The State-of-the-Art Metrology Tool for

Laser Beam Diagnostics

Optics Characterization

Real-time Monitoring of Thermal Lens Effects in Optical Elements

 

Introduction

In many laser applications it is required not only to measure the intensity distribution of the beam but also its propagation characteristics which is mainly determined by its wavefront. The knowledge about the wavefront is e.g. essential in order to optimize the beam focus or to evaluate the quality of optical beam steering components. It is possible to obtain this kind of information via a wavefront sensor that is based on the Hartmann-Shack principle.

The Hartmann-Shack wavefront sensor offers the following features:

► All parameters from single measurement

► Wide spectral range:  1064 – 1 nm

► Dynamic range:  up to 100 λ (@633 nm)

► Sensitivity (optics testing):   < 100 pm

► Various micro-lenses and sensor (10 bit – 14 bit)

► USB 3.0 camera (ideal for laptop)

► Customized solutions

 

Operation principle

The Hartmann-Shack wavefront sensor of LLG was designed for comprehensive laser beam and optics characterization. The system uses a micro-lens array for dividing the incoming wave into a large number of sub-rays (cf. figure right). Intensity and position of the individual foci are monitored with a camera, enabling the reconstruction of both beam profile and wavefront from a single measurement.

These data accomplish real-time evaluation of beam propagation parameters (such as beam width, divergence, M²) which is especially important for pulsed or fluctuating sources.

 

 

 

Applications

► Beam characterization

► Optics testing (NIR … EUV)

► Adaptive optics

► ISO beam parameters

► M2 in real-time

► Zernike analysis

► Beam propagation

 

System Specifications

The Hartmann-Shack wavefront sensor comprises a camera and a software package

Hartmann-Shack camera (VIS – NIR)

Digital camera equipped with two-dimensional quartz microlens array, C-mount adapter, absorptive ND filter (C-mount), kinematic camera mount for angular alignment, USB3.0 interface for adaptation to PC or Notebook:

Spectral sensitivity:

Dynamic range:

Active camera area:

Resolution:

Frame rate:

Exposure time:

Microlens array:

Focal length

Lens dimension:

Fill factor

Subapertures:

Wavefront repeatability:

Wavefront accuracy:

Wavefront dynamic range:

Camera size  (incl. microlens array):

Power supply:

Camera cable:

~ 320nm  – 1100nm

14 bit (digital)

8.8mm x 6.6mm (2/3“)

1,392 x 1,040 pixels (square, 6.45µm)

up to 15Hz, external trigger

50µs …. several s

10mm x 10mm, refractive

~ 3.5mm

150µm x150µm

>95%

max. 50 x 40

≈ λ/1000 (rms) at 633nm

≈ λ/30 (rms) at 633nm

≈ 5λ per subaperture, ≈ 150λ total

≈ 80mm x 60mm x 40mm

via PC, for Notebook separate power supply

5m

Software package for wavefront analysis

Beam Diagnostics Software MrBeam 3.3 (Windows)  for real-time laser beam and wavefront diagnostics according to ISO standards (ISO 11146, ISO 13694, ISO 15367, ISO 11670); near-field profile, wavefront and far-field distribution from single measurement

General features:

  • Various profile acquisition modes: ´snap´,  ´grab´,  ´averaging´,  ´floating average´,  ´summation´
  • 2D profile arithmetics (ADD, SUB, MULT, DIV, EXP, LOG etc.)
  • flexible selection of area-of-interest for analytic beam evaluations
  • real-time display and evaluation of cross-sections (horizontal / vertical),
  • including profile-fit (Gaussian, super-Gaussian, top-hat etc.), ‘history’  function
  • 3D graphics with variable resolution and viewing angle
  • various colour palettes, incl. different analytic palette functions (e.g. ´intensity threshold´,  ´intensity window´)
  • easy-to-use ‘zoom’-functions
  • ‘preview’ of profiles stored on disk
  • fast access to profiles stored in RAM
  • comprehensive user-programmable macro language
  • compatible with Labview
  • export of all data, plots and profiles to Windows clipboard or printer

Beam and wavefront analysis:

  • on-line evaluation of beam parameters from near-field profile I(x,y) at camera position, obtained by integration over Hartmann-Shack subapertures (spatial resolution given by pitch of microlens array):

Beam width (2nd moment (x,y))

Centroid

Ellipticity

Uniformity

Relative pulse energy

Pointing stability etc.

  • fast wavefront reconstruction (> 1Hz), accuracy ~ λ/10 per subaperture
  • graphical representation of wavefront w(x,y) (= OPD at camera position) as 2D or 3D plot, tabulated numerical data
  • optional correction of w(x,y) for tilt (x,y) and defocus
  • real-time modal analysis (Zernike and Legendre polynomials), Seidel aberrations, optional subtraction from displayed wavefront
  • wavefront parameters according to ISO/DIS 15367 (rms wavefront deformation, wavefront  irregularity (P-V))
  • comprehensive beam propagation analysis by Fourier algorithm: computation of profiles at arbitrary z positions, especially near-field, far-field, waist profiles
  • real-time computation and display of propagation data (absolute accuracy < 10%):

Beam propagation ratio M2 (x,y)

Divergence θx, θy

Waist diameter (x,y)

Waist position (x,y)

Rayleigh length (x,y)

Astigmatic waist separation

  • user-programmable macro codes for acquisition and evaluation of transient wavefront distortions in optical elements (thermal lenses)