Weak Gratings Research Articles

Strength properties of femtosecond-induced defects and weak Bragg gratings for distributed optical fiber sensors

One of the possible ways to increasing the sensitivity of distributed fiber sensors is to create point defects or weak Bragg gratings in a sensitive optical fiber serving as artificial reflectors. Such reflectors can be created by means of femtosecond laser radiation. However, after the inscribing such structures, a noticeable decrease in the mechanical reliability of the sensor fiber can be observed, which can be critical for the fabrication and operation of sensor cables. The paper reports the results of a study of the point defects and weak fiber Bragg gratings strength characteristics created for distributed sensor systems, depending on the parameters of the laser radiation used to inscribe them.

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.

Deformation Monitoring of Offshore Steel Pipe Piles Based on Weak Reflection Bragg Grating Technology

Deformation Monitoring of Offshore Steel Pipe Piles Based on Weak Reflection Bragg Grating Technology

Slow and stopped light in dynamic Moiré gratings

We investigate a theoretical model for a dynamic Moir\'e grating which is capable of producing slow and stopped light with improved performance when compared with a static Moir\'e grating. A Moir\'e grating superimposes two grating periods which creates a narrow slow light resonance between two band gaps. A Moir\'e grating can be made dynamic by varying its coupling strength in time. By increasing the coupling strength the reduction in group velocity in the slow light resonance can be improved by many orders of magnitude while still maintaining the wide bandwidth of the initial, weak grating. We show that for a pulse propagating through the grating this is a consequence of altering the pulse spectrum and therefore the grating can also perform bandwidth modulation. Finally we present a possible realization of the system via an electro-optic grating by applying a quasi-static electric field to a poled $\chi^{(2)}$ nonlinear medium.

Measurement of phase and group refractive indices and dispersion of thermo-optic and strain-optic coeffients of optical fibers using weak fiber Bragg gratings.

In this work we report on the measurement, with record accuracy, of the absolute modal effective refraction index (phase index) of single-mode optical fibers by using Bragg gratings. We also demonstrate a new method to measure the group index of the fibers from the grating's Bragg wavelength. We present as well the characterization of the thermo-optic and strain-optic coefficients as a function of the wavelength; the values we have obtained are the closest to those of pristine fiber measured with gratings technology so far. The phase index is measured with a set of gratings in the wavelength ranges from 1509 to 1563 nm, and the group index is obtained from the wavelength dependence of the phase index. Very weak gratings with reflectivity down to 10-3 have been used in order to minimize the perturbation of the pristine fiber. Results are presented at a temperature of 22°C and zero strain after preliminary calibration of the thermo-optic and strain-optic coefficients as a function of the wavelength. Accuracies better than 3×10-5 and 7×10-4 have been achieved for the phase and group indices, respectively. It is also shown that the main source of error relates to the uncertainty in pitch of the phase masks used for grating inscription. The technique is useful for testing different kinds of fibers including telecommunication, amplifying, and polarization maintaining.

Intracavity interrogation of an array of fiber Bragg gratings.

In this work, we explore the interrogation of an array of fiber Bragg gratings as part of a laser cavity. A semiconductor optical amplifier in a sigma-shaped fiber cavity provides gain and is gated periodically at a rate that matches the roundtrip time associated with each grating of the array. The interrogator exhibits clear laser properties such as a threshold and linewidth narrowing. Besides improving the signal-to-noise ratio and enabling the re-use of wavelengths, it is found that this interrogation scheme enables monitoring of weak gratings spaced by less than 1 cm. Intracavity grating interrogation studied here is found to be a simple and powerful way to increase the number of sensor points for industrial applications.

Numerical Technique for Study of Noise Grating Dynamics in Holographic Photopolymers.

Although the angular distribution of noise gratings in holographic photopolymer is understood to arise from Bragg matching, the details of scatter strength and dynamics are not fully understood. This confounds development of materials and recording techniques that minimize haze. Here, the kinetics are studied using a multi-physics numerical approach coupling diffraction of light from the dynamic material including scatter centers, reactions of chemical species initiated by this light, diffusion and swelling of these constituents, and the formation of the refractive index from the resulting composition. The approach is validated in the case of two-beam transmission holography by comparison to traditional harmonic series and rigorous coupled-mode approaches. Two beam holography in the presence of scatter is then used to study haze development. This reveals that haze due to weak noise gratings grows significantly above initial scatter only in reaction-limited materials, consistent with proposed Bragg-matched amplification mechanisms. Amplified haze is found to be proportional to initial scatter, quantifying the impact of clean sample fabrication. Conversely, haze is found to grow super-linearly with sample thickness, illustrating the significant challenge for applications requiring low haze in large thickness.

Measuring the shape of microbends in optical fibers.

