Modeling of Fog and Smoke Attenuation in Free Space Optical Communications Link under Controlled Laboratory Conditions

Transcription

1 Modeling of Fog and Smoke Attenuation in Free Space Optical Communications Link under Controlled Laboratory Conditions Muhammad Ijaz, Zabih Ghassemlooy, Jiri Pesek, Ondrej Fiser, Hoa Le Minh and Edward Bentley Abstract This paper theoretically and experimentally investigate the spectrum attenuation of free space optical (FSO) communication systems operating at visible and near infrared (NIR) wavelengths (0.6 μm < λ < 1.6 μm) under fog and smoke in a controlled laboratory condition. Fog and smoke are generated and controlled homogeneously along a dedicated atmospheric chamber of length 5.5 m. A new wavelength dependent empirical model is proposed to predict the fog and smoke attenuation operating at visible and NIR wavelengths. Comparison of the new proposed model with the measured continuous attenuation spectrum from visible NIR in the fog and smoke channels shows a close relationship than the semiempirical Kim and Kruse fog models. The experimental results also show the selection for the possible appropriate wavelengths from visible NIR for FSO links to achieve the maximum link span in dense fog conditions. F Index Terms Free space optics; Fog attenuation; Smoke Attenuation; Visibility I. INTRODUCTION REE space optics communications uses visible or NIR light sources to transmit a high data rate through the atmospheric channel [1]. The FSO technology is attracting attention as the contemporary and in way a complementary technology to the radio frequency (RF) to solve the last mile bottleneck issues in local area access networks [2, 3]. FSO offers higher bandwidth, a low implementation cost, a licensed free spectrum (for the time being), a relatively lower power consumption and an excellent immunity to the electromagnetic spectrum and security compared to RF technologies [4, 5]. However, the constitution of the real outdoor atmosphere (ROA), particularly aerosols (fog, smoke, dust) have similar particle size distributions compared to the propagating optical wavelengths. This can potentially result in scattering and absorption of visible and IR optical beams, thus degrading the FSO link performance and its availability [6, 7]. As a result, the study of the relation between different aerosols and M. Ijaz, Z. Ghassemlooy and H. Le Minh, are with Optical Communications Research Group, Faculty of Engineering and Environment, Northumbria University, UK. ( {muhammad.ijaz, z.ghassemlooy}@northumbria.ac.uk). J. Pesek is with the Faculty of Electronics and Informatics, University of Pardubice, Studentska 95, Pardubice, Czech Republic. O. Fiser is with the Inst. of Atmospheric Physics, Prague 4, Czech Republic Edward Bentley is with Power and Wind Energy Research (PaWER), Faculty of Engineering and Environment, Northumbria University, UK. wavelength is one of the key subjects in order to characterise the FSO link availability. In [8] a laboratory based setup is used similar to our previous work [9] showing that the terahertz (THz) signal has significantly lower fog attenuation than a 1.55 µm FSO link. In [10], a real time measurement of the fog attenuation was reported, that shows far infrared (FIR) at 10 µm offers higher transmission in fog. Despite the advantages of FSO links, to operate them at THz and FIR wavelength bands will require high cost components that are not readily available at the moment. Therefore, almost all commercially available FSO systems operate in the wavelength range of 0.60 µm 1.55 µm. Consequently, we have selected wavelengths from the visible NIR bands in order to verify the dependency of the wavelength on the fog and smoke attenuation in a controlled laboratory environment. To simultaneously investigate the entire wavelength spectrum range (0.6 μm < λ < 1.6 μm) under the real outdoor fog (ROF) condition is rather challenging, and research work reported explicitly lacks the verification of the wavelength dependent attenuation [11, 12]. This is due to various reasons mainly: (i) the unavailability of the experimental setup for outdoor links due to the long observation time and reoccurrence of dense fog events for visibility V < 0.5 km, and (ii) the difficulty in controlling and characterising aerosols in the atmosphere due to the inhomogeneous presence of aerosols along the FSO link path. Hence an indoor atmospheric laboratory chamber is designed so that the atmospheric aerosol channel can be controlled; similar to the previous attempt made for controlled atmospheric turbulence studies [13]. Therefore, specific measurements of the visibility and aerosol attenuation can be carried out for each wavelength. Moreover, to the best of our knowledge, there are no experimental data available for FSO links in diverse smoke conditions, which