DE19530461A1 - Polarising of light guided in optical waveguide - Google Patents

Abstract

The method polarises light which is guided in an optical waveguide. The light to be polarised is guided through a section of the waveguide which is expanded in the light propagation direction. This section has a structure of optical layers in the space filled with the field of the light wave. In this structure flat layers with refractive indices greater than the average refractive index along the axis if the space, are arranged next to other flat layers whose refractive indices are less than the average refractive index. The layer normal vectors form an acute angle with the waveguide axis. This angle is in the range of 20 deg. to 70 deg. Preferably the method uses a waveguide section with a quasi-continuous construction of the optical layered structure in which the transition of the optical refractive index is continuous between adjacent layers with higher and lower refractive indices over distances which are not greater than an average vacuum wavelength of the light to be polarised.

Description

The invention relates to a method for polarizing light in an optical Waveguide is guided. The invention further relates to methods of manufacture of structures for performing the polarization process.

The polarization of light that passes through a waveguide is of interest to An applications in optical metrology, optical communications and optical Si Signal processing with technical systems in optical fibers or in integrated optical Use waveguides guided light.

In these areas of application, there is often a need for one in a world Luminous flux led by the conductor, which is unpolarized or an unknown polarization is able to filter out a given polarization component. This is general possible by passing the light through a polarizer specially designed for the Operation in a waveguide is constructed. The effect of such a polarizer be is that there is a characteristic polarization state of the There is light for which the power transmitted from the waveguide input to the output becomes minimal, while at the same time for the orthogonal input polarization state the transmitted light output is maximum. You can choose between production methods a certain polarization and those for measuring a polarization are used for which "polarizers" or "analyzers". The specialist is common, however, as a method intended to generate polarization is to be modified to analyze polarization. In this respect, only in the following Methods for generating polarization mentioned, even if these methods are always flat can be used for polarization analysis.

Methods for polarizing light using optical components are common well-known open space optics. Basically absorbent and distinguish between non-absorbent processes. For the former, which is also called "dichroi table ", it is characteristic that they have the unwanted polarization Absorb on component, for example by using film polarizers. In non-absorbent polarization processes, the undesirable polarization component laterally deflected out of the luminous flux, for example by use birefringent prisms.  

Non-absorbent methods include one in which the light is passed through a stack of inclined dielectric plates [cf. WG Driscoll and W. Vaughn (Eds.), "Handbook of Optics", McGraw-Hill, New York NY (1978), pp. 10-88]. The polarizing effect occurs because the interface of two different dielectric media has a higher reflectivity for light whose electrical vector oscillates perpendicular to the plane of incidence (so-called polarization) than for light whose electrical vector is parallel to the plane of incidence vibrates (p component of polarization). The inclination of the interface directs the reflected light out of the luminous flux. Therefore, the above-mentioned higher reflectivity means a greater attenuation of the s component with respect to the transmitted light. If the incident light is unpolarized or has an unspecified polarization, its s-polarized component experiences greater attenuation than the p component when it passes through the plate stack. The p-polarized component then predominantly remains at the exit of the stack. This polarization process with a plate stack works particularly well in terms of low transmission loss if the angle of incidence Θ of the light on the plates is chosen to be approximately equal to the Brewster angle Θ _B , because at this angle the reflectivity for the p component disappears [cf. Born and E. Wolf, "Principles of Optics", Pergamon Press, Oxford (1980)]. The size of the Brewster angle is given by Θ _B = arctan (n _H / n _L ), where n _{H is} the refractive index of the plate material and n _{L is} the index of the spaces.

The described polarization method and other free-space polarization methods are not directly suitable for light which is guided in an optical waveguide. Rather, they have to be adapted to the special conditions of the optical waveguide. The latter usually have the shape of elongated fibers or strips, consisting of a core and one or more jacket materials surrounding the core. The refractive index n₀ of the core is typically somewhat larger than the refractive index n _i of each of the cladding materials (i = 1, 2, 3,...). The light spreads mainly in the area of the core, on average parallel to its longitudinal direction. This direction of propagation is selected below as the z axis of the Cartesian xyz coordinate system used. The two other coordinates (x, y) thus relate to the cross-section / plane of the waveguide that is transverse to the direction of propagation.

