The heart of the difficulty lies in the physics of the taper. A photonic lantern is a non-uniform waveguide, meaning its refractive index distribution changes along its length, and there is no clean analytical solution for how the light field evolves inside it. Physicists therefore rely on numerical methods. The team first turned to the beam propagation method, or BPM, a classic technique dating back to 1978 that steps the optical field forward through the device slice by slice. Using a five-by-one lantern built from single-mode fibers with 10-micrometer cores feeding a 30-micrometer-core multimode output, the simulations revealed the full story of mode evolution: five separate fundamental modes merge into collective supermodes inside the taper, then emerge as a broad guided mode in the multimode fiber. But the method comes at a steep price. Each simulation took one and a half to two hours, errors accumulated over distance, and every new input field required a fresh calculation, making batch design optimization or real-time adaptive control effectively impossible.

To break that bottleneck, the researchers adopted the mode expansion propagation method, which divides the waveguide into many short sections, solves for the eigenmodes of each section using Maxwell’s equations, and stitches the sections together through overlap integrals. This approach yields both forward and backward transmission matrices at once, keeps errors from compounding, and crucially allows any new input field to be computed by simple matrix multiplication once the transmission matrix is known. For a three-by-one mode-selective lantern, the method produced detailed field distributions, mode-dependent loss curves, and a concrete design rule: once the taper length exceeds roughly 17 millimeters, losses drop to negligible levels. A full lantern simulation took only about ten minutes instead of hours, a dramatic improvement that still fell short for the kind of rapid, iterative modeling that adaptive optical systems ultimately require.

The decisive leap came from machine learning. Because a photonic lantern’s behavior is a deterministic mapping between input and output light fields governed by Maxwell’s equations, it is ideally suited to supervised learning. The team built a dropout neural network whose inputs are the amplitudes and phases of each input beam and whose outputs are the complex coefficients of each output eigenmode. Each hidden layer contained 2,000 neurons with LeakyReLU activations, and dropout regularization randomly silenced 20 percent of neurons during training to prevent overfitting. Training used 1,500 experimental field pairs from a six-by-one lantern, expanded to 6,000 examples through data augmentation exploiting the symmetry of the six-fiber structure, and ran for 25,000 epochs with an adaptive learning rate schedule.

The results were striking. The best network, with three hidden layers, predicted light intensity with a mean squared error of just 9.793 times ten to the minus five and a coefficient of determination of 0.9973, while phase prediction reached an error of 3.772 times ten to the minus three. Once trained, the model produced a prediction in roughly 60 milliseconds on a standard laptop processor, an acceleration of more than five orders of magnitude compared with the hour-and-a-half beam propagation simulations. Perhaps most importantly, the approach can be trained directly on experimentally measured input-output data from a finished device, offering a way to calibrate real lanterns whose as-fabricated geometry inevitably deviates from design blueprints. The authors note that generalizing across different lantern geometries will require larger, more diverse datasets, an explicit target for future work.

Modeling was only half the battle. The team also systematically developed the fabrication of lanterns made by inserting a bundle of single-mode fibers into a low-refractive-index glass capillary and fusing the assembly while stretching it. They compared three device families: three-dimensional integrated lanterns written with ultrafast lasers, which suffer from scattering-related losses; tapered multicore fibers, which are efficient but hard to splice and difficult to address individually; and the capillary-sleeve fiber bundle approach they favored, which keeps the input fibers separated and accessible while using cheap, standard step-index fibers as raw material. Using a Vytran GPX3400 glass processing workstation and a Fujikura FSM-100P+ splicer, the researchers walked through material pretreatment, acid etching of fiber claddings, capillary pre-tapering, bundling, fusion tapering, precision cleaving, and dissimilar-fiber splicing.

The process details turned out to make or break the device. In 60 controlled tapering attempts, fabrication yield fell from about 85 percent under cleanroom conditions to roughly 35 percent with minor contamination and below 10 percent with poor drying, because residual particles ignite during fusion and become scattering centers; particles larger than 5 micrometers caused thermal runaway at just 2 watts. Bundling symmetry proved equally critical: fibers arranged in regular polygons preserve the lantern’s mode evolution capability, and unevenly distributed residual air holes introduced more than 1 decibel of loss and over 3 decibels of mode purity degradation, while symmetric holes of varying size mattered far less. The team defined an optimal collapse regime in which the capillary diameter remains about twice the circumscribed circle of the fiber bundle, achieving a splicing success rate near 90 percent. Alignment thresholds were equally strict: lateral misalignment beyond half a micrometer or tilt beyond 0.3 degrees triggered excess losses above 0.65 decibels.

