Elliptical-core Two-mode Fiber Research Articles

Controlled excitation and regulation of LP11a and LP11b modes based on long period gratings in a slightly elliptical core two-mode fiber

Controlled excitation and regulation of LP11a and LP11b modes based on long period gratings in a slightly elliptical core two-mode fiber

Characterization of Distributed Brillouin Sensors Based on Elliptical-Core Two-Mode Fiber

Distributed Brillouin sensors based on the intramodal and intermodal stimulated Brillouin scattering (SBS) of the LP01 and LP11 guided modes in an elliptical-core two-mode fiber (e-core TMF) are investigated and characterized. The strain and temperature dependence of the Brillouin frequency ( $\nu _{\mathbf {B}}$ ) for each intramodal and intermodal SBS is measured, and the feasibility of distributed sensors based on the e-core TMF is tested. As an application of intermodal SBS, the Brillouin optical time-domain analysis with an LP01 mode probe and LP11 mode pump waves is demonstrated, which can simultaneously measure the distribution of $\nu _{\mathbf {B}}$ and the propagation loss of the LP11 mode. The strain and temperature coefficients of the double-peak Brillouin gain spectrum observed in the intramodal SBS of the LP11 mode are separately measured for each peak by applying a Brillouin optical correlation domain analysis with a differential measurement scheme. The experimental results show that e-core TMF can be used as a robust platform for distributed Brillouin sensors with strain and temperature coefficients of $\nu _{\mathbf {B}}$ in the intramodal and intermodal SBS very close to each other, with a discrepancy less than 1%.

Optical time-domain reflectometry based on a Brillouin dynamic grating in an elliptical-core two-mode fiber.

Optical time-domain reflectometry based on a Brillouin dynamic grating (BDG-OTDR) in an elliptical-core two-mode fiber (e-core TMF) is proposed and experimentally demonstrated for the discriminative measurement of strain and temperature distributions. Acoustic gratings are generated by the spontaneous Brillouin scattering of a pump pulse, which is used to reflect a probe pulse. Two orthogonal polarizations of the LP01 mode are sequentially used as the pump, while single polarization of the LP11 mode is used as the probe for intermodal BDG operation. Distribution maps of intermodal and polarization birefringence are acquired by analyzing the two BDG spectra, which are used to separately measure the temperature and strain distributions. In experiments, distributed measurement of intermodal BDG spectra along a 95m e-core TMF is performed with a 2m spatial resolution, where the strain and temperature coefficients of the BDG spectral shift are measured to be -0.085 MHz/με, +7.6 MHz/°C for the pump-probe of the LP01y-LP11x mode, and -0.093 MHz/με, +4.3 MHz/°C for the LP01x-LP11x mode, respectively. Distribution maps of the strain and temperature variation along the TMF are separately obtained by matrix calculation using the four coefficients with an error of ±105 με in strain and ±1.6°C in temperature.

Mapping of intermodal beat length distribution in an elliptical-core two-mode fiber based on Brillouin dynamic grating.

Distributed measurement and characterization of the intermodal beat length between LP(01) and LP(11) modes in an elliptical-core (e-core) two-mode fiber (TMF) are demonstrated by the analysis of Brillouin dynamic grating (BDG) spectra. The BDG is generated by the stimulated Brillouin scattering (SBS) of the LP(01) mode and probed by the LP(11) mode, with four different pairs of the spatial and polarization modes in the e-core TMF applied for the pump and the probe (LP(01)x-LP(11)y, LP(01)y-LP(11)y, LP(01)x-LP(11)x, and LP(01)y-LP(11)x). A mode selective coupler (MSC) is used for selective launch and retrieval of the LP(01) and the LP(11) modes in the BDG operation, and the local reflection spectra from the BDG are obtained by an optical time-domain analysis. A distribution map of the intermodal beat length is acquired for each pair of the pump-probe modes with a 1.5 m spatial resolution along a 75 m e-core TMF. Temperature- and strain-dependence of the BDG spectrum is also evaluated for each case.

Twist effect on spectral properties of two-mode fiber acousto-optic filters

The splitting and the shift of resonance peaks in the output spectrum of two-mode fiber acousto-optic devices caused by twist perturbation are described. An elliptical-core two-mode fiber and a two-mode photonic-crystal fiber are used for the analysis. The splitting of the resonance peaks are found to be almost linearly proportional to the twist angle of the fiber with the slopes of about 0.58 nm/(rad/m) for the elliptical-core fiber and about 0.29 nm/(rad/m) for the photonic crystal fiber. The experimental results compare well with the theoretical predictions.

椭圆芯双模光纤干涉模斑中心间距的研究

By means of the weakly guiding approximation, the mode spot sizes Wx and Wy of the fundamental mode along the semimajor (x-direction) and semiminor (y-direction) axes of the fiber core in elliptical core two-mode fiber are discussed. The variation of their ratio value Wx/Wy with the operation wavelength \lambda and the length ratio a/b between the semimajor axis and the semiminor axis of the fiber core is analyzed. Based on this analysis, the distribution figures of two-lobe interferential mode patterns are evaluated and simulated quantitatively for different phase difference changes between LP01 and LP11 modes. The two-lobe interferential mode patterns have the same profile and distribute symmetrically when the phase difference equals \pi/2. Their central distance S becomes larger when Wx/Wy augments gradually. Furthermore, the equation about the central distance S of the two-lobe interferential mode patterns is given when the operation wavelength varies between 0.65 and 1.31 µm, which is important to applications shuch as sensors and coupling devices between different fibers.

