Patent Description:
Photonic integrated circuit (PIC) chips typically need optical input and output coupling from and to single-mode fiber (SMF) arrays. However, waveguides in photonic integrated circuits have typically a much smaller mode field or "spot size" compared to a standard single-mode optical fiber (SSMF). The smaller the spot size difference, the larger is the optical coupling loss between the PIC waveguide and the SMF.

Arrays of small-core fibers, lensed fibers or tapered fibers are not good for coupling light from (or to) large waveguide arrays with small spot sizes, because the fiber cores have random variation in their position with respect to each other and it is not possible to align all fibers at the same time to an array of small PIC waveguides. Variation in fiber core position in a fiber array is typically about ±<NUM>, due to variations in the core position inside each fiber, and to fiber-to fiber positioning, for example in a V-groove array.

A horizontal and vertical spot-size converter (SSC) is then usually needed between waveguides and optical fibers. Such 2D converter can be one structure that simultaneously changes the spot-size in the horizontal and vertical direction, or a combination of separate vertical and horizontal converter structures, where the waveguide is tapered from one spot size to another separately in each direction.

A separate spot-size converter chip, also called a waveguide interposer, can be used to perform vertical and horizontal spot-size conversion between a standard SMF array and a small-waveguide array at the edge of a PIC chip. The advantage is that there are no waveguide-to-waveguide variations in an array of thin waveguides, and that arrays of thin mode fields can be precisely aligned. 2D spot-size converters have been realized with various methods, such as direct writing of waveguides, step-like etching of a thick waveguide layer, gray-scale lithography, polishing of waveguides, tapering of optical fibers, lenses, grating couplers, sub-wavelength diffractive structures, inverse tapers etc. Horizontal tapering is usually not a problem on PIC's, but vertical spot size conversion is difficult to do with most optical waveguide technologies by using wafer-level processing techniques.

In some applications, there is also the need to couple light between thick and thin waveguides on different PICs or even within one multi-layer PIC, i.e. without any optical fibers being part of this coupling task. Optical fibers and PIC waveguides can both be considered as "waveguides", so the need for 2D SSC can be seen as a general need for spot-size conversions between optical mode fields in optical waveguides of different thicknesses and widths. One of those waveguides (typically the thicker one) can be a SMF or its other end can be coupled to a SMF. Publication <CIT> discloses an optical module having a first plasmonic waveguide, one end of which is composed of a first metal layer that is formed over an end of a first substrate, and the other end of which is connected to an end of a first optical waveguide. A second metal layer is formed on a lateral surface of the first substrate, said lateral surface being continued to the end, so as to be continuous to the first metal layer. A second substrate is provided with a second plasmonic waveguide that is composed of a third metal layer. A second optical waveguide is formed on the second substrateso as to be connected to the second plasmonic waveguide. The second metal layer and a part of the third metal layer are bonded with each other, thereby connecting the first substrate to the second substrate. The first plasmonic waveguide and the first optical waveguide are connected by a mode conversion unit provided on the first substrate.

There has not yet been presented a perfect solution for coupling light between an array of thin waveguides and another array of thick waveguides. An ideal coupling concept would need to have wide transmission spectrum, polarization independent operation, low insertion loss and a small size, as well as low manufacturing and assembly cost for the parts.

It is the object of the present invention to find a novel solution to the problem described above, which alleviates the drawbacks of the prior art solutions.

The invention provides an optical assembly according to independent claim <NUM>. Further embodiments are provided by the dependent claims. According to the invention, the waveguides on two different chips or in two different waveguide layers are coupled together in such a way that the width of the optical mode in one waveguide corresponds to the thickness of the optical mode in the other waveguide, and vice versa. Thus each waveguide mode needs to be enlarged only in the horizontal direction, but the optical mode field expands (or shrinks) both horizontally and vertically. Thus horizontal and vertical spot size conversion between a small and a large optical waveguide is performed by combining two horizontal spot-size conversions and by rotating the mode between the two waveguides around the optical axis. This mode rotation can be achieved with up-reflecting waveguide mirrors, with rotation of the waveguide chips, or their combination.

In practical applications of some embodiments of an inventive optical assembly the first waveguides are part of a photonic integrated circuit (PIC). A single-mode fiber array (SMF) may be optically coupled to an optical interposer between a PIC and the SMF array, said optical imposer comprising an array of said second waveguides. The light may then be coupled to the optical fiber array from the end facets of the second waveguides.

