Chapter 11 - Scanning Systems

The aim of the scanning systems is to analyze or illuminate a spectrum analyzer, point by point, the surface of a remote object. In analysis, the radiation from the scanned surface is sent to a detection system. When the aim is to illuminate an object, a radiation beam (usually a laser radiation) is projected onto the surface of the object.

Scanning systems can have several application:

• materials processing, such as cutting, marking, surface treatments (hardening,manufacture, etc..), and many others. All these procedure make mainly use of laser beams.

• medical treatments, ranging from laser therapy to active or passive imaging;

• alignments;

• collection and printing or images presentation;

• Non-contact sensors (such as remote detection of objects or substances);

• measurements of physical properties (dimensions, shape, surface properties, etc.);

• many others applications in several fields.

- You can also see our Spectrometer applications and the Spectrometer application case study page.

An optical scanning system consists of an element which deflects a radiation beam (usually in the spectral range of visible, ultraviolet or infrared), and of one or more optical systems that make the beam suitable for the application requirements.

The application can be active or passive: in the former case a light beam (usually laser) is projected onto an object in order to illuminate orto interact with its surface and, then, to allow the formationof an image or  the alteration ofsome characteristics (physical or biological).

The optical element, which deflects the radiation beam, can take many different forms in order to fit different requirements. The most commonly optical elementused is the mirror; this latter may be a flat mirror that rotates, driven by a motor; or it can constitute the faces of a rotating polygon, or, again, it can oscillate with sinusoidal motion or move by discrete steps. Other means of deflection can be acousto-optical or electro-optical cells, holographic systems, and many others.

When a laser beam is used to scan a surface (to perform some process on it or to reveal its characteristics), it is usually required the beam focusing onto the surface in order to obtain the minimum possible “spot” size. The sizes of thefocused spot mainly depend on two factors:

• the beam divergence (i.e. the maximum angle subtended between the two extreme rays of the beam);

• the focal length of the focusing optics.

The multiplication between these two factors provides the spot diameter (in the case of an ideal and perfectly set optical system). Actually, in real systems, the diameter and the shape of the spot depends mainly on the quality and the size of the optical components of the scanning system. When you choose the optical components for a particular application, you must take into account the following aspects:

• Once the laser source has been chosen because it fits the requirements of the emitted wavelength, in terms of power and of beam characteristics, the only available way to obtain the desired spot size is the choice of the focal length of the focusing optics: shorter the focal length of the focusing optics is, smaller will be the diameter of the spot.

• The minimum achievable spot size (and, consequently, the maximum achievable resolution) is limited by the "diffraction". Increasing the beam diameter, for example by means a beam expander, the spot diameter can be reduced. As a first approximation we can say that by using a focusing optics characterized by 100 mm focal length and a laser beam at a wavelength of 1µm , when the diameter of the laser beam is 1 mm, then the minimum spot size achievable cannot be less than 0.1 mm (100 µm); if the laser beam diameter is 10 mm, the minimum diameter of the spot cannot be less than 0.01 mm (10 m). By reducing the focal length to 50 mm, the spot diameter also decreases, respectively, to 0.05 and 0.005 mm (50 and 5 µm).
 

• The aberrations produced by the focusing optics introduce errors in the direction of the rays that contribute to the formation of the spot: the size and shape of the spot can be strongly affected by them. The contribution of an optical system to aberration effects depends on its design: the best corrected optics are usually the most expensive. The contribution of an optical system to aberration increases with the diameter of the laser beam crossing it. A laser beam with a larger diameter requires a larger diameter optics. The complexity and the cost of a focusing optics increase with its diameter.

• Similarly to the aberration effects introduced by the focusing optics, the lack of flatness of the scanning mirror introduces errors in the direction of the rays contributing to the formation of the spot image. Such insufficient planarity of the scanning mirror may be due to a low quality processing of its surface, or to a deformation caused by its high rotation speed.

• The useful dimensions (i.e. free aperture) of the scanning mirror must be always larger than the beam diameter impinging on it, even when the angle of incidence is large and the shape of the incident beam is elliptical. If the beam is not always entirely contained in the mirror, the diameter and shape of the spot may be strongly affected.

• An error in the positioning of the target surface with respect to the focusing optics (i.e. when the surface is out-of-focus), increases the spot diameter.

