Volume 66, number 2,3 OPTICS COMMUNICATIONS 15April 1988 OBSERVED SINGLE-MODE RESONANCE IN A MULTIMODE FIBER GENERATOR J.-L. de BOUGRENET de la TOCNAYE Groupe Oprlque et Systemes de Commumcafion, ENS. T de Bretagne, B.P. 832, 29285 Brest, France P. PELLAT-FINET, M. BONDIOU Groupe d’Etudes Signaux et SystBmes. UmversitP de Toulon et du Var, 83130 La Garde, France and M. ALGER Lehrsruhlftir Nachrichtentechmk, Technische Universitiit Miinchen, Posffach 202420, 8000 Miinchen, Fed. Rep. Germany Received 28 December 1987 We give here some experimental results concerning the resonance properties of a weakly multimode fiber generator, where a photorefractive BGO crystal is used as a light amplifier within an optical fiber ring resonator. 1. Introduction Single mode excitation of multimode fibers is of great interest for the measurement of individual or mutual mode fiber parameters. Several techniques are commonly used to perform selective mode filtering of multimode fibers [ 1,2]. These methods are generally passive. The optical ti- ber is used only as a passive waveguide and thus the excited modes are closely related to the character- istics of the excitation source. By using the optical fiber in a feed-back set-up with compensation of the resonator loss by the means of a light amplifier inserted inside the resonator field, it is possible to create an original coherent source whose modal characteristics depend only on the cho- sen fiber characteristics, Fig. 1. Optical set-up ofthe BGO multimode tibergenerator. The applied electric field E is about 8 kV/cm, the pump beam comes from an argon ion laser (50 mW/cm’ at 541.5 nm) and the BGO sample is 5 mm thick and used in the usual amplification contig- uration [ 51. 2. Description of the optical resonator The chosen optical resonator is a weakly guiding optical fiber of revolution geometry, about 40 cm length, set-up in an optical feed-back configuration (fig. 1). In the case of using a fiber with a parabolic-graded index profile, the complex amplitude of scalar line- arly polarized modes LP,,,, , (tubular modes) is given by [ 31 0 030-4018/88/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division) 97