Patent Description:
Various schemes of optical interferometry may be used to manipulate and/or detect a physical signal. For example, in "<NPL>et al. implement passive demodulation pulse interferometry in fiber optics, using a <NUM>×<NUM> fiber coupler and a phase-extraction algorithm. The authors introduce a coherence-restored pulse interferometry (CRPI) method for interferometric sensing, in which shot-noise limited sensitivity may be achieved alongside robust operation. CRPI is implemented with a fiber-based resonator, demonstrating over an order of magnitude higher sensitivity than that of conventional <NUM> intravascular ultrasound probes. The performance of the optical detector is showcased in a miniaturized all-optical optoacoustic imaging catheter.

As another example, Hazan and Rosenthal describe a passive demodulation scheme for pulse interferometry, in "<NPL>. The passive scheme is based on an unbalanced Mach-Zehnder interferometer and a <NUM>° optical hybrid, which is implemented in a dual-polarization all-fiber setup. A passive scheme is demonstrated for detecting ultrasound bursts with pressure levels for which the response of conventional, active interferometric techniques became nonlinear.

The proposed designs are based on highly symmetrical interferometers. The authors use retarders to implement <NUM>-degree optical hybrids with balanced interferometers for phase demodulation in optical communication systems.

The authors propose an approach to phase control capable of stabilization to an arbitrary phase setting by producing a linear error signal in the phase. The basic principle lies in utilizing two distinct optical modes passing through the interferometer with non-identical optical path length differences resulting in two phase offsets. The modes could consist of different transverse-spatial, temporal, frequency or polarization modes depending upon the nature of the experiment and noise involved.

In "<NPL>et al. demonstrate 4X4 multimode interference (MMI) couplers in a silicon-on-insulator rib waveguide technology that enable compact integrated fully passive optical <NUM>-degree-Hybrid devices with operation across the C-band.

Pulse interferometry was proposed by <NPL>. The authors demonstrate a method for wideband interrogation of optical sensors. The method is suggested as an alternative to conventional coherent (narrow linewidth) and incoherent (wideband) continuous-wave (CW) interrogation. The method is based on a pulse laser source whose bandwidth is significantly broader than the resonance width of the sensor and on interferometric demodulation. The authors further suggest that the use of a wideband coherent source opens new possibilities for interrogation methods with an advantageous combination of properties that cannot be found in conventional CW methods.

There is disclosed an apparatus for optical detection of ultrasound, including one or more optical resonators (OR), one or more optical passive-demodulation interferometers (OPDI), and one or more respective electro-optical readout circuits (EORC). The one or more optical resonators (OR) are configured to modulate respective carrier frequencies of optical signals indicative of US waves impinging thereon. The one or more OPDI are implemented in one or more photonic integrated circuits (PIC), wherein each OPDI is configured to demodulate the optical signal output by the respective OR, so as to generate a respective intensity-modulated optical signal. Each OPDI includes an interferometer having imbalanced arms that are recombined using an optical hybrid. The one or more respective EORC are each configured to measure the intensity demodulated optical signal produced by the respective OPDI, and to output a respective electrical signal. The optical hybrid includes a <NUM>-degree optical hybrid.

The one or more OR are implemented in the one or more PIC.

The one or more EORC are implemented in the one or more PIC.

In an embodiment, each OPDI and respective EORC are implemented on a single substrate.

In another embodiment, one or more of the OR, and the corresponding OPDI, and respective EORC, are implemented on a single substrate.

In some embodiments, the apparatus further includes a processor, which is configured to derive, from respective electrical signals output by the one or more EORC, an output indicative of the impinging US wave.

In some embodiments, the optical signals include optical pulses.

In an embodiment, the one or more OR include π-phase shifted Bragg gratings.

In some embodiments, the interferometer is configured to generate a predefined optical phase shift between the imbalanced arms, at the carrier frequency.

There is additionally provided, in accordance with an embodiment of the present invention, a method for optically detecting ultrasound (US) waves. The method includes using one or more optical resonators (OR) for modulating respective carrier frequencies of optical signals indicative of US waves impinging thereon. The optical signals are demodulated using one or more optical passive-demodulation interferometers (OPDI) implemented in one or more photonic integrated circuits (PIC), wherein each of the OPDIs includes an interferometer having imbalanced arms that are recombined using an optical hybrid, so as to generate respective intensity-modulated optical signals. The intensity demodulated optical signals are measured and outputted as respective electrical signals, using one or more electro-optical readout circuitries (EORC).

