Patent Publication Number: US-2022214456-A1

Title: Phase Difference Detection System and a Method for Detecting a Phase Difference

Description:
TECHNICAL FIELD 
     The present disclosure relates to a phase difference detection system for a single target location. The present disclosure further relates to a method for detecting a phase difference between two laser beams due to reflection at a target location. 
     BACKGROUND ART 
     Phase difference detectors are used in different areas of technology, including laser Doppler vibrometry and determining refractive index changes at different locations of a medium. 
     A laser Doppler vibrometer (LDV)  1  is an instrument used to measure the temporal velocity or displacement of a vibrating surface  2  as illustrated in  FIG. 1 . The LDV  1  sends out a laser beam  3  which is reflected by surface  2  located a distance d 0  away. The incoming reflected laser beam  4  is received by the LDV  1 . The instantaneous frequency shift of the reflection signal f Doppler (t) is proportional to the temporal velocity v(t) of the target surface. The relation is expressed as 
     
       
         
           
             
               
                 
                   
                     
                       
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     where λ 0  is the wavelength of the measurement light. This relation can also be understood as the phase shift of the reflection beam as a result of the movement of the target surface: 
     
       
         
           
             
               
                 
                   
                     
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     The expression Δd(t) is the displacement of the target surface in the direction of the laser beam. A factor of 2 is placed before Δd(t) because the optical path change of the beam corresponds to a roundtrip of the displacement. 
     Laser Doppler vibrometry shows great vibration measurement accuracy with sub-nanometer resolution and has been applied in various fields, e.g. in bio-medical engineering and mechanics. 
     In some applications, it is crucial to obtain vibration information at multiple positions simultaneously, which is not easy for a commonly used single-point LDV system, also known as a one-beam LDV. To realize such measurements, two techniques are generally used: a scanning LDV and a multi-beam LDV. 
     Scanning LDV systems are used more often thanks to their flexibility and simplicity compared to the multi-beam LDV systems. However, the performance of the scanning LDV is limited by its scanning frequency, which usually cannot reach the MHz range due to mechanics-based scanning devices. Therefore, scanning LDV systems cannot typically detect a transient movement event, such as an impact. 
     On the other hand, a multi-beam LDV system has no such limitations and it can measure transient and non-periodic events. However, because multiple separate LDV devices are implemented in the same system, a multi-beam LDV system is usually more bulky and complex. 
     Recently, there have been attempts to realize a multi-beam LDV by combining multiple one-beam LDVs on a single chip for simultaneous measurement of multiple points on a single target as discussed in Yanlu Li et al., “ Six - beam homodyne laser Doppler vibrometry based on silicon photonics technology”,  2 Feb. 2018, Optical Society of America, OPTICS EXPRESS, Vol. 26, No. 3. It is proposed to use a single photonic integrated circuit (PIC) to create a six-beam LDV. The PIC has a single laser input and a first splitter to split the laser beam into a reference beam and a measurement beam. Another two splitters are used to split the reference beam and the measurement beam into six individual reference beams and measurement beams. Each of the six phase difference detectors has the same, common, design. Specifically, the individual measurement beams are sent towards the target by a transmit antenna and the reflection is received. The received reflected signal is sent to a mixer, e.g. a 90° optical hybrid, that mixes the received reflected signal and the reference signal. By using a 90° optical hybrid as the mixer, the mixer has four optical output signals that are converted, by photo-diodes, into photo-current signals. Using a demodulator allows determining the desired movement information from the photo-current signals. 
     While such a PIC may accommodate six LDVs, it is not easy to integrate a larger number of LDVs, e.g. more than twenty, on a single chip with a limited footprint, e.g. 1 mm 2 . Especially the bonding pads and waveguide routes limit the number of LDVs realized on a single chip. 
     WO 2018/015517 A1 discloses a lens free imaging device for creating holographic images. The lens free imaging device comprises a waveguide with a first diffracting element to split a part of the waveguide light into a first light wave oriented to a target and a second light wave oriented towards an image sensor. The first light wave is scattered by the target and the resulting scattered light wave is also received at the image sensor. The scattered light wave and the second light wave are combined to form an image. 
     SUMMARY OF THE DISCLOSURE 
     It is an object of the present disclosure to provide a new LDV design which allows a large number of them to be combined on a single chip. 
     This object is achieved according to the disclosure with a phase difference detection system for a single target location, the phase difference detection system comprising: a laser source to generate a laser beam; a phase difference detector coupled to the laser source and having a multi-layer structure comprising: a planar waveguide for guiding the laser beam, the planar waveguide having a normal direction and forming a first layer of the multi-layer structure; a grating coupler to split the laser beam into a reference beam and a measurement beam, wherein the measurement beam is directed towards the target location and the reference beam is directed away from the target location, the reference beam and the measurement beam each being inclined with respect to said normal direction; and a planar imager array forming a second layer of the multi-layer structure, the imager array being configured to detect a fringe pattern generated by said reference beam and an incoming beam, the incoming beam being the result of the measurement beam having been reflected at the target location and having passed through the grating coupler; and an optical system configured to focus the measurement beam output from the phase difference detector on the single target location and to focus signals reflected at the target location to the phase difference detector. 
