When discussing the characteristics of the laser characterization tool, a really common confusion occurs between the terms “Optical Spectral Resolution” and “Wavelength Meter Resolution”. Indeed, both of these specifications contain the word “resolution” and are amongst the most common specifications of these characterization tools, but are referring to two widely different characteristics.
The aim of this article is thus to well define both of the specifications and illustrate them by an example to avoid any mix up between the 2.
Optical Spectral Resolution
The Optical Spectral Resolution (OSR), also denoted Δλ, describes the ability of an optical characterization tool, such as a Laser Spectrum Analyzer, to differentiate between two close lines in an optical spectrum. The spectral resolution is closely related to the resolving power by the equation, and is usually expressed in nm (nanometers) for low resolution tools or pm (picometers) and GHz (gigaHertz) for high resolution tools.
Several criterions can be used to determine the Optical Spectral Resolution of an instrument. The figure 1 illustrates the resolution as observed with the Dawes, Sparrow and Rayleigh criteria for two close lines meeting at the FWHM. Amongst these, the Rayleigh criterion is the most generally used since it allows an easier differentiation of the peaks by the user of the instrument.
It is also worth noticing that the OSR resolution approximately is the minimum linewidth that can be resolved.
Wavelength meter resolution
Unlike the OSR, the “wavelength meter resolution”, also known as “wavemeter resolution” or “wavelength deviation sensitivity” is a characteristic of any device that gives an accurate wavelength output such as a wavelength meter.
In optics, the resolution of a measuring device is determined by the minimum distance between two lines which are brought closer and closer together until two lines can no longer be observed in the output.
Resolution thus corresponds to the maximum deviation, i.e. RMS deviation (1σ) in case of noise limitation, between successive measurement under constant input and environmental conditions. In practice, the wavelength meter resolution also can be assimilated to the smallest wavelength change which can be observed with certainty by an instrument.
The two measurements in figure 2 and 3 illustrate the 20 MHz wavelength meter resolution performances of the LW-10 wavelength meter (by RESOLUTION Spectra Systems) on 20 MHz triangular ramps at 800 nm.
On these figures, it can be observed that the LW-10 wavelength meter offers a long-term stability better than 20 MHz (3σ = 11.7 MHz) and has the ability of resolving a scanning ramp of 20 MHz amplitude. Because it is easily fulfilling the 2 aforementioned criteria, it can therefore be defined as a 20 MHz resolution wavelength meter.
Figure 4 summarizes by a schematic the main difference between the 2 characteristics that are the wavelength meter resolution and the optical spectral resolution in order to avoid any confusion between the two. Finally, it is also important not to mix up these two characteristics with the commonly used “sampling step” also known as “ digital resolution” being the smallest spectral sampling element detectable by a detector (usually expressed in nm/pixel for the OSR).