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Multiwavelength-Switchable SOA-Fiber Ring Laser Based on Polarization-Maintaining Fiber Loop Mirror and Polarization Beam Splitter

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Yong Wook Lee at Pukyong National University
Yong Wook Lee
Jaehoon Jung
Jaehoon Jung
Jaehoon Jung
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Byoungho Lee at Seoul National University
Byoungho Lee
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In this letter, we propose a novel multiwavelength-switchable fiber ring laser based on a semiconductor optical amplifier and reflection-type interleaver composed of a polarization-maintaining fiber (PMF) loop mirror and a polarization beam splitter. In the proposed fiber laser, stable interleaved waveband switching operation at up to 17 oscillating wavelengths with 0.8 nm spacing, whose switching displacement is 0.4 nm, is successfully demonstrated at room temperature by the proper control of the polarization controller within the PMF loop mirror.
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54 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 16, NO. 1, JANUARY 2004
Multiwavelength-Switchable SOA-Fiber Ring Laser
Based on Polarization-Maintaining Fiber Loop Mirror
and Polarization Beam Splitter
Yong Wook Lee, Jaehoon Jung, and Byoungho Lee, Senior Member, IEEE
Abstract—In this letter, we propose a novel multiwave-
length-switchable fiber ring laser based on a semiconductor
optical amplifier and reflection-type interleaver composed of a
polarization-maintaining fiber (PMF) loop mirror and a polariza-
tion beam splitter. In the proposed fiber laser, stable interleaved
waveband switching operation at up to 17 oscillating wavelengths
with 0.8 nm spacing, whose switching displacement is 0.4 nm, is
successfully demonstrated at room temperature by the proper
control of the polarization controller within the PMF loop mirror.
Index Terms—Fiber laser, multiwavelength, polarization beam
splitter (PBS), polarization-maintaining fiber (PMF), semicon-
ductor optical amplifier (SOA), switching.
I. INTRODUCTION
W
ITH the fast development of dense wavelength-divi-
sion-multiplexed (WDM) systems in optical communi-
cations, wavelength-switchable lasers have been considered as
important sources in wavelength-routed WDM network systems
using reconfigurable optical cross-connects (OXCs) to avoid
channel collisions. Several techniques have been reported to re-
alize the oscillating wavelength switching in the fiber laser [1],
[2]. Recently, hierarchical cross-connect systems employing a
waveband concept have been introduced to reduce the com-
plexity of OXCs [3]. Particularly, multiwavelength-switchable
fiber laser introducing a specially defined waveband concept
(designated as interleaved waveband in [4]) has been proposed.
In [4], however, the number of interleaved channels was con-
fined to four in each waveband and there was a tradeoff between
the number of channels and their reflectivities. In this letter, we
propose a novel 17-line multiwavelength-switchable fiber ring
laser that can be used in hierarchical cross-connect systems
utilizing an interleaved waveband concept. The proposed laser
incorporates a semiconductor optical amplifier (SOA) as gain
medium and incorporates a reflection-type interleaved filter
(interleaver) composed of a polarization-maintaining fiber
(PMF) loop mirror and a polarization beam splitter (PBS) as
a wavelength-selective comb filter. Stable interleaved wave-
band (i.e., multiwavelength) switching operation at up to 17
Manuscript received April 23, 2003; revised August 14, 2003.
Y. W. Lee and B. Lee are with the School of Electrical Engineering, Seoul
National University, Seoul 151-744, Korea (e-mail: byoungho@snu.ac.kr).
J. Jung is with the School of Electrical, Electronics, and Computer Engi-
neering, Dankook University, Seoul 140-714, Korea.
Digital Object Identifier 10.1109/LPT.2003.819414
Fig. 1. Schematic diagram of the proposed fiber laser. The dotted box shows
the interleaver.
oscillating wavelengths with 0.8 nm spacing, whose switching
displacement is 0.4 nm, is successfully demonstrated at room
temperature by the proper control of the polarization controller
(PC) within the PMF loop mirror.
II. P
RINCIPLES OF OPERATION
The SOA comprises the active region of the multiwavelength
fiber laser. While erbium-doped fiber, due to the homogeneous
linewidth broadening, cannot generate simultaneous oscillation
modes with spacing of less than 1 nm at room temperature, in-
homogeneous linewidth broadening characteristics of the SOA
make multiwavelength operation with WDM ITU-grid spacing
possible at room temperature. The principal element in the pro-
posed interleaved waveband switching is the interleaver that
consists of the PMF loop mirror and PBS. The schematic di-
agram of the interleaver is shown in the dotted box of Fig. 1.
