RU2188443C2 - Process of manufacture of radiation input-output device in ring interferometer of fiber-optical gyroscope based on special twin light guide - Google Patents

Abstract

FIELD: fiber optics, design of fiber-optical gyroscopes and fiber transducers of physical quantities such as pressure pickups, temperature-sensitive elements, magnetic field sensors, etc. SUBSTANCE: process of manufacture of radiation input-output device in ring interferometer of fiber-optical gyroscope consists in formation of biconical constriction on twin light guide produced by polishing of two workstocks for single-mode light guides with large linear birefringence, in cutting of lateral grooves in polished surfaces, in alignment of these workstocks with their flat surfaces and in drawing of twin light guide from produced workstock. Prior to polishing workstocks are placed inside and then fused together with bearing quartz tube on which internal surface there is deposited layer of quartz glass with refractive index less by (2-4)10^-3 than refractive index of pure quartz glass and thickness ensuring

, where D_sh is diameter of sheath formed by layer of quartz glass with reduced refractive index; D_w is diameter of starting workstock. After drawing of twin light guide section on which biconical constriction is formed is placed in advance into etching solution to etch away external layer of twin light guide made of pure quartz glass. Later formed biconical constriction is covered with protective-reinforcing optical coat. EFFECT: reduced size and mass characteristics of radiation input-output device, raised zero stability of ring interferometer. 3 cl, 14 dwg

Description

 The invention relates to the field of fiber optics and can be used in the design of fiber-optic gyroscopes and other sensors of physical quantities (pressure sensors, temperature, magnetic field, etc.).

The fiber-optic gyroscope contains a passive ring interferometer and an electronic unit for processing rotation information received from the photodetector of the ring interferometer. A ring interferometer [1] consists of a radiation source, a fiber depolarizer, a first fiber splitter, a fiber polarizer, a second fiber splitter, a phase modulator, a fiber sensitive coil and a photodetector. A light beam from a radiation source through a fiber depolarizer enters one of the input ends of the first fiber splitter, divides this splitter into two beams, one of which passes a fiber polarizer, and then enters one of the input ends of the second fiber splitter, which divides this beam into two beam of the same intensity. These two beams pass the phase modulator and the sensing coil in two mutually opposite directions and again go to the second fiber splitter, which mixes the two beams into one beam. This mixed beam passes the fiber polarizer in the opposite direction, as well as the first fiber splitter and through the second input end of the optical fiber of the first fiber splitter enters the photodetector. Two beams interfere with each other at the photodetector, which have passed the fiber sensitive coil in two mutually opposite directions, and thus the intensity at the photodetector is proportional to:
I _Ф ~ 2Р ₀ (1 + cos Ф _С ),
where P ₀ is the power of each of the rays that have passed the fiber sensitive coil in two mutually opposite directions;
Ф _С - Sagnac phase difference.

 As a light source, optical radiation sources with a short coherence time are used. These can be either semiconductor superluminescent diodes or fiber fluorescent radiation sources based on activated optical fibers [2]. From consideration of the optical design of the ring interferometer of a fiber-optic gyroscope and the order of passage of optical rays in it, it follows that two rays carrying information about the angular velocity of rotation pass twice in mutually opposite directions to the following fiber elements of the optical design of the ring interferometer: the first fiber splitter, polarizer and second fiber splitter. Conventionally, these three elements can be combined into one device, which acts as a radiation input-output device in a ring interferometer of a fiber-optic gyroscope.

,
где "*" - операция комплексного сопряжения.The characteristics of the optical elements that make up the radiation input-output device largely determine the zero stability of the ring interferometer of a fiber-optic gyroscope, and therefore, much attention is paid to improving their characteristics. The input-output device ensures the reciprocity of the design of the ring interferometer circuit for the rays that have passed the sensitive coil in two mutually opposite directions, that is, ensures the same optical paths. The zero stability of the ring interferometer, in other words, the stability of the phase difference of the rays of the ring interferometer when exposed to external destabilizing factors, largely depends on the polarization transfer characteristics of the elements included in the input-output device. It is known that when changing the state of polarization of the rays of a ring interferometer, an error arises in measuring the angular velocity of a fiber-optic gyroscope, since a parasitic phase difference arises in the ring interferometer of a fiber-optic gyroscope due to a change in the state of polarization of rays [3]. The polarization matrix of the fiber of the sensitive coil of a fiber-optic gyroscope can be represented in a general form by a unitary unimodomeric matrix of the form:

,
where "*" is the complex pairing operation.

где S_p - операция шпур;
G_вых - матрица когерентности выходного поля на фотоприемнике;
G_вх - матрица когерентности входного поля, то есть поля, поступающего на вход второго волоконного разветвителя;
"+" - операция эрмитового сопряжения;
Т - операция транспонирования;
Ф_С - разность фаз Саньяка в кольцевом интерферометре.Assume first that the ring interferometer contains only the second fiber splitter, then the radiation intensity at the photodetector can be represented as follows:

where S _p - hole operation;
G _o - the coherence matrix of the output field at the photodetector;
G _I - the coherence matrix of the input field, that is, the field supplied to the input of the second fiber splitter;
"+" - operation of Hermitian pairing;
T - transpose operation;
Ф _С - Sagnac phase difference in a ring interferometer.

где а, b - компоненты вектора Джонса входного поля.The coherence matrix of the input field can be represented as:

where a, b are the components of the Jones vector of the input field.

With this in mind, the intensity at the photodetector can be represented as:
I _Ф ~ [aa * + bb *] • f ₁₁ f * ₁₁ - [aa * f ₁₂ f ₁₂ + bb * f * ₁₂ f * ₁₂ ] +
[ab * f ₁₁ -a * bf * ₁₁ ] • f ₁₂ ⁺ f * ₁₂ .

где Е_a, Е_b - поля х и у-поляризационных мод в световоде с большим линейным двулучепреломлением;
L₀ - длина световода чувствительной катушки волоконно-оптического гироскопа;
l - координата центра поляризационной связи мод в световоде чувствительной катушки.The polarization transfer matrix of the fiber of the sensitive coil of a fiber-optic gyroscope can be represented using a model of one center of polarization coupling [4] in a fiber with high birefringence. The radiation field along the two birefringence axes in the optical fiber having propagation constants of two polarization modes β _a and β _b at the point of polarization coupling can be represented as:

where E _a , E _b are the fields of x and y-polarization modes in a fiber with high linear birefringence;
L ₀ is the fiber length of the sensitive coil of the fiber optic gyroscope;
l is the coordinate of the center of the polarization coupling of the modes in the fiber of the sensitive coil.

