Single-mode Guidance Research Articles

Optical and fluorescent properties in Er, Yb:YAG double-line waveguide by femtosecond laser direct writing

This paper reports the double-line waveguides in Er,Yb:YAG prepared by femtosecond laser (fs-laser) writing at 1030 nm, 450 fs pulse and 100 kHz repetition rate. When light propagates at 632.8 nm along transverse electric (TE) and transverse magnetic (TM) polarization, efficient single-mode guidance is demonstrated in the double-line structures. The minimum transmission loss is as low as 0.98 dB/cm in waveguide with space of 18 μm. The structural modifications in damaged tracks and strain-affected regions were characterized by confocal micro-Raman spectroscopy, and the refractive index variations were analyzed. The corresponding fluorescence emissions of Er3+ and Yb3+ are investigated in the Er,Yb:YAG double-line structure.

Few-mode metal-free perovskite optical fiber with second-order optical nonlinearity

Semiconductor core optical fibers are highly desirable for fiber-based photonic and optoelectronic applications as they can combine strong optical nonlinearities, tight light confinement, wide transmission bands, and electronic functionality within a single platform. Perovskites have emerged as particularly exciting materials for semiconductor photonics as they have strong optical nonlinearities and tunable optoelectronic bandgaps. However, lead-based perovskites contain toxic elements and are, therefore, not environmentally friendly. Furthermore, in fiber form, their core-size is prohibitively large, making them unsuitable for nonlinear optics and applications that require single-mode guidance, such as telecommunications. Here, we report a metal-free perovskite core optical fiber where lead has been substituted for an ammonium cation in the perovskite structure. The core material has a wide bandgap greater than 5 eV, a high laser damage threshold, and a core diameter that can be produced as small as 5 µm. At this core size, the fiber supports just six modes, and the fundamental mode can readily be excited and isolated. Moreover, the metal-free perovskite has a second-order susceptibility that is absent in the archetypal lead-based perovskites and many other semiconductor core materials, such as silicon and germanium. The second-order susceptibility is important for many nonlinear optics applications, such as second-harmonic generation and quantum optics.

Three stage HCF fabrication technique for high yield, broadband UV-visible fibers.

Hollow-core optical fibers can offer broadband, single mode guidance in the UV-visible-NIR wavelength range, with the potential for low-loss, solarization-free operation, making them desirable and potentially disruptive for a wide range of applications. To achieve this requires the fabrication of fibers with <300nm anti-resonant membranes, which is technically challenging. Here we investigate the underlying fluid dynamics of the fiber fabrication process and demonstrate a new three-stage fabrication approach, capable of delivering long (∼350m) lengths of fiber with the desired thin-membranes.

Enhancing single-mode guidance using avoided crossings in anti-resonant hollow-core fibers with five nested cladding tubes

We numerically analyze the nested anti-resonant nodeless fibers (NANFs) with five tubes to determine the best single-mode guidance and low confinement loss for the fundamental mode simultaneously. We derive the empirical relation between structural parameters that lead to avoided crossings between the core higher order modes (HOMs) and the cladding tube modes in a 5-tube nested design to achieve robust single-mode guidance. We report a low confinement loss of 1.17×10−3 dB/km and 0.23 dB/km, and huge HOM extinction ratio (HOMER) of the order of 105 and 104 at wavelengths of 650 nm and at 1550 nm respectively. We also investigate the fabrication tolerances of the confinement loss and HOMER to the changes in design parameters. These 5-tube NANFs exhibit a broad loss spectrum of 200 nm with loss of fundamental mode ¡ 0.003 dB/km and HOMER of 16000 centered around 650 nm. NANFs find applications in quantum communications, sensing and telecommunications and this work can be helpful in design optimization of such fibers.

Optimization of hollow-core fibers with elliptical tubes for improved single-mode guidance

Hollow core fibers with elliptical tubes offer more design flexibility and can achieve better performance compared to their circular counterparts. We optimize the elliptical tubes of a hollow-core fiber to achieve a higher order mode extinction ratio (HOMER) exceeding 5500 along with a confinement loss of 0.012 dB/m in the telecommunication band; 4.7×10−4 dB/m with HOMER > 4300 in the visible band. We also derive an empirical relation between the structural parameters to obtain avoided crossings between the core higher order modes and the cladding modes to achieve the best single-mode guidance, besides achieving minimum confinement loss for the fundamental mode. We investigate the effect of bends on not just the confinement loss, but also on the HOMER. We have also included the dispersion and fabrication tolerances. These fibers find applications in quantum communication, sensing, and high-power laser delivery.

