Tunable switching between stable and periodic states in a semiconductor laser with feedback.

Feedback-induced switching between two nonlinear dynamical states is observed in a semiconductor laser. The single-mode laser is subject to optical feedback in the long-cavity regime. In every round-trip time τ, the feedback is found to switch the laser from a stable state to a periodic state. The stable state corresponds to a continuous-wave emission at a single optical frequency. The periodic state corresponds to emission at another optical frequency with sidebands generated from a sustained relaxation oscillation. Such regular switching between the stable and periodic states is first unveiled numerically. Experimentally, the resultant intensity time series is confirmed as comprising of a square-wave envelope repeating in τ, which is modulated on a microwave carrier near the relaxation resonance frequency. Additionally, the duty cycle for the periodic state is found as continuously tunable by adjusting the feedback strength. The tunable state switching is applicable to square-wave modulated photonic microwave generation.

Frequency-modulated microwave generation with feedback stabilization using an optically injected semiconductor laser.

Generation of frequency-modulated continuous-wave (FMCW) microwave signals is investigated using the period-one (P1) dynamics of a semiconductor laser. A modulated optical injection drives the laser into P1 oscillation with a modulated microwave frequency, while adding feedback to the injection reduces the microwave phase noise. Using simply a single-mode laser, the tunability of P1 dynamics allows for wide tuning of the central frequency of the FMCW signal. A sweep range reaching 7.7 GHz is demonstrated with a sweep rate of 0.42 GHz/ns. When the external modulation frequency matches the reciprocal of the feedback delay time, feedback stabilization is manifested as an increase of the frequency comb contrast by 30 dB for the FMCW microwave signal.

Randomness evaluation for an optically injected chaotic semiconductor laser by attractor reconstruction.

State-space reconstruction is investigated for evaluating the randomness generated by an optically injected semiconductor laser in chaos. The reconstruction of the attractor requires only the emission intensity time series, allowing both experimental and numerical evaluations with good qualitative agreement. The randomness generation is evaluated by the divergence of neighboring states, which is quantified by the time-dependent exponents (TDEs) as well as the associated entropies. Averaged over the entire attractor, the mean TDE is observed to be positive as it increases with the evolution time through chaotic mixing. At a constant laser noise strength, the mean TDE for chaos is observed to be greater than that for periodic dynamics, as attributed to the effect of noise amplification by chaos. After discretization, the Shannon entropies continually generated by the laser for the output bits are estimated in providing a fundamental basis for random bit generation, where a combined output bit rate reaching 200 Gb/s is illustrated using practical tests. Overall, based on the reconstructed states, the TDEs and entropies offer a direct experimental verification of the randomness generated in the chaotic laser.

Square-wave oscillations in a semiconductor ring laser subject to counter-directional delayed mutual feedback.

Square-wave (SW) switching of the lasing direction in a semiconductor ring laser (SRL) is investigated using counter-directional mutual feedback. The SRL is electrically biased to a regime that supports lasing in either counter-clockwise (CCW) or clockwise (CW) direction. The CCW and CW modes are then counter-directionally coupled by optical feedback, where the CCW-to-CW and CW-to-CCW feedback are delayed by τ1 and τ2, respectively. The mutual feedback invokes SW oscillations of the CCW and CW emission intensities with a period of T≈τ1+τ2. When τ1=τ2, symmetric SWs with a duty cycle of 50% are obtained, where the switching time and electrical linewidth of the SWs can be reduced to, respectively, 1.4 ns and 1.1 kHz by strengthening the feedback. When τ1≠τ2, asymmetric SWs are obtained with a tunable duty cycle of τ1/(τ1+τ2). High-order symmetric SWs with a period of T=(τ1+τ2)/n can also be observed for some integer n. Symmetric SWs of order n=13 with a period of T=10.3 ns are observed experimentally.

Contribution of off-resonant states to the phase noise of quantum dot lasers.

The phase noise of quantum dot lasers is investigated theoretically by coupling the Langevin noise sources into the rate equations. The off-resonant populations in the excited state and in the carrier reservoir contribute to the phase noise of ground-state emission lasers through the phase-amplitude coupling effect. This effect arises from the optical-noise induced carrier fluctuations in the off-resonant states. In addition, the phase noise has low sensitivity to the carrier scattering rates.

Random bit generation at tunable rates using a chaotic semiconductor laser under distributed feedback.

A semiconductor laser with distributed feedback from a fiber Bragg grating (FBG) is investigated for random bit generation (RBG). The feedback perturbs the laser to emit chaotically with the intensity being sampled periodically. The samples are then converted into random bits by a simple postprocessing of self-differencing and selecting bits. Unlike a conventional mirror that provides localized feedback, the FBG provides distributed feedback which effectively suppresses the information of the round-trip feedback delay time. Randomness is ensured even when the sampling period is commensurate with the feedback delay between the laser and the grating. Consequently, in RBG, the FBG feedback enables continuous tuning of the output bit rate, reduces the minimum sampling period, and increases the number of bits selected per sample. RBG is experimentally investigated at a sampling period continuously tunable from over 16 ns down to 50 ps, while the feedback delay is fixed at 7.7 ns. By selecting 5 least-significant bits per sample, output bit rates from 0.3 to 100 Gbps are achieved with randomness examined by the National Institute of Standards and Technology test suite.

Phase noise characteristics of microwave signals generated by semiconductor laser dynamics.

