JP4405946B2 - Optical circuit fabrication method - Google Patents

Description

  The present invention relates to an optical circuit manufacturing method, and more particularly, to an optical circuit manufacturing method including a step of adjusting optical characteristics of an optical circuit when manufacturing an optical circuit composed of optical waveguides formed on a substrate. About.

  2. Description of the Related Art Conventionally, an optical waveguide for constituting an optical circuit is manufactured using a quartz glass substrate and a silicon substrate as a main material on a quartz glass substrate. This optical waveguide has features such as low loss, high reliability and stability, good workability, and good matching with a silica-based optical fiber. Y-branch power splitter composed of this optical waveguide, Mach-Zehnder Interferometer (MZI), optical switch using MZI, arrayed waveguide type wavelength multiplexer / demultiplexer (AWG), etc. Development of optical circuits (PLC: Planar Lightwave Circuits) is in progress. These optical circuits have become important key devices for photonic network systems based on wavelength division multiplexing (WDM) optical transmission systems that are being constructed in recent years (for example, Non-Patent Documents 1 and 2). reference).

  For example, an optical circuit to which MZI is applied utilizes interference when an input optical signal is branched into two arm waveguides having different characteristics and the outputs of the arm waveguides are combined again. Since the interference depends on the phase of the optical signal to be combined, a desired function can be realized by controlling the refractive index and length of the arm waveguide through which the optical signal passes, that is, the optical path length.

  The silica-based optical waveguide is manufactured as follows. First, an under cladding layer and a core layer are deposited on a quartz substrate or a silicon substrate. Next, the core pattern is processed to deposit an overcladding layer. In this way, an optical waveguide having a core / cladding structure is formed. By processing this optical waveguide into a desired pattern, various optical circuits such as a Y-branch power splitter, MZI, and AWG can be manufactured.

M. KAWACHI, “Silica waveguides on silicon and their application to integrated-optic components”, Opt. Quantum. Electron., 1990, 22, pp.391-416 Y. HIBINO, “An array of photonic filtering advantages”, IEEE Circuits Devices Mag., 2000, 16, pp.21-27 M. Abe, et al., “Optical path length trimming technique using thin film heaters for silica-based waveguides on Si”, Electron. Lett., 1996, vol.32, pp.1818-1820 M. Abe, et al., “Mach-Zehnder interferometer and arrayed waveguide-grating integrated multi / demultiplexer with photosensitive wavelength tuning”, 2001, vol.37, pp.376-377 K. D. Simmons, et al., “Photosensitivity of solgel-derived germanosilicate planar waveguides”, Opt. Lett., Vol.18, 1993, pp.25-27 G. D. Maxwell, et al., “UV WRITTEN 1.5μm REFLECTION FILTERS IN SINGLE MODE PLANAR SILICA GUIDES”, Electron Lett., (28), 1992, pp.2106-2107 K. O. Hill, et al. OFC'93 Satoshi Inoue, “Fibre Grating”, Laser Research, Vol. 23 October (1995) pp.68-77 Raman Kashyap, “FIBER BRAGG GRATINGS”, ACADEMIC PRESS, 1999

  However, in fabricating an optical circuit, minute refractive index fluctuations, minute waveguide shape fluctuations, and stress applied to the waveguide have a great influence on the element characteristics, and the desired optical characteristics of the optical circuit can be obtained. There may not be. In particular, in recent years, in order to improve productivity in highly functional, high performance and highly integrated optical circuits, it is necessary to correct small variations in fabrication, etc., and local refractive index control. An optical circuit manufacturing method capable of realizing the above has become important. Therefore, a method of adjusting the phase of MZI with a thin film heater installed on the optical waveguide is known (for example, see Non-Patent Document 3).

  In addition, a technique using a change in refractive index caused by irradiation with visible light or ultraviolet light has been studied. Among these methods, the method using the refractive index change due to ultraviolet light irradiation is simple and does not require an additional process for producing a thin film heater for phase adjustment, and it is possible to process a plurality of optical waveguides at once. This is a practically promising technique (see Non-Patent Document 4, for example). In this method, increasing the sensitivity of the refractive index change caused by irradiation with visible light or ultraviolet light contributes to shortening of the light irradiation time, shortening of the process time, and further to the stability and reproducibility of the device. .

