Technical manual No.G006-E

Faraday Rotator ( Rare-earth Iron Garnet single crystals )6/1/2015

CONTENTS
1. Type of Faraday Rotators

2

7. Anti-Reflection Coating (AR Coating)7-1. AR coating types
7-2. Features
7-2-1. Coating ( A ) for air
7-2-2. Coating ( E ) for epoxy
7-3. Incident angle

2. Order Information
3. Properties
4. Rare-Earth Iron Garnet Single Crystal
4-1. Crystal structure
4-2. Optical properties
4-3. Magnetic properties
4-4. Lead impurities

3

5. Optical Properties
5-1. Faraday rotation angle and extinction ratio
5-1-1. Measurement method
5-1-2. Wavelength dependence
5-1-3. Temperature dependence
5-1-4. Effect of incident angle
5-1-5. Effect of stress
5-2. Insertion Loss and refractive angle
5-2-1. Measurement equipment
5-2-2. Insertion loss
5-2-3. Refractive angle and coupling loss
5-2-4. Temperature dependence
5-3. External field and optical properties
5-3-1. Faraday angle and extinction ratio
5-3-2. Diffraction phenomena by magnetic domain

6

6. Magnetic Properties
6-1. Magnetization curve
6-2. Saturation field
6-3. Coercive force
6-3-1 Magnet-free Faraday Rotator
6-3-2 How to use GMF
6-4. Magnetic Compensation temperature
and Curie temperature

14

8. Reliability Test
8-1. Environment test
8-2. Laser damage
8-2-1. Laser damage by plus laser
8-2-2. Temperature rise by light absorption

4
5

7
8
9
10
11
12
13

15
16
17
18

1

19

20

21

9. Surface Quality
9-1. Standard specifications
9-2. Influence of defect and AR pinhole
9-3. RIG scratch and AR scratch
9-4. Chipping and corner break

22

10. Appendixes
10-1. Growth Striation
10-2. Swirl
10-3. Surface Undulation

24

23

1. Type of Faraday Rotators
Wavelength coefficient, (deg/nm) for 45deg-rotator
GTD
GLB
GSF
1310nm0.085
0.085
1480nm
0.07
0.075
0.075
1550nm
0.06
0.07
0.065
1590nm
0.055
0.065
0.06

This technical document describes the properties of Faraday rotators of
GRANPT. All Faraday rotators of GRANOPT comply with RoHS (Restriction of theuse of the certain Hazardous Substances).
G-series
GTD ( TD represents Temperature Dependence )Features small temperature dependence and small wavelength coefficient. This
product is generally used for free space optical isolators.
GLB ( LB represents L-Band )Features small insertion loss in the L-band wavelength and is generally used forin-line optical isolators.
GSF ( SF represents Saturation Field )Features lower saturation field. This Faraday rotator is used to reduce the size ofan optical isolator or used for a magneto-optical switch that changes the direction ofFaraday rotation in a magnetic field.
GMF ( MF represents Magnet-Free )Dose not need an external magnetic field from permanent magnetic etc.

Temperature coefficient, T (deg/C) for 45deg-rotator
GTD
GLB
GSF
1310nm0.04
0.071480 - 1590nm
0.045
0.065

GMF
0.085
0.075
0.065
0.06

ETD
0.085
0.07
0.06
0.055

GMF

ETD
0.04
0.045

0.07

Insertion loss(dB)
1310 - 1550nm
1590nm

GTD
0.1
0.16

GLB

GSF
0.1
0.12

GMF
0.1
0.16

ETD
0.1
0.16

GTD

GLB

GSF

GMF

ETD

310
450

390

320
475

280
420

310
450

GTD
700
<-50
300

GLB
800
<-50
300

GSF
200
-50
250

GMF

ETD
700
<-50
300

0.05

Thickness(μm) (typical)

E-series
ETD ( TD represent Temperature Dependence )Has the same characteristics as the GTD but contents no lead.

1310nm1550nm

2. Order Information

Magnetic property

When you place an order, specify the product as described below.

Hs(Oe)AAABBBBCD – Other information
AAA
Garnet
BBBB
Wavelength
C
Size
D
AR-coating

ex. GTD 1550 L A
GTD, GLB, GSF, GMF, ETD
1550(nm); 1310(nm), etc.
L; 11mm square C; Chip Size
A; For air
E; For epoxy
N; non coating
X; else－ Other information
ex. Accuracy of rotation angle, chip size, etc.
* If your demand is different from the standard specifications, contact us.

0
Tcomp.(C)
230
Tc(C)
500
Hc(Oe)Hs ;Saturation field for 1mm square chip. See Section 6-2.
Tcomp. ; Magnetic compensation temperature
Tc ;Curie temperature
Hc ;Coercive force

3. Properties2

4. Rare-Earth Iron Garnet Single Crystal (RIG)
4-1. Crystal structure

Table. 4-1 Physical properties
GTD
Lattice parameter (Å)
Thermal expansion coefficient(10-6 K-1)
Refractive index (1550nm)* Fig.4-2
Mohs’ scale*
Young’s modulus*(GPa)Poisson’s ratio*
Density*(g/cm 3)
Thermal conductivity*(W/m K)
Heat capacity*(J/mole K)* Estimation based on references.

