Optical Isolator:
Application to Photonic Integrated Circuits
Tetsuya MIZUMOTO
Dept. of Electrical and Electronic Eng.
Tokyo Institute of Technology
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Outline
Bulk optical isolator
 magneto-optic (Faraday) effect
 operation principle
Waveguide optical isolator
 TE-TM mode conversion isolator
 nonreciprocal loss (active) isolator
 nonreciprocal phase shift isolator
 integration (direct bonding)
Non-magneto-optic approach
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What happens?
Photon injection
 photon-generated carrier
 disturbs carrier distribution (amplitude-noise)
 carrier-induced index change (phase-noise)
Isolator
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Magneto-optic material
Requirement
- large magneto-optic (MO) effect
--> 1-st order MO effect (Faraday rotation)
- low optical absorption
- temperature insensitive
Rare earth iron garnet (R3Fe5O12)
Y3Fe5O12 (YIG)
--> (Y3-xBix)Fe5O12, (Y3-xCex)Fe5O12
enhancement of Faraday rotation
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Characteristics of Y3-xCexFe5O12 (Ce:YIG)
Spectra of Faraday coefficient
Spectra of optical absorption
M. Gomi, et al., J. Appl. Phys., 70(11), 7065-7067 (1991).
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Bulk isolator
Bulk isolator, in either beam interface or fiber interface,
uses rotation of polarization.
Basic configuration
H
Namiki
Polarizer
45deg
Faraday rotator
Input and output : same polarization
Polarizer
H
Polarizer
H
Polarizer
Reciprocal
rotator
45deg
Faraday rotator
45deg
Faraday rotator
Polarizer
Polarizer
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Bulk isolator
birefringent plates
polarization independent operation
Fiber in-line isolator --> Walk-off
Birefringent
45deg Faraday
crystal Output fiber
rotator
/2 plate
Birefringent
crystal
FDK
Lens
Isolation>35dB, IL<0.6dB
Input fiber
H
Kyocera
Isolation>30dB, IL<2.5dB
T.Matsumoto (NTT), Trans. IECE, J62-C,
505-512 (1979).
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TE-TM mode conversion type
Polarizer
H
Reciprocal
rotator
45deg
Faraday rotator
Polarizer
Translate Faraday isolator into waveguide one.
Cotton-Mouton Mode
Faraday part
selector
part
qm
TE-TM mode conversion
Isolation:12.5 dB, =1150 nm
Length: 6.8 mm
M
K. Ando, T. Okoshi and N. Koshizuka (present AIST),
Appl. Phys. Lett., 53(1), 4 (1988).
Magnetooptic
waveguide
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TE-TM mode conversion
Phase matched: d=bTE-bTM=0
(q  q F )
ETE ( z )
 cosq F z 
ETE (0)
ETM ( z )
 sin q F z 
ETE (0)
rotates in a linearly polarized state
Phase mismatched: d  0
ETE ( z )
2
 cos q F    d 2 z - j
ETE (0)
ETM ( z )

