JP2000086274A - Grating - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a grating capable of improving the thermal stability and quality reliability for a long period of time, without need of hydrogen filling treatment. SOLUTION: The grating II is formed by irradiating a core of quartz glass co-doped with Sn and Al in addition to Ge with ultraviolet laser light without hydrogen filling treatment. In the state of the grating II, peaks (1)e, (3)e, (5)e and (6)e of Raman scattering, obtained in Raman spectral analysis, are shifted to low frequency side (low wave number side) from the state obtained before irradiating with the ultraviolet laser light. The density of bonding structure in the network structure of the glass is made to be changed to become higher and the stability to thermal load is enhanced by the shift to low frequency side.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of irradiating a core of an optical fiber or an optical waveguide with ultraviolet light to write a refractive index modulation fringe having a periodic stripe-like refractive index difference. The present invention relates to a grating used as a filter or device that reflects light of a specific wavelength corresponding to a fringe.

[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 silica-based glass (core glass) doped with germanium (Ge) is irradiated with a coherent ultraviolet laser beam to generate a light-induced refractive index change in a corresponding portion to generate (write) the grating. It is supposed to be. As described above, it is known that the Raman scattering characteristic after irradiation with ultraviolet light in a silica-based glass fiber doped with Ge is characterized in that a Raman scattering peak occurs near 580 cm ⁻¹ relating to Ge coupling (EMDianov, et al., Optics Letters, Vol.22, No.2
3, pp 1754-1756, 1997.12). In addition, an analysis result on how a Raman scattering state appears due to a difference in constituent elements of SiO ₂ glass, for example, a difference in a doping element such as Ge, B or P is known (Journal of Japan).
Non-Crystalline Solids 45, pp115-126, North-Holla
nd Publish Company, 1981).

On the other hand, the above-mentioned Ge-doped quartz glass does not have sufficient sensitivity (photosensitivity) to a photo-induced refractive index change due to ultraviolet light irradiation, so that the photosensitivity is increased, that is, relatively large light is emitted. It has been known that the Ge-doped quartz glass core glass is filled with hydrogen under a high pressure before irradiation with ultraviolet light to cause an induced refractive index change (hereinafter referred to as "hydrogen filling treatment"). (For example, see JP-A-10-82)
No. 919).

[0004]

By the way, the grating has periodic refractive index modulation fringes formed in the direction of the optical axis. By changing the period and the modulation shape of the refractive index modulation fringes, the grating is reflected. Applications to filters, duplexers, dispersion compensators, fiber laser mirrors, EDF gain equalizers, resonators, temperature sensors, and the like have been considered. It is a matter of course that gratings used for these various applications are required to have predetermined transmission characteristics (for example, reflection characteristics) as required functions. It is required to have reliability.

Here, there are several theories regarding the mechanism of causing the change in the refractive index induced by the irradiation of ultraviolet light, and the mechanism is not clear. In addition, it is not known at all what Raman scattering characteristics are when the Ge-doped quartz glass that has been subjected to the hydrogen filling treatment is irradiated with ultraviolet light.

[0006] In the hydrogen filling process, a process is usually performed in which a core glass is placed in a closed container filled with hydrogen and left at room temperature under a pressure of about 20 MPa for about 2 weeks. For this reason, in order to perform the hydrogen filling process, processing equipment and a relatively long processing time are required only for the hydrogen filling process, and the charged hydrogen is released over time. It must be performed early, which imposes restrictions on the grating fabrication timing. In addition, the increase of the photo-induced refractive index change by the hydrogen filling treatment can obtain a high refractive index change (high reflectivity) at a low irradiation energy density of ultraviolet light, but the thermal stability may not be sufficient. And may lack long-term reliability.

[0007] The present invention has been made in view of such circumstances, and it is an object of the present invention to improve the long-term reliability of quality with high thermal stability without performing hydrogen filling treatment. To provide a grating.

[0008]

Means for Solving the Problems To achieve the above object, the present inventor has proposed that by irradiating ultraviolet light, the glass network structure is significantly changed to form structurally dense refractive index modulation fringes. It has been found that the present invention has excellent thermal stability, can secure long-term reliability, and can specify the state of the structural change by observing the change in the Raman scattering state, and has conceived the present invention.

