DE102019115410B4 - optical device - Google Patents

Abstract

Optical device for bidirectional coupling of a waveguide (7) to an external medium (6) comprising at least one taper structure (2), wherein the taper structure (2) comprises a beam input segment (3), the beam input segment (3) being set up in such a way coupling 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 in such a way as to focus the light beam (5). and 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 such a way that Deflecting the light beam (5) out of the plane of the waveguide (7), characterized in that the beam input segment (3) is adjoined by a substantially conical or pyramidal area (10), the width and height of the conical or pyramidal area increasing (10) increase linearly, and the conical or pyramidal area (10) is followed by the beam output segment (4), the beam output segment (4) comprising a substantially curved area (11), the substantially curved area (11) containing the at least first reflective surface (8).

Description

The invention relates to an optical device for the 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 glass fibers is a key challenge. Waveguides correspond to electrical connections on integrated optical circuits and are therefore essential building blocks for functional photonics. In particular, they allow complex systems to be miniaturized 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. Mode converters are necessary to avoid this. Since optical signals are guided in the plane in waveguides, it is desirable to change the beam direction, since 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 in order to obtain high data rates and to 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 misalignment and therefore requires high placement accuracy, which is associated with high costs. Ongoing contacting methods are therefore lossy and not suitable for mass production. However, for all optical systems used in telecommunications applications, permanent fiber coupling is necessary for efficient packaging. Therefore, there is currently no way to efficiently couple to waveguides with high bandwidth and relaxed tolerances, especially large numbers of waveguides. Furthermore, existing coupling points cannot be tuned through. Since there are manufacturing tolerances, an adjustment after manufacture is necessary in order to achieve an optimal coupling.

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

Grating couplers use diffractive elements to allow out-of-plane coupling. These couplers typically achieve efficiencies of 30%. Higher efficiencies are possible with improved designs and more elaborate manufacturing. The primary disadvantage of grating couplers is the low bandwidth in the range of a few 10 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 aligned with respect to fibers or lenses in a complex manner. This means that automatic coupling to chips is only possible with difficulty.

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, with this method, the width of the waveguide determines the placement tolerance. This is therefore less than one micrometer and is therefore extremely demanding. In this case, the coupling bandwidth is high because the coupling is adiabatic. However, the challenging placement cannot be coupled to many waveguides since each individual fiber must be placed separately. Furthermore, no fiber arrays can be used, so that a 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 the mechanical placement, the coupling to the glass fibers cannot be further improved. However, the coupling efficiency suffers significantly, especially in the case of precise placement requirements.

From the EP 2 442 165 B1 an optical device for handling a radiation beam is known, the optical device comprising a semiconductor die comprising an integrated optical semiconductor waveguide core integrated on the semiconductor die and an at least partially overlying waveguide comprising at least one first slant shaped for coupling the radiation ray between the integrated optical semiconductor waveguide core and an external medium, the first slant having an entrance/exit side surface for receiving/ejecting a radiation beam from/to the external medium, the side surface of the first bevel being spaced from the edge of the semiconductor die 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" discloses an optical device for bidirectional coupling of a waveguide, in which the light beam is deflected out of the plane of the waveguide by an out-of-plane bend.

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

Another central 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 several connections to chips are necessary.

Therefore, a high coupling efficiency is essential. A high bandwidth is particularly necessary for applications in spectroscopy and for data transmission. In this case, it is often necessary to couple to many waveguides at the same time in order to implement several channels. This is not possible with today's methods. Likewise, the prior art cannot provide post-manufacture tuning, which is necessary to respond to long-term drift and offset.

Accordingly, the primary aim of the present invention is 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 losses and with 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 in such a way as to couple a light beam from the waveguide into the taper structure, the taper structure comprising a beam output segment, the beam output segment being set up in such a way as 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 comprising at least one first reflection surface, the first reflection surface comprising at least one surface is set up in such a way that the light beam is deflected out of the plane of the waveguide.

