In this study we propose and demonstrate a novel technique for measuring distance with high definition three-dimensional imaging. To meet the stringent requirements of various missions, spatial resolution and range precision are important properties for flash LIDAR systems. The proposed LIDAR system employs a polarization modulator and a CCD. When a laser pulse is emitted from the laser, it triggers the polarization modulator. The laser pulse is scattered by the target and is reflected back to the LIDAR system while the polarization modulator is rotating. Its polarization state is a function of time. The laser-return pulse passes through the polarization modulator in a certain polarization state, and the polarization state is calculated using the intensities of the laser pulses measured by the CCD. Because the function of the time and the polarization state is already known, the polarization state can be converted to time-of-flight. By adopting a polarization modulator and a CCD and only measuring the energy of a laser pulse to obtain range, a high resolution three-dimensional image can be acquired by the proposed three-dimensional imaging LIDAR system. Since this system only measures the energy of the laser pulse, a high bandwidth detector and a high resolution TDC are not required for high range precision. The proposed method is expected to be an alternative method for many three-dimensional imaging LIDAR system applications that require high resolution.
In this paper, a new method to improve the SNR by temporal filtering method in LADAR system using two Geiger-mode
avalanche photodiodes (GmAPDs) is proposed. The new method is implemented by using two GmAPDs with
beam splitter and employing AND process to their ends. Then, timing circuitry receives the electrical signals only if each
GmAPDs generates the electrical signals simultaneously. Though this method decreases the energy of a laser-return
pulse scattered from the target, it is highly effective in reducing the false-alarm probability because of the randomly
distributed noise on the time domain. Then it needs not any image processing steps. The experiments are performed to
prove the advantage of the new method proposed with varying the time bin size. The experimental results represent that
the improvement of SNR.
A laser radar (LADAR) system with a Geiger mode avalanche photodiode (GAPD) is used extensively due to its high
detection sensitivity. However, this system requires a certain amount of time to receive subsequent signals after detecting
the previous one. This dead time, usually 10 ns to 10 μs, is determined by the material composition of the detector and
the design of the quenching circuits. Therefore, when we measure objects in close proximity to other objects along the
optical axis using the LADAR system with GAPD, it is difficult to separate them clearly owing to the dead time problem.
One example for that is a case of hidden objects behind partially transparent blinds. In this paper, we suggested a
modified LADAR system with GAPD to remove the dead time problem by adopting an additional linear mode avalanche
photodiode (LAPD) as a complementary detector. Because the LAPD does not have dead time while still maintaining
relatively low detection sensitivity, the proposed system can measure an object placed within the dead time with high
detection sensitivity. Light is emitted from the pulsed laser of a light source and is delivered into a fast photodiode to
generate a start signal. Most of laser pulses are directed onto the target and scattered from the surfaces of targets. The
scattered light in the field-of-view of the system is divided by a polarizing beam splitter, after which it becomes incident
to two different types of APDs, the GAPD and the LAPD. The GAPD receives the signals from the target with high
sensitivity, and the signals scattered in the dead time zone are then detected by the LAPD. The obtained signals are
analyzed at the same time. In this way, the signals scattered from objects placed within the dead time can be
distinguished clearly.
A method to determine correct focus in direct detection laser radar system using Geiger-mode avalanche photodiode
focal plane array (GmAPD-FPA) is proposed. It is implemented by laser pulses with controlled beam diameter and
energy on a distant target. And the time-of-flight (TOF) of laser pulses are obtained for each pixel in GmAPD-FPA.
With multiple laser pulses, time correlated single photon counting (TCSPC) is carried out to obtain target detection
probability. Using target detection probabilities of each pixel, the photon distribution on GmAPD-FPA is acquired. The
condition to determine correct focus is the minimum photon distribution in GmAPD-FPA. In theory part, the range of
laser pulse energy is decided. The experiments are carried out with commercial 1x8 pixel GmAPD-FPA. The
experimental results show that the focus position is founded using this method and a spatial resolution of a laser radar
system is improved where the 1x8 pixel GmAPD-FPA is located in focus position.
Direct-detection three-dimensional imaging laser radar system using Geiger-mode av.alanche photodiode (GAPD) is
investigated in order to acquire three-dimensional images of objects at a long distance (more than 100m). Due to
extremely high sensitivity of the GAPD, a laser radar system using GAPD is not only advantageous in terms of ranging a
distant object but also in detecting a target screened by a sparse obstacle located in front of it. Both laser radar systems
using a single-pixel GAPD and 1x8-pixel GAPD focal plane array as detectors are built up and analyzed. Passively Q-switched
microchip laser is used as a laser source and a compact peripheral component interconnect system, which
includes a time-to-digital converter (TDC), is set up. With both the GAPD having short dead-time (45ns) and the TDC
functioning multi-stop acquisition, the system operates in a multi-hit mode. Three-dimensional images taken by the laser
radar systems are shown. Both the single-shot precision and the dependence of the precision on the effective number of
laser pulses are shown. Range walk reduction and autofocus techniques are proposed and demonstrated experimentally;
they improve the accuracy and transverse spatial resolution of the laser radar system, respectively.
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