Processing of materials by ultrashort laser pulses has evolved significantly over the last decade and is starting to reveal its scientific, technological and industrial potential. In ultrafast laser manufacturing, optical energy of tightly focused femtosecond or picosecond laser pulses can be delivered to precisely defined positions in the bulk of materials via two-/multi-photon excitation on a timescale much faster than thermal energy exchange between photoexcited electrons and lattice ions. Control of photo-ionization and thermal processes with the highest precision, inducing local photomodification in sub-100-nm-sized regions has been achieved. State-of-the-art ultrashort laser processing techniques exploit high 0.1–1 μm spatial resolution and almost unrestricted three-dimensional structuring capability. Adjustable pulse duration, spatiotemporal chirp, phase front tilt and polarization allow control of photomodification via uniquely wide parameter space. Mature opto-electrical/mechanical technologies have enabled laser processing speeds approaching meters-per-second, leading to a fast lab-to-fab transfer. The key aspects and latest achievements are reviewed with an emphasis on the fundamental relation between spatial resolution and total fabrication throughput. Emerging biomedical applications implementing micrometer feature precision over centimeter-scale scaffolds and photonic wire bonding in telecommunications are highlighted.

/pdf/ultrafast-laser-processing-of-materials-from-science-to-rducz2yjvs.pdf

Ultrafast laser processing of materials: from science to industry

Space-division multiplexing (SDM) uses multiplicity of space channels to increase capacity for optical communication. It is applicable for optical communication in both free space and guided waves. This paper focuses on SDM for fiber-optic communication using few-mode fibers or multimode fibers, in particular on the critical challenge of mode crosstalk. Multiple-input–multiple-output (MIMO) equalization methods developed for wireless communication can be applied as an electronic method to equalize mode crosstalk. Optical approaches, including differential modal group delay management, strong mode coupling, and multicore fibers, are necessary to bring the computational complexity for MIMO mode crosstalk equalization to practical levels. Progress in passive devices, such as (de)multiplexers, and active devices, such as amplifiers and switches, which are considered straightforward challenges in comparison with mode crosstalk, are reviewed. Finally, we present the prospects for SDM in optical transmission and networking.

Space-division multiplexing: the next frontier in optical communication

Single-mode fibres with low loss and a large transmission bandwidth are a key enabler for long-haul high-speed optical communication and form the backbone of our information-driven society. However, we are on the verge of reaching the fundamental limit of single-mode fibre transmission capacity. Therefore, a new means to increase the transmission capacity of optical fibre is essential to avoid a capacity crunch. Here, by employing few-mode multicore fibre, compact three-dimensional waveguide multiplexers and energy-efficient frequency-domain multiple-input multiple-output equalization, we demonstrate the viability of spatial multiplexing to reach a data rate of 5.1 Tbit s−1 carrier−1 (net 4 Tbit s−1 carrier−1) on a single wavelength over a single fibre. Furthermore, by combining this approach with wavelength division multiplexing with 50 wavelength carriers on a dense 50 GHz grid, a gross transmission throughput of 255 Tbit s−1 (net 200 Tbit s−1) over a 1 km fibre link is achieved. A few-mode, multicore fibre allows ultra-high-speed data transmission on a single wavelength of light.

/pdf/ultra-high-density-spatial-division-multiplexing-with-a-few-2noctjz1of.pdf

Ultra-high-density spatial division multiplexing with a few-mode multicore fibre

We demonstrate a 3x1 fiber-based photonic lantern spatial-multiplexer with mode-selectivity greater than 6 dB and transmission loss of less than 0.3 dB. The total insertion loss of the mode-selective multiplexers when coupled to a graded-index few-mode fiber was < 2 dB. These mode multiplexers showed mode-dependent loss below 0.5 dB. To our knowledge these are the lowest insertion and mode-dependent loss devices, which are also fully compatible with conventional few-mode fiber technology and broadband operation.

