International Journal of Scientific & Engineering Research, Volume 4, Issue 7, July-2013 2264

ISSN 2229-5518

Analysis of FSO Communication Links For Mid

And Far Infrared Wavelengths

First Author, Second Author, Third Author

Abstract— Atmospheric attenuations pose biggest challenge in implementing Free Space Optical Communication (FSO) links. Studying atmospheric attenuation as function of cumulative aerosol particle size distribution gives more reliable results rather than taking meteorological visibility as lone factor. Theoretically speaking, wavelengths of Mid IR and Far IR region may exhibit higher immunity against atmospheric attenuation against lower spectral regions but however in practical terms, the effect of totality of system and link parameters negates any such inherit advantage which means that considering the dynamic microenvironment, FSO link does not exhibit any attenuation immunity using higher spectral bands. Increase in transmitted optical power and higher receiver sensitivity along with appropriate selection of photo detector not only improves BER of system but also increases the range of communication.

Index Terms— FSO attenuation Models, Digital Signal to Noise Ratio (DSNR), Generalized Link Margin (GLM), Transmittance, Bit Error

Rate (BER) .

1 INTRODUCTION

——————————  ——————————
ree Space optical communication (FSO) is fiber less point- to-point Infrared (IR) spectrum based optical communica-
The premier work in FSO domain started with empirical for- mula given by Kruse [3] which relates the system attenuation
tion link between optical transreceivers which are separat- ed by atmosphere as physical medium. Compared with con-
with atmospheric visibility

𝛾(𝜆) = 3.912 � 𝜆(𝑛𝑚) �

−𝑞

(1)

ventional RF systems, FSO has various inherit advantages like,
IR spectrum is unregulated and hence doesn’t needs any li-

𝑉

The coefficient q depends upon

550

censing, due to point-to-point configuration laser beam gener- ated are narrow and invisible to human eye, making data im- possible to intercept. Most FSO systems are plug and play de-

1.6 𝑉 > 50 𝑘𝑘𝑘

q= � 1.3 6𝑘𝑘𝑘 < 𝑉 < 50𝑘𝑘𝑘

0.585𝑉1/3 𝑉 < 6𝑘𝑘𝑘

(2)

vices, independent of transmission protocol and data rates [1].
The performance of FSO links is significantly limited and handicapped due to absorption and scattering phenomena of atmosphere. Fog event and strong snow events are the most adverse weather conditions because they result in high specif-
where V is visibility in Kms and λ is wavelength in nm and
𝛾(𝜆) represents specific attenuation. While Kruse suggested
lower attenuation for higher wavelengths, Kim et. al[4] came
up with interesting modifications in coefficient q.

⎧ 1.6 𝑉 > 50 𝑘𝑘𝑘

1.3 6𝑘𝑘𝑘 < 𝑉 < 50𝑘𝑘𝑘

ic attenuation to optic waves [2]. Several models describing
the relationship between atmospheric visibility and related
optical attenuation have been published [3,4,5]. The Kruse [3]

q= 0.16𝑉 + 0.34 1𝑘𝑘𝑘 < 𝑉 < 6𝑘𝑘𝑘

⎨ 𝑉 − 0.5 0.5𝑘𝑘𝑘 < 𝑉 < 1𝑘𝑘𝑘

⎩ 0 𝑉 < 0.5𝑘𝑘𝑘

(3)

and Kim [4] models were based on the visibility definition for
a 0.55 μm wavelength. Al Naboulsi models were obtained by
interpolating results from FASCOD software [5]. Our core
work remained in creating understanding about wavelength
selection which could help design systems that may be im-
mune to atmospheric visibility degradations. The growing perception that wavelengths of higher order (10μm) will offer high link reliability [6,7], needs to be looked into again as this claim seems to be lopsided and considerable work has been
done by [8] to contradict the claims of advantage of higher wavelengths.
This paper has been divided in five sections. Section I con- tains brief reference to initial epmerical theories that related atmospheric attenuation as function of wavelength used and visibility. Wavelength selection from perspective of system and link parameters has been studied in Section II. In Section III results obtained from OptiwaveTM have been presented to contradict the claims the efficiency of higher order wave- lengths. Results and Conclusions have been compiled in Sec- tion IV while future research scope ideas have been listed in Section V.

2 SECTION I

[4] States, wavelength selection has no effect in case of low visibility conditions (up till 500 meters) with as shown in fig- ure 1. However beyond 500 meters of visibility, both Kim and Kruse predict same idea of decreasing attenuation with in- creasing wavelength; however Kim proposed lower dip in attenuation, figure 2(a).
Al Naboulski et. al [5] extended the attenuation and wave- length selection relation based on distribution particle and their size rather than just studying the visibility. [5] Predicted attenuation relationship based on results from FASCODE, which is software based analysis tool for real time atmospheric behaviour toward atmospheric attenuation. The fundamental of FACODE lies with Mie scattering phenomena which is again extension of aerosol particle size distribution. [9] Relates particle distribution with particle size as:
𝑛(𝑟) = 𝑎𝑟𝛼 exp(−𝑏𝑟) (4)
Where n(r) represents number of particle per unit volume
per unit increment of radius r while a,b,α represent character-
istic of particle size distribution. From figure 2(b) conclusion
can be drawn that for given visibility as the wavelength in-
creases, it approaches the particle size and hence attenuation
increases. For this, [5] categorised fog into two types based on

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International Journal of Scientific & Engineering Research, Volume 4, Issue 7, July-2013 2265

ISSN 2229-5518

their particle size density and distribution, Advection fog and

Convection Fog.

Ps is received power, R is receiver responsivity, σo is current var- iance in absence of any signal while σ1 is current variance in presence of received signal. As in case of Figure 4, the advantage of using of higher wavelength is negated on account of absorp- tion that takes over the entire link range.

Fig. 1 Kim v/s Kruse model for visibility 200 me- ters at different wavelengths

3 SECTION II

Since the studying atmosphere attenuation as function of visi- bility presents only the half picture in understanding wave- length selection hence it became it must be understood very clearly that the microenvironment in which a FSO link oper- ates is highly dynamic, unpredictable and out of human con- trol hence it becomes necessary to study the effect of atmos- phere on system and link parameters and then use the totality of these parameter aspects to decide the optimum operational levels which finally can better idea about wavelength selec- tion. The received power law model which uses receiver sensi- tivity as bases for deciding link availability provided interest- ing results. The received power also called as Generalised Link Margin is calculated as [10]
Fig. 2 (a) Kim v/s Kruse Model for visibility 3
Kms (b) Fog models using Al AB Noulski Model

𝑃𝑃 = 𝑃𝑘 𝐿×𝐷

10(−𝛼.𝑅/10) (5)

𝑑2×𝑅2×1𝑒6

Pt , Ps are transmitted and received optical power in watts,

L is transmit and receive optical losses(100%), D is receiver

aperture diameter (met), R is link Range (Km), α is specific
atmospheric attenuation in db/Km. Figure 3, describes system with visibility 8 kms while link range was studied at 0.2, 2 and
6 kms, with transmitted power 35mW. The received power showed very little or no improvement with use of wave- lengths of higher orders, however increasing transmitted power does offer promising results but then practical limita- tions of using higher powers must be kept in mind.
Another important system parameter is Signal to Noise Ra- tio (SNR) at receiver end. Since the FSO system deals with dig- ital data, hence SNR has been modified to be called as Digital Signal to Noise Ratio (DSNR) and is given as [6]:
Fig. 3 Generalized Link Margin (GLM) for differ- ent System Ranges at visibility 8 Kms.
Transmittance as given by Beer’s Law is also an important feature in context with transmission of optical data in free space. Transmittance defines the ability of optical pulse of par-

𝐷𝐷𝐷𝐷 = 𝑅Ps

𝜎0+𝜎1

(6)

ticular wavelength to penetrate through minute atmospheric

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International Journal of Scientific & Engineering Research, Volume 4, Issue 7, July-2013 2266

ISSN 2229-5518

obstacles which range from aerosol particles, fog, smoke, snow and rain etc and is given by:
𝜏 = 𝑒−𝛾𝐿 (7)
Where τ is transmission coefficient, γ is atmospheric attenu-
ation (dB/Km) and L is link range in Kms. Figure 5 describes
Beer’s Law for link range of 2 kms under different visibility
conditions of 1, 5, 10 kms, Though this law suggest better
transmittance at increasing wavelengths but it must be noticed
that for higher visibilities the somewhat transmittance satu-
rates beyond 3μm- 4μm mark while for very low visibilities

the improvement is although linear but not very appreciable.
0.785μm to 6.1μm, but in all cases the BER remained similar i.e. 3.16×10-11. However when the transmission power was raised to 45mW, the BER improved to 4.18×10-17, as shown in figure 6b, thus implying that under foggy conditions, longer wavelength does not help.
Another interesting fact that came during study was that for same system and using similar parameters NRZ modula- tion outperformed the RZ modulation scheme in terms of BER at receiver. Also if receivers’ sensitivity has improved to -
50dBm, the range could be further extended while maintain- ing optimum BER levels, figure 7. In this case the transmission range was successfully enhanced to 680 meters while BER was
8.63×10-10, under similar atmosphere as stated in above case.

Fig. 4 DSNR for a receiver at different visibilities
at different wavelengths.

