RU2451959C2 - Low switching voltage, fast time response digital optical switch - Google Patents

Abstract

FIELD: physics.

SUBSTANCE: device has an electro-optical substrate and a Y-shaped optical waveguide formed in the substrate. An electrically conductive electrode structure is formed on the substrate and is connected to the optical waveguide for selective transmission of a signal at the input of the waveguide to one of the output waveguides. The electrode structure has an internal and external electrodes. The external electrodes are supplied with a switching signal between two states. In the first state, optical energy transmission is enhanced between the input branch and the first of the output branches and is blocked in the second branch. In the second state, optical energy transmission is enhanced between the input branch and the second output branches and is blocked in the first branch. Between the electrodes and the substrate, there is an optically transparent electrically conductive film which partly covers the neighbouring output branch of the waveguide in the cross direction. The space (G₁) between each pair of neighbouring films is smaller than the space (G₀) between each pair of neighbouring electrodes. The electrically conductive films are electrically connected to corresponding electrodes through holes in dielectric buffer layers lying between each film and the corresponding electrode.

EFFECT: low switching voltage applied across electrodes and excluding the effect of dc current drift.

3 cl, 8 dwg

Description

FIELD OF THE INVENTION

In General, the present invention relates to a digital optical switch (DOS), and more particularly to a digital, electrically excited optical (electro-optical) switch with a low switching voltage, fast time response.

State of the art

As you know, from the early days of the telephone and telegraph, connected signals are traditionally transmitted through copper wires and cables. However, in recent years, connected signals in an increasing volume are transmitted in the form of light rays through optical waveguides and fibers. Based on optical waveguides, peripheral equipment of various types has been developed, such as connecting devices and switches. In particular, a technology known as integrated optics is widely used in the management of optical communication signals. Using this technology, communication signals in the form of light rays are transmitted along optical waveguides formed in substrates made of electro-optical materials such as lithium niobate (LiNbO ₃ ).

Although integrated optics are currently widely used in signal transmission, the use of this technology to perform switching and routing functions is still limited by the difficulty of manufacturing optical switching devices with characteristics and parameters that meet the requirements. In a digital optical switch, an optical signal is received at the input and selectively applied to one or more of the multiple outputs. To date, digital optical switches based on various technologies have been developed, in particular, microelectromechanical systems (MEMS), magneto-optical and electro-optical switches, the latter have faster time characteristics than the previous ones.

Figure 1 shows a standard representation of a 1 × 2 digital electro-optical switch having an optical input to which an optical input signal is supplied, an electric input to which an electrical excitation signal in the form of a switching voltage is supplied to electrically excite a digital electro-optical switch, and two optical outputs, which selectively an input optical signal is supplied as a result of an electrical excitation signal. Figure 1 also shows the step response function of the electro-optical switch, illustrating the optical power at the optical output depending on the electrical excitation signal.

An example of a known digital electro-optical switch is disclosed in EP0898197 and shown in FIG. 2. Digital electro-optical switch 1 mainly includes a Y-shaped optical waveguide 2 formed in the substrate 3 with a thickness of 0.2-1 mm from an electro-optical material, for example, lithium niobate (oriented along the X-section in the configuration shown).

The Y-shaped optical waveguide 2 comprises an input branch 4 configured to connect, when used, to an input optical waveguide (not shown), and two output branches 5, configured to connect, when used, to respective output optical waveguides (not shown). Preferably, the input optical waveguide, the output optical waveguides and the Y-shaped optical waveguide 2 are single-mode optical waveguides formed in a known manner, for example, in the case of a lithium niobate substrate, by selective diffusion of titanium into the substrate itself.

