We report a novel OFDM-transmitter operating in the emerging 2µm waveband. Sub-FEC limit transmission of a 32QAM signal over 500m of both solid and hollow-core fiber was achieved and the generation of 30Gbits 64QAM demonstrated

Alam, S.U.

Chen, Y.

Heidt, A.

Kelly, B.

Li, Z.

Liu, Z.

O'Carroll, J.

Petrovich, M.N.

Phelan, R.

Poletti, F.

Richardson, D.J.

Slavík, R.

Wheeler, N.V.

Wooler, J.P.

English

Southampton (e-Prints Soton)

Up to 64QAM (30 Gbit/s) Directly-modulated and Directly-detected OFDM at 2 µm Wavelength Z. Liu(1), Z. Li(1), Y. Chen(1), J.P. Wooler(1), B. Kelly(2), R. Phelan(2), J. O’Carroll(2), N.V. Wheeler(1), A. M. Heidt(1), F. Poletti(1), M.N. Petrovich(1), S. Alam(1), D.J. Richardson(1), and R. Slavík(1) (1) Optoelectronics Research Centre, University of Southampton, Southampton, U.K. z.liu@soton.ac.uk (2) Eblana Photonics Inc., Dublin, Ireland  Abstract We report a novel OFDM-transmitter operating in the emerging 2-µm waveband. Sub-FEC limit transmission of a 32QAM signal over 500m of both solid and hollow-core fiber was achieved and the generation of 30Gbits 64QAM demonstrated. Introduction The  2-µm  spectral  band  is  emerging  as  an attractive region for optical communications for several  reasons.  In  particular,  a  radically  new form  of  transmission  fiber,  the  Hollow-Core Photonic  Bandgap  Fiber  (HC-PBGF),  which offers  substantially  lower  nonlinearity,  lower-latency and potentially ultimately lower minimum loss in this wavelength range than conventional solid fibers operating at 1550 nm. Furthermore, the  Tm-doped  fiber  amplifier  (TDFA),  which supports  around  two  times  larger  bandwidth than  the  combined  C+L  bands  of  the  erbium doped  fiber  amplifier  operates  in  this waveband1.  However,  the  lasers,  modulators and  receivers,  required  for  efficient communications in this new spectral region, are only just starting to become available. To date OOK  signal  generation  using  an  external modulator at up to 8.5 Gbit/s2-3 and Fast-OFDM BPSK  signal  generation  via  direct  laser modulation  (5  Gbit/s)3  were  demonstrated. These signals were successfully transmitted and received  in  two  separate  four-channel  WDM experiments (1 OOK channel and 3 Fast OFDM BPSK  channels)  -  first  over  50  m  of  standard optical fiber3, and more recently over 290 m of HC-PBGF4.   Concurrently,  there  has  been  significant interest in increasing the transmission capacity while  reducing  the  cost  of  optical  transmission by using simple, low-cost optical transmitters in conjunction  with  powerful  digital  signal processing  (DSP).  A  recent  example  takes advantage of direct multi-level laser modulation using  a  sub-carrier  and  direct  detection  at  the receiver5. At the conventional 1550 nm spectral band,  operation  with  16QAM  signals  at  14 Gbaud symbol rate  was demonstrated using  a special  high-bandwidth  semiconductor  laser6. Data transmission over 4 km of standard SMF-28  fiber  was  demonstrated.  Another  report concerns  use  of  a  specially-designed semiconductor  laser  directly-modulated  under optical  injection  locking  (OIL)  that  reduces  the chirp of the directly-modulated laser (DML)  to enable  5  Gbaud  16QAM  OFDM  transmission over  25  km  of  SMF-287.  This  new  approach seems  particularly  well  suited  for  2  µm  optical communications: it is potentially low-cost (direct modulation + direct detection) and moreover the optical  components  necessary  for  ‘classical’ coherent  modulation/detection  are  still  to  be developed for the 2 µm region.   