Frequency converter with optical resonators

Abstract

The invention relates to a frequency converter (1) comprising a first surface (10) intended to receive a signal radiation (S) having a frequency fs and a pump radiation (P) having a frequency fp, the first surface comprising a set of optical resonators (11) configured to generate a converted radiation ( C) from the signal radiation (S) and the pump radiation (P), said converted radiation having the frequency fs + fp, referred to as the sum frequency and denoted f_SFG. The converter further comprises a filtering device (20) and a second surface (30) configured to receive the converted radiation (C) and to focus it outside the converter. The filtering device is configured to receive the converted radiation and the pump radiation from the first surface (10) and to limit the propagation of the pump radiation from the first surface (10) to the second surface (30).

Description

[0001] "Optical resonator frequency converter"

[0002] TECHNICAL FIELD OF THE INVENTION

[0003] The present invention relates to the field of frequency conversion. Its advantageous application lies in the field of photodetection, and particularly in the field of detecting radiation in the near-infrared range.

[0004] STATE OF THE ART

[0005] Existing photodiodes such as single-photon avalanche photodiodes (commonly referred to as SPADs) are now fully functional for detecting visible radiation. These photodiodes operate at temperatures around 250 K, which can be easily achieved with the aid of coolers. SPADs can therefore be used in numerous applications without requiring particularly complex setups for detection. While SPADs perform admirably in the visible spectrum, they are not functional in the infrared, typically for wavelengths above 1.1 pm in the case of silicon-based SPADs.

[0006] However, a very large number of application areas involve the manipulation of radiation in the infrared range, and particularly in the SWIR (Short Wavelength Infrared, which extends from 1.4 to 3 pm). This is notably the case for fiber optic communication, telemetry, laser remote sensing (commonly referred to as LiDAR, from the English "Light Detection And Ranging"), environmental analysis, defect analysis in microelectronic chips, and even the analysis of biological tissues.

[0007] Actors in these various fields are currently forced to use bulky and expensive setups to detect signals of interest. For example, it is possible to use superconducting nanowire single-photon detectors (SNSPDs). However, this type of detector only operates at extremely low temperatures (typically below 3 K), which necessitates the use of cryogenic systems. The use of SNSPDs is therefore limited to research laboratories or advanced industrial platforms.

[0008] The papers “Infrared imaging in nonlinear GaAs metasurfaces”, Camacho Morales Maria et al., SPIE PROCEEDINGS, 2019 and US 2020 / 209708 A1 disclose non-optimal infrared signal detection solutions.

[0009] There is therefore a significant need for a high-performance signal detection solution in the infrared range. Ideally, this solution can be used outside of research laboratories and will not require integration into a bulky system.

[0010] SUMMARY OF THE INVENTION

[0011] To achieve this objective, a first object of the invention relates to a frequency converter characterized in that it comprises, stacked according to a stacking direction:

[0012] - a first surface intended to receive radiation, called signal radiation, having a frequency called signal frequency f s and a radiation called pump radiation having a frequency called pump frequency f pThe first surface comprises a set of optical resonators configured to generate, from the signal radiation and the pump radiation, radiation called converted radiation having a frequency equal to the sum of the signal frequency f s and the pump frequency f p , called sum frequency and denoted fsFG,

[0013] - a filtering device,

[0014] - a second surface configured to receive the converted radiation and to focus it away from the converter.

[0015] The filtering device is advantageously configured to receive the converted radiation and the pump radiation from the first surface and to limit, preferably prevent, the propagation of the pump radiation from the first surface to the second surface.

[0016] The converter thus generates converted radiation with a wavelength shorter than that of the signal radiation. The frequency response required to detect the converted radiation is therefore distinct from that required to detect the signal radiation. The converter allows the signal frequency to be changed so that it is detectable by a common detection device such as a photodiode. In particular, the converter can change the signal frequency so that the radiation at the converter's output is detectable by a silicon-based photodiode.

[0017] The present invention exploits the phenomenon of sum frequency generation, which notably allows the input radiation to have distinct wavelengths and powers. This phenomenon can therefore be exploited even if the signal radiation is low power. The signal radiation can be mixed with a powerful pump radiation, thus enabling high frequency conversion efficiency. Furthermore, the ability to vary the wavelengths of the signal radiation and the pump radiation independently of each other expands the range of wavelengths that can be converted.