,
где

В случае, когда отсутствует скрутка световода θ_C = 0, излучение световода обладает малой длиной когерентности и на вход схемы кольцевого интерферометра поступает деполяризованное излучение, то есть

в этом случае интенсивность на фотоприемнике может быть представлена в виде:

Из этого выражения следует, что при деполяризованном излучении на входе оптической схемы кольцевого интерферометра паразитной фазовой подставки в интерферометре не возникает даже в случае, когда имеется связь между поляризационными модами в любом месте световода чувствительной катушки гироскопа, в том числе и в его центре. В рассматриваемом случае, в зависимости от величины связи α двух поляризационных мод, наблюдается так называемый "фединг" сигнала кольцевого интерферометра, то есть изменение видности интерференционной картины. В волоконно-оптических гироскопах, где используются компенсационные фазовые методы считывания информации (гироскопы с замкнутой петлей обратной связи), "фединг" сигнала не оказывает существенного влияния на точность гироскопа. Видность интерференционной картины также не может быть равна нулю, так как полной перекачки мощности из одной поляризационной моды в другую не может быть по определению. Возможна перекачка только половины мощности одной из поляризационных мод в другую, мощность которой на входе равнялась нулю.By placing the center of polarization coupling anywhere in the fiber of the sensing coil, we can evaluate the effect of polarization coupling on the stray phase shift in the ring interferometer of a fiber-optic gyroscope. Assuming that the fiber of the sensitive coil has a constant twist along the length θ _C , where θ _C is the twist angle, the transmission matrix of the fiber can be represented in the form [3]:

,
Where

In the case where there is no twisting of the fiber θ _C = 0, the radiation of the fiber has a short coherence length and depolarized radiation arrives at the input of the ring interferometer circuit, i.e.

in this case, the intensity at the photodetector can be represented as:

It follows from this expression that in case of depolarized radiation at the input of the optical circuit of a ring interferometer, a parasitic phase support in the interferometer does not occur even when there is a connection between polarization modes at any point in the fiber of the sensitive gyro coil, including in its center. In this case, depending on the magnitude of the coupling α of the two polarization modes, the so-called “fading” of the ring interferometer signal, that is, a change in the visibility of the interference pattern, is observed. In fiber-optic gyroscopes, which use compensatory phase methods of reading information (gyroscopes with a closed feedback loop), the “fading” of the signal does not significantly affect the accuracy of the gyroscope. The visibility of the interference pattern also cannot be equal to zero, since the complete transfer of power from one polarization mode to another cannot be by definition. It is possible to transfer only half the power of one of the polarization modes to another, whose power at the input was equal to zero.

Максимально возможное значение, которое может принимать паразитная фазовая подставка ΔΨ_П ≈ α. При учете малой длины когерентности источника излучения можно сделать вывод, что в рассматриваемом случае на величину фазовой подставки влияют центры поляризационной связи, находящиеся сразу за вторым волоконным разветвителем. Максимальная величина фазовой подставки в кольцевом интерферометре в случае линейно-поляризованного излучения на входе оптической схемы получается тогда, когда линейная поляризация излучения падает под углом

к осям двулучепреломления разветвителя и световода чувствительной катушки гироскопа, то есть в случае, когда aа^*-bb^*=0 и

В этом случае выражение для паразитной фазовой подставки в кольцевом интерферометре примет следующий вид:

Теперь предположим, что на вход оптической схемы кольцевого интерферометра поступает частично поляризованное излучение, что бывает наиболее часто. Частично поляризованное излучение можно представить в виде суммы полностью деполяризованной и полностью поляризованной компонент, то есть Р_ч.п=Р_п+Р_д, где Р_ч.п. - мощность частично поляризованного света; Р_п - мощность полностью поляризованной компоненты; Р_д - мощность полностью деполяризованной компоненты излучения. Исходя из условия максимально возможной паразитной фазовой подставки (полностью поляризованная компонента излучения падает под углом

к осям двулучепреломления разветвителя и световода чувствительной катушки) имеем следующее выражение:

где

остаточная степень поляризации частично поляризованного света.If a linearly polarized radiation arrives at the input of the optical circuit of a ring interferometer, i.e., one of the polarization modes of the fiber of the sensitive gyroscope coil aa * = 1, bb * = 0, a * b = ab * = 0 is excited, then spurious radiation may occur in the ring interferometer phase stand, which is expressed as follows:

The maximum possible value that the parasitic phase stand can take ΔΨ _П ≈ α. When taking into account the small coherence length of the radiation source, we can conclude that in the case under consideration, the magnitude of the phase shift is affected by the centers of polarization coupling located immediately after the second fiber splitter. The maximum value of the phase support in a ring interferometer in the case of linearly polarized radiation at the input of the optical circuit is obtained when the linear polarization of the radiation falls at an angle

to the birefringence axes of the splitter and fiber of the sensitive coil of the gyroscope, that is, in the case when aa ^* -bb ^* = 0 and

In this case, the expression for the parasitic phase stand in the ring interferometer will take the following form:

Now suppose that partially polarized radiation arrives at the input of the optical circuit of a ring interferometer, which happens most often. Partially polarized radiation can be represented as the sum of the fully depolarized and fully polarized components, that is, R _pn = R _p + R _d , where R _pp - power of partially polarized light; R _p is the power of the fully polarized component; R _d is the power of the fully depolarized radiation component. Based on the condition of the maximum possible stray phase support (the fully polarized radiation component falls at an angle

to the birefringence axes of the splitter and the optical fiber of the sensitive coil) we have the following expression:

Where

residual degree of polarization of partially polarized light.

,
где f₀ - центральная длина волны излучения источника;
Δf - ширина линии излучения источника;
L_p - длина поляризационных биений в световоде волоконного разветвителя.From the previous consideration it follows that the magnitude of the stray phase stand in the ring interferometer is mainly influenced by polarization effects in the places of the optical fibers, which are located directly behind the optical power splitter, and also, but to a much lesser extent, in the middle of the fiber of the sensitive coil. The length of these sections is limited and equal to the length of the depolarization of radiation in the fiber [4]:

,
where f ₀ is the central radiation wavelength of the source;
Δf is the width of the radiation line of the source;
L _p is the length of the polarization beats in the optical fiber of the fiber splitter.

For f ₀ = 850 nm, Δf = 20 nm, L _p = 5 • 10 ^-3, we have L _γ = 0.056 m, that is, the depolarization length of the radiation in a fiber with large linear birefringence is quite small. Therefore, the magnitude of the parasitic phase stand in the ring interferometer is mainly influenced by the sections of the optical fibers, which are the output fibers of the second fiber splitter of the radiation input / output device of the fiber-optic gyroscope ring interferometer. With a certain degree of accuracy, the coupling coefficient α of two polarization modes can be represented in the form: α = h • L _γ , where h is the intermode polarization coupling coefficient in the fiber (h-parameter).

,
где h, L_p - h-параметр и длина поляризационных биений соответственно в световодах, являющихся выходными световодами второго разветвителя устройства ввода-вывода кольцевого интерферометра.Then for the parasitic phase stand you can write:

,
where h, L _p is the h-parameter and the length of the polarization beats, respectively, in the optical fibers, which are the output optical fibers of the second splitter of the input-output device of the ring interferometer.

