Field of the invention

The present invention relates to a Mach-Zehnder Interferometer, MZI, sensor, comprising an optical input unit, an optical output unit, and a waveguide arrangement providing two optical paths from the optical input unit to the optical output unit.

Background art

EP patent publication EP3415887B1 discloses a force sensing device with a membrane, which is configured to deform upon receiving a force. A first Mach Zehnder-type interferometer device and a second Mach Zehnder-type interferometer device are provided, wherein a first measurement propagation path of the first Mach Zehnder-type interferometer device and a second measurement propagation path of the second Mach Zehnder-type interferometer device are arranged on or in the membrane, and wherein the first measurement propagation path and the second measurement propagation path are differently sensitive to applied force on the membrane.

US patent publication US7812960B2 discloses an optical ultrasonic analysis transducer device. The device includes a detection means for a reflected ultrasound signal. The detector may be incorporated into a microchip design. In one embodiment, a single element transducer exhibits a stack composed of a matching layer, a top electrode, a piezoelectric element, a bottom electrode, and a backing layer. The stack is mounted on the top surface of the substrate symmetrically over a modulated arm of a Mach-Zehnder interferometer.

EP patent publication EP1176406 discloses an optic acoustic sensor having a plurality of flexural disks. The sensor is based on spiral-wound optical fiber coils mounted onto a circular flexural disk. The sensor is fabricated as a mechanical assembly, including embodiments having two disks with two fibers on each disk.

JP patent publication JP2008-175747 discloses an optical fiber sensor having a big diaphragm. An interferometer is disclosed as part of a measurement arrangement, having an optical coupler and a mirror integrated within the diaphragm, The optical fiber sensor is provided as an optical fiber in a spiral shape adhered to an adhesive material.

International patent publication WO2014/190970 discloses a vibration sensor, in particular a hydrophone or a microphone, having a light source and an array of vibration sensors, each having a membrane carrier and a membrane. The sensor disclosed is fiber based and provided as a mechanical assembly.

The article by Boling Ouyang, Yanlu Li, Marten Kruidhof, Roland Horsten, Koen W.A. van Dongen, and Jacob Caro, "On-chip silicon Mach-Zehnder interferometer sensor for ultrasound detection," Opt. Lett. 44, 1928-1931 (2019), discloses an ultrasound sensor based on an integrated photonics Mach-Zehnder interferometer (MZI).

Summary of the invention The present invention seeks to provide an Mach-Zehnder Interferometer sensor for detecting e.g. acoustic waves in a wide frequency range, which is suitable for mass production and provides high quality of sensor operation.

According to the present invention, a Mach-Zehnder Interferometer sensor as defined above is provided, further comprising at least two sensor elements, each of the at least two sensor elements comprising a membrane and a waveguide spiral in contact with the membrane, and a waveguide arrangement providing two optical paths from the optical input unit to the optical output unit, wherein at least one of the two optical paths is provided with at least two sensor elements, and wherein the at least two sensor elements comprise an additional polymer layer applied to the membrane.

The present invention embodiments have the advantage that the operation of the Mach- Zehnder Interferometer sensor is based on an optical principle, and is, thereby, not sensitive to e.g. electromagnetic interference resulting from (external) electrical signals. This provides a solution for the reliable detection of e.g. acoustic waves without contamination of the signals of the at least two sensor elements by disturbing electromagnetic signals from the external environment, and without electrical cross talk between the at least two sensor elements. Also, the implementations using waveguide arrangements, e.g. implemented using nano-fabricated on-chip waveguides, allows to obtain much better characteristics than prior art, optical fiber based implementations. Furthermore, such a Mach- Zehnder Interferometer sensor may be used in many applications, such as, but not limited to, ultrasound medical diagnostics imaging, ultrasound materials research, non-destructive testing, acoustic microscopy, sonar. In addition, a Mach-Zehnder Interferometer sensor comprising at least two sensor elements resembles a one-dimensional array of state-of-the-art sensors, with characteristics not obtainable when using a single sensor element. The target frequency range for the present invention MZI sensor embodiments can be in the frequency range of human hearing (20Hz- 20kHz) up to ultrasound frequencies in the MHz range.

In a further aspect, the present invention relates to a measurement unit comprising an array of Mach-Zehnder Interferometer sensors according to any one of the embodiments described herein. The measurement unit comprising an array of Mach-Zehnder Interferometer sensors resembles a two- dimensional array of state-of-the-art sensors.

Short description of drawings

The present invention will be discussed in more detail below, with reference to the attached drawings, in which

Fig. 1 shows a schematic diagram of a Mach-Zehnder Interferometer sensor according to an embodiment of the present invention;

Fig. 2 shows a graph of an exemplary transmission spectrum of a Mach-Zehnder Interferometer sensor according to an embodiment of the present invention;

Fig. 3 shows a schematic diagram of a Mach-Zehnder Interferometer sensor according to a further embodiment of the present invention; Fig. 4 shows a schematic, cross-sectional view of one sensor element of the at least two sensor elements in a Mach-Zehnder Interferometer, according to an embodiment of the present invention;

Fig. 5 shows a schematic diagram of a measurement unit comprising an array of Mach- Zehnder interferometer sensors, according to an embodiment of the present invention, and

Fig. 6 shows a schematic diagram of a measurement unit comprising an array of Mach- Zehnder interferometer sensors, according to a further embodiment of the present invention.