Embodiments of the present invention that are described hereinafter provide apparatus and methods for the detection of ultrasound (US), which use optical passive-demodulation interferometry to optically detect US, implemented at least partially in photonic integrated circuits (PIC). US waves may result from various physical effects. One such effect is the thermoacoustic effect in which US waves are generated in a medium, such as human tissue, by electromagnetic radiation being absorbed by the tissue. The disclosed apparatus and methods can therefore be applied in various fields, such as, for example, in the field of minimally invasive medical diagnostics.

In some embodiments, a disclosed apparatus includes one or more optical resonators (OR), implemented in PIC, as described below, each OR is capable of optically sensing an impinging US wave. In some embodiments, each OR receives a train of optical pulses from a pulsed laser source. The OR is configured to modulate an optical carrier frequency of the optical pulses, in a way that the frequency modulation is indicative of an impinging US wave. The frequency modulated pulses are outputted, propagate, and coupled into an optically passive demodulation interferometer (OPDI), implemented in PIC, which is configured to demodulate the frequency-modulated optical pulse.

In an embodiment, the OPDI is implemented by a modified unbalanced Mach-Zehnder interferometer (MZI) that comprises an optical hybrid (i.e., an interferometer having imbalanced arms that are recombined using an optical hybrid). The OPDI outputs the intensity modulated pulses that to an electro-optical readout circuit (EORC), which measures the pulses to produce electric signals that are indicative of the impinging US wave. While the use of optical pulses is technically advantageous, the disclosed apparatus and method can be applied, with necessary changes, using a source whose bandwidth is larger than that of the OR, e.g., an amplified spontaneous emission source.

In the context of the present patent application and in the claims, the term "MZI" refers to an optical device used to determine the relative phase shift between two propagating guided wave optical pulses derived by splitting light from a single pulsed optical source. The term "unbalanced" means that the two arms of the MZI have different optical path lengths. The imbalance of the MZI enables the detection of time-dependent frequency shifts (i.e., frequency modulations) of the carrier frequency of the optical pulse inputs, which are proportional to a magnitude of a measured US wave.

In an embodiment, the OR, which is impinged upon by the US waves, comprises a π-phase shifted Bragg grating (π-BG), i.e., a Bragg grating with a narrow spectral transmission window in its bandgap created by a localized defect in the grating periodicity. The US waves modulate the center frequency of the π-BG, causing the filter to output a train of pulses having a modulated carrier frequency. The amount of frequency modulation is a good approximation of the linear function of the amplitude and timing of the US waves, and is thus directly indicative of those US waves. Alternatively, other types of OR can be used, and may be implemented in PIC, such as Fabry-Perot ORs or micro-ring resonators.

In some embodiments, the OPDI uses six out of m x n (m and n being integers) ports of a <NUM>-degree optical hybrid that is connected to the outputs of an unbalanced MZI. In another embodiment, the <NUM>-degree optical hybrid is realized by a 2x4 multimode interference (MMI) coupler, as described below. The unbalanced MZI is configured to generate an approximately <NUM>-degree (i.e., <NUM> degrees with a possible deviation of several degrees) optical phase shift at the carrier frequency between the two optical paths of the MZI. The double <NUM>-degree phase shifts, in tandem, lend themselves to large-scale implementation using planar PIC, as noted above and as further described below.

In other embodiments, the disclosed passive interferometry scheme is applied using multimode interference (MMI) couplers with optical hybrids that are not limited to a <NUM>-degree shift. In some embodiments, the different optical paths of the unbalanced MZI induce a phase difference that is different from <NUM> degrees (i.e., a predefined optical phase shift between the two optical paths of the MZI, at the carrier frequency, that is different from <NUM> degrees). The unbalanced MZI output is coupled to an m x n MMI (m and n being integers) that acts as an optical hybrid. For example, in an embodiment, a <NUM>-degree optical hybrid is implemented by a 3x3 MMI coupler.

In some embodiments, one or more arrays comprising at least part of the disclosed US optical sensing, conversion and measurement elements are fabricated as PIC on a single substrate or on different substrates. For example, in an embodiment, each OPDI and respective EORC are implemented in a PIC on a single substrate. As another example, in an embodiment, each OR, respective OPDI, and respective EORC are implemented in PIC on a single substrate. In alternative embodiments, OPDIs are implemented in a PIC, while ORs and/or EORC are implemented with other technologies, such as pigtailing the devices to optical fibers. In an embodiment, the OR and OPDI, but not the EORC, are implemented with PICS. The EORC is implemented, for example, using commercially available photodiodes.

