Experimental platform for Optical OFDM systems
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| Fig 1. Experimental platform for optical OFDM systems (EOS) within the ADRENALINE Test-bed® |
Optical OFDM: principles
OFDM is a multicarrier transmission technique where the signal data stream is transmitted over several lower-rate sub-channels, whose sub-carriers are orthogonal to each other, and their spectra partially overlap. Since the available spectrum is divided into multiple narrowband sub-channels, the use of OFDM in optical networks mitigates transmission impairments and provides high data rate transmission. The high tolerance to fiber dispersions (CD and PMD) allows extending the transmission reach before significant distortion. O-OFDM offers an alternative electronic dispersion compensation technique and also provides an increasing of the system spectral efficiency, especially if combined with high order modulation formats, orthogonal band multiplexing and/or polarization multiplexing.
The signal processing in the OFDM transceiver takes advantage of the efficient algorithm of Fast Fourier Transform (FFT) to implement the OFDM modulation/demodulation. Each transform subcarrier carries a data symbol encoded into a constellation point; usually QAM (quadrature amplitude modulation) is used. The use of cyclic prefix (CP) in transmission combined with equalization in reception allows a correct recovery of signals distorted by a linear dispersive channel.
In order to transmit the OFDM signal on an optical link, alternative solutions have been proposed. Direct detection (DD) and coherent schemes can be used, trading simplicity against increased performance.
Optical OFDM using direct detection (DD O-OFDM)
DD systems are simpler than coherent schemes, no laser is required as local oscillator at the receiver and they can be implemented by using commercial components. A simple Mach-Zehnder modulator (MZM) transmits the analog OFDM signal over the optical link and a simple photo-diode is used for detection. The amplified photocurrent is converted to a digital signal by means of an ADC (analog-to-digital converter). The simplicity of this cost-effective solution is at expenses of the spectral efficiency and its effectiveness depends on the system linearity. Only a correct mapping between the RF OFDM signal and the optical signal enables a correct detection. Therefore, additional single-side band (SSB) modulation can be adopted. If this solution is considered, an optical filter is required to transmit in the fiber channel only one side of the optical spectrum, which is symmetric with respect to the optical carrier. To reduce the ASE noise of the optical amplifiers in the optical link another filter with narrow band can be added before the photoreceiver. |
The experimental platform elements
Within the Adrenaline test-bed® we are developing an experimental platform for optical OFDM systems (EOS). Specifically, the design of flexible, power efficient and cost-effective direct detection optical OFDM transmission schemes are studied and assessed. The experimental validation of the investigated modulation schemes based on O-OFDM technique is enabled by the test-bed equipment indicated in Fig. 2. A brief description of the EOS test-bed elements follows.
Arbitrary waveform generator (AWG)
 This equipment enables to generate analog IQ signals. The input digital signal is created by using simulation software, such as Matlab. The required AWG bandwidth must be 5-10 GHz for implementing an optical OFDM. In fact, an RF OFDM signal with 5GHz bandwidth can be considered; additionally, a guard interval (to avoid intermodulation products at the detector) achieved without an external RF mixer, can be also taken into account. When one single output is required, up to 24GSample/s can be transmitted by using the interleaving function of the best commercially available AWG (state of the art). See TEKTRONIX AWG7122C for more details. |
Optical source and modulator
 In order to convert the OFDM signal to an optical signal, the optical source must be modulated by an external modulator driven by the electrical signal. The optical source used is an EXFO IQS-FLS-2600B, that is an Erbium doped fiber laser with tuning range from 1510 nm up to 1612 nm, 0 dBm of output power and OSNRs higher than 45dB at the output. After the laser, a simple Mach-Zehnder modulator (MZM) can modulate only a real OFDM signal (single I component); alternatively, the I and Q components at the outputs of the AWG must be mixed at RF. This last case requires an RF IQ mixer. Both I and Q components of the complex signal can be modulated directly in the optical domain by using a nested MZM or optical IQ/quadrature modulator. Currently, in the experimental platform, a LiNbO3 MZM with 40 GHz bandwidth and Pi voltage of 6.4 V is used. |
Fiber link
 CTTC premises has more than 600 km of standard single mode fiber (G652 and G655), which can be used for testing the transmission performance of the proposed setup. A proper inline optical amplification must be considered to correctly design the optical OFDM transmission system. |
Optical receiver
 For DD systems a single photo-detector and an electrical amplifier are needed. The photodetector used in the EOS setup is a PIN photo-diode with DC-50 GHz bandwidth and responsivity of 0.5 A/W. The amplifier in use features high linearity with 26 dB gain, 20 GHz bandwidth and 1 Vpp output voltage swing. |
Oscilloscope
 This instrument captures the received signal and converts it to digital samples to be processed by the software (e.g. Matlab). The bandwidth of the oscilloscope is 20 GHz, enabling the correct processing of the detected signal. Moreover, at maximum bandwidth this instrument is capable to provide sampling rates up to 100 GSample/s. See TEKTRONIX DPO72004C for more details. |
Future work and experimental setup extension
After the ongoing experiments (back-to-back and with fiber link), the next step is to evaluate the impact of the chromatic dispersion and the polarization mode dispersion. This will be performed with the help of our specialized instrumentation, which includes a broadband optical source (EXFO FLS-5800), a PMD emulator (EXFO EM-550) and a CD/PMD analyzer (EXFO FTB-400).
Furthermore, a straightforward extension of the experimental platform is the analysis and monitoring of coherent optical OFDM systems. The upgrade can be performed by replacing the MZM with an IQ optical modulator and by substituting the DD receiver of the optical test-bed by a coherent receiver (including local laser, optical hybrid and array of balanced detectors).
