Patent Description:
This application is based upon and claims the benefit of priority from <CIT>.

When silica-based glass to which a photosensitive material is doped is irradiated with ultraviolet light, the refractive index of the irradiated region increases. Utilizing this phenomenon, an optical fiber grating (TFG: termination fiber grating) is manufactured. Specifically, an optical fiber comprised of silica-based glass is provided with a refractive index modulated region in which the refractive index periodically varies along the longitudinal direction of the optical fiber. An optical fiber grating is used as, for example, a filter for monitoring a passive optical network (PON).

As an example of enabling higher capacity transmission in a PON system, a PON monitoring filter selectively reflects light in a <NUM>-nm wavelength band ± <NUM> for monitoring. On the other hand, the PON monitoring filter transmits not only signal light in a band (for example, C-band which is from <NUM> to <NUM>) different from the above wavelength band but also signal light in another band (for example, L-band which is from <NUM> to <NUM>), thereby enabling high capacity transmission in a wider wavelength band.

Note that the method for manufacturing an optical fiber grating is disclosed in Patent Documents <NUM> to <NUM>, for example. <CIT> discloses a structure in which a core contains a photosensitive material and further including an inner cladding and an outer cladding.

An optical fiber according to the present disclosure is defined by claim <NUM>.

In the method for manufacturing an optical fiber grating disclosed in Patent Documents <NUM> and <NUM>, an optical fiber in which both or one of a core and a cladding is comprised of silica-based glass containing a photosensitive material is prepared. This optical fiber is irradiated with ultraviolet light of a specific wavelength (for example, a second harmonic (wavelength of <NUM>) of argon ion laser light) that can increase the refractive index. This makes it possible to increase the refractive index of the silica-based glass containing the photosensitive material.

As a method for forming a refractive index modulated region having a predetermined period in the optical fiber along the longitudinal direction, there are exposure with plus/minus first-order diffracted light using a chirped grating phase mask, direct exposure with laser light, and two-beam interference exposure. Among them, the method using the phase mask is advantageous in that an optical fiber grating having the same characteristics can be manufactured with excellent reproducibility, and that alignment is relatively easy compared to other methods.

GeO<NUM> is a typical photosensitive material. GeO<NUM> is doped to both the core and the cladding, and F is doped to the cladding, whereby a difference in refractive index can be generated between the core and the cladding. However, when only GeO<NUM> is used as the photosensitive material, it is not possible to increase an amount of variation in refractive index caused by ultraviolet light irradiation. This leads to an increase in length of the optical fiber grating required to obtain a predetermined reflection characteristic, and thus entails a problem of an increase in cost for ultraviolet light irradiation.

As a method for addressing this problem, it is known to use B<NUM>O<NUM> in addition to GeO<NUM> as a photosensitive material (see Non-Patent Documents <NUM> and <NUM>). The co-doping of GeO<NUM> and B<NUM>O<NUM> can increase the amount of variation in the refractive index caused by ultraviolet light irradiation, as compared with the addition of GeO<NUM> alone. Therefore, co-doping of GeO<NUM> and B<NUM>O<NUM> enables a decrease in length of the optical fiber grating and reduction in cost for the ultraviolet light irradiation. Therefore, co-doping GeO<NUM> and B<NUM>O<NUM> as the photosensitive material is preferable.

The refractive index profile in the radial direction of an optical fiber used for manufacturing an optical fiber grating is typically a step-index profile. When the photosensitive material is doped only to the core, a refractive index modulated region in which the refractive index periodically varies along the longitudinal direction of the fiber is formed only in the core. However, an optical fiber grating having this fiber structure has a gradual increase in transmission loss on the short wavelength side of the transmission loss band (see <FIG> and <FIG>), although it can provide predetermined reflection characteristics in a wavelength band for monitoring.

<FIG> is a diagram showing an example of a gradual increase in transmission loss of an optical fiber grating. <FIG> is an enlarged view of a part of <FIG>. In <FIG>, the lower limit of transmission loss required in the L-band and the upper limit of transmission loss required in a light transmission blocking band are indicated by dotted lines. In <FIG>, the lower limit of transmission loss required in the L-band is indicated by a dotted line. In the example shown in <FIG> and <FIG>, the light transmission blocking band is from <NUM> to <NUM> inclusive, and the transmission loss required in this light transmission blocking band is -<NUM> dB or more. In this example, the loss of the optical fiber grating is so large that it cannot be ignored near the long-wavelength end (<NUM>) of the L-band.

