Er:Yb-doped waveguide laser fabricated by femtosecond laser pulses

Laser-Induced Erasable and Re-Writable Waveguides within Silver Phosphate Glasses

Femtosecond direct laser writing is a well-established and robust technique for the fabrication of photonic structures. Herein, we report on the fabrication of buried waveguides in AgPO₃ silver metaphosphate glasses, as well as, on the erase and re-writing of those structures, by means of a single femtosecond laser source. Based on the fabrication procedure, the developed waveguides can be erased and readily re-inscribed upon further femtosecond irradiation under controlled conditions. Namely, for the initial waveguide writing the employed laser irradiation power was 2 J/cm² with a scanning speed of 5 mm/s and a repetition rate of 200 kHz. Upon enhancing the power to 16 J/cm² while keeping constant the scanning speed and reducing the repetition rate to 25 kHz, the so formed patterns were readily erased. Then, upon using a laser power of 2 J/cm² with a scanning speed of 1 mm/s and a repetition rate of 200 kHz the waveguide patterns were re-written inside the glass. Scanning electron microscopy (SEM) images at the cross-section of the processed glasses, combined with spatial Raman analysis revealed that the developed write/erase/re-write cycle, does not cause any structural modification to the phosphate network, rendering the fabrication process feasible for reversible optoelectronic applications. Namely, it is proposed that this non-ablative phenomenon lies on the local relaxation of the glass network caused by the heat deposited upon pulsed laser irradiation. The resulted waveguide patterns Our findings pave the way towards new photonic applications involving infinite cycles of write/erase/re-write processes without the need of intermediate steps of typical thermal annealing treatments.

Femtosecond Laser-Fabricated Photonic Chips for Optical Communications: A Review

Integrated optics, having the unique properties of small size, low loss, high integration, and high scalability, is attracting considerable attention and has found many applications in optical communications, fulfilling the requirements for the ever-growing information rate and complexity in modern optical communication systems. Femtosecond laser fabrication is an acknowledged technique for producing integrated photonic devices with unique features, such as three-dimensional fabrication geometry, rapid prototyping, and single-step fabrication. Thus, plenty of femtosecond laser-fabricated on-chip devices have been manufactured to realize various optical communication functions, such as laser generation, laser amplification, laser modulation, frequency conversion, multi-dimensional multiplexing, and photonic wire bonding. In this paper, we review some of the most relevant research progress in femtosecond laser-fabricated photonic chips for optical communications, which may break new ground in this area. First, the basic principle of femtosecond laser fabrication and different types of laser-inscribed waveguides are briefly introduced. The devices are organized into two categories: active devices and passive devices. In the former category, waveguide lasers, amplifiers, electric-optic modulators, and frequency converters are reviewed, while in the latter, polarization multiplexers, mode multiplexers, and fan-in/fan-out devices are discussed. Later, photonic wire bonding is also introduced. Finally, conclusions and prospects in this field are also discussed.

MHz-repetition rate fs-laser-inscribed crystalline waveguide lasers inscribed at 100 mm/s

We report on fast direct laser inscription of waveguide laser structures in a crystal. For the first time, a 1 MHz-repetition rate fs-laser was utilized for this purpose. We inscribed and characterized more than 100 tracks with different inscription parameters in Yb:CALGO crystals. Waveguide lasing with slope efficiencies of up to 57% at a maximum output power of 3.4 W and more than 55% of optical efficiency was obtained under pumping with an optically pumped semiconductor laser (OPSL), even in waveguides fabricated at record-high inscription velocities of 100 mm/s. Such laser performance is similar to previously reported waveguide lasers inscribed at 1 kHz repetition rate and paves the way toward an industrial fabrication of such waveguides.

Anti-escaping of incident laser in rare-earth doped fluoride ceramics with glass forming layer

Adaptive fluoride ceramic with glass forming layer (GC_ZBL-Er) used in laser anti-escaping has been prepared by one-step synthesis, and the thickness of glass layer is identified as ~0.41 mm. Blue, green and red emissions of Er³⁺/Yb³⁺ codoped fluoride ceramic (C_ZBL-Er) and glass layer (G_ZBL-Er) have been investigated under ~980 nm laser pumping. With the forming of thin glass layer on ceramic surface, the absorption intensities on diffuse reflection of GC_ZBL-Er at 974 nm and 1.53 μm increase by 48% and 53% than those of C_ZBL-Er. Excited by a 979 nm laser, the presence of the glass layer increases the absolute absorption rate in spectral power from 75% in C_ZBL-Er to 83% in GC_ZBL-Er, which is consistent with the improvement in the absorbed photon number. In addition, the quantum yield of GC_ZBL-Er complex is raised by 28.4% compared to the case of ceramic substrate by photon quantification. Intense absorption-conversion ability and efficient macroscopical anti-escaping effect confirm the superiority of ingenious structure in the fluoride ceramics with glass forming layer, which provides a new approach for developing the absorption-conversion materials of anti-NIR laser detection.