[0018] Furthermore, the converter according to the invention does not require phase matching between the signal radiation and the pump radiation. This eliminates the need to integrate a phase-matching device, often found in prior art systems and making the assembly particularly bulky.

[0019] Furthermore, the converter can be manufactured by simply transferring and / or bonding the different layers (first surface, filter device, second surface). Specifically, the first and second surfaces can be manufactured independently on two separate substrates and then cut before assembly. The transfer / bonding of these surfaces and the filter device does not add any additional thickness to the assembly.

[0020] The converter according to the present invention is therefore an ultra-compact module that can be very easily integrated into a detection setup in any field of application. It can thus be used for ultra-fast detection of radiation, particularly in the SWIR range. It can be used at room temperature or at temperatures easily achievable with the help of compact coolers. Therefore, the converter according to the invention can be used outside of research laboratories or advanced technological platforms.

[0021] A second object of the present invention relates to a detector comprising: - a converter according to the first object of the invention,

[0022] - a photodiode, the second surface of the converter being configured to focus the converted radiation onto the photodiode.

[0023] The detector according to the present invention can therefore benefit from the high responsiveness of prior art photodiodes, such as SPADs, for the detection of radiation in a range that could not previously be analyzed using these photodiodes.

[0024] A third object of the present invention relates to an assembly comprising a converter according to the first object of the invention and an optical fiber suitable for guiding the signal radiation and the pump radiation towards the first surface of the converter.

[0025] A fourth object of the present invention relates to an assembly comprising a detector according to the second aspect of the invention and an optical fiber suitable for guiding the signal radiation and the pump radiation towards the first surface of the converter.

[0026] Advantageously, in the third and fourth objects, the optical fiber comprises two branches, each intended to carry radiation between the signal radiation and the pump radiation.

[0027] The advantages described with reference to the converter according to the first aspect of the invention apply mutatis mutandis to the other objects of the invention, and vice versa.

[0028] The invention therefore addresses the problem of detecting signals that are outside the detection range of photodetectors commonly used today.

[0029] BRIEF DESCRIPTION OF THE FIGURES

[0030] The aims, objects, features and advantages of the invention will become clearer from the detailed description of an embodiment thereof, which is illustrated by the following accompanying drawings in which:

[0031] Figure 1A represents a device according to the invention, in which the focusing of the converted radiation onto a photodiode is achieved by an array of nanoguides.

[0032] Figure 1B represents a device according to the invention, in which the focusing of the converted radiation onto a photodiode is achieved by a microlens.

[0033] Figures 2A, 2B and 2C represent different possible structures for the optical nanoresonators of the first surface.

[0034] Figure 3 represents an assembly according to the invention comprising an optical fiber carrying the signal radiation and the pump radiation to the converter according to the invention.

[0035] The drawings are provided as examples and are not intended to limit the scope of the invention. They are schematic representations of the principle intended to facilitate understanding of the invention and are not necessarily to scale with practical applications. In particular, the dimensions are not representative of reality.

[0036] DETAILED DESCRIPTION OF THE INVENTION

[0037] Before proceeding with a detailed review of embodiments of the invention, optional features that may be used in combination or alternatively are listed below:

[0038] In a preferred embodiment, the converted radiation has a wavelength called the converted wavelength, denoted ÀSFG, and the filtering device includes at least one absorption filter capable of absorbing any radiation with a wavelength strictly shorter than ÀSFG. The absorption filter thus prevents the output signal of the converter from being contaminated by frequencies unrelated to the signal radiation. In particular, the absorption filter filters out any radiation with a wavelength of À p / 2 (At p being the wavelength of the pump radiation) generated at the same time as the SFG phenomenon.

[0039] According to a preferred embodiment, the filtering device comprises at least one absorption filter capable of absorbing any radiation having a frequency equal to 2*f p This prevents the harmonic at wavelength A p / 2 (i.e., at the frequency 2*f)p ) generated during the SFG phenomenon does not propagate to the output of the converter and pollutes the detection of the converted radiation.

[0040] According to a preferred embodiment, the filtering device includes a reflector capable of reflecting any radiation having a frequency at the pump frequency f p This reflector prevents the pump radiation P from propagating to the converter output and interfering with the detection of the converted radiation. The reflector can, in particular, be a distributed Bragg reflector.