• When the focused beam impinging on the target surface, is not perpendicular to the direction of the target surface, the spot shape becomes elliptical. The spot is circular only when the incident focused beam perpendicularly impinges on the target surface. In all other cases, the spot is elliptical. This phenomenon is particularly visible at the edge of the scanned area. This defect can be resolved by making use of special targets called of "telecentric" type.

• The product between the spot diameter multiplied by its divergence is constant (for all conjugates plans, such as the input and output surfaces of a beam expander or the radiation source and its image produced by the optical system). To reduce the divergence of a beam by a factor of 10, it is necessary to enlarge its diameter by the same factor. Therefore, in order to obtain a low divergence, a large diameter beam is required; a large diameter beam implies a scanning mirror and an optical system of large dimension, and hence heavy and expensive (at least in the pre-objective configuration - see next section).

• A beam expander of high magnification represents a critical component (in terms of its mechanical and thermal stability): a small error in the distance between its two confocal optical elements increases the laser beam divergence (and, consequently, the spot size).

• A laser beam of small diameter implies a high energy density (or power), this can be dangerous if it exceeds the damage threshold of some optical component. The laser power density (or energy density, in the case of continuous laser - CW) it must not exceed the damage threshold of the optical components.

• The ability of an optical system to focus a laser beam by introducing small aberration errors, is approximately inversely proportional to its F-number. The F-number is the ratio between the optics focal length and the diameter of its entrance pupil; larger the diameter of the laser beam is, and worse is the behavior of the optical system; a better optical system is more expensive.

• The energy density distribution on the focal plane, depends on the energy distribution within the laser beam. Frequently, in the not single transverse mode laser, the lack of energy uniformity within the beam gives rise to the so-called "hot spots" on the focal plane of the optics; such hot spots present an energy density much higher than the average energy of the rest of the spot. In this case, the spot effective diameter may vary when it impinges on parts of the target surface that absorb in different way.

• In the optical configuration of the scanning system, the laser beam must never be reflected in its own cavity, because this would produce instability that would alter the energy distribution.

PRE-OBJECTIVE AND POST-OBJECTIVE SYSTEMS

The scanning systems can be divided into two types of configuration: the pre-objective and post-objective systems. In the pre-objective configuration , the collimated laser beam impinges
on the scanning mirror and the reflected beam penetrates in the optical system, which focuses it on its own focal plane. In the post-objective configuration the collimated laser beam is, firstly focused by an optical system, then it impinges on the scanning mirror, which moves the focused beam on a cylindrical surface, whose curvature center coincides with the mirror rotation axis.

As we can easily see, the depth of focus, i.e. the region along the optical axis within which the spot diameter is kept within fixed limits, becomes progressively shorter as the beam diameter increases.

F-THETA AND TELECENTRIC F-THETA LENSES

When the laser beam is angularly scanned and it impinges on an optical system well corrected by aberrations, the position of the spot on the focal plane does not move linearly with the scanning angle. If the scanning mirror rotates with a constant angular velocity, then the spot will move with a velocity greater at the edge than at the center of the field. This could represent a serious problem when the energy settled in each point of the surface shall not vary with the scanning angle, but only with the intensity modulation of the beam. To overcome this difficulty has been designed a special type of objectives: the F-theta objectives. In these objectives a linear relationship between the angular beam displacement and the position of the spot on the focal plane is maintained.

Fig. 40 - PRE-OBJECTIVE SYSTEM

The beam focused on the target by an F-theta objective (or by a “standard “ objective) impinges on the target surface with a certain angle with respect to the normal to the optical axis (see Figure 36). This condition produces an elliptical shape of the spot (especially close to the edge of the target surface). To overcome this difficulty have been designed the "F-theta telecentric " objectives (Figure 40).

In this type of objectives the focused beam is always parallel to the optical axis. The disadvantage is that the useful diameter of the objective must always be equal or greater than the target area diameter to be scanned; consequentlyits cost tends to be high.

CALIBRATION TECHNIQUES

A simple technique to test some of the main features of the spot movement is to put, on the objective focal plan, a position sensing detector (P.S.D.). This detector provides an electric signal proportional to the position,xy, of the center of gravity of the spot, with a precision of one part on 10 of the detector length side. With the help of an appropriate electronics, you can collectseveral information aboutthe scanning system behavior.

Also see our Spectrometer, spectrograph, Raman Spectroscopy and our Spectrum Analayzer - the Spectra product line.

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