Order Cladding Modes Research Articles

Analysis of Dispersion Properties of Chirped Long-Period Fiber Grating Based on Two Fiber Analysis Models

In this paper, the difference of core-mode effective refractive indices (RIs) and the long-period fiber grating (LPFG) resonant wavelength in an uniaxial-crystal fiber resulted from two fiber analysis models were demonstrated and analyzed. The two models are two-layered medium model which is normally used for conventional single-mode optical communications fiber and three-layered medium model. Using Transfer Matrix Method (TMM) based on Coupled-Mode Theory (CMT), the different dispersion properties of chirped long-period fiber gratings (CLPFG) calculated by both models were compared. The result shows that the difference of the core-mode effective RIs is significant especially when the cladding radius and cladding mode order are small, which indicates the limitation of the two-layer model, and that reducing the cladding radius and increasing the cladding mode order can improve the CLPFG dispersion compensation capability effectively. This paper provides guidance to the design and accurate analysis of CLPFG based dispersion compensator (DC).

Refractive Index Sensing Characteristics of Single-Mode Fiber-Based Modal Interferometers

We present a theoretical and experimental investigation on refractive index (RI) sensing characteristics of single mode fiber (SMF) based modal interferometers. Theoretical analysis reveals that interference between different modes in an SMF has a quite different response to the RI variation of the external medium. The interference between the core and lower order cladding modes has negative RI sensitivity whereas that between the core and higher order modes, or between two different order cladding modes have positive sensitivity. A single-mode-multimode-single-mode (SMS) fiber Michelson interferometer with a large-core step-index multimode fiber (MMF) is employed for experimental verification. In the SMS-based Michelson interferometer, the MMF acts as a mode coupler to excite cladding modes in the SMF. The RI response of the SMS-based structures with two different lengths of MMF are respectively tested in sodium-chloride water solutions. Experimental results show excellent agreements with the theoretical analysis.

Photonic Crystal Fiber-Based Modal Interferometer for Refractive Index Sensing

A core-cladding-mode interferometer is fabricated by splicing a single-mode fiber to an endlessly single-mode photonic crystal fiber (ESM-PCF). The optimum collapsed length of ESM-PCF is investigated to ensure a higher order cladding mode of ESM-PCF to be excited, serving as a sensing beam. At the end of ESM-PCF, a spherical end-facet with big radius is proposed to ensure from where the higher order cladding mode is to be reflected and then it interferes with the core mode serving as a reference beam. Such a device has an enhanced refractive index sensitivity of ~ 199 nm/RIU, offering potentials in biological and chemical applications.

High-refractive-index transparent coatings enhance the optical fiber cladding modes refractometric sensitivity

The high order cladding modes of standard single mode optical fiber appear in quasi-degenerate pairs corresponding to mostly radially or mostly azimuthally polarized light. In this work, we demonstrate that, in the presence of a high-refractive-index coating surrounding the fiber outer surface, the wavelength spacing between the orthogonally polarized cladding modes families can be drastically enhanced. This behavior can be advantageously exploited for refractometric sensing purposes. For this, we make use of tilted fiber Bragg gratings (TFBGs) as spectral combs to excite the orthogonally polarized cladding modes families separately. TFBGs were coated with a nanometer-scale transparent thin film characterized by a refractive index value close to 1.9, well higher than the one of pure silica. This coating brings two important assets: an ~8-fold increase in refractometric sensitivity is obtained in comparison to bare TFBGs while the sensitivity is extended to surrounding refractive index (SRI) values above 1.45.

Fiber modal interferometer with embedded fiber Bragg grating for simultaneous measurements of refractive index and temperature

A highly sensitive and compact fiber-optic sensor for simultaneous measurement of refractive index (RI) and temperature has been proposed and experimentally demonstrated. The device is based on a fiber modal interferometer assisted by an embedded fiber Bragg grating, which helps to resolve the crosstalk issue. The fiber modal interferometer consists of an Up-Fusion-Bitaper (UFBT) pair to excite high-order cladding-modes and re-couple them to the fiber core. For high order cladding mode involved in the interference, the implemented sensor yields high spectral sensitivities of −197.03±7.35nm/RIU (refractive index unit) for a refractive index (RI) range of 1.365–1.415 and 63.7±1.84pm/°C for a temperature range from 20 to 80°C, respectively. Simultaneous measurement of ambient RI and temperature has been experimentally demonstrated. The maximum RI and temperature measurement errors are ±0.00054RIU and ±0.77°C, respectively. These results demonstrate the feasibility of this configuration for reliable RI-based chemical and biochemical monitoring.

