Abstract:
In at least part of the layers of the integrated optical channel waveguide system, such as cladding layers, a passive light guiding layer and an active light guiding layer, an adiabatic layer thickness transition positioned next to a working zone is used to adapt the waveguide structure in the working zones to individually optimized functionality. In this way, the relative layer thickness values as located in the working area, for example a sensor window, a modulation region and a fiber-chip region, can be individually optimized, for example, for maximal evanescent field sensitivity, for minimal modulation voltage and for efficient coupling of light power without the necessity of the optical layer-thickness values elsewhere in the system having to be adjusted. Incorporated in a Mach-Zehnder interferometer this results in an exceptionally sensitive and reliable sensor. Preferably the channel waveguide system is incorporated in an electronic circuitry in which e.g. a phase shift induced by the quantity to be determined is generated.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an integrated optical channel waveguide system and, more particularly to a waveguide system with a substrate and a waveguide layer structure comprising a first cladding layer, a passive light guiding layer deposited on the first cladding layer, an electro-optical light guiding layer on the passive layer and a second cladding layer. 
     2. Description of the Related Art 
     An integrated optical channel waveguide system is known from the article “Fabrication and packaging of integrated chemo-optical sensors” published in the Journal “Sensors and Actuators B 1996, vol. 35-36, pages 234-240. In a system described there, optimization of, for example, the sensitivity of the system located in a working or active zone always influences the functioning of the channel system outside the working zone. In this way the optimization is only restrictedly feasible, which negatively influences the sensitivity of the total waveguide system 
     It is remarked that besides the article cited above similar devices are disclosed in some further documents of which “Integrated optic adiabatic devices on silicon”, published in IEEE Journal of Quantum Electronics, vol. 27 (1991) March no.3, pp. 556-566, discloses a possibility of adiabatically tapering waveguide sections for mode change in order to realise a coupler, a splitter, a multiplexer and a transformer. 
     The article “Integrated optics and new phenomena in optical waveguides”, published in Reviews of Modern Physic, vol. 49, no.2. April 1977 pp.; 361-378 discloses a similar device with the aim for a better integration in optical circuitry with a large number of devices. 
     An article in Journal of Applied Physics, vol. 69 no. 1, June 1991 pp. 7425-7429 discloses an integrated double core layer waveguide structure wherein an upper core layer comprises an adiabatic transition zone in order to examine the difference between adiabatic and non-adiabatic transition of the guided light. This article thus teaches away from optimizing a device through adaption of at least part of the transitions of an integrated waveguide system to reach a sensitive device for sensing or modulation. 
     SUMMARY OF THE INVENTION 
     Because of the thickness reduction of the active light guiding layer beside the working zone, any influence on the channel system outside the working zone is avoided. As a result of the adapted adiabatic design of the layer-thickness reduction, loss of light power in this zone is also avoided or at least reduced. A simple and reproducible coupling between a light fibre supplying the light and/or a light fibre discharging the light and the optical channel system can also be realized. In the case of the preferred design, the light guiding layer comprises an electro-optic light guiding layer consisting of ZnO and a passive light of Si 3 N 4  and the electro-optic light guiding layer exhibits an adiabatic layer-thickness reduction. Especially the adiabatic layer-thickness transition zone in the electro-optic light guiding layer reduces its layer-thickness outside the modulation working zone to at least almost zero. 
     Because the light guiding layer here is built up from an active and a passive layer, the layer-thickness reduction outside the active modulation zone can be performed without any restriction because there the passive light guiding layer will be responsible for the light guiding. Every negative influence on the channel system can therefore be avoided without restricting light-guiding throughout the channel system as such. 
     Conversely the passive light guiding layer can be provided with an adiabatic layer thickness transition zone such that the layer thickness located in the modulation working zone is eventually reduced to zero, and what is more, the space created in this way can be filled up by active light guiding material such as the already mentioned ZnO, by which its sensitivity to the applied voltage there can be further optimized. 
