Linewidth Controlled 50-W Output Polarization
Maintaining Fiber Laser
Akira Fujisaki *, Naoto Yoshizawa *, Yoshihiro Emori *,Keisuke Tominaga *2, Kanji Tanaka *3, and Jun Nishina *3

Optical fiber amplification technology, which had made vast progress in Erbium
doped fiber amplifiers (EDFAs) for communication applications, has recently
increased dramatically its optical output at the 1.0-μm wavelength by means of cladding
pumping techniques using Ytterbium (Yb) doped fibers. As the fiber laser technologies including
optical parts for fiber amplifiers and pumping semiconductor lasers have advanced year by year,
fiber lasers have developed into superior laser oscillators in terms of controllability in wavelength,
linewidth and state of polarization, as well as of output power. This paper reports on the basic
configuration of fiber lasers, discusses their wavelength and linewidth control, and presents thecharacteristics of a polarization-maintaining fiber laser of all-fiber configuration that has achieved
an optical output of 50 W at 1083-nm wavelength.

ABSTRACT1.

INTRODUCTION

Optical fiber amplification technology, which had madevast progress in Erbium doped fiber amplifiers (EDFAs)
for communication applications, has recently increased
dramatically the optical output at the 1.0-μm wavelength
by means of cladding pumping techniques, in which
pumping light is guided into the cladding of pumping fiber
1)~4). In the single-mode fiber lasers, owing to the core andclad configuration, any lightwave other than that of coreguided modes in the employed fiber is difficult to betransmitted, so that the output light forms stable transverse modes in accordance with the index profile. This
constitutes a major advantage when the output power ofa fiber laser is used in combination with an external wavelength converter. However, when application to wavelength conversion is assumed, the wavelength stability,
linewidth and polarization state of light source must be
taken into consideration in the design, and this presents
an important aspect different from using the fundamental
laser wavelength directly. In this paper, the basic operation of fiber laser will be presented together with thedesign of a fiber laser with wavelength, linewidth andpolarization state control for use in wavelength conversion
application, and output performance up to 50 W of a fiber
laser.

* FITEL Photonics Lab., R&D Div.
*2 FITEL Photonics Lab., R&D Div., currently with OFS
Laboratory, North America*3 FITEL Products Dept., Telecommunications Co.

2.

BASIC CONFIGURATION

Figure 1 shows the cross-section of a cladding mode
pumping fiber. The mode field diameter of this fiber is 6
μm for single-mode operation at 1.0-μm wavelength, andthe silica cladding is coated with a fluorine resin having alower refractive index for guiding the pumping light.
Figure 2 shows the configuration of the fiber laser studied
here. The pumping light with a wavelength of 915 nm
where Yb exhibits a broad absorption peak was
employed, taking wavelength controllability into account.
The light output from the pumping semiconductor laser
chip is coupled to a multi-mode fiber with a core diameterof 105 μm and an NA of 0.22, and is subsequently guided
into the cladding of a Yb-doped fiber. The laser cavity
Cladding of low-refractive index resin
Silica cladding
Yb-doped coreStress applying material

Figure 1

Polarization maintaining double-clad Yb-doped fiber.

YDF
PANDA

Outputconnector

915 nm MM-LD
HR FBG

Figure 2

OC FBG

Basic configuration of clad-pumped fiber laser.

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Linewidth Controlled 50-W Output Polarization Maintaining Fiber Laser

dicular to each other have two different refractive indexes.
We have realized single polarization lasing by making thepeak position of the HR on the short-wavelength side
overlap with that of the OC on the long-wavelength side.
12
10
Relectivity（%）

consists of a high-reflectivity fiber Bragg grating (HR FBG)
with a reflectivity higher than 99%, and an output coupler
FBG (OC FBG) with a reflectivity of around 10%, andlases at a wavelength that is determined by the reflected
lightwaves from the FBGs. An FBG is configured by generating periodic interference fringes directly on a fiber
glass, taking advantage of structural defects in silica.
The center wavelength λB, reflection bandwidth Δλ andreflectivity RB of an FBG can be designed as follows.
λ =2 Λ
+ 2
=
2
2∆ η
∆λ=
λ
π
∆ = 3− 2

