Development of Nano Second Pulsed Lasers
Using Polarization Maintaining Fibers
Shun-ichi Matsushita*,*2, Taizo Miyato*,*2, Hiroshi Hashimoto*,*2,
Eisuke Otani*2, Tatsuji Uchino*2, Akira Fujisaki*,*2

Pulsed lasers are increasing in their utilization in the laser marking and in thelaser micro material processing because it allow us to control input opticalpulse energy into the materials. Pulsed fiber lasers have some advantages such as stability, reliability and high brightness over CO2 lasers or YAG lasers, so that the latters are being rapidly
replaced with the pulsed fiber lasers. To improve the processing throughput and to expand theapplicable processing materials, it is necessary to increase the output power and to have someproperties such as the linear polarized output or the tunability of the pulse width and the repetition rate.
We have developed two types of polarization maintaining pulsed fiber lasers which have a configuration of master oscillator power amplifier (MOPA). One is using the external modulated seed
light source for a high power output. The other is using a direct modulated laser diode for ashort pulse generation. The seed pulse light is amplified by a polarization maintaining YDF
amplifier. We have realized more than 70 W and 13 W output power of 100 ns and 10 ns pulse at1064 nm, 1 MHz.

ABSTRACT1.

INTRODUCTION

Recently, the output power of the fiber lasers are becoming higher1)-3) so that they are replacing CO2 lasers or YAG
lasers, which have been dominant so far, in the lasermaterial processing such as welding and cutting of metals, marking and scribing on semiconductors or ceramics. Most of these laser material processings harness
melting, or changing and converting of substances byheat generated when the light is absorbed into materials.
For ceramics, semiconductors, silicon and, especially,
resins or composite materials such as plastics, the materials are easily damaged around the processing area dueto the heat effect, and therefore the quality of processing
deteriorates. It is necessary to control the injected opticalenergy precisely to improve the quality of the laser material processing, so a pulsed laser which can temporally
control the optical energy is used 4), 5).
A Q-switched pulsed fiber laser can generate several
tens of watts at several tens to several hundreds of kHz.
The Q-switched pulsed fiber laser is widely used for marking because of its advantages in small size, in simplicity* Advanced Laser Project Team, Telecommunications Company*2 Components & Advanced Interconnect Technologies
Department, FITEL Photonics Laboratory , Research &Development Divisionand in maintenance-free.
For marking on metals such as titanium or stainlesssteel, the surface of the metals can be colored by controlling processing conditions such as the pulse duration andthe pulse energy with a high precision. Therefore processing applications using a pulsed laser, such as printing and decorating on cell-phones or cameras, areexpanding.
For pulsed fiber laser, it is difficult to increase the power
and the optical pulse energy because of the non-linear
effect in the fiber and the input power limitation of thecomponents.
It is necessary to increase average power and to havehigher repetition rate in order to enhance the processing
throughput and to expand the processing condition. The
linear polarized output allows using nonlinear crystal forwavelength conversion.
In this paper, we demonstrate two types of polarization
maintaining pulsed fiber lasers which have a configuration of MOPA (Master Oscillator Power Amplifier) as
shown in Figure 1. One is using external modulation to
obtain a higher average power. The other is using directmodulated laser diode to generate a short pulse less than
100 ns.

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Development of Nano Second Pulsed Laser Using Polarization Maintaining Fibers

PM-YDFA

PM-YDFA

ISO
Seed pulsed laser (PM)Fiber laser + external mod.
Direct modulated Laser DiodePM-YDFA：Polarization Maintaining double clad Ytterbium
Doped Fiber Amplifier
ISO：Isolator

2.

The optical output pulse power from the external or the
direct modulation system is smaller than the one from theQ-switch oscillation system. So the optical fiber amplifier
is required to have high gain in order to obtain enoughpower and pulse energy required for the laser material
processing. Therefore fiber length used in the opticalamplifier tends to be longer, so that it is required to avoid
the energy diffusion and the pulse distortion by the nonlinear effect.

Figure 1 Configuration of pulsed fiber laser.
(MOPA : Master Oscillator Power Amplifier)3.

