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IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 18, NO. 21, NOVEMBER 1, 2006
All-Fiber Picosecond Laser System at 1.5 m
Based on Amplification in Short and Heavily
Doped Phosphate-Glass Fiber
Pavel Polynkin, Alexander Polynkin, Dmitriy Panasenko, N. Peyghambarian, and Jerome V. Moloney
Abstract—Amplification of ultrashort pulses in doped fibers
is limited by an onset of nonlinear effects in the fiber. At the
1.5- m wavelength, single-mode fibers typically have anomalous
dispersion. The self-phase modulation combined with dispersion
leads to instability of multinanojoule pulses in such fibers. Various
techniques developed to amplify pulses beyond the nonlinearity
limit typically rely on a delicate balance between dispersive and
nonlinear effects in different parts of the laser system. We report
a simple all-fiber alternative to these complex techniques that
utilizes a rapid amplification of pulses in a short and heavily doped
phosphate-glass active fiber. In our preliminary experiments,
picosecond pulses at 1.5 m generated by a passively mode-locked
fiber oscillator at a repetition rate of 70 MHz are amplified in a
15-cm-long heavily Er–Yb codoped fiber amplifier to the average
output power of 1.425 W. The pulse energy and peak power reach
20.4 nJ and 16.6 kW, respectively, while the pulse distortion is
minimal in both temporal and spectral domains. Further power
up-scaling is possible by using active phosphate fiber with a large
mode area, in the amplifier stage.
Index Terms—Fiber lasers, mode-locked lasers, solitons.
I. INTRODUCTION
IGH-POWER fiber sources of ultrashort pulses in the infrared have potential applications in diverse areas such
as material processing, nonlinear optics, biomedical imaging,
and optical frequency metrology. The wavelength range around
1.5 m is of a particular interest because inexpensive and reliable photonic components developed for optical communications can be utilized, and because frequency doubling of light
at 1.5 m generated by a fiber source will result in a portable
alternative to bulky and expensive Ti : sapphire laser [1], [2].
Increasing the pulse energy and peak power by straightforward amplification in an Er-doped fiber is limited by the onset
of nonlinear processes in the fiber. In particular, when the fiber
has anomalous dispersion which is typically the case around the
1.5- m wavelength, the combined action of self-phase modulation (SPM) and dispersion leads to a breakup of the pulses into
fundamental soliton components [3]. Various techniques developed to overcome the limitations imposed by fiber nonlinearity
typically rely on a delicate balance between nonlinear and dispersive effects in different parts of the optical system and frequently resort to using free-space optic elements. The examples
H
Manuscript received April 21, 2006. This work was supported by the U.S. Air
Force Office of Scientific Research under Contract F49620-02-1-0380.
P. Polynkin, A. Polynkin, D. Panasenko, and N. Peyghambarian are with the
College of Optical Sciences, The University of Arizona, Tucson, AZ 85721 USA
(e-mail: [email protected]).
J. V. Moloney is with the Arizona Center for Mathematical Science, The University of Arizona, Tucson, AZ 85721 USA.
Digital Object Identifier 10.1109/LPT.2006.884242
of such techniques previously used at the 1.5- m wavelength
include self-similar pulse evolution [4], chirped pulse amplification [2], [5], and generation of Raman-shifted solitons in the
fiber amplifier [1], [6]. Further, to reduce the aerial power density in the core of the active fiber and the associated nonlinearity,
it is common to use active fibers with large-mode areas. Such
fibers can be either multimode step-index fibers in combination
with appropriate mode converters [1] or microstructured fibers
[7].
To the best of our knowledge, the highest peak power produced by direct amplification of ultrashort pulses at 1.5 m in
a doped fiber is 54 kW [7]. It has been achieved by using a
9-m-long microstructured active fiber in the double-pass configuration. The mode size in the active fiber was 26 m, three
times as large as that in a standard single-mode step-index fiber
at that wavelength. The pulse dynamics in the experiment reported in [7] was complex and involved a change of the order
of the propagating soliton as well as the soliton self-frequency
shift resulting in a substantial distortion of the optical spectrum.
