DE102019009399B4 - Optical device - Google Patents

Abstract

In an optical device for the bidirectional coupling of a waveguide (7) to an external medium (6), the coupling bandwidth of the optical device is to be widened so that a wide spectrum of wavelengths can be coupled into an external medium via the optical device. This is achieved in that the optical device comprises at least one taper structure (2), the taper structure (2) comprising a beam input segment (3), the beam input segment (3) being set up to emit a light beam (5). the waveguide (7) into the taper structure (2), the taper structure comprising a beam output segment (4), the beam output segment (4) being set up to focus the light beam (5) and into the external medium (6 ), the taper structure (2) between the beam input segment (3) and the beam output segment (4) comprises at least one first reflection surface (8), the first reflection surface (8) being set up in such a way that the light beam (5) comes out of the plane of the waveguide (7), characterized in that the waveguide (7) is designed as a free-standing waveguide arm (14).

Description

The invention relates to an optical device for bidirectional coupling of a waveguide to an external medium.

The subject matter of the invention is defined in the appended claims.

In the field of telecommunications, bidirectional coupling to waveguides made of optical fibers is a central challenge. Waveguides correspond to electrical connections on integrated optical circuits and are therefore essential building blocks for functional photonics. They make it possible to miniaturize complex systems in particular and are therefore a key technology. However, since the guided modes in waveguides and glass fibers have very different sizes, direct coupling is associated with high losses. To avoid this, mode converters are necessary. Since optical signals are guided in the plane in waveguides, it is desirable to change the beam direction, otherwise only the edges of the chip can be used for coupling. With planar geometry, this deflection can only take place via diffractive elements or interference phenomena, which severely limits the optical bandwidth. However, a high bandwidth is necessary to maintain high data rates and maintain compatibility with today's data formats. In addition, the alignment of the glass fibers with respect to the chip is very sensitive to minimal offset and therefore requires a high level of placement accuracy, which is associated with high costs. Current contacting processes are therefore subject to losses and are not suitable for mass production. However, for all optical systems with applications in telecommunications, permanent fiber coupling is necessary for efficient packaging. Therefore, there is currently no possibility of coupling efficiently with high bandwidth and relaxed tolerances to waveguides, especially not to a large number of waveguides. Furthermore, existing coupling points cannot be tuned. Since there are manufacturing tolerances, an adjustment after manufacturing is necessary to achieve an optimal coupling.

Top-chip coupling is the preferred method. This allows many components to be addressed, which is of central importance in order to achieve a high level of integration. Two methods are known from the prior art, on the one hand coupling via grid elements and on the other hand coupling via fiber tapers in the evanescent field.

Grating couplers use diffractive elements to enable out-of-plane coupling. These couplers typically achieve efficiencies of 30%. Greater efficiencies are possible with improved designs and more complex manufacturing. The primary disadvantage of grating couplers is the narrow bandwidth in the range of a few tens of nm. This means that only specific wavelength ranges can be covered. In addition, grating couplers are very sensitive to lateral displacements and therefore have to be laboriously aligned with fibers or lenses. This makes automatic coupling to chips difficult.

Fiber tapers use reduced diameter glass fibers. These tapers are placed on the waveguides from above and couple to them in the optical near field. Therefore, in this method, the width of the waveguide is crucial for the placement tolerance. This is less than one micrometer and is therefore extremely demanding. The coupling bandwidth is high in this case because the coupling is adiabatic. However, the demanding placement cannot be used to couple to many waveguides because each individual fiber must be placed separately. Furthermore, fiber arrays cannot be used, so parallel coupling is not possible. In addition, the mechanical stability of the coupling is limited. In both cases tunability is not possible. This means that after mechanical placement, the coupling to the glass fibers cannot be further improved. However, the coupling efficiency suffers significantly, especially when precise placement requirements are required.

From the EP 2 442 165 B1 An optical device for handling a beam of radiation is known, the optical device comprising a semiconductor die comprising an integrated semiconductor optical waveguide core integrated on the semiconductor die, and an at least partially overlying waveguide comprising at least a first bevel shaped for coupling the Radiation beam between the integrated semiconductor optical waveguide core and an external medium, the first bevel having an entrance/exit side surface for receiving/emitting a radiation beam from/to the external medium, the side surface of the first bevel being of the Edge of the semiconductor chip is spaced apart by a distance d, the distance d being at least 1 µm and less than 200 µm.

