The Advantest Q8326 is an optical wavelength meter capable of high accuracy and high resolution measurement at the central wavelength of the light from laser diodes (LD) for DWDM. Using a He-Ne laser as the reference wavelength and the Michelson interference method for measurement, it assures a high accuracy measurement. This unit even provides a high sampling rate of 5 times per second, and is suitable for adjustment of oscillation wavelength from laser diodes for DWDM. Wavelength variation can also be measured with high resolution and high accuracy using a deviation display function. The Q8326 can measure wavelengths at a sampling speed of five per second so that wavelength fluctuations caused by tem-perature variations can be captured precisely.