Optical polarisation controller with waveguide structure.

p0001The invention relates to an optical polarization controller with a waveguide structure for transforming the polarization state of a guided through the optical beam and means for adjusting the coupling constant K of the TE and TM modes of the optical beam and for adjusting the difference Δβ of the propagation constant β_TE and β_TM these modes.

p0002Such by DE-OS 36 00 458 known arrangements are used to change the polarization state of an optical beam selectively. Specifically, linearly polarized light is converted into elliptically polarized light of predetermined polarization. Preferably, such formed as a strip waveguide components for the coherent reception in the optical communication technology are needed. It must have two optical beams whose frequencies differ by an intermediate frequency, are superimposed with the same polarization state as possible. The same applies for applications in which Parts with the same beams (zero intermediate frequency) to be superposed. Not only is the quotient of the run in mutually perpendicular planes TM and TE modes must be the same for both beams, but also the respective phase differences of the two modes. This can generally only by controlling the polarization state of one of the two beams, for example, the local oscillator beam can be achieved, which is to adjust the time-varying polarization state of an incoming beam via an optical line. Without readjustment fluctuations would the intensity of for receiving intermediate frequency signal occur (see. Also Electronic Letters, 1985, Vol. 21, pp 787-788).

p0003In the known monolithic device of the type mentioned above is used as a waveguide structure, an electro-optical device, in which via various complicated structure, down to electric voltage electrode systems on the one hand K and Δβ the other hand, each are adjustable for itself.

p0004The invention has for its object to provide a different type of arrangement of the type mentioned.

p0005The object is achieved in that the waveguide structure is a magneto-optical device, and that the coupling constant K acting magnetic field and are DifferenzΔβ by the waveguide structure by a force applied to the waveguide structure mechanical deformation force adjustable.

p0006An inventive polarization controller is constructed with a less expensive component than the magneto-optical strip waveguide. Even with this, not only the control of K, but also the control of Δβ in the manner specified in claim 1 is possible in a unitary body.

p0007Since suitable for magneto-optical element layer materials are also necessary for other elements of the integrated optics, the polarization controller according to the invention may be combined with other known optical elements such as in particular an insulator in a simple manner.

p0008The force required to control the phase difference Δβ pressure force can be applied in various ways to the light-guiding waveguide structure.

p0009In this case, care must be taken in the normal case, that the mechanical tension over the entire magneto-optical waveguide is evenly distributed. To achieve special effects, it may be advantageous to let the mechanical forces acting on the uneven layer surfaces.

p0010It is advantageous not only apply either positive or negative pressure forces, but to extend the control range by applying positive pressure forces to negative pressure forces. An advantageous solution for this is characterized in that the mechanical deformation force is produced by an applied to the waveguide structure bending moment.

p0011It is also possible that the mechanical deformation force acts perpendicularly on a stamp to the layer planes.

p0012A particularly compact solution that easily enables a precise and fine-grained control is characterized in that a piezoelectric element with a layer of the magneto-optical waveguide structure directly connected, in particular glued.

p0013A used for the invention magneto-optical device is preferably so constructed that on a pure or substituted from gadolinium gallium garnet (GGG) existing substrate, further layers on the basis of pure or substituted (for example with lead / bismuth / gallium) rare earth iron garnet deposited epitaxially. It may be advisable that absorption layers are applied to the substrate layer and / or on an upper cladding layer.

p0014Refractive indices of the upper and the lower cladding layer must be smaller than that of the waveguide layer located between these.

p0015The control of the coupling constant K can in a known manner (see FIG. GB-PS 15 29 374) by application of a magnetic field, the effective in the direction of the optical beam component is changed.

p0016A particularly advantageous combination of a polarization controller with a required for laser light sources optical isolator results from the fact that the polarization controller is connected on a common substrate body having a substantially a same layer structure having optical isolator.

p0017Between the insulator and polarization controller, a portion of the waveguide may be formed as a polarizer in addition.

p0018A mechanical decoupling of the polarization controller of the insulator is advantageously achieved that the combined modules are decoupled through between them and situated transverse to the direction of the optical beam notches.

p0019An influence of the insulator by the force applied to the polarization controller magnetic field can be avoided by screening which is particularly effective thereby be achieved that engage the indentations ferromagnetic shield plates which collect due to their high magnetic permeability stray fields. The invention and its advantages will become apparent from the description of illustrated in the drawing advantageous embodiments.<ul><li>Figure 1 shows an arrangement for selectively superimposing receiving transmit light of different wavelengths.</li><li>Figure 2 shows schematically a cross section through an inventive magneto-optical device in which the adjustable pressure force is generated by a piezoelectric element.</li><li>Figure 3 indicates a particularly advantageous possibility of producing the desired deformation force by a bending moment, which is generated by a directly abutting piezoelectric element.</li><li>Figure 4 shows the top view of a monolithic unit of an optical isolator and a polarization controller according to the invention.</li><li>Figure 5 shows a cross section through the arrangement according to FIG. 4</li></ul>

p0020In Figure 1 are the transmitter side with message signals encoded transmit beams of wavelengths λ₁ to λ_n to n respectively passed through optical isolators 1 in a common optical transmission line. 5 prevent the insulators that light is scattered back into the normally formed as a semiconductor laser transmitting light sources.

