Experimental Characterization of a Burst-Enabled O-OFDM TransceiverJosep M. Fabrega, Michela Svaluto Moreolo, F. Javier Vılchez, Laia Nadal

Centre Tecnologic de Telecomunicacions de Catalunya (CTTC), Castelldefels, Spain

Current wavelength switched optical networks are forecasted to evolve and acquire elastic functionalities, achieved

by using a flexible grid and deploying tunable transceivers with variable bandwidth and bitrate [1]. An interesting

approach is the combination of these gridless functionalities with time domain multiplexing techniques [2]. Ampli-

fication of the expected bursty data streams seems to be one of the challenges when using Erbium Doped Fiber Am-

plifiers (EDFAs), as the burst pattern can cause non-negligible gain transients. Employing Optical Orthogonal Fre-

quency Division Multiplexing (O-OFDM) based on electronic Digital Signal Processing (DSP), each transceiver

can be dynamically adapted to different modulation formats and/or bandwidth occupancies while achieving sub-

wavelength granularity [3]. In this paper a burst-enabled tunable intensity-modulated direct-detection O-OFDM

transceiver based on Fast Hartley Transform (FHT) is experimentally characterized.

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Fig. 1 Experimental setup (a) and system performance (b). Inset of (b) shows a sample capture of the signal at the OSC input.

The experimental setup is shown in Fig. 1a. The DSP at the transmitter/receiver is performed off-line following

the steps detailed in [4]. A stream of data randomly generated at 6 Gb/s is mapped into BPSK format and modulated

by an FHT with 64 subcarriers. The resulting OFDM symbols are serialized, clipped and converted into the analog

domain by an Arbitrary Waveform Generator (AWG) running at 12 GSa/s. The output is filtered and upconverted at

15 GHz. Afterwards the resulting waveform is injected to a Mach-Zehnder modulator biased at the quadrature point

and excited by a tunable SG-DBR laser (TLS). A Variable Optical Attenuator (VOA) is placed after the transmitter

for emulating the link losses. The received signal is optically preamplified by a commercial EDFA, filtered by a

50 GHz Optical BandPass Filter (OBPF) and detected by a PIN diode. The photodetected current is then amplified,

downconverted and lowpass filtered. The resulting signal is acquired by using a realtime oscilloscope (OSC)

at 50 GSa/s and then off-line processed. Bit Error Ratio (BER) is obtained by error counting over a total of

360448 bits. As data bursts are very long compared to the bitrate, an acquisition of an entire burst is not possible

for assessing the impact of the transients. So, the trigger of the oscilloscope is set to half of the amplitude of

the signal after electrical downconversion, for randomly acquiring the signals within the packet duration. The

photodiode used at the receiver is a broadband PIN with low responsivity and optical preamplification is needed.

The optical preamplifier delivers −5 dBm to the the photodetector, for which a BER lower than 10−5 has been

previously measured without preamplifier. The VOA is set for adapting the power at the input of the EDFA and

having a 10 % of maximum overshoot at each burst, with a relaxation time of 1.6 ms. Bursts at several points of the

setup are shown in the insets of Fig. 1a when switching between 1550.12 nm and 1550.52 nm. There it is shown

how the transients due to the burst pattern are present even when configuring the tuning sections of the laser.

Preliminary results for several wavelengths, distributed along the C-band, are depicted in Fig. 1b. There it is

shown that all the measured BER points fall below the forward error correction threshold level, set at 4 ·10−3 [5],

except for the 1530.34 nm where BER is 5.7 · 10−3. This is due to the fact that the peak of amplified stimulated

emission in EDFAs is typically between 1530 nm and 1535 nm. Regarding the achieved OSNRs, the highest value

is 52.34 dB obtained at 1550.12 nm, and the lowest value is 48.15 dB which corresponds to 1530.34 nm. Further

work to mitigate the effects of optical bursts in the proposed O-OFDM transceiver is under progress.This work was supported by the EU-FP7 integrated project IDEALIST (GA no. 317999), the spanish MINECO project FARO (TEC2012-38119),grant PTQ-11-04805 and scholarship grant BES-2010-031072.

References[1] M. Jinno et al. “Spectrum-efficient and scalable elastic optical path network,” IEEE Communications Magazine, 47, 66 (2009).

[2] N. Amaya, et al. “Experimental demonstration of gridless spectrum and time optical switching,” OSA Optics Express, 19, 182 (2011).

[3] W. Shieh and I. Djordjevic, OFDM for Optical Communications (Elsevier, 2010).

[4] M. Svaluto Moreolo et al. “Experimental Demonstration of a Cost-Effective Bit Rate Variable IM/DD Optical OFDM with Reduced Guard

Band,” OSA Optics Express, 20, B159-B164 (2012).

[5] ITU-T Rec. G.975.1, “Forward error correction for high bit-rate DWDM submarine systems,” (2004).

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