JPH08286021A - Manufacture for waveguide type optical filter - Google Patents

Abstract

PURPOSE: To provide the manufacturing method of a waveguide type optical filter where light can be fully reflected over a wide wavelength width. CONSTITUTION: A mask 60 where plural phase gratings 101-106 are serially successively formed is prepared at first, and an optical waveguide is installed so as to align the forming direction of the plural phase gratings 101-106 with a light propagating direction. Next, plural gratings 111-116 where a refractive index is periodically changed along an optical axis are formed by irradiating the optical waveguide with interference ultraviolet light through the mask 60. The center wavelength λ0 of respective gratings 111-116 and the wavelength width Δλ of a main reflection band are appropriately set, so that a waveguide type optical filter having the reflection band of high reflectance over the wide wavelength width is obtained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of manufacturing a waveguide type optical filter in which a part of the optical waveguide is provided with a region for performing an optical filter function.

[0002]

2. Description of the Related Art Optical fiber communication sometimes requires an optical component for changing the path of light according to the wavelength. For example, this is the case where the optical line is identified. In this case, in addition to the signal light for communication, the inspection light for identifying the optical line is propagated through the optical fiber for communication. In order to distinguish from the signal light, the inspection light has a wavelength different from that of the signal light.

For example, there is a method of identifying an optical line by inserting an optical component that reflects the inspection light into the optical line and measuring the time until the inspection light is reflected and returns to the input end. Optical communication, which is the original purpose, is realized by propagating signal light as it is through an optical line.

Previously, an optical filter such as a dielectric multi-layer film filter was inserted into the optical line, and the inspection light was reflected thereby to identify the optical line. This dielectric multi-layer film filter is a laminate of a plurality of types of dielectric thin films on a substrate such as glass, and has the function of reflecting or transmitting light of a specific wavelength depending on the refractive index and film thickness of the thin films. .
"Light / Thin Film Technology Manual" (Opttronics) p
Nos. 284 to 289 give various structural examples of dielectric multilayer filters.

However, since such an optical filter is manufactured by growing a thin film on a flat substrate, it is necessary to cut the optical fiber and insert the optical filter when it is installed in an optical line. Becomes Therefore, there is a problem that the signal light is lost due to the insertion and the insertion work is troublesome.

On the other hand, a region (filter region) that fulfills an optical filter function in advance in the core of the optical fiber for communication.
There is also a method of forming. In this case, the loss of signal light due to the insertion of the optical component does not occur. In this case, as the filter region, a grating having a refractive index that periodically changes along the optical axis is formed in the core. This can be formed by irradiating the germanium-doped silica-based optical fiber with ultraviolet light having a striped pattern, as described in Japanese Patent Application Publication No. 62-500052.

Instead of an optical fiber, a thin film waveguide having a filter region formed in the core is also connected to a communication optical fiber to reflect the inspection light and to branch and output the signal light passing through the filter region. You can
There is a convenient aspect.

As described above, the waveguide type optical filter in which the optical waveguide has a region for performing an optical filter function can be used as it is for optical communication as an optical line, or can be used by inserting it into an optical line as in the conventional case. It can be said that it is an easy-to-use optical filter that can also be provided with other optical functions.

[0009]

The inspection light for identifying the optical path normally uses a semiconductor laser having a single wavelength output as a light source. The inspection light emitted from this laser has a wavelength width (about 0.2 to 1.0 nm) centered on a predetermined wavelength. However, an error is likely to occur in this center wavelength, and therefore the output light of the semiconductor laser light source usually has a certain center wavelength shift.

Usually, the optical line is identified by using an optical filter having the same center wavelength as the center output wavelength of the light source. In this case, if the wavelength beyond the wavelength range that can be reflected by the optical filter becomes the central wavelength of the inspection light due to the shift of the central wavelength of the output light of the light source, part or all of the inspection light will pass through the filter. This transmitted light becomes noise for the signal light, which is not preferable for optical communication. Therefore, when the inspection light for identifying the optical line is reflected using the waveguide type optical filter, the waveguide type optical filter can reflect the inspection light to some extent so that the inspection light can be reflected even when the center wavelength shift of the inspection light occurs. It is preferable that light can be sufficiently reflected over a wide wavelength range.