ETE (0)
qF
q F 2  d 2
sin
d
q F 2  d 2
q F 2  d 2 z
q F 2  d 2 z
Birefringence-free (phase matching)
is essential to isolator operation.
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sin
Faraday rotation
in a birefringent medium
9
Waveguide isolators
type
mechanism
guided TE / guided TM
(Faraday & Cotton-Mouton)
Faraday
part
Cotton-Mouton Mode
part
selector
θm
M
Magnetooptic
waveguide
mode
transversely radiated TE / guided TM
conversion
(with TM nonreciprocal phase shift)
guided TE / radiated TM
(semi-leaky)
filed shift
θ c-axis
LiNbO3
Ce:YIG
NOG
nonreciprocal phase shift
(interferometer)
nonreciprocal loss
(active)
p-electrode
Al2O3
TiO2
Fe H
GaInAsP
MQWs
n+ InP sub.
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n-electrode
10
Mode conversion: transversely leaky mode
Nonreciprocal radiation (TM phase shift)
Propagation constant diagram
x
br
b(TE)
x
x
bc
b11
Radiation modes
Propagation constant
w
ta tc
bax
y
y
x
b11
b ay
TM mode
b cx
y
b11f
y
b11b
b cy
b(TM)
bc
TE mode
tc
ta
Film thickness
y
b11b b11f
Performance:
- Isolation: 27 dB (=1535 nm, L=4.1 mm)
- wavelength sensitive (7 dB at =1515 nm)
T. Shintaku (NTT), Appl. Phys. Lett., 73(14), 1946 (1998).
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Semi-leaky isolator: operation principle
Anisotropy of LiNbO3
 Semi-leaky waveguide
TE mode
q
c-axis
guided
Forward
-k(Ce:YIG)+k(LiNbO3)=0
Backward
k(Ce:YIG)+k(LiNbO3)≠0
LiNbO3 2.143
H
2.200
1.938
Ce:YIG
NOG
TE mode
 unidirectional mode conversion
TM mode
q
H
c-axis
radiated
LiNbO3 2.210
Ce:YIG
NOG
TM mode
LiNbO3 mode conversion
 reciprocal
Magneto-optic mode conversion
 nonreciprocal
(changes its sign for F/B)
2.200
1.938
S.Yamamoto, et al (Osaka U.), IEEE QE, 12,
764 (1976).
IEEE Photonics Soc. distinguished lecture
Semi-leaky isolator is attractive;
- relaxed fabrication tolerance
- simple mono-section structure
- easy control of magnetization
- but, uniform and tight LiNbO3 /
garnet contact is needed.
 direct bonding
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Nonreciprocal loss (active) isolator
p-electrode
Al2O3
TiO2
Fe H
GaInAsP
MQWs
n+ InP sub.
Imaginary (Loss) keff
n-electrode
0
Active group:
U.Tokyo, AIST, Ghent U.
Backward
SOA gain
Forward
Real neff
Isolation: 14.7 dB/mm
Insertion loss: 14.1 dB/mm (I=150 mA)
H.Shimizu and Y.Nakano (U.Tokyo), JLT, 24, 38-43 (2006).
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Integration with active devices
compatible waveguide structure
material & dimensions
p-electrode
Al2O3
Fe H
GaInAsP
MQWs
nonreciprocal loss (active)
excellent compatibility to active devices
TiO2
n+ InP sub.
n-electrode
4 dB isolation at =1543.8 nm
4 dB
150mA
90mA
15OC
active isolator DFB LD
0.7 mm
0.3 mm
H. Shimizu and Y. Nakano (U.Tokyo), IEEE PTL, 19, 1973-1975 (2007).
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Comparison: passive and active isolators
p-electrode
Al2O3
TiO2
Fe H
GaInAsP
MQWs
n+ InP sub.
n-electrode
type
Passive
Active
Integration
type dependent
excellent
Noise
none
ASE
Power
consumption
none
current injection
to SOA
Polarization
dependence
yes,
but can be
overcome
yes
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Waveguide isolator: nonreciprocal phase shift
Interferometer type
- Isolation: 19 dB (=1540 nm, L=8.0 mm)
J. Fujita, M. Levy and M. Osgood, Jr. (U.Columbia),
Appl. Phys. Lett., 76(16), 2158 (2000).
- Isolation: 25 dB (=1600 nm, L=4.0 mm)
Y. Shoji and T. Mizumoto (Tokyo Tech), Optics Express, 15, 13446 (2007).
- wavelength insensitive
designed to cover both 1.31/1.55 mm in a single chip
Y. Shoji and T. Mizumoto (Tokyo Tech.), Optics Express, 15, 639 (2007).
- polarization independent
not by polarization diversity scheme
Y. Shoji and T. Mizumoto (Tokyo Tech.) et al, JLT, 25(10), 3108-3113 (2007).
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Interferometric isolator: operation principle
Interferometric isolator
Forward (constructive interference)
-  /2
1/ 2
- j/ 2
/2 phase bias
1/ 2
Input
Output
1/ 2
1/ 2
Backward (destructive interference)
/2


Single polarization operation
→ No need for phase matching
→ Fabrication tolerant
-1 / 2
1/ 2
j/ 2
1/ 2
Reflected
1/ 2
Simple in-plane magnetization
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Nonreciprocal phase shift
x
y
z
 2

