Thermally-Invariant Optical Coatings

Nick Gabriel, Woo-Bin Song, and Joseph J. Talghader

In high-power laser applications, management of thermally-induced stress is a major concern, especially in thin micro-optical structures. Without proper design, elevated temperature during illumination will cause shape deformation, adversely affecting optical performance. Careful consideration of the thermo-mechanical properties of the coatings can effectively eliminate deformation. The goal of this research is to develop and demonstrate deformation-free dielectric optical coatings over a very wide temperature range. There are many uses for this technology spanning the defense, space, and medical sectors; wherever high optical power density is an asset.

This is a follow-up to previous work by colleague Wei Liu, who demonstrated a micro-mirror with a distributed Bragg reflector (DBR) optical coating stack that was invariant over a 37°C temperature range. He used a "compensation layer" on top of the DBR stack with precisely chosen material properties and thickness to balance the thermal-expansion induced shape changes in the uncompensated stack. Invariance was defined as less than λ/60 change in mirror edge deflection for HeNe (633nm), corresponding to < 0.3 nm/° C of deflection. While useful, the small temperature range has little practical use.

Figure 1

Thermally-induced deformation before (top) and after (bottom) compensation; "High T" refers to 62 ° C. Courtesy of Wei Liu.

Modeling of the system and proper choice of compensation layer parameters is done by solving the matrix equation in the following figure such that C approaches 0. C essentially represents the change in curvature per unit temperature.

Figure 2

Matrix representation of thermal curvature in the multilayer system.

Where:
α_i = thermal expansion coef. of layer i
t_i = thickness
E_i = modulus
I = moment of inertia
s_i = t_iE_i
l_i = a weighted average of layer thicknesses above layer i

A similar approach should allow demonstration thermal invariance over a much larger range of temperature, ~ 100s of °C. Optical testing at elevated temperatures becomes quite difficult and restrictive, however, and is a major roadblock to demonstration of the concept. To this end, "microheaters" have been successfully fabricated, which are isolated membranes that self-heat from room temperature to roughly 700°C. Membrane temperature is continuously monitored via the integrated resistor.

Figure 3

Optical image of a microheater plate, with integrated resistor.

Figure 4

SEM image of a microheater device, showing the etch pit and metal contact pads.

Optical coatings are then applied to the microheater by various methods. The main parameter of interest is deformation of the mirror upon heating. This is precisely measured with an interferometric microscope; an example image is shown below.

Figure 5

Sample interferometric image of microheater plate with a DBR optical coating.