GRANOPT manufactures bismuth-substituted rare-earth iron garnet (RIG)
single crystals which are material for Faraday rotator using the liquid phase epitaxial(LPE) method.
RIG, a very stable oxide with the chemical formula (C3)(A2D3)O12 is a single
crystal having the structure of a cubic system as shown Fig. 4-1. Table 4-1 andFig.4-2 show the physical properties of RIG.

GSF
12.5

11

GMF

10

１１１
４２２

2.3 – 2.4
7
200
0.29
6.7
5
420

2.40
dodecahedral sites
octahedral sites
tetrahedral sites

GLB
GTD, E TD
GS F
GMF

2.30

Oxygen ions
１ １
０
４ ２

2.20
800

1000

1200

1400

1600

W avelength (nm)Fig.4-2

Fig.4-1 Crystal structure

3

Refractive index of Faraday rotator.

ETD

11

2.50

１１
０
４２

Index

１１
０
４２

GLB

1800

4-2. Optical properties
RIG is a magneto-optical material that is transparent in the near infrared
radiation region and mid-infrared radiation region with a wavelength of 1 to 5m andhas a very large Faraday effect. The Faraday effect is a phenomenon in which thepolarization plane of the light is rotated when the light passes through a magnetizedmaterial. This rotation is caused by differences in response between the left andright circular polarization.
RIG has very large Faraday rotation coefficient of 1000 to 2000 (deg/cm) in thenear-infrared radiation region. Fig. 4-3 shows the Faraday rotation coefficient of the
GTD, that is, the Faraday rotation angle per centimeter.
The magnitude,
wavelength dependence, and temperature dependence of the Faraday rotation
coefficient vary with the product. Refer to Chapter 3 for details. In our products,
the film thickness is adjusted so that the Faraday rotation angle becomes 45 degrees
at a specified wavelength.
Letting linearly polarized light pass through RIG causes Faraday rotation, but
the ellipticity of the linearly polarized light increases more or less. This ellipticity isexpressed by extinction ratio. The extinction ratio is affected by magneto-optics,
external stress of the material (see Section 5-1-4), the performance of AR coating(see Section 5-1-4), etc. In addition, if the property (mono-chromaticity) of a lightsource used for measurement is low, the extinction ratio maybe become low due tothe wavelength dependence of the Faraday rotation angle shown in Fig. 4-3.
Because all types of RIG contain iron ions that have optical absorption, the RIG
is not opaque in the visible range. Some of our products other than the GLB may
have high insertion loss in the L band due to weak optical absorption by rare earth
ions. Refer to Section 5-2 for details.
Fig.4-3 Wavelength dependence of Faraday coefficient44-3. Magnetic properties

4-4. Lead impurities

RIG is a ferrimagnetic material, which is one of ferromagnetic materials.
Generally, a ferromagnetic material has a magnetic domain where magnetization isaligned in the same direction. The RIG manufactured by the LPE method of
GRANOPT has a maze-like domain structure in which two magnetic domains areoriented in two mutually opposite directions orthogonal to the film surface as shownin Photo 4-1. Because the light beams that pass through these two magnetic
domains rotate in mutually opposite directions, the extinction ratios of the light beams
are degraded, causing interference of the transmitted light beams (refer to Section
5-3). Therefore, it is not possible to use the RIG as a Faraday rotator without
applying a magnetic field.
When RIG is exposed to an external magnetic field greater than its saturation
field, the magnetic domain in the direction opposite to the external magnetic field
disappears and a single magnetic domain in which the direction of magnetization isaligned is formed. Since light passing through RIG with a single magnetic domain
is subject to Faraday rotation in one direction, the RIG can be used as a Faraday
rotator.
Once magnetized in a strong external magnetic field, the GMF retains its single
domain structure even if the external magnetic field is removed. Thus, the GMF
require no external magnetic field when it is used. In other words, the GMF
functions as a Faraday rotator without being exposed to an external field. Refer to
Chapter 6 for details.

GRANOPT manufactures RIG from a flux induding lead dioxide using a crystal
growth method called LPE method. Our RIG products conventionally included
about 5,000ppm lead as impurities. We have developed the technology to control
the lead content in RIG and provided the G-series Faraday rotators, including theGTD, GLB, GSF, and GMF. In addition, we have developed lead free RIG, the ETD
Faraday rotator. Fig. 4-4 shows the distribution of lead content of the G-series
products. The lead content is controlled to less than 1,000pppm so that all the
products comply with RoHS.

GMF
GTD

800

Lead Impurities ( ppm )Photo 4-1

Domain structure of RIG

Fig.4-4

5

Lead impurities

1000

900

600

500

Frequency

GLB

700

GSF

5. Optical Properties
5-1. Faraday rotation angle and extinction ratio
5-1-1. Measurement method
Fig. 5-1 shows the basic structure of the measurement equipment used tomeasure the Faraday rotation angle and extinction ratio. A DBF laser with a highmono-chromaticity is used as the light source. The rotation angle and extinction
ratio are measured using a personal computer that controls a polarizer, analyzer,
sample holder, detector, and thermometer.