[0041] In one example, the second surface comprises an array of nanoguides configured to focus the converted radiation away from the converter. Focusing by an array of nanoguides allows, in particular, for optimal compactness of the converter.

[0042] According to one example, the second surface includes at least one lens configured to focus the converted radiation out of the converter.

[0043] According to one example, the converter further includes a source generating pump radiation, and configured so that the first surface receives the pump radiation from the source.

[0044] According to a preferred embodiment, the sum frequency fsFG belongs to a predetermined frequency range of interest, for example, a high-responsiveness frequency range of a photodetector. The signal frequencies f s and pump f pcan thus be chosen such that fsFG lies within this predetermined frequency range of interest. The frequency range of interest may, in particular, correspond to a high-responsiveness frequency range of a photodetector, which is directly related, in the case of a semiconductor-based photodiode, to the absorption band of said semiconductor.

[0045] According to a preferred embodiment of the detector according to the second aspect of the invention, the photodiode is silicon-based. Silicon has an advantageous band gap for detecting radiation converted from a SWIR signal.

[0046] According to a preferred example, the photodiode is a single-photon avalanche diode. The terms "approximately", "about", "on the order of" mean, when referring to a value, "to within 10%, preferably to within 5%" of that value.

[0047] A coordinate system, preferably orthonormal, comprising the X, Y, and Z axes is shown in Figures 1A, 1B, 2A, 2B, and 2C. This coordinate system can be applied by extension to the other figures. The X-axis extends along a first direction X, the Y-axis along a second direction Y, and the Z-axis along a third direction Z, also called the stacking direction Z. The plane formed by the first and second directions X and Y is called the transverse plane XY.

[0048] In this patent application, the terms thickness for a layer and height for a structure or device will be preferred. Height is measured perpendicular to the transverse XY plane. Thickness is measured along a direction normal to the principal extension plane of the layer. Thus, a layer typically has a thickness along the Z-axis when it extends primarily along the transverse XY plane. The relative terms "on," "under," and "below" preferentially refer to positions measured along the stacking direction Z.

[0049] The converter 1 according to the present invention will now be described with reference to Figures 1A and 1B.

[0050] Converter 1 includes, in particular, the following components stacked along a stacking direction Z:

[0051] A first surface 10 intended to receive a signal radiation S and a pump radiation P and to generate a converted radiation C (which can also be designated "SFG radiation" or "SFG converted radiation") having a frequency equal to the sum of the frequencies of the signal radiation S and the pump radiation P by a process called sum frequency generation,

[0052] - A filtering device 20 designed to filter signals that could interfere with the measurement at the output of converter 1,

[0053] - A second surface 30 intended to focus the converted radiation C out of the converter 1, typically onto a detection system such as a photodiode.

[0054] The filter device 20 is thus located between the first surface 10 and the second surface 30 along the stacking direction Z. The filter device 20 preferably completely separates the first surface 10 and the second surface 30 along the stacking direction Z.

[0055] The signal radiation S received by the first surface 10 has a signal wavelength λ s and a signal frequency f s .

[0056] The pump radiation P received by the first surface 10 has a pump wavelength λ p and a pump frequency f p .

[0057] The converted radiation C generated by the first surface 10 and received and focused by the second surface 30 has a converted wavelength λSFG and a converted frequency fsFG. The converted wavelength λSFG can also be designated as the SFG wavelength λSFG. The converted frequency fsFG can also be designated as the SFG frequency fsFG. The first surface 10, the filtering device 20, and the second surface 30 will now be described in more detail.

[0058] The first surface 10 comprises a set of optical resonators 11. It also typically includes a substrate 15 on which the optical resonators 11 are formed. Within the converter 1, the substrate 15 thus extends between the optical resonators 11 and the filtering device 20. The substrate 15 can, for example, be a transparent sapphire substrate.

[0059] The optical resonators 11 are configured to be the site of a sum frequency generation (SFG) phenomenon. In particular, the optical resonators 11 have specific dimensions and shapes that allow this nonlinear optical phenomenon to be observed. These structural characteristics will be described later.

[0060] The SFG phenomenon occurring at the first surface 10 allows the generation of converted radiation C, whose wavelength ÀSFG satisfies the following equation:

[0061] "

[0062]

[0063] Either :

[0064] fsFG = fs + fp

[0065] The wavelengths of the signal radiation and the P-pump radiation can, for example, be found in the following ranges:

[0066] - HAS scan be between 1.4 and 3 pm, for example, be approximately equal to 1.55 pm, the wavelength of quantum communications. According to another example, AT s is between 2 and 2.5 pm, a range in which there are signals indicating the presence of methane (the converter can then be used in a methane detection setup).

