Laser surface and subsurface modification of sapphire using femtosecond pulses

Herein, an attention-grabbing and up-to-date review related to major multiplexing techniques is presented which includes wavelength division multiplexing (WDM), polarization division multiplexing (PDM), space division multiplexing (SDM), mode division multiplexing (MDM) and orbital angular momentum multiplexing (OAMM). Multiplexing is a mechanism by which multiple signals are combined into a shared channel used to showcase the maximum capacity of the optical links. However, it is critical to develop hybrid multiplexing methods to allow enhanced channel numbers. In this review, we have also included hybrid multiplexing techniques such as WDM-PDM, WDM-MDM and PDM-MDM. It is probable to attain <italic>N</italic>×<italic>M</italic> channels by utilizing <italic>N</italic> wavelengths and <italic>M</italic> guided-modes by simply utilizing hybrid WDM-MDM (de)multiplexers. To the best of our knowledge, this review paper is one of its kind which has highlighted the most prominent and recent signs of progress in multiplexing techniques in one place. 

https://www.oejournal.org/data/article/export-pdf?id=62cfc87b99d88171c95f55bd

Optical multiplexing techniques and their marriage for on-chip and optical fiber communication: a review

/pdf/laser-induced-crystallization-mechanisms-in-chalcogenide-2oobjvzo25.pdf

Laser-induced crystallization mechanisms in chalcogenide glass materials for advanced optical functionality

A metal–insulator–metal (MIM) waveguide consisting of a circular split-ring resonance cavity (CSRRC) and a double symmetric rectangular stub waveguide (DSRSW) is designed, which can excite quadruple Fano resonances. The finite element method (FEM) is used to investigate influences of geometric parameters on the transmission characteristics of the structure. The results show that Fano resonances are excited by the interference between the DSRSW and the CSRRC. Among them, the resonance wavelengths of the Fano resonances are tuned by the narrow-band discrete state excited by the CSRRC, and the resonance line transmittance and profiles are tuned by the wide-band continuous state excited by the DSRSW. The sensitivity (S) can be up to 1328.8 nm/RIU, and the figure of merit (FOM) can be up to 4.80 × 104. Based on these advantages, the structure has potential applications in sensing in the sub-wavelength range.

Refractive Index Sensing Based on Multiple Fano Resonances in a Split-Ring Cavity-Coupled MIM Waveguide

In this study, stress wave and damage morphology on the rear surface of silica during laser induced damage with cumulative UV pump shots is investigated. Time resolved imaging system, along with stress induced birefringence detecting, is used to demonstrate the development of stress wave. The properties of three types of stress waves, namely, compressive wave (P-wave), shear wave (S-wave), and Rayleigh wave (R-wave) are recorded and studied during the damage process. The experimental and simulated results indicated that R-wave exhibits the highest intensity among all the three stress waves. Because the R-wave is mainly localized at the region near the surface, it is responsible for the mechanical damage along the surface; in addition, it rapidly increases the damage diameter, which was observed from the front view of the damage.

Investigation of stress wave and damage morphology growth generated by laser-induced damage on rear surface of fused silica.

A silicon photonic 3-dB power splitter is one of the essential components to demonstrate large-scale silicon photonic integrated circuits (PICs), and can be utilized to implement modulators, 1 × 2 switches, and 1 × N power splitters for various PIC applications. In this paper, we reported the design and experimental demonstration of low-loss and broadband silicon photonic 3-dB power splitters. The power splitter was realized by adiabatically tapered rib waveguides with 60-nm shallow etches. The shallow-etched rib waveguides offered strong coupling and relaxed critical dimensions (a taper tip width of 200 nm and gap spacing of 300 nm). The fabricated device exhibited an excess loss as low as 0.06 dB at a 1550-nm wavelength and a broad operating wavelength range from 1470 nm to 1570 nm. The relaxed critical dimensions (≥200 nm) make the power splitter compatible with standard fabrication processes of existing silicon photonics foundries.

