RU2248022C2 - Optical element, method for controlling spectral characteristic of the latter, optical elements system and method for controlling this system - Google Patents

Abstract

FIELD: optics.

SUBSTANCE: element has Bragg phase grid, which is formed in electric-optical material or in additional layer applied to it. Grid is provided with means for forming non-homogenous non-periodic outer electric field along direction of optical radiation movement, by means of which diffraction efficiency of grid is determined. Optical elements system has at least two Bragg phase grids, having different periods and placed serially along direction of movement of optical radiation. System of optical elements is controlled by effecting on grids with non-homogenous non-periodic outside electric field.

EFFECT: higher efficiency.

4 cl, 29 dwg

Description

The invention relates to optics, in particular to optical methods and devices for spectral filtering of optical radiation based on electro-optical crystals, and can be used to create electrically controlled narrow-band filters with a wide wavelength tuning range, selective optical attenuators and light modulators, and optical equalizers.

The recent rapid growth in the volume of transmitted information has led to the emergence of new technologies that provide high bandwidth to modern telecommunication networks. One of the most promising is the spectral multiplexing of channels in fiber-optic communication lines (WDM). The transmission of several tens of spectral channels using different wavelengths of light in the range from 1530 nm to 1600 nm, along one fiber allows to increase its transmission capacity to several terabits per second.

The spectral multiplexing of transmitted channels requires a large number of optical elements, such as couplers, routers, optical filters, modulators, amplifiers, and many others. In addition, for the most efficient use of the new features that WDM provides, it is necessary to control and switch optical signals without converting them into electronic form. Thus, the role of controlled optical elements, for example, optical switches and tunable optical filters, significantly increases.

Known methods for spectral filtering of optical radiation are based on the diffraction of this radiation on a Bragg grating, previously recorded and fixed in a photorefractive crystal [1]. It is possible to use both volume and waveguide versions of the Bragg grating [2].

Actual spectral filtering is as follows. When a crystal is illuminated by a light beam in a direction almost parallel to the vector of the recorded and fixed diffraction grating, light with a wavelength that satisfies the Bragg condition is reflected from the grating in the opposite direction, and the light in the remaining spectral range passes through an optically transparent crystal. Strictly speaking, light is reflected from the grating in a certain narrow range of wavelengths, the central wavelength of which λ _r satisfies the Bragg condition:

where n is the average refractive index of the crystal;

Λ is the period of the diffraction grating, μm.

The spectral selectivity of such a filter depends on the length of the diffraction grating and is described by the following relation:

where d is the width of the spectrum of the emitted signal or the bandwidth of the spectral characteristics of the filter [3]; microns;

n ₁ is the amplitude of the change in the refractive index of the Bragg grating;

T is the length of the diffraction grating, microns.

To select the value of λ _r in the transverse direction to the direction of radiation propagation in the crystal, an electric field of intensity E can be created [4]. In photorefractive crystals, due to the linear electro-optical effect (Pockels effect), the average refractive index n depends on the electric field strength E as follows:

where Δ _n is the increment of the refractive index;

n ₀ is the average refractive index at E = 0;

r is the effective electro-optical coefficient, which depends on the direction of the electric field E with respect to the main crystallographic axes (for the chosen configuration r = const).

By changing the field strength E, the filter is rearranged, choosing a specific wavelength λ _{r of the} filtered radiation. The waveguide configuration allows you to create large control fields with a relatively low applied voltage due to the very small interelectrode distance (-10 μm).

Known holographic optical element [5], performing the function of a narrow-band optical filter. The element consists of a photorefractive crystal, inside of which a holographic reflective Bragg grating is recorded and fixed. The element has a very high spectral selectivity (it is possible to create filters with a spectral characteristic bandwidth of less than 0.01 nm), can be used to filter light with a given wavefront curvature, and also to filter several spectral lines at once.

However, when using the well-known holographic optical element in fiber-optic communication systems, the volumetric configuration of devices based on it leads to the need for additional collimating optics that require precision tuning, which makes these devices very expensive and makes it difficult to mass produce them.

A known method of electrical tuning of a holographic optical filter in a photorefractive crystal [5], by which a spatially uniform electric field is created in the crystal by applying a constant voltage to the crystal. By changing the magnitude of the applied voltage, and, consequently, the electric field E, the filter is tuned by selecting a certain central wavelength λ _r , the filtered radiation.

The disadvantages of this method are: the need for high control voltages, which are determined by the small value of the electro-optical coefficients of the photorefractive materials used, as well as a narrow tuning range limited by electrical breakdown and not exceeding 1 nm for a lithium niobate crystal.

A known system of holographic gratings for spectral multiplexing of channels in telecommunication networks [5] using spectral multiplexing of channels. The system includes a set of reflective holographic Bragg gratings with different periods corresponding to different spectral channels that are recorded in a photorefractive material. The gratings are recorded in such a way that they provide a different direction for the propagation of light diffracted on different gratings and having different central wavelengths. This system provides very high selectivity and can be used to multiplex / demultiplex a large number of closely spaced (<0.4 nm) spectral channels.

However, this system does not provide for control, has a volumetric design, which increases the dimensions of the system, and requires precision optical tuning.

A known method of electrical multiplexing [6, 7], which consists in the fact that several holographic Bragg gratings are recorded in the same volume of a photorefractive crystal at different values of the applied external electric field strength. This method allows you to expand the range of electrical adjustment of the device.

However, when using this method, there are restrictions on the number of switched spectral channels (determined by the maximum number of electrically multiplexed holograms) and on the distance between adjacent channels, due to the stringent requirements of modern communication systems in terms of crosstalk. With electrical switching, the central wavelengths of all the gratings recorded in the crystal simply shift, and the central wavelength of only one of them coincides with the center frequency of the spectral channel currently switched on, while the remaining gratings introduce additional noise.

Known optical switch [8], which includes a paraelectric photorefractive material, inside of which at least one holographic grating with two electrodes deposited on opposite sides of the material is formed to apply an external electric field.

