Linewidth characteristics of a filterless tunable erbium doped fiber ring laser
V. Deepa and R. Vijaya
Citation: J. Appl. Phys. 102, 083107 (2007); doi: 10.1063/1.2798579
View online: http://dx.doi.org/10.1063/1.2798579
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Published by the American Institute of Physics.
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JOURNAL OF APPLIED PHYSICS 102, 083107 共2007兲
Linewidth characteristics of a filterless tunable erbium doped fiber
ring laser
V. Deepa
Department of Physics, Indian Institute of Technology, Bombay, Powai, Mumbai 400076, India
and VES College of Arts, Science and Commerce, Sindhi Society, Chembur, Mumbai 400071, India
R. Vijayaa兲
Department of Physics, Indian Institute of Technology, Bombay, Powai, Mumbai 400076, India
共Received 19 July 2007; accepted 22 August 2007; published online 19 October 2007兲
The linewidth characteristics of a continuous wave erbium doped fiber ring laser whose tunability
is controlled by intracavity loss are studied in this work. The spectral linewidth of such a filterless
laser is measured experimentally, and its characteristics are analyzed using the existing analytical
model of fiber laser. The long length of the cavity results in lasing into multiple longitudinal modes,
thus leading to a broad linewidth. The extent of this broadening is found to depend on the intracavity
attenuation and pump powers. Under the given experimental conditions, for a fiber of length 12 m,
the linewidth increases up to 4.5 nm corresponding to an additional cavity loss of 5 dB, while it
increases up to 8 nm for a loss of 1.2 dB for a fiber of length 4.6 m. The linewidth decreases with
further increase in intracavity loss for both cases. In this work, the linewidth dependence on the
intracavity loss is directly linked to the spectral dependence of the threshold power for each length.
The linewidth increases with pump power irrespective of the intracavity attenuation, and this is
explained through the nature of the amplified spontaneous emission from the fiber at different pump
powers. The experimental results and the subsequent analysis are useful for the design of a filterless
tunable laser with a narrow linewidth using erbium doped fiber in the ring configuration. © 2007
American Institute of Physics. 关DOI: 10.1063/1.2798579兴
I. INTRODUCTION
Erbium doped fibers 共EDFs兲 are strong candidates for
the construction of tunable lasers in the C and L bands, thus
leading to several applications in communication, spectroscopy, and sensors. There are a variety of fiber laser designs
possible with EDF for different modes of operation such as
continuous wave 共CW兲 lasers, multiwavelength lasers, and Q
switched and mode locked lasers.1,2 Erbium doped fiber lasers offer several advantages over semiconductor lasers in
communication systems due to their inherent compatibility
with optical networks, and hence low insertion loss, high
conversion efficiency, ease of construction, and comparatively low cost. Ring cavity is the most common design for
fiber lasers due to its simplicity. The primary advantage of
this cavity is its ability to integrate a wide variety of fiber
optic components which control the mode of operation of the
laser. Tunable erbium doped fiber lasers are usually designed
with the inclusion of wavelength selective elements, such as
dielectric filters, tunable fiber Bragg gratings, and Fabry
Perot filters, in the cavity and are operated in deep saturation
to enhance the tunability.3 The effect of the length of doped
fiber, reflectivity of the cavity mirror, and the attenuation in
the cavity on tunability, as well as the techniques to enhance
tunability, have been studied by several authors in the
past.4–7
It is also possible to achieve wavelength tuning without
the use of intracavity filters. In such lasers, it is typical to
a兲
Author to whom correspondence should be addressed. Electronic mail:
rvijaya@phy.iitb.ac.in
0021-8979/2007/102共8兲/083107/6/$23.00
observe lasing at those wavelengths corresponding to the two
main peaks of the erbium gain profile, centering at 1530 and
1560 nm. Tunable action with such a laser was demonstrated
by changing the wavelength independent reflectivity of the
output coupling mirror.8,9 The underlying principle of designing a filterless fiber laser at a particular wavelength is to
choose the fiber length and the pump power such that the
round trip gain at that wavelength balances the round trip
loss at that wavelength. Hence, for a given configuration, it is
possible to obtain wavelength tuning with the change in intracavity loss. Such a filterless CW tunable laser operating in
the L band, with bidirectional pumping of the EDF, was demonstrated in the wavelength range from 1587 to 1606 nm.10
Lin and co-workers recently demonstrated a coupling ratio
controlled wavelength tunable erbium doped fiber ring laser
共EDFRL兲 using both 980 and 1480 nm pumpings and
achieved continuous tuning from 1567 to 1612 nm.11,12
The linewidth of the output of a tunable laser is usually
decided by the spectral characteristics of the filter used.
