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Preface -- Introduction -- Light
Theory -- Light Detector --
Light Emitter
Light System Configuration -- Light Processing -- Receiver Circuits --
Transmitter Circuits |
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Chapter Five
- Light
Processing Theory |
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Lenses
as Antennas |
| There is a reoccurring
analogy between optical communications and radio. Both systems use similar components
that, although made from completely different materials, perform similar functions. As an
example, a radio system will always use some kind of antenna to capture the diffuse and
often weak signals from the air. Optical systems use similar devices in the form of lenses
or mirrors to gather the weak light signals for processing. Large antennas or lenses will
allow weaker signals to be detected. |
| In microwave radio
communications, such as satellite receivers, the antenna is often a specially dish shaped
metal reflector. The microwave signals are bounced off the dish surface and are
concentrated at its focal point, where they can be more efficiently amplified. Similarly,
mirrors can be used in optical telescopes or some optical communications systems to
collect light and focus it onto special light detectors. |
| In much the same way that
the incoming radio or light signals are processed, the outgoing signals can also benefit
from specially shaped antennas or lenses. The radio or light source, when positioned at
the focal point of a reflector, can shape the outgoing signal into a narrow beam. The
larger the antenna or lens, the narrower the beam becomes. A narrow light beam insures
that more of the desired signal is directed toward the distant receiver for better
efficiency.
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Mirrors and Lenses
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| Although you can use mirrors
in through-the-air communications, lenses are more often used. Lenses are usually much
cheaper, readily available and much easier to align than mirrors. Useful lenses can be
found in hardware stores, bookstores, office supply stores and even grocery stores. All of
the discussions in this book will center on the use of lenses, although some of the
techniques used for lenses can also be applied to mirrors.
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Types of Lenses |
| Most of the lenses used in
through-the-air communications have one or two outwardly curved surfaces. Such lenses are
called "convex" lenses. Small glass or plastic lenses are great for short-range
applications. However, glass lenses larger than about 3 inches become too heavy and
expensive to be practical. Beyond the 3-inch size it is best to use a flat or
"Fresnel" lens. Fresnel lenses can be purchased with diameters ranging from one
to more than 36 inches. These lenses are made from molded plastic sheets that have small
concentric grooves on one side. When viewed close-up, they look like the grooves in a
phonograph record. These lenses are very carefully designed to bend the light just as a
convex lens would. When using a Fresnel lens always remember to keep the grooves pointing
toward the outside, away from its focal point. Using the lens in reverse will result in
lost light and a poor image. |
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Divergence
Angle |
| The outgoing light from an
optical transmitter forms a cone shaped area of illumination that spreads out from the end
of the transmitter. As illustrated in Figure 5a the specification that mathematically
describes the spreading out of the light is called the "divergence angle". It is
almost always described as a half angle or the angle from the center axis of the
illumination cone. Often the edge of the illumination cone is defined as the 1/2 power
point, relative to the center light intensity. To help illustrate the concept, imagine a
flashlight whose beam can be adjusted from a broad flood to a bright spot. The bright spot
would have a smaller divergence angle than the flood. Likewise, a red laser pointer would
be an example of light source with a very narrow divergence angle. If you have ever had a
chance to play with as laser pointer, you would have noticed that the beam does not
increase appreciably in size as it strikes a wall across a room. Such divergence angles
can be so tight, that keeping the spot on a distant target can be nearly impossible. Most
optical communications systems therefore purposely allow the beam to diverge a little so
optical alignment can be easily maintained. |
Figure 5a
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Acceptance Angle |
The incoming
light, focused onto a light detector, also has a restricted cone shaped area of
collection. Light striking the lens, outside the cone area, will not be focused onto the
detector. As illustrated in Figure 5b, the incoming angle is called the "acceptance
angle" that is also defined as a half angle. To help illustrate this concept, imagine
looking through a long and a short section of pipe. Even if the two pipes have the same
diameter the long pipe will restrict the field of view more than the shorter pipe. Pipes
that are specially made to restrict the field of view are often used to help aim an
optical system and are referred to as "bore sights" (see Figure 5c.) As in
divergence angles that are too small, an acceptance angle should also not be too narrow or
you will have problems in maintaining alignment with the distant transmitter. |
Figure 5b |
Figure 5c
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Light Collimators and Collectors |
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The light, bent by a
lens as it leaves a transmitter, is said to be "collimated". As illustrated by
Figure 5d, lenses used to collimate the emitted light from sources such as LEDs, should be
carefully selected for their diameter and focal length. A lens with a focal length that is
too long will not capture all of the light being emitted. Conversely, a lens that has a
focal length that is too short will only partially use its available diameter and will
therefore have a greater overall divergence angle. Figure 5e illustrates how a lens
affects the launched divergence angle from an LED. In a similar way, the size and focal
length of the lens used in a light receiver should be selected to insure the light
collected is focused properly onto the detector. Fortunately, most light detectors have
wide acceptance angles, so you can be use them with a much larger variety of lens shapes,
than those required by a light emitter. |
Figure 5d
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Figure 5e
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Multiple Lenses, Multiple Sources
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As
illustrated in Figures 5f, there are two methods that you can use to collimate light from
multiple emitters. If you place a single lens in front an array of light sources, multiple
images of the sources will be directed toward the receiver. The individual images will be
widely spaced with large blank areas between them. A single receiver will detect only one
of the images. This method may be useful if multiple receivers need to receive the
transmitted light, but it is not recommended if only one receiver is used. If you want to
increase the effective light intensity sent to a distant receiver, from a transmitter that
uses multiple emitters, you will need multiple lenses. As illustrated in Figure 5f an
array of lenses, each with its own light source, will appear as one light source, having a
higher intensity than a single emitter. This lens array concept is applied in nature by
most insects and can be successfully used to produce more powerful light sources that will
extend the range of a communications system. |
Figure 5f
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Optical Filters |
To increase
the separation distance between a light transmitter and a receiver, lenses are often used.
