Depolarization Compensator

Advantages vs. Alternatives

• No absorption
• Very low scattering
• Custom and continuous point-by-point patterns
• Maximum power extraction possibility without additional beam quality degradation
• Flexibility to compensate different amounts of depolarization by stacking more than one element
• Saves space, is easy to handle
• Significantly lower price
• WOP solution – depolarization compensator

Thermal effects in a high-power laser’s gain medium create predictable axially symmetric temperature gradients. Temperature gradients generate mechanical stresses in the pumped crystal, which lead to induced birefringence.

Generated optical anisotropy causes significant power losses if a laser system contains polarization-sensitive elements (e.g. Brewster plates, Faraday rotators).

Workshop of Photonics | WOP, in a joint effort with Ekspla Ltd., based on Ekspla Ltd invention EP3712664 (A1), developed and verified a solution to solve the depolarization loss issue – an optical element that compensates for distortion of original polarization in the gain medium.

A subpicosecond laser system was investigated, featuring a fiber CPA-based seed laser FemtoLux 30 (Ekspla) and a double-pass end-pumped Yb:YAG crystal power amplifier.

The key novelty of the system was the application of depolarization compensation using a specially designed spatially variable wave plate (SVWP), which allowed the extraction of nearly maximum power from such an amplifier without additional beam quality degradation.

Our proposed depolarization compensation method – a depolarization compensator – is more beneficial compared to other methods, such as intracavity quarter-wave plates, intracavity Faraday rotators, classical depolarization compensation layouts with two identically pumped and relay-imaged gain media, and different crystal cut directions.

The reasons are:

• The substrate of the spatially variable wave plate (SVWP) is fused silica, providing low bulk absorption of laser radiation and featuring a significantly lower nonlinear refractive index, as compared to a Faraday rotator, thus minimizing thermal effects and nonlinear interaction in high-intensity lasers;
• The SVWP element is compact (6 mm in thickness, usually 25.4 mm in diameter), whereas Faraday rotator material is usually at least 20 mm in length;
• There is the possibility to compensate for depolarization in the highly pumped gain medium, which is not the case using a simple approach with a quarter-wave plate;
• It is not overly sensitive to alignment and specific configuration;
• It is very practical, as induced/compensated depolarization levels can be tuned by either changing the incident laser beam size or stacking a few SVWPs in the same optical layout.