Method for manufacturing an integrated electro-optical device, and an electro-optical device obtained in the same manner.

Abstract

The present invention relates to a method for manufacturing an electro-optical device (50), the method comprising the steps of: creating an optical waveguide (5) in an electro-optical substrate (1) by proton exchange by annealing, the steps of: a) exposing a region of the surface (2) of the substrate (1) to a proton source to form a layer (3) modified by proton exchange; and b) thermal annealing the substrate to form an optical waveguide (5) in a diffusion region (4); and depositing a pair of electrodes (6, 7), wherein the optical waveguide (5) extends longitudinally between the pair of electrodes (6, 7). According to the present invention, the method further comprises the step of machining the remaining portion of the layer (3) after the thermal annealing step to form machined regions (10, 11, 12) at least partially located between the pair of electrodes (6, 7).

Description

[Technical Field]

[0001] The present invention relates to the technical field of electro-optical devices and methods for manufacturing the same.

[0002] Specifically, the present invention relates to a method for manufacturing an electro-optical device integrated on an electro-optical substrate.

[0003] The present invention also relates to an integrated electro-optic device having an electro-optic response stabilized as a function of time and modulation frequency. The present invention finds applications in optical phase modulators and Mach-Zehnder interferometer type optical intensity modulators. The present invention also finds applications in active directional couplers. [Background technology]

[0004] In the above field, it is known to manufacture and use electro-optical devices integrated on a planar electro-optical substrate, which comprises an optical waveguide and an electrode system adapted to modulate a light beam propagating through the optical waveguide by applying a voltage to the electrode system.

[0005] Various materials can be used in the manufacture of integrated electro-optic devices. Lithium niobate (LiNbO3), lithium tantalate (LiTaO3), and lithium antimonate (LiSbO3) are particularly interesting materials because they exhibit the electro-optic Pockels effect. By placing electrodes near an integrated waveguide, it is possible to modulate the refractive index of the optical waveguide and, therefore, the phase of the optical signal propagating through the waveguide. In integrated electro-optic devices where the electrodes are separated by about 10 micrometers, applying a voltage of only a few volts is sufficient to generate an electric field and induce the desired phase modulation.

[0006] Various technologies have been developed for manufacturing electro-optical devices integrated on lithium niobate substrates, including titanium (Ti) diffusion and annealed proton exchange (APE) methods.

[0007] The proton exchange technique involves immersing a birefringent lithium niobate crystal in a benzoic acid bath heated to a temperature below 250°C (below the boiling point of benzoic acid), and then releasing lithium ions (Li) from the surface exposed to the acid bath. + ) to proton (or H + The process includes a first step of replacement with ions. This proton exchange (PE) operation forms a proton exchange modified layer 3 extending from the substrate surface 2 to a shallow depth P3 in the substrate 1, in the acid-exposed region (see Figure 1). Layer 3 has an optical refractive index difference of the order of 0.1 compared to the substrate. The refractive index profile of layer 3 is stepped in the X and Z directions. Layer 3 is generally limited laterally to a width W3 of approximately 1 μm to 10 μm. The depth P3 here is less than 500 nm. In fact, due to the low temperature, the proton exchange phenomenon cannot diffuse deeply into the interior of the substrate.

[0008] Following this first step, a thermal annealing step is performed at a temperature below 500°C. During this annealing step, as the temperature increases, the protons, i.e., hydrogen ions, of the proton-reformed layer 3 can exchange with lithium ions in the diffusion region 4 around the initial position of layer 3. The annealing creates a refractive index gradient in the diffusion region 4 below the crystal surface, thereby forming an optical waveguide 5. The diffusion region 4 extends into the substrate to a depth P4, which can reach several micrometers depending on the temperature and time of the annealing (see Figure 2). The width W4 of the diffusion region 4 is greater than the width W3 of the proton-reformed layer 3. The width W4 is generally between 1 μm and 10 μm. Here, the width W4 and depth P4 are measured at the position obtained by dividing the maximum value of the refractive index gradient in the Z and X directions by the base of the natural logarithm e (or exponential function (-1)), respectively.

[0009] Following the thermal annealing step, the manufacture of the electro-optic device includes the step of depositing a pair of electrodes. This pair of electrodes allows an electric field to be applied within the optical waveguide 5, thereby controlling the electro-optic device, for example, for electro-optic modulation of light waves propagating within the optical waveguide 5.

[0010] Proton exchange in birefringent lithium niobate crystals has both the effect of increasing the anomalous refractive index of the crystal (which leads to the guidance of waves polarized along the anomalous axis of the crystal) and the effect of decreasing the normal refractive index of the crystal (which prevents waves polarized along the normal axis from being guided).

[0011] In an electro-optical device including an optical waveguide formed by proton exchange in lithium niobate, a common configuration is an X-cut, where the X-axis of the uniaxial lithium niobate birefringent crystal is perpendicular to the substrate surface, while the Y-axis and Z-axis of the crystal are parallel to the surface. The propagation direction of the waveguide is parallel to the Y-axis, and the TE mode (transverse electric field, i.e., electric field parallel to the substrate surface) is parallel to the Z-direction. In this case, the proton-exchange optical waveguide guides only the TE-polarized state, while the cross-polarized TM state (transverse magnetic field, i.e., a magnetic field parallel to the substrate surface and therefore an electric field perpendicular to the substrate surface) propagates freely within the substrate. Proton exchange technology in lithium niobate makes it possible to fabricate polarized waveguides on integrated optical circuits. An electric field parallel to the Z-axis can be applied to the optical waveguide 5 using electrodes placed on both sides of the optical waveguide 5.

