Demultiplexing Techniques of 320 Gb/s OTDM-DQPSK Signals: A Comparison by Simulation

Nhan D. Nguyen, L.N. Binh Department of Electrical and Computer Systems Engineering, Monash University,

Clayton, Victoria 3168, Australia. nhan.nguyen@eng.monash.edu.au

Abstract— We present the demultiplexing and demodulation of 320Gb/s OTDM-DQPSK signals using either ‘conventional’, i.e. demultiplexing then detection, or coherent detections with the local oscillator operating in pulsed sequence. A continuous-wave local oscillator (LO) can be replaced by a short-pulse laser source in a homodyne coherent receiver enabling us simultaneously demultiplex and detect the multiplexed channels of the ultra-high speed OTDM phase modulated signals. Simulated results also indicate at least 5 dB improvement of the receiver sensitivity over the conventional technique by the pulsed LO coherent receiver.

Keywords- OTDM; advanced modulation format; coherent detection; demultiplexing; four-wave mixing; mode-locked lasers.

I. INTRODUCTION

The demand of high capacity in communication networks has been dramatically increasing year by year. The ultra-high data rate channels toward the Tbit/s regime for Ethernet application is required in future optical networks. This demand poses several technical challenges in the transmission physical layer. One of feasible technologies for Tbs/s Ethernet is the optical time division multiplexing (OTDM). Moreover the combination of OTDM with multilevel modulation formats such as DQPSK or QAM enables easily the implementation of single channel several Tbit/s transmission [1]. The optical transmitter can be implmented without much difficulty. The most challenging issues of this advanced OTDM system rests on the receiving process that requires the realization of two principal functions: demultiplexing and demodulation.

In a ‘conventional’ OTDM receiver, two functions are separately executed. Initially, the OTDM signal sequence is demultiplexed to lower rate tributaries which are then demodulated by incoherent receivers to detect the data stream [1]. On the other hand, OTDM receivers based on homodyne coherent detection have also been recently proposed [2]. When the local oscillator is a short-pulsed optical sequence, both functions can be simultaneously performed. Thus this coherent receiver much simplifies the OTDM receiving sub-system. Furthermore the digital signal processors (DSP) can also be integrated in the coherent receiver to improve its sensitivity and possibly to mitigate the transmission impairments. However a comprehensive comparison of the performance of these OTDM receiver structures have not been conducted.

In this paper we demonstrate the implementation of both receiver structures based on the Simulink modeling platform developed for simulation of optical communication systems [3]

and evaluate the performance of these receivers for OTDM sequences under diffenetial quadrature phase shift keying (DQPSK) modulation formats. The paper is organized as follows: Section II gives us some basic principles of OTDM-DQPSK system as well as demultiplexing and demodulation functions in both incoherent and coherent receivers. Simulation models based on Simulink platforms are introduced in section III. Section IV shows the simulation results to compare the performance of both receivers which indicates a remarkable feature of the coherent OTDM receiver. Finally, the conclusions are given in section V.

Figure 1. A typical configuration of a DQPSK-OTDM transmission system

II. OPERATIONAL PRINCIPLES

Fig. 1 shows a typical configuration of an OTDM transmission system using advanced modulation format. At the transmitter, a mode-locked laser (MLL) is used to generate the ultra-short pulses before passing through the DQPSK modulator. The DQPSK signals at the output of tributary transmitters proceed to the optical time multiplexer which is structured by accurate optical delay lines to combine all tributaries into an ultra-high speed OTDM signal. The

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transmission link is characterized by the M fiber spans. Each span consists of a standard single mode fiber (SSMF), a dispersion compensating fiber (DCF) and EDFAs which fully equalize the total loss of whole span. The receiving end implements two important functions as metioned above, the demultiplexing of the OTDM signal and the demodulation of DQPSK states. The structure of an OTDM-DQPSK receiver can be classified into two types: the conventional receiver (demultiplexer+ incoherent receiver) and the coherent receiver.

