Single Polarization Transmission Research Articles

High birefringence single-polarization composite structured anti-resonant fiber

We propose a highly birefringent, single-polarization composite structured anti-resonant fiber (CS-ARF). The proposed CS-ARF has an elliptical core, two symmetrically spaced arrays of elliptical air holes, and a six-tube anti-resonant structure. The birefringence and single-polarization characteristics of the proposed CS-ARF were analyzed in detail in the mid-infrared band using the finite element analysis method. The results show that the fiber can achieve birefringence up to the order of 10−2 and single-polarization transmission with a confinement loss (CL) below 10−5 dB/m in the mid-infrared band. When the refractive index of the elliptical core is set at na = 2.70, the designed CS-ARF maintains single polarization in the range of 2.5 μm to 3.197 μm with an extinction ratio of up to 1.08 × 107. In addition, we found that by setting the refractive index of the elliptical core, the CL in the y-polarization direction can be effectively adjusted.

Focusing through scattering media by a single polarization transmission matrix with binary polarization modulation.

Wavefront distortion induced by scattering media seriously affects optical focusing. Wavefront shaping based on a transmission matrix (TM) is useful in controlling light propagation in highly scattering media. Traditional TM generally studies amplitude and phase, but the stochastic nature of the light propagation in the scattering medium also affects its polarization. Based on the binary polarization modulation, we propose a single polarization transmission matrix (SPTM) and achieve single-spot focusing through scattering media. We anticipate that the SPTM will be widely used in wavefront shaping.

Broadband polarization filter based on tellurite photonic crystal fiber with high extinction ratio

In this work, we propose an octagonal photonic crystal fiber (PCF) filter based on tellurite. Selective gold coating in air holes couples the core mode to a surface plasmon mode. The finite element method is used to analyze the structure in the mid-infrared band. The simulated results and analysis show that the losses in the y and x polarization modes at 2.55 µm are 2063.2 and 21 dB/cm, respectively. Therefore, the unwanted y-polarization mode is filtered out. The higher loss ratio optimizes the formant position and intensity of surface plasmon mode and core mode. When the fiber length of the filter is optimized to 500 µm, the extinction ratio of − 102 dB is achieved at 2.55 µm. Consequently, the single polarization transmission with a 500 nm width is realized. When the length of the fiber is greater than 500 µm, it realizes the 1500 nm (2.2–3.7 µm) ultra-wideband filtering. We also analyze the manufacturing feasibility of the proposed PCF and the influence of structural parameters on the limiting loss characteristics.

Single-polarization filter based on photonic crystal fiber with nanoscale gold-coated and liquid-filled air holes

A photonic crystal fiber (PCF) with nanoscale gold-coated and liquid-filled air-holes is proposed, which can realize anomalous single-polarization transmission. That is, the polarization filtering in the x- and y-polarization directions is achieved at wavelengths of 1.18 and 1.55 μm. The dispersion characteristics and losses of filter are analyzed by the finite element method (FEM). The results show that when the fiber length is 800 μm, the extinction ratios (ERs) at the above two wavelengths are 31.25 and -248.63 dB, and the corresponding bandwidths are 28 and 270 nm, from 1.165 to 1.193 μm and 1.43–1.70 μm, respectively. Furthermore, by changing the structural parameters, not only can the positions of resonance wavelengths in the two orthogonal polarization directions be changed simultaneously, but also the positions of resonance wavelengths in the x polarization direction can be changed singly, which is conducive to the filtering of PCF at a specific wavelength within a certain range.

Hole-Assisted Solid Core Bragg Fibers With a High-Index-Contrast Cladding for Broadband Single-Polarization Operation

We propose a hole-assisted solid core Bragg fiber (SCBF) with a high-index-contrast cladding for near-infrared broadband single-polarization transmission in this article. The circular symmetry of a common SCBF is broken by the introduced air holes in the solid core for increasing the modal birefringence and polarization-dependent loss (PDL). In the proposed single-polarization SCBF, the confinement of guided modes predominantly depends on the photonic bandgap effect from the high-index-contrast cladding, and simultaneously rests with the total internal reflection from the interface between the solid core and air holes. The numerical results show that both the enhanced modal birefringence and PDL are strongly related to the refractive index, size and locations of air holes. We demonstrate that the modal birefringence for the fundamental mode can be up to the order of 10−3, and the difference in confinement losses between both polarized fundamental modes is no less than an order of magnitude, which is sufficiently large to support single-polarization operation. Moreover, the single-polarization bandwidth with the transmission loss lower than 0.1 dB/m is as high as 277 nm and the critical bend radius is as low as 1 cm, indicating the excellent performances of high bandwidth and bending resistance in the hole-assisted single-polarization SCBF.

