An Introduction to holoOPTIX® holographic notch filters
holoOPTIX® holographic notch filters from Meta Materials Inc. (META®) offer a high-performance, environmentally sustainable, and versatile alternative to conventional dielectric solutions for rejecting undesired wavelengths of light.
Our previous blog post covered different types of notch filters and their applications. Now, let us dive into high-performing holoOPTIX® notch filters.
This family of notch filters based on conformal volume holographic grating (VHG) films is currently available both in a standard 25.4-mm rigid-mounted configuration (STRATA) and in a unique 70-mm × 375-mm flexible configuration (FLEX). Three center wavelengths (CWLs), namely, 457, 532, and 635 nm, are offered for each configuration. Note that holoOPTIX® filters’ CWLs can be easily tuned by altering the angle of incidence while maintaining consistency for both p- and s-polarizations, even at large angles of incidence, as demonstrated in Figure 1.
The same VHG film assembly design composed of a VHG film and a plastic (triacetate cellulose (TAC)) substrate constitutes the core of both formats of holoOPTIX® filters. A STRATA filter consists of a VHG film assembly sandwiched between two layers of glass with an antireflective coating, which is installed in a black anodized aluminum ring. The film assembly is secured onto the glass using optically clear adhesive (OCA). These lightweight filters can be incorporated into optical instruments with commercially available filter mounts. In comparison, instead of glass, removable protective layers are used in a FLEX filter. This flexible design allows the filter to be cut to any size and bent to any angle to meet myriad application and integration demands. Figure 2 illustrates the structure of STRATA and FLEX filters.
To date, various interference pattern recording techniques have been developed to create holographic notch filters. While similar in principle, these techniques differ vastly in terms of efficiency, adaptability, and scalability. The unique and proprietary recording and manufacturing processes designed in-house at META® enable holoOPTIX® notch filters to deliver an unrivaled combination of performance, dimensional versatility, and ease of configuration. These processes are briefly described below:
A transparent photopolymer film is used as a medium to record the desired interference pattern. The film is first placed on a mirror surface and then exposed to a collimated laser beam propagating at an angle relative to the normal to the mirror surface. The beam reflected from the mirror intersects the incident beam, forming standing waves that create an interference pattern with alternating bright and dark fringes parallel to the mirror surface due to specular reflection (meaning that the angle of incident light equals the angle of reflected light). After the recording of the interference pattern, the film is bleached and cured under ultraviolet and white light. To scale up production, META® employs a patented scanned-beam recording process that encompasses an automated motion control system. This approach is instrumental in improved manufacturing efficiency and product uniformity.
Performance, reliability, and durability
Exceeding the performance of many commercial-off-the-shelf products, holoOPTIX® notch filters are highly effective and selective at blocking the target wavelengths, as evidenced by an optical density (OD) greater than 4.0 at the CWL and a narrow spectral bandwidth (SBW) (8–16 nm @ OD1), while allowing the out-of-band wavelengths to propagate without significant attenuation (see Figure 3 for their visible transmission curves). For each target wavelength, the STRATA filter has a moderately higher transmissivity than its FLEX counterpart. The primary cause of this difference is that the glass component of the STRATA filter contains an antireflective coating that reduces broadband reflection.
Reliability and durability testing has revealed no significant changes in the performance of holoOPTIX® notch filters in terms of spectral bandwidth, OD, haze, visible light transmission, transmitted wavefront distortion, and yellowness following subjection to the temperature and humidity regimes prescribed in the MIL-C-48497 standard of the U.S. Department of Defense (USDOD). Accelerated environmental testing has also been conducted, involving thermal analysis and daily humidity cycling conducted in accordance with the USDOD’s MIL-PRF-32432(GL) standard as well as altitude testing performed under the ASTM D6653 protocol.
While sharing much optical commonality with STRATA filters, FLEX filters are distinctive in three aspects.:
They can transmit wavelengths up to 1500 nm, markedly exceeding the upper-limit capacity of most dielectric filters.
They are available in a large size of 70 mm × 375 mm, an industry’s first, while being priced the same as STRATA filters.
They feature a thin and flexible polymer-based design that imparts enormous freedom to configure to suit individual needs, including retrofitting equipment with curved surfaces. Such adaptability generally eludes dielectric filters due to their rigid glass substrates.
META® offers a number of customization options for holoOPTIX® notch filters through the aid of its highly adjustable holographic technology and laser capabilities. Many optical characteristics of these notch filters, including the target wavelength, form factor, angle of reflection, and OD, can be easily modified to meet individual optical requirements. A few examples are given below:
Target wavelength: Besides any wavelengths within the entire visible spectrum, META® can also produce notch filters targeting parts of the near-infrared spectrum up to 860 nm. These include, but are not limited to, the operating wavelengths for various lasers such as titanium-sapphire and neodymium-doped yttrium aluminum garnet and yttrium orthovanadate systems.
Dimension: The manufacturing process can be readily tailored to produce filters in many other sizes (up to 400 mm × 500 mm in the holoOPTIX® FLEX line) than the ones currently offered.
Angle of reflection: Besides STRATA and FLEX, META® also offers another line of notch filters—holoOPTIX® SLANT. Each filter of this line is individualized to reflect a light beam at a prescribed angle that is different from its incident angle. These anomalously reflective filters can enable new optical assembly designs that benefit from separation from Fresnel reflections, displacement-free transmission, a flexible beam layout, and a compact and lightweight structure.
OD: An even higher OD can be achieved for holoOPTIX® filters by increasing the thickness, or altering the chemical composition, of the holographic recording medium.
Furthermore, more than one interference pattern may be recorded in the photopolymer film to create a multiplexed VHG filter that rejects multiple wavelength ranges at once.
The potential of holographic technology from META® does not stop at notch filters but goes much beyond. This technology has already facilitated laser glare protection for pilots and law enforcement officers (see our laser glare protection article for more information). Development of other applications is currently underway, such as optical combiners for use in augmented-reality wearables.
Currently, holoOPTIX® notch filters are available for purchase in two configurations (a standard 25.4-mm rigid-mounted configuration (STRATA) and a unique 70-mm × 375-mm flexible configuration (FLEX)) and for three center wavelengths (CWLs) (457, 532, and 635 nm) through Edmund Optics or the META® estore. Each filter’s CWL can be readily tuned by adjusting the angle of incidence while maintaining consistency for both p- and s-polarizations, even at large angles of incidence. Custom solutions are available upon request.