RU2173866C2 - Filter based on fiber-optical diffraction gratings with large periods - Google Patents

Abstract

FIELD: fiber optics. SUBSTANCE: filter based on fiber-optical diffraction gratings with large periods includes core carrying fiber-optical diffraction gratings with large periods formed on it, each being formed with preset period, shell surrounding core, coat covering part of shell without fiber-optical diffraction gratings with large periods, second coat covering part of shell having fiber-optical diffraction gratings with large periods. Refractive index of core provides for displacement of coupled length of wave towards short lengths of waves with rise of temperature in correspondence with amount of impurity added into core. Refractive index of second coat diminishes with rise of temperature and coupled length of wave displaces towards long waves. Core develops negative displacement of coupled length of wave by value of positive displacement of coupled length of wave in material of second coat whose refractive index diminishes with rise of temperature. EFFECT: provision for temperature stability of parameters of filter, enhanced resistance of filter to increased humidity. 4 cl, 19 dwg

Description

 The invention relates to a filter based on fiber optic diffraction gratings, and in particular to a temperature-stabilized filter based on fiber optic diffraction gratings, which does not bias the communication parameters depending on temperature changes.

 A fiber optic diffraction grating is typically used as a filter to select an optical signal at a specific wavelength propagating along the core of the optical fiber. A fiber optic diffraction grating can remove or reflect light at a specific wavelength due to the fact that, using an ultraviolet (UV) laser, a periodic change in the refractive index of the optical fiber is induced. Fiber optic diffraction gratings are subdivided into fiber optic diffraction gratings with a short period and fiber optic diffraction gratings with a long period.

 When filtering a signal using fiber-optic diffraction gratings with small periods, light is reflected only at a certain wavelength, while fiber-optic diffraction gratings with long periods bind the core mode, on which the optical signal propagates along the core of the optical fiber, with the mode of the shell in the same direction of propagation. Fiber-optic diffraction gratings with large periods ranging from several tens of microns to several hundred microns are used as a smoothing filter of the amplifier in the VUE (an amplifier based on optical fiber with an admixture of erbium) due to their ability to remove radiation at a given wavelength by transferring radiation from core modes into sheath modes in the same direction of propagation.

 Fiber optic diffraction gratings with long periods are made by changing the refractive index of the core of the optical fiber that is sensitive to UV radiation, with a given period for each grating. In the part of the core exposed to UV radiation, the refractive index increases and does not change in that part of the core that is not exposed to UV radiation, as a result, a periodic change in the refractive index is created along the longitudinal axis of the optical fiber.

 Fiber optic diffraction gratings with large periods are temperature sensitive, and their optical characteristics are affected by the refractive index outside the sheath of the optical fiber. Microbends of an optical fiber significantly affect the central wavelength and attenuation coefficient of fiber-optic diffraction gratings with long periods, which are determined by the relationship between the core mode and the sheath mode.

 To use fiber-optic diffraction gratings with long periods, it is necessary to re-coat, providing stable optical characteristics when changing operating conditions. The parameters of the operating conditions (external environment) are temperature, humidity, dust, as well as microcracks and microbends of the optical fiber.

 A connection (between modes) in a filter based on fiber-optic diffraction gratings with large periods occurs if the phase matching condition corresponding to (1) is satisfied.

где β_co - коэффициент распространения для моды сердцевины, β $\genfrac{}{}{0ex}{}{\text{(m)}}{\text{cl}}$ - коэффициент распространения для моды оболочки m-порядка, а

период решетки.

where β _co is the propagation coefficient for the core mode, β $\genfrac{}{}{0ex}{}{\text{(m)}}{\text{cl}}$ is the propagation coefficient for the m-order cladding mode, and

lattice period.

Излучение на какой-либо длине волны может быть перенесено в моду оболочки путем установления периода

решетки и разности показателей преломления (n_c0-n_c1 ^( (m)).If β = 2π (n / λ) (n is the refractive index, and λ is the wavelength), then

Radiation at any wavelength can be transferred to the cladding mode by establishing a period

lattices and refractive index differences (n _c0 -n _c1 ^{( (m)} ).

