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Waveguide modulators
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
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Light modulators in photonics
1. Direct modulation of a light source (e.g. current of LD)
2. External modulators (for CW light sources)
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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
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Parameters of electromagnetic wave
r
r
∂ E
2
∇ E − µε
2 = 0
∂t
r
E = E0 ( x, y, z) exp[i(ω t − β z )]
2
Eo - amplitude (intensity)
Φ - phase
P - polarization
λ (ω) - wavelength (frequency)
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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
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The physical effects of light modulation
1. Absorptive effects
a. Franz-Keldysh effect
b. Quantum Confined Stark Effect
c. Band filling with free carriers
d. Stimulated emission
2. Refractive effects
a. Electro-optic
b. Magneto-optic
c. Elasto-optic
d. Acousto-optic
e. Thermo-optic
f. Free carriers depletion
g. Polarization control in liquid crystals
h. All absorptive effects through Kronig-Kramers relations
3. (Micro)mechanical modulation
a. simple mechanical choppers
b. optical scanners
c. MEMS (micro-electro-mechanical systems), MOEMS
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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.
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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)
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Physical effects utilized in light modulators - (1)
absorptive effects
Absorption (amplification)
α = 4 πn”/λ
n'’
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
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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
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Physical effects utilized in light modulators - (3)
(micro)mechanical devices
(Micro)mechanical modulation
a.
simple mechanical choppers
b.
optical scanners
c.
MEMS, MOEMS
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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
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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) chirpfree modulation is necessary.
One noteworthy exception is predistortion, intentionally introduced chirp
that cancels dispersion.
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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.
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Switching systems (multiplexer types)
Multiplexing domains
• Time division
• Space division
• Wavelength division multiplexing (WDM)
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Materials for fabrication of modulators and switches
M aterial
dielectric
semiconductor
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Physical effect
refractive index change
refractivei index or
absorption change
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Effect e-o: phase change
Phase:
φ=
2⋅π
n⋅L
λ
1
n = n0 − n03 ⋅ r ⋅ E
2
For GaAs modulator (100) when electric field
is applied in <011> direction:
φ 011
V - voltage
Γ - overlap integral
d - inter-electrode distance
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2π L 3
=
n r41VΓ
λ d
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Modulators – basic structures. Solid state
electroabsorption modulator
or phase modulator
Mach-Zehnder modulator
signal
signal
X coupler
signal
signal
acoustooptic (diffraction) modulator
Mode transformer (digital optical switch)
signal
signal
Light beam in
a planar
waveguide
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directional coupler
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few mrad
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Modulators - basic structures. Micromechanical
micromechanical modulator
signal
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Strip waveguide structures
(a )
(c)
(b)
(d)
a) strip waveguide (elevated),
b) built-in strip waveguide,
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c) ridge waveguide,
d) strip loaded waveguide
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Electro absorption modulator
Distance necessary to
obtain asumed extinction
coefficient Ξ [dB]
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Ξ
l=
4. 34 ⋅ ∆α
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Mach-Zehndera modulator
I out
Distance to obtain
phase shift of
∆β l = π
l=
I in
(1 + cos Φ )
=
2
λ
2 ⋅ neff ⋅ ∆ eq
Example characteristic length:
∆eq = 10-3 ÷ 10-8. For ∆eq ~ 10-5, L ~ 1cm
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Directional coupler
Characteristic lenght
(minimal coupling distance)
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l=
3 ⋅λ
2 ⋅ n eff ⋅ ∆ eq
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X coupler
Electrode length is
determined by an angle and
strip width.
l=
2
w
⋅
m
2 ⋅ ∆ eq
w = strip width
θ
m=
θc
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Directional coupler - basic structure
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Schotky barrier
Au-Pt
Epitaxial layer
GaAs
Substrate
GaAs
Contact
Au
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Details of modulator structure (1)
a)
Ti/Au/Au - electrodes
2 um Al
0.032
Ga
0.968
As
0.968
As
1,6 um GaAs
5 um Al
0.032
Ga
light beam 1,3 um
substrate GaAs
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Details of modulator structure (2)
b)
Ti/Au/Au - electrodes
3 um GaAs
4 um Al
0.032
Ga
0.968
As
light beam 1,3 um
substrate GaAs
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Semiconductor electrooptic modulator
Heterostructure
Waveguide
Microwave package
U-groove
Conductive epoxy
Alundum substrate
Microwave connector SMA
Fiber
Microwave microstrip line
M.-Z waveguide modulator
Modulator design
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Planar waveguide
2 µm Al0.03Ga0.97As
separator
1 µm GaAs,
Plan. waveguide
light
1,3 µm
GaAs, substrate
Heterostructure
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Semiconductor electrooptic modulator
Rib waveguide
4 µm
A ridge waveguide structure
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Simple Mach-Zehnder interferometer
Modulated output
signal
Y splitter
Strip
waveguide
GaAs
AlxGa1-xAs
Input sinal
λ=1.3 µm
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GaAs Substrate
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Details of modulator structure (2)
b)
Ti/Au/Au - electrodes
3 um GaAs
4 um Al
0.032
Ga
0.968
As
light beam 1,3 um
substrate GaAs
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Optoelectronic packaging of advanced modules
Microwave package
U-grove
Conducting glue
Alundum substrate
Fiber
waveguide
Microwave SMA connect.
microstrip line
Optoelectronic modulator
in a microwave package. Package contains modulator chip, microwave
preamplifier, impedance matching circuit.
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Semiconductor electrooptic modulator
M.-Z waveguide modulator
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M.-Z modulator speed evaluation
3.00
12.0
C/L
[pF/cm]
∆f=1/ πRC
Inter-electrode distance d = 10 µm
electrode width W = 100 µm
d/W = 0,1 ⇒ BL ~ 4
for L = 1 cm we obtain B = 4 GHz
BL
[GHz cm]
2.00
8.00
1.00
4.00
0.00
0.01
0.10
1.00
0.00
10.00
d/W
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.
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Typical parameters of photonics modulators
Parameter
Bandwidth
working wavelength
Losses
optical return loss
maximal accepted optical power
Extinction
phase modulation efficiency
Fiber waveguides
working conditions
*
Value
**
2,5 (20)
Selected telecommunications window (1300, 1500)
5
>40
<100
>20
≤1
standard singlemode or PM
standard or “typical laboratory”
Unit
GHz
nm
dB
dB
mW
dB
Rad/V
1
+ ∆Φ
2
1
− ∆Φ
2
Mach-Zehnder interferometer
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