Figure 2. Configurations of (a) nonlinear effect based OTDM demultiplexer, (b) the incoherent DQPSK receiver and (c) the coherent demultipexing-

demodulation OTDM-DQPSK receiver.

A. Conventional demultiplexing technique

1) Demultiplexing Most demultiplexing techniques are based on the

nonlinearity in optical fibers/waveguides such as highly nonlinear fiber (HNLF) [4], semiconductor optical amplifier (SOA) [5][6] and optical planar waveguides [7]. These techniques rely on the exploitation of the nonlinear effects in the nonlinear waveguide such as cross-phase modulation (XPM) or four-wave mixing (FWM) to enable switching at ultra-high speed. The most popular structure of OTDM demultiplexer is the nonlinear optical loop mirror (NOLM) using HNLF [1][4]. However its stability is still a serious obstacle. Recently, nonlinear waveguides have emergd as a promising device for ultra-high speed photonic processing [7][8]. This device is very compact and the FWM is exploited for demultiplexing as shown in Fig. 2a. The control pulses generated from a MLL at tributary rate are pumped and co-propagated with the OTDM signal through the nonlinear waveguide. Mixing process between the control pulses and the OTDM signal during propagation through the nonlinear waveguide converts the desired tributary channel to a new idler wavelength. Then the demultiplexed signal at the idler wavelength is extracted by a band pass filter (BPF) before going to a DQPSK demodulator.

2) Demodulation of DQPSK signal A DQPSK signal can be incoherently demodulated by two

Mach-Zehnder delay interferometers (MZDI) followed by a balanced detector as shown in Fig.2b. The inphase- and quadrature (I&Q) components are detected by the conversion of the differential phase modulation into intensity modulation through the interference in MZDIs [9]. In this receiver, the different length and the phase shift of the interferometer must be exactly tuned to avoid a high penalty in performance.

B. Optical coherent demultiplexing and demodulation

Instead of the incoherent receiver, a homodyne coherent optical receiver with demultiplexing function has been proposed [2]. In this scheme, an ultra-short pulse laser is used as a local oscillator (LO) laser to mix with the incoming OTDM signal. The inphase and quadrature components of the only one tributary are directly demodulated by synchronizing the LO pulses with time slots of this tributary in OTDM signal. Structure of a coherent demux-DQPSK receiver is shown in Fig.2c. Thus, the demultiplexing and demodulation functions are simultaneously implemented by using the coherent receiver with a pulsed LO source. This receiver offers a range of important advantages such as a simplification of OTDM system structure, more flexible in processing the tributaries and a remarkable reduction of pump power. Furthermore, the digital signal processing (DSP) can be easily applied to compensate deteriorative effects on the signal.

320 Gbit/s OTDM-DQPSK System with FWM base demux and incoherent demodulation

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Figure 3. Simulink models of the OTDM-DQPSK transmission system using (a) the conventional receiver, (b) the coherent receiver.

III. SIMULATION MODELS

Simulation models of the OTDM-DQPSK system are developed from Simulink modeling platform [3]. Fig. 3 shows the diagrams and key simulation blocks of the whole system using FWM based demultiplexing and incoherent receiver (Fig. 3a) and using coherent receiver with

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demultiplexing function (Fig. 3b). Simulink blocks of basic components in the system such as EDFA, transmission fibers, optical modulators and detectors are described in detail in [3]. In this section, some important parameters of main blocks and a brief description of receiving models developed for the OTDM-DQPSK system are given.

A. OTDM-DQPSK transmitter

The description of RZ-DQPSK transmitter model on Simulink platform can be found in [3]. However a MLL instead of a pulse carver is used in our setup to generate ultra-short pulses for OTDM signal. Then the outputs of tributary transmitters are time-division multiplexed to generate the OTDM signal of 160 GSymbols/s in our setup (Fig. 4).

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Figure 4. The simulink model of the OTDM-DQPSK transmitter.

