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Effect of Signal Wavelength and Aperture Area of

Detector on Performance of Free Space Optical

Link

Anil J. KshatriyaDepartment of Electronics and Communication

Government Engineering College

Chandkheda,Gujarat,India

Email:anil.kshatriya@yahoo.com

Pravin R. Prajapati

Department of Electronics and Communication

A.D.Patel Institute of Technology

New V.V. Nagar,Gujarat,India

Email:pravinprajapati05@yahoo.com

AbstractBit error rate performance of a FSO link and visi-bility range are adversely affected by, harsh weather conditions,and atmospheric turbulence like fog, rain, clouds and dry snow.Wavelength of signal and aperture area of optical detector affectsquality factor of receiver. This paper presents performance ofFSO link with different wavelengths and different aperture areaof optical detector. Effects of different wavelengths on visibilityrange and quality factor of optical receiver have been simulated.For simulation, license versions of OPTSIM 5.2 and MATLAB6.00 have been used.

Index TermsFSO, PRBS, BER, aperture area

I. INTRODUCTION

Free space optical communication has very large informa-

tion handling capacity. Free space optical (FSO) is a promising

technology capable of full duplex transmission of data, voice

and video in certain applications [1]. The link can be installed

within a day. Compared to fiber communication, FSO does not

require digging to lay the fiber and it does not require permis-

sion from the landowners. W. Popoola et al and Muhammad

Saleem Awan et al [2], [3] reported performance of FSO in

different weather conditions like thick fog, moderate fog, light

fog, heavy rain, moderate rain and light rain etc. Fang Xu et

al reported efficiency of different channel coding techniques

for different time diversity orders and turbulence conditions[4]. Performance of FSO link using MIMO transmitter and

receiver investigated by Zeinab Hajjarian et al [5]. W. O.

Popoola et al presented error performance of FSO using a

subcarrier intensity modulation based on a binary phase shift

keying scheme in turbulent atmosphere [6]. A large effective

aperture can be achieved by combining the output signals from

an array of smaller receivers.Chen [7] reported the non coher-

ently combined heterodyne receiver combined system, which

can offer superior performance for given a fixed collecting

area.M.A. Khalighi et al [8] investigated the impact of aperture

averaging on the performance of FSO systems under different

atmospheric turbulence regimes and performance evaluation

is made in terms of the average BER. Nazmi A. Mohammed

et al [9] reported that the BER performance of FSO can be

improved by using forward error correcting codes. Ivan B.

Djordjevic et al [10] reported that the LDPC coded MIMO

schemes can operate under a strong atmospheric turbulence

and at the same time provide excellent coding gains compared

with the transmission of uncoded data. Sachin M. Kale et al

[11] reported that for a 2 km FSO link, relative improvements

in SNR and BER due to combined effect of partially coherent

optical beam and the aperture averaging of the received optical

beams in different atmospheric turbulence conditions. AhmedA. Farid et al[12] derived a statistical model for the optical

intensity fluctuation at the receiver due to the combined effects

of atmospheric turbulence and pointing errors. Colin Reinhardt

et al [13] described an improved method for estimation of the

atmospheric channel impulse response function, for adverse

visibility conditions of fog to improve the performance of

terrestrial FSO system. Thomas Plank et al [14] reported some

major performance improvement obtained by employing some

specific modulation and coding schemes

II. SIMULATION

The block diagram for the simulations link of free spacecommunication is shown in Fig. 1. Transmitter section consists

of the data source of pseudo-random binary sequence (PRBS),

electrical driver, LED source and optical normalizer. The data

source is a non return to zero (NRZ) format at 1.25 Gb/s

bit rate and is indicated by PRBS generator, as shown in

Fig. 1. This model generates a binary sequence of several

different types like alternating one and zero sequence, PRBS,

only sequence of one and only sequence of zero. NRZ driver

converts an input binary signal into an output electrical signal.

The output signal may be specified as either voltage or current.

