = 0.26×10−4 (green point). In the nanostructured core fibers we assume the same effective refractive index profile. In this case the local Ge concentration in nanorods is adequately larger, and it equals 5.5 mol.%. If the photosensitivity was a linear function of the doping level, this higher local concentration would not affect the overall efficiency of the grating. Naturally, the local change in the refractive index in the nanorods after the exposure would be higher (blue point in Fig. 5, Δn = 1.29×10−4, but an averaged photo-induced refractive index change across the core would be the same as for the step-index fiber, i.e. 0.26×10−4. This claim is confirmed by the results of numerical modelling of the FBG transmission characteristics (Fig. 6(a)), where the curve corresponding to a continuous distribution of dopants (green line) and discrete distribution with linear photosensitivity (blue line) are identical. However, since in hydrogen-loaded fibers photosensitivity is expressed by a nonlinear dependence, the actual local change in refractive index is greater, and equals Δn = 1.47×10−4 (red point in Fig. 5). Thus, the averaged refractive index modulation across the entire core area is also higher (ΔnAVG = 0.29×10−4). As a consequence, we can expect that the reflectivity of FBG inscribed in nanostructured optical fiber with locally increased (in nano-rods) concentration of GeO2 is enhanced with the respect to FBG written in standard step-index optical fiber with the same average GeO2 concentration.
Fig. 5. Photosensitivity of Ge doped fibers. The dependence of the refractive index change on the concentration of the Ge dopant with the use of light of energy density 600 J/cm2, obtained for glass without (dashed line) and with (black solid line) hydrogen-loading. The dotted black line corresponds to a hypothetical case of hydrogen-loaded glass with linear photosensitivity. Points show the changes of the refractive index relative to the concentration of the dopant in the case of step-index fiber (green-point) and nanostructured core fibers with linear (blue point) and nonlinear (red point) regimes of photosensitivity.
Fig. 6. Comparison of calculated transmission spectra of Bragg gratings written in a step-index fiber (step-index), a nanostructured fiber with effective properties identical to a step-index fiber (nStruct step-index) and a fiber with non-linear dependence of refractive index changes relative to the dopant concentration (NL nStruct step-index) (a); transmission spectra of FGB inscribed in nanostructured fiber (b).
The effect of photosensitivity enhancement in the nanostructured active optical fiber has been confirmed in the experiment. The 1 cm long fiber Bragg grating was successfully inscribed in the core of nanostructured fiber using 248 nm KrF excimer laser with a pulse energy of 3 mJ and phase mask technique. Other important inscription parameters are: laser energy pulse density ∼300 mJ/cm2, pulse repetition rate 500 Hz and total number of laser pulses 60000. The period of the phase mask was 729.5 nm, which results in 1060 nm Bragg wavelength. The germanium-doped areas of 5.5 mol.%, distributed all over the core (Fig. 3), resulted with core average doping level as low as 1.1 mol.%. Thus, before the grating inscription, an optical fiber with nanostructured core had been hydrogen loaded under the high pressure (80 bars) and room temperature for one week to enhance its photosensitivity. We achieved FBG with -18 dB minimum in transmission spectrum (Fig. 6(b)), which corresponds to the reflectivity of 98.5% at designed Bragg wavelength of 1059.9 nm. In comparison to the earlier reported 8-cm long FBG in aluminosilicate 1 mol.% Ge co-doped optical step-index fiber written using UV (244 nm) radiation [42], nanostructuring has significant advantages. First of all, based on the reflection coefficients and gratings lengths, we calculated that the obtained coupling coefficient of nanostructured gratings was as high as 297 m-1 with respect to ∼100 m-1 for FBG presented by Zhang et al. [42]. Secondly, due to the spatial separation of the active and photosensitive rods, the formation of clusters did not occur, and additional co-dopants such as phosphorus or aluminum (that reduce UV absorption at 240 nm band) were not needed [43]. A further increase of FBG coupling efficiency between the propagating mode and grating structure is possible by matching the Ge rods arrangement in fiber core with the spatial distribution of propagating mode [28, 44].
