This PDF file contains the editorial “Reviewing Papers” for OE Vol. 43 Issue 05

We show a technology to overcome a known bottleneck in the communication between processor and memory or, especially in multiprocessor systems, among the processors themselves. The solution is communication with the help of fiber bundles with defined pitch. The disadvantages of microstructures based on poly-methyl-meth-acrylate are described. Attractive alternatives based on silicon for precisely structured elements are explained. A practical example of a fiber array is visualized.

This paper presents a fast K-dimensional tree-based search method to speed up the encoding process for vector quantization. The method is especially designed for very large codebooks and is based on a local search rather than on a global search including the whole feature space. The relations between the proposed method and several existing fast algorithms are discussed. Simulation results demonstrate that with little preprocessing and memory cost, the encoding time of the new algorithm has been reduced significantly while encoding quality remains the same with respect to other existing fast algorithms

Optical and electrical characteristics of InGaSb p-n photodetectors are presented at different temperatures. The device structures were grown on GaSb substrates using organic metal vapor phase epitaxy. Spectral calibration indicates peak responsivity around 2 µm, equivalent to 58% quantum efficiency, with 2.3-µm cutoff at room temperature. Reducing the device temperature increases the responsivity and shifts the cutoff wavelength to a shorter value. Current voltage measurements at different temperatures indicate that tunneling is the primary leakage current mechanism. Assuming Johnson limited performance, detectivity calculations resulted in 4×1010 cm Hz1/2/W indicating that InGaSb is a superior material for 2-µm detection applications.

This PDF file contains the editorial “Advances in Optical Components and Subsystems for Wavelength-Division Multiplexing Communications” for OE Vol. 43 Issue 05

Low-cost vertical cavity surface emitting lasers (VCSELs) may allow the development of optical interconnections and broadband optical networks. Designs suitable for micro-optics and "jisso" (a Japanese term that includes alignment, assembly, mounting, and packaging) technologies are required for low-cost and small interconnection modules. Optical interconnections based on VCSEL arrays, including both space and wavelength division multiplexing technologies, are described.

We report a strategy for the industrialization of photonic-crystal-based optical devices, starting with a review of the requirements for passive photonic crystal components. Although they have remarkable properties, such components are difficult to couple to/from optical fibers and have a relatively large propagation loss. We present an approach for meeting the requirements by using heterostructured photonic crystals fabricated by the autocloning technology. A low-loss, simple structured waveguide and sophisticated functional blocks (such as a resonator) can be integrated by a simple process. We first prepare a substrate patterned and corrugated by electron beam lithography and dry etching. Ta₂O₅/SiO₂ multilayers are repeatedly deposited on the substrate, and no other process is needed to complete the chip. After dicing and polishing, the waveguide can be butt-jointed with an optical fiber, and the loss is estimated as 0.1 dB/mm. A resonator with Q =11,700 is also demonstrated. Recent progress and the future outlooks for the technology are also discussed.

We present model calculations of gain at 1550 nm for Er-Yb double-clad amplifiers as a function of pump power, signal input power, fiber length, and temperature. The calculated gain is ~20 dB for a 6-W pump for a 20-mW input signal at 25 C. More than 10 W of amplified power can be obtained using an 8-m-long fiber with ~25 W of pump power. The gain decreases with increasing input signal. We show that a high-power Er-Yb codoped double-clad fiber amplifier also exhibits high gain for Yb transition near 1060 nm. This is not unexpected, since the input pump causes population inversion in Yb also.

Polarization mode dispersion (PMD) compensation is one of the challenges to deployment of ultra-high-bit-rate systems. PMD impact on system performance and outage probability is explained. Then both optical and electrical PMD compensation techniques are reviewed and important design considerations are discussed. Issues regarding PMD-induced distortion detection, feedback and control algorithms in the presence of local minima, and interaction between PMD and other degrading effects [such as chromatic dispersion, polarization-dependent loss (PDL), and fiber nonlinearities] are discussed.

