JP3941168B2 - Fabrication method of optical waveguide grating - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for manufacturing an optical waveguide grating that blocks guided light having a predetermined wavelength in an optical waveguide.
[0002]
[Prior art]
An optical waveguide grating is a periodic refractive index modulation region formed in an optical waveguide such as an optical fiber or a planar waveguide, and has a relatively short period (usually a period of about 0.1 to 1.0 μm). ) And a long period grating having a longer period (usually a period of about 100 to 1000 μm). Each of these gratings has a function of blocking light of a predetermined wavelength band traveling in the optical waveguide. More specifically, the Bragg grating reflects light in a wavelength band centered on Bragg wavelength light and blocks light in this wavelength band. In addition, the long-period grating blocks the light of this wavelength (so-called “loss wavelength”) by converting the core mode light having a wavelength satisfying a predetermined condition into a clad mode light and generating an optical loss. In any grating, the wavelength of light blocked by the grating (Bragg wavelength or loss wavelength; hereinafter referred to as “cutoff wavelength”) is the refractive index modulation period constituting the grating and the average refractive index of the grating. Depends on the rate.
[0003]
As a method for manufacturing an optical waveguide type grating, a method of causing light to enter a photosensitive optical waveguide and causing a photoinduced refractive index change is widely known. As a photosensitive optical waveguide, GeO₂Doped with SiO₂Glass is generally used, and ultraviolet light having a wavelength of about 240 to 250 nm or about 185 to 195 nm is often used as incident light to the glass.
[0004]
At this time, if a photosensitizer is added to the optical waveguide, the photosensitivity of the change in refractive index is increased, and the change in the refractive index when the same amount of light is incident is increased. SiO₂Hydrogen and deuterium are known as a photosensitizer for glass. This technique is disclosed, for example, in JP-A-6-118257.
[0005]
However, hydrogen and deuterium that are photosensitizers are SiO₂Since it also functions as a refractive index changing material that increases the refractive index of glass by the amount added, hydrogen and deuterium gradually diffuse into the outside air after forming a grating in the optical waveguide. If the added amount is reduced, the refractive index of the grating is lowered, and accordingly, the cutoff wavelength of the grating (the Bragg wavelength of the Bragg grating or the loss wavelength of the long period grating) is shortened. That is, there arises a problem that the characteristics of the grating change over time and are not stable. In order to prevent this, when a photosensitive improver is added, after the grating is formed, heat treatment is performed for a predetermined time, and the photosensitive improver in the optical waveguide is forcibly diffused and removed in the outside air. The characteristics of the grating are stabilized.
[0006]
[Problems to be solved by the invention]
However, since the above heat treatment is also a treatment for removing hydrogen and deuterium in the optical waveguide, it causes a change in refractive index and changes the cutoff wavelength of the grating. FIG. 9 is a diagram showing the change over time in the Bragg wavelength when a Bragg grating is formed in an optical fiber to which hydrogen is added and then the dehydrogenation treatment is performed by exposing the optical fiber to air at 80 ° C. FIG. In FIG. 9, comparing the start of the dehydrogenation process and after 48 hours, it can be seen that the Bragg wavelength is shortened by 0.5 nm or more by the dehydrogenation process. As described above, since the cutoff wavelength of the grating is changed by the dehydrogenation treatment, it is difficult to finally produce a grating having a desired cutoff wavelength with high accuracy.
[0007]
The present invention has been made in view of the above, and provides an optical waveguide grating manufacturing method including a step of removing the photosensitizer after writing the grating, and a method capable of accurately manufacturing an optical waveguide grating having desired characteristics. The issue is to provide.
[0008]
[Means for Solving the Problems]
The optical waveguide grating manufacturing method according to the present invention includes a first step of preparing a photosensitive optical waveguide that causes a change in refractive index upon irradiation with predetermined light, and adding a photosensitizer to a predetermined portion of the optical waveguide; A second step of irradiating the predetermined portion included in the predetermined portion with the light to write an optical waveguide type grating, and a third step of removing the photosensitizer from the predetermined portion of the optical waveguide, After the first step, prior to the second step, the method further includes a fourth step of predicting the amount of change in the cutoff wavelength of the optical waveguide grating in the third step, and the cutoff wavelength of the grating written in the second step is It is characterized by controlling according to the predicted value of the wavelength change amount.
[0009]
  Here, in the fourth step, the concentration of the photosensitizing material or a substitute parameter corresponding to this concentration is measured, and (1) this measured value, and (2) the change in the concentration or substitute parameter and the cutoff wavelength. This is a process of predicting the amount of change in the cutoff wavelength using the relationship previously obtained with the amount.Ru.
[0010]
In the optical waveguide grating manufacturing method according to the present invention, the amount of change in the cutoff wavelength due to the removal of the photosensitizer (third step) is predicted prior to the writing of the grating (second step). The cutoff wavelength can be appropriately set in consideration of the amount of change in cutoff wavelength due to the removal of the photosensitizing material, whereby a grating having a target cutoff wavelength can be produced with high accuracy.
