Effect of core ellipticity and core-induced thermal stress on the conversion of LP11 modes to vector vortex modes in gradually twisted highly birefringent fibers.

We study the effect of the core ellipticity and core-induced thermal stress on the conversion of LP11 modes to vortex modes in gradually twisted highly birefringent PANDA fibers using an improved perturbation-based modeling method. We show that these two technologically unavoidable factors have a significant impact on the conversion process, which manifests itself in shortening the conversion length, altering the assignment between the input LP11 modes and output vortex modes, and modifying the vortex mode structure. In particular, we demonstrate that for certain fiber geometries, it is possible to obtain output vortex modes with parallel and antiparallel spins and orbital angular momenta. The simulation results obtained using the modified method are in good agreement with recently published experimental data. Furthermore, the proposed method provides reliable guidelines for choosing fiber parameters that ensure a short conversion length and the desired polarization structure of the output vortex modes.

Rocking filter in a highly birefringent fiber for resonant coupling between different LP11 modes and generation of cylindrical vector beams.

We present a rocking filter in a highly birefringent two-mode fiber that enables resonant coupling between different modes in the LP11 group. Our simulations and experimental results prove that such a filter allows for resonant coupling between orthogonally polarized LP11 modes of the same spatial structure, as well as between modes of the same polarizations and orthogonal spatial distributions. Furthermore, we demonstrate that such rocking filters can be used to generate pure TE01, TM01 and HE21 beams or their coherent superposition.

Reducing phase errors in multimode interference coupler by side grooves formed on its top surface.

The performance of multimode interference (MMI) couplers is limited by the presence of phase errors that represent the deviation of the propagation constants of the modes from the quadratic dependence on their order. In this work, we propose a simple and effective method for reducing the phase errors of spatial modes to a relatively high order by forming rectangular grooves near the side edges of the MMI coupler along its entire length. The influence of the groove dimensions and position on the propagation constants of higher-order modes is analyzed using the perturbation method and strict vector simulations for high- and medium-index contrast material platforms. Through numerical simulations, we demonstrate the effectiveness of the proposed method in improving the performance of air-cladded dual-mode (T E 0 and T E 1) MMI-based 50:50 and 100:0 splitters for 1.31 µm wavelength made of medium- and high-contrast materials, T i O 2:S i O 2/S i O 2 and S i/S i O 2, respectively.

Modeling of the conversion of LP modes to vector vortex modes in gradually twisted highly birefringent fibers.

We present a new method for the efficient modeling of the conversion of LP modes to vortex modes in gradually twisted highly birefringent fibers, employing the coupled-mode approach in helicoidal coordinates. The method is applicable to a class of highly birefringent fibers with cylindrical cores and stress-applying elements. We analyzed the effects of refractive index contrast, birefringence, and twist rate profile on the quality of the converted vortex beams, including the intensity and polarization distributions, as well as on the crosstalk between different eigenmodes at the output of the twisted fibers. The obtained results prove the possibility of a broadband quasi-adiabatic generation of vortex beams of high purity in gradually twisted highly birefringent fibers a few centimeters long and provide hints for optimization of the conversion process.

Selective excitation of different combinations of LP01 and LP11 polarization modes in a birefringent optical fiber using a Wollaston prism.

We present an effective method for free-space selective excitation of different combinations of LP01 and LP11 polarization modes in a birefringent optical fiber using a Wollaston prism, rotatable polarizer, and achromatic half-wave plate. The method is minimally wavelength-dependent and can be used for high-power sources. The relative coupling efficiencies of different modes can be continuously tuned and the suppression rate of the unwanted modes with respect to the targeted mode exceeds 20 dB. We present input system configurations that allow for the excitation of different individual modes and groups of modes and estimate the maximum coupling efficiencies based on numerical simulations. As example applications, we show the generation of Raman sidebands in different modes, gain tunability of intermodal four-wave mixing, and broadband conversion of a supercontinuum light beam from the fundamental to the LP11 mode.

