Order Cladding Modes Research Articles

Optimization of a long-period grating-based Mach–Zehnder interferometer for temperature measurement

An approach to optimize the design of the long-period grating pair as a temperature sensor device is presented, implemented by using a long-period grating (LPG) pair with a small separation (of around 2mm) and scaling down their physical length by a factor greater than 2. The technique allows the interferometer formed not only to measure temperature variations over distances as small as the overall length of the grating pair (18mm) but also to reduce the cladding losses between the LPGs forming the pair. This approach enhances the sharpness of the interference fringes (IFs) and the pits (Pts) in the transmission spectrum and, as a result, a high resolution sensor is obtained. The LPG pair is fabricated in the appropriate photosensitive single mode/core fibres, without being restricted to the use of dual core or other special fibres, thus exploiting the sensitivities of various fibres and reducing the overall system cost. In this work, the effectiveness of this technique is demonstrated by fabricating a small-scale LPG pair in a boron–germanium co-doped single mode fibre, with particular attention being paid to the higher order cladding modes. The sensitivity of the device thus created is 0.31nm/°C with a root-mean-square (rms) deviation of 0.28nm in the wavelength measurement, which corresponds to a temperature variation of approximately 0.9°C. This was achieved while using a relatively low-resolution (0.6nm) Optical Spectrum Analyzer to detect the wavelength changes of the device and was further improved to 0.7°C when using an OSA with a resolution of 0.1nm.

Cladding mode coupling suppression in hole‐assisted fiber BRAGG gratings

AbstractWe experimentally investigate the cladding mode coupling loss of fiber Bragg gratings written in hole‐assisted fibers. The cladding mode coupling loss is remarkably suppressed due to air holes inside of the silica cladding. The reduction of cladding mode coupling loss of less than ∼0.33 dB is obtained. The wavelengths spacing between the main peak due to the core mode coupling and the first side peak due to the first order cladding mode coupling is changed by the increase of the air holes size since the effective index of cladding mode is reduced by the air hole size within the fiber. The variation of the wavelength spacing is ranged from 2.0 to 2.28 nm. © 2006 Wiley Periodicals, Inc. Microwave Opt Technol Lett 49: 74–74, 2007; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.22050

Observation of mode coupling in bitapered air-core photonic bandgap fibers

We report the fabrication and characterization of the first bitapers in air-core photonic bandgap fibers. Like conventional fiber bitapers, these devices are found to exhibit quasi-sinusoidally modulated transmission spectra, the signature of resonant coupling to a dominant cladding mode. However, unlike in conventional tapers of comparable length, the modulation period is very short (0.5–3.3nm), which suggests that coupling takes place to a very high order cladding mode. Such coupling is observed even in comparatively weak bitapers (diameter reduction as low as ∼2%), indicating that air-core fibers are highly sensitive to tapering. The loss of weak tapers is observed to be small (<0.3dB), which suggests possible applications of this simple technology as in-line filters of high resolution.

Coated long-period fiber gratings as high-sensitivity optochemical sensors

In this paper, the numerical and the experimental analyses of coated long-period fiber gratings (LPFGs) as a high-sensitivity optochemical sensor are presented. The proposed structure relies on LPFGs coated with nanoscale high refractive index chemical-sensitive overlays. The deposition of overlays with refractive index higher than the cladding one leads to a modification of the cladding-mode distribution. If the overlay features are properly chosen, a strong field enhancement within the overlay occurs, leading to an excellent sensitivity of the cladding-mode distribution to the coating properties. The effects of overlay thickness and cladding-mode order on sensor performances have been numerically and experimentally investigated. In order to provide a high-sensitivity and species-specific optochemical sensor, this mechanism has been proved with nanoscale overlays of syndiotactic polystyrene (sPS) in the nanoporous crystalline /spl delta/ form. The sensitive material has been chosen in light of its selectivity and high sorption properties towards chlorinated and aromatic compounds. Sensor probes were prepared by using dip-coating technique and an adequate procedure to obtain the /spl delta/-form sPS. Experimental demonstration of the sensor capability to perform subparts-per-million detection of chloroform in water at room temperature is also reported.