Acousto-optic devices play an important role in the field of optical information and communication. For a few years, we have worked on acousto-optic deflection in the Bragg regime at 1.55-μm wavelength, by controlling the rf signal applied to the piezoelectric transducer. Recently, still using the acousto-optic interaction, we have taken an interest in the 2×2 switching function. We present an acousto-optic switch architecture based on phased-array transducers. In the same crystal, we superimpose two diffraction gratings created by two rf signals. Generating a phase shift on a rf signal applied between successive elementary transducers allows us to tilt one grating so as to interconnect inputs to outputs. To predict and study some physical phenomena generated in this switch architecture, we present some characterizations made on a monotransducer cell. We point out the optical cross talk between output paths (influence of the intermodulation products due to the superimposition of two rf signals on the same transducer).

Morphology-dependent resonances of microspheres can provide the necessary optical feedback for applications in spectroscopy, laser science, and optical communications. The elastic scattering of focused light from dielectric microspheres is understood by the localization principle and the generalized Lorenz-Mie theory. We excited the morphology-dependent resonances of glass microspheres by a tunable distributed-feedback laser and detected the elastically scattered signal. Efficient coupling to morphology-dependent resonances is achieved using an optical fiber half coupler. Resonance peaks in the elastic scattering spectra and associated dips in the transmission spectra are observed experimentally. Simulation results of elastic scattering spectra of glass microspheres in the C-band are presented.

We present a scheme for recovering a 10-GHz clock from a 40- and 80-Gb/s time-division-multiplexed (TDM) return-to-zero (RZ) data stream. The proposed clock recovery is successfully demonstrated using an electrical phase-locked loop (PLL). The jitter of the recovered clock is estimated to be around 50 fs. The key part in the proposed clock recovery circuit is a LiNbO₃ Mach-Zehnder modulator, which is shown to be highly effective in optical to electrical down conversion.

We propose a new approach for the design of phased array (PHASAR) demultiplexers. This approach is based on cascading multimode interference (MMI) PHASAR structures with different frequency responses to obtain an optimized performance. We show that with this approach, we can improve the uniformity of the overall demultiplexer (DMUX), especially when a large number of output ports is required. The design of a 64-output DMUX with only 0.2-dB uniformity is thus demonstrated compared to a 7-dB uniformity of a two-cascaded eight-channel demultiplexer, also shown in this work. This approach allows the designer to optimize the structure design, considering both the insertion loss and the uniformity.

Fiber optic data communication networks face unique requirements for applications such as disaster recovery of multiterabyte and petabyte databases. We present experimental results from several enterprise computing testbeds that evaluate dense wavelength-division multiplexing (DWDM) performance in metropolitan area datacom networks. Many networking issues can be addressed through the components used within the DWDM platform; we discuss how current and emerging requirements can be addressed by new packages for 10-Gbit/s links, clock recovery circuits, and wavelength lockers. A comparison of three DWDM platforms evaluated in these testbeds is given, including features such as network bandwidth, scalability, performance over distance, and ease of management. We also report performance of datacom protocols such as enterprise system connection (ESCON), fiber connection (FICON), and FICON Bridge, when extended beyond industry standard distances using DWDM.

Multistage interconnection networks (MINs) are popular in switching and communication applications and have been used in telecommunication and parallel computing systems for many years. Crosstalk a major problem introduced by an optical MIN, is caused by coupling two signals within a switching element. We focus on an efficient solution to avoiding crosstalk by routing traffic through an N×N optical network to avoid coupling two signals within each switching element using wavelength-division multiplexing (WDM) and a time-division approach. Under the constraint of avoiding crosstalk, the interest is on realizing a permutation that uses the minimum number of passes for routing. This routing problem is an NP-hard problem. Many heuristic algorithms are already designed by researchers to perform this routing such as a sequential algorithm, a degree-descending algorithm, etc. The genetic algorithm is used successfully to improve the performance over the heuristic algorithms. The drawback of the genetic algorithm is its long running times. We use the simulated annealing algorithm to improve the performance of solving the problem and optimizing the result. In addition, a wavelength lower bound estimate on the minimum number of passes required is calculated and compared to the results obtained using heuristic, genetic, and simulated annealing algorithms. Many cases are tested and the results are compared to the results of other algorithms to show the advantages of simulated annealing algorithm.