2   0
 - j

tan(qd ) 
j 

0
 2 
0
2
0
q  p1 p3 
 

 2'   1  3 
linear in b
 b

p  b
p p p
 - 1 3 -  1 - 3  2 '   2 '
 1  3   1  3  n y  2  n y  2

n y2 n z2 -  2
 q
 '
2
 2' 
1st–order
MO effect
2




2
n y2
Nonreciprocal phase shift = (b+-b-) (m-1)
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Nonreciprocal phase shift
Nonreciprocal phase shift = (b+-b-) (m-1)
NPS/(/2) [mm-1]
2.0
=1550nm
TM0 mode
SiO2 (n=1.45)
d
(CeY)3Fe5O12
SGGG (n=1.94)
1.0
cutoff
0
0
0.2
0.4
0.6
0.8
1
Thickness of Ce:YIG guiding layer [mm]
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Backward loss (dB)
Forward loss (dB)
Interferometric isolator: calculated performance
0
0.1
0.2
0.3
0.4
0
10
20
30
40
50
0.5
1.45
1.5
1.55
1.6
Wavelength (mm)
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1.65
1.45
1.5
1.55
1.6
Wavelength (mm)
1.65
20
Interferometric isolator: wideband operation
3/2
q (backward)

qR
/2
0
qN (backward)
q (forward)
0
q (forward)
2
Phase shift
Phase shift
 dependences :
MO effect
waveguide dispersion

qR
q (backward)
/2
qN (forward)

-/2
qN (forward)
0
-/2
0

qN (backward)
Conventional design
wideband design
Cancellation of wavelength dependences in backward propagation
Y.Shoji and T.Mizumoto (Tokyo Tech.), Appl. Opt., 45, 7144 (2006).
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Wideband design: experimental results
•measured with a reference of
straight waveguide (5ｄB loss)
Conventional design
Wideband design
0
attenuation (dB)
attenuation (dB)
0
10
20
forward
backward
30
1500
1550
1600
Wavelength (nm)
10
20
forward
backward
30
1650
1500
1550
1600
Wavelength (nm)
1650
Larger isolation in wider wavelength range
Y. Shoji and T. Mizumoto (Tokyo Tech.), Optics Express, 15, 13446 (2007).
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Ultra-wideband design
665mm
2.0mm
26mm
300mm
665mm 3.0mm
493mm
L/2
930mm
Wideband design covers fully
1310 nm / 1550 nm bands and
more.
Isolation > 40 dB :
@ 1260-1650 nm
attenuation [dB]
0
1.55 mm Forward
1.55 mm Backward
1.31-1.55 mm Forward
1.31-1.55 mm Backward
10
20
30
40
50
1.3
Y. Shoji and T. Mizumoto (Tokyo Tech.), Optics Express, 15, 639 (2007).
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1.4
1.5
Wavelength [mm]
1.6
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Photonic integrated circuit: device and material
LD, SOA
III-V semiconductor
modulator, SW
LiNbO3,
III-V semiconductor
-MUX/DeMUX
Silica
Isolator
Magneto-optic material
- photonic integrated circuit
waveguide alignment  lithography process
materials  to be grown (deposited) on a common platform
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Our approach: integration of isolator and LD
compatible waveguide structure
material & dimensions
Single polarization operation
Direct bonding
LD integrated
with isolator
Common semiconductor guiding layer
(selective growth & mask process)
H. Yokoi and T.Mizumoto (Tokyo Tech.), Electron. Lett., 33, 1787 (1997).
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III-V waveguide isolator
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Nonreciprocal phase shift
x
y
z
0
1
0
j 

0
 1 
1st–order
MO effect

q  p1 p3

 ' 
b 
'

