Optical waveguide and its manufacture

Description: TECHNICAL FIELD [Detailed description of the invention]

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INDUSTRIAL APPLICABILITY The present invention relates to an optical waveguide in which a refractive index change region is continuously formed inside a glass material by laser irradiation, and a method for producing the same.

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PROBLEM TO BE SOLVED: An optical waveguide used in optical communication or the like is formed inside a glass material such as glass by an ion exchange method, a flame hydrolysis method or the like. In the ion exchange method, Ag is applied to the surface layer of the glass substrate from the slit-shaped opening of the metal film.⁺ Ions are thermally allowed to enter and Na in the glass⁺ Ion and Ag⁺ After forming a waveguide in the surface layer by ion exchange in the first stage of ion exchange, a uniform electric field is applied to the glass substrate to apply Na in the molten salt.⁺ Allows ions to penetrate the glass surface. Na⁺ Ion is Ag⁺ The outermost high-refractive region formed by ions is moved below the surface. As a result, the waveguide is embedded under the glass surface and low propagation loss characteristics are ensured. The core of the optical waveguide produced by this method has a semicircular or substantially circular cross section with a diameter of 10 to 200 μm, and often has a specific refractive index difference of about 1%.

In the flame hydrolysis method, two glass fine particle layers for the lower clad and the core are deposited on the surface of the silicon substrate by flame hydrolysis of silicon tetrachloride and germanium tetrachloride, and the fine particle layer is made transparent by high temperature heating. It is modified into a glass layer. Next, a core portion having a circuit pattern is formed by photolithography and reactive etching. The optical waveguide produced by this method has a thin film thickness of several μm.

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PROBLEM TO BE SOLVED: To solve a problem in an ion exchange method, since the refractive index distribution is adjusted by ion exchange, the formed waveguide structure is limited to a portion close to a glass surface. Glass that can form a waveguide is also limited to materials that can exchange ions. In addition, since it takes a long time for ion exchange, the productivity is low. On the other hand, in the flame hydrolysis method, the process of forming the waveguide is complicated, and the usable material is limited to the glass composition containing quartz as the main component. Furthermore, since the fine particles deposited on the surface of the substrate are modified into a glass layer, it is difficult to create an optical waveguide having a circular cross section.

Further, in the ion exchange method or the flame hydrolysis method, although it is possible to form an optical waveguide having various two-dimensional patterns on the same substrate, it is difficult to form an optical waveguide that is three-dimensionally combined. is there. Therefore, it is restricted when it is used as an optical waveguide circuit or the like, and cannot be applied to applications having a complicated circuit configuration. The present invention has been devised to solve such a problem, and is a structure that causes a change in the refractive index by relatively moving the focusing point of the laser beam focused inside the glass material. The purpose is to cause a change inside the glass material to form an optical waveguide.

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PROBLEM TO BE SOLVED: To achieve the object of the optical waveguide of the present invention, a portion whose refractive index is changed by focused irradiation of laser light is continuously formed inside a glass material. It is characterized by. As the glass material, halide glass, sulfide glass, chalcogenide glass and the like are used. Oxide glass includes silicate type, borate type, phosphate type, fluoride type, bismuth type, etc., and halide glass includes BeF.₂ System, ZrF₄ System, InF₃ There are systems, Cd-Zn-Cl system, etc., sulfide glass includes Ga-La-S system, etc., and chalcogenide glass includes Se-As system, etc.

This optical waveguide condenses laser light having an amount of energy that causes a photoinduced refractive index change inside the glass material, moves the condensing point relative to the inside of the glass material, and causes a continuous refractive index change region. Is manufactured by forming inside a glass material. The laser beam varies depending on the type of glass, but in order to cause a photo-induced refractive index change, 10 at the condensing point⁵ W / cm² It is preferable to have the above peak power intensity. The peak power intensity is a value obtained by expressing the peak output (W) represented by the ratio of output energy (J) / pulse width (seconds) per pulse per irradiation unit area. Peak power intensity is 10⁵ W / cm² If it is less than the above, an effective photoinduced refractive index change does not occur, and an optical waveguide is not formed. The higher the peak power intensity, the more the photoinduced change in the refractive index is promoted, and the optical waveguide is easily formed. However, it is difficult to practically obtain a laser beam having an excessively large amount of energy. Therefore, it is preferable to use a pulse laser in which the peak output is increased by narrowing the pulse width. In order to make the waveguide formed inside the glass material a smooth structure, the repetition frequency of the pulse laser is set to 10 KHz or more.

