New and different optics deliver variable spot sizes

Optical systems providing variable spot sizes have potential applications in laser material processing

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By George Nemes

Many industrial applications of laser beams require a round beam spot, of a certain size, located at a certain distance from the exit aperture of the beam delivery system–where the target is positioned (called the working distance). Specific applications can require a specific power density (irradiance) for a CW laser, or a specific energy density (fluence) for pulsed lasers, at the target plane. Beam delivery systems are designed to provide the required spot size and the necessary irradiance or fluence for a given laser power or energy.

For the same application, variation of the spot size and the laser power/energy (a factor of 1.5–4 times is a usual figure) produces certain flexibility. Spot size change is usually done either discretely, by changing the optical system, or continuously, by using zoom–type optics. In many of these cases the working distance should be changed accordingly for each different spot size, although complex optical and mechanical systems that allow a fixed working distance and a continuously adjustable spot size within the typical range of 1.5–4 times do exist. An example of beam delivery optical system with discrete spot size adjustment using image–type optics, for CW high–power Nd:YAG lasers, is presented by Björn Wedel in the March 1999 issue of Industrial Laser Solutions.

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VariSpot represents a new concept in providing a broader range of round spot sizes at a fixed working distance, with a relatively simple opto–mechanical structure. The spot size is adjusted simply by rotating a ring. The ratio between the maximum and the minimum spot size (also called the dynamic range) is typically in the range of 5–100, but special design can have a dynamic range as high as 200–400. This means the same optical system can deliver, for example, a minimum spot of 50 µm and a maximum spot of 5 mm for the same 100–mm working distance. The total laser power or energy remains constant through the optical system, which provides a variable spot size. Therefore, for a constant power/energy laser, the irradiance/fluence is varied as the square of the dynamic range of the spot size, that is, in the range 25–10,000 for a spot size dynamic range of 5–100. Typical working distances are 25 mm to 1000 mm.

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The transverse beam profile shows a 0.24?mm diameter minimum round beam (a) and a 1.2?mm diameter maximum round beam (b) after a demo focus mode VariSpot. The incoming beam is from a non?collimated 2mW He?Ne laser beam. False color indicates the local irradiance level: red = high irradiance, blue = low irradiance. Notice the almost Gaussian beam profiles.
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As a result of this huge dynamic range for irradiance/fluence, it is expected that the same type of laser might be used for different applications than those that are presently possible using conventional beam delivery optics. For example, a laser used for cutting, requiring a relatively small spot size and a high irradiance, could be used with the same optical system for surface treatment, where a larger spot size, and correspondingly a lower irradiance, is required.

Optical systems using VariSpot can be introduced into the existing optical path of the beam delivery systems, with or without fiberoptics, or they can be designed as independent beam delivery systems. However, the design and the implementation of the optics are laser specific, and for the same laser they are application(s) specific. In other words, there is no universal–type VariSpot system, although the optical principle is the same for all. It can be applied to any kind of laser, CW or pulsed, with time scales from picoseconds to seconds, and at any wavelength throughout the ultraviolet, visible or infrared spectral range. The laser characteristics and the application(s) drive the complexity, characteristics, quality and price of the optics involved.

Operation modes and characteristics
The VariSpot has three possible basic configurations and operating modes: the focus mode, the image mode and the projection mode. All use the same principle, but the optical diagram and the complexity is slightly different for each of them. All need a collimated, intrinsic stigmatic (IS) incoming beam. An IS beam is one that always can be transformed into a true stigmatic beam, for example, a beam with all propagation properties identical in x and y transverse directions. A good approximation for a beam to be IS is to have both M2 differing from each other less than 10 percent. A collimated IS beam can be either round or elliptical in shape, and specific VariSpot optical systems exist to transform each beam so as to have an adjustable round spot at the desired working distance.

The main differences between the three basic operating modes are in the dynamic range, working distance and beam profile in the adjustable round spot at the target plane. The spot size depends on the definition used and on the shape of the beam profile. Some industrial applications require steep beam profiles in the target plane; others require smooth profiles. Definitions of spot size and discussion of techniques to measure the beam profile are offered in an article by John Fleischer that appeared in the Sept.1999 issue of ILS.

The focus mode of VariSpot provides the smallest spot size compatible with the working distance and the collimation of the incoming beam (20–50 µm) and also the highest dynamic range of the round spot size, usually 20–100 or even 10–400. It works best when the beam profile at the target plane can be smooth (near Gaussian). The working distance is typically 25 mm to 1000 mm.

The image mode is used best when a steep beam profile is important at the target plane, for example, when an adjustable spot size is to be obtained from a beam delivery system using an optical fiber. It provides a lower dynamic range than the focus mode, typically 2–20, and the optical system is somewhat more complex than for the focus mode. The working distance is in the typical range of 100 mm to 2000 mm. The minimum spot size is also typically larger than in the focus mode, on the order of 0.1 mm to 0.5 mm.

The projection mode is a version of the image mode with the target plane at a very long distance (several meters). It is intended mainly for projecting images with adjustable sizes on a screen, rather than for industrial applications.

Typical dimensions of VariSpot systems are from 5 mm diameter x 50 mm in length to 50 mm diameter x 500 mm length.

User specification
Users interested in the VariSpot system should specify the following main characteristics:

  • Type of laser and its output beam characteristics: wavelength; operating regime–CW or pulsed, pulsed duration, pulsed power/ energy, average power
  • How the beam is obtained from the laser: free–space delivery or guided through an optical fiber
  • Spatial beam characteristics–waist size and location, divergence, M2, all for both x and y transverse directions
  • Desired spot size characteristics after the VariSpot optical system: minimum and maximum spot size; working distance; the requirements on the beam profile at the target plane
  • Any other specific requirements, for example, the device size (short or long options are available)

Acknowledgments
The author acknowledges Professor A. E. Siegman's interest and support dur ing his stay as a visiting scholar at Stanford University, CA. He also thanks Dr. J. Serna, Universidad Complutense, Madrid, for scientific discussions leading to several papers published together in the field closely related to these optical systems.

George Nemes is president of Astigmat, San Jose, CA. Contact him at gnemes98@hot mail.com or (408) 244–3005 (phone and fax).

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