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

The basic idea of the present invention is to provide an optical device which enables mode adaptation of planar waveguides to an external medium, for example to an optical fiber or microscope objectives. Since the optical path of the light beam can be reversed, the external medium can also be adapted to a planar waveguide. The adaptation takes place via a three-dimensional taper structure, which is generated, for example, by means of direct laser writing (DLW). The optical device performs a change in the beam direction of the in-plane beams to 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 spectrum of wavelengths can be coupled into the waveguide via the coupler. The bandwidth is only through the transparency window of the coupler and the waveguide limited. This means that the bandwidth is orders of magnitude better than that of 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 entails 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 lossless beam deflection.

In a further preferred embodiment of the invention, the height of the taper structure in the area of the beam input segment tapers continuously, while the width is kept constant, as a result of which a waveguide area is formed.

According to the invention, the beam input segment is adjoined by a substantially conical or pyramidal area, with the width and the height of the conical or pyramidal area increasing linearly.

According to the invention, the conical or pyramidal area is adjoined by the beam output segment, with the beam output segment comprising an essentially curved area, with the essentially curved area comprising the at least first reflection surface.

In a preferred embodiment of the invention, the taper structure includes at least one additional 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 to the optical device. This enables fiber coupling using industrial methods without the fine adjustment 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.

In a further preferred embodiment of the invention, the waveguide is designed as a free-standing waveguide arm. With the free-standing waveguide arm, by adjusting the inclination angle of the waveguide arm, the output angle of the optical device can be adjusted simultaneously.

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 produced by means of 3D printing, in particular by means of direct laser writing.

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

• 1 eine schematische Darstellung einer optischen Vorrichtung zur bidirektionalen Ankopplung eines Wellenleiters an ein externes Medium gemäß einem ersten Ausführungsbeispiel der Erfindung,
• 2 eine schematische Darstellung eine optische Vorrichtung zur bidirektionalen Ankopplung eines Wellenleiters an ein externes Medium gemäß einem zweiten Ausführungsbeispiel der Erfindung,
• 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 dritten Ausführungsbeispiel der Erfindung.

Show it:

• 1 a schematic representation of an optical device for bidirectional coupling of a waveguide to an external medium according to a first embodiment of the invention,
• 2 a schematic representation of an optical device for bidirectional coupling of a waveguide to an external medium according to a second embodiment of the invention,
• 3 a schematic plan 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 a third embodiment of the invention.

1 1 is a schematic representation of an optical device 1 for bidirectional coupling of a waveguide 7 to an external medium 6 according to a first exemplary embodiment of the invention. The optical device 1 consists essentially of a taper structure 2, with the taper structure 2 being roughly divided into two parts: a beam input segment 3, with the beam input segment 3 for coupling a light beam 5 from the waveguide 7 into the taper structure 2, and a beam output segment 4, with the beam output segment 4 for focusing and coupling the light beam 5 into the external medium 6. In particular, the light beam 5 can be coupled in bidirectionally. Because at the optical device 1 of the beams Since the path of the light beam 5 is reversible, the light beam 5 can thus also be coupled into the waveguide 7 from the external medium 6 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 by means of the first reflection surface 8 into the plane of the waveguide 7 towards the beam input segment 3 and is coupled into the waveguide 7 . The external medium 6 can be an optical fiber or a microscope lens, for example. The beam input segment 3 includes at the in 1 The exemplary embodiment shown has a conical or pyramidal area 10 . The beam output segment 4 comprises an essentially curved area 11 , the curved area 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 terms of beam size by the taper structure 2 . The generated Gaussian beam 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 area 11, where the curved area 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 makes it possible to achieve out-of-plane beam deflection, so that waveguides 7 become accessible from the chip surface 16 . This is done using several elements that can be created in 3D freeform writing. An adiabatic transition of the waveguide mode into a Gaussian beam propagating freely in the optical device takes place by means of the taper structure 2. This widens 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 deflection takes place 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 takes place without losses. The refractive index contrast determines the maximum tilt angle that can be achieved. If higher angles are to be achieved, multiple reflections can be used, which are also lossless. After the reflection, the beam 5 is collimated and adapted to the subsequent external medium or the mode-guiding element, which can be a glass fiber or a microscope objective.

