Apodization Profile Research Articles

Apodized chirped fiber Bragg grating for measuring the uniform and non-uniform characteristics of physical parameters

An apodized Chirped Fiber Bragg Grating (CFBG) is presented to compute and depict the sensing response for various uniform and non-uniform profiles of the temperature and the strain for different chirp parameters. The effectiveness of the CFBG can be reduced owing to the appearance of wide-scale side lobes as well as increasing group delay ripples (GDR). The introduction of apodization profile is highly impactful to enhance the sensing efficiency by minimizing the side lobes level along with the quantity of GDR. The sensing response of the apodized CFBG with different chirp parameters are evaluated for various ranges of uniform temperature (30 °C–80 °C) and strain (300 µ∊–550 µ∊). Furthermore, different non-uniform Gaussian profiles are also introduced to illustrate the overview for detecting and locating the spot size sources of measurands. This approach is highly desirable for estimating the structural integrity by ensuring the safety, efficiency, and reliability of the CFBG-based sensing outcomes.

High-performance and ultra-wide bandwidth silicon TM polarizer with suppressed reflection utilizing a tailored contra-mode conversion Bragg grating.

We present a compact TM polarizer with high polarization extinction ratio (PER), low loss, and suppressed reflection over an ultra-wide bandwidth on a 220 nm silicon-on-insulator (SOI) platform. The device utilizes a contra-mode conversion Bragg grating (CMC-BG) with strong polarization dependence embedded in a multimode waveguide. Through a sophisticated grating design incorporating tailored chirping and apodization profiles to match modal properties, we have achieved, by simulation, a compact device footprint of 34.72 × 1.22 µm2 and an ultra-wide bandwidth of 346 nm with PER > 40 dB and an insertion loss (IL) < 1 dB, a 5-fold increase over our previous design. Particularly notable is the polarizer's ability to suppress reflection to <-15 dB across an extended bandwidth exceeding 450 nm. Experimental measurements confirm the excellent performance of the fabricated TM polarizer, with IL < 1.2 dB (0.5 dB) and PER > 30 dB over a bandwidth of 336 nm (268 nm).

Optimization and analysis of apodized fiber Bragg grating properties for quasi-distributed sensing

An apodized fiber Bragg grating (FBG) is introduced with a proposed apodization function for the effective quasi-distributed sensing estimation of the temperature and the strain. FBG features such as reflectivity, side lobes, and bandwidth have been optimized for the designed apodized grating to upgrade the effectiveness of FBG for properly measuring the variations in the Bragg wavelength. Based upon the simulation, a comparative analytical study on FBG properties with different apodization function profiles has been demonstrated to achieve the optimum profile with high reflectivity, narrow bandwidth and minimal range of side lobes for quasi-distributed sensing estimation of parameters. A strong linearity has been noted for the sensitivity of designed FBG with different apodization profiles for the temperature and the strain estimation subsequently. It has been reported that the obtained sensitivity of measurands for the FBG with proposed apodization profile are higher as compared with the other profiles. The wavelength division multiplexing (WDM) based quasi-distributed network of four optimized FBGs have been implemented for different apodization profiles to illustrate the impact of apodization to prevent overlapping between neighbouring FBG spectrums in the sensing network with spatial resolution of 2 nm. The maximal detectable temperature/strain sensitivity estimation of 153 °C/1439 μϵ have been obtained for the proposed apodized FBG along with minimal detectable temperature/strain ranges of −102 °C/−1345 μϵ in the quasi-distributed network. The achieved ranges with optimum resolution can be implemented effectively in quasi-distributed based sensing application for condition observation of civil infrastructures in any complex circumstances.

Optical fiber fronthaul segment in open radio access 5G networks: enhanced performance utilizing AFBG

In open Radio access network (oRAN) 5G networks, a fronthaul segment between virtual Distribution unit (vDU) edge cloud and other external Remote radio units (RRUs) sites is very important to transmit high data rate to achieve 5G requirements. Performance parameters such as Quality factor (Q-factor) and Bit error rate (BER) of an optical fiber fronthaul segment are the main important parameters to enhance latency and bit rate. An Apodized fiber bragg grating (AFBG) is utilized with different apodization profiles to optimize linewidth of an optical signal leading to enhance performance. The designed AFBG is used with a data rate of 25 Gb/s optical Ethernet module at the vDU edge cloud to optimize the linewidth of the optical signal between the vDU and the RRU in the fronthaul optical fiber segment. The AFBG is investigated at different connections pre, post, and symmetrical connections, where different apodization functions are used in the AFBG design. It is found that the best results for the maximum Q-factor and minimum BER are 9.2 and 2.72e− 21, respectively, obtained from uniform function in the symmetrical connection. Finally, the proposed model linewidth is 0.408 nm, which is better than 1.56 nm that is evaluated by related work.