Highly-birefringent and ultra-wideband low-loss photonic crystal fiber with rhombic and elliptical holes

In this paper we propose a novel type of highly-birefringent and ultra-wideband low-loss photonic crystal fiber (PCF) incorporating elliptical holes in the cladding and rhombic holes located along the transversal axes. The PCF is simulated and analyzed with finite-difference time-domain method. The properties of PCFs were investigated for various distribution, shape and size of air holes. For the optimized PCF design the birefringence reaches 8×10−3 at 1550 nm and the confinement loss is as low as 5×10−4dB/km with single mode operation in an ultra-broad transmission band (500 nm–1800 nm). Furthermore, the single mode and single polarization transmission could be achieved in this PCF at longer wavelengths beyond 1600 nm. This kind of PCF has potentials in optical long haul communication and optical fiber sensing.

Dual-Polarization NFDM Transmission With Continuous and Discrete Spectral Modulation

Nonlinear distortion experienced by signals during their propagation through optical fibers strongly limits the throughput of optical communication systems. Recently, a strong research focus has been dedicated to nonlinearity mitigation and compensation techniques. At the same time, a more disruptive approach, the nonlinear Fourier transform (NFT), aims at designing signaling schemes more suited to the nonlinear fiber channel. In a short period, impressive results have been reported by modulating either the continuous spectrum or the discrete spectrum. Additionally, very recent works further introduced the opportunity to modulate both spectra for single polarization transmission. Here, we extend the joint modulation scheme to dual-polarization transmission by introducing the framework to construct a dual-polarization optical signal with the desired continuous and discrete spectra. After a brief analysis of the numerical algorithms used to implement the proposed scheme, the first experimental demonstration of dual-polarization joint nonlinear frequency division multiplexing (NFDM) modulation is reported for up to 3200 km of low-loss transmission fiber. The proposed dual-polarization joint modulation schemes enables to exploit all the degrees of freedom for modulation (both polarizations and both spectra) provided by a single-mode fiber (SMF).

Study on Crosstalk-Free Polarization Splitter Based on Square Lattice Single-Polarization Photonic Crystal Fibers

We propose a novel L-shaped crosstalk-free polarization splitter (PS) based on square lattice single-polarization photonic crystal fibers, which has a wide bandwidth (250 nm) for single-polarization transmission. By employing a vectorial finite-element method and finite-element beam propagation method, the numerical simulations demonstrate that an incident light polarized in any direction can be split into two orthogonal single-polarization states ( $x$ - and $y$ -polarizations) without any crosstalk through the proposed PS. Furthermore, a large structural tolerance and a wide transmission bandwidth are also demonstrated, which would lead to more easier fabrication process.

Single-Polarization Waveguiding by Low-Index Strips on the Surface of Chalcogenide Glass Film

In this paper, we propose and demonstrate a simple chalcogenide glass (ChG) waveguide structure, which is composed of a low refractive-index strip on the surface of the planar ChG film. Theoretical analysis shows that it supports single quasi-TE mode transmission at 1.55 μm with high third-order nonlinearity and negligible bending loss if the bending radius is higher than 60 μm. Then, the ChG waveguide is fabricated and measured. For the waveguide sample with a ChG film of 180 nm in thickness, the measured attenuation of the quasi-TE mode is 0.4 dB/cm at 1.55 μm. Meanwhile, the extinction ratio between the two polarization modes in the waveguide sample of 10 mm in length is higher than 25 dB over a region of 80 nm at 1.55 μm. For the waveguide sample with similar length and a smaller ChG film thickness (150 nm), the extinction ratio increases to 35 dB. These results demonstrate its property of low-loss single-polarization transmission. It provides a simple way to realize high-quality ChG waveguides, which have great potential on integrated nonlinear optical devices.

Quasi-Pilot Aided Phase Noise Estimation for Coherent Optical OFDM Systems

In this letter, a novel phase noise estimation scheme has been proposed for coherent optical orthogonal frequency division multiplexing systems, the quasi-pilot-aided method. In this method, the phases of transmitted pilot subcarriers are deliberately correlated to the phases of data subcarriers. Accounting for this correlation in the receiver allows the required number of pilots needed for a sufficient estimation and compensation of phase noise to be reduced by a factor of 2 in comparison with the traditional pilot-aided phase noise estimation method. We carried out numerical simulation of a 40 Gb/s single polarization transmission system, and the outcome of the investigation indicates that by applying quasi-pilot-aided phase estimation, only four pilot subcarriers are needed for effective phase noise compensation.

Comparison between polarization-multiplexed DPSK and single-polarization DQPSK in 640 Gbaud, 1.28 Tbit/s–500 km single-channel transmission

We describe two schemes for 1.28 Tbit/s single-channel transmission at 640 Gbaud with polarization-multiplexed DPSK and single-polarization DQPSK, and compare their performance in a 500 km transmission. We found that the polarization multiplexing of ultrahigh-speed DPSK signals, for example at 640 Gbaud, is sensitive to the PMD characteristics of the transmission link. Even when first-order PMD is fully compensated, second-order PMD, which varies over a long period and is difficult to compensate, results in inter-polarization crosstalk induced by depolarization. The single-polarization transmission of a 640 Gbaud DQPSK signal is less vulnerable to PMD and therefore more stable, as the depolarized components induced by second-order PMD can be easily rejected with a polarizer. However, its performance is limited because the OSNR must be higher than for DPSK.

