Dual-Focus Lenses

  • Update:Jun 13, 2017
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Dual-Focus Lenses (Bi-focal Lenses) are a revolutionary new type of lens for CO2 laser cutting that can be supplied as a direct replacement for your conventional lens.

Product Introduction

Dual-Focus lenses are a revolutionary new type of lens for CO2 laser cutting, that can be supplied as a direct replacement for your conventional lens. Dual-Focus lenses also allow much thicker materials to be cut at a given laser power. Additional advantages come from the fact that assist gas requirements are significantly reduced.

Advantages

How Do Dual-Focus Lenses Work?

By distributing a calculated fraction of the laser energy into a secondary (lower) focus, the lenses provide faster, cleaner cuts, with easier initiation and the elimination of sub-surface dross.

The laser beam is intercepted an focused by a lens having a central circular region with a focal length which is longer than the basic lens focal length. The secondary focal area is used to produce the concentration of laser energy further into the material, while the energy intercepted by the outer part of the lens produces the ‘upper’ focus. This results is high energy density and increased localised temperature at both the upper and the lower foci.

Technical considerations

The introduction of the secondary focal area introduces two new variables to the focusing lens:

1) The difference in focal lengths between the basic lens and the secondary focal area needs to be ‘tailored’ to the thickness of material to be cut. In general, the thicker the material, the greater should be the difference in FL.

2) The diameter of the secondary focal area, in relation to the diameter of the incident beam, determines the fraction of beam energy directed to the lower focus.

The propagation calculations for the beams forming the two foci can be approximated by the application of Gaussianformulae. An increase in the diameter of the secondary focal area has two effects:

1) Increases the fraction of energy directed to the lower focus

2) Reduces the focused waist diameter, so increasing the lower focus energy density and reducing the lower focus depth-of-focus.

For a TEM00 beam of (1/e2) diameter 2w, intercepted  by a secondary focal area of diameter 2h, the fraction of beam energy directed to the top and bottom focus are: Fraction to top focus = exp[-2(h/w)2] Fraction to bottom focus = 1 - exp[-2(h/w)2]

Applications

Successful applications include: * Cutting thick-section metals, including stainless steel and aluminium. * Improved die-board processing (with better process control). * Scribing/processing thick-section ceramics. Important note: when cutting stainless steel the (main) upper lens focus must be set near to the top material surface!

Dual-Focus product types

The main ranges of Dual-Focus lenses are:

FLDF-10.6-D28-T4.2 Series (1.1"dia 4.2mm ET lenses, 127 & 190mm FLs)

FLDF-10.6-D38-T6.2 Series (1.5"dia 6.2mm ET lenses, 127, 190 & 254mm FLs)

FLDF-10.6-D38-T7.4 Series (1.5"dia 7.4mm ET lenses, 127, 190 & 254mm FLs)

FLDF-10.6-D38-T9 (1.5"dia 9.0mm ET lenses, 127, 190 & 254mm FLs)

FLDF-10.6-D50.8-T9.6 Series (2.0"dia 9.6mm ET lenses, 127 & 190mm FLs)

Applications guideline

Based on practical trials, the following broad guidelines are offered for those customers wishing to make their own lens selection for cutting stainless steel:
Material thickness dF
Up to 6mm 5mm
8mm 7.5mm - 10mm
10mm 10mm - 12.5mm
12mm 12.5mm - 15mm
15mm 15mm - 20mm
Above 15mm Apply (experimental)

These are rough guidelines only.

Description of Part Number: FLDF-xxxx-D-T-F-dF-2h-yy

FLDF: single-element focusing lens with dual-focus

xxxx: laser wavelength, 10.6um.

D: lens diameter, mm

T: lens edge thickness, mm

F: basic focal length, mm

dF: difference in focal length, units of 0.1mm

2h: diameter 2h, in units of 0.1mm

yy: others

Example: FLDF-10.6-D28-T4.2-F095-dF100-2h060 has a diameter of 28mm (1.1”), a edge thickness of 4.2mm, a basic focal length of 95mm (3.75"), a difference of focal length of 10mm, and 2h = 6.0mm.

1. FLDF-10.6-D28-T4.2 series dual-focus lenses

FLDF-10.6-D28-T4.2 series dual-focus lenses are 1.1" diameter. They are intended to be used as direct replacements for conventional lenses in industrial laser systems with ‘full’ beam diameters up to about 19mm (0.75"). Useful for improved cutting of stainless steel and aluminium in systems with smaller beam sizes, this series of lenses are also intended for use in die-board cutting systems. In laser cutting, dual-focus lenses exhibit some or all of these advantages:- This technical data sheet presents a range of standard products. Non-standard lenses can be designed and fabricated to order.

