Photonic integrated circuit and three-dimensional laser Doppler vibrometer including the same

Abstract

A photonic integrated circuit (PIC) includes a splitter that splits a laser beam into a measurement signal and a reference signal; a phase amplitude modulator array coupled to a transmitting array to generate at least three output signals to be directed to a single target location from the measurement signal, each output signal having a different direction and a different carrier frequency; and a receiving array including at least three receiving antennas that receive reflected signals from different receiving directions. Each output signal is assigned a different carrier frequency so that any crosstalk can be distinguished in the frequency domain in signal processing to allow them to be removed and / or restored. By using the PIC, only one laser source is required for a 3D LDV, which reduces the volume and production cost of the 3D LDV device.

Description

Technical Field

[0001] The present disclosure relates to a photonic integrated circuit (PIC) for a three-dimensional (3D) laser Doppler vibrometer (LDV). The present disclosure also relates to a 3D LDV including the PIC. Background Art

[0002] A laser Doppler vibrometer (LDV) 1 is an instrument for measuring the temporal velocity or displacement of a vibrating surface 2 as shown in FIG1. ​​The LDV 1 emits a laser beam 3 which is reflected by the surface 2 located at a distance d0 away. The incident reflected laser beam 4 is received by the LDV. The instantaneous frequency shift f of the reflected signal is Doppler (t) is proportional to the time velocity v(t) of the target surface. This relationship is expressed as

[0003]

[0004] where λ0 is the wavelength of the measuring light, i.e. the wavelength of the laser beam 3. This relationship can also be understood as the phase shift of the reflected beam 4 due to the motion of the target surface:

[0005]

[0006] The expression Δd(t) is the displacement of the target surface in the direction of the laser beam as shown in Fig. 1. Since the change in the optical path of the laser beam corresponds to the round trip of the displacement, a factor of 2 is placed before Δd(t).

[0007] There are two main types of LDVs, fixed LDVs and scanning LDVs. The present disclosure relates to fixed LDVs that are able to pick up motion information of a vibrating target in one location, but typically only in one direction.

[0008] A known photonic integrated circuit (PIC) for on-chip LDV is shown in FIG2 . The PIC 10 includes an input device 12, such as a grating coupler, for coupling an external laser beam (not shown) to the PIC. The laser beam is sent via a waveguide 14 to a splitter 16 where the signal is split into a measurement signal and a reference signal. The measurement signal is sent via a waveguide 18 to a transmit-receive antenna 20, which outputs the measurement signal as an outgoing laser beam 22 to a target 24. An optical system 26 is typically used to ensure that the outgoing signal 22 is focused onto a single point (i.e., the target location 24), where the signal is reflected and sent back to the PIC 10 as a reflected signal 28. The reflected signal 28 is typically returned via the optical system 26 and received by the transmit-receive antenna 20 at the PIC 10. The received reflected signal is sent from the transmit-receive antenna 20 to a mixer 32 via a waveguide 30. The reference signal originating from the splitter 16 is sent to the mixer 32 via a waveguide 34. The received reflected signal and the reference signal are mixed by a mixer 32 (e.g., a 90° optical hybrid) on the PIC 10. By using a 90° optical bridge as the mixer 32, the mixer 32 has four optical output signals that are sent via waveguides 36 to respective photodiodes 38, which convert the optical mixed signals into photocurrent signals. The use of a demodulator (not shown) allows the desired motion information to be determined from the photocurrent signal 42.

[0009] Using LDV as explained above, it is possible to obtain one-dimensional information, i.e. the vibration behavior in the direction of the outgoing signal 3, 22. However, three-dimensional (3D) vibration information is also necessary in many applications, such as studying the movement of the hammer-anvil joint in the ear (biomechanics), monitoring the simultaneous in-plane and out-of-plane motion of surface waves (modal testing), or understanding the 3D motion of nanostructures (MEMS).

[0010] The standard way to achieve 3D LDV measurement is to use three or more separate LDV devices that measure simultaneously on the same target position. The reason for using three different LDV devices instead of three laser beams from a single laser source is to avoid beam crosstalk. Since the different laser beams are incoherent with each other, there will be no crosstalk between any of these LDV devices. However, the cost of this system is quite high due to the use of several laser sources.

[0011] It is also known in the art to use a single laser source to achieve 3D LDV measurements, as described in "Three-channel three-dimensional self-mixing thin-slice solid-state laser-Doppler measurements" by Takayuki Ohtomo et al., published in Optical Society of America, APPLIED OPTICS, Vol. 48, No. 3, January 20, 2009. In this disclosure, a single laser source is used in combination with multiple splitters and an acousto-optic modulator (AOM), also known as an optical frequency shifter, to generate three laser beams with different carrier frequencies.

