Low-loss Type II waveguide writing in fused silica with single picosecond laser pulses

Buried Depressed-Cladding Waveguides Inscribed in Nd3+ and Yb3+ Doped CLNGG Laser Crystals by Picosecond-Laser Beam Writing

Buried depressed-cladding waveguides were fabricated in 0.7-at.% Nd:Ca3Li0.275Nb1.775Ga2.95O12 (Nd:CLNGG) and 7.28-at.% Yb:CLNGG disordered laser crystals grown by Czochralski method. Circular waveguides with 100 μm diameters were inscribed in both crystals with picosecond (ps) laser pulses at 532 nm of 0.15 μJ energy at 500 kHz repetition rate. A line-by-line writing technique at 1 mm/s scanning speed was used. Laser emission at 1.06 μm (with 0.35 mJ pulse energy) and at 1.03 μm (with 0.16 mJ pulse energy) was obtained from the waveguide inscribed in Nd:CLNGG and Yb:CLNGG, respectively, employing quasi-continuous wave pumping with fiber-coupled diode lasers. The waveguide realized in RE3+-doped CLNGG crystals using ps-laser pulses at high repetition rates could provide Q-switched or mode-locked miniaturized lasers for a large number of photonic applications.

213 nm laser written waveguides in Ge-doped planar silica without hydrogen loading

In this paper we present the first example of waveguides fabricated by UV writing in non-hydrogen loaded Ge-doped planar silica with 213 nm light. Single mode waveguides were fabricated and the numerical apertures and mode field diameters were measured for a range of writing fluences. A peak index change of 5.3 x 10^-3 was inferred for the waveguide written with 70 kJ cm^-2. The refractive index change is sufficient to match the index structure of standard optical fiber. Uniformity of the written structures was measured and a propagation loss of 0.39 ± 0.03 dB cm^-1 was determined through cutback measurements.

Comparative study of quantitative phase imaging techniques for refractometry of optical waveguides

A comparative study of quantitative phase imaging techniques for refractometry of optical waveguides is presented. Three techniques were examined: a method based on the transport-of-intensity equation, quadri-wave lateral shearing interferometry and digital holographic microscopy. The refractive index profile of a SMF-28 optical fiber was thoroughly characterized and served as a gold standard to assess the accuracy and precision of the phase imaging methods. Optical waveguides were inscribed in an Eagle2000 glass chip using a femtosecond laser and used to evaluate the sensitivity limit of these phase imaging approaches. It is shown that all three techniques provide accurate, repeatable and sensitive refractive index measurements. Using these phase imaging methods, we report a comprehensive map of the photosensitivity to femtosecond pulses of Eagle2000 glass. Finally, the reported data suggests that the phase imaging techniques are suited to be used as precise and non-destructive refractive index shift measuring tools to study and control the inscription process of optical waveguides.

Modular architecture for fully non-blocking silicon photonic switch fabric

Integrated photonics offers the possibility of compact, low energy, bandwidth-dense interconnects for large port count spatial optical switches, facilitating flexible and energy efficient data movement in future data communications systems. To achieve widespread adoption, intimate integration with electronics has to be possible, requiring switch design using standard microelectronic foundry processes and available devices. We report on the feasibility of a switch fabric comprised of ubiquitous silicon photonic building blocks, opening the possibility to combine technologies, and materials towards a new path for switch fabric design. Rather than focus on integrating all devices on a single silicon chip die to achieve large port count optical switching, this work shifts the focus towards innovative packaging and integration schemes. In this work, we demonstrate 1×8 and 8×1 microring-based silicon photonic switch building blocks with software control, providing the feasibility of a full 8×8 architecture composed of silicon photonic building blocks. The proposed switch is fully non-blocking, has path-independent insertion loss, low crosstalk, and is straightforward to control. We further analyze this architecture and compare it with other common switching architectures for varying underlying technologies and radices, showing that the proposed architecture favorably scales to very large port counts when considering both crosstalk and architectural footprint. Separating a switch fabric into functional building blocks via multiple photonic integrated circuits offers the advantage of piece-wise manufacturing, packaging, and assembly, potentially reducing the number of optical I/O and electrical contacts on a single die.