Here, we demonstrate a transmitter operating at  2  µm  comprising  two  low-cost  recently-developed semiconductor lasers (a single-mode master  and  a  directly-modulated  Fabry-Perot slave)8.  We  report  5-Gbaud  directly-modulated OFDM  transmission  for  16QAM,  32QAM  and 64QAM signals. Whilst there is still substantial scope for further optimization  of these devices we have already achieved a more than 3-times higher data rate per channel than in previous 2-µm based transmission experiments2-3.     Fig. 1: (a) Experimental setup; (b) Optical spectrum of the free running directly modulated laser (DML); (c) Optical spectrum of the DML after injection locking. Experimental Setup Fig.  1  (a)  shows  our  experimental  setup.  The waveform samples of the base-band OFDM are calculated offline based on a PRBS of 218-1 for 16QAM mapping and a PRBS of 219-1 for both 32QAM  and  64QAM  mapping.  An  inverse Fourier  Transform  size  of  256  is  used.  Of  the 256 subcarriers, the central 238 subcarriers are selected  as data carriers.  A four-sample cyclic prefix (CP) is placed both before and after each OFDM symbol. The signals were designed for a symbol  rate  of  21.2  MHz  and  the  waveforms were  digitally  up-converted  to  a  3-GHz  RF carrier frequency, which results in an electrical bandwidth  of  5.2  GHz  (from  0.4  GHz  to  5.6 GHz).  The  nominal  data  rates  for  16QAM, 32QAM,  and  64QAM  mapping  are  20.2  Gb/s, 25.2  Gb/s,  and  30.3  Gb/s,  respectively.  The waveforms  are  digitally  filtered  and  pre-emphasized  to  compensate  for  the  high-frequency laser roll-off.   The  DML  operating  as  a  slave  laser8  is  an In0.75Ga0.25As Fabry-Perot laser diode (FP-LD) in a  butterfly  package  with  its  output  directly coupled  into  a  polarization-maintaining  (PM) fiber without any in-built isolator (as necessary for  the  OIL).  The  master  is  an  In0.75Ga0.25As multiple quantum-well discrete-mode laser diode emitting  3  mW  CW  light  at  2001.1  nm8  and incorporates  a  dual-stage  isolator  within  the butterfly  package.  We  characterized  its frequency  noise9  and  determined  a  1  MHz linewidth. A 60:40 split ratio PM fiber coupler is used for the OIL.   More  than  30  longitudinal  modes  are generated by the free-running slave DML (Fig. 1 (b)).  Once  injection  locked  with  0.9  mW  of master light, single mode operation with a side-mode suppression ratio of 47 dB was achieved (Fig.1 (c)). The slave DML was biased at 43 mA and  modulated  by  electrical  signals  generated using  an  arbitrary  waveform  generator  (AWG) operating at 22.4 GS/s  with 1.8  Volts peak-to-peak. The wavelength of the injected light was tuned via the bias current control of the master laser.   Due  to  the  relatively  low  sensitivity  of  our photo  receiver  (details  below),  the  transmitter power  (1.1  mW)  was  boosted  to  76  mW  in  a home-built TDFA1. The light was launched into either 500 m of SMF-28 (loss ~22 dB/km at 2 µm), or 500 m of HC-PBGF (state-of-the-art loss of 2.5 dB/km and a 3-dB transmission bandwidth of  122  nm  (1954-2076nm)).  The  PBGF  was pigtailed  with  SMF-28  fiber,  yielding  a  total insertion  loss  of  6.5dB  (with  an  estimated insertion  loss  of  around  2.5  dB  at  each  end arising primarily due to mode-mismatch).   