1
Mode-locked Er:Yb-doped double-clad fiber laser
2
with 75-nm tuning range
3 Yichang Meng,1,2Mohamed Salhi,1Alioune Niang,1Khmaies Guesmi,1,3
4 Georges Semaan,1and Francois Sanchez1,*
5 1Laboratoire de Photonique d’Angers E. A. 4464, Université d’Angers, 2 Bd Lavoisier, 49045 Angers Cedex 01, France 6 2School of Sciences, Hebei University of Science and Technology, 050018 Shijiazhuang, China
7 3Laboratoire Systèmes Electroniques et Réseaux de Communications (SERCOM), Ecole Polytechnique de Tunisie,
8 EPT, B.P. 743, 2078, Université de Carthage, Tunisie
9 *Corresponding author: francois.sanchez@univ‑angers.fr
10 Received December 5, 2014; revised February 10, 2015; accepted February 16, 2015;
11 posted February 18, 2015 (Doc. ID 229183); published 0 MONTH 0000
12 We demonstrate a widely tunable Er:Yb-doped double-clad multiple-soliton fiber laser based on nonlinear polari- 13 zation rotation (NPR). Based on both an artificial birefringent filtering effect of the cavity and population inversion 14 related gain variation, the central wavelength can be continuously tuned over 75 nm range (1545–1620 nm). Wave- 15 length tunability is achieved by controlling both the linear loss of the cavity and the polarization controllers (PC).
16 This is the widest tunable range yet reported in tunable passively mode-locked erbium-doped fiber lasers. © 2015 Optical Society of America
17 OCIS codes: (060.3510) Lasers, fiber; (140.4050) Mode-locked lasers; (140.3500) Lasers, erbium.
18 http://dx.doi.org/10.1364/OL.99.099999
19 Wavelength-tunable mode-locked fiber lasers have
20 potential application in varied subjects and fields such as
21 fiber sensing, laser measurement, spectroscopy, and op-
22 tical communication, etc. Generally, a tunable band-pass
23 filter is used in the cavity to realize mode locking at dif-
24 ferent central wavelengths. However, the restricted
25 bandwidth and tunable range of the filter greatly limit
26 the pulse width and wide-band tunability of the mode-
27 locked fiber lasers. With tunable band-pass filters, 40-nm-
28 tuning-range (1518–1558 nm) [1] and 34-nm-tuning-range
29 (1525–1559 nm) [2] mode-locked fiber lasers have been
30 reported. Using chirped fiber Bragg grating as a tunable
31 band-pass filter, 4.5-nm-tuning-range (1545.5–1550 nm)
32 mode-locked fiber laser has also been reported [3]. To
33 avoid the limitation of the tunable band-pass filter, a tun-
34 able Mach–Zehnder filter was used in mode-locked fiber
35 laser and 19-nm tuning range (1551–1570 nm) was ob-
36 served [4]. Apart from the real filter, the laser cavity
37 always has a filter effect itself [5]. It has been shown that
38 an intrinsic artificial birefringent filter exists in NPR
39 mode-locked fiber lasers [6]. One can take advantage of
40 the artificial birefringent filter to narrow down the effec- 41 tive gain bandwidth or to change the central wavelength
42 in either Yb-doped mode-locked fiber lasers [7–9] or
43 Er-doped mode-locked fiber lasers [10–14]. By using this 44 invisible birefringent filter, 30-nm tuning range (1570–
45 1600 nm) has been achieved in Er-doped mode-locked
46 fiber lasers [15,16]. However, the reported performances
47 of the birefringent filter are below the tuning range that
48 can be expected. The main reason of the limited tuning
49 range is caused by the gain profile of the laser medium.
50 In fiber laser, the gain profile of the laser medium is not
51 static and can be influenced by the parameter variations
52 of the cavity. It has been shown that the value of the pop-
53 ulation inversion at lasing threshold, which fixes the gain
54 threshold, is directly connected with the operating wave-
55 length of the erbium-doped fiber laser: low population in-
56 version ensures higher gain at long wavelength, while
57 high population inversion ensures higher gain at shorter
wavelength [17]. Actually, the linear loss of the cavity, the 58
dopant concentration, or the length of the Er-doped fiber 59
(EDF) could influence the threshold population inver- 60
sion. In practical application, control of the linear loss 61
of the cavity is the simplest way to change the population 62
inversion and then to realize the wavelength tunability. 63
Following this principle, wide-band tunable continuous- 64
wave fiber lasers based on cavity loss control have been 65
reported [18,19]. We have also achieved continuous wave 66
and mode-locked operation above 1.6μm by minimizing 67
the cavity loss in both a figure-of-eight laser and a ring 68
laser using aC-band Er:Yb doped double-clad fiber am- 69
plifier [20,21]. An active mode-locked fiber laser with tun- 70
ing range of about 40 nm (1565.1–1605.3 nm) has been 71
obtained by decreasing the output coupling ratio from 72
95% to 5% (equivalent to decrease the cavity loss) [22]. 73
Since both artificial birefringent filter and cavity loss help 74
to achieve tunability, one can wonder what will be the 75
result if both mechanisms were simultaneously used in 76
a fiber laser. In this Letter, we combine the two effects 77
in an erbium-doped double-clad fiber laser. A wide-band 78
tunable passive mode-locked fiber laser is achieved by 79
adjusting the PCs and a variable attenuator (ATT) in 80
the cavity. 81
The experimental setup is shown in Fig.1. We use here 82
a double-clad Er:Yb-doped 30-dBm fiber amplifier manu- 83
factured by Keopsys (model KPS-BT2-C-30-BO-FA) with 84
a maximum available pumping power of about 5 W. In the 85
amplifier, there is 8-m-long double-clad fiber that has a 86
second-order dispersion of −15ps2∕km. Thanks to the 87
v-groove side-pumping, high power multimode 980-nm la- 88
ser diodes can be injected to the inner clad of the double- 89
clad Er:Yb-doped fiber. A 5% output coupler is used to 90
extract the power from the cavity. It is a good compro- 91
mise allowing to minimize the cavity loss as well as to get 92
enough output power. A variable attenuator in the range 93
0–57 dB is inserted in the cavity in order to control the 94
linear losses of the cavity and consequently the threshold 95
population inversion. We use a polarization-dependent 96 April 1, 2015 / Vol. 40, No. 7 / OPTICS LETTERS 1
0146-9592/15/070001-01$15.00/0 © 2015 Optical Society of America
97 isolator (PD-ISO) placed between two polarization con-
98 trollers to achieve mode locking. Of course, they also act
99 as a flexible tunable filter to select the operating wave-
100 length of the cavity. The total cavity length is about
101 23.5 m, including 15.5-m standard telecommunications
102 single-mode fibers (SMF) with a second-order dispersion
103 of −22ps2∕km. The net cavity dispersion was about
104 −0.461ps2 (anomalous regime). The output intensity is
105 detected with a high-speed photodiode (Newport TIA
106 1200 13 GHz) coupled to a high-speed oscilloscope (Tek-
107 tronix TDS 6124C 12 GHz, 40 GS/s). Spectral properties
108 are analyzed with an optical spectrum analyzer (Anritsu
109 MS 9710C). Pulse duration is measured with an optical
110 autocorrelator (Femtochrome FR-103 XL) with a scan- 111 ning range scalable to about 200 ps.
112 Hereafter, the pump power is fixed to 4 W except when
113 specified in the figures. LetΓ(dB) be the additional loss 114 introduced by the variable attenuator ATT. With Γ0
115 and the pump power above 0.6 W, it is easy to get
116 mode-locked operation by adjusting the PCs. Because
117 the laser operates in the anomalous dispersion regime,
118 multiple pulsing occurs [23,24]. Depending on the orien-
119 tation of the PCs multiple-soliton dynamic patterns such
120 as soliton bunching, disordered multiple-soliton and har-
121 monic mode locking can be observed [24]. Figure 2
122 shows three typical temporal traces recorded with the
pump power of 4 W. There are hundreds of solitons in 123
the cavity, and the number of solitons changes when 124
the PCs are adjusted, thus implying that the total number 125
of solitons is not constant within the entire tuning range. 126
Note that it is easy to achieve high-order harmonic mode 127
locking (HML) with finely adjusting the PCs in such a way 128
that the central wavelength does not change significantly. 129
The temporal trace of 886th-harmonic mode locking is 130
shown in Fig.2(c). We have verified that HML of different 131
order can be obtained for several central operating wave- 132
lengths. Figure3shows the optical spectrum and the au- 133
tocorrelation trace. The central wavelength is 1618 nm, 134
and the spectral width is about 3 nm. The autocorrelation 135
trace shows that the pulse duration is 870 fs assuming a 136
sech2 pulse shape. The rotation of the polarization con- 137
troller results in the tuning of the birefringent filter in the 138
cavity, which causes the variation of the operating wave- 139
length. By carefully adjusting the PCs, the central wave- 140
length of the spectrum can be continuously changed 141 from 1616 to 1620 nm, only narrower spectral width 142
and broader pulse duration can be obtained at 1620 nm. 143
In mode locking operation with constant orientation 144 of the PCs, when we slightly increase the additional 145
loss, the central wavelength continuously moves toward 146
shorter wavelength but only in a very little range 147
(<1nm). Significant change of the loss destroys the 148
mode locking operation. This is because the peak of gain 149
profile is outside the bandpass of the birefringent filter. If 150
when we increase the additional loss, we finely tune the 151
PCs in order to preserve the mode-locking, the wave- 152
length can be continuously changed in a very large range. 153
Sometimes in the tuning process, a new peak at a shorter 154
wavelength appears, and even two wavelengths mode 155
locking operation can be obtained. Fortunately, the un- 156
desired wavelength can be suppressed by finely adjusting 157
one of the polarization controllers, PC2. Figure4shows 158
the process from dual-wavelength (central wavelength 159
at 1570 nm and 1615 nm) mode locking to single- 160
wavelength mode locking withΓ0.5dB. This indicates 161
the artificial birefringent filter plays an important role in 162
wavelength selection. In the dual-wavelength mode lock- 163
ing state, it may occur that each wavelength emits differ- 164
ent soliton pattern as reported in [25]. Unfortunately, it 165
was not possible to conclude in our case due to the 166
unavailability of a spectral filter. 167 F1:1 Fig. 1. Experimental setup. DCF, double-clad fiber; OC, out-
F1:2 put coupler; PC, polarization controller; PD-ISO, polarization- F1:3 dependent isolator; ATT, variable attenuator.
F2:1 Fig. 2. Temporal trace. (a) Soliton bunching. (b) Disordered F2:2 multiple-solitons. (c) 886th-harmonic mode locking.
1605 1610 1615 1620 1625 1630 1635
−70
−60
−50
−40
−30
−20
−10 0
Wavelength (nm)
Intensity (dBm)
−5 0 5
0 0.2 0.4 0.6 0.8
Delay (ps)
Intensity (a.u.)
3 nm
1.54x870 fs
Fig. 3. Optical spectrum and autocorrelation trace (inset). F3:1 2 OPTICS LETTERS / Vol. 40, No. 7 / April 1, 2015
168 Following the principle presented above, it has been
169 achieved 75-nm tuning range, from 1545 to 1620 nm. To
170 our best knowledge, this is the widest tunable range yet
171 reported in passively mode-locked Er-doped fiber laser.
172 Figure5(from bottom to top) shows the evolution of the
173 mode-locked spectrum when the linear loss is increased
174 and the PCs adjusted. Let us recall that multiple-soliton
175 operation takes place within the whole tuning range.
176 Results are summarized in Fig.6, which shows the evo-
177 lution of additional losses and pulse widths as a function
178 of the central wavelengths. As usual, the wavelength
179 monotonously decreases while the additional loss in-
180 creases. The pulse width varies from 0.64 to 1.3 ps,
181 but there is no clear evolution versus the operating wave-
182 length. The pulses are very close to their Fourier limit
in the whole tuning range. As a consequence, the pulse 183
width is mainly determined by the spectral width of the 184
variable artificial birefringent filter since the gain band- 185
width is much larger. In view of our results, we can ex- 186
pect that the spectral width of the birefringent filter does 187
not vary significantly in the tuning range. Before proceed- 188
ing, it is worth to note that slightly changing the net 189
dispersion of the cavity does not significantly modify 190
the tuning range. 191
Let us now consider the effect of the additional loss on 192
the output power. Figure7shows the evolution of output 193
power as a function ofΓfor pumping powers of 3, 4, and 194
5 W. Whatever the pumping power, the output power first 195
decreases rapidly and then decreases slowly. In our ex- 196
periments whenΓ∼12dB, no mode locking is obtained 197
for any pump power in the range 3–5 W. WhenΓchanges 198
from 0 to 12 dB, the output power decreases less than 199
3 dB. Although the output coupler is placed just after 200
the amplifier, it cannot explain in itself why the output 201
power is not more affected by the increase ofΓ. To in- 202
terpret this result we have to consider not only the var- 203
iations of the losses but also the variations of the gain 204
experienced by the operating wavelength. Indeed, while 205
the additional loss increases, the amount of pumping 206
power needed to reach the threshold also increases. 207
As a consequence, the remaining pumping power effec- 208
tively transformed in laser signal decreases. On the other 209
hand, whenΓincreases, the gain experienced by the re- 210
sulting operating wavelength increases. It is the balance 211
between these two opposite effects that finally lowers 212 30
38 46
54 1540
1560 1580
1600 1620
−40
−20 0
PC2 orientation (
°) Wavelength (nm)
Intensity (dBm)
F4:1 Fig. 4. Processes from dual-wavelength mode locking to sin- F4:2 gle-wavelength mode locking by adjusting the PCs.
1535 1555 1575 1595 1615 1635
Wavelength (nm)
Intensity (dB)
1589 nm 1568 nm
1580 nm 1573 nm 1561 nm
1620 nm 1615 nm 1545 nm
1553nm
1609nm 1603 nm 1595 nm
F5:1 Fig. 5. Wideband tunable output spectra with the central F5:2 wavelength from 1545 nm to 1620 nm by controlling the PCs F5:3 and the ATT.
1545 1565 1585 1605 1620 0
2 4 6 8 10 12
Central wavelength (nm)
Γ (dB)
1545 1565 1585 1605 1620 0.3 0.6 0.9 1.2 1.5
Pulse width (ps)
Fig. 6. Corresponding additional lossesΓand pulse widths at F6:1 different central wavelengths. F6:2
0 2 4 6 8 10 12
0 20 40 60 80 100
Γ (dB)
Output power (mW)
pump=5 W pump=4 W pump=3 W
Fig. 7. Evolution of the output power as a function of addi- F7:1 tional lossesΓwith the pump powers of 3, 4, and 5 W. F7:2
April 1, 2015 / Vol. 40, No. 7 / OPTICS LETTERS 3
213 the decrease of the output power when the additional
214 loss increases.
215 Experimental results can be conveniently summarized
216 in a tuning curve giving the evolution of the output power
217 as a function of the operating wavelength. This can be
218 easily extracted from Figs. 6 and 7. Tuning curves are
219 shown in Fig.8for the 3 pumping powers previously con-
220 sidered. The global behavior seems to be independent of
221 the pumping power. As it could be easily expected from
222 Figs.6and7, the output power is greater for longer wave-
223 length because of the smaller value of the cavity losses.
224 The spectral width and the pulse duration are not
225 strongly affected by the pumping power. On the other
226 hand, the number of solitons considerably increases
227 when the pumping power increases, although it is diffi-
228 cult to evaluate the approximate number [23,24].
229 In conclusion, using artificial birefringent filter effect
230 combined with cavity-loss-related gain variation, we have
231 demonstrated a widely tunable mode-locked Er:Yb-
232 doped double-clad fiber laser. Tuning range of the central
233 wavelength is from 1545 to 1620 nm, and variation of the
234 pulse duration is from 0.64 to 1.3 ps. We hope that such a
235 wideband tunable multiple-soliton fiber laser may pro-
236 vide a simple and cost-effective high repetition rate pulse
237 source for practical applications.
238 Yichang Meng benefits from a post-doctoral grant from
239 the Région Pays de la Loire. This work has been partially
240 supported by the European Community through FEDER
241 contract.
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1545 1565 1585 1605 1620
0 20 40 60 80 100
Central wavelength (nm)
Output power (mW)
pump=5 W pump=4 W pump=3 W
F8:1 Fig. 8. Evolution of the output power as a function of the op- F8:2 erating wavelength for different pumping powers.
4 OPTICS LETTERS / Vol. 40, No. 7 / April 1, 2015
