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Free-Space Optics Wavelength Selection: 10 µ Versus Shorter Wavelengths

  • Metawave Corporation

Abstract and Figures

This paper is written in response to many inquiries about 10 µ Free-Space Optics (FSO) performance compared to shorter wavelengths. The increasing interest in better understanding FSO weather effects is the result of carrier requests as well as recent progress in analyzing fog effects on FSO signal propagation. Extensive studies in modeling fog and simulating FSO attenuations revealed the complexity behind estimating FSO link availability in a given geographical location. There are many different types of fog that are inhomogeneous along the propagation path. Each type is characterized by the water droplet sizes and their concentrations, which are used in Mie scattering theory to compute FSO signal attenuation. As a result, some vendors are augmenting their FSO links with a microwave backup link or simply investigating other wavelengths claimed to be more resistant to fog such as 10 µ. In this paper, we analyze ways to improve FSO link availability, 10 µ improvement compared to shorter wavelengths, and challenges behind successful microwave backup installation.
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Free-Space Optics Wavelength Selection: 10 µ Versus
Shorter Wavelengths
Maha Achour, Ph.D.
UlmTech, Inc.
731 S. Hwy 101, Suite 1E,
Solana Beach, CA 92075
This paper is written in response to many inquiries about 10 µ Free-Space Optics (FSO) performance compared to
shorter wavelengths. The increasing interest in better understanding FSO weather effects is the result of carrier
requests as well as recent progress in analyzing fog effects on FSO signal propagation. Extensive studies in
modeling fog and simulating FSO attenuations revealed the complexity behind estimating FSO link availability in a
given geographical location. There are many different types of fog that are inhomogeneous along the propagation
path. Each type is characterized by the water droplet sizes and their concentrations, which are used in Mie scattering
theory to compute FSO signal attenuation. As a result, some vendors are augmenting their FSO links with a
microwave backup link or simply investigating other wavelengths claimed to be more resistant to fog such as 10 µ.
In this paper, we analyze ways to improve FSO link availability, 10 µ improvement compared to shorter
wavelengths, and challenges behind successful microwave backup installation.
Keywords: Free-space optics, optical wireless, atmospheric modeling, infrared propagation, 10 micron, 60 GHz,
Mie scattering, visibility, fog, link availability.
When comparing FSO market research published in early 2000 with recent market research [4], we notice that the
more recent publications indicate a much clearer understanding of FSO technology and emphasize less hype about
its capability. The direct competitions between FSO vendors have led to a wide variety of products supporting
different optical and microwave technologies. FSO users can leverage these FSO improvements by selecting the
product suitable for their application. Readers interested in the FSO industry may wish to frequently check the FSO
Alliance progress and publications [12].
The current FSO market addresses the need for quick enterprise connections, cellular backhaul links, fiber cable
backups, and carrier metro/access deployment. Since the enterprise market, which is sometimes referred to by its use
of a campus-like installation, was the initial market deploying FSO, a clear understanding of link availability was
not required. The situation may have slightly changed with emerging enterprise real-time applications demanding
more reliable networks than previously. However, the enterprise market still does not need requirements that are as
stringent as those in the carrier market. It is for this simple reason that FSO will still be a player in the enterprise
In the case of the fiber cable backup application, the point-to-point connection needs to comply with the FSO line-
of-sight requirement and limited distances. These types of installations usually do not require high link availability
due to the rare circumstances of fiber breaking in dense fog weather. For instance, when a few service providers
suffered network and phone service failures in New York, Free-Space Optics were quickly deployed because FSO
does not require licenses and is immune to microwave/RF interference.
Contact email:; Web:; Phone: 858-794 5322; Fax: 858-794 9628
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The concept of extending phone lines through FSO has been around since the beginning of FSO deployment. The
need to interconnect cellular base stations with FSO is increasing due to 3G deployment and cellular phone
penetration. In some developing countries, cellular-phone services exceed wired-based telephone services. In
general, developing countries are very flexible with link availability because it is their only convenient
interconnectivity choice. This convenience is primarily due to FSO being inherently license-free nearly every where
in the world.. However, there have been exceptions, recently, in some developing countries when FSO deployment
monopoly is given to government-backed agencies. It is interesting to note that in some developing countries fiber
installation is sometimes more economical than FSO deployment due to low-cost manpower and no right-of-way
The carrier market is requesting high link availability in the 99.9%-99.999% range as well as a clear understanding
of FSO capabilities. This is due to the penalties carriers have to face when part of their network goes down. Network
operators control and monitor their network under the “no-excuse” rule: failure cannot be justified by the link
nature, location, or weather conditions. As we follow service provider business evolution, we notice that they are
gradually adding QoS-based services to respond to customer’s demands increase revenue and augment their ’dumb
data’ services. These added-value services are based on Service Level Agreements (SLA) between the provider and
end-users. As an example, Gold could be the service having the minimum packet drop, while silver, bronze and
best-effort being the alternative choices. In this case, service providers may certify FSO as the technology suitable
for some SLAs, and therefore customers serviced with FSO cannot get SLAs associated with the best QoS. This is a
shift from the famous 99.999% (five nines) availability, which allows a maximum of five minutes of failure per
As we have mentioned earlier, the FSO spectrum is, in general, globally open without requiring deployment
licenses. This can be compared to unlicensed RF radios. License exempt bands, such as 0.9GHz/2.4 GHz/5
GHz/24 GHz/60 GHz, are open in a few countries. When addressing any unlicensed technology, immunity to
interference becomes the number one obstacle in dense deployment environments. Besides the 60 GHz case,
where Oxygen absorption eliminates interferences, most of the lower frequencies suffer from interferences when
adjacently deployed. Spread spectrum technologies are sometimes used to overcome interference.
Another advantage of FSO, when compared to RF, is the significant reduction in end-to-end delay. Again, with the
60 GHZ and higher frequencies exceptions, almost all radios have to process the radio signal to overcome channel
noise and interference while transmitting high throughput. This is achieved by using the so-called error correction
codes and high modulation levels, which are the two major elements in an RF system design that consumes the
highest delay. Long delay is not practical in a multi-hops deployment because of its accumulation in both transmit
and receive directions.
Most FSO products are plug-and-play units independent of the transmitted protocol and data rate. This feature is
vital to network operators since nodes are constantly upgraded to support new protocols and faster data rates. For
example, an OC3 FSO supports most of the protocols running up to 155 Mbps. Microwave, except for 60 GHz and
higher frequency radios, are limited to the transmitted protocol and speed of the signal being processed.
In order to boost network operators’ confidence in FSO compliance with their SLA offerings, FSO vendors need
to honestly present their FSO products’ performance in bad weather conditions to address users concerns and
establish realistic customer expectations. To date, two approaches have been offered to resolve the weather
problem: (1) augment FSO with an RF backup or (2) conduct thorough studies on weather conditions at each
installation. The first solution requires careful addition of the backup link, whereas the second choice calls for
extensive weather database and proper understanding of fog types in the region. In this paper, we address a
possible third choice that may eliminate the need for a backup link and exhibit improved fog penetration. It is
based on selecting 10 µ as the carrier wavelength over the current shorter 0.8 µ, 1.5 µ, 3 µ, and 5 µ wavelengths.