Cladding mode resonances of etch-eroded fiber Bragg grating for ambient refractive index sensing

In this Letter, the cladding mode resonances of a chemical etch-eroded fiber Bragg grating are proposed for highly sensitive ambient refractive index sensing. The varying characteristics of several different order cladding mode resonances with ambient refractive index are explored experimentally. The maximum sensing sensitivity of 172nm∕riu for the third-order cladding mode resonance is achieved in the experiments. Under the same cladding diameter, the cladding mode resonances have narrower bandwidth and response much more sensitive to the ambient refractive index changing than conventional Bragg core mode resonance, which would enable them to achieve a higher resolution.

Mode transition in high refractive index coated long period gratings

In this work, the numerical and experimental investigation of the cladding modes re-organization in high refractive index (HRI) coated Long Period Gratings (LPGs) is reported. Moreover, the effects of the cladding modes re-organization on the sensitivity to the surrounding medium refractive index (SRI) have been outlined. When azimuthally symmetric nano-scale HRI coatings are deposited along LPGs devices, a significant modification of the cladding modes distribution occurs, depending on the layer features (refractive index and thickness) and on the SRI. In particular, if layer parameters are properly chosen, the transition of the lowest order cladding mode into an overlay mode occurs. As a consequence, a cladding modes re-organization can be observed leading to relevant improvements in the SRI sensitivity in terms of wavelength shift and amplitude variations of the LPGs attenuation bands.

Add/drop Filter for CWDM Systems Using Side-coupled Long-period Fiber Gratings

We demonstrate a simple and effective wavelength-tunable add/drop filter suitable for coarse wavelength division multiplexing (CWDM) systems. The filter consists of two fibers in contact side by side, with identical long-period fiber gratings (LPG) in each fiber. The LPG couples the power in the fundamental core mode to one of the cladding modes, which is then coupled to the same order cladding mode in the other fiber through evanescent-field coupling between two fibers. Finally, the cladding mode in the second fiber is coupled to its core mode with the help of the other LPG. With an optimal longitudinal offset distance of 10 em, coupling efficiency as high as -1.68 dB and side lobes smaller than -24 dB were experimentally obtained. The experimental results agreed well with the theoretical ones. The operating wavelength of the proposed add/drop filter was tunable by varying the temperature. The temperature sensitivity was measured to be -0.43 nm/ $^{\circ}C$ .

High-sensitivity optical chemosensor based on coated long-period gratings for sub-ppm chemical detection in water

In this work, the use of fiber long-period gratings (LPGs)—coated with nanoscale overlays of Syndiotactic Polystyrene (sPS) in the nanoporous crystalline δ form as specific and highly sensitive chemical sensors for in water monitoring—is proposed. The approach presented here, combines the excellent sorption properties of δ form sPS as a chemosensitive layer with the excellent refractive index sensitivity of LPG-based sensors as ideal transducers. In particular, when overlays with a high refractive index compared with the cladding one are deposited along the grating region, as in this case, the refraction-reflection regime at the cladding-overlay interface occurs. As result of this mechanism, the attenuation bands of coated LPGs would respond to the optical changes induced in the sensitive overlay due to chemical sorption by a significant modification of the peak central wavelength and intensity. The sensitivity depends strongly on the overlay thickness and the grating coupled cladding mode. Here, sensor probes were prepared by using a dip coating technique and a proprietary procedure to obtain the δ form sPS. An experimental demonstration of the sensor capability to perform sub-ppm detection of chloroform in water at room temperature is reported here. Also, the effects of the overlay thickness and the cladding mode order on sensor sensitivity and response time have been numerically and experimentally investigated.

Sandwiched long-period gratings for simultaneous measurement of refractive index and temperature

A simultaneous measurement of external refractive-index (RI) and temperature is presented by using a sandwiched structure of long-period gratings (LPGs). An LPG pair with codirectional coupling between the guided mode and a low-order cladding mode acts as a temperature sensor with low RI sensitivity, whereas another in-between LPG which couples the guided mode with a higher order cladding mode performs as an RI sensing element. The experimental results show such an all-LPG structure is not only suitable for use as a compact temperature-compensated RI sensor, but also convenient for fabrication.

High relative humidity measurements using gelatin coated long-period grating sensors

A long-period grating (LPG) coated with gelatin was developed as a high relative humidity (RH) sensor. The resonance dip or coupling strength of the LPG spectrum varies with RH. Experimental investigations on the sensor yield a sensitivity of 0.833%RH/dB with an accuracy of ±0.25%RH, and a resolution of ±0.00833%RH. The LPG RH sensor also offers repeatability, hysteresis and stability errors of less than ±0.877, ±0.203 and ±0.04%RH, respectively. In addition to the characterization of the LPG RH sensor, further studies were conducted to determine the effect of grating periodicities on the sensitivity of the sensor. Results show that higher order cladding modes from smaller grating periods enable the sensor to achieve higher sensitivity to humidity. This method is proposed to be more cost effective as compared to more complex spectroscopic methods based on wavelength detection. This sensor can also help to solve problems in measuring high humidity with existing relative humidity measurement systems.

