15 August 1999 Ž . Optics Communications 167 1999 67–76 www.elsevier.comrlocateroptcom Highly unbalanced GaAlAs–GaAs integrated Mach–Zehnder interferometer for coherence modulation at 1.3 m m Sabry Khalfallah a, ),1 , Pascal Dubreuil a , Rene Legros a , Chantal Fontaine a , Antonio Munoz-Yague a , Bruno Beche b , Henri Porte b , Rafal Warno c , ¨ Miroslaw Karpierz c a Laboratoire d’Analyse et d’Architecture des Systemes du CNRS, 7, aÕenue du Colonel Roche, 31077 Toulouse Cedex, France b Laboratoire d’Optique P.M.Duffieux, URA CNRS 214, UFR Sciences et Techniques UniÕersite de Franche-Comte, 25030 Besanc ¸on Cedex, France c Warsaw UniÕersity of Technology, Pl. Politechniki 1, 00-661 Warsaw, Poland Received 15 April 1999; accepted 26 May 1999 Abstract A novel integrated III–V semiconductor waveguide modulator specially designed to generate an optical delay of 100 mm is presented. This is achieved by simultaneous propagation of the TE fundamental mode in both of the arms of a highly unbalanced Mach–Zehnder interferometer with a 29 mm length difference between both arms. In order to minimize the losses in this complex curved structure, trenches were etched on the outsides of curved sections. The modulator has a contrast of 34%, a switching voltage of 6 V with a 10 mm long electrode and is suitable for the transmission and multiplexing of signals by coherence modulation of light in an optical fiber network powered by a short coherence source, such as a 1.3 mm superluminescent diode. q 1999 Published by Elsevier Science B.V. All rights reserved. PACS: 42.79.Sz; 42.87.Bg; 42.82.Et; 85.60.Bt Keywords: Coherence modulation; Electro-optic modulation; III–V compound semiconductors; Multiplexing; Mach–Zehnder interferome- ter; Optical delay; Optical waveguide; Superluminescent diodes 1. Introduction Coherence multiplexing is an interesting tech- nique for the simultaneous transmission of several ) Corresponding author. Tel.: q81-3-3402-6231 ext. 2351, fax: q81-3-3470-5103; e-mail: khalfall@cc.iis.u-tokyo.ac.jp 1 Presently at Laboratory for Integrated Micro-Mechatronic SystemsrCNRS-IIS, Institute of Industrial Science, University of Tokyo, 7-22-1 Roppongi, Minato-ku, Tokyo 106-8558, Japan. wx signals by a single optical fiber 1 . This modulation scheme is based on the introduction of optical delays longer than the coherence length of the broadband wx source employed 2 . Demonstrations of the method were first reported using birefringent slabs and bulk wx electro-optic devices 3 , then by the realization of integrated configurations on a single LiNbO chip 3 w x 4,5 . To date, however, the monolithic integration of complex electro-optic circuits requires the use of semiconductor material platforms. We recently pre- 0030-4018r99r$ - see front matter q 1999 Published by Elsevier Science B.V. All rights reserved. Ž . PII: S0030-4018 99 00284-9