The transverse refractive index distribution is determined in an optical waveguide and the vacuum wavelength λ of the guided light, the number and the characteristics field shapes of the optical wave types that are capable of propagation in the waveguide are [cf. A.W. Snyder and J.D. Love, "Optical Waveguide Theory", Chapman and Hall, London (1983)]. In this sense, a distinction is made between so-called  th single-mode waveguides in which only one wave type can propagate, and Multimode waveguides that can carry multiple wave types. The common technical Optical waveguide perspective refers to weakly guiding waveguides, which are characterized by a small index difference between the core and cladding areas are standardized. In this way, each wave type exists in two to each other orthogonal polarization states or polarization "modes". Their characteristics The field directions are general due to the distribution of the optical refractive index n (x, y) determined over the waveguide cross section.

In the process known for the polarization of light guided in a waveguide absorbent and non-absorbent variants can also be distinguished the. The dichroic process uses a metallic or metal-like layer arranged in the immediate vicinity of the waveguide core so that the axis of the waves conductor is parallel to the layer. By excitation of plasmons on the metal layer then becomes light, the electric field vector of which does not vibrate parallel to the layer plane (p component), absorbed by the layer. The spread of this component in the The waveguide is therefore damped.

Non-absorbent process used to polarize light in optical waveguides are known, exploit anisotropic effects, in particular birefringence, by crystalline materials, by elastic stress distribution or by anisotropic geo metric arrangement, such as curvature. These effects cause the waveguide not to light a particular polarization component leads more because for this component the effective index of the waveguide is smaller than the refractive index of one of the surrounding cladding materials. This polarization comm component therefore leaks into the relevant cladding material, that is, from the waveguide emitted. For the orthogonal polarization component, the wave guide is against it normal in the sense that the effective index is larger than the index of all the kernels surrounding cladding materials. To this group of non-absorbent waveguiding Polarizers can also be used in all types of polarizing directional couplers which the unwanted polarization output coupled out of the input waveguide component is not emitted, but in the core of an adjacent waveguide is coupled. In addition to these two groups of actual waveguides described, which have a polarizing effect over a larger section of their length are also "hy bride "process. In them, the waveguide is cut and a thin one Free space polarizer placed between the waveguide ends. The light leaves the on optical waveguide, passes through the free space polarizer as freely propagating radiation through it, and then, to a greater or lesser extent, turns back into a second Coupled (output) waveguide.  

The prior art with regard to layer structures in optical waveguides and their polarization properties include the pe commonly known as "fiber grids" periodically layered structures in the core of optical fibers. They are called so-called Bragg reflectors are used to guide the light guided in a waveguide back into the To reflect the direction from which it arrives. For this application the Layer planes oriented perpendicular to the fiber axis, d. H. it becomes an angle of Θ = 0 used [cf. R. J. Campbell, R. Kashyap, "The Properties and Applications of Photo sensitive Germanosilicate fiber ", INTERNATIONAL JOURNAL OF OPTOELECTRO- NICS, Vol. 9, No. 1, pp. 33-57 (1994)]. Inclined fiber grids with Θ ≠ 0 can be used in order to deflect the light guided in a fiber laterally out of the fiber. There a particular polarization component is preferably coupled out. This can be done for the construction of a fiber polarimeter, as described in more detail by A. Bouzid, M.A.G. Abushagar, A.E-Sabe, and R.M.A. Azzam, ["Fiber-Optic Four-Detector Polarimeter ", OPTICS COMMUNICATIONS Vol. 118, pp. 329-334 (1995)].

The manufacture of these special, periodic layer structures for Bragg reflectors and fiber polarimeters takes place directly in the core of the optical fibers in that the latter are laterally irradiated with short-wave ultraviolet light in a spatially periodic (strip-like) pattern, the direction of the strips thus being inclined towards the fiber axis becomes that the above-mentioned inclination angle Θ is achieved. With this exposure technique, local increases in the optical refractive index n up to δn <10 ^-2 can be achieved [cf. JL Archambault, L. Reekie and P. St. J. Russell, "High Reflectivity and Narrow Bandwidth Fiber Gratings Written by Single Excimer Pulse", ELECTRONICS LETTERS Vol. 29, pp. 28-29 (1993)].