     In a preferential configuration the layer thickness of an existing Si 3 N 4  passive light guiding layer located in a sensor window working zone, is optimized for sensing using the evanescent field tail of the light to be used. In particular the system is designed as a sensor for measuring chemical and/or physical quantities that influence the refraction-index profile probed by the evanescent field of the light used. Especially at this point in the measurement process, use is made of the evanescent field of the light employed and the sensitivity to the quantity to be measured can be increased by making the sensor windows longer, for example by removing the second cladding layer over a greater length. Consequently, no extra properties of the light used are required for enhancing the sensitivity. 
     In a further preferential embodiment, the layer thickness of at least one of the cladding layers present in the electro-optical modulation working zone is reduced, in such a way that the active voltage modulation sorts out maximal effects in the active light-guide material, without causing intensity loss of the light used due to an underlying electric conductive substrate material, and/or a locally introduced upper electrode. To obtain a window for the sensing process, the second cladding layer is also locally completely or at least almost removed down to the passive light guiding layer. Due to the fact that in these configurations the layer thickness of one or both of the cladding layers, which preferably consist of SiO 2 , is reduced at the modulation working zone and similarly is decreased to at least almost zero at the sensor windows working zone, it is possible to optimize the functionality of the waveguide structure, so both optimal sensitivity for evanescent field sensing as well as minimal electric driving voltage modulation can be obtained, without the light confinement, as seen over the whole channel system, being reduced. 
     In a further preferential design the passive light guiding layer is provided with an adiabatic transition zone leading to a layer thickness, which at that location is optimally adjusted to the mode profile geometry of the light-supplying optic fibre. Especially the layer thickness values of both the passive light guiding layer and the first cladding layer are locally optimized to discriminate between TE and TM polarized light in such a way that the TE is optimally coupled in, and optimally transmitted as well, both with respect to the TM polarized light. 
     Optimal coupling can be realized with a minimum loss of light, without the structure of the channel set outside the zone needing to be adjusted, by adapting the layer thickness of the passive light guiding layer at the location where the light is coupled in. Discrimination between TE-polarized light and TM-polarized light and efficient TE-acceptation can be realized by a corresponding design of the geometry of the waveguide structure and of a transition zone in the passive waveguide guiding layer respectively, again also without the layer set-up outside that zone being further negatively influenced. Here, discrimination between TE and TM polarized light with the coupling of the light into the interferometer gives the substantial advantage that the whole interferometer measurement process can be carried out with only TE polarized light by which dispersion in the measuring light is reduced, and a significantly more unambiguous interferometer signal is generated. 
     In a further design a substrate preferentially made of Si is fitted with a V-shaped groove in order to realize a detachable and/or optically adjustable coupling of an optical input fibre and/or output fibre to the light guide channel system. A handy for use transit opening through which simply and with a high degree of precision the input and or output fibre can be inserted and positioned is obtained by attaching a part of the unused V-groove upside down on the V-groove already referred to. 
     Because of the exact dimensions of the V-groove in the Si, which can be very accurately obtained using etching techniques, the fibre placed there can give an optimal and good reproducible coupling of the light from the fiber into the layer in question of the channel system. As a result of the previously mentioned adiabatic thickness transition of the light guiding layer at this location, the thickness of the channel layer there can be optimally adjusted to the input and/or output fibre. Both properties together result into an optimal coupling with respect to reliability as well as to reduced loss of light. A light source equipped with an optical fibre (pigtailed), such as for example a gas laser or a solid-state (such as a diode laser) can therefore be reliably coupled to the channel system. Thereby a simple detachable coupling can be achieved, having the advantage that in the case of any technical failure, either the laser or the waveguide system can be replaced. On the other hand a permanent coupling can be realized in which case often the light loss or variation in the light loss can be further reduced. 
     In another preferential configuration, according to the invention, a light guiding system forms part of a Mach-Zehnder type interferometer. Especially such a Mach-Zehnder interferometer, including in both of its branches a sensor window as well as a modulator provided with electrodes, where one of the sensor windows is shielded against the influence of the quantity to be measured. 