Fourier decomposition method for mode characterization in metal-clad fiber with complex index profile

Fourier decomposition method in the cylindrical coordinates is used to characterize LP 0 x mode effective indexes in a step fiber with complex index profile under material dispersion, from the guided modes to the radiation mode. The method help sorting the modes in function of physical properties associated to a particular mode type LP 0 x , etc. The main findings of this work are: (1) The imaginary part of effective indexes of the guided modes and lower order cladding modes is several orders of magnitude smaller than the real part. (2) Both real and imaginary parts of metal-clad refractive indexes around the fiber have a strong influence on the real part of LP 0 x mode effective indexes. (3) In some regions of the wavelength, the fundamental mode LP 01 in an aluminum-clad fiber disappears, but the higher order cladding modes still exist and propagate.

Bending and Orientational Characteristics of Long Period Gratings Written in D-Shaped Optical Fiber

Long period gratings (LPGs) were written into a D-shaped single-mode fiber. These LPGs were subjected to a range of curvatures, and it was found that as curvature increased, there was increasingly strong coupling to certain higher order cladding modes without the usual splitting of the LPGs stopbands. A bend-induced stopband yielded a spectral sensitivity of 12.55 nm/spl middot/m for curvature and 2.2/spl times/10/sup -2/ nm/spl deg/C/sup -1/ for temperature. It was also found that the wavelength separation between adjacent bend-induced stopbands varied linearly as a function of curvature. Blue and red wavelength shifts of the stopbands were observed as the sensor was rotated around a fixed axis for a given curvature; thus, in principle, this sensor could be used to obtain bending and orientational information. The behavior of the stopbands was successfully modeled using a finite element approach.

Sensitivity characteristics of long-period fiber gratings

We present a detailed investigation into the sensitivity of long-period fiber gratings (LPFGs) as a function of temperature, strain, and surrounding refractive index, with particular attention to the higher order cladding modes and the possibilities for ultrasensitive sensors. From a general theoretical analysis, we identify a general sensitivity factor which offers new physical insight into LPFG behavior and represents a useful design aid in conjunction with a set of measurand-specific sensitivity factors. Our analysis reveals the existence of turning points in the mode dispersion characteristics at which ultrasensitive operation may be obtained. In an extensive set of coordinated experiments, we verify the theoretical predictions with close agreement and provide demonstrations of the device behavior close to the turning points. Alternative sensor schemes for temperature, strain, and refractive index based, respectively, on measurement of the dual resonance characteristic of the modes and on the transmission characteristics close to the turning points, utilizing higher order modes of the LPFG, are presented. For two variables at least, we record the highest LPFG sensitivities yet reported.

Reduction of Bragg Grating-Induced Coupling to Cladding Modes

We discuss fiber designs that have been suggested for the reduction of Bragg-grating-induced coupling to cladding modes. The discussion is based on a theoretical approach that includes the effect of asymmetry in the UV-induced index grating, made by UV-side writing. Experimental results from gratings in a depressed-cladding fiber are compared with simulations. The model gives good agreement with the measured transmission spectrum and accounts for the pronounced coupling to asymmetrical cladding modes, even when the grating is written with smallest possible blaze. The asymmetry causing this is accounted for by the unavoidable attenuation of the UV light. It is found for the considered fiber designs that a high numerical-aperture fiber increases the spectral separation between the Bragg resonance and the onset of cladding-mode losses. A depressed-cladding fiber reduces the coupling strength to the lower order cladding modes, and the UV-sensitive cladding design reduces the cladding-mode coupling loss. The analysis suggests that the UV-sensitive cladding design is the most effective in reducing the cladding-mode-coupling losses.

Dual resonant peaks of LP 015 claddingmode in long-period gratings

It is found that the coupling of the guided core mode to a higher order cladding mode may lead to dual resonant wavelengths in long-period gratings (LPGs). The dual peaks of the LP/sub 015/ cladding mode have been observed to shift towards each other during the second exposure of a 100 /spl mu/m period grating. It is also shown that the resonant wavelengths of LPGs can be tuned over a wide range by the post-exposure technique.