Revolutionizing Spectroscopy with Linear Variable Filters

Author: Dr. Christian Katzer, Product Manager at Optics Balzers Jena GmbH, Jena, Germany

As early as 1672, Isaac Newton published the findings of his first basic experiments on the refraction of sunlight into its component colors using a prism, arguably setting the foundation for modern spectroscopy. Today, spectroscopic methods have significantly advanced and are being employed in a vast range of applications. For instance, modern spectrometers allow studying the unique “fingerprint” of light emitted or absorbed by different materials, giving us insight into the basic composition of substances. Other examples include biomedical applications like pharmaceutical quality control or Raman spectroscopy as well as fields like food quality control and waste sorting.

The dispersive element and issues associated with it
No matter what application, elemental to all spectroscopic applications is that a dispersive element is needed to separate the light into its component wavelengths. The dispersive element, which for Newton was a prism, spreads out these wavelengths spatially, allowing each wavelength to be analyzed individually. Nowadays, instead of prisms often diffraction gratings are preferred as a dispersive element due to their higher dispersion and their ability to cover a broad wavelength range. This allows for compact and lightweight designs which is especially beneficial in applications where portability is key.

While gratings offer numerous advantages over prisms they do suffer from higher diffraction orders, especially when used in combination with broadband light sources (Fig. 1). When those higher-order spectra overlap with the first-order spectrum, it can become challenging to distinguish between genuine spectral features and those arising from interference between orders, complicating spectral analysis and requiring additional measures to cope with them.

Using order sorting filters to prevent overlapping diffraction orders
One method commonly employed to eliminate higher diffraction orders is to use optical filters which are placed directly in front of the detector. The ideal filter would allow transmitting the first order signal while efficiently suppressing any higher order with an optical density of OD4 or even higher (Fig. 2). While for spectrometers covering a narrow spectral range a simple longpass filter might suffice, broadband spectrometers often call for patterned multizone filters.

Figure 3: Classical order sorting filter consisting of individual zones, typically each having either a band- or a longpass performance.

A typical spectrometer covering a spectral range between 190 nm and 1100 nm may, for example, employ an order sorting filter consisting of an uncoated region as well as three additional coated zones (Fig. 3). Optical filtering in these zones would typically be done using a set of longpass filters with different cut-on wavelengths (Fig. 4).

Figure 4: A broadband spectrometer calls for different longpass filters at different sensor positions.

The advantages of using a linear variable filter instead
An elegant solution to surpass the aforementioned issues associated with transition zones is to use a linear variable longpass filter (LVF) instead. In contrast to a conventional multi-zone filter, LVFs provide a continuous and gradual variation in their spectral transmission across their surface, avoiding transition zones in the first place. Considering the LVF depicted in Figure 5, such a filter might transmit the whole visible spectrum on its one end, i.e. appearing “white” (left), while selectively allowing only the red spectrum to pass through on its other end (right). Since the cut-on wavelength of the linear variable longpass filter shifts continuously along the detector there is no need for using multiple filter zones with abrupt transitions in-between.

One important aspect to consider when employing LVFs instead of conventional order sorting filters is the dispersion or gradient. It describes how rapidly the spectral properties change as one moves along the length of the filter. A large gradient indicates that the spectral edge shifts rapidly over a short distance while a small gradient translates to a slower shift. As the dispersion of all LVFs provided by Materion Balzers Optics can be fully customized between 2nm/mm and more than 100nm/mm, they can fit almost all common detectors used in spectroscopic applications. Ideally, spectral range, layout and gradient of the LVF should be adapted to the sensor to position the 50% spectral edge between the first and second order signal (Fig. 6). This ensures that even under slight misalignment, no higher-order signals can be detected.