THE PULSED FIBER LASER

3.1. The External Modulation
The external modulation system consists of basically a
CW laser and an external modulator such as the AOM or
the LN optical intensity modulator. The system allow
design the output pulse characteristics such as pulse
shape, pulse duration or repetition rate by the combination of the CW laser and the external modulator.
The AOM generally is used as an optical shutter andcan accommodates a relatively high input power, however, there is a trade-off relation that if the operating rate ismade higher, the optical aperture becomes smaller. In thefiber inline type, the upper limits of the input power are
determined by the power tolerability of collimators usedat the input and the output to/from the AOM and by theamount of the coupling loss caused by the size of theaperture. At current status, practical values of the upper
repetition rate and input power of the AOM are in theorders of hundreds MHz and a few Watts respectively inthe case of tens ns to hundreds ns of optical pulse duration. The LN optical intensity modulator is usually used inhigh speed optical telecommunication systems. The
speed of the system is increasing the order of 10 GHz sothat the high speed electric signal generators and the driver IC became available and a pulse generation of several
100 ps became relatively easy.
However, the input power of the LN modulation is ten
times lower than that of the AOM so it leads to deteriorate
the signal to noise ratio (SNR) by the amplified spontaneous emission (ASE) in the amplifier. It is difficult to design
the amplifier because it is necessary to obtain a higher
gain to achieve the high output power.
In this paper, we used an AOM modulator to generate
an optical seed pulse source of the pulsed fiber laser.
The optical configuration of the external modulated
pulsed seed laser is shown in Figure 2. The fiber laser
resonator consists of the polarization maintaining double
clad Ytterbium doped fiber (PM-YDF), a high reflection
mirror (HR) and an output coupler (OC) both made of afiber bragg grating (FBG)6). The FBG is installed in a thermalcompensating package to stabilize a center wavelength.
The temperature dependences of the center wavelength
are shown in Figure 3, as uncompensated in red line andcompensated in blue line. With temperature compensation,
the center wavelength is nearly-constant from -40 to 80°C.

There are several methods to generate an optical pulse.
Those are principally: a) a Q-switch oscillation that sets
an optical shutter like an optical modulator into an opticalresonator and oscillating an optical pulse by temporally
changing the Q-factor of a resonator. b) an external modulation that cuts out an optical pulse from an output of a
CW laser by an AOM (Acousto-Optical Modulator) or a LN
(LiNbO3: lithium niobate) optical intensity modulator; c) a
direct modulation that directly modulates a driving currentof a seed laser and controls the oscillation temporally.
Table 1 shows typical examples of optical pulse generation methods of a pulsed fiber laser and their features.
Table 1

Methods of optical pulse generation of a pulsed fiber
laser. (Typical examples)Q-switchoscillation

External
modulation

Directmodulation

Pulse width

Tens of nm up to approx. 30 ns
hundreds of ns
or moreHundreds of ps
or moreRepetition
rate

Tens of kHz up
to 50 kHz

Tens of kHz or
more

Tens of kHz or
more

Features

High pulse
energyPulse width and repetition rate
are variable.

With a Q-switch oscillation, an optical pulse oscillation
can be obtained by setting the optical switching elements
like the AOM into the resonator. The number of components is few and it is easy to make an optical pulse with arelatively high energy. On the other hand, the repetition
rate and the pulse width depend on the property of theresonator, so that their values are limited within a certain
range.
With an external modulation, the laser oscillation andthe pulse generation are physically separated. It is possible to design its characteristics such as the center wavelength, the spectral width, the temporal pulse shape andthe repetition rate flexibly by combining them.
With a direct modulation, the driving current of thesemiconductor laser is directly modulated to generate
optical pulse. It can realize not only almost the same performance as the external modulation but also it can
reduce the number of components.

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Development of Nano Second Pulsed Laser Using Polarization Maintaining Fibers

PM-YDFAAOM

MM-LD

FBG (HR)FBG (OC)
Function
Generator

Figure 2 Configuration of external modulated pulsed seed laser.
MM-LD: Multi mode laser diode0.5

CompensatedUncompensated

0.0
1.20

-0.5

100ns
50ns
30ns
10ns
5ns

1.00
-1.0
-60

-40

-20

0

20

40

60

80

100

Temperature [℃]
Figure 3 Temperature dependence of center wavelength of FBG.
Uncompensated (Red), Compensated (Blue)The output from the polarization maintaining Ytterbium
doped fiber laser is input into the AOM of 150 MHz and iscut out into an almost 100 ns optical pulse. The repetition
rate is 1 MHz and the input power is around 1.5 W. The
electric signal and the optical pulse shape are shown in
Figure 4. An optical pulse of 88 ns is generated from theAOM driven by a rectangular electric signal of 100 ns.

