Development of Germano-silicate Optical Fiber Incorporated with Germanium
Nanoparticles and Its Optical Characteristics
Seongmook Jeonga, Seongmin Jua, Youngwoong Kima, Hyejeong Jeongb, Seongjae Boob and
Won-Taek Hana,*
a
School of Information and Communications/Department of Physics and Photon Science, Gwangju Institute of
Science and Technology(GIST), 123 Chemdangwagi-ro, Buk-gu, Gwangju, 500-712, South Korea
b
Solar City Center, Korea Institute of Industrial Technology (KITECH), 208 Chemdangwagi-ro, Buk-gu, Gwangju,
500-480, South Korea
*Corresponding author Email: wthan@gist.ac.kr
The germano-silicate optical fiber incorporated with Ge nanoparticles with enhanced optical
nonlinearity was developed by using modified chemical vapor deposition and drawing processes.
A broad photoluminescence band obtained by pumping with the 404 nm superluminescent diode
was found to appear from 540 nm to 1,000 nm. The non-resonant nonlinear refractive index, n2,
of the fiber measured by the continuous wave self-phase modulation method was 4.95×10-20
m2/W due to the incorporated Ge nanoparticles in the fiber core. The enhancement of the nonresonant optical nonlinearity may be due to the creation of the NBOs and other defects from the
incorporated Ge-NPs in the fiber core.
Keywords: Optical fiber, Germanium nanoparticles, Photoluminescence, Optical nonlinearity
1. Introduction
Recently, the optical fibers incorporated with semiconductor nanoparticles in the fiber core
have attracted much attention due to its optoelectronic and photonic applications [1-3]. Especially,
group IV semiconductor nanoparticles are being studied for applications in optical fiber
amplifiers and nonlinear optical devices for wavelength converting, all-optical switching, optical
tunable filtering, ultra-short pulse generation, and supercontinuum generation, etc [4,5].
Si and Ge nanoparticles are known to induce an efficient photoluminescence (PL) in visible and
near infrared wavelength ranges due to their quantum confinement effect or recombination of
excitons confined in the nanoparticles [6-8]. The enhancement of PL emission from Er-ions by
incorporating Si nanoparticles in the Er- doped optical fiber was demonstrated [9]. The photon
energy absorbed by Si nanoparticles in the fiber core in visible band was found to transfer to Erions by a radiative carrier recombination process [9,10]. Moreover the Si nanoparticles doped
optical fiber showed a large optical nonlinearity due to the creation of non-bridging oxygens
(NBOs) and defects of Si nanoparticles [5,11,12].
1
On the other hand, Ge nanoparticles are also known to be a good candidate as a sensitizer
because Ge nanoparticles showed stronger quantum confinement and larger PL intensity than Si
nanoparticles resulting from its direct-gap semiconductor nature and a smaller band-gap,
respectively [13-15]. In this paper, a novel germano-silicate optical fiber incorporated with Ge
nanoparticles (Ge-NPs doped fiber) was fabricated by using the modified chemical vapor
deposition (MCVD) and drawing processes and its broad PL and highly nonlinear optical
properties were investigated.
2. Experimental Details
2.1 Fabrication of the Ge-NPs doped fiber
The Ge-NPs doped fiber preform was fabricated by using the MCVD process and solution
doping technique [16]. The partially sintered porous germano-silicate core layers were deposited
onto the inner surface of a silica glass tube at 1650 oC by using vaporized SiCl4 and GeCl4. The
porous core layers were soaked with a doping solution to incorporate Ge-NPs for 2 hours at room
temperature. The doping solution was prepared by mixing high purity Ge powders (45 μm under,
Kojundo, GEE05PB) in deionized water. After the doping process, the tube was dried by flowing
a helium gas, and sintered and sealed to obtain a fiber preform at 2100~2300 oC. Finally, the
preform incorporated with Ge-NPs was drawn into an optical fiber of 125 μm diameter by using
the drawing process at 2,000 oC. The refractive index difference between the core and cladding
of the fiber was 0.006. The core diameter and cut-off wavelength of the Ge-NPs doped fiber were
11 μm and 11.8 μm, respectively.
