Optical elements that exhibit wavelength- or angular-selective properties due to diffraction or optical interference effects are often used to stabilize, point, combine or demultiplex optical beams in laser systems. Such optical elements include diffraction gratings (including holographic, Bragg, and ruled gratings) and optical thin film filters. During laser system transients, such as ramping optical power from a cold start, the residual optical absorption in such elements can cause shifts in their spectral characteristics. For example, their transmitted wavelength may change due to thermally induced changes in the optic’s refractive index or dimensions. While such transients can in principle be mitigated by aggressive heat sinking, this approach is not always practical; this is especially the case for transmissive optics with large optical apertures because their overall thermal resistance tends to be quite large.
This invention discloses a method to minimize transient variations in the wavelength- and/or pointing-behavior of an optic, without requiring a reduction in its thermal resistance, optical absorption, or operating irradiance. The invention employs a combination of a time-varying heat source and time-varying thermal resistance and/or heat sink temperature to achieve temperature stability of the optic during transient variations in the optical power to which it is exposed. To maintain a desired optic temperature at all times, independent of the optical exposure level of the optic, heating from the source and thermal resistance are reduced to compensate for changes in the optical exposure of the optic. This compensation approach does not require a feedback loop; the temporal profiles of the heat source and thermal resistance can be determined in advance of a transient, provided the optical exposure variation is known in advance.
LLNL’s novel method for stabilizing wavelength-selective optical elements during laser system startup transients has several advantages for high power laser systems using large aperture optics:
- Enables rapid achievement of full capacity operation after turning on or power change,
- Enables robust output wavelength stability and tuning,
- Enables fast and reliable laser output deflection,
- Enables efficient combination of multiple lasers output beams into a single beam.
- Enabling robust operation of high-power directed energy laser systems using large aperture optics.
- Increasing process throughput in industrial applications that employ high-power laser systems using large aperture optics.
Current stage of technology development: TRL 3 (March 2023)
LLNL has filed for patent protection on this invention.