More specifically, the present invention is based on the premise that a quartz glass to which at least Ge is added is irradiated with ultraviolet light to form a grating in which refractive index modulation fringes are formed, and a hydrogen filling treatment is not performed. Irradiation of ultraviolet light to the quartz glass in the state causes the formation of refractive index modulation fringes, and the wave number position of the Raman scattering peak in the Raman scattering state in the refractive index modulation fringes before the ultraviolet light irradiation. The particular matter is that the state is shifted to a lower frequency side than the wave number position.

[0010]

In the case of the above invention, when the wave number position of the Raman scattering peak is shifted to a lower frequency side (lower wave number side) than before the irradiation with ultraviolet light, the density becomes higher than before the irradiation with ultraviolet light. , And the connection between the glass network structures changes to a state in which it is more difficult to move, so that it tends to be more stable against thermal load. Thereby, thermal stability is improved and long-term reliability is realized. In addition, since the hydrogen-filled glass that has not been subjected to the hydrogen filling treatment is used as the quartz glass to be irradiated with the ultraviolet light, it is possible to omit the processing equipment and the processing time for the hydrogen filling treatment and to release the restriction on the timing of the ultraviolet light irradiation. This makes it suitable for mass production of gratings.

In order to write (form) refractive index modulation fringes by irradiating ultraviolet light to quartz glass that has not been subjected to hydrogen filling, the grating length is longer than that to which hydrogen-filling is applied. The irradiation energy density may be increased by setting the length to be long.
Thereby, even if the ultraviolet light sensitivity is lower than that in the case where the hydrogen filling process is performed, it is possible to compensate for the insufficient sensitivity and to ensure the same reflectance as that in the case where the hydrogen filling process is performed. Further, in order to cause the change of the Raman scattering state due to the irradiation of ultraviolet light as described above, the above-mentioned quartz glass is prepared by co-doping at least Sn in addition to Ge or co-doping Al in addition to Sn. Preferably, it is used. It is possible to compensate for the lack of sensitivity when the dopant is only Ge and to increase the change in the induced refractive index with respect to ultraviolet light. Furthermore, for example, in the case of a fiber grating, ultraviolet light may be applied to an optical fiber core in which a resin coating layer is formed on an outer surface of an optical fiber consisting of a core and a clad. In this case, the resin coating layer may be formed of an ultraviolet-transmissive resin having a transmission characteristic capable of transmitting a specific wavelength corresponding to ultraviolet light irradiated for writing a grating.

[0012]

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

FIG. 1 shows an embodiment in which the grating of the present invention is applied to a fiber grating. Reference numeral 1 denotes an optical fiber core wire having a predetermined length, and reference numeral 2 denotes a silica-based glass doped with at least Ge. Brag
g Grating) 21 is written on the core, 3 is the cladding of the optical fiber 1, 4 is a coating layer coated on the outer surface of the cladding 3, and 5 is a phase mask for writing the grating 21. The above optical fiber cable 1
As shown in FIG. 2, a coating layer 4 is coated on an optical fiber 1 'comprising a core 2 and a clad 3 manufactured by drawing from an optical fiber preform. Irradiation of ultraviolet light (ultraviolet laser light) for writing the grating 21 is usually performed in a state where the coating layer 4 is removed from the optical fiber core wire 1, but is applied to the core 2 and the coating layer 4 as described later. The above irradiation may be performed from the outside of the coating layer 4 without removing the coating layer 4 by adding a device. When the irradiation is performed with the coating layer 4 removed, the grating formation target is the optical fiber 1 ′. When the irradiation is performed from outside the coating layer 4, the grating formation target is the optical fiber core 1. In the present embodiment, a case will be described in which the ultraviolet laser light is irradiated from outside of the coating layer 4 and the grating is formed by a phase mask method in which the ultraviolet laser light is irradiated through the phase mask 5. That is, when the ultraviolet laser beam is irradiated from the outside of the coating layer 4 through the phase mask 5, the refractive index modulation fringes (grating 21) are periodically formed in the fiber axis direction with respect to the core 2 of the optical fiber 1. Will be described for the case where a fiber grating is produced by writing the symbol.

In order to effectively write the grating 21 by irradiating an ultraviolet laser beam from the outside of the coating layer 4, a special structure is adopted as the core 2 and the coating layer 4 as described below. Is preferred.