FIG. 3 is a graph showing the refractive index of the core in the grating for the waveguide type optical filter having a single grating in the core. The horizontal axis of the graph is the distance coordinate x indicating the position along the optical axis of the waveguide type optical filter, and the vertical axis of the graph is the refractive index n of the core.

As shown in this figure, the refractive index of the core in the grating periodically changes from the basic refractive index n ₀ to the maximum refractive index n ₀ + Δn. In the figure, Λ is the period of change in the refractive index, and L is the length of the grating.

A waveguide type optical filter having a single grating in the core reflects light with a relatively high reflectance over a certain wavelength range. The central wavelength λ ₀ of this reflection wavelength region is shown as λ ₀ = 2 · n ₀ · Λ.

The reflectance R when light of wavelength λ near the center wavelength λ ₀ is incident on such a waveguide type optical filter is shown as follows.

[0015]

[Equation 1]

Here, Θ = π (λ-λ ₀ ) / {(n ₀
+ Δn / 2) · Λ ² } κ = π · Δn / 2λ Based on this equation, FIG. 4 shows the relationship between the reflectance R of the waveguide type optical filter having a single grating and the wavelength λ of light, that is, the reflection. It is the graph which showed the spectrum. As shown in this figure, in the reflection spectrum of a waveguide type optical filter having a single grating, a main reflection band showing high reflectance appears near the center wavelength λ ₀ , and sub-reflection bands having low reflectance appear on both sides of the main reflection band. Occurs. The sub-reflection bands will be referred to as the first, second, ... The center wavelength λ
₀ is the wavelength at the center of the full width at half maximum of the main reflection band.

As described above, the waveguide type optical filter having a single grating reflects light with a relatively high reflectance over a certain wavelength width. However, there is a limit to the size of this wavelength width, and it is difficult to sufficiently cover the deviation of the center wavelength of the light output by a light source that is normally used.

The present invention has been made to solve the above problems, and an object of the present invention is to provide a method of manufacturing a waveguide type optical filter capable of sufficiently reflecting light over a wavelength range wider than conventional ones. I am trying.

[0019]

In order to solve the above problems, a method of manufacturing a waveguide type optical filter according to the present invention comprises:
(A) A first step of preparing a mask in which a plurality of phase gratings are sequentially formed in series, and installing an optical waveguide in such a manner that the direction of formation of the plurality of phase gratings coincides with the light propagation direction, and (b) a mask A second step of forming a plurality of gratings whose refractive index is periodically changed along the optical axis by irradiating the optical waveguide with interference ultraviolet light through the optical waveguide. .

Here, the gratings may be characterized in that the periods of change in refractive index are different from each other, and the main reflection bands of their reflection spectra are overlapped with each other.

[0021]

In the method of manufacturing the waveguide type optical filter of the present invention,
First, a mask in which a plurality of phase gratings are sequentially formed is prepared, and an optical waveguide is installed so that the forming direction of the plurality of phase gratings coincides with the light propagation direction. Then, through the mask,
The optical waveguide is irradiated with interference ultraviolet light to form a plurality of gratings whose refractive index changes periodically along the optical axis.

Here, the formed gratings may have a core having a grating in which the periods of change in the refractive index are different from each other and the main reflection bands of the reflection spectrum overlap each other.

As is clear from the relationship of λ ₀ = 2 · n ₀ · Λ, if the period Λ of the change in the refractive index of the grating is different, the center wavelength λ ₀ is also different. The main reflection band has a wavelength width Δλ, and when the main reflection bands overlap with each other with gratings having different center wavelengths λ ₀ , the main reflection bands are overlapped with each other or the main reflection band and the sub reflection band overlap each other. The light reflected by the grating is accumulated to increase the reflected light intensity. Therefore, by appropriately setting the central wavelength λ _{0 of} each grating and the wavelength width Δλ of the main reflection band, the waveguide type optical filter of the present invention has a reflection band with high reflectance over a wide wavelength width. .

