Support Centre - The Technology Explained


What is a Sub Surface Laser Engraver (SSLE)?

Types of Laser Within Common Laser Marking Systems

There is now a multitude of laser marking systems available at very affordable prices. Most commonly, such machines are lasers that react with the surface of the material for marking.

The suitability/reaction of the material to the laser is determined by the “laser wavelength”.

CO2 lasers, otherwise known as “gas lasers”, work best with materials such as wood, many types of plastic, card, leather and even glass. The typical operating wavelength of such a system is 10.6um (10,600nm) within the “Infrared Spectrum (IR)” and this type of laser is a surface marking system. If you own a laser system already then it is most likely to be a CO2 laser.

YAG lasers, otherwise known as “solid state lasers”, work best with materials such as metals, most types of opaque plastic and ceramics. The typical operating wavelength of a YAG laser is 1,064nm within the “Near Infrared Spectrum (NIR)” and this type of system is a surface marking machine. However, some materials are transparent to this laser wavelength, such as plate glass for example.

It is therefore possible to push the beam of a YAG laser at 1,064nm through a piece of plate glass and engrave stainless steel on the other side without affecting the glass at all.

IR and NIR lasers are invisible.

By using specific and additional components it’s possible to alter certain types of YAG laser by “frequency doubling” the wavelength from 1,064nm to 532nm.

Visible light ranges from 700nm to 400nm. Frequency doubled YAG at 532nm falls within the “Green Spectrum”, which is visible, and is therefore known as a “green laser”.

Green lasers can be used for high speed, fine detail surface marking and are a particular advantage when marking plastics but machines are now available that incorporate green laser technology to use for marking the inside of transparent materials, mostly crystal.

Such machines are capable of marking 3-dimensional designs with breathtaking speed and detail and so have proven very popular for marking images, logo’s, etc, to the inside of crystal giftware and promotional items as well as being used for part identification, anti-counterfeit and even architectural marking.

Sub Surface Laser Engraving (SSLE)

These types of machine are commonly known as “Sub Surface Laser Engraving” systems or “SSLE” for short.

With SSLE, images are represented as a series of co-ordinated points within the material. These points are actually micro fractures caused by rapid heating when the laser fires to a focused point. Within the laser industry such fractures are termed as “point clouds”.

How The Image Is Created

As the laser beam passes through the crystal the material remains unaltered until the beam is honed to a “focal point”. The focal point is the smallest diameter that the lens is capable of focusing the beam to. The focal point will vary with each machine from approximately 20-80um depending on the design/build/technology levels of the machine.

A fracture is created when the laser beam is set to a sufficient energy enough to alter the structure of the material.

By altering the laser energy (power) the density/clarity of the point cloud can be adjusted. It’s important to note that using too much power and/or too many point clouds without sufficient spacing can cause the micro fractures to become a visible crack. At this point the image is flawed and so the part is rejected.

High resolution images tend to use point clouds of low laser power and low resolution images tend to use point clouds of high laser power.

Images are built-up by the laser marking many layers of point clouds. The laser always starts at the base of the image working upwards.

Design, control and layout software is used to determine the number and spacing of point clouds and layers within the image. Resolution is limited by the capability of the hardware.

How The Laser Beam Is Generated – Lamp Pumped Technology

SSLE, as with all YAG laser machines, use an intense and visible light as the source of the laser to “pump” the “laser medium” and generate the laser beam.

Older technology machines use a lamp as the light source.

Although lamp pumped lasers still have their uses in laser machines used for metal welding and cutting applications, as far as marking lasers are concerned, compared to more modern alternatives a lamp pumped marking laser is very inefficient and relatively unstable, deteriorating with use/time and often providing for inconsistent performance.

Lamps require frequent replacement, sometimes within just a few hundred hours of use, and are relatively high in power consumption. Therefore, frequency and cost of maintenance for a lamp pumped laser is relatively high. As a by-product of its very low efficiency a lamp pumped laser produces a high amount of heat that has to be taken away by a relatively large chiller unit. As a consequence, such a system often has a large footprint with high energy costs.

Risk of downtime when using a lamp pumped SSLE laser is higher than alternative technology and failure to maintain the system correctly can result in unnecessary and costly damage to the machine. Lamp pumped green lasers generate a relatively large focal point of between 40 to 80um. At this size significant space is required to be between the point clouds to prevent cracking. Therefore, such machines are incapable of engraving high detail, high resolution images.

