Introduction:

Some amateur radio operators are fond of the high frequency bands.  Some
like the challenge of the VHF (Very High Frequency) or UHF (Ultra High
Frequency) bands.  Even others like the challenge of the microwave bands.
However, some like to go for the ultimate extreme in frequency and use
optical signals.  This presentation will present a simple (and cheap) way
to send and receive signals at 455 THz (TeraHertz).

Basic Theory:

Construction of equipment for the optical frequencies has little
resemblenance to equipment used for high frequencies.  The generation of
the optical signals is done in specialized devices, and these signals are
routed by waveguides, somewhat similar to the way that microwave signals
may be, only in the case of optical signals, these waveguides take the form
of optical fiber, usually made from glass.  However, since the transmitters
and detectors are so small, it's usually possible to mount them at the
"antenna" site directly, and dispense with any transmission line.  This
simplifies the system, as well as eliminating any loss from the
transmission line.

Modulation Techniques:

Due to the extremely high frequencies involved, it's usually not practical
to frequency modulate optical devices.  Thus, the only practical form of
modulation is some type of amplitude modulation.  The simplest form of
modulation is CW.  This can be achieved by switching the optical source on
and off.  Unfortunately, this is usually not the optimal modulation
technique due to contamination of the signal in the optical receiver by
ambient light.  This also has the problem of requiring a DC coupled
amplifier, which is undesirable.

An improved form of modulation is modulated CW (MCW).  In this technique,
the optical source is switched on and off at an audio rate when the key is
pressed, and is off while the key is up.  This has the advantage that the
receiver amplifier does not have to be DC coupled, as well, as allowing it
to easily differentiate the signal from ambient light.

The simplest form of modulation device to produce a modulated CW signal is
an optical chopper.  This can be as simple as a fan positioned in the
optical path such that the blades of the fan block the signal as the fan
blades rotate.  This can actually have desirable side effects such as
producing forced air cooling of the optical source.  Unfortunately, the
frequency of the modulated CW signal tends to vary depending upon the speed
of the fan.  Additionally, the vibration from the fan tends to cause
objectionable vibration of the optical signal.

However, it's very simple to produce an electrical oscillator to drive the
optical source and switch it on and off at an audio frequency.  One simple
way of doing this is to use a NE555 chip.  This approach also allows the
frequency to be varied by changing a pair of resistors, as well as allowing
the duty cycle of the optical source to be varied.

It is also possible to produce voice modulation of the optical source.
However, due to the nonlinear nature of some optical sources, and due to
the problem with audio frequency interference from ambient illumination
sources, it's usually not advisable to perform baseband audio amplitude
modulation of the optical sources.  What is usually done is that the audio
information is supplied as a modulation to a subcarrier that is used to
modulate the optical source directly.  For example, the audio information
may be made to pulse width modulate a carrier which itself is used to on
and off modulate the optical source.  It is also possible to frequency
modulate the subcarrier.  It is also possible to place multiple subcarriers
on a single optical source, so that multiple data streams can be carried by
the same optical signal.

One of the most intriguing modulation techniques involves modulating the
optical signal by a digital signal.  Due to the extreme bandwidth available
on a single optical signal, it is possible to send many billions of bits of
digital data across a single optical signal.  However, such techniques are
too advanced for this presentation.

Filtering:

One of the problems with optical systems is that the ambient light, at
least during night, is not a constant level, but is modulated at 120 Hz.
This light comes from street lights and other illumination devices.  Since
a significant number of these devices are of the Sodium Vapour type, and
since they are driven from the 60 Hz power lines, they produce a
significant amount of light varying at a 120 Hz rate (Two illumination
cycles per line frequency cycle.).  Fluorescent lamps also produce a
similar kind of light noise.  Fortunately, incandescent lamps produce very
little flickering due to the thermal inertia of their filaments.

A couple of approaches are available for filtering out such interfering
signals.  The first is a filter that can be applied at the "RF" stage, and
takes the form of a piece of glass (or other material) which only transmits
certain optical frequencies.  This may take the form of a red piece of
glass or a filter gel which only transmits red light, and absorbs the
yellow light from Sodium Vapour lamps, and the bluish light from
fluorescent lamps.  This has the advantage of preventing overload of the
detector by the ambient light, at the small expense of reduced sensitivity
of the detector to the desired light (Such filters absorb a slight amount
of the desired light in addition to absorbing large quantities of the
undesired light.).

