Describe the Difference between Coaxial and Non-coaxial Optical Paths in Laser Scanning Imaging Systems

Describe the Difference between Coaxial and Non-coaxial Optical Paths in Laser Scanning Imaging Systems

In laser scanning imaging systems, there are two configurations: coaxial and non-coaxial optical paths. What are the differences between these two imaging methods, and what are their respective advantages and disadvantages? Let Songsheng Optoelectronics explain and share this knowledge with you.

Coaxial optical path means the imaging light path shares the same optical axis with the laser beam, separated by a dichroic beam splitter in between. Off-axis imaging, on the other hand, refers to an imaging light path that is not aligned with the laser optical axis.

The coaxial optical path diagram of a laser scanning imaging system

The greatest advantage of coaxial imaging is its high positioning accuracy (this single advantage is sufficient), along with a simple external structure (the core components are all enclosed in the housing), and the ability to display the machining process in real time. Of course, the corresponding drawbacks are slightly higher costs, a somewhat more complex internal structure, and imaging clarity that is inferior to standalone off-axis imaging. Without exception, higher-end systems all adopt coaxial imaging.

Off-axis imaging comes in two types. One involves tilting the lens for oblique imaging—this structure is the simplest and lowest in cost. However, due to the oblique angle, the accuracy is very poor unless the sample can be perfectly maintained on a single plane, which is practically impossible in actual machining. Thus, oblique imaging suffers from very poor positioning accuracy. The other type also involves vertical imaging but requires a displacement stage to move the sample under the lens for imaging before retracting it for laser processing. This method is the most expensive, as a single displacement stage can cost tens of thousands, and the positioning accuracy heavily depends on the stage's precision. Additionally, real-time observation during machining is impossible.

Coaxial Optical Path

Principle: A coaxial optical path refers to a configuration where the emission axis of the laser and the reception axis of the reflected light are aligned along the same optical axis. This setup typically consists of optical components such as beam splitters, mirrors, and lenses. Taking laser ranging as an example, the laser beam emitted by a laser diode partially passes through the beam splitter, is collimated by an optical lens, and then projected onto a distant target. The diffusely reflected light from the target surface is collected and converged by the optical lens, then reflected by the beam splitter and focused onto the photosensitive surface of a photodetector. The photodetector converts the optical signal into an electrical signal, which is transmitted to a computing unit to achieve distance measurement.

Improved Accuracy: Ensures the laser beam remains perpendicular to the machining surface, enhancing processing precision and quality while minimizing errors. For example, in laser cutting, it results in straighter cut lines and smoother edges.

Enhanced Stability: Provides more stable laser transmission, reducing the impact of optical path jitter and misalignment on laser performance, thereby improving system reliability and repeatability.

Easier Alignment: With the emission and reception optical axes being coaxial, it simplifies the alignment of the laser beam with the target, reducing operational complexity and alignment errors.

Reduced Interference: The compact coaxial optical path design minimizes disturbances from external factors (e.g., dust, airflow), improving the system's resistance to interference.

The caption reads: "The scanning galvanometer focusing system integrates coaxial imaging, specifically designed for galvanometer scanning processing systems requiring high-precision positioning. The CCD observation image is fully coaxial with the laser beam focus point. When paired with an F-theta lens and light source, it enables 'what you see is what you get' machine vision positioning for laser processing."

Structural Diagram of LSH-IM-LAR Series Coaxial Optical Path Module by Songsheng Optoelectronics

The LSH-IM-LAR series is an independently developed laser coaxial vision module by our company, which can observe the working area of the scanning head through a camera. Typical applications include process monitoring or positioning the position and angle of workpieces. During laser processing, the coaxial module ensures easy integration into both new and existing systems. The mechanical interface of the module can be directly installed between the galvanometer scanning head and the laser flange.

To address the problems existing in the traditional laser industry, such as high cost of fixture production, difficulty in controlling the marking or welding accuracy of small-sized or irregular products, and the impact of product placement position and angle on marking accuracy, we have independently developed and launched marking and welding systems. Their role is equivalent to equipping traditional laser equipment with "eyes" and "brains", enabling laser equipment to "see" and "think".

Application range: Widely used in laser marking, laser welding, laser cutting, machine vision, laser micromachining, laser trimming, etc.

Non-coaxial Optical Path

Principle: The non-coaxial optical path means that the emission optical axis and the receiving optical axis of the laser are not coaxial. The emission optical path and the receiving optical path are completely isolated, thus forming two optical axes. These two optical axes are not parallel and have a certain angle between them, and this angle will change with the change of distance.