Patent Description:
Quantum computing is currently being developed with the prospective to accelerate the development of molecules for pharmaceutical applications or organic solar cells, to optimize logistics and financial markets investment, and to allow for secure cloud computing. To date, quantum computer processors with a handful of qubits to almost hundred qubits have been demonstrated. However, thousands of qubits are expected to be required for a fault-tolerant quantum correction for practical applications. It was shown theoretically that quantum computing with squeezed states of light has the potential to provide the scalability to implement thousands of qubits. The realization of this approach benefits from the physical implementation of a system which can integrate numerous sources of squeezed light. With the bulk optics commonly used as sources of squeezed lights this is a challenge. Photonic integrated circuits may solve the problem of integrating numerous optical components, in particular optical resonators, in a single device by forming the components on a continuous substrate. The continuous substrate may be an integral piece in a monolithic approach. The continuous substrate may also be constructed combining two or more substrates. The two or more substrates can comprise different materials in a hybrid approach.

In particular, the coupling of the optical resonator to a waveguide may be essential, as the waveguide mediates the coupling to other components. The optical resonator can give rise to a multipass geometry or a larger field strength of an electromagnetic wave inside the optical resonator than outside of it. The multipass geometry or the larger field strength improves the efficiency of a device comprising the optical resonator, in particular of a device adapted for a nonlinear process to take place in the optical resonator. In this context, the efficiency of the device may be characterized by a conversion rate of photons in the incoming electromagnetic wave to photons in the generated electromagnetic wave. The efficiency of the device may be characterized by an enhancement factor of the cavity. The efficiency of the device may also be a number of photons in the generated electromagnetic wave per photon in the incoming electromagnetic wave entering the resonator. Alternatively, the efficiency of the device may be characterized as the power of a generated electromagnetic wave per power of the incoming electromagnetic wave. In particular, the multipass geometry or the larger field strength can improve the efficiency of a device which comprises the optical resonator and is configured to generate squeezed light, and it can improve the squeezing factor of the squeezed light. The improved efficiency of the device and the improved squeezing factor may improve the performance of the device in applications, in particular in applications in quantum optics, for example in quantum computing.

<NPL>) and <NPL>) demonstrate the use of a lithium-niobate waveguide, i.e. of a nonlinear medium with a non-vanishing Chi2 nonlinearity, in an optical resonator for squeezed-light generation and for second-harmonic generation.

Optical resonators have been integrated into photonic integrated circuits as ring resonators, described, for example, in <CIT>, or with gratings forming grating reflectors as end elements of a optical resonator, described, for example, in <NPL>). Widely applied ring resonators are composed of nonlinear media with a vanishing Chi2 nonlinearity, for example silicon, which limits the efficiency of nonlinear processes in the ring resonator. Ring resonators with a vanishing Chi2 nonlinearity apply four-wave mixing, a Chi3 nonlinear process, which typically generates photon pairs with unequal energies of the photons (non-degenerate pair), whereas photon pairs with equal energies of the photons (degenerate pair) are desirable for many applications. Moreover, the efficiency of a device using a Chi2 nonlinear process may exceed the efficiency of a device using a Chi3 nonlinear process. The implementation of a Chi2 nonlinear process in a ring resonator is challenging. Moreover, ring resonators suffer disadvantageous bending losses, which increase with a decreasing radius of the ring resonator. Therefore, a ring resonator cannot easily have both a short optical resonator length and a high finesse. An optical resonator with a short length and a high finesse is desirable in many applications, for example for generating single frequency modes of ultra-narrow bandwidth or for wavelength-division multiplexing.

<CIT> discloses an optical wavelength conversion device with an optical waveguide type optical wavelength conversion element and with laser diodes.

An optical resonator has been integrated into a polymer-based photonic-integrated circuit using the hybrid approach, see <NPL>). The integrated optical resonator is a bulk etalon. Polymer-based photonic integrated circuits may suffer from a low thermal conduction and from high optical losses, for example due to an absorption in the polymer. The optical loss can limit a finesse of the optical resonator and reduce the efficiency of a device comprising the optical resonator. The generation of heat may be especially critical in polymer-based photonic-integrated circuits because of the low thermal conduction and a limited thermal stability.

Optical resonators have also been integrated into photonic integrated circuits using coupling elements, as described for example in <NPL>). The device described by Dietrich et al. uses coupling elements to compensate a misalignment between the optical resonator and a neighboring waveguide. The coupling elements may cause an optical loss. The optical loss can limit a finesse of the optical resonator, reduce the efficiency of an optical device comprising the optical resonator, limit a squeezing of squeezed light produced in the optical resonator, and/or may generate heat that needs to be transported away to maintain a stability of the device.

In view of the technical problems described above, there is a need for an improved optical resonator that can easily be co-integrated with other optical components on a common substrate and that can be manufactured efficiently and in large numbers.

This objective is achieved with a photonic integrated circuit according to independent claim <NUM>. Independent claim <NUM> provides a method for producing a photonic integrated circuit. The dependent claims relate to preferred embodiments.

The portion of the first waveguide within the optical resonator can advantageously be used to route a beam and/or to control a beam quality and/or to control a profile of an optical beam inside the optical resonator, in particular of a laser beam inside the optical resonator. In particular, using established techniques, the portion of the first waveguide within the optical resonator may be designed to support a specified width, diameter, divergence, transverse mode, or mode spacing of the optical beam. For example, the portion of the first waveguide within the optical resonator may be designed to route a beam and/or to control a beam quality and/or to control a profile of an optical beam inside the portion of the first waveguide within the optical resonator. The portion of the first waveguide within the optical resonator may also be designed to route a beam and/or to control a beam quality and/or to control a profile of an optical beam inside a free space or a gas volume in the optical resonator.

As the first waveguide at the first end of the first waveguide is aligned with the second waveguide at the second end of the second waveguide, the resonator and the second waveguide are aligned with respect to one another in a static and stable way. The static and stable alignment can improve the performance of a device that comprises the photonic integrated circuit. Moreover, the static and stable alignment results in a compact device geometry, which may be beneficial for a cost of production of the device and for the co-integration of other optical, electro-optical, and/or heat-conducting components. The static and stable alignment can render additional adjustment unnecessary.

The alignment of the first waveguide at the first end of the first waveguide with the second waveguide at the second end of the second waveguide can relax or eliminate the need for a coupling element which compensates an angular misalignment between the optical resonator and the second waveguide. This may be an advantage over existing integrated optical resonators as described, for example, in Smit et al. and Dietrich et al. , which may require a coupling element for compensating an angular misalignment, and possibly also for controlling a divergence of a light beam. The coupling element for compensating the angular misalignment may cause optical losses due to absorption and scattering in the coupling element and at interfaces to the coupling element. Avoiding the coupling element for compensating an angular misalignment can avoid the optical losses and improve the efficiency of a device comprising the photonic integrated circuit. Avoiding the coupling element for compensating an angular misalignment can also increase a finesse of the optical resonator, which can result in a higher electric field strength inside the optical resonator and a sharper resonance profile of the optical resonator. Avoiding the coupling element for compensating an angular misalignment can also reduce the complexity of producing the photonic integrated circuit, because the production involves producing and positioning fewer components, improving the reliability and reducing the cost of production. Avoiding the coupling element for compensating an angular misalignment can also improve the mechanical integrity and rigidity of the photonic integrated circuit, as fewer components can move with respect to each other, for example in the presence of an unintended movement such as a mechanical vibration. Moreover, the photonic integrated circuit according to Claim <NUM> advantageously allows for easy integration of a coupling element for controlling the divergence of the light beam, for example if one of the end mirrors of the optical resonator is a curved mirror.

The optical resonator can be flexibly adjusted to and optimized for specific applications by designing reflectivities and transmittances of the end mirrors. In particular, the optical resonator can act as a cavity. The advantage of flexibility is a result of the comprised first layered structure and second layered structure. The first layered structure and the second layered structure may, for example, be designed for a high reflectivity of at least one mirror of the first mirror and the second mirror. For a high finesse of the optical resonator, both the first layered structure and the second layered structure may be designed for a high reflectivity. For example, the first layered structure and the second layered structure may be designed for a high reflectivity for a first electromagnetic wave with a first frequency, and a high transmittance for a second electromagnetic wave with a second frequency. Furthermore, additional optical components may be fabricated using similar techniques or a same coating device as for the end mirrors, for example additional bending mirrors co-integrated with the photonic integrated circuit, either as part of the optical resonator or separate from the optical resonator.

In one embodiment, the first waveguide or the second waveguide comprises lithium niobate, lithium tantalate, beta barium borate, lithium triborate, potassium titanyl phosphate, silicon, silicon oxide, aluminum arsenide, gallium arsenide, aluminum gallium arsenide, or silicon nitride.

In an embodiment, the first waveguide or the second waveguide comprises titanium-indiffused lithium niobate or magnesium-indiffused lithium niobate or silicon-indiffused silicon oxide.

The first waveguide substrate and the first waveguide may form an integral piece. Here, an integral piece may refer to a piece which does not have a noticeable internal interface that would separate the piece into two pieces.

The second waveguide substrate and the second waveguide may form an integral piece.

In one embodiment, the first waveguide substrate and the second waveguide substrate form an integral piece. In the context of the present disclosure, this is referred to as a monolithic approach. The monolithic approach may be used to optimize the mechanical rigidity of the photonic integrated circuit and its resilience to unintended movements such as vibrations. In the monolithic approach, the first waveguide substrate and the second waveguide substrate may be processed together using the same processing techniques. This facilitates a fabrication of the photonic integrated circuit and reduces its cost. The monolithic approach may also be used to ensure the alignment of the first waveguide at the first end of the first waveguide and the second waveguide at the second end of the second waveguide. For example, the first waveguide and the second waveguide can initially be produced as a single, continuous waveguide and can later be separated by a recess.

In another embodiment, the first waveguide substrate is mostly composed of a first material, which is different from a second material, which the second waveguide substrate is mostly composed of. In the context of the present disclosure, this is referred to as a hybrid approach. The first material may be selected or optimized for a first specific functionality. The second material may be selected or optimized independently from the first material for a second specific functionality. The hybrid approach hence allows for combining the most suitable materials for specific functionalities. More than two materials optimized for more than two functionalities may be combined in one photonic integrated circuit.

Here and in the following, a component may be understood to be mostly composed of a specified material, if a weight of the specified material comprised in the component is at least half of a weight of the component, or in particular at least two thirds of a weight of the component.

The first mirror substrate may be different from the first waveguide substrate.

A first material, that the first mirror substrate may be mostly composed of, may be different from a second material, which the first waveguide substrate is mostly composed of.

In one embodiment, a length of the optical resonator is at least <NUM>, in particular <NUM>, or <NUM>, or <NUM>. In one embodiment, the length of the optical resonator does not exceed <NUM>, in particular <NUM>, or <NUM>, or <NUM>.

In one embodiment, a length of the portion of the first waveguide inside the optical resonator is at least <NUM>, in particular <NUM>, or <NUM>, or <NUM>. In one embodiment, the length of the portion of the first waveguide inside the optical resonator does not exceed <NUM>, in particular <NUM>, or <NUM>, or <NUM>.

In one embodiment, the length of the optical resonator does not exceed the length of the portion of the first waveguide inside the optical resonator by a factor of <NUM>, in particular a factor of <NUM>, or <NUM>.

