Design and manufacture of diffractive
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Optical Diffuser / Homogenizer Application notes

Contents Diffuser used with Multi Mode

Introduction

optical diffuser (custom)An optical Diffuser/Homogenizer (DF/HM) diffractive optical element (DOE) transforms a single or multi mode input beam into a well-defined output beam, characterized with a desired shape, and homogenized flat-top intensity (or other intensity profile, see below).
Holo/Or’s optical diffuser DOE is mainly used for high power laser applications, when uniform exposure is needed, with accurately defined sharp shaped edges, while enabling high efficiency.
The most common diffuser shapes are: round, square, rectangular, elliptical and hexagonal, however, almost any shape of image can be designed. It is also possible, to tailor the intensity distribution of the image, so that different areas receive more/less energy, as in the picture at the right.

Typical features and applications

Features
  • Works with single or multi mode input beams
  • Glass or plastic materials
  • NO centration requirement
  • Any output shape or symmetry
  • Uniform/Tailored intensity profile
  • Small transfer region
  • Exact diffusion angles
  • Single element / surface
Applications
  • Laser homogenizing/shaping
  • Laser material processing: perforation, ablation, derailing, marking, scribing and welding
  • Medical/aesthetic laser treatment
  • Beam shaping for Excimer lasers
  • Hot spot reducer

Specifications

Materials:Fused Silica, Sapphire, ZnSe, Plastics
Wavelength range:193[nm] to 10.6[um]
DOE design:Binary (2-level) and up to 16-levels
Diffraction efficiency:75% - 98%
Element size:2[mm] to 100[mm]
Coating (optional):AR/AR Coating
Custom Design:Tailored shape and intensity distribution
Pattern angles @532[nm]:Few mRad to ~41deg

General definition

A diffractive diffuser (HM) splits incident beam in semi random directions in order to achieve a desired shape of intensity profile in the far field or focal plane.
This method enables to design an element that will produce any arbitrary shape, intensity profile with precise angle and size of the output.
The diffuser performance strongly depends on the incident beam parameters. In general, one can distinguish between two qualities:

  • Highly coherent beam (and quasi-single mode) with M2 <~5
  • Multi-Mode laser beams (with M2 > 5)
Best results in terms of intensity profile, uniformity, efficiency and transfer region for single mode lasers can be achieved by using Top Hat (TH) designs like in (http://www.holoor.co.il/Diffractive_Optics_Products/Diffractive_Beam_Shapers/BeamShaper_home.htm).
When the incident beam is more complex to define – by having an arbitrary intensity distribution and not a near-to-perfect Gaussian (higher M2) one, diffractive optical diffusers become the preferred solution.

Examples of custom shapes

In addition to basic shapes as square, rectangular, round, elliptical, and line we can design any arbitrary shape. Below we show a few examples as illustration:
Optical Diffuser / Diffractive diffuser Optical Diffuser custom shape Optical Diffuser custom shape

Operation principle and design considerations

  • Minimum required parameters needed to chose/design a diffuser:
    • Wavelength
    • Output shape and intensity profile (Flat-top or customized)
    • Divergence angle (of output shape) or image size and EFL
    • Beam quality (M2)
  • Common diffuser/homogenizer elements are manufactured on a DOE window. Since the diffuser defines a certain diffusion angle, the customer is able to control the image size on the image plane by choosing a focusing lens with a correct EFL. Typical setup includes: a laser, a Homogenizer/Diffuser DOE and a focusing lens as presented below: Optical Diffuser set-up
  • The optical parameters of the diffuser can be easily calculated from geometric point of view:

    diffuser formula

    See also our interactive optical calculator for Diffuser/Homogenizer:
    http://holoor.co.il/Diffractive_Optics_Products/Calculators.php

