**6. Full-field detection for 40 Gbit/s offset DQPSK**

In addition to amplitude-modulated OOK format, FFD can also be used in phase-modulated formats, which have been widely employed for 40 Gbit/s and beyond. In conventional differential quadrature phase shifted keying (DQPSK) system, at least two AMZIs and two pairs of balanced photodiodes are required for incoherent detection (Kikuchi et al., 2006; Liu & Wei, 2007). Furthermore, the near-zero intensity during a phase shift between symbols limits the system performance unless complicated pre-distortion is used (Kikuchi & Sasaki, 2010). On the other hand, offset DQPSK format has been proposed in optical communications (Wree et al., 2004) to eliminate the near-zero intensity between symbols and this format exhibits the same spectral efficiency as conventional DQPSK. However, conventional offset DQPSK system has degraded receiver sensitivity and CD tolerance (Wree et al., 2004), which hinders its use for practical applications. In this section, we show that FFD based EDC can significantly improve the performance of the offset DQPSK system (Zhao & Ellis, 2011). The presented system uses a simpler pre-coder at the transmitter, only one AMZI, and one pair of photodiodes at the receiver, reducing the implementation cost when compared to conventional DQPSK. Consequently, it is promising for cost-sensitive 40 Gbit/s Ethernet or short metro networks.

Fig. 19. Configuration of FFD-based offset DQPSK

Fig. 19 illustrates the configuration of FFD-based offset DQPSK. The transmitted data, *a*k, is demultiplexed into the in-phase and quadrature tributaries, which are differentially encoded using exclusive OR (XOR) individually. Note that this pre-coder uses only two XOR gates and is much simpler than the conventional DQPSK pre-coder which typically requires the combination of >20 XOR, AND and NOT logic gates. The encoded quadrature signal is delayed by *T*/2 with respect to the in-phase signal (Inset in Fig. 19), where *T* is the symbol period. Consequently, the phase may possibly change every *T*/2, but each phase change can only be 0, /2. In addition, the possible zero intensity between symbols induced by instantaneous phase shift in conventional DQPSK is eliminated. At the 94 Optical Communications Systems

be degraded due to the finite MLSE compensation window (~550 km at 16 states as shown Fig. 13(b)). This figure also implies that the system was insensitive to the exact pre-set dispersion value, so a coarse estimation was sufficient. To illustrate the adaptation speed of the system, Fig. 18(b) shows the BER versus the training time for three different distances when the frequency-domain equalization was pre-set to compensate 550 km CD. The figure shows that the performance converged rapidly during the first 200 ns for all distances. After 400 ns, the BER fell below 10-3 even for the longest distance, demonstrating the potential of

In addition to amplitude-modulated OOK format, FFD can also be used in phase-modulated formats, which have been widely employed for 40 Gbit/s and beyond. In conventional differential quadrature phase shifted keying (DQPSK) system, at least two AMZIs and two pairs of balanced photodiodes are required for incoherent detection (Kikuchi et al., 2006; Liu & Wei, 2007). Furthermore, the near-zero intensity during a phase shift between symbols limits the system performance unless complicated pre-distortion is used (Kikuchi & Sasaki, 2010). On the other hand, offset DQPSK format has been proposed in optical communications (Wree et al., 2004) to eliminate the near-zero intensity between symbols and this format exhibits the same spectral efficiency as conventional DQPSK. However, conventional offset DQPSK system has degraded receiver sensitivity and CD tolerance (Wree et al., 2004), which hinders its use for practical applications. In this section, we show that FFD based EDC can significantly improve the performance of the offset DQPSK system (Zhao & Ellis, 2011). The presented system uses a simpler pre-coder at the transmitter, only one AMZI, and one pair of photodiodes at the receiver, reducing the implementation cost when compared to conventional DQPSK. Consequently, it is promising for cost-sensitive 40

Fig. 19 illustrates the configuration of FFD-based offset DQPSK. The transmitted data, *a*k, is demultiplexed into the in-phase and quadrature tributaries, which are differentially encoded using exclusive OR (XOR) individually. Note that this pre-coder uses only two XOR gates and is much simpler than the conventional DQPSK pre-coder which typically requires the combination of >20 XOR, AND and NOT logic gates. The encoded quadrature signal is delayed by *T*/2 with respect to the in-phase signal (Inset in Fig. 19), where *T* is the symbol period. Consequently, the phase may possibly change every *T*/2, but each phase change can only be 0, /2. In addition, the possible zero intensity between symbols induced by instantaneous phase shift in conventional DQPSK is eliminated. At the

FFD EDC in frequently configured optical networks.