Technical specifications

Material Lasergrade ZnSe
Diameter 27.94 +0/-0.1mm
Edge thickness 4.2 ±0.1mm
Centration Within 0.05mm ETV
Coatings AR/AR for 10.6μm (R<0.25% per surface)
Absorption <0.20% at 10.6μm
LIDT (cw) >2000W/mm
Accuracy of 2h ±0.5mm
Note: laser induced damage threshold (LIDT)

Standard parts

For 95mm focal length, part no. is FLDF-10.6-D28-T4.2-F095-dFyyy-2hzzz
For 127mm focal length, part no. is FLDF-10.6-D28-T4.2-F127-dFyyy-2hzzz
For 190mm focal length, part no. is FLDF-10.6-D28-T4.2-F190-dFyyy-2hzzz
2h (mm) Focal Difference dF (mm) %L
5.0 yyy-zzz 7.5 yyy-zzz 10.0 yyy-zzz 12.5 yyy-zzz 15.0 yyy-zzz
4.0 050-040 075-040 100-040 125-040 150-040 20%
5.0 050-050 075-050 100-050 125-050 150-050 29%
6.0 050-060 075-060 100-060 125-060 150-060 39%
7.0 050-075 075-070 100-070 125-070 150-070 49%
8.0 050-080 075-080 100-080 125-080 150-080 59%

2. FLDF-10.6-D38-T6.2 series dual-focus lenses

The FLDF-10.6-D38-T6.2 series of Dual-Focus lenses are intended to be used as direct replacements for conventional lenses in many industrial laser systems and applications. Designed to assist in a range of thick-section cutting jobs, the FLDF-10.6-D38-T6.2 lenses are intended to give some (or all!) of these advantages: This technical data sheet presents a range of standard products. Non-standard lenses can be designed and fabricated to order.

Technical specifications

Material Lasergrade
ZnSe Diameter 38.0 +0/-0.1mm
Edge thickness 6.2 +/-0.1mm
Centration Within 0.05mm
ETV Coatings AR/AR for 10.6ìm  (R<0.25% per surface)
Absorption <0.24% at 10.6ìm
LIDT (cw) >2000W/mm
Accuracy of 2h ±0.5mm

Standard parts

For 127mm focal length, part no. is FLDF-10.6-D38-T6.2-F127-dFyyy-2hzzz
For 190mm focal length, part no. is FLDF-10.6-D38-T6.2-F190-dFyyy-2hzzz
For 254mm focal length, part no. is FLDF-10.6-D38-T6.2-F254-dFyyy-2hzzz
2h (mm) Focal Difference dF (mm) %L
5.0(1) yyy-zzz 10.0 yyy-zzz 15.0 yyy-zzz 20.0(2) yyy-zzz
5.0 050-050 100-050 150-050 200-050 18%
7.0 050-070 100-070 150-070 200-070 32%
10.0 050-100 100-100 150-100 200-100 54%
12.5 050-125 100-125 150-125 200-125 70%
15.0 050-150 100-150 150-150 200-150 85%
Notes: (1) dF = 5.0 non-standard in FLDF-10.6-D38-T6.2-F254 series. (2) dF = 20.0 is standard for FLDF-10.6-D38-T6.2-F254 series only.

3. FLDF-10.6-D38-T7.4 series dual-focus lenses

The FLDF-10.6-D38-T7.4 series of Dual-Focus lenses are intended to be used as direct replacements for conventional lenses in Trumpf industrial laser systems and applications. Designed to assist in a range of thick-section cutting jobs, the FLDF-10.6-D38-T7.4 lenses will give some (or all!) of these advantages:

Technical specifications

Material Lasergrade ZnSe
Diameter 38.1 +0/-0.1mm
Edge thickness 7.4 ±0.1mm
Centration Within 0.05mm ETV
Coatings AR/AR for 10.6μm (R<0.25% per surface)
Absorption <0.24% at 10.6μm
LIDT (cw) >2000W/mm
Accuracy of 2h ±0.5mm
Note (1): An independant laser user with a 3kW Trumpf laser reports 35% process speed increase and 85% dross reduction when cutting 10mm stainless steel.