The various embodiments of the invention are characterized by what is stated in the appended claims.

In detail, exemplary embodiments shown in the drawings are based on horizontal tapering of approximately 3x3 µm (<NUM> thick and <NUM> wide) first silicon-on-insulator (SOI) waveguides to a size of approximately <NUM>×<NUM>, whereby the mode fields of the waveguides are enlarged horizontally. Throughout this description the waveguide dimensions are given as height x width, in that order. The width of the thin waveguides typically varies in the PIC. In <NUM> thick SOI waveguides, the width is typically <NUM> or narrower in at least some parts of the PIC. Reference to 3x3 µm waveguides are thus only made as exemplary input waveguides. The <FIG> are not perfectly to scale in this regard, and may show approximately 3x5 µm waveguide dimensions.

Light is then coupled to an input end of 12x3 µm second waveguides having a matching mode field when the two waveguide modes are rotated with respect to each other. The width of the second waveguide is then tapered from <NUM> to <NUM> to achieve the targeted output size of 12x12 µm. From such a 12x12 µm waveguide, the light may then be coupled to the core of a standard single-mode optical fiber (SMF). Obviously, any combination of differently sized SOI waveguides and other types of optical waveguides may be coupled together by the same inventive principle that allows coupling light from an array of thin waveguide modes to another array of thick waveguide modes without vertical tapering of any of the waveguides.

One embodiment is illustrated in <FIG>. It shows a photonic integrated circuit (PIC) <NUM>, which includes three <NUM> thick first waveguides <NUM> on a silicon substrate <NUM>. The functional parts of the PIC are not shown and three waveguides are chosen only for visualizing the invention. <FIG> also shows an interposer <NUM>, which includes three <NUM> thick second waveguides <NUM> on a silicon substrate <NUM>. The buried oxide (BOX) layer is omitted from the figure for clarity. In this embodiment, the thin <NUM> and thick <NUM> waveguides are all placed horizontally and in parallel, and the thick <NUM> waveguides are upside down on top of the thin waveguides <NUM>.

Each thin waveguide <NUM> consists of narrow, in some embodiments <NUM>×<NUM>, input parts 103a, 103b or 103c, here each having a different length, a tapered section 103d, a bent section <NUM>, and a <NUM>×<NUM> section 103e. The tapered section 103d is a horizontally broadened taper section that performs a first horizontal spot-size conversion from a narrow input width (e.g. <NUM>) to a final width (e.g. <NUM>). The horizontal tapering of the thin waveguide <NUM> can be realized in many alternative ways and in different parts of the PIC <NUM>. For example, it can be realized with continuous tapering of the waveguide width, as shown in <FIG>, or with the combination of a slab waveguide (where light horizontally expands) and a curved waveguide mirror (that collimates the expanded light).

The bent section may consist of a bent waveguide or a horizontal waveguide mirror <NUM>. The relative locations of the tapered and bent sections can also be exchanged or even overlapped. For example, the combination of a slab waveguide and a curved mirror can perform both tapering and bending. In some embodiments of the invention the bent sections may be absent, as the circuit layout, location requirements and other design considerations determine the optimum shape and size of the waveguides in each case. In the particular embodiment of <FIG> the bent section can be left out, when the straight sections 103e are not sharing the same optical axis and the waveguides <NUM> would thus not overlap each other.

After the tapered and bent sections light is guided into the <NUM>×<NUM> section 103e, which ends with an up-reflecting mirror <NUM>. Obviously, the mirror would be a down-reflecting mirror if the assembly <NUM>, <NUM> would be turned upside down.

Similar up-reflecting mirrors <NUM> are on the interposer chip <NUM>. Between the up-reflecting mirror pairs <NUM> and <NUM> light passes vertically in either direction, as shown with the arrow <NUM>. In <FIG>, the up-reflecting mirrors <NUM> at the narrow ends 105a of the thick waveguides <NUM> actually reflect light from these waveguides downwards, as the waveguides <NUM> are upside down under the silicon substrate <NUM>. The thick waveguides <NUM> also have horizontally tapered sections 105b and wide (e.g. <NUM> wide) output waveguide sections 105c. The up-reflecting mirrors <NUM>, <NUM> are preferably TIR (total internal reflection) mirrors, but they can also be metallized mirrors or other similar elements. Here we use the terms "up-reflecting mirrors" and "up-reflecting elements" to refer to any up or down reflecting structures that couple the light from the waveguides either up or down with respect to the horizontal plane where the waveguides are located.