[0013]
Next, in the optical waveguide grating manufacturing method of the present invention, after the second step, prior to the third step, the fifth step of changing the cutoff wavelength of the grating written in the second step by a predetermined fine adjustment amount. May be further provided.
[0014]
Thus, by finely adjusting the cutoff wavelength of the grating after writing the grating, a grating having a target cutoff wavelength can be manufactured with higher accuracy.
[0015]
  In the fifth step, (A1) the reduction amount of the photosensitive improvement material in the second step or a substitute parameter corresponding to this reduction amount, and (A2) the reduction amount of the photosensitive improvement material in the fifth step or this reduction amount. The fine adjustment amount is determined using at least one of the substitution parameters and (B) the concentration of the photosensitizing material or the relationship obtained in advance between the substitution parameter corresponding to this concentration and the amount of change in the cutoff wavelength.Do.
[0016]
Thus, in the fifth step, in addition to (B) the concentration of the photosensitizing material, (A1) the amount of decrease in the photosensitizing material due to the writing of the grating (second step), and (A2) the fine adjustment ( Since the fine adjustment amount is determined in consideration of at least one of the reduction amount of the photosensitive improving material in the fifth step), a very appropriate fine adjustment can be performed, and the grating having the target cutoff wavelength. Can be manufactured with extremely high accuracy.
[0017]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. In the description of the drawings, the same elements are denoted by the same reference numerals, and redundant description is omitted. Further, the dimensional ratios in the drawings do not necessarily match those described.
[0018]
(Embodiment 1)
The present embodiment aims to produce a Bragg grating, and an optical fiber is used as an optical waveguide for writing the Bragg grating. This optical fiber is made of quartz (SiO 2₂) A common single mode optical fiber mainly composed of glass, and germanium (Ge), which is a refractive index increasing material, is added only to the core. As is well known, germanium also has a function as a photosensitive material that promotes a light-induced refractive index change with respect to ultraviolet light having a wavelength of about 240 to 250 nm or about 185 to 195 nm. That is, quartz glass to which germanium is added has a property that when the ultraviolet light having the wavelength as described above is incident, the refractive index of the incident portion is increased by a magnitude corresponding to the amount of incident light. In the case of the optical fiber used in the present embodiment, since germanium is added only to the core, it is possible to write a grating in the core.
[0019]
FIG. 1 is a flowchart for explaining the grating manufacturing method of the present embodiment. In this embodiment, first, hydrogen that is a photosensitizer is added to the optical fiber in order to increase the photosensitivity of the light-induced refractive index change (step 101). Specifically, the optical fiber is exposed to a hydrogen atmosphere for a certain period of time. Thereby, hydrogen molecules can be diffused into the optical fiber through the cladding surface, and hydrogen molecules can be added to the core. As is well known, when hydrogen molecules are further added to quartz glass to which germanium is added, the photosensitivity of the light-induced refractive index change of the quartz glass is improved.
[0020]
When the hydrogenation process is completed, the concentration of hydrogen molecules in the core or a substitute parameter corresponding to the hydrogen concentration is measured (step 102). In this embodiment, light loss at a wavelength of 1240 nm in the absorption wavelength band caused by hydrogen molecules in quartz glass is measured as a substitute parameter for hydrogen concentration. Since there is a certain correspondence between the hydrogen concentration and the 1240 nm light loss, such that the higher the hydrogen molecule concentration in the core is, the larger the light loss at 1240 nm wavelength is, the 1240 nm light loss is a substitute parameter for the hydrogen concentration. Can be used. This 1240 nm light loss is caused by injecting inspection light in the 1240 nm wavelength band from one end of an optical fiber subjected to hydrogenation treatment, connecting a spectrum analyzer to the other end, passing through the hydrogenation portion, and exiting from the other end. It can be obtained by measuring the power spectrum of the inspection light.
[0021]
Next, using the measured hydrogen concentration (actually, the substitution parameter 1240 nm optical loss), the hydrogen concentration in the core at the start of writing of the grating (actually, the substitution parameter 1240 nm optical loss) A predicted value is derived (step 103), and a predicted value of the Bragg wavelength change amount due to the dehydrogenation process is derived based on the predicted value of the hydrogen concentration (step 104). These derivation methods will be described later in detail.
[0022]
Subsequently, a portion of the coating of the optical fiber is removed, the portion is irradiated with ultraviolet light, and a Bragg grating having a predetermined Bragg wavelength is written into the core (step 105). Although a general method can be arbitrarily used as a method for writing the grating, in this embodiment, the grating is written using an apparatus as shown in FIG.
[0023]
In FIG. 2, reference numeral 10 denotes an optical fiber for grating writing. As shown in the figure, a part of the optical fiber 10 has a coating removed to expose the clad, and the phase grating plate 12 is disposed so as to be spaced apart from the exposed clad surface at a constant interval. As is well known, the phase grating plate 12 irradiates a light beam from a flat surface opposite to the grating surface perpendicularly to the flat surface, thereby grating an interference fringe beam having a period approximately half of the phase grating period. The light can be emitted from the surface. The phase grating plate 12 is arranged so that its grating surface faces the cladding surface of the optical fiber 10.
