Optical Design: How to select the right laser beam expander

Each laser application, from dentistry to materials processing, has unique performance requirements. Lasers vary in wavelength, optical power output, temporal properties, and other key performance characteristics. It is possible to produce laser families of various beam diameters and divergence angles, but it is nearly always easier to select a laser based on other performance requirements, and then use a laser beam expander to produce the desired beam diameter and/or divergence.

In addition to enlarging the beam diameter, beam expanders also reduce laser-beam divergence. They are often used to compensate for source-to-source beam variation. The beam-expansion ratio, power, or magnification is the primary specification used when selecting a beam expander. Specify beam-expander characteristics carefully, because each influences the complexity of design and manufacturing. A specific application's needs may be met with an off-the-shelf, modified off-the-shelf, or entirely custom-designed laser beam expander. The overall system requirements drive the specifications of the beam expander.

Selecting a beam expander takes the same attention to detail as selecting other optical components, and can enhance the capabilities of a given laser and improve its performance in a given application. Although the system may have unique requirements, a custom beam expander may not be necessary. This is where a modified off-the-shelf beam expander or even an off-the-shelf system should be considered.

The basics of beam expansion

As the name implies, a laser beam expander is an optical system in which the input beam is expanded to a larger diameter. Beam-expander design concepts are derived from the fundamental principles of telescope design. When a collimated laser beam is input to one side of the beam expander, a collimated beam is output from the other end—that is, the object space and image space rays converge at infinity. This characteristic defines a beam expander as an afocal system.

There are two types of afocal beam expanders, named after their historical precedents. A Keplerian beam expander has two converging lenses, separated by the sum of their focal lengths. A collimated input beam converges to a focal point between the two lenses, then diverges to the output lens. A Galilean beam expander consists of one diverging lens and one converging lens. Again, the lenses are separated by the sum of their focal lengths, except that in this case the diverging lens has a negative focal length. A beam input into the diverging lens propagates to the converging lens without reaching an intermediate focal point.

The beam-expansion ratio is the fundamental performance parameter of a beam expander. The beam-expansion ratio is equal to the ratio of the focal lengths of the output and input lenses. When a laser beam is magnified by an expansion ratio m, the beam's divergence is multiplied by the inverse, 1/m. For example, if the beam expander magnification is 2, the output beam diameter will be twice that of the input beam, while the output beam will have half the divergence of the input beam. Conversely, when used in reverse, a beam expander will reduce the output beam diameter. However, when used in reverse, the divergence is increased. While beam expanders can be used in reverse, such use is highly application-dependent because the increased divergence may be detrimental to some systems.

Laser beam expanders control beam diameter and divergence. These characteristics help system designers manage collimation for optimal system performance. Beam-divergence control is especially important for long propagation lengths. In those situations, the beam diameter at long distances will typically be smaller with the use of a beam expander.

Selection characteristics

Laser beam expanders are used in interferometry, laser machining, metrology, remote sensing, and many other applications. Each application has specific requirements that influence the selection of a beam expander. A good place to start selecting a beam expander is by gathering all of the specifications related to the laser source in the system (see Fig. 1).

A logical starting point is the input laser beam diameter. Each laser beam expander has a maximum input diameter, often related to physical limitations of the optics and the housing. The primary goal in using a beam expander is often to achieve a specific output diameter, so it's important to ensure that the desired output diameter is less than the beam expander's maximum output diameter.

A beam expander will have a design input diameter as well—typically a diameter smaller than the maximum input diameter—where the optical performance of the beam expander is optimized. When the design input diameter of the beam expander matches the actual input beam diameter, system performance will be optimized.

As with any optical system, beam-expander performance varies as a function of wavelength. Material grade of the internal optics and antireflection (AR) coatings affect the transmission of the beam expander. AR coatings reduce losses at the design wavelength—in addition, the lens materials and surface figures are optimized for a given wavelength. Optical performance is optimized for the design wavelength of the beam expander, so select a beam expander with a design wavelength at or near that of the laser source.

The transmitted wavefront error quantifies the quality of the beam at the output of the beam expander. Diffraction-limited performance is most often quantified as a quarter-wave (λ/4) transmitted wavefront. Higher-quality transmitted wavefronts are possible and often specified up to λ/8 or even λ/10. Be sure to select the wavefront quality that meets the need of the system.

Beam expanders are available with many different fixed and variable magnification options. Fixed magnification beam expanders often have a collimation adjustment, typically called the "focus" or "divergence" adjustment, which allows for more compensation over the collimation and divergence of the laser beam leaving the beam expander.

Laser damage is another concern at the system level. Be sure to evaluate the laser's peak power (for continuous-wave [CW] lasers) or peak energy density (for pulsed lasers) and compare that with the published laser-induced damage threshold (LIDT) specifications. The LIDT of the laser beam expander chosen for the particular system must be greater than that of the source for guaranteed continued performance. If the source is strictly CW, consider other damage mechanisms when evaluating the LIDT requirement, such as hot spots.

The beam exiting the laser aperture is always divergent to some extent, is often not the correct size for the application, and particularly varies from unit to unit. Taking time to select the correct optic will help avoid production delays and added cost. There is no one perfect beam expander for all applications—it's necessary to consider system specifications and the application requirements when deciding on components for integration.

Laser beam expanders are typically of Galilean design, meaning that the beam does not come to an intermediate focal point. Galilean designs are relatively compact, allowing for easier system integration. Keplerian designs, with their intermediate focal point, do allow for spatial filtering at the focus. However, with an intermediate focal point, there is a potential for the air to be ionized, leading to a loss of energy and the addition of pulse distortion.

High levels of divergence can limit performance for such applications as laser ranging. Laser systems have some amount of divergence stemming from the laser source, and possibly from the optics in the optical train. Incorporating a laser beam expander can reduce beam divergence.

Properly specifying a beam expander

Laser systems have become commonplace in applications across industries from medical treatment to materials processing. Laser beam expanders are often crucial elements in the success of these systems. For high-power sources, the addition of a beam expander can provide a controlled reduction of power density. Additionally, reducing divergence can assist in alignment and reduce the spot size at the final focus of the beam. Reducing divergence to control collimation also benefits demanding laser applications, particularly in long-path-length systems. Variable laser beam expanders may be necessary to compensate for variations in laser source beam size from unit to unit.

Manufacturers produce a wide variety of stock laser beam expanders. Some, such as Edmund Optics' LC fixed YAG beam expanders, bring diffraction-limited performance at an economical price. If there is no stock item at the system's wavelength, evaluate the performance of a stock beam expander at a neighboring wavelength. For example, if the system is operating at 642 nm, perhaps a supplier has an off-the-shelf solution that matches all of the other requirements at a design wavelength of 633 nm. It is a simpler solution to modify that off-the-shelf option with different coatings rather than to use an entirely custom assembly. These semicustom product modifications will often present an economical option for meeting system requirements.

If the system requirements point to a custom design, be sure to pick a manufacturing partner with a range of stock optics. Existing optical and mechanical designs can serve as a starting point for a custom design, minimizing cost and maximizing performance.

System requirements drive the priority of the various beam-expander selection characteristics. When specifying a beam expander, use caution to not overspecify, which can lead to unnecessary cost and possibly a larger physical form factor. Conversely, ensure that the laser beam expander is not underspecified, as this leads to poor performance including poor collimation, larger spot sizes, and unwanted divergence. Successful implementations of laser-based optical systems often depend upon the right balance of beam-expander cost and performance. It all starts with an understanding of the interplay between system requirements and beam-expander characteristics.