JP2001324634A - Method for manufacturing optical waveguide having grating - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for forming a grating in an optical waveguide which is three-dimensionally formed in the inside of glass by condensing a laser beam having high peak output in the inside of glass. SOLUTION: In a method for manufacturing the optical waveguide wherein the laser beam having an energy quantity causing an optically induced change in refractive index is condensed in the inside of a glass material and the light condensing point is relatively moved along a prescribed path in the inside of the glass material to form a core in the inside of the glass material, a periodic refractive index modulation region is formed in the length direction of the core by periodically changing at least one of the intensity of the laser beam, a luminous flux diameter of the laser beam at the light condensing point and the speed of the relative movement.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical waveguide on which a grating is formed, and in particular, the optical waveguide is manufactured by continuously forming a refractive index change region inside a glass material by laser irradiation. Also, the present invention relates to an optical waveguide with a grating.

[0002]

2. Description of the Related Art An optical waveguide based on glass is formed by an ion exchange method, a flame hydrolysis method or the like.

In the ion exchange method, Ag ⁺ , Tl ⁺ , and K ^{+ are} formed through a slit-shaped opening such as a metal film provided on the surface of a glass substrate.
Alternatively, a molten salt containing Li ⁺ ions is brought into contact with the surface layer of the glass substrate so that the ions such as Ag ^{+ and} the Na ⁺
It is possible to form an optical waveguide by exchanging ions with ions to form a refractive index change region having a high concentration of the above-mentioned ions such as Ag ^{+ on the} surface layer of the glass substrate, for example, J. Lightwave Tech. Vol.
583 (1998). In order to bury the refractive index change region of this optical waveguide in glass to form a core,
The glass substrate on which the above-mentioned refractive index change region is formed is heated, and Ag ⁺ , Tl ⁺ , K ⁺ or Li ^{+ of the} surface layer of the glass substrate is heated.
The ions are diffused and moved toward the inside of the glass, or are immersed again in a molten salt containing Na ⁺ ions, and Ag ⁺ , Tl ⁺ , K ^+, or Li ⁺ ions near the glass surface side and Na
⁺ Re-exchange the ions. There is also a method of applying an electric field during this reion exchange. Na ⁺ ions are Ag ⁺ , Tl ⁺ ,
The high refractive index region on the outermost surface formed by K ⁺ or Li ⁺ ions is moved below the surface. As a result, the core is embedded under the glass surface, and low propagation loss is ensured. The core of the optical waveguide manufactured by this method often has a semicircular or substantially circular cross section with a diameter of 10 to 200 μm. In the ion exchange method, since the refractive index distribution is adjusted by ion exchange, there is a problem that the formed optical waveguide structure is limited to a portion near the glass surface.

In the flame hydrolysis method, two glass fine particle layers for lower cladding and core are deposited on the surface of a silicon substrate by flame hydrolysis of silicon tetrachloride and germanium tetrachloride, and the fine particle layer is transparent by heating at a high temperature. Modifies into a glass layer. Then, forming a core portion having a circuit pattern by photolithography and reactive etching, for example, J. Lightwave Tech. Vol. 17 (5) 771 (1999)
It is described in. The optical waveguide manufactured by this method is:
The film thickness is as thin as several μm. In addition, the flame hydrolysis method is complicated in the method of manufacturing an optical waveguide, and the usable material is limited to a glass composition containing quartz as a main component. Therefore, there is also a problem that it is difficult to manufacture an optical waveguide having a circular cross section.

Further, in the ion exchange method and the flame hydrolysis method, although optical waveguides having various two-dimensional patterns can be formed on the same substrate, it is difficult to form optical waveguides three-dimensionally combined. .