[0012] Furthermore, there are electro-optical devices equipped with optical waveguides formed by proton exchange in lithium niobate (or lithium tantalate or lithium antimonate) along a Z-cut, where the Z-axis of the uniaxial lithium niobate birefringent crystal is perpendicular to the substrate surface, while the X and Y axes of the crystal are parallel to the substrate surface. The propagation direction of the waveguide is parallel to the Y-axis, and the TE mode (transverse electric field, i.e., electric field parallel to the substrate surface) is parallel to the Z-direction. One electrode covers the optical waveguide, and the other electrode is placed near the waveguide or on the opposite side of the substrate. Using these electrodes, an electric field parallel to the Z-axis can be applied within the optical waveguide 5. Moreover, in this case, the metal electrode covering the waveguide absorbs the TM mode of the electromagnetic wave propagating within the optical waveguide 5 while transmitting the TE mode parallel to the Z-axis.

[0013] However, the behavior of integrated electro-optic devices fabricated by APE deteriorates under certain operating conditions such as vacuum and / or temperature. For example, an electro-optic phase modulator fabricated by APE deteriorates under certain voltage conditions, such as the voltage V applied to the terminals of the modulator electrodes. m The induced optical phase shift φ m It has a transfer function defined by the ratio of V, where V m / φ m = Vπ / π, and the signal Vπ is also called half-wavelength (Vπ). This signal Vπ varies depending on the environment, especially temperature and / or vacuum level.

[0014] Furthermore, the low-frequency (100 Hz to 1 kHz) and very low-frequency (0.0 Hz to 100 Hz) responses of electro-optical devices fabricated by APE can be considered equivalent to a gradual drift of the operating point or a measurable hysteresis in the transfer function of the electro-optical device in certain applications.

[0015] Various solutions have already been proposed for time drift in integrated optical phase modulators. For example, U.S. Patent No. 8,463,081 proposes combining an optical waveguide portion formed of diffuse titanium with another optical waveguide portion formed of APE in series, where these two portions have time drifts with opposite signs. However, this compensation is not perfect and depends on temperature and the frequency of the signal applied at low and very low frequencies.

[0016] One of the objectives of the present invention is to propose a means for stabilizing the electro-optic response of an integrated electro-optic device with respect to the time and / or frequency of a modulation signal applied to the electrodes of the integrated electro-optic device. It is desirable to reduce the gradual drift of the operating point in the low-frequency and very-low-frequency ranges. It is also desirable to eliminate or limit the hysteresis of the transfer function of such an integrated electro-optic device. In particular, in an electro-optic phase modulator based on an integrated electro-optic device, it is desirable to reduce fluctuations in the Vπ signal due to the environment, especially temperature and / or vacuum level.

[0017] Another object of the present invention is to propose a simple manufacturing method of such an integrated electro-optical device having an instantaneous electro-optical response that is stable with respect to time passage and / or the electrical modulation frequency and is independent of fluctuations in ambient temperature and / or pressure.

Summary of the Invention

[0018] In such a situation, the present invention proposes a manufacturing method of an electro-optical device.

Means for Solving the Problems

[0019] More specifically, a step of fabricating an optical waveguide in an electro-optical substrate by proton exchange by annealing, comprising: a) exposing a certain region of the surface of the substrate to a proton source so as to form a layer modified by proton exchange in the substrate, the layer extending over the region from the substrate surface to a shallow depth (P3) within the substrate, the exposing step, and b) thermally annealing the substrate to diffuse hydrogen ions from the layer to a deep depth (P4) up to a diffusion region extending within the substrate around the layer, and forming an optical waveguide in the diffusion layer, the step of fabricating an optical waveguide; and a step of depositing a pair of electrodes, the optical waveguide extending longitudinally between the pair of electrodes, the manufacturing method of the electro-optical device is proposed by the present invention.

[0020] According to the present invention, this method includes, after the thermal annealing step, surface machining at least a part of the substrate in the region of the optical waveguide corresponding to the remaining part of the layer from the surface, and forming a machined region that is at least partially located between the pair of electrodes over a depth P equal to the shallow depth of the layer modified by proton exchange.

[0021] Thus, by the machining step, at least a part of the residue of the layer modified by proton exchange, whose electro-optical properties are impaired by the proton exchange, can be removed. By removing this residue, when a voltage is applied between the terminals of the electrode, the time and / or frequency drift of the electro-optical device can be reduced or eliminated.

[0022] Other non-limiting and advantageous features of the system according to the invention, selected individually or according to any technically possible combination, are as follows: - The machined area extends to a depth P, where P is 0.02 * P3 or more and 1.10 * P3 or less, where P3 is the shallow depth of the layer modified by proton exchange before thermal annealing; - The machined area extends over at least a part of the deposition surface of the pair of electrodes; - The machined area extends over the entire width of the substrate measured transversely to the longitudinal direction of the optical waveguide; - The machined area is limited to the area of the waveguide between the pair of electrodes; - The machining step is adapted to form a plurality of machined areas arranged in series along the longitudinal direction of the optical waveguide, and the plurality of machined areas are at least partially located between the pair of electrodes; - The machined area extends over the entire surface of the substrate; - The machining step is performed by wet etching, plasma etching, laser, lapping, or mechanical polishing; - The method includes a step of depositing a dielectric layer, and the dielectric layer is disposed on the machined area of the surface of the substrate and / or between the substrate and at least one of the pair of electrodes.

[0023] The invention also relates to an electro-optical device obtained according to any one of the described embodiments.