Multi-nested antiresonant hollow-core fiber with ultralow loss and single-mode guidance.

We propose an antiresonant hollow-core fiber design that exhibits ultralow loss and exceptional single modedness at 1.55 µm. In this design, the confinement loss of less than 10-6 dB m-1 can be obtained with excellent bending performance even at a tight bending radius of 3 cm. At the same time, a record-high higher-order mode extinction ratio of 8 × 105 can be achieved in the geometry by inducing strong coupling between the higher-order core modes and cladding hole modes. These guiding properties make it an excellent candidate for applications in hollow-core fiber-enabled low-latency telecommunication systems.

All-Glass Single-Mode Leakage Channel Microstructured Optical Fibers with Large Mode Area and Low Bending Loss

The paper presents the results of theoretical and experimental studies of all-glass leakage channel microstructured optical fibers (MOFs) with a large mode area and low bending losses. These MOFs contain two layers of fluorine-doped silica glass elements with a reduced refractive index, different diameters, and different distances between them. A numerical analysis of the properties of these MOFs was performed using the finite element method. The leakage losses for the fundamental and higher-order modes were calculated in the spectral range from 0.65 μm to 1.65 μm. Simulation results show that the proposed MOF design allows for single-mode guidance in the spectral range from 0.92 μm to 1.21 μm with a bending radius of down to 0.08 m. The measured losses of the fabricated MOF with a core diameter of 22.5 μm and a bending radius of 0.1 m were less than 0.1 dB/m in the spectral range from 0.9 μm to 1.5 μm. It is demonstrated that the segments of this MOF longer than 5 m are single-mode.

Improved designs for short wavelength amplification based on a thulium-doped Ta2O5 waveguide

Abstract This study aims to design a thulium-doped Ta2O5 amplifier for short wavelength amplification at 1.7 µm waveband. The waveguide width of the proposed design is theoretically optimized to ensure the condition of single mode guidance and the appropriate width must be less than 3.2 µm. Finite element method based edge element is applied to compute the optical field distribution of the fundamental mode across the waveguide length and to find the values of the overlapping factor and the effective mode field area of the proposed waveguide. The numerical results display that 0.83 and 6.8 µm2 are the computed values of the overlapping factor and the effective mode field area, respectively at 1.72 µm wavelength. The amplification performance of a Tm:Ta2O5 amplifier is presented based on the rate equations and the power distribution equations. The effects of the waveguide length and the pump power are investigated on the signal amplification at 1.72 µm wavelength. The simulation results reveal that about 25 dB small signal gain and 8.7 dB noise figure could be achieved at 1.72 µm wavelength when pumped with only 100 mW pump power at 1630 nm wavelength. As a result, it is possible to achieve an efficient laser and amplifier source at the 1.7 µm waveband based on a Tm:Ta2O5 waveguide.

Designing a W-type index praseodymium-doped chalcogenide fiber for strongly-efficient MIR Laser beyond 4 μm

This study is to propose an efficient design based on W-type index praseodymium (Pr 3+ )-doped chalcogenide fiber to enhance the emission of mid-infrared (MIR) wavelength beyond 4 μm. The advantage of the proposed design is to enlarge the core diameter to 32 μm under single mode guidance in order to restrict any impacts of nonlinearity and enhance the applied pumping power before the damage threshold. The effective mode field areas are about 979 μm 2 and 812 μm 2 at 4.5μm emitted MIR wavelength and 2.04μm pump wavelength, respectively. In the considered laser layout within this paper, a single pair of Fiber Bragg Gratings FBGs lay is adopted in the overlapping area of the emitting cross-sections parts of Pr 3+ ions in the transitions ( 3 F 2 , 3 H 6 → 3 H 5 and 3 H 5 → 3 H 4 ). This selected laser layout avoids the difficulties in the fabrication of concatenated FBGs in Pr 3+ -doped chalcogenide glass fibers. In addition, the efficiency of the laser can be also enhanced at this laser layout by emitting two photons in MIR wavelengths from one excited ion. The simulation outcomes indicate the possibility to exceed 61% of slope efficiency at 4.5 μm for 1dB/m loss in the fiber.