Another approach is to improve the beamwidth of the transmitting and
receiving devices to exclude ambient light not in the desired direction.
This may take the form of a black baffle tube surrounding the receiver to
block light from any direction except the desired direction.  At the
transmitter end, the light can be focused so that more of it is directed at
the receiver.  This direction may take the form of a parabolic reflector or
a lens.

A third approach involves placing an electronic filter in the receiver
amplifier to suppress any 120 Hz signals.  Unfortunately, since the Sodium
Vapour lamps are switched on and off, their light signals also contain a
significant harmonic content, so it may also be necessary to attenuate
harmonics of 120 Hz, such as 360 Hz, 600 Hz, 840 Hz etc.  (In some countries
which us a 50 Hz power distribution system, these frequencies are 100 Hz,
300 Hz, 500 Hz, 700 Hz, etc.) [1]

Optical Sources:

Perhaps the simplest optical source is the incandescent lamp.  However,
these devices are almost entirely unsuited to use in anything except the
very lowest speed systems.  The thermal mass of their filaments means that
the optical output can not be switched very rapidly.  Additionally,
modulation of these devices typically strains the filaments due to thermal
stresses, and this can result in premature failure of the devices.  The one
advantage that these devices have is that they can be made to produce large
amounts of power.  Unfortunately, though, this optical power is mostly in
the infrared portion of the spectrum, and what small percentage that is
available in the visible portion is spread across the entire width of the
spectrum.  Thus, while it is possible to baseband modulate these at low
speed CW frequencies, there are other devices which exhibit more desirable
properties.

One of the cheapest and simplest optical sources is a Light Emitting Diode
(LED).  While these devices are rather limited in the amount of power that
they can produce, they tend to be narrow bandwidth sources, and tend to be
fairly directional with the light that they produce.  Additionally, they
can be modulated at incredibly high rates, and have lifetimes measured in
centuries, even when switched on and off at megahertz rates.  A further
advantage of LEDs is that they are compatible with solid state modulation
devices.

Unfortunately, LEDs, while being fairly directional with their light
output, still have enough divergence such that their range is limited.
Another device, similar to an LED overcomes this problem.  A solid state
laser diode produces a reasonable amount of optical energy that is
concentrated into a very narrowly divergent beam.  It is also possible to
rapidly modulate a laser diode.

Other optical sources are also possible.  Glow discharge devices produce
optical energy, although it can be difficult to modulate them due to their
high voltage requirements.  Their modulation rates, while high, are
somewhat limited.  Plus, glow discharge devices tend to be spatially
diffuse radiators, so it can be difficult to focus their light output.

Fluorescent lamps produce a large amount of optical power.  Yet they suffer
from some of the same disadvantages that glow discharge devices do in that
they require a high voltage source.  Additionally the persistence of the
phosphor in most fluorescent lamps severely limits their maximum modulation
frequency, although it is possible to obtain special fluorescent lamps with
high speed phosphors which may alleviate this problem.

Devices such as electroluminescent panels produce light, but are not
generally suitable for optical sources for communications devices.

Laser Transmitter:

The laser transmitter is based on a NE555 integrated circuit, which
supplied an audio frequency signal to modulate a laser diode.  The audio
frequency is determined by the combination of resistors R1, R2, R3, and
R4, along with the capacitor C1.  Resistors R2 and R3 may be adjusted to
select the frequency, and duty cycle of the output.  The desired frequency
is 800 Hz, which is not harmonically related to 60 Hz.

Laser Receiver:

The laser receiver is based on a PIN photodiode.  This photodiode is
transformer coupled to a preamplifer, based on (half of) a TL082CP
integrated circuit.  The transformer coupling removes any DC bias caused by
ambient light.  The output from the pre-amplifer is fed to a active filter,
based on the other half of the TL082CP integrated circuit.  This active
filter may have the peak frequency adjusted via resistor R8.  The final
stage of the receiver is an audio amplifier based on an LM386 integrated
circuit.  The audio amplifer may select its input either directly from the
preamplifier, or from the output of the active filter.  The audio amplifier
has a volume control to allow the output volume to be adjusted.

To facilitate construction, the electrical circuitry is divided into a
couple of subassemblies.  The preamplifier and filter are constructed on
one board, while the audio amplifier is constructed on another board.  This
modular construction allows different circuits to be substituted.
Additionally, the photodiode is mounted in a two inch PVC pipe cap.  This
will allow a baffle made from PVC pipe to be used to shield the photodiode
from ambient light.  Additionally, the inside of the PVC pipe may be
painted black, and a lens can be mounted to the front to focus more light
onto the photodiode.