As a method of forming a three-dimensional optical waveguide having a circular cross section in glass, for example, Japanese Patent Application Laid-Open No.
No. 31237, and "Photoinduced refractive index change inside glass by ultrashort pulse laser", Laser Research 26 (2) 150
154 (1998), there is a method of forming an optical waveguide by irradiating the inside of glass with a laser having a high peak output value. In this method, 10 ⁵ W
Laser light having a peak power intensity of / cm ² or more is condensed inside the glass, and the converging point is relatively moved to cause a structural change that causes a change in the refractive index inside the glass material. To form In this method, a three-dimensional optical waveguide can be easily produced by moving the laser condensing point three-dimensionally.

In recent years, a grating has been formed on an optical fiber or an optical waveguide as a multiplexing / demultiplexing filter for wavelength division multiplexing transmission. As an application, for example, in optical fiber, Optical Add / Drop Multiplexer is expected to be a key device in wavelength multiplex communication.
(J.Lightwave Tech.)
Vol.16 (2) 265 (1999)). This grating has good coupling with the optical line because a filter is formed directly on the optical fiber and optical waveguide, and has a large number of grating layers in the longitudinal direction, so there is a great degree of freedom in designing spectral characteristics. It has features. Thus, the inability to form a grating as an optical waveguide means that the use of the optical waveguide is extremely limited.

In order to form a grating in an optical waveguide produced by a flame hydrolysis method or another method, as described in, for example, JP-A-10-339821, a two-beam interference exposure method is applied to a core portion. Or the phase grating mask method, the core portion is doped by irradiating ultraviolet rays (GeO ₂ , P
By breaking the bond of ₂ O ₅ , TiO _2, etc.) to generate a bond defect, a grating whose refractive index changes periodically is formed in the core portion.

[0009]

However, in an optical waveguide manufactured by a method of forming an optical waveguide by irradiating a laser having a high peak output value to the inside of the glass, the glass composition in the core portion and the cladding portion is the same. Yes, selectively causing bonding defects only in the core part,
A periodic refractive index change cannot be made.

An object of the present invention is to provide a method of forming a grating in an optical waveguide formed three-dimensionally in glass by converging laser light having a high peak output inside the glass.

[0011]

The inventor of the present invention utilizes the fact that the diameter of an optical waveguide formed when the intensity of a laser beam is changed changes. It has been found that a grating can be formed in the wave path.

According to the present invention, a laser beam having an energy amount causing a photo-induced refractive index change is condensed inside a glass material, and the converging point is relatively moved along a predetermined path inside the glass material. In the method for manufacturing an optical waveguide forming a core inside a material, by periodically changing at least one of the intensity of the laser light, the luminous flux diameter of the laser light at the focal point, and the speed of the relative movement. And forming a periodic refractive index modulation region in the length direction of the core.

An optical waveguide with a grating according to the present invention comprises:
A laser beam having an energy amount that causes a photo-induced refractive index change is condensed inside the glass (the shape may be any of a flat plate, a sphere, and a lump) to cause a structural change that causes a change in the refractive index inside the glass material. The optical waveguide is manufactured by relatively moving the focal point, and the grating is formed by periodically changing the intensity of the laser light used to manufacture the optical waveguide at an arbitrary location. Can be easily manufactured.

Although the laser beam varies depending on the type of glass, it is desirable that the laser beam has a peak power intensity of 10 ⁵ W / cm ² or more at the converging point in order to cause a photoinduced refractive index change. The peak power intensity is calculated as "output energy per pulse (J)" / "pulse width (second)"
The peak output (W) expressed by the ratio is expressed per unit area of irradiation. If the peak power intensity is less than 10 ⁵ W / cm ² , no light-induced refractive index change occurs, and no optical waveguide is formed. The higher the peak power intensity, the more the light-induced refractive index change is promoted, and the easier the optical waveguide is formed. However, a very large amount of energy, for example 10 ⁵ W / cm ²
It is difficult to practically obtain the above continuous wave laser light. Therefore, it is preferable to use a pulse laser in which the peak output is increased by narrowing the pulse width.

