Impact of interferometer delay time on the performance of ODSB-SC RoF system with wavelength interleaving

Optik 125 (2014) 2057–2061

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Impact of interferometer delay time on the performance of ODSB-SC RoF system with wavelength interleaving Harjit Singh a , Anu Sheetal a,∗ , Ajay Kumar b a b

Department of Electronics and Communication Engineering, Guru Nanak Dev University, Regional Campus, Gurdaspur, Punjab, India Department of Electronics and Communication Engineering, Beant College of Engineering and Technology, Gurdaspur, Punjab, India

a r t i c l e

i n f o

Article history: Received 7 May 2013 Accepted 28 September 2013 Keywords: RoF WI MZI ODSB-SC BER

a b s t r a c t In this paper, we investigate the impact of interferometer delay time in a 5 Gb/s optical double sidebandsuppressed carrier (ODSB-SC) RoF system transmitting two wavelength interleaved radio frequency (RF) signals at 10 and 15 GHz over an optical fiber. Here, an optical Mach–Zehnder modulator is used for both optical carrier suppression and signal modulation. At the receiver, delay interferometer is used for the separation of RF frequency signals. We analyze the performance of the RoF system by varying the value of delay time of interferometer from 0.02 to 0.14 ns. The result shows that the RoF system performance is optimum for the time delay of 0.1 ns. Further, the optical spectrums, RF spectrums and eye diagrams of two interleaved RF signals have been compared. © 2013 Elsevier GmbH. All rights reserved.

1. Introduction Radio-over-Fiber (RoF) techniques are attractive for realizing high-performance integrated networks. The growth of mobile and wireless communication has fuelled the increasing demand for multimedia services with a guaranteed quality of service. This requires realization of broadband distribution and access networks. Within this framework, RoF schemes can be applied for realizing seamless wireless networks since they allow for the easy distribution of microwaves and millimeter waves over long distances along optical fibers [1,2]. RoF technology combines the capacity of optical networks with the flexibility and mobility of wireless networks. Reduction in complexity at the antenna site, reduction in installation cost of access networks, possibility of dynamic allocation of radio carriers to different antenna sites, transparency and scalability are the few advantages of RoF. The applications of RoF technology include cellular networks, satellite communication, Multipoint Video Distribution Services (MVDS), Mobile Broadband System (MBS) and Wireless LANs over optical networks [3]. RoF technology involves the use of optical components and techniques to allocate RF signals from the control stations (CSs) to the base stations (BSs). Thus, RoF makes it possible to centralize the RF signal processing function in one shared location control station with the use of single mode optical fiber that has a very low

∗ Corresponding author. Tel.: +91 9464497067. E-mail address: [email protected] (A. Sheetal). 0030-4026/$ – see front matter © 2013 Elsevier GmbH. All rights reserved. http://dx.doi.org/10.1016/j.ijleo.2013.09.075

signal loss to distribute the RF signals to the BSs [4]. In order to reduce the cost of system deployment the high frequency radio carrier (tens of gigahertz) would be generated in a CS and then transmitted to a BS via optical fiber. This scenario brings about two problems one of which is dispersion caused fading that occurs when high frequency signals travel along fiber [5]. This is caused by the fact that the intensity modulation of light normally generates two main side bands equidistant from the optical carrier by the radio frequency (double side band – DSB). These components are affected by chromatic dispersion, which introduces a phase shift between them. If this phase shift equals 180◦ the two side bands will interfere destructively in the photodiode causing the output signal to fade. Dispersion caused fading could be overcome by carefully adjusting the value of time delay of the interferometer used for signal separation at the receiver or converting the DSB signal into a single side band (SSB) format [6]. In order to improve the spectral efficiency of such a RoF system, a fairly new channel spacing technique called wavelength interleaving (WI) has been proposed [6]. In systems employing WI the channel spacing is reduced to values that are less than twice the highest modulating frequency. RoF systems having channel spacing smaller than the RF carrier frequency yields better spectral efficiency, but results in higher cost because it requires either two filters in cascade or a specially designed filter with two pass band frequencies [6,7]. From the available literature, it has been seen that the performance of RoF system can be significantly improved by WI techniques. Here interferometer has been used for separating the WI signals at the receiver. In this paper, we externally modulate

