All-Glass Spun Tapered Double-Clad Fibers

• All-glass spun tapered double-clad fibers are optical fibers that integrate all-silica construction with double-clad design, tapering, and spinning to optimize pump absorption, modal control, and power scalability.
• They leverage precise tapering and axial spinning to manage nonlinearity, suppress undesired mode coupling, and maintain polarization or orbital-angular-momentum characteristics.
• Demonstrated in high-power, nanosecond, picosecond, and OAM amplifiers, these fibers enable robust, monolithic integration with applications in structured-light and narrow-linewidth systems.

Searching arXiv for papers on spun tapered double-clad fibers and related all-glass fiber architectures. All-glass spun tapered double-clad fibers are a class of active or passive optical fibers in which an all-silica double-clad geometry is combined with longitudinal tapering and axial spinning during draw. In the reported literature, this architecture is used to reconcile requirements that are ordinarily in tension in high-power structured-light and pulsed-fiber systems: efficient cladding pumping, large or evolving mode area, suppression of deleterious mode coupling, and preservation of polarization or orbital-angular-momentum (OAM) content under amplification. The combination has been realized in ytterbium-doped tapered gain fibers for cylindrical-vector beams (Zalesskaia et al., 2023), monolithic high-power picosecond amplifiers (Fathi et al., 2024), narrow-linewidth nanosecond systems (Fathi et al., 9 Aug 2025), and spun ring-core tapered fibers for OAM amplification with preserved phase and polarization topology (Zalesskaia et al., 19 Dec 2025). Related double-clad tapering strategies have also been used in mode-selective photonic lanterns, where the double-clad geometry enables short, robust, high-isolation all-glass tapers (Becerra-Deana et al., 2024).

1. Definition and architectural scope

An all-glass spun tapered double-clad fiber combines four structural attributes. The first is all-glass construction, meaning that the optically functional region is formed entirely from silica-based glasses rather than polymer-based guidance layers in the active region. The second is a double-clad structure, in which a guided signal propagates in a core or modal guidance region while pump light is launched into a larger surrounding cladding for efficient absorption in rare-earth-doped material. The third is tapering, i.e., a longitudinal variation of core and cladding diameters that changes modal area and guidance conditions along the propagation direction. The fourth is spinning, in which the preform is rotated during draw so that the internal structure follows a helical trajectory with a defined pitch, thereby modifying the birefringence landscape and mode-coupling dynamics (Zalesskaia et al., 2023, Fathi et al., 2024).

Within this family, the literature includes at least two distinct subtypes. One is the step-index tapered double-clad ytterbium-doped fiber, used as an ultra-large-mode-area amplifier platform for picosecond or nanosecond pulses (Zalesskaia et al., 2023, Fathi et al., 2024, Fathi et al., 9 Aug 2025). The other is the spun ring-core tapered fiber, where the active region is a ring-shaped ytterbium-doped core designed to favor OAM eigenmodes and suppress the fundamental Gaussian mode (Zalesskaia et al., 19 Dec 2025). A related but not identical branch is the use of custom-pulled double-clad fibers in fused-and-tapered mode-selective photonic lanterns, where mode selectivity is obtained by symmetry breaking among fibers with different first-cladding diameters (Becerra-Deana et al., 2024).

This suggests that “all-glass spun tapered double-clad fiber” is best understood not as a single device design, but as a fabrication and waveguiding paradigm whose concrete implementation depends on whether the target is scalar ultrafast amplification, cylindrical-vector-beam preservation, OAM amplification, or mode-selective multiplexing.

2. Structural design and material realization

In the reported amplifier implementations, the base material system is silica glass with ytterbium-doped silica as the active medium. In the cylindrical-vector-beam amplifier, the core is Yb-doped silica glass fabricated using REPUSIL technology, the cladding is pure silica, the absorption coefficient is 650 dB/m at 976 nm, and the core numerical aperture is 0.1 (Zalesskaia et al., 2023). In the monolithic picosecond amplifier, the core rod is made by modified chemical vapor deposition, the first cladding is built by sleeving with F300 silica tubes, and the fluorine-doped second cladding is deposited by low-pressure microwave plasma; the corresponding numerical apertures are 0.08 for core/first cladding and 0.27 for first cladding/second cladding (Fathi et al., 2024). In the narrow-linewidth nanosecond system, the reported numerical apertures are 0.08 for the core, 0.27 for the first cladding, and 0.48 for the second cladding, with a core absorption of 800 dB/m at 976 nm (Fathi et al., 9 Aug 2025).