     The grating coupler couples the coherent light from the waveguide to both upward and downward directions of the grating coupler, i.e. it splits the coherent light from the waveguide into the reference beam and the measurement beam. The wavelength of the laser beam, the refractive index of the grating coupler and the period of the grating coupler are so chosen such as to ensure that the reference beam and the measurement beam each are inclined with respect to the normal direction of the waveguide. This is possible as these quantities are linked as discussed below. 
     The measurement laser beam is focused to the target location with the help of an optical system. Due to reflection at the target location, a reflected beam is sent back towards the phase difference detector, i.e. towards the grating coupler. The reflected beam can be considered to be parallel to the measurement beam but travelling in the opposite direction. In this way, the incoming beam that is the result of the measurement beam having been reflected at the target location and the reference beam are angled with respect to one another so they create an interference pattern, also known as a fringe pattern. The imager array detects this fringe pattern from which the phase difference between the reference beam and the incoming beam may be determined. 
     By using the fringe pattern detected by the imager to determine the phase difference, there is no need to have mixers and photo-diodes, nor are separate waveguides for the reference and measurement signals required. Consequently, a large number of them may be combined on a single chip. 
     Moreover, having the incoming beam pass through the grating coupler is advantageous as this minimizes the required area for the phase detector. In particular, when compared to the lens free imaging device disclosed in WO 2018/015517 A1, the imager may now be placed below the grating coupler instead of next to it, thus reducing the required on-chip area. Furthermore, the present inventors have found that, although the grating coupler affects light passing through it, the phase behaviour of the incoming beam which passes through the grating coupler remains unaffected. As such, the fringe pattern is not affected by the incoming beam passing through the grating coupler. 
     In an embodiment of the present disclosure, the optical system comprises a small aperture, in particular a pin-hole, provided above the grating coupler. 
     Since the target location may vary, it is not ensured that the reflected beam is parallel to the outgoing beam. In cases where the reflected beam is not parallel to the outgoing beam, the small aperture corrects this and ensures that the reflected beam, after having passed the small aperture, is parallel to the outgoing beam. 
     In an embodiment of the present disclosure, the first layer and the second layer are separated by a distance of 1 μm to 20 μm. 
     In an embodiment of the present disclosure, the reference beam and the measurement beam each are inclined with respect to said normal direction with an angle that is between 1° and 10°, which angle is preferably 2°. 
     In an embodiment of the present disclosure, said grating coupler is one of: a slanted grating, a curved grating, a compact grating, a patterned grating, an apodized grating, a 2D grating and a 2D apodized grating. 
     In an embodiment of the present disclosure, the grating coupler is formed by one of: grooves in the waveguide, which grooves may optionally be filled with a material different from the waveguide material, or protrusions from the waveguide, which protrusions may optionally be formed from a material different from the waveguide material. 
     In an embodiment of the present disclosure, the grating coupler comprises at least one additional layer separated from the waveguide. 
     The various possible designs of the grating coupler allow flexibility in designing the phase difference detector. 
     In an embodiment of the present disclosure, said imager array comprises a plurality of CCD, CMOS or NMOS imagers. 
     The various possible designs of the imager array allow flexibility in designing the phase difference detector. 
     In an embodiment of the present disclosure, the phase difference detector further comprises a processing unit configured to determine a phase shift between the reference beam and the incoming beam based on the fringe pattern. 
     In an embodiment of the present disclosure, the phase difference detector is used to perform laser Doppler vibrometry of the target location. 
     The object according to the present disclosure is also obtained by a method for detecting a phase difference between two laser beams due to reflection at a target location, the method comprising the steps of: guiding a laser beam within a planar waveguide having a normal direction; splitting the laser beam by means of a grating coupler into a reference beam and a measurement beam, wherein the measurement beam is directed towards the target location and the reference beam is directed away from the target location, the reference beam and the measurement beam each being inclined with respect to said normal direction; detecting a fringe pattern generated by said reference beam and an incoming beam, the incoming beam being the result of the measurement beam having been reflected at the target location and having passed through the grating coupler; and deriving a phase difference between the reference beam and the incoming beam based on the fringe pattern. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The disclosure will be further explained by means of the following description and the appended figures. 
         FIG. 1  shows a schematic set-up of a known one-beam LDV. 
         FIG. 2  shows a phase difference detector according to the present disclosure. 
         FIGS. 3A to 3J  show longitudinal cross-sections of waveguide-grating coupler pairs suitable for use in the a phase difference detector of  FIG. 2 . 
         FIGS. 4A to 4I  show top views of waveguide-grating coupler pairs suitable for use in the a phase difference detector of  FIG. 2 . 
         FIG. 5  shows a multi-beam phase difference detector according to the present disclosure. 
         FIG. 6  shows an alternative multi-beam phase difference detector according to the present disclosure. 
         FIG. 7  shows another alternative multi-beam phase difference detector according to the present disclosure. 
     
