The invention relates to a method of manufacturing an indiffused optical waveguide structure in a substrate, Furthermore, the invention relates to an indiffused optical waveguide manufactured by such a method and to various integrated optics devices using such indiffused waveguide structures. Such integrated optic devices may be for example an acousto-optical mode converter, an accusto-optical switch, an optical power splitter, a dual-output Mach-Zehnder modulator, a polarisation splitter and an electro-optical switch. In particular the invention relates to making an indiffused optical waveguide in birefringent substrate materials like LiNbO3. In such a birefringment substrate the refractive index neff, TE, TM for the quasi TE and quasi TM-modes in the waveguides will respectively have slightly different values with respect to the substrate index.
The manufacturing method and the waveguide structures according to the invention are superior to previously known waveguide structures in that they can be manufactured with higher dimensional precisions, for example, in order to keep the variations of the birefringence along the optical waveguides as small as possible resulting in an overall improved performance of integrated optic devices using such waveguide structures.
FIG. 1 shows several optical waveguide structures applied to integrated optic devices, for example a raised stripe waveguide (FIG. 1b), a rib waveguide or optical stripe-line (FIG. 1c), a general channel waveguide (FIG. 1d) or a ridge waveguide (FIG. 1e).
Many integrated optics devices use the so-called diffused or embedded waveguide as shown in FIG. 1a. 
Furthermore, the optical waveguide structures are not limited to any particular longitudinal geometry, i.e. any kind of straight or curved geometry as used in Y-junctions, polarising beam splatters etc. can be used. FIG. 2 shows examples of such basic structures: FIG. 2a: Y-junction, FIG. 2b: WDM.-device; FIG. 2c: star coupler and FIG. 2d: polarising beam coupler.
Furthermore, the optical waveguide and substrate materials are not limited to any particular material, That is, generally the materials can comprise isotropic, anisotropic and birefringent materials. In particular, the usage of a birefringent substrate is essential for the manufacturing of acousto-optical devices. For example, the diffused channel waveguide of FIG. 1a can comprise a substrate material of LiNbO3 with a waveguide made by a titanium indiffusion.
Whilst theoretically the geometry of the waveguide within the substrate is determined on the basis of known diffusion profiles, due to the fabrication conditions such a theoretically calculated diffusion profile or waveguide geometry is never reached in practice. Furthermore, there is no easy means to measure the actual distribution of the waveguide cross section. Therefore, the operator performs numerous experiments to find out the manufacturing conditions such that the produced integrated optics device has a performance that fulfils the theoretically calculated requirements.
Therefore, due to the imperfections during the fabrication processes used for making the waveguides in FIG. 1 (for example disuniformities in the titanium stripe dimensions, temperature gradients during diffusion, etc.) the effective waveguide birefringence varies locally over the wafer used for making a plurality of such devices at the same time and also as an averaged value from wafer to wafer. The applicant has found that the performance of single optical components (e.g. straight and curved waveguides) as well as more complex integrated optical devices, like an acousto-optical mode converter depends critically on the uniformity of the waveguide birefringence. Thus, the overall performance and reproducibility of acoustooptical devices strongly depends on the homogeneity and reproducibility of the fabrication processes.
Birefringence essentially means that the effective index (or the propagation constant) for (quasi) TE-modes and TM-modes is different and therefore the requirement of a small variation of birefringence means that the difference in propagation constants or the difference in refractive index An remains the same along the optical waveguide as much as possible. There is no known relationship between the extent of such a birefringence variation and the fabrication parameters and thus it is unpredictable how large such variations are.
The birefringence variations can have detrimental effects even in simple single waveguides. In integrated optics and also in distributed optical communication systems it is often desirable to switch the input polarisation of a TE-mode to the TM-polarisation and this can, for example, be performed by electro-optical couplers or by an acousto-optical mode converter. The latter device is based on the usage of a birefringent optical waveguide and if this waveguide has birefringent variations this will cause the performance of this device to deteriorate drastically.
