This application claims the priority benefit of Japanese application serial no. 2001-203948, filed on Jul. 4, 2001.
1. Field of the Invention
This invention relates in general to a crystallized glass in which a xcex2-quartz solid solution or a xcex2-eucryptite solid solution has been precipitated as a main crystal. More particularly, the invention relates to a crystallized glass, which is suitable for serving as a temperature compensating substrate material for various devices or a waveguide layer material for waveguide devices in the field of optical communication.
2. Description of Related Art
The crystallized glass with a thermal expansion coefficient of about zero, in which a xcex2-quartz solid solution or a xcex2-eucryptite solid solution was precipitated to serve as a main crystal is well known. Initially, it is used as window glass for stoves or fire-proof glass for architecture, and now has a variety of applications.
As the optical communication technology has become highly developed in recent years, the network using the optical fiber is in the process of being furnished promptly. In such a network, a wave multiplexing technology is used for transmitting lights with plural wavelengths in a lump, so that the wavelength filter, the optical coupler, and the waveguide etc have become important devices.
The characteristic of some of the above optical communication devices varies with the temperature, keeping away such devices from being used outdoors. Therefore, a temperature compensation technology has to be developed, i.e., a technology such that the characteristic of the optical communication device can be maintained independent of the temperature variation. A typical optical communication device requiring temperature compensation is a waveguide device, such as an arrayed waveguide (AWG) or a plane light circuit (PLC), or a fiber bragg grating (FBG).
Referring to FIG. 1, the waveguide device 1, such as the AWG or the PLC, has a waveguide layer 3 on a plane substrate 2 and a core 4 is formed within the waveguide layer 3. The waveguide device 1 can split, combine or switch the light. As shown in following equation (1), as the environmental temperature is changed, it causes a problem such that the optical path length is changed due to the variations of the refractive index and the thermal expansion coefficient. In the equation (1), S is the optical path length, n is the refractive index of the core 4, and xcex1 is the thermal expansion coefficient.
dS/dT=(dn/dT)+nxcex1xe2x80x83xe2x80x83(1) 
In addition, regarding light-transmittable material such as glass or crystal, the temperature dependence of the refractive index becomes larger as the thermal expansion coefficient becomes negatively greater. Therefore, if the glass is used as the waveguide layer, the temperature dependence of the device cannot be decreased if the thermal expansion coefficient is negatively increased. In other words, regarding the waveguide layer material for the AWG or the PLC, the refractive index variation due to the temperature variation causes a variation of the optical path length, so that the device characteristic changes.
The FBG is a device provided with a portion in which refractive index changes are given in the core of the optical fiber in grating-like, in other wards, the so-called grating portion. The FBG has the characteristic such that it reflects the light having a specific wavelength. As shown in the equation (2), the reflection wavelength will change because the refractive index and the grating spacing change. In the equation (2), xcex is the wavelength of the reflecting light, n is the effective refractive index of the core 4, xcex9 is grating spacing of the portion whose refractive index changes in grating-like.                                                                                                               ∂                    λ                                    /                                      ∂                    T                                                  =                                  xe2x80x83                                ⁢                                  2                  ⁢                                      {                                                                                            (                                                                                    ∂                              n                                                        +                                                          ∂                              T                                                                                )                                                ·                        Λ                                            +                                              n                        ⁡                                                  (                                                                                    ∂                              Λ                                                        /                                                          ∂                              T                                                                                )                                                                                      }                                                                                                                          =                                  xe2x80x83                                ⁢                                                      2                    ·                    Λ                                    ⁢                                      {                                                                  (                                                                              ∂                            n                                                    /                                                      ∂                            T                                                                          )                                            +                                                                        n                          ⁡                                                      (                                                                                          ∂                                Λ                                                            /                                                              ∂                                T                                                                                      )                                                                          /                        Λ                                                              }                                                                                      "AutoLeftMatch"                            (        2        )            
To prevent the variation of the device characteristic, such methods are provided, as applying stress according to the temperature variation against the device to cancel fluctuating factors due to the variation of refractive index, or adjusting the refractive index itself.
