METHOD FOR MODULATING A CARRIER WAVE

Abstract: A method for externally modulating an optical carrier wave by propagating the wave through a single channel waveguide structured to have an output the optical power of which in the near field is distributed in a predetermined spatial pattern, selectively changing the optical power distri­ bution pattern in correspondence with an information signal, and spatially filtering the power distribution pattern so that only the power available within selected region of the pattern is transmitted beyond the point of spatial filtering where it is available as an optical carrier wave the intensity of which is modulated in correespondence with the information signal.

Description: The invention generally relates to optical communications, and, in particular, it relates to a method of modulating electromagnetic carrier waves in the optical region of the spectrum. To convey information on an optical wave, some property of that wave has to be modulated or changed in accordance with the information and adopted coding system as can be seen in e.g. FR-A-2 379 086 or US-A-3 923 373. The modulated wave property may be intensity, phase, frequency, polarization, or direction, and the modulation format may be either analog or digital. Devices for modulating optical carrier waves are commonly classified either as direct or as external modulators. In direct modulators, the output of the carrier source, such as a semiconductor laser, is made to vary in accordance with changes in the injection current which typically serves as the electrical analog of the information signal. External modulators accept a source output as an input and then change some property of the source output for transmission along the fiber trunk line. Well known in the art are external modulators which rely on electro-optical or acousto-optical properties of crystalline birefringent materials such as lithium niobate (LiNbO₃) and lithium tantalate (LiTaO₃). Here, a light wave passing through a crystal exhibiting the electro-optic effect is divided into an ordinary and an extraordinary wave by appropriate choice of the direction of polarization. These waves travel with different velocities. When a voltage is applied to the crystal, the velocities of the waves change, and intensity modulation can be produced by using the resulting phase difference. In a typical modulator based on this scheme, the output of a laser is linearly polarized and arranged to make an angle of 45° with the electrically induced bi-refringent axes of the electro-optic crystal. A voltage applied to the crystal then changes the propagation velocities of the ordinary and extraordinary waves causing the output from the crystal to be elliptically polarized. Variations in the modulating voltage thus cause corresponding changes in the output when the output is made to pass through an analyzer. Thus, the transmitted power from the analyzer also changes. A quarter-wave plate is often inserted in the system to bias it to give optimum linearity to the modular characteristic. Waveguide modulators utilizing this scheme are also known with the waveguide taking on the function of the electro-optic crystal. In one such known waveguide modulator, the amount of coupling between two waveguides is changed by applying a modulating voltage. However, all of the known waveguides rely on relatively complex interferometric schemes to produce intensity modulation as their output. Thus, it is a primary object of the present invention to provide a method for using a simplified waveguide modulator to provide intensity modulation of an optical carrier wave. Other objects of the invention will, in part, be obvious and will, in part, appear hereinafter. SUMMARY OF THE INVENTION This invention relates in general to a method of modulating electromagnetic carrier waves in the optical region of the spectrum as claimed in claim 1 and, in particular, the invention relates to a method of externally modulating an optical carrier wave by propagating the wave through a single channel waveguide structured to produce an output the optical power of which in the near field is distributed in a predetermined pattern, selectively changing the optical power distribution pattern in correspondence with an information signal, and spatially filtering the power distribution pattern so that only the power available within a selected region of the pattern is transmitted beyond the point of spatial filtering where it becomes available as an optical carrier wave the intensity of which is modulated in correspondence with the information signal. A modulator used in the method comprises a waveguide having a single channel formed in a ferroelectric substrate through an ion-proton exchange process. The substrate is preferably an x-cut crystal of lithium niobate, but may be lithium tantalate. The substrate of either material may also be z-cut. The single channel is formed in the crystalline substrate by immersing a properly masked substrate in a bath of hot benzoic acid. The mask is defined by metallic strips which afterwards double as electrodes without additional processing after the benzoic acid proton exchange bath. When the waveguide is endfire coupled, it supports one or more propagation modes polarized or copolarized in one azimuth either parallel to or