High Peak Power Er-doped Tapered Fiber Amplifier

M. M. Khudyakov

1,2

, A. E. Levchenko

, V. V. Velmiskin

,K. K. Bobkov

, D. S. Lipatov

A. N. Guryanov

, M. M. Bubnov

and M. E. Likhachev

Moscow Institute of Physics and Technology (State University),

9 Institutskii per., Dolgoprudnyi, Moscow Region, Russian Federation

Fiber Optics Research Center of the Russian Academy of Sciences, 38 Vavilov Street, Moscow, Russian Federation

Institute of High Purity Substances of Russian Academy of Sciences,

49 Tropinin Street, Nizhny Novgorod, Russian Federation

Keywords: Tapered Fiber, Fiber Amplifier, High Peak Power, Er-doped.

Abstract: A novel tapered Er

-doped fiber design for high peak power amplification has been developed and tested.

The fiber core was based on P

-Al

-SiO

glass matrix, which allowed simultaneous achievement of

low NA and high Er content. The core diameter was changing along the fiber length from 22.5 µm (single-

mode operation) to 86 µm along 2.5 meters. Amplifier based on counter propagation signal (coupled to the

thin tapered fiber end) and pump (coupled into thick fiber end) was developed and a nearly diffraction-

limited beam quality (M

<1.27) of the output signal has been achieved. Amplification of 80 ns single

frequency Gaussian-shaped pulses has resulted in peak power of 20 kW in 55 ns pulses (1.5 mJ) limited by

available pump power.

1 INTRODUCTION

High peak power laser sources at eye-safe spectral

region near 1.55 µm are attractive for a variety of

free space applications, such as remote sensing and

LIDAR (LIght Detection And Ranging). In many

cases (for example wind LIDARs (Kotov et al.,

2016)) pulses with duration from tens to hundreds

nanoseconds with spectral width from several tens

kHz to tens of MHz are required. In this case

instability of amplified pulse caused by Stimulated

Brillouin Scattering (SBS) is the main factor that

limit maximum peak power (Kotov et al., 2014).

Nowadays, cladding pumped Er-Yb co-doped

fibers are used to obtain high average powers at 1.55

µm (Jeong et al., 2005). However, efficient energy

transfer from Yb to Er ions requires codoping of the

fiber core with large amounts of phosphorus, which

limits the diameter of the single mode core. Thus, in

single-frequency regime up to 6.6 kW of peak power

was demonstrated for multimode (M

~5) Er-Yb fiber

lasers (Codemard et al., 2006) and up to 1.2 kW for

single-mode regime (Shi et al., 2010). In addition,

parasitic Yb emission near 1 µm renders these lasers

unsafe for free space applications.

Several Yb free Er doped large mode area

cladding pumped at 976 nm fiber amplifiers were

developed by our group recently. Pump to signal

conversion efficiency of 40% in single-mode

continuous wave (CW) regime was demonstrated

(Kotov et al., 2013). The record peak power of 4

kW before the stimulated Brillouin scattering (SBS)

threshold for silica-based single-frequency single-

mode all-fiber lasers was demonstrated (Kotov et al.,

2014).

Core pumping of Er-doped Yb-free fibers at 1.48

µm by a high power Raman lasers appears

promising for obtaining high peak power due to

reduced nonlinearity. Indeed core pumping reduces

by few times the length of Er-doped used in

amplifier compared to that in cladding pumped

schemes. Moreover, single-mode pumping allows

for nearly single-mode operation in multimode

fibers, thus decreasing fiber nonlinearity (Jasapara et

al., 2009). Therefore, up to 1.1 kW of peak power

before the SBS threshold was demonstrated in the

single-mode regime using a 40µm core Er-doped

Khudyakov, M., Levchenko, A., Velmiskin, V., Bobkov, K., Lipatov, D., Guryanov, A., Bubnov, M. and Likhachev, M.

High Peak Power Er-doped Tapered Fiber Ampliﬁer.

DOI: 10.5220/0006715701050109

In Proceedings of the 6th International Conference on Photonics, Optics and Laser Technology (PHOTOPTICS 2018), pages 105-109

ISBN: 978-989-758-286-8

105

fiber (Canat et al., 2013). However, the main

problem of this approach is a high complexity of the

scheme, which results in a low overall signal-to

pump conversion efficiency and a high production

cost. Moreover it requires usage of high power

1480/1550 wavelength division multiplexers

(WDMs), which are available as experimental

samples only and are subject to rapid power

degradation.(Peng et al., 2013).

