JPH0843650A - Optical transmission line and converting method for mode field diameter - Google Patents

Abstract

PURPOSE:To obtain an optical waveguide in which the mode field diameter can be changed by irradiation with light. CONSTITUTION:In the optical transmission line essentially comprising quartz glass and having a core and a clad, the clad contains GeO2, while the amt. of GeO2 in the core is smaller than that in the clad. By adding GeO2 to the clad, the refractive index of the clad can be increased by irradiation with light of 220 to 270nm wavelength and thereby, difference in specific refractive index between the core and the clad can be decreased. Therefore, an effect to increase the mode field diameter is obtd. without changing the core diameter.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a mode field diameter conversion type optical transmission line (optical waveguide, optical fiber, etc.) and a mode field diameter conversion method thereof.

[0002]

2. Description of the Related Art Optical waveguides and optical fibers having a locally increased mode field diameter are known.
In particular, the insertion structure of the optical component can be facilitated, which is effective in reducing the diffraction loss of the integrated optical component.

The following is known as a method for locally increasing the mode field diameter in an optical waveguide. First, the core pattern is thinned to reduce the light confinement effect and increase the mode field diameter. Secondly, the core pattern is thickened to increase the effective core diameter and increase the mode field diameter. For example, it was announced by Sasaoka et al. At the 1993 Autumn Meeting of the Institute of Electronics, Information and Communication Engineers (lattice pattern). Study on reduction of reflection loss in optical waveguide). The third is to heat a part of the optical waveguide on the waveguide substrate and diffuse the dopant for increasing the refractive index from the core to the clad to effectively increase the core diameter. For example, the Institute of Electronics, Information and Communication Engineers (1994). It was announced by Yanagisawa et al. At the spring meeting (C-341).

In the first method described above, since the refractive index of the core itself does not increase, it is difficult to significantly increase the mode field diameter. In the second method, due to the increase in the core width, a large number of modes are generated in the propagating light and loss is likely to occur. In the third method, the above-mentioned drawbacks can be overcome, but high temperature (120
0-1400 ° C.) treatment is required.

On the other hand, there has been proposed a method for locally increasing the mode field diameter of an optical fiber. For example, in the drawing method, the optical fiber is locally heated and drawn, and the mode field diameter is increased by reducing the core diameter. Further, in the dopant diffusion method, the optical fiber is locally heated, the dopant for increasing the refractive index of the core is diffused, and the mode field diameter is increased.

[0006]

However, any of the above-mentioned prior arts basically increases or decreases the core diameter to increase the mode field diameter, and the refractive index itself of the core is locally increased. It is not a method to raise

On the other hand, Japanese Laid-Open Patent Publication No. 6-51147 discloses a technique for locally increasing the mode field diameter by relatively increasing the refractive index difference between the core and the clad in the mode field diameter conversion type optical fiber. It is shown. That is, focusing on the fact that the refractive index changes due to the residual stress in the optical fiber, the residual stress during drawing is released by local heating, and the mode field diameter is increased.

The present invention has been made in view of the above-mentioned conventional technique, and uses a mechanism different from the refractive index conversion mechanism disclosed in Japanese Patent Laid-Open No. 6-511147 to transmit light such as an optical waveguide or an optical fiber. The purpose is to enable mode field diameter conversion of a line.

[0009]

In the silica type optical transmission line, GeO ₂ has been widely used as a dopant for increasing the refractive index of the core. This G
For eO ₂ , a wavelength of 220-270 nm, especially 24
"Technical note (fabrication and evaluation of fiber type optical components) diffraction grating" by water waves, for example, can change the bonding state by irradiating light of 0 to 260 nm and further increase the refractive index.
Applied Physics Vol. 62 No. 1 (1993) P. Known as 53-54 and the like. Here, if the refractive index of the clad is increased, the difference in the refractive index between the core and the clad can be reduced, and the light confinement effect can be reduced to increase the mode field diameter.

Therefore, the present inventor has made it possible to increase the mode field diameter by making the cladding contain GeO ₂ which has been conventionally added to the core and irradiating with light of the above wavelength. Accordingly, the present invention is to provide an optical transmission line having a core and a clad mainly composed of quartz glass, the cladding containing GeO _2, GeO ₂ content of the core is equal to or less than that of the cladding. here,
The GeO ₂ content of the core may be less than 10% of the cladding, or the core may be characterized by the absence of GeO ₂ .

In the present invention, since the mode field diameter is converted by locally reducing the relative refractive index difference, the core diameter itself does not change, and therefore the design is easy. In addition, since no multimode region is generated, increase in loss is unlikely to occur.