решетки и УФ-излучение направляется на маску. Затем, оптическое волокно реагирует на УФ-излучение таким образом, что увеличивается показатель преломления сердцевины и связанная длина волны увеличивается в длинноволновую сторону. Для того чтобы получить требующийся спектр (т.е. требующуюся связанную длину волны и коэффициент ослабления) фильтра на основе волоконно-оптических дифракционных решеток с большими периодами, УФ-излучение должно направляться в течение соответствующего времени, точно регулируя период маскирования.The difference in refractive indices is obtained by appropriate exposure to UV radiation of an optical fiber sensitive to this UV radiation. That is, the optical fiber is covered with a mask corresponding to a certain period

gratings and UV radiation are directed to the mask. Then, the optical fiber reacts to UV radiation in such a way that the refractive index of the core increases and the associated wavelength increases in the long wavelength direction. In order to obtain the required spectrum (i.e., the required coupled wavelength and attenuation coefficient) of a filter based on fiber optic diffraction gratings with large periods, UV radiation must be directed for an appropriate time, precisely adjusting the masking period.

где T - температура.The associated wavelength of the thus obtained filter based on fiber optic arrays is affected by temperature. The shift (spectral) of the associated wavelength depending on the change in temperature is determined by changes in the refractive index and thermal expansion along the length with temperature. It can be expressed as

where T is the temperature.

в несколько десятков раз и поэтому

пренебрегают.When a filter based on fiber optic gratings with long periods is made of a conventional optical fiber for signal transmission or an optical fiber whose refractive index varies over the cross section,

several dozen times and therefore

neglected.

For example, the associated wavelength for Corcor's Flexcor 1060 is shifted by 5 nm for every 100 ^° C. In an optical fiber with a conventional cross-sectional refractive index profile, the associated wavelength is shifted by 0.3 nm for every 100 ^° C due to length extension and at 5 nm for every 100 ^o C due to changes in the refractive index. For a smoothing filter-amplifier, which is one of the applications of a filter based on fiber-optic arrays with long periods, a temperature stability of about 0.3 nm per 100 ^° C is required so that it can be applied in practice in applied systems.

в (3) имело отрицательное значение. В альтернативном случае в оптическое волокно добавляется B₂O₃ для получения dn/dТ=0.In the means known from the prior art for compensating a temperature change, an optical fiber is produced with a certain section of refractive index profile or the period of the fiber optic grating is chosen so that

in (3) had a negative value. Alternatively, B ₂ O _{3 is} added to the optical fiber to obtain dn / dT = 0.

является отрицательной величиной при традиционном способе регулирования показателя преломления фильтра путем установления

отрицательным значением. Когда Λ=40 мкм, то зависимость длины волны от температуры в волокне Flexcor 1060 составляет 0,15-0,45 нм/100^oC, а мода λ^(m) находится в области 1,1 мкм, следовательно, выходит из области систем связи. Температурно компенсированный фильтр на основе волоконно-оптических решеток с большими периодами раскрыт подробно в корейской заявке N 99-8332 на "Фильтр на основе волоконно-оптических решеток с большими периодами с температурной компенсацией", поданной заявителем настоящей заявки.If Λ <100 μm in a conventional filter based on fiber optic gratings with large periods, then

is a negative value in the traditional way of regulating the refractive index of the filter by establishing

negative value. When Λ = 40 μm, the temperature dependence of the wavelength in the Flexcor 1060 fiber is 0.15-0.45 nm / 100 ^o C, and the λ ^(m) mode is in the region of 1.1 μm, therefore, it leaves the system communication. A temperature-compensated filter based on fiber optic arrays with long periods is disclosed in detail in Korean application N 99-8332 on "A filter based on fiber-optic arrays with long periods with temperature compensation", filed by the applicant of this application.

 Although the re-coating of a filter based on fiber-optic arrays with large periods in the above application was formed from a material whose refractive index increases with increasing temperature, the refractive index of conventional re-coating, especially polymer re-coating, decreases with increasing temperature due to thermal expansion. Therefore, when a re-coating is performed on a filter based on fiber-optic gratings with long periods formed from a conventional optical fiber, the long-wave shift effect associated with the re-coating is added with the long-wave displacement characteristic of a fiber-based filter optical gratings with large periods, and therefore, for re-coating should be used material, the refractive index of which is reduced. Such material for re-coating has yet to be created.