Light is coupled from the thin waveguides 103a, 103b, 103c of the PIC circuit <NUM> to the output ends 105c of the optical interposer <NUM> as follows: Light propagates in the waveguide along the direction of the optical axis of each waveguide section. The optical axes are perpendicular to the end surfaces of each entry and exit point of light into respective waveguide portions. When encountering a horizontally bent section or an up-reflecting mirror, like <NUM>, <NUM> or <NUM>, the optical axis and the light beam make a turn.

In the case shown in <FIG>, the first parts 103a, 103b, 103c of the three waveguides <NUM> are in parallel. The waveguide sections 103a, 103b and 103c have in this example different lengths, in order to demonstrate the possibility of relatively free positioning of the waveguides on the substrate <NUM> in some embodiments of the invention. The light beam from each waveguide section 103a-c then enters a tapered section 103d, which gradually broadens the light beam to a <NUM> wide horizontal configuration.

As the light beams hit the horizontal TIR mirrors <NUM>, their optical axes are turned to become parallel with each other (which was not necessary before the bent sections). At the end of the parallel waveguide sections 103e, the up-reflecting mirrors <NUM> reflect the light vertically up to the up-reflecting mirrors <NUM> of the waveguides <NUM> that are placed orthogonally on top of the up-reflecting mirrors <NUM> to efficiently collect the light and to reflect it to the narrow ends of the thick waveguides <NUM>. Orthogonal placement of the up-reflecting mirror pairs <NUM> and <NUM> means that the polarization of the light is rotated <NUM>° when light couples between the first thin and the second thick waveguides <NUM>, <NUM>. Light then travels horizontally in the upside-downturned waveguides <NUM>, first through a tall and narrow section 105a that accommodates the horizontally enlarged mode field of the waveguide <NUM>, now turned by the up-reflecting mirrors <NUM> into a vertical direction. The horizontally tapered section 105b of the waveguide <NUM> then gradually enlarges the mode field of the light beam in an orthogonal direction with respect to the already enlarged mode field. The result is that the thin and narrow optical mode of the first waveguide <NUM> (e.g. 3x3 µm) has been efficiently coupled to the thick and wide optical mode of the second waveguide <NUM> (e.g. 12x12 µm), by a combination of two horizontal mode size expansions and one rotated interface between the two waveguides <NUM>, <NUM>. From the output end facets (not shown) of the <NUM>×<NUM>- sized waveguides <NUM> the light may be coupled to the cores of standard single-mode optical fibers (not shown), for example. The polarization of the light beam is turned, so that the horizontal TE polarization of the light beam entering the waveguide <NUM> is now vertically oriented TM polarization in the waveguide <NUM>, and vice versa.

As mentioned above, the circuit layout and other design considerations determine the optimum shape and size of the waveguides in each case. In the concept shown in <FIG>, the up-reflecting mirror pairs can be processed on the chip in various locations that don't have to be along the same line, as shown in <FIG>. When the optical axes of the optical waveguides <NUM> merge on the lower chip <NUM>, the optical waveguides on the upper chip <NUM> may be located side by side in a traditional manner, making the alignment of lower and upper up-reflecting mirrors to each other easier. Especially wet-etched up-reflecting mirrors typically need to be in specific directions, and all mirrors in <FIG> are indeed having the same orientation. However, this may in some embodiments not be necessary, as long as each mirror pair <NUM>, <NUM> is orthogonally aligned, like in <FIG>.

If the thin and thick waveguides <NUM>, <NUM> are on two separate chips <NUM>, <NUM>, as illustrated in <FIG>, the up-reflecting mirrors are preferably placed near the edges of those chips, so that the substrate <NUM> doesn't overlap too much with the substrate <NUM>. This can be beneficial if there are flip-chip integrated components, wire bonds or other parts extending on top of the substrate <NUM>. The thin and thick waveguides <NUM>, <NUM> can also be fabricated as a <NUM>-layer PIC on a common substrate <NUM> or <NUM> (depending on their relative vertical positioning), for example by using a layer-transfer technique to transfer one waveguide layer on top of the other waveguide layer.