Fiber-based vortex beam source operating in a broadband or tunable mode.

We demonstrate a fiber-based optical vortex beam source operating in broadband or tunable mode in the spectral range of 1100-1400 nm. The vector vortices of the total angular momenta equal to +2, 0, and -2 are obtained by converting the respective linearly polarized (LP11) modes of the two-mode birefringent PANDA fiber with stress-applying elements by gradually twisting its output section. At the input end, the PANDA fiber is powered by broadband supercontinuum or tunable Raman solitons generated in the LP11 polarization modes of a birefringent microstructured fiber with a specially designed dispersion profile and coupled to the respective LP11 modes of the PANDA fiber. Two pulse lasers operating in different regimes (1 ns/1064 nm and 190 fs/1037 nm) were used as the pump to generate supercontinuum or tunable solitons directly in the LP11 modes of the microstructured fiber purely excited with a special Wollaston prism-based method. The high modal and polarization purities of the beams after successive transformations were experimentally confirmed. We also proved the vortex nature of the output beams using shearing interferometry.

Multiple intermodal-vectorial four-wave mixing bands generated by selective excitation of orthogonally polarized LP01 and LP11 modes in a birefringent fiber.

This study investigates the nonlinear frequency conversions between the six polarization modes of a two-mode birefringent fiber. The aim is to demonstrate that the selective excitation of different combinations of linearly polarized spatial modes at the pump wavelength initiates distinct intermodal-vectorial four-wave mixing processes. In particular, this study shows that exciting two orthogonally polarized LP01 and LP11 modes can lead to the simultaneous generation of up to three pairs of different spatial modes of orthogonal polarizations at different wavelengths. The role of the phase birefringence of the spatial modes in the phase matching of such a four-wave mixing process is explained. Moreover, the theoretical predictions are verified through numerical simulations based on coupled nonlinear Schrödinger equations, and are also confirmed experimentally in a commercially available birefringent fiber.

Reducing bend-induced loss and crosstalk in a two-mode ridge waveguide by steplike thickness structuring.

This study demonstrated that two-step structuring of the waveguide thickness in a bent section significantly reduces bend-related inter-mode crosstalk, excess loss, and pure bending loss. A simple analytical formula was derived linking the thickness change required to compensate for bend-induced effects with geometrical parameters of the waveguide and bending radius. The effectiveness of the proposed approach was verified through rigorous numerical simulations of the loss and inter-mode crosstalk based on transformation optics formalism. The numerical results obtained for a TiO2:SiO2 medium index contrast waveguide exhibited significant improvement of all relevant parameters for TE00 and TE01 modes.

Rigorous modeling of twisted anisotropic optical fibers with transformation optics formalism.

In this study, we show that transformation optics formalism can be used to rigorously model a wide range of twisted anisotropic fibers, which could only be analyzed using perturbative methods. If the material anisotropy of fibers has an intrinsic origin or is induced by axially or helically symmetric physical factors, then they can be transformed into a form usable in rigorous two-dimensional (2D) modeling. We demonstrate the effectiveness of the proposed approach in 2D modeling of the propagation characteristics of first-order eigenmodes in twisted and spun fibers with high linear birefringence. We derive the equivalent electric permittivity tensors for such fibers in the helical coordinate system and study the evolution of the first-order modes toward vortex modes with increasing twist rate. The obtained results confirm that the proposed method can reveal phenomena that cannot be predicted by analytical approaches.

Broadband chromatic dispersion measurements in higher-order modes selectively excited in optical fibers using a spatial light modulator.

We propose an improvement of the interferometric method used up to now to measure the chromatic dispersion in single mode optical fibers, which enables dispersion measurements in higher-order modes over a wide spectral range. To selectively excite a specific mode, a spatial light modulator was used in the reflective configuration to generate an appropriate phase distribution across an input supercontinuum beam. We demonstrate the feasibility of the proposed approach using chromatic dispersion measurements of the six lowest order spatial modes supported by an optical fiber in the spectral range from 450 to 1600 nm. Moreover, we present the results of numerical simulations that confirm sufficient selectivity of higher-order mode excitation.