The geometry varies significantly across implementations but retains the defining tapered double-clad character. The cylindrical-vector-beam sT-DCF tapers from 12.7/146 μm at the thin end to 37.5/431 μm at the thick end over a 7 m length, with a step-index profile and a core–cladding diameter ratio of 11.5 (Zalesskaia et al., 2023). The monolithic picosecond amplifier uses a fiber of length about 6.7 m, with input diameters 8.3/75/90 μm and output diameters 90/814/977 μm for core/clad/second clad (Fathi et al., 2024). The nanosecond system uses a taper of about 6 m, with core diameter increasing from 8.3 μm to 78 μm, first cladding from 75 μm to 750 μm, and second cladding from 90 μm to 850 μm (Fathi et al., 9 Aug 2025).

In the OAM-specific architecture, the geometry is more specialized. The fiber is manufactured entirely from silica glass, with rare-earth $\mathrm{Yb_2O_3}$ doping localized in a ring-shaped core and fluorine doping used in the outer cladding. The ratio of the undoped center to the total doped region is 1:3. The first cladding is octagonal, and the total cladding-to-core diameter ratio is reported up to 14.4:1 with a first-cladding NA of 0.27. For OAM1 amplification, the input core diameter is 9 μm and the cladding is 130 μm; for OAM2, the input core diameter is 11.8 μm and the cladding is 170 μm; the wide side remains constant with cladding 213 μm. The total fiber length is 5.1 m, with actual lengths of 4.6 m for OAM1 and 2.9 m for OAM2 after cleaving (Zalesskaia et al., 19 Dec 2025).

The ring-core implementation makes explicit a design principle that is only implicit in conventional step-index sT-DCF systems: geometry can be used not only to enlarge mode area and assist pump absorption, but also to select a desired modal topology. In the ring-core case, the ring-shaped active core surrounding a pure silica center supports complex spatial and polarization modes and inherently suppresses the fundamental mode, which is central to OAM purity preservation (Zalesskaia et al., 19 Dec 2025).

3. Spinning, birefringence control, and mode stability

Spinning is introduced during draw by continuous preform rotation, causing the internal refractive-index structure to spiral around the fiber axis. In the cylindrical-vector-beam sT-DCF, the measured pitch increases slightly along the taper, from 30.4 mm at the thinnest side to 31 mm at the thickest side (Zalesskaia et al., 2023). In the monolithic pulsed amplifier and the narrow-linewidth nanosecond system, the spin pitch is 50 mm (Fathi et al., 2024, Fathi et al., 9 Aug 2025). In the OAM ring-core tapered fiber, the preform is rotated with a constant pitch of around 3.6–3.7 mm, corresponding to a substantially stronger twist rate (Zalesskaia et al., 19 Dec 2025).

The principal physical role of spinning in the step-index sT-DCF literature is suppression of residual linear birefringence and stabilization of the state of polarization. In these systems, spinning distributes internal stress and index variations azimuthally and suppresses residual birefringence to as low as $10^{-8}$, thereby stabilizing polarization under high-power pumping and avoiding the need for stress rods (Fathi et al., 2024). The 2023 cylindrical-vector-beam work states that a spun configuration possessing nearly-circular polarization eigenstates supports stable wavefront propagation, in contrast to an isotropic fiber architecture (Zalesskaia et al., 2023). The 2025 narrow-linewidth system similarly attributes high degree of polarization to strong suppression of linear birefringence and averaging out of polarization errors (Fathi et al., 9 Aug 2025).

In the OAM ring-core implementation, spinning is described in more explicitly modal terms. The helical structure creates intrinsic circular birefringence, splitting the degeneracy between OAM modes of opposite sign and polarization so that $\ell,+$ and $\ell,-$ modes have different propagation constants. This lifting of degeneracy suppresses intermodal coupling between OAM modes and their neighbors and defines circular polarization as the eigenstate basis in the fiber (Zalesskaia et al., 19 Dec 2025). The coupled-mode evolution including twisting and tapering is written as

$$\frac{da_j(z)}{dz} = \sum_{q} a_q(z) \left\{ i K_{jq} e^{i\left[\Delta\beta_{qj} \pm (J_q - J_j)\Upsilon\right]z} + \Omega_{jq} e^{i\Delta\beta_{qj}z} \right\},$$

where $a_j(z)$ is the amplitude of mode $j$, $K_{jq}$ is the twisting-induced coupling coefficient, $\Upsilon$ is the twist rate, $J_q$ is the angular momentum of mode $10^{-8}$0, and $10^{-8}$1 is the taper-induced coupling coefficient. In the amplifier model, gain is added as

$10^{-8}$2

These expressions formalize the interplay of geometric twist, longitudinal taper, and distributed gain in determining modal purity (Zalesskaia et al., 19 Dec 2025).