The PMF loop mirror acts as a polarization-independent wave-
length-selective filter [5], and the PBS is a compact optical ap-
paratus which can split the light into two orthogonal polarization
components or combine them into one output fiber. As can be
seen from the dotted box in Fig. 1, there are eight paths of the
returning light when the light is introduced into the input port
of the interleaver and only the reflected light returning to the
input port should be included in the theoretical reflectivity cal-
culation with the proper consideration on polarization (the eight
paths: 1–3-4–1, 1–3-4–2, 1–4-3–1, 1–4-3–2, 2–3-4–1, 2–3-4–2,
2–4-3–1, and 2–4-3–2). If we assume the PC within the PMF
1041-1135/04$20.00 © 2004 IEEE
LEE et al.: MULTIWAVELENGTH-SWITCHABLE SOA-FIBER RING LASER BASED ON PMF LOOP MIRROR AND PBS 55
loop mirror composed of two quarter-wave-plates (QWPs), the
reflectivity
of the interleaver without considering additional
losses of components is calculated as
(1)
where
and are rotation angles of each QWP of the PC with
respect to the horizontal axis of the PBS which is assumed as
the reference axis,
is the angle between the slow axis of the
PMF and the reference axis,
is the wavelength in free space,
and
and are the birefringence and the length of the PMF,
respectively. Based on (1), the function of the proposed PMF
loop mirror with PC and PBS can be classified as four cate-
gories, depending on the combination of the QWP angles. The
four categories are as follows: 1) a reflection-type comb filter
with large channel isolation (more than 30 dB); 2) a 100% re-
flector; 3) an interleaver; and 4) a reflection-type comb filter pair
which can make channel isolation of each one same but cannot
make each one interleave with the other. Among the four cate-
gories, we use the interleaver function in our laser. Depending
on the set of
and , both amplitude (modulation depth) and
maxima/minima transmission wavelengths of interference pat-
tern in reflection spectrum of the interleaver change. Especially,
two interleaved interference spectra like
phase-shifted spec-
trum pair can be achieved at suitably chosen two sets of
and
. Fig. 2(a) shows the theoretical reflection spectra of the pro-
posed interleaver at two optimal sets of (
; ) for interleaved
waveband switching:
solid line
and dotted line
where and are integers . In the calculation, ideal 50:50 cou-
pler and PBS were assumed and any insertion loss due to optical
components which comprise the interleaver was not considered.
Fig. 2(b) shows the measured reflection spectra of the proposed
interleaver at two optimal sets. Slight difference from the theo-
retical result in amplitude and wavelength location stems from
the slight unevenness in insertion loss of the 50:50 coupler and
PBS and the value of
different from that used in the calcu-
lation, respectively. In the proposed laser, therefore, the inter-
leaver can select interleaved wavelength components and make
them rotate the ring cavity by adjusting the polarization state
of light in the PMF loop mirror. For general combinations ex-
cept optimal sets, the interleaver has spectral characteristics like
those shown in Fig. 2(c) [solid line: category 1), dash-dotted flat
line: category 2), and dashed and dotted two lines: category 4)].
These cases are also confirmed from the experiments.
(a)
(b)
(c)
Fig. 2. (a) Theoretical reflection spectra of the proposed interleaver at optimal
settings. (b) Experimental reflection spectra of the proposed interleaver at
optimal settings. (c) Theoretical reflection spectra of the proposed loop mirror
at some other nonoptimal settings.
III. EXPERIMENTS AND DISCUSSIONS
Fig. 1 shows the schematic diagram of the proposed fiber
laser. The laser consists of a SOA (Alcatel A1921), optical iso-
lators, an optical circulator, a 75:25 coupler, PC 1, and the pro-
posed interleaver composed of a PBS, PMF, PC 2, and 50:50
56 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 16, NO. 1, JANUARY 2004
Fig. 3. Output spectra of both interleaved wavebands in the proposed laser.
coupler. The ring cavity is unidirectional, which is ensured by
two optical isolators and an optical circulator. The SOA com-
prises the active region of the laser. The interleaver is coupled
to the ring cavity by the optical circulator. The selected wave-
length components reflected by the interleaver rotate in the ring
cavity. The birefringence and length of the PMF is
10
and 6.25 m, respectively. The length of PMF is determined so
that the channel spacing in each waveband becomes 0.8 nm
(100 GHz @1550 nm: WDM ITU-grid spacing). The PC 1 is
used to adjust the overall gain spectrum of the cavity and the PC
2 is used to control the polarization state of the rotating light in
the PMF loop mirror. The laser output is 25% outcoupled from
the cavity through a 75:25 coupler.