Transparent Ho,Yb:YAG crystal waveguide inscribed by direct femtosecond laser writing

We report on the dual-line waveguides in Ho,Yb:YAG crystal inscribed by femtosecond laser writing. The efficient single-mode guidance is demonstrated in the type II waveguide structures at 632.8 nm. The confocal micro-Raman spectroscopy is utilized to characterize both the lattice-damaged tracks and strain-affected regions. Contributions of the refractive index changes from the two main lattice changes are further discussed in combination with beam propagation calculations. The micro-luminescence has been studied, demonstrating that the fluorescence of Ho3+ and Yb3+ ions is well preserved in Ho,Yb:YAG waveguide.

Infrared LiF ridge waveguide fabricated by carbon ion irradiation

We report on the fabrication of an optical ridge waveguide in lithium fluoride crystal using the method of C5+ ion irradiation with precise diamond blade dicing. The guiding properties of the waveguide at the infrared wavelengths of 1064 nm, 2200 nm and 4000 nm are investigated in detail. All the near-field modal profiles exhibit single-mode guidance and the minimum propagation losses of the ridge waveguide after annealing are reduced to 0.7 dB cm−1 (λ = 1064 nm), 0.4 dB cm−1 (λ = 2200 nm) and 0.2 dB cm−1 (λ = 4000 nm), respectively. The Raman spectroscopy of the waveguide shows that the ion irradiation does not obviously change the lattice structure of the crystal.

Microstructure, guiding performance and micro-spectroscopy characterization of Ce:LuAG crystal induced by direct femtosecond laser writing

Dual-line waveguides in Ce:LuAG crystal are fabricated by femtosecond laser using 1030 nm, 450 fs pulses at a 100 kHz repetition rate. The microstructure and composition of femtosecond-laser induced waveguides are studied in detail using SEM, TEM and EDX elemental composition analyses. Efficient single-mode guidance is obtained in the type II waveguide structures with relatively low propagation loss along both TE and TM polarization at 632.8 nm. Based on confocal micro-Raman spectroscopy, the strain-affected regions and the lattice-damaged tracks are demonstrated. The contribution of each effect to the refractive-index distribution are quantitatively determined in combination with beam propagation calculations. The generation of yellow-green fluorescence emission is demonstrated under excitation at 405-nm, retaining well luminescent performance in the Ce:LuAG waveguide.

Tailored vortex lasing based on hybrid waveguide-grating architecture in solid-state crystal

We report a hybrid waveguide-grating vortex laser emitter processed based on femtosecond laser direct writing of the Nd:YVO4 crystal. The cladding waveguide together with the fork grating features both excellent single-mode guidance and vortex diffraction properties. The confocal micro-Raman characterization results reveal the mechanism of the periodic refractive index modulation of the fork grating. Vortex lasers at the milliwatt level are achieved at a wavelength of 1064.7 nm under optical pump laser at 809.3 nm. The numerical simulation of the near-field diffraction propagation demonstrates the generation process of the vortex beams. Our results suggest that the waveguide-grating configuration is of great potential for optical-field steering applications in integrated photonics.

Effectively single high-order mode guidance based on selective mode filtering

Effectively single high-order mode guidance based on selective mode filtering

Femtosecond laser inscribed Yb:LuAG waveguides: guiding performance, micro-spectroscopy characterizations, and refractive index reconstruction

We report on fabrication of dual-line waveguides in Yb:LuAG crystal by 1030-nm femtosecond laser inscription, 450 fs pulses at a 100 kHz repetition rate. The efficient single-mode guidance is demonstrated in the type II waveguide structures when propagating light along both TE and TM polarization at 632.8 nm and 1064 nm. Based on the configuration, the minimum propagation loss of the 18-µm width waveguide is determined to be 0.55 dB/cm at wavelength of 632.8 nm. Both the lattice-damaged tracks and strain-affected regions are characterized by means of the confocal micro-Raman spectroscopy. In combination with beam propagation calculations, we quantitatively determine the corresponding contribution of each effect to the refractive-index field of the fabricated waveguides.