2.2 Measurements
The existence of Ge-NPs in the core of the fabricated optical fiber preform was verified by
using a transmission electron microscope (TEM; Technai, G2 S-Twin 300 KeV). The optical
absorption of the Ge-NPs doped fiber preform was also measured to confirm the formation of
Ge-NPs using the UV-VIS spectrometer (Varian, Cary 5000). The absorption of the Ge-NPs fiber
was measured by the cut-back method with the white light source (WLS; ANDO, AQ 4303B)
and the optical spectrum analyzer (OSA; Ando, AQ-6315B), and it was compared with that of
the fiber without Ge-NPs. The PL characteristics of the fibers with and without Ge-NPs of 3 meter
length were measured upon pumping with the 404 nm superluminescent diode (SLD; Sony,
SLD3239VF-51).
The non-resonant nonlinear coefficient of the Ge-NPs doped fiber was estimated by using
the continuous wave self-phase modulation (cw-SPM) method as shown in Figure 1. The
nonlinear phase shift, φSPM, and the corresponding average power of light signal, PAVG, were
measured. The φSPM, which is caused by SPM, was calculated from the ratio of measured intensity
of fundamental signal to first-order harmonic signal by using Eq. (1):
2
I 0 J 02 (  SPM )  J 12 (  SPM )

I 1 J 12 (  SPM )  J 22 (  SPM )
(1)
where Jn is the nth order Bessel function. The non-resonant nonlinear refractive index, n2, and the
effective nonlinear parameter, γ, were then obtained by using Eqs. (2) and (3):
Aeff  SPM  Aeff
ac ,


4Leff  PAVG  4Leff
(2)
2 n2
1  SPM   a c



 Aeff 2 Leff  PAVG  2 Leff
(3)
n2 

where λ = (λ1+ λ2)/2 is the center wavelength of the two pumps, κac is the slope coefficient
determined from the linear region of the function φSPM/PAVG, and Leff and Aeff are the effective
length and the effective area at 1550 nm, respectively [17-20].
3. Results and Discussion
3.1 TEM morphology
Figure 2 shows the TEM micrograph of the fabricated optical fiber preform indicating the
existence of the Ge-NPs in the core. The roughly spherical crystalline Ge-NPs with the diameters
around 3.5 nm were found to disperse homogeneously without agglomeration.
3.2 Optical absorption
The optical absorptions of the fiber preform and the fiber measured by using the UV-VIS
spectrometer and the cut-back method are shown in Figure 3. A broad and strong absorption band
with absorption peaks at 243 nm, 328 nm and 485 nm appeared in the Ge-NPs doped fiber
preform as shown in Figure 3(a) due to the incorporated Ge-NPs [21-23]. In the case of the GeNPs doped fiber, on the other hand, the broad and strong absorption band peaking at 485 nm
appeared in the preform was found to red-shift to 494 nm (Figure 3(a)), indicating that Ge-NPs
were successfully preserved in the fiber core but a little grown in size after the high temperature
drawing process at 2,000 oC [22].
3.3 Photoluminescence characteristics
To evaluate the effect of Ge-NPs on emission property, the PL characteristics of the 3 meter
long fibers with and without Ge-NPs was measured by increasing pumping power of the 404 nm
SLD from 0.01 mW to 5.30 mW at room temperature. As shown in Figure 4(a), a broad PL band
from 540 nm to 1,000 nm peaking at 690 nm was found to appear in the fiber with Ge-NPs and
it was due to the quantum confinement effect of Ge-NPs or recombination of excitons [6,7]. The
PL intensity at 690 nm increased with the increase of the pumping power from 0.01 mW to 5.30
mW. However, the fiber without Ge-NPs also showed a broad and weak PL spectrum in visible
range which was not expected and it may be due to a small amount of Ge particles formed during
3
the MCVD process even without incorporation of Ge-NPs (Figure 4(b)). Note that the emission
peaks at 404 nm and 808 nm were from the pumping source of the SLD itself.
Figure 5 compares the change in PL intensity at 690 nm with pumping power of the Ge-NPs
doped fiber and the fiber without Ge-NP. The PL intensity at 690 nm of the Ge-NPs doped fiber
increased from -75 dBm to -59 dBm with the pumping power from 0.10 mW to 5.30 mW. On the
other hand, the fiber without Ge-NPs showed a small increase from -77 dBm to -75 dBm upon
pumping from 2.95 mW to 5.30 mW. Note that the PL intensity pumping at 0.10 mW and 1.86
mW was not evaluated accurately due to the large noise level. The slope of the Ge-NPs doped
fiber was 2.7 times larger than the fiber without Ge-NPs. From the PL results, the Ge-NPs
incorporated in the fiber core were found to be beneficial to obtain an efficient PL intensity upon
pumping.