The photonic integrated circuit advantageously allows for a compact design of the optical resonator. A short length of the optical resonator not exceeding the length of the portion of the first waveguide inside the optical resonator by a factor of <NUM>, in particular a factor of <NUM>, or <NUM>, can minimize an optical loss caused by scattering and absorption in the optical resonator. The minimized optical loss can improve the efficiency of an optical device comprising the photonic integrated circuit. Reduced optical losses can also improve the finesse of the optical resonator. The improved finesse can, for instance, be used to increase an electric field strength inside the first waveguide. The short optical resonator length may also be used to generate a large spectral spacing of a frequency comb of electromagnetic frequencies supported by the optical resonator. The large spectral spacing may allow for selecting a single frequency from the frequency comb, for example using bandpass filtering or frequency-selective homodyne detection. For example, an ultra-narrow bandwidth may be achieved this way. As another example, the large spectral spacing may allow for wavelength-division multiplexing of the frequency modes into individual spatial modes for separate processing.

The first mirror substrate may be different from the second waveguide substrate.

A first material, of which the first mirror substrate is mostly composed, may be different from a second material, of which the second waveguide substrate is mostly composed.

The first mirror substrate and the second mirror substrate may be different from the first waveguide substrate or the second waveguide substrate.

A first material, of which the first mirror substrate is mostly composed, may be different from a second material, of which the first waveguide substrate or the second waveguide substrate is mostly composed; and/or a third material, of which the second mirror substrate is mostly composed, may be different from the second material.

According to the claimed invention, a portion of the first layered structure or the second layered structure is embedded within a continuous body formed by the first waveguide substrate and the second waveguide substrate. This is applied to maximize a thermal conduction between the first layered structure and the first waveguide substrate or the second waveguide substrate.

In an embodiment, a portion of the first layered structure or the second layered structure is below a top surface of the first waveguide substrate or the second waveguide substrate. For example, this embodiment may be used to maximize a thermal conduction between the first layered structure or the second layered structure and the first waveguide substrate or the second waveguide substrate.

In an embodiment, the first layered structure or the second layered structure has an orientation perpendicular to a top surface of a continuous body formed by the first waveguide substrate and the second waveguide substrate.

For instance, a width of the first waveguide and/or the second waveguide may be no smaller than <NUM>, in particular no smaller than <NUM>.

For instance, a width of the first waveguide and/or the second waveguide may be no larger than <NUM>, in particular no larger than <NUM>.

According to an embodiment, a thickness of the first waveguide substrate is at least <NUM>, in particular <NUM>, or <NUM>, or <NUM>.

According to an embodiment, a thickness of the first waveguide substrate does not exceed <NUM>, in particular <NUM>, or <NUM>, or <NUM>.

A large thickness of the first waveguide substrate of at least <NUM>, in particular <NUM>, or <NUM>, or <NUM>, can make the production of the photonic integrated circuit more reliable, as it reduces the risk of unintended breaking. The large thickness may also improve the rigidity and ruggedness of the photonic integrated circuit in the presence of unintended movements and in particular mechanical vibrations. Large thickness substrates typically also show less bowing and warping, i.e. are flatter, which may be favorable with respect to the performance of the device. This can enhance the lifetime of the photonic integrated circuit. The large thickness may also improve a thermal transport within the photonic integrated circuit, improving the thermal stability and/or homogeneity of the photonic integrated circuit.

A width of a continuous piece formed by the first waveguide substrate and the second waveguide substrate may be at least <NUM>, <NUM>, <NUM>, <NUM>, or <NUM>.

A width of a continuous piece formed by the first waveguide substrate and the second waveguide substrate may not exceed <NUM>, <NUM>, <NUM>, <NUM>, or <NUM>.

A length of a continuous piece formed by the first waveguide substrate and the second waveguide substrate may be at least <NUM>, <NUM>, <NUM>, <NUM>, or <NUM>.

A length of a continuous piece formed by the first waveguide substrate and the second waveguide substrate may not exceed <NUM>, <NUM>, <NUM>, <NUM>, or <NUM>.

A large width of a continuous piece formed by the first waveguide substrate and the second waveguide substrate or a large length of a continuous piece formed by the first waveguide substrate and the second waveguide substrate of at least <NUM>, <NUM>, <NUM>, <NUM>, or <NUM>, can support the co-integration of additional optical resonators or additional components on the photonic integrated circuit.

Both, a width and a length of the first waveguide substrate, may be at least <NUM>, and in particular <NUM>.

According to one embodiment, the photonic integrated circuit further comprises an optical bandpass filter between the first waveguide and the second waveguide.

The optical bandpass filter may be used for selecting a single frequency from a frequency comb supported by the photonic integrated circuit.

The first waveguide substrate and the second waveguide substrate may be in direct physical contact. The first waveguide substrate and the second waveguide substrate may be separated by no more than one optical component, which may in particular be the first mirror or the second mirror. The first waveguide substrate and a closest point of the second waveguide substrate may be separate by no more than <NUM>.

In an embodiment, the first waveguide and/or the first waveguide substrate comprises a nonlinear optical medium.

For example, the nonlinear medium may be adapted to support parametric down conversion, second harmonic generation, sum-frequency generation, difference-frequency generation, four-wave mixing, optical parametric amplification, optical parametric oscillation, white-light generation, Kerr lensing, two-photon absorption, or multi-photon absorption.

In an embodiment, the nonlinear medium has a non-vanishing Chi2 nonlinearity. The non-vanishing Chi2 nonlinearity can promote a high efficiency of a nonlinear process in the nonlinear optical medium, in particular compared to a reference efficiency of a reference nonlinear process in a reference nonlinear optical medium with a vanishing Chi2 nonlinearity.

The nonlinear medium with the non-vanishing Chi2 nonlinearity may comprise lithium niobate. The lithium niobate may comprise periodically poled lithium niobate. The lithium niobate may comprise titanium-indiffused lithium niobate or magnesium-indiffused lithium niobate.

Using the periodic poling, for example of periodically poled lithium niobate, a phase-matching condition of the nonlinear optical medium may be tuned. The tuning may give a maximum of flexibility of the photonic integrated circuit in terms of applications. For example, the phase-matching condition and hence the nonlinear optical medium may be tuned to support a specific nonlinear optical process. For example, the phase-matching condition and hence the nonlinear optical medium may be tuned to support a nonlinear optical process for at least one specific frequency of at least one electromagnetic wave. For example, the phase-matching condition and hence the nonlinear optical medium may be tuned to support a nonlinear optical process for a specific temperature of the nonlinear optical medium.

According to an embodiment, the nonlinear optical medium is adapted to convert an incoming electromagnetic wave with at least one incoming frequency to at least one generated electromagnetic wave with at least one generated frequency, wherein at least one generated frequency is different from at least one incoming frequency.

For example, at least one generated frequency may be smaller than at least one incoming frequency. For example, at least one generated frequency may be the double of at least one incoming frequency.

In particular, if the nonlinear medium has a non-vanishing Chi2 nonlinearity, the frequency conversion may comprise a parametric down conversion, in particular a parametric down conversion that produces degenerate photon pairs. The parametric down conversion can generate an electromagnetic wave with a generated frequency smaller than the incoming frequency of the incoming electromagnetic wave at high efficiency, in particular compared to a reference efficiency of a four-wave mixing process, which may be used in a reference nonlinear optical medium with a vanishing Chi2 linearity.

A transverse confinement of the incoming electromagnetic wave due to the waveguide structure of the first waveguide, and a confinement of the incoming electromagnetic wave along an optical axis due to the optical resonator formed by the first mirror and the second mirror may result in an enhancement of a field strength of the incoming electromagnetic wave in the first waveguide. The enhancement can improve the efficiency of a device comprising the optical resonator, in particular of a device adapted to support a nonlinear process in the nonlinear optical medium comprised in the first waveguide, for example, a frequency conversion in the nonlinear optical medium. For example, the improved efficiency of the device can relax requirements with respect to a source of optical or electric pump power, thereby reducing the technical requirements towards the device and its cost. For example, the improved efficiency can improve an output power of the generated electromagnetic wave. For example, the improved efficiency can improve a squeezing of a squeezed light, which may be generated in the nonlinear optical medium.

According to an embodiment, a field strength of the incoming electromagnetic wave inside the optical resonator is larger than a field strength of the incoming electromagnetic wave outside the optical resonator.

According to an embodiment, the generated electromagnetic wave passes through the portion of the first waveguide inside the optical resonator more than once. The generated electromagnetic wave passing through the portion of the first waveguide inside the optical resonator more than once can seed a nonlinear optical process inside the nonlinear optical medium. The seeding can improve the efficiency of a device that comprises the optical resonator and is adapted to support the nonlinear process, which can for example be a frequency conversion.

At least one incoming frequency may be in the UV or visible or IR spectral range. At least one incoming electromagnetic wave may have a wavelength of at least <NUM>, in particular of at least <NUM>. At least one incoming electromagnetic wave can have a wavelength no longer than <NUM>, in particular no longer than <NUM>. For instance, at least one incoming electromagnetic wave can have a wavelength in a wavelength range from <NUM> to <NUM>, or from <NUM> to <NUM>, or from <NUM> to <NUM>, or from <NUM> to <NUM>. The incoming electromagnetic wave with a wavelength in one of these wavelength ranges may efficiently be transported in commonly used optical fibers. This may improve the integration of the photonic integrated circuit into larger optical devices. For instance, at least one incoming electromagnetic wave can have a wavelength in a wavelength range from <NUM> to <NUM>, or from <NUM> to <NUM>, or from <NUM> to <NUM>, or from <NUM> to <NUM>; and in particular to a wavelength in a range from <NUM> to <NUM>. For example, applying an incoming electromagnetic wave with a wavelength in one of these wavelength ranges and using a degenerate parametric down conversion for the frequency conversion inside the nonlinear optical medium may generate a frequency that can be transmitted with low loss in commonly used optical fibers. This may improve the integration of the photonic integrated circuit into larger optical devices.

At least one generated frequency may be in the UV or the visible or the IR spectral range. At least one generated electromagnetic wave may have a wavelength of at least <NUM>, in particular of at least <NUM>. At least one generated electromagnetic wave can have a wavelength no longer than <NUM>, in particular no longer than <NUM>. For instance, at least one generated electromagnetic wave can have a wavelength in a wavelength range from <NUM> to <NUM>, or from <NUM> to <NUM>, or from <NUM> to <NUM>, or from <NUM> to <NUM>; in particular from <NUM> to <NUM>. The generated electromagnetic wave with a wavelength in one of these wavelength ranges may efficiently be transported in commonly used optical fibers. This may improve the integration of the photonic integrated circuit into larger optical devices.

According to one embodiment, the generated electromagnetic wave comprises an electromagnetic wave in a squeezed state.

According to an embodiment, the first layered structure or the second layered structure comprises a layer of a material different from the first substrate and/or the second substrate. For example, the first layered structure or the second layered structure may comprise a layer composed mainly of an oxide or a halide or a chalcogenide or a III-V semiconductor. For instance, the first layered structure or the second layered structure may comprise a layer composed mainly of TiO<NUM>, SiO<NUM>, MgF<NUM>, Ta<NUM>O<NUM>, ZnSe, YF<NUM>, AlGaAs, GaAs, AlAs, AlAsSb, or GaSb. In one embodiment, the first layered structure comprises an epitaxial layer, wherein the material forming the epitaxial layer is different from the first mirror substrate. For example, the epitaxial layer may comprise semiconductor material. At least one epitaxial semiconductor layer may be comprised in a first mirror or a second mirror with a high reflectivity of at least <NUM> %, <NUM> %, <NUM> % or in an ultra-performance mirror with a reflectivity of at least <NUM> %.

In an embodiment, the second waveguide and/or the first waveguide substrate comprises a nonlinear optical medium.

The first mirror and/or the second mirror may have a reflectivity of at least <NUM> %, in particular at least <NUM> % or at least <NUM> %.

The first mirror and/or the second mirror may have a transmittance of at least <NUM> %, in particular at least <NUM> % or at least <NUM> %. The transmittance may be used to couple an electromagnetic wave out of the optical resonator or into the optical resonator.

