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OC 9 Optical Sources

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					                Ch 6: Optical Sources
   Variety of sources
   LS considerations:
       Wavelength
       Dl
       Output power
       Modulation
       Coupling
       Cost/performance
   LED
       Operation
       Spontaneous emission
                             LED




   l=hc/ePH
   Recent advances:
       Porous silicon
       Semiconductor polymers
LED: Light Emission
LED: Coupling
            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
                            LD
   LASER: Light                   For stimulated emission
    Amplification by the            to happen:
    Stimulated Emission of
                                       Material capable of st.
    Radiation                           emission (have a
   Laser active media                  metastable high energy
   Laser action: Absorption,           state
    spont. Emission,                   Population inversion:
                                        pumping
    stimulated emission
                           LD
   Lasing requires:
       Active medium
       Supplying energy
       Confinement
                Fabry-Perot Laser
   LED with couple of
    mirrors
   Fabry-Perot resonator
   Operation
   L=lx/2n
                           FPL
Modes




Spectral Characteristics
Gain threshold   FPL
                          DFB Laser
   Disadvantages of FPL:
       Spectral width of 5-8 nm
       Mode hopping
       WDM
   DFB: Operation
                      DFB Laser
   DFB: Phase shifted grating




   DFB: Spectral characteristics
            DFB Characteristics
 Very narrow linewidths
 Low chirp
 Low relative intensity noise (RIN)
 Sensitive to reflections
 Temperature sensitive
 Output power fluctuations
 High cost

 Speed?
                           DFB
   Integrated Electro-
    Absorption Modulator




   Q-Switching

   Stability?
        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
                     MQW/Tunable DBR
   MQW vs SQW                       Strained layer QW
       High power                       Construction
       Low threshold                    Advantages
       Broader linewidth                     Breadth of materials
                                              Tunability
   Tunable DBR Lasers
       3 section tunable DBR
           Complex electronics
           10 nm range
                       Tunable DBR
   Sampled grating TDBR
       100 nm range




   External Cavity TDBR
       Wide range
       Slow
       Expensive
             Frequency stabilized DBR
   External fiber cavity DBR
       Low cost
       Accurate wavelength control
       Narrow linewidth
       Temepreture control is
        shifted out
       Difficult positioning

   Coherence collapse operation
    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
                       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)
Comparison
    Optical Amplifiers: EDFA
  Materials:             L Band:
Praseodymium           CoDopants
 Neodymium             Longer fiber




                           2nd generation:
                            Gain control
                                Gain
                            equalization

      Wavelengths:
         1480
          980
EDFA: Gain
EDFA
                 Raman Amplifiers




   Multistage

				
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posted:3/28/2011
language:English
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