Hence, for practical applications, the design of the filter is
improved to improve the linewidth of the laser.13 The reduction in linewidth due to the inclusion of an air gap in the
cavity is reported for a filterless laser with coupling ratio
controlled wavelength tunability.11 However, in the case of
filterless lasers, the lasing wavelength depends dynamically
on the cavity parameters, and the linewidth characteristics of
such lasers are not discussed in any of the previous works to
the best of our knowledge. Since filterless lasers are competent enough for most of the applications requiring tunable
lasers, it is important to study the linewidth characteristics of
102, 083107-1
© 2007 American Institute of Physics
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083107-2
V. Deepa and R. Vijaya
these lasers throughout the tuning range. Larger output
power requires larger pump power, and hence the study of
the dependence of linewidth on the pump power is also crucial.
In this paper, we study the influence of intracavity loss
on the linewidth characteristics of a filterless fiber laser, operating in the CW regime. The linewidth measured at the
output of such a laser is decided by the characteristics of the
fiber used, namely, reflectivity, intracavity loss, length of the
cavity, wavelength response of the intracavity components,
and the wavelength resolution of the measurement system.
Since the filterless design considered is based on the variation of intracavity loss, the linewidth variation with intracavity attenuation for typically different lengths of the fiber is
studied, and its dependence on threshold characteristics is
brought out. For a given intracavity loss, the linewidth is also
found to be strongly dependent on the pump power, beyond
the threshold. This dependence is studied experimentally and
the features are analyzed using the amplified spontaneous
emission from the doped fiber.
II. EXPERIMENTAL DETAILS
The fiber laser considered has a short length of EDF,
with an estimated dopant concentration of 3.535
⫻ 1025 ions/ m3 in a typical ring cavity configuration, with an
isolator in the cavity to ensure unidirectional traveling wave
operation.2 The pump wavelength is chosen to be 980 nm,
since this wavelength is reported to have the maximum influence in the fluorescence spectrum of a heavily doped fiber,
and the corresponding laser transition is free from excited
state absorption.14,15 The maximum pump power used is
300 mW, and all the components are fusion spliced so as to
minimize the other losses in the cavity. A variable optical
attenuator 共VOA兲 inserted in the cavity introduces an additional loss ␣. The output is analyzed using an optical spectrum analyzer 共OSA兲 with a resolution of 0.1 nm.
With the increase in length 共L兲 of the fiber and the reflectivity 共R兲 of the coupler, the lasing wavelength 共␭las兲 is
known to shift to longer wavelengths. On the other hand,
lasing occurs at shorter wavelengths when the cavity loss is
increased.1,2,16,17 Since ␭las is the one for which the cavity
gain matches the total loss, a tunable laser can be designed
by changing R or ␣. Choosing a longer L would increase the
tunability, assuming that sufficient pump power is available
for inversion. Maximum R into the cavity will push ␭las to
the largest possible value to start with and hence result in an
enhanced tunability.17 When ␣ increases, though the power
fed back to the doped fiber in a single round trip decreases,
the output power at ␭las is not expected to be significantly
different throughout the tunable range due to multiple passes
of the field within the cavity and subsequent saturation at
that wavelength. Obtaining tunability by tailoring R or by
changing ␣ are conceptually the same. However, a cavity
loss controlled tunable laser is a preferred filterless design
due to the uniform output power at different wavelengths.
Depending on L and R, the tunability is restricted to certain
wavelength ranges, and is decided primarily by the spectroscopic nature of the fiber used.18,19 It is also demonstrated
J. Appl. Phys. 102, 083107 共2007兲
FIG. 1. Output spectrum from EDFRL at different values of ␣ with EDF
length= 12 m.
experimentally and substantiated with analytical justifications that the range of tunability can be enhanced with the
increase in pump power.20
III. RESULTS AND DISCUSSION
A. Linewidth dependence on intracavity loss
The ring laser is constructed using two lengths of the
EDF, namely, 12 and 4.6 m. The fiber laser is operated with
a large pump power 共220 mW兲 in order to enhance the tunability. A tap coupler is used to reflect 99% of the power into
the cavity. The intracavity attenuation is altered in appropriate steps, and with the increase in attenuation, the lasing
wavelength is found to shift to smaller values as expected.
The output observed on the spectrum analyzer at typical values of ␣ for the two lengths is shown in Figs. 1 and 2.
FIG. 2. Output spectrum from the EDFRL at different values of ␣ with EDF
length= 4.6 m.