Planar PIC designs may be implemented by numerous fabrication techniques for photonic integrated circuits, e.g., silicon photonics, glass waveguides, and polymer waveguides. In an embodiment, a use of multiple substrates, each disposed with a PIC, enables, for example, an US probe with an optical detection array having a curved geometry.

In some embodiments, the disclosed optical US detection method maintains a linear response at high acoustic pressure levels. Furthermore, the disclosed optical US detection method is compatible with AC-coupled electrical measurements, in which the electrical signals are high-passed before they are sampled. Thus, the dynamic range of the sampler circuit may be adjusted in correspondence with the magnitude of the US signals, while ignoring potentially larger low-frequency contributions to the signal.

In some embodiments, the OPDI outputs the frequency modulated pulses to the EORC, which, as noted above, is configured to convert the frequency-modulated optical pulse into an electrical signal. The EORC may or may not be implemented in PIC, as described below. A processor that receives the electrical signals is configured to derive from the electrical signals an output indicative of the US wave, such as to produce an ultrasound image (e.g., of an organ of a patient, probed with a minimally invasive medical diagnostic system).

The disclosed US imaging systems and methods can achieve both high sensitivity and high dynamic range, due to the disclosed OR that have an extremely low noise equivalent sound pressure, and the disclosed OPDI that converts the frequency modulated optical signals into intensity modulated signals, and the OPDI that measures the intensity modulated optical signals, at least part of implemented in PIC. The high sensitivity, which is more than an order of magnitude greater than that of a piezoelectric detection element of a same area, can enable, for example, high resolution US detection. An example application of the disclosed apparatus and method is an US imaging system that employs smaller-size detection elements in PICs so as to spatially resolve anatomical features that are unresolvable with current US imaging systems employing larger-size piezoelectric US detectors.

<FIG> is a block diagram that schematically illustrates an apparatus for optical detection of ultrasound, which is implemented in photonic integrated circuits (PIC), in accordance with an embodiment of the present invention. As seen, PIC-based array <NUM> seen in detail in inset <NUM>, comprises sensing elements <NUM>, each including an OR <NUM>. PIC-based array <NUM> is implemented, by way of example, in a probe <NUM> of a minimally invasive medical diagnostic system.

PIC-based array <NUM> receives optical pulses from a pulsed source <NUM>, via a fiber optic <NUM> which runs through cable <NUM>. When impinged with US waves, PIC-based array <NUM> outputs frequency modulated pulses to a PIC-based OPDI array <NUM>, via fiber <NUM> that is also included in cable <NUM>.

In an embodiment, seen in inset <NUM>, an optical coupler <NUM> couples the output of fiber optic <NUM> into a planar array of waveguides <NUM>. Each waveguide <NUM> outputs the optical pulses to a respective optical sensing element <NUM> comprising OR <NUM> that frequency modulates the pulses. As seen in the embodiment, array <NUM> comprises multiple sensing elements 200A, 200B.

The frequency modulated pulses are outputted to an optical coupler <NUM> by OR <NUM> via a planar array of waveguides <NUM>. Coupler <NUM> outputs the pulses to optical fiber <NUM>. In an embodiment, seen in inset <NUM>, an optical coupler <NUM> couples the output of fiber optic <NUM> into a planar array of waveguides <NUM>. Each waveguide <NUM> outputs the optical pulses to a respective measurement element <NUM> that includes an OPDI <NUM>. The frequency demodulated pulses are converted by OPDI22 into light intensity modulated pulses. As seen, each measurement element <NUM> further comprises an electro-optical readout circuit (EORC) <NUM>, which is configured to measure the intensity modulated optical pulses outputted by OPDI <NUM> circuit and output the resulting electrical signals to a processor <NUM> respective, via electrical leads <NUM> and electrical cable <NUM>. In some embodiments, each OPDI <NUM> and the respective EORC <NUM> are implemented in a single PIC.

The block diagram shown in <FIG> is depicted purely by way of example. Any other suitable configuration can be used in alternative embodiments. For example, in some embodiments, OR <NUM>, OPDI <NUM> and EORC <NUM> may be implemented in a single PIC located inside a probe, like inside a hand-held US probe. As another example, various optical elements in the disclosed apparatus can be implemented in fibers additionally or alternatively to the disclosed implementation in planar waveguides in a PIC.

