Ming-Chang Lee, Integrated Photonic Devices Waveguide Loss Class: Integrated Photonic Devices Time: Fri. 8:00am ~ 11:00am. Classroom: 資電206 Lecturer: Prof. 李明昌(Ming-Chang Lee) Ming-Chang Lee, Integrated Photonic Devices Optical Loss in Waveguides • Scattering Loss – Due to surface roughness • Absorption Loss – Due to photons annihilated in materials • Radiation Loss – Due to waveguide bending Three major losses in waveguide
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Waveguide loss - 清華大學電機系-NTHUEE Photonic Devices... · Waveguide Loss Class: Integrated Photonic Devices Time: Fri. 8:00am ~ 11:00am. ... Scattering The power attenuation
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Classroom: 資電206Lecturer: Prof. 李明昌(Ming-Chang Lee)
Ming-Chang Lee, Integrated Photonic Devices
Optical Loss in Waveguides
• Scattering Loss– Due to surface roughness
• Absorption Loss– Due to photons annihilated in materials
• Radiation Loss – Due to waveguide bending
Three major losses in waveguide
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Ming-Chang Lee, Integrated Photonic Devices
Scattering Loss
• Volume Scattering
• Surface Scattering (Dominant)
voidCrystalline Defect contamination
Ming-Chang Lee, Integrated Photonic Devices
Surface Scattering Loss (Tien’s Model)
• Each reflection induce scattering light
interface
'mθ
' 24exp[ ( cos ) ]r i mP P πσ θλ
= −
iP rP
Rayleigh Criterion
σ
σ : variance of surface roughness
'
0 1
sin mm k n
βθ =
1n
m: mode number
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Ming-Chang Lee, Integrated Photonic Devices
Surface Scattering Loss
To quantitatively describe the optical loss, the exponential attenuation coefficient is generally used. In this case, the intensity (power per unit length) decays along the waveguide.
)exp()( 0 zIzI α−= 0I is the initial intensity at z = 0
12σ
13σ
'mθ
3γ
2γ
n1
n2
n3
))/1()/1(
1)(sin
cos21(
32'
'32
γγθθα
++=
gm
ms tA 2 2 1/ 2
13 124 ( )A π σ σλ
= +
efft/1Average roughness
Ming-Chang Lee, Integrated Photonic Devices
Scattering Loss Analysis by Tien’s Model
Consider a planar waveguide with TE polarization
'cos mθ
'mθ
1 cm
Substrate, n2
Waveguide, n1
The power carried by the incident beam hit on the unit length (1 cm)
yE
2 '1 cos
8 y mc n E θπ
Ey is the field amplitude
According to the Rayleigh criterion, the reflected beam from the upper film surface
22 ' '
14cos exp cos
8 y m mc n E πσθ θπ λ
⋅ −
σ: variation of surfaceroughness
Rayleigh criterion
Cover cladding, n3
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Ming-Chang Lee, Integrated Photonic Devices
Scattering Loss Analysis by Tien’s Model
Consider the two film surface
4π σλ
( )1/ 213 12
4π σ σλ
+
The power lost by surface scattering per unit length is
( ){ }22 ' '1
2 2 3 '1
cos 1 exp cos8
cos8
y m m
y m
c n E A
c n E A
θ θπ
θπ
⋅ − −
≈
The planar waveguide mode power flow
2 '1
2 3
1 1sin4 y m gc n E tθπ γ γ
⋅ + +
power flow tg
3
1γ
2
1γ
Ming-Chang Lee, Integrated Photonic Devices
Scattering
The power attenuation per unit length
3 '2
'2 3
cos1 12 sin (1/ ) (1/ )
m
m g
At
θαθ γ γ
= + +
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Ming-Chang Lee, Integrated Photonic Devices
Surface Scattering Loss
• High order modes have more reflections from the surface
Low-order mode
High-order mode
mgR t
LNθcot2
= Where m is mode no.
Ming-Chang Lee, Integrated Photonic Devices
Mode Effect
The loss for the m=3 waveguide mode is as much as 14 times that of the m = 0 waveguide mode.
Ta2O5
λ= 632.8 nm
Surface Scattering
Volume absorption
1: ( / ) 4.3 dB cm cmα −=Tien, 1971
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Ming-Chang Lee, Integrated Photonic Devices
Sidewall Scattering Loss
• Sidewall roughness is created during etching process• The propagation loss is highly related to the roughness for a
small-dimension waveguide
Rough Sidewall Smooth Sidewall Measured Result
Tsuchizawa T. JSTQE 2005
Ming-Chang Lee, Integrated Photonic Devices
Optical Loss due to Surface Roughness
• Single mode waveguide
• Surface roughness required to achieve low loss
w× t < 0.18 µm2
w
t
Waveguide Width (um)
5 nm
3 nm
1 nm
Wav
egui
de L
oss
(dB
/cm
) t = 0.25um
SurfaceRoughness
TM-like Mode
Si
δrms < 1 nm
Wavelength : 1550nm
Optical Field
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Ming-Chang Lee, Integrated Photonic Devices
Absorption Loss
• Interband absorption electron and hole pairs (photodetector)