The digital electro-optical switch 1 also contains an electrode structure including a plurality of conductive electrodes 1-30 μm thick, formed in a known manner from gold or similar metals on the surface of the substrate 3 and arranged to communicate in a working state with a Y-shaped optical waveguide 2 for selective supplying only one of the two output branches 5 of the optical signal received in the input branch 4. In particular, the electrode structure is electrically controlled so that the operation of the The electro-optical switch 1 is between two switching states: a first switching state in which the transmission of optical energy is increased between the input waveguide and the first output waveguide, while it is essentially blocked in the second output waveguide, and a second switching state in which the transmission of optical energy rises between the input waveguide and the second output waveguide, while it is essentially blocked in the first output waveguide.

In more detail, the electrode structure is located on the branching section of the optical waveguide 2 and includes an internal electrode 7 located between the output branches 5, and two external electrodes 6 located on the outer side of the output branches 5, on opposite sides and symmetrically to the inner electrode 7.

Typically, the interelectrode gap G ₀ (the distance between adjacent electrodes 6, 7) is in the range of 4 to 20 μm, the gap D ₀ between each electrode 6, 7 and the adjacent output branch 5 of the optical waveguide 2 is in the range of 3 to 10 μm, and the interaction length L ₀ (the length of the portion of the inner electrode 7 between the outer electrodes 6) is in the range from 1 to 30 mm.

The inner electrode 7 is usually grounded, while a switching voltage is applied to the outer electrodes 6 to create an electric field between the outer electrodes 6 and the inner electrode 7 passing through the output branches 5 located between them and having a direction transverse to the direction of propagation of the optical signal to the output branches 5, in this example (LiNbO ₃ X-section substrate) in a direction parallel to the Z axis of the crystal.

The electro-optical properties of the substrate 3 make it possible, when switching voltage is applied, to change the refractive indices of the output branches 5 in the opposite way, namely, to increase the refractive index in one output branch 5 and to lower the refractive index in the other output branch 5. In those cases when the threshold electric field is reached, the input the optical signal is supplied only to the output branch 5 with a higher refractive index.

Since the optical energy flowing through the digital optical switch 1 is not completely localized in the optical waveguide 2, to prevent or minimize optical losses due to absorption by the electrodes 6, 7 of the residual optical energy flowing outside the optical waveguide 2, the electrodes must be isolated from the optical waveguide 2 . As shown in Figure 3, typically provide continuous dielectric insulation (SiO ₂₎ buffer layer 8 thickness of 0.1-1.0 microns, which is formed on the optical branch portion into novoda 2 between the surface of the substrate 3 and the electrodes 6, 7 and continue along the entire length of the electrodes 6 and 7. Instead, in those applications where the dielectric buffer layer 8 does not contribute to the radio frequency response of the digital optical switch 1, the dielectric buffer layer 8 between the surface of the substrate 3 and electrodes 6, 7 are not formed, and the insulation is provided as shown in Fig. 4, namely, by placing the electrodes 6, 7 at a greater distance from the optical waveguide 2 than in the embodiment shown in Fig. 3, so that enshit absorption of optical radiation. In this embodiment, the interelectrode gap G _{0 is} usually in the range of 4 to 20 μm, and the distance D ₀ between each electrode 6, 7 and the adjacent output branch 5 of the optical waveguide 2 is in the range of 3 to 10 μm.

Another example of a known digital electro-optical switch is disclosed in JP 61198133. In particular, it discloses a waveguide-type optical switch including an X-shaped titanium optical waveguide formed in a LiNbO ₃ electro-optical substrate; two L-shaped electrodes formed on a substrate near the waveguide; and two buffer layers, each of which is formed from a material formed by combining an electrically conductive material (SnO ₂ ), within which light absorption does not increase, and a dielectric material (SiO ₂ ), and respectively located between the substrate and one of the electrodes and between the substrate and another electrode, thus suppressing the formation of direct current drift without increasing light loss.

Another example of a known digital electro-optical switch is disclosed in JP 91198133 A. In particular, a waveguide-type optical switch is disclosed, including a cross-shaped titanium optical waveguide formed on an electro-optical substrate of LiNbO ₃ , two L-shaped electrodes formed on the substrate near the waveguide, and two buffer layers formed from a material composed by synthesizing a conductive material (SnO ₂₎ within a range in which the absorption of light does not increase, and isolate its material (SiO ₂₎ and respectively placed between said substrate and one of said electrodes and between said substrate and the other electrode, thereby suppressing the formation of DC drift without increasing light loss.

