On-axis collimation of LED output using stacked volume holographic optical elements

The presence of Light Emitting Diodes (LEDs) in so many contemporary applications highlights their profound impact on our daily lives. Characterized by their cost-effectiveness, mass manufacturability, long life span, versatility and lightweight construction, LEDs have emerged as the predominant light source choice across diverse domains such as imaging, projection systems, or optical communications. However, pursuing ever more compact, lightweight, and wearable devices necessitates their integration into even more compact systems and the need to achieve collimation and other forms of beam shaping over even shorter distances. Holographic Optical elements (HOEs) have the potential to provide an effective solution to these issues, but several challenges are associated with the divergent emission characteristics of LEDs. State-of-the-art solutions for LED collimation include off-axis HOEs, which provide benefits such as compactness and reduced weight compared to traditional lenses or mirrors. However, a well-known restriction in the design and use of HOEs is the need to design systems that avoid low spatial frequency zones where the grating period or feature size becomes too large for diffraction at visible wavelengths. This means that on-axis collimating devices, one of the most commonly used refractive elements, have no efficient HOE equivalent. Indeed, a significant off-axis angle (inter-beam angle during recording) is often necessary for HOEs to avoid these zones, especially for high numerical aperture HOE lenses. In this paper, a two-step recording method that produces a stacked HOE is proposed, tested and used to produce a HOE that converts an on-axis diverging input beam into an on-axis collimated output. It is also experimentally demonstrated that a divergent LED input beam can collimated by the HOE with a very slight divergence in the output beam, showing its efficacy and applicability in practical settings and potential for the continued evolution of LED-based technologies across diverse fields.