are common in urban areas. In this research work, the real time measurements for the fog and smoke attenuation for the visible NIR spectrum is reported in line with the measured visibility (km). The visibility is measured using the wavelength of 0.55 µm along the length of an FSO channel rather than at one position as in the case of traditional outdoor FSO links [14, 15]. This approach enables us to provide the right value of fog attenuation corresponding to the measured visibility (km). Here, a new empirical model is proposed to evaluate the wavelength dependent fog and smoke attenuation by reconsidering the q value as a function of wavelength rather than the visibility [16]. Using the measured attenuation spectrum data, we provide recommendations for the best

2 wavelengths that could be adopted for links under fog conditions. We also compare the continuous attenuation spectrum from the visible NIR for the same controlled fog and smoke conditions in order to validate the laboratory-based empirical model. The rest of the paper is organised as follows: FSO evaluation in the fog channel is outlined in Section II, whereas the experimental setup for fog and smoke channel is explained in Section III. In Section IV results and discussion are presented. The conclusions are drawn in Section V. II. FSO EVALUATION IN FOG CHANNEL A. Modelling of Fog Attenuation Fog is composed of very fine spherical water particles of various sizes suspended in the air which results in light attenuation due to Mie scattering[17]. Fog particles reduce the visibility near the ground and the meteorological definition of fog is when the visibility drops to near 1 km [18]. Assuming fog particles have spherical shapes, one can apply the exact Mie theory to measure the scattering cross section C s of the particle by knowing the particle radius r. Thus, we can estimate the theoretical value of the normalized scattering efficiency Q s as [19]: Q s Cs. (1) 2 r The total attenuation induced by particles in the atmosphere is the sum of molecular absorption and scattering of light. However, wavelengths used in FSO links are almost selected in the atmospheric transmission windows where the molecular absorption cross-section C a due to gases is negligible, i.e. C a ~ 0 [19]. The atmospheric attenuation coefficient of the optical signal due to the scattering of fog particles is given by [19, 20]: 0 2 2r r Qs, nn( r) dr, (2) where n is the real part of the refractive index, 2πr/λ is the size parameter, and N(r) is the particle size distribution function. Generally this distribution is represented by analytical functions such as the modified gamma distribution for aerosols and given by [20]: N( r) ar exp( br), where, a, b and α are the parameters that characterize the particle size distribution. Since, Q s is mainly dependent on the size parameter, which is dependent on λ and r. The size parameter is ratio of the size of the fog particle to the incident wavelength. Therefore, the resultant fog attenuation will be remarkably dependent on the selected wavelength. Generally, due to the complexity involved in the physical properties of the fog, like particle size and the non-availability of particle distribution, the fog induced attenuation of the optical signal can be predicted using empirical models [21, 22]. Empirical models use the visibility data in order to estimate the fog induced attenuation. The original empirical relationship, which relates V with the (3) fog attenuation has been given by the Kruse model [21]: V q 10log10T th (4) o km, where T th is the visual threshold taken as 2%, λ o is the maximum spectrum of the solar band and q is the coefficient related to the particle size distribution in the atmosphere. The Kruse model estimates the fog attenuation from visible NIR wavelengths and the q value, which is a function of visibility is defined as: q 0.585V 1/3 for V < 6 km. However, the estimation of the fog attenuation using the Kruse model is considered to be not accurate for fog [16]. This is because the value of q has been defined from haze particles (V > 1 km) present in the atmosphere rather than fog (V = 1 km). Kim modified the Kruse model using theoretical assumptions for the fog by defining q values as follows [16]: (5) 1.6 for V > 50 km 1.3 for 6 < V < 50 km q 0.16V 0.34 for 1 < V < 6 km (6) V 0.5 for 0.5 < V < 1km 0 for V < 0.5km Values of q indicate that is wavelength independent in the fog condition for V < 0.5 km. However, recently accrued experimental data at selective wavelengths of 0.83 and 1.55μm has revealed a different behaviour than the one predicted by the especially for dense fog conditions for V < 0.5 km [22, 23] This demonstrates that in spite of a significant number of investigations, the model presented in (6) needs to be explicitly verified experimentally, not for the selective or specific