OPTICAL SWITCH MATRIX COMPRISING AN IMPROVED LEAD PATTERN

Abstract: The invention pertains to an optical device comprising a cascade of optical switches, elements (5) for driving the switches, bond pads (6) for connecting the device to peripheral equipment, and leads (7) for electrically connecting the elements to the bond pads, wherein, in top view, at least part of each lead (7) runs both substantially parallel to at least one adjacent lead (7) and obliquely with respect to the central longitudinal axis (8) of the device. The device of the present invention is more compact than cnventional devices.

Description: OPTICAL SWITCH MATRIX COMPRISING AN IMPROVED LEAD PATTERN The invention pertains to an optical device comprising a (preferably symmetrical or substantially symmetrical) cascade of optical switches, elements (numeral 5 in the Figure) for driving the switches, bond pads (6) for connecting the device to peripheral equipment, and leads (7) for electrically connecting the elements to the bond pads. Such an optical device is known from W.H.G. Horsthuis et al., "Packaged Polymeric 1x8 Digital Optical Switches," Proceedings European Conference on Optical Communication '95, pp. 1059-1062. This publication concerns a solid-state optical 1x8 switch consisting of a 3-staged cascade of 7 1x2 switches. Each of the switches comprises two heater elements for driving the switches. To facilitate connections to the outside world, the heater elements are connected to bond pads by means of leads. In turn, the bond pads are connected to the package of the switch, which has been designed to comprise a standard (48-pin) DIP connector, allowing direct Printed Circuit Board (PCB) mounting of the component. As a matter of course, the leads must be electrically isolated from one another and the lead pattern has been designed accordingly (Figure 2 of W.H.G. Horsthuis et al.). However, in the technical field at hand a constant need exists for size reduction without impairing the high quality (in terms of crosstalk, extinction, reliability, etc.) of the optical device. Also, a reduction of the variation in length of the leads is desirable, since the electrical resistance of the leads is commensurate with this length. Reduction of the length-variation will result in a reduction of the variation, e.g., in the voltage or temperature of the elements for driving the switches. Accordingly, it is an object of the present invention to significantly reduce the real estate or area occupied by the lead pattern and to decrease the variation in length of the leads. This object is achieved in the optical devices described in the first paragraph of this application, in which devices, in top view, at least part of most of the leads (7) (and preferably of each lead (7)) runs both substantially parallel to at least one adjacent lead (7) and obliquely with respect to the central longitudinal axis (8) of the device. Surprisingly, it was found that the size of the lead pattern, and hence the size of the device comprising the lead pattern, can be reduced by more than 25%, in some instances even by more than 40%. Further, the variation in length of the leads can be reduced by more than 35%. Apart from the fact that small devices are generally preferred in the technical field at hand, the size reduction also allows manufacturing more components on a single wafer or, especially for very complex structures; a reduction of the design restrictions resulting from a limited wafer size. Also, the invention solves a problem frequently encountered in larger cascaded thermo-optical devices/switches. The thermo-optical effect can induce large refractive index changes at low drive voltages (e.g., 5V). However, the heat generated in the elements for driving the switches should be sufficient to locally raise the temperature several tens of degrees Celsius and, consequently, a fairly high current is required (after all, for a given drive voltage, the generated heat is proportional to the electrical current). Owing to the efficient use of real estate or area suggested by the present invention, heat generation in the leads can be reduced, and power consumption of the device kept low, by using leads with a larger width (that would not fit in a conventional lead pattern) and, consequently, a low electrical resistance. In other words, the present invention enables the creation of an optimal balance between compactness and power consumption (heat generation) in thermo-optical devices. An integrated optical device may be built up, e.g., as follows. Underneath the waveguiding structure there is a support, e.g., a glass or silicon substrate. On the substrate the following successive layers can be identified: a lower cladding layer, a core layer (guiding layer), and an upper cladding layer. The cladding material may be glass or a polymeric material. Said cladding layers have an index of refraction lower than that of the core layer. The core layer, which comprises the actual waveguiding design, may be made of inorganic or polymeric material. Polymeric optical devices are preferred. Their production will generally involve applying a solution of the polymer used as the lower cladding to a substrate, e.g., by means of spincoating, followed by evaporating the solvent. Subsequently, the core layer, and the upper cladding layer, can be applied in the same manner. On top of the upper cladding the elements for driving the switches will be placed, e.g., by means of sputtering, chemical vapour deposition, or evaporation and standard lithographic techniques. For fixation and finishing a coating layer may be applied on top of the entire structure, so as to allow better handling of the device. Alternatively, instead of a coating layer a glue layer may be used for fixation, after which the total structure can be finished by placing an object glass on it. When making all- polymeric layered waveguide structures, it may be advantageous to apply the individual layers in the form of cross-linkable polymers. These are polymers which comprise cross-linkable monomers or polymers which comprise so-called crosslinkers such as polyisocyanates, polyepoxides, etc. This makes it possible to apply polymers on a substrate and cure the polymer, so that a cured polymeric network is formed that does not dissolve when the next layer is provided. The elements for driving the switches, the leads, and the bond pads will generally be made of a thin-film electric conductor, usually a thin metal (deposited) film. Suitable conductors are Ni/Fe, Ni or Ni/Cr (for thermo- optical devices, the latter two are preferred). Other suitable examples are noble metals, (such as gold, platinum, silver and palladium), aluminium, and those materials known as transparent electrodes, e.g., indium tin oxide. For thermo-optical devices according to the invention it is preferred that the leads are plated, so as to decrease electric resistance. Advantageous compositions are, for instance, nickel leads (having a thickness of a few hunderd nanometers) with a nickel plating (having a thickness of 3 to 4 micrometers) or nickel leads with a gold plating. Devices according to the invention can be used with advantage in optical communications networks of various kinds. Generally, the optical components either will be directly combined with other optical components such as light sources (laser diodes) or detectors, or they will be coupled to input and output optical fibres, usually glass fibres. Preferred embodiments of the devices of the present invention are 1xN and (intergrated) MxP switches (N being an integer equal to or greater than 4, preferably 8, 9, 16, 27, or 32, M and P being integers equal to or greater than 2, preferably 3, 4, 8, 9, 16, 27, or 32) preferably comprising a cascade of 1x2, and/or 1x3 switches, preferably mode-sorting adiabatic Y-junction and Ψ-junction switches, respectively. As will be clear from the above text, a cascade of switches comprises switches (e.g. 1x2 and/or 2x1 switches) arranged in series. For more details on the lay-out, characteristics, and results of the basic 1x2 switches building the preferred 1xN switch of the present invention, reference may be had to H.M.M. Klein Koerkamp et al., "Design and fabrication of a pigtailed thermo-optic 1x2 switch," Proceedings of Integrated Photonics Research, San Francisco, 1994, pp. 274-276. For more details of the basic 1x3 switches, reference may be had to K. Propstra et al., "First thermo-optic 1x3 Digital Optical Switch," 8th European Conference on Integrated Optics, Stockholm, 1997, PD3/1 - PD3/4. Given the comparatively small size of the devices according to the invention, they can be put to advantageous use in optical switch matrices. Such matrices may comprise a large number, e.g., 16 or 32, of the devices according to the present invention. Within the framework of this invention the "central longitudinal axis of the device" will usually be the axis of symmetry or a line parallel to the input waveguide channel of a switch in the first stage (1) of the cascade (i.e., the line marked "8" in the Figure). The angle between the oblique part of the leads and the central longitudinal axis depend on the specific device. However, angles ranging from 0.5° to 10°, and especially 1° to 4° are preferred. Further, it is preferred that the device according to the invention comprises clusters of at least four and preferably at least eight (substantially) parallel and oblique leads, because with such clusters the advantages of the invention are all the more noticeable. "Elements for driving the switches" are any means suitable for driving the switches at hand. As will be clear from the above description, for electro- optical switches the elements are electrodes capable of generating an electrical field in the switch. For thermo-optical switches the elements are, e.g., heaters that transform an electrical current to the required heat. Further, the terms "1xN switches" and, e.g., "1x2 switches" also comprise Nx1 and 2x1 switches, respectively. It should be noted that P. Granestand, et al., "Pigtailed Tree-Structured 8x8 LiNb03 Switch Matrix with 112 Digital Optical Switches," IEEE Photonics Technology Letters, Vol. 6, No. 1 , January 1994, discloses a lead pattern with oblique and non-parallel leads. The Figure shows an unlimitative example of a lead pattern (with an aspect ratio of 1 :10; in reality, the width of each of the leads in constant) for a cascaded 1x16 optical switch comprising 4 stages and 15 1x2 switches (the first stage (1) consists of one 1x2 switch, the second stage (2) consists of two 1x2 switches, the third stage (3) consists of four 1x2 switches, and the fourth stage consists of eight 1x2 switches). In spite of the addition of the fourth stage, which stage comprises a huge amount of leads in comparison with the other stages, the overall width of the device including the lead pattern is 7 mm and the length is 61.5 mm, which dimensions are essentially equal to the dimensions of the device according to the above- mentioned publication of W.H.G. Horsthuis et al. An additional advantage resides in that comparatively short devices exhibit a comparatively low insertion loss. Further, the difference in the length of the leads is reduced by 35.6%.

