tunable laser source ecl tls external cavity laser filters thin film tunable filters in line filters in-line filters sweeping laser stepping laser optical test equipment t&m test & measurement passive component test optical cross connect cross-connect oxc mux multiplexer dwdm wdm narrow linewidth r&d optical power meter dense wdm koshin dbm dbmoptics koshin kogaku graviton c-band l-band wavelength smsr sse stse attenuator tunable mux multiplexer polarization dependent loss pdl side mode suppression modulation fwhm ls601a return loss insertion loss 1550 nm pm raman component spectrum analyzer swept osa swept optical spectrum analyzer 2004