H. J. KBASHI
266
Feedback Two counterpropagating
m
2x2
coupler
0.3m
Ge doped
fiber
DC
1300 nm
Laser Source
Optical
Detector
Frequency
Modulato
Oscilloscope
A
Figure 1. Experimental setup of a nonlinear dynamic based
on NOLM.
wavelength at 1300 nm and threshold is 14 mA was used.
The laser is DC biased with external modulation current
source. The output power from the laser is connected to
the first input part of 2 × 2 optical directional coupler.
The two output parts of coupler are connected together
through 1 m loop length and an optical attenuator (A). 30
cm from this loop is 20% Ge doped optical fiber with 10
µm core diameter. The Ge doped fiber is located in mid
of loop as a nonlinear fiber to modified transmission
properties of NOLM and generated high nonlinear dy-
namic range with low critical power. The reflected light
from coupler is split in two ways, one reflected as feed-
back to the cavity of semiconductor laser and the other is
detected and converted into an electrical signal with a
fast amplified telecom photodiode. This electrical signal
is observed in an oscilloscope. The controllable parame-
ters of this optical feedback system are compose of the
DC bias current, modulation current source whose
modulated frequency changed, the feedback strength
using Ge doped fiber and an attenuator, as shown in Fig-
ure 2. The modulation current source provides periodic
timing slots to produce a regular pulse train, while
nonlinear fiber shortens the pulse compared to that ex-
pected from generator. By adjusting these parameters, the
system could be operated in different dynamic states.
3. Results and Discussion
The characteristics of the states found in various dynamic
NOLM feedback systems are plotted for time series on
the left-hand side of Figures 2(a)-(e), and the phase por-
traits on the right-hand side of the figure. From these set
of figures, the original periodic pulse, and its phase plane
can be observed (Figure 2(a)). With high DC bias (100
mA) and large frequency modulation (50 MHz), 2× (100
MHz) and 4× (200 MHz) multiplications were observed
(Figures 2(b) and (c)) respectively due to the competi-
tion between the effect of optical feedback and modula-
tion frequency. The multiplication times as well as the
loop frequency and loop harmonic are depends on the
optical power propagate in loop and frequency modu-
lated. As the loop frequencies are varied under strong
modulation and high nonlinearity in loop, various fre-
quency-locked pulsing states with 20 MHz repetition rate
were observed (Figures 2(d) and (e)). Here, the pulsing
frequency of the laser locks to one of the harmonics of
the delay loop frequency instead of the loop frequency
itself. Chaotic behaviours as in Figure 2(f) were ob-
served in this setup between pulse multiplications and
pulse mode locking. Weak chaotic signal was generated
near laser threshold (14 mA) and strong chaotic signal
was generated with strong feedback (–12 dBm) and high
modulation frequency (100 MHz).
From the phase plane obtained, the original periodic
pulse was stable node and the limit cycle around that
vicinity was a stable limit cycle, whereas in 2×, 4× mul-
tiplications and chaotic behavior, there is neither stable
limit cycle nor stable node on the phase planes even with
the ideal multiplication parameters. However, as the
multiplication factor increases, the system trajectories
moved away from the origin. Corresponding to the modu-
lated frequency and fundamental cavity round-trip time,
the laser operating in fundamental quasi-periodic as in
(Figure 2(d)) and in a periodic as shown in (Figure 2(e))
mode-locking regime with only a single pulse circulating
intracavity. The change from quasi-periodic to periodic
mode-locking was due to more adjusting between modu-
lated frequency and nonlinear phase shift generated in Ge
fiber. The vicinity of quasi-periodic mode-locking was
not stable cycle because the adjusting between modulated
frequency and nonlinearity in Ge doped loop mirror was
not high enough to generated uniform periodic mode-
locking, other hand; higher stability in periodic mode-
locking was result with more accuracy adjusting. In har-
monic frequency locking pulse, the laser output was
characterized using autocorrelator pulse duration meas-
urements, as shown in Figure 3. The optimum output
characteristics in terms of both shortest pulse duration
which is about 1 Psec and highest repetition rate were
observed when the modulated frequency was higher [15].
These variations in the laser dynamics was due to the
change of the modulated frequency and modulation cur-
rent and their interplay with the nonlinear phase shift in
Ge fiber. The intensity spikes correspond to ringing in
the electronic system excited by the photodiode’s im-
pulse response to a very short optical pulse.
4. Conclusion
In conclusion, the nonlinear dynamics of a semicon-
ductor laser with nonlinear optical loop mirror feedback
were investigated experimentally. The combined of mo-
dulated frequency and managed effects of the nonlinear
give rise to 2×, 4× multiplications, chaotic behaviours
and sensitive to the repetition rate mode locking mecha-
nism. This combined action of nonlinear effects inside
Ge fiber acts similar to saturable absorber action and
improving various features of the dynamic laser, such as
increasing the side mode suppression and narrowing the
ine width. l
Copyright © 2012 SciRes. OPJ