A roughness of a portion of the first mirror substrate or of the second mirror substrate may be < <NUM>, in particular < <NUM>. The lower roughness < <NUM>, in particular < <NUM>, maybe applied in a first mirror or a second mirror with a high reflectivity of at least <NUM> %, <NUM> %, <NUM> % or in an ultra-performance mirror with a reflectivity of at least <NUM> %.

The first layered structure or the second layered structure may comprise a layer composed mainly of a metal, in particular gold, silver, or aluminum. Composing the layer mainly of a metal can reduce the complexity of the first layered structure and the cost of producing the photonic integrated circuit.

The first layered structure and/or the second layered structure may be in physical contact with the first waveguide. The first layered structure and/or the second layered structure in contact with the first waveguide can minimize the length of the optical resonator. The minimized length of the optical resonator can minimize an optical loss caused by scattering and absorption in the optical resonator.

The photonic integrated circuit may comprise a space between the first mirror or the second mirror and the first optical waveguide or the second waveguide, and the space may be filled with vacuum or a filler material. The vacuum or the filler material may improve the coupling of an electromagnetic wave between the first waveguide and the second waveguide. The filler material may reduce the difference in the indices of refraction of the space and the first mirror or the second mirror. The filler material may reduce the difference in the indices of refraction of the space and the first optical waveguide or the second waveguide.

The first layered structure or the second layered structure may have a thickness of at least <NUM>, in particular of at least <NUM>.

The first layered structure or the second layered structure may have a thickness not exceeding <NUM>, in particular <NUM>, or <NUM>, or <NUM>. The small thickness of the layered structure not exceeding <NUM>, in particular <NUM>, or <NUM>, or <NUM>, may offer advantages over the much larger thickness of known Bragg gratings, which are applied as end mirrors in integrated optical resonators. In particular, the small thickness can reduce the scattering and absorption at the end mirror. Further, the small thickness can increase the density at which the first layered structure or the second layered structure can be integrated in a larger photonic integrated circuit, and ultimately the density at which the optical resonator can be integrated in a larger photonic integrated circuit.

The photonic integrated circuit may comprise at least one additional layered structure on the first mirror substrate or the second mirror substrate. The additional layered structure may comprise an antireflection coating, an optical bandpass filter, an optical shortpass filter, or an optical longpass filter. For example, the antireflection coating may have a reflectivity < <NUM> %, in particular < <NUM> %.

The photonic integrated circuit may further comprise a second optical resonator and a third mirror with a third layered structure on a third mirror substrate, wherein the third mirror substrate is provided by or fixed to the first waveguide substrate or the second waveguide substrate, and wherein the third mirror forms an end mirror of the second optical resonator, the second optical resonator being different from and/or spatially separate from the optical resonator.

The design of the photonic integrated circuit can advantageously promote the co-integration of additional optical resonators which share with the optical resonator a continuous chip formed by the first waveguide substrate and the second waveguide substrate. In particular, the co-integration of additional optical resonators on the same chip reduces the cost of production. In particular, the co-integration of additional optical resonators minimizes movement of the optical resonator and the additional optical resonators with respect to one another, for example in the presence of mechanical vibrations, thus improving the performance of a device comprising the photonic integrated circuit. More than one additional mirror may be co-integrated. More than one additional optical resonator may be co-integrated. At least one additional waveguide within an additional optical resonator of an additional optical resonator may be co-integrated. At least one second additional waveguide aligned with an additional waveguide may be integrated. The co-integration of additional optical resonators and/or waveguides can provide a scalability of the photonic integrated circuit, which is, for example, an advantage for quantum computing with continuous variables. In an advanced application, mirrors and/or resonators and/or waveguides may be co-integrated on the same chip in a large number exceeding ten or a hundred or a thousand.

The photonic integrated circuit may further comprise a fourth mirror comprising a fourth layered structure on a fourth mirror substrate, wherein the fourth mirror substrate is provided by or fixed to the first waveguide substrate or the second waveguide substrate, and wherein the fourth mirror forms an end mirror of the second optical resonator of the second optical resonator.

The photonic integrated circuit may further comprise a third waveguide, wherein at least a portion of the third waveguide is within the second optical resonator.

The photonic integrated circuit may further comprise a fourth waveguide, wherein the fourth waveguide at a fourth end of the fourth waveguide is aligned with the third waveguide at a third end of the third waveguide.

In an embodiment, the photonic integrated circuit further comprises at least one heat sink in physical contact with the first waveguide substrate or the second waveguide substrate.

A thermal conductivity of the heat sink may be at least <NUM> W/(m·K).

The heat sink may comprise diamond, or silicon carbide or aluminum nitride, copper, silver or gold. For instance, the heat sink may comprise diamond grown by chemical vapor deposition (CVD) or silicon carbide grown by CVD.

According to an embodiment, the photonic integrated circuit comprises at least two heat sinks on different sides of the first waveguide substrate.

The design of the photonic integrated circuit with the first waveguide substrate and the second waveguide substrate forming a continuous chip can provide an efficient heat transfer or thermal conduction within the chip and away from the chip, and a large contact area for heat sinks on the first waveguide substrate and the second waveguide substrate. The large contact area may be used for physically contacting at least one heat sink with the photonic integrated circuit resulting in an efficient heat transfer or thermal conduction between the chip and the heat sink. The heat sink can improve a thermal stability of the photonic integrated circuit. The improved thermal stability can improve the performance of a device comprising the photonic integrated circuit, in particular the performance of a photonic integrated circuit with a nonlinear optical medium, because a nonlinearity of the nonlinear optical medium can depend critically on temperature. For example, the improved thermal stability can make an output of the device more stable. For example, the improved thermal stability can allow for increasing an optical or electric pump power applied to the device. For example, increasing the optical or electric pump power can improve a squeezing of a squeezed light, which may be generated in the nonlinear optical medium. For example, the efficient thermal transport may be used to control the temperature of photonic-integrated light-detection modules (single-photon avalanche diodes, PIN diodes) and to reduce their optical background noise. Furthermore, the thermal properties of the heat sink, for example a low thermal capacity and a high heat conductance, may advantageously be used for faster modulation of temperature-controlled linear-optic elements, including phase shifters and variable beam splitters.

In an embodiment, the first waveguide substrate or the second waveguide substrate exhibits at least one of the following: an integrated gas cell, a microstructure, a waveguide, a phase shifter, a beam splitter, a delay line, an interferometer, an electrode, a photodetector, in particular a photodiode, an avalanche diode, a squeezed-light source, a sensor, a Peltier element, a heat sink.

The design of the photonic integrated circuit can advantageously promote the co-integration of additional components which share with the optical resonator a continuous chip formed by the first waveguide substrate and the second waveguide substrate. Additional components can comprise a microstructure, a waveguide, a phase shifter, a beam splitter, a delay line, an interferometer, an electrode, a photodetector, in particular a photodiode, an avalanche diode, a squeezed-light source, a sensor, a Peltier element, a heat sink. For example, the co-integration of additional components on the same chip may reduce the cost of production. The co-integration of additional optical resonators can improve the performance of a device comprising the photonic integrated circuit. For example, the co-integration may improve the performance by minimizing a movement of the optical resonator and the additional optical resonators with respect to one another, for example in the presence of mechanical vibrations. For example, the co-integration may improve the performance by providing a scalability of the photonic integrated circuit, which is, for example, an advantage for quantum computing with continuous variables. The additional components may benefit from the good thermal conductivity of a heat sink on the first waveguide substrate or the second waveguide substrate, improving the performance of the additional components.

The first waveguide substrate or the second waveguide substrate may comprise at least one of the following: silicon, silicon oxide, lithium niobate, beta barium borate, lithium triborate, potassium titanyl phosphate, silicon nitride, aluminum arsenide, gallium arsenide, aluminum gallium arsenide, indium phosphide, gallium phosphide, or silicon carbide. The first waveguide substrate or the second waveguide substrate may comprise crystalline material.

The photonic integrated circuit may further comprise a focusing or defocusing element, at least a portion of which is located between the first waveguide and the second waveguide. The focusing or defocusing element may improve the coupling of an electromagnetic wave between the first waveguide and the second waveguide. For example, an improved coupling improves the efficiency of a device comprising the photonic integrated circuit.

For instance, the focusing or defocusing element may comprise a curved mirror. The curved mirror may have a surface with a spherical or cylindrical or parabolic shape, or with a shape of a Cartesian oval. The shape of the surface of the curved mirror may be optimized to reduce spherical aberration below reference spherical aberration of a curved mirror with a spherical surface. The minimized spherical aberration may improve the coupling of an electromagnetic wave between the first waveguide and the second waveguide. For example, an improved coupling improves the efficiency of a device comprising the photonic integrated circuit.

According to one embodiment, the first mirror and/or the second mirror comprises or is the curved mirror. The first mirror or the second mirror comprising or being the curved mirror minimizes the number of components in or close to the optical resonator. The minimized number of components can minimize optical losses due to scattering and absorption. For example, the minimized optical losses improve the efficiency of a device comprising the photonic integrated circuit. The minimized optical losses can also improve the finesse of the optical resonator.

For instance, the curved mirror has a radius of curvature of at least <NUM>, in particular <NUM>, or <NUM>, or <NUM>. For instance, the curved mirror has a radius of curvature not exceeding <NUM>, in particular <NUM>, or <NUM>, or <NUM>.

In one embodiment, the focusing or defocusing element comprises a micro lens, a micro beam expander, a coupling grating, or a gradient-index GRIN lens. The successful integration of focusing and/or defocusing elements into photonic integrated circuits has been demonstrated in Dietrich et al. and in Happach et al. In particular, Dietrich et al. demonstrate the integration of beam expanders, whereas Happach et al. demonstrate the integration of GRIN lenses.

In another aspect, the disclosure relates to a device comprising the photonic integrated circuit with some or all of the features described above. For instance, the device may comprise an optical parametric oscillator, a cavity-enhanced photon-pair source, a second-harmonic generator, a difference-frequency generator, or a sum-frequency generator. For instance, the device may be a device for squeezed-light generation, squeezed-state encoding, quantum key distribution, quantum computing, frequency-comb generation, generation of single frequency modes of ultra-narrow bandwidth, wavelength-division multiplexing, sensing, light ranging and detection (LIDAR), spectroscopy, or mid-infrared spectroscopy.

In yet another aspect, the disclosure relates to a method for producing a photonic integrated circuit, the method comprising: providing a first waveguide substrate with a first waveguide, providing a second waveguide substrate with a second waveguide, providing a first mirror comprising a first layered structure on a first mirror substrate, providing a second mirror comprising a second layered structure on a second mirror substrate, forming a first waveguide on the first waveguide substrate, forming a second waveguide on the second waveguide substrate, characterized in that the method comprises the following steps: positioning the first mirror with respect to the first waveguide substrate or the second waveguide substrate, and fixing the first mirror to the first waveguide substrate or the second waveguide substrate, such that the first mirror and the second mirror form end mirrors of an optical resonator, and at least a portion of the first waveguide is within the optical resonator, and the first waveguide substrate is continuous with the second waveguide substrate, and the first waveguide at a first end of the first waveguide is aligned with the second waveguide at a second end of the second waveguide.

The method may comprise all the process steps, but not necessarily in the given order. At least some of the process steps may be performed at an earlier or later point in the method.

According to one embodiment, the method further comprises forming alignment structures on the first mirror substrate or the second mirror substrate. In another embodiment, the method further comprises forming alignment structures on the first waveguide substrate or the second waveguide substrate.

For example, the alignment structures may comprise alignment marks. Positioning the first mirror with respect to the first waveguide substrate or the second waveguide substrate may comprise aligning the first mirror with respect to the first waveguide substrate or the second waveguide substrate according to a visual inspection of the alignment marks.

