A method of adjusting the operation characteristics of integrated optical devices

Abstract: A method of adjusting the operation characteristics of integrated optical devices, allowing a considerable number of out-of-tolerace devices to be recovered by depositing on the surface whereupon the optical guides are manufactured, once the normal fabrication process is completed, a further layer of transparent material with suitable size and refractive index (SA).

Description: The present invention concerns technological methods of manufacturing integrated optical devices and more particularly it refers to a method of adjusting the operation characteristics of integrated optical devices, formed by a network of optical waveguides manufactured on or in the surface of a substrate wherein light signals are to be propagated, said method being carried out by depositing a further layer of transparent material on a surface area at which one or several optical waveguides of a completed optical device are exposed and by selecting the thickness of the further layer so as to adjust the operation characteristics. The fabrication of integrated optical devices demands very accurate technological methods to cope with very restrictive tolerances in the chemical, physical and geometrical characteristics of the manufactured devices. In fact even small errors can cause considerable variations in the required optical characteristics, e.g. a different spectral interval of operation, different splitting ratios in beam splitters or in directional couplers, and the like. Said disadvantages can derive not only from inaccuracy occurred in successive steps of the technological process of fabrication, but also from errors or insufficiencies of the designing techniques, especially when all the boundary conditions and their possible variations were not taken into account. Since the guiding structures forming the integrated optical devices are expensive and difficult to produce (e.g. in case of simiconductor materials, of special crystals, and the like) the manufacturing refuses highly affect the final cost. Hence it can be convenient to adjust the optical characteristics of the obtained device to return its performance within the required specifications. If the device is made of electrical-field-sensitive material, e.g. of lithium niobate (LiNbO₃), or of certain semiconductor materials, its optical characteristics modification can be obtained by using the method described in the paper entitled "Waveguide Electrooptic Modulators" by Rod C. Alferness, issued on page 1121 of IEEE Transactions on Microwave Theory and Techniques, August 1982. According to this method suitable electrodes are placed on the device for applying a potential difference of convenient value. Said potential difference generates an electrical field permitting a change in the optical characteristics of the device in its operation phase. The method requires further steps in the technological process of fabrication to manufacture the electrodes and, once the device is fabricated, it requires permanent application of a voltage of controlled value, even though the operation conditions are always the same. In Electronics Letters, Vol. 24, No. 25, 1988, p. 1584-1586, a deposition of a transparent layer of corning glass on the optical guides of an integrated optic power divider is described, deposited in order to adjust the power distribution by selecting a determined thickness of the cladding layer. The refractive index of the cladding layer is higher than that of the optical guide, what implies the possibility that the coating layer guides the light radiation which thereby could propagate in different directions causing optical losses, while the adjustment possibilities are rather restricted. Thereagainst, by the method according to the invention of adjusting the operation characteristics of the integrated optical devices, a considerable number of devices initially out of tolerance are to be recovered easily and without approaching critical limits. The present invention is characterised in that further the refractive index of the further layer is lower than that of said optical waveguides and that the extension of the further layer along the light propagation direction, is selected so as to adjust the operation characteristics, wherein the thickness of the further layer is to be chosen greater than a certain minimum value of the order of some wavelengths of the light signals to be propagated in the waveguides. Thereby, the method using a few additional technological steps besides those necessary for the fabrication provides for multiple adjustment possibilities without the risk of the light radiation passing into the further layer, it does not require the application of electrical fields for keeping the operation optical characterstics, and demands only the setup of the deposition conditions of a transparent material. The "selection" of the thickness of the further layer simply consists in the determination of a thickness which is greater than a minimum value, the exact thickness being of no further importance. The foregoing and other characteristics of the present invention will be made clearer by the following description of a preferred embodiment thereof given by way of non-limiting example, and by the annexed drawing, wherein: Fig. 1 is a plan view of a beam splitter; Fig. 2 is a cross-sectional view of the beam splitter. As an example of application of the method provided by the invention a 3 dB beam splitter is considered; it operates on the basis of the bimodal interference principle. The incoming optical signal is to be exactly divided in half between the two output branches. If the obtained device does not perform exactly the partitioning required, it is possible to intervene with the present correction method and make the changes necessary to meet the required operation specifications. The splitter shown in Fig.1 consists of two input single-mode guides, with input gates P1 and P2, which join through an Y-connection into a segment of bimodal guide of length L and width W. At the end of the bimodal guide segment there is an Y beam splitter entering two output single-mode guides, with output gates P3 and P4. This kind of device can operate equally well by exchanging the input gates with the output ones. Besides, either an only signal to be split exactly into two parts or two signals can be sent to the inputs, one per each input gate, when one wishes to combine them with one another and extract at the output the two halves of the obtained combination. The latter method can be used in a coherent receiver, when the locally-generated signal is to be combined with the signal received to effect its detection. Optical waveguides forming the device can be manufactured by epitaxial growth of a ternary or quaternary semiconductor material, such as InAIAs, InGaAIAs or InGaAsP, on a substrate of InP:Fe, denoted by SU in Fig.1. Afterwards the required guiding network is obtained by masking the grown substrate and by chemical etching the uncovered