Modelling and Simulation of Optical Communication

Modelling and Simulation of Optical Communication Systems P. L. LÓPEZ-ESPÍ, J. ALPUENTE-HERMOSILLA, F. LÓPEZ-FERRERAS Y Mª P. JARABOAMORES Dpt. of Signal Processing and Communications Escuela Politecnica. University of Alcala Ctra. Madrid-Barcelona km 33,600 28871 Alcalá de Henares (Madrid) Phone: +34 91 885 67 40 Fax: +34 91 885 67 41

Abstract.. We present in this paper a software tool that let the user simulate the overall optical system response. We can evaluate it for those ones that use either rectangular pulses or digitally modulated signals in order to transmit information by means of optical fibber, monomode or multimode, and in first, second or third optical window. This application can also evaluate point to point systems with one or more wavelengths according to ITU grid. This application characterizes the different elements of an optical system: LED and laser transmitters, the different standard fibbers and PIN and APD photodetectors. This tool can simulate the overall system behaviour or that of one of the main parts in which it has been divided: emitter, fibber and detector. Simulated responses let the user analyse in the electrical domain in order to study light generation and detection, and the optical domain to study the way of transmitting information by means of an optical fibber. This tool has been created with an educational purpose using MATLAB 5.3. Keywords: lightwave simulation, Optical communication systems.

1 Introduction

2 Optical system modelling

Optical communication systems have enormous advantages respecting to classical radio or copper systems. At First, optical fibber was used to solve congestion problems in those countries with more saturated phone networks. Now optical fibber comprises almost the overall network. In order to create mathematical models for optical systems is necessary to understand physical properties of light generation in semiconductor optoelectronic devices, light propagation in dielectric waveguides and light detection using photodetectors. First to third optical system generation employed rectangular pulses. Due to networks saturation it was necessary to improve optical systems capacity. It is possible by using more efficient modulation schemes as in fourth generation systems or by improving fibber bandwidth usage as in fifth generation systems. This way its possible to get a bandwidth of more than 1 Tbps per kilometre.

As same as in any other communication system, the overall behaviour of an optical system can be calculated employing the particular responses of each one of the subsystems that it comprises. In this case, there are three different parts: optical transmitter, communication channel, made of optical fibber, and optical receiver. Assuming that the response relative to frequency for each one of them is respectively HT(f), HC(f) y HR(f), the whole system response is given by (1): H (f) = HT(f)·HC(f)·HR(f)

(1)

We can obtain the overall bandwidth by calculating separately each particular subsystem bandwidth. The power detected by the receiver (P RX) is calculated using transmitter power (P TX), channel attenuation (α) and photodetector responsive (GRX): P RX(dB) = PTX(dB) - α(dB) + GRX(dB) (2)

3 Optical channel modelling Optical channel modelling has been realised using fibber optic attenuation and dispersion properties. In order to calculate fibber kilometric attenuation we have considered those properties due to base material (SiO 2 ) with absorptions in infrared (α IR) and ultraviolet (α UV) and Rayleigh scattering (α ER), and those properties due to the presence of impurities incorporated to the fibber during manufacturing process (metallic ions (α IM), OH groups (α OH ) and other doping materials (α EX)). These attenuations are calculated by using the numerical quantities that the user has introduced employing a numerical keyboard or mouse in the fibber menu [1]. We can also load a library file previously created with the values of the different ITU fibbers. If we want to use a particular fibber we should indicate its type (monomode or multimode) and the optical window (first to third). The user must introduce the amounts of the link distance, OH ions concentration, zero first order dispersion point and dispersion slope in that point.

attenuation (figure 1) and dispersion relatives to wavelength (figure 2).

plots

Fig. 2. Modelling results for dispersion variation with wavelength in a G.652 fibber

4 Optical transmitter modelling It is possible to use LED or LASER diodes as optical transmitters. They can be chosen from the emitter toolbar menu. First of all, we have to define the type of emitter: LED or LASER. After it, we must introduce directly or load from a library file the characteristics of the selected emitter in order to model its optical and electric properties as in [4], [5], [7] and [8]. LED and LASER communication properties can be obtained by using carrier rate equations as in [9] and [11]. This also can be used to show light emitter modulation properties. We have suppose intensity modulation and direct detection (IM/DD).

Fig. 1. Modelling results for fibber attenuation variation with wavelength.

By using these last values, the application calculates the overall dispersion. We consider this dispersion effects to obtain fibber bandwidth as in [2]. The program requires as input data the typical parameters often given by cabling manufacturers or simply those ones defined by ITU-T [6]. That is, the bandwidth-longitude product in multimode optical fibber and zero first order dispersion point and its slope in monomode fibbers. The results for second order dispersion in monomode fibbers are obtained by using Sellmeier polynomials as in [3]. They offer an expression for group delay and using it, its possible to calculate fibber bandwidth. The system calculates fibber transfer function,

Fig. 3. Results for optical power variation as function of electrical current in laser sources.

However, the required data to obtain the properties for these emitters are not often supplied by the manufacturers. So, in order to characterize their behaviour, we must use those properties usually given. These are: operation wavelength, responsivity, rise time, spectral width, bias point, and in case of laser emitters threshold current and RIN noise power. The offered answers are transfer function calculated for operation wavelength, spectral power distribution and optical power generated as function of electrical current applied (figure 3)[10][12]. By using these data we are assuming linearity in optical sources. We also have programmed the possibility of defining a second order response in LASER sources.

5 Optical detector modelling Type of detector is defined by selecting it in the detector menu toolbar. This program can be configured to use one of two types of photodetectors: PIN diodes or Avalanche diodes or APD's. As same as in emitters, detection properties can be calculated by using semiconductor photoconducting properties. But we usually do not know enough data to solve gain or time response equations. Again, we use those parameters supplied by manufacturers in order to obtain the responsive for this subsystem. We have to define the values of operating wavelength, cutoff wavelength, efficiency, rise time, maximum optical power allowed and dark current for the photodetector, and in case of an APD detector, the multiplication or gain factor [2][5].