The reason why such a gradual increase in transmission loss occurs is that, due to the formation of a refractive index modulated region in a local region of the optical fiber by ultraviolet light irradiation, the orthogonality between an LP<NUM> mode (fundamental mode) and a higher-order mode with LP<NUM> (m = <NUM>, <NUM>,. ) which is symmetrical to the LP<NUM> mode with respect to an axis is lost (as a result, a coupling loss from the LP<NUM> mode to the higher-order mode occurs).

In order to maintain the orthogonality between the LP<NUM> mode and the higher-order mode, it is necessary to form the refractive index modulated region in the entire region where light is sensed in the cross section of the fiber. As a structure that satisfies a condition for maintaining orthogonality for a preferable combination of photosensitive materials by co-doping of GeO<NUM> and B<NUM>O<NUM>, it is considered that, for example, GeO<NUM> and B<NUM>O<NUM> which are photosensitive materials are entirely co-doped in both the core and the optical cladding, and F is doped to the optical cladding. With this structure, a sufficient refractive index difference is generated between the core and the optical cladding. However, the compound of B<NUM>O<NUM> and F is one of the difficult-to-treat substances, and this method is not preferable.

On the other hand, in the invention disclosed in Patent Document <NUM> described above, an optical fiber grating is manufactured using not an optical fiber having a step-index profile but an optical fiber that includes a core having a single-peaked and graded refractive index profile. According to the disclosure of Patent Document <NUM>, due to the optical fiber to be applied having such a single-peaked and graded refractive index profile, a variation in relative refractive index difference and a variation in propagation mode in the longitudinal direction at the interface between the core and the cladding of the optical fiber can be reduced, whereby a cladding mode coupling loss can be suppressed. Further, according to the disclosure of Patent Document <NUM>, when the light transmission blocking wavelength band by the optical fiber grating is from about <NUM> to about <NUM>, it is possible to suppress the light transmission loss occurring in the <NUM>-nm wavelength band, so that the light transmission loss in the used wavelength band (about <NUM>-nm band) of the optical fiber grating can be decreased.

As a result of examining conventional optical fibers and optical fiber gratings, the inventors have found the following problems. That is, the invention disclosed in the above Patent Document <NUM> has a problem that, when the light transmission blocking wavelength band by the optical fiber grating is from about <NUM> to about <NUM>, it is possible to suppress the light transmission loss occurring in the <NUM>-nm wavelength band, whereas the transmission loss at the long-wavelength end (<NUM>) in the L-band increases to a level that cannot be ignored (at least about <NUM> dB or more). This is considered to be because the spread of the light intensity distribution in the LP<NUM> mode is larger than that in the grating region. That is, it is considered that in such a situation, the orthogonality between the LP<NUM> mode and the higher-order mode (LP<NUM> mode) is reduced, and as a result, the coupling from the LP<NUM> mode to the LP<NUM> mode occurs. Therefore, the optical fiber grating according to the invention disclosed in Patent Document <NUM> is not suitable for a PON monitoring filter of a PON system that enables high capacity transmission in a wide wavelength band using L-band signal light.

The present disclosure has been made to solve the above problems, and an object of the present disclosure is to provide an optical fiber grating in which a gradual increase in transmission loss is reduced, and an optical fiber suitable for manufacturing the optical fiber grating.

According to the present disclosure, an optical fiber grating in which a gradual increase in transmission loss is reduced, and an optical fiber suitable for manufacturing the optical fiber grating can be provided.

First, the details of an optical fiber according to the present disclosure will be individually listed and described.

In the present specification, "silica-based glass" indicates glass containing <NUM>% by mass or more of SiO<NUM>. Further, the relative refractive index nri of each region having a refractive index m with respect to pure silica glass (refractive index nsilica) is specified by the formula of <MAT> and the relative refractive index difference Δ between the region having the refractive index n<NUM> and the region having the refractive index n<NUM> is specified by the formula of <MAT>.

