Adaptive Polarization Control for Coherent Optical Links with Polarization Multiplexed Carrier

1 Adapti v e Polarization Control for Coherent Optical Links with Polarization Multiplex ed Carrier Mehul Anghan, Nandakumar Nambath ∗ , Rashmi Kamran, and Shalabh Gupta Department of Electrical Engineering, IIT Bombay , Mumbai-400076 India ∗ School of Electrical Sciences, IIT Goa, Ponda-403401 India Abstract —Self-homodyne systems with polarization multi- plexed carrier offer an LO-less coherent r eceiver with simplified signal processing r equirement that can be a good candidate f or high-speed short-reach data center interconnects. The practical implementation of these systems is limited by the requir ement of polarization control at the receiv er end f or separating the carrier and the modulated signal. In this paper , effect of polarization impairments in polarization diversity based systems is studied and modeled. A nov el and practical adaptive polarization control technique based on optical power feedback from one polarization is pr oposed f or polarization multiplexed carrier based systems and verified through simulation results. The application of the proposed concept is experimentally demonstrated also for a QPSK system with polarization multiplexed carrier . Index T erms —Coherent optical links, polarization multiplexed carrier , Adaptive polarization control. I . I N T RO D U C T I O N M ERIT of coherent modulation and demodulation tech- niques make these techniques suitable for communi- cation through optical fibres at high data rates [2]. Dual polarization Quadrature Phase Shift K eying (DP-QPSK) system has been commonly used for high data rates which utilizes diversity in both phase and polarization [3, 4]. This coherent technique uses a separate local oscillator (LO) at receiv er and also requires a carrier phase reco very (CPR) and compensation module to o vercome the effects of laser line-width and frequency of fset between the transmitter and receiv er lasers. Use of LO and CPR can be av oided in a polarization di versity based self-homodyne (SH) system, in which the carrier is polarization multiplexed with the modulated signal itself. Polarization impairments tied with optical system and channel causes mixing of the data in two orthogonal polarizations results in improper reception of message symbols. Polarization demultiplexing techniques like constant modulus algorithm and decision directed algorithm in electrical domain can be used to compensate these polarization impairments b ut it requires high speed signal processing [5, 6]. Circuit implementation of high speed signal processing will be complex and power hungry . Manual or automatic polarization controller de vice (for example EPC-15-1-1-2 from Phoenix, EPC1000 from Nov optel) can be used to correct this kind of impairments. Some of the ef fects can be minimized by properly controlling the state of recei ved polarization in optical domain itself [7, 8]. Few polarization diversity based This paper has to be submitted for publication to a journal. A significant part of this work comes from [1] SH systems are demonstrated in prior work [9, 10], although manual polarization controller has been used which is not a solution in practical scenario. In other work [11], a polariza- tion multiplexed carrier based SH system is demonstrated with direct detection that faces issue of IQ imbalance. Adaptiv e polarization control is found to be very useful for self-homodyne coherent optical links with polarization multiplex ed carrier . Proposed adaptiv e polarization control for an polarization di versity based SH system is presented in Fig. 1, in which power of one of the polarization (after con verting in electrical domain) is fed back to electronically controlled polarization controller . In this paper , we hav e car- ried out modeling of SH systems for short reach links that include the effects of polarization impairments. A technique based on minimization of the optical po wer recei ved in one of the polarizations, to control the state-of-polarization in short reach self-homodyne links, is presented with the use of feedback polarization control. A discrete time gradient descent based algorithm to achieve this minimization is presented and validated using simulations. The usefulness of polarization control has been experimentally verified for SH-QPSK system, where the proposed technique will work. I I . M O D E L L I N G O F T H E P O L A R I Z AT I O N I M PA I R M E N T S : Polarization impairments due to system components and fiber channel are discussed in this section. Polarization beam combiner (PBC) and polarization beam splitter (PBS) can mix the carrier and modulated signal due to misalignment of reference axes as explained in the Fig.2. Same phenomena can be explained for PBC also. Following equations represents the effect of PBS angle ( θ ) on the outputs ( P B S x , P B S y ):  P B S x P B S y  =  cos θ − sin θ sin θ cos θ   E x E y  , P B S x = E x cos θ − E y sin θ , P B S y = E x sin θ + E y cos θ , where E x , E y are the inputs to the PBS. Phase shift between two orthogonal polarizations due to fiber channel can also cause the mixing. φ is the angle between the reference polarizations and the principle state of polarizations (PSPs). Overall effect due to the angles of PBS, PBC and φ can be represented as: A =  cos θ − sin θ sin θ cos θ   e j φ 0 0 e − j φ   cos θ sin θ − sin θ cos θ  , 2 Fig. 1. A polarization div ersity based self homodyne system with proposed adaptive polarization control technique. PC: polarization controller, PBS/PBC: polarization beam splitter/combiner , PD: photo detector , PS: power splitter , and PR: polarization rotator . Here assumption is that de vice angles of PBS and PBC are same. Fig. 2. Effect on both polarizations with device angle. I I I . S E P A R A T I O N O F T H E C A R R I E R A N D T H E M O D U L A T I N G S I G NA L AT R E C E I V E R : This section discusses the technique for separating the carrier and the modulated signal for the polarization div ersity based SH-QPSK system. In this system as shown in Fig.1, the modulated signal is launched in one polarization and the carrier signal is launched in other orthogonal polarization. Power of the launched carrier signal should be higher than power of the launched modulated signal. The power difference is kept around 15 dB at the transmitter (which is there due to modulator insertion loss). T o separate the modulated signal and the carrier signal at the receiver , power measurement in one of the polarizations can help. Optical Channel PBS E X E Y signal carrier Optical Channel PBS E X E Y signal carrier + + carrier signal Fig. 3. Effect of polarization control on the separation of signal and the carrier at the receiver . Under ideal conditions, the PBS has two outputs, the low power modulated signal and the high power carrier as sho wn in Fig. 3. While, in practical scenario, these two signals may get mixed resulting in lower power dif ference between two output branches of the PBS. Thus, to extract the signal back in one polarization, a po wer minimization in that polarization at the recei ver can be used. The polarization div ersity based SH-QPSK system is modeled in Simulink and VPItransmissionMaker T M with three w av eplate polarization controller (PC) module which has two control parameters for changing angles. Polarization impairments due to system and channel were considered non-ideal. By varying the two controls of PC, optical power in one of the output of the PBS is measured and plotted. Fig. 4. Optical Power profile from one of the output of PBS. From the Fig. 5, it is shown that there are more than one minima with same strength. At each minima, equi valent matrix of the channel is conv erged to following matrix.  e j φ 1 0 0 e − j φ 1  From this matrix, we can see that after minimizing the optical power in one of the polarization, rotational effect is remov ed and only phase shift in individual polarization is remaining. This phase shift can be removed using carrier and phase recov ery module. Manual controlling of PC is difficult to get an y one of the minima. Feedback polarization control has been implemented in VPItransmissionMaker T M to get the desired state of polarization. Optical power from any one of the polarization is con verted in electrical domain using photodetector and based on the electrical signal it will adapt the control parameters of PC to find the minimum optical power . I V . A DA P T I V E P O L A R I Z A T I O N C O N T RO L A L G O R I T H M Discrete time based gradient descent algorithm has been used to find the minima and v alidated through simulation. Here 3 C1 and C2 are control parameters of the PC, P is optical power in one polarization and µ is stepsize. Fig. 5. Flowcharts of algorithm for minimizing power V . S I M U L A T I O N R E S U L T S : Simulation has been performed for 30 Gbps SH-QPSK sys- tem with polarization multiplexed carrier with 20 km distances in VPItransmissionMaker T M with all non-idealities on. Before minimizing po wer , po wer difference between two polarization was 6.04 dB (carrier and modulated signal are mixed) and after minimizing power in one polarization, po wer difference is 15.69 dB (carrier and modulated signal are separated). This is shown in Fig. 6-10. Fig. 6. Date rate:30 Gbps, Distance: 20 km, Without minimizing power in one polarization: X pol. Fig. 7. Date rate:30 Gbps, Distance: 20 km, Without minimizing power in one polarization: Y pol. Power profile in X and Y polarization while applying gra- dient descent algorithm is also shown in in Fig. 11. Chromatic dispersion increases with data rate and fiber length. Chromatic dispersion does not cause the mixing