On the use of the Type I and II scheme for classifying ultrafast laser direct-write photonics

The use of the Type I and Type II scheme, first introduced and used by fiber Bragg grating researchers, has recently been adopted by the ultrafast laser direct-write photonics community to classify the physical geometry of waveguides written into glasses and crystals. This has created confusion between the fiber Bragg grating and direct-write photonics community. Here we propose a return to the original basis of the classification based on the characteristics of the material modification rather than the physical geometry of the waveguide.

Expanded-core waveguides written by femtosecond laser irradiation in bulk optical glasses

Expanded-core structures based on layered increased index (type I) waveguiding traces are fabricated by ultrafast laser photoinscription in bulk optical glasses, with examples for fused silica and chalcogenide glasses. The expanded-core waveguides can serve for large-mode-area guiding concepts and their feasibility is experimentally investigated. A parametric study of the geometry, number of traces and index contrast indicates the possibility to design guided modes characteristics as exemplified in fused silica. A specific arrangement consisting of 8 traces of guiding layers with 6µm separation exhibit single-mode transport properties with mode field area of ~805µm². The condition of single mode operation is also discussed in the frame of the dispersion relation of light guiding in periodical dielectric structures. The supported supermode of expanded-core structures can be controlled by careful design of the refractive index change, the number of guiding layers and the thickness of the interlayers. Inspection of the propagation characteristics shows equally low loss features. A Y-branching splitter based on expanded-core concept conserving single mode characteristics is fabricated. The optical design is equally successfully tested in chalcogenide Gallium Lanthanum Sulfide glass. Ultrafast laser inscribed expanded-core waveguiding provides therefore an interesting path of fabricating large mode area waveguides usable in near infrared and mid-infrared region beneficial for applications requiring high power or large mode dimensions.

Embedded optical waveguides fabricated in SF10 glass by low-repetition-rate ultrafast laser

Symmetric embedded waveguides were fabricated in heavy metal oxide SF10 glass using slit-shaped infrared femtosecond laser writing in the low-repetition frequency regime. The impact of the writing parameters on the waveguide formation in the transverse writing scheme was systemically studied. Results indicate that efficient waveguides can be inscribed in a wide parameter space ranging from 500 fs to 1.5 ps pulse duration, 0.7-4.2 μJ pulse energy, and 5 μm/s to 640 μm/s scan speed and pointing out the robustness of the photoinscription process. The refractive index profile reconstructed from the measured near field pattern goes up to 10(-3). In addition, propagation losses of the waveguides are tolerable, with the lowest propagation loss estimated at 0.7 dB/cm. With a 5 μm/s scan speed and 3.5 μJ pulse energy in a high-dose regime, few-mode guiding was achieved in the waveguide at 800 nm signal injection wavelength. This is due to a combination of increased refractive index in the core of the trace and the appearance of a depressed cladding.

Femtosecond versus picosecond laser machining of nano-gratings and micro-channels in silica glass

The ability of 8 picosecond pulse lasers for three dimensional direct-writing in the bulk of transparent dielectrics is assessed through a comparative study with a femtosecond laser delivering 600 fs pulses. The comparison addresses two main applications: the fabrication of birefringent optical elements and two-step machining by laser exposure and post-processing by chemical etching. Formation of self-organized nano-gratings in glass by ps-pulses is demonstrated. Differential etching between ps-laser exposed regions and unexposed silica is observed. Despite attaining values of retardance (>100 nm) and etching rate (2 μm/min) similar to fs pulses, ps pulses are found unsuitable for bulk machining in silica glass primarily due to the build-up of a stress field causing scattering, cracks and non-homogeneous etching. Additionally, we show that the so-called "quill-effect", that is the dependence of the laser damage from the direction of writing, occurs also for ps-pulse laser machining. Finally, an opposite dependence of the retardance from the intra-pulse distance is observed for fs- and ps-laser direct writing.