Collimator

Faraday Rotator
MagnetPolarizer

SM-fiberLight Source

Analyzer

Detector

Power Meter
Themometer

Fig.5-2 Wavelength dependenceController (PC)Fig.5-1

Measurement equipment for Faraday angle and extinction ratio

5-1-2. Wavelength dependence
Figs. 5-2 and 5-3 show changes in the Faraday rotation angle of the GTD withrespect to the wavelength. The tables in Chapter 3 list the wavelength dependenceof typical Faraday rotation angles for the individual products. In the wavelength
dependency, changes in the Faraday rotation angle per 1-nm for 45-degree Faraday
rotators. There no significant differences are present in the O-band and E-band, butlarge differences are present in the C-band and L-band.
Fig.5-3 Wavelength dependence65-1-3. Temperature dependence

50

Faraday Angle （ degree)

Fig. 5-4 and 5-5 show changes in the Faraday rotation angle of the GTD withrespect to the temperature. We define the changes in the rotation angle of a 45
degree Faraday rotator per one degree as the temperature dependence by using thevalue calculated from the data measured in the range of 0 ℃ to 50 ℃ . The
temperature dependence of the GTD and ETD slightly differs between 1310nm and1550nm. In other products, the rotation angle depends on the model, but does not
greatly depend on the wavelength.
From Fig. 5-6, it is found that the extinction ratio do not depend on thetemperature.

GTD1550、ETD1550

GLB1550
GSF1550

50

40
-50

0

50

100

Fig.5-5

Temperature dependence of Faraday rotation angle
0

45

-10
Extinction Ratio (dB)Faraday Angle （degree)

Temperature （℃）

40
-50

0

50

100

Temperature （℃）

Fig.5-4

Temperature dependence of Faraday angle

-20

-30

-40

-50
-50

50

150

250

Temperature (℃)

Fig.5-6

7

Temperature dependence of extinction ratio

5-1-4. Effect of an Incident Angle

The reason why the Faraday rotation angle does not depend on the incidence
angle of light will be described with reference to Fig. 5-7. The direction ofmagnetization has an effect on the Faraday rotation angle. General RIG ismagnetized perpendicularly to the film surface because of its magnetic anisotropy.
The length of the path of light passing through RIG becomes longer even when thelight enters at the slant, but the effect of magnetization is reduced. Therefore,
certain Faraday rotation is applied.
On the other hand, the extinction ratio reduces as the incident angle increase.
This is because the performance of the AR coating is degraded. Degradation in theperformance of AR coating causes part of light to make multiple reflections in the
RIG. The multiple reflection light that transmits in the same direction as in theincident light receives the Faraday effect three to five times more than the normal
one and the multiple reflection light affects the measurement result of the extinction
ratio.
From Fig. 5-9, which shows the relationship between the incident light and theinsertion loss, it is found that generation of the multiple reflection light causes
reflection and increases insertion loss.

Filled circles in Fig. 5-8 show an example of measuring the Faraday rotation
angle by changing the incident angle of light with respect to the Faraday rotator.
Open circles show an example of measuring the extinction ratio in the same condition.
( thickness ) / cos (ψ)( Magnetization ) x cos (ψ)
incident angle : ψlightMagnetization

Fig. 5-7 Incident angle and magnetization
0

1

-10

-20
45
-30
44.5

Insertion Loss (dB)

45.5

Extinction ratio (dB)Faraday rotation (deg)

46

-40

44

0.5

-50
0

10

20

30

40

50

0
0

Incident angle (deg)

20

40

60

Incident angle of light (deg)Fig. 5-8

Measurement of Faraday rotation angle and extinction ratio
Fig. 5-9

8

Measurement of insertion loss

5-1-5. Effect of stress
The extinction ratio is also degraded by a stress. Fig. 5-11 shows themeasurement result of the extinction ratio by applying an external stress to the
Faraday rotator as shown in Fig. 5-10. Degradation in the extinction ratio is causedby photoelasticity. The horizontal axis in Fig. 5-11 represents the angle formed by
the polarization direction of incident light and the direction of stress. When thisangle is 22.5 degrees, the extinction ratio or the effect becomes minimum. This isbecause the polarization plane of the incident light rotates orthogonal to the stressdirection by -45 degrees.
00(N/mm 2)
3
6
10
12
14
17

10
Extinction Ratio (dB)

load

E

linearly polarized lightpressure sensor

20

30

40

Fig. 5-10 Measurement method
50
-67.5

-45

-22.5

0

22.5

45

polarization plane for stress (degree)

Fig. 5-11

9Degradation in extinction ratio due to external stress5-2. Insertion loss and refractive angle
5-2-1. Measurement equipment

1

Fig. 5-12 shows the structure of equipment for measuring the insertion loss andrefractive angle. The light source is a PF laser, DFB laser, or a variable wavelength
laser. A personal computer is used to control the slit for measuring the refractive
angle, the sample holder, and the detector. This equipment can measure theinsertion loss and refractive angle individually.
The insertion loss is defined as the difference in the strength of transmitted lightbetween presence and absence of the sample. The refractive angle is calculated
from the attenuation of light passing through the slit.