[0024] In particular, the present invention relates to such electro-optic devices used in electro-optic phase modulators, light intensity modulators, Mach-Zehnder interferometers, and / or active attenuated wave-directional couplers.

[0025] Naturally, the various features, alternative forms, and embodiments of the present invention can be related to each other in various combinations, provided that they are not contradictory or mutually exclusive.

[0026] Furthermore, various other features of the present invention are evident from the appended description, which has been prepared with reference to drawings illustrating non-limiting embodiments of the present invention. [Brief explanation of the drawing]

[0027] [Figure 1] This is a cross-sectional view of an electro-optic substrate in which a layer modified by proton exchange before annealing is formed on the inside. [Figure 2] This is a cross-sectional view of an electro-optic substrate after annealing, showing the formation of a diffusion region. [Figure 3] This is a cross-sectional view of an integrated electro-optical device comprising an optical waveguide and electrodes formed by APE. [Figure 4] This graph schematically shows the frequency-dependent response of electro-optic modulators fabricated by APE according to the prior art (solid line) and the present disclosure (dashed line). [Figure 5] This graph schematically shows the half-wavelength voltage, which is a characteristic of the frequency-dependent transfer function of an electro-optic modulator fabricated by an APE according to the prior art (solid line) and the present disclosure (dashed line). [Figure 6] The graph on the left schematically shows the voltage applied to the terminals of an electro-optic modulator fabricated by APE as a function of time, and the graph on the right schematically shows examples of phase shifts generated by the prior art (solid line) and the present disclosure (dashed line), respectively. [Figure 7] This is a top view of an integrated electro-optical device comprising an optical waveguide formed by APE, electrodes, and a machined region according to the first example. [Figure 8]This is a top view of an integrated electro-optical device comprising an optical waveguide formed by APE, electrodes, and a machined region according to a second example. [Figure 9] This is a cross-sectional view of an integrated electro-optical device according to the first or second example. [Figure 10] This is a top view of an integrated electro-optical device comprising an optical waveguide formed by APE, electrodes, and a machined region according to a third example. [Figure 11] This is a top view of an integrated electro-optical device comprising an optical waveguide formed by APE, electrodes, and a machined region according to a fourth example. [Figure 12] This is a top view of an integrated electro-optical device comprising an optical waveguide formed by APE, electrodes, and a machined region according to the fifth example. [Figure 13] This is a top view of an integrated electro-optical device comprising an optical waveguide formed by APE, electrodes, and a machined region according to the sixth example. [Figure 14] This is a cross-sectional view of an integrated electro-optical device according to one of the third to sixth examples. [Figure 15] This is a cross-sectional view of an integrated electro-optical device according to a modification of any one of the third to sixth examples. [Figure 16] This is a cross-sectional view of a modified integrated electro-optical device comprising an optical waveguide and electrodes formed by APE. [Figure 17] This is a cross-sectional view of another modified example of an integrated electro-optical device, comprising an optical waveguide and electrodes formed by APE. [Modes for carrying out the invention]

[0028] It should be noted that in these diagrams, structural and / or functional elements common to different alternative forms may share the same reference symbols.

[0029] Figure 3 schematically shows an integrated electro-optical device in cross-sectional view, that is, perpendicular to the surface 2 of the electro-optical substrate 1 and traversing the longitudinal direction of the optical waveguide 5. This integrated electro-optical device comprises an optical waveguide 5 formed by APE and electrodes 6 and 7.

[0030] Electro-optical substrate 1 is a substrate made of lithium niobate (LiNbO3) for electro-optical applications. As a variation, substrate 1 is a substrate made of lithium tantalate (LiTaO3) or lithium antimonate (LiSbO3). As a non-limiting example, the substrate has an X-cut configuration, where the X-axis of the crystal is perpendicular to the substrate surface, while the Y-axis and Z-axis of the crystal are parallel to the substrate surface, with the Y-axis parallel to the propagation direction of the optical waveguide. As a variation, the substrate has a Z-cut configuration, where the Z-axis of the crystal is perpendicular to the substrate surface, while the X-axis and Y-axis of the crystal are parallel to the substrate surface, with the Y-axis parallel to the propagation direction of the optical waveguide (see Figures 16-17).

[0031] As described in relation to Figures 1 and 2, a layer 3 modified by proton exchange is formed within the substrate 1. During thermal annealing, a diffusion region 4 extends into the substrate 1 around the initial position of layer 3. The diffusion region 4 has a refractive index gradient, thereby forming an optical waveguide 5. The optical waveguide 5 is constrained by the refractive index gradient and the surface 2 of the substrate 1. The width W4 of the optical waveguide 5 is generally between 1 micrometer (μm) and 10 micrometers (μm). The optical waveguide 5 extends longitudinally, for example, along the Y direction. As is well known, the optical waveguide 5 may be linear or have curved sections and / or junctions with several branches (e.g., a bifurcated Y-junction). The optical waveguide 5 is adapted to guide a single-mode light wave, thereby the light wave propagating along the longitudinal direction of the optical waveguide 5, for example, in this case along the Y axis. Figure 3 schematically shows the spatial extent of the fundamental mode 15 of the light wave propagating within the optical waveguide 5.

[0032] The electro-optic apparatus shown in Figure 3 has an X-cut and Y-propagation configuration. The integrated electro-optic apparatus includes an electrode system comprising at least one pair of conductive electrodes 6 and 7. Electrode 6 is deposited on one side of the optical waveguide 5, and electrode 7 is deposited on the other side of the optical waveguide 5. The pair of electrodes 6 and 7 are generally deposited on the surface 2 of the substrate 1.