Description of embodiments

Piezo-electric sensors constitute state-of-the-art sensors in the detection of e.g. acoustic waves in a wide frequency range in the MHz frequency range (i.e. ultrasound). Such piezo-electric sensors can usually be fabricated with the required sensitivity and frequency bandwidth, allowing them to be tailor-designed to a certain application. Furthermore, piezo-electric sensors can also be fabricated in an array. An array is often needed for efficient and high-quality imaging with ultrasound for medical diagnostics, and other applications such as materials research.

However, a drawback of piezo-electric sensors is that their operation is of an electrical nature, i.e. their sensor signal is a voltage and electrical currents are involved. As a result, these sensors are sensitive to electromagnetic interference, induced by spurious signals from the external environment. Therefore, piezo-electric sensors are susceptible to contamination of the sensing signal of individual piezo-electric sensors by disturbing electromagnetic signals from the external environment, and also to electrical cross talk between (closely-spaced) individual piezo-electric sensors. This can limit the performance of piezo-electric sensors. Owing to this susceptibility issue, the operation of piezo-electric sensors may require special precautions, and piezo-electric sensors cannot be used in specific practices, e.g. during magnetic resonance imaging.

Furthermore, the fabrication of individual piezo-electric sensors, or an array of piezo-electric sensors, is only a partially automated process. As a consequence, labour intensive work is required, and thus, the manufacturing of (an array of) piezo-electric sensors is not compatible with the desired type of mass production, and related reduction of manufacturing costs. Nevertheless, piezo-electric sensors are important for medical applications. For example, piezo-electric sensors are employed for intravascular ultrasound imaging, despite being extensively used as a disposable product. Thus, reducing manufacturing costs is highly desired.

As such, there is a need in the art to overcome these drawbacks, and provide a sensor that would enable detection of e.g. acoustic waves without electromagnetic interference issues, and while the fabrication of such a sensor exploits a series of steps suitable for (automated) mass production at low costs per sensor. Furthermore, a sensor that would also enable simultaneous detection of other physical quantities, e.g. static or quasi-static pressure and temperature, is also highly desirable, as this would increase the functionality of such a sensor in the course of the sensing of acoustic waves. This would make the sensor suitable for use in other applications than the sensing of acoustic waves.

The present invention embodiments provide a Mach-Zehnder interferometer, MZI, sensor for detection of e.g. acoustic waves. It has been established that the quality of MZI sensors for the detection of acoustic waves is comparable to, or even better, than the state-of-the-art solutions, e.g. piezo-electric sensors. Reference is made to the article by Boling Ouyang, Yanlu Li, Marten Kruidhof, Roland Horsten, Koen W.A. van Dongen, and Jacob Caro, "On-chip silicon Mach-Zehnder interferometer sensor for ultrasound detection," Opt. Lett. 44, 1928-1931 (2019), which discloses an ultrasound sensor based on an integrated photonics Mach-Zehnder interferometer (MZI). Owing to the optical operation principle of MZI sensors, they are not sensitive to electromagnetic interference signals, allowing reliable detection of e.g. acoustic waves, and detection of acoustic waves without electrical cross-talk issues when (closely-spaced) multiple sensor elements are involved. The target frequency range for the present invention MZI sensor embodiments can be in the frequency range of human hearing (20Hz-20kHz) up to ultrasound frequencies in the MHz range. Furthermore, the sensor may be implemented efficiently using nano-fabricated on-chip waveguide technology.

Fig. 1 shows a schematic view of a Mach-Zehnder Interferometer sensor according to an embodiment of the present invention.

In this exemplary embodiment, the MZI sensor 3 comprises an optical input unit 7, and an optical output unit 8. The optical input unit 7 is, e.g. via a (optical) fiber, connected to an external input source 50. The external input source 50 comprises e.g. a laser, so as to supply the optical input unit 7 with light from the external input source 50. The laser may comprise a tunable laser operating at e.g. telecom wavelengths, or a distributed feedback (DFB) laser operating at e.g. telecom wavelengths and being tunable by setting and controlling its temperature. The laser is configured to emit monochromatic light with a tunable wavelength in a suitable range, e.g. a range around 1550 nm, e.g. 1549.0 - 1551 .5 nm. In similar fashion, the optical output unit 8 is, via a fiber, connected to an external unit 51 . The external unit 51 may comprise an optical power meter.

In the exemplary embodiment shown in Fig. 1 , the optical input unit 7 and the optical output unit 8 are connected to the external input source 50 and the external unit 51 , respectively, via a grating coupler (GC) 21 . One GC 21 is connected to the optical input unit 7, and this allows light, sent from e.g. a close to vertically oriented input fiber and of a suitable wavelength, to be diffracted into the in-plane direction of e.g. a chip on which the MZI sensor 3 is located, so as to actuate the MZI sensor 3 with light via the input waveguide of the MZI sensor 3, to which GC 21 is connected. In a similar fashion, one GC 21 is connected to the optical output unit 8, so as to diffract the (output) light of the MZI sensor 3 from the in-plane direction into a (close-to) vertical direction, so as to enter e.g. an output fiber.