[0012] A similar device is described in “Two-channel self-mixing laser Doppler measurement with carrier-frequency-division multiplexing” by Kenju Otsuka et al., Optical Society of America, APPLIED OPTICS, Vol. 44, No. 9, March 20, 2005, in which an AOM is used to generate two laser beams from a single laser source.

[0013] The disadvantage of this device is its size, since it is a free-space device. In addition, this design is not easy to implement on a silicon-on-insulator (SOI) chip.

[0014] US 2013 / 083389 A1 discloses an LDV photonic integrated circuit (PIC) having an optical selector for directing light to an off-chip target area. Depending on the configuration of the optical selector, the light beam is output from different locations on the PIC. Summary of the invention

[0015] An object of the present disclosure is to provide a photonic integrated circuit (PIC) for a three-dimensional (3D) laser Doppler vibrometer (LDV).

[0016] This object is achieved according to the present disclosure by a PIC for 3D LDV, the PIC comprising: a splitter for splitting a laser beam into a measurement signal and a reference signal; a phase amplitude modulator array coupled to a transmit array to generate n output signals from the measurement signal to be directed to a single target location and output the n output signals from substantially a single location, each output signal having a different direction and a different carrier frequency, n being a natural number greater than or equal to three; a receive array comprising m receive antennas, each receive antenna being configured to receive reflection signals from a different receive direction, each reflection signal indicating one or more output signals that have been reflected at the single target location, m being a natural number greater than or equal to three; for each receive antenna, a mixer connected to the receive antenna to mix the reference signal with the received reflection signal; and for each mixer, at least one photodiode connected to the mixer to generate a photocurrent signal from the mixed signal.

[0017] The provision of an on-chip phase amplitude modulator array coupled to the transmit array allows at least three output signals with different carrier frequencies and different directions to be generated from a single laser source. Using an external optical system, the various output signals are focused on a single target position. In this way, different output signals arrive at the target position from different directions. The reflected signal is then returned to at least three receiving antennas that each receive signals from different directions via the optical system. The various received signals may include reflections caused by one or more of the output signals. The mixer creates a mixed signal with a reference signal for each received signal, while the photodiode generates a corresponding photocurrent signal, which is then further analyzed in a demodulator different from the PIC to obtain the required motion information in at least three different directions, i.e., 3D information. Each output signal is assigned a different carrier frequency so that any crosstalk can be distinguished in the frequency domain in signal processing (i.e., demodulator) to allow them to be removed and / or restored. Specifically, when signals from different output signals are received at the same receiving antenna, these signals can be demultiplexed and distinguished by the demodulator based on the different carrier frequencies of the signals.

[0018] By using this PIC, only one laser source is needed for the 3D LDV, which reduces the size and production cost of the 3D LDV device. In addition, the on-chip design is much smaller than the known free-space devices that rely on acousto-optic modulators. In addition, phase amplitude modulator arrays are easier to implement on SOI chips than acousto-optic modulators.

[0019] Yet another advantage of a phase amplitude modulator array coupled to a transmit array relates to harmonics. A known problem with phase shifting on a PIC is that it is difficult to generate a single offset to a single beam by a phase or amplitude modulator without introducing other harmonics in the frequency shifted beam. The inventors of the present invention have found that using a phase amplitude modulator array coupled to a transmit array allows such harmonics to be suppressed more easily, and in particular, completely.

[0020] In an embodiment of the present disclosure, the transmit array includes k transmit antennas positioned adjacent to each other along a substantially straight line, k being a natural number greater than or equal to three.

[0021] In this embodiment, a minimum number of transmit antennas are used, which reduces the cost of the PIC.

[0022] In a preferred embodiment of the present disclosure, k transmit antennas generate a combined near-field pattern Where j represents one of the n output signals, x represents the coordinate along the direction of the k transmitting antennas, t is the time, and a j is the angle of the jth output signal relative to the direction orthogonal to the direction of the k transmitting antennas, f j is the optical frequency of the jth output signal, and s j represents the amplitude of the jth output signal. Preferably, n is equal to three, α1=-α3, α2=0, f1=f2-df, f3=f2+df and s1=s2=s3=1.

[0023] This allows to generate three beams with equal amplitude, with the centrally directed beam having frequency f2 and the two offset beams having the same frequency shift. This is a symmetrical setup allowing easy demodulation.

[0024] In an advantageous embodiment of the disclosure, for a transmit antenna at position x, the field amplitude is 1+2·cos[2π(sin(α1·x-df·t))] and the phase is 2πf2·t. In particular, a ramp function is used in phase modulation.

