Lidar is an important equipment for the accurate detection of underwater targets. In lidar measurements of underwater target, the energy and width of the laser pulse will affect the results when using PMT as receiver, which has been proved by data processing and analysyses. To avoid the effect and enhance the accuragy of underwater, many methods such as controling the energy of the laser pulse, reducing the pulse width are taken. The experiment results of underwater target measurement are discussed.
Lidar is an important equipment for the accurate detection of underwater targets. In this paper, a compact
and remote-controlled lidar system is described. The system contains four modules which are Q-switch
Nd:YAG pulse laser module, photoelectric detector module, data collector module and remote-controlled
module. All of the modules are put into a hermetic container. The lidar system can be in operation under the
water and carried by an underwater carrier such as ROV (Remotely Operated Vehicle). The operator controls
the lidar and downloads the data from the system through a fiber. The system can automatically eliminate the
return signal due to the surface wave. The system has been used in underwater bubble detection. A brief
description of the lidar system and its operation is present in the paper. The experimental results of underwater
bubble measurement are discussed.
Laser will attenuate during its propagation in water and also be backward scattered by water when it is used to detect
bubbles in the ocean. Meanwhile backward scattering intensity of the bubbles is feeble, its dynamic range reaches to the
order of 6, which saturates PMT and its post-treatment circuit. Timely gating system is used to solve the problem. The
system contains pulsed laser and gating PMT receiver. The wavelength of the laser is 532nm, with pulse width of several
nanometers. Its operational delay is matched with the time period between laser traveling forward and back after
scattered by the target. By doing this, the light scattered by other object is eliminated, dynamic range of the signal
reduces, and consequently SNR increases. In order to avoid Signal Induced Noise(SIN), we choose PMT R1333 having
no HA coating. TTL logical level, which is used as gating signal, controls the first dynode voltage of PMT to implement
gating. Gating speed is about 100ns, of which the width is tunable. By carefully designing the electronic system, SNR is
eliminated to a level as low as possible, and the output signal of PMT is fast integrated in order to reduce the influences
of signal induced by opening the gate.
In the detection of underwater object by air-born lidar, a narrow laser beam reflected by the sea bottom or the underwater
target transfers in the ocean and refracted through the wavy sea surface, then received by the receiver carried by plane
over the sea surface. Because of the scattering by the suspended particles and the effect of the wavy sea surface, the
received signal will to some extent blurred, which will greatly decrease the image quality of the lidar system. Computer
simulation by Monte Carlo method was employed to describe the process of narrow laser beam transferring in the ocean
and refracted by the wavy sea surface. As the result, the intensity distribution on the receiver can be got as well as the
property of image blurring after transferred thought the ocean water and refracted through the wavy sea surface. The
relations between this property and the optical property of seawater, the wind speed above the sea surface were analyzed.
Near Infrared Spectroscopy (NIRS) has been widely used to access the brain functional activity non-invasively. A
portable, multi-channel, continuous-wave (CW) NIR topography instrument we designed to measure the concentration
changes of each hemoglobin species and map cerebral cortex functional activation. The probe of instrument consists of 4
LEDs operating at three-wavelength (735 nm & 805 nm & 850 nm) surrounded with 10 photodiodes mounted on a
flexible PCB with the interoptode distance up to 2.88cm. On the basis of the modified Beer-Lambert law, the ratios of
optical density changes in a ftilly oxygenated and deoxygenated state are determined by varying blood volume and
hemoglobin oxygenation state in model experiment. The average ratios are 0.56. 1.74 and 0.45. Additionally, study on
the phantoms is carried out to investigate the penetration depth (13 mm) of the sensor array.
Developmental dyslexia, a kind of prevalent psychological disease, represents that dyslexic children have unexpected difficulties in phonological processing and recognition test of Chinese characters. Some functional imaging technologies, such as fMRI and PET, have been used to study the brain activities of the children with dyslexia whose first language is English. In this paper, a portable, 16-channel, continuous-wave (CW) NIRS instrument was used to monitor the concentration changes of each hemoglobin species when Chinese children did the task of phonological processing and recognition test. The NIRS recorded the hemodynamic changes in the left prefrontal cortex of the children. 20 dyslexia-reading children (10~12 years old) and 20 normal-reading children took part in the phonological processing of Chinese characters including the phonological awareness section and the phonological decoding section. During the phonological awareness section, the changed concentration of deoxy-hemoglobin in dyslexia-reading children were significantly higher (p<0.05) than normal-reading children in the left ventrolateral prefrontal cortex (VLPFC). While in the phonological decoding section, both normal and dyslexic reading children had more activity in the left VLPFC, but only normal-reading children had activity in the left middorsal prefrontal cortex. In conclusion, both dyslexic and normal-reading children have activity in the left prefrontal cortex, but the degree and the areas of the prefrontal cortex activity are different between them when they did phonological processing.