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Femtosecond laser direct written off-axis fiber Bragg gratings for sensing applications
摘要： First order off-axis fiber Bragg gratings (FBGs) were fabricated in a standard single mode fiber (SMF-28e) through femtosecond laser direct writing. A minimum offset distance between the grating and core center of 2.5 μm was found to create a multimode section, which supports two separate fiber modes (LP0,1 and LP1,1), each split into two degenerate polarization modes. The resulting structure breaks the cylindrical symmetry of the fiber, introducing birefringence (≈10?4) resulting in a polarization dependent Bragg wavelength for each mode. Based on the modal and birefringence behavior, three off-axis FBGs were fabricated with 3.0, 4.5 and 6.0 μm offsets from the core center, and then characterized in strain, temperature, and curvature. The tested off-axis FBGs exhibited a similar strain sensitivity of ~1.14 pm/μ? and a temperature sensitivity of ~12 pm/C. The curvature and orientation angle were simultaneously monitored by analyzing the intensity fluctuation and the wavelength shift of the LP1,1 Bragg resonance. A maximum curvature sensitivity of 0.53 dB/m?1 was obtained for the off-axis FBG with a 3.0 μm offset.
关键词： optical fiber sensors,femtosecond laser direct writing,single mode fiber,off-axis fiber Bragg gratings,Birefringence
更新于2025-09-23 15:19:57
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Mass Producible Low-Loss Broadband Optical Waveguides in Eagle2000 by Femtosecond Laser Writing
摘要： Optical waveguides were fabricated in alkaline earth boro-aluminosilicate glass, by femtosecond laser direct writing, with varying pulse energy and scan velocity. A spectral characterization, from 500 nm to 1700 nm, was made in order to determine their losses and understand its dependence on the processing parameters. Three major loss mechanisms were identified. At longer wavelengths, loss is mainly due to weak coupling. On the other hand, the behavior at shorter wavelengths is governed by propagation loss due to Rayleigh scattering, which was shown to be practically eliminated (<0.05 dB· cm?1· μm4) at higher scan velocities. Bulk absorption was also found to have an influence in the propagation losses at higher wavelengths. The combination of intermediate pulse energies (between 125-250 nJ) and high scan velocities (above 6 cm/s) allowed the fabrication of optical waveguides offering low losses across the entire range of wavelengths tested, facilitating applications that require larger wavelength working bands. Furthermore, since optimal fabrication conditions are achieved at higher scanning velocities, mass production with reduced fabrication times can be achieved.
关键词： femtosecond laser direct writing,Rayleigh scattering,propagation loss,Coupling loss,Mie scattering,integrated optics,low-loss broadband optical waveguides,mass production
更新于2025-09-12 10:27:22
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[IEEE 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Munich, Germany (2019.6.23-2019.6.27)] 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Broadband Mid-Infrared Directional and Multimode Interference Couplers in GLS Glass Fabricated using Femtosecond Laser Direct-Writing
摘要： The first broadband directional and multimode interference couplers for the mid-infrared centred around 4 μm wavelength are demonstrated. The devices are inscribed into commercial chalcogenide glass (Gallium-Lanthanum-Sulfide, GLS) provided by the University of Southampton using a tightly focused 800 nm, 5.1 MHz repetition rate femtosecond laser. Low loss waveguides are formed by placing 3 individual laser written tracks next to each other, resulting in propagation losses as low as 0.22 dB/cm. These waveguides are used to create asymmetric directional couplers, where the propagation constant in one arm is tailored to flatten the chromatic response of the device. By locally changing the feedrate at which the sample is translated through the focus, a change in propagation constant is obtained in the coupling region of the device. This yields a 3 dB coupler with a wavelength flattened response over a 500 nm wavelength band for ±5% variation in the coupling ratio compared to < 200 nm for a symmetric directional coupler. Multimode interference couplers were created by placing 26 laser inscribed tracks next to each other to form a 140 μm wide multimode interference region with lengths ranging from 5 to 10 mm. Asymmetry inside the multimode interference regions induced by the sequential writing process results in port dependent cross coupling ratios. Yet, close to 3 dB coupling was found for an 8.1 mm long device when injecting into the left input. The device features nearly perfect achromaticity across 3.75 to 4.25 μm.
关键词： GLS glass,mid-infrared,multimode interference couplers,femtosecond laser direct-writing,directional couplers
更新于2025-09-12 10:27:22
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Maskless Micro/Nanopatterning and Bipolar Electrical-Rectification of MoS2 Flakes Through Femtosecond Laser Direct Writing
摘要： MoS2 micro/nanostructures are desirable for tuning electronic properties, developing required functionality, and improving existing performance of multilayer MoS2 devices. This work presents a useful method to flexibly microprocess multilayer MoS2 flakes through femtosecond laser pulse direct writing, which can directly fabricate regular MoS2 nanoribbon arrays with ribbon widths of 179, 152, 116, 98, and 77 nm, and arbitrarily pattern MoS2 flakes to form micro/nanostructures such as single nanoribbon, labyrinth array, and cross structure. This method is mask-free and simple, and has high flexibility, strong controllability, and high precision. Moreover, numerous oxygen molecules are chemically and physically adsorbed on laser-processed MoS2, attributed to roughness defect-sites and edges of micro/nanostructures that contain numerous unsaturated edge-sites and highly active centres. In addition, electrical tests of the field effect transistor fabricated from prepared MoS2 nanoribbon arrays reveal new interesting features: output and transfer characteristics exhibit strong rectification (not going through zero and bipolar conduction) of drain?source current, which is supposedly attributed to the parallel structures with many edge-defects and p-type chemical doping of oxygen molecules on MoS2 nanoribbon arrays. This work demonstrates the ability of femtosecond laser pulses to directly induce micro/nanostructures, property changes, and new device-properties of two-dimension materials, which may future enable new applications at electronic devices based on MoS2 such as logic circuits, complementary circuits, chemical sensors, and p?n diodes.