Another known technique for creating index changes inside optical Ion implantation is a waveguide. Thereby ions become electric to such high energy accelerates that it over distances of 10. . . 100 µm deep in dielectric materials penetrate when in the form of an ion beam onto the surface of the material be shot. In the area where they come to rest, the ions then increase the local refractive index if they are heavier than the atoms of the host material, and they lower the local refractive index when they're lighter. An application this technique for increasing the index is described by J. Albert et al. in the OPTICS LETTERS, Vol. 17 (1992), p. 1652.

All of the above-mentioned methods for the polarization of Light in a waveguide has disadvantages affecting its function and / or its manufacture tion of the polarizing waveguides.

An ideal method for polarizing light that is guided in a waveguide, would let the light of a certain polarization pass undiminished and that  completely block orthogonally polarized light. Furthermore, the ideal would have to be Process for use in waveguides with any given cross-section suitable, it should work over a wide range of optical wavelengths, and it should be feasible with little technical effort. The known methods are imperfect in these respects.

In the hybrid polarization methods mentioned, it is a problem that the Po Larizer transmitted light not completely back into the output waveguide can be coupled. Therefore, these polarization methods have the disadvantage of a ho forward attenuation. In the non-absorbing polarization methods mentioned it is disadvantageous that they are common because of the use of single crystalline materials are complex if the optical system in which they are to be used is because of the lower price is made of glasses. These procedures have the further Disadvantage undesirably high transmission loss, because two transitions between fiber and crystalline waveguide section are required.

High transmission loss is also the disadvantage of those polarization methods for in Waveguides light, in which the birefringence due to elastic stresses or is generated by curvature. Generally, the higher they work, the better is their birefringence. This can cause transitions to non-birefringent waveguides make necessary and then adversely affects the transmission loss.

The invention is therefore based on the object for an optical waveguide to be constructed fiber optic or integrated optical system a simple, knockout to indicate the most cost-effective method by which it is guided in a given waveguide Light can be polarized broadband, and from which the polarizing waveguide section has essentially the same core cross section as the waveguides of the rest Systems. With the method, the light is said to have a selected polarization state Propagation in the waveguide can be damped as much as possible, while light of the ortho gonal polarization state should happen as undamped as possible.

This object is achieved according to claim 1 in that the light to be polarized is passed through a section of the given waveguide, which is provided in the space filled by the field of the light wave with a structure in which, in addition to essentially planar layers, their refractive indices (n _{i, H} ) are greater than the refractive index () averaged along the axis of said space, there are other essentially planar layers whose refractive indices (n _{i, L} ) are lower than said average refractive index (), and their layer normal vectors () Form the inclination angle Θ _i with the waveguide axis, which in the range Θ _i = 20 °. . . 70 °.

Different points of view for optimized implementations of the Ver  driving in general optical waveguides form the characteristic part of the Un claims 2-12. Advantageous implementation forms that are specific to waveguides refer in the form of optical fibers, with the subclaims 13-16 bean speaks. Further advantageous implementation forms, which are specific to waveguides Obtain an integrated optical form are claimed in subclaims 17-19. The claims 20-27 relate to the production of layer structures, such as Implementation of the procedure are necessary.

The method for polarization according to the invention can be understood as the waveguide equivalent of the polarization process known from free-space optics, in which a stack of inclined dielectric plates is used. In generalization of this method on waveguides, inclined dielectric interfaces are introduced into the light-guiding cross section of the waveguide, as stated in the characterizing part of claim 1. The optical refractive index must be different on both sides of such an interface. To improve the polarization effect, it is advantageous to use a large number of such interfaces. This creates a layer structure of areas with a higher and a lower refractive index. The sequence of the refractive indices along the axis of the waveguide and the orientation of the layers can be much more general than with the discrete plates in the free-space optics. Although there the space between the plates is empty (refractive index n _L = 1), which is advantageous for the polarizing effect, this is not necessary in principle. Because the method is based only on the polarization-dependent dependence of the reflectivity of inclined interfaces, a polarizing effect is achieved in the waveguide even if the optical refractive index is only lower in the space between the high-index areas than in the interior of these areas (n _{0, L} <n _{0, H} ).

The production of the layer structures that are used to carry out the process in the Wel must be introduced in waveguides made of photosensitive material from the outside by exposure to ultraviolet light. In other waves Local index changes can be conducted by implanting atoms or ions generate, or as a result of locally acting electrical or elastic fields.