[0024]

Embodiments of the method for producing a waveguide type optical filter according to the present invention will be described in detail below with reference to the accompanying drawings.
In the description of the drawings, the same elements will be denoted by the same reference symbols, without redundant description.

FIG. 1 is an explanatory view of a method of manufacturing the waveguide type optical filter of this embodiment. In this embodiment, first, as shown in FIG. 1, a silica-based single mode optical fiber 10 is prepared as a starting material. The optical fiber 10 includes a core 11 made of quartz (SiO ₂ ) glass doped with germanium oxide (GeO ₂ ) and a clad 12 made of pure quartz glass. By doping germanium oxide,
The refractive index of the core 11 is higher than that of the cladding 12.

Next, the lattice spacings are 993 nm and 994n.
m, 995 nm, 996 nm, 997 nm, and 99
Phase gratings 101 to 1 each having a length of 8 mm and a length of 2 mm
Prepare a mask 60 in which 06 are arranged at 0.1 mm intervals,
As shown in FIG. 1, the optical fiber 10 is installed adjacent to the mask 60, and the ultraviolet light emitted from the light source 30 is made incident from the direction normal to the surface of the phase grating 60.

The light source 30 is an SHG argon laser or Kr.
Equipped with F excimer lasers, which emit coherent ultraviolet light having a predetermined wavelength. The phase grating 60 is formed by arranging gratings at a predetermined cycle. Optical fiber 10
Is a clad portion 11 and a core portion 1 made of silica glass.
It is composed of two. The core portion 12 is doped with germanium oxide as described above, and has a higher refractive index than the cladding portion 11.

According to such a process, the optical fiber 10
Since it is irradiated with ultraviolet light of a predetermined wavelength, the refractive index of the exposed region in the core portion 12 doped with germanium oxide changes.

Further, ultraviolet light is emitted from the normal direction of the surface of the phase grating 60 in which the gratings are arranged at a predetermined interval Λ ', and the 1st-order diffracted light and the -1st diffraction order are caused to interfere with each other. for that reason,
The interval Λ of the interference fringes in the exposure area of the core portion 12 is Λ = Λ ′ / 2 (1). Therefore, in the exposed region of the core portion 12, regions having different refractive indexes are arranged in the axial direction of the optical fiber 10 with the interval Λ of the interference fringes as a period, so that the grating 13 is formed.

Using the refractive index n of the core portion 12 and the period Λ of the grating 13 based on the well-known Bragg diffraction condition, the reflection wavelength λ _R of this fiber type diffraction grating is λ _R = 2nΛ = nΛ '(2 ). Further, using the length L of the grating 13 and the refractive index difference Δn, the reflectance R of this fiber type diffraction grating is as shown in the above-mentioned formula (2). Therefore, the optical fiber 10
In the core portion 12, the grating 13 is formed with a large change in the refractive index of about 10 ⁻⁴ to 10 ^−3, so that the reflectance of the reflection wavelength λ _R reaches a value close to 100%.

According to such a phase grating method, the grating period can be freely selected by the ordinary lithography technique or chemical etching, so that a complicated shape can be realized.

Thus, the gratings 111 to 116 are simultaneously written in the core 11. The maximum change amount Δn of the refractive index in each grating is 0.01. The basic refractive index n ₀ of the core is 1.46, and the refractive index of the core in each grating is 1.46 to 1.47.
Changes periodically between. The center wavelength λ ₀ of each grating is from 1450 to 1457 nm in steps of 1.6 nm. In addition, the grating 111 is 1450 nm, 1
16 is 1457 nm. As mentioned above, λ ₀ =
2 · n ₀ · Λ, and the central wavelength λ _{0 in} each grating
It can be seen that the period Λ of the change in the refractive index is different for each grating from the fact that is different.