We report on the distributed shape measurement of small deformations produced along the length of an optical fiber. The fiber contains multiple waveguiding cores, each inscribed with weak continuous Bragg gratings. The distributed Bragg-reflectivity data for the fiber cores, obtained from the optical backscatter reflectometry, are used to estimate the local curvature and the position of the fiber. We successfully demonstrate the sensing of periodic microdeformations-approximately 1 µm or less in amplitude and a few hundred µm in length. Such microbends are known to cause attenuation in optical fibers, and the approach presented here can enable a detailed measurement of these microbends in applications ranging from telecommunications cable design to biotechnology, robotics, manufacturing, aerospace, and security.

Efficient two-port electron beam splitter via a quantum interaction-free measurement

Semi-transparent mirrors are standard elements in light optics for splitting light beams or creating two versions of the same image. Such mirrors do not exist in electron optics, although they could be beneficial in existing techniques such as electron interferometry and holography and enable novel electron imaging and spectroscopy techniques. We propose a design for an electron beam splitter using the concept of quantum interaction-free measurement (IFM). The design combines an electron resonator with a weak phase grating. Fast switching gates allow electrons to enter and exit the resonator. While in the resonator, the phase grating transfers intensity from the direct beam into one of the weakly diffracted beams at each pass. To make the beam splitter an efficient two-port splitter, the intensity in all other diffracted beams is blocked by an aperture. The IFM principle minimizes the loss of total intensity by this aperture. We use a scattering matrix method to analyze the performance of the beam splitter, including the effects of inelastic scattering in the phase grating. This design can be generalized to beam splitters for not only electrons, but also photons, neutrons, atoms, and other quantum mechanical systems.

Recording of high efficiency volume Bragg gratings in a photopolymer using diffraction from very weak pre-recorded gratings

AbstractA two-step method of writing high diffraction efficiency holographic gratings in photopolymer using just one writing beam during the second step is discussed. The technique is based on the use of weak gratings (<1%) recorded during a first step which during the second step are illuminated with a single beam to form strong highly efficient holographic gratings. The weak grating provides a second interfering beam by diffracting a small portion of the illuminating light. Although effects such as beam pumping and twowave mixing arewell known in dynamic re-writable recording media such as photorefractives, the ability to write with one beam, or ‘pump’ gratings is not usually observed in photopolymer materials. Other methods of recording with a single beam have been reported, especially for data storage, but they use compact optical systems, polarization effects or lenticular devices to produce the required second beam. In this method, the second beam is produced within the photopolymer medium. Earlierwork by the authors reported increases in efficiency of weak photopolymer gratings illuminated by a single exposure beam, and more recently evidence was presented that diffraction at the weak pre-recorded grating was responsible for this effect. Here we demonstrate a strong dependence of the diffraction efficiency on grating thickness and show that the method allows the writing of high diffraction efficiency gratings in adverse conditions using a single beam. The possibility of angular multiplexing is also demonstrated. The technique may find application in writing information using pre-recorded weak holographic gratings and, potentially, in other data storage applications. In this paper it is shown that multiple new gratings can be writtenwhich are angularly separated and distinct from the pre-written grating and it is also possible to choose one grating from among a number of pre-written gratings and selectively amplify it without amplifying the others.

Multi-parameter Simultaneously Sensing Based on an Optimized Concatenated FBG with LPG

AbstractWe analyze an optical fiber sensor based on a concatenated fiber Bragg grating (FBG) with long period grating (LPG) with transfer matrix method and experiment. First, we optimize parameters of concatenated structure in the method of simulation and discuss the influence that the lengths, periods and dc induced index changes of FBG and LPG and length of optical fiber between LPG and FBG has to influence reflection spectrum. Second, based on the optimized parameters for concatenated structure, fabricate such structure and use it to monitor concentration and temperature simultaneously. Simulation result proves that increasing the lengths of FBG and LPG properly, and length of optical fiber between LPG and FBG fixed in 20 mm, more clearly reflection spectrum can be obtained. Experiment result shows a novel optical fiber sensor based on a concatenated strong fiber Bragg grating with a weak long period grating (CLBG) device can be used to monitor concentration and temperature simultaneously. The result of research has reference value in multi-parameter simultaneously sensing.

Ring Resonators With Vertically Coupling Grating for Densely Multiplexed Applications

Silicon-on-insulator photonic microring resonators are ideal components for applications that require a high degree of multiplexing because of their small size. However, when very high numbers of microrings are used in parallel, grating couplers and waveguide routing quickly take up more chip space than the actual rings. Therefore, we suggest to combine microring resonators with local weak gratings to read out light directly from the microring while still maintaining high-quality factors of 19 000. We demonstrate simulation results and an experimental analysis of different grating parameters. The resonance spectrum is recovered from the vertical emission of the microring grating, without using a traditional output waveguide.

Design of Weak 1-D Bragg Grating Filters in SOI Waveguides Using Volume Holography Techniques

To answer the growing need for more versatile integrated spectral filters, we show that weak one-dimensional gratings can be designed towards any desired target spectrum. We follow a very straightforward design procedure to demonstrate the performance of these devices. Measurements and simulations show a very good correspondence with the target spectra. By analyzing the results, we also found that the design procedure can be refined by using simulated reflections, instead of relying on the calculated Fresnel reflections.

