Passive Compensation of Thermal Lenses

Fiber Collimator with TLC Optics (Source: Laser Components)

Fiber Collimator with TLC Optics (Source: Laser Components)

One common problem in high-power lasers is the long warm-up phase and unstable operating conditions that appear if thermal lenses form in the optics used. Thermal lenses are caused by the absorption of laser light in optics. Even if it is only in the ppm range, the local temperature increase within the optics is significant if lasers with several kW of power are used. The low thermal conductivity of optical materials results in a strong temperature gradient. The resulting refractive index gradient acts as an additional lens.
To compensate for the thermal lens, so-called TLC optics are used. This thermal lensing compensation passively compensates the thermal lenses of high-power collimators and focusing optics. A special material and design selection allows the reduction of thermal lenses in kW lasers to less than the Rayleigh length.
In common substrate materials, the dependency of the refractive index n on the temperature T results in a positive value (dn/dT > 0). There are also materials with a negative dn/dT. Due to the clever combination of such materials – taking into account absorption coefficients, thickness, air gaps, and the radii of curvature – it is possible to achieve compensation of the thermal lenses across a wide power and temperature range.
Laser Components offers both: simple focusing optics and entire lens systems are available with TLC optics: such as, for example, fiber collimators.
The components are made by Haas Technologies. Their modular components design makes it possible to easily adjust the TLC optics to a specific setup. In a fiber collimator, for example, the spot size, the focal length, and the type of cooling are individually adapted. (Source: Laser Technik Journal 1 / 2015)

Links: Laser Components

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