Method for producing a polarization converter, polarization converter and polarization converter element

Abstract

It is provided a method for producing a polarization converter The method comprising the following steps: producing a first converter element which has a base side, an upper side extending parallel to the base side, and a longitudinal side oriented obliquely at an angle ε.sub.1 to the base side or a curved longitudinal side; producing a second converter element which has a base side, an upper side extending parallel to the base side, and a longitudinal side oriented obliquely at an angle ε.sub.2 to the base side or a curved longitudinal side; arranging the first and the second converter element in series in such a way that the obliquely oriented or curved longitudinal sides point in opposite directions.

Claims

1. A method for producing a polarization converter, comprising the following steps: producing a first converter element which has a base side, an upper side extending parallel to the base side, a first longitudinal side oriented perpendicular to the base side and the upper side and a second longitudinal side, wherein the second longitudinal side is a flat side oriented obliquely at an angle ε.sub.1 to the base side or a curved side; producing a second converter element which has a base side, an upper side extending parallel to the base side, a first longitudinal side oriented perpendicular to the base side and the upper side and a second longitudinal side, wherein the second longitudinal side is a flat side oriented obliquely at an angle ε.sub.2 to the base side or a curved side; and arranging the first and the second converter element in series in such a way that outward-pointing normal vectors of the first longitudinal sides of the first and the second converter element point in opposite directions, wherein: the first converter element has a length L.sub.1 measured along a longitudinal direction of the first converter element and its upper side has a width w.sub.1 measured perpendicular to the longitudinal direction of the first converter element; the second converter element has a length L.sub.2 measured along a longitudinal direction of the second converter element and its upper side has a width w.sub.2 measured perpendicular to the longitudinal direction of the second converter element, and the steps of producing the first and the second converter element comprise: determining lengths L.sub.1 and L.sub.2 different from one another and widths w.sub.1 and w.sub.2 different from one another, such that after passing through the first and the second converter element in a propagation direction parallel to the first and second longitudinal sides of the first and second converter element, a light wave polarized initially parallel to the base sides of the converter elements is polarized perpendicular to the base sides of the converter elements.

2. The method according to claim 1, wherein the determination of the different lengths L.sub.1 and L.sub.2 as well as different widths w.sub.1 and w.sub.2 is effected by using a numerical optimization method with the quantities L1, L2 and w.sub.1, w.sub.2 as variable parameters.

3. The method according to claim 2, wherein an arrangement comprising the first and the second converter element is described by a Jones matrix.

4. The method according to claim 3, wherein the components of the Jones matrix each depend on L.sub.1, L.sub.2 as well as w.sub.1, w.sub.2, wherein by means of the optimization method L.sub.1, L.sub.2 as well as w.sub.1, w.sub.2 are determined such that one of the components of a light wave exiting from the second converter element becomes zero.

5. The method according to claim 1, wherein the length L.sub.1 of the first converter element or the length L.sub.2 of the second converter element is a rational fraction of the respective beat length.

6. The method according to claim 1, wherein the longitudinal side of the first or the second converter element has a curvature.

7. The method according to claim 6, wherein the curved longitudinal side is produced by an etching method.

8. A polarization converter, comprising: a first converter element which has a base side, an upper side extending parallel to the base side, a first longitudinal side oriented perpendicular to the base side and the upper side and a second longitudinal side, wherein the second longitudinal side is a flat side oriented obliquely at an angle ε.sub.1 to the base side or a curved side; and a second converter element which likewise has a base side, an upper side extending parallel to the base side, a first longitudinal side oriented perpendicular to the base side and the upper side and a second longitudinal side, wherein the second longitudinal side is a flat side oriented obliquely at an angle ε.sub.2 to the base side or a curved side, wherein: the first and the second converter element are arranged in series such that outward-pointing normal vectors of the first longitudinal sides of the first and the second converter element point in opposite directions; and the first converter element has a length L.sub.1 measured along a longitudinal direction of the first converter element which is different from the length L.sub.2 measured along a longitudinal direction of the second converter element, wherein: the upper side of the first converter element has a width w.sub.1 measured perpendicular to the longitudinal direction of the first converter element, which is different from a width w.sub.2 of the upper side of the second converter element measured perpendicular to the longitudinal direction of the second converter element.

9. The polarization converter according to claim 8, wherein at least one of the first and the second converter element has a trapezoidal cross-section.

10. The polarization converter according to claim 8, wherein the first and the second converter element is a semiconductor component.

11. The polarization converter according to claim 10, wherein the first and the second converter element each include at least one semiconductor layer arranged on a substrate, wherein an upper side of the respective substrate defines the respective base side of the converter element.

12. The polarization converter according to claim 8, wherein the widths w.sub.1, w.sub.2 are chosen such that in each of the first and the second converter element, a mode ({right arrow over (e)}.sub.00)propagates whose polarization plane forms an angle different from 45° with the respective base side of the converter element.