Birefringence Variation in Acousto-Optic Devices
The detrimental effects of birefringent variation in the basic acousto-optical mode converter are explained with reference to FIG. 3. The working principle of an integrated acousto-optical device e.g, on LiNbO3 is based on a wavelength selective polarisation conversion between two copropagating optical waves polarised along the main birefringence axes of the LiNbO3-crystal i.e. between the xe2x80x9cTMxe2x80x9d- and xe2x80x9cTExe2x80x9d-modes, Energy can be exchanged between these orthogonal polarisation modes when they get coupled by the off-diagonal elements in the dielectric tensor. This is possible for example by the electro-optic or photo-elastic effect as explained below. A surface acoustic wave, i.e. an elastic xe2x80x9cRayleigh-wavexe2x80x9d in a photoelastic and piezoelectric material such as in LiNbO3 is an ideal means of coupling due to its tunability in frequency and in power.
As shown in FIG. 3 a straight monomodal waveguide of conventionally for example 7 xcexcm is embedded in about a 100 xcexcmm wide monomodal acoustic-waveguide (x-cut, y-propagating LiNbO3-crystal). Both optical waveguides and acoustic claddings are fabricated by a titanium indiffusion. Metalinterdigital transducers of a suitable configuration are deposited on top of the crystal at the beginning of the acoustic waveguide. By applying a RF-drive signal at the interdigital transducer electrode an acoustic wave is excited. The acoustic wave travelling along the interaction length induces the mode coupling for the optical polarisation modes. To define a certain conversion band width, the interaction length L is limited by an acoustic absorber.
A fundamental condition for energy transfer is the phase matching between the polarisation modes which results from the solution of the coupled wave equations. A conversion efficiency of 100% can only be achieved if the phase difference between the two optical modes (TE- and TM-modes) with different effective refractive indices is continuously compensated, which means a completely synchronous interaction along the interaction length. This synchronous interaction is essentially caused by means of an acoustic xe2x80x9cBraggxe2x80x9d-grating having a pre-determined period and inducing a coupling between the xe2x80x9cTExe2x80x9d- and xe2x80x9cTMxe2x80x9d-mode. The coupling effect is described by the following equation:                                                         2              ⁢              π              ⁢                              xe2x80x83                            ⁢                              n                                  eff                  ,                  TM                                                      λ                    -                                    2              ⁢              π              ⁢                              xe2x80x83                            ⁢                              n                                  eff                  ,                  TM                                                      λ                          =                                            β              TM                        -                          β              TE                                =                      Δβ            =                                          2                ⁢                π                                            Λ                                  a                  ⁢                                      xe2x80x83                                    ⁢                  c                                                                                        (        1        )            
Here neff,TM and neff,TE are the effective refractive indices for the (quasi) TE- and TM-modes, xcex2TM, xcex2TE are the propagation constants for the wavelength xcex (in vacuum) and xcex9ac is the wavelength of the acoustic ware (i.e. the periodicity of the perturbation of the dielectric censor induced for instance by a periodic electric field or a surface corrugation, i.e. the acoustic xe2x80x9cBraggxe2x80x9d-grating Typically, the xcex9ac, is about 20-21 xcexcm for xcex=1530-1570 mm. The propagation constant (wavenumber Kac) is                               K                      a            ⁢                          xe2x80x83                        ⁢            c                          =                                            2              ⁢              π                                      Λ                              a                ⁢                                  xe2x80x83                                ⁢                c                                              =                                    2              ⁢              π              ⁢                              xe2x80x83                            ⁢                              f                                  a                  ⁢                                      xe2x80x83                                    ⁢                  c                                                                    v                              a                ⁢                                  xe2x80x83                                ⁢                c                                                                        (        2        )            
where xcex9ac is the acoustic wavelength, fac is the frequency and vac is the velocity of the acoustic wave. This is a phase matched (and thus wavelength dependent) process and a variation of the waveguide birefringence has a drastic effect on the phase matching and thus negatively influences the spectral conversion characteristics. The longer the waveguide is, the more detrimental the variations of birefringence on the phase matching is.