A further example of a known electro-optical switch is disclosed in US 2006/198581 A1. In particular, an electro-optical device is disclosed having an optical waveguide, which includes two optical paths, the optical waveguide being contained in the substrate. A bias electrode layer is formed on the surface of said substrate. A buffer layer is formed in at least a portion of the bias electrode layer and the surface of said substrate. An RF electrode layer is formed on the buffer layer. A bias tee connects the bias electrode layer and the RF electrode layer.

Disclosure of invention

Applicant notes that although digital electro-optical switches of the type described above have a faster time response than digital optical switches based on other technologies, they require higher switching voltages. In fact, isolation of the optical waveguide from the electrodes limits the effective electric field applied to the optical waveguide, and as a result, the so-called electro-optical efficiency of the digital electro-optical switch, and this limitation is usually compensated by a corresponding increase in the switching voltage.

In addition, in digital electro-optical switches with a dielectric buffer layer, the electro-optical efficiency is further limited by the shielding effect due to the existence of capacitive coupling between the electrodes and the optical waveguide. In fact, during the formation of an optical waveguide (which includes the deposition and diffusion of Ti), defects are usually created in the substrate crystal and free electric charges are generated. When a DC voltage is applied between the electrodes, free electric charges move to the surface of the adjacent output branch of the optical waveguide at the interface with the dielectric buffer layer, which acts as a capacitor, denoted C _In figure 3. This capacitor blocks free electric charges, thereby leading to a screening effect of the applied switching voltage. Therefore, in order to maintain a stable electro-optical coupling (changed state of the switch), it is necessary to continuously correct (the effects of the DC drift effect) the switching voltage applied between the electrodes, thereby compensating for the screening effect.

Also, a weaker DC drift effect is observed in digital electro-optical switches without a dielectric buffer layer between the electrodes and the substrate due to the presence of the natural surface of the substrate crystal between the electrodes and the adjacent output branch of the optical waveguide. Due to non-ideal dielectric properties, this natural surface acts as a parallel connection between the capacitor and the resistor, denoted by C _S and R _S in FIG. 4, thereby leading to a weaker screening effect of the applied switching voltage.

The present invention is to provide an improved digital electro-optical switch, which can significantly reduce the switching voltage applied to the electrodes, and to completely eliminate the effect of DC drift.

This problem is solved by the present invention, since it relates to a digital electro-optical switch, as defined in the attached claims.

In the present invention, the aforementioned problem is solved by removing the buffer layer from the structure shown in FIG. 3 and forming, between each electrode and the substrate surface below, a thin film made of optically transparent and with little electrical conductivity, such as indium and tin oxide wherein each thin film is wider than the electrode located above, so that it also partially extends to the adjacent output branch of the optical waveguide. In addition, if a frequency range of the digital electro-optical switch is required, the buffer layer can be reintroduced between the electrodes and the optically transparent conductive thin film to reduce the propagation speed of the electrical signal and obtain it equal to the propagation speed of the optical signal. In this case, the electrodes are connected to an optically transparent conductive thin film through holes formed in the buffer layer.

The location of such a thin film between the substrate and the electrodes can significantly reduce the switching voltage applied between the external electrodes with the same effective electric field applied to the optical waveguide, thereby increasing the electro-optical efficiency of the digital electro-optical switch. In addition, the location of such a thin film between the substrate and the electrodes can completely eliminate the screening effect and, therefore, the effect of direct current drift.

