Patent ID: 12204147

DETAILED DESCRIPTION OF EMBODIMENTS

1. Application Scenario

FIG.1is a schematic side view of a vehicle10approaching an object12that is represented by a tree. The vehicle10has at least one scanning device14that uses light beams L11, L21, L31and L41to scan the environment ahead of the vehicle10. From the distance information generated by the scanning device14a three-dimensional image of the environment can be calculated. In addition, the scanning device14determines the relative speed to the object12. This information is particularly important if the object12is another vehicle, an animal or a pedestrian that is also moving.

As can be seen inFIG.1, the scanning device14emits light beams L11to L41in different directions in a vertical plane (inFIG.1this is the paper plane) in order to scan the environment in a vertical direction. Scanning takes place also in a horizontal direction, as this is shown inFIG.2which is a top view on the scanning device14. Four light beams L11, L12, L13and L14are shown which are emitted in different directions in a horizontal plane.

For reasons of clarity, it is assumed inFIGS.1and2that only four light beams Ln1to Ln4in four different planes—i.e. a total of 16 light beams—are generated by the scanning device14. However, in reality the scanning device14emits significantly more light beams. For example, k·2nlight beams are preferred, where n is a natural number between 7 and 13 and indicates how many beams are emitted in one of k planes, where k is a natural number between 1 and 16. In some embodiments, more than one light beam is emitted at a given time in order to achieve the desired spatial and temporal resolution.

2. Scanning Device

FIG.3schematically shows the basic design of the scanning device14according to an embodiment of the invention. The scanning device14is designed as a LiDAR system and comprises an FMCW light source16which generates measuring light in a TE0 state of polarization (SOP). The measuring light has a frequency fchirpthat varies (“chirps”) periodically over time t between a lower frequency fland a higher frequency fh.

In the embodiment shown, each measurement interval with a chirp duration T is divided into two halves of equal length T/2. During the first interval the frequency fchirpincreases linearly with a constant and positive upchirp rate rchirp=fchirp/dt. During the second interval, the frequency fchirpdecreases linearly with a constant negative downchirp rate −rchirp. The frequency of the measured light can thus be described by a periodic triangular function. However, other functional relationships are also contemplated, e.g. sawtooth functions.

The light source16is connected to a splitter22that splits the measuring light into reference light (also referred to as local oscillator) and output light. In the illustrated embodiment, the output light is amplified by an optical amplifier24and is then guided to a polarization rotator-splitter26that directs the amplified output light, which is still in a TE0 SOP, to a deflection unit28.

The deflection unit28directs the output light onto the object12—represented inFIG.3by a moving car—along different directions, as it has been explained above with reference toFIGS.1and2. To this end, the deflection unit28comprises, in the embodiment shown, a switch matrix M that selectively directs the output light to one of a plurality of output waveguides each terminating in an edge coupler29. The edge couplers29form a linear array that is arranged in a front focal plane of collimating optics31. The direction of output light emitted from the collimating optics31depends on the distance of the respective edge coupler29from the optical axis of the collimating optics31, as this is known as such in the art.

In other embodiments, the scanning device14is a multi-channel device comprising a plurality of polarization rotator-splitters26each being associated with a single edge coupler29or with a group of edge couplers29. In particular, each polarization rotator-splitter26may be directly connected to an associated edge coupler29. With respect to possible locations of the polarization rotator-splitters26in an FMCW LiDAR PIC, reference is made to European patent application No. 21168784.3 filed on Apr. 16, 2021. The full disclosure of this earlier application is incorporated herein by reference.

Referring again toFIG.3, a quarter-wave plate33is arranged between the edge couplers29and the collimating optics31. The quarter-wave plate33transforms the TE0 SOP of the emitted output light into a circular SOP, as this is indicated inFIG.3by symbols.

The output light emitted by the deflection unit28is at least partially diffusely reflected at the object12. A small portion of the reflected light thus returns to the deflection unit28, where it passes the quarter-wave plate33again and is re-coupled into the edge couplers29. The quarter-wave plate33transforms the circular SOP of the reflected light into a TM0 SOP, as this is indicated inFIG.3by symbols.

The polarization rotator-splitter26separates the reflected light TM0 mode from the output light TE0 mode traveling in the waveguide along the opposite direction. Furthermore, it simultaneously transforms the TM0 mode of the reflected light into a TE0 mode, as this will be explained in more detail below with reference toFIGS.5and6. The reflected and split-off light, now again in a TE0 SOP, is directed to a combiner30where it is superimposed with the reference light that has been separated from the measurement light by the splitter22. Since the frequencies of the superimposed light components are slightly different due to the different optical path lengths, a beat signal is generated which is detected by a symmetrical photodetector or another type of detector32. The electrical signals generated by the detector32are fed to a calculation unit34, which calculates the distance R and the relative radial velocity v to the object12on the basis of the detected beat frequencies.

With the exception of the quarter-wave plate33and the collimating optics31, all components shown inFIG.3and described in the foregoing are integrated in a photonic integrated circuit (PIC).

3. Polarization Rotator-Splitter

Reference is now made toFIG.5, which is a top view on the polarization rotator-splitter26without the cladding layer, and toFIGS.6and7, which are cross sections through the polarization rotator-splitter26along lines VI-VI and VII-VII, respectively.