8
6
4

3

0
1063

Figure 4πη ∆λλ B = 2n index of the FBG where thewhere, n2 is the refractive
n3 +UVn2 irradiation, n3 is the refracindex is increased due
n = to2 the grating writing interval ontive index of fiber core, Λ is2∆nη
in the core, and L is
the FBG, η is the confining
λB∆λ = coefficientthe grating writing length. π= n3 − ndirectly2
Since the grating is∆nwritten
on the silica glass ofπηL∆na fiber, the FBG shows a thermal
wavelength
shift as theRB ≈ tanh 2fiber elongates or contracts due λtoB the linear expansion ofsilica glass. In order to obtain high wavelength stability bycompensating for temperature changes, use of a temperature compensation package shown in Figure 3 is effective, which can eliminate elongation and contraction of an
FBG caused by temperature changes. Stabilization techniques for the lasing wavelength of a fiber laser will bepresented in the next Section.
UV

1063.2 1063.4 1063.6 1063.8

1064

1064.2 1064.4

Wavelength（nm）

Al

FBG
Inver

Reflection spectrum of FBG written on a polarization
maintaining fiber.

Figure 5 shows the lasing spectrum of this fiber laser;
Figure 6 the pumping current vs. optical output and its
temperature dependence; and Figure 7 the pumping current vs. lasing efficiency. While a conversion efficiency
from pumping light power to laser output power of 60% or
higher has been obtained including the optical loss produced by inner optical parts, this rather low efficiency
1.2
0℃
25℃

1
Normalized power（a.u.）≈ tanh2

2

50℃

0.8
0.6
0.4
0.2
0
1063.81063.91064

1064.11064.2

Wavelength（nm）

Λn2

Figure 5

n3

L

3.

8

CHARACTERISTICS OF FIBER LASERWITH 6-W OUTPUT

A fiber laser stabilized in wavelength was fabricated byusing temperature-compensated FBGs. Figure 4 shows
the reflection spectrum of an FBG written on a polarization maintaining fiber, where two reflection peaks 0.3 nmapart exist since the two optical axes in the fiber perpen-

0℃

7

Basic structure of FBG and temperature compensation package.

25℃
50℃

6
Output power（W）

Figure 3

Lasing spectrum at 6-W output.

5
4
3
2
1
0

0

1

2

3

4

5

6

7

8

9

10 11 12Current（A）

Figure 6

Pump current vs. output power characteristics.

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Linewidth Controlled 50-W Output Polarization Maintaining Fiber Laser

results from the use of a slightly shorter lasing fiber toimprove the controllability of laser linewidth.
80

0℃

75

25℃

70

50℃

60
55
50
45
40
35
30
25
20

0

1

2

3

4

5

6

7

8

9

10 11 12Current（A）

Figure 7

Pump current vs. efficiency.

The temperature dependence of the center wavelengthwas, as shown in Figure 8, 7 pm or less for a temperature
range of 0 ~ 50°C at 6-W output, and the linewidth was
found as about 35 pm. Such wavelength precision is suitable for wavelength conversion applications using second
harmonic generation (SHG) devices. Figure 9 shows theappearance of this fiber laser with 6-W output, accommodated in a compact casing measuring 100 mm x 100 mm
x 16 mm.
1064.03

The output power of a fiber laser can be increased byusing additional pumping light sources, or even when
gain saturation is reached, by increasing the length of lasing fiber.
Figure 10 shows an example configuration of a fiber
laser with high output power by CW operation. To efficiently
guide the light from multiple pumping semiconductor
lasers into the cladding of a lasing fiber, a fused-fiber typeoptical component called tapered fiber bundle is used;
specifically, the 10-W class pumping light sources of 915
nm in wavelength are each guided from the pumping ports
counting 36 in total into the cladding of a Yb-doped double
clad lasing fiber of 40 m in length. In this configuration, thecavity consists of an HR FBG having a reflectivity of 99% orhigher with a center wavelength of 1085 nm--- where thegain peak of Yb-doped fiber is located ---and an OC FBG
having a reflectivity of about 10%. As shown in Figure 11,
the output power of this laser is 250 W at a total pumping
power of 366 W, and the conversion efficiency from pumping light is 68%.
12-µm Yb-CPF

