Role of Modulators in Free Space Optical Communication

Transcription

1 Role of Modulators in Free Space Optical Communication Neha 1, Dr. Suresh Kumar 2 1 M. Tech Scholar, ECE Deptt UIET MDU Rohtak Haryana, India 2 Assistant Professor, ECE Deptt, UIET MDU Rohtak Haryana, India. Abstract- Free space optical communication is one of the remarkable technologies now-a-days. In this paper, a 10 Gbps data is transmitted through free space optical link under clear weather condition. For comparison two types of modulators are used: MZM and Electro absorb modulator.performance is measured in terms of Q factor, SNR, BER and eye diagrams. Keywords-Mach zehnder modulator Electro absorb modulator, SNR, BER. I. Introduction Free space optical communication (FSO), also known as wireless optical communication, is a cost effective and high bandwidth access technique in which optical transmitter and receivers communicate directly through the air to form point to point line of sight links. The transmitter converts the electrical signals into optical signals and sends it through the atmosphere (free space). The Receiver converts the optical signal back to an electrical signal. Communication is theoretically possible as long as the line of sight between the transmitter and the receiver is clear, and as long as the transmitted power is high enough to overcome atmospheric attenuation. With the potential high-data rate capacity, low cost and particularly high bandwidth on unregulated spectrum, FSO communication is an attractive solution for the last mile problem to bridge the gap between the end user and the fiber optic. Important features of FSO include huge modulation bandwidth, narrow beam size,low cost, easy implementationand unlicensed spectrum.fso Systems are mounted even within the building or top of the building / roofs. FSO System consists of mainly optical transceiver with a laser transmitter and receiver which allow it to transmit or receive the signal in full duplex mode. It generally looks like a video camera. Along with high power transceiver it also contains the lens which can transmits light through the atmosphere to other lens receiving informationalso contains the lens which can transmits light through the atmosphere to other lens receiving information. If transmitter source does not produce a sufficiently parallel beam to travel the required distance, the collimation can be done with lens. The receiving lens further connects to high sensitivity receiver via optical fiber. Along with advantages of FSO communication, there are some factors that limit its performance. The main disadvantages of FSO communication system are strict alignment requirement & adverse atmospheric weather condition. Achieving acceptable performance for a practical FSO link requires to overcome some major challenges at the transmitter such as determination of modulation techniques suitable light source, transmitting wavelengths and also estimating transmitting power levels.there are several challenges facing the channel performance such as effect of attenuation due to different weather condition that appears in study dealing with scattering atmospheric turbulences and scintillations. Fig 1 block diagram of FSO communication system II. OPTICAL MODULATORS As data rates in optical communication have been limited by the speed of available optoelectronics components. It is of much importance to consider practical aspects of modulation and detection hardware for designing optical modulation formats. Using the existing modulator structure in a new way has given birth to new optical modulation formats. 92 Neha, Dr. Suresh Kumar

2 The basic modulator technologies that are widely used are MZMs and EAMs. A. Mach-Zehnder Modulator MZM is an electro optic modulator (EOM). It works on the basis of Pockel effect. In Mach- Zehndermodulator the incoming optical signal is sent to two different optical paths. Applying voltage to one of the optical path produces a refractive index variation in accordance with the applied voltage introduces a phase shift in the wave travelling path. The amount of electric voltage required to produce a phase shift of 180 is called half wave voltage. The two optical paths recombine after a certain distance, causing the optical waves interfere with each other. This type of arrangement is known as an interferometer. The individual signals interfere constructively or destructively depending on their relative phase differences which determine the amplitude of an output optical signal. B. Electro-Absorption Modulator Electro absorption modulators are based on electro absorption effect, which is defined as the change of material absorption in the presence of an electric field. This modulator is used for modulating the intensity of laser beam via an electric voltage. The basic principle of EAM is Franz-Keldysh effect. It provides high data rate with low chirp. EAM have low driving voltage, large maximum extinction ratio a high figure of merit. The highest 3-dB bandwidth reported for EAM is 50 GHz. It can be monolithically integrated with driver circuitry and/or laser sources. These modulators have features such as low drive voltage(typically 2V) and are cost effective for large production. III. SIMULATION SETUP This section presents simulation setup for analyzingthe performance of optical modulators in FSO basedcommunication systems. Analysis is done at 10 Gbps for distance upto 10km. Here a CW laser source is used to transmit the data. Free space is used as a channel for transmission. Parameters like eye-diagram, quality factor, BER, SNR are used for comparison of optical modulators. At the receiver end PIN photodiode is used. Here the basic networks which are used for simulation are shown below in fig. 2and fig 3. Fig 2Simulation set-up for analysis of Mach Zehnder Modulator Fig 3Simulation set-up for analysis of electro absorb Modulator The link parameters are given below in table 1. These parameters are kept same for both the modulators to make better comparison. Serial No. Parameter Value 1. Bit Rate 10Gbps 2. Transmitter Power 1.5dBm 3. CWLaser Frequency 1550nm 4. Attenuation 0.2dB/km 5. Transmitter Aperature Diameter 6. Receiver aperature Diameter 5cm 20cm 7. Beam Divergance 2mrad Table 1: Link Parameters Outputs of both the modulators are passed through freespace atmospheric channel and performance is measured in terms of BER, SNR, and eye diagram at the receiving end. 93 Neha, Dr. Suresh Kumar

