We demonstrate a novel split-step solution for analyzing nonlinear fiber Bragg gratings. The solution is used for designing nonlinear fiber Bragg gratings with a low reflectivity. The structure of the grating is designed according to the profiles of the incident and reflected pulses. We demonstrate our method for nonlinear compression of a pulse reflected from a fiber Bragg grating. The method allows us to obtain compressed pulses with a very low wing intensity. [Read more…]
Fig. 2 Schematic description of optical pulse compression geometry.
We present a method for efficiently exciting a Bragg soliton with a spectral content located mostly within the bandgap of a one-dimensional periodic structure. The method is based on a new interaction between Bragg solitons and on a high intensity enhancement, caused owing to the reduced propagation velocity inside periodic structures. Our method can also be used for efficient compression of optical pulses. We have theoretically demonstrated pulse compression with a compression ratio of 2800—over two orders of magnitude higher than previously reported. The results open new possibilities for experimental demonstration of Bragg soliton propagation and for pulse compression in one-dimensional periodic structures. [Read more…]
Fig. 1 Bandgap diagram of the grating. The grating is divided into three sections. In the first section, the slow-light effect is used to enhance the pulse intensity. In the second region, a soliton interaction is used to form a single in-gap soliton. In the third region, the soliton is compressed owing to the shifting of the bandgap.
We demonstrate a novel method that enables one to measure the structure of highly reflecting fiber Bragg gratings. The method is based on measuring both the transmission and reflection spectra of the grating and applying an inverse-scattering algorithm. The use of the transmission spectrum significantly reduces the sensitivity of the reconstruction to measurement noise, and therefore it significantly decreases the measurement duration. We experimentally demonstrate our method for reconstructing the structure of an apodized grating with a reflectivity of 99.91%. Â [Read more…]
Fig. 1 Schematic description of experimental setup used for measuring structure of strong FBGs. FBG is the interrogated fiber Bragg grating and M is a mirror. The intensity transmission spectrum and the interference spectrum between a reflection from the grating and a reference signal, obtained by using a mirror, are measured.
We demonstrate experimentally, for the first time to our knowledge, a reconstruction of a highly reflecting fiber Bragg grating from its complex reflection spectrum by using a regularization algorithm. The regularization method is based on correcting the measured reflection spectrum at the Bragg zone frequencies and enables the reconstruction of the grating profile using the integral-layer-peeling algorithm. A grating with an approximately uniform profile and with a maximum reflectivity of 99.98% was accurately reconstructed by measuring only its complex reflection spectrum. Â [Read more…]
Image Description: Fig. 1 Intensity of the measured complex reflection spectrum.
We theoretically demonstrate what is a new method for efficient launching of in-gap solitons in fiber Bragg gratings. The method is based on generating a soliton outside the grating bandgap. Then, the soliton is adiabatically coupled into the bandgap by using its particlelike behavior. We compare our method to a previously published launching scheme that is based on generating the soliton directly within the grating bandgap. When using low-intensity incident pulses, the transmission efficiency of our method is three times higher than that of the previously published scheme. Â [Read more…]
Image generated by GPL Ghostscript (device=ppmraw)
Fig. 1 Coupling coefficient of the grating apodization. The inset shows a zoom around the second apodization segment.
Because optical systems have a huge bandwidth and are capable of generating low-noise short pulses, they are ideal for undersampling multiband signals that are located within a very broad frequency range. We propose a new scheme for reconstructing multiband signals that occupy a small part of a given broad frequency range under the constraint of a small number of sampling channels. The scheme, which we call multirate sampling (MRS), entails gathering samples at several different rates whose sum is significantly lower than the Nyquist sampling rate. The number of channels does not depend on any characteristics of a signal. In order to be implemented with simplified hardware, the reconstruction method does not rely on the synchronization between different sampling channels. Also, because the method does not solve a system of linear equations, it avoids one source of lack of robustness of previously published undersampling schemes. Our simulations indicate that our MRS scheme is robust both to different signal types and to relatively high noise levels. The scheme can be implemented easily with optical sampling systems. Â [Read more…]
Fig. 2 Illustration demonstrating how support consistency is checked. The input of the algorithm is the sampled signals whose spectra ð‘‹1(ð‘“) and ð‘‹2(ð‘“) are shown Figs. 1b, 1c, respectively; their respective indicator functions â„1(ð‘“) and â„2(ð‘“) are shown in Figs. 2a, 2b. Figure 2c shows the indicator function â„(ð‘“)=â„1(ð‘“)â„2(ð‘“) . In Figs. 2d, 2e, we check whether the subset ð’°={ð‘ˆ2}∊ð’«{ð‘ˆ} is support consistent. Figures 2d, 2e show the indicator functions for the downconversion of ð‘ˆ2 at rates ð¹1 and ð¹2:â„1ð‘ˆ2(ð‘“) and â„2ð‘ˆ2(ð‘“) , respectively. The dashed lines illustrate ð‘ˆ2 , −U2 , and their downconversions. It is evident that the functions â„1(ð‘“) and â„1ð‘ˆ2(ð‘“) are not equal. Hence, ð’°={ð‘ˆ2} is not a support-consistent combination.