Additionally, we have performed a direct experimental comparison of the grating growth during the FBGs inscription in equally hydrogen-loaded nanostructured step-index and standard step-index (SMF) optical fibers by means of phase mask with the period of 1061 nm. Using the same phase mask ensures identical UV writing conditions (e.g., phase mask diffraction efficiency), as well as gratings parameters (number of periods, Bragg wavelengths of 1537 nm) and single-mode operation for both gratings. The UV induced change in the mean refractive index of the fiber core (Δn) and thus its photosensitivity, can be deduced by measuring the shift of the central Bragg wavelength (ΔλB) during FBG inscription [41]. Dividing the ΔλB by phase mask period, we obtained the photosensitivity of SMF-28 (with c.a. 3.5 mol.% GeO2 concentration) expressed by the change in the mean refractive index of the fiber core equals Δn=7.3×10−4. It is only 15% higher than for the nanostructured step-index optical fiber with 1.1 mol.% averaged GeO2 doping level, for which Δn=6.2×10−4. This unambiguously confirms the advantage of nanostructuring, which offers similar photosensitivity with three times smaller averaged GeO2 concentration in comparison to standard SMF optical fibers.
6. Laser gain properties of the nanostructured fiber
6.1 Investigation of laser dynamics
In this part of work, we aimed at understanding whether and how the discrete distribution of gain medium around the entire core region affects the optical performance of the fiber laser, as shown in Fig. 2(b), and to expand our knowledge about the dynamics of the formation of the lasing mode in the proposed geometry.
The numerical model is based on the finite-difference time-domain (FDTD) method where the lasing dynamics of a four-level system has been integrated [45]. In this approach, we treat the atom quantum mechanically, and the electromagnetic wave classically. Although to solve this computationally-demanding problem, we calculated the electric and magnetic fields only in a subsection of the fiber laser and only for a λ/2 cavity length. The model of lasing action incorporates simplified quantized electron energies that provide four energy levels for each of two interacting electrons. Transitions between the energy levels are governed by coupled rate equations and the Pauli Exclusion Principle [45]. The model fully describes pumping and lasing dynamics. The angular frequencies corresponding to the energy differences of levels 2-1 and 3-0 in modelled ytterbium were equal 1.7770 × 1015 Hz and 1.93493 × 1015 Hz, respectively. In order to make time domain simulations feasible we have artificially reduced the levels lifetimes to 300 ps. We have also assumed optical cavity of λ/2 length, 400 nm width and periodic boundary conditions in the Y direction. The effective refractive index inside the cavity was equal 1.451455 similarly as the core of considered the step-index fiber. The averaged electron population density in the cavity area was equal to 6.25×1022 m-1. The amplitude of the electric field of the monochromatic source was equal to 106 V/m at the 973.5 nm wavelength. Used simplified model still provides useful insight into the lasing dynamics. To identify the differences between the lasing mode formation in case of homogenous or discrete distribution of active medium in the fiber core region, we present two sets of results in Fig. 7, where the left column corresponds to the first geometry and right to the latter one. In both cases, the cavity is similarly surrounded by Bragg mirrors, and the difference lies only in the distribution of the ytterbium ions (brown areas located in the first row schematics). Since the average amount of ytterbium ions per square micrometer of the fiber core cross-section is smaller in the structure with active nanorods than in the one with a uniform doping, to make the lasing efficiencies comparable, we artificially reduced the concentration of ytterbium atoms in the homogenous variant. Such a procedure can be justified since the nature of the laser transitions (3- or 4- level transitions) depends only on the population of the terminal level and not on the amount of gain [33].
Fig. 7. The numerical modelling of the lasing mode formation. The first row contains schematics of the considered structures with a) homogenous and b) discrete distribution of active ytterbium ions (brown areas). The middle row contains corresponding transient oscillations from Yb3+ doped fiber laser. The bottom row shows intensity maps calculated for four-time moments (matching dashed vertical red lines in c) and d)), which are illustrating the process of launching lasing cavities’ longitudinal optical mode for e) uniform and f) discrete dopant concentrations at 1060 nm wavelength. Dotted framed regions from the schematics correspond to the area presented in field intensity maps.
In Figs. 7(b) and 7(c), we present the transient oscillations from our cavity laser, calculated in the point located in the middle of the cavity. In general, the dynamics of the lasing process depends on many factors such as the type of the transitions, lifetimes of the decay channels, quality and size of the cavity, pump power, initial populations of states and electron density [32]. However, since these parameters are the same in both fibers considered here, and so is the volume-averaged electron density, we expect that any alteration in the dynamics would result from the discrete distribution of the ytterbium-doped nanorods. Our numerical modelling reveals that in both geometries the lasing threshold is reached at the same time point, around 5 ps. Afterwards, it is followed by quickly damped oscillations till a stable lasing performance is achieved. The amplitude, frequency and the lifetimes of the oscillations, as well as final stabilized output power, are practically the same. Moreover, the analysis of the spatial intensity distributions calculated for different time points (Figs. 7(e) and 7(f)) indicates that the process of longitudinal lasing mode formation proceeds similarly. Initially, there is only numerical noise; then the cavity mode starts to emerge. Next, when the lasing threshold is reached, the shape of the lasing mode is fully developed and remains the same until the final output power stabilizes.
