Amplification at 1400-1450 nm of the Large-Core
Nd3+-Doped Fiber by White LED Pumping
Lin Htein, Weiwei Fan, Pramod R. Watekar, and Won-Taek Han

Index Terms— Light-emitting diode, neodymium, optical fiber
amplifier.
I. INTRODUCTION
Optical fiber amplifiers are important devices in
telecommunication
systems
by
providing
effective
amplification of signals at different wavelengths. Over the last
few years, the amplification at S-band (1450-1500 nm) and
extended S-band (1410-1450 nm) have attracted considerable
attention as a means of extending the transmission bandwidth of
optical fibers [1, 2].
In this letter, with the objective of investigating amplification
by white light pumping and developing a novel cost-effective
amplifier for S-band, we demonstrated the amplification of a
Nd3+-doped fiber (NDF) using a white LED as an excitation
source. The NDF is preferred as a gain medium upon white light
pumping because Nd3+ ions can be easily excited to upper states
with relatively low threshold power due to its four-level lasing
scheme [3]. In addition, numerous absorption bands of the NDF
existing in the visible wavelength range can definitely increase
pumping efficiency.
The present demonstration of amplification of the NDF using
the LED as the excitation source was the first to our best
knowledge. The fiber showed emission bands at 918 nm, 1062
nm and 1336 nm upon pumping with the LED and an optical
Fig. 1. The experimental configuration used to measure the amplification of
the NDF upon pumping with the LED.
gain of 6 dB was obtained at 1400 nm. The wavelength
dependence of the gain over 1400-1450 nm was also
investigated.
II. EXPERIMENTS
A large-core NDF with core and cladding diameters of 73 µm
and 700 µm, respectively was drawn from an optical fiber
preform fabricated by the modified chemical vapor deposition
process. The core of the fiber preform was doped with 0.1 M%
of Nd3+ ions by the soaking method. The refractive index
difference between the core and cladding measured by the
preform analyzer was 0.011 and the numerical aperture was
0.18.
The absorption spectrum of the fiber was measured using the
white light source and the optical spectrum analyzer (OSA). The
high-power white LED (LXS8-PW30) [4] was used to excite the
NDF by coupling the LED light into the fiber core using a
pig-tailed graded-index lens (GRIN2906) [5]. A signal light
from the C+L-band source (Optoware-B200) was launched into
the NDF via a 3dB coupler and the output spectrum was
obtained by the OSA. The experimental set-up to measure
optical gain of the NDF upon the LED pumping is shown in Fig.
1.
100
Spectral power distribution (%)
2
This work was supported partially by the Ministry of Science and
Technology, the New Growth Engine Industry Project of the Ministry of
Knowledge Economy, National Research Foundation of Korea (NRF) grant
funded by the Korea government (MEST) (No. 2011-0031840), the Brain
Korea-21 Information Technology Project, and by the (Photonics 2020)
research project through a grant provided by the Gwangju Institute of Science
and Technology in 2012, South Korea.
L. Htein is with the Department of Photonics and Applied Physics, Gwangju
Institute of Science and Technology, Gwangju 500-712, South Korea.
W. Fan is with the School of Information and Communication, Gwangju
Institute of Science and Technology, Gwangju 500-712, South Korea.
P. R. Watekar is with the Sterlite Technologies Limited, Waluj, Aurangabad
431136, India.
W.-T. Han is with the Department of Photonics and Applied Physics/ School
of Information and Communication, Gwangju Institute of Science and
Technology, Gwangju 500-712, South Korea (e-mail: wthan@gist.ac.kr).
4
2
2
P1/2
60
1.0
585 nm
2
+ D3/2+ G9/2
80
2
G5/2+ G7/2
4
K15/2+ G11/2
0.8
468 nm
439 nm
2
K3/2
0.6
4
+ G7/2
40
0.4
4
+ G9/2
4
4
S3/2+ F7/2
530 nm
2
H11/2
20
754 nm
Absorption (a.u.)
