1) Researchers developed a broadband erbium-doped fiber amplifier covering the C-band (1525-1565 nm) and L-band (1570-1610 nm) using a parallel structure of optimized C-band and L-band amplifiers. 2) The amplifier achieved an amplification bandwidth of over 70nm (1524-1602nm) with C-band average gain over 30dB and L-band gain variation less than 2dB. 3) By using separate C-band and L-band amplification and combining the signals, the design effectively prevented interaction between the two bands while simplifying the configuration.

1. Experimental Study on a Broadband Erbium-doped Fiber Amplifier
Yu Ling, Zhang Hao, Liu Yange,Yuan Shuzhong, Kai Guiyun, Dong Xiaoyi
(Instituteof Modern Optics, Nankai University, Tianjin 300071 China)
Abstract: In this paper, we utilized optimized C-band and L-band amplifiers to
constitute a broadband CA-L erbium-doped amplifier with a parallel structure. This
amplifier has a simple configuration, high-flattened gain and low noise. Meanwhile,
we have realized an amplification bandwidth of over 70 nm(1 524—1 602 nm)without
any broadband gain flattening component. The C-band average gain is over 30 dB
and L-band gain variation is less than 2 dB.
Key words: erbium-doped fiber amplifier (EDFA) ; broadband; C+L band
1. Introduction
In order to meet the growing demand for communication capacity, it is a general
trend to extend the dense wavelength division multiplexing system (DWDM) from the
traditional c-band (1 525 to 1 5 6 5 nm) to the L-band (1 570, 1 610 nm). . The
development of C + L band ultra-wideband erbium-doped fiber amplifiers not only
doubles the bandwidth of communication, but also increases the dispersion of the
system compared to other methods of increasing the transmission capacity of a single
channel or reducing the channel spacing. There is no need to worry about nonlinear
effects such as four-wave mixing (FWM) that may be caused by narrow channel
spacing, which effectively avoids system performance degradation.
As the research on C-band erbium-doped fiber amplifiers is becoming more and
more mature, the research focus of current ultra-wideband amplifiers is how to

2. achieve high-gain and low-noise amplification of L-band signals. There are three main
methods: (1) development of erbium-doped fibers with different matrices. In this
application, the connection problem between the doped fiber and the ordinary
silicon-based fiber occurs in the practical application; (2) the traditional EDFA is used
in combination with the fiber Raman amplifier (FRA), because the FRA requires
multiple high-power pump lasers of different wavelengths. To obtain a wide flattening
bandwidth, increase the cost; (3) using different structures of silicon-based
erbium-doped fiber amplifiers, based on mature c-band technology is more practical,
using this method
At present, the C + L band ultra-wideband EDFA can be divided into two types:
tandem type and parallel type according to the connection mode of c-band and
L-band [i, wow series-connected structure is more flexible, but c-band and L A band of
signals together through the amplification of the fiber will inevitably affect each other.
This paper introduces a method of amplifying two bands by parallel method, and
sending the signals to the c-band and L-band amplifiers respectively for amplification.
The amplified signals are combined to form a wide-band signal output through the
combiner, which can effectively prevent the c-band. Interacting with the L-band signal.
Only the independent c-band and L-band amplification techniques are used. It is
simple and practical. As long as the amplification performance of the respective bands
is improved, the performance of the entire ultra-wideband amplifier can be greatly
improved. The development direction of the amplifier·
2. Experimental Principle and Device

3. Generally, the wavelength range of the erbium-doped fiber gain spectrum is in
the C-band (1 525, 1 565 nm), which corresponds to the transition of the erbium ion
4113/2 to 4115 /2. In 1990, Massicott et al. found that by controlling the
erbium-doped fiber. The length of the 铒 ion particle number distribution is
stabilized to a low degree, and the gain spectrum of the erbium ion can be shifted to
the L band. The gain spectrum of this displacement corresponds to the tail of the
41]3/2 to 4115/2 transition. The absorption and divergence coefficients are small, but
relatively flat due to the low absorption and emission coefficients. To obtain the high
gain spectrum of the L-band, it is necessary to increase the pump power and increase
the length of the erbium-doped fiber, which is generally about the conventional
C-band at the same doping concentration. 4 or 5 times [six not only increases the cost,
but also increases the pump power, the low population number reversal rate of the
signal input terminal will also cause waste of pump power and produce gain
saturation effect. Currently, high doping is mainly used. Low-loss erbium-doped fiber
reduces the length of the required fiber, reduces absorption loss and accumulation of
back-amplified spontaneous emission spectrum (ASE) energy, and improves power
conversion efficiency. In addition, various techniques are used to suppress backward
ASE. Production , to improve pump efficiency; such as adding a reflector [9] at the
pump input end; using a post-ASE secondary pump source to pump an unpumped
fiber 10 '11]; and using multiple different wavelengths And the fiber grating of the
bandwidth forms the pump source in multiple directions, and on the basis of this, a
front end of the ASE reflection using the fiber loop mirror is proposed, so that the
originally wasted ASE is reused. , improving power conversion efficiency,

