Optical
Connectors
This is very simple
optical fiber communication system. In this particular scenario, the optical
fiber requires one connection to the transmitter side and another connection at
the receiver side. In another scenario when it is required to communicate over
long distances where more than one length of fiber cable and other supporting
equipment are placed in the system.
Optical
Splices
Other than the
optical connectors there are other methods also which result in much lower
losses in splicing two optical fiber cables together. The two most common
methods are fusion splicing and mechanical splicing. They produce splice losses
in the range of 0.15 dB-OJ dB.
Optical
Receivers
As soon as the light
pulses reach their destination they need to be reconstructed into the original
signal and therefore, they are channelled into the optical receiver through
optical detector. As its name implies it detects the received light
incident on it and to convert it to an electrical signal containing the information impressed
on the light at the transmitting end. The electronic information is
then ready for input into electronic based communication devices, such as
a computer telephone, or TV. These detectors are light sensitive
elements, which are encased, in a special adapter similar to the one used for
the LED or LD. A photosensitive device like photo diode .of either the PIN or
the avalanche type are usually used in which optical light coming out from the
optical cable strikes an exposed section of a P-N junction of semiconductor material.
These detectors work on a principle when a photon strikes a semiconductor,
it can allow a movement of electron and thus creating an electron and hole
pair. The concentration of these electron hole pairs is dependent on the amount
of light striking the semiconductor, making the semiconductor suitable as an
optical detector. The P-N junction -usually has a large sensitive detecting
area that can be several hundred microns in diameter. The major advantage of
having large exposed area makes the alignment of fiber and the receiving device
much less critical than it is in optical transmitters. Besides, optical receivers
are equipped with high gain internal amplifiers to take care of quite small
light that exits a fiber.
PIN Diode
A PIN photo-diode is
a semiconductor P-N junction which operates on reverse biased voltage that is
the flow of current is maintained in a direction opposite to the conduction of
the diode. In this case, the diode acts as an open switch. When there is no
light incident on the diode, a current is always there which is known as dark
current.
Avalanche
Photo-diodes
An Avalanche
Photo-diode (APD) is a photo-diode that internally amplifies the current by an
avalanche process in which a very high voltage in the order of volts is applied
in the reverse direction of the conduction of the diode. This voltage initially
accelerates the electron generated by the incident photons and subsequently
they force other electrons in the semiconductor material to join them because
of heavy collision. This process is known as avalanche multiplication.
Avalanche multiplication continues to occur until the electrons move out of the
active area of the APD.
The factors affecting
the characteristics of these diodes are listed below:
Dynamic
range Dynamic range is the parameter which decides the range of optical
power levels over which the receiver operates within the specified values as in
case of digital transmission and analog transmission SNR are defined. It
usually is described by the ratio of the maximum input power to the
sensitivity.
Response
Time Response time depends upon factors such as the thickness of the
detector active area and the detector RC time constant. The thickness of P-N
junction decides the total time taken by electron to travel the junction. This
time is referred to as the electron transit time. The thicker the P-N junction
area, .the longer the transit time will be. The product of RC represents time
constant. The value of RC must be small for a better response. The capacitance
of the photo detector must be kept small to prevent the RC time constant from
limiting the response time. The photo-diode capacitance consists mainly of the
junction capacitance and any capacitance relating to packaging.
Linearity They
are highly linear devices and therefore, produce the output electrical current
in proportion to input light.
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