WO2020245259A1

WO2020245259A1 - Optical device

Info

Publication number: WO2020245259A1
Application number: PCT/EP2020/065462
Authority: WO
Inventors: Wolfram Pernice; Carsten Schuck; Fabian BEUTEL; Nicolai Walter; Helge GEHRING; Wladick HARTMANN; Martin Wolff
Original assignee: Westfälische Wilhelms-Universität Münster
Priority date: 2019-06-06
Filing date: 2020-06-04
Publication date: 2020-12-10
Also published as: DE102019115410B4; DE102019115410A1; CA3165847A1; EP3997495A1

Abstract

In an optical device for bidirectional coupling of a waveguide (7) to an external medium (6), the aim of the invention is to widen the coupling bandwidth of the optical device, so that a wide spectrum of wavelengths can be coupled into an external medium via the optical device. This aim is achieved in that the optical device comprises at least one taper structure (2), the taper structure (2) comprising a beam entry segment (3), the beam entry segment (3) being designed so as to couple a light beam (5) from the waveguide (7) into the taper structure (2), and the taper structure comprising a beam exit segment (4), the beam exit segment (4) being designed so as to focus the light beam (5) and to couple same into the external medium (6), the taper structure (2) comprising at least one first reflective surface (8) between the beam entry segment (3) and the beam exit segment (4), and the first reflective surface (8) being designed so as to deflect the light beam (5) out of the plane of the waveguide (7).

Description

Optical device

The invention relates to an optical device for the bidirectional coupling of a Wel lenleiters to an external medium.

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 subject to high losses. Mode converters are necessary to circumvent this. Since optical signals are guided in the plane in waveguides, it is desirable to change the direction of the beam, 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 a high level of placement accuracy, which is associated with high costs. Current contacting processes are therefore lossy and not suitable for mass production. For all optical systems with areas of application in telecommunications, however, permanent fiber coupling is necessary for efficient packaging. Therefore, there is currently no way to efficiently couple to waveguides with a high bandwidth and relaxed tolerances, in particular not to a large number of waveguides. Furthermore, existing coupling points cannot be voted through. Since there are manufacturing tolerances, an adjustment after manufacturing is necessary to achieve an optimal coupling.

Coupling via the top of the chip is the preferred method. This allows many components to be addressed, which is of central importance for a high degree of integration receive. To this end, 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 coupling from the plane. These couplers typically achieve efficiencies of 30%. Higher 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 10 nm. This means that only special wavelength ranges can be covered. In addition, grating couplers are very sensitive to lateral shifts and must therefore be aligned with respect to fibers or lenses. Automatic coupling to chips is therefore difficult.

Fiber tapers use glass fibers with a reduced diameter. These tapers are placed on top of the waveguides and couple to them in the optical near field. Therefore, with this method, the width of the waveguide is decisive for the placement tolerance. This is therefore less than a micrometer and is therefore extremely demanding. The coupling bandwidth is high in this case, since the coupling is adiabatic. However, the demanding placement cannot be used to couple to many waveguides, since each individual fiber has to 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, no tunability is possible. I.e. after mechanical placement, the coupling to the fiber optics cannot be further improved. However, the coupling efficiency suffers significantly when it comes to exact placement requirements.

An optical device for handling a radiation beam is known from EP 2 442 165 B1, the optical device comprising a semiconductor wafer, comprising an integrated optical semiconductor waveguide core, integrated on the semiconductor wafer, and an at least partially superimposed waveguide, comprising at least a first bevel shaped to couple the radiation beam between the integrated one optical semiconductor waveguide core and an external medium, wherein the first bevel has an entry / exit side surface for receiving / emitting a radiation beam from / to the external medium, the side surface of the first bevel from the edge of the semiconductor die by a Distance d is spaced apart, the distance d being at least 1 pm and less than 200 pm.

From the document "Low-loss fiber-to-chip couplers with ultrawide optical bandwidth" he appeared in APL Photon. 4, 010801 (2019); doi: 10.1063 / 1.5064401 a hybrid approach of a 3D coupler using 3D direct laser writing is known, which combines the advantages of a narrow-band out-of-plane coupling with a large number of devices and a broadband edge coupling with a limited number of devices . This is intended to enable broadband and low-loss coupling to waveguide components. The 3-D coupler enables stress-free mechanical alignment with respect to an optical fiber. The 3D coupler is used to rotate the direction of the incident light in the planar direction and to convert the mode field diameter of an optical fiber to the size of a nanophotonic waveguide. The 3D coupler begins on a chip with an adiabatic transition from a single-mode silicon nitride waveguide to a polymer waveguide. In order to expand the optical mode, the width of the silicon nitride waveguide in a tapered region is linearly reduced while at the same time the height of the surrounding polymer waveguide covering the taper is increased. The width of the polymer waveguide is kept constant in order to enable an adi abatic transition between the fundamental TE modes of the two waveguides.