In alternative wording, the GC 21 connected to the optical input unit 7 acts as an interface between an external input source 50 and the MZI sensor 3 via e.g. an input fiber, allowing optical energy to be efficiently coupled into the MZI sensor 3 from a laser in the external input source 50. Similarly, the GC 21 connected to the optical output unit 8 acts as an interface between the external unit 51 and the MZI sensor 3 via e.g. an output fiber, allowing optical energy to be efficiently directed out of the MZI sensor 3, into the external unit 51 .

In the exemplary embodiment shown in Fig. 1 , only one GC 21 is connected to the optical input unit 7 and one GC 21 is connected to the optical output unit 8. However, it is noted this is a non- limiting example, and the number of GCs 21 that are connected to the optical input and output unit 7, 8 may vary depending on the application.

Moreover, for the exemplary embodiment described in Fig. 1 , even though the MZI sensor 3 comprises at least two sensor elements 4A-C, e.g. three sensor elements, only one input fiber and one output fiber are used, i.e. only two fibers, for the operation of the MZI sensor 3, simplifying the assembly compared to an assembly of e.g. three piezo-electric sensors, which each need individual electrical wiring.

In the absence of a GC 21 , butt coupling can be implemented as to connect e.g. an input and output fiber to the optical input and output unit 7, 8, respectively, so as to allow light to be directed in and out of the MZI sensor 3.

Furthermore, as shown in the exemplary embodiment in Fig. 1 , the optical input unit 7 and optical output unit 8 comprise a multi-mode interferometer (MMI) 22, wherein the multi-mode interferometer 22 allows light to be split or (re)combined.

For example, the multi-mode interferometer 22 at the optical input unit 7 of the MZI sensor 3, receiving light (via e.g. an input fiber and a GC 21) from the external input source 50, comprises a 1 x 2 multi-mode interferometer splitter, allowing arriving light to be split into two separate paths, with half of the light power in each path, i.e. with a 50:50 splitting ratio.

In another example, the multi-mode interferometer 22 at the optical output unit 8 of the MZI sensor 3, comprises a 2 x 1 multi-mode interferometer (re)combiner, allowing incoming light from two separate paths to be (re)combined, resulting in a single path, guiding light (via e.g. a GC 21 and an output fiber) to the external unit 51 .

In the embodiment shown in Fig. 1 , the MZI sensor 3 further comprises at least two sensor elements 4A-C (three in this exemplary embodiment), each of the at least two sensor elements 4A-C comprising a membrane 14A-C, and a waveguide spiral 15A-C in contact with the membrane 14A-C. In further wording, each membrane 14A-C may comprise e.g. a layered structure into which a waveguide spiral 15A-C is embedded, thus forming the at least two sensor elements 4A-C. The at least two sensor elements 4A-C, comprising a membrane 14A-C and a waveguide spiral 15A-C, are arranged to detect signals from an external environment, and jointly form the ‘active components’ of the MZI sensor 3.

The MZI sensor 3 further comprises a waveguide arrangement providing two optical paths 9, 10 from the optical input unit 7 to the optical output unit 8, the at least two sensor elements 4A-C being part of at least one optical path 9 of the two optical paths 9, 10. In other wording, the optical input unit 7 and optical output unit 8 are connected by two optical paths 9, 10, wherein at least one optical path 9 of the two optical paths 9, 10 is arranged such that the connection between the optical input unit 7 and optical output unit 8 is via the at least two sensor elements 4A-C. In this regard, part of the at least one optical path 9 of the two optical paths 9, 10 is formed by the waveguide spiral 15A-C in contact with the membrane 14A-C, forming the at least two sensor elements 4A-C of the MZI sensor 3.

In general, there is a length difference between the two optical paths 9, 10 as to induce an optical path difference for the light waves travelling in the two optical paths 9, 10, whereby the optical path difference is static when the at least two sensor elements 4A-C are not influenced by a signal from the external environment, to which the at least two sensor elements 4A-C are sensitive.

To elaborate with a non-limiting example, in view of Fig. 1 , light waves arriving at the (1 x 2) multi-mode interferometer splitter 22 at the optical input unit 7, from the external input source 50, are split by the multi-mode interferometer splitter 22 into two separate paths, i.e. the two optical paths 9, 10. Since the two optical paths 9, 10 are of different lengths, the split light waves will travel different distances in the two optical paths 9, 10. As a result, when the light waves are eventually recombined (i.e. superimpose) by a (2 x 1) multi-mode interferometer recombiner 22 at the optical output unit 8, an optical phase difference will be present between the recombining light waves. Thus, in general, and in the absence of accidental complete destructive interference, a non-zero optical output power will result at the output of multi-mode interferometer recombiner 22 at the optical output unit 8. This can be detected by e.g. an optical power meter in the external unit 51 .

The optical phase difference, and, thus the optical output power detected by the optical power meter, may change or be modulated by an external signal, e.g. acoustic waves, to be sensed by the at least two sensor elements 4A-C of the MZI sensor 3. Thus, a sensor signal results from modification or modulation of the interference conditions of the light waves interfering in the multi-mode interferometer recombiner 22 at the optical output unit 8, in response to a signal from the external environment.

