20 × 960-Gb/s Space-division-multiplexed 32QAM transmission over 60 km few-mode fiber

A Multi-Layer Erbium-Doped Air-Hole-Assisted Few-Mode Fiber with Ultra-Low Differential Modal Gain

The air-hole assisted few-mode fiber (AH-FMF) enables modal intensity balance and offers a profound prospect in gain equalization with the combination of adaptive ion doping. In this paper, we proposed an AH-FM-EDF with a multi-layered erbium doping profile. In AH-FM-EDF, due to the central air hole, only the first radial order modes (LP01, LP11, LP21, and LP31) are supported, and all the modes are confined in the same high refractive index core region. The differential modal gain (DMG) is highly reduced by optimizing the erbium doping proportion in each layer. Compared with uniform doping, the DMG is reduced from 4 dB to 0.14 dB as triple-layer doping is deployed. Additionally, the proposed erbium-doped fiber performs well in gain flattening and fabrication tolerance over the whole C-band.

Intelligent gain flattening in wavelength and space domain for FMF Raman amplification by machine learning based inverse design

We propose a machine learning based approach to design few-mode DRAs by using neural networks to optimize the pump wavelengths, powers and mode content in order to obtain flat gain spectrum with low mode-dependent gain (MDG). Based on the proposed intelligent inverse design method, amplification optimization for the random fiber laser based two-mode DRA can be achieved with gain flatness of 1.0 dB and MDG of 0.6 dB at 14.5 dB on-off gain level. For backward pumping four-mode DRA, gain flatness of 0.46 dB and MDG of 0.3 dB can be achieved at 12.5 dB on-off gain.

18 km low-crosstalk OAM + WDM transmission with 224 individual channels enabled by a ring-core fiber with large high-order mode group separation

The space domain is regarded as the only known physical dimension of lightwaves left to be exploited for optical communications. Very recently, much research effort has been devoted to using orbital angular momentum (OAM) spatial modes to increase the transmission capacity in fiber-optic communications. However, long-distance low-crosstalk high-order OAM multiplexing transmission in fiber is quite challenging. Here we design and fabricate a graded-index ring-core fiber to effectively suppress radially high-order modes and greatly separate high-order OAM mode groups. By exploiting high-order OAM mode group multiplexing, together with wavelength-division multiplexing (WDM), i.e., 12.5 Gbaud 8-array quadrature amplitude modulation (8-QAM) signals over OAM₊₄ and OAM₊₅ modes on 112 WDM channels (224 individual channels), we experimentally demonstrate 8.4 Tbit/s data transmission in an 18 km OAM fiber with low crosstalk. Multiple-input multiple-output digital signal processing is not required in the experiment because of the large high-order mode group separation of the OAM fiber. The demonstrations may open a door to find more fiber-optic communication and interconnect applications exploiting high-order OAM modes.

Orbital angular momentum mode multiplexed transmission in heterogeneous few-mode and multi-mode fiber network

Mode-division multiplexing (MDM), which employs the spatial modes of light as information carriers, has been widely investigated to increase the transmission capacity. Few-mode fibers (FMFs) and multi-mode fibers (MMFs) have been used for MDM fiber transmission. One of the MDM techniques known as twisted light multiplexing using orbital angular momentum (OAM) modes has recently attracted increasing interest. In this Letter, by splicing two FMFs together with a conventional OM3 MMF, we propose and demonstrate OAM-based MDM in a heterogeneous fiber-optic network, i.e., two OAM mode (OAM₀₁ and OAM_-11) multiplexing transmission in the heterogeneous FMFs and MMF network. We transmit 20-Gbit/s quadrature phase shift keying signals over two OAM modes in different mode groups without multiple-input multiple-output equalization techniques and achieve less than 2.8 dB optical signal-to-noise ratio penalties at a bit-error rate of 2×10^-3. The experimental results show favorable transmission performance of OAM-based MDM in heterogeneous FMFs and MMF network compared to the one in FMF.

Volume holographic spatial mode demultiplexer with a dual-wavelength method

Volume holographic demultiplexers (VHDMs) provide spatial mode demultiplexing using simple optical systems. However, applying VHDM to practical optical communication systems is difficult, as typical holographic media have no sensitivity in the infrared region, which includes optical transmission bands. In this paper, we propose a VHDM scheme combined with a dual-wavelength method (DWM). Using the DWM, VHDMs are able to perform mode demultiplexing in the optical transmission bands. We experimentally demonstrated the basic operation of our proposal using experiments performed at an 850-nm wavelength. In addition, we performed numerical simulations to investigate the application of VHDM to the C-band.