Insertion Loss(dB)

0.8

0.6
GTD, E TD

0.4
GM F
GS F

0.2

Faraday Rotator
MagnetCollimator
SM-fiber

GLB

Slit

0
1300

Detector

1400

1500

1600

1700

W avelength(nm)Light Source

Fig.5-13

Power Meter

Insertion loss of individual product8

Controller (PC)6

Structure of equipment for measuring insertion loss and refractive angle

Insertion Loss(dB)

Fig.5-12

5-2-2. Insertion loss
Fig. 5-13 shows the measurement result of insertion loss in the C- and L-bands
for the individual products. In the products other than the GLB, insertion loss
increases in the L-band due to weak absorption by rare earth ion. Accordingly, inthe L-band, use of the GLB is recommended.
Fig. 5-14 shows the insertion loss of the GLB in the short wavelength. The
products other than the GLB have an insertion loss of more than 1.5 times of thisvalue. Accordingly, also in the short wavelength, use of the GLB is recommended.

4

2

0
700

Fig.5-14

10

800

900
1000
Wavelength(nm)1100

Insertion loss of GLB in short wavelength

5-2-3. Refractive angle and coupling loss
The insertion loss of the in-line optical isolator consists of insertion loss by opticalabsorption in a Faraday rotator and coupling loss by refraction of light (refractive
angle) by a Faraday rotator. GRANOPT defines insertion loss separately fromcoupling loss by refraction of light by a Faraday rotator and determine different
specifications. GRANOPT does not guarantee the coupling loss. This is because
the coupling loss is indirectly affected by the refractive angle of a Faraday rotator
through the structure of the collimator or other components.

Ｉｎｓｅｒｔｉｏｎ Loss(dB)

105-2-4. Temperature dependence
Figs. 5-11 and 5-16 show the measurement result of temperature dependenceof the insertion loss.
Fig. 5-15, which is the measurement result for the GTD1310, shows that insertion
loss is not temperature-dependent in the wavelength band in which no absorption
loss is present. However, as shown in Fig. 5-16, insertion loss is temperaturedependent in a wavelength band close to 1m in which absorption loss is present.

0

Fig. 5-16

Ｉｎｓｅｒｔｉｏｎ Loss(dB)

0.3

0.2

0.1

0
25

50

75

100

Temperature (℃）

Fig. 5-15

25

50

75

100

125

Temperature (℃ ）

0.4

0

786nm

0

GTD1310-25

51064nm

0.5

-50

973nm

Temperature dependence of insertion loss of GTD1310

11

Temperature dependence of insertion loss of GLB
in short wavelength5-3. External field and optical properties
5-3-1. Faraday angle and extinction ratio

Faraday rotation

Fig. 5-18 shows the measurement result of the Faraday rotation angle and theextinction ratio with respect to an external magnetic field. The Faraday rotation
angle apparently changes in proportion to the external magnetic field. However, it
can be see from the reduction in the extinction ratio that the Faraday rotation angle
does not change linearly. The Faraday rotation of RIG is theoretically quite different
from the magneto-optical effect proportional to the external magnetic field asexpressed by the Verdet’s constant.

50

(+f)

(-f)

50
45

Faraday Angle (deg)40domain-1
domain-2

40
30
35
20
30
10

25

d 1 d2

Saturation Field+Ms

0Fig. 5-17

substrate

200magnetization
Hex

Extinction Ratio (dB)

When RIG is not magnetically saturated, the multi-domain structure as shown in
Photo 4-1 is formed in the RIG. When light beams pass through RIG that is notmagnetically saturated, the light beams passing through the individual domains are
subject to Faraday rotation in the two opposite directions and diffraction is caused by
the interference of the individual light beams (refer to Section 5-3-2). As a result,the light beams passing through the RIG are detected as a sum of light componentsthat have passed and scattered in the magnetic domains.

-Ms

200

400

600

800

Magnetic Field Hex (Oe)Fig. 5-18

Multi-domain structure and Faraday rotation of RIG

12

Measurement of Faraday rotation angle and extinction ratio
in multi-domain structure.

5-3-2. Diffraction phenomena by magnetic domain
When light passed through a magnetically unsaturated Faraday rotator,
diffraction is caused by the multi-domain structure as shown in Fig. 5-19. As the
Faraday rotation that depends on the difference in the area of a magnetic domains
is caused in 0-dimensional transmitted light(n=0), apparently constant Faraday
rotation is observed with respect to the polarization plane of incident light while thediffracted light has the polarization plane perpendicular to the polarization plane ofincident light. For example, for a 45-degree Faraday rotator, the maximum amountof light diffracted is about -3dB or about -1dB as actual measurement data, which
depend on the multi-domain structure. The reason why the insertion loss of amagnetically unsaturated Faraday rotator appears to be larger when using a detector
with a finite size is because of this.

+2+1
incident
polarization

n=0

(0)



-1
-2

Fig. 5-19

Diffraction by multi-domain structure13

6. Magnetic properties
6-1. Magnetization curve

Nucleation magnetic field Hn :When the external magnetic field required for magnetic saturation is reduced,
the domain structure appears at the specific magnetic field. We define this specificmagnetic field as nucleation magnetic field Hn. Refer Section 6-4 for details.