Brief Description of the Drawings

Now, for a better understanding of the present invention, preferred embodiments, which are for example purposes only and should not be construed as limiting, will be described with reference to the accompanying drawings (all not to scale), in which:

figure 1 is a standard representation of a digital electro-optical switch 1 × 2 along with its response function;

figure 2 is a schematic view of a known digital electro-optical switch;

figure 3 is a cross section of a digital electro-optical switch of figure 1 along with its electrical dynamic model;

4 is a cross section of a known digital electro-optical switch without a buffer layer under the electrodes along with its electrical dynamic model;

5 is a sectional view of a portion of a digital electro-optical switch according to a preferred embodiment of the present invention;

6 and 7 are sectional views of a portion of a digital electro-optical switch according to other embodiments of the present invention; and

Fig is a table with a list of operating parameters of a digital electro-optical switch according to the present invention.

The implementation of the invention

The following discussion is presented to enable one skilled in the art to make and use the invention. Various modifications to the embodiments without departing from the claimed scope of the present invention will be fully apparent to those skilled in the art. Therefore, the present invention is not limited to the shown embodiments, but in accordance with the broadest scope should be consistent with the principles and features disclosed in this application and defined in the attached claims.

FIG. 5 shows a cross-section of a region of a digital electro-optical switch according to a preferred embodiment of the present invention, with the same positions and signs as the positions and signs in FIGS. 3 and 4, indicating similar elements that will not be described again.

As shown in FIG. 5, a dielectric buffer layer 8 is not formed, and a thin layer or film 9 made of an optically transparent and with little electrical conductivity material, such as indium and tin oxide, is formed between each electrode 6, 7 and the surface of the substrate 3, below. Each thin film 9 has a thickness of 0.05-0.3 μm, and it is dimensioned so that the inter-film gap G ₁ (the distance between adjacent thin films 9) is less than the interelectrode gap G ₀ , and in particular, depending on the working wavelength ranges from 4 to 8 microns. In more detail, each thin film 9 in the transverse direction behind the corresponding electrode 6, 7, located above, and partially covers the adjacent output branch 5 of the optical waveguide 2.

FIG. 6 shows another embodiment of a digital electro-optical switch 1 according to the present invention, in which the width of each thin film 9 is less than the width in FIG. 5, so that the thin films 9 do not partially protrude over adjacent output branches 5 of the optical waveguide 2, and a gap D _{1 is formed} between each thin film 9 and the adjacent output branch 5 of the optical waveguide 2, which is in the range from 0 to 10 μm. In this embodiment, the inter-film gap G ₁ , depending on the operating wavelength, is in the range of 8 to 20 μm.

7 shows a further implementation of the digital electro-optical switch 1 according to the present invention, in which, since the operating frequency range of the digital electro-optical switch 1 is required, a dielectric (SiO ₂ ) buffer layer 8 is provided between each electrode 6, 7 and the optically transparent conductive thin film 9 below to reduce the propagation speed of the electrical signal and equalize its propagation speed of the optical signal. In this case, the electrodes 6, 7 are connected to the optically transparent conductive thin films 9 below, through the holes formed in the buffer layers 8. In contrast to the buffer layer 8 in the embodiment shown in FIGS. 2 and 3, where the continuous dielectric buffer layer 8 is provided protruding on the surface of the substrate 3 on top of the optical waveguide 2, in this other embodiment, the dielectric buffer layer 8 is located only between each electrode 6, 7 and the thin film 9 located below, and therefore, e protrudes over the optical waveguide 2. As shown in FIG. 7, each buffer layer 8 protrudes laterally from the corresponding electrode 6, 7 to the adjacent output branch 5 of the optical waveguide 2, so that a gap D _{2 is formed} between each buffer layer 8 and the adjacent output branch 5 of the optical waveguide 2, which is in the range from 0 to 10 microns.

On Fig shows a table with a list of various operating parameters of the digital electro-optical switch 1 according to the present invention.

