Marta Nespereira

3D Finite Element Model for Writing Long-Period Fiber Gratings by CO2 Laser Radiation

In the last years, mid-infrared radiation emitted by CO2 lasers has become increasing popular as a tool in the development of long-period fiber gratings. However, although the development and characterization of the resulting sensing devices have progressed quickly, further research is still necessary to consolidate functional models, especially regarding the interaction between laser radiation and the fibers material. In this paper, a 3D finite element model is presented to simulate the interaction between laser radiation and an optical fiber and to determine the resulting refractive index change. Dependence with temperature of the main parameters of the optical fiber materials (with special focus on the absorption of incident laser radiation) is considered, as well as convection and radiation losses. Thermal and residual stress analyses are made for a standard single mode fiber, and experimental results are presented.

LOLS Research in Technology for the Development and Application of New Fiber-Based Sensors

This paper presents the research made at the Laboratory of Optics, Lasers and Systems (LOLS) of the Faculty of Sciences of University of Lisbon, Portugal, in the field of fiber-based sensors. Three areas are considered: sensor encapsulation for natural aqueous environments, refractive index modulation and laser micropatterning. We present the main conclusions on the issues and parameters to take in consideration for the encapsulation process and results of its design and application. Mid-infrared laser radiation was applied to produce long period fiber gratings and nanosecond pulses of near-infrared Q-switch laser were used for micropatterning.

Automation methodology for the development of LPFG using CO2 laser radiation

The mid-infrared radiation produced by CO2 lasers is being widely used to produce long period fiber gratings (LPFG) with several advantages over other methods. Several techniques can be used to irradiate the fiber in order to produce the necessary effect. Using a cylindrical lens to create a line of light or scanning of a spot over the fiber are the most common approaches. Usually, the period is produced either by a translation stage moving perpendicular to the incident beam or by using a two mirrors scanner that inscribes the entire period directly on the fiber. In both cases, the synchronization between the laser and the moving elements is critical. Also, when using a two mirrors scanner, the dimension of the LPFG is limited by the focusing lens diameter and its focal length. All this become critical when one needs to increase the LPFG’s length or reduce its period. The later usually implies shorter laser emission times, which is limited by the laser emission physics (at least for cheap low power CW lasers). In order to overcome the disadvantages of each method, a combined approach is presented and analyzed. A mirror scans vertically the beam over a cylindrical lens and a translation stage moves the fiber to create the different periods. The laser keeps emitting during the complete process increasing laser power stability and thus, improving grating homogeneity. To guarantee the synchronization between the translation table and the one mirror scanner, special hardware and software was created.

Nanosecond laser micropatterning of optical fibers

Towards the development of new optical fiber sensors it was studied the application of nanosecond infrared laser radiation in the micropatterning of optical fibers. Nd:YAG laser pulses were focused on silica fibers by an apparatus projected to position the fiber regarding the laser beam and analyze the interaction. Experiments allowed determining the conditions to vaporize the required amount of material. Holes with few microns and depths higher than 10 microns were accomplished with multiple shots and advancing the fiber regarding the beams focus. The results analysis demonstrated the possibility of obtaining patterns and the technique potential in the development of fiber sensors.

Writing of Long Period Fiber Gratings Using CO2 Laser Radiation

The development of optical fiber gratings (OFGs) had made significant advances both in terms of research and development of optical communications and sensors. OFGs are intrinsic devices that allow modulate the properties of light propagation within the fiber. Grating structures are comparatively simple and in its most basic form, consist on a periodic modula‐ tion of the properties of an optical fiber (usually the refraction index of the core). Its application as a sensing element is advantageous because of the intrinsic characteristics of the fiber sensors, such as remote sensing, electromagnetic immunity, weight and compactness, and capability for real time sensing and low cost [1].

Advances in Optical Fiber Laser Micromachining for Sensors Development

Both lasers and optical fibers technology appeared in the 1960s, being, from the start, close related. Even though the latter gained increased visibility in telecommunications, first ex‐ periments using optical fiber sensors are reported from early 1970s. From then on, research in optical fiber sensors has increased taking advantage of their potential when comparing with “traditional” sensors. Although there are many well established techniques to manu‐ facture optical fiber sensors, the use of laser technology as increased as their cost diminishes (at least for older, well matured laser sources technology) and new laser sources appeared. This new tool has the advantage of producing well controlled light beams.

Optical fiber tapers produced by near-infrared laser radiation

The use of CO2 lasers in the production of optical fiber sensors is a well proven application, especially in writing long period fiber gratings. The mid-infrared radiation (MIR) emitted by these lasers is highly absorbed by the glass optical fibers commonly used as fiber-based sensors, and the consequent heating easily leads to melting. Making use of this advantage, the application of this radiation in the tapering of optical fibers is demonstrated. A continuous wave emission CO2 laser is used in short duration pulses, a cylindrical lens focuses the beam on the fiber and a small weight produces the effect of pulling the fiber. The resulting tapers’ characteristics (length, degree of symmetry and central diameter, or waist) are analyzed as well as the influence of the operational parameters involved in the process (laser power, pulse duration and number of pulses). The potential of this technique is demonstrated with special attention to its use in the development of optical fiber-based sensors.

Ultrashort Long-Period Fiber Grating Sensors Inscribed on a Single Mode Fiber Using Laser Radiation

Sensing performances of ultrashort (as low as 2.4 mm) long-period fiber gratings fabricated with CO2 laser radiation using commercial single mode fibers are presented. These lengths are, to our knowledge, the shortest of those found in the literature for this kind of sensors, approaching those typical in fiber Bragg gratings. Sensitivity to temperature and refractive index are demonstrated, with performances within the range expected for a single LPFG written on a single mode fiber without any enhancing technique. Analysis on results is made based on both theoretical and experimental data.

Repeatability analysis on LPFGs written by a CO2 laser

The physical mechanisms involved in the writing process of long period fiber gratings (LPFG) using mid-infrared radiation emitted by CO2 lasers limit the obtained characteristics, in particular the minimum period that can be achieved. In order to evaluate the performances of a new methodology developed by us, we analyzed its capability to produce gratings with different periods (from 600 μm down to 300 μm). We also present a repeatability study on the obtained LPFG characteristics (mainly the resonant wavelength and grating length) for several values of the repetition period.

Short-length long period fiber grating for torsion sensing applications

The response of short-length CO2 -induced Long Period Fiber Gratings (LPFGs) sensors to torsion is reported. While engraving, the fiber is submitted to high tension allowing to obtain gratings with shorter lengths, one order of magnitude lower than the usual. Also, the fiber was only irradiated in one side, creating an asymmetrical profile leading to highly birefringent gratings. Good sensitivity to axial twists is demonstrated, with values up to 0.12 nm/(rad/m) for the resonant wavelength shift, and better than 0.03 dBm/(rad/m) for the variation in the intensity (attenuation). Discrimination between rotation direction, clockwise and counterclockwise, can be obtained.