Fig.6-1 shows the structure of equipment for measuring the magnetization curveof a Faraday rotator. When an external magnetic field is applied to a Faraday rotator,
the width of magnetic domains changes according to the external magnetic field.
Light transmitted through or diffracted in the magnetic domains is separated into two
mutually orthogonal components by the analyzer. Then, the magnetization curve of
the Faraday rotator can be drawn by taking the differences between these twocomponents. Fig. 6-2 shows the magnetization curve of the GTD measured by thisequipment.
Saturation field Hs, nucleation magnetic field Hn, and coercive force Hc can beread from this magnetization curve.

Coercive force Hc :In the GMF, the domain structure does not appear even if the applied external
magnetic field is reduced, but the direction of magnetic field is reversed when amagnetic field in the opposite direction is applied. This magnetic field at whichmagnetization is reversed is called a coercive force. Although coercive force Hc isalmost the same as nucleation magnetic field Hn theoretically, we defines this value
as coercive force Hc only for the GMF. Refer to Section 6-3 for details.

1.5

Faraday Rotator
Collimator Polarizer

Electro-magnet

SM-fiberHn1

Analyzer PD

Light Source

m/Ms

0.5

Voltmeter
Gauss-meter

Voltmeter

Hs

0
-1000

-500

0

500

-0.5

Controller (PC)
Fig. 6-1-1Structure of measurement equipment for magnetization curve-1.5

Saturation field Hs :The saturation field is the magnetic field at which a Faraday rotator becomessaturated magnetically or shifts from the multi-domain structure to the single domain
structure. When a magnetic field greater than the saturation field is applied, aFaraday rotator has a fixed Faraday rotation angle. The saturated magnetic fielddepends on the shape of the Faraday rotator. Refer to Section 6-2 for details,

Hex(Oe)Fig. 6-2

14

Magnetization curve of GTD

1000

6-2. Saturation field

250

When an external magnetic field greater than the saturation field of RIG isapplied, RIG is magnetically saturated and functions as a Faraday rotator.
Therefore, when a product other than the GMF is used, it is necessary to user apermanent magnet that can apply an external magnetic field value greater thansaturation field Hs. Fig. 6-3 illustrates the temperature dependence of saturation
field Hs of the GTD and ETD. Fig. 6-4 illustrates the temperature dependence of
saturation field Hs of the GSF. Note that the saturation field depends on thetemperature. We define the saturation field required in the temperature range from
-20℃ to 85℃ as the specification.

H s (Oe) 1.5mm □

200

150

100

Saturation Field Hs(Oe)

1500

50

0

1000

-50
GTD、ETD

Fig. 6-4500

050Temperature (℃ )100

Temperature dependence of saturation field of GSF
1.0

0.8

-50

Fig. 6-3050100Temperature(℃)

D emagnetizing F actor N

0150

Temperature dependence of saturation field of GTD and ETD

Saturation field Hs depends on the shape of a product. Fig. 6-5 shows theapproximation of the shape of a product and the demagnetization factor. The
horizontal axis represents the size-to-thickness ratio and the vertical axis represents
the demagnetization factor. The saturation field for a flat plate of infinite size (thehorizontal axis is ∞ and the demagnetization factor is 1) multiplied by thedemagnetization factor in Fig. 6-5 is the saturation field for the actual product size.
A product with a smaller size and a larger thickness has a smaller demagnetization
factor and smaller saturation field. We define the saturation field of a 1mm chip,
which is used generally, as the specification.

0.6

0.4

0.2

0.0
1

10
size / thickness

100

Fig. 6-5 Geometrical shape and demagnetization factor

15

6-3. Coercive force
6-3-1. Magnet-fee Faraday rotator
Fig. 6-7 illustrates the temperature dependence of coercive force Hc of the GMF.
The coercive force reduces as the temperature rises and loses its coercive force if
the temperature becomes high for even a moment. Note that history of temperatureapplied to the GMF. Since applying mechanical stresses to the GMF also cause
the coercive force to be lost, handle it with care.

Coercive Force (kA/m)

80

50
40

Faraday Angle (deg)

30

1000

60
typical
40

500

20

minimum

20
Hc

10

0

0
-1000

-500

-10

0

500

1000

0

100

Temperature (℃)

-20
-30

Fig. 6-7-40-50
Hex(Oe)Fig. 6-6

50

0
150

Hex -  curve of GMF

16

Temperature dependence of Hc of GMF

Coercive Force (Oe)Fig. 6-6 illustrates the magnetization curve of the magnet-free rotator GMF.
The GMF is a Faraday rotator that has a square-shaped hysteresis loop and keeps
constant Faraday rotation even when an external magnetic field in the opposite
direction is applied. If the GMF is exposed to an external magnetic field in the
opposite direction that is greater than coercive force Hc, the GMF is magnetized in
opposite direction and Faraday rotation is reversed or the GMF is demagnetizedcompletely. Note magnetic field that may be applied.
The coercive force of the chip-size GMF varies from about 400 to 2000 Oe
individually. The coercive force depends on the external stress or the shape of across section in the cutting process. We measure coercive force Hc of each chip to
see if the specification is satisfied before shipment. However, it should be notedthat if the garnet is cut into a chip size, we do not guarantee the original coercive
force.