The laser beam is focused by a condensing device such as a lens. At this time, the focusing point is adjusted so that it is located inside the glass material. By moving the condensing point relative to the inside of the glass material, a continuous refractive index change region that acts as an optical waveguide is formed inside the glass material. Specifically, the condensing point is moved relative to the condensing point of the laser light by continuously moving the condensing point of the laser light or by continuously moving the condensing point of the laser light inside the glass material. ..

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[Action] The phenomenon in which the refractive index changes due to irradiation with a pulse laser is called a photoinduced refractive index change, and examples of silica glass to which P, Ce, Ge, etc. are added are known. It is thought that this phenomenon is caused by the existence of oxygen defects having inherent absorption in the ultraviolet region in the glass, and the structural change of some of the oxygen defects by irradiating the laser beam with the absorption wavelength. Research is underway on excimer lasers whose oscillation wavelength is in the ultraviolet region. However, the laser beam used in this method has a low repetition frequency of less than 10 KHz and cannot provide sufficient energy to the irradiated portion. Therefore, the shape of the refractive index change region becomes spot-like, and it is not possible to form an optical waveguide that requires continuous refractive index change. Further, when the repetition frequency is forcibly increased while the average output is constant, the energy per pulse becomes low, and it becomes difficult to induce a change in the refractive index itself.

On the other hand, if a high peak output can be obtained by narrowing the pulse width, the refractive index will increase at the focusing point of the laser light regardless of the glass composition even in a pulsed laser having a repetition frequency of 10 KHz or higher. We confirmed the changing phenomenon. Under this condition, even if the pulsed laser has a wavelength other than the natural absorption wavelength of the glass, a phenomenon of changing the light-induced refractive index of the glass at the focusing point also occurs. Further, even a pulsed laser having a wavelength matching the inherent absorption wavelength of glass has weak absorption, and 10 at the focusing point.⁵ W / cm² When the above peak power intensity is secured, a photoinduced refractive index change occurs. Although the mechanism of occurrence of the phenomenon of change in refractive index is unknown, it is effectively used in the production of optical waveguides. Further, since the repetition period is fast , a continuous refractive index change region can be formed in the locus of the condensing portion by continuously scanning the glass material . Since this refractive index change region is higher than the initial refractive index of glass, it is used as an optical waveguide.

In order to form a smooth waveguide structure, it is necessary to narrow the pulse interval, in other words, to increase the repetition period, and to irradiate the first pulse and the second pulse as simultaneously as possible. For this reason, in the present invention, the repetition frequency of the pulse laser is set to 10 KHz, preferably 100 KHz or higher. If the repetition frequency is small, the laser beam is emitted discretely, and the continuous change in the refractive index required for forming the waveguide cannot be obtained. By slowing the scanning speed of the glass material or the condensing point of the laser light, the glass material can be continuously irradiated with the laser light. However, in this case, since the second pulse is irradiated in a state where the second pulse is overlapped after a certain period of time after the first pulse irradiation, the change in the refractive index formed by the first pulse is re-changed by the second pulse, which is sufficient. No significant change in refractive index can be obtained. The upper limit is as close to a continuous laser as possible with an infinite repetition frequency. However, when the repetition frequency is increased, the energy per pulse is generally weakened. Therefore, in reality, the upper limit of the repetition frequency is set by the threshold value at which the glass material causes a change in the refractive index and the output of the laser used.

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[Example]

Example 1: SiO₂ : 95% by weight, Geo₂ : A cube-shaped sample 1 having a size of 10 mm × 10 mm × 5 mm was cut out from quartz glass having a composition of 5% by weight. As shown in FIG. 1A, this sample was irradiated with the pulsed laser beam 2 focused by the lens 3. The pulsed laser light 2 includes Ti: Al excited by an argon laser.₂ O₃ A laser with a pulse width of 150 femtoseconds oscillated from the laser, a repetition frequency of 200 KHz, a wavelength of 800 nm, and an average output of 600 mW was used. When the pulsed laser beam 2 was focused by the lens 3 and irradiated so that the focusing point 4 was generated inside the sample 1, the refractive index of the focusing point 4 increased by 0.02. The change in refractive index occurred in a very short time on the order of nanoseconds or picoseconds. Therefore, the glass or the condensing portion was continuously moved to form a linear region having a high refractive index, that is, an optical waveguide 5, inside the sample 1 as shown in FIG. 1 (b).

The formation of the optical waveguide was confirmed by the fact that visible light was actually incident on the sample and the light was transmitted only to the portion where the refractive index was changed. Further, from the near-field image on the exit side, it was found that the cross section of the optical waveguide was circular with a diameter of 20 μm. In Example 1, an example using Ge-doped silica glass has been described, but other high-purity silica glass, phosphate glass, borate glass, fluoride glass, chloride glass, sulfide glass and the like have been described. Similarly, in glass, an optical waveguide was formed by laser irradiation. Since the obtained optical waveguide does not have a clear interface between the core and the cladding, the interface loss is extremely small, and it is expected to be used as a fine waveguide forming method in an optical integrated circuit or the like.

