Laser-induced periodic surface structures (LIPSS) formation was observed inside a microhole produced in an optical fiber, when the femtosecond laser pulse number and the laser energy were optimized to be 150 and 30μJ, respectively. We have also found that LIPSS were formed only at the bottom of the microhole, which may be responsible for the strong evanescent field produced by the femtosecond laser propagating inside the microhole.
We have evaluated inner surface roughness of inline/picoliter fiber optic spectrometer fabricated by an NUV femtosecond laser drilling. A microhole fabricated by the femtosecond laser without breaking off works as inline/picoliter fiber optic spectrometer. The attractive feature of the spectrometer is very small sensing volume which has several tens of picoliter. A second harmonic 400 nm femtosecond laser with 350 fs pulse duration launched onto the glass fiber optic. A high aspect ratio of the microhole was fabricated after 1000 pulse shots, but there was inner surface roughness. Although the repetition rate was changed 10 to 1000 Hz in order to control the inner surface roughness, the inner surface roughness was occurred in each case. It was confirmed that ablated fused silica particles deposited on the inner surface of microhole. The depth of microhole was deepened with 1 kHz of repetition rate and number of 1000 shots. In comparison to 10 Hz, the depth of microhole was increased by approximately 80%. It was assumed that heat accumulation effect enlarged the length of drilling. In order to minimize inner surface roughness, the best method is to use low number laser shots. After 100 pulse shots with 30 μJ of pulse energy, an optical inner surface quality of microhole was acquired. The optical inner surface quality of microhole was verified by measuring the transmittance of 94% of infrared light emission launched from superluminescent diode in the case of 100 pulse shots with 20 μJ. The transmittance decreased to 52% changing the microhole fabricated by 30 μJ with 100 laser shots because of increasing interaction area between the microhole and propagating light.
Inner walls of microhole in a thin fused silica plate were observed after changing ablating laser pulse shots of a focused femtosecond laser at the wavelength of 400 nm with an energy of 20 μJ in a pulse width of 350 fs. Using an objective lens with an NA of 0.28, it was revealed that the inner surface of the microhole was melted with 10 laser pulse shots. By increasing the pulse numbers to 100, however, deposition of fused silica particles on the melted inner surface was observed. In order to minimize the inner surface roughness, the objective lens was changed. After 50 laser pulse shots, the inner surface structure was brought close to optical quality using an objective lens with NA of 0.65.
Micro-processing by using an ultrashort pulsed laser has been previously reported and developed in the past decade for fabricating micro devices. Using tightly focused short pulse laser beam, the laser intensity easily can reach more than 1013 W/cm2. Under such conditions non-linear phenomena are triggered, hence multiphoton ionization and self-focusing are notably induced in a medium. Femtosecond laser enables micro-fabrication without critical heat damage owing to extremely shorter pulse width and very fast multi-photon absorption at the laser focal point, compared with longer pulse irradiation. In this report, a micro-voids array was created in optical fiber line by using a femtosecond laser to produce sensing area only at local micro-region of the fiber line. At sensing portion consisted of the voids array, transmitted light was partially scattered by voids after that the leaked light could be reflected on the interface of cladding and outsides, which held the incident angle depending on structures of the micro-voids array. Voids array played as a role of scattering sources to transmitted light and consequently it was expected that the transmitted light can be broadly leaked out from the fiber core to the cladding. Furthermore, optical losses attributed to the creation of micro voids were quantitatively obtained so as to figure out the sensor characteristics. Consequently the reflection region which was considered as a sensing area showed the re-coupling rate of 0.04 dB (3.03%) to insertion loss of 1.32 dB, and the incident angle existed between 67.2 -71.9°.
In this study, we have proposed a novel type of localized surface plasmon resonance (LSPR) fiber optic sensor based on in-line/pico-liter micro-holes which can be experimentally fabricated into the fiber waveguide by using a second harmonic 400 nm femtosecond laser. A repetitive pulse train of 1 kHz with a pulse width of 350 fs was irradiated onto a MMGI fiber optic to make three holes that penetrate through the fiber core and work as spectroscopic-microfluidic flow cells. In order to induce the interaction between transmitted light and gold nanoparticles (GNPs) adhered on the inner surface of the flow cells, micro-holes were designed to be the width of approximately 50 μm, along a direction perpendicular to an optical axis of an optical fiber. GNPs with approximately 100 nm of particle diameter adhered onto the inner surface according to 3-aminopropyltriethoxy silane treatment. The transmitted light through the micro-holes was obtained by optical instruments consisted of a white light source and an optical spectrum analyzer. In order to obtain the reference spectrum, the optical spectrum was acquired before dipping the sensor into the GNPs solution. After 30 min of immersing the sensor portion into the GNPs solution, the optical spectrum was also obtained. The reference spectrum which was considered as the baseline, was set to zero and then, the absorbance spectrum was calculated. The absorbance peak at a wavelength of 537 nm occurred in an air condition in the sensing area, which seemed like the resonance peak based on the LSPR.
In this paper, multipoint refractive index measurement is described using surface plasmon resonance (SPR) sensors
based on hetero-core structured fiber optic technique. The sensor simply consists of two different core diameters fibers,
which are connected by thermal fusion splicing, in order to deliberately leak the transmitted light wave into the cladding
layer of the sensing fiber region. Chromium film and gold film were uniformly deposited around the cladding surface
with a layer thickness of 5 nm and 45 nm, respectively, for SPR excitation. Multipoint measurement system consists of
three SPR sensors, whose hetero-core insertion length are employed as 2-mm, located in a single transmission optical
fiber and an optical time domain reflectometer (OTDR) to be used to measure refractive index. As a result, a hetero-core
insertion length has trade off between sensitivity for refractive index and the number of sensors in tandem. The proposed
multipoint refractive index measurement has been successfully demonstrated using three hetero-core SPR sensors and
OTDR.
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