Fig. 4. Results for modelling of optical detector responsivity.

By using these parameters, the application calculates the optical responsivity at the operating wavelength and the noises that are inherent to he photodetection process (shot noise and dark

noise). The program also let the use simulate the transfer function, spectral responsive (figure 4) and those plots referred to the variation of the electrical current detected as function of the optical power received and the variation of sensibility due to rise time [10][12].

6 Fourth generation systems Digital modulations are used to convert a binary pulse baseband signal into a band pass one. Depending on the variating characteristic of the carrier signal can be defined amplitude (ASK), phase (PSK) and frequency (FSK) modulations. Another type that combines amplitude and phase modulation is quadrature amplitude modulation or QAM can also be employed. Transmitted signal is created by linear combination of a set of orthogonal signals. This application assumes coherent detection when using digital modulation techniques. That is, the application simulates a matched filter in the receiver. System quality expressed by bit error rate (BER) depends on signal to noise relation as is shown in [14] and [15]. We have programmed a virtual oscilloscope to check the ideal transmitted and received signals that can be used to compare easily how the received signal degradation due to bandwidth restrictions and noise presence varies the system bit error rate. Figure 5 shows the received signal for a 16QAM modulation.

Fig. 5. Results for modelling of received 16QAM signal including bandwidth and noise effects.

The application also has another way of testing quality in fourth generation systems. It is by comparing the constellation of both expected and received signals as shown in figure 7. The positions marked as 'x' indicate the ideal signal

position for each symbol, and the positions marked as '*' show the really received signal position. These positions are calculated by using the results obtained in the bank of correlators or matched filters programmed in the receiver.

Fig. 7. Results for modelling constellations of the expected and received in a 16QAM signal.

7 WDM Systems Wavelenght Division Multiplex (WDM) systems are the most efficient fibber transmission systems. By combining this kind of multiplexing and optical amplification we can obtain the highest transmission binary rate. In this application we can simulate up to 16 λ in the values defined by ITU grid. We can also simulate the gain and noise responses of an optical amplifier [4][8][13].

8 System quality And finally, the user must complete the system physical topology before calculating the overall system simulation. It is necessary to introduce those parameters that describe transmitter (number of bits, burst time, binary rate…), receiver (Noise temperature, bias resistance…) and the link (fibber reel distance supplied, number of splices, splice losses, number of connectors and connector losses, and the number of splitters and their insertion losses and coupling ratio). It is also possible to define a margin in order to ensure system quality by preventing future increasing of losses. To obtain the whole system quality it is also necessary to calculate the received optical power, overall bandwidth, noise presence in the receiver and the effects produced when connecting the three individual subsystems already analysed [2][3][4].

By considering intrinsic photodetection noises (shot and dark noise), RIN noise due to laser emitters, amplification noise in APD detectors, and those factors that get the responsive worse such as extinction ratio, dispersion, modal noise, chirping… we obtain the system signal to noise ratio, and from it, we calculate system bit error rate (BER) [11]. The application can simulate a bi-directional behaviour, offering the possibility of defining different data bits in both ways as in CATV systems. One data stream is defined in head-end to user way (MDT) and the other one in the opposite way (AMDT). This program calculates the individual bandwidth for each subsystem and the overall bandwidth for the link, letting the user to compare and analyse them individually. It offers the transmission characteristics for codification, available channels and also the power calculations and system quality.

8 Results As an example, we have simulated a system with the following characteristics: Laser diode emitter with responsivity 0.075 w/A, operating wavelength at 1300 nm, threshold current 26 mA, rise time 0.5 ns, spectral width 2.5 nm, bias current 100 mA and RIN -140 dB/Hz. Standard monomode fibber supplied in 5 km reels, considering 2ppm for OH concentration, 1320 nm zero dispersion point and 0,091 psnm-2 km-1 slope dispersion. Detector is a PIN diode, efficiency 0.344, responsivity 0.36 A/w, rise time 3 ns, sensibility 60 nw, overload 1 mw and dark current 2.29 nA. Binary rate is 51.84 Mbps. We also consider a link distance of 20 km, splice losses 1 dB and connector losses 1 dB and QPSK transmission. The results should be: required bandwidth 51.84 MHz, BER 8.5 10-3 , peak receiver voltage 58,8 uV, constellation expected points at 8.2 nJ Figure 8 shows received signal. Due to low signal to noise relation (close to 3 dB) the waveform is quite different to the expected one. Anyway, it is possible to appreciate that peak value is similar. Figure 9 shows constellation points for this example. The expected values are marked as 'x' and received ones are marked as '*'. Scale is multiplied by 108 .

Since the system simulates each block individually, this program is a very useful tool to detect design errors. This application could also be used with didactic purposes because it approaches as much the practical operation as the foundations of the optical communications systems.

10 References

Fig. 8. Received signal.

The application also offers numerical results for the calculated link as shown in figure 8. In this case, the main results are: emitter bandwidth 354.9 MHz, fibber bandwidth 1032 MHz, receiver bandwidth 59.2 MHz, available system bandwidth 57.95 MHz, transmitted signal bandwidth 51.84 MHz. Fibber attenuation 0.86 dB/km, total link attenuation 45.55 dB. Signal to noise relation 2.56 dB, BER 0.0085.

Fig. 9. Constellation of the received signal.

9 Conclusions We have created a useful tool to simulate optical communication systems. We can simulate point to point systems from first to fifth optical generations. Therefore, we can compare the advantages that each new generation offers with regard to the previous ones.

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