WO2019145242A1

WO2019145242A1 - Device and method for determining a distance to a moving object

Info

Publication number: WO2019145242A1
Application number: PCT/EP2019/051320
Authority: WO
Inventors: Vladimir Davydenko
Original assignee: Carl Zeiss Smt Gmbh
Priority date: 2018-01-29
Filing date: 2019-01-21
Publication date: 2019-08-01
Also published as: DE102018201318A1

Abstract

The invention relates to a device and a method for determining a distance to a moving object, and to a light source for emitting an optical signal of temporally varying frequency, in particular for use in such a device. A device according to the invention comprises: a light source (110) for emitting an optical signal of temporally varying frequency, wherein the light source comprises a laser (201, 251), a first WGM resonator (210, 260), which is optically coupled to the laser, and a second WGM resonator (220, 270), which is optically coupled to the laser, a detector (160) for generating a detector signal from a superimposition of a first partial signal (111) and a second partial signal (121), wherein the first partial signal and the second partial signal are obtained by splitting the optical signal emitted by the light source (110), wherein the first partial signal reaches the detector without prior reflection at the object, and wherein the second partial signal reaches the detector (160) after reflection at the object, wherein the detector signal is characteristic of the frequency difference between the frequency of the second partial signal and the frequency of the first partial signal, and an evaluation unit for determining a distance to the object on the basis of this frequency difference.

Description

Apparatus and method for

Determining a distance of a moving object

The present application claims the priority of German Patent Application DE 10 2018 201 318.7, filed on January 29, 2018. The content of this DE application is incorporated by reference into the present application text ("incorporation by reference").

BACKGROUND OF THE INVENTION

Field of the invention

The invention relates to a device and a method for determining a distance of a moving object, as well as a light source for emitting an optical signal with a time-varying frequency, in particular for use in such a device.

State of the art

For optical distance measurement of objects u.a. a measurement principle also known as LIDAR, in which an optical signal, which is temporally changed in its frequency, is emitted towards the object in question and evaluated after back reflection on the object.

Fig. 1 a shows only a schematic representation of a known per se basic structure in which a signal emitted by a light source 110 with time-varying frequency (also referred to as "chirp") in two Partial signals is split, this splitting is done for example via a partially transparent mirror, not shown. The two sub-signals are coupled via a signal coupler 150 and superimposed on one another at a detector 160, wherein the first sub-signal 111 reaches the signal coupler 150 and the detector 160 as reference signal without reflection at the object labeled "140". The second partial signal 121 which arrives at the signal coupler 150 or at the detector 160, on the other hand, passes to the object 140 via an optical circulator 130, is reflected back by the latter and thus arrives at the signal coupler with a time delay and a correspondingly changed frequency in comparison to the first partial signal 111 150 and the detector 160.

The measuring signal supplied by the detector 160 is evaluated relative to the measuring device or the light source 110 via an evaluation device (not shown), wherein the difference frequency 131 between the sub-signals 111, 121 detected at a specific instant in the diagram of FIG is characteristic of the distance of the object 140 from the measuring device or the light source 110. According to FIG. 1b, in order to obtain additional information regarding the relative speed between the object 140 and the measuring device or the light source 110, the time-dependent frequency profile of the signal emitted by the light source 110 can also be such that two sections are present. in which the time derivative of the frequency generated by the light source 110 is opposite to each other.

To realize the tunable with respect to the frequency of the emitted light source 110, an arrangement is known in which as shown schematically in Fig. 5, a laser 501 via an optical coupler 515, for example in the form of a prism to a WGM resonator 510 (WGMR = "whispering gallery mode resonator ") is optically coupled, wherein due to the use of the WGM resonator 510 a comparatively high resonator quality, a small line width and a large coherence length can be achieved. In addition to providing the highest possible coherence length, however, there is also a need for the largest possible tunable frequency range of the light source 110 for carrying out high-precision distance measurements (eg with accuracies in the micrometer range over distances of several meters).