[0033] In a modified configuration, a dielectric layer 24, such as silicon dioxide (SiO2) or silicon nitride (SiN), is placed between the surface 2 of the substrate 1 and each electrode 6, 7. The thickness of the dielectric layer 24 is, for example, between 100 nm and 2 μm. The dielectric layer 24 acts as protection for the surface of the optical waveguide 5. The dielectric layer 24 also allows for the isolation of light waves from the metal electrodes, for example, when the optical waveguide is very close to the electrodes or when there is a risk of partial contact, especially in the case of broadband electrodes (operating in the range of 0 to tens of GHz).

[0034] For simplicity, consider a linear optical waveguide 5 and a pair of linear electrodes 6 and 7. The optical waveguide 5 extends longitudinally between the pair of electrodes 6 and 7. The distance between the two electrodes 6 and 7 is generally greater than or equal to the width W4 of the optical waveguide 5. For example, the distance D between the two electrodes 6 and 7 is between 1 micrometer (μm) and 10 μm. The length L6 of the electrodes in the Y direction is generally between a few millimeters and a few centimeters. The electrodes may have the same width or different widths. The optical waveguide 5 may or may not be located in the center between the two electrodes 6 and 7. Optionally, another electrode may be placed on the back surface 9 of the substrate 1, i.e., the surface opposite to the front surface 2 where the optical waveguide is formed by the APE.

[0035] In relation to Figure 3, the electro-optic operation of the integrated electro-optic device will be explained. An electrical voltage is applied to the terminals of a pair of electrodes 6 and 7. For example, a positive voltage is applied to electrode 6 and a negative voltage is applied to electrode 7. In a modified configuration, one of the two electrodes is grounded. An electric field is formed between the pair of electrodes 6 and 7.

number

number

[0036] However, the electro-optical operation of the electro-optical device may exhibit drift as shown in Figures 4 to 6.

[0037] Figure 4 schematically shows the frequency response curve 16 of an electro-optic phase modulator fabricated by APE according to the prior art, represented by a solid line. More precisely, curve 16 shows the ratio of the optical phase shift Δφ induced in the optical wave propagating through the optical waveguide 5 to the applied voltage V as a function of the modulation frequency f. At high frequencies, for example above 1 kHz, the ratio ±Δφ / V is constant. On the other hand, at low and very low frequencies, the ratio ±Δφ / V decreases dramatically. The response of an electro-optic phase modulator fabricated by APE according to the prior art depends on the modulation frequency, which is undesirable.

[0038] Figure 5 schematically shows, with a solid line, a curve 18 of the half-wave voltage (or signal Vπ) as a function of frequency for an electro-optic phase / intensity modulator fabricated by APE according to the prior art. More precisely, curve 18 shows the signal Vπ as a function of the modulation frequency f of the voltage applied to the terminals of electrodes 6 and 7. At high frequencies, the signal Vπ is constant. On the other hand, at low and very low frequencies, the signal Vπ increases rapidly. This curve 18 also shows that the response of an electro-optic phase / intensity modulator fabricated by APE according to the prior art is unfavorably dependent on the modulation frequency f.

[0039] On the left side of FIG. 6, a curve 20 of the voltage V applied between electrodes 6 and 7 is shown as a function of time. The voltage V gradually rises stepwise and then gradually falls stepwise as well. On the right side of FIG. 6, a curve 21 of the optical phase shift Δφ induced in the optical wave propagating in the optical waveguide 5 is shown as a function of time, that is, as a function of the applied voltage value V. In the curve 21 of the induced optical phase shift, for example, a negative exponential step is observed during voltage rise, and a positive exponential step is observed during voltage fall. In either case, for each rise or fall of the step, it takes from 1 millisecond to several hours or even more time for the induced phase shift value to stabilize.

[0040] As is clear from the present disclosure, by machining a part of the substrate within the region of the optical waveguide 5 over a shallow depth corresponding to the remaining part of the layer 3 modified by proton exchange located between the pair of electrodes 6 and 7, these slow drifts of the operating point of the electro-optic modulator with respect to time and / or modulation frequency can be significantly reduced. As schematically shown in FIG. 3, the remaining part of the layer 3 modified by proton exchange extends from the substrate surface 2 to a shallow depth within the optical waveguide 5 and is perpendicular to the longitudinal axis of the optical waveguide.

[0041] Without being bound by theory, as a physical interpretation, during the proton exchange step, a part of the lithium ions in the substrate 1, for example, lithium niobate ions, are replaced by protons or H + ions, and the layer 3 modified by proton exchange is formed. In the cross-section along the X-axis, the layer 3 modified by proton exchange shows a stepped optical refractive index up to a depth P3. The substitution rate in layer 3 may reach 60% or more H + ions. As a result, the crystal structure of the layer 3 modified by proton exchange is deteriorated compared to the crystal structure of the substrate 1 around the initial position of the layer 3 modified by proton exchange. In the region of layer 3, due to the high ion exchange rate, the electro-optic properties of lithium niobate required for electro-optic modulation are locally lost. During a thermal annealing step below 500 ° C, the diffusion region 4 forming the optical waveguide 5 extends into the substrate 1. During thermal annealing, H from layer 3+ Ions are exchanged with lithium ions at deeper locations, for example, to a depth of several microns depending on the annealing temperature and time. Annealing creates a gradient of optical refractive index within the substrate, thus forming an optical waveguide 5. In a cross-section along the X-axis, the diffusion region 4 generally exhibits an optical refractive index profile comparable to a half-Gaussian profile. The depth P4 corresponds to the maximum value of this half-Gaussian profile along the X-axis divided by the base of the natural logarithm, e (or the exponential function (-1)). Similarly, the width W4 of the optical waveguide 5 corresponds to the maximum value of this profile along the Z-axis divided by e. During thermal annealing, the ion substitution rate in the diffusion region is low, generally less than 15%. This low lithium ion substitution rate maintains the electro-optic properties of the crystal. After thermal annealing, the low lithium ion substitution rate in the region corresponding to the location of the proton-exchange-modified layer 3 decreased. Nevertheless, the crystal structure at the location of the proton-exchange-modified layer 3 is still degraded. Electro-optical devices fabricated conventionally using APE exhibit different crystalline properties in the substrate 1, in the diffusion region 4, and in the region corresponding to the proton-modified layer 3.