In order for this to function as an optical transmission line while containing a large amount of GeO ₂ in the cladding, the cladding contains a large amount of dopant for decreasing the refractive index, or the core contains a large amount of dopant for increasing the refractive index. By including it, it is necessary to compensate the increase in the refractive index due to GeO ₂ . Such a dopant does not change its refractive index with light having a wavelength that causes a change in the bonding state of GeO ₂ .
Or, there is little change. Specifically, the clad is characterized by containing B ₂ O ₃ or F as a dopant for lowering the refractive index. Further, the core is made of P ₂ O ₅ , Al ₂ O ₃ , T as a dopant for increasing the refractive index.
It may be characterized by containing iO ₂ or Si ₃ N ₄ .

Here, the optical transmission line may be an optical plane waveguide formed by depositing a glass film containing silica glass as a main component on a substrate, and a glass base material containing silica glass as a main component. It may be characterized in that the optical fiber is formed by drawing. In particular, in the case of an optical waveguide formed as a glass film on a silicon substrate or a quartz glass substrate, the mode field diameter can be converted simply by irradiating the spot light of the above wavelength, which is highly useful.

The mode-field-diameter conversion type optical transmission line according to the present invention has a core containing silica glass as a main component and a cladding containing silica glass as a main component and containing more GeO ₂ than the core. Then, the clad in at least a part of its longitudinal direction is irradiated with light having a wavelength of 220 to 270 nm to form Ge in the clad.
It is characterized in that the bonding state of O ₂ is changed, but in the production of such a device, it is desirable to expose it to a hydrogen atmosphere prior to light irradiation.

Hydrogen penetrating into quartz glass forms a photoreactive defect and promotes a change in refractive index due to light irradiation. As the treatment in a hydrogen atmosphere, any method of high temperature and short time treatment or low temperature and long time treatment may be used. However, when the treatment is carried out at a temperature higher than the melting temperature of the glass for a waveguide, the glass is deformed. Therefore, the maximum treatment temperature should be determined according to the glass composition. By the way, in a hydrogen atmosphere at atmospheric pressure, it is left at 500 ° C. for 1 hour or more and then irradiated with ultraviolet rays, or in a hydrogen atmosphere at 10 atmospheres or more, at 100 ° C. for 2 hours.
It was found that the mode field diameter can be effectively converted by irradiating with ultraviolet rays after leaving it for 4 hours or more.

According to the present invention, a large amount of GeO ₂ is doped not in the core but in the clad, which is a completely different means from the conventional concept of the optical transmission line. Since the mode field diameter can be effectively changed, it has less influence on the substrate and the like and is excellent in mass productivity as compared with the case of using the diffusion of the additive. Also, unlike the case of creating a Bragg diffraction grating, the ultraviolet light irradiated here does not have to be coherent light, so only the amount of light needs to be controlled. This uses an excimer laser, which is a laser with poor coherence. Suitable for

[0017]

EXAMPLES Examples of the present invention will be described in detail below.

The present invention relates to a technique for converting a mode field diameter of an optical transmission line. For such an optical transmission line, an optical fiber is known in addition to an optical waveguide. In order to realize the present invention in an optical fiber, first, the cladding portion contains GeO ₂ and the core portion does not contain GeO ₂ (or a content smaller than that of the cladding portion),
A base material for a silica-based optical fiber is manufactured in which the cladding has a lower refractive index than the core. Such a base material is obtained by heating a porous glass base material prepared by a known external attachment, internal attachment, VAD method or the like into a transparent glass by heating.

Here, in order to compensate the increase in the refractive index of the cladding portion due to GeO ₂ and to make the cladding portion have a lower refractive index than the core portion, a dopant for lowering the refractive index is added to the cladding portion, or the core portion is added. Is added with a dopant for increasing the refractive index. As a dopant for lowering the refractive index,
As the refractive index does not change (or hardly changes) with light having a wavelength of 220 to 270 nm, F (fluorine) or B
₂ O ₃ (boron oxide) is suitable. For the same reason, as the dopant for increasing the refractive index, P ₂ O ₅ (phosphorus oxide),
Al ₂ O ₃ (aluminum oxide), TiO ₂ (titanium oxide), Si ₃ N ₄ (silicon nitride) and the like are suitable.

This glass base material is formed into an optical fiber by drawing and is locally irradiated with light to locally increase the mode field diameter. If cut at this portion, the mode field diameter is increased at the end portion, so that the connection with the optical component can be suitably made. Note that the mode field diameter may be increased by irradiating the end portion with light after cutting into a certain length. Further, in order to improve the conversion efficiency of the mode field diameter, the treatment may be performed in a hydrogen atmosphere.

