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ENE 623 Optical Networks. Lecture 7. Tunable Semiconductor Lasers. What determines lasing frequency: Gain spectrum A function of temperature. Optical length of cavity Mirror reflectance spectrum Any perturbation which affects refractive index and/or lasing frequency.

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tunable semiconductor lasers
Tunable Semiconductor Lasers
  • What determines lasing frequency:
    • Gain spectrum
      • A function of temperature.
    • Optical length of cavity
    • Mirror reflectance spectrum
    • Any perturbation which affects refractive index and/or lasing frequency.
single frequency laser
Single frequency laser
  • DFB and DBG lasers
    • Tuning achieved by changing heat sink temperature.
    • Tuning by changing bias current which affects the number of carriers in tuning region.
modulators
Modulators
  • Mach-Zehnder modulators (electro-optic modulators)
  • Electro-absorption modulators
electrooptic modulator
Electrooptic Modulator

(A) Directional coupler geometry

(B) Mach-Zehnder configuration

mach zehnder modulator
Mach-Zehnder modulator
  • Solve wave equation for mode field distribution & propagation constant.
      • where k = constant
mach zehnder modulator1

v

Pi

Po

Mach-Zehnder modulator
  • Thus, by applying V will cause a phase shift for propagating mode.
mach zehnder modulator2
Mach-Zehnder modulator
  • By symmetry, equal amplitudes in 2 arms after passing through the first branch.
mach zehnder modulator3
Mach-Zehnder modulator
  • For the second branch, output depends on relative phases of combining waves:
    • 2 waves in phase.
    • 2 waves  rad out of phase
mach zehnder modulator4
Mach-Zehnder modulator
  • Wave amplitudes
mach zehnder modulator6
Mach-Zehnder modulator

Pout = Pin

Pout = 0 

mach zehnder modulator7
Mach-Zehnder modulator
  • V is a swiching voltage which give Pout-rad phase difference.
  • V is determined by material and electrode configuration.
  • V is different for dissimilar polarizations.
diffused optical waveguides
Diffused optical waveguides
  • Diffused optical waveguides: Ti:LiNbO3 indiffused waveguides.
  • Waveguide modes (linearly polarized or ‘LP’):
    • TE mode – light polarized in plane of substrate surface
    • TM mode – light polarized normal to plane of substrate surface.
diffused optical waveguides1
Diffused optical waveguides
  • Ti indiffused waveguides: Ti metal atoms cause refractive index increase for both TE and TM waves.
  • Proton exchanged waveguides: H atoms exchange with Li atoms in lattice. Refractive index increases for only one polarization; e.g, TE mode.
diffused optical waveguides2
Diffused optical waveguides
  • For digital transmission, different V could degrade ‘on-off radio’ or OOR. Ideally, we want OOR to be close to infinity.
  • Solutions for that are:
    • Use polarized optical input.
    • Use proton exchanged waveguides to eliminate TM modes (get Pout only for TE mode).
example
Example
  • Consider a Mach-Zehnder modulator with an electrode length of 2 cm and electrode gap width g of 12 mm, such that

with E the applied electric field, assumed to be constant between the electrodes, and KTE = 5.8 x 10-10 m/V and KTM = 2.0 x 10-10 m/V. What is VTE and VTM ?

Note: neff = n0 + Δn in one arm and neff = n0 -Δn in the other arm.