[0024]
As illustrated, a tension applying device 20 is attached to the optical fiber 10. The tension applying device 20 includes arms 21 and 22 that respectively hold the optical fiber 10 at two positions sandwiching the coating removal portion of the optical fiber 10, and these arms 21 and 22 are attached to both ends of the piezo element 23, respectively. It has been. When a voltage is applied to the piezo element 23, the piezo element 23 expands and contracts along the axial direction of the optical fiber 10 according to the polarity (positive / negative) and magnitude of the applied voltage. A control unit 24 having a built-in variable voltage source is electrically connected to the piezo element 23. The control unit 24 can control expansion and contraction of the piezo element by adjusting a voltage applied to the piezo element 23. It has become.
[0025]
In the present embodiment, the Bragg wavelength of the grating written in the optical fiber 10 can be controlled by using the tension applying device 20. Specifically, in a state where tension is applied to the grating writing portion of the optical fiber using the tension applying device 20, the flat surface of the phase grating plate 12 is irradiated with the ultraviolet light 14 in the 248 nm band for a certain period of time. The ultraviolet light 14 is a plane wave that travels perpendicular to the flat surface of the phase grating plate 12, and the light intensity distribution is substantially uniform on the wavefront. Irradiation of the ultraviolet light 14 generates ultraviolet interference fringes in a certain distance range from the grating surface of the phase grating plate 12 and irradiates a portion 16 of the coating removal portion of the optical fiber 10. As a result, periodic light-induced refractive index modulation along the axial direction of the optical fiber 10 occurs in the core included in the portion indicated by reference numeral 16, and a grating having a period approximately half the grating period of the phase grating plate 12 is formed. The After writing the grating in this way, the control unit 24 of the tension applying device 20 is operated to release the tension applied to the optical fiber 10. As a result, the optical fiber 10 contracts along the axial direction, so that the period of the grating formed in the optical fiber 10 is shortened, and the Bragg wavelength of the grating is shortened accordingly. Since the change in Bragg wavelength increases as the tension applied during writing of the grating increases, the amount of change in Bragg wavelength can be controlled by adjusting the tension applied during writing of the grating. In addition, according to the place which this inventor implemented, the Bragg wavelength was able to be shortened about 1.5 nm by providing the tension | tensile_strength of 100 g. As described above, combining the phase grating plate 12 and the tension applying device 20, preparing a plurality of phase grating plates 12 having different grating periods, and adjusting the magnitude of the tension to be applied, the gratings of various Bragg wavelengths can be obtained. It is possible to write arbitrarily.
[0026]
In the present embodiment, the Bragg wavelength of the grating written to the optical fiber is set in anticipation of the predicted value of the Bragg wavelength change amount due to the dehydrogenation process obtained in Step 104. Specifically, the Bragg wavelength of the grating to be written is a wavelength obtained by dividing the Bragg wavelength desired to be finally obtained (hereinafter referred to as the “target value” of the Bragg wavelength) by the predicted value of the Bragg wavelength change amount. Control the writing process. That is,
λ_B(Write) = λ_B(Target)-Δλ (Dehydrogenation / Prediction) (1)
It is. Where λ_B(Write) is the Bragg wavelength of the grating to be written, λ_B(Target) is the target value of the Bragg wavelength, and Δλ (dehydrogenation / prediction) is the above prediction of the Bragg wavelength change by dehydrogenation (the value obtained by subtracting the Bragg wavelength before dehydrogenation from the Bragg wavelength after dehydrogenation) Value.
[0027]
Thereafter, the optical fiber on which the grating is formed is exposed to air at 80 ° C. to diffuse and remove hydrogen molecules in the core to the outside (step 106). This dehydrogenation process is continued until the Bragg wavelength of the grating hardly changes. The reason why the optical fiber is placed in an environment at a temperature higher than room temperature of 80 ° C. is to quickly diffuse hydrogen molecules. With the above dehydrogenation treatment, the fabrication of the optical waveguide grating according to the present embodiment is completed.
[0028]
Hereinafter, the prediction of the hydrogen concentration in step 103 and the prediction of the Bragg wavelength change amount in step 104 will be described. For the prediction of the hydrogen concentration in step 103, experimental data collected in advance are used. That is, an optical fiber similar to the optical fiber for writing the grating is prepared for the experiment, and after adding a predetermined concentration of hydrogen to the core of this optical fiber, it is left in the air and left in the air at a constant hydrogen concentration ( In practice, the substitution parameter (1240 nm light loss) is measured to measure the change in the hydrogen concentration over time. FIG. 3 is a graph showing the change over time of the 1240 nm light loss determined in this way. Here, the horizontal axis represents the elapsed time from the completion of hydrogenation, and the vertical axis represents 1240 nm light loss. In the present embodiment, by using the 1240 nm optical loss measured in step 102 and the graph of FIG. 3, the reduction amount of 1240 nm optical loss over time from the measurement time to the writing start time is obtained, and this is used to start writing. The light loss at 1240 nm at the time is obtained, and this is used as the predicted value.