This paper is divided into four sections. In section one we review the different FSO technologies, commodity
components, and historic deployments. Section two will present the atmospheric effects and reviewing fog models
and their effects on the wavelengths of 0.8 µ, 1.5 µ, 3 µ, 5 µ and 10 µ. In section three we briefly review 10µ
technology and its FSO application. In section four we will compare fog attenuation of the above wavelengths based
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on UlmTech’s FSO atmospheric modeling software, Simulight™ and previously published experimental and
theoretical results.
1. FSO Technologies
Free-Space Optical links consist of two transceivers communicating by means of emitting and receiving optical
signals. It is more practical to characterize the spectrum by its wavelength instead of its terahertz-frequency
spectrum. Figure 1 illustrates the electromagnetic spectrum and corresponding applications. Unlicensed radio
frequencies fall into two regions: the low (0.9, 2.4, 5, 24 GHz) and the high (60 GHz and higher) frequencies.
1.1 Unlicensed Microwave
In order to give the reader a clear comprehension of the benefits and limitations of each unlicensed band, we
summarized their properties in Table 1. As indicated, there have been many standards and activities in the
frequencies below 10 GHz due to their potential Local Area Network (LAN) interconnections. This has led to a
continuous drop in radio costs, but at the same time increased the interference risk. In Figure 2, we show the main
building blocks of radio and free-space optical systems.
Figure 2 shows the radical differences between a typical radio design and an FSO design. The main reason behind
the complexity of a radio design is the desire to transmit more bits in a given allocation of spectrum. In order to
accomplish this goal, many bits are carried within a single pulse. Information is carried within the pulse’s amplitude
(amplitude modulation) or phase (phase modulation). Since the loss of a signal translates into missing bits, error
corrections techniques are added for bit transmission redundancy [5]. IEEE 802.11 standards define the modulation
and error correction for the popular 2.4 GHz and 5 GHz spectrum. In addition to the building blocks illustrated in
Figure (2b), spread spectrum methods are imposed in these standards to combat interference when multiple radios
operate within the same region. These spreading techniques increase the transmitter and receiver signal processing
increasing the end-to-end communication delay. Standards-based radios are attractive because of their high quality
and lower production cost. Their main disadvantage is their low bandwidth and installation in an open environment
vulnerable to interferences.
The 60 GHz band spans 7 GHz of spectrum, and therefore their radios do not need to squeeze bits in the signal but
rather send one bit per pulse as in FSO. This makes 60 GHz end-to-end delay almost zero. Another feature that these
radios enjoy is being immune to interference. This is due to the Oxygen absorbing the weaker signal (side lobes)
around the main signal (principal lobe). However, the disadvantage of Oxygen absorption is that it further limits
deployment distances to few hundred meters to one kilometer. This is, of course, in addition to rain fade and other
applications constraints.
300MHz 30 GHz 300GHz 30THz 300THz
1m 10cm 1cm 1mm 100µ 10µ 5µ 1.5µ 0.85µ 100 nm 10nm
Radio Infrared Visible UV
1m 10-1m 10-2 m 10-3m 10-4m 10-5m 10-6m 10-7m
Near InfraredThermal Infrared Far Infrared
Millimeter Wave Microwave
0.9 - 24 GHz 60 - 120 GHz
ure 1: The electroma
netic s
ectrum and corres
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In general, radios operating above 10 GHz are affected by rain fade, and therefore their link availabilities depend on
the geographical location. For example, CLECS have installed LMDS radios, which operate in the 20-40 GHz
bands, 200 meters apart in Florida to guarantee 99.999% link availability. Recently, there have been activities
regarding the release of 70-90 GHz bands [6]. Unfortunately, the process will take time to overcome many
challenges related to these bands.
Table 1: Properties of unlicensed wireless spectrum.
Spectrum Band Technology Regulations & Standards
2.4 – 2.4825 GHz Worldwide Coverage, indoor/outdoor, 11 and 54 Mbps, up to 15
miles, DS spreading and OFDM.
ISM band: FCC part 15.247 and
15.249, IEEE standard 802.11b,g
B1 5.15-5.25
B2 5.25-5.35
B3 5.725-5.825
Limited global coverage, Hiperlan in Europe (B1 and B2),
indoor/outdoor (B2 and B3), 54 Mbps using OFDM on twelve non-
overlapping 20 MHz bands, and some non-IEEE radios support 450
Mbps using QAM.
UNII band: FCC part 15.407, IEEE
standard 802.11a
5.725-5.85 GHz Open in Asia and part of Europe, DS spread spectrum, and some
radios with 25 Mbps speed.
ISM Band: FCC part 15.247 and
15.249. No IEEE standard
24.05-24.250 GHz Radios in this band are provided by Sierra Digital and are affected
by rain.
FCC part 15.249. No IEEE
57-64 GHz and
Radios with Gigabit speeds, OOK modulation, no delay, few
hundred meters in range due to Oxygen absorption.
FCC part 15.255 and 15.249. No
IEEE standards.
200-300 THz Free-Space Optics (FSO) using short (785-850 nm) and mid (1550
nm) and long wavelengths (10 µ) signals, OOK modulation, no
delay, speeds up to 2.5 Gbps and few kilometers in distances.
Eye-safety IEC, FDA and ANSI
regulation, no FCC regulation, no
IEEE standards.
Amplification & OKK
Detection, Amplification
& Conversion
(a) FSO system
(b) Radio systems (excluding 60+ GHz)
ure 2: Hi
h la
er view of FSO and Radio S
A/D and
Modulation &
Pulse Shaping
Amplification & IF/RF
Detection and RF-
Baseband conversion
& Decoding
D/A &
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1.2 Eye Safety
Eye-safe transmitted optical power standards have been created by the International Electrotechnical Commission
(IEC)[9], the Center for Devices and Radiological Health (CDRH) (which is part of the Food and Drug
Administration (FDA)), and the American National Standards Institutes (ANSI)[7]. The IEC60825-1 is in the
process of being amended, and the CDRH adopted the amended standard in May 2001, so there will be a single
worldwide standard for FSO devices. CDRH is a product safety standard, IEC combines product safety standards
and user’s safety guide, and ANSI Z136.1 is primarily a user safety standard.
In section eight of the IEC60825-1, FSO products are classified based on the energy carried within the signal pulse,
wavelength, and given transmission time period that covers consecutive pulse durations (long runs of “1” bits).
Class 1 products are considered harmless, whereas Class 1M is considered hazardous when viewed with optical
aided instruments. Since the outer layer of the eye, the cornea, acts as an optical band-pass filter passing
wavelengths ranging from .4 to 1.4 microns, light energy at wavelengths outside this range is absorbed by the cornea
and does not pass through the retina. It is for this reason that 1.5 µ and longer wavelengths are allowed to carry
higher power than the shorter wavelengths. IEC sets further restrictions on transmitted power to protect the cornea
itself from extended laser exposure. This is referred to as the Maximum Permissible Exposure (MPE). For example,
basic calculations of the IEC standard indicate that the MPE is around 1 to 2 mW/cm2 at 0.8 µ wavelengths and 100
mW/cm2 at 1.5 µ and 10µ. Some FSO vendors provide Class 1 products in the 1.5 µ and 10µ wavelengths.
1.3 FSO Systems
Free-Space Optical products are deployed in a line-of-sight point-to-point configuration. Each end consists of a
transmitter and receiver operating in a full duplex mode. That is, bits flow in both directions at the same time. FSO
systems can be installed on rooftops or behind windows, provided that glass attenuation is in the acceptable range.