Intensity (a.u.)

Wavelength deviation [nm]1.0

10 ns or less of optical pulse because a driver IC and asemiconductor LD are commercialy available similar to
the external modulator.
Even if the duty ratio is small, the direct modulation system does not need to control drifts at zero level which is
required for a LN modulator. Also, there are several
advantages available in direct modulating the LD comingfrom electrical response property, the generation of 100 ns
or less pulses, the generation of an arbitrary shape of the
optical pulse and the high repetition rate up to GHz.
The temporal pulse shape from the direct modulationof a semiconductor LD is shown in Figure 5.
Figure 5 shows that the optical pulse is well-controlled
and that the output is within from 5 ns to 100 ns of pulse
width. The pulse peak power is approx. 100 mW.

0.80
0.60
0.40
0.20
0.00

0

50

100
Time (ns)

150

200

Figure 5 Output optical pulse shape from direct modulate laser
diode. (Pulse width: 5 ns–100 ns, repetition rate: 100
kHz)

3.3. The Drive Control Circuit for a Pulsed Laser
We have developed a control board which generates a
programmable digital electric signal and controls the driving current of the seed LD and the pumping LD, to control
the AOM, the LN optical intensity modulator and the LD.
The control block diagram is shown in Figure 6.

Controller
Pulse Generator

PC

Seed LD
DriverFigure 4 Output pulse shape from external modulated pulsed
seed laser. Electric modulation signal (Yellow), Opticalpulse shape (Green)

3.2. The Direct Modulation
The direct modulation system generate an optical pulse
by the modulation of the driving current of a semiconductor LD directly. It becomes relatively easy to generate a

Pump LD
DriverPump LD
DriverFigure 6 Control block diagram of a pulsed laser control board.

The control board consists of a controller and threelaser drivers. The controller has a pulse generator to drive
the external modulator or the direct modulated laser
diode to generate an optical pulse. The Laser drivers control the LD driving current and the operation temperature.
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Development of Nano Second Pulsed Laser Using Polarization Maintaining Fibers

4.

THE OPTICAL PULSE AMPLIFIER

The output from two-types of developed pulsed seedlasers are amplified in 2-steps polarization maintaining
double clad ytterbium doped fiber amplifier as shown in
Figure 1. The configuration of an optical pulse amplifier isshown in Figure 7.
The optical input pulse is applied through a tapered
fiber bundle (PM-TFB) into the ytterbium doped fiber. The
PM-TFB is able to couple up to 18 pieces of Multi-modelaser diode (MM-LD) as a pump light source. In these
optical pulsed amplifiers, 1 pc of 10 W and 6 pcs of 25 W
MM LD are used as a preamplifier and as a booster
amplifier respectively. The pump power of the booster
amplifier reaches up to 150 W. Each amplifier has an optimized fiber length and a gain as the non-linear effectbecomes smallest to minimize the generation of SRS(Stimulated Raman Scattering) in the fiber.
PM-YDF (DC)ISO

PM-TFB…
MM-LDs up to x18
Figure 7 Configuration of the polarization maintaining ytterbium
doped double clad fiber amplifier.

20
0
-20
-40
-60
-80
1000

1050
1100
Wavelength [nm]4.2. The Amplification of the Direct Modulation
The result of the amplified power of direct modulated
pulsed seed light is shown in Figure 10. 10 ns pulse duration, 1 MHz repetition rate and more than 13 W output areachieved. The optical spectrum of 7 W output is shown in
Figure 11.
The Signal-SRS ratio of 50 dB or more is achieved by
optimizing the gain of a booster amplifier and the fiber
length. At this time, the conversion efficiency of thepumping light – optical signal power is approx. 29%.

4.1.

90
80
Output power [W]

70

1150

Figure 9 Spectrum of the output from a MOPA using an external
modulation (with 70 W output)

Output power20

Output power [W]

The Amplification of the External Modulated
Pulse
The result of the amplified power of external modulated
pulsed seed light is shown in Figure 8.

