Laser Optics FAQs

A: There are two different "types" of lens used in industrial laser machines: meniscus and plano-convex. The correct type is usually specified in the machine manufacturer's manual. For a technical explanation about the differences between the two types please refer to the Duralens section of our catalog.

Basically, Plano-Convex lenses are usually found in workshops which cut thicker materials and are more popular in American and Japanese machinery. European customers and machine manufacturers (OEM's) traditionally use the meniscus range.

A: Obviously the number of lenses used per year depends on the machine workload and the type of metal it is processing. Our studies show that in average, lenses are replaced 4-5 per year while mirrors are usually replaced once a year.

A: The higher the pressure at which the machine cuts, the thicker should the lens be.

A: When cutting mild steel thicker than 4 mm, it is best to use oxygen with a maximum pressure of 3.5 bar. If thickness is 15 mm use gas pressure of 1 bar and a small nozzle (=1 mm).

A: When cutting mild steel (thickness up to 5 mm) the focal position should be about 1/3 in the workpiece. When cutting thicker steel the focal point is moved up.

A: When cutting stainless steel use nitrogen. In this case the focal position is always under the sheet surface.

A: When cutting thin materials (less than 2 mm) you can use shop air. However, shop air could be contaminated with oil and water or other impurities requiring a high level of filtering.

When cutting Aluminum, Oxygen can be used. Note: For a quality cut proceed slowly. Nitrogen can also be used for Aluminum (cut quality improves when cutting slowly).

A: In order to check the beam quality, check the sparks under the sheet: they should follow the input laser angle to make one line. If it is lagging the focal position needs to be adjusted.

Also, pay attention to the color of the plasma in the cutting area - it should not be blue. If the plasma is blue - stop the cut and re-adjust machine constants.

A: 7.5" length is placed far from the workpiece so there is less chance for it to damage - for a longer lifetime. However, the laser power density reaching the workpiece decreases. When cutting thin material (5mm and less) one should use 5" focal length since the spot diameter is smaller.

A: These lenses absorb less heat and therefore their performance (i.e. focusing efficiency and lifetime) is better. The reduced absorption in the black-coated lenses affects lens operation as follows:

• Thermal Lansing is reduced, and therefore the focus position during operation with a high-power-laser is more stable.
• Since thermal effects also increase lens aberration, increasing the spot size, the spot size of a black-coated lens during operation in a high-power-laser is smaller than the spot size of a conventional lens.Therefore when using a black-coated lens the energy concentration is higher and enables a better and faster cut using less gas.

A: Ophir's unique product, the Black Magic lens, was created using a revolutionary black coating technology. The Black Magic lens has achieved the lowest absorption on the market - lower than 0.15%. The Black Magic lens has achieved the lowest absorption rates on the market - lower than 0.15%. The black coating is much harder, has better ability to withstand back splatter, requires less maintenance (i.e. extended lens cleaning intervals) and is RADIOACTIVE FREE.

In most applications, the Black Magic increases the cutting speed, save up to 20% of gas consumption and lasts twice the lifetime of a standard lens.

The black magic is the best solution for cutting thicker metals in high power laser cutting machines.

A: Ophir Optronics understands that beam alignment is crucial for a clean cut. We suggest operating the laser machine leaving the gas off and turning on the focus manually. After finding the right focus the laser machine can be used regularly.

A: No. The machine constants need to be re-adjusted in order to ensure its best results.

A: Clean the lens using a liquid soap and a soft tissue. Follow our cleaning instructions for fiber lasers protective windows or CO₂ optics.

A: Please perform the following steps:
Check lens mounting direction, realign the laser beam, check the focus spot size and check the color of the plasma (should be blue).
If the problem remains check for dust in the machine.
A final step is replacing mirrors (turning mirrors or resonator mirrors). Normally mirrors are replaced once a year - by a qualified person only!

A: Mirrors are arguably the most commonly used optical components. They appear industrial applications, as well as large-scale optical systems. These components utilize reflection to redirect, focus, and collect light. Optical mirrors consist of metallic or dielectric films deposited directly on a substrate such as glass, differing from common mirrors, which are coated on the back surface of the glass. Consequently, the reflective surface of an optical mirror may be subject to environmental conditions. This means that durability and damage resistance must also be considered when choosing a mirror as well as how well it reflects light at the wavelength of interest. This section introduces the physical concept of reflection and discusses the important attributes of the mirror as an optical component.

