Download Active photonics devices

Active photonics devices
- detectors
Devices on optical tract that require a source
of energy for their operation and have an
output that is a function of present and past
input signals.
Examples include controlled power supplies,
transistors, photodiodes, receiver modules.
Copying and processing permitted for noncommercial purposes, on condition that
proper reference to the source is given.
© Sergiusz Patela, 2000-5
List of active photonic devices
1. Lasers and light emitting diodes
2. Detectors
3. Amplifiers
4. Modulators
5. Switches
6. Tunable filters
+ Passive devices
Σ. Integrated optoelectronic circuits
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Detector - definition
Person or thing that detects.
UV detectors, IR detectors, …
telecommunications detectors, …
Classification !
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Optical-to-electrical converters
O/E converters: A device used to convert optical signals to
electrical signals. Also known as OEC
Optical Input
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O/E Conversion
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Electrical Output
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Detectors – main classification
Quantum detector
A photodetector in which an electrical charge is produced
when incident photons change electrons within the
detecting material from nonconducting to conducting
states. E.g. photoconductive detector; photovoltaic cell.
Thermal detector
The functional process includes absorption of infrared
radiation, which causes a temperature change,
consequently altering the physical properties of the
detector's elements
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Detectors sensitivity
Thermal detector
Ideal quantum detector
1
Sensitivity [a.u.]
Actual quantum detector
0
0
200
400
600
800
1000
wavelength [nm]
1,24
E [eV ] =
λ [µm]
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Quantum efficiency
Photocurrent is proportional to the power of incident light:
I p = RPin
where R - sensitivity [A/W]
Quantum efficiency:
Ip q
number of generated electrons
hν
=
=
R
η=
number of incident photons
Pin hν
q
sensitivity:
R=
ηq ηλ
≈
,
hν 1,24
(ν = c / λ )
λ = [µm], q = 1,602*10-19 C,
c = 3*108 m/s, h = 6,62*10-34 J.s
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Absorption coefficient of semiconductor materials
10-1
105
GaAs
GaAs
102
Si
101
In.53Ga.47As
103
101
0,4
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0,6
0,8
1,0
1,2 1,4
wavelength [µm]
Photonics devices - Active photonics devices
1,6
102
Penetration depth [µm]
100
104
In.7Ga.3As.64P.36
Absorption coefficient [cm-1]
Ge
103
1,8
9
PIN photodiode
Anti-reflection
coating
Fiber cladding
Fiber core
Incident light
Metal contact (-)
P+ region
Intrinsic region
Electron
Hole
Electron - Hole
N+ region
Metal contact (+)
PIN photodiode is constructed from materials layered by
their positive, intrinsic, and negative electron regions
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APD photodiode
Fiber cladding
Fiber core
Anti-reflection
coating
Incident light
Metal contact (+)
N+ region
Electric field
P region
P+ region
Metal contact (-)
Avalanche Photodiode (APD): A photodiode that exhibits
internal amplification of photocurrent through avalanche
multiplication of carriers in the junction region.
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Other types of semiconductor detectors
Ring contact
p+
p+
i
i
n+
π
n+
contact n
PIN
APD
(Avalanche Photo Diode)
Semitransparent contact Au
contact (n)
Interdigital electrodes
+ - + - + -
n
n+
n+
SI (semi insulating)
Schotky diode
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SI
MSM
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Photodiode parameters
1. Sensitivity
2. Spectral response
3. Capacitance
4. Dark current
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Dark current
Dark Current:
The induced current that exists in a reversed-biased
photodiode in the absence of incident optical power.
Usually caused by the shunt resistance of the photodiode.
A bias voltage across the diode (and the shunt resistance)
causes current to flow in the absence of light.
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Receiver Sensitivity
Receiver Sensitivity:
The minimum acceptable value of received power needed to
achieve an acceptable BER or performance.
It takes into account power penalties caused by use of a
• transmitter with worst-case values of extinction ratio, jitter,
pulse rise times and fall times,
• optical return loss,
• receiver connector degradations, and
• measurement tolerances.
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BER
BER (Bit Error Rate): The fraction of bits transmitted that
are received incorrectly.
The bit error rate of a system can be estimated as follows:
 I2 
MN
BER = Q 

 4 N 0 B 
Where N0 = Noise power spectral density (A2/Hz).
IMIN = Minimum effective signal amplitude (Amps).
B = Bandwidth (Hz).
Q(x) = Cumulative distribution function (Gaussian distribution).
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Dispersion penalty
Dispersion Penalty: The result of dispersion in which pulses and
edges smear making it difficult for the receiver to distinguish
between ones and zeros. This results in a loss of receiver sensitivity
compared to a short fiber and measured in dB.
The equations for calculating dispersion penalty are as follows:
Where:
ω = Laser spectral width (nm)
Dλ = Fiber dispersion (ps/nm/km)
τ = System dispersion (ps/km)
f = Bandwidth-distance product of the
fiber (Hz • km)
L = Fiber length (km)
FF = Fiber bandwidth (Hz)
C = A constant equal to 0.5
FR = Receiver data rate (b/s)
dBL = Dispersion penalty (dB)
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Spectral characteristics of semiconductor
photodiodes
UV
VIS
NIR
0.7
Sensitivity [A/W]
0.6
0.5
0.4
Blue+
0.3
0.2
Standard silicon photodiode
UV+
0.1
0
200
300
400
500
600
700
800
900
1000 1100
Wavelength [nm]
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Detector sensitivity specified in dBm
Diody PIN
Power1E-7
Photomultiplier
500 nm,
Quantum eff. 25%
[W] 1E-8
APD
1E-9
SNR = 5
BER = 10-9
1E-10
1E-11
1E-12
1E-13
10k
100k
1M
10M
100M
1G
Frequency [Hz]
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Receiver module
Receiver: A terminal device that includes a detector and signal processing
electronics. It functions as an optical-to-electrical converter.
RECEIVER
TRANSMITTER
Electrical interface
Electrical interface
• Video
• Video
• Audio
• Audio
• Data
• Data
Data Encoder/
Modulator
Data Encoder/
Modulator
• AM
Optical Fiber
• AM
• FM
• FM
• Digital
• Digital
Light Emitter
Light Detector
• LED: 780nm,
850nm, 1300nm
• PIN Diode
• Laser: 1310nm,
1550nm, 1625nm
• Materials: Si, Ge,
In, GaAs
Electrical to optical conversion
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User
output(s)
• APD
Optical to electrical conversion
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