The laser marking method has obvious advantages over other available marking methods in speed, accuracy, and flexibility. Mask marking and beam deflection marking are typical methods, each having advantages and disadvantages. In the former, an opaque mask is directly imaged to create the desired mark. This method is practical and relatively fast, but most of the marking energy is blocked, losing efficiency. Additionally, this method requires a precise and bulky lens system. In the latter method, the focused beam is steered onto the sample, writing point by point. This technique has higher flexibility between marks, but it is slow, requires micro-movements, and accurate micro-motion parts are very expensive. We propose an innovative, holographic approach in laser marking. In the new system, a holographic projection system based on a digitally designed computer-generated hologram (CGH) is employed. This specially designed, fully transparent, phase only CGH modulates the high-power writing beam to create any desired image in the far field, where the beam etches a permanent mark of that image onto the designated silicon wafer substrate. Holographic marking combines the advantages of mask and beam deflection marking methods, such as high speed and stationary operation with minimal power loss, in a relatively simple and inexpensive setup. Also, since the holographic projection maintains its image quality after a certain distance, the setup is less prone to spatial alignment errors. We believe that the proposed technique will make significant contributions in the field of laser marking.
Conventional laser resonators yield multimodal output, especially at high powers and short cavity lengths. Since highorder modes exhibit large divergence, it is desirable to suppress them to improve laser quality. Traditionally, such modal discriminations can be achieved by simple apertures that provide absorptive loss for large diameter modes, while allowing the lower orders, such as the fundamental Gaussian, to pass through. However, modal discrimination may not be sufficient for short-cavity lasers, resulting in multimodal operation as well as power loss and overheating in the absorptive part of the aperture.
In research to improve laser mode control with minimal energy loss, systematic experiments have been executed using phase-only elements. These were composed of an intra-cavity step function and a diffractive out-coupler made of a computer-generated hologram. The platform was a 15-cm long solid-state laser that employs a neodymium-doped yttrium orthovanadate crystal rod, producing 1064 nm multimodal laser output. The intra-cavity phase elements (PEs) were shown to be highly effective in obtaining beams with reduced M-squared values and increased output powers, yielding improved values of radiance. The utilization of more sophisticated diffractive elements is promising for more difficult laser systems.