F-Theta Scan Lens Application Note

Author: Neo – Principle Engineer, Christopher Lee – R&D Manager

Ronar-Smith® F-Theta Scan Lenses are meticulously designed and crafted for a broad range of laser applications. With over a decade of expertise in optical grade and coating production, Ronar-Smith® scan lenses yield some of the world’s finest optical performance in the market. We also provide customization services for customers based on their requirements.

Our F-Theta scan lenses are optimized for laser-material processes, specifically for engraving, cutting, welding, and bonding. The scan lenses are available in working wavelength telecentric (TSL-Q and TSL series) and non-telecentric (SL-Q and SL series) configurations. For a vision system that requires an additional wavelength, we provide achromatic scan lenses in telecentric (TSLA series) and non-telecentric (SLA series) configurations.

Working Principles

F-Theta Scan Lens

For most applications in laser-material processes, a planar imaging field is necessary for quality output. Traditional optics such as paraxial lenses focuses only on their spherical plane, resulting in distortions such as spherical aberration while imaging on a planar surface.

Field-flattening lenses resolve the challenges of spherical-field-orientated optics by creating a flat focal field, but at the cost of inducing a nonlinear behavior. The displacement term between the effective focal length (𝑓) and the deflection angle (𝜃) prevents a uniform movement (i.e. constant scan rate) of the scanning mirror due to this nonlinearity (𝑓 ∗ tan 𝜃). It also results in an angular field-of-view and causes inaccuracies between varying magnification and observed measurements by the vision system. To resolve this nonlinearity, F-theta lenses are designed and engineered for the beam displacement to be independent of the tangent of the deflection angle.

F-theta lens provides the linear dependence between 𝑓 and 𝜃, creating a linear displacement that is ideal for use with scanners (XY galvanometer with mirrors) rotating at a constant angular velocity. The fixed velocity of the scanners corresponds to a constant velocity of the focal point on the flat focal field, with little to no electronic noise correction required. The complex scanner algorithm for the nonlinear compensation is eliminated, hence providing an accurate, safe, and inexpensive solution to customers.

Our F-theta scan lenses are designed for a wide range of applications. It is available over a broad wavelength, ranging from UV, VIS, NIR, and CO2 Laser. We also provide custom solutions for any wavelength-specific applications.

F-theta scan lens are subject to spot size variations on the planar surface, and the spot size diagram plot provides more information about the typical variation as a result of the angle movements of both mirrors on the XY axis of a galvanometer. The spot size variations can also be calculated using the following equation.

Telecentric vs Non-Telecentric Scan Lens

Ronar-Smith® F-Theta scan lenses are designed to meet the broad industrial requirements of our customers. When laser-material processes require a constant field of view with no dependency between the lens magnification and the depth, an object-space telecentric lens is recommended. For processes that have less stringent requirements on the quality of finish at the focal plane, a non-telecentric lens is capable of delivering the job to the customers’ satisfaction.

Telecentricity describes the angle of incidence of the laser beam delivered to the surface of the material during laser processing. In general, the angle of incidence for every point on the focal plane is the same, while non-telecentric lenses have varying angles of incidence on different points of the same plane. The end result of telecentricity produces repeatable and homogeneous spot size distribution on the object space field while reducing the effects of parallax error.

Achromatic Scan Lens

Ronar-Smith® F-Theta achromatic scan lens is designed to limit spherical and chromatic aberration, and bring in two different wavelengths (working and visible) onto the same plane. This enables the transmission of wavelength-specific laser beams during laser-material processes while ensuring that the visible (feedback) and laser beam wavelengths are temporally and spatially matched. Our achromatic scan lens enables machine vision in industrial processes for automation control and feedback while ensuring the product quality is not compromised.

Important Definitions:

Aperture Stop Surface

F-Theta is usually used in laser scanning systems. The working wavelength is a single wavelength and the working piece is a plane. The F-Theta lens belongs to a large field of view and a small relative system design. Then the aperture stop diameter is equal to the laser beam diameter. In the 2D Galvo Scanner system, there is actually no optical aperture pupil.

If only one mirror is used, the aperture stop is located on the mirror. If two mirrors are used, the aperture stop is located in the middle of the two mirrors, and the beam will be skewed. Usually, they will use two galvanometers and focus the beam on a 2D plane.

In practical applications, there is no mechanical boundary to create any kind of aperture in it. When designing, they will place the aperture in the middle of the two mirrors, as shown in the figure below.

Scan Angle

Usually, the F-Theta lens has two scanning angles, one scanning angle is an optical scanning angle, and another is a mechanical scanning angle. The optical scanning angle is the field of view of the lens, which determines the diagonal length of the maximum scanning field.

The F-Theta lens specifications usually mention the optical scanning angle, the diagram is as follows:

The mechanical scanning angle is related to the scanning mirror. It is usually the rotation angle of the two mirrors, which controls the scanning range from two directions. In the Galvo Scanner system, the specifications of the scanner refer to the mechanical scanning angle of the mirror.

The schematic diagram is as follows:

We use two mechanical scanning mirrors as Mirror X angle and Mirror Y angle respectively, then the relationship between them and the optical scanning angle is: (Mirror X angle)2 + (Mirror Y angle)2 = (optical scan angle/2)2

Back Reflection

Back reflection ghosting is the surface reflection from the scanning lens. The reflected focus points appear at different positions. When using a picosecond or femtosecond pulsed laser, the reflected focus point can easily damage the coating or lens material on the lens surface.

This is a challenge for the designer. In the optimization, the designer must not only consider the performance of the design but also avoid the reflection focus point on the lens.

Ultraviolet Scan Lens: Large-Area Precision Laser Processing

UV lasers at 355nm are advantageous as micromachining tools. Light at this wavelength interacts with materials primarily through photoablation, through which high-energy photons break molecular bonds, resulting in a clean cut with minimal disruptive effects on the surrounding material. For applications ranging from microelectronics to medical equipment production, solid-state UV lasers offer high versatility at low operational costs for the micromachining industry. Demand for large area scanning range, and simplified optical system design for both laser processing and vision inspection beams, present new challenges for a critical component in a laser system, namely, the scanning lens.

Operation Principle

The two main design categories of scan lenses include telecentric and non-telecentric F-Theta scan lenses. Telecentric F-Theta scan lens is a special type of lens system whereby the deflected off-axial laser beam can be perpendicularly focused onto the workpiece like the on-axial focusing beam. The advantage of the telecentric scan lens is that it can flatten the field curvature to be least distorted while offering superb spot quality throughout the scan field. The overall design concept is shown in Figure 1.

When a vision system is being integrated into a laser machining system, our achromatic telecentric scan lenses are color-corrected between working and vision wavelengths. The achromatic telecentric scan lens offers the same benefits as the normal telecentric lens while providing accurate vision positioning. The design layout is shown in Figure 2.

The key specifications of the UV scan lens are listed below. Compared to similar products in the market, we offer a larger scanning area and flexible design of achromatic performance. For high-powered laser and ultrafast laser sources, we offer a special Q-series to minimize thermal lensing and focal shift.

Applications

The large scanning area is advantageous for high throughput precision laser processing. This is essential when display electronics require high-speed manufacturing; e.g. the laser lift-off in flexible and large-area OLED processes. These scanning lenses can work in conjunction with our customized design of beam expanders (refer to WOE application note of Versatile Beam Expansion – from tunable to automation) and new design of beam shapers (refer to WOE application note of Beam Shapers – shaping the beam from DUV to MIR).

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