The attached narrated power point presentation attempts to explain the equivalent circuit of a digital optical fiber receiving system, the function of an equalizer and the different amplifier configurations in an optical receiver. The material will be useful for KTU final year B tech students who prepare for the subject EC 405, Optical Communications.

1. Optical Receivers
MEC

2. Contents
• Digital Optical Fiber Receiver.
• Equalizer.
• Amplifier Configurations
- Low-impedance Front-end.
- High-impedance Front-end.
- Transimpedance Front-end.
• Comparison

3. Digital Optical Fiber Receiver
• Optical detector represented as a current
source idet.
• Noise sources - it, iTS and iamp , amplifier
and equalizer follows.
• Equalization compensates for signal
distortion due to combined transmitter,
medium and receiver characteristics.

4. Digital Optical Fiber Receiver
Equivalent Circuit

5. Equalizer
• Equalizer a frequency-shaping filter,
frequency response inverse of overall
system frequency response.
• Boost high-frequency components, correct
overall amplitude of frequency response.
• To acquire desired spectral shape & to
minimize intersymbol interference, system
phase frequency response to be linear.

6. Equalizer
• Applies selective
phase shifts to
particular frequency
components.
• Minimize noise
contributions from the
sources, maximize
receiver sensitivity,
maintain a suitable
bandwidth.

7. Amplifier Configurations
• Basic amplifier
configurations:
- Low-impedance
front-end.
- High-impedance
(integrating) front-
end.
- Transimpedance
front-end.

8. Low-impedance Front-end Amplifier
• Voltage amplifier
with effective input
resistance Ra.
• Detector loaded with
a bias resistor Rb
and an amplifier.
• Bandwidth
determined by
passive impedance
across detector
terminals (RL).
Modified total load
resistance

9. Low-impedance Front-end Amplifier
• RL may be modified to incorporate the
parallel resistance of detector bias
resistor Rb and amplifier input resistance
Ra.
• To achieve optimum bandwidth, Rb and Ra
to be minimized - low-impedance front-end
design.
• Design allows thermal noise to dominate
within the receiver, limits sensitivity.

10. Low-impedance Front-end Amplifier
• Trade-off between bandwidth and
sensitivity.
• Impractical for long-haul, wideband optical
fiber communication systems.

11. High-impedance (integrating)
Front-end Amplifier
• High i/p impedance
amplifier together
with a large detector
bias resistor to
reduce the effect of
thermal noise.
• Degraded frequency
response, bandwidth
relationship not
maintained for
wideband operation.

12. High-impedance (integrating)
Front-end Amplifier
• Detector output effectively integrated over
a large time constant, must be restored by
differentiation.
• Correct equalization required.
• Improvement in sensitivity over low-
impedance front-end design, creates a
heavy demand for equalization, limited
dynamic range.

13. High-impedance (integrating)
Front-end Amplifier
• Limitations on dynamic range result from
attenuation of the low-frequency signal
components by equalization, causes
amplifier to saturate at high signal levels.
• Amplifier saturates before equalization,
signal is heavily distorted.
• Reduction in dynamic range depends on
the amount of integration and subsequent
equalization employed.

14. Transimpedance Front-end
Amplifier
• Overcomes drawbacks of high-impedance
front end.
• Low-noise, high-input-impedance amplifier
with negative feedback.
• Operates as current mode amplifier, high
input impedance reduced by negative
feedback.

15. Transimpedance Front-end
Amplifier

16. Transimpedance Front-end
Amplifier
• Open loop current to voltage transfer
function HOL (ω)
G - open loop voltage gain of the
amplifier, ω - angular frequency of input.

17. Transimpedance Front-end
Amplifier
• Closed loop current to voltage transfer
function HCL(ω) (Rf - feedback resistor):
• Permitted electrical bandwidth (without
equalization)

18. Transimpedance Front-end
Amplifier
• Greater bandwidth than amplifiers without
feedback.
• When Rf << RTL, major noise contribution
is from thermal noise generated in Rf.
• Noise performance improved when Rf is
large.
• When Rf = RTL, noise performance
approaches that of high-impedance front
end.

19. Transimpedance Front-end
Amplifier
• Rf cannot be increased indefinitely due to
problems of stability with closed loop
design.
• Increasing Rf reduces bandwidth.
• Make G as large as the stability of closed
loop permits.
• Noise in transimpedance amplifier exceed
that incurred by high-impedance front-end
structure.

20. Transimpedance Front-end
Amplifier
• Greater bandwidth without equalization
than high-impedance front-end, but offset
by 13 dB noise penalty incurred.
• Optimized for noise performance at the
expense of bandwidth.
• Improvement in noise performance over
low-impedance front-end structures.
• Greater dynamic range.

21. Transimpedance Front-end
Amplifier
• Different attenuation mechanism for low
frequency components of the signal.
• Attenuation through negative feedback, low
frequency components amplified by device
closed loop rather than open loop gain.
• Improvement in dynamic range equal to
ratio of open loop to closed loop gains.
• Used in wideband optical fiber
communication receivers.

22. Thank You