of the carrier and the modulated data. It is shown that minimization of the power in one polarization is able to separate the carrier and the modulated signal ev en if there is a significant amount of chromatic dispersion. Due to chromatic dispersion, recei ved data will be dispersed (Fig. 10) Fig. 8. Date rate:30 Gbps, Distance: 20 km, W ith minimizing power in one polarization: X pol. Fig. 9. Date rate:30 Gbps, Distance: 20 km, W ith minimizing power in one polarization: Y pol. Fig. 10. Constellation diagram of received signal without minimization of power (left) and with minimization of power (right). Fig. 11. Power maximization in one polarization and minimization in other polarization by Gradient descent algorithm. V I . E X P E R I M E N T A L R E S U L T S : Block diagram of experimental setup of polarization diver - sity based SH-QPSK system is shown in Fig. 12. In this setup manually rotatable three paddle based PC is used to minimize the power in one of the polarization. SFL1550P external cavity laser (ECL) having power of 13.02 dbm is split into two polarization using PBS. Output power of the PBS branches are set using PC connected to laser source. One of the branch of PBS is connected to the QPSK Modulator LN86S-FC which is driv en by two RF signals (2 Gbps/4 Gbps) generated by arbitrary wave form generator . Optical modulator driv er- HMC788LP2E has been used to provide enough strength to RF signals. Another branch of the PBS is directly connected with output of the modulator using PBC.The received signal 4 Fig. 12. Experimental setup for SH-QPSK system with polarization multiplexed carrier . A WG: arbitrary waveform generator, PC: polarization controller, PBS/PBC: polarization beam combiner/splitter, V O A: variable optical attenuator , and PM: power meter . from the single mode fiber is split into orthogonal polarization (X and Y) using PBS. PBS branch having lower power is connected to the signal port of the receiver front end - CPR V1222A and branch having higher power is connected to the LO port of the recei ver front end. For faithful demodulation of the receive data, the carrier and the modulated signal should not be mixed. But due to non-idealites of optical setup and fiber channel, these tw o polarization signals get mixed. In this case, there will be less po wer difference between two polarization signals after splitting at the receiv er side. If these signals are directly connected to the recei ver front end, it is not possible to get proper electrical data unless there is some signal processing done on it. Manually rotatable three paddles based polarization con- troller is connected to the fiber output to separate the carrier and the modulated data. T o fulfill this requirement, two po wer meters are connected to the output branches of the PBS at the receiv er side as sho wn in the Fig. 12. Now by rotating the paddles of the polarization controller manually , maxi- mum power difference between two polarizations is achiev ed. Branch having minimum po wer is connected to the signal port of the receiv er front end and branch having maximum power is connected to the local oscillator port of the recei ver front end. Minimizing power in one branch, data are captured. Fig. 13. Data rate: 4 Gbps, Distance: 30 km, Constellation diagram: Re- ceiv ed signal without minimiziing power (Power dif ference: 2.64 dB) (Left); Receiv ed Signal after power minimizing (Power difference: 13.2 dB, EVM after phase correction: 26.66 %) (Right) Fig. 14. Data rate: 8 Gbps, Distance: 30 km, Constellation diagram: Re- ceiv ed signal without minimiziing power (Power dif ference: 5.32 dB) (Left); Receiv ed Signal after power minimizing (Power difference: 13.03 dB, EVM after phase correction: 34.16 %) (Right) Experiment has been performed for 30 km SSMF with data rate of 4 Gbps and 8 Gbps. Results are presented in Fig.13 and Fig.14. Improvement in the constellations is clearly observed in results with and without po wer minimization in one polarization. V I I . C O N C L U S I O N Proposed adaptive polarization control technique for polar- ization div ersity based SH systems is practically feasible for implementation in real time systems. This proposed method is successfully validated with simulation and practical results. This work opens a choice to employ SH systems for lo w po wer high capacity data center interconnects. A C K N O W L E D G M E N T The authors would like to thank DST and DeitY for funding the project. W e also like to thank Mr . Arvind Mishra and Mr . Madhan thollabandi from sterlite technologies for their continuous support. R E F E R E N C E S [1] M. Anghan, “ Adaptive Polarization Control for Carrier Multi- plexed Self-Homodyne Coherent Optical Receivers, ” Mtech dissertation (14307R028), IIT Bombay , Mumbai, India, June 2017. [2] K. 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