The coupling methods known from the prior art require complex alignment of the glass fibers with respect to the coupling elements. This is complex and therefore expensive. Since each component must be aligned separately with respect to the optical fibers, automatic placement is difficult and slow and therefore expensive. Furthermore, the low placement tolerances mean that the components degrade over time if there is mechanical distortion or offset with respect to the optical chip. Therefore, a placement technique is necessary that offers higher tolerances and at the same time can be used for many components in parallel. This essentially requires a 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. Coupling to many waveguides at the same time is often necessary in order to implement several channels. This is not possible with today's methods. Likewise, the state of the art cannot implement tuning after production, but this is necessary in order to respond to long-term drift and offset.

Accordingly, the present invention is primarily intended to provide an optical device, the coupling bandwidth of the optical device being very broad and a wide spectrum of wavelengths being able to 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 with achievable placement tolerances.

To achieve the above object, the present invention provides an optical device for bidirectional coupling of a waveguide to an external medium. 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 to couple a light beam from the waveguide into the taper structure, the taper structure comprises a beam exit segment, the beam exit segment being set up to focus the light beam and couple it into the external medium, the taper structure between the beam entrance segment and the beam exit segment at least one comprises first reflective surface, wherein the first reflective surface is set up to deflect the light beam out of the plane of the waveguide.

The coupling of the waveguide to the external medium can, in particular, be bidirectional. Since the beam path of a light beam is reversible, bidirectional in connection with this invention means that a light beam can not only be coupled into the external medium by means of the optical device from the waveguide, but that the light beam can also be coupled into the external medium by means of the optical device from the external medium the waveguide can be coupled. The beam output segment of the optical device described above is set up to couple the light beam from the external medium into the taper structure, the light beam being defocused, the taper structure still having at least one first reflector between the beam output segment and the beam input segment xion surface comprises, 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 optical device described above is set up in such a way as 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 objectives. Since the 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 changes the beam direction of the beams running in the plane into the vertical dimension or else the other way around. For this purpose, a reflection of the light beam on a reflection surface is used in particular.

The optical device does not use any wavelength-selective (in particular 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 limited by the transparency window of the coupler and the waveguide. 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 platform and polarization independent and can be used for coupling, for example, to any waveguide, which brings enormous flexibility with it.

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

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

In a preferred embodiment of the invention, the beam entrance segment is followed by a substantially conical or pyramidal area, the width and the height of the conical or pyramidal area increasing linearly. In a further preferred embodiment of the invention, the conical or pyramidal area is followed by the beam exit segment, the beam exit segment comprising a substantially curved area, the essentially curved area including 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. The beam collimation relaxes the alignment tolerances of the external medium with respect to the optical device. This enables fiber coupling with industrial methods without 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. Due to the free-standing waveguide arm, the output angle of the optical device can be adjusted simultaneously by adjusting the angle of inclination 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 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 shows 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,

Fig. 2 is 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,

Fig. 3 is a schematic plan view of a waveguide array with a plurality of

Waveguides and optical devices according to an embodiment of the invention,

Fig. 4 is 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. Fig. 1 is a schematic representation of an optical device 1 for the bidirectional coupling of a waveguide 7 to an external medium 6 according to a first embodiment of the invention. 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 for coupling a light beam 5 from the waveguide 7 into the taper Structure 2 is used and a beam exit segment 4, the beam exit segment 4 serving to focus and couple the light beam 5 into the external medium 6. The coupling of the light beam 5 can in particular take place bidirectionally. Since the beam path of the light beam 5 can be reversed in the optical device 1, 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 reflective 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, for example, an optical fiber or a microscope objective. In the exemplary embodiment shown in FIG. 1, the beam entrance segment 3 comprises a conical or pyramidal region 10. The beam exit segment 4 here comprises an essentially curved region 11, the curved region 11 comprising a first reflective surface 8. As shown in cross section in FIG. 1, a planar waveguide 7 is coupled to the taper structure 2. Light propagating in waveguide 7 is initially expanded in beam size by taper structure 2. The Gaussian beam generated then falls on the reflection surface 8, where total reflection takes place. This changes the beam direction and deflects it upwards. In order to obtain a collimated beam, the beam exit segment 4 comprises an essentially curved area 11, wherein the curved area can have a lens shape, for example. The taper structure 2 acts on the one hand 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 a beam deflection to be achieved out of the plane, so that waveguides 7 are accessible from the chip surface 16. This is done using several elements that can be generated in 3D free-form 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 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 takes place without losses. The refractive index contrast determines the maximum angle of inclination that can be achieved. If higher angles are to be achieved, multiple reflections can be used, which are also loss-free. After the reflection, the beam 5 is collimated and adapted to the subsequent one to the external medium or the mode guide element, which can be a glass fiber or a microscope objective.