Wavelength-division multiplexing (WDM) is a photonic technology that couples many optical channels (wavelengths) in a single fiber. Each wavelength represents a separate optical channel at a fixed bit rate. Thus, to increase the aggregate transportable bandwidth per fiber either the channel density must be increased or the bit rate per channel, or both. As a result, the signal quality for a given fiber length suffers due to the nonlinearity of the medium. Therefore, engineering and provisioning of WDM links to meet signal quality objectives is a complex task. Despite advancements in bit rate increase, not all applications require such high rates, as many applications require granularity finer than optical carrier n (OC-n) to increase efficiency. As a consequence, bandwidth efficiency, fine bit rate granularity, and diverse traffic type will be necessary characteristics of the next-generation WDM networks. We propose a WDM transmission method by which a high-bandwidth-capacity wavelength bus supports many clients, more than the number of wavelengths, each at different rate and of different traffic type. In addition, the method uses low bit rates and cost-effective transceivers. We examine strategies to minimize linear and nonlinear impairments over a given span, strategies to enhance the quality of signal and increase the fiber span, and strategies to reduce cost of transported bandwidth-kilometer in a variety of dense WDM (DWDM) applications. We also examine the applicability of the method in various network topologies, and aspects of network protection and restoration.

We study traffic grooming in synchronous optical network/wavelength-division multiplexing (SONET/WDM) bidirectional line-switched ring (BLSR) networks under the uniform all-to-all traffic model with an objective to reduce total network costs (wavelength and electronic multiplexing costs), in particular, to minimize the number of add-drop multiplexers (ADMs) while using the optimal number of wavelengths. We derive a new tighter lower bound for the number of wavelengths when the number of nodes is a multiple of 4. We show that this lower bound is achievable. We then derive new, more general, and tighter lower bounds for the number of ADMs subject to the constraint that the optimal number of wavelengths is used, and propose heuristic algorithms (the circle construction algorithm and the circle grooming algorithm) that try to minimize the number of ADMs while using the optimal number of wavelengths in BLSR networks. Both the bounds and algorithms are applicable to any value of r and for different wavelength granularity g. All previous ADM lower bounds except perhaps that in (Gerstel et al., 1999) were derived under the assumption that the magnitude of the traffic streams (r) is one unit (r = 1) with respect to the wavelength capacity granularity g. Performance evaluation shows that wherever applicable, our lower bounds are at least as good as existing bounds and are much tighter than existing ones in many cases. Our proposed heuristic grooming algorithms perform very well with traffic streams of larger magnitude. The resulting number of ADMs required is very close to the corresponding lower bounds derived in this paper.

All-optical digital sequential logic reduces the expense and bit rate constraints associated with optical-electronic-optical (OEO) conversion in telecommunications. Two semiconductor optical amplifiers (SOAs) are directly coupled by connecting the output of each to the input of the other. This new configuration provides positive feedback to create a logical NOT or inverse for complete logic. An inverted optical threshold improves the quality of bit modulation. At the same time, frequency shifting allows improvement in the quality of the optical carrier. The SOA pair is analyzed in closed form to show that the positive feedback provides an output-input slope steeper than –1, allowing connection of two pairs to form a flip-flop. Two SOA pairs are cross-connected by connecting the output of each pair to the input of the other, to construct a novel R-S flip-flop. Closed form analysis shows, for the first time, that there is an unstable state and two stable states forming bistability for this configuration. Simulation demonstrates setting and resetting the reset-set (R-S) flip-flop at more than 5 Gbps. Agreement between closed form analysis and numerical simulation provides verification of the validity of the results obtained.

The ever-increasing demand for communication bandwidth and system interconnectivity has been a motivating factor behind the integration of optoelectronics devices and conventional data processing circuitry. Based on the smart-pixel architectures first developed in the last decade, the architecture presented here monolithically integrates optical sensors with silicon CMOS-based circuitry to produce a generically programmable smart-pixel array. Two generations of the architecture are described and compared. We have proposed a reconfigurable photonic information-processing chip based on photonic VLSI device technology. Integrating detectors into a SIMD array removes the bottleneck associated with fetching slices of data. By fabricating the detectors along with logic circuits in a bulk CMOS process, the cost is minimized. The modular nature of the array organization facilitates replication of the configurable architecture for smart-pixel research (CASPR) concept into large arrays without a significant increase in design overhead. Thus, the CASPR architecture can provide the maximum flexibility associated with a reconfigurable smart-pixel array. Finally, we implement two design iterations of the CASPR architecture and show how the architecture might be used in a page-oriented optical data processing application.