Utilization of tapered fiber geometry is a novel

promising way to decrease fiber nonlinearity. The

idea of this approach is simple: the core and

cladding diameters monotonically increase along the

fiber length to several times their original size. The

fundamental mode excited at the single-mode (thin)

end propagates towards the thick end of the tapered

fiber adiabatically (without exciting higher order

modes (HOMs)) (Jung et al., 2009). Recently high

peak power Yb-doped cladding-pumped tapered

fiber amplifier was developed by our group (Bobkov

et al., 2017). Using this approach we demonstrated

increase of the stimulated Raman scattering

threshold for 28 ps pulses to the level of 760 kW,

which is an order of magnitude higher compared to

non-tapered Yb doped cladding-pumped fiber

amplifier (57 kW) (Kalaycioglu et al., 2010).

In the present paper we utilized the tapered fiber

approach for the case of Er-doped fibers.

Amplification of 80 ns pulses to peak power of 20

kW is demonstrated using Er-doped tapered

cladding-pumped at 976 nm fiber amplifier.

2 TAPERED FIBER DESIGN AND

FABRICATION

2.1 Preform Fabrication

The preform was made by Modified Chemical

Vapour Deposition technique. Similar to (Kotov et

al., 2015), the fiber core was made of P

-Al

SiO

(PAS) glass matrix which allows for higher

concentration of Er

ions due to lower clustering.

Moreover, simultaneous doping of the fiber core

with P

and

leads to formation AlPO

join,

which has refractive index nearly equal to that of

pure silica glass. The core was doped with ~0.14

mol.% Er

and had numerical aperture (NA) of

0.076. In addition, we utilized so-called W-shaped

refractive index profile (RIP) – a depressed fluorine-

doped layer was placed just outside the core (see

Figure 1). It reduced cut-off wavelength for the core

with a fixed diameter. This allowed us to achieve

single-mode propagation regime in a slightly bent

thin tapered fiber end having outer diameter of 90

µm (cut-off wavelength~1.7 µm)

Figure 1: RIP of drawn fiber with outer diameter of 90 µm

and optical image of fiber end facet.

Figure 2: Small signal absorption from the cladding

measured by cutback technique.

Fabricated preform was polished to square shape

to ensure better overlap between cladding pump

modes and the fiber core. After that, preform was

overcoated with a fluorine-doped silica layer having

NA ~ 0.3. Core to the average first cladding

diameters ratio was 1/3, which also increased pump

absorption from the first cladding – small signal

absorption at 981 nm was about 3.9 dB/m (see

Figure 2). Another important advantage of the F-

doped silica cladding was the simplicity of preparing

the thick end, which could be glued into an adapter

and angle-polished using standard equipment. Thus,

a perfect angle-cleaved thick fiber end compatible

PHOTOPTICS 2018 - 6th International Conference on Photonics, Optics and Laser Technology

106

with a high power end-pumping technique could be

routinely obtained.

Figure 3: Tapered fiber outer diameter along the length.

2.2 Fiber Drawing

To produce the tapered fiber, we utilized a non-

stationary fiber drawing process. This method was

proposed and realized for the first time at FORC

RAS in 1991 (Bogatyrev et al., 1991), When it was

used to draw relatively long (10-1000 m) tapered

fibers. Later, this method was modified to draw

short (~1 m in length) tapered fibers (Bogatyrjov and

Sysoliatin, 2001). After additional modifications, the

non-stationary fiber drawing process allowed us to

obtain a highly reproducible set of few tens tapered

fibers with a variation in the parameters of less than

10%, with a tapered region length of less than 1

meter and a tapered ratio up to 7. The diameter

distributions of outer diameter over the fiber length

for a tapered fiber cut from such a drawing are

presented in Figure 3. The maximum outer diameter

was 350 µm (core diameter of 86 µm), the

fundamental mode field diameter (MFD) was

estimated to be 53.4 µm (mode field area ~ 2240

m

) at 1550 nm.

A smooth enough transition between the thin and

the thick ends of the tapered fiber is required to

avoid excitation of HOMs during propagation of

fundamental mode to the thick end. Since our

geometry is similar to (Bobkov et al., 2017) with

lower difference between diameters of thin and thick

ends and longer taper length, we could expect that

fundamental mode propagates adiabatically in our

case too. This feature was checked by measurements

of M

at the output of the developed Er-doped

tapered fiber in continuous wave operation regime.