parts. The beam splitter operation is as follows. The optical signal is sent to an input gate, e.g. P1, and covers the single-mode guide segment. When it arrives at the bimodal guide segment, through Y-junction both the fundamental and the first higher mode are excited, wich interfere with each other. As an effect of the interference, a periodical spatial oscillation of the light intensity occurs, with a sequence of maxima and minima along the length L of the bimodal guide. The period of this oscillation depends on the difference between the effective refractive indices of the two modes, which depends in turn on the guide geometric characteristics, width and length, as well as on the refractive indices of the various materials forming the whole guiding structure more particularly on the refractive index of the substrate, of the guiding layer and of the material which possibly covers the latter layer. If the oscillation phase assumes a convenient value at the output end of the bimodal segment, in the subsequent Y-splitter and in the two single-mode output guides one exactly obtains half the power available in the input guide, possible losses excepted. The oscillation phase at the Y-splitter of course depends on length L of the bimodal segment, which hence is to be manufactured with rather stringent tolerances. The greater errors which can occur in the fabrication of such a device, due to limited accuracy obtainable in the different steps of the technological process, are encountered in the values of width and thickness of the guides, as well as on the chemical composition of the grown material. The tolerances obtained do not ensure that the phase of the oscillation of the light intensity at the output end of the bimodal guide segment is the optimum to supply the desired beam splitting. According to the invention, the technological process inaccuracy can be overcome by depositing on the surface,whereupon the optical guides are manufactured, once completed the normal fabrication process, a further transparent material layer. This covering layer, which is transparent not to increase the guide losses, is to be homogeneous, is to present a convenient refractive index and whatever thickness, provided the thickness is greater than a certain minimum value, generally of the order of some wavelengths of the processed radiation. This operation causes changes in the propagation constants of guided modes which depend on the values of the refractive indices of all the materials forming the waveguide structure, and more particularly on the refractive indices of the substrate, of the guiding layer proper and of all the possible upper and lateral boundary layers. Hence by changing the covering layer refractive index and keeping unchanged the refractive index of all the other layers, one can change within a significant and useful interval the effective refractive indices in the bimodal guide and hence the period of the oscillation caused by the interference. While fabricating the device, in case one realizes that the optical features of the finished item (or of a test sample taken from a set of identical samples) do not meet the required characteristics, an adjustment can be carried out by depositing a covering layer with a value of the refractive index apt to bring the guiding structure to the operation point. The materials used are generally to present a refractive index lower than that of the optical guide to be adjusted and can consist of resins, polymers, organic or inorganic materials, which can be deposited by whatever method. For example, in case of splitting error with an output power of 33% on a branch and 67% on the other, it is necessary to deposit a layer of material with refractive index equal to 2 and thickness greater than 0.2 µm on a guide 200 µm long of InGaAIAs. Analogously, in case of splitting error with an output power of 44% on a branch and 56% on the other, it is necessary to deposit a layer of material with refractive index equal to 1.5 and thickness greater than 0.2 µm on a guide with the same characteristics. The correction of a certain partitioning error can be performed not only by covering the whole optical guide with a layer of material with a suitable refractive index in function of the correction entity, but also by varying the length of the bimodal guide segment covered. In this way the setup of different deposition conditions and techniques, which depend on the material used for the correction, can be avoided. Hence, by varying the length M of the bimodal guide segment covered by this partial layer, denoted by SA in Fig.1, it is possible to change the phase of the intensity oscillation of the light signal at the end of the bimodal guide and, as a consequence, the splitting ratio at the device output gates P3 and P4. The variation obtained increases with the value of the refractive index of the deposited material. Fig. 2 shows a cross section of an intermediate zone of the device, wherein it is shown a simple sequence of layers forming the guiding structure, the substrate SU, the bimodal guide and correction layer SA. By covering for a length equal to 100 µm the bimodal guide, which is 200 µm long, with a material having a refractive index equal to 1.5, a power splitting error of 47% on a branch and 53% on the other can be corrected. The method above can be used to adjust the optical characteristics of other devices, e.g. directional couplers or DFB lasers (Distributed Feedback Bragg), fabricated with the above-mentioned materials or with other materials used in the integrated optics domain, such as glass, LiNbO₃,SiO₂, polymers, and the like.

Claims: A method of adjusting the operation characteristics of integrated optical devices, formed by a network of optical waveguides manufactured on or in the surface of a substrate wherein light signals are to be propagated, said method being carried out by depositing a further layer of transparent material (SA) on a surface area at which one or several optical waveguides of a completed optical device are exposed and by selecting the thickness of the further layer so as to adjust the operation characteristics, characterized in that further the refractive index of the further layer is lower than that of said optical waveguides and that the extension (M) of the further layer along the light propagation direction is selected so as to adjust the operation characteristics, wherein the thickness of the further layer (SA) is to be chosen greater than a certain minimum value of the order of some wavelengths of the light signals to be propagated in the waveguides. A method as in claim 1, characterized in that the refractive index of said further layer is selected corresponding to the adjustment the device is to be submitted to.

Inventors: CALDERA CLAUDIO, DE BERNARDI CARLO, MORASCA SALVATORE, CALDERA, CLAUDIO, DE BERNARDI, CARLO, MORASCA, SALVATORE
Applicants: CSELT CENTRO STUDI LAB TELECOM, CSELT CENTRO STUDI E LABORATORI TELECOMUNICAZIONI S.P.A.
Application year: 1991
Main classification: G02B6
IPC codes: G02B6