Sub-kHz-linewidth laser generation by self-injection locked distributed feedback fiber laser

We presented an integrated all-fiber sub-kHz-linewidth distributed feedback fiber laser (DFB-FL) assisted by self- injection locking. A π phase-shifted fiber Bragg grating (π-FBG) was employed to achieve single longitudinal mode operation, which was UV-inscribed in Er/Yb co-doped fiber with super-Gaussian apodization profile. The experiment results have shown that the apodized π-FBG based laser cavity has got the higher slope efficiency and the narrower linewidth, compared with the normal grating, which are consistent with the simulation one very well. To further compress the linewidth, we have employed the self-injection locking mechanism, which the linewidth of output laser could reach to around 350 Hz with 10-m delay fiber. After integrated encapsulation, the relative intensity noise (RIN) peak of the DFB-FL was reduced to below −125 dB/Hz. The beat frequency fluctuation was less than 15 MHz within 1000 s and power fluctuation was less than 1% in 7 h.

Accuracy of scanning interrogation of FBG sensors

Abstract Progress in the fibre optic technology spurred the development of optical fibre-based sensors. A successful fibre optics-based element for sensorial systems is fibre Bragg grating. It allows to read the measured value by the spectral shift of light, which is the variable with a good potential for accurate measurement. Various types of the interrogation of the light spectrum reflected from the measuring grating are discussed in this article and the work concentrates on the spectrum scanning that gives the good chances to diminish measuring errors by a proper choice of parameters. The focal point is the accuracy of establishing the correct value of the maximum reflectance wavelength. The significance of the noise in the photoelectric scanning signal for the measuring accuracy is analysed and the crucial factor is found in the minimizing the reflectance spectral width of the measuring grating and using the spectrum of the scanning light with the same or smaller spectral width. An important aspect is the relation of the maximal reflectance and the wavelength width of the measuring grating reflectance spectrum. The article also shows the potentiality of the apodized gratings for improving the scanning interrogation. A specific index apodization profile was analysed and its advantages are discussed.

Femtosecond laser inscribed fiber Bragg gratings based on precise spatial apodization.

Plane-by-plane femtosecond laser fabricated apodized fiber Bragg gratings (FBG) are demonstrated for the first time, to the best of our knowledge. The method reported in this work provides a fully customizable and controlled inscription that can realize any desired apodized profile. By using this flexibility, we experimentally demonstrate four different apodization profiles (Gaussian, Hamming, New, Nuttall). These profiles were chosen to evaluate their performance with regard to the sidelobe suppression ratio (SLSR). Usually, a higher reflectivity of a grating fabricated with a femtosecond laser will result in a greater difficulty to achieve a controlled apodization profile due to the nature of the material modification. Therefore, the goal of this work is to fabricate high-reflectivity FBGs without sacrificing the SLSR and provide a direct comparison with apodized low-reflectivity FBGs. In our weak apodized FBGs, we also consider the background noise introduced during the femtosecond (fs)-laser inscription process which is fundamental when multiplexing FBGs within a narrow wavelength window.

Sensitivity analysis of designed apodized fiber Bragg grating sensor using artificial neural network and tree-based models

An apodized FBG is designed to eliminate the side lobes from the reflectivity spectrum for the sensitivity measurement of physical parameters including temperature, strain, and pressure. The impact of the proposed apodization function is discussed with several other apodization profiles for the elimination of side lobes from the FBG spectrum. The sensitivity of the measurand is measured by analyzing the changes that occur in the Bragg wavelength, which is sensitive to the physical parameters. A strong linear response has been observed for the sensitivity of temperature, strain, and pressure for the designed apodized FBG. Machine Learning (ML) methods such as Artificial Neural Network (ANN) and Tree-based models are implemented for the predictive analysis of physical parameters and their respective Bragg wavelength shifts for the reliability of the designed apodized FBG sensor to achieve improved sensing outcomes, particularly to work in a hazardous environment for understanding any adverse scenario of a measurand. Decision Tree (DT) and Random Forest (RF) are used as Tree-based models in this work. Different statistical measures are introduced to evaluate the predictive analysis performance of the ML models. It has been observed that the predictive analysis performance of the ANN is better compared to the Tree-based models.