A: Generally, when light reaches a planar interface between two media (see Figure 1) , a portion of it is reflected back into the original (incident) medium and a portion is transmitted and refracted into the second medium. Absorption of the light in either medium is also possible, but non-absorbing media will be assumed here. Reflection can occur from smooth surfaces such as those found on mirrors (referred to as specular reflection) or from rough, uneven surfaces (called diffuse reflection or scattering). Although both obey the same laws of reflection, specular reflection leads to rays that reflect as a group at the same angle, whereas diffuse reflection occurs at different angles off randomly oriented surfaces. This enables specular reflection to perform the useful operations of redirecting light.

Figure 1. Illustration of the law of reflection at a planar surface

A: Mirrors made up of planar surfaces, such as that shown in Figure 1, are important components for directing light through the proper path in an optical system. Such mirrors can be combined to form optical components known as retroreflectors or corner cubes. These components consist of three mirror surfaces all perpendicular to one another. Such a geometry enables 180 degrees reflection of the light, regardless of incidence angle, and therefore requires very little alignment compared to a single flat mirror.

Curved mirror surfaces also called concave reflectors, can be exploited with the goal of collecting, focusing, and imaging light as illustrated in Figure 2. These mirrors possess an advantage over lenses in that they perform satisfactorily across a broad-wavelength range without requiring refocusing. The reason for this is that reflection occurs at the surface of these optics, rather than passing through the optic as is the case with a lens, and so the dispersion of the index of refraction does not come into play. Simple spherical reflectors can be used to collect radiation from a source at the focal point (located at half of the radius of curvature of the mirror) and reflect it as a collimated beam parallel to the axis. Since spherical mirrors possess spherical aberration, a parabolic curved surface can be used instead to either collimate light from a focal point or focus light from a collimated beam (see Figure 2). Ellipsoidal surfaces can focus light from one focal point to another (see Figure 2).

Figure 2. Concave reflectors with different surface shapes allowing for light collection and focusing. A paraboloidal reflector reflects light from the focus into a collimated beam (left). An off-axis paraboloidal reflector refocuses a collimated beam off the mechanical axis (middle). Ellipsoidal reflectors reflect light from one focus to a second focus, usually external (right).

A: Selecting the proper mirror for laser system requires consideration of a number of factors, including reflectivity, laser damage resistance, coating durability, thermal expansion of the substrate, wavefront distortion, scattered light, and cost. These mirror characteristics depend on the optical coating, the substrate, and the surface quality. The optical coating is the most critical component of a mirror as it dictates its reflectivity and durability. Optical mirror coatings are typically made up of either metallic or dielectric materials. By virtue of their conductivity, metals have a complex index of refraction with a large imaginary part over a very wide wavelength range. This gives rise to a large reflectivity that is relatively insensitive to wavelength, which gives metallic mirrors their shiny appearance. Metallic coatings are usually made of silver, gold, or aluminum and the resulting mirrors can be used over a very broad spectral range (see figure 3). Metallic coatings are relatively soft, making them susceptible to damage, and special care must be taken when cleaning. Mirrors with dielectric coatings are more durable, easier to clean, and more resistant to laser damage. However, as a consequence of their dispersive and predominantly real indices of refraction, dielectric mirrors have a narrower spectral reflectivity and are typically used in the VIS and NIR spectral region. There is greater flexibility in the design of dielectric coatings compared to metallic coatings. When compared with metallic mirrors, a dielectric mirror can offer higher reflectivity over certain spectral ranges and can offer a tailored spectral response (see figure 3).

Figure 3. Reflection spectra of silver metallic mirrors showing broadband reflectivity (left) and dielectric laser-line mirror showing two narrow reflection bands (right).

Most substrates upon which the coatings are deposited are dielectric materials and these substrates control the thermal expansion and transmission properties of mirrors. Certain materials have lower thermal expansion coefficients, e.g., fused silica, than others, e.g. N-BK7 optical glass, but the cost of the material and ease of polishing must also be considered. If light transmitted through the substrate is not required, the backside of the substrate is typically ground to prevent inadvertent transmissions. However, for transmissive mirrors, a substrate material with a homogenous index of refraction is important, e.g. fused silica.

