Symmetric Mach–Zehnder Demultiplexing Technique for Optical Time-Division Multiplexing: Comparison of EAM, MZM and AM

1 Introduction

Optical networks are expanding speedily now days due to tremendous advantages like huge bandwidth, reliability, low-cost transmission and security [1]. Multiplexing is becoming one of the key technologies capable of satisfying the growing demand of large capacity for optical transmission system [2] and also has been considered as an attractive method for achieving high transmission rates on a single wavelength [3]. The demand of increasing transmission capacities in optical fiber communication systems is challenging due to vast expansion of telecommunications over time. The bandwidth of optical systems can be increased either through wavelength division multiplexing (WDM) or optical time-division multiplexing (OTDM) or by a combination of both. OTDM is more economic than WDM due to less network management efforts and also because existing single band and narrow-band erbium-doped fiber amplifiers (EDFAs) can still work and do not need to be replaced by broadband amplifiers [4].

Over the years, lot of research has been done to develop a practical OTDM systems taking into account about its extensive potential in future high-speed photonic networks. These researches have studied all-channel multiplexer (MUX) and demultiplexer (DEMUX) and used the Periodically Poled Lithium Niobate hybrid integrated with the planer light wave circuit for multiplexing of different channels [5]. Performance of an OTDM system for the most part depends upon the switching characteristics of a demultiplexer and thus intensive study has been done on the performance of varied demultiplexing switches. Investigations disclosed that among all the switches SOA primarily based SMZ was found to be best suited due to compact size, thermal stability, and low power operation. SMZ has symmetric switching window and hence it is less venerable to jitter [6, 7]. The main advantage of the SMZ structure over different interferometric switches like Terahertz Optical Asymmetric Demultiplexer is that SMZ will be simply integrated on to single photonic chip [8, 9]. SOA is often used in telecommunication systems in the form of fiber-pigtailed components, operating at signal wavelengths between 0.85 m and 1.6 m and generating gains of up to 30 dB. The SOA has higher noise, lower gain, moderate polarization dependence and high nonlinearity with fast transient time [10].

Chen et al. [11] demonstrated the demultiplexing for OTDM system exploiting two concatenated electroabsorption modulators (EAMs). OTDM signal of 80 Gbps was successfully demultiplexed to bit rates of 20 Gbps and 10 Gbps using the pair of EAMs, respectively. Jyotsana et al. [12] investigated the performance of 40 Gbps OTDM transmission systems with pre-, post- and symmetrical dispersion compensation techniques for different fiber standards.

Feng et al. [13] demonstrated an 80-Gb/s OTDM demultiplexing system based on the cross-phase modulation (XPM) effect in high-nonlinearity fibers. The clock signal with ~11 ps pulse width is achieved by employing super continuum spectrum-slicing technique, which is distinct from that based on mode-locked lasers. The demultiplexed signal is obtained by filtering out the XPM-induced spectral sidebands of the probe signal. Kaur et al. [4] described a symmetric semiconductor amplifier and Mach–Zehnder interferometer as switch for performing demultiplexing. Further the operation of return-to-zero and non-return to zero modulated signals was demonstrated in all optical demultiplexing of an OTDM channel extending for data rates up to 160 Gbps.

In this paper, we proposed an eight channel OTDM system using SMZ. The main advantage of the SMZ structure is that it can be easily integrated on to a single photonic chip and is most suitable because of compact size, thermal stability, and low power operation. The work reported in Ref. [14] was restricted to a bit rate of 10 Gbps for four channels with transmission distance of 75 km operating in C-band (1,550 nm). The work is extended here for eight channel OTDM system to a bit rate of 20 Gbps with transmission distance of 110 km operating in L-band (1,620 nm).

This paper is structured as: Section 1 presents the introduction to OTDM system. Section 2 describes about the simulation setup for 8×20 Gbps OTDM system. Section 3 reports the results for proposed system. The Section 4 gives the conclusion.

2 Simulation setup

The system configuration is depicted in Figure 1. OTDM transmitter comprises of a pseudo-random bit sequence (PRBS) generator, continuous wave (CW) laser, pulse generator, modulators, eight time shifting blocks and an optical multiplexer. CW laser with linewidth 10 MHz is used as optical source which operates at 1,620 nm having input signal power of 5 dBm. Optical output signal of laser is then splitted using 1×8 fork into eight channels. Eight channels at wavelength 1,620 nm are modulated with an external modulator (electroabsorption, Mach–Zehnder and Amplitude) driven by RZ Gaussian pulse generator with a PRBS. The bit rate of each channel is 20 Gbps. Before being multiplexed together each consequent channel is delayed by an optical delay (1/8 of the reciprocal of the “bit rate”). The time shifts used are 0 ns, 0.00625 ns, 0.0125 ns, 0.01875 ns, 0.025 ns, 0.03125 ns, 0.0375 ns and 0.04375 ns.

Figure 1:

System configuration.

CW laser acts as a pulse train generator with same repetition rate as used at the transmitter. The state of polarization of control signal is set orthogonal to data signal in order to distinguish it from the data pulse since both are at the same wavelength. The first time delay block that is Shift 9 will set the control signal to demultiplex the channel of interest, e. g. time delay is zero if channel 1 to be demultiplexed, time delay is “1/8 of the reciprocal of the ‘bit rate×1’” ns if channel 2 to be demultiplexed, and so on. Control signal split in two parts before being coupled with data signal in two arms of SMZ as shown in Figure 1. The second time delay Shift-10 sets switching window duration.

