RU2234723C2 - Method for spectral-selective conversion of modes of optical irradiation in wave-guide and apparatus for performing the same - Google Patents

Abstract

FIELD: wave-guide optics.

SUBSTANCE: apparatus includes wave-guide having core and envelope whose part is formed of liquid crystal and is arranged between electrodes. Wave -guide mode interferes on periodic changes of refraction index along axis of wave-guide. Said changes are provided due to using chiral liquid crystal with helical symmetry with axis of helix oriented along axis of wave-guide. Spectral range of mode conversion is defined by pitch of chiral liquid crystal helix. Said pitch is changed at applying outer field.

EFFECT: enhanced characteristics of frequency-selection conversion.

5 cl, 12 dwg

Description

The invention relates to waveguide optics, to devices for controlling the parameters of electromagnetic radiation propagating in the waveguide.

In the simplest case, the dielectric waveguide consists of a core and a cladding, and the refractive index of the core is greater than the refractive index of the cladding / 1, 2 /. The properties of the waveguide vary slightly along one or two spatial directions for linear and planar waveguides, respectively. These directions are called the axes (axis) of the waveguide. If the light falls at a fairly gentle angle to the interface between the shell and the core of the latter, then there is a complete internal reflection of the light, holding it near the core. Due to this, one or several waveguide modes of radiation can be propagated in the optical waveguide, which are localized near the core of the waveguide (at a distance of the order of the wavelength) and are not radiated outward.

от этих переменных выражается формулойEach mode is characterized by a harmonic dependence on time t and the spatial coordinate z along the axis of the waveguide. For the waveguide mode, the harmonic dependence of the electric field vector

from these variables is expressed by the formula

where β is the mode propagation constant, ω is the cyclic frequency, j is the imaginary unit. Instead of the cyclic frequency ω, the usual frequency ν = ω / 2π, the wave number in free space k = ω / c and the wavelength in free space λ = 2π / k = 2πc / ω are also uniquely associated with it. Therefore, when one speaks of the wavelength of the waveguide mode, this usually means the wavelength λ in free space calculated from the mode frequency according to the above formula, while the spatial periodicity of the mode along the waveguide axis is characterized by the propagation constant β / 1, 2 /.

The frequency ω and the propagation constant β of the waveguide mode are related the dispersion equation / 2 /, depending on the type of mode, as well as on the geometry and optical properties of the waveguide, which in the general case has no explicit solution.

, где а - радиус сердцевины) существует единственная мода, обозначаемая НЕ₁₁ /1, 2/, которая при одинаковых значениях β и ω может обладать двумя различными круговыми поляризациями - правой и левой. В случае, когда волокно слабонаправляющее, то есть если n₁-n₂<<1, приближенное решение дисперсионного уравнения дает зависимость k≈β/n₁.In particular, in a two-layer cylindrical optical fiber, with the refractive indices of the core and clad n ₁ and n _2, respectively, at sufficiently low frequencies (if the waveguide parameter

Where a - radius of the core) there is a unique fashion, denoted NO _11/1, 2 / which, when the same values of β and ω may have two different circular polarizations - right and left. In the case where the fiber is weakly directing, that is, if n ₁ -n ₂ << 1, an approximate solution of the dispersion equation gives the dependence k≈β / n ₁ .

If the optical fiber is strictly uniform along the axis, then the various modes propagate independently, without passing one into another. In the case when the uniformity of the fiber along the axis is violated, various waveguide modes that differ in frequency, propagation constant, or polarization can pass one into another. This general principle is widely used to control radiation propagating through optical fiber. In particular, if the uniformity of the refractive index of the optical fiber varies periodically along its axis, this leads to frequency-selective resonance coupling of the propagating radiation modes. The connection of the two radiation modes means that when one mode is introduced into the waveguide, it is at least partially converted to another. For the coupling between the waveguide modes, it is necessary that the temporal frequencies of these modes coincide, which is assumed everywhere below. The connection between modes is subject to the equality of the beat period between the modes and the period of changes in the refractive index. Namely, if the propagation constant of one mode is β ₁ , and of the second mode β ₂ , then the resonant coupling between the modes is ensured when the relation / 2 /

where Λ is the period of change of the refractive index along the fiber axis. Since the constant interactions depend on the frequency of the e / m oscillations ν in various ways, the interaction condition (1) can be considered as an equation imposed on the frequency ν for a fixed type of waveguide modes. This equation in the general case holds only for a finite set of frequencies ν ₁ , ..., ν _N. This means that if one of the waveguide modes is introduced into a segment with an Λ-periodically changing refractive index, smoothly changing its frequency, then the effective interaction of the modes will occur only near the frequencies ν ₁ , ..., ν _N. By changing Λ, we can rearrange this set of frequencies.

One of the most important applications of resonant mode coupling is the coupling between modes propagating in opposite directions, that is, spectrally selective reflection of radiation in an optical fiber. In this case, the propagation constants are equal in magnitude and opposite in sign, β ₁ = -β ₂ , and formula (1) takes the form

The last formula coincides with the well-known Bragg-Wulf condition / 3 / for the wavelength reflected by a flat layered structure at normal incidence. Correspondingly, fiber segments with a periodic change in the refractive index along the axis, with a period Λ equal to π / β, in the literature are called Fiber Bragg Grating / 4-9 /. If, by means of periodic changes in the refractive index, unidirectional modes are coupled in the fiber, then the period of change of the refractive index obviously exceeds the wavelength 2π / β and such devices are called Long-Period Fiber Bragg Grating / 10, 11 /.

/4, 6/. Обычно периодическое вдоль оси изменение показателя преломления предварительно формируется в сердцевине оптоволокна из плавленого кварца, легированной бором и германием, с помощью интенсивного ультрафиолетового излучения /5, 7-9/. На оптоволокно воздействуют стоячей волной ультрафиолетового (УФ) излучения, что формирует области с повышенным показателем преломления в местах расположения пучностей стоячей волны. Изменения показателя преломления в веществе сердцевины волновода сохраняются после прекращения УФ воздействия. Управление длиной волны световой моды, отражаемой в обратном направлении, осуществляется за счет изменения периода неоднородности показателя преломления Λ путем растяжения/сжатия волновода вдоль его оси с помощью внешнего механического напряжения /12-16/, или через управление температурой вещества волновода /17, 18/, или посредством приложения магнитного поля к веществу волновода /19/.A known method of spectrally-selective coupling of waveguide modes of electromagnetic radiation propagating in the opposite direction in a dielectric waveguide, as a result of which their partial reflection is achieved. The method consists in the fact that in the segment of the waveguide under the action of external radiation, a change in the refractive index of the material of the core of the waveguide with a period Λ periodically along the axis of the waveguide is provided, which leads to partial reflection in the opposite direction of the waveguide modes with a propagation constant

/ 4, 6 /. Typically, a periodic change along the axis of the refractive index is preliminarily formed in the core of fused silica fiber doped with boron and germanium using intense ultraviolet radiation / 5, 7-9 /. The fiber is exposed to a standing wave of ultraviolet (UV) radiation, which forms regions with a high refractive index at the locations of the antinodes of the standing wave. Changes in the refractive index in the material of the core of the waveguide are maintained after the cessation of UV exposure. The wavelength of the light mode reflected in the opposite direction is controlled by changing the period of non-uniformity of the refractive index Λ by stretching / compressing the waveguide along its axis using external mechanical stress / 12-16 /, or by controlling the temperature of the material of the waveguide / 17, 18 / , or by applying a magnetic field to the material of the waveguide / 19 /.

