1262 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 25, NO. 13, JULY 1, 2013

Enhancing the Atmospheric Visibility and FogAttenuation Using a Controlled FSO Channel

Muhammad Ijaz, Student Member, IEEE, Zabih Ghassemlooy, Senior Member, IEEE, Joaquin Perez,Member, IEEE, Vladimir Brazda, and Ondrej Fiser

Abstract— In this letter, two methods for characterizing fogattenuation in terms of atmospheric visibility V in free spaceoptics (FSO) communications are analyzed and compared. Bothmethods measure the fog attenuation based on V using a charge-coupled device (CCD) camera and a laser-diode at 0.55 µm.The methods are evaluated in a laboratory controlled FSOatmospheric chamber operating at individual wavelengths of0.83, 1.31, and 1.55 µm using a continuous optical spectrumrange 0.6–1.6 µm. The CCD technique shows great accuracyfor V < 50 m and the laser technique for the range >100 m,thus allowing enhancement of the characterization of FSO linksin thick and dense fog conditions. The experimental resultsalso show a wavelength dependency for V < 0.5 km where theattenuation decreases from 52 dB/km at visible wavelengths to46 dB/km at near infrared wavelengths.

Index Terms— Free space optics, fog attenuation, visibility.

I. INTRODUCTION

IN recent years, the demand for FSO communications hasincreased considerably due to offering large capacity usage

(data, voice, and video) in a number of short to mediumlink range applications [1]. FSO offers a wide modulationbandwidth and an aggregate data rate of over 100 Gb/s usingwavelength division multiplexing (WDM) [2]. It consumes alow power, provides much improved security against elec-tromagnetic interference (EMI) and does not require admin-istrative licensing or tariffs [3]. Therefore, FSO is consid-ered as an alternative option in metropolitan and local areanetworks ((M/L)ANs) as well as a viable solution to theaccess network “last mile” problem in which the compromisebetween available data rates and the cost is desirable [4].However, the performance of the FSO system is dependent onatmospheric conditions particularly in the presence of aerosols,e.g., fog, smoke, and dust, which results in scattering and

Manuscript received November 7, 2012; revised April 9, 2013 and May 8,2013; accepted May 9, 2013. Date of publication May 17, 2013; date ofcurrent version June 18, 2013. This work was supported in part by EuropeanCOST actions under Grants IC801 and IC1101, and in part by the grant ofthe Agency of the Czech Republic GACR P102/11/1376.

M. Ijaz, Z. Ghassemlooy, and J. Perez are with the OpticalCommunications Research Group, Faculty of Engineering and Environment,Northumbria University, Newcastle Upon Tyne NE1 8ST, U.K. (e-mail:muhammad.ijaz@northumbria.ac.uk; z.ghassemlooy@northumbria.ac.uk;joaquin.perez@northumbria.ac.uk).

V. Brazda is with the Faculty of Electronics and Informatics, University ofPardubice, Pardubice 532 10, Czech Republic (e-mail: brazda@ufa.cas.cz).

O. Fiser is with the Department of Meteorology, Institute of AtmosphericPhysics, Prague 141 31, Czech Republic (e-mail: ondrej@ufa.cas.cz).

Color versions of one or more of the figures in this letter are availableonline at http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/LPT.2013.2264046

attenuation [5]. For example, in a dense fog condition (definedby V < 0.5 km) the FSO link failure is high due to thescattering and absorption of propagating optical beam, whichis not desirable by the end users [6]. As a result this letterof the relationship between the atmospheric aerosols and FSOwavelengths for thick and dense fog conditions are importantin order to enhance the FSO link performance and thereforeimprove the link availability [7]. However, due to the difficultyof measuring the particle density and the shape of aerosolsin the atmosphere, measurement of V is the most commontechnique used to characterize the fog attenuation in FSOlinks. Currently, the most accepted method to measure V isbased on the proportional amount of scattered light capturedby the optical receiver. Determining V from CCD imagesbased on the edge detection of various distant objects in theimage is an alternative technique that has been employed tocomplement the visibility measurement at airports [8].

In this letter both techniques are compared and experimen-tally evaluated in a dedicated indoor atmospheric chamber thatenables us to characterize the atmospheric fog and the FSOlink performance, likewise to turbulence as outlined in [9]. Themain objective of this letter is to provide a complete analysisand verification of the enhanced visibility measurements forFSO under homogeneous fog conditions. Therefore, precisemeasurements of the concurrent visibility and attenuation(dB/km) along the length of the FSO link can be carried outfor each wavelengths of 0.83, 1.31 and 1.55 μm for dense fog(V < 0.5 km) to light fog conditions (V > 0.5 km). Moreover,we have carried out real time measurements, taken in identicalconditions, for the complete visible – NIR spectrum in orderto verify the wavelength dependency of the fog attenuation forV < 0.5 km.