[0029]
As shown in FIG. 3, the hydrogen concentration rapidly decreases immediately after the addition of hydrogen is completed, but after a certain amount of time has passed, the change in the hydrogen concentration becomes gradual, and the rate of decrease in the hydrogen concentration continues to increase over time. Gradually slows as time passes. If the grating is written when the change in the hydrogen concentration is too rapid, an error occurs in the cutoff wavelength of the written grating. Therefore, the time at which the change in the hydrogen concentration becomes moderate to some extent may be obtained from the 1240 nm light loss measured in step 102 and the graph of FIG. 3, and the writing of the grating may be started at that time. In this case, the light loss at 1240 nm at the writing start time determined in this way is obtained and used as the predicted value.
[0030]
Next, the prediction of the Bragg wavelength change amount in step 104 will be described. For this prediction, experimental data collected in advance is used. That is, an optical fiber similar to the optical fiber for writing the grating is prepared for the experiment, and after adding a predetermined concentration of hydrogen to the optical fiber core, a Bragg grating with a predetermined period is written to the optical fiber core. Measure the Bragg wavelength of the grating. Thereafter, the optical fiber is dehydrogenated until the Bragg wavelength does not change, and then the Bragg wavelength is measured again. By comparing the measured values of the Bragg wavelength before and after the dehydrogenation treatment, the shift amount of the Bragg wavelength by the dehydrogenation treatment can be obtained. By repeating the above measurement for various hydrogenation concentrations, data representing the relationship between the hydrogen concentration and the Bragg wavelength change amount is collected.
[0031]
FIG. 4 is a diagram showing the relationship between the hydrogen concentration (actually, the 1240 nm light loss that is a substitute parameter) and the Bragg wavelength change amount due to the dehydrogenation process. Here, the horizontal axis represents 1240 nm light loss, and the vertical axis represents the Bragg wavelength change amount due to the dehydrogenation treatment. Each point in FIG. 4 is data obtained by experiments. In the present embodiment, a regression line indicating the relationship between the 1240 nm light loss and the Bragg wavelength variation is obtained by using the least square method based on such experimental data. When the optical loss at 1240 nm at the start of grating writing is expressed as [1240 nm], the Bragg wavelength change Δλ (dehydrogenation) due to the dehydrogenation process is expressed by₁, B₁Using,
Δλ (dehydrogenation) = a₁+ B₁× [1240 nm] (2)
It is expressed as Coefficient a₁, B₁Can be obtained by performing a regression calculation using the data of FIG. The straight line in FIG. 4 is a regression line obtained in this way. In this embodiment, based on this regression line, the Bragg wavelength change amount corresponding to the predicted value of 1240 nm optical loss obtained in Step 103 is obtained, and this value is used as the predicted value of the Bragg wavelength change amount by the dehydrogenation process.
[0032]
In this embodiment, the Bragg wavelength of the grating to be written is set as shown in the above equation (1) using the predicted value of the Bragg wavelength change amount by the dehydrogenation process thus obtained. The change can be canceled and an optical waveguide grating having a target Bragg wavelength can be manufactured with high accuracy.
[0033]
(Embodiment 2)
When light is incident on the core of the optical fiber to write the grating, a part of hydrogen molecules in the core is ionized as a result of the reaction that occurs in the core, and the hydrogen concentration is reduced. Since this affects the amount of hydrogen molecules in the core at the start of the dehydrogenation process, it also affects the amount of Bragg wavelength change caused by the dehydrogenation process. That is, the amount of decrease in hydrogen molecules due to writing of the grating has a correlation with the amount of change in Bragg wavelength due to dehydrogenation. In the present embodiment, taking this into account, the grating is manufactured with higher accuracy.
[0034]
FIG. 5 is a flowchart for explaining the grating manufacturing method of the present embodiment. As will be described in detail below, in this embodiment, after writing the grating, fine adjustment is performed by changing the Bragg wavelength of this grating by a predetermined amount (step 209).
[0035]
Steps 201 to 205 are the same as those in the first embodiment, and a description thereof will be omitted. In the present embodiment, after writing the grating in the same manner as in the first embodiment (steps 201 to 205), the Bragg wavelength of this grating is measured (step 206). Specifically, inspection light is incident from one end of the optical fiber on which the grating is written, and an optical spectrum analyzer is connected to the other end, and the spectrum of the inspection light transmitted through the grating is measured. Since light in the vicinity of the Bragg wavelength in the inspection light is reflected by the grating, a trough representing light blocking by the grating appears in the measured optical spectrum. The central wavelength of this valley represents the Bragg wavelength of the grating.
[0036]
Subsequently, in the present embodiment, the reduction amount of hydrogen molecules due to the writing of the grating, or a substitute parameter corresponding to this reduction amount is measured (step 207). In the present embodiment, the amount of light having a Bragg wavelength that is blocked by the written grating is measured as the substitute parameter. As is well known, the amount of Bragg wavelength light blocked by the grating increases as the light-induced refractive index change due to writing of the grating increases, and the magnitude of this refractive index change is a decrease in the amount of hydrogen molecules that are photosensitizers. Dependent. That is, as more hydrogen molecules in the core are ionized by the incidence of light, the photosensitivity increases and the amount of change in the refractive index increases, so the amount of Bragg wavelength light blocked by the grating written in the core also increases. Thus, since there is a correspondence between the amount of Bragg wavelength light blocked by the grating and the amount of hydrogen molecule decrease due to writing in the grating, the amount of Bragg wavelength light blocked by the grating is Can be used as a substitute parameter for the amount of decrease. The amount of Bragg wavelength light blocked by the grating can be obtained from the optical spectrum measured in step 206.
