Optical wavelength filtering apparatus with depressed-index claddings

An apparatus for separating light at a first wavelength λ1 from light at a second wavelength λ2, where λ1<λ2, said apparatus comprising:

• a) a core having a core cross-section and a refractive index no, said core comprising an optically active material having a first gain at said first wavelength λ1 and a second gain at said second wavelength λ2;
• b) a depressed cladding surrounding said core, said depressed cladding having a depressed cladding cross-section and a refractive index n1;
• and
• c) a secondary cladding surrounding said depressed cladding, said secondary cladding having a secondary cladding cross-section and a refractive index n2;
• wherein said core cross-section, said depressed cladding cross-section, said secondary cladding cross-section and said refractive indices no, n1, n2 are selected to produce:
• (i) a fundamental mode cutoff wavelength λc such that λ1<λc<λ2;
• and
• (ii) a loss at the second wavelength λ2 at least equal to 70 dB less than said gain at said second wavelength λ2.