Performance testing of the finished devices validated both the simulations and the process. At 1,064 nanometers with water cooling, a fabricated three-by-one non-mode-selective lantern transmitted more than 98 percent of the light when input power stayed below 1.5 watts per arm. At higher powers, efficiency dropped, falling to about 62 percent at 3 watts, a reversible degradation the team attributes to thermal effects in the taper waist rather than permanent damage; devices survived 5 watts per arm without harm. Mode field imaging showed output spot patterns matching simulations with overlap integrals exceeding 0.965, and multi-arm input tests revealed continuously shifting output spots driven by environmental phase drift, confirming that practical mode control demands an adaptive feedback loop with phase locking.

Mode-selective lanterns delivered on their promise with quantified crosstalk. In a three-by-one device with input fibers of different core diameters, injecting light into the 15-micrometer-core arm yielded a fundamental mode with 94.6 percent purity, while the two 10-micrometer arms produced LP11-like modes with purities of 78.6 and 84.3 percent. A larger six-by-one lantern selectively excited each of the first six LP mode groups, though with noticeably higher crosstalk, illustrating the real-world gap between ideal designs and finished hardware. Looking ahead, the authors propose double-clad input fibers to shorten the required adiabatic taper and photonic crystal structures arranged around the fiber bundle to suppress light leakage. Together, the modeling framework and the fabrication playbook bring photonic lanterns closer to routine deployment in kilowatt-class fiber lasers, capacity-crunching mode division multiplexing, and wavefront sensors that could sharpen humanity’s view of distant worlds.

Subject of Research: Transmission characteristics and fabrication of photonic lanterns for mode control in fiber optics

Article Title: Research on the transmission characteristics and fabrication process of photonic lanterns

Article References: Research on the transmission characteristics and fabrication process of photonic lanterns. (n.d.). https://doi.org/10.1016/j.isci.2026.117615

Image Credits: AI Generated

DOI: 10.1016/j.isci.2026.117615

Keywords: photonic lantern, fiber optics, mode division multiplexing, neural network, beam propagation method, mode expansion propagation, high-power fiber lasers, fused tapering, spatial mode control, wavefront sensing, optical waveguides, mode purity

Thin-film lithium niobate, or TFLN, has become one of the most celebrated materials in integrated photonics. The compound boasts a large second-order nonlinear susceptibility, excellent electro-optic properties, a wide optical transparency window, and remarkably low propagation losses. When fabricated on the lithium-niobate-on-insulator platform, the material can be patterned into nanoscale waveguides that squeeze light into extraordinarily small cross-sections. This tight confinement amplifies the intensity of the optical field, strengthening the nonlinear interactions that researchers exploit for frequency conversion, pulse shaping, and all-optical signal processing. The catch is that the same confinement that boosts nonlinearity also intensifies dispersive effects that smear pulses apart.

The heart of the new work lies in a deliberate departure from convention. Standard quasi-phase-matching uses periodically poled domains, regions of alternating ferroelectric polarization, tuned precisely to eliminate phase mismatch between interacting optical waves. The authors instead engineered quasi-phase-mismatched domains, intentionally introducing a controlled degree of mismatch to sculpt the nonlinear coupling and energy exchange along the waveguide. By varying the domain period from 5 to 20 micrometers, they could dial the effective phase mismatch from 1.26 times ten to the fifth per meter down to 0.31 times ten to the fifth per meter, gradually steering the system toward a regime where dispersion and nonlinearity balance each other.

To model the physics, the team formulated a third-order nonlinear propagation framework based on the nonlinear Schrödinger equation, incorporating linear propagation loss, group-velocity dispersion, and Kerr-induced self-phase modulation. The Kerr nonlinear coefficient, which scales inversely with the effective mode area, determines how strongly the pulse reshapes itself as it travels. Solving the governing equations numerically with the Split-Step Fourier Method, the researchers tracked the amplitude, width, spectral content, transmission efficiency, and energy retention of 100-femtosecond pulse trains injected at a wavelength of 1550 nanometers into a 600-nanometer-thick lithium niobate film.