In this respect, besides the optical path difference between the two optical paths 9,10, the optical phase difference of the split light waves, and, thus the resulting optical power of the recombined light, also depends on the wavelength used to actuate MZI sensor 3. This effect is depicted in a graph shown in Fig. 2. Fig. 2 shows, for two optical paths 9, 10 of different lengths, a graph of the typical oscillatory transmission spectrum of the sensor, i.e. the (normalized) optical power (vertical axis) of the re-combined light arriving at the external unit 51 as a function of the tunable input (laser) wavelength (horizontal axis), in the absence of any external effects, e.g. absence of impinging ultrasound waves.

In this context, the (fixed) operation wavelength for a specific length difference between the two optical paths 9, 10, is chosen to give maximum sensitivity of the MZI sensor 3, i.e. maximum response to a signal from the external environment. As a non-limiting example, in view of Fig. 2, the operation wavelength A,o is typically set at half the maximum transmitted power, i.e. 0.5 of the normalized transmission, where the slope of the transmission is maximum, and, thus, gives the maximum transmission modulation ATfor a small modulation of the position of the transmission function along the wavelength axis.

In a specific embodiment, the at least two sensor elements 4A-C are arranged to receive and detect acoustic waves 1 . For example, the at least two sensor elements 4A-C may be arranged to receive and detect acoustic waves 1 in the MHz frequency range, i.e. ultrasound, and thus, the MZI sensor 3 may be configured for ultrasound imaging purposes. As an alternative example, the at least two sensor elements 4A-C may be arranged to receive and detect sound waves in the audible hearing range e.g. frequencies less than 20 KHz, and thus, the MZI sensor 3 may be configured for microphone purposes. As a non-limiting working example of the MZI sensor 3 that relates to the present invention embodiments described herein, ultrasound waves of certain characteristics impinge on the at least two sensor elements 4A-C, each comprising a membrane 14A-C and a waveguide spiral 15A-C. A vibrational mode (typically the lowest order vibrational mode) of one of the membranes, e.g. membrane with reference numeral 14A, is excited by the impinging ultrasound waves, inducing a time- periodic strain of waveguide spiral 15A in contact with (planar) membrane 14A, in accordance with the profile and amplitude of the excited vibrational mode. This results in a modulation of the length and cross-section of waveguide spiral 15A in contact with membrane 14A. Via various physical effects, including the opto-elastic effect, this leads to a modulation of the effective index of the waveguide spiral 15A. If light is transmitted from an external input source 50 comprising e.g. a laser, through the optical input unit 7 into the two optical paths 9, 10, the phase of the light wave leaving the waveguide spiral 15A in contact with membrane 14A is modulated owing to the its modulated effective index and its modulated length. Assuming the impinging ultrasound waves have no effect on the other membranes, i.e. 14B and 14C, when the light is recombined at the multi-mode interferometer recombiner 22 at the optical output unit 8, the phase modulation of the light wave translates to a modulation of the amplitude of the light field exiting the multi-mode interferometer 22 at the optical output unit 8.

The amplitude of the phase modulation, A<p, of this light field is where A,o is the operation wavelength of the MZI sensor 3, and n _e (n _e *) and L _s (L _s ) are the effective index and the geometrical path (i.e. integration path for the integrals), respectively, of the waveguide spiral 15A in contact with a strainless and maximally strained (*) membrane 14A. The phase modulation, in turn, yields a modulation of the optical power of the light field exiting the multi-mode interferometer recombiner 22 at the optical output unit 8. In this respect, the modulation amplitude of the optical power is the transduction of the ultrasound signal impinging on membrane 14A with waveguide spiral 15A and, thus, is the sensor signal to be measured.

In more general wording, the present invention embodiments as described above relate to a Mach-Zehnder Interferometer, MZI, sensor 3, comprising an optical input unit 7, an optical output unit 8, at least two sensor elements 4A-C, each of the at least two sensor elements 4A-C comprising a membrane 14A-C and a waveguide spiral 15A-C in contact with the membrane 14A-C, and a waveguide arrangement providing two optical paths 9, 10 from the optical input unit 7 to the optical output unit 8, the at least two sensor elements 4A-C being part of at least one optical path 9 of the two optical paths 9, 10. All the embodiments as described herein provide a MZI sensor 3 with an optical operation principle that is not sensitive to electromagnetic interference signals, allowing for reliable detection of e.g. acoustic waves 1 without contamination of the signals of the at least two sensor elements 4A-C by disturbing electromagnetic signals from the external environment, and without electrical cross-talk between the at least two sensor elements 4A-C. In an exemplary embodiment, the frequency characteristics of each of at least two sensor elements 4A-C are different. In this regard, by including different frequency characteristics in the at least two sensor elements 4A-C in the MZI sensor 3, an individual MZI sensor 3 with extended functionality can effectively be obtained.

Extended functionality includes the possibility to detect, using each of the at least two sensor elements 4A-C, ultrasound waves of a frequency spectrum that is covered by the (individual) frequency characteristics of the at least two sensor elements 4A-C. For example, the extended functionality may be used advantageously for measurement of a broad band ultrasound spectrum corresponding to the combined frequency characteristics of the at least two sensor elements 4A-C, where the ultrasound waves can be continuous waves or pulsed waves. In other wording, a specific spectral response can be obtained by the combination of at least two sensor elements 4A-C with separate, specific frequency characteristics.

