April 1967 Electronics World
Wax nostalgic about and learn from the history of early electronics. See articles
Electronics World, published May 1959
- December 1971. All copyrights hereby acknowledged.
In 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.
Although 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
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 level.
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.
The lithium-tantalate 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.
The speed 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.
Fig. 2. The YIG modulator can transmit 33
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 light.
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 3 dB.
These approaches show that laser transmission systems to
replace microwave relays may not be too far off.
Fig. 3. The gallium phosphide modulator reaches