Single-mode Lamb wave excitation at high-frequency-thickness products using a conventional linear array transducer

Lamb wave excitation at high-frequency-thickness products offers a potential solution for high-resolution guided wave testing. The method is attractive for crack imaging and corrosion mapping, especially in hidden locations where direct access is limited. However, multiple modes may propagate, complicating signal interpretation, which is undesirable. In this work, a systematic approach is presented, in an effort to determine the influence of the key parameters related to single higher order Lamb wave mode excitation using a conventional linear array transducer. Specifically, a linear time delay law is used to enhance the targeted mode, while the array's length, pitch and apodisation profile remain to be optimally selected. First, an analytical solution is derived based on modal analysis. This provides a natural decomposition of the amplitude of a guided wave mode into the product of the response of a single element and the excitation spectrum. Then, a key observation is made, associating the excitation spectrum to the directivity function for bulk wave phased array steering. This allows the application of well-established phased array analysis tools to guided wave phased array excitation. In light of this fact, minimisation of the spectrum’s bandwidth, elimination of the grating lobes and derivation of an apodisation profile are performed, to enhance the purity of the targeted mode. Finally, experiments conducted on an aluminium plate verify the above theoretical results. The Full Matrix is acquired, and all signals are reconstructed synthetically.

Performance Analysis of a Linear Gaussian- and tanh-Apodized FBG and Dispersion Compensating Fiber Design for Chromatic Dispersion Compensation in Long-haul Optical Communication Networks

This paper investigates a novel compensation technique of dispersion effect mitigation using a combination of three- and four-stage-apodized fiber Bragg gratings (FBG) and dispersion compensating fiber (DCF) designs. Two designs using three-stage and four-stage FBG and DCF in combination have been proposed and compared for their performance in mitigating chromatic dispersion effects at 100 km SMF. The performance of each design has been evaluated using Q-factor results using linear Gaussian- and tanh-apodized fiber Bragg gratings. Each profile manifested different Q-factor results over a range of 5 dBm, 7.5 dBm, and 10 dBm of CW laser power over FBG grating lengths from 4 mm to 8 mm. The results obtained using the three-stage and four-stage FBG and DCF designs showed that an apodization profile using a tanh function can be used successfully with FBG lengths from 4 mm to 8 mm, regardless of the CW launched power. In contrast, the results using a Gaussian apodization profile for three- and four-stage FBG and DCF designs are applicable to FBG lengths from 5 mm to 8 mm. Designs using three-stage FBG and DCF generated higher Q-factor results than designs using only four-stage FBG and DCF, regardless of the launched power. The highest Q-factor of 18.58 was obtained for three-stage tanh-apodized FBG and DCF used in combination for an FBG length of 6 mm. The highest result obtained for a three-stage Gaussian-apodized FBG and DCF design was a Q factor of 17.13 using an FBG length of 8 mm. The proposed method was also compared to current similar works and can be successfully implemented in long-haul optical communication.

Spectral response of apodized fiber Bragg gratings as strain and temperature sensor

In this paper, the spectral response of uniform and apodized (Gaussian, hyperbolic tangent, apod1, sine, and raised sine) FBGs is analyzed for sensing applications. The reflectivity at Bragg wavelength as well as for sidelobes was assessed as a function of grating length and apodization profiles. The FBG strain and temperature sensors were simulated and a linear response between applied strain or temperature and the wavelength shift is observed. The results indicate that the sensitivity of the sensor is found to be affected both by the grating length and apodization type. The typical strain and thermal sensitivity values are 1.223 pm/[Formula: see text] and 13.60 pm/[Formula: see text]C, respectively. The results suggest that Gaussian, sine, and raised sine profiles have lower sidelobe strength and reliable sensitivities. The key finding from this study specifies that the ideal grating length must be preferably between 5 and 10 mm for a good sensing behavior.

