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Multiwavelength erbium-doped ber laser employing a nonlinear optical loop mirror
Multiwavelength erbium-doped ber laser employing a
nonlinear optical loop mirror
Xinhuan Feng
a,*
, Hwa-yaw Tam
a
, Heliang Liu
a
, P.K.A. Wai
b
a
Photonics Research Centre, Department of Electrical Engineering, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong SAR, China
b
Photonics Research Centre, Department of Electronics and Information Engineering, The Hong Kong Polytechnic University, Hung Hom,
Kowloon, Hong Kong SAR, China
Received 8 May 2006; received in revised form 5 July 2006; accepted 6 July 2006
Abstract
A stable and broad bandwidth multiwavelength erbium-doped ber laser is proposed and demonstrated successfully. A nonlinear
optical loop mirror which induces wavelength-dependent cavity loss and behaves as an amplitude equalizer is employed to ensure stable
room-temperature multiwavelength operation. Up to 50 wavelengths lasing oscillations with wavelength spacing of 0.8 nm within a 3-dB
spectral range of 15621605 nm has been achieved. The measured power uctuation of each wavelength is about 0.1 dB within a 2-h
period.
2006 Elsevier B.V. All rights reserved.
Keywords: Erbium-doped ber laser; Nonlinear optical loop mirror; Intensity-dependent loss
1. Introduction
Multiwavelength erbium-doped ber lasers (EDFLs)
attract a lot of interest due to their potential applications
in dense wavelength-division-multiplexed (DWDM) ber
communication systems. Important features of multiwave-
length sources for DWDM applications include large chan-
nel-count, uniform output power, small power uctuation,
and precise and stable wavelength spacing that complies
with the ITU-wavelength grid. Multiwavelength sources
also nd applications in ber sensors and optical instru-
mentations. Since erbium-doped ber (EDF) is a gain med-
ium with homogeneous broadening, the existence of gain
saturation causes mode competition between dierent
wavelengths, and it is dicult to achieve stable multi-wave-
length oscillations in EDFLs at room temperature. In
order to reduce the cross-gain saturation and suppress
the mode competition, dierent techniques have been pro-
posed to realize multiwavelength oscillations at room tem-
perature in EDFLs. These include the introduction of
polarization hole burning (PHB) eect
[15]
, and various
nonlinear eects such as four-wave mixing in photonic
crystal ber
[69]
and stimulated Brillouin scattering
[10,11]
, in the laser cavity. Other methods by inserting fre-
quency shifter in the laser cavity
[1215]
, incorporating a
section of multimode ber
[16,17]
or a multimode
FBG
[18]
in a laser cavity, and employing specially designed
erbium-doped bers
[19,20]
or cavity structures
[21,22]
were also reported.
Nonlinear optical loop mirrors (NOLM) have been used
as fast saturable absorbers to passively mode-lock laser
oscillators and to reshape optical pulses
[23,24]
. In this let-
ter, a NOLM is used as an amplitude equalizer to induce
intensity-dependent loss (IDL) and alleviates the mode
competition caused by homogeneous gain broadening in
EDF. This technique is quite eectively and a multiwave-
length laser source with stable and uniform output of up
0030-4018/$ - see front matter
2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.optcom.2006.07.010
*
Corresponding author. Tel.: +852 2766 4094; fax: +852 2330 1544.
E-mail addresses:
eexhfeng@polyu.edu.hk
,
xh_feng1974@hotmail.com
(X. Feng).
www.elsevier.com/locate/optcom
Optics Communications 268 (2006) 278281 to 50 wavelengths with a spacing of 0.8 nm anchored on the
ITU wavelength grid has been achieved.
2. Experimental setup and principle
The conguration of the proposed laser is shown in
Fig. 1
. It is composed of a NOLM and an amplifying uni-
directional loop. The amplifying unidirectional loop con-
sisted of a commercial high power erbium-doped ber
amplier (EDFA), a FabryPe�rot (FP) thin-lm lter, a
polarization controller (PC) and an output coupler with a
splitting ratio of 1:9. The EDFA (Amonics AEDFA-27-
B) provides the optical gain and has a saturation output
power of 500 mW. The FP lter has a free spectral range
of 0.8 nm, a nesse of better than six, and an insertion loss
less than 2 dB. The NOLM is constructed by splicing
together the outputs of a 7:3 splitting ratio fused ber cou-
pler. A PC and a 2.1-km long conventional single-mode
ber (SMF) were inserted inside the loop. PCs were used
at the input of the NOLM as well as inside the NOLM
for polarization biasing of the loop. The laser output was
taken via the 10% output port of the fused ber coupler
and was measured using an optical spectrum analyzer with
0.05 nm resolution.