Basically, this frequency sweep of the light source in the case of the structure shown in FIG. by applying to the WGM resonator 510 (made of electro-optical crystal material) with electrical voltage. A problem which arises in practice in this case is that the free spectral range (FSR = "free spectral range") of the WGM resonator 510 which is relevant for the tunable frequency range is low. In particular, the tunable frequency range ("sweep ranks") is limited by the maximum permissible electrical voltage as well as the dimensions of the WGM resonator 510.

The low free spectral range (FSR) of the WGM resonator 510 in turn requires a correspondingly strong limitation of the amplification bandwidth of the laser 501, because to ensure a one-mode operation, the free spectral range (FSR) of the WGM resonator 510 must be greater than the gain bandwidth For this reason, in the structure of Fig. 5, as the laser 501, a DFB ("distributed feedback laser") laser with a low spectral amplification bandwidth is typically used.

In view of the above background, a comparatively small tuning range results with respect to the frequency of the light emitted by the light source 110 of the order of magnitude of a few hundred MHz to a few GHz, which may prove inadequate for the abovementioned high-precision measurement applications.

For the state of the art, reference is made, by way of example only, to US 2016/0299228 A1. SUMMARY OF THE INVENTION

In view of the above background, it is an object of the present invention to provide an apparatus and a method for determining a distance of a moving object, thus providing highly accurate distance detection particularly in the distance detection of moving objects having distances of several meters a few microns (mm) can be realized.

This object is achieved by the device according to the features of the independent patent claim 1 and the method according to the features of the independent claim 8.

An apparatus according to the invention for determining a distance of a moving object has:

a light source for emitting an optical signal having a time-varying frequency, wherein the light source comprises a laser, a first WGM resonator which is optically coupled to the laser, and a second WGM resonator, which is optically coupled to the laser, having;

a detector for generating a detector signal from a superimposition of a first partial signal and a second partial signal, wherein the first partial signal and the second partial signal have arisen by decomposing the optical signal emitted by the light source, wherein the first partial signal without prior reflection on the object the second sub-signal passes to the detector after reflection at the object, wherein the detector signal for the difference frequency between the frequency of the second sub-signal and the frequency of the first sub-signal is characteristic; and

- An evaluation device for determining a distance of the object based on this difference frequency. In particular, the invention is based on the concept that in a tunable light source for use in a device for distance measurement, two WGM resonators with free spectral ranges (FSR) that can be set independently of one another (in particular to slightly, eg by 10% mutually different values) operate. In this way, as described in more detail below, in comparison to the concept described at the outset with reference to FIG. 5, with a further high coherence of the emitted light, a significant enlargement of the tuning range of the light source with respect to the frequency of the emitted light can be achieved, so that as well For objects at a distance of several meters, highly accurate distance measurements with accuracies in the micrometer range can be realized.

The provision according to the invention of two WGM resonators achieves, in particular, that by changing the free spectral range (FSR) of one of the WGM resonators or both WGM resonators (ie setting different FSR combinations), shifts of the frequencies coinciding in the frequency spectrum Modes of the two WGM resonators and thus the frequency of the emitted light across the entire amplification bandwidth of the laser medium can be realized with the result that in comparison to the use of a single WGM resonator considerably enlarged tuning range is realized.

The use according to the invention of two WGM resonators in the tunable light source further achieves the requirement described at the beginning of the conventional concept described with reference to FIG. 5 of limiting the gain bandwidth of the laser (eg by using a DFB Laser) deleted. While in the conventional approach using only a single WGM resonator, the circumstance of the comparatively small free spectral range (FSR) of this WGM resonator must be taken into account by correspondingly limiting the amplification bandwidth of the laser must, this limitation can be avoided by the inventive use of two WGM resonators.

According to one embodiment, the free spectral range of the first WGM resonator and / or the free spectral range of the second WGM resonator can be varied in order to vary the frequency of the transmitted optical signal.

According to one embodiment, by this variation of the free spectral range (FSR) of the first WGM resonator and / or the free spectral range (FSR) of the second WGM resonator is a frequency at which modes of the first WGM resonator and the second WGM resonator coincide over the entire gain bandwidth of the laser displaceable.

