SNR enhanced OAM mode division multiplexing with in-band noise modulation based on cascade delta-sigma modulation

Due to mode coupling, a high signal-to-noise ratio (SNR) is required in orbital angular momentum (OAM) modular division multiplexing (MDM) systems to improve transmission performance. In this paper, a cascade delta-sigma modulation (CDSM) scheme is proposed for OAM-MDM intensity modulation and direct detection (IM/DD) transmission. Different from the traditional DSM (TDSM) scheme, the scheme is divided into signal modulation and in-band noise modulation, in which the in-band noise modulation is used to further decrease the quantization noise generated in the signal modulation. At the same time, the output of the in-band noise modulation and the signal modulation are combined through the channel and transmitted in time division multiplexing (TDM) mode. At the receiver, the sequence generated by signal modulation and in-band noise modulation are subtracted to eliminate the in-band noise, resulting in better noise shaping performance. In this paper, an experiment of 240 Gbit/s transmission based on the CDSM scheme with two OAM modes was performed over a 2 km ring-core fiber (RCF). Similarly, the TDSM scheme and multi-stage DSM (MDSM) scheme are also employed in the same experiment setup. The experimental results show that the CDSM scheme successfully realizes 16,777,216 QAM signal transmission and the BER below the 15% soft decision forward error correction (SD-FEC) threshold in two modes. For the TDSM scheme, the BER cannot be below the SD-FEC threshold in two modes. For the MDSM scheme, the BER cannot be below the SD-FEC threshold for the OAM mode l = 2. Conversely, the BER can be below the SD-FEC threshold for the l = 3. The CDSM scheme outperforms the MDSM scheme with improvements in receiver sensitivity of 2.5 dB for the OAM mode l = 3 at the SD-FEC threshold. The results show that the CDSM scheme outperforms the TDSM scheme and MDSM scheme in two OAM modes at the same experimental conditions. Compared with the TDSM scheme and MDSM scheme, the CDSM scheme has an SNR gain of the recovered waveform with 7.1 dB and 3 dB for the two OAM modes at the SD-FEC threshold, respectively. The results also show that the OAM-MDM system based on the CDSM scheme can recover the waveform of the transmitter and realize high-fidelity transmission. This transmission scheme is advantageous for implementing high-capacity OAM-MDM optical fiber communication systems.