Fs-laser-written erbium-doped double tungstate waveguide laser

We report on the first erbium (Er³⁺) doped double tungstate waveguide laser. As a gain material, we studied a monoclinic Er³⁺:KLu(WO₄)₂ crystal. A depressed-index buried channel waveguide formed by a 60 µm-diameter circular cladding was fabricated by 3D femtosecond direct laser writing. The waveguide was characterized by confocal laser microscopy, µ-Raman and µ-luminescence mapping, confirming that the crystallinity of the core is preserved. The waveguide laser, diode pumped at 981 nm, generated 8.9 mW at 1533.6 nm with a slope efficiency of 20.9% in the continuous-wave regime. The laser polarization was linear (E || N_m). The laser threshold was at 93 mW of absorbed pump power.

Optically pumped rare-earth-doped Al2O3 distributed-feedback lasers on silicon [Invited]

This paper reviews recent progress in the field of optically pumped rare-earth-doped channel waveguide lasers, with a focus on operation utilizing distributed-feedback resonators on silicon wafers. Rare-earth-doped amorphous aluminum oxide thin films have been deposited onto silicon wafers by RF reactive co-sputtering from metallic Al and rare-earth targets, the spectroscopy and optical gain of Er³⁺, Yb³⁺, Nd³⁺, and Tm³⁺ ions has been investigated, and the near-infrared laser transitions near 1 μm in Yb³⁺, 1.5 μm in Er³⁺, and 2 μm in Tm³⁺ and Ho³⁺ have been demonstrated. Output power between a few μW and hundreds of mW have been achieved in different waveguide geometries, and ultranarrow-linewidth laser operation has been demonstrated.

Femtosecond laser writing of photonic devices in borate glasses compositionally designed to be laser writable

The design and performance of borate glass samples compositionally pre-designed to be femtosecond laser writable via laser-induced ion migration is reported in this Letter. It is demonstrated that borate glasses modified on purpose with small amounts of La₂O₃ and Na₂O can be straightforwardly used to produce high-index contrast (Δn) waveguides by femtosecond-laser-assisted ion migration. The positive Δn of the waveguides is caused by the local enrichment of La₂O₃ in the guiding region with a slope of 8·10^-3(mol.%)^-1. The value of this is consistent with numerical aperture measurements of the waveguides and local compositional measurements at the guiding region performed by energy-dispersive x-ray micro-analysis. The maximum achievable Δn values can be controlled through the initial La₂O₃ content of the glass. Maximum Δn values >10^-2 for samples with just 5.5 mol. % of La₂O₃ have been produced. This compositional design approach could be potentially used to produce highly efficient femtosecond laser writeable glasses in other glass families.

Femtosecond-laser-written Tm:KLu(WO4)2 waveguide lasers

Depressed-index channel waveguides with a circular and photonic crystal cladding structures are prepared in a bulk monoclinic Tm:KLu(WO₄)₂ crystal by 3D direct femtosecond laser writing. The channel waveguide structures are characterized and laser operation is achieved using external mirrors. In the continuous-wave mode, the maximum output power of 46 mW is achieved at 1912 nm corresponding to a slope efficiency of 15.2% and a laser threshold of only 21 mW. Passive Q-switching of a waveguide with a circular cladding is realized using single-walled carbon nanotubes. Stable 7 nJ/50 ns pulses are achieved at a repetition rate of 1.48 MHz. This first demonstration of ∼2  μm fs-laser-written waveguide lasers based on monoclinic double tungstates is promising for further lasers of this type doped with Tm³⁺ and Ho³⁺  ions.

Longitudinally diode-pumped planar waveguide YAG/Yb:LuAG/YAG ceramic laser at 1030.7 nm

Composite YAG/15 at. % Yb:LuAG/YAG transparent ceramic planar waveguide was fabricated by a tape casting method and vacuum sintering technology. Under a 970 nm diode laser pumping, the absorbed efficiency of 85.4% was achieved, and efficient CW laser operation at 1030.7 nm was accomplished with a good beam quality with Gaussian spatial profile. A maximum output power of 288 mW was obtained under a pump power of 4.69 W, corresponding to a slope efficiency of 9% and an O-O conversion efficiency of 5%.

Cascaded-focus laser writing of low-loss waveguides in polymers

Waveguide writing in poly (methyl methacrylate) (PMMA) with femtosecond laser radiation is presented. An adequate refractive index change is induced in the border area below the irradiated focal volume. It supports an almost symmetric fundamental mode with propagation losses down to 0.5  dB/cm, the lowest losses observed so far in this class of materials. The writing process with a cascaded focus is demonstrated to be highly reliable over a large parameter range.