In " GEHRING, H. [et.al.]: Low-loss fiber-to-chip couplers with ultrawide optical bandwidth. APL Photonics 4, 010801 (2019), pp. 010801-1 to 010801-7 , ISSN 2378-0967 “is an optical device for the bidirectional coupling of a world Lenleiters disclosed, in which the deflection of the light beam out of the plane of the waveguide takes place through a bend out of the plane (“out-of-plane bend”).

The coupling methods known from the prior art require a complex alignment of the glass fibers relative to the coupling elements. This is complex and therefore expensive. Since each component must be aligned separately with respect to the glass fibers, automatic placement is difficult and slow and therefore expensive. Furthermore, the low placement tolerances cause the components to degrade over time if there is mechanical distortion or offset compared to the optical chip. Therefore, a placement technique is necessary that offers higher tolerances and can be used for many components in parallel. This essentially requires coupling from the top of the chip via fiber arrays.

Another key disadvantage of the coupling methods known from the prior art is the low coupling efficiency of grating couplers. Losses cannot be accepted, especially for applications in which multiple connections to chips are necessary.

Therefore, high coupling efficiency is essential. A high bandwidth is particularly necessary for applications in spectroscopy and for data transmission. It is often necessary to connect to many waveguides at the same time in order to create multiple channels. This is not possible with current procedures. Likewise, the state of the art cannot implement tuning after production, which is necessary to react to long-term drift and offset.

Accordingly, the present invention is primarily intended to provide an optical device, wherein the coupling bandwidth of the optical device is very wide and a wide spectrum of wavelengths can be coupled into an external medium via the optical device. Furthermore, the invention is based on the object of coupling many glass fibers to a waveguide with low loss and achievable placement tolerances.

In order to achieve the above object, the present invention provides an optical device for bidirectionally coupling a waveguide to an external medium. According to the preamble, the optical device comprises at least one taper structure, the taper structure comprising a beam input segment, the beam input segment being set up to couple a light beam from the waveguide into the taper structure, the taper structure comprising a beam output segment, wherein the Beam output segment is set up to focus the light beam and couple it into the external medium, the taper structure between the beam input segment and the beam output segment comprises at least a first reflection surface, wherein the first reflection surface is set up to deflect the light beam out of the plane of the waveguide.

The waveguide can be coupled to the external medium in particular bidirectionally. Since the beam path of a light beam is reversible, bidirectional in the context of this invention means that a light beam can not only be coupled from the waveguide into the external medium by means of the optical device, but that the light beam can also be coupled from the external medium into the external medium by means of the optical device Waveguide can be coupled in. The beam output segment of the previously described optical device is set up in such a way that the light beam from the external medium is coupled into the taper structure, whereby the light beam is defocused, the taper structure further comprising at least a first reflection surface between the beam output segment and the beam input segment, wherein the first reflection surface is set up to deflect the light beam coming from the beam output segment into the plane of the waveguide. The beam input segment of the previously described optical device is set up to couple the light beam from the taper structure into the waveguide.

The basic idea of the present invention is to provide an optical device which enables the mode matching of planar waveguides to an external medium, for example to an optical fiber or microscope lenses. Since the path of the light beam is reversible, the external medium can also be adapted to a planar waveguide. The adjustment takes place via a three-dimensional taper structure, which is generated, for example, using direct laser writing (DLW). The optical device changes the beam direction of the beams running in the plane into the vertical dimension or vice versa. For this purpose, in particular, a reflection of the light beam on a reflection surface is used.

The optical device does not use any wavelength-selective (particularly diffractive) elements. Therefore, the coupling bandwidth is very wide and a wide range of wavelengths can be coupled into the waveguide via the coupler. The bandwidth is only possible through the trans The parity window of the coupler and the waveguide is limited. This means that the bandwidth is orders of magnitude better than with conventional grating couplers or other diffractive elements. In particular, the optical device is not limited by interference phenomena and therefore offers an enormous bandwidth. Furthermore, the optical device is independent of platform and polarization and can be used for coupling to any waveguide, for example, which brings enormous flexibility.

In a preferred embodiment of the invention, the first reflection surface is a total reflection surface. Total reflection has the advantage that it allows loss-free beam deflection.

In a further preferred embodiment of the invention, the height of the taper structure is continuously tapered in the region of the beam input segment, while the width is kept constant, whereby a waveguide region is formed.

In a preferred embodiment of the invention, the beam input segment is adjoined by a substantially conical or pyramid-shaped region, with the width and height of the cone-shaped or pyramid-shaped region increasing linearly.

