1967, lasers were still the things of science fiction to most people.
Real-world applications seems to be far off in the future, but in fact,
work was underway setting the stage for today's blazingly fast communications
systems. This article references to attaining 5 THz optical transmission
speeds through fiber and through the air. At the time, a laboratory
filled with bulky prototypes chassis and optical tables were required
to get those results. In 2012, devices that greatly surpass 5 THz
are available in consumer quality IC packages for a couple dollars.
Such is the way or progress.
April 1967 Electronics World
of Contents] People old and young enjoy waxing nostalgic about
and learning some of the history of early electronics. Electronics World
was published from May 1959 through December 1971. All copyrights are hereby acknowledged.
Electronics World articles.
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Laser ModulatorsAlthough still in the R&D stage, three new light modulators
allow a laser to be used as a broadband light transmitter. Bandwidths
up to 7000 MHz have been reported for one unit.
GHT modulators developed at Bell Telephone Labs., now make it possible
to modulate broadband communications signals onto laser beams, using
low-level modulators requiring less than one watt of power.
The three devices to be discussed are highly efficient modulators of
both pulsed and continuous laser light. The first two work on the well-known
principle of polarization in which a light beam is passed through two
polarizers. When the two polarization planes are rotated so that they
are 90° from each other, no light will pass through. When they are aligned
parallel to each other, the light will pass through almost undiminished.
By adjusting the polarization planes' relative angle between parallel
and 90°, it becomes possible to intensity-modulate the light beam.
Fig. 1. The lithium tantalate modulator
is capable of 896 megabits per second,
and may soon attain 5000 megabits.
This electro-optic digital transmission modulator system (Fig. 1)
has been used in an experimental system for high-speed transmission
of pulse-code modulation (PCM) signals. In PCM systems, information
to be transmitted (TV, voice, or data) is translated into a coded sequence
of electrical pulses (bits), with each bit representing a discrete signal
As shown in Fig. 1, pulses of light from the laser are
first passed through an initial polarizer that causes the light beam
to assume a particular polarization. After passing through the polarizer
the light then passes through the moulator, a thin rod of lithium tantalate
crystal (measuring 0.4 x 0.01 x 0.01 inch). The light then encounters
the analyzer filter having s a plane of polarization 90° different than
the polarizer so that the laser light will not pass through the analyzer
and be transmitted to the photo-diode detector.
crystal modulates (in this case modulation consists of polarization
changes) the incoming light and acts as a high-speed gate. Two electrodes
are plated on opposite rectangular faces of the crystal and when the
PCM terminal sends an electrical pulse (representing a "1") to these
electrodes, it causes the plane of polarization of the light passing
through the crystal to shift 90 degrees.
This change allows
the light to pass through the analyzer and be detected by the photodiode.
If no electrical pulse (representing a "0") is sent from the PCM terminal,
the light passing through the crystal is blocked at the analyzer, hence
it does not get to the photo-diode. The electro-optical modulator uses
this coded sequence of high-speed electrical pulses to modulate (gate)
an equally fast train of light pulses from the laser.
of operation of this system is about 224 million bits per second. After
some redesign of the modulator, it is expected that operational speed
will reach 896 million bits per second. This latter rate is equivalent
to a bandwidth of about 1600 MHz. It is hoped that future systems, using
a solid-state laser having extremely narrow pulse widths, may reach
speeds of 5000 million bits per second.
This modulator consists of a rod-shaped crystal of gallium-doped
yttrium iron garnet (YIG) with a small coil wound around it, and the
crystal submerged within a magnetic field. It operates on the principle
discovered by Michael Faraday in 1845 that the plane of polarization
of a light beam in a magnetic medium rotates along the magnetic lines
of force. The application of current to the coil surrounding the doped
YIG rod creates a second magnetic field in the crystal, at right angles
to the first. If the current flowing through this coil is the result
of a varying signal, the plane of polarization of the light beam passing
through the modulator will also vary in accordance with the modulation.
2. The YIG modulator can transmit 33 TV programs.
Operation is shown in Fig 2. The light output from the laser is first
sent through an initial polarizer that causes the light beam to assume
a particular polarization. A lens focuses the light beam through the
modulator and onto the analyzer filter. The analyzer has a plane of
polarization 45° away from the polarizer.
The modulation current is allowed to flow through the YIG coil,
the magnetic field within the YIG varies, thus the plane of polarization
of the light leaving the YIG varies, and is allowed to pass through
the analyzer at various light levels ranging from no light to maximum
This modulator has exhibited bandwidths of 200 MHz (sufficient to
transmit about 50,000 telephone calls or 33 TV programs). Another version
of the modulator has reached a 400-MHz bandwidth; however, maximum potential
bandwidth has not yet been determined.
This modulator, shown
in Fig. 3, consists of a semiconductor diode p-n interface, together
with mounting and input/output lenses (not shown). The incoming laser
light is divided into two equal components at the input polarizer, then
focused on the p-n interface. The light passes through the diode and
is confined within the p-n interface because of the discontinuities
in the index of refraction along both upper and lower surfaces.
When reverse bias is applied to the diode, the gallium phosphide
in the junction region changes from an optically isotropic (having the
same properties in all directions) medium to a medium having different
optical properties in different optical properties in different directions.
This anisotropy causes the two polarization components of the incoming
light beam to travel at different velocities through the p-n interface.
This change in relative velocities, in essence, phase-modulates the
passing light beam in accordance with the reverse bias (modulating signal)
across the junction. Intensity modulation results from passing the phase-modulated
components through the output polarizer.
This diode has successfully
modulated a laser beam up to 7000 MHz, with optical losses of less than
These approaches show that laser transmission systems to replace
microwave relays may not be too far off.
Fig. 3. The gallium phosphide modulator
reaches 7000 MHz.