JP3426154B2 - Manufacturing method of optical waveguide with 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 for producing an optical waveguide having a refractive index modulation grating by means of focused irradiation.

[0002]

2. Description of the Related Art An optical waveguide is formed by providing a core having a high refractive index for confining light and a clad having a low refractive index surrounding the core on a substrate. Silicon, quartz glass, etc. are used for the substrate, Ge-doped quartz glass for the core glass, and quartz glass for the clad. When irradiated with ultraviolet rays, the Ge-doped quartz glass absorbs the ultraviolet rays, and the refractive index of the portion irradiated with the ultraviolet rays increases. A Bragg grating is attached by forming a region where the refractive index changes periodically in the core of the optical waveguide.
For exposure of ultraviolet rays, two-beam interference exposure method, phase mask method,
The point exposure method is used. When the grating is formed inside the optical waveguide, only the light satisfying the Bragg reflection condition is reflected, so that it can be used as a wavelength selection device in the optical communication field and the like. The cause of the change in the refractive index is not uniform, but it is considered that it is mainly due to the interatomic bond defects generated in the Ge-doped silica-based glass due to the ultraviolet irradiation. A KrF excimer laser (wavelength 248n is used as an ultraviolet light source for forming interatomic bond defects in glass.
m), the second harmonic of the Ar ion laser (wavelength 244n
m) etc.

[0003]

In the case of Ge-doped silica glass, since the photo-induced change in refractive index due to UV irradiation is small, UV irradiation is performed by high-pressure hydrogen treatment, high concentration of Ge, addition of a photosensitive substance such as Sn, and the like. The sensitivity of the change of the refractive index to is increased. However, in the high-pressure hydrogen treatment, the productivity decreases due to the increase in man-hours, and it becomes difficult to adjust the refractive index of the core cladding when the Ge concentration is increased. The method of adding a photosensitive substance may increase light loss. It is also a characteristic required for the optical waveguide that the change in refractive index induced by ultraviolet irradiation is stable for a long period of time. However, the change in the refractive index due to the interatomic bond defect in glass may be relaxed even at room temperature. Furthermore, when an attempt is made to cause a change in the refractive index of the optical fiber by converging and irradiating the pulsed laser light, the concentrating point differs from that when converging on a flat surface because the fiber periphery is circular, and When focusing on the fiber core, it is necessary to devise a way to match the focal point with the fiber core.

[0004]

SUMMARY OF THE INVENTION The present invention has been devised to solve such a problem, and focuses and irradiates the inside of an optical waveguide with pulsed laser light in a wavelength region having a specified absorption coefficient. Thus, it is an object of the present invention to increase the refractive index of a selected portion and provide an optical waveguide with a grating excellent in stability by a simple method. In order to achieve the object, the manufacturing method of the present invention adjusts the converging point inside the core of an optical waveguide in which a high refractive index core and a low refractive index clad layer are laminated, and the optical waveguide absorption coefficient: 5 cm ⁻ Less than ¹ wave
A long region and a peak power density of 10 ⁵ to 10 ¹⁵ W / cm ² are focused and irradiated, and the refractive index at the focusing point is selectively increased. When the focused irradiation of the pulsed laser light is repeated while the focusing point is relatively moved in the length direction inside the core, a plurality of refractive index changing portions can be formed in the length direction of the optical waveguide. The grating period can be adjusted by the relative moving distance of the focusing point. Further, when changing the peak power density of the pulsed laser light, it is possible to attach a grating having a different refractive index change at each condensing point. For pulsed laser light,
A pulsed laser beam having a pulse width of 10 ^-10 seconds or less is preferable.

[0005]

The optical waveguide formed on the substrate has a structure in which a core glass having a high refractive index for confining light is surrounded by a clad glass having a low refractive index. Silicon, quartz glass, etc. are used for the substrate, and quartz glass, Ge-doped quartz glass, for core glass and clad glass,
Fluoride glass, oxide glass, sulfide glass, chalcogenide glass, etc. are used. Examples of the oxide glass include silicate glass, borate glass, phosphate glass, fluorophosphate glass, and bismuth oxide-based glass. The glass used in the present invention is not subject to material restrictions as long as the refractive index changes due to the focused irradiation of pulsed laser light. The optical waveguide is a flame deposition method, a plasma CVD method,
It is formed on the substrate by a sputtering method or the like. In the present invention, the pulsed laser light is focused and irradiated on the optical waveguide having a flat surface shape formed on the substrate to induce the change in the refractive index. The converging point can be easily adjusted inside the core glass without requiring any special measures.

