This document defines key terms and concepts related to photodetection for optical fiber communications. It discusses how photodetectors convert received optical signals to electrical signals and lists requirements for high performance. The main device types - PN photodiodes, are described. PN photodiodes work by generating electron-hole pairs when photons are absorbed in the depletion region, producing a photocurrent. Factors that determine a photodiode's response include absorption coefficient, quantum efficiency, and responsivity which is directly related to quantum efficiency. Materials properties also impact wavelength detection range.

1. 1
Photodetection –
Terms and Definitions
MEC

2. 2
Contents
• Introduction.
• Photodetector requirements.
• Device Types.
• PN Photodiodes.
• Absorption.
• Quantum Efficiency.
• Responsivity.
• Long-wavelength cutoff.

3. 3
Introduction
• Convert received optical signal to electrical
signal for amplification and further
processing.
• Signal attenuation along the link - system
performance determined at the detector.
• Improved detector characteristics &
performance - fewer repeaters, increased
repeater spacing, lower capital investment &
maintenance costs.

4. 4
Photodetector Requirements
• High sensitivity at operating wavelengths.
• High fidelity – linear response, faithful
reproduction.
• Large electrical response to received
optical signal.
• Short response time to obtain suitable
bandwidth.
• Low Noise – low dark currents, leakage
currents and shunt conductance.

5. 5
Photodetector Requirements
• Stability of performance characteristics.
• Compact – permits efficient coupling to
fiber.
• Low bias voltages/currents.
• High reliability – continuous stable
operation at room temperature.
• Economical – low cost.

6. 6
Device Types
• External and internal photoemission of
electrons.
• External photoemission devices - bulky,
high voltages operation – photomultiplier
tubes, vacuum photodiodes etc.
• Internal photoemission devices - good
performance and compatibility, relatively
low cost - semiconductor photodiodes
with/without internal (avalanche) gain.

7. 7
Device Types
• Made from semiconductors - silicon,
germanium and an increasing number of
III–V alloys.
• Internal photoemission process may take
place in both intrinsic and extrinsic
semiconductors.
• Intrinsic absorption - received photons
excite electrons from valence to
conduction bands.

8. 8
Device Types
• Extrinsic absorption involves impurity centers
created within the material.
• Intrinsic absorption for fast response and
efficient photon absorption
• Almost all detectors for optical fiber
communications use intrinsic photodetection.
• Silicon photodiodes - high sensitivity at 0.8 –
0.9 μm band, adequate speed (tens of GHz),
very low shunt conductance, low dark
current, long-term stability.

9. 9
Device Types
• Silicon - indirect bandgap energy = 1.14 eV,
loss in response above 1.09 μm.
• Germanium and III–V alloys - good response
at longer wavelengths 1.1 to 1.6 μm range.
• III–V alloys for operation at 1.3 and 1.55 μm.
• Material systems under investigation for use
at mid-infrared & far-infrared transmission (2
to 12 μm).

10. 10
Device Types
• Heterojunction phototransistor and
photoconductive detector can be
fabricated from III–V alloy materials.
• Photoconductive detector potentials for 1.1
to 1.6 μm range.
• Primary operating wavelengths of 0.8 to
0.9 μm, 1.3 μm and 1.55 μm; p–i–n and
avalanche photodiodes as major devices.

11. 11
P-N Photodiode

12. 12
P-N Photodiode
• Reverse biased.
• Electric field developed across p–n
junction sweeps mobile carriers (holes and
electrons) to their respective majority sides
(p- and n-type material).
• Depletion region created on either side of
the junction stops majority carriers
crossing the junction in opposite direction
to the field.

13. 13
P-N Photodiode
• Field accelerates minority carriers from
both sides to opposite side of the junction
– reverse leakage current.
• Intrinsic conditions created in the depletion
region.
• Incident photon in/near depletion region of
this device with energy hf ≥ Eg excite an
electron from valence band to conduction
band.

14. 14
P-N Photodiode
• Empty hole in valence band – photo
generation of electron–hole pair.
• Carrier pairs generated near the junction
separated, drift under electric field,
produce displacement by current in
external circuit in excess of reverse
leakage current.
• Thick depletion region allow large fraction
of incident light to be absorbed.

15. 15
P-N Photodiode
• Trade-off between number of photons
absorbed (sensitivity) and speed of
response.
• Long carrier drift times in depletion region
restrict speed of operation - limit its width.
d

16. 16
Absorption coefficient
• Absorption of photons in photodiode to produce
carrier pairs and photocurrent depends on
absorption coefficient.
• Photocurrent produced by incident light of
optical power Po :
e – electron charge, r - Fresnel reflection
coefficient at semiconductor - air interface,
d - width of absorption region, αo – absorption
coefficient.

17. 17
Absorption Coefficient
Dependency on
wavelength.

18. 18
Direct and Indirect Absorption

19. 19
Direct and Indirect Absorption -
Silicon
• Silicon and germanium absorb light by
direct and indirect optical transitions.
• Silicon weakly absorbing over 0.8 to 0.9
μm, transitions due only to indirect
absorption.
• Threshold for indirect absorption at 1.09
μm.
• Bandgap for direct absorption in silicon is
4.10 eV, corresponding to threshold of
0.30 μm in the ultraviolet region.

20. 20
Direct and Indirect Absorption –
Germanium
• Germanium - lowest energy absorption
takes place by indirect optical transitions.
• Threshold for direct absorption at 1.53 μm,
below which germanium becomes strongly
absorbing, indirect absorption up to a
threshold of 1.85 μm.
• Detectors at 0.8 to 1.6 μm range.
• Large dark currents due to narrow
bandgaps.

21. 21
Direct and Indirect Absorption –
III – V Alloys
• Direct bandgap III – V alloys for longer
wavelengths.
• Bandgaps can be tailored to desired
wavelength - change relative
concentrations of constituents.
• Lower dark currents.
• May be fabricated as heterojunction
structures – high speed operations.

22. 22
Quantum Efficiency
• Fraction of incident photons absorbed by
photodetector to generate electrons which
are collected at the detector terminals.
rp - incident photon rate, re – electron rate

23. 23
Quantum Efficiency
• Absorption coefficient influences quantum
efficiency.
• Function of photon wavelength, must be
quoted for specific wavelength.
• Less than unity - not all incident photons
are absorbed to create electron – hole
pairs.
• Expressed as a percentage.

24. 24
Responsivity
• Responsivity
Ip - output photocurrent, amperes, Po -
incident optical power, watts (fiber output
optical power).
Photon energy E = hf, incident photon rate

25. 25
Responsivity
Electron rate
Hence,
Therefore output photocurrent
Responsivity
Since Responsivity directly proportional to
quantum efficiency at a particular
wavelength.

26. 26
Responsivity
Silicon
Responsivity
drops rapidly
at cutoff
wavelength.
Typical
Photodiode
R = 0

27. 27
Responsivity
• Responsivity drops rapidly at cutoff
wavelength.
• Responsivity falls to zero as wavelength of
incident photon becomes longer, photon
energy eventually less than energy
required to excite electron from valence
band to conduction band.

28. 28
Long-wavelength cutoff
• Threshold for detection, applicable to
intrinsic photodetectors.
• Energy of incident photons to be greater
than or equal to bandgap energy Eg of
material used to fabricate photodetector.

29. 29
Thank You