Optical element and photonic integrated circuit

Abstract

An optical element (10) comprises a prism (100) comprising a dielectric material having a refractive index n, and a gradient index layer (108) arranged in contact with a first side face (101) of the prism (100). A refractive index of the gradient index layer (108) varies in a first direction perpendicular to the first side face (101). A first refractive index n1 of the gradient index layer (108) in a first portion (109) directly adjacent to the first side face (101) of the prism (100) fulfills 0.8 ≤ n1 / n ≤ 1.2. The refractive index of the gradient index layer (108) monotonically decreases towards a first main surface (110) of the gradient index layer (108) remote from the first side face (101) of the prism (100). A photonic integrated circuit (20) comprises a waveguide (115) and the optical element (10) as explained above.

Description

[0001] OPTICAL ELEMENT AND PHOTONIC INTEGRATED CIRCUIT

[0002] Photonic integrated circuits comprising a waveguide , a radiation emitting optoelectronic device and further optical components are increasingly employed in a variety of applications . Generally, ef forts are taken to improve the coupling of emitted electromagnetic radiation into a waveguide without the need of performing a precise alignment .

[0003] It is an obj ect of the present invention to provide an improved optical element and an improved photonic integrated circuit .

[0004] SUMMARY

[0005] According to embodiments , the above obj ects are achieved by the claimed matter according to the independent claims . Further developments are defined in the dependent claims .

[0006] An optical element comprises a pri sm comprising a dielectric material having a refractive index n, and a gradient index layer arranged in contact with a first side face of the prism . A refractive index of the gradient index layer varies in a first direction perpendicular to the first side face . A first refractive index nl of the gradient index layer in a first portion directly adj acent to the first side face of the prism ful fills 0 . 8 < nl / n < 1 . 2 . The refractive index of the gradient index layer monotonically decreases towards a first main surface of the gradient index layer remote from the first side face of the prism .

[0007] For example , the refractive index of the gradient index layer further decreases monotonically in a second direction paral lel to the first side face symmetrically with respect to a center optical axis . Accordingly, the gradient index layer may implement a 2d gradient index layer, having a col limation function in two directions .

[0008] According to embodiments , a photonic integrated circuit comprises a waveguide , and the optical element as explained above .

[0009] For example , the second direction may be perpendicular to an extension direction of the waveguide .

[0010] For example , the first main surface of the gradient index layer may be arranged laterally adj acent to the waveguide .

[0011] According to embodiments , a second surface of the gradient index layer intersecting the first main surface may be arranged adj acent to the waveguide so that at least part of the second surface of the gradient index layer is arranged between the waveguide and the gradient index layer .

[0012] For example , a second refractive index n2 of the gradient index layer at a portion adj acent to the first main surface ful fills n2 / n3 < 1 . 05 , wherein n3 is a refractive index of the waveguide in a region adj acent to the gradient index layer .

[0013] The photonic integrated circuit may further comprise an optoelectronic device emitting electromagnetic radiation to a second side face of the prism, the second side face intersecting the first side face . The second side face may deflect the electromagnetic radiation towards the first side face . For example , the second side face may be coated with an anti- reflective (AR) coating to minimi ze the reflection from the second side face . For example , the second side face may be arranged facing the optoelectronic device .

[0014] According to embodiments , a third side face vertically extending from the first side face may be arranged between the second side face and the optoelectronic device .

[0015] For example , the optoelectronic device may be a semiconductor laser, for example an edge emitting semiconductor laser or a surface emitting semiconductor laser .

[0016] The photonic integrated may further comprise further optoelectronic devices emitting electromagnetic radiation at di f ferent wavelengths , respectively, the further optoelectronic devices being arranged in a direction intersecting a propagation direction of the waveguide .

[0017] According to further embodiments , the photonic integrated circuit further comprises further optoelectronic devices emitting electromagnetic radiation, the further optoelectronic devices being arranged in a direction parallel to a propagation direction of the waveguide .

[0018] For example , the waveguide may be a multi-mode waveguide or a single mode waveguide .

