Dual-Focus Lenses (Bi-focal Lenses)

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

• 50-100% faster cutting of existing materials

• Thicker materials may be cut

• Polished finishes can be achieved

• Elimination of sub-surface dross

• Cost Savings on assist gas

• Immediate cut initiation with no top surface dross

• Reduced sensitivity to errors in setting the focus

• Direct replacement of existing lenses

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 Gaussian formulae.

An increase in the diameter of the secondary focal area has two effects:

(a) Increases the fraction of energy directed to the lower focus

(b) 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:

• The diameter, edge thickness and focal length are chosen to match their current conventional lens.

• dF is thickness of material to be cut. If a range of thicknesses is specified, dF should be towards the thinner end. If the range of thickness varies by more than a factor of 2, then perhaps more than one lens is recommended.

• 2h is selected to be 50% of the 1/e² beam diameter if the beam mode is a good ‘00’ Gaussian mode. Increase to 60% if the mode looks more like ‘01*’ with a strong central dip.

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:-

* Reduced / eliminated cut dross

* Increased thickness of cut capability

* Increased cut speed

* Improved kerf quality

* Reduced assist-gas usage

* Improved process control (die-board processing)

This technical data sheet presents a range of standard products. Non-standard lenses can be designed and fabricated to order.

Technical specifications

Note: laser induced damage threshold (LIDT)

Standard parts

• There are 3 standard basic focal lengths: 95mm, 127mm, 190mm.

• Each FL has several options for the difference in FL, chosen from the values dF = 5mm, 7.5mm, 10mm, 12.5mm, 15mm.

• Each option is available in one of five values for 2h. Standard values for 2h are: 4.0mm, 5.0mm, 6.0mm, 7.0mm, 8.0mm.

• The tabulated value ‘%L’ is the percentage of beam power in the lower focus, for a TEM00 beam of 12.0mm (1/e2) diameter.

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:
 

              * Increased thickness of cut capability

              * Improved kerf quality

              * Reduced assist-gas usage

              * Improved process control (die-board processing)

This technical data sheet presents a range of standard products. Non-standard lenses can be designed and fabricated to order.

Technical specifications

Standard parts

• There are 3 standard basic focal lengths: 127mm, 190mm, 254mm.

• Each FL has three options for the difference in FL, chosen from the values dF = 5mm, 10mm, 15mm,20mm.

• Each option is available in one of five values for 2h. Standard values for 2h are: 5.0mm, 7.0mm,10.0mm, 12.5mm, 15.0mm.

• The tabulated value ‘%L’ is the percentage of beam power in the lower focus, for a TEM₀₀ beam of16.0mm (1/e²) diameter.

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:

* Increased thickness of cut capability

* Improved kerf quality

* Reduced assist-gas usage

* Faster processing .. see note (1)

* Reduced under-surface dross .. see note (1)

Technical specifications

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

• There are 3 standard basic focal lengths: 127mm, 190mm, 254mm.

• Each FL has three options for the difference in FL, chosen from the values dF = 5mm, 10mm, 15mm, 20mm. Each option is available in one of five values for 2h. Standard values for 2h are: 5.0mm, 7.0mm, 10.0mm, 12.5mm, 15.0mm.

• The tabulated value ‘%L’ is the percentage of beam power in the lower focus, for a TEM00 beam of 16.0mm (1/e2) diameter.

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:

* Increased thickness of cut capability

* Improved kerf quality

* Reduced assist-gas usage(1)

* Faster processing

* Reduced under-surface dross

* Improved initial cut penetration

Technical specifications

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

• There are 3 standard basic focal lengths: 127mm, 190mm, 254mm.

• Each FL has three options for the difference in FL, chosen from the values dF = 5mm, 10mm, 15mm, 20mm. Each option is available in one of five values for 2h. Standard values for 2h are: 5.0mm, 7.0mm, 10.0mm, 12.5mm, 15.0mm.

• The tabulated value ‘%L’ is the percentage of beam power in the lower focus, for a TEM00 beam of 16.0mm (1/e2) diameter.

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:

* Increased thickness of cut capability

* Improved kerf quality

* Reduced assist-gas usage

* Increased cutting speed

* Reduced lower-surface dross

This technical data sheet presents a range of standard products. Non-standard lenses can be designed and fabricated to order.

Standard parts

• There are 3 standard basic focal lengths: 190.5mm, 254.0mm, and 317.5mm. (7.5", 10.0" and 12.5", respectively).

• Each FL has three options for the difference in FL, chosen from the values dF = 10mm,15mm, 20mm, 25mm. Each option is available in one of five values for 2h. Standard values for 2h are: 10.0mm, 12.5mm, 15.0mm, 19.0mm, 22.5mm.

• The tabulated value ‘%L’ is the percentage of beam power in the lower focus, for a TEM00 beam of 25.0mm (1/e2) diameter. For smaller or larger beams see eq. 23.00a, data section 23.00.

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.