A light receiver may use a lens to collect the weak light from the transmitter and focus
it onto the receiver's detector for processing. But, the lens will always collect extra
light from the environment that is not wanted. Stray light will often interfere with the
signals of interest. One method to reduce the amount of ambient light that is focused onto
a detector is to insert an optical filter between the lens and the detector.
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| You may see some optical
filters every day without realizing it. As an example, the red clear plastic covers, used
on most car taillights, are filters. These filters block most of the unwanted colors
emitted by the bulb inside and allow only the red light to pass. These single color band
filters are called optical "band pass" filters and are the most valuable type of
filter used in through-the-air communications. Other filters also exist. "High
pass" filters are used to block light of long wavelengths and pass shorter
wavelengths. Conversely, "low pass" filters block short wavelengths and allow
long wavelengths to pass. |
Figure 5g
shows the transmission spectrum of a low pass filter material. The material has been
specifically designed for near infrared use. It is nearly transparent to the near infrared
wavelengths but is very dark to most visible light. When placed in front of a silicon
detector, the filter will block much of the stray visible ambient light, which may be
collected by a lens. But as you will see in the section on light detectors, such a filter
will have a minimal effect in the reduction of interference with communications systems
that use light emitting diodes (LEDs) as light sources. This occurs because the scattered
sunlight, picked up by the lens, contains a sizable amount of infrared light as well as
visible light. The extra light, not blocked by the filter, will still be enough to cause
some interference with the signals from the LED source. Even a filter, perfectly matched
to an LEDs spectrum, would still cause problems. To filter out most of the unwanted
sunlight, a very narrow band pass filter is needed. But to take advantage of a band pass
filters they must be used with equally narrow spectrum light emitters, such as
semiconductor laser diodes. |
Figure
5g |
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One optical
band pass filter, that can be made to closely match a laser diode's emission spectrum, is
an "interference" filter. Stacking many very thin layers of special materials
onto a glass plate makes interference filters. By varying the thickness and the kind of
materials deposited, the width of the pass band and the center wavelength can be
controlled. Figure 5h is an example of such a filter. As can be seen, its bandwidth is
very narrow and happens to match the emission spectrum of a typical infrared laser diode.
If such a filter were used in a communications system, almost all the laser light
collected would be allowed to reach the detector, but it would allow only a tiny amount of
stray sunlight to pass. Narrow band pass filters can especially be useful when a single
light receiver needs to detect light from only one of many different modulated laser
sources. Different band pass filters can be moved in front of the detector to reject all
sources except one. Such techniques make it possible to have perhaps 10,000 different
light receiver bands without interference.
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Figure 5h
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Make Your Own Optical Low Pass Filter |
A pretty
good optical low pass filter can be made using a photographic film negative. As
shown in figure 5h-1, this filter works well at attenuating visible light and is pretty
transparent over much of the near infrared wave lengths. However, do note that only
light sources with wave lengths longer than 830 nanometers should be used. This
filter should not be used for detecting light from many lasers, that operate at 780
nanometers. I found that Kodak Kodacolor film with an ASA of 100 works well.
You first remove the unexposed film from the roll and expose it to the light from a cool
white fluorescent lamp for about 5 seconds. Then, you wind up the film into roll
again and take it to your favorite film developer for processing. Tell them that
your not sure if the roll has any images on it and you can usually get them to develop the
roll for free. The processed color negatives form the filter material. Keep in
mind that the film material is not very robust and should not be used if it can be
scratched or exposed to moisture. |
Figure 5h-1 |
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Inverse
Square Law |
One of the
most important principles you will discover in optics is the inverse square law. The law
defines how a light receiver's ability to collect light from a distant emitter will
decrease as the receiver is moved away from the source. To help illustrate the concept,
let's use a water analogy. Imagine light from a transmitter as a fine spray of water from
a small nozzle that produces a cone shaped pattern of water droplets. Also imagine our
water source to be in the vacuum of space so that the spray is not effected by air or
gravity and will continue to spread out evenly, forever. The gallon per minute rate of
water flowing through the nozzle would then represent the intensity of the light source.
Now, imagine moving a bucket through the spray at various distances from the nozzle, the
bucket representing a light receiver's collection area. When the bucket is near the nozzle
it would fill much faster than when it is positioned farther away. The inverse square law
predicts that if the distance between the bucket and the nozzle is doubled, the bucket
will fill 4 times slower. If it is moved 4 times farther away it will fill 16 times
slower. Such a reduction rate would continue as the bucket is moved away from the nozzle.
Conversely, if the bucket is moved, so it halved the distance, it would fill four times
faster. By knowing the flow of water from the nozzle (light intensity) and the spray
pattern (divergence angle) you can predict how fast the bucket would be filled (light
collected) at any position (range) within the spray. Such a prediction is described by the
"optical range equation" that combines the inverse square law with some simple
trigonometry.
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Range Equation |
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The equation shown in Figure 5i combines
the inverse square law with some other known information. You can use the equation to
calculate a number of factors for a typical through-the-air communications system. As in
any algebraic equation, you can solve for any unknown factor if the other factors are
known. As an example, the equation can tell you how large a light collector you will need
at the receiver or the maximum distance you can position the light receiver from the
transmitter. Of course, the equation does not take into account any other losses that may
exist within the link, such as poor air quality. Figure 5j illustrates how the divergence
angle effects the illumination area from a light source. |
Figure 5i
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Figure 5j |
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