For example, the alignment structures may comprise alignment slots. Positioning the first mirror with respect to the first waveguide substrate or the second waveguide substrate may comprise aligning the first mirror with respect to the first waveguide substrate or the second waveguide substrate according to at least one mechanical contact point provided by at least one alignment slot.

For instance, forming the first waveguide or the second waveguide can be performed prior to the positioning and the fixing.

For instance, forming the first waveguide or the second waveguide can be performed after the positioning and the fixing. In particular, forming the first waveguide or the second waveguide after the positioning and the fixing can improve the precision of the alignment. In particular, forming the first waveguide or the second waveguide after the positioning and the fixing eliminates a risk of misalignment of the first waveguide and the second waveguide during the positioning and fixing. The improved precision of the alignment may improve the coupling of an electromagnetic wave between the first waveguide and the second waveguide. For example, an improved coupling improves the efficiency of a device comprising the photonic integrated circuit. Eliminating the risk of misalignment can improve the reliability of a production of the photonic integrated circuit.

The method may comprise polishing a section of the first mirror substrate and/or the second mirror substrate to a surface roughness < <NUM>, in particular < <NUM>.

The method may further comprise depositing the first layered structure or the second layered structure. For instance, depositing the first layered structure or depositing the second layered structure may comprise ion beam sputtering or magnetron sputtering or molecular beam deposition or atomic layer deposition. According to one embodiment, depositing the first layered structure or depositing the second layered structure comprises depositing the first layered structure or depositing the second layered structure on the first waveguide or on the second waveguide.

The method may further comprise removing at least a portion of the first mirror substrate. The method may further comprise removing at least a portion of the second mirror substrate.

The positioning and fixing may comprise a microassembly step or a robotic pick-and-place step. The microassembly step or the robotic pick-and-place step may comprise picking up the first mirror with an automated device. The microassembly step or the robotic pick-and-place step may comprise positioning the first mirror with respect to the first waveguide substrate or the second waveguide substrate, and/or fixing the first mirror to the first waveguide substrate or the second waveguide substrate using an automated device. The microassembly step or the robotic pick-and-place step may comprise positioning the first mirror with a precision of no larger than <NUM>, in particular no larger than <NUM>.

The method may further comprise forming a recess in the first waveguide substrate or the second waveguide substrate, and positioning and fixing the first mirror such that at least a part of the first mirror is in the recess. Positioning the first mirror such that at least a part of the first mirror is in the recess may comprise aligning the first mirror with respect to a side or an edge of the recess.

The method may comprise direct bonding the first waveguide substrate and the second waveguide substrate. The direct bonding may for example comprise surface activated bonding, plasma activated bonding, adhesive bonding, anodic bonding, eutectic bonding, glass frit bonding, thermocompression bonding, or reactive bonding.

The method may further comprise forming an intermediate layer between the first mirror substrate and the first waveguide substrate or the second waveguide substrate. For instance, the intermediate layer may comprise silicon or silicon oxide.

The first mirror substrate may have a thickness of at least <NUM>, in particular of at least <NUM>.

The first waveguide substrate or the second waveguide substrate may have a thickness of at least <NUM>, in particular of at least <NUM>.

The method may further comprise attaching the first waveguide substrate or the second waveguide substrate to an auxiliary substrate with a thickness of at least <NUM>, in particular of at least <NUM>.

A thickness of the first mirror substrate, or the first waveguide substrate, or the second waveguide substrate, or the auxiliary substrate of at least <NUM>, in particular at least <NUM>, makes handling the respective substrate more reliable. In particular, it reduces a risk of unintendedly breaking the respective substrate during the positioning and fixing. The thickness of at least <NUM>, in particular of at least <NUM>, also improves the adhesion between the first mirror substrate and the first waveguide substrate or the second waveguide substrate. The improved adhesion reduces a danger that a joint between the first mirror substrate and the first waveguide substrate or the second waveguide substrate breaks, in particular in the presence of movements or vibrations. For example, the improved adhesion improves the reliability of the production of the photonic integrated circuit. For example, the improved adhesion enhances a lifetime of the photonic integrated circuit. Large thickness substrates typically also show less bowing and warping, i.e. are flatter, which may be favorable with respect to the performance of the device. The thickness of at least <NUM>, in particular of at least <NUM>, also improves a thermal transport between the first mirror substrate and the first waveguide substrate or the second waveguide substrate.

The method may further comprise attaching a heat sink to the first waveguide substrate or to the second waveguide substrate. For instance, a thermal conductivity of the heat sink may be at least <NUM> W/(m·K).

The heat sink may comprise diamond, silicon carbide, aluminum nitride, copper, silver or gold.

Attaching the heat sink may comprise depositing diamond or silicon carbide by chemical vapor deposition.

The method may further comprise attaching at least two heat sinks to surfaces on opposite sides of the first waveguide substrate.

The features and numerous advantages of the disclosure will become best apparent from a detailed description of exemplary embodiments with reference to the accompanying drawings, in which:.

<FIG> gives a schematic illustration of a photonic integrated circuit <NUM> according to one embodiment, which may, for example, be applied as an optical parametric oscillator <NUM>, a cavity-enhanced photon-pair source <NUM>, or a cavity-enhanced source of squeezed light <NUM>. The photonic integrated circuit is shown from a top view. In this embodiment, a first mirror <NUM> and a second mirror <NUM> are located on opposite surfaces of a first waveguide substrate <NUM>, which thus serves as a first mirror substrate <NUM> and a second mirror substrate <NUM>. According to this embodiment, the first waveguide <NUM> is fully within the optical resonator formed by the two mirrors <NUM>, <NUM>. The first waveguide substrate <NUM> comprises a nonlinear medium <NUM>, for example lithium niobate, with a portion of the nonlinear medium configured as a nonlinear gain medium, for example by periodic poling to form a section of periodically poled lithium niobate. For example, the first waveguide substrate <NUM> can be a single crystal of lithium niobate. The lithium niobate may comprise titanium-indiffused lithium niobate. The electric field of an incoming electromagnetic wave <NUM>, which can advantageously be generated in a photonic integrated circuit <NUM> according to a slightly modified embodiment adapted for second-harmonic generation, is enhanced in the optical resonator. Consequently, the electric field is enhanced in the first waveguide <NUM>, <NUM> formed in the nonlinear medium. The enhanced field strength enhances the efficiency of a device comprising the nonlinear medium. Advantageously, the periodic poling of the lithium niobate <NUM> may be optimized for a specific nonlinear optical process to occur at a specified temperature for a specific incoming frequency and incoming polarization. For example, the periodicity can be adapted to support degenerate parametric down conversion for an incoming frequency corresponding to an incoming wavelength of <NUM>. In this case, a seeded or unseeded degenerate parametric downconversion results in an electromagnetic wave <NUM> with a frequency corresponding to a wavelength of <NUM>. The photonic integrated circuit <NUM> can generate an electromagnetic wave <NUM> that comprises squeezed light, or generate photon pairs. The generated electromagnetic wave <NUM> may pass through the nonlinear optical medium more than once because of the optical resonator formed by the first mirror <NUM> and the second mirror <NUM>. Multiple passes of the generated light can be used to seed the nonlinear optical process, which can enhance an output power of the nonlinear optical process, and to implement a device which is referred to as an optical parametric oscillator in the context of this disclosure. The multiple passes can hence be beneficial for the efficiency of a device adapted to support the nonlinear optical process. In particular, the photonic integrated circuit may be applied as a source of intense squeezed light. The multiple passes and the enhanced field strength in the optical resonator can improve the performance of an optical device comprising the photonic integrated circuit, in particular of a source of squeezed light, for example by relaxing the demands on a possible source of optical or electric power which may be needed to operate the optical device. For example, the cost of the source may hence be reduced. An output power of the optical device can be improved. A quality of a squeezing of generated squeezed light can be improved. The improved efficiency of the device also reduces waste heat generated in the device, which may be beneficial for a stability of the generated light and a lifetime of the device.

<FIG> gives a schematic perspective view of a different embodiment of the photonic integrated circuit <NUM> for, for example, an optical parametric oscillator <NUM>, a cavity-enhanced photon-pair source <NUM>, or a cavity-enhanced source of squeezed light <NUM>. Like in the embodiment illustrated in <FIG>, also according to the embodiment depicted in <FIG> the first mirror <NUM>, <NUM> and the second mirror <NUM>, <NUM> form end mirrors of an optical resonator. Like in the other embodiment, a portion <NUM> of the first waveguide is in the optical resonator. In addition, this embodiment of the photonic integrated circuit <NUM> can be adapted to support a nonlinear optical process, giving the same advantages as the photonic integrated circuit shown in <FIG>. According to the embodiment shown in <FIG>, the first mirror substrate <NUM> is fixed to the first waveguide substrate <NUM>, and the second mirror <NUM> is fixed to one of the waveguide substrates <NUM>, <NUM>. In some embodiments, the first mirror substrate <NUM>, <NUM> is provided by one of the waveguide substrates, whereas the second mirror substrate <NUM> is fixed to one of the waveguide substrates. In some embodiments, the first mirror <NUM>, <NUM> and the second mirror <NUM>, <NUM> are formed on the same mirror substrate, the same mirror substrate acting as the first mirror substrate <NUM>, <NUM> and the second mirror substrate <NUM>, <NUM>. In this case, the first mirror <NUM>, <NUM> and the second mirror <NUM>, <NUM> can be formed on opposite surfaces of the same substrate. This same substrate can then be fixed to the first waveguide substrate <NUM>,<NUM> or the second waveguide substrate <NUM>, <NUM>.

The photonic integrated circuit <NUM> may comprise a second waveguide substrate <NUM> of a material different from the material of the first waveguide substrate <NUM>, thus implementing a hybrid approach to photonic integrated circuits. The hybrid approach has the advantage that the most suitable material of the second waveguide substrate <NUM> can be selected for a target functionality of the optical components on the second waveguide substrate <NUM>, which can be different from a target functionality of the optical components on the first waveguide substrate <NUM>. This allows for the design of complex photonic integrated circuits. For example, microstructures, waveguides, ring resonators, phase shifters interferometers, beam splitters, delay lines, heat sinks and electronic elements can be on the first waveguide substrate <NUM> or the second waveguide substrate <NUM> whichever has a more suitable material for the component. Additional waveguides and ring resonators are shown in the photonic integrated circuit <NUM> in <FIG> as examples. An additional third waveguide substrate or more waveguide substrates, possibly comprising a third or more different materials, can be continuous with the first waveguide substrate <NUM> or the second waveguide substrate <NUM>, forming a single chip. The chip can combine multiple materials, wherein each one may be best-suited for a specific functionality.

According to the embodiment shown in <FIG>, the second layered structure <NUM> of the second mirror <NUM> is fully enclosed in a continuous chip formed by the first waveguide substrate <NUM> and the second waveguide substrate <NUM>. This embodiment may result in an efficient optical coupling between the first waveguide and the second waveguide, as it minimizes the optical losses, for example scattering losses, associated with the optical coupling. The efficient optical coupling can improve the quality of the squeezing of a squeezed light, which can be generated in a portion of the first waveguide. The efficient coupling is also beneficial for a high finesse of the optical resonator, which can be an advantage in some applications. The continuous chip may also result in a high heat conduction between the second mirror <NUM> and the first waveguide substrate <NUM> and the second waveguide substrate <NUM>. The first mirror <NUM> and the second mirror <NUM> are in physical contact with the nonlinear medium for a high heat conduction also between the nonlinear medium and the mirrors <NUM>, <NUM>. The efficient transport of heat away from the nonlinear medium and the first mirror <NUM> and the second mirror <NUM> into the first waveguide substrate <NUM> and the second waveguide substrate <NUM> may be an advantage in applications which require a high thermal stability and homogeneity. For example, this may be the case in the generation of squeezed light with a high intensity of the pump light. Advantageously, the heat transport away from the first waveguide substrate <NUM> and the second waveguide substrate <NUM> is facilitated by using heat sinks that are attached close to or are in contact with the components in which heat originates.