Next, an embodiment of the present invention relating to an optical waveguide will be described.

The optical waveguide is composed of a quartz glass film formed on the substrate, but a quartz glass plate other than a silicon single crystal plate can be used as the substrate. In manufacturing, first, a porous glass layer to be the lower clad layer is formed on the substrate by, for example, a flame deposition method (FHD method). Then, after being vitrified into a transparent glass, a porous glass layer to serve as a core is deposited and patterned by a photolithography technique in accordance with the pattern of the optical waveguide. Next, a porous glass layer to be an upper clad layer is deposited and transparentized into vitrification. Here, when depositing the porous glass layer for cladding, GeO ₂ glass fine particles are simultaneously mixed with the dopant for lowering the refractive index.

After the waveguide substrate is formed in a wafer state as described above, the optical waveguide chip cut into a predetermined shape is locally irradiated with light. This light irradiation site is
It is a part to which other optical components are mainly connected, and the mode field diameter is increased. Note that light irradiation may be performed in a wafer state and then divided into chips, or a waveguide may be formed in a chip state and light irradiation may be performed.

Specific examples 1 and 2 of the present invention will be described below.

Example 1 A glass layer having a core-clad structure was formed on a silicon substrate by a flame deposition method (FHD method), and a core diameter was 6.5 μm and a relative refractive index difference was 0 by a reactive ion etching (RIE) method. ．
A 3% buried linear waveguide was fabricated. The core composition is SiO ₂ containing 10 wt% P ₂ O ₅ , and the clad composition is 10 wt% GeO ₂ and 17 wt% B ₂ O ₃ .
% SiO ₂ added.

This waveguide was irradiated with KrF excimer laser light (248 nm) at a irradiation intensity of 500 mJ / cm ² for 20 minutes as a Gaussian beam having a diameter of 2 mm. The refractive index distribution of the end face of the waveguide before and after irradiation is shown in FIG. In the figure, "A" is a silicon substrate, "B" is a lower clad, "C" is a core, and "D" is an upper clad. It was confirmed that the refractive index of the clad portion was increased by 0.14%. 1.3μ from the opposite end of the irradiation part of this waveguide
m LD light (laser diode output light) is incident,
Observation of the radiation field from the end face was performed with an infrared CCD camera. It was confirmed that the diameter was 10 μm before irradiation, but increased to 17 μm after irradiation.

Example 2 A glass layer having a core-clad structure was formed on a silicon substrate by a flame deposition method (FHD method), and a core diameter was 7.0 μm and a relative refractive index difference was 0 by a reactive ion etching (RIE) method. ．
A 25% buried linear waveguide was fabricated. The core composition is SiO ₂ containing 10 wt% P ₂ O ₅ , and the clad composition is 10 wt% GeO ₂ and 12 w B ₂ O ₃ .
It consists of SiO ₂ with t% added.

This waveguide was irradiated with KrF excimer laser light (248 nm) for 20 minutes with a Gaussian beam having a diameter of 3 mm and an irradiation intensity of 500 mJ / cm ² . The refractive index distribution of the end face of the waveguide before and after irradiation is shown in FIG. In addition,
Areas on the horizontal axis shown in "A" to "D" are the same as in FIG.
It was confirmed that the refractive index of the clad portion was increased by 0.13%. As a result, the relative refractive index difference between the core and the clad becomes 0.12%. The mode field diameter of this waveguide at a wavelength of 1.3 μm was 24 μm when measured by the same method as in Example 1.

After fixing single mode fibers to both ends of the waveguide (25 mm long) of this Example 2 with UV adhesive,
KrF excimer laser light (248 nm) is placed in the middle part.
Was irradiated as a Gaussian beam having a diameter of 3 mm for 20 minutes at an irradiation intensity of 500 mJ / cm ² . A groove having a width of 15 to 220 μm was formed in the center of the irradiated portion, and a refractive index matching agent was injected into the groove, and then the loss was measured. As a comparative example, the same measurement was performed using a sample that was not irradiated with excimer laser.

FIG. 3 shows the relationship between the gap distance and the loss.
In the sample (Example 1) in which the refractive index change was caused by light irradiation, only a 0.4 dB increase in loss was observed even with a gap of 220 μm, but the untreated product (Comparative Example) was 4.
It was confirmed that the loss was 5 dB, and the loss due to the gap was significantly reduced by the increase of the mode field diameter according to the present invention. Such a waveguide can be applied to reduction of insertion loss of a filter, reduction of loss due to displacement of a coupling position with a light emitting element, and the like.