The core glass has its structure, and hence its refractive index, changed by the focused irradiation of pulsed laser light. In the present invention,
Utilizing the effect of pulsed laser light on the change in the refractive index, the optical waveguide is focused and irradiated by adjusting the focal point to be located inside the optical waveguide. The change in the refractive index due to the focused irradiation of the pulsed laser light occurs not only in the Ge-doped silica glass but also in other silica glass, fluoride glass, oxide glass, sulfide glass, chalcogenide glass and the like. Therefore, unlike conventional gratings that are irradiated with ultraviolet rays, the restrictions on the materials that can be used for grating are greatly relaxed.
Further, since the amount of change in the refractive index is large as compared with the irradiation with ultraviolet rays, it is possible to omit the sensitization treatment which tends to cause another problem. As the pulsed laser light, pulsed laser light having a wavelength region in which the absorption coefficient of the optical waveguide is 5 cm ^-1 or less, which has an energy amount that causes a photo-induced refractive index change inside the optical waveguide, is used. The absorption coefficient of the optical waveguide is obtained from the absorption coefficient of the glass material forming the core and the clad of the optical waveguide. Since the intrinsic absorption of the glass material that constitutes the optical waveguide is in the ultraviolet region, when setting the wavelength of the pulsed laser light to the long wavelength side of 5 cm ^-1 or less where there is no intrinsic absorption, the change in the refractive index is only at the focal point. Occurs, and changes in the refractive index at points other than the focal point are suppressed.

The amount of energy of pulsed laser light that causes a change in the refractive index inside the optical waveguide varies depending on the type of glass, but the peak power obtained by dividing the output energy (J) per pulse by the pulse width (second). It is preferable that the peak power density represented by the density (W / cm ² ) per unit area of (W) is in the range of 10 ⁵ to 10 ¹⁵ W / cm ^{2 at} the converging point. If the peak power density does not reach 10 ⁵ W / cm ² , the increase in the refractive index at the converging point is not sufficient even by converging irradiation. On the contrary, at a peak power density of more than 10 ¹⁵ W / cm ² , an excessive amount of energy is input, so that the refractive index may increase even in a portion other than the condensing point. The repetition cycle of the pulsed laser light is not particularly limited, but is preferably in the range of 1 to 250 kHz. 250 kHz
If the repetition period exceeds, it becomes difficult to control the amount of change in the refractive index at the focal point. On the contrary, if the repetition period is less than 1 Hz, it takes a long time to form the grating.

The pulsed laser light can be condensed by a condenser such as a lens, and the condensing point can be set inside the glass material. By moving the focusing point relatively, a refractive index modulation grating is formed inside the optical waveguide. In particular,
Move the optical waveguide to the condensing point of the pulsed laser light,
The focusing point of the pulsed laser light is moved inside the optical waveguide, or both the optical waveguide and the focusing point are moved to relatively move the focusing point. At this time, the period of the grating is adjusted by the relative movement amount of the focal point,
Gratings with different periods are easily written. Further, when the intensity and / or the number of pulses of the pulsed laser light is changed for each converging point by utilizing the dependence of the variation of the refractive index on the intensity of the pulsed laser light and the number of pulses, a grating with a different amount of refractive index change is written You can also

When a refractive index modulation grating is formed by inducing an increase in the refractive index by converging irradiation of pulsed laser light, a desired grating can be manufactured by simultaneously observing the transmission characteristics of the grating. For simultaneous observation, a method is adopted in which white light is made incident from one end of the optical waveguide during fabrication of the grating, and the transmission spectrum of the light is measured by an optical spectrum analyzer connected to the other end. The characteristic measured by the optical spectrum analyzer is the wavelength dependence of the intensity of the light transmitted through the refractive index increasing region. Therefore, when the wavelength dependence of the transmitted light intensity is monitored in the process of forming multiple refractive index increase regions by repeating focused irradiation of pulsed laser light and relative movement of the optical waveguide, the transmitted light intensity is centered at a certain wavelength. Begins to decay. Therefore, by stopping the focused irradiation of the pulsed laser light at the time when the target filter characteristic is exhibited, an optical waveguide with a grating having the required characteristic can be obtained.

[0010]

[Example] A grating was attached by converging and irradiating a silica-based optical waveguide prepared by the flame deposition method with pulsed laser light. The optical waveguide W has a quartz substrate S as shown in FIG.
The core layer C having a film thickness of 8 μm deposited thereon was dry-etched to form a waveguide pattern, and then embedded by the clad layer L. The core layer C 96.5 wt% SiO ₂ -3.5 wt% GeO ₂ glass, the cladding layer L using pure SiO _2. By observing the near field pattern at a wavelength of 1.55 μm, the core layer C
It was confirmed that the light propagating in was a single mode.
A Ti sapphire laser was used as the laser light source 1, and a pulsed laser light 2 having a wavelength range of 800 nm, a pulse width of 1.2 × 10 ⁻¹³ seconds, a repetition frequency of 200 kHz and an absorption coefficient of 5 cm ⁻¹ or less was emitted. The pulsed laser light 2 was condensed by the condenser lens 3 so that the condensing point F was located inside the optical waveguide. At the focusing point F, the peak power density of the pulsed laser beam 2 was increased to 3 × 10 ¹³ W / cm ² , and the refractive index increasing region G was formed at the portion corresponding to the focusing point F.