In this letter, we report a simple alternative to the techniques
developed previously, that utilizes amplification of pulses in a
short and heavily doped active fiber made of soft phosphate
glass. When used as a gain medium in a multimode continuous-wave laser oscillator, such a fiber has been demonstrated to
generate as much as 1.33 W of power at 1.5 m per centimeter
of fiber [8]. In the preliminary experiments reported here, the
heavily doped phosphate fiber is used to amplify picosecond
pulses at 1.5 m so rapidly that no significant pulse distortion
develops in either temporal or spectral domain. Our all-fiber
fully spliced system is modular in a sense that the short phosphate-fiber amplifier is not tailored to a particular input signal
from the oscillator stage. The energy and peak power of the
pulses produced by the system are 20.4 nJ and 16.6 kW, respectively. The demonstrated peak power is not quite as high
as 54 kW reported in [7]. Further power up-scaling is feasible
by using active phosphate fiber with a large mode area in the
amplifier stage.
II. DESIGN OF THE ALL-FIBER PICOSECOND LASER SYSTEM
The high-power picosecond laser system reported here is
shown schematically in Fig. 1.
The seed pulses are generated by a passively mode-locked
fiber laser oscillator which is similar in construction to the polarization-maintaining (PM) oscillator reported in [9]. The fully
spliced fiber cavity of the oscillator consists of 1-m-long passive PM fiber, 20-cm-long passive single-mode fiber (SMF-28),
and 12.5-cm-long active phosphate fiber. The core of the active
fiber is 14 m in diameter and it is uniformly doped with
1041-1135/$20.00 © 2006 IEEE
POLYNKIN et al.: ALL-FIBER PICOSECOND LASER SYSTEM AT 1.5 m BASED ON AMPLIFICATION
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Fig. 1. Schematic diagram of the high-power all-fiber picosecond laser system
based on heavily doped phosphate-glass active fiber.
ions/cm of Er and
ions/cm of Yb . These
concentrations are by at least an order of magnitude higher than
those possible without clustering in fused silica. The phosphate
fiber has numerical aperture of 0.08 and the V-number of 2.3
at the signal wavelength, which indicates that the fiber is marginally single-mode. The active fiber is cladding-pumped by
multimode laser diodes operating at 975 nm, using a scalable
side-pumping scheme described in detail elsewhere [10].
The oscillator cavity is terminated by a butt-coupled semiconductor saturable absorber mirror (SESAM) from one side
and by a chirped fiber Bragg grating from the other. The grating
has an 8.2-nm-wide reflection bandwidth (full-width at halfmaximum) which is centered at 1558 nm, and a peak reflection coefficient of 10%. The dispersion of the chirped grating is
0.045 ps/nm. The overall cavity dispersion of the oscillator is
close to zero but slightly anomalous. The round-trip loss in the
oscillator cavity is 13 dB. It is dominated by the useful 10-dB
loss at the output coupler. The remaining loss results from the
mode mismatch at the splice points between the active phosphate fiber and the passive fibers ( 2 dB total for two splices
encountered twice per cavity round-trip) and the nonsaturable
loss of the SESAM (1 dB).
The SESAM that starts the mode-locked operation of the oscillator and shapes the pulses has a nonresonant design and a
bitemporal time-response, with the fast and slow recovery time
constants of less than 500 fs and 25 ps, respectively. The saturable absorption of the SESAM is 30%. Non-PM fiber sections
of the oscillator cavity are kept straight so that the polarization
eigenmodes of the cavity are close to the two linear polarizations along the birefringence axes of the PM-fiber. In operation,
the intracavity polarization of the oscillator locks to one of these
linearly polarized modes. Slight polarization disturbances in the
cavity as well as upon reflection off the SESAM are compensated for by adjusting a manual polarization controller mounted
on the 20-cm-long SMF section. The fundamental repetition rate
of the oscillator is 70 MHz corresponding to the total cavity
length of 1.5 m.
Optical signal generated by the oscillator is amplified in a
short phosphate-fiber amplifier. A fiber-pigtailed optical isolator is spliced between the oscillator and amplifier stages to
block spurious backreflections from the amplifier stage into the
Fig. 2. (a) Intensity autocorrelation and (b) optical spectrum on a linear scale,
for the pulses generated by the oscillator.
oscillator. A fraction of about 20% of the output power from
the oscillator is split off for monitoring. The gain fiber and the
pumping method used in the amplifier are identical to those in
the oscillator, but the length of the active phosphate fiber in the
amplifier is 15 cm.
One of the intended uses for our system is all-fiber supercontinuum generation in highly nonlinear passive fibers. Accordingly, for ease of fusion-splicing of various nonlinear fibers to
the output of the amplifier, the latter is pigtailed by a 10-cm-long
strand of a passive SMF (SMF-28). To avoid backreflections,
the splice between the passive pigtail fiber and the gain fiber is
performed on fibers cleaved at an angle. The output end of the
passive fiber pigtail is angle-cleaved as well.