That is, as the core 2, a core obtained by adding a dopant of Sn, Sn and Al, or Sn, Al and B in addition to Ge having the same concentration as that of the core of the normal specification optical fiber is used. This is preferable for constantly increasing the photo-induced refractive index change. Here, the normal specification optical fiber is an optical fiber core to be connected to the optical fiber core 1, and such an optical fiber core has a relative refractive index difference with respect to its core. It is manufactured by doping Ge with an amount of about 0.9%.
The core 2 of the optical fiber core 1 has the same amount (a relative refractive index difference of 0.9
%) And a concentration of 10,000 pp
m or more, preferably at a concentration of 10,000 to 15000 ppm
Of Sn or such a concentration of Sn and a concentration of 100
Al and the like at 0 ppm or less may be co-doped. The above dope may be carried out by various known methods, for example, when carried out by immersion, the compound of the above Ge and Sn (For Sn, for example, Sn Cl ₂ · 2H ₂ O) was mixed with methyl alcohol, What is necessary is just to immerse in the solution.

The coating layer 4 is formed so as to have a predetermined thickness by a single coating after the step of drawing the optical fiber 1 '. As a material for forming the coating layer 4, an ultraviolet transmitting resin having a property of transmitting ultraviolet light is used. As this ultraviolet transmission type resin, a specific wavelength band (for example, 240 nm to 270 n
m wavelength band), and it is particularly preferable to transmit the ultraviolet ray of the specific wavelength band with little absorption and to absorb the ultraviolet ray of a shorter or longer wavelength than the specific wavelength band. What causes a hardening reaction may be used. In other words, although the same resin has different ultraviolet absorption characteristics depending on the wavelength, it is an ultraviolet ray transmitting type in the above specific wavelength band, and is an ultraviolet curing type resin in a wavelength range shorter or longer than the above specific wavelength band. Most preferably, the coating layer 4 is formed.
Examples of such a resin include a photoinitiator (photoinitiator) that initiates and accelerates a curing reaction by receiving ultraviolet light having a wavelength range shorter than 240 nm or a wavelength range longer than 270 nm with respect to urethane acrylate or epoxy acrylate. ) May be used.

Next, the grating 21 is written on the core 2 by irradiating ultraviolet light from outside the optical fiber core 1, that is, outside the resin coating layer 4.
The writing of the grating 21 may be performed by using various well-known methods. In the case of performing the writing by the phase mask method, as shown in FIG. A grid-like phase mask 5 is provided, and an Nd-YAG laser source 6 focuses, for example, a 266 nm ultraviolet laser beam having a wavelength four times as large (4ω) from the Nd-YAG laser source 6 on the phase mask 5 by a cylindrical lens system 7. Irradiation may be performed. As a result, the ultraviolet laser light passes through the phase mask 5 and the resin coating layer 4, and the refractive index of the grating 2 corresponding to the grating pitch of the phase mask 5 is increased with respect to the core 2 so that the grating 21 is written. Will be. In FIG. 3, reference numeral 8 denotes a beam expander that expands the ultraviolet laser beam and converts it into a parallel beam.
Reference numeral 9 denotes a slit having a minute width for cutting out a portion where the power of the above-mentioned ultraviolet beam converted into a parallel beam is uniform, and these slits constitute a part of the irradiation system. Reference numeral 10 denotes a movable reflection mirror movable in the longitudinal direction of the optical fiber core wire 1 (see the dashed line arrow), 11 denotes an optical spectrum analyzer, 12 denotes an optical isolator, and 13 denotes an optical coupler. Constitutes a measurement system.

The amount of induced refractive index change is increased by setting the grating length of the grating 21 in the fiber axis direction to be relatively long so as to receive an ultraviolet laser beam having a larger irradiation energy density. The target reflectance (refractive index difference) is realized. In other words, in order to realize the same reflectance, when hydrogen filling processing is performed, shorter grating length can be achieved by shorter irradiation of ultraviolet light with smaller irradiation energy density, whereas the above hydrogen filling processing can be achieved. If not performed, the grating length may be set to be longer, and ultraviolet light may be absorbed at a larger irradiation energy density for a longer time.

As described above, by performing the ultraviolet laser light for a longer irradiation time without performing the hydrogen filling treatment, the Raman scattering peak in the Raman scattering state is shifted from the state before the irradiation to a lower frequency side. The bonding structure in the glass network structure changes to the denser side. Due to this change, even if the degree of change in the refractive index induced by the irradiation of the ultraviolet laser beam is lower than that in the case of hydrogen-filled treatment, a grating having a high thermal stability and a long-term reliability can be obtained.