At  the  receiver  side,  a  3  nm  optical  band pass filter was used to remove the out-of-band ASE noise. The filtered light was then split for simultaneous  power  monitoring  and  direct detection.  A  7-GHz,  2-µm  reverse-voltage-biased  photo-diode  (ET-5010F)  in  conjunction with  a  wide-bandwidth  (10  GHz)  RF  amplifier was  used  for  direct  detection.  The  signal  was then sampled at 40 GS/s by an Agilent DSO-X 93204A  digital  signal  analyzer.  Decoding  was implemented  using  offline  digital  signal processing,  and  the  bit-error-ratio  (BER)  was determined  based  on  offline  error  counting.  A 16%  training  overhead  was  used  for  symbol synchronization and channel estimation. After 1-tap equalization the BER was calculated. Experimental Results The BER as a function of received optical power for back-to-back operation is shown in Fig. 2 (a). Our transmitter operates error free with 16QAM and  below  the  FEC  threshold  of  3.8×10-3  for 32QAM  and  64QAM,  at  nominal  data  rates  of 20.2  Gb/s,  25.2  Gb/s,  and  30.3  Gb/s, respectively. Fig. 2 (b) shows the BER curves after transmission through the 500 m of SMF-28 or HC-PBGF. The power penalty is comparable       Fig.  2:  (a)  BER  measurements  of  back-to-back  OFDM signal at different optical powers (triangle: 16QAM, circle: 32QAM,  square:  64QAM).  (b)  BER  measurements  after transmission through PBGF and SMF-28 (c) Constellation diagrams of the demodulated signal for different subcarrier modulation.    for both fibers: 0.3 dB at the FEC threshold for 16QAM,  and  1.9  dB  for  32QAM,  with  both operating  well  below  the  FEC  limit. Whilst  the 64QAM transmission is slightly above the FEC threshold  we  believe  use  of  a  narrower-band filter  would  have  allowed  sub-threshold operation (we were using a 3 nm filter for a <12-GHz  bandwidth  signal).  Fig.  2  (c)  shows  clear demodulated  constellation  diagrams  (at  an optical  power  of  1.1mW)  with  average  error vector magnitudes (EVMs) of 7.3%, 7.4%, and 8.3%  for  subcarrier  modulation  of  16QAM, 32QAM, and 64QAM, respectively.  Fig.3 (a) compares the electrical spectra of the detected  optical  OFDM  both  with  and  without OIL.  Without OIL the DML shows a roll off in frequency  response  beyond  3.9  GHz.  By contrast  the  flat  spectrum  obtained  under properly  pre-emphasized  OIL  provides  clear evidence of a desired bandwidth enhancement under OIL, as anticipated from earlier research at  1550  nm.  Fig.  3  (b)  shows  the  optical spectrum  after  the  optical  band-pass  filtering. Fig. 3 (c) shows the electrical spectra of a 2001 nm CW signal after transmission over 500 m of SMF-28 and PBGF (without using a TDFA). In the case of HC-PBGF low frequency noise up to 80  MHz  is  observed  after  transmission.  This likely  results  from  low-level  intermodal interference  since  the  HC-PBGF  is  not  strictly single-mode2.  As  our  signal  occupies  the frequency range 0.4 GHz-5.6 GHz, the observed intermodal  interference  noise  does  not adversely affect the data signal, underlining an advantage of using such modulation formats in conjunction with HC-PBGFs. Conclusions A  low-cost,  high  spectral  efficiency  transmitter for  the  new  2  µm  spectral  region  was demonstrated  using  up  to  64QAM  OFDM  and direct  detection.  Transmission  over  500  m  of HC-PBGF with a state-of-the art 2.5 dB/km loss was demonstrated showing negligible penalty as compared  to  use  of  SMF-28.  