где ∈ - коэффициент пропускания поляризатором нежелательной поляризации по полю.The transfer polarization matrix of the optical path in the ring interferometer, provided that there is a polarizer at the input of the second fiber splitter of the input-output device, can be represented as [3]:

where ∈ is the transmittance of the undesired polarization across the field by the polarizer.

Таким образом, для уменьшения паразитной фазовой подставки в кольцевом интерферометре волоконно-оптического гироскопа необходимо обеспечивать малую длину когерентности источника излучения, малую остаточную степень поляризации излучения на входе поляризатора устройства ввода-вывода излучения, большой коэффициент поляризационной экстинкции поляризатора, малые значения коэффициента межмодовой поляризационной связи и длины поляризационных биений (большая величина двулучепреломления в световедущей жиле) в световодах, являющихся выходными световодами второго волоконного разветвителя устройства ввода-вывода излучения в кольцевом интерферометре волоконно-оптического гироскопа.The minimum possible spurious phase support in the ring interferometer in this case with partially polarized radiation at the input after the transformations will take the form:

Thus, to reduce the parasitic phase support in the ring interferometer of a fiber-optic gyroscope, it is necessary to provide a small coherence length of the radiation source, a small residual degree of radiation polarization at the input of the polarizer of the radiation input-output device, a large polarization extinction coefficient of the polarizer, small values of the inter-mode polarization coupling coefficient and lengths of polarization beats (large birefringence in a light guide conductor) in optical fibers that are odnymi waveguides second fiber coupler IO device radiation in an annular interferometer fiber optic gyroscope.

 The first fiber splitter is usually made of isotropic single-mode fibers using pull-alloy technology, that is, two segments of the fiber are fused to each other in a certain section, and then a biconical constriction is formed in this place in order to ensure communication between the light guide wires. Fiber polarizers can also be made on the basis of a biconical constriction of a single-mode fiber section with large linear birefringence [5]. It is known that in a fiber with large linear birefringence, the critical taper angle curve splits for two polarization modes, which are eigenmodes of such a fiber. For the y-polarization mode, the critical taper angle is less important than for the x-polarization mode; therefore, the biconical waist in the section of the single-mode fiber can be formed so that its taper angle slightly exceeds the critical taper angle for the y-polarization mode, but remains less than the critical taper angle for the x-polarization mode. In this case, the γ-polarization mode experiences a greater attenuation than the x-polarization mode and, as a result, the biconical constriction acts as a radiation polarizer.

 A fiber polarizer can also be manufactured using a birefringent crystal [6]. A section of a single-mode isotropic fiber is glued to the substrate and subjected to grinding. During the grinding process, part of the fiber is polished in such a way that the polished surface is very close to the light guide core. Then a birefringent crystal of potassium pentabarate is superimposed on the polished surface. This crystal has a refractive index for the x-polarization mode slightly higher than the refractive index of the light guide core material, and for the y-polarization mode its index is slightly lower. Therefore, the y-polarization mode attenuates strongly, while the x-polarization mode continues to be channelized by a light guide residential fiber.

There is also known a method of manufacturing a fiber splitter that preserves the polarization of radiation based on a two-core fiber with large linear birefringence [7]. The blank for a two-core fiber is made as follows. Two blanks for a single-mode fiber with a large linear birefringence are subjected to flat grinding from one side. Flat grinding of the preform [8] is performed in such a way that it is parallel to one of the two birefringence axes in the fiber preform. Then, transverse semicircular grooves of constant depth and width, spaced from each other at the same distance, are cut on the polished surfaces of the workpieces. After that, these two workpieces are combined with each other with their flat surfaces so that the transversely cut grooves coincide between them, after which the workpieces on the thermomechanical machine are fused with each other. A twin-core fiber is drawn from the preform thus obtained, which has alternating sections. In one section, two fused half-blanks form a double-core fiber itself, and in the second section, two fibers, each of which contains one of two light-guiding veins of a two-core fiber, are in the common sheath of the protective-reinforcing coating. The splitter is made by forming a biconical constriction in the area of the twin-core fiber. For this, a section of a two-core fiber with a length Δz [9] is heated to the melting temperature and stretched by a special mechanism. In this case, the shape of the biconical constriction is described by a parabolic law of the form:
D (z) = D _f (l + γ • z ² ),
where D (z) is the diameter of the light guide core of the original fiber;
D _f is the diameter of the light guide core in the region of the “neck” of the biconical constriction;
γ is a constant.

а длина вытяжки световода в обе стороны равна

и общая длина перетяжки l_П=Δz+l. Для определения всех параметров биконической перетяжки необходимо воспользоваться следующими соотношениями:

Угол конусности биконической перетяжки выражается следующим образом:

Критический угол θ_KP конусности конического перехода световода, при превышении которого возникают потери канализируемого излучения, определяется из условия:

где Δn - разность показателей преломления между световедущей жилой и отражающей оболочкой световода;
n₀ - показатель преломления плавленого кварца;

- параметр основной моды в оболочке световода;
V - нормализованная частота.The origin z = 0 is located in the neck region of the biconical constriction. The fiber heating section thus has the coordinates

and the length of the optical fiber hood in both directions is

and the total length of the banner l _P = Δz + l. To determine all the parameters of the biconical constriction, it is necessary to use the following relations:

The taper angle of the biconical constriction is expressed as follows:

The critical angle θ _{KP of the} taper of the conical transition of the fiber, above which losses of the canalized radiation occur, is determined from the condition:

where Δn is the difference in refractive index between the light guide core and the reflective sheath of the fiber;
n ₀ is the refractive index of fused silica;

- parameter of the main mode in the cladding of the fiber;
V is the normalized frequency.

где P₀ ¹, P₀ ² - мощности лучей на выходных концах световодов разветвителя;
θ₀ - разность фаз между основной модой и первой модой высшего порядка.In order to prevent losses in the splitter, the following condition must be fulfilled:
θ _KP > 2D _j • γ • z
To achieve the effect of communication between the light guide veins of the fused optical fibers, it is necessary that the degree of waist of the optical fibers be such that in the neck region the parameter V has a value of V <0.967. In this case, the light guide wires no longer channel the radiation, and a glass-air waveguide begins to work in the neck region, that is, the neck of the biconical waist serves as the light guide, and the surrounding air serves as a reflective sheath. It can be seen that the newly formed glass-air waveguide is multimode, but nevertheless, all the basic processes that occur in the biconical splitter are determined mainly by the interaction of the lower order mode and the next higher order mode, that is, the LP ₀₁ and LP ₁₁ modes . It is believed that the intensities of these modes are almost the same, that is, P _LP01 = P _LP11 = P ₀ . In this case, the main mode and the first higher mode are divided between the light guide veins equally in power, but for the total power through the channels of the splitter, you can write:

where P ₀ ¹ , P ₀ ² - the power of the rays at the output ends of the optical fibers of the splitter;
θ ₀ is the phase difference between the main mode and the first mode of higher order.