However, this switch uses a KLTN crystal in the paraelectric phase, operating near the phase transition, which significantly increases the requirements for temperature stabilization of this device and limits the range of operating temperatures. In addition, to date, no methods have been developed for the production of high-quality optical waveguides based on this crystal; therefore, devices based on the known electro-holography method have a three-dimensional configuration, require high switching voltage and complex optical settings.

A known method of operation of an optical switch [8]. The method is based on a quadratic electro-optical effect, which allows the electric inclusion of a holographic lattice recorded in a paraelectric crystal due to the combined action of the spatially modulated distribution of the electric field of the charge, creating a holographic lattice inside the crystal, and the spatially uniform applied external electric field. The known method allows switching light both in the direction of propagation and in the wavelength of the radiation.

The known method requires the use of high switching voltage and complex optical settings.

A known optical switch system, consisting of optical switches based on paraelectric photorefractive material [8]. This system of optical switches allows the switching of spectral channels between several input and output optical fibers.

However, this system has a volumetric configuration and is highly sensitive to temperature changes (paraelectric material works near the phase transition point).

A known method of controlling an optical switch system based on a paraelectric photorefractive material [8], which consists in applying a spatially uniform electric field to at least one of the paraelectric photorefractive crystal samples forming an optical switch system, which leads to the electrical inclusion of a holographic lattice recorded in a paraelectric crystal due to the combined action of the spatially modulated distribution of the electric field aryada creating holographic grating within the crystal, and a spatially uniform applied external electric field. Light with a certain wavelength and propagating in a certain direction experiences diffraction on the switched-on holographic grating and changes the direction of propagation. Thus, electrical switching of the optical signals is carried out.

However, the speed of the known method is limited by high control voltages. In addition, this method allows you to switch only between individual discrete states and does not have the ability to continuously scan the center wavelength over the spectral range.

Closest to the claimed invention in terms of essential features is the optical element described in [9]. An optical element consists of a substrate on which a thin film of an electro-optical dielectric material is applied with a refractive index greater than the refractive index of the substrate. A thin film of electro-optical material is used as an optical waveguide. In another embodiment, the electro-optical material (LiNbO ₃ ) is used as a substrate, and the optical waveguide is formed by diffusion of titanium ions. Longitudinal electrodes are applied to the surface of the electro-optical layer, to which a control voltage source is connected. A diffraction grating is formed inside the waveguide layer, which is a periodic perturbation of the optical properties of the waveguide.

The element has a very high spectral selectivity and performs the function of an electrically tunable narrow-band optical filter (it is possible to create filters with a spectral characteristic bandwidth of less than 0.01 nm). The waveguide configuration allows you to create large control fields at a relatively low applied voltage due to the very small interelectrode distance (~ 10 μm).

However, the tuning range of the central wavelength of such an element is limited by the breakdown voltage and, in the case of a device based on a LiNbO ₃ crystal, does not exceed 1 nm.

A known method of controlling the spectral characteristic of an optical element, adopted as a prototype [9] and consisting in the application of a control voltage to the electrodes deposited on the surface of a layer of electro-optical material. The applied control voltage creates an electric field strength homogeneous along the direction of the Bragg grating wave vector inside the electro-optical material. The created electric field causes a change in the refractive index of the electro-optical material, and consequently, a change in the light propagation constant inside the waveguide. This leads to a change in the light intensity reflected from the Bragg grating at a given fixed wavelength or, in the case of non-monochromatic light radiation, to a change in the center wavelength of the optical radiation reflected from the grating.

The tuning range of the central wavelength of the spectral characteristic in a known prototype method is limited by the breakdown voltage and in the case of a device based on a LiNbO ₃ crystal does not exceed 1 nm.

A known system of optical elements adopted for the prototype [10], including a controlled optical waveguide connected to the input and output interfaces from several connected optical waveguides, a set of electrically controlled refractive index gratings is formed inside the controlled optical waveguide, each of the gratings is formed periodically polarized (with a given period) ferroelectric material (e.g., LiNbO _3, KDP, ADP, etc.) and two electrically isolated electrodes connected to the chain RE eskogo management. This system can act as an electrically controlled optical filter, an electrically controlled selective router and a broadband multiplexer.

The disadvantages of the known prototype system are the relatively wide bandwidth of the spectral characteristic (1 nm) and the complex configuration of the input and output signals based on interfaces from several connected waveguides. The limitation on spectral selectivity is associated with the physical nature of the mechanism of formation of an electrically controlled lattice. In ferroelectric crystals, there is a restriction on the minimum size of the ferroelectric domain; a typical value is 1–10 μm; therefore, a restriction arises on the lattice period. The relationship between the bandwidth of the spectral characteristic and the lattice period is determined by expression (2). In addition to the restriction on selectivity, a large lattice period leads to the necessity of using a mechanism for transforming modes propagating in the incident direction, and hence a multimode optical waveguide. At the same time, to match a multimode waveguide with a single-mode optical fiber used in modern telecommunication systems, a complex system of input and output of an optical signal based on coupled single-mode optical waveguides is required.

A known method of controlling a system of optical elements [10], including the application of a spatially uniform electric field to a given lattice of a periodically polarized ferroelectric. At the same time, this grating is activated and the light signal modes at the corresponding central wavelength begin to transform on it. The mode conversion leads to a spatial redistribution of light intensity at this wavelength, as a result of which an optical signal at a given wavelength is output through a special matching system of waveguides to a given output fiber. This method allows electrical switching of the spectral channels at a very high speed (potentially, the switching time may be 10-9 s).

The disadvantages of the known method of controlling a system of optical elements are, firstly, that this method allows you to switch only between individual discrete states and does not provide the ability to continuously scan the central wavelength over the spectral range, and secondly, the filtering is carried out by selecting spatial modes and requires some compromise between the requirements for the level of losses and the level of crosstalk.

The objective of the present invention was to develop an optical element and a system of optical elements in integrated optical design, having a multifunctional purpose (tunable optical filters, selective optical attenuators and modulators, optical switches, etc.), and with high spectral selectivity, a wide range of operating lengths waves, a large dynamic range, low loss and crosstalk, as well as methods of control that would allow the production of electrical cal control their spectral characteristic (such as a spectral alteration, spectrally selective modulation, etc.) at high speed at relatively low control voltages.