Gbit/s Ethernet or short metro networks.

Fig. 19. Configuration of FFD-based offset DQPSK

**6. Full-field detection for 40 Gbit/s offset DQPSK** 

receiver, the optical front end and full-field reconstruction for offset-DQPSK are the same as those in the OOK format. However, an additional electrical-domain differential detection process is employed before the dispersion compensation stage, as depicted in Fig. 20. The performance of differential detection can be improved by exploiting the field differences between a symbol and its previous (*L*-1) symbols, where *L*>1, resulting in better field reference. This method is conventionally implemented in the optical domain by using (*L*-1) (or 2(*L*-1)) AMZIs (Zhao & Chen, 2007). However, this implementation is complicated. By using FFD, multiple differential fields can be obtained in the electrical domain simply using delays and multiplications while only one optical AMZI is employed. In offset DQPSK, the phase may change every *T*/2, so two samples per symbol (or one sample per bit) are used. The multiple differential fields for the *n*th bit, *I*i(*t*n), may be estimated by *conj*(*V*full(*t*ni))*V*full(*t*n), where *i (*=1,...,*L*-1) denotes the *i*th branch of the differential field detection and *conj*() represents the conjugate. These differential samples are then fed into the MLSE. The metric of MLSE, *PM*(*a*n), used by the Viterbi algorithm to estimate the most likely transmitted data sequence, is given by:

$$PM(a\_n) = PM(a\_{n-1}) - \sum\_{i=1}^{L-1} \log(p(\Re(I\_i(t\_n)), \Im(I\_i(t\_n)) \mid a\_{n-m'}, \dots, a\_n)) \tag{13}$$

where (*I*i(*t*n)) (or (*I*i(*t*n))) represents the real (or imaginary) part of the differential field *I*i(*t*n). *p*((*I*i(*t*n)), (*I*i(*t*n))*a*n-m,…,*a*n) is the joint probability of the differential field given the transmitted data *a*k-m,…,*a*k. *m* is the memory length. Eq. (13) shows that the size of the required lookup table for channel estimation and the complexity of metric computation scale approximately linearly with *L*. On the other hand, Viterbi decoding is independent of *L* and is the same as that in conventional MLSE.

Fig. 20. Multiple-reference based differential detection and MLSE. D, 2D, and 3D represent one-, two-, and three-sample delay respectively.

Simulation implemented in Matlab was performed to verify the operating principle of this scheme. The analysis model was the same as Fig. 19. Two uncorrelated 20 Gbit/s data trains using 211-1 pseudo-random binary sequence repeated nine times were differentially encoded individually. Each encoded data train generated an analogue electrical signal using raisedcosine shaped pulse with a roll-off coefficient of 0.4 and 40 samples per symbol. The response of the driving amplifier was 5th-order Bessel shaped with 20 GHz 3 dB bandwidth. The electrical signals were used to modulate a continuous wave light from a laser with 100 kHz linewidth. A piece of fiber with CD of 16 ps/km/nm was used to investigate the CD tolerance. At the receiver, the launch power into the preamplifier was adjusted to control the

Full-Field Detection with Electronic Signal Processing 97

haul systems), is of particular value for applications in DCF-free transparent access/metro networks and Ethernet. For 10 Gbit/s metro networks with transmission reach of 300-500 km, FFD MLSE is an effective approach and can exhibit 50% performance improvement when compared to DD MLSE, or exponentially reduce the required state number for a fixed transmission reach. It is also more robust to non-optimized system parameters than fullfield detection based frequency-domain equalization and FFE, and thus relaxes the system specifications. For transmission reaches longer than 500 km, the combination of costeffective and static frequency-domain equalization and adaptive FFD MLSE with parametric channel estimation can obtain a balance of performance, complexity, and adaptation speed. 0-900 km adaptive transmission with less than 400ns adaptation time is achievable at 10 Gbit/s. For higher bit rate systems, FFD based offset DQPSK offers a cost-effective solution for 40 Gbit/s Ethernet or short metro networks, and when compared to conventional DQPSK with the same spectral efficiency, it uses a simpler pre-coder at the transmitter, only one AMZI and one pair of photodiodes at the receiver, while supporting 50 km SMF