Further, in order to avoid the effect of unintended or small variations in the refractive index profile, it is determined that the "single-peaked profile" is formed, if there is only one peak in a refractive index obtained by moving average using an average of refractive indices n(r) in the interval of <NUM>, not in the measured value of the refractive index n(r).

(<NUM>) The photosensitive region contains both Ge and B as a photosensitive material.

(<NUM>) A difference between a maximum concentration and a minimum concentration of B in the photosensitive region is <NUM>% or less in terms of a variation in relative refractive index induced by the B-doping, and a concentration of Ge in an outermost region of the photosensitive region (outermost region of the inner cladding) is <NUM>% or more in terms of a variation in relative refractive index induced by the Ge-doping.

Second, details of embodiments of the present disclosure will be listed and described. (<NUM>) As one aspect of the present disclosure, it is preferable that the refractive index of the inner cladding is substantially equal to a refractive index of pure silica glass because of an offset between an amount of increase in refractive index induced by the Ge-doping and an amount of decrease in refractive index induced by the B-doping. In the present specification, "being substantially equal" means a state in which the relative refractive index between two regions to be compared is <NUM>% or less.

(<NUM>) As one aspect of the present disclosure, it is preferable that, in the inner cladding, an amount of increase in refractive index induced by the Ge-doping is greater than an amount of decrease in refractive index induced by the B-doping, and the outer cladding contains chlorine. In this configuration, it is also preferable that the refractive index of the inner cladding and the refractive index of the outer cladding are substantially equal to each other.

(<NUM>) As one aspect of the present disclosure, it is preferable that the single-peaked and graded refractive index profile is an α-profile having an exponent α greater than <NUM> and smaller than <NUM>. Further, as one aspect of the present disclosure, it is preferable that the relative refractive index difference between the core and the inner cladding is <NUM>% or more and <NUM>% or less. In the α-profile, when the maximum refractive index of the core is n<NUM>, the minimum refractive index of the core is n<NUM>, and the radius of the core is a, the refractive index n(r) at the position with the distance r (< a) from the center of the core along the radial direction is specified by the equation of <MAT>.

By adjusting the exponent α in the above equation, the shape of the refractive index profile can be set arbitrarily.

In addition, in the optical fiber according to the present disclosure having the above-described structure, the appropriate cutoff wavelength range is from <NUM> to <NUM> inclusive. In addition, the bending loss in the <NUM>-µm wavelength band, in the condition that the optical fiber is wound <NUM> times around a mandrel with a diameter of <NUM>, is preferably <NUM> dB or less.

(<NUM>) An optical fiber grating according to the present disclosure includes, as one aspect, the optical fiber having the above-mentioned structure, and has a refractive index modulated region provided along the longitudinal direction of the optical fiber. The refractive index modulated region is a region where the refractive index periodically varies along the longitudinal direction of the optical fiber, and is provided in the photosensitive region. However, the variation period of the refractive index may continuously change along the longitudinal direction.

Each of the aspects listed in [Description of Embodiment of the Present Disclosure] described above is applicable to all of the remaining aspects or all combinations of the remaining aspects.

Hereinafter, a specific structure of the optical fiber and the optical fiber grating according to the present disclosure will be described in detail with reference to the accompanying drawings. It should be noted that the present disclosure is not limited to the description below, and is intended to include all modifications within the scope as defined by the appended claims. In the following description with reference to the drawings, the same elements are denoted by the same reference numerals, and redundant description is omitted.

The structure of an optical fiber suitable for producing an optical fiber grating having a light transmission blocking wavelength band from about <NUM> to about <NUM> and having a low loss at the long-wavelength end of <NUM> in the L-band will be described below. A case where GeO<NUM> and B<NUM>O<NUM> are co-doped as a photosensitive material in order to generate a great variation in refractive index at high speed will be described. In the following, first, a comparative example will be described, and then an embodiment will be described.