The at least two sensor elements 4A-C in the MZI sensor 3 can also be designed to cover a specific frequency bandwidth tailored according to the requirements of the application. For example, in photoacoustic imaging of arteries, needed for diagnosis of atherosclerosis, a broad spectral response is required. This can be obtained by the combination of the at least two sensor elements 4A-C with close, but different frequency characteristics. As an non-limiting example, the at least two sensor elements 4A-C can be designed to cover an acoustic frequency range of 2 - 3 MHz and 3 - 8 MHz.

In a further embodiment (as shown in Fig. 1), the different frequency characteristics are determined by physical parameters of the membrane 14A-C of each of the at least two sensor elements 4A-C, i.e. the physical properties of the membrane 14A-C determine the specific frequency of the membrane’s vibrational mode and its frequency bandwidth. The membrane 14A-C can comprise many shapes e.g. square, rectangular, circular or elliptical, and have different dimensions, thus determining the frequency characteristic of the membrane 14A-C. Alternatively, the thickness or material composition of the membrane 14A-C may also be selected to determine its frequency characteristic.

As a non-limiting example, for a square-shaped membrane 14A-C with a size of 100 pm x 100 pm, the resonance frequency of the lowest vibrational mode is on the order of 1 MHz. The (lateral) side-to-side spacing between the at least two sensor elements 4A-C may reach values down to several hundred pm or smaller, for a fabrication technique adjusted to such a requirement for this side- to-side spacing.

In a specific embodiment, the at least two sensor elements 4A-C comprise an additional layer applied to the membrane 14A-C. By applying the additional layer to the membrane 14A-C, this can tune, or broaden, the spectral response, resulting in tuning and/or broadening of an individual sensor element’s 4A-C frequency characteristic. This may be achieved by depositing a material on the sensor element 4A-C exhibiting (high) internal friction under dynamic strain, e.g. a polymer. Suitable polymers may be poly methyl methacrylate and an elastomer such as Kraton, where the molecular weight distribution and layer thickness are chosen for the required width of the spectral response of the sensor, while the frequency shift of the spectral response of the at least two sensor elements 4A-C due to the polymer layer has to be taken into account as well. The layer may be applied by spin coating or spray coating. In the latter case also the backside of the membrane 14A-C may be chosen for addition of the polymer layer.

Another non-limiting method of tuning the frequency characteristic is ion implantation of the membrane 14A-C, followed by a degree of (rapid) thermal annealing, which also leads to internal friction under dynamic strain and to modification of the elastic constant of the material of the membrane 14A-C. Implantation ions of e.g. phosphorus and boron may be used. The combination of ion type, ion energy, implantation dose and annealing temperature determine the width of the resulting spectral response of the sensor. This method can be applied before or after the fabrication of the membrane 14A-C.

In a further specific embodiment, the thickness of the membrane 14A-C has a pre-determined two-dimensional profile, i.e. the membrane 14A-C is designed to have a thickness that may vary across its cross-sectional area. As described above, owing to impinging ultrasound waves, a time- periodic strain is induced in the associated waveguide spiral 15A-C in contact with the membrane 14A-C in accordance with the excited vibrational mode. In this respect, it is desirable to obtain a membrane 14A-C with a thickness profile designed to produce a strain distribution along the waveguide spiral 15A-C such that the first integral in Eq. (1) above (i.e. the contribution of the maximally strained spiral) is maximum, meaning that the amplitude of the phase modulation, and, thus, the output signal of the MZI sensor 3 is maximum.

For example, for the lowest vibrational mode of a square-shaped membrane 14A-C (by definition, the simplest mode for a square membrane and a mode without nodes, with the highest symmetry and with maximum deflection in the square center) a smooth thickness profile with a decreasing thickness from the center of the square-shaped membrane 14A-C towards the edges, and with the symmetry of a square, is expected to result in an optimized sensor signal. An optimization of this type is typically performed using detailed simulations based on finite elements techniques.

Furthermore, the pitch distribution of the waveguide spiral 15A-C may also have designed shape. The pitch distribution may be chosen such that the fraction of the total length of the waveguide spiral 15A-C that is in contact with the regions of the square-shaped membrane 14A-C where most of the deflection occurs, is maximized. This type of optimization is adjusted to the optimization of the thickness profile described above.

In an advantageous embodiment, the MZI sensor 3 is manufactured in silicon-on-insulator (SOI) technology. Since SOI technology is compatible with complementary metal-oxide-semiconductor (CMOS) - technology, it is, by definition, suitable for mass production with corresponding low manufacturing costs, per unit of wafer area, which in this case is per MZI sensor 3. As such, fabrication of the MZI sensor 3 based in SOI technology will only require well-established techniques known as such in the art of microelectronics, supplemented by a limited number of simple and CMOS compatible post-processing steps known from microelectromechanical systems (MEMS) technology for formation of the membrane 14A-C. Thus, this allows for mass production of the present invention MZI sensor 3 without labour-intensive work. Furthermore, since integrated photonics, based on SOI technology, is a very high grade technology and is highly reproducible, standardization of the MZI sensor 3 is therefore possible. This allows for manufacturing of MZI sensors 3 with repeated high quality.