Example 3 (optical fiber type) Pure SiO ₂ glass as core material and Si as clad material
An optical fiber preform was produced using O ₂ —GeO ₂ (5 wt%)-F (2.2 wt%). The clad material was SiO ₂ soot containing 5 wt% of GeO ₂ uniformly, and SiF ₄ was 50 vol. %, It was vitrified in an atmosphere containing it so as to have a refractive index reduced by 0.3% with respect to SiO ₂ by the relative refractive index difference. A hole was made in the center of the glass body for cladding with an ultrasonic hole puncher, the inner surface was etched, and a SiO ₂ glass rod was inserted and integrated to obtain an optical fiber preform. The refractive index distribution of this preform is shown in FIG. The diameter ratio of the core and the clad was set to 1:15. This preform was drawn to obtain an optical fiber having a diameter of 125 μm. The fiber loss was 0.5 dB / km (1.55 μm).

This fiber was impregnated with H ₂ under 100 atm of H ₂ for 1 week, then a part of the coating was removed, and KrF excimer laser light was irradiated on the fiber at an intensity of 500 mJ / cm ² for 20 minutes. did. When the mode field diameter was measured from the far field pattern at this tip, it was 1.55 μm.
In comparison with the untreated portion of 10.5 μm at 25 μm,
It was magnified about 2.5 times. Further, FIG. 5 shows the refractive index distribution of the fiber before and after the laser light irradiation. Although the initial refractive index difference was 0.3%, it was confirmed that the irradiation diameter was reduced to 0.12%. The refractive index distribution of the fiber was measured by the RNFP method.

[0033]

As described above in detail, according to the present invention, since the cladding contains GeO ₂ , the wavelength of 220
By irradiating light of ˜270 nm, the refractive index of the clad can be increased, whereby the relative refractive index difference between the core and the clad can be reduced. Therefore, the mode field diameter can be increased without changing the core diameter.

[Brief description of drawings]

FIG. 1 is a diagram showing a change in refractive index of Example 1.

FIG. 2 is a diagram showing a change in refractive index of Example 2.

FIG. 3 is a diagram comparing a loss change due to a gap between an example and a comparative example.

FIG. 4 is a diagram showing a refractive index distribution of the preform of Example 3.

FIG. 5 is a diagram showing changes in the refractive index of the fiber of Example 3;

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification number Reference number within the agency FI Technical display location G02B 6/13 6/16 (72) Inventor Maki 1 Taya-cho, Sakae-ku, Yokohama-shi, Kanagawa Sumitomo Electric Ki Industry Co., Ltd. Yokohama Works (72) Inventor Hiroo Kanamori 1 Tayacho, Sakae Ward, Yokohama City, Kanagawa Sumitomo Electric Ki Industries Co., Ltd. Yokohama Works

Claims (9)

[Claims] 1. An optical transmission line having a core and a clad which are mainly made of silica glass, wherein the clad contains GeO _2, and the GeO _{2 of the} core is included.
An optical transmission line characterized in that the content thereof is smaller than that of the clad. 2. The optical transmission line according to claim 1, wherein the GeO ₂ content of the core is less than 10% of the clad, or the core is GeO ₂ free. 3. The optical transmission line according to claim 1, wherein the cladding contains B ₂ O ₃ or F as a dopant that lowers the refractive index. 4. The core comprises P ₂ O ₅ , Al ₂ O ₃ , TiO ₂ or S as a dopant for increasing the refractive index.
The optical transmission line according to claim 1, further comprising i ₃ N ₄ . 5. The light according to claim 1, wherein the core and the clad form an optical plane waveguide formed by depositing a glass film containing silica glass as a main component on a substrate. Transmission line. 6. The optical transmission line according to claim 1, wherein the core and the clad form an optical fiber formed by drawing a glass base material containing silica glass as a main component. 7. A core containing silica glass as a main component, and more GeO containing silica glass as a main component than the core.
A clad containing ₂ and molded into a predetermined length, and at least a part of the region in the longitudinal direction of the clad is irradiated with light having a wavelength of 220 to 270 nm to bond GeO _{2 in} the clad. An optical transmission line characterized in that the state is changed. 8. A method for converting a mode field diameter, wherein at least a part of the optical transmission line according to claim 1 is irradiated with light having a wavelength of 220 to 270 nm. 9. The method according to claim 1, wherein at least a part of the optical transmission line is exposed to a hydrogen atmosphere, and then wavelengths 220 to 270.
A method for converting a mode field diameter, which comprises irradiating light having a wavelength of nm.