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480 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 12, NO. 5, MAY 2000

Coupled Structure for Wide-Band EDFA with Gainand Noise Figure Improvements from C to L-Band

ASE InjectionBumki Min , Student Member, IEEE , Hosung Yoon , Student Member, IEEE , Won Jae Lee , Student Member, IEEE , and

Namkyoo Park  , Member, IEEE 

 Abstract—We propose a novel structure for C plus L-band silicabased wide-band erbium-doped fiber amplifiers (W-EDFA’s),which use backward amplified spontaneous emission from theC-band EDFA as the pump-mediating injection source for theL-band amplifier unit. Experimental results show gain and noisefigure improvements of over 2.6 dB and 0.6 dB, respectively,at

3 5 

dBm of L-band input signal power. Spatially resolvednumerical analysis confirms the pump-mediating effect of C-bandbackward ASE in the L-band EDFA for the gain and noise figure

improvement, which also provides better understanding on thedynamics of C-band injection seed methods.

 Index Terms—Erbium-doped fiber amplifiers (EDFA’s), opticalfiber communication, wavelength division multiplexed (WDM)systems.

I. INTRODUCTION

ULTRAWIDE-BAND erbium-doped fiber amplifiers

(W-EDFA) have now become one of the most promising

future network elements, satisfying the ever-increasing ca-

pacity demand of wavelength division multiplexed (WDM)transmission systems [1]. Among many attempts suggested so

far, the silica-based EDFA’s in parallel configuration (C-band:1530–1560 nm, plus L-band: 1570–1610 nm) [2], [3] have

been considered to be the most immediate viable solution for

real system applications due to the maturity of the supporting

technologies such as the host material and pump sources. Still,

when compared with the C-band EDFA (C-EDFA), there has

been a lack of accumulated research efforts on L-band EDFA

(L-EDFA), except in the last few years. The most serious

research trend on L-EDFA recently has been addressed to effi-

ciency improvement to relax the requirement of a much higher

pump power when compared with C-EDFA. As a possible

approach to improve power conversion efficiency in L-band

EDFA’s [4], [5], we have suggested a structure that recycles

useless backward amplified spontaneous emission (ASE) of L-EDFA as a secondary pumping source in the passive section

of the amplifier [5]. Further extending the previous concept of 

reusing wasted ASE power, we have noticed that the backward

ASE from the C-EDFA can also be reused as the primary-pump

Manuscript recevied October 4, 1999; revised January 29, 2000.The authors are with the Optical Communication Systems Laboratory,

School of Electrical Engineering, Seoul National University, Seoul, Korea(e-mail: nkpark@plaza.snu.ac.kr).

Publisher Item Identifier S 1041-1135(00)03656-9.

Fig. 1. Structure of the C-band plus L-band silica based EDFA suggested inthis study.

to L-band photon mediating injection sources in L-EDFA with

appropriate arrangements.

In this letter, we demonstrate a novel W-EDFA structure for

L-EDFA performance improvements without the need of ad-

ditional injection sources [6]. Experimental results show con-

siderable improvements on gain and noise figure (2.6 dB and

0.6 dB, respectively, at dBm L-EDFA input signal) in ad-

dition to the limiting amplifier behavior, with negligible channel

crosstalk from C to L-EDFA’s. In addition, for the first time

to our knowledge, we provide spatially resolved W-EDFA nu-

merical analysis results, which clearly reveal the roles of the

C-band injection source in the evolution dynamics of the pri-

mary pump, L-EDFA backward ASE, C-band injection sources,

and L-EDFA signals.

II. EXPERIMENTS AND SIMULATIONS

Fig. 1 shows the coupled structure of the C-band plus L-band

silica based EDFA used in this study. As an injection source,the backward ASE from the C-EDFA was added to an L-EDFA

through a circulator and a C/L band combiner. The EDF used

in the experiment was a commercially available, Al-codoped

one with a peak absorption coefficient of 4.5 dB/m at 1530

nm, and the lengths of the EDF’s used in the C-EDFA and an

L-EDFA were 20 m and 300 m, respectively. Pump sources at

the wavelength of 980-nm were used at the power level of 90

mW, 53 mW, and 57 mW, for the forward pump of the C-band

and the forward and backward pumps of the L-band EDFA, re-

spectively. The measured losses from the circulator and C/L

1141–1135/00$10.00 © 2000 IEEE

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MIN et al.: COUPLED STRUCTURE FOR WIDE-BAND EDFA WITH GAIN AND NOISE FIGURE IMPROVEMENTS 481

Fig. 2. Output spectra of the coupled (—: solid line) and uncoupledstructure ( 1 1 1 1 1 1  : dot) amplifiers for 0  3  :  5  dBm saturating input signals at thewavelengths of 1540 nm and 1595 nm ( Resolution bandwidth = 0  :  2  nm).

Fig. 3. L-band EDFA output powers as a function of input power at variousgain levels of the C-band EDFA.

band combiner(splitter) were below 0.6 dB and 0.3 dB, respec-

tively. Two external cavity lasers tuned at 1540 and 1595 nm

were used for the evaluation of the amplifier gain and noise

figure in conjunction with the optical spectrum analyzer.

