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ENE 623 Optical Networks

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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ENE 623 Optical Networks

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  1. ENE 623 Optical Networks Lecture 7

  2. 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.

  3. 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.

  4. Modulators • Mach-Zehnder modulators (electro-optic modulators) • Electro-absorption modulators

  5. Phase Modulators

  6. Electrooptic Modulator (A) Directional coupler geometry (B) Mach-Zehnder configuration

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

  8. v Pi Po Mach-Zehnder modulator • Thus, by applying V will cause a phase shift for propagating mode.

  9. Mach-Zehnder modulator • By symmetry, equal amplitudes in 2 arms after passing through the first branch.

  10. 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

  11. Mach-Zehnder modulator • Wave amplitudes

  12. Mach-Zehnder modulator

  13. Mach-Zehnder modulator Pout = Pin Pout = 0 

  14. 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.

  15. 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.

  16. 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.

  17. 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).

  18. 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.

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