Another embodiment of the invention is illustrated in <FIG>. A photonic integrated circuit <NUM> includes three <NUM> thick first waveguides <NUM> on a silicon substrate <NUM>. An optical interposer <NUM> includes three <NUM> thick second waveguides <NUM> that are on another silicon substrate <NUM>. In this embodiment, the substrate <NUM> and the second waveguides <NUM> are placed vertically with respect to the first waveguides <NUM>.

In the embodiment shown in <FIG>, the travel of light in the first waveguides <NUM> is similar to what is described in <FIG>, i.e. the three thin waveguides <NUM> involve straight, tapered and bent waveguide sections. The tapered sections horizontally expand the mode fields of the narrow (e.g. <NUM> wide) first waveguides <NUM> to provide a targeted <NUM> width at the output end of each of the first waveguides <NUM>. The bent sections <NUM> turn the waveguides so that they at that point all share the same optical axis, before the light is coupled to the up-reflecting mirrors <NUM> at the ends of the 3x12 µm waveguide sections.

Here, light couples from the up-reflecting mirrors <NUM> up and directly into the bottom end-facets of the 3x12 µm wide portions 205a of the second waveguides <NUM>. Thus, no up-reflecting mirrors in the second waveguides <NUM> are required for the coupling of light between the waveguides <NUM> and <NUM> at the gap <NUM>. The light beams will then travel through the tapered waveguide sections 205b to reach the targeted <NUM> thick and <NUM> wide waveguide dimensions in the straight waveguide sections 205c. From there, the light can be coupled to optical fibers through the waveguide end facets 205d.

The polarization of the light beam is turned also here, so that a horizontal TE polarization of a light beam in the waveguides <NUM> becomes a vertically oriented TM polarization in the waveguides <NUM>.

In the embodiments like the one in <FIG>, a <NUM> SOI interposer chip <NUM> can be placed rather freely on top of a <NUM> SOI PIC <NUM>. Because of its vertical orientation, the chip <NUM> will cover only a relatively small area of the PIC <NUM> and its substrate <NUM>.

Further embodiments of the invention are illustrated in <FIG>. In <FIG>, a PIC <NUM> again includes three <NUM> thick first waveguides <NUM> with tapered and bent sections and up-reflecting mirrors on a silicon substrate <NUM>, like in <FIG>. Three <NUM> thick second waveguides <NUM> are realized on another silicon substrate <NUM> to form an optical interposer <NUM> that is placed vertically with respect to the <NUM> thick waveguides <NUM> and their substrate <NUM>, like in <FIG>. Each waveguide <NUM> consists of a <NUM> thick and more than <NUM> (e.g. <NUM>) wide waveguide portion 305b and of a lens 305a directed towards the up-reflecting mirror <NUM> of the corresponding waveguide <NUM>. The lens can be fabricated as part of the waveguide without any separate processing or lithography step. It is preferably planar in the vertical direction, being aligned with or being part of the waveguide side-walls, and only curved in the horizontal direction. The light beam that is reflected up from the up-reflecting mirror <NUM> diverges as it propagates in the free-space gap between the up-reflecting mirror <NUM> and the lens 305a. This divergence is much faster in the direction where the optical mode field from the thin waveguide <NUM> is narrow, which direction corresponds to the width of the thick waveguide <NUM>. The gap is preferably kept so small that the beam doesn't significantly diverge in the other direction, that corresponds to the thickness of the second waveguide <NUM> and to the enlarged width of the first waveguide <NUM>. Thus, the beam mainly diverges in one direction only. The lens 305a then collimates the diverged beam and couples it into the waveguide portion 305b.

Again, no mirrors are needed in the second waveguides <NUM>, and the polarization of light is turned also here from a horizontal TE polarization entering the waveguides <NUM> to a vertically oriented TM polarization in the waveguides <NUM>, or vice versa.

The first main difference of the embodiment shown in <FIG> with respect to the ones in <FIG> is that there is no need for a tapered section or narrow (e.g. <NUM> wide) waveguide section in the second waveguide <NUM>, because the curved input facet of the thick waveguide <NUM> acts as a lens and performs horizontal tapering of the light beam in the free-space gap outside the waveguide <NUM>. This provides for making the second waveguides <NUM> shorter, and they are also easier to fabricate, as the waveguides <NUM> no longer need to have high aspect ratio (i.e. height vs. width).