The laser light is focused by a focusing device such as a lens. At this time, the focal point is adjusted so as to be located inside the glass material. By moving the converging point relatively inside the glass material, an elongated refractive index change region (core region having a high refractive index), which functions as an optical waveguide, is formed inside the glass material. Specifically, the glass material is continuously moved with respect to the laser light focal point, or the laser light focal point is continuously moved inside the glass material, thereby relatively moving the focal point. .

The grating is manufactured by periodically changing the intensity of the laser beam. When the intensity of the laser beam is changed, the diameter of the high refractive index core of the formed optical waveguide changes so as to increase or decrease in accordance with the magnitude of the intensity. Therefore, by periodically changing the intensity of the laser beam at a position within a predetermined range while performing the relative movement, a refractive index modulation region periodically formed in the length direction of the core, that is, a change in the diameter of the optical waveguide. Is formed, and by controlling it, a grating can be formed in the optical waveguide.

The optical waveguide grating is described as follows. The Z axis is set in the length direction of the optical waveguide, the grating portion is set to 0 ≦ z ≦ L, and the refractive index n of the core is set at this portion.
(z) is the following formula (1),

## EQU1 ## It is assumed that n (z) = n ₀ + Δn · cos (2πz / Λ) (1) Here, n ₀ is the average refractive index of the core of the grating portion, Δn is the amount of change in the refractive index, and Λ is the grating period. Such a grating is given by the following equation (2):

Λ _B = 2n ₀ ２ (2) Bragg wavelength λ _B (wavelength in vacuum)
And a filter that selectively reflects light in the vicinity thereof.

Assuming that the speed at which the optical waveguide is produced, that is, the moving speed of the laser converging point inside the glass, is v, the period of the grating should be Λ [= λ _B / (2n ₀ )]. Represents the intensity of the laser light by the following equation (3)
A period T represented by

T = Λ / v = λ _B / (2n ₀ v) (3)

As described above, it is desirable to use a pulse laser in order to increase the peak power of the laser beam. The lower the pulse repetition frequency of the laser, the easier it is to increase the peak power.However, if it is too slow, only a few pulses will be irradiated in one period of the Bragg diffraction grating.In this case, the change in the optical waveguide diameter is not smooth. It does not satisfy the Bragg diffraction grating. Considering that at least 100 pulses must be emitted per one period of the Bragg diffraction grating, the pulse repetition frequency H of the laser must be equal to or larger than the numerical value represented by the equation (4). The repetition frequency H is usually 10 kHz or more.

H = 100 · v / Λ (4)

The periodic change in the intensity of the laser beam may change the output of the laser itself. However, in such a case, the oscillation of the laser becomes unstable. Externally changing the laser light periodically can be achieved by installing a device for periodically changing the intensity in the middle of the path of the laser light. As a device for changing the beam periodically, a device that periodically switches ND filters having different densities, a device that changes the beam diameter by periodically changing the diameter of the stop, and the like can be considered. Another method for periodically changing the intensity outside the laser oscillation device is as follows. First, the laser light is divided into two or more in the middle of the path, and an optical system is constructed so that they merge. A switch for blocking light in each path is installed in each of the two or more sections. A grating is produced by periodically changing the intensity of the laser light by adjusting the combination of the laser light to be combined with a switch.

The method of forming a grating by using a Bragg diffraction grating has been described above. In addition, the present invention is also applicable to a method of forming a grating by using a long-period diffraction grating utilizing coupling of a traveling direction of light into a cladding mode. Can be.

In the above description, the case where the optical waveguide with the grating is manufactured by periodically changing the intensity of the laser beam to form a periodic refractive index modulation region in the length direction of the core, Even if the laser beam diameter at the focal point is changed periodically instead of the periodic change in light intensity, an optical waveguide with grating is manufactured by forming a periodic refractive index modulation region in the length direction of the core. can do. The diameter of the laser beam at the focal point can be determined by periodically changing the beam width of the laser beam incident on the condenser lens.
Similarly, by periodically changing the speed at which the focal point is relatively moved inside the glass material, a periodic refractive index modulation region can be formed in the length direction of the core.