perpendicular to the y-z plane, and along either the y-axis or the x-axis of the crystal. Application of a variable voltage across the electrodes of the waveguide causes a predictable change in the near field energy disbribution of the coupled copolarized modes. Spatial filtering is provided in a predetermined near field location selectively to block part of the energy disbribution from travelling further downstream while the remaining part of the distribution is used as a modulated signal which can be detected by a suitably placed photodetector. DESCRIPTION OF THE DRAWINGS The invention will best be understood from the following detailed description of the illustrated embodiment and steps when read in connection with the accompanying drawings wherein like numbers have been employed in the different figures to denote the same parts and wherein: Fig. 1 is a diagrammatic perspective view of a single channel waveguide which may be used in the practice of this invention; Fig. 2 is a graph illustrating one possible near field modal power distribution for an excited state of the waveguide of Fig. 1; Fig. 3 is similar to Fig. 2 but illustrates the near field modal power distribution for another possible waveguide output; Fig. 4 is a graph illustrating the change to the near field modal power distribution of Fig. 2 when the electrodes of the waveguide of Fig. 1 have a voltage applied to them; Fig. 5 is a graph showing the change in the near field modal power distribution of Fig. 3 in response to a voltage applied to a waveguide, Fig. 6 is an enlarged diagrammatic perspective view illustrating other components used to practice the invention; and Fig. 7 is an enlarged diagrammatic perspective view illustrating an alternate embodiment which may be used to practice the invention. DETAILED DESCRIPTION This invention relates to a method of externally modulating at high rates electromagnetic carrier waves in the optical region of the spectrum ranging in wavelengths from the visible through the near infrared. The device used in the method comprises a waveguide having a single channel capable of supporting the propagation of either a preferentially polarized single mode or preferentially copolarized multimodes in such a way that the power intensity distribution of the propagating modes at the near field output location of the waveguide may be selectively changed in a controllable way by a voltage applied across the waveguide channel and afterwards spatially filtered to provide an intensity modulated output signal useful for conveying information along optical fiber. The waveguide portion of the modulator is shown diagrammatically at 10 in Fig. 1 as a generally parallelepiped, thin, planar substrate 12 having formed therein in a manner to be described a single elongated channel 14 whose index of refraction in one azimuth is higher than that of the substrate 12 in the same azimuth. On the upper side of the substrate 12 and straddling the channel 14 is a pair of electrodes 15 and 19 with attached leads 24 and 26, respectively. Substrate 12 is formed of a ferroelectric birefringent crystalline material preferably lithium niobate (LiNbO₃), but it may also be of lithium tantalate (LiTaO₃) or other suitable material having similar properties. The preferred crystalline orientation for the substrate 12 is indicated in Fig. 1 with the z-axis of the crystallographic x-y-z coordinate system coincident with the optic axis or "c"-axis of the crystalline structure of the substrate material. With this orientation, it will be understood that the preferred substrate is x-cut and thus y-propagating, although it may alternatively be z-cut for reasons to be described. The exact size for the substrate 12 will, of course, depend on a particular application; but, in general, it will be several millimeters in length and width and about one millimeter in depth. Actual substrates used in practice were approximately 10 millimeters in length, 5 millimeters in width and one millimeter in depth. The shape of the channel 14 and its optical properties may be selected in a well-known manner to support either single or multimode propagation, depending upon the intended use of the waveguide 12, and then the channel 14 is formed or reformed as necessary in the substrate 12 in the following way. The electrodes 15 and 19 are formed on the surface as layers of metallic composition. Layers 16 and 20 are laid down first; preferably they are of chromium (Cr) and approximatly 20 nm thick. Thereafter, layers 18 and 22 are applied; they are preferably made of gold (Au) in a thickness of approximately 70 nm. Leads 24 and 26 are attached to the gold layers, 18 and 22, respectively, in a well-known manner. Chromium is first deposited upon the substrate because of its good adhesion properties with both gold and lithium niobate or lithium tantalate. Gold is used, of course, because it is an excellent conductor. Furthermore, chromium and gold are the materials preferred for forming the electrodes 15 and 19 because they are resistant to degradation in the presence of other materials to which they may be exposed in the subsequent steps of forming the waveguide 10. Resistance to degradation is a general requirement for electrodes of any composition for use in the kind of waveguide under consideration. The long inboard edges of