Future work

Potential future work would be focused on the further investigation of single laser based data centre optical interconnects. The first goal of future work would be to increase the data rate per channel (wavelength) up to 50 Gb/s. This can be achieved, for example, using 25 GBaud 4 pulse-amplitude modulation (4-PAM) modulation format with IM/DD modulation-detection scheme using the Q-Dash PMLL as an optical multicarrier source. The system performance would need to be characterised in terms of RIN and OCNR, and compared to the case when OOK NRZ and OFDM modulation formats are used instead.

Furthermore, prototypes of fully integrated optical interconnect transmitters and receivers (as depicted in Chapter 5) could be manufactured, characterised and implemented in IM/DD and coherent systems. The fully integrated solution would be realised on silicon substrate, with flip-chip optical source made from III-V materials. Ring resonators could be used either to filter a desired channel or to perform filtering and modulation simultaneously. That will depend on the intended modulation formats to be used. At the receiver side, which also could be realised using silicon technology, commercially available flip-chip photodiodes would probably be used due to their reliable performance, as these components are previously tested and verified. The projected bandwidth of the chip is ~ 30 GHz.

Also, field programmable gate arrays (FPGA) implementation of transmitter and receiver hardware for the generation of various modulation formats could form part of the plans for future work. FPGA based hardware solution allows transmitter design flexibility and real-time information about the energy consumption of the electronic circuitry. Efficient power consumption management is the one of the major requirements for data centre interconnects. Therefore, the power consumption reduction enabled by the full integration of transmitter and receiver should be followed

by power efficient design of electronics for the signal generation and detection, and DSP algorithms.

Finally, photonic integration of the gain-switched comb source with demultiplexer is currently in the process. Further characterisation of the integrated optical frequency comb and its implementation in coherently detected systems is currently being planned.

Appendix A

List of Publications Arising From

This Work

A.1 Referred Journal Papers

V. Vujicic, P. M. Anandarajah, C. Browning, and L. P. Barry, "WDM-OFDM-PON

Based on Compatible SSB Technique Using a Mode Locked Comb Source", IEEE Photonics Technology Letters, vol. 25, no. 21, pp. 2058-2061, Nov. 1, 2013.

R. Zhou, T. N. Huynh, V. Vujicic, P. M. Anandarajah, and L. P. Barry, "Phase noise analysis of injected gain switched comb source for coherent communications", Optics Express, vol. 22, no. 7, pp. 8120-8125, Mar. 2014.

V. Vujicic, P. M. Anandarajah, R. Zhou, C. Browning, and L. P. Barry, “Performance

investigation of IM/DD SSB-OFDM systems based on optical multicarrier sources“, IEEE Photonics Journal, vol. 6, no. 5, 7903110, Oct. 2014.

J. Pfeifle, V. Vujicic, R. T. Watts, P. C. Schindler, C. Weimann, R. Zhou, W. Freude, L. P. Barry, C. Koos, “Flexible Terabit/s Nyquist-WDM Super-Channels using a Gain- Switched Comb Source“, Optics Express, vol. 23, no. 2, pp. 724-728, Jan. 2015. C. Calò, V. Vujicic, R. Watts, C. Browning, K. Merghem, V. Panapakkam, F. Lelarge, A. Martinez, B-E. Benkelfat, A. Ramdane, L. P. Barry, “Single-section quantum well mode-locked laser for 400 Gb/s SSB-OFDM transmission“, Optics Express, vol. 23, no. 20, pp. 26442-26449, Oct. 2015.

V. Vujicic, C. Calò, R. Watts, F. Lelarge, C. Browning, K. Merghem, A. Martinez, A.

Ramdane, L. P. Barry, “Quantum Dash Mode-Locked Lasers for Data Center Applications“, IEEE Journal of Selected Topics in Quantum Electronics, vol. 21, no. 6, pp. 1101508, Nov/Dec. 2015.

A.2 Referred Conference Papers

P. Anandarajah, R. Zhou, R. Maher, D. M. Gutierrez Pascual, F. Smyth, V. Vujicic, L. Barry, “Flexible optical comb source for super channel systems”, in Proc. Optical

Fiber Communication Conference (OFC/NFOEC), Anaheim, USA, paper, OTh3I. 8, Mar. 2013.

R. Zhou, V. Vujicic, T. N. Huynh, P. M. Anandarajah, and L. P. Barry, "Effective Phase Noise Suppression in Externally Injected Gain Switched Comb Source for Coherent Optical Communications", in Proc. European Conference on Optical Communication (ECOC), London, Sep. 2013.

V. Vujicic, P. M. Anandarajah, C. Browning, and L. P. Barry, "WDM-OFDMA Based

on Compatible SSB Employing an Optical Multi-Carrier Transmitter", in Proc. Photonics Ireland, Belfast, Sep. 2013.

V. Vujicic, J. Pfeifle, R. Watts, P. C. Schindler, C. Weimann, R. Zhou, W. Freude,

C. Koos, and L. P. Barry, “Flexible Terabit/s Nyquist-WDM Superchannels with net SE > 7 bit/s/Hz using a Gain-Switched Comb Source", in Proc. Conference on Lasers and Electro-Optics (CLEO USA), San Jose, paper SW1J.3, June 2014.

