Advanced compact 3D LIDAR using a high speed fiber coupled pulsed laser diode and a high accuracy timing discrimination readout circuit

At last year, we have been developing 3D scanning LIDAR designated as KIDAR-B25, which features the 3D scanning structure based on an optically and mechanically coupled instrument. In contrast with previous scanning LIDARs, vertical scanning is realized using two stepping motors synchronized with movement and moves in a spiral. From the results of outdoor experiments conducted last year to evaluate and measure the LIDAR performance and stability, we identified some limitations and problems that should be resolved. In the first instance, the samples per second are inefficient for use in detection, object clustering, and classification. In addition, the accuracy and precision of distance at every point is seriously affected by the reflectance and distance of the target. Therefore, we have focused on improving the 3D LIDAR range finding performance, speed of measurement, and stability regardless of environmental variation. Toward the realization of these goals, in this paper, we deal with two improvements compared with previous 3D LIDAR. First, we have stabilized the laser driver, which can drive the peak current up to 190A with a short 4ns pulse width and 55 KHz fire frequency. In addition, the TO-CAN type and low peak power laser diode is replaced with multi-channel fiber coupled diodes with high power (peak power: 60W), satisfactory duty factor (0.01 similar to 0.1%), high coupling efficiency (80%), and optimal beam shape (1 similar to 2mrad). However, due to the TO-220 package type of laser diode, the parasitic inductance and capacitance of the laser diode are increased, and hence the measured pulse width (11ns) from the Si-pin PD with 1.5GHz bandwidth is relatively long compared with the value measured by monitoring the register (4ns) in the laser driver. However, the measured rising edge of the pulse with the Pin-PD is about 2.5ns. Four channels of the fiber coupled pulsed laser diode module are designed and implemented in our advanced 3D LIDAR, and the total maximum pulse repetition frequency can be operated up to 220KHz. Second, single channel constant fraction discriminators (CFDs) based on a lumped element for accurately detecting the timing point in the part of readout circuit are developed and evaluated through experiments with the previous 3D LIDAR KIDAR-B25. We have changed the leading edge discriminator to a CFD, which can detect the exact timing crossing points of inverting and non-inverting (delayed and fracted) signals using a fast comparator. The timing discriminator is composed of a threshold level comparator and a constant fraction discriminator (CFD). This circuit can considerably reduce the random walk errors resulting from variation of the timing detection point due to the amplitude. For the scanning mechanism and optics, the previously designed optical assemblies and scanning mechanical instrument are utilized and can measure a wide range of vertical plane up to +/- 15 degrees with 0.25 degrees angular resolution and the spiral rotation of horizontal plane up to 360 degrees with 0.06 degrees angular resolution (max distant: 50m). Third, regarding system integration, previous embedded microprocessors divided into pre-processing and post-processing module are replaced to a dual-core one chip (TI OMAP-L137) module, which is operated by LINUX on an ARM core and a Real-time OS on a DSP core. In addition, a 10ps resolution TDC that supports LVPECL inputs and measures up to 500 KHz stop signals is built on the embedded system board. The experimental results show that the 3D LIDAR performance of the timing discriminator and pulsed laser diode stability, beam divergence, and maximum pulse repetition rate has been improved compared with the last version of KIDAR-B25. We anticipate that it will be used for detecting and avoiding moving objects in real time toward the realization of autonomous navigation of robots.