Transcript Document

2 光源(Optical Sources)與檢光器
(Photodetectors)重要參數量測
• LED’s and lasers are important optical sources for
the electro-optical transformation in fiber optic
communications. They are essential for optical
transmitters.
• PIN photodiodes and APD are important optical
detectors for the opto-electronic transformation in
fiber optic communications. They are essential for
optical receivers.
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2.1 Light Emitting Diodes
• LED sources are low cost.
• LED sources can be used for short-distance fiberoptic systems.
Type LED’s are:
• 2.1.1 Surface-Emitting LED (SLED)
• 2.1.2 Edge-Emitting LED (EELED)
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2.1.1 Surface-Emitting LED
• A SLED can be a hetero-junction diode. It
can be made by low band gap materials
sandwiched between high band gap
materials.
• There are no mirrors to provide optical
resonance.
• A SLED can be used as a multimode source.
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2.1.1 Surface-Emitting LED
• The electrons and holes
recombine and lose
energy spontaneously
by emitting photons in
all directions
• In the SLED, light
wave comes out of the
diode from the surface
as shown in the figure.
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Active
Layer
Spontaneous
Emission
Photons
Spontaneous
Emission
Photons
Fig 2.1. Side view of a
surface-emitting LED
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2.1.2 Edge-Emitting LED
•
The EELED can be composed of two sections: a gain section and an
absorption section.
•
The gain section is forward biased to provide an optical gain.
•
The absorber section is reverse biased to provide an optical
absorption.
: AR(Antireflection) Coating
: Optical gain
: Optical absorption
Fig 2.2. Side-view of an edgeemitting LED (EELED)
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2.1.2 Edge-Emitting LED
• The absorption section prevents the optical
amplifier from lasing.
• The output of the gain section is
antireflection-coated.
• The gain section can be a semiconductor
optical amplifier(SOA). It produces
amplified spontaneous emission (ASE).
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2.2 Lasers
• Laser means “Light Amplification by Stimulated Emission
of Radiation.”
• A laser produces coherent light by stimulated emission. It
can be used in optical transmitters for telecommunications
and data communications.
Type of lasers:
• 2.2.1 Fabry-Perot laser (FP laser)
• 2.2.2 Distributed Feedback laser (DFB laser)
• 2.2.3 Vertical Cavity Surface-Emitting laser
(VCSEL laser)
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2.2.1 Fabry-Perot Lasers
• The Fabry-Perot laser is composed of semiconductor
amplifier and reflective mirrors.
• It is low cost.
• It has substantial spectral width and can be used with
low chromatic-dispersion fibers.
Mirror
Mirror
: Current Input & Output Layer
: Gain Region & Optical Waveguide
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Fig 2.3 Cross-section of a
Fabry-Perot laser diode
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2.2.1 Fabry-Perot Lasers
40
Power (mW)
• The optical amplifier gain
increases as the current
increases.
• Lasing occurs if the
amplifier gain is equal to or
large than the mirror loss.
• The threshold current,
slope efficiency, and
spectral width are basic
parameters.
T h r e s h o ld
C u rre n t
0
0
C u rre n t (m A )
200
Fig 2.4 Light vs. Current
characteristic
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2.2.1 Fabry-Perot Lasers
• The gain, the mirror loss, and
the location of longitudinal
modes of the laser are shown in
the Fig 5 as functions of
wavelength.
• The spacing between the
possible lasing wavelengths
(mode spacing) is determined
Fig 2.5 The gain, mirror
by the distance between the
loss,and longitudinal mode
two mirrors.
location.
A m p lifie r G a in (d B )
15
M irro r L o s s (d B )
15
M ode
S p a c in g
0
1530
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0
W a v e le n g th (n m )
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2.2.2 Distributed Feedback
Lasers(DFBs)
• The DFB laser is similar to an FP laser with the
addition of a Bragg reflector structure located near
the light-emitting region.
Metal Layer
Bragg Reflector
Output
Active Layer
Metal Layer
Fig 2.6 Cross-section of a
DFB laser
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2.2.2 Distributed Feedback
Lasers(DFBs)
• The Bragg grating can be made by introducing
a periodic change in the index of the refraction
in the waveguide.
• Each period of the grating reflects a small
amount of light back in the opposite direction.
• The Bragg grating has a high reflectivity at the
wavelength where the grating period is one-half
of the wavelength of light in the semiconductor
material. It can be used as a mirror.
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2.2.2 Distributed Feedback
Lasers(DFBs)
• The reflection pass band in a
typical Bragg grating is only
a few nanometer wide.
• This frequency-dependent
reflectivity makes the laser
power to emit in a single
longitudinal mode
20
20
Amplifier Gain (dB)
0
1530
0
W a v e le n g th (n m )
1560
Fig 2.7 Bragg grating reflectivity,
amplifier gain,and longitudinal
mode location.
