Coherence transfer in an akinetic swept source OCT laser with optical feedback.

We theoretically investigate the influence of optical feedback onto the dynamics of a semiconductor swept source laser. In particular, we show that optical feedback can be used to lock the phase of the successive lasing modes of a multi-section semiconductor laser commonly used for optical coherence tomography (OCT) applications. We also identify two different regimes called sliding frequency self-mixing and sliding frequency mode locking. The second regime demonstrates sub-nanosecond sliding frequency pulses for nonlinear optics applications.

Dynamics of a class-A nonlinear mirror mode-locked laser.

Using a delay differential equation model we study theoretically the dynamics of a unidirectional class-A ring laser with a nonlinear amplifying loop mirror. We perform linear stability analysis of the continuous-wave regimes in the large delay limit and demonstrate that these regimes can be destabilized via modulational and Turing-type instabilities, as well as by an instability leading to the appearance of square-waves. We investigate the formation of square waves and mode-locked pulses in the system. We show that mode-locked pulses are asymmetric with exponential decay of the trailing edge in positive time and faster-than-exponential (superexponential) decay of the leading edge in negative time. We discuss asymmetric interaction of these pulses leading to a formation of harmonic mode-locked regimes.

[Cerebrovascular reactivity in patients with rheumatoid arthritis concurrent with and without hypertension].

AIM: To compare cerebrovascular reactivity (CVR) in patients with rheumatoid arthritis (RA) concurrent with essential hypertension (Group 1) and in those with RA and normal blood pressure (BP) (Group 2). SUBJECTS AND METHODS: During the study of Groups 1 (n = 37) and 2 (n = 12), the investigators estimated the prevalence of traditional cardiovascular risk factors, performed 24-hour BP monitoring, investigated CVR by transcranial Doppler (TCD) of the middle cerebral arteries (MCA) by hyperoxic and hypercapnic tests, and endothelium-dependent vasodilation (EDV) and endothelium-independent vasodilation of the brachial artery. The groups were matched for gender, age, RA activity and stage, and antirheumatic therapy volume. RESULTS: According to the results of MCA TCD, the hyperoxic test recorded impaired CVR in 34 (92%) and 10 (83%) patients in Group 1 and 2, respectively; the hypercapnic test revealed this condition in 19 (51%) and 6 (50%) patients in these groups, respectively. The hyperoxic test most commonly showed an insufficient decrease in MCA linear blood flow velocities (LBFV) in 31 (84%) and 8 (66%) patients in Groups 1 and 2, respectively; the hypercapnic test did an excessive increase in MCA LBFV in 12 (32%) and 4 (33%) patients, respectively. There was a high rate of impaired EDV in 32 (86%) and 9 (75%) patients in Groups 1 and 2, respectively. CONCLUSION: According to the results of MCA TCD, there were high and similar rates of impaired CVR in patients with RA concurrent with and without essential hypertension during the hyperoxic and hypercapnic tests.

Optically injected mode-locked laser.

We study analytically and numerically a delay differential model of a passively mode-locked semiconductor laser subjected to a single-frequency coherent injection. The width of the locking cone is calculated asymptotically in the limit of small injection and zero line-width enhancement factors and compared to that obtained by direct numerical integration of the model equations. The dependence of the locking cone on the laser parameters is discussed.

Optical linewidth of a passively mode-locked semiconductor laser.

We measured the optical linewidths of a passively mode-locked quantum dot laser and show that, in agreement with theoretical predictions, the modal linewidth exhibits a parabolic dependence with the mode optical frequency. The minimum linewidth follows a Schawlow-Townes behavior with a rebroadening at high power. In addition, the slope of the parabola is proportional to the RF linewidth of the laser and can therefore provide a direct measurement of the timing jitter. Such a measurement could be easily applied to mode-locked semiconductor lasers with a fast repetition rate where the RF linewidth cannot be directly measured.