Chapter 7. Light Detectors

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Chapter 7.Light Detectors
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• Introduce
• External photoelectric effect Electrons are
  freed from the surface of a metal by the energy
  absorbed from an incident stream of photons.
• ex) vacuum photodiode, photomultiplier tube
• Internal photoelectric effect The detectors
  are semiconductor junction devices in which free
  charge carriers are generated by absorption of
  incoming photons
• ex) pn junction photodiode, PIN photodiode,
  avalanche photodiode
• Important detector properties are responsivity,
  spectral response, and rise time.

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• Responsivity
• The responsive ?is the ratio of the output
  current of the detector to its optic input power.
• The units of responsivity are amperes per watt.
• Spectral response
• The spectral response refers to the curve of
  detector responsivity as a function of
  wavelength.
• Rise time
• The rise time tr is the time for the detector
  output current to change to change from 10 to 90
  of its final value when the optic input power
  variation is a step.

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• Vacuum photodiode
• A bias voltage is applied, making the anode
  positive and the cathode negative. With no light,
  the current passing through the load resistor is
  zero and the output voltage is zero.
• When the cathode is irradiated with light
  incoming photons are absorbed, giving up their
  energies to electrons in the metal.
• Some of these electrons gain enough energy to
  escape from the cathode. These free electrons
  move toward the anode, attracted by its positive
  charge.
• Current flows through the circuit. When the
  electrons strike the anode, they combine with the
  positive charge and the circuit current stops.

Vacuum photodiode
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• Vacuum photodiode
• Work function To liberate a single electron
  from the cathode requires a minimum amount of
  energy.
• Denoting the work function by F, the condition
  for release of an electron is thus
• The lowest optic frequency that can be detected
  is f F/h. This corresponds to a wavelength of
  ?hc/ F.
• If the work function is given in electron volts,
  then the cutoff wavelength became

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• Vacuum photodiode
• Not every photon whose energy is greater than
  the work function will liberate an electron. This
  characteristic is described by the quantum
  efficiency ? of the emitter.
• The optic power is the energy per second being
  delivered to the detector and hf is the energy
  per photon, then P/hf is the number of photons
  per second striking the cathode.
• With quantum efficiency ? , the number of
  emitted electrons per second is then ? P/hf.

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• Vacuum photodiode
• This is the current that flows through the load
  resistor in the external circuit. The detector
  behaves as if it were a current source for the
  receiving circuit.
• The responsivity is now
• The output voltage is

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• Photomultiplier tube (PMT)
• PMT has much greater responsivity than does the
  photodiode because of an internal gain mechanism.
  Electrons emitted from the cathode are
  accelerated toward an electrode called a dynode.
• The first dynode attracts electrons because it
  is placed at a higher voltage than the cathode.
  The electrons hitting the dynode have high
  kinetic energies. They give up this energy,
  causing the release of electrons from the dynode.
  This process is called secondary emission.
• Each dynode must be at a higher voltage than the
  preceding one in order to attract the electrons.

Photomultiplier
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• Photomultiplier tube (PMT)
• The gain at each dynode is the number of
  secondary electrons released per incident
  electron.
• Let us follow the progress of a single
  photoemitted electron through the multiplier
  tude.
• If the gain at each dynode is d, then the number
  of electrons emerging from the first dynode is
  just d. The number of electrons in the tube after
  the second dynode is d2.
• When there are N dynode, the total gain is then
• The current through the external circuit is

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• pn diode
• When reverse bised, the potential energy barrier
  between the p and n region increases. Free
  electrons and free holes cannot climb the
  barrier, it is called the depletion region.
• An incident photon being absorbed in the
  junction after passing through the p layer. The
  absorbed energy raises a bound electron across
  the bandgap from the valence to the conduction
  band. A free hole is left in the valence to the
  conduction band.
• Free charge carriers are created by photon
  absorption in this manner. The electron will
  travel down the barrier, and the hole will travel
  up the barrier. These moving charges cause
  current flow through the external circuit.
• Typical pn diodes have rise time of the order of
  microseconds, making them unsuitable for
  high-rate fiber systems.

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pn diode
Semiconductor junction photodiode
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• PIN photodiode
• The PIN diode has a wide intrinsic semiconductor
  layer between the p and n regions.
• The intrinsic layer has no free charges, so its
  resistance is high, most of the diode voltage
  appears across it, and the electrical forces are
  strong within it.
• Because the intrinsic layer is so wide, there is
  a high probability that incoming photons will be
  absorbed in it rather than in the thin p or n
  regions.
• This improves the efficiency and the speed
  relative to the pn photodiode.