Flexible photonic components in glass substrates

This paper demonstrates the fabrication and measurements of flexible photonic lightwave circuits in glass substrates. Using temporally and spatially shaped ultrafast laser pulses, highly symmetrical and low-loss optical waveguides were written in flexible glass substrates with thicknesses ranging from 25 µm to 100 µm. The waveguide propagation loss, measured by optical frequency domain reflectometry, was 0.11 dB/cm at 1550 nm telecommunication wavelength. The bend loss of the waveguide is negligible at a radius of curvature of 1.5 cm or greater. Additionally, the waveguides are thermally stable up to 400°C. This paper presents alternatives to fabricating flexible photonics in traditionally used polymeric materials.

Towards femtosecond laser written arrayed waveguide gratings

The fabrication of arrayed waveguide gratings (AWGs) using the femtosecond laser direct-write technique is investigated. We successfully demonstrate the fabrication of large planar waveguides that act as 2D free propagation zones. These slabs were found to have a highly uniform refractive index with a standard deviation of 1.97% relative to the total index contrast. The incorporation of low loss linear adiabatic tapers resulted in an increase of transmission by 90%. Strategies for manufacturing integrated laser written AWGs using continuous contouring to avoid lossy defects are discussed and demonstrated.

Laser oscillation in Yb:YAG waveguide beam-splitters with variable splitting ratio

In this Letter, we report on laser emission from Yb:YAG beam-splitters fabricated by direct femtosecond-laser inscription. These devices consist of two adjacent mirrored s-curved waveguides merged into a section allowing for multi-mode interference (MMI) and a straight waveguide. Nine-millimeter-long one-to-two beam-splitters with radii of curvature between 10 and 80 mm have been fabricated resulting in separations between the two arms of up to 0.65 mm. Due to the MMI-section, the splitting ratio was variable by slightly changing the incoupling conditions. Maximum laser output powers of 2.29 W and slope efficiencies of 52% were achieved with the best beam-splitter waveguide laser. For all radii of curvature exceeding 10 mm, similar efficiencies were demonstrated.

Theoretical and experimental study of 37-core waveguides with large mode area

The evanescently coupled multicore waveguide lattice composed of 37 linear type I cores hexagonally arranged has been theoretically studied and fabricated by low-repetition-rate femtosecond laser inscription of bulk fused silica. The effects of the single core's numerical apertures (NAs) and spacing on the mode characteristics of the 37-core waveguide were calculated by the finite-element method. It was found that the mode field areas of the fundamental mode LP₀₁ with 5 μm spacing of different NAs were all larger than 577  μm², which was confirmed by the experiments. The measured near-field mode profiles for different writing conditions and different spacing also showed that the waveguide supported both a single mode (LP₀₁) and two modes (LP₀₁ and LP₁₁). The multicore waveguide, according to our study, is particularly interesting for mode converters.

High slope efficiency and high refractive index change in direct-written Yb-doped waveguide lasers with depressed claddings

We report the first Yb:ZBLAN and Yb:IOG10 waveguide lasers fabricated by the fs-laser direct-writing technique. Pulses from a Titanium-Sapphire laser oscillator with 5.1 MHz repetition rate were utilized to generate negative refractive index modifications in both glasses. Multiple modifications were aligned in a depressed cladding geometry to create a waveguide. For Yb:ZBLAN we demonstrate high laser slope efficiency of 84% with a maximum output power of 170 mW. By using Yb:IOG10 a laser performance of 25% slope efficiency and 72 mW output power was achieved and we measured a remarkably high refractive index change exceeding Δn = 2.3 × 10(-2).

Large mode area waveguides with polarization functions by volume ultrafast laser photoinscription of fused silica

We present optical designs allowing large mode area light guiding by ultrafast laser photoinscription of bulk fused silica. If usual concepts are based on large core and depressed cladding, evanescently coupled multicore waveguides with coherent mode superposition can be effective solutions, where the introduction of nanostructured defects determines additional polarization functions.

Versatile large-mode-area femtosecond laser-written Tm:ZBLAN glass chip lasers

We report performance characteristics of a thulium doped ZBLAN waveguide laser that supports the largest fundamental modes reported in a rare-earth doped planar waveguide laser (to the best of our knowledge). The high mode quality of waveguides up to 45 um diameter (~1075 μm(2) mode-field area) is validated by a measured beam quality of M(2)~1.1 ± 0.1. Benefits of these large mode-areas are demonstrated by achieving 1.9 kW peak-power output Q-switched pulses. The 1.89 μm free-running cw laser produces 205 mW and achieves a 67% internal slope efficiency corresponding to a quantum efficiency of 161%. The 9 mm long planar chip developed for concept demonstration is rapidly fabricated by single-step optical processing, contains 15 depressed-cladding waveguides, and can operate in semi-monolithic or external cavity laser configurations.

