E. Ciaramella

G. Cossu

M. Presi

COSSU, GIULIO

PRESI, Marco

CIARAMELLA, ERNESTO

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RADIO-OVER-FIBER TRANSMISSION BASED ON SINGLE SIDEBAND CARRIERS TO OVERCOME THE DISPERSION PENALTIES USING A INJECTION-LOCKED FABRY-PERÒTG. Cossu, M. Presi and E. CiaramellaScuola Superiore Sant’Anna, via G. Moruzzi 1, 56124, Pisa, ITALY; Corresponding author:g.cossu@sssup.itAbstract:  We experimentally  demonstrate a technique for  optical  generation of  a single  sideband carrier  based on an injection-locked semiconductor  laser.  By this technique a 64-QAM 54 Mb/s OFDM signal at 6 GHz is transmitted successfully over 80 km SMF avoiding penalties associated to fading. 1. IntroductionThe distribution of radio signals through optical fibers (RoF) is an important task to enable the delivery of  wireless and wired services in future access networks.  Conventional  RoF transmitters based both on direct or external modulation produce double sideband optical signals  (DSB).  Radio  frequency  DSB  optical  signals  are  really  sensitive  to  chromatic dispersion which induces fading. This problem can be solved by using single sideband (SSB) modulators,  implemented  for  example  by  using dual-drive  Mach-Zehnder  modulators  [1]. However  this  implementation  is  rather  complex.  SSB signals  can  be  obtained  by  using narrow optical filtering (i.e., by suppressing one of the two sidebands) or by injection-locking (IL)  in  semiconductor  lasers.  In [2]  an IL Fabry-Peròt  laser (FPL) is used to obtain SSB modulation; in this configuration, the FPL is locked at a frequency corresponding to one of the sidebands of the input signal. However, this scheme suffers from mode-partition noise, and  requires  additional  saturated semiconductor  amplifiers  to  reduce  it.  In  [3]  a  simpler approach based on cavity resonance shift [4] is proposed. If the IL laser is directly modulated with an RF signal it is possible to obtain a single sideband modulation provided that the RF frequency  fRF  matches the difference between the injection-locking frequency ωinj and the shifted cavity resonance ωcav (i.e. ωinj - fRF = ωcav). In this case one modulation sidebands falls exactly on the residual shifted cavity mode and then is resonantly amplified (see inset of Fig. 1).Here  we  exploit  the  same  principle  to  demonstrate  a  Double-to-Single  Sideband Converter composed just by a DC-biased FPL: we consider an input optical signal, coming from a remote node of a network and DSB modulated at a frequency fRF . This signal is then sent into the converter (i.e., the FPL). Under proper injection conditions, i.e., if one of the modulated sidebands falls on the shifted cavity mode, the sideband is strongly amplified, thus converting the input DSB signal into a SSB signal. We report a proof of principle of this scheme by using a 54Mb/s radio signal (OFDM, 64-QAM @ 6 GHz): the conversion allows to transmit the radio signal over 80 km Single Mode Fiber (which would be not possible with a DSB signal due to the fading effect).2. Experiment and ResultsFig.  1  Experimental  setup:  ECL  External  Cavity  Laser;  MZM:  Mach-Zehnder  Modulator;  PC:  Polarization Controller; OC: Optical Circulator; FPL: Fabry-Peròt Laser; SMF: Single Mode Fiber; EDFA: Erbium Doped Fiber  Amplifier; VOA: Variable Optical Attenuator; PIN: P-I-N photodiodeFig.  1 reports  the  experimental  setup.  An  External  Cavity  Laser  (ECL)  emitting  at λS~1550 nm is externally modulated by a Mach-Zehnder Modulator (MZM) driven by a radio signal at 6 GHz. The radio signal was generated by an arbitrary waveform generator with 25 MHz  bandwidth  up  to  6 GHz.  It  consisted  of  a  54  Mb/s  64-QAM  OFDM  signal  (52 subcarriers spaced by 315KHz) carrying a PN15 data pattern. The radio signal power used to drive the MZM was –10 dBm. The double sideband signal produced by the MZM is then sent to the proposed converter. The converter comprises a Polarization Controller, an Optical Circulator and a Fabry-Peròt laser (FPL). The FPL is a commercial pigtailed device, with a front facet reflectivity of about 2 %, 44 GHz free running mode spacing and 10 dBm output power when biased at 64 mA. The injection ratio R (defined as the power ratio between the injecting signal and free running FPL) is set to -12 dB: in this condition, the cavity resonance shift  falls  approximately  at  -6 GHz  from  the  injection  frequency.  