The examples for the waveguide device such as the AWG or PLC etc are described at 2000 Communications of the Singaku Electronics Society C-13-21 or C-3-13. Stress applying pin is arranged in the device, or divided aluminum substrate is used to apply stress to the device according to the temperature variation, so that the refractive index of the waveguide can be adjusted.
An example for the FBG is provided. Alloy or quartz glass etc with a small thermal expansion coefficient and a metal (such as aluminum) with a large thermal expansion coefficient is combined as a temperature compensation material, and the temperature compensation material is fixed to the FBG. As shown in FIG. 3, aluminum brackets 11a, 11b with a relatively large thermal expansion coefficient are respectively attached to the both ends of an inver (trade mark) rod 10 with a small thermal expansion coefficient, the FBG 13 is extended to fix with a preset stress by using the clasps 12a, 12b to the aluminum brackets 11a, 11b. At this time, the grading portion 13a of the FBG 13 is wound between the two clasps 12a, 12b. 
Under the above condition, as the environmental temperature increases, the aluminum brackets 11a, 11b extend to reduce the distance between the two clasps 12a, 12b, so that the stress applied to the grating portion 13a of the FBG 13 should be decreased. As the temperature decreases, the aluminum brackets 11a, 11b contract to increase the distance between the two clasps 12a, 12b, so that the stress applied to the grating portion 13a of the FBG 13 should be increased. In this way, by varying the stress acting on the FBG based on the temperature variation, the grating spacing of the grating portion can be adjusted, so that the temperature dependence of the central wavelength of the reflection light can be reduced.
However, because the above temperature compensation device is very complicated in structure, the manufacturing is very difficult and the installation is also very difficult.
In order to solve the above problem for the FBG, as shown in FIG. 2, WO97/28480 teaches a solution. In the solution, a glass preform previously formed in a plate shape is performed in thermal process, so that the xcex2-quartz solid solution is precipitated therein to form a crystallized glass 14 with a negative thermal expansion coefficient. The FBG 16 is fixed by the adhesive 17 onto the crystallized glass 14 under a stress applying thereto with a weight 15. The stress is controlled by the expansion or the contraction of the crystallized glass 14. This method can be applied to any waveguide device. In addition, the item 16a is the grating portion of the FBG 16.
The thermal expansion coefficient of the crystallized glass disclosed in WO97/28480 is negatively large. In addition, because temperature compensation is performed by a single element, the device structure can be simply made. Because plural gaps or cracks arc intended to form at the crystal grain boundary, there is a problem that the hysteresis of the thermal expansion is large.
The hysteresis of the thermal expansion is that when the material expands or contracts due to the temperature variation, the behavior when the temperature increases and the behavior when the temperature decreases are inconsistent. Even though a material having a large hysteresis of the thermal expansion is used as a temperature compensation material, the temperature dependence of the device cannot be correctly compensated.
Additionally, in the WO97/28480, to reduce the hysteresis of the thermal expansion of the crystallized glass, a thermal process is repeatedly performed at a temperature range of 400xcx9c800xc2x0 C. However, this thermal process will reduce the productivity and the cost will be increased.
According to the foregoing description, an object of this invention is to provide a crystallized glass, wherein the crystallized glass has a negative thermal expansion coefficient required for compensating the temperature, the hysteresis of the thermal expansion is small, and the manufacturing cost can be reduced.
According to the object(s) mentioned above, the invention provides a crystallized glass. A xcex2-quartz solid solution or a xcex2-eucryptite solid solution is precipitated as a main crystal. The crystallized glass has a crystallinity not less than 70 mass %, a crystal grain size not more than 0.5 xcexcm, substantially no gap or crack occurs at boundaries of crystal grains. The crystallized glass also has a thermal expansion coefficient being negatively greater than xe2x88x9210xc3x9710xe2x88x927/xc2x0 C. within a temperature range of xe2x88x9240xc2x0 C.xcx9c100xc2x0 C., a hysteresis of the thermal expansion coefficient within the temperature range being not more than 10 ppm, and a temperature dependence of refractive index dn/dT being not more than 13xc3x9710xe2x88x926/xc2x0 C.