Patent ID: 7079322

Claim:
A wavelength division multiplexer for combining and/or separating wavelength multiplexed light comprising at least three wavelength bands, comprising: a single optical substrate or a plurality of optical substrates integrated together; and at least two optical filters, a first optical filter and a second optical filter, having different characteristics, placed on the optical substrate or substrates, wherein center wavelengths of three wavelength bands, λ 1 , λ 2 , and λ 3 , satisfy: 0.92≦λ 2 /λ 1 ≦1.08, and 0.20≦λ 3 /λ 1 ≦0.92 or 1.08≦λ 3 /λ 1 ≦5.00, the first optical filter is placed in a light path of three-wavelength multiplexed light comprising three wavelength bands with the center wavelengths of λ 1 , λ 2 , and λ 3 to combine or separate light comprising a wavelength band with the center wavelength of λ 3 and two-wavelength multiplexed light comprising two wavelength bands with the center wavelengths of λ 1 and λ 2 , and reflects the light of λ 3 and transmits the two-wavelength multiplexed light of λ 1 and λ 2 , of the three-wavelength multiplexed light, the second optical filter is placed in a light path of the two-wavelength multiplexed light to combine or separate light comprising a wavelength band with the center wavelength of λ 1 and light comprising a wavelength band with the center wavelength of λ 2 , and reflects the light of λ 2 and transmits the light of λ 1 , of the two-wavelength multiplexed light, and a refractive index n A of incident medium of the first optical filter, and an angle θ A between the three-wavelength multiplexed light incident on the first optical filter and a normal to a surface of the first optical filter satisfy: θ A ≧15° and n A *Sin θ A ≦0.95, and a refractive index n B of incident medium of the second optical filter, an angle θ B between the two-wavelength multiplexed light incident on the second optical filter and a normal to a surface of the second optical filter satisfy: θ B ≧15° and n B *Sin θ B ≦0.85.