The attached narrated power point presentation mentions the different types of noise produced by laser diode sources in a fiber optic communication link. The material will be useful for KTU final year B Tech students who prepare for the subject EC 405, Optical Communications.

1. 1
Noise in Laser Diodes
MEC

2. 2
Contents
o Noise Definitions.
o Noise in Fiber Optic Link.
o Laser Noise and its Classifications.
- Modal Noise/Speckle Noise.
- Mode Partition Noise.
- Reflection Noise.
o Impact of Noise.
o Noise Reduction.

3. 3
Noise
• Unwanted form of energy interfering with
intelligence.
• Deviation from an ideal signal.
• Associated with random processes.
• Corrupts information content and fidelity of
the signal.
• Severe impact at low signal levels.

4. 4
Noise Contributors
Additive noise terms represented by circles.
Multiplicative scale factors (efficiency terms) appear in the boxes.
All noise sources assumed to be Gaussian.

5. 5
Noise in Laser Diodes
 Two kinds of noise:
- Laser phase noise associated with
coherence effect.
- Noise associated with laser
optical power.

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Laser Phase Noise
• Laser phase noise caused by:
- small physical changes of
propagation conditions.
- shift in the spatial and temporal
output of light source caused by
modulation process.
- resembles like white noise.
- signal dependent.

7. 7
Noise due to Laser Emission
• Noise power increases with rising
optical power.
• Reaches maximum threshold.
• Decays at higher power levels.
• Shot Noise generated when light
converted to electrical signal.

8. 8
Mode Coupling
• Coupling of energy from one mode to
another.
• Due to imperfections, variations in fiber
diameter and refractive index, cabling
induced microbends.
• Increases pulse distortion.
• Tends to average out the propagation
delays associated with the modes,
reduces intermodal dispersion.

9. 9
Modal Noise
• A number of propagating modes excited
when coherent laser light launched into
the fiber.
• Constructive and destructive interference
between propagating modes at any given
phase.
• Speckle pattern seen at the end or any
point along the fiber if modes retain their
relative phase coherence.

10. 10
Modal Noise
• Number of speckles approximates the
number of propagating modes.
• Combination of mode dependent losses,
changes in phase between modes, energy
fluctuations among fiber modes to change
modal interference for light along the fiber
result in different speckle pattern.
• Also called speckle noise, occur when
speckle pattern dependent losses present
in the link.

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Modal Noise
• Losses at connectors, splices, microbends
and photodetectors with non-uniform
responsivity.
• Noise generated when speckle pattern
changes with time, falls on photodetector.
• Varies spectral power transmitted through
the loss element, degrades receiver
performance.

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Modal Noise
• Narrow band, high coherence sources (eg;
single mode lasers) produce more noise
than broadband sources.
• Incoherent sources (eg; LEDs) do not
produce modal noise.
• Reducing number of modes, single mode
fiber to eliminate the problem.

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Speckle Pattern
Produced when coherent light is
launched into multimode fiber.

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Mode Locking
• Use of a switch or absorbing region within
the cavity.
• Switch blocks light at all times except when
light pulse crosses it.
• Different longitudinal modes can have
different phases.
• If all modes are equiphase, a great narrow
pulse formed, transmitted by the switch.
• Other combinations of phases add to a
smaller amplitude and will be blocked.

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Mode Locking
Switch action is intensity dependent.
Gain Mechanism
Laser Bias Bias
Multi-quantum well
Saturable Absorber
Can be formed
with quantum
wells

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Mode Partition Noise
• Dominant in single mode fibers.
• Associated with intensity fluctuations in
longitudinal modes of a laser diode.
• Output from laser diode can come from
more than one longitudinal mode.
• Different modes or group of modes
dominate the optical output at different
times.

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Time Resolved Dynamic Spectrum
Different Modes dominate optical output at different times.
Mode Spacing

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Mode Partition Noise
• Output may arise from all modes
simultaneously or may switch from one
mode/group of modes to another.
• Intensity fluctuations likely among modes in
multimode lasers.
• Light from fluctuating modes coupled into the
fiber.
• Each longitudinal mode has a different
attenuation and time delay, each associated
with a slightly different wavelength.

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Mode Partition Noise
• Power fluctuations among different modes
can be quite large.
• Variations in signal levels at the receiver in
systems with high fiber dispersions.

20. 20
Reflection Noise
• Associated with laser diode output linearity
distortion.
• Distortion caused by part of the output
reflected back into laser cavity from fiber
joints.
• Reflected power couples with the lasing
modes, cause phase variations.
• Periodically modulated noise spectrum
peaked on the low frequency side of the
intrinsic noise profile produced.

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Reflection Noise
• Fundamental noise frequency determined
by round trip delay of laser light to the
reflecting point and back.
• Can create noise peaks in the frequency
region where data transmission systems
operate.
• Use of optical isolators, index matching
fluid at the joints to reduce noise.

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Relative Intensity Noise
• A number that aggregates the noise
contributions that can be contributed to the
laser diode.
• Ratio of variance of the intensity
fluctuations about the average to the
square of the instantaneous optical
intensity.
• Mean squared noise power divided by the
optical intensity squared.

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Thank You