[0037]
Next, prior to the fine adjustment in step 209, the amount by which the Bragg wavelength is changed by this fine adjustment is determined (step 208). The amount of black wavelength change due to dehydrogenation depends on the hydrogen concentration in the core at the start of dehydrogenation, and the hydrogen concentration at the start of dehydrogenation depends on the amount of hydrogen molecules reduced by writing the grating. The amount of Bragg wavelength change due to the elemental treatment correlates not only with the hydrogen concentration at the start of grating writing, but also with the amount of decrease in hydrogen molecules due to grating writing. In this embodiment, the hydrogen concentration at the start of grating writing (actually, the substitution parameter is 1240 nm light loss) and the amount of decrease in hydrogen molecules due to the writing of the grating (actually, the substitution parameter is Bragg due to grating) The multiple regression analysis is previously performed using the wavelength light cutoff amount) as an explanatory variable, and the fine adjustment amount of the Bragg wavelength is determined using the obtained regression line. By using the amount of decrease in hydrogen molecules due to writing of the grating as an explanatory variable in addition to the hydrogen concentration at the start of writing of the grating, a more accurate regression line can be obtained.
[0038]
Hereinafter, the method for determining the fine adjustment amount of the Bragg wavelength in step 208 will be described in detail. When the light loss at 1240 nm at the start of the grating writing is expressed as [1240 nm] and the blocking amount of the Bragg wavelength light by the grating is expressed as [blocking amount], the Bragg wavelength change Δλ (dehydrogenation) due to the dehydrogenation processing is expressed by the coefficient a₂, B₂, C₂Using,
Δλ (dehydrogenation) = a₂+ B₂× [1240nm] + c₂× [blocking amount] (3)
It is expressed as This equation (3) represents a regression line indicating the relationship between [1240 nm] and [blocking amount] and Δλ (dehydrogenation). In equation (3), coefficient a₂, B₂, C₂Represents the relationship between the light loss of 1240 nm and the wavelength change amount by the dehydrogenation process (each point in FIG. 4), and the relationship between the blocking amount of the Bragg wavelength light by the grating and the wavelength change amount by the dehydrogenation process. Data can be obtained in advance by experiments, and multiple regression calculations can be performed using these data. FIG. 6 shows data representing the relationship between the amount of Bragg wavelength light blocked by the grating and the amount of wavelength change due to dehydrogenation. Here, the horizontal axis indicates the amount of Bragg wavelength light blocked by the grating, and the vertical axis indicates the amount of Bragg wavelength change due to dehydrogenation. Each point in FIG. 6 represents each data.
[0039]
If the fine adjustment amount of the Bragg wavelength is expressed as Δλ (fine adjustment), this Δλ (fine adjustment) is
λ_B+ Δλ (fine adjustment) + Δλ (dehydrogenation) = λ_B(Target) (4)
Where λ_B: Bragg wavelength of written grating
Δλ (dehydrogenation): Bragg wavelength change by dehydrogenation
λ_B(Target): Target value of Bragg wavelength
Need to be satisfied.
[0040]
From the above formulas (3) and (4), Δλ (fine adjustment) is
Δλ (fine adjustment) = λ_B(Target)-λ_B
-(A₂+ B₂× [1240nm] + c₂× [Light blocking amount]) (5)
It is expressed as In the present embodiment, the predicted value of [1240 nm] obtained in step 203, the Bragg wavelength λ measured in step 206,_BAnd [light blocking amount], and a coefficient a determined in advance₂, B₂, C₂Is used in the above equation to determine the fine adjustment amount Δλ (fine adjustment) of the Bragg wavelength.
[0041]
Next, fine adjustment of the Bragg wavelength of the grating is performed to change the Bragg wavelength by the fine adjustment amount (step 209). FIG. 7 is a diagram for explaining the fine adjustment method. As shown in FIG. 7, this fine adjustment is performed by using the same ultraviolet light beam 30 used for writing the grating as the region including the portion 16 of the optical fiber 10 (the portion irradiated with the interference fringe beam when writing the grating). By irradiating The ultraviolet light beam 30 is a plane wave that travels perpendicular to the axis of the optical fiber, and the light intensity distribution is substantially uniform at the wavefront. Due to the light-induced refractive index change caused by the irradiation with the ultraviolet light beam 30, the refractive index of the core in the region irradiated with the ultraviolet light uniformly increases by a certain amount. Since the grating is included in the irradiation region of the ultraviolet light beam 30, the average refractive index of the grating is also increased by the irradiation of the ultraviolet light beam 30 (the amplitude of the refractive index modulation hardly changes). As is well known, the Bragg wavelength depends on the average refractive index of the Bragg grating, and the Bragg wavelength increases as the average refractive index increases. Since the amount of increase in the refractive index of the grating increases as the incident amount of the ultraviolet light beam 30 on the core increases, the shift amount of the Bragg wavelength due to the irradiation of the ultraviolet light beam 30 increases depending on the irradiation amount of the ultraviolet light beam 30. Become. Therefore, the Bragg wavelength can be increased by a desired amount by adjusting the irradiation amount of the ultraviolet light.
