What are Microlens Arrays?

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- GoPhotonics

Aug 8, 2023

Microlens arrays consist of arrays of small lenses (lenslets) organized in either one or two dimensions. These lenslets typically exhibit a periodic pattern, commonly square or hexagonal, with a lens pitch ranging from a few hundred micrometers to tens of micrometers or even smaller. Microlens arrays are commonly used to enhance the spatial resolution of optical systems. In imaging applications, each microlens captures light from a small region of the object being observed, allowing for finer details to be resolved compared to a single, large lens.

The overall shape of microlens arrays is usually square (e.g., 10 mm x 10 mm), although they can also take the form of rectangles or circles. The number of microlenses can range from thousands to potentially millions.

Microlens arrays are commercially available as separate optical components, some of which come in a mounted form. These mounted arrays are enclosed by a metal or polymer part that fits into standard optical mounts. The lenses are often coated with anti-reflection materials on both sides. In some cases, the lenses are created as thin, transparent layers with a structured design, connected to a flat, homogeneous glass or semiconductor substrate.

Also, microlens arrays can be integrated as part of a larger assembly. For instance, certain CCD and CMOS image sensors incorporate one microlens per photodetector to enhance light collection efficiency. This configuration ensures that all incident light is concentrated onto the active areas, which only occupy a portion of the chip's surface.

Features of the Microlens Arrays

  • The lenses in microlens arrays are typically circular, but there are also arrays that use cylindrical lenses. Lenses capable of focusing in both directions can still have a square shape.
  • Microlens arrays can be optimized for different spectral ranges, such as visible light and/or parts of the near-infrared. This optimization affects the choice of optical material and the application of anti-reflection coatings.
  • Microlens arrays offer various options for the lens pitch, ranging from a few micrometers (only a few times the size of an optical wavelength) to hundreds of micrometers. For certain applications, precise positioning of the lenses is crucial.
  • Many applications require a high fill factor, which is the ratio of the total usable lens aperture area to the total array area. For example, a square array of circular lenses without any space between them would have a fill factor of approximately 78.5% (π/4). Hexagonal arrays can achieve a higher fill factor, but they may not be suitable for all applications.
  • Each lens in a microlens array is characterized by its diameter and focal length. Uniformity of focal length across the entire device area can be important, especially for imaging applications. Minimizing optical aberrations is also relevant, and sometimes aspheric lenses (produced using reactive ion etching) are employed for this purpose.
  • The light throughput of microlens arrays is limited by the fill factor and potentially by less-than-perfect transmissivity of the lenses due to parasitic absorption and/or reflections.

Fabrication of Microlens Arrays

Lens arrays are typically manufactured using a micro-fabrication method that enables the production of all microlenses in a single step. One common approach is the use of photolithography, where a photolithographic mask, either grayscale or binary, defines the lens pattern. This method incorporates techniques commonly employed in semiconductor processing, known as wafer-level optics. In plastic optics, mechanical techniques like molding, often with epoxy materials, are utilized.

In some cases, the phenomenon of surface tension is harnessed to achieve smooth and consistent lens surface shapes. However, these surfaces are generally non-spherical and may not be optically ideal, leading to aberrations.

Another approach involves laser material processing, which allows for the formation of one lenslet at a time or possibly a small number of them using multiple laser beams. Although these methods offer versatility and flexibility, they tend to be slower and more expensive.

Various optical materials can be used for lens array fabrication, including fused silica, different types of glass, and polymers (plastics). The choice of material must be compatible with the chosen production technique and also impacts device properties such as the usable spectral range, as well as mechanical and thermal stability.

Working of Microlens Array

The microlens array's structure consists of an array of small lenses, arranged in a regular grid pattern. Each microlens has its focal point, and collectively, they work together to manipulate the behavior of light passing through them. This arrangement allows the microlens array to control the distribution, direction, and intensity of light, making it useful for a variety of optical applications.

When light passes through a microlens array, each microlens collects and focuses the incident light onto a specific point in space. The individual microlenses act as tiny magnifying lenses, concentrating the light and changing its direction. 

Application of Microlens Arrays

Microlens arrays are commonly used in imaging systems, such as cameras and digital image sensors. They enhance the light collection efficiency by focusing incident light onto the photodetectors, increasing image quality and sensitivity.

They are utilized in virtual reality and augmented reality headsets to create immersive visual experiences. These arrays help focus and direct light rays towards the user's eyes, improving image quality and field of view.

Microlens arrays are employed in display technologies like liquid crystal displays (LCDs) and organic light-emitting diode (OLED) displays. They enhance brightness, contrast, and viewing angles by controlling light transmission and direction.

These arrays enable the capture of 3D images by using multiple lenses to capture different perspectives simultaneously. They find applications in areas like computer vision, robotics, and medical imaging.

Microlens arrays enhance the light absorption efficiency of solar cells by focusing sunlight onto the active areas, improving energy conversion efficiency.

They are also used in microscopy to improve image resolution, depth of field, and light collection. These arrays enable high-resolution imaging of small structures and particles.

Microlens arrays are employed in optical communication systems, such as fiber optics, to couple and align light beams for efficient transmission and reception.

These arrays assist in microfluidic systems by focusing and manipulating fluid flow, enabling precise control and analysis in applications like lab-on-a-chip devices and biomedical diagnostics.

They are utilized in laser systems for beam shaping, collimation, and steering. Also, these arrays control the spatial distribution and direction of laser beams for various industrial and scientific applications.

Microlens arrays play a role in biomedical applications such as cell imaging, microarray analysis, and DNA sequencing. They aid in high-throughput analysis and imaging of biological samples.