Download Distributed-gain measurements of erbium

Distributed-Gain Measurementsof Erbium-Doped Fibers
J. P. von der Weid, A. 0.Dal Forno, J. A, Pereira da Silva and R. Passy
Centerfor Telecommunications Studies - Catholic University of Rio de Janeiro (Brazil)
Rua Marques de Siio Vicente 225 - Gavea - Rio de Janeiro, RJ22453-900
M. R.Xavier de Barros
CpqD -Telebras (Brazil)
B. Huttner and N. Gisin
Group of Applied Physics - University of Geneva (Switzerland)
Abstract
A non-desctructive optimum gain calibration of erbium-dopedfiber amplifiers was achieved
by using the Coherent Opticalfrequency Domain reflectometry. The Coherent OFDR detection technique
intrinsicallyfilters ASE, allowing precise measurements of Rayleigh Bachcattering levels.
Introduction
Optical amplifiers based on erbium-doped fiber have enabled tremendous increases in the
capacity and functionality of optical networks. One of the key parameter in an EDFA fabrication
is the ideal length of the doped fiber, which is normally calculated theoretically [1]-[2].
However, the practical determination of the optimum length for maximum gain is done with a
cutback method, which is a destructive technique and very inconvenient due to the high cost of
the Erbium doped fiber. The distributed gain in non-isolated long EDFA's could be measured
with OTDR by employing optical filters to eliminate ASE but only long, of the order of some
kilometers, and low doped fibers could be measured due to the limited resolution of the
technique [3]. Coherent Optical Frequency Domain Reflectometry (C-OFDR) is a very powerful
technique for the characterization of return loss of optical components with high dynamic range
and sensitivity [4].Because of the coherent detection technique, ASE is naturally filtered so that
C-OFDR is specially indicated for the characterization of optical circuits with gain and ASE
features. Return loss and gain measurements were indeed performed with this technique [ 5 ] .Here
we present C-OFDR measurements of the distributed gain of Erbium doped fibers for optimum
length determination for different amplifier applications.
Experiment
C-OFDR is based on the detection of the beat signal generated in a Michelson interferometer
when the optical frequency of a laser source is swept linearly. One of the arms gives a reference
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reflection, whereas the other is connected to the fiber under test. The sensitivity of the technique
is deternlined by the laser intensity and its intensity noise, whereas the maximum detectable
range is given by the laser linewidth. In our experimental setup we used a three electrodes
semiconductor laser coupled to an external cavity in order to increase its coherence length [ 6 ] .
When coupled to the external cavity the laser linewidth decreased from -2 MHz to -5 kHz, so
that Rayleigh backscattering levels could be measured in the kilometre range. The OFDR probe
power was -14 dBm, and the coherence loss due to the finite linewidth of the laser is q = 0.013
dB/m, so that the maximum coherence correction within our full 100 m measurement range will
be 1.3 dB. The spatial resolution depends on the range of the measurement, being 20 cm in the
particular 100 m range presented here. Figure 1 presents the experimental set-up, in which the
detailed configuration of theOFDR reflectometer is the same as described before [6]. The optical
isolator was used when small signal gain measurements were performed. The isolator reduced
the probe intensity to -40 dBm without any further reduction of the Rayleigh backscattering
signal detected by the OFDR. Also 20 m of a standard single mode fibre were spliced to the
WDM pigtail in order to provide a reference Rayleigh backscatter signal. The Fiber under test
was a 120 m long, 120ppm Er3+doped fibre fabricated at CpqD-Telebras.
-
Results
Figure 2 shows the small signal gain curves of a 80 m long fiber obtained from the OFDR
measurement of the Rayleigh backscattering signal. Here the probe power was -40 dBm, and the
curves clearly shows that the optimum length is strongly dependent of the pump power, the
optimum length being 40 m for 19.8 dBm pump power, and the corresponding gain is 25.7 dB.
The fiber was cut at 40 m in order to measure the gain, which was 24.5 dB, slightly less than the
OFDR result. This difference can be explained by the fact that the cutback measurement includes
splices and connections other than the OFDR, which measures the fiber gain only. Also, the
coherence lossof
40 m contributes with a -0.6 dB additional decrease in the OFDR
measurement, so that the agreement between the twomethods is remarkable.