    
    
     DESCRIPTION OF THE DISCLOSURE 
     The present disclosure will be described with respect to particular embodiments and with reference to certain drawings but the disclosure is not limited thereto but only by the claims. The drawings described are only schematic and are non-limiting. In the drawings, the size of some of the elements may be exaggerated and not drawn on scale for illustrative purposes. The dimensions and the relative dimensions do not necessarily correspond to actual reductions to practice of the disclosure. 
     Furthermore, the terms first, second, third and the like in the description and in the claims, are used for distinguishing between similar elements and not necessarily for describing a sequential or chronological order. The terms are interchangeable under appropriate circumstances and the embodiments of the disclosure can operate in other sequences than described or illustrated herein. 
     Moreover, the terms top, bottom, over, under and the like in the description and the claims are used for descriptive purposes. The terms so used are interchangeable under appropriate circumstances and the embodiments of the disclosure described herein can operate in other orientations than described or illustrated herein. 
     Furthermore, the various embodiments, although referred to as “preferred” are to be construed as exemplary manners in which the disclosure may be implemented rather than as limiting the scope of the disclosure. 
     In the following description, the phase difference detector according to the present disclosure will be explained by reference to its use in laser Doppler vibrometry. However, the phase difference detector according to the present disclosure may also be used with a static reflector which induces no phase changes in the reflected light. In this way, any phase change is due to refractive index changes at different locations of a medium (e.g. air). Based on the measured phase changes the distribution of pressure or temperature may be derived, which, in turn, may be used to map changes in sound speed. Another possible application is the movement (e.g. tilt or rotation) of an entire surface which may be derived from the phase change at each location. In particular, the phase shifts at various locations across the surface will be related to one another due to the rotational movement, which relation allows to detect the movement. 
     A phase difference detector  10  according to the present disclosure is shown in  FIG. 2 . The phase difference detector  10  comprises an incoming waveguide  12  attached to a grating coupler  14 . The incoming waveguide  12  is connected to an external laser source (not shown) either in a direct connection or through the intermediary of one or more other waveguides and/or splitters. The laser beam guided in the incoming waveguide  12  passes the grating coupler  14  as indicated by arrow  22 , which causes the laser beam to be split into at least two components, namely an upwards travelling measurement beam  16  and a downwards travelling reference beam  18 . Optionally, a part of the laser beam may also continue onwards past the grating coupler  14  to remain in an outgoing waveguide  20  that may be coupled to a subsequent phase difference detector as described in greater detail with respect to  FIG. 6 . 
     The wavelength of the laser beam, the refractive index of the grating coupler and the period of the grating coupler are so chosen such as to ensure that the reference beam  18  and the measurement beam  16  each are inclined with respect to the normal direction N of the waveguide  12 . This is possible as these quantities are linked by 
     