Another weakness of the lamp pumped system is its pulse rate, measured in Hz: this is how fast the system can fire the laser pulse per second, which has a significant impact on how long it takes to produce the engraved design. Typically a lamp pumped system will work at maximum 200Hz (200 pulses per second).

How the laser is generated – diode pumped technology

Instead of a lamp, more modern systems use diodes to pump the laser medium. Such machines are known as “Diode Pumped Solid State” lasers or “DPSS” for short. DPSS lasers are much more efficient than lamp pumped machines and therefore consume far less power. As a consequence, they are much easier and cheaper to cool.

The performance of a DPSS laser is consistently good, as is its reliability. The diodes do not last forever and will require replacement at some point but typically a good quality diode will last for many thousands of hours, which can equate to years of use.

The rate of pulse for a good DPSS system will be 2 kHz (2,000 pulses per second), which equates to 10 times the speed of a typical lamp pumped machine. Some specialist systems, such as those used for architectural work, are capable of pulse rates as high as 5kHz but most machines cannot benefit from rates above 2kHz as there are factors other than the pulse rate that effect the systems top marking speed.

These machines generate small focal points of between 20 to 60um, which is about half the size of a lamp pumped alternative. Some DPSS machines can produce up to 4 x higher resolution than when compared to some lamp pumped systems.

In short, using a diode pumped system offers minimal system footprint, minimal downtime and minimal “total cost of ownership” (TCO) with maximum performance, output quality and reliability. For these reasons most people consider that a lamp pumped SSLE system is a false economy and so DPSS machines are now the most popular type of SSLE sold Worldwide.

Types of diode laser

After choosing DPSS as the basis for a good machine there is now the type of diode to consider as not all diode lasers are the same.

Differences in type of DPSS effect laser quality and reliability. As well as differences in quality/design for the core components the way in which the diodes are presented to the laser medium can differ too.

Lower cost diode lasers will normally pump the laser medium from the side. The best, highest quality output and performance comes from machines that incorporate diodes pumping the end of the laser medium. Such systems are known as “end pumped” diode lasers.

End pumped diode lasers typically use higher grade components and are more difficult to assemble/configure. Therefore, as a consequence they are higher in cost when compared to other types of DPSS system.

The type of beam delivery

There are two ways to deliver the beam:

  1. Via fixed optics
  2. Via a galvo head
Fix optics systems move the worktable to position the workpiece. In this case the workpiece must be moved for every dot that has to be fired. For some images, even at a relatively small size this can mean moving the workpiece as much as 300,000 times to make the full engraving!

Such systems move slowly so to compensate they often incorporate multiple heads to speed-up the marking process, sometimes as many as 16. You would naturally assume that a system with multiple heads would be a highly productive system but this is a misconception.

For each working head there’s obviously a workpiece to mark. The more workpieces that there are placed upon the work table, the heavier the overall load is on the machine. So a machine with 16 heads is required to move a considerable weight of product. For example, a block 5x8x5cm weighs approximately 480g. Therefore, a table with 16 of these blocks is loaded with 7.68kg of material.

Vibration is often a result of increasing the weight of the workload. Increasing vibration often causes a significant deterioration in engraving quality as well as extra wear and tear on the moving parts of the machine. Multiple head systems are also very restrictive with regard to the size of the workpiece that can be processed.

Multiple head systems also have multiple optical components that require cleaning, alignment and replacement multiplied by the number of heads that there are. So a 16 head system requires 16 lots of optics maintenance! Fixed optic systems use comparitively low grade components. As a consequence, they are lower in cost but require more frequent maintenance/replacement and produce lower quality results.

An alternative to the fixed head design is a system that uses a galvo head. Galvanometers; otherwise known as “galvo” are extremely fast moving motors to which reflective mirrors are fixed to bounce the laser beam and direct it to mark the product through a single lens. A single mirror is fixed to a galvo for each axis (X and Y).

These systems can move as fast as 10m per second and use only one set of optics. Even at high speed because there are only two moving parts the resonance (vibration) of the system is low and so marking quality is consistently high.

The limitation with a galvo laser is the size of its marking field. Typically, for a lens with good resolution (60um focal point) the marking field is not much more than 100x100mm in work area.