The power division coefficient of the splitter is selected by selecting the waist length, since in this case the value θ ₀ changes. But under the influence of external destabilizing factors, for example, a change in temperature, instability of the power division coefficient of the splitter can be observed.

 The main disadvantage of the known input-output device is a fairly large size. Since the input-output device consists of individual elements, when assembling fiber-optic gyroscopes based on it, they occupy a rather large volume in the design of the optical unit and therefore increase the dimensions of the structure of the fiber-optic gyroscope.

 Another disadvantage of the known input-output device is that the first fiber splitter, made by pull-alloy technology, that is, a device operating on the basis of a biconical constriction, has polarizing properties. This leads to an increase in parasitic zero bias of the ring interferometer of a fiber-optic gyroscope. The polarizing properties of the splitter are explained by the fact that in the neck region the biconical constriction of two fibers fused to each other has an elliptical cross-sectional shape. This leads to the fact that the optical waveguide of the glass-air type, which is the "neck" of the biconical constriction, has linear birefringence. Since the input of the first splitter of the input-output device of the fiber-optic gyroscope receives depolarized radiation from the source in order to reduce the parasitic zero bias of the ring interferometer, due to the fact that there are two polarization modes propagating at different phase velocities in the neck region, the coefficient the division of the optical power by the splitter of the depolarized beam for its two incoherent orthogonal polarization components is somewhat different. Therefore, in one channel of the splitter comes a little more power of one state of polarization, and in another channel a little more power of another state of polarization. This leads to the fact that on the output arms of the first fiber splitter, the output radiation is already polarized, which affects the zero stability of the ring interferometer of the fiber-optic gyroscope. The difference in the division coefficients of two orthogonal polarization states of depolarized radiation occurs due to different phase relationships between the lower mode and the first higher mode of the two states of radiation polarization.

 Other disadvantages of the input-output device are quite stringent requirements for the magnitude of the polarization extinction coefficient, which is necessary to achieve the minimum spurious phase shift of the ring interferometer of a fiber-optic gyroscope. The realistic achievable polarization extinction coefficient in fiber polarizers based on biconical constrictions using optical fibers with large linear birefringence is ~ 20 dB, which is clearly not enough to achieve high precision fiber-optic gyroscopes. All other known methods for producing highly efficient fiber polarizers are based on the use of isotropic single-mode optical fibers, which is unacceptable from the point of view of using them in the optical path of a ring interferometer that preserves the state of radiation polarization.

 Another disadvantage of the known method of manufacturing a radiation input-output device in a ring interferometer of a fiber-optic gyroscope is the instability of the beam power division coefficient by the second fiber splitter of the radiation input-output device.

 To eliminate spurious phase incursions in the ring interferometer of a fiber-optic gyroscope due to the Kerr effects, as well as due to the existence of a photorefraction effect in integrated optical phase modulators, it is necessary that the division coefficient of the second fiber splitter be highly stable, as well as a ray of light entering its entrance was divided into two rays of the same intensity. Fiber splitters based on a biconical constriction with a glass-to-air waveguide in the region of its neck do not have the necessary stability of the beam power division coefficient under the influence of external destabilizing factors.

 The aim of the present invention is to reduce the size and weight of the radiation input-output device in the ring interferometer of a fiber-optic gyroscope and to increase the zero stability of the ring interferometer.

где D_ОБ - диаметр оболочки, образованной слоем кварцевого стекла с пониженным показателем преломления, D_ЗАГ - диаметр исходной заготовки. После вытяжки двужильного световода участок, на котором формируется биконическая перетяжка, помещают предварительно в травильный раствор с целью стравливания внешнего слоя двужильного световода, состоящего из чистого кварцевого стекла, а затем сформированную биконическую перетяжку покрывают защитно-упрочняющим оптическим покрытием с показателем преломления n= n_ОБ±1•10^-3, где n_ОБ - показатель преломления материала внешней оболочки двужильного световода.This goal is achieved by the fact that in the manufacture of a two-core fiber before the operation of grinding the preforms, they are preliminarily placed inside and then fused with a supporting quartz tube, on the inner surface of which a layer of quartz glass with a refractive index less than (2-4) • 10 ^-3 is applied than the refractive index of pure quartz glass, and the layer thickness providing

where D _OB is the diameter of the shell formed by a layer of quartz glass with a low refractive index, D _ZAG is the diameter of the initial billet. After the biconical constriction is drawn, the area on which the biconical constriction is formed is preliminarily placed in the etching solution to etch the outer layer of the two-conductor optical fiber, consisting of pure quartz glass, and then the formed biconical constriction is coated with a protective-strengthening optical coating with a refractive index of n = n _OB ± 1 • 10 ^-3 , where n _OB is the refractive index of the material of the outer sheath of the twin-core fiber.

This goal is also achieved by the fact that after drawing a twin-core fiber, its section, on which a biconical constriction is formed, is placed inside a capillary consisting of quartz glass with a refractive index less by (2-4) • 10 ^-3 than the refractive index of pure quartz glass .

где 2<V≤2,4;
λ_o - длина волны излучения источника;
n₀ - показатель преломления внешнего слоя стекла двужильного световода;
Δn_x - разность показателей преломления между внешним слоем стекла двужильного световода и показателем преломления двулучепреломляющего состава для х-поляризационной моды.This goal is also achieved by the fact that the biconical constriction is covered with a birefringent composition having a refractive index for the x-polarization mode of the fiber that is higher than the refractive index of the outer layer of the glass of the biconductor fiber, and for the y-polarization mode, the refractive index is lower than the refractive index of the outer layer of the glass of the biconductor fiber, moreover, a biconical constriction in the region of its "neck" is formed with such a maximum diameter in the cross section that:

where 2 <V≤2,4;
λ _o - wavelength of the radiation source;
n ₀ is the refractive index of the outer layer of glass of a twin-core fiber;
Δn _x is the difference between the refractive indices between the outer layer of glass of the biconductor fiber and the refractive index of the birefringent composition for the x-polarization mode.

 The reduction in size and weight is achieved through the integration of all the elements of the radiation input-output device, since one biconical constriction formed using a two-core fiber contains all the elements of the radiation input-output device. A biconical constriction using a two-core fiber is a fiber integrated circuit that fully functions as an input-output device of a ring interferometer of a fiber-optic gyroscope.

 An increase in the stability of zero, that is, a decrease in the parasitic phase “stand” in the ring interferometer of a fiber-optic gyroscope, is achieved due to the fact that the depolarized light from the radiation source goes directly to the input of the polarizer of the fiber integrated circuit. This increase in stability is also achieved due to the fact that after the polarizer, the remaining part of the depolarized radiation is not polarized by the first optical power divider of the fiber integrated optical circuit, since in the region of the “neck” of the biconical waist, the waveguide in which the “neck” is a light guide core is single-mode. Polarizing properties in a biconical coupler coupler appear in the coupler when the waveguide in the neck region is multimode.