The problem is solved by a group of inventions, united by a single inventive concept.

Thus, the problem is solved in that the optical element includes an electro-optical material and a Bragg phase grating, which is formed in an electro-optical material or in an additional layer deposited on an electro-optical material, while the grating is equipped with a means for creating a spatially inhomogeneous aperiodic external electric field of at least parts of the length of the grating along the direction of propagation of optical radiation.

The Bragg phase grating can be formed as a holographic refractive index grating.

The Bragg phase lattice can be formed in an optical waveguide of an electro-optical material, in particular in the form of protrusions and depressions of the surface of an optical waveguide periodically located along the direction of propagation of optical radiation.

Said lattice can be formed in an additional layer deposited on the optical waveguide made of a material whose refractive index during the formation of the lattice changes by at least 10%, for example, made of a photosensitive polymer or chalcogenide material.

In this case, said lattice can be made in the form of protrusions and depressions of the surface of said additional layer periodically arranged along the propagation direction of the optical radiation.

The means for creating a spatially inhomogeneous aperiodic external electric field can be performed in a variety of ways, for example, in the form of two electrodes located on both sides of the lattice, the distance between these electrodes aperiodically changing along the direction of propagation of optical radiation.

The means for creating a spatially inhomogeneous aperiodic external electric field can also be made in the form of four electrodes electrically isolated from each other, arranged in pairs on both sides of the said grating, while the distance between the said electrodes of each pair aperiodically varies along the direction of propagation of optical radiation.

The means for creating a spatially inhomogeneous aperiodic external electric field can be made in the form of at least three electrodes isolated from each other, located on both sides of the lattice and designed to control the strength of the external electric field at different points of the lattice along the propagation direction of optical radiation, for example, in the form of N said electrodes, while the number of said electrodes N satisfies the relation:

D is the tuning range of the central reflection wavelength of the aforementioned lattice, nm, (the variation range of the central length

waves of light reflected from the aforementioned grating when applying the maximum possible voltage to the electrodes),

d is the bandwidth of the spectral characteristics of the above-mentioned lattice, nm.

The problem is also solved by the fact that the control of the spectral characteristic of the optical element, including the electro-optical material and the Bragg phase grating, which is formed in the electro-optical material or in an additional layer deposited on the electro-optical material, the grating is equipped with a means for creating a spatially inhomogeneous aperiodic external electric field on at least part of the length of the grating along the direction of propagation of optical radiation effect on at least a part of the aforementioned grating along the direction of propagation of optical radiation by a spatially inhomogeneous aperiodic external electric field, providing a change in the diffraction of optical radiation, in particular, ensuring its maximum change.

When exposed to a spatially inhomogeneous aperiodic external electric field, the direction of the intensity vector of this electric field on one part of the lattice can be set opposite to the direction of the intensity vector of the said electric field on the other part of the lattice.

The problem is also solved by a system of optical elements, including electro-optical material and at least two Bragg phase gratings having different periods and arranged in series along the direction of propagation of optical radiation, while the above-mentioned gratings are formed in an electro-optical material or in an additional layer deposited on an electro-optical material, and each lattice is equipped with a means for creating an inhomogeneous aperiodic external electric field and controlling it on maskers along the direction of propagation of optical radiation at least on part of the screen length.

In such a system of optical elements, the central reflection wavelength of each of the gratings in the absence of an external electric field differs from neighboring gratings in the spectral range by an amount equal to or greater than D.

The Bragg phase grating in optical elements can be formed as a holographic refractive index grating.

The Bragg phase lattice can be formed in the optical waveguide of the electro-optical material of the optical element of the system, for example, in the form of protrusions and depressions of the surface of the optical waveguide periodically located along the direction of propagation of optical radiation.

This lattice can be formed in an additional layer deposited on the said waveguide made of a material whose refractive index during the formation of the lattice changes by no less than 10%, in particular, from a photosensitive polymer or chalcogenide material, and can be periodically located along the direction of propagation of optical radiation protrusions and depressions of the surface of said additional layer.

In the system of optical elements, said means for controlling the strength of the external electric field can be made in the form of at least three electrodes isolated from each other, located on both sides of the said grating and designed to control the strength of the external electric field at different points of the said grating along the propagation direction optical radiation.

For example, said means for controlling the external electric field can be made in the form of N said electrodes, wherein the number of electrodes N satisfies, as indicated above, the ratio: N≥ 2 D / d.

This embodiment allows more flexible control of the spectral characteristic of the system of optical elements.

The problem is also solved by the method of controlling a system of optical elements, including electro-optical material and at least two Bragg phase gratings having different periods and arranged sequentially along the direction of propagation of optical radiation, while these gratings are formed in an electro-optical material or in an additional layer deposited on electro-optical material, and each lattice is equipped with a means for creating an inhomogeneous aperiodic external electric field and controlling its intensity along the direction of propagation of optical radiation at least on a part of the length of the grating, in which at least one of the above-mentioned gratings is affected along the direction of propagation of optical radiation by a spatially inhomogeneous aperiodic external electric field that ensures a change in the diffraction of optical radiation, in particular its maximum change.

A method of controlling a system of optical elements may include exposing one of the gratings to the aforementioned spatially inhomogeneous aperiodic external electric field while simultaneously exposing at least one of the remaining said gratings to a uniform electric field, while the strength of the uniform electric field can be changed in time according to a given law.

Under the indicated action on one of the gratings by a uniform electric field, its intensity can increase in time from zero to the maximum allowable value, then act on this grating along the direction of optical radiation propagation by a spatially inhomogeneous aperiodic external electric field, which provides the maximum reduction in the diffraction of optical radiation, with simultaneous the action of a uniform electric field on a neighboring grating in the spectral range is mentioned oh system.