 The closest analogue is the fiber optic diffraction grating described in JP 8286040, used as a filter for selecting an optical signal at a specific wavelength, comprising a core having fiber optic diffraction gratings formed thereon; the shell surrounding the core; coating covering part of the shell.

 Thus, the aim of the present invention is to create a filter based on fiber-optic diffraction gratings having temperature compensation, which is characterized by the absence of a shift in the values of the coupling parameters (between modes) when the temperature changes.

 An additional objective of the present invention is to provide a filter based on fiber optic arrays having temperature compensation, which is resistant to high humidity and soft enough to prevent microbending.

 To achieve the above objectives, a filter based on fiber optic diffraction gratings is proposed. A filter based on fiber-optic diffraction gratings includes a core having fiber-optic diffraction gratings formed therein with predetermined periods for each grating, a shell surrounding the core, a coating covering that part of the shell that is outside the fiber-optical diffraction gratings, repeated a coating covering that part of the shell on which there are fiber optic diffraction gratings, while the refractive index of the core provides a shift in the length of the coupled wave toward shorter wavelengths with increasing temperature in accordance with the amount of impurities added to the core, and recoat the refractive index decreases as temperature increases, providing a displacement wave lengths related to longer wavelengths. Therefore, the core gives a negative offset of the associated wavelength by the amount of positive offset of the associated wavelength in the re-coating material, the refractive index of which decreases at elevated temperature.

The above and other objectives, as well as signs and advantages of the present invention will be more clearly visible from the following detailed description in combination with the accompanying drawings, in which:
FIG. 1A is a perspective view of a packaged filter based on fiber optic diffraction gratings with large periods;
FIG. 1B is a perspective view of a filter based on fiber optic arrays with long periods with remote re-coating;
FIG. 1C is a cross-sectional view of a filter based on fiber optic arrays with long periods with remote re-coating;
FIG. 2A-2D are graphs representing an offset of the associated wavelength as a function of the refractive index outside the shell;
FIG. 3 is a graph representing a shift in a coupled wavelength as a function of a change in the refractive index outside the shell;
FIG. 4 is a graph representing an offset of a coupled wavelength as a function of the refractive index outside the shell when it is less than the refractive index of the shell;
FIG. 5A is a graph representing a change in the refractive index of a re-coating with a change in temperature when the re-coating is made of ordinary polymeric material;
FIG. 5B is a graph representing a change in the refractive index of a re-coating with a change in temperature when the re-coating is made of silicone (organosilicon) resin;
FIG. 6 is a graph representing an offset of a related wavelength in a re-coating material as a function of temperature change;
FIG. 7 is a graph representing a change in the refractive index with a change in temperature at various impurity concentrations in the core of the optical fiber;
FIG. 8 is a graph showing wavelength versus temperature for various impurity concentrations in the core of an optical fiber;
FIG. 9 is a graph illustrating the effect of temperature compensation of a filter based on fiber optic arrays with long periods in accordance with the present invention;
FIG. 10A is a graph representing the temperature dependence of a filter based on fiber optic arrays with long periods with remote re-coating;
FIG. 10B is a graph representing the temperature dependence of a conventional filter based on fiber optic arrays with large re-coated periods;
FIG. 11 is a graph representing the temperature dependence of a filter based on fiber optic gratings with large periods, made according to the present invention; and
FIG. 12 is a cross-sectional view of a filter based on fiber optic arrays with long periods, made in accordance with the present invention.

 A preferred embodiment of the present invention will be described below with reference to the accompanying drawings. In the description that follows, well-known functions or features are not described in detail since they could obscure the invention with irrelevant details.

 To form fiber optic arrays with long periods on an optical fiber, a coating is removed from the optical fiber at a given length. Then, fiber optic arrays with long periods are formed on the open part using a UV laser and an amplitude mask. Uncoated fiber optic arrays with long periods are exposed to environmental conditions, including temperature, humidity, dustiness, microcracks and microbends, and therefore protection is necessary to prevent changes in optical characteristics.

 In addition, a plurality of long-period fiber optic arrays formed along the length of the optical fiber with a predetermined period function act as a filter for coupling between the core mode and the sheath mode. Therefore, the refractive index of the re-coating material must be taken into account.