Low-Loss and Broadband Silicon Photonic 3-dB Power Splitter with Enhanced Coupling of Shallow-Etched Rib Waveguides

Mode-division multiplexing (MDM) is an attractive solution for future on-chip networks to enhance the optical transmission capacity with a single laser source. A mode-division reconfigurable optical add/drop multiplexer (ROADM) is one of the key components to construct flexible and complex on-chip optical networks for MDM systems. In this paper, we report on a novel scheme of mode-division ROADM with mode-selective silicon photonic MEMS (micro-electromechanical system) switches. With this ROADM device, data carried by any mode-channels can be rerouted or switched at an MDM network node, i.e., any mode could be added/dropped to/from the multimode bus waveguide flexibly and selectively. Particularly, the design and simulation of adiabatic vertical couplers for three quasi-TE modes (TE0, TE1, and TE2 modes) based on effective index analysis and mode overlap calculation method are reported. The calculated insertion losses are less than 0.08 dB, 0.19 dB, and 0.03 dB for the TE0 mode, TE1 mode, and TE2 mode couplers, respectively, over a wavelength range of 75 nm (1515–1590 nm). The crosstalks are below −20 dB over the bandwidth. The proposed device is promising for future on-chip optical networks with flexible functionality and large-scale integration.

https://www.mdpi.com/2304-6732/7/4/80/pdf

Silicon Photonic Mode-Division Reconfigurable Optical Add/Drop Multiplexers with Mode-Selective Integrated MEMS Switches

Dynamics of a laser-induced optical breakdown in the bulk of fused silica initiated by a sub-nanosecond laser pulse of an energy fluence as high as 8.7 kJ/cm 2 was investigated by using femtosecond time-resolved shadowgraphy. Plasma ignition, growth of the damaged region and accompanying hydrodynamic motion were recorded from the moment directly before the arrival of the driving laser pulse, in the time steps adapted to the rate of the occurring processes. The growth rate of the plasma channel, curvature radii and velocities of the wave fronts were extracted from the shadowgrams. It was found that the plasma channel develops with a supersonic velocity and the first observed shock front tends to transform itself from the initial bowl-like shape to the final spherical one characterising an acoustic wave. Appearance of multiple fronts accompanying the main shock front was registered and used in more detailed analysis of the optical breakdown dynamics in the transparent dielectrics.

Time-resolved shadowgraphy of optical breakdown in fused silica

A 2 × 2 3-dB coupler is one of the essential photonic components, as a building block of Mach-Zehnder interferometers, to realize large-scale photonic integrated circuits. Unlike typical 2 × 2 3-dB couplers based on direction couplers or multimode interference couplers, adiabatic couplers offer various advantages such as broadband operation and superior fabrication tolerance thanks to their unique operating mechanism of adiabatic mode evolution. However, an adiabatic coupler typically requires a long device length for ideal adiabatic operation without the excitations of unwanted modes. Here, we report on a compact 2 × 2 3-dB adiabatic coupler designed using the shortest mode transformer method. By optimizing the profile of the waveguide widths and the gap spacing, the compact 3-dB coupler was designed with a short coupling length of 23.2 μm. The fabricated device exhibits a 3-dB splitting ratio with less than ± 0.3 dB power oscillation and a low excess loss of 0.23 dB over a broad wavelength range of 1485–1620 nm. To the best of our knowledge, our coupler has the shortest length among the adiabatic couplers with a minimum feature size of 200 nm, reported to date.

/pdf/high-performance-and-compact-silicon-photonic-3-db-adiabatic-3zsjiqt94j.pdf

High-Performance and Compact Silicon Photonic 3-dB Adiabatic Coupler Based on Shortest Mode Transformer Method

We present characterization of structural modification triggered by tightly focused single pulses of a nanosecond laser inside single-crystal sapphire. Structural changes induced in the shock compressed region were investigated using high resolution transmission electron microscopy (HRTEM). Analysis of the zone around cavity in the bulk of sapphire reveals loss of crystalline order and formation of a mixture of amorphous/poly-crystalline structure. The properties of the laser-affected solid and possible routes of material transformation to the final state long after the pulse end is discussed. The results suggest that transformations to amorphous/poly-crystalline state occur as a result of sufficient heating of the shell region. This creates a localized molten zone which solidifies so rapidly that crystallization is by-passed.

Vinh Huu Nguyen

Papers

Low-Loss and Broadband Silicon Photonic 3-dB Power Splitter with Enhanced Coupling of Shallow-Etched Rib Waveguides

Silicon Photonic Mode-Division Reconfigurable Optical Add/Drop Multiplexers with Mode-Selective Integrated MEMS Switches

Time-resolved shadowgraphy of optical breakdown in fused silica

High-Performance and Compact Silicon Photonic 3-dB Adiabatic Coupler Based on Shortest Mode Transformer Method

Laser-induced structural modification in bulk of single-crystal sapphire