The authors acknowledge M.E. McCarthy from the Photonic Systems Group at the Tyndall National Institute for invaluable assistance with the experimental demonstration, and the contribution of D. Cassidy and W. McAuliffe from BT Ireland and P. Gunning from BT Innovate and Design for ongoing support. This work was financially supported by Science Foundation Ireland under grant number 06/IN/I969 and Enterprise Ireland under grant

Alfiad, M.; Van den Borne, D.; Napoli, A.; Koonen, A.M.J & De Waardt, H. (2008). A DPSK

Bulow, H.; Buchali, F. & Klekamp, A. (2008). Electronic dispersion compensation. *IEEE/OSA Journal of Lightwave Technology*, vol. 26, no. 1, (Jan. 2008), pp. 158-167. Cai, Y. (2008). Coherent detection in long-haul transmission systems. *Proceedings of Optical Fiber Communication (OFC) conference*, OTuM1, San Diego USA, March 2008. Chandrasekhar, S.; Gnauck, A.H.; Raybon, G.; Buhl, L.L.; Mahgerefteh, D.; Zheng, X.;

Ellermeyer, T.; Mullrich, J.; Rupeter, J.; Langenhagen, H.; Bielik, A. & Moller M. (2008). DA

*IEEE Photonics Technology Letter*, vol. 20, no. 10, (May 2008), pp. 818-820. Bosco, G. & Poggiolini, P. (2006); Long-distance effectiveness of MLSE IMDD receivers. *IEEE Photonics Technology Letters*, vol. 18, no. 9, (May 2006), pp. 1037-1039. Bulow, H. & Thieleche, G. (2001). Electronic PMD mitigation – from linear equalization to

receiver with enhanced CD tolerance through optimized demodulation and MLSE.

maximum-likelihood detection. *Proceedings of Optical Fiber Communication (OFC)* 

Matsui, Y.; McCallion, K.; Fan, Z. & Tayebati, P. (2006). Chirp-managed laser and MLSE-RX enables transmission over 1200km at 1550nm in a DWDM environment in NZDSF at 10Gb/s without any optical dispersion compensation. *IEEE Photonics* 

and AD converters for 25GS/s and above. *Proceedings of IEEE Summer Topical* 

transmission without optical compensation at 40 Gbit/s.

*conference*, WAA3, Anaheim USA, March 2001.

*Technology Letters*, vol. 18, no. 14, (July 2006), pp. 1560-1562.

*Meetings*, pp. 117-118, Acapulco Mexico, July 2008.

**8. Acknowledgments** 

number CFTD/08/333.

**9. References** 

OSNR. The preamplifier was followed by an OBPF with optimized bandwidth. The AMZI had a differential phase shift of /2 and 10 ps DTD, unless otherwise stated. The signal power into the photodiodes was 3 dBm and the noise spectral power density of the photodiodes was 20 pA/Hz1/2. After detection, the signals were amplified, filtered by a 30 GHz 4th-order Bessel EF, and processed as described above. MLSE had two samples per symbol, 5-bit resolution, and 16 states (considering two (or four) adjacent symbols (or bits)). The number of differential measurements used for metric computation, *L*, was varied from two to four. The simulation was iterated ten times with different random number seeds to give a total of 184,230 simulated symbols. The performance was evaluated using the required OSNR to achieve a BER of 110-3 by direct error counting.

Fig. 21. (a) Required OSNR versus the fiber length without MLSE (pluses), and using 16 state MLSE with *L* of 2 (squares), 3 (triangles), and 4 (circles). The AMZI DTD is 10 ps. (b) OSNR penalty versus the AMZI DTD using 16-state MLSE and *L*=4. The OSNR penalty is defined as the penalty with respect to the OSNR value using optimized AMZI DTD.