Simulation of diffraction from volume holograms

Coupled wave equations that consider two-dimensional distribution and phases of gratings are derived by using weak grating and slow varying approximation. The result of angular selectivity on rand binary data page shows that each minimum is nonzero and each maximum that is not the middle maximum is a little bit smaller than that on plane wave volume hologram. Angular multiplexing is also modeled using the same method, and the middle page is read out in two different angular spacings. The relationship about PSNR versus angular spacing at different multiplexing pages is analyzed to reflect crosstalk between pages. All simulative results show this method is effective for volume holograms simulation.

Weak gratings in silicon-on-insulator for spectral filters based on volume holography

We demonstrate integrated holographic grating filters, that can be designed toward any desired target spectrum. A straightforward design method transforms this target spectrum into a weak effective refractive index variation. The filters are implemented in high-contrast silicon-on-insulator by modulating the width of the shallow etched sides of a ridge waveguide. Measurements show that this is a promising approach to integrate flexible and versatile filters on photonic chips to enable more complex applications.

Reflectivity measurement of weak fiber Bragg grating (FBG)

One method based on the end-face reflection as a reference was used to measure weak grating reflectivity of 0.01%–1%. For measuring ultra-weak grating reflectivity, the grating group with an identical wavelength was used. By using anhydrous alcohol and deionized water as the reflecting medium, the singlepulse FBG reflectivity of a highly doped-Ge photosensitive fiber was calculated to be about 0.747% and 0.739%, respectively; and the single-pulse FBG reflectivity of ordinary SMF fiber was about 0.016% and 0.015%, respectively. The consistent results by the two different reflecting media showed that the end-face reflection method on the grating group was feasible.

Theoretical Analysis of Diffraction Efficiency of Photocarrier Grating

The diffraction efficiency of the photocarrier grating (PCG) has been theoretically analyzed. Firstly, the model of the PCG has been set up, and the distribution of the carrier concentration has been studied. Secondly, the change in refractive index produced by the photocarriers has been estimated. Finally, a rigorous coupled-wave analysis has been employed to investigate the diffraction efficiency of the PCG. The results show that when the angle between the two excitation lasers increases, the change of the refractive index decreases, leading to a weak grating. The optimal incident angle of the probe laser is dependent on the angle between the two excitation lasers, and the relation between these two angles for the higher diffraction efficiency of the +1st forward-diffracted wave has been found. In terms of diffraction efficiency, the optimal parameters have been estimated, and the feasibility of the PCG to be a forward-diffracted component has been confirmed.

Impact of Sidewall Roughness on Integrated Bragg Gratings

A major issue in the fabrication of integrated Bragg grating (IBG) filters in highly confined waveguides is the average effective index fluctuations caused by sidewall roughness. In this work, we model the impact of this effect on IBG spectral responses and we identify key parameters that need to be controlled in order to minimize distortions. We show that only low spatial frequency components of the noise are relevant to the calculation of the IBG spectral response, which decreases considerably the computation time. Furthermore, we present an IBG emulator that allows estimation of expected fabrication yield of specific gratings given that the fabrication process is well characterized. The analysis of apodized gratings is used as an example to illustrate how this modeling can help to reduce development cost by first studying robustness of IBG designs to fabrication limitations. Finally, we study analytically the impact of sidewall roughness having short correlation lengths and small roughness variances on the spectral response of weak gratings.

Photoinduced mass-transport based holographic recording of surface relief gratings in amorphous selenium films

Surface relief gratings formation in amorphous selenium thin films in two recording configurations with light intensity modulation were studied in situ by real-time atomic force microscopy and diffraction efficiency measurements. We report observation of mass transport effect in films induced by band-gap irradiation when the light polarization of the recording beams has a component along the light intensity gradient (“p-p” scheme of recording) that allows obtaining giant stable gratings in this versatile chalcogenide material. On the contrary, only a pure scalar weak grating caused by photoinduced volume shrinkage is obtained in the “s-s” recording configuration, even for long-term irradiation.

Characterization of the Brillouin grating spectra in a polarization-maintaining fiber

Brillouin grating in optical fibers is of considerable interest due to its applications in optical fiber communication and distributed sensing. However, the intrinsic Brillouin grating spectrum is still unknown and several kinds of spectra were reported. In this paper, we experimentally investigate the characteristics of the Brillouin grating spectra in a polarization-maintaining fiber, and the results show that it agrees with the theory of fiber Bragg grating but not determined by the natural Brillouin gain bandwidth as thought previously, and in the case of weak grating, the bandwidth is only length-limited, i.e., inversely proportional to the length of the Brillouin grating. In addition, the apodization of the Brillouin grating induced by the acoustic wave decay, a unique feature that is different from the Bragg grating, is observed for the first time to the best of our knowledge.