[0025] This transmit antenna setup generates a symmetrical setup of the outgoing signal described above from the reference signal. The use of a ramp function in the phase modulation ensures a quick jump back to zero if the phase increases by more than 2π.

[0026] In an alternative embodiment of the present disclosure, the transmit array includes k transmit antennas positioned in a two-dimensional array, k being a natural number greater than or equal to four.

[0027] In this alternative embodiment, more transmit antennas are required to form a two-dimensional array. However, by using a two-dimensional array, a simpler external optical system can be used.

[0028] In a preferred embodiment of the present disclosure, m is equal to n and the transmitting antenna is equivalent to the receiving antenna.

[0029] Using the same antenna for both transmission and reception allows space to be saved on the PIC design by reducing the number of antennas to be included.

[0030] In a preferred embodiment of the present disclosure, the receiving antenna and the transmitting antenna are formed by one or more grating couplers.

[0031] In an embodiment of the present disclosure, m is equal to n and each receiving direction is the opposite direction of the corresponding output signal direction.

[0032] This provides a symmetrical arrangement of the directions of the exiting light beams, which makes it easier to design the external optical system.

[0033] In one embodiment of the present disclosure, the photodiode is a balanced photodiode.

[0034] Balancing the photodiode helps remove noise common in mixed signals, such as noise due to the optical system on the output signal and the transmitted signal.

[0035] In an embodiment of the present disclosure, the PIC further comprises an input device for providing an external laser beam to the PIC. Preferably, the input device is one of a grating coupler and an edge coupler, such as a cone or an inverted cone.

[0036] The grating coupler is a surface coupler which is easy to test at wafer level. The edge coupler has less insertion loss but it requires preparation at the edge of the chip.

[0037] The object according to the present invention is also achieved by a 3D LDV, which comprises: a laser source generating a laser beam; a PIC as described above, which is coupled to the laser source; an optical mirror system, which is configured to focus n output signals on a single target position and to focus a reflected signal from the single target position to the PIC; and a demodulator, which determines the instantaneous speed and direction of the single target position from the photocurrent signal.

[0038] The advantages of 3D LDV are the same as those of PIC mentioned above. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] The present disclosure will be further explained by the following description and the accompanying drawings.

[0040] FIG. 1 shows a schematic arrangement of a known LDV.

[0041] FIG. 2 shows a known PIC used in the LDV of FIG. 1 .

[0042] Figure 3A PIC for a 3D LDV according to the present disclosure is shown.

[0043] Figure 4 A 3D LDV setup according to the present disclosure is shown.

[0044] Figure 5 An alternative 3D LDV arrangement according to the present disclosure is shown.

[0045] Figures 6A to 6F It shows that it can be Figure 3 Different possible splitters used in PICs.

[0046] Fig. 7A Shows Figure 5 The beam angle at the LDV position in the device.

[0047] Figure 7B Shows Figure 5 The beam angle at the target position in the device. DETAILED DESCRIPTION

[0048] The present disclosure will be described with respect to specific embodiments and with reference to certain drawings, but the disclosure is not limited thereto but only by the claims. The drawings described are merely schematic and non-limiting. In the drawings, for illustrative purposes, the dimensions of some of the elements may be exaggerated and not drawn to scale. Dimensions and relative dimensions do not necessarily correspond to actual reductions to the implementation of the present disclosure.

[0049] In addition, the terms first, second, third, etc. in the specification and claims are used to distinguish between similar elements and not necessarily to describe a sequential or chronological order. These terms are interchangeable under appropriate circumstances, and the embodiments of the present disclosure can operate in other sequences than those described or illustrated herein.

[0050] Furthermore, the terms top, bottom, over, under, etc. in the description and claims are used for descriptive purposes. The terms so used are interchangeable under appropriate circumstances, and the embodiments of the disclosure described herein may operate in other orientations than described or illustrated herein.

[0051] Furthermore, although referred to as “preferred,” the various embodiments are to be construed as exemplary ways in which the disclosure may be implemented, rather than limiting the scope of the disclosure.

[0052] PIC according to the present disclosure will refer to Figure 3 Description. Elements or components previously described with reference to FIG. 2 have the same last two digits but are preceded by a "1".

