12-11-2012, 12:27 PM
OPTICAL AMPLIFIERS
[attachment=39067]
INTRODUCTION
Even though Optical communication system has low noise and attenuation the signal will be degraded as it travels more than 100 kms so we use power boosters in between Rxr & Txr. They are:
Regenerative Repeaters
Optical Amplifiers
REGENRATIVE REPEATERS:
It detects an optical signal and then retransmits it as a new signal using an internal laser.
Needs Conversion of signal from optical domain to electrical and vice versa so it is time consuming.
Loss is high.
Restricts operating BW.
Low Signal Quality.
Bit rate specific and modulation format specific.
It requires source , detector, pulse slicing, shaping and timing circuits.
OPTICAL AMPLIFIERS:
It is basically a laser with low feedback (medium gain) whose excited carriers amplify an incident signal but do not generate their own coherent signal.
They are used to compensate signal attenuation.
They give gain but introduce some noise also.
The amplifier requires a power source also
The power given is an optical source will excite the electrons to higher levels which will decay and emit a photon at input signal frequency.
Regarding an OA, its location, input parameters, its noise factor and its design are of importance.
Advantages of OA:
It is insensitive to bit rate, modulation formats, power and wavelength of the signal passing through the amplifier.
If amplifier is linear, then WDM is allowed.
No conversion.
Cheaper.
More reliable.
Bidirectional.
Disadvantages of OA:
There are only finite number of excited carriers. So output power is limited before gain diminishes.
Gain spectrum is not flat over the entire range of signal transmitted.
Additive noise-degradation in receiver sensitivity.
Fiber dispersion and non linear effects accumulate.
Application of SLA:
Long distance communication at 1.3 micrometer has zero dispersion loss but long distance needs periodic amplification.
SLA is small compared to FA. Also cost is less and integrability to the chip containing many optoelectronic devices is easier.
SA can be used as high speed switching element in photonic system and semiconductor will amplify if pumped and absorb if unpumped. FA has high lifetime.
SLA can be used as wavelength shifters and filters.
SLA can be used to tap optical energy from a signal passing through an amplifier. In an unpumped SLA material will absorb part or all of incoming light and generate e hole pairs and hence current. This partial detection can be used to tap off some control info in data packets.