Influence of target surfaces and speckles for eye-safe, scannerless, imaging laser radar

Due to the lack of fast switchable and sufficiently sensitive detectors for the near infrared (NIR) wavelength, the development of eye-safe, imaging, scannerless laser radar systems based on gated viewing is challenging. However, one basic approach is to use a pulsed Nd:YAG-OPO laser with 1574nm wavelength as transmitter and to gate an InGaAs-FPA camera with an electro-optical modulator (EOM) as receiver. With the EOM (a Pockels cell) the original integration time of the camera of 16ms is trimmed to the minimal exposure time (gate width) of 18ns. It is implemented in an adapted optics with a focal length of 400mm. To guide the light with low divergence through the crystal of the EOM with a diameter of 5mm and a length of 80mm, the f-number was chosen to 26. Since the aperture of the EOM causes optical limitations, it is a decisive factor for the optical performance of the system. Additionally, because of the NIR laser light in conjunction with different target surfaces and the particularities of the receiver optics, artifacts in the laser radar images arise. In this paper we discuss the combination of target surface properties (e.g. reflectivity) and speckles, which cause a degradation of the quality of the range gated intensity images of the presented system. Since this leads to a reduction of the possible achievable contrast ratio, it affects directly the possible distance and depth resolution. The correlation of the target surface structure and the system performance will be analyzed, which is necessary to improve the current system design leading to enhanced system performance.