Next, the optical waveguide W was moved by the grating period. The pulsed laser light 2 was focused and irradiated on the moved optical waveguide W under the same conditions to form the next refractive index increasing region G. Hereinafter, focused irradiation of the pulsed laser light 2 and movement of the optical waveguide W were repeated in accordance with the required grating length to manufacture an optical waveguide with a grating in which a plurality of refractive index increasing regions G were formed. The degree of change in the refractive index depends on the intensity of the pulsed laser light 2 and the number of pulses. When the intensity of the pulsed laser light 2 or the number of pulses is increased, the amount of change in the refractive index increases. Under constant intensity conditions, the required change in refractive index can be caused by extending the irradiation time or increasing the repetition period. In the present embodiment, the optical waveguide W was exposed to the focused irradiation of the pulsed laser light 2 for 5 seconds for each refractive index increasing region G.

When the period of change in the refractive index is relatively long, such as several hundreds of μm, a part of the light propagating in the core layer C of the optical waveguide W changes from the waveguide mode propagating in the core layer C to the cladding layer L.
Coupled to the propagating cladding mode. When the effective refractive index for the guided mode is n _g , the effective refractive index for the cladding mode is n _cl , and the grating period is Λ,
Light of wavelength λ that satisfies the condition of λ = ( _{ng− ncl} ) Λ is coupled from the guided mode to the cladding mode. The wavelength λ is 1.
To obtain 3 to 1.6 μm, approximately 100 to 200
A grating period Λ of 0 μm is required. The light coupled to the clad mode has a relatively wide band of 10 to several tens nm and is attenuated by leaking out of the core layer C (AM Vengsarkar et al., J. Lightwave Technol.
ogy, Vol.14, 1996, p.58). As a result, the long-period grating functions as a filter that gives a loss to light in a relatively wide band near the wavelength λ. Therefore, in this embodiment,
The grating period was set to 460 μm and the grating length was set to 30 mm. When the wavelength dependency of the transmitted light intensity of the produced optical waveguide with a grating was investigated, the transmittance was gradually attenuated over a bandwidth of about 20 nm as shown in FIG. At the peak wavelength where the transmittance was attenuated, the loss was about 12 dB.

[0013]

As described above, in the present invention, the pulsed laser light in the wavelength region where the absorption coefficient of the optical waveguide is 5 cm ^-1 or less is focused and irradiated inside the optical waveguide by adjusting the focusing point. As a result, a region having an increased refractive index is formed at the converging point, and a refractive index modulation grating is attached to the optical waveguide. Compared to the conventional grating formation by UV irradiation, this method does not require steps such as high-pressure hydrogen treatment and addition of photosensitive ions, which tend to cause an increase in optical loss, so that a grating with the required grating characteristics can be obtained. Attached to the optical waveguide. Moreover, the amount of change in the refractive index is easily controlled according to the amount of energy input, and the period and length of the grating can be freely changed, so that an optical waveguide with a grating according to required characteristics can be obtained. Further, since the pulsed laser light is focused and irradiated on the optical waveguide having a flat surface as compared with the optical fiber, the irradiation work including the adjustment of the focusing point becomes easy.

[Brief description of drawings]

FIG. 1 is a diagram illustrating a method of manufacturing an optical waveguide with a grating according to the present invention.

FIG. 2 is a graph showing the wavelength dependence of the transmission intensity of the optical waveguide with a grating obtained in the example.

[Explanation of symbols]

1: Laser light source 2: Pulse laser light 3: Condensing lens W: Optical waveguide S: Quartz substrate C: Core layer L:
Clad layer F: Focus point G: Refractive index change region

─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Kazuyuki Hirao 3-5-8, Kizugawadai, Kizu-cho, Soraku-gun, Kyoto (56) Reference JP-A-5-307118 (JP, A) JP-A-8- 286015 (JP, A) JP 9-311237 (JP, A) JP 10-288799 (JP, A) British Patent Application Publication 2210470 (GB, A) Shima Kensuke et. al. Fujikura Technical Report, May 31, 1997, No. 92, pp. 11-13 Miura et. al. , Applied Physics Letters, December 8, 1997, Vol. 71 No. 23, pp. 3329-3331 Kiyoshi Miura et. al. , Laser Research, February 15, 1998, Vol. 26, No. 2, pp. 150-154 (58) Fields surveyed (Int.Cl. ⁷ , DB name) G02B 6/10-6/16 G02B 5/18

Claims (3)

(57) [Claims] [Claim 1] to adjust the core inside the focal point of the optical waveguide formed by stacking a cladding layer having a low refractive index to a high refractive index of the core, optical
Waveguide absorption coefficient: wavelength region below 5 cm ^-1 , peak power
-Density: 10 ⁵ to 10 ¹⁵ W / cm ² of pulsed laser light is condensed and irradiated to selectively increase the refractive index at the condensing point. 2. A core having a high refractive index and a cladding layer having a low refractive index
2. The method for producing an optical waveguide with a grating according to claim 1, wherein the focused irradiation of the pulsed laser light is repeated while intermittently moving the focusing point in the length direction inside the core of the laminated optical waveguide. 3. The method for producing an optical waveguide with a grating according to claim 1, wherein pulsed laser light having a pulse width of 10 ⁻¹⁰ seconds or less is used.