<FIG> is a diagram showing a concentration distribution induced by Ge-doping and a concentration distribution induced by B-doping along the radial direction of an optical fiber according to a first comparative example. <FIG> is a diagram showing a radial refractive index profile of the optical fiber according to the first comparative example. The optical fiber according to the first comparative example has a step-index refractive index profile, and only a core out of the core and a cladding is doped with a photosensitive material. In the example shown in <FIG>, the photosensitive material is doped almost uniformly to the peripheral region of the core excluding the central region, and an amount of Ge doped to the core is <NUM>% in terms of a variation in relative refractive index induced by the Ge-doping. The amount of B doped to the core is -<NUM>% in terms of a variation in relative refractive index induced by the B-doping. The region where the relative refractive index nr indicated by the vertical axis in <FIG> and <FIG> is positive indicates a range where the refractive index is higher than the refractive index (reference) of pure silica glass, and the region where the relative refractive index nr is negative indicates a region where the refractive index is lower than the refractive index (reference) of pure silica glass.

<FIG> is a diagram showing a cross-sectional structure of an optical fiber grating <NUM> manufactured using the optical fiber according to the first comparative example. Note that <FIG> also shows the structure of an optical fiber according to a second comparative example described later. <FIG> and <FIG> are diagrams for describing reasons why a transmission loss gradually increases in the optical fiber grating <NUM> shown in <FIG>. The optical fiber according to the first comparative example includes a core <NUM> and a cladding <NUM>, and a grating region (refractive index modulated region) in which the refractive index periodically varies along the longitudinal direction of the optical fiber is formed in the core <NUM>. A gradual increase in transmission loss occurs when the LP<NUM> mode of monitoring light is coupled to the LP<NUM> mode, which is a higher-order mode and which is symmetrical to the LP<NUM> mode with respect to an axis. The relative refractive index difference between the core <NUM> and the cladding <NUM> of the optical fiber before being irradiated with ultraviolet light is defined as ΔnC. The relative refractive index difference between the core <NUM> and the cladding <NUM> when the relative refractive index of the core <NUM> varies by nrUV due to the ultraviolet light irradiation is defined as ΔnC-UV.

As shown in <FIG>, when only the core <NUM> has a periodical variation in an amount corresponding to the variation nrUV in relative refractive index along the longitudinal direction, a relative refractive index difference that periodically varies between ΔnC-UV (= nrUV + ΔnC) and ΔnC in relation to the light propagation direction appears. The MFD of the LP<NUM> mode varies depending on the periodically varying relative refractive index difference. That is, the light intensity distribution in the case where the relative refractive index difference is ΔnC (β-plane in the figure) is P<NUM>, and the light intensity distribution in the case where the relative refractive index difference is ΔnC-UV (α-plane in the figure) is P<NUM>. P<NUM> and P<NUM> have a relationship of P<NUM> ≠ P<NUM>. As a result, it is considered that scattering occurs due to variations in the light intensity distribution along the light propagation direction, and the scattering light is coupled with the high-order mode to cause a gradual increase in transmission loss as shown in <FIG> and <FIG>. Note that, on the β-plane, the light intensity distribution is the same as that of a region A in which the periodical variation in refractive index does not occur as shown in <FIG>.

Next, the structure of an optical fiber according to a second comparative example will be described. The optical fiber according to the second comparative example is comprised of silica-based glass, has a step-index refractive index profile, and includes, as shown in <FIG>, a core <NUM>, and an inner cladding and an outer cladding which surround the core <NUM> and which have refractive indices lower than the refractive index of the core <NUM>. In <FIG>, an inner cladding (optical cladding) <NUM> is shown as a part of the cladding <NUM> (a region between the core <NUM> and a broken line). Therefore, in the optical fiber according to the second comparative example, the outer cladding corresponds to a region provided outside the inner cladding <NUM> of the cladding <NUM> shown in <FIG>. The inner cladding <NUM> is adjacent to the core <NUM> and surrounds the core <NUM>. The outer cladding is adjacent to the inner cladding <NUM> and surrounds the inner cladding <NUM>. In the optical fiber according to the second comparative example, a photosensitive region constituted by the core <NUM> and the inner cladding <NUM> contains a photosensitive material. Specifically, the photosensitive region contains both Ge and B as the photosensitive material.

