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.

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 Mach-Zehnder modulators (electro-optic modulators) Electro-absorption modulators

Phase Modulators

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

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

Mach-Zehnder modulator Thus, by applying V will cause a phase shift for propagating mode. v PiPi PoPo

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

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 modulator Wave amplitudes

Mach-Zehnder modulator

P out = P in  P out = 0 

Mach-Zehnder modulator V  is a swiching voltage which give P out  -rad phase difference. V  is determined by material and electrode configuration. V  is different for dissimilar polarizations.

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 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 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 P out only for TE mode).

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 K TE = 5.8 x m/V and K TM = 2.0 x m/V. What is V  TE and V  TM ? Note: n eff = n 0 + Δn in one arm and n eff = n 0 - Δn in the other arm.