According to one embodiment, the laser has an optical semiconductor amplifier. This advantageously makes it possible to take advantage of the fact that the use of two WGM resonators according to the invention eliminates the need for limiting the gain bandwidth of the laser (for example by using a DFB laser).

According to one embodiment, the light source has a tuning range which comprises at least the wavelength range from 20 nm to 100 nm, in particular at least the wavelength range from 5 nm to 150 nm.

According to one embodiment, the optical coupling of the first WGM resonator and / or the second WGM resonator to the laser is realized via at least one prism.

According to one embodiment, the optical coupling of the first WGM resonator and / or the second WGM resonator to the laser is realized via at least one optical waveguide. The invention also relates to a method for determining a distance of a moving object, the method comprising the following steps:

Emitting an optical signal, the signal having a time-varying frequency, using a light source for emitting a time-varying frequency optical signal, the light source comprising a laser, a first WGM resonator optically transmitting to the laser and a second WGM resonator optically coupled to the laser;

- Detecting generated by decomposition of this signal, superimposed partial signals with a detector, wherein a first partial signal of these partial signals without prior reflection on the object impinges on the detector and wherein a second partial signal of these partial signals after reflection on the object impinging on the detector; and

- Determining a distance of the object based on the difference frequency between the frequency of the second partial signal and the frequency of the first partial signal.

According to one embodiment, the free spectral range (FSR) of the first WGM resonator and / or the free spectral range (FSR) of the second WGM resonator are varied to vary the frequency of the optical signal emitted by the light source.

According to one embodiment, in this case, the free spectral range (FSR) of the first WGM resonator and / or the free spectral range (FSR) of the second WGM resonator may be at least temporarily increased by at least 5%, in particular at least 10%, in each case based on the greater value, different values can be set.

The invention further relates to a light source for emitting an optical signal with a time-varying frequency, in particular for use in a device for determining a distance of a moving object, with • a laser;

A first WGM resonator, which is optically coupled to the laser; and

A second WGM resonator, which is optically coupled to the laser;

Wherein for varying the frequency of the emitted optical signal, the free spectral range of the first WGM resonator and / or the free spectral range of the second WGM resonator is variable; and

Whereby a frequency at which modes of the first WGM resonator and the second WGM resonator coincide due to this variation of the free spectral range (FSR) of the first WGM resonator and / or of the free spectral range (FSR) of the second WGM resonator , over the entire gain bandwidth of the laser is displaceable.

The invention can be used in particular for optical distance and speed measurement in LIDAR systems (LIDAR = light detection and ranging). In the context of the invention, objects measured in the context of the distance may be only examples (and without the invention being limited thereto) of objects that are relevant in the road traffic or automobile sector (eg foreign vehicles), but also robot components such as robot arms. For such robot arms, on the one hand, a distance measurement for the purpose of position tracking ("tracking") can take place. Furthermore, such robot arms can themselves also be designed for the three-dimensional scanning of components (in a 3D scanner) and for this purpose be equipped with a light source according to the invention.

In further applications, the light source according to the invention or the tuning of the WGM resonators can also be used in telecommunications for rapid switching to another frequency range or in optical coherence tomography (OCT). Further embodiments of the invention are described in the description and the dependent claims.

The invention will be explained in more detail with reference to embodiments shown in the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Show it:

Figure 1a-b is a schematic representation (Figure 1 a) and a diagram

(Figure 1 b) for explaining the structure and operation of a device for determining a distance of a moving object;

Figure 2a-b are schematic representations for explaining the basic possible structure of a light source according to the invention in exemplary embodiments;

Figure 3-4 are schematic diagrams for explaining the operation of a light source according to the invention; and

Figure 5 is a schematic representation for explaining the structure and operation of a conventional light source in a device for distance measurement.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In addition, structure and operation of an inventive Light source in an apparatus for determining a distance of a moving object in an exemplary embodiment with reference to the schematic diagrams in Fig. 2-4 described.