It is also possible, under the indicated action on one of the gratings by a uniform electric field, to reduce its intensity in time from the maximum permissible value to zero, and then to act on this grating along the direction of optical radiation propagation by a spatially inhomogeneous aperiodic external electric field, which provides the maximum reduction in the diffraction of optical radiation with simultaneous exposure by a homogeneous electric field to a neighboring grating over the spectral range crumpled system.

The essence of the invention lies in the fact that the diffraction of the Bragg grating is controlled by creating an inhomogeneous distribution of the electric field inside the electro-optical material.

When implementing the inventive control method, optical radiation can be introduced into the electro-optical crystal along the diffraction grating vector, registering as a result of filtration, the optical radiation reflected by diffraction on the grating and the optical radiation transmitted through the crystal.

In addition, it is possible to significantly reduce the control voltage and increase the control speed through the use of a waveguide configuration, where the filtered light radiation propagates inside an optical waveguide formed in an electro-optical crystal.

As indicated above, the Bragg grating inside the waveguide can be formed as a holographic refractive index grating.

In addition, the diffraction efficiency of the Bragg grating can be substantially increased by forming the grating in the form of a relief on the surface of the optical waveguide. Also, to increase diffraction efficiency, you can use an additional layer on the surface of the waveguide with the specified characteristics, then the Bragg grating is formed in this additional layer either in the form of a holographic grating of the refractive index or in the form of a relief of the surface of the additional layer.

As in the known methods, the diffraction of the filtered radiation is controlled by creating a predetermined intensity in the crystal of the electric field, which changes the refractive index of the crystal. However, a distinctive feature of the claimed invention is that the electric field is inhomogeneous in the direction of the wave vector of the lattice. By creating a given spatial distribution of the electric field inside the crystal, it is possible to obtain a certain spectral characteristic of the optical element, which makes this device multifunctional.

So, when applying an external electric field monotonically varying along the wave vector of the grating, one can significantly reduce diffraction efficiency down to a value lower than the permissible level of crosstalk in communication systems based on spectral multiplexing. Based on this, it is possible to create an electrical spectrally selective light switch. The switching speed of such a device is very high due to the electro-optical nature of the control and can be 10-100 GHz.

By varying the degree of heterogeneity of the electric field, we can control the diffraction efficiency of the Bragg grating, so that such a device will act as an electrically controlled selective attenuator or light modulator.

Additionally, it is possible to electrically control the shape of the spectral characteristic of the Bragg grating operating as an optical filter. An example is the modification from a reflective filter to a transmission filter when electric fields are applied to two equal halves of the lattice, differing by an amount providing a phase difference equal to π for light waves reflected from these halves.

The inventive system of optical elements can operate as a universal optical switch of spectral channels. In this case, a certain number of given Bragg gratings is in an inhomogeneous electric field and there is no diffraction on them, and a uniform electric field is either applied to the remaining gratings or not at all (they provide reflection of a given set of wavelengths or spectral channels).

Additionally, the mentioned system of optical elements can operate as an electrically controlled optical equalizer, where the diffraction efficiency of each individual grating is controlled by the degree of spatial heterogeneity of the external electric field applied to it.

Additionally, the mentioned system of optical elements can operate as a narrow-band optical filter with a wide range of continuous electrical tuning.

The essence of the invention is illustrated by drawings, where:

figure 1 shows in a perspective view of the optical element of the prototype (V ₁ , V ₂ - potentials supplied to the electrodes);

figure 2 shows the image in a perspective view of the inventive optical element with six electrodes (V ₁ , V ₂ , V ₃ , ... V ₆ - potentials supplied to the electrodes);

figure 3 shows in a perspective view one of the options for an optical element with two electrodes, the distance between which varies linearly along the direction of propagation of optical radiation;

figure 4 shows a top view of another variant of an optical element with two electrodes, the distance between which varies aperiodically along the direction of propagation of optical radiation;

figure 5 shows a top view of an optical element with three electrodes;

Fig. 6 is a top view of one embodiment of an optical element with four electrodes, the distance between each pair of which varies monotonically along the direction of propagation of the optical radiation;

7 shows a top view of a third embodiment of an optical element with two electrodes;

on Fig shows a graph of changes in electric field strength E (V / cm) along the direction Z (μm) of the propagation of optical radiation in the optical element shown in Fig.7;

figure 9 is a top view of another embodiment of an optical element with four electrodes, the distance between each pair of which monotonically varies along the direction of propagation of optical radiation;

figure 10 shows a graph of changes in the electric field E along the direction Z of the propagation of optical radiation in the optical element shown in figure 9;

11 shows in longitudinal section an optical element with a Bragg phase grating formed in an optical waveguide (Λ is the grating period, nm);

in Fig.12 shows a graph of changes in the refractive index n of the lattice shown in Fig.11 (n _{about the} average refractive index of the lattice, n ₁ - the amplitude of the lattice);

on Fig shows in longitudinal section an optical element with a Bragg phase grating made in the form of periodically located protrusions and troughs of the surface of the optical waveguide (h is the depth of the optical waveguide, μm; Δ h is the height difference of the protrusions and troughs, nm);

on Fig shows in longitudinal section an optical element with a Bragg phase grating formed in an additional layer deposited on an optical waveguide;

on Fig shows in longitudinal section an optical element with a Bragg phase grating, made in the form of periodically located protrusions and troughs of the surface of the additional layer deposited on the optical waveguide;

in Fig.16 shows the spectral characteristic of the reflective Bragg phase grating, λ is the wavelength of optical radiation, nm; λ _o - the Central wavelength of the reflected optical radiation, nm; R is the reflection coefficient for the intensity of optical radiation; (relative value) d is the bandwidth of the spectral characteristic of the Bragg grating, nm;

on Fig shows an optical prototype element with a Bragg phase grating in an electro-optical material, to which an external constant electric field E is applied along the length of the grating (E _bd is the value of the electric field strength at which electric breakdown of the optical element occurs (kV / cm), -E _bd - breakdown electric field of reverse polarity (kV / cm), E _o - the value of the electric field, leading to a change in the reflected central wavelength of the optical radiation by an amount equal to the spectral bandwidth hara teristics Bragg grating (kV / cm), T - lattice length mm);

on Fig shows a graph of the spectral characteristics of the optical element of the prototype depending on the magnitude of the applied external constant field along the length of the lattice of the electric field (a - no electric field, b - at E = -E _bd , c - at E = E _о , g - at E = E _bd );

on Fig shows one of the options applied to the optical element spatially inhomogeneous aperiodic external electric field (E _{π / 2} - electric field strength on the first half of the grating, providing an additional phase incidence of the optical radiation equal to π / 2, -E _{π / 2} - the electric field strength in the second half of the grating, providing an additional phase shift of the optical radiation equal to -π / 2);

