Mingming Luo

Twin-resonance-coupling and high sensitivity sensing characteristics of a selectively fluid-filled microstructured optical fiber

A twin-resonance-coupling phenomenon and the sensing characteristics of a selectively fluid-filled microstructured optical fiber (SFMOF) are proposed and demonstrated. The SFMOF is realized by selectively infiltrating refractive index fluid into a single air hole located at the second ring near the core of the MOF. Twin-resonance dips are observed in the transmission spectrum. Theoretical and experimental investigations reveal that the twin-resonance dips both result from the coupling between LP(01)(C) silica core mode and LP(01)(L) liquid rod mode. Their sensitivities strongly depend on the dispersion curves of the silica and fluid material. Sensitivities of 290 nm/°C (739,796 nm/RIU) and 591.84 nm/N (701.2 pm/µɛ) are achieved, which are the highest for a SFMOF-based device to date, to our best knowledge. Furthermore, the twin-resonance dips appear to shift in the opposite directions with changes in temperature or axial strain, providing a method to achieve two- or multi-parameter measurement in such a compact structure.

Control and design of fiber birefringence characteristics based on selective-filled hybrid photonic crystal fibers

We demonstrated a kind of birefringence-controllable hybrid photonic crystal fibers (HPCFs) by selectively infiltrating air holes of PCFs with index-tunable liquids processing higher index than silica background. Detailed theoretical investigations on mode couplings from fundamental core mode to high-index-liquid-rod modes and birefringence properties of several HPCFs were presented. Strong wavelength dependence of phase and group birefringence was found, and HPCFs with different arrangements of high index liquid rods possess distinct birefringence characteristics. Then, the Sagnac interferometers (SIs) based on two typical HPCFs with different liquid-rod arrangements were theoretically and experimentally studied. The results indicated the SIs exhibit different transmission spectra and temperature responses due to the distinct birefringence features of HPCFs. A temperature sensitivity of -45.8 nm/°C at 56.5 °C was achieved using one HPCF, and a sensitivity of -11.6 nm/°C from 65 °C to 85 °C was achieved using the other HPCF. The thermal tunable HPCFs with birefringence-controllable properties will provide great potential for a variety of tunable optical devices and sensors.

Tunable fiber polarizing filter based on a single-hole-infiltrated polarization maintaining photonic crystal fiber

A tunable fiber polarizing filter based on selectively filling a single hole of a solid-core polarization maintaining photonic crystal fiber with high index liquid are proposed and demonstrated. Two groups of polarization-dependent resonance dips in the transmission spectrum of the single-hole-infiltrated photonic crystal fiber are observed. Theoretical and experimental investigations reveal that these resonant dips result from the couplings between the silica core fundamental mode at x or y polarization and high order modes (TM(01), TE(01) and HE(11)) in the liquid core. Especially, a distinctive characteristic near the strongest resonant point (SRP) is demonstrated and revealed. The transmission loss and spectral shape at the SRP wavelength are extremely sensitive to the filling length and temperature (or Refractive Index, RI), which permits a fiber bandpass or bandstop polarizing filter with a good performance on tunability and controllability. Furthermore, the narrowband dips on both sides of the SRP wavelength have wavelength-dependent tuning velocities, providing a method to achieve flexible and controllable filters as well as two- or multi-parameter sensors with a compact structure.

Generation and excitation of different orbital angular momentum states in a tunable microstructure optical fiber.

A tunable microstructure optical fiber for different orbital angular momentum states generation is proposed and investigated by simulation. The microstructure optical fiber is composed of a high refractive index ring and a hollow core surrounded by four small air holes. The background material of the microstructure fiber is pure silica. The hollow core and the surrounded four small air holes are infiltrated by optical functional material whose refractive index can be modulated via physical parameters, leading to the conversion between circular polarized fundamental mode and different orbital angular momentum states at tunable operating wavelengths. A theoretical model is established and the coupling mechanism is systematically analyzed and investigated based on coupled mode theory. The fiber length can be designed specifically to reach the maximum coupling efficiency for every OAM mode respectively, and can also be fixed at a certain value for several OAM modes generation under tunable refractive index conditions. The proposed fiber coupler is flexible and compact, making it a good candidate for tunable OAM generation and sensing systems.

Multi-component-intermodal-interference mechanism and characteristics of a long period grating assistant fluid-filled photonic crystal fiber interferometer

A compact in-line modal interferometer based on a long period grating (LPG) inscribed in water-filled photonic crystal fiber (PCF) is proposed and demonstrated. The interferometer works from the interference between fundamental core mode and different vector components of LP(11) core mode. The LPG is especially inscribed to realize the energy exchange between the fundamental core mode and different vector components of LP(11) core mode in the PCF. We build a complete theoretical model and systematically analyze the multi-component-intermodal-interference mechanism of the interferometer based on coupled-mode theory. Due to the asymmetric index distribution over the cross section of the PCF caused by CO(2)-laser side illumination, the dispersion curves and temperature sensitivities referring to different vector components of LP(11) core mode are quite different. Thus the interferometer is polarization-dependent and the adjacent interference fringes according to different components of LP(11) mode show greatly discrimination in sensitivities of temperature and strain, making it a good candidate for multiple physics parameters measurements.

Intermodal interferometer with low insertion loss and high extinction ratio composed of a slight offset point and a matching long period grating in two-mode photonic crystal fiber

An all-fiber modal interferometer based on a long period grating (LPG) inscribed in a two-mode photonic crystal fiber (PCF) and a slight core-offset spliced end is proposed and demonstrated. The LPG is fabricated to realize energy coupling from LP01 core mode to LP11 core mode, and the two core modes will interfere at the slight core-offset spliced end. We analyze the impact of energy coupling efficiencies of the LPG and the output spliced end on the extinction ratio of the interference fringes. With an appropriate energy coupling efficiency matching condition, our modal interferometer can realize lower insertion loss and high extinction ratio. Moreover, the sensitivities of our interferometer to strain and temperature are investigated, and the good stability of this device to external refractive index change is also demonstrated. As an all-fiber interferometer made of pure silica, this device has great potential applications in high temperature sensing fields, especially in harsh conditions.

Tunable optofluidic microring laser based on a tapered hollow core microstructured optical fiber.

A tunable optofluidic microring dye laser within a tapered hollow core microstructured optical fiber was demonstrated. The fiber core was filled with a microfluidic gain medium plug and axially pumped by a nanosecond pulse laser at 532 nm. Strong radial emission and low-threshold lasing (16 nJ/pulse) were achieved. Lasing was achieved around the surface of the microfluidic plug. Laser emission was tuned by changing the liquid surface location along the tapered fiber. The possibility of developing a tunable laser within the tapered simplified hollow core microstructured optical fiber presents opportunities for developing liquid surface position sensors and biomedical analysis.

Broadband optically controlled switching effect in a microfluid-filled photonic bandgap fiber

Broadband optically controlled switching in a microfluid-filled photonic bandgap fiber (MF-PBGF) was observed and investigated. The MF-PBGF was formed by infusing a temperature-sensitive high-index fluid into all of the cladding holes of a microstructured optical fiber (MOF). The fiber was then side pumped with a 532 nm continuous wave laser. An extinction ratio of greater than 20 dB at most of the bandgap wavelengths (more than 200 nm) was obtained with a switching power of ~147 mW. Theoretical and experimental investigations revealed that the effect originated from changes in the temperature gradient induced by heat absorption of the fiber coating with laser illumination. These investigations offer a new and simple approach to achieve wideband and flexible all-optical fiber switching devices without using any photosensitive materials.

Microfluidic assistant beat-frequency interferometer based on a single-hole-infiltrated dual-mode microstructured optical fiber

A microfluidic assistant beat-frequency interferometer based on a single-hole-infiltrated dual-mode microstructured optical fiber (DM-MOF) is proposed and demonstrated. The interferometer is constructed by inserting a piece of fluid-filled DM-MOF into two sections of single-mode fiber with slight core-offset. The mode-mismatch induced interferences with high-frequency dips and low-frequency envelope are observed in the transmission spectrum. Theoretical and experimental investigations reveal that the beat-frequency spectrum works from the interferences between LP(01) core mode and the components of LP(11) core mode with close but different frequencies. The deliberately infiltrated liquid rod in the air hole located at the second ring near the core of the DM-MOF enhances the sensitivity of the envelope to temperature or axial force. However, it shows little impact on the high-frequency dips. The distinguishing sensitivities of -959.22 pm/°C (-70.59 pm/°C) and 24.26 nm/N (-3.14 nm/N) for the envelope (dips) are simultaneously achieved in experiment, allowing for dual parameter measurement in such a compact structure.

Single Longitudinal Mode Optofluidic Microring Laser Based on a Hollow-Core Microstructured Optical Fiber

A single longitudinal mode microring dye laser based on a simplified hollow-core microstructured optical fiber (SHMOF) is proposed and demonstrated. The dye laser uses a submicron thickness silica microring embedded in the SHMOF as resonator and has the characteristic of a low threshold of 664 nJ/mm2. The factors that influence the single longitudinal mode excitation are discussed and the possible operation principle is analyzed. Meanwhile, the single longitudinal mode emissions at several different wavelengths are realized by using different dyes as gain medium, which indicates that the lasing wavelength of the laser is scalable. Time domain characteristics of the lasing are also measured and they are synchronous with the lateral pumping laser. The attractive possibility of developing microfluidic single longitudinal mode dye laser within the SHMOF presents opportunities for integrated optics applications and biomedical analysis with high sensitivity.