Femtosecond laser direct-writing of high quality first-order Bragg gratings with arbitrary complex apodization by phase modulation.

Femtosecond laser direct-writing is an attractive technique to fabricate fiber Bragg gratings and to achieve through-the-coating inscription. In this article, we report the direct inscription of high-quality first-order gratings in optical fiber, without the use of an index-matching medium. A new alignment technique based on the inscription of weak probe gratings is used to track the relative position between the focal spot and fiber core. A simple and flexible method to precisely control the position of each grating plane is also presented. With this method, periodic phase modulation of grating structures is achieved and used to inscribe arbitrary apodization and phase profiles. It is shown that a burst of multiple laser pulses used to inscribe each grating plane leads to a significant increase in the grating strength, while maintaining low insertion loss, critical for many applications.

Optimum Design Sapphire-Fiber Bragg Grating for High-Temperature Sensing

Fiber Bragg grating (FBG) sensors have limitations in measuring high and extreme temperatures because in general FBG sensors are made of silica fiber, which at high temperatures can interfere with the mechanical performance of the materials. In this paper, we propose an S-FBG (Sapphire Fiber Bragg Grating) sensor which is resistant to extreme environmental influences and high temperatures. By developing S-FBG to measure high temperatures, it is found that S-FBG has high sensitivity, every 10C change is obtained and the Bragg wavelength shifts as far as 30.24 nm, this result is greatly influenced by the thermo-optic coefficient, and the coefficient of expansion-thermal. The design also evaluates the Gaussian apodization profile to improve sensor accuracy in monitoring temperature.

A reduced power budget and enhanced performance in a wdm system: a new fbg apodization function

The apodization profiles improve chirped fiber Bragg grating (CFBG) capability to compensate dispersion which enhances the performance of the wavelength division multiplexing (WDM) systems. In this paper, a new apodization profile is proposed to enhance performance and reduces the power budget for a WDM system. The newly apodization profile is implemented to a CFBG as a post compensator in a WDM system for 100 km. It is found the proposed profile enhances the performance and reduces the transmitted power at BER of 10−9. The evaluation of the proposed profile is compared with a previously published work. It is found the proposed profile enhances the Q-factor by 1.08%, decreases BER by 52.8%, and reduces the power budget by 4% than the related work.

Fiber Bragg grating employing novel apodization profile: performance optimization for quasi-distributed sensing applications

Fiber Bragg grating employing novel apodization profile: performance optimization for quasi-distributed sensing applications

Optimal parameters for fiber Bragg gratings for sensing applications: a spectral study

The spectral characteristics viz. reflectivity, bandwidth, and sidelobes’ intensity for uniform and apodized (Gaussian, hyperbolic tangent, apod1, sine, and raised sine) fiber Bragg gratings (FBGs) were studied as a function of grating length and index modulation. The optimal grating length and index modulation to obtain maximum reflectivity and minimum sidelobes were determined, as needed for sensing applications. The impact of various apodization profiles on the spectral response has also been assessed. The results indicate that out of the apodization profiles considered for the study, sine, Gaussian, and raised sine profiles offer the desired output.Article highlightsThe reflectivity (of main peak) and sidelobes’ intensity increase with grating length and index modulation.The bandwidth decreases with grating length and increases with index modulation.The ideal grating length and index modulation were found to be 5 mm and 0.0008 respectively to obtain maximum reflectivity and minimum intensity for sidelobes.Sine, Gaussian, and raised sine profiles are the best suitable apodization profiles among those considered.

Comparative analysis of silica fiber Bragg grating and chalcogenide fiber Bragg grating

Sensing performance of silica fiber Bragg grating (FBG) and chalcogenide FBG are compared in presence of apodization functions. The Bessel and Gaussian apodization functions are chosen in this analysis. These fibers are compared on the basis of their sensitivity, detection accuracy, quality parameter and sidelobe suppression ability. Reflectivity equations established from couple modes equations by matching fields at interfaces. For considered apodization profiles, it is observed that the sidelobe suppression ability and sensitivities are nearly same in both chalcogenide and silica fiber Bragg grating. Chalcogenide-based fiber Bragg grating with Bessel apodization function have narrower full width half maxima (FWHM) value that’s why its detection accuracy and quality parameter is maximum in comparison of other considered cases. Hence for sensing application purpose chalcogenide-based fiber Bragg grating in presence of Bessel apodization function is appropriate.

