The Advantest Q8344A makes use of a Fourier spectrum analysis approach based on a Michelson interferometer. Because of this, the Q8344A can measure optical coherence that cannot be obtained directly using diffraction-type optical spectrum analyzers which use monochromators to make their measurements. The features of this analyzer make it very effective for the evaluation of laser diodes used in optical data storage equipment, fiber optic gyroscopes, and related applications. The Q8344A has a built-in He-Ne laser that functions as the wavelength reference and ensures the wavelength measurement accuracy is maintained within +/- 0.1 nm (at 1.3 micro m). Because of this internal reference source, no calibration is needed, thereby allowing this analyzer to provide long-term repeatable measurements. This instrument is exceptionally well suited to the measurement of coherence length of LD and LED optical sources operated in continuous wave mode only.