Studies of neurovascular coupling (hemodynamic changes and neuronal activation) in the visual cortex using a time-domain single photon counting system have been undertaken. The system operates in near infrared (NIR) range of spectrum and allows functional brain monitoring to be done non-invasively. The detection system employs a photomultiplier and multi-channel scaler to detect and record emerging photons with sub-microsecond resolution (the effective collection time per curve point is ~ 200 ns). Localisation of the visual evoked potentials in the brain was done using knowledge obtained from electroencephalographic (EEG) studies and previous frequency-domain optical NIR spectroscopic systems. The well-known approach of visual stimulation of the human brain, which consists of an alternating black and white checkerboard pattern used previously for the EEG study of neural responses, is applied here. The checkerboard pattern is synchronized with the multi-channel scaler system and allows the analysis of time variation in back-scattered light, at different stimulation frequencies. Slow hemodynamic changes in the human brain due to Hb-HbO<sub>2</sub> changes in the blood flow were observed, which is evidence of the system's capability to monitor these changes. Monocular visual tests were undertaken and compared with those done with an EEG system. In some subjects a fast optical response on a time scale commensurate with the neural activity associated with the visual cortex was detected. Future work will concentrate on improved experimental protocols and apparatus to confirm the existence of this important physiological signal.
This study investigates the feasibility of acquiring fast optical response signals from the peripheral nervous system (PNS) and specifically to obtain knowledge about the sensory response of the median nerve through comparing electrophysiological responses with those obtained with a single photon counting system. Nerve potentials have been well studied so the primary purpose of this investigation is to better understand the conditions required for recording the optical analogue of this signal. Such action potential-correlated optical signals have been termed 'fast optical evoked responses' and their measurement in-vivo has hitherto proved fraught with difficulty. As yet measurement of these signals has been confined to evoked potential studies in the brain and so far there is no repeatable, confirmed procedure for their robust acquisition. In this work it is suggested that perhaps an easier route to acquire these elusive optical signals is through evoked potential studies centred on the PNS as opposed to the brain. Preliminary results suggest it is possible to correlate both data and draw important information from it although the most important contribution of this paper is the principle of directing the search for robust fast optical signals to the peripheral nervous system as opposed to the brain.
For the last decade sensor architectures with embedded fibers found their application in large structure monitoring and proved their capability to replace existing techniques for monitoring of linear strain, temporary or permanent none-uniform strain and load, temperature, vibrations, bending, or complex strain-temperature, vibrations-temperature influences, etc. Such sensor architectures, called smart structures, use different sensing mechanisms, in one of which - fiber Bragg grating (FBG) - is applied as a sensitive element. Because of high sensitivity, absolute measurement ability, possibility to work reliable in adverse environment, such as electromagnetic fields, radiation, extreme temperature, and quick response time, FBGs are object of numerous research of leading laboratories worldwide. Some problems are still remaining in this field, although there have been some ways found to solve part of them. This paper discusses some aspects of different fixing mechanisms of FBG and provides evaluation and comparison of methods of FBG integration in sensor housing or in sensor architecture.
The sensor described here enables measurement of temperature avoiding semiconductor-fiber coupling losses as we have in conventional fiberoptic temperature sensor. Response time of the sensor increases and it is rugged and inexpensive.
Theoretical calculations provided for further elaboration of sensitive elements for fiberoptic sensors of physical values are stated at present paper. Technological possibilities of such element creation and their further application are considered. Theoretical analysis shown that reflection coefficient from fiber cladding gratings is balanced with different types of gratings because of possibility of creation of considerable equivalent periodical change of refractive index, though optical wave amplitude on the fiber cladding less than on the fiber core. Fiber cladding gratings can be formed by holographic method using photosensitive medium and blue-green lasers. The case of corrugated boundary between fiber core and cladding or between fiber cladding and external medium, for e.g. water contaminated by particles of heavy metals, which have influence on refractive index and absorption of waves in fiber is possible.
Some of the optical devices work on the basis of light extension in the dielectric environment with refractive index as a periodical function. The following devices belongs to such class: interference filters on reflection, deep holograms, optic waveguides with Bragg gratings imprinted in the fiber core, etc. Interference filters belong to the classic optical devices and are well known. Deep holograms have no wide application at present time except holographic optic elements, and are studied for the application in holographic memory devices with high density of recording and storage of information in the volume unit of storage environment. Optical fibers with Bragg gratings wide created and applied in 1992 - 1997 now find their application in sensors, in-line Bragg reflectors, optical amplifiers, filters, laser sources, etc. But on the RC dependence on wave length spectrum we can see side maximums near the main one. Amplitude of these side tops is less than main top. These tops will deteriorate optical characteristics of devices because they become noise sources. Question arises: can we remove or decrease essentially their amplitude? In the theory of filters on the surface acoustic waves where we met the same problem this question was solved by the way of electrodes apodization. It was to be expected that provided apodization of space grating will show the same results. Under space grating apodization we understand that grating amplitude is not stable but changes according to the definite functional dependence along one of the axis.