Optical waveguides in Pr:LuAG crystal based on direct femtosecond laser writing

We report on the dual-line waveguides in Pr:LuAG crystal fabricated by femtosecond laser using 1030 nm, 450 fs pulses at a 100 kHz repetition rate. The efficient single-mode guidance is demonstrated in the type II waveguide structures when propagating light along both TE and TM polarization at 632.8 nm. The minimum propagation loss of 4.07 dB/cm is obtained in the as-fabricated waveguide in width of 18 μm. Both the lattice-damaged tracks and strain-affected regions are characterized by means of the confocal micro-Raman spectroscopy, and the refractive index modification is also analyzed. The micro-luminescence result indicates that the Pr3+ fluorescence is well preserved in the Pr:LuAG waveguide.

Photonic bandgap terahertz fibers based on honeycombed tubes

Terahertz fibers are highly applicable for short-haul stable terahertz transmissions, and thus have potential use in upgrading terahertz systems. In this paper, a photonic crystal structure consisting of honeycombed tubes is proposed. Numerical studies based on the plane wave expansion method demonstrate that, in comparison to the photonic crystal consisting of honeycombed airholes or triangulated tubes, the one proposed in this paper can deliver a broader and less dispersive bandgap, which has been further confirmed by another calculation based on the finite element method. Then a fiber structure is designed to perform the single mode guidance for a broad spectrum of terahertz waves. The second derivative of mode effective area is introduced to define the effective guiding spectrum for the bandgap terahertz fiber. Mode calculations are performed by use of the finite element method, in which the imaginary part of material refractive index is directly took into account. Obtained results show that the designed fiber suffers low transmission losses over a broad spectrum centered at 1 terahertz. Further, an optimized design is worked out to exhibit the lowest loss for the frequency of 1 terahertz, coming up to 0.3 dB/cm. Not only that, this design can also provide nearly zero flat dispersions with |β2| 1 ps2/cm over 0.81-1.12 terahertz, as well as negligible bending induced losses (&lt; 10−11 dB/cm) for bending radii larger than 1 cm , indicative of a good transmission characteristic for terahertz pulses. Finally, we discuss possible fabrication methods on the proposed fiber.

Wideband, large mode field and single vector mode transmission in a 37-cell hollow-core photonic bandgap fiber

Stable generation and propagation of ultrafast high-order mode beams has become an important research direction. A core diameter not more than 10 times the wavelength is regarded as the upper limit for single mode transmission. However, a high-power laser requires a core diameter 20 to 40 times the wavelength to achieve high-power and stable output, which exceeds the design limit of the traditional fiber. In this paper, a novel 37-cell hollow core photonic bandgap fiber (HC-PBF) that only supports pure TE01 mode over a bandwidth of 50 nm with the lowest loss of 0.127 dB/km is proposed. The HC-PBF has a core diameter of more than 40 μm. Single mode guidance is achieved by adjusting the lattice size in a particular of the cladding. The best single mode performance with a loss ratio as high as 150,000 between TE01 mode and other modes with minimum loss is obtained. The fiber also has low bend-loss and thus can be coiled to a small bend radius of 1 cm having 1.6 dB/km bend loss. The tunability of the single-mode window and the manufacturing feasibility of the proposed fiber are also discussed.

Nested compound negative curvature hollow-core fiber for single-mode operation in the infrared region

In this study, a novel tubular hollow-core fiber design with extended cladding structures aiming low transmission losses and dominant single-mode guidance in the infrared region is proposed. The fiber parameters are optimized to achieve low-loss operation at the target wavelength of 2μm, at which the emission spectrum of thulium-doped fiber lasers are centered. The confinement loss of the fundamental mode is found to be less than 0.02 dB/km in the near- and mid-infrared region, which makes the proposed design a strong alternative for conventional step-index silica optical fibers. Strong suppression of the higher order modes, and low bending sensitivity are also observed in the proposed design. The proposed fiber shows promising results to achieve low-loss transmission in the infrared spectrum.

Birefringent Anti-Resonant Hollow-Core Fiber

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 low attenuation and single-mode guidance. Here we simulated, fabricated and characterized a single-mode polarization-maintaining anti-resonant hollow-core fiber. The birefringence was achieved by introducing resonators of different thicknesses, thereby creating reduced symmetry in the structure. The measured group birefringence of 4.4x10-5 at 1550 nm is in good agreement with the calculated group birefringence from the simulations. This corresponds to a phase birefringence of 2.5x10-5 at 1550 nm. The measured loss of the fiber was 0.46 dB/m at 1550 nm. With its simple structure, low loss, and broadband operation this polarization-maintaining anti-resonant hollow-core fiber is a serious contender for applications in gas-based nonlinear optics and communications.