Abstract—A novel cost-effective fiber amplifier operating over
1400-1450 nm was developed based on pumping of a large-core
Nd3+-doped fiber with a high-power white LED. An optical gain of
6 dB at 1400 nm was obtained by simultaneously exciting the
multiple absorption bands in the visible wavelength of the
Nd3+-doped fiber.
0.2
687 nm
0
400
0.0
500
600
700
800
Wavelength (nm)
Fig. 2. Comparison of the absorption spectrum of the NDF and the relative
spectral power distribution of the white LED.
Figure 2 compares the absorption bands of the NDF in visible
wavelength with the emission spectrum of the white LED [4].
The absorption bands of the NDF at 439 nm, 468 nm, 530 nm,
585 nm, 687 nm and 754 nm corresponding to the transitions
from the ground state of 4I9/2 to the upper excited states of 2P1/2,
2
K15/2+4G11/2+2D3/2+2G9/2, 2K3/2+4G7/2+4G9/2, 4G5/2+2G7/2, 2H11/2
and 4S3/2+4F7/2, respectively were well-located within the
emission spectrum of the LED and thus it offers the multiple
wavelength pumping schemes [6, 7].
The emission bands of the NDF were found to appear peaking
at 918 nm, 1062 nm and 1336 nm upon the LED pumping in Fig.
3. Among them, the most interesting peaks were those centered
at 1062 nm and 1336 nm with the effective bandwidths of 51 nm
and 72 nm, respectively, which were about 17 nm and 32 nm
broader than those reported by others [8, 9].
For a better understanding of the energy levels involved, a
schematic energy level diagram of Nd3+ by the LED pumping
with the absorption and emission transitions is shown in Fig. 4.
The white light from the white LED excited the electrons from
the ground state of 4I9/2 to the upper excited states of 2P1/2,
2
K15/2+4G11/2+2D3/2+2G9/2, 2K3/2+4G7/2+4G9/2, 4G5/2+2G7/2, 2H11/2
and 4S3/2+4F7/2. Then the electrons in the upper levels decayed
Optical power (dBm)
-40
-45
-50
-55
-60
-65
-70
900
1000
1100
1200
1300
1400
1500
Wavelength (nm)
Fig. 3. Emission spectrum over the wavelength range of 860-1510 nm upon
pumping the NDF of 2 m in length with the LED.
2
2
21
P1/2
K15/2+4G11/2+2D3/2+2G9/2
K3/2+4G7/2+4G9/2
G5/2+2G7/2
2
H11/2
4
S3/2+4F7/2
-61
2
12
1336 nm
918 nm
1062 nm
4
687 nm
754 nm
6
Gain
3
0
Pump
Nd3+
Emission
F3/2
4
I15/2
4
I13/2
4
I11/2
4
I9/2
Fig. 4. Schematic energy levels of Nd3+ by the LED pumping. The up arrows
at left and the down arrows at right indicate the pumping and emission bands,
respectively.
5
-63
Power (dBm)
15
9
-62
4
439 nm
468 nm
530 nm
585 nm
Energy (103 cm-1)
18
6
Ps out
ASE
-64
4
-65
-66
-67
1400
3
Ps in
1410
Gain (dB)
III. RESULTS AND DISCUSSION
non-radioactively to the 4F3/2 metastable state, from which they
relaxed to the 4I9/2, 4I11/2 and 4I13/2 states and thus the emission
bands appeared peaking at 918 nm, 1062 nm and 1336 nm,
respectively.
Although the NDF exhibited three emission bands at 918 nm,
1062 nm and 1336 nm, the band at 1336 nm is of interest due to
its application to amplifiers used for telecommunication
purpose. In general, fluoride fibers are more favorable for the
amplification of signal around 1310-1360 nm [10, 11]. In the
case of the present silicate fibers, however, the gain around the
peak of the emission band was eliminated due to the strong
excited state absorption (ESA) from the 4F3/2 → 4G7/2 transition
which absorbed the ~1160-1380 nm signal and thus brought
about a significant loss [2, 12]. Thus, the gain was possible only
at the wavelength longer than 1380 nm on the edge of the ESA
band.