Phase noise of the period-one (P1) nonlinear dynamical oscillation in an optically injected semiconductor laser is numerically investigated. The P1 dynamics causes the laser output intensity to oscillate at a widely tunable frequency for photonic microwave generation, although the intrinsic spontaneous emission in the laser inevitably degrades the microwave signal and manifests as the oscillation phase noise. To characterize the phase noise, the P1 microwave linewidth is first numerically examined through the rate equations with a Langevin term. The P1 microwave linewidth is found to vary with the injection parameters. It is nearly minimized when the microwave power maximizes. Owing to the laser nonlinearities, the P1 microwave linewidth can even be smaller than the free-running optical linewidth. By adding an optical feedback to the laser, the P1 microwave linewidth is found to reduce as the feedback strength and feedback delay increase, in which an inverse-square dependency is followed asymptotically. By modification to a dual-loop feedback, noisy side peaks around the central P1 frequency are effectively suppressed through the Vernier effect. The dual-loop feedback maintains a low phase noise variance over a wide tuning range of the P1 frequency, while allowing long delay times for significant P1 microwave linewidth narrowing.

Experimental investigation of anti-colliding pulse mode-locked semiconductor lasers.

We experimentally demonstrate anti-colliding pulse mode-locking (ACPML) in an integrated semiconductor laser. The device geometry consists of a gain section and a saturable absorber (SA) section located immediately next to one of the cavity facets. After depositing a low-reflection coating on the SA facet and a high-reflection coating on the gain section facet, the threshold is unchanged, while the modulation of the SA is increased. The data presented here confirm that the ACPML configuration improves the peak output power of the pulses, reduces the amplitude fluctuation and timing jitter, and expands the biasing parameter range over which the stable mode-locking operation occurs.

Tunable photonic microwave generation using optically injected semiconductor laser dynamics with optical feedback stabilization.

The period-one (P1) nonlinear dynamics of a semiconductor laser subject to both optical injection and optical feedback are investigated for photonic microwave generation. The optical injection first drives the laser into P1 dynamics so that its intensity oscillates at a microwave frequency. A dual-loop optical feedback then stabilizes the fluctuations of the oscillation frequency. Photonic generation at 45.424 GHz is demonstrated with a linewidth below 50 kHz using a laser with a relaxation resonance frequency of only 7 GHz. The dual-loop feedback effectively narrows the linewidth by over an order of magnitude, reduces the phase noise variance by more than 500 times, and suppresses side peaks in the power spectrum.

The structural changes of the bagasse hemicelluloses during the cooking process involving active oxygen and solid alkali.

This work describes the structural changes of bagasse hemicelluloses during the cooking process involving active oxygen (O(2) and H(2)O(2)) and solid alkali (MgO). The hemicelluloses obtained from the bagasse raw material, pulp, and yellow liquor were analyzed by high-performance anion-exchange chromatography (HPAEC), gel permeation chromatography (GPC), Fourier transform infrared spectroscopy (FT-IR), and (1)H-(13)C 2D hetero-nuclear single quantum coherence spectroscopy (HSQC). The results revealed that the structure of the bagasse hemicelluloses was L-arabino-(4-O-methylglucurono)-D-xylan. Some sugar units in hemicelluloses were oxidized under the cooking conditions. Additionally, the backbones and the ester linkages of hemicelluloses were heavily cleaved during the cooking process.

Changes in alpha-L-arabinofuranosidase activity in peel and pulp of banana (Musa sp.) fruits during ripening and softening.

Arabinose is one of the most dynamic cell wall glycosyl residues released during fruit ripening, alpha-L-arabinofuranosidase (alpha-Arab) are major glycosidases that may remove arabinose units from fruit cell wall polysaccharides. To find out whether alpha-Arab plays important roles in banana fruit softening, the enzyme activities in peel and pulp, fruit firmness, respiration rate and ethylene release rate were assayed during banana softening. The results showed that alpha-Arab activities in banana pulp and peel increased slightly at the beginning of storage and reached their maxima when the fruit firmness decreased drastically, alpha-Arab activity increased by more than ten folds in both pulp and peel during ripening and alpha-Arab activities were higher in pulp than in peel. Treatment of banana fruits with ethylene absorbent postponed the time of reaching of its maxima of respiration and ethylene, enhanced the firmness of pup and decreased alpha-Arab activity in the peel and pulp. These results suggest that alpha-Arab induced the decrease of fruit firmness and played an important role in banana fruit softening, and its activity was regulated by ethylene.

Cloning and expression analysis of beta-galactosidase gene related to softening of banana (Musa sp.) fruit.

Beta-galactosidase is thought to be involved in fruit softening through cleaving beta(1-->4) galactan bonds in cell wall hemicellulose. To study the relationship between fruit softening and beta-Gal during banana (Musa sp.) fruit ripening, a beta-Gal cDNA fragment, named MA-Gal, has been cloned from banana fruit pulp using RT-PCR in this study. The results of sequence analysis showed that MA-Gal contained 927 bp, encoding a polypeptide of 309 amino acids, the deduced protein was highly homologous to plant beta-galactosidase expressed in fruit ripening. The MA-Gal putative amino acids have five homologous domains (typical eukaryotic beta-Gals have seven domains), contain the GGPIILSQIENEY(F) motif, which is a presumptive catalytic active site conserved in all beta-Gals. Phylogenetic analysis showed that MA-Gal was in plant beta-Gal group I, which was expressed in fruit tissue. The changes in beta-Gal activity and firmness in pulp during three different stages were also assayed. The Northern analysis showed that the MA-Gal transcripts in pulp were detected at low level at preclimacteric stage, while an increasing progression was observed during fruit ripening, and the highest transcript amount was found at the postclimacteric stage. These results suggest that MA-Gal may play a role during banana fruit ripening and softening.