In its simplest form, an FSO transceiver consists of an electrical sub-system and an optical sub-system. The
electrical sub-system amplifies the transmitted and received signals, modulates the transmitted signal, and monitors
and controls the unit’s operation. Most of the current FSO products support SNMP for remote monitoring and
control. The all-optical part interconnects lenses to fiber, sources and detectors. Information is transmitted between
the two transceivers using the On-and-Off Keying (OOK) method, which is a basic modulation scheme that
transmits a signal when the bit “1” is transmitted and transmits nothing for “0” bits. Long runs of consecutive zeros
and ones are important to understand when supporting different protocols not only to comply with eye-safety
regulations but also with the system optical requirements.
On the transmitter side, an LED, laser diode or fiber pigtailed laser is defocused slightly from the lens’s focal point
to incorporate beam divergence. The modulated signal is collected by the receiving lens and focused on to a
photodetector or fiber. Depending on the received signal intensity, “0” and “1” bits are recovered. Besides
photodetectors (quantum detectors), thermal detectors are common in the 3-5 µ and 8-14 µ wavelengths. Thermal
detectors respond to temperature changes from the absorbed optical energy.
In the presence of severe channel noise; the receiver introduces errors when “1” is decoded instead of “0” and vice
versa. With OOK modulation, direct detection (non-coherent) at the receiver end is used to demodulate the received
signal. In coherent detection, the signal shape is detected to derive both amplitude and phase information. The signal
carrier wavelength passes through an optical bandpass filter, which serves to reject background radiation which can
impinge on the surface of the detector. At the transmitter, lasers are commonly used to launch the optical signal, and
LEDs are sometimes used to reduce the cost of short-range FSO units. Since 0.8 µ wavelength transmitted power is
limited by the eye-safety standard (see section 1.2), off-the-shelf low power lasers fulfill most of the FSO
requirements. For 1.5 µ, however, more power can be transmitted and therefore vendors use high-power lasers or
Erbium Doped Fiber Amplifiers (EDFAs) to amplify the signal.
The photodetector operates within a certain power range of received light. That is, when the received light drops
below a certain minimum power, the receiver fails to properly operate because it is not receiving the minimum
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number of photons to convert them into electrons. Commonly, this situation is referred to as receiver starvation. On
the other hand, when the photodiode receives too much light above its maximum threshold it will get confused and
fails to properly operate as well. This situation is referred to as receiver saturation. Since this paper will focus on
atmospheric signal attenuations, we will give two other examples of channel noise. The first one is background noise
when the receiver is placed in East-West configuration. Every year, and during a short period of time, the receiver is
saturated with sunlight. This automatically confuses the receiver by either saturating the detector or simply decoding
all transmitted bits as “1”. By observing the sun spectrum, background noise at short wavelengths is much higher
than the longer 10 µ wavelength. An example of system starvation is scintillation which will divert part of the beam
away from the receiving lens as discussed in section 1.4.
On the 10 µ technology side, Maxima Corporation is currently the only provider at this wavelength. Their products
are based on small, long life CO2 lasers, as well as, Quantum Cascade Laser which is a new class of semiconductor
laser emitting signals in the 8-12 µ atmospheric windows. The company uses its proprietary high-speed, room
temperature HgCdZnTe (MCZT) detector in the receiver. Besides applications in the communication industry, long-
wavelength technologies are strongly emerging in chemical, biochemical, biological and biomedical sensing,
imaging systems illuminators, infrared counter measures, vehicle collision warning systems (adaptive cruise
control), vehicle emission and combustion diagnostics, and military applications. As fiber-based communications
drove the current FSO components market, it is believed that military and security applications will drive the longer-
wavelength component market.
1.4 Scintillation
Atmospheric optical turbulence is due to the fluctuation in the index of refraction of the air. It affects the optical
signals temporal intensity and is referred to as scintillation. The twinkling of stars is a classic example of such an
effect. In addition to scintillation, beam wander and broadening also occur when optical signals pass through
turbulent air. FSO beams are subject to spatial and temporal fluctuations when passing through turbulence cells,
which have been extensively statistically modeled using weather parameters, altitudes and refraction index [3][10].
Small temperature variations cause virtual atmospheric lenses that can redirect the incident beam by refraction
effects. The result could blur the received beam spot (scintillation) reducing the received power, image spot dancing
at the receiver focal point, or completely divert the beam direction (beam wander).
Turbulence is greater closer to the ground or large horizontal planes such as roofs. It is for this reason that most, if
not all, FSO rooftop installations are deployed at the edge of the roof or use tall tripods to avoid heated air induced
by the roof itself. In order to study optical signal propagation through turbulence, some approximations are made to
solve the electromagnetic wave propagation equation. The main approximation is that the index of refraction does
not change rapidly in either space or time. One of the most widely used models is Rytov Models. It provides signal
power and phase fluctuations due to scintillations. For optical paths parallel to the ground, the equation is simplified
and shows a (2π/λ)7/6 dependency, where λ is the transmitted wavelength. From the inverse wavelength dependence,
it is clear that longer wavelengths are less susceptible to scintillation effects.
Aperture averaging over the receiver collecting area has been known to decrease scintillation effects. That means the
larger the receiving aperture is, the more scintillation it can combat. However, increasing the size of the receiving
lens is not practical, and therefore some vendors leverage spatial diversity by using several smaller apertures that are
sufficiently far apart that each received signal experiences independent air paths and therefore uncorrelated phase
and intensity variations. The summed output from such an array of detectors provides aperture averaging. A patent
has been issued describing the use of multiple apertures in FSO systems to combat scintillation [11] and reference
[10] analyzes the effectiveness of multiple apertures versus one aperture of identical area coverage. It was found
that multiple apertures tend to exhibit better averaging results for long ranges. Furthermore, the longer the
wavelength the less aperture averaging effects occur. This may be correlated to the fact that longer wavelengths are
less susceptible to scintillation in the first place.
Beam spot image dancing at the receiver plane is described by the angle of arrival. The variance of the angle-of-
arrival is found to be independent of the optical wavelength. Under large-scale atmospheric turbulence
inhomogeneities, the optical beam will experience random deflections as it propagates. This is referred to as beam
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wander and is found to be independent of the wavelength. Experimentally, it has been shown that fast tracking can
correct the beam wander effect.
2. Atmospheric Effects
The atmosphere is a gaseous envelope surrounding the Earth. Due to gravitational force, which contributes to
atmospheric pressure, the maximum density occurs just above the surface and gradually becomes thinner as we
move away form the surface. In reference [2], the author analyzed the thermodynamics and statistics of the
atmosphere in detail.
Water vapor concentration is highest near the Earth’s surface; an indication that the Earth’s surface is the principal
source of atmospheric water vapor. Thermodynamically, water is the only substance in the atmosphere that occurs
naturally in all three phases with condensed water consisting of suspended water droplet particles (fog, cloud) and
hydrometeors (raindrops, snow). In addition, the atmosphere composition includes uncondensated aerosol particles:
dust, sea-salt particles, soil particles, volcano debris, pollen, by-products of combustion and other small particles
that arise from chemical reaction with the atmosphere. In general, aerosol particle sizes range between 0.1 and 10 µ
with drop-size concentration decreasing sharply with increasing drop sizes. Density refers to the mass of the
atmosphere per unit volume.
In the atmosphere we have a mixture of dry air and water vapor. In contrast to gases, which expand to fill the
volume of their container, condensed phases can maintain a free boundary (surface) and occupy a definite volume.