 2  1  3 1 1 
tan(qd ) 
2
 q  p1 p3
p
  - '
- 3' b
  2   1  3  1 1 3
2
2


1'  1
1
linear in b
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-1
 1

1   0
 - j

Nonreciprocal phase shift (mm )
Nonreciprocal phase shift = (b+-b-) (m-1)
0.5
=1.55mm
0.4 TM0mode
qF=-4500deg/cm
0.3
0.2
0.1
0
0
Q1.42 (n=3.45)
Q1.25 (n=3.36)
0.2
0.4
0.6
0.8
GaInAsP thickness (mm)
1
27
Bonding garnet on III-V
garnet
GaInAsP
InP
n(garnet) < n(III-V)  Evanescent field is to be used in MO garnet.
direct bonding with no gap in-between
III-V
MO garnet
crystal structure
zinc blende
garnet
lattice constant (A)
5.869 (InP)
12.54
thermal expansion (K-1)
4.56 X 10-6 (InP)
9.20 X 10-6
refractive index
3.2 – 3.5
2.2
Challenging: epitaxial growth of III-V on garnet done by Dr. M. Razeghi (Thomson),
JAP, 59, 2261 (1986) and Dr. J. Haisma (Philips), J. Cryst. Growth, 83, 466 (1987)
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Surface activated bonding
Surface activation in vacuum chamber
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Direct bonding: garnet on GaInAsP/InP waveguide
Ce:YIG / GaInAsP
Ce:YIG
GaInAsP
Bonding strength
Fracture in an InP substrate
at a tensile > 0.5 MPa
Low temperature heat treatment
T.Mizumoto, et al, ECS Meeting, 1258 (2006).
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Si-waveguide isolator
L=364mm
R=2.5mm
MMI
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Nonreciprocal phase shift in SOI WG
TM mode
Ex External
magnetic
field
External
magnetic
field
x
y
Ce:YIG
Si
z
SiO2
Ce:YIG
Si
SiO2
L/2 (Min) ~300mm
@0.2-mm-thick
Nonreciprocal phase shift (mm -1)
Nonreciprocal phase shift (NPS):b = b+ - b101
=1.55mm, TM0mode
Ce:YIG on SOI
100
10-1
0
Ce:YIG on
GaInAsP (n=3.45)
0.2
0.4
0.6
0.8
1
Thickness of guiding layer (mm)
CeY2Fe5O12 (Ce:YIG)： QF = -4500 deg/cm
H.Yokoi, et al (Tokyo Tech.)., Applied Optics, 42, 6605-6612 (2003)
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Si-waveguide optical isolator
Ce:YIG SGGG
Ce:YIG
Si rib waveguide
H.Yokoi, et al (Tokyo Tech.)., Applied Optics, 42,
6605-6612 (2003)
10nm
Ce:YIG
Si 2mm
Si 300
4.0mm
300nm
SiO2
Rib waveguide for reducing
propagation loss (trial fabrication)
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SOI
Bonding condition
Anneal: 250 oC
Press: 5 MPa, 1 hour
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Measurement setup
Sample
CW
lens
IR camera
TV
monitor
N
CCW
ASE source
S
PMF
Polarizer
S
TM mode
PMF
Optical switch
Spectrum Analyzer
 3-pole magnet --> anti-parallel magnetic field (S-N-S or N-S-N)
 2X2 optical SW --> reverses propagation direction (CWCCW)
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First demonstration of Si-waveguide isolator
Transmittance (dB)
Mag: N-S-N
CCW
-40
N-S-N
-50
-60
 The interference reverses as the
propagation direction is reversed.
w/o H field
CW
-70
1530
1540
1550
1560
Wavelength (nm)
1570
S-N-S
Transmittance (dB)
Mag: S-N-S
CW
-40
-50
-60
CCW
-70
1530
 The interference reverses as the
magnetic field directions are reversed.
Isolation:
21dB
1540
1550
1560
Wavelength (nm)
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First demonstration of Si waveguide isolator !
1570
Y. Shoji, T. Mizumoto (Tokyo Tech), et al.
APL, 92, 071117 (2008).
35
transmittance (dB)
Si-waveguide isolator: insertion loss
Ce:YIG upper clad
(a)
(b)
(c)
-40
-50
Single WG
MZI
-60
21dB Isolation
-70
1530
1540
1550
1560
wavelength (nm)
1570
2.0 mm
4.0 mm
(a) Coupling loss between fiber and waveguide x2 : 37 dB
(b) Propagation loss : 4 dB
Si waveguide (2.5 dB / 4 mm) + Absorption of Ce:YIG (0.2 dB)
+ reflection at bonding boundary (0.65 dB x2)
(c) Excess loss of MZI : 4 dB
Insertion loss of the isolator ((b)+(c)) : 8 dB
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Non-magneto-optic approach
“Indirect photonic transition”
1
-k2
k2
w (2c/a)
0.8
0.6
w2
W
-k1
k1
w1
0.4
0.2
0
Backward:
-3
-2
Mode-1 (w1, -k1) is coupled to mode-2 (w2, -k2).
(-k1 - q = -k2 , w2-w1=W : phase-matched) --> transition
mode-2 (w2, -k2) filtered out
-1
0
1
kz (2/q)
2
3
Forward:
Mode-1 (w1, k1) is uncoupled to mode-2 (w2, k2).
(k1 - q > k2 , phase-mismatched) --> no transition
Zongfu Yu and Shanhui Fan (Stanford), Nature Photonics, 3, 91-94 (2009).
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Non-magneto-optic approach
Traveling wave (dynamic) modulation
0-th
t0+t
Modulation
d
1-st
Example (=1550 nm):
d/=5x10-4, f=20 GHz
w=0.27 mm, L=2.19 mm
t0
Backward: effective coupling
(z,t)=d cos(W t - (-q)z)
-k1 - q = - k2
w2-w1=W
0
-d
0
2/q
z
Z. Yu and S. Fan (Stanford), Nature Photonics, 3, 91-94 (2009).
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Summary
Optical isolators for photonic integrated circuits
★ Mode conversion isolator
requirement of phase matching
 limited fabrication tolerances
★ Interferometric isolator
single polarization operation
 no need for phase matching
ultra-broad band operation (1.31/1.55 mm in a single chip)
integration with active devices
 Ce:YIG/ III-V, Ce:YIG/ Si low-temperature direct bonding
first demonstration of Si waveguide isolator
 21 dB isolation
★ Non-magneto-optic approach
attractive (less restricted by material), but still challenging
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Semi-leaky isolator: performance
External
magnetic field
Tunable laser
(Electromagnetic Coil)
W=3 mm
=1550 nm
PMF
20.2 dB
4.5 mm
1.5 mm
Polarizer
PMF
Power
meter
constant coupling loss (-15 dB/facet)
Measured isolation :
20.2 dB / 1.5 mm=13.5 dB/mm
T.Mizumoto et al, IEICE Trans, J89-C, 423 (2006).
T.Mizumoto et al, OFC2007, OThU4 (2007).
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Outline
Part-1: Bulk nonreciprocal devices
 magneto-optic effect (Faraday rotation)
 operation principle of isolators and circulators
Part-2: Waveguide isolators
operational principles, design and characterization
 TE-TM mode conversion isolators
 Nonreciprocal loss isolator
 Interferometric isolator
 Semi-leaky waveguide isolator
Part-3: Waveguide circulators
Part-4: Non-magneto-optic approach
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Faraday effect
Dielectric tensor
r
    0  jk
 0
- jk
r
0
0
0 
 z 
x
H
z
y
Faraday rotator
Circular polarization
CW: E  iE  j- jE 
    0 ( r - k )
b  k0  r - k
CCW: E-  iE  j jE 
 -   0 ( r  k )
b-  k0  r  k
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Faraday effect
Linearly polarized wave --> two circular polarized components
CCW circular polarized
CW circular polarized
E0
2