 As shown in FIG. 1A, 1B, and 1C, a packaged long-period fiber optic filter 100 includes a core 10 having long-period fiber optic arrays formed thereon, a sheath 12 surrounding the core 10, a coating 14 surrounding sheath 12 , and re-coating 18, made on fiber optic arrays 16 with a large period. Recoating is applied to protect the fiber optic arrays 16 with large periods on the part from which the coating 14 is removed.

 In FIG. 1C arrows indicate the direction of propagation of the light wave and indicate the relationship between the core mode and the sheath mode in the filter based on fiber optic gratings with large periods. The thickness of the arrow indicates the light intensity at the wavelength.

 An optical signal at a central wavelength propagating in the main mode of the fiber in the core 10 is scattered in the region with a varying refractive index, that is, on fiber optic gratings 16 with large periods. When the scattered light binds to the cladding 12, the light at a wavelength that satisfies the phase matching condition is coherently amplified. The light exits from the casing 12 and the filter 100 based on fiber optic arrays with long periods acts as an attenuator depending on the wavelength.

 The intensity of the light propagating in the main mode of the fiber decreases when it passes through the fiber optic gratings 16 with long periods, as indicated in FIG. 1C by decreasing the thickness of the arrows, and the light intensity at the wavelength associated with the cladding 12 increases as this is shown by an increase in the thickness of the arrows.

 The external environment of the cladding 12, namely air, has a refractive index of 1. If the cladding 12 is re-coated with a material with a refractive index n, after the fiber-optic gratings 16 are formed with long periods, the coupling condition (between modes) changes and therefore , the wavelength is shifted towards long or short waves.

 FIG. 2A-2D are graphs illustrating the offset of the associated wavelength as a function of the refractive index outside the shell.

 FIG. 2A is a graph illustrating an optical transmission characteristic when the refractive index (refractive index of air) outside the envelope surrounding the fiber optic arrays with large periods is 1.

 FIG. 2B is a graph illustrating an optical transmittance when the refractive index outside the shell is 1.400. It should be noted that the optical transmittance is increased and the associated wavelength is shifted towards short waves by about 4.8 nm compared to the graph in FIG. 2A.

 FIG. 2C is a graph illustrating the optical transmittance when the refractive index outside the shell is 1.488. The associated wavelength is shifted towards short waves by 16.5 nm compared to FIG. 2A.

 FIG. 2B is a graph illustrating an optical transmission characteristic when the refractive index outside the shell is 1.484. The associated wavelength is shifted towards long waves in comparison with FIG. 2A.

 If the refractive index outside the shell exceeds 1, but less than the refractive index of the shell, then the associated wavelength is shifted toward short waves, as shown in FIG. 2B and 2C. On the other hand, if the refractive index outside the shell exceeds the refractive index of the shell, then the associated wavelength shifts toward longer wavelengths, as shown in FIG. 2D. If the refractive index outside the shell is equal to the refractive index of the shell, the condition of total internal reflection disappears and the coupling peak (coupled wavelength) disappears.

 In FIG. 3 is a graph illustrating a shift in the associated wavelength as a function of a change in the refractive index outside the shell. The associated wavelength shifts toward shorter wavelengths as the external refractive index increases from 1.0, the associated peak disappears when the external refractive index is equal to the sheath refractive index, and then, the associated wavelength shifts toward long waves when the external refractive index exceeds refractive index of the shell.

 FIG. 4 is a graph illustrating a shift in a coupled wavelength as a function of a change in the refractive index outside the shell when the external refractive index is less than the refractive index of the shell.

 Turning to FIG. 4; as the external refractive index decreases, the associated wavelength shifts in the direction of long wavelengths only if the external refractive index is less than the refractive index of the cladding.

 The results shown in FIG. 2A-4 are described in detail in an article by the author of the present invention, “Offset of resonant peaks of a fiber optic grating with a long period caused by a change in the external refractive index” (1997, Optics Letters, December 1, 1997 / Vol. 22, No. 23).