V. Vujicic, P. M. Anandarajah, C. Browning, R. Zhou, S. O’Duill, and L. P. Barry,

“Optical Multicarrier based IM/DD DWDM-SSB-OFDM Access Networks with SOAs for Power Budget Extension”, in Proc. ECOC, Cannes, France, paper We.1.6.6, Sep. 2014.

J. Pfeifle, R. Watts, I. Shkarban, S. Wolf, V. Vujicic, P. Landais, N. Chimot, S. Joshi, K. Merghem, C. Calò, M. Weber, A. Ramdane, F. Lelarge, L. Barry, W. Freude, C. Koos, “Simultaneous Phase Noise Reduction of 30 Comb Lines from a Quantum- Dash Mode-Locked Laser Diode Enabling Coherent Tbit/s Data Transmission”, in Proc. OFC/NFOEC, Los Angeles, CA, USA, paper Tu3I.5, Mar. 2015

V. Vujicic, C. Calò, R. Watts, F. Lelarge, C. Browning, K. Merghem, A. Martinez, A.

Ramdane, L. P. Barry, “Quantum Dash Passively Mode-Locked Lasers for Tbit/s Data Interconnects”, in Proc. OFC/NFOEC, Los Angeles, CA, USA, paper Tu3I.4, Mar. 2015

C. Calò, V. Vujicic, R. Watts, F. Lelarge, C. Browning, K. Merghem, V. Panapakkam, A. Martinez, A. Ramdane, L. P. Barry, “Single-Section Quantum Well Mode-Locked Laser for 400 Gbit/s Single-Polarization IM/DD SSB-OFDM Transmission”, in Proc. CLEO/Europe-EQEC, Munich, Germany, paper CI_2_5, June 2015.

W. Freude, J. Pfeifle, R. Watts, I. Shkarban, S. Wolf, V. Vujicic, P. Landais, N. Chimot, S. Joshi, K. Merghem, C. Calò, M. Weber, A. Ramdane, F. Lelarge, L. Barry,

C. Koos, „Phase-noise Compensated Carriers from an Optical Frequency Comb Allowing Terabit Transmission“, Invited Talk, in International Conference on Transparent Optical Networks (ICTON), Budapest, Hungry, paper Tu.B5.1, Jul. 2015

V. Vujicic, C. Calò, R. Watts, F. Lelarge, C. Browning, K. Merghem, A. Martinez, A.

Ramdane, L. P. Barry, “Tbit/s transmission based on mode locked laser frequency comb sources“, Invited Talk, in Proc. IEEE Photonics Conference, Virginia, USA, Oct. 2015.

Other Publications Arisen from the PhD

Research

A.3. Journal Papers

T. N. Huynh, L. Nguyen, V. Vujicic, L. P. Barry, “Pilot-Tone-Aided Transmission of High-Order QAM for Optical Packet Switched Networks”, Journal of Optical Communications and Networking, vol. 6, no 2, pp.152-158, Feb. 2014.

V. Vujicic, R. Zhou, P. M. Anandarajah, J. O’Carroll, L. P. Barry, “Performance of a

Semi-Nyquist NRZ-DQPSK System Employing a Flexible Gain-Switched Multi-Carrier Transmitter”, Journal of Optical Communications and Networking, vol. 6, no. 3, pp. 282-290, Mar. 2014.

T. Shao, E. Martin, P. M. Anandarajah, C. Browning, V. Vujicic, R. Llorente, L. P. Barry, “Chromatic Dispersion Induced Optical Phase Decorrelation in a 60 GHz OFDM-RoF System”, IEEE Photonics Technology Letters, vol. 26, no. 20, pp. 2016- 2019, Oct. 2014.

E. P. Martin, T. Shao, V. Vujicic, P. M. Anandarajah, C. Browning, R. Llorente, L. P. Barry, “25 Gb/s OFDM 60 GHz Radio over Fibre System Based on a Gain Switched Laser“, Journal of Lightwave Technology, vol. 33, no. 8, pp. 1635-1643, Apr. 2015.

A.4. Conference Papers

T. N. Huynh, V. Vujicic, L. P. Barry, "Baudrate-Pilot-Aided Transmission for High Order QAM Modulation Format in Optical Communications", in Proc. Photonics Ireland, Belfast, Sep. 2013.

P. M. Anandarajah, R. Zhou, V. Vujicic, M. D. Gutierrez Pascual, E. Martin, L. P. Barry, “Long Reach UDWDM PON with SCM-QPSK Modulation and Direct Detection”, in Proc. OFC/NFOEC, San Francisco, CA, USA, paper W2A.42, Mar. 2014.

J. O’Carroll, V. Vujicic, N. Brochier, L. Bramerie, L. P. Barry, "Performance investigation of 112 Gb/s PDM-QPSK long-haul systems employing Discrete Mode Lasers", in Proc. Photonics Europe, Brussels, Belgium, Apr. 2014.