The disadvantage of this method is the small range of variation of the wavelength of the reflected radiation (10-15 nm), as well as high sensitivity to mechanical stress (for example, to change the central length of the reflection range by 1.5 nm, it is sufficient to stretch a quartz fiber with a diameter of 150 μm with a force of 20 g ) and to a change in the temperature of the waveguide (0.01 nm / ° C).

, где n - показатель преломления сердцевины волокна. Спектральная ширина диапазона отраженных волн определяется амплитудой периодических изменений показателя преломления сердцевины волокна. Последняя, в свою очередь, фиксируется при изготовлении устройства, когда отрезок оптоволокна из плавленого кварца, легированного бором и германием, помещается в стоячую волну УФ излучения высокой интенсивности, что приводит к увеличению показателя преломления волокна в областях расположения пучностей стоячей УФ волны. Относительное изменение показателя преломления легированного кварца пропорционально интенсивности УФ излучения. Центральная частота спектральной полосы отраженных волн может регулироваться в сравнительно узком интервале за счет управления температурой решетки и связанной с ней термической деформацией (диапазон перестройки не более 1-2 нм для длины волны 1,5 мкм /17, 18/) или же путем механического натяжения и сжатия волокна вдоль его оси (диапазон перестройки 10-20 нм для длины волны 1,5 мкм /12, 16/).A device for spectrally selective reflection of light waves propagating in an optical fiber (Bragg optical fiber lattice) is known. The Bragg fiber optic lattice is a segment of an optical fiber having a shell and a core, and the core has a periodically (with a lattice period Λ) refractive index varying along the fiber axis / 4, 6 /. In this case, the center frequency of the spectral range of the reflected waves corresponds to the wavelength λ associated with the lattice period Λ by the relation

where n is the refractive index of the fiber core. The spectral width of the range of reflected waves is determined by the amplitude of the periodic changes in the refractive index of the fiber core. The latter, in turn, is fixed during the manufacture of the device when a piece of fused silica fiber doped with boron and germanium is placed in a standing wave of high-intensity UV radiation, which leads to an increase in the refractive index of the fiber in the areas where the antinodes of the standing UV wave are located. The relative change in the refractive index of doped quartz is proportional to the intensity of UV radiation. The central frequency of the spectral band of reflected waves can be controlled in a relatively narrow range by controlling the lattice temperature and the associated thermal deformation (tuning range of not more than 1-2 nm for a wavelength of 1.5 μm / 17, 18 /) or by mechanical tension and compressing the fiber along its axis (tuning range of 10-20 nm for a wavelength of 1.5 μm / 12, 16 /).

A disadvantage of the known device is a narrow tuning band (1-2 nm) of the central frequency of the reflected light during temperature control of the central frequency, as well as the need to use mechanical components and the associated sensitivity to vibration when controlling the central frequency of the reflected waves by the compression / extension method of the optical fiber.

A known method of converting unidirectional light modes with different polarization, angular order and localization (proper waveguide modes localized near the core, and cladding modes / 2, 11 /, guided by the cladding due to total internal reflection at the cladding boundary with air) propagating in the optical waveguide with different propagation constants β ₁ and β ₂ , which consists in the formation in the core of the optical fiber of regions periodic along the fiber axis with a high refractive index / 10, 11 /. In this case, the difference between the propagation constants of the interacting modes β ₁ -β ₂ is determined by the period of change of the refractive index Λ according to formula (1), that is, to ensure the interaction of the modes, the period of non-uniformity of the refractive index must coincide with the beat length of these modes. To create refractive index changes that are periodic along the fiber axis, as well as to create Bragg fiber gratings, we use the effect of changing the refractive index of fused silica (SiO ₂ ) under intense ultraviolet irradiation in the spectral range 200-300 nm / 7-9 /. A piece of fiber is placed in a standing UV wave. At the locations of the antinodes of the standing wave, intense UV radiation causes an increase in the refractive index of quartz, which persists after the UV radiation is removed. The period of change in the refractive index Λ is determined by the wavelength of UV radiation and the relative position of the standing wave and the length of the optical fiber. To increase the UV photosensitivity, quartz is doped with boron and germanium ions.

The disadvantage of this method is the high temperature sensitivity and limited control over the period Λ (no more than 4-5%), requiring mechanical systems of application of force, and therefore, sensitive to vibration.

A device for spectrally selective coupling of unidirectional waveguide modes in an optical fiber (long-period Bragg grating), which is a segment of an optical fiber, the core refractive index of which periodically varies along the axis of the optical fiber / 10, 11 /. In this case, the difference between the propagation constants of the interacting modes satisfies condition (1). The coupled modes can differ in frequency, polarization, and radial order / 2 /. In addition, it is possible to connect the actual waveguide modes (localized in the fiber core) with the so-called shell modes, which are localized in the shell due to total internal reflection at the boundary of the shell with air / 11 /. Long-period Bragg gratings are usually formed in an optical fiber by using the effect of changing the refractive index of fused silica doped with boron and germanium under the action of UV radiation, as described above.

A disadvantage of the known device is the complexity of changing the lattice period necessary to change the spectral range of mode interaction. Changing the lattice period requires the precision application of mechanical force / 12-16 / or maintaining a given temperature of the fiber lattice with high accuracy / 17 /. In addition, for the complete conversion of the energy of one mode to another, it is necessary to control with high accuracy both the lattice strength — the relative change in the refractive index of the core — and its length, which is a complex problem with known methods for manufacturing long-period fiber gratings.

A known method of spectrally-selective conversion of optical waveguide modes, which consists in ensuring the propagation of electromagnetic radiation in a waveguide, at least partially made of a substance with the properties of an oriented nematic or smectic liquid crystal (LC) / 20 /. In smectic LCs, long rod-shaped molecules are located in planar monomolecular layers, and the molecules are oriented perpendicular to the plane of the layers. A nematic liquid crystal consists of long rod-shaped molecules and has a long-range orientational order (the axes of the molecules are directed along the marked direction — the “director” n) in the absence of a coordination order of the centers of mass of the molecules. The orientation of the LC molecules determines the principal axes of its refractive index tensor. Under the influence of an external electric field, the director of a nematic LC tends to rotate along or across the direction of the field depending on the dielectric anisotropy of the molecules, thus changing the principal axes of the refractive index / 21, 22 /. At the same time, a control electric field is applied to the liquid crystal part of the waveguide, which changes the orientation of the LC, and with it its optical properties (the direction of the principal axes of the refractive index tensor), providing the possibility of controlled conversion of the waveguide modes of electromagnetic radiation.

A disadvantage of the known method is the small spectral selectivity of the mode conversion (spectral bandwidth of more than 50 nm) associated with the uniformity of the refractive index tensor in these classes of LC substances.

A device for the controlled conversion of waveguide light modes propagating in a waveguide is an optical fiber, the core of which is made of oriented nematic or smectic liquid crystal / 20 /. The transformation of the modes of the light propagating in the optical fiber is controlled by means of a controlled change in the optical properties of the liquid crystal core by applying an electric field to the LC core, directed perpendicular to the waveguide axis. The electric field changes the orientation of the LC molecules, and with it the direction of the principal axes of the tensor of the refractive index of the core of the waveguide made of LC / 21, 22 /. In combination with polarizers, reflectors and birefringent elements, this device allows you to modulate the phase, polarization and intensity of the light propagating in the optical fiber, as well as to carry out its spectral filtering and second harmonic generation.

The main disadvantage of the known device is the impossibility of creating within the framework of these substances an LC waveguide core, the optical properties of which (in particular, the refractive index tensor) would change periodically along the axis of the waveguide and allow the period to be controlled by applying an external electric field. As a result, the known device cannot realize the Bragg reflection mode of the waveguide modes propagating through the optical fiber, as well as the spectrally selective conversion of unidirectional modes.

