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Active photonics devices
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© 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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Waveguide light sources - classification
Light emitting diodes (LED)
• surface emitting diodes
• edge emitting diodes
• RCE (resonance cavity enhanced) LEDs
Lasery (LD)
• FP (Fabry-Perota) lasers
•DFB (distributed feedback) and DBR (distributed Bragg reflector) lasers
•VCSEL (vertical cavity surface emitting) lasers
• waveguide lasers
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LED construction
125 µm
multimode
optical fiber
negative
contact
epoxy
adhesive
N-doped GaAs
substrate
P-doped
GaAs
light
emitting
region
positive contact
and heat sink
SiO2
50 µm
150 µm
surface-emitting LED
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Laser - definition
Acronym for light amplification by stimulated emission of
radiation.
A device that produces a beam of light by stimulating atoms, ions
or electrons to higher energy levels, so that when they return to
lower levels they emit energy (light).
Light – in a broad sense, for UV to X-rays
Return to lower energy levels – through stimulated emission of
radiation
The beam is collimated, coherent and monochromatic
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Laser - principle of operation
1. Presence of metastable states (ms)
2. Energy pumping (to put atom into ms)
3. Population inversion
4. Stimulated emission
5. Optical feedback (FB)
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Population inversion
absorption, n1 > n2
population
amplification, n2 > n1
n2
population
n1
n2
n1
(stimulated emission)
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Optical feedback - Fabry-Perot resonator
Optical pump
M2
M1
Laser
beam
Optical pump
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Lasers - classification
1. Solid state lasers: lasing medium plased in a soild state
matrix. Examples: neodymium laser-YAG <Yttrium Aluminum
Garnet> 1,064 um, ruby laser
2. Gas lasers
• Atomic He-Ne, 632,8 nm
• Molecular CO2 , 10,6 µm
• Ion Ar+, main wavelengths 488, 514 nm
• Excimer (ionized fluorides of noble gases) UV sources
3. Dye lasers (liquid) Liquid solution of organic dye. Possible
wavelength tuning (VIS, NIR). Tuning range depends on
selected dye– for Rodamine 6G ! 0.570-0.650 nm.
4. Semiconductor lasers (LD)
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Pumping in semiconductor lasers:
p-n junction current
n type
junction
p type
n type
Eg
Valence band
hν
Valence band
-
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p type
Conduction band
Electron energy
Conduction band
junction
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Simple semiconductor laser (homostructure)
Input current
Cleaved
mirror
Junction
Output
radiation
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Heterostructure laser (cross section)
P
i
N
N
n
Buried heterostructure (BH) laser
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Astigmatism of semiconductor laser beam
λ /w
l
λ /l
w
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Semiconductor laser types
1. Homostructure lasers, Threshold current (300K) 30000-50000 A/cm2
2. Single heterostructure. (300K) 6000-8000 A/cm2
3. Double heterostructure (300K) 500 A/cm2.
4. GRINSCH (Graded-index separate confinement heterostructure),
threshold current ~30mA
5. Mirror type: FP, DFB, DBR
6. VCSEL (Vertical Cavity Surface Emitting Laser), threshold current
~1mA
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Bragg reflection
d
d
d
θ
θ
d sin θ
2 d sin θ = ν λ, ν = 1, 2, 3, ...
Bragg grating work like reflector if θ = 90o
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DFB Laser
Distributed Feedback Laser (DFB): An injection laser diode
which has a Bragg reflection grating in the active region in
order to suppress multiple longitudinal modes and enhance a
single longitudinal mode.
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DFB laser schematic
Λ
3 µm
3 µm
GaAs
Al 0.3 Ga0.7 As
DFB (distributed feedback)
substrate GaAs
Bragg condition:
2 Λ = ν λ, ν = 1, 2, 3, ...
where: λ = λo/nświatłow
efficient reflection only for λo = 2 Λ nświatłow/ν
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Beam profiles of VCSEL and edge-emitting lasers
Elliptical beam of edge-emitting laser
Cylindrical beam of VCSEL
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VCSEL Laser
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Semiconductor laser applications
•Communications
•Printing, polygraphy
•Material processing
•Metrology, research
•Data storage (CD-ROM)
•Optical pumping
•Medicine
•Pointers, bar code scanners, leveling, geodesy
•Prototyping
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Characteristics of semiconductor CW laser diodes
5
2.0
1.0
2
0.5
intensity
3
Power [W]
4
1.5
Voltage [V]
1.0
0.5
FWHM = 2 nm
1
0.0
0
0
2
4
6
0.0
800
Current [A]
805
810
wavelength [nm]
815
Intensity
1.0
Perpend. 40 deg.