Prior to depositing the optical coating, the substrate's surface must be ground and polished to the proper shape (either planar or curved). The surface quality and flatness determine the fidelity of the mirror performance with the targeted application dictating the requirements for these parameters. Surface flatness is often specified in wavelengths, e.g. λ/10, over the entire usable area of the mirror. When preservation of the wave front is critical, a λ/10 to λ/20 mirror should be selected, while less demanding applications can tolerate a λ/2 to λ/5 mirror with the associated reduction in cost. Surface quality is usually dictated by the severity of random localized defects on the surface. These are often quantified in terms of a "scratch and dig" specification, e.g. 20-10, with a lower value indicating improved quality and therefore lower scattering. For high precision surfaces, such as those found within the cavity of a laser, a scratch-dig specification of 10-5 may be required since it would yield very little scattered light.

Surface polishing tolerances in terms of irregularity, surface roughness, and cosmetic imperfections are verified using state-of-the-art metrology equipment. These same parameters and procedures are used to assess the quality and flatness of other optical components such as lenses or windows.

A: Optical coatings typically consist of thin films made up of single or multiple layers of either metallic or dielectric materials. When properly designed and fabricated, these coatings can dramatically modify the reflection and transmission properties of an optical component. The properties can be controlled from the deep UV to the IR with narrowband, broadband, or multi-band response, and can be polarization sensitive. Optical coatings can be applied directly to the surface of an optical component to tailor its reflectivity, as in the case of an optical mirror or beam splitter. For other components, such as lenses, the applied coatings may simply improve their overall transmission properties by reducing surface reflectivity. When optical coatings are integrated into a monolithic component for the express purpose of controlling the spectral transmission of light, the component is referred to as an optical filter.

A: The individual layers that make up optical coatings are typically a few tens of nanometers to a few hundred nanometers in thickness, while a single optical coating can be comprised of several hundred layers. Consequently, the techniques used to deposit these layers require a high degree of precision. Generally, the process begins with surface fabrication to minimize surface roughness and sub-surface damage. It continues with surface cleaning and preparation and is followed by deposition of high-performance thin film designs. The deposition technologies include thermal evaporation, electron-beam, ion-assisted deposition, and advanced plasma deposition. The most appropriate coating technology for the intended product design depends on the operating environment, spectral requirements, physical characteristics, application requirements, and economic targets. The optical coating process is completed with comprehensive performance testing using sophisticated metrology tools.

Metal coatings used on optical mirrors typically consist of a single layer approximately 100 nm thick. This ensures that the broadband high reflectivity properties of the metal due to the complex index of refraction are present. In order to provide greater tuning of the reflectivity and over specific wavelengths of interest, dielectric coatings are used . These coatings consist of alternating high refractive index (n_H = 1.8 - 4.0) and low refractive index (n_L = 1.3 - 1.7) dielectric layers (see Figure 3). The thickness of each layer is chosen such that the product of the thickness and the index of refraction of the layer is λ/4.

Figure 4. Scanning electron microscope image (top) and schematic (bottom) of an optical interference coating shown on left. Reflection and transmission of light by a filter consisting of an interference coating (right).

Dielectric optical coatings are used in a myriad of ways. In addition to highly reflective dielectric mirrors (see Figure 3), these coatings are incorporated in broadband beam splitters and IR wavelength lenses. When light is incident at an angle to a surface, i.e., not normal incidence, the reflectivity becomes polarization sensitive. This allows dielectric coatings to be polarization selective and such coatings are used in polarizing beam splitters (see Section III.A.5). In addition to enhancing the reflectivity, dielectric optical coatings can also be used to reduce surface reflections in the form of broadband anti-reflection coatings. These coatings can be applied to any optical component, e.g., lens, prism, beam splitter, window, to markedly improve its transmission efficiency. The reflection from an air-glass (n2 ≈ 1.5) interface gives a reflectivity of 4%, which can be reduced considerably with a broadband anti-reflection coating (see Figure 5).

Figure 5. Typical broadband anti-reflection coating in the UV and VIS spectral regions.

These reflectivities can be reduced even more to improve transmission in laser systems with multiple optical elements, saving valuable laser energy from being lost to surface reflections. This superior performance, however, is achieved at the cost of reduced wavelength range.

A: Lenses are the optical components that form the basic building blocks of many common optical devices, including cameras, binoculars, microscopes, and telescopes. Lenses are essentially light-controlling elements and so are exploited for light gathering and image formation. Curved mirrors and lenses can accomplish many of same things in terms of light collection and image formation. However, lenses tend to be superior in terms of image formation because they are transparent, which allows light to be transmitted directly along the axis to the detector whereas mirrors require an off-axis geometry. Mirrors are typically preferred in terms of light collection as they can be made significantly more lightweight than lenses and therefore can achieve larger diameters and light collecting ability.

