external modulators

Lithium Niobate

External Modulators

-By SURESH CHAND JAT

2015PWC5398

WOC(M.Tech)

MNIT JAIPUR

Supervisor

Dr. M. Ravi Kumar

Assistant Professor

Dept. of ECE

MNIT JaipurDate:31-03-2016

Content-

Introduction to Optical Modulation

Direct Modulation

External Modulation

Lithium Niobate External Modulators

Pockel Effect

Directional Coupler

MZI Coupler & Modulator

Optical Modulation

The process of imposing data on light stream is called modulation.

The simplest and most widely used modulation scheme called On

Off Keying (OOK) Where light stream turn on and off based on the

data bits.

Two way of the OOK modulation-

1. Direct Modulation

2. Using External Modulators

Direct Modulation

The message signal is superimposed on the bias current

(dc) which modulates the laser

Robust and simple, hence widely used

Issues: chirp, turn on delay, clipping and laser nonlinearity

Direct Modulation-

External Modulation

Change the transmission characteristics

Change the power of a continuous wave laser

This is of two types-

1. Electro-optical(EO) modulation(using Lithium Niobate

Modulators) (low efficiency)

2.. Electro-absorption (EA) modulation (smaller modulation

bandwidth).

Lithium Niobate External Modulators

The Lithium Niobate modulators makes use of the electro-

optic effect, where an applied voltage induces a change in

refractive index of the material.

The device itself configured as a directional coupler or as

a Mach-Zehnder Interferometer (MZI).

Pockel Effect

The Pockel effect is the linear electro-optic

effect, where the refractive index of a medium is

modified in proportion to the applied electric

field strength.

Pockel Effect in LiNbO3

Suppose x, y and z are principal axes of a crystal with refractive indices n1, n2 and n3 along these directions

For an optically isotropic crystal, these would be the same

For a uniaxial crystal n1= n2 n3

Apply a voltage across a crystal and thereby apply an external dc field Ea along z-axis

In Pockels effect, the field will modify the optical indicatrix.

The exact effect depends on the crystal structure


(a) Cross section of optical indicatrix with no applied field n1=n2=n0

(b) Applied field along Y in LiNbO3 modified the indicatrix and changes n1

and n2 to n’1 and n’2


In the case lithium niobate (uniaxial crystal), a field Ea is applied along the y-direction

It does not significantly rotate the principal axes

changes the principal refractive indices n1 & n2 (both equal to no) to n1’ & n2’ as shown in fig (b)

Consider a wave propagating along the z-direction (optic axis) in the crystal

Before a field Ea is applied, this wave experience n1=n2=nowhatever in the polarization as fig (a)

In the presence of an applied field Ea, the light propagates as two orthogonally polarized waves (parallel to x and y) experiencing different refractive indices n1’ & n2’


The applied field thus induces a birefringence for light

traveling along the z-axis.

The field induced rotation of principal axes is neglected.

The Pockel effect gives the new refractive indices n1’ &

n2’ in the presence of Ea as

n1 ’ n1 + ½ n13 r22 Ea & n2 ’ n2 – ½ n2

3 r22 Ea

where r22 is a constant, called a Pockel coefficient that depends

on the crystal structure and the material.

Directional Coupler

In Directional coupler configuration by applying a voltage(Modulating signal or data

bits) to the coupling region changes its refractive index, which in turns determines

how much power is coupled from the input waveguide 1 to the output waveguide 1.

Shown in figure-

Mach-Zehnder Interferometer (MZI) Coupler

A Mach-Zehnder Interferometer (MZI) is an interferometric device

that makes use of two interfering paths of different lengths to resolve

different wavelengths.

MZI typically constructed in integrated optics consists of two 3dB

couplers interconnect through two paths of different lengths.

MZI Modulator

In optical switching, induced phase shift by applied voltage can be

converted to an amplitude variation by using an interferometer

Interferometer is a device that interferes two waves of the same

frequency but different phase

Compared to Directional coupler MZI offers a higher modulation

speed for a given drive voltage and provides a higher extinction

ratio.

MZI Modulator

MZI Modulator

Consider the structure shown in Fig, which has implanted single mode waveguide in a LiNbO3 substrate in the geometry.

The waveguide at the input braches out at C to two arms A and B

These arms are later combined at D to constitute the output

The splitting at C and combining at D involve a simple Y-junction waveguides

In the ideal case, the power is equally split at C so that the field is scaled into each arm

The structure acts as an interferometer because the two waves traveling through the arm A and B interfere at the output port D

The output amplitude depends on the phase difference (optical path difference) between A and B branches

MZI Modulator

Two back-to-back identical phase modulatorsenable the phase changes in A and B to be modulated. The applied field in branch A is in opposite direction

to that in branch B

The refractive index changes are opposite and phase changes in arm A and B are also opposite

If applied voltage induces a phase change of p/2in arm A, this will be –p/2 in arm B so that A & B would be out of phase by p. These two waves will interfere destructively and

cancel each other at D.

The output intensity would be zero

MZI Modulator

Since the applied voltage controls the phase difference between the two interfering waves A and B at the output

This voltage also control the output light intensity (the relationship is not linear)

The relative phase difference between the two waves A and B is doubled with respect to a phase change f in a single arm

The switching intensity can be predicted by adding waves A and B at D with A as amplitude of wave A & B:

Eoutput A cos(wt+f) + A cos(wt–f) = 2A cosf coswt

MZI Modulator

The output power is proportional to E2output which is

maximum when f = 0. Thus,

The derivation represents approximately the right relationship between the power transfer and the induced phase change per modulating arm.

The power transfer is zero when f = p/2.

In practice, the Y-junction losses and uneven splitting results in less than ideal performance

A and B do not totally cancel out when f = p/2

f=f 2cos0out

out

P

P

Thank You

external modulators

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