This letter is organised as follows: the characterization of thefog attenuation in laboratory chamber is outlined in Section II,whereas the experimental description is explained in SectionIII. In Section IV experimental results and discussions arepresented. The conclusions are drawn in Section V .

II. CHARACTERIZATION OF FOG ATTENUATION IN

LABORATORY CHAMBER

V is the visual range where a contrast ratio (C/Co) dropsto the visual threshold Tth of 2% along the propagation path,where Co is the intrinsic contrast of the object and C is thecontrast measured at the propagation distance L (km) [10].Using Koschmieder relation [11] and Beer-Lambert-Boguer’slaw, V is given by:

1041-1135/$31.00 © 2013 IEEE

IJAZ et al.: ENHANCING THE ATMOSPHERIC VISIBILITY AND FOG ATTENUATION 1263

DAQ

0.55 μm0.83 μm

1.31 μm

1.55 μm

0.36 μm to 2.5 μm

Halogen lamp

DC BIAS

Laser Source

Fog control

Fog

Atmospheric chamber

LensOSA

Optical Power meter

Lens

PRPT

Fans

Controlled fog conditionsFog machine

Air outlet

Scattering of an optical signal

Fans

Air outlet

Fans(b)

(c)

(a)

Receiver, RxTransmitter, Tx

Fig. 1. (a) The block diagram of the experimental setup, (b) the laboratorycontrolled atmospheric channel, and (c) the scattering of an optical signal dueto the fog in the chamber.

V = ln (0.02)

ln(

CCo

) L . (1)

In this case V can also be expressed in terms of theatmospheric attenuation coefficient βλ and Tth at a wavelengthof 0.55 μm and is given as [12], [13]:

V = −10 log10(Tth)

βλ, (2)

where βλ (dB/km) is the total attenuation due to the absorptionand scattering of light, which is theoretically investigatedin [14]. Generally, ROF induced optical attenuation at agiven wavelength λ can be predicted using empirical models.Kim model, relating V with the ROF attenuation for thevisible – NIR wavelengths and is defined as [15]:

V = 10 log10 Tth

βλ

(λ

λ0

)−q

, (3)

where λ0 = 0.55 μm is the maximum sensitive wavelengthfor the human eye. q is proportional to the particle sizedistribution and is related to V [16]. Kim used q = 0 to indicatethat the atmospheric attenuation coefficient βλ is wavelengthindependent for dense fog conditions (V < 0.5 km), can befound in [15].

III. EXPERIMENT DESCRIPTION

The block diagram of proposed laboratory based FSO linkis shown in Fig. 1 (a). An optical transmitter Tx and anoptical receiver Rx are separated by the channel, which isrepresented by the atmospheric chamber of a dimension of55 cm × 30 cm × 30 cm, see Fig. 1(b). Fog is generated usinga commercial fog machine (water steam) with 100% humidityto mimic ROF [17]. The amount of fog in the atmosphericchamber is allowed to settle down homogeneously and iscontrolled by fans and ventilation systems as illustrated inFig. 1(c).

Fig. 2. Comparison of the measured visibility for the outdoor and atmosphericchamber.

Following (1), the experiment was performed using a CCDcamera (model GE X5, 14 megapixels) in the presence of twooptical links at 0.83 μm and 1.55 μm. We placed a black andwhite target at the receiver. The ratio C/Co between the blackand white areas was computed by measuring the luminanceof the white Lw and black Lb parts of the target, where Co

is the contrast without fog and C is the contrast with fog[10], [12]:

C = Lw − Lb

Lw + Lb. (4)

Following (2), the experiment was performed using the laserdiode at 0.55 μm in presence of optical links at 0.83, 1.31and 1.55 μm. The normalized transmittance T was calculatedat Rx before and after the injection of fog into the chamberfor all wavelengths. We measured βλ corresponding to themeasured T from light to dense fog conditions. This allowsus to measure the link V at 0.55 μm simultaneously with thefog attenuation at 0.83, 1.31 and 1.55 μm along the length ofthe chamber.

The same experiment was performed using a continuoustungsten halogen light source, model LS-1 from Ocean OpticsInc., with a broad spectrum (0.36 to 2.5 μm) and an opticalreceiver Rx using an Anritsu MS9001B1 optical spectrumanalyzer (OSA) with a spectral response of 0.6 to 1.75 μm.This allows us to measure the fog attenuation for differentvalues of V across the spectrum of interest simultaneously.The data is acquired by an automated data acquisition (DAQ)system driven by the LabVIEW environment. The geometricand other losses were not taken into account for Tx , as thenormalized PR was measured both before and after the fog atRx to attain the wavelength dependent fog loss.