  In order to increase the sensitivity of refractive index change, it is known to increase the efficiency of light-induced refractive index change by performing heat treatment at a high temperature of about 500 ° C. in a hydrogen atmosphere (hereinafter referred to as high-temperature hydrogenation treatment). (Refer to Non-Patent Document 5, for example). However, performing high-temperature heat treatment in a hydrogen atmosphere is not safe, and the manufactured optical waveguide has a problem that loss in the near infrared region increases by several dB or more (for example, Non-Patent Document 6). reference). The room temperature and high pressure hydrogen impregnation treatment (hydrogen loading) method (see, for example, Non-Patent Documents 8 and 9) has an excellent track record in the production of fiber light-induced gratings. However, in an optical circuit where the adjustment of the absolute value of the refractive index is important, the refractive index of the waveguide changes due to the diffusion of hydrogen gas before and after light irradiation. It had the problem of becoming complicated.

  It is also known to increase the efficiency of light-induced refractive index change (hereinafter referred to as flame sweep method) by rolling the optical waveguide with a flame such as a hydrogen burner or LPG burner (refer to Non-Patent Document 7). . The flame sweep method is excellent in that the efficiency of refractive index change can be easily increased, but there is a problem in its controllability and reproducibility.

  The present invention has been made in view of such problems. The object of the present invention is to increase the sensitivity of light-induced refractive index change and to control optical characteristics with excellent safety, controllability, and reproducibility. It is an object of the present invention to provide an optical circuit manufacturing method capable of efficiently performing the above.

In order to achieve the above object, the present invention provides an optical circuit comprising an optical waveguide comprising a core and a clad formed of quartz glass as a main material on a substrate. of the manufacturing method, the optical circuit, a first step of heating in a furnace which is pressurized by a gas mainly composed of inert gas, after the furnace in the hot, holding a predetermined time at high pressure, pressure the furnace temperature was held was lowered to room temperature and a second step of lowering the reactor pressure to atmospheric pressure, at least a portion of the optical circuit, by irradiation with ultraviolet light or visible light, the light circuit And a third step of adjusting the optical characteristics.
The invention according to claim 2 is characterized in that the substrate according to claim 1 is either a silicon substrate or a quartz glass substrate.

  The invention according to claim 2 is characterized in that the substrate according to claim 1 is either a silicon substrate or a quartz glass substrate.

  The invention described in claim 3 is characterized in that the inert gas described in claim 1 or 2 is either Ar or He.

  A fourth aspect of the invention is characterized in that the temperature of the first step according to the first, second, or third aspect is increased by a carbon heater installed in the furnace.

  The invention described in claim 5 is characterized in that the first step according to any one of claims 1 to 4 is performed at a temperature of 400 ° C. or higher and lower than a melting point temperature of the quartz glass. .

  According to a sixth aspect of the present invention, in the third step according to any one of the first to fifth aspects, the refractive index of the waveguide is changed by irradiating the waveguide of the optical circuit, and the optical process is performed. The characteristic is adjusted.

  According to a seventh aspect of the present invention, in the first step according to any one of the first to sixth aspects, carbon or a reducing agent mainly composed of carbon is disposed in the furnace together with the optical circuit. It is characterized by being.

  As described above, according to the present invention, the optical characteristics of the optical circuit are adjusted (third step) after performing the HIP process (first step, second step), so that the sensitivity of the photoinduced refractive index change is adjusted. Can be increased. Further, according to the present invention, it is possible to efficiently control optical characteristics that are excellent in safety, have little change in refractive index after HIP processing, and have excellent controllability and reproducibility in that an inert gas is used. it can.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. In the present embodiment, hot isostatic pressing (HIP) processing is performed on an optical circuit composed of an optical waveguide formed using quartz-based glass as a main material. The HIP process is one of powder sintering molding techniques, and molding is performed by compressing with gas pressure without using a mechanical press. In general, an object to be processed is isotropically compressed using argon (Ar) or helium (He) gas at a high temperature (1500 ° C.) and high pressure (1500 Kgf / cm ² ) as a medium. By HIP treatment, the structure is densified and mechanical properties such as bending strength are greatly improved.