The  shifted  resonance frequency can be fine tuned by controlling the injection ratio, the frequency detuning and the polarization state of the injection light [4]. In this way it  is possible to match with a good accuracy the frequency of the input radio signal. The converted signal is then propagated through 80 km of SMF (1360 ps/nm total dispersion, 17 dB insertion loss) and then received with an optical preamplified receiver. The signal is analysed by means of a Vector Signal Analyser.In Fig. 2-a) we report the optical spectra of the input DSB signal (continuous line) and the converted SSB (dashed). Spectra are recorded with a RBW of 0.01 nm. As it can be seen, the longer wavelength sideband gets amplified in the converter. In the same picture is also possible to observe the suppressed FP modes, after the IL. Fig. 2-b) reports the link  frequency response.  This  measure is  performed by  means  of  a  Network  Analyser.  The dashed line  report  the  MZM response.  We also  show the link  frequency response after propagation through 80 km SMF. In this case, the DSB signal (dashed-dot line) shows a minimum transmission at 6.7 GHz. We note that at 6 GHz the frequency response shows a non  negligible  attenuation  of  about  10 dB.  On the  other  end,  after  performing  the  SSB conversion in the FPL, the frequency response at 6 GHz is greatly improved, by about 37 dB in respect to the DSB case, and by about 27 dB in respect to the back-to-back case. We thus conclude that the proposed converter not only compensate for the fading effect,  but also increase the link gain at the desired frequency. This is advantageous, because it allows to cope  with  signals  of  low  modulation  index:  RoF  signals  are  often  characterized  by  low modulation index in order to ensure linear operation of the electro-optic modulators.We also  measured  the  system  performance,  after  transmission  over  80  km  SMF: results are indicated in Fig 2-c), where the back-to-back curve (dashed line) is also reported for completeness.  The signal quality is reported in terms of Error Vector Magnitude (EVM). According to the gain properties of the converter, the SSB signal outperforms both the back-to-back and the DSB signals. In particular, the non converted signal shows unacceptable EVM (that prevent a correct reception of the signal). Instead, the converted signal shows an EVM improvement of about 15 dB. -70-60-50-40-30-20-101553.8 1554 1554.2 1554.4 1554.6 1554.8 1555DSBSSBOptical Power (dBm)  Wavelength (nm)-90-80-70-60-50-40-30-20-100 2 4 6 8 10MZMSingle-Sideband after 80 km SMFDouble Sideband after 80 km SMFLink Gain (dB)Frequency (GHz)-30-25-20-15-10-19 -14 -9 -4 1B2B80 km SMF DSB80 km SMF SSBEVM (dB)Received Power(dBm)(a) (b) (c)Fig.  2  a)  Input  (DSB)  and  output  (SSB)  optical  spectra  (RBW:  0.01 nm).  The  lower  frequency  modulation sideband is amplified by more than 10 dB.  b) Frequency response of the optical link, measured after the MZM, and after 80 km SMF transmission, with and without conversion.  c) System performance.3. ConclusionsIn  summary,  we  characterized the performance of  a  double-to-single  sideband  converter based on an injection-locked semiconductor laser. By using this converter it is possible to improve the performance of a Radio-over-Fiber link. In particular, a 15 dB EVM improvement has been demonstrated over 80 km SMF (1360 ps/nm total dispersion) for a 54Mb/s OFDM signal (64-QAM) at 6 GHz.4. References[1] G. H. Smith, D. Novak, Z. Ahmed, “Overcoming Chromatic-Dispersion Effects in Fiber-Wireless Systems Incorporating External Modulators”, IEEE Trans. on Microwave Theory and Techniques, 45, 8, 1997[2] T. T. Pham, H-S. Kim, Y-Y. Won, S-K. Han, “Bidirectional 1.25-Gbps Wired/Wireless Optical Transmission Based on Single Sideband Carriers in Fabry–Pérot Laser Diode by Multimode Injection Locking”, J. Lightwave Technology, 27, 13, 2009[3] H-K Sung, E. K. Lau, M. C. Wu, “Optical Single Sideband Modulation Unsing Strong Optical Injection-Locked Semiconductor Lasers”, IEEE Photonic Technology Letters, 19, 13, 2007[4] A. Murakami, K. Kawashima, K. Atsuki, “Cavity Resonance Shift and Bandwidth Enhancement in Semiconductor Lasers With Strong Light Injection”, IEEE Journal of Quantum Electronics, 39, 10, 2003

Radio-over-Fiber transmission on single sideband carriers to overcome the dispersion penalties using a injection-locked Fabry-Pérot

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