Method for increasing coupling efficiency between helical-core and standard single-mode fibers.

Helical core fibers (HCFs) suffer from low coupling efficiency and unavoidable excitation of higher order modes below the cutoff wavelength because of a core tilt with respect to the symmetry axis of the cladding. We propose an effective way of increasing the coupling efficiency to a HCF by untwisting its beginning section in a hydrogen flame. The proposed solution provides also a control over the excitation of higher order modes in HCFs and can be applied in splicing as well as in a free-space launching configuration. We experimentally demonstrate that by using the proposed method, the coupling efficiency between a SMF-28 and HCF can be increased to the level reachable for straight fibers, which is limited only by a modal fields mismatch. We also present detailed numerical and analytical studies of the coupling efficiency between a HCF and SMF versus the pitch distance in the partially untwisted input section of the HCF, which takes into account fundamental and higher-order modes.

Conversion of LP11 modes to vortex modes in a gradually twisted highly birefringent optical fiber.

We experimentally demonstrate the possibility of quasi-adiabatic conversion of LP11 modes to vortex modes in a twisted highly birefringent fiber with a gradually increasing twist rate. Based on the value of the effective indices, the LP11 modes are selectively converted to right- and left-handed circularly polarized vortex modes HE21 with a total angular momentum of ±2 and to quasi-TE01/TM01 modes with a total angular momentum of 0. Since the proposed conversion method has a purely topological origin, it is broadband in nature, in contrast to the methods based on resonant effects.

Bend-induced long period grating in a helical core fiber.

We report on a new type of long-period-grating generated in a helical core fiber by bending. The grating arises from bend-induced modulation of an equivalent refractive index in the helical core with a period equal to the helix pitch. We experimentally demonstrate that such grating induces multiple resonant couplings between the fundamental modes guided in the central core and the helical side-core. We have also shown that by varying a direction of bending, one can generate the phase shifted grating. The experimental results are supported by numerical simulations based on the coupled mode equations.

Effect of cladding geometry on resonant coupling between fundamental and cladding modes in twisted microstructured fibers.

We analyzed for the first time the effect of variations in the number of air hole rings and the filling factor of twisted microstructured optical fibers on the resonant couplings between fundamental and cladding modes. Rigorous numerical simulations show that these parameters can be used to control the spectral width of the resonance peaks, resonance loss, and relative strength of polarization effects. Furthermore, the number of air hole rings has a decisive impact on the number of twist-induced resonances and their wavelength range.

Method for direct coupling of a semiconductor quantum dot to an optical fiber for single-photon source applications.

We present an effective method for direct fiber coupling of a quantum dot (QD) that is deterministically incorporated into a cylindrical mesa. For precise positioning of the fiber with respect to the QD-mesa, we use a scanning procedure relying on interference of light reflected back from the fiber end-face and the top surface of the mesa, applicable for both single-mode and multi-mode fibers. The central part of the fiber end-face is etched to control the required distance between the top surface of the mesa and the fiber core. Emission around 1260 nm from a fiber-coupled InGaAs/GaAs QD is demonstrated and its stability is proven over multiple cooling cycles. Moreover, a single photon character of emission from such system for a line emitting above 1200 nm is proven experimentally by photon autocorrelation measurements with an obtained value of the second order correlation function at zero time-delay well below 0.5.

Polarimetric Sensitivity to Torsion in Spun Highly Birefringent Fibers.