The second main difference is that there may be a finite distance between the up-reflecting mirror <NUM> and the lens 305a. In <FIG> the free-space gap needs to be minimized, as it causes unwanted beam divergence. In the embodiment illustrated in <FIG>, the finite gap is exploited to enlarge the light beam in one direction. Such finite distance is very beneficial in PIC testing, for example, where the interposer chip can be held at a safe distance, e.g. <NUM>-<NUM>, above the PIC wafer or chip, avoiding the risk of scratching them against each other. In some embodiments of this kind, the substrate <NUM> need not extend as far down as in <FIG>. The finite distance can also be beneficial when making permanent assemblies. In both assembly and testing, the finite distance can help to avoid scratching any optical input/output facets when the two waveguide chips <NUM>, <NUM> are aligned with respect to each other using passive or active alignment.

In <FIG>, a PIC <NUM> includes three <NUM> thick first waveguides <NUM> on a silicon substrate <NUM>. Three <NUM> thick second waveguides <NUM> are realized on a silicon substrate <NUM>, and this optical interposer part <NUM> is placed vertically with respect to the <NUM> first waveguides <NUM> and their substrate <NUM>, as in <FIG>. The substrate <NUM> is provided with slots <NUM> which are etched or machined into the substrate <NUM> and designed to receive the ends of the waveguides <NUM>. For the purpose of a simple illustration of various embodiments of the invention, each waveguide <NUM> in <FIG> has a different light beam coupling device 405a, 405b and 405c. Only one of these coupling device types is preferably chosen to be used in each optical interposer <NUM>. The light beam coupling devices are lowered down to each receiving slot <NUM> during the assembly (or chip/wafer testing), in order to align the common optical axis of all the straight waveguide portions 403b on the PIC, with the optical axes of all the coupling devices 405a-c, so that all these optical axes merge together. Proper alignment of the interfacing waveguide portions may be provided by studs <NUM> or a similar mechanical arrangement. The studs <NUM> may be placed in patterned recesses <NUM> providing mechanical alignment features, or have some other alignment features, which locks them into place in the place of the substrate <NUM>.

In the embodiment of <FIG>, there are no up-reflecting mirrors required at the facets or interfaces of the interfacing waveguide portions 403b, but rather the light propagates directly into a coupling device 405a, 405b or 405c. The coupling device 405a comprises an interfacing <NUM> wide and <NUM> thick waveguide portion, a tapered waveguide section, and a TIR mirror, similarly to items 105a, 105b and <NUM> in <FIG>. The coupling device 405b has a lens like the lens 305a in <FIG> and a TIR mirror. The third exemplary coupling device 405c consists of a curved concave mirror which collects the incoming light beam and reflects it upwards through the waveguide <NUM>. All the coupling devices 405a-c collect the light from the end facet of the <NUM> thick and <NUM> wide first waveguide <NUM>, expand the light beam (spot size) in the direction that corresponds to the thickness of the waveguide section 403b, and turn the light orthogonally upwards with respect to the substrate <NUM> of the PIC <NUM>. Expanding the light beam and turning the light up occurs horizontally with respect to the surface of the optical interposer <NUM>. In all <FIG> the optical interposer only needs one lithographic patterning step that corresponds to the definition of the width of the thick waveguides <NUM>, <NUM> and <NUM>. In all embodiments, the increase of both the width and the height of the light beam (spot size) is achieved by combining the horizontal enlargement of the light beam with the first <NUM>, <NUM>, <NUM>, <NUM> and second <NUM>, <NUM>, <NUM>, <NUM> waveguides without any change in the thickness of the waveguides themselves, or their mode fields.

In the embodiments of <FIG>, it is necessary to align the chip pairs <NUM>+<NUM>, <NUM>+<NUM>, <NUM>+<NUM> and <NUM>+<NUM> to each other, in order to ensure light is coupled between waveguides and mirrors as intended with a minimum of losses. This can be done with various different methods that are not at the core of this invention, but briefly discussed here.

The most straightforward method is to use active alignment, where light transmission through the thin and thick waveguides and the coupling interface between them is maximized. A U-loop can sometimes be added to either of the waveguide chips to simultaneously maximize the transmission of two coupling interfaces, for example at the edges of a wide waveguide array, and to have both the input and output coupling through one waveguide chip only. Another common method for coupling two waveguide chips together is to use passive alignment with machine vision and suitable alignment marks.