[0042] As shown in Figure 3, the electric field line 14 passes not only through the diffusion region 4 but also through the region corresponding to the residual portion of the proton-exchange-modified layer 3 below the substrate surface. The electro-optic response of the electro-optic device depends not only on the electro-optic properties of the diffusion region 4 but also on the region corresponding to the location of the proton-exchange-modified layer 3. Crystal degradation in the layer 3 region leads to an improvement in the electrical conductivity of the electro-optic device thus manufactured when it is placed at a high temperature, for example, about 50°C. This effect is accelerated and amplified as the temperature rises and the electro-optic device is placed under a thermal vacuum.

[0043] This disclosure proposes an additional step in the manufacturing method of an electro-optical device by APE, which, after the thermal annealing step, machines at least a portion of the optical waveguide 5 to a shallow depth corresponding to the residual portion of layer 3 in a region located between or intended to be located between a pair of electrodes 6, 7, thereby reducing or removing the residual portion of layer 3 in the machined region. The depth P of the machined region is approximately the same as the shallow depth P3 of the proton-exchange-modified layer 3. In practice, the depth P may be less than, equal to, or slightly greater than the shallow depth P3. For example, the depth P of the machined region may be 0.02 * P3 ≤ P ≤ 1.10 * P3, preferably 0.50 * P3 ≤ P ≤ 1.10 * P3, more preferably 0.90 * P3 ≤ P ≤ 1.10 * This is P3. For example, if the depth P3 is 500 nm, the depth P of the machined region is approximately 500 nm ± 50 nm. The machining step is performed at a shallow depth P in order to preserve most of the diffusion region 4. The depth P of the machined region is always set to be shallower than the depth P4 of the diffusion region 4 so as not to impair the optical waveguide properties of the optical waveguide 5. Preferably, the depth P of the machined region is less than 20-25% of the depth P4 of the diffusion region 4.

[0044] This machining step of the residual portion of layer 3 can be carried out by various techniques: for example, mechanical machining such as lapping and / or polishing of the substrate surface or a portion of the substrate surface, surface ablation with laser pulses, wet etching, or dry etching with plasma.

[0045] The machining step of the residual portion of layer 3 can be performed on a plate integrating multiple electro-optical devices manufactured in a batch, or, for example, on individual chips obtained by cutting such a plate (each chip containing a single electro-optical device). The machining step of the residual portion of layer 3 can be performed before or after depositing electrodes 6, 7 onto a complete wafer or individual chips. The machining step of the residual portion of layer 3 can be performed before or after transferring the chips onto a base or into an electronic case.

[0046] Figures 7–15 illustrate various exemplary embodiments of the electro-optical apparatus according to the present disclosure. Figures 7–15 show one or more machined regions 10, 11, 12 on each electro-optical apparatus.

[0047] In Figure 7, the electro-optic apparatus 50 in the top view includes an optical waveguide 5 formed by annealed proton exchange within an electro-optic substrate 1, and a pair of electrodes 6 and 7 arranged parallel to the two longitudinal sides of the optical waveguide 5. This electro-optic apparatus includes a machined region 10 extending between the two electrodes 6 and 7 with width W and length L. The width W of the machined region 10 is here less than or equal to the distance D between the two electrodes 6 and 7. The length L of the machined region 10 is here equal to the length L6 of the electrodes 6 and 7. The width W of the machined region 10 extends in alignment with the optical waveguide. The width W is measured perpendicular to the longitudinal axis of the optical waveguide 5 and parallel to the plane of the surface 2. Advantageously, the width W is greater than or equal to the initial width W3 of the proton-exchange-modified layer 3. By machining the remaining portion of layer 3 across this width W and the entire length L6 of electrodes 6 and 7, the frequency and time drift of the electro-optical device can be completely eliminated, as detailed below. In this configuration, machining of the machined region 10 can be performed before or after electrode deposition. However, if this machining is performed after electrode deposition, care must be taken not to damage electrodes 6 and 7.

[0048] Figure 8 shows a second example of an electro-optical device according to a modification of Figure 7. The length L of the machined region 10 is here shorter than the length L6 of the electrodes 6, 7, for example as a result 0.5 * L6 ≦ L < L6. The width W of the machined region 10 is here less than or equal to the distance D between the two electrodes 6, 7. This example shows a specific configuration where it is difficult to machine the region 10 over the entire length L6 of the electrodes. In this example, the machining of the machined region 10 can be carried out before or after the deposition of the electrodes. For certain applications, machining only a part of the length of the electrodes in the region 10 is sufficient to limit the frequency and time drift of the electro-optical device. In this case, the phenomenon of time drift or frequency drift decreases in proportion to the machined region.