The essential difference of the method according to the invention compared to the known State of the art is that the latter only the properties of the fiber core decoupled light concerns, but no information about the polarization of the in the world remaining light there. In contrast, the method according to the invention allows by using the layered dielectric structure, what remains in the waveguide Polarize light.

The polarization method according to the main claim or one of the subclaims  is beneficial for several reasons.

A first general advantage of this method is that it has particularly low throughputs damping can be achieved.

Furthermore, in many cases this method can be used with any waveguide given index profile are applied so that no transitions between the connecting waveguides and a polarizer waveguide with a special cross section have to. This applies in particular to optical fibers made of quartz glass, in which the op Table layered structure made by side exposure to ultraviolet light can be. The inventive method avoids that the waveguide ground, curved or mechanically clamped.

Third, in the polarization method according to the invention, the intended us grows with the number of layers used. It can therefore be done in a simple manner be chosen large so that a predetermined blocking damping is achieved.

Moreover, it is an advantage of the polarization process of the invention that, for certain applications having a predetermined stop band attenuation A _S, the necessary length L = A _S / α _S of the polarizing waveguide section may be chosen within wide limits constructively freely by a greater length of the section by a smaller cavities modulation amplitude (n _H -n _L ) of the layer indices can be compensated. This enables the waveguide properties to be optimized for different applications.

Another advantage of the method according to the invention is that it is relative to light wide optical bandwidth works. This is due to the fact that no critical, wel length-dependent resonance effects must be exploited, but that continuously at each of the interfaces, the unwanted polarization component is coupled out takes place like a grating coupler.

Further details and features of the method according to the invention result from the following description of implementation examples with reference to the drawings. Show it

Fig. 1 is a schematic side view of an optical waveguide, in which an inclined periodic layer structure is introduced, as can be used to carry out the method according to the invention.

Fig. 2 is an enlarged view of FIG. 1 with the areas with a higher and lower refractive index.

Fig. 3 cross-section / shapes of integrated optical and fiber optic waveguides, in which the polarization method according to the invention is applicable.

Fig. 4 is a perspective view of an integrated optical strip waveguide with an applied periodic layer structure for performing the method according to the Invention.

For the following explanation of the method according to the invention, it is initially expedient to specify some of the terms used more precisely. The polarizing effect of a layer structure can be described quantitatively by the length-related damping constants of the two polarization components. Here, α _S refers to the component of maximum attenuation (s polarization) and α _p to the component of minimum attenuation (p polarization) orthogonal to the former. The optical power transmission T of a polarizing waveguide section of length L is thus

T _S = exp (-α _S L) for s polarization (1)

T _p = exp (-α _p L) for p polarization (2)

Here T _{S means} the residual transmission of the waveguide for light of the polarization to be blocked, and T _p its transmission for the polarization of the light to be used at the end of the waveguide. Accordingly, A _S = -10 log T _S denotes the blocking attenuation of the waveguide in decibels, and accordingly A _p = -10 log T _p its forward loss. The general requirement for polarization methods is that their "polarization contrast" C = A _S -A _p should be as large as possible. Depending on the application, there may also be other requirements. In optical communications technology, for example, the blocking attenuation should typically be 20. . . 30 dB in order to sufficiently suppress the crosstalk of an orthogonally polarized message channel. When using a polarization method to determine the polarization in a laser resonator, on the other hand, the main requirement is that the transmission loss should be as small as possible, for example A _p <0.5 dB.

The waveguide shown by way of example in FIG. 1 consists of a core 1 and a jacket 2 surrounding it. The layer structure according to the invention is introduced into the core area. It consists of the layers 4 with the higher optical refractive index n _{0, H} , which are represented by hatched areas, and the areas 3 in between with the lower index n _{0, L.} The layers 4 form the equivalent to the plates of the stack mentioned in the case of free space polarization, while the layers 3 correspond to the spaces between the plates. The longitudinal direction of the core is marked as the z-axis of the coordinate system, the y-axis perpendicular to it. The x-axis is perpendicular to the plane of FIG. 1. The layered structure extends in the waveguide core 1 over a section of length L. The polarization component vibrating in the plane of FIG. 1 experiences according to the effect of the plates in free space polarization minimal attenuation when propagating in the waveguide, the perpendicular component is maximally attenuated.