TABLE I. IMPORTANT PARAMETERS OF MAIN SIMULATION BLOCKS IN THE OTDM-DQPSK SYSTEM.

OTDM-DQPSK Transmitter

MLL: P0 = 1 mW, Tp = 2.5 ps, fm = 40 GHz

Modulation format: DQPSK; OTDM multiplexer: 4x40Gsymbols/s

Fiber transmission link

SMF: LSMF = 80 km, DSMF = 17 ps/nm/km, α = 0.2 dB/km

DCF: LDCF = 20 km, DDCF = -68 ps/nm/km, α = 0.2 dB/km

EDFA: Gain = 20 dB, NF = 5dB; Number of spans: 10

FWM demultiplexer

Pumped control: Pp = 500 mW, Tp = 2.5ps, fm = 40GHz, λp = 1556.55 nm

Input signal: Ps = 35 mW, λs = 1548.51 nm

Waveguide: Lw = 7cm, Dw = 28 ps/km/nm, α = 0.5 dB/cm, γ = 104 1/W/km

Coherent receiver

MLL: P0 = 12.5 mW (Pav ≈ 1 mW), Tp = 2.5 ps, fm = 40 GHz

Balanced Rx: Be = 28GHz, ieq = 20 pA/Hz1/2, id = 10 nA.

B. Fiber link

Based on the configuration in Fig. 1, the link in simulation is developed with ten spans. Blocks such as optical fibers and EDFA in each span are explained in detail in [3]. Propagation of the optical signal in fibers can be modeled by the nonlinear Schrodinger equation (NLSE) in nonlinear scheme or a transfer function in linear scheme. EDFA block with ASE noise is modeled as a black box. The important parameters of the span are summarized in Table 1.

C. Demultiplexer and receiver

Fig. 5a shows the Simulink diagram of FWM based demultiplexer. The FWM process in the nonlinear waveguide to implement the demultiplexing function is also modeled by NLSE [10]. In the block, the complex fields of the signal and the pump are frequency shifted to the corresponding wavelengths (see Table I) before launching into the waveguide. Optical Gaussian band pass filters in the model are used to reduce the ASE noise and to extract the demultiplexed signal (the idler signal). Then the demultiplexed signal is incoherently demodulated by the MZDI receivers as described in [3].

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Figure 5. The simulink models of (a) FWM based demultiplexer, (b) coherent receiver with pulsed LO laser.

A homodyne coherent receiver with demultiplexing function is developed as shown in Fig. 5b. A mode-locked laser to generate short pulses similar to that in the transmitter is used as the local oscillator. The phase noise of the laser is also included in the model and described as a complex Gaussian process with a variance fTΔ= πσ φ 42 , where Δf is the

linewidth of the longitudinal modes of pulsed laser, T is the symbol period [11]. Mixing of the OTDM signal with the LO pulses takes place in the optical 90o hybrid followed by a couple of balanced detectors to detect directly I and Q components of the desired channel. We note that the peak power of control pulses in the coherent receiver is much lower than that in the conventional receiver.

IV. SIMULATION RESULTS

A. Performance of OTDM-DQPSK receivers under coherent and incoherent detection

Fig. 6a shows the spectrum of the signals after propagating through the nonlinear waveguide. Then the demultiplexed signal is extracted by the bandpass filter as shown in Fig. 6b. Fig. 6c&d show the eyediagrams of the OTDM signal and the demultiplexed signal respectively. Once demultiplexed, the optical DQPSK signals are phase-demodulated by MZDI balanced receivers. On the other hand in the coherent receiver,

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the signal of tributaries is directly demultiplexed and demodulated by mixing with a pulsed LO laser. Fig. 7 shows the constellations and the eyediagrams of the demultiplexed and demodulated tributary signal of the incoherent receiver and coherent receiver with laser linewidth of 5 MHz. Due to nonlinear effect during propagation in the waveguide, the phase states of the DQPSK signal before the incoherent receiver are rotated in the phase plane as shown in Fig. 7a. In contrast, the phase states in the coherent receiver are only affected by the phase noise of the pulsed LO laser besides the ASE noise from EDFAs.