We report on experimental studies of polarimetric sensitivity to torsion in spun highly birefringent fibers. Two classes of spun fibers were examined, namely spun side-hole fibers and birefringent microstructured fibers with different birefringence dispersion, spin pitches, and spin directions. The polarimetric sensitivity to torsion was determined by monitoring a displacement of the spectral interference fringes arising in the output signal because of interference of polarization modes and induced by an additional fiber twist. Both the experimental results and the analytical predictions showed that the sensitivity to torsion normalized to the fringe width in the spun highly birefringent fibers increased asymptotically with the twist rate to the value of 1/ π rad-1. We have also studied the polarimetric response to temperature in the spun side-hole fibers. We have found that, in contrast to the torsional sensitivity, the temperature sensitivity decays asymptotically to zero with increasing fiber twist rate. Therefore, the spun fibers with short spin pitches are especially well suited for torsion measurements because the torsional sensitivity and the range of linear response are both enhanced in such fibers, while at the same time, the cross-sensitivity to temperature is reduced.

Scaling effects in resonant coupling phenomena between fundamental and cladding modes in twisted microstructured optical fibers.

We show that in twisted microstructured optical fibers (MOFs) the coupling between the core and cladding modes can be obtained for helix pitch much greater than previously considered. We provide an analytical model describing scaling properties of the twisted MOFs, which relates coupling conditions to dimensionless ratios between the wavelength, the lattice pitch and the helix pitch of the twisted fiber. Furthermore, we verify our model using a rigorous numerical method based on the transformation optics formalism and study its limitations. The obtained results show that for appropriately designed twisted MOFs, distinct, high loss resonance peaks can be obtained in a broad wavelength range already for the fiber with 9 mm helix pitch, thus allowing for fabrication of coupling based devices using a less demanding method involving preform spinning.

Measurement of birefringence and ellipticity of polarization eigenmodes in spun highly birefringent fibers using spectral interferometry and lateral point-force method.

We show that the spectral interferometry method and the lateral point-force method used up to now to measure spectral dependence of the group and the phase modal birefringence in highly birefringent fibers with linearly polarized eigenmodes, can be after some modifications extended for the class of spun highly birefringent fibers with elliptically polarized modes. By combining the two methods, it is possible to determine spectral dependence of the group and phase elliptical birefringence in spun highly birefringent fibers. Moreover, if the fiber spin pitch is independently measured, the spectral dependence of ellipticity angle of polarization eigenmodes as well as the built-in linear phase and group birefringence, can be also obtained using the analytical relations between the parameters of spun and non-spun fibers. We demonstrate the effectiveness of the proposed approach in spectral measurements (700-1600 nm) of the spun side-hole and microstructured highly birefringent fibers with different birefringence dispersion and spin pitches ranging from 4.1 to 200 mm.

Role of symmetry in mode coupling in twisted microstructured optical fibers.

We have studied the effect of symmetry on coupling between core and cladding modes in helical microstructured fibers, which gives rise to resonant loss peaks observed in the fiber transmission spectra. We demonstrate that the selection rules for orbital and spin angular momenta of coupled modes, proposed first for twisted conventional fibers, are universal characteristics and correctly identify the coupled cladding modes in helical microstructured optical fibers (MOFs). Moreover, we show for the first time (to our knowledge) the effect of coupling between modes of opposite polarization handedness in twisted MOFs.

Twin-Core Fiber-Based Mach Zehnder Interferometer for Simultaneous Measurement of Strain and Temperature.

In this paper we present an all-fiber interferometric sensor for the simultaneous measurement of strain and temperature. It is composed of a specially fabricated twin-core fiber spliced between two pieces of a single-mode fiber. Due to the refractive index difference between the two cores in a twin-core fiber, a differential interference pattern is produced at the sensor output. The phase response of the interferometer to strain and temperature is measured in the 850-1250 nm spectral range, showing zero sensitivity to strain at 1000 nm. Due to the significant difference in sensitivities to both parameters, our interferometer is suitable for two-parameter sensing. The simultaneous response of the interferometer to strain and temperature was studied using the two-wavelength interrogation method and a novel approach based on the spectral fitting of the differential phase response. As the latter technique uses all the gathered spectral information, it is more reliable and yields the results with better accuracy.