For optical wavelengths which do not fulfil the phase matching conditions the deviation xcex4 from the ideal phase match condition can be expressed by the following equation:                     δ        =                                            1              2                        ⁢                          (                              Δβ                -                                  K                                                            xe2x80x83                                        ⁢                                          a                      ⁢                                              xe2x80x83                                            ⁢                      c                                                                                  )                                =                                                                      π                  λ                                ⁢                Δ                ⁢                                  xe2x80x83                                ⁢                                  n                  eff                                            -                                                π                  ⁢                                      xe2x80x83                                    ⁢                                      f                                          a                      ⁢                                              xe2x80x83                                            ⁢                      c                                                                                        v                                      a                    ⁢                                          xe2x80x83                                        ⁢                    c                                                                        =                                          π                λ                            ⁢                              (                                                      Δ                    ⁢                                          xe2x80x83                                        ⁢                                          n                      eff                                                        -                                      λ                                          Λ                                              a                        ⁢                                                  xe2x80x83                                                ⁢                        c                                                                                            )                                                                        (        3        )            
where xcex94n-eff is the difference between the effective refractive indices of the guided polarisation modes. At a fixed acoustic frequency fac, the value xcex4 is a function of the optical wavelength xcex and of xcex94neff. Only for xcex4=0 a perfect phase matching exists and a complete energy transfer is possible. In a highly birefringent material as LiNbO3 (xcex94neff≈0,072) the phase mismatch xcex4 is a relatively strong function of the wavelength and hence LiNbO3 is a good candidate to fabricate components with conversion characteristics of small bandwidths. However, variations in neff,TE, neff,TM (i.e. xcex94neff) will influence whether or not a perfect phase matching can be achieved. The conversion efficiency xcex7 in case of a phase matched interaction is given by:
xcex7=sin2(xcex3{square root over (Pac)}L)xe2x80x83xe2x80x83(4) 
where the factor xcex3 contains the material constants contributing to the coupling (elastooptic and piezoelectrical coefficient), Pac is the power of the acoustic wave and L is the length of the acousto-optical mode converter. It is seen that the conversion efficiency xcex7 is, for the example of a constant coupling, approximately in the form of a (sin x/x)2-type behaviour (acoustic directional couplers with weighted coupling may for example possess a Gaussian-type behaviourxe2x80x94therefore, the constant coupling only serves as one illustrative example here).
FIG. 4 shows a typical experimental result of measuring the conversion efficiency of a LiNbO3 acousto-optical mode converter or the xe2x80x9cconstant coupling typexe2x80x9d comprising a diffused waveguide of a width of 7.0 xcexcm, a length of 60 mm and a height of 1250 xc3x85 (xc3x85=10xe2x88x9210 m). The values of width and height correspond to values before indiffusion. The graph in FIG. 4 shows many side lobes and does not very well resemble a (sin x/x)2 function due to strong variations in the birefringence of the optical waveguide,
FIG. 5 (relating to the same device as in FIG. 4) shows the phase matching frequency along an acousto-optic mode converter of 6 cm length for light of a wavelength of 1536 nm, The phase matching frequency and the effective birefringence are proportional to each other (xcex94neff=xcexxc2x7fac/vac; vac=3780 m/s for LiNbO3 in the case of the above device), FIG. 5 illustrates that the type of optical waveguide used has a strong birefringence variation since the phase matching frequency alters along the longitudinal direction. Such experimental results can be obtained by using a pulse probing technique as is described in xe2x80x9cAcoustic Pulse Measurements of Acousto-Optic Tunable Filter Propertiesxe2x80x9d by L. B. Aronson, G. Ranken, T. R. Ranganath, D. W. Dolfi in [IPR19S, DANA POINT, post-deadline paper #6-2]. Therefore, in order to improve the performance of any integrated optic device that uses an acoustic-optical mode converter, such as an acousto-optic tunable 2xc3x972 wavelength space switch (FIG. 6) or an integrated acousto-optical filter (FIG. 7), the aim is to make the conversion efficiency (shown in FIG. 4) to correspond as much as possible with the ideal curve. In the case of a constant coupling this means to make the conversion efficiency to correspond with the (sin x/x)2-type behaviour. As disclosed in a parallel patent application filed by the applicant on the same day as the present application, this is achieved by having the birefringence of the optical waveguide varying as little as possible along the Optical Waveguide (FIG. 5).