 FIG. 5A is a graph illustrating a change in the refractive index of a conventional re-coating material as a function of temperature change, and FIG. 5B is a graph illustrating a change in the refractive index of a silicone resin, taken as an example of a conventional re-coating material, as a function of temperature change.

Turning to FIG. 5A; conventional re-coating material, i.e., polymer, undergoes thermal expansion at elevated temperature and has a reduced refractive index. Turning to FIG. 5B; silicone (organosilicon) resin also experiences thermal expansion at elevated temperatures and has a reduced refractive index. The change in the refractive index of silicone resin with temperature is -2.4 • 10 ^-2 / 100 ^o C.

 FIG. 6 is a graph illustrating a shift in the associated wavelength of a re-coating material as a function of temperature change. From FIG. Figure 6 shows that the associated wavelength shifts toward longer wavelengths with increasing temperature, while the refractive index of the re-coating material decreases. An offset of the associated wavelength towards longer wavelengths means that there is a positive direction of the offset of the wavelength.

FIG. 7 is a graph illustrating a shift in the associated wavelength as a function of temperature change at various concentrations of impurity added to the core of the optical fiber. Temperature compensation by adding В ₂ О ₃ and GeO ₂ as impurities to the core is described in detail in EP 0800098 A2 "Optical waveguide diffraction grating and method for its manufacture". As shown in FIG. 7, if B ₂ O _{3 is} greater than GeO ₂ , then fiber optic gratings with long periods give a negative direction of wavelength displacement with increasing temperature. That is, a change in the refractive index with increasing temperature is negative. In the present invention, the temperature change is compensated by setting a negative value for the direction of displacement of the associated wavelength in fiber arrays with long periods and a positive value in the re-coating material.

For example, if 20 mol.% GeO ₂ and 15 mol.% B ₂ O _{3 are} added to the core, then the change in the refractive index of the fiber optic gratings formed on the core with a temperature is negative and, therefore, the associated wavelength has a negative direction of wavelength offset. This is illustrated in FIG. 8.

FIG. 8 is a graph illustrating a shift in the associated wavelength with increasing temperature, when the amount of B ₂ O ₃ in the core is greater than GeO ₂ and fiber optic arrays with long periods are not re-coated. In FIG. Figure 8 shows that the associated wavelength shifts toward short waves with increasing temperature. This means that the associated wavelength in the filter based on fiber optic gratings with large periods has a negative direction of wavelength displacement.

FIG. 9 is a graph illustrating the effect of shifting the wavelength to the long wavelength side for a re-coating material, such as silicone resin, with increasing temperature, in a filter based on fiber optic arrays with long periods and the temperature compensation resulting from the effect of shifting the wavelength to shortwave side obtained when using In ₂ About ₃ more than GeO ₂ . Position 91 indicates a shift in the associated wavelength to the long wavelength side due to a change in the refractive index of the cladding / re-coating part in accordance with a change in temperature, and position 93 indicates a shift in the associated wavelength to the short-wavelength side due to a change in the refractive index of the core / clad part in accordance with the change in temperature.

 In the filter based on fiber optic arrays with long periods, bias opposite in direction arise for the associated wavelength, in the long wavelength direction and in the short wavelength direction, as a result of which the temperature compensation is achieved in the present invention, as indicated by 92.

 FIG. 10A and 10B are graphs illustrating wavelength shift versus temperature changes in cases where a conventional filter based on fiber optic arrays with long periods, showing no short-wave bias effect in the core, does not have a re-coating and has a re-coating of silicone resin, respectively.

FIG. 8 is a graph illustrating a wavelength shift as a function of temperature when the long-period fiber-optic filter according to the present invention has no re-coating, but has a negative wavelength shift direction when using B ₂ O ₃ at a larger amount than GeO ₂ .

FIG. 11 is a graph illustrating a wavelength shift as a function of temperature when the long-period fiber-optic filter according to the present invention has a silicone resin re-coating, although it has a negative wavelength shift direction using B ₂ O ₃ in larger quantities than GeO ₂ .

 Temperature compensation according to the present invention will be described below by comparing FIG. 10A and 10B illustrating the conventional technology of FIG. 8 and 9, corresponding to the present invention.

As shown in FIG. 10A, when a conventional filter based on fiber optic arrays with long periods does not have a re-coating, the associated wavelength shifts with increasing temperature in the long wavelength direction and the temperature dependence for the wavelength is approximately 5.08 nm / 100 ^° C.

