We develop a high-precision chaos lidar system using broadband optical chaos from a semiconductor laser subjected to optical feedback. We study how the detector bandwidths, cut-off frequencies, signal-to-noise ratios, and peak sidelobe levels in correlation affect the precision in ranging. With a detector bandwidth of 1600 MHz, a precision of 0.43 mm is achieved from the chaos-modulated pulses with a pulse width of 70 ns. The demonstration and comparison of 3D imaging obtained by waveforms with bandwidths of 1600 and 400 MHz show an enhancement in image quality with broader bandwidths and higher cut-off frequencies.
The linear frequency modulated (LFM) waveforms for medical imaging have been explored previously. Although the chaotic waveforms are used for radar applications, their benefits for medical imaging applications are not adequately analyzed. In this work, we propose using chaos for microwave medical imaging. Firstly, we consider waveforms generated from two chaotic systems: the Lang-Kobayashi and the Lorenz. Through auto-correlation analysis, we show that these waveforms possess good medical imaging properties. Then, we model the received signal from a prototype of the body tissue consisting of multiple layers (media). This received signal incorporates the transmission and reflection coefficients which are a function of the intrinsic impedance of the media. Lastly, the received signal is cross-correlated with the transmitted signal, i.e., the matched filtering operation. The resultant sharp correlations peaks serve as input to the inversion algorithm that estimates the media's intrinsic impedance, which can further be used to assess the healthy/unhealthy nature of the body part.
This work reports on the optical feedback dynamics of InAs/GaAs QD lasers epitaxially grown on silicon operating in both the short and long delay regimes. Both undoped and p-doped QD lasers are considered. Whatever the external cavity length, no chaotic oscillations are observed on both samples as a result of the small α-factor observed in the silicon QD lasers. Despite that, experiments conducted in the short-cavity region raise period-one oscillation for the undoped QD laser. In addition, the transition from the short to long delay regimes can be finely covered by varying the external cavity length from 5 cm to 50 cm, and the boundaries associated to the appearance of the periodic oscillation are identified. In the short-cavity region, boundaries show some residual undulations resulting from interferences between internal and external cavity modes; whereas in the long-delay regime, the feedback ratio delimiting the boundaries keeps decreasing, until it progressively becomes rather in- dependent of the external cavity length. Overall, our results showed that the p-doped device clearly exhibits a much higher tolerance to the different external feedback conditions than the undoped one, seeing that its periodic oscillation boundaries are barely impossible to retrieve at the maximum feedback strength of -7 dB. These results show for the first time the p-modulation doping effect on the enhancement of feedback insensitivity in both short- and long-delay configurations, which is of paramount importance for the development of ultra-stable silicon transmitters for photonic technologies.
The optical feedback dynamics of two multimode InAs/GaAs quantum dot lasers emitting exclusively on sole ground or excited lasing states is investigated under the short delay configuration. Although the two lasers are made from the same active medium, their responses to the external perturbation are found not much alike. By varying the feedback parameters, various periodic and chaotic oscillatory states are unveiled. The ground state laser is found to be much more resistant to optical feedback, benefitting from its strong relaxation oscillation damping. In contrast, the excited state laser can easily be driven into very complex dynamics. While the ground state laser is of importance for the development of isolator-free transmitters, the excited one is essential for applications taking advantages of chaos such as chaos lidar, chaos radar, and random number generation.
We develop and investigate a dual-frequency Laser Doppler Velocimeter (DF-LDV) based on an optically injected
semiconductor laser. By operating the laser in a period-one oscillation (P1) state, the laser can emit light with
two coherent frequency components separated by about 11.25 GHz. Through optical heterodyning, the velocity
of the target can be determined from the Doppler shift of the beat signal of the dual-frequency light. While
the DF-LDV has the same advantages of good directionality and high intensity as in the conventional singlefrequency
LDV (SF-LDV), having an effective wavelength in the range of microwave in the beat signal greatly
reduces the speckle noise caused by the random phase modulation from the rough surface of the moving target.