关键词： micro/nanopatterning,MoS2 flakes,oxygen bonding,femtosecond laser direct writing,electrical rectification
更新于2025-09-12 10:27:22
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[IEEE 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Munich, Germany (2019.6.23-2019.6.27)] 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - An Increase in Refractive Index Contrast in a Waveguide Inscribed in Tellurite Glass by Means of Sub-Nanosecond Bursts of Femtosecond Pulses
摘要： Femtosecond laser direct writing (FLDW) of waveguides in dielectric glasses and crystals is well-recognized technique for manufacturing of compact laser sources, optical sensors, labs-on-a-chip and optical chips for quantum computing [1]. For this technology the main obstacle to compaction of optical circuits and an increase in the number of elements on an optical chip is constraint of the refractive index contrast Δn between the core and the cladding of a waveguide that is given by the nature of FLDW. Insufficient RIC also restricts application of the depressed cladding waveguides in mid-IR due to mode leakage [2]. The largest ever-obtained refractive index change is associated with ion migration in a phosphate glass under the thermal regime of laser writing, and it is as high as +0.03 [3]. Recently we found that simultaneous actions of rarefaction and electronic excitation lead to enhanced negative refractive index change in the laser modified spots in silica glass and sapphire [4], wherein the required conditions were produced by a sub-nanosecond burst of femtosecond laser pulses. Here we report on FLDW of a low loss waveguide with enhanced refractive index contrast in 70TeO2-22WO3-8Bi2O3 glass inscribed by bursts of pulses with exponentially decaying amplitudes in each burst and pulse separation of 10 ps. The depressed cladding waveguide composing of 14 parallel tracks with reduced refractive index was inscribed at wavelength of 1030 nm (Fig.1.(a)). The maximum refractive index change Δn produced by train of ordinary pulses in the investigated glass was as high as -0.002 [2]. Mapping of refractive index change in tracks inscribed by the bursts with different burst energies and pulse separation intervals is shown in Fig.1(c) The range of the parameters was found for which the burst produced smooth tracks of negative refractive index change, and its amplitude is increased by factor of 3 in comparison with tracks inscribed by ordinary pulses. The maximum index change Δn was obtained with pulse separation interval of 10 ps. Beside enhanced refractive index change the burst train inscribed tracks with reduced cross section. That is, the track height is less at least by a factor of 3 in comparison with one inscribed by ordinary pulses under the same energy for a burst and an ordinary pulse (Fig1(a),(b)). We consider that the enhanced index change and the reduced track height are inevitably accompanied by strong localization of energy deposition through a reduction in peak pulse intensity that allowed avoiding destructive Kerr self-focusing. We consider that the new technique of FLDW paves the way for better confinement of radiation in a depressed cladding waveguide writing waveguides with well deterministic architectures. The increase in the waveguide non-linearity and extension of operation range to mid-IR is inevitably expected.
关键词： sub-nanosecond bursts,tellurite glass,depressed cladding waveguide,refractive index contrast,Femtosecond laser direct writing
更新于2025-09-12 10:27:22
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[IEEE 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Munich, Germany (2019.6.23-2019.6.27)] 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Compact Femtosecond Laser Direct Written Integrated Retarders Based on Embedded Nanogratings
摘要： Femtosecond Laser Direct Writing (FLDW) is a well-known rapid prototyping method to fabricate integrated optical circuits in glass chips. These circuits have been used to show various quantum information applications, using the states of photons as qubits. Generally, when transmitting information via single photons, it is desirable to make use of all possible degrees of freedom that this photon has to offer, in order to increase the amount of information transferred per photon. One of these degrees of freedom is the photon’s polarization. To make use of this degree of freedom, devices capable of manipulating the polarization are required. Various approaches for manipulating the polarization of photons in a FLDW circuit have been demonstrated before, of which some were used for quantum information applications. In our work, we present a novel method of polarization control using embedded nanogratings as waveplates. These nanogratings are highly birefringent self-assembled structures. Due to their relatively high form birefringence on the order of Δn = 10^-3, they can be used as a compact waveplate enabling further miniaturization of integrated optical circuits. The properties of these gratings can be altered by using different laser parameters during the inscription process. We have added various waveplates made of nanogratings into FLDW waveguides. We demonstrate their functionality as waveplates of different retardation and optical axis orientations using crossed polarizer birefringence measurements. Due to the chosen writing geometry, a full control over the direction of the optical axis can be achieved. The thickness of these structures is on the order of a few hundred micrometers. Former approaches for polarization control in FLDW circuits required structures in the range of millimetres to centimeters. Some of these approaches were limited in the achievable optical axis orientations. Our waveplates can be both used for classical applications and as single qubit quantum gates, which will be demonstrated. Waveplate structures usable as Hadamard, Pauli-x, Pauli-z and Pi/8th gates have been fabricated. The transferability to fibers will be discussed.
关键词： integrated optical circuits,polarization control,quantum information applications,waveplates,nanogratings,Femtosecond Laser Direct Writing
更新于2025-09-11 14:15:04