Modulatory i prze³¹czniki œwiat³owodowe · 2016. 10. 11. · (C) Sergiusz Patela 1997-2004...

Waveguide modulators

(C) Sergiusz Patela 1997-2004 Waveguide modulators 2

Modulator of light - definition

A device that imposes signal on a carrier -telecommunications

In general, changes in one wave train caused by another wave, such as amplitude or frequency modulation in radio.

In contemporary fiber optics modulation usually means transferring information from electrical to optical domain.

In optics the term generally is used as a synonym for contrast, particularly when applied to a series of parallel lines and spaces imaged by a lens (e.g. SLM) - optics


Light modulators in photonics

1. Direct modulation of a light source (e.g. current of LD)

2. External modulators (for CW light sources)


Why do we need external light modulators?

• For some light sources direct modulation is impossible (e.g. fiber lasers)

• Semiconductor light sources chirp (change wavelength) when modulated

• Modulation speed is limited by the electrical capacitance of the source and the speed of migration of the charge carriers


Parameters of electromagnetic wave

∇ − =22

2 0r

r

EEt

µε∂∂

( )[ ]ztizyxEE βω −= exp),,(0

r

Eo - amplitude (intensity)

Φ - phase

P - polarization

λ (ω) - wavelength (frequency)


Classification of effects utilized in waveguide modulators

1. Absorptive effects: modifications of absorption coefficient (change of beam intensity).

2. Refractive effects: modifications of refractive index (resulting in changes of phase or direction of the beam, change of critical angle in total internal reflection).

3. (Micro)mechanical modulation


The physical effects of light modulation

1. Absorptive effectsa. Franz-Keldysh effectb. Quantum Confined Stark Effectc. Band filling with free carriersd. Stimulated emission

2. Refractive effectsa. Electro-opticb. Magneto-opticc. Elasto-opticd. Acousto-optice. Thermo-opticf. Free carriers depletiong. Polarization control in liquid crystalsh. All absorptive effects through Kronig-Kramers relations

3. (Micro)mechanical modulationa. simple mechanical choppersb. optical scannersc. MEMS (micro-electro-mechanical systems), MOEMS


Four types of light modulators

• Electrooptic and magnetooptic modulators. Materials change refractive index under electric or magnetic fields. Special devices (e.g. a Mach-Zehnder interferometer) required to convert phase modulation into amplitude modulation

• Electro-absorptive modulators. Material or structure changes absorption under applied electric field (e.g. reverse biased p-n junction). EA modulators are usually integrated with LDs.

• Acoustooptic modulators. High frequency sound traveling inside material or structure diffracts light.

• MOEMS modulators. Micromechanical beam deflectors or shutters change light intensity.


Complex coefficient of refraction

n = n’ - j n”

n’ - real part, in colloquial language “refractive index”

(responsible for phase changes, beam refraction, propagation speed)

n” - imaginary part, sometimes presented as extinction k (beam attenuation)


Physical effects utilized in light modulators - (1) absorptive effects

Absorption (amplification)

n'’ α = 4 πn”/λ

Physical effects responsible for attenuation

• Franz-Keldysh effect

• QCSE (quantum confined Stark effect) - shift of quantum well exciton

line

• Band filling with free carriers

• Stimulated emission


Physical effects utilized in light modulators - (2) refraction effects

Refractive index (n’)• Electro-optic effect

• elastooptical• acoustooptical• magnetooptical, Faraday effect

• free carrier injection (e.g. free electron plasma: free carrier absorption, band filling)

• Free carriers depletion• QCSE• Polarization control in liquid crystals


Physical effects utilized in light modulators - (3) (micro)mechanical devices

(Micro)mechanical modulationa. simple mechanical choppersb. optical scannersc. MEMS, MOEMS


Advantages and applications of optical modulators

Advantages of waveguide modulators: • increase modulation speed and transmission bandwidth, • improve modulation quality (lower dispersion and distortion,

eliminate chirp and crosstalk)• make optoelectronic converters obsolete.

Applications:• Telecommunications: multimedia transmission (voice, video,

data), ISDN (Integrated Services Digital Network), B-ISDN (Broad band ISDN)

• Aerial terminals• Fiber optic gyroscopes• Laser pulse forming


Laser chirp

Every time a laser diode emits a pulse of light, free carrier concentration in

active area is changed ->

which results in refraction index change->

which changes wavelength of emitted light.

The effect is called laser wavelength chirp. The result is wider spectral

linewidth and bigger fiber dispersion . In fast optical telecommunications

transmission systems (>10 Gbit/s, > 100 km inter-repeater distance) chirp-

free modulation is necessary.

One noteworthy exception is predistortion, intentionally introduced chirp

that cancels dispersion.


Bandwidth requirements

Bandwidth requirements for modulators:

• digital stereo sound 106 bit/s

• digital TV 108 bit/s (100 Mbit/s)

• high resolution TV ~1Gbit/s

• 3D TV, teleconferencing 100 Gbit/s

Speed requirements for switches:

• speed 10 kbit/s -> 100 Gbit/s

• multitude of link possibilities: point-point, point-multi point

(splitter, multiplexer), unidirectional, bi-directional.