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2.2.3 Vertical Cavity SurfaceEmitting Laser(VCSEL)
Bragg
Mirror
10
Isolation
Active
Layer
Bragg
Mirror
Substrate
• Vertical cavity lasers emit
perpendicular to the top
plane of semiconductor
wafer as shown in Fig 8.
• The VCSEL uses a
multilayer dielectric
Bragg mirror that is grown
directly on the
semiconductor surface.
Fig 2.8 Cross-section view of a
VCSEL
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2.2.3 Vertical Cavity SurfaceEmitting Laser(VCSEL)
• The length of optical
amplifier in this VCSEL is
short (on the order of 100
nm).
• This short amplifier length
limits the available gain from
the amplifier to a small value.
Fig 2.9 Mirror loss,amplifier
gain,and longitudinal mode
location
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2.2.3 Vertical Cavity SurfaceEmitting Laser(VCSEL)
• A wider spectral width is often designed into
VCSEL to avoid mode-selective loss in
multimode fiber applications.
• The VCSEL is low cost.
• The VCSEL has higher output power and
higher modulation rates than surface-emitting
LED.
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2.3 Source Measurements
Type
Data Rate
CW Spectral Width
Power(mW)
FP laser
to 10 Gb/s
5 nm, multiple line
2-100, single mode
DFB laser to 10 Gb/s
10 MHz, single line
0.2-20, single mode
VCSEL
to 5 Gb/s
1 GHz, multiple line
0.1, multiple mode
ELED
155 Mb/s
5 THz, broadband
0.1, single mode
SLED
622 Mb/s
10 THz, broadband
0.1, multiple mode
[from “Fiber Optic Test and Measurement”, edited by D. Derickson]
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2.3 Source Measurements
•Using the OSA, we can obtain the optical spectrum of an
optical source such as a LED or a laser.
• The threshold current and slope efficiency of lasers can be
obtained from the curve of power vs. pumping current,
which can be measured by a power meter.
• Linewidth and chirp measurement will be discussed latter
OSA
Optical Source
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2.4 p-i-n Photodiodes
• The optical receiver detects the
lightwave signal and then
conditions the resulting electrical
signal to the appropriate levels.
• Photodiodes are used for optical to
electrical conversion.
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2.4 p-i-n Photodiodes
• The hetero-junction p-i-n
diodes consists mainly of three
layers of semiconductors with
different band gaps and
different doping concentrations.
• A low-bandgap material is
usually used as the i (intrinsic)
layer that absorbs incoming
photons.
I
P in
Electrical
Contact
out
Light
Absorption
Layer
P (InP)
i (InGaAs)
n (InP)
n substrate
Fig 2.10 p-i-n photodetector diagram
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2.4 p-i-n Photodiodes
• The p-i-n photodiode can have a high electric
field across the intrinsic layer.
• Electron and hole pairs are created in the layer
if photons are absorbed.
• The electric field sweeps these carriers out to
an external electrical circuit.
• An InGaAs undoped layer surrounded by
doped p and n InP material can be used to make
a p-i-n photodiodes.
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2.5 APD Detectors
• A diagram of an avalanche
Electrical
I
P in
out
photodector(APD) is shown in Contact
Fig 11.
Light
• It can improve the sensitivity
Absorption
n (InGaAs)
Layer
compared to a p-i-n Multiplication
n (InP)
Layer
photodiode.
p+ (InP)
• The APD provides low-noise,
p+ substrate
high-bandwidth, and photoFig 2.11 APD diagram with
current gain.
separate absorption and
multiplication regions.
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2.5 APD Detectors
• The APD use a low-bandgap
semiconductor absorption
region.
• The photo-generated
electrons are accelerated to
high velocities and lead to
avalanche process.
• The high energy electrons
collide with the lattice to
generate new free carriers in
avalanche processes.
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• The ideal low-noise APD
multiplies the number of
electrons but does not
multiply holes in the
avalanche process.
• The APD require a high-bias
voltage to produce avalanche
conditions.
• The avalanche process is
also temperature-dependent.
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2.6 Detector Measurements
• The detector response can be measured by using light
sources of different wavelengths and an ammeter.
Pi (λ )
OUT
I0
A
input
optical
power
• Responsivity R=current (I0) / input optical power(Pi)
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2.6 Detector Measurements
• The dark current Idark is the current that flows
through the detector in the absence of light.
• The shot noise is given by 2e( Idark + Isignal).
• The noise equivalent power (NEP) is the amount
of light at a given wavelength that is equivalent to
the noise level of the detector.The noise can be
obtained from the spectrum measured by an
electrical spectrum analyzer.
• The time domain response can be obtained if the
ammeter is replaced by an oscilloscope.
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