FIG. 2a shows a merely schematic illustration for explaining the possible structure of a light source according to the invention. The light source has a laser 201, a first WGM resonator 210, which is optically coupled to the laser 201, and a second WGM resonator 220, which is likewise optically coupled to the laser 201. The optical coupling takes place (without the invention being restricted thereto) according to FIG. 2 a via a respective prism 215 or 225.

In further embodiments, the optical coupling may also be done in any other suitable manner (e.g., via optical fibers). Fig. 2b shows a schematic representation of an exemplary embodiment thereof. In this case, the optical coupling of both a first WGM resonator 260 and a second WGM resonator 270 to a laser 251 via an optical waveguide 255, wherein on the side facing away from the optical waveguide 255 of the laser 251, a mirror 280 is arranged. According to FIG. 2 b, both WGM resonators 260, 270 are arranged on one and the same side of the laser 251 (possibly in an advantageous manner under production aspects).

The respective free spectral ranges of the two WGM resonators 210, 220 (with renewed reference to FIG. 2 a) can be adjusted in a targeted manner by modifying the respective physical parameters of the respective electro-optical material, such as the refractive index or the geometric dimensions (corresponding to FIG "Tuning" of the WGM resonators 210, 220), wherein this modification of the physical parameters in turn can be done by applying an electrical voltage, by pressure and / or heat or in any other suitable manner. The laser 201 emits photons, which cause resonance excitation of both the first WGM resonator 210 and the second WGM resonator 220. By "tuning" the WGM resonators 210, 220, the frequency at which the modes of the two WGM resonators 210, 220 coincide and at which the laser 201 emits can thus be determined over the entire amplification bandwidth of the laser 201 be moved. The effect achieved by varying the free spectral range (FSR) of the WGM resonators 210, 220 is illustrated in the diagrams of FIGS. 3 and 4.

FIG. 3 shows the corresponding modes for a first setting or combination of the two free spectral ranges (FSR) of the WGM resonators 210, 220 for the two WGM resonators 210, 220 used in the light source according to the invention Fig. 3 shows a portion of the (in arbitrary units) applied frequencies is enlarged accordingly. The frequency of the light emitted by the laser 201 corresponds to the frequency at which the modes of the two WGM resonators 210, 220 coincide, as shown in FIG. 3 at the frequency of slightly less than 16 (arbitrary) frequency units highlighted by the drawn circle Case is.

4, the setting of another "FSR combination" of the two free spectral ranges (FSR) of the WGM resonators 210, 220 by corresponding "tuning" of these WGM resonators 210, 220 results in that the Herein, the coincident modes of the WGM resonators 210, 220 coincide at a different frequency value, with the result that the laser 201 emits at a different frequency (a value of about 19 arbitrary frequency units according to FIG. 4).

Since, as can be seen in FIGS. 3 and 4, the respective modes of the two WGM resonators 210, 220 exist over the entire amplification bandwidth of the laser 201, a combination of the respective free spectral ranges (FSR) can always be used for the two WGMs Resonators 210, 220 find for which a coincidence of the respective modes just at a desired frequency of the light to be emitted takes place. As a result, by modifying the free spectral range (FSR) of at least one of the WGM resonators 210, 220, a tuning of the frequency over the entire gain bandwidth of the laser 201 can be achieved, thus using the light source in the arrangement of FIG correspondingly large tuning range, which can comprise, for example, at least the wavelength range from 20 nm to 100 nm, preferably from 5 nm to 150 nm. Although the invention has also been described with reference to specific embodiments, numerous variations and alternative embodiments will be apparent to those skilled in the art, eg, by combining and / or replacing features of individual embodiments. Accordingly, it will be understood by those skilled in the art that such variations and alternative embodiments are intended to be embraced by the present invention and that the scope of the invention is limited only in terms of the appended claims and their equivalents.