The measurements were done with a Thorlabs

M2MS-BP209IR2/M measurement system and

shows the perfect beam quality – M

was equal to

1.26/1.27 for x/y axis (see Figure 5).

3 AMPLIFICATION OF 80 NS

PULSES

3.1 Experiment Setup

Experiment setup is presented in Figure 4.

Distributed feedback laser diode (DFB-LD) with

central wavelength of 1555.6 nm, output power of 1

mW and linewidth of 2 MHZ was used a seed

source. Its radiation was coupled into semiconductor

optical amplifier (SOA) through a polarization

controller (PC) driven by two channel arbitrary

waveform generator (AWG) to produce 80 ns pulses

with repetition rates of 1 kHz and average power of

0.5 µW. The polarization controller was necessary

due to SOA being polarization dependant and DFB-

LD being based on non-polarization maintaining

fiber. Resulting pulses were amplified by Er-doped

fiber amplifier (EDFA1) core pumped at 980 nm.

Since input power was far below saturation level of

EDFA1 we used single circulator (C1) with long

fiber Bragg grating (FBG1) with spectral linewidth

of 0.08 nm to filter out amplified spontaneous

luminescence (ASE) obtaining 230 µW of average

power (~3 W peak power). “Clean”, pulses were

amplified by second Er-doped core-pumped at 980

nm fiber amplifier (EDFA2). Another circulator

(C2) with long fiber Bragg grating (FBG2) with

spectral linewidth of 0.08 nm were used to filter out

additional ASE and protect EDFA2 from

backscattered light from tapered fiber amplifier.

High Peak Power Er-doped Tapered Fiber Ampliﬁer

107

Figure 4: Experiment setup.

Figure 5: M

measurements and beam intensity distribution in far field.

Up to 50 W peak power pulses (4 mW average)

limited by SBS in EDFA2 were coupled through

cladding pump stripper into tapered fiber amplifier

(TEDFA). It was pumped with wavelength-

stabilized (976±1 nm) multimode pump diode

having 105/125 delivery fiber (NA ~ 0.15). Dichroic

mirror was used to separate amplified light at 1555.6

nm from pump at 976 nm. The end facet of TEDFA

was angle cleave at ~7⁰ to prevent backscattering.

Photodiode (PD), integrating photodiode (IPD)

(Kotov et al., 2015), spectrum analyser (SA) and

power meter (PM) were used to characterize

amplified pulses.

3.2 Results

Amplification of 80 ns pulses with repetition rate of

1 kHz resulted in 1.63 W of average power. Part of

power contained in ASE was controlled by IPD and

SA. According to spectral measurements wide ASE

peak near 1530 nm contained up to 3.5% of power at

maximum pump power. Measurements by IPD

indicate that up to 9% of power was contained

between pulses at maximum pump power. Thus, we

can conclude that up to 5.5% power consisted of

amplified continuous radiation at 1555.6 nm. Pulse

peak power and energy dependencies on pump

power are presented at Figure 6. Discrepancy

between energy and peak power at high pump

powers is due to inversion depletion following

propagation of the forward front of the pulse (see

Figure 7). Obtained peak power of 20 kW (pulse

energy of 1.5 mJ) in 55 ns pulses was limited only

by available pump power. As up to 100W

wavelength stabilized pump diodes are available

now, the output power from the amplifier (and

maximum peak power) can be significantly

improved.

Figure 6: Peak power and energy vs pump power.

Figure 7: Temporal profiles at low and high pump powers.

PHOTOPTICS 2018 - 6th International Conference on Photonics, Optics and Laser Technology

108

4 CONCLUSIONS

We demonstrated amplification 80 ns pulses to up to

20 kW of SBS free peak power (1.5 mJ). The

maximum peak power is limited by pump power

available in our experiment and father power scaling

using more powerful pump diode will be discussed

at the conference.

ACKNOWLEDGEMENTS

This work was supported with a grant 16-12-10553

from the Russian Science Foundation. Optimization

of MCVD fabrication process for achieving very flat

core refractive index was made within grant 16-03-

00325 of Russian Foundation for Basic Research.

Authors would like to thank A.A. Rybaltovsky and

O.V. Butov from Kotel’nikov Institute of Radio

Engineering and Electronics of RAS for provided

narrow-linewidth fiber Bragg gratings.

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