  The inventors have found that the sensitivity of the refractive index change caused by irradiation with visible light or ultraviolet light can be increased by applying the HIP process to the manufactured optical circuit. This is presumably because the oxygen in the quartz glass escapes from the glass by the inert gas and the pressure, that is, is reduced, thereby increasing the sensitivity of the refractive index change. Since the HIP treatment uses an inert gas, the risk of explosion and the like is remarkably low as compared with a high-temperature hydrogenation treatment method using hydrogen and a room temperature high pressure hydrogen impregnation treatment method. Further, there is no change in the refractive index due to the diffusion of hydrogen gas, and the controllability and reproducibility are excellent.

  FIG. 1 shows a configuration of an optical circuit according to Example 1 of the present invention. Example 1 is an AWG composed of a silica-based optical waveguide. The AWG is an array conductor that connects the slab waveguide 2 connected to the input waveguide 1, the slab waveguide 4 connected to the output waveguide 3, and the slab waveguides 3, 4 on the silicon substrate 6. A waveguide 5 is formed. The optical path length difference of each waveguide of the arrayed waveguide 5 is set at 100 GHz intervals.

  AWG is produced as follows (for example, refer nonpatent literatures 1 and 2). First, an under cladding layer and a core layer are sequentially deposited on the silicon substrate 6 by a flame deposition method (FHD: Flame Hydrolysis Deposition). Next, the core layer is patterned by using a photolithography technique, a reactive ion etching (RIE) technique, and the like, and finally an over cladding layer is deposited by an FHD method.

  Note that a quartz-based optical waveguide can be formed not only on a silicon substrate but also on a quartz glass substrate.

  As an initial optical characteristic of the manufactured AWG, a transmission wavelength spectrum or the like is measured. The results will be described later. The optical characteristics are measured using an ASE (amplified spontaneous emission) light emitted from an erbium-doped optical fiber in a wide band and using an optical spectrum analyzer (SA). The ASE light is converted into linearly polarized light through a polarizer (polarizer) and guided to the AWG using a polarization maintaining fiber, thereby measuring the optical characteristics of each polarized light in the TE mode and the TM mode. The optical characteristics may be measured by using an external cavity laser capable of changing the wavelength in a narrow band, drawing the wavelength range, and measuring the transmitted light intensity with an optical power meter.

  After the measurement, HIP processing is performed on the manufactured AWG. FIG. 2 shows an outline of the HIP process in the first embodiment. In the HIP processing furnace 11, a container 12 for storing a sample and heaters 13a and 13b for heating are provided. In order to introduce high-pressure Ar gas into the HIP processing furnace 11, a pressurizing pump 14 and an Ar gas cylinder 15 are connected. In Example 1, AWG16 produced in the container 12 is put, the inside of the furnace is pressurized to 50 MPa in an Ar atmosphere, and the temperature is raised to 1000 ° C. (first step). After maintaining this state for 1 hour, the temperature in the furnace is gradually lowered to near room temperature while keeping the pressure substantially constant, and the inside of the furnace is lowered to normal pressure (second step). Note that the temperature in the furnace is 400 ° C. or higher and is raised to the melting point temperature or lower of the quartz glass.

  In the optical circuit manufacturing method according to Example 1 of the present invention, a container mainly composed of carbon was used as the container 12. This is because it was found that the change in refractive index obtained in the third step described later can be increased by using such a container having a strong reducing ability. Similar effects can be obtained by separately installing carbon with an optical circuit using a platinum container.

  However, the present invention is not limited to this example, and it is needless to say that the above-described carbon reducing agent is not used.

Furthermore, in the optical circuit manufacturing method according to Embodiment 2 of the present invention described below, in the second step, the furnace temperature is gradually lowered to near room temperature while keeping the pressure substantially constant, and then the inside of the furnace is The pressure was reduced to normal pressure. This is because it has been found that a large change in refractive index can be obtained in the third step described later by lowering the pressure and temperature in this order. Furthermore, the inventors have also found that it is desirable to set the temperature in the furnace near room temperature to about 100 ° C. in view of the temperature drop caused by the pressure drop thereafter. However, the present invention is not limited to this example, and it is needless to say that the above-described order of pressure and temperature drop is not used.