Claims

claims

1. A device for determining a distance of a moving object (140), with

A light source (110) for emitting an optical signal having a time-varying frequency, wherein the light source is a laser (201, 251), a first WGM resonator (210, 260) optically coupled to the laser (201, 251) and a second WGM resonator (220, 270) optically coupled to the laser (201, 251);

• a detector (160) for generating a detector signal from a superimposition of a first partial signal (111) and a second partial signal (121), wherein the first partial signal (111) and the second partial signal (121) by decomposing the of The first partial signal (111) has passed without prior reflection at the object (140) to the detector (160) and wherein the second partial signal (121) after reflection at the object (140) to the detector (160), the detector signal for the difference frequency between the frequency of the second partial signal (121) and the frequency of the first partial signal (111) being characteristic; and

• an evaluation device for determining a distance of the object (140) on the basis of this difference frequency.

2. Apparatus according to claim 1, characterized in that for varying the frequency of the emitted optical signal, the free spectral range of the first WGM resonator (210, 260) and / or the free spectral range of the second WGM resonator (220, 270) is variable.

3. Apparatus according to claim 2, characterized in that by this variation of the free spectral range (FSR) of the first WGM resonator (210, 260) and / or the free spectral range (FSR) of the second WGM Resonator (220, 270) a frequency at which modes of the first WGM resonator (210, 260) and the second WGM resonator (220, 270) coincide over the entire gain bandwidth of the laser (201, 251) is displaceable ,

4. Device according to one of claims 1 to 3, characterized in that the laser (201, 251) comprises a semiconductor optical amplifier (SOA).

5. Device according to one of claims 1 to 4, characterized in that it comprises a tuning range, which at least the wavelength range of 20nm to 100nm, in particular the wavelength range from 5nm to 150nm comprises.

6. Device according to one of the preceding claims, characterized in that the optical coupling of the first WGM resonator (210) and / or the second WGM resonator (220) to the laser (201) via at least one prism (215, 225) is realized.

7. Device according to one of the preceding claims, character- ized in that the optical coupling of the first WGM resonator (260) and / or the second WGM resonator (270) to the laser (251) via at least one optical waveguide (255). is realized.

8. A method of determining a distance of a moving object, the method comprising the steps of:

a) emitting an optical signal, the signal having a time-varying frequency, using a light source for emitting a time-varying frequency optical signal, the light source comprising a laser (201, 251), a first WGM resonator (210, 260 ) optically coupled to the laser (201, 251) and a second WGM resonator (220, 270) optically coupled to the laser (201, 251); b) acquiring superimposed partial signals generated by decomposing this signal with a detector (160), wherein a first partial signal (111) of these partial signals impinges on the detector (160) without prior reflection on the object (140) and wherein second sub-signal (121) of these sub-signals hits the detector (160) after reflection at the object (140); and

c) determining a distance of the object (140) on the basis of the difference frequency between the frequency of the second partial signal (121) and the frequency of the first partial signal (111).

9. The method according to claim 8, characterized in that the free spectral range (FSR) of the first WGM resonator (210, 260) and / or the free spectral range (FSR) of the second WGM resonator (220, 270) for varying the frequency of the light source (110) emitted optical signal can be varied.

10. The method according to claim 8 or 9, characterized in that the free spectral range (FSR) of the first WGM resonator (210, 260) and / or the free spectral range (FSR) of the second WGM resonator (220, 270) in this Variation at least temporarily by at least 5%, in particular by at least 10%, in each case based on the larger value, values different from each other are set.

11. A light source for emitting an optical signal with time-varying frequency, in particular for use in a device for determining a distance of a moving object, with

A laser (201, 251);

A first WGM resonator (210, 260) optically coupled to the laser (201, 251); and

A second WGM resonator (220, 270) optically coupled to the laser (201, 251); Wherein for varying the frequency of the emitted optical signal, the free spectral range of the first WGM resonator (210, 260) and / or the free spectral range of the second WGM resonator (220, 270) is variable; and wherein by this variation of the free spectral range (FSR) of the first WGM resonator (210, 260) and / or of the free spectral range (FSR) of the second WGM resonator (220, 270) a frequency, in which Modes of the first WGM resonator (210, 260) and the second WGM resonator (220, 270) coincide over the entire gain bandwidth of the laser (201, 251) is displaceable.