Optical isolator for pulsed fiber lasers
Koji Omichi,1 Yukihiko Takahashi,1 Kouki Shimohira,1 Jun Sato,1 Takeshi Kizaki,2Akiharu Funaki,2 and Kazunori Kabayama2
Optical isolators are required for high power pulsed fiber lasers to block reflected light fromobjects being processed. Fujikura has realized an isolator using Tb3(Sc,Lu)2Al3O12 (TSLAG)
single crystal as the Faraday rotator for the first time. In this paper, we report unique features ofthe TSLAG single crystal and typical optical quality of the isolator.

1. IntroductionPulsed fiber lasers are required further optical output power because they are expected to expand applications, such as engraving, scribing for patterning,
micro welding and micro cutting1). It is important for
the lasers to block reflected light from objects being
processed because this reflected light causes lasing
instability and damage to laser components. Therefore, an optical isolator is necessary to prevent incident reflected light from reaching the pulsed fiber lasers.
Optical isolators have been utilizing since the beginning of optical communication networks. Therefore,structures and operation principles of optical isolators
are well known, with a Faraday rotator is considered
as one of the most important parts for the isolators. In
optical communication networks, ferromagnetic garnets, such as Y3Fe5O12 (YIG), have been utilized as a
Faraday rotator because they have large Verdet constant and relatively high transparency in the 1.3-1.55
μm wavelength range. However, it is impossible forpulsed fiber lasers to employ ferromagnetic garnets
due to optical absorption in the 1 μm wavelength band.
Therefore, highly transparent garnets in the 1 μm
wavelength band have been needed for a long time.
Currently, Tb3Ga5O12 (TGG) is widely used as a Faraday rotator owing to increasing of demands for 1 μm
wavelength band pulsed fiber lasers. However, it remains a challenge for the TGG to grow large bulk crystals due to misbalance of own ion radii. Tb3(Sc,Lu)2Al3O12 (TSLAG) is one of the most promising materials to overcome the issue because its ion radiusbalance is much superior than that of the TGG. In addition, it is possible to downsize an isolator because
Verdet constant of the TSLAG is larger than that ofTGG.
Fujikura has realized an isolator using TSLAG single crystal for the first time. In this paper, we report
1 A pplied Electronics Technology Department, Optics and Electronics
Laboratory
2 New Technology Department, Environment and Energy Laboratory

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unique features of the TSLAG single crystal and typical optical quality of this new isolator.

2. TSLAG single crystal
Fujikura has developed TSLAG single crystal as aFaraday rotator in collaboration with National Institutefor Materials Science (NIMS)2). A Faraday rotator
works to rotate polarized light due to Faraday effectwhen magnetic field is applied to light propagation direction. Verdet constant is defined as the polarization
rotation angle per unit light pass length and unit magnetic flux density. It means larger Verdet constant enables downsizing of a Faraday rotator and/or a magnet in an isolator.
Figure 1 shows the crystalline structure of TSLAG.
The garnet structure is described as {C3}[A2](D3)O12,
where {C} is the octahedral site, [A] is the hexahedral
site and (D) is the tetrahedral site. The ion radii ofeach cation composing TSLAG are shown in Table 1.
TSLAG consists of Tb in the {C} site, Sc/Lu in the [A]site and Al in the (D) site; therefore, molecular formula is expressed as Tb3(Sc,Lu)2Al3O12. The key point of
this composition is small difference between the ion

ACD

Fig. 1. Crystalline structure of TSLAG.
Table 1. Cations composing TSLAG crystal and their ion radii.
Coordination site{C} site (Octahedron)[A] site (Hexahedron)(D) site (Tetrahedron)

ElementIon radius (Å)

Tb (Terbium)

1.04Sc (Scandium)

0.745

Lu (Lutetium)

0.861

Al (Aluminum)

0.390

Panel 1. Abbreviations, Acronyms, and Terms.

radius of Tb in the {C} site and Sc/Lu in the [A] site.
This superior ion radius balance enables us to growlarge bulk crystals2).
Fujikura has been growing TSLAG single crystals
by Czochralski process. We have successfully fabricated large bulk TSLAG single crystals without macro
cracks, as shown in Fig. 2. Table 2 shows Verdet constant comparison between TSLAG and TGG. WhenVerdet constant of TGG is 1.00 (a.u.), measured at1080 nm and 22˚C, the relative constant of 1.28 is obtained for TSLAG at the same measurement conditions. This result indicates the TSLAG enables downsizing of a Faraday rotator and/or a magnet comparedwith the TGG. Figure 3 shows transmission spectrumof a 10 mm-thick TSLAG single crystal without antireflection (AR) coating. High transparencies are observed not only in the 1 μm wavelength band but also
in the visible wavelength range without around 500
nm. Therefore, it is possible to apply the TSLAG forvarious lasers, such as 1 μm wavelength band pulsed
fiber lasers and visible lasers.

rotation angle, L is path length and H is magnetic
flux density in direction of light propagation.
Czochralski process
Czochralski process is one of the most popularmethods of single crystal growth. It uses a seed
crystal as the starting material and the seed
crystal is introduced into high-temperature crystal
melt. A bulk single crystal is formed by drawing
up the seed crystal from the crystal melt.
AR coating–Anti-reflection coating
An AR coating is a type of optical coating applied
to surfaces of optical crystals to reduce reflection.