- в отсутствие внешнего электрического поля,

- в присутствии внешнего электрического поля);in Fig.20 shows the change in the spectral characteristics of the optical element when applying an electric field to it, shown in Fig.19 (

- in the absence of an external electric field,

- in the presence of an external electric field);

Fig. 21 shows another embodiment of a spatially inhomogeneous aperiodic external electric field applied to the optical element (-E _bd is the electric field strength in the first half of the grating, E _bd is the electric field strength in the second half of the grating);

- в отсутствии внешнего электрического поля,

- в присутствии внешнего электрического поля);in Fig.22 shows the change in the spectral characteristics of the optical element when applying an electric field to it, shown in Fig.21 (

- in the absence of an external electric field,

- in the presence of an external electric field);

in Fig.23 shows another variant of the spatially inhomogeneous aperiodic external electric field applied to the optical element (-E _bd is the electric field strength in the first third of the grating, E _bd is the electric field in the third third of the grating;

- в отсутствие внешнего электрического поля,

- в присутствии внешнего электрического поля);on Fig shows a change in the spectral characteristics of the optical element when applying an electric field to it, shown in Fig.23 (

- in the absence of an external electric field,

- in the presence of an external electric field);

Fig. 25 shows another embodiment of a spatially inhomogeneous aperiodic external electric field applied to the optical element (the electric field strength in the first half of the grating is stepwise changed from -E _bd to 0, and in the second half of the grating the electric field strength is stepwise changing from 0 to E _bd );

- в отсутствие внешнего электрического поля,

- в присутствии внешнего электрического поля);on Fig shows a change in the spectral characteristics of the optical element when applying an electric field to it, shown in Fig (

- in the absence of an external electric field,

- in the presence of an external electric field);

Fig. 27 shows another embodiment of a spatially inhomogeneous aperiodic external electric field applied to the optical element (the electric field strength along the grating length T is stepwise changed from -E _bd to E _bd );

- в отсутствии внешнего электрического поля,

- в присутствии внешнего электрического поля);on Fig shows the change in the spectral characteristics of the optical element when applying an electric field to it, shown in Fig (

- in the absence of an external electric field,

- in the presence of an external electric field);

in Fig.29 shows one of the variants of the claimed system of optical elements (V ₁ , V ₂ , V ₃ , ... ... V ₃₂ - potentials supplied to the electrodes).

The inventive optical element includes a plate 1 of electro-optical material, which can be made of an optical waveguide 2 (see figure 2). As electro-optical material can be used: crystals of ferroelectrics, such as LiNbO ₃ , KNbO ₃ , BaTiO ₃ , SBN, or electro-optical polymer materials using various chromophores (4'-demethylamino-N-methyl-4-steelbazole, 3-methyl- 4-methoxy-4'-nitrostilbene [10]). The Bragg phase grating 3 can be formed both in the material of the plate 1 and the optical waveguide 2, as well as in an additional layer 8. On both sides of the grating 3 there is a means for creating a spatially inhomogeneous aperiodic external electric field in the form of electrodes 4 of various configurations, which through the contacts 5 apply the potentials V ₁ , V ₂ , V ₃ , ... ... V _n (depending on the number and configuration of the electrodes 4, the potentials can be equal or different in magnitude and the same or different in sign). The grating 3 can be formed both in the form of a holographic grating of the refractive index n (see Figs. 11, 12), and in the form of protrusions 6 and depressions 7 periodically located along the propagation direction of the optical radiation (see Fig. 13). The grating 3 can also be formed in an additional layer 8 deposited on the waveguide 2 (see Figs. 14, 15) made of a material whose refractive index during the formation of the grating changes by at least 10%, for example, from a photosensitive polymer (polypentafluorostyrene + polyglycetyl methacrylate + UV 16974 (Union-Carbide) [12], polymethylmethacrylate + AzoDR1 [13]) or chalcogenide material (As ₂ S ₃ , As ₃₀ S ₇₀ , As ₂ Se ₃ ). A spatially inhomogeneous aperiodic external electric field can be created by electrodes 4 of various geometries. For example, two electrodes 4, the distance between which varies aperiodically along the direction of propagation of optical radiation (see Fig.3, 4, 7); three rectangular electrodes 4 (see FIG. 5), to which various potentials V ₁ , V ₂ , V ₃ are supplied, four electrodes 4 of various geometries (see FIGS. 6, 9), six rectangular electrodes 4 (see FIG. 2) to which various potentials V ₁ , V ₂ , V ₃ , V ₄ , V ₅ , V ₆ are supplied; N electrodes, where N satisfies the ratio: N≥ 2 D / d. The above examples do not limit the choice of number of electrodes and their configuration.

The spectral characteristic of the inventive optical element is controlled as follows. Inside the electro-optical material 1 create the necessary distribution of electric field strength.

The necessary spatial distribution of the electric field strength can be specified by the geometric shape of the electrodes 4, to which the potentials V ₁ , V ₂ are applied. 7 shows an example configuration of the electrodes 4 to create a spatially inhomogeneous aperiodic electric field. The inhomogeneity of the electric field is determined by the change in the interelectrode distance. The distribution of electric field strength for the configuration of the electrodes 4 shown in Fig.7, is presented in Fig.8. The maximum possible value of the applied electric field, and therefore the maximum gradient (inhomogeneity), is determined by the electric breakdown field E _bd .

