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.     

Tuning properties of long period gratings in photonic bandgap fibers

We investigate the thermal tuning properties of long period gratings (LPGs) in a fluid-filled photonic bandgap fiber (PBGF). The combination of strong, resonant waveguide dispersion, characteristic of all PBGF modes, and the large thermo-optic coefficients of fluids yields highly tunable grating resonances. We measure grating resonances in three transmission bands with large tuning coefficients of up to -1.58 nm/degrees C, which match numerical results. We derive an analytic model for the PBGF LPG tuning coefficient to show how it depends on both the shift of the transmission bands and the dispersion of the coupled modes.     

Cladding mode coupling in long-period gratings formed in photonic crystal fibers

We extend the improved effective index method (IEIM) to analyze the cladding mode of the photonic crystal fiber (PCF) and to predict cladding mode coupling in gratings. By introducing a new step-index fiber model for the PCF, the cladding mode coupling between a LP₀₁ core mode and HE₁₁ cladding mode in long-period gratings formed in PCF is accurately predicted by the IEIM. The fiber model works well for the PCF used here not only in predicting resonant mode coupling, but also in analyzing core and low-order cladding modes.     

Rewritable densification gratings in boron-doped fibers

We show that the strength of long-period gratings recorded in boron-doped fibers by CO2 radiation can be significantly enhanced by a uniform pre-exposure by the same laser. The resultant gratings could be erased by a similar uniform exposure and then recorded again multiple times with no loss of fiber sensitivity. We suggest that such gratings are formed by reversible densification of the fiber core. These densification gratings have higher thermal stability than gratings written with ultraviolet light.     

Spectral characterization of helicoidal long-period fiber gratings in photonic crystal fibers

We report a new helicoidal long-period grating by twisting a photonic crystal fiber under CO₂ laser irradiation and experimentally investigated its novel transmission characteristics. The fabricated helicoidal PCF-LPG has a relatively short length of ∼1.4 cm with a low polarization dependent loss less than 1 dB and thermal shift of ∼3.7 pm/°C. The proposed device endows the unique resonant peaks tuning capability more than ∼18 nm by applying torsion stress, which has not been achieved in prior PCF-LPGs. From these novel thermo-optic and photo-mechanical properties, the proposed device can be applied for various components, including optical filters, attenuators, and sensors.     

Near-infrared Raman spectroscopy using hollow-core photonic bandgap fibers

We report a NIR Raman spectroscopy system incorporating hollow-core photonic bandgap fibers (HC-PBFs) in both the excitation and collection paths. Raman excitation was achieved at NIR laser wavelength of 785 nm. We demonstrate that using HC-PBFs, Raman spectroscopy can be performed without the use of an additional longpass filter on the collection side. A narrow bandpass filter on the excitation side is also not required. These results provide a framework where HC-PBF based Raman probes can be developed and used in space restricted biomedical and sensing applications.     

Arc-induced long-period gratings in aluminosilicate glass fibers

Permanent long-period gratings were written using arc discharges in two aluminosilicate fibers, one of which was doped with erbium. Reversible gratings were also mechanically induced in both fibers. The thermal behavior of the arc-induced gratings was investigated at up to 1100 degrees C. It was found that the shift of the resonant wavelengths exhibited a well-defined linear dependence on temperature up to 700 degrees C.     

Analysis of air-guiding photonic bandgap fibers

We present what is to our knowledge the first theoretical analysis of air-guiding photonic bandgap fibers. The fibers are characterized by a large hollow core and a microstructured cladding exhibiting photonic bandgap effects. Using an efficient, full-vectorial numerical method, we explain the operational principle of the fibers and obtain detailed information about the properties of the air-guided modes. This information includes accurate determination of the modes' spectral extent, cutoff properties, and mode-field distributions.     

Tunable highly birefringent photonic bandgap fibers

A novel tunable highly birefringent photonic bandgap fiber (PBGF) is designed theoretically by filling its air holes with high-index material. The transmission band can be continuously tuned by changing the refractive index of the filling material. Accordingly, the tunable modal birefringence and polarization mode dispersion of the PBGFs are investigated by adjusting the refractive index of the filling material. Furthermore, we have also analyzed the effect of surface modes in the photonic bandgap on the characteristics of the tunable PBGFs. The simulation results show the feasibility of constructing birefringence-tunable photonic crystal fibers and related fiber devices in practical applications.     

Progress in solid core photonic bandgap fibers

We investigate both experimentally and numerically confinement and bend losses in solid-core photonic bandgap fibers. We proposed two designs, based on the addition of air regions in the cladding, allowing these losses to be reduced significantly while keeping the optical properties of bandgap fibers. We also present and discuss numerical results on the impact of transversal defects on the fiber confinement loss in the case of a realistic low loss fiber.     

Directional coupling in twin-core photonic bandgap fibers

The frequency dependence of the coupling length in a twin-core photonic bandgap fiber is investigated numerically. In contrast to the case of index-guiding fibers, the coupling length exhibits a maximum if the cores are sufficiently well separated. The fiber design proposed is of interest for fabricating broadband tunable couplers.     