Capturing reflected cladding modes from a fiber Bragg grating with a double-clad fiber coupler

We present a novel measurement scheme using a double-clad fiber coupler (DCFC) and a fiber Bragg grating (FBG) to resolve cladding modes. Direct measurement of the optical spectra and power in the cladding modes is obtained through the use of a specially designed DCFC spliced to a highly reflective FBG written into slightly etched standard photosensitive single mode fiber to match the inner cladding diameter of the DCFC. The DCFC is made by tapering and fusing two double-clad fibers (DCF) together. The device is capable of capturing backward propagating low and high order cladding modes simply and efficiently. Also, we demonstrate the capability of such a device to measure the surrounding refractive index (SRI) with an extremely high sensitivity of 69.769 ± 0.035 μW/RIU and a resolution of 1.433 × 10(-5) ± 8 × 10(-9) RIU between 1.37 and 1.45 RIU. The device provides a large SRI operating range from 1.30 to 1.45 RIU with sufficient discrimination for all individual captured cladding modes. The proposed scheme can be adapted to many different types of bend, temperature, refractive index and other evanescent wave based sensors.

Polarization-dependent properties of the cladding modes of a single mode fiber covered with gold nanoparticles

The properties of the high order cladding modes of standard optical fibers are measured in real-time during the deposition of gold nanoparticle layers by chemical vapor deposition (CVD). Using a tilted fiber Bragg grating (TFBG), the resonance wavelength and peak-to-peak amplitude of a radially polarized cladding mode resonance located 51 nm away from the core mode reflection resonance shift by 0.17 nm and 13.54 dB respectively during the formation of a ~200 nm thick layer. For the spectrally adjacent azimuthally polarized resonance, the corresponding shifts are 0.45 nm and 16.34 dB. In both cases, the amplitudes of the resonance go through a pronounced minimum of about 5 dB for thickness between 80 and 100 nm and at the same time the wavelengths shift discontinuously. These effects are discussed in terms of the evolving metallic boundary conditions perceived by the cladding modes as the nanoparticles grow. Scanning Electron Micrographs and observations of cladding mode light scattering by nanoparticle layers of various thicknesses reveal a strong correlation between the TFBG polarized transmission spectra, the grain size and fill factor of the nanoparticles, and the scattering efficiency. This allows the preparation of gold nanoparticle layers that strongly discriminate between radially and azimuthally polarized cladding mode evanescent fields, with important consequences in the plasmonic properties of these layers.

Higher-order mode couplings in double-helix chiral long-period fiber gratings

All possible resonant couplings of the core modes with the cladding modes are analyzed comprehensively in double-helix chiral long-period fiber gratings (CLPGs). Couplings with higher order cladding modes that were neglected in previous works are emphasized in this paper, and thus interesting properties are revealed. Two kinds of double-helix CLPGs whose cross-section asymmetry originates from small geometric deformation and stress-induced birefringence, respectively, are analyzed in detail. The similarities and differences of mode selection and polarization selectivity of mode couplings are identified for the two structures.

Polarization maintaining fiber interferometer based on superimposed Mach–Zehnder and Sagnac interferences and its application

A novel polarization maintaining fiber interferometer based on superimposed Mach–Zehnder and Sagnac interferences is presented. Higher order cladding modes are effectively excited to construct a Mach–Zehnder interference with the low order linear polarized modes in the Sagnac interferometer when fusing the single mode fiber and polarization maintaining fiber with a peanut structure. We also demonstrate its strain and temperature tunability. The Mach–Zehnder resonance peak is strain insensitive while the Sagnac resonance dip is highly sensitive to strain, and the Mach–Zehnder resonance peak exhibits an opposite thermal response compared to the Sagnac resonance dip. This can be applied for dual parameter measurement with high resolutions.