wavelength but for the entire spectrum of the visible NIR range. B. Characterization of Fog and Smoke Attenuation in a Laboratory Chamber: In [12], measurements show that the occurrence of fog starts when the relative humidity H of the real outdoor atmosphere (ROA) approaches 80%. The density of the resulting fog reaches 0.5 mg/cm 3 for H > 95%. Thus, under high water vapour concentration conditions, the water condenses into tiny water droplets of radius 1 20 μm in the atmosphere. There are different types of real outdoor fog (ROF), which are categorized on the basis of their formation mechanism, such as convection fog, advection fog, precipitation fog, valley fog and the steam fog [24]. The steam fog is localized and is created by the cold air passing over much warmer water or a moist land [24, 25]. It is possible to simulate this form of fog in the lab by achieving H close to 95%. Hence to mimic ROF, artificial fog can be generated by means of water based steam. In addition, to demonstrate the physical similarity of the lab based fog to ROF, the mean ROF attenuation data from Prague for 0 < V < 1 km published in [26]and Metrological Institute, Czech Republic for V < 0.6 km [23] were compared with the measured attenuation data for the

3 lab generated fog at wavelength of 0.83 µm, and showed very good agreement. Thus, confirming that laboratory generated fog resembles the ROF. Smoke is generally formed in ROA from the combustion of different substances such as carbon, glycerol and house hold emission [25]. Smoke is generated in the laboratory based atmospheric chamber by imitating the natural process using a smoke machine, in which a glycerine based liquid is used to produce dry smoke particles. Further details on the inverse relationship between refractive index n and wavelength from visible NIR for water and glycerine is given in [27, 28]. The link visibility (i.e. the meteorological visual range) is used to characterize the fog and smoke attenuation. Visibility is defined in [29] as the distance to an object at which the visual contrast of the object drops to 2% of the original visual contrast (100%) along the propagation path commonly known as the Koschmieder law. This 2 % drop value is known as the visible threshold T th of the atmospheric propagation path. The 2% visual threshold value is adopted here in order to follow the Koschmieder law as opposed to the airport consideration of T th = 5% [30]. The meteorological visibility V (km) can be therefore expressed in terms of and T th at a 0.55 µm wavelength and is given as: 10 log10( Tth ) V, (7) where is normally expressed in (db/km), and is mathematically defined by knowing the transmittance T of the optical signal and the propagation distance L (km) using the Beer- Lambert law as [30, 31]: log ( T) 0. (8) 4.343L III. EXPERIMENT SETUP FOR FOG AND SMOKE CHANNEL A block diagram of a laboratory test bed for the FSO link composed of an optical transmitter end T x, an optical receiver R x, an atmospheric chamber an optical and electrical modules is shown in Fig. 1(a). The chamber representing the channel has a dimension of cm 3 as depicted in Fig. 1(b) and inset shows the scattering of the optical signal at 0.55 μm in the presence of the dense fog. We used two approaches in our experimental work to characterise fog and smoke attenuation, (i) a continuous LS-1 tungsten halogen source with a broad spectrum (0.36 to 2.5 μm) and an Anritsu MS9001B1 optical spectrum analyzer (OSA) with a spectral response of 0.6 to 1.75 μm to capture the attenuation profile and (ii) a number of laser sources at wavelengths of 0.55, 0.67, 0.83, 1.31 and 1.55 µm with the average transmitted optical powers P T of -3.0 dbm, 0 dbm, 10 dbm, 6.0 dbm and 6.5 dbm and an optical power meter, respectively. The amount of aerosols in the atmospheric chamber is controlled by a number of fans and a ventilation system. The aerosols within the chamber are very fine and light, slowly moving particles suspended within the chamber. The time duration of 30 seconds was allowed for fog/smoke particles to settle down homogeneously within the chamber before the data acquisition (DAQ). An automatic DAQ system is developed using the LabVIEW to control the MS9001B1 OSA. This process allows us to control the wavelengths under the test, the sampling frequency and optical loss estimation. In order to measure the effect of fog on different wavelengths, the average received optical power P R is measured at the receiver R x before and after the injection of the fog into the atmospheric chamber. The fog density is varied by small outlets in the chamber so that dense to very light fog conditions can readily be created and the optical power measurements are taken at a one second time interval until the chamber is free from fog. The normalized transmittance T was calculated from P R with