High-speed generation of random bits has attracted much attention recently using chaotic dynamics in laser diodes subject to different perturbations. Extraction of randomness has been investigated from a chaotic laser using selfinjection from a grating with dispersion. While the optical feedback invokes chaos, the dispersion induces different delays to different frequency components to conceal the information of the feedback delay time. Experimentally, random bits were extracted using a laser under feedback from a grating. The undesirable time-delay signature (TDS) was reduced by about 10 times. So a continuously tunable sampling rate was allowed for random bit generation (RBG), where the overall output rate tunable across 3 orders of magnitude up to 100 Gbps was demonstrated. Numerically, the concealment of TDS is further investigated using a rate-equation model incorporating self-injection through an all-pass filter (APF), which is realized by coupling the feedback path to a lossless ring cavity. While the APF provides dispersion to suppress the TDS, the APF provides no frequency selectivity so that a large chaotic bandwidth is attained. For minimal TDS, the coupling ratio to the ring is optimized at 0.5. The round-trip time of the ring also needs to be sufficiently long for inducing dispersion in the chaotic light. With comparable performance in TDS suppression, the APF feedback can generate chaos with broader bandwidth than grating feedback. The results are applicable to generating random bits for digital systems with widely tunable rates.
Semiconductor lasers in chaotic oscillations have recently been utilized for random bit generation at output rates exceeding gigabits per second, which are important for high-speed numerical simulation, encryption, and communication. Although chaotic signals were successfully invoked though optical feedback into the lasers, the feedbacks inherently led to residual autocorrelation that is detrimental to the output randomness. In this paper, we experimentally demonstrate random bit generation using an optically injected semiconductor laser without feedback. Through oversampling the signal at 10 GHz as recorded by a 2.5-GHz oscilloscope, random bit generation at 40 Gbps is attained from extracting 4 bits per sample.
When a semiconductor laser is subject to an incoming optical carrier, equivalently an external optical injection, it
can enter nonlinear period-one dynamics through Hopf bifurcation due to the radical modification in field-carrier
coupling of the injected laser which results from the dynamical competition between injection-imposed laser oscillation
and injection-shifted cavity resonance. Equally-separated spectral components appear, of which intensity
and frequency depend strongly on the injection level and frequency. This suggests that a dynamical amplitude
or frequency variation of the incoming optical signal, such as amplitude-shift keying (ASK) or frequency-shift
keying (FSK), respectively, would lead to corresponding dynamical variation in amplitude and frequency of each
spectral component. Therefore, by properly selecting the optical frequency of the output optical carrier and
by minimizing the residual ASK and FSK modulation, both ASK-to-FSK and FSK-to-ASK conversions can be
achieved, where bit-error ratio down to 10-12 is achieved with a slight power penalty. Only a typical semiconductor
laser is necessary as the key conversion unit. In addition, frequency shifts of the optical carrier can also
be achieved, which allows a simultaneous frequency conversion of the optical carrier if required.
Photonic transmission of microwave signals from a central office to remote base stations is a key functionality
in broadband radio-over-fiber access networks. Because of chromatic dispersion, a strong fluctuation of the
microwave power along fiber transmission happens to microwave-modulated optical carriers with double-sideband
features. Therefore, optical single-sideband modulation characteristics are preferred. Direct modulation of
a semiconductor laser is the simplest scheme for photonic microwave generation and transmission. However,
the symmetric property of the laser in the modulation sideband intensity makes the scheme unattractive for
radio-over-fiber applications. In this study, we apply the injection locking technique to the laser for optical
single-sideband generation. Proper optical injection can drive the laser to the stable-locking dynamical state
before entering the Hopf bifurcation. The field-carrier coupling of the injected laser is radically modified due to
the dynamical interaction between the injection-shifted cavity resonance and the injection-imposed oscillation.
Therefore, the relaxation resonance sidebands of the injected laser are considerably shifted in frequency and
asymmetrically modified in intensity, the extent of which depends strongly on the injection condition. Under the
range of our study, direct modulation of the injected laser can thus generate microwave signals that are broadly
tunable up to 4 times its free-funning relaxation resonance frequency and are highly asymmetric up to 20 dB
in modulation sidebands. The microwave frequency can be tuned over a broad range while keeping a similar
level of modulation sideband asymmetry, or different levels of modulation sideband asymmetry can be obtained
while keeping a similar microwave frequency. This adds the flexibility and re-configurability to the proposed
system. No optical phase-locking electronics, no high driving voltages, and no narrow-bandwidth optical filters
are necessary as in many other systems.
Nonlinear dynamics of semiconductor lasers have recently attracted much attention in the area of microwave photonics.
By invoking the nonlinear dynamics of an optically injected laser diode, high-speed microwave oscillation can be
generated using the period-one oscillation state. The oscillation is harnessed for application as a photonic microwave
source in radio-over-fiber (RoF) systems. It is advantageous over conventional direct current modulation because it
alleviates the modulation bandwidth limitation and naturally generates single sideband signals. The method is thus
applicable to wireless communication systems even when the subcarrier frequency increases to 60 GHz. Because RoF is
usually incorporated with standard wireless schemes that involve frequency division multiplexing (FDM), we investigate
the performance of the optical injection system under simultaneous current injection of multiple data streams. Frequency
mixings and competition for locking among subcarriers result in intermodulation distortion (IMD). The relative
weightings of different channels should be optimized to ensure acceptable signal qualities. The results illustrate the
feasibility of applying the optical injection system for FDM RoF transmission at high subcarrier frequencies.
The design and performance of a multifunction continuous wave dual-frequency lidar system is presented. The system
is based on the use of the nonlinear dynamics of an optically injected semiconductor laser. Under proper operating
conditions, the laser emits a dual-frequency beam with a broadly tunable microwave separation. The two optical lines
are coherently locked to each other using an external microwave synthesizer, resulting in a stable microwave beat
frequency. The lidar system is capable of simultaneous velocity and range measurement of remote targets. The velocity
is measured from the Doppler shift of the microwave beat frequency. The stability of the microwave beat frequency
enables accurate measurement of low velocities. In addition, the stable locking enables long-range measurements
because of the long microwave coherence length. Ranging is accomplished by extracting the time-of-flight information
carried on the residual microwave phase noise. We demonstrate preliminary measurements of velocities as low as 26
&mgr;m/s and range measurements of 7.95 km with 2 % accuracy.
Nonlinear dynamics of semiconductor lasers has found many interesting applications in microwave photonics
technology. In particular, a semiconductor laser under optical injection of proper strength and optical frequency detuning
can enter into the dynamical period-one (P1) state through Hopf bifurcation. The resulting optical output carries a
broadly tunable high-speed microwave modulation without employing any expensive microwave electronics. It is
therefore a desirable source for radio-over-fiber (RoF) applications. The P1 state can also be adjusted to have a nearly
single sideband (SSB) optical spectrum. It is an advantageous property for long distance fiber transmission because it
minimizes the microwave power penalty that is induced by chromatic dispersion. In this work, we investigate in detail
the properties of the P1 state and the effect of fiber dispersion as a function of the injection conditions. Based on a well-established
rate equation model, the results show that the generated microwave frequency can be several times higher
than the intrinsic relaxation resonance frequency of the laser. With a large injection strength and an injection detuning
frequency higher than that required for Hopf bifurcation, the generated microwave power is nearly constant and the
optical spectrum is close to SSB. We simulate the effect of fiber chromatic dispersion and the result shows a maximum
microwave power penalty of less than 2 dB. The characterization of the P1 state is useful in guiding the design of RoF
systems based on optically injected semiconductor lasers.