What is a Beam Expander

A beam expander is an optical component capable of altering the diameter and divergence angle of a light beam. It plays a crucial role in optical systems.

1. Definition of a Beam Expander

A beam expander typically consists of a set of lenses that can expand an input laser beam or other light beams, increasing their diameter and potentially altering their divergence angle. Different types of beam expanders have varying designs and structures, but their common goal is to adjust the characteristics of the beam to meet specific application requirements.

2. Functions of a Beam Expander

(1) Changing Beam Diameter

- In many optical applications, beams of specific diameters are required. For example, in laser processing, a larger beam diameter can cover a larger processing area. By using a beam expander, a narrow beam can be expanded to the desired size.

- For applications requiring uniform illumination, such as microscope lighting systems, a beam expander can enlarge the beam emitted by the light source to provide more even illumination.

(2) Adjusting Beam Divergence Angle

- The divergence angle of a beam is critical to the performance of an Optical System. A beam expander can reduce the divergence angle (formula: θ ≈ λ / (π * D)), making the beam more collimated, thereby improving transmission distance and focusing performance.

- In optical communication systems, beams with low divergence angles are needed to ensure stable signal transmission. A beam expander can adjust the input beam to meet the requirements of the optical communication system.

(3) Enabling High-Precision Optical Operations

- Some high-precision optical systems, such as optical tweezers, require precise control of beam characteristics. A beam expander can be part of the optical tweezers' beam manipulation system, working in conjunction with other Optical Components to ensure the objective's back aperture is fully illuminated while enabling trap positioning.

- In nanoscale positioning and high-precision beam shaping, beam expanders can be used with actuators like ultrasonic motors to achieve precise beam control.

(4) Adapting to Multi-Wavelength Applications

- In multi-wavelength optical systems, such as multi-wavelength lidar, traditional simple transmission beam expanders struggle to achieve beam expansion simultaneously at multiple wavelengths due to chromatic aberration. To address this, specialized beam expanders, such as off-axis reflective beam expanders, can be designed for use in multi-wavelength lidar systems.

(5) Optimizing Optical System Performance

- In the design of large-aperture aspheric Hartmann beam expanders, high-order aspheric surfaces are introduced into the objective lens to correct aberrations caused by large relative aperture lenses, thereby optimizing the optical system's performance.

- For specialized optical systems, such as Michelson interferometers in gravitational wave detectors, installing angled beam expander telescopes can reduce beam size and splitter dimensions while improving observation time efficiency, providing necessary beam diagnostic points, and facilitating beam alignment.

3. Types of Beam Expanders

Beam expanders are primarily divided into two categories: refractive (lens-based) and reflective (mirror-based).

(1) Refractive Beam Expanders (Lens-Based)

Refractive beam expanders operate based on the principle of lens refraction and typically consist of two or more lenses. Common types include Galilean beam expanders and Keplerian beam expanders.

(2) Reflective Beam Expanders (Mirror-Based)

Reflective beam expanders operate based on the principle of mirror reflection and typically consist of two or more curved mirrors. Common types include off-axis reflective beam expanders and coaxial reflective beam expanders.

(3) Comparison of Refractive and Reflective Beam Expanders

- Refractive Beam Expanders: Compact, suitable for low to medium power applications, but may introduce chromatic aberration.

- Reflective Beam Expanders: Ideal for high-power applications, free from chromatic aberration, but bulkier and more complex to align.

4. Application Examples

- Laser Processing: Refractive beam expanders are used in laser cutting and welding, while reflective beam expanders are employed in high-power laser processing.

- Astronomical Observation: Reflective beam expanders are used in telescope systems to expand the field of view.

- Optical Measurement: Refractive beam expanders are used in laser interferometers and optical experiments.

- Laser Communication: Refractive beam expanders are used for beam collimation and expansion.

Summary

Beam expanders are essential components in optical systems, enabling precise control over beam diameter and divergence angle to meet diverse application needs. Their design and selection depend on factors such as wavelength, power, and specific use cases. With advancements in technology, beam expanders continue to evolve, offering improved performance and versatility in fields ranging from laser processing to astronomical observation.