13. The polarization converter according to claim 8, wherein the widths w.sub.1, w.sub.2 are chosen such that the angles formed by polarization planes of the modes propagating in the first converter element with the base side of the first converter element are different from the angles formed by the polarization planes of modes propagating in the second converter element with the base side of the second converter element.

14. The polarization converter according to claim 8, wherein the lengths L.sub.1 and L.sub.2 are not integer multiples of each other.

15. A method for producing a polarization converter, comprising the following steps: producing a first converter element which has a base side, an upper side extending parallel to the base side, a first longitudinal side oriented perpendicular to the base side and the upper side and a second longitudinal side, wherein the second longitudinal side is a flat side oriented obliquely at an angle ε.sub.1 to the base side; producing a second converter element which has a base side, an upper side extending parallel to the base side, a first longitudinal side oriented perpendicular to the base side and the upper side and a second longitudinal side, wherein the second longitudinal side is a flat side oriented obliquely at an angle ε.sub.2 to the base; and arranging the first and the second converter element in series in such a way that outward-pointing normal vectors of the first longitudinal sides of the first and the second converter element point in opposite directions, wherein: the first converter element has a length L.sub.1 measured along a longitudinal direction of the first converter element and its upper side has a width w.sub.1 measured perpendicular to the longitudinal direction of the first converter element; the second converter element has a length L.sub.2 measured along a longitudinal direction of the second converter element and its upper side has a width w.sub.2 measured perpendicular to the longitudinal direction of the second converter element, and the steps of producing the first and the second converter element comprise determining lengths L.sub.1 and L.sub.2 different from one another, and at least one of: determining widths w.sub.1 and w.sub.2 different from one another, and determining angles ε.sub.1 and ε.sub.2 different from one another, such that after passing through the first and the second converter element in a propagation direction parallel to the first and second longitudinal sides of the first and second converter element, a light wave polarized initially parallel to the base sides of the converter elements is polarized perpendicular to the base sides of the converter elements.

16. A polarization converter, comprising: a first converter element which has a base side, an upper side extending parallel to the base side, a first longitudinal side oriented perpendicular to the base side and the upper side and a second longitudinal side, wherein the second longitudinal side is a flat side oriented obliquely at an angle ε.sub.1 to the base side; and a second converter element which likewise has a base side, an upper side extending parallel to the base side, a first longitudinal side oriented perpendicular to the base side and the upper side and a second longitudinal side, wherein the second longitudinal side is a flat side oriented obliquely at an angle ε.sub.2 to the base side, wherein: the first and the second converter element are arranged in series such that outward-pointing normal vectors of the first longitudinal sides of the first and the second converter element point in opposite directions; and the first converter element has a length L.sub.1 measured along a longitudinal direction of the first converter element which is different from the length L.sub.2 measured along a longitudinal direction of the second converter element, wherein at least one of: the upper side of the first converter element has a width w.sub.1 measured perpendicular to the longitudinal direction of the first converter element, which is different from a width w.sub.2 of the upper side of the second converter element measured perpendicular to the longitudinal direction of the second converter element, and the angles ε.sub.1 and ε.sub.2 are different from one another.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention will be explained in detail below by means of exemplary embodiments with reference to the Figures.

(2) FIG. 1 schematically shows a side view of a polarization converter according to an exemplary embodiment of the invention.

(3) FIG. 2 shows a sectional view of a polarization converter element according to the invention.

(4) FIG. 3 shows a representation of the conversion efficiency in dependence on the width of a converter element of a conventional polarization converter.

(5) FIG. 4 shows a representation of the dependence of the conversion efficiency on the width of one of the converter elements of a polarization converter according to the invention.

DETAILED DESCRIPTION

(6) The polarization converter 1 schematically shown in FIG. 1 includes a first and a second converter element 11, 12. The two converter elements 11, 12 are arranged in series (along their longitudinal axes) adjacent to each other, wherein the coupling of light into the polarization converter 1 is effected via the first converter element 11 and the outcoupling of light is effected via the second converter element 12. More exactly, the polarization converter 1 includes an input waveguide 13 via which the light is coupled into the first converter element 11 and hence into the polarization converter 1. Furthermore, a second output waveguide 14 is present, via which light can be outcoupled from the second converter element 12 and hence from the polarization converter 1. It is also possible in principle that the converter elements 11, 12 are arranged at a distance from each other.

