Generation of radially-polarized and azimuthally-polarized beams in the two-micron band using a space-variant half-wave plate

Lin, Di, Shardlow, P., Butler, A., Beresna, M., Kazansky, P.G. and Clarkson, W.A. (2015) Generation of radially-polarized and azimuthally-polarized beams in the two-micron band using a space-variant half-wave plate. SPIE Photonics West '15, , San Francisco, United States. 07 - 12 Feb 2015.

Abstract

Radially and azimuthally polarized beams have attracted growing interest for use in a variety of applications. The most popular way to obtain these beams at relatively low power levels is to transform a linearly-polarized TEM00 beam into a radially or azimuthally-polarised beam using an external polarisation mode converter. Traditionally, these converters were constructed from an arrangement of half-wave plates bonded together to form a segmented spatially-variant retardation plate and, as a consequence, they generally suffered from low polarization purity and low transformation efficiency. However, recent work on femtosecond laser writing of nanostructure gratings in silica glass has allowed the realization of a new type of polarization converter with improved performance. In these converters the grating structures induce birefringence with slow and fast axes aligned parallel and perpendicular to the grating direction respectively, allowing the construction of a continuously space-variant half-wave plate (S-waveplate). S-waveplate provides dramatically improved polarization purity compared to segmented retardation plates. Here we report on an S-waveplate designed for use in ~2µm wavelength band for use with Tm-doped and Ho-doped solid-state lasers and fiber laser. The S-waveplate was tested with a relatively low power (~1.5 W) tunable Tm fiber laser yielding a donut-shaped radially-polarized (or azimuthally-polarized) beam with a polarization extinction ratio (PER) of 18dB and a transmission efficiency of ~86%. The beam propagation factor (M2) was measured to be ~2.1 and hence in close agreement with the theory. The prospects for further scaling output power and improving efficiency will be discussed.

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Contributors

Author: Di Lin

Author: P. Shardlow

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