The waveguide can be made of silicon nitride, for example. The beam input segment 4 can consist of a polymer, for example, and can include a polymer waveguide. At the beginning of the beam input segment 4, a eigenmode source is set up to launch the light beam from the waveguide 7 to mimic the light output of a silicon nitride to polymer waveguide mode converter located immediately before the beam input segment 3. The silicon nitride waveguide 7, which is initially 0.5 μm wide, tapers linearly downwards, while the height of the polymer waveguide 4 is continuously reduced to finally 1 μm, while the width is kept constant at 1 μm. This cross-section of the polymer waveguide 3 is selected because only the basic modes are supported due to the isolation by the silicon oxide substrate 12 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 pyramidal area 10 of the taper structure 2 begins. Here the width and height of the polymer waveguide 3 is linearly increased until the conical or pyramidal area 10 meets the essentially curved area 11. In this range, the diameter of the beam increases continuously and keeps its Gaussian shape. As this extension is initially adiabatic, this scheme allows a separate optimization of the mainly wavelength tunable 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 the key parameter. After the taper, when the beam enters the bulged area 11, the beam spreads out like 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 reflecting 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 reflecting surface 8 can be chosen, for example, at 39° with respect to the chip plane 16 in order 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, unlike traditional grating couplers, the optical device can couple light of 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 arrangements by freely selecting the angle of incidence. It is thus possible to couple in 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 the popular use of polished fiber facets that are ground at angles to avoid back reflections. It is also possible to launch at shallow angles if the fiber is to be routed 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 (e.g. side coupling). Other vertical coupling methods (grating coupler) are limited to the diffraction angles of the planar structures and can therefore only be addressed poorly 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 of an optical device 1 for the bidirectional coupling of a waveguide 7 to an external medium 6 according to a second exemplary embodiment of the invention. For applications where lower refractive index contrast is encountered, for example where the couplers are immersed in a medium such as water, multiple reflections can be used to deflect the beam. By adjusting the diffraction angle of the reflecting surface 8, 9, any starting angle can be detected. It is thus possible, in particular, to couple onto the chip 16 at an angle of incidence of 90°.

3 shows a schematic plan view of a waveguide array 13 with a plurality of waveguides 7 and optical devices 1 according to an embodiment of the invention. A large number of optical devices 1 can be attached to waveguide arrays 13 via marker search and alignment, and a large number of components can thus be addressed optically. 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 a parallel and low-loss readout of many waveguide channels 7 .

4 shows a schematic representation of an optical device 1 for the bidirectional coupling of a waveguide 7 to an external medium 6 according to a third exemplary embodiment of the invention. The optical device 1 can be attached to any waveguide 7 . Due to the adiabatic widening of the beam shape, it can be coupled in with virtually no losses. 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 moveable structure, which allows the beam angle to be adjusted without losing coupling efficiency. To this end, 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 aid 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 it to be used for beam steering and lidar applications. This also enables use in phase arrays and collective emitters.

Work leading to this invention was funded by the European Union (H2020-EU.1.1.) under Grant Agreement No. 724707 (PINQS).

Reference List

1

optical device

2

taper structure

3

beam entrance segment

4

beam exit segment

5

beam of light

6

external medium

7

waveguide

8th

first reflection surface

9

second reflection surface

10

conical or pyramidal area

11

arched area

12

substrate

13

waveguide array

14

free-standing waveguide arm

15

medium

Claims (12)

Optical device for bidirectional coupling of a waveguide (7) to an external medium (6) comprising at least one taper structure (2), wherein the taper structure (2) comprises a beam input segment (3), the beam input segment (3) being set up in such a way that a light beam (5) from the waveguide (7) is coupled into the taper structure (2), the taper structure comprises a beam output segment (4), the beam output segment (4) being set up in such a way that the light beam (5) is focused and coupled 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 such a way that the light beam (5) is deflected out of the plane of the waveguide (7), characterizedthat the beam input segment (3) is adjoined by a substantially conical or pyramidal area (10), the width and height of the conical or pyramidal area (10) increasing linearly, and the conical or pyramidal area (10) is adjoined by the beam output segment (4), the beam output segment (4) comprising an essentially curved area (11), the essentially curved area (11 ) comprises the at least first reflection surface (8). Optical device 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) in the area of the beam input segment (3) tapers continuously, while the width is kept constant, whereby a waveguide area is formed. 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 waveguide (7) is designed as a free-standing waveguide arm (14). 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