In an alternative embodiment, the at least two sensor elements 4A-C are positioned in an array, wherein the array is a (linear) one-dimensional array and the at least two sensor elements 4A-C are positioned with constant spacing in-between. In light of this, in a macroscopic sense, the MZI sensor 3, comprising a one-dimensional array of at least two sensor elements 4A-C, can be thought of as a one-dimensional array of individual sensor elements. Thus, the MZI sensor 3 would closely resemble a one-dimensional array of conventional piezo-electric sensors, as described above.

In another alternative embodiment, the at least two sensor elements 4A-C are positioned in an array with non-constant spacing in-between. Such an array with non-constant spacing may be advantageous for measuring acoustic waves 1 at predetermined positions using a single MZI sensor 3.

Fig. 3 shows a schematic diagram of a Mach-Zehnder interferometer sensor according to an even further embodiment of the present invention. Elements with the same function as in the exemplary embodiment shown in Fig. 1 are indicated by the same reference numerals. The MZI sensor 3 in this exemplary embodiment further comprises a reference sensor element 4B connected to the optical input unit 7 and to the optical output unit 8, the reference sensor element 4B comprising a waveguide spiral 15B, i.e. the reference sensor element 4B is not provided with a membrane. In this regard, the waveguide spiral 15B of the reference sensor element 4B is directly attached to e.g. the substrate of the SOI component forming the MZI sensor 3. Since the reference sensor element 4B is not in contact with a membrane, the reference sensor element 4B cannot detect signals from the external environment, e.g. acoustic waves 1 , for which an underlying membrane is required.

Instead, the reference sensor element 4B may be used to e.g. set the proper value of the free spectral range of the MZI sensor 3. Further, the waveguide spiral 15B of the reference sensor element 4B may also be used to set the value of the optical path difference between the two optical paths 9,10, i.e. set the optical phase difference between the light waves split into the two optical paths 9, 10.

Fig. 4 shows of a cross-sectional view of one sensor element 4A of the at least two sensor elements 4A-C, according to an embodiment of the present invention. In general, a sensor element 4A may comprise: a glass layer 20, a first SisN4 layer 28, a Si layer 29, a first SiC>2 layer 24, a cavity 23 sealed between the glass layer 20 and the first SiC>2 layer 24, a second SiC>2 layer 25, a second SisN4 layer 26 and a Si structure 27.

As a non-limiting example, in view of Fig. 4, the fabrication of one sensor element 4A of the at least two sensor elements 4A-C, can take place on a CMOS compatible SOI platform, comprising a 220 nm Si device layer and 2 pm (buried) first SiO2 layer 24. After fabrication of the Si waveguide circuitry, which includes the waveguide spiral 15A (of which the waveguide thickness and width are e.g. 450 nm and 220 nm, respectively) of what in the fabrication process will become sensor element 4A, the first step is thinning down a 725 pm Si wafer into a 250 pm Si layer 29. After dicing, a 0.5 pm thick second SiO2 layer 25 is deposited, as waveguide spiral 15A cladding, by plasma-enhanced chemical vapour deposition on the chip level. Then, by low pressure chemical vapour deposition, a 0.15 pm first SisN4 layer 28 is deposited on the 250 pm Si layer 29, and, simultaneously, the second SisN4 layer 26 is deposited on the second SiC>2 layer 25, to act as a mask in the etch to follow. On the backside, using optical lithography and reactive ion etching in a fluorine based plasma, a square centered at the waveguide spiral 15A is opened in the first SisN4 layer 28, after which a membrane 14A of total thickness 2.65 pm is effectively created under the waveguide spiral 15A by locally removing the Si layer 29 in a KOH etch, using the BOX layer as etch stop. This yields the cavity 23 comprising a truncated pyramid shape, which results from the nature of the KOH etch. Thereafter, the glass layer 20, e.g. a glass platelet, is glued to the first SisN4 layer 28, effectively sealing the cavity 23. This results in the one sensor element 4A of the at least two sensor elements 4A-C as shown in Fig. 4, comprising a membrane 14A that is accurately aligned to the waveguide spiral 15A, whereby the waveguide spiral 15A can be thought of as being embedded in the sensor element 4A. It is noted that alternative methods can be used instead of etching in KOH for local removal of Si layer 29, for example deep reactive ion etching.

It is noted that the membrane 14A that is created after the KOH etch has a layered structure and that the layered membrane 14A created in this exemplary fabrication process, in general, will be statically buckled as a result of relief of the compressive mechanical stress of the first SiO2 layer 24, which stress dominates the overall stress of the layered structure depicted in Fig. 4 before the membrane 14A is formed. The compressive stress of the first SiO2 layer 24 is inherent to SOI wafers used in SOI technology. For optimum (control of the) operation of sensor element 4A, it is advisable to avoid the static buckling, for several reasons. For instance, the static buckling complicates prediction by simulations of the vibrational modes of the membrane 14A and the deflection amplitude of these modes. Further, the response of a sensor element 4A to impinging acoustic waves in general is different for a buckled-up membrane 14A and a buckled-down membrane 14A, since the device layer is not in the middle of the membrane 14A, while the direction of buckling cannot be predicted. Finally, the buckling affects the directivity of a sensor element 4A in an adverse way, where the directivity is defined as the dependence of the sensitivity on the angle of incidence of the impinging acoustic waves with respect to the normal to the chip plane. The static buckling of the layered membrane 14A may be avoided by depositing an additional layer with a tensile stress, to obtain a fully stress balanced layered structure. For the layered structure depicted in Fig. 4, the additional layer may be, e.g., a second silicon nitride layer deposited by chemical vapour deposition at such conditions, which include type of precursor gasses and their flow rates, and layer-deposition temperature, that the required tensile stress is obtained. Alternatively, the first silicon nitride layer 26 may be chosen thicker and may be deposited at such conditions that the required tensile stress is obtained for the thicker layer. Depending on the tensile stress level of the additional layer that can be reached, the first SiC>2 layer 24 may be made thinner, which can be done by applying, from below, an etch of the first SiC>2 layer 24 after completion of the KOH etch. Through controlling the lateral dimensions of the membrane 14A, and the thickness and mechanical properties of each of the individual layers, the resonance frequency of the (fully stress balanced) layered membrane 14A, and the sensitivity of the associated sensor element 4A, can be tailored to meet the specific requirement of certain applications, e.g. photoacoustic imaging of arteries. It is re-iterated that the embodiment shown in Fig. 4 is a non-limiting example, and that the fabrication of one sensor element 4A of the at least two sensor elements 4A-C may comprise e.g. other fabrication steps or material layers.