  E0



i
cos
w
t
b
z
w
t

j
cos
w
t
b
z

 



2  2


 b - b
Ex  E0 cos  2
b  b
 
z  cos wt - 2
 
H

z


 



i
cos
w
t
b
z

j
cos
w
t
b
z




2



b  b 
 b - b  
E y  E0 sin z  cos wt - z
2
 2
 

qF  

b- - b
2
z
k0
(  r  k -  r - k )z
2
Faraday rotator
IEEE Photonics Soc. distinguished lecture
45
Faraday effect
Reversal of propagation direction
x
y
H
x
z
Reversal of H-field
x
z
y
Faraday rotator
H
z
y
k  -k
qF
-
k0

 (  r - k -  r  k )  -q F
2
H
H
Forward
Backward
Faraday rotator
IEEE Photonics Soc. distinguished lecture
Faraday rotator
46
Waveguide Faraday rotator
E. Pross, et al. (Philips) , APL, 52(9), 682
(1988).
N. Sugimoto, et al. (NTT) , APL, 63(9),
2744 (1993).
IEEE Photonics Soc. distinguished lecture
47
Isolator
Isolator
- two-port device
- includes loss mechanism
#1
#2
 0 0

S  
 1 0
 0 0

SS  
0 1
*t
IEEE Photonics Soc. distinguished lecture
non-unitary matrix --> lossy
48
Outline
Part-1: Bulk nonreciprocal devices
 magneto-optic effect (Faraday rotation)
 operation principle of isolators and circulators
Part-2: Waveguide isolators
operational principles, design and characterization
 TE-TM mode conversion isolators
 Nonreciprocal loss isolator
 Interferometric isolator
 Semi-leaky waveguide isolator
Part-3: Waveguide circulators
Part-4: Non-magneto-optic approach
IEEE Photonics Soc. distinguished lecture
49
Circulator
Circulator
#1
- many-port device
- lossless device
0 0 1