     A Mach-Zehnder interferometer is known from U.S. Pat. No. 5,533,151 or U.S. Pat. No. 5,377,088 and always comprises two arms, where is one arm an external quantity introduces a phase change due to variations of the refractive index. This variation can be extremely accurately measured by comparing this light propagation with the light propagated through the reference arm which cannot be influenced by the measurand. 
     A practical design for a Mach-Zehnder interferometer according to the invention can be designed. This makes the interferometer less sensitive to external influences other than by the measurand. Especially if such an interferometer is incorporated in an electrical circuit for the derivation of the measurand induced phase shift from the situation at which the anti phase signals have identical intensities. Thus a signal is generated, that is even more unambiguous and more insensitive to external disturbances. 
     A practical design of an electrical circuit for that purpose is formulated herein. The electrical circuit is particularly equipped with a signal generator for activating a (couple of) modulation electrode(s) in the system, with a threshold-value switch for converting the modulated analogue output signal into a digital signal, and with a digital signal processor for an accurate determination of the value of the measurand from the modulated output signal. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     On basis of the drawings some preferential designs according to the invention will be described in the following text. For all figures holds that the layer thicknesses in the layer structure are optimized for a wavelength of 633 nm. The drawings show: 
     FIG.  1 : a well-known integrated optical-channel waveguide system, 
     FIG.  2 : a similar system in which, according to the invention, the electro-optical light guiding layer is provided with an adapted adiabatic layer-thickness transition zone, 
     FIG.  3 : a similar system in which, according to the invention, one of the cladding layers is provided with an adiabatic layer thickness transition zone, 
     FIG.  4 : a preferential design in which the passive light guiding layer on an output-side of the system is provided with an adiabatic layer adjustment, 
     FIG.  5 : a cross-section and a top view of the fibre-chip connector, 
     FIG.  6 : a preferential design in which several layers show an adiabatic layer-thickness transition zone, 
     FIG.  7 : a Mach-Zehnder type interferometer equipped with such a system, 
     FIG.  8 : a cross-section of a waveguide structure in which is shown the evanescent field of the light on which the sensor is based, 
     FIG.  9 : an electronic circuit diagram for the detection of interferometer output signals. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     An integrated optic channel waveguide system as schematically represented in FIG. 1 comprises a light guiding structure on top of a substrate  2  preferably made of Si, and a first cladding layer  4  preferable made of SiO 2  having a thickness of about 500 nm, a light guiding layer  6 , here being composed of a passive light-guiding layer  8 , preferably of Si 3 N, with a thickness, for example, about 20 nm, and preferably a ZnO electro-optical light guiding layer  10  with a thickness of about 500 nm, and a second cladding layer  12  preferably also consisting of SiO 2  with a thickness of about 500 nm. Here the system comprises a first working zone  14  with a modulation electrode  18  applied inside a recess  16  in the second enclosure layer, and a second working zone  20  with a sensor window  22 , here also formed by a recess  24  in the second enclosure layer. This latter recess can reach the light guiding layer  6 , as is shown in the figure. Such a layer system is suitable for sensing, utilizing The sensor window  22 , as well as for electro-optical modulation using the modulation electrode  18 . 
     In FIG. 2 the active electro-optical light guiding layer  10 , which also here preferably consists of ZnO and has a layer thickness of for example 500 nm, beside a working zone  14 , is provided with an adiabatic layer-thickness reduction  30  in a layer thickness transition zone  15  on both sides of the working zone  14 . The layer stack here also contains a substrate  2 , a first cladding layer  4 , a passive light guiding layer  8  and an active light guiding layer  10  on top of which a second enclosure layer  12  is found, that either has a uniform thickness, as shown by the dashed line  13 , or can continue as shown at the top right-hand side. On the second enclosure layer an electrode  18  is applied within the working zone. ZnO is especially suitable for electrical voltage modulation because this material both shows a high sensitivity to electrical voltage modulation, as well as an appropriate refractive index as a light guiding layer. The layer-thickness reduction can be taken so far that outside the adiabatic layer-thickness transition zone  15 , no more ZnO is present and only the passive light guiding layer  8  takes care of the light guiding there. Preferable the layer-thickness transition zones begin at the edges of the working zone, which is also determined by the electrode  18 . 