Low-loss, no moving parts, free-space wavelength-multiplexed optical scanner (W-MOS) modules for transmit-receive lasercom systems are proposed and experimentally demonstrated. The proposed scanners are realized using volume Bragg gratings stored in dichromated gelatin (DCG) coupled with high-speed wavelength selection, such as by a fast tunable laser. The potential speed of these scanners is in the gigahertz range using present-day state of the art nanosecond tuning speed lasers. A 940-lines/mm volume Bragg grating stored in dichromated gelatin is used to demonstrate the scanners. Angular dispersion and diffraction efficiency of the volume Bragg grating used for demonstration are studied, versus wavelength and angle of incidence to determine the free-space W-MOS angular scan and insertion loss, respectively. Experimental results show that a tunable bandwidth of 80 nm, centered at 1560 nm, delivers an angular scan of 6.25 deg. The study also indicates that an in-line scanner design realized using two similar Bragg gratings in DCG delivers a 13.42-deg angular scan, which is more than double the angular scan available from the free-space W-MOS using a single volume Bragg grating. Furthermore, a free-space W-MOS using a single Dickson grating features low (<0.15 dB) polarization-dependent loss and an average scanner insertion loss of only 0.4 dB over the 70-nm wavelength band around 1550 nm.

The formation of a doughnut-shaped laser beam is presented. To generate the beam, we use an optically addressed parallel-aligned nematic liquid-crystal phase spatial light modulator (PAL-SLM), and observe the shape of the focused beam. By using a compensating technique for wave aberration, the beam has a symmetric doughnut shape with a hole size of 1 μmφ on the focal plane. The experimental result shows that the generated beam can be expected to be applicable to super-resolving microscopy based on the fluorescence depletion process.

The systematic analysis of the relationship between temperature sensitivity of long-period fiber gratings and the doping concentrations of GeO₂ and B2O3 in the core and cladding regions, depending on their structure of the refractive index profile, is investigated. Based on this analysis, the temperature sensitivities of fiber gratings are suppressed and enhanced to 0.002 and –0.44 nm/°C, respectively. We also discuss the versatile applications of long-period fiber gratings to gain-flattening filters of optical amplifiers and strain/temperature sensors, corresponding to their temperature sensitivity. The theoretical and experimental results are very useful for the design of optical communication devices and mechanical sensors based on long-period fiber gratings.

A car window antitrapping optical fiber system based on a speckle pattern from a multimode fiber installed in the window frame is proposed and demonstrated. The speckle pattern change is measured by an array of photodiodes whose output signal data are processed by computer through an analog-to-digital (A/D) converter to control the window motor. Experimental results show that the proposed system with multiple photodiodes, along with an appropriate processing algorithm, runs well. The processing algorithm is also presented.

A novel method using a charge-coupled device (CCD) camera is developed to measure 3-D rigid-body displacement of an object. This method combines fringe projection and digital image correlation (DIC) methods into one optical system. In this method, sinusoidal fringes are projected on an object using a liquid crystal display (LCD) fringe projector. Images of the object's surface are captured by a CCD camera and stored for further processing. With the aid of the Fourier transform, the fringes in the images are removed while the background intensity variation is preserved. DIC is subsequently used to obtain in-plane displacement using the fringe-free images. The original images are also processed by fast Fourier transform (FFT) to obtain information on the shape of the object. Based on the in-plane displacement obtained by DIC, the reference and measured profiles are compared to obtain out-of-plane displacement. Experiments are conducted on a small coin, and the results demonstrate that both in-plane and out-of-plane displacements can be accurately measured using the proposed method.

A method is described to evaluate plane-wave volume hologram fringe inclination and spacing, as well as the recording material average refraction index. This evaluation allows us to estimate changes in the hologram parameters due to recording material shrinkage. The method is based on measurements of the incident and diffracted beam Bragg angles for two different wavelengths. The conditions for minimizing measurement errors are obtained. The experimental data are in good agreement with the calculated data.

A digital twin Mach-Zehnder holographic interferometer is presented and applied to the investigation of fracture mechanisms in resin concrete submitted to a three-point flexural loading. The double interferometric setup allows spatial multiplexing for a simultaneous double sensitivity measurement. The component determination is obtained from two different illumination directions giving two sensitivity vectors. The numerical reconstruction of the object is performed following the discrete Fresnel transform. We investigate the image formation by merging analogic and discrete Fourier calculations in a smooth reference plane wave scheme. We examine the complex relation between the original object and the reconstructed one and the object localization is derived. Experimental results are presented and exploited to obtain the evolution of the crack tip propagation during the test.