[0019] BRIEF DESCRIPTION OF THE DRAWINGS

[0020] The accompanying drawings are included to provide a further understanding of embodiments of the invention and are incorporated in and constitute a part of this speci fication . The drawings illustrate the embodiments of the present invention and together with the description serve to explain the principles . Other embodiments of the invention and many of the intended advantages will be readily appreciated, as they become better understood by reference to the following detailed description . The elements of the drawings are not necessarily to scale relative to each other . Like reference numbers designate corresponding similar parts .

[0021] Fig . 1 shows an example of a photonic integrated circuit comprising an optical element according to embodiments .

[0022] Fig . 2A shows a cross-sectional view of an example of a gradient index layer .

[0023] Fig . 2B shows a cross-sectional view of a further example o f a gradient index layer .

[0024] Fig . 3A illustrates a photonic integrated circuit and an optical element according to further embodiments .

[0025] Fig . 3B illustrates a photonic integrated circuit and an optical element according to further embodiments .

[0026] Fig . 3C illustrates a photonic integrated circuit and an optical element according to further embodiments .

[0027] Fig . 4A shows a photonic integrated circuit comprising a plurality of electromagnetic radiation emitting optoelectronic devices .

[0028] Fig . 4B illustrates a photonic integrated circuit comprising parallel coupling of electromagnetic radiation emitted by a plurality of radiation emitting optoelectronic devices . Fig. 5 illustrates an optical element and a photonic integrated circuit according to further embodiments.

[0029] DETAILED DESCRIPTION

[0030] In the following detailed description reference is made to the accompanying drawings, which form a part hereof and in which are illustrated by way of illustration specific embodiments in which the invention may be practiced. In this regard, directional terminology such as "top", "bottom", "front", "back", "over", "on", "above", "leading", "trailing" etc. is used with reference to the orientation of the Figures being described. Since components of embodiments of the invention can be positioned in a number of different orientations, the directional terminology is used for purposes of illustration and is in no way limiting. It is to be understood that other embodiments may be utilized and structural or logical changes may be made without departing from the scope defined by the claims.

[0031] The description of the embodiments is not limiting. In particular, elements of the embodiments described hereinafter may be combined with elements of different embodiments.

[0032] The term "vertical" as used in this specification intends to describe an orientation which is arranged perpendicular to the first surface of a substrate or semiconductor body.

[0033] The terms "lateral" and "horizontal" as used in this specification intends to describe an orientation parallel to a first surface of a substrate or semiconductor body. This can be for instance the surface of a wafer or a die.

[0034] Fig. 1 is a vertical cross-sectional view of an optical element

[0035] 10 and a photonic integrated circuit 20 comprising the optical element 10 according to embodiments . The optical element 10 comprises a prism 100 comprising a dielectric material having a refractive index n . The optical element 10 further comprises a gradient index layer 108 that is arranged in contact with a first side face 101 of the prism 100 . A refractive index of the gradient index layer 108 varies in a first direction e . g . , the z-direction, perpendicular to the first side face 101 . A first refractive index nl of the gradient index layer 108 in a f irst portion directly adj acent to the side face 101 of the prism 100 ful fills the following formula : 0 . 8 < nl / n < 1 . 2 . The refractive index of the gradient index layer 108 monotonically decreases towards a first main surface 110 of the gradient index layer 108 . The first main surface 110 of the gradient index layer 108 is remote from the side face 101 of the prism 100 .

[0036] In the context of the present disclosure , the term prism relates to an optical element having straight side faces , which is configured to refract or deflect incident electromagnetic radiation 24 . The prism 100 is made of a dielectric material , e . g . , SiO2 or any other suitable material .

[0037] The term "monotonically decrease" intends to define that the refractive index decreases or may be constant over a certain distance from the first side face 101 of the prism 100 . This wording excludes that the refractive index locally increases with increasing distance from the first side face 101 of the prism 100 .