[0042]
In addition, when fine adjustment is performed by the above method, if the irradiation with the ultraviolet light beam is not performed once, but performed in several times, the failure of making the Bragg wavelength longer than necessary can be avoided, and the Bragg wavelength can be avoided. The amount of change can be adjusted precisely.
[0043]
After changing the Bragg wavelength by the fine adjustment amount as described above, a dehydrogenation process is performed in the same manner as in step 106 of the first embodiment (step 210). Thus, the fabrication of the optical waveguide grating according to the present embodiment is completed.
[0044]
In this embodiment, fine adjustment is performed after the grating is written, and this fine adjustment amount is determined in consideration of the amount of hydrogen decrease due to the writing of the grating, so that a grating having a desired Bragg wavelength can be more accurately obtained. Can be produced.
[0045]
(Embodiment 3)
When the optical fiber is irradiated with the ultraviolet light beam by the fine adjustment performed in the second embodiment, the light-induced refractive index change occurs as described above. At this time, a part of the hydrogen molecules in the core is ionized, so the hydrogen molecular weight Decrease. The amount of decrease in hydrogen molecules due to this fine adjustment is also correlated with the amount of Bragg wavelength change due to dehydrogenation. In the present embodiment, taking this into account, the grating is manufactured with higher accuracy.
[0046]
The grating manufacturing method of this embodiment can be described with reference to FIG. Steps 201 to 207 are the same as those in the second embodiment, and a description thereof will be omitted.
[0047]
In the present embodiment, the method for determining the fine adjustment amount of the Bragg wavelength in step 208 is different from that in the second embodiment. Since the hydrogen concentration at the start of the dehydrogenation process depends on the amount of hydrogen molecule decrease due to fine adjustment, the decrease in hydrogen molecule due to fine adjustment affects the Bragg wavelength change amount due to dehydrogenation process. That is, the amount of change in Bragg wavelength due to dehydrogenation has a correlation with the amount of decrease in hydrogen molecules due to fine adjustment. Therefore, in the present embodiment, the hydrogen concentration at the start of grating writing (actually, the 1240 nm light loss that is a substitute parameter) and the reduction amount of hydrogen molecules due to the writing of the grating (actually, the grating that is the substitute parameter) In addition to the amount of blocking Bragg wavelength light), the amount of decrease in hydrogen molecules by fine adjustment is also performed as an explanatory variable in advance, and multiple regression analysis is performed in advance, and the fine adjustment amount of Bragg wavelength is determined using the obtained regression line . By adding the amount of hydrogen molecule reduction due to fine adjustment to the explanatory variable, a more accurate regression line can be obtained.
[0048]
Specifically, in this embodiment, the Bragg wavelength change amount by fine adjustment is used as a substitute parameter for the decrease amount of hydrogen molecules by fine adjustment. The amount of change in Bragg wavelength due to fine adjustment depends on the magnitude of the light-induced refractive index change during fine adjustment, and the magnitude of this refractive index change depends on the amount of decrease in hydrogen molecules due to fine adjustment. Thus, since there is a correspondence between the Bragg wavelength change amount by fine adjustment and the hydrogen molecule decrease amount by fine adjustment, the Bragg wavelength change amount by fine adjustment corresponds to the hydrogen molecule decrease amount by fine adjustment. It can be used as a substitute parameter.
[0049]
The light loss at the start of writing the grating is expressed as [1240 nm], the light blocking amount by the grating is expressed as [light blocking amount], and the Bragg wavelength change amount by fine adjustment is expressed as Δλ (fine adjustment). The Bragg wavelength change Δλ (dehydrogenation) is expressed by the coefficient a_Three, B_Three, C_Three, D_ThreeUsing,
Δλ (dehydrogenation) = a_Three+ B_Three× [1240nm]
+ C_Three× [Light blocking amount] + d_Three× Δλ (fine adjustment) (6)
It is expressed as This equation (6) represents a regression line indicating the relationship between [1240 nm], [light blocking amount] and Δλ (fine adjustment) and Δλ (dehydrogenation). In equation (6), coefficient a_Three, B_Three, C_Three, D_ThreeIs data representing the relationship between the light loss of 1240 nm and the wavelength change amount due to the dehydrogenation process (each point in FIG. 4), and the data representing the relationship between the blocking amount of Bragg wavelength light by the grating and the wavelength change amount due to the dehydrogenation process (Each point in FIG. 6), and data representing the relationship between the Bragg wavelength change amount by fine adjustment and the wavelength change amount by dehydrogenation are collected in advance by experiments, and multiple regression calculation is performed using these data. It can be determined by doing. FIG. 8 shows data representing the relationship between the Bragg wavelength change amount by fine adjustment and the wavelength change amount by dehydrogenation. Here, the horizontal axis indicates the amount of Bragg wavelength change by fine adjustment, and the vertical axis indicates the amount of Bragg wavelength change by dehydrogenation. Each point in FIG. 8 represents each data.