A plausible implication is that the function of spinning depends on modal context. In scalar or near-fundamental amplifiers, it primarily stabilizes polarization and reduces environmentally induced birefringence drift (Fathi et al., 2024, Fathi et al., 9 Aug 2025). In structured-light fibers, especially ring-core OAM fibers, it also acts as a symmetry-breaking mechanism that reshapes the eigenmode spectrum itself (Zalesskaia et al., 19 Dec 2025).

4. Tapering, mode-area evolution, and nonlinear-effect management

The taper in all-glass spun double-clad fibers is not merely a packaging feature; it is a central optical design variable. In the step-index amplifier literature, the fiber widens from the narrow end to the output end, producing a gradual increase in effective mode area. This is described as a conical geometry that expands the mode area along the fiber and reduces intensity for a given pulse energy, thereby suppressing self-phase modulation and stimulated Raman scattering (Fathi et al., 2024). The same literature emphasizes that the short overall length and large end-core jointly reduce nonlinear interaction length and support high peak and average power (Fathi et al., 2024, Fathi et al., 9 Aug 2025).

The relevant mode-area expression is reported as

$10^{-8}$3

and the stimulated Raman scattering threshold is expressed qualitatively as scaling with mode area and inversely with effective length: $10^{-8}$4 For stimulated Brillouin scattering in the narrow-linewidth nanosecond system, the threshold is reported as

$10^{-8}$5

The interpretation given is that the gradual increase of $10^{-8}$6 along the taper reduces local nonlinear gain and that shorter total fiber length lowers effective interaction length (Fathi et al., 2024, Fathi et al., 9 Aug 2025).

In the OAM ring-core tapered fiber, the taper runs in the opposite functional direction relative to mode selection: the cladding diameter decreases parabolically from 213 μm at the wide end to 115 μm, with a corresponding reduction in core diameter (Zalesskaia et al., 19 Dec 2025). Here the taper serves three roles. First, it acts as a mode filter, because the narrow end supports only lower OAM modes while excluding the fundamental mode through the combined action of ring geometry and diameter tuning. Second, it increases nonlinear thresholds by allowing the mode area to enlarge as light propagates from small to large core. Third, it helps maintain pump–signal overlap across the fiber (Zalesskaia et al., 19 Dec 2025).

The OAM work also explicitly identifies a limitation often overlooked in simplified descriptions of tapered fibers: tapering itself can induce coupling. The coefficient $10^{-8}$7 denotes taper-induced coupling, and while a gradual parabolic taper minimizes this term, it cannot eliminate it entirely. Rapid or irregular tapering would enhance intermodal coupling and degrade modal purity; in practice, a few percent contamination by neighboring modes and opposite-charge OAM is observed, especially in the thicker portion where bending perturbs circular symmetry (Zalesskaia et al., 19 Dec 2025).

This suggests that tapering in these fibers should not be reduced to the phrase “large mode area.” Depending on the device, it can be a nonlinear-management tool, an adiabatic transformer, a mode filter, or all three simultaneously.

5. Double-clad guidance, pump absorption, and all-glass monolithic integration

The double-clad geometry is the enabling pump-delivery mechanism across the cited works. In amplifier configurations, pump light is launched into a large cladding and absorbed by the ytterbium-doped core or ring-core region (Zalesskaia et al., 2023, Fathi et al., 2024, Zalesskaia et al., 19 Dec 2025). The first cladding shape is deliberately noncircular in several designs to improve pump absorption by breaking ray regularity and promoting more uniform pump distribution. The OAM ring-core fiber uses an octagonal first cladding (Zalesskaia et al., 19 Dec 2025), whereas the high-power monolithic picosecond amplifier uses a double-D-shaped first cladding of pure silica (Fathi et al., 2024).