The source of sidelobe asymmetry in integrated acousto-optic filters was studied in xe2x80x9cSource of Sidelobe Asymmetry in Integrated Acousto-Optic Filtersxe2x80x9d by D. A. Smith, A. d""Alessandro, J. E. Baran and H. Herrmann, published in Applied Physics letters, vol. 62, pages 814-818 (1993). Here, the source of the asymmetry causing a cross-talk between wavelength-multiplexed optical channels is found to be influenced by the systematic even-order variation of the effective waveguide birefringence as a function of distance from the device center. Approximate simulations are presented that indicate what degree of non-uniformity of Ti-stripes thickness, width or diffusion temperature are responsible for such typical asymmetries. It is concluded that an intrinsic An variation is most likely due to a systematic longitudinal variation in device fabrication parameters, such as waveguide widths, layer thickness or diffusion temperature. It is also found that the sidelobe asymmetry may be caused from a systematic variation of the acoustic velocity profile. As remedies for sidelobe suppression it is suggested to impose a compensating structural bias on the optical waveguide widths leading to a cancellation of the beat-length non-uniformity.
S. Schmid, M. Sottocorno, A. Appleyard, S. Bosso report in xe2x80x9cFULL WAFER SCALE FABRICATION OF ACOUSTO-OPTIC 2xc3x972 WAVELENGTH SELECTIVE SPACE SWITCHES ON LiNbO3xe2x80x9d, on pages 21-24 in the ECIO""95 proceedings (post-deadline papers) of the 7th European Conference on Integrated Optics, Apr. 3-6, 1995, Delft, The Netherlands, that optical waveguides for use in acousto-optic mode converters were manufactured using an indiffusion of an about 1000 xc3x85 thick and 7 xcexcm wide titanium stripe at a temperature of 1030xc2x0 C. for 9 h. For an acoustooptic mode converter a degradation of the band-reject characteristics is observed when light of both polarisations is launched into the device. This degradation is found to be due to small birefringence inhomogenities caused by fabrication tolerances resulting in a phase match frequency to vary between 25 and 50 kHz.
In xe2x80x9cTAPERED ACOUSTICAL DIRECTIONAL COUPLERS FOR INTEGRATED ACOUSTO-OPTICAL MODE CONVERTERS WITH WEIGHTED COUPLINGxe2x80x9d by H. Herrmann, U. Rust and K. Sch@fer, IEEE Journal of Lightwave Technology, Vol. 13, Nr. 3, March 1995, pages 364-374, a polarisation independent integrated optical wavelength filter with a tapered acoustical directional coupler is discussed. The optical waveguides were fabricated by an indiffusion of 7 xcexcm wide Ti-stripes during typically 9 h at 1030xc2x0 C. It is reported that such waveguide parameters and manufacturing parameters yield a single mode optical waveguide for both TE and TM polarisations in the spectral range round xcex=1.55 xcexcm. The conversion characteristics of the mode converters disclosed here exhibit a slight asymmetry since the main side lobe on the left side of the main maximum is about 1.3 dB higher than that on the right side of the maximum, This asymmetry is here explained with the fact that the amplitudes of the even and odd surface acoustical wave modes are not exactly equal at the location of the optical waveguide. Here, the asymmetry is not attributed to a variation of the acoustic wave vector and therefore a variation of the phase matching condition along the device. It is here concluded that it should be possible to fabricate acousto-optical mode converters with a strong side lobe suppression, if the problems with the inhomogeneity can be handled, However, no design criteria are given for the optical waveguides to be used in the acousto-optical mode converters.