Beam shaping of laser diode radiation by waveguides with arbitrary cladding geometry written with fs-laser radiation

Waveguides with arbitrary cross sections are written in the volume of Al(2)O(3)-crystals using tightly focused femtosecond laser radiation. Utilizing a scanning system with large numerical aperture, complex cladding geometries are realized with a precision around 0.5 µm and a scanning speed up to 100 mm/s. Individual beam and mode shaping of laser diode radiation is demonstrated by varying the design of the waveguide cladding. The influence of the writing parameters on the waveguide properties are investigated resulting in a numerical aperture of the waveguides in the range of 0.1. This direct laser writing technique enables optical devices which could possibly replace bulky beam shaping setups with an integrated solution.

Dynamic ultrafast laser spatial tailoring for parallel micromachining of photonic devices in transparent materials

Femtosecond laser processing of bulk transparent materials can generate localized positive changes of the refractive index. Thus, by translation of the laser spot, light-guiding structures are achievable in three dimensions. Increasing the number of laser processing spots can consequently reduce the machining effort. In this paper, we report on a procedure of dynamic ultrafast laser beam spatial tailoring for parallel photoinscription of photonic functions. Multispot operation is achieved by spatially modulating the wavefront of the beam with a time-evolutive periodical binary phase mask. The parallel longitudinal writing of multiple waveguides is demonstrated in fused silica. Using this technique, light dividers in three dimensions and wavelength-division demultiplexing (WDD) devices relying on evanescent wave coupling are demonstrated.

Transition from thermal diffusion to heat accumulation in high repetition rate femtosecond laser writing of buried optical waveguides

A variable (0.2 to 5 MHz) repetition rate femtosecond laser was applied to delineate the role of thermal diffusion and heat accumulation effects in forming low-loss optical waveguides in borosilicate glass across a broad range of laser exposure conditions. For the first time, a smooth transition from diffusion-only transport at 200 kHz repetition rate to strong heat accumulation effects at 0.5 to 2 MHz was observed and shown to drive significant variations in waveguide morphology, with rapidly increasing waveguide diameter that accurately followed a simple thermal diffusion model over all exposure variables tested. Amongst these strong thermal trends, a common exposure window of 200 mW average power and approximately 15-mm/s scan speed was discovered across the range of 200 kHz to 2 MHz repetition rates for minimizing insertion loss despite a 10-fold drop in laser pulse energy. Waveguide morphology and thermal modeling indicate that strong thermal diffusion effects at 200 kHz give way to a weak heat accumulation effect at approximately 1 microJ pulse energy for generating low loss waveguides, while stronger heat accumulation effects above 1-MHz repetition rate offered overall superior guiding. A comprehensive characterization of waveguide properties is presented for laser writing in the thermal diffusion and heat accumulation regimes. The waveguides are shown to be thermally stable up to 800 degrees C and can be written in a convenient 520 microm depth range with low spherical aberration.

Femtosecond laser direct writing of multiwavelength Bragg grating waveguides in glass

Novel Bragg grating waveguide structures have been fabricated in bulk borosilicate glass through a type II photosensitivity mechanism driven by single femtosecond laser pulses. Low-loss single-mode waveguides and narrow-linewidth Bragg gratings were generated simultaneously by forming an array of refractive index voxels in a single laser scan. Laser pulse duration is shown to significantly affect the grating strength and waveguide loss. Bragg wavelengths, defined by the periodicity of laser-modified volumes, were fully controlled by the sample scan speed to provide resonances anywhere in the 1200-1620 nm telecommunication bands. Four linear Bragg filters with distinct resonant wavelengths are presented that define the first demonstration of laser writing of multiple-wavelength and cascaded Bragg grating waveguides in a single process step.