[0049] Figure 9 shows a cross-sectional view of the electro-optical device 50 of Figure 7 or 8. The machined region 10 here corresponds to a recessed region of the substrate below the initial surface 2 of the substrate and perpendicular to the longitudinal axis of the optical waveguide. The machined region 10 has a depth P. For example, the depth P is between 10 nm and 550 nm when the depth P3 is 500 nm. The depth P of the machined region is shallower than, equal to, or slightly deeper than the shallow depth P3 of the layer 3 modified by proton exchange. In any case, the depth P of the machined region is shallower than the initial depth P4 of the diffusion region. Thus, below the machined region between the electrodes 6, 7, the diffusion region 5 has a depth equal to P4 - P. Generally, the remaining depth P4 - P of the optical waveguide 5 is much deeper than the depth P of the machined region. For example, the remaining depth P4 - P of the optical waveguide 5 exceeds 75% of the initial depth of the optical waveguide 5 before machining.

[0050] The machined region 10 of the electro-optical device 50 of Figures 7, 8 or 9 can be machined before or after the deposition of the electrodes 6, 7.

[0051] Figure 10 shows a third example of the electro-optic apparatus 50. The length L of the machined region 10 is shorter here than the length L6 of electrodes 6 and 7. The width W of the machined region 10 is greater here than the total width T of electrodes 6 and 7. The width W of the machined region 10 is the same as, for example, the width of the substrate 1, which facilitates the use of mechanical machining. The machined region is machined before electrode deposition.

[0052] Figure 11 shows a fourth example of the electro-optical apparatus 50. The length L of the machined region 10 is equal here to the length L6 of electrodes 6 and 7. The width W of the machined region 10 is greater here than the total width T of electrodes 6 and 7. The width W of the machined region 10 is equal to, for example, the width of the substrate 1, which facilitates the use of mechanical machining.

[0053] Figure 12 shows a fifth example of the electro-optical apparatus 50. The length L of the machined region 10 is here longer than the length L6 of electrodes 6 and 7. The width W of the machined region 10 is here wider than the total width of electrodes 6 and 7. The machined region 10 here extends, for example, across the entire surface of the substrate 1. The machined region 10 can be formed by etching, lapping, and / or polishing the surface so that the electro-optical apparatus has a flat area outside the electrodes. Machining the entire surface ensures that all residual parts of the proton-exchange-modified layer 3 are removed. Furthermore, machining the entire surface 2 is easy to perform because it is full-plate machining that does not require masking.

[0054] As a variation, the electro-optical apparatus 50 shown in any one of Figures 10-12 includes a machined region or several separate machined regions located outside the electrodes 6, 7. For example, in Figure 10 or 11, the apparatus can also be formed by creating a machined region located between the two electrodes 6, 7 and two machined regions located outside the electrodes 6, 7. In this case, the machined region or separate machined regions can be formed before or after the deposition of the electrodes 6, 7.

[0055] Figure 13 shows a sixth example of an electro-optical apparatus 50, which includes several machined regions 10, 11, and 12 arranged in series along the longitudinal direction Y of the optical waveguide 5. More specifically, machined region 10 extends over a length L shorter than the length L6 of electrodes 6 and 7, and over a width W greater than the total width of electrodes 6 and 7 (for example, over the width of the substrate 1). Another machined region 11 extends over a length L1 shorter than the length L6 of electrodes 6 and 7, and over a width W1 less than or equal to the distance D between electrodes 6 and 7. Yet another machined region 12 extends over a length L2, partly between electrodes 6 and 7 and partly beyond electrodes 6 and 7 in the longitudinal direction. Machined region 12 extends laterally over a width W2 greater than the distance D between electrodes 6 and 7 and less than or equal to the total length T of electrodes 6 and 7. The machined regions 10, 11, and 12 can be machined simultaneously using the same machining technique, or they can be fabricated by sequentially applying the different machining techniques described above. For example, if time drift or frequency drift remains, the machined region 11 can be formed after electrode deposition to further limit these drifts.

[0056] Figure 14 shows a cross-sectional view of an electro-optical apparatus 50, for example, as shown in one of Figures 10-13. Advantageously, the machined regions 10, 11 and / or 12 are formed before the deposition of electrodes 6 and 7. In this example, the machined regions 10, 11 and / or 12 are machined over a width W wider than the total width T of the electrodes, preferably over the total width of the substrate 1. For example, the machined regions are formed here by removing a layer of thickness P over the total width of the substrate after the thermal annealing step or before the electrode deposition step. In the cross-sectional view, it can be observed that the surface machining removes a layer of thickness P from the optical waveguide 5 where the remnants of the proton-exchange-modified layer 3 were located. Here again, beneath the machined regions between electrodes 6 and 7, the diffusion region 5 has a depth equal to P4-P, which is still greater than 75% of the initial depth of the optical waveguide 5 before machining.

[0057] Figure 15 shows a cross-sectional view of an example of an electro-optic apparatus 50, as shown in one of Figures 10-13. Here, machined regions 10, 11 and / or 12 are formed before or after the deposition of electrodes 6 and 7. In this example, the machined regions 10, 11 and / or 12 are formed over a width W less than or equal to the distance D between the electrodes. Optionally, the lateral regions outside the electrodes, between the total width T and the substrate width, are also machined. For example, here, the machined regions are formed after the thermal annealing step and before or after the electrode deposition step by removing a central band of thickness P, and possibly two lateral bands, over the entire length or part of the length of the electrodes. In the cross-sectional view, it can be observed that the surface machining removes a layer of thickness P from the optical waveguide 5 where the remnants of the proton-exchange-modified layer 3 were located. After the deposition of electrodes 6 and 7, the electro-optic apparatus has a recess between electrodes 6 and 7 in the machined region 10. In this example, electrodes 6 and 7 are located at a height greater than P relative to the optical waveguide.

