LIGHT PROCESSING THEORY:Lenses as Antennas, Light Collimators and Collectors

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Chapter Five

LIGHT PROCESSING THEORY

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.

Mirrors and Lenses

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.

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

Figure 5a

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.

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

Figure 5b

angle should also not be too narrow or you

will have problems in maintaining alignment with the distant transmitter.

Light Collimators and Collectors

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

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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 5c

Multiple Lenses, Multiple Sources

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

Figure 5d

system.

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

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the amount of ambient light that is focused

onto a detector is to insert an optical filter

between the lens and the detector.

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

Figure 5e

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.

Figure 5f

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.

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.

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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.

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 shouldn't 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 Figure 5g

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.

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 Figure 5h

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

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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

Figure 5h-1

by the "optical range equation" that

combines the inverse square law with

some simple trigonometry.

Range Equation

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

Figure 5i

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 5j

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