In the optical fiber according to the second comparative example, even if there is a periodic variation in an amount corresponding to the variation nrUV in relative refractive index induced by ultraviolet light irradiation, an amount of variation in the light intensity distribution in the LP<NUM> mode along the longitudinal direction is suppressed in order to suppress the deterioration in the orthogonality between the LP<NUM> mode and the LP<NUM> mode which is a higher-order mode. That is, the inner cladding <NUM> has an outer diameter equal to or larger than the MFD of the LP<NUM> mode in a wavelength of <NUM>. The specific outer diameter of the inner cladding <NUM> is preferably <NUM> or more and <NUM> or less, and more preferably <NUM> or more and <NUM> or less. In addition, in the optical fiber according to the second comparative example, it is preferable that the photosensitive material is substantially uniformly doped to the photosensitive region (the core <NUM> and the inner cladding <NUM>). The MFD is preferably <NUM> or more and <NUM> or less. The core <NUM> is a region having a relative refractive index higher by +<NUM>% or more than the average relative refractive index of the inner cladding <NUM>.

<FIG> is a diagram showing a refractive index profile of an optical fiber grating manufactured using the optical fiber according to the second comparative example. In the optical fiber grating produced by using the optical fiber according to the second comparative example, the periodic variation in an amount corresponding to the variation nrUV (= ΔnC-UV' - ΔnC) in relative refractive index along the longitudinal direction appears not only in the core <NUM> but also in the inner cladding <NUM>. As an ideal structure in that case, the relative refractive index difference ΔnC-UV' between the core <NUM> and the inner cladding <NUM> can be made equal to ΔnC, by which fluctuations of the LP<NUM> mode in the light propagation direction can be suppressed. That is, the optical fiber grating has a structure in which, in the entire light propagation region, periodic variation in an amount corresponding to the variation nrUV in relative refractive index occurs along the longitudinal direction, and the orthogonality between the LP<NUM> mode and the higher-order mode is maintained. In order to realize this structure, it is considered to use a method for reducing the refractive index of the inner cladding by adding F only to the inner cladding <NUM> to which Ge and B are co-doped. However, this method is not preferable in terms of production, because the compound of B and F is a difficult-to-treat substance.

Next, the structure of the optical fiber according to the embodiment of the present disclosure will be described. <FIG> is a diagram showing a cross-sectional structure of an optical fiber grating <NUM> manufactured using the optical fiber according to the embodiment. The optical fiber according to the embodiment includes a core <NUM>, an inner cladding (optical cladding) <NUM> surrounding the core <NUM> and having a refractive index lower than that of the core <NUM>, and an outer cladding <NUM> surrounding the inner cladding <NUM>. Further, a photosensitive region <NUM> constituted by the core <NUM> and the inner cladding <NUM> contains both Ge and B as a photosensitive material. In the example of <FIG>, a grating region is formed in the photosensitive region <NUM> by irradiating the optical fiber according to the embodiment with a laser beam via a phase mask <NUM> disposed apart from the optical fiber with a predetermined distance (Gap width).

While the optical fiber according to the second comparative example described above has a step-index refractive index profile, the core <NUM> of the optical fiber according to the embodiment has a single-peaked and graded refractive index profile. The inner cladding <NUM> has an outer diameter one time or more and two times or less the MFD of the LP<NUM> mode in a <NUM>-nm wavelength band.

<FIG> is a diagram showing a refractive index profile of the optical fiber grating <NUM> manufactured using the optical fiber according to the embodiment of the present disclosure. <FIG> is a diagram showing a Ge concentration distribution (concentration distribution induced by Ge-doping) and a B concentration distribution (concentration distribution induced by B-doping) along the radial direction of the optical fiber according to the embodiment of the present disclosure. <FIG> is a diagram showing a refractive index profile along the radial direction of the optical fiber according to the embodiment of the present disclosure. A dip where an amount of additive is low is generated in the central region of the core <NUM> due to reasons of the manufacturing process, but the additive diffuses when the optical fiber preform is drawn during the manufacture of the optical fiber. Therefore, an optical fiber having a single-peaked and graded refractive index profile as shown in <FIG> can be obtained.

First, as shown in <FIG>, B is substantially uniformly doped in the photosensitive region <NUM> (co-doped region constituted by the core <NUM> and the inner cladding <NUM>) excluding the central region (dip). The doped amount of B is preferably in the range of -<NUM>% to -<NUM>% in terms of the variation in relative refractive index induced by the B-doping. In order to reduce the non-uniformity of the refractive index in the cross section of the fiber, the difference between the maximum and the minimum of the doped amount of B (B concentration) in the region excluding the central dip is preferably <NUM>% or less in terms of the variation in relative refractive index induced by the B-doping.

