In a dual-spot medical laser system, the spherical biconvex lens functions as the primary collimator. It captures the naturally spreading (divergent) light exiting the optical fiber and transforms it into a straight, parallel beam. This process creates a stable, circular spot that serves as the necessary foundation for all subsequent beam manipulation and splitting.
The lens acts as the optical bridge between the raw fiber output and precise beam shaping. By converting divergent light into a parallel stream, it ensures the stability and quality required for accurate medical laser applications.
The Mechanics of Beam Transformation
The primary challenge in fiber-optic laser systems is that light exits the fiber in a cone shape, spreading out as it travels. The spherical biconvex lens resolves this geometric mismatch.
Correcting Fiber Divergence
Raw laser light emitted from an optical fiber is divergent. Without intervention, the beam energy would dissipate rapidly over distance, making precise targeting impossible.
The biconvex lens intercepts this expanding cone of light immediately after it leaves the fiber tip.
Creating a Collimated Output
Through its specific curvature, the lens refracts the incoming light rays. It bends them so they travel parallel to the optical axis.
This transformation results in a collimated circular spot. Instead of a spreading cone, the system now works with a controlled column of light that maintains its diameter over distance.
The Critical Role of Positioning
Merely having the lens is not enough; the geometry of the setup is the deciding factor in system performance.
The Focal Length Requirement
To achieve true collimation, the lens must be positioned at a precise distance from the fiber tip. This distance corresponds exactly to the lens's focal length.
If the fiber is placed anywhere other than the focal point, the output beam will either converge (focus) or continue to diverge, failing to create the necessary parallel path.
Enabling Downstream Components
A dual-spot system relies on splitting and shaping the beam later in the optical path. These complex components require a standardized input to function correctly.
The biconvex lens ensures that the subsequent beam-shaping optics receive high-quality, parallel input. This is the prerequisite for controlling the final output spot size and shape stability.
Understanding the Precision Trade-offs
While the spherical biconvex lens is effective, it introduces specific sensitivities to the system that must be managed.
Alignment Sensitivity
Because the system relies on the exact relationship between the fiber tip and the lens's focal point, mechanical tolerances are tight. Even minor misalignments along the axis can degrade collimation.
Aberration Management
Spherical lenses can introduce spherical aberrations, where rays at the edge of the lens focus differently than those at the center. In high-precision medical contexts, the lens quality and curvature must be carefully selected to minimize these distortions and maintain a uniform spot density.
Making the Right Choice for Your Design
The role of this lens is foundational: it dictates the quality of every optical process that follows it.
- If your primary focus is Beam Stability: Ensure the mechanical housing rigidly maintains the distance between the fiber and lens exactly at the focal length.
- If your primary focus is Spot Quality: Prioritize the surface quality of the biconvex lens to ensure the resulting collimated circular spot is free of distortions before it reaches the beam splitter.
Ultimately, the spherical biconvex lens transforms raw energy into a usable tool, serving as the critical gatekeeper for optical precision in medical procedures.
Summary Table:
| Component | Function | Result in Dual-Spot System |
|---|---|---|
| Input Fiber Tip | Emits raw laser energy | Divergent light cone |
| Biconvex Lens | Refracts expanding light rays | Creates collimated (parallel) beam |
| Precise Focal Alignment | Fiber-to-lens spacing | Ensures beam stability & circularity |
| Output Interface | Prepares beam for splitting | High-quality input for shaping optics |
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References
- Marcel Isper, Matthew Boyd. Design and optimization of dual-spot size optical systems for medical lasers. DOI: 10.1109/honet.2017.8102208
This article is also based on technical information from Belislaser Knowledge Base .
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