Fig. 21(a) shows the performance of the offset DQPSK with and without MLSE. The OBPF bandwidth was optimized at the back-to-back case and the optimal value when using MLSE (16.5 GHz) was smaller than that without MLSE (23.5 GHz). In common with other MLSE investigation, this was due to the capability of MLSE to compensate filtering-induced ISI such that a narrow OBPF bandwidth could be used to mitigate the impact of the noise and the CD. The figure clearly depicts the benefit of MLSE with a larger number of differential measurements *L*. When using 16-state MLSE and *L*=4, a transmission distance of around 50km could be supported for a required OSNR of 18dB (100km total dispersion tolerance range). Fig. 21(b) illustrates the low sensitivity of the system to the precise AMZI delay. Smaller DTDs gave more precise estimation of *V*f(*t*) and *V*p(*t*), and consequently resulted in reduced OSNR penalties. At 40 Gbit/s, less than 1 dB penalty was induced for an AMZI with DTD between 2.5 ps and 15 ps for both back-to-back and 30 km. Note that the DTD could not be reduced indefinitely due to the increased limit induced by thermal noise as discussed in Section 4.1.

#### **7. Conclusions**

FFD EDC, by surpassing the limited performance of current DD EDC products (300 km at 10 Gbit/s) and avoiding the high implementation cost of coherent detection EDC (for longhaul systems), is of particular value for applications in DCF-free transparent access/metro networks and Ethernet. For 10 Gbit/s metro networks with transmission reach of 300-500 km, FFD MLSE is an effective approach and can exhibit 50% performance improvement when compared to DD MLSE, or exponentially reduce the required state number for a fixed transmission reach. It is also more robust to non-optimized system parameters than fullfield detection based frequency-domain equalization and FFE, and thus relaxes the system specifications. For transmission reaches longer than 500 km, the combination of costeffective and static frequency-domain equalization and adaptive FFD MLSE with parametric channel estimation can obtain a balance of performance, complexity, and adaptation speed. 0-900 km adaptive transmission with less than 400ns adaptation time is achievable at 10 Gbit/s. For higher bit rate systems, FFD based offset DQPSK offers a cost-effective solution for 40 Gbit/s Ethernet or short metro networks, and when compared to conventional DQPSK with the same spectral efficiency, it uses a simpler pre-coder at the transmitter, only one AMZI and one pair of photodiodes at the receiver, while supporting 50 km SMF transmission without optical compensation at 40 Gbit/s.

#### **8. Acknowledgments**

96 Optical Communications Systems

OSNR. The preamplifier was followed by an OBPF with optimized bandwidth. The AMZI had a differential phase shift of /2 and 10 ps DTD, unless otherwise stated. The signal power into the photodiodes was 3 dBm and the noise spectral power density of the photodiodes was 20 pA/Hz1/2. After detection, the signals were amplified, filtered by a 30 GHz 4th-order Bessel EF, and processed as described above. MLSE had two samples per symbol, 5-bit resolution, and 16 states (considering two (or four) adjacent symbols (or bits)). The number of differential measurements used for metric computation, *L*, was varied from two to four. The simulation was iterated ten times with different random number seeds to give a total of 184,230 simulated symbols. The performance was evaluated using the

Fig. 21. (a) Required OSNR versus the fiber length without MLSE (pluses), and using 16 state MLSE with *L* of 2 (squares), 3 (triangles), and 4 (circles). The AMZI DTD is 10 ps. (b) OSNR penalty versus the AMZI DTD using 16-state MLSE and *L*=4. The OSNR penalty is defined as the penalty with respect to the OSNR value using optimized AMZI DTD.

Fig. 21(a) shows the performance of the offset DQPSK with and without MLSE. The OBPF bandwidth was optimized at the back-to-back case and the optimal value when using MLSE (16.5 GHz) was smaller than that without MLSE (23.5 GHz). In common with other MLSE investigation, this was due to the capability of MLSE to compensate filtering-induced ISI such that a narrow OBPF bandwidth could be used to mitigate the impact of the noise and the CD. The figure clearly depicts the benefit of MLSE with a larger number of differential measurements *L*. When using 16-state MLSE and *L*=4, a transmission distance of around 50km could be supported for a required OSNR of 18dB (100km total dispersion tolerance range). Fig. 21(b) illustrates the low sensitivity of the system to the precise AMZI delay. Smaller DTDs gave more precise estimation of *V*f(*t*) and *V*p(*t*), and consequently resulted in reduced OSNR penalties. At 40 Gbit/s, less than 1 dB penalty was induced for an AMZI with DTD between 2.5 ps and 15 ps for both back-to-back and 30 km. Note that the DTD could not be reduced indefinitely due to the increased limit induced by thermal noise as discussed in Section 4.1.