Based on the foregoing, it can immediately be understood that the electro-optical properties of thin films 9 under the electrodes 6, 7 can significantly reduce the switching voltage supplied between the external electrodes 6, with the same effective electric field applied to the output branches 5 of the optical waveguide 2, due to thereby increasing the efficiency of the electro-digital electro-optical switch 1. In fact, the electrical conductivity of thin films 9 and the fact that mezhplenochny gap G ₁ is less than mezhelek tional gap G ₀ in digital electro-optical switch 1 of 2, 3 and 4, are the cause of an electrical field lines between the adjacent thin films 9 and not between adjacent electrodes 6, 7, and it leads to increased overlap integral between the optical field in the waveguides 2 and an applied electric field compared with the overlap integral in the digital electro-optical switches 1 of the prior art shown in Fig.2-4. In addition, due to optical transparency (a property that allows light to pass through) of thin films 9, they do not absorb the residual optical energy flowing through the output branches 5 of the optical waveguide 2, therefore, the isolation of the optical waveguide 2 from the electrodes 6, 7 does not adversely affect.

In addition, the electrical property of the thin films 9 can completely eliminate the effect of DC drift. In fact, the free electric charges generated during the formation of the optical waveguide 2 are removed by an external voltage source connected to the electrodes 6, 7, therefore, any accumulation (C _S = 0) of electric charge on the surface of the output branch 5 of the optical waveguide 2 is excluded, and therefore, any shielding effect that causes the DC drift effect.

Finally, it is clear that numerous modifications and variations can be made to the present invention, all falling within the scope of the invention defined in the attached claims.

In particular, it should be understood that the present invention can also be theoretically applied to digital electro-optical switches of any kind, especially digital electro-optical switches with more than two output branches and operating to selectively supply an input optical signal received in the input branch to more than two output branches.

In addition, the substrate 3 can be made of another electro-optical material, such as lithium tantalate (LiTaO ₃ ) or potassium titanyl phosphate.

Claims (3)

1. Electro-optical switch (1), containing:
electro-optical substrate (3) X-section;
A Y-shaped optical waveguide (2) formed in the substrate (3) and including an input branch (4) for connecting to the input optical waveguide and output branches (5) for connecting to the corresponding output optical waveguides;
an electrically conductive electrode structure (6, 7) formed on the substrate (3) and positioned so as to be operably coupled to the optical waveguide (2) to cause selective delivery of the optical signal received on the input optical waveguide to at least one of the output optical waveguides;
the electrode structure (6, 7) includes:
an internal electrode (7) located between the output branches (5), essentially, on the branch section of the optical waveguide (2) and intended to be electrically grounded during operation; and
external electrodes (6) located on the outer side of the output branches (5), on opposite sides of the internal electrode (7), and intended to be electrically supplied with a switching signal during operation to cause the electro-optical switch (1) to switch between the first switching state, in which the transmission of optical energy is increased between the input branch (4) and the first of the output branches (5) and is essentially blocked in the second of the output branches (5), and a second switching state in which the transmission of optical energy uu increases between the inlet branch (4) and the second output branch (5) and substantially blocked in a first output branch (5);
characterized by:
optically transparent conductive film (9) located between each electrode (6, 7) and the substrate (3) and having such dimensions that the interfilm gap (G ₁ ) between each pair of adjacent optically transparent conductive films (9) is less than the interelectrode gap (G ₀ ) between each pair of adjacent electrodes (6, 7);
a dielectric buffer layer (8) located between each optically transparent conductive film (9) and the corresponding electrode (6, 7) located above it; and an electrical connection between each optically transparent conductive film (9) and the corresponding electrode (6, 7) located above;
wherein the optically transparent conductive films (9) are electrically connected to the corresponding electrodes (6, 7) through openings formed in the dielectric buffer layers (8);
and each optically transparent conductive film (9) is dimensioned so as to partially cover the adjacent output branch (5) of the optical waveguide (2) in the transverse direction. 2. The device according to claim 1, wherein each optically transparent conductive film (9) protrudes in the transverse direction behind the electrode (6, 7) located above to the adjacent output branch (5) of the optical waveguide (2). 3. The device according to claim 1 or 2, with each buffer layer (8) protruding in the transverse direction behind the corresponding electrode (6, 7) to the corresponding adjacent output branch (5) of the optical waveguide (2) to form a gap between them.

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