18x1
TFB

Co-pumping 1085-nm
using 18 LDs HR FBG

1064.01

18x1
TFB

1085-nmOC FBG

TFB: Tapered fused bundle
CPF: Clad pumping ﬁber

1064.02
Wavelength（nm）

FURTHER OUTPUT POWER
ENHANCEMENT AND LINEWIDTHEXPANSION

PoutCounter pumping
using 18 LDs

HR FBG: High-reﬂectivity FBGOC FBG: Output coupler FBG

Figure 10 Configuration of CW fiber laser.

1064
1063.991063.98
0

10

20

30

40

50

60

Temperature（℃）

Figure 8

Temperature dependence of center wavelength.

Output power（W）

1063.97
-10

260

73.0

240

72.0

220

71.0

200

70.0

180

69.0

160

68.0

140

67.0

120

66.0

100
150

200

250

300

350

Eﬃciency（%）

Eﬃciency（%）

65

4.

65.0
400

Pump power（W）

Figure 11 Output power and efficiency of CW fiber laser.

Figure 9

Appearance of compact single polarization fiber laser.

Since the cavity length of this laser is very long--- aslong as 40 m ---so that the longitudinal mode spacing,which is calculated from the allowed resonating modes
determined in number by the integral multiple of the
wavelength, becomes very narrow--- about 0.1 pm. And
since the amplification bandwidth of the Yb-doped fiber is

Furukawa Review, No. 35

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14

Linewidth Controlled 50-W Output Polarization Maintaining Fiber Laservery broad exceeding 100 nm, the lasing spectrum or
linewidth of this laser is limited by the wavelength bandwidth of the cavity.
Figure 12 shows the reflection spectrum of an FBG
designed to be equivalent to the one used in this laser,indicating that the half bandwidth of wavelength is about
100 pm.

Normalized reﬂectivity（a.u.）

1.25
1
0.75
0.5
0.25
0
1081.30 1081.35 1081.40 1081.45 1081.50 1081.55 1081.60 1081.65 1081.70
Wavelength（nm）

Figure 12 Reflection spectrum of an equivalent FBG.

On the other hand, Figures 13 and 14 show the lasing
spectrum of this laser up to 250-W output. It can be seenthat the spectrum has expanded more than ten times the
wavelength bandwidth of the FBG, and that the spectrum
width expands further as the output power increases.
Since the longitudinal mode spacing is 0.1 pm as mentioned before, about 1000 longitudinal modes are generated in the bandwidth of the FBG, i.e., 100 pm. It is considered that since these longitudinal modes originallyhave a very narrow linewidth individually, they expand inlinewidth due to self phase modulation (SPM) as they
propagate in the glass medium of fiber. Incidentally, thelasing peak due to Raman scattering can also be
observed at around 1140 nm.

5.

50-W OUTPUT POLARIZATION
MAINTAINING FIBER LASER

Although the output power of fiber lasers can be increasedby using changed laser configurations as mentioned
before, their linewidth also grows as the output powerincreases due to wavelength expansion caused by SPM.
Moreover, their center wavelength tends to shift to the longer wavelength side due to the fact that the core temperature of FBGs slightly increases, irrespective of temperaturecompensation using a temperature-compensated package, as the high-power lasing light passes through the
FBG. Since the linewidth is required to be controlled
below 200 pm for high-efficiency wavelength conversion,
to satisfy this requirement, we have employed a master
oscillator and power amplifier (MOPA) configuration, in
which the output from a fiber laser designed to produce a
relatively low output power is amplified by postpositioned
fiber amplifiers. As shown in Figure 15, the number of
postpositioned optical amplifiers is three in total.
Figure 16 plots, against output power, the lasing
915 nm Pump

TFB

Normarized power（a.u.）0
Pout=126 W
Pout=167 W
Pout=200 W
Pout=250 W

-10
-20

PM ﬁber

915 nm Pump

ILP FBG
FBG
(OC)
(HR)
Seed ﬁber laser

ConnectorISO

2 nd stage

915 nm Pump

915 nm Pump

-30
-40

1084 nm Output-50

4 th stage

3 rd stage

-60
-70
850

900

950

1000

1050

1100

1150

ILP : Inline power meter
ISO: Isolator

1200

Wavelength（nm）

Figure 15 Configuration of 50-W polarization maintaining fiber laser.