The waveguide can be made of silicon nitride, for example. The radiation input segment 4 can for example consist of a polymer and comprise a polymer waveguide. At the beginning of the beam input segment 4, a natural mode source is set for the coupling of 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 pm wide silicon nitride waveguide 7 is linearly tapered downwards, while the height of the polymer waveguide 4 is continuously tapered to finally 1 pm, while the width is kept constant at 1 pm. This cross section of the polymer waveguide 3 is selected because only the basic modes are supported due to the partitioning 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 area 10 of the taper structure 2. This is where the width and height of the polymer waveguide begins 3 is enlarged linearly until the conical or pyramidal area 10 meets the essentially curved area 11. In this area the diameter of the jet increases continuously and retains its Gaussian shape. Since this extension is initially adiabatic, this scheme enables a separate optimization of the silicon nitride polymer waveguide mode converter, which is mainly to be tuned to the wavelength, and the final taper structure 2, in which the mode field diameter of the fiber with which the light is coupled is the central parameter is. When the beam enters the curved area 11 after the taper, the beam spreads 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 has no great divergence and spreads directly to the reflective surface 8, since this distance is small compared to the Rayleigh length of the beam at the end of the cone is. The angle of the reflection surface 8 can be selected to be 39 ° with respect to the chip plane 16, for example, in order to correspond to the 12 ° angle at which the 8 ° polished fiber array emits and collects the light. After the Re flexion surface 8, the light beam 5 spreads in the direction of the curved area 11. The curved area 11 is used to focus the diverging beam and 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 63-fold objective 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 light of both conventional polarizations (TE and TM) with the same efficiency, as long as the subsequent waveguide also supports this.

The optical device 1 can be coupled to any fiber arrangement via the free choice of the angle of incidence. Coupling can thus 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 polished fiber facets that are often used and ground at angles to avoid back reflections. It can also be coupled in 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 coupling to waveguides directly 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 angle of the planar structures and can therefore only be addressed with difficulty below 90 °.

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

Fig. 2 shows a schematic representation of an optical device 1 for bidirectional coupling of a waveguide 7 to an external medium 6 according to a second exemplary embodiment of the invention. For applications in which a 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. About customizing the Any output angle can be detected at the diffraction angle of the reflection surface 8, 9. This makes it possible, in particular, to couple the chip 16 at an incidence of 90 °.

Fig. 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 inven tion. 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 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.

Fig. 4 shows a schematic representation of an optical device 1 for bidirectional coupling of a waveguide 7 to an external medium 6 according to a third 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 carried out with virtually no 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 terms of the emission angle. For example, FIG. 4 shows the use of a moveable structure, whereby the beam angle can be adjusted without losing the 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 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 the use for beam steering and lidar applications. This also enables use in phase arrays and collective emitters. The work that led to this invention was funded by the European Union (H2020-EEi.1.1.) Under grant agreement No. 724707 (PINQS).

List of reference symbols

1 optical device

2 taper structure

3 beam entrance segment

4 beam exit segment

5 light beam

6 external medium

7 waveguides

8 first reflective surface

9 second reflective surface

10 conical or pyramidal area

11 domed area

12 substrate

13 waveguide array

14 free-standing waveguide arm

15 medium

Claims

1. 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 such a way that a light beam (5) from the waveguide (7) into the taper Couple structure (2),

the taper structure comprises a beam output segment (4), the beam output segment (4) being designed to focus the light beam (5) and couple it into the external medium (6),

the taper structure (2) between the beam entrance segment (3) and the beam exit segment (4) comprises at least one first reflective surface (8), the first reflective surface (8) being set up in this way, the light beam (5) from the plane of the waveguide (7) distract out.

2. Optical device according to claim 1, characterized in that the first reflection surface (8) is a total reflection surface.

3. Optical device according to one of the preceding claims, characterized in that the height of the taper structure (2) in the region of the beam input segment (3) is continuously tapered, while the width is kept constant, whereby a hollow conductor area is formed.

4. Optical device according to one of the preceding claims, characterized in that the beam input segment (3) is followed by a substantially conical or pyramidal area (10), the width and height of the conical or pyramidal area (10) increasing linearly .

5. Optical device according to claim 4, characterized in that the ke gel-shaped or pyramid-shaped region (10) is followed by the beam output segment (4), wherein the beam output segment (4) comprises a substantially curved Be rich (11), the in Substantially curved region (11) which comprises at least the first reflection surface (8).

6. Optical device according to one of the preceding claims, characterized in that the taper structure (2) comprises at least one further reflection surface (9).

7. Optical device according to one of the preceding claims, characterized in that the beam output segment (4) comprises a collimating lens.

8. Optical device according to one of the preceding claims, characterized in that the external medium (6) is an optical fiber or a microscope objective.

9. Optical device according to one of the preceding claims, characterized in that the waveguide (7) is a planar waveguide.

10. Optical device according to one of the preceding claims, characterized in that the waveguide (7) is arranged on a substrate (12).

11. Optical device according to one of the preceding claims, characterized in that the waveguide (7) is designed as a free-standing waveguide arm (14).

12. Optical device according to one of the preceding claims, characterized in that the taper structure (2) is formed from a polymer.

13. 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.

14. Optical device according to one of the preceding claims, characterized in that the waveguide (7) is part of a waveguide array (13).