To demonstrate the speckle noise reduction, the Doppler shifted signals from a moving target covered by the
plain paper are measured both from the SF-LDV and the DF-LDV. The target is rotated to provide a transverse
velocity, where the speckle noise increases as the transverse velocity increases. The bandwidth of the Doppler
signal obtained from the SF-LDV is increased from 4.7 kHz to 9.4 kHz as the transverse velocity increases from
0 m/s to 5 m/s. In contrast, the bandwidth obtained from the DF-LDV maintains at 0.09 Hz with or without the
rotation limited by the linewidth of the P1 state used. By phase-locking the laser with a RF current modulation,
the linewidth of the P1 state can be much reduced to further improve the velocity resolution and extend the
detection range.
Some believes that the useful length of THz different frequency generation (DFG) in a highly absorptive material is
comparable to the absorption length of the THz wave. We show in theory and experiment that it is only true for
backward THz DFG. For forward DFG with strong idler absorption, the THz wave can continue to grow with the length
of a DFG crystal.
We numerically study and compare the noise suppressions in the chaos lidar (CLIDAR) and the synchronized
choas lidar (S-CLIDAR) systems with the optoelectronic feedback (OEF) and the optical feedback (OF) schemes.
The S-CLIDAR system with both OEF and OF schemes show better noise immunity than the CLIDAR system
in the low SNR region. Compare with the OEF scheme, the S-CLIDAR system with the OF scheme is more
sensitive to the phase noise. For the S-CLIDAR system with both schemes, an open-loop configuration under a
generalized synchronization condition is desired.
In this paper, we characterize and compare a quantum dot and a quantum well lasers using the four-wave mixing
analysis. The optical and power spectra of the four-wave mixing state in the quantum dot laser are studied both
numerically and experimentally. The tendency of the amplitude versus detuning in the quantum dot laser is very
similar to those seen in the quantum well laser. The four-wave mixing signals and the power spectra from both
lasers are symmetric, while asymmetry in the regenerated signal is found. Compared to the quantum well lasers,
the higher resonance peak of the regenerated signal of the quantum dot lasers appears on the opposite side of
the detuning in the optical spectra. The intrinsic parameters of the lasers are also obtained by fitting the optical
spectra and power spectra obtained experimentally with those derived directly from the rate equations. The
measured value of the linewidth enhancement factor has a good agreement with that obtained by the injection
locking method.
We numerically investigate high-frequency microwave signal generation utilizing a double injection locking technique.
A slave laser (SL) is strongly injected by a master laser 1 (ML1) and a master laser 2 (ML2) optically.
Stable locking states are observed when the SL is subject to optical injection by either the ML1 or the ML2
individually. By utilizing the hybrid scheme consists of double optical injections, the advantages of each individual
dynamical system are added and enhanced. Comparison of the performances of the spectral width, power
fluctuation, and frequency tunability between the signal generated in the double injection locking scheme and
the similar period-one (P1) oscillation signal generated in a conventional single injection scheme is studied. A
3-fold linewidth reduction is achieved by utilizing the double injection locking scheme benefitted by the strong
phase-locking and high coherence when operating at the stable injection locking state. Moreover, for the double
injection locking scheme, a wide continuous tuning range of more than 100 GHz is obtained by adjusting the
detuning frequency of the two master lasers. The performances of narrow linewidth, wide tuning range, and
frequency continuity show the great advantages of the high-frequency microwave signal generated by the double
injection locking technique.
Characteristics of the frequency-locked states generated by nonlinear dynamical behaviors of semiconductor
lasers under periodical optical injection are investigated experimentally. The periodic optical waveforms used
for injection, including repetitive pulses and sine oscillations, are generated from a laser (master laser) through
self optoelectronic feedback and direct current modulation, respectively. Under proper operational conditions,
namely the repetition frequency and injection strength of the injected light, microwave frequency combs are
observed at the output of the injected laser (slave laser). In generating the microwave frequency combs, the
pulse injection scheme shows the best performance compared to the sine modulation and cw optical injection
schemes. The potential applications of these microwave frequency combs in frequency division and multiplexing
are demonstrated.