Feed-forward frequency offset estimation for 32-QAM optical coherent detection

Due to the non-rectangular distribution of the constellation points, traditional fast Fourier transform based frequency offset estimation (FFT-FOE) is no longer suitable for 32-QAM signal. Here, we report a modified FFT-FOE technique by selecting and digitally amplifying the inner QPSK ring of 32-QAM after the adaptive equalization, which is defined as QPSK-selection assisted FFT-FOE. Simulation results show that no FOE error occurs with a FFT size of only 512 symbols, when the signal-to-noise ratio (SNR) is above 17.5 dB using our proposed FOE technique. However, the error probability of traditional FFT-FOE scheme for 32-QAM is always intolerant. Finally, our proposed FOE scheme functions well for 10 Gbaud dual polarization (DP)-32-QAM signal to reach 20% forward error correction (FEC) threshold of BER=2×10^-2, under the scenario of back-to-back (B2B) transmission.

Novel MDM-PON scheme utilizing self-homodyne detection for high-speed/capacity access networks

In this paper, we propose a cost-effective, energy-saving mode-division-multiplexing passive optical network (MDM-PON) scheme utilizing self-homodyne detection for high-speed/capacity access network based on low modal-crosstalk few-mode fiber (FMF) and all-fiber mode multiplexer/demultiplexer (MUX/DEMUX). In the proposed scheme, one of the spatial modes is used to transmit a portion of signal carrier (namely pilot-tone) as the local oscillator (LO), while the others are used for signal-bearing channels. At the receiver, the pilot-tone and the signal can be separated without strong crosstalk and sent to the receiver for coherent detection. The spectral efficiency (SE) is significantly enhanced when multiple spatial channels are used. Meanwhile, the self-homodyne detection scheme can effectively suppress laser phase noise, which relaxes the requirement for the lasers line-width at the optical line terminal or optical network units (OLT/ONUs). The digital signal processing (DSP) at the receiver is also simplified since it removes the need for frequency offset compensation and complex phase correction, which reduces the computational complexity and energy consumption. Polarization division multiplexing (PDM) that offers doubled SE is also supported by the scheme. The proposed scheme is scalable to multi-wavelength application when wavelength MUX/DEMUX is utilized. Utilizing the proposed scheme, we demonstrate a proof of concept 4 × 40-Gb/s orthogonal frequency division multiplexing (OFDM) transmission over 55-km FMF using low modal-crosstalk two-mode FMF and MUX/DEMUX with error free operation. Compared with back to back case, less than 1-dB Q-factor penalty is observed after 55-km FMF of the four channels. Signal power and pilot-tone power are also optimized to achieve the optimal transmission performance.

2-LP mode few-mode fiber amplifier employing ring-core erbium-doped fiber

A fiber amplifier supporting 2 LP modes that employs a ring-core erbium-doped fiber (RC-EDF) is investigated to reduce differential modal gain (DMG). The inner and outer radii of the ring-core of the RC-EDF are clarified for 2-LP mode operation of the amplifier, and are optimized to reduce the DMG. It is shown that using the overlap integral between the erbium-doped core area and the signal power mode distribution is a good way to optimize the inner and outer radii of the ring-core of the RC-EDF and thus minimize the DMG. A fabricated RC-EDF and a constructed 2-LP mode EDFA are described and a small DMG of around 1 dB is realized for LP01, LP11 and LP21 pumping.

Optical network scaling: roles of spectral and spatial aggregation

As the bit rates of routed data streams exceed the throughput of single wavelength-division multiplexing channels, spectral and spatial traffic aggregation become essential for optical network scaling. These aggregation techniques reduce network routing complexity by increasing spectral efficiency to decrease the number of fibers, and by increasing switching granularity to decrease the number of switching components. Spectral aggregation yields a modest decrease in the number of fibers but a substantial decrease in the number of switching components. Spatial aggregation yields a substantial decrease in both the number of fibers and the number of switching components. To quantify routing complexity reduction, we analyze the number of multi-cast and wavelength-selective switches required in a colorless, directionless and contentionless reconfigurable optical add-drop multiplexer architecture. Traffic aggregation has two potential drawbacks: reduced routing power and increased switching component size.