Similar to B, Ge is doped to the core <NUM> and the inner cladding <NUM>. The doped amount of Ge (Ge concentration) in the outermost region of the inner cladding <NUM> is preferably <NUM>% or more in terms of a variation in relative refractive index induced by Ge-doping. When the Ge concentration is less than <NUM>% in terms of the variation in relative refractive index induced by the Ge-doping, the amount of variation nrUV in relative refractive index becomes extremely small, which is not effective. On the other hand, rather than adjusting the B concentration to have an α-profile, adjusting the range where the Ge concentration is <NUM>% or more in terms of the variation in relative refractive index induced by the Ge-doping to have an α-profile is better to make the amount of variation nrUV in relative refractive index induced by ultraviolet light more uniform in the cross section of the fiber. From the above, regarding creating an α-profile, controlling the Ge concentration is more effective than controlling the B concentration. The relative refractive index difference between the core <NUM> and the inner cladding <NUM> is preferably <NUM>% or more.

As shown in <FIG>, an amount of variation nrUV1 in relative refractive index in the center of the photosensitive region <NUM> and an amount of variation nrUV2 in relative refractive index at the end of the photosensitive region <NUM> are different in a strict sense, because the concentration of doped Ge is different. However, nrUV1 can be regarded to be substantially equal to nrUV2. As a result, regarding the relative refractive index difference ΔnU-UV', ΔnU-UV' ≈ ΔnU is established, and the variation amount of the light intensity distribution P3 is negligibly small with respect to the propagation direction of the LP<NUM> mode at a wavelength of <NUM>. The relative refractive index difference ΔnU-UV' is a difference between a value obtained by adding the amount of variation nrUV1 in relative refractive index at the center of the photosensitive region <NUM> to the relative refractive index difference ΔnU between the outer cladding <NUM> in the non-ultraviolet irradiation region (outside the grating region) and the center of the photosensitive region <NUM> and a value obtained by adding the amount of variation nrUV2 in relative refractive index at the end of the photosensitive region <NUM> to the relative refractive index difference of the outer cladding <NUM>. The exponent α of the refractive index profile for suppressing the variation amount of P3 preferably satisfies <NUM> < α < <NUM>.

<FIG> shows a refractive index profile of the optical fiber according to the embodiment of the present disclosure, and <FIG> and <FIG> are diagrams for describing a co-doped region (corresponding to the photosensitive region <NUM>) in which Ge and B are co-doped in the optical fiber. The co-doped region may be set to be the same as the core <NUM> (<FIG>) or may be set to a region including the core <NUM> and wider than the core <NUM> (<FIG>). The important point is that the diameter of the co-doped region is equal to or greater than the MFD of the LP<NUM> mode at a wavelength of <NUM>. The diameter of the co-doped region may be two times or more the MFD, but if the co-doped region is too large, the absorption of ultraviolet light for writing the grating will increase, which increases variations in the amount of variation nrUV in relative refractive index in the cross section of the fiber, and thus ineffective. Therefore, it is preferable that the diameter of the co-doped region is one time or more and two times or less the MFD of the LP<NUM> mode at a wavelength of <NUM>.

In addition, when Ge and B are co-doped, the amount of variation nrUV in relative refractive index increases, as compared with the case where only Ge is doped as the photosensitive material, so that the optical fiber grating <NUM> can be decreased in length. Specifically, the optical fiber grating <NUM> can be downsized to <NUM> or less in length.

<FIG> is a diagram showing an example of transmission characteristics of the optical fiber grating <NUM> manufactured using the optical fiber according to the embodiment of the present disclosure. <FIG> is an enlarged view of a part of <FIG>. Compared to the example of <FIG> and <FIG>, a gradual increase in transmission loss within a range from <NUM> to <NUM> is reduced, so that the transmission loss at <NUM> is suppressed to about -<NUM> dB in the transmission characteristics shown in <FIG> and <FIG>.