These numerical results suggest that as long as averaged over volume electron density is maintained, and the ytterbium nanorods have the subwavelength size, the lasing performance of the nanostructured device is the same as for a system where active ions are uniformly covering the entire core region of the cavity. In other words, in nanostructured active fiber, the separation of the particular dopants in space does not influence on the laser operation.
6.2 Proof-of-concept fiber laser implementation
Next, we have verified experimentally the lasing performance of the fabricated active nanostructured fiber in setup shown in Fig. 8. The laser cavity was formed by high reflection Bragg grating written in the active fiber and Fresnel reflection of 3.4% from the surface of the output end of the fiber. The laser fiber was cladding-pumped. We used a multimode laser diode at 973.5 nm and 4 nm spectral width with an output fiber of 100 μm core and numerical aperture of 0.22. In the laser setup, we used a dichroic mirror (DM) to separate the pump radiation and the fiber laser output. The pump beam was formed with the coupling system consisted of two identical aspheric lenses with anti-reflection coating and focus length f1 = 20 mm to couple efficiently the pump beam into the internal cladding of the examined fiber. Thus the coupling loss between a pump diode delivery system and the internal cladding of the examined fiber was only a 3.4% related to Fresnel loss. We used several samples of the fiber (with lengths varying from 16.6 m to 23.6 m) to experimentally find the optimal size of the laser cavity, which is the trade-off between the background loss and pump absorption. We have obtained a single mode performance with the slope efficiency for every sample as shown in Fig. 9(a). The highest slope efficiency of 44% was obtained for the fiber length of 20.6 m, which is a significant progress in comparison with our preliminary research [46]. The threshold of the launched power was 0.34 W. The maximum power of the single-mode laser output was 3.9 W, limited by the available pump power. The obtained lasing efficiency of our proof-of-concept device is relatively low if we compare with state of the art commercially available lasers.
Fig. 8. The laser set-up with Yb3+ doped photosensitive fiber. Bragg grating is inscribed directly on active fiber.
Fig. 9. Lasing performance of nanostructured laser (a) The output power versus launched power for lasers with optical fiber lengths of 19.5 m, 20.6 m, 21.7 m, and 22.7 m . The best slope efficiency of 44% was achieved for 20.6 m fiber. (b) The spectrum of a fiber laser operating at a maximum power of 3.9 W output and corresponding registered beam profile (inset).
The relatively low efficiency can be explained mainly by high confinement losses in the developed fiber. To verify this claim, we performed measurements of the background losses. The loss of the investigated fiber was measured with the standard cut-back method. The measurement was conducted at 1310 nm wavelength, outside the absorption range of Yb ions. The measured fiber was bent with large radius of 0.5 m, not to consider bending loss in measurement. The measured losses reached 100 dB/km for the wavelength 1310 nm. Such large background losses were probably caused by several factors as quality of silica, germanium oxide and ytterbium doped silica rods, technological regime of processing (assembly of preform in research grade labs – no certified clean room). Finally nanostructuring can cause additional losses due to scattering. Detailed analysis of fiber losses requires further technological studies. Increasing absorption of the laser fiber by e.g. using nanorods with higher Yb doping level of silica or higher fill factor of Yb doped glass in nanostructure, we could use shorter fiber for maximum efficiency of lasing. Therefore, the decreased overall loss of the shorter cavity would result with higher lasing slope efficiency.
The remaining measurements confirm high quality of laser beam parameters. The FWHM of the generated laser line was 0.9 nm at the designed Bragg grating resonance wavelength of 1059.9 nm (Fig.9b), what is a typical value for lasing systems with power up to few hundred Watts [47]. The far-field pattern of the laser beam was registered (inset Fig. 9(b)). The beam quality was measured to be Mx2 = 1.15 and My2 = 1.10 (Fig.10a), which confirms the single-mode operation of the nanostructured fiber and it is in agreement with theoretical and experimental studies presented respectively in Section 3 and Section 4. The value of the numerical aperture of the generated beam was as low as 0.05, which corresponds well to the value obtained in numerical simulations NA=0.044.