 An increase in the zero stability of the ring interferometer of a fiber-optic gyroscope is also achieved due to the fact that additional polarizers appear in the fiber integrated circuit in the region of the “neck” of the biconical constriction due to the presence of a birefringent composition deposited on the biconical constriction, as well as at the two output ends of the second splitter fiber integrated circuit due to the taper of the transition of a twin-core fiber.

 An increase in the zero stability of a ring interferometer is also achieved due to the fact that due to the formation of a biconical constriction of a two-core fiber of a single-mode waveguide, the division coefficient of the first and second splitters of the integrated optical fiber circuit remains stable even under external destabilizing effects. Fibers of a fiber-optic integrated optical circuit divide the input beam into two beams of the same intensity, that is, they are analogs of single-mode Y-couplers based on channel waveguides formed in substrates, for example lithium niobate, which are most often used in ring interferometers of fiber-optic gyroscopes.

 The invention is illustrated by drawings. Figure 1 shows a structural optical diagram of a ring interferometer fiber optic gyroscope. In FIG. Figure 2 shows the sequence of technological operations of manufacturing a special two-core fiber for a fiber-optic integrated circuit. In FIG. Figure 3 shows the sequence of technological operations for the formation of a biconical constriction on a special twin-core fiber, which is a fiber integrated optical circuit of a fiber-optic gyroscope. Figure 4 shows a section of a biconical constriction formed on a special two-core fiber. Figure 5 shows the process of forming a biconical constriction on a twin-core fiber using a special capillary. Figure 6 shows the distribution profile of the refractive index in the region of the cross section of the "neck" of the biconical constriction along the x and y axes. In FIG. Figure 7 shows the equivalent optical circuit of a fiber integrated optical circuit, which is a biconical constriction formed on a special two-core fiber with large linear birefringence.

 The ring interferometer of a fiber-optic gyroscope contains a radiation source 1 (Fig. 1), a fiber depolarizer 2, a first fiber splitter 3, a fiber polarizer 4, a second fiber splitter 5, integrated optical phase modulators 6, a fiber sensitive coil 7 and a photodetector 8. B As a radiation source, sources with a short radiation coherence length are used. These are either superluminescent semiconductor emitters coupled to a segment of a single-mode fiber, or fiber fluorescent radiation sources based on activated fibers.

Semiconductor superluminescent emitting diodes have polarized light at the output, therefore a special device is used in the optical circuit of a ring interferometer - a fiber depolarizer of radiation. Depolarized radiation at the input of the optical circuit of a ring interferometer significantly increases the zero stability of the interferometer. A fiber depolarizer is two sections of a single-mode fiber with a large linear birefringence welded together with a length ratio of 2: 1, and before welding they are oriented in such a way that their main birefringence axes make an angle of 45 ^o with each other.

 The first fiber splitter, the fiber polarizer and the second fiber splitter are the so-called radiation input-output device. It is this part of the optical scheme that determines the stability of the phase difference of the rays of the ring interferometer, that is, the zero stability of the interferometer. The integrated-optical broadband phase modulator is usually two channel waveguides formed by proton exchange technology in a lithium niobate substrate. Channel waveguides are connected on the one hand with the output ends of the optical fibers of the second fiber splitter, and on the other, with the ends of the optical fiber of the sensitive gyro coil. The fiber of the sensitive coil has a so-called symmetrical winding to prevent spurious phase incursions in the ring interferometer due to temperature gradients along the fiber of the sensitive coil. A p-i-n photodiode with a small input capacitance is usually used as a photodetector.

 A special two-core fiber with a large linear birefringence, used to manufacture a fiber integrated optical circuit containing all the elements of the radiation input-output device of a ring interferometer, is made as follows. A blank for a single-mode optical fiber 8 (FIG. 2) with large linear birefringence (for example, of the “PANDA” type) comprises a light guide core 9, a reflective sheath 10 and load rods 11 [10, 11]. This shell is placed inside the supporting quartz tube 12, on the inner surface of which are deposited layers of quartz glass 13 doped with, for example, fluorine. The addition of fluorine lowers the refractive index of glass, but at the same time slightly changes the temperature coefficient of linear expansion of quartz glass. This circumstance subsequently is of great importance, since it eliminates the occurrence of additional parasitic mechanical stresses in a two-core fiber. The fluorine dopant is obtained by adding Freon 113 vapor to silicon chloride vapors during deposition of quartz glass layers by the MCVD method on the inner surface of the supporting quartz tube.

With the help of fluorine additive, when using Freon 113, it is possible to reduce the refractive index of silica glass by no more than 4 • 10 ^-3 . Then the workpiece for a single-mode fiber waveguide using a gas burner 14 on a thermomechanical machine is fused with a supporting quartz tube into a solid glass bead. As a result of fusion of the billet with the supporting quartz tube, a new billet 15 for the PANDA single-mode fiber is obtained, which additionally contains a fluorine-doped quartz glass casing 16 and an outer protective shell 17 consisting of pure quartz glass. Then the workpiece is subjected to flat grinding so that the grinding plane of the workpiece in cross section is parallel to the line connecting the centers of the light guide core and the centers of the loading rods. Grinding can be carried out on a surface grinding machine with a diamond wheel. After flat grinding of the workpiece, the transverse grooves of a semicircular shape are cut with a depth and width of ~ 0.6 mm, equally spaced from each other. It turns out half-finished 18 for a two-core fiber.

 In the same way, the second half-finished piece 19 is made for a twin-core fiber, after which these two half-finished pieces are connected to each other by their flat surfaces so that the transverse grooves of these half-finished pieces coincide with each other. After this, the half-workpieces are fused with each other on a heat-mechanical machine using a gas burner, and the final workpiece for a two-core fiber with large linear birefringence is obtained. A two-core fiber is pulled from this preform [8].

 The twin-core fiber 20 comprises an external protective quartz sheath 21, an additional sheath 22 with a low refractive index, light guide cores 23, reflective sheaths 24, loading rods 25, a protective quartz sheath 26 and a protective and reinforcing coating 27. The reflective sheaths and loading rods consist of material with the refractive index equal to the refractive index of the protective shell, that is, the refractive index of pure quartz glass. After drawing, the twin-core fiber is cut into segments of the fibers. In the middle part, each of these segments contains a two-core fiber itself, and from two ends the two-core fiber passes into two optical fibers, which are in the general protective and hardening coating, each of which contains one of two light guide veins of a two-core fiber. In the manufacture of the integrated optical circuit in the middle part of the section of the twin-core fiber 28 (Fig. 3), the protective-hardening coating is first removed and the outer protective shell consisting of quartz glass is etched using hydrofluoric acid. Then, using an arc of an electric discharge 29, for example, a certain section of a two-core fiber is heated to the melting temperature and stretched to form a biconical constriction 30. From two ends, this segment of a two-core fiber with a biconical constriction formed in its middle part terminates in two segments of optical fibers on each side 31 and 32, which are in the general protective-reinforcing coating.