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Fig. 1. Design of interferometer.

two high-speed RF signals interleaved at 10 and 15 GHz in a 5 Gb/s optical double side band-suppressed carrier RoF system. The RoF system has been optimized by adjusting the delay time of interferometer which is used for separation of RF signals at the receiver. In Section 2, simulated set up of RoF system and its parameters has been explained. In Section 3, results have been reported for varying values of delay parameter of interferometer in the form of eye diagrams, RF spectrums and optical spectrums. Finally in Section 4, conclusions are made. 2. Theory A series of Mach–Zehnder Interferometers (MZI) can be used for making a tunable optical filter. An MZI can be build up simply by connecting the two output ports of a 3-dB coupler to the two input ports of another 3-dB coupler as shown in Fig. 1. The first coupler splits the input signal equally into two parts, which acquire different phase shifts before they interfere at the second coupler if the different size arm lengths are used. The signal exits from either arm depending on arm length and signal frequency. The transfer function of MZI is: HMZ (w) =

1 [1 + ejw ] 2

(1)

where  is the added delay in the upper arm of the interferometer. Since the relative phase shift is wavelength dependent, the transmittivity T(v) is also wavelength dependent. We can use above equation to find the transmittivity: T (v) = |H(v)|2 = cos2 (v)

(2)

Fig. 3. RF spectrum of two input signals at 10 and 15 GHz.

where v = ω/2 is the frequency and  is the relative delay in the two arms of the MZ interferometer [8]. A cascaded chain of such MZ interferometers with relative delays adjusted suitably acts as an optical filter that can be tuned by changing the arm lengths slightly. Mathematically, the transmittivity of a chain of M such interferometers is given by:

T (v) =

M 

cos2 (vm )

(3)

m=1

where  m is the relative delay in the mth member of the chain. The resulting transmittivity of a 10-stage MZ chain has channel selectivity as good as that offered by a FP filter having a finesse of 1600. Moreover, such a filter is capable of selecting closely spaced channels. Tuning in MZ filters is realized through a chromium heater deposited on one arm of each MZ interferometer. Since the tuning mechanism is thermal, it results in a slow response with a switching time of about 1 ms.

Fig. 2. Schematic of 5 Gb/s ODSB-SC RoF system.

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Fig. 4. Showing the two RF signals at 10 and 15 GHz after interleaving.

3. System description and modeling The model of 5 Gb/s ODSB-SC RoF system is schematically shown in Fig. 2. It contains RF section where the two RF signals at 10 and 15 GHz are combined using electrical multiplier and combiner. The RF spectrum of two input signals as shown in Fig. 3. The RF signals are then passed through optical Mach–Zehnder modulator, which is used for both optical carrier suppression and signal modulation using a continuous wave (CW) laser at 193.1 THz, having line width 10 MHz and power 1 mW. The optical signal is then transmitted over 20 km fiber at reference wavelength 1550 nm with the other fiber parameters as: fiber loss = 0.25 dB/km, dispersion = 16.75 ps/nm km, dispersion slope = 0.75 ps/nm2 km, the nonlinear refractive index n2 = 2.6 × 10−20 m2 /W and Aeff (core effective area) = 67.56 × 10−12 m2 . The signal is then passed through delay interferometer which is used for separation of RF signals by varying delay time between the signals. The separated RF signals are then demodulated in the receiver section. It is composed of Bessel filter at 193.1 THz with 40 GHz bandwidth, optical amplifier having gain = 15 dB and noise figure = 5 dB followed by PIN photo detector having responsivity = 1 A/W and dark current = 1 nA. The signals are then demodulated using electrical AM demodulators with cut-off frequency = 1 GHz, having bandwidth of 20 and 30 GHz for 10 and 15 GHz WI signals, respectively. The signals can be subsequently visualized for eye opening, eye closure, BER and Q value.

Fig. 6. shows the output optical spectrum at 0.1 ns time delay of interferometer for (a) RF signal at 10 GHz and (b) RF signal at 15 GHz.