and without fog. We measured using (8) corresponding to the measured T from light to dense fog conditions for all wavelengths. The link visibility was measured simultaneously with along the length of the chamber using a laser diode at 0.55 µm to ensure the maximum transmission to the human eye. Optical Transmitter (T x ) DC Bias Lens LS-1 halogen source Fans clear Air outlets Fog control here Fog Atmosphere L= 5.5 m, Vol = m 3 (a) Receiver End (R x ) Fans Air Outlet With fog Fans Side to side, lengthwise Length = 5.5 m Optical Receiver (R x ) P R Lens Scattering of light Data Acquisition OSA/ Optical power meter Anritsu MS9001B1 Laser End (T x ) Lens (b) Fig. 1. (a) The experimental set up to measure the fog attenuation and visibility of an FSO link, and (b) the laboratory controlled atmospheric chamber. The inset shows the presence of fog and the scattering of light in the atmospheric chamber. Note that, the goal of the experiment is to characterise the attenuation, therefore, having identical powers at different wavelengths are not essential. The geometric and other losses were also not taken into account for T x, as P R was measured both before and after fog and smoke at R x to attain the wavelength dependent losses. IV. RESULTS AND DISCUSSIONS In this section, we report experimental results with the proposed wavelength dependent model for the fog and smoke attenuation. The atmospheric chamber allows us to control and replicate the same atmospheric conditions; therefore the procedure was repeated 10 times using the same fog and smoke conditions. A. Experimental attenuation measurements for fog and smoke The log-log plot of the measured average fog attenuation (in db/ km) against the measured visibility for 0.67 µm and 1.55

4 µm wavelengths are shown in Fig. 2(a). The measured attenuation is notably higher for 0.67 µm than at 1.55 µm for the given visibility range (0.032 km < V < 1 km). There is an attenuation difference of 50 db/ km, 10dB/ km and 7 db/ km at V of km, km and 0.5 km, respectively. This contradicts the wavelength independency of for V < 0.5 km [16]. Further, the log-log plots of the measured attenuation of smoke (in db/ km) against the visibility for 0.83 µm and 1.55 µm wavelengths are depicted in Fig. 2(b). The experimental result clearly demonstrates the dependency of the wavelength on the resultant smoke attenuation even if V is below 0.5 km. In general, the smoke attenuation difference is more than the fog attenuation. The difference for the smoke attenuation values are 108 db/ km, 23 db/km and 8 db/ km for 0.83 µm and 1.55 µm at V of 0.07 km, 0.25 km and 0.5 km, respectively as illustrated in Fig. 2(b) (inset). This depicts that selection of 1.55 µm is more favourable in the fog and smoke channels for the dense (V < 0.07 km), thick (V = 0.25 m) and the moderate fog and smoke (V = 0.5 km) conditions, respectively. Attenuation(dB/Km) db (a) 10 db Visibility (km) (b) =0.67 m =1.55 m 7 db Visibility(Km) = m = 1.55 m Fig. 2. The measured attenuation (db/ km) and visibility (km): (a) fog and (b) smoke. B. Empirical modeling of fog and smoke The attenuation due to the scattering and absorption is highly dependent on the size parameter, recalling (2). However, Kim had also defined the value of q for fog in (6), which describes the wavelength dependency of the fog attenuation and the type of scattering. Values of q are -4, -1.6 and 0 for Rayleigh scattering (r << λ), Mie scattering (r ~ λ), and geometric scattering (r >> λ), respectively [16]. In order to Table 1: Values of q obtained for different wavelength from measured fog and smoke attenuation data. For Fog Wavelength-µm q R 2 -value RMSE For Smoke predict a suitable model for the attenuation of fog and smoke based on measured data we carry out the following. Values of q in (4) are obtained for individual wavelengths from µm, respectively by using the empirical curve fitting method with a reference wavelength of 0.55 µm as shown in Table 1. The values of the root mean square error (RMSE) and R 2 confirm the curve fitting is in a good correlation with the measured data for fog and smoke. The value of q is found to be 0 to 0.14 for fog and 0 to 0.6 for smoke, indicating the predominance of the Mie scattering (r ~ λ). This verifies that in the (r ~ λ) region for dense fog conditions, the q value is the function of wavelength not the visibility. The plot of predicted q values against the wavelength and the curve of best-fit against a wavelength range of µm for fog and smoke conditions are shown in Figs. 3 and 4, respectively. The best curve fit satisfied the (9) with R 2 and RMSE values of and for the fog and similarly for the smoke with R 2 and RMSE values of and , respectively Fog q. (9) Smoke q Data Linear fit Wavelength (m) Fig. 3. The predicted q value and linear curve of best-fit against the wavelength for fog.

