Patent ID: 7091144

Claim:
A glass for a wavelength division multiplexing optical filter, comprising: 50–58 wt % of SiO 2 , 9–12 wt % of Na 2 O, 7–18 wt % of K 2 O, 11–18 wt % of CaO, 0–6 wt % of MgO, 0–6 wt % of BaO, 0–8 wt % of Al 2 O 3 , and 0–7 wt % of TiO 2 ; and having an average thermal expansion coefficient of from 110×10 −7 /° C. to 130×10 −7 /° C. within a range of 50–150° C.; having a Young's modulus of 80 GPa or lower; and having a density of 2.8 g/cm 3 or lower, wherein the glass has a bandpass, optical, multi-layer film on a surface of the glass.