     A preferable design, as illustrated in FIG. 3, shows next to a first cladding layer  4  provided with adiabatic layer-thickness transition zones  32 , a substrate  2 , a passive light guiding layer  8 , preferentially with a uniform thickness and an active light guiding layer  10  that is, as it were, fills up the space that is created by locally thinning the first cladding layer. It should be mentioned here that the geometry of an optical adiabatic layer-thickness transition zone for a light guiding layer such as ZnO layer is certainly not identical with the geometry of a similar adiabatic layer thickness transaction zone in an enclosure layer. The geometry, the slope profile and with that the length of the transition zone are, among other things dependent on the refractive index of the relevant layers and on the refractive index of neighbouring layers. Here the ZnO layer thickness can also reduce to zero as shown in FIG. 3, but that is not necessary. Otherwise this design form can be equipped with an active light guiding layer of uniform layer thickness or the passive light guiding layer can be provided with a suitable layer-thickness transition zone in order to contain the previously mentioned possible difference in the geometry. FIG. 4 shows a design form in which a passive light-conducting layer  8  at a light input side  34  is finished with an adiabatic layer-thickness transition zone  36 , resulting in a layer thickness of about 10 nm. Using this layer-thickness adaptation, an optimal coupling with an input fibre  38  having a nucleus  35  and a cladding  37  can be realized. An optimal coupling, in this case, on the one hand includes a coupling with a minimum loss of light, but certainly also a coupling with an optimal discrimination between TE polarized and TM polarized light supplied from the input fibre. This effect is enlarged by choosing an optimal thickness for cladding layer  4 . In this way the interferometer can work with only TE (or TM) polarized light by which measurement signals with a higher resolution are generated and the signal processing becomes simpler. 
     In FIG. 5 an example of a design is sketched of a fibre-to-waveguide channel system coupling showing respectively a cross-section and a top view of a fibre  38  encapsulated in a V-form  15  groove  40 . The V-groove is made in a Si-substrate  2 , and the fibre is permanently positioned and covered with another piece of the substrate  3 , preferable being a part of the same V-groove that was first used. Then, if necessary, removable, but in general fixed fibre is optically connected at the input side  42  to the channel waveguide system  1 . 
     A preferential design is given in FIG. 6, in which the first passive light guiding layer  8  as well as the active light guiding layer  10  and both cladding layers  4  and  12  are provided with an adiabatic-layer thickness transition zone. All the advantages of each of the separate layer thickness transition zones are combined in this design and can be optimized independently from each other. If it would be advantageous an additional layer, for example a (chemical) separation or adhesion layer, can be applied between the layers already mentioned, for example between the passive and the active light guiding layers. In the illustrated design form there is a sensor window  22  as well as a modulation electrode  18  included. Such an integrated optical channel waveguide can, for example form part of a Mach-Zehnder interferometer system as shown in FIG.  7 . 
     A Mach-Zehnder interferometer as depicted in FIG. 7 is already well-known, for example, from the previously mentioned U.S. patents, and comprises a light-input device, for example an optical fibre, a measuring arm  44  and a reference arm  46  each fitted with a sensor window  22  respectively  22 ′, as well as with an active zone, provided with an electrical voltage modulating electrode  18  respectively  18 ′. Both arms are connected to an optical power splitter or combiner, for example a Y-shaped junction or an multi-mode interferometer (MMI)  48  as is sketched here, with a first output  50  and a second output  52 . Thus a nearly symmetrical system is realized by which its performance can be independent of external influences such as the surrounding temperature and similar causes. The sensor window  22 ′ in the reference arm  46  is preferably shielded against the influence of the external variable to be measured, such as the relative humidity, the composition of gases and liquids, variations of the optical refractive index, temperature variations etc. Using such an interferometer two signals are generated that are, in principle, both influenced to the same extent, except for the phase shift induced by the quantity to be measured. Therefore the measurement is simple and shows a high accuracy. 