  After performing the HIP process, the optical properties of the processed AWG are measured again. After the measurement, the optical characteristics of the arrayed waveguide are adjusted. FIG. 3 shows a method for adjusting the optical characteristics of the AWG in the first embodiment. First, the measurement system will be described. As described in the measurement of the initial optical characteristics, the ASE light source 22 using the erbium-doped optical fiber (EDF), the polarizer 23, and the polarization maintaining fiber 24 are connected in cascade, via the optical axis alignment system 25a. It is connected to the input waveguide of the AWG 21 after processing. An optical spectrum analyzer (SA) 26 is connected to the output waveguide of the AWG 21.

Next, the adjustment system will be described. At least part of the optical circuit is irradiated with ultraviolet light or visible light to adjust the optical characteristics of the optical circuit (third step). Specifically, the arrayed waveguide of the AWG 21 is irradiated with ultraviolet laser light, the refractive index of the waveguide is changed to a desired value, and the center wavelength λc of the AWG is changed by the shift amount Δλc. The adjustment system is arranged so that the laser beam output from the ArF excimer laser 27 having a wavelength of 193 nm is uniformly irradiated to the entire arrayed waveguide portion of the AWG 21 via the mirror 28, the lens 29, and the cylindrical lens 30. To do. A metal light-shielding plate 31 is installed on the AWG 21 so that the entire arrayed waveguide portion is exposed and the other portions are not irradiated with laser light. The intensity of the laser light is about 100 mJ / cm ² per pulse. Laser irradiation by the adjustment system is performed by the measurement system while monitoring the optical characteristics of the AWG 21.

  FIG. 4 shows the shift amount Δλc of the center wavelength λc of the AWG with respect to the irradiation time of the laser beam. As the initial optical characteristics of the AWG without HIP processing, the wavelength shift amount with respect to the irradiation time is indicated by a small circle 41 in the TE mode and a small square 42 in the TM mode. The optical characteristics of the AWG after the HIP process are indicated by a large circle 43 in the TE mode and a large square 44 in the TM mode. When compared at the point where the laser beam irradiation time is 60 minutes, it can be seen that a wavelength shift amount of about 10 times is obtained, and the sensitivity of refractive index change is greatly increased. It has been confirmed that the sensitivity, controllability, and reproducibility of the refractive index change are maintained by the pressure, temperature, and processing time when performing the HIP processing. As shown in FIG. 4, the wavelength shift amount Δλc increases substantially in proportion to the laser light irradiation time or the integrated irradiation light energy amount. Therefore, the center wavelength λc of the AWG can be easily adjusted to a desired value by adjusting the irradiation time.

  FIG. 5 shows changes in the transmission spectrum at the center port of the AWG. A thin line shows the initial optical characteristics without HIP processing, and a thick line shows the optical characteristics after HIP processing. It can be seen that the center wavelength λc can be adjusted to a desired wavelength by irradiation with laser light for 60 minutes. Note that heat treatment can be performed to stabilize the adjustment of the center wavelength of the AWG. At this time, the irradiation time of the laser light is adjusted in view of the amount of change in the refractive index that is relaxed in the heat treatment step. Specifically, it was confirmed that the stability of the center wavelength can be significantly improved by heat-treating the optical circuit at 300 ° C. for 1 hour.

  As described above, since the HIP process can be performed on an optical circuit that has already been manufactured, for example, an optical circuit having optical characteristics that are out of specification can be corrected to desired optical characteristics after manufacturing. It can contribute to productivity improvement.

  Example 2 is an optical circuit having a light-induced grating. The HIP process is performed on the optical circuit manufactured in the same manner as the AWG of the first embodiment. In the HIP process, the furnace temperature is raised to 1000 ° C. and the pressure is increased to 100 MPa in an Ar atmosphere. After maintaining this state for 1 hour, the furnace temperature is gradually lowered to room temperature while keeping the pressure substantially constant. After performing the HIP process, a laser beam is applied to the linear portion of the optical waveguide of the optical circuit.

