Complex Index Profile Research Articles

Distributed Temperature and Strain Measurement Based on Brillouin Gain Spectrum and Brillouin Beat Spectrum

A novel fast measurement method based on Brillouin gain spectrum (BGS) and Brillouin beat spectrum (BBS) in a complex index profile fiber has been proposed to realize simultaneous measurement of distributed temperature and strain. An interesting phenomenon that the temperature/strain sensitivities of the power on the BGS slope and the power on the BBS peak are very different has been discovered and utilized for temperature and strain sensing. By means of only measuring the power trace of the BGS slope and the power trace of one BBS peak along the sensing fiber, the distributed temperature and strain can be measured by using the Brillouin optical time-domain reflectometry (BOTDR) without scanning the Brillouin spectrum. The temperature and strain measurement uncertainties of the proposed method are one order of magnitude lower than that of the BBS-based measurement method. In a proof-of- concept experiment, the temperature and strain of the 2 km large effective area fiber (LEAF) are measured at a spatial resolution of 3 m within 18 s.

Analysis of High-Index Contrast Lithium Niobate Waveguides Fabricated by High Vacuum Proton Exchange

High-index contrast waveguides fabricated with precise control and reproducibility are of high interest for nonlinear and/or electro-optical highly efficient and compact devices for quantum and classical optical data processing. Here, we present a new process to fabricate planar and channel optical waveguides on lithium niobate substrates that we called high vacuum proton exchange (HiVacPE). The main purpose was to improve the reproducibility and the quality of the produced waveguides by limiting and controlling the water traces in the melt, which is used for the ionic exchange. Moreover, we discovered that, when the acidity of the bath is increased, depending on the substrate orientation (Z-cut or X-cut) the waveguides are completely different in terms of crystallographic properties, index profiles, and nonlinearity. The best-obtained channel waveguides exhibit a refractive index contrast as high as 0.04 without any degradation of the crystal nonlinearity and state-of-the-art propagation losses (0.16 dB/cm). We have also demonstrated that the HiVacPE process allows fabricating waveguides on Z-cut substrate with high-index contrast up to 0.11 without degrading the crystal nonlinearity but high strain induced propagation losses. On top of that, we proposed an original and very useful method of analyzing waveguides with complex index profiles. This method can be used for the analysis of any waveguides, whose core contains several layers.

Tailoring Spectral Properties of Binary PT-Symmetric Gratings by Duty-Cycle Methods

We explore the frequency selective functionalities of a nonuniform PT-symmetric Bragg grating with modulated complex index profile. We start by assessing the possibility to achieve an efficient apodization of the PT-symmetric Bragg grating spectral response by using direct adaptations of the conventional apodization techniques. A particular emphasis is put on the binary type structures with different duty cycle variations. We subsequently propose a new efficient grating modulation technique allowing for largely improved reflection/transmission characteristics of the Bragg gratings. The overall intention of our approach is to adapt conventional Bragg gratings designs to improve a PT-symmetric case, fostering thus a new generation of active photonic devices.

Distributed Strain and Temperature Measurement by Brillouin Beat Spectrum

A novel distributed strain and temperature measurement method based on Brillouin beat spectrum (BBS) in a complex index profile fiber are proposed. The BBS corresponds to the optical interaction of different acoustic modes. For a large-effective-area fiber (LEAF), the first peak of BBS is contributed by the first and second acoustic modes, while the second peak in BBS is contributed by the first and third accoustic modes. Through determining power changes in the peaks of BBS due to strain and temperature variation along a LEAF, the distributed strain and temperature can be measured by the homodyne Brillouin optical time-domain reflectometry (BOTDR) without scanning the Brillouin spectrum. The strain and temperature at the end of a 4.5-km LEAF are measured at 3-m spatial resolution in 140 s.

Resonant absorption in transition metamaterials: parametric study

We perform a detailed study of the phenomenon of resonant absorption that occurs at anoblique incidence in the vicinity of the zero refractive index transition in graded indexmetamaterials with the refractive index changing from positive to negative values. Wecompare the results of numerical simulations with analytical predictions for the case of thelinear gradient index profile. Furthermore, we numerically study the transition phenomenain graded index layers with more complex index profiles. A detailed understanding of thephenomenon of resonant absorption may be of considerable importance in the context oftransformation optics as it strongly modifies the field distribution near the zero indextransition.

Propagation characteristics of single-mode optical fibers with arbitrary complex index profiles

A rapidly converging numerical technique for the evaluation mode characteristics of circularly symmetric optical fibers with an arbitrary complex refractive index profile is presented. This method is based on transmission-line principles. From Maxwell's equations, we derive a transmission-line equivalent circuit for the optical fiber refractive index profile and we demonstrate how it can be used to determine the mode effective index (normalized propagation constant) of cylindrical dielectric waveguides. To illustrate the effectiveness of the procedure, we have applied it to circularly symmetric fibers with complex step, parabolic, and segmented optical refractive index profiles. We have used this method to evaluate and manipulate the gain in a typical 980-nm pumped erbium-doped fiber as well as for calculating attenuation of optical fibers when radial loss factors are present.

Gain calculation in erbium-doped <inline-formula><math display="inline" overflow="scroll"><msub><mi>LiNbO</mi><mrow><mn>3</mn></mrow></msub></math></inline-formula> channel waveguides by defining a complex index profile

An analysis of erbium-doped titanium indiffused LiNbO3 channel waveguides under 1484-nm excitation is presented in terms of a complex refractive index profile under 1484-nm excitation (pump) to obtain modal gain and loss for signal and pump powers, respectively. Closed form analytical expressions for modal fields are used to define the complex index profile. The calculated gain characteristics are compared with available experimental results.

Fourier decomposition method for mode characterization in metal-clad fiber with complex index profile

Fourier decomposition method in the cylindrical coordinates is used to characterize LP 0 x mode effective indexes in a step fiber with complex index profile under material dispersion, from the guided modes to the radiation mode. The method help sorting the modes in function of physical properties associated to a particular mode type LP 0 x , etc. The main findings of this work are: (1) The imaginary part of effective indexes of the guided modes and lower order cladding modes is several orders of magnitude smaller than the real part. (2) Both real and imaginary parts of metal-clad refractive indexes around the fiber have a strong influence on the real part of LP 0 x mode effective indexes. (3) In some regions of the wavelength, the fundamental mode LP 01 in an aluminum-clad fiber disappears, but the higher order cladding modes still exist and propagate.

Propagation characteristics of single-mode optical fibers with arbitrary complex index profiles: a direct numerical approach

We present here a rapidly converging numerical procedure for the evaluation of the propagation characteristics of a single-mode optical fiber with an arbitrary complex refractive index profile. To illustrate the procedure, we have first applied it to a fiber with a complex step index profile. As an application, we have used it to evaluate the gain in a typical 980-nm pumped erbium-doped fiber.