Polarization-maintaining Few-mode Fiber Research Articles

Panda type separated-circles-formed elliptical ring core few-mode fiber

To support large transmission and reduce inter-mode crosstalk, we designed a polarization-maintaining few-mode fiber (PM-FMF) with a separated-circles-formed elliptical ring core and two stress rods. This fiber can support ten different polarization modes with tunable chromatic dispersion. With the stress-rods-induced birefringence and the high refractive index difference of 0.03 between the core and the cladding, the fiber can separate adjacent guided modes from each other. The effective refractive index differences between these modes all remain above 1.2 × 10−4 over the entire C + L band. In addition, the special separated-circles-formed elliptical ring structure of this few-mode fiber can provide a new degree of freedom for the regulation of chromatic dispersion. The chromatic dispersion can be simply adjusted by changing the radius of the small circles.

Modified S2 schemes for estimating differential mode group delay in polarization-maintaining few-mode fiber

In the mode-division-multiplexed (MDM) system, the few-mode fiber (FMF) replaces the single-mode fiber (SMF) as the transmission medium, to increase the per-fiber transmission capacity. The polarization-maintaining few-mode fiber (PM-FMF) can greatly enhance the optical communication system, since it can both preserve the polarization state and allow the transmission of multiple modes. To obtain the differential mode group delay (DMGD) value of the PM-FMF, which determines the computational complexity of the equalization algorithm at the receiver, the characterization of the fiber is essential. The conventional S2 method, which can estimate the DMGD of the FMF, cannot be directly applied to the PM-FMF, because it is impossible to determine which of the LP01x and the LP01y mode is the dominant mode. In this work, two new approaches are developed, for the first time to our knowledge, to measure the DMGD in the PM-FMF. The former method is to measure the DMGD in both polarization directions, and LP01x and LP01y are selected as dominant modes, respectively. The latter approach is to realize a simultaneous DMGD measurement of all LP modes, and only LP01y is selected as the dominant mode. It is demonstrated in the experiments that a good agreement has been achieved between the measurement results from both schemes.

Design of 18-mode hole-assisted elliptical-core polarization-maintaining few-mode fiber

We present a novel hole-assisted elliptical-core polarization-maintaining few-mode fiber (EC-PM-FMF) composed of three layers of air holes. The proposed fiber breaks degeneracy of modes by introducing geometry asymmetry. Through optimizing the parameters, the designed fiber could support 18 separate polarization-maintaining modes with the minimum effective refractive index difference (Δneff) between adjacent modes larger than 1.21 × 10−4 over the whole C + L band. Moreover, the designed fiber exhibits good mode and propagation properties including large effective mode area, low nonlinear coefficient, small and flat chromatic dispersion, low confinement loss and bending loss. Thus, the fiber can be used in mode division multiplexing (MDM) system without multi-input multi-output (MIMO) digital signal processing (DSP) technology to improve channel utilization.

Investigation of Spontaneous Raman Scattering in Few-Mode Fibers: Dependence on Polarization and Spatial Modes

We study Raman gain in few-mode fibers (FMFs) by measuring spontaneous Raman scattering in polarization-maintaining FMF, mode-maintaining FMF and common FMF. The results show that the intensities of spontaneous Raman scattering in different polarization and spatial modes are affected by vector mode coupling of the degenerate modes in FMFs, which weakens the polarization- and spatial-mode dependence of Raman gain. Our investigation can be used for developing the FMF-based nonlinear-optical devices and sub-systems.

Experimental demonstration of multi-parameter sensing based on polarized interference of polarization-maintaining few-mode fibers.

Polarization-maintaining few-mode fibers (PM-FMFs) have found applications in sensors with attractive features that traditional single-mode fibers do not possess. We propose a measurement method for multi-parameter sensing based on PM-FMFs. This method is mainly based on the polarized interference effect of PM-FMF. We experimentally demonstrate the principle of the simultaneous sensing of temperature, strain and transverse force. The sensor has been verified by three groups of multi-parameter measurement experiments showing that the changes of multi-parameter derived from the wavelength shift of the polarized interference spectrum are consistent with the multi-parameter changes set in the experiments. The proposed PM-FMF based sensor has a temperature sensitivity of 0.3 nm/°C, a strain sensitivity of 0.01 nm/µε and a transverse force sensitivity of 0.0065 nm/(N/m).

Simultaneous Beat-Length Measurement of a Polarization-Maintaining Few-Mode Fiber

Based on the polarization interference technique, we propose a modified method for simultaneously measuring the beat length of all guided modes in a polarization-maintaining few-mode fiber (PM-FMF). We experimentally demonstrate the effectiveness of our proposed method on a 3-mode PANDA-type PM-FMF. The simultaneous measurement results agree well with the results obtained by one-by-one measurement process. What's more, the modified measurement method is easier to be implemented and more scalable to a larger mode number.

Design of 20-polarization-maintaining-mode “pseudo-rectangle” elliptical-core fiber for MIMO-less MDM networks

Abstract We propose a polarization-maintaining few-mode fiber, which is characterized by the combination of an elliptical core structure and two stress-applying areas. Numerical simulations indicate that such fiber could support 20 distinct polarization-maintaining modes. The influences of elliptical-core size and stress-applying-area position are investigated for the 20 spatial modes. Moreover, the stress distribution, the modal birefringence distribution and the bending loss for the 20 polarization-maintaining modes of the designed fiber are also calculated. With appropriate geometrical parameters, the minimum effective refractive index difference (minΔneff) between adjacent modes is 2.01 × 10−4 at C band. The proposed fiber is a promising candidate to simplify or eliminate multiple-input-multiple-output digital-signal-processing in mode-division multiplexing networks.

