JP3262749B2 - Manufacturing method of fiber grating - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of writing a diffraction grating (grating) having a refractive index difference in a stripe pattern on a core of an optical fiber, and using the grating, a specific wavelength (Bragg reflection wavelength) corresponding to the grating. The present invention relates to a method for manufacturing a fiber grating used as a device or a filter for reflecting light.

[0002]

2. Description of the Related Art Hitherto, as this type of fiber grating, there has been known a fiber grating in which a grating is written into a core of an optical fiber by a two-beam interference method or a phase mask method (for example, Japanese Patent Laid-Open No. 6-2358).
No. 08, Japanese Patent Application Laid-Open No. H7-140311, and Patent No. 2
No. 521708). In such a fiber grating, a quartz glass (core) doped with germanium (Ge) is irradiated with a coherent ultraviolet laser beam to generate a photo-induced refractive index change in a corresponding portion to generate a Bragg grating ( Write).

[0003]

The specific wavelength (Bragg wavelength) .lambda.B reflected by the Bragg grating is expressed by the following equation (1), and the reflectance for reflecting the light having the Bragg wavelength is (2). It is represented by an equation.

[0004]

(Equation 1)

[0005]

(Equation 2)

In the above equation (2), when the grating length L is the same, the reflectance RB for a specific wavelength λB can be improved by increasing the refractive index modulation Δn. Various investigations were made with attention.

However, as an optical fiber to be written, an optical fiber having a core doped with Ge at a normal concentration (a concentration at which the relative refractive index difference between the core and the clad becomes, for example, 0.9%) is used. In addition, since the sensitivity (photosensitivity) of the core to the light-induced refractive index change is relatively small, it is not possible to obtain a refractive index modulation Δn that is too large even by irradiating ultraviolet rays.

In this regard, in order to cause a relatively large change in the photo-induced refractive index, the core to be written has a higher density than the normal density (a relative refractive index difference of, for example, 1.5%).
It has been proposed to use a core doped with Ge at a concentration of about 2.0%) or to use a core doped with normal concentration of Ge and then filled with hydrogen under high pressure (electronic information). IEICE Transactions on Electronics Vol.J79-C-1, No.11
415, p. November 1996).

However, when a fiber grating is manufactured using the core doped with Ge at a high concentration, if this fiber grating is inserted into a normal optical fiber to be used as a filter or the like, the fiber grating is Since the core of the optical fiber to be spliced (fused) is of a normal specification doped with Ge at a normal concentration, matching between the two cores cannot be achieved, and the connection loss increases due to the difference in the concentration of Ge doping. The inconvenience of doing so will occur. On the other hand, when fabricating a fiber grating using the above-described core filled with high-pressure hydrogen, the filled hydrogen diffuses with the passage of time and returns to the state before hydrogen filling in a relatively short period of time (for example, several days). Therefore, there is a disadvantage that the period of forming the Bragg grating by the irradiation of ultraviolet rays is limited to a relatively short period.

SUMMARY OF THE INVENTION The present invention has been made in view of such circumstances, and it is an object of the present invention to easily increase connection loss without being limited by a Bragg grating forming period. Another object of the present invention is to provide a method of manufacturing a fiber grating that can increase the reflectance at a specific wavelength.

[0011]

In order to achieve the above object, the invention according to claim 1 provides an optical fiber to be written.
I wrote a Bragg grating for Iva's core
Assuming the fiber grating manufacturing method,
In addition to Ge equivalent to the core of an elephant optical fiber, Sn is immersed
To be written with a core co-doped by the method
An optical fiber is manufactured, and the optical fiber
By irradiating the core of the bag with ultraviolet light,
The rating is written.

In the case of the above configuration , the optical fiber to be written is made of Sn (tin) co-doped by the immersion method.
The photo-induced refractive index change of the core of the bar is steadily increased as compared with the core simply doped with the usual concentration of Ge. As a result, when ultraviolet light is irradiated, the refractive index modulation Δn in the above equation (2) increases compared to the core simply doped with the normal concentration of Ge, and the reflectance increases. In addition, since the Ge doping concentration of the core of the obtained fiber grating is equal to that of the core of the optical fiber to be connected, even if the optical fiber is connected to an optical fiber of a normal specification, the connection loss does not increase. Further, even if the high-pressure hydrogen filling is not performed, the reflectivity can be constantly increased by the above-described Sn co-doping, so that there is a limitation on the Bragg grating formation period when the high-pressure hydrogen filling is performed. Nor.