During droplet’s initial formations, the interface between the liquid and its vapor is not a two-dimensional surface,
but rather a zone that is several molecules thick, over which the concentration of water varies continuously until the
equilibrium state is reached. Water molecules at the surface of a droplet are subject to external interaction as well as
internal interaction with neighboring molecules but cannot freely move inside the droplet. When the forces
surrounding the surface molecules are asymmetric with inward net force, surface molecules will move towards the
interior of the droplet until equilibrium is attained. This effect is due to droplet surface expansion because the
energy of the surface molecule of the liquid becomes higher than the energy in the interior. Surface effect is due to
the surface tension work, which is proportional to the surface area, and the energy necessary to extend the liquid
surface against the vapor surrounding it. Therefore, the liquid surface is created as a result of an increase in the
potential energy. Spherical shapes have the smallest surface area for a given volume, and hence the smallest surface
energy. Hence, the spherical shape of cloud and fog drops resulting from the minimum-energy principle.
A fraction of the collisions between water vapor molecules in the atmospheric are inelastic, i.e. merging molecules,
leading to the formation of molecule aggregations which have a short lifetime since they disintegrate under
continuous molecule bombardment. When the aggregated water droplet reaches a size sufficient for survival, the
nucleation of water droplet then occurs.
Fog and cloud drops are formed by heterogeneous nucleation of some aerosol particles that are hygroscopic, i.e.,
attract water vapor molecules to their surface through chemical processes or physical forces caused by the presence
of hydrogen dipole. This effect may occur below 100% humidity. For example the deliquescent point of salt (NaCl)
is about RH=75%. Another factor contributing to droplet growth is the collection model where small drops collide to
form larger drops. One efficient model of such a phenomenon is called the stochastic collection model that accounts
for the probabilistic aspects of collision and coalescence.
Cloud condensation nuclei (CCN) are considered a subset of hygroscopic aerosol particles where nucleation of pure
water drops may occur at supersaturating less than 1%. For drops below the critical nucleation radius r*, the drops
grow only in response to increase in relative humidity and are termed haze particles. A condensation nucleus is said
to be activated when the drop formed on it grows to size r*. Once the drop grows slightly beyond r*, its equilibrium
value of supersaturation is less than the supersaturation at the critical radius formation. As it will be explained later,
droplet size distribution in the atmosphere will sometime exhibits two radius peaks instead of one. This is referred to
as bimodal distributions. Stochastic collection, along with the equilibrium of droplets with its surroundings,
contributes to the bimodal size distribution explanation.
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The explanation of the observed broadening of drop size spectra n(r) remains a major challenge to cloud and fog
physicists. Fog is composed of very small water drops in suspension in the atmosphere which greatly reduces
visibility. It is mainly caused by cooling of air to its dew point and the addition of water vapor in the air. Fog may be
considered as a stratus cloud whose base is low enough to reach the ground. Fog, stratus, and stratocumulus clouds
occur within 2 km from the ground. They are not associated with precipitation, although often produce drizzle.
2.1 Aerosols
The atmospheric aerosol forms a particle spectrum which spans over four orders of magnitude sizes, and are
subdivided into four categories as described below and illustrated in Figure 3a [13]:
- Aitken particles: The smallest particles are less than or equal to 0.1 µ.
- Larger particles: Their corresponding sizes ranges between 0.1 and 1 µ.
- Giant particles: For radius greater than 1 µ.
Maritime Fog
0.02 0.064 0.2 0.64 2 6.4 8 >8
Droplet Diameter in Micron
Continental Fog
0.02 0.064 0.2 0.64 2 6.4 8 >8
Droplet Diameter in Micron
Concent ration
It is important to emphasize that Aerosol terminology in the context of our studies includes nuclei resulting from
water vapor condensation regardless of their physical or chemical properties. In Figure 3a, large particles are
responsible for scattering visible and near infrared rays. Furthermore, only large and giant nuclei become activated
as condensation nuclei because the required water vapor supersaturation is always small. Aitken nuclei, however,
remain inactive with a smooth line separating the active from the inactive nuclei [13] because of the higher
supersaturation levels required to condense Aitken particles. This is another phenomenon contributing to the
Air Electricity
Air Chemistry
Cloud Phys.
Atm Optics
10-3 10-2 10-1 100 101 103
Particle radius in
Active CCN
Large ions
Aitken Giant
Particles mainly contributing
to Aerosol Mass
Fig. 3a: Classification of Aerosol Particles [13].
Fig. 3b: Example of particle concentrations for maritime and continental regions
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bimodal water droplet distribution models. As relative humidity increases, Aerosol particles gradually grow to
become cloud, fog or ice droplets.
In general, many methods of measuring particle size distribution are simultaneously needed to cover the whole
spectrum of Figure 3a. Due to this complex measuring mechanism, frequently one method is used restricting the
outcome to a subset of the whole spectrum [13]. For instance, particles below 0.1µ are commonly derived from the
diffusion parameter whereas particles between 0.1-0.5 µ may be collected using electrical or thermal precipitation.
Particles larger than 0.5 µ are derived from light scattering instruments. Furthermore, it has been noted that Aerosol
size concentration is further classified between continental and maritime measurements as indicated in Figure 3b
The most important properties of an Aerosol mix are their size variations and relative humidity. Particle radius
grows until equilibrium between the droplet and its surroundings is attained. For soluble particles, equilibrium
depends on the initial particle size and its salt mass content. At equilibrium, the relative humidity of salt droplets
indicates that the influence of the dissolved salt decreases more rapidly with increasing size than does the influence
of surface curvature. Therefore, the growth curve passes the 100% humidity line, reaches a maximum and then
approaches the 100% humidity line asymptotically. In other word, water vapor condensation requires the particle to
be in an unstable supersaturation state with its surrounding to continuously grow. For giant and large particles, the
critical supersaturation amounts to only few tenths of a percent (for example 100.01%), whereas it increases rapidly
to several percents (for example 100.1%) for Aitken particles. As mentioned earlier, this phenomenon also
contributes to the bimodal size distribution explanation.
The supersaturation is of great importance for cloud and fog formation. When air is cooled and fog droplets start to
form, only the largest condensation particles with lowest supersaturation start to grow. Smaller particles, however,
often do not reach their peak because they require much higher supersaturation level and remain on the stable branch
of the growth curve. We schematically illustrate this effect in Figure 4 to explain the bimodal size distribution effect.
10-2 100101
RH = 70%
dN/d log(r)
per µ cm-3
Fig. 4: Schematic diagram of activated and inactivated nuclei condensation of continental
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At higher altitudes, supersaturation reaches higher values making the Aitken and large particles activated. Therefore,
Figure 4 exhibits one peak at much higher radius sizes at these altitudes. It is for this reason mid and high clouds
droplet distribution shows one narrow peak concentration at giant particle sizes.
In most literature, the basic assumption in size distribution calculations is that the temperature and humidity fields
are completely uniform throughout the fog or cloud. In reality, there are always fluctuations that are depicted in the
gap between the inactivated and activated curves in Figure 4. Due to aerosol combination of mixed nuclei and the
non-linear dependence of supersaturation with the particle size, the dividing gap between the inactivated and
activated particles is smoothed out giving birth to a second maximum, i.e. bimodal distribution size distribution
2.2 Attenuation
The atmospheric transmission of optical signals, τa, is described by the following Beer’s law equation:
Where, βabs and βscat are the absorption and scattering coefficients, respectively. In some literature, τa is also referred
to as the atmospheric transmittance or simply the transmittance of optical depth R. Absorption by atmospheric gases,
such as H2O and CO2, defines the atmospheric windows where absorption is less severe. Scattering, on the other
hand, is due to small particulates in the transmission path of electromagnetic waves, abstracting their energy and re-
radiating it in different directions. More detailed analysis and information about absorption and scattering could be
found in the author’s SPIE publications [1][2].