[0053] The PIC 100 has an input device 112 that allows the PIC 100 to be connected to an external laser 150. The input device 112 can be a grating coupler and an edge coupler, such as a cone or an inverted cone. The waveguide 114 sends the incident laser beam signal to a first splitter 1161, which splits the signal into a measurement signal and a reference signal. Based on the number of outgoing signals 122 (described in more detail below), additional splitters 1162 and 1163 are provided to the PIC 100 to further split the reference signal into the required number of reference signals, which is three in the illustrated embodiment. Each reference signal is sent to a different mixer waveguide 1321, 1322, 1323 via waveguides 1341, 1342, 1343. The reference signal is sent to the phase amplitude modulator array 140 coupled to the transmit array 120 via waveguide 118.

[0054] It will be readily appreciated that splitters 1161, 1162, and 1163 may also be combined into a single splitter. Figures 6A to 6F Given in. Fig. 6A shows a y splitter, Figure 6B shows a multimode interference coupler, Figure 6C shows a directional coupler, a star coupler in Fig.6D As shown in the figure, the combined splitter is Fig. 6E is shown in, and Fig. 6F A tunable splitter is shown. The choice of splitter will naturally depend on the application at hand, for example the total number of signals that need to be generated from the initial signal.

[0055] The phase amplitude modulator array 140 coupled to the transmit array 120 allows three output signals 1221, 1222, 1223 to be generated from the reference signal, each of the output signals 1221, 1222, 1223 having its own different carrier frequency and different output direction. As used herein, the term "different" is used to refer to a property of a signal from a plurality of signals, such as frequency or direction, that is different from the same property of all other signals from the plurality of signals.

[0056] An example of a phase amplitude modulator array coupled to a transmit array is described by Christos Tsokos et al., “Analysis of a Multibeam Optical Beamforming Network Based on Blass Matrix Architecture,” IEEE, Journal of Lightwave Technology, Vol. 36, No. 16, August 15, 2018, doi: 10.1109 / JLT.2018.2841861.

[0057] like Figure 3 As shown, the transmit array 120 is composed of three aligned transmit arrays 1201, 1202, and 1203. This transmit array 120 combines several optical signals into one field by placing the transmitters very close to each other. In other words, n output signals are actually output from a single location. The phase amplitude modulator array 140 ensures that the amplitude and phase of the combined field can be purposefully modulated in the time domain and the spatial domain. Through a suitable modulation algorithm, a combined near-field diagram can be generated to create a desired beam. In order to generate a beam with an output angle a j (with respect to the direction orthogonal to the direction of the k transmitting antennas (ie, the horizontal direction H)) and the optical frequency f j For n (e.g. three) output beams, the combined near-field diagram should be expressed as Where j represents one of the n output signals, x represents the coordinate along the direction of the k transmitting antennas (i.e., the vertical direction V), t is the time, and s j represents the amplitude of the jth output signal. In the case shown, α1=-α3, α2=0, f1=f2-df, f3=f2+df and s1=s2=s3=1.

[0058] To generate this combined near-field diagram, the transmit antenna 120 at position x i The field amplitude is 1+2·cos[2π(sin(α1·x-df·t))], and the phase is 2πf2·t. Ramp functions are used in phase modulation to handle infinitely increasing phase shifts.

[0059] In the PIC 100, a receiving antenna array 144 is provided, which is composed of three receiving antennas 1441, 1442, 1443. Each receiving antenna 1441, 1442, 1443 receives a reflection signal 1281, 1282, 1283 from a different receiving direction, and each transmission signal indicates that one or more of the output signals 1221, 1222, 1223 have been reflected at a single target position (not shown). In the illustrated embodiment, the receiving direction is the opposite direction of the output direction.

[0060] The received reflected signal is sent to different mixer waveguides 1321, 1322, 1323 via waveguides 1301, 1302, 1303. Mixers 1321, 1322, 1323 mix the received reflected signal and the reference signal and output two optical signals, which are sent to respective balanced photodiodes 1381, 1382, 1383, which convert the optical mixed signals into photocurrent signals 1421, 1422, 1423.

[0061] It will be readily appreciated that mixers 1321, 1322, 1323 may be 90 degree optical bridges (ie, 2x4 splitters) or 2x2 splitters or any other component capable of generating multiple outputs where the optical path difference between reference and measurement signals is different for different output ports.

[0062] The use of a demodulator (not shown) allows the desired motion information to be determined from the photocurrent signal 142. Since each output signal 1221, 1222, 1223 is assigned a different carrier frequency, the demodulator can distinguish any crosstalk in the frequency domain to allow it to be removed and / or restored. For example, the photocurrent from the balanced photodiode 138 can be demodulated at three different carrier frequencies f1, f2 and f3 through three bandpass filters. Thus, three velocities can be obtained as v1, v2 and v3. From the frequency f j and the output beam angle a j relationship (see Fig. 7A ) to know the measured velocity v j1 Corresponding to the output direction a j and input direction a1. Similarly, we can also get v j2 and v j3 The measured velocities are deflection velocities, which correspond to certain incident-exit direction pairs β in the target according to the optical system design. j →β m (See Figure 7B ), where m = 1, 2 or 3 and is the index of the reflected beam. In the most common setup, β1, β2 and β3 form three vectors that are orthogonal to each other. In this case, v11 、v 12 and v 13 Corresponding to the measured velocity components in three orthogonal directions.