There is a method of spectrally-selective conversion of the waveguide mode of electromagnetic radiation, involving the propagation of light in a substance with a chiral symmetry of the refractive index tensor, located in the gap between the ends of the same, coaxially arranged waveguides, and overlapping the entire cross section of the waveguide, and the axis of the spiral of the specified substance with chiral symmetry coincides with the common axis of the waveguides / 23 /. Substances with chiral symmetry - chiral liquid crystals (CLCs) consist of optically active elongated molecules that fit into monomolecular layers with the same molecular direction in the layer / 24 /. The direction of the molecules in each layer characterizes the vector n (director). The director in each layer is parallel to the plane of the layer and rotated relative to the director of the previous layer by an angle around an axis perpendicular to all layers. This leads to the formation in the CLC of an ordered twisted (spiral) structure with a helix pitch p ₀ , which corresponds to the rotation of the director n by an angle 2π around the axis of the helix. Depending on the chemical structure of the molecules, the helix can be right-handed or left-handed.

The optical properties of CLCs are determined by the described structure with helical (chiral) symmetry. Namely, the refractive index tensor of the CLC has an eigenvalue n _e along the main axis, which coincides with the direction of the director in the layer, and two equal eigenvalues n _o corresponding to the main axes perpendicular to the director. By definition, n _e > n _o . From layer to layer, only the direction of the main axes changes, repeating the spiral structure of the CLC, while the values of n _o and n _e remain constant. Thus, the refractive index tensor has helical symmetry, that is, the tensor of any two points of the CLC differs only in rotation about the axis of the CLC spiral. The period of optical properties is half the helix pitch p ₀ , since the properties of each individual layer are the same both in the n direction and in the -n direction opposite to it (the CLC structure is non-polar).

When electromagnetic radiation propagates in a substance with chiral symmetry, due to a periodic change in the refractive index tensor along the CLC axis, light diffraction occurs, similar to Wulf-Bragg diffraction for x-rays on a crystal lattice. Of particular interest are the properties of light diffraction by CLC during normal incidence, when the light beam is perpendicular to the monomolecular layers. During normal incidence, light is reflected whose circular polarization direction coincides with the CLC helix; light of opposite circular polarization passes without loss. The full reflection of light with circular polarization occurs in a certain range of the spectrum, the middle of which corresponds to the propagation constant of the waveguide mode β = p ₀ . This effect is called spectrally selective reflection of light in chiral LC / 24 /. The width of the spectral reflection interval Δλ is proportional to the optical anisotropy n _е -n ₀

When light propagates in a CLC located between the ends of coaxially oriented waveguides, spectrally selective reflection of waveguide modes of radiation with a certain circular polarization in the opposite direction occurs. In this case, the reflected light is partially introduced into the waveguide through the end face of the waveguide adjacent to the membrane.

The disadvantage of this method is the low spectral selectivity (Δλ of at least 20 nm), determined by the optical anisotropy n _e -n _{o of a} substance with chiral symmetry, which determines the width of the spectral range of reflection of the membrane located between the ends of the waveguides, as well as the impossibility of tuning the spectral range of the reflected waves. In addition, this method allows you to convert to each other only modes with opposite circular polarization. When placing a CLC substance according to the specified method at the place of its placement, the waveguide structure is violated, which requires a sheath with a refractive index greater than that of the core, which leads to undesirable light losses.

Known fiber optic device for spectrally selective reflection of a propagating waveguide mode with a specific wavelength and with a given circular polarization / 23 /. The device is a membrane of a substance with a chiral symmetry of optical properties, located in the gap between the ends of the coaxially oriented segments of the optical fiber. The specified membrane provides spectrally selective reflection of the radiation propagating in the waveguide with circular polarization, which coincides with the orientation of the spiral material of the membrane.

The disadvantages of the known device are the reflection of light with only one circular polarization, as well as the inability to control the wavelength of the reflected light. In addition, when placing a CLC membrane between the ends of the waveguides at the place of its placement, there is no waveguide structure that requires a sheath with a refractive index greater than that of the core, which leads to undesirable light emission outside the waveguide.

A known method of converting the parameters of a mode of electromagnetic radiation propagating through an optical fiber, namely, modulating the intensity of a mode, which consists in ensuring the propagation of the mode in a shell made of a substance with the properties of an LC and placed in an external electric field / 25 /. The electric field changes the orientation of the LC molecules, and with it the direction of the main optical axes of the refractive index tensor of the LC part of the shell, which leads to an increase in the effective index of the shell. This violates the waveguide property of the structure, which requires that the refractive index of the core be greater than the refractive index of the cladding, and provides a partial controlled output of the energy of the waveguide mode outside the optical fiber.

A disadvantage of the known method for converting the parameters of the waveguide mode is the low frequency selectivity (not better than 10%) due to the uniformity of the optical properties of the LC part of the waveguide at distances of the order of the wavelength of the propagating mode and, as a consequence, the impossibility of frequency-selective conversion of waveguide modes of electromagnetic radiation into each other. In particular, this method does not allow at least partial reflection of the waveguide mode in the opposite direction while maintaining waveguide properties.

A device / 25 / is known, which modulates the intensity of electromagnetic waves propagating along a cylindrical two-layer waveguide, and is a piece of fiber containing a core and a shell made of a liquid crystal in a certain interval, and electrodes placed on the liquid crystal shell in such a way that the liquid crystal part of the waveguide located between the electrodes. The described device is schematically shown in figure 1, which shows an axial section of an axisymmetric device. In figure 1, the number 1 denotes the core of the optical fiber, 2 - the sheath on the inlet sections of the fiber, 3 - the LCD sheath, 4 - the electrodes, 5 and 6 - intensity profiles of the waveguide mode before and after passing through the device, respectively. The orientation of the LC shell molecules, and with them the orientation of the principal axes of the refractive index tensor, is controlled by the electric field between the electrodes. As a result of the reorientation of the principal axes of the refractive index tensor of the LC cladding, the waveguide properties of the device are violated, namely, the value of the cladding refractive index in the direction perpendicular to the axis of the waveguide becomes equal to or greater than the refractive index of the waveguide core. As a result, a decrease in the intensity of the waveguide mode propagating in the device is controlled by the applied field.

A disadvantage of the known device is the lack of frequency-selective coupling of various modes of light propagating along the waveguide, since within the framework of these substances the effective refractive index of the LC cladding for waveguide modes is constant along the axis of the waveguide and its optical properties are determined by an external field applied to the LC, whereas radiation modes, the refractive index should periodically change along the axis of the waveguide. In particular, the known device cannot provide coupling of waveguide modes propagating in opposite directions.

A known device comprising a waveguide containing a core and a sheath made of an LC on a segment of the waveguide and equipped with electrodes placed along the waveguide in the region of the LC part of the waveguide is selected as the closest analogue. In the known device uses nematic and smectic LCD.

The known device, selected as the closest analogue, is schematically depicted in figure 2, in the context of a plane passing through the axis of the cylindrical core 1 and the cylindrical shell 2 pine to it, which is made of LC on the segment of optical fiber 3 and is equipped with electrodes 4 located on the surface of the LCD shell 3.

The disadvantages of the known device is the inability, using nematic and smectic LCs, to provide spectrally selective mode conversion with a conversion range of less than 30 nm, as well as the inability to bind waveguide modes propagating in opposite directions in the spectral range of width less than 30 nm.

The objective of the invention is to improve the characteristics of frequency-selective conversion of waveguide light modes with tunable center frequency of the conversion band.

The technical result of the invention is to enable tunable frequency-selective conversion of waveguide modes.