IV. EXPERIMENTAL RESULTS AND DISCUSSIONS

A. Characterization of Fog Attenuation

Fog is a random dynamic atmospheric phenomenon and theprobability of occurrence of the dense fog in the real outdooratmosphere is very low. Fig. 2 illustrate the typical measureddistribution of V from two field experiments at the Newcastleairport, UK and Milesovka Hill, Czech Republic over a yearand is compared to the average data generated from twoexperiments over one hour time period using the laboratory

1264 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 25, NO. 13, JULY 1, 2013

Fig. 3. (a) The characterization of measured visibility and fog attenuationusing CCD, laser diodes and Kim model, and (b) CCD fog measurementssnapshots with light (left) and dense (right) fog conditions, both at theatmospheric chamber.

atmospheric chamber. The minimum V achieved from the dataat Milesovka and Newcastle airport are 33 m for 0.1 % of theyear and 50 m for 0.01 % of the year, respectively. However,using the atmospheric chamber allows us to achieve a verydense fog condition with V = 33 m for 1% of the time, whichdemonstrates our system advantage to replicate outdoor densefog field trial conditions without waiting for a year.

Fig. 3(a) shows the plots for the concurrent measured V andthe fog attenuation (in dB/km) at 0.83 μm and 1.55 μm usingCCD and a laser diode at 0.55 μm. Moreover, Kim modelis compared with the experimental data in order to validatethe accuracy of the experiment in the atmospheric chamber.Fig 3(b) depicts the picture captured by CCD to measure V inlight (left) and dense (right) fog conditions. CCD techniqueshows higher accuracy for V < 0.05 km and the laser techniquefor the range > 0.1 km, see Fig. 3(a). Therefore, thesetechniques allow enhancement of the characterization of anFSO link in thick and dense fog conditions. The measured V ,using both methods, is in close agreement with Kim model.This validates the accuracy of visibility measurement and thecorresponding fog attenuation in the chamber.

The dependence of the wavelength on the fog attenuationis verified for V < 0.5 km by using selective wavelengths.Plots of measured fog attenuation against the measuredV for 0.83 μm, 1.31 μm and 1.55 μm wavelengths are shownin Fig. 4. The measured attenuation is wavelength dependentat 0.83 μm, 1.31 μm and 1.55 μm for the dense fog condition,i.e., V < 0.5 km. The measured attenuation at 1.31 μm is

Fig. 4. Comparison of measured fog attenuation for different wavelengths.

Fig. 5. Comparison of measured fog attenuation (dB/km) with Kim modelfor the visible - NIR spectrum at V = 0.357 km.

higher than at 1.55 μm for V < 0.5 km, contradicting thewavelength independency of Kim model. The experimentalresults show the inconsistent effect of fog attenuation on theselected wavelengths for V < 0.5 km.

B. Comparison of Attenuation Spectrum for Fog

Fig. 5 illustrates the fog attenuation for the visible – NIRspectrum at dense fog conditions (V = 0.357 km) in theatmospheric chamber. The measured fog attenuation for thevisible range is 52 dB/km, decreasing to 46 dB/km for theNIR range of the spectrum. However, the peak attenuation inthe wavelength range of 1.35-1.45 μm is due to strong waterabsorption, which is consistent with the published results asin [18]. The measured fog attenuation for the visible – NIRspectrum is also compared to the wavelength independent Kimmodel for V < 0.5 km. The experimental results contradictthe wavelength independent fog attenuation for the spectralattenuation.

The measured fog attenuation for the light fog(V ∼ 0.783 km) for the visible – NIR spectrum is depictedin Fig. 6. The measured attenuation spectrum is very closeto Kim model at V = 0.783 km. This validates that Kimmodel is more realistic to use when V > 0.5 km. This modeldoes not take into account the wavelength for V < 0.5 km.However, the experimental data shows that the spectralattenuation is wavelength dependent for V < 0.5 km.This validates that the Kim model need to be revised

IJAZ et al.: ENHANCING THE ATMOSPHERIC VISIBILITY AND FOG ATTENUATION 1265

Fig. 6. Comparison of measured fog attenuation (dB/km) with Kim modelfor the visible - NIR spectrum at V = 0.785 km.

for V < 0.5 km to predict the wavelength dependent fogattenuation.

V. CONCLUSION

In this letter, we have implemented and compared differentmethods using CCD and laser technique to measure V andcharacterize the fog attenuation measurement under labora-tory dense fog conditions. The laboratory indoor atmosphericchamber is calibrated to ROF conditions by using Kim model.The visibility measurement using CCDs and the laser tech-nique are very accurate for the link spans of < 0.05 kmand < 0.1 km, respectively. This accuracy is essential whenimplementing and installing real-time low cost visibility sys-tem where accuracy and early response are crucial, e.g. roadsurveillance, airport security, local weather applications. More-over, we have compared individual wavelengths of 0.83 μm,1.31 μm and 1.55 μm in the homogeneous fog conditions. Wehave verified that the fog attenuation is wavelength dependentfor all the spectrum range of 0.6 μm < λ < 1.6 μm under acontrolled fog condition for V < 0.5 km.