Here NRZ modulation is considered. The input data source

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modulates at the LED beam by using an LED driver.LED

source generates the LED beam at 1550 nm. The output of the

modulator is fed to an optical normalize which normalizes the

optical signal power by attenuating the input optical signal(s)

to the specified average output power level. Free space optical

length of 500 meter is considered. For attenuation constant,

different conditions of atmosphere like thick fog, moderate

fog, light fog, heavy rain, medium rain and clear conditions

have been considered The OptSim photo receiver model is

composed of several individual building blocks: the photo

detector, the preamplifier, and the post amplifier/filter complex.

Here PIN photo detector with quantum efficiency of 80%, dark

current of 10 A and ionization coefficient of 1 considered.BJT based preamplifier with noise like shot noise and thermal

noise etc considered. Bessel filter with 4th order and 1 GHz

bandwidth has been considered.

III. RESULTS AND DISCUSSION

The free space optical link is simulated to find the visibility

range for different wavelengths at given attenuation.Rayleigh

scattering and other scattering losses are inversely proportionalto wavelength and visibility () = 3.91

V

550

q

, where V is

visibility in km and q is the size distribution of the scattering

particles [2],[6].According to the Kim model, q is taken as

1.6 for V > 50 , 1.3 for 6 < V < 50, 1.6V + 0.34 for1 < V < 6, V 0.5 for 0.5 < V < 1 and 0 for V < 0.5.According to the Kruse model, q is taken as 1.6 for V > 50,1.3 for 6 < V < 500 and 0.585V

1

3 for V < 6 [2], [6]. So,attenuation decreases with the increment of wavelength,which

is shown in Fig. 2. From Fig. 3, it is observed that the

quality factor of optical receiver increases as an increase

in wavelength of signal. The lower wavelength is scattered

much more than the higher wavelength. The attenuation is

less at higher wavelength compared to the lower wavelength.

So under the environment of scattering losses, higher signal

wavelength gives improvement. So the selection of wavelength

is important in order to reduce scattering coefficient and at-

mospheric attenuation. The higher wavelengths are also better

for eye-safety. The wavelength radiation above 1.5 m is lessharmful and safe for eyes. It also allowed for carrying higher

power than the shorter wavelengths. The higher wavelength

FSO link gives opportunity to obtain better range in bad

weather conditions as compared to the currently available ones

at the shorter wavelength. As receiver aperture area increases,

sensitivity of receiver increases due to increment in received

optical power, which leads to increment in quality factor ofreceiver. The effective aperture area of the receiver improves

the quality factor of FSO link. The increase in the aperture area

of the receiver increases the sensitivity of the receiver.Figs.

4 and 5 reported that as attenuation of FSO link decreases,

quality factor of receiver improves, and also with increment

of receiver aperture area, due to increment in sensitivity of

receiver, Q factor improves.

IV. CONCLUSION

It is concluded that due to reduction in scattering loss at

higher wavelength; as wavelength increases, quality factor of

Fig. 1. Optsim 5.2 Simulation link of free space communication

Fig. 2. Relationship between visibility range (km) for different wavelengths(nm)

Fig. 3. Quality factor of receiver as function of input signal wavelength fordifferent receiver aperture area

Fig. 4. Quality factor of optical receiver as function of attenuation fordifferent receiver aperture area

receiver improves.Quality factor of optical receiver is also

improves with increment in aperture area of detector due to

increment in sensitivity of receiver due to large aperture area.

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Fig. 5. Quality factor of optical receiver as function of receiver aperture areafor different atmospheric attenuation conditions

ACKNOWLEDGMENT

The authors would like to thank the management of Charu-

tar Vidya Mandal , Vallabh Vidyanagar and management of A

D Patel Institute of Technology for their continuous support

and encouragement. The authors also like to thank the Rsoftdesign group for the OPTSIM 5.2 simulation software and

Fiber Optics Services, Bombay.

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