Standard parts

For 127mm focal length, part no. is FLDF-10.6-D38-T7.4-F127-dFyyy-2hzzz
For 190mm focal length, part no. is FLDF-10.6-D38-T7.4-F190-dFyyy-2hzzz
For 254mm focal length, part no. is FLDF-10.6-D38-T7.4-F254-dFyyy-2hzzz
2h (mm) Focal Difference dF (mm) %L
5.0(2) yyy-zzz 10.0 yyy-zzz 15.0 yyy-zzz 20.0(3) yyy-zzz
5.0 050-050 100-050 150-050 200-050 18%
7.0 050-070 100-070 150-070 200-070 32%
10.0 050-100 100-100 150-100 200-100 54%
12.5 050-125 100-125 150-125 200-125 70%
15.0 050-150 100-150 150-150 200-150 85%
Notes: (2) dF = 5.0 non-standard in FLDF-10.6-D38-T7.4-F254 series. (1)  dF = 20.0 is standard for FLDF-10.6-D38-T7.4-F254 series only.

4. FLDF-10.6-D38-T9 series dual-focus lenses

The FLDF-10.6-D38-T9 series of Dual-Focus lenses may be used as direct replacements for conventional lenses in certain Bystronic industrial laser systems normally using 9mm ET lenses. Designed to assist in a range of thick-section cutting jobs, the FLDF-10.6-D38-T9 lenses will give some (or all!) of these advantages:

Technical specifications

Material Lasergrade ZnSe
Diameter 38.0 +0/-0.1mm
Edge thickness 9.0 ±0.1mm
Centration Within 0.05mm ETV
Coatings AR/AR for 10.6μm (R<0.25% per surface)
Absorption <0.25% at 10.6μm
LIDT (cw) >2000W/mm
Accuracy of 2h ±0.5mm
Note (1) Using dual-focus lenses, the ‘main’ beam is focused near the top surface of the workpiece. As a consequence, the nozzle tip aperture can usually be smaller.

Standard parts

For 127mm focal length, part no. is FLDF-10.6-D38-T9-F127-dFyyy-2hzzz
For 190mm focal length, part no. is FLDF-10.6-D38-T9-F190-yyy-zzz
For 254mm focal length, part no. is FLDF-10.6-D38-T9-F254-dFyyy-2hzzz
2h (mm) Focal Difference dF (mm) %L
5.0(2) yyy-zzz 10.0 yyy-zzz 15.0 yyy-zzz 20.0(3) yyy-zzz
5.0 050-050 100-050 150-050 200-050 18%
7.0 050-070 100-070 150-070 200-070 32%
10.0 050-100 100-100 150-100 200-100 54%
12.5 050-125 100-125 150-125 200-125 70%
15.0 050-150 100-150 150-150 200-150 83%
Notes: (2) dF = 5.0 non-standard in FLDF-10.6-D38-T9-F254 series. (3) dF = 20.0 is standard for FLDF-10.6-D38-T9-F254 series only.

5. FLDF-10.6-D50.8-T9.6 series dual-focus lenses

The FLDF-10.6-D50.8-T9.6 series of dual-focus lenses are intended to be used as direct replacements for 2.0" diameter, conventional high-gas- pressure lenses in some (larger) industrial laser systems. Designed to assist in a range of thick-section cutting jobs, the FLDF-10.6-D50.8-T9.6 lenses will give some (or all!) of these advantages: This technical data sheet presents a range of standard products. Non-standard lenses can be designed and fabricated to order.

Technical Specifications:

Material Lasergrade ZnSe
Diameter 50.8 +0/-0.1mm
Edge thickness 9.6 ±0.1mm
Centration Within 0.05mm ETV
Coatings AR/AR for 10.6μm (R<0.25% per surface)
Absorption <0.26% at 10.6μm
LIDT (cw) >2000W/mm
Accuracy of 2h ±0.5mm

Standard parts

For 190mm focal length, part no. is FLDF-10.6-D50.8-T9.6-F190-dFyyy-2hzzz
For 254mm focal length, part no. is FLDF-10.6-D50.8-T9.6-F254-dFyyy-2hzzz
For 317mm focal length, part no. is FLDF-10.6-D50.8-T9.6-F317-dFyyy-2hzzz
2h (mm) Focal Difference dF (mm) %L
10.0(1) yyy-zzz 15.0 yyy-zzz 20.0 yyy-zzz 25.0(2) yyy-zzz
10.00 100-100 150-100 200-100 250-100 27%
12.50 100-125 150-125 200-125 250-125 39%
15.00 100-150 150-150 200-150 250-150 51%
19.00 100-190 150-190 200-190 250-190 68%
22.50 100-225 150-225 200-225 250-225 80%
Notes: (1) dF = 10.0 is non-standard in FLDF-10.6-D50.8-T9.6-F317 series. (2) dF = 25.0 is standard for FLDF-10.6-D50.8-T9.6-F254 and FLDF-10.6-D50.8-T9.6-F317 lenses only.
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