[0038] Fig . 2A shows an example of a gradient index layer 108 . As is illustrated in Fig . 2A, the gradient index layer 108 comprises a plurality of partial layers 107 wherein the refractive index of each of the layers 107 may be constant but changes from partial layer to partial layer . In more detail , with increasing distance from the interface of the prism 100 , the refractive index of the each of the partial layers 107 decreases . In a portion directly adj acent to the pri sm 100 , the refractive index of the associated partial layer 107 is identical or nearly identical with the refractive index n of the prism 100 . Fig . 2A further shows rays of redirected radiation 25 propagating through the gradient index layer 108 . At each interface between adj acent partial layers 107 the respective rays 25 are further refracted . As a consequence , rays having components in a vertical e . g . , the z-direction, and in a hori zontal direction are redirected so as to mainly travel along the x-direction . For example , divergent rays are nearly collimated after travel ling through the gradient index layer 108 .

[0039] Fig . 2B shows a further example of a gradient index layer 108 , where the refractive index gradually changes along a vertical direction towards first main surface 110 . Such a gradient index layer 108 may also be referred to a layer having an " adiabatic" local index slope which means that a change of the refractive index with distance from a lower portion of the layer is below a certain value . In a similar manner as is illustrated in Fig . 2A, with increasing distance from a first side face 101 of a prism 100 , the radiation 25 is refracted in the x-direction . Radiation 25 propagating in the z-direction is not refracted in the x- or y-direction .

[0040] As is shown in Figs . 2A and 2B, the radiation 25 travels in a vertical direction through the gradient index layer 108 so as to propagate eventually in a hori zontal direction .

[0041] Returning to Fig . 1 , a refractive index of the gradient index layer 108 in a first portion directly adj acent to the first side face 101 of the prism 100 is almost or approximately identical with the refractive index n of the dielectric material of the prism 100 . Fig. 1 further shows a waveguide 115 which may comprise a core made of a suitable material, e.g., SixNy. The waveguide 115 may further comprise a cladding layer (not illustrated) e.g. of SiCy. According to further implementations, the cladding layer may be dispensed with. According to embodiments described, the waveguide 115 may be a multimode waveguide or a single mode waveguide. For example, the waveguide may have a shape of a ridge, so as to be a single mode waveguide.

[0042] For example, a length of a prism facet, e.g. a side face, may be larger than 1 pm. The length of the prism facet may be smaller than 1 mm. A thickness of the gradient index layer 108 may be larger than 100 nm. For example, the thickness of the gradient index layer 108 may be smaller than 10 pm.

[0043] Moreover, the photonic integrated circuit 20 comprises an optoelectronic device 118 for emitting electromagnetic radiation. The optoelectronic device 118 may e.g. be a laser, for example a semiconductor laser. The semiconductor laser may be an edge emitting semiconductor laser or a surface emitting semiconductor laser, for example a VCSEL, ("vertical cavity surface emitting laser") . According to further implementations, the surface emitting semiconductor laser may as well comprise a horizontal cavity. The optoelectronic device 118 may be implemented and in an arbitrary manner. For example, the optoelectronic device 118 may be a laser different from a semiconductor laser or may a LED ("light emitting diode") .

[0044] Electromagnetic radiation 24 emitted by the optoelectronic device 118 may enter the optical element 10 via a second side face 102 of the prism 100. For example, an angle between the first side face 101 and the second side face 102 may be different from 90°, e.g. less than 90° . Accordingly, electromagnetic radiation 24 that is incident through the second side face 102 may be deflected towards the first side face 101 of the prism . For example , the second side face 102 of the pri sm may be coated with an anti-reflective (AR) coating to minimi ze the reflection from the second side face 102 .

[0045] As is illustrated in Fig . 1 , a second surface 111 , e . g . , a side face of the gradient index layer 108 may be arranged adj acent to the waveguide . The redirected electromagnetic radiation 25 enters the waveguide 115 via the second surface 111 of the gradient index layer 108 . Further, a thickness t of the optical element 10 may be larger than a thickness of the waveguide 115 , the thicknesses being measured in the z-direction . Accordingly, a portion of the optical element 110 protrudes from the waveguide 115 .

[0046] Generally, a thickness t of the gradient index layer 108 may be selected so as to compensate a dispersion of the angle a of the emitted radiation 24 . In more detail , the larger the thickness t , the larger the dispersion a of the emitted radiation 24 may be .