A summary paper of the fundamental physics and the working principle of acousto-optical tunable switches and filters is disclosed by S. Morasca, D. Scarano and S. Schmid xe2x80x9cAPPLICATION OF LiNbO3 ACOUSTO-OPTICAL TUNABLE SWITCHES AND FILTERS IN WDM TRANSMISSION NETWORKS AT HIGH BIT RATESxe2x80x9d, published in Giancarlo Prati (Ed.): xe2x80x9cPhotonics Networksxe2x80x9d, pp. 458 to 472, Springer, 1997. For an operating wavelength of around 1550 nm a straight mono-mode optical waveguide of typically 7 xcexcm is embedded in about a 100 xcexcm wide monomodal acoustic waveguide. Although it is discussed here, that the birefringent variations of the waveguide causes the phase mismatch, no design rules are indicated regarding the waveguide parameters and the fabrication parameters in order to have a small birefringence variation along the optical waveguide and a small dependency of the obtained birefringence with respect to a variation of the manufacturing parameters.
Acousto-Optic Devices Having an Improved Birefringence Variation
In the cited parallel application filed by the applicant on the same day as the present application a design rule is presented which allows to improve the birefringence variation along the waveguide (or respectively the variation of the birefringence between individual samples of waveguides on the same wafer). Essentially, this design rule particularly links the birefringence variation with the titanium stripe dimensions (before indiffusion). That is, the inventors have discovered that dimensional changes have different influence on a change of the birefringence depending on the size of the waveguide, In particular, it was found that the change in birefringence is larger for large (i.e. deep and wide) waveguides and small for small (i.e. shallow and thin) waveguides. This finding is best explained with reference to the acousto-optic mode converter since here the birefringence variation can easily be seen in terms of the conversion frequency,
FIG. 8a shows the experimental conversion frequency f plotted over the width of the waveguide (before indiffusion!) for different types of the waveguides. In FIG. 6a a diffused waveguide was employed as in FIG. 1 (a). An interesting parameter in FIG. 8a is the change of the conversion frequency xcex94f over the change of depth (height) of the waveguide xcex94"igr", where "igr" designates the height of the channel waveguide (before diffusion). It is seen from FIG. 8 that a smaller width of the waveguide together with a smaller height of the waveguide results in smaller values of xcex94f/xcex94"igr". For example, whilst the ratio xcex94f/xcex94"igr" for a 7 xcexcm is 15 kHz/xc3x85, this ratio is only 3.5 KHz/A for a 4.0 xcexcm wide waveguide. On the right vertical scale, the birefringence values are indicated (i.e. the difference xcex94neff between neff,TE and neff,TM)xe2x80x94
In FIG. 8a, the line A designates the cut-off of the optical waveguide for a wavelength xcex=1600 mm. That is, naturally, the smaller the waveguide becomes, the closer the fundamental mode of a certain wavelength will be to the cut-off condition. However, as is seen with the changing gradient of the curves near the cut-off boundary, the ratio xcex94f/xcex94"igr" becomes smaller closer to the cut-off boundary. From this a general relationship can be derived, namely that for a given desired optical wavelength of propagation (and refractive index change between the waveguide and its surrounding medium), the propagation constant of the fundamental (and only) mode should be as close to out-off as possible.
This advantage of a reduction of the birefringent variation is also seen from FIG. 8b, which respectively show against the width of the waveguide W the variation of the conversion frequency xcex94f/xcex94w with respect to the change of width and the change of conversion frequency xcex94f/xcex94"igr" with respect to a change of height, In FIG. 8b and FIG. 9 the variation xcex94f/xcex94w and xcex94f/xcex94"igr" respectively becomes smaller, in absolute value, the smaller the waveguide widths w becomes. Furthermore, it is also seen that smaller heights of waveguides (e.g. T=1050 xc3x85) lead to smaller absolute values of the changes xcex94f/xcex94w and xcex94f/xcex94"igr". The derivative of the conversion frequency with respect to the titanium layer thickness xcex94f/xcex94"igr". (FIG. 9) depends strongly from the waveguide width w. FIG. 9 also shows the values of xe2x88x9215 kHz/xc3x85 for a 7 xcexcm wide waveguide (width before indiffusion) and xe2x88x925 kHz/xc3x85 at 4 xcexcm already indicated in FIG. 8a. That is, the waveguide in FIG. 9 is about 3-times less sensitive against inhomogenities (variations or changes due to the manufacturing method) of the titanium layer thickness.
