18 October, 2021
WOP Developed a Solution to Solve Depolarization Issue

Workshop of Photonics | WOP, in a joint effort with Ekspla Ltd., developed and verified a solution to solve the depolarization issue – an optical element that compensates distortion of original polarization in the gain medium.

Due to the unique properties of precisely point-by-point inscribed nano-gratings, the depolarization compensator is flexible and versatile, and it can be widely adjusted according to customer needs.

Main 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

“In our opinion, our proposed method is more beneficial compared to others, such as intracavity quarter-wave plate [23,24], intracavity Faraday rotator [25], classical depolarization compensation layout with two identically pumped and relay-imaged gain media [19], and different crystal cut directions [20–22].”

The article – Depolarization compensation with a spatially variable wave plate in a 116 W, 441 fs, 1 MHz Yb:YAG double-pass laser amplifier – was published in Applied Optics.

Summary says:

In this work, a subpicosecond laser system was investigated, featuring 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 SVWP, which allowed the extraction of nearly maximum power from such an amplifier without additional beam quality degradation.

To the best of our knowledge, this method has been applied for the first time.

In our opinion, our proposed method is more beneficial compared to others, such as intracavity quarter-wave plate [23,24], intracavity Faraday rotator [25], classical depolarization compensation layout with two identically pumped and relay-imaged gain media [19], and different crystal cut directions [20–22].

The reasons are:

  1. the substrate of 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;
  2. 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;
  3. 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,
  4. it is not overly sensitive to alignment and specific configuration;
  5. it is very practical, as induced/compensated depolarization level can be tuned by either changing the incident laser beam size or stacking a few SVWPs in the same optical layout.
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