S  1 0 0
0 1 0


1 0 0


*t
SS   0 1 0 
0 0 1


IEEE Photonics Soc. distinguished lecture
#2
#3
unitary matrix --> lossless
50
Optical circulator
H.Iwamura, et al, Electron. Lett., 15, 830-831 (1979).
- uses rotation of polarization
- polarization independent operation
IEEE Photonics Soc. distinguished lecture
51
Outline
Part-1: Bulk nonreciprocal devices
 magneto-optic effect (Faraday rotation)
 operation principle of isolators and circulators
Part-2: Waveguide isolators
operational principles, design and characterization
 TE-TM mode conversion isolators
 Nonreciprocal loss isolator
 Interferometric isolator
 Semi-leaky waveguide isolator
Part-3: Waveguide circulators
Part-4: Non-magneto-optic approach
IEEE Photonics Soc. distinguished lecture
52
TE-TM mode conversion
Faraday rotation
Faraday
part
TE-TM mode conversion
Cotton-Mouton Mode
part
selector
θm
ETE ( z )
d
 cosq - j sin q
ETE (0)
q
M
Magnetooptic
waveguide
ETM ( z )
κ
 j sin q
ETE (0)
q
*
d
b TE - b TM
2
k0
k  j
  jq F 
2n
q  qz  k  d 2 z 
2
IEEE Photonics Soc. distinguished lecture
Phase mismatch
qF : Faraday rotation,
 : field confinement factor
q F  
 b - b TM 
  TE
 z
2


2
2
53
Outline
Part-1: Bulk nonreciprocal devices
 magneto-optic effect (Faraday rotation)
 operation principle of isolators and circulators
Part-2: Waveguide isolators
operational principles, design and characterization
 TE-TM mode conversion isolators
 Nonreciprocal loss isolator
 Interferometric isolator
 Semi-leaky waveguide isolator
Part-3: Waveguide circulators
Part-4: Non-magneto-optic approach
IEEE Photonics Soc. distinguished lecture
54
Nonreciprocal phase shift
x
y
MO perturbation
 0

   0
 - j

z
0
0
0
j 

0
0 
Nonreciprocal phase shift
b  b  - b -  2w 0


 E
2w 0
b
2b
w 0
IEEE Photonics Soc. distinguished lecture
*
E
 EdS
*

 H  E  H * z dS

2
E x dS
x
E*  H  E  H * z dS
 
 
 
 n

2
H y dS
x

*
*
E  H  E  H z dS
 
4

 
 n
2
H y dS
x
2
1
 n 2 H y dS
4
55
Interferometric isolator: polarization-independent
(a)
TM0°
MC1
2TM


