OPTICAL DETECTORS IN
FIBER OPTIC RECEIVERS.
Presenter: Julius Munyantwali.
Introduction.

A fiber optic receiver is an electro-optic
device that accepts optical signals from an
optical fiber and converts them into electrical
signals.

Consists typically of :
-Optical detector
-Low-noise amplifier
-Other circuitry.
Block diagram of fiber optic
receiver.
Optical Detectors.

These are transducers that convert optical
signals into electrical signals.

Transducers are devices that convert input
energy of one form into output energy of
another.

An optical detector does so by generating an
electrical current proportional to the intensity
of the incident optical light.
Optical Detector
Requirements.

Compatible in size to low-pass optical fibers
for efficient coupling and packaging.

High sensitivity at the operating wavelength
of the source.

Low noise contribution.

Maintain stable operation in changing
environmental conditions.
Semiconductor Photodiodes.

Generate current when they absorb photons.
The amount of current depends on ;
-Wavelength of the light and responsivity of
the photodiode
-Size of the photodiode active area relative to
the fiber core size
-Alignment of the fiber and photodiode.
Optical detector materials.

Si,GaAs, GaAlAs – 850nm

Ge, InP, InGaAs -1300nm and 1550nm.

Materials determine the responsivity of the
detector which is the ratio of the output
photocurrent to the incident optical power.

It’s a function of the wavelength and
efficiency of the device.
PIN Photodiode.

Semiconductor positive-negative structure
with an intrinsic region sandwiched between
the other two regions.

Normally operated by applying a reverse-bias
voltage.

Dark current can also be produced which is a
leakage current that flows when a reverse
bias is applied without incident light.
PIN Photodiode.
Response time factors.

Thickness of the active area.
-Related to the amount of time required for the
electrons generated to flow out of the
detector active area.

Detector RC time constant.
-Depends on the capacitance of the
photodiode and the resistance of the load.
Schematic of a Photodiode.
Advantage of PIN
photodiodes.
 The
output electrical current is linearly
proportional to the input optical power
making it a highly linear device.
 Low bias voltage(<4v).
 Low noise
 Low dark current
 High-speed response
AVALANCHE Photodiodes.
 An APD
internally amplifies the
photocurrent by an avalanche process
when a large reverse-bias voltage is
applied across the active region.
 The
gain of the APD can be changed by
changing the reverse-bias voltage.
AVALANCHE Photodiode.
Light Emitters As Detectors.

LEDs and lasers can also be used as light detectors
making them half-duplex fiber optic communication
devices.

They can be used alternately as light emitters and
detectors allowing transmission of information in
either direction over the fiber.

In order for the LED to operate as a full-duplex, the
temperatures at both ends should be carefully
chosen.
Ping-Pong(Full-Duplex) LED.
Questions And Comments.