The first waveguide <NUM>, <NUM> and the second waveguide <NUM>, <NUM> are aligned with respect to each other at corresponding ends. In particular, one end <NUM>, <NUM> of the first waveguide neighbors one end <NUM>, <NUM> of the second waveguide <NUM>, <NUM>. The alignment is such that an extension of an axis of the first waveguide <NUM>, <NUM> at an end <NUM>, <NUM> of the first waveguide coincides with an axis of the second waveguide <NUM>, <NUM> at an end <NUM>, <NUM> of the second waveguide. This way, light can couple directly along a straight line from the first waveguide <NUM>, <NUM> to the second waveguide <NUM>, <NUM>. This avoids coupling elements that would otherwise be needed for compensating angular misalignment. In some known integrated photonic circuits, coupling elements are comprised to compensate an angular misalignment or to bridge a significant gap between the axis of the first waveguide <NUM>, <NUM> and the axis of the second waveguide <NUM>, <NUM> at their corresponding ends or control the divergence of a light beam. Avoiding coupling elements reduces optical losses due to scattering and absorption in the coupling elements and at interfaces of the coupling element. This improves the efficiency of devices comprising the photonic integrated circuit. The reduced optical losses due to scattering and absorption also increase the finesse of the optical resonator, which is an advantage in many devices in which the photonic integrated circuit can be applied. The increased finesse results in a higher electric field strength inside the optical resonator and a sharper resonance profile of the optical resonator. This is an advantage in narrow-linewidth applications using the photonic integrated circuit. Moreover, avoiding the coupling elements reduces the complexity of producing the photonic integrated circuit, as fewer components are produced and/or placed with a high accuracy. Avoiding coupling elements also reduces the cost of producing the photonic integrated circuits and improves the reliability of the production. Moreover, the mechanical integrity and rigidity is improved, as fewer components can move with respect to each other, for example in the presence of a mechanical vibration. For an efficient coupling, the end <NUM>, <NUM> of the first waveguide and the end <NUM>, <NUM> of the second waveguide are located in enclosed vicinity of one another in one embodiment. In particular, the end <NUM> of the first waveguide can be in direct contact with the first mirror <NUM> or the second mirror <NUM>, and the end <NUM> of the second waveguide can also be in direct contact with the first mirror <NUM> or the second mirror <NUM>, thus minimizing the distance in between them to a minimal distance given by a thickness of the first mirror <NUM>, <NUM> or the second mirror <NUM>, <NUM>.

<FIG> illustrates another embodiment of the photonic integrated circuit, comprising a focusing or defocusing element <NUM> between the first waveguide <NUM> the second waveguide <NUM>. The focusing or defocusing element may improve the coupling between the first waveguide <NUM>, <NUM> and the second waveguide <NUM>, <NUM>. For example, the embodiments illustrated in <FIG> comprise as a focusing or defocusing element a micro-lens <NUM>, <NUM> as shown in <FIG> and <FIG>, a micro beam expander <NUM> as shown in <FIG>, or a coupling grating <NUM> or a GRIN lens <NUM> as shown in <FIG>. In one embodiment, illustrated in <FIG>, the first mirror <NUM> or the second mirror <NUM> comprises or is the curved mirror <NUM>, <NUM>. This minimizes the number of optical components in the optical resonator or close to the optical resonator, which could act as sources of scattering and/or absorption. In some embodiments, the curved mirror has a radius of curvature of <NUM> to <NUM>. The curved mirror can have any shape, including spherical, cylindrical, parabolic, or the shape of a general Cartesian oval. Parabolic and Cartesian oval shapes may be an advantage as the radius of curvature may not be much larger than the diameter of a beam of incoming radiation in an application, provoking spherical aberration. The parabolic and Cartesian oval shapes compensate the spherical aberration. The focusing or defocusing element can be configured to maximize the stability of the optical resonator. Moreover, the focusing or defocusing element can be configured to maximize the finesse of the optical resonator. In another embodiment, a space between the first waveguide <NUM> or the second waveguide <NUM> and the first mirror <NUM> or the second mirror <NUM> comprises vacuum or a filler material. The filler material reduces the difference in the optical index of refraction at an interface of the first waveguide <NUM> or the second waveguide <NUM>. This further improves the coupling between the first waveguide <NUM> and the second waveguide <NUM>. The first mirror substrate <NUM> or the second mirror substrate <NUM> may further exhibit at least one layered structure. The first mirror substrate can for example support an antireflection coating, an optical bandpass filter, an optical short pass filter, or an optical long pass filter. In particular, the antireflection coating can improve the coupling between the first waveguide and the second waveguide, for example by reducing reflection losses of a pump light or a generated light.

In one embodiment, the first waveguide <NUM>, <NUM> or the second waveguide <NUM>, <NUM> comprises lithium niobate, silicon, silicon oxide, aluminum arsenide, gallium arsenide, aluminum gallium arsenide, or silicon nitride, but the first waveguide <NUM>, <NUM> or the second waveguide <NUM>, <NUM> may be composed of any semiconductor or dielectric that is now known or developed in the future. In an embodiment, the first waveguide <NUM>, <NUM> or the second waveguide <NUM>, <NUM> comprises titanium-indiffused lithium niobate or silicon-indiffused silicon oxide. Also, in one embodiment, the first waveguide substrate <NUM>, <NUM> or the second waveguide substrate <NUM>, <NUM> comprises lithium niobate, silicon, silicon oxide, aluminum arsenide, gallium arsenide, aluminum gallium arsenide, or silica nitrite, but the first waveguide substrate <NUM>, <NUM> or the second waveguide substrate <NUM>, <NUM> may be composed of any semiconductor or dielectric that is now known or developed in the future. The first waveguide substrate <NUM>, <NUM> or the second waveguide substrate <NUM>, <NUM> can comprise crystalline material, for example in the form of a wafer.

The first layered structure <NUM>, <NUM> or the second layered structure <NUM>, <NUM> each comprise at least one layer of material different from the first waveguide substrate <NUM>, <NUM> and the second waveguide substrate <NUM>, <NUM>. The first layered structure <NUM>, <NUM> or the second layered structure <NUM>, <NUM> may comprise alternating layers of materials with a low index of refraction and materials with a high index of refraction. This combination ensures a high reflectivity (> <NUM> %, > <NUM> %, > <NUM> %, > <NUM> %, or > <NUM> %). A material forming a layer of the layered structure may comprise an oxide or a halide or a chalcogenide or a III-V-semiconductor, for example TiO<NUM>, SiO<NUM>, MgF<NUM>, Ta<NUM>O<NUM>, ZnSe, YF<NUM>, AlGaAs, GaAs, AlAs, AlAsSb, or GaSb. The layers may also be composed of any other semiconductor or dielectric that is now known or developed in the future. The first layered structure <NUM>, <NUM> and the second layered structure <NUM>, <NUM> can have thicknesses below <NUM>, in particular below <NUM>. The thicknesses can be at least <NUM>, in particular at least <NUM>. The first layered structure <NUM>, <NUM> or the second layered structure <NUM>, <NUM> can comprise a metal layer e.g. gold, silver, or aluminum coating). A metal layer has the advantage that it can be produced economically, while providing a reflectivity > <NUM> %, > <NUM> %, or > <NUM> %. A section of the mirror substrate can have a roughness of RMS < <NUM>, in particular < <NUM>. In particular, this may be useful for producing ultra-high performance mirrors (reflectivity exceeding <NUM> %), while for all other applications (reflectivity > <NUM> %, > <NUM> %, > <NUM> %, or > <NUM> %) industrial grade polishing can possibly be employed. A deposited layer may comprise an epitaxial semiconductor layer, wherein the material forming the epitaxial layer can be the same or different from the material forming the substrate. A first mirror <NUM>, <NUM> or a second mirror <NUM>, <NUM> which comprises the epitaxial layer can have a reflectivity > <NUM> % or > <NUM> %. Mirrors comprising an epitaxial semiconductor layer usually comprise a mirror substrate that comprises crystalline material.

The first layered structure <NUM>, <NUM> and/or the second layered structure <NUM>, <NUM> thus achieve a high reflectivity of the first mirror <NUM>, <NUM> and the second mirror <NUM>, <NUM>. The high reflectivity results in a high finesse of the optical resonator. The combination of a high finesse and a short length of the optical resonator is an advantage over existing ring resonators, for example when the photonic integrated circuit is used for generating single frequency modes of ultra-narrow bandwidth or for wavelength-division multiplexing.

Moreover, the reflectivity and transmittance of the first mirror <NUM>, <NUM> and/or the second mirror <NUM>, <NUM> can easily be adjusted according to the requirements of a specific application by designing the first layered structure <NUM>, <NUM> and/or the second layered structure <NUM>, <NUM> correspondingly. The first layered structure <NUM>, <NUM> and/or the second layered structure <NUM>, <NUM> can have a transmittance of at least <NUM> %, in particular at least <NUM> %, or at least <NUM> %. The transmittance of the first layered structure <NUM>, <NUM> and/or the second layered structure <NUM>, <NUM> can be used to couple the incoming electromagnetic wave into the optical resonator, or to couple the generated electromagnetic wave out of the optical resonator.

The reflectivity and transmittance of the first mirror <NUM>, <NUM> and the reflectivity and transmittance of the second mirror <NUM>, <NUM> can be configured for impedance matching. In particular, the reflectivity and transmittance of the first mirror <NUM>, <NUM> and the reflectivity and transmittance of the second mirror <NUM>, <NUM> can be configured such that a light power transmitted through the first mirror <NUM>, <NUM> or the second mirror <NUM>, <NUM> is at most as high as a light power dissipated during a round trip in the optical resonator. The reflectivity and transmittance can be adjusted individually for each frequency of an electromagnetic wave in the presence of multiple frequencies of at least one electromagnetic wave, for example if an incoming frequency differs from a generated frequency in the device using the photonic integrated circuit, or in case of multiple, differing incoming frequencies or multiple, differing generated frequencies.

The first mirror substrate <NUM>, <NUM> or the second mirror substrate <NUM>, <NUM> can be composed of materials which are the same or different from the material of the first waveguide substrate <NUM>, <NUM> or the second waveguide substrate <NUM>, <NUM>. In one embodiment, the first mirror substrate <NUM>, <NUM> or the second mirror substrate <NUM>, <NUM> comprises lithium niobate, silicon, silicon oxide, aluminum arsenide, gallium arsenide, aluminum gallium arsenide, or silicon nitride, but the first mirror substrate <NUM>, <NUM> or the second mirror substrate <NUM>, <NUM> may be composed of any semiconductor or dielectric that is now known or developed in the future. According to the claimed invention, a portion of the first layered structure <NUM> or the second layered structure <NUM> is embedded within a continuous body formed by the first waveguide substrate <NUM> and the second waveguide substrate <NUM> as illustrated in <FIG>. In another embodiment, the first layered structure <NUM> or the second layered structure <NUM> is fully embedded within the continuous body formed by the first waveguide substrate <NUM> and the second waveguide substrate <NUM> as illustrated in <FIG>. In another embodiment, at least a portion of the first layered structure <NUM> or the second layered structure <NUM> is below a top surface of the first waveguide substrate <NUM> or the second waveguide substrate <NUM> as illustrated in <FIG>. In an embodiment, the layers of the first layered structure <NUM>, <NUM> or the layers of the second layered structure <NUM>, <NUM> are perpendicular to a top surface of the first waveguide substrate <NUM>, <NUM> or a top surface of the second waveguide substrate <NUM>, <NUM>.