<Other Embodiments> The present invention is not limited to the above embodiments, but includes various other embodiments. That is, in the above embodiment,
Irradiation with ultraviolet laser light is performed from the outside of the resin coating layer 4, but the invention is not limited to this. Irradiation with ultraviolet laser light may be performed on the optical fiber 1 'from which the resin coating layer 4 has been removed. Good.

In the above embodiment, a fiber grating in which the grating is formed on the optical fiber has been described. However, the present invention is not limited to this, and the grating may be formed on a part of the optical waveguide.

Further, in the above-described embodiment, in addition to Ge, Sn
And Al are co-doped, but the present invention is not limited thereto, and Ge alone may be used as a dopant. Even in this case, by irradiating the ultraviolet laser light without performing the hydrogen filling process, the Raman scattering state is changed to a low frequency side, and the grating has a long-term stability that is not easily changed with respect to a thermal load. be able to.

[0023]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A silica glass co-doped with Sn and Al in addition to Ge is used as a core 2 (see FIGS. 1 and 2) and a cladding 3 is used.
Of an optical fiber core 1 having a predetermined length whose outer surface is covered with a resin coating layer 4 as an initial sample Gi, and irradiating an ultraviolet laser beam to a hydrogen-filled one using the initial sample Gi. A sample I as a comparative example in which a grating was formed by the method described above, and a sample II as an example in which a grating was formed by irradiating the initial sample Gi with an ultraviolet laser beam without performing hydrogen filling treatment were produced. Then, while examining the relationship between the irradiation energy density and the reflectance for the samples I and II, the initial sample G
i, samples I and II were subjected to Raman spectroscopic analysis to examine the change in Raman scattering state with and without hydrogen filling treatment.

In the above quartz-based glass, the doping amount of Ge is such that the relative refractive index difference Δ is 0.97%.
The concentration of n is 15000 ppm, and the concentration of Al is 900 pp.
m. The hydrogen filling process is performed by placing the core glass of the initial sample Gi in a closed container filled with high-pressure hydrogen of 20 MPa.
Performed by leaving for a week. As the ultraviolet laser light, the wavelength (2 times) of the Nd-YAG laser (see 6 in FIG. 3) is used.
66 nm), and the grating was formed by a phase mask method using an apparatus as shown in FIG. In this case, the grating length was set to 2 m for sample I.
m and 24 mm for Sample II.

The specifications of the grating forming method for the above-mentioned samples I and II, the reflectance of the formed grating and the estimated refractive index difference (calculated value of refractive index variation) Δnmod converted from the reflectance. Is shown in Table 1, and the reflectance characteristics represented by the irradiation energy density and the reflectance are shown in FIG. 4, respectively.

[0026]

[Table 1]

A comparison between Sample I and Sample II shows that, in Sample I, the induced refractive index change with respect to ultraviolet light is increased by the hydrogen filling treatment, so that the grating length is as short as 2 mm and the irradiation is as small as 2.25 kJ / cm ^2. A grating having a high reflectivity of 99% or more was obtained with an energy density and a short irradiation time of 15 minutes. And
The estimated refractive index difference of the grating thus formed was about 1 × 10 ⁻³ . On the other hand, in the case of the sample II not subjected to the hydrogen filling treatment, a relatively long grating length of 24 mm,
A large irradiation energy density of 7.5 kJ / cm ² ,
In addition, a long irradiation time of 450 minutes is required. Then, the estimated refractive index difference of the formed grating is 1 ×
It is only 10 ^-4 or less, which is one digit smaller than that of Sample I.

On the other hand, FIG. 5 shows the results of Raman spectroscopic analysis of the above-mentioned initial sample Gi, samples I and II, and Table 2 shows the change in the Raman scattering state of each of the samples I and II with respect to the initial sample Gi. .

[0029]

[Table 2]

In this Raman spectroscopic analysis, reflection Raman analysis was performed on the core portion where the grating was formed. Specifically, the laser Raman microprobe method was employed, and the Raman reflected light was evaluated using an excitation wavelength of 488 nm. The laser spot diameter is about 1μ.
m. FIG. 5 shows the Raman spectrum of the initial sample Gi and the sample I
And the Raman spectrum of Sample II are separately shown in the direction of the Raman scattering intensity at intervals. The right ends of the three Raman spectra coincide with each other at the same position.