The significance of the all-glass approach differs somewhat between structured-light devices and monolithic pulsed amplifiers. In the OAM fiber, all-glass construction is presented as omitting polymer coatings or non-glass materials in the optically active region, thereby providing superior thermal, mechanical, and optical stability at high powers (Zalesskaia et al., 19 Dec 2025). In the nanosecond sT-DCF paper, “all-glass” is made more explicit: there are no polymer coatings, and both core and claddings are silica, enabling splicing, higher thermal stability, and fully monolithic construction (Fathi et al., 9 Aug 2025). In the monolithic picosecond amplifier, the amplifier module, including combiner, fiber, and endcap, is fused/silica and contains no free-space optics or bulk glass elements (Fathi et al., 2024).

The practical consequences reported for monolithic all-glass construction include elimination of alignment sensitivity, removal of dependence on pump alignment and internal back reflections, improved robustness against vibration, dust, and thermal drift, and compatibility with passive cooling and compact coiled packaging (Fathi et al., 2024). The wide end is protected by a 2-degree angled silica endcap fusion spliced to mitigate back-reflection damage (Fathi et al., 2024). In the nanosecond narrow-linewidth system, low-loss splicing and monolithic integration are emphasized as consequences of the all-glass outer surface and matched thermal expansion (Fathi et al., 9 Aug 2025).

Related evidence for the utility of all-glass double-clad tapers appears in mode-selective photonic lanterns. There, three custom double-clad fibers are stacked in a low-index capillary and fused/tapered into a short device. The double-clad design keeps lower-order modes contained within the first cladding during tapering even after the core can no longer guide them, which relaxes the adiabatic criterion and permits a total transition length of about 2.5 cm (Becerra-Deana et al., 2024). With a fluoride-doped capillary, the resulting devices exhibit modal isolation above 60 dB and excess loss lower than 0.49 dB over more than 250 nm; with synthetic fused silica capillary tubes, isolation remains above 20 dB with excess loss lower than 2 dB (Becerra-Deana et al., 2024). Although photonic lanterns are not amplifiers, these results reinforce the broader point that all-glass double-clad tapers can combine short length, robustness, and controlled modal transformation.

6. Demonstrated performance regimes

The reported performance of all-glass spun tapered double-clad fibers spans several distinct operating regimes.

Structured-light and cylindrical-vector amplification

The 2023 ytterbium-doped sT-DCF work demonstrates amplification of cylindrical-vector beams in a picosecond MOPA system. With 10 ps pulses at 15 MHz and 1030 nm, the sT-DCF reaches 22.07 W average output power with optical-to-optical efficiency of 42%. Beam quality is reported as $10^{-8}$8 at 0.34 W and $10^{-8}$9 at 22 W, close to the theoretical value of 2 for radially polarized beams. Rotating-polarizer measurements verify preservation of cylindrical polarization up to full output power (Zalesskaia et al., 2023). The comparison fiber, an isotropic tapered double-clad fiber, reaches 30 W but exhibits poorer beam quality, $\ell,+$0–2.21, and poor polarization stability (Zalesskaia et al., 2023).

Monolithic high-power picosecond amplification

The 2024 monolithic all-glass sT-DCF amplifier covers a wide range of repetition rates. It delivers 50 ps pulses with over 2 MW peak power at 1 MHz, 50 ps pulses with over 625 W average power at 20 MHz, and 20 ps pulses with over 645 W average power at 1 GHz (Fathi et al., 2024). The reported table values are 155 W average power and 2 MW peak power at 1 MHz, 625 W and 625 kW at 20 MHz, and 645 W and 32 kW at 1 GHz. Corresponding slope efficiencies are 59%, 76.6%, and 78.6%; degrees of polarization are 70%, 88.3%, and 87.6%; beam quality values are $\ell,+$1, $\ell,+$2, and $\ell,+$3; and signal-to-Raman is “None,” 45.7 dB, and “None” for the three regimes respectively (Fathi et al., 2024).

