Characterisation of helical modes using Optocraft SHSLab wavefront sensors

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The SHSLab is a powerful and comprehensive wavefront measurement solution from Micro-Epsilon. Developed by Optocraft GmbH, a member of the Micro-Epsilon group of companies since 2018, SHSLab Shack-Hartmann wavefront sensors are used for applications such as optics testing, optics alignment and laser beam characterisation. Wavefront sensors are used in production and R&D in optics manufacturing, laser industries, astronomy and space applications, as well as by contact- and intra ocular lens manufacturers, including mobile phone optics, microscope and photographic lenses.

Laser beams with an orbital angular momentum (OAM) or so called optical vortex beams are light waves that can transduce rotational transversal forces, for example, the turning of a door knob. The corresponding wavefront exhibits a helical structure and is usually described by Laguerre-Gauß modes. Figure 1 shows examples of helical mode wavefronts with different helicity or topological charge.

OAM beams can be formed by spiral phase plates, computer-generated holograms, spatial light modulators, optical nano-antenna arrays or ring resonators. Applications include the rotation of particles trapped in an optical tweezer, the generation of weak microfluidic currents, STED microscopy and edge filtering by spiral phase contrast imaging, as well as improved optical data transmission and quantum cryptography. A versatile tool for the characterisation of OAM beams is the Shack-Hartman wavefront sensor.

simulated helical fields
Figure 1: Simulated helical modes with topological charge 1, 2 and 3.

Experimental setup

Arrangements of the setup
Figure 2: Experimental setup for helical wave-front measurement.

In the experimental setup shown in Figure 2, the light from a fibre coupled laser diode emitting at a wavelength of 635 nm is collimated and passed through a spiral phase plate. The Shack-Hartman wavefront sensor SHSCam-BR-110-GE with a lateral resolution of 58 x 43 microlenses is placed directly behind the phase plate  in order to minimise wavefront propagation effects. For the sake of simplicity, no relay optics were used. At first, a measurement with the phase plate removed from the beam path is taken and used as a reference for the subsequent measurements. Spiral phase plates with different topological charges of 1, 2 and 3 were measured.

Test results

software images of wavefront spot displacement
Figure 3: Spot displacements as measured (left) and with tilt and defocus subtracted (right).

Due to the phase discontinuity present in helical wavefronts, classic Shack-Hartman wavefront reconstruction algorithms fail to calculate the helical wavefront. However, in order to analyse the influence of the spiral phase plate on the transmitted wave, it is usually sufficient to evaluate the spot displacement distribution detected by the wavefront sensor, which actually represents the gradient field of the corresponding wavefront.

SHSWorks is a powerful and comprehensive wavefront analysis software package that allows complete Zernike polynomial analysis, PSF/MTF calculation, laser beam parameter calculation and refractive data. The software offers functionality to automatically remove tilt and defocus from the spot field, which is necessary for the analysis of the helical gradient field and the comparison with simulation results (see Figure 3).

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In addition, SHSWorks calculates the topological charge, l , from the gradient field by evaluating the following path integral:

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The table below shows the topological charges resulting from the measurement of 15 different spiral phase plates, consisting of three sub-groups with a topological charge of 1, 2 and 3 respectively. The deviation of lmeasured from ldesign results from the discretization due to the limited number of sampling points.

Optocraft SHSInspect instruments are widely used in R&D and production when performance, reliability and efficiency counts most. Based on the state-of-the-art wavefront sensor SHSLab, Optocraft offers measurement modules, turnkey instruments and automated tools for a great variety of applications in the field of optics and optical systems, such as:

  • objective lens testing
  • testing of optical elements and windows
  • surface shape measurement
  • inline measurement and automation

Optocraft provide wavefront sensors and inspection systems that are distinguished by their high speed, single-shot measurements, excellent unreferenced accuracy, extreme dynamics and broad spectral ranges. They are also able to measure wavefronts with very strong higher order aberrations. They can measure large tilt angles and strongly defocused beams. They offer high intrinsic stability and reliability, powerful, customisable evaluation software and are versatile and flexible in usage. Optocraft’s systems are in operation in many demanding customer applications.

For more information on the Optocraft product range, please visit www.micro-epsilon.co.uk or call the Micro-Epsilon sales department on +44 (0)151 355 6070 or email info@~@micro-epsilon.co.uk

Further Information

Wavefront Sensors
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