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 characterization. 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.
Experimental set up. Image Credit: Micro-Epsilon
For laser beam characterisation, the conventional camera-based method described in ISO 11146 requires several measurements at different locations along the beam path. When using a wavefront sensor, a single measurement is sufficient to characterise a laser beam with moderate or good beam quality. The following application describes the characterization of a diode laser using the Optocraft SHSLab wavefront sensor.
Experimental Setup
A diode laser with a wavelength of 635 nm coupled to a single mode fibre is collimated by a lens with a small focal length and then focused by a second lens with a large focal length (see figure 1).
For comparison, first a standard industrial camera is used with a pixel size of 3.75 µm to characterise the beam according to ISO 11146, taking several images of the beam cross-section at different positions along the beam. Next, the SHSLab wavefront sensor (type AR-S-130) is used to measure the wavefront at position zm = 144 mm in the divergent part of the beam.
Beam Parameter Calculation
The SHSLab measurement provides the intensity and phase distribution at the measurement position zm. This enables the wavefront curvature radius R, the 1/e2-diameter of the beam d(zm) and the beam propagation factor M2. M2 is calculated from the second spatial/angular moments σi, σa, σia of the measured intensity distribution/wavefront: (λ denotes the laser wavelength). Using this information, the position of the beam waist z0, the beam waist diameter dbw and the Rayleigh length zR can be calculated:
Comparison of Wavefront and Camera-Based Measurement
Figure 2 shows an example of the measured intensity distribution and wavefront used to calculate the beam parameters. From the wavefront/camera-based measurements the values 1.14/1.10 for M2, 0.056 mm/0.053 mm for dbw and 3.57 mm/3.52 mm for zR can be obtained. It is important to note that these results are influenced by the noise present in the camera images and by the parameters used for the wavefront reconstruction.
Stability of the Beam Parameter Measurement
In order to analyse the stability of the wavefront measurement, continuous measurements were taken with a frame rate of 3 Hz for a period of 30 minutes. The resulting mean values and standard deviations (rms) of the respective beam parameters are shown below. For all parameters, the fluctuations are reasonably small.
Beam Diameter Calculation
From the wavefront measurement taken at position zm = 144 mm the beam diameter d(z) is calculated at the positions z where the single camera images for the beam characterisation according to ISO 11146 were taken, using the equation:
The result gives a very good congruence of both methods (see plot in Figure 3). The maximum deviation between the diameter determined from the camera images and the diameter calculated from the wavefront measurement was 0.107 mm, the mean deviation was 0.034 mm.
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 provides 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.