This section discusses the mechanism of refraction that underlies the operation of a lens, issues that affect its performance, and the different lens types.

A: In addition to light reflecting off a planar interface between two media, it can also be transmitted and then refracted in the second medium (see Figure 6). Refraction refers to the change in the angle of the incident light when it enters the second medium. Since the speed of light in a medium is inversely proportional to its index of refraction, it will either slow down or speed up when it enters a different medium, resulting in the light changing its direction. Figure 6 shows an example where the index of the second medium (n2) is greater than the first (n1), which results in a bending of the light toward the normal to the interface. This phenomenon of refraction is described by Snell's law.

Figure 6. Illustration of Snell's law of refraction at an interface between media of refractive indexes n1 and n2.

A lens is typically made up of a transparent dielectric material like fused silica or optical glass with the front and back surfaces having a spherical curvature. Since the surfaces are curved, each ray of light that comes in parallel to the optical axis (as shown in Figure 7) has a different value of θi with respect to the surface normal. Each ray then refracts according to Snell's law. For a positive lens, this causes the light to converge toward its focal point on the right side of the lens while light will diverge from the focal point located on the left side of a negative lens. The ramifications of these operations are that lenses can be used for image formation as well as collection and collimation of light (see Figure 7). There are several important aspects to optical imaging with lenses, including the relationship between object and image distances and the resulting magnification as well as the quality of the resulting images. Details about these concepts can be found in . Similarly, important aspects of involving the light gathering ability of lenses including throughput and its relationship to numerical aperture (NA) or f-number (F/#) are described in.

Figure 7. Illustration of how a lens affects incoming parallel light rays (left). Applications of lenses (right) include creating a magnified image of an object (top), collimating light from a point source (middle), focusing a collimated light source (bottom).

A: Ideal lenses would form perfect images (or exact replicas of the object being imaged) and would be able to focus collimated light to a spot size limited only by diffraction. However, real lenses are not perfect and induce optical aberrations, which cause degradation in the ability to form a high-quality image, collimate a beam, or focus it tightly. Monochromatic aberrations, i.e., no wavelength dependence, are common to both mirrors and lenses and come from the inability of spherical surfaces to focus light properly when it is far from the axis.

These aberrations include spherical aberration, coma, and astigmatism. Figure 8 demonstrates the impact of spherical aberration in a lens where rays with smaller angles are effectively collimated while rays with large angles converge instead. Unlike monochromatic aberrations, chromatic aberration only occurs in lenses. Due to dispersion of the index of refraction of the lens material, different wavelengths will refract with different angles according to Snell's law (see Figure 8). This causes degradation in image quality or light gathering ability when broadband light is being used.

Figure 8. Effects of spherical aberration (left) and chromatic aberration (right) on collimation when a point source is at the focal point.

While spherical lenses do induce aberrations, choosing the proper lens shape can help minimize optical aberrations (see Figure 9). For instance, plano-convex lenses, where only one side is curved, are the best choice for focusing parallel rays of light to a single point. Bi-convex lenses (both sides have curvature that may not be equal to one another) are the best choice for imaging when the object and image are at similar distances from the lens. When a single spherical lens may be unsuitable due to spherical aberration, aspheric lenses may be used. These lenses have surfaces with tailored curvatures that help minimize the impact of aberrations but are typically expensive due to the complexities associated with fabrication. Alternatively, multiple spherical lenses can be used where one lens can cancel the aberration caused by another, as shown in Figure 9. In addition to correcting for monochromatic aberrations, an achromatic doublet can be used to minimize chromatic aberrations by choosing the dispersion of the materials in the two lenses to produce a focal length that is independent of wavelength. Microscope objectives are multi-element lens systems that can significantly reduce the impacts of aberrations but are more expensive due to the complexity of the design. All the aforementioned lenses are rotationally symmetric, that is, light focuses the same regardless of which transverse axis it passes through.

Figure 9. Using single and multiple lens systems to minimize optical aberrations for a specific imaging application.

There are many features to take into consideration when choosing a lens, including focal length, lens shape, F/#, lens material, transmission properties, wavefront distortion, scattered light, types of coating, and cost.