6-3-2. How to use GMF
The GMF retains its magnetization and operates without an external magnetic
field from a permanent magnet etc. However, the GMF is limited in the operating
temperature when an external magnetic field applied. When the GMF is placed in
an environment where the temperature is high and the magnetic field is strong, it isdemagnetized and no longer functions as a Faraday rotator. Once the GMF has
been demagnetized at high temperature, it no longer functions as a Faraday rotatoreven if it is cooled to room temperature.

Direc tio n o f Magn etization

The GMF is packed on a chip tray and magnetized before shipment. The
magnetization oriented from the top side of the chip to the bottom side, as shown in
Fig. 6-8. When light is incident on the chip in the direction of magnetization, thepolarization plane rotators counterclockwise. Magnetization can also be performedby the user.
As previously described, the value of coercive force Hc varies from chip to chip.
Fig. 6-7 illustrates the temperature dependence of Hc of the chip having a typical Hc
and the lower limit of coercive force Hc. Coercive force Hc exhibits reversible
changes with temperature. However, once the chip has been demagnetized at hightemperature, the chip no longer exhibits the original coercive force and does not
function as a Faraday rotator even if it is cooled to low temperature. In order torecover the coercive force, the chip must be magnetized again.

Magnetic Garn etSingle Cr ystal

The GMF may be demagnetized to change its coercive force due to an external
stress caused by a cutting operation etc.
The method of re-magnetizing the GMF will be described below.
The GMFcan be re-magnetized by applying a strong external magnetic field. An external
magnetic field can be applied by an electromagnet or permanent magnet. If the
GMF has been demagnetized for some reason, it need to be re-magnetized. To remagnetized the GMF, it is recommended that an external magnetic field higher than3000 Oe should be applied at a temperature of 40℃ or higher. If the temperature
is lower, it is necessary to apply a higher external field to recover the original coercive
force. If the temperature is lower than room temperature (25 ℃ ), sufficientperformance may not be obtained. The duration long enough to apply an external
magnetic field is 1 second. If the user cuts the GMF, it is necessary to confirm thatthe individual chips have sufficient coercive force by, for example, applying amagnetic field of 400 Oe in the opposite direction and making sure that the individual
chips are not magnetized in this direction.

Chip tray
Direc tio n of Magn etization
Dire ction of rotation
Direction of magn etization
Ray of light

Magnetic Garn etSingle Crystal

Po larization

Direction of Po larization

Fig.6-8

Direction of magnetization

17

60

6-4. Magnetic compensation temperature and Curie temperature

40

Faraday Angle （degree)

RIG has peculiar temperatures called the magnetic compensation temperature( Tcomp. ) and Curie temperature ( Tc ), at which the saturation field becomes zero.
At a Curie temperature of 220℃ to 300℃ or higher, which depends on the product,
the saturation magnetization and the Faraday rotation angle become zero. The
magnetic compensation temperature, which also depends on the product, is lower
than room temperature for all products. The saturation magnetization becomeszero at magnetic compensation temperature and the direction of magnetization isreversed before and after this temperature. When an external magnetic field isapplied to RIG, the sign of the Faraday rotation angle is also reversed before andafter the compensation temperature.
Fig. 6-9 shows an example of the Faraday rotation angle measured by applying
an external magnetic field larger than its coercive force to our GMF. The magnetic
compensation temperature of the GMF is about -10℃ at which the sign of the
Faraday rotation angle is reversed. This is because the direction of saturation
magnetization of RIG changes in accordance with the external magnetic field. For
the GMF to which an external magnetic field is not applied, the sign of the Faraday
rotation angle is not changed, as shown in Fig.6-10. It should be noted that an
external magnetic field greater than its coercive force cannot be applied to the GMF.

20

0
-50

0

50

100

Temperature （℃）

-20

-40

-60

Fig. 6-9

Change in sign of Faraday rotation angle near compensation temperature
with external magnetic field
60

Faraday Angle （degree)

40

20↓ Tcomp.
0
-50
-20

0

50

100

Temperature （℃）

-40

-60

Fig. 6-10

18

Change in sign of Faraday rotation angle near compensation
temperature without external magnetic field7. Anti-Reflection Coating ( AR Coating )7-1. AR coating types
Table.7-1. AR coating types
Index
Item codeA
For air
For epoxyE
N
X

Wavelength
Flexible
Fixed , 1300 – 1650 nm
No AR coatingElse

7-2. Features
7-2-1. Coating A
Coating A assumes air (refraction index n= 1) as a medium and has the minimum
reflection coefficient at a specific wavelength. Coating A is made of SiO2/TiO2 or
SiO2/Ta2O5. Fig. 7-1 shows a typical example of coating A of SiO2/Ta2O5 for 1550nm.
The GLB1064 consisting of the RIG (n = 2.39, see Table 4-1) and the substrate(n= 1.96) can be coated with the coating A, bur this coating cannot prevent thereflection caused by differences in the refractive indexes of the RIG and the substrate.
In this case, it is recommended that the RIG should face the light source.