The mechanism of the change of the photo-induced refractive index upon irradiation with ultraviolet rays is described by the Kramers-Kronig mechanism in which the bond between Ge atoms and SiO ₂ (quartz glass) is changed by irradiation with ultraviolet rays to cause a change in the refractive index. Based on the compression model, which is based on the assumption that the glass bond is broken by ultraviolet irradiation and the glass structure collapses, thereby increasing the density and increasing the refractive index, or based on the dipole model Various things have been proposed, but the fact is that it has not been completely elucidated yet. Although the mechanism by which the co-doping of Sn causes an increase in the photo-induced refractive index change is not clear, the inventor of the present application requires labor and a considerable period of time (for example, two weeks), and requires a considerable time during the formation of the Bragg grating. Without using the means of high-pressure hydrogen filling with limitations,
Moreover, assuming that the doping amount of Ge is equal to the normal specification so as not to cause connection loss, various investigations and tests were performed on the doping material for the core. It has been found that the reflectance can be increased without inconvenience in the related art.

The doping amount of Ge and Sn is such that Ge is such that the difference in the relative refractive index of the core with respect to the cladding becomes 0.9%, while Sn is used. The concentration is preferably at least 10,000 ppm. Further, as the amount of Sn to be co-doped, concentration 1
It is preferably 0000-15000 ppm.

[0015]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 illustrates a method of manufacturing a fiber grating according to an embodiment of the present invention.
Denotes a core to which a Bragg grating (diffraction grating) 21 is to be written, and 3 denotes a clad of the optical fiber main body 1.

The core 2 of the optical fiber main body 1 is manufactured by co-doping Sn in addition to Ge, which is equivalent to an optical fiber of a normal specification. Here, the optical fiber of the normal specification is an optical fiber to be connected to the optical fiber 1, and such an optical fiber has a relative refractive index difference of 0.9% with respect to its core. It is manufactured by doping a moderate amount of Ge. The core 2 of the optical fiber body 1 has the same amount of Ge as the core of the above-mentioned normal specification optical fiber (an amount such that the relative refractive index difference becomes 0.9%), and a concentration of 10,000 ppm or more. Preferably, Sn having a concentration of 10,000 to 15000 ppm is immersed.
Is co-doped with

The above-mentioned Sn doping is performed by using a compound of Sn (S
For n, for example, Sn Cl ₂ · 2H ₂ O) was mixed with methyl alcohol, it may be immersed in the solution.

Next, the Bragg grating 21 is written on the optical fiber body 1 manufactured by co-doping the core 2 with Ge and Sn as described above. This writing of the Bragg grating 21 may be performed by using various well-known methods. For example, when the writing is performed by the phase mask method, as shown in FIG. The mask 4 may be provided, and the phase mask 4 may be irradiated with ultraviolet laser light via the cylindrical lens system 5. In this case, for example, coherent ultraviolet light of 266 nm, which is four times the wavelength of the Nd-YAG laser, may be irradiated. As a result, the Bragg grating 21 having a grating pitch corresponding to the grating pitch of the phase mask 4 is written into the core 2.

When writing the Bragg grating 21, the core 2 to be written is immersed in Sn in addition to Ge.
Since it is co-doped by the immersion method, a relatively large photo-induced refractive index change is caused by the irradiation of the ultraviolet laser light (to increase the sensitivity of photosensitivity), and the Bragg grating 21 having a high reflectivity is formed. Becomes possible.
In addition, such a high reflectivity of the Bragg grating 21 can be achieved with the same Ge doping amount as that of the optical fiber of the normal specification, so that both ends of the optical fiber main body 1 constituting the fiber grating have the same reflectivity. Even if an optical fiber is connected, there is no risk of connection loss due to mismatch of the Ge doping amount, and since the above-mentioned increase in the photoinduced refractive index change is constant, high reflection due to high-pressure hydrogen filling occurs. There is no limitation on the formation period of the Bragg grating that is received in the case of efficiency.

The use of the fiber grating on which the Bragg grating 21 is written is as follows.
There are the following. For example, when used as a demultiplexer or a multiplexer, as shown in FIG. 2, when transmitting light of many different wavelengths λ1 to λn having a relatively narrow width, only the light of a specific wavelength λi is used. Is reflected and the light of the specific wavelength λi is demultiplexed via the circulator 6. Such a configuration is used in, for example, the field of Add-Drop type wavelength multiplex communication. Further, a sensor having the same configuration as that shown in FIG. 2 can be used as a sensor for detecting temperature and external force. That is, since the Bragg grating 21 has a characteristic that the reflection wavelength of the Bragg grating 21 shifts sensitively when the temperature or tension changes, it is possible to detect a change in temperature or tension (external force) due to such a change in reflection wavelength. it can.