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083107-3
V. Deepa and R. Vijaya
J. Appl. Phys. 102, 083107 共2007兲
FIG. 3. Linewidth measured at
−20 dB below the power at ␭las for
EDF lengths of 共a兲 12 m and 共b兲
4.6 m.
Figure 1 shows that for EDF length of 12 m the maximum tunability extends from 1604 to 1559.5 nm, except for
a range of wavelengths 共1577– 1587 nm兲 which is not tunable for the available pump power. Simultaneous lasing at
1577 and 1587 nm is observed corresponding to ␣ = 5.7 dB,
as shown in Fig. 1共d兲. With minimal loss in the cavity, the
lasing wavelength for L = 4.6 m is found to be 1591.3 nm, as
shown in Fig. 2共a兲. When ␣ is varied from 0 to 22 dB, the
laser is found to be continuously tunable in the wavelength
range between 1591.3 and 1557.3 nm for this length. For ␣
= 22.3 dB, simultaneous lasing is observed at 1557.3 and
1532 nm, while the wavelengths between them are not in the
tunable range. The length and power dependence of the tunable range and the forbidden ranges in tuning are discussed
elsewhere.20
It is observed in Figs. 1 and 2 that the linewidth characteristics change significantly with the change in ␣ and this
dependence is not identical for both lengths. The linewidth
measured at −20 dB below the power at ␭las for different
values of ␣ for both fiber lengths is shown in Fig. 3. The
pump power is maintained constant at 220 mW for all these
measurements.
In the case of 12 m fiber 关Fig. 3共a兲兴, as the cavity loss
increases, the linewidth is found to initially increase up to
4.5 nm corresponding to ␣ = 5 dB. With further increase in ␣,
the linewidth is found to decrease, and beyond 10 dB attenuation, the linewidth is not found to significantly change with
attenuation. A similar behavior is observed in the case of
4.6 m fiber 关Fig. 3共b兲兴. However, in this case, the linewidth
increases up to 8 nm for ␣ = 1.2 dB, beyond which, it is
found to decrease. The linewidth does not change significantly beyond 5 dB attenuation for 4.6 m EDF.
The dependence of linewidth on attenuation can be understood qualitatively by estimating the spectral dependence
of threshold power at different wavelengths for different values of ␣. The wavelength dependence of threshold power for
the EDF used in our experiments is calculated for similar
experimental conditions using the analytical model for fiber
lasers.21 The emission and absorption cross sections provided
by the manufacturer of the EDF are used for the calculations.
The threshold power calculated for different wavelengths at
different values of ␣ 共referred to as threshold curves henceforth兲 for both lengths of the fiber are shown in Fig. 4.
Considering that the lasing wavelength is the one for
which the threshold is the minimum, the linewidth characteristics of the output can be explained using Fig. 4. The total
cavity length being large 共⬃20 m兲, multiple longitudinal
modes are supported by the cavity. The linewidth at the output is due to the contribution from these modes. The number
of modes is restricted by the homogeneous broadening associated with the gain spectrum in the active medium.1,2,22
Since the output spectrum is not strictly Lorentzian at minimal ␣, it is evident that the medium is inhomogeneously
broadened at these pump powers. The extent of inhomogenity can be interpreted qualitatively using the threshold characteristics. The flatness of the threshold curves is a direct
indication of broader linewidth at the output. The larger the
change in the slope of the threshold curve at the wavelengths
closer to ␭las, the lesser is the linewidth. Figure 4共a兲 shows
the threshold curves for a 12 m long fiber. For ␣ = 0 dB, the
threshold power is minimal around 1604 nm, resulting in
lasing at that wavelength, as seen in Fig. 1共a兲. As ␣ increases
from zero, the threshold curves in the 1600 nm range become flatter, resulting in an increase in linewidth with ␣, as
FIG. 4. Wavelength dependence of the
threshold power calculated for EDF
lengths of 共a兲 12 m and 共b兲 4.6 m. The
arrow indicates increasing values of ␣.
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083107-4
V. Deepa and R. Vijaya
observed experimentally 关Fig. 3共a兲兴. Lasing occurs at two
simultaneous wavelengths for those ␣ when a double minimum appears in the threshold curve. With further increase in
␣, the lasing wavelength abruptly shifts to the 1560 nm region. Linewidth does not increase any further since the slope
of the threshold curves close to this lasing wavelength
changes significantly. The forbidden wavelengths observed
experimentally are evident in Fig. 4共a兲 in the wavelength
range of 1575– 1585 nm due to the larger threshold in that
wavelength range.
Figure 4共b兲 shows the threshold curves for an EDF
length of 4.6 m, and it is evident that the overall threshold
power decreases for the shorter length of the fiber as
expected.1 The minima of the threshold curve is found to be
flatter for ␣ = 2 dB, in comparison to the case of ␣ = 0 dB.
This accounts for the increase in linewidth when ␣ is increased from 0 to 1.2 dB, as seen in Fig. 3共b兲. For larger
values of ␣, the flatness of the threshold curve decreases,
resulting in narrower linewidths. At very large values of attenuation 共␣ ⬎ 20 dB兲, it may be noticed that the threshold
power is the same for two wavelengths, such as 1530 and
1560 nm, leading to simultaneous lasing at both wavelengths. However, larger thresholds on either side of these
two wavelengths ensure that the linewidth is not large, but
close to the ideal Lorentzian. The linewidth becomes insensitive to attenuation beyond a certain value due to larger
thresholds at the wavelengths close to the lasing wavelengths.
A very precise quantitative correspondence between the
results from the analytical model and the experimental data
is not possible since the model used does not account for the
wavelength dependence of attenuator and the other components used in the cavity. For instance, even though
1591.3 nm is a forbidden region for ␭las according to Fig.
4共b兲, lasing is observed at that wavelength for 4.6 m 关Fig.
2共a兲兴. This is because the spectral contributions due to other
passive components in the cavity is prominent for the shorter
length of EDF. Similarly, the discontinuity in the region
around 1605– 1610 nm in Fig. 4共a兲 is entirely due to the
emission and absorption characteristics of the fiber in that
region. It is also seen that though the threshold power is
slightly different for different values of ␣, the output power
at ␭las is not significantly different, as seen in Figs. 1 and 2
for both fiber lengths used in the experiment. However, due
to the differences in the linewidth, the integrated power from
the laser is not the same throughout the tuning range. For
instance, for a pump power of 300 mW, it changes from
0.7 to 0.3 mW when the wavelength is tuned from
1591 to 1532 nm for the 4.6 m fiber. For a given L and R,
the change in linewidth due to a change in ␣ depends essentially on the spectroscopic characteristics of the EDF used,
and hence could be different for different types of dopant
fibers. Whatsoever, attenuation has a direct influence on the
flatness of the spectral response of the threshold power and
hence on the linewidth of emission. An ideal Lorentzian line
shape is not obtained throughout the entire tuning range of
this filterless laser design, but the present analysis helps in
J. Appl. Phys. 102, 083107 共2007兲
FIG. 5. 共Color online兲 Output spectra at different values of attenuation for
pump power of 30 mW 共thin-blue line兲 and 300 mW 共thick-red line兲 共EDF
length= 12 m兲.
the identification of the conditions for obtaining this line
shape with appropriate change of fiber length, pump power,
and intracavity loss.
B. Linewidth dependence on pump power
The tunability of a filterless laser can be significantly
enhanced with the increase in pump power.20 However, for a
given ␣, when the pump power is increased beyond the
threshold, the linewidth of the output starts increasing. The
linewidth characteristics at different pump powers are recorded to qualitatively interpret this dependence in different
wavelength ranges of operation for both lengths of EDF. It
should be noted that the linewidth broadening discussed in
this section is for a given wavelength of operation of the
laser, originating due to the external pump power, and hence
is fundamentally different from the case discussed in the previous section. The pump power is varied from
30 to 300 mW. The output spectra at both pump power levels are shown in Figs. 5 and 6 for both lengths of the fiber.
Increase in pump power results in a slight increase in the
power at ␭las, associated with a significant increase in line-
FIG. 6. 共Color online兲 Output spectra at different values of attenuation for
pump power of 30 mW 共thin-blue line兲 and 300 mW 共thick-red line兲 共EDF
length= 4.6 m兲.
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083107-5
J. Appl. Phys. 102, 083107 共2007兲
V. Deepa and R. Vijaya
FIG. 7. ASE spectra observed at the output of the coupler for different pump
powers for fiber lengths of 共a兲 12 m and 共b兲 4.6 m.
width at the output. A simplistic explanation for this effect is
that an increase in pump power saturates the gain of the
lasing wavelength. Due to this, the gain of the neighboring
longitudinal modes increases with further increase in pump
power, resulting in a broader linewidth. The fact that the
linewidth increases with pump power shows that it does not
originate from spontaneous emission.23 Even though such an
increase is seen throughout the tunable range of the laser, the
extent of linewidth broadening is quite different for different
wavelength ranges. It is found to be the largest around
1600 nm in Fig. 5共b兲 and around 1575 nm in Fig. 6共b兲. A
Lorentzian line shape is retained even at the largest pump
power for shorter ␭las in each case 关Figs. 5共f兲 and 6共d兲兴. Since
the analytical model used for the fiber laser does not predict
pump power dependence for the ␭las and the linewidth, the
above feature can be understood by estimating the amplified
spontaneous emission 共ASE兲 at different pump powers. The
ASE spectrum can be considered as a measure of the gain at
different wavelengths to start with, and the lasing wavelength is the one for which the gain matches the cavity loss
in multiple roundtrips. The ASE spectrum is observed experimentally at the output of the coupler, with the cavity
open, so as to include the spectral effects of maximum number of elements in the cavity. The spectra observed at different power levels are shown in Fig. 7.
The undulations seen at lower pump powers are an artifact of the OSA. The stray peaks at higher pump powers in
Fig. 7共b兲 are due to lasing, with the ferrule connectors acting
as reflectors of very low R. The inflexion observed in Fig.
7共a兲 for the wavelength range of 1577– 1587 nm indicates
the inaccessible wavelengths in the tuning range for an EDF
length of 12 m, as seen in Fig. 5共d兲. The extent of inflexion
for this wavelength range has reduced considerably in the
case of shorter length of EDF 关Fig. 7共b兲兴, resulting in lasing
around 1575 nm, as seen in Fig. 6共b兲. For both lengths of
EDF considered, the degree of flatness of the ASE spectra in
the longer wavelength range 共⬎1570 nm兲 increases with the
increase in pump power. This results in more number of
longitudinal modes balancing the intracavity loss, leading to
larger linewidth at larger pump powers in the longer wavelength range. Particularly, the extent of linewidth broadening
is significantly larger around the 1600 nm region in the case
of 12 m fiber 关Fig. 5共b兲兴 and around 1575 nm 关Fig. 6共b兲兴 for
the 4.6 m fiber. This can also be directly associated with the
flatness of the corresponding ASE spectra in those wavelength ranges. For both fiber lengths, the linewidth broadening progressively decreases for ␭las less than 1570 nm, for
the same variation in pump power, since the gain changes
significantly with wavelength in this region, thus resulting in
lesser number of longitudinal modes.
It is documented in the literature that larger linewidths at
higher pump powers could be due to multiple four wave
mixing processes between the longitudinal modes in the
system.24 The contributions due to intermodal four wave
mixing cannot be isolated in our measurements. The wavelength regions for which the ASE response is flatter would
provide larger number of “seed” wavelengths for four wave
mixing and hence would further enhance the line broadening
effects. Since the wavelengths studied are significantly different from the expected zero dispersion wavelength of the
active fiber, the consequent dispersive phase would be a deterrent to the nonlinear process. The lack of a dispersion-free
mixing medium in the cavity leads to imperfect phase matching for the various four wave mixing processes.25,26 Hence
the contribution due to four wave mixing, if at all existing, is
not optimal. The phase matching occurring, if any, is due to
the contributions from the nonlinear phase due to the large
intracavity field.27
IV. CONCLUSIONS
The linewidth characteristics of an intracavity loss controlled tunable erbium doped fiber ring laser are analyzed for
its operation in the continuous wave mode. It is found that
for a given pump power the linewidth of the output is a
function of the attenuation in the cavity. The change in linewidth with attenuation is interpreted from the nature of the
spectral dependence of threshold pump power. More number
of longitudinal modes is supported in those wavelength regions which have a flatter response of the threshold curves,
resulting in larger linewidth. For a given intracavity attenuation, linewidth is found to be strongly dependent on the
pump power, and the nature of ASE curves at different pump
powers is an indicator of this dependence. A flatter region in
the ASE spectrum implies a larger linewidth for that wavelength region. Though the flatter response also indicates an
increase in the number of seed wavelengths for multiple
wave mixing processes, the consequent linewidth broadening
is not optimal due to the absence of an efficient nonlinear
mixing medium in the cavity. Since both intracavity attenuation and the pump power influence the linewidth characteristics, it is important to control both quantities in conjunction
for obtaining an ideal line shape. This analysis is useful for
the design of continuously tunable filterless fiber lasers with
a spectral output of an ideal Lorentzian line shape.
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083107-6
ACKNOWLEDGMENTS
We acknowledge the financial support from the University Grants Commission Council of Scientific and Industrial
Research 共CSIR兲 and the Department of Information Technology 共DIT兲, Government of India.
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