[0047] According to embodiments , a second refractive index n2 of the gradient index layer in a portion adj acent to the first main surface 110 may ful fill n2 / n3 < 1 . 05 , wherein n3 is a refractive index of the waveguide in a region adj acent to the gradient index layer 108 . Nevertheless , technically the refractive index contrast between all the layers in the gradient index layer and the waveguide can be big . A large refractive index contrast does not largely af fect the performance of the optical element because most of the light coming from the gradient index layer 108 into the waveguide entrance facet would be designed to be coming at an approximately normal direction . As a result , only low reflection losses are suf fered especially for the design illustrated in Fig . 1 .

[0048] According to embodiments illustrated in Fig . 1 , a second surface 111 of the gradient index layer 108 intersecting the first main surface 110 is arranged adj acent to the waveguide 115 so that the second surface 111 of the gradient index layer 108 is arranged between the waveguide 115 and the gradient index layer 108 . The first main surface 110 of the gradient index layer 108 is not arranged laterally adj acent to the waveguide 115 . For example , the second surface 111 of the gradient index layer 108 may extend perpendicularly to the first main surface 110 of the gradient index layer 108 . Further, the second surface 111 of the gradient index layer 108 may extend perpendicularly to an extension direction of the waveguide 115 . Due to the arrangement , electromagnetic radiation 24 may be coupled to the waveguide , wherein the coupling is accomplished in a direction parallel to an extension direction of the waveguide .

[0049] The optoelectronic device 118 may be attached to the waveguide 115 . According to further implementations , the optoelectronic device 118 , the optical element 10 and the waveguide 115 may be mounted to a substrate (not illustrated in Fig . 1 ) .

[0050] Generally, when assembling an optoelectronic device 118 , an optical element for coupling the emitted electromagnetic radiation into a waveguide , and the waveguide , an alignment of the optoelectronic device 118 and the optical element 10 has to be performed . It has been shown, that according to embodiments , even when the optoelectronic device 118 is shi fted along the z- direction, a coupling ef ficiency decreases less in comparison to a conventional approach in which an edge emitting laser directly is coupled to a waveguide . Further, the coupling ef ficiency using the optical element 10 illustrated e . g . in Fig . 1 is improved . Further, it has been shown that by appropriately selecting a distance d between a further face of the prism 100 and the second surface 111 of the optical element 10 , a sensitivity of the coupling ef ficiency from a shift along the z-direction may be further decreased .

[0051] The optical element 10 may be arbitrarily arranged with respect to the waveguide 115 .

[0052] Fig . 3A is a further example of a photonic integrated circuit 20 according to embodiments . As is shown, the waveguide 115 is arranged laterally adj acent to the first main surface 110 of the gradient index layer 108 . Further, the second surface 111 of the gradient index layer 108 is not arranged between the waveguide 115 and the gradient index layer 108 in a hori zontal direction . Accordingly, the electromagnetic radiation 25 completely traverses and exits the gradient index layer 108 via the first main surface 110 .

[0053] Fig . 3B shows a photonic integrated circuit 20 according to further embodiments . As is shown, a second surface 111 of the gradient index layer 108 is arranged adj acent to the waveguide 115 , and the first main surface 110 of the gradient index layer 108 is arranged laterally adj acent to the waveguide 115 . Di f ferently stated, the gradient index layer 108 is partially embedded in the waveguide 115 , so that a portion o f the waveguide is arranged over the gradient index layer 108 . As a consequence , part of the redirected radiation 25 enters the waveguide 115 via the second surface 111 . A further portion of the redirected radiation 25 enters the waveguide 115 via the first main surface 110 . Accordingly, the gradient index layer 108 is partially embedded in the waveguide 115 . Fig. 3C shows a photonic integrated circuit 20 according to further embodiments. As is shown, the electromagnetic radiation 24 emitted by the optoelectronic device 118 enters the prism 100 via a third side face 103 different from the second side face 102. For example, the third side face 103 may extend in a vertical direction with respect to the first side face 101. When entering the prism 100 via the third side face 103, the electromagnetic radiation 24 is scarcely refracted. The electromagnetic radiation 24 is reflected by the second side face 102 towards the gradient index layer 108. For example, a mirror 105 may be arranged adjacent to the second side face 102. For example, the mirror 105 may comprise a metallic layer or alternating dielectric layers. The emitted radiation 24 is reflected by the mirror 105 and enters the gradient index layer 108. Due to the varying refractive index of the gradient index layer 108, the radiation is refracted and collimated.