16QAM  and 32QAM  signals  were  received  well  below  the FEC  limit.  64QAM  transmission  is  currently impacted  by  the  lack  of  a  sufficiently  narrow-band filter for suppression of ASE from our Tm-doped  fiber  amplifier  but  should  ultimately  be possible. Acknowledgements This research has received funding from EPSRC Fellowship grant  agreement  no.  EP/K003038/1.  The  HC-PBGF,  Tm-doped  amplifier,  and  semiconductor  lasers  used  in  these experiments were developed within EU FP7 project MODE-GAP  (grant  agreement  258033)  or  EPSRC  grant EP/I061196X (HYPERHIGHWAY). References [1]  Z.  Li,  et  al.,  “Diode-pumped  wideband  thulium-doped fiber amplifiers for optical communications in the 1800 - 2050 nm window,” Opt. Express, 21(8), p. 9289 (2013).  [2]  M. N. Petrovich, et al., “Demonstration of amplified data transmission  at  2  µm  in  a  low-loss  wide  bandwidth hollow  core  photonic  bandgap  fiber,”  Opt.  Express, 21(23), p. 28559 (2013). [3]  N.  Mac  Suibhne,  et  al.,  “Wavelength  Division Multiplexing at 2µm,” Proc. ECOC, Th.3.A.3, (2012). [4]  N. Mac Suibhne, et al., “WDM Transmission at 2µm over Low-Loss Hollow Core Photonic Bandgap Fiber,” Proc. OFC, OW1I.6, (2013). [5]  E.  Giacoumidis  et  al.,  “Improved  transmission performance  of  adaptively  modulated  optical  OFDM signals over directly modulated DFB laser-based IMDD links  using  adaptive  cyclic  prefix,”  Optics  Express, 16(13), p. 9480 (2008). [6]  A.  S.  Karar,  and  J.  C.  Cartledge,  “Generation  and Detection of a 56 Gb/s Signal Using a DML and Half-Cycle  16-QAM  Nyquist-SCM”,  Photon.  Tech.  Lett., 25(8), p. 757, (2013). [7]  M.  Cheng  et  al.,  “Directly  modulating  a  long  weak-resonant-cavity  laser  diode  at  limited  bandwidth  of  5 GHz with pre-leveled 16-QAM OFDM transmission at 20 Gbit/s,” Proc. OFC, OW3B.7, (2013). [8]  R Phelan et al., “In0.75Ga0.25As/InP Multiple Quantum-Well  Discrete-Mode  Laser  Diode  Emitting  at  2  µm”, Photon. Tech. Lett., 24(8), p. 652 (2012). [9]  Z. Meng et al., “Phase noise characteristics of a diode-pumped  Nd:  YAG  laser  in  and  unbalanced  fiber-optic interferometer,” Appl. Opt. 44, p. 3425 (2005).      Fig. 3: (a) Modulation bandwidth enhancement using OIL.  (b) Optical spectrum after OBPF. (c) Electrical spectra for CW light for the two fibers. 

Demonstration of amplified data transmission at 2 µm in a low-loss wide bandwidth hollow core photonic bandgap fiber,”

Diode-pumped wideband thulium-doped fiber amplifiers for optical communications in the 1800 -2050 nm window,”

Directly modulating a long weakresonant-cavity laser diode at limited bandwidth

Generation and Detection of a 56 Gb/s Signal Using a DML and HalfCycle 16-QAM Nyquist-SCM”,

Improved transmission performance of adaptively modulated optical OFDM signals over directly modulated DFB laser-based IMDD links using adaptive cyclic prefix,”

In0.75Ga0.25As/InP Multiple QuantumWell Discrete-Mode Laser Diode Emitting at 2 µm”,

Phase noise characteristics of a diodepumped Nd: YAG laser in and unbalanced fiber-optic interferometer,”

Wavelength Division Multiplexing at 2µm,”

WDM Transmission at 2µm over Low-Loss Hollow Core Photonic Bandgap Fiber,”

Up to 64QAM (30 Gbit/s) directly-modulated and directly detected OFDM at 2µm wavelength

Up to 64QAM (30 Gbit/s) directly-modulated and directly detected OFDM at 2µm wavelength

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