Sensors and Actuators A 116 (2004) 91–94 Opto-chemical sensor on the base of two-channel tapered waveguide E.A. Janunts, A.J. Babajanyan, N.L. Margaryan, Kh.V. Nerkararyan ∗ Department of Physics, Yerevan State University, Manoogyan Str. 1, Yerevan 375049, Armenia Received 6 December 2003; received in revised form 28 March 2004; accepted 29 March 2004 Available online 28 May 2004 Abstract Electromagnetic wave propagation in a structure consisting of two parallel waveguide channels is studied. Unique feature of the structure is that width of the first channel does not change along the length while the second one changes essentially. As a result the wave energy transfer between the waveguide channels and interference distribution of the output power in the channels essentially varies at minor change of the refractive index. Estimations show, that in the structure when the length is less than 1 mm it is possible to obtain a considerable light modulation at changes of the refractive index by 4 × 10 -4 . On the basis offered structure it is possible to create an opto-chemical sensor. © 2004 Elsevier B.V. All rights reserved. Keywords: Integrated optics; Opto-chemical sensor; Tapered waveguide; Coupling modes; Modes of interference 1. Introduction The study of the opportunities of an effective control of the light in waveguide channels is one of the primary goals of modern integral optics. The improvement of character- istics of high-speed optical modulators and opto-chemical sensors is connected to the solution of this problem. For this purpose various waveguide structures have been offered. The distribution of the output power can vary essentially at minor change of the medium refractive index. Among them the most perspective have appeared the structures with two channel waveguide modulators [1,2] and Mach–Zehnder type modulators [3]. The former is based on the coupling modes effect [4,5], taking place as a result of tunneling of light between waveguides while the latter is based on the interference of phase-coherent waves with different optical lights [6,7]. We propose a new approach to the creation of a high-speed optical modulator and an opto-chemical sensor based on the wave energy transfer between waveguide modes and inter- ference. Here we are using specific feature of tapered waveg- uide [8–10]. The proposed structure has several advantages over the conventional coupling waveguide systems. It has relatively small dimensions (<1 mm), which essentially en- hances at the switching speed and has a high dept of mod- ulation. In most coupled waveguide systems a considerable ∗ Corresponding author. Tel.: +374-1-577455; fax: +374-1-554647. E-mail address: knerkar@ysu.com (Kh.V. Nerkararyan). attention is paid to the synchronization, for the realization strict conditions are put on the shape and size of structure. One of the advantages of our tapered structure is that syn- chronization is easily achieved here. 2. Theory The explored structure is shown in Fig. 1. Feature of this structure is that the width of the first channel does not change while the second channel width changes essentially. We as- sume that in the second waveguide the distance on which an essential width change takes place is much greater than the wavelength of light. It is known that the wave vector of the waveguide mode (WGM) strongly depends on the wave channel cross-sectional dimensions. Consequently in the second channel of the structure the wave vector of WGM changes essentially. As we see in Fig. 1 in a and c regions the two waveguide widths become equal, hence the wave vectors of WGMs become equal as well. According to the theory of coupled modes [5] at the vicinity of these planes an en- ergy transfer between the waveguides takes place. However in region b, where wave vectors of WGMs differ greatly, the energy transfer between the modes practically disappears. If we trace the path of the wave which enters into channel 1 we can see the following. In region a, a part of the energy transfers into channel 2, in region b two separate waves propagate independently and in region c the waves merge again. As a result we see that the value of the output power of channel 1 depends on the wave coupling efficiency in 0924-4247/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.sna.2004.03.041