The mechanism of the laser under stable multiwave-
length operation at room temperature can be easily under-
stood from the transmission characteristic of the NOLM.
The transmission of an NOLM is given in
[24]
as
T
� 1
2a
�1
a
�f1 � cos�h � �1
2a
�/ g
�1�
where a is the splitting ratio of the NOLM, h is the addi-
tional phase dierence induced by the PCs, / (=2pn
2
P
i
L/
kA
e
) is the nonlinear phase shift, n
2
is the nonlinear refrac-
tive index coecient, L is the loop length, k is the operating
wavelength, A
e
is the eective ber core area, and P
i
is the
input power.
Using Eq.
(1)
, we can obtain the transmission character-
istics of NOLM as a function of input power for dierent h,
that is, for dierent setting of PCs. Typical values of n
2
=
3.2
� 10
20
m
2
/W, A
e
= 50 lm
2
, and a = 0.3, L = 75 km,
k
= 1550 nm were used in our calculation.
Fig. 2
shows
the transmission characteristic of the NOLM as a function
of input power for h = 0.5p, 0.667p, p, 1.333p, 1.5p. As can
be seen from the gure, the transmission can be increased
or decreased with input power depending on the polariza-
tion setting of the PCs. When the PCs are set at a point
where the transmittivity of the NOLM increases with the
input signal power, the NOLM functions as a saturable
absorber, which can lead to passive mode-locking and gen-
erates pulses
[23]
. The NOLM can also be biased to a point
where it functions as an amplitude equalizer, whereby a
high intensity beam would experience larger loss than that
of a lower intensity one. The equalizing mechanism can
eectively alleviate the mode competition in the EDF. As
a result, the balance between the gain-clamping function
of the NOLM and the mode competition eect of the
EDF can lead to multiwavelength oscillations at room tem-
perature, and also ensure uniform power distribution
among wavelengths.
3. Experimental results
Using the mechanism described above and to conrm
the existence of multiwavelength operation of the laser,
we conducted experiments with the ring laser cavity cong-
uration shown in
Fig. 1
. The laser system can be easily set
to the proper biasing point by monitoring its output spec-
tra as the PCs were adjusted.
Fig. 3
(a) shows the output
spectrum of the laser emitting large number of laser lines
spaced at 0.8 nm. Power distribution over wavelengths is
very uniform and the amplitude dierence among the 41
wavelengths is less than 1.6 dB. 50-wavelength operation
was also obtained by adjusting the PCs, and the output
spectrum is shown in
Fig. 3
(b). The amplitude dierence
among the 50 oscillation wavelengths is less than 3 dB
within a spectral range of 15621605 nm. Both outputs
have a wavelength spacing of 0.8 nm, which is determined
by the FP thin-lm lter.
In addition to uniform power distribution over wave-
lengths within a wide spectral range, power stability is also
an important parameter. The output power uctuation of a
single laser line was measured by ltering out one channel
PC1
90:10
Coupler
Output
EDFA
PC2
F-P filter
70:30
Coupler
SMF
30%
Fig. 1. Schematic diagram of the proposed laser.
Fig. 2. Transmission characteristic of an NOLM as a function of input
power for dierent h.
X. Feng et al. / Optics Communications 268 (2006) 278281
279 with a bandpass lter. The measured power variation over
a 2 h period is shown in
Fig. 4
. The signal power uctua-
tion was measured to be about 0.1 dB over a 2 h period
using an optical power meter without averaging.
It can be observed from
Fig. 2
that the change in trans-
mission with input power depends on the polarization set-
tings. When the transmission rate is low (the loss is high),
fewer wavelengths will have sucient gain to compensate
the cavity loss at the corresponding wavelengths. Thus,
smaller number of wavelength will oscillate in the cavity.
In our experiments, output spectra of the laser with num-
ber of wavelengths varying from 3 to 50 were obtained
by adjusting the PCs.
Fig. 5
(a) and (b) give two examples,
respectively showing 8-wavelength and 25-wavelength
within the 3 dB spectral range. Notes that in the 8-wave-
length oscillation case, the peak powers are with values
of about
2 dBm