Fig. 2 shows the regions of higher and lower refractive index clearer. An interface normal is also entered here for one of the layers, as well as the angle of inclination Θ between the normal and the axis of the waveguide. Next 2 measured along the waveguide axis layer dimensions are indicated in Fig., G _H for the layers with higher index, g _L for the layers of lower index, and g = g _L + g _H for the overall dimension of a period of the layer structure.

In this example, the layers of the index structure are parallel to the area of the Waveguide core limited and periodic. Such properties are beneficial, however not necessary for the polarization effect of the method.

Because the light in a waveguide is mainly guided in the area of the waveguide core the layer structure should also preferably be in the core region of the waveguide be introduced. For the polarizing effect, however, it is more favorable if the Layer structure beyond the core area also at least as far as in the core extends near the edge of the waveguide jacket, like the field of the guided Light penetrates into the coat. A weaker polarizing effect is even achieved even if the layer structure does not exist in the core, but alone is introduced into the last-mentioned edge region of the jacket.

Other important features for characterizing the polarization according to the invention process are the spatial orientation of the interfaces that define the layers kidney, the thickness of the high index and low index layers, as well as the exact course the refractive index at the transition between the high index and low index range. For an efficient polarization effect, i.e. for a high polarization contrast of the light tes at the end of the structured waveguide section, one or more of the The following procedures for carrying out the inventions described below advantageous method according to the invention:

• (a) The high index layers should be oriented in space in such a way that all limits surface normals (vectors perpendicular to the interface) lie in a plane that also includes the Includes axis of the waveguide. This level is then common for all layers Plane of incidence on which the s and Obtain p components of the polarization. This orientation ensures that all interfaces preferentially pass the same polarization component and the  dampen others, orthogonal to them.
• (b) All high index layers should be oriented in space so that the two limits Surface normals of each high index layer are aligned parallel to each other are. This adds up the polarizing effects of the two interfaces a high index layer in an optimal way.
• (c) All high index layers should be oriented in space so that all of them Interface normals are aligned parallel to each other. This adds up the polarizing effects of all interfaces in an optimal way.
• (d) At each individual interface, the angle between the normal direction and the waveguide axis should preferably be Θ = 45 °. This corresponds to the "Brewster" case of the free-space optics and provides the lowest possible transmission loss. The specified angle of Θ = 45 ° is just the Brewster angle that results from the above-mentioned relationship Θ _B = arctan (n _H / n _L ) of the free-space optics in the limit case when the index difference Θ _B = n _{0, H} -n _{0, L} becomes small against the mean index n₀ of the waveguide, where Δn «.
• (e) The interfaces between the high and low index layers should be as sharp as possible in order to achieve high reflectivity. This means more precisely that the spatial distance along which a possibly continuous transition of the refractive index from n _{0, H} to n _{0, L} or vice versa should preferably not be greater than λ / (4n _{0, L} ) if it is along the Waveguide axis is measured.
• (f) The thickness g _{H of} each individual high index layer measured parallel to the waveguide axis should not be chosen too small. Preferably g _H λ / (4n _{0, H} ) should apply. Likewise, the thickness g _{L of} each individual low index layer measured parallel to the waveguide axis should not be chosen too small. For the latter, g _L λ / (4n _{0, L} ) should preferably apply. If these conditions are met, there is no mutual hindrance due to the extinguishing interference of the reflections at the two interfaces of a layer.
• (g) The thickness g = g _H + g _L of each pair of adjacent high index and low index layers measured parallel to the waveguide axis should preferably be selected in the size of g = 2λ / (n _{0, H} + n _{0, L} ). This value corresponds to the Bragg condition known from X-ray physics, in the fulfillment of which the reflection is particularly large and in which the overall length of the polarizing waveguide section required for a given polarization contrast is particularly short. The reason for this is that if the Bragg condition is met, the amplitudes of the partial waves reflected at pairs of adjacent layers interfere constructively with one another.

Further details of the implementation of the method according to the invention relate to the type of optical waveguide with which it can be used. There are no limits to the principle of using the inclined layer structure. In Fig. 3, the cross sections of three types of optical waveguides are exemplified in which the method is applicable. They could all belong to the side view chosen in FIGS. 1 and 2, but differ in their index profiles n (x, y). In detail, the cross section shown in FIG. 3a) denotes an integrated optical waveguide, in which the waveguide core 1 is arranged at the interface between the cladding material 2 and the free space 5 . FIG. 3b) shows a so-called "buried" integrated optical waveguides with core 1 and shell 2. The cross section in Fig. 3c) represents an optical fiber with a round core 1 and jacket 2 .