The waveguide type optical filter thus manufactured is
The reflectance is 99. in the band from 1450 nm to 1457 nm.
It was as good as 9%.

According to the knowledge of the present inventors, when the silica optical fiber is subjected to hydrogen treatment and hydrogen is added to the core, the amount of change in the refractive index of the core due to irradiation with ultraviolet light increases. In general, the reflectance of the grating increases as the maximum change in the refractive index due to ultraviolet light irradiation increases. For this reason, in this embodiment, the base material fiber is subjected to hydrogen treatment prior to irradiation with ultraviolet light. This hydrogen treatment is performed as follows.

That is, as shown in FIG.
The preform fiber 10 is installed therein, the valve 21 is opened, the valve 22 is closed, and hydrogen (H ₂ ) gas is introduced, so that the pressure in the core tube 20 is set to 200 atm. At this time, the injection amount of hydrogen gas is adjusted by opening and closing the valve 21. The base material fiber 10 is left under a pressure of 200 atm for one week (24 × 7 hours), and the base material fiber 10 is subjected to a pressure treatment. The above pressure is the pressure inside the core tube 20, which is measured by the pressure gauge attached to the core tube 20. In addition, since we do not heat the hydrogen atmosphere,
The temperature inside the core tube 20 remains at room temperature.

By such pressure treatment, the base material fiber 1
About 32000 ppm of hydrogen is added to the inside of 0, which increases the amount of change in the refractive index of the core due to irradiation with ultraviolet light.
Along with this, the propagation loss of light near the wavelength of 1400 nm increases, but this can be reduced by removing excess hydrogen from the waveguide type optical filter after irradiation with ultraviolet light described later. The hydrogen may be removed by leaving the waveguide type optical filter. The amount of change in the refractive index of the core does not change due to the removal of hydrogen.

According to the knowledge of the present inventors, the maximum reflectance is 5
In order to form a grating of 0% or more, it is desirable to add 1000 ppm or more of hydrogen to the optical fiber. Furthermore, 2000ppm for maximum reflectance of 90% or more
As described above, in order to achieve 99.9%, it is desirable to add 3000 ppm or more.

Next, the base material fiber that has been subjected to the above hydrogenation treatment is irradiated with ultraviolet light, and the gratings 111 to 116 are written in the core. In this example, in order to manufacture a grating with a constant period, ultraviolet light was irradiated while generating interference fringes at equal intervals.

In order to obtain a waveguide type optical filter having a reflection band that maintains a high reflectance over a wide wavelength width, as in the method of manufacturing the waveguide type optical filter of the embodiment,
It is advisable to adjust the characteristic parameters of each grating formed in the core. Hereinafter, this will be described in detail.

In the reflection spectrum by the optical filter having a single grating, the wavelength at the center of the half-value width of the main reflection band, that is, the center wavelength λ ₀ is expressed by the following equation.

Λ ₀ = 2 · n ₀ · Λ (3) where n ₀ : basic refractive index of core of waveguide type optical filter Λ: period of change of refractive index of core in grating Clear from equation (3) Thus, the center wavelength λ ₀ is different when the period Λ is different.

Next, the maximum reflectance R of the optical filter having a single grating is expressed by the following equation.

R = tanh ² (πLΔn / λ ₀ ) (4) where L: grating length, Δn: maximum change in refractive index of core Further, the wavelength width Δλ of the main reflection band is It is expressed as the following equation.

Δλ = (λ ₀ / n ₀ ) · {Δn ² + (λ ₀ / L) ² } ^1/2 (5) In order to reduce the number of gratings formed in the core of the base material waveguide, It is preferable that each grating has a main reflection band having a wide wavelength width Δλ. As shown in Expression (5), Δn may be increased and L may be decreased in order to increase the wavelength width Δλ. However, as shown in Expression (4), when L is decreased, the maximum reflectance R is increased. Get smaller.