For example, the EORC, which converts optical intensities into electrical signals, may be realized with an array of commercially available packaged photodiodes that are pigtailed to optical fibers. Similarly, the OR elements may be realized as in processing and packaging technologies different than as part of a PIC. For example, commercially available optical resonators may be assembled into an array <NUM>.

US sensor array <NUM> is shown schematically as a single monolithic unit only for clarity of presentation. In practice, array <NUM> may be implemented by adjoining separate smaller sub-arrays. Such an implementation enables, for example, a curved geometry of electro-optical US sensing array <NUM>.

<FIG> is a block diagram that schematically illustrates details of the detection apparatus appearing in <FIG>, in accordance with an embodiment of the present invention. As seen, optical pulse source <NUM> generates a train E<NUM> of filtered optical pulses, seen in inset <NUM>. In an embodiment, source <NUM> comprises an infrared pulse laser <NUM> having a <NUM> central wavelength (i.e., central frequency ν<NUM> of <NUM> THz) with a pulse repetition rate of <NUM>, pulse width of approximately <NUM> ps, and average power on the order of several tens of mW. The laser's output pulses are filtered down to a bandwidth of <NUM> around <NUM> using a band-pass filter (BPF) <NUM>, then amplified by an erbium-doped amplifier <NUM>, and filtered by an additional <NUM> band-pass filter <NUM> to reject amplified spontaneous emission from the amplifier. In alternative embodiments, any other suitable numerical values may be used.

The train E<NUM> of filtered optical pulses is coupled to optical sensing element <NUM>. As seen, sensing element <NUM> comprises an optical π-BG <NUM> that comprises an optical resonator that is highly sensitive to impinged ultrasound waves. In an embodiment, π-BG <NUM> has a resonance notch at a wavelength of <NUM>, and a bandgap width of approximately <NUM>. Alternatively, any other suitable values may be used.

The output of π-BG <NUM> is coupled to OPDI <NUM>, which is described in detail in <FIG>. In an embodiment, responsively to the output of π-BG <NUM>, demodulation circuit <NUM> outputs demodulated optical pulses (i.e., intensity modulated) to EORC <NUM>, which responsively transmits, to processor <NUM>, respective raw voltage electrical signals, such as Is and If shown in inset <NUM>, which are indicative of the US waves impinged on π-BG <NUM>. Processor <NUM> processes the raw signals to derive an US indicative signal, given in a physical unit of Hertz, seen in inset <NUM>, which is directly indicative of the US wave, for such purposes as generating an US image. The signals in insets <NUM> and <NUM> are brought by way of example and should not be assumed as being related both to each other (i.e., resulting from the same US wave).

<FIG> is a block diagram that schematically illustrates details elements of the detection apparatus appearing in <FIG>, in accordance with an embodiment of the present invention. As seen, the output of π-BG <NUM> is connected to a <NUM>/<NUM> coupler <NUM> of an MZI <NUM> of OPDI circuit <NUM> whose polarization maintaining arms <NUM> and <NUM> are coupled to a second coupler <NUM>. In an embodiment, a <NUM>-degree offset exists between the principle axes of arms <NUM> and <NUM> of MZI <NUM>.

Accordingly, in the inputs to coupler <NUM>, both the slow and fast polarizations of polarization maintaining arms are excited with equal intensities. The two outputs of MZI <NUM> (i.e., outputs of coupler <NUM>) are further split into their respective polarization modes by two polarization beam splitters, PBS <NUM> and PBS <NUM>, respectively, which realize a 2x4 optical hybrid <NUM>.

The four outputs of optical hybrid <NUM> (O1, O2, O3, and O4) are connected to EORC <NUM>. Balanced photodetectors <NUM> and <NUM>, included in EORC <NUM>, each measure the difference in optical intensity for a specific polarization mode. The EORC then transmits resulting electronic signals Is and If to processor <NUM>, for further processing as described in <FIG>.

In an embodiment, the measured electrical signals Is and If are given by: <MAT>.

A detailed derivation of signals Is and If is provided in Provisional Patent Application <CIT>, cited above. <NUM>, I<NUM> = |E<NUM>|<NUM> is the optical pulse intensity, and φ<NUM> = <MAT> is the phase difference induced between arms <NUM> and <NUM> of MZI <NUM>. Since the MZI arms <NUM> and <NUM> are weakly birefringent, Eq. <NUM> is derived assuming the approximation Δn « navg, where navg = (ns + nf)/<NUM> and Δn = (ns - nf), and where ns and nf are the refractive indices of arms <NUM> and <NUM> of MZI <NUM>, respectively.