Fig. 4 schematically shows an implementation example of the method when applied to a waveguide in integrated optical form. Here, the layer structure is not introduced into the waveguide core, but into the cladding material above it, which in the case shown is free space. Since the field of the wave guided in the waveguide core 1 extends over a short distance in the x direction into this cladding area, the expansion of the layer structure in the x direction can be correspondingly small here. This results in the indicated structure in the form of a sequence of strips 6 of dielectric material which are applied to the surface of the waveguide core at an angle Θ to the x-axis. These strips represent the high index areas of the layer structure, the free valleys in between are the low index areas.

The effectiveness of the polarization method according to the invention can be estimated from the orientation, number and thickness of the layers. For this it is assumed that the angle of inclination of the layer normal is approximately equal to the Brewster angle, i.e. that Θ = Θ _B = 45 °. Then the attenuation due to the layer structure becomes minimal (α _p → 0), and the transmission loss of the waveguide is then essentially determined by other loss mechanisms. This special feature, which was briefly justified above, forms a particularly advantageous feature of the method according to the invention for the applications mentioned at the outset.

The other important property for the polarization process is the achievable blocking attenuation A _S. It is approximately proportional to the length L of the structured waveguide section. In order to keep this length as short as possible for a given blocking damping, a layer structure with a damping constant α _S that is as high as possible should be used. Provided that Θ≈45 °, α _S becomes particularly large if the greatest possible modulation amplitude (n _H -n _L ) of the index is used, if the layer structure used is periodic, if its period g is chosen according to the Bragg condition and if the thicknesses g _H and g _{L of} the high index and low index layers are made approximately equal.

The latter optimization conditions follow from the Bragg condition in its known form G = λ / (2 cos Θ), where G is the period of the structure measured in the direction of the layer normal. For abbreviation, the size = λ _n / is introduced for the average optical wavelength in a medium with the average refractive index, and the abbreviation g = G / cos Θ for the period of the layer structure measured along the waveguide axis. The Bragg condition generally gives the optimal value g = / (2cos² Θ). In the case considered here Θ = Θ _B = 45 °, this optimal value is simply g =

In this case, the damping constant α _{S of} the blocking damping can also be estimated and the required length L can then be specified. The amplitude reflectivity of the interface of two media with the refractive indices n _H and n _L has the size r = Δn / at etwa = 45 ° and s polarization, as can be seen from the known Fresnel formulas for the limit case of a small index difference Δn = (n _H -n _L ) «1 results. In the direction y transverse to the waveguide axis, in which the reflected light leaves the waveguide, a number M of reflected partial waves overlap in a cross-section / plane xz, which emanate from the individual interfaces, which in this cross-section / plane Push through the waveguide core. The effective number M of such interfaces is M≈2D / g if D is the transverse dimension of the waveguide core. These M partial waves carry with them an optical power which is of the order of a fraction M²r² of the s-polarized power carried in the waveguide. Since they originate from a waveguide section whose length is approximately equal to the core diameter D, the damping constant sought is

In order to illustrate this relationship with a numerical example, a fiber made of quartz glass is used, which has a core diameter of D = 10 µm and conducts a luminous flux of the vacuum wavelength λ = 1.5 µm. Under favorable circumstances, a refractive index difference of Δn = 0.01 can be achieved in the fiber. The above formula thus provides an attenuation constant of α _S ≈7 dB / mm. It follows that for a blocking attenuation of A _S = 20 dB the section of the waveguide provided with the layer structure must have a length of approximately L = 3 mm. Moreover, in this embodiment, the thicknesses of the high and low index layers should be chosen according to the property (f) mentioned above for a high polarization contrast C, the values g _H = 0.5 μm, so that the periodicity measured along the fiber axis becomes about g = 1 µm. The effective number of interfaces M is of the order of magnitude M = 20.