       
         
           
             
               
                 
                   
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     where n wg  is the effective refractive index of light in the grating coupler  14 , n med  is the refractive index of light in the medium outside the grating coupler  14 , which is normally 1 for air, λ is the wavelength of the laser beam in vacuum, Λ is the period of the grating coupler  14  and θ is the angle between the split beams and the normal direction N as illustrated in  FIG. 2 . 
     Various possible designs of grating couplers  14  are possible as illustrated in  FIGS. 3A to 4I . 
       FIGS. 3A and 3B  illustrate a waveguide-grating coupler pair  12 ,  14  that is formed by removing part of the material of the waveguide  12  to form a crenelated surface which acts as the grating coupler  14 . The crenelated surface may be either the top surface ( FIG. 3A ) or the bottom surface ( FIG. 3B ) of the waveguide  12 . A similar design is shown in  FIGS. 3C and 3D  where, instead of removing material, material has been added to form the crenelated surface. 
     The waveguide-grating coupler pair  12 ,  14  of  FIGS. 3E and 3F  has the same shape as those of  FIGS. 3C and 3D , but the material that was added on top of or below the waveguide  12  to act as the grating coupler  14  is different to the material of the waveguide  12 . The waveguide may be silicon nitride, while the material surrounding silicon nitride can be silicon oxide, air or other materials with a low refractive index. 
     It will also be appreciated that the grating coupler  14  may also not be in direct contact with the waveguide  12  as shown in  FIGS. 3G and 3H . 
     It will also be appreciated that the grating coupler  14  may be formed by various layers as illustrated in  FIGS. 31 and 3J . 
     Moreover, the crenelated surface that acts as the grating coupler  14  may also have different planar configurations as shown in the top views of  FIGS. 4A to 4I . 
       FIGS. 4A to 4C  illustrate a perpendicular, slanted and curved waveguide-grating coupler pair  12 ,  14  respectively. In order to reduce the size of the waveguide-grating coupler pair  12 ,  14 , it is also possible to use a compact grating coupler  14  as illustrated in  FIGS. 4D and 4E . A patterned grating coupler  14  is illustrated in  FIG. 4F , while an apodized grating coupler  14  is shown in  FIG. 4G . Two-dimensional grating couplers  14  are also possible as shown in  FIGS. 4H and 4I . 
     Which design for the waveguide-grating coupler pair  12 ,  14  is chosen will depend on the application. However, in general, the grating coupler  14  should split the light upward and downward with a proper splitting ratio. Typically, more light should be sent to the measurement target, so that the interference pattern has a good signal quality. Another important factor is that the grating coupler  14  should be long enough to cover more than 2 fringes, but not too long to avoid distributing the optical power to a too large region. A third important factor is that most light is preferably coupled to upward or downward directions rather than being reflected or transmitted through the grating. 
     Returning to  FIG. 2 , the upwards travelling measurement beam  16  is focussed on a single target location  24  by use of an external optical focussing system  26 , e.g. one or more lenses. Due to reflection at the target location, a reflected beam is sent back towards the phase difference detector  10 , again via the external optical focussing system  26 . The optical system  26  preferably includes a small aperture, such as a pin-hole, which ensures that the reflected laser beam is parallel to the measurement beam  16  but travelling in the opposite direction when arriving back at the phase detector  10 , in particular the grating coupler  14  thereof. In this way, the incoming beam  28  that is the result of the measurement beam  16  having been reflected at the target location  24  and the reference beam  18  are angled with respect to one another. This angle is illustrated by means of lines  30 ,  32  that illustrate the wave front of the reference beam  18  and the incoming beam  28  respectively. 
     Since the wave fronts  30 ,  32  are angled with respect to one another, which angling is due to the initial angle between the beams  16 ,  18  and the normal direction N, an interference pattern  34 , also known as a fringe pattern, is created. The phase difference detector  10  is provided with an imager array  36  that detects this fringe pattern from which the phase difference between the reference beam  18  and the incoming beam  28  may be determined. 
     The phase difference of the incoming beam  28  can be determined by viewing the phase shift of the fringe pattern. To derive this phase shift, various techniques may be used on the data obtained from the imager array  36 . Examples of such techniques are a fast-Fourier transform, cross-correlation, and other methods. 
     Depending on the application, different imager arrays  36  may be used. For example, a plurality of CCD, CMOS or NMOS imagers may be used. The preference for one type of imager of the other depends on the sensitivity (the higher the better), the pixel size (the smaller the better), the frame rate (the higher the better) and the cost. 
     The distance d between the waveguide-grating coupler layer  12 ,  14  and the imager array  36  is dependent on the application. In particular, the maximal distance d determines the size of the interference pattern. Specifically, the effective length interference pattern is L inter =L grating −2d tan θ and should, typically, be greater than 2 fringes with a period of 
     