FFD EDC, by surpassing the limited performance of current DD EDC products (300 km at 10 Gbit/s) and avoiding the high implementation cost of coherent detection EDC (for long-

required OSNR to achieve a BER of 110-3 by direct error counting.

**7. Conclusions** 

The authors acknowledge M.E. McCarthy from the Photonic Systems Group at the Tyndall National Institute for invaluable assistance with the experimental demonstration, and the contribution of D. Cassidy and W. McAuliffe from BT Ireland and P. Gunning from BT Innovate and Design for ongoing support. This work was financially supported by Science Foundation Ireland under grant number 06/IN/I969 and Enterprise Ireland under grant number CFTD/08/333.

#### **9. References**


Full-Field Detection with Electronic Signal Processing 99

McCarthy, M.E.; Zhao, J.; Ellis, A.D. & Gunning, P. (2008). Full field receiver side processing

McCarthy, M.E.; Zhao, J.; Gunning, P. & Ellis, A.D. (2008). A novel field-detection

McCarthy, M.E.; Zhao, J.; Ellis, A.D. & Gunning, P. (2009). Full-field electronic dispersion

McGhan, D.; Laperle, C.; Savchenko, A.; Li, C; Mak, G. & O'Sullivan, M. (2005). 5120km RZ-

McGhan, D.; O'Sullivan, M.; Sotoodeh, M.; Savchenko, A.; Bontu, C.; Belanger, M. & Roberts

McNicol, J.; O'Sullivan, M.; Roberts, K.; Comeau, A.; McGhan, D. & Strawczynski, L. (2005).

Polley, A. & Ralph, S.E. (2007). Receiver-side adaptive opto-electronic chromatic dispersion

Savory, S.J.; Benlachtar, Y.; Killey, R.I.; Bayvel, P.; Bosco, G.; Poggiolini, P.; Prat, J. & Omella,

Savory, S.J.; Gavioli, G.; Killey, R.I. & Bayvel, P. (2007). Electronic compensation of

Schube, S. & Mazzini, M. (2007). Testing and interoperability of 10GBASE-LRM optical interfaces. *IEEE Communication Magazine*, vol. 45, (March 2007), pp. s26-s31. Taylor, M.G. (2004). Coherent detection method using DSP for demodulation of signal and

Tsukamoto, S.; Katoh, K. & Kikuchi, K. (2006). Unrepeated transmission of 20Gb/s optical

*Photonics Technology Letters*, vol. 18, no. 9, (May 2006), pp. 1016-1018.

*Comunication (OFC) conference*, OWK1, Anaheim USA, March 2006.

*Communication (OFC) conference*, OThJ3, Anaheim USA, March 2005. Poggiolini, P.; Bosco, G.; Benlachtar, Y.l; Savory, S.J.; Bayvel, P.; Killey, R.I. & Prat, J. (2008).

*Meetings*, pp. 171-172, Acapulco Mexico, July 2008.

Belgium, Sep. 2008.

Anaheim USA, March 2005.

(Aug. 2008), pp. 12919-12936.

Anaheim USA, March 2007.

(March 2007), pp. 2120-2126.

March 2007.

Proakis, J.G. (2000). *Digital communications*. McGraw-Hill, New York.

*Letters*, vol. 16, no. 2, (Feb. 2004), pp. 674-676.

5327-5334.

for electronic dispersion compensation. *Proceedings of IEEE Summer Topical* 

maximum-likelihood sequence estimation for chromatic dispersion compensation. *Proceedings of European Conference on Optical Communications*, We.2.E.5, Brussels

compensation of 10Gbit/s OOK signal over 4124km field-installed signal-mode fiber. *IEEE/OSA Journal of Lightwave Technology*, vol. 27, no. 23, (Dec. 2009), pp.

DPSK transmission over G652 fiber at 10Gbit/s with no optical dispersion compensation. *Proceedings of Optical Fiber Communication (OFC) conference*, PDP27,

K. (2006). Electronic dispersion compensation. *Proceedings of Optical Fiber* 

Electronic domain compensation of optical dispersion. *Proceedings of Optical Fiber* 

Long-haul 10Gbit/s linear and nonlinear IMDD transmission over uncompensated standard fiber using a SQRT-metric MLSE receiver. *Optics Express*, vol. 16, no. 17,

compensation. *Proceedings of Optical Fiber Communication (OFC) conference*, JThA51,