The internal structure of symmetrical Mach–Zehnder interferometer is shown in Figure 2. It consists of two couplers, two 2×1 multiplexers and two SOAs. Signal data are injected to the upper input of coupler1. The upper arm output of coupler 1 and the output of splitter are given to the 2×1 MUX 1. Similarly, the lower arm output of coupler 1 and output of shift 10 block is given to the 2×1 MUX 2 as shown in Figure 2. The output of MUX 1 is injected to the SOA 1 and of MUX 2 to the SOA 2. The resultant signals are fed to the coupler 2. The two output ports of coupler 2 correspond to “switching” (output) port and “reflective” ports. The output signal at switching port contains data information of demultiplexed channel.

Figure 2:

Internal structure of SMZ.

The demultiplexed output is fed to the receiver. Receiver consists of Photo detector, filter, regenerator and BER analyzer. The photodetector converts the optical signal into electrical signal. Avalanche photodiode is used as a detector at the receiver side having gain 3, responsivity 1 A/W and dark current 10 nA. Low pass Bessel filter with cut-off frequency 7.5 GHz removes the higher frequency components. The output of LPF is given to the 3R Regenerators followed by BER (Bit Error Rate) analyzer to visualize the BER, Q factor of the system.

3 Results and discussion

The performance of OTDM with different modulators such as EAM, MZM and AM is investigated in terms of Quality Factor (Q-factor), BER and Received Optical Output Power. SMZ is used as a demultiplexing switch due to its advantages as compact size, thermal stability, and low power operation. The results are reported for first channel by varying the input signal power (P_signal).

Figure 3 shows the plot between the Q-factor and input signal power (P_signal). It is depicted from the graph that with increase in P_signal, the quality factor increases. At minimum transmission power i. e. 5 dBm, EAM achieves Q-factor of 7.70. With MZM and AM the value of Q-factor is 7.50 and 6.69. EAM provides better signal quality as compared to MZM and AM.

Figure 3:

Quality factor versus input signal power (P_signal).

Figure 4 shows the variation in received optical power with input signal power (P_signal) for EAM, MZM and AM. The received power varies from –22.60 to –18.63, –22.97 to –19.01 and from –23.02 to –19.03 for EAM, MZM and AM, respectively. With electroabsorption modulator, the received optical output power is higher than Mach–Zehnder and amplitude modulator.

Figure 4:

Received optical output power versus input signal power (P_signal).

The effect of input signal power on BER is shown in Figure 5. The BER for EAM is in the range of 2.55×10⁻¹⁶–2.54×10⁻³² for P_signal values 5 and 9 dBm, respectively. So it is observed that with the increase in signal power (P_signal) the BER is improved. Similarly for MZM and AM this variation is in the range of 1.15×10⁻¹⁵–6.24×10⁻²³ and 4.33×10⁻¹³–2.08×10⁻²⁶ for P_signal values of 5 and 9 dBm, respectively. BER of an optical receiver is inversely proportional to SNR, which is in turn dependent on optical power of the signal. Thus BER decreases with increase in signal power. In previous work results reported for BER was in range of 10^−8.75–10⁻²⁸ for P_signal values 5 and 9 dBm whereas in this paper the range is 2.55×10⁻¹⁶–2.54×10⁻³². The results show an improvement over the results reported in [14] in terms of bit rate, number of channels and transmission distance at higher operating band (L-band).

Figure 5:

BER versus input signal power (P_signal).

The optical spectrum of demultiplexed channel 1 is depicted in Figure 6. The spectrums are drawn between wavelength (in μm) and power (in dBm) and are visualized at the output port of Symmetric Mach–Zehnder switch. It is observed that the peak power of the demultiplexed signal is obtained at wavelength 1,620 nm (or 1.620 μm). The spectrum of EAM shows less noise interference at peak power in comparison to MZM and AM. The received optical power of EAM is higher than other modulators at transmission distance of 110 km. Thus, EAM show better signal quality and is superior to MZM and AM.

Figure 6:

Optical spectrum of demultiplexed channel 1 at receiver side for (a) EAM (b) MZM (c) AM.

4 Conclusion

In this paper, the feasibility and performance of eight-channel OTDM system with different modulators such as electroabsorption, Mach–Zehnder and amplitude modulator has been investigated at bit rate of 20 Gbps in the presence of fiber nonlinearities. Symmetric Mach–Zehnder switch is used for demultiplexing of channels because of its compact size, thermal stability and less vulnerable to jitter. It is observed that EAM provides Q factor of 7.70 and BER of 2.55×10⁻¹⁶ at transmission power of 5 dBm whereas MZM and AM gives Q-factor of 7.50 and 6.69 with BER of 1.15×10⁻¹⁵ and 4.33×10⁻¹³ respectively. It is concluded that inclusion of electroabsorption modulator in OTDM transmission greatly improves the system performance. The benefit of this system is its capability to adjust the time intervals to make best use of the available bandwidth.