       
         
           
             
               
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     In case θ=2 degrees, L grating =20 μm and λ=633 nm, the distance d should be smaller than 27 μm. 
     The angle θ determines the period of fringe, which is 
     
       
         
           
             
               
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     Preferably, the pitch of the imager array  36  is smaller than ¼ of the fringes to ensure a good resolution. In other words, the pixel pitch is smaller than 
     
       
         
           
             
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     In case of a CMOS imager array  36 , the pitch may be as small as 2 μm, meaning that the angle θ has to be equal to or smaller than 2 degrees when λ=633 nm. 
       FIGS. 5 to 7  show various phase detector designs for simultaneous phase change detection at different locations. It should be noted that reference number  26  is used to denote any kind of appropriate external optical focussing system, which may include one or more convex or concave lenses. 
       FIG. 5  illustrates a chip  100  having two phase difference detectors that share a common imager array  36 . The waveguides  12  are split from one another and may be connected to a same external laser source (not shown), e.g. via splitter, or to different external laser sources. Due to the presence of two grating couplers  14 , a simultaneous measurement at two different locations is possible.  FIG. 6  shows a slight variation of the design of  FIG. 5  where the waveguides  12  are connected to one another. Another possible design is shown in  FIG. 7  where multiple external focussing systems  26  are provided to focus the output of an elongated grating coupler  14  to various locations on the target  24 . 
     It will be readily appreciated that multiple phase difference detectors may also be set-up in a two-dimensional array structure. 
     Although aspects of the present disclosure have been described with respect to specific embodiments, it will be readily appreciated that these aspects may be implemented in other forms within the scope of the disclosure as defined by the claims.