Local Target Cross Sections in Optical Band OTAKAR WILFERT, ALEŠ PROKEŠ Institute of Radio Electronics, Brno University of Technology, Purkyňova 118, 612 00 Brno, CZECH REPUBLIC Abstract: Paper deals with a mono-static experimental system for measuring of the local relative target cross sections (LRTCS) in optical band λ = 1.06 µm by using YAG: Nd 3+ laser. Requirements analysis of the optical receiver is presented. The way of the LRTCS measuring and their distribution on the airplane surface at the distance of 200 m from laser transmitter is shown. Key-Words: relative cross section, reflectivity, optical receiver, avalanche photodiode, neodymium laser 1 Introduction Clarification of the LRTCS and its definition is presented in [1, 2], where the difference between the target cross section used in microwave radar system and the target cross section used in laser radar system is also emphasized. The aim of this paper is to introduce readers to the methodology of the LRTCS experimental determination and to the experimental measuring system that was used. The term “local” means that the target (in our case the military training airplane) intercepts the entire beam and therefore as an extended target is classified [3]. Measuring of the relative value of the local target cross section is preferred to the measuring of absolute value one due to its easier determination. It is assumed that in case of an extended target the beam cross section at the target position is much smaller than the total target projection to the plane perpendicular to the direction of irradiation. For this reason the LRTCS is used with definition given by i i bs σ σ σ ′= , (1) where σ i is the absolute local cross section of the target (airplane) and σ bs is the absolute cross section of the reference surface (bead screen) placed perpendicularly to the laser beam at the same distance as the target and irradiated with the identical intensity. When the optical receiver works at the linear region of the transition characteristic, (1) can be expressed as , R i i Rbs v v σ ′= , (2) where v R,i and v Rbs are voltage levels of received signals from target and reference surface respectively. 2 Description of Measuring Chain Measuring equipment has been composed of the optical transmitter, beam splitter, target, optical receiver and two-channel storage oscilloscope as shown in Fig. 1. The total measuring chain creates so-called mono-static system. The optical transmitter consists of the neodymium laser (YAG:Nd 3+ ) and optical system (transmitted aperture diameter of 30 mm). The laser works in the pulse mode (pulse width of τ P = 20 ns). OT BS OR v M v R SO A B MT Fig. 1 Measuring chain (mono-static system) (OT – optical transmitter, BS – beam splitter, MT – measuring target, OR – optical receiver, SO – storage oscilloscope) The energy of laser radiation has been checked by using auxiliary monitoring system shown in Fig. 2. It is assembled from the beam splitter, PIN photodiode and high impedance amplifier. Output voltage v M is measured by the storage oscilloscope and used for triggering. Divergence of the laser beam is θ L = 1 mrad and average pulse energy is E L = 23,4 ± 2% mJ. 3 Analysis of the Optical Receiver Performance Optical receiver shown in Fig. 3 consists of the receiver branch and eye branch. Photocurrent of the avalanche photodiode (APD) is amplified and converted to the voltage v R in the transimpedance amplifier (TIA). Proceedings of the 5th WSEAS Int. Conf. on Signal Processing, Computational Geometry & Artificial Vision, Malta, September 15-17, 2005 (pp244-246)