We propose a new method of blind deconvolution (BD) suitable for the recovery of truncated blurred images. In practical cases we often encounter truncated images, but they are difficult to treat properly with conventional BD algorithms, since nontruncated blurred images are assumed. The proposed method is based on simulated annealing (SA), to which we introduce an adaptive masking process. After a truncated segment is cut out from the blurred image, the sharp edges surrounding the segment are removed by multiplying the initial window, whose shape is determined with the initial estimate of the point spread function (PSF). A more appropriate estimate for the PSF is obtained from the masked segment based on the SA algorithm, and according to the new estimate of the PSF, the shape of the window is varied. These procedures are iterated until we finally obtain the best possible estimate of the PSF. In the last step, the entire blurred image is deconvolved with the final estimate of the PSF to obtain the recovered image. The effectiveness of the proposed method is confirmed by computer simulation.

We propose rapid hybrid interpolation methods that employ more than one interpolation algorithm, and choose the most appropriate interpolation algorithm that provides high-quality images with a minimum number of operations. Although a complex interpolation algorithm generally outperforms a simple interpolation algorithm, the differences are negligible for most pixels, with major differences occurring around edges. Thus, in the proposed algorithm, we first apply a test to predict which interpolation is most appropriate for a given pixel in terms of complexity and performance. Then, a simple interpolation algorithm is used for pixels for which the simple interpolation algorithm provides acceptable performances, and a complex interpolation algorithm is used for pixels for which the complex interpolation algorithm significantly outperforms the simple interpolation algorithm. Consequently, it is possible to obtain high-quality images without significantly increasing the number of operations.

Coherent IR fiber optic bundles for use in IR imaging from 2 to 12 μm are fabricated from rigid hollow-glass waveguide arrays. The bore of each hollow glass tube in the bundle is coated with thin films of metallic Ag followed by AgI for enhanced reflectivity. The coating of the rigid bundle is done using liquid phase chemistry techniques applied to all tubes simultaneously. The hollow-glass arrays are composed of up to 900 individual tubes with bore sizes as small as 50 μm. Several rigid hollow-core arrays are used to transmit an IR image of a small loop of hot wire and a sample of tissue heated by a CO₂ laser.

Polarization interference filters (PIFs) could substitute for thin films in three-panel projection systems for color separation/combination. Conventional methods of designing PIFs used inverse propagation of optical networks making the design procedure very complicated. The design of PIFs requires the determination of the thickness and orientation of the birefringent crystals. We advance a method of exploring all the possible thicknesses of the crystals, and the first-used genetic algorithm to find out the orientation of the crystals.

Phase shifting interferometry is combined with wavelet-based image processing techniques to extract precise phase information for applications of moiré interferometry. Specifically, a diffraction grating identical to the specimen grating is used to introduce the additional phase shifts needed to implement phase shifting moiré interferometry. The phase map is calculated with the four-step phase shifting algorithm with 90-deg relative shifts between adjacent frames. Subsequently, continuous wavelet transform processing is applied to the derived phase map to remove the noise and extract precise phase information. This precise phase map is then used to calculate the in-plane strain of the specimen under loading. A curve-smoothing algorithm based on a discrete wavelet transform is applied to the resultant in-plane strain map to eliminate the noise generated through the derivative calculation. To demonstrate the usefulness of this technique, uniform vertical loading is applied to a three-layer ball grid array (BGA) package and the strain is experimentally obtained. The reconstructed phase and in-plane strains are compared to the simulation results obtained using Ansys®.

An interferometric method to simultaneously test plane parallel plates both for planarity and parallelism of end faces is reported. Measurements are taken by insertion of the plates in a reference cavity. The optical configuration uses a standard programmable interferometer, with an external right-angle prism folding back the probe beam. The prism error is determined by cavity calibration, and is subtracted at data reduction. The measuring procedure is discussed in detail, and results of a laboratory demonstration are presented.

A laser with its wavelength stabilized to a Fabry-Perot etalon has many industrial applications. An ideal error signal for laser stabilization is the dispersion-like signal generated without wavelength modulation. We present a technique, by which the error signal is generated by the difference of two resonance peaks of a Fabry-Perot etalon. A laser beam is split into two beams, which pass through an etalon with a small optical path length difference to generate two partially overlapped resonance peaks. Subtracting one peak from the other yields a dispersion-like error signal. The zero crossing of the differential signal is insensitive to the angle drift of the etalon if the incident angles of the two laser beams are nearly equal but with opposite sign.

