Femtosecond laser written waveguides in glass were characterized across the full telecom spectrum to gain insight into waveguide loss mechanisms, and to aid in the design of a low-loss 1300/1550 nm wavelength demultiplexer. A lambda-4 wavelength scaling of propagation loss confirms Rayleigh scattering as a principal loss mechanism. Laser exposure was optimized for generating low-loss directional couplers with high isolation between the 1300 and 1550 nm bands. Dispersive coupling in the straight and curved wavelength regions was balanced with a 1.5-fold difference in 1300 and 1550 nm beat lengths, leading to the first demonstration of 1300/1550 nm demultiplexer written with a laser. A minimum interaction length of 3.2 mm, ~2 dB insertion loss and channel isolations of 16.7 and 18.8 dB are reported.