Apodization of elliptical-core two-mode fiber acousto-optic filter based on acoustic polarization control

We propose and demonstrate a novel apodization technique for an elliptical-core two-mode fiber acousto-optic filter by twisting the fiber and adjusting the acoustic polarization. The sidelobe suppression in the filter spectrum was improved from -9.7 to -15.5 dB. A theoretical analysis that includes acoustic birefringence explains the experimental results with good agreement.

Properties of elliptical-core two-mode fiber

Polarization and modal birefringence of elliptical-core two-mode fibers are investigated. Wavelengths corresponding to zero group delay difference (GDD) between the two spatial modes and between the orthogonal polarizations are computed when the fiber parameters, i.e., the relative core/cladding index difference and the ratio of major over minor axis, are varied. Simple relationships between the zero GDD wavelengths and fiber parameters are obtained. With proper fiber design, zero GDD between the two spatial modes and the two orthogonal polarizations can be achieved at the same wavelength.

High performance fused-type mode-selective coupler using elliptical core two-mode fiber at 1550 nm

In this letter, we demonstrate a high performance fused-type mode selective coupler that couples the LP/sub 11/ mode in one fiber and the LP/sub 01/ mode in another using highly elliptical core two-mode fibers. The phase-matching condition was achieved by etching and prepulling portions of two-mode fibers. The coupling efficiency and the mode extinction ratio of 56% to 80% and 22-32 dB, respectively, were achieved with high temperature stability over 1515- to 1595-nm wavelength range.

Acousto-optic Tunable Filters in Two-Mode Fibers

We demonstrate an acousto-optic tunable filter based on a two-mode elliptical-core fiber. The filter is operated at 1090 nm as a broadband notch filter used for spectral flattening and as a bandpass filter with subnanometer bandwidth. When used for spectral flattening the FWHM bandwidth is optimized so that only one acoustic frequency is needed. In this way the spectral variation of a fiber laser source is reduced from 4 dB to less than 0.3 dB within a bandwidth of 6 nm. In the bandpass configuration we obtain a minimum bandwidth of 0.85 nm. Because of a slight polarization dependence of the beat length the bandwidth is 0.91 nm for unpolarized sources. Full acousto-optic mode conversion is obtained at an 8-mW electrical input power to the acoustic transducer. In the bandpass configuration the tuning width is limited by the bandwidth of the required static mode converter. By replacing a uniform mode converter with a linearly chirped one we can increase the tuning range from approximately 10 to 50 nm.

Optical-fiber sensor for simultaneous measurement of pressure and temperature: analysis of cross sensitivity

The use of a highly elliptical core two-mode fiber for simultaneous measurement of pressure (radial pressure or hydrostatic pressure) and temperature is presented. The sources of errors are discussed. Expressions are developed to calculate the cross sensitivities. From the numerical examples, some useful conclusions are given.

Narrow-band acousto-optic tunable filtering in a two-mode fiber

We demonstrate an optical filter that utilizes acousto-optic coupling between the spatial modes that propagate in a two-mode elliptical-core fiber. The optical bandwidth at 1090 nm is 0.85 nm, which is in agreement with predictions based on measurement of differential group delay between the modes. The filter is slightly polarization dependent, with a 0.28-nm wavelength difference between the passband peaks for the two polarization eigenstates. The optical insertion loss is dominated by 2-4-dB bending loss in nonoptimized mode strippers, and the coupling loss is negligible. Full acousto-optic mode conversion was achieved at an 8-mW electrical input power to the acoustic transducer.

A quasi-distributed optical fiber sensor

Quasi-distributed optical fiber sensors using multiple frequency shifters (FS) in a two-mode elliptical core fiber (TMECF) are presented. The sensing measurand is the phase shift between the two propagating modes in the fiber. In the present experimental configuration, four FS provide three independent sensing regions along a single TMECF. The authors construct one sensor in each region to measure localized strain and/or bending simultaneously. Cross talk between the sensors is theoretically and experimentally evaluated. Measurements were carried out using a computer system that allows a real-time data acquisition and display. The measured strain and bending sensitivity for the TMECF used is Sstrain=(101+or-5) degree (phase shift)/m epsilon and Sbending=(0.75+or-0.01) degree (phase shift)/degree (bending). The cross talk between the sensors is around -20 dB.

Perturbation effects on mode propagation in highly elliptical core two-mode fibers

The propagation of both fundamental and second-order modes in highly elliptical core two-mode fibers under external perturbations is investigated. Theoretical and experimental results are presented for the differential phase shift between two spatial modes in each eigenpolarization and the polarization behavior of each spatial mode. The perturbations include axial strain, radial strain, hydrostatic strain, temperature change, twisting, squeezing, and bending. An application of the modal behavior of these fibers to simultaneous measurement of strain and temperature is described.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

All-optical intermodal switch using periodic coupling in a two-mode waveguide

An intermodal switch based on optically induced (through the Kerr effect) periodic coupling in a two-mode waveguide is described and demonstrated. A high-power pump beam injected into the two modes of the waveguide produces a periodic modulation of the refractive-index profile with a period equal to the modal beat length, which causes coupling between the two modes of a simultaneously injected probe beam. The operating principle was successfully demonstrated in an elliptical-core two-mode fiber with counterpropagating pump and probe beams.

Strain effects on highly elliptical core two-mode fibers

Longitudinal strains in optical fibers have been found to produce a large differential phase shift between the first-and second-order spatial modes propagating in a two-mode fiber. This differential phase shift between the modes is both calculated and measured to be dispersive with respect to wavelength. For fibers with highly elliptical core geometries, which propagate only one lobe orientation of the second-order mode, the differential phase shift is also found to be polarization dependent.