The problem is solved in that in a method for spectrally selective conversion of optical radiation modes in a waveguide, providing for the propagation of at least two waveguide modes of optical radiation in a waveguide structure with periodic variation of the refractive tensor along the direction of the waveguide light propagation, the application of an electric or magnetic field of a given orientation and control the characteristics of optical radiation by changing the refractive tensor of the waveguide structure in the region the propagation of these modes when changing the magnitude of the electric or magnetic field of a given orientation, in accordance with the invention, ensures the propagation of optical radiation in a medium with the properties of a chiral liquid crystal (CLC), the axis of the spiral of which is oriented along the axis of the waveguide, analytically determine the pitch p of the CLC spiral according to the given constants propagation of the coupled modes β ₁ and β ₂ , according to the ratio

the magnitude of the spiral pitch p determines the magnitude of the external electric or magnetic field, which is applied to the waveguide in the direction perpendicular to the axis of the waveguide, and the external field is changed until the value p reaches the value corresponding to the specified values of the propagation constants of the coupled modes β ₁ and β ₂ .

In addition, an alternating electric field with a frequency lower than the dielectric relaxation frequency of the CLC is applied to a medium with CLC properties.

In addition, a constant magnetic field is applied to a medium with CLC properties.

In addition, for the coupled waveguide modes of the opposite direction of propagation, the step of the CLC helix p is determined from the relation

where β is the absolute value of the propagation constant of the reflected mode.

The problem is solved in that in the known device for spectrally selective conversion of optical radiation modes in a waveguide, comprising electrodes and a waveguide located between them, at least partially made of a liquid crystal and containing at least one core and at least , one sheath, in accordance with the invention, a part of the waveguide is made of a uniformly oriented chiral liquid crystal, the axis of the spiral of which is parallel to the axis of the waveguide.

In addition, at least one of the shells of the waveguide or part thereof is made of a uniformly oriented chiral liquid crystal, the axis of the spiral of which is parallel to the axis of the waveguide.

In addition, the core of the waveguide is made of a uniformly oriented chiral liquid crystal, the axis of the spiral of which is parallel to the axis of the waveguide.

In addition, the waveguide contains two cores, and the part of the waveguide sheath located between them is made of a uniformly oriented chiral liquid crystal, the spiral axis of which is parallel to the axis of the waveguide.

In addition, the electrodes are mounted on the outer shell of the waveguide.

In addition, the waveguide is made in the form of at least one piece of optical fiber.

In addition, single-mode fiber has been selected as the optical fiber.

The essence of the invention lies in the fact that a chiral liquid crystal (CLC) is used as an LC substance, the axis of the CLC spiral being oriented parallel to the axis of the waveguide. A segment of a waveguide with a sheath made of CLC is placed in an externally applied alternating electric or constant magnetic field directed perpendicular to the axis of the CLC spiral. Owing to the indicated orientation of the CLC in the waveguide, a change in the refractive index tensor is provided along its axis, leading to spectrally selective interaction of the waveguide modes. In this case, the propagation constants of the interacting modes are related to the period of change of the refractive index tensor by formula (1). By varying the strength of the external field applied to the CLC, the period of change of the refractive index tensor is controlled, thereby rearranging the center frequency of the spectral interval over which the mode introduced into the waveguide is converted.

CLCs consist of optically active elongated molecules that fit into monomolecular layers with the same molecular direction in the layer. The direction of the molecules in each layer characterizes the vector n (director). The director in each layer lies in the plane of the layer and is rotated relative to the director of the previous layer by an angle around an axis perpendicular to all layers. This leads to the formation in CLC of an ordered twisted (chiral) structure, characterized by a pitch of the helix p ₀ . By definition, the helix pitch p ₀ is the distance along the normal to the molecular layers of the CLC, which corresponds to the rotation of the director n by an angle of 2π. Depending on the chemical structure of the molecules, the CLC spiral can be right-handed or left-handed.

The optical properties of CLCs are determined by the described structure with helical symmetry (symmetry group ∞2 / 24 /). Namely, the CLC refractive index tensor has an eigenvalue n _e (the so-called extraordinary refractive index) along the main axis, which coincides with the direction of the director in the layer, and two equal eigenvalues n _o (the so-called ordinary refractive index), corresponding to the main axes perpendicular to the director. The refractive index tensor thus defined in the literature is called local / 24 /, since it describes the properties of the CLC substance in individual monomolecular layers and varies at distances comparable to the wavelength of the propagating light. From layer to layer, only the direction of the main axes changes, repeating the spiral structure of the CLC, while the values of n _o and n _e remain constant. Thus, the refractive index tensor has helical symmetry, that is, the tensor of any two CLC points differs only in the angle of rotation of the main axes around the axis of the spiral. The period of optical properties is half the helix pitch p ₀ , since the properties of each individual layer are the same both in the n direction and in the -n direction opposite to it (the CLC structure is non-polar).

An external electric field applied in a direction perpendicular to the axis of the CLC spiral changes the pitch of the spiral p in the range from a certain minimum spiral pitch p ₀ inherent in this substance to the complete unwinding of the spiral. Full straightening occurs at a certain critical field strength E _c and corresponds to the transition of the LC to the nematic state, when all molecules are oriented in the same direction. A gradual change in the pitch of the CLC helix p with an increase in the external field occurs due to the competition between the forces of intermolecular interaction, tending to twist the spiral to a state with a minimum pitch p ₀ for this substance, with the forces of the external field tending to completely unwind the CLC helix. The dependence of the pitch of the CLC helix p on the applied alternating electric field is given by the formula / 26-28 /

where p ₀ = const denotes the step of the spiral in the absence of an electric field, p _e denotes the step of the spiral when applying an electric field with an amplitude of intensity E, K (s) and E (s) are full elliptic integrals, tabulated, for example, in the manual / 29 /, and the dimensionless parameter s (E) is found from the equation

where E _c denotes the electric field strength at which the pitch of the spiral becomes infinite. The monotonous growth of the pitch of the spiral p flowing from the above implicit expressions with an increase in the external field E is graphically depicted in Fig. 3. An external electric field can be either variable in time or constant. When an external alternating electric field is applied, its frequency is chosen lower than the dielectric relaxation frequency HLC / 30 /. The use of a constant electric field to control the pitch of the spiral is not always advisable because of the conductivity of some types of LCs, which leads to screening of the external electric field in the LC substance.

The controlled unwinding of the CLC spiral can also be achieved by applying a constant magnetic field in the direction perpendicular to the axis of the spiral. The dependence of the step of the CLC helix p on the applied magnetic field is given by formula (4) above, in which the dimensionless parameter s (E) is replaced by a similar parameter s (H), which, in turn, can be calculated from the equation

where H denotes the intensity of the applied external magnetic field, H _c denotes the intensity of the magnetic field at which the pitch of the spiral becomes infinite.

When the optical waveguide mode propagates in the region of the CLC, the interference of electromagnetic radiation occurs at periodic changes in the refractive index tensor provided by the spiral structure of the CLC. As a result of interference, a resonant interaction occurs between those and only those waveguide modes of electromagnetic radiation whose propagation constants β ₁ and β ₂ satisfy relation (1). In this case, the period of change of the refractive index tensor Λ is half that of the helix step CLC p. By definition, the pitch of the helix p corresponds to the rotation of the orientation axis of the CLC molecules (director n) by an angle of 2π, although the period of change in the optical properties is p / 1 due to the non-polarity of the nematic, and hence the cholesteric structure. Nonpolarity is expressed in the fact that the properties of each monomolecular layer are the same both in the n direction and in the –n direction opposite to it. This is ensured by the fact that each molecule in the layer has the shape of an asymmetric stick extended along the direction of the director n, and can equally well be oriented both along the direction of the director n and the opposite direction -n. As a result, formula (1) for the case when periodic changes in the tensor of the refractive index are provided due to propagation in the CLC, takes the form

where β ₁ and β ₂ are the propagation constants of interacting modes; p is the helix step of the CLC.

