【Objective】

2

With the continuous emergence of information-based bandwidth-consuming services such as 5th Generation Mobile Communication Technology (5G)

the Internet of Things

and cloud computing

people’s demand for high-speed information transmission has increased dramatically. However

due to the inherent nonlinear effects

the transmission capacity of single-mode optical fiber has approached the Shannon limit

and it will no longer be able to meet people’s needs for ultra-high-speed and large-capacity transmission. Solving the transmission capacity problem has become a top priority. In order to solve the needs of large-capacity communication systems and long-distance high-speed transmission problems

we have built a two-mode single-channel C-band Few Mode Fiber (FMF) transmission system.

【Methods】

2

At the transmitting end

an arbitrary waveform generator is used to convert the digital signal into an electrical signal and drive the In-phase and Quadrature (IQ) modulator to modulate the optical carrier. The modulated signal is transmitted simultaneously using two multiplexing technologies: Mode Division Multiplexing (MDM) and Polarization Division Multiplexing (PDM). In order to achieve long-distance transmission of dual-mode signals of 1 000 km

we construct a dual-circulation loop system. Each time the signal passes through the loop

it will pass through a 50 km FMF. After being transmitted to the target distance

the coupler outputs the signal to the demultiplexing module

and the coherent optical receiver performs homodyne detection on the demultiplexed modulated signal. Finally

the transmitted signal is stored in an oscilloscope for offline Digital Signal Processing (DSP). The signal is sequentially subjected to frequency domain dispersion compensation

downsampling

clock recovery

and least mean square algorithm to restore the original signal.

【Results】

2

It was found that within the range of Optical Signal-to-Noise Ratio (OSNR) of each channel in the experiment

the Bit Error Rate (BER) under low Signal to Noise Ratio (SNR) is close to the theoretical channel result. Under high SNR condition

the BER is 1×10

-2

which is 2.5 dB away from the theoretical value. We test the BER of LP

11a

and LP

11b

modes at Back-To-Back (BTB) and 250

500

750 and 1 000 km transmission cases respectively. The BER at all distances are lower than the Low-Density Parity-Check (LDPC) soft decision threshold with 28% redundancy (5.2×10

-2

Soft Decision - Forward Error Correction (SD-FEC)). The BER after 1 000 km transmission in the two modes are 1.7×10

-2

and 1.8×10

-2

respectively

and the total net transmission rate is 400 Gbit/s.

【Conclusion】

2

This article demonstrates the transmission of a 32 Gbaud MDM-PDM-16 Quadrature Amplitude Modulation (QAM) C-band signal in a 1 000 km two-mode single-channel FMF system. At the receiving end

the advanced Multiple Input Multiple Output (MIMO)-DSP algorithm is used for channel equalization

and the obtained two-mode BER of 1.7×10

-2

and 1.8×10

-2

are both lower than the LDPC SD-FEC threshold with 28% redundancy. The result reachs a domestic record of 400 Gbit/s net transmission rate based on FMF transmission

and highlight the potential of FMF in large-capacity long-distance transmission.