[0058] Optionally, after fabricating the machined region, the method includes an additional step of depositing a dielectric layer 24, for example, silicon dioxide (SiO2) or silicon nitride (SiN). The dielectric layer 24 is positioned on the machined substrate surface between the teach electrodes 6, 7 and the substrate 1. In a modified form, the dielectric layer 24 extends not only beneath the electrodes but also across the entire surface of the substrate.

[0059] As an alternative or complementary method, the method includes another step of depositing a dielectric layer 24, for example, silicon dioxide (SiO2) or silicon nitride (SiN). Two electrodes 6, 7 are deposited on this dielectric layer 24. The dielectric layer 24 protects the surface of the optical waveguide 5. The dielectric layer 24 also insulates the light waves circulating within the optical waveguide from the metal electrodes.

[0060] The presence of residual layers corresponding to layer 3, which has been modified by proton exchange after thermal annealing, can be detected and measured by various characterization techniques, such as optical or contact profile measurement, Raman spectroscopy, secondary ion mass spectrometry (SIMS), X-ray rocking curve, transmission electron microscopy (TEM), Fourier transform spectroscopy (FTIR), or UV polarization analysis. These techniques allow for the evaluation of the machining depth P. The machining depth P is typically less than 500 nm. The machining depth P can be confirmed by verifying the machining depth during fabrication or by measuring the surface conductivity before / after etching. Furthermore, the electro-optical operation of the apparatus can be verified after fabrication.

[0061] An integrated electro-optic device 50 is obtained in which at least one machined region 10, 11, 12 is formed in the optical waveguide 5 over a shallow depth P, and the machined region 10, 11, 12 is at least partially located between a pair of electrodes 6, 7. The machined region is adapted to at least partially remove the residual portion of the proton-exchange-modified layer 3 in the region at least partially located between the pair of electrodes 6, 7. However, in order to maintain the optical waveguide properties of the optical waveguide 5, the depth of the machined region in the optical waveguide is limited. Preferably, the depth of the machined region in the optical waveguide is less than 25% of the initial depth of the optical waveguide 5 before machining. In certain embodiments, the machined region may remain empty and form a depression in the substrate surface. In other embodiments, the machined region may be at least partially filled by, for example, a dielectric layer. Finally, in other embodiments, the machined region extends across the entire surface of the substrate, and the substrate has the same surface relief as an unmachined electro-optic device.

[0062] The method also applies to electro-optical devices including optical waveguides formed by proton exchange on lithium niobate (or lithium tantalate or lithium antimonate) along a Z-cut, as shown in Figures 16-17. In this case, electrode 6 is in contact with the optical waveguide 5. The other electrode 7 is positioned near the optical waveguide 5 on the same plane as electrode 6 (see Figure 16) or on the opposite side 9 of the substrate 1 (see Figure 17). In this Z-cut configuration, the potential difference applied between the two electrodes generates an electric field E directed along the Z-axis within the optical waveguide 5. According to this disclosure, in the Z-cut configuration, similar to the X-cut configuration, after the thermal annealing step, at least a portion of the residual layer 3 is machined in contact with electrode 6 and in the region of the optical waveguide 5 intended to be placed between the pair of electrodes 6 and 7, thereby reducing or removing the residual layer 3 in the machined region. The depth P of the machined region is approximately the same as the shallow depth P3 of the proton-exchange-modified layer 3. In reality, the depth P may be less than, equal to, or slightly greater than the shallow depth P3. For example, the depth P of the machined area may be 0.02 * P3 ≤ P ≤ 1.10 * P3, preferably 0.50 * P3 ≤ P ≤ 1.10 * P3, more preferably 0.90 * P3 ≤ P ≤ 1.10 * This is P3. For example, if the depth P3 is 500 nm, the depth P of the machined region is approximately 500 nm ± 50 nm. The machining step is performed at a shallow depth P in order to preserve most of the diffusion region 4.

[0063] In a Z-cut configuration, the machining step is always performed before the electrode 6 deposition step. The machining step can be performed after the thermal annealing step, for example, on the entire surface 2 of the substrate to obtain a flat surface before the deposition of the electrode 6, as shown in Figure 17. In a modified form, as shown in Figure 16, machining is applied to a limited width and / or length of the surface 2 of the substrate above the optical waveguide 5. Partial or extended machining across the entire surface is applicable whether the electrode 7 is located on the same surface 2 as the electrode 6 or on the opposite surface 9.

[0064] This results in an electro-optical apparatus with an X-cut or Z-cut configuration, in which at least a portion of the residual portion of the proton-exchange-modified layer 3 is removed in a region located between or intended to be located between a pair of electrodes 6 and 7.

[0065] In an unconventional way, this surface machining can be limited locally to small portions of the substrate between a pair of electrodes, but it can significantly improve the responsiveness of electro-optical devices, whether in response to time, temperature, pressure (e.g., under vacuum), or at low or very low modulation frequencies.

[0066] Figures 4-6 schematically show the technical effects obtained by an integrated electro-optical device obtained by annealing proton exchange using the method of this disclosure, which includes a machining step.

[0067] Figure 4 schematically shows, by dashed lines, the frequency response curve 17 of an electro-optical phase modulator fabricated by APE and machined according to this disclosure. It is observed that the ratio of the optical phase shift Δφ induced by the applied voltage V is constant at low or very low frequencies. The ratio ±Δφ / V is independent of the modulation frequency f, especially at low or very low frequencies.