Hole-assisted polarization-maintaining few-mode fiber

We propose a novel polarization-maintaining few-mode fiber by introducing four identical circular side-holes surrounding an elliptical ring core. In addition to the parameters of ring core, this kind of fiber endows new degrees of freedom to adjust the modal birefringence of guided modes. Numerical simulations indicate that such fiber could support 10 distinct polarization modes, including the effectively separated fundamental modes. The influences of side-hole size and positions on the polarization-maintaining property are investigated for the 10-mode fiber. With appropriate parameters of side-holes, the minimum effective refractive difference (Δneff) between adjacent modes is 1.65 × 10−4 at 1550 nm. Compared with the fiber excluding side-holes, the modal effective refractive indexes are decreased while most Δneff values are enlarged in the proposed fiber. Moreover, all the Δneff values could be higher than 1.52 × 10−4 over a bandwidth ranging from 1510 nm to 1630 nm. The chromatic dispersion covering the broadband is analyzed subsequently. Furthermore, 16 vector modes could be guided in the proposed fiber structure with modified parameters. The proposed fiber is capable of supporting extended modes and might be a promising candidate toward high–capacity spatial-division-multiplexing communications.

4 $\times $ 10-Gb/s MIMO-Free Polarization and Mode Group Multiplexing for Data Center Applications

The $4\mathrm {\mathbf {\times }}10$ -Gb/s polarization- and mode group-multiplexed transmission without multiple-input–multiple-output equalization for data center applications is experimentally demonstrated, for the first time, over a 127-m panda-type polarization-maintaining few-mode fiber. A polarization crosstalk of less than −22.0 dB and a bending loss of <0.08 dB for one loop with a 5-mm radius were achieved.

All-fiber spatial rotation manipulation for radially asymmetric modes

We propose and experimentally demonstrate spatial rotation manipulation for radially asymmetric modes based on two kinds of polarization maintaining few-mode fibers (PM-FMFs). Theoretical finding shows that due to successful suppression of both polarization and spatial mode coupling, the spatial rotation of radially asymmetric modes has an excellent linear relationship with the twist angle of PM-FMF. Both elliptical core and panda type FMFs are fabricated, in order to realize manageable spatial rotation of LP11 mode within ±360° range. Finally, we characterize individual PM-FMF based spatial orientation rotator and present comprehensive performance comparison between two PM-FMFs in terms of insertion loss, temperature sensitivity, linear polarization maintenance, and mode scalability.

Polarization-maintaining few mode fiber composed of a central circular-hole and an elliptical-ring core

We propose a novel waveguide design of a polarization-maintaining few mode fiber (PM-FMF) supporting ≥10 non-degenerate modes, utilizing a central circular air hole and a circumjacent elliptical-ring core. The structure endows a new degree of freedom to adjust the birefringence of all the guided modes, including the fundamental polarization mode. Numerical simulations demonstrate that, by optimizing the air hole and elliptical-ring core, a PM-FMF supporting 10 distinctive polarization modes has been achieved, and the effective index difference Δneff between the adjacent guided modes could be kept larger than 1.32×10−4 over the whole C+L band. The proposed fiber structure can be flexibly tailored to support an even larger number of modes in PM-FMF (14-mode PM-FMF has been demonstrated as an example), which can be readily applicable to a scalable mode division multiplexing system.

Design of PANDA ring-core fiber with 10 polarization-maintaining modes

We present a polarization-maintaining PANDA ring-core fiber (PM-PRCF) characterized by the combination of a ring-core structure with two stress-applying rods. This special fiber design separates the adjacent modes and avoids the cutoff of the higher-order modes, which is a common problem in elliptical core polarization-maintaining few-mode fibers. Using a high-contrast index ring and stress-induced birefringence, the PM-PRCF features support for 10 vector modes, with effective refractive index separations from their adjacent modes >10−4. Broadband performance is investigated subsequently over a wide wavelength range from 1500 to 1630 nm. The proposed fiber is targeted at applications in space-division multiplexing while eliminating the complex multiple-input multiple-output signal processing.

Design of eight-mode polarization-maintaining few-mode fiber for multiple-input multiple-output-free spatial division multiplexing.

We propose a polarization-maintaining few-mode fiber (FMF) that features an elliptical ring shaped core with a high refractive index contrast ∼0.03 between the core and the cladding. This fiber design alleviates the usual trade-off between the number of guided modes and the achievable birefringence that is usually observed in conventional elliptical-core FMFs. Through numerical simulations, we show that this fiber design can support up to 10 guided vector modes over the entire C band while providing large birefringence. Except for the two fundamental modes, the eight higher-order vector modes are all separated from their adjacent modes by effective index differences >10⁻⁴, which is the typical birefringence value of single-mode polarization maintaining fibers. The designed fiber targets applications in spatial division multiplexing of optical channels, without multiple-input-multiple-output (MIMO) digital signal processing, for short-reach optical interconnects.