The optical fiber grating <NUM> according to the embodiment of the present disclosure satisfies -<NUM> dB in the transmission loss in the light transmission blocking band, and can be used up to the-<NUM> wavelength band where high capacity transmission in the L-band is enabled.

Further, in the embodiment of the present disclosure, the inner cladding <NUM> may have a refractive index substantially equal to the refractive index of pure silica glass by offsetting an amount of increase in refractive index induced by the Ge-doping and an amount of decrease in refractive index induced by the B-doping. Further, the inner cladding <NUM> may be configured to have a refractive index substantially equal to the refractive index of the outer cladding <NUM>. For example, in the configuration in which, in the inner cladding <NUM>, an amount of increase in the refractive index induced by the Ge-doping is adjusted by an amount of decrease in the refractive index induced by the B-doping, and the outer cladding <NUM> contains chlorine (Cl), the refractive index of the inner cladding <NUM> may be substantially equal to the refractive index of the outer cladding <NUM>.

In the above description, it is pointed out that the α-profile of the present disclosure (<FIG>) is superior to the conventional refractive index profile shown in <FIG> in that the transmission loss at <NUM> can be suppressed. The other advantages will be described below.

In the writing of grating via the phase mask <NUM> as shown in <FIG>, interference fringes of plus/minus first-order diffracted light are used. However, in this case, since interference fringes with different higher-order diffracted lights are also written at the same time, unnecessary transmission loss occurs near <NUM>, for example, as shown in <FIG> and <FIG>. Note that <FIG> shows transmission characteristics of the optical fiber grating <NUM> obtained by forming a grating, using a phase mask, in the optical fiber (having the step-index refractive index profile shown in <FIG>) according to the first comparative example, without varying the Gap width (distance between the phase mask and the optical fiber). <FIG> is an enlarged view of a part of <FIG>. As a countermeasure against the above problem, a manufacturing method for writing a grating while varying the Gap width (distance between the phase mask and the fiber) has been proposed (see Patent Document <NUM> and Non-Patent Document <NUM> mentioned above).

The spectra when writing of grating is performed while varying the Gap width are shown in <FIG>. Note that <FIG> shows the transmission characteristics of the optical fiber grating <NUM> obtained by forming a grating in the optical fiber (having the step-index refractive index profile shown in <FIG>) according to the first comparative example by using the phase mask. <FIG> is an enlarged view of a part of <FIG>. In <FIG> and <FIG>, a graph G141 shows the transmission characteristics when the Gap width is not varied, and a graph G142 shows the transmission characteristics when the Gap width is varied. Further, <FIG> shows the transmission characteristics of the optical fiber grating <NUM> obtained by forming a grating in the optical fiber (having the α-profile shown in <FIG>) according to the embodiment of the present disclosure by using the phase mask <NUM> as shown in <FIG>. <FIG> is an enlarged view of a part of <FIG>. In <FIG> and <FIG>, a graph G151 shows the transmission characteristics when the Gap width is not varied, and a graph G152 shows the transmission characteristics when the Gap width is varied.

It is confirmed from calculation that the unnecessary transmission loss in the vicinity of a wavelength of <NUM> is caused by the interference between plus first-order diffracted light and plus third-order diffracted light. It is found that, in the sample having the α-profile as well as in the sample having the step-index refractive index profile, the transmission loss at the wavelength of <NUM> when the Gap width is varied is suppressed compared to the case where the Gap width is not varied. The range of variation of the Gap width in the comparison is <NUM> in each sample.

In order to suppress the transmission loss, a method for improving the performance of the phase mask (the performance can be improved by mask design or manufacturing method) is also considered in addition to the method for varying the Gap width. Regarding the performance of the phase mask, having "high performance" means that the efficiency in generating plus/minus third-order diffracted light is sufficiently smaller than the efficiency in generating plus/minus first-order diffracted light. However, the point to be stressed is that it is impossible to totally eliminate higher-order diffracted light, although the efficiency in generating the higher-order diffracted light can be suppressed by preparing a high-performance phase mask, and a Bragg wavelength different from a predetermined Bragg wavelength is formed due to an interference between plus/minus first-order diffracted light and plus/minus third-order diffracted light. That is, unnecessary transmission loss occurs in the C-band. With the phase mask used in this embodiment, the transmission loss in the wavelength band including <NUM> is suppressed from -<NUM> dB to -<NUM> dB by varying the Gap width during writing of the grating. If a high-performance phase mask is prepared, further suppression is expectable, but there is a limit in suppression by improving only the phase mask.

