Tunable and switchable multiwavelength
erbium-doped fiber ring laser based on a modified
dual-pass Mach–Zehnder interferometer
Ai-Ping Luo,1,* Zhi-Chao Luo,1and Wen-Cheng Xu1,2
1Laboratory of Photonic Information Technology, School of Information and Optoelectronic Science and Engineering,
South China Normal University, Guangzhou, Guangdong 510006, China
* Corresponding author: firstname.lastname@example.org
Received April 8, 2009; revised May 26, 2009; accepted May 28, 2009;
posted June 9, 2009 (Doc. ID 109816); published July 8, 2009
A tunable and switchable multiwavelength erbium-doped fiber ring laser based on what we believe to be a
new type of tunable comb filter is proposed and demonstrated. By adjusting the polarization controllers, the
dual-function operation of the channel-spacing tunability and the wavelength switching (interleaving) can
be readily achieved. Up to 29 stable lasing lines with 0.4 nm spacing and 14 lasing wavelengths with 0.8 nm
spacing in 3 dB bandwidth were obtained at room temperature. In addition, the lasing output, including the
number of the lasing lines, the lasing evenness, and the lasing locations, can also be flexibly adjusted
through the wavelength-dependent polarization rotation mechanism. © 2009 Optical Society of America
OCIS codes: 060.2410, 140.3510, 060.2320.
Multiwavelength fiber lasers have attracted consid-
erable attention for their potential applications in the
fields of wavelength-division-multiplexing (WDM)
transmission system, optical fiber sensing, and fiber
devices testing. They can be achieved by combining
various kinds of optical filters. In some practical ap-
plications, it is also desirable to be able to enhance
the functionality and the flexibility of a multiwave-
length fiber laser. Therefore, the tunability of the
wavelength spacing, the number of channels, and the
lasing locations should be investigated. To date, vari-
ous techniques have been used to realize channel-
spacing tunable operation of multiwavelength lasing,
such as using cascaded multisection polarization
maintain fiber , incorporating high birefringence
fiber in the fiber loop mirror , and employing mul-
tiple fiber Bragg gratings in the cavity .
Generally, multiwavelength fiber laser based on a
standard Mach–Zehnder (M–Z) interferometer is
nontunable  except when incorporating an optical
variable delay line (OVDL) to change the path differ-
ence between two interferometer arms . Neverthe-
less, the accurate tuning of the OVDL must occur
through a controlling computer. In this Letter, we
propose and demonstrate a tunable and switchable
multiwavelength erbium-doped fiber (EDF) ring laser
by exploiting a new type of comb filter based on a
modified dual-pass M–Z interferometer. By adjusting
the polarization controllers (PCs), multiwavelength
lasing with the channel-spacing tunability between
0.4 and 0.8 nm as well as the wavelength switching
(interleaving) can be easily achieved. Up to 29 stable
lasing lines with 0.4 nm spacing and 14 lasing wave-
lengths with 0.8 nm spacing in 3 dB bandwidth were
obtained at room temperature. Furthermore, the las-
ing output, including the number of the lasing lines,
the lasing evenness, and the lasing locations, can
also be flexibly adjusted.
Figure 1 shows the schematic of the proposed mul-
tiwavelength EDF laser. A 4.5-m-long EDF serves as
the gain medium. A 980 nm/1550 nm WDM is used to
launch the 980 nm pumping laser into the laser cav-
ity. Two PCs are employed to adjust the polarization
statesof the circulating
dependent isolator (PD-ISO) assures the unidirec-
dependent polarization rotation mechanism [6,7]. A
phase modulator composed of a 4-m-long single-mode
fiber (SMF) wrapped around a cylindrical piezoelec-
tric transducer (PZT), which has a resonant fre-
quency of 23 kHz, is used to suppress the mode com-
petition. The tunable comb filter, which is developed
from a standard dual-pass M–Z interferometer, con-
sists of a 50:50 and a 30:70 fiber couplers with a PC3
in one arm, as shown in Fig. 1. The laser output is
measured by an optical spectrum analyzer (OSA).
First, the transmission characteristics of the indi-
vidual modified dual-pass M–Z filter are analyzed. As
light. A polarization-
Schematic of the proposed multiwavelength fiber
July 15, 2009 / Vol. 34, No. 14 / OPTICS LETTERS
0146-9592/09/142135-3/$15.00© 2009 Optical Society of America
the actual fiber is not ideal, the birefringence varies
randomly along the fiber. Consequently, for effec-
tively realizing the tunable function, we employ a
PC3 in one arm of the interferometer to control the
influence of the birefringence on the channel spacing.
The initial polarization state of the input light is
forced into aligning with the polarizer provided by
the PD-ISO. Suppose that the linearly polarized light
at an angle ? with respect to one of the principal axes
of the fiber is launched into the filter, where ? can be
adjusted by the PC1. The filter characteristics can be
analyzed from the following Jones matrix representa-
In Eq. (1), ?E1in? and ?E2in? are the input fields of
ports 1 and 2, respectively; ?E1in?=?A cos ?;A sin ??
and ?E2in?=?0;0?, where A is the amplitude of the
light. ?I? is the identity matrix. ?Cm? ?m=1,2?, ?P?,
and ?Fn? ?n=1,2? represent the matrices of the fiber
couplers, the PC3, and the two interferometer arms,
?Cm? =??1 − am?I?
where amis the coupling ratio; ? is the rotation angle
of the propagating light through the PC3; L is the
length of the shorter arm; and ? is the phase differ-
ence between the two arms, which is determined by
the path difference ?L (?L=2 mm in this experi-
ment). Then, we can obtain the transmission function
in port 2 as follows:
+ 2a2?1 − a2???cos2? − sin2? cos ??cos 2?
