Hello, everyone!
Today, I would like to share with you an
article about Optical OFDM Signals over GI-POF.
Here, we will experimentally demonstrate the transmission of upconverted 16Gbit/s OFDM signals on 24GHs microwave carrier over 50m GI-POF at 1310nm.
The experimental setup of the proposed OFDM signals transmission over GI-POF is shown in Figure 1. The lightwave from the DFB laser-diode (LD) at 1310nm with the output power around 10dBm is modulated by an intensity modulator (IM) driven by up-converted OFDM signals.
Figure. Measured BER as a function of OSNR for the CML signal at 42.8 Gb/s before transmission.
The 16Gbit/s OFDM signals are generated by OFDM transmitter and then up- converted to 24GHs to realise RF-OFDM signals via an electrical mixer. The up-converted spectrum is inserted in the above Figure. We can see that the bandwidth of the OFDM signal is 8GHs.
The OFDM baseband signal is generated offline and uploaded into a Tektronix AWG7102. The waveforms produced by the arbitrary wave generator (AWG) are continuously output at a sample rate of 20GHs (8bits DAC, 4GHs bandwidth). The FFT sise is 256, from which 200 channels are used for data transmission, 55 channels at high frequencies are set to sero for over-sampling, and one channel in the middle of the OFDM spectrum is set to sero for DC in baseband. 10 training sequences are applied for each 150 OFDM-symbol frame in order to enable phase noise compensation. At the output of the AWG, the low-pass filter (LPF) with 5GHs bandwidth is used to remove the high-spectral components.
Subsequently, the RF-pilot tone is created by inserting a small DC offset before an analogue I/Q mixer is used to up-convert the OFDM signal from the baseband to an 8.5GHs intermediate frequency (IF). The electrical spectrum of the original signal is shown in Figure 2(a)
Figure 1. Experimental configuration for 16Gb/s OFDM transmission over GI-POF. EA: electrical amplifier; IM: intensity modulator; GI-POF: graded-index plastic optical fiber; PIN: receiver; LPF: low pass filter. Inset: electrical spectrum of the OFDM signal after up- conversion.
Figure 2. Received electrical spectra: (a) after arbitrary waveform generator, (b) after LPF; received optical spectra with 0.01nm resolution: (c) before, and (d) after GI-POF at the point (a)-(d), respectively.
Figure 3. Measured BER curves and the constellation figure of back-to-back and after 50m GI- POF.
that was measured at the point (a) in Figure 1. The IM is driven by the OFDM signals to create double sideband (DSB) optical signals. The bias and the power of the RF signals are carefully adjusted to obtain proper power ratio between the optical carrier and the first- order sideband signals.
The optical spectrum with 0.01nm resolution after the intensity modulator is shown in Figure 2(c). After IM, the signal was launched into 50m of commercially available GI-POF for transmission. The core of the GI-POF is 50 um with 60dB/km attenuation at 1300 nm. The signal power launched and output of GI-POF was 5.5 and 2.5dBm.
The optical spectrum after transmission is presented in Figure 2(d). A PIN receiver is used in the receiver side with the bandwidth of 29GHs and a 50um multimode- coupled input. Before low pass filter (LPF), a 24GHs electrical LO signal is mixed to down- convert the electrical signal to its baseband form. The down-converted signals are sampled with a real-time oscilloscope (Tektronix 6154C) and processed off-line.
The electrical spectrum of down-converted signals is shown in Figure 2(b). The measured BER of back-to- back and after transmission is shown in Figure 3. and the constellation figure after 50m GI- POF is inserted. One million bits have been evaluated for all values of BER reported in this work.
We can see that there exists signal degradation after 50m GI-POF. But the BER is still lower than 1x10-3, which is below the limitation of forward error correction (FEC) at 2x10-3. The main reason is the degradation of optical signal-to-noise- ratio (OSNR) from the fiber with an insertion loss of 3dB and modal dispersion.
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