M. (2007). IMDD transmission over 1040km of standard single-mode fiber at 10Gbit/s using a one-sample-per-bit reduced complexity MLSE receiver. *Proceedings of Optical Fiber Communication (OFC) conference*, OThK2, Anaheim USA,

chromatic dispersion using a digital coherent receiver. *Optics Express*, vol. 15, no. 5,

subsequent equalization of propagation impairments. *IEEE Photonics Technology* 

quadrature phase-shift-keying signal over 200km SMF based on digital processing of homodyne-detected signal for group-velocity dispersion compensation. *IEEE* 


Ellis, A.D. & McCarthy, M.E. (2006). Receiver-side electronic dispersion compensation using

Ellis, A.D., Zhao, J. & Cotter, D. (2010). Approaching the Non-linear Shannon Limit. *IEEE Journal of Lightwave Technology*, vol. 28, no. 4, (Feb. 2010), pp 423-433. Farbert, A.; Langenbach, S.; Stojanovic, N.; Dorschky, C.; Kupfer, T.; Schulien, C.; Elbers, J.-

Franceschini, M.; Bongiorni, G.; Ferrari, G.; Raheli, R.; Meli, F. & Castoldi, A. (2007).

Gene, J.M.; Winzer, P.J.; Essiambres, R.J.; Chandrasekhar, S.; Painchaud, Y. & Guy, M.

Haunstein, H. & Urbansky, R. (2004). Application of electronic equalization and error

Iwashita, K. & Takachio, N. (1988). Compensation of 202km single-mode fiber chromatic

Jeruchim, M.C. (1984). Techniques for estimating the bit error rate in the simulation of

Kikuchi, N.; Mandai, K.; Sasaki, S. & Sekine, K. (2006). Proposal and first experimental

Kikuchi, N. & Sasaki, S. (2010). Improvement of chromatic dispersion and differential group

Liu, X. & Wei, X. (2007). Electronic dispersion compensation based on optical field

Liu, X.; Chandrasekhar, S. & Leven, A. (2008). Digital self-coherent detection. *Optics Express*,

McCarthy, M.E. & Ellis, A.D. (2007). Electronic Dispersion Compensation Utilising Full

*Communications*, Th1.5.1, Stockholm Sweden, Sep. 2004.

March 2006.

Stockholm Sweden, Sep. 2004.

2007), pp. 1742-1753.

(Aug. 2007), pp. 1224-1227.

vol. 24, no. 12, (June 1988), pp. 759-760.

vol. SAC-2, no. 1, (Jan. 1984), pp. 153-170.

Th4.4.4, Cannes France, Sep. 2006.

Diego USA, March 2010.

Anaheim USA, March 2007.

vol. 16, no. 2, (Jan. 2008), pp. 792-803.

3, no. 4, (Oct. 2007), pp. 144-151.

passive optical field detection for low-cost 10Gbit/s 600km reach applications. *Proceedings of Optical Fiber Communication (OFC) conference*, OTuE4, Anaheim USA,

P.; Wernz, H.; Griesser, H. & Glingener, C. (2004). Performance of a 10.7Gb/s receiver with digital equalizer using maximum likelihood sequence estimation. *Proceedings of European Conference on Optical Communications*, PDP Th4.1.5,

Fundamental limits of electronic signal processing in direct-detection optical communications. *IEEE/OSA Journal of Lightwave Technology*, vol. 25, no. 7, (July

(2007). Experimental study of MLSE receivers in the presence of narrowband and vestigial sideband optical filtering. *IEEE Photonics Technology Letters*, vol. 19, no. 16,

correction in lightwave systems. *Proceedings of European Conference on Optical* 

dispersion in 4Gbit/s optical CPFSK transmission experiment. Electronics Letters,

digital communication systems. *IEEE Journal of Selected Areas in Communications*,

demonstration of digital incoherent optical field detector for chromatic dispersion compensation. *Proceedings of European Conference on Optical Communications*, PDP

delay tolerance of incoherent multilevel signalling with receiver-side digital signal processing. *Proceedings of Optical Fiber Communication (OFC) conference*, OWV6, San

reconstruction with orthogonal differential direct-detection and digital signal processing. *Proceedings of Optical Fiber Communication (OFC) conference*, OTuA6,

Optical Field Estimation. *Mediterranean Journal of Electronics and Communications*, vol


**Part 2** 

**Optical Communications Systems:** 

**Amplifiers and Networks** 