(7) The sectional representations also contained in FIG. 1 each show a cross-section of the first and the second converter element 11, 12. Accordingly, the first converter element 11 has a trapezoidal cross-section with a base side 111 and an upper side 112 parallel to the base side 111 with a width w.sub.1. In addition, the first converter element 11 has two longitudinal sides 113, 114, of which the one (the longitudinal side 113) extends perpendicular to the base side and the upper side 111, 112. The other longitudinal side 114 is oriented obliquely, namely at an angle ε.sub.1 to the base side and the upper side 111, 112. The base sides 111, 112 can be defined by surfaces (e.g. of a substrate) each protruding beyond the longitudinal sides 113, 114. The converter elements 11, 12 in particular are arranged such that their base sides 111, 112 extend at one level and parallel to each other.

(8) The second converter element 12 likewise has a trapezoidal cross-section with a base side and an upper side 121, 122 as well as a longitudinal side 123 extending perpendicular thereto and a longitudinal side 124 oriented obliquely (at an angle ε.sub.2) to the base side and the upper side 121, 122. The upper side 122 has a width w.sub.2.

(9) The first and the second converter element 11, 12 are arranged in series (along the propagation direction of the light coupled into the polarization converter 1) such that the obliquely oriented longitudinal sides 114, 124 point in opposite directions.

(10) The converter elements 11, 12 have different lengths L.sub.1, L.sub.2, the length L.sub.1 being less than the length L.sub.2. In addition, the widths w.sub.1, w.sub.2 and/or the angles ε.sub.1, ε.sub.2, which the oblique longitudinal sides 114, 124 include with the base side 111, 121, are different.

(11) The lengths L.sub.1, L.sub.2, the widths w.sub.1, w.sub.2 and/or the angles ε.sub.1, ε.sub.2 were determined by means of an optimization algorithm as explained above, so that the polarization of a light wave entering into the polarization converter 1 (i.e. into the first converter element 11) is rotated by 90° when passing through the polarization converter (i.e. through the first and the second converter element 11, 12). For example, the polarization of a light wave {right arrow over (E)}.sub.TE oriented parallel to the base side 111 of the first converter element 11 on entry into the first converter element 11 is rotated when passing through the polarization converter 1 such that the light wave {right arrow over (E)}.sub.out exiting from the second converter element 12 is oriented perpendicular to the base side 121 of the second converter element 12.

(12) As likewise already explained above, the widths w.sub.1, w.sub.2 and/or the angles ε.sub.1, ε.sub.2 are determined such that the angles α.sub.1, α.sub.2 included by the polarization planes of the modes {right arrow over (e)}.sub.00 each propagating in the first and the second converter element 11, 12 with the respective base side 111, 121 are different. In particular, the angles α.sub.1, α.sub.2 are greater or less than 45°.

(13) FIG. 2 relates to a configuration of a polarization converter element 110 according to the invention, wherein FIG. 2 shows a section through the polarization converter element 110 perpendicular to its longitudinal axis. The polarization converter element 110 analogous to the converter elements of FIG. 1 has a base side 1110, an upper side 1120 parallel thereto, and a further longitudinal side 1130 which extends perpendicular to the upper side 1120 and to the base side 1110. However, the polarization converter element 110 has no purely trapezoidal cross-section.

(14) Rather, instead of a flat longitudinal side oriented obliquely to the base side 1110 a concavely curved longitudinal side 1140 is present (with a curvature 1141). The conversion properties of the polarization converter element 110, however, likewise are determined in particular by the width w of the upper side 1120. In addition, the radius R of the curvature 1141 of the longitudinal side 1140 can influence the conversion properties. Thus, the conversion behavior of the polarization converter element 110 can also be effected via an adaptation of the radius of curvature R.

(15) The polarization converter element 110 is formed from a plurality of semiconductor layers 200 which are arranged on a substrate 201. For example, the production of the curved longitudinal side 1140 is effected by an etching method, wherein via the duration of etching the radius R (and hence also the width w of the upper side 1120) can be set.

(16) FIG. 3 illustrates the relationship between the conversion efficiency (y-axis) and the width of the upper side of a converter element of a conventional polarization converter. What is shown here is the case of a polarization converter with only one converter element (broken line) and a polarization converter with two converter elements (continuous line). To obtain a conversion efficiency of at least 99%, i.e. 99% of the intensity of the input wave is present in an output wave with a polarization rotated by 90°, a tolerance of about 110 nm accordingly is required in the case of the polarization converter with two converter elements. For a conversion efficiency above only 95%, about 270 nm of tolerance still are required.

(17) FIG. 4 illustrates the dependence of the conversion efficiency on the width w.sub.1 and w.sub.2 of one of the converter elements of the polarization converter according to the invention. Accordingly, the tolerance requirements are distinctly lower. A conversion efficiency of 99% already is achieved with a tolerance of about 250 nm. For a conversion efficiency above 95%, a tolerance of about 350 nm even is sufficient. When using converter elements with curved longitudinal sides (FIG. 2) even greater tolerances can be achieved (e.g. 450 nm with a conversion efficiency of 99% and 530 nm with a conversion efficiency of 95%).