The nonlinear dynamics of a semiconductor laser (slave laser) injected by optical pulses with high repetition rate
are investigated experimentally. The pulses for injection are generated from a laser (master laser) subjected to
either an optoelectronic feedback or an optical feedback. The repetition rates of the pulses are controlled by
varying the delay time and the feedback strength of the feedback loop. By injecting the repetitive optical pulses
of different intensities and repetition frequencies into another laser (slave laser), rich dynamical states including
regular pulsations, frequency beatings, and chaotic pulsations are observed. Moreover, frequency-locked states
with different winding number, the ratio of the main pulsation frequency of the slave laser and the repetition
frequency of the injected pulses, are also found. Compared to a laser subject to a sine modulated optical injection,
the linewidths of the high-order microwave components in the output spectrum of the slave laser are substantially
narrower for the laser under repetitive optical pulse injection.
We study the nonlinear dynamics of a vertical-cavity surface-emitting laser (VCSEL) subject to a repetitive
optical pulse injection numerically using the SFM model. In our study, a linearly polarized slave laser is optically
injected by a train of optical pulses from a master laser, where the polarization of the master laser is orthogonal
to the polarization of the solitary slave laser (x-polarized). By varying the strength and the repetition frequency
of the injected pulses, different dynamical states, including regular pulsations, period-doubled pulsations, chaotic
pulsations, periodic oscillations, quasi-periodic oscillations, and chaotic oscillations, are found. Instead of having
only one polarization mode at the slave laser output, both the y- and x-polarized modes are observed for
the pulsation and oscillation states. While the pulsation states with y-polarization follow a period-doubling
route to chaotic pulsations, the oscillation states with the
x-polarization undergo a quasi-periodic route to
chaos oscillations. Then, with adequate strength of the injection, the x-polarized mode will be suppressed (i.e.
polarization switching) and eventually the slave laser will lock to the master laser with higher injection strength.
Also, the switching points, the boundary of the injection-locked, and the regions of the chaotic states are found
to be strongly influenced by the repetition frequency of the injection pulses and the detuning frequency between the two lasers.
We investigate the nonlinear dynamics of a pulse injected semiconductor laser that a laser is optically injected
by repetitive pulses. Rich dynamics, including both oscillation and pulsation states, are found by varying the
intensity and repetition rate of the injection pulses. The laser enters into chaotic pulsation (CP) and chaotic
oscillation (CO) states through individual period-double routes. Under proper injection, frequency-locked states
with different winding numbers, the ratio of the oscillation relaxation frequency and the repetition frequency
of the injection pulses, are observed. The winding numbers reveal a Devil's staircase structure, where a Farey
tree showing the relation between the neighboring states is presented. Moreover, the bandwidth enhancement
phenomenon of the chaotic states under repetitive pulse injection is also discussed.
We numerically study a novel scheme of chaotic communication based on delayed optoelectronic feedback
semiconductor laser with two time delays. By switching the route of optoelectronic feedback between two time delays
according to the message, we can generate a chaotic waveform simultaneously with the message buried inside. With the
advantage of generating the chaotic carrier while encoding message, the transmission can be much easier and more
secure than conventional chaotic communication systems. The chaotic channel message, decoded message, and filtered
message for a message bit-rate of 500 Mbits/s that has a Q-factor value of 9.45 are shown. We also study the Q-factor of
the filtered message and the channel message after filtering for different message bit-rates. The result shows that this
novel scheme can achieve communication with high bit rate and pertain desirable security.