ηTE0°
ηTE180°
180
(1-η)TM0°
N: nonreciprocal phase shifter
provides NPS only for TM mode
ηTE0°
TM0°
N
(1-η)TM0°
TM0°
MC2
(1-η)TM180°
ηTE0°
(1-η)TM0°
MC: mode converters
(b)
provide TE-TM mode conversion 2TE
ηTM0°
TE0°
ηTM0°
MC1
(1-η)TE0°
N
TE0°
(1-η)TE0°
TE0°
MC2
ηTM180°
180
(1-η)TE180°
ηTM0°
(1-η)TE0°
(c)
2ηTM
ηTM180°
MC1
(1-η)TE0°
ηTE0°
2TE
ηTM -180°
N
(1-η)TE0°
ηTM0°
MC2
Y. Shoji and T. Mizumoto (Tokyo Tech.) et al,
JLT, 25(10), 3108-3113 (2007).
TM180°
TM0°
IEEE Photonics Soc. distinguished lecture
TM0°
180
2TM
(1-η)TM0°
ηTE0°
TE0°
(1-η)TE0°
MC1
2ηTE
TE0°
TE180°
180
(1-η)TE 180°
ηTM -180°
(d)
TE180°
ηTE0°
N
(1-η)TM-180° (1-η)TM-180°
ηTE0°
MC2
(1-η)TM0°
56
Hydrophilic bonding
・Si / Si
・Si/SiO2 / Si,
・III-V(GaAs,InP) / Si,
・III-V(GaAs, GaP, InP,
InAs) / III-V
・Ce:YIG / III-V
・Ce:YIG / SiO2
・Ce:YIG / LiNbO3
Issues to be considered
・surface treatment
→ hydrophilic
・mismatch in thermal
expansion coefficient
→ low temperature
heat treatment
IEEE Photonics Soc. distinguished lecture
57
Hydrophilic bonding: fabrication
GaInAsP (λg=1.25mm)
(GdCa)3(GaMgZr)5O12(111)
InP (100)
Sputter epitaxy
(CeY)3Fe5O12
E-beam lithography
CH4/H2 RIE
O2 plasma (30s)
Surface treatment
H3PO4 (RT)
or
O2 plasma (30s)
Deionized water
Deionized water
GaInAsP
Heat treatment
in H2 atmosphere
IEEE Photonics Soc. distinguished lecture
temperature
pressure
220ºC
0.025 MPa
58
Semiconductor waveguide isolator: demonstration
S
N
N
S
Output
Input
Isolation
4.9 dB
SNS
External
magnetic field
N
S
S
N
H. Yokoi, et al (Tokyo Tech.), Appl. Opt, 39,
6158 (2000).
Output
Input
IEEE Photonics Soc. distinguished lecture
External
magnetic field
NSN
59
Calculated characteristics
perfectly balanced
Transmission loss (dB)
0
-10
-20
Lasym =111 mm
Backward
-30
-40
Lasym =0 mm
-50
1530
slightly unbalanced
Forward
1540
1550
1560
Wavelength (nm)
1570
Ideal MMI couplers
Y.Shoji, T.Mizumoto, et al., APL, 92, 071117 (2008)
IEEE Photonics Soc. distinguished lecture
60
Outline
Part-1: Bulk nonreciprocal devices
 magneto-optic effect (Faraday rotation)
 operation principle of isolators and circulators
Part-2: Waveguide isolators
operational principles, design and characterization
 TE-TM mode conversion isolators
 Nonreciprocal loss isolator
 Interferometric isolator
 Semi-leaky waveguide isolator
Part-3: Waveguide circulators
Part-4: Non-magneto-optic approach
IEEE Photonics Soc. distinguished lecture
61
Mode conversion: semi-leaky
TE mode
Semi-leaky type
proposed by S. Yamamoto, et al (Osaka U.),
IEEE QE, 12, 764 (1976).
Mode conversion
- TE-guided and TM-radiation modes
- MO and LN mode conversions
Isolation: 20.2 dB (=1550 nm, L=1.5 mm)
- fabrication tolerant
- wavelength insensitive
θ c-axis
guided
LiNbO3 2.143
H
2.200
1.938
Ce:YIG
NOG
TE mode
TM mode
θ c-axis
H
radiated
LiNbO3 2.210
Ce:YIG
NOG
TM mode
2.200
1.938
T. Mizumoto et al.(Tokyo Tech), OFC 2007, OThU4 (2007).
IEEE Photonics Soc. distinguished lecture
62
Semi-leaky isolator: design
θ c-axis
50
NOG
Ce:YIG
qF=-4500 deg/cm
To cancel mode
conversion in
forward direction
 offset angle of
LiNbO3
Mode conversion
in backward direction
 isolation
IEEE Photonics Soc. distinguished lecture
Offset angle [deg]
Ce:YIG
40
30
40
 = 1.55 mm
30
Offset angle
20
20
10
0
10
Backward loss
1.0
Guiding layer thickness [ mm]
Isolation [dB/mm]
LiNbO3
0
2.0
Isolation = 14.1 dB/mm
for 50dB isolation : L=3.5 mm
63
Backward loss
10
=1.55mm
Isolation
1.0
Foward loss
0
1
1.1
1.2
1.3
Guiding layer thickness [ mm]
0
2.0
20
Backward loss
Isolation
1.0
10
Forward loss
0
1.45
1.5