Fig. 5 and Fig. 6 show a schematic view of exemplary embodiments of a measurement unit 11 according to a further aspect the present invention. In this further aspect, the measurement unit 11 comprises an array of MZI sensors 3a-d. An array of MZI sensors 3a-d, wherein each individual MZI sensor 3 comprising at least two sensor elements 4A-C (as in the embodiments described above), would have a functionality closely resembling that of a two-dimensional array of conventional piezoelectric sensors. In other wording, it has been described above that an individual MZI sensor 3 resembles a one-dimensional array of piezo-electric sensors, and thus, an array of MZI sensors 3a-d would resemble a two-dimensional array of piezo-electric sensors.

In a further embodiment, the MZI sensors 3a-d are arranged side-by-side. Alternatively stated, the MZI sensors 3a-d are positioned laterally in the same horizontal plane. As a non-limiting example, owing to the planar character of SOI technology, the MZI sensors 3a-d can be easily arranged laterally in an array on a single chip. As such, in this non-limiting example, such an array of MZI sensors 3a-d can also be easily manufactured with corresponding low manufacturing costs, whereby the array of MZI sensors 3a-d is suitable for mass production.

In the exemplary embodiments shown in Fig. 5 and Fig. 6, an input multi-fiber 5 is connected to each of the array of MZI sensors 3a-d, and an output multi-fiber 6 is connected to each of the array of MZI sensors 3a-d. Both the input multi-fiber 5 and output multi-fiber 6 may comprise a plurality of fibers, e.g. a fiber bundle. Further, the input multi-fiber 5 may provide the array of MZI sensors 3a-d with optical input power, and the output multi-fiber 6 may be used to carry the optical output signal generated by the array of MZI sensors 3a-d.

Although there may be multiple MZI sensors 3a-d in an array, e.g. four MZI sensors 3a-d, wherein each MZI sensor 3 comprises at least two sensor elements 4A-C, only two multi-fibers, i.e. the input multi-fiber 5 and the output multi-fiber 6, are needed. This is highly advantageous in comparison to conventional piezo-electric sensors, whereby the individual elements of a piezo-electric sensor array each need separate wires, resulting in an extensive procedure of electric wiring of each individual element of the piezo-electric sensor array. Thus, in the present invention embodiment, a more simple packaging of the array of MZI sensors 3a-d is presented.

In general, the number of fibers in the input multi-fiber 5 and the output multi-fiber 6 is equal to the number of MZI sensors 3a-d of the array. As a non-limiting example, in view of Fig. 5 and Fig. 6, there is an array comprising four MZI sensors 3a-d, and thus, the input multi-fiber 5 would comprise four fibers, and the output multi-fiber 6 would also comprise four fibers.

In a specific embodiment shown in Fig. 5, the input multi-fiber 5 is connected to the optical input units 7a-d of each of the array of MZI sensors 3a-d via an input fiber array unit, FAU (17). Similarly, in another specific embodiment shown in Fig. 5, the output multi-fiber 6 is connected to the optical output units 8a-d of each of the array of MZI sensors 3a-d via an output fiber array unit (FAU) (18). Each fiber in the input multi-fiber 5 and output multi-fiber 6 is addressing, via the input fiber array unit 17 and output fiber array unit 18, respectively, a respective MZI sensor 3 in the array of MZI sensors 3a-d.

As known to the skilled person, fiber array units comprise V-grooves to which separate fibers are glued upon, allowing accurate, straight positioning of the fibers. In the context of the present invention, the plurality of fibers in the input multi-fiber 5 can be glued next to one another onto the V- grooves of the input fiber array unit 17, with appropriate spacing in-between. This effect and advantage can be similarly described of the plurality of fibers in the output multi-fiber 6.

Further, the pitch of the input fiber array unit 17 equals the pitch at the edge of the SOI chip of e.g. the input waveguides of the array of MZI sensors 3a-d, thus, enabling efficient packaging. Since the input fiber array unit 17 is used in this embodiment, grating couplers 21 are no longer necessary, since one can revert to butt coupling of the input fiber array unit 17 to the plurality of fibers in the input multi-fiber 5. This allows even more simple packaging, since the use of a grating couplers 21 is avoided. This effect and advantage can be similarly described for output fiber array unit 18.