[0050]
As already mentioned, Δλ (fine adjustment) is
λ_B+ Δλ (fine adjustment) + Δλ (dehydrogenation) = λ_B(Target) (4)
Where λ_B: Bragg wavelength of written grating
Δλ (dehydrogenation): Bragg wavelength change by dehydrogenation
λ_B(Target): Target value of Bragg wavelength
Need to be satisfied.
[0051]
Δλ (fine adjustment) that satisfies the above equations (6) and (4) can be obtained by solving the above two equations as simultaneous equations. As a result, Δλ (fine adjustment) is
Δλ (fine adjustment) = {λ_B(Target)-λ_B-(A_Three+ B_Three× [1240nm]
+ C_Three× [light blocking amount))} / (1 + d_Three(7)
It is obtained like this. [1240 nm] predicted value obtained in step 203, Bragg wavelength λ measured in step 206_BAnd [light blocking amount], and a coefficient a determined in advance_Three, B_Three, C_Three, D_ThreeIs used in the above equation to obtain Δλ (fine adjustment).
[0052]
In step 209, fine adjustment is performed by irradiating the optical fiber with an ultraviolet light beam so as to change the Bragg wavelength by Δλ (fine adjustment) determined in step 208. Thereafter, a dehydrogenation process is performed in the same manner as in Step 106 of Embodiment 1 (Step 210). Thereby, the fabrication of the optical waveguide grating according to the present embodiment is completed.
[0053]
In the present embodiment, the fine adjustment amount of the Bragg wavelength is determined in consideration of the hydrogen decrease amount by fine adjustment in addition to the hydrogen decrease amount by writing the grating, so that a grating having a desired Bragg wavelength can be more accurately obtained. Can be produced.
[0054]
The present invention is not limited to the above embodiment, and various modifications are possible. For example, although the Bragg grating is manufactured in the above embodiment, a long period grating can also be manufactured by the method of the above embodiment. In the above embodiment, an optical fiber is used as an optical waveguide for writing a grating. However, other types of optical waveguides (such as thin film waveguides) can also be used.
[0055]
In the above, (a) 1240 nm light loss and (b) a method of performing multiple regression analysis using the blocking amount of Bragg wavelength light as explanatory variables (Embodiment 2), (a) 1240 nm light loss, (b) Bragg wavelength The method of performing multiple regression analysis using the light blocking amount and (c) the Bragg wavelength change amount by fine adjustment as an explanatory variable has been described. (A) 1240 nm light loss and (c) Bragg by fine adjustment Multiple regression analysis may be performed using the wavelength change amount as an explanatory variable. In this case, the Bragg wavelength change Δλ (dehydrogenation) due to the dehydrogenation treatment is
Δλ (dehydrogenation) = a_Four+ B_Four× [1240nm] + d_Four× Δλ (fine adjustment) (8)
It is expressed as Coefficient a_Four, B_Four, D_FourIs data (representing each point in FIG. 4) between the light loss of 1240 nm and the wavelength change amount due to dehydrogenation, and data representing the relationship between the Bragg wavelength change amount due to fine adjustment and the wavelength change amount due to dehydrogenation treatment ( Each point in FIG. 8 is collected in advance by experiments and can be obtained by performing multiple regression calculations using these data.
[0056]
As in the above embodiment, Δλ (fine adjustment) is
λ_B+ Δλ (fine adjustment) + Δλ (dehydrogenation) = λ_B(Target) (4)
Need to be satisfied. By solving the above equations (8) and (4) as simultaneous equations, Δλ (fine adjustment) is
Δλ (fine adjustment) = {λ_B(Target)-λ_B
-(A_Four+ B_Four× [1240 nm])} / (1 + d_Four) ... (9)
It can be expressed as The value of [1240 nm] predicted before writing the grating and the Bragg wavelength λ measured after writing the grating_BAnd the coefficient a previously obtained_Four, B_Four, D_FourCan be used in the above equation to determine the fine adjustment amount Δλ (fine adjustment).
[0057]
Next, in the above-described embodiment, hydrogen is added to the optical fiber as the photosensitivity improving material. However, a photosensitizing material other than hydrogen may be added. Deuterium is well known as a photosensitizing material other than hydrogen. A plurality of these photosensitizers may be added. In this case, instead of the hydrogen concentration in the above-described embodiment, the total concentration (or the substitute parameter) of the photosensitizer may be measured and predicted.
[0058]
In the above embodiment, the loss of light having a wavelength of 1240 nm is measured as a substitute parameter corresponding to the hydrogen concentration, but the 620 nm band is also present as an absorption wavelength band caused by hydrogen molecules in quartz glass. It is also possible to use the loss of light having a wavelength included in this absorption wavelength band as a substitute parameter for the hydrogen concentration. It is also possible to perform the above multiple regression analysis using the loss of two lights included in these two absorption wavelength bands as substitute parameters.
