Optical device and formation of optical waveguide using the photorefractive effect

[Industrial Application Field] In the present invention, a refractive index change region is continuously formed inside a glass material by laser irradiation.<u style="single">How to make an optical waveguide</u>Regarding.

[0002] Conventional technology 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 surface high refraction region formed by the ions is moved below the surface. As a result, the waveguide is embedded under the glass surface and low propagation loss characteristics are ensured. using this method<u style="single">Production</u>The core of the optical waveguide has a semicircular or almost circular cross section with a diameter of 10 to 200 μm, and often has a specific refractive index difference of about 1%.

[0003] 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. using this method<u style="single">Production</u>The thickness of the optical waveguide is as thin as several μm.

[0004] In the ion exchange method, since the refractive index distribution is adjusted by ion exchange, the formed waveguide structure is limited to a portion close to the glass surface. The waveguide<u style="single">Production</u>Possible glass is also limited to materials capable of ion exchange. In addition, since it takes a long time for ion exchange, the productivity is low. On the other hand, the flame hydrolysis method is used for waveguides.<u style="single">Production</u>The process is complicated, and the materials that can be used are limited to the glass composition containing quartz as the main component. Furthermore, in order to modify the fine particles deposited on the substrate surface into a glass layer, an optical waveguide having a circular cross section<u style="single">Production</u>Is difficult.

[0005] 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.

[Means for Solving the Problems] In order to achieve the object, the present invention has an amount of energy that causes a photoinduced refractive index change and a repetition frequency of 100 kHz or more.<u style="single">Femtosecond laser light</u>Condensed inside the glass material<u style="single">To change the refractive index of the focusing point</u>The light-collecting point is relatively moved inside the glass material, and a continuous refractive index change region is formed inside the glass material. For glass materials<u style="single">Oxide glass,</u>Halide glass, sulfide glass, chalcogenide glass, etc. are used. Oxide glass includes silicate type, borate type, phosphate type, fluoride type, bismuth type, etc., and halide glass includes BeF.₂System, ZrF₄System, InF₃There are systems, Cd-Zn-Cl system, etc., sulfide glass includes Ga-La-S system, etc., and chalcogenide glass includes Se-As system, etc.

[0007] The laser light varies depending on the type of glass, but in order to cause a photo-induced refractive index change, 10 at the condensing point⁵W / cm²It is preferable to have the above peak power intensity. The peak power intensity is a value obtained by expressing the peak output (W) expressed by the ratio of the output energy (J) / pulse width (seconds) per pulse per irradiation unit area. Peak power intensity is 10⁵W / cm²If 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 smooth, the repetition frequency of the pulse laser is set to 10 KHz or higher.

[0008] The laser light 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. ..

[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.

[0010] On the other hand, if a high peak output is 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. Moreover, even if the pulsed laser has a wavelength that matches the natural absorption wavelength of glass, the absorption is weak and 10 at the focusing point.⁵W / cm²When 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.

[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. Therefore, in the present invention, the repetition frequency of the pulse laser is set.<u style="single">100KHz or higher</u>Set to. 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.

[0012] Example 1: SiO₂: 95% by weight, GeO₂A cube-shaped sample 1 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 is Ti: Al excited by an argon laser.₂O₃A 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 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).

[0013] 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. In addition, it was found from the near-field image on the exit side 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 was described, but other high-purity silica glass, phosphate glass, borate glass, fluoride glass, chloride glass, sulfide glass and the like were 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: ZrF₂: 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 focus the focal point 4 inside the sample 1 (Fig. 2a). When the focusing point 4 is moved in an arc shape, as shown in Fig. 2 (b), 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. It was confirmed by observation with a microscope. The formed line, the optical waveguide 5, had a circular cross section with a diameter of 12 μm.

[0015] 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 near-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. It was also 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 beam 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.

[Effect of the Invention] As described above, in the present invention, glass is irradiated with a pulsed laser having a high peak output value to a glass material such as glass to change the refractive index of the condensing point. An optical waveguide is written inside the material. This method has an advantage that an optical waveguide can be easily formed without going through a complicated process. Further, 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.

[Brief description of drawings] [Fig. 1] Quartz glass is irradiated with pulsed laser light (a), and inside the glass.<u style="single">Production</u>Optical Waveguide (b) [Fig. 2] Fluoride glass is irradiated with pulsed laser light (a) and inside the glass.<u style="single">Production</u>Optical Waveguide (b) 1: Glass Sample 2: Pulsed Laser Light 3: Condensing Lens 4: Condensing Point 5: Optical Waveguide