Fig.9 illustrates the possibility of increasing the gradient of the electric field by executing a system that creates a heterogeneous electric field, in the form of two pairs of electrodes 4 with a changing interelectrode distance. The potentials V ₁ , V ₂ are applied to each pair of electrodes, but in the opposite polarity. The distribution of the electric field strength inside the electro-optical material 1 corresponding to this configuration of the electrodes 4 is shown in FIG. 10.

The tool for creating a spatially inhomogeneous aperiodic external electric field in the form of N electrodes 4, to which potentials V ₁ , V ₂ , V ₃ ... ... V _n are applied through contacts 5, allows you to create different distributions of the electric field inside the electro-optical material 1 , and most importantly, the form of the dependence of the distribution of the electric field strength can be changed by changing the values of the applied potentials.

If the electrodes 4, located on one side of the waveguide is applied the same potential V _1, and the electrodes 4 on the other side of the potential V ₂ in electrooptical material 1 creates a spatially uniform electric field (see. Figure 17). Such a field causes a shift in the central wavelength of the spectral characteristic of the reflective Bragg grating 3 (see Fig. 16) without changing the shape (see Fig. 18). The magnitude of the shift of the central wavelength is determined by the intensity of the generated electric field. The field E ₀ corresponds to the shift of the central wavelength of the filtered radiation by the bandwidth of the spectral characteristic d (curve in in Fig. 18). The maximum displacement is achieved by applying an electric field with a strength equal to the electric breakdown strength E _bd (curves b and d in Fig. 18). The sign of the applied electric field determines the direction of displacement. The shift of the central wavelength of the spectral characteristic of the optical element by D is achieved by changing the applied uniform electric field from E _bd to -E _bd and is the full range of tuning of the central wavelength. Such a spatially uniform electric field is created in the optical element of the prototype (see figure 1).

Let us now consider the simplest form of the spatial distribution of an inhomogeneous electric field when an electric field of the same magnitude but opposite in sign is applied to the two halves of the optical element grating (see Figs. 19 and 21). Such a distribution of electric field strength can be created using the system of electrodes 4 shown in FIG. 5, when V ₁ = 0, V ₂ = -V ₃ . In this case, the Bragg grating 3 can be considered into two gratings with spectral characteristics having displaced central wavelengths. If the magnitude of the wavelength shift is much larger than the bandwidth of the spectral characteristic of the grating d, the phase relations can be neglected when summing the light radiation reflected by the two halves of the grating 3. Then the spectral characteristic of the optical element becomes the sum of the spectral characteristics of the two halves of the Bragg grating 3 with spaced central wavelengths having a half T / 2 length, and therefore a smaller reflection coefficient and a wider bandwidth of the spectral characteristic (see FIG. 22).

Of particular interest is the case where the difference in the intensity of electric fields applied to different halves of the grating 3 provides a phase difference of the light radiation reflected from these halves equal to π (see Fig. 19). In the case of small grating amplitudes 3 (n ₁ / n ₀ << Λ / Т) Е _{π / 2} ≈ Е ₀ , the central wavelengths differ only by the spectral characteristic bandwidth d. Then the amplitudes of the light waves reflected from different halves of the grating 3 are added coherently, i.e. taking into account the phase. In this case, a failure occurs in the center of the spectral characteristic of the optical element (see FIG. 20), and the element begins to play the role of a bandpass filter operating on transmission. This example clearly illustrates the possibility of electrical switching of the optical element from the operating state of the reflective optical filter to the state of the transmitting optical filter.

On Fig shows the spatial distribution of the intensity of the applied electric field, when the Bragg grating 3 is divided into three parts. Such a field distribution can be created by the electrode system 4 shown in FIG. 2 when the following relations between the applied potentials are fulfilled, V ₁ = V ₆ , V ₂ = V ₅ , V ₃ = V ₄ . In this case, the light diffracts on three independent parts of the grating 3 with spectral characteristics with shifted central wavelengths. This leads to a decrease in the total reflection coefficient and spectral selectivity, i.e. erosion of the total spectral characteristics of the optical element (see Fig.24).

The reduction in the size of the sections of the lattice 3, to which a uniform electric field is applied, leads to a further decrease in the spectral selectivity and reflection coefficient (see Figs. 25, 26, 27, 28). In the case where the means for creating a spatially inhomogeneous aperiodic external electric field consists of N electrodes 4, it is possible to create an independent electric field in N / 2 sections of the Bragg grating 3 (two electrodes 4 on both sides of the waveguide 2 in each section).

The optimal number of electrodes 4 is selected from the ratio N / 2≥ D / d, i.e. To effectively destroy diffraction (decrease the reflection coefficient and spectral selectivity), it is necessary to break the Bragg grating 3 into at least so many independent parts shifted along the central wavelength of the reflected light radiation by applying a different electric field to all parts of the grating 3 so that the central wavelength of the reflected radiation from each part differed from all the others by at least the bandwidth of the spectral characteristic of the whole lattice. In this case, the electric field is changed along the direction of light propagation in the entire range limited by electric breakdown.

Above, it was illustrated how, by applying a spatially inhomogeneous external electric field, it is possible to change the form of the spectral characteristic of an optical element, and we also considered an example of the destruction of diffraction by a Bragg grating and a decrease in the reflection coefficient and spectral selectivity. The inventive method of controlling the spectral characteristic of an optical element can be used in narrow-band optical attenuators and light modulators. However, the examples described above are not limited to the range of variation of the spectral characteristics of the optical element. It is also possible to obtain other changes in the form of the spectral characteristic of the element with different distributions of the created spatially inhomogeneous aperiodic external electric field.