As follows from the coupled mode method / 2 /, the width of the spectral range in which the mode resonance interacts is proportional to the optical anisotropy of the CLC and the fraction of the mode intensity propagating in the part of the waveguide made of CLC. By reducing this fraction of the intensity due to the location of the part of the waveguide made of CLCs at a distance from the localization region of the interacting modes, it is possible to obtain devices with a spectral band of interaction smaller than that of the bulk CLCs themselves, which is important for practical applications. As a result, the spectral selectivity of the proposed devices exceeds the spectral selectivity of the device proposed in / 23 /, which coincides with the reflection bandwidth of the used CLC, as well as the device / 20 /, which does not provide for the use of chiral LCDs.

A part of the waveguide made of CLC is placed in an external electric or magnetic field directed perpendicular to the axis of the CLC spiral, which coincides with the axis of the waveguide. In the absence of external fields applied to the CLC, the helix pitch p ₀ is determined by its composition. When an external electric or magnetic field is applied in the direction perpendicular to the axis of the CLC, there is an increase in the pitch of the CLC spiral p. Due to the non-polarity of the CLC, the helix step p, corresponding to the director rotation by 2π, is twice as long as the period of change of the tensor of the exponent Λ along the waveguide axis. By controlling an external electric or magnetic field applied to the indicated CLC, the pitch of the CLC spiral is changed, along with it is the period p / 2 of the refractive index in the waveguide and, therefore, the difference of the propagation constants β ₁ and β _{2 of the} coupled waveguide modes, defined by formula (7) . By changing the magnitude of the external field, one can control the difference in the propagation constants of β ₁ -β ₂ interacting modes. The range of tuning of the difference between the propagation constants of the coupled β ₁ -β ₂ modes in formula (7) extends from 4π / p ₀ , determined by the CLC step in the absence of external fields, to any smaller value. Such a tuning range (the period Λ changes by 100%) exceeds the tuning range of the known fiber Bragg gratings in fused quartz, in which the period Λ can be tuned by no more than 6% / 13-16 /.

In particular, it is possible to bind oppositely directed modes for which the propagation constants β ₁ and β ₂ are equal in magnitude and opposite in sign,

β ₁ = -β ₂ = β.

Since the axis of the CLC spiral coincides with the direction of propagation of the waveguide mode, the resulting diffraction is completely analogous to the Bragg-Wulf diffraction on a layered optical structure / 3 / at normal incidence, which also provides the selectivity of reflection of a plane light wave from a bulk chiral LC / 24 /. An effective mode coupling (partial reflection) takes place if the period of change of the refractive index tensor Λ satisfies the Bragg reflection condition (2), that is, if the spiral pitch p is related to the propagation constant β by the relation

до любого меньшего значения.As a result of diffraction due to periodic changes in the refractive tensor, the waveguide mode incident on the CLC region at the indicated value of the propagation constant is at least partially converted to a mode propagating in the opposite direction, while maintaining the waveguide properties. Waveguide modes with other propagation constants pass through the CLC region without reflection. For each fixed value of the external field, the CLC spiral has a period p, which is found for a known value of the electric field by formulas (4, 5), and for a magnetic field by formulas (4, 6). For each value of p, those and only those modes are reflected whose propagation constant β satisfies the Bragg condition (8). By changing the external field that unwinds the CLC spiral, one can rearrange the propagation constant β of the reflected mode, given by formula (8), in the range

to any smaller value.

The coupling of oppositely directed modes can also be carried out in a linear waveguide containing two parallel identical cores and a common cladding, the part of the cladding between the cores made of uniformly oriented CLC, the helix axis of which is directed parallel to the cores. The distance between the cores is selected in the same order as the wavelength. When radiation is introduced into one of the cores, it partially spectrally selectively transforms into a mode localized near the other core and propagating in the opposite direction. This transformation occurs when relation (8) is satisfied. Consequently, the wavelength of the introduced mode, at which it is converted into a mode localized near another core, can be tuned by changing the pitch of the spiral p using an external field. Such a mode conversion is relevant for adding / subtracting channels in modern systems with frequency division multiplexing (WDM) / 31 /.

Additional advantages of the proposed method lie in the possibility of compensating with the help of an external field the temperature dependence of the helix pitch, and, consequently, the characteristics of spectrally selective mode conversion / 17 /.

The temperature dependence of the step of the CLC helix, and hence the center of the spectral range of the converted modes, can be reduced by choosing a CLC substance to 1-0.08 nm / ° C and compensated by the application of an external field.

The mechanical sensitivity of the devices is negligible, since the step of the CLC spiral depends only on the hydrostatic pressure, and not on the shape of the received LC / 24 /.

The proposed method allows for mutual conversion of the actual waveguide modes localized near the core with cladding modes / 1 / distributed also over the fiber cladding, which are held in the cladding due to total internal reflection at the cladding boundary with air, and so on. leaky modes / 1 /, which are slowly emitted outside the optical fiber.

The invention is illustrated figure 4-11, which presents a diagram of devices that implement the proposed method.

Figure 4 schematically shows a General view of the device in section by a plane AA, perpendicular to the axis, with the removed half of the device symmetrical about the plane of the section. Figure 5 presents a schematic representation of the same device in axial section. The device comprises a segment of optical fiber consisting of a cylindrical core 1 and a sheath, the supply part of which 2 is made in the form of cylindrical rings coaxial to the core and elongated along the axis, between which a part of the sheath 3 is placed, also having the form of a cylindrical ring, coaxial to the core and elongated along the axis, and electrodes 4, between which part of the sheath 3 is located. The electrodes 4 are parallel metal plates elongated along the axis of the optical fiber. The core 1 and part of the shell 2 are made of a substance with a uniform and isotropic refractive index, and the refractive index of the core 1 is greater than that of the part of the shell 2. Part of the shell 3 is made of HLC uniformly oriented so that the axis of its spiral is parallel to the axis of the optical fiber.

Another embodiment of the device is shown schematically in FIG. 6, which shows a section of the device with a plane BB passing through the axis of the waveguide, and FIG. 7, which shows a section of the device with a plane perpendicular to the axis of the waveguide. The device comprises an optical fiber with a core 1 and a sheath, part of which 2 is made in the form of a cylindrical ring, coaxial to the core, whose profile at a certain interval along the fiber axis represents a larger segment of a circle truncated along the chord. Part of the shell 3 is in mechanical contact with part 2 and has the form of a cylinder with a profile in the form of a smaller segment of the same circle truncated along the chord. The electrodes 4, between which part of the shell 3 is located, are parallel metal plates elongated along the axis of the optical fiber. The core 1 and part of the shell 2 are made of a substance with a uniform and isotropic refractive index, and the refractive index of the core 1 is greater than that of the part of the shell 2. Part of the shell 3 is made of HLC uniformly oriented so that the axis of its spiral is parallel to the axis of the optical fiber.

The device operates as follows. In the absence of an external field, the pitch of the CLC helix p ₀ in part of the shell 3 is determined by the composition of the CLC. When the optical waveguide mode propagates in the region of the CLC in the part of the shell 3, the interference of electromagnetic radiation occurs on periodic changes in the refractive index tensor provided by the spiral structure of the CLC. As a result, the interaction of those and only those waveguide modes of electromagnetic radiation occurs, the propagation constants of which β ₁ and β ₂ satisfy relation (7), where the step of the CLC spiral in part of the shell 3 is equal to p ₀ . The interaction of the radiation modes in the part of the shell 3 made of CLC leads to spectrally selective conversion of the interacting modes into each other. The spectral selectivity of the mode conversion is ensured by the fact that for a given helix pitch and a given mode frequency, there is only a finite number of values of the propagation constant of the first mode at which it is converted to the second mode, while for all other values of the propagation constant, this conversion does not occur.