Reflection (one side)≦ 0.2 %
≦ 0.2 %

Table 7-1 lists the types of AR coatings that can be applied by GRANOPT. AR
coatings are applied by the vacuum evaporation method. Coating A is an AR
coating designed so that the reflection coefficient becomes small in the air at the
specified center wavelength. Coating E is an AR coating designed to preventreflection on a Faraday rotator in the state where the Faraday rotator is bonded toanother optical component, and reduces the reflection coefficient to a small value
over a wide range of wavelengths.
The types of AR coatings attached to both sides of Faraday rotator may bedifferent each other. Please consult us for specific applications.

1

Reflection (%)

0.8

0.6

0.4

0.2

0
1400

1500

1600

1700

W avelength (nm)Fig. 7-1 Example of coating A (example of evaluation)19

7-2-2. Coating E

8. Reliability test
8-1. Environment test

0.5

Our products are tested under the following standard conditions. The items
to be tested are the Faraday rotation angle, extinction ratio, insertion loss, surface
quality, and adherence test of AR coatings. For the GMF, the coercive force is also
evaluated.

0.4
Reflection (%)

Coating E is an AR coating
designed to prevent reflection in thestate where a Faraday rotator isbondedtoanother
opticalcomponent, and reduces thereflection coefficient to a small value
over a wide range of wavelengths
from 1300nm to 1600nm. Coating
E is made of SiO2/TiO2. Fig. 7-2
shows the result of simulating theperformance of the AR coatingwhen the refractive index of theadhesive changes.

0.3

0.2

0.1

0
1250

High temperature storage test
Low temperature storage test
Damp heat test
Heat cycling test

n=1.62n=1.58n=1.54n=1.50

1350

1450

1550

1650

W avelength (nm)Fig.7-2 Calculation results

7-3. Incident angle
0.5

0.4
Reflection (%)

The performance of an AR
coating degrades as the incident
angle of light increases. Fig. 7-3
shows the calculation results ofcoating A.

0.3

0.2

Incidente angle
5 degree10 degree (p)20 degree (s)20 degree (p)30 degree (s)30 degree (p)

0.1

0
1450

1500

1550

1600

1650

W avelength (nm)Fig.7-3 Calculation results

20

100℃、2000hr-40℃、2000hr
90℃、90％Rh、2000hr-45℃ – 90℃、1000cycles

8-2. Laser damage
8-2-1. Laser damage by plus laser

A1T1064 has a insertion loss of about 0.8dB at a wavelength of 1064nm and an
output power of 100mW or less. The marks ● represent the power of light passthrough the A1T1064 from which the substrate has been removed. The marks ■represent the transmittance. The power of transmitted light reduces significantly
when to power or incident light is 200mW or more. This is because the temperatureof RIG is raised by light absorbed by RIG and this increases the amount of lightabsorbed by RIG (refer to Section 5-1-4). This phenomenon occurs whentemperature rise due to absorption of light is faster than the rate at which generated
heat is dissipated. The marks ○ and □ represent the results of the same testwhen using the A1T1064 from which the substrate is not removed. Due to the heatsink effect of the substrate, generated heat is dissipated smoothly and an increaseby absorption dose not reach 1W. At a wavelength from 1200 to 1600nm, it isexpected that a temperature rise due to light absorption dose not occur until input
power reaches several watts.

Pulse laser

Beam diameter
Peak power
Pulse width
Pulse frequency
Exposure time

18 m
0-800 MW/cm2
20 ns
6 kHz
2 sec

10

*

The A1T1064 has been discontinued.

2

Thresholds : 100 (MW/cm )( 260W 18m )5

Output Power Po(mW)D epth of damage (  m)

P ulsed laser : 1558nm, 18 m,
6kHz, 20ns

00Fig. 8-1

200
400
600
Peak power (MW/cm2 )800

The GLB1064 is the substitute product.

1000

100

800

80
YAG
0.7~1.1mm

600

60

400

40

200

20
○□ with subs trate●■ without substrate

Laser damage by pulse laser00

8-2-2. Temperature rise by light absorption

200

400

600

800

Input Power Pi(mW)

Fig. 8-2 illustrates the relationship between the input power of light and theinsertion loss. The Faraday rotator AiT1064 and a YAG laser with a wavelength of
1064nm and a beam diameter of 1mm are used for this measurement. The

Fig. 8-2

21Power of light and transmittance

01000

Po/Pi(%)

Fig. 8-1 illustrates damage caused by the following pulse laser.
From Fig. 8-1, we estimated that the minimum power that causes opticaldamage is about 100 MW/cm2.

9. Surface Quality
9-1. Standard specifications

9-2. Influence of defect and AR pinholeFig. 9-1 shows the results of calculation (solid line) and measurement (●) of theinfluence of a defect on insertion loss. The diameters of light beams are 300µm and400µm and defect is assumed to be present at the center of the beam.