[0023]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described more specifically with reference to examples, but the present invention is not limited to the following examples unless it exceeds the gist of the present invention. Example 1 The glass composition shown in Table 1 was 20 mm long,
As shown in FIG. 1, a pulse laser beam 2 was condensed by a lens 3 and irradiated on a rectangular parallelepiped sample 1 having a width of 20 mm and a thickness of 5 mm. The pulse laser light 2 has a pulse width of 15 oscillated from a Ti: Al ₂ O ₃ laser excited by an argon laser.
Laser light having 0 femtoseconds, a repetition frequency of 200 kHz, a wavelength of 800 nm, and an average output of 600 mW was used. Laser light adjusted to 450 mW intensity through the first ND filter,
The light is condensed by an objective lens having a NA of 0.3 and a magnification of 10 times, and is irradiated so as to form a converging point inside the sample 1. The sample 1 is moved from one end to the other end by 50 μm in the direction of arrow 5. An optical waveguide was produced while moving at a speed of / s.

[0024]

[Table 1] ％ mol% Example 1 Example 2 ---------------- SiO ₂ 70.6 37.5 B ₂ O ₃ 0 12.5 Al ₂ O ₃ 14.0 25.0 P ₂ O ₃ 0.70 Na ₂ O 1.6 25.0 Li ₂ O 9.3 0 MgO 1.0 0 TiO ₂ 1.6 0 ZrO ₂ 1.2 0 ─────────── ──────

A grating was formed over a central portion of 7 mm of the length of the core of the optical waveguide manufactured by the above method. The periodic change in the intensity of the laser light
This was performed by adjusting a second ND filter provided adjacent to the filter. This ND filter uses a rotating disk-shaped filter, and the light transmittance changes smoothly,
The maximum transmission was set to increase 10% of the minimum transmission, and the change was repeated 5 times per revolution. As long as the focal point of the laser light moves relatively to the center 7 mm of the optical waveguide, the intensity of the laser light is changed by rotating the disk-shaped ND filter at 24 revolutions / second (change). Cycle T = 0.00833 seconds),
At positions other than 7 mm, the rotation of the disk-shaped ND filter was stopped, and the laser light was adjusted to pass through the lowest transmittance portion.

This optical waveguide with a grating was evaluated by the following method. The light of the LED having a central oscillation wavelength of 1.3 μm is guided to the end face of the optical waveguide formed by the single mode optical fiber for 1.3 μm, and the alignment is performed so that the intensity of the light emitted from the other end of the optical waveguide becomes maximum. The emitted light was guided to an optical spectrum analyzer through a 1.3 μm single mode optical fiber, and the transmission spectrum was measured. The transmission spectrum of the optical waveguide in which no grating was formed was measured by the same method, and the difference was evaluated to evaluate the effect of the grating. FIG. 2 shows a transmission spectrum measured for the optical waveguide on which the grating is formed. As shown in this figure, the transmittance is reduced by about 23 dB at a wavelength of 1298 nm, and it is understood that selective reflection with a half-width of about 1.5 nm centered at 1298 nm occurs. The transmission spectrum of the optical waveguide in which no grating was formed was 1298 as shown in FIG.
No sharp decrease in transmittance centered at nm was observed.

