Optoelectronics (2016/2) W.-Y. Choi

Lect. 16: Planar Waveguide Devices

Issues for practical waveguides

- Precise control of dimension and refractive index - Low loss at desired - Mass production possible

Materials used for waveguides

- Silica with Ge doping Optical fiber

- Integration desirable (Integrated Optics)- Electrical control of refractive index (Electro-Optic effect)

- Dielectric materials: LiNbO3 (Lithium Niobate)- Semiconductor-based: Si/SiO2

Optoelectronics (2016/2) W.-Y. Choi

Lect. 16: Planar Waveguide Devices

LiNbO3 waveguide

V(t)

Ea

Cross-section

LiNbO3

d

Thin buffer layerCoplanar strip electrodes

EO Substratez

y

x

Polarizedinputlight

WaveguideLiNbO 3

L

Integrated tranverse Pockels cell phase modulator in which a waveguide is diffusedinto an electro-optic (EO) substrate. Coplanar strip electrodes apply a transversefield Ea through the waveguide. The substrate is an x-cut LiNbO3 and typically thereis a thin dielectric buffer layer (e.g. ~200 nm thick SiO2) between the surfaceelectrodes and the substrate to separate the electrodes away from the waveguide.?1999 S.O. Kasap, Optoelectronics (Prentice Hall)

- Waveguide Core formation by Ti doping- Core refractive index modulation by V(t): E-field enhances refractive index

Phase modulation of incident light

Optoelectronics (2016/2) W.-Y. Choi

Lect. 16: Planar Waveguide Devices

Mach-Zehnder Interferometer:

sideI

bottomI1l

2l

2 1 2 1 2 12 1

1 2 1 2 1 21 2

2 2 2 2,

2 1 2,

2 2 2,

2 1 2,

1 1 e e2 2

sin2

2 2

cos2

l l l l l ljk jk jkjkl j klout side

out side

l l l l l ljk jk jkjkl jklout bottom

out bottom

E e e e

l lI k

j jE e e e e e

l lI k

Realize M-Z interferometer with wave devices

Optoelectronics (2016/2) W.-Y. Choi

Lect. 16: Planar Waveguide Devices

V(t)

LiNbO3 EO Substrate

AB

InOutC

DA

B

Waveguide

Electrode

An integrated Mach-Zender optical intensity modulator. The input light issplit into two coherent waves A and B, which are phase shifted by theapplied voltage, and then the two are combined again at the output.?1999 S.O. Kasap, Optoelectronics (Prentice Hall)

Mach-Zehnder Modulator (MZM)

With voltage applied, light travelling in two different arms experiences different amount of phase shift

LiNbO3 MZM is the standard optical modulator for long distance optical communication

Optoelectronics (2016/2) W.-Y. Choi

Lect. 16: Planar Waveguide Devices

Waveguides based on Si technology (Si Photonics)

- Si is transparent to 1.5m light- Si fabrication technology is very mature- Si photonics provides possibility of

integrating photonics and electronics on Si wafer

Optoelectronics (2016/2) W.-Y. Choi

Lect. 16: Planar Waveguide Devices

• Ring Resonator

Input Light Output Light Wavelength

Optical Power

λres

Optoelectronics (2016/2) W.-Y. Choi

Lect. 16: Planar Waveguide Devices

α

γ

γ

1

jκEin Eout

• Through coefficient:   (ratio for through light)• Coupling coefficient:  (field ratio of coupled light at the coupler)• Loss coefficient: α (field ratio after one round‐trip)

2 2 2 2 2 3 2 3j j jout inE E e e e

0effn k L

2 1

1

n n jnin

nE e

Optoelectronics (2016/2) W.-Y. Choi

Lect. 16: Planar Waveguide Devices

2 1

1

n n jnout

nin

Ee

E

2

out out

in in

P EP E

2

1

j

j

ee

1

j

j

ee

2 2

2

2 cos( )1 ( ) 2 cos( ) 0( )effn k L

Output Power

It can be shown min. transmission occurs when 2m

2

21out

in

PP

Wavelength-domain filter

res effm n L Resonance condition:

Optoelectronics (2016/2) W.-Y. Choi

Lect. 16: Planar Waveguide Devices

Homework: (See the figure next page. Assume Pout has cos2 dependence on Pin)

Optoelectronics (2016/2) W.-Y. Choi

Lect. 16: Planar Waveguide Devices

Homework: