Mode locking of an all-fiber laser by acousto-optic
C. Cuadrado-Laborde,1,2,* A. Diez,1M. Delgado-Pinar,1J. L. Cruz,1and M. V. Andrés1
1Departamento de Física Aplicada y Electromagnetismo, ICMUV, Universidad de Valencia, C/Doctor Moliner 50,
Burjassot 46100, Valencia, Spain
2Centro de Investigaciones Ópticas (CONICET-CIC), P.O. Box 3, Gonnet 1897, Buenos Aires, Argentina
* Corresponding author: Christian.Cuadrado@uv.es
Received January 5, 2009; revised February 25, 2009; accepted March 2, 2009;
posted March 6, 2009 (Doc. ID 105874); published March 31, 2009
Active mode locking of an erbium-doped all-fiber laser with a Bragg-grating-based acousto-optic modulator
is demonstrated. The fiber Bragg grating was acoustically modulated by a standing longitudinal elastic
wave, which periodically modulates the sidebands at twice the acoustic frequency. The laser has a Fabry–
Perot configuration in which cavity loss modulation is achieved by tuning the output fiber Bragg grating to
one of the acoustically induced sidebands. Optical pulses at 9 MHz repetition rate, 120 mW peak power, and
780 ps temporal width were obtained. The output results to be stable and has a timing jitter below 40 ps.
The measured linewidth, 2.8 pm, demonstrates that these pulses are transform limited. © 2009 Optical So-
ciety of America
OCIS codes: 140.4050, 140.3510, 140.3500, 140.3295.
Mode-locked lasers capable of generating ultrashort
pulse trains with high repetition rates  have a
number of potential applications, depending on the
wavelength, peak power, and pulse width. Actively
mode-locked fiber lasers have the ability to generate
high repetition rate pulse trains with low timing jit-
ter and small chirp, whereas passively mode-locked
fiber lasers have a simpler structure and can produce
even shorter pulses, but their poor stability some-
times become a major shortcoming. Very few all-fiber
techniques have been developed to actively mode lock
fiber lasers [2–5]. Mode locking by cavity loss modu-
lation using the acousto-optic effect is a good alterna-
tive at megahertz repetition rates. The results pre-
sented here show a stable, actively mode locked all-
fiber laser, based on a simple and low-insertion-loss
acoustically induced superlattice modulator. The su-
perposition of a standing extensional acoustic wave
along a fiber Bragg grating generates sidebands am-
plitude modulated at twice the frequency of the
acoustic wave. The modulation generated by the
standing acoustic waves produces no frequency shift,
unlike previously reported superlattice modulators
based on traveling extensional acoustic waves [6,7].
The setup used for mode locking is schematically il-
lustrated in Fig. 1. The gain was provided by 1.5 m of
erbium-doped fiber containing 300 parts per million
Er3+, with a cut-off wavelength of 939 nm and a nu-
merical aperture of 0.24. The active fiber was
pumped through a WDM coupler by a pigtailed laser
diode emitting at 976 nm, providing a maximum
pump power of 160 mW. The acousto-optically modu-
lated fiber Bragg grating (AOM-FBG) and a short de-
lay line followed by a second FBG, FBG2were fusion
spliced at each end of the active fiber. The AOM-FBG
in turn is composed of an rf source, a piezoelectric
disk, a silica horn, and a FBG ?FBG1?. The tip of the
silica horn was reduced to the same diameter of
FBG1125 ?m and subsequently fusion spliced to the
grating. Both uniform gratings were written in pho-
tosensitive fiber using a doubled argon laser and a
uniform period mask; FBG1??B
When a traveling longitudinal acoustic wave is
launched through a FBG, it leads to a coarse modu-
lation of the—otherwise uniform—grating period, as
a consequence, the reflectivity changes. The main
features of these changes depend on the ratio be-
tween the acoustical wavelength and the grating
length; in the long-wavelength regime, the reflection
band shifts as a whole to longer and shorter wave-
lengths . However, when short-wavelength acous-
tic waves are used, new reflection bands appear sym-
metrically at both sides of the original Bragg
wavelength. The position and strength of these re-
flection bands can be controlled by varying the fre-
quency ?vPZT? and voltage ?VPZT? applied to the piezo-
electric [6,7]. The reflection properties of the AOM-
through an optical circulator with a broadband light
source, and detecting the reflected light with an opti-
cal spectrum analyzer. Figure 2(a) shows the spec-
trum of FBG1, when an electrical signal of vPZT
=4.55 MHz and VPZT=16 V is applied to the piezo-
electric (whenever we refer to voltages, it is a peak-
to-peak measurement). This acousto-optic interaction
produces, in principle, no amplitude modulation of
the reflected signals, but a frequency shift [6,7]. How-
ever, we found that generating a standing acoustic
?1?=1531.15 nm? and
?2?=1528.85 nm? were 120 and 30 mm long,
Fig. 1.Mode-locked fiber-laser setup.
April 1, 2009 / Vol. 34, No. 7 / OPTICS LETTERS
0146-9592/09/071111-3/$15.00© 2009 Optical Society of America
wave by clamping the end of FBG1opposite to the
silica horn, the sidebands rise and fall at twice the
frequency of the electrical signal. Figure 2(b) shows
this effect for same condition of voltage and fre-
quency of the spectrum shown in Fig. 2(a), together
with the electrical signal applied to the piezoelectric.
The measurement was performed by tuning a laser
diode to the center of the short-wavelength sideband
and measuring the reflected light.
Now we discuss the mode-locked laser operation.
The delay line length (see Fig. 1) was selected to
match the round-trip time ??? with the reciprocal of
the optical modulation frequency, which in turn is
twice the electrical frequency applied to the piezo-
electric, see Fig. 2(b), i.e., 2L=c?n2vPZT?−1, with c be-
ing the speed of light in vacuum, n the modal index
??1.447?, and L the cavity length. Since the selected
piezoelectric operation point was the same as in Fig.
2, i.e., vPZT=4.55 MHz, this results in a cavity length
of 11.4 m. A translational stage was used for match-
ing the reflection band of FBG2 to the short-
wavelength sideband of FBG1, i.e., 1530.5 nm; see
Fig. 2(a). Figure 3(a) exemplifies the laser behavior
showing the train of optical pulses generated, at a
frequency rate of 9 MHz (50 GHz bandwidth oscillo-
scope). Figure 3(b) shows a single optical pulse; the
timing jitter was measured to be 40 ps (rms). Addi-
tionally, we have found that these pulses are best fit-
ted by a hyperbolic secant squared function, also
shown in Fig. 3(b). It is worthwhile to mention that
Gaussian or Lorentzian functions results in the worst
fittings, with higher deviation coefficients. The dis-
persion measurements of the different fibers that we
use to form the laser cavity show that both kind of
dispersion coexist, from the anomalous dispersion of
the delay line to the normal dispersion of the erbium-
doped fiber. For this type of dispersion-mixed cavi-
ties, the analytical steady-state solution is expected
to be of the sech2type, as it was recently shown .
Regarding the pulse duration, it is limited by a vari-
ety of factors, among them the modulation frequency,
the gain of the active medium, the gain bandwidth
(replaced in our setup by the modulator bandwidth,
which is considerably narrower), and the modulation
strength, as it was shown by Kuizenga and Siegman
. In our configuration, we conclude that the low
modulation frequency is likely to be the most limiting
factor. However, we think that AOM-FBGs, using
standing acoustic waves, have the potential to reach
higher modulation rates (some tens of megahertz)
with high-enough modulation strengths. The optical
pulse’s peak power and temporal width as a function
of the pump power are shown in Fig. 4(a). A smooth
variation of the pulse width with pump power can be
observed, before reaching the gain saturation. This
kind of variation has been reported before for homog-
enously broadened mode-locked lasers . A higher
available pump power would allow additional axial
modes to contribute, increasing the spectral band-
width, and thereby decreasing the pulse’s width.
Unlike other mode-locking configurations relying
on interferometric modulation —with the known
high sensitivity to frequency detuning—here the ad-
justment between the rf signal applied to the piezo-
electric and the cavity round trip did not require a
high accuracy. Although this frequency detuning be-
havior can be analyzed in the time domain, it is pref-
erable to do it in the frequency domain through an rf
spectrum analyzer (2 GHz bandwidth), because of its
higher sensitivity. Figure 4(b) shows the spectrum of
the optical pulses once mode locking is reached; in
this regime, successive harmonics appears at integer
multiples of 1/?=9 MHz. The amplitude of the Fou-
rier components are 13 dB above the noise level when
an accurate matching is achieved, i.e., 1/??2vPZT. A
small frequency detuning at this stage (e.g., by a
hundreds of hertz), does not appreciably change the
Fig. 3. (a) Mode-locked train of pulses generated at 9 MHz
repetition rate with 160 mW of pump power. (b) A single
pulse and its corresponding fitting by a hyperbolic secant
points andsolid curve,
Fig. 4. (a) Peak power and temporal width (FWHM) of the
optical pulses as a function of the pump power (solid and
dotted curves, respectively). (b) Electrical spectrum of the
optical pulses, showing discrete frequency components
spaced by 9 MHz.
cal signal reflected by the short-wavelength sideband when
standing acoustic waves are applied (solid curve) and rf
voltage (dotted curve). In both cases an rf signal of
4.55 MHz and 16 V is applied to the piezoelectric.
(a) Reflection spectrum of the AOM-FBG. (b) Opti-
OPTICS LETTERS / Vol. 34, No. 7 / April 1, 2009
pulse’s characteristics but becomes easily measurable Download full-text
in the frequency domain. When the detuning is
higher (e.g., a mismatch of ±5 kHz) the pulse’s tem-
poral parameters clearly deteriorates, whereas the
harmonics smoothly decreases. Finally, the harmonic
components are no longer present when the rf signal
is further detuned by ±11 kHz; beyond this point, the
pulses drop out. This effect agrees with the theoreti-
cal description of mode-locking frequency detuning
. Consequently, we conclude that the matching
requirements of our present configuration are just in
the kilohertz range, and it is likely to be a conse-
quence of having in our laser a modulation loss
higher than 10%, while in  they achieved only 2%.
The emission linewidth ???M-L? was measured us-
ing a high-resolution optical spectrum analyzer
(BOSA-C, Aragón Photonics, resolution of 80 fm).
Figure 5 shows the spectrum of the optical pulses
shown in Fig. 3. The high resolution of the optical
spectrum analyzer permits a direct observation of the
individual cavity modes separated by 9 MHz (see the
inset of Fig. 5). The measured 3 dB linewidth results
in ??M-L=2.8 pm, i.e., ?vM-L=360 MHz at 1530.5 nm.
On the other side, the optical pulses have a temporal
width of ??M-L=780 ps, so according to the Fourier-
transform-limited relation for a hyperbolic secant
squared pulse, i.e., ??M-L?vM-L?0.315 , its band-
width cannot be lower than 380 MHz. From the com-
parison between this last value and the linewidth
measurement, we conclude that the optical pulses of
our mode-locked laser are transform limited. The
pulses, i.e., ??M-L?vM-L?0.44 [10,12], would result in
a much higher linewidth ?564 MHz?, which is in con-
tradiction with our spectral measurements. As a con-
sequence, the fitting of the temporal pulses [see Fig.
3(b)], the application of the Fourier-transform-
limited relations, and dispersion measurements to-
gether with previous theoretical work , seems to
corroborate a non-Gaussian nature for the emitted
In conclusion, we have reported a transform-
limited, acousto-optically mode-locked all-fiber laser.
A simple and low-insertion-loss acousto-optic super-
lattice modulator based on standing extensional
waves is used for mode locking the fiber laser. In this
way, we propose a new mode-locking modulation
technique, with the potential to reach high modula-
tion strengths and operation frequencies at several
tens of megahertz. Finally, transform-limited optical
pulses of up to 120 mW peak power and 780 ps pulse
width are generated at a fixed repetition rate of
9 MHz, with an emission linewidth of 2.8 pm at
This work has been financially supported by the
Ministerio de Educación y Ciencia of Spain (project
TEC 2008-05490). C. Cuadrado-Laborde acknowl-
edges the Secretaría de Estado de Universidades e
Investigación del Ministerio de Investigación y Cien-
cia (Spain). We also acknowledge Aragón Photonics
(www.aragonphotonics.com) for their support for the
high-resolution spectral measurements.
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pulses shown in Fig. 3. The inset shows the cavity modes
spaced by 9 MHz.
High resolution optical spectrum of the output
April 1, 2009 / Vol. 34, No. 7 / OPTICS LETTERS