до любого меньшего значения. В результате изменение разности потенциалов, приложенных к электродам, управляет перестройкой значения постоянной распространения первой моды β₁, при которых происходит ее преобразование во вторую моду.When applying the potential difference to the electrodes 4 in the CLC part of the shell 3, an electric field arises perpendicular to the axis of the CLC. The potential difference, and with it the electric field strength in the CLC, varies harmonically with a frequency lower than the frequency of the dielectric relaxation of the CLC. The direction of the electric field remains constant. The application of an electric field leads to an increase in the step of the CLC helix in the part of the sheath 3, and with it, to an increase in the period of change of the refractive index tensor of the part of the sheath 3 along the axis of the waveguide. The dependence of the pitch of the spiral CLC p on the intensity of the applied electric field E is presented in figure 3. Due to the change in the field strength, the period of change of the refractive index tensor along the axis of the waveguide can be selected in the range from p _0/2 to infinity. This, in turn, leads to a rearrangement of the difference β ₁ and β _{2 of the} propagation constants of the interacting modes associated with the helix pitch according to formulas (1) and (8). Changing the potential difference at the electrodes 4, control the difference of the propagation constants β ₁ -β ₂ interacting modes, changing it in the range from

to any smaller value. As a result, a change in the potential difference applied to the electrodes controls the rearrangement of the propagation constant of the first mode β ₁ , at which it is converted to the second mode.

In particular, the device can provide a partial reflection of the mode introduced into the device. Partial reflection is equivalent to the coupling of modes with propagation constants β ₁ and β ₂ equal in magnitude and opposite in sign. As a result of diffraction due to periodic changes in the refractive tensor, the waveguide mode incident on the region 3 of the CLC location at some value of the propagation constant is at least partially reflected while maintaining the waveguide properties. Effective partial reflection occurs if the period of change of the refractive index tensor Λ satisfies the Bragg reflection condition (2), that is, if and only if the propagation constant β is related to the pitch of the spiral p by relation (8), thereby ensuring spectral reflection selectivity. Waveguide modes with other propagation constants pass through the region of location of 3 CLCs without reflection.

до любого меньшего значения. Тем самым изменение разности потенциалов, приложенных к электродам 4, приводит к перестройке постоянной распространения отражаемой моды в указанных пределах.When a potential difference is applied to the electrodes 4 in the part of the shell 3 made of CLC, an electric field arises perpendicular to the axis of the CLC. The potential difference, and with it the electric field strength in the CLC, varies harmonically with a frequency lower than the frequency of the dielectric relaxation of the CLC. The direction of the electric field remains constant. The application of an electric field leads to an increase in the step of the CLC helix in part of the sheath 3, and with it to an increase in the period of change of the refractive index tensor along the axis of the waveguide. The pitch of the spiral p is found for a known magnitude of the electric field E by the formulas (4, 5). For each value of p, those and only those modes are reflected whose propagation constant β satisfies the Bragg condition (8), where the pitch of the spiral p is given by expressions (4, 5). Mods with other propagation constants pass through the device without reflection. With an increase in the external field unwinding the CLC spiral, the propagation constant of the reflected mode decreases from

to any smaller value. Thus, a change in the potential difference applied to the electrodes 4 leads to a rearrangement of the propagation constant of the reflected mode within the indicated limits.

As follows from the coupled mode method / 2 /, the width of the spectral range of reflection is proportional to the optical anisotropy of the CLC and the fraction of the mode intensity propagating in part 3 made of CLC. By changing this fraction of the intensity due to the implementation of the devices in accordance with FIGS. 6-7, where the part 3 made of CLC is located at a greater or lesser distance from the core, it is possible to obtain devices with a spectral reflection band smaller than the selective reflection band of the bulk CLC itself , which is important for practical applications. Therefore, the spectral selectivity of the proposed device exceeds the spectral selectivity of the device proposed in / 23 /, which coincides with the reflection bandwidth of the used CLC, determined by the formula (3).

On Fig.8-9 schematically depicts a variant of the device, including a piece of fiber containing a core made of CLC. On Fig presents a General view of the device in the context of a plane perpendicular to the axis of the fiber, and part of the device, symmetrical with respect to the plane of the cut, is removed. Figure 9 shows the cross section of the device with a plane perpendicular to the axis of the fiber. The device comprises a core 1 made of a CLC, a shell 2 and electrodes 4 located on a part of the surface of the shell, the core 1 being made of a CLC uniformly oriented so that the axis of the CLC spiral is parallel to the axis of the optical fiber. To preserve the unity of the continuous numbering of functional elements in various illustrations, the electrodes are indicated by the number 4. Shell 2 is made of a substance with a uniform and isotropic refractive index less than the average refractive index of the CLC (n _e + n _o ) / 2, which ensures the existence of waveguide modes beyond due to total internal reflection at the boundary of the core and the shell.

The device operates as follows. In the absence of an external field, the step of the CLC helix 1 p ₀ is determined by its composition. When the waveguide mode propagates in the core 1 made of CLC, interference of electromagnetic radiation occurs on periodic changes in the refractive index tensor provided by the spiral structure of the CLC. As a result, the interaction of those and only those waveguide modes of electromagnetic radiation occurs, the propagation constants of which β ₁ and β ₂ satisfy relation (7), where the step of the CLC helix in core 1 is equal to p ₀ . The interaction of the radiation modes in the core 1 made of CLC leads to spectrally selective conversion of the interacting modes into each other. The spectral selectivity of the mode conversion is ensured by the fact that for a given helix pitch and a given mode frequency, there is only a finite number of values of the propagation constant of the first mode at which it is converted to the second mode, while for all other values of the propagation constant, this conversion does not occur.

до любого меньшего значения. В результате изменение разности потенциалов, приложенных к электродам, управляет перестройкой значения постоянной распространения первой моды β₁, при которых происходит ее преобразование во вторую моду с постоянной распространения β₂.When a potential difference is applied to the electrodes 4 in the core 1 made of CLC, an electric field arises perpendicular to the axis of the CLC. The potential difference, and with it the electric field strength in the CLC, varies harmonically with a frequency lower than the frequency of the dielectric relaxation of the CLC. The direction of the electric field remains constant. The application of an electric field leads to an increase in the step of the CLC spiral from which the core 1 is made, and with it to an increase in the period of change of the refractive index tensor of the core 1 along the axis of the waveguide. The dependence of the pitch of the spiral CLC p on the intensity of the applied electric field E is presented in figure 3. Due to the change in the field strength, the period of change of the refractive index tensor of the core 1 along the axis of the waveguide can be selected in the range from p _0/2 to infinity. This, in turn, leads to a rearrangement of the difference β ₁ -β _{2 of the} propagation constants of the interacting modes associated with the helix pitch according to formulas (1) and (8). Changing the potential difference at the electrodes 4, control the difference of the propagation constants β ₁ -β ₂ interacting modes, changing it in the range from

to any smaller value. As a result, a change in the potential difference applied to the electrodes controls the tuning of the values of the propagation constant of the first mode β ₁ , at which it is converted into the second mode with the propagation constant β ₂ .