Example 2 A sample having the glass composition shown in Table 1 and having a rectangular parallelepiped shape of 20 mm × 20 mm × 5 mm was prepared by using FIG.
As shown in the figure, the pulse laser beam 2 was condensed by the lens 3 and irradiated. The pulse laser beam 2 has a pulse width of 15 oscillated from an argon laser-excited Ti: Al ₂ O ₃ laser.
Laser light having 0 femtoseconds, a repetition frequency of 200 kHz, a wavelength of 800 nm, and an average output of 600 mW was used. Laser intensity is 4
After passing through the ND filter and adjusting to 70 mW, the beam is divided into two paths so that the intensity ratio becomes 1: 9 on the way, and a switch that can cut off the laser beam is installed in the middle of the weak path. did. The optical system is assembled so that the two split beams join, and the combined laser beam is assigned to NA0.
The light was condensed by a 10 × objective lens of 3 and irradiated so as to form a converging point inside the sample 1, and an optical waveguide was produced while moving the sample 1 at a speed of 50 μm / s.

A grating having a thickness of 7 mm was formed almost at the center of the optical waveguide produced by the above method. The periodic change in the intensity of the laser light was performed by controlling a switch provided in the middle. When irradiating a laser beam to form an optical waveguide core other than the central portion of 7 mm, the laser beam is cut off by turning off the switch, and the switch is turned on at a frequency of 116 times / second at the grating portion of the central portion of 7 mm.
The grating is formed by adjusting the laser intensity so that the weaker pulsed laser beam is transmitted for about half of 1/116 second and cut off for about half of the next 1/1116 second. I let it.

This optical waveguide with a grating was evaluated by the following method. The light of the LED having a central oscillation wavelength of 1.3 μm is guided to the end face of the optical waveguide formed by the single mode optical fiber for 1.3 μm, and the alignment is performed so that the intensity of the light emitted from the other end of the optical waveguide becomes maximum. The emitted light was guided to an optical spectrum analyzer through a 1.3 μm single mode optical fiber, and the transmission spectrum was measured. The transmission spectrum of the optical waveguide in which no grating was formed was measured by the same method, and the difference was evaluated to evaluate the effect of the grating. FIG. 3 shows a transmission spectrum measured for the optical waveguide on which the grating is formed. As shown in this figure, the transmittance is reduced by about 20 dB at the wavelength of 1302 nm, which indicates that selective reflection with a half value width of about 1.1 nm centered on 1302 nm occurs. The transmission spectrum of the optical waveguide without the grating was 1302 as shown in FIG.
No sharp decrease in transmittance centered at nm was observed.

[0030]

As described above, according to the present invention,
By condensing laser light having an amount of energy that causes a photoinduced refractive index change in glass and moving the condensing point relatively, the intensity of the laser light and other factors are periodically changed to provide a light guide. The waveguide core and grating can be made simultaneously.

[Brief description of the drawings]

FIG. 1A is a schematic layout view showing a method of manufacturing an optical waveguide with a grating of the present invention, and FIG. 1B is a perspective view showing an optical waveguide with a grating manufactured inside glass.

FIG. 2 is a transmission spectrum of an optical waveguide with a grating manufactured according to Example 1.

FIG. 3 is a transmission spectrum of an optical waveguide with a grating manufactured according to Example 2.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Glass sample 2 Pulsed laser beam 3 Focusing lens 4 Focusing point 5 Moving direction of glass sample 6 Optical waveguide

Claims (3)

[Claims] 1. A laser light having an energy amount causing a photo-induced refractive index change is condensed inside a glass material, and the converging point is relatively moved along a predetermined path inside the glass material to cause a change in the glass material. In the method of manufacturing an optical waveguide forming a core therein, by periodically changing at least one of the intensity of the laser light, the luminous flux diameter of the laser light at the focal point, and the speed of the relative movement, A method of manufacturing an optical waveguide with a grating, comprising forming a periodic refractive index modulation region in the length direction of a core. 2. The method according to claim 1, wherein the periodic change is such that the average value of the relative movement speed is v, the reflection wavelength of the grating is λ _B ,
If the average refractive index of the core and _{n 0, λ B / (2n} 0 v) the production method of the grating with optical waveguide according to claim 1, wherein performed in the period of. 3. The method according to claim 1, wherein the intensity of the laser beam is periodically changed, and the refractive index modulation region is a portion where the diameter of the core is periodically changed.