As seen from FIG. 8b, on the other hand, waveguides with relatively small layer thickness (height) are less sensitive with respect to inhomogenities in the waveguide width. A simple estimation considering real process related differences indicates that a 4 xcexcm wide waveguide hating a titanium layer thickness of 1050 xc3x85 is about 4- to 5-times less sensitive to typical fabrication tolerances than a conventional 7 xcexcm wide waveguide having a titanium layer thickness of 1250 xc3x85. The most preferable pair of layer height/waveguide width is 1250 xc3x85/4.1 xcexcm. Other preferable values are 1050 xc3x85/5.2 xcexcm and 1150 xc3x85/4.5 xcexcm.
FIGS. 10a, 10b are analogous to FIGS. 4, 5 (where xcex94f≈800 KHz corresponding to a birefringence variation along the waveguide of a value xcex4(xcex94n)≈3.3xc2x710xe2x88x924) and show the superior effect of using a 1050 xc3x85/4.5 xcexcm waveguide in the acousto-optical mode converter. FIG. 10a shows that the side lobe suppression is superior (in fact very similar to the theoretical assumption) and FIG. 10b indicates that only very minor variations in the order of 100 KHz of the conversion frequency occur along the longitudinal direction of the acousto-optical mode converter. As explained before, the conversion frequency is essentially a measure of the birefringence variation and thus FIG. 10b shows that only a minor variation of the birefringence (xcex4(xcex94n)≈0.4xc2x710xe2x88x924) occurs.
The inventors of the cited parallel application have clearly realised, that smaller waveguides (independent as to whether they are rib or channel waveguides etc.) perform better than strongly guiding wide waveguides. Since the conditions derived from FIGS. 8 to 11 i.e. xe2x80x9cnarrowxe2x80x9d and xe2x80x9cflatxe2x80x9d waveguides, automatically means that the optical wave is not strongly guided, an optimisation (i.e. a minimisation) of the waveguide dimensions is limited by the xe2x80x9ccut-offxe2x80x9d wavelength of the fundamental modes. The xe2x80x9cselection of waveguide dimensions (waveguide parameters)xe2x80x9d such that the propagation constant is xe2x80x9cclose to cut-offxe2x80x9d can be expressed with respect to the cut-off wavelength of the fundamental mode, If the components are operable in the wavelength window between 1530 nm and 1565 nm, then conventionally used waveguides in acousto-optical mode converters (having a width of 7-8 xcexcm as can be taken from the above mentioned prior art documents) have a cut-off beyond 1750 nm for both TE- and TMxe2x80x94polarisations. According to the invention the expression xe2x80x9coptimised waveguide parametersxe2x80x9d means, that the lowest cut-off wavelength of the TM-mode or TE-mode is as close as possible to the upper signal wavelength, but preferably greater than 1570, and smaller than 1650. This is true for straight waveguides. Curved waveguides should be kept broader since the decrease of the cut-off wavelength due to the curvature must be compensated by a larger width (for example, if the straight waveguide has a width of 5.5 xcexcm then a curved waveguide with Rc=130 mm should have a width of approximately 6.5 xcexcm).
The most important realisation from the experiments in the parallel application is that the inventors have discovered that the performance of the acousto-optical devices are most strongly influenced by the waveguide dimensions. That is, whilst previously it had not been known which factors exactly influence the non-optimal behaviour of the conversion efficiency along the optical waveguide, the inventors have discovered that the problem lies within the waveguide dimensions.
Conventional List-Off Method
In the prior art the so-called xe2x80x9clift-offxe2x80x9d method has been established as the method to use for making an indiffused channel waveguide, As for example described in Theodor Tamir (Ed.), xe2x80x9cGuided-wave optoelectronicsxe2x80x9d, Springer, 1990, pages 146 to 149, this method has been extensively studied in order to provide a set of manufacture conditions which are presently generally accepted as suitable for making indiffused channel waveguides that can be used in integrated optics. FIG. 11 shows the principle of the lift-off method.
