Ch 6: Optical Sources

1
Ch 6 Optical Sources

• Variety of sources

• LS considerations

• Wavelength

• Dl

• Output power

• Modulation

• Coupling

• Cost/performance

• LED

• Operation

• Spontaneous emission

2
LED

• lhc/ePH

• Recent advances

• Porous silicon

• Semiconductor polymers

3
LED Light Emission
4
LED Coupling
5
LED Characteristics

• Low cost

• Low power 100 uW (recently 75 mW)

• Wide spectrum 50-100 nm

• Incoherent light neither directional nor coherent

• Digital modulation up to 300 Mbps

• Analog modulation simply

6
LD

• LASER Light Amplification by the Stimulated
  Emission of Radiation

• For stimulated emission to happen

• Material capable of st. emission (have a
  metastable high energy state

• Laser active media

• Laser action Absorption, spont. Emission,
  stimulated emission

• Population inversion pumping

7
LD

• Lasing requires

• Active medium

• Supplying energy

• Confinement

8
Fabry-Perot Laser

• LED with couple of mirrors

• Fabry-Perot resonator

• Operation

• Llx/2n

9
FPL
Modes
Spectral Characteristics
10
FPL
Gain threshold
11
DFB Laser

• Disadvantages of FPL

• Spectral width of 5-8 nm

• Mode hopping

• WDM

• DFB Operation

12
DFB Laser

• DFB Phase shifted grating

• DFB Spectral characteristics

13
DFB Characteristics

• Very narrow linewidths

• Low chirp

• Low relative intensity noise (RIN)

• Sensitive to reflections

• Temperature sensitive

• Output power fluctuations

• High cost

• Speed?

14
DFB

• Integrated Electro-Absorption Modulator

• Q-Switching

• Stability?

15
Distributed Bragg Reflector (DBR)

• More stable

• Less efficient

• Quantum Well Lasers (QW)

• When light is confined to an area l, it behaves
  like a particle

• Fundamental differences of SQW over non QW

• Reduction in lasing threshold

• Low output power

• No lateral modes

• Narrower linewidth

16
MQW/Tunable DBR

• MQW vs SQW

• Strained layer QW

• High power

• Construction

• Low threshold

• Advantages

• Breadth of materials

• Broader linewidth

• Tunability

• Tunable DBR Lasers

• 3 section tunable DBR

• Complex electronics

• 10 nm range

17
Tunable DBR

• Sampled grating TDBR

• 100 nm range

• External Cavity TDBR

• Wide range

• Slow

• Expensive

18
Frequency stabilized DBR

• External fiber cavity DBR

• Low cost

• Accurate wavelength control

• Narrow linewidth

• Temepreture control is shifted out

• Difficult positioning

• Coherence collapse operation

19
Vertical Cavity Surface Emitting Laser

• Low power

• Scarce of wavelengths

• MM and SM lasers

• Easy coupling

• Low threshold current

• Simple electronics

• High modulation BWs

• Very stable

20
In Fiber Lasers

• Characteristics

• High output power

• Low noise

• Tunability

• Very narrow linewidths

• Good soliton generation

• External modulation is required

• Preselected wavelength

• Mode hopping

• Upconversion (double pumping)

21
Comparison
22
Optical Amplifiers EDFA
Materials Praseodymium Neodymium
L Band CoDopants Longer fiber
2nd generation Gain control Gain equalization
Wavelengths 1480 980
23
EDFA Gain
24
EDFA
25
Raman Amplifiers

• Multistage