The polarization rotator-splitter26comprises a substrate50which may be made of silicon. On the substrate50waveguides are formed that are embedded in a cladding layer52which may be made of SiO2, for example. The polarization rotator-splitter26has two functional units. The first unit, if seen from the left, is a waveguide polarization rotator54having a first end56and a second end58. The first end56is connected to waveguide57leading to the switch matrix M. The second functional unit is a polarization splitter61that is connected to the second end58of the waveguide polarization rotator54and comprises strip waveguides that form an asymmetric evanescent direction coupler. In the embodiment shown inFIG.3, the two outputs of the polarization splitter61are connected to the optical amplifier24and the combiner30, respectively.

In embodiments in which each edge coupler29is connected to a polarization rotator-splitter26, the waveguide57may be thickness tapered so as to form the edge coupler29. The tapered waveguide57then expands the mode field diameter of the light to an intended value and terminates in a taper tip having a square or rectangular cross-section and allowing similar coupling efficiencies for both TE0 and TM0-polarized light.

Both functional units, i.e. the waveguide polarization rotator54and the polarization splitter61, will be explained in more detail below.

a) Polarization Rotator

As shown inFIG.6, the polarization rotator54comprises a first layer62and a second layer64that together form a rib waveguide66and are made of silicon nitride. The first layer62has a thickness t1(i.e. a dimension perpendicular to the surface of the substrate50) that is larger than the thickness t2of the second layer64. In other embodiments, the thicknesses t1and t2are at least approximately equal.

The first layer62has a first section S1, a third section S3and a second section S2arranged between the first section51and the third section S3. The width w1of the first layer62linearly increases in the first section S1starting from the first end56, is constant in the second section S2and linearly decreases in the third section S3starting from the second section S2and terminating at the second end58.

The second layer64has a width w2that linearly increases from the first end56to the second end58. At the first end56the widths are equal, i.e. w1=w2. Within the polarization rotator54, w1>w2, but at the second end58the widths w1, w2are again equal, but now have a significantly larger value than at the first end56.

Typical values for the widths of the optical waveguides mentioned above are 0.1 to 1 μm for the optical waveguide57that leads either directly or via the switch matrix M (as shown inFIG.3) to an edge coupler, 1.5 to 3.5 μm for the first layer62and 1.0 to 2.0 μm for the second layer64within the second section S2, and 1.5 to 3.0 μm for the first layer62and 1.2 to 3.0 μm for the second layer64within the third section S3. The lengths of the first, second and third sections S1, S2and S3may be between 100 and 400 μm. The thicknesses t1, t2of the first layer62and the second layer64may each be between 0.1 and 0.3 μm.

The polarization rotator54is based on the concept of mode evolution. A TM0 mode entering the polarization rotator54at the first end56is first converted to a hybrid mode of TM0 and TE1 modes and then further evolves into a TE1 mode at the end of the polarization rotator54. However, a TE0 mode travelling through the polarization rotator54in either direction remains unchanged in its SOP.

b) Polarization Splitter

The polarization splitter61comprises a first strip waveguide71and a second strip waveguide72that is separated from the first strip waveguide71by a gap74. The first strip waveguide71and the second strip waveguide72are made of silicon nitride and form an asymmetric evanescent direction coupler.

The first strip waveguide71comprises a first portion76that contacts the first layer62and second layer64of the waveguide polarization rotator54, and a second portion78. The first strip waveguide71has a third width w3that is constant in the first portion76and linearly decreases in the second portion78starting from the first portion76. A non-linear increase is also possible.

The second strip waveguide72comprises a third portion80and a fourth portion82. The first portion76of the first strip waveguide71is parallel to and has the same length as the third portion80of the second strip waveguide72. The third portion80of the second strip waveguide72is arranged contiguous to the first layer62of the polarization rotator54. The second strip waveguide72has a double bend88in its fourth portion82so as to increase the width of the gap74.

The second strip waveguide72has a fourth width w4that is constant both in the third portion80and in the fourth portion82. The fourth width w4is smaller than the third width w3in the first portion76of the first strip waveguide71.

Typical values for the widths of the first strip waveguide71are 1.5 to 3 μm in the first portion76and 0.8 to 1 μm in the second portion78. The fourth width w4of the second strip waveguide72may be between 0.4 and 1.5 μm in both the third portion80and in the second portion82. The length of the first portion76and of the third portion80may be between 10 and 50 μm, and the length of the second portion78and of the fourth portion82may be between 20 and 50 μm. The gap74between the first portion76and the third portion80may be around 0.1 to 0.3 μm wide.

The asymmetric evanescent direction coupler forming the polarization splitter61selectively couples the TE1 mode guided in the first strip waveguide71into the second strip waveguide72and simultaneously transforms the TE1 mode into a TE0 mode. Therefore, only the TE0 mode propagates in the second strip waveguide72that finally leads to the detector32. A TE0 mode guided in the first strip waveguide71is unaffected by the adjacent second strip waveguide72.

Towards the end of the polarization splitter61, the first strip waveguide71and the second strip waveguide72are tapered down to a waveguide having dimensions of a single-mode waveguide. Therefore, the output light produced by the light source16in its original TE0 mode is guided unaffected through the polarization splitter61and the polarization rotator54towards the switch matrix M or, in other embodiments, directly to an edge coupler29. The SOP of the reflected light entering the edge couplers29in a TM0 mode is converted to a TE0 mode by the polarization rotator54. This reflected light then completely couples into the second strip waveguide72and is guided towards the combiner30where it is superimposed with the reference light (local oscillator).