High-power master-oscillator power-amplifier with opticalvortex output

D. J. Kim • J. W. Kim • W. A. Clarkson

Received: 25 February 2014 / Accepted: 25 April 2014

� Springer-Verlag Berlin Heidelberg 2014

Abstract A high-power master-oscillator power-amplifier

with optical vortex output is reported. The master oscillator

for an optical vortex seed beam is a simple two-mirror

Nd:YAG laser using a fiber-based pump beam conditioning

scheme. The seed is amplified in a double-clad multimode

fiber amplifier end-pumped by a high-power diode laser at

975 nm yielding 10.7 W of continuous-wave output at

1064 nm in the first-order Laguerre–Gaussian beam with

M2 & 2.11 for an absorbed pump power of 17.5 W, corre-

sponding to a slope efficiency of *59 %. The ring-shaped

intensity profile and the wave front handedness of the seed

beam were well preserved in the fiber amplifier. The pros-

pects of power scaling via this approach are discussed.

1 Introduction

Optical vortex laser beams have been attracting much interest

due to their unique properties such as a ring-shaped intensity

profile and helical wave fronts [1–4]. Since they can deliver

the orbital angular momentum (OAM) to an incident particle/

material due to the helical wave fronts [1, 2], they have been

applied to numerous areas including optical trapping and

manipulation of particles, optical imaging, quantum optics,

communication, laser material processing[5–12]. Recently,

nanoscale micromachining of a metal material was demon-

strated [10, 11], stimulating the requirement of a high-power

optical vortex laser beam with good beam characteristics. For

scaling the laser output power while preserving good beam

qualities, a master-oscillator power-amplifier (MOPA) is the

most popular configuration since it decouples the matter of

achieving good beam characteristics from scaling of the

output power. A low-power optical vortex beam with good

beam characteristics for a seed can be obtained relatively

easily via transformation of the Hermite–Gaussian (HG) laser

beam using mode transforming optics [13–15] or selection of

the exciting mode in the laser resonator [16–19]. However, its

power amplification using a bulk solid-state material is rather

challenging since it is difficult to achieve good overlap effi-

ciency between the pump beam and the ring-shaped laser

mode. Recently, a research group at Chiba University repor-

ted the generation of a high-power optical vortex laser output

in a stressed, multimode (MM) Yb fiber amplifier by off-axis

core launching of the TEM00 mode beam and controlling the

bending stress of the fiber [20, 21]. Although their configu-

ration was not a MOPA but a mode converter with power

amplification, it proved that the MM Yb fiber could support

and amplify an optical vortex beam with high efficiency.

In this paper, we report a high-power MOPA system

employing a Nd:YAG master oscillator and a multimode

Yb fiber amplifier that yield 10.7 W of optical vortex

output at 1064 nm. The optical vortex seed from the master

oscillator was successfully amplified in the MM Yb fiber

amplifier while preserving the beam characteristics. To the

best of our knowledge, this is the first demonstration of a

MOPA with optical vortex output.

2 Experiments and results

The MOPA configuration used in our experiment is shown

in Fig. 1. For the master oscillator, we built a simple two-

D. J. Kim � J. W. Kim (&)

Department of Applied Physics, Hanyang University,

Ansan 426-791, Gyeonggi-do, Republic of Korea

e-mail: jwk7417@hanyang.ac.kr

W. A. Clarkson

Optoelectronics Research Center, University of Southampton,

Southampton SO17 1BJ, UK

123

Appl. Phys. B

DOI 10.1007/s00340-014-5855-5

mirror Nd:YAG laser resonator end-pumped by a diode

laser at 808 nm. In order to generate the first-order La-

guerre–Gaussian (LG01) mode beam preferentially in the

resonator, we employed the pump beam conditioning

technique using a simple capillary fiber to produce a ring-

shaped gain distribution. The capillary fiber has a pure

silica inner cladding of a 200 lm diameter and a 130 lm

diameter airhole in the center with a calculated NA of 0.49

for the pump light [18, 19]. A diode beam with a top-hat-

shaped intensity profile was launched into the cladding of

the fiber with a launching efficiency of *67 %, resulting in

a beam with the ring-shaped intensity profile. The Nd:YAG

resonator comprised a plane pump in-coupling mirror with

high reflectivity ([99.8 %) at the lasing wavelength

(1064 nm) and high transmission ([95 %) at the pump

wavelength (808 nm), an antireflection-coated plano-con-

vex lens with a focal length of 300 mm, and a plane output

coupler with a 5 % transmission at the lasing wavelength.