In an even further exemplary embodiment shown in Fig. 6, the input multi-fiber 5 and output multi-fiber 6 comprise a multicore fiber. The multicore fiber is a fiber comprising multiple cores inside the fiber cladding. As such, the input multi-fiber 5 comprising a multicore fiber may transmit light down the multiple cores, and distribute the light into the array of MZI sensors 3a-d, and similarly, the output multi-fiber 6 comprising a multicore fiber may allow simultaneous transmission of different optical output signals, generated by the array of MZI sensors 3a-d, down the multiple cores of the multicore fiber.

The presence of multiple cores (in a plane) of an input multi-fiber 5 would allow accurate alignment with e.g. input waveguides connecting to the optical input units 7a-d of the array of MZI sensors 3a-d. The individual fibers of the multicore fiber do not need to be fanned out to the array of MZI sensors 3a-d, which otherwise would result in complicated and cumbersome packaging. As such, multicore fibers also enable efficient packaging owing to the accurate spacing of the individual collinear cores. Similar effects and advantages can also be described in this embodiment for the multicore fiber of the output multi-fiber 6.

In the embodiments shown in Fig. 5 and Fig. 6, the measurement unit 11 further comprises an optical converter 19 connected to the output multi-fiber 6. The optical converter 19 may e.g. comprise a plurality of photodetectors, allowing the optical output signals to be converted to the electrical domain, i.e. translate the optical output signal into an electrical signal. In general, the number of photodetectors in the optical converter 19 equals the number of MZI sensors 3a-d in the array. As a non-limiting example, in view of Fig. 5 and 6, there is an array comprising four MZI sensors 3a-d, and, thus, the optical converter 19 would comprise four photodetectors.

In the exemplary embodiments shown in Fig. 5 and Fig. 6, the measurement unit 11 further comprises a processing unit 12 connected to the optical converter 19, arranged to execute Fourier transformation operations on signals received from the optical converter 19. Fourier analysis of the signals received from the optical converter 19 allows easy processing of the detected signals in the frequency domain, e.g. enabling to separate output signal components attributable to the specific frequency characteristics of the at least two sensor elements 4A-C in each MZI sensor 3 in the array of MZI sensors 3a-d. Such separation is possible owing to e.g. different resonance frequencies of the at least two sensor elements 4A-C, which are sufficiently separated in frequency space as described above.

In other wording, as described above, the different frequencies of the acoustic waves 1 can be detected as a result of the vibrational (mode) frequencies of the membranes 4A-C of each MZI sensor 3. In this respect, the Fourier transformation operations yields the power spectrum of acoustic waves 1 detected by the at least two sensor elements 4A-C in each individual MZI sensor 3 in the array of MZI sensors 3a-d.

Furthermore, a MZI sensor 3, in any of the embodiments described herein, may also be arranged to detect other parameters, e.g. temperature and pressure of the medium in which the MZI sensor 3 is placed within. A single MZI sensor 3 (or an array of MZI sensors 3a-d) for sensing ultrasound can be combined with a standard MZI sensor (i.e. without pressure-sensitive elements) on the same chip to sense temperature. This temperature information can be combined with the (quasi- )static output of the standard MZI sensor for detecting ultrasound, to simultaneously detect incident ultrasound and the (quasi-)static pressure.

The output signal generated by an individual MZI sensor 3 comprises a quasi-static (low frequency) component and an AC component (high frequency), whereby the former relates to detecting temperature and quasi-static pressure, and the latter relates to detecting acoustic waves 1 . The quasi-static component and AC component of the output signal may be separated at the processing unit 12

To elaborate with a non-limiting example, a MZI sensor 3, acting as an ultrasound sensor (in miniaturized form), may enter the blood vessel of a patient. The wall of the blood vessel may be imaged with ultrasound waves, wherein the ultrasound waves are e.g. generated using the photoacoustic effect, using the MZI sensor 3 to detect the ultrasound waves emitted by the tissue of the wall. In combination with detecting the frequency characteristics of the ultrasound waves, with the operation principle as described above, the at least two sensor elements 4A-C will also respond to the varying, quasi-static blood pressure of the patient, wherein the quasi-static blood pressure is of low frequency. In this respect, the quasi-static component of the output signal generated by the MZI sensor 3 is of low frequency, and it is this low frequency quasi-static component that is measured as a means to detect (quasi-static) pressure.

To elaborate further, it is noted that the effective index of the two optical paths 9, 10 may also be temperature-dependent, which effectively influences the optical path length of the two optical paths 9, 10. Owing to this temperature dependence, this will affect the oscillatory behavior of the transmission spectrum in Fig. 2, which will experience a ‘phase shift’, i.e. the waveform depicted in the graph of Fig. 2 will shift along the wavelength axis. By measuring this ‘phase shift’, temperature information can be extracted.

In this respect, a MZI sensor 3 that simultaneously detects acoustic waves 1 , pressure and temperature, provides many advantages over conventional sensors in the art e.g. piezo-electric sensors. The present invention has been described above with reference to a number of exemplary embodiments as shown in the drawings. Modifications and alternative implementations of some parts or elements are possible, and are included in the scope of protection as defined in the appended claims.