5 q Data Linear fit needs to be considered instead of the liquid water content (LWC) and the particle radius, is a simple approach using (10). It is most suitable for the FSO link budget analysis in the urban area where fog and smoke are more likely to occur all year round Wavelength (m) Fig. 4. The predicted q value and linear curve of best-fit against the wavelength for smoke. The new proposed model for the prediction of the attenuation due to fog and smoke can be presented as: q( ) 17 ( db / km) V km. (10) o where, the wavelength with a range from 0.55 μm < λ < 1.6 µm is valid for the visibility range of km < V < 1 km. The function for q(λ) for the fog and smoke is expressed in (9). Experimental data shows that as V 0 for the very dense fog conditions, the received optical signal is significantly lower than the OSA minimum sensitivity at V of km. This validates that the measurements below km are not very practical to show the wavelength dependent fog attenuation. Thus, validating the model for the visibility range of km < V < 1 km. C. Comparison of the measured visibility and attenuation data with the proposed model Fig. 5 shows the log-log plot for the measured fog attenuation against the concurrent visibility data for the selected wavelengths of 0.67, 0.83, 1.1, 1.31 and 1.55 µm for modified and Kim fog models. The log-log plot of the attenuation curve obtained from the proposed model defined by (10) and the comparison with the measured data shows a good agreement. However, comparison of at λ of 1.55 µm with the measured data for V < 0.5 km show that the model over estimates the fog attenuation. This is because does not take into account the wavelength to estimate the fog attenuation. However, Kim model fits well with the experiment data for V > 0.5 km (see inset Fig. 5). This indicates the dependency of the fog attenuation on the wavelength. Fig. 6 displays the log-log plot for the measured smoke attenuation against the concurrent visibility data for the selected wavelengths of 0.55, 0.67, 0.83, and 1.31 µm. Comparison of the proposed smoke model shows a good agreement between the measured data for the selective individual wavelengths. This clearly indicates the dependency of attenuation on the wavelength. The plot shows a difference of 50 db/km is observed between 1.31 and 0.83 µm for the dense smoke condition (V < 0.07 km), with progressive reduction in the attenuation difference for thick and moderate smoke conditions. Thus, clearly indicating suitability of the NIR wavelengths at the dense smoke condition. The proposed model, where the available visibility data =0.67 m =0.83 m =1.31 m =1.55 m Proposed fog model Visibility (km) Fig. 5. Real time measured fog attenuation versus visibility (V = 1 km) for selected wavelengths and for modified fog and s. Attenuation(dB/km) 0.55 m 0.67 m 0.83 m 1.31 m Proposed smoke model Visibility (km) Fig. 6. Real time measured smoke attenuation versus the visibility (V = 1 km) for selected wavelengths and for proposed smoke model. D. Comparison of the proposed model with the spectrum attenuation The measured fog attenuation (in db/km) for the very dense fog (V ~ 0.05 km) for the visible NIR spectrum under test (SUT) is shown in Fig 7(a). The measured attenuation for the SUT at the very dense fog condition (V ~ 0.05 km) shows three possible attenuation windows, (i) 0.60 μm 0.85 μm, which has an attenuation range of 375 db/km 361 db/km with the peak attenuation of db/km at 0.72 μm, (ii) 0.85 μm 1.0 μm, showing a lower attenuation (360 db/km) at μm than at μm with a peak attenuation of (db/km). However, at μm the attenuation has a lower peak value of 354 db/km, and (iii) 1.0 μm 1.55 μm, with an attenuation range of 360 db/km 323 db/km. Results show that 1.33 μm has a higher attenuation peak of 357 db/km than the 1.05 μm with an attenuation dip of db/km. However, 1.55 μm has the lowest attenuation of (324 db/km) in a very dense fog at V = 0.05 km. Table 2 shows the possible wavelengths with the minimum fog attenuation suitable for use in FSO links. The behavior of the attenuation spectrum is almost the same for V = 0.3 km with the maximum attenuation of ~ 59.5 db/km to 53.5 db/km for 0.6 μm and 1.55 μm, respectively (see Fig. (7b)).

