Optical waveguides – With optical coupler – Switch
Reexamination Certificate
1998-06-19
2001-04-10
Lee, John D. (Department: 2874)
Optical waveguides
With optical coupler
Switch
C385S009000, C385S040000
Reexamination Certificate
active
06215918
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a thermo-optical switch having a layer structure on a substrate and containing, in a waveguide layer, a directional coupler waveguide structure and, above the waveguide layer, a heating electrode configured to complement the form of the coupler structure.
For the transmission of broadband optical signals without prior conversion into electrical signals, it is necessary to utilize cross-connects which may be switched to a state of optical transparency. Such optically transparent switches contain, among others, spatial switches for directing incoming optical signals to selected output fibers. The spatial switches must satisfy the following requirements: low cross-talk, low coupling attenuation, independence of signal polarization, low electric switching power, response times <10 ms. high integration density, low production costs.
2. The Prior Art
In recent years, thermo-optical switches have been developed on a polymer basis because the properties of polymeric waveguides give rise to the expectation that the above-mentioned requirements may be realized with them by way of selective structuring. Thus, polymers have a large thermo-optical coefficient, i.e. a change in temperature causes a large change of their refractive index, combined with low thermal conductivity. These properties result in low switching power for a thermo-optical switch which is below that of a comparable SiO
2
switch by a factor of about 100. Since polymers display very low birefringence they can be used for the fabrication of components which are independent of polarization. Switching times are in the range of milliseconds, 1 to 10 ms being typical.
Moreover, the use of polymer waveguides makes it possible to fabricate spatial switches by relatively simple processes which are well-known from the fabrication of microelectronic components. In addition, polymer technology makes it possible to integrate on a single substrate, as hybrid technology, a plurality of optical components, such as, for instance, III-V-lasers, photo diodes with polymer waveguides, networks and switches. Thus, components with complex functions may be fabricated in a cost-efficient manner.
Proceeding from the above-mentioned state of knowledge, solutions have been sought in recent years, to utilize as many of the above-mentioned advantages of polymers for optical elements as possible. Since the necessary switching power and switching time of thermo-optical elements are primarily dependent upon their thermal properties, i.e., their thermal conductivity, thermo-optical coefficient and the heat capacity of waveguide layer, buffer layers and substrate material, as well as upon the shape and size (dimensioning) of the waveguides and heating electrode, there are known in the art many thermo-optical elements differing in their concrete structure for optimally realizing defined functions.
In Journal of Lightwave Technology, Vol. 7, No. 3 (1989), pp. 449-453, there is described a planar thermo-optical switch in which a polymeric waveguide layer made of polyurethane is arranged upon a PMMA (polymethyl methacrylate) substrate, with a PMMA buffer layer superimposed thereon on which is provided a silver strip conducting electrode as a heating element. At a switching power of 100 mW, typical switching times are 12 ms for on-off switching and 60 ms for off-on switching.
In most thermo-optical switches based on polymer, the waveguides are formed in strips which leads to reduced switching times and switching power. Thus, a digital optical switch (DOS) which is independent of polarization is described in SPIE, Vol. 1560, Nonlinear Optical Properties of Organic Materials IV (1991), pp. 426-433 in which a gold strip electrode is provided on one of the two output branches of a symmetrically structured Y-junction. When a heating voltage is applied to the electrode it realizes an asymmetric effect upon the described switch. The change of the refractive index of the amorphous polymeric material of the waveguide is generally isotropic and, therefore, independent of any polarization as regards light propagating through the structure. The monomodal waveguide made of DANS polymer arranged upon a glass substrate was fabricated by photo bleaching the non-waveguiding areas of the waveguide layer with UV irradiation and is covered by a buffer layer. The switching times of this arrangement are in the millisecond range.
In Proc. 21st Eur. Conf. on Opt. Comm. (ECOC '95—Brussels), pp. 1063-1066, there is also described a Y-shaped waveguide in a polymer-based digital optical switch. In this case the waveguide structure has been realized by photo lithography followed by dry-etching of moats in a silicon substrate, followed by thermal oxidation in water vapor and, thus, creation of a SiO
2
buffer layer, spinning of CYCLOTENE® polymer thereon and covering the polymer layer by a further SiO
2
buffer layer. A titanium thin film electrode is divided and positioned above the two output branches. At a switching power of between 130 mW and 230 mW the extinction coefficient in the heated branch is better than 20 dB. The optical power is then fed wholly through the other—unheated—branch. But even in this technical solution the requisite switching power and switching times are still too high.
In European Patent EP 0,642,052 there is described a polymer-based digital optical switch in a layer structure consisting of a substrate, lower buffer layer, waveguide layer, upper buffer layer and heating element with a Y-shaped waveguide structure, wherein the refractive indices of the two buffer layers are lower than the refractive index of the waveguide layer. Moreover, the refractive index of the buffer layer adjacent to the heating element is lower than that of the lower buffer layer. Ranges covering the contrast of the refractive indices have been disclosed in accordance with parameters desired (optical loss, switching power) for realizing a predetermined function, and the dimensions of the output branches of the waveguide are structured symmetrically or asymmetrically, and the heating elements are also arranged symmetrically (at both output branches) or asymmetrically (at one output branch only). While precise current control is not required for the described arrangement it requires a higher switching power and leads to cross-talk of not more than about −20 dB.
Low switching power is required in a thermo-optical switch described in IEEE Photonics Technology Letters, Vol. 5, July 1993, pp. 782-784. The switch is provided with a Mach-Zehnder-interferometer above the two waveguides of which there are arranged thin-film heating elements. While this optical switch does realize low cross-talk as well, its overall length is about thrice that of a conventional directional coupler.
In connection with a different system of waveguide material there is described in Electronics Letters, Oct. 29, 1981, Vol. 17, No. 22, pp. 842-843, a thermo-optically induced waveguide based upon LiNbO
3
:Ti in which a nickel-chromium electrode is arranged on one section of the waveguide. When a voltage is applied to the electrode the refractive index of the area of the waveguide below the electrode changes thus deflecting the fed-in light.
Also, directional coupler switches with alternating &Dgr;&bgr; are known, as described, for instance, in IEEE Journal of Quantum Electronics, Vol. QE-12, No. 7, pp. 396-401, July 1976. In this case, several electrode sections are arranged upon parallel waveguides made from the previously mentioned LiNbO
3
material. In actual switching conditions the electrodes generate in the corresponding waveguide sections below them, based upon the electro-optical effect, a difference in the propagation velocities of the light of respective alternating signs. If the interactive length between the two waveguides is greater than the coupling length the desired switching state (cross-over or throughput state) may be set by way of the switching power.
A directional coupler with alternating &Dgr;&bgr; is also
Keil Norbert
Nolting Hans-Peter
Yao Huihai
Zawadzki Crispin
Heinrich-Hertz-Institut fuer Nachrichtentechnik Berlin GmbH.
Hormann Karl
Lee John D.
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