A polished substrate 1 made e.g. from LiNbO3 is cleaned and a photoresist 2 is deposited on the substrate 1 (FIG. 11a). The photoresist 2 is of a dual-tone-type and parts that are not exposed during the first exposition to UV-light are removed by a developer solution. A mask 3 with a desired waveguide pattern 4 is placed in contact with the photoresist 2 which is exposed to UV-light (FIG. 11a). A baking step of the photoresist 2 follows by heating the substrate to about 120xc2x0 C. for approximately 210 s to cause a reversal of the photoresist characteristics. The photoresist is then exposed for a second time to UV-light, without the mask 3 (FIG. 11b), to cause a reversal of the photoresist characteristics so as to achieve a negative photoresist during the development process. As shown in FIG. 11c, upon developing to remove the exposed photoresist 2, a window corresponding to a waveguide pattern is left in the photoresist 2. As shown in FIG. 11d, a titanium layer 5 is deposited over the entire structure by RF-sputtering, electron beam deposition or a resistively heated evaporator. As seen in FIG. 11d, the titanium layer 5 is deposited on the disposed region of the substrate 1 and on the photoresist 2. The entire structure is then placed in a photoresist solvent which removes the photoresist and the unwanted titanium leaving the desired strip of titanium 5 on the substrate 1 as is shown in FIG. 11e. The process from FIG. 11d to FIG. 11e is called the xe2x80x9clift-offxe2x80x9d step, The entire structure is then heated to indiffuse the titanium strip 5 into the substrate 1 to form the indiffused waveguide 6 as is shown in FIG. 11f. 
U.S. Pat. No. 5,227,011 describes a method for producing a second harmonic wave generating device. It is stated that forming optical waveguides in LiNbO3 by It diffusion is disadvantageous, since it is difficult to obtain great differences in refractive index from the bulk crystal. To produce a waveguide that is useable in such a non-linear device, a LiTaO3 layer is provided on a LiNbO3 substrate and a LiNbO3 waveguide layer is provided on the LiTaO3 layer. In order to provide a ridge waveguide (see FIG. 1e) the LiNbO3 waveguide layer is dry etched to obtain the ridge geometry. Thus, a ridge having a large refractive index change is manufactured.
U.S. Pat. No. 4,851,079 describes lithium niobate waveguide structures, where an indiffused channel waveguide is provided in the lithium niobate substrate and where additional electrodes are deposited onto the surface of the substrate. The electrode structure comprises aluminium, gold on chromium, or gold on titanium. Using a dry etching method unmasked regions of the conducting layer provided on the surface of the substrate are etched to form the electrodes. No details about the indiffused channel waveguide in the substrate are given here.
Therefore, the prior art described so far only used the liftoff method for producing the channel waveguide as is generally described with reference to FIG. 11.
The inventors found, as is shown in FIG. 11c, that by usage of a dual tone photoresist 2, invariably an undercut or negative gradient occurs. This undercut cannot be controlled in a predictable way. The inventors further found that normally the titanium strip 5 is about 0.5 xcexcm to 2 xcexcm wider than expected (desired) on the chromium mask pattern 4. For example, if the pattern 4 has a width of 7 xcexcm, then the actual strip width can be up to 8.5 xcexcm in an unpredictable way. Furthermore, the applicant has found experimentally that the width of the waveguide can vary along the length of the waveguide by xc2x10.5 xcexcm in an unpredictable way.
On the basis of the above experiments conducted in connection with the acousto-optic mode converter, where a waveguide dimension variation is found to contribute critically to the mode converter performance, and on the basis of the investigations made in connection with the lift-off method, where an unpredictability with respect to the waveguide width in particular was established, the inventors of the present application perceived a problem not known from the prior art, namely that the conventional lift-off method cannot provide waveguides with a good performance, i.e. for example a small birefringence variation along the waveguide. The applicant has established that this is mainly due to a large width variation of the titanium-stripe along the waveguide (and between several waveguide samples).