By observing the change in the Raman scattering state by the Raman spectroscopic analysis, it can be seen how the structural change of the glass in the grating forming portion before and after the irradiation of the ultraviolet laser light appears depending on the presence or absence of the hydrogen filling treatment. confirmed. Here, the Raman spectrum is information on the structural state such as crystallinity and orientation of a substance by identifying a compound by looking at its pattern or by looking at the position, shape, intensity ratio, etc. of a Raman scattering peak (Raman band). Can be obtained. Therefore, by observing the change in the Raman scattering state before and after the irradiation of the ultraviolet laser light depending on the presence or absence of the hydrogen filling treatment, the grating formed (samples I and II) was observed. ) Trying to identify the differences in characteristics.

In the case of the sample I which has been subjected to the hydrogen filling treatment, the following two points mainly change the Raman scattering state by irradiation with the ultraviolet laser light. That is,
The Raman scattering peak caused by the 6-membered ring mode of the initial sample Gi is shifted to a Raman scattering peak r toward a higher wavenumber (higher frequency side) by being irradiated with an ultraviolet laser beam, the Raman scattering peak intensity is reduced, and the wavenumber is 715 cm. ^-1
, A Raman scattering peak r is newly generated. The Raman scattering peaks r to r other than these were almost the same as the Raman scattering state of the initial sample Gi. Raman scattering peak above
r is considered to be a Raman scattering peak due to a structure bonded to hydrogen filled by the hydrogen filling treatment.

On the other hand, in the case of the sample II not subjected to the hydrogen filling treatment, by irradiating an ultraviolet laser beam, almost all of the Raman scattering peaks of the initial sample Gi are Raman scattering peaks e, e, e, and. The peak shifted to the lower wavenumber side (lower frequency side) to e, and the Raman scattering peak intensity also decreased.

To examine the change in the Raman scattering state of Sample I and Sample II due to the irradiation of the ultraviolet laser beam, it was found that in the case of Sample II, most of the Raman scattering peaks shifted to the low frequency side and the glass network structure was changed. In the case of sample I, the Raman scattering state did not change much, and the network structure shifted slightly at the Raman scattering peak to the high frequency side, while the sample I was irradiated with ultraviolet laser light. Was only. In the case of sample II, the Raman scattering peak shifts to the lower frequency side even though the refractive index change is 1 × 10 ⁻⁴ , which is an order of magnitude smaller than that of sample I. Has changed to a denser side, and each connection of the network structure has changed to a structure that is less likely to fluctuate even when subjected to an external environment, particularly a thermal load. On the other hand, in the case of sample I, a large change in the refractive index was induced by a mechanism different from that of sample II, with almost no change in the network structure of the glass caused by the hydrogen filled in the glass by the hydrogen filling treatment. it is conceivable that. In the case of Sample I, the shift to the high-frequency side causes the network structure to change correspondingly to a lower-density side, and in particular, changes to a structure that is more susceptible to fluctuations due to the thermal load. It is considered that it has become unstable. In other words, even if a grating with the same refractive index difference is formed, the one whose Raman scattering state shifts to the lower frequency side by irradiation with ultraviolet laser light changes to a more stabilized glass network structure, and longer-term stability is expected. Will be able to do it. Therefore, the sample II without hydrogen filling is superior to the sample I with hydrogen filling from the viewpoint of long-term stability, even though the degree of change in the refractive index due to ultraviolet laser light irradiation is lower. Conversely, it can be said that sample I which has been subjected to the hydrogen filling treatment lacks long-term stability although the apparent refractive index change is larger than that of sample II.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram showing a grating according to an embodiment of the present invention.

FIG. 2 is an enlarged cross-sectional view of FIG.

FIG. 3 is a schematic diagram showing a grating manufacturing apparatus.

FIG. 4 is a diagram showing a relationship between irradiation energy density and reflectance.

FIG. 5 is a diagram showing a Raman spectrum.

[Explanation of symbols]

 2 core (quartz glass) 21 grating (refractive index modulation fringe)

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification code FI Theme coat ゛ (Reference) G02B 6/10 G02B 6/10 C 6/122 6/12 A

Claims (2)

[Claims] 1. A grating in which refractive index modulation fringes are formed by irradiating at least Ge-added quartz glass with ultraviolet light, wherein the hydrogen-filled quartz glass is not treated with ultraviolet light. Irradiates to form a refractive index modulation fringe,
Further, the grating is characterized in that the wave number position of the Raman scattering peak in the Raman scattering state in the refractive index modulation fringe is shifted to a lower frequency side than the wave number position before the irradiation of the ultraviolet light. 2. The grating according to claim 1, wherein the quartz glass is formed by co-doping at least Sn in addition to Ge.