红外与激光工程 第 43 卷
0 Introduction
Passively mode locked fiber lasers are promising
as potential simple, compact, integrated sources of
ultrashort pulses for high speed telecommunications.
They are also suitable test beds for ultrafast optics
research in the infrared regime. Various passive mode鄄
locking techniques, for example, the nonlinear loop
mirror method
[1-2]
, the semiconductor saturable absorber
method
[3-4]
, and the nonlinear polarization rotation(NPR)
technique
[5-7]
, have been successfully demonstrated the
capability of generation of ultrashort pulses. Due to its
simplicity, stability, and ultrafast recovery time, the
NPR method has attracted great attention and was
widely used. It is now a routine way to use the NPR
technique to generate sub鄄picosecond optical pulses in
the erbium鄄doped fiber lasers. The physical
mechanism of the NPR technique is now well known:
when light propagates in an optical fiber with weak
birefringence, the polarization of the light will
generally become elliptically polarized in the fiber.
The orientation and ellipticity of the final light
polarization is fully determined by the fiber length and
its birefringence. However, if the intensity of the light
is strong, the nonlinear optical Kerr effect in the fiber
must be considered, which introduces extra changes to
the light polarization
[ 8 - 9 ]
. As the extra polarization
changes are proportional to the light intensity, if a
polarizer is placed behind the fiber, then through
appropriately selecting the orientation of the polarizer
and the fiber length, an effect that light with higher
intensity experiences larger transmission through the
polarizer could be obtained. Such an effect is known
as an artificial saturable absorber effect. The NPR
technique makes use of this artificial saturable
absorption to achieve the passive mode鄄locking in a
fiber laser. After mode locking a strong optical pulse
is formed in a laser and circulates in the cavity. If the
nonlinear phase modulation of the pulse could balance
the pulse width broadening caused by the cavity
dispersion, soliton is then obtained in the laser.
Soliton operation is almost a generic feature of the
fiber lasers mode鄄locked by this technique and has
been intensively investigated.
Conventionally, soliton operation of the fiber
lasers is described by the extended Ginzburg鄄Landau
Equation (GLE), which takes into account the
interaction among the cavity dispersion, nonlinear self鄄
phase modulation, and the laser gain properties
[10]
. It is
well known that soliton solutions of the GLE exist in
nonlinear dispersive media of both positive and
negative Group鄄Velocity Dispersion(GVD)
[11]
. However,
until recently gain鄄guided solitons are experimentally
observed and numerically demonstrated in fiber lasers
made of purely positive dispersion fibers
[12]
, where the
gain鄄guided solitons are generated due to the
compensated interaction between the spectral filtering
caused by the finite gain bandwidth and other effects.
In this paper we further report the dynamics of the
gain鄄guided solitons in the passively mode鄄locked
erbium鄄doped fiber ring lasers exploiting NPR
technique. We found experimentally that, apart from
the self鄄started mode locking, cavity鄄induced peak
power clamping effect
[13]
also exists in the laser comparing
with previous fiber lasers that comprise of at least an
element of negative dispersion (fibers, gratings, or
prisms). However, no multiple gain鄄guided solitons
can be achieved in the current lasers. Shifting the
cavity linear phase delay bias only makes the laser
operated either in the states of gain鄄guided soliton or
noiselike pulse emission. Pulse energy of the
generated gain鄄guided soliton is closely related to the
cavity design. Optimizing the cavity structure can
greatly improve the output pulse energy while
maintaining gain鄄guided soliton operation.
1 Experimental setup and results
The fiber laser is schematically shown in Fig.1(a).
It has a ring cavity comprising 3 .2 m, 2 880 ppm
(1 ppm = 10
-6
) erbium鄄doped fiber with GVD of about
52 ps窑nm
-1
窑km
-1
. Other fibers are Dispersion Compensation
3534