A novel design of densely dispersion-managed optical fiber transmission systems with decreasing average dispersion and decreasing local dispersion (dual-decreasing DDM) is proposed. The system is characterized by gradually decreasing not only local dispersion in each fiber segment, but also average dispersion in every dispersion compensation cell. When an optical pulse propagates along an optical fiber link, transmission loss weakens the Kerr effect to break the balance between dispersion and nonlinearity. To deal with the problem, dispersion parameters in a densely dispersion-managed system are made to vary gradually to counterbalance the nonlinearity. This novel approach has a number of advantages over ordinary densely dispersion-managed (DDM) soliton transmission systems, including allowing higher pulse power and reduction of optical pulse broadening, hence there is less interaction between neighboring pulses. Simulation results indicate that the dual-decreasing DDM has better propagation performance in comparison with the ordinary DDM in a high-speed optical communication system.

A challenging aspect of real-time infrared scene generation for the hardware-in-the-loop testing of infrared-guided weapon systems is the rendering of particle systems to represent gaseous and particulate volumes. In this work, a simulation tool is described for enabling the generation of real-time particle effects with high spatial and temporal fidelity by using many less primitives than traditional means. The tool can be applied for representing plumes and countermeasures, and enables the simulation of several key capabilities, including internal flow, turbulence, persistence, and structure, all capable of being varied dynamically as a function of power setting at the source. The principles of operation, software implementation, and general performance are discussed.

Optically realized continuously variable true-time delay of microwave pulses is demonstrated in a heterodyne system involving an acousto-optic deflector and a specially designed electro-optic modulator. The system realizes the time delay by frequency dependent phase shifting of the spectral components of the signal (FDPC technique). A new feature of the system is the real continuous variation of the time shift achieved with the continuous frequency dependent phase shift, performed by a specially designed electro-optic modulator. Continuously variable time delay values between ±200 ns are measured.

A novel self-adaptive volume reconstruction technique (SVRT) based on multiobjective optimization is proposed. Its reconstruction results for asymmetrical 3-D single-peak cosine emission coefficient fields are studied with the numerical simulation of a computer. The results show that this algorithm has faster convergences and higher reconstruction precision, and can reconstruct asymmetrical single-peak cosine emission coefficient fields perfectly with only two views. (The average error is 0.18% with no noise data.) In the experiment of argon-arc plasma diagnosis, the 3-D reconstructions of temperature and ionization coefficient fields are fulfilled with this algorithm combined with the spectrum relative-intensity method.

We describe the development of a tomographic system by employing low-cost optical sensors. This sensor selection aims at producing a low-cost solution in visualizing a low dense solid particle conveyor system. The final aim of this project is to achieve real-time monitoring of solid particles having low concentration flow when being conveyed in a vertical pneumatic conveyor. The developed tomography system consists of 32 pairs of light-emitting diodes (LEDs) and silicon PIN photodiodes. These sensors are used to monitor the emitted radiation for fluctuations caused by particles interfering with the beam when passing through it. Each sensor output depends on the position of flow regime within the sensing zone. The relationships between the particle distribution and light attenuation effects are investigated by reconstructing the cross-sectional image through computer programming. A data acquisition system is used to digitalize analog signals from the sensor system and is manipulated by a personal computer in real-time mode. The results obtained from this investigation show that low-cost optical sensors are suitable for monitoring low and medium concentration flowing materials.

We demonstrate the performance of a fiber Fabry-Perot (FFP) interferometer and show how the finesse of the interferometer can be adjusted by changing the operating wavelength. These sensors are extraordinarily sensitive and might have uses for high-density wavelength division multiplexing and high-resolution fiber sensor applications. We cannot resolve the transmission peaks using an optical spectrum analyzer and consequently use a tunable laser diode to study them. Fiber Bragg gratings act as the mirrors for the FFP interferometer and consist of an optical fiber with a periodic change in the index of refraction.1 The reflectivity spectrum depends on the grating profile. The period of the refractive index profile and therefore the reflectivity spectrum can be changed with stress and temperature. Consequently Bragg gratings are used as sensors for biosensing, civil engineering, and industrial monitoring.