In particular, the device can provide a partial reflection of the mode introduced into the device. Partial reflection is equivalent to the coupling of modes with propagation constants β ₁ and β ₂ equal in magnitude and opposite in sign. As a result of diffraction due to periodic changes in the refractive tensor, the waveguide mode incident on the region 3 of the CLC location at some value of the propagation constant is at least partially reflected while maintaining the waveguide properties. Effective partial reflection occurs if the period of change of the refractive index tensor Λ satisfies the Bragg reflection condition (2), that is, if and only if the propagation constant β is related to the pitch of the spiral p by relation (8), thereby ensuring spectral reflection selectivity. Waveguide modes with other propagation constants pass through the region of location of 3 CLCs without reflection.

When a potential difference is applied to the electrodes 4 in the core 1 made of CLC, an electric field arises perpendicular to the axis of the CLC. The potential difference, and with it the electric field strength in the CLC, varies harmonically with a frequency lower than the frequency of the dielectric relaxation of the CLC.

до любого меньшего значения. Тем самым, изменение разности потенциалов, приложенных к электродам 3, приводит к перестройке постоянной распространения отражаемой моды в указанных пределах.The direction of the electric field remains constant. This leads to an increase in the step of the CLC spiral from which the core 1 is made, and with it to an increase in the period of change of the refractive index tensor of the core 1 along the waveguide axis. The pitch of the spiral p is found for a known magnitude of the electric field E by the formulas (4, 5). For each value of p, those and only those modes are reflected whose propagation constant β satisfies the Bragg condition (8), where the pitch of the spiral p is given by expressions (4, 5). Mods with other propagation constants pass through the device without reflection. With an increase in the external field unwinding the CLC spiral, the propagation constant of the reflected mode decreases from

to any smaller value. Thus, a change in the potential difference applied to the electrodes 3 leads to a rearrangement of the propagation constant of the reflected mode within the indicated limits.

As follows from the coupled mode method / 2 /, the width of the spectral reflection range is proportional to the optical anisotropy of the CLC and the fraction of the intensity of the mode propagating in the core made of CLC. By varying this fraction of the intensity due to the implementation of the devices in accordance with Figs. 8-9 with different core radii 1, it is possible to obtain devices with a spectral reflection band smaller than the selective reflection band of the bulk CLC itself, which is important for practical applications. Thus, the spectral selectivity of the proposed device (less than 1 nm) exceeds the spectral selectivity of the device proposed in / 23 /, which coincides with the selective reflection bandwidth of the used bulk CLC and is 20-30 nm.

10-11 schematically depicts a variant of the device, including a waveguide with two identical cores 1A and 1B of rectangular cross section, parallel to each other and formed in the substrate 2, which is part of the sheath. The other part of the sheath 3 is made of CLC, uniformly oriented along the axis of the waveguide. The waveguide is located between the electrodes 4 in contact with its shell and having the form of two plates parallel to the plane separating the substrate 2 and the CLC 3. The cores 1A and 1B, as well as the substrate 2 are made of a substance with a uniform isotropic refractive index, and to provide a waveguide light propagation, the refractive index of the cores is greater than the refractive index of the substrate 2.

, где p₀ обозначает шаг спирали ХЖК 3. Взаимодействие мод излучения в области расположения ХЖК части оболочки 3 приводит к спектрально-селективному преобразованию введенной моды, локализованной около сердцевины 1А, в моду, локализованную около сердцевины 1В. Спектральная селективность преобразования мод обеспечивается тем, что при заданной длине шага спирали р₀ и заданной частоте моды ν существует лишь конечное число значений постоянной распространения первой моды, при которых происходит ее преобразование во вторую моду, тогда как при всех остальных значениях постоянной распространения такого преобразования не происходит.The device operates as follows. In the absence of an external field, the helix pitch p _{0 of the} CLC, from which part of the shell 3 is made, is determined by the composition of the CLC. During the propagation of the optical waveguide mode concentrated around the core 1A, in the region of the CLC part of the shell 3, electromagnetic radiation diffracts on periodic changes in the refractive index tensor provided by the helical structure of the CLC 3. As a result of diffraction, the introduced mode interacts with modes that are localized near the core 1B, the propagation constants of which β ₁ and β ₂ satisfy the relation

, where p ₀ denotes the helix step of the CLC 3. The interaction of radiation modes in the region of the CLC part of the shell 3 leads to spectrally selective conversion of the introduced mode localized near the core 1A to a mode localized near the core 1B. The spectral selectivity of the mode conversion is ensured by the fact that for a given spiral pitch p ₀ and a given mode frequency ν there is only a finite number of values of the propagation constant of the first mode at which it is converted to the second mode, while for all other values of the propagation constant of such a transformation going on.

до любого меньшего значения. В результате при изменении амплитуды переменной разности потенциалов, приложенных к электродам 4, происходит перестройка значения постоянной распространения первой моды β₁ сосредоточенной около сердцевины 1А, при которых происходит ее преобразование во вторую моду, сосредоточенную около сердцевины 1В.When applying the potential difference to the electrodes 4 in the CLC part of the shell 3, an electric field arises perpendicular to the axis of the CLC. The potential difference, and with it the electric field strength in the CLC, varies harmonically with a frequency lower than the frequency of the dielectric relaxation of the CLC. The direction of the electric field remains constant. The application of an electric field leads to an increase in the pitch of the helix in the CLC of part of cladding 3, and with it, to an increase in the period of change of the refractive index tensor of part of cladding 3 along the axis of the waveguide. The period of variation of the refractive index tensor along the axis of the waveguide can be selected in the range from p _0/2 to infinity. This, in turn, leads to a change in the difference β ₁ -β _{2 of the} propagation constants of the interacting modes according to formula (1). Changing the potential difference at the electrodes 4, control the difference of the propagation constants β ₁ -β ₂ interacting modes, changing it in the range from

to any smaller value. As a result, when changing the amplitude of the variable potential difference applied to the electrodes 4, the value of the propagation constant of the first mode β ₁ concentrated near the core 1A undergoes a transformation, in which it is converted into a second mode concentrated near the core 1B.

до нуля. В результате при изменении амплитуды переменной разности потенциалов, приложенных к электродам 4, происходит перестройка значения постоянной распространения первой моды β₁, сосредоточенной около сердцевины 1А, при которых происходит ее преобразование во вторую моду, сосредоточенную около сердцевины 1В и распространяющуюся в противоположном направлении.In particular, partial reflection equivalent to the coupling of oppositely directed modes can be ensured for which the propagation constants β ₁ and β ₂ are equal in magnitude and opposite in sign. In this case, a mode localized near the core 1A is transformed into a mode propagating in the opposite direction and localized near the core 1B. For each fixed value of the external field, the CLC spiral 3 has a period p, which is for a known value of the electric field according to formulas (4, 5). For each value of p, those and only those modes from the core 1A are converted, the propagation constant β of which satisfies the Bragg condition (8). With an increase in the amplitude of the external field unwinding the CLC spiral, the propagation constant of the reflected mode decreases from

to zero. As a result, when the amplitude of the variable potential difference applied to the electrodes 4 is changed, the value of the propagation constant of the first mode β ₁ concentrated near the core 1A undergoes a transformation to the second mode, concentrated near the core 1B and propagating in the opposite direction.