4. Results and discussion

Fig. 5. Input optical spectrum of ODSB-SC for two signals at 10 and 15 GHz.

The performance of ODSB-SC RoF system with wavelength interleaving has been analyzed to study the impact of variation in interferometer delay time using BER, Q value, timing jitter, optical spectrums and eye diagrams. Fig. 4 shows the two 5 Gb/s RF input signals at 10 and 15 GHz. The frequency difference of the two interleaved RF signals is clearly seen in figure where first and second part shows the signal with 15 and 10 GHz frequency, respectively. Fig. 5 shows the input optical spectrum of two RF signals showing the double side bands with suppressed carrier. It is observed that when the modulation is performed using Mach–Zehnder modulator, the two sidebands are formed. For 10 GHz signal, the frequency of upper side band is 193.11 THz and lower side band is 193.09 THz, with the total modulated signal bandwidth of 20 GHz. Similarly, for the RF signal at 15 GHz, the upper and lower sideband frequencies are 193.115 and 193.085 THz, respectively, and total modulated signal bandwidth is 30 GHz.

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Fig. 7. Showing eye diagrams for 10 GHz and 15 GHz RF signals with interferometer delay time of (a) 0.02 ns, (b) 0.06 ns, (c) 0.1 ns, and (d) 0.14 ns.

Fig. 6(a) and (b) shows the output optical spectrum of RF signals at 10 and 15 GHz, respectively, after de-interleaving at the output of interferometer with 0.1 ns delay time. The spectrum shows two distinct sidebands along with some higher frequency side lobes. This is due to the presence of nonlinearties at higher bit rates causing signal degradation owing to inter-channel cross phase modulation (XPM) and four wave mixing (FWM) effects. XPM and FWM are nonlinear optical Kerr effects, which lead to spectral broadening due to the phase variation. The generation of new frequencies also results in the increase in the bandwidth and poor reception of the signals at the receiver causing reduction in the Q value.

Fig. 7 depicts eye diagrams for two RF signals with varying delay time of interferometer from 0.02 to 0.14 ns along with various fiber nonlinearties. It is observed that the performance of system is although good for a time delay of 0.02 ns with acceptable jitter as shown in Fig. 6(a) but the eye opening is further improved when operated at 0.1 ns delay as shown in Fig. 6(c). For further increase in the time delay of interferometr the system performance gets deteriorated because of overlaping of the symbols causing inter symbol interference (ISI). So, the optimum value obtained for the interferometer time delay is 0.1 ns. These results are also endorsed by values of Q factor, BER, timing jitter as given in Table 1. Here, for

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Table 1 Showing Q factor, BER and timing jitter for different interferometer delay times. Interferometer delay time in ns

0.02 0.06 0.1 0.14

RF signal frequency = 10 GHz

RF signal frequency = 15 GHz

Q-value (dB)

BER

Jitter (ns)

Q-value (dB)

BER

Jitter (ns)

16.6 4.3 17.57 4.17

2.13 × 10−62 8.8 × 10−6 1.75 × 10−69 1.38 × 10−5

0.05 0.107 0.04 0.23

22.4 5.1 24.42 4.09

1.66 × 10−111 1.3 × 10−7 4.06 × 10−132 2.08 × 10−5

0.07 0.113 0.06 0.14

time delay of 0.1 ns the Q value obtained is maximum i.e. 17.57 and 24.42 dB for 10 and 15 GHz RF signals, respectively. Also, at this delay time, the timing jitter is minimum i.e. 0.04 and 0.06 ns, respectively. 5. Conclusions The performance of 5 Gb/s ODSB-SC RoF system using two wavelength interleaved RF signals at 10 and 15 GHz has been investigated. Since delay interferometer is used for separating the two interleave wavelengths, the system performance has been evaluated by varying the value of interferometer delay time from 0.02 to 0.14 ns. The best resolution of the wavelengths at the receiver is observed at 0.1 ns delay due to the least ISI. At this delay, the maximum Q factor obtained is 17.57 and 24.42 dB with the minimum timing jitter of 0.04 and 0.06 ns for RF signals at 10 and 15 GHz, respectively.

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