[0067] - HAS p may be between 1 and 1.4 pm, for example be approximately equal to 1.3 pm, a wavelength emitted by several commercial lasers of suitable power and dimensions.

[0068] It is understood that A s is not fixed but can vary depending on the user's needs. s will be specific to each experiment.

[0069] Converter 1 is typically used in conjunction with a source to generate pump radiation P. The wavelength λ pis therefore generally fixed. The source of the pump radiation P - and therefore its wavelength λ p - can however be chosen according to the intended applications.

[0070] Preferably, for a signal wavelength A s given (or at least for a range of values ​​in which A s (can be found), the wavelength of the pump A p will be chosen so that the wavelength ÀSFG is between 0.7 and 1.1 pm. This wavelength range corresponds to a high responsiveness range for silicon photodiodes that can be used to detect the converted radiation. Advantageously, À s and p are such that ÀSFG is approximately equal to 0.9 pm. This value corresponds to the maximum responsiveness of silicon. This ensures optimal detection of the radiation converted by the detection device that can be associated with the converter according to the invention.

[0071] The source of P-pump radiation is typically a YAG laser (Yttrium Aluminum Garnet), of which several very compact versions exist, emitting wavelengths from 1.03 pm to 1.32 pm, or a semiconductor laser diode. Furthermore, P-pump radiation can be injected continuously or in pulses. In the former case, it preferably has a power greater than or equal to 1 mW and / or preferably less than or equal to 1 W. In the latter case, it preferably has a pulse energy greater than or equal to 1 nJ and / or preferably less than or equal to 100 nJ. This allows for the development of the SFG phenomenon without excessive energy expenditure and without risking damage to the device.

[0072] The structure of optical resonators 11 will now be described in more detail. Optical resonators 11 can be based on different materials such as AI-GaAs, GaP, LiNbOa or any other material exhibiting quadratic nonlinearity.

[0073] In the case of AIGaAs, optical resonators 11 are typically formed from (100) or (110) AIGaAs substrates. The properties of (100) substrates are very well known in the technical field. (110) substrates have the advantage of possessing selection rules in nonlinear optics that are beneficial for the intended applications.

[0074] Optical resonators 11 are typically manufactured by depositing a continuous layer of AIGaAs or GaP onto the substrate 15 and then by a lithography step.

[0075] To simplify the converter manufacturing process, the optical resonators 11 are preferably all formed from the same material and / or preferably all have the same geometry. However, it is possible that they may have different geometries.

[0076] Typically, the first surface 10 contains at least 100,000 optical resonators.

[0077] Optical resonators 11 typically have a geometry that does not follow a rotational symmetry about an axis parallel to the stacking direction Z. Optical resonators 11 can, for example, follow one of the geometries illustrated in figures 2A to 2C.

[0078] Figure 2A illustrates an example of an optical resonator 11 in the form of a nanochair. In this example, the optical resonator 11 has a substantially circular shape in the transverse XY plane. It has a general shape of a cylinder with a circular base truncated over only part of its height. In other words, the general shape of the optical resonator 11 corresponds to a full circular cylinder and a truncated circular cylinder superimposed on each other and sharing the same base.

[0079] Typically, the truncated cylinder forms a semi-disk in projection in the transverse XY plane.

[0080] The characteristic dimensions of the optical resonator 11 in this first example are as follows: - hia: total height of the optical resonator 11 measured along the stacking direction Z, also corresponding to the sum of the heights of the full cylinder and the truncated cylinder,

[0081] - h2a: local height of optical resonator 11, measured along the stacking direction Z, also corresponding to the height of the entire cylinder, - Ha: width, measured along the first direction X, of the truncated cylinder.

[0082] - r2a: half the width, measured along the first Y direction, of the truncated cylinder. Typically, ri a =r2a.

[0083] - d a : characteristic dimension (typically diameter) of the base of the cylinder giving its general shape to the optical resonator 11. d a is measured in the transverse XY plane.

