High accuracy ranging with Yb 3+ -doped fiber-ring frequency-shifted feedback laser with phase-modulated seed V.V. Ogurtsov a , L.P. Yatsenko a , V.M. Khodakovskyy a , B.W. Shore b, * , G. Bonnet b , K. Bergmann b a Institute of Physics, Ukrainian Academy of Sciences Prospect Nauki 46, Kiev-39 03650, Ukraine b Technical University of Kaiserslautern, 67653 Kaiserslautern, Germany Received 25 October 2005; received in revised form 13 January 2006; accepted 26 April 2006 Abstract This paper demonstrates that a frequency-shifted feedback laser, when seeded by a phase-modulated narrow-band radiation field, is a powerful tool for distance measurements to accuracy better than 10 lm and resolution better than 100 lm, for distances of a few meters. In such measurements the unknown distance forms one arm of a Michelson interferometer, in which the intensity of the output signal is modulated at the phase-modulation frequency of the seed. The amplitude of the output-signal modulation exhibits a resonance for every distinct signal delay, i.e. for each distinct distance within the laser spot on the target. The use of a phase-modulated input seed allows one to use a very narrow-bandwidth filter when measuring the return signal. The results reported in this paper are in excellent agreement with previous theoretical predictions [L. Yatsenko et al., Opt. Commun. 242 (2004) 581] for the resolution limit and high signal-to-noise ratio for this new technique. Ó 2006 Elsevier B.V. All rights reserved. PACS: 42.55.f; 42.60.Da; 42.55.Ah Keywords: Lasers; Laser ranging; Frequency-shifted feedback 1. Introduction Laser based techniques – variants of radio-frequency RADAR – find increasing use for a variety of ranging applications, including measurements of distances with accuracy approaching or less than optical wavelengths. Such distances may be meters or kilometers, as is the case with LIDAR, or millimeters or centimeters, as is the case with typical workbench interferometers. We here describe, and demonstrate, a new technique for accurate distance measurements using a frequency-shifted feedback (FSF) laser seeded by a phase-modulated (PM) laser. As we will note, FSF lasers differ significantly from ordinary lasers, and PM seeding has particular advantages when the laser is used for ranging. In all FSF lasers, a feedback mechanism introduces a fixed frequency shift D AOM with each passage of a wave packet around the laser cavity (during the round-trip time s r ). After its first demonstration by Kowalski et al. [1], FSF lasers were further studied and developed by Berg- mann and co-workers [2]. Later Nakamura et al. further studied such systems and demonstrated the use of FSF lasers for ranging [3,4]. They pointed out that the output of a frequency-shifted feedback (FSF) laser can be regarded as a succession of chirped pulses that can be used to measure distances, in a manner akin to chirped RADAR. In such operation one measures a frequency off- set proportional to the time delay s =2h/c between a probe and a reference pulse, and from this one evaluates the incremental distance h. 0030-4018/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2006.04.070 * Corresponding author. E-mail address: bwshore@alum.mit.edu (B.W. Shore). www.elsevier.com/locate/optcom Optics Communications 266 (2006) 266–273 (4)