In terahertz (THz) time-domain spectroscopy, the excitation of THz waves by fiber optic transmission of femtosecond pulsed lasers can provide great convenience for nondestructive testing of large and medium-sized aircraft. However, due to the high peak power characteristics of the femtosecond laser, the pulse spreading and distortion caused by fiber dispersion and nonlinearity can seriously affect the output laser pulse quality and the excitation of THz waves. In this paper, a three-segment femtosecond laser transmission scheme using asymmetric dual-core fiber (ADCF)-single-mode fiber (SMF)-dispersion-compensated fiber (DCF) is proposed. The simulation study results show that the strong dispersion using a 2.8-cm-long ADCF can reduce the 1 kW peak power to 8.03 W and extend the 50 fs pulse width to 3.74 ps, respectively. The spreading pulse can be restored to the peak power of 987.20 W and pulse width of 51.80 fs after the 10m-long SMF and then transmitted by the 66.9 cm-long DCF. This shows that the proposed femtosecond laser transmission scheme can achieve good pulse recompression and simultaneous recovery of pulse waveform and power, which lays the foundation for the construction of further THz time-domain spectroscopy systems using longer fibers for femtosecond laser transmission.
A subwavelength core microstructured optical fiber (MOF) temperature sensor based on infiltration with chloroform is
proposed. Fiber core is surrounded by three large holes which can facilitate the infiltration of the chloroform. The
refractive index of chloroform is sensitive to the temperature of the environment. Mode loss of the guided mode caused
by index change of chloroform is obtained by solving Maxwell’s equation with finite element method (FEM), and is
used as a criterion for sensitivity estimation. The relationship between the sensitivity of temperature sensor and the core
diameter of MOF is investigated. Our simulation results show that the subwavelength core MOF is very promising for
developing a linear response to temperature sensor of significantly reduced core size.
An analysis of highly birefringence low losss index-guiding phptonic crystal fiber composed of different
sized circular air-holes in cladding and adjacent to the two ellipital air-holes x-diraction in the first inner rings is
carried out in this work using voul fector finite element method.The birefringence properities in terms of various
parameters,e.g,pitch length,air-hole size and ellipticity ratio are calculated.Form the numerical results confirm that
the size control of these air holes and pitch length is the key to reaching high model birefringnce.The proposed
struture at wavelength 1.55μm shows a birefringence of up to 3.02x10-2 and at the same time can get the
confinement loss of less than 1.9x10-7dB/m, which is a reference significance in useful to dessign temperature
sensors.
KEYWORDS: Gas sensors, Photonic crystal fibers, Finite element methods, Signal attenuation, Sensors, Absorption, Refractive index, Optoelectronics, Information technology, Optical fibers
A hollow-core photonic crystal fiber (HC-PCF)-based gas sensor is proposed and numerically investigated through the finite element method. To obtain higher relative sensitivity coefficient γ and the better sensing properties of the gas sensor, the floriated structure of the central hole of HC-PCF is optimized. Numerical investigation shows that γ is high and relatively stable to changes of the diameter of air holes in a wide range of wavelength of 1.5-1.7μm, which is very beneficial for sensor fabrication. A lower confinement loss and the flatness are also exhibited with the HC-PCF. And the highest γ of 0.9998 of the floriated structure of the central hole can be obtained than the circular ones of 0.9435 at the wavelength of 1.56μm.
A study of polymer photonic crystal fiber (PCF) sensor coated with smart composite materials for measurements of refractive index of analyte is presented in this paper. The proposed sensor combines the characteristics of polymer PCF and composite material, which can make the confinement loss lower than the silica PCF duo to the double interaction of the polymer and silver film. The results obtained in this study demonstrate that polymer PCF sensors coated with composite material and with the polymer PCF’s advantages, the sensor’s feasibility can be improved further and it can be applied in a broad field, especially in biosensing platforms.
We propose an intra-cavity index sensor based on double cladding ytterbium-doped photonic crystal fiber filled with liquid analyte in the air holes of optical fiber cladding. In such sensor, when pumped by 976 nm light, the output power of the ytterbium-doped photonic crystal fiber laser can be influenced by the refraction index of liquid analyte to achieve the intra-cavity fiber sensing. The numerical analysis shows that the power change is larger in a small range of loss when the end reflectivity is higher, and the sensor’s sensitivity will be higher for the change of refractive index of analyte.
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