[0084] The dimensions hi a , h2a, Ha, r2a, and d a are typically shorter than the two wavelengths A s and A p signal radiation S and pump radiation P. For example, for A s = To p =1.55 pm and ÀSFG = 0.775 pm, we can choose h2a = n a = r2a = 200nm and hi a = d a = 400nm.

[0085] According to another example illustrated in Figure 2B, the optical resonators 11 have the shape of a cylinder whose base in the transverse XY plane has the shape of a right-angled polygon, thus forming a truncated rectangle. In other words, their base has the shape of a primary rectangle attached to a smaller secondary rectangle, which protrudes from one of the sides of the primary rectangle.

[0086] The characteristic dimensions of the optical resonator 11 in this second example are as follows:

[0087] - Lib: width of the main rectangle, measured in the transverse plane XY, here along the second direction Y,

[0088] - L2b: length of the main rectangle, measured in the transverse plane XY, here along the first direction X,

[0089] - A: width of the secondary rectangle, measured in the transverse plane XY, here along the second direction Y,

[0090] - Û2b: length of the secondary rectangle, measured in the transverse plane XY, here along the first direction X,

[0091] - hb: height of optical resonator 11, measured along the stacking direction Z.

[0092] The total lengths and widths (L and L2b+Aib) of optical resonator 11 are each less than the wavelengths A s and A p signal radiation S and pump radiation P. For example, for A s = To p = 1.4 pm and ASFG = 0.7 pm, L = 315 nm, L2b = 315 nm, A = 130 nm, A2b = 210 nm, hb = 315 nm. According to another example illustrated in Figure 2C, the optical resonators 11 have the shape of a rectangular parallelepiped with a through-hole along the stacking direction Z. This through-hole can, for example, project onto the transverse XY plane in the shape of two isosceles triangles sharing a common base, as illustrated.

[0093] The characteristic dimensions of the optical resonator 11 in this third example are as follows:

[0094] - L c : length of the rectangular parallelepiped, measured in the transverse plane XY, here along the first direction X,

[0095] - t c : height of the rectangular parallelepiped, measured along the stacking direction Z,

[0096] - d c : length of the base common to the two isosceles triangles, giving the hollow its general shape. d c is measured in the transverse XY plane, here along the first X direction. d c can also be seen as the width of the recess.

[0097] - hiccup: height of the first isosceles triangle giving its general shape to the hollowing out,

[0098] - h2c: height of the second isosceles triangle giving its general shape to the hollowing.

[0099] The L dimension c is shorter than the wavelengths As and A p signal radiation S and pump radiation P. For example, for A s = 1.58 pm, AT p =1.36 pm and ÀSFG = 0.731 pm, we can choose L c = 600nm, L = 200nm, d c = 120nm, hi c = 200nm and h2 C =100nm.

[0100] The optical resonators 11 can be designated as "meta-atoms" due to their dimensions. Due to the presence of these meta-atoms and the SFG phenomenon occurring at the level of the first surface 10, the latter can be designated as a "nonlinear metasurface".

[0101] The geometries described above do not constitute an exhaustive list of possible shapes for optical resonators. An infinite number of shapes and dimensions are conceivable and can enable the SFG phenomenon. However, certain geometries and dimensions allow for a higher conversion factor of P and S radiation to converted C radiation. The structures illustrated in Figures 2B and 2C, in particular, have yielded especially interesting results. These results are explained, notably, by the symmetry breaking present in these structures.

[0102] Following its generation at the first surface 10, the converted C radiation is directed towards the filtering device 20.

[0103] The filtering device 20 has the function of limiting the propagation from the first surface 10 to the second surface 30 of radiation which could disrupt the measurement at the output of the converter 1.

[0104] Indeed, the first surface 10 generates not only the converted SFG C radiation but also other harmonics, such as parasitic radiation with a wavelength of A p / 2 (or for frequency 2*f p ).

[0105] Furthermore, part of the pump radiation P is transmitted through the first surface 10. However, this radiation P is particularly powerful and could strongly impact the measurement typically carried out at the output of converter 1.

[0106] The filtering device 20 is thus configured at least to limit, preferably completely prevent, the propagation of any radiation at wavelength A pbetween the first surface 10 and the second surface 30.

[0107] Preferably, the filtering device 20 is also configured to limit or even prevent the propagation of harmonics from non-linear phenomena occurring at the level of the first surface 10.