A tracking system utilizing tunable liquid lens is proposed and demonstrated. Adapting the concept of EWOD
(electrowetting-on-dielectric), the curvature of a droplet on a dielectric film can be controlled by varying the applied
voltage. When utilizing the droplet as an optical lens, the focal length of this adaptive liquid lens can be adjusted as
desired. Moreover, the light that passes through it can therefore be focused to different positions in space. In this paper,
the tuning range of the curvature and focal length of the tunable liquid lens is investigated. Droplet transformation is
observed and analyzed under a CCD camera. A tracking system combining the tunable liquid lens with a laser detection
system is also proposed. With a feedback circuit that maximizing the returned signal by controlling the tunable lens, the
laser beam can keep tracked on a distant reflected target while it is moving.
KEYWORDS: Telecommunications, Transmitters, Receivers, Semiconductor lasers, Radio over Fiber, Optoelectronics, Signal attenuation, Signal to noise ratio, Computer programming, Modulation
Chaotic communication in radio-over-fiber (ROF) system based on optoelectronic feedback (OEF) semiconductor
laser is studied numerically. Performance of different message encoding schemes, including additive chaotic
modulation (ACM) and on-off shift keying (OOSK), are compared. By taking into account the attenuation,
nonlinearity, dispersion, and amplifier spontaneous emission noise in the fiber module and the path loss, additive
white Gaussian noise, and multipath effect in the wireless channel, system performance including quality factor Q
and bit-error-rate for different transmission lengths and message bit-rates are investigated. While both schemes
are possible to achieve a BER < 10-9 of the benchmark set by the conventional communication, we have
demonstrated that the OOSK scheme is better suitable for long distance RoF transmission. On the contrary,
while the ACM scheme is sensitive to the perturbation in the wireless channel, it shows moderate decoding results
in the fiber-only scenario. In this paper, effects of parameters mismatch between the synchronized transmitter
and receiver lasers and the security of this communication system are also addressed.
We numerically study a novel scheme of generating linearly chirped signal utilizing the nonlinear dynamics of an optically injected semiconductor laser. With proper adjustment, the optically injected semiconductor laser can be operated in an instable region where the output of the laser exhibits periodic oscillation. The oscillation frequency of the injected laser can be controlled by simply varying the strength of the injection light. By sweeping the injection strength in time, desired chirped signal with very high linearity can readily be obtained. Without modulating the frequency of the laser through either direct current modulation or external frequency modulation, large modulation frequency exceeding 7 GHz is achieved. To suppress the amplitude modulation, a cascaded scheme is further considered that the chirp light generated from the second laser is further injected into a third laser. By tuning the third laser to an injection-locked state, the third laser reproduces the chirp signal injected but with an amplitude modulation greatly suppressed. In this paper, chirp bandwidths, chirp rates, and linearity of the chirp signals generated are studied. A chirp rate of more than 100 GHz/μs is obtained, while the bandwidth of the chirp signal exceeds 7 GHz. The relation between amplitude suppression and chirp rate is also presented. Moreover, the dependence of peak-to-peak intensity modulation suppression on the injection strength is investigated as well.
A novel chaotic lidar (CLIDAR) system utilizing optical chaos has been investigated and demonstrated. Compared
with conventional pseudo-random code-modulated continuous-wave lidars, CLIDAR has the advantages
of very high range resolution and unambiguous correlation profile benefiting from the very broad bandwidth of
the chaotic waveform used. In this paper, a CLIDAR system using an optically injected semiconductor laser as
the light source is studied both numerically and experimentally. The power spectra, phase portraits, time series,
and correlations of the chaotic states obtained at different operating conditions are compared. Chaotic states
with flat and smooth spectra are shown to have better performances. The correlation dimension and the largest
positive Lyapunov exponent for each chaotic state are computed as well, where the relation between the complexity
of chaotic states and peak sidelobe level is discussed. To show the feasibility of CLIDAR, proof-of-concept
experiments, including range finding, two-dimensional imaging, and multiple-target detection, are demonstrated.
A range resolution of 2 cm, which it is currently limited by the detection bandwidth of the real-time oscilloscope
used, is achieved.
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