In FIG. 4 shows a section through a twin-core fiber in the biconical waist along the y and z axes. The twin-core fiber 33 comprises an outer protective sheath made of pure quartz glass 34, a sheath consisting of a material with a low refractive index 35, a biconical constriction 36. After the biconical constriction is formed, it is placed in a medium 37 having a refractive index close to the refractive index of the sheath material with a reduced index refraction n _OB ³⁵ ± 10 ^-3 . As the medium in which the biconical constriction is placed, a special optical glue or liquid can act. The biconical constriction can be placed in an additional protective quartz capillary, and a liquid, for example, an aqueous solution of glycerin, can be pumped into it with a syringe.

The ratio of glycerol and water is selected so that the refractive index of the solution has a value of n _P = n _OB ³⁵ ± 10 ^-3 , but it is preferable to use a special optical glue. The biconical constriction has a diameter D _f in the neck region and conical sections 38, the shape of which obeys a parabolic law. The twin-core fiber also contains two light guide wires 39 and 40, which lose the ability to channel radiation in the region of the “neck” of the biconical waist. This is necessary in order to make it possible to exchange optical radiation powers between light guide wires.

где D_ЗАГ - диаметр исходной заготовки "PANDA", которая использовалась для изготовления двужильного световода;
D_ОБ ³⁵ - диаметр оболочки в заготовке для двужильного световода, имеющей пониженный показатель преломления;
V - нормализованная частота;
λ_C - длина волны отсечки волновода двужильный световод - оболочка с пониженным показателем преломления в области "шейки" биконической перетяжки;
n₀ - показатель преломления чистого кварцевого стекла;
Δn_x - разность показателей преломления на границе двужильный световод - отражающая оболочка с пониженным показателем преломления,
Условие для D_f, приведенное выше, в случае, если λ_C≤λ₀ (λ₀ - длина волны излучения источника, используемого в волоконно-оптическом гироскопе, означает условие одномодовости волновода двужильный световод - оболочка с пониженным показателем преломления. При Δn<2•10^-3 направляющие свойства нового волновода в области "шейки" биконической перетяжки будут очень слабыми, что приведет к увеличению потерь оптической мощности в волоконной интегрально-оптической схеме. При Δn>4•10^-3 диаметр "шейки" D_f будет достаточно малым в случае обеспечения одномодовости вновь образованного волновода в области "шейки" биконической перетяжки, что практически при формировании биконической перетяжки выполнить достаточно сложно.In FIG. 5 shows a cross section of a twin-core fiber in the neck region of a biconical constriction. In this area, a two-core fiber is a quartz filament 41, which has a somewhat elliptical shape, surrounded by a sheath with a low refractive index 42 and protected from the outside with a special optical glue 43. The distribution profiles of the refractive indices in the cross section of the “neck” along the x and y axes have View 44 and 45. The degree of drawing of a twin-core fiber in the neck region should be such that:

where D _ZAG - the diameter of the original billet "PANDA", which was used to manufacture a two-core fiber;
D _AB ³⁵ - the diameter of the sheath in the workpiece for a two-core fiber having a low refractive index;
V is the normalized frequency;
λ _C is the wavelength of the cutoff of the waveguide two-core fiber - cladding with a low refractive index in the region of the "neck" of the biconical constriction;
n ₀ is the refractive index of pure quartz glass;
Δn _x is the difference between the refractive indices at the boundary of a two-core fiber - a reflective cladding with a reduced refractive index,
The condition for D _f given above if λ _C ≤λ ₀ (λ ₀ is the radiation wavelength of the source used in a fiber-optic gyroscope means the condition of a single-mode waveguide is a two-core fiber — a clad with a low refractive index. When Δn <2 • 10 ^-3 guiding properties of the new waveguide in the neck region of the biconical constriction will be very weak, which will lead to an increase in optical power losses in the fiber integrated optical circuit. When Δn> 4 • 10 ^{-3 the} diameter of the neck D _f will be quite small if one ews newly formed waveguide in the "neck" biconical banners that practically perform quite difficult when forming biconical waist.

To ensure good channeling properties while ensuring low losses of optical power of the radiation propagating in the neck region of the biconical waist, it is also necessary that the diameter of the sheath with a low refractive index be at least 2 times the diameter of the twin-core fiber in the neck region of the biconical waist . This is accomplished in the preform manufacturing stage for a two-core optical fiber through the condition (D _ON / D _CUG) ≥2. The refractive index of the optical adhesive to protect the biconical constriction should have a value close to the refractive index of the reflecting shell of the waveguide arising in the region of the “neck” of the biconical constriction.

When all of the above conditions are met for the formation of a single-mode waveguide in the neck region of the biconical waist, the optical power division coefficient of the splitter is stabilized. That is, the division of the optical power between two beams at the output ends of the fibers of a twin-core fiber occurs exactly in half and does not depend on the influence of external destabilizing factors, or on the bending of the waist, or on the degree of drawing of the twin-core fiber, since there are no higher modes in the region of the “neck” of the waveguide orders. At λ _C = 1.5 • 10 ^-6 m, Δn = 0.002, (D _OB / D _ZAG ) = 2, the diameter of the "neck" of the biconical constriction should be ~ 21.2 μm, and at Δn = 0.004 the diameter of the neck is ~ 30 microns.

где Δα - разность коэффициентов теплового линейного расширения материалов нагружающих стержней и остального материала световода;
ΔT - разность температуры в печи установки вытяжки световодов и комнатной температуры;
d - расстояние от нагружающих стержней до центра световедущей жилы;

- угол раскрыва нагружающих стержней при наблюдении их из центра световедущей жилы.The twin-core fiber has a large linear birefringence. The magnitude of the birefringence in the light guides of the twin-core fiber of the PANDA type is proportional to the value:

where Δα is the difference between the coefficients of linear thermal expansion of the materials of the loading rods and the rest of the fiber material;
ΔT is the temperature difference in the furnace of the installation of extracting optical fibers and room temperature;
d is the distance from the loading rods to the center of the light guide core;

- the opening angle of the loading rods when observing them from the center of the light guide core.

где Δn_x - разность показателей преломления световедущей жилы и отражающей оболочки для х-поляризационной моды;
В - величина двулучепреломления в световедущей жиле.In the presence of birefringence in the light guide veins, the critical taper angle of the transition, which is a biconical constriction formed on a two-core fiber, has different meanings for the polarization modes of a two-core fiber. That is, for the γ-polarization mode, the critical taper angle has a slightly smaller value and the difference in the critical taper angles for the polarization modes increases with increasing birefringence in a two-core fiber. If the design of the two-core fiber is selected in such a way that birefringence is concentrated mainly in the light guide, then for the critical taper angles of two polarization modes, we can write:

where Δn _x is the difference in the refractive indices of the light guide core and the reflective shell for the x-polarization mode;
B is the magnitude of birefringence in the light guide vein.