[0108] To achieve this, it is possible, for example, to integrate an absorbing filter 21 configured to absorb all radiation with a wavelength strictly shorter than ÀSFG. Indeed, À p / 2 is typically less than ÀSFG and is therefore filtered by such a filter. The absorbing filter 21 is typically in the form of a semiconductor-based absorbing layer with an energy gap E g suitable, for example a commercial GaAs substrate (E g =1.42 eV) having a thickness of 300 pm.

[0109] It is also possible to integrate within the absorbing device 20 a reflector capable of reflecting the wavelength of pump A p The reflector can, for example, take the form of a multilayer dielectric mirror (see alternating layers 22, 23 in Figures 1A and 1B). This is typically a stack of quarter-wave layers with a high refractive index contrast, for example a TiO2 / SiO2 multilayer stack.

[0110] According to a preferred embodiment, both an absorbing filter 21 and a reflector are integrated within the filtering device 20.

[0111] The converted radiation SFG C passes through the filtering device 20 and thus reaches the level of the second surface 30.

[0112] The function of the second surface 30 is, in particular, to focus the converted SFG C radiation. Preferably, the second surface 30 is configured to focus the converted SFG radiation away from the converter 1, typically onto a detection device such as a photodiode, as will be described later. The second surface 30 therefore includes an element for focusing the converted SFG C radiation. This element can take several forms.

[0113] According to a first example illustrated in Figure 1A, the focusing by the second surface 30 is ensured by a set of structures which could, for example, be:

[0114] - Meta-atoms of identical shape and size but with different orientations,

[0115] - Nanoguides 31, typically cylindrical, with different cross-sections,

[0116] - Huygens nanodiscs.

[0117] This set of structures forms a meta-lens.

[0118] Preferably, the second surface 30 contains at least 100,000 structures.

[0119] This first example is particularly advantageous for miniaturizing the converter. Indeed, the structures mentioned above are very thin, typically on the order of a few hundred nanometers. This makes the converter particularly compact in the Z-stack direction.

[0120] According to a second example illustrated in Figure 1B, focusing by the second surface 30 is ensured by a lens 32. The lens 32 can, for example, be manufactured by nanoprinting or by molding.

[0121] A lens 32 is indeed thicker than a set of structures such as that described in the first example, but its thickness remains quite reasonable in the context of the present invention. As will be described later, in both cases, the thickness along the Z stacking direction of the converter is typically less than 3 mm, a dimension significantly smaller than current devices and allowing for very efficient integration into a detection system.

[0122] The second surface 30 also typically includes a substrate 35 on which the focusing element (i.e., the nanoguide array 31 or the microlens 32) is formed. The substrate 35 can, for example, be a transparent sapphire substrate.

[0123] As mentioned previously, the converter 1 according to the invention is advantageously intended to be used with a detection device on which the converted radiation SFG C is focused.

[0124] Figures 1A and 1B schematically illustrate the association of converter 1 with a detection device. The detection device can, in particular, be a photodiode 2, as illustrated.

[0125] Converter 1 and photodiode 2 thus form a detector 100.

[0126] Photodiode 2 can, for example, be based on one of the following materials: silicon, germanium, indium gallium arsenide.

[0127] Advantageously, the photodiode 2 is placed in a vacuum housing 4. A cooling system can be integrated into the detector 100 to reduce the noise of the photodiode 2 and improve the detection quality.

[0128] Another object of the invention relates to the association of the converter 1 with an optical fiber 3 for transmitting the signal radiation S and / or the pump radiation P to the first surface 10. Advantageously, the optical fiber 3 carries both radiation S and P to the first surface 10. To achieve this, a fiber 3 in the form of a "Y" junction can be used, for example. Such a fiber 3 has two branches 3', 3" in which the signal radiation S and the pump radiation P propagate, respectively. The branch 3" intended to carry the pump radiation P can be directly connected to the source of the pump radiation P.

[0129] Fiber 3 can be attached to converter 1 by a removable fastener, for example by clipping or screwing. This allows the fiber 3 to be changed when the source of the signal radiation S changes, and therefore the same converter 1 to be used for different measurements.

[0130] According to another embodiment, the fiber 3 is glued to the first surface 10 of the converter 1.

[0131] Another object of the invention, illustrated in Figure 3, relates to the association of a detector 100 with a fiber 3 as described above. Such an assembly, comprising an optical fiber 3, a converter 1, and a photodiode 2, forms a complete and functional system from the transmission of the signal radiation S to the detection of the converted radiation SFG.