0.9

Linewidth（pm）

Normarized power（a.u.）1
Pout=126 W
Pout=167 W
Pout=200 W
Pout=250 W

0.8
0.7
0.6
0.5
0.4
0.3

1083.230

1083.19525

1083.1920

1083.185

151083.18101083.175
⊿λ
λ

5

0.2
0.1
0
1080

35

0
1082

1084

1086

1088

1090

Wavelength（nm）

Figure 14 Output spectra of CW fiber laser (enlarged view).

0

0.5

1

1.5

2

1083.17
2.5

Center Wavelength（nm）

Figure 13 Output spectra of CW fiber laser.

1083.165

Output power（W）

Figure 16 Lasing wavelength and linewidth of the first-stage
seed laser.

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Linewidth Controlled 50-W Output Polarization Maintaining Fiber Laser

wavelength and the linewidth FWHM of the seed laser at
the first-stage. The output power of the seed was limited
to some 2 W maximum to ease the control of the laser
linewidth.
Figures 17 and 18 show the output power and linewidth
at the second- and third-stage amplifiers, respectively.
Figure 19 shows the output characteristics vs. pumping
power at the last stage, and Figure 20 the linewidth vs.
output power. The center wavelength was 1083.151 nm,
constant at every output power.
1083.1845

1083.17540

1083.17

Linewidth（pm）

35

1083.16530

1083.1625

1083.155

20
15

⊿λ
（pm）

10

λ（nm）

1083.145
1083.145
01083.1505101083.13520

15Output power（W）

Figure 17 Output characteristics of the second-stage amplifier.

1083.23585

Linewidth（pm）

1083.233
1083.23275

1083.231
1083.2370

1083.22965

1083.228

60
55

10

20

30

⊿λ

1083.227λ1083.226

Center wavelength（nm）

1083.23480

1083.2255040Linewidth（pm）

140
135
130
125
120
115
110
105
100

30

35

40

45

50

55

Output power（W）

Figure 20 Linewidth vs. output power.

Center wavelength（nm）

50

145

6.

CONCLUSIONS

By taking advantage of wavelength stabilizing techniques
for FBG, we have been successful in developing a fiber
laser that achieves, at a wavelength of 1064 nm, a highlevel of temperature as well as long-term stability in termsof linewidth and wavelength. Moreover, using a fiber
amplifier of MOPA configuration, we have succeeded in
obtaining a polarization-maintained output power of up to
50 W at a wavelength of 1083 nm, while controlling its
linewidth below 200 pm.
It is known that, when this laser is combined, a 532-nm
green light can be easily generated, in a postpositioning
manner with an SHG device based on periodically poledlithium niobate (PPLN), and that a conversion efficiency ofaround 30% is usually obtained.
Since the wavelength of this laser is very stable without
using a special control means, it is expected that applications will expand further. Work is now underway to investigate its applications in the 1030 ~ 1180 nm range, whichtakes advantage of the very wide gain bandwidth of Yb at
1.0 μm, since this range corresponds to the fundamental
wavelength for the 500 ~ 600 nm range that is difficult tobe generated directly using semiconductor lasers.

Output power（W）

Figure 18 Output power and linewidth of the third-stage
amplifier.

55

70

50

68

45

66

40

64

35
30

62

power
eﬃciency
0

5

10

15

20

25

30

Eﬃciency（%）

Output power（W）

REFERENCES

35

60

Pump power（W）

Figure 19 Output power and efficiency vs. pumping light powerat the fourth-stage amplifier.

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Linewidth Controlled 50-W Output Polarization Maintaining Fiber Laser

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