  • Holo/Or has the capability to design an integrated solution by combining a DOE window and a specific focusing lens into one single hybrid element. Here, the diffractive pattern will be etched on the Plano side of the focusing lens (plano-convex lens). The advantages of this solution include less optical surfaces, compact dimension and low weight.
  • The minimum diffusion angle is equivalent to approximately 5-10 times the diffraction-limit. In addition, a larger angle ratio (to a certain level) will increase the performance quality. The formula for the diffraction-limited spot diameter at 1/e2 (D.L.spotsize) follows:

    Diffraction limit equation

    L = Working distance
    λ = Wavelength
    D = Input beam size on focusing element
    M2= Beam quality value of input laser beam

  • The edges of the diffused beam are generally steep and well defined. The ratio between input divergence angle to the HM’s diffusion angle, determines the ratio of the transition region to the well-homogenized region of the output beam.
  • We recommend keeping the input beam size (1/e2) under 67% of the clear aperture, to maintain good power efficiency results. This will ensure 99% energy throughput for Gaussian intensity distribution
  • Additional improvement in the uniformity performance may be achieved by using a high M2 input beam.

Beam shaping: single-mode versus multi-mode:

Table of properties for TEM00 and Multimode beams:

  Gaussian input beam (TEM00) Multimode input beam
Image precision Very Good Very Good
Uniformity Interference pattern (Speckles) Uniform (more modes = better uniformity)
Intensity density Good Very good
Edge steepness (transfer region) Very steep Reduced steepness with larger M2
Efficiency Depends on design Depends on design
Zero Order Sensitive Reduced sensitivity with larger M2
Output image
(typical)
Diffuser used with Single Mode Diffuser used with Multi Mode
Typical 1-D intensity profile
(typical)
Diffuser used with Single Mode Diffuser used with MultiMode Mode

Additional remark: The "noisy" oscillations or speckled output intensity profile of the diffuser in the Gaussian input case is due to interference. Nevertheless, many metrology applications benefit from speckled pictures. For material processing applications, the thermal distribution of energy is more important than a point intensity value. There is also a natural smoothing effect in many (but not all) applications, which erases most signs of fluctuations.

Effect on output shape - comparison between different beam qualities (M2 values)

  • Edge of the shape becomes wider
  • Speckles are reduced and intensity profile becomes more uniform
M2 = X (low) M2 = 2*X M2 = 4*X M2 = 8*X
Diffuser M2 = X Diffuser M2 = 2*X Diffuser M2 = 4*X Diffuser M2 = 8*X

Zero-Order effect:

The figures below show the Zero-order for the same element with different M2 values of input beam. One can see that with a low M2 input beam the Zero-order is concentrated in a smaller area, causing it to look much stronger than its neighboring area.

M2 = X (low) M2 = 2*X M2 = 4*X
Diffuser / Homogenizer Diffuser / Homogenizer Diffuser / Homogenizer

High homogeneity series (RH/HH/XH)

Holo/Or developed a new class of diffusers/homogenizers with enhanced performance referred to as the high homogeneity series (Homogeneity can be defined as the average intensity over an area unit). Its advantages are higher homogeneity, and lower zero order. This corresponds well with input beams of lower M2, and for applications where it is impossible to use a Top-Hat beam shaper product.

This DOE design includes two diffractive surfaces. The first decreases the coherence of the incident beam and the second surface shapes the beam.

Advantages and disadvantages of HH relatively to HM are displayed in below table:

parameter HH vs. HM
Homogeneity Much better
Efficiency Slightly less
Zero Order Significantly reduced
Transfer region Larger

This can be illustrated with the figures below:

HM HH
Beam Homogenizer Beam Homogenizer

Application example - Optical Setup with MM fiber

The optical setup below shows a laser coupled to a MM fiber to achieve uniform intensity distribution in the far field or alternatively in the focal plane when used with a focusing lens.
The number of modes in the output is proportional to the fiber length and core diameter. Each mode of the fiber's output will propagate through the diffractive Homogenizer to create an interference pattern in the focal plane. However, in contrast to a single mode beam, the multi-mode contains many of these modes. Each mode is creating a different pattern that overlap with each other. In the focal plane, this creates a well-averaged uniform output. The setup does not require any specific property of the laser beam parameters.

