Author: General User with EDITING Role

source: © 2005 Optical Society of America

We develop a novel method that enables one to reconstruct the structure of highly reflecting fiber Bragg gratings from noisy reflection spectra. When the reflection spectrum is noisy and the grating reflectivity is high, noise in the Bragg zone of the reflection spectrum is amplified by the inverse scattering algorithms and prevents the reconstruction of the grating. Our method is based on regularizing the reflection spectrum in frequencies inside the Bragg zone by using the data on the grating spectrum outside the Bragg zone. The regularized reflection spectrum is used to reconstruct the grating structure by means of inverse scattering. Our method enables one to analyze gratings with a high reflectivity from a spectrum that contains a high level of noise. Such gratings could not be analyzed by using methods described in previous work [IEEE J. Quantum Electron. 39, 1238 (2003)]. [Read more…]

Fig. 1 Reconstruction of a uniform grating with coupling coefficient q = 500 m-1 and length L = 1 cm from a noisy reflection spectrum. The figure compares the reconstruction obtained with the method developed in Ref. 9 (solid curve) with a direct reconstruction by the ILP algorithm (dashed curve) and with a reconstruction by the method presented in this paper (dotted curve). The reflection spectrum of the grating was sampled with a bandwidth of 10 nm and resolution of 0.002 nm. The standard deviation of the noise variables, added to the complex reflection spectrum, was equal to 5 × 10-5. The ILP algorithm as well as the algorithm presented in this paper have accurately reconstructed the grating profile.

Amir Rosenthal and Moshe Horowitz, “Reconstruction of a fiber Bragg grating from noisy reflection data,” J. Opt. Soc. Am. A 22, 84-92 (2005)

source: © 2005 Optical Society of America

We demonstrate, for the first time to our knowledge, reconstruction of the structure of a long-period grating from its measured core-to-core transmission spectrum intensity. The reconstruction is obtained by writing an auxiliary grating in cascade to the interrogated grating. Our reconstruction technique is based on using the Hilbert transform and a phase-retrieval algorithm. Using our method, we have reconstructed the structure of a uniform long-period grating with a 47% coupling efficiency. [Read more…]

Fig. 1 Experimental system used to reconstruct the interrogated grating structure. An auxiliary grating is written to obtain a unique reconstruction.

Amir Rosenthal, Stephan Lange, Christian Shäffer, and Moshe Horowitz, “Experimental reconstruction of a long-period grating from its core-to-core transmission spectrum,” Opt. Lett. 30, 3272-3274 (2005)

source: © 2006 Optical Society of America

In order to reconstruct the structure of a long-period grating, both the complex core-to-core transmission function and the complex core-to-cladding transmission function should be known. However, in practice, only the core-to-core transmission function of the grating can be measured. We demonstrate theoretically the reconstruction of long-period gratings from only the core-to-core transmission function. The reconstruction is performed by extracting the complex core-to-cladding transmission function of the grating from its core-to-core transmission function. Generally, the extraction is not unique; however, we show that by writing an additional grating in cascade to the interrogated grating, a unique reconstruction can be obtained. In weak long-period gratings, only the amplitude of the core-to-core transmission function is needed to reconstruct the grating. The results of our work can enable the experimental reconstruction of long-period gratings from their transmission function as well as the development of novel distributed sensors. [Read more…]

Fig. 1 Schematic description of the grating structure analyzed in the paper. Two cascaded LPGs with lengths 𝐿1 and 𝐿2 are separated by a gap with a length 𝐿𝑓 . The fields of the core mode and cladding mode at the input end of the structure are denoted by 𝑢2(𝑘,𝑧=0) and 𝑢1(𝑘,𝑧=0) , respectively. The core-to-core and core-to-cladding transmission functions of the first grating are denoted by 𝑎1(𝑘) and 𝑏1(𝑘) , respectively, and the core-to-core and core-to-cladding transmission functions of the total structure are denoted by 𝑎tor(𝑘) and 𝑏tot(𝑘) , respectively.

Amir Rosenthal and Moshe Horowitz, “Reconstruction of long-period fiber gratings from their core-to-core transmission function,” J. Opt. Soc. Am. A 23, 57-68 (2006)

source: © 2006 Optical Society of America

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.

Amir Rosenthal and Moshe Horowitz, “Analysis and design of nonlinear fiber Bragg gratings and their application for optical compression of reflected pulses,” Opt. Lett. 31, 1334-1336 (2006)

source: ©2006 American Physical Society

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.

Amir Rosenthal and Moshe Horowitz, “Bragg-soliton formation and pulse compression in a one-dimensional periodic structure,” Phys. Rev. E 74, 066611 (2006)

source: © 2007 Optical Society of America

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%.  [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.

Anatoly Sherman, Amir Rosenthal, and Moshe Horowitz, “Extracting the structure of highly reflecting fiber Bragg gratings by measuring both the transmission and the reflection spectra,” Opt. Lett. 32, 457-459 (2007)

source: © 2007 Optical Society of America

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.  [Read more…]

Image Description: Fig. 1 Intensity of the measured complex reflection spectrum.

Amir Rosenthal, Moshe Horowitz, Sven Kieckbusch, and Ernst Brinkmeyer, “Experimental reconstruction of a highly reflecting fiber Bragg grating by using spectral regularization and inverse scattering,” J. Opt. Soc. Am. A 24, 3284-3288 (2007)

source: © 2008 Optical Society of America

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.  [Read more…]

Fig. 1 Coupling coefficient of the grating apodization. The inset shows a zoom around the second apodization segment.

Amir Rosenthal and Moshe Horowitz, “Efficient method for launching in-gap solitons in fiber Bragg gratings using a two-segment apodization profile,” Opt. Lett. 33, 678-680 (2008)

source: © 2008 Optical Society of America

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.  [Read more…]

Fig. 2 Illustration demonstrating how support consistency is checked. The input of the algorithm is the sampled signals whose spectra 𝑋1(𝑓) and 𝑋2(𝑓) are shown Figs. 1b, 1c, respectively; their respective indicator functions ℐ1(𝑓) and ℐ2(𝑓) are shown in Figs. 2a, 2b. Figure 2c shows the indicator function ℐ(𝑓)=ℐ1(𝑓)ℐ2(𝑓) . In Figs. 2d, 2e, we check whether the subset 𝒰={𝑈2}∊𝒫{𝑈} is support consistent. Figures 2d, 2e show the indicator functions for the downconversion of 𝑈2 at rates 𝐹1 and 𝐹2:ℐ1𝑈2(𝑓) and ℐ2𝑈2(𝑓) , respectively. The dashed lines illustrate 𝑈2 , −U2 , and their downconversions. It is evident that the functions ℐ1(𝑓) and ℐ1𝑈2(𝑓) are not equal. Hence, 𝒰={𝑈2} is not a support-consistent combination.

Amir Rosenthal, Alex Linden, and Moshe Horowitz, “Multirate asynchronous sampling of sparse multiband signals,” J. Opt. Soc. Am. A 25, 2320-2330 (2008)

source: © 2009 Society of Photo-Optical Instrumentation Engineers (SPIE)

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.  [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).

Thomas Jetzfellner, Daniel Razansky, Amir Rosenthal, Ralf Schulz, K.-H. Englmeier, and Vasilis Ntziachristos “Iterative finite-element-based inversion for quantified detection of molecular targets using optoacoustic tomography“, Proc. SPIE 7258, Medical Imaging 2009: Physics of Medical Imaging, 725812 (13 March 2009)