[0063] Figure 4 and 5 Two different 3D LDV devices are shown. LDV 300 includes the above reference Figure 3 LDV300 emits three output signals 2221, 2222, 2223, which are transmitted via a single optical system 260 (see Figure 4 Embodiment) or multiple optical systems 2601, 2602, 2603 (see Figure 5 Embodiment) is focused to a single target location 200. The target location 200 is moving, as shown by arrow 270. Reflected signals 2281, 2282, 2283 are sent back to the LDV using the same optical system 260, 2601, 2602, 2603.

[0064] like Figure 4 and 5 As shown, due to the output signals 2221, 2222, 2223 having different directions, the signal that hits the target position 260 also has different directions so that each output signal 2221, 2222, 2223 obtains vibration information in different directions. By deriving the phase shift in each reflection signal 2281, 2282, 2283, three-dimensional vibration information is obtained.

[0065] Although aspects of the disclosure have been described in relation to particular embodiments, it will be readily appreciated that these aspects may be embodied in other forms within the scope of the disclosure as defined by the claims.

Claims

1. A photonic integrated circuit (PIC) for a three-dimensional (3D) laser Doppler vibrometer (LDV), the PIC comprising: A splitter for dividing the laser beam into a measurement signal and a reference signal; a phase amplitude modulator array coupled to the transmit array to generate n output signals from the measurement signal to be directed to a single target location and output the n output signals from the single location, each output signal having a different direction and a different carrier frequency, n being a natural number greater than or equal to three; a receiving array comprising m receiving antennas, each receiving antenna being configured to receive reflection signals from a different receiving direction, each reflection signal indicating one or more output signals having been reflected at the single target location, m being a natural number greater than or equal to three; for each receiving antenna, a mixer connected to the receiving antenna for mixing the reference signal with the received reflected signal; as well as For each mixer, at least one photodiode is connected to the mixer to generate a photocurrent signal from the mixed signal.

2. The PIC of claim 1, wherein the transmit array comprises k transmit antennas positioned adjacent to each other along a substantially straight line, k being a natural number greater than or equal to three.

3. The PIC of claim 2, wherein the k transmit antennas generate a combined near-field pattern Where j represents one of the n output signals, x represents the coordinate along the direction of the k transmitting antennas, t is the time, a j is the angle of the jth output signal relative to the direction orthogonal to the direction of the k transmitting antennas, f j is the optical frequency of the jth output signal, and s j Represents the amplitude of the jth output signal.

4. The PIC of claim 3, wherein n is equal to three, α1 = -α3, α2 = 0, f1 = f2 - df, f3 = f2 + df and s1 = s2 = s3 = 1.

5. A PIC as claimed in claim 4, wherein for a transmitting antenna at position x, the field amplitude is 1+2·cos[2π(sin(α1·x-df·t))] and the phase is 2πf2·t.

6. A PIC as claimed in claim 5, wherein a ramp function is used in the phase modulation.

7. The PIC of claim 1, wherein the transmit array comprises k transmit antennas positioned in a two-dimensional array, k being a natural number greater than or equal to four.

8. The PIC of claim 2, wherein m is equal to n and the transmit antenna is identical to the receive antenna.

9. The PIC of claim 2, wherein the receiving antenna and the transmitting antenna are formed by one or more grating couplers.

10. The PIC of claim 1, wherein m is equal to n and each receiving direction is the opposite direction of the corresponding output signal direction.

11. The PIC of claim 1, wherein the photodiode is a balanced photodiode.

12. The PIC of claim 1, wherein the PIC further comprises an input device for providing an external laser beam to the PIC.

13. The PIC of claim 12, wherein the input device is one of a grating coupler and an edge coupler.

14. The PIC of claim 13, wherein the edge coupler is tapered or inverted tapered.

15. A three-dimensional 3D laser Doppler vibrometer LDV, comprising: a laser source for generating a laser beam; A photonic integrated circuit PIC as claimed in any one of the preceding claims, said PIC being coupled to said laser source; an optical mirror system configured to focus the n output signals onto the single target location and to focus a reflected signal from the single target location onto the PIC; as well as A demodulator determines the instantaneous speed and direction of the single target location from the photocurrent signal.

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