Optics and Photonics Journal, 2012, 2, 265-269
http://dx.doi.org/10.4236/opj.2012.24032 Published Online December 2012 (http://www.SciRP.org/journal/opj)
Nonlinear Dynamics in Directly Modulated Semiconductor
Lasers with Optical Loop Mirror Feedback
Hani J. Kbashi1,2
1Department of Physics, College of Science, University of Baghdad, Baghdad, Iraq
2School of Physics and Astronomy, University of Southampton, Southampton, UK
Email: h.kbashi@soton.ac.uk
Received August 7, 2012; revised September 12, 2012; accepted September 27, 2012
The investigation of the nonlinear dynamics of a semiconductor laser based on nonlinear optical loop mirror (NOLM)
feedback using Ge doped optical fiber was carried out experimentally. Animations of compilations of the output power
as a function of time series and phase plane with effects of optical feedback level, carrier current and modulation signal
strength are demonstrated as a tool to give insight into the laser dynamics. Different dynamic states, including 2×, 4×
multiplication and quasi-periodic and periodic frequency-locked pulsing states extended to chaotic behaviour were ob-
served by varying the parameters of modulated frequency and optical feedback strength. The frequency-locked pulsing
states were observed to exhibit a harmonic frequency-locking phenomenon and the pulsing frequency is locked to a
harmonic nonlinearity in loop instead of the modulated frequency.
Keywords: Nonlinear Dynamics; Nonlinear Loop Mirror; Pulse Multiplication; Optical Chaotic;
Pulse Mode Locking
1. Introduction
Nonlinear dynamics of semiconductor lasers have been
widely studied due to the important roles semiconductor
lasers play in conventional and chaotic optical commu-
nication systems. Under external perturbations such as
optical feedback [1], optical injection [2], and optoelec-
tronic feedback [3,4], various nonlinear dynamics and
routes to chaos have been observed and investigated in
semiconductor laser [5]. Optical feedback is performed
by re-injecting laser emission in external-cavity or by
ring-cavity geometries. Hence, numerous theoretical and
experimental studies on dynamical regimes in semicon-
ductor lasers with optical feedback by re-injecting laser
emission [6-9] and few numerical studies with ring cavity
have been reported [10,11].
The intensity-dependent transmission characteristic of
the NOLM is originally based on a nonlinear differential
phase shift between the counter-propagating beams in the
loop. This intensity is generally attributed to self-phase
modulation (SPM) and a variation of the differential
phase by π will cause a change of the NOLM from high
transmission to a high reflection. However, such differ-
ential phase shift may appear only if the NOLM power
symmetry is broken in some ways. One way to obtain a
differential phase is to use an asymmetrical coupler in the
NOLM. In such case, the dynamic range and the critical
power are directly dependent on the coupling ratio with
low insertion. A second possible way is to use a symmet-
rical coupler in the NOLM and an attenuator in the loop
which results in high dynamic range and low critical
power, even when the insertion loss is high [12]. When
applying feedback to semiconductor laser through
NOLM, it behaves as an external optical resonator and
can be used to generate bistable and chaotic output [13].
To date the reported nonlinear dynamics have been opto-
electronics and optical re-injecting feedback. While,
nonlinear dynamic with NOLM has been conventionally
modelled using complex Ginzburg-Landau in fiber lasers,
such as the figure of eight laser and the nonlinear polari-
zation rotation technique laser [14]. In this paper, we
report an experimental demonstration of nonlinear dy-
namic system of semiconductor laser using a nonlinear
optical loop mirror (NOLM) feedback. DC bias current,
the modulation frequency, and feedback strength are
taken in account as controllable parameters of this
nonlinear loop mirror feedback.
2. Experimental Setup
The schematic representation experimental setup of
nonlinear dynamics based on NOLM feedback is shown
in Figure 1. In this NOLM feedback system, an In-
GaAsP-InP single-mode DFB laser diode with a centre
opyright © 2012 SciRes. OPJ
Feedback Two counterpropagating
Ge doped
1300 nm
Laser Source
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
Copyright © 2012 SciRes. OPJ
Time (sec)
-1.20E-007-6.00E-0080.00E+000 6.00E-0081.20E-007
Time (sec)
-1.20E-007-6.00E-0080.00E+000 6.00E-0081.20E-007
Time (sec)
-0.00000030 -0.000000150.000000000.000000150.00000030
Timme (sec)
-0.0000014-0.00000070.0000000 0.0000007 0.0000014
Time (sec)
-0.000006-0.000004-0.0000020.000000 0.000002 0.000004 0.000006
Figure 2. (Left) Laser output power as a function of time, (Right) phase plane. (a) Original pulse; (b) 2× multiplication; (c)
multiplication; (d) Non periodic mode locking; (e) Periodic mode locking and (f) Chaotic behavior.
-6000 -4000 -20000200040006000
Intensity (Account)
Pulse duration (fsec)
Figure 3. Experimentally recorded autocorrelations laser pulses. The red line shows a fit to the expected hyperbolic secant
quared. The full-width of this autocorrelation, obtained from the fit curves is 1 ps. s
Copyright © 2012 SciRes. OPJ
Copyright © 2012 SciRes. OPJ
5. Acknowledgements
This work was supported by both of Institute of Interna-
tional Education/SRF and CARA.
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