The concentration of birefringence only in the light guide conductor, at least in the direction of the y axis, is possible if the distance from the loading rods to the center of the light guide conductor is d≤D _w (D _w is the diameter of the light guide conductor) and α = 90 ^o to obtain the maximum birefringence. If the biconical constriction on a two-core fiber is formed so that its taper angle slightly exceeds the critical angle for the γ-polarization mode, but remains less than the critical taper angle for
x-polarization mode, the conical transitions of the biconical constriction begin to work as filters of polarization modes, that is, as radiation polarizers. Filtration of polarization modes occurs in the biconical constriction regions closer to the middle between the beginning of the biconical constriction and its “neck”. It should be noted that a certain ellipticity of the shape of the light guide core of the waveguide formed in the neck region of the biconical waist plays a positive role, since the birefringence axes of the light guide core of the biconductor light guide and the birefringence axis of the biconical waist neck of the birefringent coincide. This is important from the point of view that the polarization of radiation in the region of the “neck” of the biconical waist is maintained due to the ellipticity of the shape of the light guide core of the waveguide in the region of the “neck” of the biconical waist.

где α₁ - коэффициент затухания х-поляризационной моды;
α₂ - коэффициент затухания у-поляризационной моды;
l - расстояние от начала световода волоконной чувствительной катушки гироскопа до центра связи.Polarizing filters formed at the second conical transition of the biconical constriction, after dividing the optical beam into two beams, add additional zero stability to the ring interferometer, i.e., significantly reduce the stray phase displacement in it. When using the model of one center of polarization coupling for two modes in a two-core fiber, one can write:

where α ₁ is the attenuation coefficient of the x-polarization mode;
α ₂ - attenuation coefficient of the γ-polarization mode;
l is the distance from the beginning of the fiber of the fiber sensitive coil of the gyroscope to the center of communication.

Введем обозначения:

С учетом этого матрица передачи приобретает следующий вид:

Если в оптической схеме кольцевого интерферометра отсутствует скрутка световода чувствительного контура и для упрощения расчетов предположим, что на входе схемы отсутствует поляризатор, то для тока фотоприемника справедливо следующее выражение:

где Δβ = β_X-β_Y/ - разность постоянных распространения двух поляризационных мод.After conversion, the transmission matrix of the fiber of the sensitive coil of the fiber optic gyroscope in general can be represented as follows:

We introduce the following notation:

With this in mind, the transfer matrix takes the following form:

If the optical circuit of the ring interferometer does not have a twist of the fiber of the sensitive circuit and, to simplify the calculations, assume that there is no polarizer at the input of the circuit, then the following expression is valid for the photodetector current:

where Δβ = β _X -β _{Y /} is the difference between the propagation constants of the two polarization modes.

где

- длина деполяризации излучения.For the maximum possible phase adjustment in the ring interferometer of a fiber-optic gyroscope using a fiber-optic integrated circuit, the following expression can be written:

Where

- the length of radiation depolarization.

Тогда при η_g = 10^-2 и (f₀/Δf)≈40 имеем ΔΨ_П = 1,1•10^-7рад. Если волоконная катушка имеет диаметр 100 мм и содержит 10³ м световода, то смещение нуля волоконно-оптического гироскопа из-за поляризационных эффектов в оптическом тракте не превысит 0,1^o/час при λ₀ = 0,83•10^-6м.
На фиг.6 показан принцип изготовления волоконной интегрально-оптической схемы с использованием двужильного световода без дополнительной оболочки с пониженным показателем преломления. Она может быть организована непосредственно при изготовлении биконической перетяжки, но с использованием специального стеклянного капилляра с пониженным показателем преломления. Она может быть изготовлена также с помощью осаждения на внутреннюю поверхность опорной кварцевой трубы слоев кварцевого стекла с пониженным показателем преломления; затем из этой трубы на установке вытяжки вытягиваются кварцевые капилляры с внутренним диаметром, соответствующим диаметру двужильного световода. После этого внешняя оболочка этих капилляров, состоящих из чистого кварцевого стекла, стравливается с помощью плавиковой кислоты, и на участок двужильного световода 46 с поперечным сечением 47 (фиг.6) одевается стеклянный капилляр 48, состоящий из материала с пониженным показателем преломления. Затем с помощью, например, дуги электрического разряда, капилляр сплавляется с двужильным световодом, после чего формируется биконическая перетяжка 49. После формирования биконической перетяжки она защищается специальным оптическим составом с показателем преломления n=n_ОБ±1•10^-3, где n_ОБ - показатель преломления материала капилляра.It follows that polarizing filters on the output arms of the fiber splitter reduce the stray phase stand in the ring interferometer of a fiber-optic gyroscope. Assume that a fiber integrated optical circuit has characteristics that are easily achievable from a practical point of view, i.e.

Then, with η _g = 10 ^-2 and (f ₀ / Δf) ≈40, we have ΔΨ _П = 1.1 • 10 ^-7 rad. If the fiber spool has a diameter of 100 mm and contains 10 ³ m of the fiber, then the zero offset of the fiber-optic gyroscope due to polarization effects in the optical path will not exceed 0.1 ^o / h at λ ₀ = 0.83 • 10 ^-6 m.
Figure 6 shows the principle of manufacturing a fiber integrated optical circuit using a two-core fiber without an additional sheath with a low refractive index. It can be organized directly in the manufacture of a biconical constriction, but using a special glass capillary with a low refractive index. It can also be made by deposition on the inner surface of the supporting quartz tube layers of silica glass with a low refractive index; then, quartz capillaries with an inner diameter corresponding to the diameter of the twin-core fiber are pulled from this pipe in the hood installation. After that, the outer shell of these capillaries, consisting of pure silica glass, is etched using hydrofluoric acid, and a glass capillary 48 consisting of a material with a low refractive index is put on a section of a twin-core fiber 46 with a cross section 47 (Fig. 6). Then, using, for example, an electric discharge arc, the capillary is fused with a two-core fiber, after which a biconical waist 49 is formed. After the biconical waist is formed, it is protected by a special optical composition with a refractive index of n = n _OB ± 1 • 10 ^-3 , where n _OB - refractive index of capillary material.