Soliton formation in hollow-core photonic bandgap fibers

The formation of solitons upon compression of linearly chirped pulses in hollow-core photonic bandgap fibers is investigated numerically. The dependence of soliton duration on the chirp and power of the input pulse and on the dispersion slope of the fiber is investigated, and the validity of an approximate scaling relation is tested. It is concluded that compression of input pulses of several ps duration and sub-MW peak power can lead to a formation of solitons with ∼100 fs duration and multi-megawatt peak powers. The dispersion slope of realistic hollow-core fibers appears to be the main obstacle for forming still shorter solitons.     

Ultrasensitive UV-tunable grating in all-solid photonic bandgap fibers

We study the shift of a long period grating’s resonance wavelength with UV induced refractive index changes in an all-solid photonic bandgap fiber. A long period grating is mechanically imprinted in an all-solid photonic bandgap fiber with Germanium doped silica high-index rods in a lower-index silica background. The index of the high-index rods is modified through UV exposure, and we observe that the long period grating’s resonance shifts with the bandgaps. With a sensitivity of 21,000 nanometers per refractive index unit and a 8.8 nm resonance width changes of refractive index of 3 × 10⁻⁶ are in principle detectable     

Modal properties of solid-core photonic bandgap fibers

The modal behavior of a 500-μm² mode area photonic bandgap fiber was theoretically and experimentally investigated. First, a full-vector finite element algorithm was used to predict the modal properties. Then, a low-coherence reflectometry technique was employed to measure the chromatic dispersion and evaluate the number of guided modes. A short length of fiber was found to guide only two LP modes with different attenuation coefficients. The measured chromatic dispersion of the fundamental mode was found in close agreement with theoretical value. The large positive dispersion is theoretically explained by the positive waveguide dispersion induced by weak a spectral variation of radial electric field distribution in Bragg fibers.     

High group birefringence in air-core photonic bandgap fibers

A numerical investigation of group birefringence is carried out on a recently reported highly birefringent hollow-core photonic bandgap fiber by use of an efficient vector finite-element method. The hollow fiber core has an area as large as that of approximately four airholes in the cladding region and assumes a rhombic shape with round corners, and the airholes in the cladding region are hexagonal and provide a high air-filling fraction. Numerical results show very high group birefringence of the order of 10(-2) and phase birefringence of the order of 10(-3).     

Strain-insensitive and high-temperature long-period gratings inscribed in photonic crystal fiber

We fabricate and demonstrate strain-insensitive and high-temperature long-period gratings in endlessly single-mode photonic crystal fiber by use of focused pulses of a CO2 laser and a periodic stress relaxation technique without geometrical deformation and elongation of the fiber. The thermal dependence of mode coupling at 1299.59 nm is 10.9 pm/degrees C from 24 to 992 degrees C, whereas the coefficient of strain sensitivity is -0.192 pm/muepsilon up to the maximum strain of 2.74%epsilon. It is found for what is believed to be the first time that, in contrast with the traditional fiber case, the coupling resonance shifts toward shorter wavelengths under applied strain, indicating that the refractive index of the core is decreased as a result of the rebuilding of tension attributed to the stress-elastic effect, and the cladding modes is highly dispersive because of airholes arranged in the fiber cladding.     

Mechanically induced long-period fiber gratings on tapered fibers

The characteristics of a mechanically induced long-period fiber grating (MLPFG) made by pressing a pair of grooved plates over single-mode fiber tapers are analyzed. Fiber tapers with a waist length of 80 mm and diameter ranging from 90 to 125 μm, fabricated using the heating and puling method, were used. We observed that the resonance wavelengths shift toward shorter wavelengths as the fiber taper waist diameter is reduced. A maximum shift of 254 nm in the position of the resonance peaks was observed when the fiber diameter was reduced to 90 μm. This technique is particularly suitable for tuning the resonance wavelengths to shorter wavelengths below the limit imposed by the grooved plate period.     

Band-selection filters with concatenated long-period gratings in few-mode fibers

We demonstrate a novel device that comprises a pair of broadband and narrowband long-period gratings written in specially designed few-mode fibers to achieve in-fiber bandpass filtering. This device configuration opens the possibility of using long-period gratings for complex spectral shaping in a band-selection as opposed to the conventional band-rejection configuration. The devices are low loss (<0.5dB) as well as tunable over large spectral ranges (26 nm). We demonstrate, for the first time to our knowledge, that the unique dispersive properties of long-period gratings allow for constructing dispersion-free bandpass filters with arbitrarily sharp spectral profiles.     

全固态光子带隙光纤第1带隙内模场分布特性

利用多极法计算了全固态光子带隙光纤第1带隙内不同波长处的基模模场分布,得到2维归一化光强分布特性.由于该光纤具有三角形(C<,6v>)的对称结构,影响了光纤内的模场分布,为此计算了该结构两个具有代表性的方向上的模场分布,分析了带隙内两个不同的结构方向上的模场分布特性.结果表明.在长波长区域,两个方向上的模场半径并不相等,但都随波长的增加而减小,其变化规律与全内反射导光的光纤不同.     

Effect of liquid crystal alignment on bandgap formation in photonic bandgap fibers

A simple analytical model is proposed to study the formation of bandgaps in liquid crystal photonic bandgap fibers. The model shows good agreement with full-vectorial plane-wave simulations. Particularly, bandgap splitting is observed due to anisotropy. If the optic axis of the liquid crystal lies perpendicular to the fiber axis, splitting of the fundamental modes of E(x) and E(y) is also observed.