Optical spectrum analyzer (OSA)
The instrument that measures optical power as a function of wavelength. The standard tool for laser spectra, SMSR, OSNR, and amplifier gain measurements.
An optical spectrum analyzer records power versus wavelength across a set span, producing the spectrum from which most source-level parameters are read: center wavelength, side-mode suppression ratio, ASE floor, OSNR, EDFA gain and noise figure.
The dominant architecture is the scanning diffraction-grating monochromator: input light disperses off a grating, a rotating element sweeps wavelengths across an exit slit, and a photodetector records power. Two specifications define what such an instrument can and cannot resolve:
Resolution bandwidth (RBW): the optical filter width, typically settable from ~2 nm down to 0.02–0.05 nm on benchtop grating OSAs. Structure narrower than the RBW is smeared to the RBW: a DFB line of MHz true width displays as the instrument's ~GHz-class resolution shape. Laser linewidth is therefore not an OSA measurement. That requires heterodyne or delayed self-heterodyne methods. What the OSA does resolve cleanly is mode structure: Fabry–Pérot mode spacings and DFB side modes sit comfortably within its grasp.
Dynamic range: the ability to see a weak feature adjacent to a strong one, typically 60–70 dB at 0.4–1 nm offsets. This is what makes SMSR and OSNR measurements trustworthy.
Two other families matter: Fabry–Pérot–based analyzers (fast, fine resolution over narrow spans) and Fourier-transform/interferometric instruments including wavelength-meter hybrids that reach sub-picometer accuracy for absolute wavelength. When comparing readings across instruments, check whether power is reported per RBW slot (affects ASE and OSNR arithmetic) and whether the instrument was wavelength-calibrated recently.
Routine settings that decide measurement quality: span and RBW appropriate to the feature, video bandwidth/averaging against noise, sensitivity mode (sweep-speed tradeoff), and single-mode fiber input discipline. Multimode or misaligned input shifts both power and apparent wavelength.
For grating-OSA anatomy, resolution/dynamic-range specifications, and the standard measurement configurations, see the optical spectrum analyzer article.