6 Table 2: Possible wavelengths for operating in the fog channel, measured at V = km. Wavelength windows (µm) Window attenuation (db/km) Peak attenuation (db/km) Suitable wavelengths (µm) Measured 300 Kruse model Proposed fog model (a) Wavelength (m) Measured 40 Kruse model Proposed fog model (b) Wavelength (m) Fig. 7. The measured fog attenuation (db/km) from the visible NIR spectrum in comparison with the selected empirical models (a) for V = km, and (b) for V = 0.3 km. The proposed model is verified experimentally for SUT using the controlled indoor chamber for fog and smoke. In order to relate the proposed model with the outdoor FSO channel, we have selected Kim and Kruse models, which are widely used in the literature. at V ~ 0.05 km and 0.3 km shows that the fog attenuation for SUT is wavelength independent contradicting the experimental data. Kruse model underestimates the fog attenuation at V ~ 0.05 km and 0.3 km for SUT (see Figs. 7 (a) and (b)). However, the new proposed fog model shows a close correlation for SUT. This verifies that the proposed model follows the profile of measured fog attenuation more precisely than both Kim and Kruse models for V < 0.5 km.the RMSE values for Kim, Kruse and the new proposed fog models are , and from the measured attenuation; and the standard deviation (SD) values are , and , respectively. The RMSE and SD values of the attenuation spectrum from visible NIR range shows a better agreement compared to RMSE and SD of published data for ROF at individual wavelengths [14, 23]. In the case of smoke, the measured smoke attenuation (in db/km) at V ~ km and km for SUT is depicted in Fig. (8). The resultant smoke attenuation at V = km is almost 90 db/km for the visible range and drops to 43 db/km at the NIR range of SUT, see Fig. 8(a). A similar behavior of the smoke attenuation is observed at V = km with the attenuation of 70 db/km at the visible range decreasing to 33 db/km at the NIR range of SUT, see Fig. 8(b). The new proposed smoke model is also compared to Kim and Kruse models for the smoke attenuation. overestimates the measured smoke attenuation and shows wavelength independent attenuation for V < 0.5 km. However, Kruse model underestimates the smoke attenuation for 0.7 μm < λ < 1 µm and overestimates the smoke attenuation for 1.1 μm < λ < 1.6 µm (see Figs. 8(a) and (b)). The new proposed model fits the experimental data showing a close correlation with the measured smoke attenuation spectrum, thus verifying the validity of the proposed model for smoke conditions in the visible NIR SUT range. The model is definitely better than the other models in predicting the smoke attenuation but not in predicting the fog attenuation Measured Kruse model Proposed smoke model (a) Wavelength (m) Measured 30 Kruse model Proposed smoke model (b) Wavelength (m) Fig. 8. The measured smoke attenuation (db/km) from the visible NIR spectrum in comparison with the selected empirical models (a) for V = km, and (b) for V = km. V. CONCLUSIONS In this paper, we have demonstrated the impact of fog and smoke on the FSO link performance using the continuous wavelength spectrum range of 0.6 μm < λ < 1.6 µm. We have proposed a wavelength dependent model for fog and smoke channels, which is valid for the visible NIR range for the visibility range of 1 km. We have experimentally demonstrated that the most robust wavelengths windows (0.83, 0.94 and 1.55 µm) that could be adopted for fog conditions in order to minimize the FSO link failure. Furthermore, to validate the behavior of the proposed empirical model for selected wavelengths, we have experimentally compared the continuous attenuation spectrum for the same fog and smoke conditions and found that the attenuation is almost linearly decreasing for both cases. The

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