4. The ultra-wideband EDFA structure is shown in Figure 1. It is mainly divided into
two parts, C and L. Both are 1 480 nm LD backward pump and 980 nm LD forward
pump.
Fig. 1 Experimental setup Of the broad band EDFA
Among them: the L-band amplification part adopts the high doping
concentration erbium-doped fiber E-knife Fl: 25 m; the circulator is used to input the
output L-band signal; the fiber loop mirror is connected by a fusion cone type with a
coupling ratio of 5:5. The device is configured to reflect the spontaneous emission of
the c-band. At the same time, the L-band signal is amplified and then
double-amplified by the optical fiber ring mirror. Therefore, the structure fully utilizes
the C-band ASE spectrum for EDFI secondary pumping. The L-band signal light is

5. amplified twice, which greatly enhances the gain of the L-band signal and improves
the power conversion efficiency.
The C-band amplifying part adopts the two-stage cascading EDFA. Due to the use
of the intermediate isolator IS02, the amplification of the forward and reverse
spontaneous radiation can be suppressed, so that the amplifier has low noise and
high gain, and the total length of the erbium-doped fiber is 21 m. The position of the
intermediate isolator is EDF2: 10 m, EDF3=11 m, and the output is equipped with a
gain flatter. Two isolators ISOI, IS03 are used to prevent self-excited oscillation due to
fiber end-face reflection. The amplified wideband signal is output through the
ultra-wideband wavelength coupler. Since the L-band itself has good flatness, only the
c-band gain flattener is used to achieve ultra-wideband gain flatness. This makes it
possible to achieve low C and L bands. Loss gain and gain unevenness can also be
improved
The c- and L-band narrowband signals are selected by the F-p waver on the basis
of the broadband source, and the amplifier gain and noise are measured by the
spectrometer.
3. Results and Discussion
Based on the broadband source, the c- and L-band signals selected by the F-p
filter are selected at 2 nm and 1 602 nm apart by 2 nm. The input signal size is 40 dBm,
and the error is between positive and negative 1 dB. Next, the gain curve after
measuring the ultra-wideband EDFA is shown in Figure 2. The black squares represent
the output gains corresponding to the respective wavelengths, where the c-band has

6. a minimum of 1 530 nm at 28 · 75 dB' and a maximum of 1 at 562 nm. 37 dB, gain
fluctuation less than 4 dB · L band minimum 1 602 nm at 20 · 39 dB, up to 1 594 nm
at 23 · 08 dB, 3 dB bandwidth over 30 nm. 1 566, 1 570 nm for gain dead zone, mainly
by c + L-wavelength division multiplexer performance determines that the actual
measured gain is attenuated a lot and the noise is quite large. Generally, this
wavelength interval is not used. Due to the instability of the C- and L-band signals
selected by the F-p filter,
Both the c and L bands have individual wavelength gain output biased averages,
but the overall trend is approximately flat. Considering the use of stable tunable fiber
lasers, better experimental results should be obtained.
Figure 3 shows the ultra-wideband EDFA spontaneous emission spectrum, where
the C-band power is 1 · 6 dB from 1 528 · 2 nm at 37 nm; the L-band is 2 · 3 dB from
25 GHz at 1 567 nm. In the experiment, the pump source was deliberately improved
the ASE spectrum at the wavelength of 1 567 nm to 1 580 nm in the L-band, so that
the L-band gain signal obtained was flatter. Several sets of L and C-band typical signal
output line spectra were measured in the experiment. It can be seen from Fig. 4 that

7. the L-band is pumped by the backward ASE and the L-band signal is amplified twice,
and the gain is significantly enhanced. At a wavelength of 1 590 nm, the signal gain is
22 · 6 dB, and the noise figure is 9 · 5 dB. The measured noise is higher because the
L-band amplifier uses a reflective structure, the L-band signal is amplified twice, and
the L-band ASE is amplified twice, and the experimental equipment (mainly flanged
and movable joints, etc.) has Reasons for non-negligible losses, etc. Figure 5 shows
that at a wavelength of 1 554 nm, the signal gain is 31.2 dB, and the noise is 3 · 66 dB.
It can be seen that since the C-band amplification part adopts a two-stage cascade
structure, The gain is increased while the noise is reduced; by using the Bragg grating
fiber grating filter, the gain flatness is also obtained on the C-band.
The gain fluctuation is less than 4 dB and the gain is not less than 22 dB at a
bandwidth of nearly 70 nm from 1 524 to 1 602 nm.