Spectral-Distortionless, Flat-Top, Drop-Filter Based on Complementarily-Misaligned Multimode-Waveguide Bragg Gratings

An integrated-optics, flat-top drop-filter with low excess loss, and high sidelobe suppression ratio (SLSR), is demonstrated. The filter is based on apodized multimode waveguide Bragg grating (AMWBG) developed on a silicon-on-insulator (SOI) platform. Antisymmetric periodic refractive-index perturbations are imposed on both sides of the waveguide to couple light contra-directionally between the fundamental (TE0) and the first-order transverse electric modes (TE1). To achieve a Gaussian apodization profile, the lateral-misalignment between the two sides of the waveguide is modulated by adjusting the positions of both sides of the grating features in a complementary manner. In this way, the undesired phase errors, which exist in conventional misalignment-modulated Bragg gratings and distort the grating responses, can be eliminated. Experimental results show that our AMWBG has a high-quality flat-top response, a high SLSR of >21 dB, a 3-dB bandwidth of 8 nm, a low excess loss of 0.24 dB, and a high roll-off rate at the filter edge. We also demonstrate a dual-stage filter that has a flat-top, drop-port response with a 3-dB bandwidth > 7.5 nm, a SLSR >40 dB, and an excess loss<0.5 dB.

Phase-shifted PT -symmetric periodic structures

We report the spectral features of a phase-shifted parity and time ($\mathcal{PT}$)-symmetric fiber Bragg grating (PPTFBG) and demonstrate its functionality as a demultiplexer in the unbroken $\mathcal{PT}$-symmetric regime. The length of the proposed system is of the order of millimeters and the lasing spectra in the broken $\mathcal{PT}$-symmetric regime can be easily tuned in terms of intensity, bandwidth and wavelength by varying the magnitude of the phase shift in the middle of the structure. Surprisingly, the multi-modal lasing spectra are suppressed by virtue of judiciously selected phase and the gain-loss value. Also, it is possible to obtain sidelobe-less spectra in the broken $\mathcal{PT}$-symmetric regime, without a need for an apodization profile, which is a traditional way to tame the unwanted sidelobes. The system is found to show narrow band single-mode lasing behavior for a wide range of phase shift values for given values of gain and loss. Moreover, we report the intensity tunable reflection and transmission characteristics in the unbroken regime via variation in gain and loss. At the exceptional point, the system shows unidirectional wave transport phenomenon independent of the presence of phase shift in the middle of the grating. For the right light incidence direction, the system exhibits zero reflection wavelengths within the stopband at the exceptional point. We also investigate the role of multiple phase shifts placed at fixed locations along the length of the FBG and the variations in the spectra when the phase shift and gain-loss values are tuned. In the broken $\mathcal{PT}$-symmetric regime, the presence of multiple phase shifts aids in controlling the number of reflectivity peaks besides controlling their magnitude.

Tailoring inhomogeneous PT -symmetric fiber-Bragg-grating spectra

The unique spectral behavior exhibited by a class of non-uniform Bragg periodic structures, namely chirped and apodized fiber Bragg gratings (FBGs) influenced by parity and time reversal ($\mathcal{PT}$) symmetry, is presented. The interplay between the $\mathcal{PT}$-symmetry and nonuniformities brings exceptional functionalities in the broken $\mathcal{PT}$-symmetric phase such as wavelength selective amplification and single-mode lasing for a wide range of variations in gain-loss. We observe that the device is no more passive and it undergoes a series of transitions from asymmetric reflection to unidirectional invisibility and multi-mode amplification as a consequence of variation in the imaginary part of the strength of modulation in different apodization profiles, namely Gaussian and raised cosine, at the given value of chirping. The chirping affords bandwidth control as well as control over the magnitude of the reflected (transmitted) light. Likewise, apodization offers additional functionality in the form of suppression of uncontrolled lasing behavior in the broken $\mathcal{PT}$-symmetric regime besides moderating the reflected signals outside the band edges of the spectra.