To compare the performances of the suggested structure

with an uncoupled one, we first measured the output power

at the signal wavelengths of 1540 nm and 1595 nm, with the

C-band backward ASE injection to the L-EDFA port being

turned off (APC connector open in Fig. 1). The measured signal

input/output powers for the W-EDFA were 3.5 dBm/13.01

dBm (@1540 nm) and 3.5 dBm/10.47 dBm (@1595 nm)

respectively, exhibiting a much lower power conversion effi-ciency for the L-band EDFA at comparable pump/input power

conditions. Still, when the seed injection port was set to the

pass-state (APC connector closed in Fig. 1), we were able to

observe a dramatic improvement on the L-EDFA output power

over 2.6 dB, with noticeable changes in the background ASE

spectral profile as well (Fig. 2). The estimated gain bandwidth

of the amplifier from the ASE profile, in this coupled configu-

ration, was larger than 80 nm.

To also investigate the possible performance variation of 

L-EDFA as a function of C-EDFA gain, we measured the

saturated output power at the wavelength of 1595 nm, while

changing the C-EDFA pump power (see Fig. 3). With the

ASE injection port set to the pass state, the L-EDFA exhibited

limiting amplifier behavior, with a considerable improvement

in the gain (2.1–2.6 dB at 3.5 dBm and 0.1–0.3 dB at 6.5 dBm

input power) and noise figure (0.6 dB), almost irrespectively

of the C-EDFA gain values (6.57–17.72 dB). Considering

the much smaller relative L-EDFA gain variation (below 0.5

dB) for much larger gain change of C-EDFA over 11.2 dB

(and correspondingly different levels of injected C-EDFAbackward ASE power), we attribute this behavior to the rapid

amplification of the injected C-band ASE signal in the front

section of the L-EDFA to a saturation level, which is then

transferred to the L-band photons in the rear portions of the

amplifier. The above explanation can also be used to explain

the negligible transient crosstalk effect to the L-EDFA from

C-band input power changes from independent measurement.

For 6–dB variation of C-band input power ( to 2 dBm), the

changes in the L-band output power stayed below 0.2 dB for a

wide range of L-EDFA input power levels ( 3.5 –6.5 dBm).

Keeping in mind previous results on a transient controlled

single C-EDFA [7], this C-EDFA to L-EDFA crosstalk of 0.2

dB would not pose a problem for most system applications.To better understand and confirm the above dynamics/expla-

nations on gain and noise figure improvements, we conducted

numerical analyses for the suggested structure. The amplifier

was modeled as a homogeneously broadened three-level system

including both spatial and spectral variations [8] with a spec-

tral grid of 1 nm and a spectral range of 120 nm between 1500

and 1620 nm. Absorption and emission cross sections, including

other additional parameters, were obtained from the fiber man-

ufacturer’s data sheets. The simulation results agreed well with

the experiments within 1.5 dB for most of the spectral range and

operating points and reproduced all of the general tendency of 

relative performance improvement, as seen in the experiments.

We attribute the discrepancies in the simulation (1.5 dB max

in terms of absolute power) to the rather inaccurate component

loss profiles and data sets in the long wavelength regime.

The evolution spectra of the forward propagating waves in the

L-EDFA part of the coupled structure are shown in Fig. 4(a). For

Fig. 4(a), the small peak at 1540 nm corresponds to the Rayleigh

backscattered signal from the C-EDFA, which was included

in the simulations. With the injection of C-EDFA backward

ASE together with the 980-nm pump from the input, the C-band

forward waves grow at a much faster rate in the input section

when compared with the uncoupled structure [Fig. 4(b)]. These

strong, amplified forward C-band waves then get absorbed over

the following sections of the EDF and transfer their energy tolonger wavelengths acting as the primary-pump to the L-band

photon mediator while at the same time suppressing the pump-

power depleting backward ASE evolution in the L-EDFA.

In contrast, the evolution spectra of the uncoupled W-EDFA

exhibit a dip at about 50 m from the input end of the L-EDFA.

Besides, the power levels of the forward propagating waves are

much weaker in most sections of the amplifier when compared

with a coupled W-EDFA [see Fig. 4(b)]. This can be explained

by the absence of injected C-band seed photons in the uncou-

pled structure, which transfer the forward 980-nm pump pho-

tons to the L-band signals, resulting in the weaker amplifica-

tion of forward waves. The backward ASE then develops at a

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482 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 12, NO. 5, MAY 2000

(a)

(b)

Fig. 4. Evolution of the forward propagating waves in the L-EDFA sections

(a) for the coupled structure (Resolution bandwidth = 1  nm) and (b) for theuncoupled structure (Resolution bandwidth = 1  nm).

much faster rate at the input section of the amplifier, depleting

the pump powers and further reducing the L-band signal gain.

Additional features for better understanding the dynamics can

also be found from the wavelength ranges of the forward waves

in the L-EDFA. After a certain point in the L-EDFA, the C-band

waves diminish, but the L-band waves begin to grow thereafter.

Considering the fact that the absorption length for a 980-nm

pump does not extend beyond several tens of meters, this fact

implies that the seeded photons in the coupled structure act as

a reservoir of primary pump photons for L-band signal amplifi-

cation and as an intermediate, pseudo pump level between the

primary pump and the L-band.

III. DISCUSSION AND CONCLUSION

In summary, we have demonstrated a novel structure for

ultra wide-band amplifiers with enhanced performances fromC-band EDFA backward ASE injection to L-band EDFA, elim-

inating the need of additional injection sources. Experimental

results showed a considerable gain improvement of over 2.6

dB on an L-band EDFA in addition to 0.6–dB noise figure

reduction without a noticeable amount of channel crosstalk 

from the C-band EDFA to the L-band EDFA. Spatially resolved

wide-band EDFA simulations were conducted, for the first time

to our knowledge, to confirm the experimental observations

and to provide better understanding on the dynamics of the

L-band EDFA.

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