[0068] Figure 5 schematically shows, by a dashed line, a curve 19 of the half-wavelength voltage (or Vπ signal) of an electro-optic phase modulator fabricated by APE and machined according to this disclosure. It is observed that the Vπ signal is kept constant at low and very low frequencies. The Vπ signal curve 19 is independent of the modulation frequency f, particularly at low and very low frequencies.

[0069] On the right side of Figure 6, the curve 22 of the optical phase shift Δφ induced in the light wave propagating within the optical waveguide 5 is shown as a function of time, i.e., as a function of the applied voltage value V along curve 20. In the curve 22 of the induced optical phase shift, instantaneous steps are observed when the voltage increases or decreases. This allows for the instantaneous acquisition of the desired optical phase shift value each time the voltage value applied to electrodes 6 and 7 changes.

[0070] The following are possible interpretations of these improvements to the electro-optic response of electro-optic devices having machined regions according to this disclosure. The electrical conductivity of the degraded crystal structure at the location of the proton-exchange-modified layer 3 can vary, for example, depending on temperature and high-temperature vacuum. As shown in Figure 3, the fundamental mode 15 of the wave guided by the optical waveguide 5 extends to the substrate surface and partially covers the location of the residual portion of the proton-exchange-modified layer 3. When a voltage is applied between electrodes 6 and 7, the field line 14 passes through various regions with different electrical conductivity: the substrate 1, the diffusion region 4, and the region where the residual portion of the proton-exchange-modified layer 3 is located. The physical properties of these regions are different. In particular, their conductivity shows different changes depending on environmental conditions, especially temperature and pressure. The increase in conductivity of the residual portion of the proton-exchange-modified layer 3 is different from that of the optical waveguide 5 and the substrate. This difference in conductivity changes modifies the electro-optic coating and therefore modifies the response as a function of the modulation frequency applied to the electrodes. By machining the residual portion in a specific shallow region of the substrate 1 between electrodes 6 and 7, the residual portion of layer 3 can be removed at least partially. This surface machining over a depth P in the ratio detailed above does not interfere with the guidance of single-mode waves in the optical waveguide 5.

[0071] The electro-optic apparatus thus obtained finds advantageous applications in electro-optic phase modulators, light intensity modulators, and / or Mach-Zehnder interferometers. The electro-optic apparatus thus obtained finds advantageous applications in active directional couplers using evanescent waves, for example, in a 2×2 active coupler on a lithium niobate substrate with electrodes arranged in the evanescent wave coupling region.

Claims

1. A step of fabricating an optical waveguide (5) in an electro-optic substrate (1) by proton exchange through annealing, comprising: a) an exposure step of a region of the surface (2) of the substrate (1) to a proton source so as to form a layer (3) modified by proton exchange within the substrate, wherein the layer (3) extends from the substrate surface (2) to a shallow depth (P3) within the substrate; and b) a thermal annealing step of the substrate to diffuse hydrogen ions from the layer (3) to a deep depth (P4) in a diffusion region (4) extending within the substrate around the layer (3), thereby forming an optical waveguide (5) within the diffusion region (4). A method for manufacturing an electro-optical device (50), comprising the steps of: fabricating; depositing a pair of electrodes (6, 7), wherein the optical waveguide (5) extends longitudinally between the pair of electrodes (6, 7), the method being characterized by comprising the step of, after the thermal annealing step, machining at least a portion of the substrate in the region of the optical waveguide (5) corresponding to the residual portion of the layer (3) from the surface (2), thereby forming machined regions (10, 11, 12) at least partially located between the pair of electrodes (6, 7) to a depth P equivalent to the shallow depth (P3) of the proton-exchange-modified layer (3).

2. The machined regions (10, 11, 12) extend within the substrate (1) to a depth P, where the depth P is 0.02 ＊ P3 or higher and 1.10 ＊ The method according to claim 1, wherein P is 3 or less.

3. The method according to claim 1 or 2, wherein the machined regions (10, 12) extend over at least a portion of the deposited surface of the pair of electrodes.

4. The method according to any one of claims 1 to 3, wherein the machined regions (10, 11, 12) extend over the entire width of the substrate (1) measured transversely to the longitudinal direction of the optical waveguide.

5. The method according to claim 1 or 2, wherein the machined region (10) is limited to the region of the waveguide (5) between the pair of electrodes (6, 7).

6. The method according to any one of claims 1 to 5, wherein the machining step is adapted to form a plurality of machined regions (10, 11, 12) arranged in series along the longitudinal direction of the optical waveguide (5), and the plurality of machined regions (10, 11, 12) are at least partially located between the pair of electrodes (6, 7).

7. The method according to any one of claims 1 to 4, wherein the machined region (10) extends over the entire surface of the substrate.

8. The method according to any one of claims 1 to 7, wherein the machining step is performed by wet etching, plasma etching, laser, lapping, or mechanical polishing.

9. The method according to any one of claims 1 to 8, comprising the step of depositing a dielectric layer (24), wherein the dielectric layer (24) is located on the machined region (10) of the surface of the substrate (1) and / or between the substrate and at least one of the pair of electrodes (6, 7).

10. An electro-optical apparatus (50) obtained by the method described in any one of claims 1 to 9.

11. The electro-optic apparatus according to claim 10, used in an electro-optic phase modulator, an optical intensity modulator, a Mach-Zehnder interferometer, and / or an actively attenuated wave-directional coupler.

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