P. M. Anandarajah , R. Zhou, V. Vujicic, M. D. Gutierrez Pascual, R. Maher, D. Lavery, M. Paskov, B. Thomsen, S. Savory, L. P. Barry, “Long Reach UDWDM PON with Direct and Coherent Detection”, in Proc. ICTON, Graz, Austria, Jul. 2014.

E. Martin, T. Shao, V. Vujicic, P. Anandarajah, C. Browning, R. Llorente, L. Barry, „25 Gb/s OFDM 60 GHz Radio over Fibre System Using an Externally Injected Gain Switched Distributed Feedback Laser“, in Proc. ECOC, Cannes, France, paper. Tu.4.2.4, Sep. 2014.

E. Martin, T. Shao, P. Anandrajah, V. Vujicic, C. Browning, Roberto Llorente, L. Barry, “Impact and Reduction of Fibre Nonlinearities in a 25 Gb/s OFDM 60 GHz Radio over Fibre System”, MWP/APMP Conference, Sapporo, Japan, Nov. 2014 P. M. Anandarajah, T. Huynh, V. Vujicic, R. Zhou, and L. P. Barry, “UDWDM PON with 62.5GBaud 16-QAM Multicarrier Transmitter and Phase Noise Tolerant Direct Detection”, in Proc. OFC/NFOEC, Los Angeles, CA, USA, paper Th2A.58, Mar. 2015. T. N. Huynh, V. Vujicic, M. D. G. Pascual, P. M. Anandarajah, L. P. Barry, “Digital coherent communications with a 1550 nm VCSEL”, in Proc. OFC/NFOEC, Los Angeles, CA, USA, paper M2D.7, Mar. 2015.

P. M. Anandarajah, M. D. G. Pascual, T. Shao, R. Zhou, V. Vujicic, F. Smyth, L. P. Barry, “Reconfigurable Optical Frequency Comb and its Applications“, Invited Talk at ICTON, Budapest, Hungry, Jul. 2015.

Appendix B

B.1. Error Vector Magnitude

Advanced modulation formats such as PSK and QAM which use the phase of the optical carrier to encode the data, require a quality metric that is different than for OOK signals. Firstly, BER and Q-factor calculations which are often used for the performance characterisation of the OOK based systems, do not offer the information about the phase error but only total error of the received signal. Secondly, in the laboratory experiments most receivers employ offline digital signal processing at much reduced clock rates [1, 2]. For reliable estimation of BER, a certain number of bits is required. For example, it is clear that a system that specifies a BER better than 10-9 must be tested by transmitting significantly more than 109 bits in order to get an accurate and repeatable measurement [3]. Therefore, reliable BER estimation using offline DSP is a very time consuming process. Hence a faster and reliable quality metric is needed, in particular when characterising multicarrier systems [1, 2].

It is well known that the complex field of a QAM signal can be represented using a constellation diagram, and an ideal constellation for square 16-QAM modulation format is shown in Figure B.1.1. The constellation of the received signal usually deviates from the ideal constellation. The level of deviation can be characterised using the error vector magnitude. The deviation between received signal vector E_R

and transmitted (ideal) signal vector E_I is denoted as an error vector E_err. Two definitions of EVM can be found in the literature. The first one uses the longest ideal constellation vector E_I,max for the normalisation. Another definition uses average power E_{I avg,} 2 of all symbol vectors within the constellation for the normalisation. When E_I,max is used for the normalisation, then the EVMmax is defined by a root mean square of E_err for a number N of randomly transmitted data [1, 2]:

max ,max err I EVM E   (B.1.1) where 2 2 , 1 1 N err err i i E N   



Figure B.1.1. Constellation diagram and error vector for 16-QAM signal.

The definition based on the normalisation using the average power of all symbol vectors within the constellation is related to the later one by the modulation format dependent factor k:

EVM_avg  k EVM_max (B.1.2)

where 2 ,max 2 2 , I I avg E k E  and 2 2 , , 1 1 N I avg I i i E E M  



The values of factor k for certain modulation formats are given in Table B.1.1 [1, 2]:

Modulation

format BPSK QPSK 8PSK 16-QAM 32QAM 64QAM

k2 1 1 1 9/5 17/10 7/3

Table B.1.1. Values of factor k for certain modulation formats.

In this thesis, EVMavg is used for the system performance characterisation in Chapter 5, whilst EVMmax is used in Chapter 6.

References

[1] R. Schmogrow, B. Nebendahl, M. Winter, A. Josten, D. Hillerkuss, S. Koenig, J. Meyer, M. Dreschmann, M. Huebner, C. Koos, J. Becker, W. Freude, J. Leuthold, “Error Vector Magnitude as a Performance Measure for Advanced Modulation Formats”, IEEE Photonics technology Letters, vol. 24, no. 1, pp. 61-63, Jan. 2012. [2] R. Schmogrow, “Real-time Digital Signal Processing for Software-defined Optical Transmitters and Receivers”, KIT Scientific Publishing, Nov. 2014

[3] HFTA-010.0: Physical Layer Performance: Testing the Bit Error Ratio (BER). Available: http://pdfserv.maximintegrated.com/en/an/3419.pdf, Oct. 2015.