Figure 6. Spectra at the outputs (a) of nonlinear waveguide, (b) of optical BPF; and the eyediagrams of (c) the OTDM signal, (d) demultiplexed signal.

Figure 7. Constellations of the demultiplexed signal and the eyediagrams of demodulated signals of (a) incoherent receiver and (b) coherent receiver.

Fig. 8 shows the back to back performance of two OTDM-DQPSK receivers. The coherent receiver with different laser linewidths offers much better performance than the incoherent receiver. In the conventional receiver, the use of many EDFAs in demultiplexing process also reduces the signal to noise ratio which degrades the performance of the receiver. Furthermore, the nonlinear propagation in the waveguide may also enhance the phase noise which causes a high error floor in the BER curve of the incoherent scheme. Fig. 9 shows the performance

after transmission through the link of 1000 km (10 fully dispersion-compensated spans). While the incoherent scheme shows a strong degradation of the performance after transmission, the coherent receiver still remains good BER performance.

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Figure 8. Back to back performance of incoherent receiver and coherent receiver with different phase noise levels of pulsed laser.

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Figure 9. Transmission performance of incoherent receiver and coherent receiver with different phase noise levels of pulsed laser.

B. Influence of synchronization

Synchronization plays a key factor in ultra-high OTDM system. To ensure a maximum efficiency of demultiplexing, the control pulses must be synchronized with the time slots of the demultiplexed tributary in the OTDM signal as shown in Fig. 10. On other hand, the performance of demultiplexing is degraded by any time mismatch between the control and the OTDM pulses as displayed in Fig. 11.

Figure 10. Synchronization between the control and the OTDM pulses.

In FWM based demultiplexer, the efficiency of demultiplexing is proportional to the power of idler signal, then the currents from the receiver depends on the square of pumped power of the control pulses

spRX PPi 2∝ , where Pp is the

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pump power, Ps is the power of OTDM signal. While the currents from the coherent receiver are proportional to the square root of LO power:

sLORX PPi ∝ , where PLO is the

power of LO. Therefore the signal level in the incoherent receiver is more sensitive to the mismatch of the synchronization than that in the coherent receiver. Fig. 12 shows the eyediagrams of demodulated signals in the incoherent and coherent receivers at different synchronization delays. They also indicate that the eye opening of the signals in the coherent receiver is better than that in the incoherent receiver.

Figure 11. The time traces of the control and the OTDM signals at different synchronization delays: (a) ~19%, (b) ~31% of the pulse period.

Figure 12. Eyediagram of the demodulated signal at two synchonization delays ~ 19% (a&c) and ~31% (b&d) pulse-period of the incoherent (above)

and coherent (below) receivers.

Fig. 13 plots the BER curves versus the synchronization delay in terms of the pulse period of both types of OTDM receivers. The results indicate a larger delay tolerance of the coherent receiver which facilitates the requirements of the OTDM-DQPSK receiver structure.

V. CONCLUSION

We have, by simulation, compared the performance of both the conventional incoherent- and coherent- receivers in 320

Gb/s OTDM-DQPSK mutli-span optical transmission system. By using a pulsed laser as a local oscillator in a homodyne coherent receiver, both functions of the OTDM-DQPSK receiver are simultaneously implemented. It also offers at least 5 dB improvement on the receiver sensitivity over that achieved by the conventional receiver in the noise-dominated long haul transmission system. Moreover, the simplification of OTDM receiver structure using the coherent detection allows a feasibility of integration of ultra-high speed OTDM technology into dense WDM system for ultra-high capacity optical networks. Other effects such as chromatic dispersion and polarization mode dispersion have been under further investigations and will be reported in the near future.

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Figure 13. BER curves vs the synchronization delay

ACKNOWLEDGMENT

N. D Nguyen acknowledges the support of his research by the Program No 322 of the Government of Socialist Republic of Vietnam.

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