By: Michael Scaggs
Athermalization of focusing objectives is a common technique for optimizing imaging systems in the infrared where thermal effects are a major concern. The athermalization is generally done within the spectrum of interest and not generally applied to a single wavelength. By applying athermalization techniques to a laser system, a significant reduction in thermal lensing of the laser system can be realized. We describe a passive method minimizing thermal lensing of high power lasers.
To date, thermal management of the lens mechanics is about the only means to mitigate thermal lensing in high power laser applications. As optical glass, in general, is not a great thermal conductor there can be large temperature gradients across the lens that impacts its performance that even thermal management cannot handle. This means that most high power fiber laser applications require several minutes for the optical system to thermally stabilize before the shift in focus stays within the lasers focusing lens Rayleigh range (depth of focus).
The TLC (Thermal Lensing Compensation) patent pending (USPTO 12/756,642) optic design passively compensates thermal lensing for high power collimators and focusing lenses. Through the use of special optical materials, optical design and thermal management of the mechanics, thermal lensing for lasers with powers in excess of 1KW can be reduced to less than the Rayleigh range of the lens design.
The Haas Laser Technologies TLC Optics™ incorporate a balancing of the temperature coefficient of refractive index (dn/dT) between two high power laser optical materials, the thicknesses, air spacing and curvatures to minimize the thermal lensing between 20°C and 250°C using a proprietary optical design merit function. This ensures a high power laser beam is in focus from the moment the laser is first turned on to when it is turned off. This technique has been applied to a CW fiber laser operating to 20 kW of average power. The all passive optical design approach required no thermal stabilization time.