Claims: CLAIMS Optical device comprising a cascade of optical switches, elements (5) for driving the switches, bond pads (6) for connecting the device to peripheral equipment, and leads (7) for electrically connecting the elements to the bond pads, characterised in that, in top view, at least part of most of the leads (7) runs both parallel or substantially parallel to at least one adjacent lead (7) and obliquely with respect to the central longitudinal axis (8) of the device. Optical device according to claim 1 , characterised in that, in top view, at least part of each lead (7) runs both parallel or substantially parallel to at least one adjacent lead (7) and obliquely with respect to the central longitudinal axis (8) of the device. Optical device according to claim 2, characterised in that the switches are thermo-optical switches. 4. Optical device according to claim 3, characterised in the leads are plated. 5. Optical device according to any one of the preceeding claims, characterised in that the switches are 1x2 and/or 1x3 switches. 6. Optical device according to claim 5, characterised in that the switches are mode-sorting adiabatic Y-junction or Ψ-junction switches. 7. 1xN or MxP switch according to any one of claims 1-6.

Inventors: HOEKSTRA TSJERK HANS, HOEKSTRA, TSJERK, HANS
Applicants: AKZO NOBEL NV, HOEKSTRA TSJERK HANS, AKZO NOBEL N.V., HOEKSTRA, TSJERK, HANS
Application year: 1998
Main classification: G02F1
IPC codes: G02F1