The inventive system 9 of the optical elements includes a plate 1 of electro-optical material in which an optical waveguide 2 can be made (see Fig. 29). As electro-optical material can be used: crystals of ferroelectrics, such as LiNbO ₃ , KNbO ₃ , ВаТiO ₃ , SBN, or electro-optical polymeric materials using various chromophores (4'-demethylamino-N-methyl-4-steelbazole, 3-methyl- 4-methoxy-4'-nitrostilbene [10]). Several Bragg 3 phase gratings (at least two), which can be formed both in the material of the plate 1 and the optical waveguide 2, as well as in the additional layer 8. The central wavelengths of the spectral characteristics of the gratings 3 differ by the size of the tuning range D when a uniform electric field. On both sides of each lattice 3 there is a means for creating a spatially inhomogeneous aperiodic external electric field in the form of electrodes 4 of various configurations, to which potentials V ₁ , V ₂ , V ₃ , ... ... V _n (depending from the number and configuration of the electrodes 4, the potentials can be equal or different in magnitude and the same or different in sign). The grating 3 can be formed both in the form of a topographic grating of the refractive index n (see Fig. 11, Fig. 12), and in the form of protrusions 6 and depressions 7 periodically located along the propagation direction of the optical radiation (see Fig. 13). The grating 3 can also be formed in an additional layer 8 deposited on the waveguide 2 (see Fig. 14, Fig. 15) made of a material whose refractive index during the formation of the grating changes by at least 10%, for example, from a photosensitive polymer ( polypentafluorostyrene + polyglycetylmethacrylate + UV 16974 (Union-Carbide) [11], polymethylmethacrylate + AzoDR1 [12]) or chalcogenide material (As ₂ S ₃ , As ₃₀ S ₇₀ , As ₂ Ce ₃ ). A spatially inhomogeneous aperiodic external electric field can be created by electrodes 4 of various geometries. For example, FIG. 29 shows a system of 9 optical elements, comprising 32 electrodes 4 (8 for each grating 3) with potentials V ₁ , V ₂ , V ₃ ... V ₃₂ supplied to them. The given example does not limit the choice for the system 9 of the number of electrodes 4 and their configuration. In fact, the claimed system of optical elements 9 represents several sequentially located the claimed optical elements described above, with shifted central wavelengths of spectral characteristics, made in an integrated optical design on one plate 1 of electro-optical material.

The spectral characteristic of the claimed system of 9 optical elements is controlled as follows. Each individual optical element of the system 9 can be independently controlled, which makes the system 9 flexible and multifunctional. Consider a control method that allows you to use system 9 as a narrow-band optical filter, electrically tunable in a wide range of wavelengths. In one optical element of the system 9 create a spatially uniform electric field using the method described previously. Moreover, this optical element reflects optical radiation in a narrow range of wavelengths d. The central wavelength of the spectral characteristic of this optical element is determined by the Bragg phase lattice period 3 and can be tuned in the spectral range D by changing the magnitude of the applied spatial spatially uniform electric field (see Fig. 17, 18). Reflection from the remaining optical elements of system 9 can be reduced to a predetermined small value by creating a spatially inhomogeneous aperiodic electric field in them (see Figs. 27, 28). A method of reducing the reflection coefficient of an optical element has been described above. Having the ability to create a spatially homogeneous electric field on different optical elements of system 9 (include reflection from this element) and at the same time, destroying the diffraction (turning off reflection) on the remaining optical elements by applying a spatially inhomogeneous aperiodic electric field, we can carry out the reconstruction of the optical filter in the wavelength range M · D, where M is the total number of optical elements in the system 9.

The given example of a method of controlling the system 9 of optical elements does not limit the possibilities of using other control methods. Any other options are possible for creating a spatially homogeneous electric field in some optical elements of the system 9, and a spatially inhomogeneous aperiodic electric field in other optical elements of the system 9, if the optical element system 9 is used to perform other operational functions other than a tunable filter, for example, system 9 as an optical switch.

Sources of information

1. G.A. Rakuljic, V. Leyva. - "Volume holographic narrow-band optical filter". - Opt. Lett. - 1993, Vol. 18, No. 6, p. P. 459-461.

2. J. Hukriede, I. Nee, D. Kip, E. Kraetzig. - "Thermally fixed reflection gratings for infrared light in LiNbO3: Ti: Fe channel waveguides." - Opt. Lett. - 1998, Vol.23, N 17, p.p. 1405-1407.

3. S.K. Madsen, J.H. Zhao. -Optical filter design and analysis: A signal processing approach. John Willey & Sons, New York, 1999.

4. R. Muller, J. V. Alvarez-Bravo, L. Arizmendi, J. M. Cabrera. - "Tuning of photorefractive interference filters in LiNbO3." - J. Phys. D: Appl. Phys. - 1994, Vol. 27, p. P. 1628-1632.

5. US patent No. 5440669, IPC G 02 B 5/32, G 03 H 1/18, G 03 H 1/26, published 08.08.1995.

6. M.P. Petrov, V. M. Petrov, A. V. Chamrai, C. Denz, T. Tschudi. - "Electrically controlled holographic optical filter". - Proc. 27th Eur. Conf. on Opt. Comm. (ECOC’01 - Amsterdam) .- Th.F.3.4, p.p.628-629 (2001).

7. M.P. Petrov, S.I. Stepanov, A.A. Kamshilin.- "Light diffraction from the volume holograms in electrooptic birefringent crystals". - Opt. Commun. - 1979, No. 29, p.p. 44-48.

8. International application No. WO 00/02098, IPC G 03 H 1/02, published January 13, 2000.

9. US patent No. 4039249, IPC G 02 B 5/14, published 02.08.1977.

10. US patent No. 5832148, IPC G 02 B 6/26, published 03.11.1998.

Nonlinear optical effects and materials. Berlin, Springer-Verlag, 2000, 540 p.eleven.

Nonlinear optical effects and materials. Berlin, Springer-Verlag, 2000, 540 p.

12. C. Pitois, A. Hull, D. Wiesmann - "Absorption and scattering in lowloss polymer optical waveguide". - J. Opt. Soc. Am. - 2001, Vol. 18, No. 7, p. P. 908-912.

13. H. Rezig, G. Vitrant. - "Feasibility of optically controlled integrated Mach-Zehnder device based on Azo dye-doped PMMA thin films." - Opt. Commun. - 2001, Vol.200, p.p.261-269.

