Abstract
Optical coherence tomography is nowadays an established imaging technique in Ophthalmology, with a key role on early detection of macular diseases, benefiting from the tremendous evolution in principles and technological developments of the last 20 years. In this paper the most important physical principles behind time-domain, spectral-domain and Fourier-domain OCT will be presented, along with examples of applications in different imaging fields, emphasizing the limitations of current systems, their performance parameters, as well as the challenges for the future within this field of development.

Abstract
We model the photocurrent of a depth-scan (A-scan) from an optical coherence tomography (OCT) system, using a linearly polarized thermal source, as an electronically filtered doubly-stochastic Poisson process, and we obtain its time-varying second-order statistics. We derive an expression for the instantaneous signal-to-noise ratio (SNR) of time-domain OCT which is more general than the previously reported time-averaged expressions. Unlike previous work, our analysis combines shot noise, due to detection of coherent light, and photon excess noise, due to fluctuations in the optical field, into a single noise source that we refer to as the photoelectron noise. Similar to previous results, our SNR is dominated by a term similar to shot-noise when the reference optical power is low and by a term similar to photon excess noise when the reference power is high. © 2005 SPIE and OSA.

Abstract
The main objective of this work is to investigate the correlation between meditation stages and the hypnagogic stages of sleep and awakening. It reports the study on the brain electric activity during Yôga Svásthya meditation stages. These data were obtained by means of scalp electroencephalograms (EEGs), and confronted with the stages of meditation and hypnagogic EEG stages. The latter were based on established criteria through the study of the duration of individual occurrences of each of the nine considered hypnagogic EEG stages. Analysis made concerns time, frequency, power and signal amplitude. The results show that meditation stages reach only the first two hypnagogic EEG stages, and the spectral analysis of the EEG confirmed a dominance of rapid rhythms (beta) while the presence of lower ones (delta and theta) are not significant.

Abstract
In optical coherence tomography (OCT), it is often assumed that the signal-to-noise ratio, at higher optical power regimes, is limited by beat noise. Nonetheless it was often noticed that the limiting value could not be experimentally achieved, despite minimizing the stray reflectance and increasing the optical source effective bandwidth. In this work we present a new model for the noise in a balanced OCT configuration, which accounts for the limited spectral response of fiber directional couplers, as well as imperfections in the photo-detectors. As a consequence, and due to different spectrum content between the two balanced-OCT outputs, excess photon noise is larger than previously considered. The model also allows for the determination of SNR optimized parameters for maximum performance of the balanced system, and can be applied to any coupler or broadband source. Results for optimized and non-optimized configurations, for different balanced detection configurations, implemented in an OCT system, are presented and discussed.

Abstract
In optical coherence tomography (OCT). it is often assumed that the signal-to-noise ratio is limited by shot noise. However, a high data acquisition rate and a low target reflectivity require operation under high optical power. Balance detection is used with the aim to bring the noise close to shot noise regime, but this is not all the time achievable in practice. If the balance detection is ideal, the limiting factor is the beat noise. This is proportional to the stray reflectance in the object arm and inverse proportional to the effective noise bandwidth. It was often noticed that the limiting theoretical value for the S/N ratio for a given stray reflectance and optical source bandwidth could not be experimentally achieved. In the present study, we develop a new model where we address this issue by taking into account the limited spectral response of fiber based balanced detection receivers. We show that due to mismatches in the balanced receiver, excess photon noise has a larger contribution than normally expected. with important implications in the maximum achievable SNR. The theoretical model developed leads to a redefinition of the effective noise bandwidth to take into account the non-flat response of the directional coupler used in the balanced stage. The model is capable to explain the limitation of SNR observed in practice when stray reflectances within the interferometer are brought to infinitesimal values. The model guides optimization of parameters in order to maximize system performance. for a given optical source power and directional coupler characteristics. In this paper we present experimental results to validate the theoretical model. Such S/N analysis is paramount in the modem OCT technology. which makes use of wide bandwidth sources in the quest for high-depth resolution.

Abstract
Two and three SLD diodes are grouped together in order to obtain a compounded source of wide band for OCT investigations. One SLD has a two-lobe spectrum around 840 nm and a compounded spectrum is achieved by using SLDs of smaller wavelengths. This has two advantages: (1) the smaller the wavelength the lower the loss of power through the vitreous due to water absorption; (2) medium band, standard SLDs if used bring a more significant reduction to the coherence length than their counterparts, of medium band but centered at longer wavelength. We show that it is possible to obtain high resolution OCT images with an inexpensive, compact, and easy to operate source.