[0059]
Similarly, when deuterium is used as the photosensitizer, there are the 1860 nm band and the 930 nm band as absorption wavelength bands due to deuterium molecules in the quartz glass. Therefore, the loss of light contained in these absorption wavelength bands Can be used as a surrogate parameter for deuterium concentration. It is also possible to perform the above multiple regression analysis using the loss of two lights included in these two absorption wavelength bands as substitute parameters.
[0060]
In the above embodiment, the Bragg wavelength light blocking amount by the grating is used as a substitute parameter for the hydrogen decrease amount by writing the grating, and the Bragg wavelength change amount by the fine adjustment is used as a substitute parameter for the hydrogen decrease amount by the fine adjustment. However, light loss at a wavelength near 1380 nm can be used as these substitute parameters. In the vicinity of 1380 nm, there is an absorption wavelength band due to OH groups in the quartz glass. Because of the incidence of light at the time of writing the grating, some of the hydrogen molecules in the quartz glass are ionized to generate OH groups, so the amount of hydrogen reduction due to the writing of the grating depends on the concentration of OH groups in the core, Since the concentration depends on the loss of light included in the absorption wavelength band near 1380 nm, this light loss can be used as a substitute parameter for the hydrogen reduction amount by writing the grating and the hydrogen reduction amount by fine adjustment.
[0061]
On the other hand, when deuterium is used as the photosensitizer, the absorption wavelength band due to the OH group generated by deuterium ionization is the 2070 nm band, so the loss of light contained in this wavelength band is written into the grating. It can be used as a substitute parameter for the deuterium reduction amount due to or by the fine adjustment.
[0062]
【The invention's effect】
As described above in detail, in the optical waveguide grating manufacturing method according to the present invention, since the amount of change in the cutoff wavelength due to the removal of the photosensitizer is predicted prior to the writing of the grating, the target cutoff wavelength It is possible to manufacture a grating having a high accuracy.
[0063]
Further, after the writing of the grating and before the removal of the photosensitizer, the grating having the target cutoff wavelength can be produced with higher accuracy by finely adjusting the cutoff wavelength of the written grating.
[Brief description of the drawings]
FIG. 1 is a flowchart for explaining a grating manufacturing method according to a first embodiment.
FIG. 2 is a schematic diagram showing a grating writing method.
FIG. 3 is a graph showing the change over time of light loss at 1240 nm after hydrogen addition to an optical fiber.
FIG. 4 is a diagram illustrating a relationship between a light loss at 1240 nm and a Bragg wavelength change amount by a dehydrogenation process.
FIG. 5 is a flowchart for explaining a grating manufacturing method according to a second embodiment.
FIG. 6 is a diagram illustrating the relationship between the amount of Bragg wavelength light blocked by a grating and the amount of wavelength change due to dehydrogenation.
FIG. 7 is a schematic diagram showing a fine adjustment method of a Bragg wavelength of a grating.
FIG. 8 is a diagram illustrating a relationship between a Bragg wavelength change amount by fine adjustment and a wavelength change amount by dehydrogenation.
FIG. 9 is a diagram showing a change with time of the Bragg wavelength by the dehydrogenation treatment.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 10 ... Optical fiber, 12 ... Phase grating plate, 14 and 30 ... Ultraviolet light beam, 16 ... Part where grating was written, 20 ... Tension applying device, 21 and 22 ... Arm, 23 ... Piezo element, 24 ... Control unit.

Claims (2)

A first step of preparing a photosensitive optical waveguide that causes a change in refractive index upon incidence of predetermined light, and adding a photosensitizer to a predetermined portion of the optical waveguide; and the light at a predetermined location included in the predetermined portion. A method for producing an optical waveguide grating comprising: a second step of writing an optical waveguide grating by making it incident; and a third step of removing the photosensitizer from the predetermined portion of the optical waveguide,
After the first step, prior to the second step, measure the concentration of the photosensitizer or a substitute parameter corresponding to this concentration, and change the measured value, the concentration or substitute parameter, and the cutoff wavelength. A fourth step of predicting the amount of change in the cut-off wavelength of the optical waveguide grating according to the third step using a previously obtained relationship with the amount;
An optical waveguide grating manufacturing method, wherein a cutoff wavelength of a grating written in the second step is controlled in accordance with a predicted value of the cutoff wavelength change amount. After the second step, prior to the third step, further includes a fifth step of changing the cutoff wavelength of the grating written in the second step by a predetermined fine adjustment amount,
In the fifth step, the reduction amount of the photosensitive improving material in the second step or a substitution parameter corresponding to the reduction amount, and the reduction amount of the photosensitive improvement material in the fifth step or a substitution parameter corresponding to the reduction amount are set. 2. The optical waveguide according to claim 1, wherein the fine adjustment amount is determined using at least one of the concentration of the photosensitizing material or a substitute parameter corresponding to the concentration and a previously determined relationship between the change amount of the cutoff wavelength. Mold grating manufacturing method.

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