Ultraviolet-inscribed long period gratings in all-solid photonic bandgap fibers.

Long period fiber gratings are fabricated in the cladding rods of all-solid photonic bandgap fibers (PBGFs) by point-by-point side UV illumination. Resonant couplings from fundamental mode to guided and radiative supermodes (rod modes), and bandgap-like modes are identified. We obtained a detailed insight over the modal and dispersive properties of the PBGF through a series of theoretical and experimental investigations on the spectral characteristics and the responses to temperature and high-index liquid of the LPGs.

Thermally tunable dual-core photonic bandgap fiber based on the infusion of a temperature-responsive liquid.

A dual-core photonic bandgap fiber (PBGF) is demonstrated by infusing a high-index liquid into a dual-core air-silica photonic crystal fiber (PCF). Extremal couplings have been experimentally observed. The temperature tunable characteristics of the dual-core PBGF's bandgap guiding and dual-core coupling are experimentally and numerically investigated. When we rise temperature, the dual-core PBGFs' bandgaps have been changed: compression of bandwidth, blue-shift and depression of the guiding band. Especially, the dual-core coupling is temperature tunable because of the tunability of the infusion liquid's index. We find that the rise of temperature increases the coupling length which results in the blue-shift of the resonant peak wavelengths with a speed of 1.9nm/degreesC, for a 20mm dual-core PBGF.

Impacts of imperfect geometry structure on the nonlinear and chromatic dispersion properties of a microstructure fiber.

We numerically investigated the impacts of the imperfect geometry structure on the nonlinear and chromatic dispersion properties of a microstructure fiber (MF). The statistical results show that the imperfect geometry structure degrades the high nonlinearity and fluctuates the chromatic dispersion in a MF. Moreover, the smaller air holes and the larger pitch are more likely to maintain the properties of nonlinearity and chromatic dispersion. Finally, the nonlinearity and chromatic dispersion are more insensitive to air-hole nonuniformity than to air-hole disorder. All of these will provide references for designing and fabricating MF.

Bragg grating resonances in all-solid bandgap fibers.

Bragg gratings are fabricated in all-solid photonic bandgap fibers by forming a longitudinal periodic index modulation over the high-index rod lattice in the cladding. Due to its unique index-modulation pattern and the modal properties of the fiber, resonance couplings to the LP(01) supermodes (also known as high-index rod modes) are observed. The corresponding peak is located at the long-wavelength side of the Bragg wavelength and presents the highest depth. The asymmetric index profile over the high-index rods, which is induced by side UV illumination, leads to broadening of the supermode resonance peaks and variation with fiber orientation in the bend response of the fiber grating.

Highly birefringent elliptical-hole photonic crystal fiber with squeezed hexagonal lattice.

We propose a novel polarization-maintaining index-guiding photonic crystal fiber (PCF). It is composed of a solid silica core and a cladding with squeezed-hexagonal-lattice elliptical air holes. Using a full-vector finite-element method, we study the modal birefringence of the fundamental modes in such PCFs. Numerical result shows that very high modal birefringence with a magnitude of the order of 10(-2) around 1550 nm has been obtained. Furthermore, large normal dispersion appears over a wide range of wavelengths in both orthogonal polarizations.

Fiber grating sensing interrogation based on an InGaAs photodiode linear array.

We present a new method of the fiber grating sensing interrogation technique by utilizing an indium gallium arsenide photodiode linear array and blazed fiber Bragg gratings. An interrogation system based on an InGaAs photodiode linear array is designed, and the system performance is analyzed. The interrogation system shows a good prospect for smart sensing.

Directional couplers operated by resonant coupling in all-solid photonic bandgap fibers.

We have proposed directional couplers operated by resonant coupling in all-solid photonic bandgap fibers structure which consist of a cladding with an array of high-index rods in silica background, two cores formed by omitting two rods, and some defect rods introduced by reducing the diameter of the high-index rods between the cores. The resonant effect induced by the avoided crossing between core-guided supermodes and defect-guided modes significant decreases the coupling length. The directional couplers proposed in this paper are almost polarization independent and have potential application in realizing integrating all-fiber communication devices.

Spectral characteristics and bend response of Bragg gratings inscribed in all-solid bandgap fibers.

In this paper, we investigate the spectral characteristics and bend response of fiber Bragg gratings (FBGs) in all-solid photonic bandgap fibers (PBGFs). We inscribe FBGs within the secondary bandgap by ultraviolet (UV) side illumination and observe the couplings to backward core mode, guided LP(01) and LP(11) supermodes and radiative LP(02) supermodes. The mechanisms of these resonant couplings in the FBG are described in detail. We demonstrate that only those supermodes with certain phase relationships and symmetric mode field profiles are responsible for the supermode resonances. When the fiber grating is bent, the guided supermode resonances become chirped as a result of the strain gradient over the fiber cross section. Meanwhile, the core resonance is enhanced, due to more energy of the core mode distributed in the cladding rods. The bend response is direction dependant owing to the nonuniform UV-induced average index raises and index modulation over the high-index rod lattice. .

Modal cutoff properties in germanium-doped photonic crystal fiber.