The results reveal a clear and striking trend. As the domain period increased from 5 to 20 micrometers, transmission efficiency climbed from 82.4 percent to 96.7 percent. Shorter domain periods produced stronger nonlinear coupling but also moderate pulse broadening and energy fluctuations, while longer periods relaxed the mismatch and allowed the pulse train to settle into a stable, low-distortion propagation regime. The pulse preservation factor, a measure of how faithfully the output amplitude matches the input, rose from 0.84 to 0.98 over the same range, edging tantalizingly close to the ideal value of unity.

Perhaps the most evocative quantity tracked in the study is the pulse area, a cornerstone of self-induced transparency theory. When a pulse area approaches integer multiples of 2 pi, the coherent interaction between the optical field and the medium allows the pulse to pass through without significant absorption. In the simulations, the pulse area evolved from 1.72 pi at the input to approximately 1.99 pi after ten millimeters of propagation, steadily approaching the transparency condition. As it did so, distortion diminished, energy retention improved, and the pulse maintained its temporal shape over ever-greater distances, evidence that the engineered domain structure actively stabilizes the coherent evolution of the light.

The transparency length itself tells a compelling story. In the optimized quasi-phase-mismatched configuration, pulses traveled about 14.8 millimeters with negligible distortion, compared with only 10.5 millimeters in a conventional quasi-phase-matched waveguide. Energy retention climbed from 81.3 percent to 96.2 percent as the domain period was tuned, and the pulse stability index improved from 0.84 to 0.97. In a head-to-head comparison under identical operating conditions, the engineered design outperformed the conventional structure on every metric: transmission efficiency of 96.7 percent versus 86.4 percent, pulse preservation of 0.98 versus 0.89, and energy retention of 96.2 percent versus 87.1 percent.

Beneath these numbers lies an important design insight. Because the nonlinear coefficient is inversely proportional to the effective mode area, tighter confinement yields stronger nonlinear interaction, and the optimized waveguide achieved a nonlinear coefficient of roughly 1.52 per watt per meter. Yet more nonlinearity does not automatically mean better pulses. Excessive self-phase modulation can distort the spectrum and temporal profile just as surely as dispersion can. The study’s central contribution is a multi-metric design philosophy: rather than maximizing any single output parameter, engineers should jointly evaluate transmission efficiency, energy retention, pulse preservation, and propagation length to find the sweet spot where nonlinear interaction is strong enough to sustain transparency but gentle enough to leave the pulse intact.

The findings align closely with a wave of recent experimental advances in TFLN photonics. The nonlinear coefficient enhancement achieved through reduced mode area echoes results from integrated femtosecond pulse generators demonstrated on the same platform, while the phase-mismatch control resonates with work on chirped periodically poled lithium niobate for broadband frequency conversion. The transparency lengths reported here, in the range of ten to fifteen millimeters, are comparable to the interaction lengths used in integrated electro-optic modulators and quantum optical phase sensors, suggesting that the numerical framework could guide real devices rather than remain a purely theoretical exercise.

The authors are careful to frame the work as a numerical feasibility and design assessment rather than an experimental demonstration. Fabrication tolerances, sidewall roughness, coupling inefficiencies, and higher-order physical effects such as Raman response and higher-order dispersion are not captured by the current model, and future work will need to incorporate experimentally measured parameters and tolerance analysis. Even so, the study delivers a practical recipe for the next generation of integrated photonic devices: choose the domain period and mismatch parameter to balance dispersion against nonlinearity, and ultrafast pulses can glide through lithium niobate channels with their shape, energy, and coherence essentially intact. For high-speed optical communication, optical switching, and nonlinear signal processing, that recipe may prove invaluable.

Subject of Research: Ultrafast nonlinear pulse propagation in periodically poled thin-film lithium niobate waveguides

Article Title: LiNoPulse: ultrafast nonlinear pulse propagation in thin-film lithium niobate waveguides

Article References: Chamundeeswari, D., Kuppusamy, B., Thiruchenduran, M., Anitha, J., & Vasanthapriya, J. (2026). LiNoPulse: ultrafast nonlinear pulse propagation in thin-film lithium niobate waveguides. Results in Optics, 25, Article 101166. https://doi.org/10.1016/j.rio.2026.101166

Image Credits: AI Generated

DOI: 10.1016/j.rio.2026.101166

Keywords: thin-film lithium niobate, self-induced transparency, nonlinear optics, quasi-phase mismatch, integrated photonics, ultrafast pulses, periodic poling, Kerr nonlinearity, group-velocity dispersion, Split-Step Fourier Method, optical waveguides, pulse preservation