The device shown in Fig.6 and 7, was implemented as follows. An optical fiber was used with a core diameter of 1 8 μm, a sheath diameter of 2 125 μm, with a core refractive index of 1.45 and a profile height parameter of Δ = 0.3%. For an oscillation frequency corresponding to a wavelength of 1.55 μm in vacuum, the fiber is in a single-mode regime with a waveguide parameter V = 1.82 / 1, 2 /. The cross section of the fiber is concentric circles centered on the axis of the fiber and the radii of the core and sheath. On a segment of the indicated fiber with a length of L = 2 mm, a part of the sheath 3 with a profile in the form of a part of the circular cross section of the optical fiber between the circle and the chord, which is 8 μm from the center of the circle, is made of uniformly oriented CLC 3 from the group of cyanbiphenyls with the chiral additive BL037 (according to the catalog / 32 /), and the axis of the CLC spiral is parallel to the axis of the optical fiber. The CLC spiral is right-handed, the CLC spiral pitch is 1004 nm at a temperature of 20 ° C, the refractive indices are n _e = 1.65, n ₀ = 1.5, the temperature dependence of the spiral pitch p in the range of 15-50 ° C is -0.2 nm / ° C, the critical electric field strength E _c , at which the complete unwinding of the spiral occurs, is 19 kV / cm. A piece of fiber with a part of the shell made of CLC is placed between plane-parallel electrodes 4, to which a potential difference is applied, which varies harmonically with a frequency of 1 kHz. A piece of optical fiber with a part of the sheath 3 made of HLC, is connected at both ends to the lead-in segments of the fiber with the same characteristics, the sheath of which is completely made of quartz. Note that, due to the small profile height Δ, the waveguide is weakly guiding / 1 /, therefore, the projection of the vectors of the electric and magnetic fields of the waveguide modes on the waveguide axis is negligible compared to the magnitude of the components of these fields perpendicular to the waveguide axis / 2, Ch. 12, p.222 /. In addition, in a weakly guiding waveguide, the propagation constant of the waveguide mode β is related to the wave number k by the ratio k≈β / n, where n is the refractive index of the core.

When a waveguide mode HE ₁₁ with a right circular polarization is introduced into the device through a lead-in fiber segment, it propagates in the part of the sheath 3 made of CLC. The waveguide mode field in the core 1 and part of the cladding 2 made of fused silica can be represented as a superposition of the incident and reflected waveguide modes. Diffraction on periodic changes in the refractive index tensor of the CLC 3, which is periodic along the fiber axis, leads to partial spectral-selective reflection of the waveguide mode in the opposite direction while maintaining waveguide properties. In the absence of a potential difference at the electrodes 4, the effective reflection of the waveguide mode occurs in the frequency range of a width of about 190 GHz with a center corresponding to a wavelength in free space of λ = 1450 nm. The dependence of the fraction of reflected light R, equal to the ratio of the intensities of the incident and reflected modes, on the wavelength in free space λ is shown in Fig. 12. The dependency is approximately rectangular in shape. The width of the reflection range, which depends on the optical anisotropy of the CLC and on the fraction of the intensity of the waveguide mode propagating in the CLC material, was about 190 GHz. The reflected wave mode has a right circular polarization. The HE ₁₁ waveguide mode with left circular polarization passes through the device without reflection.

When the potential difference ε is applied to the electrodes 4 in the CLC, an external electric field arises perpendicular to the axis of the CLC spiral 3. This field increases the pitch of the CLC spiral, and with it the central frequency of the reflection range. When the field strength changed from 0 to 14 kV / cm, the wavelength in free space corresponding to the center frequency of the spectral range of reflection changed from 1450 to 1600 nm. The shape of the spectral dependence of R on λ has not undergone significant changes. With a sharp change in the magnitude of the external electric field, the constant pitch of the spiral is set at 7 m / s.

The temperature dependence of the step of the CLC helix was compensated by changing the external electric field. As a result, the undesirable temperature dependence of the reflected light wavelength was less than 0.001 nm / ° C, while the temperature sensitivity of traditional lattices formed in fused silica is much higher and amounts to about 0.01 nm / K.

The mechanical sensitivity of this implementation is absent due to the insensitivity of the helix pitch to a change in the shape of the CLC at constant hydrostatic pressure, which was ensured by placing part of the waveguide made of CLC in a volume with at least one elastic wall. In particular, the deformations of the optical fiber itself do not affect the pitch of the spiral, and, consequently, the wavelength of the range of reflected light.

The described device allows you to change the step length of the spiral, increasing it by at least 10%, which leads to a proportional change in the center of the spectral range of reflection. In the implementation under consideration, the range varies by 150 nm, from 1450 to 1600 nm. Such a change in the wavelength of the reflected light significantly exceeds the tuning range provided by mechanical stretching / compression or by changing the temperature of the fiber gratings and constituting / 9, 13 / no more than 4% (60 nm) and 2% (30 nm), respectively.

The spectral reflection band of the described device is 190 GHz, or 1.7 nm, which is significantly less than the selective reflection bandwidth of the bulk CLC itself, equal in this case to 9.6 THz, or 80 nm. The spectral selectivity of the described device (1.7 nm) exceeds the spectral selectivity of the device proposed in / 23 /, which coincides with the selective reflection bandwidth of the used CLC (20-50 nm).

The proposed device can be used as a Bragg fiber lattice with a tunable frequency of reflected light, in fiber-optic communication systems, in particular, in systems with frequency division multiplexing (WDM) / 31 /.

The proposed device can be used as a fiber optic sensor of electric and / or magnetic fields, as well as temperature. Under the influence of these factors, the step of the CLC spiral changes, and with it the central frequency of the waves reflected by the device. Comparing the frequency of the waveguide mode reflected from the portion of the waveguide where the CLC is exposed to external influences with a certain reference frequency, one can judge the degree of change in the pitch of the CLC spiral, and find the magnitude of the external influence with it.

The invention can be used to create a frequency-selective fiber resonator with a tunable center frequency to provide feedback in a semiconductor laser.

The invention can be used to provide frequency-selective feedback in an optical pumped fiber laser (in particular, in a fiber laser, in which the working substance is fused silica fiber with a core doped with erbium ions).

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Claims (5)

1. A method of spectrally selective conversion of optical radiation modes in a waveguide, providing for the propagation of at least two waveguide modes of optical radiation in a waveguide structure with periodic variation of the refractive tensor along the direction of the waveguide light propagation, applying an electric or magnetic field and controlling the characteristics of optical radiation by changing the refractive tensor of the waveguide structure in the region of propagation of these modes when electric or magnetic field, characterized in that they provide the propagation of optical radiation in a medium with the properties of a chiral liquid crystal (CLC), the spiral axis of which is oriented along the axis of the waveguide, determine the pitch of the CLC spiral by the ratio p = 4π / (β1-β2), where β1 β2 are the values of the propagation constants of the coupled modes of optical radiation in the waveguide; the magnitude of the helix pitch p determines the strength of the external electric or magnetic field; an electric or magnetic field is applied to the waveguide direction perpendicular to the axis of the waveguide, and the external field is changed until the value p reaches the value corresponding to the values of the propagation constants of the coupled modes β1 and β2 of the optical radiation in the waveguide. 2. The method according to claim 1, characterized in that an alternating electric field with a frequency lower than the dielectric relaxation frequency of the CLC is applied to a medium with CLC properties. 3. The method according to claim 1, characterized in that a constant magnetic field is applied to a medium with CLC properties. 4. The method according to claim 1, characterized in that for the coupled modes of the opposite direction of propagation, the step of the CLC helix is determined from the ratio p = 2π / β, where β1 = -β2 = β. 5. A device for spectrally selective conversion of optical radiation modes in a waveguide, comprising electrodes and a waveguide located between them, at least partially made of liquid crystal and containing at least one core and at least one shell, characterized the fact that part of the waveguide is made of a uniformly oriented liquid crystal, the axis of the spiral of which is parallel to the axis of the waveguide, while this part is placed in an externally applied electric or magnetic field, The direction perpendicular to the waveguide axis so that the field control provides a change of the helical pitch and the variation of the difference of the propagation constants of guided modes of interacting.

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