As shown in FIG. 3, the chirped fiber grating 21 'can be used as a substitute for a multilayer filter. That is, a specific wavelength band (center wavelength λi) is selected from a plurality of wavelength bands (λ1 to λn).
The light of the above specific wavelength band is cut by reflecting the light of (2), and the light of the wavelength band excluding the specific wavelength band is transmitted. As an application example of the above-described chirped fiber grating 21 ', a delay characteristic is used in which light on the long wavelength side is reflected early in a specific wavelength band, while light with a short wavelength is reflected with a delay. It can also be used as a dispersion compensator that takes out such reflected waves with a circulator and transmits them in a dispersed state.

Further, by forming Bragg gratings 21 and 21 on both sides of the amplification medium 7 as shown in FIG. 4, each Bragg grating 21 functions as a laser mirror to form a resonator. be able to. That is, light of a specific wavelength is reflected by the Bragg gratings 21 and 21 on both sides of the amplification medium 7.

In the present embodiment, since the Bragg gratings 21 and 21 'have high reflectivity in each of the above-mentioned applications, light of a specific wavelength or a specific wavelength band can be reflected almost completely. Almost all light of a wavelength or a specific wavelength band can be extracted or cut off. <Other Embodiments> Note that the present invention is not limited to the above-described embodiments, but includes various other embodiments. That is, in the above embodiment, the metal element co-doped with Ge is Sn. However, the present invention is not limited to this. Sn and, for example, cesium (Ce), praseodymium (Pr), terbium (Tb), boron (B), or aluminum (A
l) The combination with one of the various metal elements may be co-doped.

[0025]

EXAMPLE An example in which a Bragg grating was written in a core co-doped with Ge and Sn, a first comparative example in which a Bragg grating was written in a core doped only with Ge at a concentration of a normal specification, and a normal specification A second comparative example in which a Bragg grating was written on a core doped only with a higher concentration of Ge than the comparative example was prepared, and the reflectance performance was compared.

The above embodiment and first and second comparative examples will be specifically described below. The optical fiber body 1 has an outer diameter of about 125 μm, a mode field diameter of about 6 μm, and a cutoff frequency of about 1.3 μm. In the embodiment, the amount of Ge relative to the core and the amount such that the relative refractive index difference Δ of the optical fiber body 1 becomes 0.9% and 10,000 ppm
Was co-doped, and the relative refractive index difference Δ was set to 1.0%. For the first comparative example, only the amount of Ge such that the relative refractive index difference Δ becomes 0.9%, and for the second comparative example, only the amount of Ge such that the relative refractive index difference Δ becomes 2.0%. Was respectively doped.

Further, two types of cores were prepared, one in which the above-mentioned core was subjected to high-pressure hydrogen filling and the other in which the core was not subjected to hydrogen treatment (untreated with hydrogen). A comparison was made between untreated hydrogen and high-pressure hydrogen filling. Therefore, those not treated with hydrogen were used in Examples (E-
1), a first comparative example (N-1), and a second comparative example (H-1).
A first comparative example (N-2) and a second comparative example (H-2) are provided.
The high-pressure hydrogen filling was performed by sealing the container in a sealed container filled with high-pressure hydrogen at 200 atm for a certain period of time in the range of 1 to 2 weeks.

The writing of the Bragg grating to each of the optical fiber bodies 1 of the embodiment and the first and second comparative examples is performed using an ultraviolet laser light source having a wavelength 266 nm, which is four times the wavelength of the YAG laser, a pulse width of 10 ns, and a repetition frequency of 10 Hz. The light was condensed by a lens system 5 having a laser output power of 10 mW, and the main hand time was variously changed by a phase mask method using a phase mask 4 (see FIG. 1). At this time, the embodiment (E-1), the first comparative example (N-1) and the second comparative example (N-1)
For the comparative example (H-1), a Bragg grating having a grating length of 0.3 mm was formed, and the example (E-2), the first comparative example (N-2), and the second comparative example (H-2) were formed. As for, a Bragg grating having a grating length of 2.0 mm was formed.

Differences between the above three types of dopes and six types (E-1, N-1, H-
FIG. 5 shows the reflectance characteristics of (1, E-2, N-2, H-2) with respect to the irradiation time of ultraviolet light.