REFERENCES

[1] E. Ciaramella, et al., “1.28 terabit/s (32 × 40 Gbit/s) WDM transmissionsystem for free space optical communications,” IEEE J. Sel. AreasCommun., vol. 27, no. 9, pp. 1639–1645, Sep. 2009.

[2] N. Cvijetic, Q. Dayou, Y. Jianjun, H. Yue-Kai, and W. Ting, “100 Gb/sper-channel free-space optical transmission with coherent detection andMIMO processing,” in Proc. ECOC, 2009, pp. 1–2.

[3] Z. Ghassemlooy, W. Popoola, and S. Rajbhandari, Optical WirelessCommunications: System and Channel Modelling with MATLAB. BocaRaton, FL, USA: CRC Press, 2012.

[4] E. Leitgeb, M. Gebhart, and U. Birnbacher, “Optical networks, last mileaccess and applications,” J. Opt. Fiber Commun. Res., vol. 2, no. 1,pp. 56–85, 2005.

[5] J. C. Ricklin, S. M. Hammel, F. D. Eaton, and S. L. Lachi-nova, “Atmospheric channel effects on free-space laser communica-tion,” J. Opt. Fiber Commun. Res., vol. 3, no. 2, pp. 111–158,2006.

[6] J. Perez, Z. Ghassemlooy, S. Rajbhandari, M. Ijaz, and H. L. Minh, “Eth-ernet FSO communications link performance study under a controlledfog environment,” IEEE Commun. Lett., vol. 16, no. 3, pp. 408–410,Mar. 2012.

[7] M. Grabner and V. Kvicera, “The wavelength dependent model ofextinction in fog and haze for free space optical communication,” Opt.Express, vol. 19, no. 4, pp. 3379–3386, 2011.

[8] D. Bäumer, S. Versick, and B. Vogel, “Determination of the visibilityusing a digital panorama camera,” Atmos. Environ., vol. 42, no. 11,pp. 2593–2602, 2008.

[9] Z. Ghassemlooy, H. Le Minh, S. Rajbhandari, J. Perez, and M. Ijaz,“Performance analysis of ethernet/fast-ethernet free space optical com-munications in a controlled weak turbulence condition,” J. Lightw.Technol., vol. 30, no. 13, pp. 2188–2194, Jul. 1, 2012.

[10] C. P. Colvero, M. C. R. Cordeiro, and J. P. von der Weid, “Real-timemeasurements of visibility and transmission in far-, mid- and near-IRfree space optical links,” Electron. Lett., vol. 41, no. 10, pp. 610–611,2005.

[11] H. Koschmieder, “Theorie der horizontalen Sichtweite,” Beitrage Phys.Freien Atmos., vol. 12, pp. 33–53, Jun. 1924.

[12] H. Weichel, “Laser beam propagation in the atmosphere,” Proc. SPIE,vol. TT3, pp. 25–39, Aug. 1990.

[13] A. K. Majumdar and J. C. Ricklin, Free-Space Laser Communications:Principles and Advances, vol. 233. New York, NY, USA: Springer-Verlag, 2008.

[14] M. Ijaz, Z. Ghassemlooy, S. Rajbhandari, H. Le Minh, J. Perez, andA. Gholami, “Comparison of 830 nm and 1550 nm based free spaceoptical communications link under controlled fog conditions,” in Proc.8th Int. Symp. Commun. Syst., Netw. Digit. Signal Process., 2012,pp. 1–5.

[15] I. I. Kim, B. McArthur, and E. Korevaar, “Comparison of laserbeam propagation at 785 nm and 1550 nm in fog and haze foroptical wireless communications,” Proc. SPIE, vol. 4214, pp. 26–37,Feb. 2001.

[16] P. W. Kruse, L. D. McGlauchlin, and E. B. McQuistan, Elements ofInfrared Technology: Generation, Transmission and Detection. NewYork, NY, USA: Wiley, 1962.

[17] M. S. Awan, L. C. Horwath, S. S. Muhammad, E. Leitgeb, F. Nadeem,and M. S. Khan, “Characterization of fog and snow attenuations forfree-space optical propagation,” J. Commun., vol. 4, no. 8, pp. 533–545,2009.

[18] R. N. Clark, “Spectroscopy of rocks and minerals, and principles ofspectroscopy,” in Manual of Remote Sensing, vol. 3. New York, NY,USA: Wiley, 1999, pp. 3–58.