где V - нормализованная частота;
λ₀ - длина волны излучения источника;
n₀ - показатель преломления плавленого кварца;
Δn_x - разность показателей преломления между кварцевым стеклом "шейки" биконической перетяжки и показателем преломления двулучепреломляющего кристалла для х-поляризационной моды.Another possibility of obtaining a single-mode waveguide for the x-polarization mode in the neck region of the biconical constriction of a twin-core fiber without an additional cladding with a low refractive index is sputtering on a biconical constriction of a composition with birefringence [12]. A birefringent crystal sprayed onto a biconical constriction works similarly to that described in [6]. The neck of the biconical constriction consists practically of pure quartz glass, and the birefringent crystal has a refractive index for the x-polarization mode slightly higher than the refractive index of pure quartz glass, and for the y-polarization mode it is slightly lower and therefore an additional neck is formed in the neck region polarizing filter, which significantly reduces the stray phase stand in the ring interferometer of a fiber-optic gyroscope. Before spraying a birefringent crystal on a biconical constriction, the diameter of its “neck” must satisfy the condition:

where V is the normalized frequency;
λ ₀ - wavelength of the radiation source;
n ₀ is the refractive index of fused silica;
Δn _x is the difference between the refractive indices between the quartz glass of the “neck” of the biconical waist and the refractive index of the birefringent crystal for the x-polarization mode.

 V≤2,4 is the condition of the single-mode waveguide in the region of the neck of the biconical waist, and V> 2 is the condition that there is no loss of optical power of the x-polarization mode when the biconical waist propagates along the waveguide in the neck region.

 In FIG. 7 shows an equivalent optical circuit of a fiber integrated optical circuit. It contains the input 50 and output 51 ends of the optical fibers, input 52 and output 53 polarizers, the first fiber splitter 54, spatial and polarization filters 55 at the same time, the second fiber splitter 56, the fiber polarizers 57 and 58 and the output ends of the optical fiber 59 and 60 optical fiber circuits that are connected to the optical fiber of the sensitive coil of the gyroscope and integrated optical phase modulators (figure 1).

 In the case of deposition of a birefringent crystal, a single-mode optical waveguide in the neck region of a biconical constriction acts not only as a polarizing mode filter, but also as a spatial filter, the presence of which is necessary in the optical design of the ring interferometer of a fiber-optic gyroscope. The role of the spatial filter between the first and second splitters is played by a single-mode waveguide in the neck region of the biconical constriction.

Sources of information
1. GA Pavlath "Closed-loop fiber optic gyros" SPIE v. 2837, p. 46-60, 1996

 2. Sanders et all. "Fiber optic gyros for space, marine and aviation applications" SPIE v. 2837, p. 61-71, 1996

 3. A.M. Kurbatov "Fiber optic gyroscope using single-mode fiber with high birefringence." NTS "Issues of Aviation Science and Technology" Series "On-board instruments for navigation control and management." Issue 4A, 1990, pp. 60-71.

 4. W.K. Burnus, Chin-Lm-Chen, R.P. Moeller "Fiber optic Gyroscopes with Brood-Band Surces" J of Lightwave Techology, 1983, v. LT-1, 1, p. 88-104.

 5. F. De Foruelle et all Electron. Letters, v. 20, p. 398, 1984

 6. R.A. Bergh, H.G. Lefevre and H.J. Shaw "Single-mode fiber-optic polarizer" Opt. Letters vol. 5, p. 479-481, 1980

 7. Kurbatov A.M. and others. "Single-mode fiber splitter that preserves the polarization of radiation." NTS "Issues of Aviation Science and Technology" Series "Flight-Navigation Systems". Vol. 7A, Moscow, 1989

 8. A. M. Kurbatov and others. "A method of manufacturing a single-mode fiber waveguide for a splitter that preserves the polarization of radiation" A.S. 324346, priority 04/09/90

 9. W. K. Burns, M. Abele, C. A. Villarruel Appl. Opt. v. 24, 17, p. 2753-2755, 1985. "A parabolic model of the shape of a biconical constriction."

 10. A. Kurbatov et al. "Single-mode optical fiber that preserves the polarization of radiation." NTS "Issues of Aviation Science and Technology" Series "Flight-Navigation Systems and Devices". Vol. 7A, Moscow, 1989

 11. A. Kurbatov and others. "A method of obtaining a single-mode fiber waveguide." RF patent 2043313, 09/10/95.

 12. A. Birman "Biconical all-fiber crystal polariser" SPIE vol. 2837, p. 368-374, 1996

Claims (3)

1. A method of manufacturing a fiber integrated optical circuit for an interferometer of a fiber-optic gyroscope based on a two-core fiber, which consists in the formation of a biconical constriction on a two-core fiber, made by grinding two blanks for single-mode fiber with linear birefringence, then cutting on the ground grinding surfaces, transverse grinding surfaces these blanks with their flat surfaces and hoods from the obtained blanks of a two-core fiber, characterized in that In the manufacture of pre-grinding workpieces, they are preliminarily placed inside and then fused with a supporting quartz tube, on the inner surface of which a layer of quartz glass with a refractive index less than (2 ÷ 4) • 10 ^-3 is lower than the refractive index of pure quartz glass and layer thickness providing

where D _OB - the diameter of the shell formed by a layer of quartz glass with a low refractive index,
D _ZAG - the diameter of the original billet,
and after drawing two-core fiber portion, which is formed biconical constriction is placed beforehand in the pickling solution with the purpose of etching the outer layer two-core fiber composed of pure quartz glass, and then formed biconical constriction coated protective reinforcing an optical coating with a refractive index n = n _about ± 1 • 10 ^-3 , where n _rev is the refractive index of the material of the outer sheath of the two-core fiber. 2. A method of manufacturing a fiber integrated optical circuit for an interferometer of a fiber-optic gyroscope based on a two-core fiber, which consists in forming a biconical constriction on a two-core fiber, made by grinding two blanks for single-mode fiber with linear birefringence, then cutting on the ground grinding surfaces, transverse grinding surfaces these blanks with their flat surfaces and hoods from the obtained blanks of a two-core fiber, characterized in that after drawing out a two-core fiber, its section, on which the biconical constriction is formed, is placed inside the capillary consisting of quartz glass with a refractive index less by (2 ÷ 4) • 10 ^-3 than the refractive index of pure quartz glass. 3. A method of manufacturing a fiber integrated optical circuit for an interferometer of a fiber-optic gyroscope based on a two-core fiber, which consists in the formation of a biconical constriction on a two-core fiber, made by grinding two blanks for single-mode fiber with linear birefringence, then cutting on the ground grinding surfaces, transverse grinding surfaces these blanks with their flat surfaces and hoods from the obtained blanks of a two-core fiber, characterized in that the biconical constriction is covered with a birefringent composition having a refractive index for the x-polarization mode of the fiber that exceeds the refractive index of the outer layer of the glass of the two-core fiber, and for the y-polarization mode, the refractive index is lower than the refractive index of the outer layer of the glass of the two-core fiber, and the biconical constriction in its region "form with such a maximum diameter in cross section that

where 2 <V <2.4;
λ _o - wavelength of the radiation source;
n _about - the refractive index of the outer layer of glass of a two-core fiber;
Δn _x is the difference between the refractive indices between the outer layer of glass of the two-core fiber and the refractive index of the birefringent composition for the x-polarization mode.

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