In FIG. 10B, when a conventional filter based on fiber-optic arrays with long periods has a silicone resin re-coating, the associated wavelength shifts with increasing temperature in the long wavelength direction and the temperature dependence for the wavelength is approximately 10 nm / 100 ^° C.

 From FIG. 10A and 10B, it can be seen that repeated coating of the silicone resin of a conventional filter based on fiber-optic arrays with long periods leads to a joint enhancement of the effects of long-wave displacement due to the core of the optical fiber and long-wave displacement due to silicone resin, thereby further increasing the effect long wave displacement. That is, the temperature dependence increases even more.

In FIG. 8, when in the present invention the core of the optical fiber includes B ₂ O ₃ more than GeO ₂ , and the filter based on fiber-optic gratings with long periods does not have re-coating, the associated wavelength shifts to the short-wavelength side with increasing temperature and temperature dependence wavelength is approximately -4.7 nm / 100 ^o C.

In FIG. 11, when in the present invention, the core of the optical fiber contains B ₂ O ₃ more than GeO ₂ , and the filter based on fiber-optic gratings with long periods has a re-coating of silicone resin, then opposite effects will occur - the short-wave effect due to the core and the effect of long-wave displacement due to the re-coating material, as a result of which the temperature change in the wavelength is compensated. As a result, there is no change in the associated wavelength depending on the change in temperature. Here, the temperature dependence is about 0.07 nm / 100 ^o C.

Thus, the fabricated filter based on the long-term fiber optic arrays of the present invention is shown in FIG. 12. Position 120 denotes a core in which B ₂ O _{3 is} greater than GeO ₂ , position 122 denotes a shell surrounding the core 120, position 126 denotes a plurality of fiber optic arrays with long periods formed along the length of the core 120. Position 128 denotes a repeated a silicone resin coating that covers the fiber optic arrays 126 with large periods.

It can be concluded that the change associated with temperature can be compensated with the exception of a small shift in the associated wavelength, if the associated wavelength shifts in the positive direction of the shift in wavelength with increasing temperature due to the use of a larger amount of B ₂ O ₃ optical fiber in the core than GeO ₂ , and in re-coating the refractive index decreases with increasing temperature and the associated wavelength shifts in the positive direction.

 As described above, a long-period fiber-optic filter according to the present invention includes a core in which the associated wavelength shifts in the negative direction at an elevated temperature in accordance with added impurities, and a re-coating, where the refractive index decreases with increasing temperature and the associated wavelength is shifted in the positive direction. Therefore, the offset of the associated wavelength of the filter based on fiber optic arrays with large periods associated with a change in temperature can be compensated and the temperature compensation of the filter is simplified.

 Although the invention has been described with reference to a certain preferred embodiment of the invention, it is understood by those skilled in the art that various changes in form and details can be made without departing from the spirit and scope of the invention as defined in the appended claims. inventions.

Claims (5)

 1. A filter based on fiber optic diffraction gratings, comprising a core having fiber optic diffraction gratings formed therein with periods specified for each grating; the shell surrounding the core; a coating covering a portion of the shell that is free of fiber optic diffraction gratings; a re-coating covering a portion of the shell having fiber optic arrays, wherein the core refractive index provides a shift in the wavelength of the wavelength associated with shorter wavelengths with increasing temperature in accordance with the amount of impurity added to the core, and the refractive index of the re-coating decreases with increasing temperature, providing displacement of the length of the coupled wave in the direction of long waves. 2. The filter according to claim 1, characterized in that the impurity includes B ₂ O ₃ and GeO ₂ and the sum of the displacements of the associated wavelength caused by an increase in the refractive index in accordance with the amount of GeO ₂ and a decrease in the refractive index in accordance with the amount of B ₂ O ₃ , has a negative value.  3. The filter according to claim 1, characterized in that the re-coating is made of a polymeric material, the refractive index of which decreases with increasing temperature.  4. The filter according to claim 3, characterized in that the polymer material is a silicone resin.  5. The filter according to claim 1, characterized in that the refractive index of the re-coating is less than the refractive index of the shell.

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