In optical fibers, particularly those made of quartz glass, the method can be used to advantage. The most important case is that of single-mode fibers, especially those single-mode fibers that are highly birefringent and can therefore be operated to preserve polarization. It is advantageous for them to align the azimuthal orientation of the inclined layer structure with the orientation of the main axes of the birefringence. The same applies to birefringent integrated optical waveguides. Only if all interface normals lie in one of the two planes spanned by the waveguide axis and the main axes of the birefringence, does the polarization component preferably transmitted in the method coincide with an intrinsic state of the birefringent waveguide, and the polarization state does not change any further if the light continues to propagate after passing through the structured waveguide section. This requirement is met in particular in the example of FIG. 4, where the main axes of the birefringence are parallel and perpendicular to the surface of the substrate for reasons of symmetry.

The layer structures necessary for carrying out the method according to the invention can be produced directly in that high-energy ion radiation on the side the waveguide is directed, which locally the optical bre index increased or decreased. Your energy is chosen so that this place of Index change lies in the core of the waveguide. At the same time, their spatial distribution through a mask or by moving the beam so that the described Layer structure results. This radiation can either be with light ions, which lower the index locally, or with severe ones, which cause an index increase.

A related, advantageous production method for the layer structure can be used, if the waveguide, and in particular the waveguide core, is sensitive to radiation is when it is exposed to X-ray or visible light or ultraviolet light or electron beams change its refractive index. An example are quartz glass waveguides, which become photosensitive when doped with germanium. Inside such Waveguides can be layered from the outside by exposure to ultraviolet light Structures are created. The spatial distribution of the light is determined by a Mask or an interference field predefined and then determined, similar to a photographic exposure, the resulting index structure in the waveguide.

Another well-established method of producing structures with a layered optical refractive index is photolithography. Here, too, the exposure can take place through masks or, especially for periodic structures, in an optical interference field. In the simplest case, the exposed and developed photoresist itself can serve as a dielectric high-index medium. Alternatively, it can be used in a known manner as an etching mask for structuring underlying materials. The intermediate, freely developed or etched areas can directly represent the low index layers. A resulting from this manufacturing method waveguide shows the Fig. 4.

Finally, it is also possible and of particular interest for applications to produce the layer structure in the waveguide by means of elasto-optical or electro-optical effects. For this purpose, a mechanical or electrical voltage field with the required layer structure is applied to the waveguide. For this purpose, a corresponding mechanical introduction of force by structured mechanical elements must take place, or an electrical field strength distribution must be generated by appropriate electrodes. These methods are particularly useful in the case of integrated optical waveguides according to FIG. 3a), because the elements introducing force or field can be attached to the surface of the same in direct contact with the waveguide core.

An option that is important for applications in communications and measurement technology is then to make the polarization effect achieved with the method time-controllable, for example in order to use it to modulate a luminous flux guided in the waveguide. This is possible by modulating the size n _H -n _{L of} the index difference of the layers by modulating the mechanical or electrical voltage.

Claims (27)

