Gratings In Photonic Crystal Fiber Research Articles

Detection of external refractive index change with high sensitivity using long-period gratings in photonic crystal fiber

Abstract We report that the proposed long-period gratings (LPGs), which were fabricated in an endlessly single-mode photonic crystal fiber (ESM-PCF) via point-by-point residual stress relaxation utilizing focused CO 2 laser irradiation, exhibit a high sensitivity to variations in the external refractive index, with an identified shifting of attenuation band as large as 4.4 pm for 1 × 10 −5 change in the surrounding refractive index over the range from 1.42 to 1.45. Contrary to LPGs inscribed in conventional all-solid optical fiber under similar condition, the mode-coupling resonance band of the ESM-PCF-LPG moves towards longer wavelength with increasing refractive index in the surrounding medium of the fiber cladding. The resonance wavelength has a low-temperature coefficient of 2.2 pm/°C over 30–200 °C.

Numerical analysis of refractive index sensitivity of long-period gratings in photonic crystal fiber

Abstract The modal coupling properties and resonance spectral response of long-period gratings (LPGs) in index-guiding photonic crystal fiber (PCF) are numerically investigated with respect to its sensitivity to changes in the index of refraction of the measurand in the PCF air channels using a full-vectorial mode solver combined with frequency-domain method. We show that the shift of wavelength resonance of a PCF-LPG structure can be extremely sensitive by appropriately designing the cladding air-channel dimension and geometry as well as the periodicity of the LPG inscribed in an endlessly single mode PCF (ESM-PCF-LPG). Specifically, a sensitivity of 4.3 × 10−8 refractive index unit (RIU) can be obtained in a refractive index range of 1.33 and 1.4 using an ESM-PCF-LPG with hexagonally arrayed 4-ring air channels at the grating periodicity of 1300 μm. A sensitivity of 4.1 × 10−7 RIU can be achieved in the range of 1.0–1.25 employing an ESM-PCF-LPG with similarly arrayed but 5-ring air channels at the grating periodicity of 350 μm. The predicted index sensitivity, coupled with other schemes for selectivity and specificity, opens the ESM-PCF-LPG structure for a variety of sensing and detection applications.

Bending Measurement of Long-Period Gratings in Photonic Crystal Fibers

In this study, we demonstrated the use of a long-period grating in photonic crystal fibers is for bending measurement, in which the loss peak of the long-period grating shifts toward the shorter-wavelength side and the loss intensity decreases as the radius of curvature of the photonic crystal fibers decreases. Bending sensitivity was determined to be 10.63 nm/cm in the curvature radius range of 8–12 cm and 1.41 nm/cm at a curvature radius larger than 12 cm.

Effect of macro-bending on resonant wavelength and intensity of long-period gratings in photonic crystal fiber

We report the spectral characteristics of CO(2)-laser inscribed long-period gratings (LPGs) in endlessly single mode photonic crystal fiber (PCF) subject to macro-bending. The coupling modes as a result of bending were studied by examining the shifts of resonant wavelengths, the splits of attenuation bands, and the variation in coupling strength of the transmission spectra. A bending coefficient of ~ 27.9 nm.m was determined in the PCF at 180 degrees rotational orientation relative to the point of laser inscription in the curvature range from 2.6 m(-1) to 3.5 m(-1). Compared with conventional fiber LPGs fabricated using the same method, the PCF-based LPGs possess higher sensitivity both to bending and orientation, making them promising for sensor applications.

Fluidic Sensors Based on Photonic Crystal Fiber Gratings: Impact of the Ambient Temperature

The spectral and thermooptical responses of photonic crystal fiber gratings (PCFGs) infiltrated with gaseous solutions, are investigated theoretically using an accurate semivectorial modal solver combined with exact equations for the reflection response of fiber Bragg gratings. We demonstrate numerically perhaps for the first time, that by an appropriate selection of the design parameters, it is possible to obtain compact sensing platforms based on the shift of the calculated reflectance spectra of the PCFG. Thus, our investigation adds evidence to the potential use of PCFGs as gas/liquid sensors. In addition, we show through a thermooptical sensitivity analysis that this type of sensors can easily meet the requirements for channel allocation in wavelength-division-multiplexing systems with relatively low temperature sensitivity

Femtosecond IR laser fabrication of Bragg gratings in photonic crystal fibers and tapers

Fiber Bragg gratings were written in pure silica photonic crystal fibers (PCFs) and PCF tapers with 125-fs 800-nm infrared radiation. High reflectivities were achieved with short exposure times in the tapers. Both multimode and single-mode grating reflections were achieved in the fiber tapers.

Photonic crystal fiber long-period gratings for biochemical sensing

We present experimental results showing that long-period gratings in photonic crystal fibers can be used as sensitive biochemical sensors. A layer of biomolecules was immobilized on the sides of the holes of the photonic crystal fiber and by observing the shift in the resonant wavelength of a long-period grating it was possible to measure the thickness of the layer. The long-period gratings were inscribed in a large-mode area silica photonic crystal fiber with a CO2 laser. The thicknesses of a monolayer of poly-L-lysine and double-stranded DNA was measured using the device. We find that the grating has a sensitivity of approximately 1.4nm/1nm in terms of the shift in resonance wavelength in nm per nm thickness of biomolecule layer.

Structural long-period gratings in photonic crystal fibers

We report what is believed to be the first example of structural long-period gratings written in pure silica photonic crystal fibers (PCFs). The gratings are realized by periodic collapse of the holes of the PCF by heat treatment with a CO(2) laser. The resulting periodic hole-size perturbation produces core-to-cladding-mode conversion. These results can lead to a new family of structural all-fiber devices that use the unique properties of PCFs.