Sub-millimeter resolution laser ranging at 9.3 kilometers using temporally stretched, frequency chirped pulses from a mode- locked laser Mohammad Umar Piracha 1a , Dat Nguyen a , Dimitrios Mandridis a , Tolga Yilmaz b , David Gaudiosi b , and Peter J. Delfyett 2a a CREOL, The College of Optics & Photonics, University of Central Florida, Orlando, FL 32816-2700 USA b Raydiance, Inc. 2602 Challenger Tech Court, Suite 240 Orlando, FL 32826 USA ABSTRACT A chirped fiber Bragg grating with a dispersion of 1651ps/nm is used to generate temporally stretched, frequency chirped pulses from a passively mode locked fiber laser that generates pulses of ~1ps (FWHM) duration at a repetition rate of 20MHz with 3.5mW average power (peak power of 175W). The use of a chirped fiber Bragg grating enables the generation of temporally stretched pulses with low peak power so that non-linear effects in the fiber can be avoided. A fiber based interferometeric arrangement is used for interfering a reference signal with the reflected signal from the target to realize a coherent heterodyne detection scheme. In the RF domain, the detected heterodyne beat frequency shifts as the target distance is changed. A round trip target distance of 14km in air is simulated using 9.3km of optical fiber and a resolution of less than a millimeter is observed. Keywords: Lidar, ladar, laser radar, laser ranging, mode-locked laser, chirped fiber Bragg grating, range detection, remote sensing, meteorology 1. INTRODUCTION Light detection and ranging (LIDAR) is a technology that uses scattered or reflected light from a distant target to measure its distance, speed, etc. Lidars have found applications in archaeology, geography, geology, geomorphology, seismology, remote sensing, atmospheric physics, imaging, and military and law enforcement. 1-5 Many different techniques are used for laser ranging (pulsed, frequency modulation, phase shift, time of flight (TOF), etc). 6-8 The time of flight approach is based on the concept of measuring the time interval between the transmitted laser signal and the received echo signal. The round trip distance can then be calculated by the multiplying the velocity of light with the time of flight. To achieve high range resolution at large distances however, extremely small time intervals have to be measured very accurately and high performance integrated circuits and low noise oscillators are required. 8-10 Recently, a time of flight scheme combined with synthetic wavelength interferometry and spectrally resolved interferometry has been presented. 11 In general, the performance of TOF systems is affected by noise generated timing jitter, walk, nonlinearity and drift. Laser ranging can be implemented using a heterodyne technique. The optical power is modulated at a constant frequency and the detected echo signal is photodetected. The difference in phase of the two signals is measured to calculate the target distance. However, the unambiguous range is limited due to the modulo 2π measurement of phase. 8,12 1 Mohammad Umar Piracha.: E-mail: mpiracha@creol.ucf.edu , Telephone: 407 823 0624 2 Prof. Peter J. Delfyett. : E-mail: delfyett@creol.ucf.edu , Telephone: 407 823 6812 Invited Paper Enabling Photonics Technologies for Defense, Security, and Aerospace Applications V, edited by Michael J. Hayduk, Peter J. Delfyett Jr., Andrew R. Pirich, Eric J. Donkor, Proc. of SPIE Vol. 7339, 73390I · © 2009 SPIE · CCC code: 0277-786X/09/$18 · doi: 10.1117/12.820761 Proc. of SPIE Vol. 7339 73390I-1