+ sin ? sin?2? + ??sin ? sin 2??
− 2?1 – 2a2??a2?1 − a2?sin2?
?cos?2? + ??sin ? sin ?,
?1 − am?I??,
cos ? − sin ?
= 2a2?1 − a2? + ?1 – 2a2?2sin2? sin2?
where ?=2k?nx−ny?L. From Eq. (2), we know that the
transmission depends not only on the phase differ-
ence ??,2?? but also on the polarization angle ? of
the input light and the rotation angle ?. When a2
=0.5, it is only a dual-pass M–Z interferometer.
While a2?0.5 (for this experiment, a2=0.3), the pro-
posed comb filter shows the dual-function, the
channel-spacing tunable, and the interleaving opera-
tions by appropriately rotating the PC1 ??? and the
PC3 ???. For example, when ?=0.65?, the filter can
act as single-pass ??=0.175?? and dual-pass ??
=0.425?? M–Z interferometers. Furthermore, the
wavelength switching (interleaving) operation can be
achieved (i.e., for single-pass operation, ?=0.175?,
0.675?). Figure 2 shows the measured tunable trans-
mission spectra of the filter by rotating the PC1 when
the PC3 was fixed in a proper orientation. As illus-
trated in Fig. 2, the dual-function, the comb spacing
tunable, and the wavelength switching (interleaving)
operations can be easily achieved. Therefore, it can
be employed in multiwavelength fiber lasers to real-
ize tunable and switchable lasing operation.
In the experiment, the pump power was fixed at 70
mW. When the PZT was driven by a sinusoidal signal
waveform with a frequency of 4.2 kHz, the laser is op-
erated in stable multiwavelength lasing simulta-
neously. The PC3 was first rotated in a proper posi-
tion where one could realize the tunable operation
and fix it in the following experiment. Actually, the
laser cavity incorporating a polarizer produces the
wavelength-dependent polarization rotation mecha-
nism, then the polarization states of different wave-
lengths are diversified, and each wavelength could
have different losses. This contributes to optimizing a
stable multiwavelength output. Therefore, if the PCs
were not properly set, the lasing lines were few and
uneven. In the experimental observation, we found
that the channel-spacing tunability is more sensitive
to the PC1 setting, while the number of the lasing
lines and the lasing locations are more dependent on
the PC2. It is because the polarization state launched
into the filter was mainly controlled by the PC1 and
this determines the tunability of the filter according
to the theoretical analysis, while the PC2 is more
sensitive to adjusting the polarization states of the
lasing light launched into the PD-ISO, which facili-
tates the control of the wavelength-dependent polar-
ization rotation effect. Figure 3 shows the typical out-
put spectra when the PC2 was not adjusted properly
but only rotated the PC1. The channel-spacing tun-
ability between 0.4 and 0.8 nm and the wavelength
switching operation could be obtained, which were
well consistent with the filter spectral spacing shown
able transmission spectra of the comb filter.
(Color online) Measured wavelength spacing tun-
OPTICS LETTERS / Vol. 34, No. 14 / July 15, 2009
in Fig. 2. It is worthy to note that this spacing tun- Download full-text
able process is reversible. However, in this case the
lasing lines were few and uneven.
When the PCs were further properly adjusted, the
number of the lasing lines, the lasing flatness, and
the lasing locations could be improved. As the PCs
were in optimum positions, up to 29 stable lasing
lines with 0.4 nm spacing and 14 lasing wavelengths
with 0.8 nm spacing were achieved. Figure 4 presents
the output spectra of a maximum number of 29 las-
ing wavelengths with 0.4 nm spacing in 3 dB band-
width. Then we rotated the PC1 placed before the
comb filter slightly. Finally, the stable 14 lasing
wavelengths with a channel spacing of 0.8 nm in 3 dB
bandwidth were obtained as shown in Fig. 5. In these
cases, the lasing wavelength switching operation was
also observed. To verify the stability of the proposed
multiwavelength fiber laser, we repeatedly scanned
the laser output in 30 min. Figure 6 shows the power
variation of four individual channels and the wave-
length drift in the experimental observation. The
maximum power fluctuation is less than 1.0 dB, and
the maximum wavelength drift is 0.05 nm. These re-
sults indicate that the proposed fiber laser operated
stably at room temperature.
In conclusion, we have proposed and demonstrated
a tunable and switchable multiwavelength EDF ring
laser based on a new type of comb filter. The comb fil-
ter provides the wavelength spacing tunability be-
tween 0.4 and 0.8 nm as well as the lasing wave-
length switching operation. Correspondingly, up to 29
stable lasing lines with 0.4 nm spacing and 14 lasing
lines with 0.8 nm spacing were obtained. Moreover,
the lasing output, including the number of the lasing
lines, the lasing evenness, and the lasing locations,
can also be flexibly adjusted.
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lines operation with a spacing of 0.4 nm.
(Color online) Stable output under the 29 lasing
channel spacing of 0.8 nm.
(Color online) Stable 14 lasing wavelengths with a
channels and the wavelength drift in 30 min.
(Color online) Power variation of four individual
Fig. 3. (Color online) Typical output spectra when the PC2
was not in a proper setting.
July 15, 2009 / Vol. 34, No. 14 / OPTICS LETTERS