the electrodes, 15 and 19, define between them a rectangular aperture 17 which exposes a well-defined area of the substrate material coincident with the top channel geometry (14) for further treatment. In this manner, the electrodes 15 and 19 provide a masking function as well. The unprotected part of the substrate 12 in the y-z plane is immersed in a hot melt of benzoic acid (C₆H₅COOH) or other hydrogen rich acid such as sulfuric acid (H₂SO₄). When immersed in benzoic acid, the unprotected region of the lithium niobate or lithium tantalate substrate exchanges lithium ions for free protons which are supplied by the acid. The result of this is an increase in the extraordinary index of refraction, the index in the y-z plane of the substrate 12. The amount of index change and the effective depth both in the x-direction and in the z-direction of the guiding channel 14 depend on the temperature of the acid and on the exposure time, as is well known. This is to say that the diffusion proceeds in two directions and not necessarily at the same rate. In the foregoing simple manner, the waveguide 10 is fabricated quite easily with a metallic masking function used to define the waveguiding region serving also as an electrode pattern without any additional processing. The waveguide 10 structured in this way is used to propagate only waves whose polarization azimuth is parallel to the y-z plane in Fig. 1 whether of single or multimode character. Typical index changes in this plane are up to 0.12 for lithium niobate and .14 for lithium tantalate from the substrate base material index of refraction which for both materials is approximately 2.2. Moreover, the electro-optic coefficient in this direction is usually largest for these substrate materials. This is another reason for the preferred orientation. The waveguide 10 may be made to support either single mode or multimode propagation. In the case of multimode propagation, the modes supported are the quasi-transverse electric modes for x- and z-cut crystalline materials. The term quasi is in the present context to mean that the modes are primarily polarized along one direction, but not exactly so. Some typical power distributions, depending on waveguide properties, for multimode propagation with no applied voltage across the leads 24 and 26 are given in Figs. 2 and 3. These graphs represent the spatial power distribution in the near field of the channel 14, i.e., a location close to the exit port of the channel 14. As may be seen in these graphs, the power intensity peaks more than once across the width of the channel 14, the number of peaks depending on the order of the mode. Here, Fig. 2, having 2 peaks represents a lower order than Fig. 3. Since channel 14 supports the propagation only of waves polarized in one azimuth, the modes propagating of whatever order adopted are copolarized, have spatially distributed power patterns and are believed to be weakly coupled. It has been found that even with the application of but modest voltages across the leads 24 and 26, it is possible to change the near field power distribution patterns of the propagation modes such that power from one or more modes is coupled into other modes, thereby making available in the near field predictable patterns which are exploitable for intensity modulation purposes. Such changed patterns are shown in Figs. 4 and 5; Fig. 4 corresponding to a change in the Fig. 2 pattern and Fig. 5 represents a change in the pattern of Fig. 3. Reference is now made to Fig. 6 where the waveguide 10 is shown in combination with other components, including a coherent source 30, such as a semiconductor laser, which is endfire coupled to the waveguide 10 by way of optics indicated schematically at 32. The near field output from the waveguide channel 14 is imaged by schematically indicated optics 34 onto an aperture plate 36 which serves as a means for spatially limiting a selected part only of the near field output pattern from the waveguide 10 for further transmission to a photodetector 38. The photodetector 38 operates in the usual way to convert light signals into electrical signals. An electronic signal generator 39 is connected across the waveguide leads 24 and 26, to provide the necessary information signal. The level of voltage provided by the signal generator 39 preferably does not exceed that above which the electro-optic effect for the substrate material becomes nonlinear. The voltage from the generator 39 operates to generate the electro-optic effect of the substrate 12 which, in turn, causes changes in the waveguide optical power distribution. Modulation rates in excess of video rates for a channel width of 9 micrometers, acid bath time of 5 minutes near 249°C, and at wavelengths ranging from the visible to 1.32 micrometers have been easily achieved. Furthermore, it is believed that the bulk electro-optic properties of the substate materials are such that the response of the substrate is so fast that the possible modulation rate is limited by the capability of the electronic signal generator. Instead of using a discrete spatial filter such as 36 shown in Fig. 6, spatial filtering may easily be achieved by butt coupling an optical fiber or fibers of suitable structure at an appropriate transverse location of the channel 14 to obtain an intensity modulated