Since visibility is the only weather parameter that describe different fog situations, scientists have tried to directly
relate the scattering coefficient βscat to the incident wavelength λ and visibility. Most of the early FSO publications
used the famous Kruse relation [14]:
Where, the exponent is δ = 1.6 for good visibility, 1.3 for V=6-50 Km and 0.585 V1/3 for visibility less than 6 Km.
The problem with equation (2) is that it fails to estimate attenuations for visibilities below 1.5 Km [2]. Therefore, the
author relied on the original Mie equation to estimate attenuations due to fog and rain [1][2]. For the Aerosol
distribution discussed in section 2.1, the total scattering coefficient is derived from the following equation:
Where, Qscatt is the scattering efficiency, r is the Aerosol particle radius, nr is the concentration of radius r particles
and λ is the transmitted signal wavelength. Therefore, different types of fog are described by their water droplet
concentration nr which reproduces the measured visibility V. From equation (2), βscat is related to visibility at λ= 550
nm (which is the most sensitive wavelength to the human eye) via the following equation:
The correlation between particle sizes and incident wavelength is given by the term “r2 Qscatt(x)” in equation (3). In
Figure 5 we have plotted “r2 Qscatt(x)” versus the droplet diameter “2r” for the wavelengths 0.5 µ, 1.5 µ, 3 µ, 5µ and
10µ. It is clear that for fog particles less than 5µ in diameter, 10µ light is the least affected and becomes more
affected by particles of diameters 10µ and above. The contribution of these giant Aerosol particles dominates
attenuation and visibility measurements.
(1) e R ) ( -
r 2
, )(β2
scatt x x Q nr scattr
)0.55(λ ) (βScatt
δδ λλ
== V
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From the above discussions we conclude that, in general, the 10 µ light suffers less attenuation than shorter
wavelength light. This is because the concentration of Aitkin and large particles are always larger, if not much
larger, than the giant droplets at low altitudes. In section 4 we will display some experimental as well as theoretical
results of wavelengths performance in different types of fogs.
(a) (b)
Figure 5: Attenuation versus droplet size diameter for different wavelengths
In summary, fog is the most challenging media for FSO signals to penetrate. Fog attenuation is wavelength
dependent and can severely attenuate the signal depending on the fog type. Most common fogs attenuate 10 µ
signals much less than shorter 0.8 µ - 1.5 µ wavelengths. Hence, 10 µ FSO systems are expected to perform better
than shorter wavelengths in fog. On the other hand, Rain attenuation is independent of the transmitted wavelengths.
3. “10 µ” Technology
For over 30 years, the DoD has pursued 10 µ in the battlefield because of its high penetration in different
atmospheric conditions, smoke, and dust and has deployed 10 µ CO2 lasers in spite of their previous limitations:
large size, high power consumption and inability to modulate at high speeds.
Besides applications in the communication industry, these long-wavelength technologies are strongly emerging in
the following industries:
- Military: Since 1970, the military has pursued 10 µ in the battlefield because of its high penetration in
different atmospheric conditions, smoke, and dust. The DoD has deployed 10 µ CO2 lasers in spite of their
previous limitations: large size, high power consumption and inability to modulate at high speeds. With
recent security focus, this technology may play a potential role in the chemical and biological sensors.
- Medical: Analysis of breath for the early detection of several potentially deadly diseases such as ulcer,
colon cancer and others. Its biomedical spectroscopy capability detects and analyzes common molecules of
blood plasma.
- Automotive: Vehicle collision warning system such as adaptive cruise control, monitoring and sensing
chemical and combustions in vehicle emission.
- Law enforcement: QCL-based sensors to detect explosives, emission from illicit drug production sites and
other hazardous chemical and biological substances.
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- Others: In many scientific areas such as atmospheric studies where QCL is used to determine chemical
concentration profiles.
Due to these critical applications, there have been activities in building and commercializing 10 µ lasers more
practical than the CO2 laser. One of the leading emerging long wavelength lasers is Quantum Cascade Laser (QCL).
Maxima Corporation, the only FSO vendor working in the long wavelength range, has developed a QCL laser as
small as other semiconductor-based lasers with 130milliwatts of optical power in the 8-12 µ atmospheric window.
On the receiving end, Maxima has also developed its proprietary HOT (high temperature photodetector) high-speed
mercury-cadmium-telluride HgCdZnTe (MCZT) as seen in figure 6, which operate with sensitivities near
Background Limited Performance.
Fig. 6. Maxima 10micron integrated photodetector
Semiconductor quantum cascade lasers (QCLs) achieve high power emission at room temperature and operate at
various wavelengths between 5 µ to 17 µ. The thickness of the used material determines the wavelength of the
emitted laser signal. Each QCL is a collection of alternating InGaAs and InAlAs semiconductor layers on an InP
substrate. The corresponding electrons cascade down the path with a probability of emitting laser photons every time
they hit a new layer.. MAXIMA Corporation has recently demonstrated the first commercial 10µ FSO systems
utilizing these devices with clear air link margins (fade margin) similar to 1µ micron systems. Given that 10µ
wavelengths have significantly lower attenuation in fog (discussed in the next chapter) this link margin gives 10µ
technology several times distance advantage over 1µ systems.
Reference [23] provides a good summary of Quantum Cascade Lasers.
4. Wavelength Performance Comparison
We start our wavelength comparison section by reviewing the results of reference [26]. Arnulf et al. measured
attenuations of wavelengths ranging between 0.35µ to 10µ in haze and in fog. Haze conditions are characterized by
visibilities greater than 1 Km, where droplet sizes are less than 1 micron (Figure 3a). Measured optical transmission
through haze was found to increase remarkably with increasing wavelengths. Arnulf et al. also analyzed and
measured attenuations due to selective, evolving and stable fog. Figure 7 shows some of their results. By simply
observing these two figures, we realize the complexity of quantifying fog attenuations.
Selective fogs affect some wavelengths more than others as illustrated in Figure 7a [26]. One notices that
wavelength transmission depends on the corresponding visibility. For visibilities above 200 m, 10 µ attenuation is
found to be less than 10 dB/Km, whereas 1µ attenuations varies between 10 and 60 dB/Km. This is a clear
indication of giant fog droplets absence. In the case of visibilities less than 100 m, 10µ may be subject to attenuation
as high as 100 dB/Km which is still one tenth of 1µ attenuation. At these short visibilities, some giant particles start
to formulate.
In figure 7b we display attenuation versus wavelengths in Stable fog. Unlike selective fogs, where attenuation is
more sever for some wavelengths, Stable fogs tend to attenuate most wavelengths in a similar way with some
improvement in the 10µ range. It is clear that Stable fog contains more giant particles than selective fog and the
heterojunction contacts
s ubstrate
metalliz ation
heterojunction contacts
s ubstrate
metalliz ation
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lifetime of Stable fog is short at low altitudes. In this case, 1µ attenuation is generally twice as much, on a dB/Km
log scale, as 10 µ attenuation.
Simulight™ [1][2][18] is software developed by UlmTech to model clear, rain, evolving, selective and stable fogs.