3 IV. SIMULATION RESULTS Fig4 eye diagram at 10km distance using MZM This is the eye diagram at a distance of 10km when we are using mach zehnder modulator based free space Fig 6 Q factor versus distance graph when MZM is used This is the Q factor versus distance graph when we are using mach zehnder modulator based free space It is clear from the graph that Q factor is higher at short distance and it decreases gradually as the distance increases. Fig 5 eye diagram at 10km distance using electo absorb modulator This is the eye diagram at 10km distance when we are using electro absorb modulator based free space Fig 7 Max SNR versus distance graph when MZM is used This is the Max SNR versus distance graph when we are MZM modulator in our free space optical communication circuit diagram. 94 Neha, Dr. Suresh Kumar

4 at short distances. The difference gradually decreases as the distance increases. Fig 8 min log of BER versus distance graph when using MZM modulator This is the min log of BER versus distance graph when mach zehnder modulator is used in free space Fig 10 Max SNR versus distance graph using electro absorb modulator This is the Max SNR versus distance graph when electro absorb modulator is used in free space SNR is higher when we are using electro absorb modulator. Fig 9 Q factor versus distance graph using electro absorb modulator This is the Max Q factor versus distance graph when electro absorb modulator is used in free space The difference in the q factors using MZM and electro absorb is very high Fig11 min log of BER versus distance graph when using electro absorb This is the Min log of BER versus distance graph when electro absorb modulator is used in free space 95 Neha, Dr. Suresh Kumar

5 Table 2 comparison of MZM and EAM at 10km distance at 10gbps Type of modu lator SNR Q factor Noise Gain MZM EAM V. CONCLUSION Mach zehnder modulator and Electro absorb modulator have been simulated using OPTISYSTEM. A signal at 10 Gbps data rate is passed through free space optical link using both these modulators. All the parameters are kept same for both the modulators. At the receiving end outputs of both the modulators are compared. By this comparison it is conclude that electro absorb modulator has higher SNR and Q factor. Eye diagram is also sharp in case of electro absorb modulator. REFERENCES [1] K; Sumanta Bose Dr. D. Sriram Kumar, "Analysis of Optical Modulators for Radio over Free Space Optical Communication Systems and Radio over FiberSystem" /12/$ IEEE [2] J. C. Juarez, A. Dwivedi, A. R. Mammons, S. D. JonesV.Weerackody R. A. Nichols, Free-space optical communications for next-generation militarynetworks, IEEE Communications Magazine, Vol. 44,pp.46-51,2006 [3]S. Chaudhary, P. Bansal, G. Singh, Implementation of FSO Network under the Impact of Atmospheric Turbulences International Journal of Computer Applications ( ) Volume 75 No.1, August 2013 [4] A. Amphawan, S. Chaudhary, The Role and Challenges of Free-space Optical Systems PACSR (2010) eDOI /joc [5]J. G. Proakis, Digital Communications, 4th ed. NewYork: McGraw-Hill [6] J.C.Chen, Comparison and linearization of LiNbO3 and semiconductormodulators, Journal of Optical Commuications,Vol. 22, pp.2 8, 2001 [7] J.G.M. Alvarez, L.A.Coldren, A. Alping, R. H. Yan, T. Hausken, K.Lee,et al., Analysis of depletion edge translation lightwave modulators, Journal of Lightwave Technology, Vol.6, pp , 1988 [8] R. G. Walker, High-speed III-V semiconductor intensity modulators, IEEE Journal of Quantum Electronics Vol. 27, pp , 1991 [9] D.A.B.Miller,D.S.Chemla, T.C.Damen,A.C.Gossard, W.Wiegmann,T.H.Wood, C.A.Burrus, Electric field dependence of optical absorption near the band gap of quantum-well structures, Physical Review B,Condensed Matter, Vol. 32, pp , [10] S.Sathyanandan, R.Swaminathan,R.Lavanya, S.Piramasubramanian,M.Ganesh Madhan GSM signal transmission through external modulated single mode fiber link, International Conference on Optics and Photonics 2009, Oct.-1 Nov Neha, Dr. Suresh Kumar