Switching systems (multiplexer types)

Multiplexing domains• Time division• Space division

• Wavelength division multiplexing (WDM)


Materials for fabrication of modulators and switches

Material Physical effectdielectric refractive index change

semiconductor refractivei index orabsorption change


Effect e-o: phase change

Ln ⋅λ

π⋅=φ

2

E21 3

00 ⋅⋅−= rnnn

Phase:

For GaAs modulator (100) when electric field is applied in <011> direction:

Γλπ

=φ VrndL

413

101

2

V - voltageΓ - overlap integrald - inter-electrode distance


Modulators – basic structures. Solid stateelectroabsorption modulatoror phase modulator Mach-Zehnder modulator

signaldirectional coupler

signalsignal

X couplersignal

acoustooptic (diffraction) modulator

signal

Light beam in a planar waveguide

Mode transformer (digital optical switch)signal

few mrad


Modulators - basic structures. Micromechanical

micromechanical modulator

signal


Strip waveguide structures

(d)

(c)

(b)

(a)

a) strip waveguide (elevated), b) built-in strip waveguide,

c) ridge waveguide, d) strip loaded waveguide


Electro absorption modulator

Distance necessary toobtain asumed extinctioncoefficient Ξ [dB]

l =⋅

Ξ∆4 34. α


Mach-Zehndera modulator

( )Φ+= cos12in

outII

Distance to obtainphase shift of∆β l = π

lneff eq

=⋅ ⋅

λ2 ∆

Example characteristic length:∆eq = 10-3 ÷ 10-8. For ∆eq ~ 10-5, L ~ 1cm


Directional coupler

Characteristic lenght(minimal coupling distance) l

neff eq

=⋅

⋅ ⋅3

2λ∆


X coupler

Electrode length isdetermined by an angle andstrip width.

lm

w

eq

= ⋅⋅

22 ∆

w = strip width

c

mθθ

=


Directional coupler - basic structure

Au-Pt

Epitaxial layer

Schotky barrier

GaAs

Substrate GaAs

AuContact


Details of modulator structure (1)

a)Ti/Au/Au - electrodes

2 um Al 0.032 Ga 0.968 As

1,6 um GaAs

substrate GaAs

5 um Al 0.032 Ga 0.968 As

light beam 1,3 um



b)Ti/Au/Au - electrodes

substrate GaAs

4 um Al 0.032 Ga 0.968 As

3 um GaAs

light beam 1,3 um


Semiconductor electrooptic modulator

Heterostructure Waveguide

FiberMicrowave microstrip line

Conductive epoxyAlundum substrateMicrowave connector SMA

Microwave packageU-groove

Modulator design M.-Z waveguide modulator


Planar waveguide

1 µm GaAs, Plan. waveguide

GaAs, substrate

light1,3 µm

2 µm Al0.03Ga0.97As separator

Heterostructure



Rib waveguide4 µm

A ridge waveguide structure


Simple Mach-Zehnder interferometer

Strip waveguide

Y splitter

GaAs Substrate

GaAsAlxGa1-xAs

Modulated output signal

Input sinal λ=1.3 µm



b)Ti/Au/Au - electrodes

substrate GaAs

4 um Al 0.032 Ga 0.968 As

3 um GaAs

light beam 1,3 um


Optoelectronic packaging of advanced modules

Fiber waveguide microstrip line

Conducting glue

Alundum substrateMicrowave SMA connect.

Microwave package

U-grove

Optoelectronic modulator in a microwave package. Package contains modulator chip, microwave preamplifier, impedance matching circuit.



M.-Z waveguide modulator


M.-Z modulator speed evaluation

0.01 0.10 1.00 10.000.00

1.00

2.00

3.00

0.00

4.00

8.00

12.0

d/W

C/L[pF/cm]

BL[GHz cm]

∆f=1/ πRC

Inter-electrode distance d = 10 µmelectrode width W = 100 µm

d/W = 0,1 ⇒ BL ~ 4for L = 1 cm we obtain B = 4 GHz

Capacitance per unit length for coplanar electrodes structure in GaAs as a function of electrode length /width ratio. Also shown is bandwidth-length parameter (BL) for R=50Ω.


Typical parameters of photonics modulators

Parameter Value Unit Bandwidth 2,5* (20)** GHz working wavelength Selected telecommunications window (1300, 1500) nm Losses 5 dB optical return loss >40 dB maximal accepted optical power <100 mW Extinction >20 dB phase modulation efficiency ≤1 Rad/V Fiber waveguides standard singlemode or PM working conditions standard or “typical laboratory”

∆Φ+21

∆Φ−21

Mach-Zehnder interferometer

Modulatory i prze³¹czniki œwiat³owodowe · 2016. 10. 11. · (C) Sergiusz Patela 1997-2004...

Documents

Transcript of Modulatory i prze³¹czniki œwiat³owodowe · 2016. 10. 11. · (C) Sergiusz Patela 1997-2004...