As discussed earlier, bimodal fog droplet distributions occur for very low visibilities conditions with the first
maximum occurring in the submicron to one micron region. This automatically indicates lower attenuations at the
10µ wavelength compared to shorter wavelengths. In figure 7b, we selected a few reference points to compare
Arnulf et al. Stable fog results with Simulight results, which are shown in the six black dots. We notice good
agreement between Simulight™ modeling and reference [26] measurement for Stable fogs.
Next we briefly cover backscattering effects. In section 2 we studied forward scattering, which refers to light
scattered by the droplet and aimed in the forward direction. Backscattering refers to scattered optical signals and
Figure 7: Attenuation due to (a) Selective and (b) Stable fogs [26].
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directed in the opposite direction of the incident light. Reference [17] incorporates backscattering calculations into
their theoretical attenuation models and compares them to experimental results. Wavelengths ranging from 0.7 µ to
10.6 µ are considered while focusing on the 10.6 µ improvement.
It was found that fogs with maximum droplet concentration around and above 10 µ start to affect 10.6 µ and all
other wavelengths as well. This is easily seen in figure (5b) where all lines are intersecting around that droplet sizes.
In addition to scattering coefficient, the paper also covers the backscattering coefficient and found that, unlike
forward scattering, backscattering is wavelength dependent for droplet diameters around and above 10 µ. Using two
fog models and comparing them to experimental data [17], transmission using 10.6 µ exhibited the lowest
backscattering coefficient compared to 0.7 µ, 1.06 µ, 2.8 µ, 3.5 µ, 8 µ and 5.3 µ, with the latter having the highest
backscattering value. Similar calculation and experiments were conducted [17] in rain and 10.6µ, again, showed the
lowest backscattering coefficient with 1.06 µ having highest value.
Incorporating backscattering effects is more complicated than forward scattering, and therefore it is often omitted
from most publications analyzing optical signal transmission in different weather conditions. It was also omitted in
Simulight™ [18] because it required a long run-time convergence.
In reference [19], fog droplet-size distribution was studied by measuring the transmission measurement of
wavelengths 0.53µ, 0.65µ, 1.23µ, 2.2µ, 4.01µ and 10.1µ over a 17 m path. Choosing such a short path is more
efficient for studying fog because it eliminates the path averaging of attenuation and hence droplet concentration.
Measurements were made on 29 of January 1981 and recorded four hours before fogs were formed until their
dissipation. The interesting result is that most of the fog droplet concentrations had two peaks instead of one peak
with the first peak occurring in the sub-micron region. This was in agreement with bimodal distributions reported
earlier by Eldrigde [20] and on which basis Simulight™ [18] was built. Reference [19] also concludes that fog
having concentration of droplets diameter in the 3.2-12.8µ are short-lived dense fogs.
Reference [22] provides a good summary of optical communications systems in the 1.06-3.0µ near-IR (infrared),
3.0-5.0µ mid-IR and 9-11µ far-IR wavelengths. Experimental results found that Far-IR coherent detection shows
superior performance in low visibilities. And in a most recent publication [24], a free-space optical link using 9.3µ
quantum cascade laser and an HgCdTe detector was performed over a 350m range. Under foggy conditions with
visibility below 100m, the received signal was subject to only 20%, or 21 dB/Km, attenuation in this dense fog. That
is 80% of the transmitted signal is recovered at the receiver.
On the experimental side, reference [25] conducted experiments using three types of spectral regions:
- Visible: 0.4µ to 0.7µ
- IR: 3-5µ and 8-12µ
- RF: 94 GHz, 32 GHz and 10 GHz
It was observed that attenuation due to moderate rainfall (up to 50 mm/hr) is around 14 dB/Km and was identical, as
expected, for visible and IR signals. For evolving fogs, attenuations decrease rapidly with increasing wavelength,
whereas for stable fog IR signals exhibit slight advantage over visible signals. Clouds seem to have drop-size
concentrations around and above the 10µ diameter, and therefore they affect visible and IR the same way.
As for Radio Frequency (RF) signals, it was found [25] that they are transparent to any type of fog conditions with
only 2 dB/Km (35 GHz) and 3-10 dB/Km (94 GHz) attenuations for 30-meter visibility condition. Similar results
are concluded in cloud measurements. Attenuation due to rainfall, however, is negligible for 10 GHz and could be
sever for 35 GHz and 94 GHz reaching 10 dB/Km and 30 dB/Km, respectively, for 100 mm/hr rainfall.
We conclude based on measured field data of short (0.8 µ), mid (1 µ) and long (10 µ) wavelength light, that long
wavelengths typically have better fog penetration as high as twice (stable fogs) and ten times (selective fogs) the
transmission of shorter wavelengths. This result cannot be generalized to all types of fogs because of the unknown
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distribution of particle sizes. However, from the statistical and thermodynamics studies, it is conceivable to conclude
that long lifetime fogs primarily have distributions of small particle sizes.
In this paper we analyzed the challenges, benefits and performance of different wavelengths from the visible, 1.5µ,
3µ, 5µ and 10µ wavelengths. Besides propagation through fog and rain, we analyzed scintillation effects,
backscattering and compliance with eye-safety standards. It was found that 10 µ has a significant advantage over
shorter wavelengths for FSO communications systems due to its better performance in fog and that components now
exist with adequate performance levels to produce commercially viable 10µ FSO products.
The author would like to thank Maxima Corporation [27] for discussions and providing 10 µ component references.
1. Achour Maha, Simulating atmospheric free-space optical propagation: rainfall attenuation, Proceedings of
SPIE Vol. 4635 (2002).
2. Achour Maha, Simulating Atmospheric Free-Space Optical Propagation: Part II, Haze, Fog and Low
Clouds Attenuations, Proceedings of SPIE Vol. 4873 (2002).
3. The Infrared Handbook, Chapter 6, Wolfe and Zissis [1985].
4. Bettina Tratz-Ryan and John Mazur, Free Space Optics- Focus Report, Gartner Group, September 2002.
5. Digital Communications: Fundamentals and Applications, Bernard Sklar, Prentice-Hall publisher [1988].
6. [FCC] FCC Initiatives Proceeding to Promote Commercial Development of Spectrum in the 71-76 GHz,
81-86 GHz and 92-95 GHz Band, FCC [2002].
7. [ANSI]
8. [FDA]
9. [IEC]
10. Laser Beam Scintillation with Applications, Andrews et al., SPIE Press 2001.
11. Multiple transmitter laser link, Eric Korevaar, US Patent 5,777,768 [1998].
12. Free-Space Optical Alliance,
13. C. E. Junge, Air Chemistry and Radioactivity, Academic Press, [1963].
14. P. Kruse et al., Elements of Infrared Technology, John Wiley & Sons, [1962].
15. Lasers and Electro-Optics: Fundamentals and Engineering, Christopher C. Davis, Cambridge University
Press, [1996].
16. Semiconductor-Laser Fundamentals, W. W. Chow, Spinger Publisher, 1999.
17. Comparative Studies of Extinction and Backscattering by Aerosols, Fog and Rain at 10.6µ and 0.63µ, D.
B. Rensch and R. K. Long, Applied Optics, Vol. 9 No. 7, pages 1563-1573 [1970].
18. Simuligh:FSO propagation in clear, fog and rain Software, UlmTech Inc.
19. Drop-size distribution of fog droplets determined from transmission measurements in the 0.53-10.1 µ
wavelength range, A.P. Lenham and M. R. Clay, Appled Optics Vol. 21, No. 23, pp 4191 [1982].
20. Haze and Fog Aerosol Distributions, R. G. Eldridge, Journal of Atmospheric Sciences, Volume 23, page
605, September [1966].
21. J-5 Optical Heterodyne Communications Experiments at 10.6µ,F. E. Goodwin and T. A. Nussmeier, IEEE
Journal of Quantum Electronics, pp 612, October 1968.
22. Imaging Laser Radar in the Near and far Infrared, G. R. Osche and D. S. Young, Proceedings of the IEEE
Vol. 84, No. 2, pp103 [1996].
23. Quantum Cascade Lasers, Capasso et al., Physics Today, May 2002.
24. Free-Space Optical Data Link Using Peltier-Cooled Quantum Cascade Laser, Blaser et al., Electronics
Letters, Vol. 37, No. 12 [June 2001].
25. Attenuation of Electromagnetic Radiation by Haze, Fog, Clouds and Rain, C. C. Chen, RAND Report
26. A. Arnulf et. al., Transmission by Haze and Fog in the Spectral Region 0.35 to 10 Microns, Journal of the
Optical Society of America, volume 47, number 6, [1957].
27. [Maxima Corporation]
... Atmospheric Conditions . FSO communication systems are affected by different atmospheric conditions such as haze, dust, fog, rain, smoke, and snow [31]. This results in random fluctuations of the light beam at a frequency between 10 MHz and 200Hz [28], or more. ...
... The nature of scattering depends on the optical wavelength and the size of scattering particles. Dense fog remains the most deleterious weather effect, resulting in over 100 dB/km attenuation coefficient [31]. It consequently limits the achievable link range (distance) to about 500 meters [31]. ...
... Dense fog remains the most deleterious weather effect, resulting in over 100 dB/km attenuation coefficient [31]. It consequently limits the achievable link range (distance) to about 500 meters [31]. For instance, the effects of fog on link attenuation were characterized in [28] by using outdoor recorded measurements in different countries at various wavelengths. ...
Research Proposal
Design and Model of Free Space Line of Sight system for 5G wireless Communication
... Different systems have been designed to operate between a wavelength range of 750-850 and 1300-1600 nm as a result of low attenuation in these regions [12]. The FSO link attenuates more easily at a wavelength of 1310 nm with an increase in transmission frequency [13]. ...
... Besides short wavelength, long wavelength technologies such as 10 μm has been used in applications such as communication industry, military, medical, automotive, law enforcement and many more [13]. 850, 1310, 1550 and 9993 nm wavelengths have been chosen for analysing the performance of terrestrial FSO link. Figure 2 depicts the variation of quality factor with input power, ranging from 10 to 15 dBm. ...
Over past few years, optical wireless communication has gained increased interest due to high achievable data rates, unlicensed bandwidth, inherited secure links and ease of deployment. In this paper, we analyse the performance of terrestrial free space optical (FSO) communication link considering internal parameters of the system. The performance of the system has been investigated and optimized by examining quality factor of the received signal considering different input powers, type of detectors, link ranges, beam divergence, data rates and modulation schemes employing 850 and 1310 nm transmission wavelength. Better quality of received signal has been achieved in case of different modulation schemes at a wavelength of 1310 as compared to 850 nm.
... However, detectors available for this range, which are usually made from germanium (Ge) or indium gallium arsenide (InGaAs), are more expensive when compared to silicon photodiodes. Moreover, both 800 nm and 1550 nm experience atmospheric absorption, scattering losses and scintillation effects [3], limiting their applications to indoor short-range communications. ...
... For example, the 3-5-and 8-14-μm atmospheric transmission windows can be used, for their superior penetration of atmospheric obscurants such as fog, smoke and dust [4]. Within the MWIR (3-8 μm or 37-100 THz) and LWIR (8)(9)(10)(11)(12)(13)(14)(15) μm or 20-37 THz), quantum cascade lasers (QCLs) [5][6][7] and 10 μm (or 30 THz) CO 2 lasers [3] are generally employed. However, these laser sources would be considered extravagant (non-ubiquitous) products for the domestic market, and only high-end users (e.g. ...
Full-text available
The thermal (emitted) infrared frequency bands (typically 20–40 and 60–100 THz) are best known for remote sensing applications that include temperature measurement (e.g. non-contacting thermometers and thermography), night vision and surveillance (e.g. ubiquitous motion sensing and target acquisition). This unregulated part of the electromagnetic spectrum also offers commercial opportunities for the development of short-range secure communications. The ‘THz Torch’ concept, which fundamentally exploits engineered blackbody radiation by partitioning thermally generated spectral radiance into pre-defined frequency channels, was recently demonstrated by the authors. The thermal radiation within each channel can be independently pulse-modulated, transmitted and detected, to create a robust form of short-range secure communications within the thermal infrared. In this paper, recent progress in the front-end enabling technologies associated with the THz Torch concept is reported. Fundamental limitations of this technology are discussed; possible engineering solutions for further improving the performance of such thermal-based wireless links are proposed and verified either experimentally or through numerical simulations. By exploring a raft of enabling technologies, significant enhancements to both data rate and transmission range can be expected. With good engineering solutions, the THz Torch concept can exploit nineteenth century physics with twentieth century multiplexing schemes for low-cost twenty-first century ubiquitous applications in security and defence.
... T HE ability to detect mid-infrared light efficiently is increasingly important for a range of applications, such as communications, security, medicine, and metrology [1]- [4]. Conventional semiconductor photodiodes have a limited sensitivity, especially in photon-starved and high-speed applications such as free-space optical communication and remote sensing. ...
The effectiveness of a range of alternative high-k dielectric layers as potential passivation layers for InAs avalanche photodiodes has been investigated. The suppression of surface leakage currents is investigated by analyzing the current-voltage performance of differently sized mesa diodes passivated with each oxide layer. Three potential passivation layers, such as ZnO, Al₂O₃, and MgO, have been identified, all of which enables the suppression of surface leakage in smaller sized devices of a radius of 50 μm and at lower temperatures of 175 K compared to a reference SU8 device. The influence of repeated temperature cycling on these layers has also been investigated with Al₂O₃ passivated devices, exhibiting no change in performance after multiple cooling and heating cycles.
... A new trend in the development of broadband free-space optical communication is the application of long-wave infrared radiation (LWIR: 8-14 µm) [1][2][3]. The main advantage of this solution is lower radiation scattering in aerosols and dusts. ...
In an optical like free space optical communication system, there always exits a focal or focal reflective optical antenna system working as beam expander. As optical antenna can expand and reshape the narrow and highly collimated emitted beam, it efficiently compresses divergence angle and decreases the influence of beam expansion in space optical communication. However, the optical antenna structure will decrease the transmission power for the obscuration loss caused by the secondary reflective mirror. The design of two-mirror reflective optical antenna at Tx affects the transmission efficiency in space uplink optical communication. An improved scheme with two diffractive optical elements (DOEs) can help reduce central obscuration caused by the secondary mirror of optical antenna. In order to further investigate the influence of the DOEs on communication quality, we give a bit error rate (BER) model based on space uplink optical communication system. The effect of the DOEs on the relationship curves of BER versus typical parameters at different obscuration ratios of optical antenna is the research focus. Typical system parameters include transmission power, receiving diameter, Receiver angle, divergence angle and wavelength. With the demand for data volume and data velocity growing, space optical communication has gained extensive attention. It originated in the 1960s and a large number of ground verification experiments have been carried out since 1980. Compared with traditional microwave communication, space optical communication has the advantages of small volume, low power consumption, large optical gain, and small divergence angle, strong anti-jamming and anti-interception ability.