The first waveguide <NUM>, <NUM> can comprise a nonlinear medium. The photonic integrated circuit with the nonlinear optical medium has the advantage that the strength of an electromagnetic wave in the waveguide is enhanced due to the transverse confinement in the waveguide and the confinement along the optical axis in the optical resonator enhances. The enhanced strength of the electromagnetic field improves the efficiency of a device which comprises the optical resonator, in particular of a device adapted to support a nonlinear optical process mediated by the nonlinear optical medium. For example, a required optical or electrical input power or pump power is reduced, an amount of generated heat is reduced, a generated optical output power is improved, or a squeezing of a generated light is improved. The nonlinear medium can for example comprise BBO, LBO, KDP, KTP, KTA, KBBF, BiBO, CLBO, KTA, GaAs, AgGaS2, ZnTe, InP, GaSe, CdTe, CdZnTe, LiTaO<NUM>, or LiNbO<NUM>. The nonlinear optical medium can alternatively comprise any nonlinear optical material that is now known or developed in the future. The nonlinear optical medium may for example be adapted to support parametric down conversion, second harmonic generation, sum frequency generation, difference frequency generation, four-wave mixing, optical parametric amplification, optical parametric oscillation, white-light generation, Kerr lensing, two-photon absorption, multi-photon absorption, or stimulated amplified emission. In an embodiment, the nonlinear optical medium has a non-vanishing Chi2 nonlinearity. This is realized in nonlinear optical media without inversion symmetry. In an embodiment, the nonlinear optical medium without inversion symmetry is ferroelectric. For example, the nonlinear optical medium can comprise lithium niobate, in particular it can comprise titanium-indiffused lithium niobate. The lithium niobate can comprise periodically poled lithium niobate <NUM>. Periodic poling of the ferroelectric domains in periodically poled lithium niobate <NUM> allows for tuning phase matching conditions of the nonlinear optical medium. By tuning the phase matching conditions, the nonlinear optical medium can be configured to support a specified nonlinear optical process, or can be configured to support nonlinear optical processes for specified frequencies of electromagnetic waves, or can be configured to support a nonlinear process at a specified temperature. This flexibility is advantageous in applications, as it enables an average expert in the field to design a photonic integrated circuit for a specific application.

In an embodiment, the nonlinear optical medium is adapted to support a nonlinear optical process comprising a frequency conversion. In an embodiment, the frequency conversion comprises at least one incoming electromagnetic wave <NUM> with an incoming frequency and at least one generated electromagnetic wave <NUM> with a generated frequency, and the generated frequency is different from the incoming frequency. In one embodiment, the generated frequency is smaller than the incoming frequency. For example, this is achieved by employing parametric down conversion, optical parametric amplification, optical parametric oscillation, or four-wave mixing as a nonlinear optical process. In another embodiment, at least one generated frequency is the double of at least one incoming frequency. For example, this can be achieved using second harmonic generation in the nonlinear optical medium. The optical resonator can be configured to enhance a field strength of the incoming electromagnetic wave <NUM> inside the optical resonator compared to a field strength of the incoming electromagnetic wave <NUM> outside the optical resonator. The enhanced field strength can advantageously increase the efficiency of a device which comprises the optical resonator and is adapted to support the nonlinear optical process. In one embodiment, the generated electromagnetic wave <NUM> passes through the portion <NUM>, <NUM> of the first waveguide inside the optical resonator more than once, realizing a multi-pass geometry. In the multi-pass geometry, the generated electromagnetic wave <NUM> can further seed the nonlinear optical processes. This increases the efficiency of a device comprising the optical resonator further. The increased efficiency improves the performance of the device for example by relaxing the requirements with respect to a source of optical or electrical pump power, thereby reducing the technical requirements towards the device and its cost. The increased efficiency also improves the squeezing of squeezed light which can be generated in the nonlinear optical process.

The incoming frequency of the incoming electromagnetic wave <NUM> can be in the UV, or the visible, or the infrared spectral range, with a wavelength of at least <NUM>, in particular <NUM>. The wavelength may be at most <NUM>, in particular at most <NUM>. Depending on the specific incoming frequency or frequencies, and depending on the generated frequency or frequencies, specific materials of the first waveguide <NUM>, <NUM> or the second waveguide <NUM>, <NUM> may need to be employed. In particular, the material of the first waveguide <NUM>, <NUM> and the second waveguide <NUM>, <NUM> should exhibit a high transparency to the incoming frequency or the generated frequency. The photonic integrated circuit advantageously provides the flexibility to employ a wide selection of materials such that the wavelength range may be tailored to a specific application. In particular, the incoming frequency can be in the wavelength range from <NUM> to <NUM>, from <NUM> to <NUM>, from <NUM> to <NUM>, or from <NUM> to <NUM>. Advantageously, an incoming electromagnetic wave <NUM> of such a wavelength is transported efficiently in commonly used optical fibers. Using such an incoming frequency allows to integrate the photonic integrated circuit into extended optical devices. In one embodiment, the incoming frequency can be in the wavelength range from <NUM> to <NUM>, from <NUM> to <NUM>, from <NUM> to <NUM>, or from <NUM> to <NUM>, in particular in the wavelength range from <NUM> to <NUM>. Using such an incoming frequency in combination with degenerate parametric down conversion results in generated frequencies, which can advantageously be transmitted with low loss in commonly used optical fibers. In another embodiment, the incoming frequency can be in the wavelength range from <NUM> to <NUM>, from <NUM> to <NUM>, from <NUM> to <NUM>, or from <NUM> to <NUM>, in particular in the wavelength range from <NUM> to <NUM>. Using such an incoming frequency in combination with frequency-doubling or second-harmonic generation results in generated frequencies, which can advantageously be transmitted with low loss in commonly used optical fibers.

The generated frequency of the generated electromagnetic wave <NUM> can be in the UV, or the visible, or the infrared spectral range, with a generated wavelength of at least <NUM>, in particular <NUM>. The generated wavelength may be at most <NUM>, in particular at most <NUM>. In particular, the generated frequency can correspond to a wavelength range from <NUM> to <NUM>, from <NUM> to <NUM>, from <NUM> to <NUM>, or from <NUM> to <NUM>. Advantageously, a generated electromagnetic wave <NUM> of such an outgoing frequency is transported efficiently and commonly used optical fibers. Using such an outgoing frequency allows to integrate the photonic integrated circuit into extended optical devices, for example for providing the incoming electromagnetic wave <NUM> or for processing the generated electromagnetic wave <NUM>. In an embodiment, the generated wavelength is between <NUM> and <NUM>.

In one embodiment, the generated electromagnetic wave <NUM> comprises electromagnetic wave in a squeezed state. The combined confinement along the transverse direction by the waveguide <NUM>, <NUM> comprising a nonlinear optical medium and along the optical axes by the optical resonator <NUM>, <NUM>, <NUM>, <NUM> enhances the field strength of an incoming electromagnetic wave <NUM> and improves the quality of the squeezing. Moreover, the continuous first waveguide substrate <NUM>, <NUM> and second waveguide substrate <NUM>, <NUM> provide a mechanically robust and rugged platform for squeezed light generation and for coupling squeezed light between the different components of an optical device, in particular a photonic integrated circuit. Moreover, the first mirror <NUM>, <NUM> and the second mirror <NUM>, <NUM> comprising at least one layer of material different from a material of the first waveguide substrate <NUM>, <NUM> and the second waveguide substrate <NUM>, <NUM> allow for a high finesse of the optical resonator, further improving the quality of the squeezing. The photonic integrated circuit can hence be applied for the generation of high quality or high intensity squeezed light, giving advantages in applications such as squeezed-state encoding <NUM>, quantum key distribution <NUM>, quantum computing <NUM>, or wavelength-division multiplexing. Some exemplary applications that the photonic integrated circuit can advantageously be applied in are illustrated in <FIG>.

According to the embodiments shown in <FIG> and <FIG>, the first waveguide is completely inside the optical resonator formed by the first mirror <NUM>, <NUM> and the second mirror <NUM>, <NUM>. According to the embodiment shown in <FIG>, the length of the optical resonator matches the length of the first waveguide. The first layered structure <NUM>, <NUM> and/or the second layered structure can be in physical contact with the first waveguide <NUM>, <NUM> to minimize the length of the optical resonator. This can be achieved by depositing the first layered structure <NUM>, <NUM> or the second layered structure <NUM>, <NUM> directly onto the first waveguide <NUM>, <NUM>. According to the embodiment shown in <FIG>, a gap may exist between the first mirror or the second mirror and the first waveguide, such that the length of the optical resonator slightly exceeds the length of the first waveguide. In some embodiments, the length of the optical resonator does not exceed the length of the portion <NUM>, <NUM> of the first waveguide inside the optical resonator by a factor of <NUM>, in particular of <NUM>, or <NUM>. According to some embodiments, the length of the optical resonator may not exceed <NUM>, or <NUM>, or <NUM>, or <NUM>. A longer optical resonator provides better selectivity. A short length of the optical resonator results in a compact design with the advantage of reducing optical loss. As an additional advantage, an optical resonator with a short length emits a frequency comb with a large spectral spacing. The large spectral spacing allows for selecting a single frequency, for example using bandpass filtering or frequency selective homodyne detection. In one embodiment, the photonic integrated circuit comprises an optical bandpass filter, or an optical short pass filter, or an optical long pass filter located between the first waveguide <NUM>, <NUM> and the second waveguide <NUM>, <NUM>. By selecting a single frequency from the frequency comb using the bandpass filter, an ultra-narrow bandwidth can be achieved, which is an advantage in some applications. A large spectral spacing also allows for wavelength-division multiplexing of the frequency modes into individual spatial modes for separate processing of the individual spatial modes. Therefore, the first waveguide substrate <NUM>, <NUM> or the second waveguide substrate <NUM>, <NUM> can comprise an array of waveguides or a phase array, which can be adapted to serve as a monochromator or a spectroscope. On the other hand, a larger length of the optical resonator can enhance the interaction length of a process related to an interaction between light and matter in the optical resonator. An enhanced interaction length can enhance the efficiency of a device which comprises the optical resonator and is adapted to support the process, which can for example comprise the generation of light during the interaction. A short length of the optical resonator can be used, for example if the photonic integrated circuit is used in combination with a pulsed light source. A short length of the optical resonator reduces the dispersion inside the optical resonator and can improve a pulse duration of a generated light pulse.

A typical width of the first waveguide <NUM>, <NUM> or the second waveguide <NUM>, <NUM> may be at least <NUM>, in particular <NUM>, or <NUM>, or <NUM>. In one embodiment, the width of the first waveguide <NUM>, <NUM> or the second waveguide <NUM>, <NUM> does not exceed <NUM>, in particular <NUM>, or <NUM>, or <NUM>, or <NUM>. This width is optimized for applications with electromagnetic radiation in the ultraviolet, the visible and the infrared spectral range. Different widths are possible, optimized for the application and the wavelength range used. A tapered shape of the first waveguide <NUM>, <NUM> or the second waveguide <NUM>, <NUM> is possible, for example to improve the coupling between the first waveguide <NUM>, <NUM> and the second waveguide <NUM>, <NUM>. A part of the first waveguide <NUM>, <NUM> may be embedded in the first waveguide substrate <NUM>, <NUM>. A part of the first waveguide <NUM>, <NUM> may protrude from the first waveguide substrate <NUM>, <NUM>. A part of the second waveguide <NUM>, <NUM> may be embedded in the second waveguide substrate <NUM>, <NUM>. A part of the second waveguide <NUM>, <NUM> may protrude from the second waveguide substrate <NUM>, <NUM>.