Single polarization transmission in pedestal-supported silicon waveguides

In this Letter, properties of a pedestal-supported silicon waveguide are investigated, showing that it supports single polarization transmission. The pedestal is fabricated easily through a wet-etching process on strip waveguides. Theoretical analysis shows that this property is due to the leakage of quasi-TM mode when the pedestal width is small. A polarization extinction ratio larger than 20 dB at 1550 nm is measured in the pedestal waveguide sample, demonstrating single polarization transmission property experimentally. Thanks to its large single polarization transmission bandwidth, robustness in fabrication tolerance, and simple fabrication process, pedestal waveguides will have potential applications as simple silicon-integrated polarizers.

Information Spectral Efficiency and Launch Power Density Limits Due to Fiber Nonlinearity for Coherent Optical OFDM Systems

We derive closed-form expressions for nonlinear transmission performance of dual-polarization closely spaced coherent optical orthogonal frequency-division multiplexing (CO-OFDM) systems. We find that the fiber nonlinear noise exhibits a signature of the flicker noise or 1/f noisebeyond the corner frequency that is inversely proportional to the total participating bandwidth. We derive a noise enhancement factor that captures the interference effect of nonlinear noises among different spans. For a 10 t 100-km standard single-mode-fiber (SSMF) link with no dispersion compensation at an optimal launch power density of - 15.9 dBm/GHz, the spectral efficiency of 9.90 b/s/Hz can be achieved for dual-polarization transmission, which is about 93% increase over single-polarization transmission. The closed-form expressions are also applicable to the closely spaced coherent single-carrier systems where the symbol rate is much larger than the dispersion walk-off bandwidth, and the optical dispersion is uncompensated.

Coherent Optical Single Carrier Transmission Using Overlap Frequency Domain Equalization for Long-Haul Optical Systems

In this paper, we present coherent optical single carrier transmission (COSC) using overlap frequency domain equalization (OFDE) for the chromatic dispersion (CD) compensation. Residual inter-symbol interference is suppressed by time domain equalization with small tap size. In single polarization transmission, 25-Gb/s COSC transmission with OFDE is demonstrated over 800 km to 4320 km of single-mode fiber (SMF) without optical dispersion compensation. The measured results show that the proposed configuration improves the transmission performance by setting adequate overlapped size of FFT windows and that it is suitable for long-haul optical communication systems.

Analysis/Simulation Study of Cross-Polarization Cancellation in Dual-Polarization Digital Radio

This paper analyzes cross-polarization cancellation in dual-polarization digital radio links transmitting M-ary quadrature amplitude-modulation (M-QAM) signals. We consider the use of three options in the receiver: (1) no cancellation; (2) ideal (i.e., total) cancellation; and (3) optimal nondispersive cancellation. For every option, we assume the canceler to be followed, in each polarization branch, by an ideal minimum mean-square error equalizer. By postulating a statistical model for the co-polarization and cross-polarization responses of a digital radio channel, and then simulating thousands of sets of these responses, we obtain curves that relate outage probability to the number of modulation levels. We show graphically that the no-canceler case is unthinkable; that total cancellation permits results close to those for single-polarization transmission; and that optimal nondispersive cancellation can have a limited range of application. We also examine the effects of key system parameters and the various modeling assumptions.

Coherent optical fiber transmission systems

The feasibility of coherent optical fiber transmission systems, in which laser coherence is utilized to carry information, are studied from the view point of device and system consideration and expected performance. Modulation scheme comparison among optical ASK, FSK, and PSK, transmitter and receiver configurations, single-polarization transmission through fibers, problems in providing stable local oscillator waves, and wide-band photodetectors are discussed together with alternative technologies to realize high performance systems. A study on digital system impairments caused by FM quantum noise of laser oscillators, interferometric FM-AM conversion noise, IF frequency fluctuation, and optical fiber transmission turbulence show the feasibility of the systems. Repeater spacing is estimated by considering the transmitting and receiving signal levels, optical fiber loss, and fiber transmission capacity. Repeater spacing of 240 km is feasible by 400 Mbit/s PSK homodyne-detection system and 220 km by 400 Mbit/s FSK heterodyne-discrimination detection system. The regenerative repeater spacing can be expanded further by employing intermediate repeaters with direct optical signal amplification. Based on the performance of semiconductor laser amplifiers, such as traveling wave type, Fabry-Perot cavity type, and injection locked devices, it is estimated that regenerative repeater spacing of more than 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sup> km is feasible with 50 km intermediate repeater spacing. These systems will find application in transoceanic optical fiber cable transmission as well as terrestrial long distance transmission systems.