The amplification characteristics over 1400-1450 nm arisen
from 4F3/2 → 4I13/2 transition were investigated at the fiber length
of 2 m upon pumping with the LED. Figure 5 shows the
wavelength dependence of input signal (Ps in), amplified output
(Ps out), amplified spontaneous emission (ASE) and optical gain
as well. The maximum gain of 5.8 dB was obtained at 1400 nm.
Since the significant amounts of ESA exist at longer wavelength
[13] where the optical gain decreased more rapidly than the
ASE, the gain dropped to nearly 2.5 dB at 1450 nm.
Table I compares the gain of LED-pumped NDFA with that
of others: (1) Nd-doped fluoride fiber amplifier pumped at 795
nm by a Ti-sapphire laser [10], (2) Nd-doped fluorozirconate
fiber amplifier pumped at 496 nm by an Ar laser [11] and (3)
Nd-doped silica fiber amplifier pumped at 800 nm [2]. Although
the two NDFAs (1) and (2) based on fluoride glass fibers
showed the optical gains of 5 dB at 1340 nm and 10 dB at 1330
nm upon pumping at 795 nm and 496 nm, respectively, the gain
coefficients (Gain/Ppump = 0.03-0.07 dB/mW) were rather low.
On the other hand, the NDFA (3) based on silica glass fiber was
found to have the gain of 22 dB at 1400 nm. Despite the 22 dB
gain was reported in the NDFA (3), its gain coefficient (0.11
dB/mW) was much less than that of the present work, 6 dB gain
at 1400 nm with the gain coefficient of 2.31 dB/mW upon
1420
1430
1440
1450
Wavelength (nm)
Fig. 5. (Lines, left axis) The wavelength dependence of input signal (P s in),
amplified output (Ps out) and amplified spontaneous emission (ASE).
(Squares, right axis) Optical gain as a function of wavelength.
TABLE I.
COMPARISON OF THE OPTICAL GAIN OF THE LED-PUMPED NDFA AND
OTHER AMPLIFIERS
s
(nm)
1340
1330
1400
1400
p
(nm)
795
496
800
420-750
Ppump
(mW)
190
150
200
2.6
Gain
(dB)
5
10
22
6
pumping provides a cost-effective fiber amplifier for S-band.
REFERENCES
Gain/Ppump
Ref.
(dB/mW)
0.03
[10]
0.07
[11]
0.11
[2]
2.31
This work
pumping at low power.
For the amplification arisen from the 4F3/2 → 4I13/2 transition,
the gain of the NDFA is limited not only by the presence of
strong ESA around 1336 nm but also by the existence of ASE
from the competitive transitions of much stronger emission
bands at 918 nm and 1062 nm. Nonetheless, the ASE can be
discriminated by designing the index profile, in particular,
removing the active ions from the center of the fiber [14].
Thereby, the gain improvement of up to 18 dB was achieved in
the Nd/Yb doped fiber having index profile with a multiple
layer cladding [2]. Therefore, further improvement in the
performance of the LED-pumped NDFA can be possible by
optimizing the NDF structure.
IV. CONCLUSION
A 1400-1450 nm band optical amplifier with an optical gain
of 6 dB at 1400 nm was demonstrated by simultaneously
exciting the absorption bands of the large-core NDF located at
439 nm, 468 nm, 530 nm, 585 nm, 687 nm and 754 nm with
white LED. The emission bands at 918 nm, 1062 nm and 1336
nm were found upon pumping the NDF with the LED. The
emission band at 1336 nm from the transition of 4F3/2 → 4I13/2
was shown with wide effective bandwidth of 72 nm and thus
very short pulse amplification can be possible. The present
demonstration of amplification of the NDF by the LED
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