| 摘要 |
PROBLEM TO BE SOLVED: To reduce thermal distortion and residual stress at the time of sealing to prevent the inclination of a window sheet glass and thereby increase the sealing strength and the airtight reliability by forming a ring-shaped projection which surrounds a light transmitting window hole in an inner face of a top plate section of the main body of a cap. SOLUTION: The main body 1 of a cap is formed of metal whose coefficient of thermal expansion is approximate to that of glass. The main body 1 is provided with an annular top plate section 1a having a round window hole 4 at the center and an annular flange section 1c extended nearly vertically from the periphery of the top plate section 1a. Inside the top plate section 1a, an annular projection 6 which surrounds the light transmitting window hole 4 is provided. By this structure, thermal distortion is reduced at the time of sealing. After the sealing of a window sheet glass 2, the projection 6 cuts and catches into glass solder 3 to reduce residual stress of the glass solder 3. An anchoring function of the projection 6 prevents the release of the glass solder 3. As a result, the window sheet glass 2 can be sealed with good enough sealingness and thereby the airtight reliability of the cap for an optical element can be increased.
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