Cascaded Photonic Crystal Fiber Interferometers for Refractive Index Sensing

We present a cascaded fiber device based on photonic crystal fiber (PCF) interferometers for refractive index (RI) sensing. The PCF modal interferometers have microbubbles at both sides of the splice regions. The microbubbles act as thick diverging lens that scatter light for efficient excitation of higher order cladding modes and attenuation of the core modes in the transmitted spectrum. Three resonance wavelengths are monitored, and their corresponding RI sensitivities are found to be 252, 187, and 207 nm/refractive index unit (RIU). The results, to our best knowledge, demonstrate that PCF interferometers with microbubbles are repeatable for high RI sensitivity, and the best crosstalk achievable is 0.295 nm/RIU.

Miniature Single-Mode Fiber Refractive Index Interferometer Sensor Based on High Order Cladding Mode and Core-Offset

A miniature single-mode fiber (SMF) refractive index (RI) interferometer sensor based on high order cladding mode and core offset is presented. The sensor is fabricated by first splicing a very short SMF to another SMF with core-offset to induce cladding mode propagation along the short SMF. The end of the SMF will be melted into a rounded tip to excite the propagating cladding mode into higher order at reflection which creates large phase delay. High RI sensitivity of 1.24 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-4</sup> refractive index unit (RIU) and 8.35 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-5</sup> RIU for RIU of 1.3350 and 1.3845, are obtained, respectively, for the physical length of 690- μm sensor.

An intrinsic fiber-optic single loop micro-displacement sensor.

A micro-displacement sensor consisting of a fiber-loop made with a tapered fiber is reported. The sensor operation is based on the interaction between the fundamental cladding mode propagating through the taper waist and higher order cladding modes excited when the taper is deformed to form a loop. As a result, a transmission spectrum with several notches is observed, where the notch wavelength resonances shift as a function of the loop diameter. The loop diameter is varied by the spatial displacement of one end of the fiber-loop attached to a linear translation stage. In a displacement range of 3.125 mm the maximum wavelength shift is 360.93 nm, with 0.116 nm/μm sensitivity. By using a 1,280 nm broadband low-power LED source and a single Ge-photodetector in a power transmission sensor setup, a sensitivity in the order of 2.7 nW/μm is obtained in ∼1 mm range. The proposed sensor is easy to implement and has a plenty of room to improve its performance.

Characteristics of a long-period fiber grating with reduced cladding for refractive index sensing

The sensitivity to surrounding refractive index (SRI) of a long-period fiber grating (LPFG) can be effectively improved by decreasing the cladding radius. When the cladding is reduced, a three-layer model is necessary to evaluate the effective refractive index (ERI) of the core mode. A variation of SRI can induce a greater resonant wavelength shift when the core mode is coupled to a higher-order cladding mode. However, as the cladding is reduced further, the highest-order cladding mode would be cut off, i.e. the number of cladding modes that a given fiber structure can support would be less; thus, the higher-order cladding modes that can be used for higher sensitivity are limited. Hence, the implementation of high sensitivity for SRI sensing with cladding-reduced LPFGs is dependent on the proper combination of cladding radius and cladding mode order. Based on the vector coupled-mode theory, the transmission spectrum and sensitivity are numerically analyzed with respect to the cladding radius, which shows that the SRI sensitivity of the HE12 mode with cladding radius a 2 = 20 µm is 32 times as high as that with a 2 = 62.5 µm and the SRI resolution is available to the order of 10−7.

Ultra Sensitive Fiber-Optic Hydrogen Sensor Based on High Order Cladding Mode

We demonstrate a simple but sensitive hydrogen gas sensor composed of a palladium-coated long-period fiber grating (LPG). By writing an LPG in a low core index fiber, high-order cladding modes are excited. As the palladium thin layer absorbs hydrogen, the effective refractive indexes of the cladding modes are affected, thus the resonant wavelengths of the LPG are changed with a high sensitivity. With 70-nm-thick coating, 7.5 nm of the hydrogen-induced spectral shift was achieved. The spectral response of the proposed sensor to hydrogen gas and its recovery with nitrogen gas are presented.

A novel oil level monitoring sensor based on string tilted fiber Bragg grating

In this paper, we present a novel oil level monitoring sensor based on string tilted fiber Bragg grating (TFBG). The measurement range and sensitivity of oil level monitoring can be modulated via changing the length and number of string tilted fiber gratings. The transmission spectrum of string TFBGs immersed in oil changes obviously with the oil level variation. Experiments are conducted on three 2 cm-length serial TFBGs with the same tilted angle of 10o. A sensitivity of 3.28 dB/cm in the string TFBG sensor is achieved with good linearity by means of TFBG spectrum characteristic with peak-low value. The cladding mode transmission power and the amplitude of high order cladding mode resonance are nearly linear to the oil level variation. This kind of sensor is insensitive to temperature and attributed to be employed in extremely harsh environment oil monitoring.