Femtosecond laser writing of a flat-top interleaver via cascaded Mach-Zehnder interferometers

A flat-top interleaver consisting of cascaded Mach-Zehnder interferometers (MZIs) was fabricated in bulk glass by femtosecond laser direct writing. Spectral contrast ratios of greater than 15 dB were demonstrated over a 30 nm bandwidth for 3 nm channel spacing. The observed spectral response agreed well with a standard transfer matrix model generated from responses of individual optical components, demonstrating the possibility for multi-component optical design as well as sufficient process accuracy and fabrication consistency for femtosecond laser writing of advanced optical circuits in three dimensions.

Single-pass waveguide amplifiers in Er-Yb doped zinc polyphosphate glass fabricated with femtosecond laser pulses

We have investigated the direct fabrication of subsurface waveguide amplifiers in Er-Yb zinc polyphosphate glass by utilizing the relationship between the initial glass composition and the resulting changes to the network structure after modification by fs laser pulses. Waveguides, exhibiting internal gain of 1 dB/cm at 1.53 μm when pumped with 500 mW at 976 nm, were directly fabricated using a regenerative amplified Ti:sapphire 1 kHz, 180 fs laser system. Optical properties as well as insertion losses and internal gain are reported.

Low loss depressed cladding waveguide inscribed in YAG:Nd single crystal by femtosecond laser pulses

A depressed cladding waveguide with record low loss of 0.12 dB/cm is inscribed in YAG:Nd(0.3at.%) crystal by femtosecond laser pulses with an elliptical beam waist. The waveguide is formed by a set of parallel tracks which constitute the depressed cladding. It is a key element for compact and efficient CW waveguide laser operating at 1064 nm and pumped by a multimode laser diode. Special attention is paid to mechanical stress resulting from the inscription process. Numerical calculation of mode distribution and propagation loss with the elasto-optical effect taken into account leads to the conclusion that the depressed cladding is a dominating factor in waveguide mode formation, while the mechanical stress only slightly distorts waveguide modes.

Direct femtosecond laser waveguide writing inside zinc phosphate glass

We report the relationship between the initial glass composition and the resulting microstructural changes after direct femtosecond laser waveguide writing with a 1 kHz repetition rate Ti:sapphire laser system. A zinc polyphosphate glass composition with an oxygen to phosphorus ratio of 3.25 has demonstrated positive refractive index changes induced inside the focal volume of a focusing microscope objective for laser pulse energies that can achieve intensities above the modification threshold. The permanent photo-induced changes can be used for direct fabrication of optical waveguides using single scan writing techniques. Changes to the localized glass network structure that produce positive changes in the refractive index of zinc phosphate glasses upon femtosecond laser irradiation have been studied using scanning confocal micro-Raman and fluorescence spectroscopy.

Highly efficient Yb:YAG channel waveguide laser written with a femtosecond-laser

Using a femtosecond Ti:Sapphire laser, micro-tracks of material damage were written into Yb:YAG crystals. Waveguiding was achieved in a channel between pairs of tracks with guiding losses of 1.3 dB/cm at a wavelength of 1063 nm, due to a stress induced change of the refractive index. Pumped at a wavelength of 941 nm, highly efficient laser oscillation in a Yb:YAG channel waveguide at a wavelength of 1030 nm was demonstrated. An output power of 0.8 W at 1.2 W of launched pump power was achieved, resulting in a record slope efficiency of 75%.

Spherical aberration correction suitable for a wavefront controller

We propose a simple method to correct a large amount of spherical aberration caused by a refractive index mismatch. The method is based on inverse ray tracing and can generate correction phase patterns whose peak-to-valley values are minimized. We also demonstrated spherical aberration correction in a transparent acrylic block using a liquid-crystal-on-silicon spatial light modulator (LCOS-SLM). A distorted focal volume without correction was substantially improved with correction. This method is useful in cases where a large phase modulation is needed, such as when employing a high-NA lens or focusing a beam deep inside a sample.

High-peak-power pulses from a cavity-dumped Yb:KY(WO4)2 oscillator

We report generation of 1.35 microJ femtosecond laser pulses with a peak power of 3 MW at 1 MHz repetition rate from a diode-pumped Yb:KY(WO4)2 laser oscillator with cavity dumping. By extracavity compression with a large-mode-area fiber and a prism sequence, we generate ultrashort pulses with a duration of 21 fs and a peak power of 13 MW.