The first waveguide <NUM>, <NUM> and the first waveguide substrate <NUM>, <NUM> may form one integral piece. Also the second waveguide <NUM>, <NUM> and the second waveguide substrate <NUM>, <NUM> may form one integral piece. In a monolithic approach, illustrated in <FIG>, the first waveguide substrate <NUM> and the second waveguide substrate <NUM>, <NUM> form an integral piece. This approach optimizes the mechanical rigidity of the photonic integrated circuit and hence its resilience to unintended movements such as vibrations. The monolithic approach also ensures that the first waveguide substrate <NUM> and the second waveguide substrate <NUM>, <NUM> can be processed together using the same processing techniques. This facilitates the fabrication of the photonic integrated circuit and reduces its cost. The monolithic approach can also be used to ensure the alignment of the first waveguide <NUM>, <NUM> and the second waveguide <NUM>, <NUM>, by initially producing both of them as a continuous waveguide and at a later step separating them by a recess in the waveguide.

In another embodiment, consistent with the schematic illustration in <FIG>, the first waveguide substrate <NUM> and the second waveguide substrate <NUM> are fixed to one another. In this case, the first waveguide substrate <NUM> and the second waveguide substrate <NUM> can be composed of different materials, consistent with a hybrid approach to photonic integrated circuits. In the hybrid approach, the best materials for specific functionalities can be combined. This may be an advantage in some applications, as it allows for combining various functionalities on a single chip using optimized materials for each of the functionalities. More than two materials can be combined into one photonic integrated circuit. This may be an advantage if more than two functionalities are to be combined in the photonic integrated circuit.

A thickness of the first waveguide substrate <NUM>, <NUM> or the second waveguide substrate <NUM>, <NUM> may exceed <NUM>, in particular <NUM>. A larger thickness makes the production process more reliable, as it reduces the risk of an unintended breaking of the first waveguide substrate <NUM>, <NUM> or the second waveguide substrate <NUM>, <NUM> in the production process. A larger thickness also improves the adhesion of the first waveguide substrate <NUM>, <NUM> to the second waveguide substrate <NUM>, <NUM> in embodiments in which the first waveguide substrate <NUM>, <NUM> is fixed to the second waveguide substrate <NUM>, <NUM>. The improved adhesion improves the rigidity and ruggedness of the photonic integrated circuit in the presence of mechanical vibrations. This enhances the lifetime of the photonic integrated circuit. Large thickness substrates typically also show less bowing and warping, i.e. are flatter, which may be favorable with respect to the performance of the device. This also improves the heat dissipation within the photonic integrated circuit. A small thickness is possible if material is to be saved and the production cost is to be reduced. In particular, the thickness can be reduced in embodiments where the first mirror <NUM>, <NUM> or the second mirror <NUM>, <NUM> is fixed to the first waveguide substrate <NUM>, <NUM> or the second waveguide substrate <NUM>, <NUM>, as illustrated in <FIG>.

A width of the first waveguide substrate <NUM>, <NUM> or the second waveguide substrate <NUM>, <NUM> may exceed <NUM>, in particular <NUM>. In some embodiments, a length and a width of the first waveguide substrate <NUM>, <NUM> or the second waveguide substrate <NUM>, <NUM> exceed <NUM>, in particular <NUM>. A larger width or a large width and length of the first waveguide substrate <NUM>, <NUM> or the second waveguide substrate <NUM>, <NUM> allow for the co-integration of more additional components on the photonic integrated circuit, as illustrated for some example optical applications in <FIG> and <FIG>. A smaller width and/or length of the first waveguide substrate <NUM>, <NUM> or the second waveguide substrate <NUM>, <NUM> is possible if material is to be saved to reduce a cost of production of the photonic integrated circuit or to simplify the co-integration with electrical, photonic, or micro-electro-mechanical integrated circuits.

The design of the photonic integrated circuit advantageously permits the integration of additional optical resonators, which share with the resonator device, or at least with a portion of the resonator device, one chip formed by the continuous first waveguide substrate <NUM>, <NUM> and the second waveguide substrate <NUM>, <NUM>. In one embodiment, a third mirror with a third mirror substrate forms an end mirror of a second optical resonator, and the third mirror substrate is provided by or fixed to the chip. In another embodiment, also a fourth mirror substrate of a fourth mirror forming an end mirror of the second optical resonator is provided by or fixed to the chip. The photonic integrated circuit can comprise a third waveguide, with at least a portion of the third waveguide in the second optical resonator. In yet another embodiment, the third waveguide is in contact with the chip, so that it is in contact with the first waveguide substrate <NUM>, <NUM> or the second waveguide substrate <NUM>, <NUM>. A fourth waveguide can be aligned at a fourth end of the fourth waveguide with the third waveguide at a third end of the third waveguide. The integration of multiple optical resonators in one photonic integrated circuit is not limited to two optical resonators in a single photonic integrated circuit, but a multitude of resonators can in a straightforward way be integrated in the same photonic integrated circuit. In an advanced application, tens or hundreds or thousands of optical resonators or even more may be integrated in the same photonic integrated circuit. In some embodiments, both end mirrors or only one end mirror of an additional optical resonator or multiple additional optical resonators are/is in contact with the first waveguide substrate <NUM>, <NUM> or the second waveguide substrate <NUM>, <NUM>. One embodiment comprises an additional waveguide at least part of which is located within an additional optical resonator. The additional waveguide may comprise an additional nonlinear optical medium. The additional nonlinear optical medium may be configured to support parametric down conversion, second harmonic generation, sum-frequency generation, difference-frequency generation, four-wave mixing, optical parametric amplification, optical parametric oscillation, white-light generation, Kerr lensing, two-photon absorption, or multi-photon absorption. The waveguide within an additional optical resonator of the additional optical resonator can be in contact with the first waveguide substrate <NUM>, <NUM> or the second waveguide substrate <NUM>, <NUM> in one embodiment. Additional waveguides can be aligned at their ends with corresponding ends of waveguides which are at least partially located within the additional optical resonators.

<FIG> and <FIG> illustrate examples of optical devices comprising one or more photonic integrated circuits <NUM>, <NUM>. In particular, <FIG> shows devices in which the photonic integrated circuit <NUM>, <NUM> serves as or is comprised in an optical parametric oscillator <NUM> or a photon-pair source <NUM>. <FIG> illustrate devices for frequency-comb spectroscopy <NUM>, <NUM> in particular devices for single-comb spectroscopy <NUM> or dual-comb spectroscopy <NUM>. Accordingly, a photonic chip <NUM> comprises one optical parametric oscillator <NUM> with at least one photonic integrated circuit <NUM>, <NUM>, or two optical parametric oscillators <NUM> with at least two photonic integrated circuits <NUM>, <NUM>. In case of two or more optical parametric oscillators, a multiplexer <NUM> combines the generated light from the two optical parametric oscillators. Waveguides <NUM> are used to guide the light on the chip. The light then passes through a sample <NUM> that the spectroscopy is performed on and is detected in a detector <NUM>. The detectors <NUM> may perform a homodyne detection. All detectors <NUM> may be co-integrated on the same chip as the photonic integrated circuit, or the detectors <NUM> may all be separate from the chip <NUM>, <NUM> with the photonic integrated circuit as shown in <FIG>, or at least one of the detectors <NUM> may be co-integrated while at least one of the detectors <NUM> is separate from the chip <NUM>, <NUM>. The sample <NUM> may comprise a gas cell. The sample <NUM> may be on the same chip <NUM>, <NUM> as the photonic integrated circuit, or the sample <NUM> may be separate from the chip as shown in <FIG>. The sample <NUM> or at least a portion of the sample <NUM> may be inside of a signal-enhancing optical resonator. The signal-enhancing optical resonator allows for multiple passes of an electromagnetic wave through the sample <NUM>. The signal-enhancing resonator may be or comprise a photonic integrated circuit according to Claim <NUM>.

<FIG> and <FIG> show a device in which the photonic integrated circuit <NUM>, <NUM> serves as or is comprised in a photon-pair source <NUM>. In particular, <FIG> illustrates a device for quantum-key distribution <NUM>. The generated light from the photon-pair source <NUM> passes through a demultiplexer <NUM>, and one portion of the light is detected on a reference photodetector <NUM>. The other portion is sent, possibly after passing through a phase shifter <NUM> and a polarization controller <NUM>, into a channel <NUM> for distribution. It is finally detected by a receiver <NUM> at another end of the channel.

<FIG> illustrates a device for quantum computing with photon pairs. A photonic chip <NUM> comprises several photon-pair sources <NUM>, each being or comprising a photonic integrated circuit <NUM>, <NUM>. After passing a demultiplexer <NUM>, the photon pairs are combined in a network of phase shifters <NUM> and beam splitters <NUM> to finally reach detectors <NUM>.

<FIG> illustrates optical devices in which the photonic integrated circuit <NUM>, <NUM> acts as or is integrated in a squeezed-light source <NUM>. For example, in a device for squeezed-light encoding <NUM>, the generated electromagnetic wave passes through an adjustable phase shifter <NUM> and an optical I/Q modulator <NUM> before it is sent into a channel <NUM> for communication, where it is detected by a receiver <NUM> at another end of the channel. Waveguides <NUM> are used to guide the light on the chip. <FIG> illustrates an optical device for quantum key distribution, comprising two squeezed-light sources <NUM> on a single photonic chip <NUM>, each squeezed-light source comprising or being a photonic integrated circuit <NUM>, <NUM>. A first electromagnetic wave generated in the first squeezed-light source passes a phase shifter <NUM> and a second electromagnetic wave generated in the second squeezed-light source passes another phase shifter <NUM> before they are combined at a beam splitter <NUM>. An output beam of the beam splitter <NUM> is detected at a photodiode <NUM>, whereas another output beam of the beam splitter <NUM> is coupled into a channel <NUM> for communication, where it is finally detected by a receiver <NUM> at another end of the channel. <FIG> illustrates an optical device for quantum computing with squeezed light <NUM>. Similar to the device for quantum computing with photon pairs <NUM>, the device <NUM> comprises several photonic integrated circuits <NUM>, <NUM> serving as light sources, in this case as squeezed-light sources <NUM>, and the generated electromagnetic waves are coupled into a network comprising phase shifters <NUM>, beam splitters <NUM>, and delay lines <NUM>, basically acting as interferometers <NUM>. An output state is probed at detectors <NUM>.

The optical devices illustrated in <FIG> and <FIG> are only examples of optical devices that the photonic integrated circuit can be advantageously applied in, and the comprised components are only examples of components that can be combined on a photonic chip or in a photon integrated circuit together with the photonic integrated circuit according to Claim <NUM>. For example, at least one of the following can be co-integrated: a sensor, a Peltier element, or a heat sink. Co-integration of any other optical, electrical, or micro-electro-mechanical system component is possible.

As illustrated for one embodiment in <FIG>, the photonic integrated circuit may further comprise one or several heat sinks <NUM> in contact with the first waveguide substrate <NUM> or the second waveguide substrate <NUM>. <FIG> shows a cross-sectional side view of an embodiment of the photonic integrated circuit with heat sinks <NUM> on both sides of a chip <NUM>, <NUM> formed by the continuous first waveguide substrate <NUM> and second waveguide substrate <NUM>. In some embodiments, the photonic integrated circuit can have only one heat sink <NUM> on one side of the chip <NUM>, <NUM>. In particular, the heat sink <NUM> may be at a bottom side of the chip, whereas the first waveguide <NUM>, <NUM> and the second waveguide <NUM>, <NUM> are on a top side of the chip. The design of the photonic integrated circuit with the first waveguide and second waveguide aligned allows for a large contact area between the heat sink <NUM> and the photonic integrated circuit <NUM>, <NUM>. This is an advantage in devices comprising the photonic integrated circuits, in particular in devices in which a stable temperature is critical. In particular, a strong requirement in terms of temperature stability of known sources of squeezed light, for example for squeezed state encoding <NUM>, quantum key distribution <NUM>, or quantum computing <NUM>, are fulfilled by the photonic integrated circuit. In one embodiment, the thermal conductivity of the heat sink <NUM> exceeds <NUM> W/(m·K). In one embodiment, the heat sink <NUM> comprises diamond, or silicon carbide, or aluminum nitride, copper, silver or gold. In some embodiments, the heat sink <NUM> comprises diamond grown by chemical vapor deposition (CVD) or silicon carbide grown by CVD.

