Efficient and cost effective measurement of the refractive index profile in an optical fiber is a significant technical job to design and manufacture in-fiber photonic devices and communication systems. For instance, to design fiber gratings, it is required to estimate the refractive index modulation to be inscribed by the fabrication apparatus such as ultraviolet or infrared lasers. Mach-Zehnder interferometer (MZI) based quantification of refractive index change written in single mode microfiber by femtosecond laser radiation is presented in this study. The MZI is constructed by splicing a microfiber (core diameter: 3.75 μm, cladding diameter: 40 μm) between standard single mode fibers. To measure the RI inscribed by infrared femtosecond laser, 200 μm length of the core within the MZI was scanned with laser radiation. As the higher index was written within 200 μm length of the core, the transmission spectrum of the interferometer displayed a corresponding red shift. The observed spectral shift was used to calculate the amount of refractive index change inscribed by the femtosecond irradiation. For the MZI length of 3.25 mm, and spectral shift of 0.8 nm, the calculated refractive index was found to be 0.00022. The reported results display excellent agreement between theory and experimental findings. Demonstrated method provides simple yet very effective on-site measurement of index change in optical fibers. Since the MZI can be constructed in diverse fiber types, this technique offers flexibility to quantify index change in various optical fibers.
Fabrication of fiber Bragg grating (FBG) in single mode microfiber (core diameter: 3.75 μm, cladding diameter: 40 μm) by femtosecond laser pulse radiation is presented. Femtosecond pulse filamentation technique is employed in the pointby-point writing method to inscribe single shot periodic index modification in the core of the microfiber. Prior to writing gratings, a short length (~1.5 mm) of microfiber is fusion spliced between two standard single mode fibers (SMF) in order to improve handling and ease grating fabrication. The kilohertz femtosecond laser pulses operating at center wavelength of 800 nm were tightly focused with an objective lens (40X/ NA=0.75) to confine the pulses into a very tiny focal volume and spatially control index modification. The focused femtosecond pulses create filamentary voids at focal point. For the scanning speed of 534 nm/ Sec, the partial overlapping of void structures produces a periodic index modification in the core with a period of 534 nm and constructs the Bragg reflection spectrum centered at 1550.216 nm. Fabrication of a 1 mm long FBG takes less than 2 seconds for the scanning speed of 0.534 mm/sec. The spectral position of Bragg reflection spectrum can easily be tailored simply by changing pulse scanning speed. The performance analysis of the FBG is examined for temperature and axial strain sensitivity. The grating sensor exhibits the temperature and strain sensitivity of 10 pm/ °C and ~1 pm/ micro-strain, respectively.
In this paper, we characterize the femtosecond laser filament-fringes in titanium. In order to fabricate regular arrays of filaments, we place either a pinhole or a beam shaper in the optical path of the femtosecond laser beam that originates linear diffraction of the laser beam. Soda-lime glass is used as Kerr medium to produce the filaments. As a consequence, the intensity distribution of the laser beam is modulated and fringe type of filament distributions is evident. The suitable control over the size of the diaphragms (pinhole or beam shaper) leads us to adjust the shape, orientation, and number of filaments in each irradiated spots in titanium sample. By properly adjusting the diameter of a pinhole that was placed in the optical path, we are successful in forming a single filament in titanium. By using these single filaments, we fabricated high aspect ratio periodic holes in the titanium surface by moving the translation stage in both horizontal and vertical directions. The period of the holes in the horizontal direction is controlled by varying the scanning speed, whereas the period in the vertical direction is controlled by varying the vertical scanning step. We strongly believe that, filamentation technology described in this paper will have applications in forming a variety of micro/nano-structures in various materials.
This work reports femtosecond laser based fabrication of long period fiber gratings (LPFG). Index modulation in the
core of single mode fiber (SMF) is written employing femtosecond pulse filamentation technique. Highly repeatable
filamentary voids written in line-by-line femtosecond laser inscription technique enables steady and noise free growth of LPFGs. The sharp transmission valley (with a narrow full width at half maximum of 5 nm) of long period grating offers better resolution for refractive index (RI) measurement of a solution. The LPFGs inscribed by femtosecond laser
radiation show RI sensing sensitivity of 29.199 nm/RIU which is three times higher than the sensitivity of LPFGs written
by UV radiation (sensitivity: 11.179 nm/RIU). The position of transmission dip of a grating can be tailored relatively
easily simply by varying the period of index modulation.
A sensor based on a Long Period Grating (LPG) for measurement of the refractive index (RI) of liquids with RI close to
that of water is reported. An LPG was written by a femtosecond laser (F-LPG) operating at 800nm, utilizing a point-bypoint
technique. The performance of the F-LPG is compared with an LPG written by ultraviolet irradiation (UV-LPG).
For low RIs, the sensitivities of the F-LPG and UV-LPG are 130 nm/RIU and 3.1 nm/RIU, respectively. For higher RI,
the F-LPG exhibited sensitivity as high as 249 nm/RIU. For the low range of RI, the resolution of F-LPG was 3.8 x 10<sup>-5</sup>.