In a further preferred embodiment of the invention, the beam output segment adjoins the conical or pyramid-shaped region, the beam output segment comprising a substantially curved region, the substantially curved region comprising the at least first reflection surface.

In a preferred embodiment of the invention, the taper structure comprises at least one further reflection surface.

In a further preferred embodiment of the invention, the beam output segment comprises a collimating lens. Beam collimation relaxes the alignment tolerances of the external medium relative to the optical device. This enables fiber coupling using industrial methods without the fine-tuning of individual optical devices.

In a preferred embodiment of the invention, the external medium is an optical fiber or a microscope objective.

In a further preferred embodiment of the invention, the waveguide is a planar waveguide.

In a preferred embodiment of the invention, the waveguide is arranged on a substrate.

According to the invention, the waveguide is designed as a free-standing waveguide arm. The free-standing waveguide arm allows the output angle of the optical device to be adjusted simultaneously by adjusting the inclination angle of the waveguide arm.

In a preferred embodiment of the invention, the taper structure is formed from a polymer.

In a further preferred embodiment of the invention, the optical device is manufactured using 3D printing, in particular using direct laser writing.

In a preferred embodiment of the invention, the waveguide is part of a waveguide array.

• 1 eine nicht erfindungsgemäße schematische Darstellung einer optischen Vorrichtung zur bidirektionalen Ankopplung eines Wellenleiters an ein externes Medium gemäß einem ersten Ausführungsbeispiel,
• 2 eine nicht erfindungsgemäße schematische Darstellung eine optische Vorrichtung zur bidirektionalen Ankopplung eines Wellenleiters an ein externes Medium gemäß einem zweiten Ausführungsbeispiel,
• 3 eine schematische Draufsicht eines Wellenleiterarrays mit einer Vielzahl von Wellenleitern und optischen Vorrichtungen gemäß einem Ausführungsbeispiel der Erfindung,
• 4 eine schematische Darstellung eine optische Vorrichtung zur bidirektionalen Ankopplung eines Wellenleiters an ein externes Medium gemäß einem Ausführungsbeispiel der Erfindung.

Show it:

• 1 a schematic representation, not according to the invention, of an optical device for bidirectional coupling of a waveguide to an external medium according to a first exemplary embodiment,
• 2 a schematic representation, not according to the invention, of an optical device for bidirectional coupling of a waveguide to an external medium according to a second exemplary embodiment,
• 3 a schematic top view of a waveguide array with a plurality of waveguides and optical devices according to an embodiment of the invention,
• 4 a schematic representation of an optical device for bidirectional coupling of a waveguide to an external medium according to an exemplary embodiment of the invention.

1 is a schematic representation, not according to the invention, of an optical device 1 for bidirectional coupling of a waveguide 7 to an external medium 6 according to a first exemplary embodiment. The optical device 1 essentially consists of a taper structure 2, the taper structure 2 being roughly divided into two parts: a beam input segment 3, the beam input segment 3 being used to couple a light beam 5 from the waveguide 7 into the taper structure 2 is used and a beam output segment 4, the beam output segment 4 being used for focusing and coupling the light beam 5 into the external medium 6. The light beam 5 can be coupled in, in particular, bidirectionally. Since in the optical device 1 the beam path of the light beam 5 is reversible, the light beam 5 can therefore also be coupled from the external medium 6 into the waveguide 7 by means of the optical device 1. The light beam 5 is coupled from the external medium 6 into the beam output segment 4 and defocused. The light beam 5 is then deflected into the plane of the waveguide 7 towards the beam input segment 3 by means of the first reflection surface 8 and coupled into the waveguide 7. The external medium 6 can be, for example, an optical fiber or a microscope lens. The beam input segment 3 includes in the in 1 The exemplary embodiment shown has a conical or pyramid-shaped region 10. The beam output segment 4 comprises a substantially curved region 11, the curved region 11 comprising a first reflection surface 8. As in 1 Shown in cross section, a planar waveguide 7 is coupled to the taper structure 2. Light propagating in the waveguide 7 is first expanded in beam size by the taper structure 2. The Gaussian beam generated then falls on the reflection surface 8, where total reflection takes place. This changes the direction of the beam and deflects it upwards. In order to obtain a collimated beam, the beam output segment 4 comprises a substantially curved region 11, wherein the curved region can have a lens shape, for example.