III. EXPERIMENTAL RESULTS AND DISCUSSION
The laser threshold for the oscillator occurs when it is pumped
with 2 W of launched pump power at 975 nm. When pumped
with 4 W, the oscillator is stably mode-locked and produces
60 mW of average output power at 1.5 m. The intensity autocorrelation and the optical spectrum of the pulses generated
by the oscillator at this point are shown in Fig. 2. Assuming the
sech pulse shape, the pulsewidth is estimated at 1.1 ps, indicating that the oscillator produces nearly perfect hyperbolic secant pulses with the time-bandwidth product of 0.32. The pulse
energy at the output of the oscillator is 0.86 nJ, and the peak
pulse power is estimated at 700 W. The absence of multipulsing was verified by recording the long-range intensity autocorrelation, and by detecting the output optical signal with a
fast InGaAs photodiode and observing the resulting waveform
with an oscilloscope.
Note that the oscillator stage is capable of producing a much
higher average power than 60 mW, but as we verified, above
that power level, the output pulses became unstable and started
to break up in the passive 1-m-long fiber link connecting the
oscillator and the amplifier stages. Thus, we chose to operate
the oscillator at the 60-mW power level even though pumping
it harder would better saturate the amplifier.
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IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 18, NO. 21, NOVEMBER 1, 2006
, and that from the passive fiber pigtail is
. (In
the calculation, we assumed that the nonlinear refractive index
m /W for both phosphate glass and fused
silica, and that the mode-field diameters are 14 and 10 m for the
phosphate and silica fibers, respectively.) Thus, the total nonlinear phase acquired by the pulses after propagating through
) which is close to the estimathe amplifier is equal to (
tion based on the shape of the distorted optical spectrum.
Note that the spectral distortion is very moderate even though
the aerial power density in the fiber core at the output of the amplifier is 21 GW/cm , which is to the best of our knowledge
the highest in-fiber power density ever produced by an ultrafast all-fiber system at 1.5 m. Clearly, higher average and peak
power could be generated by using a large-core active phosphate
fiber combined with an appropriate mode converter, in the amplifier stage.
IV. CONCLUSION
Fig. 3. (a) Intensity autocorrelation and (b) optical spectrum on a linear scale,
for the amplified pulses, at the highest output power.
An 80% fraction of the signal produced by the oscillator is
sent to the amplifier stage. The latter amplifies the signal to
1.425 W of average output power when pumped with maximum
available pump power of 20 W at 975 nm. At this point, the amplifier produces 14.5 dB of gain, and the output pulse energy
reaches 20.4 nJ.
The intensity autocorrelation of the pulses and the optical
spectrum at the highest output power are shown in Fig. 3. The
autocorrelation trace suggests that the pulses developed a small
satellite and became slightly narrowed. Note that these features
appeared gradually as the power level was increased. From the
data, the fraction of the total pulse energy carried by the satellite pulse can be estimated at 10%. Further, assuming that the
pulse shape remains close to hyperbolic secant, we estimate the
pulsewidth at 975 fs, and the peak pulse power at the output of
the amplifier at 16.6 kW.
Amplification of ultrashort pulses in doped silica fibers to
multinanojoule level is typically accompanied by a dramatic distortion of the optical spectrum [2], [4], [7]. In our case, the spectrum distortion is considerably less pronounced although it is
clearly noticeable. By comparing the shape of the spectrum to
the SPM-broadened spectra calculated in [11], we estimate the
amount of the SPM-induced nonlinear phase (or B-integral) at
.
For more accurate estimation of the nonlinear phase, we assume that the signal power grows in the amplifier exponentially. Then at the highest power, the amplifier gain per unit
length is 0.22/cm, and the corresponding effective length of the
amplifier is 4.5 cm. Thus, the main contribution to the nonlinear phase results from propagation of the amplified pulses in
the 10-cm-long passive fiber pigtail. Using the estimated maximum peak power of 16.6 kW, we find that the contribution
to the nonlinear phase from the active fiber in the amplifier is
We have reported a practical all-fiber system for generation
of picosecond pulses with high energy and high peak power
at 1.5 m. In our system, a rapid amplification of the modelocked pulses in a short and heavily doped phosphate-glass fiber
resulted in clean amplified pulses with a minimal amount of
distortion. This approach offers a simple alternative to the more
complex techniques developed previously.
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