0.5
Parallel 10 deg.
0.0
-60 -60 -60 -60 -60 -60 -60
Angle (degrees)
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Laser parameters
Center wavelength
FWHM, spectral line spread
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Center wavelength and FWHM
FWHM
% of Spectral Emission
100
Center Wavelength
75
50
25
1292
1296
1300
1304
1308
Wavelength Range (nm)
Center Wavelength: In a laser, the nominal value central operating
wavelength. It is the wavelength defined by a peak mode
measurement where the effective optical power resides (see
illustration). In an LED, the average of the two wavelengths
measured at the half amplitude points of the power spectrum.
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Semiconductor laser chirp
10
5
8
Light output (mW)
4
3
2
0
2
-2
-4
1
Frequency Shift (Ghz)
6
4
-6
-8
-10
0
10
20
30
40
50
Drive current (mA)
Chirp: In laser diodes, the shift of the laser’s center wavelength
during single pulse duration.
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Optical telecommunications source module
Marconi (discontinued) STM-1opt MLM Source Type
-8,5
-14,5
4
18,5
0
-34
0
1
96
1280
1335
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dBm
dBm
nm
dB
dB
dBm
dBm
dB
ps/nm
nm
nm
Mean Launch Power Maximum
Mean Launch Power Minimum
Source Width
EOL S-R Max (1e-10)
EOL S-R Min
Minimum Sensitivity
Overload
Path Penalty
Maximum Dispersion
Wavelength Minimum
Wavelength Maximum
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LD package construction
Base, package
Isolator holder
Lens holder
laser
cover
Mooving adjustment plate
lens
Fiber holder
isolator
Fiber placement
laser
lens
isolator
fiber
solder
Fiber cover
H. van Tongeren, et al., IEEE Transactions on Components, Packaging and Manufacturing Technology - Part , vol. 18, (1995) 227.
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9mm and TO18 packages
TO-18
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C-mount package
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FCA package
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Transmitter
Transmitter: A device that includes a source and driving electronics.
It functions as an electrical-to-optical converter.
RECEIVER
TRANSMITTER
User
input(s)
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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Transmitter parameters
Extinction Ratio: The ratio of the low, or OFF optical power
level (PL) to the high, or ON optical power level (PH): Extinction
Ratio (%) = (PL/PH) x 100
Jitter: Small and rapid variations in the timing of a waveform
due to noise, changes in component characteristics, supply
voltages, imperfect synchronizing circuits, etc.
Rise Time: The time taken to make a transition from one state to
another, usually measured between the 10% and 90% completion
points of the transition.
Fall Time: Also called turn-off time. The time required for the
trailing edge of a pulse to fall from 90% to 10% of its amplitude
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Laser Classifications (Safety Classes)
• Class I - lasers cannot emit laser radiation at known hazard levels.
• Class I.A. - special designation that applies only to lasers that are "not
intended for viewing," such as a supermarket laser scanner. The upper
power limit of Class I.A. is 4.0 mW.
• Class II - low-power visible lasers that emit above Class I levels but at a
radiant power not above 1 mW. The concept is that the human aversion
reaction to bright light will protect a person.
• Class IIIA - intermediate-power lasers (cw: 1-5 mW), which are
hazardous only for intrabeam viewing. Most pen-like pointing lasers are
in this class.
• Class IIIB - These are moderate-power lasers.
• Class IV - high-power lasers (cw: 500 mW, pulsed: 10 J/cm2), which are
hazardous to view under any condition (directly or diffusely scattered),
and are a potential fire hazard and a skin hazard.
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