1. A method for polarizing light which is guided in an optical waveguide, characterized in that the light to be polarized is passed through an extended in the light propagation direction section of the waveguide, which contains an optically layered structure in the space filled by the field of the light wave , in which, in addition to essentially flat layers whose refractive indices (n _{i, H} ) are greater than the refractive index () averaged along the axis of said space, there are other essentially flat layers whose refractive indices (n _{i, L} ) are lower than the mean refractive index (), and their layer normal vectors () form inclination angles Θ _i with the waveguide axis, which are in the range Θ _i = 20 °. . . 70 °. 2. A method for polarization according to claim 1, characterized by using egg Nes waveguide section with quasi-continuous structure of the optical history structure, in which the transition of the optical refractive index between neighboring strata with higher and lower index is steadily above Distan zen that are not larger than an average vacuum wavelength of the polarized Light, and in which the areas with the stated average index value () as Layer delimitation levels can be viewed. 3. Method for polarization of guided light according to one of the preceding An sayings, characterized in that uses an optically layered structure in which all normal vectors () of the layer-delimiting planes in one Lie plane, which also contains the axis of the waveguide. 4. A method for polarization of guided light according to one of the preceding claims, characterized in that an optically layered structure is used in which adjacent planes which delimit a layer with a higher refractive index (n _H ) are parallel to one another. 5. Method for polarization of guided light according to one of the preceding An sayings, characterized in that uses an optically layered structure where all normal vectors () of the layer-delimiting planes are parallel to each other. 6. A method for polarization guided light according to one of the preceding claims, characterized in that an optically layered structure is used in which the angle formed by the layer normals () and the waveguide axis Θ _{i is} in the range 40 ° Θ _i 50 ° . 7. A method for polarization guided light according to one of the preceding claims, characterized in that an optically layered structure is used in which the angle von _i formed by the layer normals e _i and the waveguide axis is _in the range 44 ° Θ _i 46 °. 8. A method for polarization of guided light according to any one of claims 5-7, because characterized in that an optically layered structure is used, at which is the arrangement existing on a section of the waveguide axis of the layers along the total extent of the layer structure on the world axis conductor axis repeated several times periodically. 9. The method for polarization of guided light according to claim 8, characterized in that an optically layered structure is used, in which each of the ge periodically arranged sections of the waveguide comprises two sub-layers, the refractive indices of all layers (n _{i , H} ) are above said average refractive index (), and in the second sub-layer the refractive indices of all layers (n _{i, L} ) are below said average refractive index (). 10. The method for polarization of guided light according to claim 9, characterized in that an optically layered structure is used, in which the layer thickness (g _H ) of the sections with the higher indices, measured between the passage points of the waveguide axis through the two layer delimitation planes Section, are not less than a tenth of an average shaft length (λ / n _H ) of the light to be polarized in a medium with the refractive index of the section concerned. 11. A method for polarization guided light according to claim 9, characterized in that an optically layered structure is used in which the layer thickness (g _L ) of the partial sections with the lower indices, measured between the points of passage of the waveguide axis through the two layer limitation levels of the section, is not less than one tenth of an average wavelength (λ / n _L ) of the light to be polarized in a medium with the refractive index of the section concerned. 12. A method for polarization of guided light according to claim 9, characterized in that an optically layered structure is used, in which each of said periodically arranged sections of the waveguide has a layer thickness (g = g _H + g _L ), between the Passage points of the waveguide axis through the respective first layer delimitation planes of two adjacent periods are measured and which satisfies the condition 0.3 <g <3, in which an average wavelength of the light to be polarized is in a medium with the said average refractive index (). 13. Method for polarization of guided light according to one of the preceding An sayings, characterized in that the waveguide used with the optical layered structure is an optical fiber. 14. A method for polarization of guided light according to claim 13, characterized indicates that the optical fiber used is a single mode fiber. 15. A method for polarization of guided light according to one of claims 13-14, characterized in that the optical fiber used is birefringent. 16. A method for polarization of guided light according to claim 15, characterized records that the normal vectors of the layer delimitation planes of the optical layered structure and the waveguide axis span a plane, the one of the two main axes of the birefringent fiber. 17. A method for polarization of guided light according to one of claims 1-13, since characterized in that the waveguide used with the optically layered Structure is an integrated optical waveguide on a substrate. 18. A method for polarization of guided light according to claim 17, characterized records that the from the normal vectors () of the layer delimitation planes and the plane spanned by the waveguide is perpendicular to the substrate plane. 19. A method for polarization of guided light according to claim 18, characterized records that the from the normal vectors () of the layer delimitation planes and the plane spanned by the waveguide lies parallel to the substrate plane.   20. Method for polarization of guided light according to one of the preceding An sayings, characterized in that the optically layered structure used in the waveguide due to inhomogeneous lateral radiation of the extended waves conductor section was generated with high-energy radiation. 21. A method for polarization of guided light according to claim 20, characterized records that the radiation used for radiation corpuscular ion beam lung is. 22. A method for polarization of guided light according to claim 20, characterized records that the radiation used for irradiation electromagnetic radiation lung is. 23. A method for polarization of guided light according to claim 21, characterized records that electron beams are used for irradiation. 24. A method for polarization of guided light according to one of claims 20-23, characterized in that the high energy radiation in one of the materials of the waveguide directly to a local change in the optical refractive index leads. 25. A method for polarization of guided light according to claim 21, characterized records that the optically layered structure used in the waveguide by means a photolithographic process is manufactured in which by development and etching parts of the waveguide material are removed. 26. A method for polarization of guided light according to one of claims 1-20, characterized in that the optically layered structure used thereby is caused in the waveguide that by means of the side of the waveguide Electrodes in its inner area, filled by the field of the guided wave, one electro-optical refractive index change is induced. 27. A method for polarization of guided light according to one of claims 1-20, characterized in that the optically layered structure used thereby is caused in the waveguide that by mechanical structuring of the Waveguide in its inner area, which is filled by the field of the guided wave, an elasto-optical refractive index change is induced.