     In order to carry out the measurement, use can be made of laser light that directly or by means of an input fibre can be coupled in. As already mentioned it is possible, according to the invention, when coupling in, to select TE polarized light. With the above mentioned adiabatic layer thickness adaptation zone of the light guiding layer and the likewise previously mentioned accurate optical input coupling, the interferometer is exceptionally sensitive. 
     Measurements with such an interferometer would usually be carried out with the input light propagating confined in and around the guiding layer. Using the adiabatic layer-thickness application it is possible, according to the invention to perform measurements with the help of the so-called evanescent tail of the used light. In order to clarify this, FIG. 8 shows how the propagation of the light in the waveguide channel system is divided into a central part  60  inside the light guiding layer  6  and an evanescent field  62  present in an cladding layer  12 . Reducing the layer thickness of the cladding layer  12  by means of an adiabatic layer-thickness zone down to zero, the evanescent field here shows a high sensitivity to external influences and in this way a window for sensing is formed. Because the power ratio between the central part  60  and the evanescent tail  62  is dependent on the layer thickness of the light guiding layer  6 , it is possible to obtain, in the sensor window, a larger part of the light in the evanescent tail and thus an even higher sensitivity for external influences, by means of a for example preceding adiabatic layer-thickness transition zone of the light guiding layer  6 . 
     FIG. 9 depicts a functional diagram of an electrical circuit with which the phase shift imposed by the measuring quantity can be derived from the output signals to the Mach-Zehnder interferometer. The generated optical output signals from the output channels  50  and  52  are first converted into electrical signals by means of photo detectors  61  and  63  and subsequently amplified by the preamplifier  64 . Afterwards, using the threshold value switch  66 , detection takes place at the point at which the anti phased output signals, have identical intensities. The advantage of this method of detection is that it is not interfered with by common fluctuations of the light power from both of the output channels  50  and  52 , regardless of the origin of these fluctuations. Besides this, the sensitivity of the detection for the imposed phase shift is maximal in the previously mentioned detection point. In order to make unambiguous detection possible, during a measurement cycle a time-periodic phase shift is applied with the help of a phase modulator  18 , for example one such as mentioned previously. Depending on the specific design form, one or more detection points can be obtained during the measurement cycle. The time dependence of this time periodic phase shift is linear, and is realized by applying an electric voltage with the same dependence to the phase modulator. In the preferential design in FIG. 7 such a voltage but with a mutually opposite polarity is applied to both electrodes  18  and  18 ′. An electrical waveform generator  68  takes care of generating and making available the mentioned voltage(s). The peak-peak value of the generated voltage waveform is adjusted by a digital signal processing and control unit  70  in such a way that during every measuring cycle the previously mentioned detection point is passed at least once. The time interval between the start of the measurement cycle and the passing of the detection point is an unambiguous and linear measure for the phase shift imposed by the measurand, and by measuring this time the phase shift can be obtained. The sampling of the detection point is quickly completed because the threshold-value switch  66  directly converts the passing of the detection point into a steep slope of a binary signal and with that, the pulse duration of this binary signal is established. The pulse-duration coding makes a simple and highly linear quantification possible by means of a digital time-interval counter  68 . A digital processing and control unit  70  carries out the reconstruction of the measurand induced phase shift on the basis of the quantified sampling times, and delivers a digital output signal. The digital signed processing and control unit  70  is also responsible for the synchronization of the waveform generation  72  with the time-interval counter  68 . The digital signal processing and control unit  70  can also optionally carry out a further final processing of the reconstructed phase shift in order, for example, to increase the measurement accuracy.