signal directly coupled into fiber which may, for example, be a trunk line. Because the ion-proton exchange process is active in raising only the extraoradinary index of refraction of the substrate material, other devices with curved or branch channels are possible with but little extra care in masking and electrode geometry placement. However, curved or branched versions of the device require a z-cut material orientation such as that shown for the device indicated at 40 in Fig. 7. Device 40 is a waveguide comprising a substrate 42, preferably of lithium niobate, having a waveguide channel at 44 including branches 60 and 62 which may serve as exit ports. Again, layered electrodes are provided on both sides of the channel branch 44 as shown at 46 and 48. In this waveguide, the polarization azimuth for propagating waves is parallel to the x-z plane, while propagation is in the direction of the x-axis. Here, two outputs are possible, and the spatial filtering is achieved by proper design of the branching configuration. Although lithium niobate is the preferred material for fabricating the devices used, lithium tantalate may be used as well as it also exhibits the required effects. However, lithium tantalate does not readily form waveguide channels with only the ion-proton exchange process. In addition, annealing at high temperatures must follow the proton exchange process to obtain suitable waveguides with this material. Those skilled in the art may make other changes to the above-described embodiments without departing from the scope of the invention. For example, it is possible to provide great variations in index profiles from extremes of graded index to nearly uniform profiles in the waveguide channels to obtain beneficial optical power patterns other than those illustrated. This is easily accomplished with known techniques for buffering benzoic acid to control the ion-proton exchange distribution across the channel both in width and depth to influence the modal characteristics propagated. In addition, the essential results may be enhanced or changed by replacing the electrodes used to define the channel with other geometries placed relative to the waveguide region. Also, devices as those described need not necessarily be confined to x or z-cut materials. y-cut materials may be used advantageously depending on the application, but they may require slightly different processing to fully exploit their efficient use.

Claims: A method of modulating an electromagnetic carrier wave in the optical region of the spectrum, comprising the steps of: propagating the carrier wave through a ferroelectric waveguide (10, Fig. 1; 40, Fig. 7) as a plurality of modes copolarized in one azimuth such that the optical power emerging from the waveguide in the near field is spatially distributed in a predetermined pattern (Fig. 2, Fig.3) containing a plurality of spatially separated intensity peaks; selectively changing said near field optical power distribution pattern in correspondence with an information signal (39, Fig. 6) by cross-coupling energy between said modes; and spatially filtering said near field optical distribution pattern so that only the optical power in a predetermined region of said pattern is transmitted beyond the point of spatial filtering to provide an optical carrier wave the intensity of which is modulated in correspondence with the information signal. The method of claim 1 wherein said plurality of modes of the carrier wave is propagated so that said near field optical energy distribution pattern (Fig. 2, Fig. 3) contains said plurality of intensity peaks spatially separated from one another across the width of the waveguide. The method of claim 1 or claim 2 wherein selectively changing said optical power distribution pattern comprises selectively redistributing said optical power distribution pattern to increase the magnitude of the intensity of selected ones of said intensity peaks while lowering the intensity of the other peaks. The method of any of the preceding claims wherein said optical power distribution pattern is selectively changed by applying across the waveguide an electric field which varies in correspondence with the information signal and operates to change the electro-optic properties of the waveguide to change said optical power distribution pattern. The method of any of the preceding claims wherein said near field optical distribution pattern is spatially filtered by imaging it onto an aperture stop (36, Fig. 6) capable of transmitting only said region of said pattern. The method of any of preceding claims 1 - 4 wherein said near field optical distribution pattern is spatially filtered by butt coupling an optical fiber to said waveguide so that only part of said pattern is transmitted along said fiber. The method of any of the preceding claims wherein said carrier wave modes are propagated along a waveguide comprising a planar substrate (12, Fig. 1; 42, Fig. 7) consisting essentially of lithium niobate (LiNbO₃) or lithium tantalate (LiTaO₃) having formed therein a channel (14;44,60,62) of higher index of refraction than that of said substrate material along one crystallographic axis thereof.

Inventors: SANFORD, NORMAN A.
Applicants: POLAROID CORPORATION
Application year: 1985
Main classification: G02F1
IPC codes: H04B10, G02F1, G02B6