Fig. 3 shows the output spectra of both interleaved wavebands
composed of 17 laser lines when the SOA is driven with the in-
jection current of 170 mA with the PC 2 set to each optimal con-
dition for interleaved waveband switching. As can be seen from
the figure, 17 laser lines (channels) in each waveband (solid and
dotted curves) oscillate with signal-to-noise ratio over 25 dB,
which is comparable to that achieved by other reported mul-
tiwavelength fiber lasers [5]. Outside these wavebands (from
to nm) unstable oscillation was observed. Each
interleaved waveband is achieved at each optimally chosen set
of
and through the PC 2. That is, interleaved waveband
switching operation can be obtained by the proper adjustment
of the PC 2. The output intensity variation between different
laser lines was measured to be less than 6 dB. The unevenness
in output intensity between different wavelengths could in any
case be overcome by the addition of a tailored broadband filter
inside or outside the laser cavity. The linewidth of each laser
line was measured to be less than
nm, whose measure-
ment was limited by the resolution bandwidth of the optical
spectrum analyzer. Fig. 4 shows repeatedly scanned output spec-
trum (10 times over 30 min) and maximum variation in the rel-
ative peak powers of the laser modes, which is thought to come
from the environmental perturbations, was measured to be less
than 1.5 dB at a fixed position of the PC 1 and 2 in each wave-
band. Particularly, the switching speed of the laser depends on
Fig. 4. Repeated scans of output optical spectrum. The time interval of each
scan was 3 min.
that of the PC because the setup time of the SOA is defined by
the round-trip time (in the order of tens of nanoseconds) of the
ring laser. By employing electrically controllable PCs such as
ones using lithium niobate structure with ns response times [6],
faster switching speed can be achieved. The proposed scheme
may have some modification. If one of the QWP’s (farther from
PMF) in PC 2 is replaced with a half-wave plate, then it is ex-
pected to be possible to finely tune the oscillating wavelength
of the laser to the WDM ITU-grid wavelength by tuning the
channel location of the interleaver, which can be achieved by
rotating the half-wave plate.
IV. C
ONCLUSION
In the proposed fiber laser, stable interleaved waveband
switching operation at up to 17 oscillating wavelengths with
0.8 nm spacing, whose switching displacement is 0.4 nm, is
successfully demonstrated at room temperature. No noticeable
variations in the relative peak powers of the laser modes were
observed and the output amplitude variation between different
laser lines was measured to be less than 6 dB.
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A novel multiwavelength fibre laser is proposed, which is based on a fibre loop mirror formed by a high-birefringence (HiBi) fibre, a polarisation controller, and a directional coupler. By setting the polarisation controller properly, the HiBi fibre loop mirror acts as a polarisation-independent wavelength filter. With this configuration, the wavelength spacing of the laser can be varied accurately by adjusting the length of the HiBi fibre. Experimentally, by cooling the erbium-doped fibre with liquid nitrogen to reduce its homogeneous linewidth, stable emission at 16 wavelengths with a spacing of 0.8 nm and an amplitude variation of <5 dB was achieved
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Broadband reset-free automatic polarisation controller
Article
  • Mar 1991
A novel electro-optic polarisation controller is proposed and demonstrated that allows reset-free endless polarisation transformations between any two varying general polarisation states. The 1.5 mu m controller is realised with standard lithium niobate waveguides, has an optical bandwidth in the order of tens of nanometres, and is electrically tunable over a wide wavelength range. Automatic polarisation control with an entirely analogue electronic feedback loop is demonstrated.
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Tunable dual-wavelength-switching fiber grating laser
Article
  • Apr 1998
We report a tunable dual-wavelength-switching fiber grating laser by exploiting the large homogeneous gain broadening in erbium (Er)-doped fiber to suppress simultaneous lasing. Wavelength switching was accomplished through a novel design which has two overlapping cavities sharing a single-gain medium. The loss corresponding to one of the lasing wavelengths can be modulated via an optical chopper. Output power of about 6 mW and extinction ratio up to 50 dB were observed. The lasing wavelengths were tunable and switching was demonstrated for wavelength separations ranging from 0.35 to 23.5 nm.
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Preciseandrapidwavelength-switching offiberlaserusingsemiconductoropticalamplifier
  • 945-946
  • D Zhao
  • S Li
  • Andk T Chan
D.Zhao,S.Li,andK.T.Chan,“Preciseandrapidwavelength-switching offiberlaserusingsemiconductoropticalamplifier,”Electron.Lett.,vol. 37, no. 15, pp. 945–946, 2001.