A numerical simulation based on the Er3+doped fiber parameters was performed in order to
evaluate the gain alongthe fiber, figure 3. The optimum length obtained bythe numerical
analysis shown to belonger than the values found experimentally with average difference around
7 meters. In the worst case, this optimum length difference produced a gain difference of 2 dB,
figure 4. Practically, theuseof numerical simulation for the optimum length determination
implies an accurate knowledge of the injected pump power in the Er3+doped fibre which can be
dificult to determinewithout a spatially resolved reflectometry technique due to splice losses and
insertion losses on the WMD coupler, filters and isolators.
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Large signal gain curves are presented in figure 5, and were measured with a modified set-up in
which a second laser was connected to the amplifier via an optical 3 dB coupler, launching - 3
dBm into the amplifier. This second laser wavelength was only -50 GHz away from the probe
wavelength (1557.6 nm) so that the depleted gain curve corresponds to the same probe
wavelength. As it can be seen in figure 5, the optimum length in this conditiondecreased to -20
m with-12 dB gain for the maximum pump power.
Conclusions
The
distributed-gain
of
Erbium-doped fibres
was
measured with coherent Optical
Frequency-Domain reflectometry. Because of its coherent detection technique, OFDR naturally
filters the ASE within the 100 kHz detection bandwidth. Although filtered, the ASE white beat
noise limits the detector amplifier
gain, increasing the noise floor of the measurement.
Our
experiments place the OFDR technique as a powerful technique for fibre Lasers ‘and Amplifier
manufacturers for WDM system applications.
Acknowledgements
This work was partially supported byCPqD Telebras under the contract JPqD 779/97, Brazilian
National ResearchCouncil CNPq, SwissCTI Project 3 160, and the European ACTS Bliss
AC065 project.
References
[l] F. F. Riihl, “Calculation of Optimum Fiber Lengths for EDFAs at arbitrary pump
wavelengths”, Electron. Lett., (1991), Vol. 27, No. 16, pp. 1443-1445.
[2] M. Lin and S. Chi, “The Gain and Optimal Length in the Erbium-Doped Fiber Amplifiers
with 1480 nm Pumping”, IEEE Photonics Technol. Lett. (1992) Vol. 4, No.2, pp. 354-356.
[3] M. Nakazawa, Y. Kimura and K. Suzuki,“Gain-Distribution Measurement along an
Ultralong Erbium Doped Fiber Amplifier
using Optical Time Domain Reflectometry”, Optics
Lett. (1990), Vol. 15,No. 1, pp. 1200-1202.
[4] J. P. von der Weid, R. Passy, G. Mussi and N. Gisin, “On the Characterization of Optical
Fiber Network Components with Optical Frequency Domain Reflectometry”, IEEE J.
Lightwave Technol. (1997), Vol. 15, No 7,pp. 1131-1141.
[5] J. P. von der Weid, R. Passy and N.Gisin, “Coherent Reflectometryof Optical Fiber
Amp1ifiers”IEEE Photonics Technol. Lett. (1997) Vol. 9,No. 9, pp 1253-1255.
[6] J. P. von der Weid, R. Passy and N. Gisin, “Mid range coherent optical frequency domain
reflectometry with a DFB laser diode coupled to an external cavity”, IEEE J. Lightwave
Technology (1995), Vol. 13, 5,954-960.
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30
Ei' fibre
-m
APC
.-c
20
IO
m
13
Figure 1 - Experimental setup for small gain measurements
0
,
0
20
1
40
60
Length [m]
Figure 2 - Small signa1 gain curves of a 120 ppm Erbium
doped fiber for co-propagating pump and signal.
Intermediated curves were taken each 3 dB pumplevel.
30
-
-A-
P = 43.6 mW
-+- P = 12.0 mW
,
20
10
0
40
30
60
50
70
0
Length (m)
Fiber end
m
'
20
I
1
40
.
I
60
~
"
80
,
,
60
,
,
80
,
,
100
1
0.0
Figure 4 - Optimum length as fuction of pump powerand
gain difference between calculated and measuredlength.
Pump 19.8 dBrn
,
,
40
Pump Power [mW]
Figure 3 - Simulated distributed gain.
I
0
20
"
100
"
'
120
140
Figure 5 - Large signal gain curves of a 120m long 120
ppm Erbiumdoped fiber.
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