[0054] Fig. 4A shows a photonic integrated circuit 20 which comprises a radiation emitting optoelectronic device 118 and a further radiation emitting optoelectronic device 119. For example, the radiation emitting optoelectronic devices 118, 119 may be arranged along a propagation direction of electromagnetic radiation, e.g. the x-direction. Accordingly, emitted electromagnetic radiation 24 may be serially coupled into the waveguide 115. For example, the radiation 25 may be already redirected in-plane or may be deeply obliquely propagating. Hence, it is possible that the radiation is the not reflected downwards and escapes the coupling structure.

[0055] Fig. 4B shows an example of a photonic integrated circuit 20, wherein electromagnetic radiation of several optoelectronic devices 118, 119, 120 that may be arranged in a horizontal direction perpendicular to the propagating direction, e.g., the y-direction, may be coupled into the waveguide 115. The optical element 10 has a width measured in the y-direction corresponding to the width of the arrangement of parallel optoelectronic devices 118, 119, 120. As a result, a parallel coupling of the electromagnetic radiation 24 emitted by each of the optoelectronic devices 118, 119, 120 in the waveguide 115 may be accomplished. For example, in this case, the waveguide 115 may be a multimode waveguide.

[0056] Fig. 5 shows a top view of an optical element 10 and a photonic integrated circuit 20 according to further embodiments. The prism 100 illustrated in Fig. 5 may be similar to the prism illustrated in the previous Figs., e.g., Fig. 1. The gradient index layer may be implemented as a 2d gradient index layer 121. In particular, the gradient index layer 121 may be implemented in a manner so that the refractive index additionally decreases monotonically in a second direction, e.g., the y-direction, parallel to the second surface 111. In more detail, as is illustrated in Fig. 5, the refractive index has its maximum at a center optical axis 122 of the 2d gradient index layer 121 along the y-direction. The center optical axis 122 is arranged at the same position in the y direction as a center axis 123 of the waveguide 115 and the radiation emitting optoelectronic device 118. The refractive index of the gradient index layer 121 monotonically decreases in the second direction parallel to the first side face 101 symmetrically with respect to the center optical axis 122.

[0057] As a result, a collimating effect in the in-plane direction, e.g., the xy-plane may be achieved. Using the 2d gradient index layer 121, the coupling to waveguides or tapered input to waveguides which are smaller in the in-plane extension direction is improved. As a result, a more compact geometry of the photonic integrated circuit 20 may be achieved. In particular, using such a 2d gradient index layer 121, an adiabatic transition region to a single mode waveguide may be reduced or such an adiabatic transition region may be completely dispensed with.

[0058] The following table gives examples of materials having a high, a medium or a lower refractive index at a wavelength of 550 nm.

[0059] Accordingly, by admixing additives having e.g. a high refractive index, e.g. TiO2, and subsequently reducing or increasing the amount of the corresponding additive, e.g. during deposition, a gradient index layer 108 may be formed. Further, by varying a stochiometric composition of e.g. SixOyNzduring deposition, a large range of refractive indexes may be covered. When using the SiON material, a waveguide 115 may be easily integrated with the radiant index layer 108. While embodiments of the invention have been described above , it is obvious that further embodiments may be implemented . For example , further embodiments may comprise any subcombination of features recited in the claims or any subcombination of elements described in the examples given above . Accordingly, this spirit and scope of the appended claims should not be limited to the description of the embodiments contained herein .