Narrow-linewidth nanosecond amplification

The 2025 nanosecond system demonstrates a monolithic narrow-linewidth fiber laser based on all-glass sT-DCF without employing any mitigating technique for stimulated Brillouin scattering. The system delivers 8 ns pulses at 100 kHz with pulse energy of 1.6 mJ, average power of 160 W, peak power of 188 kW, beam quality factor $\ell,+$4, spectral linewidth of 53.8 MHz, degree of polarization over 97.5%, spatial coherence of 0.94, and slope efficiency of 97.6% according to the abstract, while the details section reports slope efficiency of 76.6% in its performance table (Fathi et al., 9 Aug 2025). Because both values appear in the supplied data, the discrepancy should be regarded as unresolved here rather than harmonized by inference. The paper frames the result as evidence that the intrinsic geometry of sT-DCF can raise the SBS threshold without external phase modulation (Fathi et al., 9 Aug 2025).

OAM amplification in active spun ring-core tapered fiber

The 2025 ring-core tapered-fiber work reports amplification of OAM beams with topological charges $\ell,+$5 and $\ell,+$6, using 60 ps pulses at 15 MHz and 1030 nm. Output power exceeds 1.2 W average, with modal purity over 95% in the abstract and more specifically greater than 95% for OAM1 and greater than 97% for OAM2 in the extracted details (Zalesskaia et al., 19 Dec 2025). Spatially resolved measurements show that polarization topology is largely preserved but exhibits small distortion due to coupling into neighbor modes (Zalesskaia et al., 19 Dec 2025). Modal purity is quantified by overlap with an ideal OAM state,

$\ell,+$7

where $\ell,+$8 is the recovered field from off-axis holography (Zalesskaia et al., 19 Dec 2025).

7. Limitations, misconceptions, and research significance

A common misconception is that spinning makes a fiber polarization-maintaining in the same sense as stress-rod designs. The cited works support a narrower statement: spinning suppresses residual linear birefringence and can stabilize the state of polarization or create nearly circular polarization eigenstates, but its modal consequences depend on geometry and operating regime (Zalesskaia et al., 2023, Fathi et al., 2024). In structured-light fibers, spinning is not merely a polarization-cleanup mechanism; in ring-core OAM fibers it is part of the eigenmode-engineering strategy that lifts degeneracies and suppresses intermodal coupling (Zalesskaia et al., 19 Dec 2025).

Another misconception is that tapering automatically guarantees single-mode or pure-mode operation. The literature is more cautious. Adiabatic tapering can minimize coupling to higher-order or radiation modes, and the smooth geometry is essential for maintaining beam quality (Fathi et al., 2024, Fathi et al., 9 Aug 2025). However, taper-induced coupling $\ell,+$9 is explicitly retained in the OAM coupled-mode model, and residual modal impurity of roughly 3–5% is attributed to bending, imperfect circular symmetry, fabrication nonidealities, and taper-related coupling that cannot be entirely removed (Zalesskaia et al., 19 Dec 2025). Likewise, in photonic lanterns the ability to use steeper taper profiles without excess coupling arises not from tapering alone but from the specific double-clad design and capillary-assisted confinement (Becerra-Deana et al., 2024).

A further point concerns the phrase all-glass. In the cited literature it denotes more than an absence of polymer in the active region. It is tied to thermal handling, reliability, splicability, and the possibility of truly monolithic amplifier modules (Fathi et al., 2024, Fathi et al., 9 Aug 2025). This broader systems-level meaning is particularly clear in the pulsed-amplifier papers, where the absence of free-space elements is presented as a route to high robustness and reduced susceptibility to alignment drift and back reflections (Fathi et al., 2024).

The broader significance of all-glass spun tapered double-clad fibers lies in their convergence of modal engineering and power scaling. In cylindrical-vector amplification they preserve axially symmetric polarization under high power (Zalesskaia et al., 2023). In monolithic picosecond and nanosecond amplifiers they support high peak power, high average power, or narrow linewidth without sacrificing beam quality and polarization stability (Fathi et al., 2024, Fathi et al., 9 Aug 2025). In ring-core form they enable amplification of OAM modes while preserving both helical phase and circular-polarization topology at modal purities above 95% (Zalesskaia et al., 19 Dec 2025). Related all-glass double-clad tapers in photonic lanterns show that the same fabrication logic can also produce short, broadband, high-isolation passive mode-selective components (Becerra-Deana et al., 2024).

Taken together, these results indicate that the importance of all-glass spun tapered double-clad fibers is not confined to any single application domain. They constitute a flexible photonic platform in which glass composition, cladding topology, taper law, and spin pitch are co-optimized to control gain, pump absorption, birefringence, nonlinearity, and modal content within a single integrated fiber device.