Claims (30)

1. An optical element comprising electro-optical material and a Bragg phase grating, which is formed in an electro-optical material or in an additional layer deposited on an electro-optical material, wherein the grating is equipped with a means for creating a spatially inhomogeneous aperiodic external electric field of at least a part of the length of the grating along the direction propagation of optical radiation. 2. The optical element according to claim 1, characterized in that said grating is formed in the form of a holographic grating of a refractive index. 3. The optical element according to claim 1, characterized in that said grating is formed in an optical waveguide of said electro-optical material. 4. The optical element according to claim 3, characterized in that said grating is made in the form of protrusions and depressions of the surface of the optical waveguide periodically arranged along the direction of propagation of the optical radiation. 5. The optical element according to claim 3, characterized in that said additional layer is deposited on said waveguide, wherein the refractive index of the additional layer during the formation of the grating changes by at least 10%. 6. The optical element according to claim 5, characterized in that said additional layer is made of a photosensitive polymer or chalcogenide material. 7. The optical element according to claim 5, characterized in that said grating is made in the form of protrusions and depressions of the surface of said additional layer periodically arranged along the direction of propagation of the optical radiation. 8. The optical element according to claim 1, characterized in that the said means for creating a spatially inhomogeneous aperiodic external electric field is made in the form of two electrodes located on both sides of the said grating, while the distance between the said electrodes aperiodically varies along the direction of propagation of optical radiation. 9. The optical element according to claim 1, characterized in that the said means for creating a spatially inhomogeneous aperiodic external electric field is made in the form of four electrodes electrically isolated from each other, arranged in pairs on both sides of said lattice, the distance between the said electrodes of each pair aperiodically varies along the direction of propagation of optical radiation. 10. The optical element according to claim 1, characterized in that the said means for creating a spatially inhomogeneous aperiodic external electric field is made in the form of at least three electrodes isolated from each other, located on both sides of the lattice and designed to control the voltage of the external electric fields at various points of said grating along the direction of propagation of optical radiation. 11. The optical element of claim 10, characterized in that said means for creating a spatially inhomogeneous aperiodic external electric field is made in the form of N said electrodes, while the number of said electrodes N satisfies the relation N≥2 D / d, where D is the tuning range of the central reflection wavelength of said grating, nm; d is the bandwidth of the spectral characteristics of the above-mentioned lattice, nm. 12. The method for controlling the spectral characteristic of an optical element according to claim 1, comprising influencing at least a portion of said grating along the direction of propagation of optical radiation with a spatially inhomogeneous aperiodic external electric field that provides a change in the diffraction efficiency of the grating. 13. The method according to p. 12, characterized in that they act spatially inhomogeneous aperiodic external electric field, providing the maximum change in the diffraction efficiency of the lattice. 14. The method according to p. 12, characterized in that the direction of the intensity vector of said electric field on one part of said lattice is chosen opposite to the direction of the intensity vector of said electric field on another part of said lattice. 15. A system of optical elements, including electro-optical material and at least two Bragg phase gratings having different periods and arranged successively along the direction of propagation of optical radiation, said gratings being formed in an electro-optical material or in an additional layer deposited on electro-optical material, and each the lattice is equipped with a means for creating an inhomogeneous aperiodic external electric field and controlling its intensity along the direction of the races spatial optical radiation at least part of the length of the grating. 16. The system of clause 15, wherein the central reflection wavelength of each of the gratings in the absence of an external electric field differs from neighboring gratings in the spectral range by an amount equal to or greater than D. 17. The system of claim 15, wherein said lattice is formed as a holographic refractive index lattice. 18. The system of claim 15, wherein said grating is formed in an optical waveguide of said electro-optical material. 19. The system according to p. 18, characterized in that the said lattice is made in the form of protrusions and depressions of the surface of the optical waveguide periodically arranged along the direction of propagation of optical radiation. 20. The system according to p. 18, characterized in that the said additional layer is deposited on the said waveguide, and the refractive index of the additional layer during the formation of the lattice changes by at least 10%. 21. The system according to claim 20, characterized in that the additional layer is made of a photosensitive polymer or chalcogenide material. 22. The system according to claim 20, characterized in that said grating is made in the form of protrusions and depressions of the surface of said additional layer periodically arranged along the direction of propagation of the optical radiation. 23. The system according to clause 15, wherein said means for controlling the external electric field is made in the form of at least three electrodes isolated from each other, located on both sides of the lattice and designed to control the external electric field in various points of the aforementioned grating along the direction of propagation of optical radiation. 24. The system according to item 23, wherein said means for controlling the external electric field is made in the form of N said electrodes, while the number of said electrodes N satisfies the ratio N≥2 D / d. 25. The method of controlling the system of optical elements according to clause 15, including the impact on at least one of the aforementioned gratings along the direction of propagation of optical radiation spatially inhomogeneous aperiodic external electric field, providing a change in diffraction efficiency of the grating. 26. The method according A.25, characterized in that the impact along the direction of propagation of optical radiation on at least one of the above-mentioned gratings spatially inhomogeneous aperiodic external electric field, providing the maximum change in diffraction efficiency of the grating. 27. The method according A.25, characterized in that at the same time act on at least one of the other mentioned gratings with a uniform electric field. 28. The method according to item 27, wherein in said exposure to one of said gratings with a uniform electric field, its intensity is changed in time according to a given law. 29. The method according to p. 28, characterized in that when the specified impact on one of the above-mentioned gratings with a uniform electric field, its intensity increases in time from zero to the maximum allowable value, then they act on this grating along the direction of optical radiation propagation by a spatially inhomogeneous aperiodic external an electric field that provides the maximum reduction in the diffraction of optical radiation, with a simultaneous exposure to a spectrally adjacent neighboring electric field range of the lattice of the said system. 30. The method according to p. 23, characterized in that when the specified exposure to one of the above-mentioned gratings by a uniform electric field, its intensity is reduced in time from the maximum allowable value to zero, and then they are applied to this grating along the direction of propagation of optical radiation by spatially inhomogeneous aperiodic an external electric field that provides the maximum reduction in the diffraction of optical radiation, with the simultaneous exposure to a spectrally adjacent neighboring electric field in the range of the lattice of said system.

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