The germanium-doped photonic crystal fiber (PCF) has some characteristics that differentiate it from pure-silica PCF for a germanium element being doped in the core, such as the intensified nonlinearity, the enhanced photosensitivity, and so on. To pave the way for the application of the Ge-doped PCF successfully, it is necessary to study its properties. We investigated the modal cutoff properties of Ge-doped PCF quantitatively by using the beam propagation method. The numerical results show that the effective refractive indices and the normalized frequency V of Ge-doped PCF not only depend on the normalized pitch delta/lambda but also depend on the normalized hole size d/delta, the modal cutoff boundary for the single mode-multimode of the Ge-doped PCF shift to the low d/delta side in contrast to the pure-silica PCF.

Stable room-temperature multi-wavelength lasing realization in ordinary erbium-doped fiber loop lasers.

A suppressant effect for mode competition of multi-wavelength lasing oscillations induced by deeply saturated effect in an ordinary erbium-doped fiber ring laser (EDFRL) was observed and experimentally investigated. Results show that the effect is helpful to obtain stable multi-wavelength lasing at room temperature in the EDFRL, which offers a new and simple approach to achieve stable multi-wavelength EDF lasing. Stable two- and three- wavelength lasing oscillations were achieved based on the effect in the ordinary EDFRL for the first time to our best knowledge. The multi-wavelength lasing oscillations were so stable integrated over smaller than 1 ms that the maximum power fluctuation over more than 30 minutes of observation was less than 0.1 dB and 0.5 dB for two-wavelength lasing with a spacing of 1.28 nm and 0.76 nm, respectively.

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.

Coupling and decoupling of dual-core photonic bandgap fibers.

Coupling characteristics of dual-core photonic bandgap fibers with triangular photonic crystal cladding are investigated by use of a vector plane-wave expansion method and a vector finite-element method. We demonstrate the eigenmodes and the coupling length for two orthogonal polarizations. A decoupling phenomenon is found at a certain wavelength in this fiber configuration. The decoupling effect is attributed to the effect of surface modes on the eigenmodes. The decoupling wavelength decreases as the ratio of core radius to cladding air-hole pitch increases from 1.05 to 1.15.

Transformation of a transmission mechanism by filling the holes of normal silica-guiding microstructure fibers with nematic liquid crystal.

Transformation of an optical transmission mechanism was achieved when the holes of normal silica-guiding microstructure fiber (MF) were filled with nematic liquid crystal (NLC). Moreover, two photonic bandgaps (PBGs) were obtained by using a plane-wave method to create the pattern. The wavelength dependence of the effective mode area, leakage loss, and group velocity dispersion (GVD) has been theoretically investigated by using a full-vector finite-element method with anisotropic perfectly matched layers. The results reveal that the characteristics of the NLC-filled PBG-MFs are particularly wavelength dependent. This research gives a physical insight into the propagation mechanism in MFs and is crucial for future transmission applications.

High-birefringence fiber loop mirrors and their applications as sensors.

The reflection and transmission characteristics of a high-birefringence fiber loop mirror (HiBi-FLM), which is composed of a standard fiber coupler and one-section or multisection high-birefringence fibers (HBFs), are analyzed and discussed in detail. Theoretical reflectivity and transmissivity expressions for HiBi-FLMs with one-, two-, and three-section HBFs were obtained. The procedure for calculating reflectivity and transmissivity for HiBi-FLMs with n-section HBFs is given. Experimental results have verified the theoretical model. The basic characteristics of the one-section HiBi-FLM when strain and high temperature are applied to HBFs were analyzed and investigated theoretically and experimentally. The experimental results are in good agreement with the theoretical analysis. Furthermore, a strain--temperature sensor that makes use of those characteristics, which is new for applications of HiBi-FLMs, has been proposed and demonstrated.

Quadruple-wavelength actively mode-locked fiber laser with polarization-controlled wavelength switching.

An actively mode-locked fiber laser with two overlapping cavities is proposed and successfully demonstrated to generate switchable quadruple-wavelength picosecond pulses. The wavelengths are specified by two Bragg gratings in polarization-maintaining and absorption-reducing fiber. By simple adjustment of two polarization controllers, the proposed laser can be made to operate at quadruple wavelength or to switch between wavelengths at room temperature. For wavelength switching, four single-wavelength, six dual-wavelength, and four triple-wavelength operations were performed without changing the repetition rate of the output pulses.

Simultaneous four-wavelength lasing oscillations in an erbium-doped fiber laser with two high birefringence fiber Bragg gratings.

A novel multiwavelength erbium-doped fiber laser configuration is proposed and demonstrated. The laser can produce simultaneous four-wavelength lasing oscillations with a minimum wavelength spacing of only 0.36 nm in C-band via using two fiber Bragg gratings written in high birefringence fiber, while ensuring fairly stable room-temperature operation. The laser can also achieve switching modes among four wavelengths by simple adjustment of two polarization controllers in the cavities. Theconfiguration is based on the polarization hole burning and overlapping cavities principle. The laser has the advantages of simple all-fiber configuration, low cost, high stability and operating at room temperature.

L-Band switchable dual-wavelength erbium-doped fiber laser based on a multimode fiber Bragg grating.

In this letter, a simple, switchable dual-wavelength erbiumdoped fiber laser operating in L-band is successfully proposed and demonstrated. The two wavelengths are specified by a Bragg grating formed in multimode fiber and the wavelength separation is 1.7 nm. The multimode section including the multimode fiber grating selects not only the lasing wavelengths but also the polarization states of the lasing lines. Stable dualwavelength operation can be achieved due to the spatial mode beating effect induced by a single-mode /multimode/ single-mode fiber combination structure. By simple adjustment of a polarization controller, the proposed laser can operate in stable dual-wavelength or switch between two wavelengths, all at room temperature.