All-fiber Mach-Zehnder interferometer based on two liquid infiltrations in a photonic crystal fiber.

We propose a novel all-fiber Mach-Zehnder interferometer (MZI) fabricated by infiltrating two separated liquid sections along a PCF. Due to the reduced effective index difference between the core region and the liquid-filled cladding region, the guided field in the liquid sections possesses a larger mode field area and can simultaneously induce the core mode and cladding modes of the empty PCF to form a MZI. The measured results demonstrate that very clear interference spectra can be obtained. By increasing the length of the MZIs, the decreased average fringe spacing can be observed. We have also measured the temperature sensitivity of our proposed PCF-based MZIs. Due to the existence of the two liquid sections, the temperature sensitivities can be enlarged to -0.176nm/°C and -0.53dB/°C.

Using graphene nano-particle embedded in photonic crystal fiber for evanescent wave mode-locking of fiber laser.

A photonic crystal fiber (PCF) with high-quality graphene nano-particles uniformly dispersed in the hole cladding are demonstrated to passively mode-lock the erbium-doped fiber laser (EDFL) by evanescent-wave interaction. The few-layer graphene nano-particles are obtained by a stabilized electrochemical exfoliation at a threshold bias. These slowly and softly exfoliated graphene nano-particle exhibits an intense 2D band and an almost disappeared D band in the Raman scattering spectrum. The saturable phenomena of the extinction coefficient ß in the cladding provides a loss modulation for the intracavity photon intensity by the evanescent-wave interaction. The evanescent-wave mode-locking scheme effectively enlarges the interaction length of saturable absorption with graphene nano-particle to provide an increasing transmittance ΔT of 5% and modulation depth of 13%. By comparing the core-wave and evanescent-wave mode-locking under the same linear transmittance, the transmittance of the graphene nano-particles on the end-face of SMF only enlarges from 0.54 to 0.578 with ΔT = 3.8% and the modulation depth of 10.8%. The evanescent wave interaction is found to be better than the traditional approach which confines the graphene nano-particles at the interface of two SMF patchcords. When enlarging the intra-cavity gain by simultaneously increasing the pumping current of 980-nm and 1480-nm pumping laser diodes (LDs) to 900 mA, the passively mode-locked EDFL shortens its pulsewidth to 650 fs and broadens its spectral linewidth to 3.92 nm. An extremely low carrier amplitude jitter (CAJ) of 1.2-1.6% is observed to confirm the stable EDFL pulse-train with the cladding graphene nano-particle based evanescent-wave mode-locking.

Mode multiplexer for multimode transmission in multimode fibers.

We have numerically demonstrated an efficient mode multiplexer which can tailor the input field patterns by using a phase controller and a mode coupler formed by four single-mode fibers (SMFs). By connecting the mode multiplexer to a multimode fiber (MMF), two orthogonal higher-order modes of the MMF can be simultaneously excited to form two communication channels. The simulated results show that very low modal interference between the two excited modes can be achieved by using the proposed mode multiplexer. We have also discussed the effect of the distance and size of the SMFs in the mode coupler on the performance of the proposed mode multiplexer.

Reversible photo-induced long-period fiber gratings in photonic liquid crystal fibers.

A novel light-controllable long-period fiber grating (LPFG) is demonstrated by making use of a PCF infiltrated with a photoresponsive liquid crystal (LC) mixture consisting of nematic LC molecules and light-sensitive 4-methoxyazobenzene (4MAB). With the aid of the photo-induced isomerization of 4MAB, the refractive index of the LC mixture can be modulated and the periodic index perturbation along the fiber can be achieved by exposing the PCF to a blue laser through a mask. The resonance wavelength and dip depth of the LPFG can be controlled by using different blue-laser irradiation time, numbers of period, and 4MAB concentrations. In addition, the photo-induced LPFG is erasable under green-laser illumination.

Subwavelength film sensing based on terahertz anti-resonant reflecting hollow waveguides.

A simple dielectric hollow-tube has been experimentally demonstrated at terahertz range for bio-molecular layer sensing based on the anti-resonant reflecting wave-guidance mechanism. We experimentally study the dependence of thin-film detection sensitivity on the optical geometrical parameters of tubes, different thicknesses and tube wall refractive indices, and on different resonant frequencies. A polypropylene hollow-tube with optimized sensitivity of 0.003 mm/µm is used to sense a subwavelength-thick (λ/225) carboxypolymethylene molecular overlayer on the tube's inner surface, and the minimum detectable quantity of molecules could be down to 1.22 picomole/mm(2). A double-layered Fabry-Pérot model is proposed for calculating the overlayer thicknesses, which agrees well with the experimental results.

Selectively liquid-filled photonic crystal fibers for optical devices.

We have theoretically investigated the propagation properties of two kinds of selectively liquid-filled PCFs. For internally liquid-filled PCFs, the outer air-hole layers function as the second cladding to reduce the penetration of the light field while the inner liquid-hole layers can still induce the tunable PBG effect. The complementary structures, externally liquid-filled PCFs, can be used in long-period fiber gratings to decrease the utilization of the lossy liquids and remain single-mode operation for the existence of the inner air-hole layers. The confinement losses of both selectively liquid-filled PCFs are shown to be efficiently reduced due to the outer or inner air-hole layers, which is quite useful for further applications.

Tunable dual-core liquid-filled photonic crystal fibers for dispersion compensation.

We have theoretically investigated the dispersion characteristics of dispersion compensating fibers based on dual-core liquid-filled PCFs. A very high negative chromatic dispersion value D = -19000 ps/(nm-km) can be achieved at 1.55-microm wavelength by an appropriate design. By varying the geometry of the PCF and the index of the filling liquid, the phase-matching wavelength and dispersion values are shown to be well tuned to desired values. The proposed structure also demonstrates good tunable properties with operation temperature for optical communication systems.