Linearity Characterization based on single wavelength

Method of Characterizing sensor linearity and its independence of wavelengths

Scope:

This letter provides justification for specifying the linearity of a thermopile laser sensor over its entire operable wavelength range using linearity characterization results obtained at a single wavelength.

Thermopile laser sensors and sources of nonlinearity

Thermopile laser power sensors convert laser energy into heat via absorption. The flow of heat across the thermopile generates a measurable voltage which in turn is measured electronically and used to determine the power of the incident laser.

At the power densities specified for thermopile laser sensors, of the order of KW/cm2, the mechanism of absorption is linear. Nonlinearity in the absorption mechanism may in deed occur, for example, at the onset of ablation processes, but only at much higher power densities of the order of GW/cm2.

Nonlinearities of the thermopile device and the following electronics under normal operation conditions should be characterized. However, since those nonlinearities only occur after the conversion of laser energy to heat, they do not depend on the laser wavelength.

Making sure nonlinearity measurement covers the entire operating range

Nonlinearity measurement is done by conducting a series of measurements with known laser powers and comparing the measurements obtained by the DUT (Device Under Test) to the known laser values. The range of laser powers used must be such that it will generate the full range of heat flows that the sensor may encounter under normal conditions.

Because heat flow is a function of the absorbed power and not the incident power, the wavelength does carry some importance. If, for example, nonlinearity is measured at a wavelength where the sensor absorbs only 10% of the incident laser power, we can expect much less heat flow compared to a measurement performed at a wavelength where the sensor absorbs 95%, for the same laser power.

Fortunately, Ophir sensors have a much flatter spectral absorption, and nonlinearity measurements are performed at wavelengths and power levels that ensure heat flows reach their maximal values.

Revised 18.08.24, Authors: Raphael Cohen, Physics R&D team manager