Considering the above equations (3) to (5), what kind of grating is formed in the core of the base material waveguide will be concretely considered. As an example, 100
Consider the case of manufacturing a waveguide type optical filter having a maximum reflectance (R) of 99.9% or more over a wavelength width (Δλ _T ) of nm or more.

Δλ _T ≧ 100 nm, R ≧ 99.9% As shown in FIGS. 4 and 5, the main reflection band has a very high reflectance like the grating of the waveguide type optical filter of this embodiment. The shape is close to a rectangular shape, and the wavelength width Δλ of the main reflection band and the wavelength width Δλ _T having a reflectance R of 99.9% or more are substantially equal. From this, Δλ _T = N · Δ
Considering λ, Δλ ≧ 1 nm is required.

On the other hand, the central wavelength λ of one grating
_{If 0} is 1500 nm, then R.gtoreq.99.9% and the above formula (4), L..DELTA.n.ltoreq.2.0.times.10.sup.- ³ mm is required. At this time, the basic refractive index n ₀ = 1.
If it is 46, since Δλ ≦ Δn × 1.28 × 10 ⁻³ mm Δλ ≧ 1 nm from the above formula (3), Δn ≧ 7.8 × 10 ⁻⁴ is desirable. This and L · Δn ≦ 2.0 × 1
L ≧ 2.6 mm is more preferable than 0 ⁻³ mm. In this case, when the number of periods of the grating is m, m ≧ 5000 is desirable from m = L / Λ and the above equation (3).

The same consideration can be applied to other gratings to understand how to form a grating in the core of the base material waveguide. By forming a grating in the base material waveguide in consideration of the results of such consideration, it is possible to easily fabricate a waveguide type optical filter that maintains a desired reflectance over a desired wavelength width around a desired wavelength. You can

[0049]

As described above in detail, in the method of manufacturing the waveguide type optical filter of the present invention, the interference ultraviolet light is irradiated through the mask in which the plurality of phase gratings are formed, and the plurality of diffraction gratings are irradiated. Since writing is performed at the same time, it is possible to efficiently manufacture a waveguide type optical filter having a wide reflection band and a good reflectance.

[Brief description of drawings]

FIG. 1 is an explanatory diagram of a method of manufacturing a waveguide type optical filter according to an embodiment of the present invention.

FIG. 2 is a diagram showing a hydrogenation treatment of the base material fiber 10.

FIG. 3 is a graph showing the refractive index of the core in the grating for the waveguide type optical filter having a single grating in the core.

FIG. 4 is a graph showing a reflection spectrum of a waveguide type optical filter having a single grating.

[Explanation of symbols]

1 ... Core, 2 ... Clad, 10 ... Base material fiber, 20 ...
Core tube, 21, 22 ... Bulb, 30 ... Ultraviolet light source, 60
... Mask, 101-106 ... Phase grating, 111-116
… Grating.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inoue Ryo Inoue, Sakae-ku, Yokohama-shi, Kanagawa 1 Taya-cho, Sumitomo Electric Industries, Ltd. (72) Inventor Toru Iwashima 1-tani, Sakae-ku, Yokohama, Kanagawa Sumitomo Electric Ki Industry Co., Ltd. Yokohama Works

Claims (2)

[Claims] 1. A first step of preparing a mask in which a plurality of phase gratings are sequentially formed in series, and installing an optical waveguide by aligning the optical axis with the forming direction of the plurality of phase gratings, A second step of irradiating the optical waveguide with interference ultraviolet light through a mask to simultaneously form a plurality of gratings whose refractive index is periodically changed along the optical axis. A method of manufacturing a characteristic waveguide type optical filter. 2. The method for producing a waveguide type optical filter according to claim 1, wherein the plurality of the gratings have different refractive index change periods from each other, and main reflection bands of their reflection spectra are overlapped with each other. .