Resolution can be further increased by using a lens with a shorter focal length but shortening the focal length also decreases the working field of the lens. A very high resolution lens (20um focal point) would have a marking field of around 60x60mm.

Costs of SSLE machines

Over the last 10 years or so SSLE machines have evolved at a considerable pace and there is now a massive difference between the price, performance and output quality of SSLE machines of different make-up and technology. For example, prices can range from as little as £15k to as much as £350k.

Like with most things in life and business “you get what you pay for”.

Low cost SSLE machines (£15k to £25k) are typically lamp pumped and consist of 3 axis (XY for the worktable and Z to focus the head). They offer no advantages other than low initial cost. In summary low cost SSLE machines are:

  • Large in footprint
  • Low purchase cost
  • Low productivity
  • Low reliability
  • Low resolution
  • Restricted in application (no 3d photo reproduction)
  • High maintenance costs
  • High running costs
  • Short working lifetime

Systems of medium cost (£25 to £40k) are typically side pumped diode lasers with beam delivery via a galvo head.

Some machines at the upper-mid price range, such as our model SSLE-1 (approx £35k), will incorporate end pumped diode technology.

These machines deliver good quality and performance. Such systems are usually 3 axis (XY for the beam delivery via the galvo head and Z to focus the galvo head). Normally, they are only suitable for low volume batch production and have next to no capacity beyond one piece at a time.

These machines are the wise choice for retail outlets, particularly those who engrave on site at places such as shopping malls, etc, and for those who have a high quality but low production volume demand.
In summary medium cost SSLE machines are:
  • Small in footprint
  • Medium purchase costs
  • Medium productivity
  • High reliability
  • High resolution
  • Reasonable capabilities
  • Low maintenance costs
  • Low running costs
  • Long working lifetime

Systems at the upper end of the purchase price range (£40k to £350k) will always incorporate end-pumped diode technology for maximum beam quality.
Typically these machines will have a very high res lens with a 5 axis motion system (XY+XY+Z)
  • XY worktable to allow for large work areas
  • XY for the beam delivery via the galvo head
  • Z to focus the galvo head

The pulse rate of such systems is extremely high, normally from 2 kHz at minimum to 5 kHz.
Such machines will have all the qualities of the mid-range machines but with the added advantage of being capable of reproducing the highest possible resolution at the highest possible speed and over work areas that can even be several square metres in size.

Types of material suitable for SSLE

Although many transparent materials will mark to some extent with SSLE, those with minimal impurities, deformations and high thermal conductivity are best.

In general, low quality/cost products will mark better with lamp pumped laser technology and higher quality/cost products will mark better with DPSS laser technology.

Lead crystal is not suitable for SSLE apploication as the lead content of the glass will block the path of the SSLE beam.

You may often hear the term “K9” to describe a type of glass most suitable for SSLE.

Technically, K9 is a “borosilicate crown glass”. Borosilicate is commonly used to manufacture items such as test tubes and some household items like bowls for holding high temperature liquids. Pyrex, for example, is a form of borosilicate glass.

Crown glass (optical) is a term given to a type of borosilicate used in the manufacture of components used for optical purposes, such as lenses, etc. Crown glass can be worked to precise dimensions and is known to be more stable than other forms of glass.

Borosilicate crown glass has a very low “thermal expansion coefficient”. This means that it is resistant to thermal shock (when heat is applied), about one third that of normal glass. It also has “low dispersion” and a “low refractive index”. These terms are used to describe how much the glass absorbs light that enters in to it and how much the light is refracted (deflected at an angle) when it enters/exits the glass.

In short, K9 glass provides minimal beam absorption, minimal beam deflection and minimal heat reaction. It has fewer impurities and internal stresses than when compared to other types of glass.

Impurities and internal stresses prevent SSLE from producing clear, high quality results so wherever possible, for SSLE applications use K9 in preference to all other types of glass.

Which system is right for you?

As with all of the laser technology that we supply, the right system is a machine capable of performing the application, within your budget, without too many restrictions and/or additional on-costs. Purchasing the wrong SSLE system can be a very costly mistake.

As SSLE technology is a refined subject it is best you contact one of our qualified technical staff at systems@laserite.com who would be pleased to advise you after a short consultation without obligation to buy.