[0132] The paragraphs below aim to give examples of sizing for converter 1 and detector 100 according to the invention.

[0133] The following characteristic dimensions are defined:

[0134] - hconv: the total height of the converter, measured along the stacking direction Z. hconv is typically measured between the faces of the optical resonators 11 and the nanoguides 31 (or the lens 32) facing outwards from the converter 1.

[0135] - D: the distance between converter 1 and photodiode 2. D is measured between the second surface 30 and the active region of photodiode 2 on which the converted radiation C is focused.

[0136] - h2: the distance between the converter 1 and the optical window of the housing 4 in which the photodiode 2 is located. Regardless of the embodiment chosen for the second surface 30, hconv is typically less than or equal to 3 mm. The converter 1 according to the invention is therefore particularly compact.

[0137] When the focusing of the converted SFG C radiation is ensured at the level of the second surface 30 by an array of nanoguides 21, hcon v is typically less than or equal to 2 mm

[0138] When the photodiode 2 is placed in a housing 4, the latter has an opening, also called an optical window, through which the converted radiation C passes. h2 is typically less than 1 mm, therefore the addition of the converter 1 to the housing 4 does not significantly change the overall size of the latter.

[0139] In view of the different embodiments described above, it appears that the present invention offers a robust and compact solution enabling the frequency conversion and focusing of radiation, which can in particular be used to convert radiation located outside the detectable range by a conventional photodiode into radiation detectable by such a photodiode.

[0140] The invention is not limited to the embodiments previously described and extends to all embodiments covered by the invention.

Claims

DEMANDS 1. Frequency converter (1) characterized in that it comprises, stacked along a stacking direction (Z): - a first surface (10) intended to receive radiation called signal radiation (S) having a frequency called signal frequency f s and a radiation called pump radiation (P) having a frequency called pump frequency f p , the first surface (10) comprising a set of optical resonators (11) configured to generate, from the signal radiation (S) and the pump radiation (P), a radiation called converted radiation (C) having a frequency equal to the sum of the signal frequency f s and the pump frequency f p , called the sum frequency and denoted f S FG, - a filtering device (20), - a second surface (30) configured to receive the converted radiation (C) and to focus it out of the converter (1), the second surface (30) comprising an array of nanoguides (31) configured to focus the converted radiation (C) out of the converter (1), the filtering device (20) being configured to receive the converted radiation (C) and the pump radiation (P) from the first surface (10) and to limit, preferably prevent, the propagation of the pump radiation (P) from the first surface (10) to the second surface (30).

2. Converter according to the preceding claim in which the converted radiation (C) has a wavelength called converted wavelength denoted ÀS G, and the filtering device (20) includes an absorption filter (21) capable of absorbing any radiation having a wavelength strictly less than ÀS G.

3. Converter according to any one of the preceding claims, wherein the filtering device (20) comprises at least one absorption filter (21) capable of absorbing any radiation having a frequency equal to 2*f p .

4. Converter according to any one of the preceding claims, wherein the filtering device (20) comprises a reflector capable of reflecting any radiation having a frequency at the pump frequency f p .

5. Converter according to any one of the preceding claims further comprising a source generating the pump radiation (P), and configured so that the first surface (10) receives the pump radiation (P) from the source.

6. Converter according to any one of the preceding claims wherein the sum frequency fsFG belongs to a predetermined frequency range of interest, for example a high responsiveness frequency range of a photodetector.

7. Detector (100) comprising: - a converter (1) according to any one of the preceding claims, - a photodiode (2), the second surface (30) of the converter (1) being configured to focus the converted radiation (C) onto the photodiode (2).

8. Detector (100) according to the preceding claim in which the photodiode (2) is silicon-based.

9. Detector (100) according to any one of the two preceding claims wherein the photodiode (2) is a single-photon avalanche diode.

10. Assembly comprising a converter (1) according to any one of claims 1 to 6 and an optical fiber (3) capable of guiding the signal radiation (S) and the pump radiation (P) to the first surface (10) of the converter (1).

11. Assembly comprising a detector (100) according to any one of claims 7 to 9 and an optical fiber (3) capable of guiding the signal radiation (S) and the pump radiation (P) to the first surface (10) of the converter (1).

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