Referring to FIG. 5, a comparison was made between the samples not treated with hydrogen. In Example (E-1) in which Sn was co-doped in addition to Ge, the reflectance increased as the irradiation time increased.
While the reflectance reached almost 70% in the irradiation time of 0 to 50 minutes, in the first comparative example (N-1) in which only the normal concentration of Ge was doped, the reflectance increased as the irradiation time increased. However, in the second comparative example (H-1) in which only a high concentration of Ge was doped, the reflectance reached only about 30% in the irradiation time of 40 minutes to 50 minutes, and the reflectance increased with the irradiation time. It increased and reached a reflectance of almost 75% with an irradiation time of 40 to 50 minutes.

Therefore, even if the high-pressure hydrogen filling is not performed, in Example (E-1), Sn was co-doped in addition to the normal concentration of Ge, so that the first comparative example (N -1), the reflectivity can be greatly increased, and a high-concentration Ge of 2.0% in relative refractive index difference.
, A high reflectivity almost equivalent to that of the second comparative example (H-1) doped with is obtained.

In comparison with the case of high pressure hydrogen filling, in Example (E-2), the reflectivity sharply increased immediately after the start of irradiation, and reached 95% or more in several minutes of irradiation time. On the other hand, in the first comparative example (N-2) doped with only the normal concentration of Ge, the reflectance increases with the increase of the irradiation time, but the irradiation time becomes longer if the irradiation time does not exceed 30 minutes.
In the second comparative example (H-2) in which the reflectivity did not reach about 5% and only high-concentration Ge was doped, the second comparative example (E-2) was slightly inferior to the example (E-2). -2) showed almost the same tendency.

Therefore, even in the case where high-pressure hydrogen filling is performed, in Example (E-2), in addition to the normal concentration of Ge, S
By co-doping n, it is possible to obtain a high reflectance substantially equal to that of the second comparative example (H-2) doped with high concentration of Ge, and to obtain a first reflection doped only with normal concentration of Ge.
Compared to Comparative Example (N-2), a very high reflectance of 95% or more could be obtained by irradiation of ultraviolet light for a very short time.

In the above Examples (E-1) and (E-2), the concentration of Sn to be co-doped is not 10000 ppm as described above but 15000 ppm (corresponding to 0.15% in terms of relative refractive index difference). ),
The effect of increasing the reflectance was almost the same as the case of 10,000 ppm.

[0035]

As described above, according to the method for manufacturing a fiber grating according to the first or second aspect of the present invention, Sn is co-doped by the immersion method in addition to the normal concentration of Ge. It is possible to realize a fiber grating with high reflectance by greatly increasing the reflectance, and to prevent the occurrence of an increase in connection loss due to a mismatch in Ge concentration even when connected to an optical fiber to be connected. Thus, a fiber grating can be obtained . In addition, since a high reflectance is constantly obtained without filling with high-pressure hydrogen, the formation of the Bragg grating can be easily performed without being restricted by the period of forming the Bragg grating when filling with high-pressure hydrogen. Will be able to do it.

[Brief description of the drawings]

FIG. 1 is a schematic diagram showing a writing method of a Bragg grating in an embodiment of the present invention.

FIG. 2 is a schematic diagram illustrating an application of the embodiment.

FIG. 3 is a schematic view showing a use different from that of FIG. 2;

FIG. 4 is a schematic view showing an application different from those in FIGS. 2 and 3;

FIG. 5 is a diagram illustrating reflectance characteristics of various fiber gratings with respect to irradiation time of ultraviolet light.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Optical fiber main body 2 Core for writing 3 Cladding 21, 21 'Bragg grating

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-9-113729 (JP, A) JP-A-9-236720 (JP, A) IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 7 (9) (1995), p. 1048-1050 OPTICS LETTERS, VOL. 20 (19) (1995) pp. 1982-1984 (58) Field surveyed (Int.Cl. ⁷ , DB name) G02B 5/18 G02B 6/00 G02B 6/10 G02B 6/16-6/22 G02B 6/44

Claims (2)

(57) [Claims] 1. A core of an optical fiber to be written.
Fiber grating with Bragg grating
In the manufacturing method of the coating, Ge is added to the same concentration as the core of the optical fiber to be connected.
And having a core co-doped with Sn by the immersion method.
An optical fiber to be embedded is manufactured, and ultraviolet light is applied to the core of the optical fiber to be written.
Write Bragg grating by irradiating
A method of manufacturing a fiber grating. 2. The method of claim 1, the core of the optical fiber which is the write target, in addition to Ge in an amount that the relative refractive index difference of the core is 0.9% with respect to the cladding, the concentration 10000ppm or more Sn There co de
Is-loop, a fiber grating fabricating method, characterized in that.