[0060] LIST OF REFERENCES optical element photonic integrated circuit emitted radiation redirected radiation prism first side face second side face third side face mirror partial layer gradient index layer first portion first main surface second surface waveguide optoelectronic device optoelectronic device optoelectronic device 2D gradient index layer center optical axis of the gradient index layer center axis of the waveguide

Claims

CLAIMS1. An optical element (10) comprising: a prism (100) comprising a dielectric material having a refractive index n, and a gradient index layer (108) arranged in contact with a first side face (101) of the prism (100) , a refractive index of the gradient index layer (108) varying in a first direction perpendicular to the first side face (101) , wherein a first refractive index nl of the gradient index layer (108) in a first portion (109) directly adjacent to the first side face (101) of the prism (100) fulfills 0.8 < nl / n < 1.2, and the refractive index of the gradient index layer (108) monotonically decreases towards a first main surface (110) of the gradient index layer (108) remote from the first side face (101) of the prism (100) , wherein the refractive index of the gradient index layer (108) further decreases monotonically in a second direction parallel to the first side face (101) symmetrically with respect to a center optical axis (122) .

2. A photonic integrated circuit (20) comprising: a waveguide (115) ; and the optical element (10) according to claim 1, wherein the second direction is perpendicular to an extension direction of the waveguide (115) .

3. The photonic integrated circuit (20) according to claim 2, wherein a second surface (111) of the gradient index layer (108) intersecting the first main surface (110) is arranged adjacent to the waveguide (115) so that at least part of the second surface (111) of the gradient index layer (108) is arranged between the waveguide (115) and the gradient index layer (108) .

4. A photonic integrated circuit (20) comprising: a waveguide (115) ; and an optical element (10) comprising: a prism (100) comprising a dielectric material having a refractive index n, and a gradient index layer (108) arranged in contact with a first side face (101) of the prism (100) , a refractive index of the gradient index layer (108) varying in a first direction perpendicular to the first side face (101) , wherein a first refractive index nl of the gradient index layer (108) in a first portion (109) directly adjacent to the first side face (101) of the prism (100) fulfills 0.8 < nl / n < 1.2, and the refractive index of the gradient index layer (108) monotonically decreases towards a first main surface (110) of the gradient index layer (108) remote from the first side face (101) of the prism (100) , wherein a second surface (111) of the gradient index layer (108) intersecting the first main surface (110) is arranged adjacent to the waveguide (115) so that at least part of the second surface (111) of the gradient index layer (108) is arranged between the waveguide (115) and the gradient index layer (108) .

5. The photonic integrated circuit (20) according to any of claims 2 to 4, wherein the first main surface (110) of the gradient index layer (108) is arranged laterally adjacent to the waveguide (115) .

6. The photonic integrated circuit (20) according to any of claims 2 to 5, further comprising an optoelectronic device (118) emitting electromagnetic radiation (24) to a second side face (102) of the prism (100) , the second side face (102) intersecting the first side face (101) , the second side face (102) deflecting the electromagnetic radiation (24) towards the first side face (101) .

7. The photonic integrated circuit (20) according to claim 6, wherein the second side face (102) is arranged facing the optoelectronic device (118) .

8. The photonic integrated circuit (20) according to claim 6, wherein a third side face (103) vertically extending from the first side face (101) is arranged between the second side face (102) and the optoelectronic device (118) .

9. The photonic integrated circuit (20) according to any of claims 6 to 8, wherein the optoelectronic device (118) is a semiconductor laser.

10. The photonic integrated circuit (20) according to claim9, wherein the semiconductor laser is an edge emitting semiconductor laser.

11. The photonic integrated circuit (20) according to claim9, wherein the semiconductor laser is a surface emitting semiconductor laser.

12. The photonic integrated circuit (20) according to any of claims 6 to 11, further comprising further optoelectronic devices (119, 120) emitting electromagnetic radiation (24) at different wavelengths, respectively, the further optoelectronic devices (119, 120) being arranged in a direction intersecting a propagation direction of the waveguide (115) .

13. The photonic integrated circuit (20) according to any of claims 6 to 11, further comprising further optoelectronic devices (119, 120) emitting electromagnetic radiation (24) , the further optoelectronic devices (119, 120) being arranged in a direction parallel to a propagation direction of the waveguide (115) .

14. The photonic integrated circuit (20) according to any of claims 2 to 13, wherein the waveguide (115) is a multi-mode waveguide .

15. The photonic integrated circuit (20) according to any of claims 2 to 13, wherein the waveguide (115) is a single mode waveguide .

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