The range of variation of the Gap width in <FIG> described above is constant at about <NUM>, but it is estimated that there is an appropriate range of variation of the Gap width depending on the structural variation in the longitudinal direction of the fiber. In this case, the Gap width of <NUM> may not be an appropriate range. In view of this, grating writing characteristics were examined for the optical fiber having a refractive index profile of an α-profile and the optical fiber having a step-index refractive index profile, using the Gap width as a parameter (<FIG> and <FIG>). Note that <FIG> is a graph showing a relationship between a Gap width and a transmission loss in the optical fiber grating <NUM> using the optical fiber according to the first comparative example. <FIG> is a graph showing a relationship between a Gap width and a transmission loss in the optical fiber grating <NUM> using the optical fiber according to the embodiment of the present disclosure. In <FIG> and <FIG>, the horizontal axis represents a Gap width (µm) and the vertical axis represents the maximum transmission loss in the wavelength band from <NUM> to <NUM> including the C-band. During the production of the optical fiber gratings of the measurement samples, the phase mask used for producing the optical fiber grating having the transmission characteristics shown in <FIG> was used. The UV irradiation conditions during the production were set such that substantially the same level of transmission loss occurred in the <NUM>-nm wavelength band.

It is found that, in the sample having the step-index refractive index profile (<FIG>), the maximum range of suppression of the transmission loss due to the Gap width being varied was <NUM> dB. The transmission loss was suppressed by the variation in the Gap width, and the difference between the maximum value and the minimum value of the transmission loss that converged at a certain value with the Gap width of <NUM> or more was Δ0. On the other hand, in the sample having the α-profile (<FIG>), the maximum range of suppression of transmission loss was <NUM> dB, and the difference between the maximum value and the minimum value of transmission loss with a Gap width of <NUM> or more was Δ0.

It is found that the range of suppression of the transmission loss of the optical fiber grating <NUM> having the α-profile is improved by <NUM> dB as compared with the optical fiber grating <NUM> having the step-index refractive index profile, which shows that the optical fiber grating <NUM> is superior. The point to be noted is that the dependence of transmission loss on the Gap width in the sample having the α-profile is smaller than that in the sample having the step-index refractive index profile. That is, it is found that, even if an amount of variation in the Gap width (amount of deviation from the set Gap width) is unexpectedly great because of variations in the structure in the longitudinal direction of the fiber, variations in alignment, etc., the sample having the α-profile has greater manufacturing tolerance than the sample having the step-index refractive index profile, and thus is effective in manufacture.

Claim 1:
An optical fiber comprised of silica-based glass, the optical fiber comprising:
a core (<NUM>) having a single-peaked and graded refractive index profile, the single-peaked profile being a profile having only one peak in a refractive index obtained by moving average using an average of refractive indices n(r) in the interval of <NUM>, the parameter r being defined as a distance from a center of the core along a radial direction;
an inner cladding (<NUM>) surrounding the core (<NUM>) and having a refractive index lower than a maximum refractive index of the core (<NUM>); and
an outer cladding (<NUM>) surrounding the inner cladding (<NUM>) and having a refractive index lower than the maximum refractive index of the core (<NUM>),
wherein both the core (<NUM>) and the inner cladding (<NUM>) contain a photosensitive material to constitute a photosensitive region (<NUM>), characterized in that
the inner cladding (<NUM>) has an outer diameter one time or more and two times or less a mode field diameter of an LP<NUM> mode in a <NUM>-nm wavelength band,
the photosensitive region contains both Ge and B as the photosensitive material,
a difference between a maximum concentration and a minimum concentration of the B in the photosensitive region is <NUM>% or less in terms of a difference between a variation in relative refractive index induced by B-doping with the maximum concentration and a variation in relative refractive index induced by the B-doping with the minimum concentration, and
a concentration of the Ge in an outermost region of the photosensitive region is <NUM>% or more in terms of a variation in relative refractive index induced by Ge-doping.