An attractive method for low-cost mass production is passive mechanical alignment. This can be realized in various different ways. One option is to etch V-shaped alignment grooves on each chip, for example during the fabrication of up-reflecting mirrors, and to add alignment balls or similar protrusions that align the two v-grooves when brought together. It is also possible to realize some kind of monolithic protruding alignment features, such as studs, robs or pyramids, on either one of the chips, which then passively align to V-grooves, inverse pyramid holes or similar features on the other chip. One such arrangement has been discussed in connection with <FIG> where a rectangular stud <NUM> is patterned into the <NUM> thick SOI layer on the optical interposer chip <NUM> and self-aligned with the <NUM> thick waveguides <NUM> and their coupling devices 405a-c. With a flat end facet, such a stud can be used to precisely control the vertical positioning of the two chips <NUM> and <NUM>. Passive alignment in all <NUM> directions is possible by pressing the flat end facet of the stud <NUM> against a flat surface on the chip <NUM>, while pressing the top (or bottom) and left (or right) surface of a similar (preferably longer) stud against the side-walls of an etched pit that is similar to the slot <NUM> shown in <FIG>.

It is also possible, especially for the embodiments in <FIG> to wet-etch a V-shaped alignment groove on the <NUM> SOI chip and dry-etch a lithographically patterned V-shaped tip extending out from the edge of the <NUM> SOI layer. The V-shaped tip then falls into a V-groove on the <NUM> SOI when the <NUM> SOI chip is placed vertically on top of the <NUM> SOI chip.

Similar mechanical alignment features can be added to the assemblies shown in <FIG> where various different alignment features etched into the <NUM> SOI layer can penetrate into various different holes patterned into the PIC chip. In some cases this may require that the alignment features protrude beyond the edge of the substrate (<NUM>, <NUM>, <NUM>), as shown in <FIG>, see items <NUM>. This may be achieved by under-etching the Si substrate or by using a cavity-SOI wafer, for example.

In some applications, it may not be desired that the fibers are placed vertically with respect to the PIC. To avoid this, the thicker waveguides <NUM>, <NUM>, <NUM> may be bent with horizontal bend sections (similar to <NUM> in <FIG> or 405a-c in <FIG>) to turn them from the vertical to horizontal direction with respect to the PIC <NUM>, <NUM>, <NUM>. If the output facets (like 205d in <FIG>) of those bent waveguides are coupled to a SMF array then that SMF array has each fiber in horizontal direction while the individual fibers in the array are on top of each other. It is also possible to have up-reflecting mirrors in the thick waveguides to enable the orthogonal placement of fibers with respect to the substrate of the thick waveguides. This allows adding a horizontal fiber array to the embodiments of <FIG>, or a vertical fiber array to the embodiment of <FIG>.

Claim 1:
Optical assembly for realizing horizontal and vertical spot-size conversion to couple light between an expanded end of a thin waveguide (<NUM>, <NUM>, <NUM>, <NUM>) to one end of a thick waveguide (<NUM>, <NUM>, <NUM>, <NUM>), wherein the thin waveguide (<NUM>, <NUM>, <NUM>, <NUM>) is thin compared to the thick waveguide (<NUM>, <NUM>, <NUM>, <NUM>), said assembly comprising:
- at least one first thin waveguide (<NUM>) with a first section having a first optical mode field and a horizontal spot-size expansion section (103d) providing spot-size conversion for a first horizontal dimension of said first optical mode field of a light beam propagating in said first waveguide (<NUM>);
- at least one second thick waveguide (<NUM>) with a second section having a second optical mode field and a horizontal spot-size reduction section providing spot-size conversion for a second horizontal dimension of said second optical mode field of a light beam propagating in said second waveguide (<NUM>); characterized in that
the expanded end of said at least one first thin waveguide (<NUM>) is aligned and rotated to interface with said one end of said at least one second thick waveguide (<NUM>), so that the mode fields in said at least one first thin (<NUM>) and at least one second thick (<NUM>) waveguides are rotated <NUM> degrees with respect to each other, whereby the spot size of a light beam so coupled between the at least one first thin (<NUM>) and at least one second thick (<NUM>) waveguides is expanded or shrunk in both transverse dimensions, depending on the direction of the light beam.