Table. 9-1 11mm-square with AR coating
Specification
Note
Aperture
An area of at least (product size - 0.5)mm square for theuncoated triangle(1x1mm)at each corner which is masked
by a holding fixture when AR coated.
Absent in aperture
Chipping
RIG scratch2m ≦ Width ≦ 5m
Total ≦ 10mm
5m ≦ Width
AbsentAR scratch2m ≦ Width ≦ 20m
Total ≦ 10mm
20m ≦ Width
AbsentAR pinhole
50m ≦ Width
AbsentOther defects 30m ≦ Size ≦ 350m≦ 5 pieces
350m ≦ Size
AbsentTable. 9-2 Chip size with AR coating
Specification
Aperture
Product size ≦ 1mm
1mm <Product size< 2mm
Chipping
Absent in aperture
RIG scratch
Width < 2m
2m ≦ Width ≦ 5m
5m ≦ Width
AR scratch
Width < 2m
2m ≦ Width ≦ 20m
20m ≦ Width
AR pinhole
50m ≦ Size
Other defects
Size < 10m
10m ≦ Size ≦ 30m
30m ≦ Size

0.5
Calculation Results ( defect )Beam Diameter 300 m

Insertion Loss(dB)

0.4

0.3
Beam Diameter 400 m

0.2

0.1

Note
≦(Product size - 0.2mm)
≦(Product size)×0.8 

00No check(Product size) × 0.8
AbsentNo check(Product size) × 0.8
AbsentAbsentNo check≦ 5 pieces
Absent

20

40
60
80
Size of a defect (m)100

Fig. 9-1 Influence of defectFig. 9-2 shows the results of calculating the influence of an AR pinhole on
insertion loss. Fig. 9-2 also shows the influence of an AR pinhole on the reflection
coefficient. A defect and AR pinhole have an effect on the reflection coefficient andinsertion loss to some extent, but the above calculations assume that a defect andpinhole are present at the center of the light beam. In reality, it can be seen from
Fig. 9-3 that the probability of the presence of a defect or pinhole at the center of thelight beam is small.

22

0.15

30
Calcuation Results( AR-coat spot for Air )Beam Diameter = 300m

0.1

20

0.05

9-3. RIG scratch and AR scratch
Fig. 9-4 shows the resultofcalculationandmeasurement ( ● ) of theinfluence of a scratch. A
light scratch on an AR coating
does not affect insertion loss,
so we provide separate
specifications for these two
scratches.

10

0.2
Calculation Results( scratches )Beam Diameter = 300μm

0.15Insertion Loss(dB)

40

Reflection(dB)

Insertion Loss(dB)

0.2

0.1

0.05

0

0
0

10

20

30

40

50
0

Diameter of a AR pinhole(m)0Fig. 9-2 Influence of AR pinhole

calculation parameters
defect size ; 50m
beam radius ; 300m
optical effective area ; 750m

Insertion Loss (dB)

0.15

0.10

30

20

0.05

10

2
3
4
Width of a scratch (μm)

9-4. Chipping and corner break
GRANOPT defines chipping and corner break as shown in the figures below.
Chipping is defined by the depth of deficit from the edge. Corner break is definedby the broken part. The standard specification defines that there is no chipping or
corner break in the valid area.
chippingA(long)B(short)

0.00
0

100

200

300

0
400

Distance from beam center (m)Fig. 9-3

5

Fig. 9-4 Influence of RIG scratch
on insertion loss

40

existence probability(%)

0.20

1

Probability and influence of presenceof defect and pinholeFig. 9-5

23

Chipping and corner chip

chipping

10.Appendixes
10-1. Growth striation

10-2. Swirl
The V-shaped line in Photo 10-1 called swirl which is generated on RIG
manufactured by the LPE method. A swirl is a change in the refraction index that isoriginated from small defect generated in RIG and is observed by an infrared
microscope. This change is caused by a slight change in the composition underinfluence of convection in crystal growth process. We have also confirmed that the
swirl does not affect the optical properties.

The Photo 10-1 is an example of a growth striation that is observed with an
infrared microscope. GRANOPT manufactures crystals of RIG by the LPE method,using the substrate GGG as a seed crystal. Since the temperature periodicallychanges and the crystal growth rate changes in the substrate GGG, the compositionor the impurity density may change slightly, possibly causing annual ring-shapedlines. The striation of the substrate may be reflected on the RIG generated on thesubstrate GGG, and slight changes in the refraction index may be observed by an
infrared microscope as shown in Photo 10-1.
We have scanned the entire surface to measure the Faraday rotation angle,
extinction ratio, insertion loss, and other optical properties of RIG on which growth
striation is generated and confirmed that the growth striation does not affect these
optical properties.

10-3. Surface undulation
There is a nano-scale undulation on the surface of RIG.
Fig. 10-1
exaggeratedly shows effects of an undulation on reflected light. We suppose that
stripes rarely observed by a metallurgical microscope are formed when thisundulation has an effect on reflected light in the visible band, which falls outside thescope of design of AR coating.
In a crystal growth process, RIG has inner stress caused by differences thethermal expansion coefficient between RIG and the substrate. If the substrate GGG
is removed, the individual products suffer warping due to this inner stress. We
polish the while relaxing this warping and finish it in CMP. We suppose that thesurface undulation is caused by this etching of CMP.
As a result of evaluation of the effect of the undulation on optical properties, wehave confirmed that the surface undulation does not have a great effect because thesurface undulation is significantly smaller than the wavelength of light.

Fig. 10-1 Observation principle
Photo 10-1

Growth striation and swirl

Photo 10-2

24

Surface undulation