4 SECTION III

In this section the simulating environment provided by OptiwaveTM was used to ascertain the degree of correctness of theoretical results with ones obtained using Matlab as dis- cussed in previous section. OptiwaveTM helps to create a virtu- al environment using wide range of practically available la- sers, detectors, optical and electrical amplifiers and all other necessary equipment required to design a virtual system that works exactly the same way as the any real world application may have performed.
In our case a externally modulated laser along with random data bit generator was used as the transmission equipment. On receiver side PIN diode was used along with other neces- sary demodulating equipments separated by the transmission medium, atmosphere. The system was tested for wide range of parameters which include range, wavelength, atmospheric attenuation and transmission power.
Figure 6a shows eye patter for system with range
500meters, atmospheric attenuation 80dB/Km, receiver sensi-
tivity -30dBm and transmission power 35mW. This system
was simulated for wide range of wavelengths ranging from
Fig. 5 Transmittance as observed using Beers Law
Use of PIN diode as against APD in receiver section gave improved BER. This particularly because the multiplying fac- tor in APD, which not only enhances the gain but also enhanc- es other undesirable factors like shot noise, thermal noise and background noise. The performance of the two has been tabu- lated in table 1.

5 RESULTS AND CONCLUSIONS

From the above study it can be concluded that the visibility
alone cannot be taken as factor for considering the atmospher-
ic attenuation as considered in empirical methods of Kruse
and Kim. While Kruse law is extrapolation of Koschmieder’s
Law, fails to justify the link performance under severe foggy
weather. Kim’s law which came as an improvement over the
former solved the mystery of like behaviour under intense
foggy weather where visibility falls to less than 6kms but then
both of these empirical approaches fail to study the physical

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and structural composition of link environment. The dyna- mism of atmosphere which includes visibility as a factor of aerosol particle size and its physical particle distribution must be taken into account to get real time atmospheric conditions.

Table 1 Comparison of BER for PIN and APD at different transmitted power and receiver sensitivity at different rang- es

System attenuation as function of wavelength, presents on- ly the half picture from the transmitter point of view hence the selection of wavelength must be based on link and system parameters as well. Factors like link range, wind current, une- ven aerosol distribution, receiver noise etc. negate the ad- vantage lower attenuation using higher wavelength achieved during transmission. Thus has been confirmed while studying the receiver DSNR and received power, which displayed no such advantage of wavelength. However the only advantage which itself was again quite negligible was witnessed below the Mid IR region i.e. below 3 μm - 4μm.
Enhanced power at transmitter and improved receiver sen- sitivity can be very effectively used to make system immune to atmospheric vulnerabilities but it also helps to extend the system range while maintaining optimum level of BER. This can be done using Externally Modulated Laser (EML) sources coupled with optical amplifiers like EDFA at transmitter side while at receiver side PIN photodiodes can be used that offer higher receiver sensitivity.

6 FUTURE SCOPE

Since it has been very much concluded that optimization of FSO systems depend upon totality of system and link parame- ters and not on spectral variation alone. Hence our future course of action will focus be to integrate different microwave wireless modulation techniques with FSO systems along with building FSO system and test its response for different ranges of collimating lenses used in telescope so as to improve the light collimation and gathering ability at transmitter and re- ceiver respectively.

REFERENCES

[1] Z. Bielecki, W. Kolosowski and J. Mikolajczyk, “Free Space Optical Commu- nication using Quantum Cascade Lasers”, PIERS Proceedings, Cambridge, USA, July 2-6, 2008

[2] Martin Grabner, Vaclav Kvicera,” Experimental Study of Atmos- pheric Visibility and Optical Wave Attenuation for Free space optics communication”, Czech Science Foundation project No. 102/08/0851

[3] P. Kruse, L. McGlauchlin, R. McQuistan, “Elements of Infrared Technology:

Generation, transmission and detection”. John Wiley & Sons, 1962

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[4] I. I. Kim, B. McArthur, E. Korevaar, “Comparison of laser beam propagation at 785 nm and 1550 nm in fog and haze for optical wireless communications,” Proc. of SPIE, 4214, 2001, pp. 26-37

[5] M. Al Naboulsi, H. Sizun, F. de Fornel,,“Fog attenuation prediction for opti- cal and infrared waves,” Journal of SPIE, Optical Engineering, 43, 2004, pp.

319-329.

[6] Haim Manor, Shlomi Arnon,”Performance of an optical wireless communi-

cation system as function of wavelength,” Vol. 42 No. 21, Applied Optics, July

2003

[7] Maha Achour,” Free-Space Optics Wavelength Selection: 10 μ Versus”,

UlmTech.

[8] E.Korevaar, I.Kim, B.McArthur,” Debunking the recurring myth of magic wavelength for free space optics”, Optical Wireless Communication,SPIE

4873,155, 2002

[9] Maher Al Naboulsi, Frédérique de Fornel, Hervé Sizun, “Measured and predicted light attenuation in dense coastal upslope fog at 650, 850, and 950 nm for free-space optics applications”, Optical Engineering 47(3), 036001

,March 2008

[10] White paper on Generalised link margin, “Defining a Common Standard for Evaluating and Comparing Free-Space Optical Products,” by fSONA Communications, www.fsona.com.

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