Example 2: ZrF₄ : 50 mol%, LaF₃ : 5 mol%, AlF₃ : 5 mol%, BaF₂Fluoride glass having a composition of: 20 mol% and NaF: 20 mol% and having a cubic shape of 20 mm × 20 mm × 5 mm was used as sample 1. A pulsed laser beam 2 having a pulse width of 120 femtoseconds, a repetition frequency of 50 KHz, a wavelength of 1 μm, and an average output of 150 mW was focused and irradiated so as to connect the focal point 4 to the inside of the sample 1 (FIG. 2a). When the focusing point 4 is moved in an arc shape, it is optical that a line clearly different from the surroundings is formed inside the sample 1 along the trajectory of the focusing point 4 as shown in FIG. 2 (b). It was confirmed by observation with a microscope. The formed line, that is, the optical waveguide 5, had a circular cross section with a diameter of 12 μm.

When a He-Ne laser beam is incident from one end surface of the optical waveguide 5, it is confirmed from the light intensity distribution of the short-field image that the laser beam is emitted from the other end surface of the optical waveguide 5, and the laser beam is formed. It was found that the line had a higher refractive index than the surroundings and had an optical waveguide structure. Further, it was confirmed that when the optical waveguide 5 was repeatedly focused and irradiated with the laser beam, the light intensity emitted from the end face of the optical waveguide 5 also increased, and the change in the refractive index also increased due to the repeated laser irradiation. Further, the pulse width and the repetition frequency were kept constant, and the fluoride glass was irradiated while changing the wavelength of the laser light from 400 nm to 2 μm to form an optical waveguide. In this case as well, it was found from the light intensity distribution of the near-field image of the light emitting end face that a similar optical waveguide was formed inside the glass.

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INDUSTRIAL APPLICABILITY As described above, in the present invention, a pulsed laser having a high peak output value is irradiated on a glass material such as glass to change the refractive index of a condensing point, thereby changing the inside of the glass material. The optical wave guide is written in. This method has an advantage that an optical waveguide can be easily formed without going through a complicated process. In addition, the shape of the optical waveguide can be arbitrarily controlled by changing the scanning direction of the glass material with respect to the optical axis direction of the pulse laser or the moving direction of the condensing point with respect to the glass material, making it an optical integrated circuit having a complicated circuit structure or the like. Suitable three-dimensional optical waveguides are also easily produced.

[Simple explanation of drawings]

FIG. 1 is an optical waveguide (b) created inside the glass by irradiating quartz glass with a pulsed laser beam (a).

FIG. 2 is an optical waveguide created inside the glass by irradiating fluoride glass with pulsed laser light (a) (b) 1: Glass sample 2: Pulsed laser light 3: Condensing lens 4: Condensing point 5: Optical Waveguide

[Procedure amendment]

[Submission date] January 23, 1997

[Procedure amendment 1]

[Name of document subject to amendment] Statement

[Item name to be corrected] 0011

[Correction method] Change

[Correction details]

0011 In order to form a smooth waveguide structure, it is necessary to narrow the pulse interval, in other words, to increase the repetition period, and to irradiate the first pulse and the second pulse as simultaneously as possible. For this reason, in the present invention, the repetition frequency of the pulse laser is set to 10 KHz, preferably 100 KHz or higher. If the repetition frequency is small, the laser beam is emitted discretely, and the continuous change in the refractive index required for forming the waveguide cannot be obtained. By slowing the scanning speed of the glass material or the condensing point of the laser light, the glass material can be continuously irradiated with the laser light. However, in this case, since the second pulse is irradiated in a state where the second pulse is overlapped after a certain period of time after the first pulse irradiation, the change in the refractive index formed by the first pulse is re-changed by the second pulse, which is sufficient. No significant change in refractive index can be obtained. The upper limit is as close to a continuous laser as possible with an infinite repetition frequency. However, when the repetition frequency is increased, the energy per pulse is generally weakened. Therefore, in reality, the upper limit of the repetition frequency is set by the threshold value at which the glass material causes a change in the refractive index and the output of the laser used. The core diameter of the optical waveguide can be controlled by changing the power of the pulsed laser to irradiate and the diameter of the focused spot. The larger the power of the pulsed laser or the diameter of the focused spot, the larger the core diameter. Further, the amount of change in the refractive index of the core portion can be controlled by the number of scans of the pulse laser, and the larger the number of scans, the larger the amount of change in the refractive index while keeping the core diameter constant.