In addition, it is assumed that the ultrasound-induced temporal variations in v is small relative to the carrier frequency, i.e., Δν(t) « ν<NUM>, where ν = ν<NUM> + Δν(t).

In an embodiment, the lengths of arm <NUM> and <NUM> of MZI <NUM> are adjusted to create a quarter wavelength difference between the optical path difference of the two polarization arms. The disclosed optical hybrid is implemented, for example, with planar waveguides patterned on a substrate, which are utilized to create an approximate <NUM>-degree phase difference between the two polarizations. In some embodiments of the present invention, the disclosed <NUM>-degree optical hybrid is implemented with a variety of multimode interference (MMI) couplers. For example, in an embodiment, the <NUM>-degree hybrid is implemented using two MZIs with a <MAT> <MAT> (λ=c/v and m being an integer) length difference between their optical path differences. In another embodiment, the <NUM>-degree hybrid is implemented with a 4x4 MMI.

<FIG> is a flow-chart that schematically illustrates a method for optical detection of a US wave, in accordance with an embodiment of the present invention. The process begins at a receiving raw electrical signals step <NUM>, such as receiving signals Is and If, or an associated form of signals Is and If, in processor <NUM>. An example of an associated version of Eq. <NUM> occurs when the measurements are AC-coupled and are given by Ĩs = Is - sin (φ<NUM>), and Ĩf = If - cos (φ<NUM>).

Next, at a phase extraction step <NUM>, processor <NUM> performs a calculation to extract optical phase φ<NUM> that MZI <NUM> induces, as described in detail in the above cited Provisional Patent Application <CIT>. Next, in a modulation frequency calculation step <NUM>, processor <NUM> calculates, from the associated version of Eq. <NUM>, a processed signal that is indicative of the wave, such as that shown in inset <NUM> of <FIG> above.

<FIG> are block diagrams that schematically illustrate PIC-based OPDI circuits, in accordance with additional embodiments of the present invention. Each of these PIC-based OPDI circuits can be used for implementing measurement elements <NUM> shown in <FIG>.

<FIG> shows a multimode interference (MMI) coupler implementation of a 2x4 MMI <NUM>-degree hybrid using a 4x4 MMI <NUM>-degree hybrid. MMIs are robust against fabrication errors and temperature fluctuations. Alternatively, as <FIG> shows, the <NUM>-degree hybrid may be implemented by two MZIs with a λ/<NUM>+m·λ/<NUM> (m being an integer) length difference between the optical path difference of the two MZIs. The 2x2 elements in the input of both schemes, as well as the directional couplers (DCs) in Fig. 7B, may be implemented via 2x2 MMI couplers, or couplers based on evanescent-wave coupling. The optical resonator, such as π-BG <NUM>, can be connected to either of the inputs of the 2x2 MMI coupler.

The unbalanced MZI and the <NUM>-degree hybrid may be fabricated in a planar geometry with 2D waveguides, i.e., waveguides in which the mode in confined in two dimensions and propagated in the third. Such planar designs may be implemented by numerous fabrication techniques for photonic integrated circuits. In particular, silicon photonics, glass waveguides, and polymer waveguides may be used in the implementation.

In general, a passive-modulation scheme enables feeding a signal from an optical resonator, such as a π-BG, to an unbalanced MZI having several outputs, where the phase difference between the outputs of the MZI is different than <NUM> degrees.

A phase difference of <NUM> degrees leads to simple implementation, as well as a simple demodulation algorithm. MMIs, then, represent a practical method for implementing <NUM> degree-hybrids that are scalable (many MMIs can be produced in a single chip) and reproducible (the performance of the MMIs does not change between manufacturing batches). MMIs are also highly versatile and can be used to implement hybrids with various angle differences. While such implementations may not be as simple as the one embodied above with <NUM>-degree hybrid <NUM>, the advantages of MMIs make them a feasible solution.

<FIG> are block diagrams that schematically illustrate PIC-based OPDI circuits, in accordance with other embodiments of the present invention. Each of these PIC-based OPDI circuits can be used for implementing measurement elements <NUM> shown in <FIG>.

<FIG> demonstrates a general pulse-interferometry demodulation scheme in which an unbalanced two-arm interferometer is connected to an m x n MMI (m and n being integers) that acts as an optical hybrid. An example of a <NUM>-degree optical hybrid implemented by a 3x3 MMI coupler is given in <FIG>. The 2x2 elements in the input of both schemes shown in <FIG> may be implemented via 2x2 MMI couplers or couplers based on evanescent-wave coupling. With the embodiments exemplified by <FIG>, the electro-optical readout circuitry and demodulation process also differ from that exemplified in <FIG> for a <NUM>-degree implementation.