Free Space Optical (FSO) communication systems are being adapted increasingly to provide high speed data transmission. The transmission performance of a free space optical link could be severely degraded due to atmospheric conditions, which causes the temporal and spatial fluctuations of light intensity. Before establishing a FSO link, the meteorological condition of the given geographical area should be studied so that a better availability can be achieved. In this paper, we present a feasibility study of FSO link for four different cities of India representing different topological conditions as a case study.
Effects of atmospheric turbulence, on long distance endo-atmospheric propagation (over a few miles), can be seriously detrimental to free-space optical communications (FSO). The field of the optical wave undergoes phase perturbations that modulate the received intensity. These perturbations can seriously afflict FSO reliability. Pre-compensation by adaptive optics (AO) has been proposed to mitigate these effects and enable the possibility to increase propagation distance and data throughput. The purpose of this thesis is to evaluate the performance and limitations, in terms of FSO effectiveness, of different AO correction methods and to study the possibility of more efficient concepts. We demonstrate that a phase and amplitude iterative correction approach - latter described as optimal correction - enables excellent performance, among the best so far proposed. The study of this theoretical approach enables us to set boundaries to the effectiveness of AO system. We showed that an efficient correction can be achieved greatly beyond the weak perturbation regime. In strong turbulence, it appears that classical approaches - AO by wavefront measurement, phase modulation or iterative phase correction (described as sub-optimal) - are limited. These limitations are due to scintillation, phase branch points and noise. We quantify the drop of performance relative to the optimal correction and propose a solution enabling the minimization of scintillation effects on phase measurements. We finally propose a method to pre-compensation for phase and amplitude and in particular for measurement and control, that should enable the implementation of the optimal correction.
Full-text available
With recent advances and interest in Free-Space Optics (FSO) for commercial deployments, more attention has been placed on FSO weather effects and the availability of global weather databases. The Meteorological Visual Range (Visibility) is considered one of the main weather parameters necessary to estimate FSO attenuation due to haze, fog and low clouds. Proper understanding of visibility measurements conducted throughout the years is essential. Unfortunately, such information is missing from most of the databases, leaving FSO players no choice but to use the standard visibility equation based on 2% contrast and other assumptions on the source luminance and its background. Another challenge is that visibility is measured using the visual wavelength of 550 nm. Extrapolating the measured attenuations to longer infrared wavelengths is not trivial and involves extensive experimentations. Scattering of electromagnetic waves by spherical droplets of different sizes is considered to simulate FSO scattering effects. This paper serves as an introduction to a series of publications regarding simulation of FSO atmospheric propagation. This first part focuses on attenuation due to rainfall. Additional weather parameters, such as rainfall rate, temperature and relative humidity are considered to effectively build the rain model. Comparison with already published experimental measurement is performed to validate the model. The scattering cross section due to rain is derived from the density of different raindrop sizes and the raindrops fall velocity is derived from the overall rainfall rate. Absorption due the presence of water vapor is computed using the temperature and relative humidity measurements.
Full-text available
One of the biggest challenges facing Free-Space Optics deployment is proper understanding of optical signal propagation in different atmospheric conditions. In an earlier study by the author (30), attenuation by rain was analyzed and successfully modeled for infrared signal transmission. In this paper, we focus on attenuation due to scattering by haze, fog and low clouds droplets using the original Mie Scattering theory. Relying on published experimental results on infrared propagation, electromagnetic waves scattering by spherical droplet, atmospheric physics and thermodynamics, UlmTech developed a computer-based platform, Simulight, which simulates infrared signal (750 nm-12 mum) propagation in haze, fog, low clouds, rain and clear weather. Optical signals are scattered by fog droplets during transmission in the forward direction preventing the receiver from detecting the minimum required power. Weather databases describe foggy conditions by measuring the visibility parameter, which is, in general, defined as the maximum distance that the visible 550 nm signal can travel while distinguishing between the target object and its background at 2% contrast. Extrapolating optical signal attenuations beyond 550 nm using only visibility is not as straightforward as stated by the Kruse equation which is unfortunately widely used. We conclude that it is essential to understand atmospheric droplet sizes and their distributions based on measured attenuations to effectively estimate infrared attenuation. We focus on three types of popular fogs: Evolving, Stable and Selective.
Full-text available
The authors demonstrate a free-space optical data link over 350 m using a Peltier-cooled 9.3 μm quantum cascade laser operating at a duty cycle of almost 50%. On the emitter side, a Ge lens was used for beam collimation and a flat mirror for beam steering, while on the receiver side, a fast room-temperature HgCdTe detector in combination with a φ=16 cm mirror telescope detected the incoming signal. Short pulses at a maximum repetition rate of 330 MHz were successfully transmitted. The signal-to-noise ratio of the measurement was limited by the power equivalent noise of the detector
Two independent methods were used: direct spectrophotometry through the transparent windows of the atmospheric gases, and measurement of the number and diameter of the water droplets, followed by a Mie-theory calculation of the spectral transmittance. Results from the two methods are in good agreement when the media are sufficiently homogeneous, as for a quiet haze or fog and artificial smoke.The following kinds of atmospheres were considered: hazes (optical density per km in the visible spectrum is less than 2); a small number of small-drop fogs (optical density per km less than 10); evolving fogs (which have changing distributions of drop-diameters); nonevolving, slightly selective fogs; artificial smokes. In addition, some information is given on the statistical distribution of drop-diameters.It was found that the transmission of haze increased markedly with increasing wavelength, from the visible to 10 microns, but this marked increase was not found for fogs.
The report assembles, under one cover, the values of aerosol attenuation coefficients of regions in the electromagnetic (EM) spectrum containing so-called 'atmospheric windows,' in which EM radiation suffers the least amount of atmospheric gaseous absorption. The purpose is to enable rapid quantitative assessment of target acquisition terminal guidance sensors using the windows during adverse weather. Both calculated and available measured values are presented. Being a compilation drawn from numerous sources, the report is intended more as a handbook for ready use than as a theoretical treatise.
Spectral attenuation measurements through haze and fogs are used to synthesize aerosol distributions characteristic of the attenuating medium. Twelve fog drop-size distributions have been averaged into two general types of fogs for comparison with measured fog drop-size distributions and with a distribution predicted by theory. Seven time-phased fog drop-size distributions are used to illustrate the phenomena of droplet growth and fog degeneration. Finally, an empirical relationship between liquid water content and visual range is presented.
Optical Wave Propagation In Random Media - Background Review Optical Scintillation Modelling Theory Of Scintillation - Plane Wave Model Theory Of Scintillation - Spherical Wave Model Theory Of Scintillation - Gaussian-Beam Wave Model Aperture Averaging Optical Communication Systems Fade Statistics For Lasercom Systems Laser Radar Systems - Scintillation Of Return Waves Laser Radar Systems - Imaging Through Turbulence.
Theoretical calculations using continental, fog, and precipitation aerosol size distribution models are presented here for extinction and backscatter coefficients along horizontal propagation paths at sea level. The coefficients are presented for various atmospheric conditions and for wavelengths between 0.34 micro and 10.6 micro. Finally, the theoretical results are confirmed by comparison with outdoor transmission studies at 10.6 micro and 0.63 micro.