1.55
1.6
Forward loss [dB/mm]
2.0
Backward loss/Isolation [dB/mm]
20
Foward loss [dB/mm]
Backward loss / Isolation [dB/mm]
Semi-leaky isolator: calculated performance
0
1.65
Wavelength [mm]
b-diagram
LiNbO3
Ce:YIG
ne=2.143
n=2.200
Guided mode
b/k0(TE)
1500nm <  < 1600nm:
Isolation >12.5dB/mm
Forward loss < 0.09dB/mm
LiNbO3
no=2.210
b/k0(TM)
Radiation modes
IEEE Photonics Soc. distinguished lecture
64
Semi-leaky isolator: fabrication
θ c-axis
LiNbO3
Sample set
Ce:YIG
Vacuum : 6.0x10-7 Pa
NOG
High vacuum
x-cut LiNbO3
4.5mm
RF plasma : Ar + O2
Pressure : 4.0 Pa
(= 3.0x10-2 Torr)
Gas flow : O2 2 sccm
Ar 20 sccm
RF power : 250 W
Time : 5 min
Positioning : ~ 10 min
Ce:YIG waveguide&terrace
Garnet No.CY0523
IEEE Photonics Soc. distinguished lecture
Press : ~ 1MPa
Time : 3 min
Anneal : none (RT)
Bonding completed
65
Semi-leaky guiding characteristics
partially guided
TE mode
q
c-axis
LiNbO3
radiated
TM mode
q
c-axis
LiNbO3
Ce:YIG
Ce:YIG
NOG
NOG
Semi-leaky guiding characteristic
IEEE Photonics Soc. distinguished lecture
66
Outline
Part-1: Bulk nonreciprocal devices
 magneto-optic effect (Faraday rotation)
 operation principle of isolators and circulators
Part-2: Waveguide isolators
operational principles, design and characterization
 TE-TM mode conversion isolators
 Nonreciprocal loss isolator
 Interferometric isolator
 Semi-leaky waveguide isolator
Part-3: Waveguide circulators
Part-4: Non-magneto-optic approach
IEEE Photonics Soc. distinguished lecture
67
Waveguide optical circulator: TE-TM Mode conversion
N. Sugimoto, et al. (NTT), IEEE PTL, 11, 355-357 (1999).
- uses TE-TM mode conversion
(rotation of polarization plane)
IEEE Photonics Soc. distinguished lecture
68
Waveguide optical circulator: operation principle
#3
#1
#4
#2
#4
#1
#2
IEEE Photonics Soc. distinguished lecture
#4
69
Waveguide optical circulator: performance
#3
#1
#4
Measured transmittance (dB)
#2
in
out
#1
#2
#3
#4
#1
#2
#3
#4
--
17.1-19.5
3.2-3.3
3.1-3.2
17.2-18.7
-3.0-3.1 26.4-31.4
27.0-31.8 3.0-3.1
--
N. Sugimoto, et al. (NTT), IEEE PTL, 11, 355-357 (1999).
IEEE Photonics Soc. distinguished lecture
70
Waveguide optical circulator: Interferometric circulator
Direction-A (in-phase interference)
T. Mizumoto, et al. (Tokyo Tech.),
EL, 26, 199-200 (1990).
Direction-B (out-of-phase interference)
IEEE Photonics Soc. distinguished lecture
71
Outline
Part-1: Bulk nonreciprocal devices
 magneto-optic effect (Faraday rotation)
 operation principle of isolators and circulators
Part-2: Waveguide isolators
operational principles, design and characterization
 TE-TM mode conversion isolators
 Nonreciprocal loss isolator
 Interferometric isolator
 Semi-leaky waveguide isolator
Part-3: Waveguide circulators
Part-4: Non-magneto-optic approach
IEEE Photonics Soc. distinguished lecture
72
Summary 2
 Interferometric isolator
- single polarization operation
--> no need for phase matching
- ultra-wide band operation (1.31 / 1.55 mm in a single chip)
- integration with active devices
--> Ce:YIG/ III-V, Ce:YIG/ Si low-temperature direct bonding
- first demonstration of Si waveguide isolator
--> 21 dB isolation
 Semi-leaky waveguide isolator
- highly fabrication tolerant
- LN/Ce:YIG direct bonding
- 20 dB / 1.5 mm
IEEE Photonics Soc. distinguished lecture
73
Summary 3
Waveguide circulator
 Hybrid Faraday rotation type
 MZ interferometer
Non-magneto-optic approach
 dynamic modulation
- indirect photonic transition of eigen modes
dependent on propagation direction
IEEE Photonics Soc. distinguished lecture
74