3. Optical isolator
Figure 4 shows a newly developed optical isolator
using the TSLAG single crystal. This isolator cantransmit 1 μm wavelength band light launched from apulsed fiber laser with low attenuation and it can blockreflected light from the objects being processed.
Transmission loss of the laser light through the isolator is known as forward insertion loss and that of reflected light through the isolator is known as backward insertion loss. Isolation is defined as the ratio
between the backward insertion loss and the forward
insertion loss, and it is one of the most important optical parameters for isolators.
Table 3 shows typical properties of the Fujikura optical isolator for pulsed fiber lasers. This isolator has
100
Transmittance (%)

Faraday rotator
A Faraday rotator works to rotate polarized light
due to Faraday effect when magnetic field isapplied to light propagation direction. The Faraday effect is a magneto-optical phenomenon
which is interacted between light and the magnetic field in a material.
Verdet constant
Verdet constant is an optical constant that describes the strength of Faraday effect for a particular material. It is given byV = qf /l · H
where V is Verdet constant, qf is polarization

806040200300

500700900
Wavelength (nm)

1100

1300

Fig. 3. Transmission spectrum of a TSLAG crystal.

Fig. 2. Photograph of a TSLAG single crystal.

Table 2. Verdet constants of TSLAG and TGG crystals.
CrystalVerdet constant (a.u.)

TGG

1.00

TSLAG

1.28

Fujikura Technical Review, 2015

Fig. 4. photograph of a newly developed optical isolator.

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Item

Typicalproperty

Unit

NoteCenter wavelength1085nm

Center wavelength can becustomized to comply withspecification of a fiber laser.

Forward output
power80

W

Backward input
power40W

Forward insertion
loss

0.1dB

Isolation

38dB

Optical output power (W)Table 3. Typical properties of an optical isolator
for pulsed fiber lasers.

6560
55
50
45

0400

800

221,600

2,000Fig. 5. Optical output fluctuation of an optical isotatorin continuous 2000-h operation.
Before operationhigh power durability, such as 80-W forward output
power from pulsed fiber lasers and 40-W reflectedlight power from objects being processed. Superior
optical properties have also been achieved, such as
low attenuation of 0.1 dB in the forward direction andhigh isolation of 38 dB.
An optical isolator consists of a single crystal Faraday rotator and some other optical crystals in its lightpath. These crystals are coated with AR layers on their
optical surfaces. The crystals and/or the AR coatings
will be damaged in some cases when high power density laser is input into the crystals. It causes increasingof forward insertion loss and degradation of output
beam shape. Moreover, it may be possible to breakdown an isolator in the worst case. We investigatedlong-term continuous lasing operation to evaluate highpower durability of crystals, including TSLAG. Figure
5 shows the optical output fluctuation of the optical isolator in continuous 2000-h operation. Average output
power of 56 W was obtained without significant fluctuations during the 2000-h operation. Figure 6 shows
comparison of output beam patterns before and after
continuous 2000-h operation. The beam patterns werealmost identical before and after operations. In addition, actual damage of the crystals and the AR coatings
was not observed by microscope after the 2000-h operation. These results indicate Fujikura optical isolators have high reliability for high-power long-term lasing operation.

1,200Time (h)After operation

Fig. 6. Output beam patterns before and after continuous
2000-h operation.

4. Conclusion
Fujikura has realized an isolator using Tb3(Sc,Lu)2Al3O12 (TSLAG) single crystal for the first time,and it has been already utilized in pulsed fiber laserproducts. This isolator has superior optical properties
and high reliability for high-power long-term lasing operations. Optical isolators will be applicable in a varietyof fields with increasing optical output power of pulsed
fiber lasers. Therefore, further development on the optical properties and reliabilities of optical isolators will
be required in the near future.

References
1) K. Himeno : “Fiber lasers and their advanced optical technologies of Fujikura,” Fujikura Technical Review, No. 42, pp.
33-37, 2013.
2) E.G. Villora, K. Shimamura, T. Hatanaka, A. Funaki, and K.
Naoe : “Single crystals for optical isolator,” Fujikura Technical Review, No. 41, pp. 25-26, 2012.