FIG. 6 shows a method for adjusting the optical characteristics of the optical circuit in the second embodiment. Here, the adjustment system will be described. The output light of the Ar ion laser 51 is incident on the nonlinear optical element 52 to obtain second harmonic light (wavelength 244 nm). Here, a BBO (BaB ₂ O ₄ ) crystal is used as the nonlinear optical element 52. The second harmonic light is collected by an optical system including the duplexer 53, the mirror 54, the lens 55, and the cylindrical lens 56. The second-harmonic light is split into two by the half mirror 57, and the optical system 61 composed of the mirrors 58, 59, 60 is irradiated with the interference light on the waveguide of the optical circuit 61 for exposure. The intensity of the second harmonic light is 200 mW.

  FIG. 7 shows changes in the transmission spectrum of the optical circuit in the second embodiment. A thick line shows the initial optical characteristics without HIP processing, and a thin line shows the optical characteristics after HIP processing. When the HIP process is not performed, there is almost no change in refractive index due to light induction, and no grating is formed. On the other hand, it can be seen that after the HIP treatment, the refractive index change sensitivity is greatly increased, and a grating having a reflectance of approximately 90% is formed. As described above, since a larger refractive index change can be obtained by short-time laser light irradiation, it is possible to improve the characteristics of the light-induced grating.

It is a figure which shows the structure of the optical circuit concerning Example 1 of this invention. It is a figure which shows the outline of the HIP process in Example 1. FIG. 6 is a diagram illustrating a method for adjusting optical properties of AWG in Example 1. FIG. It is a figure which shows shift amount (DELTA) (lambda) c of the center wavelength (lambda) c of AWG with respect to the irradiation time of a laser beam. It is a figure which shows the change of the transmission spectrum in the center port of AWG. 6 is a diagram illustrating a method for adjusting optical characteristics of an optical circuit in Embodiment 2. FIG. It is a figure which shows the change of the transmission spectrum of the optical circuit in Example 2. FIG.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Input waveguide 2, 4 Slab waveguide 3 Output waveguide 5 Array waveguide 6 Silicon substrate 11 HIP furnace 12 Container 13 Heater 14 Pressurization pump 15 Ar gas cylinder 16, 21 AWG
22 EDF-ASE light source 23 Polarizer 24 Polarization maintaining fiber 25 Optical axis alignment system 26 Optical spectrum analyzer 27 ArF excimer laser 28, 54, 58, 59, 60 Mirror 29, 55 Lens 30, 56 Cylindrical lens 31 Light shielding plate 51 Ar ion laser 52 Non-linear optical element 53 Demultiplexer 57 Half mirror 61 Optical circuit

Claims (7)

In a method of manufacturing an optical circuit composed of an optical waveguide composed of a core and a clad formed of quartz glass as a main material on a substrate,
A first step of heating the optical circuit in a furnace pressurized with a gas containing an inert gas as a main component;
A second step of holding the inside of the furnace at a high temperature and high pressure for a certain period of time, holding the pressure to lower the furnace temperature to room temperature, and then lowering the furnace pressure to normal pressure;
And a third step of adjusting the optical characteristics of the optical circuit by irradiating at least a part of the optical circuit with ultraviolet light or visible light.   The method of manufacturing an optical circuit according to claim 1, wherein the substrate is one of a silicon substrate and a quartz glass substrate.   The method of manufacturing an optical circuit according to claim 1, wherein the inert gas is Ar or He.   4. The method of manufacturing an optical circuit according to claim 1, wherein the temperature of the first step is increased by a carbon heater installed in the furnace.   5. The method of manufacturing an optical circuit according to claim 1, wherein the temperature of the first step is 400 ° C. or higher and lower than a melting point temperature of the quartz-based glass.   6. The optical characteristic is adjusted by changing the refractive index of the waveguide by irradiating the waveguide of the optical circuit in the third step. Manufacturing method of optical circuit.   The optical circuit according to any one of claims 1 to 6, wherein in the first step, carbon or a reducing agent containing carbon as a main component is arranged in the furnace together with the optical circuit. Production method.

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