The power stability of the nanostructured fiber laser is illustrated in Fig. 10(b). During a two-hour test of laser performance at 3.9 W of the output power, we registered less than ±2.5% fluctuations of the generated power, which is an expected result in non-stabilized systems. We note that although in commercially available Yb doped single-mode fiber lasers the power stability is of ±0.5% in kW-level lasers, it is achieved in advanced systems which control the temperature of the fiber and are equipped with output feedback loop [47].
Fig. 10. Beam quality characterization of nanostructured laser a) M2 parameters calculated from the measured beam caustics are Mx2 = 1.15 and My2 = 1.10 obtained for 2 W laser output power. b) The output power during two hours stability test. The averaged power was equal to 3.9 W, and the fluctuations were up to ±2.5% of generation.
7. Conclusions
We have shown that by the arrangement of subwavelength elements any arbitrarily defined refractive index, gain and photosensitivity profiles can be obtained independently from each other. We have proved numerically and experimentally that nanostructuring can lead to enhancement of performance, as in the case of the reflectivity of our Bragg gratings inscribed in arbitrarily distributed photosensitive nanorods. Moreover, we verified that the use of nanostructuring does not degrade any passive or active properties of the fiber.
Use of rods with similar diameter and subwavelength size that fulfils effective medium condition creates a new generic platform for fiber development. A fundamental of the nanostructure fiber technology simultaneously draws the best benefits from the processes of MCVD and stack-and-draw, while avoiding their weaknesses. This approach benefit form advancement of matured MCVD technology that offers high purity highly doped rods preforms and avoid their limits related to rotational symmetry, clustering of ions, and limited gradients of refractive index. Their lack of refractive index uniformity was mitigated thanks to downscaling the elements to subwavelength sizes during the drawing process. On the other hand it benefits from the mature stack-and-draw technology, where several technological concepts of non-symmetrical, multilateral component are developed, however used mainly for development of photonic cladding structures and imaging bundles.
To our knowledge, the proof-of-concept nanostructured, silica-based fiber laser with built-in Bragg grating proposed in this work is the first type of such a fiber system reported in the literature. Although the efficiency of the fabricated device was limited to 44%, it is worth bearing in mind that this is research at its early stage. Several issues should be still verified, as the ones related to attenuation related to scattering, diffusion during fiber drawing and influence of internal stress on fiber performance. However, it is important to note that diffusion is positive effect that allows to reduce sharp borders between particular nanorods and reduce any potential scattering. Also an influence of internal stress related to various thermal expansion coefficient of various nanorods is negligible for nanoscale elements [48].
Short-cavity fiber lasers with precise control of cavity length are required for obtaining single frequency narrow linewidths, low frequency noise or, in particular, ultrashort pulsed laser operation with GHz repetition rates [49, 50]. These features are important for future development of high-precision metrology, gravitational wave detection, and coherent LIDAR, where a stable, frequency narrow linewidth is required [50].
The nanostructuring technology still requires technical improvements of the manufacturing process, such as e.g. the clean-room facility in order to decrease the background losses. Nanostructuring opens up further perspectives for improving high-power laser performance, if used with LMA fibers, as it allows to realize in practice the idea of the mode-selective confined doping. Moreover, nanostructuring might also improve thermal management within the high power lasers, as it grants precise control of absorption (gain) in active fiber though control of the filling factor of the nanostructure. Thus, we expect that the concept of nanostructuring can bring new quality to the field of fiber lasers, but at this stage, without further optimization, it is too early to judge its impact.
Funding
Politechnika Warszawska (FOTECH-1 1820/22/Z01/POB1/2020); Fundacja na rzecz Nauki Polskiej (POIR.04.04.00-00-1C74/16 Team Programme).
Acknowledgments
The authors wish to acknowledge Mr. K. Harasny and Mrs. J. Lisowska, from Institute of Electronic Materials Technology for support in preform components development and preform integration and K. Markowski for his support during the fiber hydrogen loading and FBG inscription. The authors acknowledge also Mr. M. Biduś and Dr. M. Dłubek from Fibrain for measurement of refractive index profile of developed fiber.
Disclosures
MF AA TO DP RB: Łukasiewicz Research Network - Institute of Electronic Materials Technology (P).
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