Evanescent wave sensor based on permanently bent single mode optical fiber

A novel refractive index sensing scheme based on evanescent wave interaction through locally and permanently bent single mode optical fibers is proposed. Local and permanent bends in single mode optical fibers enable significant power coupling between core and cladding modes. Order and number of excited cladding modes depend on bend features and determine the field profile at the output of the bent region. This in turn constitutes a simple mechanism to tailor the field distribution in single mode optical fibers useful for spatial light modulation. Moreover, since guided cladding modes are strongly influenced by the surrounding refractive index (SRI), the power transmitted at the output of the bent region as well as its dependence on the optical wavelength are strongly sensitive to the SRI opening new scenarios in sensing applications.

Mode degeneration in bent photonic crystal fiber study by using the finite element method

The development of highly dispersive lower and higher order cladding modes and their degeneration with respect to the fundamental core mode in a bent photonic crystal fiber is rigorously studied by use of the full-vectorial finite element method. It is shown that changes in the bending radius can modify the modal properties of large-area photonic crystal fibers, important for a number of potential practical applications.

Dual peak resonance and transmission spectrum characteristics in a coated long-period fiber grating

The dual peak resonance in a coated long-period fiber grating (LPFG) is presented and studied. By resolving the eigenvalue equation of the coated LPFG, the dual peak resonant wavelengths are determined based on a rigorous coupled mode theory. The relationships between the dual peak resonant wavelengths and the grating period for various cladding mode orders are studied. The results show that the dual resonant wavelengths of higher order cladding modes appear in a fiber within the commonly observed wavelength range. The smaller the grating period is, the higher the cladding mode order corresponding to the dual peak resonance is. Furthermore, the influence of the film optical parameters and the grating structural parameters on the wavelength interval between the dual resonant peak, the amplitude, and the bandwidth of the resonant peak is discussed. The obtained results have some similarity to those from the non-coated LPFG experiment by Shu (1999 Opt. Commun. 171, 65). In addition, the sensitivity of the coated LPFG sensor based on dual peak resonance is analyzed under the influences of the film optical parameters and the grating structural parameters. The resolution of the refractive index of this coated LPFG sensor is predicted to be 10−7, so it provides a theoretical foundation for the structural optimization of a high sensitivity dual peak resonance coated LPFG sensor.

Fiber Inline Core–Cladding-Mode Mach–Zehnder Interferometer Fabricated by Two-Point CO$_{2}$ Laser Irradiations

We report a fiber inline Mach-Zehnder-type core-cladding-mode interferometer fabricated by two-point CO2 laser irradiations on a single-mode fiber. The laser irradiations caused efficient light coupling from the core mode to the lower order cladding modes and vise versa. High-quality interference spectra with a fringe visibility of about 20 dB were observed for four different interferometer lengths (5, 10, 20, and 40 mm). The temperature sensitivity of the device with a length of 5 mm was measured to be 0.0817 nm/degC. The sensitivity for refractive index measurement of the device was comparable with a long-period fiber grating of LP04 cladding mode.

Bandwidth analysis of long-period fiber grating for high-order cladding mode and its application to an optical add-drop multiplexer

The spectrum bandwidth of long-period fiber grating (LPG) for various high-order cladding modes are analyzed in detail by two-mode coupled-mode equations and applied to design narrow bandwidth optical add-drop multiplexer (OADM) based on two parallel LPGs. In addition, in order to obtain the maximal power transmission, we further derive the structure parameters of OADM such as the distance between two parallel fibers and the length of two long-period fiber gratings according to four-mode coupled-mode equations. As far as this OADM structure is concerned, it is obvious that LPG will dominate the entire bandwidth if LPG has enough narrow bandwidth in comparison with the 2×2 coupler. In other words, we can easily use LPG to estimate the bandwidth of OADM before starting to design it. In order to survey the feasibility of the above statement, the spectrum bandwidths of LPG and OADM for the various bandwidth of high-order cladding modes are compared and analyzed. Utilizing the four steps proposed in this paper, the numerical results have demonstrated that we can use the high order cladding mode =125 to design the OADM that possesses narrow FWHM (<0.4 nm) and meets the DWDM system