The above-described prior art shows that this problem with the conventional lift-off method had not been realised before.
In the patent abstracts of Japan, Vol. 011, No. 026 (P-539) and JP 61 196 106 A, a Ti film is vapor-deposited on the surface of an electrooptic substrate composed of LiNbO3. An optical waveguide pattern is formed on the Ti film with a photoresist. The Ti film except a part forming the optical waveguide is removed by an ion etching technique such that a very thin residual Ti film is left on the surface of the electrooptic substrate at portions where no waveguide is to be formed in the substrate. Then, the remainder of the Ti film which is to be diffused is allowed to in fact diffuse into the electro-optical substrate. The extremely thin residual Ti film allows the formation of fine highly precise patterns by ionic etching and allows damage to the electro-optical substrate by ion collision to be minimized.
However, the thickness of the residual Ti film must be controlled precisely not to expose the surface of the substrate (since otherwise there is a damage of the substrate crystal) at the end of the etching process. Therefore, this type of etching process is a complicated, material-wasting- and time-consuming process.
Therefore, the problem of the invention is
to provide a method, a waveguide and integrated optics devices resulting to a high degree of accuracy, in particular in the width direction, when making indiffused optical waveguides, without requiring complicated manfacture steps.
This problem is solved by a method according to claim 1. Furthermore, this problem is solved by an indiffused optical waveguide according to claim 1. The problem is also solved by devices as mentioned in claims 8-15.
The inventors propose to use a chemical/physical etching technique. rather than the lift-off method or an ion etching in order to produce a refractive index raising-material stripe of a particular geometry on a substrate with much higher accuracy than known from the lift-off method. The combined chemical/physical etching technique has the advantage of providing for a reduced damaging effect of the substrate surface caused by the exposure of the substrate at the end of the etching process. Accordingly, the advantage is achieved that no precise control of the end of the etching process is necessary becausexe2x80x94when using the chemical/physical etching techniques according to the inventionxe2x80x94no substantial risk of damaging of the substrate surface exists when it does become exposed.
The method according to the invention is generally useable for making any kind of indiffused optical waveguide in a substrate. Thus, the method of the invention can be used for making any kind of integrated optics device having a much improved operability, in particular devices having a high sensitivity to waveguide profile variations, such as, e.g. an acousto-optical mode converter, an acousto-optical switch, an optical power splitter, a dual-output Mach-Zehnder modulator, a polarisation splitter and an electro-optical switch.
The chemical/physical etching techniques comprise downstream plasma reactor etching, an electroncyclotron resonance etching and a reactive ion etching technique. A preferred embodiment of the invention uses the reactive ion etching technique.
The reactive ion etching technique may be used for example in a CF4, SF6, CHF3, Cl2, or SiCl4 atmosphere. A preferred embodiment of the invention uses a SiCl4 gas atmosphere. Other preferred values for the manufacture conditions are a gas flow rate of 20 sccm and/or a pressure of 8 mTorr and/or a process power of 190 W and/or a RF generator frequency of 13.56 MHz and/or a bias voltage of 400 V and/or a process time of 225 s.
Furthermore, when the process gas is SiCl4, a preferred value for the pressure is 4 to 300 mTorr and preferably about 8 mTorr.
According to the invention any substrate material or any waveguide material may be used. Preferably the substrate material is a birefringent material such as LiNbO3. A preferred material for the metal layer is Ti.
A preferred embodiment of the waveguide geometry is a straight waveguide for example useable in an acousto-optical mode converter. However, the invention is not restricted to a particular geometry such that also a curved waveguide structure a branch or other shapes are within the scope of the invention.
The indiffused waveguide according to the invention is useable in any integrated optics device, such as, for example in an acousto-optical mode converter, an acousto-optical switch, an acousto-optical filter, an optical power splitter, a dualoutput Mach-Zehnder modulator, a polarisation splitter and an electro-optical switch. Also combinations of these devices on the same substrate can use the waveguide structure according to the invention.
Further advantageous embodiments and improvements of the invention may be taken from the dependent claims.
Hereinafter, the invention will be described with reference to its embodiments.