ELECTRO-OPTIC MODULATOR

Final Project

Başak Esin Köktürk 120206021

OPTİC MODULATOR TYPES

Electro-optic modulator Acousto-optic modulator Magneto-optic modulator

WHAT IS ELECTRO-OPTİC MODULATOR

An electro-optic modulator (EOM) is a device which can be used for controlling the power, phase or polarization of a laser beam with an electrical control signal.

Electro-optic effect is the modificatiın of refractive index of a nonlinear crystal by an external voltage.

POCKELS EFFECT AND KERR EFFECT

The Pockels effect is the linear electro-optic effect, where the refractive index of a medium is modified in proportion to the applied electric field strength.

The Kerr Effect or quadratic electro-optic effect is a change in refractive index of metarial refractive index with an electric field.

The difference between Pockels effect and Kerr effect; Pockels effect refractive index is changed proportionally with the square of electric field but in Kerr effect it changes with magnitude of the field.

POCKELS CELLS

A Pockels cell is a device consisting of an electro-optic crystal through which a light beam can propagate. The phase delay in the crystal can be modulated by applying a variable electric voltage.

Half-wave voltage (Vπ) is very important parameter for Pockels cells, half-wave voltage is the voltage value for requiring π phase shift.

Longitudinal devices

Transverse devices

Source : http://www.rp-photonics.com/img/pockels_cell_l.png

ELECTRO-OPTİC MODULATOR TYPES Phase Modulators

The simplest type of electro-optic modulator is a phase modulator containing only a Pockels cell, where an electric field changes the phase delay of a laser beam sent through the crystal

φ= n(E)koL=2πn(E)L/λo

Source : Uri Cummings , LINEARIZED AND HIGH FREQUENCY ELECTROOPTIC MODULATORS , CALIFORNIA INSTITUTE OF TECHNOLOGY, 2005

INTENSİTY MODULATORS Amplitude modulation with Pockels cell

and polarizers is based on Pockels cell modify the polarization state and polarizer convert this into change in transmitted optical amplitude and power

Using phase modulation in optical communication isn’t easy. Because photodetectors are only sensetive to intensity. Because of this phase modulation must be convert to intensity modulation. This can be made by using interferometer.

Source : http://www.rp-photonics.com/img/amplitude_modulator.png

MACH-ZEHNDER INTERFEROMETERS

The Mach–Zehnder interferometer benefit from the changing refractive index with different distance path.

Source : Uri Cummings , LINEARIZED AND HIGH FREQUENCY ELECTROOPTIC MODULATORS , CALIFORNIA INSTITUTE OF TECHNOLOGY, 2005

USAGE AREA OF ELECTRO-OPTIC MODULATORS

Modulating a laser beam ; e.g. laser printing,high-speed digital data recording,high-speed optical communication

In laser frequency stabilization shemes Q switching of solid-state lasers Active-mode locking Switching pulses in pulse pickers

IMPORTANT PROPERTIES OF ELECTRO-OPTIC MODULATORS BEFORE PURCHASING

There must be large open aperture Crystal length ,metarial and beam radius are

important where device is used for switching ultrashort pulses because of Kerr nonlinearity and chromatic dispersion

Depending on design an phase modulator can be generate unwanted amplitude modulations

In switching applications, unwanted ringing effect can be occured because of the material specifications,orientation and direction

The crystals should have anti-reflection coatings, designed for the required range of operation wavelength

This is an example for electro optic phase modulator

Part Number : EO-PM-NR-C3 * Weight : 0.75 kg

Price: € 2.032,10

Source : http://www.thorlabs.com/thorProduct.cfm?partNumber=EO-PM-NR-C3

OVERVIEW OF REFERENCE STUDIES

STUDY “Polarization-insensitive strip-loaded

waveguide for electro-optic modulators and switches” by Jie Sun,Changming Chen, Lei Gao, Xiaoqiang Sun,Weinan Gao,Chunsheng Ma and Daming Zhang pressed on Optics Communication 282(2009) 2255-2258.

A GLANCE TO STUDY

Study aim is design a polarisation-insensitive strip-loaded waveguide with thin EO film and SU-8 photoresist and demonstrate it.

In the study EO waveguide is introduced , needed metarials for design polarisation-insensitive strip-loaded waveguide with thin EO film and SU-8 photoresist are explained and design properties and fabrication procedures are presented

WHAT IS SU-8 PHOTORESİST?

SU-8 molecule

SU-8 is a commonly used epoxy-based negative photoresist.

SU-8 was originally developed as a photoresist for microelectronics industry, to provide a high-resolution mask for fabrication of semiconductor devices.

It is now mainly used in the fabrication of microfluidics and MEMS parts.

METARIALS AND THEIR PRODUCING STEPS

Organic and inorganic hybrid material is synthesized by sol-gel method.Sol-gel method is prefered because film fabrication is easy and it has high stability

The sol-gel TiO2 / SiO2 hybrid film is processed. However, fabricating this sol–gel TiO2/SiO2 film into a single-mode waveguide is difficult, because the material is too rigid to form thick film which is more than 2 lm. Cracks will appear if the thickness increases. To overcome this material limitation, the strip-loaded waveguide structure is devised.

SU-8 is purchased from MicroChem Corp. Choosen SU-8 has high sidewall quality,low curing temprature, simple operation procedure and appropriate film thickness.

DESİGN PROCEDURE

Normally in waveguides; core has bigger refractive index than cladding. Contrast to normal waveguide designs, in this study core divided two parts; EO film and SU-8. And also multilayer part divided four parts; the bottom cladding,the EO core,the strip-loaded rib and the top cladding.

When adjusting the waveguide size ; the effective refractive index of TE and TM modes are very close and this is good for polarisation-insensitive waveguide because of the similar propagation constant between the two modes.

In the study ; effective refractive index of TE and TM modes are 1.568 and 1.563 where the refractive index of hybrid material is 1.68

TE and TM modes are analyzed by conformal mapping theorem with using MATLAB and outputs are

Output mode profiles of the strip-loaded structure TM mode and TE mode.

FABRİCATİON PROCEDURE

MEASUREMENT

In order to measure the polarisation-insensitive characteristics, an M–Z interferometer intensity modulator based on this waveguide is fabricated and measured.

The upper signal is the voltage source and the lower is the intensity modulation response. The half-wave voltage Vπ is 7 V.

Thank You!