Core-cladding Mode Coupling Research Articles

Design of Weakly Coupled Two-Mode Hollow-Core Antiresonant Fiber With Low Loss

We first design and optimize a hollow-core conjoined-tube antiresonant fiber (CT-ARF) for weakly coupled $\mathrm{LP_{01}}$ and $\mathrm{LP_{11}}$ modes transmission. The critical core radii for the weak-coupling condition in the CT-ARFs with 4 to 8 tubes are obtained. Using numerical simulation, we demonstrate that by tuning the glass sheets positions in cladding tubes the confinement losses (CLs) of higher-order modes (HOMs) $\mathrm{LP_{21}}$ , $\mathrm{LP_{02}}$ , and $\mathrm{LP_{31}}$ can be maximized through core-cladding mode coupling. Among the CT-ARFs with 4 to 8 tubes, the 6-tube one is proved to have the lowest CLs and bending losses of $\mathrm{LP}_{01}$ and $\mathrm{LP}_{11}$ modes and the highest CL ratios between the undesired HOMs and the two guided modes. In the designed 6-tube CT-ARF, $\mathrm{LP_{01}}$ and $\mathrm{LP_{11}}$ modes have low CLs in the transmission band of 1.31–1.65 $\mu \text{m}$ . The minimum CLs of $\mathrm{LP_{01}}$ and $\mathrm{LP_{11}}$ modes are $\text{1.7}\times \text{10}^{-4} \text{ dB/m}$ and $\text{3.1} \times \text{10}^{-3} \text{ dB/m}$ in the C-band (1.53–1.565 $\mu \text{m}$ ) respectively. A 10 cm bend has a weak influence on the two guided modes. The CL ratio between the lowest loss HOM and $\mathrm{LP}_{11}$ mode keeps $>\!\!150$ in the wavelength range from 1.25–1.65 $\mu \text{m}$ , which ensures the high purity of $\mathrm{LP_{01}}$ and $\mathrm{LP}_{11}$ modes. The fabrication tolerances of the fiber are discussed.

Core-cladding mode coupling of tilted long period fiber gratings in dual-mode fibers

We present a theoretical study on the mode coupling between core modes and cladding modes of the tilted long period fiber grating (TLPFG) in dual-mode fiber. The TLPFG can be applied as mode filters at several specific wavelengths. Using the full vector complex coupled mode theory, numerical simulation shows the maximum coupling coefficients between core modes (LP01 and LP11) and cladding modes can be found near the tilted angle of 86°. The transmission peaks of modes coupling are sensitive to the titled angle. Furthermore, several peak amplitude between the LP11 mode and same azimuthal order modes increase with the growth of the titled angle. With the band-stop characteristic of specific mode, the TLPFG written in dual-mode fiber could be exploited as mode filters.

Core-offset-based fiber Bragg grating sensor for refractive index and temperature measurement

We present a fiber-optic sensor based on a fiber Bragg grating (FBG) whose core has an offset to that of the lead-in fiber from splicing for simultaneous refractive index (RI) and temperature measurement. Due to the core offset, the core mode in the lead-in fiber is coupled to both the core mode and the cladding modes of the sensing fiber with the FBG. Then the two oppositely propagating code modes, the cladding modes, and the core modes are coupled by the FBG at particular wavelengths determined by the effective indices of the modes and the FBG structure, resulting in two peaks in the FBG reflection spectrum. The peak wavelength corresponding to the core-cladding mode coupling is sensitive to both RI and temperature, while the peak wavelength corresponding to the core-core mode coupling is only sensitive to temperature. Utilizing the difference of two reflection peaks of these coupling modes, both RI and temperature can be detected simultaneously. We fabricate and characterize a sensor. The preliminary experimental results show that it can detect the refractive index in a range of 1.33 to 1.4421, with a temperature sensitivity of 10 pm/°C .

High precision micro-displacement fiber sensor through a suspended-core Sagnac interferometer

A sensing system for micro-displacement measurement based in a suspended-core fiber Sagnac interferometer is presented. The suspended-core fiber characterization was made through the use of an optical backscatter reflectometer, screening its multimodal and birefringent behavior. Its sensitivity to displacement measurements is shown to be due only to birefringence, being that core-cladding mode coupling is negligible. High precision (~0.45 μm) was obtained using three different measurement instruments, showing an extremely high stability and high insensitivity to temperature, demonstrating that the sensing system has the ability for low cost applications.

Mach-Zehnder interferometer based on core-cladding mode coupling in single mode fibers

In this paper, a novel simple but effective method is presented to construct compact all-fiber Mach-Zehnder (M-Z) interferometer based on CO2-lasermachined micro-notches in single mode fibers. Interference fringes are obtained, and temperature, force, and bending characteristics of the interferometer have been experimentally investigated. Such a compact fiber component with acceptable sensing performances makes it a good candidate for the measurement of numerous physical parameters.

Core-cladding mode coupling and recoupling in photonic crystal fiber for enhanced overlap of evanescent field using long-period gratings

Excitation of cladding modes has been achieved using long-period (LPGs) inscribed in an endlessly single-mode photonic crystal fiber (ESM PCF) by CO(2) laser irradiation. Core-cladding mode coupling and recoupling has resulted in marked improvement in the evanescent field overlap throughout the cladding air channels in the PCF-LPG, compared to the PCF alone. Our numerical simulation has shown that design optimization of the PCF-LPG configuration can lead to a field power overlap as high as 22% with a confinement loss of less than 1 dB/m in the cladding mode.

Deep-notch, ultracompact long-period grating in a large-mode-area photonic crystal fiber.

An extremely short long-period grating (LPG) with strong resonance has been developed in a large-mode-area photonic crystal fiber (PCF) by use of the heat source of a CO2 laser. We believe that such a LPG in pure silica PCF is the first example to be obtained with the point-by-point technique. The fabrication method is simple and repeatable. The resulting LPG has 8 periods, written by a CO2 laser, within a 2.8-mm length of fiber, which yields a deep notch of core-cladding mode coupling of -31.5 dB at the telecommunication wavelength of 1529.2 nm, with a FWHM of approximately 0.7 nm. The principal advantages of this LPG are that it is practical, cost effective, and compact.

Strong resonance and a highly compact longperiod grating in a large-mode-area photonic crystal fiber

A strong resonance and extremely short length long-period grating (LPG) has been fabricated in a large-mode-area photonic crystal fiber (PCF) by use of a CO2 laser heat source. We believe that such a longperiod grating in pure silica PCF is the first example of a point-by-point technique. The fabrication method is simple and repeatable. The resulting LPG has been developed with eight periodic collapses within a 2.8-mmlong period of the fiber, which gives the strong resonance of core-cladding mode coupling. The lowest mode of LP01 is at a 1529.2-nm wavelength with a full width at half-maximum of ~0.7 nm and a resonance strength of -31.5 dB. The principal advantages of this LPG are that (1) it is temperature insensitive and stable, (2) the device is compact when it is packaged, and (3) it provides practical, low-cost all-fiber filters and PCF-based devices for optical fiber communications and sensing systems.