We describe an improved optoacoustic tomography method, that utilizes a diffusion-based photon propagation model in order to obtain quantified reconstruction of targets embedded deep in heterogeneous scattering and absorbing tissue. For the correction we utilize an iterative finite-element solution of the light diffusion equation to build a photon propagation model. We demonstrate image improvements achieved by this method by using tissue-mimicking phantom measurements. The particular strength of the method is its ability to achieve quantified reconstructions in non-uniform illumination configurations resembling whole-body small animal imaging scenarios. Â [Read more…]
Fig. 3. OAT images of the 1st (a/b), 4th (c/d), 9th (e/f) and 11th (g/h) iteration of the normalization algorithm, with corresponding light distribution model (logarithmic scale).
We demonstrate experimentally an optical system for under-sampling several bandwidth-limited signals with carrier frequencies that are not known apriori and can be located anywhere within a very broad frequency region between 0–18GHz. The system is based on under-sampling asynchronously at three different sampling rates. The optical pulses required for the under-sampling are generated by a combination of an electrical comb generator and an electro-absorption optical modulator. To reduce loss and improve performance the implementation of the optical system is based on a wavelength division multiplexing technique. An accurate reconstruction of both the phase and the amplitude was obtained when two chirped signals each with a bandwidth of about 150 MHz were sampled. [Read more…]
In this letter we experimentally demonstrate the sensitivity and overall performance of iterative correction for light attenuation in optoacoustic tomography as a function of number of iterations and accuracy of the tissue optical properties estimations. Experimental optoacoustic data were obtained by circularly illuminating a tissue-mimicking phantom with a high intensity pulsed near infrared laser and measuring the subsequent acoustic waves using a broadband acoustic hydrophone. We showcase an improvement in image fidelity and quantification due to the iterative inversion but find the method sensitive to the background optical properties and of a diverging behavior when increasing the number of iterations. Â [Read more…]
Fig. 2 The reconstruction error of the algorithm as a function of iteration for different assumed reduced scattering coefficient values. In all cases μa=20 cm−1 and σ=0.001. The quality measures showed assumed (a) the standard deviation within the insertion and (b) a root-mean-square error estimate given in Eq. (4).
We demonstrate a new method based on inverse scattering theory for designing the refractive index profile of single-mode planar waveguides in order to obtain a desired TE-mode profile. The method enables a direct design of the waveguide profile without the need for iterative optimization algorithms. The design is based on a first order solution to the Gel’fand-Levitan-MarCcaronenko integral equation that gives a simple linear connection between a small change in the scattering data and the corresponding change in the kernel function. This connection reduces the design problem to a simple linear constrained minimization problem which has an explicit solution. Our design method allows adding additional constraints on the refractive index profile such as the waveguide width. The method presented in this paper can be expanded to analyze TM modes and for designing multi-mode planar waveguides. Â [Read more…]
Fig. 2 Comparison between the original and the extracted refractive index profiles that were reconstructed from the linearized GLM equation. Both profiles were obtained by changing the refractive index profile of an hyperbolic secant waveguide that corresponds to a reflectionless waveguide. An excellent agreement between the reconstructed (dotted line) and the original (solid line) profiles was obtained for (a) a truncated hyperbolic secant profile and (b) a sinusoidal perturbation.