On the one hand, the taper structure 2 acts as a mode transformer, which adapts the very small mode of a waveguide 7 to the wide beam shape of optical glass fibers or microscopes 6. On the other hand, the optical device 1 allows beam deflection to be achieved out of the plane, so that waveguides 7 become accessible from the chip surface 16. This is done via several elements that can be created in 3D freeform writing. An adiabatic transition of the waveguide mode into a Gaussian beam that propagates freely in the optical device occurs by means of the taper structure 2. This expands the beam input segment 3, which is adapted to the diameter of the waveguide 7, in order to prevent losses at the edges. The beam is deflected via reflection surfaces 8. Due to the refractive index contrast between the medium of the taper structure and the surrounding medium, total reflection occurs, which occurs without losses. The refractive index contrast determines the maximum inclination angle that can be achieved. If higher angles are to be achieved, multiple reflections can be used, which also occur without loss. After reflection, the beam 5 is collimated and adapted to the subsequent external medium or the mode guide element, which can be a glass fiber or a microscope objective.

The waveguide can be made, for example, from silicon nitride. The beam input segment 4 can, for example, consist of a polymer and include a polymer waveguide. At the beginning of the beam input segment 4, an intrinsic mode source is set for coupling the light beam from the waveguide 7 in order to imitate the light output of a silicon nitride to polymer waveguide mode converter, which is located immediately in front of the beam input segment 3. The initially 0.5 µm wide silicon nitride waveguide 7 tapers linearly downwards, while the height of the polymer waveguide 4 is continuously tapered to finally 1 µm, while the width is kept constant at 1 µm. This cross section of the polymer waveguide 3 is chosen because only the basic modes are supported due to the isolation by the substrate 12 made of silicon oxide below. Therefore, it enables an adiabatic conversion of the two basic modes of the silicon nitride waveguide 7 into the two basic modes of the polymer waveguide 3. After this interface to the polymer structure, the conical or pyramid-shaped region 10 of the taper structure 2 begins. This is where the width and height of the polymer waveguide 3 linearly enlarged until the conical or pyramid-shaped area 10 meets the essentially curved area 11. In this region, the diameter of the beam increases continuously and maintains its Gaussian shape. Since this extension is initially adiabatic, this scheme allows for separate optimization of the wavelength-tuned primarily silicon nitride polymer waveguide mode converter and the final taper structure 2, where the mode field diameter of the fiber to which the light is coupled is is the central parameter. When the beam enters the curved area 11 after the taper, the beam spreads out like a free space. Due to the relatively large beam diameter compared to the distance to the lens of the curved area 11, the beam does not have a large divergence and propagates directly to the reflection surface 8, since this distance is small compared to the Rayleigh length of the beam at the end of the cone. The angle of the reflection surface 8 can, for example, be chosen to be 39° with respect to the chip plane 16 to correspond to the 12° angle at which the 8° polished fiber array emits and collects the light. After the reflection surface 8, the light beam 5 spreads in the direction of the curved area 11. The curved area 11 serves to focus the diverging beam and to couple it into the external medium 6.

The taper structure 2 of the optical device 1 can be produced, for example, with a direct laser writing system (Nanoscribe Professional GT), a 63x lens and IP dip as a resist.

In addition, the coupling efficiency is independent of the polarization of the incident light. This means that, in contrast to conventional grating couplers, the optical device can couple in light from both common polarizations (TE and TM) with the same efficiency, as long as the subsequent waveguide also supports them.

The optical device 1 can be coupled to any fiber arrangement via the free choice of the angle of incidence. This means that coupling can be carried out directly at 90°, which is advantageous for the mechanical connection of fibers 6 and chip 16. An optimal coupling angle can also be selected for frequently used polished fiber facets that are ground at angles to avoid back reflections. Coupling can also be carried out at shallow angles if the fiber is to be guided close to the chip surface.

The shape of the taper structure 2 of the optical device 1 enables the use of total reflection for beam deflection in an integrated component. This allows direct coupling to waveguides from any angle (and especially vertically), which is a significant advantage over other coupling methods (such as side coupling). Other vertical coupling methods (grating couplers) are limited to the diffraction angles of the planar structures and can therefore only be addressed with difficulty below 90°.

Furthermore, the concept is mechanically stable and robust against shocks or external interference when used on a chip 16.