The optical passive-demodulation circuits shown above are depicted purely for the sake of conceptual clarity. In alternative embodiments, any other suitable configuration can be used.

<FIG> is a schematic, pictorial illustration of a PIC-based minimally invasive medical optoacoustic diagnostic system <NUM>, in accordance with an embodiment of the present invention. US diagnostic system <NUM>, comprises US console <NUM> that comprises an interface <NUM> to which US probe <NUM> is connected by cable <NUM>. Console <NUM> further comprises system processor <NUM>.

An interface <NUM> of system <NUM> is configured to pass electrical power, through cable <NUM>, to a source <NUM> of pulsed electromagnetic radiation, which is included in probe <NUM> and seen in inset <NUM>. Probe <NUM> is configured for insertion into a cavity of an organ of a patient, and source <NUM> is configured to, responsively to the driving power from interface <NUM>, generate electromagnetic radiation that upon being absorbed by tissue of organ, heats tissue to generate US waves in an effect known as "thermoacoustic". The effect known as "optoacoustic" is a special case of the thermoacoustic effect, in which modulated light (from visible to infrared) is used to excite US waves.

A pulsed optical source <NUM> in console <NUM> supplies, through an optical fiber included in cable <NUM>, optical pulses to PIC-based US detector array <NUM> comprised in probe <NUM> (as seen in inset <NUM>). Detector array <NUM> comprises multiple OR <NUM>, arranged in pixels, each of which is configured to optically detect the resulting opto-acoustically generated US waves using the optical pulses, as described above. Array <NUM> transmits the optical pulses to PIC-based OPDI array <NUM>, again using an optical fiber included in cable <NUM>, that were frequency modulated in response to the impinging US waves for demodulation and electrical readout of the resulting intensity modulated optical pulses.

The resulting electrical signals are received processor <NUM> via cable <NUM> and interface <NUM>. Processor <NUM> is configured to generate from the measured electrical signals an indicative information, such as an US image, and to display it on a monitor <NUM>.

Typically, processor <NUM> comprises a general-purpose computer, which is programmed in software to carry out the functions described herein.

The configuration of optical source <NUM> and array <NUM> are depicted by way of example. In an embodiment, source <NUM> is located in console <NUM> and an optical fiber transmits US exciting optical pulses generated in the console to probe <NUM>. As another example, in another embodiment, the US exciting pulse may be applied outside the body, and only the detection of the resulting US waves is performed by probe <NUM>.

As noted above, US detector array <NUM> of probe <NUM> is configured to optically detect the resulting US waves using the disclosed detection method that is based on optical passive-demodulation pulse interferometry, as described below. System <NUM> can detect US wave that are an order of magnitude weaker (i.e., of lower sound pressure) than alternative solutions. At the same time, system <NUM> can detect high intensity US waves, and achieve a dynamic range that may exceed 60dB. Thus, US detector array <NUM> may enable US system <NUM> to, for example, spatially resolve anatomical features that are unresolvable by US imaging systems employing piezoelectric US detectors. System <NUM> could therefore provide US images having a superior image quality.

As noted above, in alternative embodiments, array <NUM> of OR and/or an array of EORC are implemented using other technologies than PIC. For example, OR and/or EORC arrays are implemented using fiber optics and/or free-space electrooptical devices, such as commercially available packaged π-BG and photodiodes coupled using free-space lenslet arrays.

Claim 1:
An apparatus for optical detection of ultrasound, the apparatus comprising:
a first array of multiple optical resonators - OR - (<NUM>), configured to modulate respective carrier frequencies of optical signals indicative of US waves impinging thereon;
a second array of multiple optical passive-demodulation interferometers - OPDI - (<NUM>), implemented in one or more photonic integrated circuits (PIC), wherein each OPDI is configured to demodulate the optical signal output by the respective OR, so as to generate a respective intensity-modulated optical signal, the OPDI comprising:
an interferometer (<NUM>) having imbalanced arms (<NUM>, <NUM>) that are recombined using a <NUM>-degree optical hybrid (<NUM>); and
a third array of multiple electro-optical readout circuits - EORC (<NUM>), wherein each EORC is configured to measure the intensity demodulated optical signal produced by the respective OPDI, and to output a respective electrical signal, wherein the multiple OR are implemented in the one or more PIC and/or wherein the multiple EORC are implemented in the one or more PIC.