An antireflection-coated 1.0 at. % Nd:YAG crystal of

5 mm in length was used as the gain medium. The ring-

shaped pump beam via the capillary fiber was focused to

have a waist outer radius of *325 lm in the Nd:YAG

crystal for generation of the targeted LG01 mode since the

calculated TEM00 mode radius was *275 lm [19]. In

order to force the LG01 mode output to have the well-

determined helical wave front, we inserted an etalon and a

Brewster plate in the cavity. Under this configuration, the

laser yielded 2.1 W of the maximum output at 1064 nm in

a beam with M2 & 2.01 for an absorbed pump power of

6.7 W, corresponding to a slope efficiency of 39 %

(Fig. 2). The near-field output intensity profile and the

interference pattern with the reference beam of the

spherical wave fronts in a Mach–Zehnder interferometer

were monitored with the aid of a silicon CCD camera

(Spiricon BS-USB-SP620). As can be seen in Fig. 3, the

results proved that the generated beam was the LG01

mode with the well-determined helical wave front. The

handedness of the wave fronts, corresponding to the sign

of the OAM, could be simply controlled by tilting the

angle of the etalon in the cavity. The tilted angle of the

etalon for changing the wave front handedness was

measured to be *8�. We attributed the mechanism of the

wave front-handedness selection in our configuration to

frequency locking and induced aberration due to the

angled etalon. It is well known that the LG mode optical

vortex beam is a phase quadrature superposition of HG10

and HG01 modes with the same frequency [1, 2], but its

helicity selection is random without a special optical

device such as a spiral phase plate, a holographic phase

plate. The existence of the etalon in the resonator can

induce slight astigmatism to the resonating beam due to

the incidence of the diverging (or conversing) beam

on the angled etalon providing phase difference to the

two constituting modes. This induced phase difference

Fig. 1 Experimental setup for a

Nd:YAG master oscillator and a

MM Yb fiber amplifier. IC input

coupler, OC output coupler, BP

Brewster plate, HWP half-wave

plate

0 2 4 6 80.0

0.5

1.0

1.5

2.0

2.5

Out

put

pow

er (

W)

Absorbed pump power (W)

Fig. 2 Output power from the Nd:YAG master oscillator as a

function of incident pump power

D. J. Kim et al.

123

between the two modes allows the cavity to choose one

particular helicity depending on the angle of the etalon.

When the tilted angle was too large, the interference did

not show any spiral pattern since the output mode became

an incoherent superposition of HG10 and HG01 without

any fixed frequency or phase relationship due to too high

astigmatism. The detailed theoretical modeling and

experiments for the wave front-selection mechanism will

be dealt with in another report.

The output from the master oscillator was amplified by a

fiber amplifier employing a double-clad polarization-

maintaining (PM) large-mode-area Yb-doped fiber (LIE-

KKI Yb1200-25/250DC-PM). The core of the fiber had a

25 lm diameter with a low numerical aperture (NA) of

0.07 surrounded by a silica inner cladding of a 250 lm

diameter that can allow propagation of a few higher-order

modes. One end of the fiber was perpendicularly cleaved

and the other end was angle-polished at *13� to suppress

the self-lasing of the Yb fiber. A pump light for the

amplifier was provided by a fiber-coupled high-power

diode laser at 975 nm and was launched into one end of the

fiber with the aid of a dichroic mirror orientated at 45�. The

dichroic mirror had high transmission ([95 %) at the pump

wavelength (975 nm) and high reflectivity ([98 %) at the

signal wavelength (1.06 lm) allowing for extraction of the

amplified signal. The cladding absorption coefficient for

the pump light at 975 nm was 11.2 dB/m, so a fiber length

of *2 m was used for the amplifier. The launching effi-

ciency of the pump beam into the cladding was measured

to be *90 %. The pumped end section of the fiber was

Fig. 3 a Intensity profile and

b the spiral interference pattern

at 1 W of output from the

master oscillator

Radius (µm)0 5 10 15 20

Nor

mal

ized

inte

nsit

y (a

.u.)

0.0

0.2

0.4

0.6

0.8

1.0

1.2LP01

LP02

LP11

LP21

LP31

(a)

(b)

Fig. 4 Calculated intensity plots and the radial intensity profiles for

LP01, LP02, LP11, LP21, and LP31 modes

0 5 10 15 200

3

6

9

12

Out

put

pow

er (

W)

Absorbed pump power (W)

Fig. 5 Output power from the MM Yb fiber amplifier as a function of

absorbed pump power for 1 W of input signal power

High-power master-oscillator power-amplifier

123

mounted in a water-cooled V-groove heat sink to avoid

thermal-induced damage of the fiber coating due to the

unlaunched pump beam.

The optical vortex signal beam from the master oscil-

lator was launched into the core of the fiber at the opposite

end. Since the LG01 mode corresponds to the superposition

of the two orthogonal LP11 modes in the MM fiber, we

need to excite these modes only by adjusting the launching

conditions of the signal beam carefully, i.e., the beam size

and the incident NA at the fiber facet. In a circular step-

index fiber with the core of the radius a and the refractive

index n1 surrounded by the cladding of the radius b and the

refractive index n2, the transverse amplitude for the LP

mode in the scalar form can be expressed as [23]:

Wlmðr;/;zÞ¼ expð�iblm/Þcosðl/Þ

Jl ulmr=að ÞJl ulmð Þ ; 0�r�a

Kl wlmr=að ÞKl wlmð Þ ; a�r�b

8>><

>>:

ð1Þ

where b is the phase constant and Jl (x) and Kl (x) are the

first-kind Bessel functions and the modified Bessel func-

tions. The parameters ulm and wlm with the wave number k

are defined as

ulm ¼ a

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

ðkn1Þ2 � b2

q

ð2Þ

wlm ¼ a

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

b2 � ðkn2Þ2q

ð3Þ

and can be calculated from the boundary condition at

r = a:

ulmJl�1 ulmð ÞJl ulmð Þ ¼ �wlmKl�1 wlmð Þ

Kl wlmð Þ ð4Þ

The intensity profile of the LP mode is proportional to

the square of the amplitude, Eq. (1). The intensity plots and

the radial intensity profiles were calculated using the

software RP Fiber Power (RP Photonics Consulting

GmbH). Figure 4 shows the results for the five modes,

LP01, LP02, LP11, LP21, and LP31 modes, supported in the

MM fiber used in our experiment. As can be seen in

Fig. 4b, the outer radius of the targeted LP11 mode (i.e., the

outer position with the 1/e2 intensity of the maximum) is

13.1 lm, and hence, the signal beam was collimated and

focused to have the spot size of *26 lm in diameter at the

fiber facet using a plano-convex lens with a 125 mm focal

length and an aspheric lens with an 8 mm focal length. In

this configuration, the signal beam was launched into the

fiber amplifier with a measured coupling efficiency of

*32 %. The power of the Nd:YAG master oscillator was

fixed at 1 W to minimize the deleterious effects caused by

thermal lensing. A half-wave plate was inserted before the

fiber power amplifier to match the polarization states. The

output beam profile was monitored at all power levels, and

the bending diameter of the fiber was slightly adjusted to

maintain the LG01 mode output if necessary and remained

to be less than 15 cm for suppressing a higher-order mode.

The output power from the fiber amplifier as a function

of absorbed pump power for 1 W of incident signal is

shown in Fig. 5. The amplifier yielded a maximum output

power of 10.7 W for a launched pump power of 17.5 W.

Fig. 6 a Intensity profile and b the spiral interference patterns with

right handedness or c left handedness at 10 W of amplified output

D. J. Kim et al.

123

The slope efficiency with respect to absorbed pump power

was *59 %. The ring-shaped intensity profile was main-

tained at all power levels, as shown in Fig. 6a. The inten-

sity in the center of the beam was not zero but *8 % of the

peak. This dark offset was due to diffraction of the beam

and the undesired ASE, which might degrade the beam

characteristics. However, the measured beam quality M2

remained to be *2.1 up to the maximum output, and the

following interference patterns confirming that the output

mode was the LG01 mode and degradation due to the dark

offset in the center of the beam was negligible. Compared

to the typical fiber amplifier efficiency of over 70 %, the

slightly lower slope efficiency can be attributed to the

mismatch of the LP11 modes and the top-hat-shaped

transverse gain profile in the fiber.

Figure 6b shows the interference pattern for the ampli-

fied output power of 10 W showing that it has a clear spiral

fringe with the same handedness as the incident seed beam

(as shown in Fig. 3b). The handedness of the wave fronts in

the amplified output beam was controlled by the master

oscillator under the same operating conditions [22], con-

firming that the beam characteristics of the master oscil-

lator including the wave front handedness were well

preserved in the MM fiber amplifier (Fig. 6c). Compared to

the previous works [20, 21] which the output characteris-

tics was not determined in the master oscillator but the

bending stress of the amplified fiber, the output in our

system depended on the seed beam characteristics of the

master oscillator making the system more reliable.

In this experiment, we could not observe the nonlinear

optical phenomena such as stimulated Brillouin (or

Raman) scattering or self-phase modulation up to the

maximum output power. However, further power scaling

or pulsed operation in the fiber amplifier should induce

the nonlinear optical effects making the preservation of

the LG mode optical vortex difficult. Moreover, stronger

mode coupling between the modes in the active fiber at

higher power levels would hinder the seed from main-

taining its beam characteristics including the helical wave

fronts. Therefore, optimization of the MM active fiber

design, for example, a MM Yb fiber configuration with a

ring-shaped doping profile, and effective mode control

along with careful consideration of the optical nonlin-

earity could allow significant power scaling of the optical

vortex beam in the fiber amplifier benefiting a range of

application areas.

3 Conclusion

In summary, we have demonstrated a high-power optical

vortex master-oscillator power-amplifier with the LG01

mode output and the well-determined helical wave fronts.

The optical vortex seed beam from the Nd:YAG master

oscillator was successfully amplified in the double-clad

MM Yb fiber yielding 10.7 W of output for an absorbed

pump power of 17.5 W with a high slope efficiency of

59 %. The ring-shaped intensity profile and the wave front

handedness of the seed beam were maintained in the fiber

amplifier via careful mode matching between the incident

seed beam and the LP11 modes in the MM fiber based on

theoretical mode calculations. Therefore, it would be pos-

sible to scale the output to power levels in the hundreds of

watt regime while still maintaining the optical vortex beam

characteristics with further optimization of the gain profile

in the fiber, higher pump power, and deliberate mode

control.

Acknowledgments This research was supported by Basic Science

Research Program through the National Research Foundation of

Korea (NRF) funded by the Ministry of Education, Science and

Technology (2011-0022830).

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