p0021The transmitted beams is at the receiver end, a light beam of a local oscillator having a wavelength λ_O superimposed whose frequency is controlled to a value such that the difference frequency corresponds exactly to the beam selectively receiving the intermediate frequency to which an output electric signal of the photodetector 4-sustaining receiver circuit not shown is tuned.

p0022The beam of the local oscillator is passed through the optical isolator 6, and then additionally the polarization controller 7th By the polarization controller 7, the polarization state of the local oscillator beam is adapted to the temporally fluctuating polarization state of the receiver side, the incoming transmission beam.

p0023The polarization controller 7 consists of a built-up in a known per se magneto-optical device, which means are associated which allow the controlled adjustment of the one part of K and the other part of Δβ. The magneto-optical device is a waveguide whose optical properties can be changed by a magnetic field and by a mechanical stress.

p0024The coupling constant K, which practically corresponds to the specific Faraday rotation, can be adjusted in a known manner by the direction of the magnetic field relative to the direction of light propagation in the waveguide. The amount of the magnetic field must always have a certain minimum strength to hold the magneto-optic material of the waveguide in saturation. Critical for the Faraday effect and thus for the coupling between the modes is only the component of the magnetic field which is effective in the light propagation direction in the waveguide. If Kmax are the maximum coupling constant and α is the angle between the magnetic field and the direction of light propagation can be coupling constant K according to the relationship K_Max· Cos between the values ​​K_Max and -K_Max set to.

p0025The DifferenzΔβ, the propagation constants ß_TM and ß_TE the TM and TE modes can be set according to the invention by an externally imposed on the waveguide of the magneto-optical component stress.

p00262 shows an embodiment for a device for applying a mechanical stress is shown schematically. The waveguiding structure with the substrate 8 and the waveguide layer 9 is inserted in a rigid housing 10th By means of the piezo element 11 is connected via the plunger 12, a mechanical compressive pressure force in particular to the perpendicular to the plane of the paper embedded in the waveguide layer light-guiding strips 13 be applied. Since the measures necessary for deforming stresses are generated specifically in the light-guiding region 13, the piezoelectric element needs to generate 11 only relatively small forces.

p0027By means of a bending moment can be applied compressive or tensile stresses, as it is seen with reference to Figure 3, in which a preferred particularly compact design is shown. If bending forces 14 and 15 act in the direction of the arrows on the waveguide structure, a compressive stress is formed parallel to the film plane, with 16 the zero strain neutral layer is indicated by dashed lines in the light-conducting strips. 13 The compressive stress resulting in the expansion of the particular material of the strip 13 perpendicular to the plane of the layer 9, as if in this perpendicular direction, a tensile force would be exercised. In dotted directions 17 and 18 forces acting in the strip 13 is formed, however, a tension which causes compression of the strip 13 perpendicular to the layer 9 level. By changing the mechanical stress from a positive to a negative maximum Δ β can be varied within a wide range. This is done by the piezoelectric element 11 of known type, which may be domed concave or convex by voltage applied to its electrodes voltages.

p0028For the invention, particularly suitable waveguide structures are epitaxial layer body. On an approximately 0.5 mm thick GGG substrate layer 8 a lower high absorbing about 3 microns thick layer of Co-YIG is first applied. Then there is a lower cladding layer of particular lead substituted yttrium iron garnet (YIG) with a thickness of about 4 microns. Sandwiched between this and the lower cladding layer same upper cladding layer is about 6 microns thick waveguide layer 13 made of pure or substituted YIG. The refractive indices of the cladding layers 9 are slightly smaller than the refractive index of the waveguiding layer thirteenth

p0029On the upper cladding layer, a further highly absorbent layer be arranged, in which case, if appropriate, can be omitted, the lower absorbent layer.

p0030In the arrangement shown schematically in Figure 4 of the insulator 6 and the polarization regulator 7 are constructed as components of a magneto-optical component produced in common. A cross-section at the points of the dashed lines 19 or 20, Figure 5 shows, in which over a layer sequence of Figure 3, an additional absorption layer 21 and an only schematically indicated bending means 22 with a piezoelectric element can be seen.

p0031For the insulator 6 and the polarization controller 7 independent bending means 22 are provided, each of which act only on the assigned to them assembly. For better mechanical decoupling of components indentations 23 and 24 are provided, in which ferromagnetic shield plates 25 engage, which are designed to prevent stray lines of acting on the polarization controller 7 magnetic spread to the isolator 6 between them.

p0032In the constriction area the wave-guiding layer 13 is provided with a metallic cover layer 26, thereby there the effect of a polarizer is obtained.

p0033The annular coils 27 and 28 are intended to indicate the generation of a magnetic field in the direction of propagation of light through which the material is magnetically saturated. This magnetic field can be generated permanently magnetically. When using coils can be achieved by controlling the current fine adjustment of the Faraday effect reach.

p0034By means of the force action of the insulator associated piezoelectric device 22 can be adhered to the insulator 6, a value Δβ = 0 (phase-matching).

p0035By the polarization controller 7 associated piezoelectric device 22 on the other hand, any WertΔβ is in this area, within certain limits set (phase-tuning).

p0036The coupling constant K of the polarization controller 7 is regulated by an electrically generated magnetic field to the required value. The coaxial to the body layer aligned toroidal coils 29 and 30 produce a field component in the direction of the wave-guiding layer 13, the lateral annular coils 31 and 32 has a component perpendicular thereto in the direction of the layer plane. From angle α of formed from these two field components resulting field direction with the direction of the waveguide layer 13, the coupling constant depends on K.