3.4 Non-resonant nonlinear optical characteristics
Figure 6(a) shows the cw-SPM spectra of the Ge-NPs doped fiber of 1 km length measured
at different input powers. The fundamental signals appeared at 1549.74 nm and 1550.25 nm and
the first-order harmonic signals appeared at 1549.23 nm and 1550.76 nm. The phase shift φSPM
was calculated by comparing intensities between the fundamental signal and the first-order
harmonic signal. Figure 6(b) shows the obtained phase shift φSPM of the Ge-NPs doped fiber at
different input powers. The phase shift φSPM of the Ge-NPs doped fiber increased linearly with
the increase of input power. Then the non-resonant nonlinear refractive index, n2, and the effective
nonlinear parameter, γ, were calculated by using Eqs. (2) and (3). As summarized in Table 1, the
non-resonant nonlinear refractive index, n2, and the effective nonlinear parameter, γ, of the GeNPs doped fiber were 4.95×10-20 m2/W and 1.62 W-1km-1, respectively. The n2 and γ of the GeNPs doped fiber were 4.30 times and 3.95 times larger than those of the fiber without Ge-NPs,
respectively. The observed non-resonant optical nonlinearity of the fiber doped with Ge-NPs may
originate from the hyperpolarizabilities of non-bridging oxygens (NBOs) formed by the
incorporation of Ge-NPs. The NBOs are known to induce highly optical nonlinearity because
NBOs have higher ionicity and are distorted easily by the applied optical field [12,19,20]. The n2
and γ of the Ge-NPs doped fiber were enhanced due to the creation of the NBOs and other defects
(NBO hole centers, peroxy radicals, oxygen deficient centers) from the incorporated Ge-NPs in
the fiber core [11,12].
4. Conclusions
We fabricated the Ge-NPs doped fiber by using the MCVD and drawing processes. The
spherical Ge-NPs with the average size about 3.5 nm were identified to be embedded in the core
of the fiber preform by TEM. The optical absorption bands of the Ge-NPs doped fiber preform
measured by UV-VIS spectrometer were found to appear at 243 nm, 328 nm and 485 nm. On the
4
other hand, the absorption band of the Ge-NPs doped fiber appeared at longer wavelength of 494
nm due to the growth of Ge-NPs during the high temperature drawing process at 2,000 oC. Upon
pumping the Ge-NPs doped fiber with the 404 nm SLD, a broad PL band appeared from 540 nm
to 1,000 nm peaking at 690 nm, maybe due to the quantum confinement effect of Ge-NPs or
recombination of excitons. The non-resonant nonlinear coefficient of the Ge-NPs doped fiber was
also evaluated by using the continuous wave self-phase modulation (cw-SPM) method and the
non-resonant nonlinear refractive index, n2, and the effective nonlinear parameter, γ, of the GeNPs doped fiber were found to be 4.95×10-20 m2/W and 1.62 W-1km-1, respectively. This
enhancement of the non-resonant optical nonlinearity may be due to the creation of the NBOs
and other defects from the incorporated Ge-NPs in the fiber core.
Acknowledgments:
This work was partially supported by the New Growth Engine Industry Project of the
Ministry of Trade, Industry and Energy, Basic Science Research Program through the National
Research Foundation of Korea (NRF) funded by the Ministry of Education (No.
2013R1A1A2063250), the Korea government (MSIP) (No. 2011-0031840), the Brain Korea-21
Plus Information Technology Project through a grant provided by the Gwangju Institute of
Science and Technology, South Korea.
5
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(2010).
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J. Nanosci. Nanotechnol. 6, 3555 (2006).
17. G. P. Agrawal, Nonlinear Fiber Optics, 2nd edn., Academic Press, San Diego (2001).
18. K. Nakajima, T. Omae, and M. Ohashi, IEEE Proc.-Optoelectron. 148, 209 (2001).
19. A. Lin, B. H. Kim, D. S. Moon, Y. Chung, and W.-T. Han, Opt. Express 15, 3665 (2007).
20. S. Ju, P. R. Watekar, S. Jeong, Y. Kim, and W.-T. Han, J. Nanosci. Nanotechnol. 12, 629
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(2011).