2 shows a schematic representation not according to the invention of an optical device 1 for bidirectional coupling of a waveguide 7 to an external medium 6 according to a second exemplary embodiment. For applications in which lower refractive index contrast occurs, for example when the couplers are immersed in a medium 15 such as water, multiple reflections can be used to deflect the beam. Any starting angle can be detected by adjusting the diffraction angle of the reflection surface 8, 9. This makes it possible in particular to couple to the chip 16 at 90° incidence.

3 shows a schematic top view of a waveguide array 13 with a plurality of waveguides 7 and optical devices 1 according to an exemplary embodiment of the invention. Using marker search and alignment, many optical devices 1 can be attached to waveguide arrays 13 and thus a large number of components can be optically addressed. The optical device 1 is scalable and can be manufactured using scalable manufacturing methods and is therefore suitable for mass production. This enables direct coupling to fiber arrays 13 in which many optical fibers are bundled. The optical device 1 enables parallel and low-loss reading of many waveguide channels 7.

4 shows a schematic representation of an optical device 1 for bidirectional coupling of a waveguide 7 to an external medium 6 according to an exemplary embodiment of the invention. The optical device 1 can be attached to any waveguide 7. Due to the adiabatic expansion of the beam shape, coupling can be achieved virtually without loss. The optical device can also be easily combined with other chip elements. By fixing the optical device 1 to the waveguide 7, the coupler can be made tunable in the emission angle. For example, in 4 demonstrated the use of a movable structure, allowing the beam angle to be adjusted without losing coupling efficiency. For this purpose, the taper structure 2 is attached to a free-standing waveguide 14. This free-standing waveguide arm 14 can be moved mechanically, for example with the help of electrodes. By adjusting the angle of inclination of the waveguide 14, the output angle of the optical device 1 can then be adjusted simultaneously. This enables use for beam steering and lidar applications. This also enables use in phase arrays and collective emitters.

The work leading to this invention was funded by the European Union (H2020-EU.1.1.) under grant agreement No. 724707 (PINQS).

Reference symbol list

1

optical device

2

Taper structure

3

Beam input segment

4

Beam output segment

5

Ray of light

6

external medium

7

waveguide

8th

first reflection surface

9

second reflection surface

10

conical or pyramidal area

11

curved area

12

Substrate

13

Waveguide array

14

free-standing waveguide arm

15

medium

Claims (13)

Optical device for bidirectional coupling of a waveguide (7) to an external medium (6), comprising at least one taper structure (2), the taper structure (2) comprising a beam input segment (3), the beam input segment (3) being set up in this way is to couple a light beam (5) from the waveguide (7) into the taper structure (2), the taper structure comprising a beam output segment (4), the beam output segment (4) being set up to supply the light beam (5). focus and couple into the external medium (6), and the taper structure (2) between the beam input segment (3) and the beam output segment (4) comprises at least one first reflection surface (8), the first reflection surface (8) being set up in this way to deflect the light beam (5) out of the plane of the waveguide (7), characterized in that the waveguide (7) is designed as a free-standing waveguide arm (14). Optical device according to Claim 1 , characterized in that the first reflection surface (8) is a total reflection surface. Optical device according to one of the preceding claims, characterized in that the height of the taper structure (2) is continuously tapered in the region of the beam input segment (3), while the width is kept constant, whereby a waveguide region is formed. Optical device according to one of the preceding claims, characterized in that the beam input segment (3) is adjoined by a substantially conical or pyramid-shaped region (10), the width and height of the conical or pyramid-shaped region (10) increasing linearly. Optical device according to Claim 4 , characterized in that the beam output segment (4) adjoins the conical or pyramid-shaped region (10), the beam output segment (4) comprising a substantially curved region (11), the substantially curved region (11) being the at least first Reflection surface (8). Optical device according to one of the preceding claims, characterized in that the taper structure (2) comprises at least one further reflection surface (9). Optical device according to one of the preceding claims, characterized in that the beam output segment (4) comprises a collimating lens. Optical device according to one of the preceding claims, characterized in that the external medium (6) is an optical fiber or a microscope objective. Optical device according to one of the preceding claims, characterized in that the waveguide (7) is a planar waveguide. Optical device according to one of the preceding claims, characterized in that the waveguide (7) is arranged on a substrate (12). Optical device according to one of the preceding claims, characterized in that the taper structure (2) is formed from a polymer. Optical device according to one of the preceding claims, characterized in that the optical device (1) is produced by means of 3D printing, in particular by means of direct laser writing. Optical device according to one of the preceding claims, characterized in that the waveguide (7) is part of a waveguide array (13).

Images