Mode-selective photonic lanterns for space-division multiplexing

Preface.Introduction.Chapter 1. Selected Topics in Electromagnetic Wave Propagation.1.1. Maxwell's Equations and the Fundamental Fields.1.2. Electromagnetic Wave Propagation in Sourceless Media.1.3. Power Transmission.1.4. Group Velocity.1.5. Reflection and Transmission of Waves at Plane Interfaces.1.6. Material Resonances and Their Effects on Wave Propagation.Problems.References.Chapter 2. Symmetric Dielectric Slab Waveguides.2.1. Ray Analysis of the Slab Waveguides.2.2. Field Analysis of the Slab Waveguides.2.3. Solutions of the Eigenvalue Equations.2.4. Power Transmission and Confinement.2.5. Leaky Waves.2.6. Radiation Modes.2.7. Wave Propagation in Curved Slab Waveguides.Problems.References.Chapter 3. Weakly-Guiding Fibers with Step Index Profiles.3.1. Rays and Fields in the Step Index Fiber.3.2. Field Analysis of the Weakly-Guiding Fiber.3.3. Eigenvalue Equation for LP Modes.3.4. LP Mode Characteristics.3.5. Single Mode Fiber Parameters.3.6. Derivation of the General Step Index Fiber Modes.Problems.References.Chapter 4. Loss Mechanisms in Silica Fiber.4.1. Basic Loss Effects in Transmission.4.2. Fabrication of Silica Fibers.4.3. Intrinsic Loss.4.4. Extrinsic Loss.4.5. Bending Loss.4.6. Source-to-Fiber Coupling.Problems.References.Chapter 5. Dispersion.5.1. Pulse Propagation in Media Possessing Quadratic Dispersion.5.2. Material Dispersion.5.3. Dispersion in Optical Fiber.5.4. Chromatic Dispersion Compensation.5.5. Polarization Dispersion.5.6. System Considerations and Dispersion Measurement.Problems.References.Chapter 6. Special Purpose Index Profiles.6.1. Multimode Graded Index Fiber.6.2. Special Index Profile Optimization.Problems.References.Chapter 7. Nonlinear Effects in Fibers I: Non-Resonant Processes.7.1. Nonlinear Optics Fundamentals.7.2. Nonlinear Phase Modulation on Pulses.7.3. The Nonlinear Schrodinger Equation.7.4. Additional Non-Resonant Processes.Problems.References.Chapter 8. Nonlinear Effects in Fibers II: Resonant Processes and Amplification.8.1. Raman Scattering.8.2. Stimulated Brillouin Scattering.8.3. Rare-Earth-Doped Fiber Amplifiers.Problems.References.Appendix A: Properties of Bessel Functions.Appendix B: Notation.Index.

Fundamentals of optical fibers

Photonic lanterns are made by adiabatically merging several single-mode cores into one multimode core. They provide low-loss interfaces between single-mode and multimode systems, where the precise optical mapping between cores and individual modes is unimportant.

The photonic lantern

Simple all-fiber three-mode multiplexers were made by adiabatically merging three dissimilar single-mode cores into one multimode core. This was achieved by collapsing air holes in a photonic crystal fiber and (in a separate device) by fusing and tapering separate telecom fibers in a fluorine-doped silica capillary. In each case the LP01 mode and both LP11 modes were individually excited from three separate input cores, with losses below 0.3 and 0.7 dB respectively and mode purities exceeding 10 dB. Scaling to more modes is challenging, but would be assisted by using single-mode fibers with a smaller ratio of cladding to core diameter.

Adiabatically-tapered fiber mode multiplexers.

Photonic lanterns are made by adiabatically merging several single-mode cores into one multimode core. They provide low-loss interfaces between single-mode and multimode systems where the precise optical mapping between cores and individual modes is unimportant.

The Photonic Lantern

Measuring Raman spectra through an optical fibre is usually complicated by the high intrinsic Raman scatter of the fibre material. Common solutions such as the use of multiple fibres and distal optics are complex and bulky. We demonstrate the use of single novel hollow-core negative-curvature fibres (NCFs) for Raman and surface-enhanced Raman spectroscopy (SERS) sensing using no distal optics. The background Raman emission from the silica in the NCF was at least 1000× smaller than in a conventional solid fibre, while maintaining the same collection efficiency. We transmitted pump light from a 785-nm laser through the NCF, and we collected back the weak Raman spectra of different distal samples, demonstrating the fibre probe can be used for measurements of weak Raman and SERS signals that would otherwise overlap spectrally with the silica background. The lack of distal optics and consequent small probe diameter (<0.25 mm) enable applications that were not previously possible.

/pdf/ultra-low-background-raman-sensing-using-a-negative-4u15zo0z1s.pdf

Ultra-low background Raman sensing using a negative-curvature fibre and no distal optics.

Hollow-core fibers have demonstrated record performance in applications such as high-power pulse delivery, quantum computing, and sensing. However, their routine use is yet to become reality. A major obstacle is the ability to maintain the polarization state of light over a broad range of wavelengths, while also ensuring single-mode guidance and attenuation that is low enough for practical applications that require only a few meters of fiber length (<1 dB/m). Here we simulated, fabricated, and characterized a single-mode birefringent anti-resonant hollow-core fiber. The birefringence was achieved by introducing capillary tubes of different thicknesses, thereby creating reduced symmetry in the structure. The measured group birefringence is in good agreement with the calculated group birefringence from simulations across the fiber guidance band within the telecommunications C-band. At 1550 nm, we measured a group birefringence of 4.4 × 10−5, which corresponds to a phase birefringence of 2.5 × 10−5. The measured loss of the fiber was 0.46 dB/m at 1550 nm. The measured polarization extinction ratio of the fiber at 1550 nm was 23.1 dB (25.7 dB) along the x-(y-) polarization axis, relating to an h-parameter of 9.8 × 10−4 (5.3 × 10−4).

Stephanos Yerolatsitis

Papers

The photonic lantern

Adiabatically-tapered fiber mode multiplexers.

The Photonic Lantern

Ultra-low background Raman sensing using a negative-curvature fibre and no distal optics.

Birefringent Anti-Resonant Hollow-Core Fiber