<FIG> illustrates a method <NUM> for producing a photonic integrated circuit. The method comprises providing <NUM> a first waveguide substrate <NUM>, <NUM> and providing <NUM> a second waveguide substrate <NUM>, <NUM>. The method further comprises providing <NUM> a first mirror <NUM>, <NUM> comprising a first layered structure <NUM>, <NUM> on a first mirror substrate <NUM>, <NUM>, as well as providing <NUM> a second mirror <NUM>, <NUM> comprising a second layered structure <NUM>, <NUM> on a second mirror substrate <NUM>, <NUM>. The method further comprises forming <NUM> a first waveguide <NUM>, <NUM> on the first waveguide substrate <NUM>, <NUM>, and forming <NUM> a second waveguide <NUM>, <NUM> on the second waveguide substrate <NUM>, <NUM>. The method is characterized in that it comprises the following steps: positioning <NUM> the first mirror <NUM>, <NUM> with respect to the first waveguide substrate <NUM>, <NUM> or the second waveguide substrate <NUM>, <NUM>, and fixing <NUM> the first mirror <NUM>, <NUM> to the first waveguide substrate <NUM>, <NUM> or to the second waveguide substrate <NUM>, <NUM>. The positioning <NUM> and fixing <NUM> is done such that the first mirror <NUM>, <NUM> and the second mirror <NUM>, <NUM> form end mirrors of an optical resonator and that at least a portion <NUM>, <NUM> of the first waveguide is within the optical resonator, and that the first waveguide substrate <NUM>, <NUM> is continuous with the second waveguide substrate <NUM>, <NUM>, and that the first waveguide <NUM>, <NUM> at a first end <NUM>, <NUM> of the first waveguide is aligned with the second waveguide <NUM>, <NUM> at a second end <NUM>, <NUM> of the second waveguide.

The method may comprise all of these process steps, but not necessarily in the given order. The order of at least some of the process steps can be switched easily, without deviating from the disclosure.

<FIG> and <FIG> illustrate embodiments of the method <NUM> in which the order of some of the process steps varies. In particular, forming <NUM>, <NUM> the first waveguide <NUM>, <NUM> or the second waveguide <NUM>, <NUM> is performed prior to the positioning <NUM> and the fixing <NUM> according to one embodiment of the method <NUM> for producing the photonic integrated circuit illustrated in <FIG>. In another embodiment of the method <NUM>, <FIG>, forming <NUM>, <NUM> the first waveguide <NUM>, <NUM> or the second waveguide <NUM>, <NUM> happens after the positioning <NUM> and the fixing <NUM>. In particular, forming <NUM>, <NUM> the first waveguide <NUM>, <NUM> or the second waveguide <NUM>, <NUM> after the positioning <NUM> and the fixing <NUM> ensures the alignment of the first waveguide <NUM>, <NUM> at the first end <NUM>, <NUM> and the second waveguide <NUM>, <NUM> at the second end <NUM>, <NUM>. This removes the risk of misalignment during the positioning step. This approach has the advantage of making the production more reliable. However, forming <NUM>, <NUM> the first waveguide <NUM>, <NUM> or the second waveguide <NUM>, <NUM> after the positioning <NUM> and fixing <NUM> may involve processing routines, which can damage the first layered structure <NUM>, <NUM> or the second layered structure <NUM>, <NUM>.

The method may comprise forming alignment structures on the first mirror substrate <NUM>, <NUM> or the second mirror substrate <NUM>, <NUM>. These alignment structures may comprise alignment marks, which are configured to give an optical indication of the position of the first mirror substrate <NUM>, <NUM> or the second mirror substrate <NUM>, <NUM>. The alignment structures may also comprise alignment slots <NUM> on or in the first mirror substrate <NUM>, <NUM> or the second mirror substrate <NUM>, <NUM>. The positioning <NUM> can comprise optically aligning the mirror substrate with respect to the first waveguide substrate <NUM>, <NUM> or the second waveguide substrate <NUM>, <NUM> using the alignment structures. The positioning <NUM> can comprise moving the first mirror substrate <NUM>, <NUM> with respect to the first waveguide substrate <NUM>, <NUM> or the second waveguide substrate <NUM>, <NUM>, with the first mirror substrate <NUM>, <NUM> in contact with the alignment slots <NUM>. Alignment marks may be formed on the first waveguide substrate <NUM>, <NUM> or the second waveguide substrate <NUM>, <NUM> in addition to the alignment structures formed on the first mirror substrate <NUM>, <NUM> or the second mirror substrate <NUM>, <NUM>. Positioning the first mirror with respect to the first waveguide substrate or the second waveguide substrate may comprise aligning the first mirror with respect to the first waveguide substrate or the second waveguide substrate according to at least one mechanical contact point provided by at least one alignment slot.

The method may further comprise polishing the first mirror substrate <NUM>, <NUM> or the second mirror substrate <NUM>, <NUM>. In particular, the polishing step may comprise super polishing. The polishing step may involve polishing a section of the mirror substrates to a roughness of RMS < <NUM>, in particular < <NUM>. This may be useful for ultra-performance mirrors (reflectivity exceeding <NUM> %), while for all other applications industrial grade polishing (> <NUM> %, > <NUM> %, or > <NUM> %) could be sufficient.

The method can further comprise depositing the first layered structure <NUM>, <NUM> or the second layered structure <NUM>, <NUM>. The depositing may comprise evaporative deposition or sputter-deposition or ion-beam sputtering or magnetron sputtering. In one embodiment, at least one layer of the first layered structure <NUM>, <NUM> or the second layered structure <NUM>, <NUM> is produced using molecular beam deposition or molecular beam epitaxy (MBE). MBE results in the formation of at least one epitaxial layer, wherein the material forming the epitaxial layer can be the same or different from the material forming the substrate. If depositing the first layered structure <NUM>, <NUM> or the second layered structure <NUM>, <NUM> comprises molecular beam epitaxy, the layered structure is usually grown on a mirror substrate, which comprises crystalline material. In yet another embodiment, a layer of the layered structure of the mirror is deposited using atomic layer deposition. The production may further comprise removing at least a portion of the first mirror substrate <NUM>, <NUM> or the second mirror substrate <NUM>, <NUM> in a production step after fixing <NUM> the first mirror <NUM>, <NUM> to the first waveguide substrate <NUM>, <NUM> or the second waveguide substrate <NUM>, <NUM>. Depositing the first layered structure <NUM>, <NUM> or depositing the second layered structure <NUM>, <NUM> may comprise depositing a layer of the first layered structure <NUM>, <NUM> or the second layered structure <NUM>, <NUM> on the first waveguide <NUM>, <NUM> or the second waveguide <NUM>, <NUM>.

<FIG> illustrates an embodiment of the method <NUM>, wherein the positioning <NUM> and fixing <NUM> of the first mirror <NUM>, <NUM> comprises a microassembly step or a robotic pick-and-place step <NUM>. The microassembly step or robotic pick-and-place step <NUM> comprises picking up the first mirror substrate <NUM>, <NUM> by an automated device and placing it with a precision better than <NUM>, in particular better than <NUM>. Prior to the positioning <NUM>, <NUM> and fixing <NUM>, <NUM> a recess may be formed <NUM> in the first waveguide substrate <NUM>, <NUM> or the second waveguide substrate <NUM>, <NUM>, such that at least a part of the first mirror <NUM>, <NUM> is in the recess after the positioning <NUM> and fixing <NUM>.

<FIG> illustrates an embodiment of the method <NUM>, wherein the positioning <NUM> and fixing <NUM> comprise direct bonding <NUM> the first waveguide substrate <NUM> and the second waveguide substrate <NUM> The direct bonding may for example comprise surface activated bonding, plasma activated bonding, adhesive bonding, anodic bonding, eutectic bonding, glass frit bonding, thermocompression bonding, or reactive bonding. The direct bonding can, for example, comprise a chemical cleaning step using a solvent <NUM> as illustrated in <FIG>. The direct bonding can, for example, comprise a treatment with a plasma <NUM> as illustrated in <FIG>. The direct bonding can, for example, comprise a direct bonding of the first mirror <NUM> and the first waveguide substrate <NUM> or the second waveguide substrate <NUM> in a vacuum apparatus <NUM> as illustrated in <FIG>. An intermediate layer may be formed between the first mirror <NUM> and the first waveguide substrate <NUM> or the second waveguide substrate <NUM>. The intermediate layer can comprise silicon or silicon oxide.

The first mirror substrate <NUM>, <NUM> can have a thickness of at least <NUM>, in particular of at least <NUM>. The first waveguide substrate <NUM>, <NUM> or the second waveguide substrate <NUM>, <NUM> can have a thickness of at least <NUM>, in particular of at least <NUM>. Also, the first waveguide substrate <NUM>, <NUM> or the second waveguide substrate <NUM>, <NUM> can be attached to an auxiliary substrate with a thickness of at least <NUM>, in particular of at least <NUM>. A sufficient thickness of the first mirror substrate <NUM>, <NUM> or the first waveguide substrate <NUM>, <NUM> or the second waveguide substrate <NUM>, <NUM> or the auxiliary substrate advantageously makes handling the substrate more reliable and avoids breaking a substrate during the positioning <NUM> and fixing <NUM>. It also improves an adhesion between the first mirror <NUM>, <NUM> and the first waveguide substrate <NUM>, <NUM> or the second waveguide substrate <NUM>, <NUM>, making a joint more reliable, and reducing the danger that the joint breaks in the presence of movements and vibrations.

Claim 1:
A photonic integrated circuit (<NUM>, <NUM>), comprising:
a first waveguide (<NUM>, <NUM>) on a first waveguide substrate (<NUM>, <NUM>),
a second waveguide (<NUM>, <NUM>) on a second waveguide substrate (<NUM>, <NUM>),
a resonator device, comprising:
a first mirror (<NUM>, <NUM>), comprising a first layered structure (<NUM>, <NUM>) on a first mirror substrate (<NUM>, <NUM>), wherein the first mirror substrate (<NUM>, <NUM>) is provided by or fixed to the first waveguide substrate (<NUM>, <NUM>) or the second waveguide substrate (<NUM>, <NUM>),
a second mirror (<NUM>, <NUM>), comprising a second layered structure (<NUM>, <NUM>) on a second mirror substrate (<NUM>, <NUM>), wherein the second mirror substrate (<NUM>, <NUM>) is provided by or fixed to the first waveguide substrate (<NUM>, <NUM>) or the second waveguide substrate (<NUM>, <NUM>); wherein the first mirror (<NUM>, <NUM>) and the second mirror (<NUM>, <NUM>) form end mirrors of an optical resonator, and wherein at least a portion (<NUM>, <NUM>) of the first waveguide (<NUM>, <NUM>) is within the optical resonator;
wherein the first waveguide substrate (<NUM>, <NUM>) is continuous with the second waveguide substrate (<NUM>, <NUM>), and wherein the first waveguide (<NUM>, <NUM>) at a first end (<NUM>, <NUM>) of the first waveguide is aligned with the second waveguide (<NUM>, <NUM>) at a second end (<NUM>, <NUM>) of the second waveguide (<NUM>, <NUM>)
characterised in that the optical resonator and the second waveguide (<NUM>, <NUM>) are aligned with respect to one another, and in that a portion of the first layered structure (<NUM>, <NUM>) or the second layered structure (<NUM>, <NUM>) is embedded within a continuous body formed by the first waveguide substrate (<NUM>, <NUM>) and the second waveguide substrate (<NUM>, <NUM>).