The pulse duration is 100 ns and the repetition rate is 1
MHz. The output is more than 70 W. One of the factors
which limits the maximum value of the output power is theincrease of the Raman scattering, however the Raman
scattering can be minimized to approx. -19 dB against thesignal output by the optimizing characteristics of the optical amplifier. The optical spectrum of 70 W output isshown in Figure 9. The conversion efficiency of the pumping light – optical signal power is approx. 45%.

Power [dB]

There are functions that are monitoring each of the optical
outputs, the safety features of the controlling output constant and shuting down and performing a routine action
safely from an external ON/OFF signal and from the command input from a PC.

15
10
5

60
50

0

40
30
20

10

20
30
Pump power [W]40

Figure 10 Output power form a MOPA using a direct modulation.
10 ns, 1 MHz.

10
0

0

0

2

4

6

8

10

12

Operating current [A]Figure 8 Output power from a MOPA using an external
modulation. 100 ns, 1 MHz.

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Development of Nano Second Pulsed Laser Using Polarization Maintaining Fibers

Normalized power [dB]

10
0
-10
-20
-30
-40
-50
-60

1000

1050
1100
Wavelength [nm]1150

Figure 11 Spectrum of output from a MOPA using a directmodulation. 10 ns, 1 MHz, 7 W.

5.

THE OUTPUT POWER STABILITY AND
THE BEAM PROFILE

Output power [W]The output power stability of the pulsed fiber laser using
external modulation is shown in Figure 12. The output
power stability is ±1.6% when the output power is 70.5
W. It means that the output of the seed pulse and the optical amplification are stable and also that the polarization
fluctuation in the fiber is small. Polarization extinction
ratio of the optical output is more than 20 dB.
100
90
80
70
60
50
40
30
20
10
0

0

1

2
Time [hr]34Figure 12 Power stability of a MOPA using an external
modulation.
4 hours operation.
The deviation from the mean is ±1.6% at 100 ns,
1 MHz and 70 W.

The output beam profile from the isolator is shown in
Figure 13. The beam shape has almost a Gaussian profile
and an M2 which shows that the beam quality is less than
1.33. It is a satisfactory performance for a pulsed fiber
laser for laser material processing.

Figure 13 Beam profile of the output from a MOPA using an
external modulation. (100 cm from the isolator)

6.

CONCLUSION

We have developed 2 types of polarization maintaining
pulsed fiber lasers using an external modulation and a
direct modulation to obtain a high optical output power
and a short pulse generation.
We have realized more than 70 W output power with
100 ns, 1 MHz by using an external modulation as a seedpulse source. The output power stability is ±1.6% to themean value and a polarization extinction ratio of the optical output that becomes more than 20 dB. We have also
realized more than 13 W with 10 ns, 1 MHz by using a
direct modulated laser diode as a seed pulse source. The
booster amplifiers are optimized with respect to its fiber
length and its gain to minimize the nonlinear effects in theamplifiers.
The background of this technology comes from technologies in the optical telecommunication system such as
high-speed modulation, high power amplification, optimization of nonlinear effect in fibers7),8) and stabilizing output
power.
These pulsed fiber lasers realized linearly-polarized output by using the polarization maintaining fiber and thesplicing techniques. The linearly-polarized output allow us
to use wavelength conversion so the wavelength of thepulsed fiber laser can expand the wavelength to 532 nm
or 355 nm.
We wish this technology will be a powerful tool to realize new application technology not only in the laser processing application but also in the inspection equipments
for the bio-technology and the medical treatment etc.
and in an energy field.
ACKNOWLEDGMENT
This research and development has been carried as apart of the “High-power Pulsed Fiber Laser andProcessing Technology Project” by NEDO.

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large-core fiber laser with 1 kW continuous-wave output power”
ASSP, PDP, (2004)
3) A. Tunnermann, T. Schreiber, J. Limpert, “Fiber lasers and amplifiers: an ultrafast performance evolution” Appl. Optics, Vol. 49, No.
25, (2010)
4) “Laser Processing Handbook”, OPTRONICS
5) “Laser Handbook”, The Laser Society of Japan
6) Akira Fujisaki, “50-W output Polarization maintaining Fiber Laserand Its Linewidth Control”, Furukawa Denko Jiho No.123 (2009)7) Yoshio Tashiro, “Development of High Power Optical Amplifier”,
Furukawa Denko Jiho No.104 (1999)
		 Shun-ichi Matsushita, “Repetition-Rate Tunable Ultra-Short Pulse
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