The setup used to calibrate the wavefront measurement device is represented in the Figures 10 and 11. This setup comprises 1calibration lens LCalib and 1 calibration mirror MCalib. The image of the fiber output by L1, L2 and BS is located at the focal plane of LCalib. This lens collimates and directs the beam toward MCalib placed in auto-collimation. The beam reflected by MCalib is transmitted by BS (respectively reflected) toward L3 and WFS (respectively toward L2, L1 and the Fiber output face).
LCalib is a commercial achromatic doublet of clear aperture diameter ΦLCalib = 22 mm and focal f = 50 mm manufactured by the company Thorlabs. The irregularity of the doublet surfaces is specified to be lower than 160 nm PTV over the clear aperture (λ/4 at λ = 633 nm). MCalib is a mirror of clear aperture diameter ΦMCalib = 40 mm manufactured by the company Newport. The surface error is specified to be lower than 63 nm PTV over the full clear aperture (λ/10 at λ = 633 nm). In our case, the beam footprint on LCalib and MCalib corresponds to a circle of diameter ΦFP < 5 mm. The suface covered by the beam foot print is thus 19 time (respectively 64 time) lower than LCalib useful surface (respectively MCalib useful surface). At first approach, we thus consider that the wavefront errors induced by LCalib and MCalib surface qualities can be neglected.
The calibration of the setup comprises 3 steps described hereafter. The step 1 consists of adjusting the orientation of MCalib to superimpose the fiber output (object) and the image of this fiber output by L1, L2, BS LCalib and MCalib. The control is done visually by observing the position of the image on the fiber output face. The step 2 consists of adjusting the axial position of LCalib to minimize the Focus term of the WFE measured by the wavefront sensor. The step 3 consists of referencing the position of the spot images provided by the micro-lenses on the WFS detector plane. The wavefront measured after the spot referencing is called the “Reference wavefront”.
We have assessed the calibration error by measuring this reference wavefront before and after measuring TStar WFE. In a first time, we measured the reference wavefront obtained just after the calibration and before measuring TStar WFE . This reference wavefront is represented in the Figure 12-left and the wavefront error is WFERef,1< 1 nm RMS. In a second time, we measured the reference wavefront after measuring the TStar WFE. The result is represented in the Figure 12Right. This time, the wavefront error on the reference wavefront is WFERef,2 ≈ 10 nm RMS. This error is due to the error in the orientation of MCalib to auto-collimate the beam. Indeed, any error in the orientation of MCalib induces off-axis aberrations and thus calibration errors. We thus deduce that the calibration error due to the mirror orientation error is
equal to 10 nm RMS typically.