PIN photodiode
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• Cutoff wavelength
• To create an electron-hole pair, an incoming
  photon must have enough energy to raise an
  electron across the bandgap. This requirement,
  
• leads to a cutoff wavelength
• Materials
• Silicon is the most practical fiber optic
  detector. It cannot be used in the
  long-wavelength second window around 1.3 µm.
• Germanium and InGaAs diodes introduce more noise
  than silicon.

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• Material

Semiconductor PIN Photodiodes
Spectral response
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• Current-Voltage Characteristic
• The current-voltage characteristic curves for a
  silicon diode having responsivity 0.5 A/W are
  drawn in Fig.
• When reverse biased, the diode is said to
  operate in the photoconductive mode. In this mode
  the output current is proportional to the optic
  power.
• When no reverse bias is provided, the figure
  shows that incident optic power results in a
  forward voltage. This is the photovoltaic mode.
• Fiber communications detectors work in the
  photoconductive mode.

Current-voltage characteristic curves for a
silicon photodetetor.
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• Current-Voltage Characteristic
• Even when there is no optic power present, a
  small reverse current flows through a
  reverse-biased diode. This is called the dark
  current. Dark current is caused by the thermal
  generation of free charge carriers in the diode.
• It flows in all diodes, where it is
  conventionally called the reverse leakage
  current.
• Generally, dark currents are lowest in silicon
  detectors, somewhat larger in InGaAs diodes, and
  largest in germanium diodes.

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• Current-Voltage Characteristic
• The simplest PIN receiving circuit is drawn in
  Fig. The loop theorem (Kirch-hoffs voltage law)
  states that the sum of the voltage around a
  closed circuit must be zero.
• 20 V battery, 1 MO load resister, load line has
  a slope equal to -1/RL. It crosses the voltage
  axis at VB(-20V), and it crosses the current
  axis at VB/RL ( -20µA). A transfer
  characteristic, showing the output voltage v as a
  function of the input optic power, can easily be
  developed from Fig (b).

(a) Simple PIN circuit (b) Graphical analysis
of the circuit
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• Current-Voltage Characteristic
• Following table summarizes some of the
  calculations, and the transfer characteristic
  appear in Fig

PIN photodetection circuit transfer function
Calculating the transfer characteristic of a PIN
photodiode
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• Speed of Response
• The speed of response is limited by the transit
  time, the time it takes for free charges to
  traverse the depletion layer. The velocity of the
  free charge carriers is linearly proportional to
  the magnitude of the reverse voltage, so higher
  voltages reduce the transit time.
• This is approximately the photodiode rise time.
  Capacitance also limits the response.
• For example, Cd is mainly the junction
  capacitance. Following circuit reveals a 0-63
  rise time of RLCd and a 10-90 rise time of
• The corresponding 3-dB bandwidth can be
  calculated directly from the circuit.

Equivalent circuit of a PIN photodiode
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• Current-to-Voltage Converter
• The diode voltage diminishes when the optic
  power increases. This is because more current is
  flowing, increasing the voltage across the load
  resistor and leaving less of the battery voltage
  for the diode.
• The diode is connected to an operational
  amplifier with a feedback resister RF.
• 1. There is almost no voltage drop across the
  input terminals of high-gain operational
  amplifier.
• 2. There is almost no current flowing into the
  input terminals of the amplifier The entire diode
  current flows through the feedback resister RF.
  The voltage across this resister is RFid.

Current-to-voltage converter
Vertical load line seen by the diode In the
current-to-voltage converter
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• Avalanche photodiode (APD)
• The avalanche photodiode(APD) is a semiconductor
  junction detector that has internal gain. Having
  gain, the APD is similar to the photomultiplier
  tube.
• The gains that are available make APDs much more
  sensitive than PIN diodes.
• A photon is absorbed in the depletion region,
  creating a free electron and a free hole. The
  large electrical forces in the depletion region
  cause these charges to accelerate, gaining
  kinetic energy. When fast charges collide with
  neutral atoms, they create additional
  electron-hole.
• One accelerating charge can generate several new
  secondary charge. The second charge can
  accelerate and create even more electron-hole
  pairs.

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• Avalanche photodiode (APD)
• The accelerating forces must be strong to impart
  high kinetic energies. This is achieved with
  large reverse biases, several hundred volts in
  some instances.
• The gain increases with reverse bias vd
  according to the approximation
• where VBR is the diodes reverse breakdown
  voltage and n is an empirically determined
  parameter. Breakdown voltage of 20 to 500 V
  occur.
• The current generated by an APD with gain M is

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• Avalanche photodiode (APD)
• where ? is the quantum efficiency when the gain
  is unity.
• The responsivity is

Reach-through avalanche photodiode