MultiMode fiber

Safety

laser radiation

General recommendations when working with a laser
http://web.princeton.edu/sites/ehs/laserguide/appendixB.htm

  • Locate beam at waist level or below. Do not place beam at eye level.
  • Close and cover your eyes when stooping down around the beam (where you will pass the beam at eye level).
  • When leaning over a table, beware of beam directed upward.
  • Enclose as much of the beam as possible.
  • Do not direct beam toward doors or windows.
  • Terminate beams or reflections with fire-resistant beam stops. Anodized aluminum or aluminum painted black (which is not necessarily fire-resistant) can work well for this purpose.
  • Use surfaces that minimize specular reflections (mirror-like reflection).
  • Locate controls so that the operator is not exposed to beam hazards.
  • Make sure warning/indicator lights can be seen through protective filters.
  • If you can see the beam through your laser eyewear, you are not fully protected.
  • View applications remotely.
  • Do not wear watches or reflective jewelry around Class 3B or 4 lasers.
  • Do not wear neckties around Class 4 open beam lasers.
  • In reality, all interlocks can fail.
  • The best defense is good understanding of the hazards.

Recommendations for alignment:

  • Isolate process.
  • Use lowest practical power.
  • View diffuse reflections only.
  • Use IR/UV viewing cards/eyewear.
  • Where possible, use HeNe alignment lasers

Handling:

Use gloves Mechanical damage Chemical Dust
gloves mecha chemical dust

 

Installing / sensitivity for mechanical tolerances

Optical diffusers are very straightforward to install inside an optical system.

Parameter Effect Comments
Z-translation (along optical axis) Not sensitive As long as all diffracted rays enter the focusing element
X-Y translation Not sensitive
Tilt X,Y Zero Order slightly increases
Tilt Z Not sensitive Output shape rotates
Incident beam size Not sensitive Have to be larger than minimum defined in spec
Polarization No effect

Design and integration tools

Integrating a Diffuser has now been made easier, with the help of VirtualLab Black Box files (LPD) for selected products. This makes the design and modeling of an optical system with our products as straightforward as possible.
The VirtualLab Black Box files are available in the homogenizer page product and the instruction for using VirtualLab BlackBox models are available in the following link: VirtualLab BlackBox models Instructions.

FAQs

Q: What is the difference between Top-Hat beam Shaper and Diffuser?
A: Comparison table between TH and HM:

Parameter Top Hat element Diffuser / Homogenizer element
Incident beam M2 <1.3(TEM00) Any
Output shape Any Any
Output Shape precision Excellent Good
Uniformity Very good Depends on input beam, more uniform with larger M2 of input
Edge steepness (transfer region) <= diffraction limited spot Depends on M2 of input beam less steepness with larger M2 of input)

Q: What will happen if I use other laser wavelength than specified (designed) wavelength?
A: Using a diffuser element at a wavelength other than the design:

  1. Each diffuser element is manufactured to be wavelength-specific. The diffractive etching pattern is unique for a given diffusion angle at a given wavelength. Therefore, using a diffuser element at a wavelength other than the design will result in a new diffusion angle as well as output beam size, proportional with the ratio of   ~used λ / design λ.
  2. The etching depth is optimized for a certain design wavelength, and use at another wavelength will cause energy migration in the beam, with some portion of the diffused power moving to the non-homogenized zero order, appearing as a central peak in output beam.

Nonetheless, our mask inventory can be used flexibly to produce a wide variety of diffusion angles over a wide wavelength spectrum, and only our etching process need to be optimized per requirement/customer.

Q: What is the recommended distance between DOE to the focusing lens? Can it be few millimeters?
A: For HM elements the distance between the DOE and a Focusing system is not important, as long as the clear aperture of the Focusing system will be large enough (~1.5 x HM size at lens position) to prevent energy losses due to truncation.

formula

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Phone: +972-89409687/8, Fax: +972-89409606

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