6
23. V. N. Brudnyi and S. N. Grinyaev, Russ. Phys. J. 53, 703 (2010).
Table 1. Experimental parameters and measured optical properties of the fiber incorporated with and without
Ge-NPs
7
Refractive
Fiber
index
difference
Effective
Effective
Slope
length
core area
coefficient
Nonlinear
refractive
index
Effective
nonlinearity
Symbol
Δ
Leff
Aeff
κac
n2
γ
Unit
%
m
μm2
rad/W
m2/W
W-1km-1
The fiber without GeNPs
0.5
170.03
113.0
0.84
1.15x10-20
0.41
The Ge-NPs doped fiber
0.6
70.05
123.63
0.23
4.95x10-20
1.62
8
Figure captions
Figure 1. Schematic of the non-resonant optical nonlinearity measurement set-up using the cw-SPM
method: TLS = tunable laser source, PC = polarization controller, BPF = band-pass filter, EDFA
= erbium-doped fiber amplifier, FUT = fiber under test, VOA = variable optical attenuator, OSA
= optical spectrum analyzer.
Figure 2. TEM image of Ge-NPs in the core of the fiber preform.
Figure 3. (a) Absorption spectra of the Ge-NPs doped fiber preform, (b) Absorption spectra of the fibers
doped with and without Ge-NPs.
Figure 4. PL spectra upon pumping with the SLD at 404 nm; (a) the Ge-NPs doped fiber pumping from
0.01 mW to 5.30 mW, (b) the fibers doped with and without Ge-NPs upon pumping at 5.30 mW.
Figure 5. The change in PL intensity at 690 nm upon pumping from 0.10 mW to 5.30 mW.
Figure 6. (a) The cw-SPM spectrum of the Ge-NPs doped fiber at different input powers, (b) The phase
shift φSPM of the Ge-NPs doped fiber at different input powers.
9
Figure 1. S. Jeonget al.
10
Figure 2. S. Jeonget al.
11
Core of fiber preform (with Ge-NPs)
Cladding of fiber preform (without Ge-NPs)
2.0
243 nm
0.080
Core of fiber preform(with Ge-NPs)
-1
Absorption coefficient [cm ]
-1
Absorpton coefficient [cm ]
2.5
1.5
1.0
328 nm
485 nm
0.076
0.072
0.068
420
0.5
440
460
480
500
520
540
560
Wavelength [nm]
0.0
200
300
400
500
600
700
800
Wavelength [nm]
(a)
The fiber with Ge-NPs
The fiber without Ge-NPs
-1
Absorption coefficient [cm ]
0.008
0.006
0.004
494 nm
Cut off wavelength
0.002
0.000
400
OH absorption
OH absorption
600
800
1000
1200
1400
Wavelength [nm]
(b)
Figure 3. S. Jeonget al.
12
0
Ge-NPs doped fiber : 3 meter length
-10
PL intensity [dBm]
-20
-30
5.30 mW
3.76 mW
2.95 mW
1.86 mW
0.10 mW
0.01 mW
-40
690 nm
-50
-60
-70
-80
-90
400
500
600
700
800
900 1000 1100 1200
Wavelength [nm]
(a)
0
The fiber with Ge-NPs @ 5.30 mW
The fiber without Ge-NPs @ 5.30 mW
-10
PL intensity [dBm]
-20
-30
-40
690 nm
-50
-60
-70
-80
-90
400
500
600
700
800
900 1000 1100 1200
Wavelength [nm]
(b)
Figure 4. S. Jeonget al.
13
-55
The fiber with Ge-NPs
The fiber without Ge-NPs
Peak intensity [dBm]
-60
-65
-70
Slope = 3.2 [dBm/mW]
-75
-80
Slope = 1.2 [dBm/mW]
0
1
2
3
4
5
6
Pumping power [mW]
Figure 5. S. Jeonget al.
14
Output power [dBm]
10
1549.74nm(w1)
I0
630.96 mW
562.34 mW
467.74 mW
380.19 mW
275.42 mW
190.55 mW
112.20 mW
1550.25 nm(w2)
0
-10
-20
-30
I1
1550.76nm
(-w1+2w2)
1549.23nm
(2w1-w2)
-40
-50
1549.0
1549.5
1550.0
1550.5
1551.0
Wavelength [nm]
(a)
0.16
Phase shift [rad]
0.14
Ge-NPs doped fiber
Linear fit of the points
0.12
0.10
0.08
0.06
Slope coefficient (ac) = 0.23[rad/W]
0.04
0.02
100
200
300
400
500
600
Input power [mW]
(b)
Figure 6. S. Jeonget al.
15