Interferometer-Based vs. Grating-Based Optical Spectrum Analyzers

The fundamental difference between these instruments lies in how they resolve optical wavelengths. A diffraction-grating OSA, such as the Anritsu MS9710B or the Yokogawa AQ6370D, utilizes a physical monochromator optical front end. The incoming light is spatially dispersed by a grating, and narrow slits isolate individual wavelengths. This provides robust spectral analysis across broad telecommunications bands.

In contrast, an interferometer-based OSA, like the Advantest Q8344A, uses a Michelson interferometer to generate an interferogram. By translating a mirror and utilizing Fourier spectrum analysis, the instrument mathematically converts the interference pattern into an optical spectrum. This approach is highly precise for analyzing specific optical properties that depend on phase and path differences.

A primary functional advantage of interferometer-based systems is the capability to perform direct measurements of optical coherence length. The Advantest Q8344A utilizes its Michelson interferometer layout to calculate these coherence values natively—a capability not typically supported by spatial diffraction monochromators.

However, the spectral range differs significantly between these platforms. Interferometer-based platforms such as the Q8344A often operate in shorter wavelength bands, specifically covering 350 nm to 1000 nm. Diffraction-grating instruments excel in standard telecommunication windows. For example, the Anritsu MS9710B covers a single nominal range of 0.6 µm to 1.75 µm (600 nm to 1750 nm), while the Yokogawa AQ6370D spans 600 nm to 1700 nm.

For applications requiring precise resolution adjustments and deep dynamic ranges, diffraction-grating OSAs are highly refined. The Yokogawa AQ6370D offers seven distinct wavelength resolution settings, ranging from 0.02 nm to 2.0 nm, alongside a minimum sampling resolution of 0.001 nm. This allows technical users to fine-tune resolution according to their testing parameters.

In terms of level sensitivity and range, modern grating-based instruments demonstrate impressive performance. The Yokogawa AQ6370D measures optical levels between +20 dBm and -90 dBm with a level accuracy of ±0.4 dB at reference wavelengths of 1310 nm and 1550 nm. Similarly, the Anritsu MS9710B guarantees an optical reception sensitivity of -90 dBm, making these diffraction-grating units ideal for low-signal analysis in fiber optic links.