Stock material or miscellaneous articles – Composite – Of quartz or glass
Reexamination Certificate
2002-08-07
2004-08-24
Robertson, Jeffrey B. (Department: 1712)
Stock material or miscellaneous articles
Composite
Of quartz or glass
C428S210000, C428S304400, C428S307300, C428S312600, C428S447000, C428S448000, C385S014000, C385S037000, C385S131000, C385S137000, C385S141000
Reexamination Certificate
active
06780514
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical communication device substrate having a negative thermal expansion coefficient and an optical communication device obtained by fixing an optical component having a positive thermal expansion coefficient onto the substrate.
2. Related Art
A network using an optical fiber has been rapidly improved in company with progress in optical communication technology. In the network, there has been used a wavelength multiplexing technique transmitting light with plural wavelengths collectively, so that a wavelength filter, a coupler, a waveguide and so on become important optical communication devices.
Among such optical communication devices, some have a trouble in outdoor use due to a change in characteristic according to a temperature; therefore a necessity has arisen for a technique to sustain a characteristic of such an optical communication device at a constant level regardless of a change in temperature, so-called athermal technique.
A fiber Bragg grating (hereinafter referred to as FBG) is exemplified as a representative of optical communication devices requiring athermalization. An FBG is an optical communication device having a portion with a profile of a changed refractive index in the form of a grating, so-called grating region, in a core of an optical fiber, and features reflection of light with a specific wavelength according to a relationship given by the following formula (1). For this reason, this has drawn attention as an important optical communication device in a wavelength division multiplex transmission optical communication system in which optical signals with different wavelengths are multiplex-transmitted through a single optical fiber.
&lgr;=2n&Lgr; (Formula 1)
wherein &lgr; is a reflection wavelength, n is an effective refractive index in a core, and &Lgr; is a spacing in a region with a changed refractive index in the form of a grating.
Such an FBG has a problem, however, that a center reflective wavelength fluctuates as temperature varies. A temperature dependency of a center reflective wavelength is given by the following formula (2), which is obtained by differentiating the formula (1) with respect to a temperature T.
∂&lgr;/∂T=2{(∂
n/∂T
)&Lgr;+
n
(∂&Lgr;/∂
T
)}=2&Lgr;{(∂
n/∂T
)+
n
(∂&Lgr;/∂
T
)/&Lgr;} (Formula 2)
The second term of the right side of the formula (2), (∂&Lgr;/∂T)/&Lgr;, corresponds to a thermal expansion coefficient of an optical fiber, and the value thereof is almost 0.6×10
−6
/° C. On the other hand, the first term of the right side is a temperature dependency of a refractive index in a core portion of an optical fiber, the value thereof is almost 7.5×10
−6
/° C. That is, while the temperature dependency of a center reflective wavelength is dependent on both of a change in refractive index in a core portion and a change in spacing of the grating due to thermal expansion, most of a change in center reflective wavelength is found to be caused by a change in refractive index according to temperature.
As means for preventing a change in center reflective wavelength, a method has been known in which a tension adapted to a change in temperature is applied to an FBG to vary a spacing of a grating region, thereby canceling a component caused by a change in refractive index.
As a specific example, a device controlled with respect to a tension therein is disclosed in the Japanese Patent Laid Open No. 2000-503967, which device is fabricated this way: an FBG applied with a prescribed tension is fixed with an adhesive onto a glass-ceramic substrate having a negative thermal expansion coefficient, which is obtained by crystallizing a mother glass body molded into a plate in advance.
In the above device, the substrate shrinks with a rise in temperature, which reduces an applied tension in the grating region of an optical fiber. On the other hand, with a fall in temperature, the substrate stretches to increase an applied tension in the grating region of an optical fiber. In such a way, a tension applied to an FBG is caused to change according to a change in temperature to thereby enable a spacing of the grating in the grating region to be adjusted, with the result that a temperature dependency of a center reflective wavelength can be cancelled. It is also disclosed that while, in an optical communication device with such a substrate, glass, polymer or metal can be used for adhesion and fixing of FBG, polymer, especially, an epoxy resin adhesive, is suitable for fabrication of the device with a high efficiency.
Furthermore, in Japanese Patent Laid Open No. 2000-503967, the reason why this glass-ceramic substrate has a negative thermal expansion coefficient is described below.
Not only does the glass-ceramic substrate have a microcrack, but also includes a crystalline phase (&bgr;-eucryptite solid solution) having a large negative thermal expansion coefficient along the c axis direction and a positive thermal expansion coefficient along the a axis direction. Additionally, the crystalline phase shrinks at the time of cooling along the a axis direction of a crystalline phase, but the shrinkage of the glass-ceramic substrate at the time of cooling is suppressed since clearances in the microcracks grow. On the other hand, the crystalline phase expands at the time of cooling along the c axis direction of the crystalline phase with no respect to microcracks. As a result, the glass-ceramic substrate has a negative thermal expansion coefficient because of a small contribution of a positive thermal expansion coefficient along the a axis direction and a large contribution of a negative thermal coefficient along the c axis direction.
A problem has remained that the glass-ceramics substrate has, however, a large hysteresis in thermal expansion which causes a hysteresis of a center reflective wavelength of an FBG to be large, with the result of a great change in center reflective wavelength of an FBG according to a change in temperature. Note that a hysteresis in thermal expansion shows a phenomenon that non-coincidence arises between behaviors in the courses of a rise and fall in temperature where a material stretches and shrinks according to a change in temperature.
Contrast to this, a method is disclosed in Japanese Patent Laid Open No. 2000-503967, in which a heat cycle treatment is performed at a temperature of 400 to 800° C. in order to reduce a hysteresis in thermal expansion of a glass-ceramic substrate to stabilize an internal structure, whereas a hysteresis in thermal expansion reduced in such a method is still unstable to changes in environment such as temperature and humidity and an initial value is difficult to be maintained. Additionally, such a heat treatment causes a fabrication process to be complex, thereby leading to a problem to increase in cost.
In WO 01/04672, a disclosure is given in which if a athermal member that is made of a polycrystalline body (ceramic made of a sintered body of powder) having a major crystal of &bgr;-quartz solid solution or &bgr;-eucryptite solid solution, a spacing between crystal planes thereof that gives a major diffraction peak in X ray diffraction measurement being smaller than 3.52 Å and a negative thermal expansion coefficient is used as a substrate of an FBG, not only can a temperature dependency of a center reflective wavelength of the FBG be suppressed, but a hysteresis in thermal expansion is also reduced. Note that since this ceramic has clearances in grain boundaries in the interior thereof and further has &bgr;-quartz solid solution or &bgr;-eucryptite solid solution showing a behavior of anisotropic thermal expansion, the ceramic has a negative thermal expansion coefficient due to a mechanism similar to the above glass-ceramics.
However, when a device using the above glass-ceramic or ceramic as a substrate exposed to a high temperature and high humidi
Asai Mitsuo
Matano Takahiro
Sakamoto Akihiko
Uehara Hitoshi
Yoshihara Satoru
Arent & Fox PLLC
Nippon Electric Glass Co. Ltd.
Robertson Jeffrey B.
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